Impurity removal method for titanium dioxide by-product ferrous sulfate

Abstract

The invention relates to the technical field of titanium dioxide by-product ferrous sulfate impurity removal, and discloses a titanium dioxide by-product ferrous sulfate impurity removal method, which is based on a hydrolysis method impurity removal principle, and comprises the following steps: adjusting the pH value of a ferrous sulfate dissolving solution to 4-4.5 by adding a pH value adjusting slurry formed by calcium carbonate powder; the reaction temperature is controlled within the range of 50-70 DEG C, so that impurity metal cations are subjected to hydrolysis reaction to generate water-insoluble precipitates, and the ferrous sulfate after impurity removal is obtained after solid-liquid separation. According to the impurity removal method, the impurity precipitation and filtration speed is high, the solid-liquid separation period is short, the problem that a conventional hydrolysis method is difficult to filter is solved, the steps are simple, and operation is easy; the impurity removal cost is low, no new impurities are introduced, and the ferrous sulfate solution obtained by the impurity removal method can be directly used as an iron source raw material of battery-grade iron phosphate.

Description

Technical Field

[0001] The invention relates to the technical field of impurity removal of ferrous sulfate, a byproduct of titanium dioxide, and particularly relates to a method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide. Background Art

[0002] Ferrous sulfate heptahydrate is a byproduct of the sulfuric acid process for producing titanium dioxide. Approximately three tons of ferrous sulfate heptahydrate are produced for every ton of titanium dioxide produced. Previously, ferrous sulfate was used as a mordant, water purifier, and preservative, but usage was relatively small. Most of it was dumped as waste, making it difficult to dispose of, causing environmental pollution and increasing additional environmental disposal costs. In recent years, the global lithium battery market has seen rapid growth, and lithium-ion batteries have rapidly developed. This has driven the development of lithium batteries, including those using lithium iron phosphate as a cathode material. Iron phosphate, a precursor for lithium iron phosphate, has also garnered widespread attention. This has made using ferrous sulfate, a titanium dioxide byproduct, as a raw material for producing iron phosphate a viable approach. The electrochemical performance of lithium iron phosphate is largely dependent on the quality of the precursor, iron phosphate, which in turn is dependent on process control and raw material quality. Therefore, it is crucial to control the production of high-quality iron phosphate from the raw materials used in iron phosphate production, and to selectively remove impurities and purify the ferrous sulfate produced as a titanium dioxide byproduct to achieve the required iron source for iron phosphate synthesis and utilization.

[0003] At present, the most reported methods for removing impurities from ferrous sulfate, a by-product of titanium dioxide, include hydrolysis, colloidal precipitation adsorption, chemical precipitation, and flocculation co-precipitation. Among them, the hydrolysis method has low cost and simple operation, and is suitable for industrial mass production. The hydrolysis method uses a hydrolysis reaction to remove impurities such as aluminum and titanium in the ferrous sulfate solution. By adding a pH regulator (commonly used are phosphoric acid and ammonia water) to adjust the pH value of the system, the titanyl sulfate in the ferrous sulfate solution is hydrolyzed to form a metatitanic acid precipitate, and the aluminum sulfate is hydrolyzed to form an aluminum hydroxide precipitate. The impurities are then filtered out and the clear liquid is used. This method has a good removal effect on impurities titanium and aluminum, but because the generated metatitanic acid precipitate is fine, the filtration is very slow, which can easily lead to the Fe in the solution. 2+ Oxidation forms Fe(OH)3 colloid, which is prone to clogging during filtration, making separation difficult. 2+ To address oxidation, commonly used methods include adding a sedimentation agent to allow for static sedimentation, or adding a filter aid before filtration to increase filtration speed. This not only introduces new impurities, but also increases production costs and prolongs the solid-liquid separation cycle. Low filtration efficiency in the solid-liquid separation stage of the hydrolysis process is a key issue limiting production capacity and costs. Summary of the Invention

[0004] In view of this, the present invention provides a method for removing impurities from ferrous sulfate, a by-product of titanium dioxide, to solve the problem of low solid-liquid separation efficiency in the existing ferrous sulfate hydrolysis method for impurity removal, thereby improving production capacity and saving costs.

[0005] In order to achieve the above object, the present invention provides a method for removing impurities from ferrous sulfate, a byproduct of titanium dioxide, which comprises the following steps:

[0006] Step 1: dissolving ferrous sulfate heptahydrate, a by-product of titanium dioxide, in water to obtain a ferrous sulfate solution;

[0007] Step 2: Mixing calcium carbonate powder and water in proportion to obtain a pH adjustment slurry;

[0008] Step 3: slowly add the pH-adjusted slurry of step 2 to the ferrous sulfate solution of step 1, adjust the pH of the solution to 4-4.5, react at 50-70° C. for 90-120 min, and filter the reactant precipitate to obtain a ferrous sulfate filtrate.

