Method for removing magnesium and calcium impurities in titanium dioxide byproduct ferrous sulfate

Abstract

The invention discloses a method for removing magnesium and calcium impurities in titanium dioxide byproduct ferrous sulfate, and belongs to the technical field of battery material precursor purification. The method comprises the following steps: dissolving the titanium dioxide byproduct ferrous sulfate, and removing titanium by reducing iron powder to obtain a ferrous sulfate pretreatment solution; independently preparing and deeply washing to obtain a pure ferrous hydroxide wet filter cake of which the sodium content is lower than 100 ppm; adding a fluorine ion source into the ferrous sulfate pretreatment solution to obtain a fluorine-containing mixed solution; adding the wet filter cake into the fluorine-containing mixed solution to enable magnesium and calcium ions to form fluoride precipitates, and carrying out solid-liquid separation to obtain a ferrous sulfate purified solution; and finally, adding acid to stabilize the pH to obtain the high-purity ferrous sulfate solution. According to the method, pure ferrous hydroxide is used for replacing traditional alkali liquor, introduction of new impurities such as sodium, potassium and ammonium is avoided in principle while magnesium and calcium impurities are efficiently and synergistically removed, the technological process is simple, the cost is low, and the product purity is high.

Description

Technical Field

[0001] This invention relates to the field of hydrometallurgy and battery material precursor purification technology, specifically to a method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production. Background Technology

[0002] Ferrous sulfate is a key iron source for preparing cathode materials for lithium-ion batteries such as iron phosphate and lithium iron phosphate. Currently, it mainly comes from two sources: one is through the reaction of high-purity metallic iron powder with sulfuric acid, which has high purity but is expensive; the other is from a byproduct of titanium dioxide production (hereinafter referred to as "titanium dioxide byproduct ferrous sulfate"), which has large output and low cost, and has significant economic advantages.

[0003] However, ferrous sulfate, a byproduct of titanium dioxide production, contains various impurities such as titanium, manganese, chromium, vanadium, and high concentrations of magnesium (typically 4000-6000 ppm) and calcium. These impurities, especially magnesium and calcium, severely degrade the electrochemical performance of the final lithium iron phosphate cathode material, including capacity, cycle life, and rate performance. In high-end manufacturing processes, such as hydrothermal processes, which are extremely sensitive to raw material purity, the limits on the content of impurities such as sodium, magnesium, and calcium are even stricter. This directly limits the application of low-cost ferrous sulfate, a byproduct of titanium dioxide production, in these high-value-added fields.

[0004] To remove magnesium and calcium impurities, existing technologies are mainly based on the principle of forming fluoride precipitates. Conventional methods typically involve directly adding soluble fluorides (such as sodium fluoride or potassium fluoride) to the solution or using a combination of hydrofluoric acid with alkaline agents such as sodium hydroxide or ammonia. However, these methods inevitably introduce sodium (Na₂O₃). + ), potassium (K) + ) or ammonium ions (NH4) + New cationic impurities, such as [list of ions], are introduced. The introduction of these foreign ions makes it difficult for the purified product to meet the ultra-high purity requirements of battery-grade materials, especially for hydrothermal processes.

