Method and system for controlling oxalate in a bayer process for producing alumina

Abstract

The application provides a method and system for controlling oxalate in a Bayer process for producing alumina, and relates to the technical field of alumina production. The method comprises the following steps: mixing a solution rich in sodium oxalate in the Bayer process with lime milk to perform a causticization reaction, the amount of calcium oxide added is dynamically calculated according to the concentration of oxalate ions in the solution, solid-liquid separation is performed after the reaction, and calcium oxalate precipitate and purified solution are obtained; an oxalate crystallization form control agent is added in the treated seed solution; high-temperature and high-concentration evaporation mother liquor discharged from an evaporation station is cooled to 50-55 DEG C through heat exchange, and then is sent into an oxalate decomposition tank with an ultrasonic oscillation device, and seed crystals are added to promote the crystallization and precipitation of oxalate; subsequently, a filter press is used to perform liquid-solid separation, the separated solid is an oxalate product, and the liquid is returned to the system as a circulating mother liquor. The application has a high oxalate removal rate and controllable alumina loss.

Description

Technical Field

[0001] This invention relates to the field of alumina production technology, specifically to a method and control system for controlling oxalate in the Bayer process for alumina production. Background Technology

[0002] In the Bayer process for alumina production, organic impurities in bauxite react with alkaline solutions to generate various organic compounds, including sodium oxalate. When using high-silicon, high-iron, and high-organic-content ores from regions like Guinea, the leaching process produces large amounts of sodium oxalate. The accumulation of oxalate in sodium aluminate solutions leads to a series of serious problems: First, oxalate significantly increases solution viscosity, reducing heat transfer efficiency and equipment capacity. Second, oxalate tends to precipitate as needle-like crystals on the surface of aluminum hydroxide seed crystals, poisoning the surface and severely hindering the aggregation and growth of aluminum hydroxide crystals, resulting in a drastic reduction in product particle size and affecting filtration and washing efficiency. Furthermore, needle-like oxalate crystals easily form dense, hard scale, primarily composed of calcium oxalate, on the surfaces of pipes, evaporators, and heat exchangers, causing a decrease in equipment heat transfer coefficients, increased energy consumption, and even unplanned shutdowns. Finally, residual oxalate in the product decomposes during calcination, increasing the sodium content in the alumina product and reducing product quality.

[0003] In existing technologies, the control of oxalate mainly involves hot water washing causticization and oxalate crystallization desalination. The hot water washing causticization method is lengthy, results in significant alumina loss, and incomplete oxalate removal. The oxalate crystallization desalination method relies on natural cooling crystallization, which has low precipitation efficiency, and the precipitated oxalate is needle-shaped, making solid-liquid separation difficult and hindering effective removal from the system. Summary of the Invention

[0004] The purpose of this invention is to develop a method and control system for controlling oxalate in the Bayer process for alumina production with high oxalate removal rate and controllable alumina loss.

[0005] This invention is achieved through the following technical solution:

[0006] A method for controlling oxalate in the Bayer process for alumina production includes the following steps:

[0007] S1. The sodium oxalate enrichment solution from the Bayer process is mixed with lime slurry to carry out a causticization reaction. The reaction conditions are controlled as follows: temperature 85-95℃, stirring speed 150-250 r·min. - ¹, The amount of calcium oxide added is dynamically calculated based on the concentration of oxalate ions in the solution to control the CaO / C₂O₄ ratio. 2- The molar ratio is 1.3~1.5:1. After the reaction, solid and liquid are separated to obtain calcium oxalate precipitate and purified solution.

[0008] S2. In the seed decomposition solution after step S1, an oxalate precipitation morphology control agent is added. The amount of oxalate precipitation morphology control agent added is dynamically adjusted according to the concentration of residual oxalate in the decomposition solution. During the decomposition process, the oxalate precipitation morphology control agent acts on the surface of the seed crystals and changes the crystallization habit of oxalate, inducing the residual oxalate to precipitate as spherical or near-spherical crystals.

