A molten salt electrolysis method for extracting pure iron from red mud

Abstract

This invention discloses a molten salt electrolysis method for extracting pure iron from red mud, belonging to the field of metallurgy and solid waste resource utilization technology. The molten salt electrolysis method for extracting pure iron from red mud includes the following steps: preparing a molten salt system containing NaCl, KCl, and CaCl2; heating the molten salt system to a molten state, then adding red mud powder to obtain an electrolytic slurry; using the electrolytic slurry as the electrolyte to carry out an electrolytic reaction, obtaining a cathode deposited with metallic iron; collecting the metallic iron product from the surface of the cathode deposited with metallic iron, completing the extraction. This invention effectively controls the co-deposition of impurities during the reduction process by optimizing key parameters such as the ratio of the NaCl-KCl-CaCl2 ternary molten salt system, electrolysis temperature, and current density, thereby obtaining high-purity metallic iron deposits and achieving the goal of converting industrial solid waste into high-value resources, demonstrating good application prospects and industrialization potential.

Description

Technical Field

[0001] This invention relates to the field of metallurgy and solid waste resource utilization technology, and in particular to a molten salt electrolysis method for extracting pure iron from red mud. Background Technology

[0002] Red mud is a large-scale solid waste generated during the alumina industrial production process. Its annual discharge scale is huge, and its chemical composition is complex, containing high levels of various oxides such as iron, aluminum, titanium, calcium, and silicon, especially iron, which can typically reach 30-50%. Due to its strong alkalinity, difficulty in storage, and significant environmental hazards, the resource utilization of red mud has become one of the key technical challenges in the fields of non-ferrous metallurgy and environmental protection.

[0003] Currently, methods for recovering iron from red mud mainly include high-temperature reduction roasting and magnetic separation, acid leaching and precipitation, and atmosphere reduction. However, these methods generally suffer from high energy consumption, large pollutant emissions, complex processes, or limited iron product grade, hindering the clean, efficient, and selective extraction of iron resources from red mud and making large-scale industrial application difficult. Molten salt electrolysis, as a green metallurgical process, has advantages such as relatively low reduction temperature, good reaction selectivity, and high product purity, and has been widely used for the direct reduction of resources such as iron oxide, titanium minerals, and bauxite. However, its application to the selective extraction of iron from red mud is still in the preliminary exploration stage, especially in the construction of the electrolysis system, the selection of electrode materials, the control of the reaction process, and the effective collection of pure iron products, where a systematic and feasible technical solution is still lacking. Therefore, developing a new molten salt electrolysis method suitable for red mud systems and capable of efficiently recovering iron resources is of great significance for realizing the high-value utilization and harmless treatment of red mud. Summary of the Invention

[0004] The purpose of this invention is to provide a molten salt electrolysis method for extracting pure iron from red mud, thereby solving the aforementioned problems in the background art. This invention effectively controls the co-deposition of impurities during the reduction process by optimizing key parameters such as the ratio of the NaCl-KCl-CaCl2 ternary molten salt system, electrolysis temperature, and current density, thus obtaining high-purity metallic iron deposits. This achieves the goal of transforming industrial solid waste into high-value resources, demonstrating promising application prospects and industrialization potential.

[0005] To achieve the above objectives, the present invention provides the following technical solution: One of the technical solutions of this invention is to provide a molten salt electrolysis method for extracting pure iron from red mud, comprising the following steps: Prepare a molten salt system containing NaCl, KCl, and CaCl2; The molten salt system is heated to a molten state, and then red mud powder is added to obtain an electrolytic slurry. Using the electrolytic slurry as the electrolyte, an electrolytic reaction is carried out to obtain a cathode with deposited metallic iron; The extraction is completed by collecting the metallic iron product from the surface of the cathode where metallic iron has been deposited.

[0006] Preferably, the mass ratio of NaCl, KCl and CaCl2 is 3:3:1.

