In the case of zinc, a typical concentrate would be about 55 percent Zn, 27 percent S, and 16 percent other minerals. For lead, the corresponding numbers appear to be about 60 percent Pb, 9 percent S, and 21 percent other. Thus, assuming flux per unit of slag ratio to be the same as for copper (1.2:1), slag output should have been roughly 0.3 MT per metric ton for zinc and 0.38 MT per metric ton for lead. This implies total slag output of 0.06 MMT for zinc smelting and 0.14 MMT for lead smelting. Total slag production for the three main nonferrous metals was thus roughly 1 MMT. Carbon monoxide and carbon dioxide emissions are not known exactly, but they are quite small in comparison with other sources. The waste numbers for other metals are relatively insignificant.

Altogether, based on mass-balance considerations, we estimate smelting and refining wastes for primary metals, including CO2, to have been 43.4 MMT in 1988, including the weight of limestone, manganese, calcium fluorite, and other materials used in the blast furnaces and refineries. (This includes about 14.2 MMT of iron, or blast-furnace, slag, although most of this material is marketed commercially, mainly for road ballast.) In addition, there were about 5.2 MMT of steel-furnace slag, a denser material with a fairly high iron content.

As noted above, much of the sulfur in sulfide copper, lead, zinc, and molybdenum ores is also recovered for use and sold as sulfuric acid (1.125 MMT S content in 1988). Subtracting the blast furnace slag and the by-product sulfuric acid, we get 28.1 MMT residual waste. Of this, only about 1 MMT was solid nonferrous slag; the rest was the oxygen content of the original ores combined with carbon (from coke), released as CO2.36 We have not included the wastes from coking, which we have not estimated. The major airborne emissions other than CO2 are probably CO and particulates. In both cases, blast furnaces are the major sources. The coking quench waters and some spent acids used for pickling constitute the major waterborne wastes.

Mass flows and wastes for the metallic mineral processing industries and the metallurgical industries, taken together, are summarized in Figure 9 (ferrous) and Figure 10 (nonferrous). These values are normalized to U.S. production of the refined metals. Some of the flows are imputed from others. For example, pig iron (94 percent Fe) contains roughly 6 percent C, which implies a carbon content of 3 MMT. The oxygen required to burn this carbon away was therefore approximately 8 MMT, implying a CO2 output of 11 MMT for steel production in 1988. In the case of iron blast furnaces, we assumed that all of the input coke, less the 3 MMT of C embodied in pig iron, was converted to CO2. This consumed 63.4 MMT 02 and generated 87.1 MMT CO2. However, some oxygen was captured from the iron oxide in the ore. So, balancing inputs and outputs, we calculate that the additional oxygen taken from the air must have been 31.5 MMT. Scrap flows are very approximate, partly because scrap industry statistics are poor and partly because we have lumped stock adjustments and scrap flows for convenience.

For comparison, Science Applications International Corporation (1985) estimated the 1983 nongaseous wastes from iron and steel production at 6 MMT 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