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Figure 8.12 Global annual greenhouse gas (GHG) emissions for various primary metals.

in Figure 8.12 by a factor of 10, from 5.2% to 0.5% (i.e., 0.3% Cu, 0.2% Ni, 0.6% Pb, and 0.9% Zn), increases the estimated global annual greenhouse gas emissions from base metal production from about 140Tg to about 450 Tg, well below the steel figure of 1970 Tg. Similarly, reducing the mean base metal ore grade by a factor of 30 from 5.2-0.2% (an extreme and almost certainly uneconomic scenario) increases base metal global annual greenhouse gas emissions to 1500 Tg—still less than the current steel figure.

Thus while the greenhouse gas impacts of base metals are expected to become more significant in the future, present attempts to reduce global greenhouse gas emissions from the metal sector should focus largely on steel and aluminium. According to the U.S. Geological Survey (USGS 2008a), the world's reserves (see footnote 2) of iron ore are in the order of 150 billion tonnes, with an average grade of 49% Fe; this represents about 80 years of current mine production. A fall in iron ore grade from 64-49% Fe would increase the global greenhouse gas emissions shown in Figure 8.12 for steel by about 500 Gg CO2 equivalent/yr. Current base metal average ore grades would have to fall by a factor of more than 10 over this period to match this increase, as indicated in Figure 8.12.

The Way Forward

While it is inevitable that ore resources will deteriorate (both in ore grade and grain size) over time, there are a number of approaches which might help mitigate the energy, greenhouse gas, and water impacts of such a change. One obvious way to address the problem of deteriorating ore resources is to reduce the demand for primary metal to be produced from these resources in the first place. Dematerialization and recycling (i.e., secondary metal production), as mentioned earlier, will help achieve this goal; however, recycling is only possible for metals used in nondissipative applications, where the metals can be economically reclaimed. Another possibility is to reduce the energy consumption of primary metal production. As falling ore grades have relatively little effect on the energy (and to a lesser extent water) consumption of the metal extraction and refining stages, as pointed out earlier, the focus here is on mining and mineral processing stages.

Reducing Energy Consumption for Mining and Mineral Processing

Figure 8.13 shows the current annual energy consumption of the U.S. metal mining and mineral processing sector broken down into the various processing steps. The practical minimum energy consumption is also shown for the various steps, and it is apparent that comminution (i.e., size reduction, primarily grinding) accounts for the majority of the energy consumed by this sector. Therefore, to reduce the energy intensity consequences of deteriorating ore

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