Method for near-zero emission and resource recycling of rare earth extraction wastewater and precipitation mother liquor

Abstract

The invention provides a near-zero emission and resource recycling method for rare earth extraction wastewater and precipitation mother liquor. The method comprises the following steps: carrying out oil removal, iron-carbon micro-electrolysis, heavy removal and catalytic oxidation on the extraction wastewater, and then carrying out oxidation, evaporation and crystallization to obtain crude salt; respectively precipitating oxalic acid / carbonate washing wastewater and corresponding mother liquor to recover rare earth, recycling permeate liquid after membrane filtration, and merging concentrated liquid into evaporation inlet water; the crude salt is subjected to low-oxygen catalytic pyrolysis, salt dissolving, deep impurity removal and secondary oxidation, evaporation and crystallization to obtain regenerated industrial salt and condensed water, and the condensed water is continuously recycled; the pyrolysis flue gas reaches the standard after being subjected to quenching, dust removal and RTO-SCR. Water circulation, valuable component recovery and solid waste recycling are achieved in the whole process, and no mixed salt is discharged.

Description

Technical Field

[0001] This invention relates to a method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor. Background Technology

[0002] Rare earth hydrometallurgy generates large amounts of high-salt, high-COD, heavy-metal-containing extraction wastewater, oxalic acid mother liquor, and carbon precipitation mother liquor during the extraction, precipitation, and washing stages. Traditional "neutralization-precipitation-evaporation" processes only achieve pollutant transfer: valuable resources such as extractants, oxalic acid, and rare earth elements are locked in the neutralization residue, becoming secondary hazardous waste; the mixed salt obtained from evaporation contains organic matter and impurities, and is managed under HW18 category management, resulting in high disposal costs and significant environmental risks; the condensate water quality fluctuates, making stable reuse difficult. Current technologies have not yet formed a fully closed-loop system coupling wastewater treatment, resource recovery, and solid waste resource utilization. There is an urgent need for an integrated process with near-zero emissions that simultaneously recovers water, salt, and valuable components. Summary of the Invention

[0003] This invention provides a method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor in order to solve the problems existing in the prior art.

[0004] The technical solutions adopted in this invention are as follows:

[0005] A method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor includes the following steps:

[0006] a) After removing oil, performing iron-carbon micro-electrolysis, removing heavy metals, and catalytic oxidation on the extraction wastewater, oxidative evaporation and crystallization are carried out to obtain crude salt;

[0007] b) The oxalic acid washing wastewater is precipitated to recover rare earth oxalic acid, then filtered through a membrane. The membrane permeate is reused in production, and the membrane concentrate is combined with the oxalic acid mother liquor.

[0008] c) The carbonate washing wastewater is precipitated to recover carbon and rare earth, then filtered through a membrane. The membrane permeate is reused in production, and the membrane concentrate is combined with the carbonate precipitation mother liquor.

[0009] d) The mother liquor of oxalate is precipitated to recover rare earth oxalate, and then the pH is adjusted in stages to precipitate sodium oxalate, calcium oxalate and rare earth hydroxide in sequence. The resulting wastewater is combined with the influent from the oxidation evaporation crystallization in step a). Calcium oxalate is acidified to obtain reusable oxalic acid and gypsum.

[0010] e) The carbonate precipitate mother liquor is precipitated to recover carbon and rare earth, and then excess carbonate or ammonium salt is removed by chemical adjustment. The resulting wastewater is combined with the oxidative evaporation crystallization influent from step a).

[0011] f) The crude salt obtained in step a) is subjected to low-oxygen catalytic pyrolysis, deep salt removal, catalytic oxidation, and secondary oxidation evaporation crystallization in sequence to obtain recycled industrial salt and condensate;

[0012] g) Perform membrane filtration on the condensate obtained in step f), reuse the membrane permeate for production, and combine the membrane concentrate with the salt solution for further treatment;

[0013] h) The catalytic pyrolysis waste gas generated in step f) is subjected to rapid cooling, dust removal, alkali absorption, RTO incineration, electrostatic dust removal, and SCR denitrification in sequence before being discharged in compliance with standards.

[0014] Further, in step a), the oil removal process sequentially includes primary ultrasonic-air flotation oil separation and recovery of the organic phase, and secondary anionic surfactant demulsification-activated carbon adsorption oil removal.

