Hydrometallurgical Nickel Laterite Processing: A Review of Current SX Flowsheets and Industry Trends

Abstract

The state of the art of nickel and cobalt solvent extraction in the nickel laterite industry is assessed, covering the reagents in commercial use (organophosphorus reagents, carboxylic acids, chelating oximes and amines), research developments and chemical fundamentals that result in the different approach taken with each ofthe reagent types. lntroductionNickelore deposits are found either as sulfides or laterites. Although 40% of the known nickel reserves are associated with sulfidic ores, such ores represent approximately 60% of nickel processing [1], due mainly to their much lower processing costs. However, depletion ofthese resources and increased concern over environmental issues regarding the processing of sulfur and the handling of sulfur dioxide has made the processing of lateritic ores much more attractive, as reflected by the increasing share ofNi produced from laterites from the 1950s onwards.Exploitation of nickel laterite resources has been characterized by the diversity of the processing routes utilized. In terms ofhydrometallurgical processing, HP AL (high pressure acid leaching) is the main leaching technology adopted to date but there has been increased interest in heap leaching. The PLS resulting from heap leaching has a different composition to that derived from HPAL (higher Fe:Ni and Mg:Ni) [2,3], which would affect the downstream processes. The current methods for treating the PLS from HPAL result in either an interrnediate precipitation product or a high value-added metal or oxide product derived from solvent extraction (SX).The interrnediate precipitation products, known in the industry as MHP (mixed hydroxide product) and MSP (mixed sulfide product) contain Ni, Co and other impurity metals. For MHP, there is no selectivity for Ni and Co over Fe, Cr, Al, Cu or Zn, limited selectivity over Mn and good selectivity only against Mg, Ca and Na, while achieving about 90% Ni recovery [2]. The product also contains 35-45% moisture. For MSP, selectivity is good, but the process requires hazardous reagents under pressure. The on-site production of H2S requires first converting a hydrocarbon (oil-price dependent) to H2 and then reacting this with elemental sulfur. Furthermore, the sulfide precipitation tank requires exotic materials of construction. The main drawback of the precipitation routes is that Ni and Co are not separated, resulting in a lower value product. Low recovery and high recirculating loads ofNi and Co are also a concern.Separation of Ni and Co and rejection of the impurity metals can be accomplished by solvent extraction. The increasing application and acceptance of SX for Ni and Co extraction and 391