PRECIOUS METAL SERIES - 7
The days when miners could dig or pan for nuggets of gold are largely
gone. Today, gold is generally extracted from the Earth's crust as
microscopic particles.
Estimates put the amount of gold in the Earth's crust at just 11 parts per
billion!To recover a single ounce of gold, many tonnes of material must
therefore be blasted and processed.
Gold mines sell unrefined gold in the form of doré bars. Australian doré bars
are usually composed of between 70-80% gold and 10-15% silver. The lion's
share of these rough bars are processed into fine gold at The Perth Mint,
which operates the largest LBMA-accredited gold refinery in the Southern
Hemisphere.
Each doré bar first goes through a chlorine refining process, also known
as the Miller Process. Originally conceived by Francis Bowyer Miller in
Sydney during the 1860s, the Miller Process involves bubbling chlorine gas
through molten doré gold so that silver (and most other metals) react with
the chlorine to form silver chloride as slag on the top. The resulting gold
is 99.5% pure and typically it's cast into bars weighing about 400oz for use
in wholesale markets.
The Wohlwill process is used to increase purity further. A casting of
99.5% pure gold is lowered into a bath of hydrochloric acid and then has an
electric current passed through it. Acting as an anode in this electrolytic
refining process, the casting dissolves and then deposits on a cathode with a
purity of 99.99%.
The resulting cathodes are melted, granulated and then the granules are
used to measure out exact weights of gold for casting into bar
sizes from 1oz up to 50oz for retail investors.
