A method for removing carbon and alkali in bayer process of alumina production

Abstract

The application discloses a method for removing carbon and alkali in Bayer process of alumina production, comprising the following steps: providing bauxite and lime; mixing, grinding and milling the bauxite, the lime and a solution containing caustic alkali to obtain mixed slurry, wherein the mass of the lime is 5.0%-7.0% of the mass of the bauxite; performing a dissolution reaction on the mixed slurry to obtain a dissolution solution, wherein the temperature of the dissolution reaction is 260-270 DEG C; diluting the dissolution solution, adding aluminum hydroxide seeds to perform a decomposition reaction and solid-liquid separation, and obtaining aluminum hydroxide crystals and a solution containing sodium carbonate; evaporating and concentrating the solution containing sodium carbonate to obtain concentrated alkali liquor; and taking at least part of the concentrated alkali liquor as salt liquid to be discharged, adding composite seeds to the salt liquid to be discharged to perform evaporation concentration and solid-liquid separation, wherein the composite seeds comprise sodium carbonate and sodium sulfate, and the adding amount of the composite seeds is 0.1%-0.3% of the mass of the salt liquid to be discharged. The method can improve the removal efficiency of carbon and alkali.

Description

Technical Field

[0001] This application relates to the field of non-ferrous metal metallurgy technology, and in particular to a method for carbon and alkali removal in Bayer process alumina production. Background Technology

[0002] The Bayer process is currently the most widely used production process in the alumina industry, but the problem of carbon and alkali accumulation has long plagued the industry's development and is a technical pain point that is difficult to solve fundamentally. Carbon and alkali mainly come from the reverse causticization reaction that occurs when bauxite and minerals such as calcium carbonate and magnesium carbonate in lime are dissolved, as well as from the carbonation reaction that occurs when the production solution comes into contact with carbon dioxide in the air. When carbon-alkali solutions from the sintering system are fed into the Bayer process system, a large amount of carbon and alkali is also introduced.

[0003] Excessive carbon dioxide levels can trigger a series of chain reactions, increasing the viscosity of the system solution, making separation and filtration more difficult, reducing circulation efficiency, exacerbating scaling in equipment such as dissolvers, evaporators, pipelines, and seed tanks, and continuously declining heat exchange efficiency. Ultimately, this directly leads to a decrease in alumina production, an increase in energy consumption, and a deterioration in product quality. When the Nc / NT ratio (the ratio of sodium carbonate to total sodium oxide in the alumina production system) exceeds the critical range (approximately 5.5% for enterprises in southern regions and approximately 10% for enterprises in other regions), electricity consumption per ton of alumina increases by about 16 kWh, and steam consumption increases by 0.05 tons, significantly impacting the enterprise's economic benefits.

[0004] Currently, commonly used carbon-alkali removal technologies in industry are mainly divided into three categories. The high-concentration evaporation crystallization desalination method relies on an evaporator to concentrate the mother liquor to a high concentration, promoting the crystallization and precipitation of sodium carbonate. However, with the increasing proportion of low-grade complex ores used, the content of impurities such as organic matter and sulfur in the solution rises accordingly, leading to finer and stickier sodium carbonate crystals, significantly reducing sedimentation and filtration performance, and significantly affecting desalination efficiency. The red mud washing liquor causticization method has a simple process and is easy to operate, but its removal efficiency is low, alumina loss is significant, and the effect is slow, making it difficult to meet the needs of high-carbon-alkali load production systems. The barium salt causticization method has a relatively ideal removal effect, but the cost of barium hydroxide and barium oxide raw materials is high, and the regeneration process generates harmful gases and dust, resulting in significant environmental pressure. High equipment investment and operating costs also hinder its large-scale promotion in the industry. Summary of the Invention

[0005] The carbon and alkali removal method in Bayer process alumina production provided according to one or more embodiments of this application solves one of the technical problems of low carbon and alkali removal efficiency.

[0006] This application provides a carbon-alkali removal method in Bayer process alumina production, comprising the following steps: providing bauxite and lime, wherein the calcium oxide content of the bauxite is ≤0.5%, the inorganic carbon content of the bauxite is ≤0.20%, the effective calcium oxide content of the lime is ≥80%, and the decomposition rate of the lime is ≥90%; mixing and grinding the bauxite, the lime, and a solution containing caustic alkali to obtain a mixed slurry, wherein the mass of the lime is 5.0% to 7.0% of the mass of the bauxite; subjecting the mixed slurry to a leaching reaction to obtain a leaching solution, wherein the temperature of the leaching reaction is 2... The solution is diluted at 60–270°C, and aluminum hydroxide seed crystals are added for decomposition and solid-liquid separation to obtain aluminum hydroxide crystals and a solution containing sodium carbonate. The sodium carbonate solution is evaporated and concentrated to an Nk value of 240 g / L–250 g / L to obtain a concentrated alkaline solution. At least a portion of the concentrated alkaline solution is taken as the brine to be discharged, and composite seed crystals are added to the brine to be discharged for evaporation, concentration, and solid-liquid separation to obtain a brine discharge solution and a brine discharge filter cake. The composite seed crystals include sodium carbonate and sodium sulfate, and the amount of composite seed crystals added is 0.1%–0.3% of the mass of the brine to be discharged.

