Resourceful treatment method for stainless steel pickling waste liquid and waste water

Abstract

The invention discloses a resourceful treatment method for stainless steel pickling waste liquid and waste water. The waste liquid is divided into high-concentration waste liquid and low-concentration waste liquid according to the acid concentration and the salt concentration to be treated respectively. Sequentially recovering gypsum, chromic hydroxide, nickel sulfide, ferrous hydroxide and manganese-rich slag from the low-concentration waste liquid through gradient precipitation; and neutralizing free acid from the high-concentration waste liquid by using ferrous hydroxide generated in a low-concentration section, cooling and crystallizing to recover ferrous sulfate, deeply removing nickel by using nickel sulfide generated in the low-concentration section, and precipitating to recover chromium hydroxide. In the whole process, generation of ferric iron is strictly controlled, and high-value productization recovery of free acid and all metals is achieved. The method has the advantages of low medicament consumption, high metal recovery rate, high product value, no secondary pollution and the like.

Description

Technical Field

[0001] This invention belongs to the field of industrial wastewater treatment technology, specifically relating to a resource-based treatment method for stainless steel pickling wastewater, particularly for the treatment of pickling wastewater containing multiple heavy metal ions such as iron, chromium, nickel, and manganese, and the stepwise separation and recovery of valuable metals. Background Technology

[0002] The pickling process of stainless steel generates a large amount of waste liquid and cleaning wastewater, which contains high concentrations of free acid and heavy metal ions such as iron, chromium, nickel and manganese, and is a typical toxic and harmful industrial wastewater. The existing treatment technologies mainly have the following problems: (1) The neutralization precipitation method generates a large amount of mixed sludge, and the metal cannot be separated and recovered; (2) The sulfide precipitation method has the risk of generating H2S gas; (3) The sludge from each unit is mixed and disposed of, and the valuable metals are diluted, resulting in low resource value; (4) High-concentration waste liquid and low-concentration wastewater are not treated together, and the consumption of reagents is large. Summary of the Invention

[0003] I. Technical problems to be solved

[0004] The present invention aims to provide a resource-based treatment method for stainless steel pickling wastewater, and solve the following technical problems: (1) achieving selective stepwise separation and recovery of iron, chromium, nickel and manganese; (2) avoiding the generation of mixed metal sludge; (3) using the sludge precipitated in the low-concentration section to treat the high-concentration section of wastewater, and realizing waste treatment with waste; (4) strictly controlling the generation of ferric iron to ensure the effective recovery and utilization of ferrous iron.

[0005] II. Technical Solution

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for the resource-based treatment of stainless steel pickling wastewater includes the following steps:

[0008] (1) The waste liquid is classified into high-concentration waste liquid and low-concentration waste liquid according to the acid and salt concentrations;

[0009] (2) Low-concentration waste liquid is subjected to step-by-step precipitation treatment to recover gypsum, chromium hydroxide, nickel sulfide, ferrous hydroxide and manganese-rich slag in sequence;

[0010] (3) The high-concentration waste liquid is neutralized with the ferrous hydroxide obtained in step (2), and the ferrous sulfate is recovered by cooling and crystallization. The nickel sulfide obtained in step (2) is used as a seed crystal to remove nickel in depth, and then chromium hydroxide is recovered by precipitation.

[0011] (4) The filtrate generated in steps (2) and (3) is recycled or discharged in compliance with standards.

[0012] The treatment of low-concentration waste liquid specifically includes the following sub-steps:

[0013] S1: Add calcium-based alkaline agent to the low-concentration waste liquid, adjust the pH to 4.0, allow it to precipitate for 20-30 minutes, and then filter to obtain gypsum. The main reaction in this step is as follows:

[0014] H2SO4 + Ca(OH)2→ CaSO4↓ + 2H2O

[0015] H2SO4 + CaCO3 → CaSO4↓ + H2O + CO2↑

[0016] S2: Add a calcium-based base to the filtrate, adjust the pH to 6.0, allow it to precipitate for 20-30 minutes, and then filter to obtain chromium hydroxide. The reaction in this step is as follows:

[0017] Cr2(SO4)3+ 3Ca(OH)2→ 3CaSO4↓ + 2Cr(OH)3↓

[0018] Meanwhile, the residual sulfuric acid continues to react with the calcium-based base agent to produce gypsum.

