Method and system for removing sodium oxalate from sodium aluminate solution
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2025-07-10
- Publication Date
- 2026-08-13
Smart Images
Figure CN2025107815_13082026_PF_FP_ABST
Abstract
Description
A method and system for removing sodium oxalate from sodium aluminate solution
[0001] Cross-references to related applications
[0002] This application claims priority to Chinese patent application No. 202510137415.1, filed on February 7, 2025, the entire contents of which are incorporated herein by reference. Technical Field
[0003] This disclosure relates to the field of light metal smelting technology, and in particular to a method and system for removing sodium oxalate from sodium aluminate solution. Background Technology
[0004] In the Bayer process of alumina production, crystallization is one of the most widely used industrial methods for removing sodium oxalate in the alumina removal process. Based on the characteristic that sodium oxalate is difficult to precipitate or has low precipitation efficiency when the supersaturation of sodium oxalate in the sodium aluminate solution is low, crystallization removes sodium oxalate by evaporating and concentrating a sodium aluminate solution with a low sodium oxalate concentration. After concentration, the concentration of sodium oxalate in the sodium aluminate solution will increase to a higher level. This higher sodium oxalate concentration will result in a higher supersaturation level of sodium oxalate in the sodium aluminate solution. After a single cooling process, the supersaturated sodium oxalate in the sodium aluminate solution will crystallize out, and the sodium oxalate can then be removed by filtration. However, in this one-time cooling process, the rapid crystallization of sodium oxalate can lead to an explosive precipitation of sodium oxalate crystals. These precipitated crystals are large, individual, fine needle-like crystals that do not continue to grow with increasing crystallization time, resulting in a large number of these individual needle-like crystals during cooling. These crystals form suspended matter that mixes with the sodium aluminate solution or floats on the surface, making it difficult for them to settle. During subsequent filtration, these crystals quickly clog the filter cloth, increasing the difficulty of filtering the sodium aluminate solution and making the separation of sodium aluminate solution and sodium oxalate crystals more challenging. Excessive separation difficulty reduces the yield of the sodium aluminate mother liquor, forcing a reduction in the feed rate to the subsequent crystallization tank and affecting the overall efficiency of the sodium oxalate removal process. Therefore, avoiding the explosive precipitation of sodium oxalate during cooling is crucial for sodium oxalate removal.
[0005] To address the explosive precipitation of sodium oxalate during the cooling process, a sodium oxalate crystallization aid is typically added to the sodium aluminate solution. These aids are usually organic reagents, and their direct addition to the aluminate solution introduces new impurities, affecting the purity of the aluminate solution after sodium oxalate removal. However, without the introduction of a crystallization aid, the cooling process causes the sodium oxalate to crystallize too rapidly, inevitably leading to explosive precipitation. This explosive precipitation makes it difficult for the sodium oxalate crystals precipitated in the aluminate solution to grow into regular spherical crystals. Instead, it causes the crystals to grow into non-spherical crystal forms such as small needle-like crystals. These non-spherical sodium oxalate crystals affect the efficiency of solid-liquid separation between the sodium oxalate crystals and the aluminate solution, ultimately impacting the discharge of sodium oxalate crystals from the alumina production process.
[0006] The solutions for the explosive precipitation of sodium oxalate during the cooling process in the relevant technologies are as follows: (1) Add a crystallization aid that helps sodium oxalate crystals grow to the decomposition mother liquor, and then evaporate the sodium aluminate solution containing the crystallization aid to obtain the remaining sodium aluminate solution; then add sodium oxalate crystallization aid to the remaining sodium aluminate solution to obtain a mixed sodium aluminate solution; then add industrial alkali or caustic soda to the mixed sodium aluminate solution to make sodium oxalate crystallize and precipitate in the mixed sodium aluminate solution to obtain a suspension containing sodium aluminate and sodium oxalate crystals; then settle the suspension to obtain the underflow slurry; then filter the settled underflow slurry to remove sodium oxalate. (2) Add sodium oxalate growth promoter and sodium oxalate inducer to sodium aluminate solution to obtain a mixed solution; then cool the mixed solution once to remove sodium oxalate crystals from the mixed solution, obtaining a suspension containing sodium aluminate and sodium oxalate crystals; then separate the sodium oxalate crystals from the suspension by cyclone fractionation and solid-liquid separation. In this process, the added sodium oxalate inducer is a rare earth oxalate, such as yttrium oxalate, cerium oxalate, lanthanum oxalate, etc.; and the added sodium oxalate growth promoter is any one or more of dextran, pullulan, aminopolysaccharide, chitosan or rhamnose. Summary of the Invention
[0007] One or more embodiments of this disclosure provide a method and system for removing sodium oxalate from a sodium aluminate solution to address how to simultaneously improve the removal efficiency of sodium oxalate and the purity of the sodium aluminate solution after sodium oxalate removal.
[0008] In a first aspect, according to some embodiments of the present disclosure, a method for removing sodium oxalate from a sodium aluminate solution is provided, wherein the sodium aluminate solution includes sodium oxalate, and the method includes: performing gradient crystallization on the sodium aluminate solution to precipitate a large amount of sodium oxalate crystals, thereby obtaining a sodium aluminate mother liquor containing sodium oxalate crystals; wherein the gradient crystallization includes multiple cooling stages, and the number of cooling stages is ≥2 stages; and performing sedimentation separation on the sodium aluminate mother liquor containing sodium oxalate crystals to obtain a sodium aluminate solution free of sodium oxalate.
[0009] Secondly, according to some embodiments of this disclosure, a system for removing sodium oxalate from a sodium aluminate solution, the system being adapted to the method described in the first aspect, the system comprising: a raw material section, including a sodium aluminate solution storage tank and a raw material conveying pipe, the outlet of the sodium aluminate solution storage tank (1) being connected to the inlet of the raw material conveying pipe; a gradient crystallization section, including a seed crystal feed pipe, a multi-stage heat exchanger and a multi-stage crystallization tank, the multi-stage heat exchanger and the multi-stage crystallization tank being alternately arranged and connected in series, located in the first... The inlet of the heat exchanger in the first stage is connected to the outlet of the raw material conveying pipe; the inlet of the crystallization tank in the first stage is connected to the outlet of the heat exchanger in the first stage; the seed inlet of the crystallization tank in the first stage is connected to the seed feed pipe; and the inlet of the crystallization tank in the last stage is connected to the outlet of the heat exchanger in the last stage. A separation section, including a settling tank and a product storage tank, is also provided, with the outlet of the crystallization tank in the last stage connected to the inlet of the settling tank, and the outlet of the settling tank connected to the inlet of the product storage tank. Attached Figure Description
[0010] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure.
