Optimization of fluoride wastewater treatment process

I Fluorine containing wastewater treatment method

one Treatment method of fluoride containing wastewater

The principle of fluoride wastewater treatment is: first, from the perspective of cleaner production, reduce pollutants, prevent sewage discharge, and then comprehensive recovery and utilization. What kind of method should be used to remove fluoride should be comprehensively considered according to the quality, quantity, discharge standard of industrial wastewater and the characteristics, cost and recovery economic value of the treatment method.

Traditional industrial wastewater treatment methods can be divided into physical treatment, chemical treatment, biochemical treatment and physical chemical treatment according to the principle. Fluorine containing wastewater from photovoltaic enterprises has complex and diverse components, and there are also many treatment methods, including adsorption, sedimentation, reverse osmosis, ion exchange resin, electrocoagulation, electrodialysis, etc. In the photovoltaic wastewater treatment industry, chemical precipitation method, coagulation precipitation method and adsorption method are often used together because of their strong practicability, and the treatment effect is better.

（ one ） Adsorption method Its basic mechanism is ion exchange or surface reaction, which is a surface reaction based on the contact method. The equipment using fluoride adsorbent will exchange the fluoride in the fluoride containing wastewater with other ions or groups in the adsorbent and leave it on the adsorbent surface to be removed. The adsorbent will recover its exchange capacity through continuous regeneration.

Therefore, adsorption method is only applicable to tap water treatment with low fluoride content or deep defluorination treatment. However, the adsorption method is difficult to be popularized and used due to the problems of easy loss of adsorption bed, unstable adsorption capacity, complex bed regeneration and regeneration liquid treatment.

（ two ） Precipitation method It is a method of separating suspended pollutants in wastewater by gravity sedimentation. The chemical precipitation method is to add a certain amount of chemical reagent to the fluoride containing waste water to make it generate fluoride precipitation with the fluoride in the waste water or absorb fluoride ions by coprecipitation, and then separate the sediment from the water by filtration or natural sedimentation to achieve the purpose of fluoride removal.

At present, precipitation method is the most widely used method in fluoride removal process , applicable to treatment of mass concentration at Fluorine containing wastewater above 1000mg/L.

If the wastewater contains relatively simple fluoride ion, add lime to adjust PH value to 10? 12. CaF2 precipitation is generated, which can reduce the mass concentration of fluoride to 10mg/L? 12mg/L。 If the wastewater also contains other metal ions (such as Mg ²⁺ 、 Fe ³⁺ 、 Al ³⁺ Etc.), after adding hydrated lime In addition to CaF2 precipitation, metal hydroxide is also formed. Due to the adsorption and coprecipitation of the latter, the mass concentration of fluoride can be reduced to below 8mg/L. If lime is added to pH=11? 12. Add aluminum sulfate or polyaluminium salt to make pH=6? 8, the formation of aluminum hydroxide can reduce the fluorine content to below 5mg/L.

According to the experience of fluoride wastewater treatment, the mass concentration of fluoride ion in the treated water is 10mg/L? 15mg/L。 In order to improve the efficiency of fluoride removal, the acidity of wastewater and the dosage of inorganic flocculant and organic coagulant aid can be adjusted, and excess Ca (OH) 2 can be added to achieve the purpose of deep fluoride removal.

Common precipitants include quicklime, hydrated lime, carbide slag, calcium carbonate, stone powder, soluble calcium salt, etc.

With the exploration of combined treatment methods, the combined use of hydrated lime, calcium chloride and other calcium salts, together with inorganic coagulants and organic coagulants aids, can more effectively reduce the concentration of fluoride ions, and the precipitation effect is good, which is a major progress of chemical precipitation method. Moreover, the production of hydrated lime is convenient, and the price is preferential compared with calcium chloride and other calcium salts. In the treatment process, it can neutralize the acidity of wastewater and effectively remove fluoride, and the treatment cost is relatively low. It is a common treatment method for photovoltaic enterprises.

Although the chemical precipitation method is simple and the treatment cost is low, it has the problem of secondary pollution, which is difficult to reach the national first level discharge standard, and has the disadvantage of slow sludge sedimentation.

Coagulation sedimentation method uses water F - and Al ³⁺ 、 Fe ³⁺ 、 Mg ²⁺ It is a method of defluoridation by the formation of complex precipitation with equal cations. The selected coagulants are generally inorganic coagulants such as polyferric and polyaluminum, or organic coagulants, including polyacrylamide and natural macromolecular compounds (such as cellulose, starch, lignin and other polysaccharides and chitosan). Different coagulants have different defluoridation effects due to their different action mechanisms.

Coagulation sedimentation method can treat the fluoride content in For wastewater above 1000mg/L, the equipment is simple and the operation is easy, but there is a large dosage in the later stage, and too many metal ions and non-metallic ions are introduced into the wastewater, which is not conducive to the comprehensive reuse of the later stage. Moreover, the fluoride removal effect is not stable, resulting in more difficult to treat waste residue.

II Optimization of fluoride wastewater treatment process

Wastewater treatment process requires fluoride drainage The mass concentration (calculated by F -) is less than 5mg/L. According to the principle of fluoride removal, the index of fluoride in the final effluent (calculated by F -) of the traditional two-stage chemical precipitation method is 8mg/L? 10mg/L, it is difficult to meet the environmental discharge requirements.

A company uses calcium salt chemical precipitation in the treatment of fluoride containing wastewater +The combined three-stage defluorination process of aluminum salt adsorption, coagulation and sedimentation, and the sludge formed from the secondary sedimentation is circulated to the primary defluorination reaction tank for secondary defluorination to remove fluoride to the maximum extent, achieving good treatment effect and saving operating costs.

Add in fluoride containing wastewater Ca (OH) 2, with the increase of Ca2+and F-solubility product in the solution, when their solubility product exceeds its standard Ksp, CaF2 precipitation will be produced.

Dosing of coagulant and coagulant aid: In the formation of Add trivalent aluminum salt and anionic coagulant aid to the solution precipitated by CaF2. After hydrolysis and polycondensation, high molecular polymer is formed, and calcium fluoride precipitate generated by neutralization of calcium salt is adsorbed by colloid to form coarse floc.

When the dosage is large enough to precipitate metal hydroxides or metal carbonates rapidly, the colloid and fine suspended solids in the water are netted by these precipitates as crystal nuclei or adsorbates when they are formed, promoting the formation and growth of calcium fluoride precipitation particles, forming flocculent flowers to precipitate at the bottom of the sedimentation tank, and then discharged to the sludge tank through the mud scraper.

Three stage defluorination: Add polyaluminum chloride and polyacrylamide in the third stage defluorination section, and further reduce the concentration of fluoride in the water by the adsorption of polyaluminum chloride to ensure that the fluoride in the effluent reaches the standard.

Sludge circulation: The treatment effect of only adding calcium hydroxide in the treatment is not ideal, mainly because the crystal nucleus formed by induced precipitation is difficult to form, and the formed CaF2 precipitation is added as crystal seed, which is conducive to the formation of subsequent precipitation. It can not only save the cost of calcium salt addition, but also reduce the total hardness of water, reducing the treatment cost for the later comprehensive utilization of wastewater.

1、 The sludge formed in the secondary sedimentation tank contains a large amount of unused calcium salts and CaF2 sedimentation: according to the water pH and fluoride concentration, part of the calcium fluoride sludge in the sedimentation tank is pumped back to the primary and secondary defluorination reaction tanks for reuse.

Reducing the use of calcium hydroxide through sludge reflux 10%? 15%, the hardness of outlet water of tertiary sedimentation tank decreases by 20%? 30%。