Rinsing copper oxide heaps leached with sulfuric acid: A pilot study

Four thousand tonnes of oxide ore were leached and then rinsed in a square vat measuring 23 m x 23 m in area and 4.6 m in height. During the rinse, fresh water and alkaline solutions were sequentially dispensed with drip emitters distributed along irrigation tubes onto the spent ore. When fresh water was first applied, the effluent acidity decreased quickly as residual leach solution was displaced and diluted in the main flow routes. Later, acidity decreased more gradually due to the slow release of solutes from interstitial and intraparticulate voids located outside the main flow routes. We initially intended to rinse the spent ore until the copper concentration in the effluent met the regulated limit of 0.1 mg/L. It was found, however, that after 4.5 m 3 of rinse water per tonne of ore were passed, the effluent copper concentration was still as high as 20 mg/L. Lime solution rinsing was attempted but quickly aborted due to clogging of the emitters by undissolved lime particles. When rinsed with soda ash solution, the dissolved copper concentration decreased to about 1 mg/L at an effluent pH of 9.5; however, it rebounded to 240 mg/L soon after solution application ceased and the vat was allowed to drain down. Next, the vat was again rinsed with fresh water. Although the dissolved copper concentration decreased below 2.5 mg/L while rinsing, it rose once more to about 120 mg/L when the irrigation stopped. The difficulty in meeting effluent limits by rinsing resulted from pronounced channelling due to uneven application of the rinse solution on the surface and preferential infiltration beneath the irrigation tubes. Of the residual acidity present in the spent ore before rinsing, only an estimated 22% was contained in the main flow routes. The rebound in metal concentrations during draindowns is attributed to the slow drainage of solution with elevated metal contents from low flow routes. Consequently, long-term treatment of the heap seepage seems inevitable.