Method for synthesizing iron phosphate from waste liquid generated in chip or industrial production

Abstract

The invention relates to a method for synthesizing iron phosphate from a chip or waste liquid generated in industrial production. The method comprises the following steps: removing suspended particles through three-stage filtration, and adsorbing heavy metal ions in a solution by adopting ion exchange; the method comprises the following steps: precipitating interfering ions under a weakly acidic condition, and converting phosphoric acid into a sodium phosphate mixture under an alkaline condition; selectively adding a ferric salt compound to the obtained sodium phosphate salt solution, wherein a ferric phosphate precipitate is generated when a ferric salt is used, and a ferrous phosphate precipitate is generated when a ferrous salt is used; and carrying out centrifugal separation to obtain a ferric phosphate or ferrous phosphate precipitation product, and carrying out multi-effect evaporation on the mother liquor to recover a sodium salt byproduct. The method for synthesizing high-purity ferric phosphate and ferrous phosphate from the chip production waste liquid is realized, and sodium salt byproducts are recovered at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of hazardous waste resource utilization and new energy material preparation, and specifically provides a method for synthesizing iron phosphate from waste liquid generated in chip or industrial production. Background Art

[0002] Currently, the primary method for treating wastewater generated during chip production is neutralization and landfill. This wastewater contains a large amount of miscellaneous acids and salts, including phosphoric acid, hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid, sodium phosphate, sodium fluoride, sodium nitrate, sodium chloride, and sodium sulfate, making it a toxic and hazardous waste. Current disposal methods primarily involve adding alkali to concentrate the salt mixture into a solid, which is then landfilled. This can pose environmental risks, is costly, and consumes land resources.

[0003] The traditional process for producing ferric phosphate or ferrous phosphate uses pure chemical raw materials, namely, a process that combines finished phosphoric acid with finished ferric chloride and ferrous sulfate or ferrous sulfate. This process accounts for >70% of the raw material cost (the unit price of phosphoric acid is >6,000 yuan / ton).

[0004] There is an urgent need to develop a resource technology that can both harmlessly dispose of chip waste liquid and convert it into high-value-added products, while solving the cost and environmental pollution problems of traditional ferric phosphate or ferrous phosphate production. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for synthesizing iron phosphate from waste liquid generated by chip or industrial production, to achieve efficient directional conversion of phosphate and iron salt in the waste liquid, and to obtain the phosphate from the waste liquid through the use of impurity elements (F-, Na + ) to improve product performance, build a closed loop of the entire industrial chain from waste liquid treatment to material synthesis and by-product recycling, and solve the cost and environmental problems of traditional ferrous phosphate or ferrous phosphate production.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The method for synthesizing ferric phosphate from waste liquid generated by chip or industrial production comprises the following steps:

[0008] Step 1: Pre-treat the chip production wastewater containing phosphoric acid and impurities by removing suspended particles through three-stage filtration and using ion exchange to adsorb heavy metal ions in the solution;

[0009] Step 2: The pH value of the waste liquid is adjusted by adding alkali in stages, first precipitating interfering ions under weak acidic conditions, and then converting phosphoric acid into a sodium phosphate mixture under alkaline conditions; the process includes two stages: in the first stage, sodium hydroxide solution is added to control the pH within the range of 4-6, and stirring is continued for 30-45 minutes to preferentially precipitate aluminum and calcium ions; in the second stage, alkali is continued to be added to the pH of 8-10, and the mixture is reacted at 40±2°C for 1-2 hours to form a mixed solution containing sodium monohydrogen phosphate, sodium dihydrogen phosphate, and trisodium phosphate;

[0010] Step 3: selectively adding an iron salt compound to the obtained sodium phosphate solution, wherein when a trivalent iron salt is used, a ferrous phosphate precipitate is generated, and when a divalent iron salt is used, a ferrous phosphate precipitate is generated;

[0011] Step 4: obtain the precipitated product of ferric phosphate or ferrous phosphate by centrifugal separation, and perform multi-effect evaporation on the mother liquor to recover the sodium salt by-product.

[0012] In step 1 of the above method, the three-stage filtration includes filtering the waste liquid using a stainless steel screen, a bag filter and a ceramic membrane in sequence.

[0013] In step three of the above method, the addition process of the iron salt compound is implemented as follows: when synthesizing ferric phosphate, ferric chloride or ferric sulfate is used as the iron source, and the redox potential of the system is maintained in the range of +200mV to +300mV; when synthesizing ferrous phosphate, ferrous sulfate or ferrous chloride is used as the iron source, and nitrogen is introduced to keep the dissolved oxygen concentration below 0.5mg / L.

