A method for treating calcium-containing waste aluminum electrolyte and a method for preparing aluminum fluoride

Abstract

The present invention provides a method for treating a waste aluminum electrolyte containing calcium and a method for preparing aluminum fluoride. The method for treating the waste aluminum electrolyte containing calcium mainly comprises first reacting the waste aluminum electrolyte with potassium nitrate and a soluble aluminum salt to form a soluble calcium salt, filtering to remove most of the calcium therein at one time; then reacting with a calcium complexing agent to remove the calcium therein for a second time, and removing the calcium element in the waste aluminum electrolyte containing calcium by a two-step method to obtain a low-calcium electrolyte, wherein the calcium content does not exceed 0.5%. The present invention also provides a method for preparing aluminum fluoride, using the above-mentioned low-calcium electrolyte and aluminum sulfate as raw materials, preparing aluminum fluoride by a high-temperature calcination method, and the sulfate content in the aluminum fluoride does not exceed 0.6%.

Description

Technical Field

[0001] The invention relates to the technical field of aluminum electrolyte recycling, and in particular to a method for treating calcium-containing waste aluminum electrolyte and a method for preparing aluminum fluoride. Background Art

[0002] Calcium-containing scrap aluminum electrolyte is a hazardous solid waste generated in the production process of electrolytic aluminum. It comes from scrap aluminum electrolyte produced by salvaging the carbon slag dropped by anode carbon blocks during aluminum electrolysis, and scrap aluminum electrolyte produced by waste anode carbon blocks or overhaul slag. Theoretically, the production of 1 ton of aluminum will produce about 25 kg of excess aluminum electrolyte of fluoride salt mainly composed of cryolite. Based on the 41.59 million tons of primary aluminum production in 2023, the annual production of scrap aluminum electrolyte will be about 1.03 million tons. Part of these scrap aluminum electrolytes are consumed by filling furnaces through capacity expansion and overhaul of scrap aluminum electrolysis, and part of them are piled up in the production area. At this stage, the amount of unconsumed scrap aluminum electrolyte is increasing. If it cannot be reasonably recycled, it will not only cause a waste of resources, but also the stockpiled scrap aluminum electrolyte will occupy the effective space of the enterprise. The soluble fluorine may also leak into the ground with rainwater, causing huge pollution hazards to the environment.

[0003] The main component of waste aluminum electrolyte is 80%-90% Na3AlF6, and it also contains 9%-13% Na5Al3F 14 , 1%-5% LiF, 1.5%-8% CaF2, 1%-5% Al2O3, 0-2% MgF2. In the process of electrolytic aluminum production, anhydrous aluminum fluoride needs to be added to make up for the loss of aluminum oxide in the electrolyte, which can also reduce the molecular ratio of the electrolyte, reduce the initial crystallization temperature of the electrolyte, and control the thermal balance in the aluminum electrolysis process. Therefore, it is an important solid waste treatment idea to use a large amount of Al and F elements in waste aluminum electrolyte to prepare aluminum fluoride that can be used in electrolytic aluminum production. Efficiently recovering valuable elements such as fluorine and aluminum in waste aluminum electrolyte to prepare aluminum fluoride can not only realize the recycling of fluorine and aluminum resources, but also greatly reduce the dependence of aluminum industry raw materials on strategic resources such as fluorite.

[0004] In the Chinese invention patent application CN 113149052 A, there is a method for efficiently treating fluorine-containing waste aluminum electrolytes, which includes adding anhydrous aluminum sulfate powder and a reaction aid to the waste aluminum electrolyte, mixing them evenly and grinding them to less than 0.355 mm, roasting them, cooling them at a specific rate, crushing and washing them, obtaining an aluminum fluoride product, and concentrating the filtrate to further extract byproducts such as sodium sulfate and lithium carbonate. However, in the process of preparing aluminum fluoride using waste aluminum electrolytes, due to the addition of aluminum sulfate, calcium ions in the waste aluminum electrolytes combine with sulfate to form slightly soluble calcium sulfate, which is not easy to remove, so that the sulfate content does not meet the relevant requirements in the GB / T4292-2017 standard, affecting the purity and content of the final aluminum fluoride. Summary of the invention

[0005] In view of this, the main purpose of the present invention is to provide a method for treating calcium-containing waste aluminum electrolyte, remove most of the calcium elements in the waste aluminum electrolyte, reduce the calcium content therein, and utilize it for the subsequent preparation of high-purity aluminum fluoride.

