Abstract
The current commercial practice of processing laterite via HPAL consumes large quantities of raw materials and generates a waste material, without value, with large quantities of impure hematite, gypsum, silica and alunite reporting to tailings. Sherritt has put effort into developing an efficient process that maintains high nickel and cobalt recovery, but also recovers other constituents in the ore feed, in particular iron, as a saleable byproduct.
A process utilizing reductive leaching of laterite with sulphuric acid and an organic reductant, termed the RAPID Process, was developed and evaluated at Sherritt Technologies in an effort to selectively leach nickel and cobalt from laterites and generate a high quality hematite byproduct. Reductive leaching of Moa laterite ore samples with sulphuric acid at 150 °C resulted in extractions of 95% for Ni and 98% for Co from the limonitic ore sample, and 97% for Ni and 99% for Co from the saprolitic ore samples, while extracting almost half the iron into solution as well. Subsequent ammonium jarosite precipitation and decomposition steps allowed for recovery of iron as a hematite by-product.
The iron extracted using the reductive leach was separated from the leach solution in a jarosite precipitation step at 170°C and 500 kPag oxygen overpressure. The jarosite was then thermally decomposed at 800°C to produce a high grade hematite product, containing 70.1% Fe and <0.1% S.
Thermal decomposition was conducted to recover ammonia and sulphur from the leach residue, thereby reducing raw material requirements. More than 99% of ammonia was recovered at 450°C, and more than 97% of the sulphur was recovered at 800°C.
Cementation, cooling crystallization and sulphide precipitation were evaluated for metals recovery from the leach solution. Sulphide precipitation with hydrogen sulphide at pH 3.5 was the most promising method tested for Ni and Co recovery due to the simplicity of the process, high recovery, high selectivity and high concentration of both Ni and Co in the product. Using this method, up to 95% of Ni, more than 99% of Co and less than 3% of other metals were precipitated from the leach solution.
A conceptual engineering study was conducted by Sherritt to evaluate the RAPID Process and to estimate its capital and operating costs. While the concept is technically feasible and has been successfully demonstrated at a laboratory scale, its application at a commercial scale is deemed to be not economically viable under current market conditions.