Almost all the metals we need for everyday life are mined from the earth’s crust.
In contrast to other industries, mining cannot freely choose where to operate: it can only take place in areas where minerals are concentrated on the surface or underground. This can mean operating in sensitive or challenging environments; a trend that looks set to continue as older, easier to reach deposits run out.
The extraction of metals involves a complex and lengthy series of steps, each of which requires investment of substantial expertise and money. The exploration and development phase from initial discovery to mine production may take up to 10 years, and in some cases even longer.
Mining and metal production has evolved from being a tough and dirty activity to a high-technology operation, using computerised remote-control equipment and complex machinery. Unlike their predecessors, today’s workers are highly trained personnel performing skilled work according to the highest standards of safety and efficiency.
Before a mine is established a long process of mineral exploration is required. This involves advanced scientific techniques, including geochemical analysis of soils, or airborne surveys to measure magnetic, gravitational and electromagnetic fields, all of which help to determine if there’s a sufficient mineral deposit in that location to warrant mining. Once this preliminary investigation is carried out, rock samples are drilled and sent to a laboratory for testing.
The exploration phase brings with it great levels of uncertainty as very few initial mineral prospects result in a profitable mine. Each exploration step is marked by a succession of decisions based on information available at that time. Money and effort are spent in order to raise the degree of confidence in the measurement of the shape, size (quantity) and grade (quality) of the mineralisation held in the earth's crust.
The results of this process are measured in terms of mineral resources and ore reserves. Mineral resources refer to the concentration of materials in or on the earth's crust that have reasonable prospects for economic extraction. Ore reserves are those parts of a mineral resource that can be economically mined. For more information on reserves and resources we recommend visiting the CRIRSCO (Committee for Mineral Reserves International Reporting Standards) website.
If a company decides that a mining operation is feasible, a social and environmental impact assessment is undertaken and submitted to environmental regulatory authorities for approval. This study is usually a major undertaking and should include provisions for public hearings and submissions. The entire process can take between one and three years.
Even with the legal permissions in place there’s still no guarantee that a mine will open. A company’s final decision is based on a feasibility study, which covers every aspect of the mine’s future, including: mining method (open pit or underground), treatment plant design, waste disposal, staff, living arrangements, environmental requirements, transport, safety aspects, capital required, budget, cash flow, costs and profit. This study usually takes up to one year, but can be longer.
Overall, the start-up phase for a mine, from exploration and property development to first mine production, may take in excess of 10 years.
Surface mining involves the removal (stripping) of surface vegetation, dirt, and, if necessary, layers of bedrock to reach buried ore deposits below. Common techniques for surface mining include:
- open-pit mining and quarrying – the recovery of materials from an open pit in the ground
- strip mining – the targeted removal of surface layers to reveal the useful ore/seams underneath.
By contrast, sub-surface mining consists of digging tunnels or shafts into the ground to reach deeply buried ore deposits.
Some mining, including rare earth elements and uranium extraction, is done using less common methods, such as in-situ leaching. This involves dissolving solid minerals through the use of solutions. There is great interest in how these techniques could be applied to the extraction of more common metals as one day they may provide for less environmentally intrusive activities.
After ore is extracted from the earth’s crust, the metal bearing minerals are separated from waste material to form a concentrate using a process known as milling. This is often done by adding chemicals to the ore materials and running them through a series of processes. The waste from the mill, or tailings, is then pumped to a tailings storage facility.
Other techniques include gravity separation, where two or more minerals are separated by movement and the force of gravity and one or more other forces (such as centrifugal forces, magnetic forces or buoyant forces). This is an economic and more environmentally friendly alternative to chemical separation but isn’t commonly used due to limitations in its application. That is, it can only be used economically in locations where minerals can be easily separated from waste materials.
Smelting and refining
Once the metal-bearing minerals are separated from waste materials to form a concentrate, the metal content must be removed and refined. This can be done via a number of methods, one of the most common being smelting. The process involves the chemical breakdown of the minerals through heating and melting. Many smelters are uniquely designed to suit a specific concentrate rather than a variety of concentrates – for example, copper smelters are distinctly different from aluminium smelters and are not interchangeable.
The outputs of smelting are impure metal shapes that are shipped to refineries for purification. Here, the metal products are purified to standards set for world metal markets. While there are numerous grades of metal, the bulk of trading is done at about 99.9 per cent purity.
The other main route of processing is hydrometallurgical – where metals are separated from the minerals by dissolving them. This usually entails a chemical process using acids or cyanide. The metals are finally removed from solution using processes such as solvent extraction and electrowinning – resulting in high purity metals.
The environmental challenges of these refining processes include the handling of solid, liquid and gaseous wastes. The most wide-ranging issue is usually the airborne gases and dusts that can be emitted from the smelting process – including mercury, which is the subject of an ICMM Position Statement and a new United Nations convention. The control of cyanide in heap leaching processes is also critical and is the subject of its own management code. Regulatory controls and technology, made possible by the large capital investments of metallurgical companies, have greatly reduced environmental emissions of all kinds in the last two to three decades.