Preparation method of low-cost high-compaction iron phosphate precursor

Abstract

The invention provides a preparation method of a low-cost and high-tap-density iron phosphate precursor, and relates to the technical field of batteries, and the preparation method comprises the following steps: S1, preparing a ferrous solution; s2, preparing a phosphorus salt solution; s3, putting the ferrous solution into a reaction kettle to serve as a base solution, heating, putting the phosphorus salt solution into the reaction kettle, and reacting to obtain amorphous slurry A; s4, filtering and washing the amorphous slurry A to obtain an amorphous filter cake; s5, pulping the amorphous filter cake and ammonium salt, then adding phosphoric acid, uniformly stirring, heating and preserving heat to obtain white slurry B; s6, filtering, washing, drying and calcining the white slurry B to obtain high-compaction anhydrous iron phosphate; according to the method, the pH value of the pulp is adjusted by adding ammonium salts in different proportions in the pulping process, so that the use of a large amount of phosphoric acid is avoided, the cost of raw materials is reduced, the requirement on accurate pH adjustment of the pulp in the subsequent treatment process is reduced, and the cost is further saved.

Description

Technical Field

[0001] The invention relates to the technical field of batteries, and in particular to a method for preparing a low-cost and high-tap iron phosphate precursor. Background Art

[0002] Lithium iron phosphate has the advantages of high safety, long cycle life, and low cost. It currently has an absolute advantage in the field of positive electrode materials for power and energy storage lithium batteries. With the rapid development of new energy vehicles in recent years, lithium iron phosphate batteries are in short supply, and competition in related industries has become increasingly fierce. Only by developing lithium iron phosphate in the dimensions of low cost, high compaction, high capacity, high power, and ultra-long cycle can the competitiveness of corporate products be improved.

[0003] Iron phosphate is an important raw material for preparing lithium iron phosphate. The electrical properties of lithium iron phosphate are largely determined by the iron phosphate precursor. The iron-phosphorus ratio, tap density, and morphology of anhydrous iron phosphate play a vital role in the performance of lithium iron phosphate. A suitable iron phosphate preparation process has a direct impact on the preparation of low-cost, high-density, and high-capacity lithium iron phosphate.

[0004] At present, the mainstream preparation process in the iron phosphate industry includes a two-step ammonium method, that is, after the ferrous salt, phosphate salt and oxidant are mixed and reacted, the dihydrate iron phosphate slurry is obtained by filtration, washing, beating and crystallization reaction, and then anhydrous iron phosphate is obtained after secondary filtration, washing, drying and calcination. The iron phosphate prepared using conventional processes often has a low tap density. When using low tap density iron phosphate to prepare lithium iron phosphate batteries, the battery capacity will be unsatisfactory. In order to solve this problem, a large number of companies currently often use a large amount of phosphoric acid to maintain the stability of the iron-phosphorus ratio and increase the tap density, which undoubtedly increases production costs.

[0005] The patent with publication number CN 118324108A uses a two-step method to divide the pulping slurry into two parts in proportion, first add phosphoric acid to adjust the pH and heat up to perform a crystallization treatment, and then add another part of the slurry to perform a second crystallization treatment. Through the two-stage crystallization, an anhydrous iron phosphate product with mismatched particle size distribution and high tap density is obtained.

[0006] The patent with publication number CN 117699766A prepares basic ammonium ferric phosphate slurry first, filters and washes the filter cake, then slurries it and dissolves it in acid solution, and then adjusts the slurry pH to 1.0-1.9 and then heats it to age it to obtain dihydrated ferric phosphate. This solution uses a large amount of phosphoric acid (about 0.8 PA) to dissolve and recrystallize basic ammonium ferric phosphate to obtain a high-tapped anhydrous ferric phosphate product.