[0005] Therefore, developing a purification method applicable to ferrous sulfate byproducts of titanium dioxide production, which can remove magnesium and calcium without introducing other impurity ions, has become a key technological challenge for reducing battery material costs and enhancing resource utilization value. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production. This method generates pure ferrous hydroxide in situ within the system as a pH adjuster and co-precipitation carrier, which works synergistically with fluoride ions to achieve efficient precipitation and removal of magnesium and calcium impurities. Furthermore, the entire process does not introduce any new cationic impurities such as sodium, potassium, or ammonium, resulting in a high-purity final product, making it particularly suitable for the preparation of high-purity battery-grade ferrous sulfate.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, includes the following steps: S1. Preparation and pretreatment of raw material solution: Ferrous sulfate solid containing magnesium and / or calcium impurities is dissolved in pure water to prepare a first ferrous sulfate solution; under inert gas protection, reduced iron powder is added to the first ferrous sulfate solution to carry out a reduction and impurity removal reaction. After the reaction is completed, the solid and liquid are separated to obtain a ferrous sulfate pretreated solution. S2. Preparation of pure ferrous hydroxide: Under inert gas protection, sodium hydroxide solution is added dropwise to a separately prepared second ferrous sulfate solution to generate ferrous hydroxide colloid; flocculant is added to the ferrous hydroxide colloid to cause ferrous hydroxide to flocculate and precipitate, and after solid-liquid separation and washing, a pure ferrous hydroxide wet filter cake with sodium ion content of less than 100 ppm is obtained. S3. Fluorination and purification: Under inert gas protection, a fluoride ion source is added to the ferrous sulfate pretreatment solution obtained in step S1. The ratio of the molar amount of fluoride ions in the fluoride ion source to the total molar amount of magnesium and calcium ions in the ferrous sulfate pretreatment solution is (2.0~2.65):1, to obtain a fluorine-containing mixture. S4. Precipitation and separation: Under inert gas protection, the pure ferrous hydroxide wet filter cake obtained in step S2 is added to the fluorine-containing mixed solution obtained in step S3, the mixture is stirred and reacted, the pH of the reaction system is adjusted to 5.0-6.0, and solid-liquid separation is performed to obtain a purified ferrous sulfate solution with magnesium and calcium removed. S5. Product Stabilization: Add acid to the purified ferrous sulfate solution obtained in step S4 until the pH of the solution is 1.5-2.5 to obtain a stable high-purity ferrous sulfate solution.

[0008] Furthermore, the ferrous sulfate solid containing magnesium and / or calcium impurities is a byproduct of titanium dioxide production.

[0009] Furthermore, in S1, the mass percentage concentration of iron in the first ferrous sulfate solution is 5.0 wt% - 10.0 wt%.

[0010] Furthermore, in S1, the amount of reduced iron powder added is 0.3wt% - 1.5wt% of the mass of the first ferrous sulfate solution; the temperature of the reduction and impurity removal reaction is 70℃ to 85℃, and the time is 0.5 to 2 hours.

[0011] Furthermore, in S2, the molar ratio of NaOH in the sodium hydroxide solution to FeSO4 in the second ferrous sulfate solution is (1.95~2.10):1; the mass percentage concentration of the sodium hydroxide solution is 8 wt%-12 wt%, and the mass percentage concentration of the second ferrous sulfate solution is 1.2 wt%-6 wt%.

[0012] Furthermore, in S2, the flocculant is an anionic flocculant, and the amount of flocculant added is 0.15 wt% of the mass of ferrous hydroxide colloid.

[0013] Furthermore, in S3, the fluoride ion source is hydrofluoric acid; in S5, the acid is concentrated sulfuric acid.

[0014] Furthermore, in S4, the pH of the reaction system is adjusted to 5.0-6.0 by controlling the amount of ferrous hydroxide wet filter cake added; the stirring time is 30 minutes to 2 hours.

[0015] Furthermore, the inert gas used for inert gas protection is either nitrogen or argon.

[0016] Furthermore, in the purified ferrous sulfate solution obtained from S4, the concentration of magnesium ions was less than 200 ppm, the concentration of calcium ions was less than 100 ppm, and the concentrations of sodium, potassium, and ammonium ions were all less than 100 ppm.

[0017] Compared with the prior art, the present invention has the following beneficial effects: 1. Achieving clean purification and eliminating impurity introduction: This invention abandons the traditional method of directly introducing fluorides or alkaline solutions containing sodium, potassium, or ammonium. Instead, it uses a separately prepared and deeply washed pure ferrous hydroxide wet filter cake as the sole pH adjuster and precipitant carrier. The sodium ion content in this wet filter cake is strictly controlled below 100 ppm, thus completely avoiding the introduction of exogenous cationic impurities such as sodium, potassium, and ammonium at the source, overcoming the fundamental defect of the traditional method of "removing impurities with impurities." The resulting product has high purity and is particularly suitable for high-end applications that are extremely sensitive to impurities.

