Method for removing ferrous sulfate from titanium dioxide by-product
By adjusting the pH value with dilute phosphoric acid and ammonia water, combined with recrystallization technology, the problem of removing impurities from ferrous sulfate, a byproduct of titanium dioxide, has been solved, enabling the preparation of high-purity ferrous sulfate, which is suitable for the production of battery-grade iron phosphate.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- YUNNAN YUNTIANHUA
- Filing Date
- 2023-11-28
- Publication Date
- 2026-06-19
Smart Images
Figure BDA0004573747620000071
Abstract
Description
Technical Field
[0001] This invention relates to the field of chemical technology, and in particular to a method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide production. Background Technology
[0002] Ferrous sulfate heptahydrate is a byproduct of titanium dioxide production via the sulfuric acid process, generating approximately 3 tons of ferrous sulfate heptahydrate for every ton of titanium dioxide produced. In earlier years, ferrous sulfate was used as a mordant, water purifier, and preservative, but in small quantities; most of it was simply dumped as waste, causing environmental pollution and increasing environmental treatment costs. In recent years, ferrous sulfate has gradually been used as a raw material for the production of iron phosphate, which in turn serves as a precursor for lithium iron phosphate cathode materials. With the rapid development of lithium iron phosphate batteries, ferrous phosphate heptahydrate has also received widespread attention. However, as a byproduct of titanium dioxide production, ferrous sulfate heptahydrate has a high content of insoluble solids and metallic impurities, requiring purification before it can be used as a raw material for synthesizing iron phosphate for batteries. Therefore, research on purification methods for ferrous sulfate has significant practical application value. However, existing ferrous sulfate purification methods cannot achieve complete removal of impurity elements, especially metallic impurities such as Mg and Mn, which are almost impossible to remove effectively using conventional purification methods.
[0003] There are currently many reports on methods for removing impurities from ferrous sulfate, a byproduct of titanium dioxide production. Among them, iron powder and ammonia water are commonly used for impurity removal. The principles of these two methods are similar: both involve raising the pH value of the ferrous sulfate solution to cause hydroxide ions and impurity metal ions in the solution to precipitate, which are then removed by filtration. However, the precipitate particles generated in this impurity removal process are small, resulting in long filtration times and severe filtration through-filtering. Furthermore, this method is not very effective at removing impurities such as Mg and Mn. When ferrous sulfate obtained by this method is directly used to synthesize ferric phosphate, the product has the problem of excessive metal impurity content.
[0004] Patent CN112479174A uses soluble alkalis such as ammonia, sodium hydroxide, and potassium hydroxide. Experiments have shown that this method is ineffective at removing Mg and Mn impurities from ferrous sulfate solutions. The resulting insoluble precipitates are small, highly viscous, and difficult to filter. Furthermore, ferrous iron is easily oxidized in alkaline environments to form ferric hydroxide precipitate, leading to significant iron content loss after filtration. Patent CN115432739A mentions a recrystallization method, which can effectively reduce the content of Mg and Mn impurities in ferrous sulfate solutions after multiple recrystallizations. However, it is less effective at removing Ti, requiring the addition of a certain amount of impurity-removing agent for Ti removal. Patent CN107746082A discloses a phosphoric acid technique for removing titanium. By adding phosphoric acid to the ferrous sulfate solution, the ferric iron's effect on TiO₂ is pre-eliminated. 2+The hydrolysis effect can stabilize the pH of the solution and inhibit the oxidation of ferrous iron. The removal rate of Ti can reach more than 99%. However, even when using phosphoric acid as the only impurity remover, the overall content of other impurities in the ferrous sulfate solution is still relatively high.
[0005] In addition, sodium sulfide, magnesium fluoride, and sodium nitrite are also used as impurity removers. However, the use of sodium sulfide and sodium nitrite will introduce additional impurity elements. The generation of hydrogen sulfide gas during the process also poses certain safety risks. Moreover, they have no obvious effect on impurity removal. Ammonium fluoride has poor removal ability for Zn and Ti and its impurity removal effect is not as good as that of phosphoric acid. Experiments have verified that the above impurity removers have poor binding ability with metal impurity ions in the solution and cannot achieve a good impurity removal effect.
[0006] To address the problems of poor impurity removal efficiency, difficulty in separating precipitates after adding impurity removal agents, and low iron recovery in the current ferrous sulfate impurity removal process, there is an urgent need for a method to remove impurities from ferrous sulfate, a byproduct of titanium dioxide production. Summary of the Invention
[0007] This invention provides a method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide production.
