Method and system for synergistically removing carbon alkali and red mud alkali in Bayer process aluminum oxide production

Abstract

The invention belongs to the technical field of aluminum oxide Bayer process production, and particularly relates to a method and a system for synergistically removing carbon alkali and red mud alkali in Bayer process aluminum oxide production. According to the method, carbon alkali (Na2CO3) and red mud alkali (Na2O) are used as migratable sodium sources at the same time, and directional enrichment and recycling of sodium are completed at normal pressure through three steps of lime causticization, stepped sedimentation and countercurrent washing instead of respective independent treatment. Therefore, the carbon alkali is converted into reusable NaOH, the red mud alkali is subjected to washing-adsorption dual stripping, sodium is subjected to closed-loop migration in the whole process, and synergistic removal of the carbon alkali and the red mud alkali and closed circulation of the alkali are achieved at normal pressure and low temperature without additional evaporation.

Description

Technical Field

[0001] This application belongs to the field of alumina Bayer process production technology, and particularly relates to a method and system for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production. Background Technology

[0002] After long-term operation, the Bayer process alumina plant has seen the continuous accumulation of carbonaceous alkalis such as Na2CO3 in the circulating mother liquor. This carbonaceous alkali crystallizes and precipitates during the evaporation and cooling stages, causing scaling on heat exchangers and pipes, a sharp drop in heat transfer efficiency, increased energy consumption, and unplanned shutdowns. Simultaneously, hydrated sodium aluminosilicate in the settled red mud chemically fixes a large amount of Na2O, leading to the loss of caustic alkali and extremely high pH in the red mud, hindering its comprehensive utilization in industries such as building materials. Current technologies address both types of alkali pollution separately: on the red mud side, atmospheric lime dealkali removal is used, but this has insufficient reaction kinetics and low efficiency; or pressurized lime dealkali removal is used, resulting in high equipment investment and energy consumption; on the system side, evaporation-calcination or independent causticization are used, both with persistently high energy consumption. The two sets of equipment have overlapping processes and lack material and energy coupling, resulting in cumulative investment and operating costs, failing to achieve synergistic optimization of "waste-to-waste treatment." Summary of the Invention

[0003] This application provides a method and system for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production, in order to solve the following technical problem: how to remove carbon alkali and red mud alkali in Bayer process under normal pressure and in a closed loop.

[0004] In a first aspect, embodiments of this application provide a method for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production, characterized by comprising: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:1.5 to 1:2. The mixture is reacted at 90℃ to 95℃ for 50 min to 60 min to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 500 g / L to 600 g / L and a liquid-solid mass ratio of 1.5 to 1.6. The mixture is stirred and reacted at 75℃ to 80℃ for 0.8 to 1.2 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 500g / L to 550g / L and a liquid-to-solid mass ratio of 1.7 to 1.8. The mixture is then reacted at 80℃ to 85℃ to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 550 g / L to 600 g / L and a liquid-to-solid mass ratio of 1.5 to 1.6. The slurry is stirred and reacted at 70°C to 75°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of ≤2.5% and an alkaline filtrate with a total carbon content of ≤3.5%. All of the alkaline filtrate is returned to the Bayer process main production system.

[0005] Optionally, the effective calcium concentration of the lime slurry is 150 g / L to 180 g / L, and the solid content is 250 g / L to 300 g / L.

[0006] Optionally, the total carbon content of the highly active calcium slurry is reduced by more than 50% relative to the overflow.

[0007] Optionally, the sodium oxide mass fraction of the low-alkali red mud filter cake is reduced by ≥30% compared to the underflow of the Bayer process's third atmospheric pressure settling tank.

[0008] Optionally, the total carbon content of the alkaline filtrate is reduced by ≥80% compared to the overflow.

