The Six Most Commonly Used Raw Materials For Refractory Materials.

Refractory raw materials refer to the essential materials required for the production of refractory material products. They form the foundation for manufacturing refractory materials. The majority of refractory raw materials are natural minerals, such as refractory clay, high-alumina bauxite, silica, chromite, magnesite, kaolin, magnesia olivine, zircon, andalusite, silicon carbide, corundum, etc. With the continuous improvement in the comprehensive performance requirements of refractory materials, industrial raw materials and artificially synthesized materials, such as industrial alumina, synthetic mullite, artificial refractory fibers, and artificial refractory hollow spheres, are increasingly used in the production of refractory materials. The quality and cost-effectiveness of refractory products largely depend on the correct selection and rational utilization of raw materials.

Refractory raw materials can be classified based on their chemical properties into acidic refractory raw materials, alkaline refractory raw materials, and neutral refractory raw materials. They can also be categorized by their source into natural mineral raw materials and artificially synthesized raw materials. Generally, in the production of refractory materials, raw materials are further classified into primary materials and auxiliary materials.

The raw materials used to produce refractory products, whether natural mineral or artificially synthesized, must, from a mineralogical perspective, possess a sufficiently high refractoriness to meet the required product specifications. From a process perspective, they should meet the basic requirements of the manufacturing process. Considering the performance of the products derived from them, they should be able to meet the usage requirements of the products, especially the demands for high-temperature performance.

Refractory raw materials are commonly categorized as aluminum-silicon refractory raw materials (such as silica, clay, high-alumina, etc.), alkaline refractory raw materials, thermal insulating refractory raw materials, and other refractory raw materials.

1.Siliceous Raw Materials
Due to the volume effect of quartz variants, silica bricks are also directly produced using siliceous rocks. Siliceous rocks encompass various types such as vein quartz, quartzite, flint, and sandstone. The main component in siliceous rocks is SiO2, with other components considered impurities. Siliceous raw materials used in refractory materials are broadly categorized into crystalline granules and bonded silica rocks.

2.Clayey Raw Materials
Refractory clay is the primary raw material for producing alumino-silicate refractory materials, and its refractoriness requirements exceed various hard, soft (semi-soft) clays, and clay shale with a temperature resistance of over 1580°C, collectively referred to as refractory clay.

Natural refractory clay typically consists mainly of clay minerals, with kaolinite (Al2O3 • 2SiO2 • 2H2O) as the primary component, representing hydrated silicates. It is accompanied by free quartz, limonite, goethite, and organic matter, forming a mixture. This non-uniform mineral is predominantly composed of dispersed particles with a diameter less than 1.2μm.

Based on the different formation processes of clay, it can be classified into primary clay and secondary clay. Primary clay refers to clay formed by the weathering of parent rocks (such as feldspar) that remains in place after the weathering process. Secondary clay, also known as sedimentary clay, is clay that has been transported to other locations and redeposited under natural dynamic conditions. It has fine particle size, high dispersion, and high plasticity.

Refractory clays commonly used in the refractory materials industry can be broadly categorized into the following two types:

(1).Hard Clay:
Hard clay is characterized by a dense structure, high hardness, extremely fine particles, poor dispersibility in water, and very low plasticity. This type of clay often appears in light gray, gray-white, or gray. It has a shell-like fracture surface, some with a smooth and slippery feel, and is prone to weathering and breaking into fragments.

(2).Soft (Semi-Soft) Clay:
Soft (semi-soft) clay usually exists in block-like forms, with a loose and soft structure and relatively good plasticity. The color of this type of clay varies significantly due to differences in the types and concentrations of impurities. It can range from gray and dark gray to black, and in some cases, it may exhibit purple, light red, or white colors.

3.High-Alumina Materials

(1) Bauxite:
Bauxite is the primary raw material for producing brown fused alumina. High-alumina clinker with an Al2O3 content of 88% to 90% serves as the main material for semi-friable corundum. For the production of white fused alumina, dense corundum, etc., aluminum oxide is used as the raw material. Bauxite is also known as high-alumina shale or alumina shale, with the main minerals being diaspore (Al2O3 • H2O) and boehmite (Al2O3•3H2O).