[0008] The solution of the present invention is:
[0009] A method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide production, includes the following steps:
[0010] 1) Take ferrous sulfate, a byproduct of titanium dioxide, dissolve it at 50-90℃ and prepare a saturated solution;
[0011] 2) After adding dilute phosphoric acid and stirring thoroughly, filter to remove insoluble matter to obtain a solution;
[0012] 3) Add an appropriate amount of ammonia to the obtained solution, adjust the pH of the filtrate to 2.5-3.5, and then perform a second filtration. Evaporate the obtained filtrate and retain 60-80% of the solution.
[0013] 4) Under slow and uniform stirring conditions, the solution is cooled at a fixed rate until the temperature drops to 10°C. The solution is then separated by centrifugation to obtain primary crystals and primary crystallization mother liquor.
[0014] 5) Dissolve the primary crystal obtained in 4) again to prepare a saturated solution. Repeat steps 3) and 4) on the solution to obtain secondary crystal and secondary crystal mother liquor.
[0015] 6) Dissolve the secondary crystals to prepare a ferrous sulfate solution with an iron content of 50-60 g / L and adjust the pH. The adjusted ferrous sulfate solution is then used directly for the preparation of battery-grade iron phosphate.
[0016] As a preferred technical solution, in step 2), the amount of dilute phosphoric acid added is 0.5% to 5% of the mass of the saturated solution, and the concentration of dilute phosphoric acid is 10% to 30%.
[0017] As a preferred technical solution, after adding ammonia water to the filtrate obtained in step 2) for adjustment, stirring for 30 minutes, a second filtration is performed.
[0018] As a preferred technical solution, when cooling at a fixed rate in step 4), the fixed cooling rate is 0.2 to 0.5 °C / min.
[0019] As a preferred technical solution, the ferrous sulfate solution in step 6) has a pH of 2.5 to 3.5.
[0020] As a preferred technical solution, the slow and uniform stirring in step 4) is 30 to 80 rpm.
[0021] A method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide, using the above-mentioned technical solution includes the following steps: 1) Dissolve ferrous sulfate, a byproduct of titanium dioxide, at 50-90℃ to prepare a saturated solution; 2) Add dilute phosphoric acid, stir thoroughly, and filter to remove insoluble matter, obtaining a solution; 3) Add an appropriate amount of ammonia to the obtained solution, adjust the pH of the filtrate to 2.5-3.5, and then perform a second filtration. Evaporate the obtained filtrate, retaining 60-80% of the solution; 4) Under slow and uniform stirring conditions, cool at a fixed rate until the solution temperature drops to 10℃, and centrifuge to obtain primary crystals and primary crystallization mother liquor; 5) Dissolve the primary crystals obtained in 4) again to prepare a saturated solution, and repeat steps 3) and 4) to obtain secondary crystals and secondary crystallization mother liquor; 6) Dissolve the secondary crystals to prepare a ferrous sulfate solution with an iron content of 50-60 g / L and adjust the pH. The adjusted ferrous sulfate solution is directly used for the preparation of battery-grade iron phosphate.
[0022] Advantages of this invention:
[0023] This invention first selects dilute phosphoric acid as a purification agent, and then uses hydrolysis to remove impurities from TiO2. 2+ Hydrolysis produces metatitanic acid (H₂TiO₃). After filtration, the metallic impurities Ti in the solution are removed. Then, ammonia is added to the filtrate to adjust the pH to 2.5–3.5. Ammonia is used to adjust the pH of the ferrous sulfate solution for two reasons: first, raising the pH causes the metallic impurity ions to precipitate under a higher pH environment, further removing these ions; second, it introduces NH₄⁺. + It can consume the PO4 remaining after the phosphoric acid purification reaction. 2-This avoids the problem of unstable iron-phosphorus ratio during subsequent ferric phosphate synthesis. The filtrate after the above impurity removal steps still contains a large amount of impurity elements such as Mg and Mn that have not been removed. The filtrate is then concentrated by heating and cooled by recrystallization to obtain ferrous sulfate crystals with low impurity content. The obtained crystals are then prepared into a ferrous sulfate solution with a certain iron concentration, which can be directly used for ferric phosphate preparation.
[0024] In the impurity removal process, the concentration and amount of phosphoric acid, as well as the concentration and temperature of the ferrous sulfate solution, are controlled first. After filtration following the impurity removal reaction, impurities such as Ti are removed. Under optimal conditions, the precipitate particles formed are larger, resulting in better sedimentation and shorter filtration time. Furthermore, by adding ammonia to adjust the pH value of the solution, the pH range for impurity removal can be widened, improving the removal rate of metal ions and effectively reducing the PO4 content in the solution. 2- The content helps to precisely control the iron and phosphorus content during the subsequent iron phosphate synthesis process.
