A method for simultaneously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater

Abstract

The present invention discloses a method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater. The present invention adopts a "diffusion dialysis + three-step precipitation" coupling method to synergistically recover free acid and heavy metals in stainless steel pickling wastewater. The method comprises the following steps: firstly, a diffusion dialysis method is used to separate and recover mixed free acids (such as nitric acid, hydrofluoric acid, sulfuric acid or hydrochloric acid, etc.) in stainless steel pickling wastewater, and then a three-step precipitation method is used to successively separate and recover heavy metals (such as iron, chromium, nickel, etc.) in the dialysis residue. Compared with conventional extraction, adsorption, evaporation, membrane technology and other treatment methods that recover acid or heavy metals alone, the "diffusion dialysis + three-step precipitation" coupling method adopted by the present invention can realize the synchronous recovery of high-concentration free acid and heavy metals in stainless steel pickling wastewater, and the process is simple, easy to operate, energy-saving and efficient, low cost, and has excellent prospects for industrial application.

Description

Technical Field

[0001] The invention belongs to the field of wastewater treatment, and specifically relates to a method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater by a "diffusion dialysis + three-step precipitation" coupling method. Background Art

[0002] Pickling is an indispensable and important part of the manufacturing process in typical industries such as electroplating and metallurgy. It is used to clean the metal surface. In this process, a waste acid solution with serious environmental pollution but good recycling value is produced. The waste acid solution is rich in H + , acid radical ions and heavy metal ions. The traditional methods of adding alkali for neutralization cannot fundamentally solve the environmental impact brought about by such waste acid liquid, and it also faces problems such as high treatment cost and waste of resources.

[0003] The existing treatment methods for stainless steel pickling wastewater include spray roasting, solvent extraction, adsorption, etc. Spray roasting is a process in which the stainless steel pickling wastewater containing fluoride complexes of iron, chromium, nickel and free acid is pre-concentrated and then decomposed in a spray roasting reactor. However, the waste gas purification system of this process is expensive, and 30-40% of HNO3 is lost during the purification process. The corrosion protection and processing costs of the equipment are high, and the energy consumption is high. Lobo-Recio et al. used Cyanex-922 to extract and recover heavy metals and acids in the wastewater, among which Fe 3+ The recovery rate of 90.6%, HNO3 and HF recovery rates were 80.3% and 85.7% respectively. However, this method has complicated procedures, expensive extraction solvents, and the acid concentration after water stripping is very low, which requires evaporation and concentration to meet the acid recovery requirements ( AIChE J. 2004, 50, 1150–1155). In addition, resin adsorption can also be used to recover acids and heavy metals separately, but the specific resins used are very expensive and have high regeneration costs. Although conventional activated carbon is relatively cheap, it can only recover heavy metals and the recovery rate is very low. Membrane separation technology is an emerging separation technology with the characteristics of energy saving, high efficiency, simple operation, and small footprint. It has been widely used in wastewater resource treatment. Among them, diffusion dialysis, as a concentration difference driven membrane process, can separate and recover the free acid of waste acid solution through a specific anion exchange membrane. For example, invention patent 202211316529.5 uses a process of diffusion dialysis, neutralization precipitation and bipolar membrane electrodialysis coupling to recover nitric acid / hydrofluoric acid from stainless steel pickling wastewater. However, this method cannot simultaneously recover heavy metals in waste acid, resulting in a waste of resources and secondary pollution. In addition, the acid recovery rate of the first step of diffusion dialysis is low, 30.42%-59.78% for nitric acid and only 4.92%-12.21% for hydrofluoric acid. It needs to be concentrated by bipolar membrane electrodialysis, which greatly increases the cost. Therefore, it is necessary to design a more economical and simple method to simultaneously recover high-concentration free acid and heavy metals in stainless steel pickling wastewater, so as to maximize the resource utilization of stainless steel pickling wastewater. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater by coupling "diffusion dialysis + three-step precipitation". The method is to first recover the free acid (such as one or a mixture of any two of nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid or phosphoric acid) in the waste acid solution by diffusion dialysis process, and then use KOH / KF as a composite precipitant to precipitate and recover the heavy metal iron and chromium in the dialysis residue, and then acidify the filtrate and add KF complex precipitation to remove the residual trace Fe 3+ Finally, KOH is added to the filtrate to precipitate and recover the remaining heavy metal nickel.

[0005] A method for synchronously recovering high-concentration free acid and heavy metals from stainless steel pickling wastewater by coupling "diffusion dialysis + three-step precipitation", comprising the following steps:

[0006] (1) Add the stainless steel pickling wastewater to the dialysis chamber on the dialysis side of the diffusion dialysis device, and add pure water to the pure water chamber on the diffusion side, and then input them into the dialysis system of the diffusion dialysis device through peristaltic pumps, separate and recover the free acid in the wastewater through the diffusion dialysis membrane, and obtain the dialysis residue;

[0007] (2) The dialysis residue in step (1) is added to a KOH / KF composite precipitant. On the basis of adjusting the pH with KOH, free fluoride is further added to precipitate Fe at a low pH (4-6). 3+ and Cr 3+ Separation into Kn MF m (M refers to metal Fe 3+ and Cr 3+ ) in the form of early precipitation, reducing Ni 2+ Co-precipitation to maximize the retention of Ni 2+ In solution, Fe in wastewater 3+ and Cr 3+ Sediment recovery;

[0008] (3) After the filtrate in step (2) is acidified, KF is added to remove the small amount of Fe 3+ The precipitation is removed (forming K3FeF6) and Ni-containing 2+ of pure filtrate;

[0009] (4) Add KOH to the filtrate in step (3) to adjust the pH so that Ni 2+ All precipitates were recovered (forming Ni(OH)2).

[0010] Wherein, the free acid described in step (1) is one of nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid or phosphoric acid, or a mixture of any two of them.

[0011] Furthermore, in step (1), the diffusion dialysis device comprises a membrane stack composed of n+1 homogeneous anion exchange membranes and n acid-resistant partitions alternately formed, and dialysis chambers and diffusion chambers are alternately formed between the anion exchange membranes and the acid-resistant partitions from the dialysis side to the diffusion side, wherein the value of n is 1 to 100.

[0012] Furthermore, in the step (1), a partition with a thickness of 0.7 mm is provided between the anion exchange membranes in the diffusion dialysis device, and the effective area of ​​the anion exchange membrane is 323 cm 2 The diffusion dialysis membrane stack is respectively connected to a feed tank, a dialysis residual liquid tank, a diffusion recovery liquid tank and a pure water tank.

[0013] Furthermore, in the step (1), the feed volume ratio of the stainless steel pickling wastewater to the pure water is 1:1 to 1:2, and the peristaltic pump delivery flow rate is 2 ml / min to 8 ml / min.

[0014] Furthermore, the step (1) also includes a device for detecting the concentration of recovered acid connected to the diffusion side outlet of the diffusion dialysis in real time. When the concentration of recovered acid no longer changes significantly, it is considered that the diffusion dialysis has reached dynamic equilibrium and the diffusion dialysis is stopped.

[0015] Furthermore, the ratio of the composite precipitant KOH to KF in step (2) is (1-7):1, wherein the concentration of KOH is 1-7 mol / L, and the concentration of KF is 1 mol / L.

[0016] Furthermore, in step (2), the precipitation pH value is 4-6, the precipitation reaction temperature is 30-60°C, the reaction time is 30-60 min, and the stirring speed is 150-250 r / min.

[0017] Furthermore, before adding KF to the filtrate in step (3), the pH of the filtrate is first adjusted to 1-4 with nitric acid, and then KF is added at a concentration of 1 mol / L, and the pH of the KF solution is adjusted to 5.5.

[0018] Furthermore, the precipitation reaction in step (3) is carried out at room temperature.

[0019] Furthermore, the precipitation reaction in step (4) is carried out at room temperature, and the stirring condition is 300-350 r / min.

[0020] Furthermore, in the step (4), KOH is added to the precipitation reaction to adjust the pH of the solution to 10-11.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The newly designed "diffusion dialysis + three-step precipitation" coupling method of the present invention can simultaneously recover high-concentration free acid (such as one or a mixture of any two of nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid or phosphoric acid) and heavy metals (such as iron, chromium and nickel) in stainless steel pickling wastewater. Compared with conventional extraction, adsorption, evaporation, membrane technology and other treatment methods that only recover acid or heavy metals, the "diffusion dialysis + three-step precipitation" coupling method adopted by the present invention can achieve the simultaneous recovery of high-concentration free acid and heavy metals in stainless steel pickling wastewater, and the process is simple, easy to operate, low in energy consumption, and high in acid and heavy metal recovery rate. It is a green and efficient waste acid liquid resource treatment method with significant economic, environmental and social benefits and excellent industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the diffusion dialysis device of the present invention. Including: 1. Diffusion dialysis device; 2. Peristaltic pump; 3. Peristaltic pump; 4. Feed tank; 5. Dialysis residual liquid tank; 6. Diffusion recovery liquid tank; 7. Pure water tank; 8. Peristaltic pump tube;

[0024] Figure 2 It is a schematic diagram of the selective precipitation device of the present invention.

[0025] Figure 3 The present invention is a three-step precipitation process flow chart for recovering heavy metals. DETAILED DESCRIPTION

[0026] The following is a further detailed description in conjunction with specific embodiments to make the content of the invention more clear, but the content and scope of the invention of this patent are not limited to the following embodiments. Without departing from the content and scope of the invention, changes in implementation should be included in the technical scope of the invention.

[0027] Embodiment 1:

[0028] The "diffusion dialysis-selective precipitation combined process" of the present invention is used to recover high-concentration nitric acid and hydrofluoric acid from stainless steel pickling waste liquid. The feed composition of the stainless steel pickling waste liquid is 3 mol / L HNO3, 2 mol / L HF, 30 g / L Fe 3+ 、5g / L Cr 3+ and 5g / L Ni 2+ ; The total amount of feed liquid is 2L.

[0029] ① Diffusion dialysis to recover free acid

[0030] The diffusion dialysis device uses 19 anion exchange membranes (the anion exchange membranes are provided by Shandong Tianwei Membrane Technology Co., Ltd., model TWDDA3S) and 20 partitions. The anion exchange membranes and partitions are arranged alternately. The effective membrane area of ​​a single anion membrane is 0.017m 2 The effective membrane area of ​​the entire membrane module is 0.323m 2 2L of stainless steel pickling wastewater and pure water were added to the feed tank and pure water tank respectively, and then the wastewater and pure water were input into the diffusion dialysis system in a single-pass countercurrent manner through a peristaltic pump, wherein the wastewater flow rate on the dialysis side was 6ml / min, and the pure water flow rate on the diffusion side was 9ml / min, with a flow ratio of 1:1.5. When the solution concentrations at the dialysis residual liquid outlet and the diffusion recovery liquid outlet remained basically unchanged, it was considered to have reached dynamic equilibrium, and the diffusion dialysis process was immediately stopped. The cumulative operating time was about 210min, and the total acid recovery rate of HNO3 / HF could reach 69.5%.

[0031] Embodiment 2:

[0032] The flow rate of the feed liquid in the membrane stack in step ① was changed to 2 ml / min, and the flow ratio was 1:1 (diffusion chamber: dialysis chamber). Other operations were the same as in Example 1. After the above treatment process, the total acid recovery rate could reach 62.4%.

[0033] Embodiment 3:

[0034] The flow rate of the liquid in the membrane stack in step ① was changed to 4 ml / min, and the flow ratio was 1:1 (diffusion chamber: dialysis chamber). Other operations were the same as in Example 1. After the above treatment process, the total acid recovery rate could reach 66.7%.

[0035] Embodiment 4:

[0036] The flow rate of the liquid in the membrane stack in step ① was changed to 6 ml / min, and the flow ratio was 1: 1. Other operations were the same as in Example 1. After the above treatment process, the total acid recovery rate could reach 67.3%.

[0037] Embodiment: 5:

[0038] The flow rate of the liquid in the membrane stack in step ① was changed to 8 ml / min, and the flow ratio was 1:1 (diffusion chamber: dialysis chamber). After the above treatment process, the total acid recovery rate can reach 62.5%.

[0039] Embodiment 6:

[0040] The flow ratio of the liquid in the membrane stack in step ① was changed to 2:1 (diffusion chamber: dialysis chamber). Other operations were the same as those in Example 4. After the above treatment process, the acid recovery rate could reach 67.47%.

