Production method of low-calcium aluminum oxide

Abstract

The invention relates to a production method of low-calcium aluminum oxide, and belongs to the technical field of aluminum oxide production. The method comprises the following steps: grading aluminum hydroxide slurry obtained after seed crystal decomposition to obtain fine-particle aluminum hydroxide slurry; adsorbing the fine-particle aluminum hydroxide slurry on filter cloth to obtain an aluminum hydroxide filter aid; filtering the sodium aluminate crude liquid through the aluminum hydroxide filter aid to remove suspended matters so as to obtain sodium aluminate fine liquid; sequentially performing seed crystal decomposition and roasting on the sodium aluminate fine liquid to obtain an aluminum oxide product; the filtered aluminum hydroxide filter aid is subjected to back flushing through the sodium aluminate fine liquid, and filter residue slurry is obtained; carrying out first settling separation on the filter residue slurry to obtain underflow slurry; the underflow slurry is subjected to high-temperature dissolution, aluminum hydroxide is recycled, and dissolved-out ore pulp is obtained; and the dissolved-out ore pulp is subjected to second settling separation, and the sodium aluminate crude liquid is obtained. The method effectively reduces the content of calcium oxide in the alumina product.

Description

Technical Field

[0001] The present application relates to the technical field of alumina production, and in particular to a method for producing low-calcium alumina. Background Art

[0002] With the continuous advancement of aluminum electrolysis production technology and the growing emphasis on environmental protection, quality requirements for alumina are becoming increasingly stringent. Currently, all aluminum electrolytic companies adhere to the principle of high quality and low price. Substandard alumina chemical quality can lead to a downgrade in product quality, which in turn affects the selling price. According to the national standard "GB / T 24487-2022 Alumina," the calcium oxide content of first-grade alumina must not exceed 0.03%. The calcium oxide in metallurgical-grade alumina enters the electrolyte in aluminum electrolysis along with the alumina, reacting with cryolite to form CaF2, which accumulates continuously. While CaF2 is often used as an additive in aluminum electrolysis, and a small amount in the electrolyte can lower the crystallization point, increase the electrolyte-aluminum interfacial tension, and reduce the electrolyte vapor pressure, excessive CaF2 can also reduce alumina's solubility in the electrolyte, increase the electrolyte density, and decrease electrical conductivity, deteriorating electrolysis performance.

[0003] During the Bayer process of producing alumina, bauxite is dissolved and separated by sedimentation at high temperature and high pressure in alkaline solution to obtain crude sodium aluminate solution. Since the sedimentation process cannot completely separate the solid and liquid, the crude solution still contains a large amount of red mud suspended matter. Therefore, a filter is required to control and filter the crude sodium aluminate solution to obtain sodium aluminate concentrate with a suspended matter content of less than 0.02g / L.

[0004] Currently, vertical leaf filters are primarily used for controlled filtration of crude liquor. Before filtration, a small amount of lime milk is typically added continuously to the crude liquor tank. This reacts with the sodium aluminate solution to form calcium aluminosilicate, which serves as a filter aid to control the filtration process. The calcium oxide in the lime milk used to synthesize the leaf filter aid can enter the sodium aluminate concentrate due to factors such as the amount of lime milk added, leaf filter operating parameters, filter cloth damage, lime quality, or filter aid quality. During decomposition, it precipitates along with aluminum hydroxide into the product, resulting in excessive calcium oxide content in the alumina product. Summary of the Invention

[0005] The present application provides a method for producing low-calcium alumina to solve the following technical problem: how to reduce the calcium oxide content in Bayer process alumina products.

[0006] The present invention provides a method for producing low-calcium alumina, comprising:

[0007] The aluminum hydroxide slurry obtained after the seed crystals are decomposed is classified to obtain a fine particle aluminum hydroxide slurry;

[0008] adsorbing the fine-particle aluminum hydroxide slurry on a filter cloth to obtain an aluminum hydroxide filter aid;

[0009] filtering the crude sodium aluminate solution through the aluminum hydroxide filter aid to remove suspended solids and obtain sodium aluminate solution;

[0010] Decomposing and calcining the sodium aluminate concentrate in sequence to obtain an alumina product;

[0011] Backwashing the filtered aluminum hydroxide filter aid with the sodium aluminate semen to obtain a filter residue slurry;

[0012] performing a first sedimentation separation on the filter residue slurry to obtain an underflow slurry;

[0013] subjecting the underflow slurry to high-temperature dissolution to recover aluminum hydroxide to obtain a dissolution slurry;

[0014] The dissolved ore pulp is subjected to a second sedimentation separation to obtain the crude sodium aluminate solution.

