Refractories

materials that are manufactured chiefly from mineral ores and that exhibit a refractoriness of not less than 1580°C. The production of refractories became an important industrial activity as the demand for various types of heating units increased.

Refractories are used to make bricks and castings as well as nonmolded materials, including aggregates, bulk granular materials, and concrete mixes. In several countries, from 10 to 25 percent of nonmolded materials are made from refractories. Refractories exhibit a refractoriness between 1580° and 1700°C; high refractories, between 1770° and 2000°C; and superrefractories, above 2000°C. Refractories are divided into several groups, depending on the porosity of the material: high-density refractories have a porosity of under 3 percent; ordinary refractories, between 20 and 30 percent; and lightweight refractory products, over 45 percent.

With respect to composition, the following classes of refractories are distinguished: (1) silica refractories, from which Dinas refractory products and quartz-glass products are made; (2) aluminosilicate refractories; (3) magnesite refractories; (4) magnesian limestone refractories, including dolomite refractories; (5) magnesite-spinel refractories, including magnesiochromite refractory products and spinel refractories; (6) magnesium silicate refractories, from which forsterite refractory products are made; (7) carbon refractories; (8) silicon carbide refractories; (9) zirconium refractories, including the baddeleyite and zirconium types; (10) oxide refractories, including those made from BeO, MgO, and C_aO; and (11) oxygen-free refractories, made from nitrides, borides, and other compounds. Over 95 percent of refractories are of the aluminosilicate, magnesia, or silica classes.

Nonmolded refractories include dry or semidry powdery substances of varying granule size, refractory mortar, plastics, and liquid mixes. These materials are used as gunite coatings and to manufacture refractory linings for heating units in industrial plants. They are also applied as slushing compounds and as patching compounds for refractory masonry. Nonmolded refractories are used to make refractory concrete and often include additives to ensure hardening at ordinary temperatures or after drying. Refractory concrete can be used to manufacture large blocks that weigh from 150–300 kg to 10–20 tons and even more; these are delivered in finished form to the construction site. No separate classification of nonmolded refractories by composition and refractoriness has been defined, since the main components of nonmolded refractories are identical to those of molded refractories.

In addition to lightweight refractories, fibrous refractories, including kaolin, mullite, corundum wool, and related products, are used as heat insulators. They are characterized by extremely low thermal conductivity and are used in the external insulating layer and sometimes in the working layer of refractory masonry.

The most important properties of refractories, apart from chemical composition and refractoriness, are density, porosity, strength, thermal strain under load, thermal stability, slag resistance, thermal expansion, and thermal conductivity. The size of the granules is another important criterion for nonmolded refractories.

Standard production processes for most refractories involve preliminary preparation of the raw materials, such as refractory clay, kaolin, magnesite, and quartzite; roasting of these materials to yield a caked semifinished product (quartzite is not roasted); grinding; and adding of a binder, such as clay in fireclay refractories and milk of lime in Dinas refractories. The refractory products are then mixed, molded on presses or by other methods into blocks with an average weight ranging from 3 to 25 kg, and roasted at 1300°-1750°C in tunnel furnaces or other types of furnace. Nonroasted refractory products, for example, large blocks, and fused refractories are also manufactured. The final stages in the production of nonmolded refractories are grinding and mixing of the components.

Refractories are used in the construction of heating units; in furnaces that are designed for the extraction and smelting of metals, for the heating of semifinished products in metallurgical plants and machine-building plants, for coke extraction, and for cement firing; and in power plants and apparatus for high-temperature chemical processes. Refractories are primarily designed to insulate nonrefractory structural elements and the surrounding environment from the effects of high temperatures, melts, hot gases, and other phenomena. About 60 percent of refractories are used in ferrous and nonferrous metallurgy. The general demand for refractory products, calculated per ton of smelted steel, varies from 25–30 kg to 65–100 kg in different countries.