Wet-process phosphoric acid deep cadmium removal method based on dialkyl dithiophosphate

Abstract

The invention relates to a wet-process phosphoric acid deep cadmium removal method based on dialkyl dithiophosphate, which comprises the following steps: (1) taking mixed alcohol of one or more than one straight-chain or branched-chain alcohol as a raw material, reacting with phosphorus pentasulfide, filtering to obtain thiophosphoric acid filtrate, and reacting the thiophosphoric acid filtrate with metal oxide to synthesize dialkyl dithiophosphate (DDP); (2) roughly dividing the conventional wet-process phosphoric acid process into phosphorite pretreatment, phosphorite decomposition, dilute acid sedimentation, dilute acid concentration and concentrated acid sedimentation; and mixing the DDP with the cadmium-containing wet-process phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid for reaction to generate hydrophobic cadmium complex precipitate, and filtering after the reaction is finished to obtain purified phosphoric acid, thereby realizing removal of cadmium. According to the method, no HS gas is generated, and occupational hazards are avoided; raw material cost is low; the cadmium removal rate is gt; the phosphorus loss is as low as 0.5%; the method is suitable for wet-process phosphoric acid slurry, dilute phosphoric acid and concentrated phosphoric acid and can be directly integrated to an existing production line, the cadmium content of the cadmium-containing phosphoric acid after cadmium removal is smaller than or equal to 2 ppm, and the phosphorus loss rate is smaller than or equal to 3%.

Description

Technical Field

[0001] This invention relates to a method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate, belonging to the field of wet phosphoric acid purification. Background Technology

[0002] Phosphoric acid, a core raw material for phosphate fertilizers, food additives, and new energy materials such as lithium iron phosphate batteries, relies primarily on phosphate rock processing for its production. However, cadmium (Cd), a heavy metal commonly found in phosphate rock, is a key source of pollution. The wet-process phosphoric acid production line is the mainstream route due to its cost advantage, but 70%–80% of the cadmium enters downstream products with the phosphoric acid, leading to soil cadmium pollution through phosphate fertilizer application and ultimately threatening human health through the food chain (such as causing Itai-itai disease and kidney damage). Increasingly stringent global regulations (e.g., the EU requires phosphate products to have a cadmium content below 60 mg / kg P2O5 and promotes "low cadmium labeling" certification), coupled with the surging demand for high-purity phosphoric acid from the new energy industry, are forcing the industry to innovate cadmium removal technologies. However, existing technologies face significant bottlenecks: Although precipitation methods (such as sodium sulfide method) can combine cadmium ions to form precipitates, they easily produce highly toxic H2S gas, and the fine particulate precipitates make filtration difficult. The gypsum co-crystallization method removes cadmium by co-precipitation with calcium sulfate lattice, but requires excessive sulfuric acid, resulting in phosphorus loss of up to 10% to 30%, and produces highly viscous cadmium-containing sludge that is difficult to treat. Solvent extraction methods have low separation efficiency and are complex to operate; Although adsorption methods (such as modified biochar) have high adsorption capacity (up to 145.15 mg / g), they are difficult to integrate into existing production lines.

[0003] Recent technological trends have focused on process integration and resource utilization. For example, optimizing the amount of sulfuric acid added and the temperature (50–120°C) can improve the cadmium removal rate to >90% and reduce phosphorus loss, or utilizing the byproduct ferrous sulfate to produce lithium iron phosphate to achieve cadmium resource recycling. However, deep cadmium removal (e.g., <2 ppm), sludge reduction, and industrial compatibility remain core challenges. There is an urgent need to develop innovative solutions that are cost-effective and adaptable to existing production lines, such as dynamic control (e.g., gypsum lattice loading optimization), nano-adsorption enhancement, and intelligent parameter optimization. Summary of the Invention

[0004] The purpose of this invention is to overcome the limitations of existing cadmium removal technologies for phosphoric acid, such as phosphogypsum co-crystallization and solvent extraction, which suffer from high phosphorus loss, difficult sludge treatment, or complex processes, making it difficult to meet the requirements of low cadmium (<2 ppm), low consumption, and easy industrialization. This invention provides a wet-process deep cadmium removal method for phosphoric acid based on dialkyl dithiophosphate (DDP). DDP is used as the cadmium removal agent, and the DDP reacts with Cd in the phosphoric acid. 2+The specific chelation process generates a stable hydrophobic precipitate (Cd-DDP), which combines high selectivity with low phosphorus loss, avoiding P2O5 loss caused by excessive sulfuric acid use. Simultaneously, the generated precipitate particles are large and easily settle, significantly improving the difficulty of filtering traditional cadmium-containing sludge and reducing subsequent treatment costs. This invention achieves highly efficient cadmium removal (cadmium removal rate >95%) and improved phosphoric acid purity through a one-step reaction, without requiring complex equipment or high-temperature conditions. It can be directly integrated into existing wet-process phosphoric acid production lines and is suitable for wet-process phosphoric acid slurries, dilute phosphoric acid, or concentrated phosphoric acid, providing the industry with an economical, environmentally friendly, and highly adaptable new deep cadmium removal pathway.

