Fireproof materials. Refractory mortars, filling materials, coatings and concretes

From this article you will learn what gives ordinary solutions fire-resistant properties. We will give examples of making heat-resistant mixtures with your own hands from ordinary ones, indicating the proportions of ingredients. The article provides prices for raw materials and ready-made compositions various domestic manufacturers.

Why can't ordinary cement withstand high temperatures? The answer is simple - it uses flammable raw materials. More precisely, substances that decompose when heated. To create compounds with heat-resistant properties Scientists needed to solve only one problem - replace combustible raw materials with non-flammable ones with similar properties.

Raw materials for refractory compositions

Clay. For the manufacture of 80% of refractory materials widely used in private construction, ordinary clay is used. Even in the form of raw materials removed from the subsoil, it has fire-resistant properties that are several times greater than those of industrially produced Portland cement. At the same time, raw clay does not have the ability to reliably bind surfaces. Our ancestors perfectly studied the properties of clay and used it for laying Russian stoves and coating the walls of the house.

Fireclay, expanded clay. This is clay that has undergone heat treatment. When baked in an oven, clay evaporates moisture and loses the ability to accumulate it again. Fireclay is used to make refractory bricks, blocks and rings for laying fireplaces, chimneys, and stoves. When ground, both of these materials form the basis of most refractory factory mixtures.

Prices for refractory and auxiliary raw materials

Masonry mixture

As mentioned above, raw clay can be used to lay refractory bricks, but the wall thickness must be at least one brick (250 mm). Such masonry has weak stability, the wall does not crumble due to the static nature of the brick, the clay only distributes the load. This is quite acceptable for stoves and fireplaces inside the house, since the seams do not shrink and the walls do not deform.

Another problem with clay is that it spills out of the seam over time. To give such masonry strength, you can use a cement-clay mortar. Its proportions:

1. Clay - 2 parts.
2. Sand - 1 part.
3. Cement - 0.3 parts or 10% of the volume of the mixture.

Not a large number of cement will help the clay stay in the joints. Such a solution can be conditionally called heat-resistant, since it still contains flammable material. Cement-clay mortar will withstand temperatures of no more than 80-90 °C.

Of course, the best solution for do-it-yourself fireproof masonry would be factory-made mixtures. Up to 90% of their composition is mortars - ready-made fireproof powders, which different proportions embedded in the cement-sand mixture. The amount of mortar is calculated depending on the temperature - the higher it is, the greater the percentage of powder. Seam thickness - from 3 to 12 mm.

Fireproof masonry mixtures

Fireproof plaster

As in mixtures for general construction purposes, mortars for masonry and plaster differ little in ingredients. The functional difference between masonry mortar and plaster is that the plaster layer is protective and absorbs “impacts” external environment, which means it should be more durable (stable).

The simplest and most affordable way to impart fire-resistant properties to plaster is silicification. In practice this means adding silicon glue, colloquially " liquid glass" To achieve properties sufficient for use on the inner surface of a stove or fireplace, 20% of liquid glass by volume of the solution is required. For 1 cu. m (1000 l) of solution you need 200 l of liquid glass.

Factory-made fireproof plasters and putties are made from kaolin clay, fireclay dust (waste from the production of fireclay products) and a heat-resistant binder. They are guaranteed to withstand temperatures of 200 °C.

Factory refractory plasters

Tile adhesive and mastic

Mortar and liquid glass are also used to make glue and mastic. At its core, this mastic consists of these two components mixed together. The glue can withstand up to 1100 °C, it is used for lining visible fireplace fireplaces with heat-resistant ceramic tiles. Mastic is used for filling (grouting) joints of masonry exposed to high temperatures, because... masonry mortar has much lower heat resistance (up to 200 ° C).

Mortar can be of two varieties - hydraulic and thermal hardening. Hydraulic freezes like a regular one cement mortar. Thermal hardens when fired, forming a continuous ceramic surface (like pottery).

Factory-made heat-resistant adhesives and mastics

Name of mastic, glue	Manufacturer	Packaging, kg	Packaging price, rub.	Price 1 kg, rub.
KDP-50 (UNIVERSAL)	Ekaterinburg	25	240	9,6
NEOMID Supercontact	Saint Petersburg	4	330	82,5
XT-7200	Samara	75	1400	18,5
Nullifire F0100	Saint Petersburg	3	180	58,8
Triumph	Novosibirsk	15	675	45

Refractory cement

This type of mineral binder uses calcium aluminate, which preserves the strength characteristics of concrete and mortars. Distinctive features of mortars and concretes based on heat-resistant cement (in addition to fireproof ones):

1. Corrosion resistance. This property is achieved through the use of inorganic raw materials - calcium aluminate - which is not subject to rapid natural decomposition (unlike marl and lime in conventional cement).
2. Early hardening due to the displacement of moisture from the solution.
3. Manufacturability of application. In terms of the method of use and proportions, it is no different from Portland cement.
4. Dielectric properties due to the absence of moisture.

It is impossible to make refractory cement yourself, but it is easy to find on the open market. It allows you to prepare fire-resistant concrete, which helps to move away from the canons of fireplace masonry and embody the most daring design ideas.

Prices for refractory cement

In addition to kaolin clay, special additives based on silicon, asbestos, barium or other alumina materials give fire-resistant properties to dry mixtures. Reacting with water or a catalyst solution, the mass acquires necessary properties. It is worth noting one more property that is “attached as a bonus” to heat resistance. This is hydrophobization or waterproofing. This is especially true for solutions using liquid glass, which in smaller proportions (10-15%) provides the waterproofing properties of a regular solution.

Vitaly Dolbinov, rmnt.ru

Mortar is a crushed mixture of fire-resistant leaning and binding materials, which, after mixing with water, serve as fire-resistant solutions.

Mortars, solutions and protective coatings serve auxiliary materials, but are important and sometimes decisive in increasing the wear resistance of refractory masonry as a whole.

Refractory mortars are used when laying fireproof structures of heating installations to bind its individual elements (for example, bricks or blocks). In terms of their chemical and mineralogical composition, mortars must correspond to the refractory materials being bonded.

The solutions must be sufficiently fire-resistant, fill the recesses well, smooth out unevenness on the bricks, slowly release moisture to the bricks, create thin seams, have slight porosity after firing, gas permeability, be durable, and sinter well with the bricks during service. To ensure the durability of refractory masonry as a whole, the volumetric changes in mortar and brick in operation must be the same. A high-quality mortar should form a seam that is slightly different in strength from the masonry itself. When drying the masonry, the seam material shrinks during the evaporation of water from the solution. With excessive air shrinkage, cracks form in the drying mortar, and therefore its connection with the masonry elements is reduced. This circumstance should be taken into account when designing the compositions of mortars and solutions. Shrinkage components (clays) are introduced into them in the smallest quantities possible, but sufficient to ensure plasticity and good sintering of solutions.

As masonry operates at high temperatures, additional shrinkage (or growth) occurs. The shrinkage of solutions slightly exceeds the additional shrinkage of products. The stresses that arise at the product-solution interface can be compensated by plastic deformation in the solution due to the formation of a liquid phase in it. In this case, the shrinkage of the solution should not exceed certain limits established by practice.

Solutions usually consist of four components: the main inert mass (wetting agent) in the form of a fine-grained powder, a plastic component (binder), various additives that regulate the properties of the solution, and water.

Sometimes the masonry is done dry, that is, the thin seams remaining after grinding the products are covered with mortar - a powder of the same composition as the products (Fig. 22). Powders are made from the waters of refractory products.

The type of mortar is determined by the type of products for which it is used. Solutions are usually classified according to this criterion: fireclay, dinas, for carbon blocks, etc.

Each of these groups contains its own special classification characteristics. They usually characterize not a solution, but its solid substance - a powder consisting of inert and astringent substances - mortar.

Aluminosilicate and dinas solutions usually contain 15-20 and 5-11% binder clay, respectively. To increase plasticity, from 0.08 to 0.18% soda ash is added to them, and to reduce the amount of water required for mixing, from 0.07 to 0.15% sulfite-alcohol stillage.

Depending on the raw material and chemical composition of dinasic mortar, the following grades have been established (GOST 5338-60):

MD1 - for furnaces with operating temperatures above 1500°C;

MD2 - the same, less than 1500°C.”

Rice. 22. Thermal insulation of the furnace roof

1- lightweight fireclay; 2- dinas chips; 3-dinas

The grain composition of mortars must meet the following requirements:

For aluminosilicate mortars, depending on the raw material, chemical and grain composition, as well as on fire resistance (according to GOST 6137-61), the following grades are established:

BTl, VT2 - finely ground high-alumina mortars; ШТ1, 11ΙΤ2 - finely ground fireclay mortars; PT1 - finely ground semi-acid mortars; LLIK1, ShK.2, ShKZ - coarse fireclay mortars; PU, PK2 - coarsely ground semi-acid mortars.

Air-hardening clay and clay-free mortars contain additives that increase the strength of joints before sintering. In this case, up to 15% of liquid glass is introduced into the mortars, and 10% of bauxite, alumina hydrate or technical alumina is added to bind the alkali.

The grain composition of aluminosilicate mortars is given in Table 24.

Granulometric composition of mortars

In chromium-magnesite and chromite solutions, air hardening is ensured by the addition of periclase cement, i.e., finely ground highly calcined magnesite, mixed with an aqueous solution of MgSO4 or other salts. Such solutions are usually prepared immediately before use.

Characteristics of mortars

		Chemical composition in terms of calcined substance, %	Operating temperature, °C	Fire resistance, °C, not lower
Aluminosilicate (GOST 6137-61)		Al2O3+TiO2, not less than 60 45
Fireclay, air-solid - acting (TU-04-49)		Al2O3+TiO2, not less than 35
	Fe2O3, no more than 5
Dinas (GOST 5338-60)
Chromium-magnesite air-hardening		MgO, not less than 33

Fire resistance before mixing with liquid glass.

