A method for deep removal of heavy metals in wet phosphoric acid process

Abstract

This invention relates to a method for deep removal of heavy metals in a wet-process phosphoric acid process, comprising the following steps: (1) using a straight-chain or branched alcohol as a raw material, reacting it with phosphorus pentasulfide to obtain dialkyl dithiophosphate, and then reacting it with metal oxides, hydroxides or salts to synthesize dialkyl dithiophosphate (DDP); (2) reacting the DDP with one or more of the following: phosphoric acid slurry containing heavy metals, dilute phosphoric acid, concentrated phosphoric acid, and residual acid from phosphoric acid purification, to generate a heavy metal complex precipitate. After the reaction is complete, the purified phosphoric acid is separated to achieve the removal of heavy metals. This method has low raw material costs, and the cadmium content after heavy metal removal of phosphoric acid is ≤2ppm with a removal rate ≥96%; the copper content is ≤0.5ppm with a removal rate ≥99.5%; the arsenic content is ≤5ppm with a removal rate ≥70%; phosphorus loss is ≤0.5%; no reducing agent is required, thus avoiding hydrogen generation; it has little impact on the decrease in phosphoric acid concentration; and no H2S is generated. It is suitable for the removal of heavy metals from wet-process phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, and other systems, and can be directly integrated into existing production lines.

Description

Technical Field

[0001] This invention relates to a method for deep removal of heavy metals in a wet-process phosphoric acid process, belonging to the field of wet-process phosphoric acid purification. Background Technology

[0002] Phosphoric acid is a core raw material for the production of phosphate fertilizers, food additives, and new energy materials (such as lithium iron phosphate), and its production mainly relies on the wet processing of phosphate rock. However, heavy metals commonly found in phosphate rock, especially cadmium (Cd), copper (Cu), and arsenic (As), have become key pollutants that restrict the quality of phosphoric acid.

[0003] Although copper is an essential trace element for the human body, excessive intake can lead to acute or chronic poisoning. Acute copper poisoning mainly manifests as "metal fume fever," with symptoms including headache, dizziness, fatigue, chest tightness, and cough, followed by high fever (38–39°C). Ingestion of copper salts can cause nausea, vomiting, hemolytic jaundice, and kidney failure. Workers with long-term exposure to copper dust and fumes may develop respiratory symptoms and contact dermatitis. In phosphoric acid products, the presence of copper ions can catalyze side reactions, affecting the stability of downstream products (such as lithium iron phosphate battery materials).

[0004] Inorganic arsenic has been identified as a carcinogen by the International Agency for Research on Cancer and is the most important chemical contaminant in global drinking water. Long-term exposure to inorganic arsenic in drinking water and food can lead to skin cancer, bladder cancer, and lung cancer, as well as skin lesions (pigmentation changes, hyperkeratosis). Arsenic is also associated with cardiovascular disease, diabetes, adverse pregnancy outcomes, and infant mortality, and has a negative impact on children's cognitive development. In electronic-grade phosphoric acid applications, the arsenic content must be strictly below 1 × 10⁻⁶. -7 kg / kg, because arsenic can corrode microelectronic components and seriously affect product quality.

[0005] In the wet-process phosphoric acid production, 70%–80% of cadmium ends up in downstream products, causing soil pollution through phosphate fertilizer application. Copper and arsenic also enter the downstream industrial chain along with phosphoric acid, threatening human health and product quality. Global regulations are becoming increasingly stringent; for example, the EU requires phosphate products to have cadmium content below 60 mg / kg P2O5 and is promoting "low-cadmium labeling" certification. At the same time, the demand for high-purity phosphoric acid (requiring copper, cadmium, and arsenic ion concentrations below ppm) from the new energy industry is surging, forcing the industry to innovate technologies for deep removal of heavy metals.

[0006] For the removal of copper from phosphoric acid, domestic and international research mainly focuses on precipitation and adsorption methods. Cao Haoyu et al. (2025) studied the removal of copper and chromium from waste phosphoric acid using a coupled technology of oxalic acid chemical precipitation and activated carbon adsorption, and found that the mechanism of copper removal by oxalic acid lies in Cu 2+The two -COOH groups of H₂C₂O₄ coordinate to form a stable CuC₂O₄ precipitate, while activated carbon adsorbs copper ions through surface ion exchange; when both oxalic acid and activated carbon are added at 20 g / L, the Cu in the regenerated phosphoric acid... 2+ The concentration can be reduced to 23.85 mg / L. Mi et al. (2024) developed a chitosan-based biosorbent, which achieves Cu adsorption through the synergistic coordination of the amino groups of chitosan and the phosphate groups of phytic acid. 2+ The enhanced adsorption provides a new approach for the removal of copper from the phosphoric acid system.

