Treatment method of extraction raffinate wastewater

Abstract

The invention discloses a treatment method of extraction raffinate wastewater, which comprises the following steps: (1) carrying out acid regulation on the extraction raffinate wastewater to obtain acid-regulated wastewater, carrying out freezing air floatation on the acid-regulated wastewater, collecting an oil-containing layer on the upper layer in the freezing air floatation process through slag scraping to obtain grease, and carrying out solid-liquid separation on the wastewater on the lower layer and cooled and crystallized mirabilite to obtain an oil-containing layer; freezing mother liquor and mirabilite are obtained; (2) sequentially carrying out oxidation, alkali adjustment, precipitation and impurity removal on the frozen mother liquor, and filtering to obtain secondary clear liquid and filter residues; (3) re-dissolving the mirabilite obtained in the step (1) by adopting the secondary clear liquid obtained in the step (2) to obtain a sodium sulfate saturated solution, adding the sodium sulfate saturated solution into an evaporation crystallizer for evaporation crystallization, and generating distilled water, concentrated mother liquor and sodium sulfate crystals in the evaporation crystallization process; distilled water and sodium sulfate crystals are recycled, and concentrated mother liquor is discharged for drying. The method provided by the invention can be used for efficiently recovering oil, a neutral extracting agent, industrial salt and pure water resources.

Description

Technical Field

[0001] The present invention belongs to the technical field of industrial wastewater treatment, and particularly relates to a method for treating raffinate wastewater. Background Art

[0002] In the hydrometallurgy process or the wet recovery process of waste lithium battery black powder, there is generally a process of metal separation and purification, which generally involves extraction or stripping steps; a large amount of raffinate wastewater will be generated during extraction or stripping. The raffinate wastewater contains oil (mainly including residual organic extractants and hydrolysis products, diluents and metal complexes, etc.), fluorine, and high salt content, and the pollutant components are relatively complex, making it difficult to meet the discharge standards of the "Pollutant Discharge Standards for the Rare Earth Industry" (GB26451-2011). Therefore, it is generally necessary to further treat the raffinate wastewater.

[0003] Currently, the conventional treatment methods for raffinate wastewater include combined processes such as oil separation, acidification, flocculation, filtration, activated carbon or resin adsorption, advanced oxidation, alkali adjustment, and evaporation crystallization. For example, in the patent with the publication number CN116750924A, a combined process of oil separation, acid treatment, resin adsorption for oil removal, and evaporation crystallization is adopted. However, in this process, the resin needs to be further eluted and regenerated, and a large amount of alkali and pure water are required during the resin regeneration process, and additional eluted oily wastewater is generated. In addition, in some treatment methods including the flocculation process, impurities are easily introduced after the addition of the flocculant, affecting the quality of the evaporation crystallization product; moreover, the consumption of the flocculation agent is large, and sludge is generated, further increasing the treatment burden and treatment cost. Summary of the Invention Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a method for treating raffinate wastewater.

[0004] The present invention provides a method for treating raffinate wastewater, including the following steps: (1) Acidify the raffinate wastewater to obtain the acidified wastewater, subject the acidified wastewater to freeze flotation, and during the freeze flotation process, the upper oil-containing layer is collected by scraping the scum to obtain grease, and the lower wastewater is subjected to solid-liquid separation from the mirabilite obtained by cooling crystallization to obtain the freeze mother liquor and mirabilite; (2) Subject the freeze mother liquor to oxidation, alkali adjustment and precipitation for impurity removal in sequence, and then filter to obtain the secondary clarified liquid and filter residue; (3) Redissolve the mirabilite obtained in step (1) with the secondary clarified liquid in step (2) to obtain a sodium sulfate saturated solution, subject the sodium sulfate saturated solution to evaporation crystallization, and during the evaporation crystallization process, distilled water, concentrated mother liquor and sodium sulfate crystals will be generated; the distilled water and sodium sulfate crystals are recovered, and the concentrated mother liquor is dried.

