Method for reducing iron oxalate

Abstract

This invention provides a method for rapidly reducing iron(III) oxalate to iron(II) oxalate. [Solution] Heat an aqueous solution of iron(III) oxalate containing iron(III) oxalate to a temperature of 110°C or higher and less than 300°C.

Description

[Technical Field]

[0001] The present invention relates to a method for reducing iron(III) oxalate that can improve the rate of reduction of iron(III) oxalate to iron(II) oxalate. [Background technology]

[0002] In recent years, there has been a growing need to reduce carbon dioxide emissions. One way to achieve this is to shift from manufacturing processes that traditionally use coal and oil as raw materials to processes that do not use coal or oil. For example, in the steelmaking sector, the blast furnace method, which is currently the mainstream method for producing iron by reducing iron ore, requires the use of coke produced from coal. For this reason, a shift to new steelmaking methods to replace the blast furnace method is being considered.

[0003] As a new ironmaking method to replace the blast furnace method, for example, Non-Patent Documents 1 and 2 propose an ironmaking method using oxalic acid. This ironmaking method involves the following three steps: The first step is to dissolve iron ore in an oxalic acid aqueous solution to obtain an iron(III) oxalate aqueous solution. The second step is to irradiate the iron(III) oxalate aqueous solution obtained in the first step with light to perform photoreduction and obtain iron(II) oxalate dihydrate. The third step is to heat the iron(II) oxalate dihydrate obtained in the second step to perform thermal decomposition reduction and obtain iron.

[0004] In these steps, carbon dioxide is produced as a byproduct in the second step. In the third step, carbon monoxide and carbon dioxide are also produced as byproducts. The carbon monoxide produced in the third step is converted back into carbon dioxide through a reaction called Water-Gas Shift (WGS), which involves reacting carbon monoxide with water to produce hydrogen and carbon dioxide. Furthermore, the carbon dioxide converted from carbon monoxide and the carbon dioxide produced in the second step are reacted with hydrogen to synthesize oxalic acid, which is then reused in the first step. Through these operations, a steelmaking method is achieved in which carbon dioxide emissions are zero in the chemical reactions of the entire process.

[0005] To increase the iron productivity in the ironmaking method using oxalic acid described in Non-Patent Documents 1 and 2, it is necessary to increase the rate of the reduction reaction (hereinafter sometimes referred to as "reduction rate") in which iron(III) oxalate is reduced to iron(II) oxalate in the second step. Methods for reducing iron(III) oxalate to iron(II) oxalate include, in addition to reduction by light, methods such as electrical reduction and chemical reduction by reaction with a reducing agent. For example, Patent Document 1 discloses a method for obtaining iron(II) oxalate by adding pulverized iron as a reducing agent to iron(III) oxalate. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2022 / 204752 [Non-patent literature]

[0007] [Non-Patent Document 1] P. Santawaja, S. Kudo, A. Mori, A. Tahara, S. Asano and J. Hayashi: ACS Sustainable Chemistry & Engineering, 8 (2020), 13292. [Non-Patent Document 2] P. Santawaja, S. Kudo, A. Tahara, S. Asano and J. Hayashi: ISIJ International, 62 (2022), 2466. Summary of the Invention Problems to be Solved by the Invention

[0008] As described above, in the iron-making process using oxalic acid described in Non-Patent Documents 1 and 2, light is applied to an aqueous iron(III) oxalate solution as a method for reducing iron(III) oxalate to iron(II) oxalate. When this method is employed, in order to efficiently irradiate the entire aqueous iron(III) oxalate solution with light, it was necessary to inject the aqueous solution into a flat container with a large installation area at a shallow water depth, for example. Furthermore, the reduction rate in this method was not necessarily high. On the other hand, in the method described in Patent Document 1 in which pulverized iron is added as a reducing agent to iron(III) oxalate, the addition of the reducing agent may lower the purity of the finally obtained iron.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for reducing iron(III) oxalate to iron(II) oxalate at high speed. Means for Solving the Problems

[0010] The present inventors have studied various methods for reducing iron(III) oxalate with the aim of increasing the rate of reducing iron(III) oxalate to iron(II) oxalate. As a result, they found that the reduction of iron(III) oxalate does not necessarily need to be performed by light irradiation, and that the reduction can also be achieved by heating an aqueous solution of iron(III) oxalate. They also found that switching from reduction by light to reduction by heat can significantly improve the reduction rate of iron(III) oxalate.