[0018] 2. High removal efficiency and simultaneous deep removal of magnesium and calcium: This invention provides optimal thermodynamic and kinetic conditions for the precipitation of magnesium fluoride and calcium fluoride by precisely controlling the dosage of fluoride ions (based on the stoichiometric ratio to the total molar amount of magnesium and calcium ions) and the final pH value of the reaction system (preferably 5.0–5.5). Simultaneously, the newly generated ferrous hydroxide colloid acts as an effective nucleus and adsorption medium, significantly promoting the aggregation, co-precipitation, and separation of trace fluoride precipitates. This method can reduce the concentration of magnesium ions in ferrous sulfate solution to below 200 ppm and the concentration of calcium ions to below 100 ppm simultaneously, achieving highly efficient and synergistic deep removal of magnesium and calcium impurities.

[0019] 3. Integrated Process Flow with Titanium Removal Function: This invention organically combines the reduction-based titanium removal pretreatment with the fluorination-based magnesium and calcium removal main process. Firstly, the titanium impurity content can be reduced to below 5 ppm through iron powder reduction, achieving synergistic purification of multiple impurity elements and simplifying the overall process flow.

[0020] 4. Flexible process layout and significant economic benefits: In this invention, the key reagent, namely the second ferrous sulfate solution, can be prepared independently and transported in a sealed manner, while the ferrous sulfate by-product of titanium dioxide production can be dissolved and purified on-site. This mobile, on-site raw material processing design significantly reduces the transportation cost of the raw material solution and facilitates direct connection with the downstream ferric phosphate synthesis section, resulting in outstanding overall economic benefits. It provides a feasible process route for the high-value-added utilization of ferrous sulfate by-products of titanium dioxide production at low cost.

[0021] 5. Excellent product stability: By adjusting the pH to a stable range of 1.5-2.5 in the final step, the oxidation of ferrous ions is effectively inhibited, ensuring the chemical stability of the high-purity ferrous sulfate solution during storage and transportation. Attached Figure Description

[0022] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0024] In the description of this invention, it should be noted that the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0025] like Figure 1 As shown, the method provided by this invention mainly includes five core steps: raw material liquid preparation and pretreatment, preparation of pure ferrous hydroxide, fluorination and impurity removal, precipitation separation and product stabilization.

[0026] S1. Raw material liquid preparation and pretreatment specifically involves: preparing ferrous sulfate solid, a byproduct of titanium dioxide production with magnesium, calcium, and titanium as the main impurities, into iron (in the form of Fe). 2+ A ferrous sulfate solution with a mass percentage concentration of 5.0 wt% to 10.0 wt% was prepared. Subsequently, an inert gas was continuously applied to the reaction system to isolate oxygen and prevent oxidation of the ferrous sulfate. While stirring, reduced iron powder was added to the solution in an amount of 0.3 wt% to 1.5 wt% of the total mass of the ferrous sulfate solution. The reaction system was gradually heated to 70°C to 85°C, and the reaction was continuously stirred at this temperature for 0.5 to 2 hours. This process is a reduction and impurity removal reaction, the core purpose of which is to deeply remove titanium impurities from the solution and ensure that all iron exists in its divalent form.

[0027] In the reduction and impurity removal reaction: initially, due to the Fe contained in the raw materials... 3+ The solution is yellowish-green. As the reaction proceeds, Fe... 3+ Preferred to be reduced to Fe 2+ The yellowish-green color gradually faded. Subsequently, the Ti in the solution... 4+ Reduced to Ti 3+ The system exhibits a characteristic blue-purple color. Accompanied by H + When reduced, hydrogen bubbles are continuously produced. Towards the end of the reaction, unstable Ti... 3+ The water is further reduced to generate hydrogen gas, which itself is converted into titanium hydroxide or titanium oxide hydrate precipitate. The blue color of the solution fades, and finally a mixture containing grayish-white or grayish-black flocculent precipitate is obtained.

