View source for Engineering:Refining (metallurgy)

{{Short description|Process of purifying metals}}

In [[Chemistry:Metallurgy|metallurgy]], '''refining''' consists of purifying an impure metal.  It is to be distinguished from other processes such as [[Engineering:Smelting|smelting]] and calcining in that those two involve a chemical change to the raw material, whereas in refining, the final material is usually identical chemically to the original one, only it is purer.{{clarify|date=March 2021}} The processes used are of many types, including [[Engineering:Pyrometallurgy|pyrometallurgical]] and [[Chemistry:Hydrometallurgy|hydrometallurgical]] techniques.

==Lead==

===Cupellation===
{{Main|Engineering:Cupellation}}

One ancient process for extracting the silver from [[Chemistry:Lead|lead]] was [[Engineering:Cupellation|cupellation]].  Lead was melted in a bone ash 'test' or 'cupel' and air blown across the surface.  This oxidised the lead to [[Chemistry:Litharge|litharge]], and also oxidised other base metals present, the silver (and gold if present) remaining unoxidised.<ref>Metallurgy - An Elementary Text Book, E.L. Rhead F.I.C F.C.S, Longmans, 1895, pp225-229</ref>

In the 18th century, the process was carried on using a kind of reverberatory furnace, but differing from the usual kind in that air was blown over the surface of the molten lead from [[Engineering:Bellows|bellows]] or (in the 19th century) blowing cylinders.<ref name= "Tyle"/>

===Pattinson process===
{{Main|Chemistry:Pattison's Process}}
The Pattinson process was [[Social:Patent|patent]]ed by its inventor, Hugh Lee Pattinson, in 1833 who described it as "An improved method for separating silver from lead". It exploited the fact that in molten lead containing traces of silver the first metal to solidify out of the melt is lead, leaving the remaining liquid richer in silver. Pattinson's equipment consisted basically of nothing more complex than a row of up to 13 iron pots, which were heated from below. Some lead, naturally containing a small percentage of silver, was loaded into the central pot and melted. This was then allowed to cool. As the lead solidified it is removed using large perforated iron ladles and moved to the next pot in one direction, and the remaining metal which was now richer in silver was then transferred to the next pot in the opposite direction.  The process was repeated from one pot to the next, the lead accumulating in the pot at one end and metal enriched in silver in the pot at the other.<ref name=Tylecote>{{cite book |last=Tylecote |first=R. F. |title=A history of metallurgy |publisher=Institute of Materials |location=London |year=1992 |pages=157–158}}</ref><ref name=Rowe189>Rowe, 1983. pp 189–190.</ref> The level of enrichment possible is limited by the lead-silver [[Physics:Eutectic|eutectic]] and typically the process stopped around 600 to 700 ounces per ton (approx 2%), so further separation is carried out by cupellation.<ref>Metallurgy - An Elementary Text Book, E.L.Rhead F.I.C F.C.S, Longmans, 1895, pp193-195</ref>

The process was economic for lead containing at least 250 grams of silver per ton.<ref name= "Tyle">Tylecote, 1992. pp 157-158.</ref>

===Parkes process===
The [[Chemistry:Parkes process|Parkes process]], patented in 1850 uses molten [[Chemistry:Zinc|zinc]]. Zinc is not miscible with lead and when the two molten metals are mixed the zinc separates and floats to the top carrying only some 2% lead. However silver preferentially dissolves in zinc, so the zinc that floats to the top carries a significant proportion of the silver. The melt is then cooled until the zinc solidifies and the zinc crust is skimmed off.  The silver is then recovered by volatalising{{verify spelling|reason=volatilising?|date=November 2022}} the zinc.<ref name= "Tyle"/> The Parkes process largely replaced the Pattinson process, except where the lead contained insufficient silver, in which case the Pattinson process provided a method to enrich it in silver to about 40 to 60 ounces per ton, at which concentration it could be treated using the Parkes' process.<ref>Metallurgy - An elementary text-book, E.L. Rhead F.I.C. F.C.S., Longmans, 1895, p195</ref>

==Copper==

===Fire refining===
The initial product of [[Chemistry:Copper|copper]] [[Engineering:Smelting|smelting]] was impure black copper, which was then repeatedly melted to purify it, alternately oxidizing and reducing it. In one of the melting stages, lead was added.  Gold and silver preferentially dissolved in this, thus providing a means of recovering these precious metals.  To produce purer copper suitable for making copper plates or hollow-ware, further melting processes were undertaken, using charcoal as fuel.  The repeated application of such fire-refining processes was capable of producing copper that was 99.25% pure.

