Method for preparing ferrous phosphate octahydrate by taking pyrite cinder as raw material

Abstract

The invention relates to the technical field of industrial solid waste resource utilization, in particular to a method for preparing ferrous phosphate octahydrate by taking pyrite cinder as a raw material, which comprises the following steps: 1) taking pyrite cinder as a raw material, and carrying out ball milling to obtain fine powder; 2) rinsing the fine powder with dilute acid and water respectively to obtain red slag; 3) mixing the red slag with a reducing agent, and calcining to obtain reducing slag; (4) under the protection of inert gas, leaching the reducing slag with sulfuric acid, filtering to obtain a leaching solution, and adjusting the iron ion concentration and the iron-phosphorus ratio to obtain an iron-phosphorus solution; (5) under the protection of inert gas, dilute alkali is added into the iron-phosphorus solution to adjust the pH value, filtering is performed after full reaction, and an iron-phosphorus purified solution is obtained; (6) under the protection of inert gas, dilute alkali is added into the iron-phosphorus purified liquid to adjust the pH value, filtering is performed after full reaction, and a filter cake is obtained; and 7) washing the filter cake, and carrying out vacuum drying to obtain ferrous phosphate octahydrate. The problems of environmental pollution and serious resource waste caused by difficult recycling of pyrite cinder can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial solid waste resource utilization, and particularly to a method for preparing ferrous phosphate octahydrate using pyrite cinder as a raw material. Background Art

[0002] Lithium iron phosphate batteries are widely used in the field of new energy vehicles, and have the advantages of good cycle performance, good thermal stability, wide raw material sources, low price, environmental friendliness and no pollution. Their installed capacity has gradually exceeded that of ternary batteries. At present, lithium iron phosphate cathode materials are usually prepared by a high-temperature carbothermal reduction method using ferric phosphate and lithium carbonate as raw materials. This preparation method requires the complete reduction of trivalent iron in the raw materials to divalent iron, with a high required sintering temperature, long time and prone to side reactions during the reaction process; at the same time, ferric phosphate also needs to be synthesized through steps such as reaction, aging, drying and calcination from a phosphorus source and by-product ferrous sulfate heptahydrate of titanium white slag, and the process stability is difficult to control, which is not conducive to ensuring the high quality of lithium iron phosphate products.

[0003] As another precursor for preparing lithium iron phosphate, ferrous phosphate octahydrate has the difference from the synthesis of lithium iron phosphate with ferric phosphate that there is no reduction process of trivalent iron in the high-temperature solid-phase reaction and fewer side reactions. While shortening the process, it also reduces costs, and the product quality is stable and reliable. In addition, ferrous phosphate octahydrate does not produce harmful by-products during the reaction process, meeting the current requirements for green chemistry and sustainable development.

[0004] Pyrite cinder is the waste residue produced during the production of sulfuric acid from pyrite. China is a major sulfuric acid producer, and 0.8 - 1.3t of cinder is produced for every 1t of sulfuric acid produced. The composition of pyrite cinder is complex. In addition to containing main components such as Fe2O3 and SiO2, it also contains elements such as S, Pb, Cu, K, Al, Ca, etc., and it is difficult to recycle; but if directly discarded, on the one hand, it will cause environmental pollution, and on the other hand, it will cause serious waste of resources. Therefore, realizing the comprehensive recycling with high added value of cinder is the only way to solve the problems of occupying land and polluting the environment.

[0005] Chinese Patent with the application publication number CN1114638A discloses a process for producing ferrous sulfate by sulfuric acid leaching of pyrite cinder. The raw material cinder is successively subjected to the following steps: sulfuric acid leaching → filtration → reduction of the leaching solution → filtration to obtain a reduced solution, and then ferrous sulfate products are obtained from the reduced solution. In this technical solution, sulfuric acid is directly reacted with pyrite cinder. Due to the limited reactivity of trivalent iron, only a small amount of Fe2O3 can be dissolved in sulfuric acid, and the leaching and utilization efficiency of iron is low. Moreover, the cinder leaching solution needs to be reduced with a large amount of iron powder before impurity removal, increasing the production cost.

[0006] A Chinese patent with the application publication number CN114684801A discloses a method for preparing high-purity iron phosphate from pyrite cinder, which includes the following steps: (1) cleaning, drying, and pulverizing the pyrite cinder to remove soluble components in the pyrite cinder; (2) mixing the pyrite cinder with a carbon source and calcining it under an inert atmosphere; (3) mixing the calcined pyrite cinder with dilute phosphoric acid for reaction to obtain an acid leaching solution, and filtering the acid leaching solution to obtain a filtrate and a filter residue; (4) adjusting the pH value of the filtrate, adding a flocculant to the filtrate to purify the filtrate; adding hydrogen peroxide to the purified filtrate; washing and drying the obtained product to obtain iron phosphate. In this technical solution, glucose or sucrose is used as a reducing agent, the calcination temperature is above 700°C, the reduction time is long, and the energy consumption is high. At the same time, because phosphoric acid is weakly acidic, the use of phosphoric acid leaching requires a relatively high temperature for the reaction system, and the leaching efficiency is limited, which is not conducive to cost reduction and efficiency improvement.

[0007] A Chinese patent with the application publication number CN118702079A discloses a method for preparing iron phosphate octahydrate, which includes the following steps: 1) preparing a ferrous sulfate solution and a phosphorus source solution, protecting with an inert gas, adding the ferrous sulfate solution and the phosphorus source solution to a surfactant bottom solution, stirring and dropping an alkali solution until the pH of the reaction system is 2.5 - 4.0, stirring the first-stage reaction and filtering to obtain a filter cake; 2) re-slurrying the filter cake according to the original solid content to obtain a reaction solution, heating to 40 - 50°C under the protection of an inert gas and then dropping an alkali solution to adjust the pH value of the solution to 4.5 - 6.0, stirring the second-stage reaction, filtering, washing, and drying to obtain iron phosphate octahydrate. In this technical solution, the prepared iron phosphate octahydrate product contains more impurities and is difficult to meet the quality standards, and further impurity removal is required. During the impurity removal process, the crystal structure of iron phosphate octahydrate is likely to change, affecting its crystallinity and stability, and even causing a change in the iron-phosphorus ratio, affecting the product performance. At the same time, the added surfactant increases the difficulty of wastewater treatment and the post-treatment cost is high.

[0008] In summary, finding a new process method to fully utilize the iron resources in pyrite cinder to prepare high-purity iron phosphate octahydrate material is one of the directions for realizing the high-value reuse of cinder. Summary of the Invention

[0009] The present invention provides a method for preparing iron phosphate octahydrate from pyrite cinder, which can solve the problems of difficult reuse of pyrite cinder, environmental pollution, and serious waste of resources.

[0010] The present application provides the following technical solutions:

[0011] A method for preparing iron phosphate octahydrate from pyrite cinder, which includes the following steps:

[0012] (1) Using pyrite cinder as raw material, ball milling and separation are carried out to obtain fine powder;

[0013] (2) The fine powder is leached with dilute acid and water respectively to obtain red residue;

[0014] (3) The red residue is mixed with a reducing agent and calcined under the protection of inert gas to obtain reduced residue;

[0015] (4) Under the protection of inert gas, the reduced residue is leached with sulfuric acid, filtered to obtain leaching solution, the iron ion concentration is adjusted by adding water, and the iron to phosphorus ratio is adjusted by adding phosphoric acid to obtain iron-phosphorus solution;

[0016] (5) Under the protection of inert gas, dilute alkali is added to the iron-phosphorus solution to adjust the pH to 2.8 - 3.6, and after sufficient reaction, it is filtered to obtain iron-phosphorus purification solution;

[0017] (6) Under the protection of inert gas, dilute alkali is added to the iron-phosphorus purification solution to adjust the pH to 4.0 - 5.5, and after sufficient reaction, it is filtered to obtain filter cake;

[0018] (7) The filter cake is washed and vacuum dried to obtain ferrous phosphate octahydrate.

[0019] A ferrous phosphate octahydrate prepared from pyrite cinder as raw material and its application in the preparation of lithium iron phosphate.

[0020] Beneficial effects:

[0021] 1) Convert low-value pyrite cinder into higher-value ferrous phosphate octahydrate, the precursor of the cathode material for lithium batteries, realizing the comprehensive utilization of industrial solid waste and solving problems such as the large amount of space occupied by the accumulation of pyrite cinder and its difficult treatment. Using pyrite cinder as raw material makes full use of industrial waste, reduces resource waste, reduces its potential environmental pollution, and reduces the risk of heavy metals infiltrating into the soil and water bodies.

