A method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue

Abstract

The present invention belongs to the technical field of industrial waste slag resource utilization, and specifically discloses a method for hydrothermally synthesizing dihydrate ferric phosphate from iron-phosphorus waste slag, the steps of which are as follows: (1) first placing the iron-phosphorus waste slag in an oven at 50-80°C for 4-6 hours, then crushing and ball-milling it for standby use; (2) placing the iron-phosphorus waste slag obtained in (1) in a hydrothermal reactor, then adding a regulator to the reactor, and finally adding industrial-grade phosphoric acid and hydrogen peroxide to the reactor, reacting at 160-180°C for 12-24 hours, cooling and filtering to obtain a dihydrate ferric phosphate crude product and a filtrate 1; washing and drying the dihydrate ferric phosphate crude product to obtain a dihydrate ferric phosphate product and a washing liquid 1. The method of the present invention fully recycles and utilizes the iron and phosphorus resources in the iron-phosphorus waste slag through the hydrothermal synthesis technology; the method does not require the adjustment of pH value, does not introduce additional anions and cations, and does not generate wastewater, thereby providing a new way to utilize the iron-phosphorus waste slag resources.

Description

Technical Field

[0001] The invention relates to the technical field of industrial waste residue resource utilization, and in particular to a method for utilizing iron-phosphorus waste residue resources produced by iron salt dephosphorization. The method of the invention is suitable for treating iron-phosphorus waste residue produced by iron salt dephosphorization of phosphorus-containing wastewater in urban sewage treatment plants or winery wastewater treatment plants. Background Art

[0002] With the booming development of the new energy industry, lithium iron phosphate batteries have become the most promising positive electrode material in lithium-ion batteries due to their high safety, long cycle life, environmental friendliness and low cost. Iron phosphate, as its precursor, has the characteristics of non-toxicity, low cost and stable structure, and has also become the focus of attention.

[0003] The method of synthesizing ferric phosphate from waste residue in China is mainly to first add strong acid to the waste residue to dissolve the iron and phosphorus in the waste residue, then add a certain proportion of industrial-grade phosphorus source, and synthesize ferric phosphate under the action of oxidant (to oxidize divalent iron), ammonia water (to adjust the pH value to greater than 2) and reaction temperature of 40-100°C. This method produces ammonium sulfate as a by-product, and the process flow is very long. The cost of phosphorus source accounts for more than 50% of the production cost of ferric phosphate.

[0004] At present, the phosphorus resources on the earth are in a state of polarization. On the one hand, the large-scale mining of phosphate rocks and the difficulty of phosphorus recycling in nature have led to an increasingly serious shortage of phosphorus resources. On the other hand, with the rapid development of industrialization and the world economy, the large-scale discharge of phosphorus-containing wastewater has caused phosphorus pollution in the natural water environment. Phosphorus pollution has become the primary pollutant in the Yangtze River.

[0005] With the implementation of a series of strict environmental protection policies such as the Yangtze River Protection, iron salt dephosphorization, as a mature sewage dephosphorization technology, is being widely used in urban sewage treatment plants or winery wastewater treatment plants. How to synthesize iron phosphate from the iron-phosphorus waste residues produced in the above industries and realize the resource utilization of iron-phosphorus waste residues and turn waste into treasure is of great significance. Summary of the invention

[0006] The purpose of the present invention is to solve the problems existing in the above-mentioned prior art and to provide a method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue.

[0007] The present invention achieves its invention object by adopting the following technical scheme.

[0008] A method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue, the steps of which are as follows:

[0009] (1) Pretreatment of iron-phosphorus waste residue: First, place the iron-phosphorus waste residue in an oven at 50-80°C and dry it for 4-6 hours, then crush and ball-mill it for later use;

[0010] (2) Hydrothermal synthesis of ferric phosphate dihydrate: the iron-phosphorus waste residue obtained in (1) is placed in a hydrothermal reactor, a regulator is added to the reactor, and finally industrial-grade phosphoric acid and hydrogen peroxide are added to the reactor. After reacting at 160 to 180° C. for 12 to 24 hours, the reaction is cooled and filtered to obtain a crude ferric phosphate dihydrate product and a filtrate 1; the crude ferric phosphate dihydrate product is washed with water and dried to obtain a ferric phosphate dihydrate product and a washing liquid 1.

