'(iii) in the case of metal mines, aplite, bauxite, fluorspar, flake, graphite, vermiculite, beryl, garnet, feldspar, mica, talc (including pyrophyllite), lepidolite, spodumene, barite, clay, ball clay, sagger clay, china clay, phosphate rock, rock asphalt, mines, trona, bentonite, gilsonite, thenardite, borax, fuller's earth, tripoli, refractory and fire clay, quartzite, diatomaceous earth, metallurgical grade limestone, chemical grade limestone and potash, mines or deposits, 15 per centum * * *.' (Strike out indicates deletion from former section and italics indicates additions by the Revenue Act of 1951).
REFRACTORIES. One of the definitions of the word 'refractoriness' is 'the capacity of a material to resist a high temperature.' The non-metallic materials which have this property, and which are used for the construction of furnaces, are known as 'refractories.' Without them, high temperatures could not be confined or harnessed for industrial use, and it would be impossible to reduce iron, copper, aluminum and other metals from their ores. Without refractories, it would be impossible to produce chinaware, porcelain, glass, portland cement, steam power or steam generated electric power.
Refractories are made of a wide variety of raw materials, of which refractory fire clay and quartzite (a type of silica rock) are the ones used in largest quantities. Other refractory raw materials include kaolin, magnesite, chrome ore, the minerals olivine, diaspore, bauxite, kyanite, sillimanite, zircon, zirconia and others. Many of these are produced from mines and quarries in various parts of the United States; others come from foreign lands. Chrome ore, for example, is imported from Cuba, the Philippines, and South Africa.
Most refractories are used in the form of brick shapes, which vary greatly in size, form and composition, depending upon the service for which they are intended. The most widely used size is the so-called '9-inch straight,' measuring 9 inches by 4 1/2 inches by 2 1/2 inches. Bricks of this size are made of many different raw materials and formed by several processes. The weight of a 9-inch straight varies from as little as 2 pounds for some heat-insulating refractories to as much as 12 pounds for certain special products.
In addition to the preformed shapes, refractory products include finely ground refractories, used mainly as mortar for laying the shapes; and granular materials of various compositions, which serve mainly for forming furnace bottoms. They are also used for making repairs in furnace walls and bottoms.
In service, the one condition which practically all refractories must withstand is exposure to high temperatures, which may vary from about 1,000 degrees to 4,000 degrees F., depending upon the type of furnace, and the part of the furnace in which they are used. In addition, they may be called upon to withstand the sand blast action of dusts in rapidly moving furnace gases, the abrasive effects of moving furnace charges, the tendency toward cracking caused by rapid changes in temperature, and chemical attack by solids, liquid slags, or other molten materials, or furnace gases. Usually refractories must also sustain pressure.
To meet the various furnace conditions, refractories of many types have been developed. The types most widely used include fireclay, high-alumina, silica, basic and silicon carbide brick, and insulating firebrick.
Refractory fireclay brick of several classes are used commercially, classified in accordance with their 'refractoriness' or ability to withstand high temperatures. The two types of silica refractories available commercially are referred to as 'conventional' and 'superduty.' High-alumina refractories, which consist essentially of alumina and silica, are classed as 50, 60, 70, 80, 90, or 99 per cent alumina products. Of these, the refractories with the highest alumina content have the greatest resistance to high temperatures. Basic refractories include those made of magnesite, of chrome ore, of the mineral fosterite, or of mixtures of these. Insulating firebrick, of which there are several classes, are used mainly to retard the flow of hear through furnace walls.
In the United States, the state which produces the largest among of refractories is Pennsylvania; Missouri comes second. Other important products are Maryland, New Jersey, Ohio, Kentucky, Indiana, Illinois, Alabama, Texas, Colorado, and California. In 1953, the value of the refractories used in the United States amounted to more than $ 324 million.
Bibliography. -- Wilson, H., Ceramics-Clay Technology (New York 1928); Chesters, J.H., Steel Plant Refractories (Sheffield, Eng. 1941); Norton, F.H., Refractories (New York 1949); American Iron and Steel Institute and American Ceramic Society, Refractories Bibliography (Columbus 1950); Harbison-Walker Refractories Company. Modern Refractory Practice (Pittsburgh 1950).
J. Spotts McDowell,
Harbison-Walker Refractories Company,