Harmless treatment process and equipment for chromium-containing waste liquid

Abstract

This invention relates to the field of industrial wastewater treatment, specifically to a process and apparatus for the harmless treatment of chromium-containing wastewater. The process, provided by this invention, involves S1 pretreatment; S2 photoelectrocatalytic reduction; S3 precipitation separation; S4 low-temperature pyrolysis; and S5 tail gas resource recovery treatment. This not only efficiently removes highly toxic Cr(VI) and converts it into less toxic Cr(III), ultimately transforming it into economically valuable chromium oxide, thus achieving waste resource recovery. Simultaneously, the closed-loop process design recovers ammonia generated during pyrolysis as a precipitant, significantly reducing chemical consumption and the risk of secondary pollution, thus achieving both environmental and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment, specifically to a process and apparatus for the harmless treatment of chromium-containing waste liquid. Background Technology

[0002] Chromium and its compounds are widely used in industry. Industries such as metallurgy, chemical industry, electroplating, pigments, textiles, and chromium salt production all generate large amounts of chromium-containing wastewater, with the electroplating sector being particularly problematic. Chromium commonly exists in trivalent (Cr(III)) and hexavalent (Cr(VI)) forms. The latter is extremely toxic, approximately 100 times more so than the former. It easily migrates in the environment and is absorbed by the human body, exhibiting strong carcinogenicity and posing a serious threat to ecology and health.

[0003] my country's chromium resource dependence on foreign sources exceeds 99%, posing a significant risk to resource supply. Currently, the treatment of chromium-containing wastewater still relies primarily on chromium slag landfill, which not only leads to a serious waste of chromium resources but also poses long-term environmental pollution risks. At the same time, the variety and quality of high-value-added chromium products in China are limited, further highlighting the urgent need to develop efficient recycling and resource utilization technologies.

[0004] Existing technologies for treating chromium-containing wastewater can be divided into physicochemical and biological methods, and further categorized by treatment objectives into "achieving emission standards" and "recirculation and recovery." Chemical reduction, electrolysis, and sulfur dioxide reduction methods, while effectively reducing wastewater toxicity, aim to achieve emission standards, but fail to achieve the resource recovery of chromium. Barium salt methods, ion exchange methods, and activated carbon adsorption methods, although intended to recover chromic acid, still suffer from low treatment efficiency, high costs, and insufficient purity of recovered products in practical applications.

[0005] Taking patent CN109942106A as an example, although the proposed chromium water recycling system claims to achieve chromium recovery and water reuse, it still fails to overcome the limitations of the traditional reduction-precipitation process. This process has not made substantial breakthroughs in terms of chromium precipitation efficiency, reaction rate, and product usability. The generated chromium hydroxide precipitate has low purity and is difficult to utilize at high value. Overall, it is still a case of "treating waste with waste" and has failed to truly achieve a closed-loop resource system.

[0006] Therefore, there is an urgent need to develop a harmless treatment process and corresponding equipment for chromium-containing wastewater that combines efficient treatment and resource recovery, so as to achieve high-value recovery of chromium and safe reuse of wastewater, and promote the green and sustainable development of related industries. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a process and apparatus for the harmless treatment of chromium-containing waste liquid with mild conditions, fast reaction rate, and no harmful gas generation during the reaction process.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a process for the harmless treatment of chromium-containing waste liquid, comprising the following steps:

[0009] S1. Pretreatment: The chromium-containing waste liquid is filtered through multiple stages to remove suspended solids and colloids, and then subjected to electrodialysis desalination pretreatment to reduce the salt content and conductivity of the waste liquid.

[0010] S2, Photoelectrocatalytic reduction: The pretreated waste liquid is pumped into the photoelectrocatalytic reactor. Under the conditions of constant bias voltage and ultraviolet light irradiation of a specific wavelength, the titanium dioxide nanotube array electrode with palladium nanoparticles loaded on the surface is used as the working electrode to catalytically reduce hexavalent chromium (Cr(VI)) in the waste liquid to trivalent chromium (Cr(III)).

[0011] S3. Precipitation and separation: A composite precipitant is added to the reduced waste liquid in multiple stages, and the pH value of the reaction system is controlled to be 7.5~8.5. Under stirring conditions, the reaction is fully carried out to generate chromium hydroxide precipitate. After the reaction, the slurry is separated into solid and liquid by a silicon carbide ceramic membrane filter unit to obtain chromium hydroxide filter cake.

[0012] S4. Low-temperature pyrolysis: The chromium hydroxide filter cake is placed in a microwave pyrolysis furnace and subjected to a low-temperature pyrolysis reaction at 400~500℃ under an inert atmosphere to generate chromium oxide powder.

[0013] S5. Waste gas resource utilization treatment: After the ammonia-containing waste gas generated by the thermal decomposition in step S4 is condensed to recover moisture, it is introduced into the ammonia absorption tower and absorbed by the dilute sulfuric acid solution to generate ammonium sulfate solution. The ammonium sulfate solution is reused in the pH adjustment and precipitation process of step S3.

[0014] Furthermore, the multi-stage filtration in step S1 includes sequentially connected quartz sand filtration, anthracite packing filtration, and sintered organic membrane filtration.

[0015] Furthermore, the constant bias voltage in step S2 is 1.0~3.0 V, and the center wavelength of the specific wavelength ultraviolet light is 254 nm or 365 nm.

[0016] Furthermore, in step S3, the composite precipitant is composed of chitosan-modified attapulgite and ammonia, wherein the mass-to-volume ratio of the chitosan-modified attapulgite to ammonia is (0.5-1.5) g: 10 mL.

[0017] Furthermore, the multi-stage addition of the composite precipitant involves adding the composite precipitant in three stages: first, the waste liquid is pumped into the first sedimentation reaction tank, and 50% of the total amount of composite precipitant is added, controlling the pH of the reaction system to 7.5-8.5, and reacting for 4 hours under stirring conditions; then, the slurry overflows or is pumped into the second sedimentation reaction tank, and the remaining 30% of the total amount of precipitant is added, and the reaction continues to be stirred for 4 hours; finally, the slurry enters the third sedimentation reaction tank, and the remaining 20% ​​of the precipitant is added, allowing for a full reaction to ensure complete sedimentation.

[0018] Furthermore, the inert atmosphere in step S4 is nitrogen or argon.

[0019] The device for harmless treatment of chromium-containing waste liquid uses the above-mentioned harmless treatment process for chromium-containing waste liquid, including a pretreatment unit connected sequentially to the pumping equipment through pipelines along the material processing flow, including a multi-stage filtration assembly and an electrodialysis unit arranged in series.

[0020] The photoelectrocatalytic reduction unit includes at least one reactor chamber, which is equipped with an ultraviolet light source and a catalytic electrode system and is electrically connected to a DC power supply.

[0021] The sedimentation and membrane separation unit includes one or more sedimentation reaction tanks equipped with mechanical stirring components and silicon carbide ceramic membrane filter units connected thereto;

[0022] The pyrolysis and product collection unit includes a microwave pyrolysis furnace body, an inert gas source connected to the furnace body's air inlet, a gas treatment component connected to the furnace body's air outlet, and a product collection chamber located below the furnace body.

[0023] The exhaust gas resource recovery unit includes a condenser and a packed absorber, wherein the absorber is equipped with an absorbent circulation pump.

[0024] The outlet of the gas treatment component is connected to the inlet of the tail gas resource recovery unit, and the absorbent storage tank of the packed absorber is connected to the precipitation reaction tank through a reflux pipeline.

[0025] Furthermore, the working electrode in the catalytic electrode system is a TiO2 nanotube array electrode grown in situ by anodic oxidation on a titanium plate substrate, and palladium nanoparticles are loaded in the nanotube cavity by electrochemical deposition.

[0026] Furthermore, the filtrate outlet of the silicon carbide ceramic membrane filter unit is connected to the feed inlet of the photoelectrocatalytic reduction unit through a return pipeline equipped with an online water quality monitor, forming a secondary treatment cycle.

[0027] Furthermore, the precipitation reaction tank includes a first precipitation reaction tank, a second precipitation reaction tank, and a third precipitation reaction tank, which are arranged in series, and each of them is equipped with a centrifugal pump on its outlet pipe; the outlet of the third precipitation reaction tank is connected to the inlet of the silicon carbide ceramic membrane filter unit through the centrifugal pump.

[0028] As described above, the harmless treatment process for chromium-containing waste liquid provided by this invention involves: 1. Pretreatment (S1); photoelectrocatalytic reduction (S2); precipitation separation (S3); low-temperature pyrolysis (S4); and tail gas resource recovery treatment. This process not only efficiently removes highly toxic Cr(VI) and converts it into less toxic Cr(III), ultimately transforming it into economically valuable chromium oxide products, thus realizing the resource recovery of waste. Simultaneously, the closed-loop process design recovers ammonia generated during pyrolysis as a precipitant, significantly reducing the risk of chemical consumption and secondary pollution, thus achieving both environmental and economic benefits.

[0029] 2. This invention avoids localized oversaturation by adding the composite precipitant in a three-stage gradient: the initial addition of 50% of the base amount allows Cr³⁺ ions to slowly nucleate, generating a large number of crystal nuclei, avoiding the instantaneous oversaturation caused by a single addition, which would result in a large number of amorphous fine colloids that are difficult to settle; it optimizes crystal growth: the subsequent two additions provide continuous "nutrients" for the existing crystal nuclei, enabling them to grow into larger, denser, and more easily filtered crystal particles (chromium hydroxide); it improves precipitation efficiency and purity: the gradient addition ensures a more thorough reaction, reducing the adsorption and encapsulation of other impurities in the wastewater by the chromium hydroxide colloid, thereby improving the purity of the final chromium hydroxide filter cake and the quality of subsequent chromium oxide products; and it saves reagents: through step-by-step control, the appropriate amount of reagent can be added more precisely according to the reaction progress, avoiding waste caused by a single over-addition.

[0030] 3. This invention uses microwave low-temperature pyrolysis technology, which has high energy utilization efficiency, fast reaction speed, and the inert atmosphere protection avoids the re-oxidation of Cr(III), thus ensuring the quality of chromium oxide products.

[0031] 4. This invention innovatively utilizes the ammonia tail gas generated by pyrolysis to prepare ammonium sulfate, which is then recycled in the upstream precipitation process. This achieves internal recycling of the ammonia reagent, greatly reducing the consumption of fresh chemicals and the emission of secondary pollutants, and embodies the principles of green chemistry.

[0032] 5. The composite precipitant of this invention is composed of chitosan-modified attapulgite and ammonia water, wherein the mass-to-volume ratio of chitosan-modified attapulgite to ammonia water is (0.5-1.5) g: 10 mL. Chitosan is a natural polymeric flocculant, and the amino groups on its chains have a strong complexing effect on Cr(III). Attapulgite has a huge specific surface area and adsorption capacity. The modified material formed by the combination of the two has multiple synergistic effects of netting and sweeping, adsorption and complexation, which can form larger and denser chromium hydroxide flocs, greatly improving precipitation efficiency and settling speed, reducing the total dosage of precipitant, and facilitating subsequent filtration and separation.

[0033] 6. The device of this invention materializes the optimized process steps into an integrated, continuous equipment system. Each unit is precisely connected through pipelines and pumping equipment, achieving fully automated operation from waste liquid inlet to product collection and tail gas recovery. This reduces manual intervention, improves processing efficiency and stability, and provides a hardware foundation for the large-scale industrial application of the process. Attached Figure Description

[0034] Figure 1 This is a flowchart of the harmless treatment process for chromium-containing waste liquid according to the present invention.

[0035] Figure 2 This is a schematic diagram of the structure of the chromium-containing waste liquid harmless treatment device of the present invention.

[0036] Reference numerals: 1. Pretreatment unit; 2. Photoelectrocatalytic reduction unit; 3. Precipitation and membrane separation unit; 301. First precipitation reaction vessel; 302. Second precipitation reaction vessel; 303. Third precipitation reaction vessel; 4. Pyrolysis and product collection unit; 401. Microwave pyrolysis furnace body; 402. Inert gas source; 403. Gas treatment assembly; 404. Product collection bin; 5. Tail gas resource recovery unit; 501. Condensation recovery unit; 502. Packed absorption tower; 503. Absorbent liquid circulation pump; 6. Water quality monitor; 7. Silicon carbide ceramic membrane filter unit; 8. Mechanical stirring assembly. Detailed Implementation

[0037] The present invention will be further described below through specific embodiments.