[0011] The iron-phosphorus waste slag is waste slag produced by an iron salt dephosphorization process.

[0012] The iron-phosphorus waste residue is derived from the waste residue produced by phosphorus removal with trivalent iron salts in the terminal treatment process of urban sewage treatment plants or winery wastewater treatment plants, and the solid suspended matter in the phosphorus removal process with trivalent iron salts is ≤0.1%. The trivalent iron salts used are polyferric sulfate, ferric chloride, etc.

[0013] In the step (1), the pH value of the iron-phosphorus waste slag is 5-9; the iron content in the iron-phosphorus waste slag is between 30-50%, the phosphorus content is between 5-10%, the TOC content is between 0-1%, and the chloride ion content is between 0-0.05%; if the TOC or chloride ion content in the iron-phosphorus waste slag is greater than or equal to 1% and 0.05% respectively, the iron-phosphorus waste slag needs to be washed with water to reduce the TOC and chloride ion contents to the target values ​​before resource utilization.

[0014] Preferably, in the step (1), the iron-phosphorus waste slag after crushing and ball milling can pass through a 260-mesh sieve.

[0015] In the step (2), the regulator is water.

[0016] In the step (2), the order of adding the industrial grade phosphoric acid and the hydrogen peroxide is not particular. The mass fraction of the industrial grade phosphoric acid is 85%, and the mass fraction of hydrogen peroxide in the hydrogen peroxide is 27.5%.

[0017] In the step (2), the molar ratio of iron and phosphorus in the hydrothermal process is 1:4.4 to 1:9, preferably, the molar ratio of iron and phosphorus is 1:4.4. The molar ratio of iron and phosphorus refers to the ratio of the amount of iron in the iron-phosphorus waste slag to (the amount of phosphorus in the iron-phosphorus waste slag + the amount of phosphorus in the industrial-grade phosphoric acid).

[0018] In the step (2), the molar ratio of iron element to hydrogen peroxide (measured as hydrogen peroxide therein) is 1:0.145-0.3.

[0019] In the step (2), the pH value of the system reaction before cooling and filtration is ≤1.

[0020] In the step (2), the solid-liquid ratio in the reaction system is 1 g:10 mL, and the solid-liquid ratio refers to the ratio of the mass of the iron-phosphorus waste slag to the sum of the volumes of (regulator+phosphoric acid) (the volume of hydrogen peroxide added to the reaction system is small and can be ignored).

[0021] Preferably, in step (2), the hydrothermal reaction temperature is 180° C. and the reaction time is 12 h.

[0022] In the step (2), the filtrate 1 and the washing liquid 1 generated can be reused, and no waste water is generated. The order of adding the raw materials for hydrothermal synthesis of ferric phosphate dihydrate by reusing the filtrate 1 and the washing liquid 1 generated in the step (2) is as follows:

[0023] The iron-phosphorus waste slag obtained by (1) is placed in a hydrothermal reactor, and then a regulator, water washing liquid 1 and filtrate 1 are added to the reactor, and finally industrial-grade phosphoric acid and hydrogen peroxide are added to the reactor. The order of adding the regulator, water washing liquid and filtrate 1 is not specific, and the order of adding the industrial-grade phosphoric acid and hydrogen peroxide is not specific. The water washing liquid 1 can be used as a regulator of the solid-liquid ratio; the filtrate 1 can replace part of the industrial-grade phosphoric acid, and can also be used as a regulator; when the amount of water washing liquid 1 and filtrate 1 is sufficient, the regulator water can be omitted. At this time, the iron-phosphorus molar ratio in the system refers to the ratio of the amount of iron in the iron-phosphorus waste slag to (the amount of phosphorus in the iron-phosphorus waste slag + the amount of phosphorus in the industrial-grade phosphoric acid + the amount of phosphorus in the filtrate 1).