[0011] The gist of the present invention, which the inventors have completed based on the above findings, is as follows.

[0012] [1] A method for reducing iron(III) oxalate to obtain iron(II) oxalate by reducing iron(III) oxalate, comprising heating an aqueous iron(III) oxalate solution containing iron(III) oxalate to a temperature of 110°C or higher and lower than 300°C. A method for reducing iron(III) oxalate.

[0013] [2] The method for reducing iron(III) oxalate according to claim 1, wherein the molar concentration of all iron ions contained in the aqueous iron(III) oxalate solution before heating is 0.080 mol / L or more and 1.60 mol / L or less.

[0014] [3] The method for reducing iron(III) oxalate according to [1] or [2] above, wherein the aqueous iron(III) oxalate solution is heated to a temperature of 120°C or higher.

[0015] [4] The method for reducing iron(III) oxalate according to any one of [1] to [3] above, wherein the aqueous iron(III) oxalate solution is heated to a temperature of 150°C or lower.

[0016] [5] The method for reducing iron(III) oxalate according to any one of [1] to [4] above, wherein the temperature of the aqueous iron(III) oxalate solution before heating is controlled to be lower than 110°C. Effects of the Invention

[0017] According to the present invention, the reduction of iron(III) oxalate to iron(II) oxalate can be performed at high speed. In addition, in the method for reducing iron(III) oxalate according to the present invention, there is no need to irradiate the aqueous iron(III) oxalate solution with light, so the installation area of the reaction vessel used for the reduction reaction can be greatly reduced. Brief Description of the Drawings

[0018] [Figure 1] It is a graph showing the relationship between the heating temperature of the aqueous iron(III) oxalate solution and T80. Mode for Carrying Out the Invention

[0019] The embodiments for carrying out the present invention will be described in detail below.

[0020] In one embodiment, the present invention relates to a method for reducing iron(III) oxalate to obtain iron(II) oxalate, wherein an aqueous solution of iron(III) oxalate containing iron(III) oxalate is heated to a temperature of 110°C or higher and less than 300°C.

[0021] [Iron(III) oxalate aqueous solution] In the method for reducing iron(III) oxalate according to the present invention, the reduction of iron(III) oxalate is carried out by heating an aqueous solution of iron(III) oxalate. In this specification, "aqueous solution of iron(III) oxalate" means an aqueous solution containing iron(III) oxalate as a solute. Other solutes besides iron(III) oxalate may be dissolved in the aqueous solution of iron(III) oxalate.

[0022] The method for obtaining the iron(III) oxalate aqueous solution used in the present invention is not particularly limited, but generally, the iron(III) oxalate aqueous solution is obtained by dissolving iron oxide in an oxalic acid aqueous solution. Iron ore can be used as the iron oxide. Depending on the type of iron oxide it contains, iron ore may include hematite ore, magnetite ore, etc.

[0023] Hematite, as a compound, is also known as red iron ore. Its chemical name is iron(III) oxide, and its chemical formula is Fe2O3. All the iron in hematite as a compound is trivalent. Hematite is both the name of the compound and the name of a natural iron ore that contains a large amount of iron(III) oxide. An aqueous solution of iron(III) oxalate can be obtained by dissolving hematite ore in an aqueous solution of oxalic acid.

[0024] Magnetite, as a compound, is also known as iron(II,III) oxide or triiron tetroxide. Its chemical name is iron(II,III) oxide, and its chemical formula is Fe3O4. Magnetite contains divalent and trivalent iron in an atomic ratio of 1:2. Magnetite is both the name of the compound and the name of a natural iron ore rich in iron(II,III) oxide. Dissolving magnetite ore in an oxalic acid solution produces iron(II) and iron(III) oxalate in approximately a 1:2 molar ratio. Of these, iron(II) oxalate dissolves only slightly in the solution and precipitates along with other impurity metals that do not dissolve in oxalic acid. By removing the precipitate, an aqueous solution rich in iron(III) oxalate can be obtained.

[0025] In the method for reducing iron(III) oxalate according to the present invention, when using an aqueous solution of iron(III) oxalate obtained by dissolving iron oxide in an aqueous solution of oxalic acid, the iron oxide is not limited to iron ore, but can be any iron oxide that can produce an aqueous solution of iron(III) oxalate. For example, slag and mill scale generated as by-products in steel mills can also be used as iron oxide.

