Research on the preparation and application of collaborative turbidity and fluorine removal agents for typical coal mine water in the Yellow River basin

The Yellow River basin is the region with the greatest coal potential in China. Although the amount of coal mine water inflow is large, the resource utilization rate is not high and, especially in arid and semi-arid areas with high fluorine content, the excessive fluoride ions in coal mine water have become one of the main factors restricting the improvement of mine water resource utilization rate. Orthogonal experiments were used to screen out five components of efficient defluorination agents, including polyaluminium chloride (PAC), poly-aluminum silicate, magnesium nitrate, poly-ferric chloride (PFC), and carboxyl methyl starch sodium. The single factor experimental method was used to explore the effects of different preparation and reaction conditions on the defluorination effect, and the defluorination mechanism was preliminarily explored through X-ray energy spectrum analysis (EDS), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) characterization. The experimental results showed that the fluoride removal agent developed under the conditions of the total amount of metal to Si molar ratio of 43, Al/Mg molar ratio of 40, and Al/Fe molar ratio of 40 could reduce the concentration of fluoride ions in wastewater from 20 mg/L to below 1.0 mg/L, meeting the Class Ⅲ water fluoride limit requirements (1.0 mg/L) of Environmental Quality Standards for Surface Water (GB 3838-2002). When the dosage of the fluoride removal agent was 1.25 g/L, the initial pH was 2-12, the suspended solids concentration was 100-2 000 mg/L, and the polyacrylamide (PAM) dosage was 0.4 mg/L, the residual fluoride ion concentration in the supernatant after treatment could be controlled below 1.0 mg/L, and the fluoride removal rate could reach over 95%. Al and Si elements of the defluorination agent played an important role in defluorination, and their mechanism was mainly through the formation of metal complexes such as Al-F-Al for precipitation and removal. Applying defluorination agents to the treatment of fluorinated mine water in a certain coal mine in the Yellow River Basin, the concentration of fluoride ions decreased from 5.6 mg/L to 0.85 mg/L when the dosage of defluorination agents was 400 mg/L and PAM was 0.2 mg/L. Simultaneously, the turbidity of mine water could be reduced from 500 NTU to 4.59 NTU. The cost of water treatment agents per ton was 1.602 yuan. The defluorination agent has good application potential in the treatment of mine water containing fluorine in the Yellow River basin.