[0009] Secondly, embodiments of this application provide a system for the synergistic removal of carbon and alkali from red mud during the Bayer process for alumina production, characterized in that it includes: An atmospheric pressure causticizing tank is provided with an overflow inlet, a lime slurry inlet, and a calcium slurry outlet. The overflow inlet is connected to the overflow pipeline of the Bayer process third atmospheric pressure settling tank, and the lime slurry inlet is connected to the lime slurry supply pipeline. The fourth atmospheric pressure settling tank is provided with a calcium slurry inlet, an underflow inlet, an overflow outlet, and an underflow outlet. The calcium slurry inlet is connected to the calcium slurry outlet, the underflow inlet is connected to the underflow pipeline of the third atmospheric pressure settling tank of the Bayer process, and the overflow outlet is connected to the overflow return pipeline of the third atmospheric pressure settling tank. The fifth atmospheric pressure settling tank is provided with an upstream underflow inlet, a filtrate inlet, a final overflow outlet, and a final underflow outlet. The upstream underflow inlet is connected to the underflow outlet of the fourth atmospheric pressure settling tank, the filtrate inlet is connected to the filter press filtrate return pipeline, and the final overflow outlet is connected to the overflow replenishment pipeline of the fourth atmospheric pressure settling tank. The filter press is equipped with a final underflow inlet, a low-alkali red mud filter cake outlet, and a high-alkali filtrate outlet. The final underflow inlet is connected to the final underflow outlet of the fifth atmospheric pressure settling tank, and the high-alkali filtrate outlet is connected to the alkali return pipeline of the Bayer process main production system.

[0010] Optionally, the atmospheric pressure causticizing tank, the fourth atmospheric pressure settling tank, and the fifth atmospheric pressure settling tank are all atmospheric pressure containers, and each is independently equipped with a stirring device.

[0011] Optionally, the overflow return pipeline, the underflow return pipeline, and the filtrate return pipeline are all closed pipelines and equipped with variable frequency speed control pumps.

[0012] Optionally, the filter press is a diaphragm filter press, with the washing water inlet connected to the plant's fresh water pipeline and the washing liquid outlet connected to the filtrate return pipeline.

[0013] Optionally, the low-alkali red mud filter cake outlet is connected to the building material raw material warehouse via a conveyor belt, and the high-alkali filtrate outlet is connected to the raw liquid tank of the Bayer process evaporation via a pipeline.

[0014] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production. This method treats both "carbon alkali (Na2CO3)" and "red mud alkali (Na2O)" as migratory sodium sources, and completes the directional enrichment and recycling of sodium under normal pressure through three steps: "lime causticization - step-sequence sedimentation - countercurrent washing", instead of treating them separately.

[0015] Causticization decarburization: In the third settling overflow, Na2CO3 reacts with excess Ca(OH)2 at a ratio of 1:1.5~2. Within 1 hour at 90℃, Na2CO3 → NaOH, CO3. 2- →CaCO3↓, sodium remains in the liquid phase as NaOH, and carbon is fixed into highly active CaCO3 crystal nuclei, simultaneously providing a support for subsequent "adsorption-co-precipitation".

[0016] Cascade dealkalization: The above-mentioned highly active calcium slurry is mixed with the third settling underflow (red mud), utilizing the adsorption and replacement of Na in the red mud surface and interstitial spaces by the newly formed CaCO3 / Ca(OH)2 surface. + Al(OH)4 - Stir gently at 75–80℃ for 1 hour to desorb sodium into the liquid phase; gradually tighten the liquid-solid ratio (1.6→1.5) so that each sedimentation pushes the "clean liquid" forward and the "sodium-rich liquid" backward, thus achieving reverse enrichment of sodium.

[0017] End-of-line sealing: The fifth stage adds filter press filtrate as the final washing liquid, and washes again at 70℃ to completely squeeze out residual sodium; the resulting alkaline-rich filtrate has a total carbon content of <3.5%, and is directly returned to the main process to dissolve the next batch of ore, with no alkaline wastewater discharged from the system. The red mud filter cake has Na2O ≤2.5%, meeting the requirements for stockpiling and comprehensive utilization of red mud.