[0005] The technical solution of this invention is as follows: 1. A method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate, characterized by comprising the following steps: (1) Using a mixture of two or more straight-chain or branched alcohols as raw materials, react with phosphorus pentasulfide and filter to obtain thiophosphate filtrate. The thiophosphate filtrate is then reacted with metal oxides to synthesize dialkyl dithiophosphate (DDP). (2) Conventional wet phosphoric acid process is roughly divided into phosphate rock pretreatment (including ore mining, crushing and flotation), phosphate rock decomposition (sulfuric acid and phosphate rock powder are stirred and reacted at 80-85℃ for 30min-1h to obtain wet phosphoric acid slurry), dilute acid precipitation (the slurry after phosphate rock decomposition is filtered and separated to obtain dilute phosphoric acid, which is slowly stirred at 60-70℃ and allowed to settle naturally), dilute acid concentration (the clarified dilute phosphoric acid is concentrated to obtain crude concentrated phosphoric acid product) and concentrated acid precipitation (the dilute phosphoric acid is slowly stirred at about 70-80℃ and allowed to settle naturally to remove impurities, and the final supernatant is the concentrated phosphoric acid product); the DDP is mixed and reacted with cadmium-containing wet phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid to generate hydrophobic cadmium complex precipitate, and after the reaction is completed, it is filtered to obtain purified phosphoric acid, thus achieving the removal of cadmium.

[0006] The carbon chain length of the mixed alcohol in step (1) is 4-18, and the molar ratio of the mixed alcohol to phosphorus pentasulfide is 4:1-1.12.

[0007] The synthesis of DDP in step (1) specifically includes: (i) stirring and reacting a mixed alcohol with phosphorus pentasulfide at 90-100°C for 4-8 hours, and filtering to obtain a thiophosphate filtrate; (ii) reacting the thiophosphate filtrate with a metal oxide at 85-97°C for 4-8 hours, drying under reduced pressure for 1-4 hours after the reaction is complete, and filtering while hot to obtain dialkyl dithiophosphate DDP; the metal oxide is a mixture of one or more oxides of Na, Mg, Ca, Zn, and Fe.

[0008] The molar ratio of the thiophosphate filtrate to the metal oxide is 1:1.05-1.20.

[0009] In step (2), the DDP is added to the wet phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid in the wet phosphoric acid production line. The amount of DDP added is 0.1%-1% of the mass of the wet phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid. The reaction temperature is 60-85℃, the stirring rate is 80-300r / min, and the reaction time is 10-30 minutes.

[0010] The cadmium content after cadmium removal by wet phosphoric acid is ≤2ppm, and the phosphorus loss rate is ≤3%.

[0011] Existing phosphate chemical processes primarily remove cadmium from phosphoric acid using inorganic sulfides such as sodium hydrosulfide. While this method is relatively inexpensive, sodium hydrosulfide can generate large amounts of toxic hydrogen sulfide gas during use, posing a significant health threat to factory workers. This invention primarily removes cadmium from phosphoric acid using DDP (dihydrogen sulfide), eliminating the threat of hydrogen sulfide during use. Traditional organic sulfides use single alcohols, which generally have low removal efficiency and high synthesis costs. This method uses mixed alcohols, costing less than half that of traditional single alcohols, while achieving comparable or slightly higher performance than traditional organic thiophosphates. Furthermore, by altering the water solubility and reducing properties of thiophosphates through the properties of different metal cations, the removal capacity of organic thiophosphates for cadmium ions is enhanced. This method also offers advantages such as being non-toxic, highly selective, and low-cost.

[0012] Beneficial effects of this invention: 1. Safety: No H2S gas is generated, avoiding occupational hazards; 2. Economic efficiency: The cost of mixed alcohols is only 50% of that of traditional single alcohols; 3. 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.

