Pressure acid leaching of Çaldağ lateritic nickel ore: An alternative to heap leaching Mehmet Ali Recai Önal, Yavuz Ali Topkaya ⁎ Department of Metallurgical and Materials Engineering, Middle East Technical University (METU), 06531 Ankara, Turkey abstract article info Article history: Received 3 April 2013 Received in revised form 5 November 2013 Accepted 18 November 2013 Available online 6 December 2013 Keywords: Hydrometallurgy Çaldağ Nickel Pressure acid leaching Scandium In this study, the leachability of Çaldağ 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 extrac- tion efficiencies of Çaldağ ore sample were more than 90% for nickel and cobalt. After the optimization of the pro- cess 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 °C leaching temperature, 1 h leaching duration and 100% -1 mm par- ticle 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. © 2013 Elsevier B.V. All rights reserved. 1. Introduction It is well known that HPAL is one of the two common hydrometallur- gical processes that are utilized for extraction of nickel and cobalt from limonitic ores in industrial applications. Although both heap and agita- tive atmospheric acid leaching (AL) processes offer some advantages by means of lower capital cost, simpler process equipment, rapid ramp-up period and easier maintenance with high on-stream availabil- ity over HPAL, it also suffers from several weaknesses. The most important superiority offered by HPAL over AL is that under high temperature reaction conditions most of the iron and alumi- num leached are re-precipitated as hematite and alunite respectively, regenerating almost all of the acid that was consumed in leaching these metals. This results in significantly lowered acid consumption. Moreover, faster reaction kinetics at higher temperatures (230 °C to 270 °C versus ambient to 90 °C) offer reduced residence time (i.e., from about 12 h in the case of agitative leaching to 0.5–1.5 h) and hence increased productivity (Griffin et al., 2002). Being the first heap leaching project, Çaldağ lateritic nickel deposit has achieved worldwide renown along with its previous owner ENK PLC (previously known as European Nickel PLC) and is now owned by parent company VTG Holding. Having the highest amount of proven re- serve, which is about 33 million tonnes, this lateritic deposit is still one of the most important nickel reserves of Turkey (European Nickel PLC, 2009). The major advantage of atmospheric heap leaching is its lower capital cost but some researchers questioned this by introducing a rough economic balance between revenue obtained from heap leaching and actual expenses of the process (Willis, 2012). Due to its low metal extraction values and high acid consuming behavior with respect to HPAL option, researchers agree that it is suitable for low grade laterites or low reserve ore deposits that could not meet the capital costs of HPAL or other AL operations (i.e., agitation leaching). In 2004 acid leaching of the first Çaldağ heap commenced and was followed by the irrigation of two more heaps within 4 months. After 548 days of leaching of the first heap, that was the most representative of the ore site, the recoveries of nickel and cobalt were 79.4% and 82.7%, respectively with calculated 528 kg/tonne of dry ore acid consumption (Oxley et al., 2006, 2007). The resultant pregnant leach solution (PLS) was then treated to produce mixed hydroxide precipitate (MHP). The optimum conditions for this study were also selected based on this op- tion as the expected downstream process. Currently, META Nickel and Cobalt Company is constructing a HPAL plant for nickel reserves in Gördes, Manisa neighboring the Çaldağ site. Although Çaldağ nickel laterite cannot be considered as a true limonite due to its relatively high silica content, it is still expected that this later- ite would also be a suitable ore feed for an HPAL plant. This alternative option to heap leaching was investigated at laboratory scale in the present study. Furthermore, scandium, a potential by-product not con- sidered by previous studies was also examined for its extraction behav- ior. This by-product could provide a cost relief for a HPAL process if it is recovered at the end of the process train. For economic reasons, values greater than 90% are generally re- quired for nickel and cobalt extractions in HPAL operations. The higher these values the more will be the obtainable revenue from the ore. However, higher acid requirements will be more costly if not balanced against the additional valuable metal extractions. The terminal free acidity of the resultant PLS after HPAL is also dependent on the acid Hydrometallurgy 142 (2014) 98–107 ⁎ Corresponding author. Tel.: +90 312 2102538; fax: +90 312 2102518. E-mail address: topkaya@metu.edu.tr (Y.A. Topkaya). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.11.011 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet