Harmless purification and recycling method for waste acid through low-temperature evaporation and concentration

Abstract

The invention discloses a waste acid low-temperature evaporation and concentration harmless purification and recycling method. The method comprises the following steps: S1, industrial waste acid impurity pre-removal treatment: S1-1, industrial waste acid is subjected to pre-filtration treatment; s1-2, carrying out crystallization treatment on the filtered waste acid; s1-3, after crystallization treatment, solid-liquid separation is achieved through low-temperature centrifugal separation; according to the method disclosed by the invention, an ultrasonic and vacuum pulse evaporation mechanism is designed, an interface is updated through ultrasonic membrane rupture and vacuum pulse, the core problems of low low-temperature evaporation mass transfer efficiency and easiness in scaling are solved, and efficient low-temperature concentration of waste acid is realized; according to the invention, a pretreatment process combining composite membrane pre-interception with low-temperature crystallization is designed, heavy metal ions are accurately captured by using a composite crystallization inducer, and the disadvantage that new impurities are introduced by a traditional chemical precipitation method is avoided.

Description

Technical Field

[0001] This invention relates to the field of industrial waste acid treatment technology, specifically a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. Background Technology

[0002] Industrial waste acid refers to acidic waste liquid containing high concentrations of inorganic or organic acids, generated during industrial production processes. It is commonly found in industries such as chemical, metallurgical, electroplating, pharmaceutical, and metal processing. Its main components may include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, and hydrofluoric acid, and it usually also contains pollutants such as heavy metal ions, organic matter, and oils, exhibiting strong corrosiveness, toxicity, and environmental hazards.

[0003] The treatment of industrial waste acid should follow the principles of "reduction, resource recovery, and harmlessness." However, existing treatment methods still have shortcomings in terms of recovery rate and treatment effect, and need to be further improved and optimized. Summary of the Invention

[0004] The purpose of this invention is to provide a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. An ultrasonic-assisted vacuum pulsed evaporation mechanism is designed to make the low-temperature concentration of waste acid more efficient.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid includes the following steps: S1. Pre-treatment of industrial waste acid for impurity removal: S1-1. Industrial waste acid is first pre-filtered through a modified polytetrafluoroethylene composite membrane. S1-2. The filtered waste acid enters a low-temperature crystallization tank, and a composite crystallization inducer is added to the tank for crystallization treatment. S1-3. After crystallization treatment, solid-liquid separation is achieved by low-temperature centrifugation at 5℃ to obtain the purified industrial waste acid. S2. Low-temperature evaporation concentration and ultrasonic-assisted vacuum pulsed evaporation treatment: The pre-treated industrial waste acid is transported to a low-temperature evaporation kettle and evaporated at 35-55°C under vacuum conditions. The industrial waste acid is concentrated to 70-80% by mass fraction to obtain concentrated acid solution. S3, Gradient membrane separation and low-temperature recrystallization: S3-1, Gradient membrane separation treatment: The concentrated acid solution is filtered and concentrated by two stages of ceramic ultrafiltration membranes with different retention capacities; The concentrated acid solution passing through the ceramic ultrafiltration membrane forms a preliminarily purified acid solution; S3-2, Low-temperature recrystallization treatment: The preliminarily purified acid solution is transported to a low-temperature recrystallization tank and cooled to -5~0℃ at a cooling rate of 1~2℃ / min. The solution is then kept at a constant temperature for 4~6 hours to allow the acid components in the preliminarily purified acid solution to form crystals. Finally, these crystals were filtered and separated from the mother liquor at a low temperature of -2 to 0℃ to obtain high-purity acid crystals. S4. Acid regeneration and waste material harmless treatment: High-purity acid crystals are fed into a dissolving tank, and the condensate obtained in step S2 is added to dissolve them. The acid concentration is adjusted to the required standard for production to obtain regenerated acid solution, which can be reused in the original production process.

[0006] Preferably, in step S1-1, the pore size of the modified polytetrafluoroethylene composite membrane is controlled at 0.1~0.5μm, the filtration pressure is controlled at 0.2~0.3Mpa, and the filtration temperature is 25~30℃.

[0007] Note: This treatment process can accurately remove suspended particulate matter and colloidal impurities from waste acid, achieving a removal rate of ≥98% for suspended particulate matter and colloidal impurities.

[0008] Preferably, during the crystallization process in step S1-2, the temperature is controlled at 0~5℃, the stirring rate is 50~80r / min, and the constant temperature crystallization is carried out for 2~3h. The composite crystallization inducer is composed of trisodium citrate and sodium pyrophosphate in a mass ratio of 2:1. The mass fraction of the composite crystallization inducer is 0.5~1.0%.

[0009] Note: The use of composite crystallization inducers to precisely capture heavy metal ions avoids the drawbacks of introducing new impurities in traditional chemical precipitation methods.

[0010] Preferably, in steps S1-3, during the low-temperature centrifugation separation, the rotation speed is controlled at 8000~10000 r / min, and the centrifugation process lasts for 20~30 min; The heavy metal content in industrial waste acid after pre-purification treatment is ≤5mg / L.

[0011] Note: Solid-liquid separation is achieved through low-temperature centrifugation to obtain industrial waste acid after impurity removal.

[0012] Preferably, in step S2, an ultrasonic transducer with a frequency of 20-40 kHz and a power density of 100-150 W / L is installed in a low-temperature evaporation kettle to continuously apply ultrasonic vibration energy to the industrial waste acid. The vacuum level inside the low-temperature evaporator is periodically pulsated and adjusted within a 60-second cycle, ranging from -0.08 to 0.09 MPa.

[0013] Note: Ultrasonic combined with vacuum pulse regulation for evaporation treatment solves the industry pain points of low mass transfer efficiency and easy scaling in low-temperature evaporation, and the evaporation rate is increased by 40-50% compared with conventional vacuum evaporation.

[0014] Preferably, in step S2, the water vapor generated by evaporation is transported to a condenser through a pipeline. The condenser liquefies the water vapor at an environment of 5~10℃ to obtain condensate. The conductivity of the condensate is ≤50μS / cm, which can be directly reused for production water or discharged after further treatment.

[0015] Preferably, in step S3-1, the primary membrane is an Al2O3-TiO2 composite ceramic membrane with a molecular weight cutoff of 500~1000 Da, which removes residual macromolecular organic impurities and trace suspended matter in the concentrated acid solution. The secondary membrane is a modified SiO2 ceramic membrane with a molecular weight cutoff of 100~300 Da, which can selectively retain trace metal ions and anionic impurities in acid solutions.

[0016] Preferably, in step S3-1, during gradient membrane separation, the membrane separation pressure is controlled at 0.4~0.6MPa and the temperature is 30~40℃.

[0017] Explanation: By selectively retaining the acid solution through two-stage membranes and reconstructing it at low temperatures, the purity of the acid solution is significantly improved, enabling the high-value reuse of waste acid.

[0018] Preferably, in step S4, the crystalline composite produced by the pre-purification treatment is mixed with the mother liquor produced by low-temperature recrystallization and disposed of as tailings. 10-15% by mass of modified zeolite powder and 5-8% by mass of calcium hydroxide powder are added to the tailings, and the mixture is stirred continuously at 25-35°C for 2-3 hours to allow the heavy metal ions in the tailings to form hydroxides or silicate coordination compounds, and the fluoride ions to form calcium fluoride precipitates. After the reaction is completed, the solidified tailings can be obtained by pressure filtration. The filtrate produced by pressure filtration is adjusted to neutral before being discharged.

[0019] Preferably, in step S4, the modified zeolite powder is zeolite powder that has been activated with 3wt% hydrochloric acid; The pressure during filter press treatment is 1.0~1.2 MPa, which ensures that the moisture content of the solidified tailings is ≤20%.

[0020] Description: The development of modified zeolite and calcium hydroxide synergistic coordination solidification technology achieves stable solidification of harmful impurities in tailings, while simultaneously enabling resource utilization of tailings and achieving the goal of harmless disposal throughout the entire process.

[0021] Compared with the prior art, the beneficial effects of the present invention are reflected in the following aspects: 1. This invention designs an ultrasonic-assisted vacuum pulsed evaporation mechanism, which solves the core problems of low mass transfer efficiency and easy scaling in low-temperature evaporation by ultrasonic membrane breaking and vacuum pulsed interface renewal, and realizes efficient low-temperature concentration of waste acid. 2. This invention designs a pretreatment process combining composite membrane pre-retention and low-temperature crystallization, which uses a composite crystallization inducer to accurately capture heavy metal ions, avoiding the drawbacks of introducing new impurities in traditional chemical precipitation methods. 3. This invention constructs a two-stage purification system of gradient membrane separation and low-temperature recrystallization. Through the selective retention of the two-stage membrane and the reconstruction of the low-temperature crystal, the purity of the acid solution is greatly improved, and the high-value reuse of waste acid is realized. 4. This invention develops a synergistic coordination and solidification technology for modified zeolite and calcium hydroxide to achieve stable solidification of harmful impurities in tailings, while simultaneously utilizing tailings as resources and achieving the goal of harmless disposal throughout the entire process. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0023] The following is combined with Figure 1 The present invention will be described in detail below.

[0024] Example 1: A method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid, such as... Figure 1 As shown, it includes the following steps: S1. Pre-treatment of industrial waste acid for impurity removal: S1-1. Industrial waste acid is first pre-filtered through a modified polytetrafluoroethylene composite membrane to remove suspended particulate matter and colloidal impurities from the waste acid. Modified polytetrafluoroethylene composite membranes are commercially available products based on existing technology; for example, they could be PTFE series hydrophilic composite membranes from Maiborui Company. The removal rate of suspended particulate matter and colloidal impurities in waste acid is ≥98%.

[0025] The pore size of the modified polytetrafluoroethylene composite membrane is controlled at 0.1 μm, the filtration pressure is controlled at 0.2 MPa, and the filtration temperature is 25℃.

[0026] S1-2. The filtered waste acid enters a low-temperature crystallization tank, and a composite crystallization inducer is added to the tank for crystallization treatment. The mass fraction of the composite crystallization inducer is 0.5%; During the crystallization process, the temperature was controlled at 0℃, the stirring rate was 50r / min, and the crystallization was carried out at a constant temperature for 2 hours. The composite crystallization inducer is composed of trisodium citrate and sodium pyrophosphate in a mass ratio of 2:1.

[0027] S1-3, After crystallization treatment, heavy metal ions (such as Fe) in the waste acid 3+ Cu 2+ Pb 2+ The mixture forms a stable crystalline complex with the inducing agent. Solid-liquid separation is achieved by low-temperature centrifugation at 5°C to obtain the purified industrial waste acid. In the low-temperature centrifugation separation, the rotation speed is controlled at 8000 r / min, and the centrifugation process lasts for 20 min; The heavy metal content in industrial waste acid after pre-purification treatment is ≤5mg / L.

[0028] S2. Low-temperature evaporation concentration and ultrasonic-assisted vacuum pulsed evaporation treatment: The pre-treated industrial waste acid is transported to a low-temperature evaporation kettle and evaporated at 35°C under vacuum to concentrate the industrial waste acid to 70% by mass fraction, thus obtaining concentrated acid solution. The continuous action of the ultrasonic field helps to break the gas film blockage at the evaporation interface, and the vacuum pulsation regulation can promote the renewal of the evaporation surface and avoid local overheating and scaling of the acid solution. The water vapor produced by evaporation is transported to the condenser through pipelines. The condenser liquefies the water vapor at 5°C to obtain condensate. The conductivity of the condensate is ≤50μS / cm, which can be directly reused for production water or discharged after further treatment. An ultrasonic transducer with a power density of 100W / L was installed in a low-temperature evaporation kettle to continuously apply ultrasonic vibration energy to industrial waste acid. The vacuum level inside the low-temperature evaporator is periodically pulsated. Within a 60-second cycle, the vacuum level inside the low-temperature evaporator is periodically adjusted between -0.08 and -0.09 MPa. That is, within the first 30 seconds, the vacuum level inside the low-temperature evaporator changes uniformly from -0.08 MPa to -0.09 MPa, and within the next 30 seconds, the vacuum level inside the low-temperature evaporator changes uniformly from -0.09 MPa back to -0.08 MPa, and so on.

[0029] S3, Gradient membrane separation and low-temperature recrystallization: S3-1, Gradient membrane separation treatment: The concentrated acid solution is filtered and concentrated by two stages of ceramic ultrafiltration membranes with different retention capacities; The primary membrane is an Al2O3-TiO2 composite ceramic membrane with a molecular weight cutoff of 500 Da, which removes residual macromolecular organic impurities and trace suspended matter in concentrated acid solution. The secondary membrane is a modified SiO2 ceramic membrane with a molecular weight cutoff of 100 Da, capable of selectively retaining trace metal ions and anionic impurities (such as F) in acid solutions. - ClO3 - ); The concentrated acid solution passing through the ceramic ultrafiltration membrane forms a preliminarily purified acid solution; In gradient membrane separation, the membrane separation pressure is controlled at 0.4 MPa and the temperature at 30℃.

[0030] S3-2, Low-temperature recrystallization treatment: The preliminarily purified acid solution is transported to a low-temperature recrystallization tank and cooled to -5℃ at a cooling rate of 1℃ / min. The solution is then kept at a constant temperature for 4 hours to allow the acid components in the preliminarily purified acid solution to form crystals, such as sulfuric acid crystals and hydrochloric acid hydrate crystals. Finally, these crystals were filtered and separated from the mother liquor at a low temperature of -2℃ to obtain high-purity acid crystals. However, trace impurities remain dissolved in the mother liquor; S4. Acid regeneration and waste material harmless treatment: High-purity acid crystals are fed into a dissolving tank, and the condensate obtained in step S2 is added to dissolve them. The acid concentration is adjusted to the required production standard, such as 98% sulfuric acid and 31% hydrochloric acid, to obtain regenerated acid solution that can be reused in the original production process. The crystalline composite produced by the pre-purification treatment is mixed with the mother liquor produced by low-temperature recrystallization and treated as tailings. 10% by mass of modified zeolite powder and 5% by mass of calcium hydroxide powder are added to the tailings and stirred continuously at 25°C for 2 hours to allow heavy metal ions in the tailings to form hydroxides or silicate coordination compounds and fluoride ions to form calcium fluoride precipitates. After the reaction is completed, the solidified tailings can be obtained by pressure filtration. The filtrate produced by pressure filtration is adjusted to neutral before being discharged. Modified zeolite powder is zeolite powder that has been activated by 3wt% hydrochloric acid; The pressure during the filter press treatment is 1.0 MPa, which ensures that the moisture content of the solidified tailings is ≤20%.

[0031] Tests showed that the leaching toxicity of the solidified tailings met the requirements of the "Identification Standard for Hazardous Waste: Leaching Toxicity Identification" (GB5085.3-2007), and it can be used as an additive in building materials or sent to a hazardous waste disposal center for co-processing.

[0032] Example 2: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. The difference from Embodiment 1 is that in step S1-1, the pore size of the modified polytetrafluoroethylene composite membrane is controlled at 0.3 μm, the filtration pressure is controlled at 0.3 MPa, and the filtration temperature is 28°C.

[0033] In steps S1-2, a composite crystallization inducer is added to the tank for crystallization treatment, and the mass fraction of the composite crystallization inducer is 0.7%. During the crystallization process, the temperature was controlled at 2℃, the stirring rate was 60r / min, and the crystallization was carried out at a constant temperature for 2.5h. In steps S1-3, during the low-temperature centrifugation separation, the rotation speed is controlled at 9000 r / min, and the centrifugation process lasts for 25 min.

[0034] Example 3: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. The difference from Embodiment 1 is that in step S1-1, the pore size of the modified polytetrafluoroethylene composite membrane is controlled at 0.5 μm, the filtration pressure is controlled at 0.3 MPa, and the filtration temperature is 30°C.

[0035] In steps S1-2, a composite crystallization inducer is added to the tank for crystallization treatment, and the mass fraction of the composite crystallization inducer is 1.0%. During the crystallization process, the temperature was controlled at 5℃, the stirring rate was 80r / min, and the crystallization was carried out at a constant temperature for 3 hours. In steps S1-3, during the low-temperature centrifugation separation, the rotation speed is controlled at 10000 r / min, and the centrifugation process lasts for 30 min.

