The main crystalline phase in high-aluminum products is mullite and corundum. The content of corundum increases with the increase of alumina content. Corundum products containing more than 95% alumina are a kind of high-quality refractory material with wide application. The chrome brick is mainly made of chrome ore and the main crystalline phase is chromite. It has good corrosion resistance to steel slag, but has poor thermal shock resistance and low temperature deformation temperature. The chrome-magnesia bricks made of chrome ore and magnesia in different proportions have good thermal shock resistance and are mainly used as alkaline flat roof bricks.
Carbonaceous products are another type of neutral refractory materials. They are classified into carbon bricks, graphite products and silicon carbide products according to the composition of carbonaceous raw materials and the mineral composition of the products. Carbon bricks are made of high-grade petroleum coke, tar and asphalt are used as binders, and fired at 1300 ° C under air. Graphite products (except natural graphite) are obtained by graphitizing carbonaceous materials in an electric furnace at 2500~2800 °C. The silicon carbide product is made of silicon carbide as a raw material, and is baked at a temperature of 1350 to 1400 ° C by adding a binder such as clay or silicon oxide. Silicon carbide-silicon carbide powder can also be made into a silicon nitride-silicon carbide product in an electric furnace under a nitrogen atmosphere.
Carbonaceous products have a low coefficient of thermal expansion, high thermal conductivity, good thermal shock resistance, and high temperature strength. It is not softened under long-term use at high temperature, is not attacked by any acid and alkali, has good salt resistance, is not wetted by metal and slag, and is light in weight. It is a high-quality high-temperature resistant material. The disadvantage is that it is easily oxidized at high temperatures and should not be used in an oxidizing atmosphere. Carbonaceous products are widely used in high temperature furnace linings (bottom, hearth, lower part of the furnace, etc.) and lining of smelting non-ferrous metal furnaces. Graphite products can be used as reaction tanks and petrochemical autoclave linings. Silicon carbide and graphite products can also be used to make bismuth copper with gold and light alloys.