Photovoltaic fluoride wastewater treatment technology
Fluorine-containing wastewater, fluorine-containing waste gas, and fluorine-containing waste residue are the "three wastes" containing fluorine discharged from industrial production, and are the main sources of fluorine pollution in the environment. Under normal circumstances, their mass concentration is above 10mg/L, which mainly involves semiconductor industry, non-ferrous metal smelting, fluorite ore, silicon electrical parts cleaning and so on. The common feature of these industries is that fluorine-containing minerals and hydrofluoric acid are the main raw materials or auxiliary raw materials, and in the process of smelting and production, fluorine-containing substances decompose and enter the environment, causing fluorine pollution. In recent years, a large amount of hydrofluoric acid is used in emerging photovoltaic enterprises, and the fluorine-containing wastewater discharged in industrial production often contains fluoride with a mass concentration of 10mg/L ~ 10000mg/L, seriously polluting the environment and endangering human health. Because of the strong activity of fluoride ion, it is one of the difficult substances to remove in industrial wastewater. And there may be other nonmetallic ions in the photovoltaic wastewater, so it becomes more difficult to remove fluorine. In order to protect the living environment of human beings, the defluorination process of fluorine-containing wastewater is an important task in the field of environmental protection at home and abroad, and it is also an environmental problem.
1, fluorine-containing wastewater treatment methods
The principle of treatment of fluorine-containing wastewater is: First, from the perspective of clean production, reduce pollutants, prevent sewage discharge, and then comprehensive recovery and utilization. The method of defluorination should be considered according to the quality, quantity, discharge standard, characteristics of treatment method, cost and recovery economic value of industrial wastewater.
Traditional industrial wastewater treatment methods are divided into physical treatment method, chemical treatment method, biochemical method and physical chemical treatment method according to the principle. The composition of fluoride wastewater in photovoltaic enterprises is complex and diverse, and there are a variety of treatment methods, commonly used mainly adsorption method, precipitation method, reverse osmosis method, ion exchange resin method, electric coagulation method, electrodialysis method and so on. In the photovoltaic wastewater treatment industry, the chemical precipitation method, coagulation precipitation method and adsorption method are often combined because of their strong practicability, and the treatment effect is better.
The basic mechanism of the adsorption method is ion exchange or surface reaction, which is a surface reaction based on the contact method. The fluorine in the fluorine-containing wastewater is exchanged with other ions or groups in the adsorbent by the equipment using the fluorine adsorbent, and is removed on the surface of the adsorbent, and the adsorbent is restored by continuous regeneration to restore the exchange ability. Therefore, the adsorption method is only suitable for tap water treatment with low fluoride content or deep defluorination treatment. However, it is difficult to popularize the adsorption method because of the easy loss of adsorption bed, the instability of adsorption capacity, the complexity of bed regeneration and the treatment of regenerated liquid.
Precipitation method is a method of separating suspended pollutants from wastewater by gravity sedimentation. Chemical precipitation method is to add a certain amount of chemical reagents to the fluorine-containing wastewater, so that it can generate fluoride precipitation with the fluorine in the wastewater or adsorb fluorine ions by co-precipitation, and then separate the sediment from the water by filtration or natural sedimentation, so as to achieve the purpose of fluoride removal.
At present, precipitation is the most widely used method in the defluorination process, which is suitable for the treatment of fluorine-containing wastewater with a mass concentration of more than 1000mg/L. If the wastewater contains relatively simple fluoride ions, add lime, adjust the pH value to 10-12, generate CaF2 precipitation, which can reduce the fluorine mass concentration to 10mg/L ~ 12mg/L. If the wastewater also contains other metal ions (such as Mg2+, Fe3+, Al3+, etc.), after adding hydrated lime, in addition to forming CaF2 precipitation, metal hydroxide is also formed. Due to the adsorption co-precipitation of the latter, the mass concentration of fluorine can be reduced to less than 8mg/L. If lime is added to pH=11 ~ 12, and aluminum sulfate or polyaluminum salt is added to pH=6 ~ 8, the formation of aluminum hydroxide can reduce the fluorine mass concentration to less than 5mg/L. In terms of the treatment experience of fluorine-containing wastewater, the mass concentration of fluoride ions in the treated water is 10mg/L ~ 15mg/L. In order to improve the efficiency of defluorination, the acidity of wastewater, the dosage of inorganic flocculant and organic coagulant aid can be adjusted, and excessive Ca(OH)2 can be added to achieve the purpose of deep defluorination.
