Process for purifying raffinate acid with a complexing extractant

Abstract

This invention specifically relates to a process for purifying residual raffinate using a composite extractant, comprising the following steps: diluting, filtering, and pH adjusting the residual raffinate to obtain a pretreated solution; using an aluminum-containing solution to perform aluminum loading treatment on an acidic phosphorus extractant to obtain an aluminum-loaded organic phase; mixing the obtained aluminum-loaded organic phase with the obtained pretreated solution for multi-stage countercurrent extraction to obtain a loaded organic phase and a residual extractant phase; mixing the obtained loaded organic phase with a boron-based back-extractant for back-extraction and complex-breaking reactions to obtain a regenerated organic phase and a back-extraction aqueous phase; and performing stepwise crystallization treatment on the obtained back-extraction aqueous phase to recover aluminum salts, magnesium salts, and fluoride salts, respectively. This invention prepares an "aluminum-loaded extractant" to achieve highly selective active capture of fluorine, and for the first time introduces a boron source (H3BO3) into the back-extraction stage, utilizing BF4. ‑ The highly stable "directional complex breaking" technology solves the problem of fluorine-aluminum complexes, transforming fluorine from an "impurity" into the high-value-added product potassium fluoroborate with a yield of over 90%, while simultaneously achieving the separate recovery of aluminum and magnesium.

Description

Technical Field

[0001] This invention relates to the field of chemical technology, specifically to a process for purifying residual raffinate using a composite extractant. Background Technology

[0002] Wet-process phosphoric acid purification methods mainly include chemical precipitation, aging and clarification, physical adsorption, cooling crystallization, solvent precipitation, and solvent extraction. Solvent extraction, due to the recyclability of the extractant and its advantages of low energy consumption, good purification effect, and the ability to achieve large-scale continuous production, is currently widely used in the industrial production of wet-process phosphoric acid. During production, the phosphoric acid remaining after extraction becomes raffinate. In the process of purifying phosphoric acid, a large number of impurities from the wet-process phosphoric acid enter the raffinate. Therefore, in addition to containing 40%–50% P2O5, phosphoric acid raffinate also contains various impurities: F, Mg, Fe, Al, and rare earth elements. Extracting fluorides from the remaining raffinate during the wet-process phosphoric acid production process is a crucial step in balancing resource recovery and environmental protection. This not only turns a "harm" into a "treasure" but also solves many problems in the production process. If the fluorine in the raffinate is not recovered, it will enter the circulating water and slag dump through the production process, causing environmental pollution. The state has strict standards for exhaust gas emissions (e.g., fluorine concentration ≤9 mg / m³). 3 Fluorine recovery is a necessary means to achieve emission standards. If fluorine accumulates in the system, it will severely corrode equipment and cause blockages in pipes and scrubbing nozzles, affecting the continuous and stable operation of production.

[0003] Fluorine is a strategic resource of equal importance to rare earth elements. With the increasing sophistication of fluorine recovery technology from phosphate rock, phosphate rock is gradually replacing the increasingly depleted fluorite as the primary source of fluorine resources. Extracting fluorine from raffinate is akin to mining in an "urban mine." The fluorosilicic acid process (for treating magnesium-removing slag) uses fluorosilicic acid to remove magnesium from raffinate. The resulting magnesium-removing slag then reacts with concentrated sulfuric acid to release fluorine-containing gases (SiF4, HF), which are then recovered. However, this process is economically unsustainable and difficult to industrialize. While the defluorination rate can reach over 95% under laboratory conditions, the long process and low-value byproducts result in high overall costs, making industrial profitability currently difficult.

[0004] The concentrated gas-phase absorption method utilizes the property that fluorine evaporates in gaseous form at high temperatures during phosphoric acid concentration. A scrubbing and absorption system recovers the fluorine-containing gas into fluorosilicic acid. However, this method requires sophisticated equipment and is prone to clogging. Fluorine-containing gases are highly corrosive and easily form scale and blockages in pipes and equipment. The scrubbing effect is affected by various factors such as nozzle design, gas velocity, and circulating water volume; improper operation can result in low recovery rates.