[0009] S3. The high-temperature, high-concentration evaporation mother liquor discharged from the evaporation station is cooled to 50~55℃ through heat exchange and then sent to an oxalate decomposition tank equipped with an ultrasonic oscillation device. Seed crystals are added to promote the crystallization and precipitation of oxalate. Subsequently, a filter press is used for liquid-solid separation. The separated solid is the oxalate product, and the liquid is returned to the system as circulating mother liquor.

[0010] Optionally, in step S1, the reaction time is controlled at 60~120 min. Online particle size monitoring or timed sampling and sedimentation observation are adopted. The reaction is terminated when the D50 particle size distribution of the reaction slurry transitions from a single peak to a double peak or the sedimentation rate slows down significantly, in order to prevent the generated calcium aluminate double salt from coating the unreacted lime.

[0011] Optionally, in step S2, the oxalate precipitation form control agent is one or more of polyacrylic acid, sodium polyacrylate, or hydrolyzed polymaleic anhydride. The amount of oxalate precipitation form control agent added is adjusted according to the residual oxalate concentration in the decomposition solution. When the liquid phase oxalate concentration is ≥1.5 g / L, the amount of oxalate precipitation form control agent added is 10~30 ppm, which uses polymer adsorption to inhibit the needle-like growth of sodium oxalate until the liquid phase oxalate concentration drops below 1.5 g / L. When the liquid phase oxalate concentration is <1.5 g / L, based on the particle size feedback of the aluminum hydroxide product, 5~10 ppm of oxalate precipitation form control agent can be added or addition can be stopped.

[0012] Optionally, step S2 also includes a process parameter adjustment step linked to the addition of oxalate precipitation form control agent: when the concentration of oxalate in the concentrate is ≥1.8g / L during the particle size coarsening stage, while adding oxalate precipitation form control agent, the temperature of the first tank of the seeding process is reduced by 1~2℃ and the temperature of the last tank is maintained at ≥55℃, so as to synergistically promote the precipitation of a large number of oxalate pellets.

[0013] Optionally, in step S3, the frequency of the ultrasonic oscillation device is 15~25kHz, and the power density is 1.0~1.5W / cm².

[0014] Optionally, in step S3, the filter press includes a vertical filter press and a horizontal filter press. The vertical filter press is used to separate oxalate seed crystals, and its filter plate is made of stainless steel. The horizontal filter press is used to process most of the mother liquor after salt precipitation, and 1.0~1.5wt% of active dissolution residue is added to the mother liquor to be filtered as a filter aid before processing.

[0015] Optionally, in step S3, the heat exchange and cooling process adopts a two-stage heat exchange process. The first stage achieves pre-cooling by using circulating cooling water to cool the 110°C evaporated mother liquor to 75~80°C. The second stage achieves waste heat recovery by using the cold filtrate with a temperature of 45~50°C after pressure filtration to further cool the mother liquor to 50~55°C, while simultaneously raising the filtrate to 60~70°C and returning it to the system to recover heat.

[0016] A control system for oxalate production in the Bayer process for alumina production includes:

[0017] The chemical precipitation unit is used to receive a solution containing sodium oxalate and react it with lime slurry. It includes a reaction vessel, an online oxalate concentration analyzer, a stirring control mechanism, a lime slurry precision dosing mechanism, and a reaction endpoint monitoring mechanism.

[0018] The morphology control precipitation unit, located before the seeding tank, is used to regulate the morphology of oxalate crystals, including an OCA addition mechanism and an online liquid phase oxalate concentration analyzer.

[0019] The enhanced separation and removal unit includes a staged heat exchange and waste heat recovery mechanism, an ultrasonic crystallization tank group, a filter aid mixing tank and a pressure filtration separation mechanism connected in sequence.

[0020] The reactor is equipped with a heating and insulation jacket, and the lime slurry precise dosing mechanism is based on the C2O4 concentration in the incoming liquid monitored by an online oxalate concentration analyzer. 2- The concentration is adjusted to control the flow rate of lime slurry. The reaction endpoint monitoring device includes an online particle size analyzer or a sedimentation rate probe to prevent calcium aluminate double salt from encapsulating the solution.

