How to extract gold

to prepare

Gold is usually found alone or alloyed with mercury or silver, but it can also be found in ores such as calaverite, sylvanite, nagyaite, bitzit, and kernite.


Most of the gold ore now comes from either open pit mines or underground mines. Ores sometimes contain less than 5/100th of an ounce of gold per ton of rock.


In all gold ore refining methods, the ore is usually washed and filtered at the mine, and then sent to the plant. In the mill, the ore is ground into smaller particles with water, and then ground again in a ball mill to further crush the ore.

cyanide

Several processes can then be used to separate the gold from the ore. The most common techniques in the United States use cyanide in various ways. In one, ground ore is placed in a tank containing a weak cyanide solution and zinc is added. The zinc causes a chemical reaction that separates the gold from the ore. The gold is then removed from the solution by pressure filtration.


For the pulping carbon method, ground ore is mixed with water before cyanide is added. Then carbon is added to bond with the gold. Carbon and gold particles are placed in a caustic carbon solution, and the gold separates.


In the mound leaching process, the ore is placed on pads in the open air and cyanide is sprayed over it, taking several weeks for it to reach an impermeable base. The solution from the pad is then poured into a pond and pumped from there to a recovery plant where the gold is recovered. Heap leaching helps recover gold from ore that would otherwise be too expensive to process.

Other <methods

Another process involves passing ground ore over mercury-coated plates. The gold and mercury form an amalgam that leads to the name of the process, amalgamation. Once the amalgam is formed, it is heated until the mercury gas boils away, leaving the gold. Mercury gas is highly toxic and must be handled with care.


Another gold removal process is flotation, in which ground ore is placed in a solution containing a frothing agent with a bundling agent and organic chemicals. A foaming agent turns the solution into foam. The aggregating agent binds to the gold, forming an oily film, which subsequently adheres to the surface of the air bubbles. Organic chemicals prevent gold from binding to other substances. Air is then passed through the solution and the gold-laden film affixes itself to the bubbles. The bubbles rise to the top and the gold is removed from the skim.


Gold refining or separation is used to separate gold from impurities and other metals such as silver. Gold and silver, which are often extracted from the same ores, are chemically similar, making them difficult to separate. Before the advent of silver-gold separations, an alloy of gold and silver called electrum was used frequently. Technological advances have resulted in improved gold refining techniques. Removing as many impurities from gold as possible adds to its value, both in its raw form and in fine jewelry.

Miller operation

Used to refine gold on an industrial scale, pioneered by Francis Bower Miller, the Miller process refining gold to 99.95% purity. This technique involves passing chlorine gas through molten unrefined gold, causing the silver and other base metals to solidify and float to the top from where they are skimmed. The result is 98% pure gold, which is then electrolytically refined to remove platinum and palladium

Wohlwill operation

Another large-scale gold refining technology, the Wohlwill process refines gold to 99.999% purity - the highest purity possible. Developed by Emil Wohlwill in 1987, this electrochemical process uses a piece of 95% pure gold as the anode and small sheets of pure 24k gold as the cathode. A current runs through the system which uses chlororic acid as the electrolyte solution; Pure gold collects on the negative electrode, which can then be melted or processed.

pelleting

A process that has been around since at least the Early Bronze Age, annealing involves treating ores under high temperatures to separate noble metals, such as gold and silver, from the base metals. Base metals, such as copper, zinc, and lead, are oxidized, while noble metals are not. The pelletizing process involves melting the ore at temperatures in excess of 960 °C; At this point the base metals are oxidized while silver and gold remain above the mixture.

Do it yourself

It is possible to purify gold without using extensive and expensive chemical procedures. The process involves first adding nitric acid to the gold, then adding hydrochloric acid or muriatic acid. After letting this mixture sit, it is filtered to remove contaminants, and then treated to neutralize the acids in the solution. The result will be what looks like slurry at the bottom of the container; This "mud" is actually gold. Rinse the muddy pieces three or four times with water instead of treating with aqueous ammonia. After the formation of white fumes, rinse the gold again with water and let it dry.