A deep purification process for aluminum hydroxide seeds in alumina production

Abstract

This invention relates to a deep purification process for aluminum hydroxide seeds in alumina production, belonging to the field of alumina production technology. The process includes: primary hot water washing, secondary hot water washing, and stop determination. The deep purification process for aluminum hydroxide seeds protected in this application innovatively performs precise washing of aluminum hydroxide seeds, ensuring that the aluminum hydroxide seeds are free of oxalate before decomposition, thereby improving seed purity and decomposition activity, and completely preventing the negative impact of oxalate accumulation on product quality and production efficiency.

Description

Technical Field

[0001] This invention relates to the field of alumina production technology, and in particular to a deep purification process for aluminum hydroxide seed crystals in alumina production. Background Technology

[0002] The Bayer process for alumina production has advantages such as simple process, low energy consumption, good product quality and low cost. Therefore, more than 95% of the world's alumina is produced using the Bayer process.

[0003] In the existing Bayer process, organic impurities in the ore enter the production system and circulate with the solution. During the high-pressure leaching process, a series of chemical reactions occur, causing the organic impurities in the solution to decompose from high-molecular-weight compounds into low-molecular-weight compounds, ultimately producing sodium oxalate, sodium carbonate, and other low-molecular-weight sodium salts. Among these, sodium oxalate has the most significant impact on the decomposition process in alumina production and is the core impurity that restricts production stability.

[0004] The decomposition process, a key step in the Bayer process, typically involves cooling the sodium aluminate solution and adding aluminum hydroxide seed crystals to induce the decomposition and precipitation of aluminum hydroxide. Ideally, large, uniform aluminum hydroxide crystals are desired to ensure the efficiency of subsequent filtration and washing processes. However, organic impurities such as sodium oxalate preferentially adsorb onto the surface of the aluminum hydroxide seed crystals, occupying active growth sites and inhibiting normal crystal growth. This ultimately leads to an increase in the proportion of fine particles smaller than 45 μm in the system.

[0005] For fine particles that have adsorbed oxalate, their subsequent fate presents a dual problem: if discharged with the aluminum hydroxide product, the oxalate decomposes to produce CO2 during the subsequent roasting process, polluting the environment; if returned to the high-pressure leaching process with the mother liquor, oxalate will accumulate in the system, further exacerbating equipment scaling and increasing alkali consumption. Therefore, addressing the adverse effects of oxalate on production, especially blocking its accumulation pathway, has become a key requirement in Bayer process alumina production.

[0006] Chinese invention patent application CN111573699A, published on August 25, 2020, discloses a method for removing sodium oxalate during alumina production. The process steps are as follows: first, aluminum hydroxide containing crystalline sodium oxalate is washed with soft water at 50℃~100℃ to obtain a finished product and a first filtrate; then, seed aluminum hydroxide is washed with the first filtrate. The washing conditions for the seed aluminum hydroxide are: washing temperature 40℃~90℃, washing time 10min~60min, and stirring speed 100r / min~150r / min, resulting in washed seed aluminum hydroxide and a second filtrate. This method achieves sodium oxalate removal through a heated washing process.

[0007] Regarding the aforementioned method for removing sodium oxalate, the inventors discovered that this technology, by maintaining a constant stirring speed, can lead to insufficient speed in the later stages of washing, making it impossible to effectively rinse the high-concentration oxalate liquid film on the seed crystal surface, resulting in poor washing uniformity. In addition, this technology focuses more on removing sodium oxalate impurities from the seed crystal surface, without considering the residual oxalate in the pores of the aluminum hydroxide seed crystal. This residual oxalate accumulates continuously after the seed crystal is returned to the decomposition process, leading to a decline in the effectiveness of oxalate concentration control during long-term production. Summary of the Invention

[0008] To address the problem of residual oxalate impurities on the surface and within the pores of aluminum hydroxide seed crystals after vertical plate filtration, which cause these impurities to enter the decomposition process along with the aluminum hydroxide seed crystals, affecting the purity of the seed crystals and the stability of subsequent decomposition reactions, this invention provides a deep purification process for aluminum hydroxide seed crystals in alumina production.

[0009] The present invention provides a sodium aluminate solution purification process, which adopts the following technical solution: A sodium aluminate solution purification process includes the following steps: S1: First-stage hot water washing: The aluminum hydroxide seed filter cake containing oxalate impurities is washed countercurrently with hot water at a temperature of 96±3℃, with the liquid-solid ratio controlled at (3~5):1. The stirring rate is initially set to 120~140r / min, and is increased by 20% relative to the initial setting rate every 8min. After stirring for 2min, the initial setting rate is restored. The washing time is 10~30min. S2: Secondary hot water washing and stop determination: The aluminum hydroxide seed filter cake after the initial washing in S1 is further washed countercurrently with hot water at a temperature of 96±3℃, maintaining the same liquid-solid ratio, initial setting value of stirring rate and speed change rule as in S1; the oxalate concentration in the secondary washing waste liquid is monitored in real time, and when the concentration is ≤0.1g / L, washing is stopped, and the obtained purified aluminum hydroxide seed is dried.

[0010] By adopting the above technical solution, oxalate impurities adhering to the surface of aluminum hydroxide seed crystals after vertical plate filtration cannot be efficiently dissolved and removed by conventional washing. This causes the impurities to enter the decomposition process with the seed crystals, affecting the purity of the seed crystals and the stability of subsequent decomposition reactions. Countercurrent washing with water at 93-99℃ ensures efficient dissolution of oxalate. The liquid-to-solid ratio is controlled at (3-5):1 to ensure sufficient dissolution of oxalate while avoiding over-washing that would reduce production efficiency. A basic stirring rate of 120-140 r / min is used to maintain the integrity of the seed crystals while promoting uniform suspension of the seed crystals and continuously rinsing the seed crystal surface, thereby improving production efficiency. The stirring speed is increased every 8 minutes and maintained for 2 minutes to quickly rinse the high-concentration oxalate liquid film on the seed crystal surface into the main solution, improving the uniformity of purification. In addition, basic stirring can quickly remove free oxalate, and the strong turbulence generated by the 2-minute speed increase can destroy the weak bond between bound oxalate and the hydroxyl groups on the crystal surface.

