An Introduction to Refractory Aggregates

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Any aggregate can potentially be used in the formulation of a monolithic refractory. They are chosen based on their stability at the temperature of application, mechanical strength, and corrosion resistance.

Temperature Limits

As a guide, Table 1 illustrates the temperature limits of operation for castables with different aggregates.

Table 1. Upper limits of service temperatures for various aggregates (Calcium aluminate cement (CAC))

Table 2. Pyrometric cone equivalent of calcium aluminate cement

Refractory Aggregates

Bauxite

Bauxite is an ore that mainly contains either Gibbsite (alumina trihydrate, Al₂O₃.3H₂O) or Boehmite (a monohydrate of alumina, Al₂O₃.H₂O). Raw bauxite may have other impurities such as ferrous oxide, silica, and titania. Refractory grade bauxite has low iron but high alumina content.

A majority of the materials mined in Europe contain a higher proportion of Boehmite while bauxites mined in South America and Asia contain high quantities of Gibbsite. The Bayer process is applied to bauxite to create higher grades of alumina that can be used in elevated temperature applications.

Bauxite used in refractory applications is usually calcined in a rotary kiln, creating a material that mostly contains mullite (3Al₂O₃.2SiO₂), corundum (alpha-Al₂O₃), and a small quantity of a glassy phase.

Calcined and Sintered Alumina

Calcined aluminas are made from bauxite processed through the Bayer process. The resulting material contains very low levels of impurities. Calcined alumina is produced by heating bayerite (Al(OH)₃) in a rotary kiln. Calcined alumina remains stable even at extremely high temperatures.

Sintered alumina is produced by sintering calcined alumina at 1,800 °C in a rotary kiln. Later, it is crushed and categorized based on grain size.

Fused Alumina

When aluminous raw materials are fused electrically in an electric arc furnace, fused alumina is produced. The fused product is cooled into ingots, crushed, and then categorized. There are two kinds of fused alumina—white and brown. The brown fused alumina is produced from bauxite. Its impurities are minimized to precipitate as Fe-Si-Ti system iron alloys, but some of the titanium produces a solid solution with alumina. This gives the brown color to the material.

Fused alumina is known to have a very high degree of toughness, and is often used as a grinding material. White fused alumina is developed from calcined alumina. Sodium is the only impurity in fused alumina which exists in the form of alpha-alumina. Fused alumina, with its ideal crystallization, cannot be sintered easily. It is not only inactive but also does not react freely with other raw materials. Fused alumina is mostly used in refractories which are exposed to extremely severe conditions.

Fused Bubble Alumina

Fused bubble alumina is utilized in high-temperature insulating monolithic refractories. Bubble alumina is produced by blowing a stream of high-pressure air into molten alumina. This creates bubbles in the material and the imperfect bubbles are then segregated.

Spinel

Spinels are used in refractories and are usually synthesized from trivalent and bivalent oxides. These oxides are mixed at equivalent mole ratios, forming materials of general formula XY₂O₄. The double oxide of magnesia and alumina (MgAl₂O₄) is a common refractory spinel product. More examples of spinels are given in Table 3.

Table 3. Examples of spinels

Spinel (Mg.Al₂O₃) as well as dichromite (MgO.Cr₂O₃) are employed in refractory castable formulations. Their melting points are 2135 °C and 2350 °C, respectively. At high temperature, spinel is more neutral than alumina but its corrosion resistance against standard slags is high. Against Fe₂O₃, Al³⁺ may be replaced by Fe³⁺ that causes corrosion. Spinel’s thermal conductivity as well as coefficient of thermal expansion is smaller than that of magnesia. It also has good resistance to spalling.

Magnesia

Magnesia clinker is a standard aggregate and has high corrosion resistance to regular slags. It has high thermal conductivity as well as a large coefficient of thermal expansion. Calcining magnesite (MgCO₃) in a rotary kiln yields magnesia clinker. Iron oxide forms a solid solution with periclase (MgO) which serves as a sintering aid.

Fused magnesia is another magnesia raw material acquired by electrically fusing seawater magnesia clinker. Magnesia has a propensity to react with water to form magnesium hydroxide (Mg(OH)₂), thus magnesia-based castables must be stored carefully, and exposure to steam must be avoided.

Dolomite

Dolomite is a double salt of MgCO₃ and CaCO₃. The CaO.MgO system contains mixtures that do not melt below 2300 °C, and are highly refractory. Dolomite refractories are employed in steel making. Dolomite has exceptional corrosion resistance to regular slags.

Silicon Carbide

Silicon carbide is manufactured by heating silica sand and petroleum coke, packed around electrodes in an electric resistance furnace to over 2200 °C. This material is highly resistant to corrosion and abrasion with a molten slag. It also has superior resistance to thermal spalling. Since it is a carbide, it will oxidize instantly. Regarding other refractory aggregates, silicon carbide has a moderately high conductivity.

Chamotte

Chamotte is made from clay, which is sintered to the point that further shrinkage is no longer possible. It is usually made by extruding raw clay and then firing in either a rotary kiln or tunnel. Mullite is the key phase in chamotte at high temperatures. Some chamottes are also produced from shale clays, which are calcined in either a rotary kiln or shaft.

Vermiculite

This clay mineral is more or less similar to mica. It comprises a three-layered structure with a MgO.6H₂O layer between sheets. When heated to a temperature of about 350 °C, vermiculite starts to shrink. At over 400 °C, the combined water in the material is discharged and it exfoliates.

In this process, vermiculite swells as much as 10–20 times its initial volume. Exfoliated vermiculite is moderately poor in strength but it has a very low thermal conductivity which makes it a superior insulating material. Vermiculite is typically used in insulating castables.