Process analysis of wastewater from the production of ammonium iron phosphate precursor-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Process analysis of wastewater from the production of ammonium iron phosphate precursor

Published Time:

2023-11-07

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Background

Lithium iron phosphate is currently the most commonly used cathode material for lithium-ion batteries, and the mainstream production process of lithium iron phosphate uses iron phosphate as a raw material. In recent years, the rapid development of the new energy vehicle industry has driven a rapid increase in the production capacity of upstream lithium iron phosphate and iron phosphate. The production of iron phosphate generates a large amount of wastewater with a high concentration of mother liquor, making it difficult to treat. New factories have high wastewater discharge standards and strict environmental monitoring. How to efficiently solve the wastewater treatment problem of new iron phosphate factories has become a focus of iron phosphate production enterprises. Iron phosphate is a salt produced from iron and phosphorus sources, and is a precursor to lithium iron phosphate, a new energy cathode material. Its current main use is in the preparation of lithium iron phosphate.

Ammonium Iron Phosphate Production Process

Mainstream processes for iron phosphate precursors: "ferrous sulfate + ammonium phosphate," "iron block + phosphoric acid"

There are two main ways to industrially produce iron phosphate:

0 1 Method 1

Iron phosphate is prepared by reacting phosphate with ferrous sulfate. The commonly used phosphates are ammonium phosphate monobasic (ammonium dihydrogen phosphate) and diammonium phosphate (diammonium hydrogen phosphate), with ammonium phosphate monobasic being the mainstream.

0 2 Method 2

Iron phosphate is prepared by reacting purified phosphoric acid (thermal or wet method) with ferrous sulfate. These are classified as "ferrous sulfate + ammonium phosphate monobasic" and "iron block + phosphoric acid" processes.

Comparing the process routes, the main differences lie in the raw materials (phosphorus source, iron source), product quality, energy consumption, and by-products. The main by-product is ammonium sulfate. Ammonium sulfate is one of the important raw materials for fertilizers and has economic value, with a current price of about 1200-1500 yuan/ton.

Wastewater Discharge Standards and Product Requirements

Based on project experience, the wastewater discharge standards for iron phosphate factories are mainly determined by the factory's drainage conditions. Generally speaking, iron phosphate factories require zero discharge, and all wastewater needs to be recycled. When there is a wastewater treatment plant downstream of the iron phosphate factory, some of its emission standards can be in accordance with the intermediate indirect emission standards of "Discharge Standards for Pollutants from Inorganic Chemical Industry" (GB31573-2015). The main control indicators are as follows: PH 6-9, suspended solids 100mg/L, COD 200mg/L, ammonia nitrogen 40mg/L, total nitrogen 60mg/L, total phosphorus 2mg/L (other indicators refer to the requirements of the downstream wastewater treatment plant). All freshwater obtained during wastewater treatment is used as production recycled water, with a conductivity of less than 10μS/cm.

Concentrated brine is evaporated and crystallized to obtain Type I ammonium sulfate, Type II ammonium sulfate, and compound fertilizer. The sludge produced by chemical reaction precipitation in the pretreatment contains a large amount of P and N, and can be used to make compound fertilizer after drying.

Resource Utilization Treatment Process for Ammonium Iron Phosphate Wastewater

Based on previous engineering experience, the production of 1 ton of iron phosphate generates approximately 12-15 m³ of mother liquor and about 50 m³ of washing liquid. The treatment process is as follows:

Core Water Treatment Process

The main process uses an evaporation unit process using "pretreatment + membrane concentration + two-stage falling film series evaporation concentration + ammonium sulfate evaporation crystallization (mother liquor discharges miscellaneous salts) + ammonium phosphate monobasic flash cooling crystallization (mother liquor returns to ammonium sulfate system)" Process package design, where the secondary falling film concentration and ammonium sulfate crystallizer share one steam compressor to reduce system energy consumption. High-quality, high-yield ammonium phosphate monobasic and first-grade ammonium sulfate can be obtained separately, achieving higher economic benefits.

