Ceramic materials and products. Application of ceramic materials Types of ceramic materials and products
Ceramics as a polycrystalline solid generally consists of three main phases:
- crystalline, consisting of grains,
- glassy (amorphous) - in the form of layers located between the grains,
- gas - in the form of pores between grains surrounded by layers of the amorphous phase.
Porcelain
Faience
Thin stone products
Majolica
Terracotta
Pottery ceramics
Fireclay ceramics
The main difference between ceramic materials is the different composition and relationship between the three phases that determine the properties of ceramic products. Structure, i.e. The structure of the ceramic body depends on the composition of the raw materials and the technology of the material. By dispersion (size) of structure elements ceramic materials There are fine ceramic and coarse ceramic. If the ceramics consists of finely dispersed grains, its fracture is uniform and the particles are indistinguishable, then such a material is classified as fine ceramic (primarily porcelain, earthenware, majolica, etc.). If large grains are observed in the structure of the ceramic, the structure itself is heterogeneous, then we have a coarse ceramic product (fireclay products, pottery ceramics, terracotta). Pottery and terracotta, made from high-quality clays without the admixture of large particles, can also be classified as fine ceramic products, which indicates the conventions of such a division.
The main types of ceramic materials: porcelain, faience, fine stone products, majolica, terracotta, pottery ceramics, fireclay ceramics.
Porcelain is a type of white ceramic with a dense conchoidal fracture, the highest achievement of ceramic technology. To make porcelain, refractory white-burning clays and kaolins, quartz and feldspars are used (the ratio of plastic and waste materials is 1:1). There are soft and hard porcelain. The distinctive features of porcelain are: whiteness, translucency, mechanical strength, hardness, thermal and chemical resistance. Scope of application: from the manufacture of dishes and technical products to the creation of unique works of art.
Faience (from the name of the Italian city of Faenza) is a type of white ceramic with a finely porous fracture. To make faience, refractory white-burning clays, quartz and various additives are used. Unlike porcelain, it has an opaque porous shard; the temperature of the waste firing exceeds the temperature of the poured one. There are soft and hard faience. Scope of application: manufacturing of tableware, technical products, decorative products, building ceramics.
Fine-stone products are a type of ceramics characterized by a white or colored sintered shard with a uniform conchoidal fracture. For the manufacture of fine-stone products, refractory and refractory clays are used, the chemical composition of which varies within fairly wide limits. Fine-stone products are distinguished between low-temperature and high-temperature sintering. Depending on the raw materials used, the degree of sintering and color of the shard, and the features of the technology, thin-stone products have different names: semi-porcelain, low-temperature porcelain, “stone goods”, etc. Thin-stone products are characterized by low water absorption (0.5...5.0%). Their area of application: manufacturing of tableware, decorative and interior ceramics.
Majolica (from the name of the island of Mallorca) is a type of ceramic with a porous, naturally colored shard from light cream to red (brick) color, covered with a transparent or dull (opaque) glaze. To make majolica, low-melting clays are used in pure form or with the addition of thinning and fluxing additives. Often majolica products are covered with a layer of white clay, engobe, which hides the natural color of the shard. The low temperature of glaze firing of majolica (960–1050? C) allows the use of a wide palette of colored glazes and enamels for decoration. Scope of application: manufacturing of dishes, facing tiles, decorative ceramics.
Terracotta (terra (Italian) – earth, cotta – burnt) is a type of ceramic, unglazed ceramic products with a porous shard. To make terracotta, high-quality low-shrinkage clays that have a uniform color and a relatively high melting point are used. Sometimes terracotta is covered with engobe. Area of application: making sculptures, tiles, tiles, etc.
Pottery ceramics are ceramic products with the natural color of fired clay, relatively high porosity, fine-grained, usually unglazed. To make this type of ceramics, local fusible pottery clays are used without the use of any other components except for small additions of quartz sand. Sometimes products are covered with a layer of engobe or glaze. Area of application: making dishes, jewelry, souvenirs.
Fireclay ceramics is a type of coarse ceramic products that has a porous, coarse-grained, often light-colored shard. Chamotte is burnt ground clay. To bind chamotte grains in chamotte products, clays are used, kneading them until a plastic mass is formed. Small sculptures, floor vases, bricks and some other types of architectural ceramics are made from fireclay masses.
All of the above ceramic materials, no matter how different they may be in the composition of raw materials and, consequently, in the final chemical composition and properties of the products, they are united by technology that determines the sequence of operations.
Schematic flow diagram for ceramic production
- Procurement of raw materials (clay, fireclay, sand, etc.)
- Preparing the molding material
- Molding
- Drying
- Burning
Ceramic are stone products obtained from mineral raw materials by molding and firing at high temperatures, as a result of which the raw materials irreversibly transform into a durable, water-resistant state.
The term "ceramics" comes from the Greek word "kerameia", which in Ancient Greece was used to describe the art of making objects from clay. Ceramics is perhaps the first artificial building material obtained by mankind. The age of ceramic brick as a building material exceeds 5000 years.
In modern construction, ceramic products are used in almost all structural elements of buildings and structures.
Based on their intended purpose, ceramic materials and products are divided into the following types:
- wall products (bricks, hollow stones and blocks);
- roofing products (tiles);
- floor elements;
- products for cladding facades (facing bricks, small-sized and other tiles, typesetting panels, architectural and artistic details);
- products for internal wall cladding (glazed tiles and shaped parts for them - cornices, corners, corbels);
- fillers for lightweight concrete (expanded clay, agloporite);
- thermal insulation products (perlite ceramics, cellular ceramics, diatomaceous earth, etc.);
- sanitary products (wash tables, bathtubs, toilets);
- floor tiles;
- road brick;
- acid-resistant products (bricks, tiles, pipes and fittings for them);
- refractories;
- products for underground communications (sewage and drainage pipes).
Based on their structure, ceramic materials are divided into porous having a water absorption by mass of more than 5%, on average 8...20% (wall, roofing and facing materials, etc.), and dense, having a water absorption by weight of less than 5% (floor tiles, road bricks, some types of pipes, etc.).
2. Raw materials
The raw materials for the production of ceramic materials are divided into plastic And non-plastic. Clays are used as plastic components, and additives are used as non-plastic components, which are introduced to regulate various properties of both the molding mass and the finished products.
CLAY MATERIALS
Clay is a sedimentary rock of a fine-earth structure, capable of forming a plastic dough when mixed with water, which, after firing, irreversibly turns into a stone-like state.
An important property of clays is their granulometric (grain) composition. Depending on the particle size, clay contains different fractions. Clay substances are scaly particles that have a size of less than 0.005 mm. Dust particles range in size from 0.005 to 0.16 mm, sand - from 0.16 to 2 mm, larger particles are called stony inclusions. The ratio between the fractions included in the composition of clays affects the basic properties of clays (will be discussed below) as raw materials for the production of ceramic materials.
Another important characteristic of clays is their chemical composition, which includes various clay minerals, the main of which is kaolinite Al2O3 2SiO2 2H2O. In addition, clays may contain related minerals: halloysite Al2O3 2SiO2 4H2O, montmorillonite Al2O3 4SiO2 n H2O, etc. The following impurities in clay may include: crystalline silica SiO2, calcium carbonates CaCO3, iron compounds Fe(OH)2, Fe2O3, alkali metal oxides (Na2O, K2O), etc.
PROPERTIES OF CLAY RAW MATERIALS
Clay mixed with a certain amount of water forms clay dough, which has a number of physical, physicochemical and chemical properties, collectively called ceramic.
Plastic- the property of clay dough to deform under load without the formation of cracks and tears and to retain its given shape after removing the load.
When dry clay is wetted, water molecules are drawn between the scaly particles of the clay substance, wedge them, form a hydration shell on the surface of the particles and cause swelling of the clay. Hydration shells act as a lubricant, facilitating the sliding of clay particles.
Plasticity depends on the content of clay substance in the clay and on the particle size. The higher the clay content and the finer the particles, the more plastic the clay. According to the degree of plasticity, clays are divided into: highly plastic, whose water requirement is more than 28%; medium-plastic, with a water requirement of 20...28%, and low-plastic with a water requirement of less than 20%.
Connectedness- the force required to separate clay particles. Clays containing a high amount of clay fractions have high cohesion.
Bonding capacity- the ability of clays in a moistened state to easily mix with non-plastic materials and, when dry, to bind them into a fairly strong product - raw material.
Air shrinkage- reduction in linear dimensions and volume of clay upon drying. During the drying process, water evaporates, the thickness of the water shells around the clay particles decreases, and individual clay particles move closer to each other. Air shrinkage is related to the plasticity of clays: the higher the plasticity, the greater the air shrinkage. Highly plastic clays have an air shrinkage of 10...15%; medium plasticity - 7...10% and low plasticity - 5...7%.
Fire shrinkage- reduction in linear dimensions and volume of clay during firing. During the firing process, the most fusible clay compounds transform into a melt, which envelops the unmelted particles, fills the gaps between them and, due to the action of the surface tension forces of the liquid phase, causes the particles to approach each other. Fire shrinkage is 2…6%.
Complete shrinkage- the sum of air and fire shrinkage.
NON-PLASTIC MATERIALS
As noted above, these materials are introduced as additives to regulate the properties of both clay raw materials and finished products.
Leaning Supplements- are introduced to reduce the plasticity of clays and, as a result, to reduce air shrinkage. Fireclay, dehydrated clay, thermal power plant ash, crushed granulated slag, and natural sand are used as waste additives.
Chamotte is clay that is pre-fired and crushed to the required size (less than 2 mm). Dehydrated clay is clay fired at a temperature of 500…600 °C. At this temperature, chemically bound water is removed from clay minerals and the clay irreversibly loses its plasticity properties.
Plasticizing additives- are introduced to improve the plasticity of clays. For these purposes, highly plastic clays, surfactants, and electrolytes are used.
Burn-out additives- are introduced into the molding mass in order to obtain highly porous products: sawdust, ground coal, peat, husks, etc. These additives are also leaning.
Plavni- are introduced to reduce the sintering temperature and, as a result, save fuel and energy resources. Sintering means the appearance of a partial melt of the raw material mixture during the firing process. Feldspars, dolomite, magnesite, etc. are used as fluxes.
To impart increased resistance to external influences, water resistance and a certain decorative appearance, the surface of some ceramic products is coated with glaze or engobe.
A glassy layer of glaze applied to the surface of the ceramic material is secured by firing. Glazes can be transparent or opaque in various colors. The main raw materials of the glaze are quartz sand, kaolin, feldspar, salts of alkali and alkaline earth metals, various oxides, etc.
Engobe is made from white or colored clay and applied in a thin layer to the surface of an unfired product. Unlike glaze, engobe does not melt during firing, so the surface is matte. In terms of its properties, the engobe should be close to the main shard.
3. General scheme for the production of ceramic products
Ceramic materials and products produced by industry have a variety of sizes, shapes, physical and mechanical properties and various purposes, but the main stages of the technological process of their production are approximately the same and consist of the extraction of raw materials, their transportation to the plant, preparation of the raw material mass, and molding of the product (raw), drying and firing.
EXTRACTION AND DELIVERY OF CLAY
Clay for the production of ceramic materials and products is mined in quarries, usually located in close proximity to the plant. For mining, single- or multi-bucket excavators are used, and it is also possible to use hydraulic mechanization equipment. Clay is delivered to the plant along rail tracks in trolleys with tipping bodies, dump trucks, conveyor belts, cable car trolleys and other types of transport.
PREPARATION OF RAW MASS
Clay extracted from a quarry and delivered to a factory in its natural state is usually unsuitable for molding products. It is necessary to destroy the natural structure of the clay, remove harmful impurities from it, crush or remove large inclusions, mix the clay with additives, and also moisten it to obtain a moldable mass. For this purpose, various mechanisms are used: rollers, disintegrators, runners, clay cutters, clay grinders, mixers, etc. These mechanisms will be discussed below.
Clay is processed using semi-dry, plastic and wet methods. The choice of one method or another depends on the properties of the raw materials, the composition of the ceramic masses and the method of molding the products, as well as their size and purpose.
With the semi-dry (dry) method raw materials are dried, crushed, ground and thoroughly mixed. Clay is usually dried in drying drums, crushed and ground in dry grinders, disintegrators or ball mills, and mixed in paddle mixers. The moisture content of the press powder is 8…12% (4…6%). Moisten the press powder with water or steam.
