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CN113279048B - Method for preparing high-purity iron phosphate from iron-containing slag - Google Patents

Method for preparing high-purity iron phosphate from iron-containing slag Download PDF

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CN113279048B
CN113279048B CN202110505319.XA CN202110505319A CN113279048B CN 113279048 B CN113279048 B CN 113279048B CN 202110505319 A CN202110505319 A CN 202110505319A CN 113279048 B CN113279048 B CN 113279048B
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马保中
王成彦
曹志河
陈永强
张文娟
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University of Science and Technology Beijing USTB
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Abstract

The invention discloses a method for preparing high-purity iron phosphate from iron-containing slag, and belongs to the technical field of industrial solid waste comprehensive utilization. The method comprises the following steps: selectively leaching the iron-containing slag by using high-concentration phosphoric acid, wherein the concentration of the phosphoric acid is 4-8mol/L, performing solid-liquid separation after the reaction is finished, and selling the leached slag as a raw material for manufacturing cement ceramics; adding water into the leaching solution for dilution, and then controlling crystallization to prepare high-purity iron phosphate; the crystallized residual liquid is subjected to membrane separation-evaporation concentration process to realize regeneration and cyclic utilization of phosphoric acid. The method has the advantages of short flow, low cost, high environmental friendliness and high iron utilization rate, the prepared high-purity iron phosphate can be used for preparing materials such as lithium ion batteries, ceramics, catalysts and the like, and the macro-element iron in the iron-containing industrial solid waste is utilized in a high-value manner. The invention not only solves the problems of environmental pollution and resource waste caused by the iron-containing slag, but also relieves the environmental protection pressure of metallurgical enterprises and improves the economic benefit of the metallurgical enterprises.

Description

Method for preparing high-purity iron phosphate from iron-containing slag
Technical Field
The invention belongs to the technical field of industrial solid waste comprehensive utilization, and particularly relates to a method for preparing high-purity iron phosphate from iron-containing slag.
Background
With the continuous development of metallurgical industry and the increase of social demand, the capacity of non-ferrous metals such as nickel, aluminum and the like is continuously expanded, the global refined nickel yield is 242 ten thousand tons in 2020, and the raw aluminum yield is 6527 ten thousand tons. The huge output is accompanied by the generation of a large amount of smelting slag, and the generation of the industrial solid wastes not only causes serious environmental burden to smelting enterprises, but also threatens the safety of ecological environment more seriously. Therefore, it is urgent to develop a comprehensive and efficient method for treating such industrial solid wastes.
In the nickel smelting process, along with the continuous expansion of global nickel demand and the increasing exhaustion of nickel sulfide ores, the laterite-nickel ore gradually becomes the main raw material for nickel smelting, and the proportion of the laterite-nickel ore in the nickel yield is over 50 percent at present. The smelting process of the laterite-nickel ore mainly comprises a pyrogenic process and a wet process, wherein the pyrogenic process can produce a large amount of smelting slag while producing ferronickel, and the wet process can produce a large amount of iron-containing leaching slag while using various nickel salts or nickel oxides as product guides. The Rumex nickel plant located in Babuxingui adopts a sulfuric acid pressure leaching process to treat laterite-nickel ore, but the plant adopts a pipeline to directly convey iron-containing leaching slag to deep sea, so that serious marine pollution and ecological damage are caused, and therefore the problem of indefinite adjustment and improvement under the command of the Baxin government is solved. The encounter of the Rumex plant is that laterite-nickel ore smelting enterprises sound an alarm clock, and the treatment of smelting slag becomes an important part for determining the development and survival of the smelting enterprises, so the resource utilization of the smelting slag is the necessary requirement for clean production of the metallurgical process.
In the aluminum production process, the bayer process is the main process for producing alumina, and studies have reported that 1.0-2.0 tons of red mud are additionally produced for every 1 ton of alumina produced. The red mud is named because of large iron oxide content and similar appearance to red soil. Because the red mud has large component fluctuation, complex treatment process and higher alkali content, the main treatment mode of the red mud at present is landfill, which not only causes the waste of valuable metal resources in the red mud, but also causes serious environmental pollution and ecological damage.