[0022] 2) The main available component in pyrite cinder is Fe2O3, and the impurities include Si, S, Ca, Al, Mg, Mn, Na, etc. In addition to impurity removal, the main problem is that the Fe2O3 contained in the red residue is mainly in the spinel inverse phase structure, which + has poor reaction activity, so it is difficult to directly react with acid to obtain crude iron ion solution. In this technical solution, a reducing agent is added for high-temperature calcination to first reduce the Fe 3+ in pyrite cinder to Fe 2+ , and then the activation energy that the system can provide is increased through high temperature and high H + concentration, etc. to achieve efficient leaching. Compared with direct acid leaching using concentrated sulfuric acid, the iron leaching rate is increased from 40% to over 95%; at the same time, it avoids using a large amount of iron powder to reduce Fe 3+ in the leaching solution, which is beneficial to cost saving.

[0023] 3) The purity of ferrous phosphate octahydrate is ensured by means of ball milling separation, pickling, water washing and precise pH adjustment. After ball milling and sieving, large pieces of SiO2 impurities without reaction activity are separated. At the same time, Fe2O3 ground into fine powder can also participate in the reaction more fully. Pickling and water washing are beneficial to removing impurities such as Mg, S, Mn, etc. in the calcined cinder. Precisely adjusting the pH of the leaching solution of the reduced cinder is beneficial to reducing the Al content in the product and reducing the losses of Fe and P in the impurity removal process.

[0024] 4) An inert gas is introduced into the reaction vessel during the reaction process for protection to ensure a slightly positive pressure environment in the vessel, reduce the oxygen content in the reaction environment, and reduce the oxidation of Fe during the reaction 2+ to ensure the stability of the product and help improve the product purity.

[0025] Preferably, the zircon beads are used for ball milling in step (1), the mass of the zircon beads is 30 - 40% of the mass of the pyrite cinder, the ball milling time is 30 - 60 min, and the ball milling speed is 450 - 550 r / min; the separation in step (1) is carried out by using a sieve with 50 - 150 meshes.

[0026] The pyrite cinder is pulverized by the ball milling process, which improves the efficiency of subsequent leaching and reaction, ensures the full dissolution of components and the effective removal of impurities. At the same time, the components in the pyrite cinder have different hardnesses and different particle sizes after ball milling. After separation by the sieve, large pieces of SiO2 impurities without reaction activity can be effectively removed. Moreover, the proportion of SiO2 impurities in the pyrite cinder is relatively large. Through ball milling and separation, a large amount of impurities can be removed in the initial stage, simplifying the subsequent impurity removal process, reducing the usage amount of subsequent reagents, and thus reducing the cost.

[0027] Preferably, the dilute acid in step (2) is one or more of sulfuric acid, phosphoric acid and oxalic acid; the concentration of the dilute acid is 2 - 10% wt, the dosage of the dilute acid is 1 - 3 times the mass of the fine powder, and the dosage of water is 2 - 5 times the mass of the fine powder.

[0028] By leaching with dilute acid and water respectively, impurities (such as Al, Mn, Mg, etc.) in the red cinder are effectively removed, making the iron ion concentration purer and improving the quality of the iron - phosphorus solution.

[0029] Preferably, the reducing agent in step (3) is one or more of oxalic acid, citric acid or ascorbic acid; the dosage of the reducing agent is added according to the molar ratio of carbon element in the reducing agent to Fe2O3 in the pyrite cinder raw material being 1.2 - 2:1; the calcination temperature is 500 - 600 °C, the calcination time is 120 - 180 min; the inert gas is N2.

[0030] The selection and proportion control of the reducing agent enable the effective reduction of iron oxide to ferrous ions, improving the utilization rate of iron. Compared with using carbon sources such as glucose and sucrose, using oxalic acid, citric acid, etc. as reducing agents can lower the calcination reduction temperature to below 600 °C, improve the calcination efficiency, and reduce energy consumption.

[0031] Preferably, in step (4), the concentration of sulfuric acid is 10% - 40% wt, sulfuric acid and the reduced slag are mixed according to the molar ratio of H2SO4 to Fe of 1 - 2:1, the acid leaching temperature is 65 - 90 °C, and the acid leaching time is 1 - 3 h; water is added to adjust the concentration of ferrous ions in the leaching solution to 4% - 6%; the concentration of phosphoric acid is 80 - 90% wt, and the iron - phosphorus ratio is adjusted to 0.95 - 1.50.

[0032] Sulfuric acid ensures sufficient acid strength to dissolve iron oxide and provides appropriate thermal energy, making it easier for iron ions to be released into the solution, accelerating the reaction rate, and shortening the reaction time. By precisely adjusting the concentration of ferrous ions and the iron - phosphorus ratio, not only the purity and stability of the final product, ferrous phosphate octahydrate, are improved, but also its electrochemical performance is significantly enhanced.

[0033] Preferably, the dilute base in step (5) is one or more of dilute ammonia water, (NH4)2CO3, and NH4HCO3 solutions, the concentration of the dilute base is 5% - 15%; the reaction temperature is 40 - 70 °C, the time is 60 - 120 min; the pH at the end of the reaction is 2.8 - 3.6.

[0034] Adjusting the pH value to 2.8 - 3.6 ensures that impurity ions (such as Al3+, Mn2+, etc.) in the iron - phosphorus solution precipitate out, improving the purity of the solution.

[0035] Preferably, the dilute base in step (6) is one or more of dilute ammonia water, (NH4)2CO3, and NH4HCO3 solutions, the concentration of the dilute base is 5% - 15%; the reaction temperature is 40 - 70 °C, the time is 90 - 150 min; the pH at the end of the reaction is 4.0 - 5.5.

[0036] Adjusting the pH value to 4.0 - 5.5 effectively promotes the precipitation of ferrous phosphate octahydrate, while ensuring that the Fe content, iron - phosphorus ratio, etc. meet the quality requirements.

[0037] Preferably, in step (7), the vacuum drying temperature is 40 - 60 °C, the drying time is 8 - 12 h, and the relative vacuum degree is - 100 - - 80 KPa.

[0038] Vacuum drying can efficiently remove moisture, protect the structure and quality of the product, improve production efficiency, and save energy. Description of the Drawings

[0039] Figure 1Process flow diagram of a method for preparing ferrous phosphate octahydrate from pyrite cinder according to the present invention.

[0040] Figure 2 XRD pattern of the ferrous phosphate octahydrate prepared in Example 2 of the present invention. Detailed description of specific embodiments

[0041] The following is a further detailed description through specific embodiments:

[0042] A method for preparing ferrous phosphate octahydrate from pyrite cinder includes the following steps:

[0043] (1) Using pyrite cinder as the raw material, weigh the pyrite cinder raw material and place it in a planetary ball mill for ball milling. Separate it with a 50-150 mesh sieve to obtain the lumps on the sieve (mainly containing SiO2) and the fine powder under the sieve (mainly containing Fe2O3). Zirconium beads are used for ball milling. The mass of the zirconium beads is 30-40% of the mass of the pyrite cinder. The diameter of the large zirconium beads is 5 mm, and the diameter of the small zirconium beads is 1 mm. The mass ratio of large balls to small balls in the grinding medium is 3-4:7. The ball milling time is 30-60 min, and the ball milling speed is 450-550 r / min.

[0044] (2) Wash the fine powder obtained in step (1) with dilute acid and water respectively to obtain red slag. The dilute acid is one or more of sulfuric acid, phosphoric acid or oxalic acid. The concentration of the dilute acid is 2-10% wt, the amount of the dilute acid is 1-3 times the mass of the fine powder, the amount of water is 2-5 times the mass of the fine powder, the leaching temperature is 60-80 °C, and the leaching time is 2-3 h.

[0045] (3) Mix the red slag in step (2) with a reducing agent and calcine it in a tubular atmosphere furnace under the protection of an inert gas to obtain reduced slag. The reducing agent is one or more of oxalic acid, citric acid or ascorbic acid. The amount of the reducing agent is added according to the molar ratio of carbon in the reducing agent to Fe2O3 in the pyrite cinder raw material of 1.2-2:1. The calcination temperature is 500-600 °C, and the calcination time is 120-180 min; the inert gas is N2.

[0046] (4) Under the protection of an inert gas, leach the reduced slag in step (3) with sulfuric acid, filter to obtain a leachate, add water to adjust the Fe ion concentration, and add phosphoric acid to adjust the iron-phosphorus ratio to obtain an iron-phosphorus solution. The mixing ratio of sulfuric acid and the reduced slag is: the molar ratio of H2SO4 to Fe is 1-2:1, and the concentration of sulfuric acid is 10%-40% wt; the acid leaching temperature is 65-90 °C, and the acid leaching time is 1-3 h. Add pure water to adjust the concentration of ferrous ions in the leachate to 4%-6%. The concentration of phosphoric acid is 80-90% wt, and the iron-phosphorus ratio is adjusted to 0.95-1.50.

[0047] (5) Under the protection of inert gas, dilute alkali is added to the iron phosphate solution obtained in step (4) to adjust the pH to 2.8 - 3.6. After sufficient reaction, filtration is carried out to obtain a purified iron phosphate solution. The dilute alkali is one of dilute ammonia water, (NH4)2CO3 or NH4HCO3 solution, and the concentration of the dilute alkali is 5% - 15%; the reaction temperature is 40 - 70 °C, the time is 60 - 120 min; the pH at the end point of the reaction is 2.8 - 3.6.

