A Novel Process for Al2O3 and KCl Production from Mechanically Activated K-Feldspar via HCl Leaching and Roasting

Abstract This study investigates the recovery of potassium and aluminum from a Turkish K-feldspar ore via a combined process of intensive milling for mechanical activation, hydrochloric acid (HCl) leaching, and evaporation–roasting for the production of potassium chloride (KCl) and alumina (Al2O3). Mechanical activation was performed in a planetary ball mill at a ball-to-ore ratio of 30, 400 rpm for 120 min, which resulted in complete amorphization of microcline and albite, while quartz remained partially crystalline (13.91%) due to its homogeneous Si–O–Si network. The activated ore exhibited a sixfold increase in specific surface area (from 4.68 to 28.05 m2/g) and a significant increase in loss on ignition (LOI) from 0.46% to 2.88%, attributed to the formation of surface hydroxyls (Si–OH, Al–OH), confirmed by FT-IR analysis. Under identical leaching conditions (4 M HCl, 100 °C, 24 h, L/S = 20, 600 rpm), the raw ore yielded only 2.83% Al extraction, whereas the optimally milled sample achieved 87.72%, demonstrating the critical role of mechanical activation. Subsequently, response surface methodology (RSM) was employed to optimize the leaching parameters, yielding optimum conditions of 4.88 M HCl, 100 °C, and 36 h, which resulted in 82.08% Al and 84.20% K recoveries. The negative quadratic coefficient for HCl concentration (C2 = −8.65) reflects the decreased solubility of AlCl3·6H2O in concentrated HCl due to chloro-aluminum complex formation (AlCl3(aq), AlCl4–). The loaded leach solution was directly evaporated and roasted at 1150–1250 °C to separate Al2O3 from KCl, which volatilized and condensed as a salt mixture containing 81.3% KCl and 17.3% NaCl; the KCl fraction can be further purified to a high-purity product. Contamination from WC–Co milling media was fully eliminated by changing to sintered alumina media, which also improved Al2O3 purity to 94.1% and produced a white product. The silica-rich leach residue (90.87% SiO2) was valorized into high-purity SiO2 (97.7%), calcium silicate (CaSiO3), and silicon carbide (SiC) via biochar-driven carbothermal reduction. Techno-economic analysis of the integrated POTASSIAL process (100 ton/day) indicated marginal baseline economics (IRR ≈ 6.1%, payback ≈ 12 years) but became highly attractive (IRR > 22%, payback <4 years) with a 30% increase in SiC price, while LCA confirmed long-term environmental benefits from closed-loop operation and zero-waste generation. This work demonstrates a sustainable, zero-waste pathway for the simultaneous valorization of K and Al from feldspathic ores, offering a viable alternative for regions lacking soluble potash resources.