[0018] Therefore, carbon alkali is converted into reusable NaOH, red mud alkali is stripped away by water washing and adsorption, and sodium migrates in a closed loop throughout the process. At normal pressure, low temperature and no additional evaporation are achieved, realizing the synergistic removal of carbon alkali and red mud alkali and the closed-loop circulation of alkali. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0021] In a first aspect, embodiments of this application provide a method for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production, characterized by comprising: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:1.5 to 1:2. The mixture is reacted at 90℃ to 95℃ for 50 min to 60 min to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 500 g / L to 600 g / L and a liquid-solid mass ratio of 1.5 to 1.6. The mixture is stirred and reacted at 75℃ to 80℃ for 0.8 to 1.2 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 500g / L to 550g / L and a liquid-to-solid mass ratio of 1.7 to 1.8. The mixture is then reacted at 80℃ to 85℃ to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 550 g / L to 600 g / L and a liquid-to-solid mass ratio of 1.5 to 1.6. The slurry is stirred and reacted at 70°C to 75°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of ≤2.5% and an alkaline filtrate with a total carbon content of ≤3.5%. All of the alkaline filtrate is returned to the Bayer process main production system.

[0022] The overflow from the Bayer process's third atmospheric settling tank is mixed with lime slurry in an atmospheric causticizing tank at a carbonate to calcium ion molar ratio of 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, and 1:2.0. The mixture is then reacted at 90℃, 91℃, 92℃, 93℃, 94℃, and 95℃ for 50 min, 52 min, 54 min, 56 min, 58 min, and 60 min, respectively, to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The newly generated calcium hydroxide in this highly active calcium slurry has a high specific surface area and many lattice defects, thus exhibiting stronger replacement activity for sodium ions of hydrated sodium aluminosilicate on the surface of subsequent red mud particles. This leads to the premature release of sodium oxide from the red mud into the liquid phase, reducing the alkali content of the red mud.

[0023] High-activity calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank. Overflow from the fifth atmospheric pressure settling tank is added to the fourth atmospheric pressure settling tank to prepare dealkali slurries with solid contents of 500 g / L, 520 g / L, 540 g / L, 560 g / L, 580 g / L, and 600 g / L, and liquid-to-solid mass ratios of 1.5, 1.52, 1.54, 1.56, 1.58, and 1.6. These slurries are then processed at 75℃. The mixture was stirred at 76℃, 77℃, 78℃, 79℃, and 80℃ for 0.8h, 0.9h, 1.0h, 1.1h, and 1.2h, respectively, to obtain the fourth sedimentation overflow and the fourth sedimentation underflow. The solid content and liquid-solid mass ratio ensured that the highly active calcium slurry and red mud particles were in full contact, thereby increasing the sodium ion replacement reaction rate. This, in turn, caused sodium carbonate in the liquid phase to undergo a causticization reaction with calcium hydroxide to form calcium carbonate precipitate, reducing the total carbon ratio in the liquid phase.

[0024] The fourth settling overflow is returned in reverse to the third atmospheric pressure settling tank, where it mixes with the underflow from the second atmospheric pressure settling tank to form a first-stage dealkali removal slurry with solid contents of 500 g / L, 510 g / L, 520 g / L, 530 g / L, 540 g / L, and 550 g / L, and liquid-to-solid mass ratios of 1.7, 1.72, 1.74, 1.76, 1.78, and 1.8. This slurry continues to react at 80℃, 81℃, 82℃, 83℃, 84℃, and 85℃ to obtain the third settling overflow and the third settling underflow. This reverse flow sends the residual calcium hydroxide and low sodium carbonate concentration liquid in the fourth settling overflow back upstream, thereby pre-desalting the fresh red mud and further absorbing liquid sodium carbonate, thus simultaneously reducing the carbon-alkali content and red mud alkali content in the system, achieving a closed-loop circulation.

[0025] The fourth settling underflow is fed into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare final slurries with solid contents of 550 g / L, 560 g / L, 570 g / L, 580 g / L, 590 g / L, 600 g / L, and liquid-to-solid mass ratios of 1.5, 1.52, 1.54, 1.56, 1.58, and 1.6. The slurries are stirred and reacted at 70℃, 71℃, 72℃, 73℃, 74℃, and 75℃ for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. This final slurry exposes the remaining free alkali and residual sodium silicate slag in the red mud to the surface of calcium hydroxide, thereby further reducing the sodium oxide mass fraction and enabling the final red mud to meet the requirements for building materials.