[0005] Chemical precipitation involves adding precipitants such as sodium or potassium salts to the residual acid, causing fluorine to precipitate as fluorosilicates (such as sodium fluorosilicate). This method results in low product purity and generates waste residue. Furthermore, the precipitation process can introduce a large amount of other impurities, leading to low quality of the recovered fluoride products. Additionally, if the precipitate residue cannot be utilized, it will create new solid waste.

[0006] Solvent extraction, which uses specific extractants to selectively extract fluoride from phosphoric acid, is costly and complex. Extractants are expensive and may be lost through dissolution, requiring sophisticated extraction, back-extraction, and solvent regeneration systems, resulting in high investment and operating costs.

[0007] How to improve the recovery rate of fluoride in raffinate at low cost has become an urgent industry problem to be solved. Summary of the Invention

[0008] To address the problems in fluorine recovery in existing technologies, this invention proposes a process for purifying residual raffinate using a composite extractant. Overcoming the shortcomings of high cost and low yield in fluorine recovery, the method of this invention effectively improves the fluorine yield and enhances economic efficiency. The extraction of fluorine from residual raffinate is shifting from a single objective of "impurity removal" to a multi-objective, synergistic system engineering approach encompassing "resource recovery, environmental protection, and energy conservation."

[0009] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A process for purifying residual acid using a composite extractant includes the following steps: (1) Pretreatment: The residual acid is diluted, filtered and pH adjusted to obtain the pretreated solution; (2) Preparation of aluminum-loaded extractant: Acidic phosphorus extractant was subjected to aluminum loading treatment with an aluminum-containing solution to obtain an aluminum-loaded organic phase; (3) Synergistic extraction: The aluminum-loaded organic phase obtained in step (2) is mixed with the pretreatment liquid obtained in step (1) and subjected to multi-stage countercurrent extraction to obtain the loaded organic phase and the extract residue phase; (4) Back-extraction-complex breaking: The loaded organic phase obtained in step (3) is mixed with a boron-containing back-extraction agent to carry out back-extraction and complex breaking reactions to obtain a regenerated organic phase and a back-extraction aqueous phase; (5) Stepwise crystallization recovery: The back-extraction aqueous phase obtained in step (4) is subjected to stepwise crystallization treatment to recover aluminum salt, magnesium salt and fluoride salt products respectively.

[0010] Further, in step (1), the dilution is to dilute the residual acid to a P2O5 mass fraction of 15-30%, preferably 20-25%; the pH adjustment is to adjust to 1.0-3.0, preferably 1.5-2.0.

[0011] Further, the acidic phosphorus extractant in step (2) is selected from at least one of D2EHPA (P204), P507, and Cyanex272, preferably D2EHPA; the aluminum-containing solution is aluminum sulfate, aluminum chloride, or aluminum nitrate solution, and the aluminum ion concentration is 0.1~1.0 mol / L, preferably 0.3~0.6 mol / L.

[0012] Further, the operating conditions for the aluminum loading treatment in step (2) are: organic phase to water phase volume ratio of 1:1 to 5:1, mixing time of 5 to 30 minutes, and temperature of 20 to 50°C.

[0013] Further, the multi-stage countercurrent extraction in step (3) has 3 to 8 stages, an O / A ratio of 1:1 to 6:1, an extraction temperature of 30 to 60°C, and a mixing time of 5 to 15 minutes per stage. F extraction rate: >90%, Al extraction rate: >75%, Mg extraction rate: 50%, Fe extraction rate: 20%.

[0014] Further, the stripping agent in step (4) is an acidic aqueous solution containing a boron source, wherein the acid is at least one of sulfuric acid, hydrochloric acid or nitric acid, and the concentration is 0.5~3.0 mol / L; the boron source is at least one of boric acid, borax or tetrafluoroborate, and the concentration is 0.1~0.8 mol / L based on boron element.

[0015] Furthermore, the conditions for the back-extraction-complex breaking reaction in step (4) are: a ratio (O / A) of 1:1 to 5:1, a temperature of 40 to 80°C, and 2 to 5 back-extraction stages. The back-extraction rate is F > 95% and Al > 90%.

