Treatment of process water from molybdenum flotation

摘要

The Boliden copper mine in Aitik, north of Sweden is aiming to produce molybdenum as a new product. Due to the use of sodium hydrogen sulphide (NaHS) as a depressant of copper concentrate during the molybdenum flotation, it was a concern that the tailings pond cannot buffer the potential acid generation when free hydrogen sulphide ions (HS~-) and degradation products such as thiosulphate ions (S_2O_3~(2-)) are further oxidized downstream to sulphate ions (SO_4~(2-)). There was also a concern regarding possible dissolution of heavy metals into the process water, for example mercury. Test work carried out aiming to generate data for the design of a suitable process for treatment of the process water before discharging it from the molybdenum flotation circuit into the tailings pond, demonstrates that: A significant portion of the Chemical Oxygen Demand (COD) can be removed by aeration of the pulp through oxidation of HS- and polysulphides to elemental sulphur and thiosulphate. Subsequent flocculation test work on the oxidised pulp has shown that elemental sulphur as well as precipitated mercury or fine mercury minerals will go to the final copper concentrate by sedimentation. The thiosulphate content in the overflow from the solid-liquid separation of the oxidised pulp can be completely converted to sulphate using hydrogen peroxide based on the Fenton procedure. The overflow was finally neutralized using slaked lime. Acceptable water quality to the tailings pond could be accomplished.