Gold out of Rocks
By “Awake!” correspondent in Zaire
GOLD, known as a “noble” metal, has long been highly prized. Its malleability, its resistance to the corrosion of time and its beautiful yellow luster have made gold a favorite for fine jewelry and other ornamentation. Because of its rarity, gold is still treasured the world over, even though it is no longer an international monetary standard as in the past.
Small nuggets or grains of this metal may be found in the sand of certain riverbeds. This is known as alluvial gold. Though containing between 2 and 3 percent silver, alluvial gold has the characteristic yellow color. Since it is not physically attached to the grains of sand that surround it, this fine metal may easily be obtained by means of gravity separation. How is this possible? Gold is over 19 times as heavy as water and about seven times as heavy as sand. Therefore, if a mixture of gold and sand passes through a flowing stream, the sand is carried away but the heavy gold falls to the bottom.
Gold may also be locked in seams of rock, often far underground. Today, in the heart of Africa, man is obliged to dig deep to find narrow veins of quartz containing minute grains of gold alloyed with a low percentage of silver or mixed with sulfides. Five tons of ore may contain but one ounce of gold! How, then, is this small quantity of gold extracted?
The first stage in mining is to break the rock mass into transportable chunks. This is done with special pneumatic drills and dynamite. Since the quartz vein is sometimes only a foot or two (30 to 60 centimeters) thick, a tremendous amount of material surrounding the vein also has to be broken up to get out the gold-bearing quartz. This ore is then loaded into small cars running on narrow-gauge rails and is transported to the mill, where the gold is separated from the quartz and the sulfides.
The ore cars are unloaded into a huge hopper with a capacity of more than 400 tons of rock. From the opening at the bottom of this great concrete pit, 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 crushed ore is now put into a ball mill. What is that? From the outside all that one can see is a huge cylinder or drum rotating horizontally with a thunderous din. Inside, the mill is perhaps 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 of grinding in the ball mill, the ore particles are reduced to the size of sand.
The larger particles of gold have by now been unlocked from the rock that held them. These particles can now be separated by such methods as jigging. A jig is a machine that causes a stream of water to move up and down. The current of water carries the sand mixed with gold over the bed of the jig. On the upward pulse of the water in the jig, the “bed,” composed of small steel balls or small pebbles, is lifted on the rising water and opens out. This allows the particles of gold, being denser than either the water or the jig “bed,” to drop into a “hutch,” or collecting compartment, below. Particles of sand without any gold pass over the jig and remain suspended in the water, to be carried away by the current. The sulfides, however, are also collected by the jig. They have a density somewhere between that of the sand (quartz) and gold and may still contain minute particles of gold. The portion of the ore collected by the jig, consisting of particles of now liberated gold, sulfides and a little sand, is called the “jig concentrate.” This concentrate still is not marketable but must be submitted to a process known as “amalgamation.”
This is accomplished with mercury. Though a metal, mercury is a liquid that absorbs gold, forming what is called an “amalgam.” But the mercury does not form amalgams with quartz or sulfide. Therefore, if the jig concentrate is mixed with mercury under the right conditions, 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? Gold particles have to be large enough to be absorbed by mercury. However, the particles of gold found in the sulfides are very, very small, perhaps only a few microns in diameter. A micron is one millionth of a meter, or about one five-hundredth of the diameter of the period at the end of this sentence. Think of that! Since this gold escapes amalgamation, the precious sulfides must be ground very fine and then the gold can be dissolved.
The deadly poison cyanide, when in very dilute solution, has the amazing ability to dissolve gold. For this reason 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 stirring is stopped and the now barren sulfides are allowed to settle. The solution full of gold, known as “pregnant” solution is decanted or siphoned off. Then, to recover the gold in a solid state, zinc dust is added. This precipitates 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 extracted substance does 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. Hence, both of these substances have to be sent from the concentrating mill to the chemical plant for treatment.
Final Microns Recovered
How is the transformation of the gold-mercury amalgam accomplished? Mercury boils at 357 degrees Celsius (675 degrees Fahrenheit), whereas gold does not even melt until it reaches 1,063 degrees Celsius (1,945 degrees Fahrenheit). So the first step in treating the amalgam is distillation. 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.
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.
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. The liquefied gold is stirred and then poured quickly into cast-iron ingot molds. 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. Having been analyzed for purity or fineness, the ingots are stamped with a number and packed for shipping.
After going to such tremendous efforts to wrest a few ounces of gold from many tons of ore, it is little wonder that this yellow metal carries a high price tag. And what a paradox that much of it is then hidden away in bank vaults, where it can be of no direct use or service to man!