The semi-dry method is used in the production of semi-dry pressing building bricks, floor tiles, facing tiles, etc.
With the plastic method raw materials are mixed at natural humidity or with the addition of water to obtain clay dough with a moisture content of 18...25%. Rollers and runners of various types are used for grinding and processing raw materials, and clay mixers are used for mixing.
The plastic method of preparing the raw material mixture is widely used in the production of plastic molded ceramic bricks, ceramic stones, tiles, pipes and other types of building ceramics.
With the wet (slip) method raw materials are first crushed into powder and then thoroughly mixed in the presence of a large amount (more than 40%) of water, obtaining a homogeneous fluid mass (slip). This method is used in the production of porcelain and earthenware products, facing tiles, etc.
FORMATION OF PRODUCTS
Ceramic products are formed in various ways: plastic, semi-dry, dry and wet. The choice of molding method depends on the type of product, as well as the composition and physical and mechanical properties of the raw materials.
Plastic molding method is the most common in the production of ordinary and hollow bricks, ceramic stones and blocks for various purposes, tiles, facing slabs and other products. With this molding method, the prepared clay mass with a moisture content of 18...25% is sent to the receiving hopper of a belt press. Using an auger, the mass
it is additionally mixed, compacted and extruded in the form of a bar through the outlet of a press equipped with a replaceable mouthpiece. By changing the mouthpiece, you can get a beam of various shapes and sizes. So, for example, when molding a brick, it has a rectangular cross-section. The timber continuously coming out of the press is cut into separate parts according to the dimensions of the manufactured products by an automatic cutting device. Modern belt presses are equipped with vacuum chambers in which air is partially removed from the clay mass. Vacuuming the mass increases its plasticity and reduces molding moisture, reduces the drying time of the raw material and at the same time increases its strength.
Semi-dry molding method has become widespread in modern factories in the production of facing tiles, floor tiles and other thin-walled ceramic products. This method can be used to produce bricks and other products from low-plasticity clays, which expands the raw material base for the production of building ceramics products. In addition, a significant advantage of the semi-dry molding method compared to the plastic one is the use of clay mass with lower humidity (8...12%), which significantly reduces or even eliminates the drying of the raw material.
With the semi-dry method, each product is molded separately on high-performance presses of various designs, providing double-sided pressing into clay powder forms under a pressure of more than 15 MPa.
Semi-dry pressed raw material has a clear shape, precise dimensions, strong corners and edges. Its strength is quite sufficient for subsequent loading and transportation for drying and firing.
Dry molding method used mainly for the manufacture of dense ceramic products, for example, floor tiles, road bricks. The raw material for pressing products is clay powder with a moisture content of 4 to 6%. The molded raw material does not require drying, which saves fuel and energy resources.
Wet molding used for the manufacture of sanitary faience, mosaic tiles, etc. With this method, a clay mass with a moisture content of more than 40% is poured into special porous molds.
DRYING PRODUCTS
Molded products (raw material) must be dried to reduce their moisture content to 8...10%. Drying increases the strength of the raw material, and also prevents cracking and deformation during the firing process. Drying can be natural(in drying sheds) and artificial(in special dryers).
Natural drying does not require fuel, but lasts a very long time (10...15 days) and depends on the temperature and humidity of the surrounding air. In addition, natural drying requires large areas. Currently, large factories, as a rule, artificially dry raw materials in batch or continuous dryers.
Batch dryers are separate chambers in which raw material is placed on rack shelves. The raw material is supplied to the chambers on carts. In chamber dryers, all operations for loading, drying and unloading raw materials are repeated at certain intervals.
Continuous dryers are tunnels in which raw material, laid on trolleys, gradually passes through different temperature and humidity zones and is dried.
The raw material is dried in chamber and tunnel dryers according to the regime chosen for this type of product, taking into account the raw materials used. Flue gases from kilns, as well as gases produced in special furnaces, are used as a coolant in dryers. Thin ceramics are dried with hot air from heaters. The duration of artificial drying of raw materials ranges from one to three days.
FIRMING OF PRODUCTS
Firing is the final stage of the technological process for the production of ceramic products. The firing process can be divided into three periods: heating of the raw material, firing itself, and cooling of the fired products.
During high-temperature firing, clay undergoes complex physical and chemical changes.
With a gradual rise in temperature to 100...120 °C, the remaining free moisture is removed from the clay and the ceramic mass becomes non-plastic, but if water is added, the plastic properties of the mass are restored. With an increase in temperature to 500...700 °C, organic impurities burn out and chemically bound water is removed from clay minerals, while the ceramic mass irrevocably loses its plasticity property. At a temperature of 700...900 °C, decomposition of anhydrous clay minerals occurs and an amorphous mixture of alumina Al2O3 and silica SiO2 is formed. With a further increase in temperature to 1000...1300 °C, reactions occur in the solid phase and artificial minerals are formed, for example sillimanite (Al2O3SiO2) and mullite (3Al2O32SiO2). At the same time, the most fusible compounds of the ceramic mass pass into the melt, creating a certain amount of the liquid phase. The melt envelops the unmelted particles, fills the voids between them and, having the force of surface tension, pulls the particles together. After cooling, a hard stone-like shard is formed.
The maximum firing temperature for ceramic products depends on the composition of the clays. Firing of products made from low-melting clays is carried out at a temperature of 900...1000 °C, from refractory and refractory clays - at a temperature of 1200...1400 °C.
Ceramic products are fired in batch or continuous kilns using solid (coal), liquid (fuel oil) or gaseous fuel.
Batch ovens are chambers into which molded and dried products are loaded on racks, after which a gradual rise in temperature begins, which is brought to the required maximum, then the products are held at maximum temperature and gradually decreased.
Continuous furnaces have different designs. Ring kilns have an ellipsoidal firing channel covered by a semicircular vault. The fired products are loaded into the channel and remain motionless, and the temperature zones move relative to the fired material. Tunnel kilns have a straight channel along which trolleys with products laid on them move slowly, which successively pass through the heating, firing and cooling zones.
In slot kilns, ceramic products placed in one row in height move slowly in the firing channel along a roller or other conveyor. In such furnaces, uniform firing is ensured, its duration is reduced and fuel consumption is reduced.
PROPERTIES OF BASIC CERAMIC MATERIALS
Wall ceramic products are intended for masonry and cladding of load-bearing and self-supporting walls and other elements of buildings and structures, as well as for the manufacture of wall panels and blocks. They are made in the form of a regular parallelepiped. Depending on their size, they are divided into the types indicated in table. 1. Brick is made solid and hollow, stone - only hollow. Voids in products can be through or non-through, they can be located perpendicular (vertical) or parallel to the bed (horizontal). According to the molding method, wall ceramic products are divided into products obtained by plastic molding and semi-dry pressing. In accordance with regulatory documents, wall products are divided into ordinary and front. Ordinary products are designed to ensure the operational characteristics of the masonry; facing products, in addition to ensuring the operational characteristics of the masonry, perform the functions of a decorative material.
Table 1
Nomenclature and nominal dimensions of wall products
| Type of products | Type designation | Nominal dimensions, mm | Size designation | ||
| Length | Width | Thickness | |||
| Normal format brick (single) | KO | 250 | 120 | 65 | 1 SF |
| Eurobrick | KE | 250 | 85 | 65 | 0.7 NF |
| Thickened brick | KU | 250 | 120 | 88 | 1.4 NF |
| Single modular brick | KM | 288 | 138 | 65 | 1.3 NF |
| Thickened brick with horizontal voids | KUG | 250 | 120 | 88 | 1.4 NF |
| Stone | TO | 250 | 120 | 140 | 2.1 NF |
| 288 | 288 | 88 | 3.7 NF | ||
| 288 | 138 | 140 | 2.9 NF | ||
| 288 | 138 | 88 | 1.8 NF | ||
| Stone | TO | 250 | 250 | 140 | 4.5 NF |
| 250 | 180 | 140 | 3.2 NF | ||
| Large format stone | QC | 510 | 250 | 219 | 14.3 NF |
| 398 | 250 | 219 | 11.2 NF | ||
| 380 | 250 | 219 | 10.7 NF | ||
| 380 | 255 | 188 | 9.3 NF | ||
| 380 | 250 | 140 | 6.8 NF | ||
| 380 | 180 | 140 | 4.9 NF | ||
| 250 | 250 | 188 | 6.0 NF | ||
| Stone with horizontal voids | KG | 250 | 200 | 70 | 1.8 NF |
Based on strength, bricks are divided into grades M100, M125, M150, M175, M200, M250, M300; large-format stones - M35, M50, M75, M100, M125, M150, M175, M200, M250, M300; brick and stone with horizontal voids - M25, M35, M50, M75, M100.
In terms of frost resistance, brick is produced in four grades: F15, F25, F35, F50.
Based on average density, products are divided into classes 0.8; 1.0; 1.2; 1.4; 2.0, which must correspond to the values given in table. 2.
table 2
Classes of wall products by average density
Depending on thermal conductivity and average density class, wall products are divided into groups given in table. 3.
Table 3
Product groups by thermal characteristics
TO roofing ceramic materials include tiles. It must have high durability, water resistance, resistance to various atmospheric factors and aesthetics, have a uniform fracture structure and a dry fracture strength of at least 7 MPa, a weight of 1 m 2 of roofing of no more than 45 kg, and also have frost resistance of not less than 25 cycles of alternating freezing and thawing, water absorption of no more than 10% by weight.
Facade ceramic tiles used for cladding facades and plinths of buildings, external surfaces of reinforced concrete wall panels, and underground passages.
The main indicators characterizing the quality of facade tiles are frost resistance, water absorption, accuracy of geometric dimensions and appearance. Frost resistance of ordinary tiles with a thickness of more than 9 mm must be at least 35 cycles, with a thickness of less than 7 mm - at least 40 cycles with a water absorption of up to 12%. For special-purpose tiles, frost resistance must exceed 50 cycles, and water absorption is allowed no more than 5%.
Floor tiles can be unglazed and glazed, single- and multi-colored, with a smooth, rough (embossed) or grooved front surface. The shape of the tiles is square, rectangular, triangular, four-, five-, six- and octagonal, figured. Their water absorption should be no more than 3.8...5%, abrasion no more than 0.07...0.06 g/cm 2.
Tiles for interior cladding Designed for cladding internal surfaces of walls and partitions. They differ in shape, texture and type of material forming the textured layer (50 types). Ceramic tiles must have a water absorption of no more than 16%, a flexural strength of at least 15 MPa, and the glaze coating must have a heat resistance of at least 150 ° C and a hardness of at least 5 on the Mohs scale.
Ceramics are polycrystalline materials obtained by sintering natural clays and their mixtures with mineral additives, as well as metal oxides and other refractory compounds.Ceramics have been known to mankind since ancient times. Thus, during excavations in Mesopotamia, ceramic products made about 15 thousand years BC were found. In Egypt, starting from the 5th millennium BC. e., ceramics become an industrial product.
Ceramics were also widespread in our homeland. A significant amount of ceramic products were discovered during excavations of ancient settlements in the Kyiv region, dating back to the period of the formation of Kievan Rus.
In the XVI-XVIII centuries. The development of ceramics production in Rus' is intensified, a special Stone Decree is issued, which regulates the requirements for it. In the 19th century The ceramic industry in Russia continues to develop intensively: large factories are being built in Moscow, St. Petersburg, Kharkov, Kyiv, and Yekaterinoslav.
After the Great October Socialist Revolution in 1919, the State Scientific Research Ceramic Institute (GIKI) was created in Leningrad. In the pre-war years, Soviet specialists developed designs for continuous tunnel kilns and dryers, completed the creation of a scientific base for the ceramic and refractory industry, and subsequently created a number of research institutes.
The ceramic industry is still developing rapidly. Particular attention is paid to accelerating the development and implementation of high-speed firing of ceramic products and technical re-equipment of production. The production of ceramic colored wall tiles and large-size floor tiles is increasing.
At construction ceramics factories, new conveyor lines with increased capacity (up to 1 million m2 per year) are being created for the production of tiles with full automation of the entire production process, right down to sorting and packaging.