[0007] In the above existing technologies, the use of large amounts of phosphoric acid will significantly increase the cost, and the preparation of iron phosphate by large and small particle grading has the disadvantages of complex steps and long reaction cycle. Therefore, a low-cost iron phosphate production technology that can prepare high-tap iron phosphate with a stable iron-phosphorus ratio is very important. Summary of the invention

[0008] In order to solve the above technical problems, the present invention provides a method for preparing a low-cost and high-tapped iron phosphate precursor, by adding ammonium salts in different proportions to pulping water to increase the pH after pulping, thereby obtaining pure iron phosphate dihydrate, or a mixture of basic ammonium ferric phosphate and dihydrate iron phosphate, or a slurry of pure basic ammonium ferric phosphate, and finally calcining to obtain a pure phase of high-tapped anhydrous iron phosphate; on the one hand, the invention does not need to add a large amount of phosphoric acid and accurately adjust the pH of the slurry for multiple times, which saves costs and reduces control nodes; on the other hand, the invention can be directly applied to the existing ammonium process production line without complicated technical transformation, and is suitable for production line introduction, as follows: A method for preparing a low-cost and high-tap iron phosphate precursor comprises the following steps: S1: preparing a ferrous solution, adding an inorganic acid to adjust the pH to 1.5-2.5; the iron concentration of the ferrous solution is 0.6-1.8 mol / L; S2: preparing a phosphate solution, wherein the phosphate solution has a phosphorus concentration of 0.6-1.8 mol / L, adding a pH adjuster to the phosphate solution to adjust the pH to 6.0-8.0, and adding an oxidant to mix; S3: the ferrous solution of step S1 is put into a reactor as a primer and the temperature is increased, and the phosphate solution of step 2 is put into the reactor under continuous stirring and reacted for a period of time to obtain an amorphous slurry A; S4: filtering and washing the amorphous slurry A to obtain an amorphous filter cake; S5: beating the amorphous filter cake and the ammonium salt, then adding phosphoric acid and stirring evenly, heating and keeping the temperature, to obtain a white slurry B; the molar ratio of the phosphoric acid to the ammonium salt is 1.5-2:1; S6: The white slurry B is filtered, washed, dried and calcined to obtain anhydrous iron phosphate with high tap density.

[0009] Preferably, in step S1, the ferrous source for preparing the ferrous solution is any one of ferrous sulfate heptahydrate, ferrous nitrate, and ferrous chloride; and the inorganic acid is one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0010] Preferably, the ferrous source is ferrous sulfate heptahydrate, and the inorganic acid is sulfuric acid.

[0011] Preferably, in step S2, the phosphorus source for preparing the phosphate solution is one of phosphoric acid, ammonium phosphate, ammonium monohydrogen phosphate, and ammonium dihydrogen phosphate; the pH adjuster is any one of ammonia water, ammonia gas, and sodium hydroxide; the oxidant is any one of sodium persulfate, hydrogen peroxide, and perchloric acid; and the molar ratio of the ferrous source to the oxidant is 1:0.4-1:1 Preferably, the phosphorus source is diammonium phosphate.

[0012] Preferably, in step S3, the Fe / P of the ferrous solution and the phosphate solution is 1:0.5-1:2 (molar ratio).

[0013] Preferably, in step S3, the base solution is heated to 30-80°C, the stirring rate is 200-300 rpm, the feeding time of the phosphate solution is 20-40 min, and the reaction time is 50-70 min.

[0014] Preferably, in step S4, the condition for washing to end is that the conductivity of the washing water is ≤2000 μs / cm.

[0015] Preferably, in step S5, the amount of phosphoric acid and the ferrous source P / Fe=0.1-0.3 (molar ratio), the heating temperature is 80-100°C, and the insulation time is 1-2 h; the ammonium salt includes any one of ammonium phosphate, monoammonium hydrogen phosphate, diammonium phosphate, ammonia water, ammonium sulfate, and ammonium chloride.

[0016] Preferably, the ammonium salt is diammonium phosphate or aqueous ammonia.