[0024] In the step (2), the ferric phosphate dihydrate product is a nearly white powder, and the iron-phosphorus molar ratio is 0.96-1.02.

[0025] Compared with the prior art, the method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue of the present invention has the following advantages:

[0026] 1. No need to adjust pH value, no additional anions or cations are introduced

[0027] In the traditional process, the synthesis of iron phosphate requires adjusting the pH value to 2-3, and the reagents for adjusting the pH value include ammonia and other alkaline substances. This introduces a large number of cations into the system, prolongs the process route, and increases by-products. This method is a direct solid-liquid reaction. The reagents added to the iron-phosphorus waste slag are phosphoric acid and hydrogen peroxide. The iron phosphate is generated under strong acidic conditions (pH ≤ 1). There is no process of adjusting the pH value, and no additional anions and cations are introduced.

[0028] 2. No wastewater generated

[0029] In this method, the filtrate 1 contains a large amount of phosphoric acid which can be reused, and the water washing liquid is used as a regulator of the solid-liquid ratio. In this method, no wastewater treatment is required and no by-products are generated.

[0030] 3. Realize resource utilization of iron-phosphorus waste slag

[0031] The iron-phosphorus waste residue produced by urban sewage treatment plants or winery wastewater treatment plants is a hazardous solid waste and needs to be safely disposed of. The method achieves 100% recovery of iron and phosphorus elements in the iron-phosphorus waste residue and produces a high-value-added iron phosphate dihydrate product.

[0032] 4. Low processing cost

[0033] Compared with the traditional process of synthesizing ferric phosphate from ferrous sulfate, the cost of iron and phosphorus in the iron-phosphorus waste slag in the method can be ignored, the cost of phosphorus source can be reduced by 20-50%, and the amount of hydrogen peroxide added is only to prevent TOC and chloride ions in the iron-phosphorus waste slag from reducing trivalent iron under strong acidic and high temperature conditions, so the amount of hydrogen peroxide added can be reduced by more than 80%; if the TOC or chloride ion content in the iron-phosphorus waste slag is greater than 1% and 0.05% respectively, the iron-phosphorus waste slag needs to be washed with water to reduce the TOC and chloride ion contents to target values ​​before resource utilization. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 The schematic diagram of the basic process of hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste slag of the present invention.

[0035] Figure 2 This is the XRD pattern of the ferric phosphate dihydrate product prepared in Example 1.

[0036] Figure 3 This is the XRD pattern of the mixture prepared in Comparative Example 3.

[0037] Figure 4 This is the XRD pattern of the basic iron phosphate prepared in Comparative Example 4.

[0038] Figure 5 These are product appearance pictures, a is the product prepared in Example 1, b is the product prepared in Comparative Example 3, c is the product prepared in Comparative Example 4, and d is the product prepared in Comparative Example 7. DETAILED DESCRIPTION

[0039] The applicant will further describe the technical solution of the present invention in conjunction with specific embodiments below, but the scope of protection claimed by the present invention is not limited to these embodiments. Any changes or equivalent substitutions that do not deviate from the concept of the present invention should be included in the protection scope of the present invention.

[0040] Example 1

[0041] This embodiment uses the wastewater from a winery in Hubei Province as the treatment object, and uses a polyferric sulfate dephosphorization process to produce iron-phosphorus waste slag after the effluent from the secondary sedimentation tank. The pH value of the iron-phosphorus waste slag is 6.5; the iron content in the iron-phosphorus waste slag is 31%, the phosphorus content is 9%, the TOC content is 0.5%, and the chloride ion content is 0.02%.

[0042] Figure 1 The schematic diagram of the basic process of hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste slag of the present invention.

[0043] A method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue, the steps of which are as follows:

[0044] (1) Pretreatment of iron-phosphorus waste slag:

[0045] First, place the iron-phosphorus waste slag in a 60°C oven to dry for 4 hours, and then crush and ball-mill it until it passes through a 260-mesh sieve.

[0046] (2) Hydrothermal synthesis of iron phosphate dihydrate:

[0047] The above 10g sieved iron-phosphorus waste residue was placed in a hydrothermal reactor, and 86.9mL of water, 13.1mL of industrial-grade 85% phosphoric acid and 0.68mL of industrial-grade hydrogen peroxide with a mass fraction of 27.5% were added to the reactor in sequence. The system reaction pH value was 1. After reacting at 180°C for 12h, the reaction was cooled and filtered to obtain a crude product and filtrate 1 (phosphorus molar concentration was 1.65mol / L); the crude product was washed and dried to obtain 10.36g of a nearly white powder product and a washing liquid 1. The obtained nearly white powder product has an appearance shown in FIG. Figure 5 a, XRD pattern of the product is shown in Figure 2 The product has an iron-phosphorus molar ratio of 0.99 by elemental analysis. By comparing with the standard card and combining with elemental analysis, it can be confirmed that the product prepared in Example 1 is ferric phosphate dihydrate.

[0048] Step (2) The iron-phosphorus molar ratio of the hydrothermal process is 1:4.4, which refers to the ratio of the amount of iron in the iron-phosphorus waste slag to (the amount of phosphorus in the iron-phosphorus waste slag + the amount of phosphorus in the industrial-grade phosphoric acid). (Calculation formula: the amount of iron in the iron-phosphorus waste slag = 10*31% / 56 = 0.055 mol, the amount of phosphorus in the iron-phosphorus waste slag = 10*9% / 31 = 0.029 mol, the molar concentration of phosphoric acid = 1.874*0.85 / 98 = 0.01625 mol / mL, the amount of phosphorus in the industrial-grade phosphoric acid = 13.1*0.01625 = 0.213 mol, therefore, the Fe / P molar ratio

[0049] =0.055mol / (0.029mol+0.213mol)=1 / 4.4)

[0050] Step (2) The molar ratio of iron to hydrogen peroxide in the hydrothermal process is 1:0.145 (calculation formula: molar concentration of hydrogen peroxide = 1.463*0.275 / 34 = 0.0118 mol / mL, amount of hydrogen peroxide = 0.68*0.0118 = 0.008 mol, therefore, Fe / H 2 O2 Molar ratio = 0.055 mol / 0.008 mol = 1 / 0.145)

[0051] In order to prevent the process from generating wastewater, the filtrate 1 and the washing liquid 1 generated in the above step (2) are reused. The pretreated iron-phosphorus waste residue can be further hydrothermally synthesized into ferric phosphate dihydrate according to the following step (3):

[0052] (3) Hydrothermal synthesis of iron phosphate dihydrate:

[0053] The above 10g of sieved iron-phosphorus waste residue was placed in a hydrothermal reaction kettle, and 5.22mL of water washing solution 1, 90.9mL of filtrate 1, 3.88mL of industrial-grade 85% phosphoric acid and 0.68mL of industrial-grade hydrogen peroxide with a mass fraction of 27.5% were added into the kettle in sequence. The system reaction pH value was 1. After reacting at 180°C for 12h, the reaction was cooled and filtered to obtain a crude ferrous phosphate dihydrate product and a filtrate 2. The crude ferrous phosphate dihydrate product was washed and dried to obtain 10.35g of ferrous phosphate dihydrate product (nearly white powder, iron-phosphorus molar ratio of 0.99) and water washing solution 2.

[0054] The iron-phosphorus molar ratio of the hydrothermal process in step (3) is 1:4.4, which refers to the ratio of the amount of iron in the iron-phosphorus waste slag to (the amount of phosphorus in the iron-phosphorus waste slag + the amount of phosphorus in the industrial-grade phosphoric acid + the amount of phosphorus in the filtrate 1). (Calculation formula: The amount of iron in the iron-phosphorus waste slag

[0055] =10*31% / 56=0.055mol, the amount of phosphorus in iron-phosphorus waste slag =10*9% / 31=0.029mol, the amount of phosphorus in industrial-grade phosphoric acid =3.88*0.01625=0.063mol, the amount of phosphorus in filtrate 1

[0056] =1.65*0.0909=0.15mol, therefore, Fe / P molar ratio=0.055mol / (0.029mol+0.063mol+0.15mol)=1 / 4.4)

[0057] In step (3) the molar ratio of iron to hydrogen peroxide in the hydrothermal process is 1:0.145.