[0005] Preferably, in the step (1), the salt content of the raffinate wastewater is 8-12 wt%, and the COD value is 2000-5000 mg / L.

[0006] More preferably, in the step (1), the extractant contained in the raffinate wastewater is one or more of P507, P204 or Cyanex272.

[0007] Preferably, in the step (1), acid adjustment is carried out by using dilute sulfuric acid to adjust the pH of the raffinate wastewater to 2-4.

[0008] Preferably, in the step (1), the acid-adjusted wastewater is precooled before freeze flotation.

[0009] More preferably, the precooling is carried out by heat exchange between the freezing mother liquor or freezing water and the acid-adjusted wastewater, so that the acid-adjusted wastewater is precooled to a temperature of 8-10 °C.

[0010] Preferably, in the step (1), the freeze flotation is carried out in a freeze flotation device, and the freeze flotation device combines a freezer and a flotation device.

[0011] Preferably, in the step (1), the temperature of the freeze flotation is -2-2 °C, and the gas used for freeze flotation aeration is nitrogen; the time of the freeze flotation is 30-60 min.

[0012] Preferably, in the step (1), the COD value of the freezing mother liquor is less than 500 mg / L.

[0013] Preferably, in the step (2), the oxidant used for oxidation is one or two of hydrogen peroxide and ozone.

[0014] Preferably, in the step (2), the oxidation time is 30-60 min; the oxidation is carried out until the COD value of the freezing mother liquor is less than 50 mg / L and the oil content is less than 1 mg / L.

[0015] Preferably, in the step (2), calcium hydroxide is used to adjust the pH of the freezing mother liquor to 10-11 for alkali adjustment; after alkali adjustment, the precipitation reaction is carried out for 40-70 min.

[0016] Preferably, in the step (2), part of the secondary clarified liquid is used for redissolving sodium sulfate crystals in the step (3); the remaining secondary clarified liquid is directly discharged.

[0017] Preferably, in the step (3), all of the saturated sodium sulfate solution generated by the primary redissolution is added to the evaporation crystallizer for evaporation crystallization. After the system runs stably, the generated saturated sodium sulfate is continuously introduced into the evaporation crystallizer, and the concentrated mother liquor is continuously discharged, so that the evaporation crystallization process maintains a dynamic balance to obtain high-quality sodium sulfate crystals.

[0018] Further preferably, the discharge rate of the concentrated mother liquor is related to the purity of the sodium sulfate crystals. When the purity of the sodium sulfate crystals decreases, it is necessary to increase the discharge rate of the concentrated mother liquor.

[0019] Preferably, in step (3), before the concentrated mother liquor is dried, it is first heat-exchanged with the frozen mother liquor in step (2).

[0020] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: (1) Through the combined process of acid adjustment, freezing and flotation, oxidation, alkali adjustment and precipitation, and evaporation crystallization, the present invention can efficiently recover oil, neutral extractant, industrial salt and water resources. Moreover, the process is simple and easy to operate, with less dosage of chemicals, low wastewater volume after treatment, and can be directly discharged, which is safe, green and environmentally friendly.

[0021] (2) In the treatment method of the present invention, the resin adsorption process or the flocculation process is cancelled, which can reduce the dosage of chemicals and / or reduce the discharge of elution adsorption regeneration wastewater.

[0022] (3) In the treatment method of the present invention, the frozen mother liquor is heat-exchanged with the wastewater after acid adjustment and the concentrated mother liquor, which can improve the utilization rate of heat in the system and reduce the treatment cost.

[0023] (4) In the method of the present invention, the secondary clear liquid is used to redissolve mirabilite, which can reduce the use of pure water. The prepared saturated mirabilite solution can be directly evaporated and crystallized, reducing the evaporation and concentration process section, and obtaining sodium sulfate of Class II and Grade I at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is the process flow chart of the present invention.

