A Facile and Green Synthesis of White Carbon Black from Coal Gangue Acid Leaching Residue by Low-Temperature Ammonium Fluoride Activation

The extraction of critical elements from coal gangue has resulted in rapid accumulation of silica-rich byproducts, imposing a significant burden on the environment. In this study, a novel and environmentally sustainable approach was proposed for synthesizing high-purity white carbon black from coal gangue acid leaching residue via ammonium fluoride (NH 4 F)-assisted activation. The F/Si molar ratio, roasting temperature, and roasting time were investigated in detail to elucidate their impacts on fluorination roasting process. Systematic characterization mechanistically revealed that siloxane (Si-O-Si) bonds in amorphous silica were cleaved through F⁻ ions nucleophilic attack. This selective bond cleavage fragments the tetrahedral framework into discrete [SiO 4 ] units, which subsequently react with NH 4 F to form water-soluble (NH 4 ) 2 SiF 6 intermediates. Through the integrated process of fluorination roasting, water leaching, and ammonia precipitation, the leaching efficiency of (NH 4 ) 2 SiF 6 reached 99.76%, while the white carbon black product with purity and recovery rate of 98.16% and 95.20%, respectively. The closed-loop design achieved 81.45% recovery of high-purity NH₄F (99.57%) from fluoride-rich filtrate via evaporative crystallization, coupled with capture of 74.34% roasting-derived ammonia gas as 0.2313 mol/L aqueous ammonia (200 mL DI water) for precipitation reuse, thereby largely eliminating fluoride and ammonia discharge as environmental pollutants. This work offers a unique approach for the further study on the facile synthesis of white carbon black with similar mineral composition. Graphical