Workers in the building materials industry have been given a large and responsible task - to increase, first of all, the volume of production of building materials by improving the use of existing production capacities and technical re-equipment of existing enterprises.
The ceramic industry of the Ukrainian SSR, which has significant reserves of clay raw materials, will receive further development. The main direction of its development is the reconstruction and expansion of existing enterprises, the introduction of high-performance technological equipment.
Cladding ceramics include materials for external cladding (facing bricks and facing stones, facade slabs and tiles, terracotta), for internal cladding of buildings (slabs and tiles), for roads and floors (clinker, slabs and tiles).
Products intended for artistic decoration of buildings, interiors, transitions belong to architectural and artistic ceramics, the peculiarity of which is a wide variety of unglazed (terracotta), glazed, engobed and decorated products with complex profiles and large sizes.
2.4.2. Product range
Brick and ceramic face stones Depending on the purpose, they are ordinary (for smooth walls) and profiled (for cornices, corbels, etc.). These products must have a given configuration and at least two adjacent faces (ordinary brick). For profiled products, the front sides are, in addition to the profiled one, the upper and lower sides adjacent to it for 73 times the length. Dimensions of brick are 250x120x65 mm, ceramic facing stones are 250x120x140 mm.According to GOST 7484-78, bricks are produced in grades 300, 250, 200, 150, 125, 100 and 75. The bending strength limits are respectively equal to: 4; 3.6; 3.4; 2.8; 2.5; 2.2; 1.8 MPa, water absorption - from 6 to 14% and for white-burning clays - no more than 12%. In terms of frost resistance, the brick must satisfy the grades Mrz 25, Mrz 35 and Mrz 50.
Brick and facing stones designed for cladding buildings and have dimensions 250x120x65; 250x120x88; 250x138x120 mm, brick grades - 300, 250, 200, 150, 125, 100 and 75. If it is necessary to obtain colored products, various additives are used to color the entire mass of products during their production or a thin layer of engobe or glaze is applied to the surfaces of the pokes and spoons. Surfaces are textured by knurling using rollers, combs, and shotcrete.
Facade slabs They are produced in ordinary, corner and lintel types. Based on the type of front surface, they are divided into flat, rusticated and profiled, and based on their design - into solid and hollow. During production, they can be painted in various colors. According to GOST 13996-84, slabs are produced in the following sizes: 50x50x(2-4); 25x25x(2-4); 20x20x(2-4); 48x48x4; 20x20x4; (90-120)x(40-60)x(5-6) mm. Water absorption of products should not be more than 14%, and for tiles made of white-burning clays - no more than 10%. Frost resistance - at least 35 cycles. Plates of plastic molding are characterized by a compressive strength of at least 14.7 MPa, and for semi-dry - at least 9.9 MPa. The bending strength is no less than 2.74 and 1.57 MPa, respectively.
Terracotta products- These are plain, unglazed, naturally colored ceramic products. Terracotta includes all unglazed ceramic products that have artistic and decorative properties.
Faience glazed tiles used for interior cladding. They are made from earthenware masses and covered on the front side with transparent or solid glaze.
The shape of the tiles is produced square, having dimensions of 150x150x5 and 100x100x5 mm, rectangular - 75x150x5 mm and shaped, which are divided into corner, cornice and plinth.
According to GOST 6141-82, tiles are characterized by a compressive strength of 98-127.4 MPa, and an impact bending strength of 0.16-0.19 MPa; water absorption should not be more than 16%. Glazed tiles must be gas and waterproof.
Floor tiles, according to GOST 6787-80, are available in the following sizes, mm 50x50x(10-15); 100x100x10; 150x150x10; 150x150x13; 150x74x13; 100x115x10 (hexagonal); 150X50X80X13 (octagonal), etc. The compressive strength of the tile is 180-250 MPa, water absorption - no more than 5%, Mohs hardness - 7-8.
In accordance with GOST 6787-80, tiles with dimensions 48x48x(4-6) and 48x22x(4-6) mm can be glued to paper and produced in the form of carpets.
2.4.3. Characteristics of raw materials
The raw materials for the production of finishing ceramic products are clays and additives.Clays- sedimentary, cohesive, unconsolidated rocks consisting primarily of clay minerals. In terms of fractional composition, these are fine powders containing more than half of the particles with a size of less than 0.01 mm, including at least 25% of particles with a size of less than 0.001 mm.
For the production of coarse building ceramics, including facing ceramics, an important feature is the melting temperature of clays, according to which they are divided into fusible (up to 1350°C), refractory (up to 1580°C) and refractory (above 1580°C).
Most often, in the production of building finishing ceramics, fusible clays are used, which have a fairly varied mineralogical composition and contain no more than 18% alumina and up to 80% silica.
The oxides that make up clays have different effects on the production process and the final properties of the product.
Silicon oxide SiO 2 can be present in both free and bound states. With a significant content of free silica in the form of quartz, a shard with increased porosity and low mechanical strength is formed.
Aluminum oxide Al 2 O 3 with an increased amount in clay leads to an increase in the firing temperature and the interval between the temperatures of the onset of sintering and melting. Products with low alumina content have low strength.
Iron oxides Fe 2 O 3 +FeO are fluxes; they reduce the temperature range of clay sintering. Depending on their content in the clay, after firing the products range from light cream to cherry red in color.
Calcium oxide CaO lowers the melting point of clay, reduces the sintering temperature range, and whitens the shard.
Magnesium oxide MgO acts similarly to calcium oxide, but its effect on the clay sintering interval is less.
Alkali metal oxides significantly reduce the sintering temperature, promote bleaching, increase shrinkage, compaction and hardening of the shard.
The presence of sulfates in clays causes the appearance of efflorescence on the surface of products after firing. Clays have plasticity, i.e. the ability to maintain the shape taken by a clay product when wet. According to this criterion, clays are divided into highly plastic, medium plastic, moderately plastic, low plastic and non-plastic.
Additional materials in the production of ceramics they are used to regulate the properties of both the raw material mass and the product. These include: surfactants and highly plastic clay, which improve the molding properties of the mass; thermal power plant ashes, fuel and metallurgical slags, coal, improving firing conditions; chamotte, sand, dehydrated clay, sawdust, facilitating the drying process; coal, sawdust, which are burnable additives and reduce the density of the product; broken glass, mustard cinders, iron ore, which increase the strength and frost resistance of products; dyes, liquid glass, table salt, which improve the color of products, prevent efflorescence, and neutralize limescale inclusions.
Leaning additives should not have large particles (more than 2 mm), while the content of particles up to 0.25 mm in size should not exceed 20%.
Glazes- suspensions of low-melting charge, fixed to the product by firing at high temperatures. According to the sintering temperature, they are divided into refractory (1250-1400°C) and low-melting (900-1250°C), according to the manufacturing method - into raw (or feldspathic), applied to products in their raw form, and fritted, subjected to fritting, i.e. e. preliminary fusion of the charge.
Raw glazes are refractory and are used mainly for porcelain production. Fritted ones are fusible and contain, in addition to feldspar and quartz, chalk, marble, dolomite, soda, potash, borax, barium and lead compounds, and sometimes compounds of strontium, tin, lithium, zinc, and bismuth. Since some components of glazes are toxic and soluble in water, the mixture is partially or completely pre-fused and a glassy alloy (frit) is obtained, which is the basis of the glaze.
Grind the glaze in a mill until the residue on a sieve of 10,000 holes/cm 2 is no more than 0.3% and prepare a suspension. The suspension of the prepared glaze should spread in an even layer over the surface of the product, not peel off from it during subsequent cooling or heating, and not form local swellings or a network of cracks.
Before glazing, some products are pre-fired to fix the shape of the shard.
The main methods of glazing are immersing products in a glaze suspension, pouring a suspension on products using special machines, spraying the suspension with a spray bottle, applying with a brush, dusting products with dry glazed powder.
After glazing, the pieces are fired again at the melting temperature of the glaze. The resulting glaze film interacts with the shard of the product, creating an intermediate layer of a smooth transition from the sintered shard to the glassy glaze cover.
Glazes are colorless, colored, transparent and opaque (dull).
Engob- A white or colored clay coating on pottery that disguises the pottery's rough texture or color. Products can be engobed using a plastic method, applying a textured layer simultaneously with molding the products on belt presses, as well as spraying, dipping, watering and coating. In the production of two-layer facade ceramics, the textured layer is applied plastically.
Decoration of products- a technical operation consisting of applying decor in order to improve the aesthetic qualities of the product.
There are the following types of decoration of products: relief, monochromatic, marble, as well as stamping, printing (seriography), decalcomania, applying decor in an electrostatic field.
Relief decoration is formed when a relief pattern is applied during pressing of products.
Colored plain products are obtained by ordinary glazing, and marble-like tiles are obtained by sprinkling various glazes, which, when mixed on the shard, give a marble-like pattern.
The finishing of stamping is carried out with a roller with a relief pattern on it, which is rolled over a tile with freshly applied glaze. During this operation, part of the glaze is removed with a roller and a contrasting pattern is formed. The stamp method allows paint to be applied to fired glazed tiles, which are then re-fired.
Printing (seriography) involves producing single-color or multi-color designs. It includes the following basic technological operations: obtaining a photograph of a design (transparency), making meshes (stencils), preparing a binder and mastics, applying a design to tiles using stencils, glazing and firing. Transparencies corresponding to each color element are obtained from a given drawing. Then, using a photomechanical method, stencil meshes coated with a photosensitive emulsion are made on nylon or silk meshes. The slide is photocopied using a contact method using a special machine onto a stencil mesh, which is processed to fix the picture with special compounds. In this way, one grid is prepared for a single-color pattern, and several for multi-color patterns, for each color separately. Then, by pressing the paint through each stencil mesh, the design is applied to the tile, which is then fired.
The electrostatic field allows one-color paint to be applied to the tiles. This creates an electrostatic voltage of 1-10 kV.
Decalcomania (transferring a design from paper to a ceramic product) allows you to obtain colored tiles with designs of any complexity. Drawings are applied to paper tape in the form of a roll using special glue. Then they are pressed against a hot plate with a temperature of 125-145°C. At this temperature, the glue softens and the design is transferred to the tile.
2.4.4. Technology Basics
There are several methods for producing veneer ceramics. At the same time, as already noted, the main technological stages are the preparation of raw materials, molding, drying of raw materials and firing of products. The preparation of materials and the molding method depend to the greatest extent on the properties of the raw materials, the type of products and the volume of production. In subsequent operations (drying and firing) the differences are insignificant.The method of preparing raw materials can be plastic, semi-dry and slip.
Plastic method is most widely used; highly plastic, fatty clays are processed with its help.
In Fig. Figure 2.4 shows a basic technological diagram of the plastic method for preparing mass when introducing burnable additives (sawdust and coal preparation waste) with subsequent operations - plastic molding, drying and firing of products. The main technological stages are: coarse grinding of clay with simultaneous separation of rocky inclusions on coarse grinding rollers; mixing clay with sawdust, dried coal preparation waste and bringing the mass to molding moisture content (18-25%); fine grinding of the mass on fine grinding rollers; curing the mass with subsequent molding of products; drying and firing. The need to dry coal enrichment waste is determined by its high humidity, especially in winter.
Semi-dry method preparation of raw materials is used for clay raw materials of low plasticity and humidity. In Fig. 2.5 shows a basic technological diagram of semi-dry processing of mass, which involves semi-dry pressing and firing of products. The main technological operations are coarse grinding of raw materials, drying in a drying drum, fine grinding in disintegrators, a rotary mill or on runners. Fine grinding of clay raw materials can be combined with drying in a shaft mill. After grinding, the crushed mass is moistened to 12% and sent for semi-dry pressing, followed by firing.
By using a less wet molding mass with the semi-dry method, compared to the plastic method, a significant economic effect is achieved: metal consumption is almost 3 times, and labor intensity is 26-30% less. Drying of raw materials is excluded. The duration of production of products is also reduced.
Slip method It is most advisable to use the preparation of raw materials for clays that have high humidity or soak well in water and contain rocky inclusions that must be removed.