[0017] Preferably, in step S6, the washing end condition is that the conductivity of the washing water is ≤400 μs / cm, the drying temperature is 150-200°C, the drying time is 5-7 h, the calcination temperature is 500-650°C, and the calcination time is 3-5 h.

[0018] On the other hand, the present invention provides an anhydrous ferric phosphate product obtained by a method for preparing a low-cost and high-tapped ferric phosphate precursor.

[0019] Preferably, the tap density of the prepared anhydrous ferric phosphate can reach up to 1.17 g / cm according to different ratios. 3 .

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention adjusts the pH value of the slurry by adding ammonium salts in different proportions during the pulping process, thereby avoiding the use of a large amount of phosphoric acid, which not only reduces the cost of raw materials, but also reduces the need for precise pH adjustment of the slurry in subsequent processing, further saving costs.

[0021] 2. The technical solution of the present invention can significantly improve the tap density of the finished iron phosphate, without the need to use a large amount of phosphoric acid, reducing cost investment, and the process is simple, suitable for introduction into existing production lines; at the same time, the increase in tap density helps to improve the energy density and cycle performance of lithium iron phosphate batteries, thereby meeting the market demand for high-performance batteries.

[0022] 3. The preparation method of the present invention can be directly applied to the existing ammonium process production line without complicated technical transformation, which reduces the adjustment cost of the production line and makes the method easier to promote and apply in the industry.

[0023] 4. By precisely controlling the reaction conditions and the ratio of raw materials, the present invention can prepare an iron phosphate precursor with a stable iron-phosphorus ratio, thereby ensuring that the performance of the final product is stable and reliable; moreover, reducing the use of phosphoric acid not only reduces production costs, but also helps to reduce wastewater discharge and environmental pollution, which is in line with the current concept of green production and sustainable development.

[0024] 5. With the rapid development of new energy vehicles and energy storage markets, high-performance, low-cost, and environmentally friendly lithium iron phosphate batteries will have broad market prospects. The preparation method of the low-cost and high-touch iron phosphate precursor provided by the present invention will help promote the further development and application of lithium iron phosphate battery technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the SEM image of anhydrous iron phosphate obtained in Experiment 1; Figure 2 This is the SEM image of anhydrous iron phosphate obtained in Experiment 2; Figure 3 This is the SEM image of anhydrous iron phosphate obtained in Experiment 3; Figure 4 This is the SEM image of anhydrous iron phosphate obtained in Experiment 4; Figure 5 This is the SEM image of anhydrous iron phosphate obtained in Experiment 5; Figure 6 This is the SEM image of anhydrous iron phosphate obtained in Experiment 6; Figure 7 This is the SEM image of anhydrous iron phosphate obtained in Experiment 7; Figure 8 This is the SEM image of anhydrous iron phosphate obtained in Experiment 8. DETAILED DESCRIPTION

[0026] Example 1 A method for preparing a low-cost and high-tap iron phosphate precursor comprises the following steps: S1: preparing a ferrous solution, adding an inorganic acid to adjust the pH to 1.5; the iron concentration of the ferrous solution is 0.6 mol / L; S2: preparing a phosphate solution, wherein the phosphate solution has a phosphorus concentration of 0.6 mol / L, adding a pH adjuster to adjust the pH of the phosphate solution to 6.0, and adding an oxidant to mix; S3: the ferrous solution of step S1 is put into a reactor as a primer and the temperature is increased, and the phosphate solution of step 2 is put into the reactor under continuous stirring and reacted for a period of time to obtain an amorphous slurry A; S4: filtering and washing the amorphous slurry A to obtain an amorphous filter cake; S5: beating the amorphous filter cake and the ammonium salt, then adding phosphoric acid and stirring evenly, heating and keeping the temperature, to obtain a white slurry B; the molar ratio of the phosphoric acid to the ammonium salt is 1.5:1; S6: The white slurry B is filtered, washed, dried and calcined to obtain anhydrous iron phosphate with high tap density.