[0038] like Figure 1 As shown, the harmless treatment process for chromium-containing wastewater according to the present invention includes the following steps:

[0039] S1. Pretreatment: The chromium-containing waste liquid is filtered through multiple stages to remove suspended solids and colloids, and then subjected to electrodialysis desalination pretreatment to reduce the salt content and conductivity of the waste liquid.

[0040] This multi-stage filtration system comprises sequentially connected quartz sand filtration, anthracite packing filtration, and sintered organic membrane filtration, achieving a gradient filtration approach from coarse to fine. Quartz sand and anthracite packing remove most suspended solids and larger colloidal particles, reducing the load on subsequent precision filtration units. The final sintered organic membrane is used for deep removal of fine colloids and ultrafine particles. This combination ensures effective pretreatment, effectively preventing membrane fouling and electrode passivation in subsequent electrodialysis and photoelectrocatalytic reactors, and guaranteeing the long-term stable operation of the core process.

[0041] S2, Photoelectrocatalytic reduction: The pretreated waste liquid is pumped into the photoelectrocatalytic reactor. Under the conditions of constant bias voltage and ultraviolet light irradiation of a specific wavelength, the titanium dioxide nanotube array electrode with palladium nanoparticles loaded on the surface is used as the working electrode to catalytically reduce hexavalent chromium (Cr(VI)) in the waste liquid to trivalent chromium (Cr(III)).

[0042] The constant bias voltage is 1.0~3.0V, and the center wavelength of the specific wavelength ultraviolet light is 254nm or 365nm. Choosing a low bias voltage of 1.0~3.0V can significantly reduce the energy consumption of the reaction process, while avoiding side reactions (such as water electrolysis) caused by excessive voltage, and improving current efficiency. The specific wavelength (254nm corresponds to strong absorption in TiO2, and 365nm corresponds to commonly used ultraviolet light sources) of ultraviolet light can effectively match the band gap of TiO2, greatly improving the separation efficiency of photogenerated electron-hole pairs, thereby significantly enhancing the catalytic reduction rate and efficiency of Cr(VI).

[0043] S3. Precipitation and separation: A composite precipitant is added to the reduced waste liquid in multiple stages, and the pH value of the reaction system is controlled to be 7.5~8.5. Under stirring conditions, the reaction is fully carried out to generate chromium hydroxide precipitate. After the reaction, the slurry is separated into solid and liquid by a silicon carbide ceramic membrane filter unit to obtain chromium hydroxide filter cake.

[0044] The composite precipitant consists of chitosan-modified attapulgite and ammonia, with a mass-to-volume ratio of (0.5-1.5) g:10 mL. Chitosan is a natural polymeric flocculant, and its amino groups have a strong complexing effect on Cr(III). Attapulgite has a large specific surface area and adsorption capacity. The modified material formed by the combination of the two has multiple synergistic effects of trapping, sweeping, adsorption, and complexation, which can form larger and denser chromium hydroxide flocs, greatly improving precipitation efficiency and settling speed, reducing the total dosage of precipitant, and facilitating subsequent filtration and separation.

[0045] The multi-stage addition of the composite precipitant involves a three-stage gradient addition process: First, the waste liquid is pumped into the first precipitation reaction tank, and 50% of the total composite precipitant is added. The pH of the reaction system is controlled at 7.5-8.5, and the reaction is carried out for 4 hours under stirring. Then, the slurry overflows or is pumped into the second precipitation reaction tank, and the remaining 30% of the total precipitant is added (i.e., the cumulative addition is 80%), and the reaction continues for another 4 hours under stirring. Finally, the slurry enters the third precipitation reaction tank, and the remaining 20% ​​of the precipitant is added, ensuring complete precipitation. The final slurry after the three-stage reaction undergoes solid-liquid separation using a silicon carbide ceramic membrane filter unit to obtain a chromium hydroxide filter cake with higher purity and more uniform particle size. This multi-stage addition is controllable, improves precipitation efficiency, and addresses the problem of long reaction times leading to the formation of coarse chromium hydroxide, thus solving the difficulty in filtering fine chromium hydroxide.

[0046] Because of the multi-stage feeding method, after the first addition of the composite precipitant, fine chromium hydroxide will be formed after the reaction. Then, 30% more feed is added after entering the second precipitation reaction tank. The fine chromium hydroxide generated in the second precipitation reaction tank acts as crystal nuclei, which shortens the reaction time of the second reaction and allows the fine chromium hydroxide to grow, making it easier to filter. This solves the problem of the fine chromium hydroxide being difficult to filter and also promotes the formation of large chromium hydroxide crystals.