[0026] The particle size of iron oxide when dissolving it in an aqueous solution of oxalic acid is not particularly limited. However, from the viewpoint of increasing the reaction surface area and promoting dissolution, it is preferable that the particle size of the iron oxide be 2.0 mm or less. On the other hand, there was little difference in the dissolution rate between iron ore with a particle size of 1.0 mm to 2.0 mm and iron ore that had been further crushed to a particle size of 106 μm or less. Therefore, it is not necessary to excessively finen the iron oxide before dissolution or to remove fine particles. When adjusting the particle size of the oxide raw material, oxide raw material whose particle size has been reduced by a normal crushing operation may be used as is without classification.

[0027] According to the description in Non-Patent Document 2, in order to completely dissolve iron oxide in an aqueous oxalic acid solution, oxalic acid is required in a molar amount that is 1.82 times or more the molar amount of iron in the added iron oxide. Therefore, in a preferred embodiment, in the method for reducing iron(III) oxalate according to the present invention, let M be the total molar amount of all iron contained in the iron oxide T.Fe (mol), and let M be the molar amount of oxalic acid contained in the aqueous oxalic acid solution OxA (mol), then the molar ratio M of the two OxA / M T.Fe is 1.8 or more. On the other hand, M OxA / M T.Fe has no particular limitation on the upper limit. However, when M OxA / M T.Fe is excessively large, an increase in cost caused by adding excess oxalic acid becomes a problem. Therefore, M OxA / M T.Fe is preferably 5.4 or less.

[0028] When dissolving iron oxide in an aqueous oxalic acid solution, if the temperature of the aqueous oxalic acid solution is too low, the solubility of oxalic acid in water will decrease, and the dissolution rate of iron oxide will also decrease. Therefore, the temperature of the aqueous oxalic acid solution is preferably 90°C or higher. On the other hand, when the temperature of the aqueous oxalic acid solution reaches 100°C or higher, as will be described later, the reduction of iron(III) oxalate to iron(II) proceeds, which may reduce the yield of iron(III) oxalate, and it also becomes necessary to use a special pressure-resistant container for the reaction vessel. Therefore, the temperature of the aqueous oxalic acid solution is preferably less than 100°C.

[0029] [Reduction by Heat] In the method for reducing iron(III) oxalate according to the present invention, an aqueous solution of iron(III) oxalate containing iron(III) oxalate is heated to a temperature of 110°C or higher and less than 300°C. This reduces the iron(III) oxalate contained in the aqueous solution to iron(II) oxalate. However, if the temperature of the aqueous solution of iron(III) oxalate is too low, the thermal reduction of iron(III) oxalate hardly proceeds. Also, the reduction reaction of iron(III) oxalate accelerates as the temperature increases. Therefore, in the method for reducing iron(III) oxalate according to the present invention, the aqueous solution of iron(III) oxalate is heated to a temperature of 110°C or higher, preferably 120°C or higher. On the other hand, if the temperature of the aqueous solution of iron(III) oxalate is 300°C or higher, there is a risk that the iron(II) oxalate produced by the reduction of iron(III) oxalate will undergo thermal decomposition. Therefore, in the method for reducing iron(III) oxalate according to the present invention, the iron(III) oxalate aqueous solution is heated to a temperature of less than 300°C.

[0030] In a preferred embodiment, the method for reducing iron(III) oxalate according to the present invention involves heating the iron(III) oxalate aqueous solution to a temperature of 150°C or lower. As mentioned above, the reduction reaction of iron(III) oxalate is accelerated as the temperature of the aqueous solution increases. Therefore, if the temperature of the iron(III) oxalate aqueous solution is too high, the reduction reaction will be completed during the heating process. In this case, the energy used for heating after the reduction reaction is completed is wasted. For this reason, under normal circumstances, it is sufficient to heat the iron(III) oxalate aqueous solution to a temperature of 150°C or lower.

[0031] While the exact reason why the reduction rate of iron(III) oxalate increases with increasing temperature is unclear, the inventors believe the following: Iron(III) and iron(II) oxalate in the iron(III) oxalate solution exist in various chemical forms. Therefore, the reduction reaction of iron(III) oxalate is unlikely to be a simple elementary reaction, but rather a multi-step reaction involving multiple reaction intermediates and a combination of multiple elementary reactions. The main chemical reactions in the reduction reaction from iron(III) oxalate to iron(II) oxalate are the reduction of iron accompanied by the transfer of charge from the ligand (oxalic acid) to trivalent iron, and the production of CO2. The transfer of charge from the ligand to trivalent iron is accelerated by the application of thermal energy. Therefore, it is thought that the reduction rate of iron(III) oxalate increases with increasing temperature.

