Pickling is one of the important processes of quartz beneficiation and purification. It is widely used in the actual production process, but how to treat the acid waste liquid is also a headache for quartz processing enterprises.

The difficulty in the treatment of quartz pickling waste liquid is the effective removal of fluoride ions. Fluoride is an accumulation of poisons. Plant leaves and pastures can absorb fluorine. When cattle and sheep eat this contaminated forage, they can cause joint swelling, osteoporosis, and even lie. Excessive intake of fluoride can interfere with the activity of enzymes, destroy the metabolic balance of calcium and phosphorus , and cause fluorosis of symptoms such as tooth plaque and joint deformation.

1. Common treatment methods for fluorine-containing wastewater

The first-level emission standard for fluoride ion in industrial wastewater in China is 10mg/L. The treatment of fluorine-containing wastewater is an urgent task for environmental protection and related departments at home and abroad. At present, the treatment methods of fluorine-containing wastewater mainly include chemical precipitation method, adsorption method, ion exchange method and electrodialysis.

The adsorption method is suitable for the treatment of low-concentration fluorine-containing wastewater, and has high requirements on the use environment and low adsorption capacity;

The electrodialysis adsorption process is simple, the analysis is troublesome, and the operation cost is high;

The chemical precipitation method produces calcium fluoride during the reaction process, the particle size is very fine, it is difficult to settle, and the concentration is difficult to reach the fluoride ion discharge standard of the wastewater.

2. Fluorine-containing wastewater purification test

The fluorine-containing ore dressing wastewater is treated by a combination of chemical precipitation and coagulation and coagulation. The test flow is shown in the following figure.

Figure 1 Fluoride wastewater purification test process

(1) Effect of the amount of calcium chloride added on the purification of fluorine-containing wastewater

100 mL of 100 mg/L fluorine-containing wastewater was placed in a plastic cup, and calcium chloride was added to carry out the reaction. Theoretically, n(Ca)/n(F)=0.5, and the fluoride ion was completely precipitated. Due to the complex and variable reaction conditions, the reaction is incomplete, so the theoretical addition is often insufficient. Calcium chloride having a molar ratio of 0.5, 0.6, 0.75, 0.85, 1.0 was added to the test. The mass concentration of fluoride ions was tested at intervals of 10 min, and the results are shown in Fig. 2.

Figure 2 Effect of calcium chloride addition and reaction time on fluoride ion purification

It can be seen from Fig. 2 that the reaction between calcium ions and fluoride ions is a rapid process. As the amount of calcium chloride increases, the concentration of residual fluoride ions decreases gradually, and eventually tends to be stable when n(Ca)/ is added. n (F) = 0.75, the removal rate of fluorine is close to 85%.

Calcium chloride is a soluble neutral salt that does not affect the initial pH of the system. The amount of calcium chloride added is increased, calcium fluoride is slightly soluble in water, the concentration of calcium ions in the water becomes large, and calcium fluoride reaches a dissolution equilibrium in water. If excessive calcium ions participate in the reaction, calcium fluoride precipitation will occur prematurely, and the chemical activity and adsorption time of adsorbing fluorine will be lowered, and the fluoride ion removal rate will be lowered. The molar ratio of calcium chloride was 0.75, and the reaction time was 40 min.

(2) Effect of pH of chemical precipitation on purification of fluorine-containing wastewater

Take 100mL of 100mg/L fluorine-containing wastewater in a plastic cup, adjust the pH of the solution to 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13, and add chlorine with a molar ratio of 0.75. Calcium, the mass concentration of fluoride ion in the 40min test system, the results are shown in Figure 3.

Figure 3 Effect of pH of chemical precipitation on fluoride ion purification

It can be seen from Fig. 3 that in the process of fluoride ion removal, the pH value of the system has obvious influence on the test results. The residual mass concentration of fluoride ion decreases first and then rises with the increase of pH value. When the pH value is 6-8, fluorine The ion removal rate is relatively high. When the pH value in the system is less than 5, the acidity is strong, and the fluorine mainly exists in the form of HF and HF-, which is not conducive to the reaction of fluoride ion and calcium ion; on the contrary, the pH value is too high, the system is alkaline, and the calcium ion is alkaline. Under the conditions, calcium hydroxide is formed, and the solubility of calcium hydroxide in water is small, which hinders the reaction of fluoride ions with calcium ions. Therefore, in the case of strong acidity and strong alkalinity, it is not conducive to the removal of fluoride ions by calcium chloride. In the test, the pH value was chosen to be about 7, that is, the pH was not adjusted at the beginning of the reaction, and the test procedure was made simple.

(3) The effects of the amount of aluminum chloride to effect fluorine-containing waste water purification

After the fluorine-containing wastewater is chemically precipitated by calcium chloride, 100, 200, 400, 600, 800, 1000 mg/L of polyaluminum chloride is added thereto, and the mixture is rapidly stirred for 60 s on a magnetic stirrer, and then stirred slowly for 15 minutes. After standing, the fluoride ion mass concentration was measured every 0.5 h, and the results are shown in Fig. 4.