[0015] Optionally, the solid content of the fine particle aluminum hydroxide slurry is 100 g / L to 200 g / L.

[0016] Optionally, in the fine-particle aluminum hydroxide slurry, the mass fraction of solid aluminum hydroxide with a particle size of less than 45 μm is 25% to 50%, and the D50 of the solid aluminum hydroxide is 30 μm to 60 μm.

[0017] Optionally, the mass concentration of suspended matter in the crude sodium aluminate solution is 0.1 g / L to 2.0 g / L.

[0018] Optionally, the mass concentration of suspended matter in the sodium aluminate semen is 0.005 g / L to 0.020 g / L.

[0019] Optionally, the solid content of the filter residue slurry is 30 g / L to 100 g / L.

[0020] Optionally, the solid content of the underflow slurry is 400 g / L to 800 g / L.

[0021] Optionally, the temperature of the high-temperature dissolution is 130°C to 280°C.

[0022] Optionally, the calcination temperature is 1000°C to 1200°C.

[0023] Optionally, the mass fraction of calcium oxide in the alumina product is 0.005% to 0.015%.

[0024] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0025] An embodiment of the present application provides a method for producing low-calcium alumina, which includes: classifying the aluminum hydroxide slurry obtained after seed crystal decomposition to obtain fine-particle aluminum hydroxide slurry; adsorbing the fine-particle aluminum hydroxide slurry on a filter cloth to obtain an aluminum hydroxide filter aid; filtering a crude sodium aluminate solution through the aluminum hydroxide filter aid to remove suspended matter to obtain a sodium aluminate semen; sequentially subjecting the sodium aluminate semen to seed crystal decomposition and roasting to obtain an alumina product; backwashing the filtered aluminum hydroxide filter aid through the sodium aluminate semen to obtain a filter residue slurry; subjecting the filter residue slurry to a first sedimentation separation to obtain an underflow slurry; subjecting the underflow slurry to high-temperature dissolution to recover aluminum hydroxide to obtain a dissolved ore slurry; and subjecting the dissolved ore slurry to a second sedimentation separation to obtain the crude sodium aluminate solution. Using fine-particle aluminum hydroxide obtained from the seed crystal decomposition process instead of lime milk as a filtration aid not only ensures the filtration efficiency of the crude sodium aluminate solution, achieving efficient interception of red mud suspended matter in the crude sodium aluminate solution, and ensuring that the suspended matter content of the sodium aluminate concentrate meets the standard, but also fundamentally eliminates the introduction of calcium impurities due to the use of a calcium-free filter aid. During the seed crystal decomposition process, the free calcium entering the product can be significantly reduced, effectively reducing the calcium oxide content in the alumina product. Finally, the aluminum hydroxide filter aid undergoes a high-temperature dissolution process to effectively recover the aluminum hydroxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0027] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0028] Figure 1 A schematic flow chart of a method for producing low-calcium alumina provided in an embodiment of the present application. DETAILED DESCRIPTION

[0029] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0030] The range descriptions described in this article, such as numerical ranges, ratio ranges, etc., include all possible sub-ranges and single numerical values within the range. For example, the range description of "1 to 6" or "1~6" covers all sub-ranges from 1 to 6 (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6). Unless otherwise specified, the terms "including", "comprising", etc. used in this article mean "including but not limited to"; relational terms such as "first" and "second" are only used to distinguish different entities or operations, and do not imply an actual sequence or association relationship; "and / or" means that multiple situations can exist alone or at the same time; expressions such as "at least one", "multiple", and "at least one" refer to any combination of corresponding objects, including a combination of single or multiple objects. The proportional relationships involved in the article, such as mass ratios, molar ratios, etc., should be understood as the corresponding relationship between the first and second terms of the proportional formula in the order of description. The raw materials, reagents, instruments and equipment used in this article can be purchased on the market or prepared by existing methods.

[0031] Figure 1 A schematic flow chart of a method for producing low-calcium alumina provided in an embodiment of the present application.

[0032] See Figure 1 , the present application embodiment provides a method for producing low-calcium alumina, the method comprising:

[0033] S1. Classifying the aluminum hydroxide slurry obtained after decomposing the seed crystals to obtain fine-particle aluminum hydroxide slurry;

[0034] After the seed decomposition process, the obtained aluminum hydroxide slurry is classified by a classifier (such as a hydrocyclone). The large particles (bottom flow) are used as finished products in the roasting process, while the fine particle aluminum hydroxide slurry (overflow) is used as a filter aid to achieve accurate classification and utilization of aluminum hydroxide particles.

