Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 279

releases. Ivanov et al. (1983) suggested that the present river sulphur transport (from air
pollution, mining, erosion, and other sources) of 200 Mt/y is about double that of pre-
industrial conditions. Excess acidic deposition is likely to cause changes in rock weath-
ering, and in mobilization of metals in soils and sediments. Thus, global cycles of heavy
metals are coupled to the sulphur cycle primarily by liberation from metal sulphide ores
by mining, metal-sulphide precipitation in anoxic conditions, and metal mobilization due
to sulphate-induced acidification in terrestrial environments (Benjamin and Honeyman
1992). Human-induced emissions have led to a net flux from land to sea through the
atmosphere, where 100 years ago the net flux was the reverse. The global cycle of sulphur
is not at a natural steady state at present and probably never has been. Schlesinger (1992)
concludes that although human activities have caused only a minor change in global pools
of sulphur, very large changes have occurred to the annual flux through the atmosphere.
On the other hand, there are large areas of the land surface where sulphur is deficient.
Much of Australia is in this category. The argument can be made that such areas benefit
from deposition of sulphur from the atmosphere.

Solid Emissions

The production of metals results in the generation of a wide variety of wastes and resi-
dues. They are a result of the metals separation that is necessary for the production of pure
metals from complex sources. These wastes and residues arise from the different stages of
processing as well as from the off-gas and water treatment systems. The key solid wastes
and residues which result from production of non-ferrous metals are: (1) slag; (2) drosses
and skimmings; (3) spent linings and refractories; (4) wastes/residues/by-products of air and
water pollution abatement systems; and (5) wastes and residues from hydrometallurgical
processes.
Slag is produced by the reaction of slag-forming elements (e.g. iron) in the ore with
added fluxes. In the smelting process, the slag is liquid and has a different density than
the melted metal and separates on top of the metal-rich matte. The slag can, therefore, be
tapped-off separately. It is either rapidly quenched with water to form granules that can be

Slag is produced by the reaction
of slag-forming elements (e.g.
iron) in the ore with added
fl uxes.

CASE 6.5
Coal Combustion and Acid Rain

In Europe acid rain started to make the front page in the
1970s when large forest areas started to die. While much
of the damage was concentrated around the coal-fi red
power plants in Eastern Europe burning low quality, high
sulphur coal, conifer trees also started to shed their nee-
dles in the picturesque Black Forest, a forest close to the
heart of Germans and tourists alike. Forest damage due to
acid rain, of course, also occurred elsewhere. Maple trees in
Quebec started to produce less syrup, largely attributed by
the maple syrup producers to the sulphur dioxide

emissions generated in the US by the extensive coal
and steel industry in the Appalachian region. The issue
of trans-boundary pollution, and its political fallouts
emerged. As with the current debate on global warm-
ing, the controversy about acid rain was emotionally
charged and highly polarized. Nevertheless, the severe
environmental consequences of acid rain were soon
recognized, and, among other measures, more stringent
SO 2 emission standards were implemented.