In Fig. 2.6 shows a basic technological scheme for preparing raw clay using the slip method. The main technological stages are: coarse grinding of clay with simultaneous removal of rocky inclusions; dissolving clay in clay mixers or grinding in a ball mill to obtain slip with a moisture content of 68-95% and a density of 1.12-1.18 g/cm 3 ; removal of large particles using sieves and obtaining a suspension characterized by a residue on a sieve of 10,000 holes/cm 2 of no more than 2%. The resulting slip is dewatered in a tower spray dryer and sent to a mixer, where it is moistened to a moisture content that ensures plastic or semi-dry pressing. When molding products using slip casting, the clay suspension may not be dehydrated.
In table 2.10 shows comparative cost calculations (according to the Keramik plant, Kiev) for tiles using semi-dry and slip methods for preparing raw materials. Due to the different thicknesses of tiles produced by semi-dry and slip methods, costs should be compared per 1 m 3 of products. From the above data it follows that the slip method is characterized by high costs of labor, energy and fuel.
Coarse crushing of clay is carried out on destoning rollers or destoning disintegrator rollers. If there are no rocky inclusions or more thorough coarse grinding is required, then planers, disintegrators, rotary crushers and runners can be used for this.
The destoning rollers have one smooth roller and the other one with a helical spiral. The principle of their operation is that when the rolls operate, rocky inclusions fall into the grooves of the helical spiral and are removed from the rolls.
Stone separating disintegrator rollers have one large smooth roller with a diameter of 900 mm, rotating at a frequency of up to 1 s -1, and a smaller roller (diameter 600 mm), rotating at a frequency of 10 s -1. There are 6-8 steel bars on the surface of the smaller roll. With their help, rocky inclusions are either thrown out of the mass or crushed.
Clay can be dried in drum dryers, spray dryers (Figure 2.7) or shaft mills.
The principle of operation of a tower spray dryer is that the clay slurry flows through a pipeline onto a disk atomizer, which is a rapidly rotating disk. The atomized fine clay suspension is blown by hot flue gases coming from the bottom of the dryer. During the passage from the top of the dryer to its bottom, the clay is completely dried and precipitated. The precipitated dried clay is transported for storage. Flue gases undergo a purification system to remove tiny particles of clay and are released into the atmosphere.
Fine grinding of raw materials is usually carried out on smooth fine grinding rollers. The best grinding performance is achieved by sequential grinding through 2-3 pairs of rollers.
It is advisable to moisten the clay mass twice: once at the beginning of processing, the second time before molding.
To mix, homogenize and moisten masses, single-shaft and twin-shaft mixers are used, in which the material is moved using blades located on the shaft. The productivity of the mixers is 18-35 m 3 /h.
To improve the physical and mechanical properties of both the raw materials themselves and ceramic products by 18-25%, the clay must be aged.
The molding of ceramic masses is carried out using the plastic method, semi-dry pressing or casting.
Plastic molding of masses is carried out under the condition that the cohesion of the clay mass is greater than its adhesion to the surface of the molding equipment. This is ensured by the use of highly plastic clays or the use of plasticizing additives.
For plastic molding, belt presses are used - non-vacuum and vacuum with a capacity of 5...7 thousand pcs./h, providing a specific pressing pressure of up to 1.6 MPa. When the mass is evacuated in a belt press, air is removed from it, as a result of which the density of the raw material increases by 6-8%, and the molding moisture content decreases by 2-3%. This allows you to reduce the drying time of products, increase the strength of baked brick by almost 2 times and reduce its water absorption by 10-15%.
On the SMK-168 belt press (Fig. 2.8), using a screw mechanism, the mass is fed, compacted and pressed through the head and mouthpiece, which gives shape and size to the clay beam, which is then cut into raw brick.
During semi-dry pressing, lean clays and significant amounts of ash and slag are added. During semi-dry pressing of raw materials, complex physical and chemical processes occur.
At the initial stage of pressing, particles move, weak film contacts between them are destroyed, the mass is compacted, air is partially removed, and the number of these contacts increases.
A further increase in pressing pressure increases the density of the mass, and plastic, elastic and irreversible deformations of the particles develop. Molding water envelops the particles with a thin film and serves as a structure-forming element. As a result of compaction of the mass, air is pinched. The trapped air, together with deformed elongated particles and excess moisture, elastically counteracts the growing pressure. At the final stage of pressing, the most dense raw brick with film-based, non-waterproof contacts is formed. After the pressure is removed, the volume of the pressed material partially increases under the influence of reversible elastic deformation.
Trapped air and excess moisture in the molded mass are one of the reasons for delamination of products, and therefore there is a need to use presses of increased power. In addition, to avoid entrapment of air and excess moisture, they increase the pressing time, implement double-sided pressure with multi-stage action, correctly select the granulometry of the mass, introduce leaning additives, and use the method of vacuuming the powder.
The duration of pressing products is on average 0.5-3.5 s.
The parameters of the impact load during pressing depend on the type of clay. For plastic clays, the pressure is 7.35-9.8 MPa, for heavy loams - 11.76-14.76, for loams, loess and loess-like loams - 12.74-14.7 MPa.
The productivity of semi-dry pressing presses is from 2 to 5 thousand pieces/hour.
The quality of pressed products is determined not only by the pressing parameters, but also by the properties of the powders.
Press powders must have a certain granulometry that ensures a minimum air content in the mixture and the required flowability. With an increased content of large fractions (up to 1.5 mm), a free-flowing powder is obtained that is evenly compacted during pressing, but requires increased pressure when molding the product. The content of francium less than 0.06 mm in an amount of 10% in relation to particles measuring 0.5-0.75 mm increases the mobility of the mass. With a significant content of fine fractions, air is slowly removed during pressing, the viscosity of the mass increases, and uneven compaction occurs.
Casting method(slip casting) is based on the property of clays to form coagulation structures with thixotropic properties in the form of suspensions capable of delivering a dispersion medium to the capillaries of the mold with the formation of a solid layer on its surface. The rate of increase in the wall thickness of the product depends on the rate of absorption of the liquid phase of the slip by the mold, the granulometric composition of the solid phase, the ratio of solid and liquid phases, as well as the rate of water diffusion through the layer of the resulting product.
The casting method is used to produce small ceramic tiles and corrosion-resistant products of complex shapes.
Products formed by plastic or casting methods are dried and then fired. Semi-dry pressed products are usually not dried, but directly sent for firing.
Drying raw material and firing ceramic products. Excess moisture in the material during firing can lead to a decrease in the physical and mechanical characteristics of the shard, cracking, i.e., to defects, and therefore the firing of products is usually preceded by drying them.
Effective drying modes should ensure the minimum duration of the operation, as well as minimum coolant consumption.
Clean air, flue gases, or a mixture of heated air and flue gases are used as a coolant with a certain humidity, which regulates the rate of evaporation of moisture from the material.
During the drying process, three main periods can be distinguished (Fig. 2.9): heating, constant and decreasing drying rates.
During heating, the maximum temperature rise is determined by the moisture content of the coolant. Such a coolant is characterized by the dry-bulb temperature, i.e., the temperature to which it is heated, and the wet-bulb temperature, i.e., the temperature at which the coolant becomes saturated with moisture. Therefore, the maximum temperature of the material at the initial stage of heating is determined by the temperature of the wet thermometer placed in the coolant, i.e., the dew point.
The difference between the dry and wet bulb temperatures determines the drying intensity. The greater this difference, the faster the drying goes and the more stringent the mode can be set. The smaller the temperature difference, the slower the drying proceeds and the softer the mode should be. The drying speed does not depend on the amount of water in the product, but depends on the difference in the partial pressures of water vapor on the surface of the material and in the environment. In this regard, the speed increases abruptly from zero to a sharp break in the drying curve, which means the end of its first period (curve 2, Fig. 2.9).
The constant drying rate is numerically equal to the rate of moisture evaporation from the surface to which it comes from the deep parts of the molded products. Thus, the drying rate in the second period is determined by the rate of water diffusion in the material. The surface temperature of the material practically does not increase (curve 3, Fig. 2.9).
As a result of drying the material and, accordingly, reducing its moisture content (curve 1, Fig. 2.9), the rate of diffusion of water from the deep layers to the surface of the material decreases. Drying speed drops. This moment on the drying curves is fixed by a fracture at point K. At the same moment, the second drying period ends and the third begins. The humidity of the material at point K is called critical for the given parameters of the coolant.
The period of decreasing drying rate can be divided into three phases:
- 1. Evaporating moisture reaches the surface of the product only from small pores. The moisture evaporation surface decreases. The temperature of the material becomes higher than the wet-bulb temperature, but lower than the dry-bulb temperature.
- 2 Equilibrium humidity is established on the surface of the product, corresponding to the parameters of the coolant. The moisture evaporation surface continues to decrease and move deeper into the material. The temperature of the material increases.
- 3. The temperature of the dried material becomes equal to the dry bulb temperature. The drying speed drops to zero. An equilibrium moisture content is established in the material between the moisture content of the material and the parameters of the coolant.
The drying duration is determined by the initial and final moisture content of the material, its shape, size, coolant parameters, etc.
It is believed that drying speeds up to 4 kg/(m 2 h) are safe. Drying time can be reduced by introducing lean additives into the mass, increasing the temperature and speed of the coolant, and drying the semi-finished product with large volumes of coolant.
Drying is carried out in periodic and continuous drying units. Its duration is determined by their design, coolant parameters and properties of the dried product.
In periodic dryers, the parameters of the coolant change over time; in continuous dryers, these parameters do not change over time, but change along its length. Based on the nature of the coolant movement, dryers are divided into recirculating and non-recirculating, and depending on their design, the material can be stationary or moving.
According to their design features, dryers can be chamber, tunnel, single- and two-tier, conveyor, radiation and slot. Efficiencies of some of them, %:
- Chamber dryer using waste heat or flue gases from furnaces - 15-30
- Chamber dryer with steam heating and recirculation - 37-51
- Tunnel dryer - 23-43
Burning. The purpose of firing is to acquire the product's water resistance and the required physical and mechanical properties.
During firing, complex physicochemical processes occur, the essence of which is the transition of reversible coagulation structures with film non-waterproof contacts or pseudo-condensation irreversible structures with point non-waterproof contacts into condensation-crystallization irreversible structures with hard phase water-resistant sintering contacts.
The firing process can be divided into four periods: 1) drying (up to 200°C); 2) heating or smoking (700-800°C); 3) actual firing or boiling (900-1050°C); 4) cooling (cooling to 40°C).
During the first period, complete drying of the products occurs and the formation of pseudo-condensation non-waterproof structures in which the substance is in state 5 ().
During the second period, organic impurities and additives burn out, chemically bound water is removed from the clay (at 500-600 °C), which is accompanied by amorphization of the substance, and limestone begins to decompose (at 700-800 °C). The porosity of products increases by the end of the second period, the substance passes into state 6 ().
The third period is associated with the beginning of crystallization of the substance amorphized during the second period, which is accompanied by an increase in its density. At the same time, the processes of crystallization of anhydrous formations develop. They may be accompanied by the formation of a melt rich in oxides of calcium, iron, and alkali metals. An increase in the density of the substance leads to intense shrinkage, a decrease in the viscosity of the mass and the porosity of the product. The substance goes from state 6 to the submicrocrystalline state 7, and partially to the crystalline state 8 ().
Fire shrinkage is 4-8% - depending on the type of raw material, its humidity, degree of compaction and firing temperature.
During the last firing period, the temperature is lowered gradually to avoid the appearance of internal stresses and cracking of the products.
Firing is carried out in continuous furnaces - ring, tunnel, slot. The duration of firing, depending on the type of product and the design of the furnace, ranges from 1.5 to 60 hours.
Automation of the drying and firing process involves maintaining the required parameters of the coolant in heating units while maintaining the rhythm of supplying products to them. The automated drying and firing control system includes such functional subsystems as information and control. Information subsystems, using sensors, collect the necessary information: temperature, humidity of the environment, type of environment (oxidizing or reducing), rate of change of parameters, fuel consumption, degree of combustion, etc. The received signals are used as initial data for a set of computational and logical operations. As a result of these operations, the control subsystems determine the current and predicted values of the measured quantities, calculate technical and economic indicators, and detect violations during drying or firing.
Control subsystems, designed to develop optimal solutions, prepare a control action during the drying or firing period, and then implement it, automatically changing the positions of the control elements.