[0035] In some embodiments, the solid content of the fine particle aluminum hydroxide slurry is 100 g / L to 200 g / L.

[0036] The solid content of the fine-particle aluminum hydroxide slurry is controlled within the range of 100g / L to 200g / L, which can ensure that the overflow and underflow particle size distribution of the classifier are reasonable. The function of the classifier is to separate the coarser particles (as alumina products) from the finer particles (used as filter aids). If the solid content is lower than 100g / L or higher than 200g / L, it will affect the classification effect, resulting in the underflow and overflow particle size not meeting the requirements, thereby affecting the subsequent process steps. Exemplarily, the solid content of the fine-particle aluminum hydroxide slurry can be 100g / L, 120g / L, 140g / L, 160g / L, 180g / L, 200g / L, etc.

[0037] In some embodiments, in the fine-particle aluminum hydroxide slurry, the mass fraction of solid aluminum hydroxide having a particle size of less than 45 μm is 25% to 50%, and the D50 of the solid aluminum hydroxide is 30 μm to 60 μm.

[0038] In the fine-particle aluminum hydroxide slurry, solid aluminum hydroxide with finer particle size (<45μm, accounting for 25% to 50%) can be better adsorbed on the filter cloth surface of the filter to form a uniform and dense filter aid layer, which helps to effectively intercept the suspended matter (such as red mud particles, etc.) in the sodium aluminate crude liquid, thereby significantly improving the filtration effect of the crude liquid. D50, that is, the particle size at 50% of the cumulative distribution, is an important parameter in the particle size distribution. It means that among all the particles, 50% of the particles have a diameter smaller than D50, while the remaining 50% of the particles have a diameter larger than D50. Controlling D50 between 30μm and 60μm can not only avoid the excessive porosity of the filter aid layer (insufficient filtration accuracy) due to excessively large particle size, but also prevent the filter layer from being overly dense (surge in filtration resistance) due to excessively fine particles, thereby achieving the best balance between filtration efficiency and accuracy. Illustratively, in the fine-particle aluminum hydroxide slurry, the mass fraction of solid aluminum hydroxide with a particle size less than 45 μm can be 25%, 30%, 35%, 40%, 45%, 50%, etc.; the D50 of the solid aluminum hydroxide can be 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, etc.

[0039] S2, adsorbing the fine particle aluminum hydroxide slurry on a filter cloth to obtain an aluminum hydroxide filter aid;

[0040] In the embodiments of the present application, a leaf filter can be used to trap solid particles in the fine-particle aluminum hydroxide slurry on the surface of the filter cloth through vacuum adsorption and filtration through a filter cloth, forming a filter cake (i.e., a filter aid), while the liquid is discharged through the filter cloth. Using fine-particle aluminum hydroxide slurry as a filter aid instead of traditional lime milk filter aid avoids the introduction of calcium impurities due to factors such as the amount of lime milk added and unstable quality.

[0041] S3, filtering the crude sodium aluminate solution through the aluminum hydroxide filter aid to remove suspended solids to obtain sodium aluminate semen;

[0042] During the alumina production process, crude sodium aluminate liquor undergoes refined filtration to produce a purer sodium aluminate solution, also known as semen. During this process, aluminum hydroxide filter aids are used in the filter to remove suspended matter from the crude sodium aluminate liquor, ensuring that the semen meets standards for suspended matter. Specifically, fine-particle aluminum hydroxide slurry enters the filter and is adsorbed onto the filter cloth, forming a filter aid layer. As the crude sodium aluminate liquor passes through this filter aid layer, suspended matter is trapped on the filter layer. The clear filtered solution, known as semen, is collected and stored for subsequent use. Furthermore, since aluminum hydroxide filter aids contain no calcium impurities, they significantly reduce the calcium content in semen, thereby reducing the calcium oxide content in the final alumina product. This process not only ensures efficient filtration of the crude liquor but also avoids the potential calcium contamination associated with traditional lime milk filter aids. Furthermore, the aluminum hydroxide after filtration can be recycled and reused in a subsequent high-temperature dissolution process, further improving resource utilization.

[0043] In some embodiments, the mass concentration of suspended matter in the crude sodium aluminate solution is 0.1 g / L to 2.0 g / L.