[0036] Example 4: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. The difference from Embodiment 3 is that in step S2, low-temperature evaporation is carried out at 45°C to concentrate the industrial waste acid to 75% by mass fraction, thereby obtaining concentrated acid solution. The condenser liquefies water vapor at 7°C to obtain condensate, and the conductivity of the condensate is ≤50μS / cm; An ultrasonic transducer with a power density of 120 W / L was installed in a low-temperature evaporation kettle to continuously apply ultrasonic vibration energy to industrial waste acid.

[0037] Example 5: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. The difference from Embodiment 3 is that in step S2, low-temperature evaporation is carried out at 55°C to concentrate the industrial waste acid to 80% by mass fraction, thereby obtaining concentrated acid solution. The condenser liquefies water vapor at 10℃ to obtain condensate, and the conductivity of the condensate is ≤50μS / cm; An ultrasonic transducer with a power density of 150 W / L was installed in a low-temperature evaporation kettle to continuously apply ultrasonic vibration energy to industrial waste acid.

[0038] Example 6: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification and reuse of waste acid. The difference from embodiment 5 is that in step S3-1, the primary membrane is an Al2O3-TiO2 composite ceramic membrane with a molecular weight cutoff of 800 Da. The secondary membrane is a modified SiO2 ceramic membrane with a molecular weight cutoff of 200 Da; In gradient membrane separation, the membrane separation pressure is controlled at 0.5 MPa and the temperature at 35℃.

[0039] In step S3-2, the preliminarily purified acid solution is transported to a low-temperature recrystallization tank, where it is cooled to -2°C at a cooling rate of 2°C / min and kept at a constant temperature for 5 hours.

[0040] Example 7: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification and reuse of waste acid. The difference from embodiment 5 is that in step S3-1, the primary membrane is an Al2O3-TiO2 composite ceramic membrane with a molecular weight cutoff of 1000 Da. The secondary membrane is a modified SiO2 ceramic membrane with a molecular weight cutoff of 300 Da; In gradient membrane separation, the membrane separation pressure is controlled at 0.6 MPa and the temperature at 40℃.

[0041] In step S3-2, the preliminarily purified acid solution is transported to a low-temperature recrystallization tank and cooled to 0°C at a cooling rate of 2°C / min, and then kept at a constant temperature for 6 hours.

[0042] Example 8: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. The difference from Embodiment 7 is that in step S4, 12% by mass of modified zeolite powder and 7% by mass of calcium hydroxide powder are added to the tailings. The mixture is stirred continuously at 30°C for 3 hours to allow heavy metal ions in the tailings to form hydroxides or silicate coordination compounds, and fluoride ions to form calcium fluoride precipitates. After the reaction is completed, the solidified tailings can be obtained by pressure filtration. The pressure during pressure filtration is 1.1 MPa, so that the moisture content of the solidified tailings is ≤20%.

[0043] Example 9: This embodiment describes a method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid. The difference from Embodiment 7 is that in step S4, 15% by mass of modified zeolite powder and 8% by mass of calcium hydroxide powder are added to the tailings. The mixture is stirred continuously at 35°C for 3 hours to allow heavy metal ions in the tailings to form hydroxides or silicate coordination compounds, and fluoride ions to form calcium fluoride precipitates. After the reaction is completed, the solidified tailings can be obtained by pressure filtration. The pressure during pressure filtration is 1.2 MPa, so that the moisture content of the solidified tailings is ≤20%.