[0006] Another object of the present invention is to provide a method for preparing aluminum fluoride, which mainly comprises first treating waste aluminum electrolyte to reduce the calcium content therein; and then preparing aluminum fluoride using the waste aluminum electrolyte with low calcium content as a raw material. The sulfate content in the aluminum fluoride prepared by this method does not exceed 0.6%, which meets the requirements for sulfate content of aluminum fluoride products in the current GB / T 4292-2017 standard.

[0007] Specifically, the technical solution provided by the present invention is as follows:

[0008] A method for treating calcium-containing waste aluminum electrolyte comprises the following steps:

[0009] Primary decalcification: firstly, potassium nitrate and aluminum salt are uniformly mixed to obtain a first mixed solution; then, waste aluminum electrolyte is added to the first mixed solution, calcium fluoride therein is dissolved under stirring at 60°C-95°C, filtered, and dried to obtain a primary decalcified electrolyte; wherein, the aluminum salt includes aluminum chloride, aluminum nitrate, or any combination thereof;

[0010] Secondary decalcification: firstly, the electrolyte after primary decalcification is uniformly mixed with the calcium complex solution in an alkaline environment, and a complex reaction occurs under stirring; then, it is filtered to obtain a low-calcium electrolyte.

[0011] The primary decalcification step mainly utilizes the reaction of aluminum salt and calcium fluoride in the electrolyte at 60-95°C under the action of potassium nitrate to generate soluble calcium salt, which is removed by filtering to remove part of the calcium in the waste aluminum electrolyte. The following reactions mainly occur during the primary decalcification process: 2Al 3+ +3CaF2→2AlF3↓+3Ca 2+ ; AlF3 → AlF2 + +F - . Therefore, in a primary decalcification process, the dosage relationship between potassium nitrate, aluminum salt and waste aluminum electrolyte has an important influence on the dissolution rate of calcium fluoride. In order to ensure a high dissolution rate of calcium fluoride, the potassium nitrate concentration in the first mixed solution is 1.0-3.5 mol / L, the aluminum ion concentration is 2.5-4.0 mol / L, and the solid-liquid ratio of the waste aluminum electrolyte and the first mixed solution is 1:2-8. In order to keep calcium fluoride dissolved as much as possible, the stirring reaction time is preferably 0.3-1.0h. Preferably, the aluminum salt includes aluminum chloride hexahydrate, anhydrous aluminum chloride, aluminum nitrate nonahydrate or any combination of the two.

[0012] The main component of the waste aluminum electrolyte is 80%-90% Na3AlF6, in which the CaF2 content is 1.5%-8%. After the waste aluminum electrolyte is treated with calcium removal once, the main component of the electrolyte obtained after calcium removal is sodium fluoroaluminate Na3AlF6, in which the calcium content is 0.4-2.1%.

[0013] Obviously, during the primary decalcification process, a portion of AlF3 will also dissolve, so that while filtering and removing soluble calcium salts, a portion of aluminum and fluorine are also removed, which not only causes the loss of aluminum and fluorine, but also increases the fluorine content in the discharged wastewater, which may not meet the wastewater discharge standard. When the pH value of the reaction system exceeds 10, the aluminum element in the electrolyte is converted into soluble aluminate. Therefore, in order to reduce the loss of fluorine and aluminum elements and make the discharged wastewater meet the discharge standard, in the primary decalcification step, after the calcium fluoride is dissolved and before the filtration step, an alkaline substance is added to adjust the pH value of the reaction system to no more than 10, so that the fluorine generated by the partial dissolution of aluminum fluoride is converted into hydroxyaluminum fluoride precipitation. The reaction process is mainly as follows: Al 3+ +OH - +2F - =AlF2OH↓;AlF2 + +OH - =AlF2OH↓. Wherein, the alkaline substance is sodium hydroxide, potassium hydroxide or ammonia water. In addition, in the primary decalcification step, the pH value of the reaction system is adjusted to not exceed 10, and the aluminum fluoride product finally obtained not only has a sulfate content that meets the requirements of GB / T 4292-2017 standard, but also has an aluminum fluoride content that meets the requirements of GB / T 4292-2017 standard.

[0014] The secondary decalcification step mainly utilizes the calcium fluoride in the electrolyte and the calcium complex to undergo a complexation reaction at 75°C-100°C in an alkaline environment to further remove the calcium in the electrolyte. Therefore, in the secondary decalcification process, the concentration of the calcium complex solution is preferably 0.1-0.4 mol / L, the solid-liquid ratio of the electrolyte to the calcium complex solution after the primary decalcification is 1:2-8, and the complexation reaction time is preferably 6-24h. The process parameters of this step are specifically determined according to the calcium content in the electrolyte after the primary decalcification.

[0015] The calcium complex solution is a mixture of one or more of disodium ethylenediaminetetraacetate solution, sodium citrate solution, citric acid solution and sodium nitrilotriacetate solution. The substance providing the alkaline environment in this step is sodium hydroxide, potassium hydroxide and the like.

[0016] While ensuring the smooth progress of the calcium complexation reaction, the conversion of the aluminum element into soluble metaaluminate is reduced or avoided, and the pH value of the alkaline environment is preferably 8-10.

[0017] After the waste aluminum electrolyte is subjected to secondary decalcification treatment, the main component of the low-calcium electrolyte obtained is sodium fluoroaluminate Na3AlF6, in which the calcium content is 0.2-0.5%.

[0018] A low-calcium electrolyte obtained by the above treatment method, the low-calcium electrolyte comprises 81-87% by mass of sodium fluoroaluminate Na3AlF6 and 0.2-0.5% by mass of calcium element.

[0019] A method for preparing aluminum fluoride comprises: preparing aluminum fluoride using the low-calcium electrolyte prepared as above as a raw material.

[0020] Furthermore, the preparation method comprises: using the low-calcium electrolyte and aluminum sulfate as raw materials, calcining at 450° C.-950° C. for 1-6 hours to prepare aluminum fluoride, wherein the molar ratio of the low-calcium electrolyte to aluminum sulfate is 1:0.7-1.3.

[0021] According to experiments, the method for treating the calcium-containing waste aluminum electrolyte provided by the present invention removes the calcium element in the waste aluminum electrolyte twice, so that the calcium removal rate reaches more than 70%, or even can be completely removed, thereby improving the purity of the waste aluminum electrolyte.

[0022] In the method for preparing aluminum fluoride provided by the present invention, calcium in the waste aluminum electrolyte is first removed. Therefore, in the process of preparing aluminum fluoride by reacting the waste aluminum electrolyte that has undergone two calcium removal treatments with aluminum sulfate, it is difficult for calcium and sulfate to combine, thereby increasing the F content in the final product aluminum fluoride and reducing the sulfate content. The sulfate content is below 0.6%, meeting the GB / T 4292-2017 standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 : is the XRD pattern of aluminum fluoride prepared by Example 2 of the present invention. DETAILED DESCRIPTION

[0024] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0025] In the present invention, unless otherwise specified and / or described, all numerical values ​​involving the amount of components are "weight". Unless otherwise specified, the terms used in the present invention are all commonly used terms in the relevant field. The preparation processes, test methods, etc. used in each embodiment are conventional means well known to those skilled in the art unless otherwise specified, and the raw materials and equipment used can be obtained from public commercial channels. Among them, in the present invention, the detection methods for the components of various electrolytes and the finally obtained aluminum fluoride are XRD and ICP (inductively coupled plasma) methods.

[0026] The invention mainly comprises the following steps: firstly reacting waste aluminum electrolyte with potassium nitrate and soluble aluminum salt to form soluble calcium salt, filtering and removing most of the calcium element therein at one time; then reacting with a calcium complexing agent to remove the calcium element therein for a second time, removing the calcium element in the calcium-containing waste aluminum electrolyte by a two-step method, and obtaining a low-calcium electrolyte, wherein the calcium element content does not exceed 0.5%; and preparing aluminum fluoride by a high-temperature calcination method using the low-calcium electrolyte as a raw material, wherein the sulfate content in the prepared aluminum fluoride does not exceed 0.6%, meeting the requirement for the sulfate content in aluminum fluoride in the GB / T4292-2017 standard.

[0027] Specifically, the main technical solutions provided by the present invention are as follows:

[0028] A method for treating calcium-containing waste aluminum electrolyte comprises the following steps:

[0029] Primary decalcification: firstly, potassium nitrate and aluminum salt are uniformly mixed to obtain a first mixed solution; then, waste aluminum electrolyte is added to the first mixed solution, and calcium fluoride therein is dissolved into a soluble calcium salt under stirring at 60°C-95°C, filtered, and dried to obtain a primary decalcified electrolyte; wherein, the aluminum salt includes aluminum chloride, aluminum nitrate, or any combination thereof;

[0030] Secondary decalcification: firstly, the electrolyte after primary decalcification is uniformly mixed with the calcium complex solution in an alkaline environment, and a complex reaction occurs under stirring; then, it is filtered to obtain a low-calcium electrolyte.

[0031] A method for preparing aluminum fluoride comprises: preparing aluminum fluoride using the low-calcium electrolyte prepared as above as a raw material.

[0032] In order to make the purpose, technical solution and advantages of the present invention more clear, the technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention.

[0033] The present invention provides a method for treating calcium-containing waste aluminum electrolyte, comprising the steps of:

[0034] (I) One-time decalcification

[0035] The primary decalcification step mainly utilizes the reaction of aluminum salt and calcium fluoride in the electrolyte at 60-95°C under the action of potassium nitrate to generate soluble calcium salt, which is removed by filtering to remove part of the calcium in the waste aluminum electrolyte. The following reactions mainly occur during the primary decalcification process: 2Al 3+ +3CaF2→2AlF3↓+3Ca 2+ ; AlF3 → AlF2 + +F - Therefore, the dosage relationship between potassium nitrate, aluminum salt and waste aluminum electrolyte in the primary decalcification step has an important influence on the dissolution rate of calcium fluoride. The following single factor analysis test further illustrates the influence of the above factors on the dissolution rate of calcium fluoride, as shown in Table 1.

[0036] Experimental conditions: first, potassium nitrate, aluminum chloride hexahydrate and water are uniformly mixed to prepare a first mixed solution; then, waste aluminum electrolyte is added to the first mixed solution, and stirred continuously at 80°C for 0.5h to convert the calcium fluoride therein into calcium chloride, so that the calcium fluoride is fully dissolved. After the stirring is completed, the product is filtered and dried to obtain a decalcified electrolyte; wherein, the composition of the waste aluminum electrolyte includes: Na3AlF6 content of 80.2%, CaF2 content of 6.2%.

[0037] Table 1 Process parameters and calcium fluoride dissolution rate in the primary decalcification step

[0038]

[0039] It can be seen from Table 1 that the potassium nitrate concentration in samples 1-5 is 0.5-4 mol / L, the aluminum ion concentration is 2-4 mol / L, and other conditions are the same. When the potassium nitrate concentration is 1.0-3.5 mol / L and the aluminum salt concentration is 2.5-4.0 mol / L, the calcium fluoride dissolution rate is relatively high, and the 2.2 mol / L potassium nitrate and 3.3 mol / L aluminum ion concentrations in sample 3 are the best.

[0040] The solid-liquid ratio of the waste aluminum electrolyte and the first mixed solution in samples 3 and 6-9 is 1:1-9, and other conditions are the same. As the amount of water increases, the solid-liquid ratio decreases, and the calcium fluoride dissolution first increases rapidly, and reaches the highest when the solid-liquid ratio reaches 1:5, and then as the amount of water continues to increase, the calcium fluoride dissolution rate slowly decreases. Therefore, considering the comprehensive cost, the solid-liquid ratio is preferably 1:2-8, and 1:5 is the best.

[0041] Obviously, during the primary decalcification process, a portion of AlF3 will also dissolve, so that while filtering and removing soluble calcium salts, a portion of aluminum and fluorine will also be removed, which not only causes the loss of aluminum and fluorine, but also increases the fluorine content in the discharged wastewater, which may not meet the wastewater discharge standard. Therefore, in order to reduce the loss of fluorine and make the discharged wastewater meet the discharge standard, in the primary decalcification step, after the calcium fluoride is dissolved, an alkaline substance is added to adjust the pH value of the reaction system to not more than 10, so that the fluorine generated by the partial dissolution of aluminum fluoride is converted into hydroxyaluminum fluoride precipitation. The reaction process is mainly as follows: Al 3+ +OH - +2F - =AlF2OH↓;AlF2 + +OH - =AlF2OH↓. The alkaline substance is sodium hydroxide, potassium hydroxide or ammonia water. The reason why the pH value of the reaction system does not exceed 10 is mainly because when the pH value does not exceed 10, the aluminum element in the electrolyte is converted into soluble aluminate, resulting in the loss of aluminum element.

[0042] Therefore, the electrolyte after one decalcification obtained in Sample 3 is the electrolyte obtained under the optimal conditions, and its composition includes: NaAlF6 content of 82.7% and CaF2 content of 2.9%.

[0043] (II) Secondary decalcification

[0044] The secondary decalcification step mainly utilizes the complexation reaction between calcium fluoride in the electrolyte and EDTA-2Na in an alkaline environment at 75°C-100°C to further remove calcium from the electrolyte. The following reactions mainly occur during the secondary decalcification process: 2+ +EDTA 4- →Ca(EDTA) 2- .

[0045] The concentration of EDTA-2Na in the secondary decalcification step, the solid-liquid ratio of the electrolyte after primary decalcification and the concentration of EDTA-2Na, etc. have an important influence on the total calcium removal rate in the electrolyte, as shown in Table 2. The specific test conditions are as follows: the electrolyte after primary decalcification obtained from sample 3 is uniformly mixed with disodium ethylenediaminetetraacetic acid (EDTA-2Na) solution and sodium hydroxide solution, and the pH of the reaction system is adjusted to 9, and stirred at 90°C for 15 hours; after the stirring is completed, it is filtered, washed with water and dried to obtain a low-calcium electrolyte.

[0046] Table 2 Process parameters and calcium removal rate in the secondary decalcification step

[0047] sample EDTA-2Na concentration (mol / L) Solid-liquid ratio (g / mL) Total removal rate of calcium element (%) 10 0.10 1:5 86.98 11 0.15 1:5 90.55 12 0.25 1:5 92.11 13 0.4 1:5 89.90 14 0.5 1:5 87.05 15 0.25 1:1 85.10 16 0.25 1:2 85.99 17 0.25 1:8 91.98 18 0.25 1:9 91.57

[0048] It can be seen from Table 2 that the EDTA-2Na concentration in samples 10-14 is 0.10-0.50 mol / L, and other conditions are the same. When the EDTA-2Na concentration is 0.15-0.4 mol / L, the calcium removal rate is relatively high, and the highest is at 0.25 mol / L.

[0049] The solid-to-liquid ratio of electrolyte and EDTA-2Na concentration after primary decalcification in samples 12 and 15-18 is 1:1-9, and other conditions are the same. As the amount of water increases, the solid-to-liquid ratio decreases, and the total removal of calcium element first increases rapidly, and reaches the highest when the solid-to-liquid ratio reaches 1:5, and then as the amount of water continues to increase, the total removal rate of calcium element does not change significantly. Therefore, considering the comprehensive cost, the solid-to-liquid ratio is preferably 1:2-8, and 1:5 is the best.

[0050] Therefore, the low-calcium electrolyte obtained by sample 12 has the highest total calcium removal rate under the optimal conditions, which can reach 92%, wherein the NaAlF6 content is about 84.2% and the CaF2 content is 0.8%.

[0051] A method for preparing aluminum fluoride comprises: preparing aluminum fluoride using the low-calcium electrolyte prepared as above as a raw material.

[0052] Furthermore, the preparation method comprises: using the low-calcium electrolyte and aluminum sulfate as raw materials, calcining at 450°C-950°C for 1-6 hours to prepare aluminum fluoride. The molar ratio of the low-calcium electrolyte to aluminum sulfate is 1:0.7-1.3, the calcination temperature is preferably 500°C-850°C, and the calcination time is preferably 2-6 hours.

[0053] Among them, aluminum sulfate and the low-calcium electrolyte are calcined to obtain a calcined product, and the calcined product is ground by wet grinding to obtain a mixed slurry with a concentration of 5% to 60%; the mixed slurry is subjected to solid-liquid separation treatment to obtain the filter residue and the filtrate; the filter residue is subjected to a high-temperature dehydration treatment at 450°C-800°C to obtain an aluminum fluoride product.

[0054] The present invention will be further described below with reference to specific embodiments.

[0055] Example 1

[0056] This embodiment provides a method for treating calcium-containing waste aluminum electrolyte, comprising the steps of:

[0057] Primary decalcification: firstly, potassium nitrate, aluminum chloride hexahydrate and water are uniformly mixed to obtain a first mixed solution, wherein the potassium nitrate concentration is 2.2 mol / L and the aluminum ion concentration is 3.3 mol / L; then, waste aluminum electrolyte is added to the first mixed solution, at which time the pH value of the system is about 6.0, and after continuous stirring for 0.5 hours, the waste aluminum electrolyte no longer dissolves, and the calcium fluoride therein reacts with aluminum chloride to form water-soluble calcium chloride, and then filtered and dried to obtain a decalcified electrolyte; wherein, the composition of the waste aluminum electrolyte includes: the composition of the waste aluminum electrolyte includes: Na3AlF6 content is 81.0%, Na5Al3F 14 The content of the electrolyte is 9.0%, the content of Al2O3 is 1.5%, the content of CaF2 is 6.0%, the content of MgF2 is 1%, and the content of LiF is 1.5%; the composition of the electrolyte after one decalcification includes: the content of Na3AlF6 is 82.1%, the content of Na5Al3F 14 The content is 9.9%, the Al2O3 content is 1.8%, the CaF2 content is 2.7%, the MgF2 content is 0.9%, the LiF content is 1.4%, and the AlF3 content is 1.2%;

[0058] Secondary decalcification: The electrolyte after primary decalcification was uniformly mixed with 0.25 mol / L alkaline ethylenediaminetetraacetic acid disodium (EDTA-2Na) solution and stirred for 15 hours, wherein the pH of the alkaline solution was 9; after the stirring, the electrolyte was filtered and dried to obtain a low-calcium electrolyte, wherein the content of the low-calcium electrolyte was about 82.5% of Na3AlF6, Na5Al3F 14 The content is 10.1%, the Al2O3 content is 1.9%, the CaF2 content is 0.8%, the MgF2 content is 0.7%, the LiF content is 1.3%, the AlF3 content is 1.8%, and the AlF2OH content is 0.9%.

[0059] The present embodiment also provides a method for preparing aluminum fluoride using the above-mentioned low-calcium electrolyte, comprising the steps of: uniformly mixing the low-calcium electrolyte (calculated as the molar number of cryolite) and aluminum sulfate in a molar ratio of 1:0.9, and calcining at 850°C for 3 hours to obtain a calcined product, grinding the calcined product by wet grinding to obtain a mixed slurry with a concentration of 30%; performing solid-liquid separation treatment on the mixed slurry to obtain the filter residue and the filtrate; and performing high-temperature dehydration treatment on the filter residue at 800°C to obtain an aluminum fluoride product.

[0060] Example 2

[0061] This embodiment provides a method for treating calcium-containing waste aluminum electrolyte and a method for preparing aluminum fluoride, which are basically the same as the method provided in the embodiment, and the main difference is that:

[0062] In the primary decalcification step, when the waste aluminum electrolyte is added to the first mixed solution and stirred for 0.5 hours until the waste aluminum electrolyte is no longer dissolved, a 2 mol / L sodium hydroxide solution is added to adjust the pH value of the system to 9, and the stirring is continued for 0.5 hours, and the product is filtered and dried to obtain a decalcified electrolyte; wherein the composition of the decalcified electrolyte includes: Na3AlF6 content of 81.9%, Na5Al3F 14 The content is 9.4%, the Al2O3 content is 2.2%, the CaF2 content is 2.4%, the MgF2 content is 0.8%, the LiF content is 1.2%, the AlF3 content is 1.3%, and the AlF2OH content is 0.8%;

[0063] The low calcium electrolyte obtained after the secondary decalcification treatment includes Na3AlF6 with a content of about 82.3%, Na5Al3F 14 The content of aluminum fluoride is 9.8%, the content of Al2O3 is 2.3%, the content of CaF2 is 0.4%, the content of MgF2 is 0.6%, the content of LiF is 1.2%, the content of AlF3 is 2.2%, and the content of AlF2OH is 1.2%. The XRD pattern of the aluminum fluoride product prepared in this embodiment is as follows: Figure 1 shown.

[0064] Example 3

[0065] This embodiment provides a method for treating calcium-containing waste aluminum electrolyte, comprising the steps of:

[0066] Primary decalcification: firstly, potassium nitrate, aluminum nitrate nonahydrate and water are uniformly mixed to obtain a first mixed solution, wherein the potassium nitrate concentration is 3.5 mol / L and the aluminum ion concentration is 4 mol / L; then, waste aluminum electrolyte is added to the first mixed solution, and the mixture is stirred for 1 hour, and then 2 mol / L sodium hydroxide solution is added to adjust the pH value of the system to 9, and the mixture is stirred for 0.5 hours, and then the product is filtered and dried to obtain a decalcified electrolyte; wherein, the waste aluminum electrolyte is composed of: Na3AlF6 content is 80.5%, Na5Al3F 14 The content is 9.6%, Al2O3 content is 1.8%, CaF2 content is 6.8%, MgF2 content is 1.3%, and the composition of the electrolyte after one decalcification is: Na3AlF6 content is 81.1%, Na5Al3F 14 The content is 9.9%, the Al2O3 content is 2.6%, the CaF2 content is 2.9%, the MgF2 content is 1.2%, the AlF3 content is 1.4%, and the AlF2OH content is 0.9%;

[0067] Secondary decalcification: The electrolyte after primary decalcification was uniformly mixed with 0.25 mol / L sodium citrate solution, and stirred continuously for 20 hours. The pH of the reaction system was maintained at 9 by adding 2 mol / L sodium hydroxide solution. After stirring, the electrolyte was filtered and dried to obtain a low-calcium electrolyte. The composition of the low-calcium electrolyte was about 81.9% Na3AlF6, 14 The content is 10.1%, the Al2O3 content is 2.7%, the CaF2 content is 1.1%, the MgF2 content is 1.0%, the AlF3 content is 2.0%, and the AlF2OH content is 1.2%.

[0068] The present embodiment also provides a method for preparing aluminum fluoride using the above-mentioned low-calcium electrolyte, comprising the steps of: uniformly mixing the low-calcium electrolyte (calculated as the molar number of cryolite) and aluminum sulfate in a molar ratio of 1:0.7, and calcining at 650°C for 2 hours to obtain a calcined product, grinding the calcined product by wet grinding to obtain a mixed slurry with a concentration of 30%; performing solid-liquid separation treatment on the mixed slurry to obtain the filter residue and the filtrate; and performing high-temperature dehydration treatment on the filter residue at 600°C to obtain an aluminum fluoride product.

[0069] Example 4

[0070] This embodiment provides a method for treating calcium-containing waste aluminum electrolyte, comprising the steps of:

[0071] Primary decalcification: firstly, potassium nitrate, aluminum nitrate nonahydrate and water are uniformly mixed to obtain a first mixed solution, wherein the potassium nitrate concentration is 1.0 mol / L and the aluminum ion concentration is 2.5 mol / L; then, waste aluminum electrolyte is added to the first mixed solution, and the mixture is stirred for 1.0 h, and then 2 mol / L sodium hydroxide solution is added to adjust the pH value of the system to 10, and the mixture is stirred for 0.5 h, and then the product is filtered and dried to obtain a decalcified electrolyte; wherein the waste aluminum electrolyte is composed of: Na3AlF6 content is 84.1%, Na5Al3F 14 The content of calcium is 11.4%, the content of Al2O3 is 1.7%, the content of CaF2 is 2.2%, and the content of MgF2 is 0.6%. The composition of the electrolyte after one calcium removal is: the content of Na3AlF6 is 84.6%, the content of Na5Al3F 14 The content is 11.5%, the Al2O3 content is 1.8%, the CaF2 content is 1.2%, the MgF2 content is 0.3%, the AlF3 content is 0.4%, and the AlF2OH content is 0.2%;

[0072] Secondary decalcification: The electrolyte after primary decalcification is uniformly mixed with 0.25 mol / L sodium trinitrilate solution, and stirred for 10 hours. A 2 mol / L sodium hydroxide solution is added to maintain the pH of the reaction system at 9. After stirring, the electrolyte is filtered, washed with water and dried to obtain a low-calcium electrolyte. The low-calcium electrolyte comprises: Na3AlF6 with a content of 84.8%, Na5Al3F 14 The content is 11.7%, the Al2O3 content is 1.8%, the CaF2 content is 0.3%, the MgF2 content is 0.4%, the AlF3 content is 0.7%, and the AlF2OH content is 0.3%.

[0073] The present embodiment also provides a method for preparing aluminum fluoride using the above-mentioned low-calcium electrolyte, comprising the steps of: uniformly mixing the low-calcium electrolyte (calculated as the molar number of cryolite) and aluminum sulfate in a molar ratio of 1:1.3, and calcining at 500°C for 4 hours to obtain a calcined product, grinding the calcined product by wet grinding to obtain a mixed slurry with a concentration of 30%; performing solid-liquid separation treatment on the mixed slurry to obtain the filter residue and the filtrate; and performing high-temperature dehydration treatment on the filter residue at 500°C to obtain an aluminum fluoride product.

[0074] Comparative Example

[0075] Comparative Examples 1-3 respectively provide a method for preparing aluminum fluoride, and the main difference between them and the method for preparing aluminum fluoride provided in Examples 1-4 is that: each comparative example does not perform calcium removal treatment on the waste aluminum electrolyte, that is, aluminum fluoride is directly prepared using the waste aluminum electrolyte as raw material, and other processes and parameters are the same.

[0076] The calcium content and F content in the waste aluminum electrolyte used in Examples 1-4 and Comparative Examples 1-3, and the calcium content, F content, and sulfate content in the obtained aluminum fluoride products were tested, and the results are shown in Table 3.

[0077] Table 3 Content of some ingredients in raw materials and products in Examples and Comparative Examples

[0078]

[0079] Therefore, the method for preparing aluminum fluoride provided in the embodiment of the present invention can achieve a calcium removal rate of more than 92% by first removing calcium from the waste aluminum electrolyte, and it is even possible to remove the calcium almost completely. Therefore, in the process of preparing aluminum fluoride by reacting the waste aluminum electrolyte that has undergone two calcium removal treatments with aluminum sulfate, it will be difficult for calcium and sulfate to combine, thereby increasing the F content in the final product aluminum fluoride and reducing the sulfate content, meeting the GB / T 4292-2017 standard.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or some technical features can be replaced by equivalents without departing from the spirit of the technical solution of the present invention, which should be included in the scope of the technical solution for protection of the present invention.

Claims

1. A method for treating calcium-containing waste aluminum electrolyte, comprising the steps of: Primary decalcification: firstly, potassium nitrate and aluminum salt are uniformly mixed to obtain a first mixed solution; then, waste aluminum electrolyte is added to the first mixed solution, calcium fluoride is dissolved therein at 60°C-95°C under stirring, filtered, and dried to obtain a decalcified electrolyte; wherein, The aluminum salt includes aluminum chloride, aluminum nitrate or any combination thereof; Secondary decalcification: firstly, the electrolyte after primary decalcification is uniformly mixed with the calcium complex solution in an alkaline environment, and a complex reaction occurs under stirring; then, it is filtered to obtain a low-calcium electrolyte.

2. The processing method according to claim 1, characterized in that The potassium nitrate concentration in the first mixed solution is 1.0-3.5 mol / L, and the aluminum ion concentration is 2.5-4.0 mol / L.

3. The processing method according to claim 1, characterized in that: The solid-to-liquid ratio of the waste aluminum electrolyte to the first mixed solution is 1:2-8.

4. The processing method according to any one of claims 1 to 3, characterized in that: The primary decalcification step further comprises: adding alkaline substances after the calcium fluoride dissolution step and before the filtering step to adjust the pH value of the reaction system to not exceed 10, so that the dissolved aluminum fluoride is converted into hydroxyaluminum fluoride precipitate.

5. The processing method according to claim 1, characterized in that In the secondary decalcification step, the concentration of the calcium complex solution is 0.1-0.4 mol / L, the solid-liquid ratio of the electrolyte to the calcium complex solution after the primary decalcification is 1:2-8, and the pH of the alkaline environment is 8-10.

6. The processing method according to claim 1, characterized in that: The complexation reaction temperature is 75°C-100°C.

7. The processing method according to claim 5 or 6, characterized in that: The calcium complex solution is one or a mixture of disodium ethylenediaminetetraacetate solution, sodium citrate solution, citric acid solution and sodium nitrilotriacetate solution.

8. A low-calcium electrolyte obtained by the treatment method according to any one of claims 1 to 7.

9. A method for preparing aluminum fluoride, comprising: Aluminum fluoride is prepared using the low-calcium electrolyte described in claim 8 as a raw material.

10. The preparation method according to claim 9, characterized in that: The low-calcium electrolyte and aluminum sulfate are used as raw materials and calcined at 450° C.-950° C. for 1-6 h to prepare aluminum fluoride.

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