Geology is the study of the Earth and its component parts. What does our bedrock look like and how do we best extract metals? What does a mine look like, and what types are there?

Return to navigation

Mineral dEPOSITS FORM THE MINE

Our bedrock was formed several million years ago. Different processes have contributed to the formation of different types of rock in different places. Mineralisations, concentrations of valuable minerals, are formed during certain processes. If these mineralisations are considered economically viable to extract, they are called ore.

Several factors are considered when calculating the mineability of a deposit. Important factors are the cost of extracting the deposit, the metal content and its location. Copper ore with a content of 0.3 per cent is probably not worth extracting if it lies at a depth of 1000 m, but can be interesting if it is at the surface. This is because extraction costs are lower in an open pit (at the surface) than in an underground mine. In an open pit, more low-grade ore can be extracted with a lower content, while underground mines mean more expensive operation. If deposits with a higher concentration are found at great depth however, underground mines can be profitable. In Sweden there are mines reaching depths of more than 1,400 m.

The location of the deposit decides which type of mine can be set up. For example, an open pit mine cannot be started if the deposit is at a depth of 1,000 m.

The quantity of ore extracted every year, the rate of extraction, differs between different types of mine. An open pit means that more ore can be extracted at a lower extraction cost. The offer also means that lower ore content can be extracted, which means that more ore must be extracted every year to achieve profitability in the mine. This is the case for example with the open pit called Aitik, where large quantities of low-grade ore are extracted. In the small underground Lovisa mine a small quantity is extracted each year, but comprises more high-grade ore.

The size of the mine depends on several factors, among other things how large the deposits are, how large the investment is, the mining method and so on.

Facts: Sulphide mineral and the environment

THE MINE’S ECONOMY

A crucial factor in the mine’s economy is the value of the ore, which is often given in kronor per ton. The value is determined above all by the metal content of the ore, and usually amounts to between 200 and 2,000 SEK/ton. Metals of low value like iron therefore require ores with a high metal content (30-65 per cent Fe) for extraction to be economically viable. Deposits of gold and platinum which have a very high metal value on the other hand are profitable to extract even at a very low content (1 – 10 g metal/ton ore).

Depending on the character of the deposit, the ore is extracted from an open pit or underground. If the ore occurs near the surface, it is usually extracted in (with large machines, giving high productivity and lower production costs. Further ore bodies are located deeper, the ore is extracted in an underground mine, which brings higher production costs and lower productivity. An underground mine also takes longer to setup, and demands considerably higher investment costs.

The nature of the deposit and the choice of mining method also affect how large a proportion of the metal content in the ore body can be extracted. There is a certain loss of ore in all ore extraction, which means that between 5 and 30 percent of the ore is not extracted, just as the proportion of waste rock in mined ore reduces the metal content. Ore losses arise if for example parts of the ore must be left as a pillar to stabilise the extraction area, through irregularities of the ore boundary, so that all ore is not blasted away, or when all the blasted ore cannot be loaded up. The dilution by waste rock is caused above all by irregular ore boundaries, or poor strength of the surrounding rock.

Several different mining methods are used for underground mines, depending on the character and value of the ores. For smaller ore bodies with a high ore value, backfilling is often applied, which brings small ore losses, small waste rock dilution but higher production costs. Deposits with lower ore value require that extraction can be carried out at a lower cost through sub level caving or similar, which means a more large-scale process, which is thereby suitable only for large ore bodies. This often gives greater ore loss and greater waste rock dilution.

EXTRACTION AND THE ROUTE TO METAL VIA THE CONCENTRATION PLANT

Ore extraction can be carried out in an open pit or underground. Ore bodies are therefore often mined in open pits down to a depth of 300 – 400 m, below which normal extraction changes to underground extraction. In Sweden, it is underground extraction that dominates, but the majority of the world’s extraction is in open pits. Extraction underground entails bigger costs and greater complexity, and there a richer deposit is required to achieve profitability. As an example where extraction takes place at deeper levels, the distance the ore needs to be transported to reach the surface gradually increases in length, and costs and energy needs thereby increase. Great depths often also mean increased rock stress, which lead to more stringent demands for rock reinforcement, which also increase costs. Great depth also means increased costs for and need of ventilation in the mine.

Underground extraction

In underground mines the ore is accessed by driving ramps or shafts down to the level where the ore is extracted. Many underground mines lack ramps, and people and machines must be transported in shafts. At the extraction levels, production areas are prepared, i.e. drifts (tunnels) are driven to the ore face. A development tunnel can be up to 200 m long. Where necessary, the walls and roof of the tunnel are reinforced with bolts, nets and shotcreting. The choice of mining method is made after consideration of a number of parameters, such as the placing of the ore body, the geology, geometry, rock mechanical conditions and the environment and surroundings. The form and appearance of the ore, which becomes apparent through prospecting, is of vital importance when selecting a mining method. A large compact or body can often be extracted cheaper and more effectively than a small irregular ore body, since one can then use more large-scale methods such as sub level caving. Sub level caving means that you create a cavity in the ore body by drilling and blasting, then the blasted material collapses of its own accord, the ore is driven away from underneath and the rocks surrounding the ore body is allowed to fill the cavity created. All infrastructure in the form of roads, shafts, ventilation etc. is placed in the rock alongside the ore body.