[0009] Furthermore, in step 1, the mass ratio of ferrous sulfate heptahydrate to water is 1:2.

[0010] Furthermore, in step 2, the mass ratio of calcium carbonate powder to water is 1:5.

[0011] Furthermore, the calcium carbonate powder is heavy calcium powder.

[0012] Furthermore, in step 3, the pH adjusting slurry is added to the ferrous sulfate solution under stirring, and stirring is performed to ensure that the calcium carbonate powder is evenly dispersed in the pH adjusting slurry, thereby enhancing the effect of the pH adjusting slurry.

[0013] Furthermore, the amount of the pH adjustment slurry added in step 3 is such that the mass of the calcium carbonate powder in the pH adjustment slurry accounts for 0.5 to 1.1% of the mass of the ferrous sulfate heptahydrate dissolved in the ferrous sulfate solution.

[0014] Furthermore, in step 3, the pH adjusting slurry is added to the ferrous sulfate solution at a speed such that the aforementioned amount of pH adjusting slurry is added uniformly within 20 to 30 minutes.

[0015] Furthermore, the water in step 1 and step 2 is desalted water.

[0016] The impurity removal method provided by the present invention is based on the principle of removing impurities by hydrolyzing ferrous sulfate, a byproduct of titanium dioxide. After adjusting the pH of the ferrous sulfate solution to 4-4.5 by adding a pH adjustment slurry formed by calcium carbonate powder, the reaction temperature is controlled (50-70° C.) to allow impurity metal cations (titanium ions, aluminum ions) to undergo a hydrolysis reaction to form a water-insoluble precipitate. After solid-liquid separation, the impurity-removed ferrous sulfate is obtained. The reaction equation is as follows:

[0017] TiOSO4+2H2O=H2SO4+TiO(OH)2↓;

[0018] Al2(SO4)3+6H2O=3H2SO4+2Al(OH)3↓.

[0019] The impurity removal method of the present invention can remove more than 95% of the impurities titanium and aluminum in ferrous sulfate.

[0020] Since the ferrous sulfate solution obtained by dissolving the titanium dioxide byproduct ferrous sulfate in water is highly acidic (the solution contains more sulfuric acid), and the low pH environment will inhibit the hydrolysis reaction of titanyl sulfate, affecting the impurity removal effect, the present invention uses calcium carbonate slurry as a pH regulator to adjust the pH value of the ferrous sulfate solution. On the one hand, CaCO3 can undergo a double decomposition reaction with the free H2SO4 in the ferrous sulfate heptahydrate, thereby consuming the H in the solution. + , causing the pH value of the system to rise, which is conducive to the hydrolysis of titanyl sulfate to form insoluble metatitanic acid. On the other hand, by slowly adding calcium carbonate slurry to the dilute sulfuric acid environment, the nucleation rate of calcium sulfate can be effectively controlled, resulting in large-sized calcium sulfate crystals and improving the filtration efficiency of solid-liquid separation. Due to the low solubility of CaCO3, the CaSO4 precipitate nuclei generated by the reaction of CaCO3 and H2SO4 mainly nucleate in situ on the surface of calcium carbonate. By slowly adding calcium carbonate slurry, calcium sulfate is conducive to forming a coarse flaky crystal structure. This structure can make the filter cake structure more uniform and loose, reduce the formation of dense filter cake, and produce a "bridging" effect during the separation process, reducing filtration resistance and improving filtration rate.

[0021] Furthermore, the ferrous sulfate filtrate obtained after impurity removal of the present invention has a removal rate of more than 95% for the impurities titanium and aluminum, and can be directly used for the preparation of battery-grade iron phosphate. Specifically, phosphoric acid solution is added to the ferrous sulfate filtrate until the pH of the filtrate is 1 to 1.5 to obtain a ferrous sulfate solution, which is directly used for the preparation of battery-grade iron phosphate. The purpose of adjusting the pH value of the ferrous sulfate filtrate with phosphoric acid is to prevent Fe 2+ Furthermore, the concentration of the phosphoric acid solution is 70-85 wt%.

