Lithium iron phosphate production zero discharge process of sulfur-containing wastewater

Abstract

The present application relates to a kind of lithium iron phosphate production sulfur-containing wastewater zero discharge process, the present application can effectively recycle phosphorus iron in ammonia sulfate and first washing water, while large particle calcium sulfate is prepared, after calcination, building gypsum powder can be produced as product, and obtain the purity of 20-30% ammonia water as product, for new energy industry lithium iron phosphate production sulfur-containing wastewater comprehensive utilization and energy saving and emission reduction develop a new way, the method of the present application is applicable to the treatment of sulfur-containing wastewater in lithium iron phosphate battery production process, can effectively control the reuse of enhanced phosphorus iron, effectively control the water content and purity of generated calcium sulfate, prepare building gypsum powder, while ammonia water can be produced, realize the zero emission resource treatment of lithium iron phosphate sulfur-containing wastewater.

Description

Technical Field

[0001] This invention relates to a zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production, belonging to the field of wastewater treatment technology. Background Technology

[0002] In recent years, the new energy vehicle industry has developed rapidly, and lithium iron phosphate (LFP) batteries have been widely used in this field. LFP production processes include solid-phase and liquid-phase methods, with the carbothermic reduction method within the solid-phase synthesis method being widely applied. Production involves reacting water-soluble monoammonium phosphate (MAP) and ferrous sulfate to produce iron phosphate. Then, iron phosphate, lithium carbonate, and an organic carbon source are mixed, coarsely ground, finely ground, spray-dried, sintered, and pulverized to obtain lithium iron phosphate. LFP production generates a large volume of wastewater, primarily containing high concentrations of ammonia nitrogen, sulfate, and hardness ions. The wastewater has a low organic content, consisting mostly of inorganic ions.

[0003] Traditional lithium iron phosphate wastewater treatment processes only recover some components from the wastewater, such as recovering sulfate to prepare calcium sulfate, and recovering ammonia nitrogen to prepare ammonium salts or ammonia water. The calcium sulfate obtained by this method can basically only be treated as solid waste, so it is difficult to fully utilize the water as a resource. Alternatively, the wastewater can be pretreated and then evaporated and crystallized to obtain ammonium sulfate, but the investment and operating costs are high, which is not in line with the development direction of energy conservation, environmental protection and circular economy. Summary of the Invention

[0004] The purpose of this invention is to provide a process for treating sulfur-containing wastewater from lithium iron phosphate production in the new energy industry to prepare high-quality calcium sulfate and ammonia. This process breaks away from the traditional wastewater treatment model of evaporation and crystallization to produce ammonia sulfate, solving the high energy consumption problem of traditional treatment methods. Simultaneously, the solution provided by this invention avoids the costs of neutralization and sedimentation in the front-end water treatment and the increased cost of reverse osmosis membranes. This invention avoids the high costs of traditional treatment methods by producing calcium sulfate as a byproduct, while ensuring the quality of the byproduct calcium sulfate and ammonia. In this method, an ultrafiltration membrane is first used to remove small-particle inorganic materials from the production wastewater, while also removing residual lithium iron phosphate, thus reducing the impact of other inorganic substances during the calcium sulfate precipitation and crystallization process. Furthermore, by adding calcium sulfate seed crystals during the neutralization and precipitation process, the obtained calcium sulfate has a better morphology and can be used in building materials.

[0005] A zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production includes the following steps:

[0006] Step 1: Use an ultrafiltration membrane to filter the ammonium sulfate mother liquor and the first wash water generated in the lithium iron phosphate production to remove iron phosphate and obtain the ultrafiltration membrane clear liquid.

[0007] Step 2: Concentrate the ultrafiltration membrane supernatant using a reverse osmosis membrane;

[0008] Step 3: Add calcium hydroxide and crystallization promoter to the obtained reverse osmosis membrane concentrate to make the pH of the system reach 10-12, and perform solid-liquid separation on the obtained first precipitate to obtain calcium sulfate seed crystals and ammonia water.

[0009] Step 4: Add calcium sulfate seed crystals to the ultrafiltration membrane supernatant, and continue to add calcium hydroxide to adjust the pH to 3-5, carry out the precipitation reaction and solid-liquid separation to obtain acidic calcium sulfate solid and reaction solution;

[0010] Step 5: Add calcium hydroxide to the reaction solution obtained in step 4 to adjust the pH to 6-8, carry out the precipitation reaction and solid-liquid separation to obtain neutral calcium sulfate and ammonia.

[0011] In step 1, the ultrafiltration membrane is a ceramic membrane with a pore size of 4-10 nm.

[0012] In step 1, before the ultrafiltration membrane is used for filtration, larger suspended particles are removed by a pre-filter, which is a ceramic membrane with a pore size of 50-500 nm.

[0013] In step 2, the concentration factor during the concentration process of the reverse osmosis membrane is 2-10 times.

[0014] The concentration of the ammonia solution is 20-30%.

[0015] It also includes: washing the neutral calcium sulfate in step 5 and / or the acidic calcium sulfate in step 4 with water and then drying them for use in building materials.

[0016] The crystallization promoter is a mixture of calcium sulfate, sodium sulfate and sodium citrate in a weight ratio of 3-5:1-2:0.5-0.8.

[0017] The solid-liquid separation is achieved by one of centrifugal separation, plate and frame filtration, or membrane separation.

[0018] A zero-discharge device for sulfur-containing wastewater from lithium iron phosphate production includes:

[0019] Ultrafiltration membranes are used to filter sulfur-containing wastewater containing lithium iron phosphate to remove lithium iron phosphate.

[0020] A reverse osmosis membrane is connected to the permeate side of an ultrafiltration membrane and is used to concentrate the supernatant from the ultrafiltration membrane.

[0021] The first precipitation reactor is connected to the concentration side of the reverse osmosis membrane and is used to carry out a precipitation reaction on the concentrated liquid to produce calcium sulfate.

[0022] The first solid-liquid separation device is connected to the first precipitation reactor and is used to wash and separate the generated calcium sulfate to obtain calcium sulfate seed crystals.

[0023] The second precipitation reactor is connected to the concentration side of the reverse osmosis membrane and is used to carry out a precipitation reaction on the concentrated liquid to produce calcium sulfate.

[0024] The second solid-liquid separation device is connected to the second precipitation reactor and is used to wash and separate the generated calcium sulfate to obtain acidic calcium sulfate.

[0025] The third precipitation reactor is connected to the concentration side of the reverse osmosis membrane and is used to precipitate the concentrated liquid to produce calcium sulfate; and the third precipitation reactor is connected to the clear liquid side of the second solid-liquid separation device.

[0026] The third solid-liquid separation unit, connected to the third precipitation reactor, is used to wash and separate the generated calcium sulfate to obtain neutral calcium sulfate.

[0027] It also includes: a first addition tank and a crystallization promoter addition tank, connected to the first precipitation reactor, used to add calcium hydroxide and crystallization promoter to the first precipitation reactor, respectively.

[0028] It also includes a second addition tank and a seed crystal addition tank, connected to the second precipitation reactor, used to add calcium hydroxide and the seed crystals obtained from the first solid-liquid separation device to the second precipitation reactor, respectively.

[0029] It also includes a pre-filter, located upstream of the ultrafiltration membrane inlet, used to filter and remove large suspended particles from wastewater.

[0030] It also includes: a third dosing tank, connected to the third precipitation reactor, used to add calcium hydroxide to the third precipitation reactor.

[0031] The ultrafiltration membrane is a ceramic membrane with a pore size of 4-10 nm.

[0032] The pre-filter is a ceramic membrane with a pore size of 50-500nm or a plate and frame filter press.

[0033] The first solid-liquid separation device, the second solid-liquid separation device, and the third solid-liquid separation device are any one of a centrifuge, a plate and frame filter, or a membrane filter.

[0034] Beneficial effects

[0035] This invention provides a process for zero discharge of sulfur-containing wastewater from new energy sources. It primarily employs ultrafiltration membranes to filter the water, recovering lithium iron phosphate. Subsequent concentration and stepwise precipitation processes can then be used to recover seed crystals and acidic / neutral calcium sulfate products, achieving zero discharge and resource utilization of the wastewater.