[0048] (6) Under the protection of inert gas, dilute alkali is added to the purified iron phosphate solution obtained in step (5) to adjust the pH to 4.0 - 5.5. After sufficient reaction, filtration is carried out to obtain a filter cake. The dilute alkali is one or several of dilute ammonia water, (NH4)2CO3 or NH4HCO3 solution, and the concentration of the dilute alkali is 5% - 15%; the reaction temperature is 40 - 70 °C, the time is 90 - 150 min; the pH at the end point of the reaction is 4.0 - 5.5.

[0049] (7) The filter cake is washed and vacuum dried to obtain iron phosphate octahydrate. The vacuum drying temperature is 40 - 60 °C, the drying time is 8 - 12 h, and the relative vacuum degree is - 100 - - 80 KPa.

[0050] In this example, the main components (dry basis) of the pyrite cinder raw material used are: Fe2O3 68.70%, SiO2 14.51%, SO3 6.14%, CaO 2.38%, Al2O3 2.35%, MgO 0.75% and about 5.17% of other impurities. The specific contents are shown in Table 1. The phosphoric acid is industrial wet - purified phosphoric acid, and the phosphoric acid concentration is 85.32% wt. The purity of the carbon source and other raw materials are all of analytical purity, and the process water is pure water.

[0051] Table 1 Main substances, impurities and their contents in pyrite cinder

[0052] Name <![CDATA[Fe2O3]]> <![CDATA[SiO2]]> <![CDATA[SO3]]> CaO <![CDATA[Al2O3]]> MgO Content, % 68.70 14.51 6.14 2.38 2.35 0.75 Name <![CDATA[K2O]]> MnO ZnO <![CDATA[TiO2]]> <![CDATA[Na2O]]> Others Content, % 0.47 0.29 0.17 0.17 0.20 12.04

[0053] Example 1

[0054] (1) Take 100.64 g of pyrite cinder and place it in a ball - milling tank. Add about 33.73 g of zirconium beads (11.52 g of large balls and 22.21 g of small balls) according to 33% of the raw material mass. Ball - mill at a rotation speed of 500 r / min for 45 min, and then carry out ball - material separation with an 80 - mesh sieve. The mass of the fine powder under the sieve is 83.17 g.

[0055] (2) Place the sifted fine powder in a 500 mL beaker, add 252.12 g (liquid-solid mass ratio of 3:1) of 5% wt dilute sulfuric acid, stir for 2 h at 60 °C and then filter to obtain 273.96 g of pickling solution (Al content 369.41 ppm, Mn content 52.44 ppm, Mg content 81.68 ppm) and pickling filter residue; then place the pickling filter residue in a 500 mL beaker, add 250.47 g of pure water (liquid-solid mass ratio of 3:1), stir for 2 h at 60 °C and then filter to obtain 241.08 g of washing solution (Al content 69.41 ppm, Mn content 25.66 ppm, Mg content 50.30 ppm) and 108.30 g of wet residue. After drying, 79.81 g of pretreated red residue is obtained.

[0056] (3) Add 24.68 g of oxalic acid to the red residue (the stoichiometric ratio of Fe2O3:C is 1:1.4), mix evenly and then place it in a tubular atmosphere furnace. Set the N2 inlet rate to 15 L / h and the heating rate to 5 °C / min. Calcinate at 550 °C for 180 min and then cool naturally to obtain 76.48 g of reduced residue.

[0057] (4) Place the reduced residue in a three-necked flask, introduce N2 for protection, add 380.28 g of sulfuric acid with a mass fraction of 25% (the molar ratio of H2SO4 to Fe is 1.1:1), set the reaction temperature to 70 °C and the stirring paddle speed to 200 rpm. After reacting for 2 h, filter to obtain 484.97 g of leaching solution. After diluting with 306.04 g of pure water, add 74.61 g of purified phosphoric acid to the solution, adjust ω(Fe 2+ ) to 5.01% and the iron-phosphorus ratio to 1.2:1 (the Al content in the solution is 864.7 ppm) to obtain an iron-phosphorus solution. The mass of the acid leaching filter residue after drying is 4.07 g.

[0058] (5) Under N2 atmosphere, add 389.29 g of 10% wt dilute ammonia water to the iron-phosphorus solution, adjust the pH to 3.2, keep the reaction temperature at 55 °C during this period, control the reaction time to 90 min, and filter to obtain 1097.38 g of iron-phosphorus purified solution (Al content 25.41 ppm) and 105.44 g of purified wet filter residue.

