Resourceful treatment process for sodium hydroxide wastewater in photovoltaic industry

Abstract

The invention belongs to the technical field of photovoltaic industry sodium hydroxide wastewater treatment, and particularly relates to a photovoltaic industry sodium hydroxide wastewater resourceful treatment process, which comprises silicon removal and alkali extraction, ultrafiltration SS removal, softening calcium removal, nanofiltration silicon removal, electrodialysis concentration, MVR evaporation concentration, electrocatalytic oxidation COD removal, activated carbon adsorption of trace COD, and seawater desalination membrane reclaimed water reuse. NaOH in wastewater is concentrated and recycled through calcium precipitation-nanofiltration combination, the purchase cost of new alkali is reduced, TDS in water is reduced, the water recycling cost is reduced, electrocatalytic oxidation and activated carbon adsorption are combined, the problem that COD in a high-PH environment cannot be removed through a traditional biochemical method is solved, meanwhile, the content of silicon dioxide in outlet water is smaller than 5 mg / L through a nanofiltration membrane, the scaling risk of recycled water is avoided, and in addition, through ultrafiltration and softening resin pretreatment, the recycling rate of the recycled water is greatly increased. The service life of the RO membrane can be prolonged and the maintenance cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of sodium hydroxide wastewater treatment technology in the photovoltaic industry, specifically to a resource-based treatment process for sodium hydroxide wastewater in the photovoltaic industry. Background Technology

[0002] The photovoltaic industry (especially in the production of polycrystalline silicon cells) generates a large amount of sodium hydroxide wastewater during processes such as texturing and cleaning. This wastewater is divided into high-concentration and low-concentration sodium hydroxide wastewater, and has the following characteristics: High alkalinity and high silicon content: High-concentration sodium hydroxide wastewater often has a pH value of 12-14, a sodium hydroxide content of about 5000 mg / L, and contains about 4000 mg / L of dissolved silicon (silicon dioxide). Traditional neutralization treatment will generate silica gel, which will block the pipes and affect subsequent processes. Complex pollutants: Containing COD (from additives), existing processes struggle to balance resource recovery and COD removal; Resource waste: The concentration of sodium hydroxide in wastewater can reach 0.5%. Direct neutralization wastes alkali resources and causes excessive salt content in the water, which affects the municipal sewage system and the ecological environment. The shortcomings of existing processing technologies: The existing treatment process involves neutralization followed by biochemical treatment. Chemical neutralization method: sulfuric acid is added for neutralization, but NaOH is not recovered, resulting in a large amount of salt in the wastewater, which affects the biological system and the ecological environment; Biological treatment: Biological systems remove COD, but due to the high salt content in the water, the biological efficiency and equipment / pipeline lifespan are affected. In subsequent water reuse, reverse osmosis (RO) alone is prone to membrane fouling due to silicon and calcium-magnesium scaling, which shortens the lifespan by more than 50%. Insufficient separation of processing: Existing processes (such as patent CN202621013U) only focus on NaOH recycling and have not achieved the synergistic resource utilization of silicon, water and alkali; Therefore, there is an urgent need to provide a resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry. Summary of the Invention

[0003] The purpose of this section is to outline some aspects of the embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0004] Therefore, the purpose of this invention is to provide a resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry. By using a combination of calcium precipitation and nanofiltration, the NaOH in the wastewater is concentrated and reused, reducing the cost of purchasing new alkali. Electrocatalytic oxidation and activated carbon adsorption solve the problem that traditional biochemical methods cannot remove COD under high pH conditions. At the same time, the nanofiltration membrane ensures that the silica in the effluent is <3mg / L, avoiding the risk of scaling in the reused water. Furthermore, ultrafiltration and softening resin pretreatment can extend the life of the RO membrane and reduce maintenance costs.

[0005] To address the aforementioned technical problems, according to one aspect of the present invention, the present invention provides the following technical solution: A resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry includes the following steps: (1) Desiliconization and alkali removal: Add calcium-based precipitant to wastewater, control the dosage, and calcium silicate precipitate is generated after reaction. After solid-liquid separation, a low-silicon, high-alkali solution is obtained. (2) Ultrafiltration to remove SS: The alkaline solution obtained in step (1) is passed through an ultrafiltration membrane system to retain suspended solids with a particle size > 0.1 μm, and the permeate enters the softening unit; (3) Softening and calcium removal: Ion exchange resin is used to soften and remove residual calcium in the water. , The hardness decreased to <50mg / L; (4) Nanofiltration for silicon removal: The remaining dissolved silicon is separated by nanofiltration membrane, and silicates with a molecular weight >200 Da are retained, and the silicon content in the produced water is <10 mg / L; (5) Electrocatalytic oxidation: Under the action of titanium-coated electrodes, a voltage of 3-5V is applied to degrade organic matter in wastewater, with an oxidation time of 30-60min; (6) Activated carbon adsorption: The effluent treated by electrocatalysis is passed through an activated carbon bed to further adsorb residual COD to <30mg / L; (7) Seawater desalination membrane water reuse: The purified water is desalinated and concentrated using reverse osmosis or electrodialysis membranes, the produced water is reused, and the concentrated water is discharged or used for salt distillation.

[0006] As a preferred embodiment of the resource-based treatment process for sodium hydroxide wastewater in the photovoltaic industry described in this invention, wherein: the calcium-based precipitant in step (1) is... The dosage is 1-1.2 times the silicon molar ratio, and the reaction temperature is 25-60℃.

[0007] As a preferred embodiment of the resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry described in this invention, in step (3), the ion exchange resin is a sodium-type cation exchange resin, and the regeneration uses 5-10%... Solution.

[0008] As a preferred embodiment of the resource-based treatment process for sodium hydroxide wastewater in the photovoltaic industry described in this invention, wherein: the nanofiltration membrane in step (4) is a polyamide composite membrane, with an operating pressure of 0.8-1.5 MPa, for... Retention rate ≥ 95%.

[0009] As a preferred embodiment of the resource-based treatment process for sodium hydroxide wastewater in the photovoltaic industry described in this invention, the activated carbon in step (6) is steam-activated coal-based activated carbon made from anthracite coal with a particle size of 1-3 mm and an adsorption contact time of ≥20 min.

[0010] As a preferred embodiment of the resource-based treatment process for sodium hydroxide wastewater in the photovoltaic industry described in this invention, wherein: the reverse osmosis membrane in step (7) is a spiral wound aromatic polyamide membrane with a recovery rate ≥70%, and the concentrated water TDS >50g / L is returned to step (1) for recycling.

[0011] According to any one of the above-mentioned photovoltaic industry sodium hydroxide wastewater resource utilization processes, the final product water has a COD of <30mg / L, SiO2 of <3mg / L, and hardness of <5mg / L, and can be reused as raw water for pure water preparation in photovoltaic production.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: The NaOH in wastewater can be concentrated and reused by calcium precipitation-nanofiltration combination, reducing the cost of purchasing new alkali. COD is degraded in two stages: electrocatalytic oxidation + activated carbon adsorption, which solves the problem that traditional biochemical methods cannot remove COD in high pH environments. Nanofiltration membranes ensure that the silica content in the effluent is <5mg / L, avoiding the risk of scaling in reclaimed water. Furthermore, ultrafiltration h+ softening resin pretreatment can extend the life of the RO membrane and reduce maintenance costs. Attached Figure Description

[0013] To more clearly illustrate the technical solutions of the embodiments of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and detailed embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is a flowchart of the process steps of the present invention; Figure 2 This is a schematic diagram of the process flow of the present invention. Detailed Implementation

[0014] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0015] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0016] Secondly, the present invention is described in detail with reference to the schematic diagrams. When describing the embodiments of the present invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not according to the usual scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of the present invention. In addition, actual fabrication should include the three-dimensional spatial dimensions of length, width, and depth.