[0022] In summary, compared with the prior art, the impurity removal method of ferrous sulfate, a by-product of titanium dioxide, provided by the present invention has a fast impurity precipitation and filtration speed and a short solid-liquid separation cycle, which solves the problem of Fe 2+The ferrous sulfate solution is oxidized to ferric hydroxide colloid upon contact with air, and the metatitanic acid precipitate is an extremely fine amorphous colloid, making filtration difficult. The process is simple and easy to operate. The calcium carbonate powder used is readily available and relatively inexpensive, reducing impurity removal costs. Furthermore, the calcium carbonate reacts with free sulfuric acid to form calcium sulfate precipitate, carbon dioxide gas, and water. The addition of calcium carbonate does not introduce new impurities into the ferrous sulfate solution. The present invention achieves a removal rate of over 95% for titanium and aluminum impurities in ferrous sulfate. The ferrous sulfate solution obtained by the impurity removal method can be directly used as an iron source for preparing ferric phosphate, a precursor to lithium iron phosphate, a positive electrode material for lithium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The graph is a filtrate quality-filtration time curve of the embodiments of the present invention and the comparative example. DETAILED DESCRIPTION

[0024] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the present invention is further described in detail below with reference to specific embodiments and accompanying drawings.

[0025] Unless otherwise specified, the materials and equipment involved in the following examples are all commercially available products and can be purchased through commercial channels.

[0026] The raw material titanium dioxide by-product ferrous sulfate heptahydrate involved in the following examples has a FeSO4.7H2O content of 86.22%; a TiO2 content of 0.26%; and an Al content of 88.5 ppm.

[0027] Example 1:

[0028] 500 g of ferrous sulfate heptahydrate was dissolved in 1000 g of desalted water to obtain a ferrous sulfate solution;

[0029] Take 2.5g of heavy calcium powder and mix it evenly with 12.5g of desalted water to obtain a pH adjustment slurry;

[0030] The pH adjustment slurry was uniformly added to the ferrous sulfate solution for 20 minutes. After the addition was completed, the pH of the ferrous sulfate solution was measured to be 4.0. The solution temperature was maintained at 50°C for 120 minutes. The reactant precipitate was filtered to obtain the ferrous sulfate filtrate (the filtrate mass was recorded over time during the filtration process, and a filtrate mass-filtration time curve was drawn. The results are shown in the figure. Figure 1 ),from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this example is 218 s. The ferrous sulfate filtrate was sampled and subjected to ICP detection. The results are shown in Table 2 below.

[0031] Example 2:

[0032] 500 g of ferrous sulfate heptahydrate was dissolved in 1000 g of desalted water to obtain a ferrous sulfate solution;

[0033] Take 5.5g of heavy calcium powder and mix it evenly with 27.5g of desalted water to obtain a pH adjustment slurry;

[0034] The pH adjustment slurry was uniformly added to the ferrous sulfate solution for 30 minutes. After the addition was completed, the pH of the ferrous sulfate solution was measured to be 4.5. The solution temperature was maintained at 70°C for 90 minutes. The reactant precipitate was filtered to obtain the ferrous sulfate filtrate (the filtrate mass was recorded over time during the filtration process, and a filtrate mass-filtration time curve was drawn. The results are shown in FIG. Figure 1 ),from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this example is 219 s. The ferrous sulfate filtrate was sampled and subjected to ICP testing. The results are shown in Table 2 below.

[0035] Comparative Example 1:

[0036] Same as Example 1, except that:

[0037] 7 g of heavy calcium powder was mixed evenly with 35 g of desalted water to obtain a pH-adjusted slurry; after the ferrous sulfate solution was added to the pH-adjusted slurry, the pH of the solution was 4.9.

[0038] from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this comparative example is 221 s.

[0039] Comparative Example 2:

[0040] Same as Example 1, except that:

[0041] The pH adjustment slurry is added to the ferrous sulfate solution at a uniform speed for 5 minutes.

[0042] from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this comparative example is 267 s.

[0043] Comparative Example 3:

[0044] Same as Example 2, except that:

[0045] The pH adjustment slurry was added to the ferrous sulfate solution at a uniform speed for 40 minutes.

[0046] from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this comparative example is 216 s.

[0047] Comparative Example 4: Same as Example 1, except that the reaction temperature is 40°C.

[0048] from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this example is 252 s.

[0049] Comparative Example 5: Same as Example 2, except that iron powder is used instead of heavy calcium powder.

[0050] from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this example is 309 s.

[0051] Comparative Example 6:

[0052] 500 g of ferrous sulfate heptahydrate was dissolved in 1000 g of desalted water to obtain a ferrous sulfate solution. Ammonia water with a concentration of 25 wt % was added to the ferrous sulfate solution to adjust the pH value of the solution to 5. The solution temperature was maintained at 30 ° C. and the reaction was continued for 120 min. The reactant was filtered to obtain a ferrous sulfate filtrate (the filtrate mass was recorded over time during the filtration process, and a filtrate mass-filtration time curve was drawn. The results are shown in FIG. Figure 1 ), samples were taken for ICP testing, and the results are shown in Table 2 below.