[0036] This method uses a reverse osmosis membrane to concentrate wastewater. The concentrated mother liquor is then mixed with calcium hydroxide and a crystallizing agent to prepare large-particle calcium sulfate dihydrate. The resulting gypsum is used to make building gypsum powder, solving the problem that existing methods of directly adding calcium precipitants to sulfur-containing lithium iron phosphate wastewater cannot yield reusable gypsum. The supernatant obtained in the above steps can be used to recover ammonia nitrogen and prepare ammonia water. This process is reasonable, and the recovered ammonia water and calcium sulfate can be recycled. Furthermore, this process has advantages such as low energy consumption and small footprint, achieving zero-discharge treatment and low-energy resource utilization of lithium iron phosphate production wastewater. Attached Figure Description

[0037] Figure 1 This is a flow chart of a zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production in the new energy industry, as described in this invention.

[0038] Figure 2 This is a diagram of a zero-discharge process device for producing sulfur-containing wastewater in the production of lithium iron phosphate in the new energy industry, as described in this invention.

[0039] Figure 3 This is a micrograph of the acidic calcium sulfate obtained in Example 1.

[0040] Figure 4 This is a photomicrograph of the acidic calcium sulfate obtained in Comparative Example 1.

[0041] Figure 5 This is a photomicrograph of the neutral calcium sulfate obtained in Comparative Example 2.

[0042] Among them, 1. Ultrafiltration membrane; 1a. Pre-filter; 2. Reverse osmosis membrane; 3. First precipitation reactor; 4. First solid-liquid separation device; 5. Second precipitation reactor; 6. Second solid-liquid separation device; 7. Third precipitation reactor; 8. Third solid-liquid separation device; 9. First dosing tank; 10. Second dosing tank; 11. Third dosing tank. Detailed Implementation

[0043] This invention relates to a zero-discharge process for sulfur-containing wastewater in the production of lithium iron phosphate (LiFePO4) for new energy applications. In the production of iron phosphate and ammonium sulfate using the reaction of water-soluble monoammonium phosphate and ferrous sulfate, the ammonium sulfate mother liquor and the washed iron phosphate are treated to obtain a first wash. This invention uses an ultrafiltration membrane to recover iron phosphate from the mother liquor and the first wash water, and uses a reverse osmosis membrane to concentrate the obtained ammonium sulfate solution 2-3 times, with the resulting pure water being reused in the process. At this point, the pH of the ammonium sulfate solution is approximately 2, and it enters the downstream reaction tank. Calcium hydroxide and gypsum crystal growth agent are added to the reaction tank, and the pH of the reaction system is approximately 11. Solid-liquid separation is further performed using a plate separator, yielding an ammonia solution with a purity of approximately 20-30%, and a solid mixture containing small amounts of calcium hydroxide and calcium sulfate. Add ammonium sulfate mother liquor to the solid mixture of calcium hydroxide and calcium sulfate again until the reaction system is balanced to 4-5. Stop the experiment to obtain calcium sulfate with a purity greater than 90% and a water content of less than 10%, as well as a solution with a pH of 4. Return this solution to the ammonium sulfate solution at the front end to neutralize it to a pH of 11, so that all the ammonium sulfate in the solution is reacted into calcium sulfate and ammonia water.

[0044] Based on the above processing technology, the device provided by the present invention is as follows: Figure 2 As shown, it specifically includes:

[0045] Ultrafiltration membrane 1 is used to filter sulfur-containing wastewater containing lithium iron phosphate to remove lithium iron phosphate.

[0046] Reverse osmosis membrane 2 is connected to the permeate side of ultrafiltration membrane 1 and is used to concentrate the ultrafiltration membrane supernatant.

[0047] The first precipitation reactor 3 is connected to the concentration side of the reverse osmosis membrane and is used to carry out a precipitation reaction on the concentrated liquid to generate calcium sulfate.

[0048] The first solid-liquid separation device 4 is connected to the first precipitation reactor 3 and is used to wash and separate the generated calcium sulfate to obtain calcium sulfate seed crystals.

[0049] The second precipitation reactor 5 is connected to the concentration side of the reverse osmosis membrane and is used to carry out a precipitation reaction on the concentrated liquid to generate calcium sulfate.

