A method for preparing battery-grade iron phosphate and its by-product sulfate based on phosphorus-sulfur mixed acid waste liquid and titanium dioxide residue.

Abstract

This invention discloses a method for preparing battery-grade ferric phosphate and its by-product sulfate based on the purification of mixed phosphorus-sulfur acid waste liquid and titanium dioxide slag, comprising the following steps: S1, titanium dioxide slag purification; S2, mixed acid purification and preparation of ferric phosphate; S3, preparation of sulfate by-product; S4, salting-out agent recovery. This invention targets waste liquid containing complex metal impurities for impurity removal. It utilizes a two-step impurity removal process to remove metal ions and uses ammonium dihydrogen phosphate to wash the filter cake, reducing the sulfate content. Furthermore, by first adjusting the pH to convert hydroxide ions to hydrogen phosphate ions in the solution and then fully dispersing them before adding an iron source, the localized formation of ferric hydroxide can be prevented, ensuring that the prepared ferric phosphate product meets the standard HG / T 4701-2021 "Battery-grade Ferric Phosphate".

Description

Technical Field

[0001] This invention relates to the field of chemical processing technology, and in particular to a method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphoric acid and sulfuric acid waste liquid and titanium dioxide residue. Background Technology

[0002] The semiconductor industry commonly uses mixed phosphorus and sulfuric acids for acid cleaning. The waste liquid from this cleaning process mainly contains small amounts of metal ions, such as aluminum, iron, and magnesium; direct discharge leads to resource waste. Furthermore, titanium dioxide slag is a solid waste byproduct generated during the sulfuric acid process for titanium dioxide production. Its main component is ferrous sulfate, with slight variations in impurity content depending on the raw materials and processes used. Currently, most titanium dioxide slag cannot be directly utilized and is mostly disposed of through stockpiling, creating serious safety and environmental hazards and hindering the sustainable development of titanium dioxide production enterprises.

[0003] Patent CN201910412494.7 discloses a method for preparing battery-grade iron phosphate by extracting phosphoric acid from industrial wastewater. The method includes the following steps: using industrial wastewater with a fixed phosphorus content as a phosphorus source, adding a flocculant for impurity removal, then adding an iron source and alkaline solution, and reacting under specific conditions to obtain an iron phosphate precursor. This method removes impurities through simple flocculation and precipitation, resulting in a low removal rate of metal ions. Therefore, although the iron phosphate prepared in this paper meets the metal quality standards, this is mainly because the metal impurity content in the phosphoric acid wastewater is relatively low. Thus, this patent targets wastewater with higher requirements. Furthermore, adding alkaline solution after adding ferrous sulfate can easily lead to the localized formation of ferric hydroxide, affecting the color and quality of the iron phosphate.

[0004] Patent 201010253200.X discloses a method for preparing ferric phosphate and its product. The method involves introducing a trivalent iron source solution into an acid-resistant reactor, followed by the addition of a certain amount of phosphoric acid solution, stirring for 1 hour, and then adding a certain amount of organic solvent. The resulting ferric phosphate slurry is washed with water, filtered, and spray-dried to obtain the final ferric phosphate product. This method is for pure phosphoric acid solution, which can be directly washed with water several times to obtain the battery-grade ferric phosphate product. However, if the solution is a mixed phosphoric acid and sulfuric acid waste liquid, the metal ion content cannot meet the standards under this method.

[0005] Patent CN106892415B discloses a method for preparing ferrous phosphate from ferrous sulfate, a byproduct of titanium dioxide production. The steps are as follows: Ferrous sulfate is prepared into a saturated solution; undissolved solids are removed by filtration; the solution is then heated to 80-95°C, air is introduced and stirred; dilute sulfuric acid is added to adjust the pH to 3-4, stirring until turbidity is produced; a flocculant is added, and the solids are removed by filtration; a small amount of sodium dihydrogen phosphate is added, stirring until a large amount of turbidity is produced; another flocculant is added, and the solids are removed by filtration; sodium phosphate is added to the liquid, and the pH is controlled at approximately 1-3 using dilute phosphoric acid; the solids are filtered out, and the solution is washed with deionized water to obtain ferrous phosphate. This method first prepares a saturated solution of ferrous sulfate, and during the heating process, some solution evaporates. Therefore, during the filtration process after adding dilute sulfuric acid, a large amount of ferrous sulfate crystals precipitate and remain in the precipitate, resulting in a significant loss of ferrous sulfate.

