Geology Reference
In-Depth Information
Moreover, its carbon footprint for primary and secondary production is 2.18 t CO 2 =t
Sn and 0.03 CO 2 =t Sn, respectively. In the Ecoinvent compilation (Classen et al.,
2007), obtained from Richter et al. (1996), the energy consumption for the benefi-
ciation stage is 12.6 GJ/t plus an additional 15.2 GJ/t of metal produced. In the
refining process, the energy reported is 6.4 GJ/t plus 11.4 GJ/t of tin produced.
Table 8.3 shows additional energy data such as that compiled by Chapman and
Roberts (1983) for alluvial and vein ores.
8.7 Nickel and Cobalt
Two ore minerals may be used to obtain primary nickel: sulphide and lateritic
(limonite and saprolite) ores. Sulphide deposits, often found with copper ores and
associated with volcanic rock, are mined underground. Lateritic nickel ores are a
mix of oxides and silicates that have been exposed to weathering in tropical climates.
They are found in superficial deposits. Such deposits are cheaper to mine and allow
for on-site hydrometallurgical technologies with valuable cobalt co-production.
Around 60% of the world's nickel is produced from sulphide ores with the rest
originating from lateritic ores. And whilst conventional pyro-metallurgical process
like that of copper sulphide can be done with nickel sulphides, consisting of roasting,
electric smelting, converting and electrolytic refining, half of the world nickel sul-
phidic ores are smelted using an Outotec flash furnace. This is because the Outotec
renders converters obsolescent and in doing so saves energy(Mäkinen and Taskinen,
2006; Outotec, 2011).
8.7.1 Nickel sulphides process
Prior to smelting, the iron sulphide content of ore is decreased via a floatation
pretreatment which concentrates the ore 20-fold, obtaining a feed material of 7-25%
Ni which is then dried until the moisture content is less than 1%. The flash smelting
process itself occurs when electricity and oxygen are combined quickly to reach the
very high temperatures needed (nickel melts at 1453 o C). This highly exothermic
process oxidises the iron and sulphur in an oxygen-rich atmosphere. The liberated
heat then produces the nickel matte and a fluid slag. As the furnace matte contains
some iron and sulphur, additional air or oxygen is injected in the molten bath
to increase its nickel concentration. This high grade matte (35-70% Ni;Co and
Cu) is tapped off at the bottom of the furnace and directed to water-granulation.
Furthermore, the slag layer, which is composed of different oxides, is channelled
out to an electric furnace and reduced with coke and a sulphurising agent, an
operation which separates the slag from its metal content. This additional matte
is subsequently tapped-off and then granulated for a further hydrometallurgical
treatment in atmospheric leaching. The heat of the process gases, rich in SO 2 , is
recovered in a waste heat boiler and any particles are then removed by electrostatic
 
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