[0058] The filtrate 2 and the water washing liquid 2 produced in step (3) can be further reused according to the operation of step (3), without generating waste water and other by-products.

[0059] Comparative Example 1:

[0060] Compared with Example 1, the only difference is that: in step (1), the iron-phosphorus waste slag pretreatment process is to place the iron-phosphorus waste slag in an oven at 100° C. and dry it for 7 hours. The results are as follows:

[0061] The product obtained in step (2) after washing and drying is a yellow powder, and the iron-phosphorus molar ratio of the product is 1.44:1 according to elemental analysis.

[0062] Comparative Example 2:

[0063] Compared with Example 1, the only difference is that the reaction temperature of the hydrothermal reaction in step (2) is 100° C. The results are as follows:

[0064] The product obtained in step (2) after washing and drying is a yellow powder, and the iron-phosphorus molar ratio of the product is 1.15:1 according to elemental analysis.

[0065] Comparative Example 3:

[0066] Compared with Example 1, the only difference is that the reaction temperature of the hydrothermal reaction in step (2) is 200° C. The results are as follows:

[0067] The product obtained in step (2) after washing and drying is a light yellow powder, and its appearance is shown in FIG. Figure 5 b, XRD pattern of the obtained product is shown in Figure 2 XRD analysis showed that the product was basic iron phosphate (Fe 5 (PO 4 ) 4 (OH) 3 ·2H 2 O) and iron phosphate dihydrate (FePO 4 ·2H 2 O), the iron-phosphorus molar ratio is 1.15:1.

[0068] Comparative Example 4:

[0069] Compared with Example 1, the only difference is that: in the hydrothermal reaction process of step (2), the iron-phosphorus molar ratio is 1:2.3, and the added materials are: 10g iron-phosphorus waste slag, 94.05mL water, 5.95mL industrial grade 85% phosphoric acid, and 0.68mL industrial grade hydrogen peroxide with a mass fraction of 27.5%.

[0070] The results are as follows:

[0071] The product obtained in step (2) after washing and drying is a dark yellow powder, and its appearance is shown in FIG. Figure 5 c, XRD pattern of the obtained product is shown in Figure 3 XRD analysis showed that the product was basic iron phosphate (Fe 5 (PO 4 ) 4 (OH) 3 ·2H 2 O), the product has an iron-phosphorus molar ratio of 1.3:1 according to elemental analysis.

[0072] Comparative Example 5:

[0073] Compared with Example 1, the only difference is that the order of adding the raw materials in step (3) is iron-phosphorus waste residue, industrial-grade phosphoric acid, water washing liquid 1, filtrate 1, and hydrogen peroxide. The results are as follows:

[0074] The product obtained in step (3) after washing and drying is a dark yellow powder. The iron-phosphorus molar ratio of the product is 1.26:1 according to elemental analysis.

[0075] Comparative Example 6:

[0076] Compared with Example 1, the only difference is that the order of adding the raw materials in step (3) is iron-phosphorus waste residue, hydrogen peroxide, water washing liquid 1, filtrate 1, and industrial-grade phosphoric acid. The results are as follows:

[0077] The product obtained in step (3) after washing and drying is a dark green powder, and the iron-phosphorus molar ratio of the product is 1.08:1 according to elemental analysis.

[0078] Comparative Example 7:

[0079] Compared with Example 1, the only difference is that no hydrogen peroxide is added in step (2). The results are as follows:

[0080] The product obtained in step (2) after washing and drying is a dark green powder, and its appearance is shown in FIG. Figure 5 d. The elemental analysis of the product showed that the iron-phosphorus molar ratio was 1.35:1.

