Graphite purification waste liquid treatment method and treatment system

Abstract

The present application belongs to the technical field of waste liquid treatment, and particularly relates to a graphite purification waste liquid treatment method and a treatment system. The present application introduces a pre-acid pickling system to utilize waste acid twice, reduces the amount of acid used in the acid pickling process of graphite purification, improves the use efficiency of acid liquid, thereby reducing the cost of the graphite purification process and significantly enhancing the overall economy. The present application also introduces a sodium fluorosilicate production line, effectively realizing the resource utilization of residual fluorosilicic acid and fluorosilicate in waste water. Not only can the operation cost of reagents in the subsequent water treatment process be effectively reduced, but also the produced sodium fluorosilicate can be sold to improve economic benefits. An evaporation crystallization system is used to crystallize and recover valuable salts in waste water. The obtained crystalline salt can be further used as a raw material for other products. The treatment system provided by the present application can realize the resource utilization and recycling of waste acid and waste water, and achieve zero emission.

Description

[0001] This invention is a divisional application based on the application filed on December 7, 2023, with application number 202311667004.0 and entitled "A graphite purification waste liquid treatment system and a graphite purification waste liquid treatment method". Technical Field

[0002] This invention belongs to the field of waste liquid treatment technology, specifically relating to a method and system for treating graphite purification waste liquid. Background Technology

[0003] With the continuous development of industry, the demand for graphite materials with wide applications is constantly increasing. In the purification process of natural graphite, hydrofluoric acid, nitric acid, and hydrochloric acid are used in a certain order and proportion to acid-wash the graphite, generating a large amount of waste acid. After acid washing, the graphite needs to be washed with water to remove residual acid, which in turn generates a large amount of acid-containing washing wastewater. This wastewater is abundant, complex in composition, and highly acidic. If the wastewater generated from graphite purification is discharged directly without treatment or with only simple treatment, it will seriously pollute the environment.

[0004] Currently, the main method for treating wastewater generated during graphite purification is to collect and transport it to a wastewater treatment system. However, due to the excessive use of acid during the production process, a large amount of unused acid is present in the pickling wastewater. Directly introducing this wastewater into the wastewater treatment process not only increases the difficulty and cost of treatment (such as reagent costs and sludge treatment costs), but also results in a serious waste of acid and water resources.

[0005] Therefore, developing a clean production process that can properly dispose of the waste liquid generated in the graphite purification process while realizing the resource utilization of waste acid and wastewater and achieving zero wastewater discharge is an urgent problem to be solved in the graphite purification industry. Summary of the Invention

[0006] In view of this, the present invention provides a method and system for treating graphite purification waste liquid. By using the treatment method and system provided by the present invention to treat graphite purification waste liquid, waste acid and wastewater can be recycled and reused.

[0007] To address the aforementioned technical problems, this invention provides a method for treating graphite purification waste liquid, comprising the following steps: Hydrofluoric acid waste liquid is passed into the hydrofluoric acid pre-washing unit to pre-wash graphite with hydrofluoric acid, and the pre-washed graphite and hydrofluoric acid waste liquid are obtained respectively. The pre-acid-washed hydrofluoric acid waste liquid is passed into the sodium fluorosilicate production unit to obtain sodium fluorosilicate and the first wastewater, respectively. The nitric acid / hydrochloric acid mixed acid waste liquid is passed into the nitric acid / hydrochloric acid mixed acid pre-acid washing unit to pre-acid wash the graphite pickled with hydrofluoric acid, and the pre-acid washed graphite and the pre-acid washed nitric acid / hydrochloric acid mixed acid waste liquid are obtained respectively. The first wastewater and the pre-washed nitric acid / hydrochloric acid mixed acid waste liquid are passed into the first neutralization sedimentation tank and mixed with the first calcium-containing substance to carry out the first precipitation reaction, and the first sludge and the first supernatant are obtained respectively. The first supernatant is passed into the first softening sedimentation tank for first softening, and the second sludge and the second supernatant are obtained respectively. The second supernatant is passed into a filtration unit for filtration to obtain the liquid phase; The liquid phase is passed into an evaporation and crystallization unit for evaporation and crystallization to obtain crystalline salt; The first and second sludge were transferred to a sludge tank for solid-liquid separation to obtain sludge cake.

[0008] Preferably, the method further includes the following steps: The washing wastewater is fed into a wastewater pH adjustment tank and mixed with alkaline substances for pH adjustment. Then it is transferred to a second neutralization sedimentation tank and mixed with a second calcium-containing substance for a second precipitation reaction, resulting in a third sludge and a third supernatant. The third supernatant is passed into the second softening sedimentation tank for second softening, to obtain the fourth sludge and the fourth supernatant respectively. The fourth supernatant is passed into a V-type filter tank for filtration to obtain filter water; The water produced by the filter bed is subjected to ultrafiltration, first-stage reverse osmosis and second-stage reverse osmosis in sequence to obtain recycled water; The concentrate obtained from the first-stage reverse osmosis is subjected to two-stage reverse osmosis to obtain two-stage reverse osmosis concentrate and two-stage reverse osmosis desalination, respectively. The concentrated water from the two-stage reverse osmosis process is softened in the third stage and then evaporated and crystallized.

