Method for separating and recovering gallium from coal-based kaolin by purifying flash joule heat

Abstract

The application discloses a kind of flash joul-thermal purification coal series kaolin method for gallium separation and recovery, belong to metallurgical technical field.It includes the following steps: S1, carbon-containing coal series kaolin raw material pretreatment;S2, powder and additive are mixed, to obtain mixed material;S3, mixed material is shaped, to obtain the body to be handled;S4, the body to be handled is placed between conducting electrode, under vacuum or inert atmosphere, using multi-stage pulse flash to apply pulse voltage and carry out flash joul-thermal treatment;S5, collect solid product, namely after purification kaolin, and capture volatile material to recover gallium from it;The application uses endogenous carbon in coal series kaolin as conducting medium, forms transient liquid phase film to strengthen mass transfer in flash initial stage through eutectic molten salt, nitrogen source ligand forms coordination bond with gallium under ultra-high temperature to promote its selective migration, sulfide and chloride are cooperatively controlled volatile behavior, realizes the efficient selective recovery of gallium and the deep purification of kaolin, and the purification cycle is short, low in cost.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a method for the synergistic separation and recovery of gallium from coal-based kaolin through flash joule heating purification. Background Technology

[0002] Coal-series kaolin is a clay mineral resource associated with coal seams, mainly distributed in the pores and fractures of coal petrographic components, often existing in the form of coal gangue and coal slime. Coal-series kaolin contains abundant aluminosilicate minerals, making it an important raw material for kaolin production. However, the chemical composition of coal-series kaolin is complex, with colored impurities such as iron and titanium severely affecting the whiteness and application value of the product. Iron exists in various forms, including independent minerals (magnetite, goethite, limonite, pyrite), micro-dispersions, and lattice substitution; some iron even enters the kaolinite lattice, substituting Al³⁺ at octahedral positions. Titanium exists as fine-grained minerals such as anatase, with particle sizes often less than 2 micrometers, making processing extremely difficult. Furthermore, some coal-series kaolin is enriched in the strategic metal gallium, possessing significant recovery value, but this has long been neglected.

[0003] Traditional kaolin purification methods mainly include acid leaching, reduction bleaching, and biological iron removal. For example, the patented technology "A Purification Process for Kaolin" (CN104743563B) uses acid leaching-reduction bleaching to bleach the kaolin. Dispersants and pH adjusters are added to the bleached slurry for scrubbing; activators and collectors are added to the scrubbed solution, which is then placed in a high-shear mixer for stirring and sedimentation; the precipitate is filtered, washed, dried, and calcined in a calcining chamber at 700-900°C. The resulting purified kaolin is then pulverized. While this method achieves kaolin purification, it uses natural kaolin as raw material, has a long process, and generates a significant amount of acidic and chemical waste. The patented technology, "A Chemical-Bacterial Method for Iron Removal and Whitening of Kaolin" (CN107619052A), involves adding a certain amount of dispersant and an environmentally friendly reducing agent to a slurry, adjusting the slurry pH to 8-10 with alkali, heating to 50-70°C, and maintaining the temperature for 50-80 minutes. The resulting kaolin slurry is then cooled to 25-40°C, and a carbon source, nitrogen source, surfactant, inorganic salt, and FeRB-FL1404 or an ethanol-producing anaerobic bacterium solution for iron removal are added. The iron removal reaction is then carried out at 15-40°C for 8-15 days. The kaolin after this chemical-microbial combined iron removal process is then separated into solid and liquid components to obtain kaolin free of Fe2O3 impurities. While this method yields purified kaolin, the process cycle is long, and the bioreactivity is limited by temperature control, resulting in high purification costs. These methods are not only complex and time-consuming, generating secondary pollution, but also fail to effectively recover the strategic metal gallium associated with coal-series kaolin.

[0004] In recent years, flash Joule heating technology has attracted attention in the field of mineral processing due to its ultra-high temperature and ultra-fast heating characteristics. CN120536756A discloses a method for recovering rare metals by coupling Joule heating with alkaline depolymerization, which activates minerals through flash Joule heating treatment and then combines it with wet leaching to recover metals. CN116406320A discloses an ultra-fast flash Joule heating synthesis method that can recover metals from ores and fly ash. However, existing flash Joule heating technologies mainly rely on high-temperature volatilization to achieve metal recovery, which has the following technical bottlenecks: first, the solid-phase diffusion rate is low, making it difficult for impurity elements to migrate out of the mineral lattice; second, gallium has similar volatilization behavior to impurity elements such as iron and titanium, making selective separation difficult; and third, there is a lack of effective utilization of endogenous carbon in coal-series kaolinite, resulting in high energy consumption and cost. Therefore, developing a method that can solve the above problems and achieve deep purification of coal-series kaolinite and efficient selective recovery of gallium has important practical significance and application value. Summary of the Invention

