Almost all the raw materials used in the manufacture of refractories are minerals found in nature. Raw materials that lose a great deal of volume or weight during heating are pre-heated in rotary kilns in order to obtain dense and stable raw materials.
Because of the increased requirements regarding the purity and properties of refractory materials, synthetic raw materials are being used more and more frequently.
Main raw materials
- Silica
The raw materials used to manufacture silica bricks must have an SiO2 content above 95 %. Quartzite raw materials are compact granular rocks found in nature underground.
- Fireclay / Mullite
Fireclays with variable alumina content are produced by blending raw mineral clays rich in alumina (generally kaolinite-based materials) and calcining them at a temperature of 1500 to 1600°C. At this temperature, the clays react together to produce the refractory material, which is generally a mixture of mullite, quartz, cristobalite and a glassy phase containing the impurities of the raw materials. By controlling the quantity of each mineral clay in the original mixture, it is possible to produce a refractory product with the desired alumina content. However, for fireclays with a high alumina content, bauxitic minerals may be blended with the kaolinitic materials, or a mixture of bauxitic minerals and siliceous sand may be used to produce a refractory product with an alumina content of 60 to 70 %. The main sources of high-quality fireclays are the United States, China, France and South Africa.
- Sillimanite, andalusite and kyanite
Sillimanite, andalusite and kyanite minerals are commonly encountered in volcanic rocks and their alteration products. Sillimanite, andalusite and disthene decompose into a mixture of mullite and silica during firing. They are extracted mainly in South Africa, the United States and France.
- Bauxite
The bauxite ore is a lateritic rock, characterised by its high alumina (Al2O3) content. It is formed by continental weathering in hot, humid climates. Bauxite contains varying proportions of alumina hydrate, kaolinite, silica and iron oxides, which often give it a red colouring. Australia is the leading producer of bauxite with almost a third of world production, followed by China and Brazil.
Refractory-grade bauxite must have a high alumina content and a very low alkali content. The levels of iron oxide and titanium in the ore are also very important, as they affect the high-temperature resistance properties of the refractory material. To produce the bauxite used in refractories, the ores are calcined at 1600 – 1650°C to produce dense aggregates that are more stable in volume. GCC aluminium smelters already produce this type of alumina for their electrolytic cell requirements.
- Zircon sand
Zircon sand is a natural sand with excellent refractory properties. Zircon sand is also known as zirconium silicate (ZrSiO4).
synthetic raw materials
- Sintered/fused alumina
The refractories richest in alumina are synthetic materials formed at very high temperature. There are three general categories: sintered alumina, Brown Fused Alumina (BFA) and White Fused Alumina (WFA).
Brown fused alumina is obtained by melting bauxite in an electric arc furnace. This makes it possible to remove impurities as a ferro-alloy. The fused alumina is then cooled and solidified before being processed into fractions of the required size.
WFA is formed in the same way as BFA, but the alumina source in this case is calcined alumina from the Bayer process. In the Bayer process, bauxite is treated with caustic soda to precipitate alumina hydroxide, which is then heated to 1200-1300°C to produce calcined alumina. Because of the greater purity of the starting material, the WFA product is purer than BFA, with an Al2O3 content of 99.5 %.
- Magnesia
Refractory materials extracted from magnesite (MgCO3) or magnesium chloride (MgCl2) from seawater or brines form the basis of sintered or fused magnesia. Sintered magnesia (MgO) is generally fired in shaft kilns or rotary kilns in order to remove the carbonate and to sinter it to a low porosity; separate kilns are often used for calcination and sintering. Fused magnesia is produced by electric melting, either directly from the carbonate or from calcined magnesia. Magnesia is mainly produced in China. The other producing countries are Brazil, Russia, Turkey, Poland and Austria.
- Carbon and graphite
Graphite and carbon are natural minerals present in various metamorphic rocks and require an enrichment process by grinding and flotation before being used as flake graphite in refractory products. The main suppliers are located in China.
In summary, refractory materials, indispensable in high-temperature industries, are mainly produced from natural minerals. In order to optimise their properties and to meet the ever more demanding requirements of industrial processes, these raw materials undergo complex thermal and physico-chemical treatments. At the same time, the emergence of synthetic materials, such as sintered alumina or fused magnesia, opens up new prospects in terms of purity, resistance and durability. The choice of raw materials and manufacturing processes depends closely on the intended applications and on the thermal and chemical constraints to which the refractories will be subjected.
Xavier Schuster
Consultant at SXE Consulting. Industrial consulting firm based in Luxembourg, 25 years of experience in operational excellence.
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