In order to reduce the time spent on drying, as well as the labor costs for transferring raw material, drying and firing of products made from clays that are slightly and moderately sensitive to drying are often combined in one unit. In this case, savings in labor costs are achieved by 35%, fuel savings by 20-25%, and the cost of products is reduced by 25-30%. The combined process of drying and firing lasts up to 63 hours, of which drying - 28 hours, firing - 21 hours (including heating - 8 hours 45 minutes), cooling - 14 hours.
Saving fuel and energy resources when drying and firing ceramic products is possible due to:
- the use of energy-containing waste recorded in metastable states 6, 7, 9, 10 (), and less wet raw material mixtures;
- use of high-speed methods;
- combining drying and firing;
- replacing conventional firing (with combined drying and firing of products) with hydrothermal treatment in an environment of superheated steam and high pressure (with this firing method, the temperature decreases by almost 200 ° C);
- development and implementation of new designs of drying and furnace units with high efficiency;
- the use of additives (fluxes) in ceramic mixtures that reduce the firing temperature;
- carrying out measures to ensure intensive heat exchange in the channels of furnaces and drying units.
The creation of waste-free technologies provides an effective solution to such problems as environmental protection. At the same time, devices are provided for dust removal and purification of exhaust gases, water, restoration of land in places where raw materials are produced, planting green spaces around the enterprise, etc. This creates conditions for effective labor protection. Thus, the problems of creating waste-free technologies, labor protection and the environment are comprehensively solved.
The implementation of waste-free technologies expands the areas of application of ceramic materials. Thus, the waste (broken, defective) generated during the production of ceramic products can be used not only in the main production as waste additives, but also in the technology of binders as active hydraulic additives.
Indispensable conditions that increase the technical and economic efficiency of the production of ceramic products in industrial construction are improving the quality of products and reducing labor intensity in their production and use. This is achieved by reducing and stopping the production of small-piece products and increasing the production of front large-sized lightweight (with increased voids) ceramic stones and slabs, as well as the production of large blocks and wall panels from them at factories. Thus, when using large blocks, labor costs are reduced by 15-20%, construction time is reduced by 10-15%, and labor productivity increases by 2-3 times. The use of ceramic panels instead of piece bricks reduces the consumption of bricks and cement, reduces the weight and cost of the wall.
2.4.5. Ceramic tiles
Ceramic tiles are divided into three groups according to their intended purpose: 1) facade (glazed and unglazed), used for external cladding; 2) glazed faience tiles used for interior cladding; 3) floor tiles.
The main raw materials for the production of facade tiles are light-burning clays and additional materials - fireclay, dehydrated clays or quartz sand. Approximate compositions of molding compounds are given in table. 2.11.
For the manufacture of earthenware tiles, light-burning refractory clays and kaolins, thinning additives (quartz sand, broken products, burnt kaolin, broken chamotte), and fluxes (feldspar, nepheline, syenite, perlite) are used.
They are usually fired twice: the first - long-term (bisque), the second - poured, during which the glaze is fixed on the previously fired shard. A number of factories have already mastered one-time firing of tiles, which has a number of advantages compared to double firing. During a single firing, the compositions of the ceramic masses are adjusted to increase the content of fired kaolin, which increases the strength and water resistance of the tiles after drying. Approximate compositions of masses for single firing are given in table. 2.12.
For the manufacture of floor tiles, high-quality, highly plastic, low-caking clays are used. The compositions of the masses are given in table. 2.13.
For the production of façade unglazed ceramics, raw materials are usually prepared using the semi-dry or slip method. For tiles molded by the semi-dry method, toggle, rotary, hydraulic and friction presses are used, in which the pressure is 7-20 MPa.
For tiles molded plastically, screw belt, vacuum and vertical (pipe) presses are used. After molding, the tiles are sent to tunnel or radiation dryers, where they are dried to a residual moisture content of 3-4% with a drying time of about 24 hours.
Firing is carried out in tunnel or roller kilns at a temperature depending on the type of raw material: for products made from refractory clays - 1200-1300°C, refractory clays - 1080-1160°C, low-melting clays - 950-1000°C. Firing duration - 40-120 hours.
Glazed facade tiles can be produced on production lines developed by PKB Stroykeramika (Fig. 2.10). The mass prepared by the slip method, after drying in a tower spray dryer, enters a hopper with a moisture content of 6-8%. From the hopper, the press powder is loaded into the press through a burat sieve. The pressed tiles are transported via a roller conveyor to dryers, where they are dried to a moisture content of 2.5%. After drying, they are glazed using disc sprayers and dolls and fed back into the dryer via a roller conveyor for drying. Excess glaze is poured into a special container and returned again for glazing. After secondary drying at a temperature of 30-40 °C to a residual moisture content of 0.5%, the tiles are stacked on special pallets and fed into a roller tunnel kiln for firing. After firing, they are calibrated and transported to the warehouse.
Glazes of various compositions are used for tiles. For example, at the Kharkov Tile Factory they use glazes based on frits of the following compositions, %:
1. Quartz sand - 10: borax - 30; boric acid - 3.2; zinc oxide - 7; chalk - 4.9; dolomite - 2.5; quartz-feldspathic raw materials - 20.1; strontium carbonate - 3; zircon - 13; barium carbonate - 6.3.
2. Quartz sand-17; borax - 32; sodium nitrate - 3; cryolite-10; soda - 7; quartz-feldspathic raw materials - 31.
The production of glazed facade tiles is also possible using the casting method. Tiles produced by this method have a thickness (depending on the standard size) from 1 to 3.5 mm (GOST 18623-82).
The technological process for manufacturing cast ceramic products lasts 2-2.5 hours instead of 48-50 hours when producing tiles using the semi-dry method.
To produce ceramic tiles using the casting method, you need blades (stands), a separating layer, a tile layer and a glaze.
Flakes are ceramic stands made of fireclay mass, designed for installing tiles on them and absorbing moisture from them. They are subject to a number of requirements: precise dimensions, flat smooth surface, high filtering capacity, low coefficient of thermal expansion, sufficient mechanical strength, low abrasion, minimal change in the rate of moisture absorption from the slip during repeated use.
A separating layer up to 0.25 mm thick, usually made from a mixture of limestone (90%) and bentonite (10%) is applied to the platforms to hold the tiles on them. The raw materials for the separation layer are wet-ground to a residue of 0.5-2% on a sieve of 10,000 holes/cm 2 (0.063 mm). The humidity of the mixture is 68-95%, the average density of the resulting slip is 1100-1300 kg/m3. Excess moisture is absorbed by the blade.
The main layer of tiles is tiled. It is prepared from lean masses and applied in two steps after the moisture has disappeared from the previous layer. The thickness of the layers is 1.5-2 mm.
Approximate composition of the tile layer,%:
- Chasov-Yarskaya clay - 4-8
- Chamotte - 30-42
- Nepheline syenite - 20-35
- Broken glass - 18-34
- Sodium pyrophosphate (over 100%) - 0.02-0.1
The glaze is prepared from frit (Table 2.14) followed by the addition of 9% kaolin during grinding. It is applied either by watering or spraying. Excess moisture is absorbed by the blade. Glaze thickness 0.25 mm.
The formation time of the separating layer is 25-30 s, the tile layer is 180-270 s, the glaze layer is 180-240 s.
As a result of sequential application of layers, an array is formed, which, before drying, is cut into tiles of the required sizes using knives.
The tiles are dried in dryers equipped with a mesh conveyor and injection multi-jet gas burners. Drying time 14-35 minutes, residual moisture 0.2-2%.
The tiles are fired in multi-channel slot kilns at a temperature of 930-1080°C for 2 hours. The temperature of the blades and tiles after leaving the kiln is 35-40°C.
Glazed ceramic tiles are produced on the SM-725A or KPL-4 conveyor (Fig. 2.11).
The cost of tiles produced by casting is 20-40% less than the production of conventional tiles, labor costs are 2 times less, fuel consumption is 3.8 kg/m2 instead of 11.4 kg/m2, raw material consumption is 4 kg/m2 2 instead of 8-10 kg/m2.
Small tiles are usually assembled into carpets using special machines. The tiles are laid out in a given pattern with the back side down. Kraft paper is glued onto the resulting pattern of tiles using Galerta carpentry (bone) glue or flour glue. The main requirements for glue are low water resistance, good adhesion to tiles and paper, pot life of at least 4 hours, low cost. The resulting carpets measuring 400x560 or 615x407 mm are sent for drying at a temperature of 50-60 °C for 8-12 hours.
Earthenware facing tiles are made from press powders obtained by wet (slip) or dry methods.
The slip method for preparing raw materials is the most widely used.
With the dry method of preparing raw materials, both separate and joint grinding of the components is carried out. In Fig. Figure 2.12 shows a basic technological diagram of dry preparation of raw materials with separate grinding.
The properties of press powders obtained by dry or slip method are different. The quality of the powder obtained by the slip method using a spray dryer is higher than that of the powder obtained by the dry method. In the first case, the bulk of the powder, in which there is no dust function, contains grains 0.2-0.5 mm in size. The resulting granulometric composition ensures high flowability in a wide range of humidity. To avoid powder sticking to the mold, it must be kept in hoppers for 8-18 hours before pressing.
The tiles are pressed at a powder moisture content of 6.5-9.5% and then sent either to conveyor shelf dryers or tunnel dryers. Drying time is 28-40 hours. After drying, the tiles are glazed or decorated.
Single firing is usually carried out in tunnel kilns at a temperature of 1140-1160°C and a duration of up to 29 hours.
Floor tiles are made on the basis of single- or multi-component compositions. Depending on this, the raw material is prepared using the dry method, if only clay is used, or the slip method, if multicomponent compositions are used.
Pressing floor tiles has its own peculiarity, which is that the degree of compaction should be 1.9-2.2. Pressing pressure in order to remove air and prevent it from being pressed in, as well as to prevent delamination of the tiles, is applied only in steps. The first holding is done at a pressure of 3-6 MPa, and then further pressing at 20-30 MPa. The duration of application of pressure depends on the granulometric composition of the mixture: for coarse grains - 2-3 s, for fine grains - up to 4 s.
The pressed tiles are dried and fired.
2.4.6. Facing bricks and stones
Facing bricks and stones are molded using plastic or semi-dry pressing. The raw materials are the same materials that are used to produce ordinary ordinary bricks, but they are subject to more thorough preparation.To eliminate efflorescence on the surface of products, barium carbonate is additionally introduced into the charge, which converts soluble compounds such as sodium sulfate and calcium into insoluble barium sulfate. Another active additive that eliminates efflorescence is amorphous silica, which, at high temperatures, forms calcium or magnesium silicate with the release of sulfate gas.
When plastic molding bricks and stones, evacuated masses with a vacuum of at least 93.5 Pa are used. The moisture content of the mass during molding should be no more than 20%.
The drying mode for plastic molded products must prevent moisture condensation on its surface. For this purpose, the coolant is recirculated. The moisture content of the brick after drying should not exceed 8%.
The use of semi-dry pressing of mixtures with a moisture content of 6-9% makes it possible to obtain products of the highest quality.
To improve the appearance of facing bricks and stones, they are often engobed. Such products belong to two-layer ceramics, in which the textured (angobic) layer is applied using plastic molding.
The economic feasibility of producing two-layer ceramics lies in the production of highly decorative materials consisting of more than 90% non-scarce raw materials. Expensive raw materials, which form a thin textured layer, make up 8% of the total mass of the product.
A number of special requirements are imposed on engobed products: strong adhesion of the front layer applied to the spoon and butt sides; identical color and uniform thickness of the engobe layer; the proximity of the fire and air shrinkage indicators of the front layer and the main mass of the brick; the permissible difference in shrinkage between different layers is no more than 1.5%.
The base layer contains low-melting clays that do not contain harmful inclusions. The engob layer contains light-burning clay, quartz, and dyes (oxides of cobalt, iron, chromium).
Two-layer molding is based on feeding two masses into a transition head with a forming L-shaped frame, which ensures the distribution of a textured layer with a thickness of 3...3.5 mm along the spoon and butt sides. In the press head, the mass is compacted and a two-layer beam is obtained. For better adhesion of the layers, furrows are applied to the top layer with special liners in the form of combs.
The molding pressure on the spoon and butt sides is not the same and varies from 1 to 0.55 MPa with distance from the engobe insertion site. If the pressure is insufficient, the texture layer may shift. If the pressure is sufficient, then the textured layer diffuses to a depth of 0.2-0.3 mm and there is strong adhesion to the main layer.
