A method for preparing lithium iron phosphate

Abstract

The application discloses a method for preparing lithium iron phosphate, which is prepared by comprehensively processing all phosphorus-containing materials generated in the process of phosphate rock mining and processing, solves the problem of how to prepare high-purity yellow phosphorus from existing low-grade phosphate rock and further prepare lithium iron phosphate, makes a large amount of low-grade phosphate rock effectively utilized, improves the utilization efficiency of phosphorus resources, and relieves the demand pressure of lithium iron phosphate.

Description

Technical Field

[0001] This invention relates to the preparation of new energy materials, specifically to a method for preparing lithium iron phosphate, belonging to the field of lithium-ion battery and key material preparation technology. Background Technology

[0002] With the increasing severity of environmental pollution and the growing imbalance between fuel supply and demand, developed countries worldwide have made the development of new energy sources a national strategy, accelerating the technological development and industrialization of high-energy-density batteries. The continuous upgrading of lithium-ion batteries and their key materials has driven the vigorous development of many related upstream and downstream industries. Among them, lithium iron phosphate, as a cathode material for lithium-ion batteries, is experiencing ever-increasing demand.

[0003] Phosphate rock is a non-renewable resource. Due to its wide range of uses, relatively low output, and lack of suitable substitutes, it is considered a scarce resource. The scarcity and irreplaceability of phosphate rock resources determine that its valuation should be higher than that of other resources. With the increasing consumption of high-quality phosphate rock, existing production methods cannot meet the growing demand for phosphorus resources.

[0004] Meanwhile, during the production process of obtaining natural phosphate rock lumps, enterprises inevitably generate a large amount of waste phosphate rock powder. This high-quality phosphate rock powder cannot be directly used for electric arc furnace phosphorus production, resulting in the idleness of high-quality phosphate rock resources and causing resource waste. Furthermore, the large-scale stockpiling of this phosphate rock powder, which cannot be directly used for yellow phosphorus production, occupies a significant amount of space in stockpiles, wasting land resources and easily causing environmental pollution. In addition, the domestic steel industry has overcapacity, and with increasingly stringent environmental protection requirements, the steel industry will gradually phase out existing vertical shaft furnaces in the sintering and pelletizing field. The direct dismantling of these vertical shaft furnaces will inevitably lead to the waste of resources and equipment. How to further utilize these phased-out vertical shaft furnaces has become a current technical challenge.

[0005] Given the decreasing availability of high-grade phosphate rock and the increasing demand for phosphorus resources, how to utilize low-grade phosphate rock to meet the growing demand for lithium iron phosphate has become a current technical challenge. Summary of the Invention

[0006] To address the imbalance between supply and demand caused by the decreasing availability of high-grade phosphate rock and the increasing demand for lithium iron phosphate in existing technologies, this invention provides a method for preparing lithium iron phosphate. By comprehensively processing all phosphorus-containing materials generated during the mining and processing of phosphate ore, the utilization efficiency of phosphorus resources is improved, alleviating the pressure on lithium iron phosphate demand. At the same time, it also enables the secondary reuse of vertical shaft furnaces used in steelmaking, reducing waste of resources and equipment and further improving economic efficiency.

[0007] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing lithium iron phosphate includes: first, pretreating raw phosphate ore to obtain phosphate ore powder; then, mixing the phosphate ore powder, biomass straw pellets, quartz powder, and magnetite powder to form pellets and heat-treating them to obtain phosphorus-containing briquettes; then, mixing the phosphorus-containing briquettes with coke and performing reduction treatment to obtain yellow phosphorus and iron-containing solid slag; finally, using yellow phosphorus, lithium carbonate, iron-containing solid slag, and coke as raw materials, performing sintering treatment to obtain lithium iron phosphate.

[0009] Preferably, the pretreatment of the phosphate rock includes:

[0010] A1: Preliminary grading: After mixing open-pit phosphate mine ore and deep-buried phosphate mine ore, the ore is successively refined, washed, dried and screened to obtain raw ore blocks, large-particle raw ore and fine-particle raw ore.

[0011] A2: Deep classification: Large-particle raw ore is ground and classified to obtain raw ore coarse powder, raw ore fine powder and raw ore micro powder.

[0012] A3: Impurity Removal and Mixing: The fine ore powder of the raw ore is subjected to flotation, magnetic separation, photoelectric separation and calcination in sequence, and then mixed with fine-particle raw ore, coarse ore powder of the raw ore, micro-ore powder of the raw ore and waste phosphate rock powder to obtain phosphate rock powder.

[0013] In this invention, open-pit phosphate ore generally has low impurity content and hard texture, and is often mined by blasting; deep-buried phosphate ore generally has high impurity content and soft texture, and is often mined by drilling and electric scraper; the mixing ratio of the two can be any ratio, such as 1:1, 1.5:1, 2:1, 3:2, etc.

[0014] Preferably, in step A1, the refining involves crushing the mixture of open-pit and deep-buried phosphate ore to a particle size ≤30mm. The washing involves repeatedly washing the refined ore mixture with water, with a liquid-to-solid mass ratio of 1-6:1 in each wash. The drying involves drying the washed ore mixture at 110-300℃ until the moisture content is <10wt%. Preferably, the particle size of the raw ore lumps is 20-30mm. The particle size of the large-particle raw ore is between 5-20mm. The particle size of the fine-particle raw ore is no greater than 5mm.

[0015] Preferably, in step A2, the grinding process involves grinding large-particle raw ore to a particle size ≤1mm, with at least 95% of the particles being pulverized. Preferably, the coarse raw ore powder has a particle size >1mm and ≤5mm. The fine raw ore powder has a particle size ≥0.5mm and ≤1mm. The micro-raw ore powder has a particle size <0.5mm.

[0016] Preferably, in step A3, the waste phosphate rock powder includes phosphate rock powder generated during the extraction of open-pit and buried phosphate rock ore, as well as phosphate rock powder generated during the reprocessing of open-pit and buried phosphate rock ore. Preferably, the calcination temperature is 400-600℃. The P2O5 content in the phosphate rock powder is 10-15%.

