Process and system for treating stainless steel wastewater containing transition metal ions and acids

Abstract

This invention relates to the field of wastewater treatment technology, and more particularly to a treatment process and system for stainless steel wastewater containing transition metal ions and acids. The treatment process includes the following steps: adding sodium hydroxide to the wastewater to convert the contained transition metal ions into various hydroxide precipitates under different pH conditions, achieving separation; adding alkaline earth metal oxides or hydroxides and coagulants to the wastewater to convert the contained fluoride ions into fluoridated alkaline earth precipitates, achieving separation; first, thoroughly removing the transition metal ions from the wastewater using a cation exchange resin, then using a reduction technology to reduce sodium nitrate in the wastewater to nitrogen gas and sodium hydroxide; finally, using an evaporation technology to obtain a concentrated sodium hydroxide solution from the wastewater. The above process can not only completely remove transition metal ions, fluoride ions, and nitrate ions from the wastewater, but also recover and reuse the sodium hydroxide consumed in the acid neutralization reaction and the excess sodium hydroxide added due to the precipitation of transition metal ions.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, and in particular to a stainless steel wastewater treatment process and system containing transition metal ions and acids. Background Technology

[0002] The stainless steel pickling line mainly produces two types of wastewater: one is neutral salt wastewater containing hexavalent chromium produced by the neutral salt electrolysis section, and the other is acidic wastewater produced by the mixed acid pickling section.

[0003] Currently, stainless steel plants use traditional neutralization methods to treat pickling wastewater. Neutral saline wastewater undergoes a chemical reduction reaction to reduce hexavalent chromium to trivalent chromium, and then enters a neutralization aeration tank along with acidic wastewater. A large (excessive) amount of lime is added to adjust the wastewater to a strongly alkaline state, causing heavy metal ions and fluoride ions to react with the lime, forming a mixture of heavy metal hydroxides and fluorides that precipitate. This precipitate is classified as hazardous solid waste, and its generation and subsequent treatment are extremely difficult, especially due to the large amount of fluorine present, making direct resource recovery of its heavy metal components impossible.

[0004] Traditional lime neutralization methods suffer from high treatment costs and the inability to recover heavy metals from sludge. Currently, methods using sodium hydroxide to partially replace lime as a neutralizing and precipitating agent (a new sludge reduction process) are emerging in China. For example, patent 200710067749.8, "Treatment Method for Stainless Steel Wastewater," discloses a method where sodium hydroxide is first added to the wastewater to adjust the pH to 9.0–9.5, precipitating heavy metal hydroxides. Then, hydrochloric acid, lime, and polyaluminum chloride are added, and calcium fluoride precipitate is formed at pH 8–9 to remove fluoride ions. Finally, the wastewater is adjusted to neutral with hydrochloric acid before discharge. The main drawbacks of this method are twofold: firstly, the resulting sludge-heavy metal hydroxide mixture limits its resource utilization; secondly, to increase the defluorination effect, a much higher amount of lime is used (the actual amount used is more than 1.5 times the theoretical amount), leading to increased fluoride-containing hazardous waste generation and lime costs. For example, the technology mentioned in patent CN200910101868.X, "A Method for Treating Stainless Steel Cold Rolling Pickling Wastewater," involves first adding lime slurry to adjust the pH of the wastewater to 5-6, then adding liquid alkali to adjust the pH to 9.0-9.5 to remove heavy metal ions from the wastewater, causing them to precipitate as heavy metal hydroxides. Then, by adding hydrochloric acid, polyaluminum chloride, and lime slurry, fluoride ions are precipitated and removed under pH conditions of 8-9. Finally, the wastewater is adjusted to neutral with hydrochloric acid before discharge. The main drawback of this method is that, because a large amount of calcium ions are introduced when the pH of the wastewater is adjusted to 5-6 with lime slurry, the subsequent addition of liquid alkali to adjust the pH to 9.0-9.5 inevitably generates calcium fluoride along with the heavy metal hydroxides, resulting in a large amount of fluoride mixed in with the heavy metal hydroxides. Therefore, this heavy metal hydroxide actually still belongs to solid hazardous waste.

[0005] Furthermore, existing stainless steel pickling processes all use a mixture of nitric acid and hydrofluoric acid as the pickling solution, resulting in wastewater containing a large amount of nitrate ions. Neither of the methods disclosed in the two patents addresses how to treat these nitrate ions in the wastewater. Additionally, both methods add hydrochloric acid in the defluorination step, but do not mention how to remove chloride ions from the wastewater. According to relevant environmental regulations, wastewater discharge standards have strict requirements regarding the content of nitrate and chloride ions. Therefore, neither of these methods can effectively achieve the harmless treatment of stainless steel wastewater. Summary of the Invention

[0006] The main objective of this invention is to provide a stainless steel wastewater treatment process and system containing transition metal ions and acids, aiming to improve the technical problems of existing stainless steel wastewater treatment methods where the precipitates (sludge and waste residue) obtained are difficult to recycle and utilize, and where effective and harmless treatment of stainless steel wastewater cannot be achieved.

[0007] To achieve the above objectives, this invention proposes a stainless steel wastewater treatment process containing transition metal ions and acid, comprising the following steps:

[0008] (1) Sodium hydroxide was added to stainless steel wastewater to adjust the pH value to 4.5-5.0. After solid-liquid separation and water washing, the resulting precipitate was used to obtain the first filtrate and iron hydroxide filter residue.

[0009] (2) After the first filtrate and the wastewater obtained during the water washing in step (1) are mixed, sodium hydroxide is added to adjust the pH value to 5.3-5.8. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the second filtrate and chromium hydroxide filter residue.

[0010] (3) After the second filtrate and the wastewater obtained during the water washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to ≥9.0. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the third filtrate and filter residue containing transition metal hydroxides.

[0011] (4) After the third filtrate and the wastewater obtained during the water washing in step (3) are mixed, alkaline earth metal alkali and coagulant are added to it. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the fourth filtrate and filter residue containing alkaline earth metal fluoride.

[0012] (5) The fourth filtrate and the wastewater obtained during the water washing in step (4) are mixed, treated with cation exchange resin first, and then reduced to obtain the fifth filtrate.

[0013] (6) After the fifth filtrate is evaporated, a concentrated alkaline solution containing sodium hydroxide and steam are obtained. The concentrated alkaline solution containing sodium hydroxide is recycled for adjusting the pH value in steps (1)-(3). The steam is discharged directly after condensation.

[0014] 1. The traditional lime method and the lime + sodium hydroxide precipitation method used in patent CN200910101868.X produce sludge containing a large amount of difficult-to-remove calcium fluoride after treating stainless steel wastewater. This makes it impossible to directly utilize the sludge as a resource, resulting in a high amount of hazardous waste. The heavy metal ion precipitation method at pH 9.0-9.5 used in patent 200710067749.8 produces a mixture of hydroxides of all transition metal elements contained in the wastewater, which dilutes the content of high-value elements, such as nickel, making it difficult to utilize these metal elements separately and reducing their value.

[0015] This invention effectively solves the aforementioned problems. By adding sodium hydroxide in stages, the acidity of stainless steel wastewater is gradually reduced, resulting in the stepwise precipitation of transition metal hydroxides with different compositions, thus enabling resource utilization. Firstly, sodium hydroxide is added to adjust the acidity to pH 4.5-5.0, achieving a 60-80% precipitation rate of iron ions in the stainless steel wastewater. When the acidity is adjusted to pH 5.3-5.8, the chromium ion precipitation rate reaches 70-90%, at which point most of the transition metal ions (mainly iron / chromium) have been removed. When the acidity is adjusted to pH ≥ 9.0, almost all transition metal ions, including iron, chromium, and nickel, are precipitated from the stainless steel wastewater. The three types of precipitates obtained using this process mainly contain >80wt% iron hydroxide (Fe(OH)3), >40wt% chromium hydroxide (Cr(OH)3), and >15wt% nickel hydroxide (Ni(OH)2). Because the contents of fluorine, calcium, and sodium are very low (all less than 1%), they can be directly utilized as resources. For example, the obtained ferric hydroxide and chromium hydroxide are high-quality raw materials for smelting iron and chromium, respectively. The appropriate combination of the two is also a high-quality raw material for preparing ceramic pigments. Nickel is an element with higher value than iron and chromium. Slag containing more than 4% nickel can be recycled.

[0016] 2. In patents CN200910101868 and CN200710067749.8, after adding alkali to neutralize the strongly acidic wastewater and adjusting the pH to 9.0-9.5 to precipitate hazardous solid slag containing transition metals and fluorine, the resulting process wastewater is strongly alkaline (provided by the alkali composed of sodium hydroxide and calcium hydroxide). To make it suitable for discharge, hydrochloric acid is added to the wastewater to neutralize the alkali until it becomes neutral. However, this results in the waste of alkali in the wastewater and the introduction of high concentrations of chloride ions that cannot be directly discharged. Furthermore, stainless steel wastewater generally contains large amounts of nitrate (5000-25000 mg / L). The aforementioned patents do not mention the treatment of nitrate in the wastewater, and according to the wastewater treatment methods provided in these patents, the nitrate content in the wastewater cannot be reduced in practice. Therefore, the treated stainless steel wastewater has both high nitrate content and alkalinity, making it unsuitable for direct discharge.

[0017] This invention also effectively solves the aforementioned problems. In this invention, after adding sodium hydroxide to stainless steel wastewater to adjust the acidity to pH ≥ 9.0 to remove transition metal ions and adding a slightly excessive amount of alkaline earth metal alkali, the resulting process wastewater still contains a small amount of alkaline earth metal ions. First, by using a cation exchange resin (as described in existing technology), the alkaline earth metal ions in the process wastewater are removed to avoid their potential adverse effects on the subsequent reduction reaction process. Then, a reduction technology is used to convert sodium nitrate in the process wastewater into nitrogen gas and sodium hydroxide. Finally, a high-efficiency evaporation technology is used to concentrate the process wastewater, obtaining a concentrated alkaline solution with a NaOH content of 20-40 wt%. This alkaline solution does not contain calcium or nitrate ions and can be directly reused to adjust the acidity of the stainless steel wastewater to precipitate transition metal ions such as iron, chromium, and nickel. This effectively reduces the consumption of sodium hydroxide during the stainless steel wastewater treatment process. The condensate obtained from the evaporation has extremely low levels of various metals, nitrogen, and fluorine (< 0.5 mg / L) and can be directly discharged.

[0018] This invention discloses a wastewater treatment process for stainless steel, specifically a process for treating wastewater generated during the pickling process of stainless steel. It is applicable to the recovery of valuable metal elements in the pickling wastewater treatment process of the stainless steel industry, significantly reducing the amount of hazardous waste residue generated, thereby reducing and saving the disposal costs of hazardous waste sludge and achieving harmless treatment of wastewater. First, sodium hydroxide is added to the wastewater, and a multi-stage stepwise neutralization and precipitation process is used to remove and recover the transition metal ions contained therein. Then, alkaline earth metal (compound) alkali and a coagulant are added to the wastewater to precipitate and remove the fluoride ions contained therein. Next, a reduction technology is used to remove sodium nitrate from the wastewater and convert it into sodium hydroxide. Subsequently, a concentrated sodium hydroxide solution is obtained from the wastewater using a high-efficiency evaporation technology, ultimately achieving resource recovery of transition metal ions, significantly reducing the amount of fluoride-containing hazardous waste residue generated, and achieving clean treatment and discharge of wastewater.

[0019] Preferably, the alkaline earth metal alkali in step (4) is at least one of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, barium oxide, and barium hydroxide. The alkaline earth metal alkali used is one or more of the oxides or hydroxides of magnesium, calcium, strontium, and barium; more preferably, the alkaline earth metal alkali is one of the oxides or hydroxides of calcium and magnesium, or a mixture thereof.

[0020] Preferably, the amount of alkaline earth metal alkali added in step (4) is such that the molar ratio of alkaline earth metal to fluoride ions in the third filtrate is 0.5 to 0.6. When the amount added is within the above range, the extraction effect on transition metals and fluoride is better.

[0021] Preferably, the coagulant in step (4) is at least one of calcium chloride, polyaluminum ferric chloride, polyaluminum chloride, polyferric sulfate, ferrous sulfate, aluminum sulfate, and ferric chloride. This solution allows for the adaptive selection of different coagulants based on actual conditions, achieving excellent treatment results for the filtrate, significantly accelerating sedimentation, and facilitating rapid removal of waste residue.

[0022] Preferably, the amount of coagulant added in step (4) is 0.05 to 0.2‰ of the mass of the third filtrate.

[0023] Preferably, the cation exchange resin in step (5) is a macroporous weak acid cation exchange resin.

[0024] Preferably, the reduction treatment in step (5) is any one of homogeneous catalytic hydrogenation treatment, heterogeneous catalytic hydrogenation treatment, low-valence metal reducing agent reduction treatment, and organic reducing agent reduction treatment.

[0025] Preferably, the evaporation process in step (6) is any one of multi-effect evaporation, mechanical vapor compression evaporation, or thermal vapor compression evaporation.

[0026] Preferably, the alkaline earth metal content and fluorine content of the iron-containing hydroxide filter residue, chromium-containing hydroxide filter residue, and transition metal-containing hydroxide filter residue are all <1 wt%. The hydroxide filter residues mainly composed of iron hydroxide, chromium hydroxide, and other transition metal elements such as nickel and manganese obtained through the above stepwise precipitation steps have sodium, alkaline earth metal, and fluorine content of less than 1 wt%, which is beneficial for resource utilization such as metal smelting and raw materials for ceramic glazes.

[0027] This invention also proposes a stainless steel wastewater treatment system, which includes a wastewater mixing tank, an iron precipitation tank, a chromium precipitation tank, a nickel precipitation tank, a fluoride removal tank, a cation exchange resin column, a reduction reaction tower, and an evaporator, connected in sequence. This stainless steel wastewater treatment system employs all the technical solutions described above, and therefore possesses at least all the effects of the aforementioned technical solutions, which will not be elaborated further here. Furthermore, operating the above treatment process through this system results in low cost and achieves good industrial-scale wastewater treatment.

[0028] Compared with the prior art, the stainless steel wastewater treatment process and system containing transition metal ions and acids of the present invention have the following beneficial effects:

[0029] (1) Compared with the traditional stainless steel wastewater lime treatment process, the stainless steel wastewater treatment process of this scheme can reduce the amount of sludge and hazardous waste disposal residue by more than 80 wt%.

[0030] (2) Through the stainless steel wastewater treatment process of this scheme, more than 99 wt% of heavy metal ions in stainless steel wastewater can be recovered as resources.

[0031] (3) Through the stainless steel wastewater treatment process of this scheme, more than 80 wt% of the transition metal nickel, which is present in small amounts but has higher value, can be recovered from the stainless steel wastewater.

[0032] (4) The stainless steel wastewater treatment process of this scheme can significantly reduce the amount of fluoride-containing hazardous waste residue generated during the treatment of stainless steel wastewater.

[0033] (5) The stainless steel wastewater treatment process of this scheme can effectively reduce the amount of sodium hydroxide used when treating stainless steel wastewater, and significantly reduce the total cost of wastewater treatment.

[0034] (6) Through the stainless steel wastewater treatment process of this scheme, the treated stainless steel wastewater can be discharged in a clean manner. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a flow chart of the stainless steel wastewater treatment process containing transition metal ions and acids according to this application.

[0037] In the attached diagram: 1-Wastewater mixing tank, 2-Iron precipitation tank, 3-Chromium precipitation tank, 4-Nickel precipitation tank, 5-Fluoride removal tank, 6-Cation exchange resin column, 7-Reduction reaction tower, 8-Evaporator separator.

[0038] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0039] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Furthermore, the technical solutions of the various embodiments can be combined with each other, but only if they are feasible for those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by this invention.

[0041] A process for treating stainless steel wastewater containing transition metal ions and acids includes the following steps:

[0042] (1) Add sodium hydroxide to stainless steel wastewater to adjust the pH value to 4.5-5.0 (the iron ions in the wastewater undergo hydrolysis to generate iron hydroxide precipitate). After solid-liquid separation and water washing, the precipitate is used to obtain the first filtrate and filter residue containing iron hydroxide.

[0043] (2) After mixing the first filtrate and the wastewater obtained during the water washing in step (1), sodium hydroxide is added to adjust the pH value to 5.3-5.8 (chromium ions in the wastewater undergo hydrolysis to generate chromium hydroxide). The resulting precipitate is separated into solid and liquid and washed with water to obtain the second filtrate and the filter residue mainly containing chromium hydroxide.

[0044] (3) After the second filtrate and the wastewater obtained during the water washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to ≥9.0 (the various transition metal ions in the wastewater undergo hydrolysis to generate a mixture of hydroxides mainly composed of nickel, manganese and other transition metals). After solid-liquid separation and water washing, the resulting precipitate yields the third filtrate and a mixed hydroxide filter residue mainly containing transition metals (nickel, manganese, etc.). After this step, the transition metal ions in the wastewater (filtrate) are completely removed, and the pollutant content in the filtrate is (T-Cr<1.5mg / L, Cr(VI)<0.5mg / L, Ni<1.0mg / L, Fe<10mg / L, copper<0.5mg / L).

[0045] (4) After the third filtrate and the wastewater obtained during the water washing in step (3) are mixed, an alkaline earth metal alkali and a coagulant are added to it. After solid-liquid separation and water washing, the resulting precipitate is used to obtain a fourth filtrate and a filter residue mainly containing alkaline earth metal fluorides, so as to remove fluoride ions (F<10mg / L) from the wastewater. The alkaline earth metal alkali is at least one of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, barium oxide, and barium hydroxide. The amount of alkaline earth metal alkali added is equal to the molar ratio of alkaline earth metal to fluoride ions in the third filtrate, which is 0.5 to 0.6. The coagulant is at least one of calcium chloride, polyaluminum ferric chloride, polyaluminum chloride, polyferric sulfate, ferrous sulfate, aluminum sulfate, and ferric chloride. The amount of coagulant added is 0.05 to 0.2‰ of the mass of the third filtrate.

[0046] (5) After the fourth filtrate and the wastewater obtained during the water washing in step (4) are mixed, they are first treated with a cation exchange resin to remove the alkaline earth metal ions contained therein; then a reduction treatment is performed to convert the sodium nitrate contained therein into nitrogen gas and sodium hydroxide, thereby removing nitrate ions (total nitrogen <20mg / L, ammonia nitrogen <8mg / L) from the wastewater. This step yields the fifth filtrate; wherein, the cation exchange resin is a macroporous weak acid cation exchange resin; the reduction treatment is any one of homogeneous catalytic hydrogenation treatment, heterogeneous catalytic hydrogenation treatment, low-valent metal reducing agent reduction treatment, and organic reducing agent reduction treatment;

[0047] (6) After evaporation, the fifth filtrate yields a concentrated alkaline solution containing sodium hydroxide (concentration 20-40%). wt The concentrated alkaline solution containing sodium hydroxide is recycled to the multi-stage stepwise precipitation process in steps (1)-(3) for acidity adjustment, effectively reducing the amount of sodium hydroxide used. The steam is discharged after condensation to meet the standards. The evaporation treatment is any one of multi-effect evaporation treatment, mechanical vapor compression evaporation treatment, and thermal vapor compression evaporation treatment.

[0048] The metal and fluorine contents of the filter residues containing iron hydroxide, chromium hydroxide, transition metal hydroxide, and alkaline earth metal fluoride obtained after treatment by the stainless steel wastewater treatment process in this scheme are all <1wt%.

[0049] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the present invention.

[0050] The stainless steel wastewater used in the following examples or comparative studies was fluoride-containing waste acid water generated from the pickling process of a large stainless steel enterprise. The composition of the raw water was analyzed by ICP and ion chromatography as follows (unit: mg / L, except pH):

[0051] name pH Ni Fe Mn Cr Ca Na <![CDATA[F － ]]> <![CDATA[NO 3－ ]]> Waste acid water 1.05 118 3327 80 470 231 65 3172 17045

[0052] Example 1

[0053] A process for treating stainless steel wastewater containing transition metal ions and acids includes the following steps:

[0054] (1) Sodium hydroxide was added to stainless steel wastewater to adjust the pH value to 4.9. The resulting precipitate was separated by solid-liquid separation and washed with water to obtain the first filtrate and iron hydroxide filter residue.

[0055] (2) After the first filtrate and the wastewater obtained during the water washing in step (1) are mixed, sodium hydroxide is added to adjust the pH value to 5.4. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the second filtrate and chromium hydroxide filter residue.

[0056] (3) After the second filtrate and the wastewater obtained during the water washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to 9.0. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the third filtrate and filter residue containing transition metal hydroxides.

[0057] (4) After the third filtrate and the wastewater obtained during the water washing in step (3) are mixed, an alkaline earth metal alkali (calcium hydroxide) and a coagulant (polyaluminum chloride) are added to it. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the fourth filtrate and filter residue containing alkaline earth metal fluoride. The amount of alkaline earth metal alkali added is 6.52 kg, and the amount of coagulant added is 0.15 kg (all of which are based on 1 ton of wastewater treated).

[0058] (5) The fourth filtrate and the wastewater obtained during the water washing in step (4) are mixed and then treated with a macroporous weak acid cation exchange resin, and then reduced (homogeneous catalytic hydrogenation) to obtain the fifth filtrate.

[0059] (6) After the fifth filtrate is evaporated (MVR high-efficiency evaporation treatment), a concentrated alkaline solution containing sodium hydroxide and steam are obtained. The concentrated alkaline solution containing sodium hydroxide is recycled for adjusting the pH value in steps (1)-(3). The steam is discharged directly after condensation.

[0060] Example 2

[0061] A process for treating stainless steel wastewater containing transition metal ions and acids includes the following steps:

[0062] (1) Sodium hydroxide was added to stainless steel wastewater to adjust the pH value to 4.5. After solid-liquid separation and water washing, the resulting precipitate was used to obtain the first filtrate and iron hydroxide filter residue.

[0063] (2) After the first filtrate and the wastewater obtained during the water washing in step (1) are mixed, sodium hydroxide is added to adjust the pH value to 5.6. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the second filtrate and chromium hydroxide filter residue.

[0064] (3) After the second filtrate and the wastewater obtained during the water washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to 9.2. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the third filtrate and filter residue containing transition metal hydroxides.

[0065] (4) After the third filtrate and the wastewater obtained during the washing in step (3) are mixed, an alkaline earth metal alkali (calcium oxide) and a coagulant (polyaluminum ferric chloride) are added to it. After solid-liquid separation and washing, the resulting precipitate is used to obtain a fourth filtrate and a filter residue containing alkaline earth metal fluoride. The amount of alkaline earth metal alkali added is 6.2 kg, and the amount of coagulant added is 0.1 kg.

[0066] (5) The fourth filtrate and the wastewater obtained during the water washing in step (4) are mixed and then treated with a macroporous weak acid cation exchange resin, and then reduced (heterogeneous catalytic hydrogenation treatment) to obtain the fifth filtrate.

[0067] (6) After the fifth filtrate is evaporated (thermal steam compression evaporation), a concentrated alkaline solution containing sodium hydroxide and steam are obtained. The concentrated alkaline solution containing sodium hydroxide is recycled for adjusting the pH value in steps (1)-(3). The steam is discharged directly after condensation.

[0068] Example 3

[0069] A process for treating stainless steel wastewater containing transition metal ions and acids includes the following steps:

[0070] (1) Sodium hydroxide was added to stainless steel wastewater to adjust the pH value to 4.7. After solid-liquid separation and water washing, the resulting precipitate was used to obtain the first filtrate and iron hydroxide filter residue.

[0071] (2) After the first filtrate and the wastewater obtained during the water washing in step (1) are mixed, sodium hydroxide is added to adjust the pH value to 5.8. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the second filtrate and chromium hydroxide filter residue.

[0072] (3) After the second filtrate and the wastewater obtained during the water washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to 9.5. After solid-liquid separation and water washing, the resulting precipitate yields the third filtrate and filter residue containing transition metal hydroxides.

[0073] (4) After the third filtrate and the wastewater obtained during the water washing in step (3) are mixed, an alkaline earth metal alkali (barium hydroxide) and a coagulant (polyferric sulfate) are added to it. After solid-liquid separation and water washing, the resulting precipitate is used to obtain a fourth filtrate and a filter residue containing alkaline earth metal fluoride. The amount of alkaline earth metal alkali added is 7.1 kg, and the amount of coagulant added is 0.2 kg.

[0074] (5) The fourth filtrate and the wastewater obtained during the washing in step (4) are mixed and then treated with a macroporous weak acid cation exchange resin and then reduced (organic reducing agent reduction) to obtain the fifth filtrate.

[0075] (6) After the fifth filtrate is evaporated (multi-effect evaporation), a concentrated alkaline solution containing sodium hydroxide and steam are obtained. The concentrated alkaline solution containing sodium hydroxide is recycled for adjusting the pH value in steps (1)-(3). The steam is discharged directly after condensation.

[0076] Comparative Example 1

[0077] Traditional lime method for treating stainless steel wastewater: Lime slurry is added to the stainless steel wastewater to adjust the pH to 9.5; the wastewater is then introduced into a flocculation sedimentation tank, and then polyaluminum chloride and polyacrylamide are added in sequence and mixed. The treated purified water is obtained by separation in a vertical flow sedimentation tank.

[0078] Comparative Example 2

[0079] All parameters and preparation steps in this comparative example are consistent with those in Example 1, except that sodium hydroxide is replaced with calcium hydroxide as the neutralizing and precipitating agent.

[0080] The various test data for stainless steel wastewater treatment in Examples 1-3 and Comparative Examples 1-2 were recorded, as shown in the table below:

[0081] Sludge production (sludge cake with 50% moisture content)

[0082] Step (1) Filter residue / kg Step (2) Filter residue / kg Step (3) Filter residue / kg Step (4) Filter residue / kg Example 1 20 8.6 1.24 6.2 Example 2 14.6 15.1 0.96 7.8 Example 3 16.3 13.1 0.62 7.4 Comparative Example 2 30.7 33.4 11.2 0

[0083] Hazardous waste lime slag containing heavy metals and fluorine / kg Comparative Example 1 75.9

[0084] The test results of Comparative Example 1 and Example 1 show that, compared with the traditional lime method, the treatment process of the present invention reduces the amount of sludge generated by 75.9 kg - 36.04 kg (20 + 8.6 + 1.24 + 6.2) = 39.86 kg per ton of stainless steel wastewater treated, even without deducting the recyclable transition metal hydroxide slag. That is, the sludge production reduction rate is approximately 52.5%. If the recyclable transition metal hydroxide slag is deducted, the sludge production reduction rate is as high as 91.8% per ton of stainless steel wastewater treated, which is 75.9 kg - 6.2 kg = 69.7 kg per ton of stainless steel wastewater treated. The comparison results of Comparative Example 2 and Example 1 show that when calcium hydroxide is used as a neutralizing precipitant, a relatively large amount of sludge is also generated, with a total sludge volume of approximately 75.3 kg.

[0085] Main impurities in the filter residue after water washing (%)

[0086]

[0087]

[0088] As can be seen from the table above, the sludge metal filter residue produced by the process of treating stainless steel wastewater according to this scheme has very low impurity content (Na2O and F) after water washing (washing). In particular, the fluorine in the impurities can be basically washed away, so that the sludge metal filter residue can have resource recycling value. For example, the sludge metal oxide residue can be returned to the steelmaking furnace for remelting, or it can be used as a raw material for the preparation of other composite metal materials.

[0089] Example 1: Percentage of each ion in wastewater (mg / L) when sodium hydroxide was added to adjust pH.