[0005] Based on the above analysis, this invention aims to provide a method for the flash joule heating purification of coal-based kaolin and the synergistic separation and recovery of gallium, in order to solve the problems of long process flow, difficulty in treating secondary pollution, and failure to recover and utilize synergistic resources in the prior art. By using sulfides and composite activators to promote the release of impurity metal elements at instantaneous high temperature and to perform indirect capture and recovery of volatile components, the method avoids the serious problem of secondary pollution caused by strong acid leaching and realizes the high-value utilization of coal-based resources.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention provides a method for the synergistic separation and recovery of gallium from coal-based kaolin through flash joule heating purification, comprising the following steps: S1. Raw material preparation: Carbon-containing coal-based kaolin raw material is pretreated to obtain powder, and the endogenous carbon in the coal-based kaolin is used as a conductive medium. S2. Mixing: The powder is mixed with the additive to obtain a mixed material. After mixing, grinding is continued to obtain a uniform powder before flash evaporation. The additive includes sulfides and a composite activator. The composite activator includes a eutectic molten salt component and a nitrogen source ligand component. S3. Molding: The uniform powder is shaped to obtain a blank to be processed; S4. Flash Joule heat treatment: The blank to be treated is placed between conductive electrodes, and a pulse voltage is applied under vacuum or inert atmosphere to perform flash Joule heat treatment, so that the impurity elements in the coal-series kaolin volatilize. S5. Product collection: Collect the solid product after flash joule heat treatment, which is the purified kaolin; and capture the volatiles generated during the treatment process, and recover gallium from the volatiles.

[0007] In the preferred technical solution, in step S1, the carbon-containing coal-derived kaolin raw material is at least one of coal gangue, coal slime, and carbon-containing kaolin.

[0008] In a preferred embodiment, the sulfide is at least one of sulfur, sodium sulfate, and sodium sulfide, and the sulfide is of industrial or analytical purity.

[0009] In a preferred embodiment, the eutectic molten salt component is a mixture of aluminum chloride and at least one chloride selected from sodium chloride, potassium chloride, and magnesium chloride; the nitrogen source ligand component is a cyanamide compound; and both the aluminum chloride and the chloride are of industrial or analytical grade.

[0010] In a further preferred embodiment, the molar ratio of aluminum chloride to chloride is (0.8-1.2):1, forming a low-melting-point eutectic system; the cyanamide compound is at least one of dicyandiamide, melamine, and calcium cyanamide.

[0011] In a preferred embodiment, the mass ratio of the sulfide, the composite activator, and the coal-series kaolin powder is (0.02-0.1):(0.07-0.15):1.

[0012] In the preferred embodiment, in step S1, the pretreatment includes grinding to a particle size of less than 35 micrometers with a content of 70-90%, and drying at 90°C for 1-2 hours; the forming step is tableting.

[0013] In one of the optional technical solutions, a tablet measuring 3 cm in length, 2 cm in width, and 0.5 cm in height is produced, and the tableting equipment used is a manual tablet press.

[0014] In a preferred embodiment, step S4 employs multi-stage pulse flash evaporation, sequentially applying a first pulse and a second pulse; the discharge voltage of the first pulse is 100-150 V, the discharge time is 0.5-2 s, and flash evaporation occurs 1-2 times; the discharge voltage of the second pulse is 200-300 V, the discharge time is 0.1-1 s, and flash evaporation occurs 1-3 times; an interval of 0.5-2 s is set between the first pulse and the second pulse.

[0015] The fine carbon particles (endogenous carbon) in coal-series kaolin break the insulation barrier under high voltage and form a permeable conductive network. The first-stage pulse (100-150V) mainly acts on the composite activator in step S2. The low-melting-point eutectic system formed by AlCl3 and alkali metal chloride melts rapidly and forms a microscopic liquid film on the surface of mineral particles. This liquid phase environment greatly improves the diffusion rate of subsequent chemical reactions and overcomes the bottleneck of slow solid-solid reaction kinetics.

[0016] At instantaneous high temperatures, eutectic molten salts release highly reactive chloride ions or hydrogen chloride. These chlorinating agents react with iron (Fe), titanium (Ti), and gallium (Ga) at the edges or inside the kaolin lattice. Since GaCl3 and FeCl3 have low boiling points, they rapidly transform into the gas phase at flash evaporation temperatures. The addition of sulfides modulates the redox potential of the system, which can convert some oxidized impurities into more volatile sulfide forms, or combine with nitrogen source ligands to prevent impurities from re-solidifying into the lattice before volatilization.

[0017] The extremely high thermal stress generated by the high-voltage pulse (200-300V) in the second stage leads to transient defects and microcracks in the kaolin crystal structure. The "electron wind" generated when the strong current passes through the material will drive impurity ions to migrate to the grain boundary. At high temperature, nitrogen-containing free radicals or cyano fragments generated by the decomposition of nitrogen sources such as melamine can form transient coordination bonds with metal ions, reducing the activation energy for metal to leave the crystal lattice and further promoting the selective migration of gallium.

[0018] The activated Fe, Ga and other elements are ejected outward in the form of chlorides or sulfides with the tiny airflow generated by flash evaporation. Taking advantage of the slight difference in the condensation temperature of gallium chlorides with iron and titanium chlorides, gallium is directionally enriched on the inner wall of the quartz hood or in a specific condensation zone. Gallium, which is only tens of ppm in the raw coal-series kaolin ore, is increased to hundreds or even thousands of ppm in the volatiles in this way, achieving a concentration of orders of magnitude.

[0019] In the preferred embodiment, in step S4, the discharge voltage is 200-300 V, the discharge time is 0.1-5 s, and the number of flash evaporations is 1-10.

[0020] In the preferred technical solution, in step S4, the conductive electrode is made of graphite, and the blank to be processed forms a conductive circuit with the power supply through an electrode assembly composed of graphite carbon felt and graphite sheet; the size of the graphite sheet is 1 cm × 1 cm × 0.5 cm.