[0059] (6) Under N2 atmosphere, continue to add 161.12 g of 10% wt dilute ammonia water to the iron-phosphorus purified solution, adjust the pH to 4.5, keep the reaction temperature at 55 °C during this period, control the reaction time to 100 min, and filter to obtain 406.12 g of filter cake and 856.70 g of mother liquor.

[0060] (7) The filter cake is vacuum dried after washing. The drying temperature is controlled at 50 °C and the drying time is 10 h. Finally, 105.97 g of ferrous phosphate octahydrate product is obtained and tested. The test results are shown in Table 2 below.

[0061] Example 2

[0062] (1) Take 100.51 g of pyrite cinder and place it in a ball mill tank. Add about 33.68 g of zirconium beads (11.47 g of large balls and 22.21 g of small balls) according to 33% of the raw material mass. Ball mill at a speed of 500 r / min for 60 min, and then separate the ball material with a 100-mesh sieve. The mass of the fine powder under the sieve is 79.55 g.

[0063] (2) Place the fine powder under the sieve in a 500 mL beaker, add 238.66 g (liquid-solid mass ratio of 3:1), 5% wt dilute sulfuric acid, stir at 70 °C for 2 h and then filter to obtain 244.44 g of pickling solution (Al content 308.56 ppm, Mn content 46.19 ppm, Mg content 70.42 ppm) and pickling filter residue; then place the pickling filter residue in a 500 mL beaker, add 319.63 g of pure water (liquid-solid mass ratio of 4:1), stir at 70 °C for 2 h and then filter to obtain 327.40 g of washing solution (Al content 45.38 ppm, Mn content 12.66 ppm, Mg content 36.87 ppm) and 99.52 g of wet residue. After drying, 74.02 g of pretreated red residue is obtained.

[0064] (3) Add 28.28 g of oxalic acid to the red residue (the stoichiometric ratio of Fe2O3:C is 1:1.6), mix evenly and place it in a tubular atmosphere furnace. Set the N2 inlet rate to 15 L / h and the heating rate to 5 °C / min. Calcinate at 600 °C for 180 min and then cool naturally to obtain 69.53 g of reduced residue;

[0065] (4) Place the reduced residue in a three-necked flask, introduce N2 for protection, add 414.86 g (the molar ratio of H2SO4 to Fe is 1.2:1) of 25% sulfuric acid, set the reaction temperature to 80 °C, the stirring paddle speed to 200 rpm, react for 2 h and then filter; the mass of the filtrate is 490.11 g. After diluting with 228.52 g of pure water, add 68.30 g of purified phosphoric acid to the solution, adjust ω(Fe 2+ ) to 5.52% and the iron-phosphorus ratio to 1.3:1 (the Al content in the solution is 927.6 ppm) to obtain an iron-phosphorus solution. The mass of the acid leaching filter residue after drying is 5.45 g.

[0066] (5) Under a nitrogen atmosphere, 365.04 g of 10% wt dilute ammonia water was added to the iron phosphate solution, and the pH of the system was adjusted to 3.4. During this period, the reaction temperature was maintained at 60 °C, the reaction time was controlled for 90 min. After filtration, the mass of the purified iron phosphate solution (Al content was 16.77 ppm) was 918.44 g, and the mass of the wet purified filter residue was 137.84 g;

[0067] (6) Under a nitrogen atmosphere, 137.04 g of 10% wt dilute ammonia water was continuously added to the purified iron phosphate solution, and the pH of the system was adjusted to 5.0. During this period, the reaction temperature was maintained at 60 °C, the reaction time was controlled for 100 min. After filtration, 367.03 g of filter cake and 788.00 g of mother liquor were obtained.

[0068] (7) After washing, the filter cake was dried under vacuum. The drying temperature was controlled at 50 °C and the drying time was 12 h. Finally, 98.42 g of ferrous phosphate octahydrate product was obtained and tested. The test results are shown in Table 2 below.

[0069] Example 3

[0070] (1) Take 100.07 g of pyrite cinder and place it in a ball mill tank. Add about 33.73 g of zirconium beads (11.52 g of large balls and 22.21 g of small balls) according to 33% of the raw material mass. Ball mill at a rotation speed of 520 r / min for 60 min, and then separate the ball material with a 100-mesh sieve. The mass of the fine powder under the sieve was 81.25 g.