[0081] Example 2

[0082] In this embodiment, the wastewater from a winery in Guizhou is treated by using a ferric chloride dephosphorization process after the effluent from the secondary sedimentation tank to produce iron-phosphorus waste slag, and the pH value of the iron-phosphorus waste slag is 7.3; the iron content in the iron-phosphorus waste slag is 38%, the phosphorus content is 6%, the TOC content is 0.4%, and the chloride ion content is 0.06%.

[0083] A method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue, the steps of which are as follows:

[0084] (1) Pretreatment of iron-phosphorus waste slag:

[0085] First, place the iron-phosphorus waste slag in a 70°C oven and dry it for 6 hours, then crush it and ball-mill it until it passes through a 300-mesh sieve.

[0086] (2) Hydrothermal synthesis of iron phosphate dihydrate:

[0087] The above 10g of sieved iron-phosphorus waste residue was placed in a hydrothermal reaction kettle, and 78.23mL of water, 21.77mL of industrial-grade 85% phosphoric acid and 1.34mL of industrial-grade hydrogen peroxide with a mass fraction of 27.5% were added to the kettle in sequence. The system reaction pH value was 0.91. After reacting at 180°C for 12h, the reaction was cooled and filtered to obtain a crude product and filtrate 1 (phosphorus molar concentration was 2.76mol / L); after washing and drying the crude product, 12.69g of a nearly white powder product and a washing liquid 1 were obtained. The obtained nearly white powder product had an iron-phosphorus molar ratio of 0.98 according to elemental analysis, and combined with the XRD spectrum of the product, the product prepared in Example 2 was ferrous phosphate dihydrate.

[0088] Step (2) hydrothermal process: the amount of iron in the iron-phosphorus waste slag = 10*38% / 56 = 0.0678 mol, the amount of phosphorus in the iron-phosphorus waste slag = 10*6% / 31 = 0.019 mol, the amount of phosphorus in the industrial grade phosphoric acid

[0089] =21.77*0.01625=0.354mol, the amount of hydrogen peroxide =1.34*0.0118=0.0158mol, therefore, Fe / P molar ratio =0.0678mol / (0.019mol+0.354mol)=1 / 5.5; molar ratio of iron to hydrogen peroxide

[0090] =0.0678mol / 0.0158mol=1:0.23.

[0091] In order to prevent the process from generating wastewater, the filtrate 1 and the washing liquid 1 generated in the above step (2) are reused. The pretreated iron-phosphorus waste residue can be further hydrothermally synthesized into ferric phosphate dihydrate according to the following step (3):

[0092] (3) Hydrothermal synthesis of iron phosphate dihydrate:

[0093] The above 10g of sieved iron-phosphorus waste residue was placed in a hydrothermal reaction kettle, and 11.76mL of water washing solution 1, 80mL of filtrate 1, 8.24mL of industrial-grade 85% phosphoric acid and 1.34mL of industrial-grade hydrogen peroxide with a mass fraction of 27.5% were added into the kettle in sequence. The system reaction pH value was 0.91. After reacting at 180°C for 12h, the reaction was cooled and filtered to obtain a crude ferric phosphate dihydrate product and a filtrate 2. After the crude ferric phosphate dihydrate product was washed and dried, 12.69g of ferric phosphate dihydrate product (nearly white powder, iron-phosphorus molar ratio of 0.98) and water washing solution 2 were obtained.

[0094] The iron-phosphorus molar ratio of the hydrothermal process in step (3) is 1:5.5, which refers to the ratio of the amount of iron in the iron-phosphorus waste slag to (the amount of phosphorus in the iron-phosphorus waste slag + the amount of phosphorus in the industrial-grade phosphoric acid + the amount of phosphorus in the filtrate 1). (Calculation formula: the amount of iron = 10*38% / 56 = 0.0678 mol, the amount of phosphorus in the iron-phosphorus waste slag = 10*6% / 31 = 0.019 mol, the amount of phosphorus in the industrial-grade phosphoric acid = 8.24*0.01625 = 0.134 mol, the amount of phosphorus in the filtrate 1 = 2.76*0.08 = 0.221 mol, therefore, the Fe / P molar ratio = 0.0678 mol / (0.019 mol + 0.134 mol + 0.221 mol) = 1 / 5.5)

[0095] In step (3) the molar ratio of iron to hydrogen peroxide in the hydrothermal process is 1:0.23.

