Impurity removal method and impurity removal apparatus

Abstract

Provided in the present invention are an impurity removal method and an impurity removal apparatus. The impurity removal method comprises: subjecting a delithiated iron phosphate slag system to a trivalent iron electrolytic reduction treatment to obtain a reduced system; and subjecting the reduced system to an impurity removal treatment to obtain a divalent iron system.

Description

A method and apparatus for removing impurities

[0001] This application claims priority to Chinese Patent Application No. 202510103540.0, filed on January 22, 2025, entitled "A Method and Apparatus for Removing Impurities", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to a method and apparatus for removing impurities, and particularly to a field of material recycling technology. Background Technology

[0003] Batteries are currently widely used in various fields. However, since the average lifespan of a battery is 5-8 years, improper disposal of used batteries can lead to serious environmental and safety problems. Meanwhile, due to the scarcity of energy metals and the continuous growth in market demand, the lithium, iron, and other energy metals abundant in used batteries have become an important source of supplementary energy. Therefore, the recycling of used batteries has become one of the research hotspots in recent years. However, because battery performance is closely related to the quality of materials, the battery industry has high requirements for materials. For example, the content of certain impurities (such as aluminum, titanium, and copper) in the cathode material must be below 100 ppm, which further increases the difficulty of recycling metal elements.

[0004] Currently, some researchers electrolyze spent lithium iron phosphate to induce oxidation at the anode, releasing lithium ions and the oxidation product, iron phosphate. The lithium ions can be recycled into lithium hydroxide or lithium carbonate as a lithium source for lithium cathode active materials, while the iron phosphate oxidation product is calcined with the recovered lithium source to produce recycled lithium iron phosphate cathode active materials. However, the recycled iron phosphate often contains significant amounts of impurities such as aluminum and titanium, which inevitably reduces the purity of the recycled lithium iron phosphate cathode active material, making it unsuitable for battery applications.

[0005] Therefore, it is essential to develop an economical, environmentally friendly, and efficient method for removing impurities suitable for recycling waste battery cathode materials. Summary of the Invention

[0006] This invention provides a method for removing impurities, which can effectively differentiate the iron element in phosphate slag from other impurity elements, thereby improving the impurity removal effect. On the one hand, it can effectively remove impurities, resulting in a product with high iron purity; on the other hand, it can reduce the loss of iron element caused by the impurity removal operation, thereby improving the iron element recovery rate. It also has the advantage of low cost.

[0007] The present invention also provides a purification device that can be used to implement the above-described purification method.

[0008] This invention provides a method for removing impurities, comprising the following steps:

[0009] The lithium-free phosphorus iron slag system was subjected to trivalent iron-electrolytic reduction treatment to obtain a reduced system.

[0010] The reduction system is subjected to impurity removal treatment to obtain a divalent iron system.

[0011] The impurity removal method of this invention first reduces the lithium-phosphorus iron slag system by electrolysis, which efficiently and cost-effectively reduces ferric iron to ferrous iron, thereby differentiating iron from other impurity elements. Because the impurity metals are different from iron, the subsequent impurity removal process can effectively remove impurity metal ions without affecting the iron, resulting in a high-purity ferrous iron system. This provides a higher-purity iron source for downstream applications, making it applicable to fields with high impurity requirements and broadening the application range of iron-containing materials. Attached Figure Description

[0012] Figure 1 is a schematic diagram of one embodiment of the impurity removal device of the present invention;

[0013] In the diagram, 1. Electrolysis unit, 1a. Diaphragm, 1b. Cathode chamber, 1c. Anode chamber, 2. Impurity removal unit. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0015] The first aspect of this invention provides a method for removing impurities, comprising the following steps:

[0016] The lithium-free phosphorus iron slag system was subjected to trivalent iron-electrolytic reduction treatment to obtain a reduced system.

[0017] The reduction system is subjected to impurity removal treatment to obtain a divalent iron system.

