In many South African mines (though not in East Driefontein), the rock is sorted before processing. Electronic sorting devices are now being developed to separate gold-bearing ore from non-gold rock automatically on the conveyors.
Not all the rock can be replaced in galleries which have been worked dry. Some is removed and trans-ported directly to huge dumps many feet high and sometimes many hundreds of feet long. These dumps have become an emblem of South African gold mining.
Ore brought to the surface is hammered into chunks no bigger than six cubic inches. Non-gold ore is separated from the rest and removed. The remaining ore is directed through various mills, into which water is fed: The result is a thin kind of mud.
Sodium cyanide is then added and the combination goes into a mixing tank. Under intense air pressure, the gold dissolves. Then extraction begins. This cyanide process is remarkably effective. In East Driefontein, a ton of ore contains about 19.3 grams of metallic gold that is, two parts gold to a hundred thousand parts of rock. The cyanide process extracts some ninety-five percent of the gold. In many mines, the mud is amalgamated before the cyanide process begins by being poured over copper beds coated with mercury. Most of the gold amalgamates with the mercury and then, as described earlier, the gold can be extracted in pure form by simple distillation. Amalgamation followed by the cyanide process can extract as much as ninety-seven percent of the gold in the mud.
After the cyanide treatment, the solution is fed into rotating filter drums. The particles of mud are almost completely cleared of gold which has joined a solution with sodium cyanide and oxygen. Thus, after filtering, only lumps of mud are left. Of no further value, they are liquefied again and pumped to dumps where the water gradually evaporates.
From Gold Mud to Gold
Gold is removed from the gold-bearing cyanide solution by precipitation with zinc dust. A new mud is formed, one which is very rich in gold. The particles of mud are separated in filter presses from the solution which is then pumped back into the production cycle.
Sodium cyanide and mercury are highly poisonous materials. In the old days, poisoning of personnel handling the process used to occur regularly and was often fatal. That danger has now been largely eliminated. A virtue of automation is that relatively few people have to handle the poisonous materials. In East Driefontein, three men a shift usually handle the entire cycle of processing and separation. The operation is electronically controlled from a central chamber.
Cakes of mud, still damp after the filtering process, are dried in an oven, a process which oxidizes most of the other metals present in the mud. The conglomerate, hardened by heat treatment, is placed in an electric arc furnace together with slag-forming aggregates. The smelting operation, which takes about, two hours, disperses most of the remaining impurities. Bars of raw gold, the end product of the mining process are now cast.
A recent half-yearly reckoning of gold production at East Driefontein showed extraction of about 9,200 pounds of raw gold, making for a corresponding yield of about 7,900 pounds of fine gold. South African gold is very pure. On average, it contains eighty-eight percent pure gold, only nine percent silver and three percent other impurities.
South African mines are not permitted to sell their own gold on the world market. They are obliged by law to deliver all they produce to the Rand Re-finery in Germiston, near Johannesburg, the largest gold refinery in the world.
In refining, gold bars are melted down once more in crucibles and chlorine gas is piped into the solution. The gas combines with other metals and impurities to form chlorides which cling to the slag and the crucible walls. Gold emerging from this process is at least 99.6 percent pure.
That is adequate for the manufacture of gold bars and mint currency. So-called Good Delivery bars, weighing 400 troy ounces and taking the form of small bricks (the usual form in which gold is marketed), are sold exclusively by South African mines to the South African government. Depending on market conditions, the government either stores the bars in its gold reserves or places them on the gold markets of London and Zurich.