[0044] Controlling the concentration of suspended solids in the crude liquor within the range of 0.1g / L to 2.0g / L helps reduce impurities in subsequent processing and ensures that the calcium oxide content of the final product is below the national standard. If the mass concentration of suspended solids in the crude sodium aluminate liquor exceeds 2.0g / L, it may clog the filter cloth, increase filtration resistance, and affect efficiency.

[0045] In some embodiments, the mass concentration of suspended matter in the sodium aluminate semen is 0.005 g / L to 0.020 g / L.

[0046] The concentration of suspended matter in sodium aluminate semen is within the range of 0.005g / L to 0.020g / L, ensuring the filtration efficiency of the filter. This effectively removes suspended matter from the crude liquor, thereby ensuring the quality of the low-calcium alumina product in subsequent processes. Furthermore, by strictly controlling the suspended matter in sodium aluminate semen within the range of 0.005g / L to 0.020g / L, calcium impurities are effectively reduced from entering the decomposition process, significantly reducing the calcium oxide content in the final alumina product. Effective filtration and low calcium impurity levels contribute to the production of high-quality alumina products, meeting the national standard for a calcium oxide content of no more than 0.03% for first-grade alumina, thereby enhancing the product's market competitiveness. Through filtration with filter aids, the suspended matter content in sodium aluminate semen is significantly reduced, ensuring that semen quality meets standards. For example, the mass concentration of suspended matter in sodium aluminate semen can be 0.005 g / L, 0.008 g / L, 0.011 g / L, 0.014 g / L, 0.017 g / L, 0.020 g / L, etc.

[0047] S4, sequentially decomposing and calcining the seed crystals on the sodium aluminate semen to obtain an alumina product;

[0048] The diluted sodium aluminate concentrate is fed into a decomposition tank, where an appropriate amount of aluminum hydroxide seed crystals are added. Under continuous stirring and gradually decreasing temperature, the aluminum oxide in the solution precipitates as aluminum hydroxide. The aluminum hydroxide precipitate is filtered, washed, and then fed into a rotary kiln for high-temperature calcination to produce the alumina product.

[0049] In some embodiments, the calcination temperature is 1000°C to 1200°C.

[0050] By calcining, moisture and other impurities in aluminum hydroxide can be removed to obtain high-quality aluminum oxide. For example, the calcining temperature can be 1000°C, 1050°C, 1100°C, 1150°C, 1200°C, etc.

[0051] In some embodiments, the mass fraction of calcium oxide in the alumina product is 0.005% to 0.015%.

[0052] Alumina is the main component, and its purity is a key indicator for measuring product quality. Calcium oxide is one of the impurities, and its content must be strictly controlled within a certain range to ensure the purity of the alumina. In the examples of this application, a calcium-free filter aid is used to avoid calcium impurities introduced by lime milk, successfully controlling the mass fraction of calcium oxide in the alumina within the range of 0.005% to 0.015%, thereby improving the purity and performance of the product. For example, the mass fraction of calcium oxide in the alumina product can be 0.005%, 0.007%, 0.009%, 0.011%, 0.013%, 0.015%, etc.

[0053] S5, backwashing the filtered aluminum hydroxide filter aid with the sodium aluminate semen to obtain a filter residue slurry;

[0054] Backflushing removes filter aids and impurities from the filter cloth, helping to keep it clean, thereby ensuring the efficiency of subsequent filtration processes and improving filtration precision. Secondly, the filter residue slurry obtained through backflushing contains aluminum hydroxide, which can be recycled and reused through a subsequent high-temperature dissolution process, improving resource utilization and reducing production costs. Furthermore, this step helps reduce waste emissions during the production process, complying with environmental protection requirements, while also helping to improve the continuity and stability of the entire production process.

[0055] In some embodiments, the solid content of the filter residue slurry is 30 g / L to 100 g / L.

[0056] After a cycle of filtration, the filter aid needs to be discharged and replaced with a new one. The filter aid adsorbed on the filter cloth needs to be rinsed with semen to obtain a filter residue slurry. Therefore, the filter residue slurry is composed of semen from the backwash filter cloth, aluminum hydroxide filter aid, and red mud suspended matter adsorbed by the filter aid. Controlling the solid content of the filter residue slurry within the range of 30g / L to 100g / L can ensure that the filter cloth is rinsed clean in the shortest possible time, improve the rinsing efficiency, and thus shorten the production cycle. A solid content range of 30g / L to 100g / L is beneficial to the solid-liquid separation effect of the filter residue slurry during the sedimentation separation process. A solid content higher than 100g / L may slow the sedimentation rate and even clog the sedimentation equipment; a solid content lower than 30g / L may increase the processing capacity and reduce equipment utilization. Illustratively, the solid content of the filter residue slurry can be 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, etc.

[0057] S6, subjecting the filter residue slurry to a first sedimentation separation to obtain an underflow slurry;

[0058] Through the first sedimentation separation, the solid components in the residue slurry (such as aluminum hydroxide filter aid, red mud floats, etc.) are concentrated into the underflow slurry, which significantly increases the solid content of the underflow slurry. The high-solid content underflow slurry can react more effectively in the subsequent high-temperature dissolution process, thereby improving the processing efficiency. The first sedimentation separation realizes solid-liquid separation through gravity sedimentation, separating solid particles containing valuable components (such as aluminum hydroxide) from the liquid, providing convenience for subsequent resource recovery and utilization, which helps to reduce resource waste and improve overall resource utilization. Finally, the first sedimentation separation can remove some fine impurities in the residue slurry, thereby reducing the impurity content in the underflow slurry, which helps to reduce the interference of impurities in the subsequent high-temperature dissolution and alumina production process, thereby improving the purity and quality of the final product.

[0059] In some embodiments, the solid content of the underflow slurry is 400 g / L to 800 g / L.

[0060] After the filter residue slurry is separated by sedimentation, the high solid content (400g / L~800g / L) of the underflow slurry can ensure a high concentration of aluminum hydroxide, which helps to achieve efficient recovery of aluminum hydroxide in the subsequent high-temperature dissolution process and avoid waste of resources. At the same time, the underflow slurry with high solid content (400g / L~800g / L) is mixed with the ore pulp and sent to the high-temperature dissolution process, which can reduce the dilution effect and improve the dissolution efficiency. If the solid content of the underflow slurry is lower than 400g / L, the efficiency of the dissolution reaction will decrease, and if it is higher than 800g / L, it may increase the difficulty of pumping and equipment wear. For example, the solid content of the underflow slurry can be 400g / L, 500g / L, 600g / L, 700g / L, 800g / L, etc.

[0061] S7, subjecting the underflow slurry to high-temperature dissolution to recover aluminum hydroxide to obtain a dissolution slurry;

[0062] The main purpose of this process is to effectively recover the aluminum hydroxide in the underflow slurry and reintegrate it into the production process through high-temperature dissolution to improve resource utilization and production efficiency. Specifically, the underflow slurry comes from the part of the filter residue slurry after sedimentation separation, and its solid content is relatively high (400g / L~800g / L). This part of the underflow slurry is mixed with the ore slurry and enters the dissolution process to achieve complete dissolution of aluminum hydroxide under high temperature conditions. Through this high-temperature dissolution process, aluminum hydroxide is effectively recovered and re-formed into a dissolution slurry, which is then further separated by sedimentation to obtain a crude sodium aluminate solution, which enters the production cycle again.

[0063] In some embodiments, the temperature of the high temperature dissolution is 130°C to 280°C.

[0064] Within the temperature range of 130℃ to 280℃, the aluminum hydroxide in the underflow slurry can be fully dissolved, thereby achieving efficient recovery, which not only improves resource utilization but also reduces raw material waste in the production process. The high-temperature dissolution temperature of 130℃ to 280℃ helps the chemical substances in the slurry to fully react, promotes the dissolution of aluminum hydroxide and the formation of sodium aluminate, which is conducive to obtaining high-quality dissolution slurry and laying a solid foundation for the subsequent production of high-quality alumina products. The high-temperature dissolution temperature of 130℃ to 280℃ can accelerate the dissolution reaction process, shorten the production cycle, and thus improve overall production efficiency. Finally, high-temperature dissolution within the temperature range of 130℃ to 280℃ helps maintain process stability. Temperatures above 280℃ or below 130℃ may result in poor dissolution effect or equipment damage. For example, the temperature of high-temperature dissolution can be 130℃, 160℃, 190℃, 220℃, 250℃, 280℃, etc.

[0065] After backwashing, sedimentation separation and high-temperature dissolution of the spent filter aid, the recovery rate of aluminum hydroxide can reach 100%, forming a closed circulation process.

[0066] S8. Subjecting the dissolved ore pulp to a second sedimentation separation to obtain the crude sodium aluminate solution.