[0025] Figure 2 is the structural schematic diagram of the freezing and flotation equipment in the embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0026] For the convenience of understanding the present invention, the following will describe the present invention more comprehensively and meticulously with reference to the accompanying drawings of the specification and preferred embodiments, but the protection scope of the present invention is not limited to the following specific embodiments. Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention. Unless otherwise specifically stated, various raw materials, reagents, instruments and equipment used in the present invention can be obtained through market purchase or can be prepared by existing methods.

[0027] As described above, the present invention provides a method for treating raffinate wastewater, comprising the following steps: (1) Acidify the raffinate wastewater to obtain acidified wastewater. Subject the acidified wastewater to freeze flotation. During the freeze flotation process, the upper oil-containing layer is collected by skimming to obtain grease, and the lower wastewater is subjected to solid-liquid separation from the cooled and crystallized mirabilite to obtain a freeze mother liquor and mirabilite; (2) Subject the freeze mother liquor to oxidation, alkali adjustment and precipitation for impurity removal in sequence, and then filter to obtain a secondary clear liquid and filter residue; (3) Redissolve the mirabilite obtained in step (1) with the secondary clear liquid in step (2) to obtain a saturated sodium sulfate solution. Subject the saturated sodium sulfate solution to evaporation crystallization. During the evaporation crystallization process, distilled water, concentrated mother liquor and sodium sulfate crystals will be generated. The distilled water and sodium sulfate crystals are recovered, and the concentrated mother liquor is dried.

[0028] In the present invention, the raffinate wastewater is first acidified. The main reason for acidification is that the oil-based extractant has lower solubility under acidic conditions, which is conducive to the precipitation of grease. The acidified wastewater is subjected to freeze flotation. During the freeze flotation process, with continuous aeration, the oil and oil-based extractant continuously float to form an oil layer, and the skimmer collects and recovers the grease. The upper clear liquid is continuously cooled by a freezer, and mirabilite crystals continuously form in the clear liquid layer and settle to the bottom layer. Mirabilite and freeze mother liquor can be obtained through solid-liquid separation. The freeze mother liquor enters the oxidation process. The oxidation process can remove the dissolved oil contained in the wastewater. At the same time, with the removal of organic matter, the complexation between organic matter and heavy metals is destroyed, which is conducive to alkali adjustment and precipitation for impurity removal in the next section. During the alkali adjustment and precipitation for impurity removal process, fluoride ions and heavy metal ions will form precipitates, which can be removed by filtration. The secondary clear liquid after removing fluorine and heavy metal ions meets the discharge standard and can be directly discharged. However, in the present invention, part of the secondary clear liquid is further used to redissolve mirabilite, which can purify industrial salt and recover the residual salt in the secondary clear liquid at the same time. During the evaporation crystallization process, as evaporation proceeds, water will be evaporated to form distilled water. As the water evaporates, the saturated sodium sulfate solution will become supersaturated and thus sodium sulfate crystals will precipitate and deposit at the bottom layer. The impurities in the remaining solution will be concentrated to form a concentrated mother liquor.

[0029] The method in the present invention can efficiently recover oil, neutral extractant, industrial salt and pure water resources through a combined process of acidification, freeze flotation, oxidation, alkali adjustment and precipitation, and evaporation crystallization.

[0030] In the method of the present invention, air flotation is carried out under freezing conditions, which can increase the adsorption capacity of bubbles and oil, improve the separation effect of oil, and thus achieve efficient separation of oil without the processes of flocculation or resin adsorption. In addition, during the freezing process, mirabilite in the wastewater will freeze and crystallize, which can reduce the salt content in the freezing mother liquor and help improve the subsequent oxidation and alkali adjustment precipitation impurity removal effects; further, freezing air flotation can avoid the crystallization and caking of the precipitated mirabilite, improve the dispersion of mirabilite and reduce impurity inclusion; freezing air flotation can also promote the rapid precipitation of mirabilite crystals.

[0031] The sodium sulfate solution in step (3) of the present invention is a saturated sodium sulfate solution, which can remove the concentration step and improve the treatment efficiency.

[0032] Preferably, in step (1), the salt content of the raffinate wastewater is 8-12 wt%, and the COD value is 2000-5000 mg / L.