[0007] Optionally, the mass ratio of sodium carbonate to sodium sulfate in the composite seed crystal is (1~2):1.

[0008] Optionally, if the Nc / NT ratio of the brine to be discharged is greater than 10%, the brine to be discharged is evaporated and concentrated to an Nk value of 300 g / L to 324 g / L, or... The Nc / NT ratio of the brine to be drained is 5.5% to 10%, and the brine to be drained is evaporated and concentrated to an Nk value of 284 g / L to 300 g / L.

[0009] Optionally, the step of evaporating and concentrating the brine to be discharged to an Nk value of 300 g / L to 324 g / L further includes adding a solid caustic alkali to the brine to be discharged, and the step of evaporating and concentrating the brine to be discharged to an Nk value of 284 g / L to 300 g / L further includes adding a solid caustic alkali to the brine to be discharged.

[0010] Optionally, the caustic alkali is added to the brine to be discharged in a continuous and uniform manner, and the addition rate of the caustic alkali is proportional to the discharge rate of the brine.

[0011] Optionally, the brine to be discharged is 10% to 20% of the mass of the concentrated alkali solution.

[0012] Optionally, the carbon-alkali removal method further includes reusing the concentrated alkali solution and / or the discharged brine that were not used as the brine to be discharged.

[0013] Optionally, the carbon-alkali removal method further includes adding water to the desalination filter cake to adjust the Nk value to 30 g / L to 50 g / L and the Nc / NT to 50% to 70%, adding lime slurry at a Causticization reaction with a Ca(OH)2 to Na2CO3 molar ratio of 1.0 to 1.2, and separating the reaction products into solid and liquid phases to obtain solid and liquid phase products.

[0014] Optionally, the causticizing reaction is carried out at a temperature of 85–98°C for 2–4 hours.

[0015] Optionally, the liquid product can be reused.

[0016] Optionally, the sulfur content of the bauxite is ≤0.1%.

[0017] Compared with the prior art, the technical solution provided in this application has the following beneficial effects: The carbon-alkali removal method in the Bayer process alumina production disclosed in this application controls the formation of a low calcium-to-carbon ratio in the raw materials bauxite and lime by controlling the calcium oxide content, inorganic carbon content, and the ratio of bauxite to lime. Simultaneously, a high dissolution temperature of 260℃–270℃ is used to synergistically suppress the reverse causticization reaction between carbonates and caustic alkali (CaCO3 + 2NaOH). The reaction Na₂CO₃ + Ca(OH)₂, at high temperatures (260-270℃), shifts the equilibrium of the reverse causticization reaction to the left (consuming carbon alkali and generating caustic alkali). A low calcium-to-carbon ratio reduces the concentration of the reactant Ca(OH)₂. Both factors synergistically inhibit the formation of carbon alkali and promote the entry of carbon impurities into the red mud in the form of CaCO₃. Furthermore, after obtaining a concentrated alkaline solution with an Nk value of 240-250 g / L, at least a portion of this concentrated alkaline solution is used as the salt solution to be discharged. Composite seed crystals are added to this solution for further evaporation and concentration to promote sodium carbonate crystal growth and improve crystal morphology. This significantly enhances the sedimentation and filtration performance of sodium carbonate in the subsequent salt discharge process (i.e., sodium carbonate removal), further reducing the carbon alkali content in the system, improving carbon alkali removal efficiency, and achieving highly efficient decarbonization. The carbon and alkali removal method in the Bayer process alumina production disclosed in this application reduces carbon simultaneously at the production source, production process, and production end, achieving precise control and efficient removal of carbon and alkali, and greatly improving the carbon and alkali removal efficiency.

[0018] Furthermore, the carbon-alkali removal method in the Bayer process alumina production of this application also controls sulfur at the source, ensuring that the sulfur content of bauxite is ≤0.1%. The high temperature (260~270℃) and low calcium-to-carbon ratio further promote the entry of sulfur impurities into the red mud in the form of embedded sodium silicate slag, achieving synergistic desulfurization. Moreover, controlling the sulfur content of bauxite to ≤0.1% avoids the simultaneous entry of both carbon and sulfur impurities into the production system, reducing the carbon-alkali regulation load for subsequent processes. The total sulfur content of the leaching solution is reduced by 25%–30%, reducing the interference of sulfur impurities on sodium carbonate crystallization and further improving the carbon-alkali removal efficiency (this method can improve efficiency by more than 30% compared to the traditional evaporation and desalination method). Attached Figure Description

[0019] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and disclosure, and together with the description serve to explain the principles of this application and disclosure.