[0007] This invention optimizes the ternary molten salt system of NaCl-KCl-CaCl2 to a ratio of 3:3:1. The rationale is that at this ratio, the ternary molten salt system can form a stable eutectic mixture with an actual melting temperature of approximately 580–620℃. Compared to molten salt systems such as the NaCl-KCl binary system, it exhibits lower viscosity and higher ion mobility, which is beneficial for improving Fe... 2+ / Fe 3+ The diffusion rate of species in molten salt. Secondly, adding an appropriate amount of CaCl2 (approximately 1 / 7 by mass) can significantly improve the structural relaxation of the molten salt, resulting in higher ionic conductivity and thus reducing the ohmic voltage drop required for electrolysis, making it easier to maintain the cathode potential stably within the iron reduction range. Furthermore, Ca... 2+ With a large radius, it is less prone to reduction during electrolysis, effectively diluting the interactions between Fe, Al, Si, and other metal ions. This makes the coordination environment of Fe ions in the molten salt more stable, improving its selective reduction rate. More importantly, K in this system... + With Ca 2+ The synergistic effect can significantly inhibit Al 3+ Si 4+ The solubility of difficult-to-reducible impurities in molten salt.

[0008] Preferably, the red mud powder has a particle size of less than 75 micrometers and an iron content of not less than 30 wt%.

[0009] Preferably, the amount of red mud powder added is 35 wt% of the molten salt system.

[0010] Preferably, the current density of the electrolysis reaction is 0.2-1.5 A / cm². 2 More preferably, it is 0.5-1.2 A / cm. 2 The voltage is 2.2-3.6V, the temperature is 950-970℃, and the time is 2-6 hours.

[0011] Preferably, the anode of the electrolysis reaction is an inert anode, more preferably a graphite electrode, a SnO2-based electrode, or a Pt-based electrode.

[0012] Preferably, the cathode of the electrolysis reaction is a metal cathode, more preferably a metal sheet or metal rod made of stainless steel, nickel-based alloy or molybdenum-based alloy.

[0013] Preferably, the molten salt system further includes a slag remover to improve fluidity and electrical conductivity; the slag remover is AlF3 and / or MgF2, and the amount added is 5-15 wt% of the molten salt system.

[0014] Preferably, the temperature at which the material is heated to a molten state is 600-900°C.

[0015] Preferably, the red mud powder is added in stages and intermittently to avoid local agglomeration and electrode short circuits.

[0016] Preferably, the electrolysis reaction is controlled by a constant current or constant voltage power supply, and the temperature, voltage and current changes of the electrolytic cell are monitored in real time, so that the iron oxides in the red mud are reduced to metallic iron at the cathode and oxygen is released at the anode.

[0017] Preferably, during the electrolysis reaction, the electrolytic cell is equipped with a stirring device or a gas bubbling device to enhance the dispersibility and mass transfer effect of the red mud powder; the stirring device is a mechanical stirring device or a magnetic stirring device.

[0018] Preferably, the method for collecting the metallic iron product is as follows: after the electrolysis reaction is completed, the electrolytic cell is cooled to below 500°C, the cathode with the deposited metallic iron is removed, the metallic iron deposit layer on the cathode is peeled off and collected, and after washing and drying, the purity of the obtained metallic iron is not less than 99 wt%.

[0019] Preferably, after the electrolysis reaction is completed, the electrolyte is collected, and insoluble impurities are removed by sedimentation or filtration before being recycled.

[0020] The technical principle of this invention is as follows: This invention proposes a method for extracting high-purity metallic iron from red mud using high-temperature molten salt electrolysis. The core concept is to selectively reduce iron oxides in red mud in an optimized chloride molten salt system, so that iron is deposited in a high-purity metallic state on the cathode, while other impurity elements (such as Al, Ti, Si, etc.) are effectively separated because they have a more negative reduction potential or are insoluble in this electrolysis system, thereby achieving the preparation of high-purity iron.