[0047] S4. Low-temperature pyrolysis: The chromium hydroxide filter cake is placed in a microwave pyrolysis furnace and subjected to a low-temperature pyrolysis reaction at 400~500℃ under an inert atmosphere to generate chromium oxide powder.

[0048] The inert atmosphere is nitrogen or argon. Microwave pyrolysis under an inert atmosphere such as nitrogen or argon can effectively prevent chromium hydroxide from being oxidized by oxygen at high temperatures and regenerating Cr(VI), thus ensuring the purity and stability of the final product, chromium oxide. At the same time, the inert atmosphere is also a key condition for achieving low-temperature pyrolysis, avoiding increased energy consumption and product sintering caused by high temperatures.

[0049] S5. Waste gas resource utilization: The ammonia-containing waste gas generated from the thermal decomposition in step S4 is condensed to recover moisture and then introduced into an ammonia absorption tower. It is absorbed by a dilute sulfuric acid solution to generate an ammonium sulfate solution. This ammonium sulfate solution is reused in the pH adjustment and precipitation process in step S3. This innovative approach utilizes the ammonia waste gas generated from pyrolysis to prepare ammonium sulfate and reuses it in the previous precipitation process, realizing the internal circulation of ammonia reagent. This greatly reduces the consumption of fresh chemicals and the emission of secondary pollutants, reflecting the principles of green chemistry.

[0050] The process of this invention not only efficiently removes highly toxic Cr(VI) and converts it into less toxic Cr(III), ultimately transforming it into an economically valuable chromium oxide product, thus realizing the resource utilization of waste. Simultaneously, the closed-loop process design recovers the ammonia gas generated during pyrolysis as a precipitant, significantly reducing the risk of chemical consumption and secondary pollution, thus achieving both environmental and economic benefits.

[0051] like Figure 2 As shown, the chromium-containing waste liquid harmless treatment device uses the above-mentioned chromium-containing waste liquid harmless treatment process, including a pretreatment unit 1 connected sequentially to a pumping device via pipelines along the material processing flow, including a multi-stage filtration assembly and an electrodialysis unit arranged in series.

[0052] The photoelectrocatalytic reduction unit 2 includes at least one reactor chamber, which is equipped with an ultraviolet light source and a catalytic electrode system and is electrically connected to a DC power supply.

[0053] The sedimentation and membrane separation unit 3 includes one or more sedimentation reaction tanks with mechanical stirring components 8 and silicon carbide ceramic membrane filter units 7 connected thereto;

[0054] The pyrolysis and product collection unit 4 includes a microwave pyrolysis furnace body 401, an inert gas source 402 connected to the furnace body inlet, a gas treatment component 403 connected to the furnace body outlet, and a product collection chamber 404 disposed below the furnace body.

[0055] The exhaust gas resource recovery unit 5 includes a condenser recovery unit 501 and a packed absorption tower 502, wherein the packed absorption tower 502 is equipped with an absorption liquid circulation pump 503.

[0056] The outlet of the gas treatment component 403 is connected to the inlet of the tail gas resource recovery unit 5, and the absorbent storage tank of the packed absorption tower 502 is connected to the sedimentation reaction tank through a return pipeline. This realizes fully automated operation from waste liquid inlet to product collection and tail gas recovery, reduces manual intervention, improves processing efficiency and stability, and provides a hardware foundation for the large-scale industrial application of the process. The gas treatment component 403 is a dust collector that removes most of the solid particles and dust carried in the pyrolysis gas.

[0057] In this embodiment, the working electrode in the catalytic electrode system is a TiO2 nanotube array electrode grown in situ via anodic oxidation on a titanium plate substrate, with palladium nanoparticles loaded within the nanotube cavities through electrochemical deposition. This TiO2 nanotube array provides a large specific surface area and well-ordered electron transport channels; the loaded palladium (Pd) nanoparticles, acting as a highly efficient co-catalyst, effectively capture photogenerated electrons from TiO2, suppress electron-hole recombination, and significantly reduce the activation energy of the Cr(VI) reduction reaction as active sites. The synergistic effect of these two elements greatly enhances the quantum efficiency and reduction rate of the photoelectrocatalytic reaction, which is key to the high efficiency of this process.