[0011] To more clearly illustrate the technical solutions in the embodiments or related technologies of this disclosure, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0012] Figure 1 shows a schematic flowchart of a method for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure;
[0013] Figure 2 shows a detailed flowchart of a method for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure; and
[0014] Figure 3 shows a system logic diagram for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure.
[0015] In the attached diagram, 1-sodium aluminate solution storage tank, 2-raw material conveying pipe, 3-seed crystal feed pipe, 4-heat exchanger, 5-crystallization tank, 6-sedimentation tank, 7-product storage tank, 8-underflow pump, 9-vibrating screen, 10-transfer pump, 11-filter press, 12-filtrate conveying pipe, 13-seed crystal conveying pipe. Embodiments of the present invention
[0016] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this disclosure. Based on the embodiments of this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.
[0017] Various embodiments of this disclosure may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this disclosure; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range such as 1, 2, 3, 4, 5, and 6, regardless of the range; furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.
[0018] In this document, terms such as “comprising” mean “including but not limited to”. Relational terms such as “first” and “second” are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. “And / or” describes the relationship between related objects, indicating that there can be three relationships, for example, A and / or B can mean: A alone, A and B simultaneously, or B alone; where A and B can be singular or plural. “At least one” means one or more, “more” means two or more; “at least one,” “at least one of the following,” or similar expressions refer to any combination of these items, including any combination of single or plural items; for example, “at least one of a, b, or c,” or “at least one of a, b, and c,” can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts representation" such as parts by weight or parts by mass indicates the proportional relationship between components. In the proportional relationships discussed in this article, the parameters that need to be described by proportion should be understood as the first term of the proportion in the order of description, and the proportion figures should be understood as the second term of the proportion. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substances A, B, and C should correspond one-to-one with the proportion figures in the proportion in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.
[0019] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this article can be purchased from the market or prepared by existing methods.
[0020] It should be noted that, regarding the related technology (1) described in the background art, the inventors have found that although this method can simultaneously precipitate sodium carbonate and sodium oxalate from the sodium aluminate solution, the sodium oxalate crystallization aid added in this method is generally an organic reagent. Directly adding it to the sodium aluminate solution will introduce new impurities, affecting the purity of the sodium aluminate solution after sodium oxalate removal. Regarding the related technology (2) described in the background art, the inventors have found that the sodium oxalate growth agent and sodium oxalate inducer contain a large number of impurity elements. For example, the sugars used in the sodium oxalate growth agent are difficult to degrade in the sodium aluminate solution, affecting the purity of the sodium aluminate solution after sodium oxalate crystallization. The sodium oxalate inducer introduces rare earth elements, which will also affect the purity of the sodium aluminate solution after sodium oxalate crystallization.
[0021] Figure 1 shows a schematic flowchart of a method for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure;
[0022] As shown in Figure 1, a method for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure, wherein the sodium aluminate solution comprises sodium oxalate, the method comprising:
[0023] S1. The sodium aluminate solution is subjected to gradient crystallization to precipitate a large amount of sodium oxalate crystals, resulting in a sodium aluminate mother liquor containing sodium oxalate crystals; wherein the gradient crystallization includes multiple cooling stages, and the number of cooling stages is ≥2 stages; and,
[0024] S2. The sodium aluminate mother liquor containing sodium oxalate crystals is subjected to sedimentation separation to obtain a sodium aluminate solution free of sodium oxalate.
[0025] It should be noted that sodium oxalate crystals are sodium oxalate that precipitates from sodium aluminate solution in various crystalline forms.
[0026] It should be noted that the caustic alkali concentration of this sodium aluminate solution is 220 g / L to 230 g / L.
[0027] It should be noted that the mass concentration of oxalate ions in this sodium aluminate solution can be 1.70 g / L to 1.90 g / L.
[0028] It should be noted that, according to some embodiments of this disclosure, a method for removing sodium oxalate from a sodium aluminate solution achieves the large-scale and uniform precipitation of sodium oxalate through steps such as gradient cooling crystallization, thereby improving the removal efficiency of sodium oxalate from the sodium aluminate solution and the purity of the sodium aluminate solution after sodium oxalate removal.
[0029] In some embodiments, the procedure for removing sodium oxalate from a sodium aluminate solution is as follows:
[0030] (1) This method employs a gradient crystallization process with two or more cooling stages. By gradually reducing the temperature of the sodium aluminate solution in batches, the solubility of sodium oxalate in the sodium aluminate solution can be progressively reduced, allowing sodium oxalate crystals to precipitate from the solution in stages. This method avoids the explosive precipitation of sodium oxalate crystals in the sodium aluminate solution, thus reducing the probability of sodium oxalate precipitating in the form of needle-like crystals and ensuring uniform precipitation of sodium oxalate crystals.
[0031] (2) In addition, this method does not introduce any additional crystallization aids throughout the process, thus avoiding the introduction of new impurities into the sodium aluminate solution. This feature ensures that the sodium aluminate solution after sodium oxalate removal maintains a high purity, which is beneficial for subsequent production and application.
[0032] In summary, the method for removing sodium oxalate from sodium aluminate solution according to some embodiments of this disclosure achieves efficient removal of sodium oxalate from sodium aluminate solution through innovative steps such as gradient cooling crystallization, while ensuring the purity of the sodium aluminate solution after sodium oxalate removal, and has high industrial application value.
[0033] In some optional embodiments, the cooling stage has 2 to 4 stages, and the cooling range between two adjacent cooling stages is 10°C to 35°C.