The iron content of both the laterite-nickel ore smelting slag and the red mud is high and can reach 65 percent at most, so for the treatment of the two industrial solid wastes, the resource utilization and recovery of the macro element iron must be considered preferentially. Iron phosphate compounds are widely used in iron-based materials. In recent years, iron phosphate has been an important electrode material for electric vehicle batteries because of its properties such as good thermal stability and easy recycling, while overcoming the problem of electrical conductivity. In addition, the iron phosphate is bonded to the metal surface, which prevents the metal from being further oxidized. Iron phosphate can also be used as a coating as a base coat to increase the adhesion of iron or steel surfaces, and is commonly used for rust prevention treatment; or for bonding surfaces of facings, wood, or other materials. If the iron-containing slag can be directly prepared into the high-purity iron phosphate, the economic benefit of smelting enterprises is improved, the environmental protection burden of the enterprises is relieved, and the problems of resource waste and environmental pollution caused by industrial solid waste are solved.
Therefore, the development of a preparation process for directly producing high-purity iron phosphate by using the iron-containing slag as a raw material has important significance.
Disclosure of Invention
Aiming at the defects and shortcomings in the prior art, the invention provides a method for preparing high-purity iron phosphate from iron-containing slag, which comprises the steps of firstly carrying out selective leaching on the iron-containing slag by using high-concentration phosphoric acid, carrying out solid-liquid separation after the reaction is finished, and selling the leached slag as a raw material for manufacturing cement ceramics; adding water into the leaching solution for dilution, and then controlling crystallization to prepare high-purity iron phosphate; the crystallized residual liquid is subjected to membrane separation-evaporation concentration process to realize regeneration and cyclic utilization of phosphoric acid. The process method has the advantages of short flow, low cost, high environmental friendliness and high iron utilization rate, the prepared high-purity iron phosphate can be used for the production of lithium ion batteries, ceramics, catalysts and the like, and the macro-element iron in the iron-containing industrial solid waste is utilized with high value. The invention not only solves the problems of environmental pollution and resource waste caused by the iron-containing slag, but also relieves the environmental protection pressure of metallurgical enterprises and improves the economic benefit of the metallurgical enterprises.
The purpose of the invention is realized by the following technical scheme:
a method for preparing high-purity iron phosphate from iron-containing slag comprises the following steps:
(1) selective leaching: under the condition of adding an oxidant, carrying out high-concentration phosphoric acid selective leaching reaction on iron-containing slag to be treated, and carrying out solid-liquid separation after the reaction is finished to obtain leachate and leaching slag;
(2) and (3) controlling crystallization: diluting the leachate, adding an iron phosphate seed crystal and a surfactant for crystallization reaction, carrying out solid-liquid separation after the reaction is finished to obtain a crystallization residual liquid and a crystallization product, and drying the crystallization product to obtain iron phosphate;
(3) membrane separation/concentration: performing membrane separation on the crystallized residual liquid to obtain primarily concentrated phosphoric acid and phosphoric acid containing impurities;
(4) and (3) evaporation and concentration: further concentrating the primarily concentrated phosphoric acid obtained in the step (3) by adopting an evaporation concentration process to obtain high-concentration phosphoric acid, and returning to the step (1) for recycling;
(5) removing impurities: and (3) when the impurity ions in the crystallized residual liquid reach a certain concentration, opening the crystallized residual liquid and introducing the impurity-containing phosphoric acid obtained in the step (3) into an impurity removal process, and returning the low-concentration phosphoric acid obtained after impurity removal to the step (4) for continuous evaporation and concentration.
Further, the iron-containing slag in the step (1) refers to iron-containing slag materials generated in smelting or leaching processes in the process production process, and the iron-containing slag materials include but are not limited to one or more of red mud, laterite-nickel ore pyrometallurgical slag, laterite-nickel ore wet leaching slag, iron-containing slag generated in the recovery process of lithium iron phosphate batteries, or iron-phosphorus slag generated after alloy extraction; the wet leaching process comprises a nitric acid leaching method, a sulfuric acid leaching method or a hydrochloric acid leaching method, and the leaching mode comprises pressure leaching or atmospheric pressure leaching.
