This is my alternate assignment project for Chemistry 2202.
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One way of mining for gold, the old-fashioned way, is the simplest way.
“Panning” is the process of filling a circular dish, usually with a pocket, with
gold bearing sand or gravel. Once the dish was partially filled, it was placed
under a gentle stream of water and rotated in the pan, resulting in the lighter
sand and gravel particles being washed away, leaving the heavier gold
particles either in the center of the pan or in the pocket. This was a slow and
tedious process producing only small quantities of gold.
Today, modern scientific advances help in the search for gold. Gold can
be obtained from the Earth’s crust, and from copper ores. To produce gold,
geologists use satellite surveys and geochemistry to locate a deposit of
copper ore. When this is achieved, geologists use computers to make a
“blue-print” of a possible mine which needs exact and precise measurements
of the deposit of ore. Holes are then drilled for blasting, so that samples of
the ore can be examined, and the grade of the metallurgical characteristics (a
procedure used to separate metals from their ores) determined. Based upon
the metallurgical makeup, a dispatcher tells truck operators where the correct
processing location is found to deliver the ore.
To continue the processing of gold it must be divided into low grade ore
and high grade ore. Low grade ore is roughly broken into small chunks and
carefully placed upon lined pads to diluted cyanide solution can be added.
The cyanide in the solution dissolves the gold so that it can be collected.
High grade ore must be delivered to a grinding mill so the ore can be grinded
into a powder. Metallurgical characteristics of the ore are used to decide how
the ore is treated in a recovery circuit. The first circuit is refractory ore. It
contains carbon, which is heated to near 540 ºC (1 000 ºF) to burn off the
sulfide and the carbon, which produces oxide ore. The oxide ore is then sent
to a leaching circuit where, again, cyanide can dissolve the gold. Finally,
sulfide refractory ore, without carbon, is oxidized in an autoclave to free the
gold from sulfide minerals, which is then sent to a leaching circuit.
The following processing step has the gold absorbed, so it can be
collected from the solution and placed on activated carbon. This created
carbon loaded with gold. It is moved to a vessel where the gold can be
chemically stripped from the carbon. Electrically or by chemical solution, gold
is precipitated from the solution. Adding zinc powder to the resulting solution
is what precipitates out the gold. These are the chemical equations of what
happens:
4Au + 8NaCN + O2 + 2H2O ---» 4Na [Au(CN)2] + 4NaOH,
2Na [Au(CN)2] + Zn ---» 2NaCN + Zn(CN)2 + Au (solid)
The pure gold that now remains is now ready to be melted into dore’ bars
which can contain upwards of 90% gold. These bars are sent to an external
refinery to be refined into new bars of 999.9 parts per thousand pure gold.
Worldwide about 1 500 tones are mined each year, mainly in Australia, South
Africa and Russia.
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