Decommissioning of copper heap-leach residues by rinsing with water and alkaline solutions - A pilot scale study

Acid heap leaching is widely applied around the world for the purpose of recovering copper from copper oxide ores. Nevertheless, environmental issues associated with the large volumes of spent ore left by leaching have received only limited attention from the mining and research communities. At sites with a positive water balance, seepage contaminated with residual metals resulting from infiltration of rain and snow melt through the spent heap is expected after the leaching operation ceases. This contamination could require treatment of the seepage for centuries. In this study, sequential rinsing of spent copper oxide ore with fresh water and alkaline solutions was tested for accelerating the removal and the neutralization of contaminants. Four thousand tons of ore contained in a vat were initially leached by applying acid solution onto the ore with drip emitters mounted on irrigation tubes, a common method for solution distribution in full-scale operations. Next, rinsing was carried out first with drip emitters and later with sprinklers. Compliance of the vat effluent with discharge criteria under conditions of natural precipitation (i.e., after rinsing had ceased) was the final goal of decommissioning. Copper was the most enduring contaminant in the effluent. Rinsing with fresh water was only able to reduce its concentration to 20 mg/L. Lime solution was ineffective, but application of soda ash solution quickly reduced the dissolved copper concentration in the effluent to less than 1 mg/L. Geochemical modeling of the effluent suggests that copper precipitated as malachite and tenorite in the vat during the application of soda ash solution. When rinsing stopped, however, the copper concentration in the draindown water rebounded to 240 mg/L, This rebounding effect is attributed to flow heterogeneities in the ore caused by precipitation of secondary minerals between drip emitters during leaching. Flow heterogeneities also reduced the copper recovery. Distribution of the leaching solution with sprinklers, where practical, is recommended to attenuate this problem. Alternatively, the spacing between drip emitters should be optimized to make solution application more uniform.