Air-hardening aluminosilicate mortars on an aluminophosphate binder (a.f.s.) are obtained by adding 3-5% aluminum oxide hydrate and 10-15%, respectively, orthophosphoric acid. These solutions produce thin joints of great strength at normal and high temperatures. For the preparation of. starting components use the same equipment as for obtaining fine-grained constituent powders in the corresponding production of refractory products. Solutions from mortars are prepared in mobile batch mortar mixers, immediately before laying. In table Table 25 shows the main indicators of some mortars.

When testing mortars, their chemical and grain compositions, consistency of solutions, water-holding capacity, strength and gas permeability are determined.

Mortars are used for laying linings of industrial furnaces, ladles, recuperators, etc. Fireclay-alumina and fireclay air-hardening mortars are used for laying blast furnaces and air heaters. Dinas - for masonry of coke ovens. Air-hardening chromium-magnesite mortars are used for laying basic refractories in steel-smelting and other furnaces.

The theoretical foundations of the production of refractory materials were first outlined by Academician A. A. Baikov, who considered the process of converting a powdery mass into a solid crystalline aggregate as a process of recrystallization of refractory material in the liquid phase at a certain temperature. In basic terms, this process is similar to the process of hardening cement mixed with water. Therefore, refractory materials can be called “high-temperature cements,” and finished refractory products made from them can be called “high-temperature concretes.”

In the production of refractory products, a mass consisting of a refractory of a certain chemical composition and a binder is subjected to molding, drying and firing. During the molding process, the product is given a given shape using special molding presses. When drying, excess moisture is removed and the product acquires some initial strength. The firing process can be divided into three periods: during the first period, the temperature gradually rises to a certain fairly high level, determined by the chemical and mineralogical composition of the mass; in the second period, which is quite long, the temperature is maintained at a given level; in the third period, the temperature drops to normal, and the fired products are cooled.

The second period is of greatest importance for the quality of the product. At the beginning, the product being fired is a mass consisting of individual grains or grains of refractory material, impregnated and moistened with a small amount of melt. This liquid phase was formed by the interaction of the main oxide, which is a refractory material, with all the impurities present in the mass. The amount of melt formed depends on the temperature and the amount of impurities, and the higher the firing temperature in the second period and the more impurities, the more melt is formed. As a result of recrystallization in the melt at the end of the second period particulate matter form a dense crystalline aggregate. In this case, the mass loses its looseness and acquires mechanical strength. This transformation occurs at a constant temperature (which is below the melting point of the refractory) by recrystallization of the refractory material in a small amount of the liquid phase.

The degree of dissolution of the main oxide in the melt, and therefore the completeness of its recrystallization, depends on the degree of crushing of the starting material, since as the grain size decreases, their solubility increases. Solid with a regular crystal lattice has less solubility than a body with a deformed lattice. Deformation of the crystal lattice can occur during firing either as a result of a polymorphic transformation accompanied by a significant change in volume, or as a result of the decomposition of a chemical compound included in the starting material.

The conditions that must be observed to obtain high-quality refractory products, formulated by A. A. Baikov, are as follows:

• the presence in the charge of such impurities with which the refractory material can produce a melt and can dissolve in it;
• firing at a temperature that ensures the formation of the required amount of melt;
• holding at firing temperature for a time sufficient to complete the recrystallization process.

Classification of refractory materials

Refractories are building materials that deform at temperatures not lower than 1580° C and can withstand prolonged exposure to high temperatures without changing their physical and mechanical properties.

When constructing metallurgical furnaces along with conventional building materials- reinforced concrete, concrete, building bricks - materials are widely used special purpose- fire-resistant, heat-insulating, heat-resistant metals. Of them highest value in metallurgy they have refractories, since metals and alloys in most cases are obtained by high temperature, and the productivity of furnaces largely depends on the quality of the refractories used.

According to chemical and mineralogical composition

Based on their chemical and mineralogical composition, refractories are divided into the following groups.

• Siliceous- dinas (not less than 92% SiO 2), made from quartzite materials (mainly quartzite).
• Aluminosilicate, made from refractory clays and kaolins, which include fireclay (up to 45% Al 2 O 3) and high-alumina refractories (over 45% Al 2 O 3).
• Magnesian, made from minerals containing magnesite with various binding additives. This includes magnesite (at least 85% MgO), dolomite (at least 35% MgO and 40% CaO), forsterite (from 35 to 55% MgO and Cr 2 O 3), spinel (MgO and Al 2 O 3 in molecular ratio ) refractories.
• Chrome, which include chromite (about 30% Cr 2 O 3) and chromium-magnesite (10 - 30% Cr 2 O 3 and 30 - 70% MgO) products.
• Carbon, which contain carbon in varying quantities - graphite (30 - 60% C), coke (70 - 90% C).
• Zirconium: zirconium, made from ZrO 2 and zircon, made from the mineral Zr 2 O 3 SiO 2 .
• Oxidic- products made from beryllium oxide, thorium oxide and cerium oxide.
• Carbide and nitride, which include carborundum (30-90% SiC) refractories and refractories made from nitrides, carbides and sulfides.

According to the degree of fire resistance

According to the degree of fire resistance, materials are divided into three groups:

• fireproof (1580-1750° C);
• highly refractory (1770-2000° C);
• highest fire resistance (>2000° C).

According to GOST 4385 - 68, fireproof materials are in turn divided into classes:

• Class 0 - fire resistance not less than 1750° C;
• Class A - fire resistance of at least 1730° C;
• Class B - fire resistance not less than 1670° C;
• Class B - fire resistance of at least 1580° C.

By heat treatment

According to heat treatment, refractory products are divided into:

• fired (fired after molding);
• non-firing;
• cast fused.

By manufacturing method

According to the manufacturing method, refractories are divided into:

• molded - the shape is given during manufacture (fire-resistant and heat-insulating products);
• unshaped - the shape is acquired during the process of application (refractory concrete, ramming masses, coatings);
• refractory mortars - fillers for refractory masonry joints.

According to the complexity of shape and size

According to the complexity of shape and size, piece refractory products are divided into the following types:

• normal brick;
• shaped product;
• large blocks;
• special products (crucibles, tubes, etc.).

Basic properties of refractory materials

The suitability of certain refractories in each individual case is assessed depending on their basic physical and working properties.

Workers are the properties of refractories that meet the requirements in a given particular case. The main properties of refractories are fire resistance, thermal resistance, chemical resistance, deformation under load at high temperatures and constancy of shape and volume, porosity, gas permeability, thermal conductivity, electrical conductivity.

Fire resistance

Fire resistance is the ability of materials to withstand high temperatures without deforming under the influence of their own weight. When heated, the refractory material first softens due to the melting of its fusible component. With further heating, the bulk begins to melt, and the viscosity of the material gradually decreases. The process of melting refractories is expressed in a gradual transition from a solid to a liquid state, and the temperature range from the beginning of softening to melting sometimes reaches several hundred degrees. Therefore, the softening temperature is used to characterize fire resistance.

For this purpose, when determining the fire resistance of materials, ceramic pyroscopes (PCs) are used. Pyroscopes are triangular truncated pyramids up to 6 cm high with a base in the form of an equilateral triangle with sides equal to 1 cm.

Each pyroscope corresponds to a certain softening temperature, that is, the temperature at which the pyroscope softens so much that its top touches the stand (Fig. 84). The marking of pyroscopes indicates its fire resistance, reduced by ten times. To determine the fire resistance of a material, a pyramid is made from it according to the size of a pyroscope. The test sample, together with several pyroscopes of different numbers, is installed on a stand and placed in an electric oven. The fire resistance test comes down to observing the softening (falling) of samples in comparison with pyroscopes at certain conditions heating The fire resistance of the material is indicated by the number of the pyroscope with which the sample fell at the same time.

Deformation under load at high temperatures

In the furnace masonry, refractories experience mainly compressive force, which increases as the furnace heats up. For assessments of the mechanical strength of refractories usually determine the dependence of the change in deformation on temperature under constant load (Fig. 85).

Tests are carried out on a cylindrical sample with a height of 50 and a diameter of 36 mm under a constant load of 1.96 10 5 Pa. The test results are presented as a graph of the change in sample height versus temperature. To characterize the deformation, note the temperature at which softening begins, when the height of the sample decreases by 4%, the temperature corresponding to a change in height by 40%, and the temperature range of softening, which represents the difference between these two temperatures.

Constancy of shape and volume

When refractories are heated in furnaces, their volume changes under the influence of two factors - thermal expansion and shrinkage (or growth). The thermal expansion of most refractories is low. The change in the volume of the refractory at high temperatures is much more significant due to the transformations that occur. Thus, fireclay products shrink as a result of the formation of a certain amount of liquid phase and compaction of the shard. Usually this reduction in volume is greater than its thermal expansion and leads to enlargement of the seams. Dinas products increase in volume when heated due to additional recrystallization processes. The increase in the volume of the product during service helps to seal the masonry seams. The change in volume of refractories is assessed by heating precisely measured samples in an oven.

Thermal resistance

Thermal resistance is the ability of refractories not to collapse under sudden temperature changes. This is especially important for refractories operating in batch furnaces. The higher the thermal resistance of refractories, the higher the thermal conductivity coefficient of the material, its porosity and grain size, and the lower the temperature coefficient of linear expansion, density, dimensions of the product and changes in volume during allotropic transformations.

To determine thermal resistance, a brick-shaped sample is used. The sample is heated for 40 minutes at 850°C, then cooled for 8-15 minutes. The cycle of heating and cooling is called heat cycle. Cooling can only be in air (air heat exchanges) or first in water for 3 minutes, then in air for 5-10 minutes (water heat exchanges). Heating and cooling are carried out until the mass loss of the sample (due to breaking off pieces) reaches 20%. Thermal resistance is assessed by the number of heat cycles sustained.

Chemical resistance

The chemical resistance of refractory materials is understood as their ability to resist destruction from the chemical and physical effects of the products formed in the furnace - metal, slag, dust, ash, vapors and gases. Slag has the greatest effect on refractories in melting furnaces. In relation to the action of slags, refractories can be divided into three groups - acidic, basic and neutral.

