Mining and processing of sulphide orebodies has the potential to solubilise sulphate. Gypsum in the host rock, as well as oxidation of sulphides during milling and processing can solubilise sulphate. Acid rock drainage (ARD) also has the potential to generate waters high in sulphate. Even after treatment of the ARD with lime, the treated water can have sulphate present in gypsum saturation levels.
Most mine-sites reuse sulphate impacted water but as a mine nears closure, reuse options may be limited and water treatment becomes an option. Furthermore, sulphate discharge requirements, particularly for anti-degredation narrative standards, can be very strict. Freeport has investigated many new technologies to economically remove sulphate from these types of water.
Dutch company Paques has developed one of these technologies and it used biological reactors to convert sulphate into elemental sulphur in a two-step process. In 1992 the first industrial reference of the SULFATEQTM technology was commissioned at the Nystar Budel zinc refinery in the Netherlands.
Biological sulphate removal – a case study from Sierrita pilot plant operations was a paper presented at last week’s SME Annual Conference in Phoenix by V. R. K. Paruchuri, R. Collins and B. Waterman, all of Freeport-McMoRanm and two Paques specialists, P. Gonzalez and H. Dijkman.
The bio sulphate pilot plant demonstration has been a success in terms of sulphate removal.
Modifications in the process controls increased the process efficiency. Major modifications in the process were:
- Hydrogen gas was added based on the stoichiometric ratio instead of anaerobic gas buffer tank level
- Aerobic air compressor control strategy programmed based on an algorithm that considers all possible scenarios of deviations in ORP, pH and rH. This control strategy has provided a much tighter control in limiting over oxidation in the system.
The authors conclude that with further clean-in-place provisions for scaled pipes and aeration in the post treatment phase a much higher efficiency and reduced downtime can be achieved. Operational and technical knowledge has been gained to size equipment for optimised biological activity. The aerobic compressor even at its lower speed still provided excess air causing over oxidation of sulphide in the aerobic reactor. Sizing this compressor within range will increase the plant efficiency by almost 7% and sulphate concentrations in the effluent could potentially reach levels of around 250 mg/litre after equipment optimisation.