[0025] Different cooling rates during subsequent recrystallization directly affect the crystal growth rate. Faster crystal growth makes it easier for impurities to coat the crystals, resulting in poorer impurity removal. Furthermore, the cooling process requires stirring to maintain the solution's fluidity and prevent impurity coating during crystal growth. Simultaneously, the evaporation rate significantly impacts recrystallization impurity removal. Insufficient evaporation leads to low recrystallization yield, while excessive evaporation causes some impurities to precipitate with the crystal product at lower temperatures, thus affecting the purity of ferrous sulfate. Through these optimizations, the goal is twofold: first, to improve the ferrous sulfate product yield during recrystallization; and second, to prevent impurity element coating during crystallization, thereby obtaining high-purity ferrous sulfate crystals.
[0026] This method can reduce the concentration of high-content impurities such as Ti, Mg, Mn, Al, and Zn in ferrous sulfate solution to a lower level, and the resulting ferrous sulfate can be directly used for the synthesis of battery-grade iron phosphate.
[0027] Compared with existing processes, this method removes metal impurities from ferrous sulfate solution more thoroughly, and the process is simple. The resulting ferrous sulfate solution has a low impurity content and can be directly used for the preparation of battery-grade iron phosphate. Detailed Implementation
[0028] This invention provides a method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide production.
[0029] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.
[0030] Example 1:
[0031] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 13.5g of 20% dilute phosphoric acid to the solution, keep the temperature at 60℃ and stir for 30 minutes. Then filter. The filtrate is green.
[0032] Add ammonia to the filtrate to adjust the pH to 2.5. Place the filtrate in a programmable thermostat and heat it to 70°C to evaporate the solution. Reduce the liquid level to about 80% and cool it down at a rate of 0.2°C / min. During the cooling process, keep the stirring speed at 80 rpm until the solution temperature drops to 10°C. Green ferrous sulfate crystals slowly precipitate during the cooling process. Centrifuge the solid-liquid mixture to obtain primary crystals and primary crystallization mother liquor.
[0033] The obtained crystals were repeatedly dissolved, evaporated, and cooled to crystallize. After separation, secondary crystals were obtained. The secondary crystals were dissolved and prepared into a ferrous sulfate solution with an iron content of 56 g / L for later use. The pH of the ferrous sulfate solution was adjusted to 2.5 and subsequently used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0034] Example 2:
[0035] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 6.75g of 40% dilute phosphoric acid to the solution, keep the temperature at 60℃ and stir for 30 minutes. Then filter. The filtrate is green.
[0036] Add ammonia to the filtrate to adjust the pH to 3.5. Place the filtrate in a programmable thermostat and heat it to 70°C to evaporate the solution. Reduce the liquid level to 80% and cool it down at a rate of 0.2°C / min. During the cooling process, keep the stirring speed at 80 rpm until the solution temperature drops to 10°C. Green ferrous sulfate crystals slowly precipitate during the cooling process. Centrifuge the solid-liquid mixture to obtain primary crystals and primary crystallization mother liquor.
[0037] The obtained crystals were repeatedly dissolved, evaporated, and cooled to crystallize. After separation, secondary crystals were obtained. The secondary crystals were dissolved and prepared into a ferrous sulfate solution with an iron content of 56 g / L for later use. The pH of the ferrous sulfate solution was adjusted to 3.5 and subsequently used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0038] Example 3:
[0039] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 13.5g of 20% dilute phosphoric acid to the solution, keep the temperature at 60℃ and stir for 30 minutes. Then filter. The filtrate is green.
[0040] Add ammonia to the filtrate to adjust the pH to 3.0. Place the filtrate in a program-controlled thermostat and heat it to 70°C to evaporate the solution. Reduce the liquid level to about 80% and cool it down at a rate of 0.5°C / min. During the cooling process, keep the stirring speed at 80 rpm until the solution temperature drops to 10°C. Green ferrous sulfate crystals slowly precipitate during the cooling process. Centrifuge the solid-liquid mixture to obtain primary crystals and primary crystallization mother liquor.
