Although feasible, oxidation processes for cyanide degradation in settling ponds are
expensive and can bring about environmental problems.
Despite the willingness of mining companies to spend a lot of money on the breaking down and replacement of cyanide
compounds, a by-product of the cyanide oxidation, i.e. CNOÀ, may generate ammonia, a
potentially toxic compound (Mosher and Figueroa, 1996; Miltzarek et al., 2002).
Environmental concerns in mining include cyanide in aqueous discharges from mills
and heap leach operations.
The biological treatment methods have a higher capital cost,
but a significantly lower operating cost, so that the present-worth cost is significantly
lower for the biological method (Nelson et al., 1998).
Biological processes, which might satisfy both the needs of extraction and environmental control, have been proven at large scale in well-understood and engineered systems in several countries.
A stronger trade-off than initially foreseen exists between conventional processes and
biological processes in relation to environmental friendliness.
The balance of costs that currently favor biotechnology in specific cases only are frequently tipped in the favor of
biotechnology when a greater emphasis is put on environmental factors.
In many cases,cleaner effluents would then be achievable at lower cost using biotechnology.
However, if minimum effluent standards are currently being met by using conventional treatment, a
switch to biological treatment is unlikely to be economic.
Biotechnological treatment approaches are under various stages of development;
however, many proposed processes cannot currently compete with conventional technologies and are not likely to do so unless regulatory standards change.
Processes which have been applied commercially are as follows: in-plant cyanide destruction; in situ cyanide
destruction of spent heap leach piles; metal and sulfate removal using active (in-plant)
sulfate reduction; and limited use of passive processes such as wetlands and ecological
engineering for metals polishing. Biotreatment of cyanide and associated species such as
ammonia and thiocyanate in gold mill effluents is a natural process and can be readily
exploited to handle large flows and cyanide concentrations found in commercial gold
operations. Commercial applications involving both in-plant treatment as an alternative to
chemical processes and in situ cyanide destruction of spent heap leach piles are in use
(