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the related prior art, the accompanying drawings used in the description of the embodiments or the related prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic flow diagram of a carbon and alkali removal method in Bayer process alumina production according to some embodiments of this application; Figure 2 This is a schematic flow diagram of a carbon and alkali removal method in Bayer process alumina production according to some other embodiments of this application. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. 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.

[0023] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. The term "implementation" as used herein has a similar understanding.

[0024] In this application, except where expressly stated, any matters or issues not mentioned are directly applicable to those known in the art without any modification. Furthermore, any implementation described in this application can be freely combined with one or more other implementations described in this application, and the resulting technical solutions or concepts shall be considered part of the original disclosure or original record of this application, and should not be regarded as new content not disclosed or anticipated in this application, unless those skilled in the art consider the combination to be clearly unreasonable.

[0025] Any method steps, processes, and operations described in this application should not be construed as necessarily requiring them to be performed in a specific order as discussed or shown, unless explicitly specified. It should also be understood that additional or alternative steps may be used unless otherwise stated.

[0026] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0027] Any specific numerical values ​​disclosed herein (including the endpoints of numerical ranges) are not limited to their exact values, but should be understood to also include values ​​close to the exact value, such as all possible values ​​within ±5% of the exact value. Furthermore, with respect to the disclosed numerical ranges, one or more new numerical ranges can be obtained by arbitrarily combining the endpoint values ​​of the range, the endpoint values ​​with specific point values ​​within the range, and the specific point values ​​themselves; these new numerical ranges should also be considered as specifically disclosed herein.

[0028] Unless otherwise stated, the terms used herein have the same meaning as commonly understood by those skilled in the art, and if a term is defined herein and its definition differs from the common understanding in the art, the definition herein shall prevail.

[0029] In this article, "caustic alkali" has a commonly known meaning in the field, specifically referring to NaOH.

[0030] In this article, "carbon alkali" has a commonly known meaning in the field, specifically referring to Na2CO3.

[0031] In this article, "Nc / NT" represents the proportion of sodium carbonate (carbon alkali) in the total alkali, reflecting the degree of carbon alkali accumulation in the system.

[0032] In this article, "Nk" refers to the "caustic alkali concentration" in the solution, specifically: the mass concentration of caustic alkali (NaOH) in the solution converted to sodium oxide (Na2O), usually expressed in g / L.

[0033] First aspect Please see Figure 1 This application provides a method for carbon and alkali removal in Bayer process alumina production, comprising the following steps: S1 provides bauxite and lime, wherein the calcium oxide content of the bauxite is ≤0.5%, the inorganic carbon content of the bauxite is ≤0.20%, the effective calcium oxide content of the lime is ≥80%, and the decomposition rate of the lime is ≥90%. S2, bauxite, lime, and a solution containing caustic alkali are mixed and ground to obtain a mixed slurry, wherein the mass of lime is 5.0% to 7.0% of the mass of bauxite; S3, the mixed slurry is subjected to a leaching reaction to obtain a leaching solution. The temperature of the leaching reaction is 260℃~270℃. S4, after diluting the leachate, add aluminum hydroxide seed crystals to carry out decomposition reaction and solid-liquid separation to obtain aluminum hydroxide crystals and a solution containing sodium carbonate; S5, the solution containing sodium carbonate is evaporated and concentrated to an Nk value of 240 g / L~250 g / L to obtain a concentrated alkaline solution; and S6. Take at least a portion of the concentrated alkaline solution as the brine to be discharged, add composite seed crystals to the brine to be discharged for evaporation, concentration and solid-liquid separation to obtain the brine to be discharged and the brine to be discharged filter cake. The composite seed crystals include sodium carbonate and sodium sulfate, and the amount of composite seed crystals added is 0.1% to 0.3% of the mass of the brine to be discharged.

[0034] The carbon-alkali removal method in the Bayer process alumina production of this application controls the formation of a low calcium-to-carbon ratio in the raw materials bauxite and lime by controlling the calcium oxide content, inorganic carbon content, and the ratio of bauxite to lime. Simultaneously, it combines this with a high dissolution temperature of 260°C–270°C to synergistically suppress the reverse causticization reaction (CaCO3 + 2NaOH) between carbonates and caustic alkali. The reaction Na₂CO₃ + Ca(OH)₂, at high temperatures (260-270℃), shifts the equilibrium of the reverse causticization reaction to the left (consuming carbon and alkali, generating caustic alkali). A low calcium-to-carbon ratio reduces the concentration of the reactant Ca(OH)₂. Both factors synergistically inhibit the formation of carbon and alkali and promote the entry of carbon impurities into the red mud in the form of CaCO₃. Furthermore, after obtaining a concentrated alkali solution with an Nk value of 240-250 g / L, at least a portion of the concentrated alkali solution is used as the salt solution to be discharged. Composite seed crystals are added to this solution for further evaporation and concentration to promote sodium carbonate crystal growth and improve crystal morphology. This significantly improves the sedimentation and filtration performance of sodium carbonate in the subsequent salt discharge process (i.e., sodium carbonate removal), further reducing the carbon and alkali content in the system, increasing carbon and alkali removal efficiency, and achieving highly efficient decarbonization. The carbon and alkali removal method in the Bayer process alumina production of this application simultaneously reduces carbon at the production source, during the production process, and at the production end, achieving precise control and efficient removal of carbon and alkali, and greatly improving the carbon and alkali removal efficiency.

