Treatment system for alkali-acid process graphite purification wastewater

Abstract

The invention provides a treatment system for graphite purification wastewater through an alkali-acid method, and relates to the technical field of graphite purification wastewater treatment. The treatment system for the graphite purification wastewater by the alkali-acid method is divided into a route I and a route II according to different acid-base concentrations in water; the route I is used for treating primary and secondary washing water and comprises a weighted flocculation efficient precipitation unit, a vertical pressure filter, an ultrafiltration unit and a bipolar membrane device which are connected in sequence; the route II is used for treating three times or more than three times of low-concentration washing water and comprises a mixing neutralization tank, a secondary neutralization tank, a contact filter, an ultrafiltration unit and an ion exchange unit. A complete and highly-targeted graphite purification wastewater resourceful treatment scheme is formed by combining bipolar membrane electrodialysis with a grading treatment process of flow division according to quality and line selection according to needs and matched pretreatment and deep treatment systems, and the method is remarkably superior to the prior art in the aspects of acid-base recovery rate, water reuse rate, treatment cost and the like, and has a wide application prospect. The method has remarkable technical advantages and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of graphite purification wastewater treatment technology, and in particular to a treatment system for graphite purification wastewater using the alkali-acid method. Background Technology

[0002] Graphite is an important strategic mineral resource and a fundamental raw material for many industrial sectors, playing a crucial role in modern industrial production and economic development. The purity of graphite determines the performance and overall quality of processed graphite products; higher purity graphite has higher application value. Graphite purification is fundamental to the preparation of all graphite materials and is a common issue in the development of graphite materials. While flotation can yield graphite with a fixed carbon content of 80.0%-95.0% from raw graphite ore, it still contains a significant amount of impurities. To obtain high-carbon graphite with a carbon content of over 99%, further secondary purification using physical or chemical methods is necessary.

[0003] Among various purification processes, the alkali-acid method is advantageous due to its lower energy consumption and lack of toxic wastewater generation, making it suitable for achieving fluoride-free purification requirements. However, the alkali-acid method generates large amounts of acidic and alkaline wastewater. Especially in order to improve graphite purity, excessive amounts of alkali and acid are added during alkali fusion and acid leaching. This unused, surplus alkali and acid remains in the washing water of the purification process, resulting in high-concentration alkaline and acidic wastewater. Simultaneously, the wastewater contains salts formed from the reaction of impurities in the graphite with the acid or alkali during purification. The common treatment method for this wastewater is neutralization, which not only wastes reagents but also generates high-salt wastewater, increasing the difficulty of wastewater treatment and product costs.

[0004] Currently, bipolar membrane electrodialysis is a green and pollution-free technology for producing acids and alkalis from saline solutions. Traditional bipolar membrane applications often combine bipolar membranes with cation exchange membranes and anion exchange membranes. Under the influence of an electric field, cations in the salt chamber migrate towards the cathode, pass through the cation exchange membrane into the alkali chamber, and combine with OH- ions ionized from the bipolar membrane to form alkali. Conversely, anions in the salt chamber migrate towards the anode, pass through the anion exchange membrane into the acid chamber, and combine with H+ ions ionized from the bipolar membrane to form acid, thus converting salt into the corresponding acid and alkali. However, this traditional application method is mainly for treating saline solutions and lacks a targeted treatment solution for the complex wastewater system containing residual acids and alkalis and reacted salts generated during the alkali-acid graphite purification process.

[0005] Diffusion dialysis is another common method for acid and alkali recovery, but its recovery efficiency is generally only 50%-70%, and the effluent is still acidic or alkaline, requiring further treatment before reuse, which increases treatment costs and process complexity. Furthermore, while there are cases of wastewater staged treatment in fields such as dyeing and printing wastewater and papermaking wastewater, there is a lack of systematic staged treatment process design to address the significant differences in concentration between the primary, secondary, and tertiary washing water in graphite purification wastewater. Mixing all wastewater for treatment would lead to a decrease in alkali and acid concentrations, an increase in the amount of water to be treated, and an increase in the equipment and operating costs of the bipolar membrane electrodialysis unit.

