feldspar leucite PAC activation method alkali fusion alums bauxite residue bentonite dynamic light scattering (DLS) extraction hydrated alumina import lime-soda sinter process meso porous silica milling nano silica particle size particle size analyzer (PSA) phosphate rock planetary ball mill (PBM) potassium potassium fertilizer roasting selective leaching silica sand synthesizing ultrafine grinding utilization