[0006] Patent CN110980678A discloses a low-cost, low-impurity method for preparing ferric phosphate, comprising the following steps: S1, taking a titanium dioxide ferrous sulfate solution, heating, adjusting the pH, adding phosphoric acid and nitrite, stirring evenly to obtain a reaction base solution; taking a phosphorus-containing compound fertilizer, dissolving it, stirring and filtering to obtain a phosphate salt solution; S2, adding the phosphate salt solution to the reaction base solution, then adding hydrogen peroxide to react and obtain ferric phosphate slurry; S3, washing the ferric phosphate slurry and calcining it to obtain the ferric phosphate. This method is simple to operate and the synthesized ferric phosphate has a low impurity content, especially the content of Mn, Mg and Ti, which are all reduced to below 50 ppm. However, this method introduces nitrite ions into the purified ferrous sulfate solution, affecting the quality of the ferric phosphate in the later stages. In addition, due to the simple chemical precipitation, it is impossible to purify ferrous sulfate to analytical grade, and the prepared ferric phosphate cannot meet the standard of lithium battery precursor battery-grade ferric phosphate. Summary of the Invention

[0007] The main objective of this invention is to provide a method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphoric acid waste liquid and titanium dioxide residue.

[0008] To achieve the above objectives, this invention provides a method for preparing battery-grade iron phosphate and its by-product sulfate based on phosphorus-sulfur mixed acid waste liquid and titanium dioxide residue, comprising the following steps:

[0009] S1. Purification of titanium dioxide residue:

[0010] S11: Dilution: Prepare a saturated aqueous solution of titanium dioxide residue at room temperature, and then dilute it with water to form an unsaturated aqueous solution;

[0011] S12: Impurity removal: Add iron powder and phosphate to the unsaturated aqueous solution of titanium dioxide residue, and stir at 60-80℃ for 1-2 hours to obtain the impurity removal solution;

[0012] S13: Filtration: Filter the impurity-removing liquid and collect the filtrate;

[0013] S14: Salting out: Add sulfuric acid to the filtrate under stirring. When crystals precipitate in the filtrate, add a salting-out agent and continue stirring to crystallize, thus obtaining a crystalline solution.

[0014] S15: Filtration: The crystallization liquid is filtered to obtain purified ferrous sulfate crystals as the solid.

[0015] S2. Purification and preparation of ferric phosphate using mixed acid:

[0016] S21: Aluminum removal: Add water to the phosphorus-sulfur mixed acid waste liquid for dilution, then add alkali solution to adjust the pH to 5-6, stir and react for 0.5-1h, and then filter. The filtrate obtained is the phosphorus-sulfur mixed acid after aluminum removal.

[0017] S22: Reaction 2: Add sulfide to the phosphorus-sulfur mixed acid after aluminum removal, stir the reaction and then filter. The filtrate is the phosphorus-sulfur mixed acid after deep metal removal.

[0018] S23: pH adjustment: Add alkaline solution to the phosphoric acid-sulfur mixed acid after deep metal removal to adjust the pH to 7-8, and after stirring, obtain purified phosphoric acid;

[0019] S24: Reaction 3: Add hydrogen peroxide and a saturated solution of ferrous sulfate crystals prepared in step S1 to purified phosphoric acid, and react at 60-90℃ for 1-3 hours to obtain a reaction mixture.

[0020] S25: Pressure filtration: The reaction mixture is pressure filtered to obtain crude ferric phosphate in the filter cake, and the filtrate proceeds to the next step.

[0021] S26: Washing: Wash the crude ferric phosphate until the TDS value is between 500 and 2000 mg / L, and then filter to obtain wet ferric phosphate.

[0022] S27: Drying: Dry the wet ferric phosphate to obtain anhydrous ferric phosphate;

[0023] S3, Sulfate preparation by-products:

[0024] S31: pH adjustment: Add alkaline solution to the filtrate obtained in step S25 to adjust the pH to 5-7, and obtain the adjusted mother liquor;

[0025] S32: Evaporation and concentration: The mother liquor is evaporated and concentrated under the conditions of temperature 80~100℃ and pressure -0.06~-0.08MPa;

[0026] S33: Centrifugation: Centrifugation is used to process the concentrated and evaporated mother liquor;

[0027] S34: Drying: The solid obtained by centrifugation is dried at a temperature of 110-120℃ to obtain sulfate byproduct.

