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Six Raw Materials For Refractories

What is refractory aggregate?

Refractory aggregate is the main material in the organizational structure of refractory castable. Its skeleton function also affects its physical mechanics and high-temperature service performance, especially for wear-resistant and refractory castable. Therefore, the variety, critical particle size and particle gradation of refractory aggregate should be reasonably selected according to the actual situation and technical formula requirements, so as to obtain high-strength and wear-resistant products.


What is the mass ratio of aggregate to matrix?

When the matrix composition is the same, different varieties of refractory aggregates are used to make refractory castable, and the mass ratio of aggregate to matrix is 7:3. Brown fused alumina has high bulk density, low water absorption, smooth and dense particle zoom, so the mixture needs less water at the same fluidity.


The bulk density of super grade bauxite clinker is small, the water absorption is in the middle, the particles have edges and corners and more pores, so the water demand for mixing is large; The bulk density of white corundum is in the middle, the water absorption is large, the particles are angular and have shrinkage holes, so the water demand for mixing is large.


Size of refractory aggregate

Refractory aggregate is divided into coarse aggregate and fine aggregate. Generally, the particle size greater than 5mm is called coarse aggregate; 5mm and below to 0.09mm is called fine aggregate. The critical particle size of aggregate is determined according to the thickness of lining, which is generally 10mm or 15mm. 20mm or 25mm particles can be used for large structures.

refractory products

In addition, the critical particle size of refractory aggregate is also different with different construction methods. At present, the critical particle size of refractory aggregate tends to increase. It should be pointed out that when preparing amorphous refractory materials, the particle gradation of refractory aggregate should meet the technical requirements in the table, that is, when using unified materials, it should also be screened and inspected to meet the requirements before it can be used.


Refractory powder is the matrix material of amorphous refractory material, and its grade shall be higher than or equal to that of refractory aggregate. The fineness required to be less than 0.09mm or 0.088mm shall be greater than 85%. For ultrafine powders, 5 μ M below shall account for more than 80%.


Modern amorphous refractories use high-grade materials such as corundum, synthetic mullite, spinel and alumina. There are many refractory aggregates and powders. Some iron is brought into the crushing process. In order to ensure the product quality, iron removal treatment should be carried out. Raw materials used for refractory aggregate and powder shall be subject to quality inspection and can be processed into refractory aggregate and powder only after passing the inspection.


Types of refractory materials

Refractory raw materials refer to the materials necessary for the production of refractory products. It is the basis for the production of refractory materials. Most refractory raw materials are natural minerals (such as refractory clay, high alumina bauxite, silica, chrome ore, magnesite, dolomite, forsterite, zircon, kyanite, sillimanite, andalusite, etc.).


With the continuous improvement of the requirements for the comprehensive performance of refractories, industrial raw materials and synthetic raw materials (such as industrial alumina, synthetic mullite, artificial refractory fiber, artificial refractory hollow ball, etc.) are increasingly used in the production of refractories. The quality and cost of refractory products largely depend on the correct selection and rational use of raw materials.


Refractory raw materials can be divided into acidic refractory raw materials, alkaline refractory raw materials and neutral refractory raw materials according to their chemical properties; According to the source, it can be divided into natural mineral raw materials and synthetic raw materials; In general, the raw materials for the production of refractory materials are divided into main raw materials, auxiliary raw materials and so on.


The raw materials used to produce refractory products, whether natural mineral raw materials or synthetic raw materials, must have high enough fire resistance from the perspective of mineralogy; From the viewpoint of technology, it should meet the basic requirements of technology; In terms of the performance of the products made with it, it should meet the requirements of the service performance of the products, especially the service performance at high temperature.


Generally, refractory raw materials are divided into aluminum silicon refractory raw materials (siliceous, clayey, high alumina, etc.), alkaline refractory raw materials, thermal insulation refractory raw materials and other refractory raw materials.


1. Siliceous raw material

Due to the volume effect of quartz variants, silica bricks are also made directly from silica, which is a general term for vein quartz, quartzite, chert and sandstone. The main component of silica is SiO2, and others are impurities. The siliceous raw materials used for refractories include crystalline crushed stone and cemented silica.

refractory materials

2. Clay material

Refractory clay is the main raw material for the production of aluminum silicate refractories. All kinds of hard clay, soft (semi soft) clay and clay shale with a fire resistance higher than 1580 ℃ are collectively referred to as refractory clay. Natural refractory clay is usually a mixture composed of clay minerals, mainly kaolinite (Al2O3 2SiO2 2H2O), that is, aqueous silicate, mixed with free quartz, pyrite, rutile and organic matter. This non single mineral is mostly composed of less than 1.2 in diameter μ M particles.