[0034] In these embodiments, the cooling stages can be 2 to 4, and the cooling range between adjacent cooling stages can be 10°C to 35°C. By reducing the temperature of the sodium aluminate solution in batches and controlling the cooling range of each cooling stage, the solubility of sodium oxalate crystals in the sodium aluminate solution can be gradually reduced, allowing the sodium oxalate crystals to precipitate from the solution in stages. This avoids the explosive precipitation of needle-shaped sodium oxalate crystals, which would affect the subsequent sieving of sodium oxalate crystals and sodium aluminate solution.
[0035] The cooling stage can be 2, 3, or 4 stages.
[0036] The temperature drop range of the two adjacent cooling stages can be 10℃, 15℃, 20℃, 25℃, 30℃ or 35℃.
[0037] It should be noted that when the number of stages in the cooling section is less than two, for example, one stage, the sodium aluminate solution is directly and rapidly cooled to 35℃~55℃. At this time, sodium oxalate crystals in the sodium aluminate solution will rapidly precipitate due to the rapid cooling. This process will cause the sodium oxalate crystals to precipitate explosively, forming needle-like sodium oxalate crystals, making it difficult to separate the sodium oxalate crystals from the sodium aluminate solution later. When the number of stages in the cooling section is greater than four, more cooling and crystallization stages are required. This not only prolongs the gradient crystallization time but also increases the overall energy consumption of the method.
[0038] It should be noted that when the temperature drop between two adjacent cooling stages is less than 10°C, the precipitation rate of sodium oxalate crystals in different cooling stages is not significantly different, making it difficult to achieve effective precipitation of sodium oxalate crystals. When the temperature drop between two adjacent cooling stages is greater than 35°C, sodium oxalate crystals in the sodium aluminate solution will rapidly cool and precipitate. This process will lead to an explosive precipitation of sodium oxalate crystals, resulting in needle-shaped sodium oxalate crystals. These needle-shaped sodium oxalate crystals are difficult to separate from the sodium aluminate solution and form a relatively pure sodium aluminate solution in the subsequent screening process.
[0039] In some optional embodiments, the cooling stage includes a first cooling stage and a final cooling stage, wherein the initial temperature of the first cooling stage is 80°C to 100°C, the final temperature of the first cooling stage is 65°C to 75°C, and the final temperature of the final cooling stage is 35°C to 55°C.
[0040] In these embodiments, the cooling stage may include an initial cooling stage, with an initial temperature of 80°C to 100°C, to ensure the sodium aluminate solution has sufficient temperature, thereby providing sufficient solubility and concentration of sodium oxalate in the solution. This facilitates the initial precipitation of a certain amount of sodium aluminate crystals after the initial cooling stage. Furthermore, the final temperature of the initial cooling stage may be 65°C to 75°C, ensuring the cooling range is controlled below 35°C to prevent explosive precipitation of sodium oxalate crystals and promote uniform precipitation. Additionally, the cooling stage may include a final cooling stage, with an final temperature of 35°C to 55°C, allowing sufficient precipitation of sodium oxalate crystals in the solution during the final cooling stage, effectively removing sodium oxalate crystals from the sodium aluminate solution.
[0041] The initial temperature of this first cooling phase can be 80℃, 85℃, 90℃, 95℃, or 100℃.
[0042] The final temperature of this initial cooling phase can be 65℃, 66℃, 67℃, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃, or 75℃.
[0043] The final temperature of this last cooling stage can be 35℃, 40℃, 45℃, 50℃, or 55℃.
[0044] It should be noted that if the final temperature of the first cooling stage is less than or equal to 65℃, the temperature drop of the sodium aluminate solution before and after the first cooling stage will be too large. This will cause sodium oxalate in the sodium aluminate solution to precipitate explosively in the form of needle-shaped sodium oxalate crystals. These needle-shaped sodium oxalate crystals will be difficult to separate from the sodium aluminate solution and obtain a pure sodium aluminate solution in the subsequent screening process. If the final temperature of the first cooling stage is greater than 75℃, it will be difficult for sodium oxalate crystals to precipitate effectively from the sodium aluminate solution. This will result in a large amount of sodium oxalate crystals remaining in the sodium aluminate solution, affecting the removal efficiency of sodium oxalate and the purity of the sodium aluminate solution after sodium oxalate crystal removal.
[0045] It should be noted that when the final temperature of the last cooling stage is less than 35℃, the sodium aluminate solution at this low temperature has a high viscosity, and sodium carbonate impurities in the sodium aluminate solution will also precipitate. These precipitated sodium carbonate impurities and precipitated sodium oxalate crystals will form large particles in the relatively viscous sodium aluminate solution, affecting the subsequent separation effect of sodium oxalate crystals from the sodium aluminate solution, thus affecting the purity of the sodium aluminate solution after sodium oxalate crystal removal. When the final temperature of the last cooling stage is greater than 55℃, the precipitation rate of sodium oxalate crystals in the sodium aluminate solution is low in the last cooling stage, resulting in insufficient removal of sodium oxalate from the sodium aluminate solution.
[0046] In some alternative embodiments, the gradient crystallization further includes multiple crystallization stages, with each cooling stage followed by a single crystallization stage in series, the crystallization time of the crystallization stage being negatively correlated with the temperature of the cooling stage.
[0047] In these embodiments, gradient crystallization may also include multiple crystallization stages, with a single crystallization stage connected in series after each cooling stage, and the crystallization time of the crystallization stage being negatively correlated with the temperature of the cooling stage. By connecting the cooling stage and the crystallization stage in series, sufficient crystallization time can be provided for sodium oxalate crystallization after the sodium aluminate solution is cooled, thereby effectively removing sodium oxalate crystals from the sodium aluminate solution.
[0048] In some optional embodiments, when the end temperature of the cooling section is greater than 65°C, the crystallization time of the crystallization section connected in series after the cooling section is 5h to 10h.
[0049] When the end temperature of the cooling section is less than or equal to 65°C and greater than or equal to 55°C, the crystallization time of the crystallization section connected in series after the cooling section is 2h to 10h.
[0050] When the final temperature of the cooling section is less than 55°C, the crystallization time of the crystallization section connected in series after the cooling section is 2h to 5h.
