Method for purifying an aqueous ferrous sulfate solution suitable for producing a lithium iron phosphate (LFP) precursor

Abstract

A purification method for an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor according to one embodiment of the present invention comprises the steps of preparing an aqueous iron sulfate solution, adjusting the pH of the prepared aqueous iron sulfate solution to a range of 2.0 to 3.0, adding 2 to 4 weight percent of iron powder relative to the reaction solution to the pH-adjusted aqueous iron sulfate solution and stirring for 4 to 6 hours under conditions of 40 to 60°C, and removing a precipitate formed after stirring by solid-liquid separation, thereby purifying the process-derived raw solution containing iron sulfate to a high purity so that it can be used for manufacturing a lithium iron phosphate (LFP) precursor regardless of the initial impurity level.

Description

Technology Field

[0001] The present invention relates to a method for purifying an aqueous iron sulfate solution suitable for the production of a lithium iron phosphate (LFP) precursor. Background Technology

[0002] Iron sulfate is widely generated during processes such as the pickling process in the steel industry, the manufacturing of titanium dioxide (TiO2), and the dissolution of ferrous sulfate (FeSO4·7H2O) crystals, and is used in aqueous solution for various chemical applications. However, these aqueous iron sulfate solutions are contaminated with metallic impurities originating from the raw materials, such as Al, Cr, Ti, Cu, and Mn. These impurities can interfere with subsequent precipitation reactions, destabilize the redox states of metal ions, and degrade the quality of the final product.

[0003] Generally, the purification of aqueous iron sulfate solutions is carried out by combining processes such as pH adjustment, alkaline precipitation, iron powder addition, and solid-liquid separation; however, it has limitations such as difficulty in selectively removing impurities and instability depending on the amount of iron powder added.

[0004] With the recent increase in demand for lithium iron phosphate (LFP) cathode materials, the required purity level for aqueous iron sulfate solutions used in the precursor synthesis step is rising significantly. In particular, LFP precursors contain Fe within their crystal structure 2+ The oxidation state of ions must be maintained, and in this process, metal ions present in metal impurities cause inhibition of crystal growth, reduced conductivity, and performance degradation due to residual impurities; therefore, they must be removed to an ultra-low concentration level in the ppm range. Accordingly, unlike general industrial iron sulfate, high-purity iron sulfate is required for the manufacture of LFP precursors, in which the concentration of specific metal impurities is precisely controlled to a level of 10 ppm or less. The problem to be solved

[0005] Based on the technical background described above, the present invention aims to provide a method for purifying an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor, which can purify a process-derived raw solution containing iron sulfate to a high purity regardless of the initial impurity level. means of solving the problem

[0006] A purification method for an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor according to one embodiment of the present invention may include the steps of: preparing an aqueous iron sulfate solution; adjusting the pH of the prepared aqueous iron sulfate solution to a range of 2.0 to 3.0; adding 2 to 4 weight percent of iron powder based on the total weight of the pH-adjusted aqueous iron sulfate solution and stirring for 4 to 6 hours under conditions of 40 to 60 ℃; and removing a precipitate formed after stirring by solid-liquid separation.

[0007] The stirring step may further include a step of aging the iron sulfate solution for 30 to 90 minutes after adding the iron powder. Additionally, the amount of iron powder added can be adjusted so that the oxidation-reduction potential (ORP) of the iron sulfate solution is in the range of +250 to +120 mV.

[0008] The adjusting step can control the amount of acid or alkali injected so that the pH change rate (dpH / dt) is 0.05 / min or less.

[0009] The purified aqueous iron sulfate solution obtained after the step of solid-liquid separation and removal is Fe 2+ / Fe total The ratio can be maintained at 97% or higher. Effects of the invention

[0010] A purification method for an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor according to one embodiment of the present invention can purify a process-derived stock solution containing iron sulfate to a high purity so that it can be used for manufacturing a lithium iron phosphate (LFP) precursor regardless of the initial impurity level. Brief explanation of the drawing

[0011] FIG. 1 is a flowchart of a purification method for an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor according to one embodiment of the present invention. Specific details for implementing the invention

[0012] Hereinafter, embodiments of the present invention are described in detail with reference to the attached drawings so that those skilled in the art can easily implement the invention. The present invention may be embodied in various different forms and is not limited to the embodiments described herein. In the drawings, parts unrelated to the explanation have been omitted to clearly explain the present invention, and the same reference numerals have been used throughout the specification for identical or similar components.

