All-substance recycling method for preparing high-quality cryolite from waste electrolyte by alkaline process

Abstract

The invention relates to an all-substance recycling method for preparing high-quality cryolite by using a waste electrolyte alkaline process, which adopts CO2 to adjust the pH value of a leaching solution and realizes the recovery of all substances of fluorine, aluminum and sodium in the electrolyte. Sodium hydroxide is recycled through causticization reaction, so that the production cost is reduced. Calcium oxide and carbon dioxide are recycled through a calcium carbonate calcining method, and recycling of all substances is achieved. The method solves the problems that in the prior art, a large amount of hydrogen fluoride harmful gas is generated in the acid leaching process, and the content of impurity elements in leachate is high, the prepared cryolite product is high in purity, and the cryolite yield and the cryolite molecule ratio are high by taking fluorine, aluminum and sodium elements as references.

Description

Technical Field

[0001] The invention belongs to the field of resource utilization of waste electrolytes, and in particular relates to a full material recycling method for preparing high-quality cryolite from waste electrolytes. Background Art

[0002] Waste electrolyte is a hazardous waste that accompanies the electrolytic aluminum, aluminum processing and recycled aluminum industries. Waste electrolyte includes excess electrolyte that is fished out to stabilize the electrolyte height during electrolytic aluminum production, waste aluminum electrolyte obtained by salvaging carbon slag and separation, and electrolyte entrained in overhaul slag and residual anodes.

[0003] There are two common methods for treating waste electrolyte: high temperature pyrolysis and wet treatment. The wet treatment is further divided into alkaline leaching and acid leaching. Compared with acid leaching, alkaline leaching has lower requirements on the corrosion resistance of equipment, reduces the content of impurity elements in the leaching solution, and avoids the generation of toxic and harmful HF gas during the process, showing great application potential.

[0004] Cryolite is a resource product of fluorine, which has a wide range of uses and high added value. High molecular weight cryolite has excellent physical and chemical properties and is mainly used in the aluminum electrolysis industry. As a cosolvent, it can reduce production costs. Cryolite can be used as an additive for grinding products, as an insecticide in agriculture, as a sunscreen and cosolvent in glass and enamel production, as a deoxidizer in casting, and as a catalyst for olefin polymerization. It can also be used as a wear-resistant filler for resin rubber. In addition, cryolite is also an ingredient for the electrolyte and grinding wheel of aluminum alloys and boiling steel. This shows that the market demand for cryolite is huge. The synthesis of cryolite using waste aluminum electrolyte alkaline leaching solution is not only economical and environmentally friendly, but also very practical.

[0005] At present, there are several main methods for industrial production of cryolite: dry method, soda ash fluoroaluminate method, phosphate fertilizer byproduct method, coal gangue method, etc. These methods can basically meet the needs of the cryolite market, but there are also various problems, such as environmental pollution and high cost. At present, how to use low-cost methods to prepare high-purity high-molecular-weight cryolite is a hot issue in the industry and has great research significance.

[0006] In addition, patent CN1225897A proposes a method for preparing high molecular weight cryolite using a mixed solution of sodium fluorosilicate and sodium aluminate. The process uses sodium fluorosilicate, sodium aluminate and ammonia water as raw materials, decomposes sodium fluorosilicate with ammonia water under normal temperature and pressure to prepare a fluorine-containing solution, and then reacts the solution with sodium aluminate to prepare cryolite. The more obvious problem of this method is that the cost is relatively high. Patent CN102976383A proposes a process for producing high-quality cryolite using waste liquid containing fluorosilicic acid. However, the process includes ammonolysis reaction and synthesis reaction, centrifugal separation and drying and roasting, and industrial aluminum hydroxide is used in the process, which is also relatively costly.

[0007] Therefore, how to prepare high-purity high-molecular-weight cryolite by a low-cost method while making full use of waste electrolytes has become an urgent problem to be solved. The present invention is proposed. Summary of the invention

[0008] In view of the above-mentioned state of the prior art, the present invention aims to provide a method for the recycling of all substances in the preparation of high-quality cryolite by alkaline method, so as to solve the problems in the prior art that a large amount of harmful hydrogen fluoride gas is generated during the acid leaching process and the content of impurity elements in the leachate is high. The present invention uses CO2 to adjust the pH of the leachate to achieve the recovery of all substances of sodium fluoride aluminum in the electrolyte. The sodium hydroxide is recycled through the causticizing reaction, thereby reducing the production cost. Calcium oxide and carbon dioxide are recovered by calcining calcium carbonate to achieve the recycling of all substances.

[0009] The technical solution of the present invention is as follows:

[0010] The method for preparing high-quality cryolite by a waste electrolyte alkali process with full material recycling comprises the following steps:

[0011] S1: Take sodium hydroxide solution, add waste electrolyte to leaching reaction, and obtain reaction material I;

[0012] S2: adding deionized water to the reaction material I in step S1 for washing to obtain reaction material II;

[0013] S3: filtering the reaction material II in step S2 to obtain filter residue I and filtrate I;

[0014] S4: introducing carbon dioxide gas into the filtrate I in S3, adjusting the pH of the solution, and obtaining reaction material III after reaction;

[0015] S5: filtering the reaction material III in S4 to obtain the filter residue II which is the cryolite product and the filtrate II;

[0016] S6: adding calcium oxide to the filtrate II of S5 for causticization reaction to obtain reaction material IV;

[0017] S7: filtering the reaction material IV in S6 to obtain a filter residue III which is calcium carbonate and a filtrate III which is a dilute alkali solution;

[0018] S8: Evaporating and concentrating the filtrate III of S7, collecting the evaporated water, returning to the water washing step of S2 for recycling; after the filtrate III is concentrated, a concentrated alkaline solution is obtained, which is returned to step S1 for recycling;

[0019] S9: calcining the filter residue III in S7 to obtain calcium oxide as a calcined product, and returning to step S6 for recycling; collecting the carbon dioxide generated during the calcination process, and returning to step S4 for recycling.

[0020] According to the present invention, preferably, the mass fraction of the sodium hydroxide solution in step S1 is 5-30%; the liquid-solid ratio of the sodium hydroxide solution to the electrolyte is 3-6 mL: 1 g;

[0021] Preferably, the leaching reaction temperature is 60°C to 90°C, and the reaction time is 1 to 2 hours;

[0022] The leaching reaction is preferably carried out under stirring conditions, with a stirring speed of 200 to 400 rpm.

[0023] According to the present invention, preferably, the mass of water added in step S2 is 18 to 25 times the mass of the electrolyte powder added in step S1.

[0024] According to the present invention, preferably, in step S4, the filtrate I is heated to 20-90° C. and then carbon dioxide gas is introduced, the pH is adjusted to 8-11 by the carbon dioxide, and the reaction time is 0.5-1 h.

[0025] According to the present invention, preferably, the volume ratio of the mass of calcium oxide added in step S6 to the filtrate II is 1.1-2 g:100 mL.

[0026] According to the present invention, preferably, in step S9, the calcination temperature is 950-1000° C., and the calcination time is 0.5-1 h.

[0027] Beneficial effects of the present invention:

[0028] 1. The present invention utilizes solid waste electrolyte from the aluminum industry as raw material, adopts alkaline leaching to remove aluminum, fluorine and sodium elements from the electrolyte, and prepares high-quality cryolite, thereby realizing the recycling and cyclic utilization of resources within the industry, greatly reducing water consumption, low production cost and significant economic benefits.

[0029] 2. The present invention solves the problem in the prior art that a large amount of harmful hydrogen fluoride gas is generated during the acid leaching process and the content of impurity elements in the leaching solution is high.

[0030] 3. The present invention uses CO2 to adjust the pH of the leaching solution to achieve the recovery of all the sodium fluoride aluminum substances in the electrolyte. The sodium hydroxide is recycled through the causticization reaction, which reduces the production cost. Calcium oxide and carbon dioxide are recovered by calcining calcium carbonate to achieve the recycling of all substances.

[0031] 4. The cryolite product prepared by the present invention has high purity. Calculated based on fluorine, aluminum and sodium elements, the cryolite yield is high and the cryolite molecular weight is relatively high. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The present invention is a process flow chart of the method for preparing high-quality cryolite by the waste electrolyte alkali method with full material recycling.

[0033] Figure 2 Preparation of high quality cryolite by waste electrolyte alkaline method of the present invention DETAILED DESCRIPTION

[0034] The present invention provides a method for recycling all substances for preparing high-quality cryolite by an alkaline process, which solves the problems in the prior art that a large amount of harmful hydrogen fluoride gas is generated during the acid leaching process and the content of impurity elements in the leaching solution is high. The present invention uses CO2 to adjust the pH of the leaching solution to achieve the recovery of all substances of sodium fluoride aluminum in the electrolyte. The sodium hydroxide is recycled through the causticizing reaction, thereby reducing the production cost. Calcium oxide and carbon dioxide are recovered by calcining calcium carbonate to achieve the recycling of all substances.

[0035] The method for preparing high-quality cryolite by a waste electrolyte alkali process of the present invention comprises the following steps:

[0036] S1: Take sodium hydroxide solution, add waste electrolyte to leaching reaction, and obtain reaction material I;

[0037] S2: adding deionized water to the reaction material I in step S1 for washing to obtain reaction material II;

[0038] S3: filtering the reaction material II in step S2 to obtain filter residue I and filtrate I;

[0039] S4: introducing carbon dioxide gas into the filtrate I in S3, adjusting the pH of the solution, and obtaining reaction material III after reaction;

[0040] S5: filtering the reaction material III in S4 to obtain filter residue II (cryolith) and filtrate II;

[0041] S6: adding calcium oxide to the filtrate II of S5 for causticization reaction to obtain reaction material IV;

[0042] S7: filtering the reaction material IV in S6 to obtain filter residue III (calcium carbonate) and filtrate III (dilute alkaline solution).

[0043] S8: Evaporation and concentration treatment is performed on the filtrate III (diluted alkaline solution) of S7, and the evaporated water is collected and returned to the water washing step of S2 for recycling; after the filtrate III is concentrated, a concentrated alkaline solution (sodium hydroxide solution) is obtained, which is returned to step S1 for recycling;

[0044] S9: calcining the filter residue III in S7 to obtain calcium oxide as a calcined product, and returning to step S6 for recycling; collecting the carbon dioxide generated during the calcination process, and returning to step S4 for recycling.

[0045] According to the present invention, in step S1, the main components of the spent electrolyte are cryolite (Na3AlF6), alumina (Al2O3), β-alumina (NaAl 11 O 17 ), calcium fluoride (CaF2), sodium lithium cryolite (Na2LiAl3F6), and carbon (C).

[0046] The main chemical reactions of spent electrolyte and sodium hydroxide solution are:

[0047] Na3AlF6+4NaOH=Na[Al(OH)4]+6NaF

[0048] In one or more preferred embodiments, the mass fraction of the sodium hydroxide solution in step S1 is 5-30%; preferably, the liquid-solid ratio of the sodium hydroxide solution to the electrolyte is 3-6 mL: 1 g;

[0049] Preferably, the leaching reaction temperature is 60°C to 90°C, and the reaction time is 1 to 2 hours;

[0050] Preferably, the leaching reaction is carried out under stirring conditions, and the stirring speed is 200-400 rpm.

[0051] According to the present invention, after the S1 reaction is completed, the reaction material I is washed with water to ensure that the product sodium fluoride and sodium tetrahydroxyaluminate are fully dissolved in the solution.

[0052] In one or more preferred embodiments, the mass of water added in step S2 is 18 to 25 times the mass of the electrolyte powder added in step S1.

[0053] According to the present invention, carbon dioxide is introduced into S4 to carry out a carbonization reaction, and the equation for the carbonization reaction is as follows:

[0054] Na[Al(OH)4]+2CO2+6NaF=Na3AlF6+2Na2CO3+2H2O

[0055] Cryolite is synthesized through carbonization reaction, with mild reaction conditions and high yield, no new impurities are introduced during the process, and the product purity is high.

[0056] In one or more preferred embodiments, in step S4, the filtrate I is heated to 70-90° C. and then carbon dioxide gas is introduced, the pH is adjusted to 8-11 by the carbon dioxide, and the reaction time is 0.5-1 h.

[0057] According to the present invention, the causticizing reaction equation in S6 is as follows:

[0058] CaO+H2O+Na2CO3=CaCO3↓+2NaOH

[0059] The sodium carbonate in the solution is converted into sodium hydroxide through causticization reaction, so that the sodium hydroxide can be recycled, which greatly improves the economy and environmental protection.

[0060] In one or more preferred embodiments, the ratio of the mass of calcium oxide added in step S6 to the volume of filtrate II is 1.1-2 g:100 mL.

[0061] According to the present invention, the water collected in the evaporation process in step S8 can be used for water washing in S2, and the concentrated alkaline solution obtained by evaporation and concentration can be used for leaching in S1.

[0062] According to the present invention, the carbon dioxide collected in step S9 can be used for the carbonation reaction in S4, and the obtained calcium oxide can be used for the causticization reaction in S6.

[0063] The calcination reaction equation in S9 is CaCO3=CaO+CO2↑

[0064] The present invention will be further described below by means of specific examples in conjunction with the accompanying drawings, but is not limited thereto.

[0065] The waste electrolyte in the embodiment is taken from a waste electrolyte of an electrolytic aluminum oxide plant, and its main chemical composition is as follows:

[0066] Na3AlF6, Al2O3, NaAl 11 O 17 , CaF2, Na2LiAl3F6, C, etc. Due to the different pre-treatment effects, there are certain differences in the composition and content of the raw materials.

[0067] Cryolite molecular ratio, the molecular ratio is the molar ratio of sodium fluoride to aluminum fluoride in cryolite. Cryolite with a molecular ratio greater than 2.6 is called high molecular ratio cryolite.

[0068] Embodiment 1:

[0069] The method for preparing high-quality cryolite by a waste electrolyte alkali process with full material recycling comprises the following steps:

[0070] S1: Weigh 30g of waste electrolyte and 18g of sodium hydroxide to prepare 180ml of solution. Mix the electrolyte and the sodium hydroxide solution, and stir the mixture at 70°C for 1.5 hours to obtain reaction material I.

[0071] S2: Add 600 ml of water to the reaction material I of S1 for washing, and stir for half an hour to obtain the reaction material II.

[0072] S3: filtering the reaction material II in step S2 to obtain filter residue I and filtrate I.

[0073] S4: Heat the filtrate I in S3 to 80°C, adjust the pH to 8 by passing carbon dioxide, and react for half an hour to obtain the reaction material III.

[0074] S5: Filter the reaction material III in S4 to obtain filter residue II (cryolith) and filtrate II.

[0075] S6: Add 8 g of calcium oxide to the filtrate II of S5 at 90°C, and causticize for half an hour to obtain reaction material IV;

[0076] S7: filtering the reaction material IV in S6 to obtain filter residue III (calcium carbonate) and filtrate III (dilute alkaline solution).

[0077] S8: The filtrate III in S7 is evaporated and concentrated, and the evaporated water is retained and returned to the water washing step S2 for recycling; the concentrated liquid (sodium hydroxide solution) is returned to step S1 for alkaline leaching and recycling.

[0078] S9: The filter residue III calcium carbonate generated in S7 is calcined at 950°C to finally generate calcium oxide, which is returned to step S6 for recycling; the carbon dioxide generated in the process is collected and returned to step S4 for carbonation reaction for recycling.

[0079] In this embodiment, the alkali leaching efficiency can reach 70%, and the cryolite recovery rate in the alkali leaching solution can reach 99%. The molecular ratio of the cryolite product obtained is 2.65.

[0080] Embodiment 2:

[0081] The method for preparing high-quality cryolite by a waste electrolyte alkali process with full material recycling comprises the following steps:

[0082] S1: Weigh 20g of waste electrolyte and 14g of sodium hydroxide to prepare 100ml of solution. Mix the electrolyte and the sodium hydroxide solution, and stir the mixture at 70°C for 1.5 hours to obtain reaction material I.

[0083] S2: Add 500 ml of water to the reaction material I of S1 for washing, and stir for half an hour to obtain the reaction material II.

[0084] S3: filtering the reaction material II in step S2 to obtain filter residue I and filtrate I.

[0085] S4: Heat the filtrate I in S3 to 80°C, adjust the pH to 9 by passing carbon dioxide, and react for half an hour to obtain the reaction material III.

[0086] S5: Filter the reaction material III in S4 to obtain filter residue II (cryolith) and filtrate II.

[0087] S6: Add 8g of calcium oxide to the filtrate II of S5 at 90°C, and causticize for half an hour to obtain reaction material IV;

[0088] S7: filtering the reaction material IV in S6 to obtain filter residue III (calcium carbonate) and filtrate III (dilute alkaline solution).

[0089] S8: The filtrate III in S7 is evaporated and concentrated, and the evaporated water is retained and returned to the water washing step S2 for recycling; the concentrated liquid (sodium hydroxide solution) is returned to step S1 for alkaline leaching and recycling.

[0090] S9: The filter residue III calcium carbonate generated in S7 is calcined at 850°C to finally generate calcium oxide, which is returned to step S6 for recycling; the carbon dioxide generated in the process is collected and returned to step S4 for recycling by carbonation reaction.

[0091] In this embodiment, the alkali leaching efficiency can reach 72%, and the cryolite recovery rate in the alkali leaching solution can reach 99.4%. The molecular weight ratio of the cryolite product obtained is 2.67.

[0092] Test Example 1

[0093] The yield, purity and molecular ratio of the cryolite product of Example 1-2 were tested. The results are shown in Table 1.

[0094] Table 1

[0095] Project / No. Example 1 Example 2 Cryolite purity 99.6％ 99.7％ Cryolite yield 99％ 99.4％ Molecular ratio 2.65 2.67

[0096] Test Example 2

[0097] The element content of the cryolite product obtained in Example 1 was tested, and the test results are shown in Table 2.

[0098] Table 2

[0099]

[0100]

[0101] The XRD pattern of cryolite obtained in Example 1 is as follows Figure 2 shown.

[0102] From Table 2, Figure 2It can be seen that the cryolite product obtained by the present invention has high purity and good quality.

Claims

1. A method for the full material recycling of waste electrolyte alkali method for preparing high-quality cryolite, comprising the following steps: S1: Take sodium hydroxide solution, add waste electrolyte to leaching reaction, and obtain reaction material I; S2: adding deionized water to the reaction material I in step S1 for washing to obtain reaction material II; S3: filtering the reaction material II in step S2 to obtain filter residue I and filtrate I; S4: introducing carbon dioxide gas into the filtrate I in S3, adjusting the pH of the solution, and obtaining reaction material III after reaction; S5: filtering the reaction material III in S4 to obtain the filter residue II which is the cryolite product and the filtrate II; S6: adding calcium oxide to the filtrate II of S5 for causticization reaction to obtain reaction material IV; S7: filtering the reaction material IV in S6 to obtain a filter residue III which is calcium carbonate and a filtrate III which is a dilute alkali solution; S8: Evaporating and concentrating the filtrate III of S7, collecting the evaporated water, returning to the water washing step of S2 for recycling; after the filtrate III is concentrated, a concentrated alkaline solution is obtained, which is returned to step S1 for recycling; S9: calcining the filter residue III in S7 to obtain calcium oxide as a calcined product, and returning to step S6 for recycling; collecting the carbon dioxide generated during the calcination process, and returning to step S4 for recycling.

2. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: The mass fraction of the sodium hydroxide solution in step S1 is 5-30%; the liquid-solid ratio of the sodium hydroxide solution to the electrolyte is 3-6mL:1g.

3. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: The leaching reaction temperature in step S1 is 60°C to 90°C.

4. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: The leaching reaction time in step S1 is 1 to 2 hours.

5. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: In step S1, the leaching reaction is carried out under stirring conditions, and the stirring speed is 200-400 rpm.

6. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: The mass of water added in step S2 is 18 to 25 times the mass of the electrolyte powder added in step S1.

7. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: In step S4, the filtrate I is heated to 20-90°C and then carbon dioxide gas is introduced.

8. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: In step S4, carbon dioxide is introduced to adjust the pH to 8-11.

9. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: The mass ratio of calcium oxide added in step S6 to the volume ratio of filtrate II is 1.1-2 g:100 mL.

10. The method for full material recycling of waste electrolyte alkaline process for preparing high-quality cryolite according to claim 1, characterized in that: In step S4, the calcination temperature in step S9 is 950-1000°C.

Images