[0051] In these embodiments, the precipitation of sodium oxalate crystals in sodium aluminate solution initially increases rapidly with the increase of crystallization time in the crystallization stage, but the precipitation rate decreases after a certain crystallization stage. Based on this characteristic and the relationship between the end temperature of the cooling stage and 65°C and 55°C, the crystallization time of the crystallization stage after each cooling stage can be clearly defined. This ensures that the precipitation rate of sodium oxalate crystals in the sodium aluminate solution in each cooling stage is at a high level, thereby allowing sufficient precipitation of sodium oxalate crystals in the sodium aluminate solution. For example, when the end temperature of the cooling stage is greater than 65°C, the crystallization time of the crystallization stage following the cooling stage can be 5h to 10h, allowing for rapid precipitation of sodium oxalate crystals in the sodium aluminate solution at the higher temperature cooling stage. Furthermore, when the final temperature of the cooling stage is less than or equal to 65°C and greater than or equal to 55°C, the crystallization time of the crystallization stage following the cooling stage can be 2h to 10h. This allows for the rapid precipitation of sodium oxalate crystals in the sodium aluminate solution at the intermediate temperature cooling stage, resulting in a high precipitation rate and effective removal of sodium oxalate crystals from the sodium aluminate solution. Additionally, when the final temperature of the cooling stage is less than 55°C, the crystallization time of the crystallization stage following the cooling stage can be 2h to 5h. This allows for the rapid precipitation of sodium oxalate crystals in the sodium aluminate solution at the lower temperature cooling stage, resulting in a high precipitation rate and effective removal of sodium oxalate crystals from the sodium aluminate solution.
[0052] When the final temperature of the cooling section is greater than 65℃, the crystallization time of the crystallization section connected in series after the cooling section can be 5h, 6h, 7h, 8h, 9h or 10h.
[0053] When the final temperature of the cooling section is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization section connected in series after the cooling section can be 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h.
[0054] When the final temperature of the cooling section is less than 55℃, the crystallization time of the crystallization section connected in series after the cooling section can be 2h, 3h, 4h or 5h.
[0055] In some alternative embodiments, the total time for gradient crystallization is 10h to 25h.
[0056] In these embodiments, the total time for gradient crystallization can be 10h to 25h, allowing sodium oxalate in the sodium aluminate solution to crystallize out rapidly, thereby effectively removing sodium oxalate crystals from the sodium aluminate solution.
[0057] The total time for gradient crystallization can be 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h, 24h or 25h.
[0058] It should be noted that when the total gradient crystallization time is less than 10 hours, some sodium oxalate crystals will remain in the sodium aluminate solution, resulting in a low precipitation rate of sodium oxalate crystals in the sodium aluminate solution. When the total gradient crystallization time is greater than 25 hours, if the hourly processing capacity of sodium aluminate solution and the crystallization temperature are constant, extending the total gradient crystallization time is unlikely to improve the precipitation rate of sodium oxalate crystals in the sodium aluminate solution. In addition, additional equipment such as crystallization tanks is required, which will increase the production cost of this method.
[0059] Figure 2 shows a detailed flowchart of a method for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure;
[0060] In some optional embodiments, as shown in Figure 2, the step of sedimenting and separating the sodium aluminate mother liquor containing sodium oxalate crystals to obtain a sodium aluminate solution free of sodium oxalate includes the following steps:
[0061] S201. The sodium aluminate mother liquor containing sodium oxalate crystals is subjected to sedimentation and solid-liquid separation to obtain sediment and sodium aluminate supernatant, respectively.
[0062] S202. The sediment is screened to obtain the undersize material;
[0063] S203. The undersize material is filtered by pressure to obtain filtrate and sodium oxalate fine crystals;
[0064] S204. The filtrate and the sodium aluminate supernatant are combined to obtain a sodium aluminate solution free of sodium oxalate; and,
[0065] S205. The sodium oxalate seed crystals are returned to the gradient crystallization process as a crystallization aid.
[0066] In these embodiments, the ammonium aluminate mother liquor containing sodium oxalate crystals is first subjected to sedimentation and solid-liquid separation to completely separate the sediment and sodium aluminate supernatant. The sediment is then sieved to remove larger sodium oxalate crystals, yielding undersize material containing fine sodium oxalate seed crystals with high seed activity. The undersize material is then pressure filtered to separate the sodium aluminate solution and fine sodium oxalate seed crystals, resulting in a filtrate containing sodium aluminate solution. Finally, the filtrate and sodium aluminate supernatant are combined to obtain a pure sodium aluminate solution. The fine sodium oxalate seed crystals are then returned to the gradient crystallization section as a crystallization aid to increase the crystallization rate of sodium oxalate in the sodium aluminate solution, thereby increasing the precipitation rate of sodium oxalate crystals from the sodium aluminate solution.
[0067] In some optional embodiments, the sodium oxalate fine seed crystals have a particle size ≤1 mm and are spherical crystals.
[0068] In these embodiments, the sodium oxalate crystals precipitated by the gradient crystallization described above are regular spherical crystals. However, when the particle size is greater than 1 mm, the activity of these spherical sodium oxalate crystals is poor, and they cannot be used as crystallization aids. When the particle size is less than or equal to 1 mm, these spherical sodium oxalate crystals are in the growth stage. When used as a crystallization aid in the gradient crystallization process, the sodium oxalate in the sodium aluminate solution will continue to grow on the surface of these sodium oxalate crystals, thereby increasing the precipitation rate of sodium oxalate crystals from the sodium aluminate solution.
[0069] Figure 3 shows a system logic diagram for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure.