Further, the mass fraction of the iron element in the iron-containing slag in the step (1) is 25-65%.
Further, the concentration of the phosphoric acid in the step (1) is 4-8 mol/L.
Further, the solid-to-liquid ratio of the leaching reaction in the step (1) is 1:6-1:10g/mL, the oxidant is hydrogen peroxide or oxygen, the leaching temperature is controlled at 70-90 ℃, the stirring speed is set at 200-600rpm, and the leaching time is 0.5-4 h.
Furthermore, the concentration of the hydrogen peroxide is 0.1-0.5mol/L, and the flow rate of the oxygen is 5-100 mL/min.
Furthermore, the main components of the leaching slag in the step (1) comprise silicate and silicon dioxide, and the leaching slag can be sold as raw materials for manufacturing cement or ceramics.
Further, in the step (2), the addition amount of water in the dilution process is 0.5-5 times of the volume of the leachate, and the pH control range is 0.2-2.0.
Further, the adding amount of the iron phosphate seed crystal in the step (2) is 10-100 g/L.
Furthermore, the type of the surfactant in the step (2) is one or more of CTAB, SDS, SDBS and PEG6000, and the addition amount is 0.1-0.5% of the mass fraction of the iron element in the leachate in the step (1).
Further, the temperature of the crystallization reaction in the step (2) is 80-95 ℃, the time is 1-24h, and the stirring speed is 50-400 rpm.
Further, the drying temperature of the crystallized product in the step (2) is preferably 80 ℃ and the time is 12 hours.
Further, the membrane separation in the step (3) and the impurity removal in the step (5) adopt one or more combined processes of microfiltration, ultrafiltration, nanofiltration, bipolar membrane, reverse dialysis and electrodialysis.
Further, in the membrane separation process in the step (3), the temperature of phosphoric acid is controlled to be 25-45 ℃, the pressure of phosphoric acid is controlled to be 1.5-5.0MPa, the flow rate of phosphoric acid is controlled to be 50-200L/h, and phosphoric acid is primarily concentrated to the mass fraction of 40-65%.
Further, an MVR evaporator or a multi-effect evaporator is adopted for evaporation concentration in the step (4), and the mass fraction of the concentrated phosphoric acid is 65-85%.
Further, when the concentration of impurity ions in the crystallized residual liquid in the step (5) is enriched to be 5.0g/L or more, the impurity removal is carried out by opening the circuit with the volume fraction of the crystallized residual liquid being 20-40%.
The technical scheme of the invention is realized by the following principle:
the selective leaching reaction in the step (1) comprises the processes of phosphoric acid ionization, iron slag dissolution and the like, and the efficient selective leaching of the iron slag is realized by utilizing the characteristic that the iron phosphate can be dissolved in high-concentration phosphoric acid, and the main reactions are as follows:
Figure BDA0003058138250000041
Fe2O3+6H+→2Fe3++3H2O(2)
Figure BDA0003058138250000042
Figure BDA0003058138250000043
Figure BDA0003058138250000044
and (3) the crystallization is controlled to reduce the concentration of phosphoric acid in the leachate obtained in the step (1) through dilution, and the preparation of the iron phosphate is realized by utilizing the characteristic that iron phosphate in low phosphoric acid concentration is low in solubility and easy to separate out. This step involves, in addition to the conventional direct iron phosphate formation reaction, the formation of Fe2(HPO4)3The process of mesophase, where the main reactions taking place are:
Figure BDA0003058138250000045
Figure BDA0003058138250000046
Figure BDA0003058138250000047
and (3) in the membrane separation/concentration, the selective removal of impurity elements is realized by mainly utilizing the selectivity difference of the separation membrane on ions and substances with different particle sizes. In addition, under the action of an electric field, the polarity moving directions of the anions and the cations are different, and the separation and purification of the anions and the cations can be realized.