===Electrolytic refining===
The purest copper is obtained by an [[Chemistry:Electrolysis|electrolytic]] process, undertaken using a slab of impure copper as the [[Physics:Anode|anode]] and a thin sheet of pure copper as the [[Physics:Cathode|cathode]].  The [[Physics:Electrolyte|electrolyte]] is an acidic solution of copper sulphate.  By passing [[Physics:Electricity|electricity]] through the cell, copper is dissolved from the anode and deposited on the cathode.  However impurities either remain in solution or collect as an insoluble sludge.  This process only became possible following the invention of the [[Engineering:Dynamo|dynamo]]; it was first used in South Wales in 1869.

==Iron==

===Wrought iron===
{{Main|Chemistry:Wrought iron}}

The product of the [[Engineering:Blast furnace|blast furnace]] is [[Chemistry:Pig iron|pig iron]], which contains 4–5% [[Chemistry:Carbon|carbon]] and usually some [[Chemistry:Silicon|silicon]].  To produce a forgeable product a further process was needed, usually described as ''fining'', rather than ''refining''.  From the 16th century, this was undertaken in a [[Chemistry:Finery forge|finery forge]].  At the end of the 18th century, this began to be replaced by puddling (in a puddling furnace), which was in turn gradually superseded by the production of mild steel by the [[Engineering:Bessemer process|Bessemer process]].{{Citation needed|date=March 2009}}

===Refined iron===

The term refining is used in a narrower context.  Henry Cort's original [[Engineering:Puddling (metallurgy)|puddling process]] only worked where the raw material was white [[Chemistry:Cast iron|cast iron]], rather than the grey [[Chemistry:Pig iron|pig iron]] that was the usual raw material for finery forges.  To use grey [[Chemistry:Pig iron|pig iron]], a preliminary refining process was necessary to remove silicon.  The pig iron was melted in a ''running out furnace'' and then run out into a trough.  This process oxidised the silicon to form a slag, which floated on the iron and was removed by lowering a dam at the end of the trough.  The product of this process was a white metal, known as ''finers metal'' or ''refined iron''.

==Precious metals==
Precious metal refining is the separation of [[Chemistry:Precious metal|precious metal]]s from [[Chemistry:Noble metal|noble-metalliferous]] materials.   Examples of these materials include used [[Chemistry:Catalysis|catalysts]], [[Engineering:Electronics|electronic assemblies]], [[Earth:Ore|ore]]s or metal [[Chemistry:Alloy|alloy]]s.

===Process===
In order to isolate noble-metalliferous materials, [[Chemistry:Pyrolysis|pyrolysis]] and/or [[Chemistry:Hydrolysis|hydrolysis]] procedures are used. In pyrolysis, the noble-metalliferous products are released from the other materials by solidifying in a melt to become cinder and then poured off or [[Chemistry:Redox|oxidized]]. In hydrolysis, the noble-metalliferous products are dissolved either in [[Chemistry:Aqua regia|aqua regia]] (consisting of [[Chemistry:Hydrochloric acid|hydrochloric acid]] and [[Chemistry:Nitric acid|nitric acid]]) or in hydrochloric acid and [[Chemistry:Chlorine|chlorine]] gas in solution.  Subsequently, certain metals can be precipitated or reduced directly with a salt, gas, organic, and/or nitro hydrate connection. Afterwards, they go through cleaning stages or are [[Physics:Recrystallization (metallurgy)|recrystallized]]. The precious metals are separated from the metal [[Chemistry:Salt|salt]] by [[Chemistry:Calcination|calcination]].  The noble-metalliferous materials are hydrolyzed first and thermally prepared ([[Chemistry:Pyrolysis|pyrolysed]]) thereafter.  The processes are better yielding when using catalysts that may sometimes contain precious metals themselves. When using catalysts, the recycling product is removed in each case and driven several times through the cycle.{{Citation needed|date=November 2008}}

== See also ==
* {{annotated link|Engineering:Beneficiation}}
* List of alumina refineries

==Bibliography==
* J. Day and R. F. Tylecote, ''The Industrial Revolution in Metals'' (The Institute of Metals, London 1991).
* Söderberg, A. 2011. [http://web.comhem.se/vikingbronze/soderberg_situne_dei_2011.pdf Eyvind Skáldaspillir's silver - refining and standards in pre-monetary economies in the light of finds from Sigtuna and Gotland]. Situne Dei 2011. Edberg, R. Wikström, A. (eds). Sigtuna.
* R. F. Tylecote, ''A history of metallurgy'' (Institute of materials, London 1992).
* [http://www.ncl.ac.uk/library/specialcollections/collections/daguerreotypes/pattinson.php Newcastle University: Hugh Lee Pattinson]

==References==
{{Reflist}}


[[Category:Metallurgical processes]]

{{Sourceattribution|Refining (metallurgy)}}