In
metallurgy
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the sc ...
, refining consists of purifying an impure metal. It is to be distinguished from other processes such as
smelting
Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ch ...
and
calcining
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), genera ...
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. The processes used are of many types, including
pyrometallurgical
Pyrometallurgy is a branch of extractive metallurgy. It consists of the thermal treatment of minerals and metallurgical ores and concentrates to bring about physical and chemical transformations in the materials to enable recovery of valuable ...
and
hydrometallurgical techniques.
Lead
Cupellation
One ancient process for extracting the silver from
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
was
cupellation
Cupellation is a refining process in metallurgy where ores or alloyed metals are treated under very high temperatures and have controlled operations to separate noble metals, like gold and silver, from base metals, like lead, copper, zinc, arsen ...
. Lead was melted in a bone ash 'test' or 'cupel' and air blown across the surface. This
oxidise
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
d the lead to
litharge
Litharge (from Greek lithargyros, lithos (stone) + argyros (silver) ''λιθάργυρος'') is one of the natural mineral forms of lead(II) oxide, PbO. Litharge is a secondary mineral which forms from the oxidation of galena ores. It forms as coa ...
, and also oxidised other base metals present, the silver (and gold if present) remaining unoxidised.
In the 18th century, the process was carried on using a kind of
reverberatory furnace
A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases. The term ''reverberation'' is used here in a generic sense of ''rebo ...
, but differing from the usual kind in that air was blown over the surface of the molten lead from
bellows
A bellows or pair of bellows is a device constructed to furnish a strong blast of air. The simplest type consists of a flexible bag comprising a pair of rigid boards with handles joined by flexible leather sides enclosing an approximately airtigh ...
or (in the 19th century) blowing cylinders.
Pattinson process
The Pattinson process was
patent
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
ed by its inventor,
Hugh Lee Pattinson
Hugh Lee Pattinson FRS (25 December 1796 – 11 November 1858) was an English industrial chemist. He was also an entrepreneur, sharing the risk of major industrial developments with famous ironmaster Isaac Lowthian Bell and cable manufacturer Ro ...
, 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.
[Rowe, 1983. pp 189–190.] The level of enrichment possible is limited by the lead-silver
eutectic and typically the process stopped around 600 to 700 ounces per ton (approx 2%), so further separation is carried out by cupellation.
The process was economic for lead containing at least 250 grams of silver per ton.
[Tylecote, 1992. pp 157-158.]
Parkes process
The
Parkes process
The Parkes process is a pyrometallurgical industrial process for removing silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-Europ ...
, patented in 1850 uses molten
zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
. 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 the zinc.
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.
[Metallurgy - An elementary text-book, E.L. Rhead F.I.C. F.C.S., Longmans, 1895, p195]
Copper
Fire refining
The initial product of
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
smelting
Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ch ...
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
electrolytic
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
process, undertaken using a slab of impure copper as the
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and a thin sheet of pure copper as the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
. The
electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
is an acidic solution of copper sulphate. By passing
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
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
dynamo
file:DynamoElectricMachinesEndViewPartlySection USP284110.png, "Dynamo Electric Machine" (end view, partly section, )
A dynamo is an electrical generator that creates direct current using a commutator (electric), commutator. Dynamos were the f ...
; it was first used in South
Wales
Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...
in 1869.
Iron
Wrought iron
The product of the
blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...
is
pig iron
Pig iron, also known as crude iron, is an intermediate product of the iron industry in the production of steel which is obtained by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with silic ...
, which contains 4–5%
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and usually some
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
. 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
finery forge
A finery forge is a forge used to produce wrought iron from pig iron by decarburization in a process called "fining" which involved liquifying cast iron in a fining hearth and removing carbon from the molten cast iron through oxidation. Finery ...
. At the end of the 18th century, this began to be replaced by puddling (in a
puddling furnace
Puddling is the process of converting pig iron to bar (wrought) iron in a coal fired reverberatory furnace. It was developed in England during the 1780s. The molten pig iron was stirred in a reverberatory furnace, in an oxidizing environment, ...
), which was in turn gradually superseded by the production of mild steel by the
Bessemer process
The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is steelmaking, removal of impurities from the iron by ox ...
.
Refined iron
The term refining is used in a narrower context.
Henry Cort
Henry Cort (c. 1740 – 23 May 1800) was an English ironware producer although formerly a Navy pay agent. During the Industrial Revolution in England, Cort began refining iron from pig iron to wrought iron (or bar iron) using innovative producti ...
's original
puddling process only worked where the raw material was white
cast iron
Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
, rather than the grey
pig iron
Pig iron, also known as crude iron, is an intermediate product of the iron industry in the production of steel which is obtained by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with silic ...
that was the usual raw material for finery forges. To use grey
pig iron
Pig iron, also known as crude iron, is an intermediate product of the iron industry in the production of steel which is obtained by smelting iron ore in a blast furnace. Pig iron has a high carbon content, typically 3.8–4.7%, along with silic ...
, 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
precious metal
Precious metals are rare, naturally occurring metallic chemical elements of high economic value.
Chemically, the precious metals tend to be less reactive than most elements (see noble metal). They are usually ductile and have a high lustre. ...
s from
noble-metalliferous materials. Examples of these materials include used
catalysts
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
,
electronic assemblies,
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...
s or metal
alloy
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
s.
Process
In order to isolate noble-metalliferous materials,
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
and/or
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
procedures are used. In pyrolysis, the noble-metalliferous products are released from the other materials by solidifying in a melt to become
cinder
Cinder is an alternate term for scoria.
Cinder or Cinders may also refer to:
In computing
*Cinder (programming library), a C++ programming library for visualization
*Cinder, OpenStack's block storage component
* Cyber Insider Threat, CINDER, a ...
and then poured off or
oxidized
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
. In hydrolysis, the noble-metalliferous products are dissolved either in
aqua regia (consisting of
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
and
nitric acid
Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...
) or in hydrochloric acid and
chlorine
Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
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
recrystallized. The precious metals are separated from the metal
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
by
calcination
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), genera ...
. The noble-metalliferous materials are hydrolyzed first and thermally prepared (
pyrolysed) thereafter. The processes are better yielding when using catalysts that may sometimes contain precious metals themselves. When using catalysts, the
recycling product
Recycling is the process of converting waste materials into new materials and objects. The Energy recycling, recovery of energy from waste materials is often included in this concept. The recyclability of a material depends on its ability t ...
is removed in each case and driven several times through the cycle.
See also
*
*
*
List of alumina refineries
This is a list of alumina refineries in the world. The list is incomplete and missing some data.
Smelter-grade alumina refineries
Specialty alumina plants
See also
*List of aluminium smelters
This is a list of primary aluminium smelters ...
Bibliography
* J. Day and R. F. Tylecote, ''The Industrial Revolution in Metals'' (The Institute of Metals, London 1991).
* Söderberg, A. 2011
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).
Newcastle University: Hugh Lee Pattinson
References
{{Authority control
Metallurgical processes