Method for reducing sulfur content in photovoltaic wastewater defluorination sludge

Abstract

The application discloses a method for reducing the sulfur content in photovoltaic wastewater defluorination sludge, and the wastewater generated in the production process of photovoltaic cells is shunted in the following manner: high-sulfur mixed acid etching wastewater is neutralized with lime, and calcium sulfate is precipitated for resource utilization, and the wastewater enters an anaerobic denitrification tank; sulfide alkali washing denitration wastewater directly enters the anaerobic denitrification tank to provide a denitrification electron donor to promote heterotrophic and autotrophic synergistic denitrification; high ammonia-nitrogen wastewater enters the anaerobic denitrification tank after the ammonia-nitrogen is converted into nitrate-nitrogen through nitrification treatment; after defluorination, other wastewater (alkali washing wastewater, acid washing wastewater and the like) obtains high-quality calcium fluoride sludge, and the remaining wastewater is subjected to anaerobic denitrification in the anaerobic denitrification tank; after denitrification, the wastewater is sequentially subjected to solid-liquid separation in a sedimentation tank, COD degradation in an aerobic tank and solid-liquid separation in a sedimentation tank, and the treatment is completed. The application avoids the co-precipitation of calcium fluoride and calcium sulfate caused by the coexistence of fluorine and sulfur in the wastewater through shunting from the source.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology for the photovoltaic industry and high-fluoride wastewater treatment technology for the electronics industry. Background Technology

[0002] In recent years, the photovoltaic, electronics and semiconductor industries have needed to use large amounts of alkaline solutions, hydrofluoric acid and their mixtures with sulfates, nitric acid and hydrochloric acid to etch and clean elemental silicon materials, as well as to wash acid mist and exhaust gases in the workshop, resulting in the discharge of large amounts of wastewater with high fluoride content. For example, in the production of solar cells, the main processes that discharge wastewater include silicon wafer texturing and cleaning, diffusion junction formation, wet etching and oxidation, layer etching, and silicon nitride film formation. Specifically, this includes: monocrystalline silicon wafers first undergo a primary alkaline wash with a 1-2% sodium hydroxide solution to discharge strongly alkaline wastewater; pure water washing to discharge weakly alkaline wastewater; a secondary alkaline wash with a 2-3% sodium hydroxide solution to discharge strongly alkaline wastewater; a mixed acid wash with hydrochloric acid and hydrofluoric acid to discharge strongly acidic mixed acid wastewater; etching of monocrystalline silicon wafers with a mixed acid of sulfuric acid, hydrofluoric acid, and nitric acid to discharge highly sulfur-containing strongly acidic etching wastewater; pure water washing after etching to discharge weakly acidic cleaning wastewater; a third alkaline wash with approximately 5% sodium hydroxide solution to discharge strongly alkaline wastewater; and a second hydrofluoric acid wash to discharge highly fluorine-containing strongly acidic wastewater. In addition, there is wastewater from acid washing workshop, wastewater from washing silicon wafers with sodium sulfide solution, high ammonia nitrogen wastewater from washing waste gas in silicon nitride film deposition workshop, and wastewater from acid washing of unqualified silicon solar cells, graphite frame acid washing, and quartz boat acid washing.

[0003] Currently, the wastewater treatment method in the photovoltaic industry involves feeding acidic and alkaline fluoride-containing wastewater from various sections and workshops into a regulating tank, where the acidic and alkaline wastewaters neutralize each other. Then, calcium hydroxide and calcium chloride are added. Utilizing the low solubility of calcium fluoride, calcium ions precipitate the fluoride in the wastewater, reducing the residual fluoride concentration to below 8 mg / L, thus meeting the fluoride concentration limit for wastewater discharge. However, because the mixed wastewater contains very high levels of sulfate ions, the calcium sulfate becomes supersaturated during the calcium ion precipitant removal process, leading to the co-precipitation of calcium fluoride and calcium sulfate. This results in calcium fluoride sludge containing a large amount of calcium sulfate, excessively high sulfur content, and reduced calcium fluoride content, hindering the resource utilization of calcium fluoride sludge.

[0004] The main applications of calcium fluoride sludge include cement raw material additives, ceramic raw material additives, and metallurgical fluxes. Among these, its use as a metallurgical flux to replace natural fluorite ore is in high demand and has high value, making it a very promising application area for calcium fluoride sludge. Calcium fluoride can lower the melting point of refractory materials in steelmaking raw materials, improve slag fluidity, separate slag from molten metal, and desulfurize and dephosphorize. However, high-quality steelmaking fluxes require a calcium fluoride mass fraction of no less than 85% and a sulfur content of no more than 0.2% (YB / T5217-2019).

[0005] Wang Bainian et al. (Research on Desulfurization Process of Calcium Fluoride Sludge, Modern Chemical Industry, 2022) reported a method for desulfurizing calcium fluoride sludge in photovoltaic industry wastewater treatment. After removing calcium carbonate and calcium silicate by hydrochloric acid leaching, further treatment with sodium D-gluconate reduced the sulfur content in the sludge to below 0.1%. Ji Dongliang et al. (Nonferrous Metallurgy, 2023) studied the optimization and mechanism of impurity removal process for fluoride-containing sludge. A four-step treatment consisting of salinization, acidification, fluorination, and alkalization effectively removed impurities such as silicon, non-fluoride calcium, and heavy metals. The mass fraction of calcium fluoride in the fluoride-containing sludge after the four-step treatment could reach 85%, and the final yield could reach 72.53%. However, these methods for purifying calcium fluoride sludge with chemical agents are not only costly but also generate a large amount of waste liquid that needs to be treated, making them impractical. Summary of the Invention

[0006] To address the issues of low calcium fluoride content and high sulfur content in calcium fluoride sludge obtained from existing photovoltaic industry wastewater treatment methods, this invention, based on research into the generation and discharge characteristics of photovoltaic industry wastewater, wastewater treatment, fluoride recovery effects, and quality experiments, provides a method to reduce the sulfur content in defluorinated sludge from photovoltaic wastewater. By separating wastewater from different stages and workshops according to its quality and pre-treating it separately, fluoride and sulfur are separated at the source, thereby achieving high-efficiency recovery and high-quality resource utilization of fluoride in high-fluoride wastewater from industries such as photovoltaics and electronics.

[0007] To achieve its objectives, the present invention employs the following technical solution:

[0008] A method for reducing the sulfur content in fluoride removal sludge from photovoltaic wastewater is characterized by the following steps: The wastewater generated during photovoltaic cell production is diverted and pretreated in the following manner:

[0009] (1) High sulfur content etching wastewater: is the etching tank waste liquid for etching single crystal silicon wafers with a mixed acid of sulfuric acid, hydrofluoric acid and nitric acid;

[0010] Therefore, the pretreatment method for high sulfur content etching wastewater is as follows: first, neutralize it with lime to pH 6-10, remove sulfate ions by precipitation in the form of gypsum, separate calcium sulfate from solid and liquid and utilize it as a resource, and then mix the liquid with other wastewater after defluorination and enter the anaerobic-aerobic biological treatment system.

[0011] (2) Sodium sulfide denitrification washing wastewater: Wastewater generated from washing the exhaust gas in the silicon wafer etching workshop with sodium sulfide solution;

[0012] The pretreatment method for the sodium sulfide denitrification washing wastewater is as follows: it is directly added to the influent of the anaerobic-aerobic biological treatment system to promote autotrophic denitrification.

[0013] (3) High ammonia nitrogen wastewater: This refers to high ammonia nitrogen wastewater generated from the waste gas scrubbing process in the silicon nitride membrane deposition workshop;

[0014] The pretreatment method for the high ammonia nitrogen wastewater is as follows: after nitrification to convert ammonia nitrogen into nitrate nitrogen, it is mixed with other wastewater after fluoride removal and then enters the anaerobic-aerobic biological treatment system.

[0015] (4) Other wastewater: including silicon wafer alkaline washing wastewater, acid washing wastewater, texturing cleaning wastewater, etching cleaning wastewater, rework cleaning wastewater of unqualified battery cell semi-finished products, acid washing wastewater of graphite frame and quartz boat, and alkaline washing wastewater of acid washing workshop exhaust gas.