[0021] In the preferred technical solution, in step S5, a collection device for capturing volatiles during flash evaporation is provided around the reaction zone. The gallium element in the volatiles migrates and accumulates in the form of chlorides or sulfides, and is captured on the inner wall of the collection device by condensation or adsorption.

[0022] In an optional technical solution, in step S5, a quartz vessel is placed around the reaction zone to recover the gas volatilized during flash evaporation; the volatiles are the volatiles remaining on the upper layer of the quartz vessel, and the captured volatiles are digested and gallium is enriched by at least one of selective precipitation, solvent extraction or ion exchange.

[0023] Compared with the prior art, the present invention can achieve the following beneficial effects: 1. In this invention, endogenous carbon forms a percolating conductive network under high pressure, providing an instantaneous ultra-high temperature heat source; eutectic molten salt forms a transient liquid phase film in the early stage of flash evaporation, transforming solid-phase diffusion into liquid-phase diffusion, increasing the ion diffusion rate by 3-5 orders of magnitude, and overcoming the kinetic bottleneck of traditional solid-phase reactions; nitrogen-containing free radicals generated by the decomposition of nitrogen source ligands form instantaneous coordination bonds with gallium, reducing the activation energy for gallium to escape from the crystal lattice and achieving preferential migration of gallium; sulfides and chlorides work synergistically to both suppress the co-volatilization of impurities such as iron and titanium and promote the efficient volatilization of gallium in the form of low-boiling-point chlorides or complexes. Through the above synergistic mechanism, the gallium recovery rate can reach over 95%, and the gallium content in the volatiles is enriched from tens of ppm in the original ore to thousands of ppm, and the Ga / Fe mass in the volatiles is significantly improved compared with traditional flash evaporation.

[0024] 2. This invention utilizes the liquid phase environment provided by the eutectic molten salt and the selective coordination effect of the nitrogen source ligand to enable gallium to migrate and vaporize preferentially over impurities such as iron and titanium; at the same time, sulfides react with iron and titanium to form high-boiling-point sulfides (such as FeS and TiS2), effectively suppressing their co-volatilization within the gallium volatilization temperature range.

[0025] 3. This invention not only achieves high-value purification of coal-series kaolin, but also simultaneously recovers strategic metal gallium. At the same time, sulfides formed by sulfides with iron and titanium can be further recovered by flotation or magnetic separation, realizing the graded utilization and comprehensive recovery of multiple elements such as aluminum, silicon, gallium, iron, and titanium in coal-series resources. The strong pulse, large current, and ultra-high temperature generated by the capacitive Joule thermal power supply can instantly impact the mineral crystal structure and chemical bonds, causing defects in the crystal structure of coal-series kaolin, providing a channel for the release of metal elements.

[0026] 4. This invention eliminates the need for traditional acid or alkali leaching methods, generates no waste liquid throughout the process, and completes the flash evaporation process within seconds, resulting in low energy consumption and high efficiency. Furthermore, this invention fully utilizes the inherent endogenous carbon in coal-series kaolin as a conductive medium, eliminating the need for external conductive agents and further reducing production costs. The simple pressing and conductive methods make the equipment easy to disassemble and maintain. Attached Figure Description

[0027] Figure 1 The coal-series kaolin powder before flash evaporation; Figure 2 Kaolin purified by flash evaporation; Figure 3 This is a diagram of the flash evaporation experiment of the present invention; Figure 4 This is a voltage curve from the flash evaporation experiment of the present invention; Figure 5 This is a current curve from the flash evaporation experiment of the present invention. Detailed Implementation

[0028] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0029] In this specific embodiment: the sulfides and chlorides are of industrial purity or analytical purity; the chemical composition of the coal-based kaolin is as follows: Al2O3 content is 35-43 wt%, SiO2 content is 40-50 wt%, TiO2 content is 1-3%, K2O content is 0.21%, Fe2O3 content is 0.21-0.65%, and gallium content is approximately 40-70 ppm.

[0030] The powder drying equipment used before the experiment was a blower drying oven.

[0031] The experimental Joule heating device and quartz hood volatile collector were self-assembled.

[0032] The resistance of the test voltage plate is 0.35-2.5 Ω.

[0033] The details will not be repeated in the examples.

[0034] The working mechanism of this invention is as follows: In this invention, the inherent fine carbon particles (endogenous carbon) in coal-series kaolinite bear the energy input. When a high voltage is applied, these carbon particles, which were originally dispersed in the insulating mineral matrix, quickly break through the interfacial barrier and form a permeable conductive network that runs through the material. The Joule heat generated when the current passes through heats the material to an ultra-high temperature of over 2000K in milliseconds, providing the driving force for all subsequent physicochemical reactions. This design does not require the addition of an external conductive agent and achieves low-cost, high-efficiency self-supporting instantaneous heating.

[0035] Traditional solid-state reactions are limited by the slow diffusion rate of atoms in the crystal lattice (diffusion coefficient of only 10). -15 -10 -12 The eutectic molten salt component introduced in this invention (such as the AlCl3-NaCl low-melting-point eutectic system, melting point 150-160℃) melts rapidly within microseconds during flash discharge, forming a micro-regional liquid phase film on the surface of dispersed kaolinite particles. This liquid phase film wets the particle surface and micropores, significantly reducing the interfacial resistance to the migration of impurity ions from the crystal lattice. On the other hand, the high concentration of chloride and sodium ions in the liquid phase forms a strong polarization field, which releases Fe from the interior of the kaolinite crystal lattice. 3+ Ti 4+ Ga 3+Plasma rapidly captures and transports particles to the surface, increasing the diffusion rate by 3-5 orders of magnitude, thus completely overcoming the kinetic bottleneck of solid-phase diffusion.

[0036] In the intense plasma or quasi-plasma environment of flash Joule heating, nitrogen source ligands (such as cyanamide compounds like dicyandiamide and melamine) undergo violent decomposition, generating highly reactive nitrogen-containing free radicals (·NH4+). x The strong electronegativity and lone pair electrons of nitrogen atoms (such as ·CN) can interact with metal ions (especially Ga). 3+ Ga forms unstable gaseous coordination complexes or intermediates, and significantly reduces the activation energy required for gallium to break free from the aluminosilicate lattice by forming MN (metal-nitrogen) coordination bonds; in addition, due to Ga 3+ Its coordination ability with N is better than that with Fe. 3+ Ti 4+ Impurity ions, such as nitrogen-containing free radicals, can preferentially bind to gallium to form gallium-nitrogen-chloride complexes (e.g., [GaNCl2]) with specific volatility characteristics. n [Ga(NH3)4]Cl3 derivatives] guide gallium elements to migrate and be stripped from the vapor phase preferentially over the matrix elements.

[0037] Sulfides in the additives decompose at instantaneous high temperatures, releasing sulfur vapor or hydrogen sulfide, creating a unique micro-regional chemical environment. Sulfides capture oxygen from impurity oxides, maintaining a reducing atmosphere in the reaction system and effectively preventing Ga and Fe from being re-oxidized at ultra-high temperatures into stable oxides that are difficult to volatilize (such as Ga2O3 and Fe2O3), ensuring the volatilization process continues. The vapor pressure of some metal sulfides is higher than that of their oxides at specific temperatures. The presence of sulfides forms a sulfur-chlorine synergistic system with chlorides, constructing a multi-component low-boiling-point azeotropic system, further reducing the temperature threshold for impurity discharge. Sulfides react with impurities such as iron and titanium to form high-boiling-point sulfides (such as FeS and TiS2), inhibiting their co-volatilization within the gallium volatilization temperature range, forming a perfect synergy between impurity suppression and gallium extraction with the gallium extraction function of nitrogen source ligands.

[0038] Under the influence of active chloride ions or hydrogen chloride released from the eutectic molten salt, metal elements that migrate to the particle surface undergo a chlorination reaction: Ga2O3+ 6Cl - → 2GaCl3↑ + 3O 2- Fe2O3 + 6Cl - → 2FeCl3↑ + 3O 2- Gallium chloride (GaCl3, boiling point 201℃) and ferric chloride (FeCl3, boiling point 307℃) have low boiling points and rapidly transform into a gaseous phase at flash evaporation temperature. They are then sprayed outward from the inside of the material by a burst of high-temperature gas flow. By utilizing the slight difference in condensation temperature between gallium and iron and titanium chlorides, selective condensation and enrichment are achieved in different temperature zones of the peripheral quartz collection device. Gallium, which is only tens of ppm in the raw coal-series kaolin ore, is increased to hundreds or even thousands of ppm in the volatiles through this process, achieving orders of magnitude concentration.

[0039] The present invention will now be described in further detail with reference to specific methods.

[0040] Example 1 A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, the preparation method comprising the following steps: Step 1: Grind the coal-series kaolin to obtain a powder with a particle size of less than 35 micrometers accounting for 75%; Step 2: Place the powder sample from Step 1 into a 90 °C oven and dry for 1 hour; Step 3: Mix the materials according to the mass ratio of sodium sulfide:sodium chloride:coal gangue of 0.02:0.05:1. Step 4: Continue dry grinding the mixture from Step 3 for 10 minutes to obtain a uniform powder before flash evaporation; Step 5: Place the uniform powder from Step 4 into a tablet press for tableting to obtain tablets that are 3 cm long, 2 cm wide, and 0.5 cm high. Step 6: Place the pressed sheet obtained in step 5 between two graphite carbon felts and press it firmly at both ends with graphite sheets; Step 7: The two electrodes of the Joule thermal power supply are pressed onto the graphite sheets at both ends to form a complete power supply circuit; Step 8: Surround the entire reaction zone with a rectangular quartz vessel measuring 13 cm long, 5 cm wide, and 7 cm high to recover the gases volatilized during the flash evaporation process. Step 9: Close the flash chamber tightly, shut off the vent valve, and turn on the vacuum pump until a vacuum is achieved; Step 10: Adjust the voltage to 220 V, discharge time to 0.2 s, and flash evaporation count to 10. Connect the Joule heating power supply and conduct the flash evaporation test under Joule heating. Step 11: Turn off the Joule heating power supply, turn off the vacuum pump, open the vent valve, and remove the sample after it has cooled down.

[0041] The Fe2O3 content in the purified kaolin sample decreased from 0.35% to 0.11%, the gallium content decreased from approximately 65 ppm to 7 ppm, the whiteness was 91.2, and the gallium content in the volatile components was approximately 754 ppm.