[0041] According to the above results, it can be seen that Example 1 is the best condition for the diffusion dialysis process of the present invention: when the dialysis chamber flow rate is 6 ml / min, and the flow ratio of the diffusion chamber to the dialysis chamber is 1.5:1. Under this optimal condition, the HNO3 / HF total acid recovery rate exceeds 69%. Therefore, the subsequent step, based on Example 1, continues to use the first step precipitation to recover heavy metal iron and chromium from its dialysis raffinate.

[0042] ②First step precipitation selectively recovers heavy metal iron and chromium

[0043] The above 20 ml dialysis residue (containing about 0.5 mol / L HNO3, 1.6 mol / L HF, 29.1 g / L Fe 3+ 、4.8g / LCr 3+ and 4.9g / L Ni 2+ ) was added to a 30°C polytetrafluoroethylene beaker containing 300 ml of pure water, and then a composite precipitant of KOH:KF=6:1 was added dropwise to the beaker. When the solution pH was adjusted to 5, the addition of the precipitant was stopped. After stirring for 30 minutes, the heavy metals iron and chromium were filtered and recovered, as well as the Ni 2+ and a small amount of residual Fe 3+ The precipitation results are listed in Table 1.

[0044] Embodiment 7:

[0045] The heating temperature in step ② was changed to 40° C., and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 1.

[0046] Embodiment 8:

[0047] The heating temperature in step ② was changed to 50° C., and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 1.

[0048] Embodiment 9:

[0049] The heating temperature in step ② was changed to 60° C., and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 1.

[0050] Table 1: Precipitation results at different reaction temperatures

[0051] Example <![CDATA[Fe 3+ Recovery rate%]]> <![CDATA[Cr 3+ Recovery rate%]]> <![CDATA[Ni 2+ Retention Rate %]]> Example 1 56.01 59.82 57.37 Example 7 68.75 62.69 61.07 Example 8 73.10 66.03 62.43 Example 9 78.13 72.87 55.24

[0052] Embodiment 10:

[0053] The heating temperature in step ② was changed to 50° C., the precipitation endpoint pH was changed to 4, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 2.

[0054] Embodiment 11:

[0055] The heating temperature in step ② was changed to 50° C., and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 2.

[0056] Embodiment 12:

[0057] The heating temperature in step ② was changed to 50° C., a precipitant prepared from potassium hydroxide and potassium fluoride (KOH:KF=6:1) was added dropwise into the beaker, and the precipitation endpoint pH was changed to 6. Other operations were the same as those in Example 1. The precipitation results are listed in Table 2.

[0058] Table 2: Precipitation results at different precipitation pH

[0059] Example <![CDATA[Fe 3+ Recovery rate%]]> <![CDATA[Cr 3+ Recovery rate%]]> <![CDATA[Ni 2+ Retention Rate %]]> Example 10 6.68 34.27 68.41 Embodiment 11 73.10 66.03 62.43 Example 12 98.68 96.56 28.69

[0060] Embodiment 13:

[0061] The heating temperature in step ② was changed to 40° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 3:1, and the other operations were the same as those in Example 1. The precipitation results obtained are listed in Table 3.

[0062] Embodiment 14:

[0063] The heating temperature in step ② was changed to 40° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 1:1, and the other operations were the same as those in Example 1. The precipitation results obtained are listed in Table 3.

[0064] Table 3: Precipitation results under different precipitant compositions

[0065] Example <![CDATA[Fe 3+ Recovery rate%]]> <![CDATA[Cr 3+ Recovery rate%]]> <![CDATA[Ni 2+ Retention Rate %]]> Embodiment 13 58.74 56.86 59.74 Embodiment 14 19.46 36.55 60.39

[0066] In order to further explore the optimal conditions for the first step of metal precipitation recovery, a three-factor three-level orthogonal experiment was carried out based on the above single-factor experiment. The implementation examples are shown in 15-23.

[0067] Embodiment 15:

[0068] The heating temperature in step ② was changed to 40° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 5:1, and the other operations were the same as those in Example 1. The precipitation results obtained are listed in Table 4.

[0069] Embodiment 16:

[0070] The heating temperature in step ② was changed to 50° C., and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0071] Embodiment 17:

[0072] The heating temperature in step ② was changed to 60° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 7:1, and the other operations were the same as those in Example 1. The precipitation results obtained are listed in Table 4.