China has extremely abundant reserves of bauxite, with production areas extending from Shanxi, Hebei, and Shandong north of the Yellow River, through Henan and Guangxi in the central region, to Guizhou and Yunnan in the southwest. The main production areas for high-alumina clinker in China are currently in Shanxi, Henan, and Guizhou. There are also some smaller mines under development in Hunan. The main minerals of high-alumina bauxite in China include diaspore, boehmite, kaolinite, and pyrophyllite. Based on their mineral composition, they are classified into three types: diaspore-kaolinite type (DK), boehmite-kaolinite type (BK), and diaspore-pyrophyllite type (DP). Among them, the DK-type high-alumina bauxite is the most widely used. The DK-type high-alumina clinker is further classified based on its Al2O3 content into grades S, I, IIA, IIB, III, etc.

(2) Sintered Corundum and Fused Corundum

Artificial production of corundum utilizes industrial alumina or high-alumina bauxite as the main raw materials and is melted in an electric arc furnace. In addition to this, corundum plate-like aluminum oxide can be produced using the sintering method. In this method, industrial alumina powder is the main raw material, and the process involves calcination, fine grinding, pelletizing, and sintering. This production method poses technical challenges, but the resulting products exhibit high strength, strong erosion resistance, and good thermal shock stability.

The term "semi-friable corundum" essentially refers to dense fused corundum based on high-alumina bauxite, with an Al2O3 content exceeding 98% and an apparent porosity of less than 4%. It is produced by electric melting high-alumina bauxite under reducing atmospheres and controlled conditions. The corundum crystals are granular, typically ranging from 1 to 15 mm, with main impurities including hematite, aluminum titanate, and their solid solutions.

(3) Mullite

Mullite is a refractory material primarily composed of the crystalline phase 3Al2O3•2SiO2. Mullite can be classified into two categories: natural mullite and synthetic mullite. Natural mullite is rare, and it is generally produced synthetically. Mullite exhibits stable chemical properties and is insoluble in hydrofluoric acid. It possesses excellent high-temperature mechanical and thermal properties.

Synthetic mullite and its products are characterized by high density, high purity, high temperature structural strength, low creep rate at high temperatures, small thermal expansion coefficient, strong resistance to chemical erosion, and resistance to thermal shock.

(4) Silimanite Group Minerals

The silimanite group minerals include kyanite, andalusite, and sillimanite, commonly referred to as the "three stones." These minerals share the same chemical composition but have different crystal structures, classifying them as polymorphs. When heated to high temperatures, they all transform into mullite, producing a small amount of molten SiO2 accompanied by volume expansion.

Due to variations in the degree of thermal expansion among these minerals, their direct utilization differs. Because andalusite exhibits minimal volume change during heating, it is used directly in its raw state, either for brick making or as an additive. On the other hand, sillimanite and kyanite are often added in the form of expanding agents to the mix, particularly in the production of unshaped refractory materials. When used for brick making, they need to be fired into clinker, especially in the case of kyanite, which must be sintered into clinker form.

4.Alkaline Refractory Materials
4.1 Magnesia Materials

(1) Magnesite Ore

In China, magnesite ore is primarily classified into two types: crystalline magnesite ore and amorphous magnesite ore. The main distribution areas for magnesite ore are in the provinces of Liaoning and Shandong. The main impurity in magnesite ore is talc, and some magnesite ores also contain higher levels of CaO, with dolomite being the secondary mineral. In China, magnesite ore is graded into five levels (S, I, II, III, IV) based on its chemical composition. Only S and I grades are used for the calcination of magnesia sand to produce magnesia bricks.

Using a two-step flotation method and a two-step calcination method to prepare high-purity magnesia sand, the high-purity magnesia sand obtained through this process can be utilized as a raw material to develop various high-performance refractory products.

(2) Other Magnesium-Containing Minerals

In magnesia refractory materials, products made from forsterite, the main mineral components are forsterite (2MgO·SiO2) and periclase (MgO). These products are characterized by a strong resistance to molten iron oxidation, and their thermal shock stability is superior to ordinary magnesia bricks. The primary raw materials for producing these products are dunite and serpentinite.