[0016] The pretreatment method for the other wastewater is as follows: First, lime and calcium chloride are used to precipitate and remove fluoride. Solid-liquid separation is performed to obtain high-quality calcium fluoride sludge, which is then utilized at a high value. The remaining liquid is the other wastewater after fluoride removal.

[0017] (5) Integrated wastewater biochemical treatment: Other wastewater after fluoride removal is mixed with pretreated high-sulfur etching wastewater and high-ammonia nitrogen wastewater and then enters the anaerobic-aerobic biochemical treatment system. The sulfide denitrification washing wastewater is directly added to the anaerobic-aerobic biochemical treatment system. First, it undergoes heterotrophic and autotrophic synergistic denitrification in the anaerobic denitrification tank to remove nitrogen. Then, it undergoes COD biodegradation in the aerobic tank and solid-liquid separation in the sedimentation tank. The effluent meets the wastewater discharge standards of the photovoltaic industry.

[0018] The technical principles and beneficial effects of this invention are reflected in:

[0019] (1) Strict separation of high-sulfur wastewater and high-fluoride wastewater at the source avoids the co-precipitation of calcium fluoride and calcium sulfate caused by the coexistence of fluoride and sulfur in the wastewater. The sulfide denitrification washing wastewater is directly added to the denitrification tank to promote autotrophic denitrification, bypassing the wastewater defluorination system and avoiding the mixing of this part of high-sulfur wastewater with high-fluoride wastewater. The high-sulfur mixed acid etching wastewater is separately neutralized with lime and pretreated for desulfurization. Sulfate is removed by precipitation in the form of gypsum. The solid after solid-liquid separation and washing is high-whiteness chemical gypsum, containing a small amount of calcium fluoride, which can be used as building material raw material, acid-resistant plastic filler, etc. to achieve resource utilization. The wastewater after desulfurization retains nitrate ions from the original wastewater. It is mixed with other wastewater after defluorination and enters the anaerobic-aerobic biological treatment system for denitrification through heterotrophic-autotrophic synergistic denitrification.

[0020] (2) The sodium sulfide denitrification washing wastewater is generated during the washing of waste gas from the mixed acid etching section using sodium sulfide solution. The mixed acid wet etching tank produces waste gas containing a large amount of nitrogen oxides and a small amount of hydrofluoric acid. This waste gas is washed and washed with alkaline sodium sulfide solution to absorb the nitrogen oxides and hydrofluoric acid. During the circulating spraying process, sulfides reduce some nitrogen oxides to nitrogen gas, and absorb some nitrogen oxides to form nitrates. During the spraying process, sulfur ions react with oxygen in the air to form elemental sulfur, sulfites, thiosulfates, and sulfates. Solid elemental sulfur reacts with dissolved sulfides to form dissolved polysulfides (S...). n 2-The investigation and analysis of this invention revealed that the silicon wafer etching waste gas from alkaline sulfide denitrification washing contains nitrates (15,000-25,000 mg / L), sulfates (30,000-40,000 mg / L), and COD (reduced sulfur) of 80,000-100,000 mg / L. The reduced sulfur exists in various forms, including polysulfides, sulfides, thiosulfates, and sulfites, with polysulfides being the predominant species. When the alkaline alkaline sulfide denitrification washing wastewater is mixed with acidic fluoride-containing wastewater, the polysulfides undergo transformation, releasing hydrogen sulfide and elemental sulfur solids. This elemental sulfur mixes into the calcium fluoride sludge formed during the defluorination process, contributing to the high sulfur content of the calcium fluoride sludge. This invention discovers and solves this problem through experimental research by directly adding sodium sulfide denitrification washing wastewater to the denitrification tank. This avoids the problems caused by the mixing of fluoride and sulfur, and provides a large number of electron donors for wastewater denitrification, reducing the carbon source requirements for denitrification. A 5GW silicon cell manufacturing plant consumes approximately 2,800 tons of 68% nitric acid annually, most of which is denitrified through heterotrophic denitrification in the water treatment process, consuming approximately 5,000 tons of composite carbon source annually. Based on experimental results, calculations show that using reduced sulfur in sodium sulfide denitrification washing wastewater as an electron donor for denitrification can save 50-80% of organic carbon source, significantly reducing water treatment costs and greenhouse gas emissions. Attached Figure Description

