Depression by Hydrosulphide Ion

The depression by hydrosulphide ion is in a similar manner as hydroxyl depression, i.e. there is a critical pH for each HS ion concentration at a constant xanthate concentration above which no flotation is possible. In the case that the hydrophobic entity is disulphide, the mineral will be depressed when the reaction (3-5a) or (3-6a) occurs before the reaction (1-3). Thus for the pyrite /ethyl xanthate system, pyrite will be depressed if the oxidation reaction (3-5a) takes place prior to the oxidation reaction (4-35). 

Assuming that sulphur could be oxidized to sulphate at overpotential 0.5 V and the concentration of sulphate is 10 mol/L, then Eq. (5-13) become 

For the galena/ethyl xanthate system, if the reaction Eq. (3-5a) occurs before the reaction (4-12a), the critical pH condition determined by the reaction Eqs. (3-5b) and (4-12a) is 

Therefore, the critical pH of hydrosulphide ion depression of pyrite and galena could be calculated using Eqs. (5-15) and (5-16). The results are plotted in Fig. 5.13. The same figure is the results of contact curves reported by Wark and Cox (1933). It can be seen from Fig. 5.13 that the critical pH condition defined by electrochemical equilibrimns of Eqs. (5-15) and (5-16) is correlated reasonably well with the contact curves. The calculated concentration of sodium sulphide required for preventing the interaction between xanthate and galena or pyrite at 

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