[0070] Based on a general inventive concept, as shown in Figure 3, a system for removing sodium oxalate from a sodium aluminate solution according to some embodiments of the present disclosure, the system being adapted to the method, the system comprising:
[0071] The raw material section includes a sodium aluminate solution storage tank 1 and a raw material conveying pipe 2, wherein the outlet of the sodium aluminate solution storage tank 1 is connected to the inlet of the raw material conveying pipe 2;
[0072] The gradient crystallization section includes a seed crystal feed pipe 3, a multi-stage heat exchanger 4, and a multi-stage crystallization tank 5. The multi-stage heat exchangers 4 and the multi-stage crystallization tanks 5 are arranged alternately and connected in series. The inlet of the first-stage heat exchanger 4 is connected to the outlet of the raw material conveying pipe 2; the inlet of the first-stage crystallization tank 5 is connected to the outlet of the first-stage heat exchanger 4; the seed crystal inlet of the first-stage crystallization tank 5 is connected to the seed crystal feed pipe 3; and the inlet of the last-stage crystallization tank 5 is connected to the outlet of the last-stage heat exchanger 4.
[0073] The separation section includes a settling tank 6 and a product storage tank 7. The outlet of the crystallization tank 5, located at the last stage, is connected to the inlet of the settling tank 6, and the outlet of the settling tank 6 is connected to the inlet of the product storage tank 7.
[0074] The system is implemented based on the above method. The specific steps of the method can be referred to the above embodiments. Since the system adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.
[0075] In some optional embodiments, the separation unit further includes: an underflow pump 8, a vibrating screen 9, a transfer pump 10, a filter press 11, a filtrate transfer pipe 12, and a seed crystal transfer pipe 13. The underflow pump 8 is connected to the bottom outlet of the settling tank 6, the vibrating screen 9 is connected to the outlet of the underflow pump 8, the bottom outlet of the vibrating screen 9 is connected to the inlet of the transfer pump 10, the outlet of the transfer pump 10 is connected to the inlet of the filter press 11, the outlet of the filter press 11 is connected to the inlet of the filtrate transfer pipe 12, the outlet of the filtrate transfer pipe 12 is connected to the inlet of the product storage tank 7, the outlet of the filter press 11 is connected to the inlet of the seed crystal transfer pipe 13, and the outlet of the seed crystal transfer pipe 13 is connected to the inlet of the seed crystal feed pipe 3.
[0076] In these embodiments, the separation unit may further include: an underflow pump 8, a vibrating screen 9, a transfer pump 10, a filter press 11, a filtrate transfer pipe 12, and a seed crystal transfer pipe 13. The underflow pump 8 is connected to the bottom outlet of the settling tank 6, the vibrating screen 9 is connected to the outlet of the underflow pump 8, the bottom outlet of the vibrating screen 9 is connected to the inlet of the transfer pump 10, the outlet of the transfer pump 10 is connected to the inlet of the filter press 11, the outlet of the filter press 11 is connected to the inlet of the filtrate transfer pipe 12, the outlet of the filtrate transfer pipe 12 is connected to the inlet of the product storage tank 7, the outlet of the filter press 11 is connected to the inlet of the seed crystal transfer pipe 13, and the outlet of the seed crystal transfer pipe 13 is connected to the seed crystal feed pipe. The feed inlet of the filter press 3 allows the sediment in the settling tank 6 to be transported to the vibrating screen 9 for screening via the underflow pump 8. The undersize material is then transported to the filter press 11 via the transfer pump 10 for filtration. This separates the filtrate containing sodium aluminate solution from the sodium oxalate fine crystals, resulting in filtrate and sodium oxalate fine crystals. The filtrate is then transported to the product storage tank 7 via the filtrate conveying pipe 12, allowing the filtrate and the sodium aluminate supernatant after gradient crystallization to merge and form a relatively pure sodium aluminate solution. Meanwhile, the sodium oxalate fine crystals are returned to the seed feed pipe 3 via the seed conveying pipe 13 and used as a crystallization aid in gradient crystallization, thus achieving material recycling.
[0077] It should be noted that the vibrating screen 9 can use a square hole screen with a side length of ≥1mm or a round hole screen with a diameter of ≥1mm as the screen mesh.
[0078] The technical solutions of this disclosure are further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.
[0079] Example 1
[0080] Select an infeed flow rate of 100m³ / h 3 A sodium aluminate solution with an initial temperature of 80℃~100℃ and a caustic alkali concentration of 220g / L~230g / L.
[0081] As shown in Figure 2, this embodiment describes a method for removing sodium oxalate from a sodium aluminate solution, wherein the sodium aluminate solution includes sodium oxalate. The method includes:
[0082] S1. A sodium aluminate solution is subjected to gradient crystallization to precipitate a large amount of sodium oxalate crystals, resulting in a sodium aluminate mother liquor containing sodium oxalate crystals; wherein, the gradient crystallization includes multiple cooling stages, with two cooling stages; the mass concentration of oxalate ions in the sodium aluminate solution is 1.70 g / L to 1.90 g / L.
[0083] S201. The mother liquor containing sodium aluminate crystals is subjected to sedimentation and solid-liquid separation to obtain sediment and sodium aluminate supernatant, respectively.
[0084] S202. Screen the sediment to obtain the undersize material;
[0085] S203. The sieved material is filtered by pressure to obtain filtrate and sodium oxalate fine crystals; wherein the particle size of the sodium oxalate fine crystals is ≤1mm;
[0086] S204. Combine the filtrate and the sodium aluminate supernatant to obtain a sodium aluminate solution free of sodium oxalate; and,
[0087] S205. Sodium oxalate fine seed crystals are returned to gradient crystallization as a crystallization aid.
[0088] The cooling process includes an initial cooling phase and a final cooling phase. The initial temperature of the initial cooling phase is 80℃ to 100℃, the final temperature of the initial cooling phase is 75℃, and the final temperature of the final cooling phase is 50℃.
[0089] Gradient crystallization also includes multiple crystallization stages, with each cooling stage followed by a single crystallization stage in series. The crystallization time of a crystallization stage is negatively correlated with the temperature of the cooling stage.
[0090] When the final temperature of the cooling section is greater than 65℃, the crystallization time of the crystallization section connected in series after the first cooling section is 10h.
[0091] When the final temperature of the cooling section is less than 55℃, the crystallization time of the crystallization section connected in series after the last cooling section is 5 hours.
[0092] The total time for gradient crystallization was 15 hours.