[0021] I. Basic Electrochemical Reaction Principles Iron in red mud mainly exists in the form of Fe2O3 oxide, which undergoes a multi-step reaction of dissolution, dissociation, and reduction in high-temperature molten salt. The core reaction of electrolysis can be represented as follows: Cathode reaction: Fe2O3+6e - →2Fe + 3O 2- Anode reaction: 2O 2- →O2↑+4e- Overall response: Fe₂O₃→2Fe + 3 / 2O₂↑ By applying an external DC electric field, iron oxide is gradually reduced to metallic iron in the cathode region, and oxygen ions migrate to the anode to discharge and form oxygen, thereby achieving an efficient conversion of oxide to metallic iron.

[0022] II. Selective Reduction and High Purity Mechanism Obtaining high-purity iron depends on the effective suppression of impurity elements. This invention achieves selective electrochemical reduction through the following multiple mechanisms: 1. Potential difference control principle The standard reduction potentials of various metal oxides differ significantly: the reduction potential of Fe₂O₃→Fe is approximately -0.44 V (relative to Cl₂ / Cl₂). - (The reduction potentials of Al2O3 and TiO2 are lower than this value. By precisely controlling the cell voltage and current density, the system potential is made sufficient to reduce iron ions without triggering the reduction of impurities such as aluminum and titanium, thus ensuring product purity.)

[0023] 2. Differences in ion migration and dissolution In the NaCl-KCl-CaCl2 ternary system, iron ions can be converted to Fe through oxide-chloride equilibrium. 2+ / Fe 3+ Soluble ions; while Al 3+ Ti 4+ High-valence ions hardly form stable solutions in this system, making it difficult for them to migrate to the cathode region. This "discretionary solubility" mechanism ensures iron selectivity in the cathode reduction process.

[0024] 3. Impurity Collection The addition of small amounts of CaF2 and / or AlF3 to the molten salt system can generate high-melting-point compounds (such as CaTiO3 and CaAl2O4) with impurity oxides, which are suspended or deposited in solid form, preventing them from participating in the electrochemical process and further improving the purity of iron.

[0025] III. Electrochemical Kinetics of Pure Iron Formation The cathodic deposition process includes the following stages: 1. Activation and nucleation: Fe 2+ Active nucleation sites are first formed on the cathode surface; 2. Growth and densification: Current density controlled between 0.2-1.5 A / cm² 2 Within this range, iron grains grow along the crystal orientation to form a dense deposition layer; 3. Recrystallization and purification: Under high temperature conditions (950℃), local thermal drive causes impurity elements (such as small amounts of oxygen and chlorine) inside the deposited layer to migrate out, thereby improving the purity of the metal.

[0026] IV. Analysis of the Thermodynamic and Electrochemical Behavior of Impurity Elements The reduction potentials of common associated components in red mud, Al2O3, TiO2, and SiO2, in molten salt are approximately: Al₂O₃ → Al: approximately -1.2 V TiO2→Ti: Approximately -0.9 V SiO2→Si: Approximately -1.6 V These values ​​are all significantly lower than the reduction potential of Fe₂O₃ to Fe. Therefore, when the control cell voltage is below 3.6 V, only iron undergoes electrochemical reduction, while other components remain in a stable oxidized state and are difficult to co-deposit. This principle constitutes the potential window control mechanism for high-purity iron extraction in this invention. Furthermore, to avoid contamination of the molten salt system by anodic oxidation products, this invention employs an inert anode structure, which can stably release oxygen at high temperatures without reacting with the molten salt, thereby achieving long-cycle electrolysis and low impurity introduction.

[0027] The electrolytic iron prepared by this invention, after XRD analysis, was found to be mainly composed of α-Fe with extremely low levels of impurity elements. The high purity is attributed to the synergistic effect of three factors: electrochemical selective control (reducing only iron), impurity fixation mechanism (Al and Ti forming stable oxides that remain in the slag), and stable oxygen ion concentration in the molten salt system. This ensures highly selective iron deposition and high purity at the electrochemical, thermodynamic, and materials levels.