[0019] S3: Under online monitoring and control of TDS and ORP, a sulfide precipitant is added to the filtrate, and the pH is controlled between 6.5 and 7.5. Nickel sulfide is obtained through circulating filtration. The reaction tank is sealed and connected to an alkaline spray absorption tower. The reaction in this step is as follows:

[0020] NiSO4 + Na2S → NiS↓ + Na2SO4

[0021] Sulfide precipitants include sodium sulfide, sodium hydrosulfide, or hydrogen sulfide. Hydrogen sulfide is suitable for simplified treatment under zero-emission requirements and can be directly introduced into the reaction system, reducing reagent storage and preparation steps.

[0022] S4: Under air-isolated conditions (such as nitrogen protection), add a calcium-based base to the filtrate to adjust the pH to 8.8, then filter to obtain ferrous hydroxide. The reaction in this step is as follows:

[0023] FeSO4 + Ca(OH)2 → CaSO4↓ + Fe(OH)2↓

[0024] The recovered ferrous hydroxide is used to neutralize high-concentration waste liquid.

[0025] S5: Add calcium-based alkaline agent to the filtrate to adjust the pH to 10.0, and simultaneously add oxidant for aeration oxidation for 30-40 minutes. Solid-liquid separation yields manganese-rich slag. The reaction in this step is as follows:

[0026] MnSO4 + Ca(OH)2 → CaSO4↓ + Mn(OH)2↓

[0027] 2Mn(OH)2 + O2 → 2MnO2↓ + 2H2O (under the action of oxidizing agent)

[0028] The oxidant is hydrogen peroxide or sodium persulfate, and the dosage is 1.2-1.5 times the molar amount of Mn2+.

[0029] S6: Adjust the pH of the purified water to 6-9, filter it, and then reuse or discharge it to control residual sulfur. 2- ≤1.0mg / L.

[0030] The treatment of high-concentration waste liquid specifically includes the following sub-steps:

[0031] T1: The ferrous hydroxide obtained from S4 is used to neutralize the free acid in the high-concentration waste liquid, adjusting the pH to 4.0, and then filtered to obtain gypsum and the first filtrate. The reaction in this step is as follows:

[0032] H2SO4 + Fe(OH)2 → FeSO4 + 2H2O

[0033] The ferrous sulfate generated at the same time remains in the solution.

[0034] T2: Cool the first filtrate to crystallize, and filter to obtain ferrous sulfate crystals (FeSO4·7H2O) and the second filtrate.

[0035] T3: Add nickel sulfide (as a seed crystal) and sulfide precipitant generated by S3 to the second filtrate. The nickel ions react completely, and the mixture is then filtered through a cycle to obtain nickel sulfide and the third filtrate.

[0036] T4: Adjust the pH of the third filtrate to 6.0, filter to obtain chromium hydroxide and the fourth filtrate.

[0037] T5: Return the fourth filtrate to the low-concentration waste liquid treatment system for further processing.

[0038] The above steps ultimately yield gypsum, ferrous sulfate, nickel sulfide, chromium hydroxide, and manganese-rich slag. The entire process strictly controls the formation of ferric iron to ensure that ferrous iron is recovered as ferrous hydroxide and used for neutralization.

[0039] III. Beneficial Effects

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] (1) Stepwise separation, high product purity: By precisely controlling pH and reaction conditions, selective stepwise precipitation of iron, chromium, nickel and manganese is achieved, and the purity of each product meets the requirements for resource utilization.

[0042] (2) Waste treatment with waste, low reagent consumption: Ferrous hydroxide produced in the low concentration section is used to neutralize the free acid in the high concentration section, greatly reducing the consumption of alkali agent; nickel sulfide produced in the low concentration section is used as seed crystal to improve the nickel deposition efficiency in the high concentration section.

[0043] (3) Control iron throughout the process to avoid interference: strictly control the generation of ferric iron and ensure that ferrous iron is recovered in the form of ferrous hydroxide to avoid contamination of chromium and nickel products by iron.

[0044] (4) Closed-loop treatment with no waste discharge: the filtrate from each step is recycled and reused, and the final effluent meets the standards for reuse, achieving true reduction and resource utilization.