The technical scheme shows that: the method for preparing high-purity iron phosphate by using the iron-containing slag utilizes the difference of the solubility of the iron phosphate in different phosphoric acid concentrations to realize the selective leaching of the iron-containing slag and the preparation of the high-purity iron phosphate. Selectively leaching iron by using high-concentration phosphoric acid, controlling crystallization in low-concentration phosphoric acid, and strictly controlling crystallization conditions to prepare the high-purity iron phosphate. The crystallized residual liquid can be recycled by membrane separation/concentration-evaporative concentration. The method flexibly utilizes the solubility difference of the iron phosphate in different phosphoric acid concentrations, realizes high-valued comprehensive utilization of the iron-containing slag, and has the advantages of simple process flow, high utilization rate of the iron slag and small influence on the environment. Meanwhile, the cyclic regeneration of the leaching agent phosphoric acid is realized, and the green and environment-friendly development of the metallurgical industry is further promoted.
Compared with the prior art, the method for preparing high-purity iron phosphate from the iron-containing slag provided by the invention has the following advantages:
(1) the method for preparing high-purity iron phosphate from iron-containing slag provided by the invention has the advantages that the cost of raw materials is low, the environmental burden of non-ferrous smelting enterprises is reduced, and the problems of serious resource waste and environmental pollution caused by iron-containing slag are solved;
(2) the method flexibly utilizes the difference of the ferric phosphate in different phosphoric acid concentrations, and realizes the high-efficiency separation of iron from impurities such as silicon, calcium and the like in the iron-containing slag through the selective leaching of the high-concentration phosphoric acid;
(3) under the condition of low phosphoric acid concentration, the high-purity ferric phosphate is prepared by controlling crystallization, and can be used for preparing a lithium ferric phosphate battery;
(4) the invention has the advantages of cyclic utilization of the leaching agent phosphoric acid, low process cost and high environmental friendliness.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the description of the embodiments are briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.
FIG. 1 is a process flow diagram of a method for preparing high-purity iron phosphate from iron-containing slag according to the present invention;
fig. 2 is a phase analysis diagram of iron phosphate prepared in example 1 of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without inventive step, are within the scope of the present invention.
Example 1
According to the process flow diagram shown in figure 1, 7mol/L phosphoric acid is added into sulfuric acid pressure leaching slag of laterite nickel ore containing 48.2% of iron at a solid-to-liquid ratio of 1:6g/mL, the concentration of hydrogen peroxide is 0.1mol/L, the selective leaching temperature is controlled to be 85 ℃, the leaching time is 3h, and the stirring speed is set to be 200 rpm; solid-liquid separation, and the leached residue can be sold as a raw material for preparing cement ceramics. Adding 2 times volume of water into the leachate for dilution, maintaining the pH value of the system at 0.5, adding 80g/L of ferric phosphate serving as a seed crystal and CTAB (cetyl trimethyl ammonium bromide) with the iron content of 0.1% serving as a surfactant, wherein the crystallization temperature is 80 ℃, the crystallization time is 24 hours, and the stirring speed is 100 rpm; after the crystallization is finished, solid-liquid separation is carried out, the crystallized product is dried in an oven at the temperature of 80 ℃ for 12 hours, and the ferric phosphate dihydrate is obtained, and the phase analysis is shown in figure 2. Performing membrane separation/concentration on the crystallization residual liquid by adopting an electrodialysis and bipolar membrane combined process under the conditions that: the phosphoric acid feeding temperature is 25 ℃, the phosphoric acid feeding pressure is 1.8MPa, and the phosphoric acid production flow is 80L/h. The electrodialysis can selectively remove impurity cations, and the bipolar membrane can realize further purification and concentration of the phosphoric acid. And (3) continuously evaporating and concentrating the phosphoric acid prepared by membrane separation/concentration, passing through a multi-effect evaporator to obtain the phosphoric acid with the concentration of 85%, and continuously returning to the selective leaching for use.
