Glass foams from acid-leached phlogopite waste

XRD analysis and exfoliation behaviour of Palabora phlogopite flakes supplied by PMC indicate a heterogeneous mineralogical composition dominated by phlogopite and hydrobiotite. 4 M nitric acid leaching for 6 h at 65 °C resulted in amorphous silica flakes with a very high surface area (< 450 m² g⁻¹). This was used to produce lightweight and high-strength glass foams via a direct foaming method. The best results were obtained with a formulation with borax:waterglass:amorphous silica 23:46:31 wt%. A closed-cell foam with a density of 0.326 g cm⁻³ and compressive strength of 2.09 MPa was obtained by heating a compacted mixture at ca. 6 K min⁻¹ to 525 °C and keeping it at that temperature for 30 min before allowing it to cool down. These results suggest that the silica waste, obtained from acid leaching of phlogopite waste from mine tailings, could provide a valuable raw material for the manufacture of low-density glass foams.