[0021] Figure 1 A schematic diagram of the wastewater diversion and treatment process in photovoltaic silicon cell production. Detailed Implementation

[0022] The embodiments of the present invention are described in detail below. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0023] Example 1

[0024] The main wastewater from photovoltaic silicon cell production includes: 1-2% sodium hydroxide solution primary washing wastewater W1-1; pure water rinsing wastewater W2; 2-3% sodium hydroxide solution secondary alkaline washing wastewater W1-2; hydrochloric acid-hydrofluoric acid mixed acid rinsing wastewater W3-1; nitric acid-sulfuric acid-hydrofluoric acid mixed acid etching wastewater W3-2; pure water rinsing wastewater after etching W4; third sodium hydroxide solution alkaline washing wastewater W1-3; second hydrofluoric acid acid rinsing wastewater W3-3; nitric acid-sulfuric acid-hydrofluoric acid mixed acid etching workshop exhaust gas washing wastewater (i.e., wastewater generated from washing silicon wafer etching workshop exhaust gas with sodium sulfide solution) W1-4; acid washing workshop exhaust gas alkaline washing wastewater W5; high ammonia nitrogen wastewater generated from silicon nitride film deposition workshop exhaust gas washing W6; and wastewater from reprocessing unqualified silicon cells, graphite frame acid washing, and quartz boat acid washing W7.

[0025] First, the wastewater from photovoltaic silicon cell production should be strictly separated and pretreated separately according to the following method (see...). Figure 1 The wastewater is divided into four parts: high sulfur mixed acid etching wastewater (W3-2); wastewater generated from washing silicon wafer etching workshop exhaust gas with sodium sulfide solution (W1-4); high ammonia nitrogen wastewater (W6); and other wastewater (alkali washing wastewater, acid washing wastewater, W1-1, W1-2, W1-3, W2, W3-1, W3-3, W4, W5, W7).

[0026] High-sulfur etching wastewater W3-2: Pre-treated separately, it first enters an equalization tank to regulate flow, then proceeds to a lime neutralization and sedimentation tank. The pH in the neutralization tank is controlled at 7-9 to remove sulfate ions through precipitation. The wastewater is then filtered through a plate and frame filter press and washed once with water to obtain a solid calcium sulfate filter cake with a moisture content of 43%. The filter cake is dried, dehydrated, and pulverized at 150-500℃. The product is used as an acid-resistant plastic packing material. Wastewater discharged from the sludge dewatering equipment and washing drainage, mixed with other wastewater after fluoride removal, enters the anaerobic denitrification tank.

[0027] High ammonia nitrogen wastewater: First, it enters the equalization tank to adjust the flow rate, and then enters the nitrification tank for separate treatment. It relies on nitrifying bacteria in the tank to convert ammonia nitrogen into nitrate nitrogen. The effluent is mixed with other wastewater after fluoride removal and then enters the anaerobic denitrification tank.

[0028] Other wastewater (including W1-1, W1-2, W1-3, W2, W3-1, W3-3, W4, W5, and W7): First, it enters the equalization tank to regulate the flow rate and neutralize the acidic and alkaline wastewater. Then, it enters the defluoridation tank, where lime and calcium chloride are added to precipitate and remove fluoride. The amount of lime used is controlled by monitoring the pH of the effluent to around 7. The concentration of dissolved calcium ions in the defluoridated effluent is greater than 150 mg / L, and the amount of calcium chloride used is controlled accordingly.

[0029] The effluent from the defluorination tank enters sedimentation tank A. The settled sludge is dewatered by plate and frame filter press to obtain calcium fluoride sludge with a moisture content of approximately 50%. After drying, high-quality calcium fluoride product is obtained, with a calcium fluoride content of 87% and a sulfur content of <0.1%, meeting the quality requirements for steel smelting additives. The effluent from sedimentation tank A and the effluent from the calcium fluoride sludge plate and frame filter press are mixed with the pretreated wastewater and then enter the anaerobic denitrification tank.