[0018] The impurity removal method of this invention can be used as a downstream process after lithium recovery from waste lithium iron phosphate, efficiently removing iron from the waste lithium iron phosphate to facilitate iron recovery. The delithiated iron phosphate slag is the residual iron phosphate residue after lithium recovery. In addition to iron, the delithiated iron phosphate slag also contains impurity metals such as titanium, aluminum, and copper. This invention's impurity removal method achieves efficient separation of iron from impurity metals at low cost, thus improving the recovery rate and purity of iron.

[0019] Specifically, the ferric ions in the delithiated phosphorus iron slag system are subjected to ferric electrolytic reduction treatment to reduce the ferric ions in the delithiated phosphorus iron slag to ferrous ions, thus obtaining a reduced system.

[0020] It is understandable that as the ferric electrolytic reduction process proceeds, a large number of ferric ions in the delithiated ferric slag system are reduced to ferrous ions, and the resulting reduced system is a system containing a large number of ferrous ions.

[0021] Subsequently, by removing impurities from the reduction system, a ferrous iron system is obtained. The purpose of the impurity removal process in this invention is to separate iron, phosphorus, and other impurities. This invention does not limit the specific method of impurity removal; the specific method depends on the differences between ferrous iron and the impurity metal elements. For example, differences in valence, solubility, etc., can be used to separate ferrous iron from other impurity metal elements.

[0022] This invention uses the lithium-removed phosphorus-iron slag system as the cathode reduction target. By maximizing the reduction of trivalent iron ions in the lithium-removed phosphorus-iron slag, and then utilizing the differences between divalent iron and other impurity elements (such as differences in valence and solubility potential) to remove other impurity metals, it lays the foundation for high iron recovery rate.

[0023] This invention does not limit the location for performing the above-mentioned trivalent iron-electrolytic reduction treatment. For example, it can be carried out in an electrolytic cell including an anode chamber and a cathode chamber. The cathode chamber is used to contain the cathode electrolyte and the cathode, and the anode chamber is used to contain the anolyte and the anode.

[0024] In this process, the lithium-free phosphorus slag system gains electrons in the cathode chamber and undergoes trivalent iron electrolytic reduction. The type of system in the anode chamber is not specifically limited in this invention, as long as it is a system capable of losing electrons and undergoing an oxidation reaction. For example, the system could be capable of one or more reactions such as oxygen evolution reaction, chlorine evolution reaction, oxidation of low-valence ions, reaction to generate metal ion oxides, methanol oxidation, and formic acid oxidation.

[0025] In detail, using water as the electrolyte allows the oxygen evolution reaction to occur in the anode chamber, where water is oxidized to produce oxygen and hydrogen ions. The ionic equation is as follows: 2H₂O → O₂ + 4H⁺+ +4e -

[0026] The oxygen generated by the oxygen evolution reaction can be collected and used not only for the chemical oxidation of ferrous iron, but also as a combustion aid in a range of applications that require oxygen as a combustion environment.

[0027] Alternatively, an electrolyte solution containing chloride ions can be used as the electrolyte in the anode chamber to undergo a chloride evolution reaction, where chloride ions are oxidized to chlorine gas. The ionic equation is as follows: 2Cl₂ - →Cl2+2e -

[0028] The chlorine gas generated by the chlorine evolution reaction can be collected and used as a raw material for the chlorination treatment of fine organic chemicals.

[0029] Alternatively, an electrolyte solution containing reducing low-valence ions can be used as the electrolyte in the anode chamber to induce a low-valence ion oxidation reaction. For example, sulfate ions (SO42-) can be oxidized. 2- ) is oxidized to persulfate (S2O8) 2- or persulfate free radicals (SO4) - These free radicals can further participate in various chemical reactions, such as in wastewater treatment, to oxidize organic pollutants and promote their degradation.

[0030] Alternatively, a solution containing metal ions can be used as the anolyte. The metal ions will not only deposit as metal, but may also combine with oxygen or other anions to form metal oxides or composite oxide layers. The resulting metal oxides or composite oxides can be used to manufacture semiconductor materials and ceramic coatings, thus enabling their application in sensor manufacturing or semiconductor manufacturing.

[0031] Alternatively, methanol (CH3OH) can be used as the anolyte to undergo oxidation, producing formaldehyde (HCHO) or other basic raw materials used in organic synthesis.

[0032] Alternatively, formic acid (HCOOH) can be used as an anolyte to oxidize the anode, converting it into carbon dioxide (CO2) and other oxidation products, thus providing a key reaction intermediate for the synthesis of chemical products.

[0033] It is understandable that to obtain a high-purity ferrous iron system, the lower the impurity content, the better. Through the inventors' research, it has been found that when the impurity content in the ferrous iron system is less than 100 ppm, it can basically meet the requirements for downstream applications of the iron source. The impurities mentioned here refer to aluminum, titanium, and copper impurities.

[0034] Generally, the impurity removal process includes at least one of the following steps: titanium ion removal, aluminum ion removal, and copper ion removal.

[0035] As mentioned above, the present invention does not limit the specific method of impurity removal treatment, and may include at least one of impurity metal resin adsorption treatment, reduction treatment and pH adjustment treatment.

[0036] In one specific embodiment, since the oxidation states of iron in the ferrous iron system differ from those of some impurity metal elements (trivalent aluminum, trivalent titanium, and tetravalent titanium), this difference in oxidation states can be used to separate high-valence impurity metal ions from the reduction system. In practical applications, a metal ion adsorption resin can be used to adsorb the impurity metals into the reduction system, thereby separating the high-valence metal ions from the ferrous iron by adsorbing them into the resin.

[0037] Alternatively, the pH value of the divalent system can be adjusted. For example, a pH adjuster can be added to the divalent iron ion solution to adjust the pH to 2, so that Ti, Al, etc. can form precipitates with smaller Ksp values, thereby achieving removal.

[0038] For divalent or monovalent impurity metal ions, in one embodiment, a reducing agent can be added to the divalent iron system for reduction treatment. During the reduction treatment, the reducing agent reduces the metal impurity ions with a higher oxidation potential than divalent iron to elemental form, which is then separated by filtration. For example, taking copper ions as an example, the divalent iron system is reduced using a reducing agent, and the copper ions are reduced to elemental copper, which is then separated from the divalent iron system.

[0039] This invention does not limit the subsequent applications of the ferrous iron system. It is understood that, due to the low chemical stability of ferrous iron, in order to facilitate downstream applications, this invention may also include an oxidation treatment of the ferrous iron system after the impurity removal process to obtain a ferric iron system.

[0040] This invention does not limit the specific method for oxidizing the ferrous iron system. In one specific embodiment, the ferrous iron system can be oxidized using an oxidizing agent to obtain a ferric iron system. The oxidizing agent includes at least one selected from hydrogen peroxide, oxygen, ozone, potassium dichromate, potassium permanganate, and sodium persulfate.

[0041] In some embodiments of this application, the oxidation treatment method is as follows: electrolytic oxidation treatment of the ferrous iron system: using the ferrous iron system as the anolyte, and adjusting the voltage of the electrolytic oxidation treatment to 2V to 10V. For example, the voltage of the electrolytic oxidation treatment can be 2V, 3V, 4V, 5V, 6V, 7V, 8V, 9V, 10V, or a range of any two of these values. This method can oxidize ferrous ions in the solution to ferric ions, resulting in high conversion efficiency and low cost. Furthermore, the oxidized ferric iron system can be used as an iron source to generate iron-containing materials, such as iron phosphate, lithium iron phosphate, lithium manganese iron phosphate, etc.

[0042] Specifically, by adding phosphorus and other metal sources (such as manganese) to the trivalent iron system, iron phosphate or iron phosphate doped with other metals can be prepared. Then, iron phosphate is used as a precursor and undergoes a reduction reaction with lithium source to obtain lithium iron phosphate, lithium manganese iron phosphate, etc.

[0043] In the impurity removal method of the present invention, the delithiated iron phosphate slag system can participate in the trivalent iron-electrolytic reduction treatment in any form, for example, the delithiated iron phosphate slag leachate can be used as the electrolyte, or the delithiated iron phosphate slag can be used as the electrode.