[0006] Therefore, there is an urgent need to develop an economical and efficient treatment method for the wastewater from the alkaline-acid graphite purification process, which can not only efficiently recover the acid and alkali reagents in the wastewater, but also realize the recycling of treated water, so as to promote the sustainable development of the graphite industry. Summary of the Invention

[0007] In view of this, the present invention provides a treatment system for graphite purification wastewater using the alkali-acid method. The present invention employs bipolar membrane electrodialysis technology combined with a graded treatment process based on quality separation and demand-based flow selection, along with supporting pretreatment and advanced treatment systems, forming a complete and highly targeted resource-based treatment solution for graphite purification wastewater. This solution significantly outperforms existing technologies in terms of acid and alkali recovery rate, water reuse rate, and treatment cost, demonstrating significant technical advantages and economic benefits.

[0008] The treatment system for graphite purification wastewater using the alkaline-acid method of the present invention is divided into Route 1 and Route 2 according to the different acid and alkali concentrations in the water. Route 1 is used to treat primary and secondary washing water and includes a gravity flocculation high-efficiency sedimentation unit, a vertical pressure filter, an ultrafiltration unit and a bipolar membrane device connected in sequence. The high-efficiency sedimentation unit with enhanced flocculation consists of a rapid stirring reaction tank, a medium-speed stirring reaction tank, and an inclined plate sedimentation tank; the vertical pressure filter is filled with a double layer of filter media of activated carbon and quartz sand; the bipolar membrane device uses an alternating arrangement of bipolar membranes and cation membranes when extracting alkali, and an alternating arrangement of bipolar membranes and anion membranes when extracting acid; Route 2 is used to treat washing water with low concentrations after three or more treatments, and includes a mixing and neutralization tank, a secondary neutralization tank, a contact filter, an ultrafiltration unit, and an ion exchange unit.

[0009] Preferably, the NaOH concentration of the primary washing alkaline solution is 60-120 g / L, and the HCl concentration of the primary washing acid solution is 40-70 g / L; the NaOH concentration of the secondary washing water is 20-40 g / L, and the HCl concentration is 10-30 g / L; the pH value of the tertiary washing water is 2-12. More preferably, the pH value of the tertiary washing water is 4-10.

[0010] Preferably, during the wastewater treatment process, an inorganic coagulant is added to the pipeline of the rapid stirring reaction tank of the high-efficiency sedimentation unit with enhanced flocculation. The effluent from the pipeline flows into the rapid reaction tank, where polyacrylamide and fine sand are added to form large settleable flocs. The effluent then flows into the inclined plate sedimentation tank to separate the suspended solids and the carried-out graphite powder, which then settle.

[0011] Preferably, the inorganic coagulant is at least one selected from iron salts, aluminum salts, polyaluminum chloride, and polyferric sulfate, with a dosage of 10-20 mg / L; the polyacrylamide dosage is 0.5-3 mg / L; the sedimentation time is 10-20 min; and the fine sand dosage ratio is 3 g / m³. 3 .

[0012] Preferably, the vertical pressure filter has a filtration rate of 4-8 m / h and a backwashing cycle of 24-48 h.

[0013] Preferably, the operating voltage of the bipolar film device is 9-18 V. More preferably, the operating voltage of the bipolar film device is 12-15 V.

[0014] Preferably, the ultrafiltration unit uses a PVDF ultrafiltration membrane with a molecular weight cutoff of 50,000-80,000 Da and an effluent turbidity of less than 2 NTU.

[0015] Preferably, the contact filter uses quartz sand filter media with a particle size of 0.5-1.2 mm.

[0016] Preferably, the effluent from the feed chamber of the bipolar membrane device is neutral and reused in the graphite purification and washing process; the acid in the recovery chamber is reused in the acid leaching process; the alkali in the recovery chamber is reused in the alkali fusion process; and the hydrogen generated in the electrode chamber is recovered as a clean energy source. The water reuse rate of both Route 1 and Route 2 is greater than or equal to 80%.

