A method and system for treating stainless steel pickling wastewater

Abstract

This invention relates to the field of wastewater treatment technology, specifically to a method and system for treating stainless steel pickling wastewater. The treatment method includes: adding concentrated sulfuric acid to the stainless steel pickling wastewater, heating and distilling to obtain defluorinated wastewater; heating the defluorinated wastewater and adding sulfate to obtain first wastewater; adding alkali to the first wastewater and performing solid-liquid separation to obtain second wastewater; subjecting the second wastewater to constant voltage deposition to obtain third wastewater; passing the third wastewater through a first nanofiltration module and a second nanofiltration module to obtain retentate retained by the first nanofiltration module and permeate passing through the second nanofiltration module; evaporating and crystallizing the retentate to obtain sodium sulfate; and evaporating and crystallizing the permeate to obtain sodium chloride. This invention also provides a system for treating stainless steel pickling wastewater. This invention solves the problems of low pollutant removal efficiency, poor resource utilization, and serious secondary pollution associated with existing stainless steel pickling wastewater treatment methods.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a method and system for treating stainless steel pickling wastewater. Background Technology

[0002] Stainless steel pickling produces highly polluting industrial wastewater during the cleaning process using mixed acids (usually HNO3 / HF or H2SO4 / HNO3 / HF systems). This wastewater is characterized by its complex composition, high pollutant concentration, and high salt content. It primarily contains heavy metal ions such as iron, nickel, and chromium, which are produced under strongly acidic conditions, as well as high concentrations of sulfate and fluorides. Because the wastewater contains multiple difficult-to-treat pollutants, traditional single-treatment processes are insufficient for efficient separation and resource recovery.

[0003] For example, while conventional chemical precipitation can remove some metal ions, its effectiveness in removing fluorides is limited, and it easily leads to co-precipitation, resulting in complex sludge composition. This not only affects the efficiency of subsequent metal recovery but also produces fluoride-containing heavy metal sludge, which is hazardous solid waste with high disposal costs. Membrane separation technology is easily contaminated in high-salt environments, experiences rapid flux decline, and cannot effectively distinguish between ions of different valence states. Furthermore, existing methods often use calcium salt precipitation for fluoride removal, which can reduce fluoride content but introduces a large number of calcium ions, forming calcium sulfate and calcium fluoride complex scale, interfering with subsequent electrodeposition processes and producing sludge that is difficult to dispose of. Meanwhile, valuable metals such as nickel and chromium in wastewater usually exist in ionic form. If recovered solely through precipitation or adsorption, only low-value compounds are often obtained, which is insufficient to meet the requirements for high-purity material preparation. Direct evaporation crystallization, on the other hand, results in insufficient product purity due to impurity co-precipitation and cannot achieve zero discharge.

[0004] In summary, existing treatment methods cannot simultaneously resolve the multiple contradictions of efficient separation of fluoride and heavy metals, high-purity recovery of valuable metals, and zero wastewater discharge. There is an urgent need to develop a clean treatment process that can achieve stepwise selective extraction and high-value utilization. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method and system for treating stainless steel pickling wastewater, so as to solve the problem of low pollutant removal efficiency in existing stainless steel pickling wastewater treatment methods, and also to solve the problems of poor resource utilization and serious secondary pollution in existing stainless steel pickling wastewater treatment methods.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for treating stainless steel pickling wastewater includes the following steps: S1. Defluorination: Concentrated sulfuric acid is added to stainless steel pickling wastewater, and the mixture is heated and distilled to allow the fluorine to evaporate and be recovered in the form of hydrogen fluoride, thus obtaining defluorinated wastewater. S2. Iron removal: The defluorination wastewater is heated, and sulfate containing monovalent cations is added to precipitate iron ions in the form of potassium ferric sulfate. Solid-liquid separation is performed to obtain the first precipitate and the first wastewater. S3. Chromium removal: Add alkali to the first wastewater to adjust the pH to the second preset value, so that chromium precipitates while nickel remains in the solution, and solid-liquid separation is achieved to obtain the second precipitate and the second wastewater. S4. Nickel removal: Constant voltage deposition is performed on the second wastewater to obtain metallic nickel and the third wastewater; S5. Nanofiltration desalination: The third wastewater is sequentially passed into the first nanofiltration module and the second nanofiltration module. The first nanofiltration module retains sulfate ions, and the second nanofiltration module enriches chloride ions on the permeation side, resulting in the retentate retained by the first nanofiltration module and the permeate that passes through the second nanofiltration module. S6. Evaporation and crystallization: Evaporate and crystallize the retentate to obtain sodium sulfate; evaporate and crystallize the permeate to obtain sodium chloride.

[0007] Based on the aforementioned technical methods, firstly, deep removal of fluoride via distillation effectively avoids interference from fluoride ions in subsequent metal separation processes. Fluorine is selectively volatilized and recovered in the form of hydrogen fluoride, not only realizing the resource utilization of fluorine but, more importantly, eliminating the possibility of fluorine forming stable complexes with metals such as iron, chromium, and nickel. This provides a clean reaction environment for subsequent potassium ferric sulfate precipitation, selective precipitation of chromium, and electrochemical deposition of nickel. Secondly, the potassium ferric sulfate method efficiently removes iron ions under strongly acidic conditions while avoiding the introduction of foreign impurities such as calcium and aluminum. This achieves directional separation of iron without loss of chromium and nickel, significantly reducing the load on subsequent processing units and preventing co-precipitation or co-precipitation of iron during electrodeposition, thereby improving the purity of the heavy metal recovery product. Thirdly, the synergistic effect of deep fluoride removal and efficient iron removal creates the necessary conditions for high-value nickel recovery. The solution obtained after fluoride, iron, and chromium removal has a high nickel ion concentration and extremely low impurity content, making the constant voltage electrodeposition process stable and efficient. High-purity metallic nickel can be directly obtained without additional purification steps, significantly enhancing the resource recovery value. It effectively solves the problem of low pollutant removal efficiency in existing stainless steel pickling wastewater treatment methods, and also solves the problems of poor resource utilization and serious secondary pollution in existing stainless steel pickling wastewater treatment methods.

[0008] Preferably, in step S1, the heating and distillation temperature is 160~180℃.

[0009] Preferably, in step S2, the defluoridated wastewater is heated to 90~98°C.

[0010] By controlling the heating temperature of the defluorination wastewater between 90 and 98°C, it is possible to ensure the rapid precipitation of potassium ferric sulfate while avoiding the transformation of potassium ferric sulfate into hematite due to excessively high temperatures.

[0011] Preferably, in step S2, the pH of the defluoridated wastewater is 1.5 to 2.0.