[0044] In detail, the aforementioned delithiated iron phosphate slag leachate refers to the solution system obtained by dissolving and leaching the delithiated iron phosphate slag with a solvent. For example, an acidic solvent can be used to dissolve the delithiated iron phosphate slag to maximize the leaching of metals in ionic form, resulting in a leaching system including phosphate, ferric, copper, titanium, and aluminum ions. The leaching system is then filtered to remove insoluble substances, yielding the delithiated iron phosphate slag leachate. This delithiated iron phosphate slag leachate participates in the electrolysis reaction as an electrolyte in a conventional electrolysis process.

[0045] When lithium-free phosphorus iron slag is required as an electrode, the electrolysis method is not specifically limited. For example, it can be ordinary electrolysis, solid-phase electrolysis, or suspension electrolysis.

[0046] For example, a solid cathode sheet can be obtained by mixing lithium-free iron phosphate slag with a conductive agent and solvent to form a slurry, and then coating the slurry onto a conductive substrate and drying it; or by coating the slurry onto a substrate, drying it, pressing it into a sheet, and then placing the sheet onto the conductive substrate to obtain a cathode sheet. This cathode sheet is then subjected to a trivalent iron-electrolytic reduction treatment with a solid electrolyte (such as zirconium oxide, polymer electrolyte, etc.) to obtain a reduced divalent iron electrode sheet, which is then leached with acid to obtain a leachate rich in divalent iron.

[0047] Alternatively, the delithiated iron phosphate slag can be dispersed in a solution containing a supporting electrolyte (a conductive inorganic salt solution, such as a sulfate solution or sodium chloride solution) to form a suspended electrode. This method typically requires a stirring device to keep the particles suspended and requires highly conductive electrode materials as the working and counter electrodes. Specifically, the delithiated iron phosphate slag particles are prepared, ensuring they have suitable particle size and morphology; then, the particles are mixed with a solution containing a supporting electrolyte to form a stable suspension. The working and counter electrodes are placed in the electrolytic cell, ensuring an appropriate distance between them. A magnetic or mechanical stirring device is used to maintain the uniform dispersion of the delithiated iron phosphate slag in the solution. A constant voltage or current is applied by an electrochemical workstation to achieve the electrolytic reduction of ferric iron. After the reaction is complete, the leachate rich in ferrous iron is collected by methods such as filtration and centrifugation.

[0048] To improve the efficiency of ferric iron electrolytic reduction treatment, the molar concentration of ferric ions in the lithium-free ferric slag system can be controlled to be no less than 0.01 M. This results in faster reduction of ferric iron, higher Faraday efficiency, and lower reduction energy consumption. For example, concentrations can be within the range of 0.01 M, 0.02 M, 0.05 M, 0.1 M, 0.5 M, 1 M, 2 M, 3 M, and any other molar concentration.

[0049] Of course, the reduction efficiency of ferric iron can also be improved by limiting the mass percentage of iron. When the mass percentage of iron is not less than 0.1%, it is beneficial to further achieve efficient reduction of ferric iron. Here, the mass percentage of iron refers to the mass percentage of iron in the delithiated ferric slag.

[0050] During the ferric iron-electrolytic reduction process, the concentration of ferric ions in the cathode electrolyte (current reduction system) can be monitored in real time to determine the current degree of electrolysis. It can be understood that a higher concentration of ferrous ions in the reduction system indicates a more thorough ferric iron-electrolytic reduction process; conversely, a lower concentration of ferric ions in the reduction system indicates a more thorough ferric iron-electrolytic reduction process.

[0051] The above-mentioned detection of ferric iron content or molar concentration can be performed using redox potentiometric titration.

[0052] Furthermore, the voltage for the trivalent iron-electrolytic reduction treatment is 1-10V.

