I. How are refractory bricks classified?

　　According to the preparation process, they can be divided into burnt bricks, unburnt bricks, electrofused bricks (cast bricks), and refractory heat-insulating bricks; according to shape and size, they can be divided into standard bricks, ordinary bricks, and special bricks, etc. They can be used as high-temperature building materials and structural materials for building kilns and various thermal equipment, and can withstand various physical and chemical changes and mechanical actions at high temperatures. For example, refractory clay bricks, high-alumina bricks, silica bricks, magnesia bricks, etc.

　　II. What are the types of refractory bricks?

　　According to the different components of refractory bricks, they can be divided into five categories: silica-alumina refractory bricks, alkaline series refractory bricks, carbon-containing refractory bricks, zirconia-containing refractory bricks, and heat-insulating refractory bricks. Any furnace is not built with only one kind of refractory brick, but requires the combination of different refractory bricks. The components of these five categories of refractory bricks are described in detail below.

　　1. Silica-alumina refractory bricks

　　It is understood that it is based on the AL2O3—SiO2 binary system phase diagram as the basic theory, mainly including the following types.

　　① Silica bricks

　　Refers to refractory bricks with SiO2 content above 93%, which is the main type of acid refractory bricks. It is mainly used for building coke ovens, and also for various glass, ceramic, carbon calcination furnaces, and the arch tops and other load-bearing parts of refractory brick thermal kilns. It is also used in the high-temperature load-bearing parts of hot blast stoves, but it is not suitable for use in thermal equipment below 600℃ with large temperature fluctuations.

　　② Clay bricks

　　Clay bricks are mainly composed of mullite (25%～50%), glass phase (25%～60%), and quartz and quartz (up to 30%). Usually, hard clay is used as the raw material, pre-calcined into mature materials, and then mixed with soft clay, and formed by semi-dry method or plastic method, and the temperature is 1300～1400C to burn clay brick products. It can also be made into unburnt products and monolithic materials by adding a small amount of water glass, cement and other binders. It is a commonly used refractory brick in blast furnaces, hot blast stoves, heating furnaces, power boilers, lime kilns, rotary kilns, ceramic and refractory brick firing kilns.

　　③ High-alumina refractory bricks

　　The mineral composition of high-alumina refractory bricks is corundum, mullite and glass phase, the content of which depends on the AL2O3/SiO2 ratio and the type and quantity of impurities, and the grade of refractory bricks can be classified according to the AL2O3 content. The raw materials are high-alumina bauxite and sillimanite-type natural minerals, and some also add electrofused corundum, sintered alumina, synthetic mullite, and clinker calcined from alumina and clay in different proportions. It is mostly produced by sintering. However, the products also include cast bricks, fused grain bricks, unburnt bricks and monolithic refractory bricks. High-alumina refractory bricks are widely used in the steel industry, non-ferrous metal industry and other industries.

　　④ Corundum refractory bricks

　　Corundum bricks refer to a kind of refractory bricks with AL2O3 content not less than 90%, with corundum as the main phase, which can be divided into sintered corundum bricks and electrofused corundum bricks.

　　2. Alkaline series refractory bricks

　　Alkaline refractory bricks refer to refractory products with alkaline oxides such as Mg0 and CaO as the main components, and the main varieties are as follows.

　　① Magnesite refractory bricks

　　Refers to refractory bricks with magnesite as the raw material, periclase as the main crystalline phase, and MgO content above 80%～85%. Its products are divided into two categories: metallurgical magnesia sand and magnesia products. According to different chemical compositions and uses, there are Martin sand, ordinary metallurgical magnesia sand, ordinary magnesia bricks, magnesia-silica bricks, magnesia-alumina bricks, magnesia-calcium bricks, magnesia-carbon bricks and other varieties. Magnesite refractory bricks are the most important products among alkaline refractory bricks, with high refractoriness and good resistance to alkaline slag and iron slag, which is an important high-grade refractory brick. Mainly used in open-hearth furnaces, oxygen converters, electric furnaces and non-ferrous metal smelting.

　　② Dolomite bricks

　　It is a kind of alkaline refractory brick produced with dolomite as the main raw material. Widely used in alkaline converters, it can also be used as the lining of some out-of-furnace refining vessels.

　　③ Forsterite refractory bricks

　　It is a kind of refractory brick with forsterite 2MgO·SiO2 as the main component. Mainly used for checker bricks in open-hearth furnace regenerators, bricks for casting ingots, furnace bottoms of heating furnaces, and also has good use effects in copper smelting furnaces.

　　3. Carbon-containing refractory bricks

　　Carbon-containing refractory bricks refer to refractory brick products made of carbon or carbon compounds, with carbon in different forms as the main component, mainly divided into the following types.

　　① Carbon bricks

　　It is a high-temperature neutral refractory brick product made of carbonaceous materials as the main raw material and an appropriate amount of binder. Carbon bricks are widely used in the construction of blast furnace hearths, furnace bottoms, furnace bodies and lower parts of the furnace body. It can improve the continuous operation time of the blast furnace and prolong its service life. In addition, carbon bricks are also widely used in the electrochemical industry, chemical industry, petrochemical industry, electroplating industry, ferroalloy industrial furnaces (or equipment linings), acid and alkali liquid tank linings and pipelines, and furnace linings for smelting non-ferrous metals (such as lead, aluminum, tin, etc.).

　　② Graphite refractory products

　　It is a refractory material made of natural graphite as the raw material and clay as the binder. Such products include graphite clay crucibles, distillation tanks, stopper bricks for casting steel, tuyere bricks and steel ladle lining bricks. Among them, the most produced and widely used are graphite clay crucibles for steelmaking and smelting non-ferrous metals.

