This paper describes the experimental findings of the extraction of nickel and cobalt by high pressure acid leaching (HPAL) of a refractory limonitic nickel laterite ore from the Gordes region of Manisa in Turkey. By optimizing the basic HPAL process parameters: leaching at 255 degrees C with 0.30 sulfuric acid to ore weight ratio with a particle size of 100%-850 mu for 1 h of leaching, it was found that 87.3% of nickel and 88.8% of cobalt present in the ore could be extracted into the pregnant leach solution (PLS). However, these extraction results were found to be relatively low compared with other similar studies. In order to understand the possible reasons for this relatively lower extraction, further investigations have shown that together with a problem related to the kinetics of the dissolution reactions, a persistent acid resistant refractory mineral present in this sample also limited the leaching process. Attempts were made with different additives to solve this problem. The effects of chemical additives such as HCl, Na(2)SO(4), FeSO(4), Cu(+) and sulfur were tested and the effect of each addition on the degree of extraction of nickel and cobalt was determined. (C) 2011 Elsevier Ltd. All rights reserved.