[0017] Preferably, the mixture pelletizing and heat treatment include:

[0018] B1: Mixing and pelletizing: First, phosphate rock powder, biomass straw pellets, quartz powder, and magnetite powder are mixed and ground to obtain a pelletizing mixture. Then, according to the set pelletizing index requirements, rice slurry aqueous solution is added to the mixture for preliminary granulation. After the preliminary granulation is completed, calcium hydroxide solution is added to pelletize, resulting in phosphorus-containing green pellets.

[0019] B2: Heat treatment: After using natural phosphate ore blocks as the base material, the phosphorus-containing green pellets are successively dried, preheated, roasted and cooled to obtain phosphorus-containing cooked pellets.

[0020] Preferably, in step B1, the mixing mass ratio of the phosphate rock powder, biomass straw pellets, quartz powder, and magnetite powder is 60~75:4~7:18~25:8~15, and more preferably 65~73:5~6:20~23:10~13.

[0021] Preferably, the pelleting index is greater than 0.8, and more preferably 0.8 to 0.9. The pelleting index is the ratio of the maximum molecular water mass to the maximum capillary water fill value in the mixture, wherein the maximum capillary water fill value refers to the difference between the maximum capillary water mass and the maximum molecular water mass.

[0022] Preferably, the concentration of the rice slurry aqueous solution is 1~9 g / L. The concentration of the calcium hydroxide solution is 0.01~0.08 mol / L. The amount of rice slurry aqueous solution and calcium hydroxide solution added results in an overall moisture content of 6.5~8.5 wt% for the phosphorus-containing green pellets and a moisture content of 9~10 wt% for the outer layer of the phosphorus-containing green pellets.

[0023] Preferably, the particle size of the pelletizing mixture is ≤0.074mm. The particle size of the material after preliminary granulation is 2~9mm. The particle size of the phosphorus-containing green pellets is 12~30mm.

[0024] Preferably, in step B2, the drying process specifically involves: first drying the material composed of natural phosphate ore lumps and phosphate-containing green pellets using microwave at 200-300°C for 30-50 minutes, then drying it with hot air at 350-500°C for 15-45 minutes, and finally drying it with hot air at 500-700°C for 8-20 minutes. The preheating process involves preheating the dried material at 750-950°C for 5-20 minutes. The calcination process involves calcining the preheated material at 1100-1350°C for 0.2-5 hours. The cooling process involves first cooling the calcined material to 600-800°C with room temperature air, and then further cooling it to 150-250°C with room temperature air.

[0025] Preferably, the P2O5 content in the natural phosphate ore lump is not less than 18%. The thickness of the base material is 50-70 mm. The thickness of the phosphorus-containing raw pellet layer is 100-160 mm. The particle size of the phosphorus-containing cooked pellets is not less than 5 mm.

[0026] Preferably, the hot air generated from the second cooling is circulated and used as the hot air for the second drying. The hot air generated from the first cooling is circulated and used as the hot air for preheating. The hot air generated from roasting, after dust removal treatment, is used as the hot air for the third drying. The hot air generated from the third drying and preheating is discharged after dust removal, desulfurization, and denitrification treatment.

[0027] Preferably, the high-temperature reduction treatment includes:

[0028] C1: Ingredients: Phosphorus-containing cooked pellets are mixed with coke to obtain the mixture to be reduced.

[0029] C2: Reduction: The mixture to be reduced is added to a vertical furnace and subjected to reduction treatment under the action of high-temperature gas flow generated by the combustion of combustible gas to obtain phosphorus-containing flue gas and iron-containing solid slag.

[0030] C3: Phosphorus precipitation: Phosphorus-containing flue gas is passed into water for cooling and precipitation of fixed precipitate. After solid-liquid separation, the fixed precipitate is dried to obtain yellow phosphorus.

[0031] Preferably, in step C1, the mass ratio of phosphorus-containing cooked pellets to coke is 75~90:10~25, more preferably 80~88:12~20.

[0032] Preferably, in step C2, the reduction treatment temperature is 1300~1500℃, more preferably 1350~1450℃. The reduction treatment time is 5~9h, more preferably 7~8h.

[0033] Preferably, in step C3, the drying is hot air drying, preferably using phosphorus-containing flue gas as a heat source for drying, and the dried phosphorus-containing flue gas is then cooled by passing it through water.

[0034] Preferably, the sintering process includes:

[0035] D1: Pretreatment: Yellow phosphorus is oxidized and hydrolyzed sequentially to obtain phosphoric acid, which is then reacted with lithium carbonate to obtain lithium dihydrogen phosphate. The iron-containing solid slag is acid-dissolved and impurity-removed to obtain ferrous salt.

[0036] D2: Tableting: Lithium dihydrogen phosphate, ferrous salt and coke are mixed and then ground, moistened and compressed in sequence to obtain mixed tablets.

[0037] D3: Sintering: Under a nitrogen atmosphere, the mixed material sheets are sintered to obtain sintered material. The sintered material is then crushed and demagnetized to obtain lithium iron phosphate.

[0038] Preferably, in step D1, the oxidation is performed by mixing and burning yellow phosphorus with oxygen. The hydrolysis is performed by hydrating the P2O5 flue gas generated from the combustion of yellow phosphorus with oxygen and then demisting it. The mixing reaction is performed by mixing phosphoric acid and lithium carbonate, heating and reacting them, followed by evaporation and crystallization. The impurity removal treatment is performed by dissolving iron-containing solid slag with sulfuric acid, followed by hardening removal and evaporation crystallization of the leachate through precipitation.

[0039] Preferably, in step D2, the mixing amount of lithium dihydrogen phosphate and ferrous salt is such that the molar ratio of Li, P, and Fe is consistent (close to 1:1:1, with either one slightly in excess), and the amount of coke added is 8-15% of the total mass of lithium dihydrogen phosphate and ferrous salt. The grinding involves grinding the mixture to a particle size ≤0.074 mm. The wetting involves ensuring the moisture content of the mixture is 1-5 wt%. The tableting is performed using a 3-7 t / cm³ pressing method. 2 The pressure compresses the mixture into a regular sheet-like structure.

[0040] Preferably, in step D3, the sintering temperature is 600~900℃, more preferably 650~800℃. The sintering time is 10~21h, more preferably 13~18h. The crushing involves crushing the sintered material to a particle size of 3-10μm. The demagnetization is performed using a magnet for magnetic attraction.

[0041] This invention utilizes both open-pit and deep-buried phosphate rock powders for the production of yellow phosphorus, thus broadening its applicability and expanding resource utilization. It also solves the problem of how to prepare high-purity yellow phosphorus from existing low-grade phosphate rock and further process it to obtain lithium iron phosphate, enabling the effective utilization of large quantities of low-grade phosphate rock. Furthermore, this invention utilizes abandoned vertical shaft furnaces in the steel industry for yellow phosphorus production, avoiding the waste of resources and equipment caused by the direct dismantling of these furnaces. By using these abandoned furnaces to produce phosphorus, resource reuse is achieved. Since vertical shaft furnace phosphorus production has low energy consumption, it further reduces phosphorus production costs, thereby lowering the production cost of lithium iron phosphate.

[0042] In this invention, the method for obtaining open-pit phosphate ore is generally as follows: the open-pit phosphate mine is first blasted, and then mined to obtain the open-pit phosphate ore. The method for obtaining deep-buried phosphate ore is generally as follows: the deep-buried phosphate mine is first drilled, and then the ore is extracted using an electric scraper to obtain the deep-buried phosphate ore. In this process, it is necessary to collect and store the phosphate ore powder (one type of waste phosphate ore powder) generated during the blasting and mining of the open-pit phosphate mine, as well as the phosphate ore powder (one type of waste phosphate ore powder) generated during the drilling and electric scraper extraction of the deep-buried phosphate mine.

[0043] In this invention, the mixing of open-pit phosphate ore and deep-buried phosphate ore broadens its applicability and expands the scope of resource utilization. By washing the refined ore mixture, wet mud adhering to the phosphate ore can be removed, along with soluble substances (such as sodium chloride and free lime), thereby improving the grade of the phosphate ore and reducing the difficulty of subsequent processing.

[0044] In the process of this invention, low-grade phosphate rock powder undergoes pretreatment through a series of measures including washing, filtration, rapid drying, and grinding. This not only yields a particle size distribution with good pelletizing properties but also improves the grade of the phosphate rock powder. In this invention, the washed and dried raw ore mixture is graded and screened into three grades: mixed raw phosphate rock with a particle size of 20mm~30mm (including the endpoint value) is classified as irregular raw ore lump (which can be used as a base material for subsequent sintering); mixed raw phosphate rock with a particle size between 5mm and 20mm (excluding the endpoint value) is classified as large-particle raw ore; and mineral powder with a particle size ≤5mm is classified as fine-particle raw ore (which can be used in subsequent mixing to obtain phosphate rock powder for sintering and briquetting). The purpose of grading and screening is to refine and classify the raw ore mixture to facilitate full utilization in subsequent processes, thereby improving the utilization efficiency of phosphorus resources and avoiding the production of solid waste.

[0045] In this invention, the large-particle raw ore with a particle size between 5 mm and 20 mm is further ground (the proportion of particles with a particle size ≤ 1 mm is not less than 95%), thereby obtaining raw ore coarse powder with a particle size of 1 mm to 5 mm, raw ore fine powder with a particle size of 0.5 mm to 1 mm, and raw ore micro powder with a particle size < 0.5 mm. The fine ore powder with a particle size of 0.5mm~1mm undergoes impurity removal treatment (including flotation, magnetic separation, photoelectric separation, and calcination). Specifically, the process begins with the use of flotation agents (for phosphate rock flotation, the flotation agents include collectors, depressants, and synergists. Collectors can be long-chain fatty acids and their soaps. In flotation, long-chain fatty acids and their soaps are used to float phosphate and carbonate minerals, exhibiting strong collecting ability and sensitivity to pulp and certain ions. Depressants can be carboxymethyl cellulose, citric acid, etc. Phosphate ore often contains carbonate minerals such as dolomite and calcite; depressants can remove these impurities. Synergists can be surfactants). As an alkanolamide, in phosphate rock flotation, the flotation performance can be improved by adding a small amount of activator to a long-chain fatty acid collector, causing the phosphate minerals to float while gangue and other impurities sink, thus obtaining initially enriched phosphate rock. Then, the initially enriched phosphate rock undergoes magnetic separation to remove the magnetic minerals, followed by photoelectric separation. Utilizing the different colors of the phosphate rock and impurity minerals, photoelectric elements are used for identification, and compressed air jets are controlled to separate the phosphate rock from the impurity minerals, resulting in enriched phosphate rock. Finally, the enriched phosphate rock is calcined by burning a mixture of combustible gas and air to obtain the final enriched phosphate rock powder. Generally, the calorific value of the combustible gas is ≥3000 kcal / Nm³. 3 The calcination temperature is 400~600℃. Through calcination, organic matter, carbon dioxide and some fluorine in the enriched phosphate rock are removed. The hot gas after calcination can be returned to the raw ore drying step to dry the mixed raw phosphate rock, thereby improving the grade of this part of the phosphate rock powder. This not only improves the utilization value of this part of the phosphate rock powder, but also helps to improve the grade of the final artificial finished product, phosphate lump ore.

[0046] It should be noted that in the process of this invention, phosphate rock powder (such as large-particle raw ore) can be mixed with fine bulk materials (bulk materials collected from various processes) and low-temperature calcined pellets for grinding. Phosphate rock powder has low hardness and is relatively easy to grind, but its pelletizing ability is weak; fine bulk materials and low-temperature calcined pellets have high hardness and are relatively difficult to grind, but after fine grinding, they have good hydrophilicity and better pelletizing ability. After fine grinding, the pelletizing index is improved, and the green pellet strength can be met without the addition of binders; the combination of high hardness and low hardness, and the grinding process of material grinding material, reduces grinding energy consumption, improves the efficiency of fine grinding, and enhances the effect of fine grinding.

[0047] In this invention, the sintering mixture includes phosphate rock powder, quartz powder, biomass straw pellets, and magnetite powder, etc. Small pellets are obtained by spraying a rice slurry aqueous solution for mixing and granulation, followed by spraying a calcium hydroxide solution to form larger pellets. In this invention, the addition of quartz powder during the batching process significantly increases the silicon content of the phosphate rock pellets, adjusts the acidity, and increases the acidity of the pellets (e.g., acidity greater than 0.85), which is beneficial for improving the roasting strength of the pellets. In subsequent phosphorus production, quartz can also act as a flux to promote phosphorus reduction. At high temperatures, quartz can react with calcium oxide in the phosphate rock to form easily fusible slag, while simultaneously lowering the melting temperature of the reactants and reducing energy consumption. It should be noted that the amount of quartz added should not be too high, as excessive amounts will form a large liquid phase, damaging the furnace body. In the process of this invention, by adopting layered pelletizing, the outer layer of the pellets has high moisture content and the inner layer has low moisture content. Therefore, the vapor pressure is reduced in the later stage of pellet drying, which can not only prevent the pellets from bursting in the later stage of drying, but also reduce the overall moisture content of the pellets and save drying energy consumption.

