- Exploration: Finding minerals
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This is to get the minerals choc chips out. The next step is to separate the choc chips from the crumbs - also a step in the mining process. Mineral crystals are spread throughout rocks, just like chocolate chips are spread throughout a choc chip biscuit. Sometimes we can see the chocolate chips from the outside, but most of the time the chips are not visible because they are inside the biscuit. The only way to find out how many choc chips there are is to crush the biscuit.
In the same way we can sometimes see mineral crystals from the outside of the rock, but mostly we don't know what minerals there are and and what concentrations are inside the rock. The only way to find out is to break the rock into smaller and smaller pieces. Once we have crumbled the choc chip biscuit, the chocolate pieces can be separated from the crumbs. In the same way in the mining process the valuable minerals can be separated from the unwanted rock. The unwanted rock is called waste rock. Let's look at an example. You have learnt in the previous chapter that copper minerals are found in rocks.
Igneous rock with high mineral content is found here. Here they mine for PGMs, chromium, iron, tin, titanium, vanadium and other minerals using open pit and underground mining. The rocks from the mines are transported by conveyor belts to crushers. Jaw crushers and cone crushers break the huge rocks into smaller rocks.
Short videos showing jaw crushers at work and a ball mill turning. The smaller rocks are then moved to mills where large rod mills and ball mills grind them further into even smaller pieces until it is as fine as powder. You can demonstrate this to your class by placing some pieces of broken up biscuit into a plastic container with some marbles or ball bearings.
Place the lid on the container and then shake it so that the marbles help to crush and break up the biscuit pieces even further. This process of reducing the size of the rocks requires a lot of energy. Just image how hard it is to break a rock. How much more energy do you think is needed to crush a rock until it is like sand? This is one of the steps in the mining process that is very expensive because energy is needed to drive the process.
Exploration: Finding minerals
Most minerals are found as compounds in rocks. Only a few minerals are found in their pure form, in other words not bound to any other element. Examples of minerals found in their pure form are gold and diamonds diamonds consist of the element carbon. Some rocks are used as is, and do not need to be crushed into powder, or involved in minerals extraction. For example phosphate rock itself can be used as a fertiliser, or it can be used to make phosphoric acid. Sand, or the mineral silicon dioxide SiO 2 is used in the building industry.
Coal found in sedimentary rock, is crushed into the appropriate size and used as fuel for electricity generation or the iron-making process. Before the minerals can be used, they need to be separated from the waste rock. A number of different separation techniques are used. These techniques are based on the properties of the minerals. Different minerals are often found together, for example copper and zinc, gold and silver or the PGMs. A combination of techniques are used to separate the minerals from the waste and then the minerals from each other.
You might remember some of the different methods of physical separation from previous grades. This was covered in Matter and Materials. 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.
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.
An electromagnet, as we have learnt in Energy and Change, is a type of magnet in which the magnetic field is produced by electric current. 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 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. 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.
When gold was discovered in Pilgrim's Rest, Mpumalanga in the s they mostly used panning to separate the gold nuggets ore ore from sand and stones in rivers. 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.
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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.
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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.
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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. 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.
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Learner-dependent answer. Learners could have used the mesh and let the smaller beads fall through.
For example, some beads might have been flat and would therefore stick to the carpet, whereas the round ones would roll off. 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.
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. 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.
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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.