Optmization of acid and lime consumption at Nkana oxide leach plant.

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Bwalya, Chishimba S.
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Nkana Leach Plant (NLP) of the Mopani Copper Mines Pic Nkana mine site treats ore from Mindola open pit mine. Leaching is employed in the treatment of the oxide ore which is followed by solvent extraction (SX). However the leach circuit was characterized by low metal recoveries in the pregnant leach solution (PLS), while acid consumed was not equivalent to the amount of metal recovered. In an effort to lower acid consumption at the leach tanks test works were conducted on laboratory scale on the feed material to the ball mill. Sampling was done from the stock pile which feeds the belt to the sag and ball mills respectively. Parameter considered for study in leach experiments were, densities, Particle size and its distribution, Optimal pH for leeching, Rate of agitation. In order to see what effect these parameters have on acid consumption, laboratory tests were conducted on some of these parameters, the parameters selected were, densities and pH which showed significant response to acid consumption. Leaching experiments were done in a baffled tank and an agitator was used to agitate the slurry, a pH meter was used monitor the pH. Results obtained showed that an increase in the pH from 1.8-1.9 show a reduction in acid consumption from 87ml-75ml with a deference in the efficiency variations averaging 67.38%-48.79 respectively. The densities show significant change with reduction in densities, a change from 24%-27% solids showed acid consumption changes from 360ml-377ml. the metal recoveries in both cases averaged 4-6gpl Cu in solution. The ore was averaging 1.0-1.89% total copper and 0.8-1.2% acid soluble Cu, this was the ore used in the experiments. The plant maximum efficiency of about 90% on average is usually obtained, with acid consumptions of about 200tonnes per day on average. Lime was used for nuetralisation of tails after the leaching process; the two types of lime which were in use are hydrated lime and quick lime. Experiments were conducted using the two types of lime in a baffled tank and an agitator, without an agitator and using water when and during addition to the tailing. It was discovered that with quick lime, if an agitator is used the consumption averaged 23.21g in 200ml volume of tails and hydrated lime showed 25.48g in 200ml while in non agitated systems it was shown that quick lime and hydrated lime showed increases of about 26g and 30g respectively. And reaction kinetics is faster in agitated systems as opposed to non agitated systems as shown in the tables of the appendix. It was concluded from experiments that to have a good balance between acid consumed and good leach efficiency the pH of between 1.9 and 2.0 should be used with densities of below 27% solids. Tail tanks should be agitated to allow for faster kinetics and low lime consumption as experiments show that lime consumption is low when agitated tanks were used. Experiments also showed that when water is added to the lime consumption is greatly reduced hence the need to add water when adding to the tails.
Oxide minerals