[0028] After the reaction was completed, the reaction mixture was subjected to solid-liquid separation under an inert gas atmosphere. The resulting filtrate was a light green or nearly colorless transparent liquid, which was the ferrous sulfate pretreatment solution. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis confirmed that the titanium (Ti) content in this pretreatment solution was less than 5 ppm, and the content of ferric iron was extremely low, with iron mainly existing in the form of ferrous iron, providing a qualified high-purity ferrous sulfate feedstock for subsequent steps.

[0029] S2. The preparation of pure ferrous hydroxide is as follows: First, weigh high-purity ferrous sulfate crystals and dissolve them in deoxygenated deionized water to prepare a second ferrous sulfate solution with an iron (FeSO4) mass percentage concentration of 1.2wt% to 6wt%. This solution should ensure that the content of impurity ions such as magnesium, calcium, sodium, and potassium is extremely low.

[0030] In another container, prepare a sodium hydroxide solution with a mass percentage concentration of 8 wt% to 12 wt%. Place the reactor containing the second ferrous sulfate solution in an environment continuously purged with inert gas to ensure the system is isolated from air. Under constant stirring, slowly and uniformly add the sodium hydroxide solution to the second ferrous sulfate solution. The amount of sodium hydroxide added must be strictly controlled, and the molar ratio of its NaOH content to the FeSO4 content in the second ferrous sulfate solution should be (1.95~2.10):1. During the addition, a dark green or light green ferrous hydroxide colloid gradually forms in the system. This process is exothermic, and the dropping rate must be controlled to maintain a suitable temperature (usually room temperature to 40°C).

[0031] After the sodium hydroxide solution has been added dropwise, continue stirring for a period of time (e.g., 10-30 minutes) to ensure complete reaction. Then, add a flocculant, such as anionic polyacrylamide, to the resulting ferrous hydroxide colloid. The amount added is 0.15 wt% of the mass of the ferrous hydroxide colloid. After adding the flocculant, the colloid will rapidly aggregate to form large, easily settling flocculent clumps. Stop stirring and allow it to stand for a period of time (e.g., 0.5-2 hours) to allow the flocculent ferrous hydroxide to precipitate completely.

[0032] After precipitation, solid-liquid separation is performed under inert gas protection (e.g., by suction filtration, pressure filtration, or centrifugation). The resulting filter cake is repeatedly washed (preferably 3 to 5 times) with a large amount of deoxygenated pure water or deionized water to thoroughly remove soluble impurities such as sodium ions and sulfate ions adsorbed on the precipitate pores and surface.

[0033] After thorough washing and separation, the obtained product was a pure ferrous hydroxide wet filter cake. Samples of this wet filter cake were taken and, after appropriate treatment (such as dissolution and acidification), detected by atomic absorption spectrometry or inductively coupled plasma atomic emission spectrometry. The results showed that the wet filter cake contained sodium ions (Na₂O₃). + The content of [unspecified substance] is less than 100 ppm, meeting the requirement of being a pure precipitant that does not introduce impurities. The wet filter cake should be sealed and stored in an inert atmosphere for use in step S4.

[0034] S3. Fluorination and depurification specifically involves: taking the ferrous sulfate pretreated solution obtained in S1 and transferring it to a reactor equipped with a stirrer, thermometer, and inert gas inlet pipe. Stirring is started, and inert gas is continuously introduced above the liquid surface to maintain a reducing atmosphere throughout the system and prevent the oxidation of ferrous ions.

[0035] Based on the magnesium ions (Mg) in the ferrous sulfate pretreatment solution 2+ ) and calcium ions (Ca 2+ ) total content, calculate the required fluoride ions (F - The molar amount of fluoride ions (F) is controlled. An aqueous solution of hydrofluoric acid (HF) is used as the fluoride ion source. - The molar ratio of fluoride ions to the total molar ratio of magnesium and calcium ions in the pretreatment solution is (2.0~2.65):1. This ratio ensures that sufficient fluoride ions react fully with all magnesium and calcium ions to form fluoride precipitates, while leaving an appropriate margin to overcome kinetic factors and achieve deep removal.

