Method for preparing low-impurity aluminum hydroxide from guinea bauxite

Abstract

The invention provides a method for preparing low-impurity aluminum hydroxide from guinea bauxite, and belongs to the field of aluminum oxide production. The method comprises the following steps: crushing guinea bauxite; the ore particles and the blending liquid are mixed and ground into ore pulp, and then the ore pulp is subjected to first-stage dissolution treatment; the first-stage dissolution ore pulp is mixed with a blending solution, then second-stage dissolution treatment is conducted, and a sodium hydroxycarboxylate complexing agent is added in the second-stage dissolution treatment process for complexing and impurity removal; diluting the second-stage dissolved-out ore pulp to a set concentration, and then adding an alkaline precipitator into the diluted pulp to carry out a co-precipitation reaction so as to remove impurities; and carrying out seed decomposition on the refined liquid by adopting a double-process impurity removal and decomposition process in which a main process and an auxiliary process are connected in parallel to obtain a low-impurity aluminum hydroxide product. Through a process chain of ore pretreatment, gradient temperature control dissolution and impurity interception in a decomposition process, deep removal of trace elements is realized in a guinea ore Bayer process flow, and low-impurity high-purity aluminum hydroxide is produced.

Description

Technical Field

[0001] This application relates to the field of alumina production technology, and more particularly to a method for preparing low-impurity aluminum hydroxide using Guinean bauxite. Background Technology

[0002] With metallurgical-grade alumina production capacity becoming increasingly surplus, alumina producers are increasingly focusing their transformation and upgrading efforts on the fine alumina sector. Against this backdrop, alumina products with lower impurity content, suitable for producing high-purity aluminum, are gaining increasing market competitiveness. However, domestically produced alumina generally fails to meet the requirements for high-purity aluminum production due to excessive levels of trace metals such as vanadium, gallium, calcium, iron, and zinc. Simultaneously, due to changes in resource supply patterns, alumina producers using the low-temperature Bayer process are shifting their primary bauxite source from Indonesia and Australia to the Bofa mine in Guinea.

[0003] Guinea's Baufa ore is a high-iron, low-silicon type, characterized by fine-grained silica minerals that are tightly intercalated with aluminum and are mostly low-activity minerals, making aluminum-silicon separation difficult. The ore also contains high levels of trace elements such as gallium, chromium, manganese, vanadium, and zinc, making direct use for producing high-quality raw materials challenging. Existing low-temperature Bayer processes for this type of ore face a series of significant challenges: First, the high hardness and large size of the ore lead to low grinding efficiency, and the high-chromium casting balls used to improve grinding efficiency experience increased wear, introducing additional impurities such as chromium into the system. Second, during the leaching stage, despite using lower temperatures, impurities such as iron, gallium, calcium, and zinc still have a high leaching rate, entering the sodium aluminate solution. Third, and more critically, during the decomposition and mother liquor recycling stages, there is a lack of effective removal methods for impurity ions such as calcium (Ca), zinc (Zn), and gallium (Ga). These ions are prone to co-precipitation or adsorption during the decomposition process, and subsequently, some enter the product alumina during the roasting process, while others return to the system with the circulating mother liquor, resulting in a difficult-to-eliminate closed-loop cycle and continuous enrichment throughout the production process. This "cyclic enrichment effect" of impurities is a bottleneck restricting the improvement of the purity of the final aluminum hydroxide product (especially the content of impurities such as calcium, zinc, and gallium). Existing technologies mostly improve the leaching or solution purification stages, but none can fundamentally cut off this cycle within the Bayer process main flow. Therefore, developing a process that can systematically inhibit impurity leaching and efficiently remove circulating impurities is crucial for the economically feasible production of low-impurity aluminum hydroxide from Guinea ore. Summary of the Invention

[0004] This application provides a method for preparing low-impurity aluminum hydroxide using Guinean bauxite, which achieves source suppression of impurities and deep removal of trace elements in the Bayer process using Guinean ore, producing high-purity aluminum hydroxide with low impurities.

[0005] This application provides a method for preparing low-impurity aluminum hydroxide using Guinean bauxite, the method comprising: Guinean bauxite is crushed to obtain ore particles; The ore particles are mixed with the blending solution and ground into a slurry. The slurry is then subjected to a first-stage leaching treatment to obtain a first-stage leaching slurry. The first-stage leaching slurry is mixed with the conditioning solution, and the mixed slurry is subjected to a second-stage leaching treatment. During the second-stage leaching treatment, sodium hydroxycarboxylate complexing agent is added to complex and remove impurities, resulting in a second-stage leaching slurry. The two-stage leaching slurry is diluted to a set concentration, and then an alkaline precipitant is added to the diluted slurry to carry out a co-precipitation reaction to remove impurities and obtain a refined solution; the alkaline precipitant includes at least one of magnesium-based materials, calcium-based materials and barium-based materials; A dual-process impurity removal and decomposition process, consisting of a main process and a secondary process running in parallel, is employed to perform seed separation and decomposition on the purified liquid, yielding a low-impurity aluminum hydroxide product. The secondary process, by controlling decomposition conditions and using induced active seed crystals, ensures that Ca... 2+ Zn 2+ Ga 3+ Impurities are enriched and separated; the main process employs seed crystal addition in stages and low-temperature gradient temperature change decomposition.

[0006] Optionally, the particle size of the ore particles is 50 mm to 80 mm; The fineness of the slurry produced by grinding the ore particles with the blending solution meets the following requirement: ≤2% residue on a 35# sieve.

[0007] Optionally, the conditions for the first stage of leaching treatment include: a temperature of 80℃~100℃, a time of 8h~14h, and a slurry solid content of 380g / L~420g / L.

[0008] Optionally, the conditions for the two-stage dissolution treatment include: a temperature of 135℃~150℃, a Na2O concentration of 170g / L~200g / L, an Al2O3 concentration of 200g / L~230g / L, a caustic ratio of 1.3~1.5, and a dissolution time of 40min~80min. The dissolution rate α of the two-stage dissolution treatment is 83% to 87%, and the molecular ratio MR is 1.35 to 1.45.

[0009] Optionally, the sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate, wherein the mass ratio of sodium citrate to sodium gluconate is 1:(1.5-2.5).

[0010] Optionally, the set concentrations for the dilution of the second-stage leaching slurry include: Na2O concentration of 150g / L to 170g / L, Al2O3 concentration of 160g / L to 175g / L, and caustic ratio of 1.35 to 1.55. The alkaline precipitant is a slurry with a concentration of 100g / L to 300g / L and a solid content of 200g / L to 500g / L.

[0011] Optionally, the dual-process impurity removal and decomposition process employing a main process and a secondary process in parallel decomposes the purified liquid to obtain a low-impurity aluminum hydroxide product, comprising: The purified liquid is divided into two parts, which are introduced into the main process and the auxiliary process that are set in parallel, respectively; Controlling the decomposition conditions of the sub-process and adding induced active impurity removal seed crystals to the sub-process, so that Ca 2+ Zn 2+ Ga 3+ Impurities are enriched and separated in the sub-process; the decomposition conditions of the sub-process include: a solid content of 100 g / L to 300 g / L, an initial decomposition temperature of 75℃ to 85℃, and a decomposition time of 15 h to 20 h; the particle size D50 of the induced active impurity removal seed crystals is 2 μm to 10 μm, and the chemical composition is a compound rich in the target impurity element. The main process is controlled by seed crystal graded addition and low-temperature gradient temperature change decomposition process to obtain the low-impurity aluminum hydroxide product. The solution obtained after the sub-process is decomposed is incorporated into the main process.

