Study on two-stage treatment of fluoride in coking concentrated brine with CaCl₂+defluorination agent

Due to the high background fluoride content in part of China's Inner Mongolia, Shaanxi, Shanxi and other high-fluorine regions, the concentrated brine with high concentrations of fluoride ions was produced after membrane treatment of coking industry wastewater using coal as raw material. Crystallization and salt separation of brine salt is a necessary method to achieve "zero emission", but the excessive fluoride concentration can have adverse effects on the treatment of concentrated brine. In order to reduce the corrosion of evaporator caused by high concentration of fluoride ions in concentrated brine and its impact on the purity of salt separation, this paper took concentrated brine from a coking enterprise in Hebei, discussed the effects of co-existing ions SO₄ ²⁻, NO₃ ⁻ and Cl⁻ on defluorination agents based on the water quality characteristics of concentrated brine and the defluorination agent developed by our research group. Meanwhile, SEM and EDS were used to characterize the defluorination agents and treated flocs, and the mechanism of defluorination was analyzed. The defluorination effect of one stage treatment process with CaCl₂, PAC and defluorination agent was compared. The combined process of CaCl₂ and defluorination agent applied in the actual concentrated brine was designed and optimized, then analyzed the economic costs of different treatment schemes. The results showed that the influence order of the three co-existing ions on the effect of defluorination agent was SO₄ ²⁻>NO₃ ⁻>Cl⁻. SEM and EDS analysis indicated that F element was uniformly distributed in the floc and could be efficiently removed through the surface adsorption, surface complexation, and surface hydroxyl exchange mechanism. One stage treatment process with defluorination agent was better than the effect of CaCl₂ or PAC, when the dosage of defluorination agent was 8 g/L, the removal rate reaches 92.3%. The combined process of CaCl₂ and defluorination agent could reduce the concentration of fluoride ions from 238.27 mg/L to less than 10 mg/L. The combination cost of CaCl₂ dosage of 6 g/L and fluoride removal agent dosage of 2 g/L was the lowest, which was 14.03 yuan per ton of coking concentrated brine. The two-stage defluorination process with CaCl₂+defluorination agent can economically and efficiently remove high concentration fluoride ions from coking concentrated brine which can provide a technical reference for zero discharge of wastewater in the coking industry.