As gold is perhaps the best known noble metal, it is a bit ironic that often its extraction from ore can cause pollution in the processes used. Maurice Jones reports
These days concentrated gold is much less easy to find, with large placer deposits requiring only gravity separation very rare. Mining companies are faced with the task of freeing relatively minute quantities of the precious metal from intimate association with other metals and the holding waste hard rock.
After crushing and milling the extraction process is largely chemical, and the processes can cause potential pollution of water resources and the surrounding ground, plus possible harm to life. To avoid the contamination caused by the excesses of the past precautions have to be taken to dispose of and/or contain potential contaminants safely, and to prevent workers and other life forms coming into contact with them.
The leading pollutants possibly released by gold ore processing are cyanide and mercury. With the former, sodium cyanide solution is mixed with the milled ore and the gold is removed as gold cyanide solution. The same can also apply to some other metals. Zinc is added to precipitate out the required metal as sludge after the zinc has been removed by sulphuric acid. The gold can then be smelted into ingots for further refining.
Cyanide and related compounds have been released in ore processing areas, whether accidentally or carelessly, causing possible long-term pollution (see below).
A preferable alternative to straight cyanide processing has been introduced since the 1970s and introduced by most large gold extraction operations. This involves using activated carbon to ‘absorb’ the fine gold and extracting the gold again by a solution of caustic soda (sodium hydroxide) and cyanide. Electrowinning is used to plate steel wool with the gold.
Orica, as one of the world’s largest producers of sodium cyanide for mining processing, promotes its Sparge delivery system (see video). In this sodium cyanide is delivered in sealed ISO tankers that are flushed with water connected to the process plant to remove the material without leakage. The company is a signatory of the International Cyanide Management Code, a voluntary industry working practice procedure. The company offers storage recommendations and reviews, round-the-clock emergency response service and site-based product awareness and safety training.
One example of the hazards of cyanide-based processes is the failure of a waste pond holding dam at Baia Mare in Romania in 2002. The huge loss of tailings sludge contaminated the Tisza River with 122t of cyanide. Most countries now require the cyanide to be destroyed before it is stored in process tailings, and others ban it altogether.
Even Orica has had mishaps with several reported leakages at its Gladstone, Queensland manufacturing facility resulting in a large fine in 2012, although there have been no reported losses during delivery and transport of the product.
In small gold winning operations, commonly so-called artisan mining, where separation by gravity alone is not achievable, it is common, but inadvisable, to use mercury to form an amalgam with gold and so extract it from rock. The gold is released by boiling off the mercury from the amalgam, so not only can the workers be contaminated by absorbing the toxic mercury through the skin, but more dangerously by inhaling mercury vapour. If mercury has to be used it should be recycled in a sealed, monitored process.
Video: Description of the Orica Sparge system for sodium cyanide delivery to mines