Acidic refractories resistant to acidic slags containing large amounts of SiO 2, but are corroded by basic slags. Dinas is an acidic refractory. Dinas is resistant to oxidizing and reducing gases.

Basic refractories resistant to the action of basic slags, but corroded by acidic ones. These include refractories containing lime, magnesia and alkali oxides (dolomite, magnesite, etc.).

Neutral (intermediate) refractories, which include amorphous oxides, react with both acidic and basic slags, but to a much lesser extent than acidic and basic ones. These include chromium iron ore, containing FeO·Cr 2 O 3 as the main component.

Slag resistance

The slag resistance of refractories depends on the speed chemical reactions refractory with slag and on the viscosity of the slag. With viscous slags and low reaction rates, a refractory product can work well. With increasing temperature, the rate of chemical reactions increases, and the viscosity of slag decreases, so even a slight increase in temperature (by 25-30 ° C) leads to a significant increase in corrosion of refractories. Porous products with open pores are less slag resistant than denser ones. Outdoor smooth surface brick crusts resist the action of slag better than the rough surface of fractures. Cracks in the product also reduce its slag resistance.

To determine slag resistance, two methods are used - static and dynamic. With the static method, a cylindrical hole is drilled in a refractory product into which finely ground slag is poured. The product is heated in an oven until it operating temperature(but not lower than 1450 ° C) and kept at this temperature for 3-4 hours. Slag resistance is judged qualitatively by the degree of dissolution of the product in the slag and the depth of its penetration into the product. In the dynamic method, powdered slag (1 kg) is poured onto the test refractory brick, installed vertically in a furnace, at a temperature of 1450° C for 1 hour. Melting and flowing down the surface of the brick, the slag eats grooves in it. Slag corrosion is determined by volume loss (in cubic centimeters) taking into account additional shrinkage of the brick.

Thermal conductivity

Depending on the purposes for which the refractory is used, its thermal conductivity should be high or low. Thus, materials intended for lining furnaces must have low thermal conductivity to reduce heat losses to the surrounding space and increase the efficiency of the furnace. However, materials for the manufacture of crucibles and muffles must have high thermal conductivity, which reduces the temperature difference in their walls.

As the temperature increases, the thermal conductivity of most refractories increases (Fig. 86). The exception is magnesite and carborundum products, whose thermal conductivity is reduced. The thermal conductivity of all refractories decreases with increasing porosity. However, at high temperatures (above 800-900° C), an increase in porosity has little effect on thermal conductivity. The configuration and size of the pores, which determine the convective heat transfer inside the pores, become influential. An increase in the content of the crystalline phase in the material leads to an increase in thermal conductivity.

Electrical conductivity

Electrical conductivity is a determining parameter of refractories used for lining electric ovens. At normal temperatures, generally all refractory materials are good dielectrics. As the temperature rises, their electrical conductivity increases rapidly and they become conductors. The electrical conductivity of materials with high porosity decreases at high temperatures.

Heat capacity

The heat capacity of refractories determines the rate of heating and cooling of the lining and the heat consumption for heating. This is especially important when operating batch furnaces. The heat capacity depends on the chemical and mineralogical composition of the refractories. It is determined by the calorimetric method. Heat capacity usually increases slightly with increasing temperature. Its average value lies in the range of 0.8-1.5 kJ/(kg K).

Porosity

All refractory products are porous. The size of pores, their structure and number are very diverse. Individual pores are either connected to each other and to the atmosphere, or represent closed spaces inside the product. Hence they distinguish porosity open, or apparent, in which the pores communicate with the atmosphere, porosity closed, when the pores have no outlet, and porosity true, or general, i.e. total.

Open porosity is calculated based on measurements of water absorption and bulk density of refractory products.

Gas permeability

Gas permeability depends on the nature of the refractory, the amount of open porosity, the homogeneity of the structure of the product, temperature and gas pressure. With increasing temperature, the gas permeability of refractories decreases, since the volume of gas increases and its viscosity increases. Refractories should have the lowest possible gas permeability, especially those used for the manufacture of retorts, muffles, and crucibles. The greatest gas permeability is for fireclay products, the lowest for dinas.

Density and bulk mass

The density of a material is the ratio of the mass of a sample to the volume it occupies minus the pore volume. Volumetric mass is the ratio of the mass of a sample dried at 105°C to the volume it occupies, including pore volume.

Appearance and structure

All refractory products are divided into grades in accordance with developed standards. The grade of refractory products is determined by the size of the deviation from the established dimensions, curvature, broken corners, bluntness of the ribs, the presence of individual melts, slagging, notches and cracks. Deviations in sizes are allowed within the limits specified in the relevant standards, depending on the grade. The curvature of products is determined by the deflection arrow. Obviously, the greater the curvature, the less dense the masonry will be. Broken corners and dull edges also negatively affect the quality of the masonry.

Melting is a local melting of the refractory surface with the formation of a “cavity”. The reason for melting is insufficiently good mixing of the charge during the manufacture of the refractory. In places of smelting, rapid destruction by slag occurs even at relatively low temperatures, so the number of smeltings on the surface of the product is strictly limited.

Slagging is formed on the surface of the product in the form of growths as a result of its contamination during firing with sand, clay, etc. The presence of slagging on the surface of products is also limited.

Notches (gaps up to 0.5 mm wide) and cracks (gaps more than 0.5 mm wide) on the surface of refractory products increase corrosion by slag and reduce their mechanical strength. They are formed during the firing process due to careless heating or cooling of the product.

A good quality refractory material should have a uniform structure at the fracture, without voids or delaminations. Grains of different fractions should be evenly distributed over the fracture surface, without falling out or easily chipping.

When choosing a particular material, you must be guided by the basic requirements for it in each specific case. Thus, the material for the walls and roof of the melting furnace must first of all have high mechanical strength. For furnace slopes, a refractory should be used that is more resistant to the action of slags formed during this metallurgical process.

When choosing refractories, their cost should be taken into account. Comparative Cost 1 ton of some 1st grade refractory bricks in relation to the cost of silica bricks is as follows:

Transportation and storage of refractory products

Upon delivery to the consumer correct transportation and storage of finished refractory products ensure their safety, good quality masonry and consistency of performance characteristics. When transported in wagons, refractory bricks are laid in rows tightly across the entire area of ​​the wagon with wedging. Straw or wood shavings are laid between the rows. When transported in vehicles, bricks are also tightly laid in rows with wedging using wooden wedges. IN Lately bricks are transported in containers, which improves their safety and facilitates loading and unloading operations. When transporting bricks to work places on conveyors and trays, they should not hit each other or parts of transport devices.

Mortars and powders are transported in containers, paper bags, or in bulk in clean wagons.

Warehouses for storing fireproof products must be closed. When stored at outdoors Due to alternate wetting and drying, freezing and thawing, the performance characteristics of refractories deteriorate. The decrease in compressive resistance after a year of storage in the open air is 27-30% for fireclay, 35% for silica, and 30% for magnesite products. Allowed in summer time store fireclay and silica products in semi-closed warehouses. Fireproof powders and mortars are stored in closed warehouses in separate bins.

Unshaped refractories and refractory mortars

Unshaped refractories are mixtures of powdered refractory filler and a binder additive.

The use of unmolded refractory materials makes it possible to simplify the process of lining metallurgical furnaces, including complex elements, increase the chemical resistance of the lining and reduce its gas permeability due to the absence of seams, speed up the repair of furnaces. They have found widespread use in
the design of the hearth and roof of furnaces, lining of induction furnaces, chutes for melt release and other elements of complex configuration.

Unshaped refractories include refractory concrete, plastic and non-plastic ramming masses.

Refractory concrete, in which cements are used as a binding material, harden in air at normal temperature in the presence of water. Concrete is laid with slight compaction. The resulting high strength in air does not have a stable ceramic bond, like refractory products, so concrete changes its structure and properties when heated. This explains a slight decrease in the strength of concrete when heated. Portland cement, aluminous, magnesian and high-alumina cements are used as cements. Fillers can be various refractory materials, selected depending on the working conditions and the cement material. The fire resistance of concrete is determined by the fire resistance of the filler.

When using Portland cement in concrete, one should take into account the decrease in their strength and destruction when heated above 600° C due to polymorphic transformations of the cement component 2CaO SiO 2. The introduction of stabilizing additives containing SiO 2 or Al 2 O 3 makes it possible to obtain concrete with sufficient mechanical strength
strength when heated. Concrete based on stabilized Portland cement with fireclay filler can be used up to a temperature of 1400°C, and with chromium-magnesite filler - up to 1700°C.

The most widely used in the production of concrete is alumina cement, which has high speed hardening. Since concrete becomes very hot during the hardening process, it must be watered. This concrete is characterized by a significant loss of mechanical strength when heated in the temperature range of 500-1100 ° C, so it should be used at higher temperatures. Concrete based on alumina cement with fireclay filler is recommended to be used at a temperature of 1150-1400° C. Concrete based on high-alumina and chromium-magnesite filler is used at a temperature of 1400-1700° C.

Magnesia cement is used for the production of highly refractory concrete with magnesite or chromium-magnesite filler. The fire resistance of such concrete is 1900° C.

Recently, concrete has begun to be used with phosphate binders - orthophosphoric or phosphoric acids. In this case, high-quality fully fired refractories are used as fillers: high-alumina fireclay, high-purity fused silica, etc. Phosphate binder concretes have increased fire resistance, high heat resistance and wear resistance. These concretes quickly harden and acquire mechanical strength at low temperatures and adhere well to various refractories.

IN plastic ramming masses The binder is plastic refractory clay. Fillers can be any fire-resistant materials. Most wide application received fireclay, high-alumina, chromite and, in especially critical cases, carbon materials. Plastic ramming masses are characterized by significant shrinkage when heated, which is explained by the high clay content. Their strength increases as the temperature increases due to changes occurring in the clay binder. The placement of ramming masses is carried out by manual compaction or pneumatic compaction.

