Market Survey on Copper - Indian Bureau of Mines

viscosity of slag during smelting affecting matte-slag separation leading to
increased copper losses in slag.
Roasting is generally performed employing multiple hearth of fluid bed
roasters. Fluid bed roasters are preferred because of their high production rates,
excellent temperature and chemical control and high SO2 concentration in
effluent gases.
iii) Matte Smelting : Smelting or matte smelting consists of melting
concentrates or partially roasted concentrates at 1,150 - 1,200 o C to produce the
separable liquid phases known as copper matte (Cu 30-60% sp, gravity > 5) and
slag (sp. gravity < 3). The matte (Cu2S, FeS) form feed for production of
blister copper. The slag are formed from the oxides in furnace charge
(impurities in the concentrate: SiO2, Al2O3, CaO, MgO and oxides from fluxing
material) and iron oxides produced by oxidation during smelting. Smelting slag
generally contains 30-40% iron oxides (FeO, Fe2O3), 35-40% SiO2, 0-10%
Al2O3 and 0-10% CaO.
The reaction between cuprous oxide (Cu2O) and iron sulphide to
produce copper sulphide (Cu2S) proceeds rapidly and fully as sulphur has
greater affinity for copper than iron. This is the main smelting reaction and this
is the reason why copper is concentrated in matte and iron is partially slagged
off.
Smelting operations are carried out in blast, reverberatory, electric and
flash furnaces. As blast furnace requires lumpy feed, fine flotation concentrates
are smelted in the other three furnaces only.
iv) Reverberatory Smelting : The reverberatory furnace is the most widely
used unit for matte smelting. It is a fuel fired hearth furnace in which
concentrates or roaster calcines are melted in presence of flux to produce
separate layers of molten matte and slag. The heat for smelting is provided by
burning fuel in the furnace and by passing hot combustion gases over the
charge. All types of material (lumpy or fine, wet or dry) can be readily smelted.
However, everything is not well with the reverberatory furnace. It requires
large quantity of hydrocarbon fuel because it makes little use of the energy
which is potentially available from sulphide oxidation. Thermal efficiency is in
the range of 25 - 30% only. The reverberatory furnace gas presents serious
pollution problem because SO2 concentration is too low (0.5 to 1.5% SO2) to
form a feed for sulphuric acid manufacture, the minimum SO2 concentration
required is 5%. The principal products from reverberatory furnace are liquid
matte (30-50% Cu; 1150 o C), which collects gold and silver, is sent forward for
converting and a liquid slag (0.3 to 0.8% Cu: 1200 o C) which is discarded as
effluent gas (1250 o C).
v) Electric Furnace Smelting : In the regions where abundant electricity is
available at low price and fossil fuels are costly, electric furnaces are frequently
used in place of reverberatory furnace for copper smelting. The advantages of
electric furnace smelting are:
38