A hydrometallurgical comparison between the caustic and sulphuric acid cracking of steenkampskraal (skk) monazite

Abstract

The demand for rare earth elements (REEs) is continuously increasing since all high-tech products contain them. The Steenkampskraal (SKK) mine holds the full spectrum of REEs including high-value neodymium (Nd) used in the manufacture of high strength magnets, computers and hard drives. The REEs and thorium (Th) deposit at the SKK mine in the Western Cape has been confirmed to be the highest grade in the world, at an average of 14.4% Total Rare Earth Oxide (TREO) and 2.14% ThO2 respectively, with some locations of the ore body having as high as 45% TREO. These rare earths are hosted in the dominant mineral called monazite. In order to free the REEs from the monazite crystal, it requires “cracking” from the lattice such that it may be soluble in aqueous solution from which it undergoes purification. Two methods of cracking are usually employed in processing of monazite concentrate: caustic and sulphuric acid cracking. The aim of this study was to decide which route was more suitable to the SKK monazite in terms of factors which are usually considered in the setting up of such a processing plant, such as: recovery, capital expenditure (CAPEX); operating expenditure (OPEX); Safety, Health, Radiation, Environment and Quality (SHREQ); and lastly the effects on downstream processes. Experiments were firstly done to develop a flow sheet based on the optimum conditions at which each of these processes were to be carried out. These results would firstly indicate the recovery obtained from each of the processes and additionally impact on our choice of equipment (hence CAPEX), the amount of reagents and power (thus OPEX) and other factors such as SHREQ and effects on downstream processes. It was found that the TREO recovery from the caustic crack was 89% and that of the sulphuric acid crack 90%. However, the sulphuric acid crack required more process stages since the test results implied that the feed to the crack required drying and the product from the crack required conversion from a sulphate into a hydroxide salt. This resulted in a CAPEX of R 13,54M as opposed to the lower value of R 10,03M in the caustic circuit. Also, based on approximately 3 tons of feed to the hydrometallurgical circuit it was found that the sulphuric acid crack required almost twice the amount of OPEX since the power requirement was double that of the caustic cracking method and the reagent requirements were approximately 22% greater. Both of the CAPEX and OPEX mentioned, indicate that the project NPV for the caustic cracking route is ~12.5% higher (~R200M) when treating SKK monazite. The effect on downstream processes was also more negative in the sulphuric acid crack since there was a 5% loss during the double salt precipitation (DSP), 70% of which was Y. Were Y to be recovered at a later stage, its recovery, necessitating further process steps, would have further implications in the plant's CAPEX and OPEX. In addition to this, Th splits at an almost 1:1 ratio after the DSP step and would require an introduction of two new processing legs. The SHREQ aspects that were considered identified minimal impact in the caustic crack route. These, however, were quite extreme in the acid crack route, due to the latter containing high volumes of chemically altered effluents as well as the handling of fine dust particles resulting from its numerous drying stages. In addition to this, the main motivation to favour the caustic cracking route was to avoid the SO3 gas emissions that occurred when using the sulphuric acid cracking route. Hence, it was concluded that the caustic cracking was superior to the sulphuric acid cracking route when treating SKK monazite