[0007] Research on arsenic removal from phosphoric acid has focused on advanced purification technologies. Xiao et al. (2024) systematically reviewed the research progress in the preparation of electronic-grade phosphoric acid using wet-process arsenic removal from phosphoric acid, pointing out that electronic-grade phosphoric acid requires an arsenic mass fraction of less than 1×10⁻⁶. -7 While arsenic and phosphoric acid have similar physicochemical parameters, achieving deep separation of arsenic and phosphoric acid is crucial for the preparation of electronic-grade phosphoric acid. This review summarizes the research progress and advantages and disadvantages of techniques such as chemical precipitation, crystallization, electrodeposition, electrodialysis, and adsorption, providing a technical pathway reference for the deep removal of arsenic.

[0008] However, existing technologies still face significant bottlenecks when processing copper, cadmium, and arsenic: Precipitation methods (such as sodium sulfide method): Although they can combine with various heavy metal ions to form precipitates, they are prone to producing highly toxic H2S gas, posing a significant safety hazard; moreover, the generated sulfide precipitates are mostly fine particles, making filtration difficult and hindering the deep removal of arsenic.

[0009] Gypsum co-crystallization method: Cadmium and calcium sulfate are co-precipitated by adjusting the conditions. However, this method has low removal efficiency for copper and arsenic, requires excessive sulfuric acid, resulting in phosphorus loss of up to 10% to 30%, and produces a large amount of heavy metal-containing sludge that is difficult to treat.

[0010] Solvent extraction: It has low separation efficiency for copper, cadmium and arsenic, is complicated to operate, and the extractant is easily degraded in a strong acid environment, resulting in high cost.

[0011] Adsorption methods (such as modified biochar and activated carbon): Although they have high adsorption capacity, they have poor adsorption selectivity, especially when multiple heavy metal ions coexist, competitive adsorption is obvious, and they are difficult to integrate seamlessly into existing continuous production lines.

[0012] In recent years, technological trends have focused on process integration and resource utilization, but the deep removal of copper, cadmium, and arsenic to trace levels (such as Cd≤2ppm, Cu≤0.5ppm, As≤5ppm), sludge reduction, and improved industrial compatibility remain core challenges.

[0013] Several existing patents have proposed improvements to address the above issues, but shortcomings still exist: American company SITECH disclosed a patent, CN121311439A, entitled "Removal of Heavy Metals from Phosphoric Acid Streams." This patent uses an ammonium salt of C8-C12 dialkyl dithiophosphate, overcoming the problems of high viscosity and inability to be pumped by sodium and potassium salts, and maintaining good flowability at room temperature. However, its fatal flaws are its high dependence on the ammonium salt system; high cost; insufficient cadmium removal rate (up to 63%) and easy dissociation; significant phosphorus loss; and, in particular, the negative impact of generating H2S.

[0014] The patent with US4986970A, titled "Method for removal of heavy metals, especially cadmium, from phosphoric acid containing solutions," primarily uses dithiocarbonates or organothiophosphorus compounds in combination with solid adsorbents and reducing agents to remove heavy metals. This patented process has extremely demanding conditions, requiring pre-cooling of phosphoric acid to 5-40°C; partial neutralization to adjust the pH; and the addition of large amounts of adsorbent, resulting in a lengthy process chain and substantial equipment investment, material, and energy costs.

[0015] A patent with US10865110B2, titled "Compositions and processes for removing heavy metals from phosphoric acid solutions," attempts to combine organic dithiophosphonic acid with high-dose surfactants to enhance the capture of heavy metals. The drawback is the need for strict control of the compounding ratios of various agents, significantly increasing the precision requirements for dosing during production and raising overall reagent costs.

[0016] The patent No. CN120423507B issued by Shenyang Nonferrous Metals Research Institute Co., Ltd., entitled "A Removal Agent and Method for Cadmium from Phosphoric Acid," discloses a method for removing cadmium from phosphoric acid using dithiophosphates (mainly dibutyl dithiophosphate, dibutyl dithiophosphate ammonium, dibutyl dithiophosphate sodium, diisobutyl dithiophosphate, diisobutyl dithiophosphate ammonium, and diisobutyl dithiophosphate sodium). This method achieves a high cadmium removal rate, but suffers from drawbacks such as requiring extremely large amounts of cadmium removal additives, resulting in residual H2S in the finished product, significant phosphorus loss, and a marked decrease in phosphoric acid concentration.

[0017] Canada has published a patent, CN112930321A, entitled "Method for Removing Cadmium and Other Metals and Impurities from Phosphate-Containing Materials." This patent discloses a technique using dialkyldithiophosphonic acid (or its alkali metal / ammonium salt). The advantage is a relatively simplified process; the disadvantages are a high dependence on alkali metals or ammonium salts, difficulty in phase separation in complex slurries, and the potential for some derivatives to release toxic H2S gas under high temperature and strong acid conditions. Summary of the Invention

[0018] Based on a comprehensive study of the aforementioned patented technologies and in consideration of customer needs, in order to overcome the shortcomings of these patents, such as high cost, generation of toxic gases like H2S, low efficiency in removing heavy metals, and significant phosphorus loss or even dissociation, as well as the high phosphorus loss, difficult sludge treatment, or complex processes associated with phosphogypsum co-crystallization and solvent extraction, we have developed a method for deep removal of heavy metals in wet-process phosphoric acid. This invention utilizes dialkyl dithiophosphate (DDP), a non-sodium, non-ammonium salt, as a heavy metal removal agent, based on the characteristics of different metal ions. This allows DDP to react with Cd in phosphoric acid. 2+ Cu 2+ As 3+ As 5+ The specific chelation process generates stable hydrophobic precipitates, offering the advantages of high selectivity and low phosphorus loss, thus avoiding P2O5 loss caused by excessive sulfuric acid use. Simultaneously, the generated precipitates are large particles and easily settle, significantly improving the difficulty of filtering traditional heavy metal-containing sludge and reducing subsequent treatment costs. This invention achieves highly efficient removal of heavy metals (removal rate ≥96%) and improved phosphoric acid purity through a one-step reaction, with minimal impact on phosphoric acid concentration. It requires no complex equipment or high-temperature conditions and can be directly integrated into existing wet-process phosphoric acid production lines. It is applicable to one or more scenarios involving wet-process phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid from phosphoric acid purification, providing the phosphoric acid chemical industry with a low-cost, environmentally friendly, and highly adaptable deep cadmium removal technology.

[0019] The technical solution of this invention is as follows: A method for deep removal of heavy metals in a wet-process phosphoric acid process, characterized by comprising the following steps: (1) Using a straight-chain or branched alcohol as a raw material, react with phosphorus pentasulfide and filter to obtain dialkyl dithiophosphate filtrate. The dialkyl dithiophosphate filtrate is then reacted with metal oxide to synthesize dialkyl dithiophosphate (DDP). (2) The DDP is mixed and reacted with wet phosphoric acid slurry containing heavy metals, dilute phosphoric acid, concentrated phosphoric acid or residual acid produced by wet phosphoric acid purification, wherein the wet phosphoric acid slurry containing heavy metals is a slurry containing heavy metals and phosphogypsum, and heavy metal complex precipitates are generated. After the reaction is completed, the slurry or purified phosphoric acid is obtained by filtration, thereby achieving the removal of heavy metals.

[0020] The alcohol in step (1) has a carbon chain length of 4-18, and the molar ratio of the alcohol to phosphorus pentasulfide is 4:1-1.12.

[0021] The synthesis steps of DDP in step (1) specifically include: (i) stirring and reacting alcohol with phosphorus pentasulfide at 80-120°C for 4-8 hours to obtain dialkyl dithiophosphate; (ii) reacting the dialkyl dithiophosphate with metal oxides, hydroxides or salts at 0-120°C for 4-8 hours, drying under reduced pressure for 1-4 hours after the reaction is completed, and filtering while hot to obtain dialkyl dithiophosphate DDP; the metal oxides, hydroxides or salts are one or more of Fe, Mn, Zn, Al, Li, Sn, Mo.