[0090]

[0091] The table above shows that when the pH is adjusted to 5.4, the proportion of nickel ions in the filtrate increases significantly. At this point, adjusting the pH to 9.0 with sodium hydroxide can basically precipitate and extract all the nickel in the wastewater. Moreover, the nickel content in the filter residue obtained in this step can reach more than 15%, which has high recycling value.

[0092] Effluent water quality (mg / L) without steps 5 and 6.

[0093] pH Ni Fe Mn Mg Cr Cu Ca Na K <![CDATA[F - ]]> <![CDATA[NO3 - <!-- 8 -->]]> Example 1 11.9 0.00 0.00 0.00 0.00 0.639 0.00 25.8 6403 15.10 8.21 16973 Example 2 11.8 0.00 0.00 0.00 0.00 0.72 0.00 15.1 7201 15.10 9.2 16802 Example 3 11.6 0.00 0.00 0.00 0.00 0.54 0.00 20.1 7603 15.10 7.8 16734

[0094] As can be seen from the table above, the stainless steel wastewater treatment technology of this invention can ensure stable effluent quality (except for nitrate).

[0095] The quality of the effluent after completing the full treatment process (mg / L)

[0096]

[0097] As shown in the table above, the condensate after reduction and evaporation is essentially equivalent to distilled clean water, containing almost no impurities or salts. It can be directly discharged or retained for reuse. The concentrated liquid after multi-effect evaporation is mainly sodium hydroxide solution, which can be reused as a (NaOH) alkali neutralizing agent. Furthermore, the stainless steel wastewater treatment process of this invention eliminates the need for the complex subsequent microbial denitrification process.

[0098] Example 4

[0099] All parameters and preparation steps in this embodiment are the same as in Example 1, except that the alkaline earth metal alkali used in step (4) is different, as detailed in the table below:

[0100] Alkaline earth metal alkali Example 4-1 magnesium oxide Example 4-2 Calcium oxide Example 4-3 Strontium hydroxide, calcium hydroxide Example 4-4 Calcium hydroxide, barium hydroxide

[0101] The various test data during the stainless steel wastewater treatment in Example 4 were recorded, as shown in the table below:

[0102] Sludge production (sludge cake with 50% moisture content)

[0103] Step (4) Filter residue / kg Example 4-1 14.6 Example 4-2 11.2 Example 4-3 9.7 Example 4-4 7.6

[0104] Fluoride content (mg / L) in water after defluorination with different alkaline earth metal alkalis

[0105] <![CDATA[F - ]]> Example 4-1 25.6 Example 4-2 16.9 Example 4-3 13.5 Example 4-4 8.21

[0106] The quality of the effluent after completing the full treatment process (mg / L)

[0107]

[0108]

[0109] The table above shows that all the different alkaline earth metal alkalis used in this scheme have the effect of removing fluoride. Considering the comprehensive analysis of fluoride removal performance and cost, the combination of calcium hydroxide and barium hydroxide has the best fluoride removal effect (the effect of calcium hydroxide alone and in combination is also similar).

[0110] Example 5

[0111] A process for treating stainless steel wastewater containing transition metal ions and acids includes the following steps:

[0112] (1) Sodium hydroxide was added to stainless steel wastewater to adjust the pH value to 4.6. The resulting precipitate was separated by solid-liquid separation and washed with water to obtain the first filtrate and iron hydroxide filter residue.

[0113] (2) After the first filtrate and the wastewater obtained during the water washing in step (1) are mixed, sodium hydroxide is added to adjust the pH value to 5.6. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the second filtrate and chromium hydroxide filter residue.

[0114] (3) After the second filtrate and the wastewater obtained during the water washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to 9.0. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the third filtrate and filter residue containing transition metal hydroxides.

[0115] (4) After the third filtrate and the wastewater obtained during the washing in step (3) are mixed, an alkaline earth metal alkali (calcium hydroxide) and a coagulant (polyaluminum ferric chloride) are added to it. After solid-liquid separation and washing, the resulting precipitate is used to obtain a fourth filtrate and a filter residue containing alkaline earth metal fluoride. The amount of alkaline earth metal alkali added is 5.8 kg, and the amount of coagulant added is 0.15 kg.

[0116] (5) The fourth filtrate and the wastewater obtained during the water washing in step (4) are mixed and then treated with a macroporous weak acid cation exchange resin, and then reduced (homogeneous catalytic hydrogenation) to obtain the fifth filtrate.

[0117] (6) After the fifth filtrate is evaporated (MVR high-efficiency evaporation treatment), a concentrated alkaline solution containing sodium hydroxide and steam are obtained. The concentrated alkaline solution containing sodium hydroxide is recycled for adjusting the pH value in steps (1)-(3). The steam is discharged directly after condensation.