[0012] Actual experiments have shown that when the acid concentration in defluoridation wastewater increases, the precipitation rate of synthesized ferrous sulfate decreases, while other iron compounds precipitate at higher pH levels. As temperature increases, the pH value at which ferrous sulfate forms and stabilizes decreases. According to the equilibrium equation, an increase in acid concentration in defluoridation wastewater inhibits the formation of ferrous sulfate. Improper pH control may lead to over-neutralization, causing the precipitate to become semi-gel-like, making it difficult to filter and wash. Therefore, it is crucial to precisely control the pH of defluoridation wastewater between 1.5 and 2.0.

[0013] Preferably, in S2, the monovalent cation in the sulfate containing a monovalent cation is selected from Na. + NH4 + and K + At least one of them.

[0014] Preferably, the sulfate containing a monovalent cation is selected from at least one of sodium sulfate, ammonium sulfate, and potassium sulfate.

[0015] Preferably, in step S2, the first precipitate is potassium ferric sulfate. Potassium ferric sulfate is used for recycling.

[0016] In S1, the fluoride removal and iron removal processes aim to remove fluoride and iron separately as much as possible, achieving a removal rate of over 95%; while nickel and chromium are retained in the solution with a loss rate of no more than 10%.

[0017] In addition, calcium salt precipitation can be used to remove fluoride.

[0018] Lime can both precipitate fluoride ions and act as a neutralizing agent to adjust the pH value during the defluorination process. The reaction formula is: Iron removal using the jaundice alum method; the general formula of jaundice alum is AFe3(SO4)2(OH)6. The A-site is a monovalent cation A. + (can be Na) + NH4 + K + Ag + 、Rb + Et. Potassium ferricyanide is a sulfate mineral, insoluble in water but soluble in hydrochloric acid. In industrial production, the solution pH is typically adjusted to around 1.5 at approximately 95°C, and a monovalent cation source A is added. + A large amount of iron in the leachate precipitates as potassium ferric sulfate. The main reaction equation is: 3Fe 3+ +A + +2SO4 2-+6H₂O→AFe₃(SO₄)₂(OH)₆ +6H + Preferably, in S3, the alkali is selected from sodium hydroxide.

[0019] Preferably, in step S3, the first preset value is 6.0~7.0.

[0020] Preferably, in step S3, the second precipitate is chromium hydroxide. The chromium hydroxide is recycled.

[0021] Preferably, S4 includes: The pH of the second wastewater was adjusted to 3.5-4.5 using concentrated sulfuric acid, and the temperature was raised to 25-35℃. Then, a constant voltage of -1.0 V to -1.3 V was applied for constant voltage deposition, so that nickel ions in the second wastewater were deposited as metallic nickel on the cathode surface.

[0022] Preferably, a three-electrode system is used for the constant voltage deposition, wherein an iridium-tantalum-titanium electrode is selected as the counter electrode, a saturated calomel electrode is selected as the reference electrode, and a stainless steel substrate is selected as the working electrode.

[0023] Preferably, in step S5, the first nanofiltration module is a polyamide thin-layer composite nanofiltration membrane (TFC NF membrane).

[0024] Preferably, in step S5, the second nanofiltration module is a negatively charged polyamide TFC nanofiltration membrane.

[0025] Preferably, in step S6, the mother liquor obtained from evaporation and crystallization is returned to step S2 as a source of sulfate replenishment.

[0026] Preferably, the stainless steel pickling wastewater contains 54 g / L of iron (Fe), 16 g / L of nickel (Ni), 28 g / L of chromium (Cr), and SO42-24 g / L. 2- The content is 95 g / L, F - The content is 59 g / L, and the TDS (total dissolved solids / total dissolved solids) content is 254 g / L; The pH value of the stainless steel pickling wastewater is 1.85.

[0027] Preferably, in step S6, the retentate is evaporated and crystallized to obtain sodium sulfate; the permeate is evaporated and crystallized to obtain sodium chloride, specifically including: The retentate is sequentially passed into a first-effect evaporator, a second-effect evaporator, and a third-effect evaporator for multi-stage evaporation and concentration. The first-effect evaporator is heated by external steam, and the generated secondary steam is used as the heat source for the second-effect evaporator and the third-effect evaporator in turn. The concentrated sodium sulfate solution enters a crystallizer to precipitate sodium sulfate decahydrate crystals. After solid-liquid separation, sodium sulfate product and mother liquor are obtained. The mother liquor is returned to S2 as a supplementary source of monovalent cations and sulfate ions.

[0028] Preferably, step S2 further includes: detecting the concentrations of fluoride ions and iron ions in the first wastewater; If the fluoride ion concentration is higher than the first threshold, the first wastewater is returned to S1 for reheating and distillation to remove fluoride; the first threshold is 0.05 g / L. If the iron ion concentration is higher than the second threshold, the first wastewater will be subjected to the S2 operation again for further precipitation of potassium ferric sulfate; the second threshold is 1.0 g / L.

[0029] The method for treating stainless steel pickling wastewater of the present invention has the following advantages: 1) This invention achieves full utilization of steel pickling waste. The treatment method of this invention achieves a recovery rate of over 95% for heavy metals such as Cr and Ni, and F... - and SO4 2- The recycling rate reaches over 90%, the Fe recycling rate reaches over 95%, and the overall comprehensive utilization rate of steel pickling waste reaches over 90%.

[0030] 2) Various resources are classified and graded for recycling, maximizing the utilization value of waste. This project can achieve a high proportion of recovery of high-value heavy metals Cr and Ni from pickling waste; low-toxicity Fe is made into high-value water treatment phosphorus removal packing; F ions are separated and recovered as CaF2 for use as a flux in metal smelting; and waste acid is reused in the preparation of pickling solutions.

[0031] 3) Develop new technologies for heavy metal recovery and utilization using electroreduction methods to achieve highly selective separation and high-proportion recovery of metals, enabling the resource utilization of metal ions. A new approach to the comprehensive utilization of low-toxicity, iron-rich sludge in the preparation of phosphorus removal packing materials for water treatment is proposed, realizing the high-value utilization of iron-rich sludge and providing a new solution for the current treatment of eutrophication in water bodies.

