Editing Copper extraction (section)

==Roasting==
{{See also|Roasting (metallurgy)}}
The roasting process is generally undertaken in combination with [[reverberatory furnace]]s. In the roaster, the copper concentrate is partially oxidised to produce "[[calcine]]". [[Sulfur dioxide]] is liberated. The [[stoichiometry]] of the reaction is:
:{{chem2|CuFeS2  +  3 O2  ->  2 FeO  +  2 CuS  +  2 SO2}}

Roasting generally leaves more sulfur in the calcined product (15% in the case of the roaster at [[Mount Isa Mines]]<ref>B V Borgelt, G E Casley and J Pritchard (1974) "Fluid Bed Roasting at Mount Isa," ''The Aus. I.M.M. North West Queensland Branch, Regional Meeting, August 1974''. The Australasian Institute of Mining and Metallurgy: Melbourne, pp. 123–130.</ref>) than a sinter plant leaves in the sintered product (about 7% in the case of the Electrolytic Refining and Smelting smelter<ref>P J Wand (1980) "Copper smelting at Electrolytic Refining and Smelting Company of Australia Ltd., Port Kembla, N.S.W.", in: '' Mining and Metallurgical Practices in Australasia: The Sir Maurice Mawby Memorial Volume'', Ed J T Woodcock. The Australasian Institute of Mining and Metallurgy: Melbourne. pp. 335–340.</ref>).

As of 2005, roasting is no longer common in copper concentrate treatment because its combination with reverberatory furnaces is not energy efficient and the SO<sub>2</sub> concentration in the roaster offgas is too dilute for cost-effective capture. Direct smelting is now favored, and uses the following smelting technologies: [[flash smelting]], [[Isasmelt]], Noranda, Mitsubishi or El Teniente furnaces.<ref name="Davenport"/>

===Smelting===
[[file:Evolution copper smelting.svg|thumb|upright=1.3|Replacement of reverberatory furnace smelting by flash smelting, related to the number of copper smelters using this technology. |link=File:Evolution_copper_smelting.svg]]
[[File:Inco flash smelting furnace.png|thumb|upright=1.3|Flash smelting furnace from Inco]]
The initial melting of the material to be smelted is usually referred to as the ''[[Smelting#Copper and bronze|smelting]]'' or ''matte smelting'' stage. It can be undertaken in a variety of furnaces, including the largely obsolete [[Water jacket furnace (metallurgy)|blast furnaces]] and [[reverberatory furnace]]s, as well as [[Flash smelting|flash furnaces]], [[Isasmelt]] furnaces, etc. The product of this smelting stage is a mixture of copper, iron and sulfur that is enriched in copper, which is called ''[[Matte (metallurgy)|matte]]'' or ''copper matte''.<ref name="Davenport"/> The term ''matte grade'' is normally used to refer to the copper content of the matte.<ref name="Smelting">W G Davenport, M King, M Schlesinger and A K Biswas, ''Extractive Metallurgy of Copper, Fourth Edition'' (Elsevier Science Limited: Kidlington, Oxford, England, 2002), pp. 57–72.</ref>

The purpose of the matte smelting stage is to eliminate as much of the unwanted iron, sulfur and ''gangue'' minerals (such as silica, magnesia, alumina and limestone) as possible, while minimizing the loss of copper.<ref name="Davenport"/> This is achieved by reacting iron sulfides with oxygen (in air or oxygen enriched air) to produce iron oxides (mainly as [[FeO]], but with some [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>)) and [[sulfur dioxide]].<ref name="Smelting"/>

Copper sulfide and iron oxide can mix, but when sufficient silica is added, a separate [[slag]] layer is formed.<ref name="Hayes173">P C Hayes, ''Process Principles in Minerals and Materials Production'' (Hayes Publishing Company: Brisbane, 1993), pp. 173–179.</ref> Adding silica also reduces the melting point (or, more properly, the [[liquidus]] temperature) of the slag, meaning that the smelting process can be operated at a lower temperature.<ref name="Hayes173"/>

The slag forming reaction is:

:FeO + SiO<sub>2</sub> → FeO.SiO<sub>2</sub><ref name="Smelting"/>

Slag is less dense than matte, so it forms a layer that floats on top of the matte.<ref>C B Gill, ''Non-ferrous Extractive Metallurgy'' (John Wiley & Sons, New York, 1980) p. 19</ref>