4.2 Dolomite Materials

Dolomite is a refractory material primarily composed of a complex salt of magnesium carbonate (MgCO3) and calcium carbonate (CaCO3). Its chemical formula is CaMg(CO3)2 or MgCO3 • CaCO3, with a theoretical composition of CaO 30.41%, MgO 21.87%, CO2 47.72%. The CaO/MgO ratio is 1.39, and its hardness is 3.5 to 4.

China has abundant and widely distributed dolomite resources, known for their relative purity. The region around Dashiqiao in Liaoning Province has particularly rich reserves. Provinces such as Shandong, Hubei, Shaanxi, Guangxi, Gansu, Jiangxi, Anhui, Sichuan, Yunnan, and Hunan all boast abundant deposits. Dolomite deposits are often associated with limestone and magnesite.

5、Zirconium-based Product Raw Materials

(1) Zircon

Zircon (ZrO2·SiO2 or ZrSiO4) is the primary raw material for producing zirconium-based products and zirconia products. The main production site for zircon in China is Hainan Province. It is also found 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 trace elements such as Ti, Fe, and other rare earth oxides, imparting varying degrees of radioactivity. Therefore, necessary protective measures should be taken when using this raw material for product manufacturing.

Zircon has a relatively low thermal conductivity, measuring 3.72 W/(m·K) between 20 and 1000℃. Its coefficient of expansion is also comparatively low, reaching 4.6 × 10-6/℃ at 1000℃. The expansion coefficients in two directions, perpendicular and parallel to the main axis (C-axis), exhibit significant differences in single crystals. Zircon demonstrates high chemical inertness, resisting reactions with acids. It reacts to a lesser extent with glass melts and is commonly used in refractory materials for the metallurgical and glass industries.

(2) Monoclinic Zirconia

Natural monoclinic zirconia (ZrO2) often appears as irregular blocks in black, brown, yellow, or colorless forms. Natural deposits of monoclinic zirconia are rare in China. The industrial-grade ZrO2, a chemical raw material, is obtained through chemical methods from zircon (ZrO2·SiO2) and appears as a white or slightly yellow powder.

Pure ZrO2 has three crystal phases at atmospheric pressure: monoclinic, tetragonal, and cubic, in ascending order of temperature.

Stable ZrO2 can be further classified into partially stabilized ZrO2 and fully stabilized ZrO2, with the latter exhibiting a larger coefficient of thermal expansion and lower thermal shock stability than the former. Therefore, partially stabilized ZrO2 is often used as a toughening agent in ceramics and refractory materials.

(3) Desilicated Zirconia

In the production of fused cast zirconia corundum (AZS) refractory materials abroad, in addition to using zirconium silicate concentrate, a certain amount of "desilicated zirconia" raw material is mostly added. The purpose is twofold: to adjust and stabilize the formula, and to improve and optimize product performance.

(4) Zirconia Corundum Mullite

The original materials for this product are industrial alumina, kaolin, and zircon. They are finely ground, mixed uniformly, semi-dry pressed into balls, and sintered at 300 to 1700°C. Studies show that increasing the content of zircon leads to a higher sintering temperature, reduced total shrinkage, and increased closed pores. These reactions contribute to the sintered zirconia corundum mullite having higher density and strength, as well as better resistance to thermal shock stability.

6.Chromium-based Product Raw Materials

One of the primary raw materials for producing chromium-based refractory materials such as chrome bricks, chrome-magnesia bricks, and magnesia-chrome bricks is chromium ore or chromite. Chromite is a mixture of various minerals, and its composition fluctuates significantly, leading to variations in both chemical and physical properties. It typically consists of chromic grain minerals, with these minerals often being magnesium silicates, such as serpentine, forsterite, and olivine. In addition to Cr2O3, chromium iron ore also contains Al2O3, Fe2O3, MgO, etc. The general representation for chromite, due to the presence of magnesium and iron, is often expressed as (Mg, Fe) Cr2O3.

The mentioned materials are commonly used refractory raw materials. With the continuous advancement of refractory technology, the variety of raw materials has become more extensive. In recent years, there has been a focus on developing better-performing artificial synthetic materials and more environmentally friendly resource-recycled raw materials (such as silicon nitride iron and seelon), driven by environmental concerns and the depletion of natural resources.