Method for recovering titanium dioxide byproduct ferrous sulfate

Abstract

The invention discloses a method for recovering a titanium dioxide byproduct ferrous sulfate. The method comprises the following steps: S1, dissolving a titanium dioxide byproduct ferrous sulfate in water, and filtering to obtain a first filtrate; s2, adjusting the pH value of the first filtrate to 4.5-5.5, and filtering to obtain a second filtrate; s3, adding sulfide into the second filtrate, and filtering after reaction to obtain third filtrate; s4, adding an alkaline liquid into the third filtrate, and filtering after reaction to obtain a fourth filtrate and a first filter cake, so as to obtain ferrous hydroxide with first purity; and S5, preparing ferrous oxalate dihydrate from the ferrous hydroxide with the first purity. According to the method, through the synergistic effect of accurate pH value control, sulfide impurity removal and fractional precipitation, efficient separation of iron and manganese is achieved, the complex impurity removal process of multiple oxidation reduction is avoided, the operation process is simple, ferrous oxalate dihydrate is prepared after high-purity ferrous hydroxide is prepared, and the impurity removal cost is low.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial solid waste resource utilization, and in particular to a method for recovering ferrous sulfate produced as a by-product of titanium dioxide. Background Art

[0002] Amid the rapid development of the new energy industry, market demand for lithium iron phosphate (LIFP) as a battery cathode material continues to grow. Ferrous sulfate, a byproduct of titanium dioxide, is a potential high-quality raw material for the production of LFP due to its high iron content and widespread availability. However, this byproduct contains significant impurities, such as titanium, manganese, magnesium, and calcium, which severely impact its application in battery materials. Efficient impurity removal has become a key technical challenge. Currently, in the process of recovering ferrous sulfate, a by-product of titanium dioxide, to prepare ferrous oxalate, when removing manganese impurities in the by-product ferrous sulfate, sulfuric acid and an oxidant are needed to completely oxidize the ferrous sulfate into ferric sulfate. Then, a strong oxidant such as permanganate or ozone is used to oxidize divalent manganese into manganese dioxide for removal. Finally, the ferric sulfate needs to be reduced back to ferrous sulfate. This process is cumbersome and not only increases reagent costs and operational complexity, but also multiple redox reactions can easily lead to a decrease in product purity. In terms of removing calcium and magnesium impurities, traditional methods usually use fluoride to separate them by forming insoluble calcium fluoride and magnesium fluoride precipitates. However, fluoride is highly corrosive and toxic, posing a major safety hazard during production, storage, and transportation. Once leaked, it can easily cause serious harm to the environment and human health. At the same time, the use of fluoride for impurity removal is expensive and has poor economic efficiency. Therefore, it is urgent to develop a safer, more efficient and economical process for removing impurities from ferrous sulfate, a by-product of titanium dioxide, to solve the problems of the traditional method in which the ferrous sulfate is first oxidized and then reduced, and the use of fluoride is dangerous and uneconomical, so as to improve the impurity removal effect and provide guarantees for the preparation of high-quality chemical products such as battery-grade ferrous oxalate and battery-grade ferric phosphate. Summary of the Invention

[0003] The invention discloses a method for recovering ferrous sulfate produced as a by-product of titanium dioxide, so as to solve the technical problems in the prior art of recovering ferrous sulfate produced as a by-product of titanium dioxide to prepare ferrous oxalate, such as complicated impurity removal steps and high cost.

[0004] In order to solve the above problems, the present invention adopts the following technical solutions: In a first aspect, the present application provides a method for recovering ferrous sulfate produced as a by-product of titanium dioxide, comprising the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water and filter to obtain a first filtrate; S2, adjusting the pH of the first filtrate to 4.5-5.5, filtering, and obtaining a second filtrate; S3, adding sulfide to the second filtrate, filtering after the reaction, to obtain a third filtrate; S4. Adding an alkaline liquid to the third filtrate, the amount of the alkaline liquid added is based on the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of hydroxide in the alkaline liquid of 1:0.1-1.2. After the reaction, filtering is performed under the protection of an inert gas to obtain a fourth filtrate and a first filter cake, and the first filter cake is washed under the protection of an inert gas to obtain ferrous hydroxide of a first purity.

[0005] S5. Dispersing the ferrous hydroxide of the first purity in pure water, adding oxalic acid, and filtering after the reaction to obtain a fifth filtrate and a second filter cake. After the second filter cake is dried, ferrous oxalate dihydrate is obtained.

[0006] The technical solution adopted by the present invention can achieve the following beneficial effects: (1) The method for recovering ferrous sulfate produced as a by-product of titanium dioxide provided in this application achieves efficient separation of iron and manganese through the synergistic effect of precise control of pH value, sulfide impurity removal, and step-by-step precipitation. The impurity removal step is simple, avoiding the defects of traditional processes. Specifically: First, the ferrous sulfate produced as a by-product of titanium dioxide is dissolved and filtered to remove insoluble impurities such as slag and TiO2 particles. Then, an alkaline liquid is added to adjust the pH for the second step of impurity removal. When the pH is 4.5-5.5, TiO2 4+ 、Al 3+ Cr 3+ The solubility of the hydroxide of the plasma is extremely low and will precipitate first; while Fe 2+ The precipitation pH is about 8.5. At pH 4.5-5.5, Fe 2+ Still remain in the second filtrate; therefore, by controlling the pH value, the initial separation of impurities such as titanium, aluminum, chromium and iron can be achieved, avoiding the influence of these impurities on the purity of the product in the subsequent steps; then, sulfide is added for the third step of impurity removal. Sulfide (such as Na2S, BaS, etc.) can react with most transition metal ions (such as Cu 2+ , Pb 2+ 、Zn 2+ 、Ni 2+ , etc.) to form an extremely insoluble sulfide precipitate, while MnS still has a high solubility under weakly acidic conditions; therefore, by controlling the amount of sulfide added, other transition metal impurities can be selectively removed, while manganese will remain in the third filtrate; finally, the iron is subjected to step-by-step precipitation. Since there are still impurity manganese ions in the third filtrate, the present application adopts a reverse approach, firstly recovering the iron by step-by-step precipitation, while retaining the manganese in the solution, avoiding the complex impurity removal process of multiple oxidation-reduction processes, not only simplifying the operating process, but also the obtained first-purity ferrous hydroxide can be used to prepare battery-grade ferrous oxalate dihydrate, and the purity of the obtained ferrous oxalate dihydrate reaches more than 99.5%; and the first-purity ferrous hydroxide can also be used to prepare high-purity iron oxide.

[0007] (2) The method for recovering ferrous sulfate produced as a by-product of titanium dioxide provided in this application avoids the complex impurity removal process of multiple oxidation-reduction reactions, reduces the amount of reagents added, and thus reduces the reagent addition cost, thereby greatly reducing the production cost of battery-grade ferrous oxalate dihydrate.