[0009] Preferably, the method further includes the following steps: Two stages of reverse osmosis freshwater are fed into a second-stage reverse osmosis unit for second-stage reverse osmosis. The concentrate from the second-stage reverse osmosis is returned for ultrafiltration; The third sludge, the fourth sludge, and the third softened sludge were transferred to a sludge tank for solid-liquid separation to obtain sludge cake.

[0010] Preferably, the pH value of the system after pH adjustment is 6 to 9.

[0011] Preferably, the first calcium-containing substance and the second calcium-containing substance each independently include lime and / or calcium chloride.

[0012] Preferably, the V-shaped filter pool has a single-layer sand filter media structure, and the filter sand used in the V-shaped filter pool is quartz sand with an average particle size of 1-2 mm.

[0013] Preferably, the ultrafiltration is used to remove impurities from water, the impurities including organic macromolecules and colloids with a particle size of 1 to 10 μm.

[0014] Preferably, the two-stage reverse osmosis adopts a disc tube reverse osmosis system with an operating pressure of 70–200 bar.

[0015] Preferably, the evaporation crystallization temperature is 35–70°C, the vapor compression ratio is 2.5–3, and the gas flow purging rate is 15–40 m / s.

[0016] Preferably, the softening agents used for the first, second, and third softening processes each independently include lime, sodium carbonate, or caustic soda.

[0017] Preferably, the hydrofluoric acid pre-washing process further includes filtering the hydrofluoric acid pre-washing system.

[0018] Preferably, the process after the nitric acid / hydrochloric acid mixed acid pre-washing further includes filtering the system after the nitric acid / hydrochloric acid mixed acid pre-washing.

[0019] The present invention also provides a graphite purification waste liquid treatment system corresponding to the above method, comprising a hydrofluoric acid pre-washing unit and a sodium fluorosilicate production unit connected in sequence. The nitric acid / hydrochloric acid mixed acid pre-washing unit, the first neutralization precipitation tank, the first softening precipitation tank, the filtration unit and the evaporation crystallization unit are connected in sequence. The inlet of the first neutralization sedimentation tank is also connected to the sodium fluorosilicate production unit; The sedimentation outlets of the first neutralization sedimentation tank and the first softening sedimentation tank are connected to a sludge tank; the sludge tank is connected to a solid-liquid separation unit.

[0020] Preferably, it also includes a wastewater pH adjustment tank, a second neutralization sedimentation tank, a second softening sedimentation tank, a V-type filter tank, a filter product water tank, an ultrafiltration unit, a first-stage reverse osmosis unit, a second-stage reverse osmosis unit, and a water storage tank connected in sequence. The concentrate outlet of the first-stage reverse osmosis unit is sequentially connected to the concentrate tank, the two-stage reverse osmosis unit, and the third softening sedimentation tank; the concentrate outlet of the two-stage reverse osmosis unit is connected to the inlet of the second-stage reverse osmosis unit, and the concentrate outlet of the second-stage reverse osmosis unit is connected to the inlet of the ultrafiltration unit. The sedimentation outlets of the second neutralization sedimentation tank, the second softening sedimentation tank, and the third softening sedimentation tank are connected to the sludge tank; The supernatant outlet of the third softening sedimentation tank is connected to the evaporation crystallization unit.

[0021] Preferably, the filtration unit is a multi-media filter.

[0022] Preferably, the solid-liquid separation unit is a plate and frame filter press.

[0023] This invention provides a method and system for treating graphite purification wastewater. By introducing a pre-acid washing system for secondary utilization of waste acid, the amount of acid used in the graphite purification process is reduced (by 30-50%), significantly improving the efficiency of acid utilization and thus lowering the cost of the graphite purification process, significantly enhancing overall economic efficiency. This invention also introduces a sodium fluorosilicate production line, effectively realizing the resource utilization of residual fluorosilicic acid and fluorosilicates in the wastewater, thereby effectively reducing the operating costs of reagents in subsequent water treatment processes. Furthermore, this invention employs an evaporation crystallization system to crystallize and recover valuable salts from the wastewater, and the resulting crystalline salts can be further used as raw materials for other products. Using the treatment system provided by this invention to treat graphite purification wastewater enables the resource-based reuse of waste acid and wastewater, achieving zero discharge. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the graphite purification waste liquid treatment system provided by the present invention; Figure 2 This is a schematic diagram of the graphite purification waste liquid treatment system used in the embodiment; The system consists of the following components: 1 is the hydrofluoric acid pre-washing unit; 2 is the sodium fluorosilicate production unit; 3 is the nitric acid / hydrochloric acid mixed acid pre-washing unit; 4 is the first neutralization sedimentation tank; 5 is the first softening sedimentation tank; 6 is the filtration unit; 7 is the evaporation crystallization unit; 8 is the sludge tank; 9 is the solid-liquid separation unit; 10 is the wastewater pH adjustment tank; 11 is the second neutralization sedimentation tank; 12 is the second softening sedimentation tank; 13 is the V-type filtration tank; 14 is the filter product water tank; 15 is the ultrafiltration unit; 16 is the first-stage reverse osmosis unit; 17 is the second-stage reverse osmosis unit; 18 is the water storage tank; 19 is the concentrate tank; 20 is the two-stage reverse osmosis unit; and 21 is the third softening sedimentation tank. Figure 3 This is a schematic diagram of the process for purifying graphite in the example. Detailed Implementation

[0025] This invention provides a graphite purification waste liquid treatment system, comprising a hydrofluoric acid pre-washing unit 1 and a sodium fluorosilicate production unit 2 connected in sequence; The following components are connected in sequence: nitric acid / hydrochloric acid mixed acid pre-washing unit 3, first neutralization sedimentation tank 4, first softening sedimentation tank 5, filtration unit 6, and evaporation crystallization unit 7. The inlet of the first neutralization sedimentation tank 4 is also connected to the sodium fluorosilicate production unit 2; The sedimentation outlets of the first neutralization sedimentation tank 4 and the first softening sedimentation tank 5 are connected to a sludge tank 8; the sludge tank is connected to a solid-liquid separation unit 9.

[0026] This invention provides a graphite purification wastewater treatment system, comprising a hydrofluoric acid pre-washing unit 1 and a sodium fluorosilicate production unit 2 connected in sequence. In one embodiment, the hydrofluoric acid pre-washing unit 1 is connected to a filter. This invention uses hydrofluoric acid wastewater as raw material to pre-wash graphite with hydrofluoric acid in the pre-washing unit 1, followed by filtration. The wastewater after pre-washing mainly contains fluorosilicic acid. Sodium fluorosilicate is then produced in the sodium fluorosilicate production unit 2 using this wastewater as raw material, thus realizing the resource utilization of the hydrofluoric acid wastewater. The sodium fluorosilicate produced by this invention can be sold externally, improving economic efficiency.

[0027] This invention provides a graphite purification wastewater treatment system, comprising a nitric acid / hydrochloric acid mixed acid pre-washing unit 3, a first neutralization sedimentation tank 4, a first softening sedimentation tank 5, a filtration unit 6, and an evaporation crystallization unit 7, connected in sequence. In one embodiment, the nitric acid / hydrochloric acid mixed acid pre-washing unit 3 is connected to a filter. In another embodiment, the first softening sedimentation tank 5 is a high-efficiency clarifier. In another embodiment, the filtration unit 6 is a multi-media filter. In another embodiment, the evaporation crystallization unit 7 is a vapor compression system (MVR).

[0028] As an embodiment of the present invention, the outlet of the first neutralization sedimentation tank 4 is connected to the inlet of the first softening sedimentation tank 5 through a water pipe, and the outlet of the first softening tank 5 is connected to the inlet of the filtration unit 6.

[0029] In one embodiment of the present invention, the inlet of the first neutralization sedimentation tank 4 is also connected to the liquid outlet of the sodium fluorosilicate production unit 2. The present invention uses nitric acid / hydrochloric acid mixed acid waste liquid as raw material and utilizes the nitric acid / hydrochloric acid mixed acid pre-washing unit 3 to pre-wash graphite after hydrofluoric acid pre-washing, followed by filtration. The present invention transfers the waste liquid after nitric acid / hydrochloric acid mixed acid pre-washing and the waste liquid after sodium fluorosilicate production to the first neutralization sedimentation tank 4 to remove residual fluorine from the waste liquid. Then, the wastewater after fluorine removal is softened to reduce water hardness. The softened wastewater is filtered to remove suspended particulate matter. Finally, evaporation and crystallization are performed to obtain mixed salt crystals for post-treatment.

[0030] In this invention, the sedimentation outlets of the first neutralization sedimentation tank 4 and the first softening sedimentation tank 5 are connected to a sludge tank 8; the sludge tank is connected to a solid-liquid separation unit 9. As an embodiment of this invention, the solid-liquid separation unit 9 is a plate and frame filter press.

[0031] As an embodiment of the present invention, the graphite purification waste liquid treatment system further includes a wastewater pH adjustment tank 10, a second neutralization sedimentation tank 11, a second softening sedimentation tank 12, a V-type filter tank 13, a filter product water tank 14, an ultrafiltration unit 15, a first-stage reverse osmosis unit 16, a second-stage reverse osmosis unit 17, and a water storage tank 18, which are connected in sequence. In one embodiment of the present invention, the outlet of the wastewater pH adjustment tank 10 is connected to the inlet of the second neutralization sedimentation tank 11 via a water pipe. The outlet of the second neutralization sedimentation tank 11 is connected to the inlet of the second softening sedimentation tank 12. The outlet of the second softening sedimentation tank 12 is connected to the inlet of the V-type filter tank 13 via a water pipe. The outlet of the V-type filter tank 13 is connected to the inlet of the filter product water tank 14. The outlet of the filter product water tank 14 is connected to the inlet of the ultrafiltration unit 15. The outlet of the ultrafiltration unit 15 is connected to the inlet of the first-stage reverse osmosis unit 16. The freshwater outlet of the first-stage reverse osmosis unit 15 is connected to the inlet of the second-stage reverse osmosis unit 16. The freshwater outlet of the second-stage reverse osmosis unit 17 is connected to the inlet of the water storage tank 18. In this invention, the washing wastewater generated during graphite purification is stored in the wastewater pH adjustment tank 10 for subsequent pH adjustment.

[0032] In one embodiment of the present invention, the second softening sedimentation tank 12 is a high-efficiency clarifier. In another embodiment of the present invention, the V-shaped filter tank 13 has a single-layer sand filter media structure, and the filter sand used in the V-shaped filter tank 13 is quartz sand with an average particle size of 1-2 mm. The present invention has no special requirements for the reverse osmosis components used in the primary reverse osmosis unit 16; conventional products in the art can be used. The present invention has no special requirements for the reverse osmosis components used in the secondary reverse osmosis unit 17; conventional products in the art can be used.

[0033] In this invention, the washing wastewater from the graphite purification process is pH adjusted in a pH adjustment tank 10, and fluoride is removed from the wastewater by precipitation in a second neutralization sedimentation tank 11. The wastewater is then softened in a second softening sedimentation tank 12 to reduce its hardness. After softening, the wastewater is filtered and then passed through an ultrafiltration unit 15 to remove impurities, including organic macromolecules and colloids with a particle size of 1-10 μm.

[0034] In one embodiment of the present invention, the concentrate outlet of the first-stage reverse osmosis unit 16 is sequentially connected to the concentrate tank 19, the two-stage reverse osmosis unit 20, and the third softening sedimentation tank 21; the concentrate outlet of the two-stage reverse osmosis unit 20 is connected to the inlet of the second-stage reverse osmosis unit 17, and the concentrate outlet of the second-stage reverse osmosis unit 17 is connected to the inlet of the ultrafiltration unit 15. In another embodiment of the present invention, the concentrate outlet of the first-stage reverse osmosis unit 16 is connected to the inlet of the concentrate tank 19, the outlet of the concentrate tank 19 is connected to the inlet of the two-stage reverse osmosis unit 20, the desalination outlet of the two-stage reverse osmosis unit 20 is connected to the inlet of the second-stage reverse osmosis unit 17, and the concentrate outlet of the two-stage reverse osmosis unit is connected to the inlet of the third softening sedimentation tank 21.

[0035] In one embodiment of the present invention, the third softening sedimentation tank 21 is a high-efficiency clarifier. In another embodiment of the present invention, the two-stage reverse osmosis unit 20 is a disc tube reverse osmosis system; the operating pressure of the two-stage reverse osmosis unit 20 is preferably 70-200 bar, more preferably 100-150 bar.

[0036] As an embodiment of the present invention, the sedimentation outlets of the second neutralization sedimentation tank 11, the second softening sedimentation tank 12 and the third softening sedimentation tank 21 are connected to the sludge tank 8.

[0037] As an embodiment of the present invention, the supernatant outlet of the third softening sedimentation tank 21 is connected to the evaporation crystallization unit 7.

[0038] The present invention also provides a method for treating graphite purification waste liquid using the waste liquid treatment system described above, comprising the following steps: Hydrofluoric acid waste liquid is passed into hydrofluoric acid pre-washing unit 1 to pre-wash graphite with hydrofluoric acid, and pre-washed graphite and hydrofluoric acid waste liquid are obtained respectively. The pre-acid-washed hydrofluoric acid waste liquid is passed into sodium fluorosilicate production unit 2 to obtain sodium fluorosilicate and first wastewater, respectively. The nitric acid / hydrochloric acid mixed acid waste liquid is passed into the nitric acid / hydrochloric acid mixed acid pre-acid washing unit 3 to pre-acid wash the graphite pickled with hydrofluoric acid, and the pre-acid washed graphite and the pre-acid washed nitric acid / hydrochloric acid mixed acid waste liquid are obtained respectively. The first wastewater and the pre-washed nitric acid / hydrochloric acid mixed acid waste liquid are passed into the first neutralization sedimentation tank 4 and mixed with the first calcium-containing substance to carry out the first precipitation reaction, and the first sludge and the first supernatant are obtained respectively. The first supernatant is passed into the first softening sedimentation tank 5 for first softening, and the second sludge and the second supernatant are obtained respectively. The second supernatant is passed into the filtration unit 6 for filtration to obtain the liquid phase; The liquid phase is passed into the evaporation and crystallization unit 7 for evaporation and crystallization to obtain crystalline salt; The first sludge and the second sludge were transferred to sludge tank 8 for solid-liquid separation to obtain sludge cake.

[0039] This invention involves passing hydrofluoric acid waste liquid into a hydrofluoric acid pre-washing unit 1 to pre-wash graphite with hydrofluoric acid, obtaining pre-washed graphite and pre-washed hydrofluoric acid waste liquid. In this invention, the hydrofluoric acid waste liquid is preferably the washing liquid after hydrofluoric acid washing of the graphite. Preferably, the process after pre-washing with hydrofluoric acid further includes filtering the pre-washed system; the filtration is preferably pressure filtration. This invention further purifies the pre-washed graphite with hydrofluoric acid by performing additional hydrofluoric acid washing.

[0040] The present invention reduces the amount of hydrofluoric acid used in hydrofluoric acid pickling by pre-acid washing.

[0041] After obtaining the pre-acid-washed hydrofluoric acid waste liquid, this invention introduces the pre-acid-washed hydrofluoric acid waste liquid into sodium fluorosilicate production unit 2 to obtain sodium fluorosilicate and the first wastewater, respectively. This invention has no special requirements for sodium fluorosilicate production unit 2, as long as it can react to produce sodium fluorosilicate.

[0042] In this invention, the purity of the sodium fluorosilicate is preferably above 98.5%, which meets the first-grade standard in GB / T23936-2018.

[0043] This invention involves passing a nitric acid / hydrochloric acid mixed acid waste liquid into a nitric acid / hydrochloric acid mixed acid pre-washing unit 3 to pre-wash graphite that has been acid-washed with hydrofluoric acid, thereby obtaining pre-washed graphite and the nitric acid / hydrochloric acid mixed acid waste liquid after pre-washing. In this invention, the nitric acid / hydrochloric acid mixed acid waste liquid is preferably the washing liquid after acid washing of graphite with nitric acid / hydrochloric acid mixed acid. In this invention, the hydrofluoric acid-washed graphite is the product of pre-washing graphite with hydrofluoric acid followed by hydrofluoric acid washing. In this invention, the acid / hydrochloric acid mixed acid pre-washing preferably further includes filtering the system; the filtration is preferably pressure filtration. This invention further purifies the graphite by subjecting the pre-washed graphite with acid / hydrochloric acid mixed acid washing.

[0044] After obtaining the first wastewater and the pre-acidified nitric acid / hydrochloric acid mixed waste liquid, this invention introduces the first wastewater and the pre-acidified nitric acid / hydrochloric acid mixed waste liquid into a first neutralization sedimentation tank 4 and mixes them with a first calcium-containing substance to undergo a first precipitation reaction, yielding a first sludge and a first supernatant, respectively. In this invention, the first calcium-containing substance preferably includes lime and / or calcium chloride, more preferably lime and calcium chloride. In this invention, the calcium-containing substance reacts with fluorine in the waste liquid to form a precipitate.

[0045] The first supernatant is passed into the first softening sedimentation tank 5 for first softening, yielding a second sludge and a second supernatant. In this invention, the softening agent for the first softening preferably includes lime, sodium carbonate, or caustic soda, more preferably sodium carbonate. This invention does not have special requirements on the amount of the softening agent used; conventional amounts in the art are sufficient.

[0046] After obtaining the second supernatant, the present invention passes the second supernatant into the filtration unit 6 for filtration to obtain the liquid phase. The present invention has no special requirements for the filtration process; conventional methods in the art can be used.

[0047] After obtaining the liquid phase, the present invention introduces the liquid phase into the evaporation crystallization unit 7 for evaporation crystallization to obtain crystalline salt. The present invention has no special requirements for the evaporation crystallization; conventional methods in the art can be used. In embodiments of the present invention, the evaporation crystallization temperature is 35–70°C, the vapor compression ratio is 2.5–3, and the gas flow rate is 15–40 m / s. In the present invention, the metal cation in the crystalline salt is preferably sodium, calcium, magnesium, or aluminum ions, and the anion in the crystalline salt is preferably chloride, carbonate, or nitrate ions.

[0048] After obtaining the first sludge and the second sludge, the present invention transfers the first sludge and the second sludge to sludge tank 8 for solid-liquid separation to obtain sludge cake. In the present invention, the solid-liquid separation is preferably performed by pressure filtration. Preferably, the sludge cake is transported to a cement plant for resource recovery as an auxiliary material.

[0049] In this invention, the method for treating graphite purification waste liquid preferably further includes the following steps: The washing wastewater is fed into the wastewater pH adjustment tank 10 and mixed with alkaline substances for pH adjustment. Then it is transferred to the second neutralization sedimentation tank 11 and mixed with the second calcium-containing substances for the second sedimentation reaction, resulting in the third sludge and the third supernatant. The third supernatant is passed into the second softening sedimentation tank 12 for second softening, to obtain the fourth sludge and the fourth supernatant respectively; the fourth supernatant is passed into the V-type filter tank 13 for filtration to obtain filter water; The water produced by the filter bed is subjected to ultrafiltration, first-stage reverse osmosis and second-stage reverse osmosis in sequence to obtain recycled water; The concentrate obtained from the first-stage reverse osmosis is subjected to two-stage reverse osmosis to obtain two-stage reverse osmosis concentrate and two-stage reverse osmosis desalination, respectively. The concentrated water from the two-stage reverse osmosis process is softened in the third stage and then evaporated and crystallized.

[0050] This invention involves passing washing wastewater into a wastewater pH adjustment tank 10, mixing it with an alkaline substance for pH adjustment, and then transferring it to a second neutralization sedimentation tank 11. This sedimentation is then mixed with a second calcium-containing substance for a second precipitation reaction, yielding a third sludge and a third supernatant. In this invention, the alkaline substance preferably includes lime or caustic soda, more preferably lime. In this invention, the pH of the system after pH adjustment is preferably 6-9, more preferably 7-8. In this invention, the second calcium-containing substance preferably includes lime and / or calcium chloride, more preferably lime and calcium chloride. In this invention, the calcium-containing substance reacts with fluorine in the wastewater to form a precipitate.

[0051] In this invention, the third sludge is preferably transferred to sludge tank 8 and then subjected to solid-liquid separation to obtain sludge cake.

[0052] After obtaining the third supernatant, the present invention introduces the third supernatant into the second softening sedimentation tank 12 for a second softening, thereby obtaining the fourth sludge and the fourth supernatant. In the present invention, the softening agent for the second softening preferably includes lime, sodium carbonate, or caustic soda, more preferably sodium carbonate. The present invention has no special requirements on the amount of the softening agent used, and conventional amounts in the art are sufficient.

[0053] In this invention, the fourth sludge is transferred to sludge tank 8 and then subjected to solid-liquid separation to obtain sludge cake.

[0054] After obtaining the fourth supernatant, the present invention filters the fourth supernatant into a V-type filter tank 13 to obtain filter water. The present invention has no special requirements for the filtration process.

[0055] After obtaining the filter permeate, the present invention sequentially processes the filter permeate through ultrafiltration, primary reverse osmosis, and secondary reverse osmosis to obtain recycled water. The present invention does not impose any particular limitations on the ultrafiltration, primary reverse osmosis, and secondary reverse osmosis processes.

[0056] This invention preferably involves subjecting the concentrate obtained from the first-stage reverse osmosis to a second-stage reverse osmosis process, yielding two stages of reverse osmosis concentrate and two stages of reverse osmosis desalination. This invention does not impose any special requirements on the two-stage reverse osmosis process. Preferably, the two-stage reverse osmosis concentrate is then softened in a third stage followed by evaporation and crystallization. In this invention, the softening agent used for the third softening preferably includes lime, sodium carbonate, or caustic soda, more preferably caustic soda. This invention does not impose any special requirements on the amount of the softening agent used; conventional amounts in the art are sufficient. Preferably, the sludge after the third softening is transferred to a sludge tank 8 for solid-liquid separation to obtain a sludge cake.

[0057] In this invention, the two-stage reverse osmosis desalination water is preferably fed into the second-stage reverse osmosis unit 17 for second-stage reverse osmosis. In this invention, the concentrate from the second-stage reverse osmosis is preferably returned for ultrafiltration.

[0058] This invention introduces a sodium fluorosilicate production line into the waste acid process, using hydrofluoric acid pickling waste acid as raw material to prepare commercially valuable sodium fluorosilicate for resource utilization of waste acid. In addition, the introduction of a pre-pickling system effectively reduces pickling costs and the amount of lime added during subsequent waste acid treatment, relatively reducing sludge production and treatment costs.

[0059] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0060] according to Figure 3 The flowchart shown illustrates the purification of graphite, generating hydrofluoric acid waste liquid, nitric acid / hydrochloric acid mixed acid waste liquid, and washing wastewater. The example uses... Figure 2 The system shown treats the wastewater generated during the graphite purification process.