[0050] The second solid-liquid separation device 6 is connected to the second precipitation reactor 5 and is used to wash and separate the generated calcium sulfate to obtain acidic calcium sulfate.

[0051] The third precipitation reactor 7 is connected to the concentration side of the reverse osmosis membrane and is used to precipitate the concentrated liquid to generate calcium sulfate; and the third precipitation reactor 7 is connected to the clear liquid side of the second solid-liquid separation device 6.

[0052] The third solid-liquid separation device 8 is connected to the third precipitation reactor 7 and is used to wash and separate the generated calcium sulfate to obtain neutral calcium sulfate.

[0053] It also includes: a first addition tank 9-1 and a crystallization promoter addition tank 9-2, which are connected to the first precipitation reactor 3 and are used to add calcium hydroxide and crystallization promoter to the first precipitation reactor 3, respectively.

[0054] It also includes: a second addition tank 10-1 and a seed crystal addition tank 10-2, which are connected to the second precipitation reactor 5 and are used to add calcium hydroxide and the seed crystals obtained from the first solid-liquid separation device 4 to the second precipitation reactor 5, respectively.

[0055] It also includes: a pre-filter 1a, located upstream of the feed inlet of the ultrafiltration membrane 1, used to filter and remove large suspended particles in the wastewater.

[0056] It also includes: a third dosing tank 11, connected to the third precipitation reactor 7, for adding calcium hydroxide to the third precipitation reactor 7.

[0057] The ultrafiltration membrane 1 is a ceramic membrane with a pore size of 4-10 nm.

[0058] The pre-filter 1a is a ceramic membrane or a plate and frame filter press with a pore size of 50-500nm.

[0059] The first solid-liquid separation device 4, the second solid-liquid separation device 6, and the third solid-liquid separation device 8 are any one of a centrifuge, a plate and frame filter, or a membrane filter.

[0060] Example 1

[0061] The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production involves the following steps:

[0062] (1) Ultrafiltration membrane clarification: The mother liquor obtained after filtration through a 500 nm ceramic membrane was sent to a 5 nm pore size ultrafiltration membrane for clarification, resulting in a clarified ammonium sulfate mother liquor, and ferric phosphate was recovered. The ammonium sulfate mother liquor contained 35,000 ppm of ammonium and 9,000 ppm of sulfate.

[0063] (2) Neutralization with calcium hydroxide: Add calcium hydroxide and crystallizing agent (calcium sulfate, sodium sulfate and sodium citrate mixed in a weight ratio of 4:1:0.6) to the reaction vessel, add washing water and slurry, add 2% crystallizing agent as required for neutralization, add ammonium sulfate solution obtained in step (1) to carry out neutralization reaction, and stop the reaction when the pH of the reaction system is about 11.

[0064] (3) First-stage plate and frame separation: The solution obtained in the above steps is separated by plate and frame separation to obtain ammonia water with a purity of 23.5% and calcium sulfate with a free water content of 24.65% and a purity of 82.52%, which are used as seed crystals.

[0065] (4) Calcium sulfate crystal growth: The seed crystal solid obtained in the above steps is placed in a reaction vessel, washing water is added and slurry is added, and then the ammonium sulfate solution obtained in step (1) is added again, and calcium hydroxide is added to continue the reaction until the pH of the system is about 4. The reaction is then stopped to obtain acidic calcium sulfate.

[0066] (5) Secondary plate and frame separation: The solution obtained in the above steps is separated by centrifugation to obtain calcium sulfate with a purity of more than 97.91% and a free water content of 11.11%.

[0067] (6) Wastewater reuse: The solution with pH 4 obtained in the above steps is reused. It is mixed with the ammonium sulfate solution obtained in step (1), and calcium hydroxide is added to neutralize the reaction. Finally, all of it is made into ammonia water.

[0068] (7) Washing with calcium sulfate: Add water to wash the neutral calcium sulfate obtained in step (6) above until the pH of the system is about 7;

[0069] (8) Three-stage plate and frame separation: The solution obtained in the above steps is dehydrated and separated by centrifugation to obtain calcium sulfate and washing water; (9) Washing water reuse: The washing water obtained in the above steps is used in steps (2) and (4) to pulp the neutralizing agent.

