GOLD MINING
Read through the steps below, or take a
look at the illustrated production chart How
Gold Is Produced.
EXPLORING
Explorers have searched for gold for thousands of years. It was
first found in the beds of rivers and creeks. Our image of the great
California gold rush of 1848 is of hardy prospectors panning for
gold, sometimes in water up to their waists. Even today, individual
prospectors still pan for gold in remote locations. Gold can still
be found this way. The gold in riverbeds is known as alluvial gold.
But over the years, most of the gold close to the surface has been
found.
Today, to
meet growing world demand, gold has to be mined. It is found in
veins of rocks, sometimes thousands of feet below ground. How
do we know it is there? How do we know where to mine?
Exploring
for gold today is very different from in the the past. It involves
the very latest technology, sometimes beginning with infrared
photographs of the earth taken from space satellites. The U.S.
Geological Survey makes these maps avilable to geologists who
specialize in looking for anomalies in the earth's surface that
indicate gold might be present below the surface. Close-up aerial
photos of these areas are then taken and carefully analyzed. Eventually,
drilling equipment is moved to the most promising sites.
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DRILLING AND ENGINEERING
The drills bring up samples of rock which are carefully examined
to see if they contain gold, at what depth, and of what quality
or grade. The samples are chemically analyzed in a laboratory.
The drilling takes place in several locations to help deterine
the size of the deposit and how it varies in depth and quality.
When the areas have been mapped in this way, mining engineers
decide whether or not the value of the gold deposit will be greater
than the cost to develop and operate the mine.
Then they
decide which would be the best kind of mine to design for this
location. They take many things into account, including the depth
of the gold deposits, the surrounding terrain, potential difficulties
in reaching and bringing out the gold, the presence of water,
where to put in roads and buildings, and - among the most important
of today's considerations - the potential impact on wildlife and
the environment. The mining engineers work closely with surveyors,
environmental scientists and government officials.
When the gold
deposits are not too far below the surface, the engineers usually
design an open pit mine, in which the pit is worked downwards
in a series of steps or benches. When the deposits are deeper,
they may design an underground mine. In the western United States,
open pit mining is usually chosen whereas in South Africa, most
mines are underground. The engineers use computers to help them
determine the precise design and measurements of the eventual
mine.
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CONSTRUCTION
Even for an open pit mine, the construction phase is complicated
and can take well over a year. Roads have to be built. Buildings
have to be erected to house and maintain all the engineering and
processing equipment. Offices have to be built for the technical
and administrative personnel. And the processing equipment - in
which the raw rock is crushed and then processed to release the
gold - must be installed.
None of this
construction can begin right away. First, to ensure the plans
meet zoning regulations and adequately protect the environment,
they must be submitted to the authorities - the federal, state
and local agencies that govern mining operations. The mining company
also must set aside adequate funds to pay for reclaiming the land
after the mining is eventually finished. Only when all this is
completed and all the necessary permits have been issued can construction
begin.
The time between
first discovering gold deposits and actually beginning to mine
the gold can be as long as five years. This planning and preparation
process is also expensive, often requiring an investment of hundreds
of millions of dollars before the first ounce of gold can be produced.
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MINING AND
PROCESSING
1) Once the
mine has been designed and built, holes are drilled for blasting,
and samples of ore are examined to determine grade and metallurgical
characteristics. The broken rock is marked by type for efficient
processing.
2) Based on
its metallurgical make-up, a dispatcher directs truck operators
to deliver the ore to the correct processing location.
3a) LOW GRADE
ORE is roughly broken into small chunks and placed on carefully
lined pads where a dilute cyanide solution is distributed over
the surface of the heap. The solution percolates through the heap
and the cyanide dissolves the gold. This solution containing dissolved
gold is then collected.
3b) HIGH GRADE
ORE is delivered to a grinding mill where the ore is pulverized
to a powder. Depending on its metallurgical characteristics the
ore may be treated in one of three recovery circuits.
1. Oxide ore
is sent directly to the leaching circuit where cyanide dissolves
the gold.
2. Refractory ore containing carbon is roasted to over 1,000 degrees
Fahrenheit, burning off the sulfide and carbon. The product of
this process is an oxide ore, which is routed to the leaching
circuit.
3. Sulfide refractory ore without carbon is oxidized in an autoclave
to liberate the gold from sulfide minerals, then it is sent to
the leaching circuit.
4)Treated,
high-grade ore is leached with cyanide. The gold is absorbed (collected)
out of solution onto activated carbon. The remaining cyanide solution
is recycled.
5) The gold-loaded
carbon is moved into a vessel where the gold is chemically stripped
from the carbon which is then recycled.
6) Gold is
precipitated from the solution electrolytically or by chemical
substitution.
7) The impure
gold is then melted into dore bars containing up to 90 percent
gold. Dore bars are then sent to an external refinery to be refined
to bars of 999.9 parts per thousand pure gold.
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REFINING
Refining is the separation and purification of gold from other
metals, as distinct from smelting which is the separation of gold
from mineral impurities. Gold going though refineries may either
be recycled scrap being purified and upgraded or on the final
stage of its transformation from ore in the mine to bullion bars.
Final refining
of gold is normally a two-step process:
First, the
crude gold is melted and chlorine is bubbled through the molten
metal converting all the other metals to chlorides which are floated
off. This yields 99.5 percent pure gold which is cast into anodes.
The anodes
are placed in an electrolytic cell and on passing a current, 99.99
percent pure gold is deposited at the cathode.
Most gold
products and alloys require 99.99 percent gold as their starting
material.
RECLAMATION
Every mine has a limited working life. After a number of years,
when most of the available gold has been removed, a mine is said
to be exhausted. In the old days, exhausted mines were just abandoned.
But today is very different. Stringent environmental and mining
standards require that the land on which mining has taken place
be reclaimed, or restored as closely as possible to its natural
state.
Detailed plans
for reclamation have to be submitted to the government before
permists are issued for mining to begin in the first place, and
progress is closely monitored by environmental scientists and
engineers. During the life of the mine, strict measures are taken
to protect wildlife, particularly any endangered species. Wetlands
and other natural resources are fully protected. And even before
mining has ended, work begins on reclamation - recontouring the
land, planting trees and grasses, and reintroducing wildlife.
Reclamation
of the land has become such an important part of the modern mining
process that, when completed, it is often impossible to tell that
a mine was ever there. Some reclaimed areas are often left more
beautiful than before, with woods, lakes, and abundant wildlife.
Reclamation is the final step in today's gold mining process.
For more information
on environmental issues, click on the icon below.
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