[0028] Furthermore, it also includes the following steps:

[0029] S4. Salting-out agent recovery:

[0030] S41: pH adjustment: Adjust the pH of the filtrate obtained in S15 to neutral to obtain a neutral mother liquor;

[0031] S42: Evaporation and concentration: The neutral mother liquor is evaporated and concentrated at 80-110℃, and the distillate water can be used to dilute the titanium dioxide residue;

[0032] S43: Centrifugation: The neutral mother liquor after evaporation and concentration is centrifuged to obtain a solid as a salting-out agent. The centrifuged liquid is then recycled into the unsaturated solution of titanium dioxide slag in step S11 for crystallization.

[0033] Furthermore, in step S11: first, a saturated aqueous solution of titanium dioxide slag is prepared at room temperature, and then 0.1 to 0.3 times the amount of water is added to dilute it into an unsaturated aqueous solution to prevent crystallization during filtration.

[0034] Further, in step S12, the amount of iron powder added is 0.3% to 0.7% of the titanium dioxide slag, and the amount of phosphoric acid or phosphate added is a phosphate solution with a concentration of 30 to 90 wt%, added at a mass ratio of phosphate ions to metallic impurities in the titanium dioxide slag of 3 to 5:1. The addition of iron powder prevents ferrous ions from being oxidized to ferric ions, which are more easily precipitated in this solution; therefore, adding iron powder can reduce the loss rate of ferrous sulfate. The phosphate can be selected from any one of ammonium dihydrogen phosphate, potassium dihydrogen phosphate, or sodium dihydrogen phosphate.

[0035] Further, in step S14, the salting-out agent is any one or a combination of two or more of ammonium sulfate, ammonium bisulfate, ammonium dihydrogen sulfate, sodium sulfate, sodium bisulfate, potassium sulfate, and potassium bisulfate, the mass ratio of salting-out agent to filtrate is 0.1 to 0.3:1, and the stirring and crystallization time is 3 to 6 hours.

[0036] Furthermore, in step S21, aluminum ions in the phosphorus-sulfur mixed acid waste liquid are removed by coagulation and precipitation. The alkaline solution used is any one of sodium hydroxide solution, sodium carbonate solution, potassium hydroxide solution, potassium carbonate solution, or ammonia water, with a concentration of 10-35 wt%.

[0037] Furthermore, in step S22, the amount of sulfide added is 0.5 to 2 times the total mass of metallic impurities other than aluminum in the phosphorus-sulfur mixed acid waste liquid.

[0038] Furthermore, in step S24, the amount of each raw material added is ensured to maintain a phosphorus:ferrous sulfate:hydrogen peroxide molar ratio of 1-1.05:1:0.75-1, and the pH is controlled at 1.5-2.5. This ensures sufficient precipitation of ferric phosphate and avoids the formation of ferric hydroxide during synthesis, which would affect the quality of ferric phosphate.

[0039] Further, in step S26, the washing process involves first washing with a 0.05-0.2 wt% ammonium dihydrogen phosphate solution 3-5 times, followed by washing with pure water 3-5 times. The temperature for each wash is 60-80°C, the stirring speed is 200-400 rpm, and the stirring time is 30-60 minutes.

[0040] Furthermore, in step S27, the drying process involves first flash evaporation at 100–400°C to remove surface water, followed by calcination in a rotary kiln at 500–700°C to remove water of crystallization. Since wet ferric phosphate requires the removal of both surface water and water of crystallization to obtain anhydrous ferric phosphate, this drying method provides more stable results.

[0041] Furthermore, the alkali solution used in the other steps is the same as that used in step S21, in order to reduce the introduction of impurities.

[0042] Furthermore, in step S32, during evaporation and concentration, the mother liquor is controlled to remain at 10-20% before proceeding to the next step of centrifugation.