The engobic layer can be applied to the clay beam by spraying immediately after molding.
Engobe products are dried with a coolant with a humidity of 85-90% and a temperature of up to 90 °C for 35-40 hours.
2.4.7. Cladding materials for aggressive environments
Chemically resistant facing materials include acid-resistant and alkali-resistant materials, which are sometimes classified into a group of corrosion-resistant ones. These materials are obtained as a result of high-temperature processes and are conventionally classified as ceramic.There are two types of acid-resistant materials: metallic and non-metallic.
Metal alloys include iron alloys, as well as non-ferrous metals (nickel, copper, titanium, gold) and their alloys (nickel-silicon, silumin).
Non-metallic acid-resistant materials usually include materials based on silicate acid salts, the increased acid resistance of which is caused by the presence of a significant amount of acid oxide. These are stone castings from diabase and basalt, fused quartz, glassy carbon, glass, acid-resistant enamels and putties, acid-resistant concrete, ceramic materials, slag glass, granite, asbestos, etc.
Alkali-resistant materials are also divided into metallic and non-metallic. Alkali-resistant metal materials include many metals and alloys (steel, cast iron, nickel, brass), and non-metallic materials include materials containing a significant amount of basic oxides. Such materials are: limestone, magnesite, Portland cement, slag-alkaline cements, etc. These also include glassy carbon, enamels, silicate glasses with the addition of boron, etc. Organic polymer materials also have high alkali resistance.
Ceramic products having an approximate composition: 20-40% Al 2 O 3; 01-0.8% CaO; 0.3-1.4% MgO; 50-75% SiO 2; 0.5-3% Na 2 O+K 2 O; 0.3-1.6% F 2 O 3, stable in alkalis of low and medium concentrations.
Corrosion-resistant materials are required not only not to enter into chemical interaction with the external environment, but also not to be destroyed as a result of physical, physico-chemical, biological and other types of external influence.
Physical factors of influence include processes of heat and mass exchange with the environment, phase and other transformations.
Physico-chemical factors are electrochemical processes, temperature and humidity effects in the presence of chemical reagents, etc.
Biological corrosion is that an aggressive environment, which is created as a result of the vital activity of organisms, leads to physical destruction of the material.
Non-metallic corrosion-resistant materials, in addition to being acid- or alkali-resistant, must have high density and smooth surfaces of the product.
Among ceramic materials, thin ceramics, including porcelain, semi-porcelain and earthenware, characterized by density and low porosity, have the greatest corrosion and chemical resistance. The water absorption of porcelain is 0.2-0.5%, semi-porcelain - no more than 5 and unglazed earthenware - up to 12%.
The raw materials for the production of fine ceramics are plastic refractory white-burning clays and kaolins, fluxes and waste additives - feldspar, pegmatite, quartz sand.
The preparation of raw materials is carried out using the slip method, and molding is carried out using the slip casting method. After drying the raw material, a glaze composition is applied to its surface. Firing is carried out at temperatures: 1160-1280°C - for earthenware, 1270-1280°C - for fireclay products, 1230-1250°C - for semi-porcelain and 1170-1280°C - for porcelain. During firing, a liquid phase and mullite (Al 2 O) are formed in significant quantities, providing high density, strength and corrosion resistance of products.
The economic efficiency of facing materials for aggressive environments lies in the protection of structural materials from destruction, extending the service life of chemical technology equipment, as well as the possibility of using industrial methods of construction and repair of chemical and thermal equipment.
Clay is a fine-grained sedimentary rock, dust-like when dry, plastic when moistened. Clay consists of one or more minerals of the kaolinite group (derived from the name of the locality Kaolin in China), montmorillonite or other layered aluminosilicates (clay minerals), but may also contain sand and carbonate particles.
Ceramics (ancient Greek kesbmpt - clay) - products from inorganic, non-metallic materials (for example, clay) and their mixtures with mineral additives, manufactured under the influence of high temperature with subsequent cooling.
Slip (German: Schlicker) is a mushy, soft porcelain mass used in the production of porcelain, consisting of kaolin, quartz and feldspar. Clay mixed with water and colored, used in ancient times to paint pottery, is also called slip. Currently, slip is a name for aqueous suspensions of clay-based compositions used for molding ceramic products by casting into porous, usually gypsum, molds. Typical slip humidity for porcelain casting is 30-33%, for earthenware casting - 33-37%, slips based on red-burning clays can have a humidity of more than 40%. The lower the moisture content of the slip, the faster the formation of a layer of ceramic mass on the surface of the gypsum mold, the less shrinkage during drying and deformation of the products. To prepare a slip with low humidity, deflocculants (thinners) are added to its composition - liquid glass, soda ash, carbon-alkali reagent - in an amount of 0.1-0.5%.
Engobe is a type of decorative refractory coating for ceramics that does not have glare. Typically used to disguise the dark color of the shard. Engobe can be white or colored. Akunova, L.F., Krapivin, V.A. Technology of production and decoration of artistic ceramic products./ L.F. Akunova, V.A. Krapivin. - M.: House. - 75 s.
Glaze is a glassy coating on ceramics that protects it from external influences and also serves as decoration. Modern industrial glazes are usually colorless and transparent (for example, on porcelain) or colored and opaque (on tiles). But visiting any art museum, you can see that glazes have a much wider “repertoire” of visual effects. The glazes on ancient Egyptian amulets shine as blue as the day they were taken out of the kiln. The scenes depicted on ancient Greek vases covered with red and black glaze have not faded at all. Bright tricolor lead glazes, “glowing” celadon and porcelain testify to the taste and power of the Chinese imperial court.
A potter's wheel is a machine for shaping dishes and ceramic products, which allows you to use rotational inertia to create the shape of products and increase labor productivity. A hand potter's wheel is rotated on a vertical axis with one hand and the product is formed with the other hand. The foot potter's wheel is driven by a flywheel located at the bottom, which is rotated by the feet.
Types of ceramics
Depending on the structure, a distinction is made between fine ceramics (vitreous or fine-grained shards) and coarse ceramics (coarse-grained shards). The main types of fine ceramics are porcelain, semi-porcelain, faience, majolica. The main type of coarse ceramics is pottery ceramics. Porcelain has a dense sintered shard of white color (sometimes with a bluish tint) with low water absorption (up to 0.2%), when tapped it produces a high melodic sound, and can be translucent in thin layers.
The glaze does not cover the edge of the bead or the base of the porcelain piece. The raw materials for porcelain are kaolin, sand, feldspar and other additives.
Faience has a porous white shard with a yellowish tint, the porosity of the shard is 9 - 12%. Due to the high porosity, earthenware products are completely covered with a colorless glaze of low heat resistance. Earthenware is used to produce tableware for everyday use. The raw materials for the production of earthenware are white-burning clays with the addition of chalk and quartz sand.
Semi-porcelain in properties occupies an intermediate position between porcelain and earthenware, the crock is white, water absorption is 3 - 5%, it is used in the production of tableware.
Majolica has a porous shard, water absorption is about 15%, the products have a smooth surface, shine, thin walls, are covered with colored glazes and can have decorative relief decorations. Casting is used to make majolica. Raw materials - white-burning clay (faience majolica) or red-burning clay (pottery majolica), flux, chalk, quartz sand.
Pottery ceramics have a red-brown shard (red-burning clays are used), high porosity, water absorption up to 18%. Products can be covered with colorless glazes or painted with colored clay paints - engobes. Kitchen and household utensils, decorative items. Tretyakov, Yu.D., Lepis, Kh.L. Chemistry and technology of solid-phase materials./ Yu.D Tretyakov. H.L. Lepis. - M.: MSU. -203 s.
According to the method of preparation, ceramic masses are divided into powdery, plastic and liquid. Powdered ceramic masses are a mixture of crushed and dry mixed initial mineral components, moistened or with the addition of organic binders and plasticizers. By mixing clays and kaolins with lagging additives in a wet state (18-26% water by weight), plastic molding masses are obtained, which, with a further increase in water content and with the addition of electrolytes (peptizers), are transformed into liquid ceramic masses (suspensions) - foundry slips. In the production of porcelain, earthenware and some other types of ceramics, plastic molding mass is obtained from slip by partial dewatering in filter presses, followed by homogenization in vacuum mass grinders and screw presses. In the manufacture of some types of technical ceramics, the casting slip is prepared without clays and kaolins by adding thermoplastic and surfactants (for example, paraffin, wax, oleic acid) to the finely ground mixture of the initial raw materials, which are then removed by preliminary low-temperature firing of the products.
Firing of ceramics is the most important technological process that ensures a given degree of sintering. Exact observance of the firing regime ensures the required phase composition and all the most important properties of the ceramics. With rare exceptions, sintering of crystalline phases occurs with the participation of liquid phases formed from eutectic melts. Depending on the composition of the ceramic mass and the firing temperature in densely sintered porcelain and steatite products, the content of the liquid phase during the sintering process reaches 40-50% by weight or more. Due to surface tension forces arising at the boundary of the liquid and solid phases, the grains of crystalline phases (for example, quartz in porcelain) are brought closer together, and gases distributed between them are forced out of the capillaries. As a result of sintering, the dimensions of the products are reduced, their mechanical strength and density increase. Sintering of some types of technical ceramics (for example, corundum, beryllium, zirconium) is carried out without the participation of the liquid phase as a result of volumetric diffusion and plastic flow, accompanied by crystal growth. Sintering in solid phases occurs when using very pure materials and at higher temperatures than sintering with the participation of a liquid phase, and therefore has become widespread only in the production of technical ceramics based on pure oxides and similar materials. In accordance with the set of requirements, the degree of sintering of different types of ceramics varies widely.
The production of ceramic products includes the following basic operations: preparing the mass, molding the products, drying, firing and decorating.
Materials used for ceramic production are usually divided into basic and auxiliary. The main ones include materials used for the preparation of ceramic masses, glazes, ceramic paints; auxiliary materials - materials used for the manufacture of plaster molds and capsules.
Basic materials are divided into plastic, thinning, flux, glaze-forming and ceramic paints.
Plastic materials are clays and kaolins. Clays and kaolins are formed as a result of the breakdown of rocks such as granite, gneiss, and feldspar. Kaolins differ from clay in having a purer chemical composition, less plasticity, and greater fire resistance.
The thinning materials are quartz and pure quartz sands; they help reduce the plasticity of clays, reduce shrinkage and deformation of products during drying.
Fluxes lower the melting and sintering temperature of clay materials, impart density, translucency, and mechanical strength to the ceramic shard; these include feldspar, pegmatite, chalk, limestone, and dolomite.
Glaze-forming materials (glaze) are a thin glassy layer on the surface of ceramic products. It protects the crock from mechanical influences, improves its hygiene, and gives the surface of the product a better appearance. Glazes can be transparent or opaque (solid), colorless or colored.
Ceramic paints are used to decorate porcelain, earthenware, majolica and other products. The basis of ceramic paints are metals and their oxides, which, when heated, form colored compounds with silicates, aluminates, borates and other substances on the ceramic shard. Based on the nature of their application, ceramic paints are divided into underglaze and overglaze.
Underglaze paints are applied to an unglazed, exposed shard, then the piece is glazed and fired.
Overglaze - applied to a shard covered with glaze, secured by special firing at a temperature of 600-850°C.
The preparation of ceramic mass is carried out by sequentially performing a number of technological processes: cleaning raw materials from harmful mineral inclusions, crushing, breaking, sifting through sieves, dosing and mixing.
Products are formed from plastic and liquid (slip) ceramic masses. Products of simple shapes - (cups, plates) are molded from plastic mass with a moisture content of 24-26% in plaster molds using steel templates on automatic and semi-automatic machines.
Method: casting from a liquid mass - slip with a moisture content of 30-35% gypsum molds is indispensable in the production of ceramic products, where the complexity and variety of shapes exclude the use of other molding methods. Casting is carried out manually or automatically.
Drying helps to increase the strength of ceramic products formed from plastic mass or cast from slip. Drying is carried out in convection (conveyor, chamber and tunnel) and radiation dryers at a temperature of 70-90 0C.
Firing is the main technological process. As a result of complex physical and chemical transformations taking place at high temperatures, ceramic products acquire mechanical strength.