[0032] In the method for reducing iron(III) oxalate according to the present invention, it is preferable to carry out the reduction while the iron(III) oxalate aqueous solution remains in a liquid state during heating. By heating the iron(III) oxalate aqueous solution in a reaction vessel, at least a portion of the iron(III) oxalate aqueous solution heated to a temperature of 110°C or higher and less than 300°C can be kept in a liquid state. The reaction vessel used for heating the iron(III) oxalate aqueous solution can be any type of reaction vessel, as long as it has pressure resistance against the internal pressure rise due to the generation of water vapor and corrosion resistance to the heated iron(III) oxalate aqueous solution. If, for example, a sealable container is used as the reaction vessel, the reduction of iron(III) oxalate can be carried out in batch processing. Alternatively, for example, the stirred tank of a continuous stirred-tank reactor (CSTR) can be used as the reaction vessel. In this case, the reduction of iron(III) oxalate can be carried out in continuous processing.

[0033] When heating an aqueous solution of iron(III) oxalate in a reaction vessel, any heating method is acceptable as long as it can heat the aqueous solution to a temperature of 110°C or higher and less than 300°C. The temperature of the aqueous solution of iron(III) oxalate can be measured using a known temperature sensor. The temperature of the aqueous solution of iron(III) oxalate in the reaction vessel does not necessarily have to be maintained at a constant temperature; an appropriate temperature profile may be set within the above temperature range depending on the size of the reaction vessel, etc.

[0034] The solubility of iron(II) oxalate in water is significantly lower than that of iron(III) oxalate. Therefore, iron(II) oxalate produced by the reduction of iron(III) oxalate by heat precipitates as iron(II) oxalate dihydrate. High-purity iron(II) oxalate can be obtained by recovering the resulting precipitate.

[0035] The reduction rate of iron(III) oxalate to iron(II) oxalate in this invention can be evaluated by taking a sample of the supernatant of the aqueous solution after heating and cooling, and measuring the molar concentration of all iron ions contained in the supernatant at intervals of heating time. The molar concentration of all iron ions in the aqueous solution can be measured according to the method described in Non-Patent Document 2 below. Generally, aqueous solutions contain divalent iron ions (Fe 2+ (Ions) and trivalent iron ions (Fe 3+ It is thought that ions are present among them. Of these, Fe 3+ Regarding ions, first, using hydroxylamine hydrochloride, Fe 2+ Reduce to ions. Next, Fe 2+ By complexing the ion with 1,10-phenanthroline, ferroin ([Fe(phen)3] 2+ Next, the concentration of ferroin contained in the aqueous solution is quantified using a UV-Vis-Near-Infrared spectrophotometer and a calibration curve created by the analysis of standard reagents, thereby measuring the molar concentration of all iron ions dissolved in the aqueous solution. The unit of the molar concentration of all iron ions measured here is mol / L (moles per liter).

[0036] As mentioned above, Fe is derived from iron(II) oxalate. 2+ The ions precipitate in aqueous solution by forming iron(II) oxalate dihydrate. Therefore, the supernatant of the aqueous solution after heating and cooling contains Fe (ferrous iron(II) oxalate). 2+ It contains almost no ions and is derived from iron(III) oxalate that remained unreduced. 3+ It mainly contains ions. First, the molar concentration of all iron ions in the aqueous solution before heating is measured, and then the molar concentration of all iron ions in the supernatant collected from the aqueous solution after heating is measured. This allows for the determination of Fe ions originating from iron(III) oxalate present in the aqueous solution before heating. 3+ Among the ions, Fe originates from iron(II) oxalate. 2+ The percentage of ions reduced to ions, i.e., the reduction rate, can be determined. By repeatedly performing this operation while varying the heating time of the iron(III) oxalate aqueous solution, the reduction rate of iron(III) oxalate can be evaluated.

[0037] In this specification, "T" is used as an indicator of the reduction rate, which is the heating time required for the reduction rate of an aqueous solution of iron(III) oxalate to reach 80% while the temperature is kept constant. 80 Use "T 80 The unit may be minutes. The heating time of the iron(III) oxalate aqueous solution is preferably longer than the time required for the iron(III) oxalate contained in the aqueous solution to be reduced to iron(II) oxalate. As for the heating time, for example, the T mentioned above. 80 It may be (minutes) or more.

