A method for preparing aluminum fluoride by low-temperature wet process using waste aluminum electrolyte alkali leaching residue

Abstract

The application relates to a method for preparing aluminum fluoride by low-temperature wet process of waste aluminum electrolyte alkali leaching residue, which comprises the following steps: drying, crushing, grinding and sieving the waste aluminum electrolyte alkali leaching residue to obtain residue powder to be treated; dispersing the residue powder in water to form a slurry, adding oxalic acid and a silicon source at 40-90 DEG C to make calcium and magnesium into insoluble oxalate and make fluorine-containing components into liquid phase; performing hot filtration on the reaction slurry to obtain first filter residue and first filtrate; heating the first filtrate to 90-100 DEG C, adding aluminum hydroxide to react, separating out the separated silicon dioxide to obtain second filtrate containing aluminum fluoride; cooling the second filtrate to crystallize, aging, solid-liquid separation, drying and calcining to obtain anhydrous aluminum fluoride product. The method has the advantages of mild reaction condition, short process, high fluorine extraction rate, low energy consumption, no fluorine-containing waste gas, and suitability for fluorine resource recycling of the waste aluminum electrolyte alkali leaching residue.

Description

Technical Field

[0001] This invention belongs to the field of hazardous solid waste resource recycling technology, specifically relating to a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process. Background Technology

[0002] Hazardous wastes such as waste carbon slag and waste aluminum electrolyte generated in the aluminum electrolysis industry are often treated using the "alkaline leaching" process to recover lithium resources. However, alkaline leaching produces a large amount of alkaline leaching residue. The main components of this residue are calcium fluoride, calcium hydroxide, and magnesium hydroxide. If inorganic acid leaching is used directly, impurities such as calcium hydroxide in the residue will consume a large amount of acid, and simple acid leaching has limited ability to decompose calcium fluoride, resulting in low fluoride recovery rates. In addition, hydrogen fluoride (HF) is inevitably generated during the acid leaching process.

[0003] Currently, the treatment of this type of waste residue faces significant challenges: (1) serious waste of resources. The residue contains a large amount of valuable fluorine resources (mainly in the form of stable and insoluble calcium fluoride). However, due to the lack of economically feasible extraction technology, it is currently mostly treated by stockpiling or landfilling, which not only wastes resources but also poses a serious threat of fluorine pollution to soil and groundwater; (2) conventional acid treatment is difficult: if inorganic acids such as hydrochloric acid and sulfuric acid are used for leaching directly, the large amount of alkaline calcium-based impurities (such as calcium hydroxide) in the residue will preferentially consume a large amount of acid, resulting in extremely high acid consumption costs; (3) simple acid leaching is difficult to completely open the high lattice energy structure of fluorite (calcium fluoride), resulting in a low fluorine leaching rate. In addition, uncontrollable hydrogen fluoride (HF) gas will be released during the reaction, which will corrode equipment and harm the environment. Currently, the mainstream process for industrial production of aluminum fluoride is the "dry method" route, which first prepares HF intermediates. Usually, acid-grade fluorite and concentrated sulfuric acid are reacted in a high-temperature (>300°C) rotary kiln to produce anhydrous hydrogen fluoride (HF) gas. The purified HF gas then reacts with aluminum hydroxide in a fluidized bed to produce aluminum fluoride. This route is highly dependent on anhydrous HF, which is extremely toxic, corrosive, and volatile, as an intermediate. Its production, storage, and transportation are all considered major hazards, resulting in extremely high safety management costs. This is a highly endothermic, high-temperature calcination process with enormous energy consumption. Furthermore, it places extremely stringent corrosion resistance requirements on the reactor material, leading to expensive equipment investment. Summary of the Invention

[0004] To address the aforementioned problems, this invention aims to provide a mild, safe, and low-energy-consumption process that, in a low-temperature (<100°C) aqueous solution system, directly and efficiently converts calcium fluoride in alkaline leaching residue into high-value-added aluminum fluoride through chemothermodynamic driving, while simultaneously producing industrial-grade calcium oxalate as a byproduct, completely avoiding the production and transportation of the highly hazardous intermediate HF gas. Therefore, this invention provides a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process. The method employs a "dual-acid synergistic activation-precipitation-driven" strategy, using oxalic acid (Brønsted acid) and silica (fluoro Lewis acid) to construct a dual-acid synergistic system. Utilizing the extremely low solubility of calcium oxalate (precipitation-driven), a strong "calcium ion trap" is constructed to continuously reduce the calcium ion concentration in the solution, thermodynamically driving the dissolution equilibrium. The strong bond energy of the Si-F bond and subsequent Al-F bond stabilizes the fluoride ions for transfer in the liquid phase. Finally, by leveraging the "metastable supersaturation" characteristic of aluminum fluoride at specific high temperatures and the kinetic difference between the "rapid gelation and precipitation" of silica, effective separation of silicon and aluminum is achieved.

[0005] The present invention discloses a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, comprising the following steps: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue is dried, crushed, ground, and sieved to 100-200 mesh to obtain uniform residue powder to be treated. Step S2: Co-conversion reaction of oxalic acid and silicon source The residue powder to be treated is dispersed in water to prepare a slurry. Oxalic acid dihydrate and silica are added under stirring conditions to carry out the first reaction. The temperature of the first reaction is 40~90 ℃ and the time of the first reaction is 10~24 h. Immediately after the reaction is completed, the reactants of the first reaction at a temperature of 40~90°C are filtered to obtain the first filter residue and the first filtrate. The components of the first filter residue include calcium oxalate, magnesium oxalate and unreacted carbon residue. The first filter residue is washed and dried and sold as an industrial-grade calcium oxalate by-product, or it can be calcined to recover calcium oxide. Step S3: Separation reaction of silicon and aluminum The first filtrate is heated to 90-100°C, and aluminum hydroxide is added under vigorous stirring to carry out a second reaction. The second reaction time is controlled within 30-60 minutes. When the second reaction reaches its endpoint, the second reaction system at 90-100°C is immediately filtered to separate the second filter residue and the second filtrate. The vigorous stirring speed is 500-900 rpm, and the temperature of the second filtrate is 80-95°C. The second filter residue is a silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate includes a metastable supersaturated solution of aluminum fluoride and unreacted fluorosilicic acid; Step S4: Crystallization, aging, and calcination The second filtrate, at a temperature of 80-95°C, is transferred to a crystallization vessel and linearly cooled to 20-40°C within 2-6 hours. Aluminum fluoride trihydrate is added as a seed crystal to induce crystallization. After standing and aging, aluminum fluoride crystals precipitate. After solid-liquid separation, the filtrate is returned to the oxalic acid-silicon source synergistic conversion reaction stage to prepare an oxalic acid solution. The solid phase is calcined and dehydrated at a temperature of 350-500°C to obtain anhydrous aluminum fluoride product.

[0006] The waste aluminum electrolyte alkaline leaching residue of the present invention is a hazardous waste generated in the aluminum electrolysis industry. The components of the waste aluminum electrolyte alkaline leaching residue include calcium fluoride, aluminosilicate, calcium aluminum hydroxide, unreacted cryolite, carbon, and quartz.

[0007] Furthermore, in the method for preparing aluminum fluoride using the alkaline leaching residue of waste aluminum electrolyte according to the present invention, in step S1, the total calcium content in the alkaline leaching residue of waste aluminum electrolyte is 3wt.% to 10wt.%, and the total fluorine content is 10wt.% to 25wt.%.

[0008] Furthermore, in the method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue by low-temperature wet process according to the present invention, in step S2, the mass fraction of the residue powder to be treated in the slurry is 10wt.% to 25wt.%.

[0009] Furthermore, in the method for preparing aluminum fluoride using the low-temperature wet process of alkaline leaching residue of waste aluminum electrolyte according to the present invention, in step S2, the amount of oxalic acid dihydrate added is 1.0 to 1.5 times the total molar amount of calcium and magnesium in the residue powder to be treated.

[0010] Furthermore, in the method for preparing aluminum fluoride using the low-temperature wet process of alkaline leaching residue of waste aluminum electrolyte according to the present invention, in step S2, the molar ratio of the amount of silicon dioxide added to the total fluorine content in the residue powder to be treated is 1:4 to 1:6.

[0011] Furthermore, in the method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue by low-temperature wet process according to the present invention, in step S2, the first reaction temperature is 50~70 ℃.

[0012] Furthermore, in the method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue by low-temperature wet process according to the present invention, in step S2, the first filtrate contains a fluorosilicic acid solution.

[0013] Furthermore, in the method for preparing aluminum fluoride using the low-temperature wet process of waste aluminum electrolyte alkaline leaching residue described in this invention, the mass fraction of calcium oxalate in the first filter residue is 40wt.%~60wt.%, the mass fraction of magnesium oxalate is 5wt.%~15wt.%, and the mass fraction of unreacted carbon residue is 20wt.%~40wt.%.

[0014] Furthermore, in the method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue by low-temperature wet process according to the present invention, in step S3, the molar ratio of aluminum hydroxide to fluorosilicic acid is in the range of 2:1 to 3:1.

[0015] Furthermore, in the method for preparing aluminum fluoride using the low-temperature wet process of waste aluminum electrolyte alkaline leaching residue described in this invention, in step S3, when the second reaction reaches the reaction endpoint, the volume of the precipitated silica solid phase is 30% to 50% of the volume of the second reaction system, and aluminum fluoride is not precipitated.

[0016] In the low-temperature wet process for preparing aluminum fluoride using alkaline leaching residue from waste aluminum electrolyte of this invention, oxalic acid acts as a Brønsted acid (proton donor) and silica as a fluoro Lewis acid (electron docking acceptor) in the aqueous phase system. Both act synergistically on the calcium fluoride component in the alkaline leaching residue. Utilizing the thermodynamic property of the extremely low solubility product constant of calcium oxalate, oxalate ions can rapidly capture trace amounts of calcium ions dissociated from the surface of the calcium fluoride lattice, forming a stable calcium oxalate precipitate. This process significantly reduces the concentration of free calcium ions in the solution, forcing the dissolution equilibrium of calcium fluoride to continuously shift to the right according to Le Chatelier's principle, thereby achieving complete breaking of the Ca-F bond. Simultaneously with the removal of calcium ions by precipitation, fluoride ions are rapidly captured by the fluorophilic silica. Due to the extremely strong Si-F bond energy (approximately 570 kJ / mol), stable water-soluble fluorosilicic acid is formed in the acidic aqueous solution. This step not only prevents the leakage of highly toxic and corrosive HF gas but also transfers fluorine from the solid phase to the liquid phase, achieving a gentle leaching of fluorine resources.

[0017] Simultaneously, after obtaining the filtrate containing fluorosilicic acid, aluminum hydroxide is added. Due to the extremely high stability of the Al-F bond (superior to the Si-F bond) and the strong thermodynamic tendency of the reaction, fluoride ions in the fluorosilicic acid transfer to aluminum ions, forming aluminum fluoride, while releasing silica and water. Furthermore, aluminum fluoride readily forms a metastable supersaturated solution; that is, in the initial stage of the reaction, although the concentration exceeds the solubility, it will not immediately crystallize within a certain temperature and time. In contrast, silica will dehydrate and polymerize instantly upon formation, forming a colloid or precipitate. Utilizing the metastable supersaturation characteristics of aluminum fluoride in the reaction system and the difference in precipitation rate of silica, silica residue is separated by hot filtration, and the filtrate is cooled and crystallized to obtain aluminum fluoride.

[0018] The beneficial effects of this invention are: This invention presents a low-temperature wet process for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue. The process is mild and safe, completely eliminating the traditional high-temperature concentrated sulfuric acid / HF route. The main wet reaction is carried out in an aqueous solution system at atmospheric pressure and below 100°C, completely avoiding the production, transportation, and use of highly toxic, high-pressure, and highly corrosive anhydrous hydrofluoric acid (HF). This not only eliminates the potential for major safety accidents but also significantly reduces the enterprise's safety supervision and environmental compliance costs.

[0019] The method of this invention is thermodynamically driven for efficient extraction. It cleverly utilizes the extremely small solubility product of calcium oxalate to construct a strong thermodynamic "calcium ion trap". Combined with the formation of strong Si-F bonds, it can also force the stable calcium fluoride lattice to completely disintegrate even at low temperatures, resulting in a high fluoride extraction rate.

[0020] This invention utilizes hazardous solid waste (alkali leaching residue) from the aluminum electrolysis industry as a low-cost fluorine source, turning waste into treasure. It not only eliminates hazardous waste disposal costs but also produces high-value aluminum fluoride (a raw material for electrolytic aluminum) and calcium oxalate (a raw material for ceramic glazes / reagents). Through the resource-based transformation of impurities (calcium), it achieves a complete utilization of all components.

[0021] The method of this invention has low equipment requirements, low investment, mild reaction conditions, no high-temperature calcination process, and significantly reduced energy consumption, meeting the "dual carbon" target. Since the reaction medium is mainly water and a weak acid environment, there is no need for expensive nickel-based alloys or other equipment resistant to hydrogen fluoride corrosion; conventional stainless steel or even plastic-lined reactors can meet the requirements, significantly reducing equipment investment and maintenance costs. Attached Figure Description

[0022] Figure 1 A process flow diagram illustrating a specific embodiment of the present invention; Figure 2 The image shown is the XRD pattern of the anhydrous aluminum fluoride product in Example 1 of this invention. Figure 3 This is a SEM image of the anhydrous aluminum fluoride product in Example 1 of the present invention. Detailed Implementation

[0023] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments are now described in detail with reference to the accompanying drawings. The described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the detailed embodiments, conventional conditions or conditions provided by the manufacturer shall apply.

[0024] In the following description, when referring to the accompanying drawings, the same numbers in different drawings denote the same or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this disclosure. The terminology used in this disclosure is for the purpose of describing particular embodiments only and is not intended to be limiting of this disclosure.

[0025] like Figure 1 As shown, this invention provides a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, comprising the following steps: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue is dried, crushed, ground, and sieved to 100-200 mesh to obtain uniform residue powder to be treated. Step S2: Co-conversion reaction of oxalic acid and silicon source The above-mentioned residue powder to be treated was dispersed in water to prepare a slurry. Under stirring conditions, oxalic acid dihydrate and silica were added to carry out the first reaction. The temperature of the first reaction was 40~90 ℃ and the time of the first reaction was 10~24 h. Immediately after the reaction is complete, the reactants from the first reaction at a temperature of 40-90 °C are filtered to obtain the first filter residue and the first filtrate. The first filter residue consists of calcium oxalate, magnesium oxalate, and unreacted carbon residue. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or it can be calcined to recover calcium oxide. The main reaction formula is as follows: 3CaF2 + 3H2C2O4 + SiO2 → 3CaC2O4↓ + H2SiF6 + 2H2O 3MgF2 + 3H2C2O4 + SiO2 → 3MgC2O4↓ + H2SiF6 + 2H2O CaC2O4 + 1 / 2O2 → CaO + 2CO2↑ Step S3: Separation reaction of silicon and aluminum The first filtrate was heated to 90-100 °C, and aluminum hydroxide was added under vigorous stirring to carry out the second reaction. The second reaction time was controlled within 30-60 min. After the second reaction reached the endpoint, the second reaction system at a temperature of 85-100 °C was immediately filtered to separate the second filter residue and the second filtrate. The vigorous stirring speed was 500-900 rpm, and the temperature of the second filtrate was 80-95 °C. The second filter residue is a silica precipitate, which, after washing, is recycled back to step S2 as a raw material. The second filtrate consists of a metastable supersaturated solution of aluminum fluoride and unreacted fluorosilicic acid, and its main reaction formula is as follows: H2SiF6 + 2Al(OH)3 → 2AlF3 + SiO2↓ + 4H2O Step S4: Crystallization, aging, and calcination The second filtrate, at 80-95°C, is transferred to a crystallization vessel and linearly cooled to 20-40°C over 2-6 hours. Aluminum fluoride trihydrate is added as a seed crystal to induce crystallization. After standing and aging, aluminum fluoride crystals precipitate. After solid-liquid separation, the filtrate is returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. The solid phase is calcined and dehydrated at 350-500°C to obtain anhydrous aluminum fluoride product. The main reaction formula is as follows: AlF3 + 3H2O → AlF3·3H2O↓ AlF3·3H2O → AlF3 + 3H2O↑ In some embodiments, in step S1 above, the total calcium content in the waste aluminum electrolyte alkaline leaching residue is 3wt.% to 10wt.%, and the total fluorine content is 10wt.% to 25wt.%.

[0026] In some embodiments, in step S2 above, the mass fraction of the slag powder to be treated in the slurry is 10wt.%~25wt.%.

[0027] In some embodiments, in step S2 above, the amount of oxalic acid dihydrate added is 1.0 to 1.5 times the total molar amount of calcium and magnesium in the slag powder to be treated.

[0028] In some embodiments, in step S2 above, the molar ratio of the amount of silica added to the total fluorine content in the slag powder to be treated is 1:4 to 1:6.

[0029] In some embodiments, in step S2 above, the first reaction temperature is 50~70 °C.

[0030] In some embodiments, in step S2 above, the first filtrate contains fluorosilicic acid.

[0031] In some embodiments, the first filter residue contains calcium oxalate at a mass fraction of 40 wt.% to 60 wt.%, magnesium oxalate at a mass fraction of 5 wt.% to 15 wt.%, and unreacted carbon residue at a mass fraction of 20 wt.% to 40 wt.%.

[0032] In some embodiments, in step S3 above, the molar ratio of aluminum hydroxide to fluorosilicic acid is in the range of 2:1 to 3:1.

[0033] In some embodiments, in step S3 above, when the second reaction reaches the reaction endpoint, the volume of the precipitated silica solid phase is 30% to 50% of the volume of the second reaction system, and aluminum fluoride is not precipitated.

[0034] Figure 1 This is a process flow diagram of the method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, as described in this invention. Figure 1 As shown, the waste aluminum electrolyte alkaline leaching residue is dried, crushed, ground, and sieved to obtain the residue powder to be treated. The residue powder to be treated is mixed with water to form a slurry, and oxalic acid and silicon source are added to carry out the oxalic acid-silicon source synergistic conversion reaction. After hot filtration, the first filter residue and the first filtrate are obtained. Aluminum hydroxide is added to the first filtrate to carry out the silicon and aluminum separation reaction. After hot filtration, silica filter residue and aluminum fluoride-containing second filtrate are obtained. The second filtrate is subjected to cooling crystallization, aging, solid-liquid separation, drying, and calcination to dehydrate, and anhydrous aluminum fluoride product is obtained.

[0035] The present invention will be further described in detail below with reference to specific embodiments. Example 1

[0036] The present invention discloses a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, comprising the following steps: Step S1: Raw material pretreatment Take the residue from the alkaline leaching of waste aluminum electrolyte. The waste aluminum electrolyte alkaline leaching residue of the present invention is a hazardous waste generated in the aluminum electrolysis industry. The components of the waste aluminum electrolyte alkaline leaching residue include calcium fluoride, aluminosilicate, calcium aluminum hydroxide, unreacted cryolite, carbon, and quartz.

[0037] Its composition includes: a total fluorine content of 15.2 wt.%, a total silicon content of 8.1 wt.%, a total calcium content of 5.3 wt.%, a total magnesium content of 2.1 wt.%, and a carbon slag content of 35 wt.%.

[0038] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 120°C for 4 hours, crushed, and then ball-milled for 2 hours. It was then passed through a 150-mesh sieve (screening rate ≥95%) to obtain the residue powder to be treated.

[0039] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 500 L of deionized water to prepare a slurry with a solid content of 16.7 wt.%. Under stirring at 500 rpm, 9.8 kg of oxalic acid dihydrate (1.2 times the total molar amount of calcium and magnesium) and 6.5 kg of silica powder (molar ratio to total fluorine of 1:5) were added to initiate the first reaction. The temperature of the first reaction was controlled at 60 ± 2°C, and the reaction time was 12 h. Immediately after the reaction was completed, the reactants from the first reaction at 60°C were hot-filtered to obtain 42 kg of the first filter residue (dry basis) and 480 L of the first filtrate.

[0040] The first filter residue consists of 51.2 wt.% calcium oxalate, 9.8 wt.% magnesium oxalate, 28.5 wt.% unreacted carbon residue, and 10.5 wt.% other impurities. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or calcined to recover calcium oxide.

[0041] Step S3: Separation reaction of silicon and aluminum The first filtrate was heated to 95°C, and 12.5 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.5:1) was added while stirring at 700 rpm to carry out the second reaction. The reaction time of the second reaction was 45 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 40% of the volume of the second reaction system, the second reaction system at 90°C was immediately subjected to hot filtration to separate the second filter residue (8.2 kg) and the second filtrate (450 L at 85°C).

[0042] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate includes a metastable supersaturated solution of aluminum fluoride and unreacted fluorinated silica.

[0043] Step S4: Crystallization, aging, and calcination The second filtrate was transferred to a crystallization vessel at 85°C and linearly cooled to 25°C within 4 hours. 0.35 kg of aluminum fluoride trihydrate seed crystals (1.0 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 3 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation and filtration, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. The solution was then washed with solid phase (3 times with 50 L of deionized water each time), dried at 80°C for 8 hours, and then calcined at 420°C for 5 hours to obtain 13.8 kg of anhydrous aluminum fluoride product.

[0044] like Figure 2 The image shown is the XRD pattern of the anhydrous aluminum fluoride product of Example 1. Figure 3 The image shown is a SEM image of the anhydrous aluminum fluoride product of Example 1.

[0045] Characterization of the anhydrous aluminum fluoride product in Example 1: Purity of anhydrous aluminum fluoride: 98.5 wt.%; Main impurities: SiO2 0.8 wt.%; CaO 0.3 wt.%; Other impurities: 0.4 wt.%; Total yield of aluminum fluoride: 92.3% (calculated based on total fluorine in raw materials).

[0046] In this embodiment, the unit product energy consumption is approximately 420 kWh / ton, with no fluorine-containing waste gas emissions.

[0047] Example 2: The present invention discloses a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, comprising the following steps: Step S1: Raw material pretreatment The alkaline leaching residue of waste aluminum electrolyte is composed of the following components: total fluorine content of 20.5 wt.%, total silicon content of 10.2 wt.%, total calcium content of 7.8 wt.%, total magnesium content of 3.2 wt.%, and carbon slag content of 58.3 wt.%.

[0048] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 120°C for 4 hours, crushed, and then ball-milled for 2 hours. After passing through a 120-mesh sieve, the residue powder to be treated was obtained.

[0049] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 456 L of deionized water to prepare a slurry with a solid content of 18 wt.%. Under stirring at 450 rpm, 14.2 kg of oxalic acid dihydrate (1.3 times the total molar amount of calcium and magnesium) and 8.5 kg of silica (molar ratio to total fluorine of 1:4.5) were added to initiate the first reaction. The temperature of the first reaction was controlled at 55 ± 2°C, and the reaction time was 16 h. Immediately after the reaction was completed, the reactants from the first reaction at 55°C were hot-filtered to obtain 48 kg of the first filter residue and 420 L of the first filtrate.

[0050] The first filter residue consists of calcium oxalate, magnesium oxalate, and unreacted carbon residue. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or it is calcined to recover calcium oxide.

[0051] Step S3: Separation reaction of silicon and aluminum The first filtrate was heated to 92°C, and 18.5 kg of aluminum hydroxide (molar ratio of aluminum hydroxide to fluorosilicic acid was 2.2:1) was added under stirring at 650 rpm to initiate a second reaction. The reaction time for the second reaction was 40 min. After the second reaction reached its endpoint, and the volume of silica solid phase precipitated in the reaction system reached 35% of the volume of the second reaction system, the second reaction system at 88°C was immediately subjected to hot filtration to separate the two, yielding 10.5 kg of the second filter residue and 395 L of the second filtrate (at 85°C).

[0052] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0053] Step S4: Crystallization, aging, and calcination The second filtrate was transferred to a crystallization vessel at 85°C and linearly cooled to 30°C within 3 hours. 0.48 kg (0.8 wt.%) of seed crystals were added, and the mixture was allowed to stand for 2 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation, filtration, washing, and drying, the product was calcined at 400°C for 4 hours to remove water, yielding 18.0 kg of anhydrous aluminum fluoride.

[0054] Characterization of the anhydrous aluminum fluoride product in Example 2: Anhydrous aluminum fluoride purity: 98.1%, Total aluminum fluoride yield: 88.5%.

[0055] Example 3: The present invention discloses a method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, comprising the following steps: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue has the following composition: total fluorine content of 18.0 wt.%, total silicon content of 9.0 wt.%, total calcium content of 6.0 wt.%, total magnesium content of 2.5 wt.%, and the remainder is carbon slag.

[0056] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 120°C for 4 hours, crushed, and then ball-milled for 2 hours. After passing through a 180-mesh sieve, the residue powder to be treated was obtained.

[0057] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 400 L of deionized water to prepare a slurry with a solid content of 20 wt.%. Under stirring at 550 rpm, 11.5 kg of oxalic acid dihydrate (1.3 times the total molar amount of calcium and magnesium) and 7.5 kg of silica (the molar ratio of silica to total fluorine was 1:4) were added to initiate the first reaction. The temperature of the first reaction was controlled at 65 ± 2°C, and the reaction time was 14 h. Immediately after the reaction was completed, the reactants from the first reaction at 65°C were hot-filtered to obtain the first filter residue and the first filter media.

[0058] The first filter residue consists of calcium oxalate, magnesium oxalate, and unreacted carbon residue. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or it can be calcined to recover calcium oxide.

[0059] Step S3: Separation reaction of silicon and aluminum The first filtrate was heated to 96°C, and 12.5 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.8:1) was added while stirring at 750 rpm to carry out the second reaction. The reaction time of the second reaction was 35 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 45% of the volume of the second reaction system, the second reaction system at 92°C was immediately subjected to hot filtration to separate the second filter residue and the second filtrate (at a temperature of 90°C).

[0060] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0061] Step S4: Crystallization, aging, and calcination The second filtrate was fed into a crystallization vessel at 90°C, cooled to 28°C after 5 hours, and seed crystals (1.2 wt.%) were added. The mixture was then aged for 4 hours and calcined at 450°C for 3 hours.

[0062] The second filtrate was transferred to a crystallization vessel at 90°C and linearly cooled to 28°C within 5 hours. 0.52 kg of aluminum fluoride trihydrate seed crystals (1.2 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 4 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation and filtration, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. After solid-phase washing and drying, the solution was calcined at 450°C for 3 hours to obtain 16.4 kg of anhydrous aluminum fluoride product.

[0063] Characterization of the anhydrous aluminum fluoride product in Example 3: Anhydrous aluminum fluoride purity: 97.8%, Total aluminum fluoride yield: 90.2%.

[0064] Example 4: Step S1: Raw material pretreatment The alkaline leaching residue of waste aluminum electrolyte has the following composition: total fluorine content of 16.8 wt.%, total silicon content of 8.7 wt.%, total calcium content of 5.8%, total magnesium content of 2.3 wt.%, and carbon slag content of 31.0 wt.

[0065] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 110°C for 5 hours, crushed, and then ball-milled for 1.5 hours. After passing through a 160-mesh sieve, the residue powder to be treated was obtained.

[0066] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 470 L of deionized water to prepare a slurry with a solid content of 17.5 wt.%. Under stirring at 480 rpm, 10.8 kg of oxalic acid dihydrate (1.2 times the total molar amount of calcium and magnesium) was added, followed by 7.0 kg of silica powder (the molar ratio of silica to total fluorine was 1:4.8) to initiate the first reaction. The temperature of the first reaction was controlled at 58 ± 2°C, and the reaction time was 13 h. Immediately after the reaction was completed, the reactants from the first reaction at 58°C were hot-filtered to obtain 44.6 kg of the first filter residue (dry basis) and 452 L of the first filtrate.

[0067] The first filter residue consists of 48.6 wt.% calcium oxalate, 10.4 wt.% magnesium oxalate, 29.7 wt.% unreacted carbon residue, and 11.3 wt.% other impurities. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or calcined to recover calcium oxide.

[0068] Step S3: Silicon and aluminum separation reaction The first filtrate was heated to 94°C, and 13.9 kg of aluminum hydroxide (molar ratio of aluminum hydroxide to fluorosilicic acid was 2.4:1) was added under stirring at 680 rpm. The reaction was carried out for 42 min. When the volume of solid phase precipitated in the system reached 38%, hot filtration was performed at 89°C to obtain 8.9 kg of second filter residue and 428 L of second filtrate.

[0069] The first filtrate was heated to 94°C, and 13.9 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.4:1) was added under stirring at 680 rpm to carry out the second reaction. The reaction time of the second reaction was 42 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 38% of the volume of the second reaction system, the second reaction system at 89°C was immediately subjected to hot filtration separation to obtain 8.9 kg of second filter residue and 428 L of second filtrate (at 86°C).

[0070] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0071] Step S4: Crystallization, aging, and calcination The second filtrate was fed into a crystallization vessel at 86°C and cooled to 27°C over 4.5 hours. 0.40 kg of aluminum fluoride trihydrate seed crystals (0.9 wt.% of the theoretical product mass) were added, and the mixture was allowed to stand for 3 hours. After filtration, washing, and drying at 80°C for 6 hours, the mixture was calcined at 430°C for 4 hours to obtain 15.1 kg of anhydrous aluminum fluoride product.

[0072] The second filtrate was transferred to a crystallization vessel at 86°C and linearly cooled to 27°C within 4.5 hours. 0.40 kg of aluminum fluoride trihydrate seed crystals (0.9 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 3 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation and filtration, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. After solid-phase washing and drying at 80°C for 6 hours, the solution was calcined at 430°C for 4 hours to obtain 15.1 kg of anhydrous aluminum fluoride product.

[0073] Characterization of the anhydrous aluminum fluoride product in Example 4: The purity of the anhydrous aluminum fluoride is 98.3%, and the total yield of aluminum fluoride is 91.1%.

[0074] Example 5: Step S1: Raw material pretreatment The alkaline leaching residue of waste aluminum electrolyte is composed of the following components: total fluorine content of 21.0 wt.%, total silicon content of 10.8 wt.%, total calcium content of 8.2 wt.%, total magnesium content of 3.4 wt.%, and carbon slag content of 27 wt.%.

[0075] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 120°C for 4 hours, crushed, ground, and passed through a 140-mesh sieve to obtain the residue powder to be treated.

[0076] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 430 L of deionized water to prepare a slurry with a solid content of 18.9 wt.%. Under stirring at 500 rpm, 15.1 kg of oxalic acid dihydrate (1.25 times the total molar amount of calcium and magnesium) and 9.2 kg of silica powder (molar ratio to total fluorine of 1:4.6) were added to initiate the first reaction. The temperature of the first reaction was controlled at 56 ± 2°C, and the reaction time was 15 h. Immediately after the reaction was completed, the reactants from the first reaction at 56°C were hot-filtered to obtain 50.2 kg of the first filter residue (dry basis) and 405 L of the first filtrate.

[0077] The first filter residue consists of 53.4 wt.% calcium oxalate, 11.1 wt.% magnesium oxalate, 24.6 wt.% unreacted carbon residue, and 10.9 wt.% other impurities. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or calcined to recover calcium oxide.

[0078] Step S3: Silicon and aluminum separation reaction The first filtrate was heated to 93°C, and 19.3 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.3:1) was added under stirring at 700 rpm to carry out the second reaction. The reaction time of the second reaction was 38 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 36% of the volume of the second reaction system, the second reaction system at 90°C was immediately subjected to hot filtration to separate the second filter residue (11.2 kg) and the second filtrate (381 L at 88°C).

[0079] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0080] Step S4: Crystallization, aging, and calcination The second filtrate was transferred to a crystallization vessel at 88°C and linearly cooled to 29°C within 3.5 hours. 0.52 kg of aluminum fluoride trihydrate seed crystals (0.8 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 2.5 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation and filtration, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. After solid-phase washing and drying, the solution was calcined at 410°C for 4.5 hours to remove water, yielding 18.4 kg of anhydrous aluminum fluoride product.

[0081] Characterization of the anhydrous aluminum fluoride product in Example 5: Anhydrous aluminum fluoride purity: 98.0 wt.%, Total aluminum fluoride yield: 89.6% (calculated based on total fluorine in the raw materials).

[0082] Example 6: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue has the following composition: total fluorine content of 14.5 wt.%, total silicon content of 7.6 wt.%, total calcium content of 4.9 wt.%, total magnesium content of 1.8 wt.%, and carbon slag content of 36 wt.%.

[0083] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 105°C for 6 hours, crushed, and then ball-milled for 2 hours. After passing through a 180-mesh sieve, the residue powder to be treated was obtained.

[0084] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 500 L of deionized water to prepare a slurry with a solid content of 16.7 wt.%. Under stirring at 520 rpm, 8.7 kg of oxalic acid dihydrate (1.2 times the total molar amount of calcium and magnesium) and 6.0 kg of silica powder (molar ratio to total fluorine of 1:5.2) were added to initiate the first reaction. The temperature of the first reaction was controlled at 62 ± 2°C, and the reaction time was 12 h. Immediately after the reaction was completed, the reactants from the first reaction at 62°C were hot-filtered to obtain 41.5 kg of the first filter residue (dry basis) and 476 L of the first filtrate.

[0085] The first filter residue consists of 47.9 wt.% calcium oxalate, 8.7 wt.% magnesium oxalate, 32.8 wt.% unreacted carbon residue, and 10.6 wt.% other impurities. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or calcined to recover calcium oxide.

[0086] Step S3: Silicon and aluminum separation reaction The first filtrate was heated to 96°C, and 11.8 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.7:1) was added while stirring at 760 rpm to carry out the second reaction. The reaction time of the second reaction was 35 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 42% of the volume of the second reaction system, the second reaction system at 92°C was immediately subjected to hot filtration to separate the second filter residue (7.6 kg) and the second filtrate (446 L at 90°C).

[0087] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0088] Step S4: Crystallization, aging, and calcination The second filtrate was transferred to a crystallization vessel at 90°C and linearly cooled to 25°C within 4 hours. 0.32 kg of aluminum fluoride trihydrate seed crystals (1.0 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 3.5 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation and filtration, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. After solid-phase washing and drying at 80°C for 8 hours, the solution was calcined at 440°C for 3.5 hours to remove water, yielding 12.9 kg of anhydrous aluminum fluoride product.

[0089] Characterization of the anhydrous aluminum fluoride product in Example 6: Anhydrous aluminum fluoride purity: 98.6 wt.%, Total aluminum fluoride yield: 92.8%.

[0090] Example 7: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue has the following composition: total fluorine content of 19.2 wt.%, total silicon content of 9.5 wt.%, total calcium content of 6.7 wt.%, total magnesium content of 2.7 wt.%, and carbon slag content of 30 wt.%.

[0091] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 115°C for 4 hours, crushed, ground, and passed through a 150-mesh sieve to obtain the residue powder to be treated.

[0092] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 445 L of deionized water to prepare a slurry with a solid content of 18.4 wt.%. Under stirring at 460 rpm, 12.6 kg of oxalic acid dihydrate (1.18 times the total molar amount of calcium and magnesium) and 7.8 kg of silica powder (molar ratio to total fluorine of 1:5) were added to initiate the first reaction. The temperature of the first reaction was controlled at 60 ± 2°C, and the reaction time was 14 h. Immediately after the reaction was completed, the reactants from the first reaction at 60°C were hot-filtered to obtain 46.8 kg of the first filter residue (dry basis) and 430 L of the first filtrate.

[0093] The first filter residue consists of 50.1 wt.% calcium oxalate, 9.6 wt.% magnesium oxalate, 28.3 wt.% unreacted carbon residue, and 12.0 wt.% other impurities. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or calcined to recover calcium oxide.

[0094] Step S3: Silicon and aluminum separation reaction The first filtrate was heated to 95°C, and 16.4 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.6:1) was added under stirring at 720 rpm to carry out the second reaction. The reaction time of the second reaction was 40 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 40% of the volume of the second reaction system, the second reaction system at 91°C was immediately subjected to hot filtration to separate the second filter residue (9.7 kg) and the second filtrate (408 L at 87°C).

[0095] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0096] Step S4: Crystallization, aging, and calcination The second filtrate was fed into a crystallization vessel at 87°C and cooled to 26°C within 4 hours. 0.46 kg of aluminum fluoride trihydrate seed crystals (1.1 wt.% of the theoretical product mass) were added, and the mixture was aged for 3 hours. After solid-liquid separation, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. After washing and drying the solid phase, it was calcined at 425°C for 4 hours to obtain 16.7 kg of anhydrous aluminum fluoride product.

[0097] The second filtrate was transferred to a crystallization vessel at 87°C and linearly cooled to 26°C within 4 hours. 0.46 kg of aluminum fluoride trihydrate seed crystals (1.1 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 3 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation, filtration, washing, and drying, the product was calcined at 425°C for 4 hours to remove water, yielding 16.7 kg of anhydrous aluminum fluoride product.

[0098] Characterization of the anhydrous aluminum fluoride product in Example 7: The purity of the anhydrous aluminum fluoride is 98.4%, and the total yield of aluminum fluoride is 91.7%.

[0099] Example 8: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue has the following composition: total fluorine content of 23.0 wt.%, total silicon content of 11.0 wt.%, total calcium content of 9.1 wt.%, total magnesium content of 3.6 wt.%, and carbon slag content of 25 wt.%.

[0100] The above-mentioned waste aluminum electrolyte alkaline leaching residue was dried at 120°C for 5 hours, crushed, ground, and passed through a 130-mesh sieve to obtain the residue powder to be treated.

[0101] Step S2: Co-conversion reaction of oxalic acid and silicon source 100 kg of the residue powder to be treated was added to 410 L of deionized water to prepare a slurry with a solid content of 19.6 wt.%. Under stirring at 540 rpm, 16.8 kg of oxalic acid dihydrate (1.30 times the total molar amount of calcium and magnesium) and 9.8 kg of silica powder (molar ratio to total fluorine of 1:4.4) were added to initiate the first reaction. The temperature of the first reaction was controlled at 54 ± 2°C, and the reaction time was 18 h. Immediately after the reaction was completed, the reactants from the first reaction at 54°C were hot-filtered to obtain 52.6 kg of the first filter residue and 390 L of the first filtrate.

[0102] The first filter residue consists of 54.8 wt.% calcium oxalate, 11.5 wt.% magnesium oxalate, 22.9 wt.% unreacted carbon residue, and 10.8 wt.% other impurities. After washing and drying, the first filter residue is sold as an industrial-grade calcium oxalate byproduct, or calcined to recover calcium oxide.

[0103] Step S3: Silicon and aluminum separation reaction The first filtrate was heated to 94°C, and 21.2 kg of aluminum hydroxide (molar ratio of aluminum hydroxide to fluorosilicic acid was 2.5:1) was added while stirring at 780 rpm. The reaction was carried out for 45 min. The volume of the precipitated silica solid phase reached 43% of the volume of the second reaction system. The mixture was then hot-filtered at 93°C to obtain 12.3 kg of the second filter residue and 365 L of the second filtrate.

[0104] The first filtrate was heated to 94°C, and 21.2 kg of aluminum hydroxide (the molar ratio of aluminum hydroxide to fluorosilicic acid was 2.5:1) was added under stirring at 780 rpm to carry out the second reaction. The reaction time of the second reaction was 45 min. After the second reaction reached the reaction endpoint and the volume of silica solid phase precipitated in the reaction system reached 43%, the second reaction system at 93°C was immediately subjected to hot filtration to separate the second filter residue (12.3 kg) and the second filtrate (365 L).

[0105] The second filter residue is silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate is a metastable supersaturated solution of aluminum fluoride, and the second filtrate also contains unreacted fluorosilicic acid.

[0106] Step S4: Crystallization, aging, and calcination The second filtrate was fed into a crystallization vessel at 90°C and cooled to 28°C within 5 hours. 0.60 kg of aluminum fluoride trihydrate seed crystals (1.0 wt.% of the theoretical product mass) were added, and the mixture was allowed to stand for 4 hours. After filtration, washing, and drying, the mixture was calcined at 450°C for 3 hours to obtain 20.1 kg of anhydrous aluminum fluoride product.

[0107] The second filtrate was transferred to a crystallization vessel at 90°C and linearly cooled to 28°C within 5 hours. 0.60 kg of aluminum fluoride trihydrate seed crystals (1.0 wt.% of the theoretical product) were added, and the mixture was allowed to stand for 4 hours, resulting in the precipitation of aluminum fluoride crystals. After solid-liquid separation and filtration, the filtrate was returned to the oxalic acid-silicon source co-conversion reaction stage for preparing the oxalic acid solution. After solid-phase washing and drying, the solution was calcined at 450°C for 3 hours to obtain 20.1 kg of anhydrous aluminum fluoride product.

[0108] Characterization of the anhydrous aluminum fluoride product in Example 8: The purity of the anhydrous aluminum fluoride is 97.9%, and the total yield of aluminum fluoride is 89.1%.

[0109] The methods described in Examples 1-8 above for preparing aluminum fluoride using the low-temperature wet process with alkaline leaching residue from waste aluminum electrolytes avoid the use of highly toxic, high-pressure, and highly corrosive anhydrous hydrofluoric acid (HF), eliminating the potential for major safety accidents and significantly reducing the safety supervision and environmental compliance costs for enterprises.

[0110] The unit product energy consumption of the preparation methods in Examples 1-8 above is 380-520 kWh / ton of anhydrous aluminum fluoride product.

[0111] Comparative Example 1: Comparative Example 1 uses a traditional high-temperature dry process to treat waste aluminum electrolyte alkaline leaching residue. The polymerization steps are as follows: 1): High-temperature roasting Take 100 kg of the same waste aluminum electrolyte alkaline leaching residue as in Example 1, and roast the waste aluminum electrolyte alkaline leaching residue at 850°C for 3 hours to partially decompose the calcium fluoride, obtaining the roasted material. Energy consumption: approximately 600 kWh.

[0112] 2): Sulfuric acid leaching After roasting, the material is leached with concentrated sulfuric acid (98%) at 120°C for 2 hours to obtain a leachate with a liquid-to-solid ratio of 3:1. This step generates a large amount of fluorine-containing waste gas HF and SiF4, which requires alkaline absorption treatment.

[0113] 3): Preparation of fluorosilicic acid The leachate was cooled to 60°C, and silicon dioxide was added, which reacted to produce fluorosilicic acid.

[0114] 4): Preparation of aluminum fluoride Fluorosilicic acid and aluminum hydroxide were reacted at 95°C for 4 hours, filtered, washed, dried, and calcined at 500°C for 6 hours to obtain aluminum fluoride product.

[0115] Product characterization of Comparative Example 1: Aluminum fluoride yield: 10.8 kg, purity: 95.8%, total yield: 78.5%, total energy consumption: approximately 1520 kWh / ton of product, fluorine-containing waste gas emission: approximately 8.5 kg (calculated as HF), wastewater volume: approximately 650 L (fluorine concentration 2500 mg / L).

[0116] Compared with Comparative Example 1, the method of the present invention shows significant advantages in terms of total aluminum fluoride yield, product purity, energy consumption, and wastewater discharge. Taking Example 1 as an example, the total aluminum fluoride yield of the method of the present invention reaches 92.3%, which is significantly higher than 76.8% of Comparative Example 1; the product purity is 98.5%, which is higher than 95.8% of Comparative Example 1; the energy consumption per unit product of Example 1 is approximately 420 kWh / ton, which is about 49% lower than 820 kWh / ton of Comparative Example 1; at the same time, the method of the present invention does not introduce sodium impurities, produces no sodium silicate byproducts, and reduces wastewater discharge by about 65%, demonstrating good economic efficiency and environmental friendliness.

[0117] Comparative Example 2: Comparative Example 2 uses the traditional sodium fluorosilicate intermediate method to treat the alkaline leaching residue of waste aluminum electrolyte. The polymerization steps are as follows: 1): Sulfuric acid leaching Take 100 kg of the same waste aluminum electrolyte alkaline leaching residue as in Example 1, and leach the waste aluminum electrolyte alkaline leaching residue with 20% sulfuric acid at 80°C for 2 hours with a liquid-to-solid ratio of 4:1 to obtain a fluorine-containing and silicon-containing solution.

[0118] 2): Preparation of sodium fluorosilicate 25 kg of sodium chloride was added to a fluorine-containing and silicon-containing solution and reacted at 80°C for 2 h to produce sodium fluorosilicate precipitate; 18.5 kg of sodium fluorosilicate solid was obtained by filtration (wet basis).

[0119] 3): Preparation of aluminum fluoride Sodium fluorosilicate is reacted with aluminum hydroxide at 90°C for 3 hours to produce aluminum fluoride and sodium silicate; approximately 180 L of sodium silicate solution is produced as a byproduct, which is difficult to handle.

[0120] 4) Product separation The aluminum fluoride product is obtained by filtration, washing (5 times, 80L of water each time), drying, and calcination.

[0121] Product characterization of Comparative Example 2: Aluminum fluoride yield: 11.2 kg, purity: 96.2%, total yield: 76.8%, total energy consumption: approximately 820 kWh / ton of product, by-product sodium silicate: 180 L (COD 15000 mg / L, difficult to treat), total wastewater: approximately 580 L (containing 1800 mg / L fluoride and 8500 mg / L sodium), sodium impurity introduction: Na₂O content in the product: 0.8%. Compared to the sodium fluorosilicate intermediate method of Comparative Example 2, the method of Example 1 of this invention has the following advantages: the process is shortened by 40% (by omitting the intermediate step), the yield is increased by 15.5 percentage points (92.3% in Example 1 vs. 76.8% in Comparative Example 2), the purity is increased by 2.3 percentage points (98.5% in Example 1 vs. 96.2% in Comparative Example 2), the energy consumption is reduced by 49% (420 kWh / ton in Example 1 vs. 820 kWh / ton in Comparative Example 2), there is no sodium silicate byproduct, the wastewater volume is reduced by about 65%, and no sodium impurities are introduced.

[0122] This invention has been described through the specific embodiments described above. Those skilled in the art should understand that various modifications and equivalent substitutions can be made to this invention without departing from its scope. Parts not described in detail in this specification are well-known to those skilled in the art. Furthermore, various modifications can be made to this invention for specific situations or circumstances without departing from its scope. Therefore, this invention is not limited to the specific embodiments disclosed, but should include all embodiments falling within the scope of the claims.

Claims

1. A method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process, characterized in that, Includes the following steps: Step S1: Raw material pretreatment The waste aluminum electrolyte alkaline leaching residue is dried, crushed, ground, and sieved to 100-200 mesh to obtain uniform residue powder to be treated. Step S2: Co-conversion reaction of oxalic acid and silicon source The slag powder to be treated is dispersed in water to prepare a slurry. Under stirring conditions, oxalic acid dihydrate and silica are added to carry out the first reaction. The temperature of the first reaction is 40~90 ℃, and the reaction time of the first reaction is 10~24 h. Immediately after the reaction is completed, the reactants of the first reaction at a temperature of 40~90 ℃ are filtered to obtain the first filter residue and the first filtrate. The components of the first filter residue include calcium oxalate, magnesium oxalate and unreacted carbon residue. The first filter residue is washed and dried and sold as an industrial-grade calcium oxalate by-product, or it can be calcined to recover calcium oxide. Step S3: Separation reaction of silicon and aluminum The first filtrate is heated to 90-100°C, and aluminum hydroxide is added under vigorous stirring to carry out a second reaction. The reaction time of the second reaction is controlled within 30-60 minutes. After the second reaction reaches the endpoint, the second reaction system at a temperature of 90-100°C is immediately filtered to separate the second filter residue and the second filtrate. The vigorous stirring speed is 500-900 rpm, and the temperature of the second filtrate is 80-95°C. The second filter residue is a silica precipitate, which is washed and recycled back to step S2 as a raw material; the second filtrate includes a metastable supersaturated solution of aluminum fluoride and unreacted fluorosilicic acid; Step S4: Crystallization, aging, and calcination The second filtrate is transported to a crystallization vessel at a temperature of 80-95°C and linearly cooled to 20-40°C within 2-6 hours. Aluminum fluoride trihydrate is added as a seed crystal to induce crystallization. After standing and aging, aluminum fluoride crystals precipitate. After solid-liquid separation, the filtrate is returned to the oxalic acid-silicon source synergistic conversion reaction stage to prepare oxalic acid solution. The solid phase is calcined and dehydrated at a temperature of 350-500°C to obtain anhydrous aluminum fluoride product.

2. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S1, the total calcium content in the waste aluminum electrolyte alkaline leaching residue is 3wt.% to 10wt.%, and the total fluorine content is 10wt.% to 25wt.%.

3. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S2, the mass fraction of the slag powder to be treated in the slurry is 10wt.% to 25wt.%.

4. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S2, the amount of oxalic acid dihydrate added is 1.0 to 1.5 times the total molar amount of calcium and magnesium in the slag powder to be treated.

5. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S2, the molar ratio of the amount of silica added to the total fluorine content in the slag powder to be treated is 1:4 to 1:

6.

6. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S2, the first reaction temperature is 50~70 ℃.

7. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S2, the first filtrate contains fluorosilicic acid.

8. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In the first filter residue, the mass fraction of calcium oxalate is 40wt.%~60wt.%, the mass fraction of magnesium oxalate is 5wt.%~15wt.%, and the mass fraction of unreacted carbon residue is 20wt.%~40wt.%.

9. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S3, the molar ratio of aluminum hydroxide to fluorosilicic acid is in the range of 2:1 to 3:

1.

10. The method for preparing aluminum fluoride using waste aluminum electrolyte alkaline leaching residue via a low-temperature wet process according to claim 1, characterized in that, In step S3, when the second reaction reaches its endpoint, the volume of the precipitated silica solid phase is 30% to 50% of the volume of the second reaction system, and aluminum fluoride is not precipitated.

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