GALVANIC INTERACTIONS
Application of electrochemical techniques and measurements in Sulphide flotation systems has established that the flotation is partly governed by electrochemical interaction between flotation agents (collectors) and the minerals. Interaction of a mineral with a sulfhydryl collector (xanthates) corresponds to a specific potential, the value of the potential depends upon the chemical reaction.
Two main redox reactions have been recognized. The first is adsorption of xanthate ion at the mineral surface:
- X- + M M(X) + e- .............(1a)
- ½ O2 + H2O + 2e- 2OH- ...(1b)
The potential of this reaction is in the range 60-70 mV, vs. SHE, [3].
The second reaction is oxidation of xanthate to dixanthogen at the mineral surface.
- X- ½ X2 + e- …………….(2a)
- ½ O2 + H2O + 2e- 2OH- …(2b)
The potential of this reaction is in the range 200-220 mV, vs. SHE.
In both cases the presence of oxygen is essential as electron acceptor. The reactions can be hindered by bringing its activity down to extremely low values (< 0.5 ppm).
A convenient method of lowering the activity of oxygen is by nitrogenation. Lower activity of oxygen leads to lower pulp potentials, Ep. It the potential is brought into the range 60<Ep<200 mV, reaction (2) is hindered while reaction (1) still occurs. The formation of a hydrophobic film at pyrite is governed by reaction (2) while for a mineral like galena it is governed by reaction (1). Therefore, when the potential is in the range 60<Ep<200 mV flotation of pyrite should be hindered while flotation of galena should be occur.
Depression of pyrite lowering the potential by using nitrogen has been demonstrated with single minerals or when mixed with a nonsulphide minerals. With complex ores containing two or more Sulphide minerals the situation is complicated by electron transfer between two Sulphide minerals. In this interaction a more cathodic mineral draws electrons from a less cathodic one [3]. Pyrite is known to be the most cathodic among the four principal Sulphide minerals, galena, chalcopyrite, sphalerite and pyrite. As a result the pyrite surface acquires a film of OH- ions as the electrons drawn by the mineral are transferred to oxygen (reaction (3)) making the surface hydrophobic.
- MS M++ + So + 2e-.............................……… (3a)
- FeS2 + H2O + ½ O2 + 2e- FeS2 + 2 OH- ……(3b)
Galvanic interaction can de weakened by lowering the activity of oxygen and this would tend to promote flotation of pyrite. The introduction of nitrogen helps to weaken the galvanic interaction between pyrite and sphalerite. In Fig. 1 is shown the galvanic mechanism [4,5] Since pyrite is nobler than sphalerite acts as a cathode draws electrons when both are in contact. The dissolved oxygen accepts these electrons forming OH- ions. The hydroxides compete with xanthate for adsorption sites. In a nitrogen environment the oxygen content is reduced and the depression mechanism of pyrite is interrupted. Thus, the xanthate is able to adsorb on pyrite surface.
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