[0027] Moreover, in the step S1, the ferrous source for preparing the ferrous solution is any one of ferrous sulfate heptahydrate, ferrous nitrate, and ferrous chloride; and the inorganic acid is one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0028] Moreover, in step S2, the phosphorus source for preparing the phosphate solution is one of phosphoric acid, ammonium phosphate, monoammonium hydrogen phosphate, and diammonium phosphate; the pH adjuster is any one of ammonia water, ammonia gas, and sodium hydroxide; the oxidant is any one of sodium persulfate, hydrogen peroxide, and perchloric acid; and the molar ratio of the ferrous source to the oxidant is 1:0.4.

[0029] Moreover, in step S3, the Fe / P ratio of the ferrous solution and the phosphate solution is 1:0.5.

[0030] Moreover, in the step S3, the base solution is heated to a temperature of 30-80°C, the continuous stirring rate is 200 rpm, the feeding time of the phosphate solution is 20 min, and the reaction time is 50 min.

[0031] Furthermore, in step S4, the condition for washing to end is that the conductivity of the washing water is ≤ 2000 μs / cm.

[0032] Moreover, in the step S5, the amount of phosphoric acid added is such that P / Fe=0.1, the heating temperature is 80°C, and the insulation time is 1 h; the ammonium salt includes any one of ammonium phosphate, monoammonium hydrogen phosphate, diammonium hydrogen phosphate, ammonia water, ammonium sulfate, and ammonium chloride.

[0033] Moreover, in step S6, the conditions for finishing washing are that the conductivity of washing water is ≤400 μs / cm, the drying temperature is 150° C., the drying time is 5 h, the calcination temperature is 500° C., and the calcination time is 3 h.

[0034] Example 2 A method for preparing a low-cost and high-tap iron phosphate precursor comprises the following steps: S1: preparing a ferrous solution, adding an inorganic acid to adjust the pH to 2.5; the iron concentration of the ferrous solution is 1.8 mol / L; S2: preparing a phosphate solution, wherein the phosphorus concentration of the phosphate solution is 1.8 mol / L, adding a pH adjuster to the phosphate solution to adjust the pH to 8.0, and adding an oxidant to mix; S3: the ferrous solution of step S1 is put into a reactor as a primer and the temperature is increased, and the phosphate solution of step 2 is put into the reactor under continuous stirring and reacted for a period of time to obtain an amorphous slurry A; S4: filtering and washing the amorphous slurry A to obtain an amorphous filter cake; S5: slurrying the amorphous filter cake and the ammonium salt, then adding phosphoric acid and stirring evenly, heating and keeping the temperature, to obtain a white slurry B; the molar ratio of the phosphoric acid to the ammonium salt is 2:1; S6: The white slurry B is filtered, washed, dried and calcined to obtain anhydrous iron phosphate with high tap density.

[0035] Moreover, in the step S1, the ferrous source for preparing the ferrous solution is any one of ferrous sulfate heptahydrate, ferrous nitrate, and ferrous chloride; and the inorganic acid is one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0036] Moreover, in step S2, the phosphorus source for preparing the phosphate solution is one of phosphoric acid, ammonium phosphate, monoammonium hydrogen phosphate, and diammonium phosphate; the pH adjuster is any one of ammonia water, ammonia gas, and sodium hydroxide; the oxidant is any one of sodium persulfate, hydrogen peroxide, and perchloric acid; and the molar ratio of the ferrous source to the oxidant is 1:1.

[0037] Moreover, in step S3, the Fe / P ratio of the ferrous solution and the phosphate solution is 1:2.

[0038] Moreover, in the step S3, the base solution is heated to a temperature of 30-80°C, the continuous stirring rate is 300 rpm, the feeding time of the phosphate solution is 40 min, and the reaction time is 70 min.

[0039] Furthermore, in step S4, the condition for washing to end is that the conductivity of the washing water is ≤ 2000 μs / cm.