[0030] After pretreatment by sorting and separation of wastewater from the above-mentioned sections, all wastewater enters the anaerobic denitrification tank. The sodium sulfide denitrification washing wastewater (W1-4) is directly added to the anaerobic denitrification tank, where the reduced sulfur provides the electron donors required for the denitrification reaction; any shortfall is supplemented by the addition of a composite organic carbon source. In the anaerobic denitrification tank, denitrification is achieved through the synergistic action of heterotrophic and sulfur-autotrophic microorganisms. The effluent undergoes solid-liquid separation in sedimentation tank B, with the settled sludge returned to the inlet of the anaerobic denitrification tank. The effluent then enters the aerobic biological treatment tank for COD degradation and sedimentation tank C for solid-liquid separation. The effluent's TN, F, TP, and COD levels meet the wastewater discharge standards for the photovoltaic industry. Part of the sludge discharged from sedimentation tank C is returned to the front end of the aerobic tank to maintain the sludge concentration and increase the rate of aerobic organic matter degradation. The remaining sludge is dewatered by plate and frame filter press and transported as excess sludge. The filter press effluent is returned to the inlet of the anaerobic denitrification tank.

[0031] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing the sulfur content in fluoride removal sludge from photovoltaic wastewater, characterized in that, Wastewater generated during photovoltaic cell production is diverted and pretreated as follows: (1) High sulfur content etching wastewater: is the etching tank waste liquid used for etching single crystal silicon wafers with a mixed acid of sulfuric acid, hydrofluoric acid and nitric acid; The pretreatment method for the high-sulfur etching wastewater is as follows: first, it is neutralized to pH 6-10 with lime, then sulfate is removed by precipitation in the form of gypsum, calcium sulfate is separated from the solid and liquid and utilized as a resource, and the liquid is mixed with other wastewater after defluorination and then enters the anaerobic-aerobic biological treatment system. (2) Sodium sulfide denitrification washing wastewater: Wastewater generated from washing silicon wafer etching workshop exhaust gas with sodium sulfide solution; The pretreatment method for the sodium sulfide denitrification washing wastewater is as follows: it is directly added to the influent of the anaerobic-aerobic biological treatment system to promote autotrophic denitrification. (3) High ammonia nitrogen wastewater: This refers to the high ammonia nitrogen wastewater generated from the waste gas scrubbing process in the silicon nitride membrane deposition workshop; The pretreatment method for the high ammonia nitrogen wastewater is as follows: after nitrification to convert ammonia nitrogen into nitrate nitrogen, it is mixed with other wastewater after fluoride removal and then enters the anaerobic-aerobic biological treatment system. (4) Other wastewater: including silicon wafer alkaline washing wastewater, acid washing wastewater, texturing cleaning wastewater, etching cleaning wastewater, rework cleaning wastewater of unqualified battery cell semi-finished products, graphite frame and quartz boat acid washing wastewater, acid washing wastewater of acid washing workshop exhaust gas; The pretreatment method for the other wastewater is as follows: First, lime and calcium chloride are used to precipitate and remove fluoride. Solid-liquid separation is performed to obtain high-quality calcium fluoride sludge, which is then utilized at a high value. The remaining liquid is the other wastewater after fluoride removal. (5) Comprehensive wastewater biochemical treatment: Other wastewater after fluoride removal is mixed with pretreated high sulfur etching wastewater and high ammonia nitrogen wastewater and then enters the anaerobic-aerobic biochemical treatment system. The sulfide denitrification washing wastewater is directly added to the anaerobic-aerobic biochemical treatment system. First, it undergoes heterotrophic and autotrophic synergistic denitrification in the anaerobic denitrification tank to remove nitrogen. Then, it undergoes COD biodegradation in the aerobic tank and solid-liquid separation in the sedimentation tank. The effluent meets the wastewater discharge standards of the photovoltaic industry.

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