Market Survey on Copper - Indian Bureau of Mines

a large extent by volatisation of their sulphides during slag forming stage of
converting i.e. before metallic copper is formed. In continuous operations,
since metallic copper is always present, these impurities get dissolved in copper
before volatilisation can occur. The elimination of these impurities is difficult
by further pyrometallurgical operations because of their low chemical activities.
This is the reason why the anodes from continuous operation have higher level
of As, Bi and Sb necessitating more careful electro-refining. Therefore, the
continuous smelting route is restricted to smelting of concentrate with less
content of these impurities.
3.2.4 New and Emerging Processes
3.2.4.1 Foam Smelting
A stationary furnace with water cooled side walls, similar to
zinc fuming furnace is employed. A layer of slag-matte foam is
maintained above a quiescent bath of slag and matte/metal. Concentrate,
flux and carbonaceous fuel are fed into the foam layer maintained in a
highly turbulent condition by oxygen-enriched air blown through tuyers.
When blowing stops, the foam subsides exposing the tuyers directly
above the bath. The slag and matte/metal continuously overflow from
the furnace through weirs. Provision is made for cleaning the slag by
reduction in a separate section of the furnace.
3.2.4.2 Electrolytic Refining
Electrolytic refining involves electrochemically dissolving
copper from impure anodes and selectively plating the dissolved pure
copper on to copper cathodes. During the process, it eliminates
electrically and mechanically harmful impurities and at the same time
separate valuable impurities which can be recovered as byproducts.
The principal impurities in a copper anode are As, Bi, Ni, Pb, Sb, Se, Te,
Au, Ag and Pt. Gold and platinum do not dissolve in the sulfate electrolyte and
hence they remain as anode residue/slimes. Silver partially dissolves but is
precipitated from the electrolyte as AgCl by dissolving a small amount of NaCl
or HCl in the electrolyte. Sulphur, selenium and tellurium being present as
Ag2Se, Cu2Se, Ag2Te4 and Cu2S are not dissolved in electrolyte and therefore
report in anode residue/slimes in a similar fashion as noble metals. Lead and tin
forming insoluble sulphates do not enter electrolyte to any appreciable extent.
The other impurities (As, Bi, Co, Fe, Ni and Sb) being less noble than copper,
dissolve in the electrolyte and therefore must be removed from the electrolyte to
avoid contamination of cathode copper by occlusion.
Cleaning of electrolyte involves three main sequential steps:
a) Removal of copper from the electrolyte by electro-winning using inert
antimonial lead anodes and normal copper starting sheets.
b) Removal of arsenic antimony and bismuth by electro-winning them into an
impure copper cathode deposit.
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