[0070] The 2-hour flexural and compressive strengths of the acidic calcium sulfate obtained above are 3.5 MPa and 8.4 MPa, respectively, while those of the neutral calcium sulfate are 3.7 MPa and 9.2 MPa, respectively, meeting the specifications for building gypsum powder. A micrograph of the acidic calcium sulfate is shown below. Figure 3 As shown, the particles are plump and have a uniform particle size distribution.

[0071] Example 2

[0072] The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production involves the following steps:

[0073] (1) Ultrafiltration membrane clarification: The mother liquor obtained after filtration through a 500 nm ceramic membrane was sent to a 5 nm pore size ultrafiltration membrane for clarification, resulting in a clarified ammonium sulfate mother liquor, and ferric phosphate was recovered. The ammonium sulfate mother liquor contained 35,000 ppm of ammonium and 9,000 ppm of sulfate.

[0074] (2) Neutralization with calcium hydroxide: Add calcium hydroxide and crystallizing agent (calcium sulfate, sodium sulfate and sodium citrate mixed in a weight ratio of 4:1:0.6) to the reaction vessel, add washing water and slurry, add 2% crystallizing agent as required for neutralization, add ammonium sulfate solution obtained in step (1) to the neutralization reaction, and stop the reaction when the pH of the reaction system is about 11.

[0075] (3) First-stage plate and frame separation: The solution obtained in the above steps is separated by plate and frame separation to obtain ammonia water with a purity of 24.6% and calcium sulfate with a free water content of 22.48% and a purity of 85.79%, which are used as seed crystals.

[0076] (4) Calcium sulfate crystal growth: The seed crystal solid obtained in the above steps is placed in a reaction vessel, washing water is added and slurry is added, and then the ammonium sulfate solution obtained in step (1) is added again, and calcium hydroxide is added to continue the reaction until the pH of the system is about 4. The reaction is then stopped to obtain acidic calcium sulfate.

[0077] (5) Secondary plate and frame separation: The solution obtained in the above steps is separated by centrifugation to obtain calcium sulfate with a purity of more than 96.53% and a free water content of 10.57%.

[0078] (6) Wastewater reuse: The solution with pH 4 obtained in the above steps is reused. It is mixed with the ammonium sulfate solution obtained in step (1) and added to calcium hydroxide to neutralize the ammonium sulfate solution and react with the calcium hydroxide. Finally, all of it is made into ammonia water.

[0079] (7) Washing with calcium sulfate: Add water to wash the neutral calcium sulfate obtained in step (6) above until the pH of the system is about 7;

[0080] (8) Three-stage plate and frame separation: The solution obtained in the above steps is dehydrated and separated by centrifugation to obtain calcium sulfate and washing water; (9) Washing water reuse: The washing water obtained in the above steps is used in steps (2) and (4) to pulp the neutralizing agent.

[0081] The 2-hour flexural and compressive strengths of the acidic calcium sulfate obtained above are 2.9 MPa and 8.3 MPa, respectively, and the 2-hour flexural and compressive strengths of the neutral calcium sulfate are 3.9 MPa and 9.3 MPa, respectively, which meet the specifications of building gypsum powder.

[0082] Example 3

[0083] The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production involves the following steps:

[0084] (1) Ultrafiltration membrane clarification: The mother liquor obtained after filtration through a 500 nm ceramic membrane was sent to a 5 nm pore size ultrafiltration membrane for clarification, resulting in a clarified ammonium sulfate mother liquor, and ferric phosphate was recovered. The ammonium sulfate mother liquor contained 35,000 ppm of ammonium and 9,000 ppm of sulfate.

[0085] (2) Neutralization with calcium hydroxide: Add calcium hydroxide and crystallizing agent (calcium sulfate, sodium sulfate and sodium citrate mixed in a weight ratio of 4:1:0.6) to the reaction vessel, add washing water and slurry, add 3% crystallizing agent as required for neutralization, add ammonium sulfate solution obtained in step (1) to neutralize the reaction, and stop the reaction when the pH of the reaction system is about 11.

[0086] (3) First-stage plate and frame separation: The solution obtained in the above steps is separated by plate and frame separation to obtain ammonia water with a purity of 23.7% and calcium sulfate with a free water content of 22.64% and a purity of 87.31%, which are used as seed crystals.