[0032] This invention also provides a treatment system for stainless steel pickling wastewater, comprising: The distillation defluorination unit is used to add sulfuric acid to stainless steel pickling wastewater, heat and distill it to make the fluorine volatilize and be recovered in the form of hydrogen fluoride, and output defluorinated wastewater. The jaundice iron alum iron removal unit is connected to the outlet of the distillation defluorination unit. It is used to heat the defluorination wastewater and adjust the pH value to a first preset value, add sulfate containing monovalent cations, so that iron ions precipitate in the form of jaundice iron alum, separate solid and liquid, and output the first wastewater. The chromium precipitation unit is connected to the outlet of the potassium ferric sulfate iron removal unit. It is used to add alkali to the first wastewater, adjust the pH to a second preset value, so that chromium precipitates while nickel remains in the solution, thus achieving solid-liquid separation and outputting the second wastewater. The electrodeposition unit for removing nickel is connected to the outlet of the chromium precipitation unit. It performs constant voltage deposition on the second wastewater, causing nickel to precipitate as metallic nickel on the cathode surface, and outputs the third wastewater. The nanofiltration salt separation unit includes a first nanofiltration module and a second nanofiltration module connected in series. The inlet of the first nanofiltration module is connected to the outlet of the electrodeposition nickel removal unit, and the permeate outlet is connected to the inlet of the second nanofiltration module. The first nanofiltration module is configured to retain sulfate ions, and the second nanofiltration module is configured to enrich chloride ions on the permeate side. The evaporation crystallization unit includes a first evaporation crystallization submodule and a second evaporation crystallization submodule; the first evaporation crystallization submodule is connected to the retentate outlet of the first nanofiltration module and is used for evaporation crystallization to obtain sodium sulfate and mother liquor; the second evaporation crystallization submodule is connected to the permeate outlet of the second nanofiltration module and is used for evaporation crystallization to obtain sodium chloride; the mother liquor outlet of the first evaporation crystallization submodule is connected to the inlet of the jaundice iron alum iron removal unit, so that the mother liquor serves as a supplementary source of sulfate.

[0033] Preferably, the first evaporation crystallization submodule is a triple-effect evaporation crystallization device, comprising a first-effect evaporator, a second-effect evaporator, and a third-effect evaporator connected in sequence; The single-effect evaporator is connected to an external steam pipeline; The second-effect evaporator receives secondary steam generated by the first-effect evaporator through a steam pipe. The triple-effect evaporator receives secondary steam generated by the double-effect evaporator through a steam pipe.

[0034] The beneficial effects of this invention are: The present invention provides a method for treating stainless steel pickling wastewater. First, fluoride is deeply removed by distillation, effectively avoiding interference from fluoride ions in subsequent metal separation processes. Fluorine is selectively volatilized and recovered in the form of hydrogen fluoride, not only realizing the resource utilization of fluorine but, more importantly, eliminating the possibility of fluorine forming stable complexes with metals such as iron, chromium, and nickel. This provides a clean reaction environment for subsequent potassium ferric sulfate precipitation, selective precipitation of chromium, and electrochemical deposition of nickel. Second, the potassium ferric sulfate method is used to efficiently remove iron ions under strongly acidic conditions, while avoiding the introduction of foreign impurities such as calcium and aluminum. This achieves directional separation of iron without loss of chromium and nickel, significantly reducing the load on subsequent treatment units and preventing co-deposition of iron during electrodeposition or co-precipitation during precipitation, thereby improving the purity of the heavy metal recovery product. Third, the synergistic effect of deep defluorination and efficient iron removal creates the necessary conditions for the high-value recovery of nickel. The solution obtained after defluorination, iron removal and chromium removal has a high concentration of nickel ions and extremely low impurity content, which makes the constant voltage electrodeposition process stable and efficient, and can directly obtain high-purity metallic nickel without additional purification steps, significantly improving the resource recovery value.

[0035] The present invention provides a method for treating stainless steel pickling wastewater. The overall process flow achieves step-by-step removal of pollutants and tiered recovery of valuable components, with each unit exhibiting a strong synergistic effect. Front-end purification ensures the stable operation of the back-end membrane separation and evaporation crystallization systems. Nanofiltration achieves high salt separation efficiency, and the purity of the resulting sodium sulfate product meets industrial standards. Simultaneously, the evaporation crystallization mother liquor, having already had interfering components such as fluoride and iron removed at the front end, can be safely reused in the potassium ferric sulfate reaction process, achieving the recycling of monovalent cations and sulfate ions and preventing impurity accumulation. Furthermore, the entire process generates no process wastewater discharge, and fluoride, iron, chromium, nickel, and sodium sulfate are all utilized as resources, truly achieving zero liquid discharge and demonstrating significant environmental and economic benefits. This method has significant application value in the field of wastewater treatment technology. Attached Figure Description

[0036] Figure 1 A flowchart illustrating the treatment method for stainless steel pickling wastewater; Figure 2 A roadmap for the treatment of stainless steel pickling wastewater; Figure 3 This is a process flow diagram for defluorination and iron removal in the treatment of stainless steel pickling wastewater. Figure 4 This is a process flow diagram for chromium and nickel recovery in the treatment of stainless steel pickling wastewater. Figure 5 A summary chart of pH values ​​for metal ion precipitation; Figure 6 This is a schematic diagram of the treatment system for stainless steel pickling wastewater. Figure 7This is a process flow diagram for the evaporation and crystallization of the retentate in the first nanofiltration module; Among them, 1-distillation defluorination unit; 2-potassium ferric sulfate deferral unit; 3-chromium precipitation unit; 4-electrodeposition denickel removal unit; 5-nanofiltration salt separation unit; 51-first nanofiltration module; 52-second nanofiltration module; 6-evaporation crystallization unit; 61-first evaporation crystallization sub-module; 62-second evaporation crystallization sub-module. Detailed Implementation

[0037] The following description, with reference to preferred embodiments, illustrates the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are merely illustrative of the present invention and not intended to limit the scope of protection of the present invention.

[0038] The present invention aims to disclose a method and system for treating stainless steel pickling wastewater, so as to simultaneously solve the problems of low pollutant removal efficiency, poor resource utilization, and serious secondary pollution in existing stainless steel pickling wastewater treatment methods.

[0039] like Figures 1 to 4 As shown, the treatment method for stainless steel pickling wastewater includes the following steps: S1. Defluorination: Concentrated sulfuric acid is added to the stainless steel pickling wastewater and heated to the first preset temperature for distillation, so that the fluorine evaporates and is recovered in the form of hydrogen fluoride, resulting in defluorinated wastewater. The distillation method can efficiently remove fluorine from the stainless steel pickling wastewater, realize the recycling of fluorine resources, and avoid secondary pollution.

[0040] S2. Iron removal: The defluorination wastewater is heated to the second preset temperature, and sulfate containing monovalent cations is added to precipitate iron ions in the form of potassium ferric alum. Solid-liquid separation is performed to obtain the first precipitate potassium ferric alum and the first wastewater. S3. Chromium Removal: Add alkali to the first wastewater to adjust the pH to the first preset value, causing chromium to precipitate while nickel remains in the solution, thus separating the solid and liquid to obtain the second precipitate, chromium hydroxide, and the second wastewater. By adjusting the pH value, trivalent chromium in the stainless steel pickling wastewater precipitates as Cr(OH)3, while nickel remains in the solution, achieving selective separation of chromium and ensuring high-purity recovery of nickel at the same time. S4. Nickel Removal: Constant voltage deposition is performed on the second wastewater to precipitate nickel ions as metallic nickel on the cathode surface, yielding metallic nickel and the third wastewater. Based on the deep purification at the front end, precise electrochemical reduction is carried out on the nickel-containing second wastewater with low impurity content, avoiding co-deposition interference and directly obtaining high-purity metallic nickel. This breaks through the technical bottleneck of traditional chemical precipitation methods, which can only recover low-value hydroxides or mixed sludge, and provides an efficient and clean new path for heavy metal resource utilization.