[0016] Furthermore, the stepwise crystallization recovery described in step (5) specifically includes: (a) First crystallization: Ammonium salt was added to the back-extraction aqueous phase, the pH was adjusted, and the crystals were cooled and separated to obtain ammonium aluminum sulfate crystals with a purity greater than 90%; (b) Second crystallization: The mother liquor from the first crystallization was further cooled and crystallized to obtain magnesium ammonium sulfate crystals with a purity >90%; (c) Third crystallization: Add potassium salt to the mother liquor of the second crystallization, precipitate, filter, and dry to obtain potassium fluoroborate product with a purity >95%.

[0017] Further, the ammonium salt mentioned in step (a) is ammonium sulfate, ammonium chloride, or ammonium nitrate, and the amount added is such that n(NH4) + ):n(Al³ + The ratio of 2.5 to 4:1 is used; the crystallization temperature is 15 to 30℃.

[0018] Furthermore, the cooling crystallization temperature in step (b) is 0~10℃.

[0019] Further, the potassium salt mentioned in step (c) is potassium chloride, potassium sulfate, or potassium nitrate, and the amount added is such that n(K) + ):n(BF4 - The ratio is 0.8 to 1.2:1.

[0020] Furthermore, the residual phase obtained in step (3) is returned to the phosphoric acid production system for the preparation of phosphate fertilizer; the regenerated organic phase obtained in step (4) is returned to step (2) or directly returned to step (3) for recycling.

[0021] The potassium fluoroborate product prepared by the method described above has a purity of ≥98% and a fluorine recovery rate of ≥95%.

[0022] A system for the synergistic recovery of fluorine and metals from residual acid includes: a pretreatment unit, an aluminum-loaded extractant preparation unit, a multi-stage countercurrent extraction unit, a back-extraction-complex breaking unit, and a stepwise crystallization unit; wherein, a fluid communication pipeline is provided between the back-extraction-complex breaking unit and the stepwise crystallization unit, and an organic phase circulation pipeline is provided between the multi-stage countercurrent extraction unit and the back-extraction-complex breaking unit.

[0023] The beneficial effects of this invention are: 1. This invention pioneers the concept of "aluminum-loaded extractant," which utilizes Al... 3+ Pre-loaded onto D2EHPA to achieve highly selective active capture of fluorine; for the first time, a boron source (H3BO3) is introduced into the back-extraction section, utilizing BF4. - The highly stable "directional complex breaking" technology solves the problem of fluorine-aluminum complexes, transforming fluorine from an "impurity" into a high-value-added product (potassium fluoroborate), while simultaneously enabling the separate recycling of aluminum and magnesium.

[0024] 2. The extraction-washing-back-extraction-crystallization multi-unit coupling forms a closed-loop process, the extractant is recycled, and the waste gas is nearly zero-emission. This reveals the molecular mechanism of "aluminum loading-fluorine capture-complex breaking" and provides a new approach for fluorine recovery in complex systems. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1 This embodiment provides a process for purifying residual acid using a composite extractant, comprising the following steps: (1) Pretreatment: Reduce the viscosity of residual acid, reduce impurity interference, and improve extraction efficiency. Based on the original concentration of residual acid, dilute to a P2O5 mass fraction of 20-25%, filter using a 5 μm filter cartridge, and adjust to pH 1.5 with ammonia or dilute sulfuric acid (the fluorine-aluminum complex is relatively stable within this pH range, which is beneficial for extraction). (2) Preparation of aluminum-loaded extractant When traditional extractants come into direct contact with residual acid, the Al in the solution needs to be "captured" on the spot. 3+ The formation of active complexes is inefficient and unstable. This process pre-processes Al... 3+ It is loaded onto the extractant so that it already has the ability to capture fluoride when it enters the system.

[0027] Preparation method: Extractant composition: 30% D2EHPA (P2O4) and 70% kerosene (volume ratio); Aluminum-supported solution: 0.5 mol / L Al2(SO4)3 solution, pH adjusted to 2.0 with dilute sulfuric acid; The organic phase and aqueous phase were mixed at a 1:1 ratio and extracted for 10 minutes. After standing and phase separation, an aluminum-supported organic phase was obtained. The POH groups in the D2EHPA molecule reacted with Al. 3+ A cation exchange reaction occurs, forming the AlL3HL structure. In this structure, Al³⁺ + It is "anchored" to the extractant molecule, but still retains some coordination vacancies, which can further interact with F. - Combine.