IN non-plastic ramming masses the binders are aqueous solutions of salts: magnesium sulfate and magnesium chloride, phosphoric acid, various phosphates, boric acid, liquid glass and some organic substances. They provide temporary low strength of the material at normal temperatures and form fluxes at high temperatures, accelerating the recrystallization of the main refractory material to obtain high strength. The use of coal tar pitch and resin as a binder allows, when heated, to create a carbon binder that increases the resistance of the ramming masses to the corrosive action of melts.

Laying of non-plastic refractory ramming mass is carried out under high pressure using a pneumatic rammer, and when lining large areas - with a vibrator. Refractory ramming compounds are used in places with severe working conditions, where high wear resistance of the lining and slag resistance are required, as well as in places where high dimensional accuracy is required. They are widely used for lining induction furnaces, making furnace bottoms, for smelting non-ferrous metals, charging holes for rotary kilns, and holes in the roofs of arc furnaces.

Fireproof solutions- these are masses used to fill the seams in the masonry of the furnace, which provides it with mechanical strength and solidity. Based on their density, solutions are divided into liquid, semi-thick and thick. The thicker the joint, the thicker the solution for filling it should be. Liquid solutions are used for
seam thicknesses of 1-2 mm, which occurs with very dense masonry. The requirements for the properties of mortars are high fire resistance, close to the fire resistance of the masonry material, a high softening temperature and good slag resistance.

The main components of the solutions are refractory material powder and plastic refractory clay, mixed with water. For silica masonry, the solution is made up of finely ground silica powder (85-90%) and high-quality refractory clay (10-15%); fireclay solution contains fireclay powder (70-85%) and
refractory clay (15-30%), etc. At temperatures above 800° C, the solution sinteres with the masonry material. Solutions can be prepared by mixing ready-made dry mixtures with water - mortars, the composition of which is established by GOST. In some cases, it may be necessary to obtain durable masonry at normal temperatures. This is ensured by the use of air-hardening solutions and mortars obtained by adding cement to their composition.

Solutions are not used only for magnesite and chromium-magnesite refractories. They are laid dry with the seams filled with magnesite or chromium-magnesite powder.

Fireproof coatings. To compact the masonry and reduce its gas permeability, as well as to protect the masonry from the effects of the oven environment and as an insulating coating, fire-resistant coatings are used. Hence, according to their purpose, coatings can be divided into three groups - sealing, insulating and protective.

Sealing and insulating coatings applied to the previously cleaned outer surface of the masonry in a layer of 2-4 mm at a surface temperature not exceeding 100°C. Cover with protective coatings with a layer of 2-3 mm inner surface masonry mainly heating and thermal furnaces. They can be used to seal small holes in masonry during hot repairs, when they are applied under pressure using special shotcrete machines. Refractory coatings consist of finely dispersed refractory powders, refractory clays and adhesives, usually liquid glass. Asbestos is also added to the sealing and insulating coatings in quantities of 15 and 40%, respectively. Setting and hardening of coatings occurs as a result of drying and sintering of the mass when heated.

Products of the highest fire resistance

Products of the highest fire resistance are products made from pure oxides, as well as some nitrides, carbides, borides and sulfides. The need for them was determined by their use in modern technology refractory rare metals such as titanium, zirconium, tantalum, niobium, molybdenum, uranium, high purity thorium.

Oxide refractories. Beryllium oxide(BeO) has a melting point of 2530° C. BeO products fired at 1900° C are distinguished by high heat resistance and thermal conductivity, low porosity (apparent porosity is less than 6%, and there is no open porosity). Their gas permeability is insignificant, so they can be used in installations for the distillation of metals in a vacuum.

Thorium oxide(ThO 2) has a melting point of 3300° C. Products made from ThO 2, fired at a temperature of 1500° C, have high density and high fire resistance (3000° C), but low thermal resistance, since with low thermal conductivity they have a large coefficient of linear expansion . Thorium oxide is used for the manufacture of high-temperature heaters for electric resistance furnaces.

Carbides. Carbides of many metals have high melting points and significant chemical resistance. Titanium carbide (TiC) has a melting point of 3140° C. Titanium carbide crucibles with the addition of 1% Na 2 SiO 3 and 2.5% iron powder are used for melting refractory and chemically active metals (sodium, etc.).

Borides. Products made from zirconium and chromium borides have been used in metallurgy. Zirconium boride (ZrB 2) has a melting point of 3040° C. Products made from zirconium boride are resistant to nitrogen and hydrochloric acid, as well as molten metals and salts.

Chromium boride has a melting point of 1850° C. Products made from chromium boride are also resistant to chemically active metals. It is used as a material for the manufacture of crucibles, thermocouple covers, nozzles of high-temperature burners, etc.

Sulfides. Thorium sulfide has a melting point of more than 2500° C. Barium sulfide crucibles are used for melting cerium, thorium, magnesium, and aluminum.

Zirconium and zircon refractories

Refractories containing zirconium dioxide can be divided into two groups - zirconium refractories and zircon refractories. Zirconium refractories, consisting predominantly of zirconium dioxide (ZrO 2), are made from natural rocks- badelite mineral or from zirconium ore containing 80-99% ZrO 2 and up to 20% impurities and oxides various metals. Zirconium dioxide can also be produced artificially by chemical processing of its natural compounds. The charge for the production of zirconium refractories is made up of well-ground zirconium mass, pre-fired in briquettes, and raw zirconium dioxide as a binder (up to 10%). Since products made from zirconium dioxide are characterized by volume variability during heating and cooling, lime is introduced into the charge for stabilization. Products are molded by pressing or cast from a liquid mass, fired at a temperature of 1700 ° C.

Zirconium products are characterized by high fire resistance (about 2500 ° C), high heat resistance (more than 25 water heat cycles), and chemical resistance to the action of both acidic and basic slags. At high temperatures (about 2000 ° C), zirconium dioxide can interact with nitrogen and carbon, forming brittle carbides and nitrides, and with the main slag. Zirconium refractories are used in the manufacture of crucibles for melting non-ferrous metals.

Zircon refractories are made from zirconium silicate - zircon (ZrO 2 SiO 2). Zircon rocks contain 56-67% ZrO 2 and 33-35% SiO 2. Impurities are usually metal oxides - Al 2 O 3, TiO 2, Fe 2 O 3, etc. The production of zircon refractories is similar to the production of zirconium refractories. Zircon products retain a constant volume when heated and cooled, so stabilizers are not introduced into the charge for their manufacture. The main properties of zircon products are a higher softening temperature under load than zirconium (1650° C) and high thermal resistance, fire resistance 1900-2000° C.

Carborundum products

Carborundum - silicon carbide - is obtained by calcining a mixture of pure quartz sand with petroleum coke or anthracite, sawdust and table salt in an electric furnace. The process of carborundum formation begins at 1600 and ends at 2000°C, proceeding through the following reactions:

SiO 2 + 2C = 2CO + Si (steam)
Si + C = SiC
SiO 2 + 3C = SiC + 2CO.

First, amorphous carborundum is formed, which at temperatures above 1900 ° C becomes almost completely crystalline. Wood sawdust are introduced into the mixture to increase the porosity of carborundum and more completely remove volatiles. The presence of table salt helps remove impurities that, forming chloride compounds with NaCl, evaporate when heated. Pure carborundum corresponds to the formula SiC (70.4% Si and 29.6% C). Technical carborundum contains iron carbide, colloidal carbon and various resins as impurities. Carborundum does not melt, but at temperatures above 1900-2000 ° C it decomposes into silicon (steam) and carbon (graphite). Fire resistance of carborundum products is ~ 2000-2200° C.

Depending on the source material and production method, two types of carborundum products are distinguished:

1. products based on clay binders, ferrosilicon or other mineral binders (carbofrax);
2. products recrystalized without binding (refracted).

Raw materials for manufacturing carbofrax products Crushed crystalline carborundum (60-90%) and refractory clay (binder) are used. Products are formed by semi-dry pressing or compaction.

After drying, the products are fired at a temperature of 1380-1450° C.

Carbofrax products are characterized by fairly high heat resistance (at least 20 air heat cycles), high thermal conductivity, which decreases with increasing clay in the charge, high apparent porosity, and high mechanical strength. The temperature at which softening begins under load depends on
amount of clay binder, when it is contained in an amount of 10-20%, the beginning of softening occurs at 1750 ° C. It resists the effects of acidic siliceous slags and the action of acids (except HF and HNO 3), but under the influence of alkalis and oxides of heavy metals, carborundum quickly decomposes. Low stability in an oxidizing atmosphere, oxidizing according to the reaction 2SiC + 3O 2 = 2SiO 2 + 2CO (the SiO 2 film formed on the product somewhat protects it from further oxidation).

Carborundum products with a ferrosilicon binder are characterized by lower porosity (about 10%), and hence lower gas permeability and greater slag resistance.

Refraction products are made from finely ground crystalline carborundum in an organic binder and fired at a temperature of 2300° C. During firing, the carborundum recrystallizes, as a result of which the product acquires strength. Refractive products are characterized by a higher temperature
the onset of deformation under load, high thermal resistance (up to 150 water thermal cycles), significantly higher thermal conductivity, however, they are easily oxidized, since they have significant porosity.

Carborundum is used to make plates for muffles, lining of electric furnaces and electron beam melting furnaces, molds for aluminum casting, distillation columns for zinc production, heaters for electric resistance furnaces, recuperators.

Carbon refractories

Carbonaceous refractories contain at least 30% C and are characterized by high fire resistance, heat resistance, slag resistance, thermal conductivity and electrical conductivity. Carbonaceous refractories can be divided into two groups - coke refractories, consisting mainly of carbonaceous materials (coke, etc.), and graphite refractories, containing graphite and clay materials.

