Method for recovering phosphate radicals, fluorine ions and sulfate radicals from ardealite washing water step by step

Abstract

The invention provides a method and system for recovering phosphate radicals, fluorine ions and sulfate radicals from phosphogypsum washing water step by step, and belongs to the technical field of industrial wastewater resourceful treatment. The method comprises the following steps: (1) adjusting the pH value of the wastewater to 3.0 + / -0.5, adding ferric sulfate according to the molar ratio of Fe to PO4 < 3-> of (1.0-1.5): 1, and selectively precipitating phosphate radicals to generate iron phosphate with the purity of more than or equal to 97%; (2) adjusting the pH value of the first filtrate to 4.7 + / -0.5, adding calcium fluoride seed crystals with the particle size of 50-200mm and CaCl2 (the molar ratio of Ca to F <-> is 0.5: 1-0.6: 1), and carrying out induced crystallization to recover calcium fluoride with the purity of more than or equal to 96%; and (3) adding Ca (OH) 2 (Ca: SO4 < 2-> molar ratio of 1.0: 1-1.2: 1) into the second filtrate, and directionally precipitating sulfate radicals to generate calcium sulfate dihydrate with the purity of more than or equal to 98%. According to the method, the pH and the precipitation sequence are controlled step by step, so that the problem of polyanion competition is solved, the recovery rates of phosphate radical, fluorine ion and sulfate radical respectively reach 98.8%, 96.3% and 97.5%, the content of PO4 < 3-> in purified water is less than or equal to 10 mg.L <-1 >, the content of F <-1 > is less than or equal to 5 mg.L <-1 >, and the content of SO4 < 2-> in purified water is less than or equal to 100 mg.L <-1 >, and the purified water can be directly recycled.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial wastewater resource treatment, specifically a method for recovering phosphate (PO4) from phosphogypsum washing water in steps. 3- ), fluoride ion (F-) and sulfate (SO4 2- ) method and system to achieve efficient separation and high-value utilization of multivalent anions. Background Art

[0002] Phosphogypsum is the main by-product in the production of wet-process phosphoric acid. Every ton of phosphoric acid produced produces about 4.5-5 tons of phosphogypsum. Its washing wastewater has a complex composition, a pH value of about 2, and contains a high concentration of PO4 3- (200-800mg / L), F-(50-300mg / L) and SO4 2- (2000-5000mg / L). Existing treatment technologies have the following problems:

[0003] 1. Resource waste and environmental risks

[0004] Existing technologies typically use neutralization precipitation methods (such as lime neutralization) to co-precipitate phosphate and fluoride ions, resulting in difficult-to-treat fluoride-containing phosphogypsum slag, which renders phosphorus and fluoride resources unrecoverable. It is estimated that over 30,000 tons of recoverable P2O5 are lost annually through phosphogypsum washing water in my country. Furthermore, high sulfate concentrations in wastewater can easily form acid mine drainage in the natural environment, causing long-term pollution to soil and water bodies.

[0005] 2. Separation technology bottleneck

[0006] (1) Ion interference problem: Phosphate and fluoride ions compete with each other during conventional precipitation. Existing processes (such as the fluoride-phosphorus step-by-step precipitation method adopted in patent CN110642348A) require the addition of excessive calcium salts, resulting in low purity of the precipitate and difficulty in subsequent sulfate recovery.

[0007] (2) Process coupling defects: The "phosphate-fluoride co-precipitation + evaporation crystallization of sodium sulfate" solution proposed in the patent CN112499716A does not solve the problem of calcium fluoride encapsulating calcium phosphate, resulting in a phosphoric acid recovery rate of less than 60% and excessive evaporation energy consumption;

[0008] (3) Limitations of end-of-pipe treatment: The "chemical precipitation + flocculation" combined process (such as CN114956344A) currently used in the industry can only achieve pollutant discharge standards, but fails to realize the resource utilization of components. Summary of the Invention

[0009] To address the deficiencies of the prior art, the present invention provides a method for recovering phosphate, fluoride and sulfate from phosphogypsum washing water in steps. Through the three-step process of "phosphate priority precipitation - fluoride ion induced crystallization - sulfate directional precipitation", combined with pH gradient control and seed crystal strengthening, PO4 3- 、F-、SO4 2- The recovery rate is greater than 95%, and the purity of the recovered product is greater than 96%.