[0041] The obtained crystals were repeatedly dissolved, evaporated, and cooled to crystallize. After separation, secondary crystals were obtained. The secondary crystals were dissolved and prepared into a ferrous sulfate solution with an iron content of 56 g / L for later use. The pH of the ferrous sulfate solution was adjusted to 3.0 and subsequently used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0042] Example 4:
[0043] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 13.5g of 20% dilute phosphoric acid to the solution, keep the temperature at 60℃ and stir for 30 minutes. Then filter. The filtrate is green.
[0044] Add ammonia to the filtrate to adjust the pH to 2.5. Place the filtrate in a programmable thermostat and heat it to 70°C to evaporate the solution. Reduce the liquid level to about 80% and cool it down at a rate of 0.2°C / min. During the cooling process, keep the stirring speed at 30 rpm until the solution temperature drops to 10°C. Green ferrous sulfate crystals slowly precipitate during the cooling process. Centrifuge the solid-liquid mixture to obtain primary crystals and primary crystallization mother liquor.
[0045] The obtained crystals were repeatedly dissolved, evaporated, and cooled to crystallize. After separation, secondary crystals were obtained. The secondary crystals were dissolved and prepared into a ferrous sulfate solution with an iron content of 56 g / L for later use. The pH of the ferrous sulfate solution was adjusted to 3.5 and subsequently used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0046] Example 5:
[0047] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 13.5g of 20% dilute phosphoric acid to the solution, keep the temperature at 60℃ and stir for 30 minutes. Then filter. The filtrate is green.
[0048] Add ammonia to the filtrate to adjust the pH to 3.5. Place the filtrate in a program-controlled constant temperature bath and heat it to 70°C to evaporate the solution. Reduce the liquid level to about 60% and cool it down at a rate of 0.2°C / min. During the cooling process, keep the stirring speed at 80 rpm until the solution temperature drops to 10°C. Green ferrous sulfate crystals slowly precipitate during the cooling process. Centrifuge the solid-liquid mixture to obtain primary crystals and primary crystallization mother liquor.
[0049] The obtained crystals were repeatedly dissolved, evaporated, and cooled to crystallize. After separation, secondary crystals were obtained. The secondary crystals were dissolved and prepared into a ferrous sulfate solution with an iron content of 56 g / L for later use. The pH of the ferrous sulfate solution was adjusted to 2.5 and subsequently used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0050] Comparative Example 1:
[0051] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved and filter out insoluble impurities.
[0052] The filtrate was placed in a program-controlled thermostatic bath and heated to 70°C for solution evaporation. The liquid level was then reduced to about 80%, and the solution was cooled at a rate of 0.2°C / min. During the cooling process, the stirring speed was kept at 80 rpm until the solution temperature dropped to 10°C. Green ferrous sulfate crystals slowly precipitated during the cooling process. The solid-liquid mixture was then centrifuged to obtain primary crystals and primary crystallization mother liquor.
[0053] The obtained crystals were repeatedly dissolved, evaporated, and cooled to crystallize. After separation, secondary crystals were obtained. The secondary crystals were dissolved and prepared into a ferrous sulfate solution with an iron content of 56 g / L for later use in the preparation of battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0054] Comparative Example 2:
[0055] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved and filter out insoluble impurities.
[0056] Add ammonia to the filtrate to adjust the pH to 3.0. Place the filtrate in a programmable thermostat and heat it to 70°C to evaporate the solution. Reduce the liquid level to about 80% and cool it down at a rate of 0.2°C / min. During the cooling process, keep the stirring speed at 80 rpm until the solution temperature drops to 10°C. Green ferrous sulfate crystals slowly precipitate during the cooling process. Centrifuge the solid-liquid mixture to obtain crystalline crystals and mother liquor.
[0057] The crystalline crystals were dissolved to prepare a ferrous sulfate solution with an iron content of 56 g / L for later use. The pH of the ferrous sulfate solution was adjusted to 3.0 and it was subsequently used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0058] Comparative Example 3:
[0059] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 13.5g of 20% dilute phosphoric acid to the solution. Stir at 60℃ for 30 minutes and then filter. The filtrate is green and will be used to prepare battery-grade iron phosphate products. The filtrate was sent for ICP testing.
[0060] Comparative Example 4:
[0061] Take 700g of ferrous sulfate heptahydrate, a byproduct of titanium dioxide production, and add it to 2L of deionized water at 60℃. Stir until fully dissolved. Add 25% ammonia solution to adjust the pH of the solution to 5.0. Stir at 30℃ for 30 minutes and then filter. The filtrate is green and has a pH of 4.52. Add 98% sulfuric acid to adjust the pH of the solution to 3.0 and set it aside for later use in the preparation of battery-grade iron phosphate products. Send a sample of the filtrate for ICP testing.