[0043] This invention solves the problem of crystal precipitation and reduced recovery rate during impurity removal filtration by preparing an unsaturated solution from titanium dioxide slag. Furthermore, preliminary impurity removal using iron powder and phosphoric acid reduces the content of impurities such as titanium, manganese, and magnesium in the solution, and reduces the amount of impurities encapsulated by ferrous sulfate crystals, achieving further purification. Then, a small amount of concentrated sulfuric acid is added, utilizing its water-absorbing effect to further prepare a saturated solution of ferrous sulfate. A sulfate salting-out agent is then added to promote further crystallization of ferrous sulfate, obtaining purified ferrous sulfate, which is then used to prepare ferric phosphate. Additionally, this invention utilizes chemical precipitation with alkaline solution for preliminary aluminum removal and utilizes sulfides to deeply remove metallic impurities from waste acid, ensuring the subsequent preparation of qualified ferric phosphate. Moreover, adjusting the pH of the metal-removed solution to 7-8 ensures that the pH remains within the range of 1.5-2.5 after a period of reaction with hydrogen peroxide and ferrous sulfate, reducing the localized generation of ferric hydroxide during further pH adjustment with alkali. The resulting crude ferric phosphate, containing all metals except sodium, is prepared after reacting with the purified ferrous sulfate solution and hydrogen peroxide. In addition, washing ferric phosphate with a 0.05–0.2% ammonium dihydrogen phosphate solution can prevent hydrolysis of ferric phosphate and further remove sodium ions and sulfate ions, enabling the ferric phosphate to meet the HG / T 4701-2021 standard for "Ferric Phosphate for Batteries". Simultaneously, the mother liquor remaining after ferric phosphate production can be further reacted with alkali, and sulfate byproducts can be obtained through evaporation, concentration, and centrifugation, achieving the goal of resource utilization of hazardous waste.

[0044] The beneficial effects of this invention are reflected in:

[0045] This invention first purifies the phosphorus-sulfur mixed acid and titanium dioxide slag, and then uses a two-step chemical precipitation method to remove metals based on the properties of the phosphorus-sulfur mixed acid. Then, it reacts to prepare battery-grade iron phosphate, and washes it with a certain concentration of ammonium dihydrogen phosphate. The washed iron phosphate meets the standard of "Iron Phosphate for Batteries" HG / T 4701-2021, which not only prevents resource waste but also obtains high-quality, high-value iron phosphate products and sulfate by-products.

[0046] This invention targets wastewater containing complex metal impurities for impurity removal. It utilizes a two-step impurity removal process to remove metal ions and uses ammonium dihydrogen phosphate to wash the filter cake, reducing the sulfate content. Furthermore, by first adjusting the pH to convert hydroxide ions in the solution into hydrogen phosphate ions and then dispersing them completely before adding an iron source, the local formation of ferric hydroxide can be prevented, ensuring that the prepared ferric phosphate product meets the standard of HG / T 4701-2021 "Ferric Phosphate for Batteries".

[0047] This invention prepares an unsaturated solution of ferrous sulfate, removes impurities and crystals it through precipitation and salting out, and then prepares a saturated solution to react with phosphoric acid and hydrogen peroxide to prepare lithium battery precursor iron phosphate, thereby reducing the loss of ferrous sulfate.

[0048] This invention purifies titanium dioxide slag to analytical grade ferrous sulfate using chemical precipitation and salting out without introducing new ions. The ferric phosphate prepared by adding purified phosphoric acid and hydrogen peroxide meets the standards for lithium battery precursors. Attached Figure Description

[0049] Figure 1 This is a process flow diagram of the preparation method of the present invention. Detailed Implementation

[0050] The present invention will be further described clearly and in detail below with reference to specific embodiments. Those skilled in the art will be able to implement the present invention based on these descriptions. Furthermore, the embodiments of the present invention described below are generally only some, not all, of the embodiments of the present invention. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

[0051] Unless otherwise specified, all raw materials used in the embodiments of this invention are commercially available or obtainable by those skilled in the art; unless otherwise specified, all methods used in the embodiments of this invention are methods mastered by those skilled in the art. Unless otherwise specified, all solutions mentioned below refer to aqueous solutions, and all concentrations mentioned below refer to wt% concentrations.

[0052] Example 1:

[0053] Preparation of battery-grade ferric phosphate and its by-product sulfate based on the purification of phosphoric acid-sulfuric acid waste liquid and titanium dioxide residue

[0054] Take 500g of phosphoric acid-sulfur mixed acid waste liquid (phosphoric acid concentration 58%, sulfuric acid concentration 12%, metallic aluminum 2%, and other metallic impurities content 0.5%), add 1000mL of water for dilution, then add 10% ammonia water to adjust the pH to 5, stir and react for 0.5h, then filter. Add 1.25g of ammonium sulfide to the filtrate, stir and react for another 1.5h, then filter. The filtrate weighs 1518g, and the phosphorus content is 4.35%. Finally, add 10% ammonia water to adjust the pH to 7.0, stir for 0.5h, and the purified phosphoric acid is obtained for later use.