[0028] (3) Extraction: The ratio (O / A) = 4:1. The higher the ratio, the more thorough the extraction. However, it is necessary to balance the processing capacity of the equipment. The temperature is 50℃. Appropriate heating can reduce the viscosity of the solution and accelerate mass transfer. The number of stages is 5-stage countercurrent extraction. The more stages, the higher the extraction rate, but the investment increases. Each stage takes 8 minutes.

[0029] Al in aluminum-supported organic phase 3+ With F in residual acid - A coordination reaction occurs, forming an Al-F complex that enters the organic phase. Simultaneously, D₂EHPA in the organic phase also reacts with free Al in the solution. 3+ Mg 2+ Cation exchange occurs, enabling metal co-extraction.

[0030] F extraction rate: >95%, Al extraction rate: >80%, Mg extraction rate: 55%, Fe extraction rate: 30% (Fe extraction rate is relatively low at low pH).

[0031] (4) Washing: Removes phosphate and other impurities entrained in the organic phase to improve product purity. The washing solution is 0.1 mol / L dilute sulfuric acid, with a ratio of (O / A) of 5:1, number of stages: 1~2, and temperature: room temperature.

[0032] (5) Back-extraction-complex breaking treatment: Traditional back-extraction can only transfer fluorine and metal from the organic phase to the aqueous phase, but they still exist in the aqueous phase as stable fluorine-aluminum complexes, which are difficult to separate later. This process introduces a boron source (H3BO3) into the back-extraction agent and utilizes the stronger binding force between boron and fluorine to "directively break the complex".

[0033] Boric acid (H3BO3) can react with F under acidic conditions. - Formation of stable fluoroborate ions (BF4) - ), BF4 - Its stability is much higher than that of aluminum fluoride complexes, therefore the reaction tends to produce BF4. - proceed in that direction. The reaction equation is: AlFx (3-x) + 4H + + 4F - +H3BO3→ [Al(H2O)6] 3+ + BF4 - +Other products The stripping agent composition is: 1.5 mol / L H₂SO₄ + 0.3 mol / L H₃BO₃, with an O / A ratio of 3:1. The temperature was 70℃ (high temperature promotes complex-breaking reaction), the number of stages was 3, and the back-extraction rate was F>98% and Al>95%; Regenerated organic phase: D2EHPA + kerosene, which can be recycled to the extraction unit.

[0034] Back-extraction aqueous phase: containing Al 3+ Mg 2+ F - (with BF4) - (form) SO4 2- .

[0035] (6) Stepwise crystallization, utilizing the differences in solubility of different metal salts, to achieve the separate recovery of aluminum, magnesium, and fluorine: Step 1: Recovering ammonium aluminum sulfate Ammonium sulfate ((NH4)2SO4) is added to the aqueous phase of the back-extraction to make n(NH4)2SO4 ... + ):n(Al 3+ The ratio of sodium sulfate to sodium sulfate is approximately 3:1; adjust the pH to 3.0, cool to 25°C, crystallize for 18 hours, and filter to obtain NH4Al(SO4)2 (ammonium aluminum sulfate) crystals with a purity >95%. Step 2: Recover magnesium ammonium sulfate The mother liquor from step 1 was further cooled to 5°C and crystallized for 12 hours. After filtration, (NH4)2Mg(SO4)2·6H2O (magnesium ammonium sulfate) crystals were obtained with a purity >90%.

[0036] Step 3: Recover potassium fluoroborate In step 2, add KCl or K2SO4 to the mother liquor to make n(K + ):n(BF4 - The ratio of potassium fluoroborate to potassium fluoroborate was approximately 1:1. The mixture was stirred at room temperature for 1 hour, the precipitate was filtered, and dried to obtain KBF4 (potassium fluoroborate) with a purity >98%.

[0037] Example 2 This embodiment provides a process for purifying residual acid using a composite extractant, comprising the following steps: (1) Pretreatment: Reduce the viscosity of residual acid, reduce impurity interference, and improve extraction efficiency. Based on the original concentration of residual acid, dilute to a P2O5 mass fraction of 20-25%, filter using a 10 μm filter element, and adjust to 2.0 with ammonia or dilute sulfuric acid; (2) Preparation of aluminum-loaded extractant When traditional extractants come into direct contact with residual acid, the Al in the solution needs to be "captured" on the spot. 3+ The formation of active complexes is inefficient and unstable. This process pre-processes Al... 3+ It is loaded onto the extractant so that it already has the ability to capture fluoride when it enters the system.

