A process for the preparation of crystalline hematite by direct iron precipitation from an iron sulfate solution

Abstract

The present application relates to the technical field of hydrometallurgy, and is a method for preparing crystalline hematite by directly precipitating iron from an iron sulfate solution, comprising the following steps: solution pretreatment: adjusting the Fe3+ containing sulfate solution so that the initial pH value is controlled in the range of 3.0-4.5; seed addition and mixing: adding seed to the pretreated solution and mixing thoroughly to form the mixed ore slurry, wherein the seed is hematite powder; high-temperature and high-pressure reaction; post-treatment: after the reaction is completed, the hematite precipitate is filtered for solid-liquid separation to obtain hematite slag and post-iron precipitation solution, and the hematite slag is washed and dried to obtain a high-grade crystalline hematite product; through the innovative short process, especially the seed induction and the synergistic control of multiple parameters, the present application successfully realizes the direct and efficient preparation of high-quality crystalline hematite from the Fe3+ solution, solves the common technical problems in the industry, and has extremely high industrial application value.

Description

Technical Field

[0001] This invention relates to the field of hydrometallurgical technology, and in particular to a method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution. Background Technology

[0002] In the hydrometallurgical processes of non-ferrous metals such as zinc, copper, and nickel, comprehensive resource utilization and clean production have become essential requirements for industry development. Taking zinc smelting as an example, conventional leaching produces a large amount of leaching slag with high iron content (such as zinc ferrite). While traditional methods such as the jaundice process and goethite process can remove iron, the resulting iron slag has poor stability, low iron content (usually 30%-40%), and contains harmful impurities, making it difficult to use directly as raw material for ironmaking. Most of it needs to be stockpiled, which not only occupies land but also poses environmental risks.

[0003] The hematite (Fe2O3) process is considered one of the ideal iron removal technologies, producing iron slag with high iron content (>58%), few impurities, and good filtration performance. It can be sold as a cement corrective agent or ironmaking raw material, truly turning waste into treasure. However, existing hematite processes, such as the Japanese Iijima process, generally employ a two-step "reduction-oxidation" route. That is, first, the ferric iron (Fe³⁺) in the leachate is reduced to ferrous iron (Fe²⁺), and then Fe²⁺ is oxidized and hydrolyzed under high temperature and pressure to generate hematite. This process is lengthy, requiring large amounts of reducing and oxidizing agents, and the high-temperature, high-pressure reactor needs to process the entire leachate, resulting in high equipment investment and energy consumption.

[0004] To address these issues, researchers explored the possibility of directly hydrolyzing iron from Fe³⁺ solutions. However, Fe³⁺ is readily hydrolyzed in acidic solutions, and the hydrolysis pathway is complex. Under conventional conditions, direct hydrolysis easily produces colloidal amorphous precipitates with extremely poor filtration performance or basic ferric sulfate [Fe(OH)SO₄] with high sulfur content, failing to yield well-crystallized and easily separable hematite products, thus becoming a bottleneck restricting the development of this technology.

[0005] Therefore, there is an urgent need to develop a new method that can effectively control the Fe³⁺ hydrolysis pathway, avoid the formation of colloids and impurities, and directly obtain high-quality crystalline hematite. Summary of the Invention

[0006] This invention provides a method for preparing crystalline hematite by directly precipitating iron from ferric sulfate solution, which overcomes the shortcomings of the prior art and effectively solves the problem of complex process in existing methods for preparing crystalline hematite using a "reduction + oxidation" process.

[0007] To address the above problems, the present invention provides a method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution, comprising the following steps: Solution pretreatment: Adjust the sulfate solution containing Fe³⁺ to control its initial pH value within the range of 3.0~4.5; Seed crystal addition and mixing: Add seed crystals to the pretreated solution and mix thoroughly to form a mixed slurry, wherein the seed crystals are hematite powder; High-temperature and high-pressure reaction: The mixed slurry is fed into a high-pressure reactor and undergoes a hydrothermal reaction under stirring to generate hematite precipitate; Post-processing: After the reaction is completed, the hematite precipitate is filtered to separate solid and liquid, resulting in hematite slag and liquid after iron precipitation. The hematite slag is washed and dried to obtain high-grade crystalline hematite product.