[0044] Experimental example: Test subject: A plating plant containing sulfuric acid and heavy metals (Fe) was selected. 3+ Cu 2+ The waste acid solution has the following initial parameters: acid concentration: 15%, Fe 3+ Concentration: 500 mg / L, Cu 2+ Concentration: 200 mg / L; Suspended solids content: 800 mg / L; Experimental Groups: Group 1: Using all the process steps of Example 9 of the present invention; Group 2: The conventional vacuum evaporation method was used, which combined traditional vacuum evaporation with chemical precipitation. The evaporation temperature was 70℃, without ultrasonic assistance, and the chemical precipitant was NaOH. Group 3: Membrane separation method is used, with only ultrafiltration membrane combined with reverse osmosis membrane treatment. The membrane molecular weight cutoff is 1000 Da, and there are no low-temperature crystallization and recrystallization steps. The test results are recorded in Tables 1-5 below: Table 1. Comparison of Pretreatment Effects

[0045] Analysis of the data in Table 1 shows that the pretreatment process of this invention is significantly superior to traditional chemical precipitation and single membrane separation methods in terms of removal rates of suspended solids and heavy metals, especially for Fe. 3+ Cu 2+ The removal rate was close to 99%, proving that the composite crystallization inducer can efficiently and selectively capture heavy metal ions without introducing new impurities, laying the foundation for subsequent purification.

[0046] Table 2. Comparison of Evaporation and Concentration Efficiency

[0047] As can be seen from the data analysis in Table 2, the present invention achieves faster concentration speed at lower temperatures through the ultrasonic-assisted vacuum pulsed evaporation mechanism, reducing time by about 40%, energy consumption by about 35%, and completely avoiding scaling problems.

[0048] Table 3. Comparison of the purity of recovered acid

[0049] Analysis of the data in Table 3 shows that after two-stage purification via gradient membrane separation and low-temperature recrystallization, the recovered acid concentration reaches as high as 98%, heavy metal residue is ≤1 mg / L, TOC is extremely low, and the purity is significantly higher than that of traditional processes. This indicates that the combined process can deeply remove trace metal ions, anions, and organic impurities, realizing the high-value resource utilization of waste acid.

[0050] Table 4. Effects of harmless treatment of waste materials

[0051] As can be seen from the data analysis in Table 4, the process of synergistic curing of modified zeolite and calcium hydroxide in this invention results in low moisture content of the tailings, complete compliance with leaching toxicity standards, and good long-term stability. In contrast, the cured body of the traditional method is prone to releasing heavy metals and fluoride ions, posing environmental risks.

[0052] Table 5. Condensate quality

[0053] Analysis of the data in Table 5 shows that the conductivity of the evaporation condensate of the present invention is ≤50μS / cm and the pH is close to neutral, which can meet the standard for direct reuse and realize the closed-loop utilization of water resources. In contrast, the condensate of the traditional method still contains a certain amount of acidic ions and requires secondary treatment, which increases the cost and environmental burden.

[0054] Experimental conclusion: 1. In the pretreatment stage, the present invention achieves efficient removal of heavy metals (>98%) through a composite crystallization inducer, which is superior to the traditional chemical precipitation method and does not introduce new impurities.

[0055] 2. In the evaporation and concentration stage, the ultrasonic-assisted vacuum pulsation mechanism increases evaporation efficiency by about 40%, reduces energy consumption by about 40%, and eliminates scaling.

[0056] 3. The purity of the recovered acid is significantly improved, reaching over 98%, and the heavy metal residue is ≤1mg / L, meeting the requirements for high-value reuse.

[0057] 4. The solidification effect of the tailings is stable, and the leaching toxicity meets the standards, achieving the dual goals of harmlessness and resource utilization.

[0058] 5. The entire process is a closed loop, and the condensate can be directly reused, achieving zero wastewater discharge.