[0021] The OCA addition mechanism includes a metering pump and a storage tank, and the online liquid oxalate concentration analyzer detects C2O4 in semen and circulating mother liquor in real time. 2- concentration;

[0022] The staged heat exchange and waste heat recovery mechanism includes a first-stage heat exchanger and a second-stage heat exchanger connected to each other. The first-stage heat exchanger uses circulating cooling water as a cold source to pre-cool the mother liquor from the evaporation station to 75~80℃. The second-stage heat exchanger uses cold filtrate with a temperature of 45~50℃ after pressure filtration as a cold source to further cool the 75~80℃ mother liquor to 50~55℃. At the same time, the cold filtrate is heated to 60~70℃ and returned to the production process.

[0023] The ultrasonic crystallization tank assembly receives cooled mother liquor at 50~55℃ and is equipped with a seed crystal addition port and an insertion-type ultrasonic oscillator.

[0024] Optionally, in the morphology control precipitation unit, the online liquid oxalate concentration analyzer is interlocked with the OCA addition mechanism and the seed tank temperature control system. Based on the monitoring value of the online liquid oxalate concentration analyzer, the OCA addition amount and the seed tank temperature are interlocked and controlled. The online liquid oxalate concentration analyzer detects C2O4... 2- When the concentration remains above 2.0 g / L, an alarm signal is issued, prompting the chemical precipitation unit to check the lime activity or increase the residence time.

[0025] Optionally, it also includes an anti-scaling and cleaning unit, which includes a dilute acid solution pipeline and an industrial water pipeline connected to the first-stage heat exchanger, the second-stage heat exchanger and the filter press separation mechanism, for periodically performing online water washing on the equipment to eliminate oxalate and carbonate scaling. The dilute acid solution pipeline delivers sulfuric acid or hydrochloric acid with a concentration of 5-10%.

[0026] The beneficial effects of this invention are:

[0027] This invention achieves high oxalate removal rate and controllable alumina loss through an optimized high-temperature causticization precipitation process (85~95℃) and a precise lime addition strategy based on oxalate concentration (CaO / C2O4). 2- With a molar ratio of 1.3 to 1.5:1, the oxalate removal rate was ≥85%, while the alumina loss was strictly controlled to ≤1.8 kg Al2O3 per kg of oxalate removed, thus avoiding the drawback of large alumina loss in the traditional causticization method.

[0028] To address the hazards of needle-like crystals at their source, this innovative approach introduces an oxalate precipitation morphology control agent (OCA) into the seeding process, combined with stepwise concentration control and temperature fine-tuning. This successfully transforms the precipitation behavior of oxalate from harmful needle-like or dendritic induction to low-surface-activity spherical or near-spherical crystals. This completely eliminates the poisoning effect of needle-like crystals on aluminum hydroxide seed crystals, significantly improves product particle size distribution, and effectively extends the overhaul cycle for system particle size refinement.

[0029] The separation efficiency and processing capacity are greatly improved. The system integrates ultrasonic oscillation to enhance crystallization (frequency 15~25kHz) and a special alkali-resistant pressure filtration system, which increases the oxalate crystallization rate by 30~50%, causing the precipitated oxalate crystals to be significantly coarsened. The oxalate removal efficiency is high, which greatly improves the filtration and washing performance of the slurry. With the use of red mud slurry filter aid, the system's processing capacity for evaporation mother liquor is increased to no less than 330m³ / h, achieving efficient physical removal of oxalate.

[0030] It has outstanding energy-saving, emission-reduction and anti-scaling effects. It has constructed a two-stage heat exchange and waste heat recovery network. It uses the cold filtrate after pressure filtration to recover the sensible heat of the evaporated mother liquor. While precisely cooling the mother liquor to the crystallization window (50~55℃), the filtrate is heated and returned to the system, which effectively reduces the steam consumption of the whole process. At the same time, the combination of ultrasonic anti-agglomeration effect and staged cooling strategy significantly reduces the oxalate scaling rate on the surface of heat exchangers and pipes, extends the equipment operation cycle and reduces the frequency of cleaning and maintenance.