01 Pretreatment + Membrane Concentration

For zero wastewater discharge, evaporation crystallization is required for the end-of-pipe treatment. However, the concentration of pollutants in iron phosphate mother liquor and washing water is quite different. The washing water has a large volume but low salt concentration, and direct evaporation crystallization is energy-intensive. The washing water is first concentrated by reverse osmosis membrane, and the produced fresh water is reused. The concentrated liquid is mixed with the mother liquor and then concentrated, which can effectively reduce the energy consumption of the evaporation crystallization stage. The washing water first enters the equalization tank to balance the water quality, then adjusts and adds coagulant. Iron phosphate forms a precipitate under the action of the coagulant, and is removed by the sedimentation tank. The bottom sediment is concentrated in the sludge tank and then enters the filter press system, and the filtrate is returned to the sedimentation tank. The supernatant from the sedimentation tank is finely filtered and ultrafiltered to remove particles in the liquid, meeting the reverse osmosis feed water requirements (SDI ≤ 3). The clarified liquid passes through a primary reverse osmosis system, and the primary reverse osmosis clarified liquid enters a secondary reverse osmosis system. The secondary reverse osmosis clarified liquid is used as recycled water. The secondary reverse osmosis concentrated liquid is returned to the primary reverse osmosis system. The primary reverse osmosis concentrated liquid enters the mother liquor equalization tank. The mother liquor and the washing water primary reverse osmosis concentrated liquid balance the water quality in the mother liquor equalization tank, and undergo chemical precipitation to remove metal ions such as iron, calcium, magnesium, manganese, and titanium. The bottom sludge from the sedimentation tank enters the sludge tank, and the sludge is filtered, and the filtrate is returned to the mother liquor equalization tank, and the mud cake is disposed of externally. The supernatant from the sedimentation tank is micro-filtered and then ultrafiltered before entering the high-concentration reverse osmosis concentration system for further concentration. The high-concentration reverse osmosis membrane clarified liquid is treated by the secondary reverse osmosis system for washing water. The high-concentration reverse osmosis concentrated liquid is treated in a steam mechanical recompression evaporator to recover the salts, and the condensate is recycled.

0 2 Falling Film Evaporator Process Description

1. Material Flow: Raw material liquid from the pretreatment section enters the regulating storage tank, where scale inhibitors are added to reduce the risk of scaling in the evaporator heat transfer tubes and subsequent crystallization system heat transfer tubes. The raw material liquid is then pumped at the rated flow rate into the falling film condenser preheater and condenser heat exchanger (falling film and crystallization total condensate) and non-condensable gas preheater for preheating. The preheated raw material enters the primary falling film evaporator sump, where it mixes with the concentrated liquid in the primary evaporator sump for the first concentration step. After primary concentration, the raw material liquid is pumped into the secondary falling film evaporator sump, where it mixes with the concentrated liquid in the secondary evaporator sump for further concentration. The system uses a vertical falling film evaporator, with the concentrated liquid in the sump circulated by a circulation pump. After being distributed by a distributor, it falls like a thin film into the inside of the tube. The falling film evaporator uses a special distribution system to ensure that the brine is evenly and fully distributed on the inner wall of each tube, keeping the inside of each tube moist and preventing scaling. The falling brine film absorbs latent heat from the condensing steam in the shell side and evaporates. The concentrated brine after two-stage evaporation is sent to the ammonium sulfate crystallizer for crystallization to produce the product.

2. Steam Flow: Secondary steam generated in the primary falling film evaporator separator enters the secondary falling film heater for heating. The secondary steam generated in the secondary evaporation separator enters the mixed steam compressor (secondary steam from both the secondary falling film separator and the sodium sulfate crystallizer enters one compressor), where its temperature and saturated steam pressure are increased. Part of the steam enters the shell side of the primary evaporator as the system heating source, reusing the latent heat of the steam. Therefore, the evaporator only requires live steam during startup; after stable operation, the secondary steam is recovered by the compressor as the system heat source.

3. Condensate Flow: The distillate generated by the primary and secondary falling film evaporators and the ammonium sulfate heater is stored in the condensate tank and pumped into the falling film condensate preheater, where heat is transferred to the incoming water to recover as much heat as possible. The cooled condensate enters the water pool.

0 3 MVR Ammonium Sulfate Evaporation Crystallization Unit Process Description

1. Characteristics of the Forced Circulation Crystallizer: The liquid in the forced circulation evaporator is circulated by a forced circulation pump, flowing at high speed in the tubes of the forced circulation heat exchanger. The liquid exchanges heat with the steam in the shell side through the forced circulation heat exchange tube wall, raising the liquid temperature. Because the temperature of the heat-exchanged liquid is higher than the flash evaporation temperature in the forced circulation separator, when the liquid reaches the separator, it will flash evaporate. During the continuous evaporation and concentration process, the concentration of ammonium sulfate is gradually increased. When the ammonium sulfate concentration reaches supersaturation, it will crystallize in the evaporator. Then, it is pumped to the thickener by the discharge pump for further thickening, and ammonium sulfate wet material is obtained after centrifugal separation. After drying, ammonium sulfate product is obtained.