[0013] In this specification, terms such as "comprising" or "having" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof. Furthermore, when a part such as a layer, film, region, or plate is described as being "above" another part, this includes not only the case where it is "immediately above" another part, but also the case where there is another part in between. Conversely, when a part such as a layer, film, region, or plate is described as being "below" another part, this includes not only the case where it is "immediately below" another part, but also the case where there is another part in between.

[0015] FIG. 1 is a flowchart of a method for purifying iron sulfate using iron powder according to one embodiment of the present invention.

[0016] Referring to FIG. 1, a purification method for an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor according to the present invention comprises the steps of preparing an aqueous iron sulfate solution (S100), adjusting the pH of the aqueous iron sulfate solution (S200), adding iron powder to the aqueous iron sulfate solution and stirring (S300), and removing a precipitate by separating the solid and liquid (S400).

[0017] The step (S100) of preparing an aqueous iron sulfate solution may use a solution containing iron sulfate of various forms. Specifically, the aqueous iron sulfate solution used in the present invention is not particularly limited in the source of its raw materials or its manufacturing method. It may be used not only as a solution prepared by directly dissolving commercially available ferrous sulfate (FeSO4·7H2O) solid, but also as a solution obtained by appropriately pre-treating an iron sulfate-containing stock solution generated in various industrial processes.

[0018] Commercially available ferrous sulfate crystals or solid iron sulfate are typically provided in the form of heptahydrate (FeSO4-7H2O), and can be used as the iron sulfate aqueous solution in the present invention by completely dissolving it using deionized water or industrial water. This method has the advantage that the composition of the iron sulfate is relatively uniform and the impurity concentration is low, making it easy to prepare the initial aqueous solution.

[0019] Accordingly, the aqueous iron sulfate solution prepared in this step (S100) may be either a solution obtained by dissolving commercially available solid ferrous sulfate in water or a solution obtained by appropriately adjusting and diluting an iron sulfate-containing stock solution generated in an industrial process, and various forms of raw material solutions can be used in the present invention. Furthermore, impurities such as Ti, Al, and Cr can be selectively removed to a level of ppm or less by the subsequent purification steps (S200~S400) of the present invention, so that differences in the initial composition of the raw material do not substantially affect the quality of the final product.

[0020] The step (S200) of adjusting the pH of the iron sulfate solution can adjust the pH range to 2.0 to 3.0. Generally, the initial pH of the iron sulfate solution obtained in the step (S100) of preparing the iron sulfate solution is located in a strongly acidic region of about 1.3 to 1.5. Therefore, if the initial pH is less than 2.0, the pH can be raised using an alkaline aqueous solution such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or ammonium hydroxide (NH4OH). Conversely, if the initial pH of the source solution exceeds 3.0, the pH can be adjusted to lower the pH to 3.0 or lower using a sulfuric acid such as sulfuric acid (H2SO4) or ammonium hydrogen sulfate ((NH4)HSO4).

[0021] pH adjustment can be performed by stirring at 40 to 60 ℃, and alkali or acid can be injected dropwise or in small fractions rather than all at once. For example, the pH change rate (dpH / dt) of the solution can be controlled to be 0.05 / min or less. Accordingly, by gradually adjusting the pH, impurities such as Al, Cr, and Ti slowly grow into fine hydroxides or sulfate complex precipitates, forming a precipitate with excellent filterability, and at the same time Fe 2+ It can prevent excessive hydrolysis of ions or co-precipitation.