[0118] The various test data during the stainless steel wastewater treatment in Example 5 were recorded, as shown in the table below:

[0119] Sludge production (sludge cake with 50% moisture content)

[0120] Step (1) Filter residue / kg Step (2) Filter residue / kg Step (3) Filter residue / kg Step (4) Filter residue / kg Example 5 20.4 7.4 1.5 5.8

[0121] Main impurities in the filter residue after water washing

[0122]

[0123] The quality of the effluent after completing the full treatment process (mg / L)

[0124]

[0125] The table above shows that, through the optimization of parameters such as pH value and alkaline earth metal alkali in this scheme, and based on the test results of Comparative Example 1 and Example 1, even without deducting the recyclable transition metal hydroxide slag, the amount of sludge generated can be reduced by 75.9 kg – 35.1 kg (20.4 + 7.4 + 1.5 + 5.8) = 40.8 kg per ton of stainless steel wastewater treated, i.e., the sludge production reduction rate is about 53.75%. If the recyclable transition metal hydroxide slag is deducted, the amount of sludge generated can be reduced by 75.9 kg - 5.8 kg = 70.1 kg per ton of stainless steel wastewater treated, with a sludge production reduction rate as high as 92.35%. Simultaneously, the heavy metal ion content in the filtered filtrate is also significantly increased.

[0126] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A process for treating stainless steel wastewater containing transition metal ions and acids, characterized in that, Includes the following steps: (1) Sodium hydroxide was added to stainless steel wastewater to adjust the pH value to 4.5-5.

0. The resulting precipitate was separated by solid-liquid separation and washed with water to obtain the first filtrate and iron hydroxide filter residue. (2) After mixing the first filtrate and the wastewater obtained during the water washing in step (1), sodium hydroxide is added to adjust the pH value to 5.3-5.

8. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the second filtrate and chromium hydroxide filter residue. (3) After the second filtrate and the wastewater obtained during the washing in step (2) are mixed, sodium hydroxide is added to adjust the pH value to ≥9.

0. After solid-liquid separation and washing, the resulting precipitate is used to obtain the third filtrate and filter residue containing transition metal hydroxides. (4) After the third filtrate and the wastewater obtained during the water washing in step (3) are mixed, alkaline earth metal alkali and coagulant are added to it. After solid-liquid separation and water washing, the resulting precipitate is used to obtain the fourth filtrate and filter residue containing alkaline earth metal fluoride. (5) The fourth filtrate and the wastewater obtained during the water washing in step (4) are mixed and first treated with a macroporous weak acid cation exchange resin, and then reduced to obtain the fifth filtrate; the reduction treatment in step (5) is either homogeneous catalytic hydrogenation treatment or heterogeneous catalytic hydrogenation treatment. (6) After the fifth filtrate is evaporated, a concentrated alkaline solution containing sodium hydroxide and steam are obtained. The concentrated alkaline solution containing sodium hydroxide is recycled for adjusting the pH value in steps (1)-(3). The steam is discharged directly after condensation.

2. The stainless steel wastewater treatment process containing transition metal ions and acid according to claim 1, characterized in that, The alkaline earth metal alkali mentioned in step (4) is at least one of magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, barium oxide, and barium hydroxide.

3. The stainless steel wastewater treatment process containing transition metal ions and acid as described in claim 1 or 2, characterized in that, The amount of alkaline earth metal alkali added in step (4) is such that the molar ratio of alkaline earth metal to fluoride ions in the third filtrate is 0.5 to 0.

6.

4. The stainless steel wastewater treatment process containing transition metal ions and acid as described in claim 1, characterized in that, The coagulant in step (4) is at least one of calcium chloride, polyaluminum ferric chloride, polyaluminum chloride, polyferric sulfate, ferrous sulfate, aluminum sulfate, and ferric chloride.

5. The stainless steel wastewater treatment process containing transition metal ions and acid according to claim 1, characterized in that, The amount of coagulant added in step (4) is 0.05 ~ 0.2‰ of the mass of the third filtrate.

6. The stainless steel wastewater treatment process containing transition metal ions and acid according to claim 1, characterized in that, The evaporation process in step (6) is any one of multi-effect evaporation, mechanical vapor compression evaporation, or thermal vapor compression evaporation.

7. The stainless steel wastewater treatment process containing transition metal ions and acid as described in claim 1, characterized in that, The metal and fluorine contents in the iron hydroxide filter residue, chromium hydroxide filter residue, transition metal hydroxide filter residue, and alkaline earth metal fluoride filter residue are all <1wt%.

8. A stainless steel wastewater treatment system, characterized in that, Using the stainless steel wastewater treatment method according to any one of claims 1-7, the stainless steel wastewater treatment system includes a wastewater mixing tank (1), an iron precipitation tank (2), a chromium precipitation tank (3), a nickel precipitation tank (4), a fluoride removal tank (5), a cation exchange resin column (6), a reduction reaction tower (7), and an evaporator (8) connected in sequence.

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