[0026] The fifth settling underflow is filtered and washed in a filter press to obtain low-alkali red mud filter cakes with sodium oxide mass fractions of 2.5%, 2.4%, 2.3%, 2.2%, 2.1%, 2.0%, 1.9%, 1.8%, 1.7%, 1.6%, and 1.5%, and alkaline filtrates with total carbon content of 3.5%, 3.2%, 2.9%, 2.6%, 2.3%, 2.0%, 1.7%, 1.4%, 1.1%, 0.8%, and 0.5%. All alkaline filtrates are returned to the Bayer process main production system. This filtration and washing process completely separates the alkaline solution adhering to the surface of the red mud, thereby ensuring that the sodium oxide mass fraction of the low-alkali red mud filter cake is ≤2.5%, and that the total carbon content of the alkaline filtrate is ≤3.5%, and that the filtrate is returned to the main system in a closed loop. This ultimately solves the technical problem of "how to remove carbon and alkali from the Bayer process and red mud alkali under normal pressure and achieve a closed loop".

[0027] In some embodiments, the effective calcium concentration of the lime slurry is 150 g / L to 180 g / L, and the solid content is 250 g / L to 300 g / L.

[0028] When the effective calcium concentration of lime slurry is 150 g / L, 155 g / L, 160 g / L, 165 g / L, 170 g / L, 175 g / L, or 180 g / L, and the solid content is 250 g / L, 260 g / L, 270 g / L, 280 g / L, 290 g / L, or 300 g / L, the calcium hydroxide particles have high dispersion, which increases the contact area with carbonate ions. This allows the causticizing reaction to be completed within 50 to 60 minutes, ensuring a sufficient amount of highly active calcium slurry to be generated, directly supporting the calcium hydroxide requirement for the dealkalization reaction in claim 1.

[0029] In some embodiments, the total carbon content of the highly active calcium slurry is reduced by more than 50% relative to the overflow.

[0030] When the total carbon content of the high-activity calcium slurry is reduced by 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, and 90% in the overflow, the concentration of sodium carbonate in the liquid phase decreases significantly, thereby reducing the risk of scaling in the subsequent evaporator and extending the system's operating cycle. This directly quantifies the carbon and alkali removal effect.

[0031] In some embodiments, the sodium oxide mass fraction of the low-alkali red mud filter cake is reduced by ≥30% compared to the underflow of the Bayer process's third atmospheric settling tank.

[0032] When the sodium oxide mass fraction of the low-alkali red mud filter cake is reduced by 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 46%, 48%, and 50% compared to the bottom flow of the Bayer process's third atmospheric pressure settling tank, the pH value of the red mud decreases simultaneously, thus meeting the building materials industry standard for alkali content limits. This opens up a channel for large-scale resource utilization of red mud and directly solves the problem of red mud alkali pollution.

[0033] In some embodiments, the total carbon content of the alkali-rich filtrate is reduced by ≥80% compared to the overflow.

[0034] When the carbon content of the alkali-rich filtrate is reduced by 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, and 90% in the overflow, the carbon and alkali load of the alkali solution returned to the main system is significantly reduced, thereby inhibiting the accumulation of carbon and alkali in the closed-loop circulation, which in turn reduces the scaling rate of the evaporator and directly quantifies the carbon and alkali removal limit of the system.

[0035] Secondly, embodiments of this application provide a system for the synergistic removal of carbon and alkali from red mud during the Bayer process for alumina production, characterized in that it includes: An atmospheric pressure causticizing tank is provided with an overflow inlet, a lime slurry inlet, and a calcium slurry outlet. The overflow inlet is connected to the overflow pipeline of the Bayer process third atmospheric pressure settling tank, and the lime slurry inlet is connected to the lime slurry supply pipeline. The fourth atmospheric pressure settling tank is provided with a calcium slurry inlet, an underflow inlet, an overflow outlet, and an underflow outlet. The calcium slurry inlet is connected to the calcium slurry outlet, the underflow inlet is connected to the underflow pipeline of the third atmospheric pressure settling tank of the Bayer process, and the overflow outlet is connected to the overflow return pipeline of the third atmospheric pressure settling tank. The fifth atmospheric pressure settling tank is provided with an upstream underflow inlet, a filtrate inlet, a final overflow outlet, and a final underflow outlet. The upstream underflow inlet is connected to the underflow outlet of the fourth atmospheric pressure settling tank, the filtrate inlet is connected to the filter press filtrate return pipeline, and the final overflow outlet is connected to the overflow replenishment pipeline of the fourth atmospheric pressure settling tank. The filter press is equipped with a final underflow inlet, a low-alkali red mud filter cake outlet, and a high-alkali filtrate outlet. The final underflow inlet is connected to the final underflow outlet of the fifth atmospheric pressure settling tank, and the high-alkali filtrate outlet is connected to the alkali return pipeline of the Bayer process main production system.

