Gold cyanidation is also known as the cyanide process or the MacArthur-Forrest Process, and is a method used for extracting gold from poor grade ore ("tailings") by converting it to soluble complex aurocyanide anions. Although the procedure is that most commonly used for gold extraction it is attended with controversy on account of the toxicity of cyanide and the perceived potential for contamination by it, since there have been a number of environmental catastophes involving cyanide, e.g. in Romania where fish stocks in rivers were devastated some years ago. The process was originally invented in 1783 by the Swedish chemist Carl Wihelm Scheele, who was also the discoverer of chlorine, a gas used on a large scale in industry, e.g. to make bleach, but which was also used as the world's first chemical gas-weapon in the trenches of WW1. The underlying chemical reaction is called the Elsner reaction, and can be written as:
4Au + 8NaCN + O2 + 2H2O --> 4NaAu(CN)2 + 4 NaOH.
The ore is finely ground (comminuted) and is often further concentrated using froth floatation or centifugal concentration, and the resulting alkaline ore slurry is then mixed with a solution of cyanide anions (obtained by dissolving 250 - 500 parts per million of sodium cyanide or potassium cyanide in water). The negatively charged cyanide anions extract the gold from the ore in the form of positively charged gold cations to form the soluble aurocyanide complex, NaAu(CN)2 as shown above. In general, the finer the gold particles, the more quickly they will dissolve. For instance, a 45 micron gold particle might dissolve in 10 - 13 hours, while a 45 micron particle might take from 20 to 44 hours to dissolve in the same solution. It has been found that the addition of lead nitrate can increase the rate at which the gold is leached from rocks and the quantity recovered, particularly in processing partially oxidized ores. Indeed, oxygen (since it is consumed in the reaction shown above) is a critical factor in the gold cyanidation process. Air or pure oxygen gas can be bubbled through the mineral pulp to increase the dissolved oxygen concentration. Oxygen can also be "added" by adding hydrogen peroxide solution to the pulp.
The gold is then recovered from the "pregnant" solution (as it is called) using a number of different processes, but passage through highly porous carbon is most commonly used. So high is the internal surface area of the material that around six grams of it would equal the area of the Melbourne cricket ground! The carbon contains micropores (pores of molecular dimensions, similar in size or smaller than those in zeolites) to filter out the gold, and somewhere around 8 kilograms of gold can be extracted by a tonne of carbon. The gold can be removed from the carbon by using a strong solution of caustic soda and cyanide. This is known as elution. The gold is then plated out onto steel wool through electrowinning. Resins that are specific for gold can also be used in place of activated carbon, or where selective separation of gold from copper or other dissolved metals is required.
However, cyanide is a highly toxic material, which is why the process is controversial. For example, one teasponful of 2 per cent cyanide solution can kill a human adult (although it would take probably several hours to die. It is hydrogen cyanide, "Prussic Acid" of Nuremberg Trials and James Bond movies who's lethal effects are "instantaneous"). The toxic effects on fish occur at far smaller concentrations that this. Indeed, the worst environmental catastrophe caused by mining in the history of the U.S. was at the Summitville mine, where 27 miles of a Colorado river were left "dead" by cyanide poisoning. There have also been disasters in Kyrgyzstan, French Guiana and Romania, where spillage of cyanide in Baia Mare resulted in widescale contamination of the river Tisza. In the United States, the state of Montana along with several other countries have banned gold mining using cyanide. Although cyanide is toxic, it is readily broken down when exposed to sunlight in the presence of oxygen (air), although this is little comfort when contaminated waters have leaked into groundwater, or other underground sources of freshwater that are protected from the sun, on dank cloudy days, or in winter especially in Eastern Europe (Romania) when rivers are largely covered by ice and snow.
In fairness, most of the operations have been conducted without obvious incident, but clearly there is leakage of cyanide, for example from plastic-lined ponds, and the areas surrounding some mines in the U.S. have been found to have elevated levels of cyanide, even decades after they were first processed. The following examples, however, show there is no call for complacency.
A History of Accidents
Ten miners were killed when a disused slime dam at the Harmony mine in South Africa, operated by Randgold, burst its banks and buried a housing complex in cyanide contaminated mud in Feburary 1994.
Cyanide and heavy metal leaks from the Summitville gold mine killed all aquatic life along a 27 kilometer stretch of the Alamosa river in the San Juan mountains of southwestern Colorado by the time the mine was shut down in December 1992. The total clean-up costs have exceeded US$150 million.
Failure of a leach pad structure at the Gold Quarry mine in Nevada released about a million liters of cyanide-laden wastes into two creeks in 1997.
Over 11,000 fish were killed along an 80
kilometer stretch of the Lynches River by a cyanide spill from the Brewer gold mine in South Carolina in 1992.
On May 20,1998, a truck transporting cyanide to the Kumtor mine in Kyrgyzstan plunged off a bridge spilling 1762 kilograms of sodium cyanide into local surface waters. Local people have reported at least four deaths that they claim resulted from the spill. Hundreds of people also checked into local hospitals complaining of health problems following the spill.
More than 3.2 billion litres of cyanide-laden tailings were released into the Essequibo river in Guyana when a dam collapsed at the Omai gold mine in August 1995. Studies by the Pan American Health Organization have shown that all aquatic life in the four kilometer long creek that runs from the mine to the Essequibo has been killed.
On May 29, 1998 six to seven tons of cyanide-laden tailings spilled into Whitewood Creek in the Black Hills of South Dakota from the Homestake Mine, killing a substantial number of fish.
On the night of January 30th, 2000, spillage of 120 tonnes of cyanide at a gold reprocessing facility near the town of Baia Mare in Romania, resulted in widescale contamination of the rivers Tisza and Danube. 150 tonnes of dead fish were recovered, and the drinking water supplies of around 3 million people were threatened. Claims for compensation in Romania, Hungary and Slovakia remain outstanding.