[0035] The calcium oxide content of bauxite refers to the percentage (%) of all calcium elements in bauxite converted into calcium oxide (CaO), determined according to the national standard GB / T 3257.5-1999. For example, the calcium oxide content of bauxite is ≤0.5%, ≤0.4%, ≤0.3%, ≤0.2%, or ≤0.1%.

[0036] The inorganic carbon content of bauxite refers to the mass percentage of total carbon minus the mass percentage of organic carbon in bauxite, determined according to the national standard GB / T 3257.5-1999. For example, the inorganic carbon content of bauxite is ≤0.20%, ≤0.15%, ≤0.10%, or ≤0.05%.

[0037] The effective calcium oxide content of lime refers to the mass fraction of chemically active free calcium oxide (f-CaO) in lime, determined according to the industry standard method (sucrose method). For example, the effective calcium oxide content of lime is ≥80%, ≥85%, ≥90%, or ≥95%.

[0038] The decomposition rate of lime refers to the percentage by mass of calcium carbonate that has decomposed into calcium oxide after limestone is calcined, and is calculated using the following formula: Decomposition rate = (1 - mass of residual calcium carbonate in limestone / mass of total calcium carbonate in limestone) × 100%. For example, the decomposition rate of lime is ≥90%, ≥92%, ≥95%, or ≥98%.

[0039] For example, the mass of lime is 5.0%, 5.5%, 6.0%, 6.5%, or 7.0% of the mass of bauxite. Furthermore, a low calcium-to-carbon ratio can reduce calcium salt side reactions and lower the risk of scaling.

[0040] In some embodiments, the bauxite is gibbsite-type bauxite. In other embodiments, the calcium oxide content and inorganic carbon content of the bauxite are controlled by blending. For example, the bauxite comprises a first bauxite and a second bauxite, wherein the first bauxite has a higher calcium oxide content and / or inorganic carbon content than the second bauxite. The ratio of the first and second bauxite is adjusted to ensure that the calcium oxide content of the bauxite is ≤0.5% and the inorganic carbon content is ≤0.20%. In still other embodiments, high-calcium ore segments are removed by sorting to ensure that the calcium oxide content in the bauxite is ≤0.5%.

[0041] In some embodiments, the mass ratio of sodium carbonate to sodium sulfate in the composite seed crystal is (1~2):1. A mass ratio of sodium carbonate to sodium sulfate within this range is more conducive to the precipitation of sodium carbonate crystals. Exemplarily, the mass ratio of sodium carbonate to sodium sulfate in the composite seed crystal is 1:1, 1.2:1, 1.4:1, 1.5:1, 1.6:1, 1.8:1, or 2:1.

[0042] The amount of brine to be evaporated and concentrated is controlled according to different Nc / NT values ​​to ensure that the system's Nc / NT value can be stably controlled below the safety threshold. This improves carbon and alkali removal efficiency while reducing energy consumption.

[0043] In some embodiments, the Nc / NT ratio of the brine to be drained is greater than 10%, and the brine is evaporated and concentrated to an Nk value of 300 g / L to 324 g / L. For example, if the Nc / NT ratio of the brine to be drained is greater than 10%, the brine is evaporated and concentrated to an Nk value of 300 g / L, 305 g / L, 310 g / L, 315 g / L, 320 g / L, or 324 g / L.

[0044] In some embodiments, the Nc / NT ratio of the brine to be drained is 5.5% to 10%, and the brine is evaporated and concentrated to an Nk value of 284 g / L to 300 g / L. Exemplarily, the Nc / NT ratio of the brine to be drained is 5.5% to 10%, and the brine is evaporated and concentrated to an Nk value of 284 g / L, 285 g / L, 290 g / L, 295 g / L, or 300 g / L.

[0045] In some embodiments, the step of evaporating and concentrating the brine to be discharged to an Nk value of 300 g / L to 324 g / L further includes adding a solid caustic alkali to the brine. By adding a caustic alkali, the evaporation and concentration process can be partially replaced, thereby further reducing energy consumption and lowering costs.

[0046] In some embodiments, the step of evaporating and concentrating the brine to be discharged to an Nk value of 284 g / L to 300 g / L further includes adding a solid caustic alkali to the brine. By adding a caustic alkali, the evaporation and concentration process can be partially replaced, thereby further reducing energy consumption and lowering costs.

