Printable Documents: Extracting Metal From Ore
Mining History
There are many plentiful materials on this planet. Early humans
used stone, wood, and any natural fibers that were available.
Useful, implements could be made of these naturally occurring
materials. With the discovery of fire, metals could be extracted
from special rocks and forged into implements. Copper was
certainly one of these earliest materials used and may have been
around since 8000 B.C. Copper can be used for making tools,
weapons, cooking utensils and decorations. Some 5800 years ago
bronze, an alloy of copper and tin, was invented. During this
same time the first extraction of gold and silver from the rocks
was also developed.
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.
Extraction Methods
There are three basic ways of treating metallic ores in order to
extract their metal content. Pyrometallurgy is the treatment of
metals and their ores by heat. Pyrometallurgy includes Roasting,
which is the heating of ores in air or heating it with a reducing
agent such as carbon (coke) or carbon monoxide. Pyrometallurgy is
the oldest extractive process. In early day Nevada, carbon was
obtained from pinyon pines, juniper trees or sagebrush. The wood
was collected and heated in ovens which resembled beehives and
these ovens became the "Beehive Ovens" found today near
numerous old mining camps.
The second basic way of treating metallic ore is
electrometallurgy. In this method the metals are processed using
electricity such as electroplating and electrorefining. Electric
arc furnaces are used to make steel. Electrometallurgy is used
when very high purity metal is needed.
The third basic way of treating metallic ore is
hydrometallurgy. This is a wet process as the name implies, where
reactants are used in a water solution. This method has not been
used on an industrial scale due to the high costs involved.
However, as the concentration of desired metal becomes less in
the ores mined, the wet processing of the more valuable metal
ores becomes more feasible.
Geology
Project Homepage
Please direct questions to: Tom
Lugaski
Last Modified May 20, 1997
Copyright University of Nevada, Reno 1996
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