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processes and machinery required to extract gold from lithosphere

mining and mineral processing | the lithosphere | siyavula

mining and mineral processing | the lithosphere | siyavula

Now that we know where the minerals that mankind uses can be found, we can look at how he accesses the minerals. We have seen that often the minerals are not found just lying around waiting to be picked up, but rather are embedded in rocks and combined with other elements.

As man learnt more and made new discoveries, the techniques used to extract the ores improved and the amount of mineral that could be extracted increased greatly. Large tunnels started to be cut into the Earth to access minerals buried deep underground. New processing methods meant new kinds of metals could be mined.

Ancient humans did not worry about whether they could make money from the ores they mined. All they cared about was accessing metals and minerals that would help them eat, keep warm and beat the neighbouring tribes.

In modern times money matters and so the first step in mining is to find a suitably sized deposit of ore. For example, diamonds were found in Kimberley and gold ore was found in Johannesburg. These deposits were large enough to make mining for the gold and diamonds profitable.

As the number and size of known deposits is shrinking, geologists (who study the lithosphere) are developing new ways of finding suitable deposits. Geologists will often spend years learning how to find ores and then spend years exploring for the ores.

Once a suitably sized deposit of a mineral has been found, mining can begin. In ancient times mining and mineral processing were very primitive and the main method used to extract ores was digging by hand.

Mining is largely divided into surface and underground mining. Minerals are often found in river beds, beach sands and other sandy areas. These are known as alluvial deposits. These minerals can be fairly easily removed by surface mining techniques. Other minerals occur in long streaks known as veins or in pipes, and underground mining techniques are used to access these minerals.

Underground mining mainly consists of digging tunnels and shafts into the Earth's crust. Gold is often mined in this way. Underground mining is more risky than surface mining as tunnels can collapse and dangerous gases build up underground.

Flotation involves the use of air bubbles to separate the valuable minerals from the unwanted rock. The valuable mineral becomes attached to the air bubbles and rises to the top of the mixture from where they can be removed.

Gold had long been known in Africa by the indigenous people, but in the late 1800's colonists found gold reefs and started exploiting the resource. Since then it has played a very important role (and often a controversial one too) in South Africa's history and economy. Its discovery brought many foreigners into South Africa, lured by the promises of wealth. They set up small mining villages which later grew into larger settlements, towns and cities. One of the first of these settlements was the beginning of present-day Johannesburg, also known as Egoli or Place of Gold.

Most of South Africa's gold is concentrated in the Golden Arc which stretches from Johannesburg to Welkom. Geologists believe that, millions of years ago, this area was a massive inland lake. Rivers feeding into this lake brought sand, silt, pebbles and fine particles of gold, depositing them over a long period of time. Eventually these deposits accumulated and became compacted to form gold-bearing sedimentary rock or gold reefs. It is because of this complex but unique set of circumstances that South Africa's gold deposits are so concentrated in that area. In other countries like Zimbabwe, gold occurs in smaller pockets which are scattered over a much greater area.

Panning for gold is a manual technique that is used to sort gold from other sediments. Wide, shallow pans are filled with sand and gravel (often from river beds) that may contain gold. Water is added and the pans are shaken so that the gold is sorted from the rock and other materials. Because gold is much more dense, it settles on the bottom of the pan. Pilgrim's Rest in Mpumalanga was the first site for gold panning in South Africa.

This type of mining takes place from the surface of the Earth. In open pit mining, the topsoil is removed first and placed on the side of a trench. Then the surface layers of rock are removed to expose the deeper, valuable mineral layers. The valuable rocks are then blasted into smaller rocks using explosives. The rocks are loaded onto huge trucks and taken away for further crushing and processing. Any mineral that is found close to the surface (even up to \(\text{1000}\) \(\text{m}\)) can be mined using surface mining techniques. If minerals are found deeper, as is the case with most of the gold in South Africa, underground mining is used.

South Africa's thin but extensive gold reefs slope at an angle underneath the ground, and this means that some deposits are very deep and often difficult to reach. Shaft mining is needed to reach the gold ore. After the initial drilling, blasting and equipping of a mine shaft, tunnels are built leading outwards from the main shaft so that the gold reef can be reached. Shaft mining is a dangerous operation, and roof supports are needed so that the rock does not collapse. In addition the intense heat and high pressure below the surface make shaft mining very complex, dangerous and expensive. A diagram illustrating open cast and shaft mining is shown in Figure 14.3.

