Direct Methods for Lithium Extraction from Lepidolite and Spodumene Minerals

Kudzai Angeline Mchibwa

Conventional processing technologies currently in use for lithium extraction from spodumene (LiAlSi2O6) and lepidolite (K(Li,Al)3(Al,Si)4O10(F,OH)2) are based on the sulphuric acid process. These technologies involve energy-intensive steps such as decrepitation (1100 °C) and sulphuric acid baking (250 °C) in spodumene processing and roasting (750-1000 °C) for lepidolite processing. In this project, lithium extraction from lepidolite using direct acid leaching at temperatures less than 100°C was investigated. Lepidolite is a mineral that contains Li and valuable elements such as Cs, Rb and K. The acids; H2SO4, HNO3, HClO4 and H3PO4 were studied to assess their extraction efficiency of Li and other valuable elements. The results show that high Li leach efficiencies (above 90%) regardless of anion type were achieved within 3 h of leaching. The Li extraction followed the descending order of HClO4 > H2SO4 > HCl > H3PO4 indicating that Li+ formed stable complexes with various ligands in solution and this factor was more influential than the effect of proton activity. Anion participation played a more crucial role in determining the complexation behaviour of the valuable elements such as Rb, Cs and K. The sulphuric acid leach process results in highest extractions of the valuable by products Rb (99%), Cs (100%) and K (90%) whilst the perchloric process resulted in extractions of Rb (21%), Cs (90%) and K (20%), showing the effect of HSO4- and ClO4- anions. The X-ray diffraction (XRD) analysis confirmed the presence of tainiolite (KLiMg2Si4O10F2), quartz (SiO2) and fluoro phlogopite (KMg3(AlSi3O10)F2 in the residues. The direct extraction of Li from α-spodumene in a single roasting step resulted in fair to moderately high Li extractions of (45 -70 %) with various fluorinating and sulphating additives. Direct roasting of refractory α-spodumene with NaF, NaHSO4, (NaF + NaHSO4) mixtures and lepidolite, followed by leaching was also investigated and resulted in moderate to relatively high Li extractions of 58%, 71%, 43% and 41 % respectively. The effectiveness of F in lithium extraction from α-spodumene roasting with the mixed (NaF + NaHSO4) additives was however, slightly lower than that from roasting with NaF alone and was possibly due to the formation of F-depleting eutectics such as Na3SO4F double salts in the Na2SO4-NaF segregation melt. The direct leaching of α- and β spodumene with H2SO4 and H3PO4 and about 40% Li was extracted after 4 h of leaching β-spodumene in 4 M H3PO4. The use of lepidolite additives during leaching was investigated and proved beneficial in the direct acid leaching of β-spodumene as lepidolite additions of 5%, 10% and 15% (w/w) increased the Li extraction to 47%, 77% and 80% respectively in phosphoric acid. The use of lepidolite additives improved Li extraction from α- spodumene vastly in a sulphuric acid leach. Li extraction from α-spodumene leaching in H2SO4 increased from a very low 2.5 % to 58%, 67% and 84% with lepidolite additions of 5%, 10% and 15% (w/w) respectively. An ion-exchange mechanism between Li+ and H+ in a heterogenous solid-liquid reaction on the mineral surface was proposed for both α- and β-spodumene direct leaching, informed partly by the XRD identification of HAlSi2O6 and analcite, NaAl (SiO3)2 phases in leach residue. The purification of typical lithium sulphate leach liquors by several precipitants was studied via pH control, highlighting the effect of ions such as oxalate, phosphate, hydroxide and carbonates on the precipitation efficiencies of various impurity elements. The use of LiOH as a precipitant in a one-step batch purification step and subsequent lithium recovery resulted in lower lithium losses (2.4%) at pH 12 and (10.6%) at pH 7 compared to traditional processes (23 %) during purification. The direct extraction of Li through the elimination of one or more calcination/roasting steps reduces the energy intensity of the process and translates to lower capital costs. The findings of this work align well with the United Nations sustainable development goals (SDGs), particularly SDG 7: Affordable and Clean Energy, SDG 12: Responsible Consumption and Production and SDG 13: Climate Action.

Direct Methods for Lithium Extraction from Lepidolite and Spodumene Minerals

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