[0028] The beneficial technical effects of the present invention are as follows: (1) This invention provides a molten salt electrolysis method for extracting pure iron from red mud. From the perspective of efficient resource utilization, red mud, as a large amount of solid waste discharged during alumina production, has a high iron content and is a secondary resource with great development value. Traditional methods often fail to achieve complete iron recovery or can only obtain low-grade iron concentrate. However, this invention adopts a direct electrolysis method. Through process control, the iron oxides in red mud can be reduced to high-purity metallic iron in the molten salt system, which greatly improves resource utilization efficiency and truly realizes the goal of transforming industrial solid waste into high-value resources.

[0029] (2) This invention effectively controls the co-deposition of impurities during the reduction process, thereby obtaining high-purity metallic iron deposits. This method can stably obtain metallic iron products with a purity of over 99%, and has the potential to be used as high-performance alloy materials, electromagnetic materials, or powder metallurgy raw materials, providing a new source of iron for high-end metal products.

[0030] (3) The electrolysis method of this invention is green and environmentally friendly throughout the entire process. The electrolysis medium is a non-toxic, recyclable molten salt, eliminating the need for corrosive chemicals such as acids and alkalis, thus avoiding waste liquid discharge. The anode product is oxygen, with no harmful gas release. The molten salt can be recycled multiple times, and the solid residue can be further used for the recovery of elements such as aluminum and titanium without generating new secondary pollution. This invention is suitable for batch or continuous operation and has industrial scale-up capabilities. Therefore, this invention not only achieves efficient and clean extraction of iron resources from red mud, but also provides a new technical path for the harmless and high-value utilization of heavy metal waste, which is in line with the development direction of green and circular economy, and shows good application prospects and industrial promotion potential. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a process flow diagram of the molten salt electrolysis method for extracting pure iron from red mud according to the present invention.

[0033] Figure 2 This is a schematic diagram of the electrolysis apparatus of Example 1.

[0034] Figure 3 This is a macroscopic image of the metallic iron deposit prepared in Example 1.

[0035] Figure 4 The image shows the XRD pattern of the metallic iron deposit prepared in Example 1. Detailed Implementation

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0037] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, are also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0038] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. It should be noted that any aspects of this invention not described in detail are conventional practices in the art and are not the focus of this invention.

[0039] The terms “comprising,” “including,” “having,” “containing,” etc., used in this invention are all open-ended terms, meaning that they include but are not limited to.

[0040] This invention discloses a molten salt electrolysis method for extracting pure iron from red mud, comprising the following steps: (1) The red mud raw material is crushed and pretreated to obtain dry red mud powder suitable for electrolysis; (2) Mix NaCl, KCl and CaCl2 to obtain a molten salt system; (3) The molten salt system is heated to 600-900℃ to maintain the molten state, and then the red mud powder is added to form a suspension or sedimentation system to obtain an electrolytic slurry; (4) Using the electrolytic slurry as the electrolyte, a direct current is applied between the inert anode and the metal cathode at a current density of 0.2-1.5 A / cm². 2 Under certain conditions, an electrolytic reaction is carried out, and the iron oxides in the red mud are reduced to metallic iron through electroreduction and deposited on the cathode, resulting in a cathode with deposited metallic iron. At the same time, oxygen is released at the anode. (5) Collect the metallic iron product from the surface of the cathode on which metallic iron is deposited, and wash and dry it.

[0041] Furthermore, the impurity removal pretreatment includes the following steps: crushing the red mud raw material, then sieving particles with a diameter of less than 75 micrometers to remove large particles and impurities; and then drying at 100-120℃ for 4-6 hours to remove moisture.

[0042] Furthermore, the mass ratio of the molten salt system to the red mud powder is 20:7.

[0043] Furthermore, the current density of the electrolysis reaction is 0.2-1.5 A / cm². 2 More preferably, it is 0.5-1.2 A / cm. 2 The voltage is 2.2-3.6V, the temperature is 950-970℃, and the time is 2-6 hours.