For smaller and more irregular or bodies, other mining methods are used, for example backfilling.

For large ore bodies, and flat ore bodies, the room and pillar mining method is used.

Regardless of the mining method, extraction is through a process that involves a number of basic operations, a so-called blasting cycle.

The blasting cycle for the mining method called sub level caving is as follows: After making a drift to reach the ore in the rock, so-called development, caving is carried out, where long vertical upward holes are drilled through the ore body in a fan shaped pattern. When the whole drift has been drilled, blasting agent is injected into the drill holes, which is called charging. After that, often once a day and at night when the mine is empty, blasting takes place. When the blasting has been done, the explosion gas is ventilated out, and in the morning, loading begins. In the LKAB mines in Kiruna and Malmberget the ore is removed from the drifts with loaders using buckets having a capacity of 17-30 tons. The ore is then dropped into a steep shaft, a so-called ore pass. The ore falls through the shaft and is collected in rock pockets just above the main level.

From the rock pockets the ore is transported in large trucks (Malmberget) or by train (Kiruna) and tipped into large rock pockets above the crushers (transport). In the crushers, the ore is ground down to 10 cm and then transported on long belt conveyors to the ore hoists or skips, which lift the ore to the surface. The ore is loaded automatically into the skips. Each skip can transport about 40 t of ore and travels at a speed of 17 m/s.

Open pit mining

One precondition for open pit extraction is that the ore body extends up to the surface or is not covered by an excessively thick layer of soil or rock. In most open pit mines, ore is extracted by so-called benching. The mining method is based on the ore being extracted in "benches" at successively lower levels. These benches give the open pit a characteristic stepped appearance.

Extraction in open pits is done in a number of productions stages. First the soil and rock layer lying over the ore is transported away, and then the ore is extracted in horizontal discs called stopes. Blasting holes are drilled by downward drilling, and when the blasted ore has been unloaded, production is successively moved to greater depths. The blasted rock is loaded on to tracks by loaders, and waste rock is transported by truck to a waste rock dump, while the ore is transported to a crusher, either in the open pit or on the surface. After crushing, the ore is transported to a concentration plant for grinding, flotation, thickening and drying.

THE ROUTE THROUGH THE CONCENTRATION PLANT

The ore extracted in the mine needs to be treated to separate economically valuable mineral from mineral with little or no economic value. This is done in a concentration plant. How large a proportion of the material extracted has commercial value varies from mine to mine, but as an example, iron mines contain around 50 per cent iron bearing mineral, while a copper mine perhaps contains only 0.5-1.5 per cent copper minerals (e.g. chalcopyrite). A goldmine can be profitable with as little as one or a few grams of gold per ton of ore, i.e. in the order of 0.0001 per cent. Whatever is not commercially valuable mineral is separated out from the process as "waste rock".

Dressing, or the concentration process, normally begins with crushing and grinding. In certain cases, pre-dressing is done, which means that coarse waste particles are separated from the ore flow at an early stage of the process. In iron mines this can be done by dry magnetic separation and in other cases there are also optical separation processes. Separating, which is normally done after crushing but before grinding, has both economic and environmental advantages. The production capacity of the plant increases in following stages so that the need of energy, water and chemicals per ton of extracted ore decreases, at the same time as coarser waste is more inert than if it were fine-grained, and additionally causes fewer problems with e.g. damming. Pre-dressing is not always possible, however, for example if the commercial mineral has small-sized grains. Generally, pre-dressing also leads to some loss of commercially valuable minerals. The goal when crushing and grinding is to reduce the ore to a particle size where commercially valuable mineral can be separated from the waste in subsequent separation processes. The most common separation processes are based on differences in the density, magnetic or surface properties of different minerals. There are also methods based on differences in electrical conductivity or optical properties. For iron bearing minerals, the magnetic separation method is chosen, depending on the mineral type, between high- or low-magnetic methods. Magnetite ores react strongly to magnetic fields and can be separated by low magnetic methods, while haematite is magnetically weaker and therefore needs high magnetic methods. Gravity based methods can be used when there are great differences in density between the minerals to be separated. For example, gold, tin, lead and tungsten-bearing minerals are usually heavy in relation to waste rock and can be separated using such methods. For the base metals copper, zinc and lead, the methods used are often based on surface properties. This is done through selectively adsorbing chemicals (called collectors) on specific mineral surfaces to make them hydrophobic, or water repellent. In flotation cells, the mineral suspension is mixed with air bubbles which attach themselves to the hydrophobic particle surfaces and lift the particles up to the froth phase, while waste mineral is not lifted by the bubbles and instead sinks to the bottom of the cell. There are also processes based on so-called reverse flotation, where instead the waste mineral is floated away from the commercial mineral. The process flows from the concentration plant which constitute waste are transported to tailings storage or mixed with cement and used in underground mines to fill mine tunnels, so-called backfilling. The commercial minerals concentrated (or dressed) through different separation processes need to be dewatered in the last stage of the concentration process. This is often done in several different stages, first through sediment based methods and later through pressure or vacuum filtration. The end product after concentration is then transported in powder form to smelting plants.