[0036] Under continuous stirring and inert gas protection, a calculated amount of fluoride ion source (e.g., slowly adding hydrofluoric acid using a dropping device made of hydrofluoric acid-resistant material) is added to the ferrous sulfate pretreatment solution. During the addition process, the system temperature should be maintained between room temperature and 50°C to avoid drastic temperature rise. After the fluoride ion source is completely added, continue stirring for a period of time (e.g., 15-60 minutes) to allow the fluoride ions to be evenly distributed in the solution and to undergo initial combination with some magnesium and calcium ions, resulting in a homogeneous and clear fluoride-containing mixture.

[0037] S4. Precipitation and separation specifically involves transferring the fluorine-containing mixture prepared in S3 to a reactor equipped with a powerful stirrer, an online pH monitor, and an inert gas protection system. Inert gas is continuously introduced to ensure the reaction proceeds in a strictly anaerobic environment.

[0038] Under stirring, the pure ferrous hydroxide wet filter cake obtained in S2 is added in batches or continuously to the fluoride-containing mixture. The addition of the ferrous hydroxide wet filter cake is crucial for pH adjustment and impurity precipitation in this step. With the addition of ferrous hydroxide, it undergoes a neutralization reaction with the acid in the solution (Fe(OH)2 + 2H+). + → Fe 2+ (+ 2H2O), the pH value of the reaction system gradually increases accordingly. By controlling the total amount of wet filter cake added, the final pH value of the reaction system is precisely adjusted and maintained within the range of 5.0 to 6.0, preferably between 5.0 and 5.5. This pH range is the most suitable range for the precipitation of magnesium fluoride (MgF2) and calcium fluoride (CaF2), which can ensure the complete precipitation reaction and minimize the dissolution of ferrous hydroxide or the generation of other side reactions.

[0039] Under these pH conditions, the fluoride ions (F) in the solution - ) and magnesium ions (Mg 2+ ), calcium ions (Ca 2+ The reaction proceeds rapidly, producing precipitates of MgF2 and CaF2 with extremely low solubility. Simultaneously, the newly dissolved Fe... 2+ When added to the solution, the ferrous hydroxide colloids or microcrystals present in the system can act as heterogeneous crystal nuclei, significantly promoting the aggregation and growth of fine fluoride particles, forming precipitates that are easy to settle.

[0040] Maintain the system at the set pH and continue stirring for a period of time (usually 30 minutes to 2 hours) to ensure the precipitation reaction reaches equilibrium and aging is complete. Then, stop stirring and allow to settle (e.g., 0.5-1 hour) or directly use a solid-liquid separation device under an inert atmosphere for solid-liquid separation. The resulting filtrate is a clear purified ferrous sulfate solution. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis shows that the magnesium ion concentration in this purified solution is below 200 ppm, and the calcium ion concentration has simultaneously decreased to a low level (usually below 100 ppm). No significant increase in the concentrations of sodium, potassium, or ammonium ions was detected due to this step, proving that impurities were effectively removed and no new impurities were introduced. The resulting filter residue mainly consists of magnesium fluoride, calcium fluoride, and a small amount of entrained iron compounds, which can be used as raw material for solid waste treatment or valuable element recovery.

[0041] S5. Product stabilization specifically involves transferring the purified ferrous sulfate solution obtained after solid-liquid separation in S4 to a clean, acid-resistant container. Under normal temperature and pressure conditions, and with stirring, concentrated acid is slowly added to the purified solution for acidification. The concentrated acid is preferably concentrated sulfuric acid, as it does not introduce new cationic impurities and provides a high concentration of hydrogen ions (H+). + This allows for rapid pH adjustment. The amount of acid added needs to be precisely controlled using an online pH meter or by taking timed samples until the final pH of the solution stabilizes within the range of 1.5 to 2.5, preferably around 2.0.

[0042] Adjusting the solution to the range of 1.5 to 2.5 has a dual key function: firstly, by lowering the oxidation potential of dissolved oxygen and increasing the hydrogen ion concentration, it effectively suppresses ferrous ions (Fe2+). 2+ ) to trivalent iron ions (Fe 3+ The oxidation tendency of Fe is thus avoided. 3+ The potential precipitation from hydrolysis and its negative impact on the quality of downstream products; secondly, an acidic environment can prevent trace amounts of fluoride ions (F) remaining in the solution. - (Other impurity ions may precipitate due to local pH fluctuations during subsequent storage, ensuring that the solution remains clear and homogeneous at all times.)