[0047] In some embodiments, caustic alkali is added to the brine to be discharged in a continuous and uniform manner, and the addition rate of caustic alkali is proportional to the discharge rate of the brine. Specifically, the addition rate of caustic alkali = k × the discharge rate of the brine. In some embodiments, the k value (i.e., the ratio of the caustic alkali addition rate to the discharge rate of the brine) is dynamically adjusted by online monitoring of the Nk value, so that Nk accurately tracks the target value. The K value is a dynamic value, k = k_evaporation replacement + k_normal alkali replenishment, where k_evaporation replacement is dynamically calculated by the intelligent control model based on the current load limit of the evaporator, scaling trend, target Nk, etc., to determine how much evaporation volume to replace with solid alkali. k_normal alkali replenishment is the basic amount of alkali replenishment required to maintain the alkali balance of the system. The intelligent control model can be constructed based on the carbon-alkali balance data statistics table. The carbon-alkali balance data statistics table monitors the Nc / NT index of the system in real time and dynamically matches the concentration of the brine to be discharged based on the monitoring results.

[0048] In some implementations, the caustic alkali is added to the brine to be discharged in solid form.

[0049] In some implementations, the brine to be discharged is 10% to 20% of the mass of the concentrated alkali solution. Further carbon and alkali removal from the 10% to 20% concentrated alkali solution can balance the demand for carbon and alkali removal with the energy consumption demand, so as to achieve better overall benefits.

[0050] In some embodiments, the above-described carbon and alkali removal method further includes reusing the concentrated alkali solution and / or the discharged brine that was not used as the brine to be discharged. The concentrated alkali solution that was not used as the brine to be discharged has an Nk value of 240 g / L to 250 g / L, which meets the requirements for dissolving bauxite to produce alumina, and therefore can be directly reused. The discharged brine after further carbon and alkali removal can also be directly reused.

[0051] Please see Figure 2 In some embodiments, the above-mentioned carbon-alkali removal method further includes step S7, which involves chemically treating the desalination filter cake with lime slurry. In some specific embodiments, the desalination filter cake is slurried with water to a Nk value of 30 g / L–50 g / L and an Nc / NT ratio of 50%–70%. Lime slurry is added at a Causticization reaction with a Ca(OH)₂ to Na₂CO₃ molar ratio of 1.0–1.2. The reaction products are then separated into solid and liquid phases to obtain a solid product and a liquid product. The causticization reaction with lime slurry can also be called lime slurry chemical treatment. The purpose of this step is to convert sodium carbonate into calcium carbonate precipitate and sodium hydroxide. Lime slurry is a Ca(OH)₂ suspension obtained after lime and water digestion. Using lime slurry allows for precise control of the addition amount and ensures a uniform and controllable reaction. The liquid product obtained in this step can be directly reused, while the solid product is treated as solid waste.

[0052] In some embodiments, the causticizing reaction temperature is 85°C to 98°C, and the causticizing reaction time is 2 hours to 4 hours. Exemplarily, the causticizing reaction temperature is 85°C, 88°C, 90°C, 92°C, 95°C, or 98°C. Exemplarily, the causticizing reaction time is 2 hours, 2.5 hours, 3 hours, 3.5 hours, or 4 hours.

[0053] In some embodiments, the carbon-alkali removal method also includes the reuse of the liquid phase product.

[0054] In some implementations, the sulfur content of bauxite is ≤0.1%. This implementation controls sulfur at the source. High temperature (260~270℃) and low calcium-to-carbon ratio further promote the entry of sulfur impurities into the red mud in the form of embedded sodium silicate slag, achieving synergistic desulfurization. Moreover, controlling the sulfur content of bauxite to ≤0.1% avoids the simultaneous entry of carbon and sulfur impurities into the production system, reducing the carbon-alkali regulation load for subsequent processes. The total sulfur content of the leaching solution is reduced by 25%~30%, reducing the interference of sulfur impurities on sodium carbonate crystallization and further improving carbon-alkali removal efficiency. Especially for high sulfur and high carbon ores, the sulfur content of bauxite can be ≤0.1% through ore blending. The preferred principle is to control sulfur and assist in carbon regulation through ore blending. By properly mixing with low-sulfur ores, the sulfur content is controlled, avoiding the simultaneous entry of both types of impurities into the production system and reducing the carbon-alkali regulation load for subsequent processes.

[0055] The sulfur content of bauxite refers to the percentage (%) of all calcium elements in bauxite converted to calcium oxide (CaO), determined according to the national standard GB / T 3257.5-1999. For example, the inorganic carbon content of bauxite is ≤0.10%, ≤0.05%, ≤0.025%, or ≤0.01%.

[0056] In some embodiments, the carbon-alkali removal method also includes the use of intelligent closed-loop control equipment to monitor key indicators such as Nc / NT and Nk in the leaching solution, sodium carbonate-containing solution, brine to be discharged, and brine in real time online. Based on the daily carbon-alkali balance database, an intelligent control model is constructed to automatically, in real time, and dynamically adjust process parameters at each step, such as the temperature of the leaching reaction, the amount of lime used, and the amount of composite seed crystals added. This forms a closed-loop control of the entire carbon-alkali removal process, ensuring that the carbon and alkali levels in the system remain stable below the critical value for a long period of time, and avoiding adverse effects of carbon and alkali fluctuations on production indicators.

[0057] In some embodiments, before evaporating and concentrating the sodium carbonate solution to an Nk value of 240 g / L to 250 g / L in step S5, the method further includes diluting the sodium carbonate solution with water and allowing it to settle and separate to remove solid impurities such as red mud.

[0058] This is achieved based on the preparation method described above. The specific characteristics described above can be referred to in the above embodiments. Since the prebaked anode carbon block 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.

[0059] Example To better understand this application, the following description, in conjunction with embodiments, further illustrates this application. However, the scope of protection claimed in this application is not limited to the scope of the embodiments.

[0060] In the following examples, unless otherwise specified, all experimental instruments, raw materials, and quantities involved are commercially available products or can be prepared by known methods. Experimental methods not specifying particular conditions in the examples were performed under conventional conditions, such as those described in literature, books, or methods recommended by the manufacturer.

[0061] Unless otherwise specified, the specific parameters used in each step of the material preparation process in each embodiment and comparative example are the same.

[0062] Example 1 This embodiment is applicable to a high-carbon alkali load system, taking the Henan Mine alumina system with an Nc / NT ratio of 14.15% as an example, with a critical value of 10%. The bauxite conventionally used in this system is the first type of bauxite, which contains 0.8% calcium oxide, 0.25% inorganic carbon, and 0.12% total sulfur.

[0063] S1 provides bauxite and lime. Bauxite is obtained by blending primary bauxite and secondary bauxite at a mass ratio of 7:3. The secondary bauxite contains 0.26% calcium oxide, 0.11% inorganic carbon, and 0.05% total sulfur. After blending, the calcium oxide content of the bauxite is ≤0.5%, and the inorganic carbon content is ≤0.20%. The effective calcium oxide content of the lime is controlled at 85%, and the decomposition rate of the lime is ≥90%.

[0064] S2, bauxite, lime and a caustic alkali solution (Nc / NT = 14.15%) are mixed and ground to obtain a mixed slurry, wherein the mass of lime is 6.0% of the mass of bauxite.

[0065] S3. The mixed slurry was subjected to a leaching reaction to obtain a leachate. The leaching reaction temperature was 265℃. Online monitoring showed that the Nc / NT ratio of the leachate was 12.2%, and the total sulfur content in the solution before and after the reaction decreased from 1.18 g / L to 0.85 g / L, a reduction of 28%.

[0066] S4. After diluting the leachate, add aluminum hydroxide seed crystals to carry out decomposition reaction and solid-liquid separation to obtain aluminum hydroxide crystals and a solution containing sodium carbonate. S5, the solution containing sodium carbonate is evaporated and concentrated to an Nk value of 240 g / L to 250 g / L to obtain a concentrated alkaline solution.

[0067] S6. Take approximately 10% of the concentrated alkaline solution as the brine to be discharged. Add composite seed crystals to the brine and evaporate and concentrate it to an Nk value of 320 g / L. Then, perform solid-liquid separation to obtain the discharged brine and the discharged filter cake. The composite seed crystals consist of sodium carbonate and sodium sulfate, and the amount of composite seed crystals added is 0.2% of the mass of the brine to be discharged. The mass ratio of sodium carbonate to sodium sulfate in the composite seed crystals is 1:1. The mass percentage of the adsorbed liquid in the discharged filter cake is 25%, indicating that the brine filtration performance of this method is significantly improved. The remaining 90% of the concentrated alkaline solution and the discharged brine are recycled back to the system for reuse. The Nc / NT ratio of the brine to be discharged is 14.15%, the Nc / NT ratio of the discharged brine is 8.5%, and the brine discharge rate is 40%. Brine discharge rate = (Nc / NT of brine to be discharged - Nc / NT of discharged brine) / Nc / NT of brine to be discharged × 100%.

[0068] S7. The desalination filter cake was mixed with water to a slurry with an Nk value of approximately 40 g / L and an Nc / NT ratio of approximately 60%. Lime slurry was added at a Causticization reaction with a Ca(OH)2 to Na2CO3 molar ratio of 1.1. The reaction products were then subjected to solid-liquid separation to obtain solid and liquid products. The causticization reaction temperature was 90℃, and after a reaction time of 3 hours, the Nc / NT ratio of the reaction solution stabilized at 0.07, achieving efficient recovery of the caustic alkali.

[0069] Example 2 The process was basically the same as in Example 1, except that, after continuous operation and fine-tuning, the temperature of the leaching reaction was adjusted to 268°C and the amount of composite seed crystals added was adjusted to 0.22%. After 30 days of continuous operation, the Nc / NT ratio of the system stabilized at 9.5%, the power consumption per ton of alumina was reduced by 16 kWh, the steam consumption was reduced by 0.05 tons, and the production indicators were improved.

