Editing Cyanide (section)

===Mining===
{{Main|Gold cyanidation}}
Cyanide is mainly produced for the [[mining]] of [[silver]] and [[gold]]: It helps dissolve these metals allowing separation from the other solids. In the ''[[cyanide process]]'', finely ground high-grade ore is mixed with the cyanide (at a ratio of about 1:500 parts NaCN to ore); low-grade ores are stacked into heaps and sprayed with a cyanide solution (at a ratio of about 1:1000 parts NaCN to ore). The precious metals are complexed by the cyanide [[anion]]s to form soluble derivatives, e.g., {{chem2|[Ag(CN)2]-}} (dicyanoargentate(I)) and {{chem2|[Au(CN)2]-}} (dicyanoaurate(I)).<ref name=Ullmann>{{cite book|doi=10.1002/14356007.a08_159.pub3 |chapter=Cyano Compounds, Inorganic |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |last1=Gail |first1=Ernst |last2=Gos |first2=Stephen |last3=Kulzer |first3=Rupprecht |last4=Lorösch |first4=Jürgen |last5=Rubo |first5=Andreas |last6=Sauer |first6=Manfred |last7=Kellens |first7=Raf |last8=Reddy |first8=Jay |last9=Steier |first9=Norbert |last10=Hasenpusch |first10=Wolfgang |isbn=978-3-527-30385-4 }}</ref> Silver is less [[Noble metal|"noble"]] than gold and often occurs as the sulfide, in which case redox is not invoked (no {{chem2|O2}} is required). Instead, a displacement reaction occurs:
:{{chem2|Ag2S + 4 NaCN + H2O → 2 Na[Ag(CN)2] + NaSH + NaOH}}
:{{chem2|4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOH}}
The "pregnant liquor" containing these ions is separated from the solids, which are discarded to a [[tailing pond]] or spent heap, the recoverable gold having been removed. The metal is recovered from the "pregnant solution" by reduction with [[zinc]] dust or by [[adsorption]] onto [[activated carbon]]. This process can result in environmental and health problems. A number of [[List of gold mining disasters|environmental disasters]] have followed the overflow of tailing ponds at gold mines. Cyanide contamination of waterways has resulted in numerous cases of human and aquatic species mortality.<ref>{{cite journal |last1=Kumar |first1=Rahul |last2=Saha |first2=Shouvik |last3=Sarita |first3=Dhaka |last4=Mayur B. |first4=Kurade |last5=Kang |first5=Chan Ung |last6=Baek |first6=Seung Han |last7=Jeong |first7=Byong-Hun |title=Remediation of cyanide-contaminated environments through microbes and plants: a review of current knowledge and future perspectives |journal=Geosystem Engineering |date=2016 |volume=70 |issue=1 |pages=28–40 |doi=10.1080/12269328.2016.1218303 |s2cid=132571397 |url=https://www.tandfonline.com/doi/full/10.1080/12269328.2016.1218303 |access-date=24 April 2022}}</ref>

Aqueous cyanide is hydrolyzed rapidly, especially in sunlight. It can mobilize some heavy metals such as mercury if present. Gold can also be associated with arsenopyrite (FeAsS), which is similar to [[iron pyrite]] (fool's gold), wherein half of the sulfur atoms are replaced by [[arsenic]]. Gold-containing arsenopyrite ores are similarly reactive toward inorganic cyanide.<ref>{{Cite journal |last1=Konyratbekova |first1=Saltanat Sabitovna |last2=Baikonurova |first2=Aliya |last3=Akcil |first3=Ata |date=2015-05-04 |title=Non-cyanide Leaching Processes in Gold Hydrometallurgy and Iodine-Iodide Applications: A Review |url=http://www.tandfonline.com/doi/abs/10.1080/08827508.2014.942813 |journal=Mineral Processing and Extractive Metallurgy Review |language=en |volume=36 |issue=3 |pages=198–212 |doi=10.1080/08827508.2014.942813 |bibcode=2015MPEMR..36..198K |issn=0882-7508}}</ref><ref>{{Cite journal |last1=Zhang |first1=Yan |last2=Cui |first2=Mingyao |last3=Wang |first3=Jianguo |last4=Liu |first4=Xiaoliang |last5=Lyu |first5=Xianjun |date=2022 |title=A review of gold extraction using alternatives to cyanide: Focus on current status and future prospects of the novel eco-friendly synthetic gold lixiviants |url=https://linkinghub.elsevier.com/retrieve/pii/S0892687521005653 |journal=Minerals Engineering |language=en |volume=176 |pages=107336 |doi=10.1016/j.mineng.2021.107336|bibcode=2022MiEng.17607336Z}}</ref>