[0041] S5. Nanofiltration for salt separation: The third wastewater is sequentially passed through the first and second nanofiltration modules. Specifically, the third wastewater is passed through the first nanofiltration module, and the permeate from the first nanofiltration module is then passed through the second nanofiltration module. The first nanofiltration module retains sulfate ions, while the second nanofiltration module enriches chloride ions on the permeate side, resulting in a retentate retained by the first nanofiltration module and a permeate that has passed through the second nanofiltration module. By retaining sulfate ions in the first nanofiltration module and allowing chloride ions to pass through in the second nanofiltration module, the salts in the wastewater are effectively separated and concentrated, which not only improves the purity of the salt product but also provides conditions for subsequent evaporation and crystallization.

[0042] S6, Evaporation and Crystallization: The retentate from the first nanofiltration module is evaporated and crystallized to obtain sodium sulfate; the mother liquor from the evaporation and crystallization is returned to S2 as a source of sulfate replenishment; Sodium chloride is obtained by evaporating and crystallizing the permeate from the second nanofiltration module. The mother liquor from the evaporation and crystallization of the first nanofiltration module is returned to the iron removal process as a sulfate replenishment source, realizing resource recycling within the system, reducing raw material consumption and waste generation, and conforming to the principles of green chemistry.

[0043] The stainless steel pickling wastewater contained 54 g / L of iron (Fe), 16 g / L of nickel (Ni), 28 g / L of chromium (Cr), and SO42-4 g / L. 2- The content is 95 g / L, F - The content is 59 g / L, and the TDS (total dissolved solids / total dissolved solids) content is 254 g / L; The pH value of the stainless steel pickling wastewater is 1.85.

[0044] The above-mentioned treatment method for stainless steel pickling wastewater firstly removes fluorides through distillation, effectively avoiding interference from fluoride ions in subsequent metal separation processes. Fluorine is selectively volatilized and recovered as hydrogen fluoride, not only realizing the resource utilization of fluorine but, more importantly, eliminating the possibility of fluorine forming stable complexes with metals such as iron, chromium, and nickel. This provides a clean reaction environment for subsequent potassium ferric sulfate precipitation, selective precipitation of chromium, and electrochemical deposition of nickel. Secondly, the potassium ferric sulfate method efficiently removes iron ions under strongly acidic conditions while avoiding the introduction of foreign impurities such as calcium and aluminum. This achieves directional separation of iron without loss of chromium and nickel, significantly reducing the load on subsequent treatment units and preventing co-deposition of iron during electrodeposition or co-precipitation during precipitation, thereby improving the purity of the heavy metal recovery products. Third, the synergistic effect of deep defluorination and efficient iron removal creates the necessary conditions for the high-value recovery of nickel. The solution obtained after defluorination, iron removal and chromium removal has a high concentration of nickel ions and extremely low impurity content, which makes the constant voltage electrodeposition process stable and efficient, and can directly obtain high-purity metallic nickel without additional purification steps, significantly improving the resource recovery value.

[0045] The overall treatment process achieves step-by-step removal of pollutants and tiered recovery of valuable components, with each unit exhibiting a strong synergistic effect. Front-end purification ensures the stable operation of the back-end membrane separation and evaporation crystallization systems. Nanofiltration achieves high salt separation efficiency, and the purity of the resulting sodium sulfate product meets industrial standards. Simultaneously, the evaporation crystallization mother liquor, having already had interfering components such as fluorine and iron removed at the front end, can be safely reused in the potassium ferric sulfate reaction process, achieving the recycling of monovalent cations and sulfate ions and preventing impurity accumulation. Furthermore, the entire process generates no process wastewater discharge, and fluorine, iron, chromium, nickel, and sodium sulfate are all utilized as resources, truly achieving zero liquid discharge and demonstrating significant environmental and economic benefits.

[0046] In some embodiments, in S1, the first preset temperature is set to 160~180℃. Within this temperature range, fluoride ions in the stainless steel pickling wastewater are fully converted into volatile hydrogen fluoride (HF) under the action of strong acid (sulfuric acid) and effectively evaporated with water vapor. This temperature is higher than the boiling point of the HF azeotrope, ensuring that HF ​​is fully released, resulting in a fluoride removal rate of over 99.9%, creating clean conditions for subsequent metal recovery. However, in actual experimental research, it was found that if the temperature is set <160℃, incomplete HF volatilization will occur, and residual fluoride will interfere with subsequent precipitation and electrodeposition of potassium ferric sulfate; if the temperature is set >180℃, it will easily exacerbate the corrosion of the equipment by sulfuric acid and promote the decomposition or coking of some metal sulfates, increasing the difficulty of system maintenance. 160~180℃ is the optimal balance range between fluoride removal efficiency, energy consumption control, and equipment tolerance. Furthermore, within this temperature range, the HF gas concentration is high and impurities are low, making it easy to obtain hydrofluoric acid through condensation absorption or further use in the synthesis of fluoride salts, achieving high-value utilization of fluoride.

[0047] One method for recovering acid is by distillation, which utilizes the principle that higher-boiling-point acids displace lower-boiling-point acids. Sulfuric acid is added to the waste acid, and hydrofluoric acid is distilled off. The recovered gaseous hydrofluoric acid can be condensed and reused in the pickling line. The sulfuric acid reacts with metal ions in the pickling wastewater in a metathesis reaction, producing metal sulfates. This method can recover over 90% of the hydrofluoric acid. The acids in stainless steel pickling wastewater are hydrofluoric acid and sulfuric acid. Since H₂SO₄ has a boiling point of 338℃, distillation is a feasible method for purifying HF.

[0048] In some embodiments, such as Figure 2 and Figure 3 As shown, fluoride can also be removed by calcium salt precipitation.

[0049] Lime can both precipitate fluoride ions and act as a neutralizing agent to adjust the pH value during the defluorination process. The reaction formula is: In some embodiments, S2, the monovalent cation includes Na. + NH4 + and K + At least one of them.

[0050] The general formula for potassium ferrous sulfate is AFe3(SO4)2(OH)6, where the A-site is a monovalent cation. This monovalent cation must have a suitable radius and stable charge to embed into the crystal lattice and form a stable precipitate. The main reaction equation is: 3Fe 3+ +A + +2SO4 2- +6H₂O→AFe₃(SO₄)₂(OH)₆ +6H + .

