PT Newmont Nusa Tenggara plans to stockpile approximately 500Mt of copper ore for up to 20 years at the Batu Hijau mine on Sumbawa Island, Indonesia. Cu recovery from unoxidised ore is about 85%.
It was not clear what the impact of oxidation would be on recovery after ore was stockpiled for 20 years. The company decided to undertake a field measurement and modelling program to identify the main controls on oxidation, predict the rate of chalcopyrite oxidation and simulate how alternative stockpile designs could reduce oxidation.
To accomplish this, SRK Australia designed a field program to measure oxygen concentration and temperature distributions within the existing stockpile. The objectives were to determine the oxidising regions and the rates of oxidation. SRK made predictions of the extent of chalcopyrite oxidation using the numerical model SULFIDOX. Integrating these results with outcomes from flotation studies made it possible to predict future oxidation levels and develop improved stockpile management strategies.
The study concluded that if 500Mt of ore was stockpiled for 15 years, the expected recovery is about 58%. This is equivalent to a loss of about 400,000t of copper, due to stockpiling.
The approaches developed for preserving the value of the stockpiled ore included:
• Increasing the flotation recovery of oxidised ore using Controlled Potential Sulfidisation (CPS). When Newmont conducted testing using CPS, they found that an average 10% increase in copper recovery could be achieved
• Reducing oxidation while the ore is stockpiled by reducing the amount of oxygen that could enter the copper stockpile by selective material placement. The oxidation of higher value ore can be reduced by surrounding it with waste sulfidic material that acts as an oxygen consuming rind. When SRK constructed a model of the stockpile covered in a rind, the results showed that oxidation could be reduced by up to 50%; a saving of 200,000t Cu