Copper can be lost from the matte in three ways: as [[cuprous oxide]] (Cu<sub>2</sub>O) dissolved in the slag,<ref>R Altman and H H Kellogg, "Solubility of copper in silica-saturated iron silicate slag," ''Transactions of the [[Institution of Mining and Metallurgy]] (Section C: Mineral Processing and Extractive Metallurgy),'' '''81''', September 1972, C163–C175.</ref> as sulfide copper dissolved in the slag<ref name="Nagamori">{{cite journal |author=M. Nagamori|doi=10.1007/BF02644646|title=Metal loss to slag: Part I. Sulfidic and oxidic dissolution of copper in fayalite slag from low grade matte|year=1974 |journal=Metallurgical Transactions|volume=5|issue=3 |pages=531–538|bibcode=1974MT......5..531N |s2cid=135507603}}</ref> or as tiny droplets (or ''[[prills]]'') of matte suspended in the slag.<ref>A Yazawa and S Nakazawa, "Evaluation of non-equilibrium minor components in pyrometallurgy," in: ''EPD Congress 1998'', Ed. B Mishra (The Minerals, Metals and Materials Society: Warrendale, Pennsylvania, 1998), pp. 641–655.</ref><ref name="Elliott">B J Elliott, J B See, and W J Rankin, "Effect of slag composition on copper losses to silica-saturated iron silicate slags," ''Transactions of the Institution of Mining and Metallurgy (Section C: Mineral Processing and Extractive Metallurgy),'' September 1978, C–C211.</ref>

The amount of copper lost as oxide copper increases as the oxygen potential of the slag increases.<ref name="Elliott"/> The oxygen potential generally increases as the copper content of the matte is increased.<ref>{{cite journal|first=J |last=Matousek |title=Oxygen potentials of copper smelting slags|journal=[[Canadian Metallurgical Quarterly]]|volume=32|issue=2|year=1993|pages=97–101 |doi=10.1179/cmq.1993.32.2.97|bibcode=1993CaMQ...32...97M}}</ref> Thus, the loss of copper as oxide increases as the copper content of the matte increases.<ref name="Mackey">{{cite journal|author=P J Mackey|title=The Physical Chemistry of Copper Smelting Slags and Copper Losses at the Paipote SmelterPart 2 – Characterisation of industrial slags |year=2011|journal=Canadian Metallurgical Quarterly |volume=50|issue=4|pages=330–340 |bibcode=2011CaMQ...50..330C |s2cid=137350753 |doi=10.1179/000844311X13112418194806}}</ref>

On the other hand, the solubility of sulfidic copper in slag decreases as the copper content of the matte increases beyond about 40%.<ref name="Nagamori"/> Nagamori calculated that more than half the copper dissolved in slags from mattes containing less than 50% copper is sulfidic copper. Above this figure, oxidic copper begins to dominate.<ref name="Nagamori"/>

The loss of copper as prills suspended in the slag depends on the size of the prills, the viscosity of the slag and the settling time available.<ref name="Rosenqvist"/> Rosenqvist suggested that about half the copper losses to slag were due to suspended prills.<ref name="Rosenqvist">T Rosenqvist (2004) ''Principles of Extractive Metallurgy, Second Edition'', Tapir Academic Press: Trondheim, p. 331, {{ISBN|82-519-1922-3}}.</ref>

The mass of slag generated in the smelting stage depends on the iron content of the material fed into the smelting furnace and the target matte grade. The greater the iron content of the feed, the more iron that will need to be rejected to the slag for a given matte grade. Similarly, increasing the target matte grade requires the rejection of more iron and an increase in the slag volume.

Thus, the two factors that most affect the loss of copper to slag in the smelting stage are:
* matte grade
* mass of slag.<ref name="Hayes173"/>

This means that there is a practical limit on how high the matte grade can be if the loss of copper to slag is to be minimized. Therefore, further stages of processing (converting and fire refining) are required.

The following subsections briefly describe some of the processes used in matte smelting.

====Reverberatory furnace smelting====

Reverberatory furnaces are long furnaces that can treat wet, dry, or roasted concentrate. Most of the reverberatory furnaces used in the latter years treated roasted concentrate because putting dry feed materials into the reverberatory furnace is more energy efficient, and because the elimination of some of the sulfur in the roaster results in higher matte grades.<ref name="Davenport"/>

The reverberatory furnace feed is added to the furnace through feed holes along the sides of the furnace, and the solid charge is melted.<ref name="Davenport"/> Additional silica is normally added to help form the slag. The furnace is fired with burners using pulverized coal, fuel oil or natural gas.<ref name="Gill">C B Gill, ''Non-ferrous Extractive Metallurgy'' (John Wiley & Sons, New York, 1980) pp. 29–35</ref>

Reverberatory furnaces can additionally be fed with molten slag from the later converting stage to recover the contained copper and other materials with a high copper content.<ref name="Gill"/>

Because the reverberatory furnace bath is quiescent, very little oxidation of the feed occurs (and thus very little sulfur is eliminated from the concentrate). It is essentially a melting process.<ref name="Rosenqvist"/> Consequently, wet-charged reverberatory furnaces have less copper in their matte product than calcine-charged furnaces, and they also have lower copper losses to slag.<ref name="Gill"/> Gill quotes a copper in slag value of 0.23% for a wet-charged reverberatory furnace vs 0.37% for a calcine-charged furnace.<ref name="Gill"/>