[0055] Take 600g of titanium dioxide slag (metallic impurity content 0.698%), dissolve it at room temperature to prepare a saturated aqueous solution, then add 200ml of water to dilute it into an unsaturated aqueous solution. Add 50.72g of 30% ammonium dihydrogen phosphate solution and 1.8g of iron powder, stir at 60℃ for 1h, then filter. Add 85% sulfuric acid to the filtrate while stirring. Stop stirring and adding sulfuric acid when crystals precipitate. Add 180g of ammonium sulfate, continue stirring and crystallizing for 3h, and finally filter. The solid obtained is purified ferrous sulfate crystals (619g of crystals with an iron content of 19.31%). Adjust the pH of the final filtered filtrate to 5.5, then evaporate and concentrate it to 20% at 80℃ and -0.06MPa. After centrifugation, the solid obtained is used as a salting-out agent with an ammonium sulfate concentration of 85%. The centrifuged liquid is recycled in the unsaturated solution of titanium dioxide slag for cyclic crystallization.

[0056] 617.73g of ferrous sulfate crystals were dissolved in water to prepare a saturated solution, which was then slowly added to purified phosphoric acid. 181.06g of 30% hydrogen peroxide was then added dropwise. The pH was measured to be 1.5. After stirring at 60℃ for 2 hours, the mixture was filtered. The pH of the filtrate was adjusted to 5 with 10% ammonia solution. The solution was then evaporated and concentrated to 15% at 80℃ and -0.08MPa. The solution was then centrifuged, and the resulting solid was dried at 110℃ for 4 hours to obtain ammonium sulfate byproduct. The indicators for the ammonium sulfate byproduct are shown in Table 1-2, and it meets the requirements of the "Industrial Ammonium Sulfate" (HGT) standard. According to the 5744-2020 standard, the filter cake obtained by pressure filtration was first washed 5 times with a 0.05% ammonium dihydrogen phosphate solution, and then washed 5 times with pure water. The mud-to-water ratio was 1:5 for each wash, the temperature was 60℃, the stirring speed was 200rpm, and the stirring time was 60min. The TDS was stabilized at about 2000mg / L. Then, it was filtered to obtain wet ferric phosphate. The wet ferric phosphate was flash-dried at 100℃ for 1h, and then dried in a rotary kiln at 500℃ for 4h to obtain anhydrous ferric phosphate. The test indicators are shown in Table 1-1.

[0057] Example 2:

[0058] Preparation of battery-grade ferric phosphate and its by-product sulfate based on the purification of phosphoric acid-sulfuric acid waste liquid and titanium dioxide residue

[0059] Take 700g of phosphoric acid waste liquid (phosphoric acid concentration 58%, sulfuric acid concentration 12%, metallic aluminum 2%, and other metallic impurities content 0.5%), add 1400ml of water for dilution, then add 20% sodium hydroxide solution to adjust the pH to 5.5, stir for 40min and filter. Add 3.5g of sodium sulfide to the filtrate, stir for another 1h and filter. The filtrate mass is 2061g, and the phosphorus content is detected to be 4.219%. Finally, add 20% sodium hydroxide solution to adjust the pH to 7.5, stir for 1.5h, and the purified phosphoric acid is obtained for later use.

[0060] Take 770g of titanium dioxide slag (metallic impurity content 0.698%), dissolve it at room temperature to prepare a saturated aqueous solution, then add 300ml of water to dilute it into an unsaturated aqueous solution. Add 54.29g of 50% sodium dihydrogen phosphate solution and 3.85g of iron powder, stir at 70℃ for 1.5h, then filter. Add 85% sulfuric acid to the filtrate while stirring. Stop stirring and adding sulfuric acid when crystals precipitate. Add 434g of sodium sulfate, continue stirring to crystallize for 3h, and finally filter. The solid obtained is purified ferrous sulfate crystals (798.2g of crystals with an iron content of 19.28%). Adjust the pH of the final filtered filtrate to 6.0, then evaporate and concentrate it to 15% at 90℃ and -0.07MPa. Then centrifuge and reuse the solid as a salting-out agent. The sodium sulfate concentration is 87%. The centrifuged liquid is reused in the unsaturated solution of titanium dioxide slag for cyclic crystallization.

