Energy and greenhouse gas impacts of mining and mineral processing operations

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Abstract

Life cycle assessments of the mining and mineral processing of iron ore, bauxite and copper concentrate were carried out, focussing on embodied energy and greenhouse gas emissions. The results showed that loading and hauling make the largest contributions to the total greenhouse gas emissions for the mining and processing of iron ore and bauxite. In the case of copper ore, the crushing and grinding steps make the largest contribution to the total greenhouse gas emissions for the production of copper concentrate. These results indicate that efforts to reduce the increased greenhouse gas emissions from mining and mineral processing, anticipated in the future as a result of falling ore grades and more finer-grained deposits, should focus on loading and hauling for iron ore and bauxite, while for copper ore the focus should be on grinding. There are a number of new and emerging technologies that could be expected to assist in this task, and these include high pressure grinding rolls and stirred mills for grinding, and further advances in diesel engine technology for loading and hauling applications.

Introduction

The ever increasing global demand for consumer goods means that the production of primary metals can be expected to increase well into the future, despite society's best efforts in recycling and dematerialisation (broadly defined as the reduction in the amount of energy and materials required to serve economic functions, e.g. production of consumer goods or the provision of services). Thus well into the future, metals will be supplied from a combination of primary metal produced from newly mined ores and recycled metals, though the amount of metal recycled will continue to increase. The mining, mineral processing and metal production sector, like other industrial sectors, is coming under increased pressure to reduce the amounts of energy it consumes and greenhouse gases it emits. This has led to the application of life cycle assessment methodology (discussed later) to the production life cycles of most metals [1]. However, most life cycle assessments of metal production processes do not consider the mining and mineral processing stages in any detail, largely due to a lack of publicly available data. This generally means that mining and mineral processing are lumped together as a single stage in the metal life cycle. This approach does not usually introduce any significant errors due to the relatively small contribution that the mining and mineral processing stages make to the ‘cradle-to-gate’ environmental impacts of many metal production processes, particularly with regard to impacts such as embodied energy and greenhouse gas emissions. This is illustrated in Fig. 1, which compares the embodied energy of the mining and mineral processing stages to the downstream metal extraction (smelting and refining) stages for iron, aluminium and copper [1].

However, it has been pointed out previously [2], [3] that the grades (i.e. metal content) of metallic ores have been falling globally for some time, and that this will have a significant effect on the amount of energy required for mining and processing of these lower grade ores due to the additional amount of material that must be treated in these stages. On the other hand, lower ore grades will not significantly increase the energy consumption of the downstream metal extraction and refining stages of many metals (e.g. copper), as a concentrate of fixed grade is produced for downstream processing, irrespective of the initial ore grade. This is illustrated in Fig. 2 for copper produced pyrometallurgically1. In addition to falling ore grades, many of the newly discovered ore deposits are complex and finer-grained, requiring grinding to finer sizes to liberate the valuable or waste minerals in order to achieve separation and concentration. This will also increase the energy consumption of the mineral processing stage, and the combined effects of declining ore grades and finer grind sizes on the embodied energy of copper metal produced pyrometallurgically are shown in Fig. 3.

Thus it can be expected that the environmental impacts, particularly energy consumption and greenhouse gas emissions, of mining and mineral processing for many metals will become much more significant in the future than they currently are. It is therefore important that the contributions of the various processing steps that make up these stages be quantified, with the major contributing steps being identified in order that efforts to reduce these environmental impacts be focussed on these steps. With this objective in mind, life cycle assessments (LCAs) of the mining and mineral processing of iron, aluminium (bauxite) and copper ores were carried out. These three ores were chosen for this study mainly due to the significant amount of these ores mined annually in Australia, as shown in Table 1. In the case of iron ore and bauxite, these ores are largely transported to downstream metal extraction plants without any significant beneficiation because of their relatively high ore grades as mined (e.g. typically 60% for iron ore and 22% for bauxite in Australia). However, copper ore generally undergoes beneficiation (concentration) to produce a concentrate (typically in the order of 30% Cu) for downstream metal extraction as pointed out above. This paper presents the results of these LCAs, with breakdown of the results to show the contributions of the various processing stages to the overall impact. Some possible technologies to reduce the energy and greenhouse gas impacts of mining and mineral processing operations are also described in the paper.

Section snippets

Mining and mineral processing operations

The extraction of metallic ores involves both surface (open-pit) and underground mining techniques. The method selected depends on a variety of factors, including the nature and location of the deposit, and the size, depth and grade of the deposit. Underground mining requires more energy than surface mining due to greater requirements for hauling, ventilation, water pumping and other operations. Surface mining accounts for the majority of mining, although most of the copper ore produced in

Life cycle assessment

Life Cycle Assessment (LCA) is a relatively new methodology that can be used to assess the environmental impact of various activities, products and processes objectively. LCA covers the consecutive and interlinked stages of a product or process system, from raw material acquisition or generation from natural resources through to final disposal. It essentially involves the compilation of an inventory of relevant environmental exchanges during the life cycle of a product and evaluating the

Results

The energy and greenhouse gas results from the LCA study of the various mining and mineral processing operations are given in Table 3 and shown graphically in Fig. 4, Fig. 5, Fig. 6. The greenhouse gas emissions were 11.9 and 4.9 kg CO2e/t ore for iron ore and bauxite respectively, while for copper concentrate they were 628 kg CO2e/t concentrate. Based on the inventory data in Table 2, the latter figure corresponds to 38.8 kg CO2e/t ore, which is not too different to the value of 32 kg CO2e/t ore

Technologies to reduce energy and greenhouse gas impacts of mining

It has been reported [4] that the metal mining industry in the United States has the potential to reduce energy consumption by about 61% from current practice to the best-estimated practical minimum energy consumption. This reduction was made up of a 21% reduction by implementing best practices and a 40% reduction from research and development that improves energy efficiency of mining and mineral processing technologies. Reported [4] current and practical minimum energy consumptions, including

Future demand for metals

Increasing demand for primary metals, together with falling ore grades and more complex ore bodies can be expected to lead to an increase in global energy consumption and greenhouse gas emissions from primary metal production (particularly in the mining and mineral processing stages) in the future, as pointed out earlier. Furthermore, though land used for the extraction of primary metals represents less than 0.1% of the terrestrial surface of the earth [41], exploration and mining activity can

Conclusions

Most life cycle assessments of metal production processes do not consider the mining and mineral processing stage in any detail, largely due to a lack of publicly available data and the relatively small contribution that the mining and mineral processing stages make to the ‘cradle-to-gate’ environmental impacts of many metal production processes, particularly with regard to impacts such as embodied energy and greenhouse gas emissions. However, falling ore grades together with the likelihood of

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