[0015] Furthermore, in step a), the catalytic oxidation employs a two-stage oxidation process using hydrogen peroxide and ozone.

[0016] Further, in step d), the pH is adjusted in stages as follows: first stage pH 1–3 precipitates sodium oxalate, second stage pH 3–6 precipitates calcium oxalate, and third stage pH 6–8 precipitates rare earth hydroxides.

[0017] Furthermore, in step f), the temperature of the low-oxygen catalytic pyrolysis is 300–550 °C, and the oxygen volume fraction is <5%.

[0018] Furthermore, in step f), the deep purification of salt includes denitrification, defluorination, dephosphorization, debromination, and heavy removal.

[0019] Further, in step g), the membrane filtration is reverse osmosis.

[0020] Furthermore, in step h), the dust removal is performed using a bag filter, and the alkali absorption uses a 10 wt% sodium hydroxide solution.

[0021] The present invention has the following beneficial effects:

[0022] (1) Through the process design of “differentiated treatment + targeted recycling”, the step-by-step recycling of multiple valuable resources is realized. Extraction wastewater is de-oiled in two stages to recover organic phase extractant. Oxalic acid washing wastewater and oxalic acid precipitation mother liquor are used to recover oxalic acid, rare earth and oxalic acid (oxalic acid is acidified and regenerated for reuse). Carbonate washing wastewater and carbonate precipitation mother liquor are used to recover carbon precipitation rare earth. The total recovery rate of rare earth and the recovery efficiency of organic phase and oxalic acid are significantly improved, greatly reducing the replenishment consumption of fresh extractant, oxalic acid and rare earth raw materials, and improving the utilization value of resources.

[0023] (2) Breaking through the dilemma of treating mixed salt obtained from traditional evaporation as HW18 hazardous waste, the crude salt is refined into recycled industrial salt that meets the standards of "Salt for Caustic Soda with Ion-Exchange Membrane" (QB / T5270) through a combination process of "low-oxygen catalytic pyrolysis + deep salt removal + catalytic oxidation + secondary evaporation and crystallization". The waste salt is no longer treated as hazardous waste, which not only avoids the environmental risks of hazardous waste storage, transportation and disposal, but also significantly reduces the end-of-life disposal costs and realizes the resource recycling of waste salt.

[0024] (3) Constructing a closed-loop water resource system throughout the entire process—the permeate from oxalic acid washing wastewater and carbonate washing wastewater after membrane filtration is directly reused in production, the condensate from secondary evaporation and crystallization is reused after reverse osmosis membrane filtration, and the membrane concentrate is combined with the corresponding mother liquor or the desalination liquid for treatment, thus avoiding water waste. The water resource reuse rate throughout the entire process is significantly improved, effectively reducing the amount of fresh water replenishment, which meets the environmental protection requirements of water conservation and emission reduction.

[0025] (4) For the waste gas generated by catalytic pyrolysis, a multi-stage synergistic treatment process of "rapid cooling + bag filter + alkaline absorption + RTO incineration + electrostatic dust removal + SCR denitrification" is adopted: rapid cooling avoids dioxin synthesis, alkaline absorption removes acidic gases, RTO incineration thoroughly degrades organic matter, and SCR denitrification removes nitrogen oxides. Finally, all indicators of the waste gas meet the emission standards and there is no secondary white smoke phenomenon.

[0026] (5) Achieve full-process control of wastewater, waste salt and waste gas - no mixed salt discharge, wastewater is either reused or enters the evaporation crystallization system after treatment, and there are no new hazardous waste items; the process operation conditions are similar to conventional evaporation processes, and all equipment used are existing conventional devices, without the need for complex special equipment investment, which is easy to transform and upgrade existing rare earth smelting devices and lower the threshold for industrial application.

[0027] (6) Through processes such as iron-carbon micro-electrolysis, two-stage catalytic oxidation, and sodium sulfide degrafting, the recalcitrant organic matter and heavy metal ions in wastewater are deeply removed, and the removal rates of pollutants such as COD and heavy metals are high. The control indicators of toxic and harmful substances in the recycled industrial salt meet the relevant limits of the "Surface Water Environmental Quality Standard" (GB3838), solving the secondary pollution problem caused by the "pollutant transfer" of traditional processes, and taking into account both economic and environmental benefits. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0029] The invention will now be further described with reference to the accompanying drawings.