[0051] By limiting monovalent cations to include Na + NH 4+ and K + At least one of them, namely A + Selected from Na + NH4 + and K + At least one of them, Na + NH4 + or K + The ionic radius and charge characteristics of this substance both meet the requirements of the jaundice iron alum crystal structure, and can effectively promote Fe... 3+ Under acidic conditions, it forms a dense, easily settling alum precipitate, significantly improving iron removal efficiency while avoiding the formation of colloidal or amorphous ferric hydroxide. Furthermore, Na... + NH4 + and K+ The corresponding sulfates, such as Na₂SO₄, (NH₄)₂SO₄, and K₂SO₄, do not introduce difficult-to-remove impurities into the system. This is especially true when using Na… + or K + When it is returned to process S2 along with the mother liquor, it can be directly used as A. + Supplementary source; if NH4 is used + During distillation or evaporation, it can also be partially converted into ammonia for recovery, enhancing the flexibility of resource utilization. Meanwhile, Na... + NH4 + and K + Widely available, inexpensive, non-toxic, and harmless, it is suitable for large-scale industrial applications. Compared to other monovalent cations, it significantly reduces reagent costs. Because the selected cation can be safely reused in the iron removal process with the triple-effect evaporation mother liquor, it will not accumulate harmful byproducts in the system, effectively supporting the closed-loop process goals of mother liquor recycling, reagent reduction, and zero liquid discharge.

[0052] In some embodiments, in S2, the second preset temperature is set to 90~98℃, and the pH value of the defluoridated wastewater is 1.5~2.0. Using a sulfate containing a monovalent cation at a specific pH value can promote the formation of a stable precipitate of ferrous sulfate ions, thereby effectively separating iron ions and reducing the co-precipitation problem that may occur during subsequent treatment.

[0053] The formation of potassium ferrous sulfate (AFe3(SO4)2(OH)6) is highly sensitive to temperature. Within the temperature range of 90–98 °C, Fe… 3+ Optimal equilibrium between hydrolysis and crystallization kinetics favors the formation of dense alum precipitates with large particles, rapid settling, and good filterability, significantly improving solid-liquid separation efficiency and iron removal rate. In actual experimental research, it was found that if the temperature is set below 90℃, a colloidal precipitate easily forms, making settling difficult; if the temperature is set above 98℃, it easily leads to the decomposition of some hydroxysulfates or structural disorder. A temperature range of 90~98℃ combined with strongly acidic conditions at pH 1.5~2.0 can ensure the removal of Cr. 3+ and Ni 2+ Maintaining a dissolved state avoids the co-precipitation of non-target metals caused by local overheating or alkalinity fluctuations, thus providing high-purity influent water for subsequent S3 selective chromium removal and S4 high-purity nickel electrodeposition.

[0054] In some embodiments, to ensure the complete precipitation of S2, a neutralizing agent needs to be continuously added during the reaction to neutralize the hydrogen ions formed during the reaction and to ensure that the pH is maintained at 1.5~2.

[0055] In some embodiments, the neutralizing agent is selected from calcium carbonate (CaCO3).

[0056] In some embodiments, after S2, the method further includes: detecting the concentrations of fluoride ions and iron ions in the first wastewater; if the fluoride ion concentration is higher than a first threshold, returning the first wastewater to S1 for redistillation to remove fluoride; if the iron ion concentration is higher than a second threshold, repeating the S2 operation on the first wastewater for further precipitation treatment with potassium ferric sulfate.

[0057] Because the composition of stainless steel pickling wastewater fluctuates significantly, single-stage defluoridation or iron removal may fail to achieve the desired effect due to sudden changes in influent load. By monitoring the concentration of fluoride and iron ions in real-time / batch and setting reasonable thresholds, it is possible to dynamically determine whether secondary treatment is necessary. This effectively avoids residual fluoride complexing with metal ions or residual iron interfering with subsequent chromium removal and nickel electrodeposition, ensuring the stability of the entire process. Furthermore, this preferred implementation method only initiates the rework process when the detected levels exceed the limits, avoiding the energy and time waste caused by indiscriminate multiple treatments. It achieves on-demand control and precise treatment, optimizing operating costs while ensuring effluent quality.

[0058] In some embodiments, the first threshold is set to 0.05 g / L and the second threshold is set to 1.0 g / L.

[0059] In some embodiments, the first wastewater after iron removal enters an equalization tank, where the pH is adjusted to 6-7 with NaOH to prevent nickel precipitation. The precipitated Cr(OH)3 is recovered. The filtrate enters a nickel electrodeposition reactor to recover elemental Ni, and the supernatant enters a nanofiltration system. The chromium recovery rate is ≥98%, and the nickel recovery rate is ≥95%.

[0060] The pH values ​​for metal ion precipitation are summarized as follows: Figure 5 As shown.

[0061] In some embodiments, in S3, the first preset value is set to 6.0~7.0. When the first preset value is set in the range of 6.0~7.0, trivalent chromium ions (Cr)... 3+ It can be completely hydrolyzed to form Cr(OH)3 precipitate, with a high precipitation rate and good settling performance. Meanwhile, nickel ions (Ni...) 2+ Within this pH range, it remains stable in solution, effectively avoiding nickel co-precipitation loss and ensuring a high-concentration, high-purity nickel-containing solution for subsequent electrodeposition processes. In actual experimental research, it was found that when the pH value is below 6.0, Cr(OH)3 precipitation is incomplete, and residual chromium may enter the subsequent system, affecting the quality of nickel deposition. When the pH value is above 7.0, it easily leads to Ni… 2+ Partial hydrolysis forms mixed hydroxide sludge, reducing nickel recovery and increasing the burden of solid waste treatment. Therefore, by precisely controlling the pH at 6.0–7.0, high-purity Cr(OH)3 precipitate can be obtained, facilitating resource utilization, such as the preparation of chromium salts or pigments.

[0062] In some embodiments, the base is selected from sodium hydroxide (NaOH).

[0063] In some embodiments, S4 specifically includes: adjusting the pH of the second wastewater to 3.5 to 4.5, and applying a constant voltage of -1.0 V to -1.3 V at a temperature of 25 to 35°C for electrodeposition.

[0064] In some embodiments, a three-electrode system is used for constant voltage deposition, wherein an iridium-tantalum-titanium electrode is selected as the counter electrode, a saturated calomel electrode is selected as the reference electrode, and a stainless steel substrate is selected as the working electrode.

[0065] By controlling the pH within the range of 3.5 to 4.5, both the violent hydrogen evolution reaction at excessively low pH (<3.5) leading to a decrease in current efficiency and the initiation of nickel ion (Ni) reaction at excessively high pH (>4.5) are avoided. 2+ Hydrolysis generates Ni(OH)₂ precipitate, ensuring stable and dense metallic nickel deposition at the cathode, thus improving product purity. Furthermore, electrodeposition at a mild temperature of 25–35°C promotes ion migration and electrode reaction equilibrium, preventing excessive corrosion from high temperatures or excessively slow deposition rates from low temperatures. Simultaneously, a constant voltage of -1.0 V to -1.3 V effectively drives nickel ion reduction while avoiding excessively negative voltage that could lead to impurity co-deposition or a loose coating, resulting in dense, high-purity metallic nickel. This optimal parameter window is optimized based on the water quality characteristics (low impurities, high nickel ion concentration) of stainless steel pickling wastewater after previous defluorination, iron removal, and chromium removal processes. This allows the electrodeposition system to operate stably for extended periods with low energy consumption, requiring minimal maintenance or adjustments, making it suitable for continuous industrial operation.