[0071] (2) Place the fine powder under the sieve in a 500 mL beaker, add 162.89 g (liquid-solid mass ratio 2:1), 7% wt dilute sulfuric acid. Stir at 80 °C for 3 h and then filter to obtain 180.48 g of acid washing solution (Al content 428.55 ppm, Mn content 50.17 ppm, Mg content 86.37 ppm) and acid washing filter residue; then place the acid washing filter residue in a 500 mL beaker, add 326.18 g of pure water (liquid-solid mass ratio 4:1), stir at 80 °C for 3 h and then filter to obtain 337.94 g of water washing solution (Al content 15.61 ppm, Mn content 10.40 ppm, Mg content 20.18 ppm) and 114.62 g of wet residue. After drying, 77.71 g of pretreated red residue was obtained;

[0072] (3) Add 22.01 g of citric acid to the red residue (the stoichiometric ratio of Fe2O3:C is 1:1.6). After mixing evenly, place it in a tubular atmosphere furnace. Set the nitrogen inlet rate to 15 L / h and the heating rate to 5 °C / min. Calcinate at 570 °C for 150 min and then cool naturally to obtain 70.36 g of reduced residue.

[0073] (4) Place the reduced slag in a three-necked flask, introduce N2 for protection, add 374.52 g of sulfuric acid with a mass fraction of 30% (the molar ratio of H2SO4 to Fe is 1.3:1), set the reaction temperature to 85 °C, the stirring paddle speed to 200 rpm, and filter after reacting for 3 h; the mass of the filtrate is 471.89 g. After adding 425.92 g of pure water for dilution, add 63.47 g of purified phosphoric acid to the solution, and adjust ω(Fe 2+ ) in the system to 4.50% and the iron-phosphorus ratio to 1.4:1 (the Al content in the solution is 628.17 ppm) to obtain an iron-phosphorus solution. The mass of the acid-leaching filter residue after drying is 6.81 g.

[0074] (5) Under a N2 atmosphere, add 347.49 g of a 12% wt NH4HCO3 solution to the iron-phosphorus solution, adjust the pH of the system to 3.6, maintain the reaction temperature at 60 °C during this period, control the reaction time to 120 min, and filter to obtain an iron-phosphorus purification solution (with an Al content of 11.84 ppm) with a mass of 1185.55 g and a purified wet filter residue with a mass of 167.83 g.

[0075] (6) Under a N2 atmosphere, continue to add 128.69 g of a 12% wt NH4HCO3 solution to the iron-phosphorus purification solution, adjust the pH of the system to 5.5, maintain the reaction temperature at 60 °C during this period, control the reaction time to 120 min, and filter to obtain 425.38 g of filter cake and 804.60 g of mother liquor.

[0076] (7) After washing, the filter cake is dried under vacuum, controlling the drying temperature at 55 °C and the drying time at 12 h, and finally 93.84 g of ferrous phosphate octahydrate product is obtained, and it is tested. The test results are shown in Table 2 below.

[0077] Comparative Example 1

[0078] The difference between this comparative example and Example 1 is that in step (2), the fine powder under the sieve is not subjected to water washing and acid washing treatments. The contents of relevant elements in the finally obtained ferrous phosphate octahydrate product are shown in Table 2 below.

[0079] Comparative Example 2

[0080] The difference between this comparative example and Example 2 is that in step (5), the pH of the system is adjusted to 2.5. The contents of relevant elements in the finally obtained ferrous phosphate octahydrate product are shown in Table 2 below.

[0081] Comparative Example 3

[0082] The difference between this comparative example and Example 3 is that in step (6), the pH of the system is adjusted to 6.5. The contents of relevant elements in the finally obtained ferrous phosphate octahydrate product are shown in Table 2 below.

[0083] Comparison of the Contents of Relevant Elements in the Products Obtained in Each Example and Comparative Example in Table 2

[0084]

[0085]

[0086] As can be seen from Table 2, the contents of impurity elements in the products of each example can meet the quality requirements of downstream manufacturers, and the main content indexes such as Fe, P, Fe / P, Fe 3+ and other main content indexes such as particle size and crystal water meet the standards. Calculated based on the product, the recovery rate of Fe element is ≥85%, the loss rate during the impurity removal process is controllable, and the antioxidant measures have excellent effects. The XRD pattern of the synthetic product in Example 2 is completely consistent with the standard card of the octahydrate ferrous phosphate pattern, and the diffraction peaks of the pattern are sharp, indicating that the product has a good crystal form and high crystallinity. The impurity content of the product in Comparative Example 1 is generally high, indicating that increasing the water washing and acid washing processes of the raw material cinder has a significant effect on reducing product impurities; the impurity removal pH in Comparative Example 2 is too low, resulting in the content of impurities such as Al in the product exceeding the standard, while the pH of the ferrous phosphate precipitation in Comparative Example 3 is too high, resulting in a high Fe content in the product and impure phase. Therefore, controlling the pH within a suitable range is the key to obtaining the target product.