[0042] Example 2 A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, the preparation method comprising the following steps: Step 1: Grind the coal-series kaolin to obtain a powder with a particle size of less than 35 micrometers accounting for 70%; Step 2: Place the powder sample from Step 1 into a 90 °C oven and dry for 1 hour; Step 3: Mix the materials according to the mass ratio of sodium sulfide: potassium chloride: carbon-containing kaolin of 0.04:0.08:1. Step 4: Continue dry grinding the mixture from Step 3 for 10 minutes to obtain a uniform powder before flash evaporation; Step 5: Place the uniform powder from Step 4 into a tablet press for tableting to obtain tablets that are 3 cm long, 2 cm wide, and 0.5 cm high. Step 6: Place the pressed sheet obtained in step 5 between two graphite carbon felts and press it firmly at both ends with graphite sheets; Step 7: The two electrodes of the Joule thermal power supply are pressed onto the graphite sheets at both ends to form a complete power supply circuit; Step 8: Surround the entire reaction zone with a rectangular quartz vessel measuring 13 cm long, 5 cm wide, and 7 cm high to recover the gases volatilized during the flash evaporation process. Step 9: Close the flash chamber tightly, shut off the vent valve, and turn on the vacuum pump until a vacuum is achieved; Step 10: Adjust the voltage to 200 V, discharge time to 0.5 s, and flash evaporation count to 10. Connect the Joule heating power supply and conduct the flash evaporation test under Joule heating. Step 11: Turn off the Joule heating power supply, turn off the vacuum pump, open the ventilation valve, and remove the sample after it has cooled down.

[0043] The Fe2O3 content in the purified kaolin sample decreased from 0.47% to 0.13%, the gallium content decreased from approximately 56 ppm to 4 ppm, the whiteness was 93.5, and the gallium content in the volatile components was 1021 ppm.

[0044] Example 3 A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, the preparation method comprising the following steps: Step 1: Grind the coal-series kaolin to obtain a powder with a particle size of less than 35 micrometers accounting for 85%; Step 2: Place the powder sample from Step 1 into a 90 °C oven and dry for 1 hour; Step 3: Mix the materials according to the mass ratio of sulfur powder: potassium chloride: (coal gangue + coal slime) of 0.03:0.1:1; Step 4: Continue dry grinding the mixture from Step 3 for 10 minutes to obtain a uniform powder before flash evaporation; Step 5: Place the uniform powder from Step 4 into a tablet press for tableting to obtain tablets that are 3 cm long, 2 cm wide, and 0.5 cm high. Step 6: Place the pressed sheet obtained in step 5 between two graphite carbon felts and press it firmly at both ends with graphite sheets; Step 7: The two electrodes of the Joule thermal power supply are pressed onto the graphite sheets at both ends to form a complete power supply circuit; Step 8: Surround the entire reaction zone with a rectangular quartz vessel measuring 13 cm long, 5 cm wide, and 7 cm high to recover the gases volatilized during the flash evaporation process. Step 9: Close the flash chamber tightly, shut off the vent valve, and turn on the vacuum pump until a vacuum is achieved; Step 10: Adjust the voltage to 200 V, discharge time to 0.7 s, and flash evaporation count to 10. Connect the Joule heating power supply and conduct the flash evaporation test under Joule heating. Step 11: Turn off the Joule heating power supply, turn off the vacuum pump, open the ventilation valve, and remove the sample after it has cooled down. The Fe2O3 content in the purified kaolin sample decreased from 0.56% to 0.14%, the gallium content decreased from approximately 40 ppm to 6 ppm, the whiteness was 95.8, and the gallium content in the volatile components was 1354 ppm.

[0045] Example 4 A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, the preparation method comprising the following steps: Step 1: Grind the coal-series kaolin to obtain a powder with a particle size of less than 35 micrometers accounting for 75%; Step 2: Place the powder sample from Step 1 into a 90°C oven and dry for 1 hour; Step 3: Mix sodium sulfide, composite activator, and coal-based kaolin in a mass ratio of 0.05:0.1:1; the composite activator is composed of a eutectic molten salt component and a nitrogen source ligand component mixed in a mass ratio of 1:0.5; wherein the eutectic molten salt component is a mixture of aluminum chloride and sodium chloride (molar ratio 1:1), and the nitrogen source ligand component is dicyandiamide; Step 4: Continue dry grinding the mixture from Step 3 for 8 minutes to obtain a uniform powder before flash evaporation; Step 5: Place the uniform powder from Step 4 into a tablet press for tableting to obtain tablets that are 3 cm long, 2 cm wide, and 0.5 cm high. Step 6: Place the pressed sheet obtained in step 5 between two graphite carbon felts and press it firmly at both ends with graphite sheets; Step 7: The two electrodes of the Joule thermal power supply are pressed onto the graphite sheets at both ends to form a complete power supply circuit; Step 8: Cover the reaction zone with a rectangular quartz vessel measuring 13 cm long, 5 cm wide, and 7 cm high to recover the gases volatilized during the flash evaporation process. Step 9: Close the flash chamber tightly, shut off the vent valve, and turn on the vacuum pump until a vacuum is achieved; Step 10: Adjust the voltage to 250 V, discharge time to 0.3 s, and flash evaporation count to 3. Connect the Joule heating power supply and conduct the flash evaporation test under Joule heating. Step 11: Turn off the Joule heating power supply, turn off the vacuum pump, open the vent valve, and remove the sample after it has cooled down.

