2.3. Effect of cyanide on the flotation of copper sulphide minerals
The addition of free cyanide depresses the flotation of chalcopyrite
with high natural floatability after coating with graphitic
carbon on the surface (Grano, 1990). A slightly higher concentration
of NaCN is required to reduce chalcopyrite hydrophobicity in
the presence of xanthate compared to pyrite. Much higher NaCN
concentrations are required to completely depress the hydrophobicity
of chalcocite as can be seen from Fig. 5 (Wark, 1938).
Chalcocite is known to float at potential values more negative than
pyrite due to the formation of cuprous xanthate species which are
thermodynamically more stable than the oxidised form of collector
such as dixanthogen, while both cuprous xanthate and dixanthogen
attribute to the hydrophobicity on chalcopyrite surfaces. The
depression effectiveness of NaCN on different sulphide minerals
may be related to the specific potential range required for the
flotation of each mineral as shown in Fig. 4.
Castro and Larrondo (1981) found that the depression effect of
free cyanide on chalcocite flotation was dependent on the conditioning
time. At the initial period with the addition of 150 mg/L
NaCN, the completely depressive action of free cyanide on
chalcocite flotation was similar to that in pyrite flotation with
the potential of chalcocite electrode dropped almost instantly from
150 mV to 300 mV (SHE) at which the xanthate oxidation was
inhibited according to spectrophotometric evidence. The leaching
of chalcocite by free cyanide in flotation and the occurrence of
reactions between cyanide and copper imply that free cyanide
can be extensively consumed, which also explains the possible
attenuation of the depressant action of cyanide on chalcocite.