[0008] (3) The method for recovering ferrous sulfate produced as a by-product of titanium dioxide provided in this application avoids the use of fluoride, thus avoiding the potential safety hazards of using fluoride, and is also beneficial to the environment. DETAILED DESCRIPTION

[0009] To make the objectives, technical solutions, and advantages of the present invention more apparent, the technical solutions of the present invention will be described in detail below. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other implementations obtained by those of ordinary skill in the art without inventive effort are within the scope of protection of the present invention.

[0010] The terms "first," "second," and the like in the specification and claims of this application are used to distinguish similar objects, and are not used to describe a specific order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, so that the embodiments of this application can be implemented in an order other than that described herein, and that the objects distinguished by "first," "second," and the like are generally of the same type, and do not limit the number of objects; for example, the first object can be one or more. In addition, the term "and / or" in the specification and claims refers to at least one of the connected objects, and the character " / " generally indicates that the objects connected are in an "or" relationship.

[0011] The present application provides a method for recovering ferrous sulfate produced as a by-product of titanium dioxide, comprising the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water and filter to obtain a first filtrate; In this step, the ferrous sulfate (mainly FeSO4·H2O) produced as a by-product of titanium dioxide is dissolved in water and dissociated into iron ions and sulfate ions. Insoluble impurities (such as slag and TiO2 particles) are removed by filtration.

[0012] S2. Add an alkaline liquid to the first filtrate to adjust the pH to 4.5-5.5, and filter to obtain a second filtrate; In this step, the pH is adjusted to remove impurities. When the pH is between 4.5 and 5.5, Ti 4+ 、Al 3+ Cr 3+ The solubility of hydroxides in plasma is very low (such as K sp ≈10 -54), will be precipitated first. 2+ The precipitation pH is about 8.5. At pH 4.5-5.5, Fe 2+ Still remain in the second filtrate. Therefore, by controlling the pH value, it is possible to achieve a preliminary separation of impurities such as titanium, aluminum, and chromium from iron, preventing the impact of these impurities on product purity in subsequent steps. Taking ammonia as an example, the following chemical reaction occurs in this step: TiO 2+ +2H2O→Ti(OH)4↓+2H + Al 3+ +3NH3·H2O→Al(OH)3↓+3NH4 + Cr 3+ +3NH3·H2O→Cr(OH)3↓+3NH4 + S3, adding sulfide to the second filtrate, filtering, and obtaining a third filtrate; In this step, impurities are removed by sulfide. Sulfide (such as Na2S, BaS, etc.) can react with most transition metal ions (such as Cu 2+ , Pb 2+ 、Zn 2+ 、Ni 2+ etc.) to form extremely insoluble sulfide precipitates (such as CuS K sp ≈10 -36 ), while K of MnS sp About 10 -13 ), which still has high solubility under weakly acidic conditions. Therefore, by controlling the amount of sulfide added, most transition metal impurities can be selectively removed, while manganese, calcium, and magnesium, whose solubility products are comparable to or slightly smaller than those of ferrous sulfide, will remain in the third filtrate. Taking sodium sulfide as an example, the following chemical reaction occurs in this step: Na2S→2Na + +S 2- Cu 2+ +S 2- →CuS↓ Pb 2+ +S 2- →PbS↓ Zn 2+ +S 2- →ZnS↓ S4, add alkaline liquid to the third filtrate, the amount of alkaline liquid added is 1:0.1-1.2 according to the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of hydroxide in the alkaline liquid, and after the reaction, filter under the protection of an inert gas to obtain the fourth filtrate and the first filter cake, and wash the first filter cake under the protection of an inert gas to obtain ferrous hydroxide of the first purity; This step is the first precipitation of iron. Under the conditions of this step, the Fe 2+ With OH - The reaction generates Fe(OH)2 precipitate. Due to the relatively low hydroxide concentration, Ca 2+ Mg 2+ 、Mn 2+ Remaining in the solution, the iron can be precipitated first, obtaining high-purity (first purity) Fe(OH)2. Taking ammonia as an example, the following chemical reaction occurs in this step: Fe 2+ +2OH - →Fe(OH)2↓ S5, dispersing the first purity ferrous hydroxide in pure water, adding oxalic acid, and filtering after reaction to obtain the fifth filtrate and the second filter cake, and drying the second filter cake to obtain ferrous oxalate dihydrate. The following chemical reaction occurs in this step: Fe(OH)2+H2C2O4→FeC2O4·2H2O+2H2O.

[0013] In some embodiments, in step S1, the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:2-4.

[0014] In some embodiments, in step S2, the alkaline liquid selected to adjust the pH of the first filtrate is any one of ammonia water and sodium hydroxide solution.

[0015] In some embodiments, in step S2, the alkaline liquid is ammonia water. Using ammonia water to adjust the pH to 4.5-5.5 is an ideal solution for removing titanium, aluminum, and chromium impurities. + The system can stabilize the pH and avoid local over-alkalinity, which can ensure the complete precipitation of impurities without introducing new metal ion impurities.

[0016] In some embodiments, in step S2, when adding the alkaline liquid for adjusting the pH to the first filtrate, the first filtrate is stirred. After the alkaline liquid is added, the stirring is continued and then the filtration is performed.

[0017] In some embodiments, in step S2, when the alkaline liquid for adjusting the pH is added to the first filtrate, the stirring rate of the first filtrate is 100-300 rpm. After the alkaline liquid is added, stirring is continued at a stirring rate of 100-300 rpm for 20-60 minutes, and then filtration is performed.

[0018] In some embodiments, in step S3, the sulfide is selected from any one of sodium sulfide, barium sulfide, hydrogen sulfide and ammonium bisulfide.

[0019] In some embodiments, in step S3, the mass of the added sulfide is 0.8%-1.5% of the mass of the ferrous sulfate by-product of titanium dioxide.

[0020] In some embodiments, in step S3, when the sulfide is added to the second filtrate, the second filtrate is stirred. After the sulfide is added, the stirring is continued and then the filtration is performed.

[0021] In some embodiments, in step S3, when sulfide is added to the second filtrate, the stirring rate of the second filtrate is 100-300 rpm. After the sulfide is added, the stirring rate is continued at 100-300 rpm for 20-60 minutes, and then filtration is performed.

[0022] In some embodiments, in step S4, the amount of alkaline liquid added is such that the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of hydroxide in the alkaline liquid is 1:1.0-1.2.

[0023] In some embodiments, in step S4, the alkaline liquid is any one of aqueous ammonia and sodium hydroxide solution.

[0024] In some embodiments, in step S4, when adding the alkaline liquid to the third filtrate, the third filtrate is stirred. After the alkaline liquid is added, the stirring is continued and then the filtration is performed.

[0025] In some embodiments, in step S4, when adding the alkaline liquid to the third filtrate, the stirring rate of the third filtrate is 100-300 rpm. After the alkaline liquid is added, the stirring rate is continued at 100-300 rpm for 20-60 minutes, and then the filtration is performed.