[0058] In this embodiment, the filtrate outlet of the silicon carbide ceramic membrane filter unit 7 is connected to the feed inlet of the photoelectrocatalytic reduction unit 2 via a return pipeline equipped with an online water quality monitor 6, forming a secondary treatment cycle. This creates an internal quality feedback and control loop. If the chromium ion concentration in the filtrate (clarified liquid) does not meet the standard, it can automatically return to the photoelectrocatalytic unit for further treatment, ensuring that the final effluent water quality meets 100% of the standards. This improves the overall system's processing reliability and fault tolerance, and achieves precise control of high-risk pollutants.

[0059] In this embodiment, the precipitation reaction tank includes a first precipitation reaction tank 301, a second precipitation reaction tank 302, and a third precipitation reaction tank 303, which are arranged in series, and each of their outlet pipes is equipped with a centrifugal pump. The outlet of the third precipitation reaction tank 303 is connected to the inlet of the silicon carbide ceramic membrane filter unit 7 via the centrifugal pump, thereby allowing for the gradual addition of composite precipitant, improving precipitation efficiency, and addressing the problem of long reaction time leading to the formation of coarse chromium hydroxide, thus solving the problem of difficult filtration of fine chromium hydroxide. The mechanical stirring assembly 8 includes a motor mounted on the reaction tank, with a rotating shaft at the output end of the motor. A rotating rod is fixedly connected to the rotating shaft, and the rotating rod extends into the precipitation reaction tank and is equipped with stirring blades. This allows the composite precipitant to fully react with the wastewater.

[0060] In this embodiment, the microwave pyrolysis furnace body 401 operates at a frequency of 2290MHz, has a rated power of 3~5kW, and is equipped with an infrared temperature measurement and power adaptive adjustment system. This microwave heating method offers advantages such as internal heating, high speed, and high efficiency. Combined with real-time monitoring of the furnace temperature via infrared temperature measurement and feedback adjustment through the power adaptive system, the pyrolysis temperature can be precisely controlled within the optimal range (400-500℃). This avoids incomplete decomposition due to excessively low temperatures or increased energy consumption and product sintering due to excessively high temperatures, ensuring the quality stability of the chromium oxide product and the energy efficiency of the production process.

[0061] The device of this application also includes a control unit, which comprises: an online chromium concentration analyzer installed at the outlet of the photoelectrocatalytic reduction unit; pH sensors and temperature sensors installed in each reaction tank; and a programmable logic controller (PLC) that receives signals from each sensor and outputs control commands to the DC power supply, metering pump, and stirring motor for real-time feedback and optimization control of various process parameters. This achieves intelligent and refined monitoring and management of the entire process. By real-time monitoring of key parameters (such as Cr concentration, pH value, and temperature) and feedback control of the actuators (power supply, pump, and stirring), the system can always operate under optimal conditions, significantly improving the stability of the treatment effect, the consistency of the product, and the automation level of the entire production line, while minimizing labor costs and the risk of operational errors. Furthermore, the PLC, pH sensor, temperature sensor, chromium concentration analyzer, power supply, pump, and stirring assembly of this application can be directly purchased from the existing market, and their circuit connections can be implemented using existing technology; therefore, this embodiment will not elaborate on them individually.

[0062] The following is a description using specific examples.

[0063] Example 1:

[0064] The wastewater containing chromium from an electroplating plant was to be treated. The initial Cr(VI) concentration was 150 mg / L, the pH was 3.5, and it contained a small amount of suspended solids and high salt content (conductivity of about 5 mS / cm).

[0065] S1. Pretreatment: Wastewater first enters a quartz sand filter to remove larger suspended particles; then it flows through an anthracite packing filter for deep filtration; finally, it passes through a sintered organic filter membrane (0.45 μm pore size) to ensure the removal of colloidal particles. The pretreated wastewater enters an electrodialysis unit, where its conductivity drops to below 1.5 mS / cm after desalination.

