is usually fed from the first compartment inlet of the autoclave. This paper describes how the recovery of Ni and Co and the scale formation process in the autoclave are affected by the staged-addition of acid at different dosages. In addition to a single dose, sulfuric acid was fed to the leaching process performed in a novel designed autoclave experimental set-up at ascending, descending, and equal amounts in two and three stages. The results showed that the staged-addition of acid into the autoclave both increased the Ni and Co metal recovery and almost eliminated the formation of scales. It was found that when the acid was fed to the autoclave in three stages, Ni and Co recoveries increased from 85% to 87% and from 86% to 89%, respectively, compared to singledose and two-stage acid addition tests. The improvement in recovery also increased the selectivity of Ni and Co relative to impurities. Besides, it was determined that the scale formation in the autoclave was 11 times less in two-stage acid addition and 31 times less in three-stage acid addition compared to single-dose feeding.