[0026] In some embodiments, in step S4, the filtration is performed using a membrane filter press, an inert gas protection type vacuum drum filter, a sealed horizontal spiral unloading filter centrifuge, or an inert gas protection type pressurized plate and frame filter. It is understood that in the process of producing ferrous hydroxide, because it has strong reducing properties and high purity requirements, the filtration process needs to be strictly isolated from air and ensure low moisture content. Suitable filtering equipment can be selected according to the production scale and other specific requirements.

[0027] In some embodiments, in step S4, the washing is performed under the protection of an inert gas, and the first filter cake is first washed with a NaOH solution having a pH of 7.5-8.5 for two or more washings, and then washed with pure water.

[0028] In some embodiments, in step S4, the washing is performed under the protection of inert gas, and a NaOH solution with a pH of 8 is added to the first filter cake at a solid-liquid ratio of 1:3-5, and the washing is performed three times; then, pure water is used to wash the first filter cake once at a solid-liquid ratio of 1:3-5.

[0029] In some embodiments, in step S5, the first purity ferrous hydroxide is dispersed in pure water at a solid-liquid ratio of 1:3.5-4.5, and oxalic acid is added under stirring. The amount of oxalic acid added is added at a molar ratio of ferrous hydroxide to oxalic acid of 1:1. After the addition of oxalic acid, stirring and reacting are continued for 0.8-1.5 hours. After the reaction, filtering is performed to obtain a second filter cake, and the second filter cake is dried at a temperature of 95-105° C. for 2-3 hours to obtain ferrous oxalate dihydrate; In some embodiments, in step S5, the fifth filtrate is reused for dispersion of first purity ferrous hydroxide.

[0030] In some embodiments, the recovery method further comprises: step S6, secondary precipitation of iron and co-production of ferric phosphate dihydrate, specifically comprising: S61. Adding an alkaline liquid to the fourth filtrate, wherein the amount of the alkaline liquid added is based on a ratio of the molar amount of ferrous iron in the fourth filtrate to the molar amount of hydroxide in the alkaline liquid of 1:0.1-1.9. After the reaction, filtering is performed under the protection of an inert gas to obtain a sixth filtrate and a third filter cake, and the third filter cake is washed under the protection of an inert gas to obtain ferrous hydroxide of a second purity. Taking the alkaline liquid as aqueous ammonia as an example, the following chemical reaction occurs in this step: Fe 2+ +2OH - →Fe(OH)2↓; S62, then dispersing the second purity ferrous hydroxide in pure water, adding phosphoric acid and hydrogen peroxide to react, filtering, washing, and drying after the reaction to obtain ferric phosphate dihydrate. The following chemical reaction occurs in this step: Fe(OH)2+H3PO4→FeHPO4+H2O 2FeHPO4+H2O2→2FePO4·2H2O↓+2H2O.

[0031] It is understood that this step is to carry out the second precipitation of iron to generate ferrous hydroxide of the second purity for the production of ferric phosphate dihydrate. 2+ This method is to further precipitate the remaining iron by increasing the hydroxide ratio, but at this time the manganese ion concentration in the solution is relatively high, which increases the possibility of co-precipitation with ferrous ions, resulting in the purity of the third filter cake being slightly lower than that of the first filter cake.

[0032] In some embodiments, in step S61, the amount of the alkaline liquid added is such that the ratio of the molar amount of the ferrous iron in the fourth filtrate to the molar amount of the hydroxide in the alkaline liquid is 1:1.6-1.9.

[0033] In some embodiments, the alkaline liquid includes any one of aqueous ammonia and sodium hydroxide solution.

[0034] In some embodiments, in step S61, when adding the alkaline liquid to the fourth filtrate, the fourth filtrate is kept stirred, and after the alkaline liquid is added, the stirring is continued and then the filtration is performed.

[0035] In some embodiments, in step S61, when adding the alkaline liquid to the fourth filtrate, the stirring rate of the fourth filtrate is 100-300 rpm. After the alkaline liquid is added, the stirring is continued at a stirring rate of 100-300 rpm for 20-60 minutes, and then filtering is performed.

[0036] In some embodiments, in step S61, after the second precipitation of iron, a second purity of ferrous hydroxide is obtained, and therefore the filtration is performed using a membrane filter press, an inert gas protection type vacuum drum filter, a sealed horizontal spiral unloading filter centrifuge, or an inert gas protection type pressurized plate and frame filter. It is understood that in the process of producing ferrous hydroxide, because it has strong reducing properties and high purity requirements, the filtration process needs to be strictly isolated from air and ensure low moisture content. Suitable filtering equipment can be selected according to the production scale and other specific requirements.

[0037] In some embodiments, in step S61, the third filter cake is washed under the protection of inert gas, firstly using a NaOH solution with a pH of 7.5-8.5 for more than two times, and then washing with pure water.

[0038] In some embodiments, in step S61, the third filter cake is washed under inert gas protection, and a NaOH solution with a pH of 8 is added at a solid-liquid ratio of 1:3-5 to wash the third filter cake three times; then, the third filter cake is washed once with pure water at a solid-liquid ratio of 1:3-5.

[0039] In some embodiments, in step S62, the second purity ferrous hydroxide is dispersed in pure water at a solid-liquid ratio of 1:3.5-4.5; the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.05-1.15. After the reaction, the filter cake is washed with pure water and washed in steps of 2-3 times at a solid-liquid ratio of 1:3-5, and then dried at 95-105°C for 2-3 hours to obtain ferric phosphate dihydrate.

[0040] In some embodiments, the recovery method further comprises: step S7, co-producing calcium sulfate, specifically comprising: S71. An alkaline liquid is added to the sixth filtrate to adjust the pH to 8.5-9.5, and the sixth filtrate is aerated with an oxidizing gas to react the iron ions and manganese ions in the sixth filtrate with the oxidizing gas to form a precipitate, which is then filtered to obtain a seventh filtrate. For example, the alkaline liquid is aqueous ammonia. The following chemical reaction occurs in this step: 2Fe 2+ +[O]+4OH - +H2O → 2Fe(OH)3↓ Mn 2+ +[O]+2OH - +H2O → MnO(OH)2↓ S72: Add lime to the seventh filtrate to obtain calcium sulfate after reaction, and recover ammonia water by gas stripping. The following chemical reaction will occur in this step: Calcium sulfate precipitation reaction: Ca(OH)2+(NH4)2SO4→CaSO4↓+2NH3·H2O Ammonia stripping reaction: In some embodiments, in step S71, the oxidizing gas is selected from any one or more of air, oxygen, and ozone.

[0041] In some embodiments, in step S71, the alkaline liquid includes any one of aqueous ammonia and sodium hydroxide solution.

[0042] In some embodiments, in step S71, Fe in the sixth filtrate 2+ and Mn 2+ The total molar amount is a; the amount of oxidizing gas added, calculated as oxygen atoms, is a:oxygen = 1:0.56-1.11.