[0014] In step 3 of the above method, 0.08-0.12% of polyethylene glycol is added as a crystal modifier during the synthesis of ferric phosphate, and 0.04-0.06 mol / L of ascorbic acid is added as a reducing protective agent during the synthesis of ferrous phosphate.

[0015] In step 4 of the above method, the centrifugal separation process uses a horizontal spiral unloading centrifuge, and the operating parameters include: the drum speed is controlled at 4000-4500 rpm; the differential speed is maintained at 10-15 rpm; dilute hydrochloric acid and deionized water are used for three-stage countercurrent washing, and the liquid-to-solid ratio of each washing is 5:1.

[0016] In step 4 of the above method, the multi-effect evaporation process for recovering sodium salt specifically includes: a first-effect evaporation at 80° C. and −90 kPa to precipitate sodium chloride crystals; and a second-effect evaporation at 110° C. and −98 kPa to precipitate sodium sulfate crystals.

[0017] In step 4 of the above method, the finally obtained ferric phosphate product is subjected to a rotary flash drying treatment, and the drying conditions include: inlet temperature 210-230° C.; outlet temperature 80-90° C.; and product moisture content ≤0.5%.

[0018] The beneficial effects of the present invention are:

[0019] The present invention converts harmful substances in traditional processes into beneficial additives through the synergistic utilization of impurity elements in waste liquid; realizes visual control of the reaction process through precise potential-pH regulation; and transforms waste liquid treatment from a cost center to a profit center through a complete resource circulation chain. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0022] refer to Figure 1 The present invention provides an innovative method for resource-resourced synthesis of ferric phosphate or ferrous phosphate from chip production wastewater, achieving efficient wastewater conversion and the preparation of high-value-added products. The specific steps include wastewater pretreatment → pH gradient control → selective precipitation → precise addition of iron salts → crystal form control → solid-liquid separation → product purification. The entire process can be divided into four major steps, detailed below:

[0023] Step 1: Waste liquid pretreatment. The waste liquid pretreatment stage is the basic link of the entire process. The waste liquid generated during the chip production process contains a large amount of suspended matter and heavy metal impurities, and needs to be initially purified through a three-stage filtration system. The first stage uses a stainless steel screen to intercept large particles of impurities larger than 500 microns; the second stage uses a bag filter to further remove particles of 50 microns; the last stage uses a ceramic membrane for precision filtration to ensure the clarity of the waste liquid. For heavy metal ions in the waste liquid, ion exchange is specially used. Under the condition of strictly controlling the pH value, harmful metals such as copper and nickel can be efficiently removed. The gradient filtration scheme shown in Table 1 is adopted.

[0024] Table 1 Filtration system parameters

[0025] Filtration stages Filter method Accuracy Flow Control Differential pressure range Level 1 stainless steel screen 500μm <![CDATA[3m 3 / h]]> ≤0.1MPa Level 2 Bag filter 50μm <![CDATA[2.5m 3 / h]]> 0.1-0.3MPa Level 3 Ceramic membrane 10kDa <![CDATA[2m 3 / h]]> 0.3-0.5MPa

[0026] Step 2: pH gradient control. This step adopts the staged alkali addition method. First, the pH value of the waste liquid is adjusted to the weak acid range of 4-6, and the stirring is continued for 30-45 minutes to allow interfering ions such as aluminum and calcium to precipitate preferentially. Then continue to add alkali to the alkaline range of pH 8-10, and react at 40±2°C for 1-2 hours to form a mixed solution containing sodium monohydrogen phosphate, sodium dihydrogen phosphate and trisodium phosphate; the phosphoric acid is completely converted into sodium phosphate salt. A high-precision pH online monitoring system is used throughout the process, combined with a pulsed alkali addition method to ensure the stability of the reaction conditions. It is particularly noteworthy that the temperature needs to be strictly controlled at around 40°C in the secondary control stage to prevent the volatilization loss of fluoride.

[0027] Step 3: Selectively add an iron salt compound to the obtained sodium phosphate solution, wherein when a trivalent iron salt is used, a ferrous phosphate precipitate is generated, and when a divalent iron salt is used, a ferrous phosphate precipitate is generated; including three steps: selective precipitation, precise addition of iron salts, and crystal form control.

