Modern Gold Mining
Three Basic Steps ~ Today, commercial gold mining involves the expertise of geologists and chemists in strategic maneuvers, some similar to those used during the Gold Rush era.
When gold has been located, either near the surface or deeper undergroundsome modern mines are three miles deepthree basic operations are undertaken.
- First, the ore is extracted by conventional methods.
- Next, the gold is separated by chemical and/or mechanical processes.
- Then the gold is refined, or purified, and cast into bars.
Small nuggets or grains of gold may be found in riverbed sand. Since it is not physically attached to the grains of sand that surround it, the gold can be easily separated by gravity. This is possible because gold is over 19 times as heavy as water and about seven times as heavy as sand. Therefore, the sand is carried away by the flow of water, but the heavy gold falls to the bottom.
Gold can also be locked in seams of rock, often far underground. Today, man often has to dig deep to find narrow veins of quartz containing minute grains of gold, sometimes alloyed with a low percentage of silver or mixed with sulfides. It takes five tons of ore to produce just an ounce of gold. How is this done?
Extraction ~ The first stage in mining is to break the rock into transportable chunks with special pneumatic drills and dynamite. Since the quartz vein is sometimes only a foot or two thick, a tremendous amount of material surrounding the vein also has to be broken up to get the gold-bearing quartz out. This ore is then loaded into small cars running on narrow-gauge rails and is carried to the mill, where the gold is separated from the quartz and the sulfides. The ore is dumped into a large hopper able to hold more than 400 tons of rock.
The Crusher ~ From the opening at the bottom of this big concrete hopper, the ore drops into a crusher that reduces the size of the rock. Often two or more crushers work in succession, gradually reducing the size of the rock.
The Ball Mill ~ The crushed ore is now put into a ball mill, which is a huge cylinder or drum that rotates horizontally and is about one third full of steel balls weighing several tons. As the mill rotates, the ore is gradually pulverized under the tremendous pressure of the steel balls as they roll over the rock. After many hours, the ore is reduced to sand. The larger particles of gold have by now been released and can now be separated by such methods as jigging.
Jigging ~ A jig is a machine that uses a stream of water to separate gold and sulfides from most of the sand made by the ball mill. The gold, sulfide (because it is also heavy), and a little sand settle into the hutch of the jig and this mixture is called the "jig concentrate." It now must be subjected to the amalgamation process.
Amalgamation ~ This is accomplished with mercury, which absorbs gold, forming what is called an "amalgam." But the mercury does not form amalgams with quartz or sulfide, so the mercury extracts the gold and leaves the rest. Such amalgamation accounts for the extraction of over 60 percent of the total gold in the ore. What about the rest of the gold that is locked inside the sulfides?
Cyanidation ~ The precious sulfides must be ground very fine so the gold can be dissolved. A very dilute solution of the deadly poison cyanide has the amazing ability to dissolve gold. So the finely ground sulfides are stirred in huge vats for a day or two in a solution containing cyanide and a bit of lime. When the gold dissolves, the now barren sulfides are allowed to settle. The gold solution, known as "pregnant" solution, is siphoned off. To recover the gold in a solid state, zinc dust is added. This precipitates, or separates, the gold from the solution.
By means of amalgamation and cyanidation over 90 percent of the gold contained in the original ore is extracted. But at this stage the extracts do not even look like gold. The gold-mercury amalgam is in the form of silver-gray balls and the zinc-gold precipitate is a brownish-black sludge. So both of these substances have to be sent from the concentrating mill to the chemical plant for treatment.
Treating Amalgam ~ The first step in treating the amalgam is distillation. Mercury boils at 675 degrees Fahrenheit, whereas gold does not even melt until it reaches 1,945 degrees Fahrenheit. The amalgam is put into an iron retort fitted with an outlet pipe that is cooled by running water. The retort is heated to the point where the mercury boils off, leaving the gold behind. The mercury, however, is collected and reused.
Treating Precipitate ~ The gold-zinc precipitate has to be handled differently. It is treated with acid, which dissolves the zinc and thus frees the gold. Then the gold residue is washed and dried.
The Melt Down ~ At this point gold from both concentrates is ready for melting down, along with any gold that may have been retained on top of the jig "bed." The gold is put into large graphite crucibles, along with various chemicals to facilitate the melting and also the formation of slag. These crucibles are heated in an oil-fired open-hearth refractory furnace.
Casting ~ The liquefied gold is stirred and then poured quickly into cast-iron ingot molds.
Slagging ~ The impurities, being lighter than the precious metal, float to the surface as a scum that solidifies into a crust known as slag. After cooling a few minutes, the slag is hammered off and the ingots are scrubbed clean.
Finishing Touches ~ Having been analyzed for purity or fineness, the ingots are stamped with a number and packed for shipping.
After going to such great lengths to wrest a few ounces of gold from many tons of ore,
is it any wonder that this yellow metal carries such a hefty price tag:
U.S. Price of Gold
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