Two techniques are usually used to recover gold from its place in nature—placer mining, used
to extract gold from rivers and streams; and lode mining, used to take
gold out of hard rock.
Placer mining, named after the alluvial deposits (or “placers”) of
gold found in the beds of streams, was the most common and productive
method of gold mining until the 1920s. The principle is to separate
gold from river gravel by washing it with water. The gold, mixed with
the river gravel, is washed through a series of sluice boxes, each
containing crossbars at the bottom. As the water and gravel pass
through the boxes, which are slanted downward to employ the force of
gravity, the gold sinks to the bottom and lodges behind the crossbars.
An important derivative of this basic placer mining is dredging. A
continuously working chain of buckets brings gravel up from the
riverbed onto a ship, where it is broken, screened and washed through sluices.
MORE: CoinWorld.com's precious metals basics
Extracting gold from solid rock, or lode mining, uses the same
processes common to all underground mining. Entry into the ground is
gained by breaking the rock, by drilling and blasting. The gold ore is
broken away from the surrounding rock in a transportable size and
hauled out of the mine.
These processes of mining yield an impure gold, containing amounts
of silver, copper and other metals. Two methods are most commonly used
to extract gold from its metal relatives—a chemical process using
chlorine, and the electrolytic process, first used in United States Mints.
In the chlorine process, the impure gold is heated until it becomes
molten. Chlorine is bubbled through the molten gold, converting the
silver to silver chloride, which can be skimmed off the top of the
liquid solution. Unlike the electrolytic process, this chlorine method
does not extract platinum from the impure gold.
The electrolytic process involves suspending plates of pure gold
alternately with plates of impure gold in a cell containing a solution
of gold chloride and hydrochloric acid. The impure gold plates are the
anode, the pure gold plates are the cathode. Through the liquid, a
current of electricity is passed from the anode plates to the cathode
plates. The gold dissolves from the anodes and is precipitated on the
cathodes. The other metals that were combined with the impure gold
dissolve in the liquid, except the silver, which is converted into
insoluble chloride and falls to the bottom of the cell. The gold that
precipitates on the cathode plates is washed and melted into bars.
This process yields gold that is more than .999 fine, or 99.9 percent pure.
The above is an excerpt from the eighth edition of the
Coin World Almanac
, published by Amos Media Company in 2011.