[0036] The atmospheric pressure causticizing tank receives the overflow from the Bayer process's third atmospheric pressure settling tank through the overflow inlet and the lime slurry supply pipeline through the lime slurry inlet, thus completing the causticizing reaction of sodium carbonate and calcium hydroxide in the same atmospheric pressure space. The resulting highly active calcium slurry is continuously output through the calcium slurry outlet, providing reactants for subsequent dealkali removal.

[0037] The fourth atmospheric pressure settling tank receives the highly active calcium slurry from the calcium slurry outlet through the calcium slurry inlet, and receives the underflow from the underflow pipeline of the Bayer process third atmospheric pressure settling tank through the underflow inlet. The two substances are mixed and reacted internally, thereby displacing sodium oxide from the sodium silicate slag on the surface of the red mud particles into the liquid phase. This causes the overflow from the fourth settling overflow outlet to return to the third atmospheric pressure settling tank in reverse, completing the first alkali recovery.

[0038] The fifth atmospheric pressure settling tank receives the underflow from the fourth atmospheric pressure settling tank through the upstream underflow inlet, and receives the filter press filtrate returned to the pipeline through the filtrate inlet, thereby performing final dealkalization of the residual red mud. In turn, the overflow from the final overflow outlet is added to the fourth atmospheric pressure settling tank to form an internal circulation and reduce the introduction of fresh water.

[0039] The filter press receives the underflow from the fifth atmospheric pressure settling tank through the final underflow inlet and performs filtration and washing on the final underflow, thereby completely separating the solid and liquid phases. This results in the discharge of low-alkali red mud filter cake with a sodium oxide mass fraction ≤2.5%, and the discharge of high-alkali filtrate with a carbon content ≤3.5%. The high-alkali filtrate is then returned to the main system through the Bayer process main production system alkali return pipeline, ultimately solving the technical problem of "how to remove carbon and alkali from the Bayer process and red mud alkali under atmospheric pressure in a closed loop".

[0040] In some embodiments, the atmospheric pressure causticizing tank, the fourth atmospheric pressure settling tank, and the fifth atmospheric pressure settling tank are all atmospheric pressure containers, and each is independently equipped with a stirring device.

[0041] The atmospheric pressure causticizing tank, the fourth atmospheric pressure settling tank, and the fifth atmospheric pressure settling tank are each equipped with independent stirring devices and are all kept at atmospheric pressure, thereby avoiding investment in pressure vessels and enabling the reaction of calcium hydroxide with sodium carbonate and red mud with calcium hydroxide to proceed efficiently under atmospheric pressure.

[0042] In some embodiments, the overflow return line, the underflow return line, and the filtrate return line are all closed pipelines and equipped with variable frequency speed control pumps.

[0043] The overflow return pipeline, underflow return pipeline, and filtrate return pipeline are all closed pipelines equipped with variable frequency speed control pumps, thereby precisely controlling the flow rate of each material stream, stabilizing the internal circulation ratio, ensuring a constant liquid-solid mass ratio and residence time in each settling tank, and directly improving the dealkali removal efficiency.

[0044] In some embodiments, the filter press is a diaphragm filter press, with the washing water inlet connected to the plant's fresh water pipeline and the washing liquid outlet connected to the filtrate return pipeline.