For coke refractories feedstock Foundry cake or petroleum coke, which does not contain ash, is used to increase electrical conductivity. Anthracene oil and pitches with the addition of bitumen are used as a binder. After molding and drying, the products are fired in a reducing atmosphere at a temperature of 1000-1320° C. Coke refractories are characterized by high fire resistance (over 3000° C), high thermal resistance and constant volume. There is virtually no deformation under load at high temperatures. Coke refractories are not wetted by slag, therefore they are not destroyed by them, and have high thermal and electrical conductivity. The main disadvantage of carbon products is their rapid oxidation, so they can only be used in a reducing atmosphere or under a layer of other refractories.

Products cylindrical used as electrodes in arc furnaces.

Graphite occurs in its natural state and is obtained artificially by heating anthracite or petroleum coke in electric furnaces at a temperature of 2300 ° C. Of the graphite refractories, the most widely used in non-ferrous metallurgy are graphite-chamotte refractories, used for the manufacture of crucibles for melting metals and
alloys The charge for their production consists of 30-35% flake graphite, 30-45% fireclay and 30-40% refractory clay. Crucibles are molded in plaster or metal molds, carefully dried and fired in a reducing atmosphere in special capsules filled with coal at a temperature of 700-900 ° C. Before use, crucibles must be calcined at a temperature of 1200 ° C to remove hygroscopic moisture. The fire resistance of graphite products is about 2000° C. They do not deform under load up to a temperature of 2000° C, and are characterized by constant volume (only slight expansion is observed when heated). Graphite products are neutral and have high slag resistance, but at high temperatures carbon interacts with both acidic and basic slags, reduces oxides and oxidizes itself. Therefore, crucibles are corroded by slag mainly at the upper level. A characteristic property of graphite crucibles is their high thermal and electrical conductivity, which determines their use in induction crucible furnaces.

Graphite electrodes used in electric arc furnaces are made by graphitizing carbon electrodes. To do this, a current is passed through electrodes covered with coke in a furnace, heating them to 2000° C. At this temperature, graphitization of carbon products occurs.

Chromite, chromium-magnesite and magnesite-chromite refractories

Chromite, or chromium iron ore, in its pure form corresponds to the chemical compound Cr 2 O 3 FeO with a content of 67.9% Cr 2 O 3 and 32.1% FeO. In addition, it always contains a certain amount of impurities, mainly MgO, Al 2 O 3, SiO 2, etc. Being the most valuable ore for chromium, chromium iron ore is also used as a refractory material. The production scheme for chromite products is fundamentally the same as for magnesite products. When chromite products are fired, as a result of reactions between chromite and other refractory oxides, forsterite, highly refractory spinels and other compounds are formed, which increases the refractory properties of the products. The main properties of chromite products are as follows: relatively high fire resistance (~ 1850° C), but low temperature at which deformation begins (~ 1470° C), heat resistance not exceeding 20 air heat cycles, good resistance to the action of both acidic and basic slags, but they are destroyed with formation of ferrochrome in a reducing atmosphere.

Chromium-magnesite refractories are made from chromite and metallurgical magnesite, with the charge containing 50-60% chromite and 40-50% metallurgical powder.

Magnesite-chromite refractories contain 25-30% chromite and 65-70% magnesite in the charge composition. An increase in magnesite content increases the temperature at which deformation begins and the heat resistance of products. The manufacturing scheme for chromium-magnesite and magnesite-chromite products is similar to the manufacturing scheme for magnesite products.

The main properties of chromium-magnesite products are high fire resistance (~ 1950° C), relatively low temperature at which deformation begins (1450-1530° C), low heat resistance, relatively high porosity, high resistance to the action of basic and acidic slags. The properties of magnesite-chromite refractories are determined by the granulometric composition of the charge, the pressure during pressing of products and the firing temperature.

The properties of products made from a charge consisting of small fractions, manufactured by pressing at a pressure of 80-130 MPa and fired at a temperature of 1500-1600 ° C, are the same as those of chromium-magnesite products, with a slightly higher temperature for the onset of deformation and a significantly higher heat resistance. Magnesite-chromite
high-density products, for which the charge is made up of finely ground magnesite sinter and large fractions of chromite, are pressed at a pressure of at least 130 MPa and fired at a temperature of 1700-1750 ° C. The main properties of such products are high fire resistance (~ 2000 ° C) and heat resistance and high density (low porosity), which increases the service life of these products by 1.5 times.

Chromomagnesite and magnesite-chromite products are used for laying walls and vaults of high-temperature furnaces - arc, heating and smelting.

Forsterite and talc refractories

Forsterite refractories are materials whose main component is chemical compound- forsterite 2MgO SiO 2. The raw materials for the manufacture of forsterite refractories are magnesium-silicate rocks - olivinites, slivinites, serpentinites, etc. When making refractories, MgO is added to the charge to convert low-melting magnesium silicates into forsterite, and iron oxides into magnesium ferrite. An excess of MgO in the charge increases the slag resistance of products and accelerates the formation of shards. The charge is made up of fine fractions of components (<0,5 мм). В качестве связки добавляют сульфатно-спиртовую барду или патоку. Процесс изготовления такой же, как и при изго­товлении магнезиальных огнеупоров. Форстеритовые изделия обла­дают высокой огнеупорностью (1830-1880° С) и температурой начала деформации под нагрузкой (1580-1620° С). Термическая стойкость невысока (14 воздушных теплосмен) и соответствует тер­мической стойкости магнезитовых изделий, но коэффициент тепло­проводности их значительно ниже. По химической стойкости они являются слабоосновными. В изделиях возможно структурное рас­трескивание при поглощении окислов железа. Форстеритовые изде­лия, обладающие сравнительно высокими рабочими характеристика­ми, могут во многих случаях заменить магнезитовые.

The main component of talc is magnesium silicate (3MgO×4SiO 2 H 2 O). Natural talc has a crystalline structure and light gray color, and is easy to machine. Fireproof products are cut from talc stone and fired at a temperature of 1000-1300°C, and when heated to 900°C, talc decomposes:

3MgO 4SiO 2 H 2 O = 3MgSiO 3 + SiO 2 + H2O.

Silica is released mainly in the form of cristobalite. The formation of cristobalite, which has a low density, prevents shrinkage during firing. Therefore, the volume of talc products almost does not change when heated. Talc products resist the action of ferrous slags and iron oxide well, have high heat resistance, and a low temperature for the onset of deformation (1350-1400°C), and above this temperature deformation occurs quickly and sharply.

In non-ferrous metallurgy, talc products are used for lining copper smelting reverberatory furnaces up to the slag hole.

Dolomite refractories

Dolomite refractories are made from the mineral dolomite, which in its pure form is a double carbon dioxide salt of magnesium and calcium (MgCO 3 CaCO 3). Natural dolomite also contains SiO 2, Al 2 O 3, Fe 2 O 3 and some other impurities. In metallurgy, dolomites containing less than 4% impurities are used. Dolomite refractories are used both in the form of fired metallurgical powder and in the form of piece products. As a result of firing dolomite raw materials at a temperature of 850° C, caustic dolomite is obtained.

A feature of dolomite products is the impossibility of firing “tightly”, since only MgO, which forms periclase during firing, practically loses its ability to hydrate. Free calcium oxide CaO can be hydrated after firing. Therefore, burnt dolomite can only be stored indoors and for no more than 2-2.5 months. Firing of dolomite “tightly” and its sintering with loss of hydration ability is achievable only due to fluxing impurities that bind active calcium oxide. The best results are obtained by introducing silica into the charge, which forms tricalcium silicate 3CaO SiO 2 with CaO. To stabilize 3CaO SiO 2, compounds P 2 O 3 and B 2 O 3 are added to it. The mixture is pressed into briquettes, which are fired until sintered. After firing, clinker is obtained, which consists of periclase, tricalcium silicate, crystalline calcium oxide, calcium ferrite (2Fe 2 O 3 CaO) and glass. Products are formed from crushed clinker under a pressure of 50-60 MPa, which, after drying, are fired at a temperature of about 1550 ° C. The fired products are waterproof and allow long-term storage.

It is also known to produce resin-dolomite products, which can be used both fired and unfired. For the manufacture of such products, burnt dolomite is used, crushed to a grain size of less than 8 mm. The binder is a dehydrated resin consisting of 60-70% pitch and 40-30% anthracene oil. The masses are mixed at a temperature of 50-100° C. The prepared mass is pressed and fired at a temperature of 1000-1100° C in a reducing environment. Since in these products MgO and CaO remain mainly in a free state and are capable of hydration, resin-dolomite products are water unstable and can be destroyed during long-term storage. The same applies to unfired resin and dolomite products.

Dolomite waterproof products have a fairly high fire resistance (1780-1800°C), but a low temperature for the onset of deformation (1540-1550°C), are resistant to the effects of basic slags, and have great strength at high temperatures. Their thermal conductivity coefficient is almost three times less than the thermal conductivity coefficient of magnesite products. Resin-dolomite products are characterized by good resistance to the effects of basic slags, a high temperature at the onset of deformation and fairly high heat resistance.

Dolomite refractories, like magnesite refractories, are used in the form of metallurgical powder for welding hearths and products in the construction of furnaces.

Magnesite refractories

Magnesite refractories are those that contain 90% or more MgO. The raw material for the production of magnesite refractories is the mineral magnesite MgCO 3 or magnesium oxide hydrate Mg(OH) 2 obtained from sea water. Magnesite occurs in nature in amorphous form and in the form of crystalline magnesite sword. Amorphous magnesite is almost pure magnesium carbonate, crystalline contains impurities in the form of CaCO 3, FeCO 3, Al 2 O 3, SiO 2, etc. The FeCO 3 content in magnesite reaches 8%, and iron acts as a mineralizer during firing.

Deposits of crystalline magnesite are located in the USSR in the Southern Urals near the Satka station. In some countries that do not have magnesite deposits, the extraction of magnesium salts from sea water and the production of magnesium oxide hydrate by precipitation according to the reactions are organized:

MgCl 2 + Ca(OH) 2 = Mg(OH) 2 + CaCl 2 ;
MgSO 4 + Ca(OH) 2 = Mg(OH) 2 + CaSO 4.

After extraction, magnesite is fired at a temperature of 800-900 ° C to completely remove CO 2 and possibly more complete sintering:

MgCO 3 = MgO + CO 2 - 117780 kJ.