[0043] After acidification, continue stirring to ensure homogeneous mixing, resulting in a stable, high-purity ferrous sulfate solution. This solution can be directly used in downstream iron phosphate or lithium iron phosphate synthesis processes, or further processed through concentration, crystallization, etc. The product should be stored in a sealed, light-protected container to minimize air contact and the effects of light.

[0044] Example 1. Ferrous sulfate, a byproduct of a titanium dioxide plant, was used as raw material. Its magnesium content was approximately 5300 ppm and its calcium content was approximately 50 ppm.

[0045] S1. Weigh 81g of solid ferrous sulfate (ferrous sulfate byproduct of a titanium dioxide plant), dissolve it in pure water, and dilute to 120ml to prepare a solution with an iron content of approximately 9.8wt% (first ferrous sulfate solution). Under nitrogen protection, add 0.81g of reduced iron powder to the solution, heat to 80℃, and stir for 2 hours. During the reaction, the solution color changes from yellow-green to blue-purple and then to nearly light blue, with a grayish-white precipitate forming. After the reaction is complete, filter under nitrogen protection to obtain approximately 120mL of clear ferrous sulfate pretreated solution, with a titanium content of <5 ppm.

[0046] S2. In another reactor, under nitrogen protection, a 1.3 wt% pure ferrous sulfate solution (second ferrous sulfate solution) prepared with 290 g of pure water was used. Based on the molar ratio of NaOH in the sodium hydroxide solution to FeSO4 in the pure ferrous sulfate solution being 2:1, the required amount of sodium hydroxide was calculated and prepared into a 10 wt% solution. This solution was slowly added dropwise to the pure ferrous sulfate solution with stirring until precipitation was complete, producing approximately 310 g of dark green ferrous hydroxide colloid. Flocculant was added, and after standing for flocculation, the mixture was filtered. The filter cake was washed multiple times with pure water. The washed wet ferrous hydroxide filter cake was tested, and the Na... + The content is 85 ppm.

[0047] S3. Under nitrogen protection, based on the total magnesium and calcium content in the pretreatment solution of S1 (calculated from the magnesium content in 81g of ferrous sulfate solid raw material), 2+ With Ca 2+ The total molar amount is approximately 0.0179 mol), according to F⁻:(Mg 2+ +Ca 2+ With a molar ratio of 2.6:1, add approximately 2.33g of 40% hydrofluoric acid to the pretreatment solution and stir for 10 minutes to obtain a fluorine-containing mixture.

[0048] S4. Add all the wet filter cake of ferrous hydroxide obtained in S2 (containing approximately 2.25 g of Fe(OH)2) to the fluorine-containing mixture in S3, and stir the mixture for 1 hour under nitrogen protection. The pH of the reaction system eventually stabilizes at 5.2. Stop stirring, allow the mixture to settle for 2 hours, and then perform pressure filtration to obtain a clear purified ferrous sulfate solution.

[0049] S5. Slowly add concentrated sulfuric acid to the purified solution to adjust the pH to 2.0, thus obtaining a stable high-purity ferrous sulfate solution.

[0050] Detection results: Inductively coupled plasma atomic emission spectrometry (ICP-OES) analysis of the ferrous sulfate solution obtained from S5 showed the following main elemental contents: Fe: ~7.5 wt%, Mg: 190 ppm, Na: <10 ppm, Ca: <10 ppm, K: <5 ppm, NH4+ + <5 ppm. Magnesium removal rate is greater than 95%, and no new cationic impurities are introduced.

[0051] Comparative example. Using the traditional method: A calculated amount of hydrofluoric acid and 10% sodium hydroxide solution were directly added to the ferrous sulfate pretreatment solution obtained from S1, and the pH was adjusted to 5.5 to induce precipitation. After precipitation separation, the solution analysis results were: Mg: 150 ppm, Ca: 110 ppm, but Na... + The content is as high as 1200 ppm. Although this method can remove some magnesium and calcium, it introduces serious sodium contamination, making it unsuitable for the preparation of high-purity products.

