A method for preparing high-purity alumina using kaolin

Abstract

This invention provides a method for preparing high-purity alumina using kaolin. The method comprises the following steps: S1: Pre-treating kaolin to obtain pre-treated kaolin; S2: Adding hydrochloric acid solution to the pre-treated kaolin for acid purification, followed by activation to obtain activated kaolin; S3: Adding hydrochloric acid solution to the activated kaolin for acid dissolution, followed by concentration and sedimentation to obtain crude aluminum chloride solution and silica filter cake; S4: Purifying the crude aluminum chloride solution to obtain a high-purity aluminum chloride solution; S5: Preparing high-purity alumina using the high-purity aluminum chloride solution. This invention can reduce the production cost of high-purity alumina, and the obtained high-purity alumina has a purity ≥99.9%, which is beneficial for expanding the industrial application of high-purity alumina.

Description

Technical Field

[0001] This invention relates to the field of alumina preparation technology, and in particular to a method for preparing high-purity alumina using kaolin. Background Technology

[0002] The Bayer process is a widely used industrial chemical process for producing alumina from bauxite. The basic principle is to use a concentrated sodium hydroxide solution to convert aluminum hydroxide into sodium aluminate, and then dilute and add aluminum hydroxide seed crystals to allow aluminum hydroxide to precipitate again. The remaining sodium hydroxide solution is reused to process the next batch of bauxite, thus achieving continuous production.

[0003] High-purity alumina (purity ≥ 99.9%) is a key inorganic functional material widely used in high-end fields such as sapphire substrates, lithium-ion battery separator coatings, LED phosphors, and integrated circuit substrates. However, the traditional Bayer process has high production costs and only achieves an alumina purity of 99.8%, which is insufficient to meet the application requirements of high-end fields. Therefore, there is an urgent need to develop a high-purity alumina preparation technology with lower costs and higher alumina purity.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-purity alumina using kaolin. This method can reduce the production cost of high-purity alumina and produce high-purity alumina with a purity of ≥99.99%, which is beneficial for expanding the industrial application of high-purity alumina.

[0006] This invention provides a method for preparing high-purity alumina using kaolin, comprising the following steps: S1: Kaolin is pretreated to obtain pretreated kaolin; S2: Add hydrochloric acid solution to the pretreated kaolin for acid purification, and then activate it to obtain activated kaolin. S3: Add hydrochloric acid solution to activated kaolin for acid dissolution. After acid dissolution, concentrate and settle to obtain crude aluminum chloride solution and silica mud filter cake. S4: Purify the crude aluminum chloride solution to obtain a high-purity aluminum chloride solution; S5: High-purity aluminum oxide is prepared using high-purity aluminum chloride solution.

[0007] In step S1, the pretreatment includes: adding water to kaolin to crush and slurry, dispersing and screening the slurry at high speed, and performing hydraulic classification, mineral processing and iron removal on the screened slurry.

[0008] In step S2, the concentration of the hydrochloric acid solution is 25-35%; the temperature for acid purification is 80-110 ℃, and the time is 4-6 h. Activation includes: first heating the acid-purified kaolin to above 400 ℃ using a microwave tunnel kiln, and then using plasma gas to decompose the kaolin into hydroxyl groups.

[0009] In step S3, the concentration of the hydrochloric acid solution is 25-35%; the temperature for acid dissolution is 100-110 ℃, and the time is 5-7 h; furthermore, the silica mud filter cake can be dried to prepare silicon-containing products.

[0010] In step S4, purification includes: filtering and adsorbing the crude aluminum chloride solution, followed by oxidation with a strong oxidant, and then filtering, adsorbing, ultrafiltration, electrolysis, and ion exchange on the oxidized crude aluminum chloride solution. Further, quartz sand can be used for filtration, activated carbon for adsorption, and the pore size of the ultrafiltration membrane can be 10-100 nm; the strong oxidant can be hydrogen peroxide, etc. In addition, ion exchange can be performed sequentially using an iminodiacetic acid-type chelating resin (e.g., TP207 resin) and an aminomethylphosphonic acid-type chelating resin (e.g., TP260 resin).