[0008] In the above-mentioned sulfate solution containing Fe³⁺, the concentration of Fe³⁺ is 15~25 g / L.

[0009] The above-mentioned adjustment of the Fe³⁺-containing sulfate solution to control its initial pH value within the range of 3.0 to 4.5 includes adjusting the pH of the solution by adding zinc oxide.

[0010] The particle size of the above-mentioned seed crystals is <10μm, and the amount of seed crystals added is 1%~10% of the mass of Fe³⁺.

[0011] In the above hydrothermal reaction, the reaction temperature is 180~200℃; the reaction time is 3~5 hours.

[0012] The stirring speed is 400~800 rpm.

[0013] In the above solution pretreatment step, a neutralizing agent is added, wherein the neutralizing agent is ZnO or limestone.

[0014] The above hydrothermal reaction is: Fe2(SO4)3 + 3H2O = Fe2O3↓ + 3H2SO4, which is a high temperature and high pressure reaction. The H2SO4 generated in the hydrothermal reaction is partially neutralized by a neutralizing agent added in the solution pretreatment step.

[0015] Compared with the prior art, the present invention has the following advantages: 1. Short process and low cost: For the first time, high-quality hematite can be directly prepared from Fe³⁺ solution without a reduction step. This completely eliminates the traditional “Fe³⁺→Fe²⁺” reduction process and the subsequent “Fe²⁺→Fe³⁺” oxidation process, which significantly shortens the process and reduces the consumption of reducing agent and oxygen.

[0016] 2. High product quality and easy separation: By introducing seed crystals to induce heterogeneous nucleation, the formation of colloids caused by homogeneous nucleation is effectively suppressed. The resulting hematite has high crystallinity, large particles, and regular morphology. The product has an iron content of over 64% and a sulfur content that can be controlled below 1.5%. It has excellent filtration performance and can be directly sold as a product.

[0017] 3. Low equipment investment and low energy consumption: Since the reduction step is eliminated and the high-pressure reactor mainly processes seed crystals and Fe³⁺, its processing load is optimized compared to the traditional process for processing the whole solution at the optimized concentration, which helps to reduce equipment size and energy consumption.

[0018] 4. Environmentally friendly: The produced iron slag is a high-grade resource product, eliminating the environmental pollution risks caused by iron slag stockpiling. Attached Figure Description

[0019] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0020] Figure 1 This is a flowchart of the method in Embodiment 1 of the present invention.

[0021] Figure 2 This is a phase analysis diagram of the product in Embodiment 1 of the present invention. Detailed Implementation

[0022] The present invention is not limited to the following embodiments, and the specific implementation can be determined according to the technical solution of the present invention and the actual situation.

[0023] Example 1: As Figure 1 As shown in the figure, this invention discloses a method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution, comprising the following steps: Step 1, Solution pretreatment: Adjust the sulfate solution containing Fe³⁺ to control its initial pH value within the range of 3.0~4.5; The second step is seed crystal addition and mixing: add seed crystals to the pretreated solution and mix thoroughly to form a mixed slurry, wherein the seed crystals are hematite powder; The third step is high-temperature and high-pressure reaction: the mixed slurry is sent into a high-pressure reactor and hydrothermal reaction is carried out under stirring to generate hematite precipitate. The fourth step is post-processing: After the reaction is completed, the hematite precipitate is filtered to separate the solid and liquid phases, resulting in hematite slag and the liquid after iron precipitation. The hematite slag is then washed and dried to obtain a high-grade crystalline hematite product.

[0024] In the first step above, the Fe³⁺ concentration in the sulfate solution is 15~25 g / L. The Fe³⁺-containing sulfate solution is adjusted to maintain its initial pH within the range of 3.0~4.5, including by adding zinc oxide to adjust the pH of the solution.

[0025] In the second step above, the seed crystal size is <10μm to provide a larger specific surface area and enhance the induction effect. The seed crystal addition amount is 1%~10% of the Fe³⁺ mass. More preferably, the seed crystal addition amount is 3%~6% of the Fe³⁺ mass.

[0026] In the third hydrothermal reaction described above, the reaction temperature is 180~200℃, and the reaction time is 3~5 hours. More preferably, the reaction temperature is 190~200℃, and the reaction time is 4 hours.