[0052] Finally, it should be noted that the above embodiments are merely preferred embodiments of the present invention used to illustrate the technical solutions of the present invention, and are not intended to limit the invention, nor are they intended to limit the patent scope of the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention. That is to say, any changes or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but whose technical problems are still consistent with the present invention, should be included within the protection scope of the present invention. In addition, the direct or indirect application of the technical solutions of the present invention to other related technical fields are similarly included within the patent protection scope of the present invention.

Claims

1. A method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, characterized in that, Includes the following steps: S1. Preparation and pretreatment of raw material solution: Ferrous sulfate solid containing magnesium and / or calcium impurities is dissolved in pure water to prepare a first ferrous sulfate solution; under inert gas protection, reduced iron powder is added to the first ferrous sulfate solution to carry out a reduction and impurity removal reaction. After the reaction is completed, the solid and liquid are separated to obtain a ferrous sulfate pretreated solution. S2. Preparation of pure ferrous hydroxide: Under inert gas protection, sodium hydroxide solution is added dropwise to a separately prepared second ferrous sulfate solution to generate ferrous hydroxide colloid; flocculant is added to the ferrous hydroxide colloid to cause ferrous hydroxide to flocculate and precipitate, and after solid-liquid separation and washing, a pure ferrous hydroxide wet filter cake with sodium ion content of less than 100 ppm is obtained. S3. Fluorination and purification: Under inert gas protection, a fluoride ion source is added to the ferrous sulfate pretreatment solution obtained in step S1. The ratio of the molar amount of fluoride ions in the fluoride ion source to the total molar amount of magnesium and calcium ions in the ferrous sulfate pretreatment solution is (2.0~2.65):1, to obtain a fluorine-containing mixture. S4. Precipitation and separation: Under inert gas protection, the pure ferrous hydroxide wet filter cake obtained in step S2 is added to the fluorine-containing mixed solution obtained in step S3, the mixture is stirred and reacted, the pH of the reaction system is adjusted to 5.0-6.0, and solid-liquid separation is performed to obtain a purified ferrous sulfate solution with magnesium and calcium removed. S5. Product Stabilization: Add acid to the purified ferrous sulfate solution obtained in step S4 until the pH of the solution is 1.5-2.5 to obtain a stable high-purity ferrous sulfate solution.

2. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... Ferrous sulfate solid containing magnesium and / or calcium impurities is a byproduct of titanium dioxide production.

3. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In S1, the mass percentage concentration of iron in the first ferrous sulfate solution is 5.0 wt% - 10.0 wt%.

4. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In S1, the amount of reduced iron powder added is 0.3wt% - 1.5wt% of the mass of the first ferrous sulfate solution; the temperature of the reduction and impurity removal reaction is 70℃ to 85℃, and the time is 0.5 to 2 hours.

5. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In S2, the molar ratio of NaOH in the sodium hydroxide solution to FeSO4 in the second ferrous sulfate solution is (1.95~2.10):1; the mass percentage concentration of the sodium hydroxide solution is 8 wt%-12 wt%, and the mass percentage concentration of the second ferrous sulfate solution is 1.2 wt%-6 wt%.

6. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that, In S2, the flocculant is an anionic flocculant, and the amount of flocculant added is 0.15 wt% of the mass of ferrous hydroxide colloid.

7. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In S3, the fluoride ion source is hydrofluoric acid; in S5, the acid is concentrated sulfuric acid.

8. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In S4, the pH of the reaction system is adjusted to 5.0-6.0 by controlling the amount of ferrous hydroxide wet filter cake added; the stirring time is 30 minutes to 2 hours.

9. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that, The inert gas used for inert gas protection is nitrogen or argon.

10. The method for removing magnesium and calcium impurities from ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that, The purified ferrous sulfate solution obtained from S4 has a magnesium ion concentration of less than 200 ppm, a calcium ion concentration of less than 100 ppm, and sodium, potassium, and ammonium ion concentrations of less than 100 ppm.

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