Metallurgy of Key Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 202

and casting of zinc. The reported range of dust emissions derived from such pro-
cesses varies from between 200 and 900 mg=Nm 3 in the raw gas. Such figures fall
greatly after fume collection and gas cleaning and can be as low as 10 mg=Nm 3 .
Wastewaters meanwhile come principally from the smelter gas cleaning system,
fluidised bed roasting stage, e uents from Zn electrolysis and the washing stages
of the Pb battery breaking. Modern wet gas cleaning systems operate with acidic
recycling and remove chlorines, fluorides and most of the mercury and selenium.
Finally, iron oxides of which jarosite and goethite are two, constitute the greatest
volume of solid waste. Both are hazardous because they contain leachable elements
such as Cd;Pb and As. They subsequently need to be treated or disposed of in
lined ponds or isolated areas. Typically, the production of jarosite and goethite is
between 0.35 to 0.80 t/t Zn and 0.3 to 0.35 t/t Zn, respectively.
The degree of emissions (solid, liquid or gas) released depends on the age of
the plant and the technology used. In the case of solids, any with a reasonable
concentration of metal(s) is returned to the smelter or placed back into the leach
circuit for recovery. If there are any with an important proportion of a given metal,
then it is recovered and sold (IPPC, 2009).
In regards to energy consumption, Norgate et al. (2007) obtained a life cy-
cle gross energy requirement for flash furnace smelting production of 20 GJ/t Pb,
whereas for roasting, leaching and electrolytic of primary Zn production a figure
of 48 GJ/t. Their figure for the GWP of lead was 2.1 tCO 2 e/t Pb whilst for
zinc, 4.6 tCO 2 e/t Zn. Barkas (2009) gives slightly different figures for lead and
zinc, with an energy consumption of 14 GJ/t Pb and 25 GJ/t Zn, respectively. The
GWP measured was 1.4 tCO 2 e/t Pb and 2.3 tCO 2 e/t Zn (Fthenakis et al.,
2007).
In the Ecoinvent database (Classen et al., 2007) energy consumption values at
the beneficiating stage of zinc-lead ores are calculated. Given that the percentage is
proportioned at 37.4% Pb and 62.6% Zn, the energy associated with lead is around
0.89 GJ/t whilst zinc's is 1.49 GJ/t. For the refining stage, the energy consumption
should approximate at 40.4 GJ/t of Zn and 3.28 GJ/t of lead (IPPC, 2002).
Furthermore, according to the Bureau of International Recycling (Grimes et al.,
2008), the energy requirement for the production of 1 tonne of primary lead (from
ore concentrate) is 10 GJ, whereas for the same quantity of secondary (from lead
scrap) this figure drops to only 0.13 GJ. The carbon footprint for primary production
is 1.63 t CO 2 and again this is lower for secondary at 0.015 t CO 2 . Correspondingly,
the energy requirement for the production of 1 tonne of primary zinc is 24 GJ,
whereas for its secondary production this figure drops by a quarter to only 18 GJ.
For secondary vaporisation of zinc, the energy needed is much reduced at 4.7 GJ.
Likewise the carbon footprint for primary production is 2.36 t CO 2 , for secondary
1.40 t CO 2 , and for secondary vaporisation 0.56 t CO 2 .
A summary of the aforementioned figures is provided in Table 8.3.