Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 261

The SX/EW process operates at ambient temperatures. Copper is kept in either an aque-
ous environment or an organic environment during its processing until it is reduced to its
metallic form. Because of its dependence on sulphuric acid, the SX/EW process is often not
a substitute for, but rather an adjunct to conventional smelting, using sulphuric acid pro-
duced from smelter gases. It is also applicable in locations where smelter acid is not avail-
able by importation of sulphuric acid or through importation of sulphur or pyrite, which
are used to manufacture sulphuric acid on-site. Such acid plants are found associated with
many hydrometallurgical plants including lateritic nickel plants and uranium plants.
Electrowinning - In electrowinning, copper is reduced electrochemically from copper
sulphate in the aqueous solution to a metallic copper cathode. When electrically charged,
pure copper ions migrate directly from the aqueous solution produced by solvent extrac-
tion to starter cathodes made from pure copper foil. The inert (non-dissolving) anodes
are made of lead (alloyed with calcium and tin) or stainless steel, referred to as sheets (US
Congress, Office of Technology Assessment 1988). Electrowon copper cathodes are as pure
as electro-refined cathodes from the smelting process.
The electrochemical reaction at the lead-based anodes produces oxygen gas and sulphu-
ric acid by electrolysis. Spent acid is either pumped to the solvent extraction process via
electrolyte heat exchangers (to balance the tank house temperature by heating up incom-
ing copper bearing electrolyte from the solvent extraction process), or is recycled and
pumped back to the leaching operation (US Congress, Office of Technology Assessment
1988; Engineering and Mining Journal 1990).
While smelting is more energy consuming than applying SX/EW technology, elec-
trowinning consumes more energy than electro-refining. The electrowinning of copper
requires considerably more electrical energy than does the electro-refining process - an
average of about 8 MJ/kg for electrowinning compared to about 1.5 MJ/kg for electro-
refining: in electrowinning the copper must be reduced from the cupric form to metal;
whereas, in electro-refining the copper is already in metallic form and is merely trans-
ported from the anode to the cathode to purify it.
In recent years hydrometallurgical routes have become more popular in metallurgical
research because: total energy costs are lower; increased environmental awareness favours
'zero discharge' conditions; hydrometallurgical processes can separate impurities better;
operating temperatures are lower making plants easier to operate; while pyrometallurgi-
cal processes generate sulphur dioxide, a gas that has to be converted into sulphuric acid,
hydrometallurgical processes use and even form sulphuric acid. However, apart from the
SX/WE process, industrial leaching plants for metals such as copper or nickel at the scale
required for modern mineral processing have yet to establish a track record comparable
with that of smelters. The hydrometallurgical route at industrial plants has proved to be
challenging and in many respects is still in its infancy ( Case 6.2 ).

Because of its dependence
on sulphuric acid, the SX/EW
process is often not a substitute
for, but rather an adjunct to
conventional smelting, using
sulphuric acid produced from
smelter gases.

While smelting is more energy
consuming than applying SX/
EW technology, electrowinning
consumes more energy than
electro-refi ning.

CASE 6.2
Leaching Nickel using Pressure, Temperature, and Acid

Most of today's nickel is produced in ferro-nickel smelt-
ers that process nickel sulphide ore. Advances in leaching
technology make it increasingly attractive to process
laterite nickel ore, which result from preferential leaching
and enrichment of nickel under prolonged tropical weath-
ering. Lateritic nickel ores are found in abundance along a

geological belt stretching from New Caledonia to Southern
Philippines, and also in Western Australia and Cuba.
As the name suggests, in the High Pressure Acid Leaching
(HPAL) process, nickel and cobalt, its common companion, are
dissolved from ore under high pressure, and high tempera-
ture, using concentrated sulphuric acid solution. At present,

only a few HPAL plants are in commercial production,
and even fewer plants operate without technical problems.
The HPAL operation at Coral Bay in Southern Philippines
demonstrates that the technology can work successfully, the
Goro HPAL plant in New Caledonia and its past challenges
highlights the perils associated with this technology.