According to the formation of clay, it can be divided into primary clay and secondary clay. Primary clay refers to the clay formed by weathering of parent rock (such as feldspar) and still remains in place. Secondary clay, also known as drift clay, is a clay that is transferred from primary clay to other places under natural dynamic conditions and then deposited. It has small particle size, large dispersion and high plasticity.


Refractory clay used in refractory industry mainly includes the following two categories.

① Hard clay is characterized by dense structure, high hardness, very fine particles, difficult to disperse in water and low plasticity. The appearance of this kind of clay is often light gray, gray white or gray. Shell shaped fracture, some surfaces are greasy and easy to be weathered and broken into fragments.


② Soft (semi soft) clay is often in the shape of soil block, with soft tissue and good plasticity. The color of this clay varies greatly due to the different types and contents of impurities. From gray, dark gray to black; Others are purple, light red or white.


3. High alumina raw materials

3.1 Bauxite

Bauxite is the main raw material for producing brown corundum, and high alumina clinker with Al2O3 content of 88% ~ 90% is the main raw material of sub white corundum; Alumina is used as raw material for the production of white corundum and dense corundum. Bauxite, also known as high alumina bauxite or bauxite, is mainly composed of boehmite (Al2O3 H2O) and gibbsite (Al2O3 H2O).


The reserves of high alumina bauxite in China are extremely rich: the origin ranges from Shanxi, Hebei and Shandong in the north of the Yellow River, through Henan and Guangxi in the middle, to Guizhou and Yunnan in the southwest. At present, the main producing areas of high alumina clinker are Shanxi, Henan and Guizhou. There are also some smaller mines under development in Hunan.


The main minerals of high alumina bauxite in China are boehmite, boehmite, kaolinite and pyrophyllite, which can be divided into three types according to their mineral composition: boehmite kaolinite type (DK); Boehmite kaolinite type (BK); Boehmite pyrophyllite type (DP). At present, DK type high alumina bauxite is most widely used. According to its Al2O3 content, DK type high alumina bauxite clinker is divided into s, I, IIA, IIB, III, etc.


3.2 Sintered corundum and fused corundum

Alumina is mainly produced by melting alumina in electric arc furnace. In addition, corundum tabular can also be obtained by sintering. This method still takes industrial aluminum oxide powder as the main raw material and is prepared by calcination, fine grinding, balling and sintering. The production technology of this method is difficult, but the product has high strength, strong corrosion resistance and good thermal shock stability.


The so-called "sub white corundum" is actually a dense fused corundum based on high alumina, with Al2O3 content greater than 98% and apparent porosity less than 4%; It is made of high alumina bauxite under reducing atmosphere and controlled conditions. Corundum crystal is granular, generally 115mm; The main impurities are rutile, aluminum titanate and their solid solutions.


3.3 Mullite

Mullite is a refractory raw material with 3al2o32sio2 crystalline phase as the main component. Mullite is divided into natural mullite and synthetic mullite. There are few natural mullites, which are generally synthetic. Mullite is chemically stable and insoluble in hydrofluoric acid. It has good high-temperature mechanical and high-temperature thermal properties. Therefore, synthetic mullite and its products have the advantages of high density and purity, high high high-temperature structural strength, low high-temperature creep rate, low thermal expansion rate, strong chemical corrosion resistance, thermal shock resistance and so on.

aluminum oxide grit

3.4 Sillimanite group minerals

Sillimanite group minerals include kyanite, andalusite and sillimanite, commonly known as "three stones". The chemical composition of the three stones is the same, but the crystal structure is different. They belong to homogeneous and heterogeneous crystals. When heated to high temperature, they are transformed into mullite to form a small amount of molten SiO2, accompanied by volume expansion.


Due to the different heating expansion of the three stones, their direct utilization degree is also different. Due to the small volume change of andalusite, raw materials are directly used whether it is used to make bricks or as additives. Sillimanite and kyanite are often added to the ingredients in the form of expansion agent, especially for amorphous refractories. When used to make bricks, clinker must be calcined, especially kyanite.


4. Basic refractory material

4.1 magnesium raw materials

(1) Magnesite

There are two main types of magnesite in China: crystalline magnesite and amorphous magnesite. China's magnesite is mainly distributed in Liaoning and Shandong provinces. The main impurity of magnesite is talc, and some magnesites contain high Cao, followed by dolomite.


China's Magnesite can be classified into five levels according to chemical composition: s, I, II, III and IV. only grade s and I are used for calcining magnesia for brick making. High purity magnesia is prepared by two-step flotation and two-step calcination. Using the high-purity magnesia prepared by this method as raw material, a variety of high-performance refractory products can be developed.