[0030] Traditional, single-stage acid-base neutralization processes are inefficient, consuming large amounts of reagents, leaving valuable resources solidified in the neutralization residue, resulting in significant resource waste and secondary pollution. This invention provides a comprehensive and optimized plant-wide water usage approach that deeply couples wastewater treatment with resource recovery, and performs deep purification of the final concentrated salt. This overcomes these challenges. Figure 1 This invention provides a detailed explanation of the near-zero discharge and resource recovery method for rare earth extraction wastewater and precipitate mother liquor, including operating parameters and synergistic principles. This embodiment is for illustrative purposes only and does not constitute a limitation on its scope of protection. It should be noted that the ultrasonic-air flotation integrated equipment, reaction vessel, iron-carbon micro-electrolysis reactor, catalytic oxidation tower, evaporation crystallizer, membrane filtration system, pyrolysis furnace, waste gas treatment tower (quench tower, alkali absorption tower), RTO regenerative thermal oxidizer, SCR denitrification device, etc., used in this application are all existing conventional devices, and their structures and working principles are common knowledge in the field; therefore, the structure and principle of the devices will not be described in detail.

[0031] I. Pretreatment stage: Resource recovery and preliminary purification of classified wastewater.

[0032] (a) Treatment of extraction wastewater;

[0033] Extraction wastewater generated during rare earth hydrometallurgical processes is characterized by high oil content, recalcitrant organic matter, heavy metal ions, and high salt concentrations. The treatment process is as follows:

[0034] Primary oil removal: The extraction wastewater is pumped into an integrated ultrasonic-air flotation device. The ultrasonic device is turned on and compressed air is introduced to generate microbubbles. The air flotation effect causes the organic phase extractant in the wastewater to adhere to the surface of the bubbles and float to the surface. The organic phase is separated and recovered by the top oil separator. The recovered organic phase can be reused in the rare earth extraction process after simple purification.

[0035] Secondary oil removal: The wastewater after primary oil removal is introduced into a reactor, and an anionic surfactant is added at 5‰~1% of the wastewater mass as a demulsifier. The mixture is stirred thoroughly for 30~60 minutes to achieve chemical demulsification of residual oil. Subsequently, activated carbon or carbon slag (which can be used alone or in combination) is added at 5%~10% of the wastewater mass, and the mixture is stirred and adsorbed for 60~90 minutes. The adsorbent that has adsorbed oil and some organic matter is then removed by filtration to obtain the oil-removed wastewater.

[0036] Iron-carbon micro-electrolysis (iron-carbon reduction): The de-oiled wastewater is introduced into the iron-carbon micro-electrolysis reactor. The reactor is filled with a packing material composed of iron-carbon / iron scrap / iron powder and granular carbon. The mass ratio of iron source to carbon source is controlled at 4~6:1 to ensure that the wastewater completely covers the packing material. The reaction is carried out for 24 hours under gas-liquid two-phase flow conditions. The micro-electrolysis degrades recalcitrant organic matter and removes the color of the wastewater.

[0037] Heavy metal removal reaction: Adjust the pH of the wastewater after micro-electrolysis to 4-8, add sodium sulfide as a heavy metal removal agent at 5‰-5% of the wastewater mass, stir thoroughly for 30-45 minutes to form sulfide precipitates with the heavy metal ions in the wastewater, filter to remove the precipitates and obtain the filtrate after heavy metal removal.

[0038] Catalytic oxidation: The filtrate is pumped into a two-stage catalytic oxidation tower. Ozone is introduced into the first-stage tower and hydrogen peroxide is added at 1% to 5% of the wastewater mass. Ozone is added to the second-stage tower to enhance oxidation. The total reaction time is controlled at 60 to 90 minutes. Through the synergistic effect of Fenton reaction and ozone oxidation, residual organic matter in the wastewater is deeply removed.

[0039] Oxidation evaporation crystallization: The catalytically oxidized wastewater is sent to an evaporator crystallizer, and hydrogen peroxide is added as an oxidant at a mass of 1% to 5% of the wastewater. The evaporation temperature is controlled at 80 to 100℃ and the vacuum degree is -0.06 to -0.08 MPa. After evaporation and crystallization, crude salt is obtained, and the crystallization mother liquor is returned to the evaporation process for recycling.

[0040] (ii) Oxalic acid washing wastewater treatment.

[0041] The oxalic acid washing wastewater generated during the oxalic acid washing process contains rare earth oxalate, unreacted oxalic acid, and salts. The treatment is as follows:

[0042] Wastewater is introduced into a sedimentation tank, where natural sedimentation or centrifugal separation is used to precipitate and separate rare earth oxalate from the wastewater. The precipitate is collected to obtain crude rare earth oxalate, which can be used for rare earth refining after washing and drying. The precipitated wastewater is then passed through a membrane module for concentration and filtration. The membrane module uses a combination of two or three types of membranes: ultrafiltration, nanofiltration, and reverse osmosis. The permeate water meets the requirements for production water and is directly reused for oxalate washing or other production processes. The membrane concentrate is then sent to the oxalate sedimentation mother liquor treatment system for combined treatment.

[0043] (iii) Treatment of carbonate washing wastewater.

[0044] The carbonate washing wastewater generated during the carbonate precipitation process contains trace amounts of carbon, rare earth elements, carbonates, and salts. The treatment process is as follows:

[0045] Wastewater is sent to a sedimentation tank, where carbon and rare earth precipitates are recovered through natural sedimentation or centrifugal separation. The recovered carbon and rare earth precipitates can be returned to the sedimentation process for further treatment. The supernatant enters a membrane module for concentration and filtration. The membrane module uses a combination of ultrafiltration, nanofiltration and reverse osmosis membranes. The permeate is reused in production, and the concentrate is sent to the carbonate precipitation mother liquor treatment system for combined treatment.

[0046] II. Mother liquor treatment stage: in-depth resource recovery and wastewater homogenization.

[0047] (a) Treatment of mother liquor from grass sedimentation;

[0048] The mother liquor from grass sedimentation contains oxalate, rare earth ions, sodium ions, etc., and the treatment process is as follows:

[0049] Rare earth recovery: The mother liquor of oxalic acid is introduced into the sedimentation tank, and the residual rare earth oxalate precipitate is recovered by natural sedimentation or centrifugal separation. The precipitate is collected and incorporated into the rare earth oxalate refining process.

[0050] Primary pH adjustment: Add a 10%~30% sodium hydroxide solution to the mother liquor after precipitation to adjust the pH to 1~3. Stir and react for 30 minutes to allow the oxalate ions in the mother liquor to combine with sodium ions to form sodium oxalate precipitate. Filter and separate to obtain the sodium oxalate product.

[0051] Secondary pH adjustment: Add calcium chloride solution with a concentration of 100 g / L to 200 g / L to the filtrate after primary adjustment, while continuing to adjust the pH to 3 to 6 with sodium hydroxide solution. Add slowly and stir, repeat the dropwise addition and standing operation until no new precipitate is formed, and filter to obtain calcium oxalate precipitate.

[0052] Third-stage pH adjustment: Add a 10%~30% sodium hydroxide solution to the filtrate after the second-stage adjustment to adjust the pH to 6~8, so that the residual rare earth ions form rare earth hydroxide precipitate, and the rare earth hydroxide is separated and recovered by filtration. The filtered wastewater is sent to the inlet of the extraction wastewater oxidation evaporation crystallization process for combined treatment.

[0053] Oxalic acid recovery: The obtained calcium oxalate precipitate is introduced into an acidification reactor, sulfuric acid is added for acidification treatment, and after full reaction, it is filtered. The filter residue is gypsum (which can be used as a raw material for building materials), and the filtrate is an oxalic acid solution. After concentration and purification, it is reused in the rare earth precipitation process.

[0054] (ii) Treatment of mother liquor from carbonate precipitation.

[0055] The mother liquor from carbonate precipitation contains excess carbonate or ammonium salts, trace amounts of rare earth ions, and salts. The treatment is as follows:

[0056] Rare earth recovery: The mother liquor of carbonate precipitation is sent to a sedimentation tank, and the carbon and rare earth precipitates are recovered by natural sedimentation or centrifugal separation. The recovered rare earth precipitates are then used for rare earth refining.