[0045] The filter press is a diaphragm filter press. The washing water inlet is connected to the plant's fresh water pipeline, and the washing liquid outlet is connected to the filtrate return pipeline. This allows a small amount of fresh water to replace and wash the low-alkali red mud filter cake, thereby collecting all the residual alkali on the surface of the filter cake into the alkali-rich filtrate and returning it to the main system. This directly reduces the consumption of fresh water and maintains the system's alkali balance.

[0046] In some embodiments, the low-alkali red mud filter cake outlet is connected to the building material raw material warehouse via a conveyor belt, and the high-alkali filtrate outlet is connected to the raw liquid tank of the Bayer process evaporation process via a pipeline.

[0047] The low-alkali red mud filter cake outlet is connected to the building material raw material warehouse via a conveyor belt, while the high-alkali filtrate outlet is connected to the raw liquid tank of the Bayer process evaporation process via a pipeline. This allows the low-alkali red mud filter cake with a sodium oxide mass fraction of ≤2.5% to be directly sent to the building material production line, while the high-alkali filtrate with a carbon content of ≤3.5% is directly sent to the evaporation process. This achieves the dual goals of red mud resource utilization and closed-loop alkali circulation in the system, ultimately solving the technical problem of "how to remove carbon and alkali from the Bayer process and red mud alkali under normal pressure and achieve closed-loop circulation".

[0048] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0049] Example 1 A method for the synergistic removal of carbon alkali and red mud alkali in the Bayer process for alumina production includes: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:2. The mixture is reacted at 95°C for 60 minutes to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 600 g / L and a liquid-solid mass ratio of 1.6. The mixture is stirred and reacted at 80°C for 1.0 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 550 g / L and a liquid-solid mass ratio of 1.8. The reaction continues at 85°C to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 600 g / L and a liquid-solid mass ratio of 1.6. The slurry is stirred and reacted at 75°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of 2.3% and an alkali-rich filtrate with a total carbon content of 3.2%. All of the alkali-rich filtrate is returned to the Bayer process main production system.

[0050] Example 2 A method for the synergistic removal of carbon alkali and red mud alkali in the Bayer process for alumina production includes: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:1.5. The mixture is reacted at 90°C for 50 minutes to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 500 g / L and a liquid-solid mass ratio of 1.5. The mixture is stirred and reacted at 75°C for 0.8 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 500 g / L and a liquid-solid mass ratio of 1.7. The reaction continues at 80°C to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 550 g / L and a liquid-solid mass ratio of 1.5. The slurry is stirred and reacted at 70°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of 2.5% and an alkali-rich filtrate with a total carbon content of 3.5%. All of the alkali-rich filtrate is returned to the Bayer process main production system.

[0051] Example 3 A method for the synergistic removal of carbon alkali and red mud alkali in the Bayer process for alumina production includes: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:1.8. The mixture is reacted at 93°C for 55 minutes to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 570 g / L and a liquid-solid mass ratio of 1.55. The mixture is stirred and reacted at 78°C for 1.1 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 530 g / L and a liquid-to-solid mass ratio of 1.75. The reaction continues at 83°C to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 580 g / L and a liquid-to-solid mass ratio of 1.58. The slurry is stirred and reacted at 73°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of 2.4% and an alkali-rich filtrate with a total carbon content of 3.0%. All of the alkali-rich filtrate is returned to the Bayer process main production system.

[0052] Comparative Example 1 A method for the synergistic removal of carbon alkali and red mud alkali in the Bayer process for alumina production includes: The overflow from the third atmospheric pressure settling tank of the Bayer process is directly sent into the fourth atmospheric pressure settling tank without adding lime slurry; The overflow and the underflow from the third atmospheric pressure settling tank of the Bayer process are mixed in the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added to the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 600 g / L and a liquid-solid mass ratio of 1.6. The mixture is stirred and reacted at 80°C for 1.0 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 550 g / L and a liquid-solid mass ratio of 1.8. The reaction continues at 85°C to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 600 g / L and a liquid-solid mass ratio of 1.6. The slurry is stirred and reacted at 75°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of 4.1% and an alkaline filtrate with a total carbon content of 15.9%. All of the alkaline filtrate is returned to the Bayer process main production system.

