Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 253

high-grade ores in form of concentrates can be economically smelted. Froth flotation is the
usual method of producing copper concentrate, prior to smelting. In most cases the concentra-
tion processes take place at a plant located adjacent to the mine. After crushing and milling,
one or more flotation agents are added to the slurry. The slurry is agitated to maintain the sol-
ids in suspension and air is pumped into the tank. The flotation reagent binds to the sulphide
minerals which float to the surface as bubbles, which are then skimmed from the surface of the
tank, leaving the unwanted gangue minerals. For ores containing different sulphides, more
than one flotation stage can be used to produce concentrates of different minerals.

Roasting

Sometimes roasting precedes smelting. Roasting of the concentrate at temperatures between
500ºC and 700ºC in air converts some of the CuFeS 2 to copper sulphide, and removes some
of the sulphur as sulphur dioxide. The roasting temperature is too low to actually melt the
concentrate. The product from the roaster is called calcine, which is a solid mixture of oxides,
sulphides and sulphates. The roasting reaction may be expressed typically as

(6.1) 2MS 3O 2 → 2MO 2SO 2

where M represents a metal. One such reaction when chalcopyrite (CuFeS 2 ) is roasted is:

(6.2) 2CuFeS 2 3O 2 → 2FeO 2CuS 2SO 2

Smelting with Fluxes-producing Matte

The initial letter 's' of the word 'smelting' implies that, in addition to the physical process
of melting, a chemical change also occurs. The calcine is heated to over 1,200ºC with fluxes
such as silica and limestone. In all smelting operations the kind of flux depends on the ore:
limestone flux for iron; iron flux for siliceous ore; and siliceous flux for sulphide concen-
trates. The calcine melts and reacts with silica and limestone. Some impurities combine
with the flux to form a slag which floats on the surface of the molten metal (like oil on
water) and is easily removed ( Figure 6.3 ) . The reaction is:

In all smelting operations the
kind of fl ux depends on the ore:
limestone fl ux for iron; iron fl ux
for siliceous ore; and siliceous
fl ux for sulphide concentrates.

SO 2

2CuFeS 2 heat O 2 → Copper-iron matte

(6.3) CaCO 3 Al 2 O 3 SiO 2 iron → slag

The remaining liquid is a mixture of copper sulphides and iron sulphides called matte.

Conversion of Matte to Copper Blister

The liquid matte is oxidized with air in a converter to form blister copper. The reactions are
twofold. First is the elimination of iron sulphide by oxidation to iron oxide which forms a slag:

(6.4)

2FeS 5O 2 → 2Fe 2 O 3 2SO 2

Second is the formation of blister copper by reduction of copper sulphide:

(6.5)

CuS O 2 → Cu SO 2