Geology Reference
In-Depth Information
precipitators. These gases are oxidised to produce sulphuric acid for the leaching
process.
Once granulated and ground, the matte coming from either the flash furnace
or from the electric furnace undergoes atmospheric leaching stage. The leaching
solvent recovers the sulphate anolyte of the nickel electro-winning process. The
sulphide matte is then leached in vessels
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where air or oxygen is bubbled to oxidise
the sulphide iron which precipitates. Subsequently, copper is precipitated as copper
sulphide in a pressure leaching stage. Any cobalt is then removed by a solvent ex-
traction process. High pressure acid leaching is generically named HPAL technology.
After these steps, nickel is finally obtained in an electro-winning process whereby
the nickel matte is cast into anodes containing a nickel sulphate electrolyte that
circulates through the cathode in which nickel (II) is reduced to yield pure nickel.
The sulphuric acid produced is neutralised with ammonia to produce ammonium
sulphate as a byproduct. The slime in the anode also contains precious metals that
are further recovered.
An alternative route, known as the Sherritt ammonia leaching process (IPPC,
2009), consists of leaching, in an ammoniacal ammonium sulphate solution, the
matte in pressure autoclaves with air as an oxidant. Here the copper sulphide
precipitates and nickel powder is then obtained by reduction with hydrogen. Cobalt
can be obtained in the same way.
There are various options for the matte refining process. Regardless of the
method, the idea centres round the optimal recovery of not only nickel but also the
co-existing cobalt and precious metals. Therefore a multistage refining process is
needed, using different leaching, reduction, solvent extraction, hydrogen reduction
and electro-winning techniques. One of the most conventional methods involves
roasting nickel matte in a fluid bed with lime to agglomerate particles and liberate
the metal from roasted sulphides. Following this, the matte is subjected to high
pressure chloride leaching, including a final hydrogen reduction as described next.
The chloride leach of matte, known as the Falconbridge process (IPPC, 2009),
utilises chlorine gas as an oxidant at high pressure and temperature (850-950
o
C).
Step by step copper is precipitated as a sulphide, iron and arsenic as hydroxides and
arsenates. Thereafter lead and manganese are removed and cobalt is extracted using
an organic solvent that acts as a reversible and selective chelating agent. Copper,
cobalt and nickel are electro-won separately. The obtained electrolyte is a chloride
instead of a sulphate and is used to produce the chlorine gas for the leaching process.
The solvent extraction explained above (Eramet process), is a clean route since
it produces precipitated sulphur (which is roasted to produce sulphuric acid) whilst
dissolving ferric, cobalt and nickel chlorides. After oxidation and selective solvent
extraction, all become separated. Typical extractants are tri-butyl phosphate and
tri-iso octyl amine. The process needs to take care of some heavy metal impurities
such as Al, Cr, Mn and Pb and is the chemical basis of the Eramet process.
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The temperature of the autoclaves is commonly 230-280
o
C and its pressure 45-55 bar.
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