The resulting calcined MgO, called caustic magnesite, is able to hydrate and reabsorb CO 2. Therefore, caustic magnesite is not used as a raw material for the manufacture of refractories, but is used as a binder, since it has good cementing properties.
To obtain a material that is resistant to water and CO2, magnesite must be fired until completely sintered (“tightly”) at a temperature not lower than 1600 ° C. In this case, crystallization of MgO occurs in the form of periclase - a modification of magnesite that is much more resistant to water and CO 2.

Sintered magnesite serves as a raw material for the production of metallurgical powder and fused magnesite. In the first case, the magnesite cake is crushed to a grain size of 5 mm to fine dust and sifted into fractions. In this form it is called metallurgical powder.

To obtain fused magnesite, its sinter is melted in electric arc furnaces. When the melt cools, coarse-crystalline magnesite without impurities is formed. Fused magnesite contains 95% or more MgO. Cast beams and bricks, which have high density and slag resistance, are made from melts. To manufacture products by molding or stuffing, fused magnesite is crushed and sieved and classified into fractions.

When making magnesite products from metallurgical powder or crushed fused magnesite, a charge of a certain granulometric composition is prepared. Since burnt magnesite does not have plasticity, a binder is added to the charge, which is used as sulfate-alcohol stillage, finely ground clay (no more than 2%) or caustic magnesite. The mass is moistened to 3-5% moisture content, thoroughly mixed and placed in special storage for 4-5 days for aging. In this case, some hydration of the dust particles occurs, which gives the mass greater plasticity.

Magnesite products are formed on hydraulic presses under a pressure of at least 90 MPa, and the higher the pressing pressure, the more dense and heat-resistant the products are. After drying, during which an increase in mechanical strength occurs due to the transition of colloidal magnesium hydroxide to crystalline magnesium hydroxide, the products are fired at a temperature of 1600 ° C for 6-7 days.

Along with fired magnesite products, non-fired ones are also used. When manufacturing them, chromium iron ore and a binder - sulfate-alcohol stillage, molasses, etc. - are added to metallurgical powder with a grain size of up to 2-3 mm. Non-firing products are pressed under pressure up to 100 MPa. After drying at a temperature of 200-300° C, the products acquire sufficient mechanical strength without subsequent firing.

Magnesite products have very high fire resistance (above 2000 ° C), are resistant to the action of basic slags, but at high temperatures they are destroyed by iron oxide, carbon and heavy metal carbides, and are not very resistant to water vapor. Magnesite products have high thermal conductivity, but with an increase
the temperature decreases. The temperature at which deformation begins is relatively low (1500-1600° C), however, with an increase in the firing temperature and a decrease in the amount of impurities, it can be increased.

A big disadvantage of magnesite products is their low thermal resistance - the products can withstand only 4-9 air heat cycles, so furnaces with magnesite lining should be heated and cooled very slowly. The low heat resistance of magnesite products is due to the difference in the linear expansion coefficients of periclase and monticellite binder. Replacing the monticellite binder with an aluminous one makes it possible to obtain heat-resistant magnesite products, since the linear expansion coefficients of periclase and aluminous spinel (MgO Al 2 O 3) are close. These products have a lower linearity coefficient
expansion and heat resistance 20 times higher than conventional products. To obtain dense and high-density magnesite products, 3% TiO 2 is additionally introduced into the charge, which increases the density of the scoop. The apparent porosity of these products is 10-15%.

Products with a high temperature of the onset of deformation can be obtained by replacing the monticellite binder with forsterite binder (2MgO SiO 2). Products made from a charge into which 80-85% of metallurgical powder contains 10-15% quartz sand or other siliceous materials and 5% caustic magnesite contain 8-10% silica after firing, which increases the temperature at which the softening begins to 1600-1630° C, but their heat resistance is low.

Products made from fused magnesite are distinguished by a high temperature at which deformation begins (1660°C), low porosity and significant heat resistance, but their cost is high and, therefore, their use is limited.

The main use of magnesite refractories in non-ferrous metallurgy is the laying of walls and bottoms of melting furnaces of mixers. Metallurgical powder is used for welding hearths.

High alumina refractories

High-alumina refractories are those containing more than 45% Al 2 O 3. For their production, minerals of the sillimanite group are used (kyanite, andalusite, sillimanite, containing aluminosilicates such as Al 2 O 3 SiO 2), alumina hydrates (hydrargillite Al 2 O 3 3H 2 O, bauxite Al 2 O 3 nH 2 O, Al 2 O diaspores 3 H 2 O) and artificial raw materials - technical alumina and electrocorundum. Technical alumina, which is a product of chemical processing of bauxite followed by calcination at a temperature of 1000-1200 ° C, contains more than 90% Al 2 O 3. Electrocorundum is produced by smelting materials containing Al 2 O 3 in electric furnaces, followed by purification from
impurities.

The main crystalline phases of high-alumina refractories are mullite and corundum. When the raw material contains less than 72% Al 2 O 3, the only stable solid phase is mullite (3Al 2 O 3 ×2SiO 2). All excess silica and impurities form a glassy substance that turns into a liquid at high temperatures. As the Al 2 O 3 content increases, another stable solid phase appears - corundum. At the same time, there is an increase in the content of the solid phase (see Fig. 88) and a decrease in the content of the liquid phase, which causes an increase in the fire resistance of the products.

There are two methods for producing high-alumina products: molding followed by firing (sintered products) and melt casting (cast products).

When molding sintered products, high-alumina fireclay is used, fired at 1500-1600 ° C. The purest refractory clays and kaolins or temporarily binding organic substances (for example, paraffin), which burn out during firing, are used as a binding material. Organic binder products have a higher softening point. After molding and drying, the products are fired at a temperature of 1600-1650° C

The density of sintered products increases significantly, and the sintering temperature decreases to 1500° C when 2-3% TiO 2 is introduced into the molding mass.

Cast products are made from melts obtained by melting raw materials in arc furnaces. The charge for the manufacture of cast mullite products is made up of a sillimanite group mineral, coke and steel scrap. When the charge melts, mullite is formed according to the reaction 3(Al 2 O 3 SiO 2) + Fe + 2C = FeSi + 3Al 2 O 3 × 2SiO 2 + 2CO.

The molten mullite poured into special molds is cooled very slowly (for 4-10 days), which relieves internal stress in the products, then it is ground to the desired size.

High-alumina products have high fire resistance (1770-1920 ° C), good slag resistance, high mechanical strength, high density, high thermal conductivity and heat resistance. Corundum products have a high temperature at which deformation begins.

High-alumina cast products have very high mechanical strength and slag resistance for any slag composition, but are susceptible to cracking at high temperatures.

Fireclay and fireclay products

Chamotte - an aluminosilicate refractory material - is a mass of refractory clay or kaolin that has been fired to a constant volume and has lost its plasticity. Clay is the product of the destruction of certain rocks, mainly granite, gneiss, and porphyry. The resulting aqueous aluminosilicate Al 2 O 3 2SiO 2 2H 2 O, called kaolinite, is the main component of refractory clays and kaolins. Kaolins contain fewer impurities than refractory clays, so they are used to make higher-quality products.

The most important properties of clays are plasticity, binding ability and sinterability.

Plasticity is the ability of moistened clay in a doughy state to take a given shape, which does not change after the pressure is removed and the water is removed. Depending on plasticity, clays are distinguished between plastic (fat) and lean.

Bonding capacity- the ability of clay with the addition of a certain amount of non-plastic material in the dried state to produce a durable material. Plastic clays have greater binding capacity than lean clays.

Water in clays is contained in the form of hygroscopic water, mixing water and chemically bound. Hygroscopic called water that clay absorbs from the environment. Air-dry clay always contains hygroscopic water. Mixing water- this is the amount of water added that corresponds to the optimal plasticity of the clay. Chemically bound water is found mainly in kaolinites.

When drying, due to partial loss of mixing water, products made from refractory clay decrease in volume by 12-15% for lean clays and by 25-30% for fatty clays. When the clay is heated to 150° C, the remaining mixing water and hygroscopic water are removed. With further heating in the temperature range of 450-650 ° C, chemically bound water is released, and plasticity is completely lost. Heating above 930° C is accompanied by the formation of mullite, and fire shrinkage occurs, which is irreversible.

Caking ability- the ability of clays at certain firing temperatures to form a dense, durable shard called fireclay. Fireclay does not shrink and has high mechanical strength, slag resistance, and chemical resistance.

The fire resistance of clays depends mainly on their composition and lies in the range of 1580-1770 ° C. In Fig. 88 shows a state diagram of the SiO 2 - Al 2 O 3 system, which shows that increasing the alumina content above the eutectic composition increases refractoriness. All impurities reduce the fire resistance of clay. Alkalies K 2 O and Na 2 O cause a particularly strong decrease in fire resistance, so their content in clays above 1% is undesirable.

Depending on the ratio of Al 2 O 3 and SiO 2 in the clay composition, semi-acid, fireclay or high-alumina refractories are obtained.

Fireclay products, most widely used in the construction of metallurgical furnaces, are made from a mixture of unfired plastic refractory clay powder and ground fireclay as a waste component. The presence of fireclay in the charge reduces shrinkage and cracking of the product when heated. The production of fireclay products includes the production of fireclay, the preparation of plastic clay and the manufacture of products from their mixture.

The process of producing fireclay consists of firing clay onto fireclay at a temperature of 1300-1400 ° C. After firing, the fireclay is first subjected to coarse crushing, then fine grinding. The ground fireclay is sifted and divided into fractions according to the size of the grains.

The preparation of refractory clay consists of cleaning it from mechanical impurities and drying it in drying drums. The dried clay is ground in ball mills.

There are two ways to manufacture products - plastic molding and semi-dry pressing. At plastic molding products, fireclay of a certain granulometric composition is mixed with clay in a dry mixer, and for ordinary fireclay products, the mixture is made up of 50-60% fireclay and 50-40% refractory clay. After dry mixing, the mass is sent to a wet mixer, moistened to 16-24% (dry mass), and more for oily clays. Products are molded on presses under a pressure of 1500-2000 kPa.