[0031] The product quality has been significantly improved. Through the synergistic control of oxalate throughout the entire process, the inclusion and adsorption of sodium oxalate on the surface of aluminum hydroxide particles have been reduced. As a result, the content of alkali metal impurities (such as Na2O) in the final calcined alumina product has been effectively reduced, and the purity and whiteness of the product have been improved. Detailed Implementation

[0032] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the following description is to be considered exemplary in nature and not restrictive.

[0033] The embodiments of the present invention will be described in detail below.

[0034] This invention discloses a method for controlling oxalate in the Bayer process for alumina production, comprising the following steps:

[0035] S1. Chemical precipitation removal step: The solution enriched with sodium oxalate in the Bayer process (such as seed crystal washing solution) is mixed with lime slurry for a causticization reaction. The reaction conditions are controlled as follows: temperature 85-95℃, stirring speed 150-250 r·min. - ¹, The amount of calcium oxide (effective CaO) added is based on the amount of oxalate ions (C₂O₄) in the solution. 2- Dynamic concentration calculation to control CaO / C2O4 2- The molar ratio is 1.3~1.5:1 to compensate for the consumption of calcium aluminate by the side reaction of lime and sodium aluminate solution. After the reaction, solid and liquid are separated to obtain calcium oxalate precipitate and purified solution. The calcium oxalate formation rate is ≥85%, ensuring that the alumina loss per unit removal of 1 kg of oxalate is ≤1.8 kg of Al2O3.

[0036] S2. Morphology control precipitation step: Oxalate precipitation morphology control agent (OCA) is added to the seed decomposition solution after step S1. The amount of OCA added is dynamically adjusted according to the concentration of residual oxalate in the decomposition solution. During the decomposition process, OCA acts on the surface of the seed crystals and changes the crystallization habit of oxalate, inducing the residual oxalate to precipitate as spherical or near-spherical crystals.

[0037] S3. Enhance the separation and removal steps: After the high-temperature and high-concentration evaporation mother liquor discharged from the evaporation station is cooled to 50~55℃ through heat exchange, it is sent to the oxalate decomposition tank equipped with an ultrasonic oscillation device, and seed crystals are added to promote the crystallization and precipitation of oxalate. Subsequently, a high-temperature and high-alkali resistant filter press is used for liquid-solid separation. The separated solid is the oxalate product (which can be stored or recycled), while the liquid is returned to the system as circulating mother liquor.

[0038] In step S1, the reaction time is controlled between 60 and 120 minutes. Online particle size monitoring or timed sampling and sedimentation observation are used. The reaction is terminated when the D50 particle size distribution of the reaction slurry transitions from a single peak to a double peak or the sedimentation rate slows down significantly, in order to prevent the generated calcium aluminate double salt from coating the unreacted lime.

[0039] In step S2, the oxalate precipitation speciation control agent is one or more of polyacrylic acid, sodium polyacrylate, or hydrolyzed polymaleic anhydride. The amount of oxalate precipitation speciation control agent (OCA) added is based on the residual oxalate concentration in the decomposition solution (liquid phase C2O4). 2- ) Regulation, when liquid phase C2O4 2- When the concentration is ≥1.5 g / L, the dosage of oxalate precipitation speciation control agent is 10~30 ppm. This utilizes polymer adsorption to inhibit the needle-like growth of sodium oxalate until it reaches the liquid phase C2O4. 2- When the concentration drops below 1.5 g / L, when the liquid phase C2O4 2- When the concentration is <1.5 g / L, based on the particle size feedback of the aluminum hydroxide product, you can choose to add 5~10 ppm of oxalate precipitation form control agent for fine-tuning the form or stop adding it.

[0040] Step S2 also includes a process parameter control step linked to OCA addition: when the system is in the particle size coarsening stage and the concentration of oxalate in the concentrate is ≥1.8g / L, while adding OCA, the temperature of the first tank of the seeding process is reduced by 1~2℃ and the temperature of the last tank is maintained at ≥55℃, so as to synergistically promote the precipitation of a large number of oxalate pellets.

