Large amounts of nonmetallic minerals are mined or quarried domestically. By weight, stone (1,150 MMT, including limestone) and sand and gravel (863 MMT) top the list of these minerals. Imports and exports are comparatively small. Unlike the case of metals, overburden wastes are small in relation to production (except for clay, for which waste amounts to 35.7 MMT). Domestic concentration wastes are also negligible, except for phosphates (179 MMT), potash (8.9 MMT), and soda ash from trona (7.0 MMT).
Phosphate rock mining and processing are extremely important activities, because phosphate fertilizers are absolutely essential for modern agriculture. Unfortunately, the ore is not of very high grade and is contaminated, especially with cadmium, fluorine, and uranium. In the United States, 451.8 MMT of raw materials were handled to produce 224.1 MMT of crude phosphate rock in 1988. The difference was presumably overburden, which was mostly left in previously mined areas. The crude ore was concentrated, mainly by flotation, to 45.4 MMT of concentrated fluorapatite mineral—roughly (CaF)·Ca4(PO4)3—which was, in turn, treated by sulfuric acid to yield fertilizer-grade phosphoric acid (13.8 MMT phosphorus pentoxide [P2O5]). This refining operation is considered to be part of the chemical industry and is not discussed further here.
Uranium mining in the United States produced about 15 MMT of ore in 1980. This was reduced, mostly by flotation, to 19,500 metric tons (MT) of U3O8 concentrate, or yellow cake, which yielded 4,740 MT of refined uranium oxide (nuclear fuel). About 3,200 MT of ore are needed to produce 1 MT of concentrated UO2 pellets (LeBel, 1982, table 6.1). Uranium production has been declining sharply; production in 1991 was 0.58 MMT of ore and 1,150 MT of yellow cake, a decrease of 96 percent from 1980. Uranium mining added 15 MMT to the 1980 figure for concentration waste, but due to declining demand, waste amounted to about 1 MMT in 1988. (We do not have an exact figure for that year.)
Mine wastes from metallic and nonmetallic mineral production within the United States in 1988 can be summarized as follows. For metal ores, overburden wastes were 1,192 MMT and concentration wastes were 784 MMT, if alumina and crude phosphate rock processing is included, or about 965 MMT and 600 MMT, respectively, if it is not (alumina and phosphate rock are considered products of the chemical industry). The metals system is discussed further below and summarized diagrammatically in connection with metals smelting and refining.
For nonmetallic minerals excluding coal, overburden wastes were 47 MMT and concentration wastes were 15.9 MMT (excluding phosphates). The nonmetallic minerals that are ultimately transformed into inorganic chemicals (phosphates, potash, and soda ash) are discussed further below in connection with chemicals.
The total of overburden wastes for metals and nonmetallic minerals amounted to 1,239 MMT; total concentration wastes were 800 MMT, essentially dry weight, including alumina and phosphates (Tables 3 and 4). Total mineral mining wastes in the United States are actually about 2,050 MMT, excluding water used for