Figure 4 Effect of the amount of polyaluminum chloride added and reaction time on the purification of fluoride ion

It can be seen from Fig. 4 that with the increase of the amount of polyaluminum chloride added, the residual fluoride ion concentration first decreases and then tends to be stable. When the addition amount is 200 mg/L, the reaction time is 60 min, and the fluoride ion mass concentration is the lowest. Near 10mg/L. The amount of polyaluminum chloride added is too small to be completely reacted; if the amount is too much, the pH of the system changes greatly, which is unfavorable to the reaction process.

The effect of polyaluminium chloride on fluoride ion in water comes from the polymerization product of A1(III) salt hydrolysis, mainly including adsorption, ion exchange and complexation sedimentation. The coagulation process produces amorphous Al(OH)3 flocs with a large surface area that produces hydrogen bond adsorption to fluoride ions. The fluorine ion has a small radius and a strong electronegativity, and this adsorption method is easy to occur. The amount of polyaluminum chloride added was 200 mg/L.

(4) Effect of pH value of coagulation on purification of fluorine-containing wastewater

The coagulation effect of polyaluminium chloride is closely related to the pH value of the system. It is found in the data that the polyaluminum chloride has the best effect at pH 5-10, so the fluorine-containing wastewater is firstly adjusted after being treated with calcium chloride. The pH value of the system (5, 6, 7, 8, 9, 10), then 200 mg / L of polyaluminum chloride was added, coagulation treatment for 15 min, and the fluoride ion mass concentration was measured every 0.5 h. The results are shown in Fig. 5.

Figure 5 Effect of pH value and reaction time on fluoride ion purification

It can be seen from Fig. 5 that as the pH value increases, the fluoride ion mass concentration first decreases and then tends to be stable. When the pH value is 8, the residual fluoride ion concentration in the effluent drops below 10 mg/L. The pH value affects the morphology of Al in polyaluminium chloride. When the pH is low, Al mainly exists in the form of low-charged monomer aluminum. As the pH value of the system increases, the content of low-charged polymer decreases gradually, and the high-charge poly-polymerization The content of the material increases; under alkaline conditions, the polyaluminum chloride is gradually converted into an Al(OH)3 gel by hydrolysis, and the effect of removing fluoride ions is further enhanced by adsorption, complex precipitation and the like. The pH should be maintained at the appropriate pH value during the test.

(5) Effect of the amount of polyacrylamide added on the purification of fluorine-containing wastewater

Adding a certain amount of coagulant can solve the problem that the fluoride ion sedimentation rate is slow and the effluent is unstable. The polyacrylamide added in the test is a macromolecular substance, and is configured into a solution having a mass fraction of 0.5% when used. Add polyacrylamide quantitative, stir for 15min, settling, test the concentration of fluoride ion. With the increase of polyacrylamide, the concentration of fluoride ion in the system will decrease. When the amount is more than 2mL, it can reach 10mg/L. Emission standards, but little change with the increase in the amount of coagulant added.

Polyacrylamide is a high-molecular substance with flocculation, adsorption bridging, surface adsorption and reinforcement, lowering the dynamic potential in water, and fixing it on the surface of different particles by molecular chain, and forming a polymer bridge between the particles. The particles are allowed to form aggregates and settle.

The removal of fluoride ion by polyacrylamide promotes the formation of floc by adsorption bridging, improves sedimentation speed, improves sedimentation performance, and enhances the effect of defluoridation. As the addition amount increases, it has little effect on the whole stable system. The effect was basically not improved, so the amount of polyacrylamide added in the test was 3 mL and treated for 60 min.

3. Industrial practice of fluoride ion removal in quartz pickling waste liquid

The fluorine-containing beneficiation wastewater produced by pickling and purifying quartzite in a place of Huangshi was selected. The concentration of fluoride ion was 2242mg/L with a composite fluorine electrode. After dilution 20 times, the concentration of fluoride ion was 110.5mg/L. 5.

Taking the above optimal test conditions, the final concentration of fluoride ion in the water body is 8.82 mg/L, which meets the first-level discharge standard of fluoride ion in industrial wastewater. The precipitate is separated, washed, dried, and analyzed by XRD. The results are shown in Fig. 7. . The test pattern was compared to the characteristic diffraction peak on the CaF2 standard card 77-2093, the composition of which was calcium fluoride.

Figure 7 XRD pattern of precipitated calcium fluoride

4 Conclusion

(1) The chemical precipitation and coagulation assisted coagulation method for fluorine-containing wastewater treatment test can reduce the fluoride ion mass concentration from 100 mg/L to 8 mg/L, and the removal rate is 92%. The optimum conditions are: n(Ca)/n(F) molar ratio 0.75, that is, the actual amount of calcium chloride is 1.5 times of the theoretical addition amount, the pH value is adjusted to 6-8, the reaction time is 40 min, and the fluoride ion removal rate is 85. %; Add 200 mg / L of polyaluminum chloride, adjust the pH value of 8, add 3 mL of 0.5% polyacrylamide, stir for 15 min, let stand for 60 min.

(2) Selecting the fluorine-containing ore dressing wastewater produced by pickling and purifying quartz ore in Huangshi City for industrial practice. After purification, the fluoride ion mass concentration can reach the national industrial wastewater first-level discharge standard, which is used to purify industrial fluorine-containing wastewater. Purification treatment is of great significance.

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October 28, 2024