[0066] In some embodiments, in S5, the first nanofiltration module is a polyamide thin-layer composite nanofiltration membrane (TFC NF membrane), which can achieve high sulfate rejection and high chloride ion permeability; in S5, the second nanofiltration module is a negatively charged polyamide TFC nanofiltration membrane, which can achieve high chloride ion permeability.

[0067] In some embodiments, in S6, the retentate from the first nanofiltration module is evaporated and crystallized to obtain sodium sulfate. The mother liquor from the evaporation and crystallization is returned to S2 as a source of sulfate replenishment. Specifically, this includes: passing the retentate from the first nanofiltration module sequentially through a first-effect evaporator, a second-effect evaporator, and a third-effect evaporator for multi-stage evaporation and concentration. The first-effect evaporator is heated by external steam, and the generated secondary steam is used as a heat source for the second-effect evaporator and the third-effect evaporator in sequence. The concentrated sodium sulfate solution enters a crystallizer to precipitate sodium sulfate decahydrate crystals. After solid-liquid separation, sodium sulfate product and mother liquor are obtained. The mother liquor is returned to S2 as a source of monovalent cations and sulfate ions.

[0068] By using a series design of first-effect, second-effect, and third-effect evaporators, the secondary steam generated in the first effect is used sequentially as the heat source for the second and third effects, achieving cascaded utilization of thermal energy. Compared to single-effect evaporation, steam consumption can be reduced by more than 60%, significantly reducing operating costs and meeting the requirements of green and low-carbon processes. Under multi-stage concentration and temperature-controlled crystallization conditions, sodium sulfate decahydrate crystals can be stably precipitated with high purity, meeting the GB / T6009-2014 industrial-grade standard, possessing resource utilization value, and avoiding the problem of mixed salt disposal.

[0069] The mother liquor obtained from evaporation and crystallization is rich in uncrystallized monovalent cations and sulfate ions, which perfectly match the reaction of potassium ferric sulfate in S2 with A. + and SO4 2- To meet the demand, it is directly returned to S2, which not only replaces part of the purchased sulfate reagent, but also avoids the accumulation of impurities and ensures the long-term stable operation of the entire process.

[0070] In some embodiments, such as Figure 6 As shown, a treatment system for stainless steel pickling wastewater is also provided, comprising: Distillation defluorination unit 1 is used to add sulfuric acid to stainless steel pickling wastewater, heat it to a first preset temperature for distillation, so that fluorine volatilizes and is recovered in the form of hydrogen fluoride, and outputs defluorinated wastewater. The 2nd iron removal unit of potassium ferric alum is connected to the outlet of the distillation defluorination unit 1. It is used to heat the defluorination wastewater to the second preset temperature, adjust the pH to the first preset value, and add monovalent cation sulfate to precipitate iron ions in the form of potassium ferric alum, thereby achieving solid-liquid separation and outputting the first wastewater. Chromium precipitation unit 3 is connected to the outlet of jaundice iron alum iron removal unit 2. It is used to add alkali to the first wastewater, adjust the pH to the second preset value, so that chromium precipitates while nickel is retained in the solution. After solid-liquid separation, the second wastewater is output. Electrodeposition nickel removal unit 4 is connected to the outlet of chromium precipitation unit and is used to perform constant voltage deposition on the second wastewater, so that nickel is deposited as metallic nickel on the cathode surface and the third wastewater is output. The nanofiltration salt separation unit 5 includes a first nanofiltration module 51 and a second nanofiltration module 52 connected in series. The inlet of the first nanofiltration module 51 is connected to the outlet of the electrodeposition nickel removal unit 4, and the permeate outlet of the first nanofiltration module 51 is connected to the inlet of the second nanofiltration module 52. The first nanofiltration module 51 is configured to retain sulfate ions, and the second nanofiltration module 52 is configured to enrich chloride ions on the permeate side. The evaporation crystallization unit 6 includes a first evaporation crystallization sub-module 61 and a second evaporation crystallization sub-module 62; The first evaporation and crystallization sub-module 61 is connected to the retentate outlet of the first nanofiltration module 51 and is used for evaporation and crystallization to obtain sodium sulfate and generate mother liquor; The second evaporation crystallization submodule 62 is connected to the permeate outlet of the second nanofiltration module 52 and is used for evaporation crystallization to obtain sodium chloride; wherein, the mother liquor outlet of the first evaporation crystallization submodule 61 is connected back to the inlet of the jaundice iron alum removal unit 2 as a supplementary source of sulfate.

[0071] In some embodiments, the first evaporation crystallization submodule 61 is a triple-effect evaporation crystallization device, including a first-effect evaporator, a second-effect evaporator and a third-effect evaporator connected in sequence. The first-effect evaporator is connected to an external steam pipeline, and the second-effect evaporator and the third-effect evaporator receive secondary steam generated in the previous stage through steam pipelines, respectively.

[0072] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the method for treating stainless steel pickling wastewater of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. Obviously, the specific embodiments described are only some embodiments of this application, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this application or its application. Based on the specific embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0073] Where specific techniques or conditions are not specified in the detailed embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products. Example 1

[0074] The stainless steel pickling wastewater used below contains 54 g / L of iron (Fe), 16 g / L of nickel (Ni), 28 g / L of chromium (Cr), and SO42-. 2- The content is 95 g / L, F - The content was 59 g / L, and the TDS (total dissolved solids / total dissolved solids) content was 254 g / L. The specific water quality indicators of stainless steel pickling wastewater are shown in Table 1.

[0075] Table 1 shows the specific water quality indicators for stainless steel pickling wastewater. The determination of iron (Fe) content in stainless steel pickling wastewater is based on GB 11911-89 "Determination of Iron and Manganese in Water - Flame Atomic Absorption Spectrophotometry", while the determination of nickel (Ni) and chromium (Cr) content is based on HJ 1453-2026, SO4 2- The determination of sulfate content was performed in accordance with HJ / T 342-2007 "Determination of Sulfate in Water - Barium Chromate Spectrophotometric Method (Trial)", F -The determination of fluoride content is based on GB / T 7484-1987 "Determination of Fluoride in Water - Ion Selected Electrode Method". The determination of TDS (Total Dissolved Solids) content is based on GB / T 6908-2018 conductivity conversion method. The same applies to subsequent wastewater determination.

