Enhanced Lithium Extraction from Nevada Claystone via Low-Temperature Binary Ammonium Salt Roasting and Sulfuric Acid Leaching

This study presents a binary ammonium salt (BAS)-assisted low-temperature roasting (LTR) process for lithium (Li) extraction from Nevada Claystone (NC), a calcite-rich sedimentary clay-hosted Li deposit. Conventional high-temperature roasting (HTR) approaches operate at 850–1000 °C, making them energy-intensive and environmentally burdensome due to the emission of SO2, Cl2, and related toxic gases. The proposed LTR method roasts NC with a 1:1 (w/w) mixture of NH4Cl and NH4F (BAS) at 320 °C under a 10 psi O2 atmosphere, followed by H2SO4 leaching at room temperature (25 °C). Under optimized conditions (NC:BAS = 1:1 (w/w), 15% (w/v) H2SO4, liquid-to-solid ratio of 10:1 mL/g, and 6 h leaching duration), 88.5% Li extraction was achieved, compared to 65.5% by direct acid leaching alone, corresponding to an absolute increase of 23 percentage points and a relative improvement of 35.1%. BAS-assisted roasting promotes structural transformation of the Li-bearing clay matrix, as supported by X-ray diffraction (XRD), Fourier transform infra-red (FTIR) spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) analyses, while the formation of CaF2 and (NH4)2SiF6 supports the participation of fluoride-containing species in the roasting reactions. Among BAS-based roasting studies reported for clay-hosted Li deposits, this roasting temperature is the lowest to date; single-salt NH4F systems have achieved lower temperatures on other feedstocks (e.g., 155 °C on coal fly ash), but a BAS approach has not previously been demonstrated on a calcite-rich sedimentary claystone. While the approach demonstrates meaningful reduction in roasting temperature and energy input relative to HTR, acid input remains substantial (~1500 kg H2SO4 per ton of clay, based on acid charged rather than measured net consumption), primarily driven by Ca, Fe, and Al-bearing gangue minerals. Process gases including HF, NH3, and HCl require appropriate capture systems. The transferability of this approach to other clay-hosted Li deposits would require direct validation given compositional variability across feedstocks.