[0070] Example 3 This embodiment is applicable to the implementation scenario of a medium carbon-alkali load system. Taking the Shanxi Lvliang Mine alumina system with an Nc / NT ratio of 12.5% ​​as an example, the critical value is 10%.

[0071] It is basically the same as Example 1, except that: In step S1, the high-calcium ore section is removed by sorting, reducing the calcium oxide content of the bauxite entering the mill to 0.48%, which can meet the index requirements without large-scale ore blending.

[0072] In step S2, the mass of lime is 6.5% of the mass of bauxite.

[0073] In step S3, the dissolution reaction temperature is 260°C. The Nc / NT ratio of the dissolution solution is 10.8%.

[0074] In step S6, composite seed crystals are added to the brine to be drained and then evaporated and concentrated until the Nk value reaches 284 g / L. The amount of composite seed crystals added is 0.15% of the mass of the brine to be drained, and the mass ratio of sodium carbonate to sodium sulfate in the composite seed crystals is 2:1. The Nc / NT ratio of the drained brine decreases from 7.5% to 6.2%, the draining rate increases by 18%, and the crystallization performance of sodium carbonate is improved.

[0075] In step S7, the causticization reaction temperature is 85℃, and after a reaction time of 2 hours, the Nc / NT ratio of the reaction solution stabilizes at 0.06, achieving efficient recovery of the caustic alkali.

[0076] After continuous operation and verification, the system's Nc / NT ratio was stably controlled at 5.2%, equipment scaling was reduced, evaporator heat exchange efficiency was improved, and production stability was significantly enhanced.

[0077] Example 4 This embodiment is applicable to the implementation scenario of low-critical carbon-alkali systems in southern China. Taking the Guangxi alumina system with Nc / NT of 5.8% as an example, the critical value is 5.5%.

[0078] It is basically the same as Example 1, except that: In step S1, through ore blending adjustments, the inorganic carbon content of the ore fed into the mill is reduced to 0.19%, the total sulfur content is controlled below 0.09%, and the effective calcium oxide content of the lime fed into the mill is 85%.

[0079] In step S2, the mass of lime is 7% of the mass of bauxite.

[0080] In step S3, the dissolution reaction temperature is 268°C. The Nc / NT ratio of the dissolution solution is 5.6%.

[0081] In step S6, composite seed crystals are added to the brine to be drained and then evaporated and concentrated to an Nk value of 320 g / L. The amount of composite seed crystals added is 0.3% of the mass of the brine to be drained. The salt removal rate is increased by 15%, and the filtration performance of sodium carbonate crystallization is significantly improved.

[0082] Example 5 This embodiment is an industrial-scale test, applying the method of Example 1 to a Bayer process production line with an annual output of 1 million tons of alumina.

[0083] The original production system had maintained an Nc / NT ratio of 12.5% ​​for a long time, and had problems such as severe equipment scaling, high energy consumption, and low salt removal efficiency.

[0084] After applying the method of Example 1, the Nc / NT ratio of the system stabilized at 9.2% within 30 days, and the synergistic removal effect of carbon and sulfur impurities was significant; the heat exchange efficiency of the evaporator was significantly improved, the amount of scaling on the dissolution unit, pipes and seed tank was reduced, and the equipment maintenance cycle was extended from 1 month to 2 months; the power consumption per ton of alumina was reduced by 18 kWh and the steam consumption was reduced by 0.06 tons; there was no additional environmental pollution, and it completely replaced the use of barium salts, meeting the requirements of green production.

[0085] Comparative Example 1 The Henan mine alumina production system in Example 1 was treated using the traditional high-concentration evaporation crystallization desalination method. Unlike Example 1, this comparative example did not use a second bauxite for blending; the mass of lime was 8.0% of the bauxite mass; the leaching reaction temperature was 250°C; and no composite seed crystals were added in step 6, with liquid caustic alkali added only during the evaporation and concentration stage.

[0086] The test results showed that the Nc / NT ratio of the system only decreased from 14.15% to 11.8%, and the carbon and alkali removal rate was 32% lower than that of Example 1; the sodium carbonate crystal particle size was refined, and the mass ratio of the attached liquid in the salt discharge filter cake was 35%, making sedimentation and filtration difficult; the power consumption and steam consumption per ton of alumina were not significantly reduced, and the evaporator required frequent water washing.

[0087] The carbon and alkali removal method in the Bayer process alumina production disclosed in this application can significantly reduce production energy consumption, with a 16 kWh reduction in electricity consumption per ton of alumina and a 0.05 ton reduction in steam consumption. Equipment operating conditions are significantly improved, with a substantial reduction in scaling on the leaching unit, evaporator, pipelines, and seed tank. Equipment heat exchange efficiency is increased by 15% to 20%, and maintenance cycles are extended by more than two times, effectively reducing maintenance costs and the frequency of unplanned downtime.