[0053] This invention does not limit the specific selection of electrode materials for electrolytic reduction and electrolytic oxidation treatments. For example, the anode is a titanium plate or a titanium electrode with a cathode coating, and the anode coating includes at least one of ruthenium-iridium alloy, ruthenium-iridium coating, iridium-tantalum alloy, ruthenium-iridium-tantalum alloy, titanium suboxide, platinum, gold, and silver; the cathode is a stainless steel electrode, a copper plate, a nickel plate, or a stainless steel electrode with a cathode coating, and the cathode coating includes a tin-iron alloy or carbon material.

[0054] Furthermore, the cathode and anode can be separated by a diaphragm, which can be a proton membrane or an ion membrane, such as polybenzimidazole (PBI), perfluorosulfonic acid (PFSA), sulfonated polyether ether ketone (SPEEK), nonwoven fabric, polyphenylene ether (PPO), etc.

[0055] Of course, a diaphragm may not be required between the anode and cathode chambers. In this case, other compensation methods are needed to suppress unnecessary material migration between the chambers. These methods include, but are not limited to, optimizing electrode arrangement, selecting electrode materials, adjusting electrode spacing, controlling flow rate and stirring speed, setting up in-tank flow guides, and adjusting electrolysis parameters through online analysis.

[0056] A second aspect of the present invention provides an impurity removal apparatus for performing the aforementioned impurity removal method of the first aspect. As shown in FIG1, the impurity removal apparatus includes an electrolysis unit 1 and an impurity removal unit 2. The electrolysis unit 1 includes a diaphragm 1a, a cathode chamber 1b, and an anode chamber 1c; the outlet of the cathode chamber 1b is connected to the inlet of the impurity removal unit 2. The cathode chamber 1b is used to contain a cathode electrolyte and a cathode, the anode chamber 1c is used to contain an anolyte and an anode, and the impurity removal unit 2 is used to remove impurities from the reduction system from the cathode chamber 1b, separating the impurity elements from the ferrous ions.

[0057] In practical applications, by applying voltage to the cathode and anode, the delithiated iron-phosphorus slag system at the cathode undergoes a trivalent iron-electrolytic reduction treatment, gradually decreasing the trivalent iron content to obtain a reduced system containing divalent iron ions. After exiting the cathode chamber, the reduced system enters the impurity removal unit 2 through the inlet for impurity removal treatment.

[0058] The present invention does not limit the specific material selection of the cathode and anode, or the specific selection of the impurity removal unit 2, as long as the above-mentioned processing can be completed efficiently.

[0059] In one specific embodiment, the electrolysis unit 1 is a common electrolytic cell in the art, the specific materials of the cathode and anode are the same as described above, and the impurity removal unit 2 is a non-divalent metal adsorption resin, such as a cation exchange resin or a chelating resin.

[0060] It is understood that, in order to improve the recovery efficiency, the above-mentioned device may also include an electrical control unit and a monitoring unit. The monitoring unit is used to monitor at least one of the following: the content of ferric ions in cathode chamber 1b, the liquid level in cathode chamber, the liquid level in anode chamber, and the content of metal ions in the impurity removal unit. The electrical control unit is electrically connected to the monitoring unit, the inlet and outlet valves of cathode chamber 1b, the inlet valve of impurity removal unit 2, and the inlet and outlet valves of anode chamber 1c.

[0061] For example, when the monitoring unit detects that the content of ferric ions in the cathode chamber 1b has decreased to the target content, it will release an electrical signal to the electronic control unit. After receiving the electrical signal, the electronic control unit will control the outlet valve of the cathode chamber 1b and the inlet valve of the impurity removal unit 2 to open them, so that the reduction system of the cathode chamber 1b enters the impurity removal unit 2 for impurity removal treatment.

[0062] Of course, the monitoring unit can also be used to monitor the liquid level of cathode chamber 1b. When the liquid level of cathode chamber 1b is lower than the target liquid level, an electrical signal can be released to make the electronic control unit open the inlet valve of cathode chamber 1b, so that more fresh lithium-phosphorus iron slag system to be treated can enter cathode chamber 1b for trivalent iron-electrolytic reduction treatment.

[0063] Of course, a flow pump can also be installed between the cathode chamber 1b and the impurity removal unit 2. While driving the flow of the material, the flow rate of the material can be further monitored to further improve the impurity removal efficiency.