[0045] (5) Significant economic benefits: The gypsum, ferrous sulfate, nickel sulfide, chromium hydroxide, and manganese-rich slag obtained are all valuable products that can be sold to relevant enterprises, turning waste into treasure. Attached Figure Description

[0046] Figure 1 This is a process flow diagram of the stainless steel pickling waste liquid and wastewater treatment method of the present invention. The left side shows the low-concentration waste liquid treatment process, and the right side shows the high-concentration waste liquid treatment process. The dashed arrows indicate the material application paths.

[0047] Low-concentration process: After the cleaning water is mixed with some of the high-concentration waste liquid from series 2, it enters the following stages in sequence: S1 neutralization (pH 4.0, add CaCO3 / Ca(OH)2) → S2 chromium precipitation (pH 6.0, add Ca(OH)2) → S3 nickel precipitation (pH 6.5-7.5, add sulfide) → S4 iron precipitation (pH 8.8, add Ca(OH)2, isolate from air) → S5 manganese precipitation (pH 10.0, add Ca(OH)2 + oxidant) → S6 effluent adjustment and reuse.

[0048] High-concentration process: 3 series of high-concentration waste liquid enters T1 for neutralization (Fe(OH)2 obtained by adding S4, pH4.0) → T2 for cooling and crystallization to obtain ferrous sulfate → T3 for nickel precipitation (NiS obtained by adding S3 as seed crystals, and sulfide replenishment) → T4 for chromium precipitation (pH6.0) → T5 for filtrate returned to the low-concentration system.

[0049] The gypsum, chromium hydroxide, nickel sulfide, ferrous hydroxide, manganese-rich slag, and ferrous sulfate crystals produced in each step are collected and utilized separately. Detailed Implementation

[0050] Example 1 (Taking pickling wastewater from a stainless steel plant as an example)

[0051] This embodiment treats pickling wastewater and cleaning water generated by a stainless steel plant. The wastewater includes:

[0052] 3 series high-concentration waste liquid: 20 m 3 / d, free sulfuric acid 130 g / L, Fe 2+60 g / L, Cr 3+ 15 g / L, Ni 2+ 5g / L, Mn 2+ 4 g / L, specific gravity 1.273.

[0053] 2 series high-concentration waste liquid: 10 m 3 / d, free sulfuric acid 30 g / L, Fe 2+ 38 g / L, Cr 3+ 5 g / L, Ni 2+ 1.2g / L, Mn 2+ 5 g / L, specific gravity 1.1523.

[0054] Low-concentration cleaning water: 120 m 3 / d, pH 3.5, Fe 2+ 750 mg / L, Cr 3+ 120 mg / L, Ni 2+ 50 mg / L, Mn 2+ 80 mg / L, SO4 2- 3000 mg / L.

[0055] Treatment strategy: 10 m 3 / d 2 series high-concentration waste liquid and 120 m 3 / d Cleaning water mixture (total 130 m) 3 / d), as low-concentration system influent; 20 m 3 / d 3 series high-concentration waste liquid is used as the influent of the high-concentration system.

[0056] Low concentration system treatment (130 m 3 / d Mixed influent) Water quality after mixing: Free sulfuric acid 2.31 g / L, Fe 2+ 3.62 g / L, Cr 3+ 0.495 g / L, Ni 2+ 0.139 g / L, Mn 2+ 0.459 g / L, SO4²⁻ 10.04 g / L, pH ≈ 1.9.

[0057] S1: Add lime milk (calcium hydroxide) to the mixed wastewater, adjust the pH to 4.0, stir and react for 25 minutes, and filter to obtain about 2200 kg of gypsum.

[0058] S2: Add lime milk to the filtrate of S1 to adjust the pH to 6.0, stir for 25 minutes, and filter to obtain 120 kg of chromium hydroxide (dry basis, Cr2O3 content 42%).

[0059] S3: Under online ORP monitoring, add 10% sodium sulfide solution to the S2 filtrate, control the pH to 7.0, and continue until the ORP drops sharply, indicating a slight excess of sulfide. Stir the reaction for 20 minutes, and then circulate and filter to obtain 26 kg of nickel sulfide (Ni content 38%). The reaction tank is sealed, and the tail gas is introduced into the alkaline spray tower.

[0060] S4: Under nitrogen protection, lime milk was added to the S3 filtrate to adjust the pH to 8.8. The mixture was stirred and reacted, and then filtered to obtain 720 kg of ferrous hydroxide (dry basis, Fe content 45%), which was immediately used to neutralize the high-concentration waste liquid.