[0087] (4) Calcium sulfate crystal growth: The seed crystal solid obtained in the above steps is placed in a reaction vessel, washing water is added and slurry is added, and then the ammonium sulfate solution obtained in step (1) is added again, and calcium hydroxide is added to continue the reaction until the pH of the system is about 4. The reaction is then stopped to obtain acidic calcium sulfate.

[0088] (5) Secondary plate and frame separation: The solution obtained in the above steps is separated by centrifugation to obtain calcium sulfate with a purity of more than 97.42% and a free water content of 9.62%.

[0089] (6) Wastewater reuse: The solution with pH 4 obtained in the above steps is reused. It is mixed with the ammonium sulfate solution obtained in step (1), and calcium hydroxide is added to neutralize the reaction. Finally, all of it is made into ammonia water.

[0090] (7) Washing with calcium sulfate: Add water to wash the neutral calcium sulfate obtained in step (6) above until the pH of the system is about 7;

[0091] (8) Three-stage plate and frame separation: The solution obtained in the above steps is dehydrated and separated by centrifugation to obtain calcium sulfate and wash water; (9) Wash water reuse: The wash water obtained in the above steps is used in steps (2) and (4) to slurry the neutralizing agent. The 2-hour flexural and compressive strengths of the acidic calcium sulfate obtained above are 3.1 MPa and 7.5 MPa, and the 2-hour flexural and compressive strengths of the neutral calcium sulfate are 3.8 MPa and 8.9 MPa, which meet the specifications of building gypsum powder.

[0092] Compare with Example 1

[0093] The difference from Example 1 is that an ultrafiltration membrane was not used to filter and remove small-diameter suspended solids in the wastewater.

[0094] The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production involves the following steps:

[0095] (1) Ultrafiltration membrane clarification: The mother liquor obtained after filtration through a 500nm ceramic membrane is ammonium sulfate mother liquor with an ammonium content of 35,000ppm and a sulfate content of 9,000ppm.

[0096] (2) Neutralization with calcium hydroxide: Add calcium hydroxide and crystallizing agent (calcium sulfate, sodium sulfate and sodium citrate mixed in a weight ratio of 4:1:0.6) to the reaction vessel, add washing water and slurry, add 2% crystallizing agent as required for neutralization, add ammonium sulfate solution obtained in step (1) to carry out neutralization reaction, and stop the reaction when the pH of the reaction system is about 11.

[0097] (3) First-stage plate and frame separation: The solution obtained in the above steps is separated by plate and frame separation to obtain ammonia water with a purity of 23.5% and calcium sulfate with a free water content of 24.65% and a purity of 82.52%, which are used as seed crystals.

[0098] (4) Calcium sulfate crystal growth: The seed crystal solid obtained in the above steps is placed in a reaction vessel, washing water is added and slurry is added, and then the ammonium sulfate solution obtained in step (1) is added again, and calcium hydroxide is added to continue the reaction until the pH of the system is about 4. The reaction is then stopped to obtain acidic calcium sulfate.

[0099] (5) Secondary plate and frame separation: The solution obtained in the above steps is separated by centrifugation to obtain calcium sulfate with a purity of more than 97.91% and a free water content of 11.11%.

[0100] (6) Wastewater reuse: The solution with pH 4 obtained in the above steps is reused. It is mixed with the ammonium sulfate solution obtained in step (1), and calcium hydroxide is added to neutralize the reaction. Finally, all of it is made into ammonia water.

[0101] (7) Washing with calcium sulfate: Add water to wash the neutral calcium sulfate obtained in step (6) above until the pH of the system is about 7;

[0102] (8) Three-stage plate and frame separation: The solution obtained in the above steps is dehydrated and separated by centrifugation to obtain calcium sulfate and washing water; (9) Washing water reuse: The washing water obtained in the above steps is used in steps (2) and (4) to pulp the neutralizing agent.

[0103] The 2-hour flexural and compressive strengths of the acidic calcium sulfate obtained above were 2.6 MPa and 5.3 MPa, respectively, while those of the neutral calcium sulfate were 2.7 MPa and 5.6 MPa, respectively. A micrograph of the acidic calcium sulfate is shown below. Figure 4 As shown, it exhibits poor crystallinity and is in the form of fine fibers.

