Gold cyanidation
[1] The highly toxic nature of cyanide has led to controversy regarding its use in gold mining, with it being banned in some parts of the world.
[3] One critical factor in its safe use is maintaining an alkaline pH level above 10.5, which is typically controlled using lime in industrial-scale operations.
Lime plays an essential role in gold processing, ensuring that the pH remains at the correct level to mitigate risks associated with cyanide use.
Through the work of Bagration (1844), Elsner (1846), and Faraday (1847), it was determined that each gold atom required two cyanide ions, i.e. the stoichiometry of the soluble compound.
The expansion of gold mining in the Rand of South Africa began to slow down in the 1880s, as the new deposits found tended to contain pyritic ore.
[8] Around 1900, the American metallurgist Charles Washington Merrill (1869–1956) and his engineer Thomas Bennett Crowe improved the treatment of the cyanide leachate, by using vacuum and zinc dust.
Lead nitrate can improve gold leaching speed and quantity recovered, particularly in processing partially oxidized ores.
Studies show that residual cyanide trapped in the gold-mine tailings causes persistent release of toxic metals (e.g. mercury ) into the groundwater and surface water systems.
Although aqueous solutions of cyanide degrade rapidly in sunlight, the less-toxic products, such as cyanates and thiocyanates, may persist for some years.
According to Romanian authorities, in the Someș river below Baia Mare, the plankton returned to 60% of normal within 16 days of the spill; the numbers were not confirmed by Hungary or Yugoslavia.
Although cyanide is cheap, effective, and biodegradable, its high toxicity & increasingly poor impact on a mine's political and social license to operate, has incentivized alternative methods for extracting gold.
These technologies are at a pre-commercialisation stage and compare favourably to equivalent cyanidation methods, including gold recovery percentage.
The European Commission rejected a proposal for such a ban, noting that existing regulations (see below) provide adequate environmental and health protection.
[32] In response to the 2000 Baia Mare cyanide spill, the European Parliament and the Council adopted Directive 2006/21/EC on the management of waste from extractive industries.
This in particular may affect smaller companies wanting to build gold mines in the EU, as they are less likely to have the financial strength to give these kinds of guarantees.