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0018] This invention provides a resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry. Please refer to [link / reference]. Figure 1 It includes the following steps: (1) Desiliconization and alkali removal: Add calcium-based precipitant to wastewater, control the dosage, and calcium silicate precipitate is generated after reaction. After solid-liquid separation, a low-silicon, high-alkali solution is obtained. (2) Ultrafiltration to remove SS: The alkaline solution obtained in step (1) is passed through an ultrafiltration membrane system to retain suspended solids with a particle size > 0.1 μm, and the permeate enters the softening unit; (3) Softening and calcium removal: Ion exchange resin is used to soften and remove residual calcium in the water. , The hardness decreased to <50mg / L; (4) Nanofiltration for silicon removal: The remaining dissolved silicon is separated by nanofiltration membrane, and silicates with a molecular weight >200 Da are retained, and the silicon content in the produced water is <10 mg / L; (5) Electrocatalytic oxidation: Under the action of titanium-coated electrodes, a voltage of 3-5V is applied to degrade organic matter in wastewater, with an oxidation time of 30-60min; (6) Activated carbon adsorption: The effluent treated by electrocatalysis is passed through an activated carbon bed to further adsorb residual COD to <30mg / L; (7) Seawater desalination membrane water reuse: The purified water is desalinated and concentrated using reverse osmosis or electrodialysis membranes, the product water is reused, and the concentrated water is discharged or used for salt distillation; The calcium-based precipitant mentioned in step (1) is The dosage is 1-1.2 times the silicon molar ratio, and the reaction temperature is 25-60℃; In step (2), the ultrafiltration membrane is made of polyvinylidene fluoride, with an operating pressure of 0.1-0.3 MPa and a membrane permeate flux of 20-60 L / (㎡·h). In step (3), the ion exchange resin is a sodium-type cation exchange resin, and the regeneration uses 5-10% [regeneration process]. Solution; In step (4), the nanofiltration membrane is a polyamide composite membrane, with an operating pressure of 0.8-1.5 MPa. Retention rate ≥ 95%; In step (5), the electrocatalytic oxidation uses a Ti / RuO2-IrO2 anode and a graphite cathode, with a current density of 10-30 mA / cm². In step (6), the activated carbon is steam-activated coal-based activated carbon made from anthracite coal, with a particle size of 1-3 mm and an adsorption contact time of ≥20 min. In step (7), the reverse osmosis membrane is a spiral wound aromatic polyamide membrane with a recovery rate ≥70%. When the concentrate TDS > 50 g / L, it is returned to step (1) for recycling. According to any of the above-mentioned resource-based treatment processes for sodium hydroxide wastewater in the photovoltaic industry, the final produced water has a COD of <30mg / L, SiO2 of <3mg / L, and hardness of <5mg / L, and can be reused as raw water for pure water preparation in photovoltaic production. Example 1: Treatment of concentrated alkaline wastewater from photovoltaic cell etching (high silicon, high COD type): Wastewater quality: pH=13.2, COD=380mg / L, SiO2=3750mg / L, TDS=17.5g / L To remove silica and extract alkali, Ca(OH)2 was added, and the mixture was stirred for 40 minutes. The conversion rate of sodium silicate to sodium hydroxide was >90%.

[0019] Softening and calcium removal are achieved by passing the solution through a sodium-type cation exchange resin at a flow rate of 15 BV / h. The effluent Ca²⁺ is less than 5 mg / L. The resin is regenerated using 8% NaCl.

[0020] For nanofiltration to remove silicon, a polyamide nanofiltration membrane (NF270) is selected, with a pressure of 1.2 MPa, a SiO2 rejection rate of 98%, and SiO2 in the produced water < 3 mg / L.

[0021] Activated carbon adsorption, using coconut shell activated carbon (particle size 2mm), with an empty bed contact time of 30min, further reduced COD to 25mg / L.

[0022] Seawater desalination membrane water reuse, reverse osmosis membrane (SW30HR), operating pressure 4.5MPa, water recovery rate 70%, product water conductivity <50μS / cm, concentrate TDS=58g / L returned to the desiliconization process.

[0023] Final result: The quality of the produced water is as follows: COD=25mg / L, SiO2=3mg / L, Ca²⁺=2mg / L, which meets the requirements of GB / T19923-2005 "Industrial Reclaimed Water Standard".

[0024] The NaOH recovery rate is >85%, and the purity of the silicon slag (CaSiO3) is >90%, making it suitable as a building material raw material.

[0025] Example 2: Treatment of low-COD, low-silicon wastewater from photovoltaic cells using dilute alkaline solutions (with an emphasis on resource recovery) Wastewater quality: pH=12.1, COD=80mg / L, SiO2=258mg / L, TDS=0.25g / L Key adjustment steps: Electrocatalytic oxidation: reducing the current density to 10 mA / cm² and the reaction time to 30 min, COD decreased from 80 mg / L (after nanofiltration) to 40 mg / L.

