Recovery of Silica from Phlogopite by Acid Leaching

Phlogopite-rich mine tailings from the Siilinjärvi carbonatite complex mine in Finland provide a secondary resource rich in magnesium and silica. These tailings are attractive for mineral-based CO2 capture and the circular economy concept. This study investigates the recovery of silica from phlogopite through an acid leaching process. Leaching efficiency was studied by considering the acid type, leaching parameters including time and temperature, and particle size. Batch leaching experiments were conducted using sulfuric, hydrochloric, and nitric acids at different concentrations under controlled conditions. The solid residues were characterised by X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS), and inductively coupled plasma spectrometry (ICP-SFMS). Silica selectivity parameter, and removal efficiencies for the major elements were evaluated. Among the three acids tested, nitric acid exhibited the highest silica selectivity. The highest mass reduction was about 43 wt% with the highest silica selectivity, 0.97, and achieved by 3 M HNO₃ at 60 °C and 1 h, reflecting the preferential dissolution of Mg, Fe, K, Ca, and Al. Increasing temperature and/or time increased mass reduction up to about 45–47 wt%, but the differences between 60 °C for 2 h and 80 °C for 30 min were minor, and both conditions produced very similar elemental compositions and removal efficiencies. Particle size analysis indicated that the tailings are mainly coarse (>125 μm). Although pre-sieving to 53–125 μm slightly increased mass reduction, it changed the bulk composition; therefore, subsequent leaching tests were performed on non-sieved material. XRD results indicate the disappearance of phlogopite peaks and the dominance of feldspar and quartz, with sharp peaks indicating that the residues are predominantly crystalline and contain no significant amorphous phase. Overall, the results show that phlogopite tailings from Siilinjärvi can be used effectively to recover the silica-rich residues by nitric acid leaching. This process offers a promising route for sustainable silica recovery alongside magnesium extraction for CO₂ mineralisation applications.