[0022] Accordingly, the preferred pH range in the present invention is a range of pH 2.0 to 3.0, which is a preferred operating range for the preparation of an aqueous iron sulfate solution for LFP precursors, and among these, 2.0 may be the most preferred operating range. If the pH is below 2.0, the hydrolysis of Al and Cr does not proceed sufficiently, resulting in a decrease in the removal rate, and conversely, if the pH exceeds 3.0, Fe 2+Hydrolysis and a rapid increase in sludge volume may lead to reduced filtration performance and process stability. This will be examined in more detail through the following examples. To determine a desirable pH range, a process-derived stock solution containing iron sulfate was taken and stirred while heating to 40°C. The initial impurity concentrations in the stock solution were Ti 957 ppm, Al 10 ppm, and Cr 2 ppm. Since the initial pH was measured at 1.4, the pH was adjusted by drop-injecting a 10 wt% aqueous solution of ammonium hydroxide (NH4OH) into the reaction vessel to reach the pH values ​​according to the following examples. While monitoring the pH in real time using a pH electrode, the injection rate of ammonium hydroxide was controlled so that the pH change rate (dpH / dt) of the solution was 0.05 / min or less. Subsequently, once pH adjustment was completed according to each example, 2 wt% of iron powder was added based on the total weight of the aqueous iron sulfate solution. The reaction was carried out by stirring at 40°C for 4 hours, and after the reaction was completed, the generated precipitate was removed by filtration and each supernatant was recovered. In addition, impurities contained in the supernatant were measured using an ICP analyzer and listed in Table 1 below.

[0024] item unit Initial iron sulfate solution pH 1.5 (Example 1) pH 2.0 (Example 2) pH 2.5 (Example 3) pH 3.0 (Example 4) pH 3.5 (Example 5) Al ppm 10 8 0.2 0.5 1 3 Ca ppm 23 23 24 24 25 25 Cr ppm 2 1.8 0.05 0.1 0.3 1 Cu ppm 4 4 2 3 3 3 Mg ppm 4 4 5 5 5 5 Mn ppm 79 78 78 78 79 80 Na ppm 43 42 39 38 37 36 Pb ppm 4 4 4 4 4 4 Zn ppm 2 2 2 2 2 2 Ti ppm 957 800 1 5 10 25

[0025] Referring to Table 1 above, it was confirmed that when the pH of the aqueous iron sulfate solution is adjusted to 2.0 to 3.0, more preferably to pH 2.0, Ti, Al, and Cr are removed to a level of 1 to several ppm, respectively, while other impurities (Ca, Cu, Mg, Mn, Na, Pb, Zn) are maintained without significant change, thereby allowing for the stable production of a high-purity aqueous iron sulfate solution for LFP precursors. On the other hand, when the pH is outside the above range, such as 1.5 or 3.5, the residual amounts of Ti, Al, and Cr increase significantly, and the effectiveness is reduced to the point where they are distinguishable even by the naked eye; thus, it can be seen that pH 2.0 to 3.0 forms a critical range for securing the removal efficiency of Ti, Al, and Cr in the purification method of the present invention.

[0026] The step (S300) of adding iron powder to an aqueous iron sulfate solution and stirring can be performed by stirring for 4 to 6 hours at a temperature of 40 to 60°C after adding the iron powder. This will be explained in more detail through the examples in Table 2 below. Table 2 below is based on Example 2, which showed the best impurity removal performance in Table 1 above, and experiments were conducted by varying the reaction temperature. All examples were adjusted to pH 2.0, and 2% by weight of iron powder was added relative to the total weight of the initial aqueous iron sulfate solution, and the experiment was carried out by stirring for 4 hours while varying only the reaction temperature.

[0027] item Initial iron sulfate solution 30℃ (Example 6) 40℃ (Example 2) 50℃ (Example 7) 60℃ (Example 8) 70℃ (Example 9) Al (ppm) 10 5 0.2 0.3 0.7 2.5 Ca (ppm) 23 23 24 24 25 25 Cr (ppm) 2 1.2 0.05 0.08 0.2 1.2 Cu (ppm) 4 4 2 2.2 2.5 5 Mg (ppm) 4 4 5 5 5 6 Mn (ppm) 79 79 78 78 79 85 Na (ppm) 43 42 39 38 37 35 Pb (ppm) 4 4 4 4 4 4.5 Zn (ppm) 2 2 2 2 2 2.5 Ti (ppm) 957 200 1 2 4 20

[0028] Referring to Table 2, in the case of Example 2 and Examples 7 to 8, where the reaction temperature was maintained in the range of 40 to 60°C, it can be seen that the concentrations of Ti, Al, and Cr impurities decreased rapidly, satisfying the required level for the aqueous iron sulfate solution for the preparation of LFP precursors. In particular, at 40°C, Ti decreased by approximately 1,000 times (957→1 ppm), Al by 50 times (10→0.2 ppm), and Cr by 40 times (2→0.05 ppm), showing the most excellent purification effect. On the other hand, in comparative examples that fall outside the above preferred temperature range, such as at 30°C or 70°C, the residual amounts of Ti, Al, and Cr increased to the level of tens to hundreds of ppm, and the purification effect was significantly reduced.