[0022] The molar ratio of the dialkyl dithiophosphate filtrate to the metal oxide, hydroxide, or salt is 1:1.05-1.20.

[0023] First, mix DDP and water in step (2) at a ratio of 1:0-100 until homogeneous, and then add it to one or more places in the wet process phosphoric acid production line, such as phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid produced by phosphoric acid purification. The amount of DDP added is 0.1%-1% of the mass of phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid produced by phosphoric acid purification. The reaction temperature is 50-90℃, the stirring rate is 50-3000r / min, and the reaction time is 0.01-30 minutes.

[0024] The P2O5 content of the phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid produced from phosphoric acid purification in the wet phosphoric acid production line described in step (2) is as follows: 5%-20% in phosphoric acid slurry; 18%-36% in dilute phosphoric acid; 36%-65% in concentrated phosphoric acid; and 18%-46% in residual acid produced from phosphoric acid purification.

[0025] The heavy metal content after heavy metal removal by wet phosphoric acid is ≤2ppm, copper content ≤0.5ppm, arsenic content ≤5ppm, and phosphorus loss ≤0.5%.

[0026] Existing phosphate chemical processes mainly remove harmful cadmium metal from phosphoric acid using inorganic sulfides such as sodium hydrosulfide. Although this method is inexpensive, sodium hydrosulfide generates a large amount of toxic hydrogen sulfide gas during use, posing a significant threat to the health of factory workers. This invention mainly removes harmful cadmium metal from phosphoric acid using DDP, which eliminates the threat of hydrogen sulfide during use. Traditional organic sulfides, such as butanol or isobutanol, have low removal efficiency, require a high amount of cadmium removal agent to achieve deep purification, significantly reduce the phosphoric acid concentration, and the resulting organic cadmium complexes are prone to dissociation, and the investment is relatively high.

[0027] The beneficial effects of this invention are as follows: 1. Economic efficiency: Improved efficiency in removing heavy metals, reduced dosage of additives, and minimal impact on phosphoric acid concentration reduction, thereby lowering the overall cost of heavy metal removal with phosphoric acid.

[0028] 2. Stability: After complexing with heavy metal ions, the structure is stable and not easily dissociated.

[0029] 3. Safety and environmental protection: No H2S gas is generated, avoiding occupational hazards; it can be used directly without adding reducing agents, avoiding the generation of explosive gases (such as H2).

[0030] 4. Universality: Applicable to wet-process phosphoric acid slurry systems, dilute phosphoric acid systems, and concentrated phosphoric acid systems, and can be directly integrated into existing production lines. Detailed Implementation

[0031] The present invention will be further described below with reference to embodiments. It should be noted that the technical effect of the present invention stems from the specific coordination chemistry of metal ions.

[0032] Example 1 DDP Synthesis: Take 0.4 mol of isooctanol (C8 straight chain) and add it to a three-necked flask. Add P2S5 at a molar ratio of 4:1 (p2S5 = 0.1 mol). Stir the reaction at 90℃ for 5.5 h and filter to obtain dialkyl dithiophosphate filtrate. Mix the dialkyl dithiophosphate filtrate with ZnSO4 at a molar ratio of 1:1.1 and react at 90℃ for 4 h. Dry under reduced pressure for 3 h and filter while hot to obtain DDP-Zn.

[0033] Application test: Take a wet-process phosphoric acid slurry with a P2O5 mass concentration of 20% and a cadmium content of 50 ppm, add 0.5% DDP-Zn by mass, stir at 85℃ and 210 r / min for 20 min, and after the reaction is complete, filter to obtain purified dilute phosphoric acid; the P2O5 mass concentration in the purified phosphoric acid is determined to be 19.86%; the cadmium content is 0.8 ppm, the removal rate is 98.4%; and the phosphorus loss is 0.18%.

[0034] Example 2 DDP Synthesis: 1.0 mol of isoamyl alcohol (C5 branched) was used, and P2S5 was added at a ratio of 4:1.10 (0.275 mol of P2S5). The mixture was reacted at 80 °C for 8 h, and filtered to obtain dialkyl dithiophosphate filtrate. The dialkyl dithiophosphate filtrate was mixed with MnO at a molar ratio of 1:1.05 and reacted at 120 °C for 4 h. The mixture was then dried under reduced pressure for 1 h to obtain DDP-Mn.