[0038] [Molar concentration of all iron ions in an aqueous solution of iron(III) oxalate] In a preferred embodiment, the method for reducing iron(III) oxalate according to the present invention is such that the molar concentration of all iron ions in the iron(III) oxalate aqueous solution before heating is 0.080 mol / L or more and 1.60 mol / L or less. In this specification, "molar concentration of all iron ions in the iron(III) oxalate aqueous solution" refers to the molar concentration of the iron(III) oxalate aqueous solution before thermal reduction, measured by the method described in Non-Patent Document 2 mentioned above. The molar concentration of all iron ions measured by this method is the molar concentration of divalent iron ions (Fe) contained in the iron(III) oxalate aqueous solution. 2+ Molar concentration of ions and trivalent iron ions (Fe 3+ This corresponds to the sum of the molar concentrations of ions.

[0039] If the molar concentration of all iron ions in an aqueous solution of iron(III) oxalate is too low, the yield of iron(II) oxalate per unit volume of the solution will be low, resulting in reduced productivity. Therefore, it is preferable that the molar concentration of all iron ions be 0.080 mol / L or higher. On the other hand, if the molar concentration of all iron ions in an aqueous solution of iron(III) oxalate is too high, the molar concentration of iron oxalate in the solution will exceed the solubility, potentially causing problems such as the precipitation and accumulation of solid iron oxalate. Therefore, it is preferable that the molar concentration of all iron ions be 1.60 mol / L or lower. A more preferable molar concentration of all iron ions is 1.00 mol / L or lower.

[0040] [Temperature control of iron(III) oxalate solution before heating] In the method for reducing iron(III) oxalate according to the present invention, when using an aqueous solution of iron(III) oxalate obtained by dissolving iron oxide in an aqueous solution of oxalic acid, impurity precipitates that do not dissolve in the aqueous solution of oxalic acid among the components contained in iron oxide can be removed by filtering the aqueous solution of iron(III) oxalate after dissolving the iron oxide in the aqueous solution of oxalic acid.

[0041] In a preferred embodiment, the method for reducing iron(III) oxalate according to the present invention controls the temperature of the iron(III) oxalate aqueous solution to less than 110°C before heating. As mentioned above, the reduction of iron(III) oxalate in aqueous solution proceeds at temperatures of 110°C or higher. If the temperature of the iron(III) oxalate aqueous solution before reduction by heating exceeds 110°C, the reduction of iron(III) oxalate progresses, and a precipitate consisting of iron(II) oxalate dihydrate is formed. In this case, the precipitate consisting of iron(II) oxalate dihydrate is also removed during the process of removing impurities that do not dissolve in the oxalic acid aqueous solution by filtration, and the yield of iron obtained in the end decreases.

[0042] Furthermore, for example, when introducing an aqueous iron(III) oxalate solution into a reaction vessel using piping, if the temperature of the aqueous iron(III) oxalate solution in the piping exceeds 110°C, a precipitate consisting of iron(II) oxalate dihydrate will adhere to the inner wall of the piping, causing problems such as pipe blockage. Therefore, it is preferable to maintain the temperature of the aqueous iron(III) oxalate solution below 110°C from the time of production until it is introduced into the reaction vessel. More preferably, the temperature of the aqueous iron(III) oxalate solution should be maintained below 100°C.

[0043] As mentioned above, when dissolving iron oxide in an oxalic acid aqueous solution, from the viewpoint of dissolution rate, the temperature of the oxalic acid aqueous solution is preferably 90°C or higher and less than 100°C. In this case, from the viewpoint of energy saving, it is preferable to introduce the heated iron(III) oxalate aqueous solution into the reaction vessel without lowering its temperature. [Examples]

[0044] The following describes embodiments of the present invention. However, the embodiments of the present invention are not limited to the following embodiments and can be modified as appropriate without departing from the spirit of the invention.

[0045] Hematite ore, used as a raw material for sintered ore in steel mills, was prepared as iron oxide. The prepared iron ore was weighed out and dissolved in an oxalic acid aqueous solution heated to between 90°C and 100°C. Undissolved impurity precipitates were filtered to prepare five different iron(III) oxalate aqueous solutions with varying iron ion concentrations. Next, the molar concentration of all iron ions in the obtained aqueous solutions was measured using the method described above. A Shimadzu UV-1900i ultraviolet-visible-near-infrared spectrophotometer was used. Table 1 shows the molar concentrations of all iron ions contained in the iron(III) oxalate aqueous solutions before reduction by heating.