[0064] Furthermore, the impurity removal device of the present invention also includes an oxidation unit; the inlet of the oxidation unit is connected to the outlet of the impurity removal unit. The oxidation unit is used to receive the divalent iron system from the impurity removal unit 2 and oxidize the divalent iron system to obtain a trivalent iron system.

[0065] The impurity removal method of the present invention will be described below through specific embodiments.

[0066] Example

[0067] Example 1

[0068] The impurity removal method in this embodiment includes the following steps:

[0069] 1) The lithium-phosphate slag was mixed with sulfuric acid at a molar ratio of 1:1.2, dissolved, and filtered to obtain a lithium-phosphate slag leachate with a trivalent iron molar concentration of 1M; the composition of the lithium-phosphate slag leachate was shown in Table 1 by ICP analysis.

[0070] 2) The leaching solution of delithiated phosphorus iron slag is placed in the cathode chamber of the electrolytic cell as the cathode electrolyte, and the leaching solution of delithiated phosphorus iron slag is placed in the anode chamber of the electrolytic cell as the anode electrolyte. Electrolysis is performed by applying a cell voltage of 4V.

[0071] In the electrolytic cell, the cathode is a stainless steel plate, and the anode is a ruthenium-iridium coated titanium plate; the diaphragm separating the cathode chamber and the anode chamber is a perfluorosulfonic acid proton exchange membrane;

[0072] 3) When the molar concentration of ferric iron C2 in the cathode chamber is 0.001M, electrolysis is stopped, and a reduced system is obtained;

[0073] 4) Utilizing metal adsorption resin columns (resin used) The reduction system was purified by T-62MP to obtain a solution of ferrous iron.

[0074] Table 1

[0075] Example 2

[0076] The impurity removal method in this embodiment is basically the same as that in embodiment 1, except that in step 2), the anolyte is a 1M NaCl solution.

[0077] Example 3

[0078] The impurity removal method in this embodiment is basically the same as that in embodiment 1, except that in step 2), the anolyte is a 1M ferrous sulfate solution.

[0079] Example 4

[0080] The impurity removal method in this embodiment is basically the same as that in embodiment 1, except that in step 2), the anolyte is a 1M manganese sulfate solution.

[0081] Example 5

[0082] The impurity removal method in this embodiment is basically the same as that in Example 1, except that in step 2), the anolyte is a 1M KOH solution containing 30 vol% methanol.

[0083] Example 6

[0084] The impurity removal method in this embodiment is basically the same as that in Example 1, except that in step 2), the anolyte is a 1M KOH solution containing 30 vol% formic acid.

[0085] Example 7

[0086] The impurity removal method in this embodiment is basically the same as that in embodiment 1. The difference is that in step 4), iron powder is added to 100 mL of reduction system at a ratio of 2 g iron powder per liter of reduction system for reduction treatment so that some impurities are reduced and precipitated. After filtration, the solution is then adsorbed and removed by a non-divalent impurity metal adsorption resin column to obtain a solution of divalent iron system.

[0087] Example 8

[0088] The impurity removal method in this embodiment is basically the same as that in Embodiment 7, except that the delithiated iron phosphate slag is replaced. The composition of the delithiated iron phosphate slag and the leachate from the delithiated iron phosphate slag in this embodiment is shown in Table 2.

[0089] Table 2

[0090] Example 9

[0091] The recycling method in this embodiment uses the same apparatus as in Embodiment 1. The main difference lies in the cathode electrolyte, requiring adaptive adjustments to the electrolysis method. Specifically, the method includes the following steps:

[0092] 1) The delithiated iron phosphate slag from Example 1 was added to a 1M sulfuric acid solution and stirred thoroughly to form a solid suspension slurry. This suspension slurry was used as the cathode electrolyte, and a 1M ferrous sulfate solution was used as the anode electrolyte. Stainless steel was used as the reduction electrode at the cathode, and a titanium electrode was used as the anode. Electrolysis was carried out at 4V.