[0027] In the third step described above, the stirring conditions include a stirring rate of 400-800 rpm. This ensures uniform seed crystal suspension and enhances mass transfer, while preventing excessive shear force that could lead to crystal breakage.

[0028] In the first step of solution pretreatment mentioned above, a neutralizing agent is added, wherein the neutralizing agent is ZnO or limestone.

[0029] The third hydrothermal reaction step described above is: Fe2(SO4)3 + 3H2O = Fe2O3↓ + 3H2SO4, which is a high-temperature and high-pressure reaction. The H2SO4 generated in the hydrothermal reaction is partially neutralized by a neutralizing agent added during the solution pretreatment step. This facilitates maintaining the pH of the reaction system within the target range. However, the neutralization rate needs to be controlled during the neutralization process to avoid excessively high local pH values.

[0030] The XRD phase analysis diagram of the product produced in the fourth step above is as follows: Figure 2 As shown in the figure, the analysis of the physical phenomena shows that the iron in the hematite slag mainly exists in the form of hematite, with a small amount of basic ferric sulfate. This indicates that the product basically meets the requirements for high-quality crystalline hematite, but there is still room for further optimization.

[0031] Example 2: The first step involves taking a simulated leaching solution from a wet zinc smelting process. Its main components are: Fe³⁺ 20 g / L and Zn²⁺ 80 g / L. The initial pH of the solution is adjusted to 4.0 using ZnO.

[0032] The second step is to add hematite seed crystals with a particle size D90 < 10 μm to the pH-adjusted solution. The amount added is 5% of the mass of Fe³⁺ in the solution. After stirring evenly, the solution is pumped into a 2L high-pressure reactor.

[0033] The third step is to seal the reactor, turn on the stirrer, set the speed to 600 rpm, heat to 195°C, and set the reaction pressure to 1.8-2.2 MPa (autogenous pressure). Maintain the temperature for 4 hours.

[0034] Fourth step: After the reaction is complete, cool, depressurize, and filter to obtain a reddish-brown filter cake, wash with hot water and dry at 105℃.

[0035] Results: The iron removal rate of Example 2 was 97.3%, and the resulting hematite slag contained 65.1% iron and 1.2% sulfur. XRD analysis of the slag phase showed characteristic peaks of well-crystallized hematite, with a small amount of basic ferric sulfate phase and no other impurities. Sedimentation experiments showed that the average particle size D50 of this hematite slag was 15-20 μm, and the filtration rate was more than 5 times higher than that of traditional amorphous iron slag.

[0036] Example 3: This example examines the effect of different initial pH values. Take the same solution as in Example 2 and adjust the initial pH to 2.5, 3.0 and 4.5 respectively, while keeping all other conditions exactly the same as in Example 2.

[0037] Results: When pH=2.5, the iron removal rate was only 82%. XRD analysis of the product showed that, in addition to hematite, there were obvious characteristic peaks of basic ferric sulfate, and the sulfur content of the slag increased to 3.5%.

[0038] When the pH was 3.0 and 4.5, the iron removal rate was higher than 96%, and the product was pure phase hematite, with sulfur content in the slag of 1.4% and 1.3%, respectively. This indicates that controlling the initial pH above 3.0 is the key to obtaining high-quality hematite.

[0039] Example 4: This example examines the effect of different seed crystal addition amounts. Take the same solution as in Example 2 and adjust the initial pH to 4.0. Add 0%, 1%, 3%, 6%, and 10% Fe³⁺ hematite seeds (particle size <10 μm) respectively, while keeping all other conditions exactly the same as in Example 2. The results show: Without seed crystals, the product is a colloidal amorphous precipitate that is difficult to filter. XRD shows it as a mixed phase of amorphous material and a small amount of goethite.

[0040] When the addition amount is 1%, the product still has a certain amount of colloid, and the improvement in filtration performance is not obvious.

[0041] When the addition amount is 3%, 6% and 10%, the product is well-crystallized hematite with excellent filtration performance. Moreover, the average particle size of the product with 6% and 10% addition amount is slightly smaller than that with 3% addition amount, indicating that the amount of seed crystal added can control the particle size of the final product.

