Refractory castable is a kind of refractory material which is composed of a certain grade of refractory aggregate and powder material mixed with binder and admixture, and is used directly without forming and firing process.

In recent years, the output of non-shaped refractory products has been increasing, and the application range has become wider and wider. 

The refractory castable is a new type of refractory material that does not require high temperature firing, is directly used after construction and baking, and has simple production process, saves energy and labor, can be mechanized, has good integrity, is easy to repair, and has a long service life.

01

Classification of refractory castables

Refractory castables are usually classified according to the size of the porosity, the selected binder or bonding method, the type of aggregate, and the construction method.

According to the porosity, it can be divided into dense refractory castables and heat-insulating refractory castables.

According to the cementation method, refractory castables can be divided into four types: hydrated-bonded refractory castables, chemically-bonded (including polymerized-bonded) refractory castables, hydrated and polymerized refractory castables (typically represented as low-cement-bonded refractory castables), and condensed-bonded refractory castables.

Classified according to the special effects of binders and certain materials, they can be divided into the following seven categories:

(1) clay combined with refractory castable;

(2) Ultrafine powder (such as silica fume, etc.) combined with refractory castables;

(3) Cement-bonded refractory castables;

(4) Chemically bonded refractory castables;

(5) γ-Al 2O (hydrated Al 2O) combined with refractory castables;

(6) Low cement combined with refractory castables;

(7) silicon, aluminum sol (sol-gel) combined with refractory castables.

According to the combination of raw materials, refractory castables can be divided into oxide refractory castables, non-oxide refractory castables and composite refractory castables. Oxide refractory castables can be subdivided into non-alkaline refractory castables and alkaline refractory castables.

According to the construction method, refractory castables can be divided into two categories: vibration construction refractory castables and self-flowing refractory castables.

According to whether the refractory castables contain cement or CaO content, they can be simply divided into four categories: ordinary refractory castables, low cement refractory castables, ultra-low cement refractory castables, and no cement refractory castables.

The refractory aggregate is the + 0.88mm or + 0.1mm part of the refractory castable, which is the main material in the refractory castable's organizational structure and plays the role of the skeleton. The refractory powder is the matrix part of the castable. After high temperature action, the refractory aggregate is joined or cemented to fill the pores, achieve tight accumulation, ensure the fluidity, volume stability and promote sintering of the mixture, and ultimately improve the density, strength and high temperature performance of the refractory castable.

Therefore, the performance of refractory castables mainly depends on the raw materials used in the formula. In order to improve the performance of refractory castables, most manufacturers have used high-purity raw materials, homogeneous materials, fused materials, synthetic materials, transformation materials and ultrafine powders in the main raw materials. Let's talk about the 24 kinds of raw materials that are often used in the formulation of refractory castables.

02

Main raw material of refractory castable

(1) Sintered corundum.

Sintered corundum, also known as sintered alumina or semi-molten alumina, is a refractory clinker made of calcined alumina or industrial alumina as raw material, finely ground into balls or blanks, and sintered at a high temperature of 1750-1900 ° C. Sintered alumina containing more than 99% aluminum oxide is mostly directly combined with homogeneous fine-crystal corundum. The gas yield is below 3.0%, the volume density reaches 3.60%/m3, the refractoriness is close to the melting point of corundum, and it has good volume stability and chemical stability at high temperature. It is not eroded by reducing atmosphere, molten glass and molten metal. It has good mechanical strength and wear resistance at room temperature and high temperature.

(2) Electrofused corundum.

Fused corundum is artificial corundum made by melting pure alumina powder in a high temperature electric furnace. It has the characteristics of high melting point, high mechanical strength, good thermal shock resistance, strong corrosion resistance and small linear expansion coefficient. Fused corundum is the raw material for the manufacture of advanced special refractory materials. Mainly include fused white corundum, fused brown corundum, sub-white corundum, etc.

(3) fused white corundum.

Fused white corundum is made of pure alumina powder as raw material and smelted at high temperature, which is white in color. The smelting process of white corundum is basically the process of melting and recrystallization of industrial alumina powder, and there is no reduction process. The content of Al 2O O is not less than 9%, and the content of impurities is very small. The hardness is slightly smaller and the toughness is slightly lower than that of brown corundum. It is often used in the production of abrasives, special ceramics and advanced refractory materials.

(4) Fused brown corundum.

Fused brown corundum is made of high bauxite as the main raw material and coke (anthracite), which is smelted in a high-temperature electric furnace above 2000 ° C. The main chemical composition is Al 2O, TiO 2O and contains a small amount of SiO 2O and Fe 2O O. Fused brown corundum has a dense texture and high hardness, and is commonly used in ceramics, precision casting and advanced refractory materials.

(5) sub-white corundum.

Sub-white corundum is made by electrofusion of extra-grade or first-grade bauxite under reducing atmosphere and controlled conditions. When melting, a reducing agent (carbon), a settling agent (iron filings) and a decarburizing agent (iron scales) are added. Because its chemical composition and physical properties are close to white corundum, it is called sub-white corundum. Its bulk density is above 3.80g/cm3, and the apparent porosity is less than 4%. It is an ideal material for manufacturing advanced refractory materials and wear-resistant materials.