[0038] Preparation method: Extractant composition: 30% D2EHPA (P2O4) and 70% kerosene (by volume). Aluminum-supported solution: 0.5 mol / L Al2(SO4)3 solution, adjusted to pH 2.0 with dilute sulfuric acid. The organic phase and aqueous phase were mixed at a 1:1 ratio and extracted for 15 minutes. After standing and phase separation, an aluminum-supported organic phase was obtained. The POH groups in the D2EHPA molecule reacted with Al. 3+ A cation exchange reaction occurs, Al 3+ It is "anchored" to the extractant molecule, but still retains some coordination vacancies, which can further interact with F. - Combine.

[0039] (3) Extraction: ratio (O / A) = 2:1, temperature 50℃; number of stages: 3-stage countercurrent extraction, 5 minutes per stage.

[0040] Al in aluminum-supported organic phase 3+ With F in residual acid - A coordination reaction occurs, forming an Al-F complex that enters the organic phase. Simultaneously, D₂EHPA in the organic phase also reacts with free Al in the solution. 3+ Mg 2+ Cation exchange occurs, enabling metal co-extraction.

[0041] F extraction rate: >95%, Al extraction rate: >75%, Mg extraction rate: 50%, Fe extraction rate: 40% (Fe extraction rate is relatively low at low pH).

[0042] (4) Washing: The washing solution is 0.1 mol / L dilute sulfuric acid, with a ratio of (O / A) = 5:1, number of stages: 1~2, and temperature: room temperature.

[0043] (5) Back-extraction-complex breaking treatment Boric acid (H3BO3) can react with F under acidic conditions. - Formation of stable fluoroborate ions (BF4) - ), BF4 - Its stability is much higher than that of aluminum fluoride complexes, therefore the reaction tends to produce BF4. - proceed in that direction. The reaction equation is: AlFx (3-x) + 4H + + 4F - +H3BO3→ [Al(H2O)6] 3+ + BF4 - +Other products The stripping agent composition is: 1.5 mol / L H₂SO₄ + 0.3 mol / L H₃BO₃, with an O / A ratio of 3:1. The temperature is 60℃, the number of stages is 2, and the back-extraction rate is F>92% and Al>90%. The regenerated organic phases D2EHPA and kerosene are recycled back to the extraction unit for use.

[0044] Back-extraction aqueous phase: containing Al 3+ Mg 2+ F - (with BF4) - (form) SO4 2- .

[0045] (7) Stepwise crystallization, utilizing the differences in solubility of different metal salts, to achieve the separate recovery of aluminum, magnesium, and fluorine: Step 1: Recovering ammonium aluminum sulfate Ammonium sulfate ((NH4)2SO4) is added to the aqueous phase of the back-extraction to make n(NH4)2SO4 ... + ):n(Al 3+ The ratio of aluminum sulfate to sodium sulfate is approximately 3:1; the pH is adjusted to 2.5, cooled to 25°C, crystallized for 12 hours, and filtered to obtain NH4Al(SO4)2 (ammonium aluminum sulfate) crystals with a purity >93%.

[0046] Step 2: Recover magnesium ammonium sulfate The mother liquor from step 1 was further cooled to 5°C and crystallized for 12 hours. After filtration, (NH4)2Mg(SO4)2·6H2O (magnesium ammonium sulfate) crystals were obtained with a purity >89%.