[0028] Selective precipitation specifically treats silicates and fluorides in wastewater. By adding specific concentrations of calcium chloride and sodium carbonate solutions, they form insoluble compounds with impurity ions. Using highly efficient cyclone separation technology, rapid solid-liquid separation is achieved at an operating pressure of 0.3 MPa, ensuring an impurity removal rate exceeding 98%. This successful step creates a pure reaction environment for the subsequent iron salt reaction.

[0029] The precise addition of iron salt adopts differentiated process control according to the different target products. The product-oriented synthesis is achieved by controlling the process parameters in Table 2. For the synthesis of ferric phosphate, ferric chloride is used as the iron source to control Fe 3 + with PO43 - The molar ratio of is 1:1.00-1.05, and the redox potential of the system is maintained in the range of +200mV to +300mV; the oxidation state of the reaction system is precisely controlled by the redox potential (ORP) real-time monitoring system. At the same time, a special crystal modifier is added to ensure the uniformity of the product morphology. For example, the reaction system is kept at 65°C and the redox potential is +265mV, and 0.1% polyethylene glycol is added as a crystal modifier. In the preparation process of ferrous phosphate, ferrous sulfate or ferrous chloride is used as the iron source to control Fe 2+ The molar ratio of PO43- to PO43- is 3:2.00-2.10. A strict inert gas protection system needs to be established. At the same time, nitrogen is introduced to keep the dissolved oxygen concentration below 0.5 mg / L to maintain the reducing environment of the reaction system and prevent the oxidation of divalent iron.

[0030] Table 2 Synthesis process comparison table

[0031] parameter Ferric phosphate process Ferrous phosphate process Iron source <![CDATA[FeCl3·6H2O(≥96%)]]> <![CDATA[FeSO4·7H2O(≥98%)]]> molar ratio <![CDATA[Fe 3+ :PO43-=1:1.02]]> <![CDATA[Fe 2+ :PO43-=3:2.05]]> Temperature control 65±0.5℃ (PID control) 55±0.5℃ (PID control) Key additives PEG-4000 (0.1 wt%) Ascorbic acid (0.05 mol / L)

[0032] Crystal form control is a key step in ensuring product performance. During the synthesis of ferric phosphate, polyethylene glycol is added as a crystal form modifier at a concentration of 0.08-0.12% of the total mass of the system, and 0.04-0.06 mol / L of ascorbic acid is added as a reducing protective agent during the synthesis of ferrous phosphate. Ultrasound-assisted technology is used to precisely control the crystal growth process. An online laser particle size analyzer monitors particle growth in real time to ensure that the particle size distribution of the final product meets the requirements. This invention combines traditional crystallization technology with modern process analysis technology (PAT), greatly improving product consistency.

[0033] Step 4: obtaining ferric phosphate or ferrous phosphate precipitation products by centrifugal separation, and subjecting the mother liquor to multi-effect evaporation to recover the sodium salt by-product, including solid-liquid separation of the ferric phosphate or ferrous phosphate precipitation, product refining and by-product recovery.

[0034] Solid-liquid separation utilizes an advanced horizontal spiral discharge centrifuge, rotating at a high speed of 4000-4500 rpm to efficiently separate the ferric phosphate or ferrous phosphate precipitate, with the differential speed maintained at 10-15 rpm. The subsequent countercurrent washing system utilizes three stages of countercurrent washing using dilute hydrochloric acid and ultrapure water, with a liquid-to-solid ratio of 5:1 per wash cycle, to thoroughly remove impurity ions adsorbed on the product surface. This step is designed with full consideration for both the economic and environmental requirements of industrial production, and the washing liquid is recycled for reuse.

[0035] The product is refined using rotary flash drying technology. The resulting wet material follows a three-stage temperature control process (see Table 3): pre-drying at 100°C, followed by main drying at 150°C, and final drying at 200°C, ensuring a moisture content below 0.5%. Subsequently, airflow milling and ultrasonic screening are performed to obtain a final product that meets battery-grade requirements. The entire drying and milling process occurs within a closed system, preventing external contamination while ensuring operator safety.

[0036] Table 3 Drying process parameters

[0037] stage Temperature control Moisture requirements Pre-drying 100℃ ≤15％ Main drying 150℃ ≤5％ Final drying 200℃ ≤0.5％

[0038] Finally, the process's byproduct recovery system uses multiple-effect evaporation technology to crystallize and recover the sodium salts from the reaction mother liquor. This process involves: the first evaporation effect at 80°C and -90kPa precipitates sodium chloride crystals; the second evaporation effect at 110°C and -98kPa precipitates sodium sulfate crystals; and the evaporated mother liquor is recycled back into the chip production cleaning process. These byproducts can be reused as industrial raw materials, truly realizing waste resource recovery.