[0040] Moreover, in the step S5, the amount of phosphoric acid added is such that P / Fe=0.3, the heating temperature is 100°C, and the insulation time is 2 h; the ammonium salt includes any one of ammonium phosphate, monoammonium hydrogen phosphate, diammonium hydrogen phosphate, ammonia water, ammonium sulfate, and ammonium chloride.

[0041] Moreover, in step S6, the conditions for finishing washing are that the conductivity of washing water is ≤400 μs / cm, the drying temperature is 200° C., the drying time is 7 h, the calcination temperature is 500-650° C., and the calcination time is 5 h.

[0042] Example 3 A method for preparing a low-cost and high-tap iron phosphate precursor comprises the following steps: S1: preparing a ferrous solution, adding an inorganic acid to adjust the pH to 2.0; the iron concentration of the ferrous solution is 1.2 mol / L; S2: preparing a phosphate solution, wherein the phosphate concentration of the phosphate solution is 1.2 mol / L, adding a pH adjuster to adjust the pH of the phosphate solution to 7.0, and adding an oxidant to mix; S3: the ferrous solution of step S1 is put into a reactor as a primer and the temperature is increased, and the phosphate solution of step 2 is put into the reactor under continuous stirring and reacted for a period of time to obtain an amorphous slurry A; S4: filtering and washing the amorphous slurry A to obtain an amorphous filter cake; S5: beating the amorphous filter cake and the ammonium salt, then adding phosphoric acid and stirring evenly, heating and keeping the temperature, to obtain a white slurry B; the molar ratio of the phosphoric acid to the ammonium salt is 1.8:1; S6: The white slurry B is filtered, washed, dried and calcined to obtain anhydrous iron phosphate with high tap density.

[0043] Moreover, in the step S1, the ferrous source for preparing the ferrous solution is any one of ferrous sulfate heptahydrate, ferrous nitrate, and ferrous chloride; and the inorganic acid is one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

[0044] Moreover, in the step S2, the phosphorus source for preparing the phosphate solution is one of phosphoric acid, ammonium phosphate, monoammonium phosphate, and diammonium phosphate; the pH adjuster is any one of ammonia water, ammonia gas, and sodium hydroxide; the oxidant is any one of sodium persulfate, hydrogen peroxide, and perchloric acid; the molar ratio of the ferrous source to the oxidant is 1:0.7 Moreover, in step S3, the Fe / P ratio of the ferrous solution and the phosphate solution is 1:1.

[0045] Moreover, in the step S3, the base solution is heated to 50°C, the continuous stirring rate is 250 rpm, the feeding time of the phosphate solution is 30 min, and the reaction time is 60 min.

[0046] Furthermore, in step S4, the condition for washing to end is that the conductivity of the washing water is ≤ 2000 μs / cm.

[0047] Moreover, in the step S5, the amount of phosphoric acid added is such that P / Fe=0.2, the heating temperature is 90°C, and the insulation time is 1.5 h; the ammonium salt includes any one of ammonium phosphate, monoammonium hydrogen phosphate, diammonium hydrogen phosphate, ammonia water, ammonium sulfate, and ammonium chloride.

[0048] Moreover, in the step S6, the conditions for finishing washing are that the conductivity of washing water is ≤400 μs / cm, the drying temperature is 180° C., the drying time is 6 h, the calcination temperature is 600° C., and the calcination time is 4 h.

[0049] Experimental Section

[0050] Experiment 1 In this experiment: the molar ratio of phosphoric acid to ammonium salt is 4:1, as follows A method for preparing a low-cost and high-tap iron phosphate precursor comprises the following steps: S1: Dissolve ferrous phosphate heptahydrate in deionized water to prepare a ferrous sulfate solution with an Fe concentration of 1.2 mol / L. Take 4 L of ferrous sulfate solution and add concentrated sulfuric acid to adjust the pH of the solution to 2.2.