[0012] Optionally, the seeding process in the main flow includes: At the start of the main process decomposition, high-purity aluminum hydroxide seed crystals are added to the main process; the particle size D50 of the high-purity aluminum hydroxide seed crystals is 15 μm, and the addition amount is 30 g / L; After the main process decomposition has been carried out for 24 hours, a crystallization aid is added to the main process; the crystallization aid consists of polyacrylate polymers and surfactants.

[0013] Optionally, the low-temperature gradient temperature-variable decomposition process of the main process includes: Within 0–24 hours after the start of decomposition, the initial decomposition temperature of the main process is controlled at 75±2℃, and the temperature is reduced in a 5℃ gradient. Within 24 to 50 hours after the start of decomposition, the main process is controlled to cool down at a temperature gradient of 3°C until the final decomposition temperature reaches 55°C. The total decomposition time of the main process shall not be less than 40 hours.

[0014] Optionally, the total content of trace elements Fe, Si, Ca, Mg, Ti, Cr, Ga, Mn, V, and Zn in the low-impurity aluminum hydroxide product is ≤250ppm.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing low-impurity aluminum hydroxide from Guinean bauxite. By systematically integrating and optimizing a series of unit operations with specific impurity removal functions within the Bayer process main flow, deep removal of various trace elements from Guinean bauxite is achieved, thereby producing a low-impurity aluminum hydroxide product. The technical path follows a process sequence, specifically intercepting specific impurities at each stage. The specific implementation is as follows: First, source control and preliminary separation are carried out before leaching. By grinding the ore particles into a slurry and then performing a first-stage leaching process, aluminum and silicon separation is preferentially achieved under relatively mild conditions. This aims to remove silicon in advance as sodium silicon slag to the maximum extent possible and to inhibit the excessive leaching of other impurity minerals at low temperatures, thereby reducing the total amount of impurities entering the liquid phase from the source.

[0016] Secondly, targeted chemical blocking is implemented during the main dissolution stage. Sodium hydroxycarboxylate complexing agent is added simultaneously during the second-stage dissolution process. This additive can bind with the Fe atoms that dissolve into the solution. 3+ Ga 3+ The metal ions undergo a complexation reaction to form stable water-soluble complexes. This process achieves "molecular-level sealing" of key metal impurities such as iron and gallium during the dissolution stage, effectively preventing them from co-precipitating with aluminum hydroxide in subsequent processes.

[0017] Next, the solution undergoes deep purification after dissolution. The second-stage dissolution slurry is diluted, and an alkaline precipitant is added for co-precipitation. Cations such as magnesium, calcium, or barium can combine with residual anionic impurities in the solution, such as silicate, titanate, and vanadate, to form insoluble complex salt precipitates. This step achieves synergistic and deep removal of various anionic impurities, including silicon, titanium, phosphorus, and vanadium, resulting in a highly purified solution.

[0018] Finally, a dual impurity interception mechanism is established during the decomposition and crystallization stage. An innovative dual-process impurity removal and decomposition process is adopted, with the main process and auxiliary process running in parallel. The auxiliary process, through dedicated decomposition conditions and induced active impurity-removing seed crystals, selectively enriches and separates impurities such as zinc, iron, calcium, gallium, and chromium from the system, thereby cutting off the cyclic accumulation path of these impurities in the process. Simultaneously, the main process employs graded seed crystal addition and low-temperature gradient temperature decomposition. By providing pure growth nuclei, suppressing fine crystal formation, and optimizing the decomposition temperature regime, a kinetic environment conducive to the pure growth of aluminum hydroxide crystals is created, maximally suppressing the adsorption of residual impurity ions on the crystal surface or their entry into the crystal lattice.

[0019] In summary, this application utilizes a synergistic technology chain embedded in the Bayer process, namely "first-stage leaching treatment for source reduction, second-stage leaching treatment for complexation and blockade, co-precipitation reaction for deep purification, and dual-process decomposition and crystallization interception," to achieve deep removal of various trace elements such as Fe, Si, Ca, Ga, Ti, and V throughout the entire process without relying on external independent purification units, ultimately producing low-impurity aluminum hydroxide products. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0021] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the 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.

[0022] Figure 1 A schematic flowchart illustrating a method for preparing low-impurity aluminum hydroxide from Guinean bauxite, provided as an embodiment of this application. Figure 2 This is a schematic diagram of the actual process for preparing low-impurity aluminum hydroxide using Guinean bauxite, as provided in an embodiment of this application. Detailed Implementation

[0023] 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.

[0024] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1 to 6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0025] Figure 1 A schematic flowchart illustrating a method for preparing low-impurity aluminum hydroxide from Guinean bauxite, provided as an embodiment of this application. Figure 2 This is a schematic diagram of the actual process for preparing low-impurity aluminum hydroxide using Guinean bauxite, as provided in an embodiment of this application.

[0026] like Figure 1 and Figure 2 As shown in the embodiments of this application, a method for preparing low-impurity aluminum hydroxide using Guinean bauxite is provided, the method comprising: S1. Crushing Guinean bauxite to obtain ore particles; S2. The ore particles are mixed with the blending solution and ground into a slurry. The slurry is then subjected to a first-stage leaching treatment to obtain a first-stage leaching slurry. S3. Mix the first-stage leaching slurry with the conditioning solution, and then perform a second-stage leaching treatment on the mixed slurry. During the second-stage leaching treatment, add sodium hydroxycarboxylate complexing agent to complex and remove impurities to obtain a second-stage leaching slurry. S4. Dilute the second-stage leaching slurry to a set concentration, then add an alkaline precipitant to the diluted slurry for co-precipitation reaction to remove impurities and obtain a refined solution; the alkaline precipitant includes at least one of magnesium-based materials, calcium-based materials and barium-based materials; S5. A dual-process impurity removal and decomposition process with the main process and the auxiliary process in parallel is adopted to decompose the refined liquid into low-impurity aluminum hydroxide product. The auxiliary process controls the decomposition conditions and uses induced active impurity removal seed crystals, while the main process adopts seed crystal graded addition and low-temperature gradient temperature change decomposition.

[0027] It should be noted that the induced active impurity removal seed crystals used in the secondary process are highly active seed crystals made by mechanically grinding aluminum hydroxide using a sand mill (with a D50 particle size of 2μm~10μm, possessing high specific surface area, high surface defect density, and strong surface activity). The decomposition regime is precisely controlled (e.g., maintaining high solids content and suitable low temperature) to create a kinetic environment conducive to impurity co-extrusion but unfavorable to crystal growth. The aluminum hydroxide generated under these conditions exhibits a loose structure, fine and uniform particle size, and high activity, and can efficiently enrich Ca in the solution through various mechanisms such as surface adsorption, encapsulation, and lattice substitution. 2+ Zn 2+ Ga 3+ Impurity ions. Fine-grained aluminum hydroxide enriched with impurities can be used as a high-impurity carrier to be discharged separately from the system, achieving deep removal of impurities.

[0028] This method is a low-temperature Bayer process specifically designed for high-iron, low-silica Guinean bauxite. It aims to deeply remove impurities through a synergistic process chain, ultimately producing a low-impurity aluminum hydroxide product with a total trace element content ≤250 ppm. Its core operating principle lies in: maximizing aluminum-silicon separation efficiency and initially inhibiting impurity leaching through optimized pretreatment and gradient leaching; deeply purifying the solution through co-precipitation reaction; and finally, actively intercepting and rejecting impurities during the crystal growth stage through an innovative dual-process decomposition process, thereby achieving a significant improvement in product purity within the main Bayer process flow. The role and principle of each step are explained in detail below.

[0029] The purpose of the S1 ore pretreatment step is to prepare raw materials with suitable physical properties for subsequent hydrometallurgical processes. Crushing yields uniformly sized ore particles, providing a foundation for efficient and stable grinding operations. Mixing the ore particles with the blending solution and then finely grinding them aims to fully dissociate aluminum minerals from tightly associated silica-containing minerals and other impurity minerals, increasing their reaction surface area. This step is a physical prerequisite for subsequent efficient chemical separation and impurity control.

[0030] The S2 stage of leaching treatment (pre-desiliconization) aims to preferentially dissolve some of the active alumina in bauxite under relatively mild conditions, and promote the reaction of some silica to form sodium silicate slag, thereby achieving preliminary aluminum-silicon separation. This process is carried out at a lower temperature, which effectively inhibits the large-scale dissolution of impurities such as iron, titanium, and gallium. By removing some silicon in advance, the burden on the subsequent main leaching process can be reduced, the concentration of silicon in the solution can be lowered, and a foundation can be laid for obtaining a low-silicon product.

[0031] The S3 two-stage leaching treatment (main leaching and complexation removal) step aims to achieve efficient leaching of the main alumina component at a higher temperature, while using chemical additives to inhibit impurity elements from entering the solution or altering their form. Under optimized leaching conditions, the aluminum minerals are fully dissolved. Simultaneously, the added sodium hydroxycarboxylate complexing agent reacts with the dissolved iron, gallium, and other metal ions to form stable, soluble complexes. This effectively "locks in" these ions, preventing them from precipitating with aluminum hydroxide or being adsorbed in subsequent processing stages. This is a crucial step in the deep removal of specific metallic impurities from the solution.

[0032] The S4 co-precipitation purification step is designed to deeply purify the complex slurry after dissolution, focusing on removing various impurity ions such as silicon, titanium, phosphorus, and vanadium. First, the solution concentration is adjusted by dilution to create a more suitable thermodynamic environment for impurity precipitation. Then, an alkaline precipitant is added, which provides cations (Mg²⁺, ... 2+ Ca 2+ Ba 2+ These substances (such as silicate, titanate, phosphate, and vanadate) can react with anions in solution to form various complex salt precipitates with extremely low solubility. Through this co-precipitation effect, multiple impurities existing in anionic form are simultaneously converted into a solid phase, thus being effectively removed in subsequent solid-liquid separation to obtain a purified liquid.

[0033] The S5 dual-process impurity removal and decomposition step is the core of final product purity control. Impurity interception is achieved by setting up parallel purification and production routes during the crystal growth stage. The secondary process, as a dedicated purification loop, utilizes induced active impurity removal seed crystals under specific conditions to preferentially promote the solid-phase enrichment of impurity ions such as zinc, iron, calcium, gallium, and chromium, thereby removing them from the circulation system. The main process receives the solution purified by the secondary process and, under optimized seed crystals and temperature regimes, focuses on growing large-particle, high-purity aluminum hydroxide crystals. The low-temperature, variable-temperature decomposition strategy aims to control crystal growth kinetics and minimize the adsorption of impurity ions on the crystal surface or their entry into the crystal lattice. The synergy of the main and secondary processes achieves active interception and rejection of impurities during product precipitation.

[0034] In some embodiments, the particle size of the ore particles is 50 mm to 80 mm; The fineness of the slurry produced by grinding the ore particles with the blending solution meets the following requirement: ≤2% residue on a 35# sieve.

[0035] The ore particle size is controlled between 50mm and 80mm. This specification sets a suitable upper limit for the particle size of the material fed into the mill, which is conducive to achieving stable operation and efficiency optimization of the grinding process. For example, the particle size of the ore can be 50mm, 55mm, 60mm, 65mm, 70mm, 75mm, 78mm, 80mm, etc.

[0036] The required fineness of the slurry is ≤2% residue on a 35# sieve. This indicator is a core quality control point in the grinding process, ensuring that the vast majority of solid particles are smaller than 0.5 mm. Achieving this fineness means that the minerals in the bauxite, especially the tightly intercalated silica-containing minerals, have been fully liberated, thus providing the necessary and large specific surface area for subsequent wet chemical leaching reactions. This is a fundamental physical condition for ensuring overall reaction efficiency and separation effect. For example, the residue on a 35# sieve of the slurry can be 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.7%, 1.9%, 2.0%, etc.

[0037] In some embodiments, the conditions for a single leaching treatment include: a temperature of 80°C to 100°C, a time of 8 hours to 14 hours, and a slurry solids content of 380 g / L to 420 g / L.

[0038] The leaching process temperature is set between 80℃ and 100℃. This temperature range provides the necessary activation energy for the pre-desiliconization reaction between active silica in the ore and the alkaline solution, while effectively inhibiting the large-scale leaching of impurity minerals such as iron and titanium, achieving selective reaction under mild conditions. The pre-desiliconization time is controlled between 8h and 14h to ensure sufficient reaction time, allowing the reactive silicon components to be fully converted into sodium silicate slag and removed. The slurry solids content is maintained between 380g / L and 420g / L. This concentration range is beneficial for maintaining a stable material concentration and good mass transfer conditions in the reaction system, balancing reaction efficiency and equipment processing capacity. For example, the temperature of a leaching treatment can be 80℃, 83℃, 86℃, 90℃, 93℃, 96℃, 98℃, 100℃, etc.; the time can be 8h, 9h, 10h, 11h, 12h, 13h, 13.5h, 14h, etc.; and the slurry solids content can be 380g / L, 385g / L, 390g / L, 395g / L, 400g / L, 405g / L, 415g / L, 420g / L, etc.

[0039] In some embodiments, the conditions for the two-stage dissolution treatment include: a temperature of 135°C to 150°C, a Na₂O concentration of 170 g / L to 200 g / L, an Al₂O₃ concentration of 200 g / L to 230 g / L, a caustic ratio of 1.3 to 1.5, and a dissolution time of 40 min to 80 min. The dissolution rate α of the two-stage dissolution treatment is 83%–87%, and the molecular ratio MR is 1.35–1.45.

[0040] The second-stage dissolution temperature is set at 135℃–150℃. This range provides sufficient energy for efficient dissolution of the main alumina minerals and, compared to traditional high-temperature processes, better balances the dissolution rate with impurity suppression. Na₂O concentration is controlled at 170 g / L–200 g / L, and Al₂O₃ concentration at 200 g / L–230 g / L. Together, they create the high-alkalinity chemical environment required for dissolution, ensuring sufficient thermodynamic driving force for the dissolution reaction. The caustic ratio is controlled at 1.3–1.5, ensuring high solubility of alumina in the leachate while maintaining good chemical stability. The dissolution time is set at 40 min–80 min to guarantee the full progress of the main dissolution reaction under the above conditions. For example, the temperature of the two-stage dissolution treatment can be 135℃, 138℃, 140℃, 143℃, 146℃, 148℃, 150℃, etc.; the Na2O concentration can be 170g / L, 178g / L, 185g / L, 190g / L, 193g / L, 195g / L, 198g / L, 200g / L, etc.; the Al2O3 concentration can be 200g / L, 206g / L, etc. 210g / L, 215g / L, 220g / L, 224g / L, 227g / L, 230g / L, etc.; caustic ratios can be 1.3, 1.34, 1.38, 1.4, 1.42, 1.45, 1.47, 1.5, etc.; dissolution times can be 40min, 45min, 50min, 55min, 60min, 68min, 75min, 80min, etc.

[0041] Controlling the dissolution rate α between 83% and 87% and the molecular weight ratio MR between 1.35 and 1.45 directly reflects the pursuit of high efficiency and controllability in the process. The former ensures a high aluminum recovery rate, while the latter determines the supersaturated state of the solution after dissolution, suitable for subsequent decomposition. For example, the dissolution rate α can be 83%, 84%, 85%, 86%, 87%, etc.; the molecular weight ratio MR can be 1.35, 1.37, 1.39, 1.40, 1.41, 1.42, 1.44, 1.45, etc.

[0042] In some embodiments, the sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate, wherein the mass ratio of sodium citrate to sodium gluconate is 1:(1.5 to 2.5).

[0043] The sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate, with a mass ratio controlled at 1:(1.5–2.5). This specific ratio aims to utilize the synergistic complexing effect of the two organic acid salts. Within this ratio range, the complexing agent has a significant effect on Fe in the solution. 3+ Ga 3+The metal ions exhibit optimized complexing ability and complex stability, thereby effectively "blocking" these impurity ions and preventing their precipitation in subsequent processes. For example, the mass ratio of sodium citrate to sodium gluconate can be 1:1.5, 1:1.7, 1:1.9, 1:2.0, 1:2.1, 1:2.3, 1:2.4, 1:2.5, etc.

[0044] In some embodiments, the set concentrations for the dilution of the second-stage leaching slurry include: Na2O concentration of 150 g / L to 170 g / L, Al2O3 concentration of 160 g / L to 175 g / L, and caustic ratio of 1.35 to 1.55. The alkaline precipitant is a slurry with a concentration of 100g / L to 300g / L and a solid content of 200g / L to 500g / L.

[0045] The second-stage leaching slurry was diluted to a Na₂O concentration of 150 g / L–170 g / L, an Al₂O₃ concentration of 160 g / L–175 g / L, and a caustic ratio of 1.35–1.55. The dilution aimed to reduce the free alkalinity of the solution, creating more favorable thermodynamic conditions for the subsequent co-precipitation reaction, and simultaneously adjusting the solution composition to closely approximate the feed requirements of the decomposition process. For example, the Na2O concentration after dilution of the second-stage leaching slurry can be 150 g / L, 154 g / L, 158 g / L, 160 g / L, 163 g / L, 165 g / L, 168 g / L, 170 g / L, etc.; the Al2O3 concentration after dilution can be 160 g / L, 163 g / L, 166 g / L, 169 g / L, 170 g / L, 172 g / L, 174 g / L, 175 g / L, etc.; and the caustic ratio after dilution can be 1.35, 1.40, 1.43, 1.45, 1.48, 1.50, 1.52, 1.55, etc.

[0046] The alkaline precipitant is formulated into a slurry with a concentration of 100 g / L to 300 g / L and a solid content of 200 g / L to 500 g / L. This concentration and solid content range facilitates accurate metering and delivery of the precipitant and ensures rapid and uniform mixing and dispersion with the bulk slurry. For example, the concentration of the alkaline precipitant can be 100 g / L, 150 g / L, 200 g / L, 250 g / L, 300 g / L, etc.; and the solid content can be 200 g / L, 250 g / L, 300 g / L, 350 g / L, 400 g / L, 450 g / L, 500 g / L, etc.

[0047] In some embodiments, a dual-process impurity removal and decomposition process with the main process and the auxiliary process running in parallel is used to decompose the purified liquid into a low-impurity aluminum hydroxide product, including: The refined liquid is divided into two parts, which are introduced into the main process and the auxiliary process set in parallel respectively. The decomposition conditions of the sub-process are controlled, and induced active impurity removal seed crystals are added to the sub-process to enrich and separate zinc, iron, calcium, and chromium impurities in the sub-process. The decomposition conditions of the sub-process include: a decomposition solid content of 100 g / L to 300 g / L, an initial decomposition temperature of 75℃ to 85℃, and a decomposition time of 15 h to 20 h. The particle size D50 of the induced active impurity removal seed crystals is 2 μm to 10 μm, and the chemical composition is a compound rich in the target impurity elements. The main process control adopts seed crystal graded addition and low-temperature gradient temperature change decomposition process to decompose the seed crystals and obtain low-impurity aluminum hydroxide products. The solution obtained after the sub-process is decomposed is incorporated into the main process.

[0048] It should be noted that the induced active impurity removal seed crystals are micron-sized active particles prepared by mechanical grinding, with a median particle size D50 controlled between 2 μm and 10 μm. This particle size range has been optimized: on the one hand, sufficiently small particles provide a large specific surface area, exposing abundant active sites; on the other hand, this size ensures good suspension and dispersibility of the particles in the slurry, avoiding excessively rapid sedimentation. The chemical composition of the seed crystals is preferably a compound rich in the target impurity element (exemplarily, it may be calcium oxalate, iron phosphate, chromium hydroxide, aluminum hydroxide, or a complex thereof).

[0049] The solid content in the sub-process decomposition is controlled at 100 g / L to 300 g / L. This higher solid concentration (including induced active seed crystals and some aluminum hydroxide seed crystals) effectively increases the collision frequency between impurity ions and the solid surface. Induced active impurity removal seed crystals (aluminum hydroxide is mechanically ground into highly active seed crystals with a particle size of 2-10 μm using a sand mill) are added to further increase the collision and adsorption frequency of impurity ions on the solid surface, thus enhancing the enrichment process of impurities (zinc, iron, calcium, gallium, chromium, etc.). The initial decomposition temperature is controlled at 75℃ to 85℃. This temperature maintains a certain decomposition reaction rate to promote impurity precipitation while preventing excessive precipitation of the main component, aluminum hydroxide. The decomposition time is set at 15 h to 20 h, providing the necessary reaction duration for the selective precipitation and enrichment of impurities in the sub-process. For example, the solid content of the sub-process decomposition can be 100 g / L, 130 g / L, 160 g / L, 200 g / L, 230 g / L, 260 g / L, 280 g / L, 300 g / L, etc.; the initial decomposition temperature of the sub-process can be 75℃, 77℃, 79℃, 80℃, 81℃, 82℃, 84℃, 85℃, etc.; and the decomposition time of the sub-process can be 15h, 16h, 17h, 17.5h, 18h, 18.5h, 19h, 20h, etc.