[0017] Preferably, the wastewater treatment system further includes a security filter and a multi-stage reverse osmosis (RO) unit, used to mix and treat the effluents from Route 1 and Route 2 for high-purity graphite purification; the multi-stage RO unit consists of 2-4 stages; the effluent from the security filter is pumped into the first stage of the multi-stage RO unit, and the operating pressure of the multi-stage RO unit is 1.0-4.0 MPa, with a membrane flux of 10-15 L / m³. 2The recovery rate of the first-stage RO unit is 45%-50%, and the effluent from the first-stage RO unit is reused for purifying high-purity graphite. The concentrate from the first-stage RO unit enters the second-stage RO unit, the effluent from the second-stage RO unit is reused for purifying high-purity graphite, the concentrate from the second-stage RO unit enters the third-stage RO unit, and the concentrate from the third-stage RO unit enters the fourth-stage RO unit. More preferably, the operating pressure of the multi-stage reverse osmosis unit is 1.5-2.8 MPa, and the membrane flux is 15 L / m³. 2 ·h.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows: This invention addresses the unique characteristics of graphite purification wastewater from the alkali-acid process by employing a combination of bipolar membranes with separate combinations of cation exchange and anion exchange membranes, rather than the traditional simultaneous combination. During alkali extraction, bipolar and cation exchange membranes are arranged alternately; during acid extraction, bipolar and anion exchange membranes are arranged alternately. This allows for the efficient extraction of waste alkali or acid containing other ions from the raw solution chamber into clean acids and alkalis. This configuration not only recovers remaining unreacted acids and alkalis from the wastewater but, more importantly, converts the reacted salts back into acids and alkalis, thereby maximizing the utilization rate of reagent raw materials. Compared to diffusion dialysis, this invention significantly improves recovery efficiency. More importantly, the effluent from the raw solution chamber after bipolar membrane electrodialysis treatment is neutral and can be directly reused in the washing and other processes of the purification process, while the effluent from diffusion dialysis remains acidic or alkaline and requires further treatment. This advantage greatly simplifies the subsequent treatment process and reduces treatment costs.

[0019] This invention employs a wastewater classification treatment method based on "quality-based diversion and demand-based route selection." High-concentration primary and secondary wastewater generated during alkali fusion and acid leaching are pre-treated to remove impurities before entering a bipolar membrane device to extract acid and alkali for reuse in the process. Low-concentration washing water from tertiary or higher treatments is treated separately, undergoing neutralization, contact filtration, and ion exchange before reuse. This diversion method avoids the concentration dilution problem caused by mixed treatment. Demand-based route selection refers to choosing different treatment routes based on different graphite purity requirements, reducing treatment costs and minimizing treatment paths while meeting product requirements. For high-carbon graphite purification, the effluent from routes one and two can be directly reused; for high-purity graphite purification, the effluent from the two routes is mixed and further treated by a security filter and a multi-stage reverse osmosis device to ensure that the washing water does not introduce impurity ions that affect graphite purity. This flexible route selection method gives the system high treatment efficiency, flexible operation, and low treatment costs.

[0020] This invention designs a pretreatment system specifically for the conditions of graphite purification wastewater. By combining a gravity flocculation high-efficiency sedimentation unit, a vertical pressure filter, and an ultrafiltration unit, it effectively removes graphite powder and suspended solids from the wastewater, ensuring the quality of the influent to the bipolar membrane device, extending the membrane's lifespan, and improving system operational stability. This pretreatment system, combined with bipolar membrane electrodialysis technology, forms a complete treatment process for alkali-acid graphite purification wastewater.

[0021] The wastewater treatment system of this invention achieves triple benefits in terms of environment, society, and economy. First, it removes pollutants from wastewater, and the treated water is reused in the washing process, saving fresh water consumption and reducing wastewater discharge, thus avoiding external wastewater discharge. Second, it extracts acids and alkalis from the wastewater for reuse in the purification process, improving reagent utilization. Compared with the traditional neutralization method, the consumption of fresh reagents can be reduced by more than 20%, significantly reducing reagent consumption per ton of product and production costs. Third, clean energy hydrogen is generated in the electrode chamber during the treatment process, which can be recycled as a clean energy source. Through the alkali-acid method graphite purification wastewater treatment system proposed in this invention, process water and reagents can be recycled, significantly reducing water and reagent consumption per ton of product, providing an effective technical solution for the sustainable development of the graphite industry. Attached Figure Description

[0022] The present invention will be further described below with reference to the accompanying drawings.