In the case of calcine-charged furnaces, a significant portion of the sulfur has been eliminated during the roasting stage, and the calcine consists of a mixture of copper and iron oxides and sulfides. The reverberatory furnace acts to allow these species to approach chemical equilibrium at the furnace [[operating temperature]] (approximately 1600&nbsp;°C at the burner end of the furnace and about 1200&nbsp;°C at the flue end;<ref>C B Gill, ''Non-ferrous Extractive Metallurgy'' (John Wiley & Sons, New York, 1980) p. 23</ref> the matte is about 1100&nbsp;°C and the slag is about 1195&nbsp;°C<ref name="Gill"/>). In this equilibration process, oxygen associated with copper compounds exchanges with sulfur associated with iron compounds, increasing the iron oxide content of the furnace, and the iron oxides interact with silica and other oxide materials to form the slag.<ref name="Gill"/>

The main equilibration reaction is:

:Cu<sub>2</sub>O + FeS → Cu<sub>2</sub>S + FeO<ref name="Gill"/>

The slag and the matte form distinct layers that can be removed from the furnace as separate streams. The slag layer is periodically allowed to flow through a hole in the wall of the furnace above the height of the matte layer. The matte is removed by draining it through a hole into ladles for it to be carried by crane to the converters.<ref name="Gill"/> This draining process is known as ''tapping'' the furnace.<ref name="Gill"/> The matte taphole is normally a hole through a water-cooled copper block that prevents erosion of the [[refractory|refractory bricks]] lining the furnace. When the removal of the matte or slag is complete, the hole is normally plugged with clay, which is removed when the furnace is ready to be tapped again.

Reverberatory furnaces were often used to treat molten converter slag to recover contained copper.<ref name="Gill"/> This would be poured into the furnaces from ladles carried by cranes. However, the converter slag is high in magnetite<ref name="Casley">G E Casley, J Middlin and D White, "Recent developments in reverberatory furnace and converter practice at the Mount Isa Mines copper smelter," in: ''Extractive Metallurgy of Copper, Volume 1,'' (The Metallurgical Society: Warrendale, Pennsylvania, 1976), pp. 117–138.</ref> and some of this magnetite would precipitate from the converter slag (due to its higher melting point), forming an accretion on the hearth of the reverberatory furnace and necessitating shut downs of the furnace to remove the accretion.<ref name="Casley"/> This accretion formation limits the quantity of converter slag that can be treated in a reverberatory furnace.<ref name="Casley"/>

While reverberatory furnaces have very low copper losses to slag, they are not very energy-efficient and the low concentrations of sulfur dioxide in their off-gases make its capture uneconomic. Consequently, smelter operators devoted a lot of money in the 1970s and 1980s to developing new, more efficient copper smelting processes.<ref name="Mackey1983">P J Mackey and P Tarassoff, "New and emerging technologies in sulphide [sic] smelting," in: ''Advances in Sulfide Smelting Volume 2: Technology and Practice'', Eds H Y Sohn, D B George and A D Zunkel (The Metallurgical Society of the American Institute of Mining, Metallurgical and Petroleum Engineers: Warrendale, Pennsylvania, 1983), pp. 399–426.</ref> In addition, flash smelting technologies had been developed in earlier years and began to replace reverberatory furnaces. By 2002, 20 of the 30 reverberatory furnaces still operating in 1994 had been shut down.<ref name="Davenport"/>

====Flash furnace smelting====

In [[flash smelting]], the concentrate is dispersed in an air or oxygen stream and the smelting reactions are largely completed while the mineral particles are still in flight.<ref name="Mackey1983"/> The reacted particles then settle in a bath at the bottom of the furnace, where they behave like calcine in a reverberatory furnace.<ref name="Flash">W G Davenport, M King, M Schlesinger and A K Biswas, ''Extractive Metallurgy of Copper, Fourth Edition'' (Elsevier Science Limited: Kidlington, Oxford, England, 2002), pp. 73–102.</ref> A slag layer forms on top of the matte layer, and they can separately be tapped from the furnace.<ref name="Flash"/>

====ISASMELT====
{{Excerpt|ISASMELT}}

===Converting===
[[File:Tough-Pitch Copper Containing Antimony And Nickel.jpg|thumb|[[Oxygen-free copper]] aka "Tough-pitch" copper (ca. 98% pure), containing antimony and nickel]]
The matte, which is produced in the smelter, contains 30–70% copper (depending on the process used and the operating philosophy of the smelter), primarily as copper sulfide, as well as iron sulfide. The sulfur is removed at a high temperature as sulfur dioxide by blowing air through molten matte:
:2 CuS + 3 O<sub>2</sub> → 2 CuO + 2 SO<sub>2</sub>

:CuS + O<sub>2</sub> → Cu + SO<sub>2</sub>

In a parallel reaction the iron sulfide is converted to slag:

:2 FeS + 3 O<sub>2</sub> → 2 FeO + 2 SO<sub>2</sub>
:2 FeO + SiO<sub>2</sub> → Fe<sub>2</sub>SiO<sub>4</sub>

The purity of this product is 98%, it is known as ''blister'' because of the broken surface created by the escape of sulfur dioxide gas as blister copper ''pigs'' or [[ingot]]s are cooled. By-products generated in the process are sulfur dioxide and [[slag]]. The sulfur dioxide is captured and converted to sulfuric acid and either sold on the open market or used in copper leaching processes.