[0067] After high-temperature dissolution treatment, the slurry undergoes a secondary sedimentation separation, which effectively removes insoluble impurities from the slurry. This results in a lower impurity content in the resulting crude sodium aluminate solution, helping to ensure the final alumina product has a high purity and quality, meeting market demand for high-quality alumina. As a crucial step in the production process, the secondary sedimentation separation achieves solid-liquid separation of the slurry, providing clear raw materials for subsequent sodium aluminate concentrate processing and alumina production. This helps optimize the entire production process and improve production efficiency. During the sedimentation separation process, insoluble solid impurities are effectively separated, while the solution containing sodium aluminate is recovered and re-enters the production cycle. This resource recycling method not only reduces raw material waste, but also lowers production costs and improves resource utilization. The secondary sedimentation separation reduces waste and pollutants generated during the production process, helping to reduce negative environmental impacts. This is in line with current environmentally friendly production concepts and contributes to the sustainable development of enterprises.

[0068] The present application uses the fine-particle aluminum hydroxide slurry obtained after the decomposition of the seed crystal as a filter aid to replace the traditional calcium-containing lime milk filter aid. This change fundamentally eliminates the path of calcium impurities introduction, because calcium-containing substances are no longer used as filter aids in the entire production process. The fine-particle aluminum hydroxide slurry has excellent adsorption properties and can be evenly and densely adsorbed on the surface of the filter cloth to form an effective filter aid layer. This filter aid layer can efficiently intercept suspended matter (such as red mud particles, etc.) in the crude sodium aluminate solution, thereby ensuring that the suspended matter content in the filtered sodium aluminate semen is extremely low, meeting production requirements. The used aluminum hydroxide filter aid is not discarded, but is effectively recovered through processes such as backwashing, sedimentation separation and high-temperature dissolution. This closed-loop process not only improves resource utilization, but also reduces the waste of raw materials in the production process. This application precisely controls multiple key parameters during the production process, such as the solid content and particle size distribution of the fine-particle aluminum hydroxide slurry, the suspended solids concentration of the crude and refined sodium aluminate solution, the solid content of the filter residue slurry and underflow slurry, and the temperature of high-temperature dissolution and roasting. These control measures ensure the stability of the production process and the reliability of product quality.

[0069] In summary, this application addresses the technical issue of excessive calcium oxide content in Bayer process alumina products by employing calcium-free filter aids, optimizing the filtration process, achieving recycled aluminum hydroxide, and precisely controlling production parameters. This approach not only improves product purity and quality, but also simplifies the production process and reduces production costs, offering significant technical advantages and economic benefits.

[0070] The present application is further described below with reference to specific examples. Experimental methods in the following examples where specific conditions are not specified are generally measured in accordance with national / industry standards. If there are no corresponding national / industry standards, the methods are carried out in accordance with commonly used international standards, conventional conditions, or conditions recommended by the manufacturer.

[0071] Example 1

[0072] 100mL of overflow from a classifier at an alumina plant was collected, with a solids content of 150g / L. The solid aluminum hydroxide particle size was 25% below 45μm, and the D50 was 60μm. The fine-particle aluminum hydroxide slurry was poured into a suction flask funnel and adsorbed onto a filter cloth. 500mL of crude sodium aluminate solution was then added to the funnel. The suspended matter content of the crude sodium aluminate solution was 0.1g / L. After filtration, the suspended matter content of the sodium aluminate concentrate was measured to be 0.011g / L. After seed decomposition and product calcination testing of the sodium aluminate concentrate, the resulting alumina product had a calcium oxide content of 0.008%. The filter cake was rinsed with the filtered sodium aluminate concentrate, the resulting solution was separated by sedimentation, and the resulting solid phase was dried and fed into a dissolution furnace for a high-temperature dissolution reaction at 130°C. Chemical analysis of the resulting red mud revealed the absence of aluminum hydroxide.

[0073] Example 2

[0074] 100mL of overflow from a classifier at an alumina plant was collected, with a solids content of 150g / L. The solid aluminum hydroxide had a particle size of less than 45μm, 38%, and a D50 of 44μm. The fine-particle aluminum hydroxide slurry was poured into a suction funnel and adsorbed onto a filter cloth. 500mL of crude sodium aluminate solution was then added to the funnel. The suspended matter content of the crude sodium aluminate solution was 0.1g / L. After filtration, the suspended matter content of the sodium aluminate concentrate was measured to be 0.006g / L. After seed decomposition and product calcination testing of the sodium aluminate concentrate, the resulting alumina product had a calcium oxide content of 0.012%. The filter cake was rinsed with the filtered sodium aluminate concentrate, and the resulting solution was separated by sedimentation. The resulting solid phase was dried and then fed into a dissolution furnace for a high-temperature dissolution reaction at 130°C. Chemical analysis of the resulting red mud revealed the absence of aluminum hydroxide.

