Methods of exploration

In all these methods we use the properties of the minerals and our knowledge of the lithosphere to locate them underground, without going underground ourselves. For example, iron is magnetic so instruments measuring the changes in the magnetic field can give us clues as to where pockets of iron could be.

Exploration methods are used to find, and assess the quality of mineral deposits, prior to mining. Generally a number of explorative techniques are used, and the results are then compared to see if a location seems suitable for mining.

Remote sensing is the term used to gain information from a distance. For example, by using radar, sonar and satellite images, we can obtain images of the Earth's surface. These images help us to locate possible mining sites, as well as study existing mining sites for possible expansion.

Geophysical methods make use of geology and the physical properties of the minerals to detect them underground. For example, diamonds are formed deep in the Earth at very high temperatures, in kimberlite pipes of igneous rock. The kimberlite pipe is a carrot shape. The first kimberlite pipe to be detected was in Kimberley in South Africa. The pipe was mined, eventually creating the Big Hole.

Geochemical methods combine the knowledge of the chemistry of the minerals with the geology of an area to help identify which compounds are present in the ore and how much of it is present. For example, when an ore body is identified, samples are taken to analyse the mineral content of the ore.

Surface mining

Surface mining is exactly what the word says - digging rocks out from the surface, forming a hole or pit. In South Africa, this method is used to mine for iron, copper, chromium, manganese, phosphate and coal.

Let's look at coal as an example. For surface mining, the minerals need to be close to the surface of the Earth. Most of the coal found in South Africa is shallow enough for surface mining. Usually the rocks are present in layers. To expose the coal layer, the layers above it need to be removed. The vegetation and soil, called the topsoil, is removed and kept aside so that it can be re-deposited in the area after mining. If there is a layer of rock above the coal face, called the overburden, this is also removed before the coal can be excavated. Once all the coal has been removed, the overburden and topsoil are replaced to help in restoring the natural vegetation of the area. This is called rehabilitation.

In Phalaborwa in Limpopo province, copper ore is mined using open pit mining. This open pit is one of the world's largest open pit mines. It is 2 km across and the largest man-made hole in Africa.

When you mine you are digging into solid rock. The rock needs to be broken up into smaller pieces before it can be removed. Holes are drilled in the rock and explosives, like dynamite, are placed inside the holes to blast the rock into pieces. The pieces are still very large and extremely heavy. The rocks are loaded onto very large haul trucks and removed. Sometimes the rocks (ore) are crushed at the mining site to make them easier to transport.

Do you remember learning about how coal is formed? What is coal made from?

Underground mining

Shaft mining

Often the minerals are not found close to the surface of the Earth, but deeper down. In these cases underground mining, also called shaft mining, is used. Examples of underground mining in South Africa are mining for diamonds, gold and sometimes the platinum group metals (PGM).

The PGMs are six transition metals usually found together in ore. They are ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt). South Africa has the highest known reserves of PGMs in the world.

Igneous and metamorphic rock is found where gold is mined.

Sometimes the ore is very deep, which is often the case with diamonds or gold ore. In these cases mine shafts go vertically down and side tunnels make it possible for the miners and equipment to reach the ore. A structure called the headgear is constructed above the shaft and controls the lift system into the vertical shaft. Using the lift, it can take miners up to an hour to reach the bottom of the shaft.

Coal miners used to take a canary with them down the mines. If the canary died, they knew that oxygen levels were being dangerously depleted and that it was not safe to remain underground.

The TauTona Mine in Carletonville, Gauteng is the world's deepest mine. It is 3,9 km deep and has 800 km of tunnels. Working this deep underground is very dangerous. It is very hot, up to 55°C. To be able to work there, the air is constantly cooled to about 28 °C using air-conditioning vents.

South Africa is a leader in the field of deep underground mining as we have several mines deeper than 3 km.

Gold mining in South Africa

South Africa is a world leader in the gold mining industry. We have been doing gold mining for more than a century and our mines are the deepest in the world. Until 2010 we were the leading producer of gold in the world. Gold is a lustrous, precious metal which has a very high conductivity.

QUESTIONS:

What mining method is used to mine for gold?

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Underground mining or shaft mining.

What type of rock is found where gold is mined?

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Igneous rock, or metamorphic rock.

What is gold used for?

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Jewelry, ornaments and decoration, electronics in computers and cell phones.

Do you think gold mining is dangerous? Why do you say so?

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Yes it is, the mines are very deep, of the deepest in the world. Mining deep underground is difficult and dangerous because of the heat and lack of oxygen. Rocks can also collapse because of the pressure.

Provide labels for numbers 1-7 in the following diagram.

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1. Air supply and air conditioners/heat control

2. Lift to take miners into the mine or down the vertical shaft

3. Headgear

4. Horizontal tunnel

5. Removal of ore

6. Mine dump

7. Ore body

Underground mining.

