Effect on ceramic grade CaF2 recovery quality from the etching wastewater under the optimum sulfate content

Abstract

The improvement of fluorite quality is an attractive measure to recover fluoride from high F⁻ content wastewater and thus reduces the fluorite sludge generation rate, and the removal of contaminant pollutants could be helpful for the production of ceramic grade fluorite (>85%). In this work, typical etching wastewater was collected and characterized, and SO₄²⁻ was found to be the main barrier for high quality CaF₂ production, while CaSO₄ could also contribute to CaF₂ precipitation. The optimum SO₄²⁻ content was adjusted by the introduction of barium salts, to keep a balance between the CaF₂ quality and the settlement process. The ceramic grade CaF₂ of 88% was obtained under the conditions: [Ca]/[F] of 1.29, pH 8.14 and C_sulfate of 117.4 mg L⁻¹. The residual SO₄²⁻ content should be kept in the range of 100–130 mg L⁻¹, and the formation of CaSO₄, combined with CaF₂ could accelerate the CaF₂ precipitation by the co-precipitation method, with a larger particle size from 758 nm (without SO₄²⁻) to 898 nm. The gradient settlement of CaF₂ contributes to the recovery of fluoride from etching wastewater, while CaF₂ preparation should be carried out in a dynamic experiment, such as a fluidized bed, for further research.