[0048] In this invention, the phosphate rock powder may be optionally washed and dried again before mixing and pelletizing (for example, by using rapid high-temperature hot air flow or microwave rapid drying, which can greatly shorten the drying time and save time; at the same time, it can also prevent some minerals in the phosphate powder from crystallizing and affecting the subsequent crushing).

[0049] It should be noted that the powder, quartz powder, biomass straw pellets and magnetite powder can be ground separately to a particle size of 0~0.074mm before mixing, or they can be ground together to a particle size of 0~0.074mm after mixing.

[0050] In this invention, the P2O5 content of the phosphate rock powder is no more than 15 wt%, and its chemical composition includes: SiO2 content 10%~15%, CaO content 45%~55%, Fe2O3 content 0.5%~1%, Al2O3 content 3%~5%, MgO content 1%~2%, F content 0.1%~0.3%, and S content 0.5%~0.8%.

[0051] In this invention, the moisture content (mass content) of the magnetite is generally 3% to 7% (preferably 4% to 5%), and the chemical composition of the magnetite includes: P2O5 content 0.01% to 0.03%, SiO2 content 5% to 7%, CaO content 0.3% to 0.7%, Fe2O3 content 60% to 65%, Al2O3 content 0.8% to 0.9%, MgO content 0.1% to 0.2%, F content 1% to 2%, and S content 0.1% to 0.2%.

[0052] In this invention, during the mixing and granulation process, adding a rice slurry aqueous solution (made by mixing rice and water, heating, crushing, and stirring to obtain a rice slurry aqueous solution, which is a type of starch solution and generally uses expired waste rice as raw material, thus recycling resources) can improve the granulation effect, resulting in better particle size distribution and stronger pellets. This solution can also provide some heat during subsequent roasting, further improving the high-temperature consolidation strength of the green pellets. During pelleting, adding a calcium hydroxide solution not only improves the strength of the green pellets but also adjusts their acidity, enhancing their physicochemical and metallurgical properties. In this invention, the calcium hydroxide aqueous solution is used as the pelleting raw material and mixed with the pelletizing material. During mixing, the calcium hydroxide aqueous solution adsorbs onto the surface of the pelletizing material particles, existing in the form of molecular water. This results in a uniform distribution of a large number of calcium ions on the surface of the pelletizing material particles, avoiding the situation in conventional pelletizing materials where the calcium component is mainly in the calcium hydroxide particles, and there are virtually no calcium ions in the molecular water on the surface of the pelletizing material particles. Molecular water does not flow on the surface of the particles. During the process of adding water in pelletizing, water is adsorbed on the surface of the pelletizing material particles and migrates between the particles. When the calcium ions in the molecular water that are uniformly dispersed on the surface of the pelletizing material particles come into contact with the carbonate ions in the water, the calcium ions and carbonate ions undergo a carbonation reaction to generate calcium carbonate. During the precipitation of the newly formed calcium carbonate, the pelletizing material particles that are in contact with each other will be bonded together, which will increase the carbonation and consolidation rate of the green pellets and the strength of the green pellets.

[0053] In this invention, fine phosphate rock powder and biomass straw are mixed, granulated into small balls, and then made into large balls. The moisture content of the small balls and the large balls (outer layer) are different. By adopting layered pelletizing, the outer layer of the pellets has high moisture content and the inner layer has low moisture content, which reduces the vapor pressure in the later stage of the pellet drying time and avoids the pellets from bursting in the later stage of drying. At the same time, it also reduces the overall moisture content of the pellets and saves drying energy consumption.

[0054] It should be noted that in this invention, the sphericity index K is a comprehensive parameter that reflects the particle size, particle size distribution, and specific surface area of ​​the material. Its calculation formula is as follows:

[0055] W 分 The percentage of the maximum molecular weight of water, %; W 毛 This represents the maximum capillary water mass percentage, %.

[0056] In this invention, the process does not require the addition of a special binder, and P2O5 is not lost during roasting. After removing impurities such as carbonates, the P2O5 content can be increased by approximately 1.5%, thus improving the phosphorus grade of the pellets and lumps. This invention yields a large quantity of phosphorus-containing roasted pellets with a particle size of 18-30 mm, exhibiting a concentrated and stable particle size distribution and good permeability. Under the high-temperature reducing atmosphere of subsequent yellow phosphorus preparation, the pulverization rate is low, significantly reducing dust generation during phosphorus production. Furthermore, the phosphorus-containing roasted pellets have high grade, good chemical composition, and high strength, facilitating subsequent transportation and improving transportability, thereby reducing transportation costs. In addition, their low moisture content and low carbonate content effectively reduce power consumption in subsequent lump ore phosphorus production, improving economic efficiency.

[0057] In this invention, the process of obtaining finished phosphate ore lumps from phosphorus-containing green pellets through sintering requires transportation and heat treatment, with roasting temperatures exceeding 1100℃. This necessitates that the phosphorus-containing green pellets possess sufficient strength to prevent excessive breakage during transportation and significant bursting during heat treatment. Therefore, this invention incorporates biomass straw pellets into the feed formulation, distributing them throughout the phosphorus-containing green pellets. This provides diffusion channels for internal moisture evaporation during drying, significantly increasing the rate of water vapor escape. Furthermore, it effectively prevents the rapid evaporation of water molecules at high temperatures, which could lead to bursting of the phosphorus-containing green pellets. Simultaneously, the fibers of the biomass straw in the phosphorus-containing pellets enhance the adhesion between phosphate ore particles, thereby increasing the strength of the pellets. Additionally, the biomass straw provides heat during subsequent roasting, aiding in the high-temperature consolidation of the phosphorus-containing pellets and further improving their physical strength and chemical properties.

