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A novel alkaline hydrothermal approach for low-temperature conversion of α-spodumene into Li 2 SiO 3 residue was proposed, providing a promising method for extracting lithium from α-spodumene as a pretreatment process. This work proposed a systematic investigation for extracting lithium from the residue by acid leaching and preparing lithium carbonate. The reaction feasibility between Li 2 SiO 3 and acids (HCl and H 2 SO 4 ) was first evaluated through thermodynamic calculation. Compared with the leaching effects of hydrochloric acid and sulphuric acid, sulphuric acid is the preferred leaching agent due to its higher extraction efficiency for lithium and lower acid consumption. Lithium extraction efficiency from the residue achieved up to 87.48% under the following optimized conditions: 0.75 mol/L H 2 SO 4 , 0.4 times the theoretical amount of acid, 10 min, 30 C, and 100 rpm. Based on the optimized conditions, the lithium-

The conventional process of lithium extraction from α-spodumene requires high-temperature calcination to transform α-spodumene into β-spodumene, and concentrated acid roasting followed by water leaching. This study proposed the direct preparation of water-soluble lithium salts from α-spodumene by roasting with different sulfates to avoid the use of concentrated acid and simplify the process flow. In this study, the theoretical feasibilities of the roasting of α-LiAlSi2O6 with different sulfates to produce water-soluble lithium salts were first verified using HSC Chemistry 10. Then, the roasting of spodumene concentrate with CaSO4·2H2O, MgSO4·7H2O, and Na2SO4 followed by water leaching was investigated to extract lithium from α-spodumene. The complete transformation from α-spodumene to γ- and β-spodumene without additives occurred at 1000 °C for 30 min. CaSO4·2H2O did not significantly promote the phase transformation or the reaction with α-spodumene. MgSO4·7H2O and Na2SO4 could react with α-spodumene to generate water-soluble Li2Mg2(SO4)3 and LiNaSO4, respectively, but the roasting results with Na2SO4 were better than MgSO4·7H2O. Here, 93.8% Li was converted into LiNaSO4 when roasting with Na2SO4 at 1000 °C for 60 min with an ore/Na2SO4 mass ratio of 1:0.7. Low-temperature roasting of the concentrate with Na2SO4 and CaO was proposed to produce LiNaSO4 at 850–875 °C for 60 min with ore/Na2SO4/CaO mass ratios of 1:0.7:(0.05 to 0.1), and the lithium extraction efficiencies were about 86%.

Lithium extraction from α-spodumene in a potassium hydroxide solution was proposed to provide a new green metallurgical process for spodumene concentrate. The structure of α-spodumene could be destroyed directly by a KOH solution, and new solid-phase products of Li2SiO3 and KAlSiO4 were generated simultaneously. The total lithium extraction efficiency could reach 89.9%, of which 84.1% was converted into Li2SiO3 and 5.8% converted into the liquid phase under optimal conditions: initial KOH concentration of 50 wt %, stirring speed of 500 rpm, mass ratio of KOH/ore of 2:1, leaching temperature of 523.15 K, and leaching time of 16 h. The experimental data fitted well with the Avrami–Erofeev equation model, –ln­(1 – X) = (Kt) n . The apparent activation energy and Avrami index were calculated as 106.37 kJ·mol–1 and 0.80, respectively.