Mining and metal production has evolved from being manual, unsafe, dirty and small-scale to being partially- or fully-automated, safer, cleaner and large-scale. Unlike their predecessors, today’s workers are highly trained personnel performing skilled work in accordance with the highest health, safety and environmental standards.
Before a mine is established a long process of metal and mineral exploration is required. Advanced scientific techniques (eg geochemical analysis of the Earth’s crust or airborne surveys to measure magnetic, gravitational and electromagnetic fields) are used to help determine whether a location has a sufficient mineral deposit to warrant mining. Once this preliminary investigation has been carried out, more and larger rock samples are drilled and sent to a laboratory for testing.
Very few rock samples contain metals or mineral of a high enough grade (quality) to be worth mining. Each exploratory step is based on the information available at that time. Money and effort are spent to raise the degree of confidence in the measurement of the shape, size (quantity) and grade of the mineralisation held in the Earth's crust.
The result of the exploration phase is measured in terms of mineral resources and ore reserves.
- Mineral resources: The concentration of materials in or on the Earth's crust that have reasonable prospects for economic extraction.
- Ore reserves: Those parts of a mineral resource that can be economically mined.
For more information on resources and reserves visit CRIRSCO (Committee for Mineral Reserves International Reporting Standards).
When a company decides that a mining operation is feasible, a social and environmental impact assessment is undertaken and submitted to the relevant environmental regulatory authorities for approval. This important process should include provisions for public hearings and submissions. It takes one to three years.
Even with legal permission in place there’s still no guarantee that a mine will be built. A company’s final decision is based on a feasibility study which builds on the pre-feasibility study. A feasibility study covers every aspect of the mine’s future, including:
- Mining method (open pit or underground).
- Processing plant design.
- Waste treatment and disposal.
- Labour agreements.
- Safety, health and environmental requirements.
- Logistics and supply chain.
- Risk controls.
- Capital costs investment.
- Duration of construction.
- Operating costs.
- Production volume.
- Cash flow.
A feasibility study is also concerned with mine closure, ie what happens to the mine when there is no more economically-mineable ore to extract.
A feasibility study typically takes from one to three years, but it can be longer. It depends on the need to test at small- and large-scale the planned mining and processing technologies, and the nature of the mine being built, ie whether it is a brownfield project (an extension to an existing mine) or a greenfield project (a new mineral deposit).
In total, the start-up phase for a mine (from exploration to first mine production) may take more than ten years.
Surface mining involves the stripping (removal) of surface vegetation, soil and (if necessary) layers of bedrock to reach buried ore deposits. Common techniques for surface mining include:
- Open-pit mining and quarrying: The recovery of ore from a progressively wider and deeper hole in the ground using large vehicles to haul the ore from the pit to the processing facility.
- Strip mining: The targeted removal of surface layers to reveal the useful seams underneath.
By contrast, subsurface mining requires mining a vertical shaft into the ground, from which lateral tunnels are excavated at different depths to reach the ore body. Vehicles move the ore from the mining face onto conveyors which carry the ore to the shaft. The ore is then lifted to the surface for further processing.
The Innovation for Cleaner Safer Vehicles (ICSV) is focused on improving the safety of mining vehicles and reducing their undesirable diesel emissions.
Some mining, including rare earth elements and uranium extraction, uses less common methods, such as in situ leaching. This involves injecting solutions into fissures and cracks (some natural, some manmade) in the ground to dissolve the minerals (which are naturally in solid form). This technique involves significantly less material movement and is less intrusive. As a result of this, there is great interest in how this technique could be applied to the extraction of more common metals and minerals.
The processing method chosen to separate the mineral or metal resource from the ore depends on the type of minerals or the grade of the metal.
The first part of the process is to reduce the size of the pieces of mined material. It might be crushed into pebble-sized pieces or ground into a fine powder.
The mineral or metal resource is then extracted by one of the following processes, or by a combination of them:
- Using heavy oils.
- Mechanically with cyclones.
- Chemically with water and reagents (eg sulfuric acid or cyanide).
- Mechano-chemically in flotation vessels.
After extraction, the metal or mineral product (in solid, slurry or dissolved form) may require further processing to increase its purity.
The waste material is sent to long-term storage. Coarse waste is moved to a residue pad, and tailings (fine waste) is pumped to the tailings storage facility.
The state (solid, liquid or dissolved) or the metal or mineral determines how it is purified. Some products, like iron ore, only need to be sized and shaped. To produce London Metal Exchange (LME) grade copper cathodes the copper must first be changed from a dissolved to a sold state by solvent extraction or activated carbon adsorption. This is then followed by electrowinning and electrorefining.
Metals in a liquid (slurry) form are filtered and dried before being smelted or roasted. Smelting is a high-temperature process that extracts the molten metal from the slag (unwanted molten waste). Roasting is also a high-temperature process. It produces metal oxide particles. The metals are further chemically refined to increase their purity to the standards set for world metal markets. Most trading takes place at 99.9 per cent purity.
The environmental challenges of these refining processes include the handling of solid, liquid and gaseous waste. The biggest issue is the airborne gases and dust that can be emitted from the smelting and roasting processes. In addition, mercury risk management is the subject of an ICMM Position Statement and a UNEP convention (Minamata Convention on Mercury). The control of cyanide used for the recovery of gold and silver and 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 mining companies, have greatly reduced environmental emissions of all kinds over the last five decades.