Chemistry EXTRACTION OF GOLD

Extraction of Gold :

`text(Occurrence of Gold)` : Gold is mainly found in native state either as vein good, placer gold or alluvial gold. It is also present to a small extent in the combined state as sulphide, telluride and arsenosulphide. Some important ores of gold are :

(i) Calaverite, `AuTe_2` (ii) Sylvanite, `AuAgTe_2` and (iii) Bismuthaurite, `BiAu_2`

(I) `text(Mac-Arthur-Forest Cyanide process)` : The powdered gold ore, after concentration by Forth-floatation process, is roasted to remove easily oxidisable impurities of tellurium, arsenic and sulphur. The roasted ore is then treated with a dilute solution of `KCN` in presence of atmospheric oxygen when gold dissolves due to the formation of an aurocyanide complex.

`4Au + 8KCN + 2H_2O + O_2 -> undersettext(Soluble)(4K[Au(CN)_2]) + 4KOH`

The metal is then extracted by adding zinc dust.

`2 K[Au(CN)_2] + Zn -> K_2 [Zn(CN)_4] + undersettext(ppt.)(2Au) downarrow`

(II) `text(Plattner's chlorine process)` : The roasted ore is moistened with water and placed in wooden vats with false perforated bottoms. It is saturated with current of chlorine, gold chloride thus formed is leached with water and the solution is treated with a reducing agent such as `FeSO_4` or `H_2S` to precipitate gold.

`AlCl_3 + 3FeSO_4 -> Au downarrow + FeCl_3 + Fe(SO_4)_3`

`2AuCl_3 + 3H_2S -> 6HCl + 3S + 2Au downarrow`

The impure gold thus obtained contains impurities of `Ag` and `Cu`. The removal of `Ag` and `Cu` from gold is called parting. This is done by heating impure gold with conc. `H_2SO_4` (or `HNO_3`) when `Ag` and `Cu` dissolve leaving behind `Au`.

`Cu + 2H_2SO_4 -> CuSO_4 + SO_2 + 2H_2O`

`2Ag + 2H_2SO_4 -> Ag_2SO_4 + SO_2 + 2H_2O`

Properties of Gold :

Gold is a yellow, soft and heavy metal. Gold and `Ag` are called noble metals since they are not attacked by atmospheric oxygen. However, `Ag` gets tarnished when exposed to air containing traces of `H_2S`. Gold is malleable, ductile and a good conductor of heat and electricity. Pure gold is soft. It is alloyed with `Ag` or `Cu` for making jewellry. Purity of gold is expressed in terms of carats. Pure gold is `24` carats. Gold `14` `text(carats)` means that it is an alloy of gold which contains `14` parts by weight of pure gold and `10` parts of copper per `24` parts by weight of the alloy.

Thus the percentage of gold in `14` `text(carats)` of gold is `= 100/24 xx 14 = 58.3%`

Most of the jewellery is made from `22` carat gold (`91.66%` pure gold). Gold is quite inert. It does not react with oxygen, water and acids but dissolves in aqua regia (`3` parts concentrated `HCl` + `1` part concentrated `HNO_3`)

`3HCl + HNO_3 -> 2H_2O + 2Cl] xx 3`

`Au + 3Cl -> Au Cl_3] xx 2`

`2Au + 9 HCl + 3HNO_3 -> undersettext(Auric chloride)(2AuCl_3) + 6H_2O + undersettext(nitrosyl chloride)(3NOCl)`

Oxidation States of Gold :

The principal oxidation states of gold are `+1` and `+3` though `+1` state is more stable than `+3`.

Compounds of Gold :

(i) Auric chloride `AuCl_3`.It is prepared by passing dry `Cl_2` over finely divided gold powder at `573` `K`.

`2 Au + 3Cl_2 overset(`573` `K`)-> 2AuCl_3`

It is a red coloured crystalline solid soluble in water and decomposes on heating to give gold (I) chloride and `Cl_2`

`AuCl_3 oversettext(Heat)-> AuCl + Cl_2`

It dissolved in conc. `HCl` forming chloroauric acid

`AuCl_3 + HCl -> H [AuCl]`

Chloroauric acid is used in photography for toning silver prints and as an antidote for snake poisoning.

(ii) Aurous sulphide, `Au_2S` : lt is prepared when `H_2S` is passed through an acidified solution of potassium aurocyanide, `K[Au(CN)_2]`

`2K [Au (CN)_2] + H_2S -> Au_2S + 2KCN+2HCN`

It is dark brown solid, not attacked by dilute mineral acids and hence is probably the most stable gold (I) compound.