132 / Basics of Environmental Science
Mineral processing begins with separation of the ore mineral from the crushed rock. If the ore mineral is
denser than the unwanted rock mixed with it, called ‘gangue’, water will separate them, the mineral being
precipitated first from a suspension. Other minerals are separated by froth flota-tion. A compound with a
strong affinity for the mineral is mixed with water and agitated to make a froth; when the crushed rock is
added, the desired mineral adheres to the bubbles, the gangue sinks, and the froth is skimmed from the
surface. The separated mineral is removed and dried, ready for the next stage in its preparation, leaving
behind the wet gangue.
Most ores are then heated to a temperature at which the metal melts, and the elements with which it was
combined react to form compounds which float above the molten metal and can be removed as ‘slag’.
This is smelting and it usually proceeds as a series of chemical reactions. In the smelting of iron ore in a
blast furnace, for example, the oxide ore is mixed with coke to supply carbon and limestone as a ‘flux’
that reacts to bind the slag (see Figure 3.19). The carbon is partly oxidized to carbon monoxide (CO), then
oxidized further by reducing the iron oxide (Fe 2 O 3 + 3CO → 2Fe + 3CO 2 ). The impure iron may then be
mixed with any of a range of alloying metals and heated again in a ‘converter’ to make steel.
Some metals are purified by electrolysis. Copper, for example, is obtained by passing an electric current
through a copper sulphate solution. The anode (positive electrode) is made of ore, the cathode (negative)
of pure copper. Copper ions move from the solution to the cathode and sulphate irons recombine with
copper at the anode. Aluminium is also purified by electrolysis because, although it occurs as oxide ores,
its affinity for oxygen is so great that heating cannot reduce it without also reducing all its impurities.
At every stage, from cutting the ore from the ground to extracting the metal from its ore mineral, the
pollution risk is obvious and high. The pollution is contained by sealing tailings so dust cannot blow from
them, or noxious liquors leach from them, removing gases and dust from smelters before they reach the
outside air, and by treating liquid effluents.
A completely different technology now exists for extracting some metals. Bacteria that either possess the
ability to isolate particular metals from their compounds or can be genetically modified to make them do
so allow metals to be obtained with much less environmental disruption. Thiobacillus ferroxidans, for
example, separates copper into an acid solution containing about 50 parts of copper per million. Sulphuric
acid containing the bacteria is sprayed on to the ore rock, the liquor is collected, and the metal is removed,
in this case at about one-third the cost of conventional processing, and yielding nickel as a by-product.
Uranium can also be ‘mined’ in this way.
The limits to growth
In 1968 a group of 30 industrialists, economists, scientists, civil servants, and
others met in Rome at the invitation of Dr Aurelio Peccei, an industrialist, to
discuss ‘the predicament of man’. The meeting resulted in the formation of the
Club of Rome, which eventually grew to a body with about 70 members drawn
from 25 countries. Members met at intervals and collaborated to sponsor
studies. Their first, launched in 1970, was the construction of a computer model
to trace the consequences of interactions among five factors: population growth;
agricultural production; depletion of natural resources; industrial production;
and environmental pollution. The model was developed on computers at the
Massachusetts Institute of Technology (MIT) by a team of 16 people led by
Professor Dennis L.Meadows. Its results were published in 1972 as a non-
technical account, called The Limits to Growth. The technical information experts
needed to evaluate the methods used was published later.