Political and commercial geology and the world's mineral resources

by Unknown

USES OF CHROMIUM

Chromite is the principal ore from which metallic chromium and chromium products are obtained. The theoretical composition of chromite is represented by the formula FeO Cr₂O₃, which represents 32 per cent. ferrous oxide and 68 per cent. chromic oxide. In many ores, however, the ferrous oxide is partly replaced by magnesia, up to 30 per cent., and the chromic oxide by alumina and ferric oxide, up to 20 per cent. Thus the composition of chromite varies considerably. The percentage of chromic oxide may be as low as 10 per cent.; that of ferrous oxide may range from 10 to 50 per cent. Other common minerals of chromium are picotite (chrome spinel), uvarovite (chrome garnet), chrome diopside and crocoite (lead chromate).

Chrome ore is consumed mainly in the manufacture of special steels and in tanning leather. The special steels comprise chrome steel, chrome-nickel steel, chrome-tungsten steel, and chrome-vanadium steel. Metallic chromium is added to such steel in the form of ferrochrome, an alloy of chromium and iron containing 60 to 70 per cent. metallic chromium. Chrome steel is tough and hard to break. It hardens rapidly and has a fine grain and a fibrous fracture; it does not break readily upon concussion and because of its hardness is difficult or impossible to cut with ordinary machine tools. Metallic chromium is present in percentages varying from 1 to 5 per cent. Special steels containing chromium are used for guns, armor plate, armor-piercing projectiles, automobile parts, machine tools, bars for prisons, burglar-proof safes, shoes, cutlery, crusher jaws, stampmills, springs and for other articles in which hardness and toughness are necessary. During the war, when a considerable shortage of chromite threatened, less chromium was used in chrome steels, and certain other hardening materials were used in its place. The results are said to have been unsatisfactory, however.

Ferrochrome used in the manufacture of chrome steels is produced in the electric furnace by smelting a mixture, in proper proportions, of chromite, coal or coke, lime, and fluorspar or silica.

Much chrome ore is used in the steel industry for refractory materials in lining open-hearth furnaces. Some of the ore thus used is first manufactured into chrome brick and some is utilized in the crude form. Chrome brick is used in open-hearth furnaces as a lining along the slag line between the magnesite bottom of the furnace and the silica-brick sides and roof. Chrome brick is used also to cover the ports of gas-fired furnaces. It is desirable for these purposes on account of its neutral reaction, which reduces the wear due to corrosion. Lump or crushed chromite is used for patching the bottoms of open-hearth furnaces, particularly the toe or apex of the bottom, the ore being either hammered in place as lump or crushed and mixed with a little water or tar and clay and then tamped into place. The use of lump chrome in repairing such furnaces is desirable on account of the rapidity with which the furnaces can be repaired and on account of the greater wear that chromite will stand. The amount of chromite used for this purpose ranges from 2 pounds to 10 pounds per ton of steel manufactured. Magnesite has been used to replace chromite for repairing furnaces, but has been found to be more expensive and to stand less wear.

Chrome brick is used in a minor way in electric furnaces for manufacturing steel, in a belt along the slag line and in the area around the pouring lips. It is also being used in furnaces manufacturing steel by the duplex process.

Chrome brick, besides being used in steel-making furnaces, is used in lining furnaces for making copper, nickel, and other metals. In these furnaces it is used in the bottoms and around the tap holes. Magnesite brick, as well as bauxite brick, have been used to replace chromite brick for this purpose.

Chromium chemicals used for tanning are mainly sodium or potassium bichromates. About half of the bichromates produced in the United States is commonly used for tanning, the remainder being used for paints, pigments, dry colors, and dyes in the paint, printing and engraving, and textile industries. Chrome yellow, chrome orange, and chrome black are used in calico printing and dyeing. Chromic oxide, or chrome green, is an indelible pigment employed in printing banknotes. Various other chrome colors are used for paints and pigments as well as in the ceramic arts. The minor uses of chrome chemicals are many.