[0051] S2: Dissolve ammonium dihydrogen phosphate in deionized water, add hydrogen peroxide first, then add ammonia water to adjust the pH to 7.3 to prepare a phosphate solution with a P concentration of 1.4 mol / L. Take 3.53 L of the solution.

[0052] S3: Pour all the ferrous sulfate solution in S1 into a 10L reactor as a base liquid. Heat the base liquid to 50°C at a stirring rate of 250rpm and then start adding the phosphate solution. Use a peristaltic pump to set the phosphate solution feeding time to 30min. After the feeding is completed, perform the synthesis reaction for 60min to obtain amorphous slurry A.

[0053] S4: The amorphous slurry A is washed and centrifuged using cold pure water. The washing is terminated when the conductivity of the washing water is ≤2000 μs / cm, and an amorphous filter cake is finally obtained.

[0054] S5: Weigh 41.4 g of ammonium dihydrogen phosphate and dissolve it in deionized water, dilute to 4.5 L to obtain beating water, and beat the amorphous filter cake with the beating water for 30 min at a stirring rate of 400 rpm. After beating, add 166 g of phosphoric acid and continue stirring for 20 min to mix evenly. Heat the slurry to 95 °C and keep it warm for 1.5 h to obtain white slurry B.

[0055] S6: The white slurry B is washed and centrifuged with cold pure water. The washing is completed when the conductivity of the washing water is ≤400μs / cm to obtain a second-wash filter cake. The second-wash filter cake is dried (180°C, 6 h) to obtain a dried product. The dried product is then calcined at high temperature (580°C for 4 h) and cooled to obtain anhydrous iron phosphate.

[0056] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 1 shown.

[0057] Experiment 2 The difference between this experiment and Experiment 1 is that the amounts of the components are different, the amount of ammonium dihydrogen phosphate dissolved in the beating water in S5 is 82.8 g, and the molar ratio of phosphoric acid to ammonium salt is 2:1.

[0058] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 2 shown.

[0059] Experiment 3 The difference between this experiment and Experiment 1 is that the amount of ammonium dihydrogen phosphate dissolved in the pulping water in S5 is 110.4 g, and the molar ratio of phosphoric acid to ammonium salt is 1.5:1.

[0060] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 3 shown.

[0061] Experiment 4 The difference between this experiment and Experiment 1 is that the amount of diammonium phosphate dissolved in the pulping water in S5 is 165.6 g, and the molar ratio of phosphoric acid to ammonium salt is 1:1.

[0062] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 4 shown.

[0063] Experiment 5 The difference between this experiment and Experiment 1 is that the amount of ammonium dihydrogen phosphate dissolved in the pulping water in S5 is 248.5 g, so that the molar ratio of phosphoric acid to ammonium salt is 1:1.5.

[0064] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 5 shown.

[0065] Experiment 6 The difference between this experiment and Experiment 1 is that the amount of ammonium dihydrogen phosphate dissolved in the pulping water in S5 is 331.3 g, and the molar ratio of phosphoric acid to ammonium salt is 1:2.

[0066] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 6shown.

[0067] Experiment 7 The difference between this experiment and Experiment 1 is that the ammonium salt dissolved in the pulping water in S5 is ammonia water, the amount used is 101 g, and the molar ratio of phosphoric acid to ammonium salt is 2:1.

[0068] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 7 shown.

[0069] Experiment 8 The difference between this experiment and Experiment 1 is that the pulping water in S5 is deionized water, and no additional ammonium salt is added.

[0070] The anhydrous ferric phosphate obtained in this experiment was analyzed by electron microscope scanning. The results are as follows: Figure 8 shown.

[0071] Depend on Figure 1-6 It can be seen that when the ammonium salt in the pulping water is diammonium hydrogen phosphate, as the proportion of ammonium salt increases, the morphology of anhydrous iron phosphate changes from partially melted and dispersed to almost non-melted, dense, and rich in pores; however, when the ammonium salt is added in excess, the morphology of anhydrous iron phosphate will become melted and sparse again, which indicates that the proportion of ammonium salt can significantly affect the morphology of the calcined sample.