[0058] In this invention, to further improve the strength of phosphorus-containing green pellets, the biomass straw pellets are pretreated before mixing. Specifically, the biomass straw pellets are soaked in a calcium hydroxide solution of a certain concentration (e.g., 0.01-0.5 mol / L) for 0.1-5 hours. After soaking, the biomass straw pellets are filtered and dried before being added to the batch. Because the biomass straw adsorbs calcium hydroxide, it can improve its bonding performance with other materials during the batching process, increasing the strength of the phosphorus-containing green pellets and significantly reducing the breakage rate during handling. Simultaneously, during subsequent heat treatment, the biomass straw decomposes upon heating, releasing carbon dioxide and water. The released carbon dioxide, under the action of water vapor, reacts with the internal calcium hydroxide to form a compound that acts as a binder (the adsorbed calcium hydroxide solidifies internally as calcium carbonate), further improving the bonding strength between the biomass straw and other raw materials. This helps prevent high-temperature cracking while greatly ensuring and improving the strength of the finished phosphorus-containing cooked pellets and reducing the ore powder rate. It should be noted that the amount of biomass straw added should not be too much or too little. Too much straw will reduce the proportion of phosphate rock powder and quartz powder, thus reducing the yield. At the same time, too much straw particles will cause more large pores inside the finished phosphorus-containing pellets after heat treatment, which will easily lead to the collapse and pulverization of the finished phosphorus-containing pellets, which is not conducive to improving the strength of the finished phosphorus-containing pellets. On the other hand, if the amount added is too little, it will not be conducive to improving the internal bonding strength of the phosphorus-containing green pellets, and the green pellets will easily break apart before heat treatment.

[0059] In this invention, magnetite with special chemical composition is added during the batching process. This not only improves the strength of the pellets during roasting, but also produces high-reduced iron solid blocks during the production of phosphorus in the vertical shaft furnace. This also serves as a high-quality iron source for subsequent lithium iron phosphate production.

[0060] In this invention, the process does not require the addition of a special binder, and there is no loss of P2O5 during the calcination process. After removing impurities such as carbonates, the P2O5 content can be increased by about 1.5%, thereby improving the phosphorus grade of the phosphorus-containing cooked pellets.

[0061] In this invention, the drying process of the sintered material includes intermittent microwave drying (primary drying) and two-stage hot air drying (secondary and tertiary drying). Microwave heating is used first, allowing simultaneous heating of the green pellets' interior and exterior. This causes both internal and external moisture to vaporize and diffuse outwards simultaneously, preventing cracking due to uneven drying. Furthermore, the three-stage drying process of this invention is a variable-temperature drying method. The drying stages are divided into multiple sections arranged in a sequence from low to high temperature and from long to short time. Compared to conventional drying, this increases the drying speed, shortens the drying time, avoids material cracking during drying, and improves the strength of the material.

[0062] In this invention, during the sintering heat treatment process, the dust after the hot air is purged is collected. This dust is first humidified and then returned to the sintering mixing step to participate in the briquetting process.

[0063] In this invention, the phosphorus-containing cooked pellets obtained after sintering heat treatment also need to be screened to select finished phosphorus-containing cooked pellets that meet the requirements for yellow phosphorus production (generally requiring a particle size of not less than 5 mm, preferably not less than 8 mm). The bulk materials generated in the above screening steps are recycled and returned for batching, granulation and pelletizing, thereby saving resources and minimizing solid waste discharge.

[0064] In this invention, natural phosphate rock lumps (i.e., irregular lumps with a P2O5 content of approximately 18% to 20%) with a grade higher than that of low-grade phosphate rock powder are used as a base material. This mixing method can improve the average grade of the finished product lumps. At the same time, the metallurgical properties of the natural phosphate rock lumps are further improved after heat treatment. Using natural phosphate rock lumps as a base material increases permeability and protects the roasting equipment. By selecting an appropriate base material thickness, the production capacity is also increased.

[0065] In this invention, to reduce heat emissions and save energy, the hot air emitted from each node of the system is selectively recycled based on the characteristics of each operating condition within the system. Specifically, the hot air generated during cooling is recycled to provide heat for processes such as drying and preheating. Furthermore, waste heat is fully recovered through cascaded utilization, significantly reducing additional heating energy consumption. In addition, this invention also recycles and utilizes various bulk materials and dust generated during the process. The cascaded utilization of waste heat further significantly reduces additional heating energy consumption, achieving the recycling of valuable resources and contributing to environmental protection.

[0066] In this invention, phosphorus is produced in a vertical shaft furnace using coke as a reducing agent, resulting in low cost and good reducing properties. Water is used as a cooling medium to cool and precipitate yellow phosphorus from the flue gas, achieving low cost and good results. Furthermore, a portion of the yellow phosphorus flue gas is used to dry the wet phosphorus, allowing for recycling, reducing energy consumption, and improving the purity of the yellow phosphorus to obtain high-quality yellow phosphorus. This yellow phosphorus can be sold separately as a product and is also one of the raw materials for subsequent lithium iron phosphate production. In other words, this invention utilizes intermediates generated in its process as iron and phosphorus sources, and adds lithium and carbon sources, thus significantly reducing the cost of lithium iron phosphate production.

[0067] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0068] 1. This invention reuses phosphate rock powder from phosphate mining and processing, solving the problem of resource waste caused by the inability to reuse phosphate rock powder in existing phosphate mining and processing. It also achieves full utilization of phosphate rock, greatly alleviating the pressure on phosphate resource production and use, and solving the problem of land resource waste caused by the large-scale stockpiling of phosphate rock powder in existing phosphate mining and processing. At the same time, the use of vertical shaft furnaces for phosphorus production improves equipment resource utilization efficiency, avoids equipment resource waste, and produces high-quality yellow phosphorus products. Furthermore, it utilizes the intermediate products generated in the process to produce high-quality lithium iron phosphate products.

[0069] 2. The finished phosphorus-containing calcined pellets obtained by this invention have a concentrated and stable particle size distribution, good air permeability, and low pulverization rate under the high-temperature reducing atmosphere of subsequent yellow phosphorus preparation, which can greatly reduce the amount of dust in phosphorus production. Furthermore, these phosphorus-containing calcined pellets have high grade, good chemical composition, and high strength, facilitating subsequent transportation and exhibiting strong transportability. Their low moisture content and low carbonate content effectively reduce energy consumption in subsequent phosphorus production and improve the purity of the yellow phosphorus product, ensuring the excellent quality of the subsequent lithium iron phosphate product. Attached Figure Description

[0070] Figure 1 This is a schematic diagram of the overall process of the method described in this invention.

