(MERAF) for the Base Metals Smelting Sector - CCME

Metallic impurities either dissolve in the electrolyte or precipitate out and form a
sludge. These anode slimes contain precious metals such as silver, gold and
tellurium and are recovered. The cathode deposits are washed, then cast into
bars, ingots, or slabs for sale.
2.3.8. Carbonyl Refining
The carbonyl process is used for refining crude nickel oxide. Carbon monoxide
and the crude nickel react to form nickel carbonyl at high pressure. The volatile
and highly toxic nickel carbonyl is refined by separation of solid impurities. With
further heating, the carbon monoxide separates and high purity nickel powder or
pellets are formed. The solid impurities contain copper and precious metals
which are recovered. The generated carbon monoxide off-gases are recycled
back to the process.
2.3.9. Leaching
Leaching requires the use of an acid or other solvent to dissolve the metal
content of ores and concentrates before refining and electrowinning. Leaching is
usually conducted using material in the form of an oxide, either an oxidic ore or
an oxide produced by roasting. Sulphidic ores can also be leached but require
conditions which promote oxidation of the ore or concentrate, such as high
pressure, presence of bacteria and/or the addition of oxygen, chlorine or ferric
chloride. The pregnant leach solution is processed by solvent extraction and is
purified. The purified solution is then used for electrowinning and refining of the
metal.
2.3.10. Electrowinning
Electrowinning is used to capture metal dissolved in the pregnant solution
produced during leaching (purified electrolyte). Electrowinning is used for
refining zinc, copper, nickel and cobalt in Canada. The process is conducted in
tank houses, with electrolytic cells containing the purified electrolyte, inert anodes
and starter cathodes of the pure metal (for copper refining) or permanent
cathodes of stainless steel or aluminum. Electric current is passed through the
cell and the dissolved metal ions (metal of value) are deposited onto the cathode.
Oxygen gas, acid mist, and spent electrolyte (acid) are generated through the
electrowinning process. The spent electrolyte is returned to the leaching
process. After a sufficient time lapse the cathodes are removed. The cathodes
are either sold directly, or the metal is stripped from the cathode, is melted and
cast.
2.3.11. Casting
The casting process involves melting the metal and passing it through a holding
furnace and into the caster where billets, blocks, slabs or cakes, and rods are
produced. Casting can be done continuously or in batches. Stationary casting
uses a casting wheel with a series of molds which can be on the circumference
of a rotating table that passes through a series of cooling water jets. Continuous
casting involves the production of a continuous bar or rod for reduction to wire
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