:Parkes process

{{Short description|Pyrometallurgical industrial process for removing silver from lead}}

The Parkes process is a pyrometallurgical industrial process for removing silver from lead during the production of bullion. It is an example of liquid–liquid extraction.

The process takes advantage of two liquid-state properties of zinc. The first is that zinc is immiscible with lead, and the other is that silver is 3000 times more soluble in zinc than it is in lead. When zinc is added to liquid lead that contains silver as a contaminant, the silver preferentially migrates into the zinc. Because the zinc is immiscible in the lead it remains in a separate layer and is easily removed. The zinc-silver solution is then heated until the zinc vaporizes, leaving nearly pure silver. If gold is present in the liquid lead, it can also be removed and isolated by the same process.Pauling, Linus General Chemistry W.H.Freeman 1947 ed.

The process{{cite encyclopedia|url=http://www.britannica.com/EBchecked/topic/444136/Parkes-process |title=Parkes process (chemistry) |accessdate=2009-08-20 |encyclopedia=Britannica Online Encyclopedia }} was patented by Alexander Parkes in 1850.{{cite book | page = 158 | url = https://books.google.com/books?id=4ZdTAAAAMAAJ | title = A history of metallurgy | isbn = 978-0-901462-88-6 | last1 = Tylecote | first1 = R. F | year = 1992 | publisher=Institute of Materials }}{{cite book | url = https://archive.org/details/metallurgyleadi01percgoog| pages = [https://archive.org/details/metallurgyleadi01percgoog/page/n168 148] | title = The metallurgy of lead: Including desilverisation and cupellation | publisher = J.Murray| last1 = Percy | first1 = John | year = 1870}}{{cite book| url = https://archive.org/details/patentsforinven63offigoog| page = [https://archive.org/details/patentsforinven63offigoog/page/n197 177]| title = Patents for inventions. Abridgments of specifications| publisher = Patent Office| last1 = Office| first1 = Patent| year = 1861}}{{cite book| url = https://books.google.com/books?id=AXZUAAAAYAAJ&pg=PA246| title = Patents for inventions: Abridgments of specifications : Class| last1 = Patent Office| first1 = Great Britain| year = 1867}} Parkes received two additional patents in 1852.{{cite book| url =https://archive.org/details/patentsforinven12offigoog| page =[https://archive.org/details/patentsforinven12offigoog/page/n222 197]| title =Patents for inventions: Abridgments of specifications : Class| publisher =Patent Office| last1 =Patent Office| first1 =Great Britain| year =1861}}

The Parkes process was not adopted in the United States, due to the low native production of lead.{{cite journal

| last =Eurich

| first =Ernst

| authorlink =

| editor =Joseph Struthers

|date=December 1912

| title =The Development of the Parkes Process in the United States

| journal =Bulletin of the American Institute of Mining Engineers

| volume =

| issue =72

| pages =1531–1540

| publisher =The American Institute of Mining Engineers

| format =

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| url =https://books.google.com/books?id=XhIOAAAAYAAJ&q=%22balbach+process%22&pg=RA10-PA1532

}} The problems were overcome during the 1880s and by 1923 only when the Parkes process was used.{{cite book | url = https://books.google.com/books?id=ZL4OAAAAQAAJ&pg=PA189 | pages = 189–190 | title = Lead Manufacturing in Britain: A History | isbn = 978-0-7099-2250-6 | last1 = Rowe | first1 = David John | date = 1983-07-01| publisher = Croom Helm }}