Method for preparing aluminum hydroxyl fluoride from cryolite

Abstract

The invention provides a method for preparing aluminum hydroxyl fluoride from cryolite, and relates to the technical field of aluminum hydroxyl fluoride production. The method comprises the following steps: (1) dissolving the cryolite through a leaching solution, and carrying out solid-liquid separation to obtain an acidic dissolution solution; wherein the leachate is a mixed solution of ammonium sulfate and sulfuric acid; and (2) adding an aluminum sulfate solution into the acidic digestion solution, adjusting the pH value to 3.5-5.5, carrying out a neutralization reaction, and carrying out solid-liquid separation to obtain the aluminum hydroxyl fluoride. According to the method, cryolite is used as a raw material, hydroxyl fluoroaluminum is prepared through a chemical method, and the hydroxyl fluoroaluminum is applied to the industrial application fields of cement accelerators, aluminum fluoride and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of hydroxyaluminum fluoride production, and in particular to a method for preparing hydroxyaluminum fluoride from cryolite. Background Art

[0002] Hydroxyfluoroaluminum is a compound formed by the complexation of fluoride ions with aluminum ions in an acidic liquid phase, ultimately combining with a certain amount of hydroxide and water of crystallization. Hydroxyfluoroaluminum is readily soluble in acid. At a pH of 3-4, it can maintain aluminum ions at concentrations exceeding 50g / l without hydrolysis, making it a suitable raw material for cement accelerators. Calcination of hydroxyfluoroaluminum at temperatures above 600°C removes most of the water of crystallization, resulting in a mixture of aluminum fluoride and alumina, which can be used as a raw material for dry-process aluminum fluoride production.

[0003] CN119143164A discloses a method for preparing hydroxyaluminum fluoride and aluminum fluoride using fluorine-containing waste materials, comprising first mixing fluorine-containing waste powder and a mixed solution of aluminum chloride and dilute hydrochloric acid, leaching, and solid-liquid separation to obtain a leachate; adjusting the pH value of the leachate to 3.5-6, aging, and solid-liquid separation to obtain a crude hydroxyaluminum fluoride; and secondly mixing the crude hydroxyaluminum fluoride with an aqueous solution of aluminum chloride, and solid-liquid separation to obtain a pure hydroxyaluminum fluoride. It directly uses a mixed solution of soluble aluminum salts and acids to leach fluorine-containing waste electrolytes, and then, by controlling the conditions, a fluoride salt with aluminum fluoride as the main component will be obtained in the leachate. However, experiments have found that when aluminum salts and acids are mixed for leaching, the pH in the solution is about 1, and the aluminum fluoride in the solution is mainly in the form of AlF 2+ and AlF2 + Exists in the form of complex ions; due to AlF 2+ and AlF2 + The solubility product constant of the complex ion combined with the hydroxide is high (pKsp is 24.52 at 25℃), which leads to the 2+ and AlF2 + The complex ion groups are more inclined to hydrolyze at high temperatures of 50-100°C to obtain hydroxyaluminum fluoride precipitates, so that the filter residue after leaching and separation contains a certain amount of hydroxyaluminum fluoride, which affects the aluminum fluoride recovery rate. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing hydroxyaluminum fluoride from cryolite, which can solve the problem that aluminum fluoride is precipitated as hydroxyaluminum fluoride during the leaching process and improve the recovery rate of aluminum fluoride.

[0005] The present invention is achieved through the following technical solutions: A method for preparing hydroxyaluminum fluoride from cryolite comprises the following steps: (1) dissolving cryolite in a leaching solution and performing solid-liquid separation to obtain an acidic solution; Wherein, the leachate is a mixed solution of ammonium sulfate and sulfuric acid; (2) Aluminum sulfate solution is added to the acidic solution to adjust the pH to 3.5-5.5, and a neutralization reaction is carried out. The solid-liquid separation is performed to obtain hydroxyfluoroaluminum.

[0006] Furthermore, the concentration of ammonium sulfate in the mixed solution is 50-80 g / l, and the pH value of sulfuric acid is 1-2.

[0007] Furthermore, the process of dissolving cryolite in the leachate includes: preparing a slurry by slurrying the cryolite with water, wherein the solid-liquid ratio of cryolite to water is 5-15:1, and mixing the slurry with the leachate while maintaining the pH value of the mixed system at 1-4.5, preferably 1.5-2.5.

[0008] Furthermore, in (1), the reaction temperature is 50-60°C and the reaction time is 6-10h.

[0009] Furthermore, in (2), the molar ratio of F to Al in the mixture of the acidic eluent and the aluminum sulfate solution is 1.5-3:1.

[0010] Furthermore, in (2), when alkaline solution is used to adjust the pH, the raw materials of the alkaline solution include one or more of sodium hydroxide, ammonia water, sodium carbonate or sodium aluminate.

[0011] Furthermore, in (2), the reaction temperature is 20-30°C and the reaction time is 4-6h.

[0012] Furthermore, (2) further includes cooling the filtrate after solid-liquid separation to 0-5°C, allowing Na2SO4 in the filtrate to crystallize out in the form of Glauber's salt, and then returning the filtrate to step (1).

[0013] Research has found that using aluminum salts alone as the leachate increases the aluminum ion concentration in the solution. When the aluminum ion concentration in the solution is high, the aluminum fluoride produced under acidic conditions has a stronger binding capacity for hydroxyl groups than hydrogen ions. Fluorine precipitates as basic aluminum fluoride, lowering the leachate pH. This converts the leached aluminum fluoride back into precipitation, reducing the yield of hydroxyl aluminum fluoride. Using acid alone as the leachate can cause fluorine loss due to the volatility of the generated hydrogen fluoride.

[0014] Therefore, in the present invention, a mixed solution of ammonium sulfate and sulfuric acid is used as the leachate. Sulfuric acid reacts with cryolite to generate aluminum sulfate, sodium sulfate and hydrogen fluoride. Since hydrogen fluoride is volatile and may cause fluorine loss, ammonium sulfate is added as an auxiliary. Ammonium sulfate reacts with cryolite to generate aluminum sulfate, sodium sulfate and ammonium fluoride. The generated ammonium fluoride forms a soluble fluoroaluminum-ammonium complex with F ions and aluminum ions. On the one hand, it can prevent fluoride ions from being lost in the form of hydrogen fluoride. On the other hand, it can inhibit the hydrolysis of aluminum ions, prevent the formation of hydroxyaluminum fluoride, and improve the aluminum-fluoride recovery rate.

[0015] Dissolution process: The dissolution rate of F and Al elements in cryolite can reach more than 85%. The main chemical reactions are:

[0016] The cryolite slurry after acid dissolution is filtered to remove a small amount of undissolved residue to obtain an acidic dissolution solution.

[0017] Neutralization process: The yield of F and Al elements in the liquid phase can reach more than 98%. The main chemical reactions are:

[0018]

[0019] Aluminum sulfate solution is used to adjust the molar ratio of F and Al in the mixture of acidic solution and aluminum sulfate solution, and then alkali solution is added to neutralize the mixture, so that the free fluoride ions, aluminum ions and AlF2 in the mixture are + The ions will hydrolyze to form hydroxyaluminum fluoride precipitate, and at the same time, the fluorinated aluminum in the ammonium fluorinated aluminum complex will hydrolyze to form hydroxyaluminum fluoride precipitate. In the above chemical reaction equation, the final hydroxyaluminum fluoride contains crystal water, and its chemical formula is: Al[(OH) 0.5 F 0.5 ]3·0.5H2O, with a F:Al complexation ratio of 1.3:1. Due to variations in reaction conditions, the actual F:Al complexation ratio can range from 1.5:1 to 3:1, and the corresponding amount of water of crystallization will also vary. After the reaction is complete, solid-liquid separation and washing are performed to obtain the finished product of hydroxyfluoroaluminum containing water of crystallization.

[0020] The cryolite of the present invention can be industrial cryolite or other cryolite-rich materials, such as electrolytes used in aluminum electrolysis production, electrolytes obtained by flotation of aluminum electrolysis carbon slag, and other fluorine-containing products produced by the disposal of aluminum electrolysis hazardous waste.

[0021] The technical solution of the present invention has at least the following advantages and beneficial effects: The present invention uses a mixed solution of ammonium sulfate and sulfuric acid as the leachate. Sulfuric acid reacts with cryolite to generate aluminum sulfate, sodium sulfate and hydrogen fluoride. Since hydrogen fluoride is volatile and may cause fluorine loss, ammonium sulfate is added to react with cryolite to generate aluminum sulfate, sodium sulfate and ammonium fluoride. The generated ammonium fluoride forms a soluble fluoroaluminum-ammonium complex with F ions and aluminum ions. On the one hand, it can prevent the loss of fluoride ions in the form of hydrogen fluoride, and on the other hand, it can inhibit the hydrolysis of aluminum ions and prevent the formation of hydroxyaluminum fluoride. Aluminum sulfate solution is used to adjust the molar ratio of F and Al in the mixture of acidic dissolution solution and aluminum sulfate solution, and then alkali solution is added for neutralization to allow the free fluoride ions, aluminum ions and AlF2 in the mixture to be released. + The ions will hydrolyze to form a hydroxyaluminum fluoride precipitate, and the aluminum fluoride in the ammonium fluoroaluminum complex can also be hydrolyzed to form a hydroxyaluminum fluoride precipitate, thereby improving the aluminum fluoride recovery rate. The present invention uses cryolite as a raw material to prepare hydroxyaluminum fluoride through a chemical method, and ultimately uses it in industrial applications such as cement accelerators and aluminum fluoride. It has the advantages of a simple production process and readily available raw materials from a wide range of sources. DETAILED DESCRIPTION

[0022] The present invention will be further described below in conjunction with specific embodiments, but the examples do not limit the present invention in any form. Unless otherwise stated, the raw materials and reagents used in the examples of the present invention are conventionally purchased raw materials and reagents.

[0023] Example 1 A method for preparing hydroxyaluminum fluoride from cryolite comprises the following steps: (1) Acid dissolution: Industrial cryolite and water were mixed in a stirred reactor at a liquid-to-solid mass ratio of 15:1. The leachate was added to the reactor and the mixture was mixed. The pH value of the mixed system was maintained at 2.5. The mixture was reacted at 60°C for 10 h. The concentration of ammonium sulfate in the leachate was 80 g / l and the concentration of sulfuric acid was 100 g / l.

[0024] (2) Solid-liquid separation of acid-soluble slurry: After the acid-soluble reaction is completed, the slurry will contain a small amount of undissolved cryolite and trace amounts of Ca, Mg, Fe, etc. contained in the raw cryolite to form insoluble sulfates. The above insoluble substances are filtered out to obtain a colorless and transparent acidic solution.

[0025] (3) Neutralization precipitation reaction: In a stirred reactor, aluminum sulfate solution is added to the acidic dissolution liquid to make the molar ratio of F and Al elements in the mixed liquid be 3:1, and then alkali solution is continuously added to maintain the pH value of the stirred reactor at 5.5. The reaction is carried out at 30°C for 6 hours, and then solid-liquid separation is carried out. The solid phase is washed to obtain hydroxyfluoroaluminum, and the washing water is returned to the front-end acid dissolution process for slurry adjustment.

[0026] (4) Freezing crystallization of filtrate to remove salt: the temperature of the filtrate from the neutralization precipitation reaction is lowered to 0℃, and the Na2SO4 in the filtrate is crystallized in the form of Glauber's salt. The Glauber's salt is filtered out, and the SO4 in the filtrate is 2- Reduce to below 55g / l and return to the front-end acid dissolution process.

[0027] Example 2 A method for preparing hydroxyaluminum fluoride from cryolite comprises the following steps: (1) The electrolyte used in aluminum electrolysis production (mainly composed of Na3AlF6) is crushed and ball-milled, and more than 70% of the particles after ball milling reach a particle size of 200 mesh.

[0028] (2) Acid dissolution: Electrolyte powder and water were mixed in a stirred reactor at a liquid-to-solid mass ratio of 10:1. The leachate was added to the reactor and mixed. The pH value of the mixed system was maintained at 1.5. The reaction was carried out at 50°C for 6 hours. The concentration of ammonium sulfate in the leachate was 50 g / l and the concentration of sulfuric acid was 50 g / l. (3) Liquid-solid separation of acid-soluble slurry: After the acid-soluble reaction is completed, the slurry will contain a small amount of undissolved electrolytes and trace amounts of Ca, Mg, Fe, etc. contained in the raw materials to form insoluble sulfates. The above insoluble substances are filtered out to obtain a colorless and transparent acidic solution.

[0029] (4) Neutralization precipitation reaction: In a stirred reactor, aluminum sulfate solution is added to the acidic solution to make the molar ratio of F and Al in the mixed solution 1.5:1, and then alkali solution is continuously added to maintain the pH value of the stirred reactor at 4.5. The reaction is carried out at 20°C for 5 hours, and then solid-liquid separation is carried out. The solid phase is washed to obtain hydroxy fluoroaluminum, and the washing water is returned to the front-end acid dissolution process for slurry preparation; (5) Freezing crystallization of filtrate to remove salt: the temperature of the filtrate from the neutralization precipitation reaction is lowered to 5℃, and the Na2SO4 in the filtrate is crystallized in the form of Glauber's salt. The Glauber's salt is filtered out, and the SO4 in the filtrate is 2- Reduce to below 55g / l and return to the front-end acid dissolution process.

[0030] Example 3 A method for preparing hydroxyaluminum fluoride from cryolite comprises the following steps: (1) The electrolyte (mainly Na3AlF6) flotated from the aluminum electrolysis carbon slag is crushed and ball-milled, and more than 70% of the particles after ball milling reach a particle size of 200 mesh.

[0031] (2) Acid dissolution: Electrolyte powder and water were mixed in a stirred reactor at a liquid-to-solid mass ratio of 5:1. The leachate was added to the reactor and mixed. The pH value of the mixed system was maintained at 1.5. The reaction was carried out at 55°C for 8 hours. The concentration of ammonium sulfate in the leachate was 60 g / l and the concentration of sulfuric acid was 20 g / l. (3) Liquid-solid separation of acid-soluble slurry: After the acid-soluble reaction is completed, the slurry will contain a small amount of undissolved electrolytes and trace amounts of Ca, Mg, Fe, etc. contained in the raw materials to form insoluble sulfates. The above insoluble substances are filtered out to obtain a colorless and transparent acidic solution.

[0032] (4) Neutralization precipitation reaction: In a stirred reactor, aluminum sulfate solution is added to the acidic solution to make the molar ratio of F and Al in the mixed solution 2:1, and then alkali solution is continuously added to maintain the pH value of the stirred reactor at 3.5. The reaction is carried out at 25°C for 4 hours, and then solid-liquid separation is carried out. The solid phase is washed to obtain hydroxy fluoroaluminum, and the washing water is returned to the front-end acid dissolution process for slurry preparation; (5) Freezing crystallization of filtrate to remove salt: the temperature of the filtrate from the neutralization precipitation reaction is lowered to 2 ° C, and the Na2SO4 in the filtrate is crystallized in the form of Glauber's salt. The Glauber's salt is filtered out, and the SO4 in the filtrate is 2- Reduce to below 55g / l and return to the front-end acid dissolution process.

[0033] Test Case The cryolite of Example 1 was treated with sulfuric acid, aluminum sulfate, and sulfuric acid + aluminum sulfate as leaching solutions, and the obtained product, hydroxyaluminum fluoroaluminum, was tested. The results are as follows: Table 1 Test results of hydroxy fluoroaluminum products

[0034] As shown in Table 1, Example 1 uses sulfuric acid + ammonium sulfate, and the hydroxyaluminum fluoride prepared in Example 1 has a higher yield and better purity. This shows that the preparation method of hydroxyaluminum fluoride of the present invention solves the problem of aluminum fluoride loss during the leaching process and improves the aluminum fluoride recovery rate.

[0035] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing hydroxy fluoroaluminum from cryolite, characterized in that: The following steps are involved: (1) dissolving cryolite in a leaching solution and performing solid-liquid separation to obtain an acidic solution; Wherein, the leachate is a mixed solution of ammonium sulfate and sulfuric acid; (2) Aluminum sulfate solution is added to the acidic solution to adjust the pH to 3.5-5.5, and a neutralization reaction is carried out. The solid-liquid separation is performed to obtain hydroxyfluoroaluminum.

2. The method for preparing hydroxyfluoroaluminum from cryolite according to claim 1, characterized in that: The concentration of ammonium sulfate in the mixed solution is 50-80 g / l, and the concentration of sulfuric acid is 20-100 g / l.

3. The method for preparing hydroxyaluminum fluoride from cryolite according to claim 1, characterized in that: The process of dissolving cryolite with leaching solution includes: preparing slurry by slurrying the cryolite with water, wherein the solid-liquid ratio of cryolite to water is 5-15:1, mixing the slurry with the leaching solution and maintaining the pH value of the mixed system at 1-4.

5.

4. The method for preparing hydroxyaluminum fluoride from cryolite according to claim 1, characterized in that: In (1), the reaction temperature is 50-60°C and the reaction time is 6-10h.

5. The method for preparing hydroxyfluoroaluminum from cryolite according to claim 1, characterized in that: (2) In the mixture of the acidic eluent and the aluminum sulfate solution, the molar ratio of F to Al is 1.5-3:

1.

6. The method for preparing hydroxyaluminum fluoride from cryolite according to claim 1, characterized in that: In (2), when alkaline solution is used to adjust the pH, the raw materials of the alkaline solution include one or more of sodium hydroxide, ammonia water, sodium carbonate or sodium aluminate.

7. The method for preparing hydroxyfluoroaluminum from cryolite according to claim 1, characterized in that: (2), the reaction temperature is 20-30°C and the reaction time is 4-6h.

8. The method for preparing hydroxyaluminum fluoride from cryolite according to claim 1, characterized in that: (2) also includes cooling the filtrate after solid-liquid separation to 0-5°C, allowing Na2SO4 in the filtrate to crystallize out in the form of Glauber's salt, and then returning the filtrate to step (1).

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