A method for separating and enriching lithium-aluminum electrolyte

Abstract

The present invention belongs to the field of environmental protection, and in particular to a separation and enrichment method of a lithium-aluminum electrolyte. First, the ratio of the auxiliary agent is adjusted according to the electrolyte composition so that the molecular molar ratio of alkali metal fluoride to aluminum fluoride is 8-20; then oxygen is evenly introduced into the insulation tank filled with the electrolyte through a conduit, and the temperature is kept at 900-1000 ° C to remove most of the carbon elements, while converting most of the Li elements in the electrolyte into liquid lithium fluoride; then the temperature is lowered to 880-990 ° C to separate lithium fluoride from other electrolytes; then a mechanical device is used to pass most of the LiF melt through the reserved hole of the tank body and put it into a hopper for cooling and storage; finally, the temperature is kept at 890-950 ° C, with a negative pressure of 0-1500 Pa, and the residual LiF is separated from other electrolytes by vacuum distillation. The separated LiF enriched material can be used as an aluminum electrolysis raw material auxiliary agent, or as a lithium extraction raw material, and the remaining electrolyte is returned to the aluminum electrolysis tank for recycling. This invention relies on existing electrolytic aluminum processes, which have low energy consumption, short process, low cost, and convenient operation, and will not cause secondary pollution, thus facilitating industrial application and promotion.

Description

Technical Field

[0001] This invention relates to the field of environmental protection, and more particularly to a method for separating and enriching lithium-containing aluminum electrolytes. Background Technology

[0002] In the electrolytic aluminum production process, to improve current efficiency and electrolyte properties, in addition to adding cryolite, certain fluoride or chloride salts are also added. One commonly used additive is lithium fluoride. This can effectively reduce the initial temperature of the electrolyte, reduce fluoride emissions, and promote energy conservation and consumption reduction in electrolytic aluminum enterprises. A large amount of lithium is enriched in the ash of electrolytic aluminum waste, typically reaching 1%-3% (calculated as Li+). Combined with the fact that some bauxite itself contains Li, nearly 180,000 tons of lithium-containing electrolyte are generated annually, which is approximately 1800-4900 tons per year based on lithium content.

[0003] Initially, aluminum plants typically disposed of their waste through stockpiling, landfilling, or incineration, which presented challenges in emissions control, polluting the environment and wasting resources. Subsequently, a series of advanced technologies were developed.

[0004] The current mainstream process is: pretreatment, acid hydrolysis, leaching, neutralization precipitation, carbonization, and roasting to prepare lithium carbonate, with cryolite prepared from other impurities. A representative example is the Chinese Patent Publication No. CN 105293536 B, which discloses "A Method for Extracting Lithium from Electrolytic Aluminum Waste." This method includes the following steps: reacting lithium-containing electrolytic aluminum waste with concentrated sulfuric acid, then adding water for leaching and filtering to obtain filtrate and residue; adding sodium carbonate to the filtrate for alkaline hydrolysis, then filtering to obtain filtrate and residue; adding water to the residue to make a slurry, then adding lime for causticization, then filtering to obtain filtrate and residue; then passing CO2 through the filtrate for carbonation, followed by filtration, washing, and drying to obtain battery-grade lithium carbonate. However, the fluorine content in the lithium electrolytic aluminum waste raw material is too high, reaching over 50%, making fluorine treatment an unavoidable environmental problem and a serious issue of equipment corrosion. Other problems include low leaching rates, product contamination, wastewater, and secondary waste. Later improvements focused on three main aspects:

[0005] First: The roasting section involves adding sulfates, chlorides, fluorides, and alkalis containing elements such as Ca, Mg, and Ba during roasting. This process achieves both fluorine fixation and modification to increase the leaching rate.

[0006] Second: In the leaching section, the leaching system is changed, such as by using different leaching solution formulations, such as mixed acids, mixed alkalis, and acid salts, to increase the leaching pressure and supplement it with microwave, ultrasonic and other technical means.

[0007] Third: Use different precipitating reagents to improve product purity.

[0008] While the above-mentioned improvements have increased the utilization efficiency of lithium in waste and improved the quality of corresponding products to some extent, the processing costs have not decreased much, energy consumption has not been reduced, the process is generally long, the conversion rate is low, other elements besides lithium are not fully utilized, and wastewater, waste gas and waste residue are generated, resulting in secondary pollution and environmental problems.

[0009] Therefore, how to efficiently, environmentally friendly, and cost-effectively utilize lithium-containing aluminum electrolytes has become an industry challenge that the aluminum electrolysis industry needs to address. Summary of the Invention

[0010] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for separating and enriching lithium-containing aluminum electrolytes. This method relies on existing aluminum electrolysis equipment, with appropriate modifications, fully utilizing the waste heat of the lithium-containing aluminum electrolyte, using pure oxygen for carbon removal, alkaline enrichment of liquefied LiF, and finally achieving the separation and enrichment of lithium fluoride from the electrolyte using mechanical vacuum.

[0011] Its features include the following steps:

[0012] (1) Transfer the lithium-aluminum electrolyte from the electrolytic cell or dust collection device to the heat preservation tank and keep it at 950-1100℃ to maintain the molten state;

[0013] (2) Adjust the proportion of additives according to the electrolyte composition so that the molecular molar ratio of alkali metal fluoride to aluminum fluoride is between 8 and 20.

[0014] (3) Oxygen is uniformly introduced into the insulated tank containing the electrolyte through a conduit and kept at 900-1000℃ to remove most of the carbon elements, while converting most of the Li elements in the electrolyte into liquid lithium fluoride.

[0015] (4) Cool down to a certain temperature point of 880-990℃ and keep it at that temperature for 2-5 hours to allow LiF to separate from other electrolytes.

[0016] (5) Using mechanical devices, most of the LiF melt is placed into the hopper through the pre-reserved holes in the tank and cooled for storage.

[0017] (6) Keep warm to 890-950℃, negative pressure 0-1500pa, and use vacuum distillation to separate the residual LiF from other electrolytes.

[0018] According to the invention, the electrolyte needs to be kept at a temperature of 950-1100℃ in an insulated bath to maintain its molten state.

[0019] According to the invention, the additive is one or more of sodium oxide, potassium oxide, sodium fluoride, and potassium fluoride.

[0020] According to the invention, after the electrolyte is kept at a stable temperature, the molar ratio of alkali metal fluoride to aluminum fluoride is 8-20.

[0021] According to the invention, oxygen is uniformly introduced into the electrolyte through multiple conduits to remove carbon, and the oxygen purity is above 99.5%.

[0022] According to the invention, after carbon removal, most of the Li element in the electrolyte is converted into liquid lithium fluoride. The temperature is lowered to 880-990℃ and held for 2-5 hours, so that LiF is released from specific holes in the tank in liquid form using a mechanical device, thus achieving preliminary separation.

[0023] According to the invention, the residual LiF is separated from other electrolytes by distillation using a vacuum distillation apparatus at a temperature of 890-950℃ and a negative pressure of 0-1500pa.

[0024] Beneficial effects:

[0025] This invention relies on existing aluminum electrolysis processes and can be implemented directly in aluminum electrolysis plants, effectively reducing the amount of electrolyte stored and eliminating the hassle of transportation. It utilizes pure oxygen for carbon removal while fully leveraging the waste heat of the electrolyte, resulting in low energy consumption, no introduction of new nitrogen and sulfides, and no leakage of fluorine-containing gases. The electrolyte, after the addition of additives and mechanical and vacuum extraction of lithium fluoride, can be used as a new electrolyte for aluminum electrolysis.

[0026] The entire process is energy-efficient, has a short process, low cost, and is easy to operate, with no secondary pollutant emissions, making it conducive to industrial application and promotion. Detailed Implementation

[0027] This invention provides a method for the separation and enrichment of lithium-containing aluminum electrolytes. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0028] The present invention will be described in detail below with reference to specific embodiments:

[0029] Example 1

[0030] Take 100 kg of aluminum electrolyte from a certain electrolytic aluminum plant. The molecular molar ratio of NaF to ALF3 is 2.5. The content of KF is 2.82%, LiF is 2.92%, CaF2 is 3.8%, MgF2 is 0.5%, C is 0.66%, and the remainder is cryolite and AlF3.

[0031] The electrolyte was placed in a sealed, insulated bath and heated to 950°C, then kept at that temperature for 1.5 hours. NaF and KF were added separately to make the molar ratio of (NaF+KF) / AlF3 in the electrolyte 9, where NaF / KF = 4. Oxygen was then continuously introduced, and the temperature was maintained at 970-1000°C. Samples were taken at 0.5 hours, 1 hour, and 1.5 hours to monitor the carbon removal rate.

[0032] The removal rate was 54% after 0.5 hours, 92% after 1 hour, and 94% after 1.5 hours. Therefore, it was chosen to keep the temperature for 1 hour.

[0033] Example 2

[0034] Take 100 kg of aluminum electrolyte from a certain electrolytic aluminum plant, with the same composition as in Case 1.

[0035] The electrolyte was placed in a sealed, insulated bath and heated to 950°C for 1.5 hours. NaF and KF were added separately to make the molar ratio of (NaF+KF) / AlF3 molecules in the electrolyte 9, where NaF / KF = 4. Oxygen was then continuously introduced, and the temperature was maintained at 970-1000°C for one hour. The temperature was then lowered to 950°C and maintained for 1 hour, 2 hours, 3 hours, and 5 hours, respectively.

[0036] Samples were taken vertically from the tank and rapidly cooled to determine the content and phase distribution of Na, Al, F, Li, Ca, and K, thus identifying the distribution location of the LiF melt, which was then released through a pre-drilled hole in the tank. After three hours of holding at this temperature, a stable LiF melt formed in the electrolyte, and its quantity remained unchanged. Therefore, a holding time of 3 hours was selected.

[0037] Example 3

[0038] Take 4 portions of 100 kg of aluminum electrolyte from a certain electrolytic aluminum plant, with the same composition as in Case 1.

[0039] The electrolytes were placed in four identical sealed insulated baths and heated to 950°C for 1.5 hours. NaF and KF were added respectively to make the (NaF+KF) / AlF3 molar ratio in the electrolyte 9, 12, 15, and 18, where NaF / KF = 4. Oxygen was then continuously introduced, and the temperature was maintained at 970-1000°C for one hour, then cooled to 950°C and held for 3 hours.

[0040] The LiF melt was released from the pre-reserved hole in the tank. The test showed that the best effect was achieved when the (NaF+KF) / AlF3 molecular molar ratio was 12. The Li content in the melt was 26.1%, accounting for 64% of the total lithium mass in the electrolyte.

[0041] Example 4

[0042] Take 3 portions of 100 kg of aluminum electrolyte from a certain electrolytic aluminum plant, with the same composition as in Case 1.

[0043] The electrolytes were placed in three identical sealed insulated baths and heated to 950℃ for 1.5 hours. NaF and KF were added respectively to make the molar ratio of (NaF+KF) / AlF3 molecules in the electrolyte 1:2, where NaF / KF = 4. Oxygen was then continuously introduced, and the temperature was maintained at 970-1000℃ for one hour. The temperature was then lowered to 980℃, 930℃, and 900℃ respectively, and held for 3 hours.

[0044] The LiF melt was released from the pre-reserved holes in the tank. The test showed that when the temperature dropped to 930℃, the Li content was the highest at 26.8%, accounting for 72.4% of the total lithium mass in the electrolyte.

[0045] Example 5

[0046] Take 3 portions of 100 kg of aluminum electrolyte from a certain electrolytic aluminum plant, with the same composition as in Case 1.

[0047] The electrolyte was placed in three identical sealed, insulated tanks and heated to 950°C for 1.5 hours. NaF and KF were added respectively to achieve a (NaF+KF) / AlF3 molar ratio of 1:2, where NaF / KF = 4. Oxygen was then continuously introduced, maintaining the temperature at 970-1000°C for one hour, followed by cooling to 930°C and holding for three hours. LiF melt was discharged through pre-drilled holes in the tanks. The remaining electrolyte, along with the electrolytic cell, was then held at a vacuum pressure of 50 Pa, 890°C, 950°C, and 980°C for three hours each, and lithium fluoride was collected. Testing revealed that at a vacuum of 50 Pa and a temperature of 950°C, 90% of the residual lithium fluoride, representing 24.8% of the total lithium mass in the electrolyte, could be recovered.

[0048] In this invention, the electrolyte is kept at 950°C for 1.5 hours, the molar ratio of (NaF+KF) / AlF3 molecules is adjusted to 12, where NaF / KF = 4, oxygen is continuously introduced, and the temperature is maintained at 970-1000°C for one hour, which can remove most of the carbon.

[0049] The temperature was lowered to 930℃ and held for 3 hours. The LiF melt was then released from the pre-reserved holes in the tank to obtain lithium fluoride. The highest Li content was 26.8%, and the lithium mass accounted for 72.4% of the total lithium mass in the electrolyte.

[0050] The remaining electrolyte, along with the electrolytic cell, was kept at a vacuum pressure of 50 Pa and 950 °C for three hours, and lithium fluoride was collected. Testing revealed that at a vacuum level of 50 Pa, 90% of the residual lithium fluoride, representing 24.8% of the total lithium mass in the electrolyte, could be recovered.

[0051] Two recycling processes can yield 97.2% lithium fluoride, which can eliminate most of the lithium in the electrolyte. The lithium-enriched electrolyte can then be recycled in aluminum electrolysis.

[0052] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Those skilled in the art can modify the technical solutions described in the above embodiments, or make equivalent substitutions for some of the technical features; and all such modifications and substitutions should fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for separating and enriching lithium-containing aluminum electrolytes, characterized in that, Includes the following steps: (1) Transfer the lithium-aluminum electrolyte from the electrolytic cell or dust collection device to the heat preservation tank and keep it at 950-1100℃ to maintain the molten state; (2) Adjust the proportion of additives according to the electrolyte composition so that the molecular molar ratio of alkali metal fluoride to aluminum fluoride is 8-20; (3) Oxygen is uniformly introduced into the insulated tank containing the electrolyte through a conduit and kept at 900-1000℃ to remove most of the carbon elements, while converting most of the Li elements in the electrolyte into liquid lithium fluoride. (4) Cool down to a certain temperature point of 880-990℃ and keep it at that temperature for 2-5 hours to allow LiF to separate from other electrolytes; (5) A mechanical device is used to put most of the LiF melt into the hopper through the reserved hole in the tank, and then cool and store it. (6) Keep warm to 890-950℃ and negative pressure 0-1500pa, and use vacuum distillation to separate the residual LiF from other electrolytes.

2. The method for separating and enriching lithium-containing aluminum electrolytes as described in claim 1, characterized in that, The additives are one or more of sodium oxide, potassium oxide, sodium fluoride, and potassium fluoride.

3. The method for separating and enriching lithium-containing aluminum electrolytes as described in claim 1, characterized in that, It uses oxygen to remove carbon, and the oxygen purity is over 99.5%.

4. The method for separating and enriching lithium-containing aluminum electrolytes as described in claim 1, characterized in that, Oxygen is evenly introduced into the electrolyte through multiple tubes.