[0042] Example 5: This example investigates the effect of different Fe³⁺ concentrations on a sulfate solution containing Fe³⁺. A simulated leaching solution from a hydrometallurgical zinc refining process was taken, with the main components being Zn²⁺ 80 g / L and Fe³⁺ concentrations of 15 g / L, 20 g / L, and 25 g / L, respectively. The remaining conditions were the same as in Example 2. The results showed: When the Fe³⁺ concentration is low (15 g / L), the iron removal rate is 97.5%, and the resulting hematite slag contains 66.2% iron and 1.1% sulfur.

[0043] With an Fe³⁺ concentration of 20 g / L and an iron removal rate of 97.3%, the resulting hematite slag contained 65.1% iron and 1.2% sulfur.

[0044] With an Fe³⁺ concentration of 25 g / L and an iron removal rate of 92.3%, the resulting hematite slag contained 62.1% iron and 2.3% sulfur.

[0045] Note: Maintaining a lower Fe(III) concentration in the solution is beneficial for the reaction of iron ions to form hematite precipitate. Example 6: This example examines the effect of different reaction temperatures in a hydrothermal reaction: The same solution as in Example 2 was used, and the operating conditions were the same as in Example 2 except that the reaction temperature was changed to 180°C, 190°C and 200°C. The results showed that as the reaction temperature increased, the iron removal rate and the iron content of the hematite slag increased, indicating that high temperature helps to produce hematite products with fewer impurities.

[0046] Example 7: Investigating the effect of hydrothermal reaction time A simulated zinc sulfate solution containing Fe³⁺ (Fe³⁺ 20 g / L, Zn²⁺ 80 g / L), identical to that used in Example 2, was taken. The initial pH was adjusted to 4.0 with ZnO. Hematite seed crystals with a particle size <10 μm (5% of the Fe³⁺ mass) were added, mixed thoroughly, and then pumped into a 2 L high-pressure reactor. The reactor was sealed, the stirring speed was set to 600 rpm, and the temperature was raised to 195 °C. The reaction was maintained at this temperature for 2 hours, 3 hours, 5 hours, and 6 hours under autogenous pressure, respectively. After the reaction was completed, the solution was cooled, depressurized, filtered, washed with hot water, and dried at 105 °C.

[0047] The results showed that after 2 hours of reaction, the iron removal rate was only 86.4%, the sulfur content in the slag was as high as 3.8%, XRD showed obvious impurity peaks in basic ferric sulfate, and the filtration rate was only 0.4 times the normal value. After 3 hours of reaction, the iron removal rate increased to 96.5%, the sulfur content in the slag decreased to 1.5%, XRD showed a pure hematite phase, and the filtration performance was significantly improved. After 5 hours of reaction, the iron removal rate reached 97.1%, and all indicators were excellent. Extending the reaction to 6 hours did not further improve the iron removal rate and purity, but the median particle size of the product coarsened from 18 μm to 32 μm, which is not conducive to maintaining the number of fine-grained seed crystals in continuous production and reduces the production efficiency of the reactor. Therefore, 3-5 hours is a necessary time window to achieve full conversion and ensure a balance between product quality and production efficiency.

[0048] Example 8: Investigating the effect of stirring rate The same feed solution, seed crystals, and pretreatment conditions as in Example 2 were used. The mixed slurry was fed into a 2L high-pressure reactor, heated to 195°C, and held for 4 hours, during which the stirring speed was set to 300 rpm, 400 rpm, 800 rpm, and 900 rpm respectively. After the reaction was completed, post-processing and analysis were performed according to the same steps.

[0049] The results showed that at a stirring speed of 300 rpm, the iron removal rate was only 94.2%, the slag contained 2.9% sulfur, and there was obvious particle deposition and agglomeration at the bottom of the vessel, indicating poor filtration performance. At a stirring speed of 400 rpm, the iron removal rate increased to 97.0%, the slag contained 1.4% sulfur, the particles were uniform, and the filter cake was loose. At a stirring speed of 800 rpm, the iron removal rate was 96.8%, and the product quality remained excellent. When the stirring speed increased to 900 rpm, the median particle size of the product dropped sharply to 6.5 μm, the mother liquor became turbid, and the filtration rate decreased significantly, indicating that excessive shear force had broken the crystals and inhibited their growth. Therefore, 400~800 rpm is the necessary stirring range to ensure uniform suspension of the seed crystals, avoid deposition, and prevent excessive crystal breakage. Comparative example (simulation of existing technology): The same solution as in Example 2 was taken and processed according to the traditional two-stage "reduction-oxidation" process.