Example 2
According to the process flow chart shown in figure 1, 5mol/L phosphoric acid is added into 25.0% red mud containing iron according to the solid-to-liquid ratio of 1:10g/mL, the concentration of hydrogen peroxide is 0.5mol/L, the selective leaching temperature is controlled to be 80 ℃, the leaching time is 4h, and the stirring speed is set to be 500 rpm; solid-liquid separation, and the leached residue can be sold as a raw material for manufacturing cement ceramics. Adding 5 times volume of water into the leachate for dilution, maintaining the pH of the system at 2.0, adding 50g/L of ferric phosphate serving as a seed crystal and SDS (sodium dodecyl sulfate) with the iron content of 0.2% serving as a surfactant, wherein the crystallization temperature is 85 ℃, the time is 18h, and the stirring speed is 50 rpm; and after the crystallization is finished, carrying out solid-liquid separation, and drying the crystallized product in an oven at the temperature of 80 ℃ for 12 hours to obtain the ferric phosphate dihydrate. Performing membrane separation/concentration on the crystallization residual liquid by adopting an ultrafiltration-electrodialysis-bipolar membrane combined process under the following conditions: the phosphoric acid feeding temperature is 70 ℃, the phosphoric acid feeding pressure is 3.0MPa, and the phosphoric acid production flow is 150L/h. The removal of large-particle-size impurity ions is realized through ultrafiltration, and then the separation of other impurities and the preliminary concentration of phosphoric acid are realized by utilizing the combination process of electrodialysis and a bipolar membrane. And (3) continuously evaporating and concentrating the phosphoric acid prepared by membrane separation/concentration, and passing through an MVR evaporator to obtain phosphoric acid with the concentration of 65%, wherein the phosphoric acid can be continuously returned to the selective leaching for use.
Example 3
According to the process flow diagram shown in figure 1, adding 8mol/L phosphoric acid into the pyrometallurgical slag of the laterite-nickel ore containing 32.0% of iron according to the solid-liquid ratio of 1:8g/mL, introducing oxygen into the system, controlling the oxygen flow at 5mL/min, controlling the selective leaching temperature at 70 ℃, the leaching time at 0.5h, and setting the stirring speed at 400 rpm; solid-liquid separation, and the leached residue can be sold as a raw material for manufacturing cement ceramics. Adding 0.5 volume times of water into the leachate for dilution, maintaining the pH value of the system at 0.2, adding 10g/L of ferric phosphate serving as seed crystal and SDBS (sodium dodecyl benzene sulfonate) serving as surfactant and having iron content of 0.5%, wherein the crystallization temperature is 90 ℃, the time is 6h, and the stirring speed is 300 rpm; and after the crystallization is finished, carrying out solid-liquid separation, and drying the crystallized product in an oven at the temperature of 80 ℃ for 12 hours to obtain the ferric phosphate dihydrate. 20% of crystallization residual liquid is subjected to an impurity removal process, and a membrane separation process is still adopted for impurity removal; and performing membrane separation/concentration on the rest of the crystallization residual liquid by adopting a nanofiltration and electrodialysis combined process under the following conditions: the phosphoric acid feeding temperature is 35 ℃, the phosphoric acid feeding pressure is 5.0MPa, and the phosphoric acid production flow is 200L/h. Nanofiltration and electrodialysis can accomplish the purification and concentration of phosphoric acid. And (3) continuously carrying out evaporation concentration on the phosphoric acid prepared by membrane separation/concentration and the low-concentration phosphoric acid obtained by impurity removal, obtaining phosphoric acid with the concentration of 85% by virtue of an MVR (mechanical vapor recompression) evaporator, and continuously returning to the selective leaching for use.