[0096] The filtrate 2 and the water washing liquid 2 produced in step (3) can be further reused according to the operation of step (3), without generating waste water and other by-products.

[0097] Example 3

[0098] In this embodiment, the wastewater from a certain town sewage treatment plant in Jiangxi Province is treated by using a ferric chloride dephosphorization process after the effluent from the secondary sedimentation tank to produce iron-phosphorus waste slag, and the pH value of the iron-phosphorus waste slag is 7.2; the iron content in the iron-phosphorus waste slag is 42%, the phosphorus content is 5%, the TOC content is 0.05%, and the chloride ion content is 0.005%.

[0099] A method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue, the steps of which are as follows:

[0100] (1) Pretreatment of iron-phosphorus waste slag:

[0101] First, place the iron-phosphorus waste slag in a 60°C oven to dry for 4 hours, and then crush and ball-mill it until it passes through a 325-mesh sieve.

[0102] (2) Hydrothermal synthesis of iron phosphate dihydrate:

[0103] The above 10g sieved iron-phosphorus waste residue was placed in a hydrothermal reaction kettle, and 60.27mL of water, 39.73mL of industrial-grade 85% phosphoric acid and 1.85mL of industrial-grade hydrogen peroxide with a mass fraction of 27.5% were added to the kettle in sequence. The system reaction pH value was 0.8. After reacting at 180°C for 12h, the reaction was cooled and filtered to obtain a crude product and filtrate 1 (phosphorus molar concentration was 5.72mol / L); after washing and drying the crude product, 14.03g of a nearly white powder product and a washing liquid 1 were obtained. The obtained nearly white powder product had an iron-phosphorus molar ratio of 0.99 according to elemental analysis, and combined with the XRD spectrum of the product, the product prepared in Example 3 was ferrous phosphate dihydrate.

[0104] Step (2) hydrothermal process: the amount of iron in the iron-phosphorus waste slag = 10*42% / 56 = 0.075 mol, the amount of phosphorus in the iron-phosphorus waste slag = 10*5% / 31 = 0.016 mol, the amount of phosphorus in the industrial grade phosphoric acid

[0105] =39.73*0.01625=0.645mol, the amount of hydrogen peroxide = 1.85*0.0118=0.0218mol, therefore, Fe / P molar ratio = 0.075mol / (0.016mol+0.645mol) = 1 / 8.8; molar ratio of iron to hydrogen peroxide

[0106] =0.075mol / 0.0218mol=1:0.29.

[0107] In order to prevent the process from generating wastewater, the filtrate 1 and the washing liquid 1 generated in the above step (2) are reused. The pretreated iron-phosphorus waste residue can be further hydrothermally synthesized into ferric phosphate dihydrate according to the following step (3):

[0108] (3) Hydrothermal synthesis of iron phosphate dihydrate:

[0109] The above 10g of sieved iron-phosphorus waste residue was placed in a hydrothermal reaction kettle, and 20.85mL of water washing solution 1, 61mL of filtrate 1, 18.15mL of industrial-grade 85% phosphoric acid and 1.85mL of industrial-grade hydrogen peroxide with a mass fraction of 27.5% were added into the kettle in sequence. The system reaction pH value was 0.8. After reacting at 180°C for 12h, the reaction was cooled and filtered to obtain a crude ferric phosphate dihydrate product and a filtrate 2. The crude ferric phosphate dihydrate product was washed and dried to obtain 14.02g of ferric phosphate dihydrate product (nearly white powder, iron-phosphorus molar ratio of 0.99) and water washing solution 2.