[0010] The technical solutions provided by the present invention are as follows:

[0011] A method for recovering phosphate, fluoride and sulfate from phosphogypsum washing water in steps, characterized by comprising the following steps:

[0012] Step 1: Adjust the pH of the phosphogypsum washing water to 3.0±0.5, add ferric sulfate, stir and react for a certain period of time, and separate the solid and liquid to obtain ferric phosphate and the first filtrate;

[0013] In step 1:

[0014] The ferric sulfate is Fe2(SO4)3·xH2O (x=0-9), and the dosage of ferric sulfate is based on the ratio of Fe to PO4. 3- Molar ratio (1.0-1.5): 1 addition;

[0015] PO4 in the first filtrate 3- Residual concentration ≤10mg·L -1 ;

[0016] Step 2: Adjust the pH of the first filtrate to 4.7±0.5, add calcium fluoride seed crystals and calcium chloride, stir and react for a certain time, and separate the solid and liquid to obtain calcium fluoride and the second filtrate;

[0017] In step 2:

[0018] The purity of the calcium fluoride seed crystals is ≥95%, the particle size of the calcium fluoride seed crystals is 50-200 mm, and the amount of the seed crystals added is 1 / 5-1 / 2 of the mass of the fluoride ions in the filtrate;

[0019] The dosage of the calcium chloride is such that the molar ratio of Ca to F is (0.5-0.6):1;

[0020] The reaction endpoint is when the concentration of F- in the second filtrate is ≤5 mg·L -1 For the judgment criteria;

[0021] Step 3: Add calcium hydroxide to the second filtrate, stir and react for a certain period of time, and separate the solid and liquid to obtain calcium sulfate and purified water;

[0022] In step 3:

[0023] The calcium hydroxide is industrial grade slaked lime, and the dosage is based on the ratio of Ca and SO42- Molar ratio (1.0-1.2): 1 addition;

[0024] Purify SO4 in water 2- Concentration ≤ 100 mg·L -1 .

[0025] Specifically, the pH adjusting agent in step 1-2 is a combination of an inorganic acid such as dilute sulfuric acid, hydrochloric acid, nitric acid, and an inorganic base such as sodium hydroxide and sodium carbonate;

[0026] Specifically, the stirring reaction time of steps 1-3 is 0.5-3h, the stirring speed is 150-400rpm, and the reaction temperature is 10-40°C;

[0027] Specifically, the solid-liquid separation in steps 1-3 is performed by suction filtration, filter press, screw dehydration or centrifugation;

[0028] Specifically, the resources prepared by the method are products, including: iron phosphate with a purity of ≥97%, which is used to prepare lithium-ion battery positive electrode materials, food / feed additives, catalysts, ceramic glazes, etc.; calcium fluoride with a purity of ≥96%, which is used in the fields of fluorine chemical raw materials, metallurgy and glass manufacturing; calcium sulfate with a purity of ≥98%, which is used as an industrial raw material for the manufacture of cement, calcium sulfate hemihydrate and sulfuric acid;

[0029] Specifically, the method includes a phosphogypsum washing water treatment system, characterized in that it includes: a pH adjustment device, a reactor and a solid-liquid separation device connected in sequence; the reactor is divided into a ferrous phosphate precipitation unit, a calcium fluoride crystallization unit and a calcium sulfate precipitation unit; the calcium fluoride crystallization unit is provided with a seed crystal dosing device, and the seed crystal dosing device includes a particle size grading screen.

[0030] In the above technical solution:

[0031] Step 1: Selective precipitation and recovery of phosphate

[0032] This step achieves efficient and selective recovery of phosphate by precisely controlling the reaction conditions:

[0033] Reaction mechanism: Based on Fe 3+ With PO4 3- The specific precipitation reaction of FePO4 is used to generate Fe 3+ With F-, SO4 2- Solubility difference: At 25°C, the solubility of FePO4 is 0.07 mg / L, which is much lower than 910 mg / L of FeF3 and 7200 mg / L of Fe2(SO4)3, achieving preferential separation of phosphate.

[0034] pH control: The system pH is strictly controlled within the range of 3±0.5, mainly because under this condition, more than 98% of PO43- Precipitate in the form of FePO4 while avoiding large amounts of Fe 3+ Hydrolysis generates Fe(OH)3 precipitate, thus ensuring the purity of iron phosphate ≥97%.

[0035] Advantages of this method: Compared with the traditional calcium salt precipitation method, it avoids the generation of impurities such as Ca3(PO4)2.

[0036] Step 2: Fluoride ion induced crystallization recovery

[0037] This step achieves high-purity recovery of fluoride ions through seed induction:

[0038] Technical principle: 50-200mm calcium fluoride seed crystals (purity ≥95%) are added in advance to provide heterogeneous nucleation sites; CaCl2 is added according to the ratio of Ca:F-=(0.5-0.6):1 to induce the fluoride in the solution to grow directional on the seed crystal surface.

