Abstract:

Impurity ions are one of the key factors affecting the crystallization and precipitation of fluorine-containing wastewater.In order to investigate the influence of impurity ions（Al³⁺ and Si⁴⁺） on the calcium salt crystallization and precipitation process of fluorine-containing wastewater and improve the treatment effect of fluorine-containing wastewater，the reaction of NaF and CaCl₂ was used to simulate the sedimentation process of fluorine-containing wastewater.Combined with Visual MINTEQ software simulation，XRD characterization，and SEM-EDS testing，the influence of Al³⁺ and Si⁴⁺ on the turbidity of precipitation crystallization，residual fluoride ion concentration，and precipitation product characteristics was systematically studied.The research results indicated that low concentrations of Al³⁺ and Si⁴⁺ had little effect on the precipitation and defluorination of calcium fluoride crystals.When the concentration of Al³⁺ was 200.00 mg/L，impurities such as aluminum hydroxide and cryolite precipitated under alkaline conditions，leading to an increase in reaction turbidity and a decrease in residual fluoride ion concentration with increasing pH.When the concentration of Si⁴⁺ was 200.00 mg/L，the silicon impurities in the crystal precipitation product were mainly quartz and calcium silicate，and the optimal defluorination conditions were n（Ca）/n（F）=2.5，pH=9.At this point，the residual fluoride ion concentration in the solution was 5.62 mg/L.SEM-EDS testing showed that the precipitate products were in the form of block aggregates，and the embedding relationship between calcium fluoride and impurity products was complex，making separation difficult.The research results provided reference for the treatment of complex multi ion fluorine-containing wastewater and the recycling of crystallization products in actual production processes.

Key words: fluorinated wastewater, calcium fluoride, impurity ions, crystalline precipitation, fluoride removal effect