[0072] Depend on Figure 7 It can be seen that when the ammonium salt is ammonia water, the morphology of anhydrous iron phosphate is spherical with pores and is sparsely dispersed.

[0073] Depend on Figure 8 It can be seen that when no ammonium salt is added to the pulping water, that is, under the conventional process, the anhydrous ferric phosphate obtained is mainly partially melted and sparsely dispersed.

[0074] In summary, Figure 1-8 It is further explained that adding ammonium salt to the pulping water can inhibit the formation of dihydrate ferric phosphate during the conversion process. The appropriate input ratio can make the conversion produce a mixture of dihydrate ferric phosphate and basic ammonium ferric phosphate, which can maintain the normal iron-phosphorus ratio of the finished ferric phosphate on the one hand, and improve its specific surface area and tap density on the other hand.

[0075] Experiment 9 The physical and chemical indicators of the anhydrous ferric phosphate prepared in the above experiments 1-8 were measured respectively, specifically TD (tapped density), BET (specific surface area), Fe content, P content, n Fe / P (iron-phosphorus ratio), as shown in Table 1.

[0076] Table 1 Physical and chemical indicators of anhydrous ferric phosphate

[0077] It can be seen from Table 1 that from experiments 1 to 6, when the ammonium salt is diammonium hydrogen phosphate, as the proportion of ammonium salt in the pulping water gradually increases, the tap density of the anhydrous ferric phosphate product is improved, and the increase in the ratio of TD to ammonium salt basically conforms to the trend of first increasing and then decreasing.

[0078] Compared with Experiment 8, the specific surface area in all experiments increased, indicating that the addition of ammonium salts to the pulping water can improve the burning resistance of the finished ferric phosphate product.

[0079] From the iron-phosphorus ratios of Experiments 1-6 and 8, it can be seen that when a small amount of ammonium salt is added to the pulping water (phosphate / ammonium salt is greater than 1, Experiments 1-3), the iron-phosphorus ratio of the finished product is basically normal and has no significant effect. However, if the ammonium salt ratio is further increased (phosphate / ammonium salt is less than or equal to 1), the iron content of the finished product will increase significantly and the phosphorus content will decrease significantly, ultimately resulting in a higher iron-phosphorus ratio of the finished product.

[0080] It can be seen from Experiments 7 and 8 that when the ammonium salt in the pulping water is ammonia water, the TD and specific surface area of ​​the finished product are improved. However, the addition of ammonia water will not only react with a portion of the phosphoric acid, but also generate a portion of diammonium phosphate, which will lead to an increase in the iron content and a decrease in the phosphorus content of the final product, resulting in a high iron-phosphorus ratio.

[0081] This is because the main reaction involved in the conversion stage is the reaction of basic ammonium ferric phosphate and phosphoric acid to produce dihydrate ferric phosphate and diammonium phosphate. Adding ammonium salt (dihydrate ammonium phosphate or ammonia water) to the pulping water can partially inhibit the reaction (the addition of ammonia water will lead to more severe inhibition), so that the conversion ends and finally forms a mixture of dihydrate ferric phosphate and basic ammonium ferric phosphate. The anhydrous ferric phosphate generated by high-temperature dehydration of basic ammonium ferric phosphate has the characteristics of high specific surface area and high iron-phosphorus ratio (more than 1). Therefore, the performance of the finished anhydrous ferric phosphate can be improved by retaining a small amount of basic ammonium ferric phosphate in the conversion stage.

[0082] In summary, it can be determined from Table 1 that the anhydrous iron phosphate in Experiments 2 and 3 has the best performance, and the maximum tap density can reach 1.1671 g / cm 3 , 1.1704 g / cm 3 Adding too little or too much ammonium salt will reduce the performance. The optimal ratio of adding ammonium salt (especially diammonium phosphate) to pulping water is 1.5-2:1.