[0057] Chemical conditioning: Add conditioning agents to the mother liquor after precipitation. The conditioning agents are selected from calcium chloride (concentration 100g / L~200g / L) and sodium sulfate (concentration 100g / L~200g / L) or a combination of both. Use chemical precipitation, chemical precipitation + air flotation or chemical precipitation + ultrasound + air flotation to remove excess carbonate or ammonium salts from the mother liquor. After the reaction is completed, filter and send the resulting wastewater to the inlet of the extraction wastewater oxidation evaporation crystallization process for combined treatment.

[0058] III. Crude Salt Refining Stage: Waste Salt Resource Utilization and Condensate Reuse.

[0059] (a) Low-oxygen catalytic pyrolysis;

[0060] The crude salt obtained by oxidation evaporation crystallization is fed into a pyrolysis furnace in batches. Nitrogen gas is introduced into the furnace to replace the air, so that the oxygen volume fraction in the furnace is less than 5%. The pyrolysis temperature is controlled at 300℃~550℃ and the pyrolysis time is 30~60 minutes. The organic matter adsorbed in the crude salt is removed by low-oxygen or anaerobic catalytic pyrolysis to obtain pyrolyzed salt.

[0061] (ii) Deep salt removal;

[0062] The pyrolytic salt was dissolved in deionized water at a solid-liquid ratio of 1:4. After thorough stirring until completely dissolved, the solution was filtered to remove insoluble impurities. Subsequently, the salt solution underwent further purification.

[0063] 1. Denitrification treatment: Removing nitrogen-containing impurities such as ammonia nitrogen from salt solutions through stripping or chemical reduction;

[0064] 2. Removal of fluoride, phosphorus, and bromine: Targeted impurity removal agents are added sequentially to cause fluoride ions, phosphate ions, and bromide ions to form insoluble compounds that precipitate and are then separated by filtration.

[0065] 3. Heavy metal removal treatment: Adjust the pH of the salt solution to a suitable range, add sodium sulfide to remove residual heavy metal ions, and filter to obtain a subsaturated salt solution after deep impurity removal.

[0066] (III) Catalytic oxidation and secondary evaporation crystallization;

[0067] The subsaturated salt solution is fed into a catalytic oxidation reactor, ozone is introduced, and hydrogen peroxide is added at 1% to 5% of the salt solution mass. The reaction is carried out for 60 minutes to remove residual organic matter. The solution is then introduced into a secondary oxidation evaporation crystallizer, where hydrogen peroxide is added as an oxidant at 1‰ to 1% of the salt solution mass. The evaporation temperature is controlled at 90 to 110°C and the vacuum degree is -0.07 to -0.09 MPa. After evaporation and crystallization, the recycled industrial salt product is obtained. The condensate generated during the crystallization process is collected for later use.

[0068] (iv) Condensate reuse treatment;

[0069] The condensate produced by secondary evaporation and crystallization is fed into a reverse osmosis (RO) membrane filtration system. The permeate is directly reused in the production process. The membrane concentrate is combined with the subsaturated salt solution after salting and returned to the deep purification process for reprocessing.

[0070] IV. Waste gas treatment stage: pyrolysis waste gas is discharged in compliance with standards.

[0071] The waste gas generated during catalytic pyrolysis contains organic matter, particulate matter, acidic gases, and nitrogen oxides. The treatment process is as follows:

[0072] Rapid cooling treatment: The pyrolysis waste gas is introduced into a rapid cooling tower and cooled quickly by water or air cooling to prevent the formation of dioxins during the cooling process. The temperature of the waste gas after cooling is controlled below 200℃.

[0073] Baghouse dust collector: After rapid cooling, the exhaust gas enters the baghouse dust collector to capture particulate matter in the exhaust gas, with a dust removal efficiency of ≥99%.

[0074] Alkali absorption: The dust-removed waste gas is passed into an alkali absorption tower. A 10wt% sodium hydroxide aqueous solution is used as the absorbent in the tower. The acidic gases in the waste gas are absorbed in a countercurrent contact, and the acid content of the waste gas is significantly reduced after absorption.