[0093] 2) When the concentration of ferric iron in the solution is detected to be 0.001M, electrolysis is stopped; after filtration and centrifugation, the leachate rich in ferrous iron is collected to obtain the reduction system.

[0094] 3) After removing impurities using the method described in Example 1, a solution of the divalent iron system is obtained.

[0095] Comparative Example 1

[0096] The lithium-phosphorus iron slag leachate in Table 1 was recovered using an iron powder reduction method. The specific method includes the following steps:

[0097] 1) Add 7.8g of reduced iron powder to 100mL of ferric phosphate slag leachate to carry out a reduction reaction, and obtain a reduction system with a ferric ion molar concentration of 0.001M;

[0098] 2) The reduction system was purified according to the purification method in Example 1 to obtain a divalent iron system.

[0099] The reduction cost for treating 1 ton of ferric phosphate slag leachate using the above method is approximately 308 yuan. Converted to reducing each ton of trivalent iron, the reduction cost is 5130 yuan. The price of iron powder is 4700 yuan per ton.

[0100] Comparative Example 2

[0101] The lithium-phosphorus iron slag leachate in Table 2 was recovered using an iron powder reduction method. The specific method includes the following steps:

[0102] 1) Add 6g of reduced iron powder to 100mL of ferric phosphate slag leachate to carry out a reduction reaction, and obtain a reduction system with a ferric ion molar concentration of 0.001M;

[0103] 2) The reduction system was purified according to the purification method in Example 8 to obtain a divalent iron system.

[0104] The reduction cost for treating 1 ton of ferric phosphate slag leachate using the above method is approximately 235 yuan. Converted to reducing each ton of trivalent iron, the reduction cost is 4895 yuan. Of this, the price of iron powder is 4700 yuan per ton.

[0105] Comparative Example 3

[0106] The impurity removal method in this comparative example includes the following steps:

[0107] NaOH solution was added to the delithiated phosphorus iron slag leachate with the composition shown in Table 1 to adjust the pH of the system to 3. After precipitation, the solution was filtered to obtain the purified system solution.

[0108] Test case

[0109] 1. The reduction cost (electricity consumption and electricity cost) of a unit mass of trivalent iron recovered in the example was calculated, and the results are shown in Table 3.

[0110] The electrical energy consumption E (kW·h· / ton) for recovering a unit mass of ferric iron refers to the electrical energy required to reduce 1 ton of ferric iron. E is calculated according to Formula 1. The cost of electricity is calculated at 0.67 yuan / kWh.

[0111] 2. The iron content in the purified solutions of the examples and comparative examples was determined by ICP, and the iron recovery rate w was calculated according to the following formula. The results are shown in Table 4. w(%) = Iron content in the purified solution / Iron content in the reduced system

[0112] 3. Using the purified systems from the various embodiments and comparative examples as the iron source, ferric phosphate was prepared. The preparation steps of ferric phosphate are as follows: The reduced and purified system solution was used as the synthesis mother liquor. Ammonia was used to adjust the pH to 1.8–2.0, and phosphoric acid was added as needed to make the phosphorus-to-iron ratio close to 1.05. After stirring evenly, the temperature was raised to 95°C. After the yellow material turned white, it was kept at this temperature for 4 hours to obtain the dihydrate material. After drying, sintering, and dehydration, ferric phosphate was obtained. The impurity metals in the prepared ferric phosphate solid phase were analyzed by ICP, and the composition is shown in Table 4 below.

[0113] Table 3

[0114] Table 4

[0115] As shown in Tables 3 and 4, compared to Comparative Examples 1 and 2, the recovery method of the present invention can achieve high-purity iron recovery at a lower cost and has an excellent recovery rate. The iron recovery rates in Examples 7 and 8 and Comparative Examples 1 and 2 exceed 100% because iron powder was added during the impurity removal process. Furthermore, when the delithiated iron phosphate slag is added to sulfuric acid solution as a cathode electrolyte, not only does a reduction reaction of ferric iron occur, but iron in the delithiated iron phosphate slag also continuously leaches out. Therefore, the iron content in the reduction system is higher than that in the delithiated iron phosphate slag. The iron content in the reduction system of Example 4 in Table 4 is the result of ICP detection of the delithiated iron phosphate slag and calculation based on the mass of the sulfuric acid solution.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for removing impurities, characterized in that, Includes the following steps: The lithium-free phosphorus iron slag system was subjected to trivalent iron-electrolytic reduction treatment to obtain a reduced system. The reduction system is subjected to impurity removal treatment to obtain a divalent iron system.