[0061] S5: Add lime slurry to the S4 filtrate to adjust the pH to 10.0, and simultaneously add Mn... 2+ Add 30% hydrogen peroxide at a molar ratio of 1.3, aerate and stir for 35 minutes, and then separate the solid and liquid to obtain 90 kg of manganese-rich slag (Mn content 38%).

[0062] S6: Adjust the pH of the S5 filtrate to 7.5 with dilute sulfuric acid, then reuse it after sand filtration. Analyze the effluent: Cr 3+ 0.2 mg / L, Ni 2 + 0.1 mg / L, Mn 2+ 0.4 mg / L, Fe 2+ 0.5 mg / L, S 2- 0.3 mg / L.

[0063] High concentration system treatment (20 m 3 / d 3 series high concentration waste liquid)

[0064] T1: Add the ferrous hydroxide (720 kg / d) obtained from S4 to 20 m 3 The high-concentration waste liquid was stirred and neutralized to pH 4.0, and then filtered to obtain approximately 1800 kg of gypsum, yielding the first filtrate.

[0065] T2: Cool the first filtrate to 5°C, crystallize for 8 hours, and filter to obtain 8200 kg of ferrous sulfate crystals (FeSO4·7H2O, purity 92%), thus obtaining the second filtrate.

[0066] T3: Add 26 kg of nickel sulfide obtained from S3 to the second filtrate as seed crystals, and add a small amount of sodium sulfide until the nickel ions react completely. Circulate and filter to obtain 150 kg of nickel sulfide (Ni content 42%), and obtain the third filtrate.

[0067] T4: Adjust the pH of the third filtrate to 6.0, stir the reaction, filter to obtain 570 kg of chromium hydroxide (Cr2O3 content 45%), and obtain the fourth filtrate.

[0068] T5: Return the fourth filtrate to the low-concentration system for further processing.

[0069] Daily resource recovery statistics: 4000 kg gypsum, 8200 kg ferrous sulfate crystals, 176 kg nickel sulfide, 690 kg chromium hydroxide, and 90 kg manganese-rich slag. All products meet the resource utilization standards.

[0070] Example 2 (Effect of staged sulfide addition)

[0071] The difference from Example 1 is that sodium sulfide in S3 is added in stages. First, 65% of the total amount is added quickly, and after stirring for 8 minutes, the remaining 35% is added slowly. The results show that compared with one-time addition, staged addition increases the settling speed of sulfide sludge by 30%, reduces the turbidity of the supernatant by 40%, and reduces the amount of hydrogen sulfide gas produced by 60%.

[0072] Example 3 (Hydrogen sulfide for zero-emission treatment)

[0073] In a zero-emission industrial park, hydrogen sulfide gas was used instead of sodium sulfide for the nickel precipitation process in step S3. The hydrogen sulfide gas was directly introduced into the reaction tank through microporous aeration pipes, and the aeration rate was controlled online using ORP (Optical Resource Management). Results showed that: there was no need to prepare and store sodium sulfide solution, reducing reagent storage capacity; the tail gas after the reaction was absorbed and reused with alkali solution, and the system generated no saline wastewater; the nickel precipitation efficiency was comparable to that of sodium sulfide, but the operating cost was reduced by 15%.

[0074] Comparative Example 1 (Traditional One-Step Neutralization Method)

[0075] Low-concentration cleaning water was treated using a traditional one-step neutralization method: lime slurry was directly added to adjust the pH to 9.5, followed by sedimentation and filtration. Results: Cr in the effluent... 3+ 2.8 mg / L, Ni 2+ 1.5 mg / L, Mn 2+ The concentration was 8.5 mg / L, which failed to meet the emission standards, and the sludge was precipitated as mixed sludge, making it impossible to utilize as a resource.