[0023] Figure 1 This is a flow chart of the wastewater treatment system for graphite purification using the alkali-acid method. The "reuse" indicates that the extracted acid or alkali is reused in the alkali fusion or acid leaching process; "reuse A" indicates that the treated water is reused in the washing process for purifying high-carbon graphite; and "reuse B" indicates that the treated water is reused in the washing process for purifying high-purity graphite. Explanation of reference numerals in the attached diagram: 1. Weighted flocculation high-efficiency sedimentation unit; 2. Vertical pressure filter; 3. Ultrafiltration unit; 4. Bipolar membrane device; 5. Mixing and neutralization tank; 6. Secondary neutralization tank; 7. Contact filter; 8. Ultrafiltration unit; 9. Ion exchange unit; 10. Security filter; 11. Multi-stage reverse osmosis (RO) device. Detailed Implementation

[0024] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Unless otherwise specified, all experiments were repeated three times, and the results are expressed as averages.

[0026] Example 1: A treatment system for wastewater from alkali-acid graphite purification. (1) The collected water from the first alkali fusion process (NaOH concentration of 80 g / L) and the second washing water (NaOH concentration of 22 g / L) are transported through pipelines into the rapid stirring reaction tank of the high-efficiency sedimentation unit for precipitating flocculation. An inorganic coagulant, polyaluminum chloride, is added to the pipeline at a dosage of 10 mg / L. The effluent from the pipeline flows into the rapid reaction tank, where 1.0 mg / L of polyacrylamide is added at a dosage of 3 g / m³. 3 The fine sand is added at a certain ratio to form large settleable flocs. The effluent flows into the inclined plate sedimentation tank to separate the settled suspended solids and the carried-out graphite fine powder. The sedimentation time is 20 min. The effluent from the flocculation and high-efficiency sedimentation unit flows into a vertical pressure filter via a connected pipe. This vertical pressure filter is filled with a double layer of filter media—activated carbon (0.8-2.0 mm particle size) and quartz sand (0.5-1.2 mm particle size)—at a volume ratio of 1:3. The filtration rate is 4 m / h, and the backwashing cycle is 24 h. After filtration, the effluent enters an ultrafiltration unit via a connected pipe. This ultrafiltration unit uses a PVDF ultrafiltration membrane with a molecular weight cutoff of 50,000 Da, further removing fine suspended solids, resulting in an effluent turbidity of less than 2 NTU. The effluent from the ultrafiltration unit is then fed into a bipolar membrane device employing alternating bipolar and cation exchange membranes for alkali extraction. The bipolar membrane device operates at 12 V. Operation stops when the water in the feed chamber is neutral. The alkali in the recovery chamber is reused in the alkali fusion process, achieving an alkali recovery rate of 98%. The water in the feed chamber is reused as washing water after alkali fusion for the purification of high-carbon graphite. Simultaneously, clean energy hydrogen generated in the polar chambers can be collected throughout the process.

[0027] (2) The washing acid water (pH 2.5) and washing alkaline water (pH 12) with lower concentrations are treated three times using Route 2. The three lower concentration washing acid water and washing alkaline water flow into a mixing and neutralization tank, where the two streams are mixed and neutralized to make the effluent neutral. The neutralized water is then sent to a contact filter, which uses quartz sand filter media with a particle size of 0.5-1.2 mm to filter out fine suspended solids and other substances in the water. The effluent can be reused as washing water for purifying high-carbon graphite. After three cycles, the effluent passes through an ion exchange unit to remove dissolved ions in the water. The effluent from the ion exchange unit can be reused for purifying high-carbon graphite. In this embodiment, the water reuse rate is 85%.

[0028] Example 2: A treatment system for graphite purification wastewater using the alkali-acid method. The difference from Example 1 is that the treated water is the washing water from the first (HCl concentration 52 g / L) and second (HCl concentration 20 g / L) acid leaching processes. The effluent from the ultrafiltration unit is sent to a bipolar membrane device with alternating bipolar and anion membranes for acid extraction. The operating voltage of the bipolar membrane device is 12 V. Operation stops when the water in the feed solution chamber is neutral. The acid in the recovery chamber is reused in the acid leaching process, with an acid recovery rate of 95%. The water in the feed solution chamber is reused as washing water after acid leaching for the purification of high-carbon graphite. The treatment process of Route 2 is the same as that of Example 1, with a water reuse rate of over 80%.