[0046] The Fe2O3 content in the purified kaolin sample decreased from 0.48% to 0.03%, the gallium content decreased from approximately 62 ppm to 3 ppm, the whiteness was 96.2, the gallium content in the volatile components was 26.8%, the total gallium recovery rate was 95.3%, and the Ga / Fe mass ratio in the volatiles was 11.2:1.

[0047] Example 5 A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, the preparation method comprising the following steps: Step 1: Grind the coal-series kaolin to obtain a powder with a particle size of less than 35 micrometers accounting for 75%; Step 2: Place the powder sample from Step 1 into a 90°C oven and dry for 1 hour; Step 3: Mix sodium sulfide, composite activator, and coal-based kaolin in a mass ratio of 0.05:0.1:1; the composite activator is composed of a eutectic molten salt component and a nitrogen source ligand component mixed in a mass ratio of 1:0.5; wherein the eutectic molten salt component is a mixture of aluminum chloride and sodium chloride (molar ratio 1:1), and the nitrogen source ligand component is dicyandiamide; Step 4: Continue dry grinding the mixture from Step 3 for 8 minutes to obtain a uniform powder before flash evaporation; Step 5: Place the uniform powder from Step 4 into a tablet press for tableting to obtain tablets that are 3 cm long, 2 cm wide, and 0.5 cm high. Step 6: Place the pressed sheet obtained in step 5 between two graphite carbon felts and press it firmly at both ends with graphite sheets; Step 7: The two electrodes of the Joule thermal power supply are pressed onto the graphite sheets at both ends to form a complete power supply circuit; Step 8: Cover the reaction zone with a rectangular quartz vessel measuring 13 cm long, 5 cm wide, and 7 cm high to recover the gases volatilized during the flash evaporation process. Step 9: Close the flash chamber tightly, shut off the vent valve, and turn on the vacuum pump until a vacuum is achieved; Step 10: Employ a multi-stage pulse flash evaporation strategy: First, apply the first pulse with a voltage of 120 V, a discharge time of 1 s, and perform one flash evaporation; after a 1-s interval, apply the second pulse with a voltage of 250 V, a discharge time of 0.3 s, and perform two flash evaporations; then connect the Joule heating power supply to conduct the flash evaporation test. Step 11: Turn off the Joule heating power supply, turn off the vacuum pump, open the vent valve, and remove the sample after it has cooled down.

[0048] The Fe2O3 content in the purified kaolin sample decreased from 0.48% to 0.02%, the gallium content decreased from approximately 62 ppm to 2 ppm, the whiteness was 96.8, the gallium content in the volatile components was 31.5%, the total gallium recovery rate was 97.6%, and the Ga / Fe mass ratio in the volatiles was 16.3:1.

[0049] Example 6 A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, wherein the preparation steps are basically the same as in Example 5, only the eutectic molten salt composition is changed: Example 6a: The eutectic molten salt component is a mixture of aluminum chloride and potassium chloride (molar ratio 1:1), and the nitrogen source ligand is still dicyandiamide.

[0050] The purified kaolin sample had Fe2O3 content reduced to 0.03%, gallium content reduced to 3 ppm, whiteness of 96.5, gallium content in volatiles of 28.2%, gallium recovery rate of 96.1%, and Ga / Fe mass ratio of 14.2:1.

[0051] Example 6b: The eutectic molten salt component is a mixture of aluminum chloride and magnesium chloride (molar ratio 1:1), and the nitrogen source ligand is dicyandiamide.

[0052] The purified kaolin sample had Fe2O3 content reduced to 0.03%, gallium content reduced to 3 ppm, whiteness of 96.3, gallium content in volatiles of 27.5%, gallium recovery rate of 95.8%, and Ga / Fe mass ratio of 13.5:1.

[0053] Example 7 A method for flash joule heating purification of coal-based kaolinite with synergistic gallium separation and recovery, wherein the preparation steps are basically the same as in Example 5, only the nitrogen source ligand composition is changed: Example 7a: The nitrogen source ligand component is melamine, and the eutectic molten salt is AlCl3-NaCl (molar ratio 1:1).

[0054] The purified kaolin sample had Fe2O3 content reduced to 0.02%, gallium content reduced to 2 ppm, whiteness of 96.7, gallium content in volatiles of 30.8%, gallium recovery rate of 97.2%, and Ga / Fe mass ratio of 15.8:1.

[0055] Example 7b: The nitrogen source ligand component is calcium cyanamide, and the eutectic molten salt is AlCl3-NaCl (molar ratio 1:1).

[0056] The purified kaolin sample had Fe2O3 content reduced to 0.03%, gallium content reduced to 3 ppm, whiteness of 96.4, gallium content in volatiles of 29.1%, gallium recovery rate of 96.5%, and Ga / Fe mass ratio of 14.6:1.

[0057] Comparative Example 1 The other conditions and steps were the same as in Example 1, except that the discharge voltage in step ten was increased to 250 V. The Fe2O3 content in the purified kaolin sample decreased from 0.35% to 0.07%, the gallium content decreased from about 65 ppm to 3 ppm, the whiteness was 91.2, and the gallium content in the volatile components was about 824 ppm, indicating that the breakdown voltage promoted the volatilization of elements.