[0066] S2. Photoelectrocatalytic reduction: The pretreated wastewater was pumped into a photoelectrocatalytic reactor. A Pd-loaded TiO2 nanotube array electrode was used as the working electrode in the reactor, with a constant bias voltage of 2.0 V applied, and a 365 nm ultraviolet lamp was simultaneously turned on for irradiation. After 60 minutes of reaction, samples were taken for analysis, and the Cr(VI) concentration dropped below 0.05 mg / L, with a reduction efficiency exceeding 98%.

[0067] S3. Precipitation Separation: A composite precipitant (1.0 g of chitosan-modified attapulgite mixed with 10 mL of concentrated ammonia) was added to the reduced wastewater in three stages. The pH was adjusted to 8.0 with dilute ammonium sulfate solution, and mechanical stirring was performed. Finally, the generated chromium hydroxide slurry was pumped into a silicon carbide ceramic membrane filter unit for solid-liquid separation, yielding a chromium hydroxide filter cake with low water content. The total chromium concentration of the filtrate was found to be below 0.1 mg / L, meeting the discharge standards.

[0068] S4. Low-temperature pyrolysis: The chromium hydroxide filter cake is transferred to a microwave pyrolysis furnace, and nitrogen is introduced as a protective gas. The microwave power is set to 4 kW, and the pyrolysis temperature is controlled at 450℃ and held for 30 minutes. After cooling, dark green chromium oxide (Cr2O3) powder is obtained.

[0069] S5. Tail Gas Resource Utilization: The ammonia-containing tail gas generated during pyrolysis, after water vapor is removed by a condenser, enters the bottom of a packed absorber tower, where it comes into countercurrent contact with a 10% dilute sulfuric acid solution sprayed from the top of the tower. The ammonia is absorbed by the sulfuric acid to form an ammonium sulfate solution, which is stored in an absorbent storage tank. This ammonium sulfate solution is partially used to supplement the pH adjustment and precipitant compounding in step 3, achieving internal circulation within the system.

[0070] Comparative Example 1:

[0071] The difference between this embodiment and Example 1 is that the composite precipitant is added in a single step. Within the same timeframe, Example 1 yielded significantly more chromium oxide (Cr2O3) powder than this embodiment.

[0072] Comparative Example 2:

[0073] The same batch of wastewater was treated using a traditional chemical reduction precipitation method. Sodium metabisulfite was used as the reducing agent, and sodium hydroxide was used to adjust the pH to 8.5 after reduction to generate precipitate. Although the total chromium concentration in the supernatant after treatment met the standards, a large volume of hazardous chromium-containing sludge (approximately 85% water content) was generated. This sludge needs to be disposed of as hazardous waste, which is costly, and no resources are recovered.

[0074] In summary, the process of this invention not only efficiently removes highly toxic Cr(VI) and converts it into less toxic Cr(III), ultimately transforming it into an economically valuable chromium oxide product, thus realizing the resource utilization of waste. Simultaneously, the closed-loop process design recovers the ammonia gas generated during pyrolysis as a precipitant, significantly reducing the risk of chemical consumption and secondary pollution, thus achieving both environmental and economic benefits.

[0075] The above are merely some specific embodiments of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial modifications made to the present invention using this concept shall be considered as infringing upon the protection scope of the present invention.

Claims

1. A process for the harmless treatment of chromium-containing wastewater, characterized in that, Includes the following steps: S1. Pretreatment: The chromium-containing waste liquid is filtered through multiple stages to remove suspended solids and colloids, and then subjected to electrodialysis desalination pretreatment to reduce the salt content and conductivity of the waste liquid. S2, Photoelectrocatalytic reduction: The pretreated waste liquid is pumped into the photoelectrocatalytic reactor. Under the conditions of constant bias voltage and ultraviolet light irradiation of a specific wavelength, the titanium dioxide nanotube array electrode with palladium nanoparticles loaded on the surface is used as the working electrode to catalytically reduce hexavalent chromium (Cr(VI)) in the waste liquid to trivalent chromium (Cr(III)). S3. Precipitation Separation: A composite precipitant is added to the reduced waste liquid in multiple stages. The composite precipitant is composed of chitosan-modified attapulgite and ammonia water, and the mass-volume ratio of chitosan-modified attapulgite to ammonia water is (0.5-1.5) g: 10 mL. The pH value of the reaction system is controlled at 7.5~8.5, and the reaction is carried out under stirring conditions to generate chromium hydroxide precipitate. After the reaction, the slurry is subjected to solid-liquid separation by a silicon carbide ceramic membrane filter unit to obtain chromium hydroxide filter cake. S4. Low-temperature pyrolysis: The chromium hydroxide filter cake is placed in a microwave pyrolysis furnace and subjected to a low-temperature pyrolysis reaction at 400~500℃ under an inert atmosphere to generate chromium oxide powder. S5. Waste gas resource utilization treatment: After the ammonia-containing waste gas generated by the thermal decomposition in step S4 is condensed to recover moisture, it is introduced into the ammonia absorption tower and absorbed by the dilute sulfuric acid solution to generate ammonium sulfate solution. The ammonium sulfate solution is reused in the pH adjustment and precipitation process of step S3.

