Chemistry_Grade_10-12 (1).pdf

2153 processing the gold ore for every ton of ore

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21.5.3 Processing the gold ore For every ton of ore that is mined, only a very small amount of gold is extracted. A number of different methods can be used to separate gold from its ore, but one of the more common 401
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21.5 CHAPTER 21. THE LITHOSPHERE - GRADE 11 1 2 3 4 5 1 2 3 4 5 Gold seam close to surface Open cast mining at shallow gold seam Sloping gold seams Tunnel from main shaft to access gold Main underground shaft Gold deposits Figure 21.2: Diagram showing open cast and shaft mining methods is called gold cyanidation . In the process of gold cyanidation, the ore is crushed and then cyanide solution is added so that the gold particles are chemically dissolved from the ore. In this step of the process, gold is oxidised. Zinc dust is then added to the cyanide solution. The zinc takes the place of the gold, so that the gold is precipitated out of the solution. This process is shown in figure 21.3. Interesting Fact erest Fact Another method that is used to process gold is called the ’carbon-in-pulp’ (CIP) method. This method makes use of the high affinity that activated carbon has for gold, and there are three stages to the process. The first stage involves the absorption of gold in the cyanide solution to carbon. In the elution stage, gold is removed from the carbon into an alkaline cyanide solution. In the final stage, electro-winning is used to remove gold from the solution through a process of electrolysis. Gold that has been removed is deposited on steel wool electrodes. Both the carbon and the acid are then treated so that they can be re-used. 21.5.4 Characteristics and uses of gold Gold has a number of uses because of its varied and unique characteristics. Below is a list of some of these characteristics that have made gold such a valuable metal: Shiny Gold’s beautiful appearance has made it one of the favourite metals for use in jewelery. 402
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CHAPTER 21. THE LITHOSPHERE - GRADE 11 21.5 STEP 1 - The ore is crushed until it is in fine pieces STEP 2 - A sodium cyanide (NaCN) solution is mixed with the finely ground rock 4 Au + 8 NaCN + O 2 + 2 H 2 O 4 NaAu ( CN ) 2 + 4 NaOH Gold is oxidised . STEP 3 - The gold-bearing solution can then be separated from the remaining solid ore through filtration STEP 4 - Zinc is added. Zinc replaces the gold in the gold-cyanide solution. The gold is precipitated from the solution. This is the reduction part of the reaction. Figure 21.3: Flow diagram showing how gold is processed Durable Gold does not tarnish or corrode easily, and therefore does not deteriorate in quality. It is sometimes used in dentistry to make the crowns for teeth. Malleable and ductile Since gold can be bent and twisted into shape, as well as being flattened into very thin sheets, it is very useful in fine wires and to produce sheets of gold.
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