[0029] In the step (S300) of adding iron powder to an aqueous iron sulfate solution and stirring, 2 to 4 weight percent of iron powder can be added based on the total weight of the aqueous iron sulfate solution. Accordingly, based on Example 2, which was the most preferred example in Tables 1 and 2, the pH of the aqueous iron sulfate solution was adjusted to 2.0, the reaction temperature was fixed at 40 ℃, and the reaction time was fixed at 4 hours. Then, experiments were conducted by varying only the amount of iron powder added, and the results are listed in Table 3 below.

[0030] In addition, to determine the change in redox potential with the addition of iron powder, measurements were taken and the values ​​were also recorded in Table 3.

[0031] Oxidation Reduction Potential (ORP) is a potential value representing the relative difficulty and directionality of electron transfer reactions in a solution, generally expressed in mV units. A higher ORP indicates a stronger oxidizing agent, while a lower ORP indicates a stronger reducing agent. In the case of the removal reaction of impurities such as Ti, Al, and Cr in an aqueous iron sulfate solution, iron powder (Fe 0 The process can proceed efficiently when the solution maintains a certain level of reduction potential due to the addition of ). In particular, since trace metal impurities have high solubility in the oxidized state and are more likely to precipitate and adsorb in the reduced state, the redox potential can be considered a key indicator determining the quality of the iron powder-based purification process.

[0033] item Initial iron sulfate solution 1 wt% (Example 10) 2 wt% (Example 2) 3 wt% (Example 11) 4 wt% (Example 12) 5 wt% (Example 13) ORP (mV) - 380 250 180 120 0 Al (ppm) 10 4.5 0.2 0.3 0.7 2.5 Cr (ppm) 2 1 0.05 0.08 0.15 1 Ti (ppm) 957 150 1 2 4 20 Ca (ppm) 23 23 24 24 25 25 Cu (ppm) 4 4 2 2.5 3 5 Mg (ppm) 4 4 5 5 5 6 Mn (ppm) 79 79 78 78 79 85 Na (ppm) 43 42 39 38 37 35 Pb (ppm) 4 4 4 4 4 4.5 Zn (ppm) 2 2 2 2 2 2.5

[0034] Referring to Table 3, in the example where the amount of iron powder added was 2 to 4 wt% based on the total weight of the initial iron sulfate solution, the redox potential was stably maintained in the range of +250 to +120 mV, and under these conditions, Ti, Al, and Cr were effectively removed to levels of 1.0 ppm, 0.2 to 0.7 ppm, and 0.05 to 0.15 ppm, respectively.

[0035] On the other hand, when the iron powder was 1 wt% or less, the oxidation-reduction potential maintained an oxidative environment of +380 mV, so the reduction and precipitation of Ti, Al, and Cr did not occur sufficiently and remained at levels of 150 ppm, 4.5 ppm, and 1.0 ppm, respectively.

[0036] In addition, when iron powder is added in excess at 5% by weight or more, the redox potential changes to a super-reduced state of 0 mV or less, which increases the Ti concentration to 20 ppm and causes sludge formation and redissolution reactions, resulting in a significant decrease in purification efficiency.

[0037] Therefore, 2 to 4 weight percent of iron powder constitutes the critical addition range required to remove impurities (Ti, Al, Cr) from the aqueous iron sulfate solution in the present invention, and among them, it is evident that Example 2, corresponding to 2 weight percent, is the most preferred example.