Firing is carried out in two stages. For porcelain products, the first firing (waste) occurs at a temperature of 900-950°C, and the second (watered) - at a temperature of 1320-1380°C. For earthenware products, the first firing is carried out at a temperature of 1240-1280 °C, and the second - at a temperature of 1140-1180 °C. Two types of furnaces are used: tunnel furnaces (continuous operation) and batch furnaces).
Decorating products is the final stage of production of porcelain and earthenware products, which consists of applying special cuts to linen (unpainted semi-finished product) using two methods: manual and semi-mechanized.
Tendril, layering, tape are continuous circular strips (tendril 1 mm wide, layering - from 1 to 3 mm, tape from 4 to 10 mm).
The stencil is applied with an airbrush using plates made of thin tin or foil that have cutouts, the contours of which correspond to the applied pattern. It can be single-color or multi-color.
The roof is divided into the following types: solid - the entire product is covered with an even layer of paint; half-covered - the product is covered with paint with a width of 20 mm and above, downward - the paint is applied with a weakening tone towards the bottom of the product; roofing with cleaning - the pattern has been cleaned along the continuous roof; covering with cleaning and painting with paints and gold.
Printing is applied to a product from a printed impression on paper, thereby obtaining a graphic one-color design, which is usually painted with one or more colors.
A stamp is the simplest way to decorate. The design is applied with a rubber stamp. More often, stamps are applied in gold.
Decalcomania (decal) occupies a major place in the decoration of products. The design is transferred to the product using a decal made lithographically. Currently, a sliding decal is used. A cellulose acetate film is applied to the lining paper, on which a design is printed. When wetted, the film with the pattern separates from the paper and remains on the product. During the muffle firing process, the film burns and the paint fuses with the surface of the product.
Silk-screen printing is a promising way to decorate ceramic products. The design is printed through a silk mesh on which a stencil is applied. The item to be decorated is placed under a silk mesh. A rubber roller, carrying paint through the mesh, presses it into the cutouts of the stencil, and thus the design is transferred to the product.
Painting works are performed manually with a brush or pen. Depending on the complexity of painting, it can be simple or highly artistic.
Photoceramics reproduces portraits of famous people and views of cities on the product; it is especially effective in color.
The main properties of ceramic products are physical and chemical. The properties of ceramic products depend both on the composition of the masses used and on the technological features of their production.
The main properties are bulk density, whiteness, translucency, mechanical strength, hardness, porosity, thermal resistance, speed of propagation of sound waves, chemical resistance.
The volumetric mass of porcelain is 2.25-2.4 g/cm³, and that of earthenware is 1.92-1.96 g/cm³.
Whiteness is the ability of a material to reflect light falling on it. Whiteness is especially important for porcelain products. Whiteness is determined visually by comparing the test sample with a standard or using a spectrophotometer.
Translucency is characteristic of porcelain, which is translucent even with a large thickness of the product, since: it has a dense sintered shard. Earthenware products are not translucent because the shard is porous.
Mechanical strength is one of the most important properties on which the durability of the product depends. Specific mechanical strength, i.e. the ratio of the applied force to a unit of bottom thickness is determined by the method of free fall of a steel ball along the bottom of the product. In faience it is higher than in porcelain. Impact strength using the pendulum method, on the contrary, is lower for earthenware products than for porcelain ones.
The hardness of the glaze layer on the Mohs mineralogical scale for porcelain is 6.5-7.5, and for earthenware - 5.5-6.5, microhardness is determined by indentation of a diamond pyramid (according to Vickers). Porcelain glazes are hard, majolica are soft, and earthenware are medium.
Porosity is determined by the method of water absorption, which for porcelain is 0.01-0.2%, and for earthenware - 9-12%.
Thermal resistance characterizes the ability of a product to withstand sudden changes in temperature. The thermal resistance of porcelain products is higher than that of earthenware. So, in accordance with current GOSTs, the glaze for porcelain products must withstand temperature changes from 205 to 20 ° C, and for earthenware - from 145 to 20 ° C (for colorless glazes) and from 135 to 20 ° C (for colored glazes).
The speed of propagation of sound waves in porcelain products is 3-4 times higher than in earthenware, therefore, when hitting the edge with a wooden stick, porcelain products produce a high-pitched sound, while earthenware produce a dull sound.
The chemical resistance of glazes and ceramic paints used for household porcelain and earthenware products must be high, since they should not be destroyed when treated with weak acids and alkalis at ordinary temperatures or when heated to 60-65°C.
All ceramic products are divided into coarse and fine ceramic products. Products of coarse ceramics have a heterogeneous structure of the shard, distinguishable by the naked eye; in addition, the shard has a natural color from yellow to brown tones.
Fine ceramic products are characterized by a sintered, finely porous shard with a uniform, dense structure.
Rice.
1 - layering; 2 - tape; 3 - stencil; 4 - stamp; 5 g - continuous roof, 6 - descending roof; 7 - print; 8 - print with coloring; 9 - decalcomania; 10 and 11 painting; 12 and 13 - photos on ceramics; 14 and 15 - cutting the relief.
Fine ceramics products include two groups:
1- products with a shard sintered in a fracture (hard porcelain, soft, bone and frit china, fine stone products);
2 - products with porous shards (faience, majolica, semi-porcelain).
Hard porcelain is characterized by high mechanical strength, chemical and thermal resistance. Russian factories produce mainly porcelain products from hard porcelain, which is prepared from a mass containing 50% clay substances, 25% feldspar and 25% quartz.
Soft porcelain has high translucency, but lower thermal and mechanical strength. The masses used in the production of soft porcelain contain 30% clay materials, 30-36% feldspar and 20-45% quartz. Soft porcelain is used in the manufacture of artistic products.
Bone china is made from a mass that, in addition to the usual components, includes 20-60% bone ash. Bone china is characterized by high translucency, but at the same time low mechanical and thermal strength. Used for making souvenir dishes.
Fritted porcelain is similar in composition to glass because it does not contain clay materials. Due to the insufficient hardness of the glaze and the labor-intensive technological process, this type of porcelain is rarely used for making tableware.
Fine stone products have a color depending on the natural properties of the clay (light gray, cream). These products have high thermal stability. They produce chemical fine stone utensils, as well as mugs, coffee and tea sets.
Earthenware has a white porous shard, the water absorption of which ranges from 9-12%. Earthenware products are coated with a low-melting glaze. The composition of the earthenware mass includes 65% clay materials, 30% quartz or quartz sand and 2-5% feldspar.
Majolica is a type of earthenware that is highly porous. Majolica products are usually coated with colored glaze.
Semi-porcelain, in its properties, occupies a middle position between porcelain and earthenware and is mainly used for the manufacture of sanitary products. Semi-porcelain products are cheaper than porcelain and are of higher quality than earthenware.
Ceramic products are divided into tableware and artistic and decorative products. In turn, tableware can be used for tableware, tea and coffee purposes.
Based on wall thickness, porcelain products are divided into ordinary ones with a wall thickness of 2.5 (cup) - 4 mm and thin-walled 1.4 (cup) - 2.5 mm, all others.
Depending on the size, ceramic products are divided into small and large.
Based on their shape, products are divided into hollow and flat.
Flat ones include saucers, dishes, plates, herring bowls and others; hollow - glasses, cups, mugs, bowls, teapots, coffee pots, sugar bowls, jugs and others.
Depending on the presence of the glaze layer, porcelain products are distinguished between glazed and unglazed (biscuit).
Depending on the completeness of the product, there are either piece or complete sets (sets, sets, sets). A special feature of the products included in the set is the unity of decorative design, design and shape.
Classification of porcelain tableware
Based on their intended purpose, the range of household porcelain products is divided into tableware, teaware, household tableware, and others.
Artistic and decorative items are especially highlighted.
Porcelain tableware (see Appendix 1) is represented by a wide variety of products, both by name and by style and size.
Dishes are produced in round and oval sizes of 300, 350,400 and 450 mm.
Vases for soup or compote are made with lids of different styles with a capacity of 2000-3500 cm 3.
Gravy jugs are available without a tray and with a tray (on a plate)
with a capacity from 80 to 400 cm3.
Salad bowls are characterized by different styles (round, oval, quadrangular) and a capacity of 1200-1400 cm3, quadrangular ones have a capacity from 120 to 1000 cm3.
Herring bowls are available in lengths of 135 and 250-270 mm.
Plates are the main type of tableware. They come in deep and shallow, for adults and children. Deep plates are produced with a diameter of 240 and 200 mm and small plates of 240 mm (stand for a deep plate of 240 mm), 200 mm (for second courses), 175 mm (snack bar) and 158 mm (pie). Children's plates, deep and shallow, with a diameter of 178 mm, are included in children's sets. In addition to the listed products, this group includes products for spices - mustard pots, salt shakers, pepper shakers and horseradish shakers.
Tea and coffee utensils (see Appendix 2) are very diverse in styles, sizes and decor. Cups and saucers occupy the main place in the assortment of this group. Tea cups differ from coffee cups in capacity. So coffee cups have a capacity of 60, 85 and 100-130 cm3. The capacity of tea cups is 200-250 cm 3 (ordinary), 260-275 cm 3 (medium-sized), 300-350 cm 3 (large-sized) and 400 and 500 cm 3 (gift).
Kettles are divided into brewing kettles (for brewing tea) with a capacity of 250, 350 - 375, 450, 500 - 700, 735 - 800 cm3 and top-up kettles (for boiling water) with a capacity of 1000-1250, 1400 and 3000 cm3.
Glasses are available in different styles with saucers with a capacity of 375 - 400, 500 and 600 cm³.
Coffee pots are made in different styles with a capacity of 500, 750,
1000-1250, 1400 cm.
Mugs are produced with or without a handle, with a thickened shard, and special flat mugs with a hole in the handle. The capacity of the mugs ranges from 90 to 500 cm³.
Bowls are conical in shape, without handles, with a capacity of 140-150, 220-250, 350-400 cm3.
The group of tea and coffee utensils also includes vases for fruit and jam on a stem.
Other products are cheesecake bowls, napkin holders, etc.
Complete tableware is produced in the form of services, sets, and sets; it is characterized by unity of form (style) and cutting.
Sets and sets according to their intended purpose are dining rooms, tea and coffee shops for 6 and 12 persons. The set includes a larger number of items than a service for the same purpose.
Artistic and decorative products - household tableware (see Appendix 3) occupy a significant place in the group of porcelain products. The range of artistic and decorative products includes sculpture (figurines of people, animals, birds, fish, etc.), busts, wall bas-reliefs, flower vases, various products (powder compacts, ashtrays, pencil boxes, dishes and wall plates, decanters for wines, commemorative medals, etc.).
Products of applied art are characterized by a combination of utilitarian properties and high aesthetic properties. These products are varied in shape, they are more carefully finished and decorated (usually by painting).
Crockery.
The range of earthenware products is simpler and less varied than similar porcelain products. A significant share is occupied by flat products (plates, bowls, herring bowls, etc.). The assortment of earthenware products does not include tea cups, teapots, or coffee pots. The range of earthenware is mainly represented by tableware products. Earthenware consists of piece and complete products. Complete products include tableware, sets of plates (different sizes and children's sets).
Artistic and decorative items occupy a small place in the assortment of earthenware products, mainly sculptures, flower vases and ashtrays of various styles.
Majolica and pottery.
The range of majolica products includes tableware and artistic and decorative items.
Majolica products are characterized by cutting with various colored glazes (majolica glazes) and underglaze paints.
The range of majolica products is presented in both piece and complete tableware. They produce mugs, butter dishes, coffee pots, rusk bowls, ashtrays, egg cups, cheese bowls, salad bowls, honey pots; The assortment includes especially wide selection of appliances for fruit, pancakes, salad, eggs, water, jam, compote, tea, spices, as well as coffee and children's appliances.
Artistic and decorative items include flower vases, wall dishes and plates, ashtrays, sculptures and others.
Pottery belongs to rough ceramics. The main raw materials are low-melting clays of medium plasticity. These products are formed on a pottery wheel or in plaster molds. After drying and glazing, they are fired in furnaces at a temperature of 900-1000 °C.