Room and pillar method

One of the methods used in underground mining is called room and pillar, and is often used for mining coal. Part of the mine is open to the surface and part of it is underground. The coal face is dug out, but pillars of coal are left behind to keep the tunnels open and support the roof. Machines called continuous miners are used to remove the coal. The coal is loaded onto conveyor belts and taken up to the surface for further crushing.

What happens once the ore has been removed from the crust by mining?

Hand sorting

Sorting by hand is not a very effective method to separate out the minerals you want. It can only be used in exceptional situation or by individuals, for example many people mine for alluvial diamonds by hand in rivers in Angola. It is a cheap and easy process to do individually, but it is not feasible on an industrial scale.

Magnetic separation

Iron is a metal with magnetic properties. Iron ore can be separated from waste rock by using magnetic separation techniques. Conveyor belts carry the ore past strong electromagnets which remove the magnetic pieces (containing the iron) from the non-magnetic waste. How do you think this works? Study the following diagram

Which container, the left or the right, will contain the magnetic iron ore and which one will contain the non-magnetic waste? Label this on the diagram and provide a reason for your answer below.

The labelled diagram is provided here:

The magnetic iron ore will fall into the container on the right as it is attracted to the magnetic roller and travels around the bend of the magnet for a longer period, whereas the non-magnetic waste drops straight down due to gravity, as the magnet turns, and falls into the first container on the left.

Density separation

One of the first methods for mining gold was that of panning, a technique where ore is mixed with water and forms a suspension. When it is shaken, the dense particles of gold sink to the bottom and could be removed.

Separating beads

Let the learners work in groups of three. The value of the activity is the process of doing it, and not so much the end product. Learners will want to separate every single bead in the process and this is not possible, nor does it happen in the mining industry. Valuable materials do end up as waste.

When choosing beads to separate, ensure that there are a variety of shapes, round and flat, small and large. Most plastic beads will float on water, but metallic ones will sink. The piece of carpet is provided to make the tray rough, but still smooth enough for round beads to roll off, and flat beads to stick. Choose the smallest flattest beads to represent the valuable materials.They will remain on the carpet in the tray more easily.

Learners might want to use the cups to separate by size. This is a great option. If magnetic beads are included, provide the learners with a magnet.

To separate by density, learners can drop the beads into water - some beads will float and others will sink. To separate by size, learners can use the mesh and let the smaller beads fall through into the cup, with the larger ones staying behind.

As an extension, include some beads which are identical in shape and size, but different colours. At this point, learners will want to hand sort them. Tell learners that hand sorting, although effective and is used by individuals, it is a very time-consuming process and therefore almost never done in the mining industry. Ask learners if they have any other ideas. This is where chemical properties come in. For example, tell learners that one colour bead reacts with an acid and the other does not. Get learners to discuss how they would then separate the beads knowing this. A real world example is that silver reacts with chlorine, but gold does not.

This activity can also be done as a class demonstration and some flow diagrams can be designed. The learners can then vote which one they think would work best.

In this activity you are going to separate beads as an analogy for separating minerals in the mining industry.

MATERIALS

• collection of beads, different shapes, sizes, densities and magnetic properties
• paint tray
• piece of carpet
• plastic cup and mesh
• magnet
• water

Optional is a sheet of paper for each learner for planning and making observations.

INSTRUCTIONS:

1. Work in groups of three.
2. Your teacher will indicate to you which bead is the valuable mineral. You need to design a process to separate the valuable mineral from the waste rock.
3. Draw a flow diagram for the process you have designed. Consider using a number of steps in different orders. You may use the same technique more than once.
4. Also remember that repeating a technique improves the efficiency of it. Think about changing the order in which you separate the beads to see if you can find a more efficient process.
5. Hand sorting may NOT be used.

Although hand sorting is an effective method, it is very time consuming which makes it an expensive process, so it is almost never used in the mining industry, except for diamond sorting.

Use the following space to draw a final flow diagram of the process your group designed.

QUESTIONS:

How did you sort the beads based on size?

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Learner-dependent answer. Learners could have used the mesh and let the smaller beads fall through.

How did you sort the beads based on shape?

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Learner-dependent answer. For example, some beads might have been flat and would therefore stick to the carpet, whereas the round ones would roll off.

How did you sort the beads based on density?

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Some beads float and others sink.

How did you sort the beads based on magnetic properties?

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The magnet can be used to lift magnetic beads out of the other beads.

As you have seen in the activity, separating a mixture can be done using different properties, depending on the different properties of the beads. There could be a number of different ways to separate the beads depending on which type of bead you want to select (considered to be the most valuable ones).

Size separation is used frequently in mining to classify ore. For example, when iron ore is exported, it needs to be a certain size to be acceptable to the world market. Coal that is used in power stations also needs to be a certain size so that it can be used to generate electricity effectively.