[0029] Example 3: A treatment system for wastewater from alkali-acid graphite purification. This embodiment is used for wastewater treatment in the purification of high-purity graphite. Because high-purity graphite requires a carbon content of over 99.9% and impurity content of the purified graphite to be less than 0.1%, strict requirements are placed on the dissolved ion content in the washing water. It is necessary to minimize the introduction of other impurities into the washing water to avoid affecting the purity of the graphite. This embodiment differs from Embodiments 1 and 2 in that the effluents from routes one and two of Embodiments 1 and 2 are mixed and sent together to a security filter to further protect the reverse osmosis membrane and reduce membrane fouling.

[0030] The effluent from the security filter is pumped into the first stage of a multi-stage reverse osmosis (RO) system. The operating pressures of the three RO stages are 1.5, 2.0, and 2.8 MPa, respectively, and the membrane flux is 15 L / m³. 2 The recovery rate of the first-stage RO unit is 50%, and the effluent from the first-stage RO unit is reused for purifying high-purity graphite. To improve the water reuse rate to over 80%, the concentrate from the first-stage RO unit enters the second-stage RO unit, the effluent from the second-stage RO unit is reused for purifying high-purity graphite, and the concentrate from the second-stage RO unit enters the third-stage RO unit.

[0031] This embodiment uses a 3-stage RO device, with a product water conductivity of 125 μS / cm, a total dissolved solids of 69 mg / L, and a water reuse rate of 85%.

[0032] Example 4: A treatment system for wastewater from alkali-acid graphite purification. The difference from Example 1 is that the operating voltage of the bipolar film device is 15 V.

[0033] Experimental results show that the alkali recovery rate is 99%, the acid recovery rate is 97%, and the water reuse rate is 65%.

[0034] Comparative Example 1 The difference from Example 1 is that: instead of using a treatment method that separates wastewater by quality, all wastewater is mixed and then treated using the scheme of Route 1.

[0035] Because the concentrations of alkali and acid decrease after mixed treatment, and the volume of water treated increases, the equipment and operating costs of the bipolar membrane electrodialysis device increase. Experimental results show that, compared to the quality-separated treatment method in Example 1, the equipment investment for mixed treatment increases by approximately 30%, and the operating cost increases by approximately 25%.

[0036] Comparative Example 2 The difference from Example 1 is that the bipolar membrane electrodialysis device is replaced with a diffusion dialysis device.

[0037] In the treatment of alkaline wastewater from primary and secondary washing processes, diffusion dialysis achieved an alkali recovery rate of only 70%, far lower than the 98% recovery rate of bipolar membrane electrodialysis. In the treatment of acidic wastewater from primary and secondary washing processes, diffusion dialysis achieved an acid recovery rate of approximately 78%, lower than the 95% recovery rate of bipolar membrane electrodialysis. Furthermore, the effluent from diffusion dialysis remains acidic or alkaline, requiring further neutralization before reuse, while the effluent from bipolar membrane electrodialysis can reach a neutral pH and can be directly reused in the washing process.

[0038] Comparative Example 3 The traditional method of wastewater treatment involves mixing and neutralizing acidic and alkaline wastewater before further treatment and discharge.

[0039] Experimental results show that, compared with the wastewater treatment system of this invention, the traditional neutralization method cannot recover acids and alkalis from wastewater, wastes more than 20% of unreacted reagents, thus requiring a large amount of purification reagents and generating a large amount of high-salt wastewater, increasing the difficulty and cost of wastewater treatment. Furthermore, most of the treated water does not meet reuse requirements and is directly discharged, which, compared to the over 80% water reuse rate of this invention, also results in a waste of water resources.

