Method for improving the whiteness of regenerated cryolite and the reagent used

Abstract

The present application belongs to the technical field of comprehensive utilization of solid waste resources, and particularly relates to a method for improving the whiteness of regenerated cryolite. The method comprises the following steps: dispersing cryolite powder and a surfactant in water to obtain a suspension slurry, then adding a composite reagent dropwise, removing impurity agglomerates by gradient shear stirring and using a physical sorting device, and then filtering, washing and drying to obtain high-whiteness cryolite. The composite reagent comprises an A component and a B component, the A component is a thiol-modified polyethyleneimine, and the B component is polyacrylamide. Through integrated agglomeration treatment of carbon, iron, sulfur, silicon and other impurities, the present application breaks the technical bottleneck that traditional pickling cannot efficiently remove fine carbon powder, not only removes multiple impurities at one time, but also significantly improves the whiteness. The whitened regenerated cryolite can be directly returned to the electrolytic cell for recycling, which meets the environmental protection requirements.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource comprehensive utilization technology, specifically relating to a method for improving the whiteness of recycled cryolite and the reagents used. Background Technology

[0002] Waste cryolite from aluminum electrolysis cell residue is a highly hazardous waste, with an annual production of approximately 5 million tons, most of which is disposed of through landfill. However, with the development of the circular economy, higher demands are being placed on the comprehensive utilization of waste resources. Cryolite (sodium hexafluoroaluminate, Na3AlF6) is a crucial flux in the aluminum electrolysis industry. Waste cryolite contains significant amounts of fluorine and aluminum resources. Replacing primary cryolite with it can significantly reduce carbon emissions in the aluminum electrolysis industry chain, reduce dependence on primary fluorite, enhance the industry's green competitiveness, and drive the development of the fluorochemical and metallurgical solid waste co-utilization industries.

[0003] Currently, cryolite recovered from electrolytic aluminum cell residues often uses alcohol esters, diesel oil, etc. as flotation agents to remove carbon. However, trace amounts of oily flotation agents often leave behind carbonaceous powder impurities. The presence of these impurities not only causes the regenerated cryolite product to appear gray, brown, or even dark in macroscopic terms, but more seriously, when cryolite with low whiteness and high impurities is put into the electrolytic cell, the impurities will continuously accumulate in the electrolyte system, inducing a decrease in current efficiency and a surge in power consumption. Furthermore, they may enter the molten aluminum through the electrochemical reduction process, thereby negatively impacting the grade of the primary aluminum and its downstream processing performance.

[0004] In existing technologies, the removal of impurities from cryolite typically employs traditional techniques such as physical washing, grading, and acid leaching. Acid leaching utilizes a strong acid solution to chemically react with metallic impurities like iron and aluminum oxide on the cryolite surface, converting them into soluble salts that are removed with the washing solution. However, this method is inefficient at removing hydrophobic fine carbon powder and complex substances like silicon and sulfides. This is because the acid solution lacks effective interfacial wettability and affinity with the fine carbon particles, causing the carbon powder to adhere tightly to the cryolite crystal surface under interfacial tension, even becoming trapped within crystal clusters, making effective removal impossible.

[0005] Patent CN119176575A provides a method for separating and purifying cryolite from electrolytic aluminum slag. The method involves sequentially removing impurities and crushing the electrolytic aluminum slag to obtain fragmented electrolytic aluminum slag, and then removing the carbon components from the fragmented slag to obtain carbon-free electrolytic aluminum waste residue. This carbon-free waste residue is then further crushed into fragmented electrolytic aluminum waste residue, which is then formulated into a slurry. This slurry is then diluted to prepare a leaching solution. The leaching solution is subjected to acidification and solid-liquid separation to extract the precipitated solids. The solids are then preliminarily washed and extracted to obtain a cryolite extract. The cryolite extract is then deeply washed and dried to obtain a cryolite product, which is then melted and solidified to obtain a refined cryolite product. This method adds extraction and calcination after the acid washing step, making the process lengthy, cumbersome, and costly, resulting in poor overall economic efficiency. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this application provides a method and reagent for improving the whiteness of recycled cryolite, aiming to solve the technical problems in the prior art, such as low whiteness, incomplete impurity removal, and serious environmental pollution caused by residual impurities such as carbon, iron, silicon, and sulfur in the production or recycling process of cryolite.

[0007] To achieve the above-mentioned objectives, the technical solution adopted by the present invention is as follows:

[0008] This invention provides a method for improving the whiteness of recycled cryolite, the method comprising the following steps:

[0009] Cryolite powder and surfactant are dispersed in water to prepare a suspension slurry. Then, a composite agent is added dropwise, and the mixture is subjected to gradient shearing and stirring at 25-60℃ for 15-40 minutes. Impurities and agglomerates are removed using a physical sorting device, followed by filtration, washing, and drying to obtain high-whiteness cryolite. The composite agent includes component A and component B. Component A is thiol-modified polyethyleneimine, and component B is a nonionic bridging agent, preferably polyacrylamide.

[0010] Furthermore, the amount of the composite agent added is 0.01-0.5% of the dry weight of the cryolite powder.

[0011] Furthermore, the rate at which the compound agent is added is 5-10% of the total drug dose per minute.

[0012] Furthermore, the solid content of the suspension slurry is 20-40%wt, and the D50 of the cryolite powder is 45-75μm.

[0013] Furthermore, the preferred surfactants are fatty alcohol polyoxyethylene ether (AEO-9) or sodium lignosulfonate, used at 0.15-0.45% of the cryolite powder mass. Pre-addition to water can further reduce the wetting angle of the cryolite surface, promote the dissociation of fine oily carbon powder from the cracks in the cryolite crystal surface, and provide a cleaner reaction interface for the directional adsorption of subsequent reagents.

[0014] Furthermore, the gradient shear stirring includes two stages: a high-shear contact stage and a low-shear agglomeration growth stage.

[0015] During the high-shear contact stage, the stirring speed is 400-600 rpm, maintained for 5-15 minutes. In this stage, the high turbulence intensity is used to break the weak physical adsorption between impurities and the cryolite matrix, and to promote the coordination chelation between the thiol functional groups of component A and the metal impurity ions. At the same time, the hydrophobic backbone of component A is used to coat the surface of the fine carbon powder.

[0016] The low-shear agglomeration growth is carried out by stirring at 80-150 rpm for 10-25 minutes. During this stage, the long-chain bridging effect of component B causes the impurity particles modified by component A to collide and bridge with each other, forming stable impurity agglomerates with a particle size between 150-500 μm.

[0017] Furthermore, the physical sorting equipment is a hydrocyclone classifier. The feed pressure is controlled at 0.1-0.35 MPa. Through the difference in centrifugal force field, the carbonaceous agglomerates with lower density and fine-grained impurities are discharged from the overflow port, while high-purity cryolite particles are discharged from the underflow port.

[0018] Furthermore, the temperature for heating and drying the cryolite product is 105-120℃.

[0019] The composite agent provided by this invention is composed of component A and component B in a mass ratio of 3-8:1. When the proportion of component A is too high, although the chelating ability is strong, the initial nuclei formed are too small, which is not conducive to subsequent separation; when the proportion of component B is too high, excessive bridging is likely to occur, resulting in cryolite particles being mechanically entrained.

[0020] Further, component A is thiol-modified high-molecular-weight polyethyleneimine. Component A can be obtained directly from external purchase, or it can be prepared by the following method: using branched polyethyleneimine (PEI) as a backbone, a thiol group (-SH) is introduced into the polymer side chain through an amidation reaction with mercaptoacetic acid or mercaptopropionic acid under acid catalysis. This component has multidentate chelating sites, and the chelating field formed by its nitrogen and sulfur atoms can bind with Fe... 3+ Cu 2+ Transition metal ions can form highly stable five- or six-membered chelate rings.

[0021] Furthermore, component B is a nonionic bridging agent, preferably polyacrylamide (PAM). Component B exhibits a highly extended linear conformation in aqueous solution, and achieves long-range bridging by forming hydrogen bonds or van der Waals forces with the surface of impurity particles adsorbed by component A through its amide groups.

[0022] Mechanism of Action: The above-mentioned impurity removal process involves the synergistic effect of selective adsorption and bridging. Specifically, due to the strong hydrophobicity of carbon powder impurities on the cryolite surface, they are extremely difficult to remove in an aqueous system. However, the hydrophobic carbon chain skeleton of component A interacts with the carbon powder surface through π-π bonds, achieving directional adsorption. At the same time, the thiol groups have a high affinity for impurities such as iron and sulfur. When the impurity surface is covered by component A, component B, through the entanglement and pulling of its long chains, overcomes the electrostatic repulsion between the fine impurity particles, inducing the impurity particles to undergo non-spontaneous aggregation. This greatly increases the particle size difference and sedimentation velocity difference between the impurities and the cryolite matrix.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This invention is applicable to the whitening and impurity removal of recycled cryolite. Through integrated agglomeration treatment of impurities such as carbon, iron, sulfur, and silicon, it overcomes the technical bottleneck of traditional acid washing methods, which cannot efficiently remove fine carbon powder. It not only removes multiple impurities in one step but also simplifies the process and significantly improves whiteness. The amount of reagent used is only 0.01-0.5% of the cryolite's weight, far lower than the acid consumption in traditional acid washing methods. Furthermore, the composite reagent used can be removed along with the impurity agglomerates in subsequent processes or biodegraded in water circulation. The whitened recycled cryolite can be directly returned to the electrolytic cell for reuse, meeting environmental protection requirements. Attached Figure Description

[0025] Figure 1 This is a photograph of the whitened cryolite after impurity removal in Example 1.

[0026] Figure 2 This is a photograph of cryolite, the raw material, before impurity removal in Example 1. Detailed Implementation

[0027] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. These embodiments are only for illustrating the present invention and should not be regarded as limiting the scope of the present invention or the method of implementing the present invention. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The cryolite raw material used in this embodiment is recycled cryolite from a cryolite factory in Henan Province. The initial whiteness is 51.2%, and the chemical composition is as follows: iron (Fe) content 0.28%, total carbon (C) content 0.85%, silicon dioxide (SiO2) content 0.15%, and sulfur (S) content 0.08%. The thiol-modified polyethyleneimine (PEI-SH) used is purchased from a biotechnology company in Xi'an, with a Mw of 20000 and a degree of substitution of 18%.

[0028] Example 1

[0029] A method to improve the whiteness of recycled cryolite.

[0030] (1) Take 3.6g of PEI20K-SH type thiol-modified polyethyleneimine and Mw of 12×10 6 0.7g of PAM was mixed to obtain 4.3g of compound agent, which was then prepared for use.

[0031] (2) Add 5 kg of purified water and 4.3 g of sodium lignosulfonate to a beaker and stir to dissolve. Then add 2.15 kg of cryolite powder and stir at 600 rpm for 10 min to obtain a suspension (solid content 30% wt). Then slowly add the above-mentioned compound reagent dropwise, and complete the addition in 10 min. First stir at 500 rpm for 10 min at 40℃, and then stir at 100 rpm for 20 min at 42℃. Remove the suspended matter with a hydrocyclone classifier, then filter, wash, and dry at 110℃ for 2 h to obtain 2.09 kg of whitening cryolite.

[0032] Example 2

[0033] A method to improve the whiteness of recycled cryolite.

[0034] (1) Take 4.9g of PEI20K-SH type thiol-modified polyethyleneimine and Mw of 12×10 6 1.4g of PAM was mixed to obtain 6.3g of compound agent, which was then prepared for use.

[0035] (2) Add 5 kg of purified water and 4.9 g of sodium lignosulfonate to a beaker and stir to dissolve. Then add 1.4 kg of cryolite powder and stir at 600 rpm for 10 min to obtain a suspension (solid content 22% wt). Then slowly add the above compound agent dropwise over 13 min. First stir at 30℃ at 450 rpm for 15 min, then stir at 35℃ at 120 rpm for 15 min. Remove the suspended solids using a hydrocyclone classifier, then filter, wash, and dry at 105℃ for 2 h to obtain 1.35 kg of whitening cryolite.

[0036] Example 3

[0037] A method to improve the whiteness of recycled cryolite.

[0038] (1) Take 4.2g of PEI20K-SH type thiol-modified polyethyleneimine and Mw of 11×10 6 0.6g of PAM was mixed to obtain 4.8g of compound agent, which was then prepared for use.

[0039] (2) Add 5 kg of purified water and 9.6 g of AEO-9 to a beaker and stir to dissolve. Then add 3.2 kg of cryolite powder and stir at 600 rpm for 10 min to obtain a suspension (solid content 39% wt). Then slowly add the above composite reagent dropwise, completing the addition in 12 min. First stir at 500 rpm for 12 min at 30℃, then stir at 140 rpm for 15 min at 35℃. Remove the suspended matter using a hydrocyclone classifier, then filter, wash, and dry at 115℃ for 2 h to obtain 3.12 kg of whitening cryolite.

[0040] Example 4

[0041] A method to improve the whiteness of recycled cryolite.

[0042] (1) Take 4.4g of PEI20K-SH type thiol-modified polyethyleneimine and Mw of 15×10 6 0.9g of PAM was mixed to obtain 5.3g of compound agent, which was then prepared for use.

[0043] (2) Add 5 kg of purified water and 3 g of AEO-9 to a beaker and stir to dissolve. Then add 1.75 kg of cryolite powder and stir at 600 rpm for 10 min to obtain a suspension (solid content 26% wt). Then slowly add the above-mentioned composite reagent dropwise, and complete the addition in 16 min. First stir at 500 rpm for 10 min at 40℃, and then stir at 100 rpm for 20 min at 45℃. Remove the suspended matter with a hydrocyclone classifier, then filter, wash, and dry at 120℃ for 2 h to obtain 1.7 kg of whitening cryolite.

[0044] Comparative Example 1 (Conventional pickling method)

[0045] 5 kg of 15%wt hydrochloric acid and 2.15 kg of cryolite powder were added to a beaker and stirred at 600 rpm for 120 minutes at 60°C. The mixture was then filtered, washed, and dried at 110°C for 2 hours to obtain 2.11 kg of whitened cryolite.

[0046] Comparative Example 2 (using only component A)

[0047] The only difference from Example 1 is the type of reagent used. Only component A, PEI-SH, was used, and the dosage was maintained at 3.6g, resulting in 2.1kg of whitening cryolite.

[0048] Comparative Example 3 (using component B only)

[0049] The only difference from Example 1 is the type of reagent used; only component B, PAM, is used, and the dosage is maintained at 4.3g, resulting in 2.11kg of whitening cryolite.

[0050] To visually demonstrate the technical effects of the above embodiments and comparative examples, the experimental data are summarized in Table 1 below.

[0051] Table 1. Comparison of Impurity Removal Effect and Whiteness Improvement Data for Cryolite

[0052] Experimental group Fe content (%) C content (%) S content (%) <![CDATA[SiO2(%)]]> Whiteness Whiteness increase Example 1 0.031 0.045 0.009 0.042 83.4 32.2 Example 2 0.018 0.032 0.015 0.038 85.6 34.4 Example 3 0.042 0.052 0.012 0.055 87.1 35.9 Example 4 0.055 0.025 0.010 0.062 81.5 30.3 Comparative Example 1 0.125 0.780 0.065 0.130 59.4 8.2 Comparative Example 2 0.068 0.520 0.035 0.095 65.7 14.5 Comparative Example 3 0.220 0.410 0.058 0.115 62.3 11.1

[0053] The data in the table show that Examples 1-4 all achieved a significant increase in whiteness, with the highest whiteness reaching 87.1. In contrast, the traditional acid method (Comparative Example 1) contributed very little to the whiteness, only 7.2. Comparative Example 2 (using only component A) lacked long-chain bridging effects. Although it had a good chelating and removal effect on iron and sulfur, the resulting micro-chelates were difficult to effectively retain in the physical sorting process, resulting in a final whiteness of only 65.7. Comparative Example 3 (using only component B) reduced some carbon content through physical capture, but because the carbon powder surface was not directionally modified by component A, the binding force between the carbon powder and PAM was extremely weak, making it easily broken and re-dispersed in the shear flow field, resulting in an unsatisfactory impurity removal effect. In contrast, Example 2, through the molecular-level synergy of A and B, reduced the iron content by nearly 90% and the carbon content by about 95%, fully demonstrating the synergistic effect of selective adsorption and bridging in the impurity removal process.

Claims

1. A method for improving the whiteness of recycled cryolite, characterized in that, Includes the following steps: Cryolite powder and surfactant are dispersed in water to prepare a suspension slurry. Then, a composite agent is added dropwise, and the mixture is stirred for 15-40 minutes. Impurities and agglomerates are removed using a physical sorting device. The mixture is then filtered, washed, and dried to obtain high-whiteness cryolite. The composite agent includes component A and component B. Component A is thiol-modified polyethyleneimine, and component B is polyacrylamide.

2. The method according to claim 1, characterized in that, The amount of the composite agent added is 0.01-0.5% of the dry weight of cryolite powder, and the mass ratio of component A to component B is 3-8:

1.

3. The method according to claim 2, characterized in that, The rate at which the compound drug is added is 5-10% of the total drug dosage per minute.

4. The method according to claim 1, characterized in that, The suspended slurry has a solid content of 20-40%wt, the cryolite powder has a D50 of 45-75μm, and the amount of surfactant is 0.15-0.45% of the mass of the cryolite powder.

5. The method according to claim 1, characterized in that, The stirring is performed under gradient shear stirring at 25-60℃.

6. The method according to claim 5, characterized in that, The gradient shear stirring includes two stages: a high-shear contact stage and a low-shear agglomeration growth stage.

7. The method according to claim 6, characterized in that, During the high-shear contact stage, the stirring speed is 400-600 rpm and maintained for 5-15 minutes.

8. The method according to claim 6, characterized in that, For the low-shear agglomeration growth, the stirring speed is set to 80-150 rpm and maintained for 10-25 minutes.

9. The method according to claim 1, characterized in that, The physical sorting equipment is a hydrocyclone classifier, and the drying temperature is 105-120℃.

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