Density separation is used widely in mining, and you will see why in the next section.

Flotation

Flotation makes use of density separation, but in a special way. Chemicals are added to change the surface properties of the valuable minerals so that air bubbles can attach to them. The minerals are mixed with water to make a slurry, almost like a watery mud. Air bubbles are blown through the slurry and the minerals attach to the bubbles. The air bubbles are much less dense than the solution and rise to the top where the minerals can be scraped off easily.

Separating peanuts and raisins

You will be working in pairs for this activity. You need to observe carefully and explain your observations.

A video of this activity can be found here.

The focus of this activity is to illustrate the principle of flotation and for learners to practice explaining their observations. They will have to apply what they know about density to be able to explain what they see. This activity can also be modified by letting the learners predict what they think will happen before they add the peanuts and raisins to the tap water; and again before they add it to the soda water. The outcome might not be what they expected and the value of the activity is for them to try to explain what they see.

The peanuts will float because they are oily and the bubbles stick to them. This reduces the density of the peanuts so that they are able to float.

The activity can be done as a classroom demonstration, but it is more effective if done by the learners in pairs. The one learner can use the tap water, and the other the soda water. A suggestions is to buy packets of peanuts and raisins separately, otherwise oil from the peanuts can coat the raisins, causing some of the raisins to rise. The raisins can also be rinsed in acidulated water because they are often dressed with oil before sale for visual enhancement.

MATERIALS:

• peanuts
• raisins
• soda water
• tap water
• two tall glasses or beakers

INSTRUCTIONS:

1. Pour tap water into the first glass until it is about ¾ full.
2. Add a handful of the peanuts and raisin mixture to the water and note what happens.
3. Pour soda water into the second glass until it is about ¾ full.
4. Add a handful of the peanuts and raisin mixture to the soda water and note what happens.
5. Write down your observations.
6. Explain your observations.

Use the following space to record and explain your observations.

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Learners should observe that the peanuts and raisins sink to the bottom in the tap water and remain there since they are more dense than water. However, in the soda water, the peanuts and raisins initially sink to the bottom, but then the peanuts start to rise. Small bubbles from the soda water attach to the peanuts' oily surface and cause them to rise to the surface.

The methods mentioned so far are all physical separation methods. Sometimes they are sufficient to separate minerals for use, like coal or iron ore. But more often the element that we are looking for is found as a chemical compound, and so will have to be separated by further chemical reactions. For example, copper in Cu₂S or aluminium in Al₂O₃. What is the name for the force that is holding atoms together in a compound?

Once the compound is removed from the ore, the element we want needs to be separated from the other atoms by chemical means. This process forms part of refining the mineral, as you will see in the next section.

Refining iron

The iron that is formed in the blast furnace often contains too much carbon - about 4% where it should contain not more than 2%. Too much carbon makes the iron brittle. To improve the quality of the iron, it needs to be refined by lowering the amount of carbon. This is done by melting the metal and reacting the carbon with pure oxygen to form carbon dioxide gas. In this way the carbon is burned off and the quality of the iron improves. The iron can now be used in the steel-making process. Carbon reacts with oxygen according to the following chemical equation:

C + O₂ → CO₂

Most minerals go through chemical extraction and refining processes to purify them for use in making materials and other chemical products. These are then distributed to where they are needed, for example, coal is distributed to coal power stations and slag is distributed to construction groups for building roads. The mining industry supplies the manufacturing industry and the chemical industry with its raw materials, for example iron is distributed to steel manufacturing industries.

The impact of mining

Mining has played a major role in the history of South Africa. It accelerated technological development and created infrastructure in remote areas in South Africa. Many small towns in South Africa started because of mining activity in the area. It also created a demand for roads and railways to be built. Most importantly it created job opportunities for thousands of people. Even today many households are dependent on the mining activities for jobs and an income. Mining is an important part of our economic wealth. We export minerals and ore to many other countries in the world.

Mining activities also have a negative impact on the environment. In many cases the landscape is changed. This applies particularly to surface mines (open pit mines), where large amounts of soil and rock must be removed in order to access the minerals. 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 unsightly and dangerous holes (pits) in the ground that change the shape of the land.

Air and water pollution can take place if care is not taken in the design and operation of a mine. Dust from open pit mines, as well as harmful gases such as sulphur dioxide and nitrogen dioxide, could be released from mining processes and contribute to air pollution. Mining activities produce carbon dioxide. Trucks and other vehicles give off exhaust gases.

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

An example are pollutants (dangerous chemicals), called tailings, left over from gold mining which pose a threat to the environment and the health of nearby communities. Dangerous waste chemicals can leak into the groundwater and contaminate water supplies if the tailings are not contained properly.