Depression of Galena and Pyrite

The underlying principle of depression by hydroxyl ion is that for each concentration of collector there is a pH below which any given mineral will float and above which it will not float. This is the so-called critical pH which can be determined according to the relative extent of oxidation of the mineral surface and collector or the formation of the metal collector salt. 

For the case that the hydrophobic entity is disulphide, the mineral will be depressed when the reaction of the type (2-3) or (2-4) occurs before the reaction (1-3). Thus for the pyrite /diethyl dithiophosphate (DTP) system, pyrite will be depressed if the oxidation reaction 

Assuming that the sulphur could be oxidized to sulphate at overpotential 0.5 V and the concentration of sulphate is 10 mol/L, then Eqs. (5-lb), (5-2b) and (5-3a) become 

8548 - 0.07473pH 0.252 - 0.059 Ig [DTP ] Thus, the critical pH above which pyrite will not float is 

In the case of the galena/diethyl/dithiophosphate system, if the reaction Eq. (2-22a) occurs before the following reaction 

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