[0011] In one embodiment, step S5 includes: adding ammonia water to a high-purity aluminum chloride solution to react, evaporating the reaction product to obtain an ammonium chloride solution and an aluminum hydroxide slurry, drying and calcining the aluminum hydroxide slurry to obtain high-purity alumina; wherein the calcination temperature is 900-1100 ℃, the calcination time is 5-10 min, and the purity of the high-purity alumina is above 99.9%.

[0012] In another embodiment, step S5 includes: heating a high-purity aluminum chloride solution to 95-105 °C until saturated by evaporation, followed by cooling and crystallization to obtain aluminum chloride crystals; drying and calcining the aluminum chloride crystals to obtain high-purity alumina. Further, the calcination includes sequential primary calcination, secondary calcination, and tertiary calcination; wherein the primary calcination temperature is 200-400 °C for 10-20 min; the secondary calcination temperature is 700-900 °C for 15-25 min; the tertiary calcination temperature is 1050-1150 °C for 15-25 min; and the purity of the high-purity alumina is 99.99% or higher.

[0013] The method of this invention can reduce the production cost of high-purity alumina, and the purity of the obtained high-purity alumina can reach more than 99.9%, which is conducive to expanding the industrial application of high-purity alumina. Attached Figure Description

[0014] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of the kaolin acid purification process. Figure 2 This is a schematic diagram of the aluminum chloride extraction process; Figure 3 This is a schematic diagram of the aluminum chloride solution purification process; Figure 4 Schematic diagram of the process for preparing aluminum hydroxide and aluminum oxide by reducing ammonia water; Figure 5 Schematic diagram of the process for preparing alumina by evaporation and cold crystallization; Figure 6 A schematic diagram of the process for preparing high-purity alumina by plasma catalysis. Detailed Implementation

[0016] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0017] It should be noted that the terminology used herein is for the purpose of describing particular implementations only and is not intended to limit the exemplary implementations according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this description, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1 This embodiment describes a method for preparing high-purity alumina using kaolin, and the steps are as follows: 1. Pretreatment Kaolin ore is placed in a box crusher, water is added and it is crushed into a slurry. The slurry is then injected into a high-speed disperser for high-speed mixing and dispersion. Under the high-speed mixing of the impeller, the un-powdered particles in the slurry are further sheared into fine particles.

[0020] After being dispersed by high-speed stirring, the kaolin slurry is then injected into a probability water screen for screening. The larger particles on the screen are mostly non-kaolin particles (such as rutile, ilmenite, quartz sand, etc.). The larger particles on the screen enter the wastewater treatment process under hydraulic action. Finally, the slurry is settled in a deep cone thickener and dewatered by plate and frame filter press. The filter cake is treated as general solid waste, and the clean water produced by plate and frame filter press and deep cone thickener is recycled.

[0021] The slurry after screening still contains large kaolin particles, which may contain ilmenite, pyrite, and associated spinel, mullite, feldspar, and other particles, requiring further separation. Therefore, the slurry is hydraulically classified. Under hydraulic action, most of the fine particles are preferentially separated, while the large mineral particles at the bottom of the hydraulic classification settle and enter the wastewater treatment process, followed by filter press dewatering.

[0022] After hydraulic classification, the slurry is injected into the primary and secondary spiral sluices for mineral processing. During the spiral sluice mineral processing, smaller particles of pyrite, ilmenite, and calcium magnesium are stripped out of the kaolin slurry. Most of the high-purity kaolin slurry enters the next magnetic separation process for final pretreatment. The minerals stripped out in the spiral sluice process enter the wastewater treatment process.

[0023] After the spiral sluice concentrator treatment, the kaolin slurry is subjected to electromagnetic iron removal. The deep magnetic separation removes weakly magnetic minerals (such as iron, nickel, cobalt, and ilmenite with high iron content). Deep iron removal helps reduce the acid consumption in the next process and ensures the purity of kaolin. This reduces acid consumption and waste acid recovery pressure in the next kaolin acid purification process.

[0024] After the slurry is removed by electromagnetic iron removal, it settles in a deep cone thickener and then passes through a plate and frame filter press to form kaolin wet cakes, which are then pretreated kaolin and placed in a kaolin cake stockpile.

[0025] In the above-mentioned kaolin pretreatment process, all water used in each stage adopts a closed-loop recycling method, generating a small amount of harmless non-metallic mineral solid waste, from which minerals such as titanium and iron can be recovered as needed.

[0026] II. Acid purification and activation like Figure 1As shown, a hydrochloric acid solution with a mass content of about 30% is added to the pretreated kaolin wet mud cake and dispersed by high-speed stirring. Under the action of high-speed stirring, an acidic slurry of hydrochloric acid and kaolin with a slurry concentration of 35% is formed. The slurry is then pumped into an aeration reaction tank for aeration reaction (i.e., acid purification). Multiple aeration reaction tanks can be set up in parallel to carry out acid purification at the same time according to actual needs.

[0027] Compressed air is electrically heated to heat the aeration reactor, with simultaneous aeration and stirring to prevent the deposition of wet kaolin cake within the reactor. The slurry temperature inside the aeration reactor is controlled at 80-110 ℃ to enhance the solubility of soluble minerals in the kaolin (such as iron, calcium, magnesium, potassium, and other free metallic minerals) in the hydrochloric acid solution. Acid purification is maintained at 80-110 ℃ for 4-6 hours to maximize the solubility of the minerals in hydrochloric acid and prevent the dissolution of alumina in the kaolin during the purification process, thus reducing the alumina extraction rate. The acid mist generated during the acid purification process is collected through a closed pipeline to an acid mist absorption system and an acid mist condensation system for absorption and condensation. The acid mist mixes with supercooled air generated by compressed air and vortex tubes and condenses to form acid liquid, which is then recovered to the hydrochloric acid storage tank. Excess air enters a spray tower for alkaline washing and is subsequently discharged harmlessly by an induced draft fan.

[0028] After acid purification, the slurry is pumped to a closed deep cone thickener for concentration and sedimentation. Inside the thickener, the slurry settles to the bottom, while acid precipitates from the top, and the overflowing acid is recovered. Subsequently, the high-concentration slurry at the bottom is injected into a ceramic filter for filtration, removing the acid which is then recovered to an intermediate tank. After acid removal, the kaolin filter cake contains approximately 6% residual acid. This acid-containing kaolin filter cake is then dried in a drying device. During the drying process, the filter cake volatilizes at high temperatures, releasing a mixture of water vapor and hydrogen chloride. This mixture is then cooled into pure hydrochloric acid in a condenser and recovered to a hydrochloric acid storage tank.

[0029] After acid purification and drying, the kaolin then enters the activation process, which adopts a combined process of microwave tunnel kiln heating and microwave plasma decomposition. The microwave tunnel kiln is heated to above 400 ℃, and the microwave plasma decomposition includes multi-stage tower plasma continuous activation and fluidized bed continuous activation.

[0030] After acid purification, the kaolin is rapidly heated to over 400 ℃ in a continuous negative pressure microwave tunnel kiln. During the heating process, the mixed gas of hydrogen chloride and water vapor volatilized is cooled and condensed in a heat exchange station under high negative pressure. The supercooled air separated by compressed air through a vortex tube is aerated and diffused through a ceramic membrane in the heat exchange device to mix and cool the mixed gas of hydrogen chloride and water vapor. The cooled mixed gas enters a cyclone to collect and settle out liquid hydrochloric acid. The settled liquid hydrochloric acid is reduced to the initial pure hydrochloric acid concentration. The tail gas is washed with alkaline water aeration, and the harmless gas is discharged from the vacuum station.

[0031] Since some kaolin was not fully activated during the microwave tunnel kiln heating process, the kaolin heated in the microwave tunnel kiln underwent further multi-stage tower plasma continuous activation and fluidized bed continuous activation. Activation was performed using a plasma generator, with compressed air forming a plasma source in the multi-stage microwave cavity. The multi-stage microwave plasma gas source used a mixture of compressed air and argon, with argon accounting for 1.5%. The initial plasma arc of the multi-stage microwave plasma used a 3 kW microwave source, and the secondary plasma used a 1 kW microwave source to maintain the plasma electron density within the pipeline. The plasma activation temperature was below 300℃. A mixture of room temperature compressed air and argon was introduced between the initial plasma arc and the secondary plasma, and the activation inlet temperature was controlled by the output plasma gas temperature variable.

[0032] After activation, hydroxyl groups are removed from the kaolin, alumina is activated, and residual hydrogen chloride in the kaolin is further released. The gas released during the activation process is cooled by heat exchange, and the exhaust gas is washed by an alkaline water spray tower.

[0033] All the waste acid generated from the above-mentioned kaolin acid purification and activation process is recycled to the hydrochloric acid purification system for purification. After filtration and ion exchange resin adsorption, dissolved impurities are removed to obtain pure hydrochloric acid. The concentration of the purified hydrochloric acid solution does not decrease. The high hydrochloric acid wastewater generated by the backwashing of the hydrochloric acid purification system enters the sewage treatment system.

[0034] III. Acid Dissolution and Extraction of Aluminum Chloride like Figure 2 As shown, a hydrochloric acid solution with a mass content of about 30% is added to the activated kaolin and stirred at high speed to form an acidic slurry of hydrochloric acid and kaolin with a slurry concentration of 25%. The slurry is then pumped into an aeration reaction tank for acid dissolution to extract aluminum chloride.

[0035] Compressed air is electrically heated to heat the aeration reactor while simultaneously aerating and stirring. The slurry temperature inside the reactor is controlled at 100-110℃ to enhance the dissolution of alumina in kaolin with hydrochloric acid. The acid dissolution process is maintained at 100-110℃ for 5-7 hours. The acid mist generated during the dissolution process is collected through a closed pipeline to an acid mist absorption system and an acid mist condensation system. The acid mist is mixed with supercooled air generated by compressed air and vortex tubes and condensed into acid liquid, which is then recovered to a hydrochloric acid storage tank. Excess air is sent to a spray tower for alkaline washing and then discharged harmlessly via an induced draft fan.

[0036] After acid dissolution, the slurry is pumped to a closed deep cone thickener for settling. The aluminum chloride solution precipitated at the top overflows and is recovered to obtain crude aluminum chloride solution. The high-concentration slurry at the bottom is injected into a ceramic filter to remove and recover the acid solution. After acid removal, the solid phase is acidic silica filter cake, which is dried in a drying device and can be used to prepare sodium silicate, silica, and other silica products for various applications in the glass industry. The water vapor and hydrogen chloride mixture released during the drying process at high temperature is cooled into pure hydrochloric acid in a condenser and recovered to a hydrochloric acid storage tank.

[0037] While hydrochloric acid solution dissolves alumina in kaolin, hydrochloric acid-soluble minerals in the original kaolin (such as weakly magnetic iron oxide, calcium, magnesium, potassium, sodium, free titanium oxide, manganese, nickel, etc.) and some alumina are also dissolved by the hydrochloric acid solution. Some acidic and basic metal oxides exist in colloidal form in aluminum chloride solution.

[0038] IV. Purification of Crude Aluminum Chloride Solution like Figure 3 As shown, the crude aluminum chloride solution is first filtered with quartz sand, and then adsorbed with activated carbon to effectively intercept larger colloidal particles. After quartz sand filtration and activated carbon adsorption, a 20-micron ceramic membrane and a 5-micron ceramic membrane are used for further filtration to remove colloidal substances.

[0039] Subsequently, without changing the pH value of the crude aluminum chloride solution, a certain amount of hydrogen peroxide was added to the solution. Under the strong oxidizing effect of hydrogen peroxide, the iron, calcium, magnesium, and titanium ions in the crude aluminum chloride solution were reduced to ferric and tetravalent iron and high-valent hydroxides. The solution was then filtered again with quartz sand and adsorbed with activated carbon. Finally, an ultrafiltration system resistant to strong acids (sulfonic acid-based ultrafiltration) was used to remove fine colloidal minerals by using an ultrafiltration membrane with a pore size of 10-100 nm. At this point, only ionic states (ferrous divalent and other divalent metal ions) existed in the crude aluminum chloride solution.

[0040] The purity of the ultrafiltration-treated aluminum chloride crude solution is improved, and it then enters the high-frequency electrolysis iron removal process. The aluminum chloride crude solution is injected into the electrolytic cell, maintaining the electrolysis potential at the standard electrode potential (approximately -0.44 V vs. SHE), and the electrolysis frequency at 28-32 kHz. Iron ions in the aluminum chloride crude solution are deposited as metal ions at the cathode of the electrolysis electrode, while chloride ions are released at the anode (with simultaneous electrolysis ventilation). The electrolytic cell can be used in multi-stage series to achieve continuous operation, effectively removing most of the metal ions. The high-frequency electrolyzed aluminum chloride crude solution can then undergo ultrafiltration again (sulfonic acid-based ultrafiltration).

[0041] Subsequently, the crude aluminum chloride solution enters the ion exchange resin adsorption process. Before entering the ion exchange column, the crude aluminum chloride solution is preferentially filtered using ultrafiltration (10-50 nm) to ensure that the ion exchange resin is not contaminated by colloidal substances. The ion exchange resins used are an iminodiacetic acid-type chelating resin (Lanxess TP207) and an aminomethylphosphonic acid-type chelating resin (TP260) used in series. TP207 effectively adsorbs transition metal ions (iron, nickel, copper, etc.), and after removing transition metal ions, TP260 has a good adsorption effect on other alkali metal ions dissolved in the aluminum chloride solution. Both TP207 and TP260 resins can be used in a two-stage series. TP207 preferentially uses two-stage filtration to remove transition metal ions (especially iron ions) to ensure the safety of TP260 resin. After TP207 completes the two-stage series filtration adsorption, the aluminum chloride solution then enters the TP260 two-stage exchange column to complete the adsorption and interception of alkali metal ions, thus obtaining a high-purity aluminum chloride solution. When using ion exchange resin to adsorb and remove metal impurity ions from crude aluminum chloride solution, the temperature of the crude aluminum chloride solution should be controlled at 25-30 ℃ to ensure the service life of the resin.

[0042] Both TP207 and TP260 ion exchange resins can be used in a 2-for-2 configuration. When one resin group reaches its adsorption saturation threshold, it can be switched to the standby resin group to continue operation, while the original resin group enters the backwashing and regeneration process. During resin backwashing, a low-concentration pure hydrochloric acid solution is first injected to decompose and dissolve the metal ions adsorbed by the resin. Then, the waste acid water is discharged, and EDI ultrapure water is injected to rinse the resin until the pH value is around 6. The discharged waste acid water can be neutralized by alkaline washing and then filtered.

[0043] After removing metal ion impurities from the crude aluminum chloride solution, all filtration equipment (except ion exchange resin) is cleaned online during the aluminum chloride solution purification process. Quartz sand and activated carbon filtration can be backwashed intermittently with hydrochloric acid, while ultrafiltration and ceramic membrane filtration can be backwashed simultaneously with low-concentration hydrochloric acid. The acidic wastewater generated by the simultaneous backwashing of ultrafiltration and ceramic membrane filtration is collected and reused after removing solids through independent quartz sand filtration and microfiltration to reduce the consumption of acidic water and the amount of acidic wastewater to be treated.

[0044] V. Preparation of Alumina by Ammonia Reduction like Figure 4 As shown, a high-purity aluminum chloride solution is injected into a dispersion tank, and ammonia water is added and stirred at high speed to fully disperse and mix evenly, maintaining a pH of 7-8. The ammonia water reacts with the high-purity aluminum chloride solution to produce aluminum hydroxide colloid and ammonium chloride solution. Subsequently, microwave heating and vacuum evaporation are performed, forcing the aluminum hydroxide colloid to settle and concentrate in the ammonium chloride solution. The precipitated ammonium chloride solution overflows to a recovery tank. The concentrated aluminum hydroxide colloid and ammonium chloride mixture enters a falling film evaporator for falling film evaporation, evaporating some water and ammonium chloride vapor. The concentration of the aluminum hydroxide mixture increases to over 50%, and it is then injected into a deep cone concentration tank to settle the aluminum hydroxide, while the overflowing ammonium chloride solution is recovered.

[0045] The aluminum hydroxide slurry that settles in the deep cone concentrator is injected into the spray drying tower via a plunger pump for continuous spray drying and granulation. The dry aluminum hydroxide powder is collected by a cyclone dust collector and a pulse dust collector. The water vapor and ammonium chloride vapor generated during the drying process are sequentially fed into the primary spray scrubbing tower and the secondary spray scrubbing tower for water washing treatment. The resulting ammonium chloride solution is stored in an intermediate tank.

[0046] Aluminum hydroxide powder is first decomposed with argon plasma to form hydrogen and oxygen, which then escape. It is then transported to a microwave continuous tunnel kiln for continuous calcination. The calcination temperature is adjusted to 900-1100 ℃ and the calcination time is 5-10 min, depending on the product. This completes the conversion of aluminum hydroxide into high-temperature alumina, resulting in high-purity alumina with a purity of over 99.9%.

[0047] The ammonium chloride waste gas released during the roasting process enters the spray scrubbing tower for water washing to produce ammonium chloride solution. The produced ammonium chloride solution is evaporated under reduced pressure to precipitate ammonium chloride crystals. The ammonium chloride crystals are washed with water and fluidized dry to obtain ammonium chloride crystals, which can be used for agricultural nitrogen fertilizer production and industrial applications such as metal processing and welding, realizing the effective use of solid waste.

[0048] The process described in this embodiment is applicable to all mineral sources with an alumina content higher than 7% (kaolin, coal gangue powder, and various high and low grade bauxite). Microwave heating and plasma activation rapidly and efficiently achieve the exfoliation of silicon-aluminum hydroxyl groups. Taking kaolin as an example, the activation cost per ton of kaolin can be reduced to 37 kW of power consumption. Based on an average alumina content of 25% in kaolin, four tons of kaolin are required for extraction per ton of alumina, and the power consumption for raw material activation before extraction per ton of alumina is 148 kW.

[0049] After kaolin activation, the hydrochloric acid extraction of alumina to prepare aluminum chloride achieves a solubility of over 98% in the activated kaolin. At room temperature, the concentration of aluminum chloride solution in hydrochloric acid is 6.5%, which translates to a net alumina content of 2.2%. Solid-liquid separation is then performed after hydrochloric acid extraction. The first stage of filtrate separation uses ceramic membrane filtration, with a liquid phase loss rate of 5%. Hydrochloric acid is added to the solid phase to adjust the slurry concentration to 50% for a second stage of solid-liquid separation. This second stage uses a vacuum ceramic filter to separate the filtrate, with a liquid phase loss rate (solution content) of 8%. The final total loss per ton of alumina is approximately 14 kg, which translates to an alumina loss of approximately 1.2-1.5% per ton of kaolin during the secondary hydrochloric acid extraction process. This loss rate is significantly lower than the average 12% loss rate of the Bayer process. The extraction rate of alumina from kaolin using hydrochloric acid extraction reaches over 95%, and the power consumption for aluminum hydroxide phase inversion (high-temperature alumina) is approximately 150 kW per ton.

[0050] Based on this, the production cost of alumina is calculated as follows: Based on a market price of 350 yuan per ton of hydrochloric acid, each ton of kaolin requires 6 tons of hydrochloric acid to prepare the slurry.

[0051] The comprehensive cost of activated kaolin is 300 yuan / ton. It is calculated that about 4.2 tons of kaolin are used to extract one ton of alumina, and 3 tons of high-purity silica powder are produced simultaneously. The initial input cost of alumina production is 3360 yuan / ton.

[0052] The hydrochloric acid recovery and reuse rate reaches over 95%, which means that the hydrochloric acid consumption is about 5%.

[0053] The resulting silica powder has a market price of 500 yuan / ton, which offsets the cost of kaolin.

[0054] In summary, the comprehensive raw material cost per ton of alumina is calculated as follows: hydrochloric acid consumption cost is 105 yuan, and the comprehensive power consumption is as follows: kaolin drying-activation 37 kw × 4 = 148 kw, aluminum chloride extraction (heating) 55 kw × 4 = 220 kw, and alumina phase transformation (calcination) 150 kw. The comprehensive production cost of alumina can be controlled within 1000 yuan / ton.

[0055] Example 2 Except for step five, the rest is the same as in Example 1; step five in this example is the preparation of alumina by evaporation and cold crystallization, as detailed below: like Figure 5As shown, a high-purity aluminum chloride solution is injected into a hydrothermal falling film vacuum evaporation system. In this system, made of acid-resistant non-metallic materials such as PP, the high-purity aluminum chloride solution is heated to 95-105 °C (the azeotropic point of hydrochloric acid and water), and evaporates under reduced pressure to produce hydrogen chloride gas and water vapor. Once the high-purity aluminum chloride solution reaches its maximum saturation concentration, it enters a cooling crystallizer. Ultra-low temperature air generated by compressed air and a vortex tube is introduced to rapidly cool the high-purity aluminum chloride solution, precipitating aluminum chloride hexahydrate crystals. The low-temperature aluminum chloride solution containing the mixed aluminum chloride hexahydrate crystals is injected into a vacuum filtration device to remove the aluminum chloride solution. The aluminum chloride solution is then returned to the mother liquor tank for further hydrothermal falling film vacuum evaporation. The aluminum chloride hexahydrate crystals are first subjected to low-temperature negative pressure fluidized bed drying, and then subjected to continuous microwave calcination in stages.

[0056] The mixture of hydrogen chloride and water vapor produced during the hydrothermal falling film depressurization evaporation process, along with the low-temperature hydrogen chloride gas overflowing from the vortex tube cold air in the cooling crystallizer, is mixed and collected before entering the hydrogen chloride gas collection tank. It is then directly mixed with the vortex tube cold air and condensed into pure hydrochloric acid.

[0057] Microwave high-temperature calcination of aluminum chloride hexahydrate crystals releases high-temperature hydrogen chloride gas, which is then cooled in a quartz glass heat exchanger using circulating water. The heated circulating water is then fed into an aluminum chloride mother liquor tank for further heating of the aluminum chloride solution, thus reducing heat loss during the hydrothermal falling film evaporation process.

[0058] Part of the hot water generated during the high-temperature gas heat exchange and cooling process is injected into a steam generator to produce steam (steam temperature 120 ℃) ​​in a hydrothermal falling film depressurization evaporation system. The steam tail gas is piped into an aluminum chloride mother liquor tank to further preheat the aluminum chloride mother liquor.

[0059] Based on the requirements for the crystal morphology of high-purity alumina, a stepped continuous microwave calcination process was performed at temperatures ranging from 300 to 1150 ℃ to obtain alumina products with different crystal forms. The stepped continuous microwave calcination steps are as follows: The first-stage roasting was carried out in a negative pressure microwave tunnel kiln, with the negative pressure set at 20 kPa, the temperature at 300 ℃, and the roasting time at 10-20 min, to completely release hydrogen chloride gas and water vapor and prepare aluminum hydroxide.

[0060] The secondary calcination temperature is 700-900 ℃ and the calcination time is 20 min to complete the transition of the aluminum hydroxide alpha phase.

[0061] The three-stage calcination uses microwave heating at a temperature of 1050-1150 ℃, which is maintained for 20 min to complete the alpha alumina phase transformation.

[0062] The obtained high-purity alumina has a purity higher than 99.99%.

[0063] Example 3 Except for step five, the rest is the same as in Example 1; step five in this example is the preparation of high-purity alumina by plasma catalysis, as detailed below: like Figure 6 As shown, ultrafine spherical alumina was prepared by atomizing a high-purity aluminum chloride solution and then using plasma catalysis. The high-purity aluminum chloride solution was atomized using a 60 kHz ultrasonic spray device, with droplet size of 1-5 micrometers, and the flow rate of the high-purity aluminum chloride solution was set to 50 liters / hour.

[0064] The atomized solution droplets mix with compressed air and enter the plasma device under air pressure. The compressed air input pressure is set to 0.015 MPa, and the air flow rate is set to 2.2 cubic meters per minute under this input pressure.

[0065] The plasma arc initiation microwave source has an output power of 3000-6000 W, and there are 3 sets of 3000 W secondary plasma excitation microwave sources and 3 sets of 3000 W tertiary plasma excitation microwave sources. The plasma range is constrained by both electromagnetic and permanent magnet magnetic fields.

[0066] The permanent magnet in the confinement magnetic field is installed between the second-stage plasma excitation front section and the second- and third-stage plasma excitation magnetic fields. The electromagnetic field confinement is installed after the third-stage plasma excitation. By adjusting the strength of the electromagnetic confinement magnetic field, spherical high-purity alumina with different particle sizes can be obtained. The purity of the spherical high-purity alumina reaches more than 99.99%.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing high-purity alumina using kaolin, characterized in that, Includes the following steps: S1: Kaolin is pretreated to obtain pretreated kaolin; S2: Add hydrochloric acid solution to the pretreated kaolin for acid purification, and then activate it to obtain activated kaolin. S3: Add hydrochloric acid solution to activated kaolin for acid dissolution. After acid dissolution, concentrate and settle to obtain crude aluminum chloride solution and silica mud filter cake. S4: Purify the crude aluminum chloride solution to obtain a high-purity aluminum chloride solution; S5: High-purity aluminum oxide is prepared using high-purity aluminum chloride solution.

2. The method according to claim 1, characterized in that, In step S1, the pretreatment includes: adding water to kaolin to crush and slurry, dispersing and screening the slurry at high speed, and performing hydraulic classification, mineral processing and iron removal on the screened slurry.

3. The method according to claim 1, characterized in that, In step S2, the concentration of the hydrochloric acid solution is 25-35%; the temperature for acid purification is 80-110 ℃, and the time is 4-6 h.

4. The method according to claim 1, characterized in that, In step S2, activation includes: first heating the acid-purified kaolin to above 400 °C using a microwave tunnel kiln, and then using plasma gas to decompose the kaolin into hydroxyl groups.

5. The method according to claim 1, characterized in that, In step S3, the concentration of hydrochloric acid solution is 25-35%; the temperature for acid dissolution is 100-110 ℃ and the time is 5-7 h; after drying the silica mud filter cake, silicon-containing products are prepared.

6. The method according to claim 1, characterized in that, In step S4, purification includes: filtering and adsorbing the crude aluminum chloride solution, then adding a strong oxidant for oxidation, and then filtering, adsorbing, ultrafiltration, electrolysis, and ion exchange the oxidized crude aluminum chloride solution.

7. The method according to claim 6, characterized in that, Ion exchange was performed sequentially using iminodiacetic acid-type chelating resin and aminomethylphosphonic acid-type chelating resin.

8. The method according to claim 1, characterized in that, Step S5 includes: adding ammonia water to a high-purity aluminum chloride solution to react, evaporating the reaction product to obtain an ammonium chloride solution and an aluminum hydroxide slurry, and drying and calcining the aluminum hydroxide slurry to obtain high-purity aluminum oxide.

9. The method according to claim 1, characterized in that, Step S5 includes: heating a high-purity aluminum chloride solution to 95-105 °C until it is saturated by evaporation, followed by cooling and crystallization to obtain aluminum chloride crystals; drying and calcining the aluminum chloride crystals to obtain high-purity alumina.

10. The method according to claim 9, characterized in that, The roasting process includes three stages: primary roasting, secondary roasting, and tertiary roasting. The primary roasting temperature is 200-400 ℃ and the time is 10-20 min. The secondary roasting temperature is 700-900 ℃ and the time is 15-25 min. The tertiary roasting temperature is 1050-1150 ℃ and the time is 15-25 min.

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