[0033] More preferably, in step (1), the extractant contained in the raffinate wastewater is one or more of P507, P204 or Cyanex272.

[0034] Preferably, in step (1), the acid adjustment is to adjust the pH of the raffinate wastewater to 2-4 with dilute sulfuric acid.

[0035] Preferably, in step (1), the acid-adjusted wastewater is precooled before freezing air flotation.

[0036] More preferably, the precooling is to exchange heat between the freezing mother liquor or freezing water and the acid-adjusted wastewater to precool the acid-adjusted wastewater to a temperature of 8-10 °C.

[0037] In the present invention, the acid-adjusted wastewater is precooled by exchanging heat with the freezing mother liquor, which can improve the heat utilization of the entire process system and reduce energy consumption.

[0038] Preferably, in step (1), the freezing air flotation is carried out in a freezing air flotation device, and the freezing air flotation device combines a freezer and a flotation device.

[0039] Preferably, in step (1), the temperature of the freezing air flotation is -2-2 °C, and the gas used for air flotation aeration is nitrogen; the time of the freezing air flotation is 30-60 min, including but not limited to 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, etc.

[0040] In the present invention, better separation of oil, freezing mother liquor and mirabilite can be achieved by controlling the freezing air flotation temperature and the freezing air flotation time; in addition, nitrogen is used for air flotation aeration, which will not destroy the freezing temperature in the flotation device and improve the effect of freezing air flotation.

[0041] Preferably, in the step (1), the COD of the frozen mother liquor is less than 500 mg / L.

[0042] Preferably, in the step (2), the oxidant used for oxidation is one or more of hydrogen peroxide and ozone.

[0043] In the present invention, hydrogen peroxide and ozone are used as the oxidants, and no new pollutants will be added.

[0044] More preferably, hydrogen peroxide is used as the oxidant.

[0045] In the present invention, hydrogen peroxide is preferably used as the oxidant. The heavy metal ions contained in the frozen mother liquor can play a catalytic role. Therefore, without adding a catalyst, the catalytic oxidation effect can be well achieved, and the removal of dissolved oil can be realized.

[0046] Preferably, in the step (2), the oxidation time is 30 - 60 min, including but not limited to 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, etc.; the oxidation is carried out until the COD of the frozen mother liquor is less than 50 mg / L and the oil content is less than 1 mg / L.

[0047] Preferably, in the step (2), calcium hydroxide is used to adjust the pH of the frozen mother liquor to 10 - 11; after adjusting the alkali, the precipitation reaction time is 40 - 70 min, including but not limited to 40 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, etc.

[0048] In the present invention, calcium hydroxide is used to adjust the alkali. Calcium ions can form calcium fluoride precipitate with fluoride ions; hydroxide ions can form precipitates with heavy metal ions. Therefore, fluoride ions and heavy metal ions in the frozen mother liquor can be removed.

[0049] In the present invention, by controlling the pH of the supernatant and the precipitation reaction time after adjusting the alkali, sufficient reaction can be ensured, and impurity ions can be efficiently removed.

[0050] Preferably, in the step (3), all the saturated sodium sulfate solution generated by the primary re - dissolution is added to the evaporation crystallizer for evaporation crystallization. After the system runs stably, the generated saturated sodium sulfate is continuously introduced into the evaporation crystallizer, and the concentrated mother liquor is continuously discharged, so that the evaporation crystallization process maintains a dynamic balance, and high - quality sodium sulfate crystals are obtained.

[0051] During the evaporation crystallization process of the present invention, after the system is stabilized, a saturated solution of redissolved sodium sulfate is continuously introduced, and the concentrated mother liquor is continuously discharged to maintain a dynamic balance in the evaporation crystallization system. Through such a treatment method, sodium sulfate crystals and distilled water can be efficiently recovered, while other impurities in the solution are removed to ensure the purity of the sodium sulfate crystals.

[0052] Preferably, the discharge rate of the concentrated mother liquor is related to the purity of the sodium sulfate crystals. When the purity of the sodium sulfate crystals decreases, it is necessary to increase the discharge rate of the concentrated mother liquor.

