Desulfurization of calcium fluoride sludge for reuse by selective removal of gypsum using replacement and chelating...

Fluorite (CaF₂) sludge in this research was from a chip-producing industry with a CaF₂ content of about 88%. However, the sludge also contained about 1.82 wt% of sulfur in the form of sulfate which made it unsuitable for reuse as flux in iron smelting. This study focused on desulfurization of the sludge. Na₂CO₃ and sodium D-gluconate were employed as replacement and chelating reagents respectively to replace sulfate with carbonate or transfer CaSO₄ into calcium D-gluconate. Results showed that, at a solid/liquid ratio of 30 g/L and molar ratio of Na₂CO₃:S of 1.2:1, after reaction of 4 h, the S content of CaF₂ sludge decreased from the original 1.82 wt% to around 0.10 wt%. For sodium D-gluconate, the optimal condition is solid/liquid ratio of 30 g/L, molar ratio of 2.0:1, and reaction time of 60 min. A desulfurization efficiency of more than 88.9% was achieved. S contents in the treated sludge dropped to 0.20 wt%. CaF₂ contents after desulfurization reached 95%. Characterization of sludge after desulfurization showed that desulfurization process did not significantly change the physical structure of CaF₂ particles. Results from this research provide two effective methods for potential recycle of CaF₂ sludge.