360 



UNITED STATES MINERAL RESOURCES 



terials, such as shale and siUca sand (Ericksen and 

 Thomson, 1968, p. 165). Suitable limestone contains 

 as much as 25 percent clay and sand, generally not 

 more than 6.5 percent dolomite, less than 0.5 percent 

 phosphorous pentoxide, and only very small amounts 

 of sulfur. In the Chicago, Pittsburgh, and Birming- 

 ham steel-producing areas, blast-furnace slag is 

 used for making cement; on the Gulf Coast and in 

 California, large deposits of sea shells are used as 

 the source of CaCOa. Cement made with slag and 

 shells, however, accounts for only a small part of 

 the total production. 



FLUXES 



Flux for blast and open-hearth furnaces consumed 

 about 5 percent of carbonate rocks produced in the 

 United States in 1969. During smelting, the mag- 

 nesia (MgO) and lime (CaO) of these rocks combine 

 with impurities, such as silica and alumina, to form 

 a liquid slag that separates from the molten metal. 

 About half the limestone used for flux comes from 

 Michigan, and about 20 percent from Ohio, and 20 

 percent from Pennsylvania (U.S. Bur. Mines, 1953- 

 1971, Minerals Yearbooks for 1950 to 1969). Lime- 

 stone suitable for blast-furnace flux ranges from 

 high-calcium limestone to calcareous dolomite con- 

 taining as much as 31 percent MgCOs and not more 

 than 5 percent silica, 2 percent AUOs, and 0.05 per- 

 cent sulfur (Dever and McGrain, 1969, table 22). 

 It should contain at least 95 percent total carbonate ; 

 however, where the transportation cost of such stone 

 is high, lower grade stone may be used. Limestone 

 for open-hearth furnaces should contain at least 97 

 percent CaCOa, less than 3 percent silica, less than 

 1.5 percent AI2O3, and low sulfur. Limestone suit- 

 able for fluxing nonferrous metals is similar to 

 high-calcium limestone suitable for blast-furnace 

 flux. 



LIME 



Lime manufacture consumed about 5 percent of 

 the limestone produced in the United States in 1969. 

 Lime is calcium oxide or calcium and magnesium 

 oxide made by calcining (heating) limestone or dolo- 

 mite to drive off the carbon dioxide. About one- 

 third of the manufactured lime is used in metallurgy 

 for smelting and for concentration or beneficiation 

 of metalliferous ores, another third is used by the 

 chemical industry, and the remainder is used in 

 many ways, principally for water softening, pulp 

 and paper manufacture, sugar manufacture, and 

 sewage and waste treatment. Limestone suitable 

 for high-calcium lime contains at least 97 percent 

 CaCOa. Low-magnesium lime contains 6-40 percent 

 MgCOa, whereas high-magnesium lime contains more 



than 40 percent MgCOa ; both types should have less 

 than 3 percent noncarbonate impurities. 



REFRACTORY DOLOMITE 



Dead-burned or refractory dolomite, a fairly 

 stable lime made by calcining dolomite at especially 

 high temperature, used about one-half a percent of 

 the carbonate rocks produced in 1969 in the United 

 States. It is used chiefly for lining and patching of 

 linings of open-hearth steel furnaces. Dolomite for 

 this purpose should contain more than 37.5 percent 

 MgCOa and at least 97 percent total carbonate 

 minerals. 



AGRICULTURAL LIMESTONE 



Agricultural limestone consumed about 4 percent 

 of the limestone produced in the United States in 

 1969. Agricultural limestone must have a calcium 

 carbonate equivalent value of 80 or more. Pure lime- 

 stone has a value of 100 and pure dolomite 108.6. 

 (Dolomite has a larger value because magnesium is 

 lighter than calcium.) 



OTHER USES 



About 8 percent of the limestone produced in the 

 United States in 1969 had a wide variety of uses, 

 each too small to be shown in figure 43. Among 

 these are for alkali works, about 0.5 percent; glass 

 manufacture, about 0.15 percent; and limestone 

 sand, about 1.5 percent. Some other uses were not 

 tabulated by the U.S. Bureau of Mines in 1969 but 

 in past years have consumed comparable amounts of 

 limestone. These include the manufacture of cal- 

 cium carbide, beet sugar, pulp and paper, and in- 

 dustrial filler (whiting) , and uses as pulverized lime- 

 stone or dust that is applied to walls, roofs, and 

 floors of coal mines to prevent or check coal-dust 

 explosions, and as filter beds for water purification. 

 The requirements for these uses vary, but, in gen- 

 eral, limestone with a CaCOs content of at least 97 

 percent is required. 



Limestone and dolomite probably will continue to 

 be consumed in increasing quantities in the future 

 and undoubtedly, new uses, particularly for high- 

 purity stone, will become important. 



GEOLOGIC ENVIRONMENTS 



Limestone and dolomite form in a variety of en- 

 vironments, but they generally accumulate in warm 

 shallow marine water. High-calcium limestone 

 forms in those areas of free circulation with the 

 open sea that are free of clastic sediments from 

 nearby streams. Dolomite, however, is chiefly a rock 

 of secondary origin, formed during diagenesis of 

 limestone by partial replacement of CaCOa with 

 MgCOa to form the mineral dolomite. 