[0061] 790.98g of ferrous sulfate crystals were dissolved in water to prepare a saturated solution, which was then slowly added to purified phosphoric acid. 326.79g of 30% hydrogen peroxide was then added dropwise. The pH was measured to be 2.0. After stirring at 70℃ for 2 hours, the mixture was filtered. The filtrate was adjusted to pH 6 with a 20% sodium hydroxide solution. The solution was then evaporated and concentrated to 20% at 90℃ and -0.07MPa. The solution was then centrifuged, and the resulting solid was dried at 115℃ for 3 hours to obtain sodium sulfate as a byproduct. The indicators for the sodium sulfate byproduct are shown in Tables 1-3, and the indicators meet the requirements of the "Industrial Anhydrous Sodium Sulfate" standard (GB / T). (6009-2014) Class II qualified product standard: The filter cake obtained by pressure filtration is first washed 4 times with a 0.1% ammonium dihydrogen phosphate solution, and then washed 4 times with pure water. The mud-to-water ratio is 1:5 for each wash, the temperature is 70℃, the stirring speed is 300rpm, the stirring time is 40min, and the TDS is stabilized at about 1000mg / L. Then it is filtered to obtain wet ferric phosphate. The wet ferric phosphate is flash-dried at 400℃ for 1h, and then dried in a rotary kiln at 600℃ for 3h to obtain anhydrous ferric phosphate. The test indicators are shown in Table 1-1.

[0062] Example 3:

[0063] Preparation of battery-grade ferric phosphate and its by-product sulfate based on the purification of phosphoric acid-sulfuric acid waste liquid and titanium dioxide residue

[0064] Take 800g of phosphoric acid waste liquid (phosphoric acid concentration 58%, sulfuric acid concentration 12%, metallic aluminum 2%, and other metallic impurities content 0.5%), add 2000mL of water for dilution, then add 30% potassium hydroxide to adjust the pH to 6, stir and react for 1h, filter, add 8g of potassium sulfide to the filtrate, stir and react for another 1h, filter again, the filtrate mass is 2594g, the phosphorus content is detected to be 3.96%, finally add 30% potassium hydroxide to adjust the pH to 8, stir for 1h, and obtain purified phosphoric acid for later use.

[0065] Take 950g of titanium dioxide slag (metallic impurity content 0.698%), dissolve it at room temperature to prepare a saturated aqueous solution, then add 400ml of water to dilute it into an unsaturated aqueous solution. Add 52.77g of 90% potassium dihydrogen phosphate solution and 3.8g of iron powder, stir at 80℃ for 2h, then filter. Add 85% sulfuric acid to the filtrate while stirring. Stop stirring and adding sulfuric acid when crystals precipitate. Add 825g of potassium sulfate, continue stirring to crystallize for 3h, and finally filter. The solid obtained is purified ferrous sulfate crystals (965.26g of crystals with an iron content of 19.45%). Adjust the pH of the final filtered filtrate to 6.5, then evaporate and concentrate it to 15% at 100℃ and -0.06MPa. Then centrifuge and reuse the solid as a salting-out agent. The potassium sulfate concentration is 88%. The centrifuged liquid is reused in the unsaturated solution of titanium dioxide slag for cyclic crystallization.

[0066] 954.04 g of ferrous sulfate crystals were dissolved in water to prepare a saturated solution, which was then slowly added to purified phosphoric acid. 300.435 g of 30% hydrogen peroxide was then added dropwise. The pH was measured to be 2.5. After stirring at 90℃ for 2 hours, the mixture was filtered. The pH of the filtrate was adjusted to 7 with 30% potassium hydroxide solution. The solution was then evaporated and concentrated to 10% at 100℃ and -0.06 MPa. The solution was then centrifuged, and the resulting solid was dried at 120℃ for 2 hours to obtain potassium sulfate byproduct. The potassium sulfate byproduct indicators are shown in Tables 1-4, and meet the requirements of the "Agricultural Use..." The potassium sulfate meets the Class II qualified product standard in HG / T3279-1990. The filter cake obtained by pressure filtration is first washed three times with a 0.2% ammonium dihydrogen phosphate solution, and then washed three times with pure water. The mud-to-water ratio is 1:5 for each wash, the temperature is 80℃, the stirring speed is 400rpm, and the stirring time is 30min. The TDS is stabilized at about 500mg / L. Then it is filtered to obtain wet ferric phosphate. The wet ferric phosphate is flash-dried at 200℃ for 1h, and then dried in a rotary kiln at 700℃ for 2h to obtain anhydrous ferric phosphate. The test indicators are shown in Table 1-1.