[0039] Key parameters: pH control 4.7 ± 0.5: below 4.2, CaF2 dissolution increases (Ksp = 4 × 10 -11 ), when it is higher than 5.2, the risk of CaSO4 co-precipitation increases; the seed crystal dosage is 20-50% of the mass of F- in the filtrate to ensure that the crystal growth rate is greater than the secondary nucleation rate.

[0040] Effect: The average particle size of the obtained calcium fluoride crystals is increased by 3-5 times (compared with the seedless process), and the purity is increased to ≥96%.

[0041] Step 3: Sulfate ion directional precipitation recovery

[0042] This step achieves clean conversion of sulfate:

[0043] Reaction characteristics: PO4 has been removed in the previous step 3- and F-interference, the system is mainly SO4 2- With Ca 2+ Single precipitation reaction (CaSO4·2H2O); according to Ca:SO4 2- =(1.0-1.2):1 Add Ca(OH)2 to avoid the introduction of impurities by excessive calcium ions.

[0044] The beneficial effects of the present invention are:

[0045] (1) Efficient separation and resource utilization of multiple components

[0046] Through the gradient separation process of "phosphate priority precipitation-fluoride ion induced crystallization-sulfate directional precipitation", the efficient conversion of three target components is achieved: phosphate is converted into iron phosphate, fluoride ion is converted into calcium fluoride, and sulfate is converted into calcium sulfate dihydrate. The waste conversion rate in traditional processes reaches more than 95%, and the resource value is increased by 3-5 times.

[0047] (2) Good recycling performance

[0048] The examples show that the phosphate recovery rate is ≥98.2%, which is 38.2 percentage points higher than that of the conventional process; the fluoride ion recovery rate is ≥95.7%, and the seed crystal induction increases the recovery rate by 17.7%; and the sulfate recovery rate is ≥96.8%.

[0049] (3) Deep purification effect of wastewater

[0050] Key indicator of final effluent: Phosphate residue ≤ 10 mg·L -1 , compliance rate 100%; fluoride ion concentration ≤ 5mg·L -1 , lower than the first-level standard of GB 8978-1996; sulfate content ≤100mg·L -1 , meeting the requirements of HG / T3923-2007 circulating cooling water.

[0051] (4) Comprehensive technical and economic advantages

[0052] Compared with the evaporation crystallization method, energy consumption is reduced by 52%; precise dosing avoids overdosage, and chemical consumption is reduced by 45%; high-purity precipitates are directly recycled, and sludge is reduced by 80%. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 The present invention provides a flow chart for recovering phosphate, fluoride and sulfate in steps from phosphogypsum washing water;

[0054] Figure 2 is the XRD characterization spectrum of the product iron phosphate in step 1;

[0055] Figure 3 is the XRD characterization spectrum of calcium fluoride, the product of step 2;

[0056] Figure 4 This is the XRD characterization spectrum of calcium sulfate dihydrate, the product of step 3. DETAILED DESCRIPTION

[0057] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0058] Unless otherwise specified, the test methods used in the examples are conventional methods; the materials and reagents used are commercially available reagents and materials unless otherwise specified.

[0059] Example

[0060] A method for recovering phosphate, fluoride and sulfate from phosphogypsum washing water in steps, comprising the following steps:

[0061] Take the phosphogypsum washing wastewater, pH = 2, the initial wastewater phosphate, fluoride and sulfate concentrations: PO4 3 -500mg / L, F - 150mg / L, SO4 2- 3000mg / L.

[0062] Step 1: Selective precipitation of phosphate

[0063] Take 1L of phosphogypsum washing water, adjust the water sample pH to 3 with sodium hydroxide solution, add ferric sulfate, react at room temperature for a certain time, filter to separate the solid and liquid, collect the solid and the first filtrate, dry the solid and perform phase analysis using XRD, and determine the phosphate concentration in the first filtrate using ion chromatography. The results are as follows:

[0064]

[0065]

[0066] Step 2: Fluoride ion-induced crystallization

[0067] Take 1L of the first filtrate, adjust the pH of the water sample to 4.7 with sodium hydroxide solution, add calcium fluoride seeds with different particle sizes and 20% fluorine mass and calcium chloride solid, react at room temperature for a certain time, filter to separate the solid and liquid, collect the solid and the second filtrate, dry the solid and use XRD to analyze the phase, and use ion chromatography to determine the fluoride ion concentration in the second filtrate. The results are as follows:

[0068]

[0069] Step 3: Directed precipitation of sulfate

[0070] Take 1L of the second filtrate, add calcium hydroxide, react at room temperature for a certain time, filter to separate the solid and liquid, collect the solid and purified water, dry the solid and use XRD to analyze the phase, and use ion chromatography to measure the sulfate concentration in the purified water. The results are as follows:

[0071]

[0072] Comparative Example

[0073] The initial wastewater conditions are the same as those in the examples, and conventional industry methods or simplified step-by-step processes are adopted.