Claims

1. A method for the low-temperature evaporation, concentration, harmless purification, and reuse of waste acid, characterized in that, Includes the following steps: S1. Pre-treatment of industrial waste acid for impurity removal: S1-1. Industrial waste acid is first pre-filtered through a modified polytetrafluoroethylene composite membrane. S1-2. The filtered waste acid enters a low-temperature crystallization tank, and a composite crystallization inducer is added to the tank for crystallization treatment. S1-3. After crystallization treatment, solid-liquid separation is achieved by low-temperature centrifugation at 5℃ to obtain industrial waste acid after impurity removal. S2, Low-temperature evaporation concentration and ultrasonic-assisted vacuum pulsed evaporation treatment: The pre-treated industrial waste acid is transported to a low-temperature evaporation kettle and evaporated at 35-55°C under vacuum conditions. The industrial waste acid is concentrated to 70-80% by mass fraction to obtain concentrated acid solution. S3, Gradient membrane separation and low-temperature recrystallization: S3-1, Gradient membrane separation treatment: The concentrated acid solution is filtered and concentrated by two stages of ceramic ultrafiltration membranes with different retention capacities; The concentrated acid solution passing through the ceramic ultrafiltration membrane forms a preliminarily purified acid solution; S3-2, Low-temperature recrystallization treatment: The preliminarily purified acid solution is transported to a low-temperature recrystallization tank and cooled to -5~0℃ at a cooling rate of 1~2℃ / min. The solution is then kept at a constant temperature for 4~6 hours to allow the acid components in the preliminarily purified acid solution to form crystals. Finally, these crystals were filtered and separated from the mother liquor at a low temperature of -2 to 0℃ to obtain high-purity acid crystals. S4. Acid regeneration and waste material harmless treatment: High-purity acid crystals are fed into a dissolving tank, and the condensate obtained in step S2 is added to dissolve them. The acid concentration is adjusted to the required standard for production to obtain regenerated acid solution, which can be reused in the original production process.

2. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, In step S1-1, the pore size of the modified polytetrafluoroethylene composite membrane is controlled at 0.1~0.5μm, the filtration pressure is controlled at 0.2~0.3Mpa, and the filtration temperature is 25~30℃.

3. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, During the crystallization process in step S1-2, the temperature is controlled at 0~5℃, the stirring rate is 50~80r / min, and the constant temperature crystallization is carried out for 2~3h. The composite crystallization inducer is composed of trisodium citrate and sodium pyrophosphate in a mass ratio of 2:

1. The mass fraction of the composite crystallization inducer is 0.5~1.0%.

4. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, In steps S1-3, during the low-temperature centrifugation separation, the rotation speed is controlled at 8000~10000 r / min, and the centrifugation process lasts for 20~30 min; The heavy metal content in industrial waste acid after pre-purification treatment is ≤5mg / L.

5. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, In step S2, an ultrasonic transducer with a frequency of 20-40 kHz and a power density of 100-150 W / L is installed in a low-temperature evaporation kettle to continuously apply ultrasonic vibration energy to the industrial waste acid. The vacuum level inside the low-temperature evaporator is periodically pulsated and adjusted within a 60-second cycle, ranging from -0.08 to 0.09 MPa.

6. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, In step S2, the water vapor generated by evaporation is transported to a condenser through a pipeline. The condenser liquefies the water vapor at an environment of 5~10℃ to obtain condensate. The conductivity of the condensate is ≤50μS / cm, which can be directly reused for production water or discharged after further treatment.

7. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, In step S3-1, the primary membrane is an Al2O3-TiO2 composite ceramic membrane with a molecular weight cutoff of 500~1000 Da, which removes residual macromolecular organic impurities and trace suspended matter in the concentrated acid solution. The secondary membrane is a modified SiO2 ceramic membrane with a molecular weight cutoff of 100~300 Da, which can selectively retain trace metal ions and anionic impurities in acid solutions.

8. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 7, characterized in that, In step S3-1, during gradient membrane separation, the membrane separation pressure is controlled at 0.4~0.6MPa and the temperature is 30~40℃.

9. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 1, characterized in that, In step S4, the crystalline composite produced by the pre-purification treatment is mixed with the mother liquor produced by low-temperature recrystallization and disposed of as tailings. 10-15% by mass of modified zeolite powder and 5-8% by mass of calcium hydroxide powder are added to the tailings. The mixture is stirred continuously at 25-35°C for 2-3 hours to allow heavy metal ions in the tailings to form hydroxides or silicate coordination compounds, and fluoride ions to form calcium fluoride precipitates. After the reaction is completed, the solidified tailings can be obtained by pressure filtration. The filtrate produced by pressure filtration is adjusted to neutral before being discharged.

10. The method for low-temperature evaporation, concentration, harmless purification, and reuse of waste acid according to claim 9, characterized in that, In step S4, the modified zeolite powder is zeolite powder that has been activated by 3wt% hydrochloric acid. The pressure during filter press treatment is 1.0~1.2 MPa, which ensures that the moisture content of the solidified tailings is ≤20%.

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