[0040] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A treatment system for wastewater from the alkaline-acid graphite purification process, characterized in that, Based on the different acid and alkali concentrations in the water, it is divided into Route 1 and Route 2; Route 1 is used to treat primary and secondary washing water, including a gravity flocculation high-efficiency sedimentation unit, a vertical pressure filter, an ultrafiltration unit and a bipolar membrane device connected in sequence. The weighted flocculation high-efficiency sedimentation unit consists of a rapid stirring reaction tank, a medium-speed stirring reaction tank, and an inclined plate sedimentation tank; the vertical pressure filter is filled with a double layer of filter media consisting of activated carbon and quartz sand. The bipolar membrane device uses an alternating arrangement of bipolar membranes and cation membranes when extracting alkali, and an alternating arrangement of bipolar membranes and anion membranes when extracting acid. Route 2 is used to treat washing water with low concentrations after three or more treatments, and includes a mixing and neutralization tank, a secondary neutralization tank, a contact filter, an ultrafiltration unit, and an ion exchange unit.

2. The treatment system for graphite purification wastewater using the alkali-acid method according to claim 1, characterized in that, The NaOH concentration of the primary washing alkaline water is 60-120 g / L, and the HCl concentration of the primary washing acid water is 40-70 g / L; the NaOH concentration of the secondary washing water is 20-40 g / L, and the HCl concentration is 10-30 g / L; the pH value of the tertiary washing water is 2-12.

3. The treatment system for graphite purification wastewater using the alkali-acid method according to claim 1, characterized in that, In the wastewater treatment process, inorganic coagulant is added to the pipeline of the rapid stirring reaction tank of the high-efficiency sedimentation unit with enhanced flocculation. The effluent from the pipeline flows into the rapid reaction tank, where polyacrylamide and fine sand are added. The effluent then flows into the inclined plate sedimentation tank to separate the suspended solids and the carried-out graphite powder, which then settle.

4. The treatment system for graphite purification wastewater using the alkali-acid method according to claim 3, characterized in that, The inorganic coagulant is at least one selected from iron salts, aluminum salts, polyaluminum chloride, and polyferric sulfate, with a dosage of 10-20 mg / L; the polyacrylamide dosage is 0.5-3 mg / L; the sedimentation time is 10-20 min; and the fine sand dosage ratio is 3 g / m³. 3 .

5. The treatment system for graphite purification wastewater according to claim 1, characterized in that, The vertical pressure filter has a filtration rate of 4-8 m / h and a backwashing cycle of 24-48 h.

6. The treatment system for graphite purification wastewater according to claim 1, characterized in that, The operating voltage of the bipolar membrane device is 9-18 V.

7. The treatment system for graphite purification wastewater according to claim 1, characterized in that, The ultrafiltration unit uses a PVDF ultrafiltration membrane with a molecular weight cutoff of 50,000-80,000 Da and an effluent turbidity of less than 2 NTU.

8. The treatment system for graphite purification wastewater according to claim 1, characterized in that, The contact filter uses quartz sand filter media with a particle size of 0.5-1.2 mm.

9. The treatment system for graphite purification wastewater according to claim 1, characterized in that, The effluent from the feed chamber of the bipolar membrane device is neutral and reused in the graphite purification and washing process. The acid in the recovery chamber is reused in the acid leaching process, the alkali in the recovery chamber is reused in the alkali melting process, and the hydrogen generated in the electrode chamber is recovered as a clean energy source.

10. The treatment system for graphite purification wastewater using the alkali-acid method according to claim 1, characterized in that, The wastewater treatment system also includes a security filter and a multi-stage reverse osmosis (RO) unit, used to mix and treat the effluents from Route 1 and Route 2 for high-purity graphite purification; the multi-stage RO unit consists of 2-4 stages; the effluent from the security filter is pumped into the first stage of the multi-stage RO unit, and the operating pressure of the multi-stage RO unit is 1.0-4.0 MPa, with a membrane flux of 10-15 L / m³. 2 •h, the effluent from the first RO unit is recycled for purifying high-purity graphite; the concentrate from the first RO unit enters the second RO unit, the effluent from the second RO unit is recycled for purifying high-purity graphite, the concentrate from the second RO unit enters the third RO unit, and the concentrate from the third RO unit enters the fourth RO unit.

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