[0041] In step S3, the frequency of the ultrasonic oscillation device is 15~25 kHz and the power density is 1.0~1.5W / cm², so as to increase the crystallization rate of oxalate by 30~50%, significantly coarsen the particle size of the precipitated oxalate crystals, and improve the filtration performance.

[0042] In step S3, the high-temperature and high-alkali resistant filter press includes a vertical filter press and a horizontal filter press. The vertical filter press is used to separate oxalate seed crystals, and its filter plate is made of stainless steel. The horizontal filter press is used to treat most of the mother liquor after salt precipitation. Before treatment, 1.0~1.5wt% of active leaching residue (red mud slurry) is added to the mother liquor to be filtered as a filter aid to improve the porosity and permeability of the filter cake.

[0043] In step S3, the heat exchange and cooling process adopts a two-stage heat exchange process. The first stage achieves pre-cooling by using circulating cooling water to cool the 110°C evaporated mother liquor to 75~80°C. The second stage achieves waste heat recovery by using the cold filtrate (45~50°C) after pressure filtration to further cool the mother liquor to 50~55°C, while simultaneously raising the filtrate to 60~70°C and returning it to the system to recover heat.

[0044] A control system for oxalate production in the Bayer process for alumina production includes:

[0045] The chemical precipitation unit receives a solution containing sodium oxalate and reacts it with lime slurry. It includes a reaction vessel, an online oxalate concentration analyzer, a stirring control mechanism, a precise lime slurry dosing mechanism, and a reaction endpoint monitoring mechanism. The reaction vessel has a heating and insulation jacket. The precise lime slurry dosing mechanism uses the C2O4 concentration in the incoming solution monitored by the online oxalate concentration analyzer. 2- The concentration is adjusted to control the flow rate of lime slurry. The reaction endpoint monitoring device includes an online particle size analyzer or a sedimentation rate probe to prevent calcium aluminate double salt from encapsulating the solution.

[0046] The morphology control precipitation unit, located before the seed tank, is used to regulate the morphology of oxalate crystals. It includes an OCA addition mechanism and a liquid-phase oxalate concentration online analyzer. The OCA addition mechanism includes a metering pump and a storage tank. The liquid-phase oxalate concentration online analyzer detects C2O4 in semen and circulating mother liquor in real time. 2- Concentration (g / L);

[0047] The online liquid oxalate concentration analyzer is interlocked with the OCA addition mechanism and the seed tank temperature control system, and the OCA addition amount and seed tank temperature are interlocked based on the monitoring value of the online liquid oxalate concentration analyzer.

[0048] The online liquid oxalate concentration analyzer detected C2O4. 2- When the concentration remains above 2.0 g / L, an alarm signal is issued, prompting the chemical precipitation unit to check the lime activity or increase the residence time;

[0049] The enhanced separation and removal unit includes a staged heat exchange and waste heat recovery mechanism, an ultrasonic crystallization tank group, a filter aid mixing tank, and a pressure filtration separation mechanism connected in sequence. The staged heat exchange and waste heat recovery mechanism includes a first-stage heat exchanger and a second-stage heat exchanger connected in sequence. The first-stage heat exchanger uses circulating cooling water as a cold source to pre-cool the mother liquor from the evaporation station to 75~80℃. The second-stage heat exchanger uses the cold filtrate with a temperature of 45~50℃ after pressure filtration as a cold source to further cool the 75~80℃ mother liquor to 50~55℃, while heating the cold filtrate to 60~70℃ and returning it to the production process.

[0050] The ultrasonic crystallization tank group receives cooled mother liquor at 50~55℃, and is equipped with a seed crystal addition port and an insertion ultrasonic oscillator.

[0051] The filter press separation mechanism includes vertical filter presses and horizontal filter presses. The vertical filter press has a filtration area of ​​50~60m² and uses stainless steel filter plates. It is used to separate oxalate crystals and perform fine filtration. The horizontal filter press has a total filtration area of ​​1000~1500m² and is used to treat the mother liquor after the main salt precipitation. Its processing capacity is not less than 330m³ / h.