[0073] Embodiment 18:

[0074] The heating temperature in step ② was changed to 40° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 7:1, the precipitation endpoint pH was changed to 5.5, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0075] Embodiment 19:

[0076] The heating temperature in step ② was changed to 50° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 5:1, the precipitation endpoint pH was changed to 5.5, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0077] Embodiment 20:

[0078] The heating temperature in step ② was changed to 60° C., the precipitation endpoint pH was changed to 5.5, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0079] Embodiment 21:

[0080] The heating temperature in step ② was changed to 40° C., the precipitation endpoint pH was changed to 6, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0081] Embodiment 22:

[0082] The heating temperature in step ② was changed to 50° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 7:1, the precipitation endpoint pH was changed to 6, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0083] Embodiment 23:

[0084] The heating temperature in step ② was changed to 60° C., the ratio of the precipitant prepared by potassium hydroxide and potassium fluoride was changed to 5:1, the precipitation endpoint pH was changed to 6, and the other operations were the same as in Example 1. The precipitation results obtained are listed in Table 4.

[0085] Table 4: Test results of metal ion selective precipitation in orthogonal experiment

[0086] Example <![CDATA[Fe 3+ Recovery rate%]]> <![CDATA[Cr 3+ Recovery rate%]]> <![CDATA[Ni 2+ Retention Rate %]]> Embodiment 15 54.91 43.07 52.55 Example 16 73.10 66.03 62.43 Embodiment 17 79.03 57.92 43.71 Embodiment 18 91.01 77.83 82.07 Embodiment 19 94.08 80.12 64.90 Embodiment 20 95.80 86.09 57.54 Embodiment 21 97.91 89.74 49.73 Embodiment 22 99.46 93.43 38.98 Embodiment 23 99.57 97.78 29.00

[0087] According to the above results, it can be seen that Example 18 is the best process condition for the first step of precipitation of the present invention: the precipitation temperature is 40°C, the molar ratio of potassium hydroxide to potassium fluoride is 7:1, and the precipitation end point pH is 5.5. Under this optimal condition, the recovery rates of heavy metal iron and chromium in stainless steel pickling wastewater exceed 91% and 77% respectively, while the retention rate of nickel is greater than 82%, the separation effect is good, and the loss is small. Therefore, based on Example 18, the second step is continued to remove the remaining small amount of Fe from the filtrate. 3+ , in order to increase the Ni 2+ purity.

[0088] ③ The second step is precipitation to remove residual Fe in the filtrate 3+ , increase the Ni content in the filtrate 2+ Purity

[0089] Embodiment 24:

[0090] The filtrate of Example 18 in step ② was first acidified with 1 mol / L HNO3 to adjust the pH to 1, and then 1 mol / L KF solution was added under stirring at 300 r / min until the solution pH was 5.5, and then the addition was stopped and the filtrate was obtained by filtration. 3+ The precipitation rate of Ni 2+ The purity is as high as 96.68%.

[0091] Embodiment 25:

[0092] The filtrate of Example 18 in step ② was acidified with 1 mol / L HNO3, and the pH was adjusted to 2. Then, 1 mol / L KF solution was added under stirring at 300 r / min until the pH of the solution was 5.5, and then the addition was stopped and the filtrate was obtained by filtration. 3+ The precipitation rate reached 99.84%, Ni 2+ The purity is 95.28%.

[0093] Embodiment 26:

[0094] The filtrate of Example 18 in step ② was acidified with 1 mol / L HNO3, and the pH was adjusted to 3. Then, 1 mol / L KF solution was added under stirring at 300 r / min until the pH of the solution was 5.5, and then the addition was stopped and the filtrate was obtained by filtration. 3+ The precipitation rate of Ni 2+ The purity is 91.65%.

[0095] According to the above results, it can be seen that Example 25 is the best process condition for the second step precipitation of the present invention: solution acidification pH = 2, KF concentration is 1 mol / L, precipitation end point pH = 5.5, stirring speed is 300r / mi. Under this optimal condition, the residual Fe in the filtrate of Example 18 3+ The precipitation rate exceeds 99%, and Ni 2+ The purity of the product is also as high as 95%. Therefore, based on Example 25, the third step of precipitation is continued to be used to recover the remaining heavy metal nickel from the filtrate.