Commonly used precipitating agents are quicklime, hydrated lime, calcium carbide slag, calcium carbonate, stone powder, soluble calcium salt and so on. With the exploration of the combined treatment method, the combined use of calcium salts such as hydrated lime and calcium chloride, combined with inorganic coagulants and organic coagulants, can more effectively reduce the concentration of fluoride ions, and the precipitation effect is good, which is a great progress of chemical precipitation method. Moreover, hydrated lime production is convenient, the price is relatively favorable compared with calcium chloride and other calcium salts, in the treatment process can both neutralize the acidity of wastewater and effectively defluorination, the treatment cost is relatively low, is a commonly used 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, it is difficult to reach the national level discharge standard, and there are shortcomings of slow settling of mud.
Coagulation sedimentation method is a method of defluorination by using F- in water to form complex precipitation with Al3+, Fe3+, Mg2+ and other cations. The coagulant selected is generally inorganic coagulant such as polyiron and polyaluminum, and organic coagulant can also be used. Including polyacrylamides and natural polymer compounds (such as cellulose, starch, lignin and other glycans and chitosan). Different coagulants have different defluorination effects due to their different mechanism of action.
The coagulation precipitation method can treat the wastewater with fluorine content above 1000mg/L, the equipment is simple and the operation is easy, but there is a large dosage of late dosing, too many metal ions and non-metal ions are introduced into the wastewater, which is not conducive to the comprehensive reuse of the latter section. And the effect of fluoride removal is unstable, resulting in more difficult to treat waste residue.
2. Treatment process of fluorine-containing wastewater of a company
The wastewater treatment process requires the mass concentration of drained fluoride (measured as F-) to be less than 5mg/L. From the principle of fluoride removal, the index of final effluent fluoride (measured by F-) of the traditional two-stage chemical precipitation method is 8mg/L ~ 10mg/L, which is difficult to meet the requirements of environmental protection discharge. A company used a combined three-stage defluorination process of calcium salt chemical precipitation + aluminum salt adsorption coagulation precipitation in the treatment of fluorine-containing wastewater, and recycled the sludge formed by secondary precipitation to the primary defluorination reaction tank for secondary defluorination, maximizing the removal of fluoride, achieving good treatment results and saving operating costs.
When Ca(OH)2 is added to fluorine-containing wastewater, with the increase of Ca2+ and F-solubility products in the solution, CaF2 precipitate will be produced when their solubility products exceed their standard Ksp.
Dosage of coagulants and coagulants: trivalent aluminum salts and anionic coagulants are added to the solution forming CaF2 precipitate. The polymer is formed by hydrolysis and polycondensation. The calcium fluoride precipitates produced by calcium salt neutralization are adsorbed by colloid to form coarse floc. When the amount is large enough to precipitate metal hydroxide or metal carbonate rapidly, the colloids and fine suspended substances in the water are caught by these sediments as crystal nuclei or adsorbent during formation, promoting the formation and growth of calcium fluoride precipitated particles, forming floss and then deposited at the bottom of the sedimentation tank and discharged to the sludge tank through the mud scraper.
Three-stage defluorination: add polyaluminum chloride and polyacrylamide in the third-stage defluorination section, and use the adsorption of polyaluminum chloride to further reduce the concentration of fluoride in water and ensure that the effluent fluoride meets the standard.
Sludge cycle: The treatment effect of only adding calcium hydroxide in the treatment is not ideal, mainly because it is difficult to generate crystal nuclei formed by induced precipitation. Adding the formed CaF2 precipitation as crystal seeds is conducive to the subsequent precipitation formation, which can not only save the cost of adding calcium salt, but also reduce the total hardness of water, and reduce the treatment cost for the comprehensive utilization of wastewater in the later stage. The sludge from the primary and secondary sedimentation tanks contained a large amount of unused calcium salt and CaF2 precipitation. According to the pH and fluoride concentration of the water quality, part of the calcium fluoride sludge from the sedimentation tanks was pumped back to the primary and secondary defluorination reaction tanks for reuse. Through sludge reflux, the amount of calcium hydroxide can be reduced by 10% ~ 15%, and the hardness of the discharge water of the tertiary sedimentation tank can be reduced by 20% ~ 30%.
3. Conclusion
Calcium salt precipitation method is used to treat fluorine-containing wastewater of photovoltaic enterprises, and the fluorine removal rate is high, and the discharge standard can be reached below 5mg/L. After the three-stage system is defluorinated, the sludge is returned, the effluent fluoride concentration is reduced, and the effluent fluoride ion is less than 5mg/L. Under the condition that the flocculant and coagulant aid are unchanged, the pH value of fluorine-containing wastewater, the influent F- concentration and the residence time will affect the defluorination effect.
For photovoltaic enterprises to optimize the treatment of fluorine-containing wastewater, reduce the cost of tons of water treatment, and improve the competitiveness of photovoltaic cells, in-depth research is needed to find out the process that both improves the effect of fluoride removal and energy saving is the next direction of efforts.