[0052] The anti-scaling and cleaning unit includes dilute acid solution pipelines and industrial water pipelines connected to the first-stage heat exchanger, the second-stage heat exchanger, and the vertical and horizontal filter presses. These pipelines are used to periodically perform online water washing on the equipment to eliminate oxalate and carbonate scale. The dilute acid solution pipelines deliver sulfuric acid or hydrochloric acid with a concentration of 5-10% to dissolve calcium oxalate scale. The industrial water pipelines are used to rinse soluble sodium carbonate scale and residues after acid washing.

[0053] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the technical solutions of the present invention. Any technical solution that can be implemented based on the above embodiments without creative effort should be considered to fall within the scope of protection of the patent of the present invention.

Claims

1. A method for controlling oxalate in the Bayer process for alumina production, characterized in that, Includes the following steps: S1. The sodium oxalate enrichment solution from the Bayer process is mixed with lime slurry to carry out a causticization reaction. The reaction conditions are controlled as follows: temperature 85-95℃, stirring speed 150-250 r·min. - ¹, The amount of calcium oxide added is dynamically calculated based on the concentration of oxalate ions in the solution to control the CaO / C₂O₄ ratio. 2- The molar ratio is 1.3~1.5:

1. After the reaction, solid and liquid are separated to obtain calcium oxalate precipitate and purified solution. S2. In the seed decomposition solution after step S1, an oxalate precipitation morphology control agent is added. The amount of oxalate precipitation morphology control agent added is dynamically adjusted according to the concentration of residual oxalate in the decomposition solution. During the decomposition process, the oxalate precipitation morphology control agent acts on the surface of the seed crystals and changes the crystallization habit of oxalate, inducing the residual oxalate to precipitate as spherical or near-spherical crystals. S3. The high-temperature, high-concentration evaporation mother liquor discharged from the evaporation station is cooled to 50~55℃ through heat exchange and then sent to an oxalate decomposition tank equipped with an ultrasonic oscillation device. Seed crystals are added to promote the crystallization and precipitation of oxalate. Subsequently, a filter press is used for liquid-solid separation. The separated solid is the oxalate product, and the liquid is returned to the system as circulating mother liquor.

2. The method for controlling oxalate in the Bayer process for alumina production according to claim 1, characterized in that, In step S1, the reaction time is controlled between 60 and 120 minutes. Online particle size monitoring or timed sampling and sedimentation observation are used. The reaction is terminated when the D50 particle size distribution of the reaction slurry transitions from a single peak to a double peak or the sedimentation rate slows down significantly, in order to prevent the generated calcium aluminate double salt from coating the unreacted lime.

3. The method for controlling oxalate in the Bayer process for alumina production according to claim 1, characterized in that, In step S2, the oxalate precipitation form control agent is one or more of polyacrylic acid, sodium polyacrylate, or hydrolyzed polymaleic anhydride. The amount of oxalate precipitation form control agent added is adjusted according to the residual oxalate concentration in the decomposition solution. When the liquid phase oxalate concentration is ≥1.5 g / L, the amount of oxalate precipitation form control agent added is 10~30 ppm. The polymer adsorption effect is used to inhibit the needle-like growth of sodium oxalate until the liquid phase oxalate concentration drops below 1.5 g / L. When the liquid phase oxalate concentration is <1.5 g / L, according to the particle size feedback of the aluminum hydroxide product, the amount of oxalate precipitation form control agent added is 5~10 ppm or the addition is stopped.

4. The method for controlling oxalate in the Bayer process for alumina production according to claim 1, characterized in that, Step S2 also includes a process parameter adjustment step linked to the addition of oxalate precipitation form control agent: when the concentration of oxalate in the concentrate is ≥1.8g / L during the particle size coarsening stage, while adding oxalate precipitation form control agent, the temperature of the first tank of the seeding process is reduced by 1~2℃ and the temperature of the last tank is maintained at ≥55℃, so as to synergistically promote the precipitation of a large number of oxalate pellets.

5. The method for controlling oxalate in the Bayer process for alumina production according to claim 1, characterized in that, In step S3, the frequency of the ultrasonic oscillation device is 15~25kHz, and the power density is 1.0~1.5W / cm².