[0104] Compare with Example 2

[0105] The difference from Example 1 is that no seed crystals were added during the precipitation process of acidic calcium sulfate.

[0106] The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production involves the following steps:

[0107] (1) Ultrafiltration membrane clarification: The mother liquor obtained after filtration through a 500 nm ceramic membrane was sent to a 5 nm pore size ultrafiltration membrane for clarification, resulting in a clarified ammonium sulfate mother liquor, and ferric phosphate was recovered. The ammonium sulfate mother liquor contained 35,000 ppm of ammonium and 9,000 ppm of sulfate.

[0108] (4) Calcium sulfate crystal growth: Add ammonium sulfate solution to the reaction vessel and add calcium hydroxide at the same time to continue the reaction until the pH of the system is about 4. Stop the reaction to obtain acidic calcium sulfate.

[0109] (5) Secondary plate and frame separation: The solution obtained in the above steps is separated by centrifugation to obtain calcium sulfate with a purity of more than 97.91% and a free water content of 11.11%.

[0110] (6) Wastewater reuse: The solution with pH 4 obtained in the above steps is reused. It is mixed with the ammonium sulfate solution obtained in step (1), and calcium hydroxide is added to neutralize the reaction. Finally, all of it is made into ammonia water.

[0111] (7) Washing with calcium sulfate: Add water to wash the neutral calcium sulfate obtained in step (6) above until the pH of the system is about 7;

[0112] (8) Three-stage plate and frame separation: The solution obtained in the above steps is dehydrated and separated by centrifugation to obtain calcium sulfate and washing water; (9) Washing water reuse: The washing water obtained in the above steps is used in steps (2) and (4) to pulp the neutralizing agent.

[0113] The 2-hour flexural and compressive strengths of the acidic calcium sulfate obtained above are 3.5 MPa and 8.4 MPa, respectively, while those of the neutral calcium sulfate are 3.7 MPa and 9.2 MPa, respectively, meeting the specifications for building gypsum powder. A micrograph of the neutral calcium sulfate is shown below. Figure 4 As shown, it exhibits poor crystallinity and is in the form of fine fibers.

Claims

1. A zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production, characterized in that, Includes the following steps: Step 1: A ceramic ultrafiltration membrane with a pore size of 4-10 nm is used to filter the ammonium sulfate mother liquor and the first wash water generated in the lithium iron phosphate production to remove iron phosphate and obtain ultrafiltration membrane clear liquid; before filtration, the ultrafiltration membrane is also filtered by a pre-filter to remove larger suspended particles, and the pre-filter is a ceramic membrane with a pore size of 50-500 nm. Step 2: Concentrate the ultrafiltration membrane supernatant using a reverse osmosis membrane. The concentration factor during the reverse osmosis membrane concentration process is 2-10 times. Step 3: Add calcium hydroxide and crystallization promoter to the obtained reverse osmosis membrane concentrate to make the pH of the system reach 10-12, and perform solid-liquid separation on the obtained first precipitate to obtain calcium sulfate seed crystals and ammonia water. Step 4: Add calcium sulfate seed crystals to the ultrafiltration membrane supernatant, and continue to add calcium hydroxide to adjust the pH to 3-5, carry out the precipitation reaction and solid-liquid separation to obtain acidic calcium sulfate solid and reaction solution; Step 5: Add calcium hydroxide to the reaction solution obtained in step 4 to adjust the pH to 6-8, carry out precipitation reaction and solid-liquid separation to obtain neutral calcium sulfate and ammonia water. The crystallization promoter is a mixture of calcium sulfate, sodium sulfate, and sodium citrate in a weight ratio of 3-5:1-2:0.5-0.8; the concentration of the ammonia water is 20-30%.

2. The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production according to claim 1, characterized in that, Also includes: The neutral calcium sulfate in step 5 and / or the acidic calcium sulfate in step 4 are washed with water and then dried for use in building materials.

3. The zero-discharge process for sulfur-containing wastewater from lithium iron phosphate production according to claim 1, characterized in that, The solid-liquid separation is achieved by one of centrifugal separation, plate and frame filtration, or membrane separation.

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