[0050] First, zinc concentrate or iron powder is added to the solution, and Fe³⁺ is reduced to Fe²⁺ at high temperature to obtain a solution containing 20 g / L of Fe²⁺. Then, this solution is sent to an autoclave and oxidized and precipitated at 200°C and an oxygen partial pressure of 200 kPa for 4 hours.

[0051] The results showed that the iron removal rate was 98.0%, and the resulting hematite slag contained 64.8% iron. However, the process was significantly prolonged and consumed reducing agent and high-pressure oxygen.

[0052] Compared with the prior art, the present invention has the following advantages: 1. Short process and low cost: For the first time, high-quality hematite can be directly prepared from Fe³⁺ solution without a reduction step. This completely eliminates the traditional “Fe³⁺→Fe²⁺” reduction process and the subsequent “Fe²⁺→Fe³⁺” oxidation process, which significantly shortens the process and reduces the consumption of reducing agent and oxygen.

[0053] 2. High product quality and easy separation: By introducing seed crystals to induce heterogeneous nucleation, the formation of colloids caused by homogeneous nucleation is effectively suppressed. The resulting hematite has high crystallinity, large particles, and regular morphology. The product has an iron content of over 64% and a sulfur content that can be controlled below 1.5%. It has excellent filtration performance and can be directly sold as a product.

[0054] 3. Low equipment investment and low energy consumption: Since the reduction step is eliminated and the high-pressure reactor mainly processes seed crystals and Fe³⁺, its processing load is optimized compared to the traditional process for processing the whole solution at the optimized concentration, which helps to reduce equipment size and energy consumption.

[0055] 4. Environmentally friendly: The produced iron slag is a high-grade resource product, eliminating the environmental pollution risks caused by iron slag stockpiling.

[0056] In summary, this invention, through an innovative short-process technology, particularly seed induction and multi-parameter synergistic control, successfully achieves the direct and efficient preparation of high-quality crystalline hematite from Fe³⁺ solution, solving a common technical problem in the industry and possessing extremely high industrial application value.

Claims

1. A method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution, characterized in that, Includes the following steps: Solution pretreatment: Adjust the sulfate solution containing Fe³⁺ to control its initial pH value within the range of 3.0~4.5; Seed crystal addition and mixing: Add seed crystals to the pretreated solution and mix thoroughly to form a mixed slurry, wherein the seed crystals are hematite powder; High-temperature and high-pressure reaction: The mixed slurry is fed into a high-pressure reactor and undergoes a hydrothermal reaction under stirring to generate hematite precipitate; Post-processing: After the reaction is completed, the hematite precipitate is filtered to separate solid and liquid, resulting in hematite slag and liquid after iron precipitation. The hematite slag is washed and dried to obtain high-grade crystalline hematite product.

2. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 1, characterized in that, In a sulfate solution containing Fe³⁺, the concentration of Fe³⁺ is 15~25 g / L.

3. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 1, characterized in that, Adjusting the pH of a sulfate solution containing Fe³⁺ to a range of 3.0–4.5 includes adjusting the pH of the solution by adding zinc oxide.

4. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 1, characterized in that, Seed particle size: <10μm, seed addition amount: 1%~10% of Fe³⁺ mass.

5. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 1, characterized in that, In the hydrothermal reaction, the reaction temperature is 180~200℃; the reaction time is 3~5 hours.

6. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 1, characterized in that, The stirring speed is 400~800 rpm.

7. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 1, characterized in that, In the solution pretreatment step, a neutralizing agent is added, wherein the neutralizing agent is ZnO or limestone.

8. The method for preparing crystalline hematite by direct precipitation of iron from ferric sulfate solution according to claim 7, characterized in that, The hydrothermal reaction is as follows: Fe2(SO4)3 + 3H2O = Fe2O3↓ + 3H2SO4, which is a high-temperature and high-pressure reaction. The H2SO4 generated in the hydrothermal reaction is partially neutralized by a neutralizing agent added in the solution pretreatment step.

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