[0074]

[0075] Result analysis:

[0076] The results of Examples 1-5 show that the step-by-step recovery process significantly improves the recovery rate and product purity (PO4 3->97%,F->95%,SO4 2- >95%). In particular, Example 3 shows the best recovery effect: PO4 3 - Recovery rate 98.8%, calcium fluoride purity 97%, gypsum purity 99%.

[0077] The results of Comparative Examples 1-4 show that the recovery and purity are significantly lower than those of the present invention due to competitive precipitation, lack of seed induction or high energy consumption.

[0078] Conclusion: The present invention solves the problem of multi-anion separation by controlling pH and addition sequence in steps, achieving the dual goals of resource utilization and environmental protection.

[0079] 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 in the scope of protection of the present invention.

Claims

1. A method for recovering phosphate, fluoride and sulfate from phosphogypsum washing water in steps, characterized in that: The following steps are involved: Step 1: Adjust the pH of the phosphogypsum washing water to 3.0±0.5, add ferric sulfate, stir and react for a certain period of time, and separate the solid and liquid to obtain ferric phosphate and the first filtrate; In step 1: The ferric sulfate is Fe2(SO4)3·xH2O (x=0-9), and the dosage of the ferric sulfate is based on a molar ratio of Fe to PO43- (1.0-1.5):1; The residual concentration of PO43- in the first filtrate is ≤10 mg·L-1; Step 2: Adjust the pH of the first filtrate to 4.7±0.5, add calcium fluoride seed crystals and calcium chloride, stir and react for a certain time, and separate the solid and liquid to obtain calcium fluoride and the second filtrate; In step 2: The purity of the calcium fluoride seed crystals is ≥95%, the particle size of the calcium fluoride seed crystals is 50-200 mm, and the amount of the seed crystals added is 1 / 5-1 / 2 of the mass of the fluoride ions in the filtrate; The dosage of the calcium chloride is such that the molar ratio of Ca to F is (0.5-0.6):1; The reaction endpoint was determined by the F- concentration in the second filtrate ≤ 5 mg·L-1; Step 3: Add calcium hydroxide to the second filtrate, stir and react for a certain period of time, and separate the solid and liquid to obtain calcium sulfate and purified water; In step 3: The calcium hydroxide is industrial grade slaked lime, and the dosage is based on a molar ratio of Ca to SO42- (1.0-1.2):1; The concentration of SO42- in the purified water is ≤100 mg·L-1.

2. The method according to claim 1, characterized in that The agent for adjusting pH in step 1-2 is a combination of an inorganic acid such as dilute sulfuric acid, hydrochloric acid, nitric acid, and an inorganic base such as sodium hydroxide, sodium carbonate, etc.

3. The method according to claim 1, characterized in that The stirring reaction time of steps 1-3 is 0.5-3h, the stirring speed is 150-400rpm, and the reaction temperature is 10-40°C.

4. The method according to claim 1, wherein The solid-liquid separation in steps 1-3 is carried out by suction filtration, filter pressing, screw dehydration or centrifugation.

5. The resource prepared by the method of claim 1 is a product, comprising: Iron phosphate, purity ≥97%, used to prepare lithium-ion battery cathode materials, food / feed additives, catalysts, ceramic glazes, etc.; Calcium fluoride, with a purity of ≥96%, is used as a raw material for fluorine chemicals, metallurgy, and glass manufacturing. Calcium sulfate, with a purity of ≥98%, is used as an industrial raw material for the manufacture of cement, calcium sulfate hemihydrate, sulfuric acid, and other industries.

6. The method according to claim 1 comprises a phosphogypsum washing water treatment system, characterized in that: include: A pH adjustment device, a reactor and a solid-liquid separation device are connected in sequence; the reactor is divided into an iron phosphate precipitation unit, a calcium fluoride crystallization unit and a calcium sulfate precipitation unit; The calcium fluoride crystallization unit is provided with a seed crystal dosing device, and the seed crystal dosing device includes a particle size classification screen.

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