[0076] like Figures 1 to 4 As shown, the stainless steel pickling wastewater treatment system is used for the treatment of stainless steel pickling wastewater, and the method includes the following steps: S1. Defluorination: In the distillation defluorination unit 1, to investigate the effect of the amount of pure sulfuric acid added on the fluoride removal rate, multiple experimental groups were set up. That is, 0 mL, 10 mL, 15 mL, 20 mL, 25 mL and 30 mL of 98% concentrated sulfuric acid were added to 100 mL of stainless steel pickling wastewater, respectively, and evaporated at 170℃ for 1 h to obtain defluorinated wastewater. After evaporation, the concentration of fluoride in the defluorinated wastewater was measured, and the results are shown in Table 2.

[0077] Table 2 shows the fluoride (F) content in the defluorination wastewater after evaporation. - concentration of) Analysis of Table 2 shows that when the sulfuric acid addition is 25 mL / 100 mL for stainless steel pickling wastewater, boiling at atmospheric pressure for 1 hour achieves a 96% HF evaporation removal rate, and the fluoride concentration decreases from the initial 59 g / L to 2.45 g / L, demonstrating a significant removal effect. At this point, the solution is almost completely evaporated, forming sulfate crystals, and the residual liquid has a pH below -0.4, making it suitable for acid recovery. These conditions represent the optimal parameters for the fluoride removal process, balancing efficiency and economy.

[0078] Table 3 shows a comparison of the concentrations of fluoride, iron, chromium, and nickel in the defluorinated wastewater after defluorination of stainless steel pickling wastewater with a sulfuric acid addition of 25 mL / 100 mL and the original stainless steel pickling wastewater.

[0079] Table 3 shows the comparison results of the content of each element in the wastewater before and after distillation. S2. Iron Removal: In the iron removal unit 2 (potassium ferric alum), the defluorinated wastewater obtained after defluorination of stainless steel pickling wastewater (25 mL / 100 mL sulfuric acid added in S1) is heated to 95°C. The pH is adjusted to 1.5 using calcium carbonate (CaCO3) and sulfuric acid as neutralizing agents, and the stirring speed is adjusted to 350 r / min. Sodium sulfate (Na2SO4) is added to initiate precipitation, causing iron ions to precipitate as potassium ferric alum. During the precipitation process, the pH is continuously stabilized using calcium carbonate as a neutralizing agent. Solid-liquid separation is achieved, yielding the first precipitate and the first wastewater. Based on the iron ion concentration in the defluorinated wastewater, the excess sodium sulfate (Na2SO4) coefficient is 1.5, the precipitation time is 3 hours, and the iron removal rate is 98.84%. The first precipitate, potassium ferric alum, is used for recycling.

[0080] Table 4 shows a comparison of the concentrations of iron, chromium, and nickel in the first wastewater after defluorination and iron removal from stainless steel pickling wastewater with a sulfuric acid addition of 25 mL / 100 mL, and the original stainless steel pickling wastewater.

[0081] Table 4 shows the comparison of element content between the raw wastewater and the first wastewater after iron removal. S3, Chromium Removal: In chromium precipitation unit 3, NaOH is added to the first wastewater obtained in S2 to adjust the pH to 6.0, causing chromium to precipitate while nickel remains in the solution. Solid-liquid separation yields the second precipitate and the second wastewater. Measurements show that the chromium removal rate is greater than 90%.

[0082] Table 5 shows a comparison of the concentrations of iron, chromium, and nickel in the second wastewater after defluorination, iron removal, and chromium removal of stainless steel pickling wastewater with a sulfuric acid addition of 25 mL / 100 mL, and the original stainless steel pickling wastewater.

[0083] Table 5 shows the comparison of element content between the raw wastewater and the second wastewater after chromium removal. S4. Nickel Removal: In the nickel removal electrodeposition unit 4, the second wastewater obtained in S3 is subjected to constant voltage deposition, including: adjusting the pH of 50 mL of the second wastewater to 4.0 with sodium hydroxide, and then using a three-electrode system with an iridium-tantalum-titanium electrode as the counter electrode, a saturated calomel electrode as the reference electrode, and a stainless steel substrate as the working electrode, applying a constant voltage of -1.2 V at a temperature of 30℃ for electrodeposition, setting the distance between each electrode to 1 cm, and the electrodeposition time to 1~4 h, so that nickel ions are deposited as metallic nickel on the cathode surface, and the third wastewater is obtained.

[0084] The concentration of nickel in the wastewater was measured after electrodeposition for 1 h, 2 h, 3 h and 4 h, and the results are shown in Table 6.

[0085] Table 6 shows the nickel concentration in the wastewater after 1, 2, 3, and 4 hours of electrodeposition. As shown in Table 6, the nickel electrodeposition process exhibits a significant time dependence. Under pH conditions of 4.06–4.16, the nickel ion recovery rate gradually increased from 22.69% after 1 hour to 91.31% after 4 hours with prolonged deposition time. The deposition phenomenon evolved from the initial formation of a small amount of solid to the formation of a dense nickel film, indicating good film formation performance on the electrode surface. Considering both recovery efficiency and energy consumption, it is advisable to control the electrodeposition time to 3–4 hours.

[0086] Table 7 shows a comparison of the concentrations of chromium and nickel in the third wastewater after defluorination, iron removal, chromium removal, and nickel removal of stainless steel pickling wastewater with a sulfuric acid addition of 25 mL / 100 mL, and the original stainless steel pickling wastewater.

[0087] Table 7 shows the comparison of element content between the raw wastewater and the third wastewater after nickel removal. S5, Nanofiltration and Salt Separation: In nanofiltration and salt separation unit 5, the third wastewater obtained in S4 after electrodeposition for 4 hours is passed into the first nanofiltration module (NF1) 51 made of a polyamide thin-layer composite nanofiltration membrane (TFC NF membrane). The permeate of the first nanofiltration module 51 is then passed into the second nanofiltration module 52 made of a negatively charged polyamide TFC nanofiltration membrane. The first nanofiltration module 51 retains sulfate ions, and the second nanofiltration module (NF2) 52 enriches chloride ions on the permeation side.

[0088] The two-stage nanofiltration design for salt separation is shown in Table 8.

[0089] S6, Evaporation and Crystallization: In evaporation and crystallization unit 6, such as... Figure 7 As shown, the retentate from the first nanofiltration module 51 is evaporated and crystallized to obtain sodium sulfate decahydrate Na2SO4·10H2O. The purity of the obtained sodium sulfate is ≥98%, meeting the Class II standard of GB / T 6009-2014. The mother liquor from the evaporation and crystallization is returned to S2 as a supplementary source of sulfate. The permeate from the second nanofiltration module 52 is evaporated and crystallized by MVR (Mechanical Vapor Recompression) in the second evaporation and crystallization sub-module 62 to obtain sodium chloride.