[0096] ④The third step is precipitation to recover heavy metal nickel

[0097] 1.5 mol / L KOH solution was added to the filtrate of Example 25, and the solution pH was adjusted to 11 before stopping the dropwise addition. Ni(OH)2 was obtained by filtration. At this time, the Ni(OH)2 precipitation rate exceeded 99%, and the final recovery rate reached 77.35%.

[0098] According to the above results, it can be seen that the "diffusion dialysis + three-step precipitation" coupling method designed by the present invention can simultaneously realize the recovery of high-concentration free acid and heavy metals in stainless steel pickling wastewater compared with the existing stainless steel pickling wastewater treatment method, among which the recovery rate of HNO3 / HF total free acid is as high as 69%, and the recovery rates of heavy metals iron, chromium and nickel are as high as 91.01%, 77.83% and 77.35%, respectively. In addition, the process is simple, the operation is easy, the energy consumption is low, and the acid and heavy metal recovery rates are high. It is a green and efficient waste acid liquid resource treatment method with significant economic, environmental and social benefits, and has huge potential for industrial application.

Claims

1. A method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater, characterized in that: The following steps are involved: (1) Add the stainless steel pickling wastewater to the dialysis chamber on the dialysis side of the diffusion dialysis device, and add pure water to the pure water chamber on the diffusion side, and then input them into the dialysis system of the diffusion dialysis device through peristaltic pumps, separate and recover the free acid in the wastewater through the diffusion dialysis membrane, and obtain the dialysis residue; (2) Add KOH / KF composite precipitant to the dialysis residue in step (1) to remove Fe 3+ and Cr 3+ Precipitation recovery, while retaining Ni 2+ And a small amount of residual Fe 3+ The filtrate; (3) After acidification, KF is added to the filtrate obtained in step (2) to remove the small amount of Fe 3+ The precipitate is removed to obtain Ni-containing 2+ of pure filtrate; (4) Add KOH to the filtrate of step (3) to adjust the pH so that Ni 2+ All precipitates are recovered; The free acid described in step (1) is one of nitric acid, hydrofluoric acid, hydrochloric acid, sulfuric acid or phosphoric acid, or a mixture of any two of them; the concentration of KOH in the composite precipitant in step (2) is 1-7 mol / L, the concentration of KF is 1 mol / L, and the ratio of KOH to KF is (1-7):1; the precipitation pH value in step (2) is 4-6, the precipitation reaction temperature is 30-60°C, and the reaction time is 30-60 min; before adding KF in step (3), the pH of the filtrate is first adjusted to 1-4 with nitric acid; after adjusting the pH, the concentration of KF added is 1 mol / L, and the solution after adding KF is further adjusted to pH=5.5; in step (4), KOH is added during the precipitation reaction to adjust the solution pH to 10-11.

2. A method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater according to claim 1, characterized in that: In the step (1), the membrane stack of the diffusion dialysis device is composed of n+1 homogeneous anion exchange membranes and n acid-resistant separators alternately formed, and dialysis chambers and diffusion chambers are alternately formed between the anion exchange membranes and the acid-resistant separators from the dialysis side to the diffusion side, wherein the value of n is 1 to 100.

3. A method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater as claimed in claim 2, characterized in that: In the step (1), a partition with a thickness of 0.7 mm is provided between the anion exchange membranes in the diffusion dialysis device, and the effective area of ​​the anion exchange membrane is 323 cm 2 The diffusion dialysis membrane stack is respectively connected to a feed tank, a dialysis residual liquid tank, a diffusion recovery liquid tank and a pure water tank.

4. A method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater according to claim 1, characterized in that: In the step (1), the feed volume ratio of the stainless steel pickling wastewater to the pure water is 1:1 to 1:2, and the peristaltic pump delivery flow rate is 2 ml / min to 8 ml / min.

5. A method for simultaneously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater according to claim 1, characterized in that: The stirring speed in step (2) is 150-250 r / min.

6. A method for synchronously recovering high-concentration free acid and heavy metals in stainless steel pickling wastewater according to claim 1, characterized in that: The precipitation reaction in step (4) is carried out at room temperature with a stirring condition of 300 to 350 r / min.

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