[0043] In some embodiments, step S72 is specifically as follows: adding lime with a purity of 80%-95% to the seventh filtrate to adjust the pH to 10.5-11.5; filtering to obtain a fourth filter cake and an eighth filtrate, wherein the fourth filter cake is calcium sulfate; heating the obtained eighth filtrate to 55-65°C, performing negative pressure gas stripping to obtain ammonia-containing gas, cooling the ammonia-containing gas generated by the gas stripping to 20-30°C, and finally recovering the ammonia water using deionized water or dilute ammonia water as an absorbent.

[0044] The present application provides a method for recovering ferrous sulfate, a by-product of titanium dioxide, which is recovered through two precipitations of iron to produce high-purity ferrous oxalate dihydrate and ferric phosphate dihydrate, respectively. During the iron precipitation process, the efficient separation of iron and manganese is achieved with a simple impurity removal step through precise control of pH value, sulfide impurity removal, and the synergistic effect of step-by-step precipitation, thereby greatly reducing the production cost of ferrous oxalate dihydrate. Moreover, since the complex impurity removal process of multiple oxidation-reduction steps is avoided, the recovery process is simpler. At the same time, calcium sulfate can be co-produced, and ammonia water can be recovered by gas stripping, which can increase the recovery income, maximize the economic benefits, and avoid waste of resources.

[0045] The following is a specific example of producing ferrous hydroxide from ferrous sulfate produced as a by-product of titanium dioxide.

[0046] 1. Raw materials: Source of raw materials: Ferrous sulfate, a by-product of titanium dioxide, was purchased from Longbai Sichuan Titanium Co., Ltd. The main impurity content of ferrous sulfate, a by-product of titanium dioxide, is shown in Table 1 below: Table 1: Main element content of ferrous sulfate as a by-product of titanium dioxide 2. Preparation Example Example 1: A method for recovering ferrous sulfate produced as a by-product of titanium dioxide comprises the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water (the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:3), filter, and obtain a first filtrate; S2, maintaining the stirring rate of the first filtrate at 200 rpm, adding 25% by mass of ammonia water to the first filtrate to adjust the pH to 5, after the addition of the ammonia water, continuing to stir at a stirring rate of 200 rpm for 40 minutes, and then filtering to obtain a second filtrate; S3, keeping the second filtrate stirring speed at 200rpm, sodium sulfide was added to the second filtrate, the added mass of sodium sulfide being 1.2% of the mass of the titanium dioxide by-product ferrous sulfate, after the sodium sulfide was added, the stirring reaction was continued at a stirring speed of 200rpm for 40min, and then filtered to obtain a third filtrate; S4, keeping the third filtrate stirring speed (S.S.) at 200rpm, adding mass fraction of 25% ammoniacal liquor to the third filtrate, the addition amount of ammoniacal liquor, is to add according to the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of ammoniacal liquor as 1:1.2, after ammoniacal liquor is added, continue to stir the reaction for 40min with a stirring speed (S.S.) 200rpm, then adopt inert gas protection type pressurized plate and frame filter to filter, obtain the fourth filtrate and the first filter cake, the first filter cake is washed under inert gas protection, NaOH solution with pH=8 is added at a solid-liquid ratio of 1:4 to wash the first filter cake, washing number is twice, then adopt pure water to wash once at a solid-liquid ratio of 1:4; obtain ferrous hydroxide of the first purity; S5, maintaining a stirring rate of 200 rpm, dispersing the first purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4; maintaining a stirring rate of the third filtrate at 200 rpm, adding oxalic acid, the amount of oxalic acid added is added in a molar ratio of ferrous hydroxide to oxalic acid of 1:1, continuing to stir and react at a stirring rate of 200 rpm for 1 hour, and then filtering to obtain a fifth filtrate and a second filter cake, and drying the second filter cake in an air atmosphere at 100° C. for 2.5 hours to obtain ferrous oxalate dihydrate; S6. Secondary precipitation of iron and co-production of ferric phosphate dihydrate S61, maintaining the fourth filtrate stirring rate at 200 rpm, adding 25% mass fraction of ammonia water to the fourth filtrate, the amount of ammonia water added is according to the molar amount of ferrous iron in the fourth filtrate to the molar amount of ammonia water is 1:1.9 added, after the addition of ammonia water, continue to stir the reaction at a stirring rate of 200 rpm for 40 min, then filter using an inert gas protection type pressurized plate and frame filter to obtain a sixth filtrate and a third filter cake, the third filter cake is washed under inert gas protection, and a NaOH solution with a pH of 8 is added at a solid-liquid ratio of 1:4 to wash the third filter cake, the number of washings is twice, and then washed once with pure water at a solid-liquid ratio of 1:4; ferrous hydroxide of the second purity is obtained; S62, dispersing the second purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4; adding phosphoric acid and hydrogen peroxide to react, the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.1, filtering after the reaction, and washing the obtained filter cake with pure water: the first washing: adding pure water at a solid-liquid ratio of 1:5, stirring at a stirring rate of 400 rpm for 15 minutes, letting it stand and settle for 30 minutes, and separating the supernatant; the second washing: adjusting the solid-liquid ratio to 1:4, stirring at a stirring rate of 300 rpm for 15 minutes, letting it stand and settle for 15 minutes, and separating the supernatant; the third washing: adjusting the solid-liquid ratio to 1:3, stirring at a stirring rate of 300 rpm for 8 minutes, letting it stand and settle for 20 minutes, and separating the supernatant, and then drying in an air atmosphere at 100°C for 2h to obtain ferric phosphate dihydrate; S7, co-production of calcium sulfate S71, maintaining the stirring rate of the fourth filtrate at 200 rpm, adding 25% by mass of ammonia water to the sixth filtrate to adjust the pH to 9; then aerating the sixth filtrate with ozone to allow the iron ions and manganese ions in the sixth filtrate to react with the oxidizing gas to form a precipitate, and the Fe 2+ and Mn 2+ The total molar amount is a; the amount of ozone added, calculated as oxygen, is in a molar ratio of a:oxygen = 1:0.8; after the reaction is completed, the reaction is filtered to obtain a seventh filtrate; S72. Add 85% pure lime to the seventh filtrate to adjust the pH to 11; filter to obtain a fourth filter cake and an eighth filtrate, where the fourth filter cake is calcium sulfate; heat the obtained eighth filtrate to 60°C, perform gas stripping at a negative pressure of -0.03 MPa to obtain ammonia-containing gas, cool the ammonia-containing gas produced by the gas stripping to 25°C, and finally recover the ammonia solution using deionized water as an absorbent.