[0038] In preferred Example 2, the performance of removing impurities according to reaction time was investigated. Based on Example 2, which was the most preferred example, the pH of the iron sulfate solution was adjusted to 2.0, the reaction temperature was 40 ℃, the amount of iron powder added was 2% by weight based on the total weight of the initial iron sulfate solution, and only the reaction time was varied. The results are listed in Table 4 below.

[0040] item Initial iron sulfate solution 2 hr (Example 14) 4 hr (Example 2) 6 hr (Example 15) 8 hr (Example 16) Al (ppm) 10 2 0.2 0.3 1.5 Ca (ppm) 23 23 24 24 25 Cr (ppm) 2 0.5 0.05 0.1 0.8 Cu (ppm) 4 3.5 2 2.2 3 Mg (ppm) 4 4 5 5 5.5 Mn (ppm) 79 79 78 78 80 Na (ppm) 43 41 39 38 37 Pb (ppm) 4 4 4 4 4.2 Zn (ppm) 2 2 2 2 2.1 Ti (ppm) 957 50 1 2 10

[0041] Referring to Table 4, when the reaction time was maintained at 4 to 6 hours, Al, Cr, and Ti decreased to levels of 0.2 to 0.3 ppm, 0.05 to 0.10 ppm, and 1 to 2 ppm, respectively, showing the best impurity removal efficiency. In particular, in Example 2, where the reaction time was 4 hours, Ti decreased from 957 to 1 ppm, showing the maximum purification effect.

[0042] On the other hand, in the case of Example 14, with a reaction time of 2 hours, the purification efficiency was significantly low as impurities remained at relatively high concentrations, with Ti at 50 ppm, Al at 2 ppm, and Cr at 0.50 ppm, and in the case of Example 16, with a reaction time of 8 hours, some impurities were redissolved, resulting in an adverse effect where Ti increased to 10 ppm and Cr to 0.80 ppm.

[0043] Therefore, it is clearly confirmed that a reaction time of 4 to 6 hours is a critical range for securing impurity removal efficiency in the purification process of the present invention, and among these, 4 hours is the most desirable reaction time condition.

[0044] After adding iron powder, a step (S310) of aging the iron sulfate solution may be further included.

[0045] Aging refers to the process of inducing reaction equilibrium by maintaining a constant pH of the composition while stirring or remaining stationary for a certain period of time after adding iron powder. Aging involves iron powder (Fe 0 The impurity reduction reaction by ) can proceed uniformly throughout the solution, and the nucleation and growth of metal hydroxides / oxides can be stabilized to maximize the precipitation efficiency of impurities. In addition, through aging, local pH deviations, unreacted iron particles, and fine precipitated sludge generated during the reaction are stabilized, and consequently, the sludge particles become denser and filtration performance is improved. Therefore, in the purification process of the present invention, the aging step can be considered an essential auxiliary step for improving impurity removal efficiency and ensuring process stability.

[0046] Specifically, aging can be carried out for 30 to 90 minutes. This will be explained in more detail through the examples in Table 5 below. Experiments were conducted by varying the aging time based on the most preferred Example 2 conditions across Tables 1 to 4.

[0047] item Initial iron sulfate solution 10 min (Example 17) 30 min (Example 18) 60 min (Example 19) 90 min (Example 20) 120 min (Example 21) Al (ppm) 10 1.5 0.5 0.2 0.4 1 Ca (ppm) 23 23 24 24 24 25 Cr (ppm) 2 0.4 0.12 0.05 0.1 0.5 Cu (ppm) 4 3 2.5 2 2.3 3 Mg (ppm) 4 4 5 5 5 5.5 Mn (ppm) 79 79 78 78 79 80 Na (ppm) 43 42 40 39 38 37 Pb (ppm) 4 4 4 4 4.1 4.2 Zn (ppm) 2 2 2 2 2 2.2 Ti (ppm) 957 35 5 1 3 15

[0048] Referring to Table 5, it was confirmed that when an aqueous iron sulfate solution is aged at a constant pH for 30 to 90 minutes after the addition of iron powder, metal impurities such as Ti, Al, and Cr are effectively removed, and the particle size of the precipitated sludge is stabilized, thereby improving filtration performance. In particular, the 60-minute aging condition exhibited the best impurity removal efficiency and is considered the most preferred embodiment of the present invention, and 30 to 90 minutes constitutes the effective range of the aging time. On the other hand, under conditions outside the above range, such as 10 minutes or 120 minutes, the precipitation efficiency decreases or redissolution occurs, resulting in a significant drop in purification efficiency; therefore, the above range can be considered the preferred aging time range in the present invention.