[0071] Figure 2 This is a flowchart illustrating the pretreatment of raw phosphate rock by the method described in this invention.

[0072] Figure 3 This is a flowchart of the mixed pelletizing, heat treatment, and reduction phosphorus production process described in this invention.

[0073] Figure 4 This is a flowchart illustrating the process of preparing lithium iron phosphate by sintering yellow phosphorus and other materials according to the method described in this invention. Detailed Implementation

[0074] The technical solution of the present invention will be illustrated below with examples. The scope of protection sought by the present invention includes, but is not limited to, the following embodiments.

[0075] Example 1

[0076] Open-pit phosphate mines are first blasted and then mined to obtain open-pit phosphate ore. Deep-buried phosphate mines are first drilled and then extracted using electric scrapers to obtain deep-buried phosphate ore. The open-pit and deep-buried phosphate ores are mixed at a mass ratio of 1.5:1 and then finely crushed to a particle size ≤30mm to obtain a raw ore mixture. This mixture is then washed three times with water at a liquid-to-solid mass ratio of 4:1, and dried at 210℃ until the moisture content is below 8wt%. After drying, it is screened to obtain raw ore lumps with a particle size of 20-30mm (collected for later use), large particles with a particle size between 5-20mm, and fine particles with a particle size ≤5mm (collected for later use). Further, the large particles of raw ore with a particle size between 5 and 20 mm are ground until the proportion of particles with a particle size ≤ 1 mm is greater than 97%, and then screened again to obtain raw ore coarse powder with a particle size > 1 mm and ≤ 5 mm (collected for later use), raw ore fine powder with a particle size ≥ 0.5 mm and ≤ 1 mm, and raw ore micro powder with a particle size < 0.5 mm (collected for later use).

[0077] First, the fine ore powder of the raw ore undergoes beneficiation treatment, using flotation agents to float phosphate minerals while gangue and other impurities sink, obtaining pre-enriched phosphate rock powder. Then, the pre-enriched phosphate rock is subjected to magnetic separation to remove magnetic minerals. Finally, the different colors of the phosphate rock and impurity minerals are used for identification with photoelectric elements, and compressed air jets are controlled to separate the phosphate rock from the impurity minerals, obtaining the final enriched phosphate rock powder. The final enriched phosphate rock powder is calcined at 520℃ to remove organic matter, carbon dioxide, and some fluorine. The calcined phosphate rock powder is then mixed with fine particles of the raw ore, coarse powder of the raw ore, micro powder of the raw ore, and waste phosphate rock powder, and ground until the particle size is less than 0.0074 mm, resulting in phosphate rock powder with a P2O5 content of approximately 14.16%.

[0078] Corn stalk pellets, quartz powder, and magnetite powder are all ground to a particle size of less than 0.0074 mm. Then, the phosphate rock powder, corn stalk pellet powder, quartz powder, and magnetite powder are mixed evenly in a mass ratio of 65:5:20:10 to obtain a mixture. Then, according to the requirement of a pelleting index of 0.85, a rice slurry aqueous solution with a concentration of 7 g / L is first added to the mixture for granulation treatment. After obtaining small balls with a particle size of about 4 mm, a calcium hydroxide solution with a concentration of 0.03 mol / L is added for pelletizing treatment to obtain phosphorus-containing green pellets with a particle size of about 25 mm (the overall moisture content is about 7.6 wt%, and the outer layer moisture content is about 9.6 wt%).

[0079] The raw ore lump (P2O5 content approximately 19.30%) was laid as the base material for the sintering machine, with a thickness of 65 mm. Then, a 150 mm thick layer of phosphorus-containing raw pellets was laid on top of the base material. After laying, the sintering material was first dried using microwave at 260℃ for 40 min, then dried with hot air at 400℃ for 30 min, and finally dried with hot air at 630℃ for 10 min. After drying, the sintering material was preheated with hot air at 850℃ for 15 min. After preheating, the sintering material was roasted at 1200℃ for 50 min using a mixture of natural gas and air. After roasting, the roasted clinker was cooled once to approximately 700℃ using room temperature air, and then further cooled a second time to approximately 200℃ using room temperature hot air. After cooling, the clinker was sieved, and clinker blocks ≥5 mm in size were collected as the finished phosphorus-containing roasted pellets (P2O5 content approximately 40.37%).

[0080] The finished phosphorus-containing calcined pellets and coke are mixed at a mass ratio of 85:15 and then added to a vertical shaft furnace. Combustion of a mixture of combustible gas and air provides heat for phosphorus production in the furnace. At 1400℃, the phosphorus in the calcined phosphate rock pellets is reduced using the reducing properties of coke at high temperatures. The phosphorus is released from the top of the furnace as yellow phosphorus-containing flue gas, while the remaining iron-containing solid slag containing calcium silicate is discharged from the bottom. The yellow phosphorus flue gas released from the top of the furnace is collected and cooled in a water tank. The yellow phosphorus in the flue gas precipitates as a solid underwater when cooled in the water, while soluble impurities in the flue gas dissolve in the water. Solid-liquid separation and filtration yield wet yellow phosphorus. A portion of the collected high-temperature yellow phosphorus flue gas is used to dry the wet yellow phosphorus, obtaining high-quality yellow phosphorus (purity 99.93%). The dried yellow phosphorus flue gas is then cooled again in the water tank, creating a continuous cycle.

[0081] First, yellow phosphorus is mixed with oxygen and burned to obtain high-purity P2O5 flue gas. This high-purity P2O5 flue gas is then hydrated and demisted to obtain phosphoric acid. Lithium carbonate is then added to the phosphoric acid and heated to a boiling reaction to obtain a mixed solution of lithium dihydrogen phosphate. This solution is then evaporated and crystallized to obtain lithium dihydrogen phosphate. The iron-containing solid slag obtained from the vertical shaft furnace is crushed and acid-dissolved with sulfuric acid. The solution is then filtered, and carbon dioxide is introduced into the filtrate to remove magnesium, calcium, and other metals, resulting in a high-concentration ferrous sulfate solution. Finally, this high-concentration ferrous sulfate solution is evaporated and crystallized to obtain solid ferrous sulfate.