[0075] RTO incineration: The waste gas after alkali absorption is sent to a regenerative thermal oxidizer (RTO) and incinerated at a high temperature of 800~900℃ to completely degrade the organic matter in the waste gas, with an organic matter removal rate of ≥99.5%.

[0076] Electrostatic precipitator: The exhaust gas after incineration enters the electrostatic precipitator to further capture trace amounts of particulate matter in the exhaust gas, ensuring that the particulate matter content in the exhaust gas meets the standards.

[0077] SCR denitrification: The waste gas after electrostatic dust removal is passed into a selective catalytic reduction (SCR) denitrification device. Under the action of the catalyst, nitrogen oxides are reduced to nitrogen and water by injecting ammonia. The treated waste gas meets the national emission standards and is discharged through the chimney in compliance with the standards.

[0078] Using this method to treat rare earth extraction wastewater and precipitate mother liquor, the sodium chloride content (on a wet basis) in the regenerated industrial salt product is ≥99.2%, and the content of impurities such as calcium and magnesium meets the requirements of the "Salt for Caustic Soda Using Ion-Exchange Membrane" (QB / T5270) standard. The control indicators of toxic and harmful substances meet the relevant limits of the "Surface Water Environmental Quality Standard" (GB3838), realizing the recycling of resources such as water, salt, rare earth, and organic phases. There is no discharge of mixed salts and no new hazardous waste throughout the entire process.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor, characterized in that: The steps are as follows: a) After removing oil, performing iron-carbon micro-electrolysis, removing heavy metals, and catalytic oxidation on the extraction wastewater, oxidative evaporation and crystallization are carried out to obtain crude salt; b) The oxalic acid washing wastewater is precipitated to recover rare earth oxalic acid, then filtered through a membrane. The membrane permeate is reused in production, and the membrane concentrate is combined with the oxalic acid mother liquor. c) The carbonate washing wastewater is precipitated to recover carbon and rare earth, then filtered through a membrane. The membrane permeate is reused in production, and the membrane concentrate is combined with the carbonate precipitation mother liquor. d) The mother liquor of oxalate is precipitated to recover rare earth oxalate, and then the pH is adjusted in stages to precipitate sodium oxalate, calcium oxalate and rare earth hydroxide in sequence. The resulting wastewater is combined with the influent from the oxidation evaporation crystallization in step a). Calcium oxalate is acidified to obtain reusable oxalic acid and gypsum. e) The carbonate precipitate mother liquor is precipitated to recover carbon and rare earth, and then excess carbonate or ammonium salt is removed by chemical adjustment. The resulting wastewater is combined with the oxidative evaporation crystallization influent from step a). f) The crude salt obtained in step a) is subjected to low-oxygen catalytic pyrolysis, deep salt removal, catalytic oxidation, and secondary oxidation evaporation crystallization in sequence to obtain recycled industrial salt and condensate; g) Perform membrane filtration on the condensate obtained in step f), reuse the membrane permeate for production, and combine the membrane concentrate with the salt solution for further treatment; h) The catalytic pyrolysis waste gas generated in step f) is subjected to rapid cooling, dust removal, alkali absorption, RTO incineration, electrostatic dust removal, and SCR denitrification in sequence before being discharged in compliance with standards.

2. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step a), the oil removal process sequentially includes primary ultrasonication-air flotation oil separation and recovery of the organic phase, and secondary anionic surfactant demulsification-activated carbon adsorption oil removal.

3. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step a), the catalytic oxidation employs a two-stage oxidation process using hydrogen peroxide and ozone.

4. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step d), the pH is adjusted in stages as follows: first stage pH 1–3 precipitates sodium oxalate, second stage pH 3–6 precipitates calcium oxalate, and third stage pH 6–8 precipitates rare earth hydroxides.

5. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step f), the temperature of the low-oxygen catalytic pyrolysis is 300–550 °C, and the oxygen volume fraction is <5%.

6. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step f), the deep purification of salt includes denitrification, defluorination, dephosphorization, debromination and heavy removal.

7. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step g), the membrane filtration is reverse osmosis.

8. The method for near-zero discharge and resource recovery of rare earth extraction wastewater and precipitate mother liquor as described in claim 1, characterized in that: In step h), dust removal is performed using a bag filter, and alkali absorption is achieved using a 10 wt% sodium hydroxide solution.

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