(6) Mullite.

Mullite is a refractory raw material with 3Al _ 2O _ 2 SiO _ 2 as the main crystal phase. Natural mullite is very rare and is usually synthesized by sintering or electrofusion. Mullite has the characteristics of uniform expansion, good thermal shock stability, high load softening point, small high temperature creep value, high hardness and good chemical corrosion resistance.

(7) Zirconium corundum mullite.

Zircon corundum mullite is synthesized from industrial alumina, kaolin and zircon as main raw materials, through fine grinding, uniform mixing, semi-dry pressure ball, and high temperature calcination at 1600-1700 ℃. Increasing the content of zircon will lead to an increase in sintering temperature, a decrease in total shrinkage, and an increase in closed pores. These reactions make sintered zircon corundum mullite have higher density and strength, as well as better thermal shock stability and slag resistance.

(8) magnesium aluminum spinel.

Magnesium-aluminum spinel is synthesized from industrial alumina and light-burned magnesium oxide by sintering or electrofusion at high temperature. The chemical formula of magnesium-aluminum spinel is MgO · Al 2O O, in which the content of MgO is 28.2% and the content of Al 2O O is 71.8%. It has the advantages of high temperature resistance, wear resistance, corrosion resistance, high melting point, small thermal expansion, low thermal stress, good thermal shock stability, strong resistance to alkaline slag erosion and good electrical insulation properties.

(9) sillimanite, andalusite, kyanite.

The chemical formula is Al 2O O < unk > · SiO < unk >, and the theoretical composition is Al 2O O < unk > 63.1%, SiO < unk > 36.9%. After heating, they are irreversibly converted into mullite and calcite, which have the advantages of good slag corrosion resistance, good thermal shock stability, and high load softening point. Kyanite group mineral products are high-quality raw materials for amorphous refractories. Silicite and andalusite can be directly made into bricks or used as refractory aggregates due to small volume changes during heating. Kyanite expands greatly when heated, and can be used directly as an expansion agent for amorphous refractories.

(10) high bauxite.

China's high bauxite resources are mainly distributed in Shanxi, Henan, Guangxi and Guizhou. The high-bauxite clinker calcined at high temperature is mainly used for high-alumina refractory materials, and can also be used to make fused brown corundum and sub-white corundum. In recent years, the homogenized bauxite clinker produced in China has achieved good results in the application of amorphous refractories due to its low absorption rate and stable performance.

(11) Soft clay.

The mineral composition of soft clay is mainly kaolinite or multi-hydrated kaolinite, mixed with other impurity minerals, the content of Al _ 2O _ O can range from 22% to 38%, and the average refractoriness is about 1600 ° C. Soft clay is mostly soil-like, with fine particles, easily dispersed in water, and has strong plasticity and adhesion. It is widely used in plastics, ramming materials, spraying materials, refractory slurries, and medium and low-grade refractory materials.

(12) Clay clinker.

According to the different raw materials and production methods used, refractory clay clinker can be divided into two types: one is obtained by calcining hard clay blocks directly in a kiln; the other is made of kaolin or hard clay, which is finely ground, homogenized, filtered, dehydrated, dried, and finally burned in a kiln. It is a high-quality clay clinker. The main mineral phase of hard clay clinker is mullite, accounting for 35% to 55%, followed by glass phase and calcite. Clay clinker is the main raw material for ordinary aluminum silicate refractories.

(13) Magnesite.

Magnesite is a natural alkaline mineral raw material mainly composed of magnesium carbonate (MgCO3). China is rich in magnesite resources with high quality and large reserves. Magnesite is mainly distributed in Liaoning Province. Magnesite is mainly used in the production of sintered magnesia, fused magnesia and the production of basic refractory materials.

(14) sintered magnesia.

Sintered magnesia is the product of fully sintering magnesite at 1600-1900 ° C. The main mineral is periclase. The MgO content of high-quality magnesia is generally above 95%, and the particle volume density is not less than 3.30g/cm3, which has excellent resistance to alkaline slag erosion. Sintered magnesia is one of the main raw materials for the production of alkaline refractories.

(15) Fused magnesia.

Fused magnesia is made by melting selected magnesite or sintered magnesia in an electric arc furnace at a high temperature of 2500 ° C. Compared with sintered magnesia, the main crystal phase periclase has coarse grains and direct contact, high purity, dense structure, strong alkaline slag resistance, and good thermal shock stability. It is a good raw material for advanced carbon-containing unfired bricks and amorphous refractories.

(16) Silicon carbide.

Silicon carbide is usually made of a mixture of coke and silica sand as the main raw materials by high temperature melting in an electric furnace. β-SiC (cubic crystal) is formed at a temperature of 1400-1800 ° C, and α-SiC (hexagonal crystal) is formed when the temperature is higher than 1800 ° C. Silicon carbide has high hardness, high thermal conductivity, low thermal expansion rate and excellent resistance to neutrality and acidic slag. The composition of commercial silicon carbide ranges from 90% to 99.5% SiC. Silicon carbide with higher purity is often used in refractory castables, spraying materials, ramming materials and plastics.