[0035] Application test: 0.3% DDP-Mn was added to a wet-process phosphoric acid slurry with a P2O5 mass concentration of 16% and a cadmium content of 65 ppm. The mixture was stirred at 65℃ and 150 r / min for 25 min. After the reaction was completed, the slurry was filtered to obtain purified dilute phosphoric acid. The P2O5 mass concentration in the purified phosphoric acid was determined to be 15.97%, the cadmium content was 1.48 ppm, the removal rate was 97.7%, and the phosphorus loss was 0.42%.

[0036] Example 3 DDP Synthesis: Take 0.2 mol of n-hexanol (C6 straight chain), add 0.051 mol of P2S5 at a molar ratio of alcohol to P2S5 of 4:1.02, react at 102℃ for 5 h, filter to obtain dialkyl dithiophosphate filtrate; the molar ratio of dialkyl dithiophosphate filtrate to Fe2O3 is 1:1.10, react at 0℃ for 8 h, dry under reduced pressure for 2.5 h to obtain DDP-Fe.

[0037] Application Test: 1.0% DDP-Fe was added to a wet-process phosphoric acid slurry with a P2O5 mass concentration of 14%, a cadmium content of 80 ppm, a copper content of 45 ppm, and an arsenic content of 20 ppm. The mixture was stirred at 90℃ and 300 r / min for 15 min. After the reaction was complete, the slurry was filtered to obtain purified dilute phosphoric acid. The P2O5 mass concentration in the purified phosphoric acid was determined to be 13.99%; the cadmium content was reduced to 1.20 ppm, with a removal rate of 98.5%; the copper content was reduced to 0.16 ppm, with a removal rate of 99.64%; the arsenic content was reduced to 4.83 ppm, with a removal rate of 75.85%; and the phosphorus loss was 0.43%.

[0038] Example 4 DDP Synthesis: 2 mol of n-butanol (C4 straight chain) was added to 0.53 mol of P2S5 at a molar ratio of alcohol to P2S5 of 4:1.06. The mixture was reacted at 90℃ for 6.5 h and filtered to obtain dialkyl dithiophosphate filtrate. The molar ratio of dialkyl dithiophosphate filtrate to LiOH was 1:1.17, and the mixture was reacted at 34℃ for 7 h and dried under reduced pressure for 4 h to obtain DDP-Li.

[0039] Application Test: 0.1% DDP-Li was added to purify the residual acid produced by phosphoric acid purification to a P2O5 mass concentration of 34%, cadmium content of 30 ppm, copper content of 50 ppm, and arsenic content of 15 ppm. The mixture was stirred at 60℃ and 50 r / min for 30 min. After the reaction was complete, the purified phosphoric acid was obtained by filtration. The P2O5 mass concentration in the purified phosphoric acid was determined to be 33.79%; cadmium content was 0.91 ppm, with a removal rate of 96.97%; copper content was 0.32 ppm, with a removal rate of 98.93%; arsenic content was 2.76 ppm, with a removal rate of 81.60%; and phosphorus loss was 0.39%.

[0040] Example 5 DDP Synthesis: Take n-decyl alcohol (C 10 0.9 mol of linear (straight-chain) total substance was added to 0.252 mol of P2S5 at a ratio of 4:1.12 (alcohol to P2S5), and the reaction was carried out at 120 °C for 4 h to obtain dialkyl dithiophosphate; the molar ratio of dialkyl dithiophosphate to Al2O3 was 1:1.12, and the reaction was carried out at 110 °C for 4 h, followed by drying under reduced pressure for 1.5 h to obtain DDP-Al.

[0041] Application Test: 0.8% DDP-Al was added to concentrated phosphoric acid with a P2O5 mass concentration of 49%, cadmium content of 100 ppm, copper content of 60 ppm, and arsenic content of 35 ppm. The mixture was stirred at 75℃ and 1250 r / min for 5 min. After the reaction was complete, the purified concentrated phosphoric acid was obtained by filtration. The P2O5 mass concentration in the purified phosphoric acid was determined to be 48.66%; the cadmium content was 1.94 ppm, with a removal rate of 98.06%; the copper content was 0.10 ppm, with a removal rate of 99.83%; the arsenic content was 4.86 ppm, with a removal rate of 86.11%; and the phosphorus loss was 0.05%.