[0013] 4. Enhanced hydrophobic effect: Long-chain alkyl groups form micelles to coat heavy metal precipitates, reducing phosphorus loss caused by phosphorus entrainment in the slag phase. The phosphorus loss rate of short-chain alkyl dithiophosphate process is ≥2%, while that of long-chain dithiophosphate can be reduced to ≤0.5%. Detailed Implementation

[0014] The present invention will be further described in conjunction with specific embodiments.

[0015] Example 1 DDP Synthesis: Isooctyl alcohol (C8 straight chain) and n-butanol (C4 straight chain) were mixed at a mass ratio of 3:1, with a total amount of 0.5 mol of mixed alcohol. The mixture was added to a three-necked flask, and P2S5 was added at a molar ratio of 4:1, with an amount of 0.125 mol of P2S5. The mixture was stirred at 95 °C for 6 h, and filtered to obtain a thiophosphate filtrate. The thiophosphate filtrate was mixed with ZnO at a molar ratio of 1:1.10, and reacted at 90 °C for 5 h. After drying under reduced pressure for 2 h, the mixture was filtered while hot to obtain DDP-Zn.

[0016] Application test: Take wet phosphoric acid slurry with cadmium content of 50 ppm, add 0.5% DDP-Zn by mass, stir at 85℃ and 200 r / min for 20 min, filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid is determined to be 1.8 ppm, the cadmium removal rate is 96.4%, and the phosphorus loss is 0.9%.

[0017] Example 2 DDP Synthesis: Mixed isoamyl alcohol (C5 branched) and isodecyl alcohol (C5 branched) 10 Branched alcohols were mixed at a mass ratio of 1:1, with a total amount of 0.8 mol of mixed alcohol. P2S5 was added at a ratio of 4:1.08 (P2S5 amount of 0.216 mol), and the mixture was reacted at 98℃ for 4 h. The mixture was then filtered to obtain a thiophosphoric acid filtrate. The thiophosphoric acid filtrate was then reacted with a MgO-CaO mixture (where the mass ratio of MgO to CaO was 1:1) at a molar ratio of 1:1.15 at 85℃ for 7 h, and then dried under reduced pressure for 3 h to obtain DDP-Mg / Ca.

[0018] Application test: Add 0.3% DDP-Mg / Ca to the wet phosphoric acid slurry with a cadmium content of 65 ppm, stir at 65℃ and 150 r / min for 25 min, and filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid was measured to be 2.1 ppm, the cadmium removal rate was 96.8%, and the phosphorus loss was 1.2%.

[0019] Example 3 DDP Synthesis: Hexanol (C6 straight chain) and isooctol (C8 branched chain) were mixed at a mass ratio of 2:1, with a total amount of 1.0 mol of mixed alcohol. P2S5 was added at a molar ratio of 4:1.1, with an amount of 0.275 mol of P2S5. The mixture was reacted at 92℃ for 7 h, and filtered to obtain a thiophosphate filtrate. The thiophosphate filtrate was reacted with Fe2O3 at a molar ratio of 1:1.20 at 97℃ for 4 h, and dried under reduced pressure for 1.5 h to obtain DDP-Fe.

[0020] Application test: Add 1.0% DDP-Fe to the wet phosphoric acid slurry with a cadmium content of 80 ppm, stir at 80℃ and 300 r / min for 15 min, and filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid was measured to be reduced to 1.2 ppm, the cadmium removal rate was 98.5%, and the phosphorus loss was 2.5%.

[0021] Example 4 DDP Synthesis: A mixture of n-butanol (C4 straight chain) and isoamyl alcohol (C5 branched chain) was prepared at a mass ratio of 5:1, with a total amount of 0.6 mol of mixed alcohol. P2S5 was added at a molar ratio of 4:1.12, with an amount of 0.168 mol of P2S5. The mixture was reacted at 100 °C for 5 h, and filtered to obtain a thiophosphate filtrate. The thiophosphate filtrate was reacted with Na2O at a molar ratio of 1:1.05 at 88 °C for 6 h, and dried under reduced pressure for 4 h to obtain DDP-Na.

[0022] Application test: Add 0.1% DDP-Na to the wet phosphoric acid slurry with a cadmium content of 30 ppm, stir at 60℃ and 80 r / min for 30 min, and filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid is measured to be 0.9 ppm, the cadmium removal rate is 97.0%, and the phosphorus loss is 0.5%.