[0047] Table 2 Test data of Example 1 Example 2: A method for recovering ferrous sulfate produced as a by-product of titanium dioxide comprises the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water (the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:4), filter, and obtain a first filtrate; S2, maintaining the stirring rate of the first filtrate at 300 rpm, adding 30% by mass of ammonia water to the first filtrate to adjust the pH to 5, after the addition of the ammonia water, continuing to stir at a stirring rate of 300 rpm for 30 minutes, and then filtering to obtain a second filtrate; S3, keeping the second filtrate stirring speed at 300 rpm, ammonium bisulfide was added to the second filtrate, the mass of ammonium bisulfide added was 1.5% of the mass of the titanium dioxide by-product ferrous sulfate, after the ammonium bisulfide was added, the stirring reaction was continued at a stirring speed of 300 rpm for 30 min, and then filtered to obtain a third filtrate; S4, keeping the third filtrate stirring speed (S.S.) at 300rpm, adding ammoniacal liquor having a mass fraction of 30% to the third filtrate, the addition amount of ammoniacal liquor, according to the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of ammoniacal liquor, is added in 1:1.1, after the addition of ammoniacal liquor, the reaction is continued with stirring at a stirring speed (S.S.) of 300rpm for 30min, then filtered using an inert gas protection type pressurized plate and frame filter to obtain the fourth filtrate and the first filter cake, the first filter cake is washed under inert gas protection, a NaOH solution having a pH of 8 is added at a solid-to-liquid ratio of 1:5 to wash the first filter cake, the number of washings is more than twice, then washed once with pure water at a solid-to-liquid ratio of 1:5; ferrous hydroxide of the first purity is obtained; S5, maintaining a stirring rate of 300 rpm, dispersing the first purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4.5; maintaining a stirring rate of the third filtrate at 300 rpm, adding oxalic acid, the amount of oxalic acid added is added in a molar ratio of ferrous hydroxide to oxalic acid of 1:1, continuing to stir and react at a stirring rate of 300 rpm for 1 hour, and then filtering to obtain a fifth filtrate and a second filter cake, and drying the second filter cake in an air atmosphere at 105° C. for 2 hours to obtain ferrous oxalate dihydrate; S6. Secondary precipitation of iron and co-production of ferric phosphate dihydrate S61, keeping the fourth filtrate stirring speed at 300rpm, adding 30% ammonia water by mass fraction to the fourth filtrate, the addition amount of 30% ammonia water by mass fraction is added according to the ratio of the molar amount of divalent iron in the fourth filtrate to the molar amount of ammonia water of 1:1.8, after the addition of ammonia water, continuing to stir the reaction at a stirring speed of 300rpm for 30min, then filtering using an inert gas protection type pressurized plate and frame filter to obtain a sixth filtrate and a third filter cake, washing the third filter cake under inert gas protection, adding a NaOH solution with a pH=8 at a solid-liquid ratio of 1:3 to wash the third filter cake, washing three times, and then washing once with pure water at a solid-liquid ratio of 1:5; obtaining ferrous hydroxide of the second purity; S62, maintaining a stirring rate of 300rpm, dispersing the second purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4.5; adding phosphoric acid and hydrogen peroxide to react, the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.05, filtering after the reaction, and washing the obtained filter cake with pure water: the first washing: adding pure water at a solid-liquid ratio of 1:5, stirring at a stirring rate of 400rpm for 10 minutes, letting it stand and settle for 20 minutes, and separating the supernatant; the second washing: adjusting the solid-liquid ratio to 1:4, stirring at a stirring rate of 300rpm for 10 minutes, letting it stand and settle for 20 minutes, and separating the supernatant; the third washing: adjusting the solid-liquid ratio to 1:3, stirring at a stirring rate of 100rpm for 6 minutes, letting it stand and settle for 20 minutes, and separating the supernatant, and then drying in an air atmosphere at 105°C for 2h to obtain ferric phosphate dihydrate; S7, co-production of calcium sulfate S71, maintaining the stirring rate of the sixth filtrate at 300 rpm, adding 30% by mass of ammonia water to the sixth filtrate to adjust the pH to 9.5; then aerating the sixth filtrate with ozone to allow the iron ions and manganese ions in the sixth filtrate to react with ozone to form a precipitate, and the Fe in the sixth filtrate 2+ and Mn 2+ The total molar amount is a; the amount of ozone added, calculated as oxygen, is in a molar ratio of a:oxygen = 1:1.11; after the reaction is completed, filtration is performed to obtain a seventh filtrate; S72. Add 95% pure lime to the seventh filtrate to adjust the pH to 11.5; filter to obtain a fourth filter cake and an eighth filtrate, where the fourth filter cake is calcium sulfate; heat the obtained eighth filtrate to 65°C, perform gas stripping at a negative pressure of -0.04 MPa to obtain ammonia-containing gas, cool the ammonia-containing gas produced by the gas stripping to 30°C, and finally recover the ammonia water using deionized water or dilute ammonia water as an absorbent.

[0048] Table 3 Test data of Example 2 Example 3: A method for recovering ferrous sulfate produced as a by-product of titanium dioxide comprises the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water (the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:4), filter, and obtain a first filtrate; S2, maintaining the stirring rate of the first filtrate at 100 rpm, adding a 10% mass fraction of sodium hydroxide solution to the first filtrate to adjust the pH to 4.5, after the sodium hydroxide solution is added, continuing to stir at a stirring rate of 100 rpm for 60 min, and then filtering to obtain a second filtrate; S3, keeping the second filtrate stirring speed at 100 rpm, sodium sulfide was added to the second filtrate, the added mass of sodium sulfide being 0.8% of the mass of the titanium dioxide by-product ferrous sulfate, after the sodium sulfide was added, the stirring reaction was continued at a stirring speed of 100 rpm for 60 min, and then filtered to obtain a third filtrate; S4, keeping the 3rd filtrate stir speed (S.S.) at 100rpm, adding mass fraction to the 3rd filtrate is 10% sodium hydroxide solution, the addition amount of sodium hydroxide solution, is to add according to the ratio of the molar amount of ferrous iron in the 3rd filtrate and the molar amount of hydroxide radical in sodium hydroxide solution for 1:1.2, after sodium hydroxide solution adds, continue to stir the reaction for 60min with stir speed (S.S.) 100rpm, then adopt inert gas protection type pressurized plate and frame filter to filter, obtain the 4th filtrate and the first filter cake, the first filter cake is washed under inert gas protection, press solid-liquid ratio 1:3 and add the NaOH solution of pH=8 to carry out washing of the first filter cake, washing number is three times, then press solid-liquid ratio 1:3 and adopt pure water to wash once; Obtain the ferrous hydroxide of the first purity; S5, maintaining a stirring rate of 100 rpm, dispersing the first purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:3.5; maintaining a stirring rate of the third filtrate at 100 rpm, adding oxalic acid, the amount of oxalic acid added is added in a molar ratio of ferrous hydroxide to oxalic acid of 1:1, continuing to stir and react at a stirring rate of 100 rpm for 1 hour, and then filtering to obtain a fifth filtrate and a second filter cake, and drying the second filter cake in an air atmosphere at 95° C. for 3 hours to obtain ferrous oxalate dihydrate; S6. Secondary precipitation of iron and co-production of ferric phosphate dihydrate S61, keeping the fourth filtrate stirring speed at 100rpm, adding a sodium hydroxide solution with a mass fraction of 10% to the fourth filtrate, the addition amount of the sodium hydroxide solution with a mass fraction of 10%, is added according to the ratio of the molar amount of ferrous iron in the fourth filtrate to the molar amount of hydroxide in the sodium hydroxide solution is 1:1.9, after the sodium hydroxide solution is added, the stirring reaction is continued at a stirring speed of 100rpm for 40min, and then filtered using an inert gas protection type pressurized plate and frame filter to obtain a sixth filtrate and a third filter cake, the third filter cake is washed under inert gas protection, and a NaOH solution with a pH=8 is added at a solid-liquid ratio of 1:5 to wash the third filter cake, the number of washings is twice, and then washed once with pure water at a solid-liquid ratio of 1:3; ferrous hydroxide of the second purity is obtained; S62, maintaining a stirring rate of 100 rpm, dispersing the second purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:3.5; adding phosphoric acid and hydrogen peroxide to react, the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.05, filtering after the reaction, and washing the obtained filter cake with pure water: the first washing: adding pure water at a solid-liquid ratio of 1:5, stirring at a stirring rate of 400 rpm for 15 minutes, letting it stand and settle for 30 minutes, and separating the supernatant; the second washing: adjusting the solid-liquid ratio to 1:4, stirring at a stirring rate of 300 rpm for 15 minutes, letting it stand and settle for 15 minutes, and separating the supernatant, and then drying in an air atmosphere at 95°C for 3 hours to obtain ferric phosphate dihydrate; S7, co-production of calcium sulfate S71, maintaining the stirring rate of the sixth filtrate at 100 rpm, adding a 10% by mass sodium hydroxide solution to the sixth filtrate to adjust the pH to 8.5; then aerating the sixth filtrate with air to allow the iron ions and manganese ions in the sixth filtrate to react with the oxidizing gas to form a precipitate, and the Fe in the sixth filtrate 2+ and Mn 2+ The total molar amount is a; the amount of air added, calculated as oxygen, has a molar ratio of a:oxygen = 1:0.56; after the reaction is completed, filtration is performed to obtain a seventh filtrate; S72. Add 95% pure lime to the seventh filtrate to adjust the pH to 10.5; filter to obtain a fourth filter cake and an eighth filtrate, wherein the fourth filter cake is calcium sulfate; heat the obtained eighth filtrate to 55°C, and perform gas stripping at a negative pressure of -0.02 MPa to obtain ammonia-containing gas, cool the ammonia-containing gas produced by gas stripping to 20°C, and finally recover the ammonia water using deionized water or dilute ammonia water as an absorbent.