[0011] The washing of aluminum hydroxide seed filter cake is divided into a primary hot water washing process and a secondary hot water washing process. The primary hot water washing dissolves most of the oxalate on the surface and breaks down the filter cake structure. The secondary hot water washing maintains the same washing parameters as the primary hot water washing, which facilitates the dissolution and washing of the bound oxalate deep in the seed pores, thereby achieving the purpose of completely removing oxalate.

[0012] After separation, the seed crystals are dried and returned directly to the decomposition process to avoid the wet seed crystals carrying excessive water to dilute the sodium aluminate solution and to ensure the stability of the decomposition system.

[0013] The overall synergistic effect described above makes oxalate accumulation controllable, completely preventing the chain reaction of negative impacts of oxalate accumulation on product quality and production efficiency.

[0014] Optionally, during the hot water countercurrent washing processes S1 and S2, the seed crystal washing system is subjected to ultrasonic treatment, with the ultrasonic frequency set to 20–30 kHz and the power density to 0.2–0.4 W / cm³. 2 The treatment method is to pause for 2 minutes after every 8 minutes of continuous ultrasound.

[0015] By adopting the above technical solution, the cavitation effect generated by ultrasound can destroy the liquid film boundary layer on the surface of aluminum hydroxide, and accelerate the diffusion rate of oxalate from the surface of aluminum hydroxide into hot water. By utilizing the 2-minute window of ultrasound pause to coordinate with the increase in stirring rate, the oxalate that has detached from the seed crystal surface can be diffused into the bulk solution in a timely manner, avoiding the re-adsorption process caused by excessive local concentration, and ensuring the activity of the seed crystals returning to the decomposition process.

[0016] Optionally, in step S2, the primary washing waste liquid and the secondary washing waste liquid are combined to obtain a seed washing waste liquid rich in oxalate. The hot water used in S1 and S2 is the treated seed crystal washing waste liquid, and the treatment of the seed crystal washing waste liquid includes the following steps: Targeted causticization: By detecting the concentration of oxalate in the seed crystal washing waste liquid, calcium hydroxide is added to the oxalate-rich seed crystal washing waste liquid. The molar ratio of the amount of calcium hydroxide added to sodium oxalate in the washing waste liquid is (1.05~1.1):1. The reaction temperature is controlled at 40~50℃ and the stirring rate is 100~120r / min, so that the sodium oxalate in the washing waste liquid is converted into calcium oxalate precipitate. Precipitation separation: The causticized slurry is subjected to sedimentation separation to obtain an alkaline supernatant and calcium oxalate precipitate; Supernatant treatment: The alkaline supernatant is filtered through a ceramic membrane with a pore size of 1 μm to remove suspended impurities. The concentration of oxalate in the supernatant after filtration is detected to ensure that the concentration is ≤0.02 g / L. The supernatant is then heated to 96±3℃ to obtain the final product.

[0017] By adopting the above technical solution, the separated alkaline supernatant is mainly composed of NaOH solution. The alkaline environment can improve the solubility of sodium oxalate. At 25°C, the solubility of sodium oxalate in water with a pH of 9 is about 20% higher than that in neutral water. The higher the temperature, the more significant the increase in solubility. Combined with ultrasound and stirring, the oxalate removal rate is further improved. This not only greatly reduces the consumption of fresh hot water and maintains the water balance of the system, but also allows the recovered alkali to be reused in the system, avoiding alkali loss.

[0018] Overall, the alkaline environment provided by the sodium hydroxide used comes from the internal circulation within the process, requiring no additional purchase and without introducing new impurities. This achieves a further closed loop of water and alkali within the system, reducing fresh water consumption and wastewater discharge, making the process greener and more environmentally friendly.

[0019] Optionally, in the directional causticization step, the added calcium hydroxide is a modified calcium hydroxide composite, which includes calcium hydroxide particles and nano-hydroxyapatite nanosheets loaded on its surface, wherein the mass ratio of calcium hydroxide to nano-hydroxyapatite nanosheets is 19:1 to 32:1. The modified calcium hydroxide composite is prepared by mixing calcium hydroxide and nano-hydroxyapatite nanosheets according to the above mass ratio, adding deionized water to prepare a 15% mass fraction mixed slurry, ultrasonically treating it for 15-20 minutes at an ultrasonic frequency of 25-30 kHz and a power density of 0.3-0.4 W / cm², and then filtering and drying it.

[0020] By adopting the above technical solution, calcium hydroxide is modified with nano-hydroxyapatite. The nanosheets form a two-dimensional reaction interface, which increases the contact area with sodium oxalate and accelerates the binding rate of sodium oxalate and calcium ions. At the same time, the porous structure of hydroxyapatite can adsorb some tiny calcium oxalate particles, thereby improving the precipitation and separation efficiency.

[0021] Optionally, in the directional causticization step, polyacrylamide is added to the causticized slurry, wherein the mass fraction of the polyacrylamide is 0.005% of the total mass of the causticized slurry and the molecular weight is 8 million Da; after adding the polyacrylamide, the mixture is stirred for 1 to 2 minutes, and then proceeds to the precipitation separation step.

[0022] By adopting the above technical solution, the directional causticization reaction completely removes oxalate by converting soluble sodium oxalate into insoluble calcium oxalate, blocking the circulation and accumulation in the system, fundamentally cutting off the impurity circulation path, and reducing the occurrence of scaling in the decomposition tank and pipelines; the caustic alkali generated by the reaction, mainly NaOH, can be recovered into the supernatant, reducing the system's alkali consumption.

[0023] Polyacrylamide, a coagulant aid, is used to promote the aggregation of fine calcium oxalate particles, thereby improving sedimentation efficiency and avoiding the reuse of the alkaline supernatant obtained after precipitation separation in the seed washing or leaching process, which would cause secondary recycling of oxalate and reduce removal efficiency.

[0024] Optionally, the hot water used in S1 and S2 is desalinated soft water that has been softened, wherein 0.05 to 0.1 wt% of dodecyl dimethyl betaine has been added to the desalinated soft water; wherein the total content of calcium and magnesium ions in the softened desalinated soft water is ≤5 mg / L.

[0025] By adopting the above technical solution, a hydrophobic layer is formed on the surface of aluminum hydroxide seed crystals due to the adsorption of sodium oxalate, which is difficult for traditional hot water to fully penetrate. The hydrophobic groups of the surfactant will combine with the sodium oxalate on the seed crystal surface, while the hydrophilic groups will face the washing liquid, breaking the adsorption force between sodium oxalate and seed crystals and promoting the rapid dissolution of sodium oxalate into hot water.

[0026] The added betaine, as an amphoteric surfactant, combined with ultrasound assistance, reduces the solid-liquid interfacial tension and promotes the desorption of oxalate from the surface of aluminum hydroxide seed crystals. Ultrasonic vibration can generate a micro-jet effect, which enhances the dissolution of oxalate in the seed crystal pores by the washing solution. In addition, the use of softened desalinated water can prevent the introduction of calcium and magnesium ions and avoid the formation of new impurity precipitates.

[0027] Using dodecyl dimethyl betaine ensures that no new impurities are introduced while removing impurities. On the one hand, it will not chemically react with the Al-OH groups on the surface of aluminum hydroxide seed crystals, avoiding surface corrosion or crystal structure damage to the aluminum hydroxide seed crystals and ensuring the decomposition activity of the seed crystals when used for subsequent decomposition of sodium aluminate solution. On the other hand, it will not introduce impurities itself, thus avoiding the formation of new precipitates such as calcium carbonate during the subsequent causticization of washing waste liquid.

[0028] Optionally, after obtaining the purified aluminum hydroxide seed crystals in S2 and before drying, a cold water spray washing step is also included: the purified aluminum hydroxide seed crystals are sprayed and washed with cold water at 25-30°C; the amount of cold water used is 0.8-1 times the total mass of the mixed system composed of the seed crystal filter cake and residual washing water after hot water washing, and the washing time is 15-20 minutes. During the cold water washing process, the spray pressure is controlled at 0.2–0.3 MPa; After being washed with cold water, the aluminum hydroxide seed crystals that have been further purified are separated and then dried before being returned to the decomposition process for use as seed crystals.

[0029] By adopting the above technical solution, after dissolving sodium oxalate in hot water, the surface of the aluminum hydroxide seed filter cake after washing will be covered with a hot aqueous solution containing sodium oxalate. When the filter cake cools down naturally, the sodium oxalate dissolved in the hot water will re-precipitate due to the decrease in solubility and re-adhere to the surface of the filter cake, resulting in incomplete removal of sodium oxalate. When washing with hot and cold water in combination, the cold water can quickly rinse away the residual hot aqueous solution containing sodium oxalate on the surface of the filter cake, preventing the sodium oxalate from re-precipitating and adhering to the filter cake after cooling, thus reducing the back mixing of impurities from the source.

[0030] Optionally, the hot water in S1 and S2 with a temperature of 96±3℃ is heated by the last-effect secondary steam from the evaporation process. The heat source is used by directly introducing the last-effect secondary steam into a dedicated hot water preparation tank to mix and heat the washing water to be heated, or by indirectly exchanging heat with the washing water to be heated through a plate heat exchanger.

[0031] By adopting the above technical solution, the secondary steam at the end of the evaporation process, which is usually 90-100℃, is a typical waste heat resource. Direct discharge would cause energy waste. Using it to heat the washing water can achieve the dual benefits of waste heat recovery and energy consumption reduction.

[0032] Optionally, the hot water used in S1 and S2 to heat to 96±3℃ is heated by a dilution tank of high-pressure leaching slurry. The hot water is heated by introducing the washing water to be heated into the dilution tank through a heat exchange pipe, preheating it with the heat of the high-pressure leaching slurry in the dilution tank, and then heating it to 96±3℃ with auxiliary steam.

[0033] By adopting the above technical solutions, the combination of dilution tank preheating and auxiliary steam can further achieve the goals of deep energy saving and system thermal balance optimization, fully utilize waste heat resources, and realize energy saving and cost reduction.

[0034] Optionally, the aluminum hydroxide seed crystals in S2 are dried by using a gas bath to dehydrate and dry them with a carbon dioxide-nitrogen mixture with a volume ratio of 1:9; after the gas bath dehydration and drying is completed, the aluminum hydroxide seed crystals are purged with nitrogen at room temperature for 30-40 seconds.

[0035] By adopting the above technical solution, the weak acidity of carbon dioxide can be used to neutralize the trace amounts of alkali remaining on the seed crystal surface, reducing pH interference to subsequent decomposition processes. The use of an inert nitrogen gas bath can reduce the formation of hydrogen bonds from the dehydration of hydroxyl groups on the surface of aluminum hydroxide seed crystals at high temperatures, further reducing secondary agglomeration and ensuring the dispersibility of the seed crystals. In addition, the nitrogen atmosphere isolates oxygen, preventing oxidation and deterioration of the seed crystal surface at high temperatures and protecting the decomposition activity of the aluminum hydroxide seed crystals. After drying, the seed crystals are gently blown with room temperature nitrogen for 30 seconds to remove any trace amounts of dust that may be attached to the surface, avoiding impurity residue.

[0036] In summary, the present invention has at least one of the following beneficial technical effects: 1. By using countercurrent washing with hot water at 93-99℃, efficient dissolution of oxalate is ensured, and the uniformity of purification is improved. Innovative high-temperature precision washing of aluminum hydroxide seeds is carried out to ensure that the aluminum hydroxide seeds are free of oxalate before decomposition, thereby improving the activity of the seeds. The seeds are mainly used in the decomposition process. The combination of hot / cold water washing of the seeds with causticization reaction completely prevents the chain of negative impacts of oxalate accumulation on product quality and production efficiency. The entire process has zero wastewater discharge, which contributes to the construction of a green factory.

[0037] 2. The cavitation effect generated by ultrasound can disrupt the liquid film boundary layer on the surface of aluminum hydroxide, accelerate the diffusion rate of oxalate from the surface of aluminum hydroxide into hot water, and shorten the washing time; the 2-minute window of ultrasonic pause is used to increase the stirring rate and ensure the activity of the seed crystals returning to the decomposition process.

[0038] 3. By adding dodecyl dimethyl betaine and combining it with ultrasound assistance, the dissolution of oxalate in the seed crystal pores by the washing solution is enhanced; the alkaline supernatant separated by precipitation is used in the seed crystal washing step, and the alkaline environment can improve the solubility of sodium oxalate, further improving the oxalate removal rate; at the same time, it greatly reduces the consumption of fresh hot water, maintains the water balance of the system, and can also reuse the recovered alkali in the system, avoiding alkali loss.

[0039] 4. By using nano-hydroxyapatite to modify calcium hydroxide, the nanosheets form a two-dimensional reaction interface, increasing the contact area with sodium oxalate and accelerating the binding rate of sodium oxalate and calcium ions. At the same time, the porous structure of hydroxyapatite can adsorb some tiny calcium oxalate particles, improving the precipitation separation efficiency. Attached Figure Description

[0040] Figure 1 This is a flow chart of the deep purification process for aluminum hydroxide seed crystals provided in Embodiment 1 of the present invention. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the embodiments.

[0042] Unless otherwise specified, the experimental methods used in the embodiments of this application are conventional methods, and the materials used are commercially available unless otherwise specified.

[0043] Example 1: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0044] A deep purification process for aluminum hydroxide seeds in alumina production, as shown in the attached figure. Figure 1 As shown, the process comprises three core stages: primary hot water washing, secondary hot water washing, and drying. Specifically, it includes the following steps: 1) Raw material preparation: Aluminum hydroxide seed filter cake after vertical plate filtration in Bayer process is used as initial raw material. In order to verify the technical effect uniformly, analytical grade oxalate reagent is added in this embodiment to stabilize the initial oxalate adhering amount in the filter cake at 1.8 g / kg (based on the cake mass of the filter cake).

[0045] 2) S1: First-stage hot water washing: The aluminum hydroxide seed filter cake containing oxalate impurities is washed countercurrently with hot water at a temperature of 96°C. The washing conditions are: liquid-solid ratio 4:1, stirring rate initially set to 130 r / min, stirring rate increased by 20% relative to the initial set rate every 8 min, stirring for 2 min and then returned to the initial set rate, for a duration of 20 min, to obtain the preliminarily washed aluminum hydroxide seed filter cake and first-stage washing waste liquid.

[0046] 3) S2: Secondary hot water washing and stop determination: The aluminum hydroxide seed filter cake after preliminary washing is further washed countercurrently with hot water at 96℃, under the same washing conditions as S1; the oxalate concentration in the secondary washing waste liquid is detected every 2 minutes using ion chromatography. When the oxalate concentration in the waste liquid drops to 0.08 g / L, washing is stopped, and the net weight filter cake is obtained; the obtained aluminum hydroxide seed crystals are dried at 120℃ and a wind speed of 1.5 m / s until the seed crystal moisture content is less than 1.5%. The dried aluminum hydroxide seed crystals are returned to the decomposition process as seed crystals to participate in the decomposition reaction of sodium aluminate solution.

[0047] In other embodiments, the hot water in S1 and S2, which has a temperature of 96°C, is heated by the secondary steam from the final effect of the evaporation process in the alumina production process. The secondary steam has a temperature of 98°C and is indirectly heated by the washing water to be heated through a plate heat exchanger, thereby raising the temperature of the washing water to 96°C and achieving a steam utilization rate of over 85%.

[0048] In other embodiments, the hot water in S1 and S2, which has a temperature of 96°C, is heated by a high-pressure leaching slurry dilution tank. The washing water to be heated is introduced into the high-pressure leaching slurry dilution tank through a heat exchange pipe, preheated to 85°C by the heat of the slurry in the tank, and then heated to 96°C by auxiliary steam.

[0049] The main testing indicators are the residual oxalate content in the seed crystals and the seed crystal decomposition activity.

[0050] ① Oxalate residue (g / kg): Used to quantify the removal effect of impurities on the surface and in pores of aluminum hydroxide seed crystals, and is the core indicator for measuring the purification of aluminum hydroxide seed crystals.

[0051] The specific detection method is as follows: Take 5.0 g of dried aluminum hydroxide seed sample, add 50 mL of deionized water, place it in an ultrasonic cleaner, and extract ultrasonically for 30 min at a frequency of 25 kHz and a power of 300 W. Then transfer it to a centrifuge tube and centrifuge at 5000 r / min for 10 min. Take the supernatant and filter it through a 0.22 μm aqueous filter membrane. Detect it using an ion chromatograph. The mobile phase is a mixed solution of 3.5 mmol / L sodium carbonate and 1.0 mmol / L sodium bicarbonate, the flow rate is 1.0 mL / min, the column temperature is 30 ℃, and the injection volume is 20 μL. Plot a standard curve with sodium oxalate standard solution. Calculate the oxalate concentration in the supernatant based on the peak area of ​​the sample, and convert it to obtain the residual oxalate in the seed crystal.

[0052] ② Seed crystal decomposition activity (%): Directly related to the output of the decomposition process in alumina production. The higher the activity of aluminum hydroxide seed crystals, the higher the efficiency of aluminum hydroxide decomposition from sodium aluminate solution.

[0053] The specific detection method is as follows: Prepare 100 mL of sodium aluminate standard solution with an Al2O3 concentration of 120 g / L, a caustic ratio of 1.5, and a temperature of 60℃. Place the solution in a constant temperature water bath reactor with a stirrer, add 2.0 g of aluminum hydroxide seed crystals to be tested, control the stirring rate at 150 r / min and the reaction temperature at 60℃, and filter immediately after reacting at a constant temperature for 4 h. The Al2O3 concentration in the filtrate is determined by EDTA complexometric titration. The decomposition activity is calculated according to the formula: Decomposition activity = (initial Al2O3 concentration - Al2O3 concentration after reaction) / initial Al2O3 concentration × 100%. At the same time, the unwashed original aluminum hydroxide seed crystals are used as a blank control. The determination is performed in parallel for 3 times and the average value is taken.

[0054] In this application, the decomposition activity value of the unwashed original aluminum hydroxide seed crystals is 15%.

[0055] Example 2: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0056] Based on Example 1, this implementation adds ultrasonic synergistic washing treatment to the primary and secondary hot water washing stages to enhance the desorption and diffusion efficiency of oxalate. Specifically, it includes the following: S1: First-stage hot water washing: In addition to the parameters of Example 1, ultrasonic treatment was applied to the washing system. The ultrasonic frequency was set to 25kHz and the power density to 0.3W / cm². The treatment method was to pause for 2 minutes after every 8 minutes of continuous ultrasonic treatment, which was synchronized with the stirring speed change cycle. The washing was continued for 20 minutes to obtain the preliminary washed seed filter cake and the first-stage washing waste liquid.

[0057] S2: Secondary hot water washing and stop determination: Ultrasonic parameters are the same as in S1 of this embodiment.

[0058] Everything else is exactly the same as in Example 1.

[0059] Example 3: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0060] This embodiment, based on Embodiment 2, pre-treats the washing wastewater and reuses it as washing hot water to achieve the recycling of water and alkali. Specifically, the primary washing wastewater and the secondary washing wastewater are combined to obtain oxalate-rich seed crystal washing wastewater. The obtained seed crystal washing wastewater is then pre-treated, and the treatment steps include: Targeted causticization: By detecting the concentration of oxalate in the seed crystal washing waste liquid, calcium hydroxide is added to the oxalate-rich seed crystal washing waste liquid. The molar ratio of the amount of calcium hydroxide added to sodium oxalate in the washing waste liquid is 1.08:1. The reaction temperature is controlled at 45℃ and the stirring rate is 110r / min, so that the sodium oxalate in the washing waste liquid is converted into calcium oxalate precipitate.

[0061] Precipitation separation: The causticized slurry is subjected to sedimentation separation to obtain an alkaline supernatant and calcium oxalate precipitate.

[0062] Supernatant treatment: The alkaline supernatant is filtered through a ceramic membrane with a pore size of 1 μm to remove suspended impurities. The concentration of oxalate in the supernatant after filtration is detected to ensure that its concentration is ≤0.02 g / L. The supernatant is then heated to 96°C and used as washing hot water for S1 and S2.

[0063] Everything else is exactly the same as in Example 2.

[0064] Example 4: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0065] In this embodiment, the calcium hydroxide added in the directional causticization step mentioned in Example 3 is a modified calcium hydroxide composite. The modified calcium hydroxide composite includes calcium hydroxide particles and nano-hydroxyapatite nanosheets loaded on its surface, wherein the mass ratio of calcium hydroxide to nano-hydroxyapatite nanosheets is 25:1.

[0066] The modified calcium hydroxide composite is prepared by mixing calcium hydroxide and nano-hydroxyapatite nanosheets according to the above mass ratio, adding deionized water to prepare a 15% mass fraction mixed slurry, and then ultrasonicating at a frequency of 27 kHz and a power density of 0.35 W / cm². 2 The mixture was ultrasonically treated for 18 minutes under the specified conditions, filtered, and dried at 105℃ for 2 hours to obtain the modified calcium hydroxide composite.

[0067] Everything else is exactly the same as in Example 3.

[0068] Example 5: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0069] Based on Example 4, this embodiment adds polyacrylamide to the causticized slurry. The mass fraction of polyacrylamide is 0.005% of the total mass of the causticized slurry, and the molecular weight is 8 million Da. After adding polyacrylamide, the mixture is stirred for 1.5 min before proceeding to the precipitation separation step.

[0070] Everything else is exactly the same as in Example 4.

[0071] Example 6: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0072] In this embodiment, the hot water used in S1 and S2 is desalinated soft water that has been softened, with the total content of calcium and magnesium ions controlled to be ≤5mg / L. 0.08wt% of dodecyl dimethyl betaine is added to the softened desalinated soft water and heated to 96℃ as the washing hot water for S1 and S2.

[0073] Everything else is exactly the same as in Example 2.

[0074] Example 7: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0075] In this embodiment, after obtaining the purified aluminum hydroxide seed crystals in S2 and before drying, a cold water spray washing step is added: the purified aluminum hydroxide seed crystals are sprayed and washed with cold water at 28°C. The amount of cold water used is 0.9 times the total mass of the mixed system consisting of the seed crystal filter cake and residual washing water after hot water washing. The spray pressure is 0.25 MPa, and the washing time is 18 min. After cold water spray washing, the purified aluminum hydroxide seed crystals are obtained by filtration and separation. After drying, they are returned to the decomposition process for use as seed crystals.

[0076] Everything else is exactly the same as in Example 6.

[0077] Example 8: This example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0078] In this embodiment S2, the aluminum hydroxide seed crystals are dried by gas bath dehydration drying with a carbon dioxide-nitrogen mixed gas with a volume ratio of 1:9 at a temperature of 120°C and a gas flow rate of 1.5 m / s. After drying, the seeds are purged with nitrogen at room temperature for 35 seconds to obtain dried seed crystals, which are then returned to the decomposition process.

[0079] Everything else is exactly the same as in Example 7.

[0080] The deep purification process for aluminum hydroxide seed crystals described in Examples 1-8 was used respectively, and the key performance indicators were tested. The test results are shown in Table 1: Table 1: serial number Oxalate residue (g / kg) Seed decomposition activity (%) Example 1 0.083 22.4 Example 2 0.058 31.1 Example 3 0.056 30.8 Example 4 0.047 32.5 Example 5 0.039 32.7 Example 6 0.021 36.3 Example 7 0.012 41.3 Example 8 0.011 43.2 As can be seen from the data in Example 1, Example 1 is a basic process of primary and secondary hot water washing and conventional drying. By using variable speed stirring to wash the surface of the seed crystal, the weakly bound oxalate in the seed crystal pores can be initially broken.

[0081] By comparing Example 2 with Example 1, it can be seen that Example 2, by incorporating an ultrasonic synergistic treatment process, utilizes the cavitation effect generated by ultrasonic treatment to break the liquid film boundary layer on the surface of the seed crystal. Simultaneously, through micro-jet action, it penetrates into the pores of the seed crystal, accelerating the diffusion of oxalate into the hot water and achieving a deep purification effect. The reduced amount of oxalate residue attached to the seed crystal will cause the aluminum hydroxide seed crystal to release more active sites, ultimately achieving a significant improvement in seed crystal activity. This directly proves that reducing oxalate residue is a key method to improve seed crystal activity.

[0082] A comparison between Example 3 and Example 2 shows that Example 3, while maintaining the advantages of seed purification effect and decomposition activity in Example 2, achieves a breakthrough in water resource utilization and environmental protection. This indicates that the recycled pre-treated washing waste liquid used in Example 3 does not introduce impurities that interfere with the active sites of the seed crystals. Although the trace amount of NaOH remaining in the waste liquid can theoretically increase the solubility of sodium oxalate, the trace amount of oxalate may remain in the supernatant alkaline solution after causticization treatment, which to some extent offsets the solubility gain of the alkaline solution, resulting in a small difference in values.

[0083] In addition, in terms of fresh water consumption, Example 3 reduces it by 90% compared to Example 2, which not only achieves a significant saving of water resources, but also reduces wastewater discharge through closed-loop recycling of waste liquid, which is more in line with the industrial production demand for low cost and low pollution.

[0084] By comparing Example 4 with Example 3, it can be seen that Example 4 replaces the calcium hydroxide in the directional causticization step of Example 3 with a modified composite of calcium hydroxide-nanohydroxyapatite nanosheets. The nanosheets of the modified composite can form a two-dimensional reaction interface, which significantly increases the contact area with sodium oxalate in the washing waste liquid and accelerates the binding rate of calcium ions and oxalate ions. At the same time, the porous structure of hydroxyapatite can adsorb some tiny calcium oxalate particles, improve the precipitation separation efficiency, and thus reduce the oxalate residue in the recycled washing water, making the washing process more effective at eluting bound oxalate in the seed pores.

[0085] By comparing Example 5 with Example 4, it can be seen that Example 5, through the coagulation aid step of adding 0.005wt% polyacrylamide to the causticized slurry, achieved the goal of low oxalate residue in the recycled washing water and slight optimization of the oxalate residue in the seed crystals. This result confirms that polyacrylamide can effectively promote the aggregation of tiny calcium oxalate particles generated by the causticization reaction, improve precipitation separation efficiency, reduce secondary pollution of the seed crystals by suspended calcium oxalate particles in the supernatant with the recycled washing water, further consolidate the purification stability of waste liquid recycling, and at the same time retain the advantages of low water consumption and high activity in Example 4.

[0086] By comparing Example 6 with Example 2, it can be seen that Example 6, based on the ultrasonic synergistic variable speed stirring of Example 2, adds 0.08wt% dodecyl dimethyl betaine to soften and desalinate the water, which improves both the residual amount of oxalate and the seed crystal decomposition activity, indicating that it enhances the dissolution and removal of bound oxalate in the seed crystal pores; the improved seed crystal decomposition activity indicates that the surfactant does not react with the Al-OH groups on the seed crystal surface and does not damage the crystal structure.

[0087] By comparing Example 7 with Example 6, it can be seen that Example 7 uses a combination of hot and cold washing. Cold water spray quickly washes away the oxalate-containing solution remaining on the surface of the seed filter cake after hot water washing, avoiding the re-precipitation and re-attachment of oxalate due to decreased solubility during the cooling process of the filter cake, thus avoiding the risk of secondary pollution. The improved seed activity indicates that the cold water spray only acts on the residual liquid on the surface of the filter cake and does not damage the crystal structure and surface active sites of the seed, so that the active sites released by oxalate retain their normal decomposition activity.

[0088] By comparing Example 8 with Example 7, it can be seen that Example 8 uses a carbon dioxide-nitrogen mixed gas bath for dehydration and drying with a volume ratio of 1:9 and nitrogen purging at room temperature. The carbon dioxide, being weakly acidic, neutralizes the trace amounts of alkali remaining on the seed surface, avoiding interference with the pH of the decomposition process. The inert atmosphere of nitrogen reduces the dehydration and agglomeration of hydroxyl groups in the seed at high temperatures and oxidative degradation. This achieves the goal of further releasing the active sites for seed growth and improving decomposition activity without reducing the purification effect of oxalate.

[0089] In addition, since the causticization step is added in Examples 3-5, the oxalate removal rate of the waste liquid after causticization and the alkali content of the supernatant are added to the existing detection indicators.

[0090] Oxalate removal rate (%) of causticized waste liquid: refers to the proportion of oxalate removed from the waste liquid after the causticization reaction. The calculation formula is: (oxalate concentration of waste liquid before causticization - oxalate concentration of waste liquid after causticization) / oxalate concentration of waste liquid before causticization × 100%, which reflects the causticization step's effect on oxalate removal.

[0091] Alkali content in supernatant (g / L, as NaOH): refers to the concentration of sodium hydroxide in the supernatant after precipitation separation. It reflects the recovery efficiency of the alkali generated by the causticization reaction and is related to the auxiliary effect of subsequent hot water reuse for seed washing. An alkaline environment increases the solubility of oxalate.

[0092] The test results are shown in Table 2: Table 2: Indicator Name Example 3 Example 4 Example 5 Oxalate removal rate (%) in causticization waste liquid 96.8 98.7 99.1 Alkali content of supernatant (g / L) 2.1 2.2 2.4 Example 4 replaces conventional calcium hydroxide with a calcium hydroxide-nanohydroxyapatite complex, which accelerates the binding rate of calcium ions and oxalate ions, making the causticization reaction more thorough, and thus converting more sodium oxalate into sodium hydroxide.

[0093] In Example 5, 0.005 wt% polyacrylamide coagulant was added to the causticized slurry. Its polymer chains agglomerated the tiny calcium oxalate particles into larger particles through bridging, which accelerated the precipitation and further improved the removal rate of oxalate. In Example 4, the unagglomerated tiny calcium oxalate particles adsorbed a small amount of supernatant containing NaOH, which caused alkali entrainment loss during precipitation and separation.

[0094] Comparative Example 1: This comparative example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0095] In the seed washing of S1 and S2 in this comparative example, hot water at a temperature of 80℃ was used, which is a temperature commonly used in traditional processes.

[0096] Everything else is exactly the same as in Example 1.

[0097] Comparative Example 2: This comparative example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0098] In this comparative example, the aluminum hydroxide seed crystals were washed using a constant stirring rate of 130 r / min, and the other parameters were the same as in Example 1.

[0099] Everything else is exactly the same as in Example 1.

[0100] Comparative Example 3: This comparative example discloses a deep purification process for aluminum hydroxide seeds in alumina production.

[0101] In the comparative examples S1 and S2, the aluminum hydroxide seed crystals were washed with hot water at a temperature of 80°C; at the same time, a constant stirring rate of 130 r / min was used, and the other parameters were the same as in Example 1.

[0102] Everything else is exactly the same as in Example 1.

[0103] The aluminum hydroxide seed deep purification process described in Comparative Examples 1-3 was used respectively, and the key performance indicators were tested. The test results are shown in Table 3: Table 3: serial number Oxalate residue (g / kg) Seed decomposition activity (%) Comparative Example 1 0.131 15.3 Comparative Example 2 0.184 11.6 Comparative Example 3 0.216 7.7 By comparing Comparative Example 1 with Example 1, it can be seen that the decrease in hot water temperature in Comparative Example 1 leads to a decrease in oxalate solubility, resulting in an increase in the amount of oxalate residue. At the same time, the higher oxalate residue occupies more growth active sites in the seed crystals, which significantly reduces the decomposition activity of the seed crystals in Comparative Example 1.

[0104] By comparing Comparative Example 2 with Example 1, it can be seen that the constant stirring rate in Comparative Example 2 resulted in a higher residual amount of oxalate and a lower seed decomposition activity compared to Example 1. This data difference indicates that the variable speed stirring in Example 1, by periodically increasing the speed, can efficiently flush the high-concentration oxalate liquid film on the seed surface and avoid the residue of bound oxalate, while the constant stirring in Comparative Example 2 cannot overcome the liquid film resistance.

[0105] By comparing Comparative Example 3 with Example 1, the residual oxalate content in Comparative Example 3 was higher than that in Example 1, and the seed crystal decomposition activity was lower than that in Example 1. This indicates that Example 1 effectively solved the problems of low low-temperature dissolution efficiency, poor uniformity of constant stirring and washing, and inadequate cleaning of oxalate in pores in Comparative Example 3 by increasing the solubility of oxalate at high temperature and breaking the high-concentration oxalate liquid film on the seed crystal surface through variable speed stirring.

[0106] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A deep purification process for aluminum hydroxide seeds in alumina production, characterized in that, Includes the following steps: S1: First-stage hot water washing: The aluminum hydroxide seed filter cake containing oxalate impurities is washed countercurrently with hot water at a temperature of 96±3℃, with the liquid-solid ratio controlled at (3~5):

1. The stirring rate is initially set to 120~140r / min, and is increased by 20% relative to the initial setting rate every 8min. After stirring for 2min, the initial setting rate is restored. The washing time is 10~30min. S2: Secondary hot water washing and stop determination: The aluminum hydroxide seed filter cake after the initial washing in S1 is further washed countercurrently with hot water at a temperature of 96±3℃, maintaining the same liquid-solid ratio, initial setting value of stirring rate and speed change rule as in S1; the oxalate concentration in the secondary washing waste liquid is monitored in real time, and when the concentration is ≤0.1g / L, washing is stopped, and the obtained purified aluminum hydroxide seed is dried.

2. The deep purification process for aluminum hydroxide seeds in alumina production according to claim 1, characterized in that, During the hot water countercurrent washing processes S1 and S2, the seed crystal washing system was subjected to ultrasonic treatment. The ultrasonic frequency was set to 20–30 kHz, and the power density was 0.2–0.4 W / cm³. 2 The treatment method is to pause for 2 minutes after every 8 minutes of continuous ultrasound.

3. The deep purification process for aluminum hydroxide seeds in alumina production according to claim 2, characterized in that, In step S2, the primary washing waste liquid and the secondary washing waste liquid are combined to obtain a seed washing waste liquid rich in oxalate. The hot water used in S1 and S2 is the treated seed crystal washing waste liquid, and the treatment of the seed crystal washing waste liquid includes the following steps: Targeted causticization: By detecting the concentration of oxalate in the seed crystal washing waste liquid, calcium hydroxide is added to the oxalate-rich seed crystal washing waste liquid. The molar ratio of the amount of calcium hydroxide added to sodium oxalate in the washing waste liquid is (1.05~1.1):

1. The reaction temperature is controlled at 40~50℃ and the stirring rate is 100~120r / min, so that the sodium oxalate in the washing waste liquid is converted into calcium oxalate precipitate. Precipitation separation: The causticized slurry is subjected to sedimentation separation to obtain an alkaline supernatant and calcium oxalate precipitate; Supernatant treatment: The alkaline supernatant is filtered through a ceramic membrane with a pore size of 1 μm to remove suspended impurities. The concentration of oxalate in the supernatant after filtration is detected to ensure that the concentration is ≤0.02 g / L. The supernatant is then heated to 96±3℃ to obtain the final product.

4. The deep purification process for aluminum hydroxide seeds in alumina production according to claim 3, characterized in that, In the directional causticization step, the added calcium hydroxide is a modified calcium hydroxide complex, which includes calcium hydroxide particles and nano-hydroxyapatite nanosheets loaded on its surface, wherein the mass ratio of calcium hydroxide to nano-hydroxyapatite nanosheets is 19:1 to 32:

1. The modified calcium hydroxide composite is prepared by mixing calcium hydroxide and nano-hydroxyapatite nanosheets according to the above mass ratio, adding deionized water to prepare a 15% mass fraction mixed slurry, ultrasonically treating it for 15-20 minutes at an ultrasonic frequency of 25-30 kHz and a power density of 0.3-0.4 W / cm², and then filtering and drying it.

5. The deep purification process for aluminum hydroxide seeds in alumina production according to claim 3, characterized in that, In the directional causticization step, polyacrylamide is added to the causticized slurry. The mass fraction of the polyacrylamide is 0.005% of the total mass of the causticized slurry, and the molecular weight is 8 million Da. After adding the polyacrylamide, the mixture is stirred for 1 to 2 minutes before proceeding to the precipitation separation step.

6. The deep purification process for aluminum hydroxide seeds in alumina production according to claim 2, characterized in that, The hot water used in S1 and S2 is desalinated soft water that has been softened. The desalinated soft water contains 0.05 to 0.1 wt% dodecyl dimethyl betaine. The total content of calcium and magnesium ions in the softened desalinated soft water is ≤5 mg / L.

7. A deep purification process for aluminum hydroxide seeds in alumina production according to any one of claims 1-6, characterized in that, After obtaining the purified aluminum hydroxide seed crystals in S2 and before drying, a cold water spray washing step is also included: the purified aluminum hydroxide seed crystals are sprayed and washed with cold water at 25-30°C; the amount of cold water used is 0.8-1 times the total mass of the mixed system composed of the seed crystal filter cake and residual washing water after hot water washing, and the washing time is 15-20 minutes. During the cold water washing process, the spray pressure is controlled at 0.2–0.3 MPa; After being washed with cold water, the aluminum hydroxide seed crystals that have been further purified are separated and then dried before being returned to the decomposition process for use as seed crystals.

8. A deep purification process for aluminum hydroxide seeds in alumina production according to any one of claims 1-6, characterized in that, The hot water in S1 and S2 has a temperature of 96±3℃, and its heat source is the final-effect secondary steam of the evaporation process. The heat source is utilized in the following ways: the final-effect secondary steam is directly introduced into a dedicated hot water preparation tank and mixed with the washing water to be heated, or the heat is indirectly exchanged with the washing water to be heated through a plate heat exchanger.

9. A deep purification process for aluminum hydroxide seeds in alumina production according to any one of claims 1-6, characterized in that, The hot water used in S1 and S2 to heat to 96±3℃ is heated by a dilution tank of high-pressure leaching slurry. The hot water is heated by introducing the washing water to be heated into the dilution tank through a heat exchange pipe. After preheating by the heat from the high-pressure dissolving slurry in the dilution tank, it is then heated to 96±3℃ by auxiliary steam.

10. A deep purification process for aluminum hydroxide seeds in alumina production according to any one of claims 1-6, characterized in that, The drying method for aluminum hydroxide seeds in S2 is as follows: gas bath dehydration drying is performed using a carbon dioxide-nitrogen mixed gas with a volume ratio of 1:9; after the gas bath dehydration drying is completed, the aluminum hydroxide seeds are purged with nitrogen at room temperature for 30-40 seconds.

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