　　③ Silicon carbide refractory products

　　This is an advanced refractory material produced using silicon carbide (SiC) as a raw material. It has good wear resistance and corrosion resistance, high high-temperature strength, high thermal conductivity, low coefficient of linear expansion, and good thermal shock resistance. In steel smelting, it can be used in steel ladle linings, nozzles, stoppers, blast furnace bottoms and hearths, tapping spouts, converter and electric furnace tapping spouts, and waterless cold sliding rails for heating furnaces. In non-ferrous metal (zinc, copper, aluminum, etc.) smelting, it is widely used in distillers, rectification tower trays, electrolytic cell side walls, molten metal pipelines, suction and delivery pumps, and melting metal crucibles. In the silicate industry, it is widely used as shelf plates and flame-resistant materials for various kilns, such as muffle furnace linings and crucibles. In the chemical industry, it is mostly used in oil and gas generators, organic waste calcination furnaces, petroleum gasifiers, and desulfurization furnaces. In addition, it can be used in space technology as rocket nozzles and high-temperature hot gas turbine blades.

　　4. Zirconia-containing refractory bricks

　　This is an acidic material, a product made from natural zircon sand (ZrSiO2) as a raw material. Zirconia refractory bricks are different from silica-alumina refractory bricks and carbon-containing refractory bricks. They have good slag resistance, low thermal expansion rate, thermal conductivity decreases with increasing temperature, high load softening temperature, high wear resistance, and good thermal shock resistance. They have become important materials in various industrial fields. With the development of continuous casting and vacuum degassing technology in the metallurgical industry, the application of zircon refractory bricks is becoming increasingly widespread, mainly including the following categories.

　　① Zircon bricks

　　In the steelmaking process, the steelmaking degassing method is used, and the zircon refractory material has good corrosion resistance and thermal shock resistance to slag and molten steel, and is suitable for working under reduced pressure. It is widely used in the metallurgical industry for building degassing steel ladle linings, and is also used as stainless steel ladle linings, continuous casting ladle linings, nozzle bricks, stopper bricks, sleeve bricks, and high-temperature induction furnace linings. Zircon refractory bricks have high resistance to acidic slag and glass, and are therefore widely used in severely damaged parts of glass kilns that melt the most active glass. Zircon refractory materials also have the property of not being penetrated by metallic aluminum, aluminum oxides, and their slags, and have therefore achieved good results on the furnace bottoms in aluminum smelting.

　　② AZS electrofused bricks

　　Also known as electrofused zircon-alumina bricks, with the development of the glass industry, AZS electrofused bricks have become essential refractory materials for key parts of glass kilns. They have strong resistance to glass liquid corrosion.

　　③ AZS re-sintered electrofused bricks

　　Mainly used for building the bottom, walls, and regenerative checker bricks of glass melting furnaces and other thermal equipment.

　　④ Zircon mullite electrofused bricks

　　This type of brick is characterized by a dense crystal structure, high load softening temperature, good thermal shock resistance, high mechanical strength at room temperature and high temperature, good wear resistance, good thermal conductivity, and excellent resistance to slag corrosion. This type of product has a wide range of uses, and is used in metallurgical heating furnaces, homogenizing furnaces, calcium carbide furnaces, glass melting furnace walls, etc., with good results.

　　5. Insulation refractory bricks

　　These refer to refractory materials with high porosity, low bulk density, and low thermal conductivity. Insulation refractory materials are also called lightweight refractory materials. They include the following categories.

　　① High-alumina insulating lightweight refractory bricks

　　These are insulating lightweight refractory bricks with alumina (Al2O3) content of not less than 48%, mainly using bauxite. The production process uses the foam method, or the burnout additive method. High-alumina insulating lightweight refractory bricks can be used for building insulation layers and parts without strong high-temperature molten material erosion and scouring. When in direct contact with flames, the surface contact temperature of general high-alumina insulating refractory bricks should not exceed 1350℃. Mullite insulating refractory bricks can be in direct contact with flames, and have the characteristics of high temperature resistance, high strength, and significant energy saving. They are suitable for the linings of pyrolysis furnaces, hot blast stoves, ceramic roller kilns, electric porcelain drawer kilns, and various resistance furnaces.

　　② Clay insulating lightweight refractory bricks

　　These are insulating refractory bricks with an Al2O3 content of 30% to 48%, made mainly from refractory clay. The production process uses the burnout additive method and the foam method. Using refractory clay, microspheres, and refractory clay clinker as raw materials, adding a binder and sawdust, after batching, mixing, molding, drying, and firing, products with a bulk density of 0.3 to 1.5 g/cm3 are obtained. The output of clay insulating bricks accounts for more than half of the total output of insulating refractory bricks.

　　The Chinese standard (GB 3994-1983) divides clay insulating bricks into 10 grades according to bulk density: NG-1.5, NG-1.3a, NG-1.3b, NG-1.0, NG-0.9, NG-0.8, NG-0.7, NG-0.6, NG-0.5, and NG-0.4.

　　③ Diatomaceous earth insulating lightweight refractory bricks

　　These are insulating refractory products made mainly from diatomaceous earth. They are mainly used in insulation layers below 900℃. The Chinese standard (GB 3996-1983) divides diatomaceous earth insulating products into six grades according to bulk density: GG-0.7a, GG-0.7b, GG-0.6, GG-0.5a, GG-0.5b, and GG-0.4.

　　④ Microsphere bricks

　　These are insulating refractory products made mainly from microspheres. Microspheres are hollow aluminosilicate glass beads floated from the fly ash of thermal power plants. They are light, thin-walled, hollow, smooth-surfaced, high-temperature resistant, and have good insulation properties. Using these excellent properties of microspheres, lightweight insulating refractory materials with excellent insulation performance can be manufactured. Semi-dry pressing can be used to produce microsphere bricks.