[0061] Example 1 S1. Hydrofluoric acid pickling wastewater is fed into hydrofluoric acid pre-pickled unit 1 to pre-pickle graphite with hydrofluoric acid. After pressure filtration, the pre-pickled hydrofluoric acid waste liquid is fed into sodium fluorosilicate production unit 2 to obtain sodium fluorosilicate and the first wastewater. The first wastewater is fed into the first neutralization sedimentation tank 4. The pre-pickled graphite with hydrofluoric acid is then pickled with hydrofluoric acid to obtain hydrofluoric acid-pickled graphite. S2. The nitric acid / hydrochloric acid pickling wastewater is fed into the nitric acid / hydrochloric acid mixed acid pre-washing unit 3 to pre-wash the graphite that has been pickled with hydrofluoric acid with nitric acid / hydrochloric acid. The pre-washed graphite and the pre-washed nitric acid / hydrochloric acid mixed acid waste liquid are obtained by pressure filtration. The pre-washed nitric acid / hydrochloric acid mixed acid waste liquid is fed into the first neutralization sedimentation tank 4. S3. Add lime and calcium chloride to the first neutralization sedimentation tank 4 to carry out a sedimentation reaction. Pass the supernatant after sedimentation into the first softening sedimentation tank 5 and add sodium carbonate for softening. S4. After the softened supernatant is filtered through a multi-media filter 6, it enters a vapor compression system 7 for evaporation and crystallization (temperature 70℃, vapor compression ratio 2.5, airflow purging rate 15m / s) to obtain crystalline salt. S5. Collect the washing wastewater into the wastewater pH adjustment tank 10, and add sodium hydroxide to adjust the pH to 8; S6. After adjusting the pH value, the wastewater is fed into the second neutralization sedimentation tank 11, lime and calcium chloride are added to carry out the precipitation reaction, and the supernatant after precipitation is fed into the second softening sedimentation tank 12, and sodium carbonate is added to soften it. S7. After softening, the supernatant is passed into the V-type filter 13 for pre-filtration, and then the filtrate is passed into the filter product water tank 14. S8. The wastewater in the filter bed product water tank 14 is passed into the ultrafiltration unit 15 for ultrafiltration; S9. The ultrafiltration effluent is fed into the first-stage reverse osmosis unit 16 for first-stage reverse osmosis. The concentrated effluent from the first-stage reverse osmosis is fed into the concentrated water tank 19. The desalinated effluent from the first-stage reverse osmosis is fed into the second-stage reverse osmosis unit 17 for second-stage reverse osmosis. The desalinated effluent from the second-stage reverse osmosis is stored in the product water tank. The concentrated effluent from the second-stage reverse osmosis is returned to the ultrafiltration unit 15. S10. The concentrated water in the concentrated water tank 19 is fed into the two-stage reverse osmosis unit 20 for two-stage reverse osmosis concentration. The two-stage reverse osmosis desalination water is fed into the second-stage reverse osmosis unit 17. The two-stage reverse osmosis concentrated water is fed into the third softening sedimentation tank 3 for softening. The supernatant is fed into the steam compression system 7 for evaporation and crystallization (temperature is 70℃, steam compression ratio is 2.5, and airflow purging rate is 15m / s) to obtain crystalline salt.

[0062] S11. The sludge produced by the first neutralization sedimentation tank 4, the second neutralization sedimentation tank 11, the first softening sedimentation tank 5, the second softening sedimentation tank 12 and the third softening sedimentation tank is transferred to the sludge tank 8, and after being filtered by the plate and frame filter press 9, the sludge cake is transported off-site for disposal.

[0063] The processing system provided by this invention can treat graphite purification waste liquid, enabling the reuse of waste acid and wastewater resources and achieving zero discharge.

[0064] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for treating graphite purification waste liquid, characterized in that, Includes the following steps: Hydrofluoric acid waste liquid was passed into the hydrofluoric acid pre-washing unit (1) to pre-wash the graphite with hydrofluoric acid, and the pre-washed graphite and the pre-washed hydrofluoric acid waste liquid were obtained respectively. The pre-acid-washed hydrofluoric acid waste liquid is passed into the sodium fluorosilicate production unit (2) to obtain sodium fluorosilicate and the first wastewater, respectively. The nitric acid / hydrochloric acid mixed acid waste liquid is passed into the nitric acid / hydrochloric acid mixed acid pre-acid washing unit (3) to pre-acid wash the graphite pickled with hydrofluoric acid, and pre-acid washed graphite and pre-acid washed nitric acid / hydrochloric acid mixed acid waste liquid are obtained respectively. The first wastewater and the pre-washed nitric acid / hydrochloric acid mixed acid waste liquid are passed into the first neutralization sedimentation tank (4) and mixed with the first calcium-containing substance to carry out the first precipitation reaction, and the first sludge and the first supernatant are obtained respectively. The first supernatant is passed into the first softening sedimentation tank (5) for first softening, and the second sludge and the second supernatant are obtained respectively. The second supernatant is passed into the filtration unit (6) for filtration to obtain the liquid phase; The liquid phase is passed into the evaporation crystallization unit (7) for evaporation crystallization to obtain crystalline salt; The first sludge and the second sludge were transferred to the sludge tank (8) for solid-liquid separation to obtain sludge cake.