At semi-dry pressing products, the moisture content of the pressed mass is significantly less than 6-9%. The ratio of chamotte to clay is the same as for plastic molding, but part of the plastic clay is pre-mixed with water to form a slip with which the chamotte grains are moistened. The fireclay, moistened with the slip, and the remaining clay are sent for mixing (when the slip is added to the fireclay, the fireclay grains are well coated with clay). All the necessary mixing water is added to the mixture with the slip. The semi-dry mass is pressed on mechanical presses under a pressure of 10-60 MPa. The semi-dry pressing method has become widespread, since the products have less shrinkage during drying and firing (about 2-3%) and are more dense, mechanically strong and heat-resistant. However, it is difficult to produce complex and massive products using semi-dry pressing. The advantage of plastic molding is its comparative low cost, especially when producing products of complex shapes.

Molded or pressed products are dried. During the drying process, most of the mixing water is removed, and at the same time the volume of the product decreases (shrinkage occurs). To prevent warping and cracking of the product, drying is carried out with gradual and uniform heating. Typically, drying is carried out in special devices at a temperature of 110-120 ° C.

After drying, raw fireclay with a moisture content of 3-5% is sent for firing, which is necessary to convert all the clay included in the raw material into fireclay. During the first firing period, with a slow increase in temperature to 200°C (at a rate of 5°C/min), the remaining mixing water and hygroscopic moisture are removed. In the second period, when the temperature rises from 200 to 900°C, chemically bound water is released. Next, the temperature is increased to 1350°C at a rate of 10-12°C per minute. During this period, the formation of mullite and complex processes of formation of iron silicates, alkali metals and other compounds occur. After firing, the temperature is slowly reduced to 40-50°C.

The general properties of fireclay products are low fire resistance (1610-1730°C depending on the class), relatively low temperature at which deformation begins under load (1200-1400°C), increased apparent porosity (13-28%), relatively high heat resistance, low thermal conductivity , good resistance to acidic (with increased SiO 2 content) and basic (with increased Al 2 O 3 content) slags, high wear resistance and low cost. The main characteristics of fireclay products are given in Appendix IV.

The types of fireclay products include multi-chamotte, non-chamotte, kaolin and semi-acid products. Multi-chamotte products are made from a charge with a high chamotte content of 80-95% and 20-5% of binder refractory clay. The granulometric composition of chamotte is selected so as to obtain the most dense arrangement of grains. Clay is added in the form of a slip. To increase the binding capacity of clay, adhesive additives are introduced into the mixture (sulfite-alcohol stillage about 0.4%). Pressure during molding is 40-50 MPa. Almost no drying is required. Firing is carried out according to the program usual for fireclay. Firing temperature is 1400° C. Products made from multi-chamotte refractories are characterized by high mechanical compressive strength, low porosity, high thermal resistance (up to 100 or more heat cycles), low shrinkage and, therefore, high dimensional and shape accuracy.

Chamotteless products, in which fireclay is replaced by dried sulfate clays, have low porosity, high mechanical strength and thermal resistance. Chamotte-free products are produced by semi-dry pressing.

Kaolin products are made from a charge consisting of 70% kaolin pre-calcined at a temperature of 1400°C, 15% raw kaolin and 15% plastic refractory clay. They are made by semi-dry pressing at a pressure of 40-60 MPa. The firing temperature is 1450-1500° C. Compared to fireclay, kaolin products have higher fire resistance, a higher deformation temperature under load, as well as greater thermal resistance and slag resistance.

Semi-acid products in their composition they are intermediate between dinas and fireclay. They are made from lean or artificially lean clays or kaolins and contain 15-30% Al 2 O 3 and at least 65% SiO 2. Since clay shrinks during firing, and silica increases in volume, then with a certain quantitative ratio of clay and silica it is possible to obtain products that practically do not change in size during prolonged heating. Semi-acid products have fire resistance close to that of fireclay, reduced heat resistance, but an increased temperature at which softening begins under load and low shrinkage. Kaolin increases the heat resistance of semi-acid refractories. Semi-acid products have relatively low porosity.

Dinas refractories

Dinas is a refractory material made from quartzite or quartz rocks and containing at least 93% SiO 2.

Silica can exist in one amorphous and seven crystalline modifications, which, having the same chemical composition, differ from each other in some properties (crystal shape, density, refractive index, etc.). The crystalline modifications of silica are referred to as the crystals found in nature: quartz, tridymite and cristobalite, with each of the major forms subdivided into α-, β- and γ-phases.

In nature, β-quartz is the most common. It is found independently under the name “quartz” and as a component of many rocks: granites, gneisses, sandstones, etc. When heated, silica changes from one modification to another. The transformations of SiO 2 can proceed in two ways, significantly different from each other. The first includes transformations between various modifications within the main forms of silica: quartz, tridymite and cristobalite (Fig. 87). These transformations are reversible and occur quickly.

The second group includes transformations between the main forms of silica - such transformations occur very slowly, and the transformation of quartz into tridymite or cristobalite is practically irreversible.

The rate of slow-moving transformations increases with increasing temperature, increasing grinding, and also in the presence of mineralizers (smoothies). In the production of dinas, they are lime and substances containing ferrous oxide. During the firing of silica, CaO and FeO form fusible silicates with silica, which dissolve silica at high temperatures. From a supersaturated solution, silica crystallizes in the form of the modification that is less soluble at the crystallization temperature.

Since silica modifications have different densities, volumes change during transformations (see Fig. 87).

The degree of transition of quartz into tridymite and cristobalite can be judged by the density of fired products. The lower the density, the more complete the transition. When firing, it is desirable to convert quartz as much as possible into tridymite, which has a smaller change in volume upon cooling. If you lay a furnace of lightly fired brick, in which quartz has not transformed into cristobalite or tridymite, then these transformations will occur in the masonry when the furnace is heated. In this case, the volume of bricks will increase significantly, and the masonry may collapse. Dinas products in which, during firing, most of the quartz turned into tridymite or cristobalite are called tridymite or tridymite-cristobalite.

The raw materials for the production of dinas are quartzites containing at least 95% SiO 2. Quartzites consist of small and microscopic grains of quartz cemented by silica with small amounts of other compounds. The fire resistance of quartzites depends on their chemical and mineralogical composition, but should not be lower than 1750° C.

After crushing and grinding on runners, quartzites are sifted into several fractions. The granulometric composition of the charge depends on the nature of the raw material, the methods of its processing and the purpose of the products. The dinas mixture is made up of quartzite grains ranging in size from the finest flour to 5-6 mm. To bind quartzite grains in the raw material, as well as to accelerate the transformation of quartz, 1.5-3% lime is usually added in the form of milk of lime. A mixture of quartzite and lime milk is crushed using runner rollers. After molding on presses and drying, the raw material is fired in tunnel kilns.

Firing dinas is the most important operation. The temperature rise should be uniform and slow, especially at the points of transition of quartz from one modification to another. With a rapid rise in temperature, the quartz grains crack, the brick greatly increases in volume and loosens. In addition, the faster the temperature rises, the less liquid phase is formed. If there is a sufficient amount of liquid phase, it fills the space between the recrystallizing quartz grains and absorbs the resulting stresses. If the amount of liquid phase is insufficient, the so-called dry transformation of α-quartz into α-cristobalite occurs, and the raw material swells and cracks due to a strong increase in volume.

The maximum firing temperature should not exceed 1460° C, since at higher temperatures not only α-quartz, but also α-tridymite is converted into α-cristobalite. A large amount of cristobalite in dinas is undesirable, since the volume will change greatly during heating and cooling. When cooling fired silica, care must also be taken, especially when silica changes from one modification to another. The conditions for firing dinas must also be observed when heating the furnaces.

Dinas products are characterized by relatively low fire resistance (1710-1720° C), but a high temperature at which deformation begins under load (1620-1660° C). The main characteristics of dinas are given in Appendix IV.

Tridymite-cristobalite dinas retains mechanical strength and does not change shape until almost the melting point. Therefore, silica brick is widely used in metallurgy, especially where high mechanical strength at high temperatures is required. The heat resistance of dinas is very low, no more than two heat cycles, however, with slow heating and cooling, dinas is able to withstand multiple heat changes well without losing mechanical strength.

In terms of chemical resistance, dinas is a typically acidic refractory. The change in size upon heating of well-fired, completely recrystallized silica is insignificant. But since during the manufacture of bricks the complete transformation of quartz is not achieved, a certain increase in volume occurs during repeated heating. Thus, when heated to 1450° C, the change in linear dimensions reaches 1.6 - 2.1%, and subsequent expansion can reach 0.7%. This should be taken into account when laying the stove, providing expansion joints.

Dinas refractories are widely used for laying the roofs of smelting furnaces due to their lack of additional shrinkage during long service life at high temperatures.

High-density dinas with a content of at least 98% SiO 2 and an apparent porosity of about 10% is made from high-silica pure quartzites, and the raw material is subjected to strong pressing before firing. High-density dinas has increased fire resistance (up to 1740 ° C) and heat resistance. Having less porosity, it is more resistant to slag. Used for lining high-temperature melting furnaces. Electrodynas has characteristics close to high-density dynas. Used for lining the roofs of electric melting furnaces.

Conventional solutions are not designed to withstand temperature changes, as a result of which they crack and quickly crumble. The refractory mixture for furnaces is distinguished by its strength, resistance to high temperatures, and durability. A simple solution almost does not stick to the smooth surface of the oven and areas made of metal. Refractory mixtures, on the contrary, adhere perfectly to all surfaces and are convenient for working with stoves.

Types of plaster

Fireproof plaster for stoves and fireplaces is divided into two types: simple and complex. A simple one consists of two components; for example, for a Russian stove, clay and sand are usually used. Complex plaster consists of three ingredients, and sometimes more:

• asbestos, clay, sand;
• sand, clay, cement, asbestos;
• sand, fiberglass, lime, gypsum.

