Energy and Water Impacts
Terry E. Norgate
It is almost inevitable that the ore resources used to satisfy society's ongoing demand for primary metals will deteriorate over time. This deterioration in the quality of metallic ore resources will bring about significant interactions with other resources, such as energy and water. These interactions occur essentially because of the additional gangue (or waste) material that must be moved and treated in the mineral processing stage of the metal's life cycle as the grade falls. The downstream metal extraction and refining stages are virtually unaffected by ore grade, as output streams of relatively constant metal concentration are generally produced in the mineral processing stage, irrespective of initial ore grade.
Results from life cycle assessments on the production of various metals by a number of different processing routes, particularly copper and nickel, have been used to quantify the likely magnitude of the increases in energy and water consumption as ore grade falls for these metals. The need to grind finer-grained ores to finer sizes to liberate valuable minerals has been examined, and the likely magnitude of the increase in energy consumption resulting from this eventuality quantified for copper and nickel production. In light of the need to produce primary metals from ores well into the future, a number of possible approaches to mitigate these effects of increased energy and water consumption as ore quality falls are discussed. Given the significance of the mineral processing stage to these energy and water impacts, these approaches focus primarily on this stage of the metal production supply chain. Additional issues that arise from the deterioration in ore quality are discussed (e.g., greenhouse gas emissions and the possible utilization of metals in the future).
As developing countries strive to improve their standard of living, the anticipated growth in these economies means that there will be an ongoing need for primary metals1 for at least many decades, even with increased levels of dematerialization (i.e., the reduction in the amount of energy and materials required for the production of consumer goods or the provision of services) and recycling. While the potential for the discovery of new high grade (i.e., metal content) resources2 exists, it is almost inevitable that ore resources will deteriorate over time as higher grade resources are exploited and progressively depleted. Figure 8.1 charts, for example, the decline in grade of copper, lead, and gold ores in the United States and Australia over the past century. In addition, many of the newer ore resources are fine-grained, requiring finer grind sizes to achieve mineral liberation. Both of these effects, either combined or in isolation, increase the amount of energy and water resources required for primary metal production. This interaction between energy, water, and metallic ore resources will have a significant impact on the sustainability of these resources. In this chapter I describe how these interactions happen, their likely magnitude and impacts, and some possible approaches to mitigate these impacts.
From a geological perspective, metals, along with other elements, are clas-sifi ed as either being geochemically abundant or geochemically scarce. The first group consists of 12 elements (of which four are widely used metals: aluminium, iron, magnesium, and manganese) and comprises 99.2% of the mass of Earth's continental crust. The other elements, including all other metals, account for the remaining 0.8% of crustal mass. It has been suggested that certain geochemically scarce elements tend to have a bimodal distribution (see Figure 7.4a), in which the smaller peak (corresponding to relatively high concentrations) reflects geochemical mineralization, while the main peak reflects atomic substitution in more common minerals (Skinner 1976). Using copper as an example, Figure 8.1a shows how the average grade of copper ore mined in the U.S. has fallen over the last century to a current value of about 0.5% (globally 0.8%). However, copper present as atomic substitutions in common crustal rocks has an average grade of around 0.006%. Separating the copper atoms from the surrounding mineral matrix would require significantly more energy than current extraction processes. Thus to mine Earth's crustal rock for copper, after the reserves of mineralized copper are exhausted, would increase energy requirements (per tonne of copper metal extracted) by a factor
Metal produced from ores extracted from Earth's crust.
A resource is a concentration of naturally occurring material in the Earth's crust in such form and amount that extraction of a commodity from the concentration is currently or potentially feasible. A reserve, on the other hand, is that part of an identified resource which could be economically extracted or produced at the time of determination. Hence reserve availability changes dynamically with improved geological knowledge, advances in production technology and increased price expectations.
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