[0067] Table 1-1 Detection Indicators of Ferric Phosphate

[0068] Iron (Fe) w / % 35.9 36.46 36.5 Phosphorus (P) w / % 20.11 20.74 20.79 Iron-to-phosphorus ratio (Fe:P) w / % 0.9899 0.9749 0.9736 Na / % 0.0052 0.0166 0.0068 Mg / % 0.0421 0.0229 0.0003 Al / ％ 0.0130 0.0026 0.0001 K / ％ 0.0088 0.0002 0.0167 Ca / % 0.005 0.002 0.0007 Ti / % 0.0351 0.0103 0.0032 Cr / ％ 0.0011 0.0012 0.0001 Mn / % 0.0024 0.0018 0.0009 Co / % 0.0000 0.0000 0.0000 Cu / % 0.0000 0.0001 0.0000 Zn / % 0.0008 0.0007 0.0001 Pb / % 0.0000 0.0000 0.0000 S / ％ 0.0101 0.0139 0.022 Moisture / % 0.00 0.00 0.00 Sulfate ions / % 0.008 0.005 0.00 <![CDATA[Tap density / (g / cm 3 )]]> 0.8 0.9 0.8 Particle size (D50) / μm 4 5 5 <![CDATA[Specific surface area / (m 2 / g)]]> 14 13 12

[0069] Conclusion: The higher the sulfide content, the lower the impurity content in the final ferric phosphate. However, the requirement for sulfur content in ferric phosphate specifies that the mass of sulfide added should be approximately 0.5 to 2 times the impurity content. The higher the phosphorus content, the lower the iron-to-phosphorus ratio in the resulting ferric phosphate. During the washing process of ferric phosphate, the lower the TDS value of the final wash, the lower the sulfate content in the ferric phosphate.

[0070] Sulfate by-product indicators:

[0071] Table 1-2 Ammonium sulfate by-product indicators

[0072] Nitrogen (N) content (on a dry basis) w / % ≥ 20.71 19.5 Moisture content w / % ≤ 0.000076 1.5 Free acid (calculated as H2SO4) content w / % ≤ 0.0000 2.0 Zinc (Zn) w / % ≤ 0.00028 0.001 Mercury (Hg) w / % ≤ 0.000 0.0001 Cobalt (Co) w / % ≤ 0.00026 0.0005 Manganese (Mn) w / % ≤ 0.0002 0.0005 Nickel(Ni)w / %≤ 0.0001 0.0005 Chromium (Cr) w / % ≤ 0.0030 0.001 Titanium (Ti) w / % ≤ 0.0081 0.0005 Copper (Cu) w / % ≤ 0.0007 0.0015 Iron (Fe) w / % ≤ 0.000803 0.002 Lead (Pb) w / % ≤ 0.000000 0.003

[0073] Table 1-3 By-product indicators of industrial sodium sulfate

[0074] <![CDATA[Sodium sulfate (Na2SO4) w / %≥]]> 97.2 97.0 Water insoluble matter w / %≤ 0.13 0.20 Calcium and magnesium (as Mg) w / % ≤ 0.35 0.40 Chloride (as Cl) w / % ≤ 0.76 0.90 Iron (Fe) w / % ≤ 0.002 0.040 Moisture content w / % ≤ 0.5 1.0

[0075] Table 1-4 Potassium sulfate by-product indicators

[0076]

[0077]