[0049] The step of removing precipitates by solid-liquid separation (S400) can remove precipitates generated after the stirring step (S300) by solid-liquid separation. This refers to a process of physically separating insoluble precipitates, such as metal hydroxides / oxides formed in the stirring and aging steps (S300, S350), from the iron sulfate aqueous solution. This step is a final purification step to secure a high-purity iron sulfate aqueous solution, which is a purified supernatant, and solid-liquid separation is a technique commonly used in wet purification processes.

[0050] Solid-liquid separation refers to separating a solid phase (precipitate) from a liquid phase using methods such as gravity sedimentation, centrifugation, and filtration (filter pressing, housing filter, membrane filtration, etc.). In particular, in purification processes where a large amount of fine metal impurity precipitates are formed, such as in the present invention, the applicability of the filtration process is excellent, and filtration may be performed in conjunction with preliminary sedimentation if necessary.

[0051] In this way, precipitates of impurities such as Ti, Al, and Cr can be effectively removed through solid-liquid separation, and the supernatant can be obtained as a high-purity aqueous iron sulfate solution suitable for the production of LFP precursors. Furthermore, since the presence of precipitates in the supernatant leads to reaction non-uniformity and reduced productivity in subsequent processes, the solid-liquid separation step can function as an essential process step to ensure purification efficiency and uniformity in the purification process of the present invention.

[0052] Accordingly, the step of separating solids and liquids to remove them (S400) is an essential purification step for obtaining a high-purity aqueous iron sulfate solution in which Ti, Al, Cr, etc. are maintained at a level of a few ppm or less by reliably removing metal impurity precipitates generated in the preceding steps of stirring (S300) and aging (S310), and any solid-liquid separation technology commonly adopted in the industry, such as filtration, sedimentation, or centrifugation, may be applied.

[0053] In addition, the purified aqueous iron sulfate solution obtained after the solid-liquid separation and removal step is Fe 2+ / Fe total The ratio can be maintained above a certain content.

[0054] Fe in iron sulfate solution 2+ / Fe total The ratio is divalent iron (Fe) present in the solution. 2+ ) and trivalent iron (Fe 3+ It refers to the relative concentration of ) and is an important indicator of the purity and reaction stability of iron sulfate.

[0055] In the purification process of the present invention, acidic conditions of pH 2.0 and iron powder (Fe 0 Since a reducing atmosphere is established by ), Fe according to the reaction of Chemical Formula 1 below 3+ Ga Fe 2+ It is reduced to, consequently, Fe 2+ / Fe total The ratio can be maintained at 97.5% or higher.

[0057] <Chemical Formula 1>

[0058] Fe 3+ + Fe 0 -> 2Fe 2+

[0059] According to the reaction of Chemical Formula 1 above, under strongly acidic conditions of pH 2.0, Fe 2+ A reducing environment is formed where oxidation is difficult, and Fe 3+ The formation of the hydroxide (Fe(OH)3) is suppressed, thereby reducing reoxidation. Furthermore, as previously experimented, the redox potential is maintained at +250 to +120 mV under desirable conditions, and this range corresponds to Fe 2+ It is known as a dominant dislocation range and can be described as a stable dislocation band that naturally forms under conditions of 2 to 4 weight percent iron powder addition. That is, the aqueous iron sulfate solution obtained after purification of the present invention is Fe 2+ is the most thermodynamically stable state, and Fe 3+ Fe 2+ / Fe total The ratio can be maintained at 97.5% or higher. More specifically, regarding Fe depending on the iron powder content 2+ / Fe total I will explain this through Table 6 below, which shows the ratios.

[0060] Accordingly, the Fe of the purified aqueous iron sulfate solution obtained according to the present invention 2+ / Fe total The ratio was measured based on ICP. Fe total The concentration of is directly measured by ICP, and Fe 2+ In the case of, the ratio was calculated using the two values ​​after measuring through the potassium permanganate titration method.

