Page 39 - Curriculum Visions Dynamic Book

Panning
The accumulation of gold in river sediments is known as a placer deposit. The gold can be separated out by hand in a method known as gold panning. Because it is heavy, gold resists being washed away. The gold particles can be separated from sand by swirling the pan around in a little water. The gold will remain in the centre of the pan while the sand swirls off the edges.
amalgam: a liquid alloy of mercury with another metal.
placer deposit: a kind of ore body made of a sediment that contains fragments of gold ore eroded from a mother lode and transported by rivers and/or ocean currents.
sediment: material that settles out at the bottom of a liquid when it is still.
Chemical refining
Chemical refining is the last stage of the refining process. It can be applied to gold sediment that has already been partly refined using amalgamation or to the sludge produced in copper and zinc refining.
Like silver, gold can be dissolved in sodium cyanide. When metallic zinc is added to the solution, the gold is precipitated.
 Panning gold in a river in Alaska.
Amalgamation: using mercury to refine gold
Another way to recover gold from placer deposits is by making the gold into a liquid alloy with mercury, known as an amalgam.
In this age-old process, the mercury and gold-containing sediment are heated, causing the gold to amalgamate with the mercury. The amalgam can then be drawn off and dissolved in dilute sodium cyanide. When zinc is added to the solution, gold is precipitated to the bottom of the vessel.
Amalgamation recovers about two-thirds of the gold in an ore. To extract the remainder, other chemical methods have to be used.
This process uses a range of highly poisonous substances and has to be carried out in carefully controlled conditions. Nevertheless, despite the risks, refining using mercury is done on a small scale by prospectors, for example, in the depths of the Brazilian rainforest. The waste products
of this process are flushed into the rivers, where they pollute whole stretches of river water that are used as drinking supplies.
Many of the prospectors breathe in the mercury vapour as the amalgam is heated, causing long-term (and even fatal) damage to their health.
EQUATION: Precipitation of gold using sodium cyanide and zinc
Sodium cyanide + gold amalgam + water + oxygen ➪ cyanide complex + sodium hydroxide 8NaCN(s) + 4Au(s) + 2H2O(l) + O2(g) ➪ 4NaAu(CN)2(aq) + 4NaOH(aq) Cyanide complex + zinc ➪ cyanide complex + gold
2NaAu(CN)2(aq) + Zn(s) ➪ Na2Zn(CN)4(aq) + 2Au(s)
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