At high temperatures, asbestos begins to release toxic substances, for this reason it is replaced in fire-resistant solutions with fibrous reinforcing additives.

Properties of refractory mortars

To make it easier to understand why heat-resistant mixtures are used in the construction of stoves and fireplaces, you need to consider their properties:

Ready-made fire-resistant mixtures

Today, the building materials market surprises with the variety of fireproof products. The most popular of them, which have received many good reviews:

1. There are three types of heat-resistant mixtures produced by this company:

• smooth;
• simulated;
• for tiled surfaces.

When using a modeled one, it is possible to obtain a textured surface on which any design can be applied. A smooth surface is perfect for painting the stove.

1. "Terracotta". The dry mixture consists of clay and fine fireclay sand and fireproof additives. The solution is intended for ceramic and fireclay bricks and can withstand temperatures up to 200 ºC.
2. "Stove maker." It includes:

• lime;
• sand;
• gypsum;
• asbestos;
• cement;
• clay;
• mineral supplements.

The mixture can withstand temperatures up to 600 ºC. The only negative side is the asbestos included in the composition, which is not an environmentally friendly product.

1. Plitonit-SuperFireplace FireUpor. This mixture consists of fire-resistant fibers and an adhesive base; it is resistant to high temperatures during the use of the oven and does not crack.

Ready-made dry mixtures are popular and, according to reviews about them, are practically free of drawbacks, but their cost is not cheap.

Preparing the mixture

If you want to save money on building a stove or fireplace, you can avoid purchasing expensive materials and prepare the solution yourself.

Simple mixture

A regular solution can be prepared from sifted sand, which must be moved with clay. After this, you need to add the required amount of water and alternating eat until smooth. The proportions of sand and clay depend on the fat content of the second component. If the clay is oily, you need to add three parts of sand. To determine the fat content of the clay, place a small piece of it at the bottom of the bucket and fill it with water, continuously stirring until you get a homogeneous mass. Next, immerse any board in the resulting mixture. If the board is covered with a 1 mm layer of clay, it means it is non-greasy. A thicker layer suggests the opposite.

Complex solution

Complex mixtures consist of different components, which are selected depending on which area of ​​the fireplace they will be applied to. To make preparation easier, it is advisable to use a construction mixer.

Mixture No. 1. Composition:

• clay – 1 tsp;
• lime – 1 tsp;
• sand – 2 hours;
• reinforcing additives – 1/10 part.

To prepare the solution, you need to sift all the components and mix them dry. Next, add water and stir until smooth.

No. 2. Composition:

• clay – 1 tsp;
• sand – 2 hours;
• cement – ​​1 hour;
• reinforcing additive – 1/10 part.

To prepare, you also need to mix clay and sand, add water and stir until smooth. Then you need to add cement and add a reinforcing additive, add water and mix until a thick, homogeneous mass.

No. 3. Composition:

• gypsum – 1 hour;
• sand – 1 hour;
• lime - 2 hours;
• reinforcing additive – 2/10 parts.

After sifting the components, you need to mix dry lime, sand and reinforcing additive. Next you need
Next, pour the mixture with water and bring it to a homogeneous state, add gypsum and stir again until the solution becomes thick.

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To lay the furnace, you must have materials with fire-resistant properties on hand. As a rule, this is brick and fireproof mortar for the stove. If the brick is a piece material, then the mortar still needs to be prepared. You can purchase ready-made masonry mixture in specialized stores or prepare the mixture yourself, following a certain set of technical rules and technological recommendations.

Clay is the main component of the mortar for laying the furnace.

Clay deposits are found in almost all regions of our country. The main places where you can find high-quality clay are steep banks of rivers and ravines. When carrying out excavation work at a depth of more than 500 mm, you can also stumble upon layers of clay. Clay can be oily, medium fat (normal) and lean. The best option would be to use normal clay, since if a solution with fatty clay is used, when it dries, significant shrinkage will occur, followed by the formation of cracks in the furnace masonry. Using lean clay will reduce the plasticity and increase the fragility of the mixture, which will lead to the destruction of the seams. Both options are fraught with the unpleasant penetration of acrid smoke into your home.

Checking the properties of the main ingredient of the refractory mixture

There are several proven methods for checking the quality of clay:

1. Take about 1 kg of dry clay (0.5 l) and pour water into it portionwise, mixing it with your hands. The clay should completely absorb water and be a solution with a thick consistency. The next step is to roll balls with a diameter of 4-5 cm. A flat cake with a diameter of 9-10 cm is made from the resulting ball. All this is dried naturally for 3-4 days. Next, inspect for surface cracks. Identification of cracks on the ball and cake indicates increased fat content of the material. If no cracks are found on the ball and cake, then it is necessary to drop the ball from a height of no more than 1 m. The preservation of the integrity of the ball after the fall indicates the quality of the clay, and destruction shows that the clay is thin.
2. Take approximately 3.6-5.4 kg of clay (2-3 l) and pour it into a container, thoroughly mixing with a wooden spatula and kneading the lumps. If the clay sticks to the shovel quite well, it means it has a high fat content. You need to add a little sand to this solution. If the clay remains partially on the blade, then such material is considered high quality and suitable for use. Poor adhesion of the mixture indicates that the mixture is lean and requires the addition of fatty clay.
3. Take up to 1 kg of dry clay (about 0.5 l) and prepare a thick solution, mixing thoroughly with your hands. Balls with a diameter of 4-5 cm are prepared from the resulting composition. Next, take two smooth plates of chipboard or wood, place the ball on one of them, cover with the other and squeeze until cracks appear on the ball. Test control:

• if the ball collapses at the slightest pressure, it means the clay is thin;
• if cracks appear when squeezing up to 1/4-1/5 of the diameter of the ball, then the clay has a low fat content;
• if cracks appear when compressed to 0.3 of the ball diameter, then the mixture is normal and suitable for further use;
• Oily clay cracks when compressed to 0.5 times the diameter of the ball.

1. A ball is made from the resulting steep solution and rolled out until sausages with a diameter of 1-1.5 cm and a length of 160-200 mm are formed. Next, they are stretched until they break. A sample made of lean clay practically does not stretch and gives a rather uneven break. Normal clay is characterized by smooth stretching and breaks when thinned to 20% of the original sample. Oily clay, on the contrary, stretches out gradually and gives a smooth break with the formation of sharp ends at the point of break.

Preparation of components for preparing masonry mixture

Checking the plasticity of the solution: 1 - plastic, 2 - insufficiently plastic, 3 - loose.

To achieve the required fat content, different types of clays are mixed or sand is added, controlling the fat content using the methods described above. The refractory clay selected for the preparation of the solution must be sifted through a sieve with a mesh size of 2 to 3 mm to remove impurities and large particles. This is justified by the fact that the standard thickness of the seam when laying a furnace should be 3 mm. Therefore, large particles in the solution will interfere with the masonry.

There is another method for cleaning clay. Take an oblong trough and set it at an angle of 5-10°. A layer of clay is placed on the raised part, and water is poured into the lower part. Then, with a ladle or trowel, water is poured onto the layer of clay until the latter is completely dissolved. The resulting solution is filtered into a separate container, the clay is settled and dried.

According to the technology, sand must be added to fatty clays, which requires preparation. Sand can be of three types: river, sea and regular quarry (mountain) sand, which is mined in industrial quarries and on the slopes of natural ravines. It is better to prepare the solution with the addition of quarry sand. It provides better adhesion of the mating surfaces of the brickwork and mortar components. The sand also needs to be sifted on a grid with a mesh size of 1.5 mm. After sifting, the sand is washed to remove impurities. To do this, take burlap and stretch it onto a rectangular frame 70-100 mm thick. The frame is placed on a stand. Sand is poured onto the surface of the burlap and washed with water from a hose.

Preparation of fire-resistant masonry mixture

Once all the preparatory work is completed, you can prepare the mixture for laying the stove. There are several cooking methods:

1. The prepared clay is soaked for 3 days in an airtight container. Next, put on waterproof shoes (rubber boots) and knead until smooth, adding sand in the required proportions. Unmixed clay clumps are broken up with a tamper. Next, the homogeneous clay composition is probed by hand for the presence of foreign particles and pieces of clay. A properly mixed solution should flow easily from the metal surface of the spatula or trowel without sticking to it. A high-quality solution should begin to set within 4-6 minutes. The surface of a wooden cutting dipped into the mixture should have slight traces of clay. A greasy clay composition will leave significant marks, while a thin clay composition will not stick to the handle at all.
2. The second method is used only when the clay composition does not require additional sand and has normal fat content. To prepare, you need to have a wooden shield on hand. Clay is placed on the shield and watered. As soon as the clay is saturated with moisture and softens, it is shoveled. To do this, narrow elevations of different lengths and a height of 30-40 cm are formed. These elevations are struck with a shovel, cutting off parts of the ridge. Such manipulations break up the lumps. Insoluble particles and stones are removed manually. Then the mass is mixed again and the operation is repeated 4 to 6 times until the stones are completely mixed and removed.
3. Preparing a mixture for masonry by adding sand to the clay. To do this, sand is poured into a bed in which depressions are made. Clay is brought into these depressions, filled with water, sprinkled with a layer of sand and wait until the clay absorbs the water. Next, the ridge is mixed and kneaded with a shovel in the same way as in the previous case, until a homogeneous consistency. The proportions of sand and clay should be such that the clay completely binds all the grains of sand. To improve the quality of the solution, filter it through a sieve.

Fireproof finishing mixture

After finishing the laying of the stove, it is necessary to finish the outer surface of the stove. To do this, you need to make a solution for plastering work. There are several recipes for preparing dry plaster mixture:

1. Mix 1 part refractory clay, 1 part lime, 2 parts sand and 1/10 part asbestos fluff.
2. Mix 1 part dry clay, 2 parts sand, 1 part 400 grade cement and 1/10 part asbestos fluff.
3. Mix 1 part ordinary gypsum, 1 part fine sand, 2 parts lime and 2/10 parts asbestos fluff.