[0023] Example 5 DDP synthesis: using isooctyl alcohol and n-decanol (C 10 Straight-chain alcohol and isoamyl alcohol were mixed in a mass ratio of 2:2:1, with a total amount of 1.2 mol of mixed alcohol. P2S5 was added at a ratio of 4:1.05 (P2S5 amount of 0.315 mol), and the mixture was reacted at 90℃ for 8 h. The mixture was then filtered to obtain a thiophosphoric acid filtrate. The thiophosphoric acid filtrate was then reacted with a ZnO-Fe2O3 mixture (ZnO and Fe2O3 were mixed in a mass ratio of 2:1) at a molar ratio of 1:1.18. The mixture was then reacted at 93℃ for 8 h and dried under reduced pressure for 1 h to obtain DDP-Zn / Fe.

[0024] Application test: Add 0.8% DDP-Zn / Fe to the wet phosphoric acid slurry with a cadmium content of 100 ppm, stir at 75℃ and 250 r / min for 10 min, and filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid is measured to be 3.0 ppm, the cadmium removal rate is 97.0%, and the phosphorus loss is 1.8%.

[0025] Example 6 DDP Synthesis: Take n-dodecyl alcohol (C 12 Straight-chain) and isohexadecanol (C 16 Branched ZnO was mixed at a mass ratio of 3.5:1, with a total amount of 0.9 mol of mixed alcohol. The mixture was added to a three-necked flask, and P2S5 was added at a molar ratio of 4:1.10, with a P2S5 amount of 0.248 mol. The mixture was stirred at 90 °C for 7 h, and filtered to obtain a thiophosphate filtrate. The thiophosphate filtrate was mixed with ZnO at a molar ratio of 1:1.15, and reacted at 92 °C for 4.5 h. After drying under reduced pressure for 3 h, the mixture was filtered while hot to obtain DDP-Zn.

[0026] Application test: Take wet phosphoric acid slurry with 40 ppm cadmium content, add 0.4% DDP-Zn by mass, stir at 75℃ and 220r / min for 18 min, filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid is 0.8 ppm, the cadmium removal rate is 98%, and the phosphorus loss is 0.5%.

[0027] Example 7 DDP Synthesis: Mixed Isotetradecyl Alcohol (C 14 Branched chain) and isooctadecyl alcohol (C 18 Branched alcohols were mixed at a mass ratio of 1:1.5, with a total amount of 1.0 mol of mixed alcohol. P2S5 was added at a ratio of 4:1.12 (0.28 mol of P2S5). The mixture was reacted at 95°C for 5 h, and filtered to obtain a thiophosphate filtrate. The thiophosphate filtrate was reacted with CaO at a molar ratio of 1:1.20 at 95°C for 5 h, and dried under reduced pressure for 2 h to obtain DDP-Ca.

[0028] Application test: 0.4% DDP-Mg / Ca was added to dilute phosphoric acid with a cadmium content of 70 ppm, stirred at 65℃ and 210 r / min for 20 min, and filtered after the reaction was completed to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid was measured to be 3.2 ppm, the cadmium removal rate was 95.4%, and the phosphorus loss was 1.6%.

[0029] Example 8 DDP Synthesis: Take n-tetradecyl alcohol (C 14 Straight-chain) and isohexadecanol (C 16 Branched alcohols were mixed at a mass ratio of 3:1, with a total amount of 0.8 mol. P2S5 was added at a molar ratio of 4:1 (0.20 mol) and reacted at 96 °C for 6 h. The mixture was then filtered to obtain a thiophosphoric acid filtrate. The thiophosphoric acid filtrate was then reacted with Fe2O3 at a molar ratio of 1:1.15 at 90 °C for 5 h and dried under reduced pressure for 2.5 h to obtain DDP-Fe.

[0030] Application test: 0.9% DDP-Fe was added to concentrated phosphoric acid with a cadmium content of 68 ppm, and stirred at 85℃ and 250 r / min for 20 min. After the reaction was completed, the purified concentrated phosphoric acid was obtained by filtration. The cadmium content in the purified phosphoric acid was measured to be reduced to 2.1 ppm, the cadmium removal rate was 96.9%, and the phosphorus loss was 1.8%.

[0031] Example 9 DDP Synthesis: Mixed n-dodecyl alcohol (C 12 Straight-chain) and isooctadecyl alcohol (C 18Branched alcohols were mixed at a mass ratio of 1:2, with a total amount of 0.9 mol of mixed alcohol. P2S5 was added at a molar ratio of 4:1.12, with an amount of 0.252 mol of P2S5. The mixture was reacted at 95 °C for 5 h, and filtered to obtain a thiophosphoric acid filtrate. The thiophosphoric acid filtrate was reacted with Na2O at a molar ratio of 1:1.10 at 85 °C for 8 h, and dried under reduced pressure for 4 h to obtain DDP-Na.