(2) Other magnesium bearing minerals

Forsterite products in magnesia refractories are mainly composed of forsterite (2MgO · SiO2) and periclase (MgO). This product is characterized by strong resistance to molten iron oxide and better thermal shock stability than ordinary magnesia brick. The main raw materials for producing this product are peridotite and serpentine.


4.2 dolomitic raw materials

Dolomite is a refractory raw material with the double salt of magnesium carbonate (MgCO3) and calcium carbonate (CaCO3) as the main component. Its chemical formula is CaMg (CO3) 2 or MgCO3 CaCO3. Its theoretical composition is cao3 0.41%, MgO2 1.87% and CO2 47 72%。 Cao / MgO = 1.39, hardness 3.54.


China's dolomite is rich in raw materials, widely distributed and relatively pure. Liaoning Dashiqiao area is rich in reserves. Shandong, Hubei, Shaanxi, Guangxi, Gansu, Jiangxi, Anhui, Sichuan, Yunnan, Hunan and other provinces are rich in mineral resources. This ore body is often associated with limestone and magnesite.


5. Raw materials for zirconium products

(1) Zircon

Zircon (ZrO2 · SiO2 or ZrSiO4) is the main raw material for the production of zirconium products and zircon products. The origin of zircon in China is Hainan Province. It is distributed in Guangdong Province, Guangxi Zhuang Autonomous Region, Shandong Province, Fujian Province and Taiwan Province.


The theoretical composition of zircon is ZrO2 67 01%、SiO2 32. 99%。 It often contains ticfe and other trace rare earth oxides, which are radioactive to varying degrees due to the existence of these elements. Therefore, necessary protective measures should be taken when using this raw material to produce products.

mullite

Zircon has low thermal conductivity, which is 3.72w / (m.k) at 201000 ℃. When the main axis of zirconium (x10-6 ℃) is parallel to that of other single crystals, the expansion coefficient of zirconium (x10-6 ℃) is also higher than that of quartz at 1000 ℃. Zircon has high chemical inertia and is difficult to react with acid. It reacts with glass melt to a lesser extent. It is commonly used in refractories for metallurgy and glass industry.


(2) Oblique zircon

Natural oblique zircon (ZrO2) is often irregular block, black, brown, yellow or colorless. There are few natural oblique zircon ore bodies in China. ZrO2 used in industry is a chemical raw material. It is a white or yellowish powder made of zircon (ZrO2 · SiO2) by chemical method. Pure ZrO2 has three crystal forms at atmospheric pressure: monoclinic phase, tetragonal phase and cubic phase from low temperature to high temperature.


Stable ZrO2 can be divided into partially stable ZrO2 and fully stable ZrO2 according to the degree of stability. Due to the large coefficient of thermal expansion of fully stable ZrO2, its thermal shock stability is not as good as that of partially stable ZrO2, so the latter is often used as the toughening material of ceramics and refractories.


(3) Desiliconized zirconium

In the manufacture of fused cast zirconium corundum (AZS) refractories abroad, in addition to zircon concentrate, a certain amount of "desiliconized zirconium" raw materials are mostly added. The purposes are: first, to adjust and stabilize the formula; Second, improve and optimize product performance.


(4) Zirconium corundum mullite

The raw materials are industrial alumina, kaolin and zircon, which are finely ground and mixed evenly, pressed by semi dry method and calcined at 3001700 ℃. The results show that increasing the content of zircon will increase the sintering temperature, reduce the total shrinkage and increase the closed pores. These reactions make the sintered zircon corundum mullite have higher density and strength and better thermal shock resistance.


6. Raw materials for chromium products

Chrome ore or chromite is one of the main raw materials for producing chrome refractory (chrome brick, chrome magnesia brick and magnesia chrome brick). Chromite is a mixture of many minerals. Because its mineral composition fluctuates greatly, it also changes greatly in chemical composition and physical properties.


It is usually composed of gangue minerals with chromic grains. These gangue minerals are usually magnesium silicate, such as serpentine, forsterite and olivine. In addition to Cr2O3, there are also Al2O3, Fe2O3 and MgO in chromite. General chromite is often represented by (Mg, Fe) Cr2O3 due to the existence of magnesium and iron.


The above are common refractory raw materials. With the development of refractory technology, there are more kinds of raw materials. Especially in recent years, due to a series of reasons such as environmental protection problems and raw ore resources, synthetic materials with better performance and more environmentally friendly resource renewable raw materials (such as ferrosilicon nitride, Cylon, etc.) have been continuously developed.


white aluminium oxide
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