[0053] Preferably, in step (3), before the concentrated mother liquor is dried, it is first heat-exchanged with the frozen mother liquor in step (2).

[0054] In the present invention, heat exchange between the concentrated mother liquor and the frozen mother liquor is adopted, which can improve the utilization rate of the heat in the system.

[0055] The process flow chart of the embodiment of the present invention is as Figure 1 shown, and the structural schematic diagram of the freeze-flotation equipment can be seen Figure 2 ; the specific steps can be seen in the embodiment.

[0056] Example 1 In this embodiment, the COD value of the raffinate wastewater is about 3210 mg / L, the salt content is 10.6 wt%, and the main extractants contained are Cyanex272 and P507.

[0057] (1) The pH of the raffinate wastewater is adjusted to about 3 with 30 wt% dilute sulfuric acid to obtain the acid-adjusted wastewater; the acid-adjusted wastewater is heat-exchanged with the frozen mother liquor and precooled to 8-10 °C (the initial precooling uses chilled water, and after the system operates normally, the frozen mother liquor separated after freeze-flotation is used), obtaining the precooled wastewater.

[0058] (2) The precooled wastewater enters the freeze-flotation unit (wherein, the freeze-flotation unit is as Figure 2 shown, which combines a refrigerator, a flotation device, and a centrifuge); the temperature in the flotation device is controlled at -2-2 °C by the refrigerator, and nitrogen is introduced into the flotation device for aeration; during the freeze-flotation process, the solution in the flotation device will be stratified (the upper oil-containing layer, the middle frozen mother liquor layer, and the bottom mirabilite crystal layer, and the schematic diagram can be seen Figure 2 ); the upper oil layer is continuously scraped and separated by a slag scraper to obtain grease; after 40 minutes of freeze-flotation, the COD value of the frozen mother liquor drops to 487 mg / L, and the middle frozen mother liquor layer and the bottom mirabilite crystal layer are centrifugally separated to obtain the frozen mother liquor and mirabilite.

[0059] (3) After the system operates normally, the chilled mother liquor first exchanges heat with the acid-adjusted wastewater, and then exchanges heat with the concentrated mother liquor in step (5) (when operating for the first time, the chilled mother liquor only exchanges heat with the acid-adjusted wastewater) to obtain a primary clear liquid; hydrogen peroxide is added to the primary clear liquid for oxidation treatment for 30 minutes until the COD value in the primary clear liquid drops to 45 mg / L and the oil content is less than 1 mg / L, obtaining the oxidized wastewater.

[0060] (4) Calcium hydroxide is added to the oxidized wastewater to adjust the pH to 10 - 11. After a precipitation reaction for 40 minutes, filtration is carried out to obtain a secondary clear liquid and filter residue; the concentration of heavy metal ions in the secondary clear liquid is reduced to about 0.5 mg / L, and the concentration of fluoride ions is reduced to about 9 mg / L; it already meets the discharge standards of the "Pollutant Discharge Standards for the Rare Earth Industry", and part of it is used for the subsequent preparation of saturated sodium sulfate solution, and part is directly discharged.

[0061] (5) The secondary clear liquid in step (4) is used to prepare a saturated sodium sulfate solution from mirabilite in step (1); then the saturated sodium sulfate solution is subjected to evaporation crystallization. During the evaporation crystallization process, as evaporation proceeds, the sodium sulfate solution will become supersaturated and sodium sulfate crystals will precipitate at the bottom. After the evaporated water vapor cools, distilled water is obtained, and the remaining part of the solution is the concentrated mother liquor; at the initial stage of system operation, all the prepared saturated sodium sulfate solution is heated in the evaporation crystallizer, and the subsequently generated saturated sodium sulfate solution is continuously fed into the evaporation crystallizer, and the concentrated mother liquor is continuously discharged (the discharge rate of the concentrated mother liquor is mainly adjusted according to the purity of the generated sodium sulfate crystals. When the quality of the sodium sulfate crystals deteriorates, the discharge rate of the concentrated mother liquor is increased), so that the solution in the evaporation crystallizer maintains a dynamic balance; the distilled water and sodium sulfate crystals generated during the evaporation crystallization process are recovered. After the sodium sulfate crystals are dried, sodium sulfate is obtained; the discharged concentrated mother liquor exchanges heat with the chilled mother liquor and then is discharged from the system for drying. In this example, the purity of the prepared sodium sulfate is 98.3%.