[0062] item Amount of iron powder added (wt%) pH Reaction temperature (°C) Reaction time (h) Aging time (min) Fe² (g / L) Fetotal (g / L) Fe² / Fetotal (%) initial solution - 2 25 - - 88 100 88% Example 22 1 wt% 2 40 4 60 87 100 87% Example 19 2 wt% 2 40 4 60 98 100 98% Example 23 3 wt% 2 40 4 60 97.5 100 97.50% Example 24 4 wt% 2 40 4 60 98.2 100 97% Example 25 5 wt% 2 40 4 60 93 100 93%

[0063] Referring to Table 6, when 2 wt% iron powder was added and the reaction was performed for 4 hours at pH 2.0 and 40 ℃ followed by aging for 60 minutes, Fe² / Fe totalThe ratio was maintained at the 98% level. This indicates that under the purification process conditions of the present invention, almost no Fe³ is generated, and

[0064] This means that the atmosphere of reduction was maintained stably.

[0065] Furthermore, Fe² / Fe also under 3 wt% and 4 wt% conditions total It can be seen that the ratios are maintained at 97.5% and 97%, respectively, and that high stability of over 97% is maintained.

[0066] In contrast, 1 wt% iron powder lacks reducing power, so Fe³ is not completely reduced to Fe², resulting in Fe² / Fe total The ratio was found to be relatively low at 87%. In addition, it is judged that the 5% by weight iron powder dropped to the 93% level due to an excessive reducing environment and side reactions such as sludge surface reoxidation.

[0067] Therefore, it can be seen that the range of 2 to 4 weight percent of added iron powder is a desirable range for ensuring Fe² stability, and the aqueous iron sulfate solution obtained through the purification process of the present invention is Fe² / Fe total It can be seen that the ratio is stably maintained at over 97%, which is because the reduction environment caused by the addition of iron powder, acidic conditions of pH 2, and the aging stage inhibit the regeneration of Fe³ and stabilize the Fe² state.

[0069] Thus, according to the purification method of an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor according to one embodiment of the present invention, a process-derived stock solution containing iron sulfate can be purified to a high purity so that it can be used for manufacturing a lithium iron phosphate (LFP) precursor regardless of the initial impurity level.

[0071] Although embodiments of the present invention have been described above, the spirit of the present invention is not limited to the embodiments presented in this specification. Those skilled in the art who understand the spirit of the present invention may easily propose other embodiments within the scope of the same spirit by adding, changing, deleting, or adding components, and such embodiments shall also be considered to fall within the scope of the spirit of the present invention.

Claims

Claim 1 A method for purifying an aqueous iron sulfate solution prepared by dissolving a solid ferrous sulfate (FeSO4·7H2O) or a process-derived stock solution containing iron sulfate in water to a high purity suitable for manufacturing a lithium iron phosphate (LiFePO4) precursor, comprising: a step of preparing said aqueous iron sulfate solution; a step of adjusting the pH of the prepared aqueous iron sulfate solution to a range of 2.0 to 3.0 by controlling the amount of acid or alkali injected so that the rate of change (dpH / dt) of the prepared aqueous iron sulfate solution is 0.05 / min or less; a step of adding 2 to 4 weight% of iron powder and stirring for 4 to 6 hours under conditions of 40 to 60 ℃, wherein the amount of iron powder added is adjusted so that the oxidation-reduction potential (ORP) of said aqueous iron sulfate solution is in the range of +250 to +120 mV based on the total weight of the pH-adjusted aqueous iron sulfate solution; and a step of stirring said aqueous iron sulfate solution for 30 to 90 minutes after adding said iron powder The method comprises an aging step; and a step of removing a precipitate generated after stirring by solid-liquid separation, wherein the purified aqueous iron sulfate solution obtained after the step of removing by solid-liquid separation is Fe 2+ / Fe total A method for purifying an aqueous iron sulfate solution suitable for manufacturing a lithium iron phosphate (LFP) precursor, wherein the ratio is maintained at 97% or higher and impurities are reduced to levels of Ti 5 ppm, Al 0.7 ppm, and Cr 0.15 ppm or lower. Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete

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