Converting Minerals to Metals - Mining and the Environment: From Ore to Metal - page 271

cyanide into surface water, dilution may be the most important determinant of the fate of
cyanide and the environmental damage that results ( Case 6.4 ).
The most common method of cyanide destruction from process waste streams is the
INCO SO 2 /Air™ detoxification process in which sodium metabisulphite solution is used
in the presence of a copper sulphate catalyst to oxidize the cyanide in accordance with the
following reaction:

CN CNO (6.16) SO 2 H 2 O O 2 → H 2 SO 4

This process can oxidize weak and moderately strong metal cyanide complexes as well as
free cyanide (CN ). Depending on a variety of considerations including water balance,
likely presence of birds, and applicable regulations, the process may be applied to the total
tailings stream before discharge to the tailings storage facility (TSF), or it may be applied
only to excess supernatant water removed from the TSF, for subsequent release to the
environment. Clearly, cyanide destruction prior to discharge to the TSF removes any risk
to wildlife or to downstream communities or ecosystems in the event of overflow from
the TSF. However, this represents a much more expensive approach as it decreases the
extent to which the cyanide-bearing solution can be recycled as well as requiring higher
consumption of detoxifying reagents.
As shown in Table 6.3 , there are several processes to recover cyanide for reuse. These
non-oxidative processes involve acidification to form HCN, volatilization of the HCN and
absorption into solution. One such process is CYANISORB ® , which is used at the Golden
Cross Gold Mine in New Zealand and at a mine in Argentina. In this process, the tailings
are first acidified using sulphuric acid to lower the pH to 7.5, following which the tailings
are contacted with high volumes of turbulent air which strips the HCN. The HCN vapour
is then redissolved in caustic solution which flows to the leach circuit. Subsequently,
lime is added to restore alkaline conditions, thereby precipitating de-complexed cations.
At Golden Cross, this process reportedly recovers 80% to 90% of cyanide. The process is
reported to be cost-effective compared to other cyanide control technologies. In view of its
reported efficacy and cost benefits, it is somewhat surprising that this technology has not
been more widely applied.
As an environmental threat, the use of cyanide to leach gold has been much discussed in
recent years, but that cyanide leaching is a problem is perception rather than fact. The min-
ing industry has been using cyanide for gold extraction for more than 100 years without

The mining industry has been
using cyanide for gold extraction
for more than 100 years without
human fatalities.

CASE 6.4
Comparing Two Cyanide Spills

failure caused a spill of 3 million m 3 containing cyanide
at 25 - 30 mg/L. However, in this case, fi sh kills were
limited to the Omai River River and, possibly, a short
portion of the larger Essequibo River, into which it fl owed.
The much more serious impact of the Romanian event can
partly be explained by the cold temperature, the presence
of ice shielding the contaminated water from sunlight,
and the low rate of mixing with uncontaminated water
(dilution).

A notorious example of a serious cyanide spill was the case
of the tailings overfl ow from the Baia Mare gold mining
operation in Romania, further detailed in Case 18.3. The tail-
ings overfl ow resulted in the sudden discharge of an
estimated 100,000 m 3 of mud and wastewater with a
126 mg/L cyanide load into the Lapus River, a tributary to
the Szamos River, which both were largely frozen at the time.
As a result, the plume or 'slug' of contaminated water fl owed
downstream along the bed of this river and, subsequently,

the Danube River with relatively low rates of dilution.
Cyanide decomposition was minimized by the very low tem-
perature and the absence of sunlight beneath the river ice.
Undoubtedly, free cyanide would have been generated progres-
sively from complex cyanide ions as mixing lowered the pH.
As a result of this spill event, toxic effects, notably fi sh kills,
extended many kilometres downstream of the spill site.
This contrasts with another notorious incident at the
Omai Gold Mine in Guiana in 1995, where a tailings dam