[0075] Example 3

[0076] 100mL of overflow from a classifier at an alumina plant was collected, with a solids content of 150g / L. 50% of the solid aluminum hydroxide had a particle size of less than 45μm, and a D50 of 30μm. The fine-particle aluminum hydroxide slurry was poured into a suction funnel and adsorbed onto a filter cloth. 500mL of crude sodium aluminate solution was then added to the funnel. The suspended matter content of the crude sodium aluminate solution was 0.1g / L. After filtration, the suspended matter content of the sodium aluminate concentrate was measured to be 0.005g / L. After seed decomposition and product calcination testing of the sodium aluminate concentrate, the resulting alumina product had a calcium oxide content of 0.005%. The filter cake was rinsed with the filtered sodium aluminate concentrate, and the resulting solution was separated by sedimentation. The resulting solid phase was dried and then fed into a dissolution furnace for a high-temperature dissolution reaction at 130°C. Chemical analysis of the resulting red mud revealed the absence of aluminum hydroxide.

[0077] Example 4

[0078] 100mL of overflow from a classifier at an alumina plant was collected, with a solids content of 150g / L. The solid aluminum hydroxide particle size was less than 45μm, accounting for 25%, and the D50 value was 60μm. The fine-particle aluminum hydroxide slurry was poured into a suction funnel, washed with hot water to remove the attached liquid, and 500mL of crude sodium aluminate solution was added to the funnel. The suspended matter content of the crude sodium aluminate solution was 0.1g / L. After filtration, the suspended matter content of the sodium aluminate concentrate was measured to be 0.020g / L. After seed decomposition and product calcination testing of the sodium aluminate concentrate, the resulting alumina product had a calcium oxide content of 0.015%. The filter cake was rinsed with the filtered sodium aluminate concentrate, the resulting solution was separated by sedimentation, and the resulting solid phase was dried and sent to a dissolution furnace for a high-temperature dissolution reaction at 180°C. Chemical analysis of the resulting red mud showed no aluminum hydroxide.

[0079] Example 5

[0080] 100mL of overflow from a classifier at an alumina plant was collected, with a solids content of 150g / L. The solid aluminum hydroxide particle size was less than 45μm, at 38%, and the D50 was 44μm. The fine-particle aluminum hydroxide slurry was poured into a suction flask funnel and adsorbed onto a filter cloth. 500mL of crude sodium aluminate solution was then poured into the funnel. The suspended matter content of the crude sodium aluminate solution was 2.0g / L. After filtration, the suspended matter content of the sodium aluminate concentrate was measured to be 0.015g / L. After seed decomposition and product calcination testing of the sodium aluminate concentrate, the resulting alumina product had a calcium oxide content of 0.013%. The filter cake was rinsed with the filtered sodium aluminate concentrate, and the resulting solution was separated by sedimentation. The resulting solid phase was dried and then fed into a dissolution furnace for a high-temperature dissolution reaction at 230°C. Chemical analysis of the resulting red mud revealed the absence of aluminum hydroxide.

[0081] Example 6

[0082] 100mL of overflow from a classifier at an alumina plant was collected, with a solids content of 150g / L. 50% of the solid aluminum hydroxide had a particle size of less than 45μm, and a D50 of 30μm. The fine-particle aluminum hydroxide slurry was poured into a suction flask funnel and adsorbed onto a filter cloth. 500mL of crude sodium aluminate solution was then poured into the funnel. The suspended matter content of the crude sodium aluminate solution was 2.0g / L. After filtration, the suspended matter content of the sodium aluminate concentrate was measured to be 0.013g / L. After seed decomposition and product calcination testing, the resulting alumina product had a calcium oxide content of 0.012%. The filtered filter cake and the filtered sodium aluminate concentrate were then rinsed, the resulting solution separated by sedimentation, and the resulting solid phase dried and fed into a dissolution furnace for a high-temperature dissolution reaction at 280°C. Chemical analysis of the resulting red mud revealed the absence of aluminum hydroxide.