[0053] Comparative Example 2 A method for the synergistic removal of carbon alkali and red mud alkali in the Bayer process for alumina production includes: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:1. The mixture is reacted at 95°C for 60 minutes to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 600 g / L and a liquid-solid mass ratio of 1.6. The mixture is stirred and reacted at 80°C for 1.0 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 550 g / L and a liquid-solid mass ratio of 1.8. The reaction continues at 85°C to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 600 g / L and a liquid-solid mass ratio of 1.6. The slurry is stirred and reacted at 75°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of 3.45% and an alkaline filtrate with a total carbon content of 6.25%. All of the alkaline filtrate is returned to the Bayer process main production system.

[0054] Experimental methods for evaluating results: Determination of sodium oxide mass fraction: In accordance with GB / T5484-2012 "Chemical Analysis Methods for Bauxite and Red Mud", the sodium oxide content of low-alkali red mud filter cake was determined by hydrochloric acid-fluoric acid decomposition-flame photometry after calcination at 700℃, and the result was converted into mass fraction.

[0055] Carbon-to-total ratio determination: Take the alkali-rich filtrate and determine the total amount of alkali by HCl titration. Use sodium salicylate to complex aluminum-barium chloride to precipitate interfering ions such as sulfate and carbonate. Determine the amount of caustic alkali (OH-) by hydrochloric acid titration. Subtract the caustic alkali from the total alkali to get the carbon-to-alkali ratio. Calculate the carbon-to-total ratio NC / NT.

[0056] Table 1. Results data for both the examples and comparative examples.

[0057] As shown in Table 1, the technological advancements of this application's technical solution include: 1. The sodium oxide mass fraction decreased from 4.1% in Comparative Example 1 to a minimum of 2.3% in Example 1, with the maximum reduction reaching 43.9%. This proves that this application can achieve deep dealkali removal of red mud under normal pressure conditions, breaking through the industry consensus that "the dealkali removal efficiency of the normal pressure lime method is low".

[0058] 2. The total carbon ratio decreased from 15.9% in Comparative Example 1 to a minimum of 3.0% in Example 3, with the maximum reduction reaching 81.1%. This demonstrates that this application simultaneously achieves efficient removal of carbon and alkali from the system, solving the problems of evaporator scaling and decreased heat transfer efficiency.

[0059] 3. Examples 1–3 all meet the requirements of sodium oxide mass fraction ≤2.5% and total carbon ratio ≤3.5%, while Comparative Examples 1 and 2 both exceed these thresholds. This proves that only by adopting the combination of "carbonate ion to calcium ion molar ratio 1:1.5–1:2" and "reverse overflow-multi-stage series" can the synergistic goals of "red mud alkali ≤2.5%" and "system carbon alkali reduction ≥78%" be achieved simultaneously, thus obtaining a synergistic effect of "1+1>2".

[0060] 4. Examples 1–3 can achieve or approach the dealkali removal level of the "pressurized lime method" under normal pressure, no external activator, and fully closed circulation conditions, thereby eliminating the cost of pressurized containers and activators, achieving a double reduction in investment and energy consumption, and possessing the feasibility of directly carrying out in-situ technical transformation of existing alumina plants.

[0061] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application 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 claimed herein.

Claims

1. A method for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production, characterized in that, include: The overflow from the third atmospheric pressure settling tank of the Bayer process is mixed with lime slurry in an atmospheric pressure causticizing tank. The molar ratio of carbonate ions in the overflow to calcium ions in the lime slurry is 1:1.5 to 1:

2. The mixture is reacted at 90℃ to 95℃ for 50 min to 60 min to generate a highly active calcium slurry containing calcium hydroxide and calcium carbonate. The highly active calcium slurry and the underflow from the third atmospheric pressure settling tank of the Bayer process are fed into the fourth atmospheric pressure settling tank, and the overflow from the fifth atmospheric pressure settling tank is added into the fourth atmospheric pressure settling tank to prepare a dealkali slurry with a solid content of 500 g / L to 600 g / L and a liquid-solid mass ratio of 1.5 to 1.