[0044] Furthermore, the anode of the electrolysis reaction is an inert anode, more preferably a graphite electrode, a SnO2-based electrode, or a Pt-based electrode.

[0045] Furthermore, the cathode of the electrolytic reaction is a metal cathode, more preferably a metal sheet or metal rod made of stainless steel, nickel-based alloy or molybdenum-based alloy.

[0046] Furthermore, the molten salt system also contains a slag remover to improve fluidity and electrical conductivity; the slag remover is of the type AlF3 and / or MgF2, and the amount added is 5-15 wt% of the molten salt system.

[0047] Furthermore, the red mud powder is added intermittently in multiple stages to avoid local agglomeration and electrode short circuits.

[0048] Furthermore, the electrolysis reaction is controlled by a constant current or constant voltage power supply, and the temperature, voltage, and current changes of the electrolytic cell are monitored in real time, so that the iron oxides in the red mud are reduced to metallic iron at the cathode and oxygen is released at the anode.

[0049] Furthermore, during the electrolysis reaction, the electrolytic cell is equipped with a stirring device or a gas bubbling device to enhance the dispersibility and mass transfer effect of the red mud powder; the stirring device is a mechanical stirring device or a magnetic stirring device.

[0050] Furthermore, the method for collecting the metallic iron product is as follows: after the electrolytic reaction is completed, the electrolytic cell is cooled to below 500°C, the cathode with the deposited metallic iron is removed, the metallic iron deposit layer on the cathode is peeled off and collected, and after washing and drying, the purity of the obtained metallic iron is not less than 99wt%.

[0051] Furthermore, after the electrolysis reaction is completed, the electrolyte is collected, and insoluble impurities are removed by sedimentation or filtration before being recycled.

[0052] The composition of the red mud raw materials used in the following embodiments and comparative examples of the present invention is shown in Table 1.

[0053] Table 1 All raw materials used in the following embodiments and comparative examples of the present invention are commercially available products.

[0054] Figure 1 This is a process flow diagram of the molten salt electrolysis method for extracting pure iron from red mud according to the present invention.

[0055] Example 1 A molten salt electrolysis method for extracting pure iron from red mud includes the following steps: (1) The red mud raw material was ground with a ball mill until the particle size was less than 75 micrometers, and then dried at 110°C for 4 hours to obtain dry red mud powder suitable for electrolysis; (2) Mix NaCl, KCl and CaCl2 in a mass ratio of 3:3:1, and then add 5g of AlF3 to obtain a molten salt system (total mass is 100g, and the proportion of AlF3 in the molten salt system is 5wt%). (3) Place the molten salt system in an electrolytic cell (high-purity alumina crucible), heat it to 900°C to maintain the molten state, and then add the above-mentioned red mud powder (add it to the bottom of the electrolytic cell in multiple batches of 2g each time, and stir it to make it evenly distributed. The total amount of red mud added is controlled to be about 35wt% of the mass of the molten salt system) to obtain the electrolytic slurry. (4) Using the above-mentioned electrolytic slurry as the electrolyte, a high-density graphite rod (6 mm in diameter) as the anode, and a stainless steel sheet (10 mm × 50 mm × 1 mm) as the cathode, with a minimum distance of 2 cm between the two electrodes, the anode is suspended above the molten salt, and the cathode is immersed in the molten salt to obtain the electrolytic device (see Figure 2 ); (5) Apply a DC regulated power supply between the anode and cathode of the electrolysis device, and control the current density at 0.8 A / cm². 2 Electrolysis was carried out under the conditions of voltage controlled at 2.8V and temperature controlled at 950℃ to obtain a cathode with deposited metallic iron, while oxygen was evolved at the anode; the electrolysis time lasted for 4 hours, and the temperature fluctuation was kept within ±10℃ during the process. (6) After the electrolysis reaction is completed, wait for the electrolytic cell to cool to below 500°C, take out the cathode with deposited metallic iron, shake to peel off and collect the metallic iron deposit layer on the cathode, wash with distilled water, dry and obtain brown metallic iron deposit, weigh and analyze. (7) After the electrolysis reaction is completed, the molten salt can be recycled after the insoluble residue is removed by natural clarification; the insoluble residue can be used as raw material for further extraction of aluminum, titanium and other polymetallic resources.