[0093] As shown in Figure 3, this embodiment provides a system for removing sodium oxalate from a sodium aluminate solution. This system is adapted to the above method and includes:
[0094] The raw material section includes a sodium aluminate solution storage tank 1 and a raw material conveying pipe 2, with the outlet of the sodium aluminate solution storage tank 1 connected to the inlet of the raw material conveying pipe 2.
[0095] The gradient crystallization section includes a seed crystal feed pipe 3, a multi-stage heat exchanger 4, and a multi-stage crystallization tank 5. The multi-stage heat exchanger 4 and the multi-stage crystallization tank 5 are arranged alternately and connected in series. The inlet of the first-stage heat exchanger 4 is connected to the outlet of the raw material conveying pipe 2; the inlet of the first-stage crystallization tank 5 is connected to the outlet of the first-stage heat exchanger 4; the seed crystal inlet of the first-stage crystallization tank 5 is connected to the seed crystal feed pipe 3; and the inlet of the last-stage crystallization tank 5 is connected to the outlet of the last-stage heat exchanger 4.
[0096] The separation section includes a settling tank 6 and a product storage tank 7. The outlet of the crystallization tank 5, located at the last stage, is connected to the inlet of the settling tank 6, and the outlet of the settling tank 6 is connected to the inlet of the product storage tank 7.
[0097] The separation section also includes: an underflow pump 8, a vibrating screen 9, a conveying pump 10, a filter press 11, a filtrate conveying pipe 12, and a seed crystal conveying pipe 13. The underflow pump 8 is connected to the bottom outlet of the settling tank 6, the vibrating screen 9 is connected to the outlet of the underflow pump 8, the bottom outlet of the vibrating screen 9 is connected to the inlet of the conveying pump 10, the outlet of the conveying pump 10 is connected to the inlet of the filter press 11, the outlet of the filter press 11 is connected to the inlet of the filtrate conveying pipe 12, the outlet of the filtrate conveying pipe 12 is connected to the inlet of the product storage tank 7, the outlet of the filter press 11 is connected to the inlet of the seed crystal conveying pipe 13, and the outlet of the seed crystal conveying pipe 13 is connected to the inlet of the seed crystal feed pipe 3.
[0098] Example 2
[0099] Based on the content disclosed in Example 1, the following modifications are made:
[0100] The final temperature of the last cooling stage is 45℃.
[0101] Example 3
[0102] Based on the content disclosed in Example 1, the following modifications are made:
[0103] The final temperature of the last cooling stage is 40℃.
[0104] Example 4
[0105] Based on the content disclosed in Example 1, the following modifications are made:
[0106] The final temperature of the first cooling phase is 70℃.
[0107] If the final temperature of the first cooling stage is greater than 65°C, the crystallization time of the crystallization stage connected in series after the first cooling stage is 8 hours.
[0108] Example 5
[0109] Based on the content disclosed in Example 4, the following modifications are made:
[0110] The final temperature of the last cooling stage is 45℃.
[0111] Example 6
[0112] Based on the content disclosed in Example 4, the following modifications are made:
[0113] The final temperature of the last cooling stage is 40℃.
[0114] Example 7
[0115] Based on the content disclosed in Example 1, the following modifications are made:
[0116] The final temperature of the first cooling phase is 65℃.
[0117] If the final temperature of the first cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the first cooling stage is 5 hours.
[0118] Example 8
[0119] Based on the content disclosed in Example 1, the following modifications are made:
[0120] The final temperature of the last cooling stage is 45℃.
[0121] Example 9
[0122] Based on the content disclosed in Example 1, the following modifications are made:
[0123] The final temperature of the last cooling stage is 40℃.
[0124] Example 10
[0125] Based on the content disclosed in Example 1, the following modifications are made:
[0126] The cooling stage has three stages.
[0127] The final temperature of the first cooling phase is 75℃.
[0128] If the final temperature of the first cooling stage is greater than 65°C, the crystallization time of the crystallization stage connected in series after the first cooling stage is 10 hours.
[0129] The final temperature of the intermediate cooling section is 65℃.
[0130] When the final temperature of the intermediate cooling section is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization section connected in series after the intermediate cooling section is 5 hours.
[0131] The final temperature of the last cooling stage is 55℃.
[0132] When the final temperature of the last cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the last cooling stage is 5 hours.
[0133] Example 11
[0134] Based on the content disclosed in Example 1, the following modifications are made:
[0135] The cooling stage has three stages.
[0136] The final temperature of the first cooling phase is 70℃.
[0137] If the final temperature of the first cooling stage is greater than 65°C, the crystallization time of the crystallization stage connected in series after the first cooling stage is 8 hours.
[0138] The final temperature of the intermediate cooling section is 60℃.
[0139] When the final temperature of the intermediate cooling section is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization section connected in series after the intermediate cooling section is 5 hours.
[0140] The final temperature of the last cooling phase is 50℃.
[0141] If the final temperature of the last cooling stage is less than 55℃, the crystallization time of the crystallization stage connected in series after the last cooling stage is 5 hours.
[0142] Example 12
[0143] Based on the content disclosed in Example 1, the following modifications are made:
[0144] The cooling stage has three stages.
[0145] The final temperature of the first cooling phase is 65℃.
[0146] If the final temperature of the first cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the first cooling stage is 5 hours.
[0147] The final temperature of the intermediate cooling section is 55℃.
[0148] When the final temperature of the intermediate cooling section is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization section connected in series after the intermediate cooling section is 3 hours.
[0149] The final temperature of the last cooling stage is 45℃.
[0150] If the final temperature of the last cooling stage is less than 55℃, the crystallization time of the crystallization stage connected in series after the last cooling stage is 3 hours.
[0151] Example 13
[0152] Based on the content disclosed in Example 1, the following modifications are made:
[0153] The cooling stage has three stages.
[0154] The final temperature of the first cooling phase is 75℃.
[0155] If the final temperature of the first cooling stage is greater than 65°C, the crystallization time of the crystallization stage connected in series after the first cooling stage is 10 hours.
[0156] The final temperature of the intermediate cooling section is 65℃.
[0157] When the final temperature of the intermediate cooling section is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization section connected in series after the intermediate cooling section is 5 hours.
[0158] The final temperature of the last cooling stage is 55℃.