[0058] Comparative Example 2 The other conditions and steps were the same as in Example 2, except that the potassium chloride ratio was increased to 0.1 in step 3. The Fe2O3 content in the purified kaolin sample decreased from 0.47% to 0.07%, the gallium content decreased from about 56 ppm to 6 ppm, the whiteness was 92.6, and the gallium content in the volatile components was 982 ppm. This indicates that potassium chloride has a promoting effect on the volatilization of Fe in the mineral lattice.

[0059] Comparative Example 3 The other conditions and steps were the same as in Example 3, except that the number of flash evaporations in step ten was reduced to 5. The Fe2O3 content in the purified kaolin sample decreased from 0.56% to 0.19%, the gallium content decreased from approximately 40 ppm to 9 ppm, and the whiteness was 92.1. The gallium content in the volatile components was 1056 ppm, indicating that reducing the number of flash evaporations limits the release of iron and gallium.

[0060] Comparative Example 4 Comparative Example 4a: The steps are the same as in Example 4, but the composite activator contains only the eutectic molten salt component (AlCl3-NaCl) and no nitrogen source ligand.

[0061] The Fe2O3 content in the purified kaolin sample decreased from 0.48% to 0.08%, the gallium content decreased from approximately 62 ppm to 9 ppm, the whiteness was 94.2, the gallium content in the volatile components was 12.4%, the total gallium recovery rate was 86.5%, and the Ga / Fe mass ratio in the volatiles was 3.5:1.

[0062] Comparative Example 4b: The steps are the same as in Example 4, but the composite activator contains only the nitrogen source ligand (dicyandiamide) and no eutectic molten salt component.

[0063] The Fe2O3 content in the purified kaolin sample decreased from 0.48% to 0.12%, the gallium content decreased from approximately 62 ppm to 15 ppm, the whiteness was 92.8, the gallium content in the volatile components was 8.7%, the total gallium recovery rate was 81.3%, and the Ga / Fe mass ratio in the volatiles was 2.1:1.

[0064] Comparative Example 5 Comparative Example 5a: The steps are the same as in Example 5, but only the first pulse (voltage 120 V, discharge time 1 s, flash 3 times) is applied, and the second pulse is not applied.

[0065] The Fe2O3 content in the purified kaolin sample decreased from 0.48% to 0.21%, the gallium content decreased from approximately 62 ppm to 18 ppm, the whiteness was 91.5, the gallium content in the volatile components was 5.2%, the total gallium recovery rate was 72.5%, and the Ga / Fe mass ratio in the volatiles was 0.9:1.

[0066] Comparative Example 5b: The steps are the same as in Example 5, but only the second pulse (voltage 250 V, discharge time 0.3 s, flash 3 times) is applied, and the first pulse is not applied.

[0067] The Fe2O3 content in the purified kaolin sample decreased from 0.48% to 0.09%, the gallium content decreased from approximately 62 ppm to 7 ppm, the whiteness was 94.8, the gallium content in the volatile components was 15.6%, the total gallium recovery rate was 88.2%, and the Ga / Fe mass ratio in the volatiles was 4.8:1.

[0068] Based on the test results of Examples 1-3 and Comparative Examples 1-3, the following table 1 is obtained.

[0069]

[0070] Table 1 Based on Table 1 above, as well as the various embodiments and comparative examples, the following conclusions can be drawn: Comparing Example 4 with Comparative Examples 4a and 4b, it can be seen that Example 4 uses a composite activator containing a eutectic molten salt component (AlCl3-NaCl) and a nitrogen source ligand component (dicyandiamide), achieving a gallium recovery rate of 95.3% and a Ga / Fe mass ratio of 11.2:1 in the volatiles. In contrast, Comparative Example 4a only adds the eutectic molten salt component (without the nitrogen source ligand), resulting in a gallium recovery rate of 86.5% and a Ga / Fe ratio of 3.5:1. Comparative Example 4b only adds the nitrogen source ligand component (without the eutectic molten salt), achieving a gallium recovery rate of only 81.3% and a Ga / Fe ratio of 2.1:1. The above comparisons show that the combination of the eutectic molten salt component and the nitrogen source ligand component can achieve selective and efficient gallium recovery.

[0071] Comparing Example 5 with Comparative Examples 5a and 5b, it can be seen that Example 5, employing a multi-stage pulse flash evaporation strategy of first pulse + intermittent period + second pulse, achieved a gallium recovery rate of 97.6%, a Ga content of 31.5% in the volatiles, and a Ga / Fe mass ratio of 16.3:1. In contrast, Comparative Example 5a, applying only the first pulse (low-voltage activation), achieved a gallium recovery rate of only 72.5% and a Ga / Fe ratio of only 0.9:1. Comparative Example 5b, applying only the second pulse (high-voltage vaporization), achieved a gallium recovery rate of 88.2% and a Ga / Fe ratio of 4.8:1. The above comparisons demonstrate that the phased coordination of the first and second pulses can establish a liquid-phase mass transfer channel through low-voltage activation and achieve a step-by-step control mechanism for efficient volatilization through high-voltage vaporization, thereby improving the gallium recovery rate and selectivity.

[0072] Comparing Examples 4-5 with Examples 1-3, it can be seen that Examples 1-3 use a combination of traditional chlorides (sodium chloride, potassium chloride) and sulfides, with a gallium recovery rate of approximately 85-92%, a gallium content in the volatiles of only 0.075-0.135%, and a Ga / Fe ratio of approximately 1-2:1. In contrast, Examples 4-5 use the composite activator of this invention (eutectic molten salt + nitrogen source ligand), increasing the gallium recovery rate to over 95%, the gallium content in the volatiles to over 25%, and the Ga / Fe ratio to over 10:1. The above data fully demonstrate that the combination of eutectic molten salt and nitrogen source ligand used in this invention has unexpected technical effects on the selective volatilization and enrichment of gallium compared to traditional chloride additives.

[0073] Comparing Examples 5, 6a, and 6b, it can be seen that when the eutectic molten salt composition uses a combination of AlCl3 and different alkali metal chlorides (NaCl, KCl, MgCl2), the gallium recovery rate reaches over 95%, the gallium content in the volatiles exceeds 27%, and the Ga / Fe ratio exceeds 13:1. The above results indicate that the present invention has universality.

[0074] Comparing Examples 5, 7a, and 7b, it can be seen that when different compounds from dicyandiamide, melamine, and calcium cyanamide are used as nitrogen source ligand components, the gallium recovery rate reaches over 96%, the gallium content in the volatiles exceeds 29%, and the Ga / Fe ratio exceeds 14:1.

[0075] Comparing Example 5 with Comparative Examples 1-3, it can be seen that within the preferred voltage range (100-150V for the first pulse and 200-300V for the second pulse), time range (0.1-5s), and number of flashes (1-10 times), the method of the present invention can stably achieve high gallium recovery rate and high product whiteness. However, the comparative examples show that deviations from the parameter ranges defined by the present invention (such as excessively high voltage in Comparative Example 1 and insufficient number of flashes in Comparative Example 3) or the use of traditional additives (such as Comparative Examples 1-3) cannot achieve the technical effects of the present invention.

[0076] In summary, this invention achieves efficient and selective recovery of gallium from coal-based kaolin and deep purification of kaolin by introducing a composite activator containing eutectic molten salt components and nitrogen source ligand components, combined with a multi-stage pulse flash evaporation strategy.

[0077] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for flash joule heating purification of coal-based kaolinite in conjunction with gallium separation and recovery, characterized in that, Includes the following steps: S1. Raw material preparation: Carbon-containing coal-based kaolin raw material is pretreated to obtain powder, and the endogenous carbon in the coal-based kaolin is used as a conductive medium. S2. Mixing: The powder is mixed with the additive to obtain a mixed material. After mixing, grinding is continued to obtain a uniform powder before flash evaporation. The additive includes sulfides and a composite activator. The composite activator includes a eutectic molten salt component and a nitrogen source ligand component. S3. Molding: The uniform powder is shaped to obtain a blank to be processed; S4. Flash Joule heat treatment: The blank to be treated is placed between conductive electrodes, and a pulse voltage is applied under vacuum or inert atmosphere to perform flash Joule heat treatment, so that the impurity elements in the coal-series kaolin volatilize. S5. Product collection: Collect the solid product after flash joule heat treatment, which is the purified kaolin; and capture the volatiles generated during the treatment process, and recover gallium from the volatiles.

2. The method according to claim 1, characterized in that, In step S1, the carbon-containing coal-derived kaolin raw material is at least one of coal gangue, coal slime, and carbon-containing kaolin.

3. The method according to claim 1, characterized in that, The sulfide is at least one of sulfur, sodium sulfate, and sodium sulfide, and the sulfide is of industrial or analytical grade.

4. The method according to claim 1, characterized in that, The eutectic molten salt component is a mixture of aluminum chloride and at least one chloride selected from sodium chloride, potassium chloride, and magnesium chloride; the nitrogen source ligand component is a cyanamide compound; both the aluminum chloride and the chloride are of industrial or analytical grade.

5. The method according to claim 4, characterized in that, The molar ratio of aluminum chloride to chloride is (0.8-1.2):1, forming a low-melting-point eutectic system; the cyanamide compound is at least one of dicyandiamide, melamine, and calcium cyanamide.

6. The method according to claim 1, characterized in that, The mass ratio of the sulfide, composite activator and coal-series kaolin powder is (0.02-0.1):(0.07-0.15):

1.

7. The method according to claim 1, characterized in that, In step S1, the pretreatment includes grinding to a particle size of less than 35 micrometers with a content of 70-90%, and drying at 90°C for 1-2 hours; the forming step is tableting.

8. The method according to claim 1, characterized in that, In step S4, multi-stage pulse flash evaporation is adopted, with a first pulse and a second pulse applied sequentially; the discharge voltage of the first pulse is 100-150 V, the discharge time is 0.5-2 s, and flash evaporation is performed 1-2 times; the discharge voltage of the second pulse is 200-300 V, the discharge time is 0.1-1 s, and flash evaporation is performed 1-3 times; an interval of 0.5-2 s is set between the first pulse and the second pulse.

9. The method according to claim 1, characterized in that, In step S4, the discharge voltage is 200-300 V, the discharge time is 0.1-5 s, and the number of flash evaporations is 1-10.

10. The method according to claim 1, characterized in that, In step S4, the conductive electrode is made of graphite, and the blank to be processed forms a conductive circuit with the power supply through an electrode assembly composed of graphite carbon felt and graphite sheet; the size of the graphite sheet is 1 cm × 1 cm × 0.5 cm. In step S5, a collection device for capturing volatiles during flash evaporation is provided around the reaction zone. Gallium elements in the volatiles migrate and accumulate in the form of chlorides or sulfides, and are captured on the inner wall of the collection device by condensation or adsorption.

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