Fluoride is an important trace element in the human body and an indispensable component of teeth and bones. However, excessive intake can cause symptoms of fluorosis and dental fluorosis.
There are various treatment methods for fluorine-containing wastewater, and the commonly used methods are roughly classified into two types, namely, a precipitation method and an adsorption method. At present, for high-concentration fluorine-containing industrial wastewater, a calcium salt precipitation method is generally employed, that is, lime milk is added to waste water to cause precipitation of fluoride ions and calcium ions to form CaF2. However, after the treatment, the water discharge is difficult to reach the standard, the sludge is slowly settled and the dehydration is difficult. The flocculation sedimentation method and the adsorption method are mainly used for low-concentration fluorine-containing wastewater. For high-concentration fluorine-containing wastewater, in order to ensure the quality of the effluent, it is often necessary to carry out two-step treatment, first using lime to precipitate, so that the fluorine content is reduced to 20-30 mg·L-1, and then the adsorbent is used to reduce the fluorine content to 10 mg. · L-1 or less.
Based on the precipitation of calcium salt and the various factors such as the coagulation and precipitation of different aluminum salts and the type of alkali, the paper conducts a small test on the fluorine-containing wastewater of a chemical plant and finds that NaOH is used. The pH of the wastewater was adjusted, and CaCl2 was used as a precipitation reaction agent to assist the coagulation and precipitation of PAC. The fluoride ion concentration of the effluent was less than 4 mg·L-1, which reached the discharge standard and the effect was stable.
1 test part
1.1 Reagents and instruments
JJ-4 six-coupled electric stirrer, PHS-25 pH meter, PXS-270 ion activity meter, E-201-C type pH electrode, PF-1 type fluorine electrode, 217 type double salt bridged calomel electrode.
Ca (OH) 2 was formulated into a 10% emulsion, and CaCl 2 , PAC, and Al 2 (SO 4 ) 3 were formulated into a 10% solution. NaF (analytical grade) is dried at 105 ° C ~ l10 ° C for 2 hours and then stored in a desiccator to prepare the desired concentration of fluorine-containing aqueous solution for calibration of the fluoride ion electrode. The wastewater used in the test is a fluorine-containing industrial wastewater from a chemical plant in Fujian. The chemical plant is a fluorine chemical company that integrates fluorite mining, processing, and fluoride production and sales. The main products are fluoride salts such as hydrogen fluoride, ammonium hydrogen fluoride and ammonium fluoride. .
1.2 Test methods
Take a certain amount of fluorine-containing wastewater, the fluoride ion concentration is 975 ~ 1094mg · L-1, the pH value is 2.95 ~ 3.23, the test is carried out by the following method:
The pH value was adjusted to neutral or alkaline with Ca(OH)2, and the reaction was carried out for 1 hour. The coagulant such as PAC or Al2(SO4)3 was added for 10 min, and the fluoride ion concentration of the supernatant was determined after 2 h of precipitation.
The pH was adjusted to neutral or alkaline with NaOH, CaCl2 was added for 1 h, PAC was added as a coagulant for 10 min, and the fluoride ion concentration of the supernatant was determined after 2 h of precipitation.
2 Results and discussion
2.1 Effect of calcium ion concentration on fluoride ion removal
The lime precipitation treatment process has low operating cost and is currently the most used treatment method. The pH value of the wastewater was adjusted by adding Ca(OH)2, and CaF2 precipitated with calcium ions and fluoride ions. After 1 h of reaction, PAC was added as a coagulant, the concentration was 400 mg·L-1, and the reaction was carried out for 10 min. , the supernatant fluoride concentration was measured, and the experimental results are shown in the following table:
The electrostatic attraction between fluoride ion and calcium ion is strong, the lattice energy is high, and the solubility of calcium fluoride is small. Its solubility product is Ksp = 4 × 10 -11 (25 ° C).
From the reaction equation, the greater the concentration of calcium ions, the smaller the concentration of fluoride ions in the solution. The test results are consistent with the theoretical analysis. As the calcium ion concentration increases, the concentration of fluoride ions in the wastewater decreases. However, when adding lime milk, even if the amount of the wastewater reaches 12, the fluoride ion concentration in the wastewater can be reduced to about 15 mg/L, and the suspended matter content in the water is high.