[0032] Application test: Add 0.4% DDP-Na to the wet phosphoric acid slurry with a cadmium content of 45 ppm, stir at 65℃ and 180 r / min for 25 min, and filter after the reaction is complete to obtain purified dilute phosphoric acid; the cadmium content in the purified phosphoric acid is measured to be 0.5 ppm, the cadmium removal rate is 98.9%, and the phosphorus loss is 0.4%.

[0033] Example 10 DDP Synthesis: Using n-tetradecyl alcohol (C 14 Straight-chain), isooctadecyl alcohol (C 18 Branched chain), n-dodecyl alcohol (C 12 The straight-chain alcohols were mixed at a mass ratio of 3:2:1, with a total amount of 1.0 mol of mixed alcohol. P2S5 was added at a ratio of 4:1.10 (P2S5 amount of 0.275 mol), and the mixture was reacted at 90℃ for 7 h. The mixture was then filtered to obtain a thiophosphoric acid filtrate. The thiophosphoric acid filtrate was then reacted with a ZnO-Fe2O3 mixture (ZnO and Fe2O3 were mixed at a mass ratio of 1:1) at a molar ratio of 1:1.05. The mixture was then reacted at 97℃ for 6 h and dried under reduced pressure for 2 h to obtain DDP-Zn / Fe.

[0034] Application test: 1.0% DDP-Zn / Fe was added to concentrated phosphoric acid with a cadmium content of 80 ppm, stirred at 80℃ and 300 r / min for 12 min, and filtered after the reaction was completed to obtain purified concentrated phosphoric acid; the cadmium content in the purified phosphoric acid was measured to be 2.2 ppm, the cadmium removal rate was 97.25%, and the phosphorus loss was 2.0%.

[0035] The main feature of this invention is that the raw material is changed from the traditional single alcohol to a lower-priced mixed alcohol, and the water solubility and reducing properties of the thiophosphate are enhanced by changing the metal cations in the thiophosphate. It replaces the traditional inorganic sulfide cadmium removal method such as sodium hydrosulfide, and has the advantages of being non-toxic, highly selective and low-cost.

Claims

1. A method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate, characterized in that, Includes the following steps: (1) Using a mixture of two or more straight-chain or branched alcohols as raw materials, react with phosphorus pentasulfide and filter to obtain thiophosphate filtrate. The thiophosphate filtrate is then reacted with metal oxides to synthesize dialkyl dithiophosphate (DDP). (2) Conventional wet phosphoric acid process is generally divided into phosphate rock pretreatment, phosphate rock decomposition, dilute acid precipitation, dilute acid concentration and concentrated acid precipitation; the DDP is mixed with cadmium-containing wet phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid to generate hydrophobic cadmium complex precipitate, and after the reaction is completed, it is filtered to obtain purified phosphoric acid, thereby achieving the removal of cadmium.

2. The method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate according to claim 1, characterized in that, The carbon chain length of the mixed alcohol in step (1) is 4-18, and the molar ratio of the mixed alcohol to phosphorus pentasulfide is 4:1-1.

12.

3. The method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate according to claims 1 and 2, characterized in that, The synthesis of DDP in step (1) specifically includes: (i) stirring and reacting a mixed alcohol with phosphorus pentasulfide at 90-100°C for 4-8 hours, and filtering to obtain a thiophosphate filtrate; (ii) reacting the thiophosphate filtrate with a metal oxide at 85-97°C for 4-8 hours, drying under reduced pressure for 1-4 hours after the reaction is complete, and filtering while hot to obtain dialkyl dithiophosphate DDP; the metal oxide is a mixture of one or more oxides of Na, Mg, Al, Ca, Zn, and Fe.

4. The method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate according to claim 3, characterized in that, The molar ratio of the thiophosphate filtrate to the metal oxide is 1:1.05-1.

20.

5. The method for deep cadmium removal from wet phosphoric acid based on dialkyl dithiophosphate according to claim 1, characterized in that, In step (2), the DDP is added to the wet phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid in the wet phosphoric acid production line. The amount of DDP added is 0.1%-1% of the mass of the wet phosphoric acid slurry, dilute phosphoric acid or concentrated phosphoric acid. The reaction temperature is 60-80℃, the stirring rate is 80-300r / min, and the reaction time is 10-30 minutes.