Geology Reference
In-Depth Information
Box 2.2 The Mining and Refining of Nickel Ores
Mining
Nickel is found in two different types of ore-, magmatic sulfide and lateritic.
The former are usually mined by underground techniques or in large and deep
open pits for some new deposits; the latter are mined in shallow pits using
heavy earth-moving equipment such as shovels, draglines, and front-end
loaders The ore is usually ground to coarse aggregate and then it
is
transported to the refinery.
Refining and Smelting
The ore is first ground in large mills to powder that is fine enough that the
particle size is less than that of individual grains of the ore minerals. Sulfide
grains are separated from the gangue by the floatation process. The ground
ore is mixed in large vats with water and chemicals such as fatty acids and oils
that increase the hydrophobicity of the sulfide particles. Mechanical and
pneumatic devices stir the mixture and produce air bubbles which are injected
at the base of the vats. Sulfide particles adhere to the bubbles and float to the
surface where they are recovered by scraping away the froth. The magnetic
properties of Fe-Ni sulfides are used to aid their extraction.
The nickel concentrates are then dried, mixed with flux, and heated to
about 1,350 C in an oxidizing environment in smelters. The reaction of
oxygen with iron and sulfur in sulfide ore supplies some of the heat required
for smelting. The product is an artificial nickel-iron sulfide known as matte,
which contains 25-45% nickel. The iron is then converted to an oxide, which
combines with silica flux to form a slag. When the slag is drawn off, the matte
contains 70-75% nickel.
The nickel matte is either leached at high pressure with ammonia and the
metal is recovered from solution, or the matte is roasted to produce high-
grade nickel oxide. The final stage is electrorefining: the nickel oxide is
dissolved in sulfate or chloride solutions in electrolytic cells and pure nickel
metal is deposited on the cathode. Sulfur is released in large quantities at
several stages of the process. Some is recovered to be used in industry or as a
fertilizer, but a large fraction is lost in smelter fumes and constitutes a serious
pollutant.
Laterite nickel ores do not contain sulfur and cause less of a pollution
problem, but their refinement requires much higher energy input. The ore
minerals are oxide or silicates that are not amenable to floatation and other
conventional processes. Large tonnages or untreated ore must therefore be
smelted. In addition, the reactions of oxide ores from laterite deposits are not
 
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