[0053] from Figure 1 It can be seen that the time for completely obtaining the ferrous sulfate filtrate by filtration of the reactant precipitation in this comparative example is 398 s.

[0054] Table 2. ICP test results of ferrous sulfate filtrates obtained in various examples and comparative examples

[0055]

[0056] See Table 2 and Figure 1 It can be seen that the impurity removal methods provided in Examples 1 and 2 have low iron loss, fast filtration rate, and the removal rate of impurity Ti is over 99%, and the removal rate of Al is over 95%.

[0057] In Comparative Example 1, the amount of pH adjustment slurry added was increased to 1.4% of the mass of calcium carbonate powder in the pH adjustment slurry based on the mass of ferrous sulfate heptahydrate. After adjustment by the adjustment slurry, the pH value of the ferrous sulfate solution was adjusted to 4.9. From the results, it can be seen that although the amount of calcium carbonate added was increased, the impurity removal rate was not significantly improved, and the filtration rate of the ferrous sulfate solution was not further improved.

[0058] When the addition time of the pH adjustment slurry in Comparative Example 2 was shortened to 5 min, the filtration rate of the ferrous sulfate solution was lower than that in Example 1, and the impurity removal rate of the filtrate was also lowered; when the addition time of the pH adjustment slurry in Comparative Example 3 was extended to 40 min, the filtration rate of the ferrous sulfate solution was not significantly changed compared with Example 2, and the impurity removal rate of the filtrate was not significantly changed.

[0059] When the reaction temperature in Comparative Example 4 was 40° C., the filtration rate of the ferrous sulfate solution was lower than that in Example 1, and the impurity removal rate of the filtrate was reduced.

[0060] In Comparative Examples 5 and 6, iron powder and ammonia water were used as pH regulators, respectively. It can be seen that the removal rate of impurities Ti and Al reached more than 90%, but the filtration rate was significantly lower than that of Examples 1 and 2.

[0061] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be construed as limiting the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. Persons skilled in the art will appreciate that improvements and modifications may be made without departing from the spirit and scope of the present invention, and such improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for removing impurities from ferrous sulfate, a by-product of titanium dioxide, characterized by: The following steps are involved: Step 1: dissolving ferrous sulfate heptahydrate, a by-product of titanium dioxide, in water to obtain a ferrous sulfate solution; Step 2: Mixing calcium carbonate powder with water to obtain a pH-adjusted slurry; Step 3: slowly add the pH-adjusted slurry of step 2 to the ferrous sulfate solution of step 1, adjust the pH of the solution to 4-4.5, react at 50-70° C. for 90-120 min, and filter the reactant precipitate to obtain a ferrous sulfate filtrate.

2. The impurity removal method according to claim 1, wherein: In the step 1, the mass ratio of ferrous sulfate heptahydrate to water is 1:

2.

3. The impurity removal method according to claim 1, wherein: In step 2, the mass ratio of calcium carbonate powder to water is 1:

5.

4. The impurity removal method according to claim 1 or 3, characterized in that: The calcium carbonate powder is heavy calcium powder.

5. The impurity removal method according to claim 1, wherein: In step 3, the pH-adjusted slurry is added to the ferrous sulfate solution under stirring.

6. The impurity removal method according to claim 1, characterized in that: The amount of the pH adjustment slurry added in step 3 is such that the mass of the calcium carbonate powder in the pH adjustment slurry accounts for 0.5 to 1.1% of the mass of the ferrous sulfate heptahydrate dissolved in the ferrous sulfate solution.

7. The impurity removal method according to claim 6, characterized in that: The time for adding the pH adjusting slurry into the ferrous sulfate solution in step 3 is controlled to uniformly add the amount of pH adjusting slurry according to claim 6 within 20 to 30 minutes.

8. The impurity removal method according to any one of claims 1 to 3, characterized in that: The water in step 1 and step 2 is desalted water.

9. The impurity removal method according to claim 1, characterized in that: A phosphoric acid solution is added to the ferrous sulfate filtrate until the pH value of the filtrate is 1 to 1.5 to obtain a ferrous sulfate solution, which is directly used for the preparation of battery-grade ferric phosphate.

10. The impurity removal method according to claim 9, characterized in that: The concentration of the phosphoric acid solution is 70-85 wt %.

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