Example 4
Adding 4mol/L phosphoric acid into the laterite nickel ore nitric acid pressurized leaching slag containing 65.0% of iron according to the solid-liquid ratio of 1:6g/mL according to the process flow diagram shown in the figure 1, introducing oxygen into the system, controlling the oxygen flow at 100mL/min, controlling the selective leaching temperature at 75 ℃, the leaching time at 2h, and setting the stirring speed at 600 rpm; solid-liquid separation, and the leached residue can be sold as a raw material for preparing cement ceramics. Adding 3 times volume of water into the leachate for dilution, keeping the pH of the system at 1.2, adding 100g/L of ferric phosphate serving as seed crystal and PEG6000 with the iron content of 0.2% serving as surfactant, wherein the crystallization temperature is 95 ℃, the time is 3h, and the stirring speed is 400 rpm; and after the crystallization is finished, carrying out solid-liquid separation, and drying the crystallized product in an oven at the temperature of 80 ℃ for 12 hours to obtain the ferric phosphate dihydrate. The crystallization residual liquid is subjected to membrane separation/concentration by adopting an ultrafiltration-nanofiltration combined process, and the conditions are as follows: the phosphoric acid feeding temperature is 35 ℃, the phosphoric acid feeding pressure is 1.5MPa, and the phosphoric acid production flow is 50L/h. The ultrafiltration can remove impurity metal ions with large particle size, and then the purification and concentration of the phosphoric acid are realized through nanofiltration. And (3) continuously evaporating and concentrating the phosphoric acid prepared by membrane separation/concentration, and passing through an MVR evaporator to obtain phosphoric acid with the concentration of 65%, wherein the phosphoric acid can be continuously returned to the selective leaching for use.
Example 5
According to the process flow diagram shown in figure 1, adding 6mol/L phosphoric acid into laterite nickel ore hydrochloric acid normal pressure leaching slag containing 48.3% of iron according to a solid-to-liquid ratio of 1:10g/mL, wherein the concentration of hydrogen peroxide is 3.5mol/L, the selective leaching temperature is controlled to be 90 ℃, the leaching time is 1h, and the stirring speed is set to be 200 rpm; solid-liquid separation, and the leached residue can be sold as a raw material for preparing cement ceramics. Adding 1 volume of water into the leachate for dilution, maintaining the pH value of the system at 0.2, adding 30g/L of ferric phosphate serving as seed crystal and SDBS (sodium dodecyl benzene sulfonate) serving as surfactant, wherein the iron content is 0.2%, the crystallization temperature is 95 ℃, the time is 1h, and the stirring speed is 200 rpm; and after the crystallization is finished, carrying out solid-liquid separation, and drying the crystallized product in an oven at the temperature of 80 ℃ for 12 hours to obtain the ferric phosphate dihydrate. 40% of the crystallization residual liquid enters an impurity removal process after being opened, and the impurity removal still adopts a membrane separation process; and performing membrane separation/concentration on the rest of the crystallized residual liquid by adopting a reverse dialysis process under the conditions of: the phosphoric acid feeding temperature is 45 ℃, the phosphoric acid feeding pressure is 4.5MPa, and the phosphoric acid production flow is 150L/h. And (3) continuously evaporating and concentrating the prepared phosphoric acid by membrane separation/concentration and the low-concentration phosphoric acid obtained by impurity removal, passing through a multi-effect evaporator to obtain the phosphoric acid with the concentration of 85%, and continuously returning to the selective leaching for use.
In conclusion, in the embodiment of the invention, the iron-containing slag is treated by using the high-concentration phosphoric acid, so that the selective leaching of iron in the iron-containing slag is realized, the leachate is used for preparing the high-purity iron phosphate by adopting the controlled crystallization process, the regeneration and cyclic utilization of the phosphoric acid is realized by using the membrane separation/concentration-evaporation concentration process of the crystallized residual liquid, the whole process is free of the introduction of a neutralizer and a precipitator, the high-valued comprehensive utilization of iron resources in the iron-containing slag is realized, the economic benefit of the iron-containing slag treatment process is improved, the environmental pressure caused by the iron-containing slag is relieved, and a new thought is provided for the comprehensive utilization of the solid waste of the iron-containing industry in the metallurgical industry.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.

