Extraction of Lithium from Spodumene

Thamsanqa Ncube

The conventional process of lithium extraction from α-spodumene (LiAlSi2O6) is energy-intensive and associated with high by-product management cost. In this work, we investigate an alternative process route that uses potassium sulfate (K2SO4) to extract lithium while producing leucite (KAlSi2O6), a slow-release fertiliser and potash alum (KAl(SO4)2∙12H2O), which can be used in water purification, and medical drugs. This work presents the first-ever high temperature in situ record of the reaction of α-spodumene with potassium sulfate, using synchrotron X-ray diffraction (S-XRD) and differential scanning calorimetry (DSC) to document the reaction sequence during prograde heating. During in-situ studies, we observe that the reaction of potassium sulfate and spodumene proceeds through ion exchange between lithium and potassium in the spodumene structure, followed by phase conversion of α-(Li,K)-spodumene into leucite. Once conversion of spodumene to leucite reaches 90 %, a lithium sulfate- melt starts to appear. We optimised the potassium sulfate process at a K2SO4:α-SC 7.6 ratio of 0.6:1 (w/w), 1050 oC and 30 min roasting time, achieving 96.3 ± 2 % (w/w) lithium extraction efficiency, similar to the conventional sulfuric acid process (96.7 ± 0.6 % (w/w)). Using OLI systems modelling we found that purification of the leach liquor from the potassium sulfate requires the addition of aluminium sulfate to recover potassium as potash alum. Comparing the two processes based on 200 kt y-1 of spodumene concentrate, we estimate that the operating profit of the potassium sulfate is 5 % lower than that of the sulfuric acid process. Because of its potassium content, leucite can be converted into a slow-release fertiliser through further processing. For the potassium sulfate process to be more profitable than the sulfuric acid process, the fertiliser market price should exceed US$34.5 t-1. This work demonstrates that K2SO4 process could be a feasible alternative to the conventional sulfuric acid process.

Extraction of Lithium from Spodumene

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