6. The method for controlling oxalate in the Bayer process for alumina production according to claim 1, characterized in that, In step S3, the filter press includes a vertical filter press and a horizontal filter press. The vertical filter press is used to separate oxalate crystals, and its filter plate is made of stainless steel. The horizontal filter press is used to process most of the mother liquor after salt precipitation, and 1.0~1.5wt% of active leaching residue is added to the mother liquor to be filtered as a filter aid before processing.

7. The method for controlling oxalate in the Bayer process for alumina production according to claim 1, characterized in that, In step S3, the heat exchange and cooling process adopts a two-stage heat exchange process. The first stage achieves pre-cooling by using circulating cooling water to cool the 110°C evaporated mother liquor to 75~80°C. The second stage achieves waste heat recovery by using the cold filtrate with a temperature of 45~50°C after pressure filtration to further cool the mother liquor to 50~55°C, while simultaneously raising the filtrate to 60~70°C and returning it to the system to recover heat.

8. A control system for oxalate in the Bayer process for alumina production, characterized in that, include: The chemical precipitation unit is used to receive a solution containing sodium oxalate and react it with lime slurry. It includes a reaction vessel, an online oxalate concentration analyzer, a stirring control mechanism, a lime slurry precision dosing mechanism, and a reaction endpoint monitoring mechanism. A morphology control precipitation unit, located before the seeding tank, is used to regulate the morphology of oxalate crystals, including an OCA addition mechanism and an online liquid phase oxalate concentration analyzer. The enhanced separation and removal unit includes a staged heat exchange and waste heat recovery mechanism, an ultrasonic crystallization tank group, a filter aid mixing tank and a pressure filtration separation mechanism connected in sequence. The reactor is equipped with a heating and insulation jacket, and the lime slurry precise dosing mechanism is based on the C2O4 concentration in the incoming liquid monitored by an online oxalate concentration analyzer. 2- The concentration is adjusted to control the flow rate of lime slurry. The reaction endpoint monitoring device includes an online particle size analyzer or a sedimentation rate probe to prevent calcium aluminate double salt from encapsulating the solution. The OCA addition mechanism includes a metering pump and a storage tank, and the online liquid oxalate concentration analyzer detects C2O4 in semen and circulating mother liquor in real time. 2- concentration; The staged heat exchange and waste heat recovery mechanism includes a first-stage heat exchanger and a second-stage heat exchanger connected to each other. The first-stage heat exchanger uses circulating cooling water as a cold source to pre-cool the mother liquor from the evaporation station to 75~80℃. The second-stage heat exchanger uses cold filtrate with a temperature of 45~50℃ after pressure filtration as a cold source to further cool the 75~80℃ mother liquor to 50~55℃. At the same time, the cold filtrate is heated to 60~70℃ and returned to the production process. The ultrasonic crystallization tank assembly receives cooled mother liquor at 50~55℃ and is equipped with a seed crystal addition port and an insertion-type ultrasonic oscillator.

9. The control system for oxalate in the Bayer process for alumina production according to claim 8, characterized in that, In the morphology-controlled precipitation unit, the online liquid oxalate concentration analyzer is interlocked with the OCA addition mechanism and the seed tank temperature control system. Based on the monitoring values ​​of the online liquid oxalate concentration analyzer, the OCA addition amount and the seed tank temperature are interlocked and controlled. The online liquid oxalate concentration analyzer detects C2O4... 2- When the concentration remains above 2.0 g / L, an alarm signal is issued, prompting the chemical precipitation unit to check the lime activity or increase the residence time.

10. The control system for oxalate in the Bayer process for alumina production according to claim 8, characterized in that, It also includes an anti-scalding and cleaning unit, which includes a dilute acid solution pipeline and an industrial water pipeline connected to the first-stage heat exchanger, the second-stage heat exchanger and the filter press separation mechanism. The industrial water pipeline is used to periodically perform online water washing on the equipment to eliminate oxalate and carbonate scale. The dilute acid solution pipeline delivers sulfuric acid or hydrochloric acid with a concentration of 5-10%.