The forced circulation evaporation crystallizer unit consists of a heater, crystallizer, circulation pump, feed pump, and slurry pump.

2. Forced Circulation Crystallizer and Mother Liquor Drying Process Description: Material flow: The concentrated liquid from the falling film evaporator enters the ammonium sulfate evaporation system. The discharge from the falling film system is divided into two streams: one directly enters the crystallizer circulation pipe, and the other enters the crystallizer salt leg. The liquid entering the circulation pipe mixes with the circulating concentrated liquid and then enters the ammonium sulfate heater. The concentrated liquid in the flash tank is pumped into the evaporator by a circulation pump, obtaining heat from the condensing steam in the shell side of the forced circulation heat exchanger, forming an overheated fluid. The hydrostatic head of the water level in the heat exchanger can inhibit the boiling of the concentrated liquid in the heat exchange tubes. This is to prevent scaling. If dry spots appear on the heater pipes (this happens when boiling occurs in the pipes), scaling will occur. The overheated fluid enters the crystallizer, where flash evaporation occurs, further concentrating the brine and producing ammonium sulfate crystals. The salt slurry is discharged to the thickening and centrifugation system via the salt leg. After dehydration by the ammonium sulfate centrifuge, the salt cake is discharged into the drying and packaging system to obtain a better quality for sale. The centrifuge mother liquor enters the ammonium sulfate mother liquor tank and is returned to the ammonium sulfate crystallizer by the mother liquor pump.

3. Mother Liquor Drying Unit: Due to the presence of certain impurities in the incoming water, such as COD, sodium ions, calcium ions, and magnesium ions, these impurities will gradually accumulate as the liquid is concentrated. When the impurity concentration exceeds a certain limit, it will affect the purity of ammonium sulfate and monoammonium phosphate crystalline salts. To maintain the impurity concentration in the entire evaporation crystallization system within a certain range, a certain amount of mother liquor needs to be discharged into the salt drying system for mother liquor drying to obtain a product for outsourcing. The mother liquor drying heat source uses fresh steam for indirect drying of the material. The mother liquor drying condensate is stored in the condensate tank.

4. Monoammonium Phosphate Flash Cooling Crystallization:

Material Flow

This project plans to adopt a cold (hot) salt separation process: The saturated concentrated liquid is taken from the ammonium sulfate crystallizer, buffered in the feed tank, and then pumped into the flash cooling crystallization unit. The cooling crystallizer uses vacuum flash evaporation to maintain low-temperature evaporation. Under this condition, the solubility of monoammonium phosphate decreases, while the solubility of ammonium sulfate changes little. Monoammonium phosphate is crystallized by cooling, with inorganic matter and ammonium sulfate remaining in the cooling crystallization mother liquor. The crystallizer is equipped with a density detector. The monoammonium phosphate obtained by cooling crystallization then enters the subsequent centrifugal separation unit for separation from the mother liquor. The mother liquor after centrifugation enters the settler for sedimentation, and the smaller fine crystals are filtered and then enter the filtrate tank. The filtrate pump pumps it into the ammonium sulfate crystallizer for mother liquor reflux. The slurry at the bottom of the settler is pumped into the clear water pool to continue participating in cooling crystallization.

Condensate Flow

The secondary steam flashed from the flash tank enters the indirect condenser, where it is cooled with low-temperature water to maintain the system vacuum. The condensed secondary steam becomes liquid condensate, and the remaining condensate is pumped out of the system for process water reuse.

The treatment of wastewater from iron phosphate production varies depending on water quality. Some wastewater, during evaporation, directly produces ammonium sulfate type I salt; however, wastewater of poorer quality requires the production of ammonium sulfate type II salt. Treatment of iron phosphate production wastewater can be approached by considering the raw water quality and discharge standards to select an appropriate technical route. Proper route selection ensures stable and compliant wastewater discharge and helps to reasonably save on wastewater treatment operating costs.

Since the company's establishment over 20 years ago, we have gained a deep understanding of various water treatment processes. By introducing advanced zero-discharge technology from abroad and through continuous technological innovation and practical experience accumulation, we have become a leader in China, boasting the highest level and best operational results for zero-discharge systems. Based on our rich operational experience and unique patented technologies, we provide clients with cost-effective and highly profitable process systems, helping them reduce water treatment costs and improve operational efficiency.