[0050] In some implementations, the seeding process in the main flow includes: At the start of the main process decomposition, high-purity aluminum hydroxide seed crystals are added to the main process; the particle size D50 of the high-purity aluminum hydroxide seed crystals is 15μm, and the addition amount is 30g / L; After the main process decomposition has been carried out for 24 hours, a crystallization aid is added to the main process; the crystallization aid consists of polyacrylate polymers and surfactants.

[0051] The high-purity aluminum hydroxide added at the beginning of the main process has a particle size (D50) controlled at 15 μm. This medium-to-fine particle size provides a large number of active growth sites for the decomposition process, which is beneficial for obtaining a good product particle size distribution. The seed crystal addition amount is 30 g / L. This addition amount ensures that there is sufficient seed crystal surface in the solution during the initial stage of decomposition to receive the precipitated aluminum hydroxide, thereby effectively inhibiting excessive secondary nucleation.

[0052] Adding a crystallization aid during the initial stage of decomposition, the effective components of which include polyacrylate polymers and their compounded surfactants, allows the aid to selectively adsorb onto the surface of microcrystals and newly formed crystal nuclei in the solution through its molecular structure characteristics, thereby effectively inhibiting secondary nucleation and the generation of fine particles, and achieving optimized control of product particle size.

[0053] In some implementations, the low-temperature gradient temperature-variable decomposition process of the main flow includes: Within 0–24 hours after the start of decomposition, the initial decomposition temperature of the main process is controlled at 75±2℃, and the temperature is reduced in a 5℃ gradient. Within 24 to 50 hours after the decomposition begins, the main process is controlled to cool down at a temperature gradient of 3°C until the final decomposition temperature reaches 55°C. The total decomposition time for the main control process shall not be less than 40 hours.

[0054] The initial decomposition temperature is controlled at 75±2℃. This relatively high initial temperature helps accelerate the initial decomposition reaction rate and promotes early crystal growth. A 5℃ cooling gradient is used from 0 to 24 hours to rapidly increase and maintain the supersaturation of the solution in the early stages of decomposition, providing a strong driving force for crystal growth. From 24 to 50 hours, a smaller cooling gradient of 3℃ is applied to maintain supersaturation gradually in the later stages of decomposition, allowing crystals to grow uniformly under more stable conditions and reducing the co-crystallization and adsorption rates of impurity ions. The final decomposition temperature is controlled at 55℃, which is the set endpoint temperature. A total decomposition time of at least 40 hours is required to ensure a high decomposition rate (≥50%) and coarse-grained product. For example, the total decomposition time of the main process can be 40 hours, 43 hours, 45 hours, 47 hours, 50 hours, 53 hours, 55 hours, or 60 hours.

[0055] In some embodiments, the total content of trace elements Fe, Si, Ca, Mg, Ti, Cr, Ga, Mn, V, and Zn in the low-impurity aluminum hydroxide product is ≤250ppm.

[0056] This application's method constructs a tightly linked and highly targeted collaborative process chain, systematically controlling impurities from multiple dimensions, including physical pretreatment, chemical leaching, deep solution purification, and crystal growth interception. Ultimately, it achieves the goal of a total trace element content of ≤250 ppm for Fe, Si, Ca, Mg, Ti, Cr, Ga, Mn, V, and Zn in low-impurity aluminum hydroxide products. The specific implementation path is as follows: Firstly, in the ore pretreatment and primary leaching stage, the main focus is on source control and preliminary separation of impurities such as silicon and iron. By crushing the ore to 50mm-80mm and grinding it to a 35# sieve residue of ≤2%, sufficient mineral dissociation is achieved, creating the physical conditions for efficient chemical separation. Subsequently, a primary leaching treatment is carried out at a low temperature of 80℃-100℃ for 8h-14h. This condition preferentially promotes the reaction and removal of active silica, while maximally inhibiting the leaching of impurity minerals such as iron and titanium, thereby reducing the amount of these impurities entering the liquid phase from the source.

[0057] Secondly, in the second-stage dissolution process, the core focus is on suppressing the effective dissolution and subsequent migration of metallic impurities such as gallium and iron. At a main dissolution temperature of 135℃–150℃, combined with a Na₂O concentration of 170 g / L–200 g / L, the overall relatively low dissolution temperature system itself helps limit impurity dissolution while efficiently dissolving alumina. More importantly, a mixture of sodium citrate and sodium gluconate at a mass ratio of 1:(1.5–2.5) is added as a sodium hydroxycarboxylate complexing agent. This complexing agent specifically binds to the dissolved Fe. 3+ Ga 3+ Plasma forms a stable water-soluble complex, "locking" it in the solution and effectively preventing it from precipitating out as hydroxide in the subsequent cooling and dilution process, thus laying the foundation for deep impurity removal in subsequent processes.

[0058] Next, in the co-precipitation impurity removal stage, various anionic impurities such as silicon, titanium, and vanadium are removed at a concentrated depth. After diluting the dissolved slurry to a caustic ratio of 1.35–1.55, magnesium-based, calcium-based, or barium-based alkaline precipitants are added. These precipitants provide cations (Mg... 2+ Ca 2+ Ba 2+ It can undergo co-precipitation reactions with silicate, titanate, vanadate and other ions in the solution to form insoluble double salt precipitates, thereby removing a large amount of impurities such as silicon, titanium, phosphorus and vanadium in one go through solid-liquid separation, and obtaining a highly purified liquid.

[0059] Finally, in the dual-process impurity removal and decomposition stage, efforts are focused on solving the problems of deep removal of calcium and the co-crystallization and adsorption of other impurities during product precipitation. The secondary process is specifically designed to intercept impurities such as zinc, iron, calcium, and chromium: by controlling a high decomposition solids content (100g / L~300g / L) and adding induced active impurity removal seed crystals, conditions are created that favor the preferential precipitation and enrichment of zinc, iron, calcium, and chromium, ensuring that the vast majority of recycled calcium is captured and discharged from the system in this process. The main process focuses on crystal growth in a pure environment: high-purity aluminum hydroxide seed crystals are added to guide product growth; crystallization aids are added in the early stages of decomposition to suppress fine particle formation and optimize particle size; a low-temperature gradient decomposition regime is adopted, starting at an initial temperature of 75℃, decreasing in a 5℃ gradient initially, and then in a 3℃ gradient later, down to 55℃. This variable-temperature strategy, through precise control of supersaturation and growth kinetics, significantly inhibits the co-crystallization of impurity hydroxides such as iron and gallium and their adsorption on the surface of aluminum hydroxide crystals.

[0060] In summary, this application achieves targeted and deep removal of various trace elements through the synergistic effect of the entire process of "pretreatment to control silicon and limit impurities, dissolution and complexation to block iron and gallium, co-precipitation to deeply purify the solution, and decomposition to intercept calcium and other impurities in a dual process", thereby stably preparing low-impurity aluminum hydroxide products with a total impurity content of ≤250ppm.

[0061] This application provides a method for preparing low-impurity aluminum hydroxide from Guinean bauxite, which has the following advantages: (1) Effective removal of circulating impurities: The dual-process decomposition process is the most prominent advantage of this method. The secondary process is designed specifically for removing circulating impurities. Through the selective adsorption and co-precipitation of induced active impurity removal seeds, it can directionally enrich and discharge impurities such as calcium and zinc that accumulate in the mother liquor, cutting off the circulation path of impurities. This is the fundamental guarantee for obtaining products with low calcium content.

[0062] (2) A synergistic effect of impurity removal throughout the entire process is formed: the method is not a simple superposition of isolated units. For example, the specific proportion of sodium citrate-sodium gluconate complexing agent added in the second-stage dissolution effectively "locks in" iron and gallium ions, preventing them from precipitating prematurely and affecting subsequent purification; while the pre-treatment deep co-precipitation purification provides a purified solution with a lower impurity load for the final dual-process decomposition, improving the impurity removal efficiency of the sub-process and the product purity of the main process. Each step is interconnected and synergistically enhances the effect.

[0063] (3) Highly targeted and applicable to Guinean ore: The optimization of all process parameters (such as leaching temperature, complexing agent ratio, and precipitant selection) is aimed at the impurity occurrence and leaching characteristics of Guinean high-iron low-silicon ore, solving the practical problems of difficult silicon separation and high impurity leaching rate.

[0064] (4) Achieve deep impurity removal without changing the main process framework: This method is deeply integrated into the existing Bayer process. There is no need to introduce complex and expensive external purification equipment. Through structural optimization and process enhancement within the process, the purity of the product can be improved, and the industrialization feasibility is strong.

[0065] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards / industry standards / the disclosure herein; if there are no corresponding national standards / industry standards / the disclosure herein, they are performed according to generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer.

[0066] Example 1 This embodiment provides a method for preparing low-impurity aluminum hydroxide using Guinean bauxite, specifically including the following steps: S1. Guinean ore is crushed using a double-toothed roller crusher to reduce the particle size of the ore entering the mill to 50mm-60mm. The main process is as follows: A loader transports the ore to the scraper conveyor's receiving hopper, which then transports it to the crusher. The crusher crushes the 300mm feed material to 50mm, after which it falls into the discharge hopper and is transferred to the grinding head hopper. The scraper conveyor is equipped with movable guide rails, which automatically move out in case of emergencies without affecting production. The crushed ore particles are then mixed with a blending solution (with concentrations of ak 2.92, NK 195g / L, AO 110g / L, NT 239g / L) and ground to obtain a slurry. The fineness of the slurry is controlled to be 1.5% residue on a 35# sieve. After classification by a hydrocyclone, the overflow portion of the slurry enters a first-stage leaching treatment process.

[0067] S2. Perform a first-stage leaching treatment: control the solid content of the slurry in the first-stage leaching treatment to be 400 g / L, the temperature to be 85℃, and the pre-desilicon leaching time to be 12 h, so as to achieve the initial separation of aluminum and silicon and obtain a first-stage leaching slurry.

[0068] S3. Second-stage leaching treatment: The first-stage leaching slurry is mixed with the conditioning solution (the temperature of the mixed slurry is calculated to be above 85℃), and then fed into a pipeline leaching apparatus. The leaching temperature is controlled at 141℃, the Na₂O concentration in the leaching solution is 170 g / L, and the leaching residence time is 60 min. During this process, sodium hydroxycarboxylate complexing agent is added for complexation and impurity removal. The sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate in a mass ratio of 1:2. The leaching rate α of this step reaches 85.2%, and the molecular ratio MR is 1.405, resulting in the second-stage leaching slurry.

[0069] S4. Co-precipitation for impurity removal: The second-stage leaching slurry is diluted, and the Na2O concentration of the diluted slurry is controlled to be 160 g / L, the Al2O3 concentration to be 168 g / L, and the caustic ratio ak to be 1.45. Then, an alkaline precipitant is added to the diluted slurry for co-precipitation reaction, which lasts for 30 minutes to remove impurities such as silicon, titanium, phosphorus, and vanadium, yielding a refined solution. The alkaline precipitant is calcium oxide, added in the form of a slurry with a concentration of 200 g / L and a solid content of 300 g / L.

[0070] S5. Seed decomposition using a dual-process impurity removal decomposition process: (1) The purified liquid is divided into two parts and introduced into the main process and the auxiliary process set in parallel. (2) The decomposition conditions of the auxiliary process are controlled as follows: the solid content of decomposition is 200 g / L, the initial decomposition temperature is 85℃, the decomposition time is 18h, and induced active impurity removal seed crystals (aluminum hydroxide is ground by mechanical methods such as sand milling with a D50 particle size of 2~10μm) are added to it to enrich and separate the calcium impurities. The solution after decomposition in the auxiliary process is filtered and then incorporated into the main process. (3) The main process adopts seed crystal grade addition and low temperature gradient temperature change decomposition: Seed crystal grade addition: At the beginning of decomposition, high purity aluminum hydroxide seed crystals are added to the main process. The seed crystals have a particle size of D50 of 15μm and the addition amount is 30 g / L; when the decomposition is carried out for 24h, a crystallization aid (polyacrylate polymer and its compound surfactant, the polyacrylate polymer is cationic polyacrylamide, the surfactant is sodium polyacrylate, and the mass ratio of the two is 5:1) is added. Low-temperature gradient temperature-variable decomposition: The decomposition process is controlled within 0–24 hours, with an initial temperature of 75±2℃ and a cooling gradient of 5℃; within 24–48 hours, the cooling gradient is 3℃, until the final temperature reaches 55℃; the total decomposition time is 48 hours. Through the above process, a low-impurity aluminum hydroxide product is finally obtained.

[0071] Example 2 This embodiment provides a method for preparing low-impurity aluminum hydroxide using Guinean bauxite, specifically including the following steps: S1. The Guinean ore is crushed to obtain ore particles with a diameter of 50mm to 60mm. The ore particles are then mixed with a blending solution and ground to obtain a slurry. The fineness of the slurry is controlled to be 1.0% residue on a 35# sieve. After the slurry is classified, the overflow portion enters a first-stage leaching treatment process.

[0072] S2. Perform a first-stage leaching treatment: control the solid content of the slurry in the first-stage leaching treatment to be 380 g / L, the temperature to be 80℃, and the pre-desiliconization leaching time to be 8 h to obtain a first-stage leaching slurry.

[0073] S3. Second-stage leaching treatment: After mixing the first-stage leaching slurry with the conditioning solution, a second-stage leaching treatment is performed. The leaching temperature is controlled at 135℃, the Na₂O concentration in the leaching solution is 170 g / L, the Al₂O₃ concentration is 200 g / L, the caustic ratio is 1.3, and the leaching residence time is 40 min. During this process, sodium hydroxycarboxylate complexing agent is added for complexation and impurity removal. The sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate in a mass ratio of 1:1.5. The leaching rate α of this step is 83.5%, the molecular ratio MR is 1.35, and the second-stage leaching slurry is obtained.

[0074] S4. Co-precipitation for impurity removal: The second-stage leaching slurry is diluted, and the Na2O concentration of the diluted slurry is controlled to be 150 g / L, the Al2O3 concentration to be 160 g / L, and the caustic ratio to be 1.35. Then, an alkaline precipitant is added to the diluted slurry for co-precipitation reaction, which lasts for 30 minutes to obtain a purified solution. The alkaline precipitant is magnesium hydroxide, added in the form of a slurry with a concentration of 100 g / L and a solid content of 200 g / L.

[0075] S5. Seed decomposition using a dual-process impurity removal decomposition process: (1) The purified liquid is divided into two parts and introduced into the main process and the auxiliary process set in parallel. (2) The decomposition conditions of the auxiliary process are controlled as follows: the solid content of the decomposition is 300 g / L, the initial decomposition temperature is 75℃, the decomposition time is 15 h, and induced active impurity removal seed crystals are added to it to enrich and separate the impurities. The solution after decomposition in the auxiliary process is filtered and then incorporated into the main process. (3) The main process adopts seed crystal grade addition and low temperature gradient temperature change decomposition: Seed crystal grade addition: at the beginning of decomposition, high-purity aluminum hydroxide seed crystals are added to the main process. The seed crystals have a particle size D50 of 15 μm and the addition amount is 30 g / L; when the decomposition has proceeded for 24 h, crystallization aids are added. Low-temperature gradient temperature-variable decomposition: The decomposition process is controlled within 0–24 hours, with an initial temperature of 75±2℃ and a cooling gradient of 5℃; within 24–40 hours, the temperature is further reduced by a gradient of 3℃ until the final temperature reaches 55℃; the total decomposition time is 40 hours. Through the above process, a low-impurity aluminum hydroxide product is finally obtained.

[0076] Example 3 This embodiment provides a method for preparing low-impurity aluminum hydroxide using Guinean bauxite, specifically including the following steps: S1. The Guinean ore is crushed to obtain ore particles with a diameter of 50mm to 60mm. The ore particles are then mixed with a blending solution and ground to obtain a slurry. The fineness of the slurry is controlled to be 1.0% residue on a 35# sieve. After the slurry is classified, the overflow portion enters a first-stage leaching treatment process.

[0077] S2. Perform a first-stage leaching treatment: control the solid content of the slurry in the first-stage leaching treatment to be 420 g / L, the temperature to be 100℃, and the pre-desilicon leaching time to be 14 h to obtain a first-stage leaching slurry.

[0078] S3. Two-stage leaching treatment: After mixing the first-stage leaching slurry with the conditioning solution, a second-stage leaching treatment is performed. The leaching temperature is controlled at 150℃, the Na₂O concentration in the leaching solution is 200 g / L, the Al₂O₃ concentration is 230 g / L, the caustic ratio is 1.5, and the leaching residence time is 80 min. During this process, sodium hydroxycarboxylate complexing agent is added for complexation and impurity removal. The sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate in a mass ratio of 1:2.5. The leaching rate α of this step is 86.2%, the molecular ratio MR is 1.45, and the two-stage leaching slurry is obtained.

[0079] S4. Co-precipitation for impurity removal: The second-stage leaching slurry is diluted, and the Na2O concentration of the diluted slurry is controlled to be 170 g / L, the Al2O3 concentration to be 175 g / L, and the caustic ratio to be 1.55. Then, an alkaline precipitant is added to the diluted slurry for co-precipitation reaction, which lasts for 30 minutes to obtain a purified solution. The alkaline precipitant is barium carbonate, added in the form of a slurry with a concentration of 300 g / L and a solid content of 500 g / L.

[0080] S5. Seed decomposition using a dual-process impurity removal decomposition process: (1) The purified liquid is divided into two parts and introduced into the main process and the auxiliary process set in parallel. (2) The decomposition conditions of the auxiliary process are controlled as follows: the solid content of the decomposition is 100 g / L, the initial decomposition temperature is 85℃, the decomposition time is 20 h, and induced active impurity removal seed crystals are added to it to enrich and separate the impurities. The solution after decomposition in the auxiliary process is filtered and then incorporated into the main process. (3) The main process adopts seed crystal grade addition and low temperature gradient temperature change decomposition: Seed crystal grade addition: at the beginning of decomposition, high purity aluminum hydroxide seed crystals are added to the main process. The seed crystals have a particle size D50 of 15 μm and the addition amount is 30 g / L; when the decomposition has proceeded for 24 h, crystallization aids are added. Low-temperature gradient temperature-variable decomposition: The decomposition process is controlled within 0–24 hours, with an initial temperature of 75±2℃ and a cooling gradient of 5℃; within 24–50 hours, the temperature is further reduced by a gradient of 3℃ until the final temperature reaches 55℃; the total decomposition time is 50 hours. Through the above process, a low-impurity aluminum hydroxide product is finally obtained.

[0081] Comparative Example 1 This comparative example provides a comparative method for processing Guinean bauxite, specifically including the following steps: S1. Guinean ore is milled by adding a blending solution to the slurry grinding process. After classification by a hydrocyclone, the overflow portion enters a first-stage pre-dissolution process. The solid content of the slurry in the first-stage pre-dissolution process is controlled at 200 g / L, and the temperature is 85℃.

[0082] S2. The slurry after the first stage of pre-dissolution is subjected to a second stage of dissolution treatment. The temperature of the second stage of dissolution is controlled at 140℃, the Na2O concentration in the dissolution solution is 172g / L, and the dissolution residence time is 60min. The dissolution rate α of this step is 83%, the molecular ratio MR is 1.395, and the slurry after dissolution is obtained.

[0083] S3. After dilution, sedimentation, separation, and purification, the dissolved slurry enters the seed decomposition process. Decomposition process control: initial decomposition temperature is 75℃, and final decomposition temperature is 50℃. At the beginning of decomposition, ordinary aluminum hydroxide seed crystals are added. The particle size index of the ordinary seed crystals is -45μm (25%), particle size D50 is 90.34μm, and the addition amount is 400g / L.

[0084] The trace element index (ppm) in the aluminum hydroxide products obtained in Examples 1-3 and Comparative Example 1 was determined, and the results are shown in Table 1.

[0085] Table 1. Trace element indicators (ppm) in aluminum hydroxide products of Examples 1-3 and Comparative Example 1

[0086] As shown in Table 1, the aluminum hydroxide product prepared using the traditional process in Comparative Example 1 had a high impurity content, with Si at 115 ppm, Fe at 90 ppm, Ca at 220 ppm, Ti at 22 ppm, Ga at 85 ppm, Cr at 2.5 ppm, Mn at 2.1 ppm, V at 2.0 ppm, and Zn at 3.0 ppm. However, using the methods in Examples 1, 2, and 3, the impurity content in the aluminum hydroxide product was significantly reduced and stably controlled within the following ranges: Si ≤30 ppm, Fe ≤33 ppm, Ca ≤105 ppm, Ti ≤15 ppm, Ga ≤40 ppm, Cr ≤1 ppm, Mn ≤1.5 ppm, V ≤1.5 ppm, and Zn ≤1.5 ppm.

[0087] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: (1) It effectively solves the problem of high grinding media wear and impurity introduction caused by the characteristics of Guinea ore in traditional processes. By pre-crushing and stably controlling the particle size of the ore entering the mill to 50mm-80mm, and precisely controlling the grinding fineness (≤2% residue on 35# sieve), abnormal wear of grinding media (such as high-chromium cast balls) caused by the hardness and large size of the ore during the grinding process is significantly reduced. This not only reduces the introduction of metallic impurities such as chromium and iron due to wear, but also extends the life of the grinding media and reduces production costs.

[0088] (2) Significantly inhibited the excessive dissolution of various trace metal elements in Guinean ores during the leaching process. By adopting a gradient temperature-controlled leaching process that combines low-temperature pre-desiliconization at 80℃~100℃ and relatively low-temperature two-stage leaching at 135℃~150℃, and by adding a specific ratio of sodium hydroxycarboxylate complexing agent in the two-stage leaching, the leaching rate of metal ions such as iron (Fe) and gallium (Ga) was synergistically inhibited from a thermodynamic and kinetic perspective. The leached ions were "locked" by the complexing effect, thereby achieving effective control of the content of key impurities at the leaching source.

[0089] (3) The pre-desiliconization efficiency of the slurry was greatly improved and the synergistic deep removal of various impurities was achieved. By optimizing the temperature, time and solid content conditions of pre-desiliconization, and by innovatively introducing a co-precipitation impurity removal step based on alkaline substances such as magnesium, calcium and barium after leaching, not only was the removal of silica more thorough, but also impurity ions such as titanium, phosphorus and vanadium, which are difficult to remove individually in traditional processes, were precipitated and separated in an integrated manner, which greatly purified the solution entering the decomposition process.

[0090] (4) The core bottleneck of the cyclic enrichment of impurity ions (especially calcium ions) during the decomposition process has been innovatively solved. By establishing a dual-process impurity removal decomposition process with the main process and the auxiliary process running in parallel, the auxiliary process is made to operate specifically as an "impurity enricher". Under optimized solid content, temperature and time conditions, induced active impurity removal seed crystals are used to directionally enrich and separate the zinc, iron, calcium, chromium and other coprecipitated impurities accumulated in the system, thereby cutting off the circulation path of impurities in the process and creating a pure solution environment for the main process to produce low-impurity products.

[0091] (5) Through multi-stage synergistic impurity removal throughout the entire process, a fundamental reduction in product impurity content was achieved within the Bayer process main flow. This solution does not rely on additional expensive purification steps, but rather achieves precise impurity interception at every stage, from ore pretreatment, leaching, solution purification to crystal growth, through adaptive optimization and innovative combination of existing Bayer process procedures. Ultimately, this makes it possible to directly and industrially produce low-impurity aluminum hydroxide products with total impurities (Fe, Si, Ca, etc.) ≤250ppm using high-iron, low-silica Guinean bauxite, providing qualified raw materials for the production of high-purity aluminum and enhancing resource value and product competitiveness.

[0092] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing low-impurity aluminum hydroxide using Guinean bauxite, characterized in that, The method includes: Guinean bauxite is crushed to obtain ore particles; The ore particles are mixed with the blending solution and ground into a slurry. The slurry is then subjected to a first-stage leaching treatment to obtain a first-stage leaching slurry. The first-stage leaching slurry is mixed with the conditioning solution, and the mixed slurry is subjected to a second-stage leaching treatment. During the second-stage leaching treatment, sodium hydroxycarboxylate complexing agent is added to complex and remove impurities, resulting in a second-stage leaching slurry. The two-stage leaching slurry is diluted to a set concentration, and then an alkaline precipitant is added to the diluted slurry to carry out a co-precipitation reaction to remove impurities and obtain a refined solution; the alkaline precipitant includes at least one of magnesium-based materials, calcium-based materials and barium-based materials; A dual-process impurity removal and decomposition process, consisting of a main process and a secondary process running in parallel, is employed to perform seed separation and decomposition on the purified liquid, yielding a low-impurity aluminum hydroxide product. The secondary process, by controlling decomposition conditions and using induced active seed crystals, ensures that Ca... 2+ Zn 2+ Ga 3+ Impurities are enriched and separated; the main process employs seed crystal addition in stages and low-temperature gradient temperature change decomposition.

2. The method according to claim 1, characterized in that, The particle size of the ore is 50mm to 80mm; The fineness of the slurry produced by grinding the ore particles with the blending solution meets the following requirement: ≤2% residue on a 35# sieve.

3. The method according to claim 1, characterized in that, The conditions for the first stage of leaching treatment include: a temperature of 80℃~100℃, a time of 8h~14h, and a slurry solid content of 380g / L~420g / L.

4. The method according to claim 1, characterized in that, The conditions for the two-stage dissolution treatment include: a temperature of 135℃~150℃, a Na2O concentration of 170g / L~200g / L, an Al2O3 concentration of 200g / L~230g / L, a caustic ratio of 1.3~1.5, and a dissolution time of 40min~80min. The dissolution rate α of the two-stage dissolution treatment is 83% to 87%, and the molecular ratio MR is 1.35 to 1.

45.

5. The method according to claim 1, characterized in that, The sodium hydroxycarboxylate complexing agent is a mixture of sodium citrate and sodium gluconate, wherein the mass ratio of sodium citrate to sodium gluconate is 1:(1.5-2.5).

6. The method according to claim 1, characterized in that, The set concentrations for the dilution of the two-stage leaching slurry include: Na2O concentration of 150g / L to 170g / L, Al2O3 concentration of 160g / L to 175g / L, and caustic ratio of 1.35 to 1.

55. The alkaline precipitant is a slurry with a concentration of 100g / L to 300g / L and a solid content of 200g / L to 500g / L.

7. The method according to claim 1, characterized in that, The dual-process impurity removal and decomposition process, employing a main process and a secondary process in parallel, decomposes the refined liquid to obtain a low-impurity aluminum hydroxide product, including: The purified liquid is divided into two parts, which are introduced into the main process and the auxiliary process that are set in parallel, respectively; Controlling the decomposition conditions of the sub-process and adding induced active impurity removal seed crystals to the sub-process, so that Ca 2+ Zn 2+ Ga 3+ Impurities are enriched and separated in the sub-process; the decomposition conditions of the sub-process include: a solid content of 100 g / L to 300 g / L, an initial decomposition temperature of 75℃ to 85℃, and a decomposition time of 15 h to 20 h; the particle size D50 of the induced active impurity removal seed crystals is 2 μm to 10 μm, and the chemical composition is a compound rich in the target impurity element. The main process is controlled by seed crystal graded addition and low-temperature gradient temperature change decomposition process to obtain the low-impurity aluminum hydroxide product. The solution obtained after the sub-process is decomposed is incorporated into the main process.

8. The method according to claim 7, characterized in that, The main process of seed crystal addition includes: At the start of the main process decomposition, high-purity aluminum hydroxide seed crystals are added to the main process; the particle size D50 of the high-purity aluminum hydroxide seed crystals is 15 μm, and the addition amount is 30 g / L; After the main process decomposition has been carried out for 24 hours, a crystallization aid is added to the main process; the crystallization aid consists of polyacrylate polymers and surfactants.

9. The method according to claim 7, characterized in that, The main process of low-temperature gradient temperature-variable decomposition includes: Within 0–24 hours after the start of decomposition, the initial decomposition temperature of the main process is controlled at 75±2℃, and the temperature is reduced in a 5℃ gradient. Within 24 to 50 hours after the start of decomposition, the main process is controlled to cool down at a temperature gradient of 3°C until the final decomposition temperature reaches 55°C. The total decomposition time of the main process shall not be less than 40 hours.

10. The method according to claim 1, characterized in that, The total content of trace elements Fe, Si, Ca, Mg, Ti, Cr, Ga, Mn, V, and Zn in the low-impurity aluminum hydroxide product is ≤250ppm.

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