Mechanical, chemical, and Thermal processing of acid-grade fluorspar concentrates:

Abstract

This study investigates the chemical purification of fluorite (CaF₂) concentrate toward acid-grade quality (≥97% CaF₂) using sequential hydrochloric acid (HCl) and sodium hydroxide (NaOH) leaching. The feed material, obtained as a flotation concentrate from a Mongolian deposit, contained approximately 93.02% CaF₂, with silica (2–4%), iron (0.11–0.13%), and carbonate minerals as the primary impurities. The purification process was evaluated through a series of controlled experiments, varying NaOH concentration (1–4 M) and temperature (70–90°C) while maintaining consistent HCl leaching conditions (1 M, 50°C). Chemical and mineralogical characterization was conducted using X-ray fluorescence (XRF), supported by titration and gravimetric analysis for accurate determination of CaF₂, CaCO₃, and SiO₂ contents. HCl leaching was found to be highly effective in removing carbonate and iron impurities, resulting in a consistent decrease in iron content and an apparent increase in calcium concentration due to gangue dissolution. However, silica remained largely unaffected during acid treatment, confirming the chemical stability of quartz. In contrast, NaOH leaching demonstrated that temperature is the dominant factor controlling silica removal. At 70°C, silica reduction was negligible, whereas at 90°C, a maximum reduction of approximately 16.5% was achieved, indicating enhanced dissolution of silica under elevated temperature conditions. The combined treatment resulted in an increase in CaF₂ grade to a maximum of 95.18%, representing a significant improvement over the feed material. However, the acid-grade specification was not achieved, primarily due to incomplete silica removal, with residual silica remaining in the range of 3–4%. The results confirm that sequential HCl and NaOH leaching is an effective approach for fluorite purification, particularly for the removal of iron and carbonate impurities. Nevertheless, further optimization of alkaline treatment conditions, including increased temperature, extended reaction time, or improved mineral liberation, is required to achieve complete silica removal and meet industrial acid-grade specifications.