[0082] Lithium dihydrogen phosphate and ferrous sulfate were weighed according to a stoichiometric ratio of Li:P:Fe = 1:1:1, and then 10 wt% (based on the total mass of lithium dihydrogen phosphate and ferrous sulfate) of coke was added and mixed. The mixture was then ground to a particle size ≤0.074 mm to obtain a powder mixture. The powder mixture was then spray-treated with distilled water to obtain a wetted powder mixture. Finally, a 6 t / cm...2 The wet mixture is compressed under pressure to obtain regular sheet-like mixture material. The mixture material is sintered at 700℃ for 16 hours under nitrogen protection, and then cooled to room temperature under nitrogen protection. Finally, the cooled material is crushed and demagnetized to obtain powdered lithium iron phosphate product (purity of 99.69%).

[0083] Example 2

[0084] Example 1 was repeated, except that the corn stalk pellets were modified stalk pellets obtained by soaking in a 0.02 mol / L calcium hydroxide solution for 1 hour and then filtering them off.

[0085] Example 3

[0086] Example 1 was repeated, except that the mixing mass ratio of phosphate rock powder, corn stalk pellet powder, quartz powder, and magnetite powder was 67.5:1:21:10.5.

[0087] Example 4

[0088] Example 1 was repeated, except that the mixing mass ratio of phosphate rock powder, corn stalk pellet powder, quartz powder, and magnetite powder was 62.5:10:19:8.5.

[0089] Example 5

[0090] Example 1 was repeated, except that the mixing mass ratio of phosphate rock powder, corn stalk pellet powder, quartz powder, and magnetite powder was 69.2:5.3:15:10.5.

[0091] Example 6

[0092] Example 1 was repeated, except that the mixing mass ratio of phosphate rock powder, corn stalk pellet powder, quartz powder, and magnetite powder was 56.5:4.5:30:9.

[0093] Example 7

[0094] Example 1 was repeated, except that the mixing mass ratio of phosphate rock powder, corn stalk granules powder, quartz powder, and magnetite powder was 68.7:5.3:21:5.

[0095] Example 8

[0096] Example 1 was repeated, except that the mixing mass ratio of phosphate rock powder, corn stalk pellet powder, quartz powder, and magnetite powder was 57.7:4.5:18:20.

[0097] Comparative Example 1

[0098] Repeat Example 1, except that corn stalk pellets are not added during the mixing and pelletizing process.

[0099] Comparative Example 2

[0100] Repeat Example 1, except that no quartz powder is added during the mixing and pelletizing process.

[0101] Comparative Example 3

[0102] Repeat Example 1, except that magnetite powder is not added during the mixing and pelletizing process.

[0103] Comparative Example 4

[0104] Using natural phosphate rock (P2O5 content of approximately 19.30%) as raw material, finished products containing phosphorus, yellow phosphorus, and lithium iron phosphate were obtained using the same process as in Example 1.

[0105] Result detection

[0106] The final phosphorus-containing pellets, yellow phosphorus, and lithium iron phosphate obtained from the above embodiments and comparative examples were subjected to various quality tests, and the test results are shown in the table below:

[0107] .

Claims

1. A method of preparing lithium iron phosphate, characterized by: The method comprises the following steps: firstly, pretreating phosphate ore raw ore to obtain phosphate ore powder; then, mixing the phosphate ore powder, biomass straw particles, quartz stone powder and magnetite powder to form pellets and performing heat treatment to obtain phosphorus-containing sintered pellets; then, performing reduction treatment on the phosphorus-containing sintered pellets and coke to obtain yellow phosphorus and iron-containing solid residue; finally, performing sintering treatment on the yellow phosphorus, lithium carbonate, iron-containing solid residue and coke to obtain lithium iron phosphate; the mixing mass ratio of the phosphate ore powder, biomass straw particles, quartz stone powder and magnetite powder is 60-75:4-7:18-25:8-15; The pretreatment of the phosphate ore raw ore comprises the following steps: A1: preliminary classification: mixing open phosphate ore raw ore and deep-buried phosphate ore raw ore, and then performing the following steps of refining, washing, drying and screening to obtain raw ore lump ore, large-particle raw ore and fine-particle raw ore; the particle size of the raw ore lump ore is 20-30 mm; the particle size of the large-particle raw ore is 5-20 mm; the particle size of the fine-particle raw ore is not more than 5 mm; A2: deep classification: performing grinding and classification on the large-particle raw ore to obtain raw ore coarse powder, raw ore fine powder and raw ore micro powder; the particle size of the raw ore coarse powder is >1 mm and ≤5 mm; the particle size of the raw ore fine powder is ≥0.5 mm and ≤1 mm; the particle size of the raw ore micro powder is <0.5 mm; the grinding is to grind the large-particle raw ore to a particle size of ≤1 mm, and the proportion is not less than 95%; A3: impurity removal and mixing: performing the following steps of flotation, magnetic separation, photoelectric separation and calcination on the raw ore fine powder, and then mixing the fine-particle raw ore, raw ore coarse powder, raw ore micro powder and waste phosphate ore powder to obtain phosphate ore powder.

2. The method of claim 1, wherein: In step A1, the refining is to crush the mixed ore composed of open phosphate ore raw ore and deep-buried phosphate ore raw ore to a particle size of ≤30 mm; the washing is to wash the refined ore mixture with water for multiple times, and the liquid-solid mass ratio during single washing is 1-6:1; the drying is to dry the washed ore mixture at a temperature of 110-300 ℃ to a water content of <10 wt%; and / or In step A3, the waste phosphate ore powder comprises phosphate ore powder generated in the mining process of open phosphate ore raw ore and deep-buried phosphate ore raw ore, and phosphate ore powder generated in the reprocessing of open phosphate ore raw ore and deep-buried phosphate ore raw ore.

3. The method of claim 1, wherein: The temperature of the calcination treatment is 400-600 ℃; the content of P2O5 in the phosphate ore powder is 10-15%.