[0062] The main metallic impurity contents of the ferrous sulfate filtrate in the above embodiments are shown in the table below:
[0063]
[0064] Examples 1-5 compared the effects of different solution pH values, phosphoric acid concentrations, cooling rates, stirring speeds, and evaporation rates on the impurity removal process. Experimental data showed that when adjusting the pH of the impurity removal solution with ammonia, a balance must be struck between the impurity removal effect and Fe loss; a pH of 2.5 was the optimal condition. Adding 20% phosphoric acid to a ferrous sulfate solution achieved a good impurity removal effect, and the resulting precipitate particles were large, with good settling properties, effectively shortening the filtration time. The cooling rate during recrystallization significantly affected the impurity removal effect; different cooling rates directly influenced the crystal growth rate. Faster crystal growth led to impurities easily coating the crystals, resulting in poorer impurity removal. Different stirring speeds during cooling also varied the impurity removal effect; maintaining a certain level of fluidity was necessary to prevent impurities from coating the crystals during growth. The evaporation rate significantly affected recrystallization impurity removal; too little evaporation resulted in a low recrystallization yield, while too much evaporation caused some impurities to precipitate with the product at lower temperatures, thus affecting the purity of the ferrous sulfate.
[0065] In Comparative Example 1, phosphoric acid was not used as a purification agent; purification was achieved solely through recrystallization. Analysis showed that the Ti content was significantly affected. Therefore, the conclusion is that adding dilute phosphoric acid can significantly reduce the Ti content in the ferrous sulfate filtrate.
[0066] In Comparative Example 2, phosphoric acid was not used as a purifying agent. After adjusting the pH of the solution with ammonia, it was recrystallized once. The resulting ferrous sulfate crystals, when made into a solution of a certain concentration, had a higher impurity content than those in Example 1. This suggests that secondary recrystallization can further remove impurities from the solution.
[0067] In Comparative Example 3, only phosphoric acid was used as a purification agent, and recrystallization was not used for purification. The analysis data shows that phosphoric acid can achieve a good purification effect on Ti, but the contents of other impurities such as Mg and Mn are at a high level.
[0068] In Comparative Example 4, ammonia was used for impurity removal. This method resulted in a large loss of iron, and the impurity removal effect was not much different from that of ferric phosphate. However, the ammonia method could completely remove Al and Ti.
[0069] After comparison, different impurity removal methods have their own advantages and disadvantages. However, based on the impurity content as the criterion, Example 1 can reduce the impurities in ferrous sulfate, a by-product of titanium dioxide, to the lowest level.
[0070] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.
Claims
1. A method for removing ferrous sulfate by-products from titanium dioxide, characterized in that, Includes the following steps: 1) Take ferrous sulfate, a byproduct of titanium dioxide, dissolve it at 50-90℃ and prepare a saturated solution; 2) After adding dilute phosphoric acid and stirring thoroughly, filter to remove insoluble matter to obtain a solution; 3) Add ammonia to the obtained solution, adjust the pH of the filtrate to 2.5-3.5, and then perform a second filtration. The obtained filtrate is then evaporated and concentrated, retaining 60-80% of the solution. 4) Under uniform stirring conditions, the solution is cooled at a fixed rate until the temperature drops to 10°C. The solution is then separated by centrifugation to obtain primary crystals and primary crystallization mother liquor. 5) Dissolve the primary crystal obtained in step 4) again to prepare a saturated solution. Repeat steps 3) and 4) on the resulting solution to obtain secondary crystal and secondary crystal mother liquor. 6) Dissolve the secondary crystals to prepare a ferrous sulfate solution with an iron content of 50-60 g / L and adjust the pH. The adjusted ferrous sulfate solution is then used directly for the preparation of battery-grade iron phosphate. In step 2), the amount of dilute phosphoric acid added is 0.5% to 5% of the mass of the saturated solution, and the concentration of dilute phosphoric acid is 10% to 30%. In step 4), when cooling at a fixed rate, the fixed cooling rate is 0.2 to 0.5 °C / min. In step 6), the pH of the ferrous sulfate solution is 2.5 to 3.
5.
2. The method for removing ferrous sulfate byproduct from titanium dioxide according to claim 1, characterized in that: After adding ammonia to the solution obtained in step 2), stir for 30 minutes and then filter a second time.
3. The method for removing ferrous sulfate byproduct from titanium dioxide according to claim 1, characterized in that: The stirring speed in step 4) is 30-80 rpm.