[0110] The iron-phosphorus molar ratio of the hydrothermal process in step (3) is 1:8.8, which refers to the ratio of the amount of iron in the iron-phosphorus waste slag to (the amount of phosphorus in the iron-phosphorus waste slag + the amount of phosphorus in the industrial-grade phosphoric acid + the amount of phosphorus in the filtrate 1). (Calculation formula: the amount of iron = 10*42% / 56 = 0.075 mol, the amount of phosphorus in the iron-phosphorus waste slag = 10*5% / 31 = 0.016 mol, the amount of phosphorus in the industrial-grade phosphoric acid = 18.15*0.01625 = 0.295 mol, the amount of phosphorus in the filtrate 1 = 5.72*0.061 = 0.349 mol, therefore, the Fe / P molar ratio = 0.075 mol / (0.016 mol + 0.295 mol + 0.349 mol) = 1 / 8.8)

[0111] In the hydrothermal process of step (3), the molar ratio of iron to hydrogen peroxide is 1:0.29.

[0112] The filtrate 2 and the water washing liquid 2 produced in step (3) can be further reused according to the operation of step (3), without generating waste water and other by-products.

Claims

1. A method for hydrothermally synthesizing ferric phosphate dihydrate from iron-phosphorus waste residue, characterized in that: The steps are as follows: (1) Pretreatment of iron-phosphorus waste residue: first place the iron-phosphorus waste residue in an oven at 50-80°C to dry, then crush and ball-mill it for later use; the iron-phosphorus waste residue is derived from the waste residue produced by the phosphorus removal of trivalent iron salts in the terminal treatment process of a municipal sewage treatment plant or a winery wastewater treatment plant, and the solid suspended matter in the water of the phosphorus removal process of trivalent iron salts is ≤0.1%; (2) Hydrothermal synthesis of ferric phosphate dihydrate: the iron-phosphorus waste residue obtained in (1) is placed in a hydrothermal reactor, a regulator is added to the reactor, and finally industrial-grade phosphoric acid and hydrogen peroxide are added to the reactor. After reacting at 160-180° C. for 12-24 hours, the reaction is cooled and filtered to obtain a crude ferric phosphate dihydrate product and a filtrate 1; the crude ferric phosphate dihydrate product is washed and dried to obtain a ferric phosphate dihydrate product and a washing liquid 1; In the step (2), the regulator is water, and in the reaction system: the molar ratio of iron to phosphorus is 1:4.4-1:9, the molar ratio of iron to hydrogen peroxide in hydrogen peroxide is 1:0.145-0.3, and the pH value is ≤1.

2. The method according to claim 1, characterized in that In the step (1), the pH value of the iron-phosphorus waste slag is 5-9; the iron content in the iron-phosphorus waste slag is between 30-50%, the phosphorus content is between 5-10%, the TOC content is between 0-1%, and the chloride ion content is between 0-0.05%.

3. The method according to claim 1, characterized in that In the step (1), the iron-phosphorus waste slag after crushing and ball milling can pass through a 260-mesh sieve.

4. The method according to claim 1, characterized in that: In the step (2), the mass fraction of industrial-grade phosphoric acid is 85%, the mass fraction of hydrogen peroxide is 27.5%, and the solid-liquid ratio is 1 g:10 mL.

5. The method according to any one of claims 1 to 4, characterized in that: The filtrate 1 and the water washing solution 1 produced in step (2) are repeatedly used to hydrothermally synthesize the dihydrate ferric phosphate. The order of adding the raw materials is as follows: The iron-phosphorus waste slag obtained in (1) is placed in a hydrothermal reactor, and then a regulator, water washing liquid 1 and filtrate 1 are added to the reactor. Finally, industrial-grade phosphoric acid and hydrogen peroxide are added to the reactor. The regulator, water washing liquid and filtrate 1 are added in any order, and the industrial-grade phosphoric acid and hydrogen peroxide are added in any order.

6. The method according to claim 5, characterized in that In the step (2), the hydrothermal reaction temperature is 180°C.

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