Editing Gold (section)

=== Extraction and refining ===
{{Main|Gold extraction}}
[[File:Gold nuggets from Arizona.jpg|thumb|left|Gold Nuggets found in [[Arizona]].]]
[[Gold extraction]] is most economical in large, easily mined deposits. Ore grades as little as 0.5&nbsp;parts per million (ppm) can be economical. Typical ore grades in [[open-pit mining|open-pit]] mines are 1–5&nbsp;ppm; ore grades in underground or [[Underground mining (hard rock)|hard rock]] mines are usually at least 3&nbsp;ppm. Because ore grades of 30&nbsp;ppm are usually needed before gold is visible to the naked eye, in most gold mines the gold is invisible.

The average gold mining and extraction costs were about $317 per troy ounce in 2007, but these can vary widely depending on mining type and ore quality; global mine production amounted to 2,471.1 tonnes.<ref>{{Cite news |last=O'Connell |first=Rhona |date=13 April 2007 |title=Gold mine production costs up by 17% in 2006 while output fell |url=http://www.mineweb.net/mineweb/view/mineweb/en/page33?oid=19485&sn=Detail |url-status=dead |archive-url=https://web.archive.org/web/20141006084904/http://www.mineweb.net/mineweb/view/mineweb/en/page33?oid=19485&sn=Detail |archive-date=6 October 2014}}</ref>

After initial production, gold is often subsequently refined industrially by the [[Wohlwill process]] which is based on [[electrolysis]] or by the [[Miller process]], that is chlorination in the melt. The Wohlwill process results in higher purity, but is more complex and is only applied in small-scale installations.<ref>{{Cite book |last=Noyes |first=Robert |url=https://books.google.com/books?id=__lqGczo9TwC&pg=PA342 |page=342 |title=Pollution prevention technology handbook |publisher=William Andrew |date=1993 |isbn=978-0-8155-1311-7}}</ref><ref>{{Cite book |last1=Pletcher |first1=Derek |first2=Frank |last2=Walsh |url=https://books.google.com/books?id=E_u9ARrm37oC&pg=PA244 |page=244 |title=Industrial electrochemistry |name-list-style=amp |publisher=Springer |date=1990 |isbn=978-0-412-30410-1}}</ref> Other methods of assaying and purifying smaller amounts of gold include parting and inquartation as well as [[cupellation]], or refining methods based on the dissolution of gold in aqua regia.<ref>{{cite book |last1=Marczenko |first1=Zygmunt |last2=Balcerzak |first2=María |url=https://books.google.com/books?id=0NE1KjVISyAC&pg=PA210 |page=210 |title=Separation, preconcentration, and spectrophotometry in inorganic analysis |name-list-style=amp |publisher=Elsevier |date=2000 |isbn=978-0-444-50524-8}}</ref>