4. The method of claim 1, wherein: The mixing and pelletizing and heat treatment comprise the following steps: B1: mixing and pelletizing: mixing the phosphate ore powder, biomass straw particles, quartz stone powder and magnetite powder to obtain a mixed material, then adding rice slurry solution to the mixed material to perform preliminary granulation according to the set pelletizing index requirement, and then adding calcium hydroxide solution to the mixed material to perform pelletizing, to obtain phosphorus-containing green pellets; B2: heat treatment: performing the following steps of drying, preheating, roasting and cooling on the phosphorus-containing green pellets to obtain phosphorus-containing sintered pellets.

5. The method of claim 4, wherein: In step B1, the mixed mass ratio of the phosphate ore powder, the biomass straw particle, the quartz stone powder and the magnetite powder is 65-73:5-6:20-23:10-13; and / or The balling index is greater than 0.8; the balling index is the ratio of the maximum molecular water mass to the maximum capillary water excess value, wherein the maximum capillary water excess value refers to the difference between the maximum capillary water mass and the maximum molecular water mass; and / or The concentration of the rice slurry aqueous solution is 1-9 g / L; the concentration of the calcium hydroxide solution is 0.01-0.08 mol / L; the adding amount of the rice slurry aqueous solution and the calcium hydroxide solution is such that the overall moisture content of the phosphorus-containing green ball is 6.5-8.5 wt%, and the moisture content of the outer layer of the phosphorus-containing green ball is 9-10 wt%.

6. The method of claim 5, wherein: The balling index is 0.8-0.9; and / or The particle size of the balling mixture is ≤0.074 mm; the particle size of the material after the preliminary granulation is 2-9 mm; and the particle size of the phosphorus-containing green ball is 12-30 mm.

7. The method of claim 4, wherein: In step B2, the drying is specifically: the material composed of the natural phosphate ore lump ore and the phosphorus-containing green ball is first dried at a temperature of 200-300 ℃ for 30-50 min by using a microwave, then dried at a temperature of 350-500 ℃ for 15-45 min by using hot air, and finally dried at a temperature of 500-700 ℃ for 8-20 min by using hot air; the preheating is specifically: the dried material is preheated at a temperature of 750-950 ℃ for 5-20 min; the roasting is specifically: the preheated material is roasted at a temperature of 1100-1350 ℃ for 0.2-5 h; and the cooling is specifically: the roasted material is first cooled to 600-800 ℃ by using room temperature air; and then further cooled to 150-250 ℃ by using room temperature air.

8. The method of claim 7, wherein: The content of P2O5 in the natural phosphate ore lump ore is not less than 18%; the thickness of the bedding material is 50-70 mm; the thickness of the phosphorus-containing green ball layer is 100-160 mm; and the particle size of the phosphorus-containing sintered ball is not less than 5 mm.

9. The method of claim 8, wherein: The hot air generated by the second cooling is recycled as the hot air for the second drying; the hot air generated by the first cooling is recycled as the hot air for the preheating; the hot air generated by the roasting is treated by dust removal and then used as the hot air for the third drying; and the hot air generated by the third drying and the preheating is treated by dust removal and desulfurization and denitrification and then discharged.

10. The method of claim 1, wherein: The reduction treatment includes: C1: batching: mixing the phosphorus-containing sintered ball with coke to obtain a to-be-reduced mixture; C2: reduction: adding the to-be-reduced mixture into a shaft furnace for reduction treatment under the action of a high-temperature gas flow generated by combustion of combustible gas, to obtain phosphorus-containing flue gas and iron-containing solid slag; C3: phosphorus precipitation: passing the phosphorus-containing flue gas into water for cooling and precipitating a fixed deposit, and after solid-liquid separation, drying the fixed deposit to obtain yellow phosphorus.

11. The method of claim 10, wherein: In step C1, the mixed mass ratio of the phosphorus-containing sintered ball and the coke is 75-90:10-25; and / or In step C2, the temperature of the reduction treatment is 1300-1500 ℃; and the time of the reduction treatment is 5-9 h; and / or In step C3, the drying is hot air drying, and the phosphorus-containing flue gas after the drying is passed into water for cooling.

12. The method of claim 11, wherein: In step C1, the mass ratio of the phosphorus-containing matured pellet to the coke is 80-88:12-20; and / or In step C2, the temperature of the reduction treatment is 1350-1450℃; the time of the reduction treatment is 7-8h; and / or In step C3, the drying is performed by using the phosphorus-containing flue gas as a heat source.

13. The method of claim 1, wherein: The sintering treatment comprises: D1: pretreatment: obtaining phosphoric acid by sequentially oxidizing and hydrolyzing yellow phosphorus, and then obtaining lithium dihydrogen phosphate by mixing and reacting the phosphoric acid with lithium carbonate; obtaining a ferrous salt by acid-dissolving and impurity-removing the iron-containing solid residue; D2: tabletting: mixing the lithium dihydrogen phosphate, the ferrous salt and the coke, and then sequentially grinding, wetting and tabletting to obtain mixed material tablets; D3: sintering: sintering the mixed material tablets under the protection of a nitrogen atmosphere to obtain sintered material; crushing the sintered material and removing the magnetism to obtain lithium iron phosphate.

14. The method of claim 13, wherein: In step D1, the oxidation is mixing and combusting yellow phosphorus with oxygen; the hydrolysis is mist-removing treatment after hydrating the P2O5 flue gas generated by combusting yellow phosphorus with oxygen; the mixing reaction is evaporation crystallization treatment after mixing and heating phosphoric acid with lithium carbonate to react; the impurity-removing treatment is hard-removing and evaporation crystallization treatment of the dissolved solution by precipitation after dissolving the iron-containing solid residue with sulfuric acid; and / or In step D2, the mixed amount of lithium dihydrogen phosphate and ferrous salt is such that the molar ratio of Li, P and Fe is consistent, and the added amount of coke is 8-15% of the total mass of lithium dihydrogen phosphate and ferrous salt; the grinding is to grind the mixture to a particle size of ≤0.074 mm; the wetting is such that the moisture content of the mixture is 1-5 wt%; the tabletting is to press the mixture into a regular sheet structure using a pressure of 3-7 t / cm 2 ; and / or In step D3, the temperature of the sintering is 600-900℃; the time of the sintering is 10-21h; the crushing is crushing the sintered material to a particle size of 3-10um; and the removing the magnetism is magnetic attraction treatment by using a magnet.

15. The method of claim 14, wherein: In step D3, the temperature of the sintering is 650-800℃; the time of the sintering is 13-18h.

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