Mining Today
Mining Today
The precious metal deposits that are currently being mined in
Nevada are called bulk-mineable deposits because they can be
mined using open-pit methods in which large volumes (tens of
thousands to hundreds of thousands of tons) of rock are moved
each day, creating large piles or "heaps" of ore and
waste rock. The metal content of the ore rock is often less than
5/100ths of an ounce of gold per ton of rock. 
A hundred years ago the ore mined in Nevada
contained gold, silver and other metals in high percentages and
waste rock from these mines contained some 15-40% of the original
value of the ore. Existing methods did not allow for the
extraction of the rest of the metal. Waste rock in these old
mining camps became ore rock when science and technology advanced
the extraction methods to a point where 10ths of a percent gold
per ton could be economically utilized. The mine dumps of today
will become the ore rocks of tomorrow.
Today, in Nevada, the wet processing of precious metal ores
uses a sodium cyanide solution that is placed on the heaps and
allowed to percolate through the ore. The cyanide dissolves the
gold and silver from the rock. The resulting fluid is said to be
pregnant with the metal ion. The use of cyanide to extract
metallic ions from ore is not a new process, but one that has
been actively used since the late 1890's. The cyanide process
replaced the amalgam process that was the preferred method of
extraction. The amalgam process involves the use of metallic
mercury. 
In this process, up to 300
pounds of mercury is combined with a ton of metallic ore, an
amalgamation of the mercury and other metallic minerals is
formed. This method, for example, extracted some 50-85% of the
precious metals from the Comstock Lode ores it was used to treat.
In the process, however, 1-3 pounds of mercury were usually lost
to the environment in a typical operation. In the case of the
Comstock Lode, several million pounds (up to 14 million pounds by
some estimates) of mercury now exists in the Carson River down
stream from the mills. Mercury is still used in many Third World
countries to treat precious metal ores. Cyanide on the other hand
will rapidly degrade into carbon and nitrogen in sunlight or in
contact with the atmosphere. Cyanide can also be rapidly
neutralized by hydrogen peroxide and other chemicals.
It is important that students know where the metals come from they use in their daily lives. However, it is also important that any demonstration of these methods be safe and reasonable for a variety of grade levels. The use of cyanide is therefore completely out of the question in demonstrating the hydrometallurgy process. Also, volume and scale become important considerations in demonstrating the leaching of gold from ores with 5/100ths of an ounce of metal per ton of rock.
Geology
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Extraction Methods