[0039] This invention fully considers the actual needs of industrial production. Each process is equipped with an automated control system to ensure precise control of process parameters. Experimental verification shows that this method can generate direct economic benefits of more than 1,500 yuan per ton of waste liquid treated. The purity of the resulting ferrous phosphate or ferrous phosphate product exceeds 98%, and the tap density reaches 1.48g / cm 3 , fully meeting the requirements for lithium battery positive electrode materials. In particular, in practice at an industrialization base in Jiangsu, this process has achieved continuous and stable operation for over 8,000 hours. The process exhibits excellent stability and scalability, maintaining consistent product quality from pilot trials to industrial scale-up. This invention not only solves the difficult problem of chip wastewater treatment but also creates significant economic value, providing a high-quality source of raw materials for the new energy materials industry.

[0040] The present invention converts harmful substances in traditional processes into beneficial additives through the synergistic utilization of impurity elements in waste liquid; realizes visual control of the reaction process through precise potential-pH regulation; and transforms waste liquid treatment from a cost center to a profit center through a complete resource circulation chain.

[0041] The above is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solutions and inventive concepts of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for synthesizing iron phosphate from waste liquid generated in chip or industrial production, characterized in that: The following steps are involved: Step 1: Pre-treat the chip production wastewater containing phosphoric acid and impurities by removing suspended particles through three-stage filtration and using ion exchange to adsorb heavy metal ions in the solution; Step 2: The pH value of the waste liquid is adjusted by adding alkali in stages, first precipitating interfering ions under weak acidic conditions, and then converting phosphoric acid into a sodium phosphate mixture under alkaline conditions; the process includes two stages: in the first stage, sodium hydroxide solution is added to control the pH within the range of 4-6, and stirring is continued for 30-45 minutes to preferentially precipitate aluminum and calcium ions; in the second stage, alkali is continued to be added to the pH of 8-10, and the mixture is reacted at 40±2°C for 1-2 hours to form a mixed solution containing sodium monohydrogen phosphate, sodium dihydrogen phosphate, and trisodium phosphate; Step 3: selectively adding an iron salt compound to the obtained sodium phosphate solution, wherein when a trivalent iron salt is used, a ferrous phosphate precipitate is generated, and when a divalent iron salt is used, a ferrous phosphate precipitate is generated; Step 4: obtain ferric phosphate or ferrous phosphate precipitation product by centrifugal separation, and perform multi-effect evaporation on the mother liquor to recover the sodium salt by-product.

2. The method according to claim 1, characterized in that In step 1, the three-stage filtration includes filtering the waste liquid using a stainless steel screen, a bag filter and a ceramic membrane in sequence.

3. The method according to claim 1, characterized in that In step three, the addition process of the iron salt compound is implemented as follows: when synthesizing ferric phosphate, ferric chloride or ferric sulfate is used as the iron source, and the redox potential of the system is maintained in the range of +200mV to +300mV; when synthesizing ferrous phosphate, ferrous sulfate or ferrous chloride is used as the iron source, and nitrogen is introduced to keep the dissolved oxygen concentration below 0.5mg / L.

4. The method according to claim 3, characterized in that During the synthesis of triferric phosphate, 0.08-0.12% of polyethylene glycol, which accounts for the total mass of the system, is added as a crystal modifier, and during the synthesis of diferric phosphate, 0.04-0.06 mol / L of ascorbic acid is added as a reducing protective agent.

5. The method according to claim 1, wherein In step 4, the centrifugal separation process uses a horizontal spiral unloading centrifuge, and the operating parameters include: the drum speed is controlled at 4000-4500 rpm; the differential speed is maintained at 10-15 rpm; dilute hydrochloric acid and deionized water are used for three-stage countercurrent washing, and the liquid-solid ratio of each washing is 5:

1.

6. The method according to claim 1, characterized in that In step 4, the multi-effect evaporation process for recovering sodium salt specifically includes: a first-effect evaporation at 80° C. and −90 kPa to precipitate sodium chloride crystals; a second-effect evaporation at 110° C. and −98 kPa to precipitate sodium sulfate crystals.

7. The method according to claim 1, characterized in that In step 4, the final ferric phosphate or ferrous phosphate product is subjected to rotary flash drying, and the drying conditions include: inlet temperature 210-230° C.; outlet temperature 80-90° C.; and product moisture content ≤0.5%.

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