[0026] Activated carbon adsorption: shorter retention time, better water quality, and COD finalized to 15 mg / L.

[0027] Concentrate circulation: The reverse osmosis concentrate with a TDS of 62 g / L is directly used to prepare Ca(OH)2 slurry, reducing reagent consumption.

[0028] Resource-based output: Recycled water is reused in the pure water purification process; Silica slag (CaSiO3) with a purity >90% can be used as a building material raw material; Working principle: When in use, this invention concentrates and reuses NaOH in wastewater through a combination of calcium precipitation and nanofiltration, reducing the cost of purchasing new alkali. Electrocatalytic oxidation and activated carbon adsorption solve the problem that traditional biochemical methods cannot remove COD in high pH environments. At the same time, the nanofiltration membrane ensures that the silica in the effluent is <5mg / L, avoiding the risk of scaling in the reused water. Furthermore, ultrafiltration and softening resin pretreatment can extend the life of the RO membrane and reduce maintenance costs.

[0029] Although the present invention has been described above with reference to embodiments, various modifications can be made and components can be replaced with equivalents without departing from the scope of the invention. In particular, as long as there is no structural conflict, the features in the disclosed embodiments can be combined with each other in any manner. The lack of an exhaustive description of these combinations in this specification is merely for the sake of brevity and resource conservation. Therefore, the present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry, characterized in that, Includes the following steps: (1) Desiliconization and alkali removal: Add calcium-based precipitant to wastewater, control the dosage, and calcium silicate precipitate is generated after reaction. After solid-liquid separation, a low-silicon and high-alkali solution is obtained. (2) Ultrafiltration to remove SS: The alkaline solution obtained in step (1) is passed through an ultrafiltration membrane system to retain suspended solids with a particle size > 0.1 μm, and the product water enters the softening unit; (3) Softening and calcium removal: Ion exchange resin is used to soften and remove residual calcium in the water. 2+ Mg 2+ The hardness decreased to <50mg / L; (4) Nanofiltration for silicon removal: The remaining dissolved silicon is separated by nanofiltration membrane, and silicates with a molecular weight >200 Da are retained, and the silicon content in the produced water is <10 mg / L; (5) Electrocatalytic oxidation: Under the action of titanium-coated electrodes, a voltage of 3-5V is applied to degrade organic matter in waste alkali, with an oxidation time of 30-60min; (6) Activated carbon adsorption: The effluent treated by electrocatalysis is passed through an activated carbon bed to further adsorb residual COD to <30mg / L; (7) Seawater desalination membrane water reuse: The purified water is desalinated and concentrated using reverse osmosis or electrodialysis membranes, the produced water is reused, and the concentrated water is discharged or used for salt distillation.

2. The resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry according to claim 1, characterized in that, The calcium-based precipitant in step (1) is Ca(OH)2, and the dosage is 1-1.2 times the silicon molar ratio, with a reaction temperature of 25-60℃.

3. The resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry according to claim 1, characterized in that, In step (3), the ion exchange resin is a sodium-type cation exchange resin, and regeneration is performed using a 5-10% NaCl solution.

4. The resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry according to claim 1, characterized in that, In step (4), the nanofiltration membrane is a polyamide composite membrane with an operating pressure of 0.8-1.5 MPa and a SiO2 rejection rate of ≥95%.

5. The resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry according to claim 1, characterized in that, In step (6), the activated carbon is steam-activated coal-based activated carbon made from anthracite coal, with a particle size of 2-10mm and an adsorption contact time of ≥20min.

6. The resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry according to claim 1, characterized in that, In step (7), the reverse osmosis membrane is a spiral wound aromatic polyamide membrane with a recovery rate of ≥70%, and the concentrate is discharged or the salt is distilled.

7. The resource-based treatment process for sodium hydroxide wastewater from the photovoltaic industry according to any one of claims 1-6, characterized in that: The final product water has a COD of <30mg / L, SiO2 of <3mg / L, and hardness of <5mg / L, and can be reused as raw water for pure water preparation in photovoltaic production; the final liquid alkali concentration is about 30%, which can be reused as industrial liquid alkali.

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