2. The method for treating graphite purification waste liquid according to claim 1, characterized in that, It also includes the following steps: The washing wastewater is fed into the wastewater pH adjustment tank (10) and mixed with alkaline substances for pH adjustment. Then it is transferred to the second neutralization sedimentation tank (11) and mixed with the second calcium-containing substance for the second precipitation reaction, to obtain the third sludge and the third supernatant respectively. The third supernatant is passed into the second softening sedimentation tank (12) for second softening, and the fourth sludge and the fourth supernatant are obtained respectively. The fourth supernatant is passed into a V-type filter tank (13) for filtration to obtain filter water; The water produced by the filter bed is subjected to ultrafiltration, first-stage reverse osmosis and second-stage reverse osmosis in sequence to obtain recycled water; The concentrate obtained from the first-stage reverse osmosis is subjected to two-stage reverse osmosis to obtain two-stage reverse osmosis concentrate and two-stage reverse osmosis desalination, respectively. The concentrated water from the two-stage reverse osmosis process is softened in the third stage and then evaporated and crystallized.

3. The method for treating graphite purification waste liquid according to claim 2, characterized in that, It also includes the following steps: Two stages of reverse osmosis freshwater are fed into the second stage reverse osmosis unit (17) for second-stage reverse osmosis; The concentrate from the second-stage reverse osmosis is returned for ultrafiltration; The third sludge, the fourth sludge and the third softened sludge were transferred to the sludge tank (8) for solid-liquid separation to obtain sludge cake.

4. The method for treating graphite purification waste liquid according to claim 2, characterized in that, The pH value of the system after pH adjustment is 6-9; The first and second calcium-containing substances each independently include lime and / or calcium chloride.

5. The method for treating graphite purification waste liquid according to claim 2, characterized in that, The V-shaped filter pool (13) has a single-layer sand filter material structure. The filter sand used in the V-shaped filter pool (13) is quartz sand, and the average particle size of the quartz sand is 1-2 mm. The ultrafiltration is used to remove impurities from water, including organic macromolecules and colloids with a particle size of 1 to 10 μm. The two-stage reverse osmosis system adopts a disc tube reverse osmosis system with an operating pressure of 70–200 bar. The evaporation crystallization temperature is 35–70°C, the vapor compression ratio is 2.5–3, and the gas flow rate is 15–40 m / s.

6. The method for treating graphite purification waste liquid according to claim 3, characterized in that, The softening agents used for the first, second, and third softening processes are each independently lime, sodium carbonate, or caustic soda.

7. The method for treating graphite purification waste liquid according to any one of claims 1-6, characterized in that, The process after hydrofluoric acid pre-washing also includes: filtering the hydrofluoric acid pre-washing system; The process of pre-washing with the nitric acid / hydrochloric acid mixture also includes filtering the system after pre-washing with the nitric acid / hydrochloric acid mixture.

8. A graphite purification waste liquid treatment system, characterized in that, It includes a hydrofluoric acid pre-washing unit (1) and a sodium fluorosilicate production unit (2) connected in sequence; The nitric acid / hydrochloric acid mixed acid pre-washing unit (3), the first neutralization sedimentation tank (4), the first softening sedimentation tank (5), the filtration unit (6) and the evaporation crystallization unit (7) are connected in sequence. The inlet of the first neutralization sedimentation tank (4) is also connected to the sodium fluorosilicate production unit (2); The sedimentation outlet of the first neutralization sedimentation tank (4) and the sedimentation outlet of the first softening sedimentation tank (5) are connected to a sludge tank (8); the sludge tank is connected to a solid-liquid separation unit (9).

9. The graphite purification waste liquid treatment system according to claim 8, characterized in that, It also includes a wastewater pH adjustment tank (10), a second neutralization sedimentation tank (11), a second softening sedimentation tank (12), a V-type filter tank (13), a filter product water tank (14), an ultrafiltration unit (15), a first-stage reverse osmosis unit (16), a second-stage reverse osmosis unit (17), and a water storage tank (18) connected in sequence. The concentrate outlet of the first-stage reverse osmosis unit (16) is sequentially connected to the concentrate tank (19), the two-stage reverse osmosis unit (20), and the third softening sedimentation tank (21); the concentrate outlet of the two-stage reverse osmosis unit (20) is connected to the inlet of the second-stage reverse osmosis unit (17), and the concentrate outlet of the second-stage reverse osmosis unit (17) is connected to the inlet of the ultrafiltration unit (15). The sedimentation outlets of the second neutralization sedimentation tank (11), the second softening sedimentation tank (12) and the third softening sedimentation tank (21) are connected to the sludge tank (8); The supernatant outlet of the third softening sedimentation tank (21) is connected to the evaporation crystallization unit (7).

10. The graphite purification waste liquid treatment system according to claim 8, characterized in that, The filtration unit (6) is a multi-media filter; The solid-liquid separation unit (9) is a plate and frame filter press.

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