[0078] 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 method for preparing battery-grade iron phosphate and its by-product sulfate based on phosphorus-sulfur mixed acid waste liquid and titanium dioxide residue, characterized in that, Includes the following steps: S1. Purification of titanium dioxide residue: S11: Dilution: Prepare a saturated aqueous solution of titanium dioxide residue at room temperature, and then dilute it with water to form an unsaturated aqueous solution; S12: Impurity removal: Add iron powder and phosphate to the unsaturated aqueous solution of titanium dioxide residue, and stir at 60-80℃ for 1-2 hours to obtain the impurity removal solution; S13： Filtration: The impurity-removed liquid is filtered, and the filtrate is collected; S14: Salting out: Add sulfuric acid to the filtrate under stirring. When crystals precipitate in the filtrate, add a salting-out agent and continue stirring to crystallize, thus obtaining a crystalline solution. In step S14, the salting-out agent is any one or a combination of two or more of ammonium sulfate, ammonium bisulfate, ammonium dihydrogen sulfate, sodium sulfate, sodium bisulfate, potassium sulfate, and potassium bisulfate. The mass ratio of the salting-out agent to the filtrate is 0.1 to 0.3:1, and the stirring and crystallization time is 3 to 6 hours. S15: Filtration: The crystallization liquid is filtered to obtain purified ferrous sulfate crystals as the solid. S2. Purification and preparation of ferric phosphate using mixed acid: S21: Aluminum removal: Add water to the phosphorus-sulfur mixed acid waste liquid for dilution, then add alkali solution to adjust the pH to 5-6, stir and react for 0.5-1h, and then filter. The filtrate obtained is the phosphorus-sulfur mixed acid after aluminum removal. S22: Add sulfide to the phosphorus-sulfur mixed acid after aluminum removal, stir and react, then filter to obtain the filtrate as phosphorus-sulfur mixed acid after deep metal removal. S23: pH adjustment: Add alkaline solution to the phosphoric acid-sulfur mixed acid after deep metal removal to adjust the pH to 7-8, and after stirring, obtain purified phosphoric acid; S24: Add hydrogen peroxide and a saturated solution of ferrous sulfate crystals prepared in step S1 to purified phosphoric acid, and react at 60-90℃ for 1-3 hours to obtain a reaction mixture. S25: Pressure filtration: The reaction mixture is pressure filtered to obtain crude ferric phosphate in the filter cake, and the filtrate proceeds to the next step. S26: Washing: Wash the crude ferric phosphate until the TDS value is between 500 and 2000 mg / L, and then filter to obtain wet ferric phosphate. In step S26, the washing process involves first washing with a 0.05-0.2 wt% ammonium dihydrogen phosphate solution 3-5 times, followed by washing with pure water 3-5 times. The temperature for each wash is 60-80°C, the stirring speed is 200-400 rpm, and the stirring time is 30-60 minutes. S27: Drying: Dry the wet ferric phosphate to obtain anhydrous ferric phosphate; S3, Sulfate preparation by-products: S31: pH adjustment: Add alkaline solution to the filtrate obtained in step S25 to adjust the pH to 5-7, and obtain the adjusted mother liquor; S32: Evaporation and concentration: The mother liquor is evaporated and concentrated under the conditions of temperature 80~100℃ and pressure -0.06~-0.08MPa; S33: Centrifugation: Centrifugation is used to process the concentrated and evaporated mother liquor; S34: Drying: The solid obtained by centrifugation is dried at a temperature of 110-120℃ to obtain sulfate byproduct.

2. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide slag as described in claim 1, characterized in that, It also includes the following steps: S4. Salting-out agent recovery: S41: pH adjustment: Adjust the pH of the filtrate obtained in S15 to 5.5-6.5 to obtain a neutral mother liquor; S42: Evaporation and concentration: Evaporate and concentrate the neutral mother liquor at 80-110℃; S43: Centrifugation: The neutral mother liquor after evaporation and concentration is centrifuged to obtain a solid as a salting-out agent. The centrifuged liquid is then recycled into the unsaturated solution of titanium dioxide slag in step S11 for crystallization.

3. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide residue as described in claim 1 or 2, characterized in that, In step S12, the amount of iron powder added is 0.3% to 0.7% of the titanium dioxide slag, and the amount of phosphate added is 30 to 90 wt% phosphate solution with a mass ratio of phosphate to metallic impurities in titanium dioxide slag of 3 to 5:

1.

4. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide slag as described in claim 1 or 2, characterized in that, In step S21, the alkaline solution is any one of sodium hydroxide solution, sodium carbonate solution, potassium hydroxide solution, potassium carbonate solution, or ammonia water, with a concentration of 10-35 wt%.

5. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide slag as described in claim 1 or 2, characterized in that, In step S22, the amount of sulfide added is 0.5 to 2 times the total mass of metallic impurities other than aluminum in the phosphorus-sulfur mixed acid waste liquid.

6. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide slag as described in claim 1 or 2, characterized in that, In step S24, the amount of each raw material added is such that the molar ratio of phosphorus element: ferrous sulfate: hydrogen peroxide is 1-1.05:1:0.75-1, and the pH is controlled at 1.5-2.

5.

7. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide slag as described in claim 1 or 2, characterized in that, In step S27, the drying process involves flash evaporation at 100–400°C followed by calcination in a rotary kiln at 500–700°C.

8. The method for preparing battery-grade iron phosphate and its by-product sulfate based on the purification of phosphorus-sulfur mixed acid waste liquid and titanium dioxide slag as described in claim 4, characterized in that, The alkaline solution used in other steps is the same as that used in step S21.

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