[0049] Table 4 Test data of Example 3 Example 4: A method for recovering ferrous sulfate produced as a by-product of titanium dioxide comprises the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water (the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:2), filter, and obtain a first filtrate; S2, maintaining the stirring rate of the first filtrate at 150 rpm, adding 20% ​​by mass fraction of ammonia water to the first filtrate to adjust the pH to 5.5, and after the addition of the ammonia water, continuing to stir at a stirring rate of 150 rpm for 30 minutes, and then filtering to obtain a second filtrate; S3, keeping the second filtrate stirring speed at 150rpm, adding sodium sulfide to the second filtrate, the added mass of sodium sulfide being 1.0% of the mass of ferrous sulfate by-product of titanium dioxide. After the addition of sodium sulfide, the reaction was continued at a stirring speed of 150rpm for 30min, and then filtered to obtain a third filtrate; S4, keeping the third filtrate stir speed (S.S.) at 150rpm, adding mass fraction and being 20% ​​ammoniacal liquor to the third filtrate, mass fraction and being 20% ​​ammoniacal liquor are added in an amount, it is to add according to the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of ammoniacal liquor being 1:1.2, after ammoniacal liquor is added, stirring reaction is continued with stir speed (S.S.) 150rpm for 20min, then inert gas protection type pressurized plate and frame filter is adopted to filter, and the fourth filtrate and the first filter cake are obtained, and the first filter cake is washed under inert gas protection, and the NaOH solution of pH=8 is added by solid-liquid ratio 1:4 to wash the first filter cake, and the number of washings is three times, and then by solid-liquid ratio 1:4, pure water is adopted to wash once; Ferrous hydroxide of the first purity is obtained; S5, maintaining a stirring rate of 150 rpm, dispersing the first purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4; maintaining a stirring rate of the third filtrate at 150 rpm, adding oxalic acid, the amount of oxalic acid added is added in a molar ratio of ferrous hydroxide to oxalic acid of 1:1, continuing to stir and react at a stirring rate of 150 rpm for 1 hour, and then filtering to obtain a fifth filtrate and a second filter cake, and drying the second filter cake in an air atmosphere at 100° C. for 3 hours to obtain ferrous oxalate dihydrate; S6. Secondary precipitation of iron and co-production of ferric phosphate dihydrate S61, maintaining the fourth filtrate stirring speed at 150rpm, adding 20% ​​ammonia water by mass fraction to the fourth filtrate, the amount of ammonia water added is according to the molar amount of divalent iron in the fourth filtrate to the molar amount of ammonia water is 1:1.9 added, after the addition of ammonia water, continue to stir the reaction at a stirring speed of 150rpm for 60min, then filter using an inert gas protection type pressurized plate and frame filter to obtain a sixth filtrate and a third filter cake, the third filter cake is washed under inert gas protection, and a NaOH solution with a pH of 8 is added at a solid-liquid ratio of 1:4 to wash the third filter cake, the number of washings is three times, and then washed once with pure water at a solid-liquid ratio of 1:4; ferrous hydroxide of the second purity is obtained; S62, maintaining a stirring rate of 150rpm, dispersing the second purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4; adding phosphoric acid and hydrogen peroxide to react, the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.15, filtering after the reaction, and washing the obtained filter cake with pure water: the first washing: adding pure water at a solid-liquid ratio of 1:5, stirring at a stirring rate of 400rpm for 10 minutes, standing and settling for 30 minutes, and separating the supernatant; the second washing: adjusting the solid-liquid ratio to 1:4, stirring at a stirring rate of 100rpm for 10 minutes, standing and settling for 10 minutes, and separating the supernatant, and then drying in an air atmosphere at 100°C for 3h to obtain ferric phosphate dihydrate; S7, co-production of calcium sulfate S71, maintaining the stirring rate of the sixth filtrate at 150 rpm, adding 20% ​​by mass of ammonia water to the sixth filtrate to adjust the pH to 9.5; then aerating the sixth filtrate with oxygen, so that the iron ions and manganese ions in the sixth filtrate react with the oxygen to form a precipitate, and the Fe in the sixth filtrate is reduced to 1.5%. 2+ and Mn 2+ The total molar amount is a; the molar ratio of a to oxygen is a: oxygen = 1: 1.11; after the reaction is completed, filtration is performed to obtain a seventh filtrate; S72. Add 95% pure lime to the seventh filtrate to adjust the pH to 11; filter to obtain a fourth filter cake and an eighth filtrate, where the fourth filter cake is calcium sulfate; heat the obtained eighth filtrate to 65°C, perform gas stripping at a negative pressure of -0.04 MPa to obtain ammonia-containing gas, cool the ammonia-containing gas produced by the gas stripping to 30°C, and finally recover the ammonia solution using deionized water as an absorbent.

