metallurgical industry, and combustion wastes are discussed separately in connection with fossil fuel combustion.
The Bureau of Mines estimates that mineral exploration and mine development activities (not including those targeting energy fuels) in 1988 generated 190 MMT of waste material, mostly from overburden stripping. Total overburden moved by U.S. mines (excluding coal mines) was 1,241 MMT; overburden moved to supply U.S. metals consumption, adjusted for both imports and exports, was 1,431 MMT.
Mining and quarrying activities consumed 1.87 MMT of industrial explosives, of which 85 percent was ammonium-nitrate based. We have calculated that about 5 percent of the explosive mass was probably converted to N2O, 14 percent to NO, and 5 percent to NO2. This implies N2O emissions of 0.09 MMT and NOx emissions of about 0.36 MMT from explosives.
Total concentration wastes for metals mined and concentrated domestically, including uranium (discussed below), were about 780 MMT. Adjusting for imports and exports reduces the total to 730 MMT. The most common physical concentration process is froth flotation. It is used, especially, to separate sulfide minerals of copper, lead, zinc, molybdenum, and silver from lighter minerals such as silicates, aluminates, and carbonates. Froth flotation is also used to concentrate phosphate rock and, to a minor extent, to clean coal. The most recent data available are for 1985 (Bureau of Mines, 1987). In that year, 380 MMT of mineral ores were concentrated by flotation, yielding 71.5 MMT of concentrates and 309 MMT of dry equivalent mineral wastes. There were 947 billion gallons, or 3,580 MMT, of water used in the process. Wastes from flotation are generally disposed of in ponds, mostly in dry areas. There was 0.63 MMT of chemical reagents used in concentration activities. Grinding mills required 8 billion kW of electricity and 0.134 MMT of iron rods and balls to break up the lumps of ore.
Aluminum and iron ores are not concentrated by flotation. In the case of aluminum, the ore is bauxite, which is a relatively pure mineral that already contains about 30 percent aluminum. Aluminum is further concentrated to relatively pure Al2O3 by the so-called Bayer process, then reduced by electrolysis rather than carbothermic smelting. In 1988, 8.2 MMT of bauxite ore were concentrated to 4.6 MMT of alumina (and some calcined bauxite) in the United States. Almost all of the bauxite was imported. Primary aluminum production in the United States consumed 3.2 MMT of imported alumina.
Iron ore mined in the United States is concentrated for blast furnaces by two processes: pelletizing at the mine and sintering. The latter process is carried out near the blast furnace because it utilizes significant quantities of iron-rich reverts, such as mill scale and dust, from later stages in the iron and steel production process. Blast furnace feed, or concentrates, average 63 percent iron, whereas domestic iron ore is only about 20 percent iron. In 1988, 197 MMT of crude iron ore were concentrated into 57.5 MMT of furnace feed, leaving 140 MMT of wastes.