Separation of Gold

The industrial separation process that I have decided to do is gold separation. Gold separation can be done in a number of ways but there is not always one way to do it, but the combination of a couple of these processes to extract pure gold. Gold is obtained form the lithosphere. It is separated from the ore found in the lithosphere.

Gold's symbol is Au. Its atomic number is 79 and has an atomic mass of 196.96655 amu. Its melting point is 1064.43oC, while its boiling point is 2807.0 oC. It is a transition metal and is yellow or gold in colour.

There are also many principles used in the separation of gold. One of these is boiling points in the process of smelting. In Froth floatation, the principle is the organic chemicals in the gold which causes it to float when disturbed by air in a tank.

When gold is separated, there is also a bi-product or waste. The waste that comes from the mining of the gold is often used as land fill. This is after further minerals are extracted from the gangue. Waste products are also found in the froth flotation process. Here, the wet gangue is taken to a dam, usually lined with cement, called a tailings dam. The water is allowed to evaporate and the solid waste (tailings) is disposed of. It must however be disposed of safely as it may contain small amounts of toxic lead and zinc. Polluted water from the dam must be kept out of local water ways and prevented from entering the food chain. The problem with this is that it is very time consuming and costly.

Liberation:

Usually the first stage in the liberation of ore minerals from gangue (waste) minerals is crushing. Ore broken by explo¬sives in an open cut mine is usually carried to the primary crusher in large rear-end dump trucks and tipped into a "jaw or cone crusher".

In an underground mine, primary crushing often takes place in a crusher located near the bottom of the main shaft. Ore is tipped down "ore-passes" and is fed to the crusher before being hoisted to the surface.

After secondary crushing, further reduction in size is car¬ried out in rod or ball mills. The ore is ground to a pulp and liberation of gold or gold-bearing minerals is complete.

In the case of alluvial gold deposits, liberation has already been achieved by the natural surface processes of weathering and erosion.

Separation:

Gold's high density compared to most gangue minerals pro¬vides a prompt method of separation, particularly with alluvial material. Cyclones and/or spirals are used to force a slurry of gold and gangue minerals to spin which allows the separa¬tion of the heavy fraction.

More complex ores, where the gold may be thoroughly mixed with other metals and metal sulfides, require more sophisticated techniques of separation. Froth flotation uses the different surface tension properties of metals and metal sulfides compared with gangue minerals. Organic chemicals cause the required gold and metal sulfides to float as a froth on water when disturbed by air in a tank or cell. The gangue minerals sink to the bottom of the cell where they are taken off to a tailings dam. The gold and metal sulfide froth, called concentrate, is skimmed off the top of the cell and taken to cleaners, thickeners and filters to remove water and chemicals.

Roasting:

Roasting may be required for the concen¬trates of "refractory ore" (an ore from which it is difficult or expensive to recover the valuable element) to convert base metal sulfides to sulfur dioxide and metal oxides. Gold trapped in base metal sulfides is more easily recovered than oxide forms.

The next stage in separation involves the recovery of gold from a slurry of gold bearing material (either ore, concentrate or roasted concentrate). Initially this is affected by 'cyanidation'. Here, gold reacts with an aqueous solution of sodium cyanide (NaCN), to form a soluble gold cyanide, Au(CN)2.

Most other non-metallic minerals in the slurry do not react with sodium cyanide.

The solution containing the gold-cyanide complex is fed into tanks in processes called carbon in pulp (CIP) or carbon in leach (CIL). In these processes activated carbon, manufac¬tured from charred coconut husk and in the form of pea-sized pellets, is cycled through a series of tanks counter-current to the flow of gold-complex bearing slurry or leach. The carbon pellets have very large surface areas and the gold-complex ions are adsorbed onto these surfaces.

The carbon pellets with their load of gold are removed from the process tanks and the gold is washed off their surfaces with a hot cyanide solution. The carbon is then reactivated at high temperatures and returned to the process.

Gold is finally removed from solution by electrolysis. A steel wool cathode becomes coated with gold as electricity is passed through the solution in an electrolytic cell. Excess steel wool is dissolved in hydrochloric acid and the gold passes to a smelter.

Smelting:

Smelting involves heating the crude gold until it melts, at which point most impurities float to the surface as a slag. The slag is poured off and the molten gold which still contains 15% silver and 10% base metals (e.g. cop¬per, nickel) is poured into moulds to cool. The resulting gold bars, called mine bars and usually each weighing about 400 ounces, are then ready for refining.

At the refinery, the mine bars are re-melted in a cru¬cible and chlorine gas is bubbled through the melt. Silver and base metal impurities form chlorides which float on the molten gold and are skimmed off for later reprocessing. The gold, now 99.9% pure, is poured into moulds and allowed to cool as bullion. The ingots turned out of the moulds are termed 'good delivery bars' and are acceptable for sale on world gold markets.

Smelting and roasting are classified as pyrometallurgy, which is, the use of heat for the treatment of an ore

Heap Leach Process:

Low grade gold-bearing ore is mined in an open cut, then crushed and screened. Lime is mixed with the ore to control the pH of the leaching operation. The ore is piled into heaps on polyethylene pads and cyanide solution is sprayed over the heaps. The cyanide dissolves most of the gold as it trickles through the ore, producing a "pregnant" solution which is collected in drains at the edge of the heap and pumped to a gold recovery circuit,