[0042] Example 6 DDP Synthesis: Take 3.0 mol of n-dodecyl alcohol (C 12 The linear chain was added to a three-necked flask, and 0.81 mol of P2S5 was added at a molar ratio of n-dodecyl alcohol to P2S5 of 4:1.08. The mixture was stirred at 90 °C for 6.5 h to obtain dialkyl dithiophosphate. Dialkyl dithiophosphate was mixed with Li2O at a molar ratio of 1:1.2 and reacted at 60 °C for 6.5 h. After drying under reduced pressure for 2 h, the mixture was filtered while hot to obtain DDP-Li.

[0043] Application Test: 0.4% DDP-Li was added to dilute phosphoric acid with a P2O5 concentration of 30%, cadmium content of 43 ppm, copper content of 18 ppm, and arsenic content of 5 ppm. The mixture was stirred at 75℃ and 3000 r / min for 0.01 min. After the reaction was complete, the mixture was filtered to obtain purified phosphoric acid. The P2O5 concentration in the purified phosphoric acid was determined to be 30.02%; the cadmium content was 0.8 ppm (cadmium removal rate 98.14%); the copper content was 0.04 ppm (copper removal rate 99.77%); the arsenic content was 0.84 ppm (arsenic removal rate 83.2%); and the phosphorus loss was 0.15%.

[0044] Example 7 DDP Synthesis: Take 1.8 mol of 4-isopropylundecyl alcohol (C 14 Branched chain), add 0.459 mol of P2S5 at a ratio of 4:1.02 of alcohol to P2S5, react at 115 °C for 4.5 h to obtain dialkyl dithiophosphate; dialkyl dithiophosphate and SnCl2 at a molar ratio of 1:1.14 are reacted at 80 °C for 7 h and dried under reduced pressure for 2 h to obtain DDP-Sn.

[0045] Application Test: 0.4% DDP-Sn was added to the raffinate produced from wet phosphoric acid purification with a P2O5 mass concentration of 40%, cadmium content of 20 ppm, copper content of 20 ppm, and arsenic content of 10 ppm. The mixture was stirred at 65℃ and 2100 r / min for 1 min. After the reaction was complete, the purified raffinate was obtained by filtration. The P2O5 mass concentration in the purified raffinate was determined to be 39.94%, the cadmium content was 0.84 ppm with a removal rate of 95.80%, the copper content was 0.03 ppm with a removal rate of 99.85%, the arsenic content was 1.64 ppm with a removal rate of 83.60%, and the phosphorus loss was 0.17%.

[0046] Example 8 DDP Synthesis: Take n-tetradecyl alcohol (C 14 2.0 mol of the total amount of linear (linear) alcohol was added to 0.50 mol of P2S5 at a molar ratio of 4:1. The mixture was reacted at 85 °C for 8 h and filtered to obtain dialkyl dithiophosphate filtrate. The molar ratio of the dialkyl dithiophosphate filtrate to Mo(OH)3 was 1:1.13. The mixture was reacted at 100 °C for 5.5 h and dried under reduced pressure for 2.5 h to obtain DDP-Mo.

[0047] Application test: 0.9% DDP-Mo was added to concentrated phosphoric acid with a P2O5 mass concentration of 50%, a copper content of 72 ppm, and an arsenic content of 23 ppm. The mixture was stirred at 85℃ and 50 r / min for 30 min. After the reaction was completed, the purified concentrated phosphoric acid was obtained by filtration. The P2O5 mass concentration in the purified phosphoric acid was measured to be 49.87%; the copper content was reduced to 0.43 ppm, with a removal rate of 99.40%; the arsenic content was reduced to 4.43 ppm, with a removal rate of 80.74%; and the phosphorus loss was 0.08%.

[0048] Example 9 DDP Synthesis: Take n-octadecanol (C 18 The total amount of linear (1.6 mol) alcohol was added to 0.412 mol of P2S5 at a molar ratio of 4:1.03, and the reaction was carried out at 105 °C for 5 h. The mixture was then filtered to obtain dialkyl dithiophosphate filtrate. The molar ratio of dialkyl dithiophosphate filtrate to MnCO3 was 1:1.07, and the mixture was reacted at 0 °C for 8 h. After drying under reduced pressure for 3.5 h, DDP-Mn was obtained.