[0062] Example 2 In this example, the COD value of the raffinate wastewater is 2210 mg / L, the salt content is 8.2 wt%, and the extractants contained are P507 and P204.

[0063] (1) The pH of the raffinate wastewater is adjusted to about 3 with 30 wt% dilute sulfuric acid to obtain the acid-adjusted wastewater; the acid-adjusted wastewater exchanges heat with the chilled mother liquor to precool the acid-adjusted wastewater to 8 - 10 °C (for the initial precooling, chilled water is used, and after the system operates normally, the chilled mother liquor separated by chilled air flotation is used), obtaining the precooled wastewater.

[0064] (2) The precooled wastewater enters the chilled air flotation unit (wherein, the chilled air flotation unit is as Figure 2As shown in the figure, a refrigerator, a flotation device and a centrifuge are used in combination; the temperature in the flotation device is controlled by the refrigerator to be -2 to 2 °C, and nitrogen is introduced into the flotation device for aeration; during the refrigerated flotation process, the solution in the flotation device will be stratified (the upper oil-containing layer, the middle refrigerated mother liquor layer, and the bottom mirabilite crystal layer, for the schematic diagram, see Figure 2 ); the upper oil layer is continuously scraped and separated by a slag scraper to obtain grease; after 35 minutes of refrigerated flotation, the COD value of the refrigerated mother liquor drops to 465 mg / L, and the middle refrigerated mother liquor layer and the bottom mirabilite crystal layer are separated by centrifugation to obtain refrigerated mother liquor and mirabilite.

[0065] (3) After the system operates normally, the refrigerated mother liquor first exchanges heat with the acid-adjusted wastewater, and then exchanges heat with the concentrated mother liquor in step (5) (when operating for the first time, the refrigerated mother liquor only exchanges heat with the acid-adjusted wastewater) to obtain a primary clarified liquid; ozone is added to the primary clarified liquid for oxidation treatment for 30 minutes until the COD value in the primary clarified liquid drops to 43 mg / L and the oil content is less than 1 mg / L to obtain the oxidized wastewater.

[0066] (4) Calcium hydroxide is added to the oxidized wastewater to adjust the pH to 10 - 11, and after a precipitation reaction for 40 minutes, filtration is carried out to obtain a secondary clarified liquid and filter residue; the concentration of heavy metal ions in the secondary clarified liquid is reduced to about 0.3 mg / L, and the concentration of fluoride ions is reduced to about 7.8 mg / L. The secondary clarified liquid already meets the discharge standards of the "Pollutant Discharge Standards for the Rare Earth Industry", and part of it is used for the subsequent preparation of saturated sodium sulfate solution, and part is directly discharged.

[0067] (5) The mirabilite in step (1) is used to prepare a saturated sodium sulfate solution with the secondary clarified liquid in step (4); then the saturated sodium sulfate solution is subjected to evaporation crystallization. During the evaporation crystallization process, as evaporation proceeds, the sodium sulfate solution will become supersaturated and sodium sulfate crystals will precipitate at the bottom. After the evaporated water vapor cools, distilled water is obtained, and the remaining part of the solution is the concentrated mother liquor; at the initial stage of system operation, all the prepared saturated sodium sulfate solution is heated in the evaporation crystallizer, and the subsequently generated saturated sodium sulfate solution is continuously fed into the evaporation crystallizer, and the concentrated mother liquor is continuously discharged (the discharge rate of the concentrated mother liquor is mainly adjusted according to the purity of the generated sodium sulfate crystals. When the quality of the sodium sulfate crystals decreases, the discharge rate of the concentrated mother liquor is increased) to keep the solution in the evaporation crystallizer in a dynamic balance; the distilled water and sodium sulfate crystals generated during the evaporation crystallization process are recovered. After the sodium sulfate crystals are further dried, sodium sulfate is obtained. The discharged concentrated mother liquor exchanges heat with the refrigerated mother liquor and then is discharged from the system for drying. The purity of the sodium sulfate obtained in this example is 98.5%.