[0047] Step 3: Recover potassium fluoroborate In step 2, add KCl or K2SO4 to the mother liquor to make n(K + ):n(BF4 - The ratio of potassium fluoroborate to potassium fluoroborate was approximately 1:1. The mixture was stirred at room temperature for 1 hour, the precipitate was filtered, and dried to obtain KBF4 (potassium fluoroborate) with a purity >92%.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for purifying residual acid using a composite extractant, characterized in that, Includes the following steps: (1) Pretreatment: The residual acid is diluted, filtered and pH adjusted to obtain the pretreated solution; (2) Preparation of aluminum-loaded extractant: Acidic phosphorus extractant was subjected to aluminum loading treatment with an aluminum-containing solution to obtain an aluminum-loaded organic phase; (3) Co-extraction: The aluminum-loaded organic phase obtained in step (2) is mixed with the pretreated liquid obtained in step (1) and subjected to multi-stage countercurrent extraction to obtain the loaded organic phase and the extract residue; (4) Back-extraction-complex breaking: The loaded organic phase obtained in step (3) is mixed with a boron-containing back-extraction agent to carry out back-extraction and complex breaking reactions to obtain a regenerated organic phase and a back-extraction aqueous phase; (5) Stepwise crystallization recovery: The back-extraction aqueous phase obtained in step (4) is subjected to stepwise crystallization treatment to recover aluminum salt, magnesium salt and fluoride salt products respectively.

2. The process for purifying residual acid using the composite extractant according to claim 1, characterized in that, The dilution in step (1) is to dilute the residual acid to a P2O5 mass fraction of 15-30%, and the pH adjustment is to adjust it to 1.0-3.

0.

3. The process for purifying residual acid using the composite extractant according to claim 1, characterized in that, The acidic phosphorus extractant in step (2) is selected from at least one of P204, P507, and Cyanex272; the aluminum-containing solution is aluminum sulfate, aluminum chloride, or aluminum nitrate solution, and the aluminum ion concentration is 0.1~1.0 mol / L.

4. The method according to claim 1 or 3, characterized in that, The operating conditions for the aluminum loading treatment in step (2) are: organic phase to water phase volume ratio of 1:1 to 5:1, mixing time of 5 to 30 minutes, and temperature of 20 to 50°C.

5. The process for purifying residual acid using the composite extractant according to claim 1, characterized in that, The multi-stage countercurrent extraction in step (3) has 3 to 8 stages, the ratio of organic phase to aqueous phase is 1:1 to 6:1, the extraction temperature is 30 to 60°C, and the mixing time for each stage is 5 to 15 min.

6. The process for purifying residual acid using the composite extractant according to claim 1, characterized in that, The stripping agent in step (4) is an acidic aqueous solution containing a boron source, wherein the acid is at least one of sulfuric acid, hydrochloric acid or nitric acid, and the concentration is 0.5~3.0 mol / L; the boron source is at least one of boric acid, borax or tetrafluoroborate, and the concentration is 0.1~0.8 mol / L based on boron element.

7. The process for purifying residual acid using the composite extractant according to claim 1 or 6, characterized in that, The conditions for the back-extraction-complex breaking reaction in step (4) are: ratio (O / A) of 1:1 to 5:1, temperature of 40 to 80°C, and back-extraction stages of 2 to 5.

8. The process for purifying residual acid using the composite extractant according to claim 1, characterized in that, The stepwise crystallization recovery described in step (5) specifically includes: (a) First crystallization: Ammonium salt was added to the back-extraction aqueous phase, the pH was adjusted, and the crystals were cooled and separated to obtain ammonium aluminum sulfate crystals; (b) Second crystallization: The mother liquor from the first crystallization is further cooled and crystallized to obtain magnesium ammonium sulfate crystals; (c) Third crystallization: Add potassium salt to the mother liquor of the second crystallization, precipitate, filter, and dry to obtain potassium fluoroborate product.

9. The process for purifying residual acid using the composite extractant according to claim 8, characterized in that, The ammonium salt mentioned in step (a) is ammonium sulfate, ammonium chloride, or ammonium nitrate, and the amount added is such that n(NH4) + ):n(Al³ + The ratio of potassium chloride to potassium sulfate is 2.5 to 4:1; the crystallization temperature is 15 to 30°C; the cooling crystallization temperature in step (b) is 0 to 10°C; the potassium salt in step (c) is potassium chloride, potassium sulfate, or potassium nitrate, and the amount added is such that n(K) + ):n(BF4 - The ratio is 0.8 to 1.2:

1.

10. In the process of purifying residual acid with the composite extractant according to claim 1, the regenerated organic phase obtained in step (4) is returned to step (2) or directly returned to step (3) for recycling.