[0159] When the final temperature of the last cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the last cooling stage is 5 hours.
[0160] Example 14
[0161] Based on the content disclosed in Example 1, the following modifications are made:
[0162] The cooling stage has four stages.
[0163] The final temperature of the first cooling phase is 70℃.
[0164] If the final temperature of the first cooling stage is greater than 65℃, the crystallization time of the crystallization stage connected in series after the first cooling stage is 8 hours.
[0165] The final temperature of the second cooling phase is 60℃.
[0166] If the final temperature of the second cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the second cooling stage is 5 hours.
[0167] The final temperature of the third cooling phase is 50℃.
[0168] If the final temperature of the third cooling stage is less than 55℃, the crystallization time of the crystallization stage connected in series after the third cooling stage is 3 hours.
[0169] The final temperature of the fourth cooling phase was 45℃.
[0170] If the final temperature of the fourth cooling stage is less than 55℃, the crystallization time of the crystallization stage connected in series after the third cooling stage is 3 hours.
[0171] Example 15
[0172] Based on the content disclosed in Example 1, the following modifications are made:
[0173] The cooling stage has four stages.
[0174] The final temperature of the first cooling phase is 65℃.
[0175] If the final temperature of the second cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the second cooling stage is 5 hours.
[0176] The final temperature of the second cooling phase was 55℃.
[0177] If the final temperature of the second cooling stage is less than or equal to 65℃ and greater than or equal to 55℃, the crystallization time of the crystallization stage connected in series after the second cooling stage is 5 hours.
[0178] The final temperature of the third cooling phase was 45℃.
[0179] If the final temperature of the third cooling stage is less than 55℃, the crystallization time of the crystallization stage connected in series after the third cooling stage is 2 hours.
[0180] The final temperature of the fourth cooling phase was 35℃.
[0181] If the final temperature of the fourth cooling stage is less than 55℃, the crystallization time of the crystallization stage connected in series after the third cooling stage is 2 hours.
[0182] Comparative Example 1
[0183] Based on the content disclosed in Example 1, the following modifications are made:
[0184] Instead of using gradient crystallization, the temperature is directly lowered to 35°C in one step for crystallization.
[0185] Comparative Example 2
[0186] Based on the content disclosed in Example 1, the following modifications are made:
[0187] The temperature drop between two adjacent cooling stages is 5℃, meaning the final temperature of the last cooling stage is 70℃.
[0188] Comparative Example 3
[0189] Based on the content disclosed in Example 1, the following modifications are made:
[0190] The temperature drop between two adjacent cooling stages is 40℃, meaning the final temperature of the last cooling stage is 30℃.
[0191] Relevant experimental and effect data:
[0192] The sodium aluminate solutions obtained in each embodiment and comparative example were tested to determine the mass concentration of residual oxalate ions and calculate the corresponding oxalate ion removal rate. In addition, the yield of oversize material (sodium oxalate crystals with a diameter > 1 mm) obtained from vibrating screen sieving within one day was statistically analyzed. The results are shown in Table 1. The formula for calculating the oxalate ion removal rate is:
[0193] Oxalate ion removal rate = 1 - (mass concentration of residual oxalate ions × volume of sodium aluminate solution) / (mass concentration of oxalate ions in sodium aluminate solution × volume of sodium aluminate solution)
[0194] It should be noted that the removal rate of oxalate ions = 1 - the residual rate of oxalate ions. That is, the residual rate of oxalate ions is calculated by taking the residual rate of oxalate ions, which is the ratio of the mass of residual oxalate ions in the sodium aluminate solution after oxalate removal to the initial mass of oxalate ions in the sodium aluminate solution. Therefore, when calculating the mass of residual oxalate ions in the sodium aluminate solution after oxalate removal, the volume of the sodium aluminate solution after oxalate removal and the mass concentration of oxalate ions in the sodium aluminate solution after oxalate removal should be used as the basis for calculation. Thus, the removal rate of oxalate ions = 1 - (mass concentration of residual oxalate ions in the sodium aluminate solution after oxalate removal × volume of the sodium aluminate solution after oxalate removal) / (mass concentration of oxalate ions in the sodium aluminate solution × volume of the sodium aluminate solution).
[0195] Table 1. Residual oxalate ions in sodium aluminate solution
[0196] Project Group: Residual oxalate ion mass concentration (g / L); Oxalate ion removal rate (%); Oversize material yield (t / d); Example 1: 0.7–0.947–646–10; Example 2: 0.5–0.758–747–12; Example 3: 0.3–0.570–859–15; Example 4: 0.7–0.947–646–10; Example 5: 0.5–0.758–747–12; Example 6: 0.3–0.570–859–15; Example 7: 0.7–0.947–646–10; Example 8: 0.5–0.759–747–12; Example 9: 0. 3~0.570~859~15 Example 10 0.9~1.041~535~9 Example 11 0.7~0.947~646~10 Example 12 0.5~0.758~747~12 Example 13 0.4~0.665~799~16 Example 14 0.3~0.476~8510~18 Example 15 0.2~0.382~9012~21 Comparative Example 1 0.2~0.3 (requires secondary filtering) 82~90<1 Comparative Example 2 1.4~1.65~271.5~3 Comparative Example 3 <0.2 (requires secondary filtering) Greater than 85<1
[0197] As shown in Table 1, the method for removing sodium oxalate from sodium aluminate solution according to some embodiments of this disclosure achieves highly efficient removal of sodium oxalate through an innovative gradient cooling crystallization step. This ensures a removal rate of oxalate ions of over 40%, meaning a sodium oxalate removal rate of over 40%. Furthermore, the gradient cooling crystallization method controls the morphology of the generated sodium oxalate crystals, resulting in regular spherical crystals precipitated from the sodium aluminate solution. These regularly spherical sodium oxalate crystals can be separated from the sodium aluminate solution by vibrating sieve separation, improving the efficiency of solid-liquid separation between the sodium oxalate crystals and the sodium aluminate solution. Simultaneously, this method avoids introducing new organic impurities into the sodium aluminate solution, ensuring the purity of the sodium aluminate solution after sodium oxalate removal, and has high industrial application value.