[0076] Comparative Example 2 (without pH stepwise control)

[0077] The difference from Example 1 is that pH-step precipitation was not performed; instead, sulfide precipitation was performed directly. Results: A distinct hydrogen sulfide odor was generated during the precipitation process, the H2S concentration in the work area exceeded the standard, posing a safety hazard; furthermore, chromium, nickel, and manganese were mixed in the precipitate, making separation and recovery impossible.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for the resource-based treatment of stainless steel pickling wastewater, characterized in that, Includes the following steps: (1) The waste liquid is classified into high-concentration waste liquid and low-concentration waste liquid according to the acid and salt concentrations; (2) Low-concentration waste liquid is subjected to step-by-step precipitation treatment to recover gypsum, chromium hydroxide, nickel sulfide, ferrous hydroxide and manganese-rich slag in sequence; (3) The high-concentration waste liquid is neutralized with the ferrous hydroxide obtained in step (2), and the ferrous sulfate is recovered by cooling and crystallization. The nickel sulfide obtained in step (2) is used as a seed crystal to remove nickel in depth, and then chromium hydroxide is recovered by precipitation. (4) The filtrate generated in steps (2) and (3) is recycled or discharged in compliance with standards.

2. The method according to claim 1, characterized in that, The treatment of low-concentration waste liquid in step (2) includes the following sub-steps: S1: Add calcium-based alkaline agent to low-concentration waste liquid, adjust pH to 4.0, and filter to obtain gypsum; S2: Add calcium-based alkaline agent to the filtrate, adjust the pH to 6.0, and filter to obtain chromium hydroxide; S3: Under the monitoring and control of TDS and ORP, add sulfide precipitant to the filtrate, control the pH to 6.5-7.5, and filter to obtain nickel sulfide; S4: In the absence of air, add calcium-based alkaline agent to the filtrate to adjust the pH to 8.8, and filter to obtain ferrous hydroxide; S5: Add calcium-based alkaline agent to the filtrate to adjust the pH to 10.0, and at the same time add oxidant for aeration oxidation. Solid-liquid separation yields manganese-rich slag. S6: Adjust the pH of the purified water to 6-9, filter it, and then reuse or discharge it.

3. The method according to claim 1, characterized in that, The high-concentration waste liquid treatment in step (3) includes the following sub-steps: T1: Use the ferrous hydroxide obtained in step (2) to neutralize the free acid in the high-concentration waste liquid, adjust the pH to 4.0, and filter to obtain gypsum and the first filtrate; T2: Cool the first filtrate to crystallize, and filter to obtain ferrous sulfate crystals and the second filtrate; T3: Add the nickel sulfide obtained in step (2) as seed crystals to the second filtrate, and add sulfide precipitant until the nickel ions react completely. Filter to obtain nickel sulfide and the third filtrate. T4: Adjust the pH of the third filtrate to 6.0, filter to obtain chromium hydroxide and the fourth filtrate; T5: Return the fourth filtrate to the low-concentration waste liquid treatment system of step (2).

4. The method according to claim 2 or 3, characterized in that, The calcium-based alkaline agent includes one or more of calcium carbonate, calcium oxide, and calcium hydroxide, and is selected according to the treatment stage and pH requirements; the sulfide precipitant includes one or more of sodium sulfide, sodium hydrosulfide, or hydrogen sulfide, wherein hydrogen sulfide is suitable for simplified treatment under zero-emission requirements.

5. The method according to claim 2, characterized in that, The sulfide precipitation process in S3 uses online TDS and ORP monitoring to control the addition endpoint. The reaction tank is sealed and connected to the alkaline spray absorption tower. The sulfide precipitant is added in stages. First, 60-70% of the total amount is added quickly, stirred for 5-10 minutes, and then the remaining amount is added slowly.

6. The method according to claim 2, characterized in that, The oxidant mentioned in S5 is hydrogen peroxide or sodium persulfate, and the dosage is Mn in the wastewater. 2+ The molar amount is 1.2-1.5 times, and the aeration oxidation time is 30-40 minutes.

7. The method according to claim 2, characterized in that, The precipitation process of ferrous hydroxide in S4 is carried out in the absence of air to prevent Fe from being released. 2+ Oxidation.

8. The method according to claim 2, characterized in that, S6 residual S in purified effluent 2- Concentration should be controlled at ≤1.0 mg / L.

9. The method according to claim 1, characterized in that, The entire process strictly controls the formation of ferric iron, ensuring that ferrous iron is recovered as ferrous hydroxide and used for neutralization.

10. The method according to claim 1, characterized in that, The final recycled products include gypsum, ferrous sulfate, nickel sulfide, chromium hydroxide, and manganese-rich slag, all of which meet the standards for resource utilization.

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