The assortment of pottery consists of jars, pots, bowls, jugs, butter dishes, crackers, containers for sour cream and butter, and flower pots. The following artistic and decorative items are produced from pottery mass: flower vases, flowerpots, wall dishes, sculptures, toys, etc.
Assessing the quality of ceramic tableware.
Ceramic products must be durable, easy to use, and have a beautiful appearance. They are manufactured in accordance with samples approved in accordance with the established procedure. When assessing the quality of ceramic goods, attention is paid to the quality indicators of the shard, glaze and decoration. Depending on the appearance, physical and technical indicators, nature, size and number of defects, according to current GOSTs, dishes are divided into 1st and 2nd grades.
Whiteness, thermal resistance, water absorption, acid resistance are determined according to the methods set out in GOSTs.
The whiteness of porcelain products for grade 1 must be at least 64%, for grade 2 - 58%. Whiteness is not regulated for earthenware products.
Translucency is characteristic only of porcelain products, which are translucent in layers up to 2.5 mm thick. Porcelain and earthenware plates and saucers are considered mechanically strong; they do not collapse when stored in stacks of them for five days (the first of 120 pieces, and the second of 100 and 150 pieces).
The presence of defects is determined during an external inspection of the product. All the variety of defects found on ceramic products are divided into defects in the shard and glaze and decoration defects.
Defects in shard and glaze. The deformation of the product is expressed in its curvature. This defect occurs as a result of targeted drying and firing processes. Especially typical for flat products. The deformation is measured using a stepped template in millimeters and for the main types of products it has tolerances in accordance with GOST.
Potholes and chips on the product are formed during the production, transportation and storage process.
Glazed one-sided cracks represent non-through cuts of the shard.
The glaze spread on the shards should be smooth and uniform. Minor leaks are allowed. Small, dispersed punctures that do not affect the presentation of the product are allowed. Matte glaze in grade 1 is not allowed.
Bald spots and glaze build-up appear as areas not covered with glaze. On the front side of porcelain products in grade 1, earthenware products in grades 1 and 2 are not allowed.
Dry glaze and flying edges are found on earthenware products. Dry glaze occurs as a result of insufficient thickness of the glaze layer on the product. A flying edge sharply reduces the hygienic properties of the product; it is characterized by rebound of the glaze along its edges in grade 1 is not allowed.
Ceks and hairs are cracks in the glaze layer. Products with these defects are rejected.
Clogging appears on products as a result of chipping of chamotte grains from capsules. It can be underglaze, as well as overglaze, which can be sanded.
The front sight appears as dark dots on the product. This defect appears due to the ingress of iron oxides into the ceramic mass.
Traces from runners are typical only for earthenware products and are allowed on the reverse side to be sanded or cleaned. Incorrect installation of product parts is their asymmetrical arrangement, deviation of attached parts (spouts, handles) from the vertical and horizontal planes.
Undermining of attached parts is allowed if it is hairline and non-through and does not violate the mechanical strength of the product. However, blowing up the spouts of teapots is not allowed.
Decoration defects. Burnt or underburned paints are formed when muffle firing is disrupted. The paint should not rub off.
Assembly of the decal is allowed if it does not violate the design.
Overglaze paint marks on the front side of the product are not allowed in the first grade.
Peeling paint renders the product defective.
According to GOST, the number of permissible defects should not exceed for porcelain products for grade 1 - 3, for grade 2 - 6; earthenware products - 3 and 6, respectively.
Labeling, packaging, transportation and storage of ceramic tableware.
Each porcelain and earthenware product is marked with a trademark, which is applied to the center of the bottom of the product with ceramic paint and secured by firing. The trademark must be clear.
When packaging dishes, consumer containers are used (boxes made of cardboard, paper and combined materials); packs of cardboard, paper and combined materials and bags of paper and combined materials, auxiliary materials (wrapping and cushioning paper, corrugated cardboard, heat-shrinkable materials, polyethylene film, polystyrene, wood shavings, etc.); transport containers (wooden boxes and corrugated cardboard boxes).
Cups and saucers are placed in the following way: the cup is placed upside down on the saucer on the front side, previously covered with paper, and wrapped in paper. Then a stack containing from two to twelve products is formed, which is also wrapped in paper. It is permissible to mold feet made up separately from cups and saucers. Flat products are wrapped in paper through one product, and then into a bag of 25-40 pieces. The enlarged package is tied with twine or sealed with paper tape and a label is pasted indicating the manufacturing plant and its address, product name, number of products in the package, grade, packaging date, packer number and GOST or TU number. When packing packages of services, sets, and sets, products of the same type and decorative design are placed: each item is wrapped in paper. Then the dishes are placed in consumer and transport containers. Products for souvenir and gift purposes are placed in corrugated cardboard boxes, onto which artistically designed labels are pasted.
Dishes are transported by all types of transport. Basically, dishes are transported in railway cars and containers, the floor of which is lined with wood shavings in an even and dense layer. The rows of bags are also lined with shavings. On containers and railway cars, the manufacturer must put the inscription “Caution glass”.
Dishes sent to the Arctic, Far North and remote areas are packaged in accordance with special technical conditions.
In the narrow sense, the word ceramics means clay that has been fired.
The earliest ceramics were used as dishes made from clay or mixtures of it with other materials. Currently, ceramics is used as a material in industry (mechanical engineering, instrument making, aviation industry, etc.), construction, art, and is widely used in medicine and science. In the 20th century, new ceramic materials were created for use in the semiconductor industry and other areas.
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Types of ceramics
Depending on the structure, a distinction is made between fine ceramics (vitreous or fine-grained shards) and coarse ceramics (coarse-grained shards). The main types of fine ceramics are porcelain, semi-porcelain, stone ceramics, earthenware, majolica. The main type of coarse ceramics is pottery ceramics. In addition, there are carbide ceramics (tungsten carbide, silicon carbide), aluminum oxide, zirconium (based on ZrO 2), nitride (based on AlN), etc.
Pottery ceramics have a red-brown shard (red-burning clays are used), high porosity, and water absorption up to 18%. Products can be coated with colorless glazes or painted with colored clay paints - engobes.
Story
Ceramics have been known since ancient times and are perhaps the first artificial material created by man. The emergence of pottery was believed to be directly related to the transition of man to a sedentary lifestyle, so it occurred much later than baskets. Until recently, the first examples of ceramics known to us belonged to the Upper Paleolithic era (Gravettian culture). The oldest object made of baked clay dates back to 29-25 millennia BC. This is the Vestonice Venus, kept in the Moravian Museum in Brno.
Pots found in Xianrendong Cave in 1993 (English) in the Jiangxi province in the southeast of China they were molded 20-19 thousand years ago. Shards from a pointed vessel found in Yuchanyan Cave (English) in Hunan province in southeast China, date back to 18.3-17.5 thousand years ago.
The oldest ceramic dishes (12 thousand years ago) in Russia were discovered in Transbaikalia (at archaeological sites of the Ust-Kareng culture) and in the Far East (Gromatukha, Osipovskaya, Selemdzhinskaya cultures; see Siberian Neolithic).
Pottery with a thick layer of vegetable wax and fatty sediment from Libyan sites in the Sahara (Yuan Afuda ( Uan Afuda) and Takartori (Takarkori) dates back to the period 8200-6400 BC. e.
Initially, ceramics were shaped by hand. The invention of the potter's wheel in the third millennium BC (late Chalcolithic - early Bronze Age) made it possible to significantly speed up and simplify the process of forming a product. In the pre-Columbian cultures of the Americas, Native American pottery was made without a potter's wheel until the arrival of Europeans.
Certain types of ceramics were formed gradually as production processes improved, depending on the properties of the raw materials and the resulting processing conditions.
The oldest types of ceramics are a variety of vessels, as well as spindle whorls, weaving weights and other objects. This household ceramics was ennobled in various ways - relief was applied by stamping, tracing, and applied elements. The vessels received different colors depending on the firing method. They could be polished, painted or painted with ornaments, covered with engobe, a glossy layer (Greek ceramics and Roman Terra sigillata), or colored glaze (“Hafnerceramics” of the Renaissance).
By the end of the 16th century, majolica (depending on its origin, also often called faience) appeared in Europe. Featuring a porous shard of iron- and lime-containing but white earthenware, it was coated with two glazes: an opaque, high-tin glaze, and a transparent, shiny lead glaze.
Stoneware was also produced by Wedgwood in England. Fine faience, as a special type of ceramic with a white porous shard covered with white glaze, appeared in England in the first half of the 18th century. Depending on the strength of the shard, earthenware is divided into soft, thin earthenware with a high lime content, medium - with a lower lime content, and hard - without lime at all. This latter shard often resembles stone pottery or porcelain in composition and strength.
Making pottery forms with and without using a pottery wheel
The history of the appearance of ceramics in Rus'
Archaeological finds in many ancient Russian cities indicate the widespread development of pottery in Rus'. In Ancient Rus', they mostly used two-tier pottery kilns (the lower, furnace tier was buried in the ground), but there were also single-tier ones.
The Mongol-Tatar invasion influenced the development of ancient Russian culture. The history of one of its branches - ceramics, shifted from the southern regions to the northern and western border cities, to the Moscow lands, so it is no coincidence that the revival of tiled art in Ancient Rus' was destroyed by many works of Russian potters of the 9th-12th centuries. For example, two-handled amphora pots, vertical lamps, the art of cloisonne enamel, glaze (the simplest one was yellow, survived only in Novgorod) disappeared, and the ornament became simpler.
A separate direction of Russian, and then modern Russian ceramics, is Gzhel (named after the city). These products are made in white and blue style.
Transparent ceramics
The original ceramic materials are opaque due to the characteristics of their structure. However, sintering nanometer-sized particles has made it possible to create transparent ceramic materials with properties (range of operating wavelengths, dispersion, refractive index) that lie outside the standard range of values for optical glasses.
Nanoceramics
Technology for the production of ceramic products
The technological scheme for the production of ceramic tiles includes the following main phases:
- Preparation of slip;
- Product molding;
- Drying;
- Preparation of glaze and glazing (enamelling);
Raw materials for ceramic masses are divided into plastic (clays and kaolins) and non-plastic. Additions of fireclay and quartz reduce product shrinkage and the likelihood of cracking at the molding stage. Lead and borax are used as glass formers.
Preparation of slip
The preparation of slip takes place in three phases:
- First phase: grinding of feldspar and sand (grinding lasts from 10 to 12 hours);
- In the first phase, clay is added;
- In the second phase, kaolin is added. The finished slip is poured into containers and aged.
Transportation from the raw materials warehouse is carried out using a loader to the receiving bunkers. From there it is sent along a conveyor either to a ball mill (for grinding) or to turbo solvents (for dissolving clay and kaolin)
Glaze preparation area
Glazes are glossy alloys that melt on a ceramic shard in a layer 0.12 - 0.40 mm thick. Glaze is applied to cover the shard of the product with a dense and smooth layer, as well as to give the product with a dense shard increased strength and attractive appearance, to guarantee dielectric properties and protect the decor from mechanical and chemical influences.
The glaze contains finely ground zircon, chalk, and white. The finished glaze is loaded into one of the containers determined by the technologist. It is passed several times through vibrating screens and magnetic traps to remove metal impurities, the presence of which in the glaze can lead to the formation of defects during production. Glue is added to the composition, and the glaze is sent to the line.
Molding
Before molding, the slip is loaded into one of the containers. Three containers are used alternately (changing approximately once a day) for a specific stand. The mold is pre-cleaned of slip residues from the previous molding, treated with slip water and dried.
The slip is poured into dried molds. The forms are designed for 80 fills. When molding, the pouring method is used. The mold absorbs some of the water, and the volume of the slip decreases. Slip is added to the mold to maintain the required volume.
After hardening, the products are dried, and the products are initially rejected (cracks, deformations).
Manual processing of products
After molding, the products are sent to the manual processing workshop.
After applying the glaze, the product is sent to the oven for firing. The oven is equipped with a pre-drying module, dust removal and blowing chambers. Heat treatment is carried out at a temperature of 1230 degrees, the length of the furnace is about 89 meters. The cycle from loading to unloading a trolley is about one and a half days. Firing of products in the kiln takes place throughout the day.
After firing, sorting is carried out: dividing into groups of similar products, identifying defects. If the defects are removable, they are sent for revision and removed manually at the restoration site. Otherwise, the product is considered defective.