2. The impurity removal method according to claim 1, characterized in that, The impurity content in the divalent iron system is less than 100 ppm.

3. The impurity removal method according to claim 1 or 2, characterized in that, The impurity removal process includes at least one of the following steps: titanium ion removal, aluminum ion removal, and copper ion removal.

4. The method for removing impurities according to any one of claims 1-3, characterized in that, The impurity removal treatment includes at least one of the following: impurity metal resin adsorption treatment, reduction treatment, and pH adjustment treatment.

5. The method for removing impurities according to any one of claims 1-4, characterized in that, It also includes the step of oxidizing the divalent iron system.

6. The impurity removal method according to claim 5, characterized in that, The oxidation treatment includes: oxidizing the divalent iron system with an oxidizing agent to obtain a trivalent iron system; the oxidizing agent includes at least one of hydrogen peroxide, oxygen, ozone, potassium dichromate, potassium permanganate, and sodium persulfate.

7. The impurity removal method according to claim 5, characterized in that, The oxidation treatment includes: oxidizing the divalent iron system using electrolytic treatment to obtain a trivalent iron system.

8. The impurity removal method according to claim 6 or 7, characterized in that, The impurity removal method further includes the step of using the trivalent iron system as an iron source to generate iron phosphate, lithium iron phosphate, or lithium manganese iron phosphate.

9. The method for removing impurities according to any one of claims 1-8, characterized in that, The delithiated iron phosphate slag system is a delithiated iron phosphate slag leachate or an electrode containing delithiated iron phosphate slag.

10. The method for removing impurities according to any one of claims 1-9, characterized in that, In the lithium-free phosphorus-iron slag system The molar concentration of ferric ions is not less than 0.01 M; and / or, The iron content by mass is not less than 0.1%.

11. The method for removing impurities according to any one of claims 1-10, characterized in that, The voltage for the trivalent iron-electrolytic reduction treatment is 1 to 10 V.

12. The method for removing impurities according to any one of claims 1-11, characterized in that, In the aforementioned trivalent iron-electrolytic reduction treatment The anode is a titanium plate or a titanium electrode with an anode coating, wherein the anode coating includes at least one selected from ruthenium-iridium alloy, ruthenium-iridium coating, iridium-tantalum alloy, ruthenium-iridium-tantalum alloy, titanium suboxide, platinum, gold, and silver; and / or, The cathode is a stainless steel electrode, a copper plate, a nickel plate, or a stainless steel electrode with a cathode coating, wherein the cathode coating includes at least one of tin-iron alloy and carbon material; and / or, The membrane separating the cathode and anode is a proton membrane or an ion membrane.

13. A purification device, characterized in that, It includes an electrolysis unit and a purification unit, wherein the electrolysis unit includes a cathode chamber and an anode chamber; The outlet of the cathode chamber is connected to the inlet of the impurity removal unit.

14. The impurity removal device according to claim 13, characterized in that, It also includes a monitoring unit and an electronic control unit; the monitoring unit is used to monitor at least one of the following: the content of ferric ions in the cathode chamber, the liquid level in the cathode chamber, the liquid level in the anode chamber, and the content of metal ions in the impurity removal unit; The electrical control unit is electrically connected to the monitoring unit, the inlet and outlet valves of the cathode chamber, the inlet valve of the impurity removal unit, and the inlet and outlet valves of the anode chamber.

15. The impurity removal device according to claim 13 or 14, characterized in that, It also includes oxidation units; The inlet of the oxidation unit is connected to the outlet of the impurity removal unit.

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