[0068] Example 3 In this embodiment, the COD of the raffinate wastewater is 4980 mg / L, the salt content is 11.8 wt%, and the extractants contained are P507 and Cyanex272.

[0069] (1) The pH of the raffinate wastewater is adjusted to about 3 with 30 wt% dilute sulfuric acid to obtain the acid-adjusted wastewater; the acid-adjusted wastewater is heat-exchanged with the freezing mother liquor, and the acid-adjusted wastewater is precooled to 8 - 10 °C (the first precooling uses chilled water, and after the system operates normally, the freezing mother liquor separated by freezing air flotation is used), to obtain the precooled wastewater.

[0070] (2) The precooled wastewater enters the freezing air flotation unit (wherein, the freezing air flotation unit is as Figure 2 shown, which combines a freezer, a flotation device, and a centrifuge); the temperature in the flotation device is controlled at -2 - 2 °C by the freezer, and nitrogen is introduced into the flotation device for aeration; during the freezing air flotation process, the solution in the flotation device will have stratification (the upper layer is the oil-containing layer, the middle layer is the freezing mother liquor layer, and the bottom layer is the mirabilite crystal layer, and a schematic diagram can be seen in Figure 2 ); the oil layer on the upper layer is continuously scraped and separated by a slag scraper to obtain grease; after 60 minutes of freezing air flotation, the COD value of the freezing mother liquor drops to 493 mg / L, and the middle freezing mother liquor layer and the bottom mirabilite crystal layer are centrifuged for solid-liquid separation to obtain the freezing mother liquor and mirabilite.

[0071] (3) After the system operates normally, the freezing mother liquor is first heat-exchanged with the acid-adjusted wastewater, and then heat-exchanged with the concentrated mother liquor in step (5) (when operating for the first time, the freezing mother liquor is only heat-exchanged with the acid-adjusted wastewater), to obtain a primary clear liquid; ozone is added to the primary clear liquid for oxidation treatment for 60 minutes until the COD value in the primary clear liquid drops to 48 mg / L and the oil content is less than 1 mg / L, to obtain the oxidized wastewater.

[0072] (4) Calcium hydroxide is added to the oxidized wastewater to adjust the pH to 10 - 11, and after a precipitation reaction for 60 minutes, filtration is carried out to obtain a secondary clear liquid and filter residue; the concentration of heavy metal ions in the secondary clear liquid is reduced to about 0.8 mg / L, and the concentration of fluoride ions is reduced to about 9.6 mg / L. The secondary clear liquid already meets the discharge standards of the "Pollutant Discharge Standards for the Rare Earth Industry", and part of it is used for the subsequent preparation of saturated sodium sulfate solution, and part is directly discharged.

[0073] (5) Prepare a saturated sodium sulfate solution from mirabilite in step (4); then evaporate and crystallize the saturated sodium sulfate solution. During the evaporation and crystallization process, as evaporation proceeds, the sodium sulfate solution will become supersaturated and sodium sulfate crystals will precipitate at the bottom. After the evaporated water vapor cools, distilled water is obtained, and the remaining part of the solution is the concentrated mother liquor to be discharged. At the initial stage of system operation, all the prepared saturated sodium sulfate solution is heated in the evaporation crystallizer, and the subsequently generated saturated sodium sulfate solution is continuously fed into the evaporation crystallizer, and the concentrated mother liquor to be discharged is continuously discharged (the discharge rate of the concentrated mother liquor is mainly adjusted according to the purity of the generated sodium sulfate crystals. When the quality of the sodium sulfate crystals deteriorates, the discharge rate of the concentrated mother liquor is increased) to keep the solution in the evaporation crystallizer in dynamic balance; the distilled water and sodium sulfate crystals generated during the evaporation and crystallization process are recovered. After the sodium sulfate crystals are dried, sodium sulfate is obtained; the discharged concentrated mother liquor is heat-exchanged with the frozen mother liquor and then discharged from the system for drying. The purity of the sodium sulfate obtained in this step is 98.4%.