For every ton of ore that is mined, only a very small amount (about \(\text{5}\) \(\text{g}\)) of gold is extracted. A number of different methods can be used to separate gold from its ore, but one of the more common methods is called gold cyanidation.

In the process of gold cyanidation, the ore is crushed and then cyanide (\(\text{CN}^{-}\)) solution is added so that the gold particles are chemically separated from the ore. In this stage of the process, gold is oxidised. Zinc dust is then added to the cyanide solution. The zinc then takes the place of the gold, so that the gold is precipitated out of the solution. This process is shown in Figure 14.4.

Because gold reflects heat very effectively, it is used in space suits and in vehicles. It is also used in the protective outer coating of artificial satellites. One of the more unusual applications of gold is its use in firefighting, where a thin layer of gold is placed in the masks of the firefighters to protect them from the heat.

In Mapungubwe (in the Limpopo Province) there is evidence of gold mining in South Africa as early as 1200. Today, South Africa is a world leader in the technology of gold mining. The following flow diagram illustrates some of the most important steps in the recovery of gold from its ore.

Refer to chapter 13, types of reactions to calculate oxidation numbers. In the reactants gold has an oxidation number of \(\text{0}\). In the products gold occurs as \(\text{NaAu}(\text{CN})_{2}\). The cyanide ion (\(\text{CN}^{-}\)) has an oxidation number of \(-\text{1}\). The sodium ion (\(\text{Na}^{+}\)) has an oxidation number of \(\text{+1}\). The overall compound must have a total oxidation number of \(\text{0}\). Let gold's oxidation number be \(x\). Then:

So gold has an oxidation number of \(\text{+1}\) in the products. Since this is more positive (greater than) the oxidation number of gold in the reactants gold has lost electrons and so being oxidised.

However, despite the incredible value of gold and its usefulness in a variety of applications, all mining is undertaken at a cost to the environment. The following are just a few of the environmental impacts of gold mining:

If the mining process is not monitored properly, acid and other chemicals from gold processing can leach into nearby water systems such as rivers. This causes damage to animals and plants, as well as to humans who may rely on that water for drinking.

This applies particularly to surface mines (open pit mines), where large amounts of soil and rock must be displaced in order to access the mineral reserves. The shape of the landscape can be changed when large amounts of rocks are dug up from the Earth and stacked on the surface. These are called mine dumps. Open pit mines also create very large holes (pits) in the ground that change the shape of the land.

There is a growing emphasis on the need to rehabilitate old mine sites that are no longer in use. If it is too difficult to restore the site to what it was before, then a new type of land use might be decided for that area. Any mine rehabilitation programme should aim to achieve the following:

There are different ways to achieve these goals. For example plants can be used to remove metals from polluted soils and water, and can also be used to stabilise the soil so that other vegetation can grow. Land contouring can help to restore drainage in the area.

One rehabilitation project that has received a lot of publicity is the rehabilitation of dunes that were mined for titanium by Richards Bay Minerals (RBM). As a group, carry out your own research to try to find out more about this project.

Because of the history preserved at the site, Mapungubwe was declared a National Heritage Site in the 1980's and a World Heritage Site in 2003. The area surrounding it was declared a National Park in 1995. Unfortunately, the site is currently under huge threat from an Australian mining company who have been granted rights to construct an opencast and underground coal mine less than 6 km from the border of the National Park. There is much concern that mining operations will have a negative impact on the ecosystems, flora and fauna around Mapungubwe, and that it will hinder preservation of the site. Several environmental agencies have taken the matter to court in order to prohibit the planned mine and protect the environment around Mapungubwe.

As a class discuss, whether or not the proposed mining should go ahead. Divide into two teams. Assign one team to be in favour of the mining and one team against it. Each team should find facts relevant to their argument and then present it to the other team. At the end of the discussion, draw conclusions about whether or not the mining should take place.

Choose one of the minerals given in the list below and find information on how that mineral is mined. Use the information given in this chapter about gold mining and try to find similar information for the mineral you have chosen. Write up your findings in a report. If possible, split the class into groups and assign a different mineral to each group.

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