(17) silica fume.

Silica fume is a by-product of the production of ferrosilicon and silicon products. The appearance is white to dark gray fine powder, its particles are round, the particle diameter is generally 0.02~ 0.45μm, the specific surface area is about 15~ 25m2/g, and the volume density is 0.15~ 0.25g/cm3. In recent years, some silica fume has been used as a leading product, rather than a by-product. It has high purity, white color, and stable composition. Application in artesian castables shows good rheology.

(18) Graphite.

Graphite is divided into artificial graphite and natural graphite. Artificial graphite is made in two ways by sintering petroleum coke (heated to above 2800 ° C) or by the process of graphite electrodes. Natural graphite crystals are hexagonal crystals with rhomboid symmetry. There are usually three forms: amorphous graphite, amorphous graphite and pure crystals. Amorphous graphite (without morphology) and artificial graphite have better fluidity than amorphous graphite and crystalline graphite in castable applications.

(19) Asphalt.

Coal tar asphalt has a higher amount of residual carbon than petroleum asphalt, which can effectively provide carbon components to refractory materials. Depending on the formulation design requirements of the material, it can be used in the form of fine powder or granules. The use of asphalt in amorphous refractory applications is superior to other forms of carbon (such as graphite) because asphalt has a low melting temperature and can wrap particles, thus providing a good protective layer against slag erosion.

(20) calcium aluminate cement.

The main method of producing high alumina cement is sintering. Pure limestone is the calcium oxide raw material for the production of all calcium aluminate cement. Sintered alumina is used for the production of high-grade calcium aluminate cement, while low iron, low silicon bauxite is used as the alumina raw material for medium-grade and low-grade high alumina cement. Pure calcium aluminate cement or high alumina cement is the most important hydraulic cement used in the combination of refractory castable and spraying material. During the construction of refractory castable lining, the water temperature and amount of water added, the mixing strength and time, the temperature and the heating rate must be strictly controlled. Among them, the temperature is the most important parameter, which significantly affects the formation of cement binding phase and the discharge of moisture in the initial heating.

(21) Silica sol.

Silica sol is an aqueous colloid dispersed with silica particles. It is a milky white liquid that is somewhat viscous to the touch and has a high specific surface. Silica sol can be cemented by dehydration, changing pH, adding salts or organic solvents that are miscible with water. When dried, silica (Si-0-Si) binding is formed on the surface of the particles by rapid dehydration, resulting in polymerization and internal bonding. Silica sol is converted from a solution to a solid, commonly known as cementation. It is commonly used in coatings, castables, pumping materials, ramming materials, and spraying materials.

(22) Sodium silicate.

The commonly used silicates are sodium silicate (Na _ 2O · mSiO · nH _ 2O, potassium silicate, and lithium silicate. The dehydration of sodium silicate is usually transparent like glass and soluble in water, so it is also called water glass. The molar ratio of SiO _ 2O/N _ 2O in industrial products (called the modulus of water glass) is between 0.5 and 4.0, and the molar ratio of sodium silicate for refractory materials is 2.2 to 3.35. The viscosity of sodium silicate aqueous solution is affected by its molar ratio and concentration, and changes significantly with temperature. Sodium silicate hydrates in aqueous solution, and the solution is alkaline. The smaller the molar ratio, the more obvious the hydration of sodium silicate, and the pH value decreases with the decrease of molar ratio. The hydration reaction of sodium silicate with higher moles is slow. The choice of curing agent for sodium silicate combined with refractory materials needs to be determined according to the application of refractory materials. Commonly used curing agents are sodium fluorosilicate, polyaluminum chloride, phosphoric acid, sodium phosphate, polyaluminum phosphate

(23) Phosphoric acid and phosphate

Orthophosphoric acid itself has no adhesion. When it comes into contact with refractories, it exhibits good adhesion due to the rapid reaction between the two to form phosphate. Different forms of phosphate can be used as binders. The most commonly used salt in refractory materials is aluminum phosphate. As a binder, aluminum dihydrogen phosphate is famous for its solubility, bonding strength and stability in water. Sodium phosphate is mainly used in refractory materials for coagulation, depolymerization and as a binder for alkaline spraying. Sodium polyphosphate is often used as a water reducer in castables. In addition, sodium phosphate can react with alkaline earth metal compounds (such as CaO and MgO) to produce coagulation. It is based on this property of sodium phosphate that it is used in magnesium basic spraying materials.

(24) δ-Al 2O O

It is different from other crystalline states of Al 2O O < unk > and is the worst crystallized variant of Al 2O O < unk >. Among the various crystalline states of Al 2O O < unk >, only p-Al 2O O < unk > has a spontaneous hydration reaction at room temperature, and the hydrated boehmite and boehmite sol can play a cementing and hardening role. At high temperatures, p-Al 2O O < unk > eventually transforms into an excellent refractory α-Al 2O O < unk > (corundum). Therefore, this castable combined with p-Al 2O O < unk > can be regarded as a refractory material