[0088] The carbon and alkali removal method for Bayer process alumina production disclosed in this application is highly adaptable and economically efficient. It can be adapted to bauxite of different grades and calcium-magnesium contents, requiring no new large-scale main equipment; it can be achieved simply by optimizing and upgrading the traditional process. The raw materials used for carbon and alkali removal (composite seed crystals and caustic alkali) are inexpensive and can completely replace barium salts, with no harmful gas emissions. The overall cost per ton of alumina is reduced by 10-16 yuan, aligning with the development direction of green metallurgy. The entire process exhibits high stability and intelligence, achieving real-time monitoring of carbon and alkali indicators and dynamic adjustment of process parameters through intelligent closed-loop control. This avoids the deterioration of production indicators due to carbon and alkali fluctuations, further improving the continuity and stability of alumina production.

[0089] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

[0090] Although preferred embodiments have been described in this specification, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this specification.

[0091] Obviously, those skilled in the art can make various modifications and variations to this specification without departing from its spirit and scope. Therefore, if such modifications and variations fall within the scope of the claims and their equivalents, this specification is also intended to include such modifications and variations.

Claims

1. A method for carbon and alkali removal in Bayer process alumina production, characterized in that, Includes the following steps: The facility provides bauxite and lime, wherein the bauxite has a calcium oxide content ≤0.5%, an inorganic carbon content ≤0.20%, an effective calcium oxide content ≥80%, and a decomposition rate ≥90%. The bauxite, lime, and a caustic alkali-containing solution are mixed and ground to obtain a mixed slurry, wherein the mass of the lime is 5.0% to 7.0% of the mass of the bauxite. The mixed slurry is subjected to a leaching reaction to obtain a leaching solution, and the temperature of the leaching reaction is 260℃~270℃; The leachate was diluted and then aluminum hydroxide seed crystals were added to carry out a decomposition reaction and solid-liquid separation to obtain aluminum hydroxide crystals and a solution containing sodium carbonate. The sodium carbonate-containing solution is evaporated and concentrated to an Nk value of 240 g / L to 250 g / L to obtain a concentrated alkaline solution; and at least a portion of the concentrated alkaline solution is taken as a brine to be discharged, and composite seed crystals are added to the brine to be discharged for evaporation, concentration, and solid-liquid separation to obtain a brine discharge solution and a brine discharge filter cake, wherein the composite seed crystals include sodium carbonate and sodium sulfate, and the amount of composite seed crystals added is 0.1% to 0.3% of the mass of the brine to be discharged.

2. The carbon-alkali removal method according to claim 1, characterized in that, The mass ratio of sodium carbonate to sodium sulfate in the composite seed crystal is (1~2):

1.

3. The carbon-alkali removal method according to claim 1, characterized in that, The Nc / NT ratio of the brine to be discharged is >10%. The brine to be discharged is evaporated and concentrated to an Nk value of 300 g / L to 324 g / L, or... The Nc / NT ratio of the brine to be drained is 5.5% to 10%, and the brine to be drained is evaporated and concentrated to an Nk value of 284 g / L to 300 g / L.

4. The carbon-alkali removal method according to claim 3, characterized in that, The step of evaporating and concentrating the brine to be discharged to an Nk value of 300 g / L to 324 g / L further includes adding a solid caustic alkali to the brine to be discharged. The step of evaporating and concentrating the brine to be discharged to an Nk value of 284 g / L to 300 g / L further includes adding a solid caustic alkali to the brine to be discharged.

5. The carbon-alkali removal method according to claim 4, characterized in that, The caustic alkali is added to the brine to be discharged in a continuous and uniform manner, and the addition rate of the caustic alkali is proportional to the discharge rate of the brine.

6. The carbon-alkali removal method according to claim 1, characterized in that, The brine to be drained is 10% to 20% of the mass of the concentrated alkali solution.

7. The carbon-alkali removal method according to any one of claims 1 to 6, characterized in that, It also includes reusing the concentrated alkaline solution and / or the discharged brine that are not intended for use as brine.

8. The carbon-alkali removal method according to any one of claims 1 to 6, characterized in that, It also includes adding water to the salt-removing filter cake to adjust the Nk value to 30 g / L to 50 g / L and the Nc / NT to 50% to 70%, adding lime milk at a Causticization reaction with a Ca(OH)2 to Na2CO3 molar ratio of 1.0 to 1.2, and separating the reaction products into solid and liquid phases to obtain solid and liquid phase products.

9. The carbon-alkali removal method according to claim 8, characterized in that, The causticizing reaction is carried out at a temperature of 85–98°C for 2–4 hours; and / or, the carbon-alkali removal method further includes reusing the liquid phase product.

10. The carbon-alkali removal method according to claim 1, characterized in that, The sulfur content of the bauxite is ≤0.1%.

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