[0050] Table 5 Test data of Example 4 Example 5: A method for recovering ferrous sulfate produced as a by-product of titanium dioxide comprises the following steps: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water (the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:2.5), filter, and obtain a first filtrate; S2, maintaining the stirring rate of the first filtrate at 250 rpm, adding 25% by mass of ammonia water to the first filtrate to adjust the pH to 5, after the addition of the ammonia water, continuing to stir at a stirring rate of 250 rpm for 20 minutes, and then filtering to obtain a second filtrate; S3, keeping the second filtrate stirring speed at 250rpm, sodium sulfide was added to the second filtrate, the added mass of sodium sulfide being 0.8% of the mass of the titanium dioxide by-product ferrous sulfate, after the sodium sulfide was added, the stirring reaction was continued at a stirring speed of 250rpm for 60min, and then filtered to obtain a third filtrate; S4, keeping the third filtrate stir speed (S.S.) at 250rpm, adding mass fraction and being 25% ammoniacal liquor to the third filtrate, the addition amount of ammoniacal liquor, is to add according to the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of ammoniacal liquor for 1:1.2, after ammoniacal liquor is added, continue to stir the reaction for 50min with a stir speed (S.S.) 250rpm, then adopt inert gas protection type pressurized plate and frame filter to filter, obtain the fourth filtrate and the first filter cake, the first filter cake is washed under inert gas protection, the NaOH solution of pH=8 is added at a solid-liquid ratio of 1:3 to wash the first filter cake, the number of washings is three times, then adopts pure water to wash once at a solid-liquid ratio of 1:3; Obtain ferrous hydroxide of the first purity; S5, maintaining a stirring rate of 250 rpm, dispersing the first purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:3.5; maintaining a stirring rate of the third filtrate at 250 rpm, adding oxalic acid, the amount of oxalic acid added is added in a molar ratio of ferrous hydroxide to oxalic acid of 1:1, continuing to stir and react at a stirring rate of 250 rpm for 1 hour, and then filtering to obtain a fifth filtrate and a second filter cake, and drying the second filter cake in an air atmosphere at 100° C. for 2 hours to obtain ferrous oxalate dihydrate; S6. Secondary precipitation of iron and co-production of ferric phosphate dihydrate S61, maintaining the fourth filtrate stirring rate at 250rpm, adding 25% ammonia water by mass fraction to the fourth filtrate, the amount of ammonia water added is based on the ratio of the molar amount of divalent iron in the fourth filtrate to the molar amount of ammonia water is 1:1.9, after the addition of ammonia water, the stirring reaction is continued at a stirring rate of 250rpm for 30min, and then filtered using an inert gas protection type pressurized plate and frame filter to obtain a sixth filtrate and a third filter cake, the third filter cake is washed under inert gas protection, and a NaOH solution with a pH of 8 is added at a solid-liquid ratio of 1:4 to wash the third filter cake, the washing number is more than twice, and then washed once with pure water at a solid-liquid ratio of 1:4; ferrous hydroxide of the second purity is obtained; S62, maintaining a stirring rate of 250rpm, dispersing the second purity ferrous hydroxide in pure water at a solid-liquid ratio of 1:4; adding phosphoric acid and hydrogen peroxide to react, the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.15, filtering after the reaction, and washing the obtained filter cake with pure water: the first washing: adding pure water at a solid-liquid ratio of 1:5, stirring at a stirring rate of 400rpm for 10 minutes, letting it stand and settle for 20 minutes, and separating the supernatant; the second washing: adjusting the solid-liquid ratio to 1:4, stirring at a stirring rate of 300rpm for 10 minutes, letting it stand and settle for 20 minutes, and separating the supernatant; the third washing: adjusting the solid-liquid ratio to 1:3, stirring at a stirring rate of 50rpm for 8 minutes, letting it stand and settle for 20 minutes, and separating the supernatant, and then drying in an air atmosphere at 100°C for 2h to obtain ferric phosphate dihydrate; S7, co-production of calcium sulfate S71, maintaining the stirring rate of the sixth filtrate at 250 rpm, adding 25% by mass of ammonia water to the sixth filtrate to adjust the pH to 9.5; then aerating the sixth filtrate with air, so that the iron ions and manganese ions in the sixth filtrate react with the air to form a precipitate, and the Fe in the sixth filtrate 2+ and Mn 2+ The total molar amount is a; the amount of air added, calculated as oxygen, has a molar ratio of a:oxygen = 1:0.8; after the reaction is completed, filtration is performed to obtain a seventh filtrate; S72. Add 90% pure lime to the seventh filtrate to adjust the pH to 10.5; filter to obtain a fourth filter cake and an eighth filtrate, wherein the fourth filter cake is calcium sulfate; heat the obtained eighth filtrate to 60°C, perform gas stripping at a negative pressure of -0.02 MPa to obtain ammonia-containing gas, cool the ammonia-containing gas produced by gas stripping to 30°C, and finally recover the ammonia water using deionized water or dilute ammonia water as an absorbent.

[0051] Table 6 Test data of Example 5 From Table 2 to Table 6, we can see that: In Examples 1-5 of the present application, the purity of the obtained ferrous oxalate dihydrate is above 99.5%, and the purity of the ferric phosphate dihydrate is above 98.5%, indicating that in the examples of the present application, the efficient separation of iron and manganese is achieved with a simple and low-cost impurity removal step through the synergistic effect of precise control of pH value, sulfide impurity removal and step-by-step precipitation.

[0052] The above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field can easily think of changes or replacements within the technical scope disclosed by the present invention, which should be covered by the scope of protection of the present invention.

Claims

1. A method for recovering ferrous sulfate produced as a by-product of titanium dioxide, characterized in that: The steps include: S1. Dissolve ferrous sulfate produced as a by-product of titanium dioxide in water and filter to obtain a first filtrate; S2, adjusting the pH of the first filtrate to 4.5-5.5, filtering, and obtaining a second filtrate; S3, adding sulfide to the second filtrate, filtering after the reaction, to obtain a third filtrate; S4, add alkaline liquid to the third filtrate, the amount of alkaline liquid added is 1:0.1-1.2 according to the ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of hydroxide in the alkaline liquid, and after the reaction, filter under the protection of an inert gas to obtain the fourth filtrate and the first filter cake, and wash the first filter cake under the protection of an inert gas to obtain ferrous hydroxide of the first purity; S5. Dispersing the ferrous hydroxide of the first purity in pure water, adding oxalic acid, and filtering after the reaction to obtain a fifth filtrate and a second filter cake. After the second filter cake is dried, ferrous oxalate dihydrate is obtained.

2. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 1, wherein: The recovery method further comprises: step S6, secondary precipitation of iron and co-production of ferric phosphate dihydrate, specifically comprising: S61. Adding an alkaline liquid to the fourth filtrate, wherein the amount of the alkaline liquid added is based on a ratio of the molar amount of ferrous iron in the fourth filtrate to the molar amount of hydroxide in the alkaline liquid of 1:0.1-1.

9. After the reaction, filtering under the protection of an inert gas to obtain a sixth filtrate and a third filter cake; washing the third filter cake under the protection of an inert gas to obtain ferrous hydroxide of a second purity; S62. Dispersing the second purity ferrous hydroxide in pure water, adding phosphoric acid and hydrogen peroxide to react, filtering, washing, and drying after the reaction to obtain ferric phosphate dihydrate.

3. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 2, wherein: In step S61, the amount of alkaline liquid added is based on a ratio of the molar amount of ferrous iron in the fourth filtrate to the molar amount of hydroxide in the alkaline liquid of 1:1.6-1.9; And / or, in step S61, the alkaline liquid is selected from any one of ammonia water and sodium hydroxide solution; and / or, in step S61, when the alkaline liquid is added to the fourth filtrate, the fourth filtrate is stirred, and after the alkaline liquid is added, the stirring is continued and then filtered; And / or, in step S61, the third filter cake is washed under the protection of inert gas, firstly using a NaOH solution with a pH of 7.5-8.5 for washing more than twice; and then washing with pure water; And / or, in step S62, the second purity ferrous hydroxide is dispersed in pure water at a solid-liquid ratio of 1:3.5-4.5; the molar ratio of ferrous hydroxide to the added phosphoric acid is 1:1; the molar ratio of ferrous hydroxide to the added hydrogen peroxide is 2:1.05-1.15; after the reaction, the mixture is filtered, and the obtained filter cake is washed with pure water, washed in steps of 2-3 times at a solid-liquid ratio of 1:3-5, and then dried at 95-105°C for 2-3 hours to obtain ferric phosphate dihydrate.

4. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 3, wherein: The recovery method further comprises: step S7, co-producing calcium sulfate, specifically comprising: S71, adding an alkaline liquid to the sixth filtrate to adjust the pH to 8.5-9.5, then aerating the sixth filtrate with an oxidizing gas to allow the iron ions and manganese ions in the sixth filtrate to react with the oxidizing gas to form a precipitate, which is then filtered to obtain a seventh filtrate. S72. Add lime to the seventh filtrate to obtain calcium sulfate after reaction, and recover ammonia water by gas stripping.

5. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 4, wherein: In step S71, the oxidizing gas is selected from any one or more of air, oxygen and ozone; And / or, in step S71, the alkaline liquid includes any one of ammonia water and sodium hydroxide solution; And / or, in step S71, Fe 2+ and Mn 2+ The sum of the molar amounts is a; the amount of oxidizing gas added, calculated as oxygen, is a:oxygen = 1:0.56-1.11; And / or, step S72 specifically comprises: adding lime with a purity of 80%-95% to the seventh filtrate to adjust the pH to 10.5-11.5; filtering to obtain a fourth filter cake and an eighth filtrate, wherein the fourth filter cake is calcium sulfate; heating the obtained eighth filtrate to 55-65° C., performing negative pressure gas stripping to obtain ammonia-containing gas, cooling the ammonia-containing gas produced by the gas stripping to 20-30° C., and finally recovering the ammonia solution using deionized water or dilute ammonia solution as an absorbent.

6. The method for producing ferrous hydroxide using ferrous sulfate as a by-product of titanium dioxide according to claim 5, wherein: In step S1, the mass ratio of ferrous sulfate produced as a by-product of titanium dioxide to water is 1:2-4; And / or, in step S2, the alkaline liquid selected for adjusting the pH of the first filtrate is any one of ammonia water and sodium hydroxide solution.

7. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 6, wherein: In step S3, the sulfide is selected from any one of sodium sulfide, barium sulfide, hydrogen sulfide and ammonium bisulfide; And / or, in step S3, the mass of the added sulfide is 0.8%-1.5% of the mass of the ferrous sulfate by-product of titanium dioxide; And / or, in step S3, when adding sulfide to the second filtrate, the second filtrate is kept stirred, and after the sulfide is added, the stirring is continued and then filtered.

8. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 7, wherein: In step S4, the amount of alkaline liquid added is based on a ratio of the molar amount of ferrous iron in the third filtrate to the molar amount of hydroxide in the alkaline liquid of 1:1.0-1.2; And / or, in step S4, the alkaline liquid is any one selected from ammonia water and sodium hydroxide solution; and / or, in step S4, when adding the alkaline liquid to the third filtrate, the third filtrate is stirred, and after the alkaline liquid is added, the stirring is continued and then filtered; And / or, in step S4, the washing is performed under the protection of an inert gas, firstly washing the first filter cake with a NaOH solution having a pH of 7.5-8.5 for more than two times, and then washing with pure water.

9. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 8, wherein: In the step S5, the first purity ferrous hydroxide is dispersed in pure water at a solid-liquid ratio of 1:3.5-4.5, and the amount of oxalic acid added is a molar ratio of ferrous hydroxide to oxalic acid of 1:

1. After the reaction, the filter is filtered, and the second filter cake is dried at a temperature of 95-105° C. for 2-3 hours to obtain ferrous oxalate dihydrate; And / or, in step S5, the fifth filtrate is reused for dispersion of first purity ferrous hydroxide.

10. The method for recovering ferrous sulfate produced as a by-product of titanium dioxide according to claim 9, characterized in that: In step S2, when adding the alkaline liquid to adjust the pH of the first filtrate, the stirring rate of the first filtrate is 100-300 rpm. After the alkaline liquid is added, the stirring rate is continued at 100-300 rpm for 20-60 minutes, and then the filtration is performed; and / or, in step S3, when adding the sulfide to the second filtrate, the stirring rate of the second filtrate is 100-300 rpm, and after the sulfide is added, the stirring rate is continued at 100-300 rpm for 20-60 minutes, and then the filtration is performed; and / or, in step S4, when adding the alkaline liquid to the third filtrate, the stirring rate of the third filtrate is 100-300 rpm, and after the alkaline liquid is added, the stirring rate is continued to be 100-300 rpm for 20-60 minutes, and then the filtration is performed; And / or, in step S4, the washing is performed under inert gas protection, and a NaOH solution with a pH of 8 is added to the first filter cake at a solid-liquid ratio of 1:3-5, and the washing times are three times; and then the filter cake is washed once with pure water at a solid-liquid ratio of 1:3-5; and / or, in step S61, when adding the alkaline liquid to the fourth filtrate, the stirring rate of the fourth filtrate is 100-300 rpm, and after the alkaline liquid is added, the stirring rate is continued at 100-300 rpm for 20-60 minutes, and then the filtration is performed; And / or, in step S61, the third filter cake is washed under inert gas protection, and a NaOH solution with a pH of 8 is added at a solid-liquid ratio of 1:3-5 to wash the third filter cake three times; then, the third filter cake is washed once with pure water at a solid-liquid ratio of 1:3-5.

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