1. Why is it worth mining gold?
:: source of economics
:: economic value in the jewelry industry as well as in high level electronics
:: highly profitable
2. Where are the locations of the major mining activities in South Africa?
:: Witwatersrand
:: Gauteng
:: Welkom
3. Discuss each of the following steps:
3.1. deep level underground mining
:: gold bearing rock called quartz conglomerate is found deep underground (app. 4km)
:: Hence a vertical shaft is sunk to the lowest level and horizontal tunnels are made for access.
:: Lifts called skips are used to hoist the ore to the surface.
3.2. separation of the ore from other rock
:: the crushed powder is washed over copper plates coated with mercury
:: the gold hence forms an amalgam with mercury
:: the amalgam is distilled in iron vessels to recover the gold
3.3. the need to crush the ore bearing rock
:: this makes the ore more manageable and increases the
exposed surface area for the chemical reaction to have a higher rate
3.4. separation of the finely divided gold metal in the ore by dissolving in sodium cyanide oxygen mixture (oxidation)
The gold (Au) is too fine and embedded in the ore to be physically extracted with hammer and chisel!
Hence a chemical means is used.
Step 1
:: The fine gold ore is mixed with water.
:: This mixture is called SLIME.
:: The mixture is made to react with a certain chemical (potassium or sodium cyanide)to make a gold compound.
:: The gold (Au) on the left hand side of the equation is the gold that is still in the ore.
2Au + 4KCN + 1/2 O2 + H2O → 2KAu(CN)4 + 2KOH
The gold forms potassium (or sodium) cyanaurite.
potassium cyanaurite : KAu(CN)4
Notice that the gold is not yet free. It is trapped in the KAu(CN)4
Step 2
Then the gold compound is reacted with another compound (zinc) to finally free the gold as a pure element.
The pure gold is finally recovered by precipatition :
:: Zn reducing agent
2KAu(CN)4 + Zn → ZnK2(CN)4 + 2Au
This is treated with Zn in the absence of oxygen, and the precipitate is
collected in filters.
The gold is finally extracted
3.5. smelting
Smelting is heating ore with carbon (COKE) to release the metal.
(sounds like melting by heating, but this heating includes using carbon)
Smelting is also known as Chemical Reduction
The main use of smelting is to release a metal from its ore by means of a REDOX REACTION.
The CARBON acts as a REDUCING AGENT.
The carbon is oxidized, producing carbon dioxide and carbon monoxide.
The carbon or carbon monoxide derived from it causes oxygen from the ore to leave the metal.
By means of a redox reaction, the oxidation state of the metal ore is changed.
As most ores are impure, it is often necessary to use flux, such as limestone to remove the accompanying rock gangue as slag (also called scoria or cinder).
A common mistake is to think that by using high temperatures only (without the reducing agent carbon), the metal would melt from the ore, and just leak out.
Just heating the ore would create molten (liquid) ore! The metal would still be trapped in the ore.
By using carbon, the redox reaction frees the metal.
The presence of carbon also allows some ores to smelt at temperatures lower than the melting point of the metal in the ore.