2.2 Effect of different coagulants on fluoride ion concentration
When Ca(OH)2 is used alone as a chemical precipitant, the CaF2 particles formed are fine and difficult to precipitate, and it is considered to add a coagulating precipitant to assist in the precipitation of CaF2. The flocculating agent commonly used in the flocculation precipitation method of fluoride ion wastewater is aluminum salt. After the aluminum salt is added to water, the complexation of Al3+ with F- and the hydrolysis of the aluminum salt intermediate and the finally formed Al(OH)3(am) quinone are used for ligand exchange, physical adsorption and sweeping of fluoride ions. Remove fluoride ions from the water. In this experiment, Ca(OH)2 was first added to the wastewater as a chemical precipitant. After 1 h of reaction, PAC and Al2(SO4)3 were added as a coagulant. The concentration was 400 mg·L-1, and the reaction was carried out for 10 min. The precipitate was precipitated for 2 h, and the fluoride ion concentration of the supernatant was measured. The experimental results are as follows:
It can be seen from Table 2 that Al2(SO4)3 acts as a coagulant, and the removal effect of fluoride ions is superior to that of PAC even under the condition that the amount of Ca2+ is small. Studies have shown that ion adsorption is an important mode of action during the flocculation and precipitation of fluoride by PAC. When the concentration of SO42-, Cl- and other anions in water is high, it will form during flocculation due to competition. The adsorption capacity of Al(OH)3(am) strontium for fluoride ions is significantly reduced. In addition, F- can form a total of 6 complexes from AlF2+, AlF2+, AlF3 to AlF63- with Al3+, etc. These aluminum fluoride complex ions form aluminum fluoride complex (AlFx(OH)(3-) during flocculation. x) and Na(x-3)AlFx) or inclusions settle in the newly formed Al(OH)3(am) flocs.
On this basis, the effect of the concentration of Al2(SO4)3 on the removal of fluoride ions was investigated. The experimental results are shown in Figure 1.
In this test, the dosage of Al2(SO4)3 was increased, and the concentration of fluoride ions in the effluent was lowered. When the concentration of Al2(SO4)3 was 400mg·L-1, the fluoride ion concentration of the effluent reached 11.4mg·L-1, which was higher than the corresponding data in Table 2. The removal effect of fluoride ion in the flocculation method of aluminum salt is affected by the operation factors such as stirring conditions and sedimentation time and the anions such as SO42- and Cl- in water, and the effluent water quality is not stable enough.
2.3 Adjusting pH with NaOH CaCl2 as a chemical precipitant on fluoride ion
The wastewater was adjusted to pH neutral or alkaline with 25% NaOH. After adding CaCl2 (2240 mg·L-1) for 1 hour, PAC was added as a coagulant, and the concentration was 400 mg·L-1. After 10 minutes of reaction, the reaction was carried out for 10 minutes. The precipitate was precipitated for 2 h, and the fluoride ion concentration of the supernatant was measured. The experimental results are shown in Table 3:
With CaCl2 as the chemical precipitant, the fluoride ion concentration in the effluent is less than 4mg·L-1, which is much smaller than the 10mg·L-1 required in the discharge standard, and also less than the solubility of calcium fluoride 8.9mg·L-1, and the effect is stable. This is because when the water contains a soluble calcium salt such as calcium chloride or calcium sulfate, the solubility of calcium fluoride is lowered by the same ion effect, and the concentration of fluoride ions in the effluent is greatly lowered.
3 Summary and conclusion
Through the small test of fluorine-containing wastewater from a chemical plant in Fujian, the following conclusions were drawn:
3.1 As the concentration of calcium ions increases, the concentration of fluoride ions in the wastewater decreases.
3.2 When Ca(OH)2 is used as a chemical precipitant, the addition of Al2(SO4)3 as a coagulant is better than the addition of PAC as a coagulant. As the dosage of Al2(SO4)3 increases, the removal efficiency of fluoride ions increases. However, the removal of fluoride ions from wastewater by aluminum salts is not stable.
3.3 Adjust the pH value of wastewater with NaOH, use CaCl2 as the precipitation reagent and assist the coagulation and sedimentation of PAC. The fluoride ion concentration of effluent is less than 4mg·L-1, and the emission standard is achieved, and the effect is stable.
In engineering practice, Ca(OH)2 is hardly soluble in water and is mostly added in the form of an emulsion. Since the fluoride ion concentration of the effluent decreases with the increase of the calcium ion concentration, Ca(OH)2 is used as the calcium salt to ensure the water discharge effect, and the Ca(OH)2 dosage is required to be large, because the CaF2 precipitate produced is wrapped in Ca ( The surface of the OH) 2 particles is not fully utilized, so the amount is further increased and the pH of the effluent is adjusted back. In addition, during the addition of Ca(OH)2 emulsion, the solvent-dissolving process is difficult to operate, the pipeline is easily blocked, and maintenance is frequent. The pH value of the wastewater is adjusted by NaOH, and CaCl2 is used as the calcium salt. The solubility is large, the dissolution and the addition are convenient, the operation is convenient, the equipment investment is small, and the power consumption is small. At the same time, the same ion effect produced by CaCl2 effectively reduces the fluoride ion concentration of the effluent and stabilizes the effluent effect.
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