[0056] Figure 2 This is a schematic diagram of the electrolysis apparatus of Example 1.

[0057] Figure 3 This is a macroscopic image of the metallic iron deposit prepared in Example 1.

[0058] Figure 4 The image shows the XRD pattern of the metallic iron deposit prepared in Example 1.

[0059] pass Figure 4The XRD results showed that the metallic iron deposit in Example 1 was mainly composed of metallic Fe with a purity of 99.5 wt%. X-ray diffraction was performed in the range of 20–90° with a step size of 0.02°. The analysis results showed that the characteristic peaks of Fe (110), (200), and (211) in the metallic iron deposit were located at 44.68°, 65.03°, and 82.35°, respectively, and no FeO or Fe3O4 peaks were detected; indicating that the product was high-purity metallic iron.

[0060] Example 2 The only difference from Example 1 is that the addition of AlF3 in the molten salt system in step (2) is omitted.

[0061] Comparative Example 1 The only difference from Example 1 is that the electrolysis voltage in step (5) is changed from 2.8V to 4.0V.

[0062] Comparative Example 2 The only difference from Example 1 is that the electrolysis temperature in step (5) is changed from 950°C to 850°C.

[0063] Comparative Example 3 The only difference from Example 1 is that the addition of CaCl2 in the molten salt system in step (2) is omitted.

[0064] Effect verification By changing the electrolysis voltage, electrolysis temperature, and molten salt composition, the chemical composition of the product was analyzed, and the test results are shown in Table 2.

[0065] Table 2 As can be seen from Table 2, the purity of the product decreases significantly after changing the preferred parameters (electrolysis voltage and electrolysis temperature), while the preferred electrolysis conditions of this invention can obtain metallic iron with a purity of over 99%.

[0066] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A molten salt electrolysis method for extracting pure iron from red mud, characterized in that, Includes the following steps: Prepare a molten salt system containing NaCl, KCl, and CaCl2; The molten salt system is heated to a molten state, and then red mud powder is added to obtain an electrolytic slurry. Using the electrolytic slurry as the electrolyte, an electrolytic reaction is carried out to obtain a cathode with deposited metallic iron; The extraction is completed by collecting the metallic iron product from the surface of the cathode where metallic iron has been deposited.

2. The molten salt electrolysis method according to claim 1, characterized in that, The mass ratio of NaCl, KCl and CaCl2 is 3:3:

1.

3. The molten salt electrolysis method according to claim 1, characterized in that, The red mud powder has a particle size of less than 75 micrometers.

4. The molten salt electrolysis method according to claim 1, characterized in that, The amount of red mud powder added is 35 wt% of the molten salt system.

5. The molten salt electrolysis method according to claim 1, characterized in that, The current density of the electrolysis reaction is 0.2-1.5 A / cm². 2 The voltage is 2.2-3.6V, the temperature is 950-970℃, and the time is 2-6 hours.

6. The molten salt electrolysis method according to claim 1, characterized in that, The anode of the electrolysis reaction is a graphite electrode, a SnO2-based electrode, or a Pt-based electrode.

7. The molten salt electrolysis method according to claim 1, characterized in that, The cathode of the electrolytic reaction is made of stainless steel, nickel-based alloy, or molybdenum-based alloy.

8. The molten salt electrolysis method according to claim 1, characterized in that, The molten salt system also contains a slag remover; the slag remover is AlF3 and / or MgF2, and the amount added is 5-15 wt% of the molten salt system.

9. The molten salt electrolysis method according to claim 1, characterized in that, The temperature at which the material is heated to a molten state is 600-900℃.

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