[0049] Application Test: 0.4% DDP-Mn was added to the phosphoric acid purification residue to achieve a P2O5 mass concentration of 47%, a cadmium content of 45 ppm, and a copper content of 60 ppm. The mixture was stirred at 50℃ and 180 r / min for 25 min. After the reaction was complete, the purified dilute phosphoric acid was obtained by filtration. The P2O5 mass concentration in the purified phosphoric acid was determined to be 46.88%, the cadmium content was 0.51 ppm (removal rate 98.87%), the copper content was 0.22 ppm (removal rate 99.63%), and the phosphorus loss was 0.27%.

[0050] This invention mainly enhances the water solubility and reducing properties of thiophosphate by using metal cations; it replaces traditional methods of removing cadmium with inorganic sulfides such as sodium hydrosulfide, and has advantages such as being non-toxic, highly selective, and low-cost.

Claims

1. A method for deep removal of heavy metals in a wet-process phosphoric acid process, characterized in that, Includes the following steps: (1) Using a straight-chain or branched alcohol as a raw material, reacting it with phosphorus pentasulfide to obtain dialkyl dithiophosphate, and then reacting it with metal oxides, hydroxides or salts to synthesize dialkyl dithiophosphate DDP; (2) The DDP is mixed and reacted with wet phosphoric acid slurry containing heavy metals, dilute phosphoric acid, concentrated phosphoric acid or residual acid produced by wet phosphoric acid purification, wherein the wet phosphoric acid slurry containing heavy metals is a slurry containing heavy metals and phosphogypsum, and heavy metal complex precipitates are generated. After the reaction is completed, purified phosphoric acid is obtained by separation, thereby achieving the removal of heavy metals.

2. The method for deep removal of heavy metals in a wet-process phosphoric acid process according to claim 1, characterized in that, The alcohol in step (1) has a carbon chain length of 4-18, and the molar ratio of the alcohol to phosphorus pentasulfide is 4:1-1.

12.

3. A method for deep removal of heavy metals in a wet-process phosphoric acid process according to claim 1 or 2, characterized in that, The synthesis steps of DDP in step (1) specifically include: (i) stirring and reacting alcohol with phosphorus pentasulfide at 80-120°C for 4-8 hours to obtain dialkyl dithiophosphate; (ii) reacting the dialkyl dithiophosphate with metal oxides, hydroxides or salts at 0-120°C for 4-8 hours, drying under reduced pressure for 1-4 hours after the reaction is completed, and filtering while hot to obtain dialkyl dithiophosphate DDP; the metal oxides, hydroxides or salts are one or more of Fe, Mn, Zn, Al, Li, Sn, Mo.

4. The method for deep removal of heavy metals in a wet-process phosphoric acid process according to claim 3, characterized in that, The molar ratio of the dialkyl dithiophosphate filtrate to the metal oxide, hydroxide, or salt is 1:1.05-1.

20.

5. A method for deep removal of heavy metals in a wet-process phosphoric acid process according to claim 4, characterized in that, First, mix DDP and water in step (2) at a ratio of 1:0-100 until homogeneous, and then add it to one or more places in the wet process phosphoric acid production line, such as phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid produced by phosphoric acid purification. The amount of DDP added is 0.1%-1% of the mass of phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid produced by phosphoric acid purification. The reaction temperature is 50-90℃, the stirring rate is 50-3000r / min, and the reaction time is 0.01-30 minutes.

6. The method for deep removal of heavy metals in a wet-process phosphoric acid process according to claim 1, characterized in that, The P2O5 content of the phosphoric acid slurry, dilute phosphoric acid, concentrated phosphoric acid, or residual acid produced from phosphoric acid purification in the wet phosphoric acid production line described in step (2) is as follows: 5%-20% in phosphoric acid slurry; 18%-36% in dilute phosphoric acid; 36%-65% in concentrated phosphoric acid; and 18%-46% in residual acid produced from phosphoric acid purification.

7. The method for deep removal of heavy metals in a wet-process phosphoric acid process according to claim 1, characterized in that, The heavy metal content after heavy metal removal by wet phosphoric acid is ≤2ppm, copper content ≤0.5ppm, arsenic content ≤5ppm, and phosphorus loss ≤0.5%.