[0090] Specifically, the retentate from the first nanofiltration module (NF1) 51 is sequentially passed into the first-effect evaporator, second-effect evaporator, and third-effect evaporator of the first evaporation and crystallization sub-module 61 for multi-stage evaporation and concentration. The first-effect evaporator is heated by external steam, and the generated secondary steam is used as the heat source for the second-effect evaporator and the third-effect evaporator in turn. The sodium sulfate solution is concentrated to supersaturation and then enters the crystallizer to precipitate sodium sulfate decahydrate crystals. After solid-liquid separation, sodium sulfate product and mother liquor are obtained. The mother liquor is returned to the jaundice iron alum process of S2 as a supplementary source of monovalent cations and sulfate ions.

[0091] Table 8 shows the two-stage nanofiltration design. In summary, the stainless steel pickling wastewater treatment method of this invention, through a synergistic process of deep defluorination, iron removal with potassium ferric sulfate, selective precipitation for chromium removal, nickel recovery by electrodeposition, and nanofiltration-evaporation for salt separation, achieves a recovery rate of over 90% for heavy metals such as chromium and nickel in the wastewater, a recovery rate of over 90% for fluoride and sulfate ions, and an iron recovery rate of over 95%. The overall resource utilization rate reaches over 90%, significantly superior to traditional precipitation-landfill or simple neutralization treatment methods. Specifically, fluorine is recovered in the form of high-purity HF, which can be used in fluorochemical production, and the purified acidic solution can be partially reused in the pickling process after adjustment, significantly reducing fresh acid consumption and enhancing resource utilization value. Iron is separated in the structurally stable form of potassium ferric sulfate, possessing potential application as a phosphorus removal filler in water treatment. High-value heavy metals chromium and nickel are recovered in the forms of Cr(OH)3 and metallic nickel, respectively, which can be directly used as chemical raw materials or battery materials, demonstrating significant application value in the field of wastewater treatment technology.

[0092] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

Claims

1. A method for treating stainless steel pickling wastewater, characterized in that, Includes the following steps: S1. Defluorination: Concentrated sulfuric acid is added to stainless steel pickling wastewater, and the mixture is heated and distilled to obtain defluorinated wastewater; S2, Iron removal: The defluorination wastewater is heated, and sulfate containing monovalent cations is added. Solid-liquid separation is performed to obtain the first precipitate and the first wastewater. S3. Chromium removal: Add alkali to the first wastewater to adjust the pH to the first preset value, and separate the solid and liquid to obtain the second precipitate and the second wastewater. S4. Nickel removal: Constant voltage deposition is performed on the second wastewater to obtain the third wastewater; S5. Nanofiltration desalination: The third wastewater is sequentially passed through the first nanofiltration module and the second nanofiltration module to obtain the retentate retained by the first nanofiltration module and the permeate that passes through the second nanofiltration module. S6. Evaporation and crystallization: Evaporate and crystallize the retentate to obtain sodium sulfate; evaporate and crystallize the permeate to obtain sodium chloride.

2. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, In S1, the heating and distillation temperature is 160~180℃.

3. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, In step S2, the defluoridated wastewater is heated to 90~98℃; And / or, in S2, the pH value of the defluoridated wastewater is 1.5~2.0; And / or, in S2, the monovalent cation in the sulfate containing a monovalent cation is selected from Na. + NH4 + and K + At least one of them; And / or, in S2, the first precipitate is jaundice.

4. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, In S3, the base is selected from sodium hydroxide; And / or, in S3, the first preset value is 6.0~7.0; And / or, in S3, the second precipitate is chromium hydroxide.

5. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, S4 includes: The pH of the second wastewater was adjusted to 3.5-4.5 using concentrated sulfuric acid, and the temperature was raised to 25-35℃. Then, a constant voltage of -1.0V to -1.3V was applied for constant voltage deposition, so that nickel ions in the second wastewater were deposited as metallic nickel on the cathode surface.

6. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, In S5, the first nanofiltration module is a polyamide thin-layer composite nanofiltration membrane (TFC NF membrane). And / or, in S5, the second nanofiltration module is a negatively charged polyamide TFC nanofiltration membrane; And / or, in S6, the mother liquor obtained from evaporation and crystallization is returned to S2 as a source of sulfate replenishment.

7. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, The stainless steel pickling wastewater contained 54 g / L of iron (Fe), 16 g / L of nickel (Ni), 28 g / L of chromium (Cr), and SO42-. 2- The content is 95 g / L, F - The content is 59 g / L, and the TDS (total dissolved solids / total dissolved solids) content is 254 g / L; The pH value of the stainless steel pickling wastewater is 1.

85.

8. The method for treating stainless steel pickling wastewater according to claim 1, characterized in that, S2 further includes: detecting the concentrations of fluoride ions and iron ions in the first wastewater; If the fluoride ion concentration is higher than the first threshold, the first wastewater is returned to S1 for reheating and distillation; the first threshold is 0.05 g / L. If the iron ion concentration is higher than the second threshold, the first wastewater is subjected to the S2 operation again; the second threshold is 1.0 g / L.

9. A treatment system for stainless steel pickling wastewater, characterized in that, include: The distillation defluorination unit is used to add sulfuric acid to stainless steel pickling wastewater, heat and distill it, and output defluorinated wastewater. The jaundice iron alum iron removal unit is connected to the outlet of the distillation defluorination unit. It is used to heat the defluorination wastewater and adjust the pH value to a first preset value, add sulfate containing monovalent cations, separate solid and liquid, and output the first wastewater. The chromium precipitation unit is connected to the outlet of the potassium ferric sulfate iron removal unit. It is used to add alkali to the first wastewater, adjust the pH to a second preset value, so that chromium precipitates while nickel remains in the solution, thus achieving solid-liquid separation and outputting the second wastewater. The electrodeposition unit for removing nickel is connected to the outlet of the chromium precipitation unit. It performs constant voltage deposition on the second wastewater, causing nickel to precipitate as metallic nickel on the cathode surface, and outputs the third wastewater. The nanofiltration desalination unit includes a first nanofiltration module and a second nanofiltration module connected in series. The inlet of the first nanofiltration module is connected to the outlet of the electrodeposition nickel removal unit, and the permeate outlet is connected to the inlet of the second nanofiltration module. The evaporation crystallization unit includes a first evaporation crystallization submodule and a second evaporation crystallization submodule; the first evaporation crystallization submodule is connected to the retentate outlet of the first nanofiltration module and is used for evaporation crystallization to obtain sodium sulfate and mother liquor; the second evaporation crystallization submodule is connected to the permeate outlet of the second nanofiltration module and is used for evaporation crystallization to obtain sodium chloride.

10. The stainless steel pickling wastewater treatment system according to claim 9, characterized in that, The first evaporation crystallization submodule is a triple-effect evaporation crystallization device, comprising a first-effect evaporator, a second-effect evaporator, and a third-effect evaporator connected in sequence; The single-effect evaporator is connected to an external steam pipeline; The second-effect evaporator receives secondary steam generated by the first-effect evaporator through a steam pipe. The triple-effect evaporator receives secondary steam generated by the double-effect evaporator through a steam pipe.

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