6. The mixture is stirred and reacted at 75℃ to 80℃ for 0.8 to 1.2 h to obtain the fourth settling overflow and the fourth settling underflow. The fourth settling overflow is returned to the third atmospheric pressure settling tank in reverse direction and mixed with the underflow of the second atmospheric pressure settling tank to form a first-stage dealkali slurry with a solid content of 500g / L to 550g / L and a liquid-to-solid mass ratio of 1.7 to 1.

8. The mixture is then reacted at 80℃ to 85℃ to obtain the third settling overflow and the third settling underflow. The fourth settling underflow is sent into the fifth atmospheric pressure settling tank, and filter press filtrate is added to the fifth atmospheric pressure settling tank to prepare a final slurry with a solid content of 550 g / L to 600 g / L and a liquid-to-solid mass ratio of 1.5 to 1.

6. The slurry is stirred and reacted at 70°C to 75°C for 1 hour to obtain the fifth settling overflow and the fifth settling underflow. The fifth settling underflow is filtered and washed in a filter press to obtain a low-alkali red mud filter cake with a sodium oxide mass fraction of ≤2.5% and an alkaline filtrate with a total carbon content of ≤3.5%. All of the alkaline filtrate is returned to the Bayer process main production system.

2. The method according to claim 1, characterized in that, The effective calcium concentration of the lime slurry is 150 g / L to 180 g / L, and the solid content is 250 g / L to 300 g / L.

3. The method according to claim 1, characterized in that, The carbon content of the highly active calcium slurry is reduced by more than 50% relative to the overflow.

4. The method according to claim 1, characterized in that, The sodium oxide mass fraction of the low-alkali red mud filter cake is ≥30% lower than that of the bottom flow of the Bayer process's third atmospheric pressure settling tank.

5. The method according to claim 1, characterized in that, The total carbon content of the alkaline filtrate is reduced by ≥80% compared to the overflow.

6. A system for the synergistic removal of carbon alkali and red mud alkali in Bayer process alumina production, characterized in that, include: An atmospheric pressure causticizing tank is provided with an overflow inlet, a lime slurry inlet, and a calcium slurry outlet. The overflow inlet is connected to the overflow pipeline of the Bayer process third atmospheric pressure settling tank, and the lime slurry inlet is connected to the lime slurry supply pipeline. The fourth atmospheric pressure settling tank is provided with a calcium slurry inlet, an underflow inlet, an overflow outlet, and an underflow outlet. The calcium slurry inlet is connected to the calcium slurry outlet, the underflow inlet is connected to the underflow pipeline of the third atmospheric pressure settling tank of the Bayer process, and the overflow outlet is connected to the overflow return pipeline of the third atmospheric pressure settling tank. The fifth atmospheric pressure settling tank is provided with an upstream underflow inlet, a filtrate inlet, a final overflow outlet, and a final underflow outlet. The upstream underflow inlet is connected to the underflow outlet of the fourth atmospheric pressure settling tank, the filtrate inlet is connected to the filter press filtrate return pipeline, and the final overflow outlet is connected to the overflow replenishment pipeline of the fourth atmospheric pressure settling tank. The filter press is equipped with a final underflow inlet, a low-alkali red mud filter cake outlet, and a high-alkali filtrate outlet. The final underflow inlet is connected to the final underflow outlet of the fifth atmospheric pressure settling tank, and the high-alkali filtrate outlet is connected to the alkali return pipeline of the Bayer process main production system.

7. The system according to claim 6, characterized in that, The atmospheric pressure causticizing tank, the fourth atmospheric pressure settling tank, and the fifth atmospheric pressure settling tank are all atmospheric pressure containers, and each is independently equipped with a stirring device.

8. The system according to claim 6, characterized in that, The overflow return pipeline, the underflow return pipeline, and the filtrate return pipeline are all closed pipelines and equipped with variable frequency speed control pumps.

9. The system according to claim 6, characterized in that, The filter press is a diaphragm filter press, with the washing water inlet connected to the plant's fresh water pipeline and the washing liquid outlet connected to the filtrate return pipeline.

10. The system according to claim 6, characterized in that, The low-alkali red mud filter cake outlet is connected to the building material raw material warehouse via a conveyor belt, and the high-alkali filtrate outlet is connected to the raw liquid tank of the Bayer process evaporation via a pipeline.

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