[0046] Next, 2.0 mL of an iron(III) oxalate aqueous solution was placed in a reaction vessel with an internal volume of 4.0 mL. The temperature of the aqueous solution was always controlled to be below 100°C from the time the iron ore was dissolved until it was placed in the reaction vessel. Next, the reaction vessel was placed in an oil bath heated to the heating temperature shown in Table 1, and the iron(III) oxalate aqueous solution was heated at the heating temperature for a predetermined time. After that, the reaction vessel was removed from the oil bath and cooled to room temperature. Next, the aqueous solution was removed from the reaction vessel, and the molar concentration of all iron ions in the aqueous solution was measured using the same method as described above, and the reduction rate of iron(III) oxalate was determined. These operations were repeated while varying the heating time. The heating time at which the reduction rate reached 80% or more was determined. 80 (minutes) was used. The obtained T 80 The values ​​are shown in Table 1. Also, the heating temperature and T of the iron(III) oxalate aqueous solution. 80 Figure 1 shows a graph illustrating the relationship between the two.

[0047] [Table 1]

[0048] Furthermore, it takes approximately 5 minutes for the temperature of the iron(III) oxalate aqueous solution to reach the heating temperature after the reaction vessel is placed in the oil bath. Therefore, the T obtained by the method described above 80If the value is less than 5 minutes, it means that the reduction rate reached 80% before the temperature of the iron(III) oxalate solution reached the heating temperature, so T 80 The exact value of T is unknown. Therefore, in such a case, 80 In Table 1, this is indicated as "<5", and in Figure 1, it is plotted assuming "0 (minutes)".

[0049] Next, as a control experiment, the reduction of iron(III) oxalate by light was performed using the following procedure. Of the five types of iron(III) oxalate aqueous solutions prepared by the method described above, 50 mL of the aqueous solution in which the total molar concentration of iron ions contained in the iron(III) oxalate aqueous solution before reduction by irradiation with sunlight was placed in a glass container and sealed. Next, the container was left outdoors without heating and reduced to iron(II) oxalate by irradiation with sunlight, and the irradiation time T was determined by the same method as described above until the reduction rate reached 80% or more. 80 We calculated the value of T. 80 The values ​​of T are shown in Table 1. 80 This is the total amount of daylight hours only; for the time when the sun is not up, T 80 It is not included.

[0050] According to Table 1 and Figure 1, when the heating temperature is 100°C, the reduction reaction of iron(III) oxalate does not proceed, but when the heating temperature is 110°C or higher, T proceeds more rapidly than reduction by light. 80 The value of decreases, indicating an improvement in the reduction rate. Also, when the heating temperature is 120°C or higher, T 80 The reduction rate is less than 1 / 3 compared to reduction by light, indicating a significant improvement in the reduction rate. These results show that the reduction method for iron(III) oxalate according to the present invention improves the reduction rate of iron(III) oxalate compared to conventional reduction by light. [Industrial applicability]

[0051] The present invention provides a method for reducing iron(III) oxalate, which improves the rate of reduction from iron(III) oxalate to iron(II) oxalate. This increases the productivity of iron production in ironmaking processes using oxalic acid.

Claims

1. A method for reducing iron(III) oxalate to obtain iron(II) oxalate, Heat an aqueous solution of iron(III) oxalate containing iron(III) oxalate to a temperature of 110°C or higher but less than 300°C. A method for reducing iron(III) oxalate.

2. The method for reducing iron(III) oxalate according to claim 1, wherein the molar concentration of all iron ions contained in the iron(III) oxalate aqueous solution before heating is 0.080 mol / L or more and 1.60 mol / L or less.

3. The method for reducing iron(III) oxalate according to claim 1 or 2, wherein the iron(III) oxalate aqueous solution is heated to a temperature of 120°C or higher.

4. The method for reducing iron(III) oxalate according to claim 1 or 2, wherein the iron(III) oxalate aqueous solution is heated to a temperature of 150°C or lower.

5. A method for reducing iron(III) oxalate according to claim 1 or 2, wherein the temperature of the iron(III) oxalate aqueous solution before heating is controlled to be below 110°C.

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