[0198] In addition, according to some embodiments of this disclosure, a method for removing sodium oxalate from a sodium aluminate solution not only ensures a high sodium oxalate removal efficiency but also improves the solid-liquid separation efficiency of the sodium aluminate solution, while avoiding the introduction of new impurities into the sodium aluminate solution. Therefore, it can simultaneously improve the sodium oxalate removal efficiency and the purity of the sodium aluminate solution after sodium oxalate removal.
[0199] In summary, the method for removing sodium oxalate from a sodium aluminate solution according to some embodiments of this disclosure has the following advantages compared with related technologies:
[0200] This method employs a gradient crystallization process with two or more cooling stages. This allows for the phased reduction of sodium oxalate's solubility in sodium aluminate solution, enabling the sodium oxalate to crystallize out of the solution in stages. This avoids explosive precipitation of sodium oxalate crystals, preventing the formation of needle-like crystals. The goal is to achieve a large and uniform precipitation of sodium oxalate from the solution, thereby improving the removal efficiency. Furthermore, this method does not introduce any additional crystallization aids, preventing the introduction of new impurities into the sodium aluminate solution and thus increasing the purity of the sodium aluminate solution after sodium oxalate removal.
[0201] The above description is merely a specific embodiment of this disclosure, enabling those skilled in the art to understand or implement it. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined in this disclosure may be implemented in other embodiments without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. A method for removing sodium oxalate from a sodium aluminate solution, wherein the sodium aluminate solution comprises sodium oxalate, the method comprising: The sodium aluminate solution is subjected to gradient crystallization, causing a large amount of sodium oxalate to crystallize out, resulting in a sodium aluminate mother liquor containing sodium oxalate crystals; wherein the gradient crystallization includes multiple cooling stages, and the number of cooling stages is ≥2 stages; and, The sodium aluminate mother liquor containing sodium oxalate crystals was subjected to sedimentation separation to obtain a sodium aluminate solution free of sodium oxalate.
2. The method according to claim 1, wherein, The cooling stage has 2 to 4 stages, and the cooling range between two adjacent stages is 10°C to 35°C.
3. The method according to claim 1, wherein, The cooling section includes an initial cooling section and a final cooling section. The initial temperature of the initial cooling section is 80℃~100℃, the final temperature of the initial cooling section is 65℃~75℃, and the final temperature of the final cooling section is 35℃~55℃.
4. The method according to claim 1, wherein, The gradient crystallization also includes multiple crystallization segments, with each cooling segment followed by a single crystallization segment connected in series. The crystallization time of each crystallization segment is negatively correlated with the temperature of the cooling segment.
5. The method according to claim 4, wherein, When the final temperature of the cooling section is greater than 65°C, the crystallization time of the crystallization section connected in series after the cooling section is 5h to 10h. When the end temperature of the cooling section is less than or equal to 65°C and greater than or equal to 55°C, the crystallization time of the crystallization section connected in series after the cooling section is 2h to 10h. When the final temperature of the cooling section is less than 55°C, the crystallization time of the crystallization section connected in series after the cooling section is 2h to 5h.
6. The method according to claim 1, wherein, The total time for gradient crystallization is 10h to 25h.
7. The method according to claim 1, wherein, The step of sedimentation separation of the sodium aluminate mother liquor containing sodium oxalate crystals to obtain a sodium aluminate solution free of sodium oxalate includes the following steps: The sodium aluminate mother liquor containing sodium oxalate crystals was subjected to sedimentation and solid-liquid separation to obtain sediment and sodium aluminate supernatant, respectively. The sediment is screened to obtain the undersize material; The undersize material is filtered to obtain filtrate and sodium oxalate fine crystals. The filtrate and the sodium aluminate supernatant were combined to obtain a sodium aluminate solution free of sodium oxalate. as well as, The sodium oxalate seed crystals are returned to the gradient crystallization process as a crystallization aid.
8. The method according to claim 7, wherein, The sodium oxalate fine seed crystals have a particle size ≤1mm and are spherical crystals.
9. A system for removing sodium oxalate from a sodium aluminate solution, the system being adapted to the method of any one of claims 1 to 8, the system comprising: The raw material section includes a sodium aluminate solution storage tank (1) and a raw material conveying pipe (2), wherein the outlet of the sodium aluminate solution storage tank (1) is connected to the inlet of the raw material conveying pipe (2); The gradient crystallization section includes a seed feed pipe (3), a multi-stage heat exchanger (4), and a multi-stage crystallization tank (5). The multi-stage heat exchangers (4) and the multi-stage crystallization tanks (5) are arranged alternately and connected in series. The inlet of the first-stage heat exchanger (4) is connected to the outlet of the raw material conveying pipe (2), the inlet of the first-stage crystallization tank (5) is connected to the outlet of the first-stage heat exchanger (4), the seed inlet of the first-stage crystallization tank (5) is connected to the seed feed pipe (3), and the inlet of the last-stage crystallization tank (5) is connected to the outlet of the last-stage heat exchanger (4). The separation section includes a settling tank (6) and a product storage tank (7). The outlet of the crystallization tank (5) located at the last stage is connected to the inlet of the settling tank (6), and the outlet of the settling tank (6) is connected to the inlet of the product storage tank (7).
10. The system according to claim 9, wherein, The separation section further includes: an underflow pump (8), a vibrating screen (9), a conveying pump (10), a filter press (11), a filtrate conveying pipe (12), and a seed crystal conveying pipe (13). The underflow pump (8) is connected to the bottom outlet of the settling tank (6). The vibrating screen (9) is connected to the outlet of the underflow pump (8). The bottom outlet of the vibrating screen (9) is connected to the inlet of the conveying pump (10). The outlet of the conveying pump (10) is connected to the inlet of the filter press (11). The outlet of the filter press (11) is connected to the inlet of the filtrate conveying pipe (12). The outlet of the filtrate conveying pipe (12) is connected to the inlet of the product storage tank (7). The outlet of the filter press (11) is connected to the inlet of the seed crystal conveying pipe (13). The outlet of the seed crystal conveying pipe (13) is connected to the inlet of the seed crystal feed pipe (3).