In this study, the leachability of Caldag lateritic nickel ore under high pressure acid leaching (HPAL) conditions was investigated. Several HPAL parameters were tested in a comparative manner since in many cases, the extraction efficiencies of Caldag ore sample were more than 90% for nickel and cobalt. After the optimization of the process parameters, it was found that 94.1% nickel and 94.0% cobalt could be extracted under the conditions of 325 kg sulfuric acid/tonne of dry ore, 250 degrees C leaching temperature, 1 h leaching duration and 100% - 1 mm particle size. Under these conditions, scandium also present in the ore was highly extracted into the pregnant leach solution. The results obtained were compared with the reported pilot plant testing results of atmospheric heap leaching by European Nickel PLC. (C) 2013 Elsevier B.V. All rights reserved.