MMT of limestone and dolomite were used, mostly in blast furnaces. Data published by the Bureau of Mines indicate that these materials were calcined on site and consumed as lime (4.8 MMT). Other inputs reported by the Bureau of Mines include salt (0.33 MMT), manganese ore (0.123 MMT), and fluorspar (0.137 MMT).

The production of primary metals from concentrates is normally accomplished by carbothermic reduction (smelting with coke) or electrolysis. By far the major product by weight is pig iron. U.S. blast-furnace output in 1988 was 50.9 MMT. This material has an iron content of 94 percent and it is almost entirely used for carbon steel production by the basic oxygen process. (There was a small amount of open-hearth production in 1988, which has now ended. Electric steel "minimills" use scrap almost exclusively.)

Blast furnace inputs in 1988 included about 3 MMT of scrap iron and steel, whereas sinter also utilized about 6 MMT of upstream reverts (dust, mill scale). Therefore, accounting for the virgin ore is somewhat complex. However, the iron content of U.S. ores in 1988 was reported as 57.515 MMT. Blast furnace inputs (pellets) averaged about 63 percent iron, 5 percent silica, 2 percent moisture, and 0.35 percent other minerals (phosphorus, manganese, alumina). The remainder was oxygen.

In the reduction process, the oxygen combines with carbon (actually carbon monoxide) from the coke. About 0.5 MT of coke was used per metric ton of pig iron, along with 0.142 MT of miscellaneous materials, mostly fluxes (lime and limestone) for the sinter plants and to make the molten slag flow easily. Slag consists of the silica and other nonferrous minerals in the sinter and pellets and the materials in the fluxes. Total iron blast furnace slag production in the United States was 14.2 MMT, or 0.28 MT of slag per metric ton of pig iron. However, slag is no longer considered a waste, because virtually all slag produced is marketed for a variety of uses. Subsequent refining of pig iron and scrap iron to carbon steel is done in a later refining stage, normally the basic oxygen furnace. Steel furnaces produced an additional 5 MMT of slag in 1988.

As noted, the oxygen in the iron-bearing concentrates reacts in the blast furnace with carbon monoxide. The reduction process requires excess CO, so the emissions (blast-furnace gas) consist mostly of unreacted CO. Although combustible, it is of relatively low heating value. Currently, most blast-furnace gas is used elsewhere in the integrated steel complex as fuel (e.g., for preheating blast air), although some is used as fuel by nearby electric power plants. The capture of gaseous emissions from blast furnaces is not 100 percent efficient, so some CO escapes. However, considering the iron/steel process as a whole, all of the carbon (from coke) is eventually oxidized to CO2. In 1988, the steel industry accounted for 182 MMT of CO2 from coke, which is included in the grand total from fossil fuel combustion, discussed below. (In addition, the steel industry used some other hydrocarbon fuels.)

Coke ovens and steel-rolling mills are significant sources of hazardous wastes, even though the coke-oven gas is efficiently captured for use as fuel, and



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement