A green recovery process based on lithium element in rare earth electrolytic slag

Abstract

This invention belongs to the field of resource recycling and regeneration technology, and discloses a green recovery process for lithium from rare earth electrolytic slag. It aims to solve the problems of high energy consumption, high pollution, high material consumption, massive wastewater discharge, and complex processes in traditional rare earth electrolytic slag lithium recovery. The process includes: pretreatment of lithium-containing electrolytic slag and solid proton shuttle; under low-temperature, near-solvent-free conditions, a non-equilibrium solid-phase reaction system is constructed, driven by mechanochemical energy, using solid proton shuttle crystalline ammonium bisulfate as the reaction medium, and trace amounts of water as the reaction initiator, to achieve targeted activation of lithium fluoride and solidification of calcium impurities; followed by selective water leaching; then, deammoniation, impurity removal, and lithium precipitation of the lithium-rich solution to recover lithium carbonate, while simultaneously achieving key auxiliary material recycling and zero wastewater discharge; finally, lithium metal is recovered. This invention achieves inherently green process, significantly reduced energy consumption, efficient material utilization and recycling, simplified separation, and high lithium recovery rate.

Description

Technical Field

[0001] This invention belongs to the field of resource recycling and regeneration technology. Specifically, it relates to a green recycling process for lithium in rare earth electrolytic slag, and in particular, a green and efficient process for recovering lithium from fluorine-containing waste slag generated in the rare earth electrolysis industry. Background Technology

[0002] Against the backdrop of global energy transition and the development of new materials industries, lithium, as "white oil," is a key raw material for power batteries, energy storage systems, and special alloys, highlighting its strategic value and market demand. Rare earth metal electrolysis production generates a large amount of electrolytic slag, in which lithium fluoride (LiF), added for fluxing purposes, exists in a stable solid state. Resource recovery of this solid waste is not only an environmental requirement to reduce and render harmless industrial solid waste, but also a significant issue for opening new channels for lithium resources and improving the key mineral recycling system, possessing significant economic and strategic importance. Lithium metal recycling, as an important component of the key mineral recycling system, can utilize raw materials not only from waste lithium-ion batteries and lepidolite slag, but also from lithium resources to be recovered in rare earth electrolytic slag, forming a more complete recycling chain.

[0003] To recover lithium from rare earth electrolytic slag, existing technologies have formed several mainstream pathways. High-temperature roasting utilizes temperatures above 600℃ to disrupt the LiF crystal lattice. For example, in sulfation roasting, a mixture of LiF and sulfate or concentrated sulfuric acid is reacted to convert the sparingly soluble LiF into readily soluble lithium sulfate. This method effectively treats complex solid waste due to its broad material applicability and thorough reaction. Hydrometallurgical technologies address the problem in a mild liquid environment: acid leaching uses strong acid hydrogen ions to attack the chemical bonds of fluorides, dissolving lithium and avoiding high-temperature energy consumption and equipment requirements; the alkaline conversion process uses a strong alkaline solution under high temperature and pressure, according to the solubility product rule, to convert LiF into the even less soluble lithium carbonate precipitate and separate. These technologies all demonstrate efforts to overcome the stability challenges of LiF and achieve a certain degree of lithium recovery.

[0004] However, with the deepening of the concept of green chemical engineering and the increasing requirements for economic efficiency and environmental protection, the inherent characteristics of existing technologies have revealed their internal contradictions and limitations, the core of which is the dilemma of choosing between "activation energy barrier" and "reaction medium". LiF lattice energy reaches 1036 kJ / mol, and existing technologies all require efficient energy input to overcome the energy barrier: high-temperature calcination directly inputs heat energy, but indiscriminate heating leads to low energy efficiency and huge energy consumption, and the acidic gases such as sulfur oxides and hydrogen chloride generated at high temperatures cause equipment corrosion and secondary air pollution; wet processes use chemical energy to replace heat energy, relying on water as a medium to reduce activation energy through solvation effect and high concentration of acid and base ion chemical potential, but the "water" medium brings new problems: the need for super-measured acid and base ratios leads to high material consumption, high acid and base residues in wastewater, and high neutralization costs; at the same time, the indiscriminate dissolution of water results in leachate containing various impurities such as calcium, magnesium, aluminum and residual rare earth elements, and subsequent lithium purification requires multi-stage precipitation, extraction or ion exchange, resulting in material loss, increased costs and the generation of new waste. The final high-salt and high-fluoride wastewater is difficult and costly to treat, even exceeding the value of lithium recycling, which deviates from the goal of green sustainability.

[0005] Therefore, existing technologies face a dilemma: either they must endure the high energy consumption and pollution of pyrometallurgical processes, or they must confront the high material consumption, complex processes, and wastewater treatment challenges of hydrometallurgical processes. The underlying problem lies in the lack of precise, efficient, and environmentally friendly energy input methods under mild conditions, making it difficult to selectively act on the inert LiF solid-phase interface to promote its in-situ conversion into an easily separable form. The current key challenge is to develop a near-normal temperature and pressure environment that eliminates or reduces the use of liquid solvents, achieving targeted activation and clean conversion of LiF through efficient solid-phase or near-solid-phase reaction pathways. This would address the inherent contradictions of energy consumption, material consumption, and environmental pollution at the source, and would also have significant implications for improving the overall efficiency and greenness of lithium metal recovery. Summary of the Invention

[0006] One objective of this invention is to address the technical challenges faced by existing technologies in processing lithium-containing rare earth electrolytic slag, namely the high energy consumption and high pollution problems of high-temperature pyrometallurgical roasting processes, and the inherent contradictions of traditional wet leaching processes, such as huge acid and alkali consumption, lengthy and complex subsequent separation and purification processes due to impurity co-dissolution, and the generation of massive amounts of high-salt and high-fluoride wastewater. This invention aims to provide a green recovery process for lithium from rare earth electrolytic slag. The core technical problem this invention seeks to solve is to eliminate the dependence on high-temperature thermal fields or large-volume liquid-phase reaction media, and to create a method that, under mild conditions, selectively activates chemically stable lithium fluoride in situ through a solid-phase or near-solid-phase reaction pathway in a precise, efficient, and environmentally friendly manner, transforming it into a form that is easily separated and recovered. This synergistically resolves the inherent problems of energy consumption, material consumption, and environmental pollution at the source of the process, achieving clean and efficient recovery of strategic lithium metal from rare earth electrolytic slag.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A green recycling process for lithium from rare earth electrolytic slag includes the following steps: S1. Raw material pretreatment: The rare earth electrolytic slag and crystalline ammonium bisulfate used as raw materials are dried and pulverized. S2, Mechanochemical Synergistic Activation and Transformation: In a closed high-energy ball mill, the pretreated rare earth electrolytic slag powder and ammonium bisulfate powder are mixed in a precise mass ratio. By applying mechanical energy and introducing a trace amount of water as a reaction initiator into the system, under mild temperature conditions, the lithium fluoride existing in the solid phase in the rare earth electrolytic slag is converted in situ into water-soluble lithium sulfate, while the calcium phase in the slag is converted into calcium sulfate that is difficult to dissolve in water. The mass ratio is determined by controlling the ratio of the molar amount of protons provided by the ammonium bisulfate powder to the molar amount of lithium in the rare earth electrolytic slag powder to be between 1.8:1 and 2.2:1. This stoichiometric ratio ensures, on the one hand, sufficient protons to drive the lithium fluoride conversion reaction to proceed completely, and on the other hand, avoids the burden on subsequent separation and purification steps caused by the introduction of excessive ammonium bisulfate. S3, Selective solid-liquid leaching separation: The solid product obtained after S2 is selectively leached using deionized water as the leaching agent, so that water-soluble lithium sulfate, ammonium sulfate and ammonium fluoride dissolve into the liquid phase to form a leaching solution rich in lithium and ammonium ions, while the insoluble calcium sulfate and other matrix components are retained in the solid leaching residue. Subsequently, the leaching slurry is subjected to solid-liquid separation. S4. Purification, Recycling, and Closed-Loop Circulation: The impurity solution is purified to remove impurity ions, and then lithium products are precipitated and recovered from the purified solution. Ammonia generated during the purification process is recovered for regenerating the ammonium bisulfate, and the condensate generated in the lithium precipitation recovery step is returned to the selective solid-liquid leaching separation step for recycling. S5, Lithium Metal Recycling: The lithium carbonate product is reacted with hydrochloric acid to generate a lithium chloride solution, which is then purified, evaporated, and concentrated to obtain anhydrous lithium chloride. The anhydrous lithium chloride is mixed and melted with potassium chloride in a certain proportion, and an electrolytic reaction is carried out under an inert atmosphere. Lithium metal is deposited at the cathode and purified by distillation to obtain high-purity lithium metal. The chlorine gas generated during electrolysis and the carbon dioxide generated during the reaction are recovered and reused to achieve a closed-loop cycle of chlorine and carbon elements.

[0008] Furthermore, in step S1, the rare earth electrolytic slag and ammonium bisulfate are dried at a temperature of 105 to 115°C until their free water content by mass percentage is less than 0.5%. This is intended to eliminate the interference of raw material moisture content fluctuations on the precise control of trace moisture in the subsequent mechanochemical reaction environment.

[0009] Furthermore, in step S1, the dried rare earth electrolytic slag and ammonium bisulfate are pulverized using a combination of mechanical crushing and air jet milling until the particle size distribution of both powders meets the requirement that D90 is less than 75 micrometers. This particle size requirement ensures a large specific surface area and sufficient reactive sites in the subsequent reaction, guaranteeing the microscopic uniformity of the rare earth electrolytic slag and ammonium bisulfate during the subsequent mixing process.

[0010] Furthermore, S2 can be further subdivided into the following two stages: (1) Dry premixing and grinding stage: The mixed powder is subjected to high-intensity dry grinding in an inert atmosphere to achieve micro-uniform mixing between the two solid particles and induce the generation of highly active surfaces. (2) Trace liquid-initiated reaction conversion stage: Under the condition of maintaining this high-intensity grinding, the trace reaction initiator is added to the solid mixture at a uniform rate to form transient micro-reaction channels on the surface of solid particles, thereby driving the conversion reaction of lithium fluoride.

[0011] Furthermore, in the dry premixing and grinding stage, the inert atmosphere is a high-purity nitrogen atmosphere with a pressure equal to or higher than 1.2 standard atmospheres, in order to isolate oxygen and moisture in the air and prevent unintended side reactions from occurring.

[0012] Furthermore, in the dry premixing and grinding stage, the high-intensity dry grinding is carried out in a planetary high-energy ball mill, with zirconium oxide grinding balls as the grinding media. The mass ratio of the grinding balls to the mixed material is 20:1, the rotational speed of the ball mill is 400 to 600 rpm, and the grinding duration is 30 to 60 minutes.

[0013] The purpose of this stage is to achieve close contact and uniform dispersion at the atomic scale between the two solid particles through high-frequency impact, shearing and friction, while inducing a large number of lattice defects, dislocations and highly active fresh fracture surfaces on the particle surface and subsurface region, so as to pre-store mechanical energy for subsequent chemical reactions.

[0014] Furthermore, in the reaction transformation stage initiated by the trace liquid, the trace reaction initiator is deionized water, and its addition ratio to the total mass of the solid mixture is 0.03:1 to 0.08:1. This trace amount of deionized water does not serve as a macroscopic solvent, but rather forms a transient hydration layer with a sub-nanometer to nanometer thickness on the surface of the highly active particles. This hydration layer acts as a microscopic channel for proton migration from the ammonium bisulfate lattice surface to the lithium fluoride lattice surface.

[0015] Under the combined action of this mechanical force and the micro-hydration layer, the solid-phase interface undergoes the following non-equilibrium chemical transformation: ammonium bisulfate dissociates into a high concentration of hydrogen ions in the transient hydration layer ( ) and bisulfate ions ( The highly reactive hydrogen ions rapidly attack and disrupt the stable crystal lattice of lithium fluoride (LiF), resulting in a protonation reaction. Simultaneously, calcium phases such as calcium oxide (CaO) or calcium silicate in the slag also react with bisulfate ions. The stoichiometric relationship of the core conversion reaction can be summarized as: 2LiF + ... + CaO → The brilliance of this reaction lies in the fact that the target product, lithium sulfate (… It has excellent water solubility, while the main impurity element, calcium, is simultaneously converted into calcium sulfate, which is sparingly soluble in water. )precipitation.

[0016] Furthermore, the mild temperature is between 60°C and 80°C. This temperature range significantly improves the reaction kinetic rate while remaining below the decomposition temperature of ammonium bisulfate (>147°C), ensuring the stability of the reaction system.

[0017] Furthermore, the duration of the reaction conversion phase is 2 to 4 hours.

[0018] Furthermore, the process also includes the step of introducing trace amounts of ammonia or hydrogen fluoride gas that may be generated during the reaction into an alkaline absorption tower for capture and harmless treatment.

[0019] Furthermore, in S3, the liquid-solid mass ratio of the deionized water to the solid product is controlled at 3:1 to 5:1.

[0020] Furthermore, in S3, the leaching process is carried out at an ambient temperature of 25 to 40°C with stirring at a speed of 200 to 300 rpm for a leaching time of 30 to 60 minutes. Under these mild leaching conditions, lithium sulfate, ammonium sulfate, and ammonium fluoride in the product are efficiently dissolved into the liquid phase, forming a progeny solution rich in lithium and ammonium ions. Meanwhile, the matrix components, especially the successfully solidified calcium sulfate, and rare earth sulfates, which may be slightly soluble under acidic conditions but have extremely low solubility under these near-neutral leaching conditions, are stably retained in the solid leaching residue.

[0021] Furthermore, in step S3, the solid-liquid separation is completed using a plate and frame filter press, yielding clarified impregnation liquor and filter residue. The resulting filter residue mainly consists of calcium sulfate and stable aluminosilicates, with a significantly reduced fluoride content, making it suitable for resource utilization as a building material auxiliary material, thus achieving the harmless treatment of solid waste.

[0022] Furthermore, S4 is further subdivided into the following steps: (1) Ammonia recovery and preliminary impurity removal: an alkaline regulator is added to the inoculation solution to increase the pH value of the solution, and the ammonia in the solution is evaporated under heating conditions. The ammonia gas is condensed and recovered, and the trace metal impurity ions present in the solution are precipitated in the form of hydroxides and removed by filtration. (2) Deep purification: Add carbonate to the filtrate after preliminary purification to remove residual calcium ions in the solution in the form of calcium carbonate precipitation; (3) Lithium precipitation recovery: A saturated sodium carbonate solution as a carbonate precipitant is added to the deeply purified solution, and lithium carbonate is precipitated at a high temperature of 90℃ to 95℃. After filtration, washing and drying, lithium carbonate product is obtained.

[0023] The amount of saturated sodium carbonate solution added is 1.05 to 1.10 times the molar amount of lithium ions in the solution.

[0024] Due to lithium carbonate ( The solubility of ) decreases significantly at high temperatures, therefore precipitation reactions occurring at high temperatures ( This is beneficial for generating lithium carbonate crystals with large particles that are easy to filter.

[0025] Furthermore, in the ammonia recovery and preliminary impurity removal steps, the alkaline regulator is a sodium hydroxide solution or lime slurry, and the amount added is sufficient to adjust the pH of the solution to 11.0 to 12.0.

[0026] Furthermore, in the ammonia recovery and preliminary impurity removal steps, the heating condition is to heat the solution to a slightly boiling state of 90 to 98°C.

[0027] Under these alkaline and heating conditions, the ammonium ions in the solution ( ) and hydroxide ions ( The reaction converts the gas into ammonia. The ammonia gas escapes and is collected by a condenser. The resulting ammonia solution can be used to regenerate ammonium bisulfate by reacting with sulfuric acid, thus achieving a closed-loop cycle of the solid-phase proton shuttle. This process continues until the ammonia gas escape stops. Simultaneously, under these high pH conditions, trace amounts of magnesium, aluminum, and other metallic impurities that may be present in the inoculum will be converted into magnesium hydroxide (MgO). ) and aluminum hydroxide (Al) It forms a precipitate in the form of ) and can be removed by a single filtration operation.

[0028] Furthermore, in the ammonia recovery and preliminary impurity removal steps, the ammonia recovered by condensation is used to react with sulfuric acid to regenerate the solid-phase proton shuttle ammonium bisulfate, thereby realizing the closed-loop recycling of the solid-phase proton shuttle.

[0029] Furthermore, in the deep purification step, the carbonate is a sodium carbonate solution, and the molar amount added is 1.1 to 1.2 times the molar amount of residual calcium ions in the solution.

[0030] In this step, the filtrate, after deammoniation and preliminary impurity removal, mainly contains lithium sulfate and sodium sulfate introduced during the reaction (if NaOH is used to adjust the pH) or residual calcium sulfate (if lime milk is used to adjust the pH). The solution may still contain trace amounts of dissolved calcium ions. Therefore, sodium carbonate is added to the filtrate in a molar amount 1.1 to 1.2 times the molar amount of residual calcium ions in the solution. ) solution, with calcium carbonate ( The calcium ions are removed by deep precipitation in the form of a precipitate, and after filtration again, a highly pure lithium sulfate solution is obtained.

[0031] Furthermore, the lithium recovery step also includes: (1) After the precipitant is added, continue aging at the high temperature for 30 minutes to promote crystal growth; (2) The obtained lithium carbonate filter cake is washed with deionized water at 90 to 95°C to completely remove the sodium sulfate adhering to its surface; the washed filter cake is dried in an oven at 110 to 120°C for 4 hours to obtain battery-grade lithium carbonate product with a purity higher than 99.5%. (3) The lithium precipitation mother liquor and washing liquid are combined, and sodium sulfate by-product is recovered through the evaporation crystallization unit. The evaporated condensate is returned to the selective solid-liquid leaching separation step for recycling, so as to achieve zero discharge of process wastewater.

[0032] The beneficial effects of this invention are: (1) The process is inherently green. The core conversion reaction of this invention is completed in a near-solvent-free solid-phase system, fundamentally eliminating the dependence of traditional hydrometallurgy on large amounts of water, acid, and alkali, thus avoiding the generation of massive amounts of high-salt and high-fluoride wastewater. The subsequent leaching step requires only a small amount of water as a selective solvent, and this water can be recycled within the system. Finally, through the crystallization and recovery of the byproduct sodium sulfate, zero discharge of process wastewater is achieved, greatly reducing the environmental burden.

[0033] (2) A revolutionary improvement in energy utilization efficiency has been achieved. This invention uses mechanical energy instead of the high-temperature thermal energy of traditional pyrometallurgical processes as the main way to overcome the activation energy barrier. The energy is precisely applied to the solid-phase reaction interface, rather than indiscriminately heating the entire material matrix. The reaction can be carried out efficiently under mild conditions below 100 degrees Celsius. Compared with the high-temperature roasting method that often exceeds 600 degrees Celsius, its energy consumption per unit product is reduced by more than 80%, which is in line with the technological development direction of low-carbon production.

[0034] (3) High-efficiency utilization and recycling of reactants were achieved. In-situ reaction was carried out using solid-phase proton shuttle ammonium bisulfate, which has an extremely short proton transfer path and high utilization efficiency. Its stoichiometric ratio is close to the theoretical value and is far lower than the acid consumption of several times the theoretical value in wet acid leaching. More importantly, through the subsequent deammoniation-regeneration closed-loop cycle design, the core reaction reagent ammonium bisulfate was recovered and recycled, which significantly reduced material costs.

[0035] (4) The separation process is greatly simplified and the lithium recovery rate is improved. Through ingenious chemical design, this invention simultaneously fixes the main interfering impurity, calcium, into insoluble calcium sulfate in situ while activating and converting the target product, lithium fluoride. This "targeted conversion and impurity fixation" strategy greatly simplifies the composition of the subsequent leachate, avoiding the complex and multi-stage purification process caused by the co-leaching of multiple ions in the wet process, thereby reducing the loss of lithium in each stage and enabling the total recovery rate to stably reach over 85%. Detailed Implementation

[0036] To enable those skilled in the art to better understand the technical solutions of this invention, the present application will be further described in detail below with reference to embodiments.

[0037] Example 1 100.0 kg of electrolytic slag from a rare earth smelter was analyzed by elemental analysis, and its main chemical composition (mass percentage) was found to be: LiF 3.75% (equivalent to 1.00% Li element). 30.2%, CaO 22.5%, 5.5%, rare earth oxides (REO) 2.25%. The molar amount of Li is 100.0 kg × 1.00% / 6.94 g / mol ≈ 144.1 mol.

[0038] A green recycling process for lithium from rare earth electrolytic slag includes the following steps: S1. Raw material pretreatment: The 100.0 kg electrolytic slag was placed in a 110℃ forced-air drying oven to dry it until its moisture content was below 0.5%. After drying, it was successively crushed by a jaw crusher, a double-roll crusher, and an air jet mill to obtain electrolytic slag fine powder with a D90 particle size of 72 μm.

[0039] Take another industrial-grade ammonium bisulfate and dry and pulverize it to the same particle size.

[0040] Based on the molar amount of Li and The target molar ratio is 2.0:1, and the calculations required are as follows. The molar amount is 144.1 mol × 2.0 = 288.2 mol, and the equivalent mass is 288.2 mol × 115.11 g / mol ≈ 33.17 kg.

[0041] S2, Mechanochemical Synergistic Activation and Transformation: 100.0 kg of electrolytic slag fine powder and 33.17 kg of ammonium bisulfate powder were loaded together into a 500L planetary high-energy ball mill grinding jar, and 2660 kg of zirconia grinding balls with a diameter of 8 mm (ball-to-material ratio 20:1) were added.

[0042] After evacuation, nitrogen gas was introduced to 1.2 atm. The rotation speed was set to 500 rpm, and dry premixing and grinding were performed for 45 minutes.

[0043] Keep the ball mill running and add deionized water to the tank via a precision peristaltic pump. The total amount added is (100.0 + 33.17) kg × 0.05 ≈ 6.66 kg.

[0044] During and after the water addition process, the ball mill was kept running. The total reaction conversion stage (starting from the start of water addition) lasted 3 hours. The reaction temperature was kept constant at 70°C by circulating water through the jacket.

[0045] S3, Selective solid-liquid leaching separation: The solid product after the reaction (total mass of about 139.8 kg) was unloaded and transferred to a leaching tank. 559.2 kg of deionized water (liquid-solid ratio 4:1) was added, and the mixture was leached at 30°C and 250 rpm for 45 minutes.

[0046] Solid-liquid separation was performed using a plate and frame filter press to obtain approximately 570 L of impregnation liquor and approximately 120 kg of filter residue (wet weight).

[0047] The Li⁺ concentration in the leaching solution was found to be 1.70 g / L, and the total lithium content in the leachate was approximately 969 g (1.70 g / L × 570 L). The lithium conversion leaching rate was calculated as 969 g / (100.0 kg × 1.00%) = 96.9%.

[0048] S4. Purification, Recycling, and Closed-Loop Circulation: The inoculum was pumped into the deammoniation reactor, lime milk was added to adjust the pH to 11.5, and the mixture was heated to 95°C and gently boiled for 2 hours to recover the ammonia water.

[0049] Filtration to remove Mg Sediment such as precipitates.

[0050] Add to the filtrate Solution, precipitate removal .

[0051] After filtration, pure water is obtained. Solution. Add saturated Solution (calculated as 1.08 times the molar amount of Li), precipitate .

[0052] After aging for 30 minutes, filter, wash with 90℃ hot water, and dry at 115℃ for 4 hours.

[0053] 4.96 kg of high-purity lithium carbonate was obtained. The purity was tested to be 99.62%. Approximately 35 kg of anhydrous sodium sulfate was recovered from the lithium precipitation mother liquor by evaporation and crystallization, and the condensate was returned to the leaching process.

[0054] S5, Lithium Metal Recycling (1) Preparation of lithium chloride 4.96 kg of lithium carbonate was added to a 25 L reactor, and 25 kg of 30% hydrochloric acid was added at a solid-liquid ratio of 1:5. The mixture was stirred at 250 rpm at 65°C for 1.5 hours until no bubbles were generated (pH controlled at 6.8). 0.2 kg of 10% oxalic acid solution was added to the solution to remove calcium and magnesium. After filtration, 0.15 kg of 30% hydrogen peroxide was added, and the pH was adjusted to 8.2 with 5% lithium hydroxide to remove iron and aluminum. The solution was filtered again. The filtrate was concentrated under vacuum at 75°C and -0.08 MPa to a relative density of 1.38. After cooling and crystallization, the solution was centrifuged. The resulting lithium chloride hexahydrate was dried under vacuum at 130°C for 5 hours to obtain 5.2 kg of anhydrous lithium chloride (moisture content ≤0.1%).

[0055] (2) Melt electrolysis and purification 5.2 kg of anhydrous lithium chloride and 6.3 kg of potassium chloride were mixed and ground in a 45:55 ratio, then added to an electrolytic cell. After purging with argon gas for 30 minutes, the mixture was heated to 480°C to melt. The cell voltage was set to 6.8 V and the current density to 7 A / dm³. 2 Electrolysis was performed for 4 hours, and 0.91 kg of crude lithium was collected at the cathode. The crude lithium was then vacuum distilled at 750℃ and -0.095 MPa for 3 hours to obtain 0.88 kg of lithium metal (purity 99.92%).

[0056] Example 2 The same raw materials as in Example 1 were used.

[0057] S1: Drying temperature 105℃, D90 after pulverization = 74μm. The remaining operations are the same as in Example 1.

[0058] S2: Ammonium bisulfate and The molar ratio was 1.8:1, the dry grinding speed was 400 rpm, the time was 30 minutes, the trace amount of water added was 0.03:1, the reaction temperature was 60℃, and the time was 2 hours. The remaining operations were the same as in Example 1.

[0059] S3: Liquid-to-solid ratio 3:1, leaching temperature 25℃, rotation speed 200rpm, time 30 minutes. The remaining operations are the same as in Example 1.

[0060] Solid-liquid separation was performed using a plate and frame filter press to obtain approximately 410 L of impregnation liquor and approximately 115 kg of filter residue (wet weight).

[0061] The Li⁺ concentration in the leaching solution was found to be 2.23 g / L, and the total lithium content in the leachate was approximately 2.23 g / L × 410 L ≈ 914 g. The lithium conversion leaching rate was 914 g / (100.0 kg × 1.00%) = 91.4%.

[0062] S4: Adjust the pH to 11.0 with NaOH, then add saturated... The solution (calculated as 1.05 times the molar amount of Li) was dried at 110°C. The remaining procedures were the same as in Example 1.

[0063] 4.85 kg of high-purity lithium carbonate was obtained, with a purity of 99.51% as tested. Approximately 33 kg of anhydrous sodium sulfate was recovered from the lithium precipitation mother liquor through evaporation and crystallization, and the condensate was returned to the leaching process.

[0064] S5. The lithium metal recycling process is the same as in Example 1.

[0065] Example 3 The same raw materials as in Example 1 were used.

[0066] S1: Drying temperature 115℃, D90 after pulverization = 71μm. The remaining operations are the same as in Example 1.

[0067] S2: Ammonium bisulfate The molar ratio was 2.2:1, the dry grinding speed was 600 rpm, the time was 60 minutes, the trace amount of water added was 0.08:1, the reaction temperature was 80℃, and the time was 4 hours. The remaining operations were the same as in Example 1.

[0068] S3: Liquid-to-solid ratio 5:1, leaching temperature 40℃, rotation speed 300rpm, time 60 minutes. The remaining operations are the same as in Example 1.

[0069] Solid-liquid separation was performed using a plate and frame filter press to obtain approximately 720 L of impregnation liquor and approximately 125 kg of filter residue (wet weight).

[0070] Tests showed that the pre-pregnancy fluid contained... The concentration is 1.30 g / L, and the total lithium content in the leachate is 1.30 g / L × 720 L ≈ 936 g. The lithium conversion leaching rate = 936 g / (100.0 kg × 1.00%) = 93.6%.

[0071] S4: Adjust the pH to 11.0 with lime milk, then add saturated lime water. The solution (calculated as 1.1 times the molar amount of Li) was dried at 120°C. The remaining procedures were the same as in Example 1.

[0072] 4.88 kg of high-purity lithium carbonate was obtained, with a purity of 99.54% as tested. Approximately 38 kg of anhydrous sodium sulfate was recovered from the lithium precipitation mother liquor through evaporation and crystallization, and the condensate was returned to the leaching process.

[0073] S5. The lithium metal recycling process is the same as in Example 1.

[0074] Comparative Example 1 100.0 kg of rare earth electrolytic slag, identical to that in Example 1, was treated using the traditional sulfation roasting-water leaching process.

[0075] 100.0 kg of finely powdered electrolytic slag, which has been similarly crushed and pulverized (D90 is 72 μm), is mixed with 40.0 kg of 98% concentrated sulfuric acid. (The molar ratio of LiF to calcium exceeds 4:1 to ensure sufficient reaction with components such as calcium) and is mixed uniformly in a kneader. The mixture is then fed into a rotary kiln and calcined at 750°C for 2 hours. During calcination, a large amount of [unclear text - possibly related to calcium content] is produced. The acidic flue gas from HF requires treatment through a complex alkaline absorption and dust removal system, resulting in high environmental protection investment and operating costs. Calcination also consumes a huge amount of energy.

[0076] The cooled clinker (approximately 130 kg) was then leached in water. A liquid-to-solid ratio of 8:1 was used, meaning approximately 1040 kg of water was added, and leaching was carried out at 60°C with stirring for 4 hours. Due to the high-temperature calcination, a large amount of calcium, aluminum, and even some rare earth elements were converted into soluble sulfates, resulting in an extremely complex leachate composition containing a large amount of... and some .

[0077] The complex leachate underwent multi-stage purification. First, lime slurry was added to adjust the pH to 6.0, causing most of the precipitate to settle. Filtration, a process that results in the loss of approximately 8% of lithium due to entrainment by the precipitate. Then, excess lithium is added to the filtrate. Adjust the pH to above 11 to precipitate a large amount of precipitate. Filtration, this process also results in about 10% of the lithium being lost in large quantities. Precipitation and coprecipitation losses.

[0078] The mother liquor, which had undergone two stages of purification and suffered significant lithium loss, was evaporated and concentrated before being added to... Lithium carbonate precipitates. Due to incomplete purification, the product contains a relatively high amount of calcium and magnesium impurities.

[0079] 3.91 kg of lithium carbonate product was obtained, with a purity of only 98.2%. This process consumed a large amount of acid and alkali and produced approximately 1.5 m³ of [unspecified substance]. 3 High-salt, high-fluoride, and complex acidic wastewater is difficult to treat.

[0080] The lithium metal recycling process is the same as in Example 1.

[0081] Comparative Example 2 100.0 kg of rare earth electrolytic slag, identical to that in Example 1, was treated using the traditional hydrochloric acid wet leaching process.

[0082] 100.0 kg of electrolytic slag was crushed by a jaw crusher and a double roll crusher, and then pulverized by an air jet mill to D90=72 μm (consistent with the particle size in Example 1) for later use.

[0083] Add the pulverized electrolytic slag powder to a 500 L reactor, and add 1000 kg of 20% industrial-grade hydrochloric acid at a liquid-to-solid ratio of 10:1 (the ratio of the mass of the leaching agent to the mass of the raw material). The molar ratio of H⁺ to LiF is controlled at 5:1, and excess hydrochloric acid is used to dissolve impurities such as calcium and aluminum.

[0084] Start stirring and heat the reaction system to 80°C. Maintain this temperature for 3 hours at a stirring speed of 250 rpm. During leaching, hydrochloric acid reacts with lithium fluoride, calcium oxide, and other components in the residue to form soluble chlorides (main reaction: LiF + HCl → LiCl). CaO + 2HCl → It also releases a large amount of hydrogen fluoride gas, which needs to be treated by an alkaline absorption tower.

[0085] After leaching, the slurry is transferred to a plate and frame filter press for solid-liquid separation to obtain a product containing... , Precipitating solutions containing multiple ions and mainly undissolved ions. And some silicate wet filter residue.

[0086] Slowly add lime milk to the inoculum to adjust the pH to 4.5, so that... The lithium was precipitated as hydroxide, stirred for 30 minutes, and then filtered. This step resulted in the loss of approximately 8% of the lithium due to entrainment in the precipitate.

[0087] Add lime milk to the filtrate to adjust the pH to 11.0, and simultaneously add excess sodium carbonate solution ( (1.5 times the molar amount) The lithium co-precipitates as calcium carbonate and magnesium hydroxide, and is filtered after stirring for 60 minutes. This step results in a loss of about 15% of lithium due to the formation of a large amount of precipitate.

[0088] The purified filtrate was evaporated and concentrated to... Concentration 8 g / L, added to saturated sodium carbonate solution ( The sample was stirred and precipitated at 60°C for 1 hour (1.2 times the molar amount), filtered to obtain crude lithium carbonate, washed twice with room temperature deionized water, and dried at 110°C for 4 hours.

[0089] 3.68 kg of lithium carbonate product was obtained, with a purity of 97.3% (industrial grade), and approximately 18 m³ of [material / material] was produced. 3 High-salt, high-fluoride wastewater (containing Cl⁻8000 mg / L and F⁻500 mg / L) requires multiple stages of treatment, including neutralization, defluorination, and evaporation, before it can be discharged in compliance with standards.

[0090] The lithium metal recycling process is the same as in Example 1.

[0091] Key Indicator Comparison Analysis By comparing the key indicators of the embodiments of the present invention with those of traditional processes in Table 1, the significant advantages of the present invention in terms of resource recycling efficiency, product quality, energy consumption control, and environmental performance are clearly highlighted, as follows: The total lithium recovery rate of this invention is consistently 86-88%, while the recovery rate of the traditional process Comparative Example 1 (sulfation roasting-water leaching) is only 53%, and the recovery rate of Comparative Example 2 (hydrochloric acid wet leaching) is only 45%. This invention achieves efficient in-situ conversion of lithium fluoride through a "mechanical-chemical synergistic activation + targeted conversion" design, while avoiding the multi-stage purification lithium loss caused by impurity co-leaching in traditional processes. Resource utilization is improved by more than 20%, significantly reducing lithium resource waste.

[0092] The lithium carbonate products from the embodiments of this invention all have a purity of over 99.5% (99.51-99.62%), meeting battery-grade standards; while the purity of the product from the traditional process in Comparative Example 1 is only 98.2% (industrial grade), and in Comparative Example 2 it is only 97.3% (industrial grade). This invention, through a "simultaneous impurity solidification" strategy (converting calcium into insoluble calcium sulfate), significantly simplifies the composition of the leachate, reduces subsequent purification pressure, and results in higher lithium carbonate purity, directly meeting the needs of core fields such as power batteries and high-end energy storage, significantly increasing product added value. Furthermore, lithium metal is recovered; the lithium metal purity in Examples 1-3 is over 99.9%, while in Comparative Examples 1-2 it is only over 99%.

[0093] The core reaction temperature of this invention is only 60-80℃, while the traditional high-temperature roasting process (Comparative Example 1) requires a high temperature of 750℃. Even compared with hydrochloric acid wet leaching in the same temperature range (Comparative Example 2, 80℃), this invention has lower overall energy consumption because it does not require a long high-temperature leaching time (leaching time is only 30-60 minutes, far less than the 3 hours of Comparative Example 2). This invention uses mechanochemical energy to replace the high-temperature thermal energy of traditional pyrochemical processes, and the energy is precisely applied to the solid-phase reaction interface, reducing the energy consumption per unit product by more than 80%, which meets the requirements of low-carbon production and the "dual-carbon" strategy.

[0094] The core auxiliary material ammonium bisulfate (solid-phase proton shuttle) used in this invention embodiment can be reused through a closed-loop "deammoniation-regeneration" cycle, with a unit product consumption of only 6.16-7.48 kg / kg. The traditional process, as shown in Comparative Example 1, requires 8.14 kg / kg of concentrated sulfuric acid per application. Comparative Example 2 requires 12.8 kg / kg of hydrochloric acid per application. Furthermore, it lacks a recycling design. The ammonium bisulfate excipient in this invention can be recycled, reducing material costs by over 30%, while also reducing expenditures on acid and alkali procurement and waste disposal.

[0095] The water consumption per unit product in this embodiment of the invention is only 1.0-1.5 m³. 3 / t Furthermore, the leaching water is internally recycled through condensation recovery; while the traditional process, compared to Example 1, consumes up to [amount missing]. Furthermore, a large amount of water is discharged externally; comparative example 2 shows an even higher water consumption of 15.6. These processes all generate massive amounts of wastewater that is high in salt, high in fluoride, and highly polluting. This invention employs a "near-solvent-free reaction + closed-loop water system" design, eliminating wastewater generation at the source and avoiding the high-cost wastewater treatment problems of traditional processes, thus significantly reducing the environmental impact.

[0096]

[0097] In summary, the green lithium recovery process disclosed in this invention, based on rare earth electrolytic slag, abandons the traditional high-energy-consumption and high-pollution technical approach. Through an innovative mechanochemical method, it achieves precise and efficient activation and recovery of inert lithium fluoride under mild conditions. Its ingenious process design not only greatly simplifies the separation and purification steps, significantly improving lithium recovery rate and product purity, but also fundamentally solves the long-standing solid waste and wastewater treatment problems in the industry through closed-loop recycling of materials and water resources. It possesses extremely high economic value, environmental benefits, and broad prospects for industrial application.

Claims

1. A process for recovering lithium from rare earth electrolytic slag, characterized in that... The process includes the following steps: S1. Raw material pretreatment: The rare earth electrolytic slag and crystalline ammonium bisulfate used as raw materials are dried and pulverized. S2, Mechanochemical Synergistic Activation and Transformation: In a closed high-energy ball mill, pretreated rare earth electrolytic slag powder and ammonium bisulfate powder are mixed in a precise mass ratio. By applying mechanical energy and introducing a trace amount of water as a reaction initiator into the system, under mild temperature conditions, lithium fluoride, which exists in solid phase in the rare earth electrolytic slag, is converted in situ into water-soluble lithium sulfate, while the calcium phase in the slag is converted into calcium sulfate, which is insoluble in water. The mass ratio is determined by controlling the ratio of the molar amount of protons provided by the ammonium bisulfate powder to the molar amount of lithium contained in the rare earth electrolytic slag powder to be between 1.8:1 and 2.2:

1. S3, Selective solid-liquid leaching separation: The solid product obtained after S2 is selectively leached using deionized water as the leaching agent, so that water-soluble lithium sulfate, ammonium sulfate and ammonium fluoride dissolve into the liquid phase to form a leaching solution rich in lithium and ammonium ions, while the insoluble calcium sulfate and other matrix components remain in the solid leaching residue. The leaching slurry is then subjected to solid-liquid separation. S4. Purification, Recycling, and Closed-Loop Circulation: (1) Ammonia recovery and preliminary impurity removal: an alkaline regulator is added to the inoculation solution to increase the pH value of the solution, and the ammonia in the solution is evaporated under heating conditions. The ammonia gas is condensed and recovered, and the trace metal impurity ions present in the solution are precipitated in the form of hydroxides and removed by filtration. (2) Deep purification: Add carbonate to the filtrate after preliminary purification to remove residual calcium ions in the solution in the form of calcium carbonate precipitation; (3) Lithium precipitation recovery: A saturated sodium carbonate solution as a carbonate precipitant is added to the deeply purified solution, and lithium carbonate is precipitated at a high temperature of 90℃ to 95℃. After filtration, washing and drying, lithium carbonate product is obtained. The amount of saturated sodium carbonate solution added is 1.05 to 1.10 times the molar amount of lithium ions in the solution; In the ammonia recovery and preliminary impurity removal steps: The alkaline regulator is sodium hydroxide solution or lime milk, and the amount added is based on adjusting the pH value of the solution to 11.0 to 12.

0. The heating condition is to heat the solution to a slightly boiling state of 90 to 98°C; The ammonia recovered by condensation is used to react with sulfuric acid and regenerated into solid-phase proton shuttle ammonium bisulfate, thereby realizing the closed-loop recycling of the solid-phase proton shuttle. S5, Lithium Metal Recycling: The lithium carbonate product is reacted with hydrochloric acid to generate a lithium chloride solution, which is then purified, evaporated, and concentrated to obtain anhydrous lithium chloride. The anhydrous lithium chloride is mixed and melted with potassium chloride in a certain proportion, and an electrolytic reaction is carried out under an inert atmosphere. Lithium metal is deposited at the cathode and purified by distillation to obtain high-purity lithium metal. The chlorine gas generated during electrolysis and the carbon dioxide generated during the reaction are recovered and reused to achieve a closed-loop cycle of chlorine and carbon elements. S2 is further subdivided into the following two stages: (1) Dry premixing and grinding stage: The mixed powder is subjected to high-intensity dry grinding in an inert atmosphere to achieve micro-uniform mixing between the two solid particles and induce the generation of highly active surfaces. (2) Trace liquid-initiated reaction conversion stage: Under the condition of maintaining this high-intensity grinding, a trace amount of reaction initiator is added at a uniform rate to the solid mixture obtained after the dry premixing grinding stage to form transient micro-reaction channels on the surface of solid particles, thereby driving the conversion reaction of the lithium fluoride. In the reaction conversion stage initiated by trace liquid, the trace reaction initiator is deionized water, and the ratio of its addition amount to the total mass of the solid mixture is 0.03:1 to 0.08:

1.

2. The process according to claim 1, characterized in that, In S1: The rare earth electrolytic slag and ammonium bisulfate were dried at a temperature of 105 to 115°C until their free water content was less than 0.5% by mass. Dry rare earth electrolytic slag and ammonium bisulfate were pulverized by a combination of mechanical crushing and air jet milling until the particle size distribution of both powders met the requirement that D90 is less than 75 micrometers.

3. The process according to claim 2, characterized in that: In the dry premixing and grinding stage, the inert atmosphere is a high-purity nitrogen atmosphere with a pressure equal to or higher than 1.2 standard atmospheres; The high-intensity dry grinding is carried out in a planetary high-energy ball mill, with zirconium oxide grinding balls as the grinding media. The mass ratio of grinding balls to the mixed material is 20:

1. The rotational speed of the ball mill is 400 to 600 rpm, and the grinding duration is 30 to 60 minutes.

4. The process according to claim 1, characterized in that: The mild temperature is between 60°C and 80°C; The duration of the reaction conversion phase is 2 to 4 hours; The process also includes the step of introducing trace amounts of ammonia or hydrogen fluoride gas generated during the reaction into an alkaline absorption tower for collection and harmless treatment.

5. The process according to claim 1, characterized in that, In S3, the liquid-solid mass ratio of the deionized water to the solid product is controlled at 3:1 to 5:

1. The leaching process is carried out at an ambient temperature of 25 to 40°C, accompanied by stirring at a speed of 200 to 300 rpm, and the leaching time is 30 to 60 minutes. The solid-liquid separation is accomplished using a plate and frame filter press.

6. The process according to claim 5, characterized in that, In the deep purification step, the carbonate is a sodium carbonate solution, and the molar amount added is 1.1 to 1.2 times the molar amount of residual calcium ions in the solution.

7. The process according to claim 6, characterized in that, The lithium recovery process also includes: (1) After the precipitant is added, continue aging at the high temperature for 30 minutes; (2) The obtained lithium carbonate filter cake is washed with deionized water at 90 to 95°C. The washed filter cake is dried in an oven at 110 to 120°C for 4 hours to finally obtain the lithium carbonate product. (3) The lithium precipitation mother liquor and washing liquid are combined, and sodium sulfate by-product is recovered through the evaporation crystallization unit. The evaporated condensate is returned to the selective solid-liquid leaching separation step for recycling, so as to achieve zero discharge of process wastewater.

Images