Claims (9)

1. The method for preparing high-purity iron phosphate from iron-containing slag is characterized by comprising the following steps:
(1) selective leaching: under the condition of adding an oxidant, carrying out high-concentration phosphoric acid selective leaching reaction on iron-containing slag to be treated, and carrying out solid-liquid separation after the reaction is finished to obtain leachate and leaching slag, wherein the concentration of phosphoric acid is 4-8 mol/L;
(2) and (3) controlling crystallization: diluting the leachate, adding an iron phosphate seed crystal and a surfactant for crystallization reaction, performing solid-liquid separation after the reaction is finished to obtain a crystallization residual liquid and a crystallization product, drying the crystallization product to obtain iron phosphate, wherein the addition amount of water in the dilution process is 0.5-5 times of the volume of the leachate, and the pH control range is 0.2-2.0;
(3) membrane separation/concentration: performing membrane separation on the crystallized residual liquid to obtain primarily concentrated phosphoric acid and phosphoric acid containing impurities;
(4) and (3) evaporation and concentration: further concentrating the primarily concentrated phosphoric acid obtained in the step (3) by adopting an evaporation concentration process to obtain high-concentration phosphoric acid, and returning to the step (1) for recycling;
(5) removing impurities: and (3) when the impurity ions in the crystallized residual liquid reach a certain concentration, opening the crystallized residual liquid and introducing the impurity-containing phosphoric acid obtained in the step (3) into an impurity removal process, and returning the low-concentration phosphoric acid obtained after impurity removal to the step (4) for continuous evaporation and concentration.
2. The method for preparing high-purity iron phosphate from the iron-containing slag according to claim 1, wherein the iron-containing slag in the step (1) comprises one or more of red mud, laterite-nickel ore pyrometallurgical slag, laterite-nickel ore wet leaching slag, iron-containing slag produced in the lithium iron phosphate battery recovery process or iron-phosphorus slag produced after alloy extraction.
3. The method for preparing high-purity iron phosphate from the iron-containing slag according to claim 1 or 2, wherein the mass fraction of iron element in the iron-containing slag in the step (1) is 25 to 65%.
4. The method for preparing high-purity iron phosphate from the iron-containing slag as claimed in claim 1, wherein the solid-to-liquid ratio of the leaching reaction in the step (1) is 1:6-1:10g/mL, the oxidant is hydrogen peroxide or oxygen, the leaching temperature is controlled at 70-90 ℃, the stirring speed is set at 200-600rpm, and the leaching time is 0.5-4 h; the concentration of the hydrogen peroxide is 0.1-0.5mol/L, and the flow rate of the oxygen is 5-100 mL/min.
5. The method for preparing high-purity iron phosphate from iron-containing slag according to claim 1, wherein the addition amount of the iron phosphate seed crystal in the step (2) is 10-100 g/L; the type of the surfactant is one or more of CTAB, SDS, SDBS and PEG6000, and the addition amount of the surfactant is 0.1-0.5% of the mass fraction of the iron element in the leachate obtained in the step (1); the temperature of the crystallization reaction is 80-95 ℃, the time is 1-24h, and the stirring speed is 50-400 rpm.
6. The method for preparing high-purity iron phosphate from iron-containing slag according to claim 1, wherein the drying temperature of the crystallized product in the step (2) is preferably 80 ℃ for 12 hours.
7. The method for preparing high-purity iron phosphate from the iron-containing slag according to claim 1, wherein the membrane separation in step (3) and the impurity removal in step (5) adopt one or more combined processes of microfiltration, ultrafiltration, nanofiltration, bipolar membrane, reverse dialysis and electrodialysis; controlling the temperature of phosphoric acid feeding to be 25-45 ℃, the pressure of phosphoric acid feeding to be 1.5-5.0MPa, the flow rate of phosphoric acid to be 50-200L/h and the primary concentration of phosphoric acid to the mass fraction of 40-65% in the membrane separation process in the step (3).
8. The method for preparing high-purity iron phosphate from iron-containing slag according to claim 1, wherein an MVR evaporator or a multi-effect evaporator is adopted for evaporation and concentration in the step (4), and the mass fraction of concentrated phosphoric acid is 65-85%.
9. The method for preparing high-purity iron phosphate from iron-containing slag according to claim 1, wherein, in the step (5), when the concentration of impurity ions in the crystallized raffinate is enriched to 5.0g/L or more, the impurity removal is carried out by opening the crystallized raffinate with a volume fraction of 20-40%.
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