[0074] The above embodiments are only for illustrating the technical concept and features of the present invention, and the purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and it should not be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for treating raffinate wastewater, characterized in that, It includes the following steps: (1) Acidify the raffinate wastewater to obtain the acidified wastewater. Subject the acidified wastewater to freeze flotation. During the freeze flotation process, the upper oil-containing layer is collected by scraping the scum to obtain grease. The lower wastewater is subjected to solid-liquid separation from the cooled and crystallized mirabilite to obtain the frozen mother liquor and mirabilite; (2) Oxidize the frozen mother liquor, adjust the pH with alkali for impurity removal by precipitation, and then filter to obtain the secondary clarified liquid and filter residue; (3) Redissolve the mirabilite obtained in step (1) with the secondary clarified liquid in step (2) to obtain a saturated sodium sulfate solution. Add the saturated sodium sulfate solution to an evaporation crystallizer for evaporation crystallization. During the evaporation crystallization process, distilled water, concentrated mother liquor, and sodium sulfate crystals are generated. The distilled water and sodium sulfate crystals are recovered, and the concentrated mother liquor is dried; 2. The treatment method of the raffinate wastewater according to claim 1, wherein, In step (1), the salt content of the raffinate wastewater is 8 - 12 wt%, and the COD value is 2000 - 5000 mg / L.

3. The treatment method of the raffinate wastewater according to claim 1 or 2, characterized in that, In step (1), the extractant contained in the raffinate wastewater is one or more of P507, P204, or Cyanex272.

4. The treatment method of the raffinate wastewater according to claim 1, wherein In step (1), acidification is carried out by using dilute sulfuric acid to adjust the pH of the raffinate wastewater to 2 - 4. Before freeze flotation of the acidified wastewater, precooling is carried out. The precooling is carried out by using the frozen mother liquor or chilled water to exchange heat with the acidified wastewater to precool the acidified wastewater to a temperature of 8 - 10°C.

5. The treatment method of the raffinate wastewater according to claim 1, characterized in that, In step (1), the temperature of the freeze flotation is -2 - 2°C, and the gas used for flotation aeration is nitrogen. The time of the freeze flotation is 30 - 60 min.

6. The treatment method of the raffinate wastewater according to claim 1, characterized in that, In step (1), the COD value of the frozen mother liquor is less than 500 mg / L.

7. The treatment method of the raffinate wastewater according to claim 1, characterized in that In step (2), the oxidant used for oxidation is one or more of hydrogen peroxide and ozone.

8. The treatment method of the raffinate wastewater according to claim 1 or 8, characterized in that, In step (2), the oxidation time is 30 - 60 min; after oxidation, the COD value of the frozen mother liquor is less than 50 mg / L, and the oil content is less than 1 mg / L; the pH of the frozen mother liquor is adjusted to 10 - 11 with calcium hydroxide for alkali adjustment; after alkali adjustment, the precipitation reaction is carried out for 40 - 70 min.

9. The treatment method of the raffinate wastewater according to claim 1, characterized in that, In step (3), all of the saturated sodium sulfate solution generated by the initial redissolution is added to the evaporation crystallizer for evaporation crystallization. After the system operates stably, the generated saturated sodium sulfate solution is continuously introduced into the evaporation crystallizer, and the concentrated mother liquor is continuously discharged to keep the evaporation crystallization process in dynamic equilibrium.

10. The treatment method of the raffinate wastewater according to claim 1, wherein, In step (3), before the concentrated mother liquor is dried, it is first heat-exchanged with the frozen mother liquor in step (2).

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