[0087] The above are only the embodiments of the present invention. The present invention is not limited to the fields involved in this embodiment case. Common knowledge such as the specific structures and characteristics known in the solution is not described in detail here. It should be noted that for those skilled in the art, without departing from the structure of the present invention, several deformations and improvements can still be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicability of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing ferrous phosphate octahydrate using pyrite slag as raw material, characterized in that: The steps include: (1) Using pyrite slag as raw material, ball milling, separation, and obtaining fine powder; (2) The fine powder is washed with dilute acid and water respectively to obtain red slag; (3) mixing the red slag with a reducing agent and calcining the mixture under the protection of an inert gas to obtain reduced slag; (4) under the protection of inert gas, leaching the reduced slag with sulfuric acid, filtering to obtain a leachate, adding water to adjust the iron ion concentration, adding phosphoric acid to adjust the iron-phosphorus ratio, and obtaining an iron-phosphorus solution; (5) Under the protection of inert gas, add dilute alkali to the iron-phosphorus solution to adjust the pH to 2.8-3.6, filter after sufficient reaction, and obtain an iron-phosphorus purified solution; (6) Under the protection of inert gas, add dilute alkali to the iron-phosphorus purification liquid to adjust the pH to 4.0-5.5, filter after sufficient reaction, and obtain a filter cake; (7) The filter cake is washed and vacuum dried to obtain ferrous phosphate octahydrate.

2. The method according to claim 1, characterized in that: The ball milling in step (1) uses zirconium beads, the mass of the zirconium beads is 30-40% of the mass of the pyrite slag, the ball milling time is 30-60 minutes, and the ball milling speed is 450-550r / min; the separation in step (1) is carried out using a 50-150 mesh sieve.

3. The method according to claim 1, characterized in that: The dilute acid in step (2) is one or more of sulfuric acid, phosphoric acid and oxalic acid; the concentration of the dilute acid is 2-10%wt, the amount of the dilute acid is 1-3 times the mass of the fine powder, and the amount of water is 2-5 times the mass of the fine powder.

4. The method according to claim 1, characterized in that: The reducing agent in step (3) is one or more of oxalic acid, citric acid or ascorbic acid; the reducing agent is added according to the molar ratio of carbon element in the reducing agent to Fe2O3 in the pyrite slag raw material of 1.2 to 2:1; the calcination temperature is 500 to 600°C, and the calcination time is 120 to 180 minutes; and N2 is used as the inert gas.

5. The method according to claim 1, characterized in that: In step (4), the concentration of sulfuric acid is 10% to 40%wt; sulfuric acid and reducing slag are mixed according to a molar ratio of H2SO4 to Fe of 1 to 2:1, the acid leaching temperature is 65 to 90°C, and the acid leaching time is 1 to 3h; water is added to adjust the concentration of ferrous ions in the leachate to 4% to 6%; the concentration of phosphoric acid is 80 to 90%wt, and the iron-phosphorus ratio is adjusted to 0.95 to 1.

50.

6. The method according to claim 1, characterized in that: The dilute alkali in step (5) is one or more of dilute ammonia water, (NH4)2CO3 and NH4HCO3 solution, and the concentration of the dilute alkali is 5% to 15%; the reaction temperature is 40 to 70°C and the reaction time is 60 to 120 min; the reaction endpoint pH is 2.8 to 3.

6.

7. The method according to claim 1, characterized in that: In step (6), the dilute alkali is one or more of dilute ammonia water, (NH4)2CO3 and NH4HCO3 solution, and the concentration of the dilute alkali is 5% to 15%; the reaction temperature is 40 to 70°C, and the reaction time is 90 to 150 min; the reaction endpoint pH is 4.0 to 5.

5.

8. The method according to claim 1, characterized in that: In step (7), the vacuum drying temperature is 40 to 60° C., the drying time is 8 to 12 hours, and the relative vacuum degree is -100 to -80 KPa.

9. Ferrous phosphate octahydrate prepared by the method according to any one of claims 1 to 8.

10. Use of the ferrous phosphate octahydrate according to claim 9 in the preparation of lithium iron phosphate.

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