[0083] Comparative Example 1

[0084] Take 100mL of lime milk from an alumina enterprise with a solid content of 150g / L, pour it into the funnel of a suction flask, and pour 500mL of crude sodium aluminate liquid into the funnel. The suspended matter content of the crude sodium aluminate liquid is 2.0g / L. After filtration, the suspended matter content of the sodium aluminate liquid is measured to be 0.011g / L. After the sodium aluminate liquid is subjected to seed decomposition and product roasting tests, the calcium oxide content of the obtained alumina product is 0.028%.

[0085] It can be seen from the examples and comparative examples that the examples adopt the method provided by the present application, which can ensure the filtration effect of the crude liquid and make the suspended matter of the semen meet the standards; it can effectively reduce the calcium impurities in the semen and the calcium oxide content in the product; it can effectively recover the aluminum hydroxide in the filter residue, and the optimization effect is obvious.

[0086] One or more technical solutions in the embodiments of the present invention may have at least the following technical effects or advantages:

[0087] Significantly reduce the calcium oxide content in alumina products: By using a calcium-free filter aid (fine-grained aluminum hydroxide slurry) instead of the traditional lime milk filter aid, the introduction path of calcium impurities is fundamentally eliminated, so that the calcium oxide content in the alumina product can be reduced to 0.005% to 0.015%, which is far lower than the 0.03% required by the national standard "GB / T 24487-2022 Alumina", significantly improving the purity of the product.

[0088] Optimize production processes and reduce production costs: Eliminating the lime milk preparation process not only reduces lime consumption, but also simplifies the production process and reduces production costs. After the high-temperature dissolution process, the aluminum hydroxide filter aid can effectively recover the aluminum hydroxide, reducing raw material waste and further improving resource utilization.

[0089] Improve filtration efficiency and ensure semen quality: Fine-grained aluminum hydroxide slurry as a filter aid can form a uniform and dense filter aid layer, effectively intercepting the suspended matter in the sodium aluminate crude liquid, ensuring that the suspended matter content of the sodium aluminate semen meets the standard, and improving filtration efficiency.

[0090] Environmentally friendly and reduces pollution: Since the use of lime milk is avoided, the environmental pollution problems caused by improper addition of lime milk or unstable quality are reduced, which is more in line with the environmental protection requirements of modern industrial production.

[0091] The foregoing is merely a detailed description of the present invention, intended to enable those skilled in the art to understand or implement the present invention. 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 the present invention. Therefore, the present invention is not limited to the embodiments shown herein, but rather is intended to conform to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for producing low-calcium alumina, comprising: The aluminum hydroxide slurry obtained after the seed crystals are decomposed is classified to obtain a fine particle aluminum hydroxide slurry; adsorbing the fine-particle aluminum hydroxide slurry on a filter cloth to obtain an aluminum hydroxide filter aid; filtering the crude sodium aluminate solution through the aluminum hydroxide filter aid to remove suspended solids and obtain sodium aluminate solution; Decomposing and calcining the sodium aluminate concentrate in sequence to obtain an alumina product; Backwashing the filtered aluminum hydroxide filter aid with the sodium aluminate semen to obtain a filter residue slurry; performing a first sedimentation separation on the filter residue slurry to obtain an underflow slurry; subjecting the underflow slurry to high-temperature dissolution to recover aluminum hydroxide to obtain a dissolution slurry; The dissolved ore pulp is subjected to a second sedimentation separation to obtain the crude sodium aluminate solution.

2. The method according to claim 1, characterized in that The solid content of the fine particle aluminum hydroxide slurry is 100 g / L to 200 g / L.

3. The method according to claim 1, characterized in that In the fine-particle aluminum hydroxide slurry, the mass fraction of solid aluminum hydroxide with a particle size of less than 45 μm is 25% to 50%, and the D50 of the solid aluminum hydroxide is 30 μm to 60 μm.

4. The method according to claim 1, wherein The mass concentration of suspended matter in the sodium aluminate crude solution is 0.1 g / L to 2.0 g / L.

5. The method according to claim 1, characterized in that The mass concentration of suspended matter in the sodium aluminate semen is 0.005 g / L to 0.020 g / L.

6. The method according to claim 1, wherein The solid content of the filter residue slurry is 30 g / L to 100 g / L.

7. The method according to claim 1, characterized in that The solid content of the underflow slurry is 400 g / L to 800 g / L.

8. The method according to claim 1, characterized in that The temperature of the high-temperature dissolution is 130°C to 280°C.

9. The method according to claim 1, characterized in that The calcination temperature is 1000°C to 1200°C.

10. The method according to claim 1, characterized in that The mass fraction of calcium oxide in the alumina product is 0.005% to 0.015%.

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