Claims

1. A method for preparing a low-cost and high-tap iron phosphate precursor, characterized in that: The following steps are involved: S1: preparing a ferrous solution, adding an inorganic acid to adjust the pH to 1.5-2.5; the iron concentration of the ferrous solution is 0.6-1.8 mol / L; S2: preparing a phosphate solution, wherein the phosphate solution has a phosphorus concentration of 0.6-1.8 mol / L, adding a pH adjuster to the phosphate solution to adjust the pH to 6.0-8.0, and adding an oxidant to mix; S3: the ferrous solution of step S1 is put into a reactor as a primer and the temperature is increased, and the phosphate solution of step 2 is put into the reactor under continuous stirring and reacted for a period of time to obtain an amorphous slurry A; S4: filtering and washing the amorphous slurry A to obtain an amorphous filter cake; S5: beating the amorphous filter cake and the ammonium salt, then adding phosphoric acid and stirring evenly, heating and keeping the temperature, to obtain a white slurry B; the molar ratio of the phosphoric acid to the ammonium salt is 1.5-2:1; S6: The white slurry B is filtered, washed, dried and calcined to obtain anhydrous iron phosphate with high tap density.

2. The method for preparing a low-cost and high-tapped iron phosphate precursor according to claim 1, characterized in that: In the step S1, the ferrous source for preparing the ferrous solution is any one of ferrous sulfate heptahydrate, ferrous nitrate, and ferrous chloride; and the inorganic acid is one of sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid.

3. A method for preparing a low-cost and high-tapped iron phosphate precursor as claimed in claim 2, characterized in that: In step S2, the phosphorus source for preparing the phosphate solution is one of phosphoric acid, ammonium phosphate, monoammonium hydrogen phosphate, and diammonium phosphate; the pH adjuster is any one of ammonia water, ammonia gas, and sodium hydroxide; the oxidant is any one of sodium persulfate, hydrogen peroxide, and perchloric acid; and the molar ratio of the ferrous source to the oxidant is 1:0.4-1:

1.

4. The method for preparing a low-cost and high-tapped iron phosphate precursor according to claim 1, characterized in that: In step S3, the Fe / P ratio of the ferrous solution and the phosphate solution is 1:0.5-1:

2.

5. The method for preparing a low-cost and high-tapped iron phosphate precursor according to claim 1, characterized in that: In the step S3, the base solution is heated to 30-80°C, the stirring rate is 200-300 rpm, the feeding time of the phosphate solution is 20-40 min, and the reaction time is 50-70 min.

6. The method for preparing a low-cost and high-tapped iron phosphate precursor according to claim 1, characterized in that: In step S4, the condition for washing to end is that the conductivity of the washing water is ≤2000 μs / cm.

7. The method for preparing a low-cost and high-tapped iron phosphate precursor according to claim 1, characterized in that: In the step S5, the amount of phosphoric acid added is such that P / Fe=0.1-0.3, the heating temperature is 80-100°C, and the insulation time is 1-2 h; the ammonium salt includes any one of ammonium phosphate, monoammonium hydrogen phosphate, diammonium hydrogen phosphate, ammonia water, ammonium sulfate, and ammonium chloride.

8. The method for preparing a low-cost and high-tapped iron phosphate precursor according to claim 1, characterized in that: In step S6, the conditions for finishing washing are that the conductivity of washing water is ≤400 μs / cm, the drying temperature is 150-200°C, the drying time is 5-7 h, the calcination temperature is 500-650°C, and the calcination time is 3-5 h.

9. An anhydrous ferric phosphate product obtained by the method for preparing a low-cost and high-tapped ferric phosphate precursor according to any one of claims 1 to 7.

10. The anhydrous ferric phosphate product according to claim 9, characterized in that: The tap density of the anhydrous ferric phosphate product is 1.1704 g / cm 3 .

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