2. The process for harmless treatment of chromium-containing wastewater according to claim 1, characterized in that: The multi-stage filtration in step S1 includes sequentially connected quartz sand filtration, anthracite packing filtration, and sintered organic membrane filtration.

3. The process for harmless treatment of chromium-containing wastewater according to claim 1, characterized in that: The constant bias voltage in step S2 is 1.0~3.0 V, and the center wavelength of the specific wavelength ultraviolet light is 254 nm or 365 nm.

4. The process for harmless treatment of chromium-containing wastewater according to claim 1, characterized in that: The multi-stage addition of the composite precipitant involves adding the composite precipitant in three stages. First, the waste liquid is pumped into the first precipitation reaction tank, and 50% of the total amount of composite precipitant is added. The pH of the reaction system is controlled at 7.5-8.5, and the reaction is carried out for 4 hours under stirring. Then, the slurry overflows or is pumped into the second precipitation reaction tank, and the remaining 30% of the total amount of precipitant is added. The reaction is continued for another 4 hours under stirring. Finally, the slurry enters the third precipitation reaction tank, and the remaining 20% ​​of the precipitant is added to ensure complete precipitation.

5. The process for harmless treatment of chromium-containing wastewater according to claim 1, characterized in that: In step S4, the inert atmosphere is nitrogen or argon.

6. A device for the harmless treatment of chromium-containing waste liquid, characterized in that: The harmless treatment process for chromium-containing waste liquid according to any one of claims 1-5 includes a pretreatment unit connected sequentially to a pumping device via pipelines along the material processing flow, including a multi-stage filtration assembly and an electrodialysis unit arranged in series. The photoelectrocatalytic reduction unit includes at least one reactor chamber, which is equipped with an ultraviolet light source and a catalytic electrode system and is electrically connected to a DC power supply. The sedimentation and membrane separation unit includes one or more sedimentation reaction tanks equipped with mechanical stirring components and silicon carbide ceramic membrane filter units connected thereto; The pyrolysis and product collection unit includes a microwave pyrolysis furnace body, an inert gas source connected to the furnace body's air inlet, a gas treatment component connected to the furnace body's air outlet, and a product collection chamber located below the furnace body. The exhaust gas resource recovery unit includes a condenser and a packed absorber, wherein the packed absorber is equipped with an absorbent circulation pump. The outlet of the gas treatment component is connected to the inlet of the tail gas resource recovery unit, and the absorbent storage tank of the packed absorber is connected to the precipitation reaction tank through a reflux pipeline.

7. The device for harmless treatment of chromium-containing waste liquid according to claim 6, characterized in that: The working electrode in the catalytic electrode system is a TiO2 nanotube array electrode grown in situ by anodic oxidation on a titanium plate substrate, and palladium nanoparticles are loaded in the nanotube cavity by electrochemical deposition.

8. The device for harmless treatment of chromium-containing waste liquid according to claim 6, characterized in that: The filtrate outlet of the silicon carbide ceramic membrane filter unit is connected to the feed inlet of the photoelectrocatalytic reduction unit through a return pipeline equipped with an online water quality monitor, forming a secondary treatment cycle.

9. The device for harmless treatment of chromium-containing waste liquid according to claim 6, characterized in that: The precipitation reaction tank includes a first precipitation reaction tank, a second precipitation reaction tank, and a third precipitation reaction tank, which are arranged in series, and each of them is equipped with a centrifugal pump on its outlet pipe; the outlet of the third precipitation reaction tank is connected to the inlet of the silicon carbide ceramic membrane filter unit through the centrifugal pump.

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