Device and method for recycling valuable metals from fly ash in stepped manner

Abstract

The invention discloses a device and a method for recycling valuable metals from fly ash in a stepped manner. The device comprises a slag-salt separation unit and molten salt electrolysis purification units, the interior of the slag-salt separation unit is configured to be capable of receiving and melting fly ash, molten slag and molten salt are layered in a molten pool by means of the density difference between the molten slag and the molten salt, and an inlet of the first-stage molten salt electrolysis purification unit communicates with a molten salt discharge port through a molten salt runner; each stage of molten salt electrolysis purification unit is provided with an independent electrode pair, a power supply system for controlling electrolysis voltage and a purified molten salt outlet, and all stages of purified molten salt outlets are communicated in sequence, so that molten salt can sequentially flow through all stages of molten salt electrolysis purification units to be subjected to electrolysis purification; and the slag-salt separation unit is physically connected with at least the first-stage molten salt electrolysis purification unit to form an integrated furnace body structure. According to the method, efficient molten state separation of the molten slag and the molten salt is achieved firstly, and then the high-purity valuable metal is selectively recycled from the molten salt in a stepped mode through multi-stage controllable voltage electrolysis.

Description

Technical Field

[0001] This invention relates to the field of waste treatment technology, and in particular to an apparatus and method for the cascade recovery of valuable metals from fly ash. Background Technology

[0002] Fly ash from municipal solid waste incineration is a harmful byproduct generated during the incineration process. It contains high concentrations of heavy metals (such as Pb, Zn, Cu, Cr, and Cd) and soluble chloride salts (such as NaCl and KCl), and is classified as hazardous waste. Currently, high-temperature melting technology is considered one of the most effective technologies for achieving fly ash harmlessness (heavy metal solidification) and volume reduction. Related technical solutions mainly focus on fly ash pretreatment, melting processes, and byproduct disposal, for example: The core technical approach is to promote chloride volatilization. This involves high-temperature melting (typically exceeding 1400℃) to accelerate the volatilization of chlorides in fly ash, which is then collected and recovered as part of the flue gas purification system. However, to achieve full chloride volatilization, this approach requires maintaining extremely high melting temperatures and long residence times, resulting in huge energy consumption, low melting efficiency, and the simultaneous entry of volatile heavy metals into the flue gas system, increasing the load and complexity of flue gas purification.

[0003] The technical approach based on water washing pretreatment involves first using water washing to remove soluble salts from fly ash, and then melting or pyrolyzing the dechlorinated fly ash for resource recovery. While this approach avoids the salt damage problem during the melting process, it introduces a complex water treatment system, generates a large amount of high-salt wastewater, has a long process flow, high costs, and poses a potential risk of secondary pollution from heavy metal ions.

[0004] Optimization of the melting technology itself: Some related technologies have improved the melting process itself, such as using granulation before furnace feeding to improve material properties, or adding preheating sintering units to improve thermal efficiency. However, these improvements often increase the complexity of the process and equipment investment, and fail to fundamentally solve the problems of separating molten salt and slag during the melting process and their respective resource utilization. Although a few technologies mention the physicochemical processes within the molten pool, their core remains carbothermic reduction or promoting salt volatilization, and they do not construct a complete process chain for slag and salt separation and their respective deep resource utilization.

[0005] The main resource-based products of related technologies are mostly concentrated in the preparation of glassy slag (used in building materials) or the recovery of mixed chloride salts. For the multiple valuable metals contained in fly ash, either no recovery is involved, or additional auxiliary materials are required for reduction, or only complex and difficult-to-separate metal-containing dust is obtained through flue gas capture. There is no efficient integrated technology for the direct, selective, and tiered recovery of multiple valuable metals.

[0006] In summary, the fly ash melting and disposal technologies generally suffer from problems such as high energy consumption, complex processes, limited resource recovery pathways, and particularly low efficiency or lack of methods for recovering valuable metals. Summary of the Invention

[0007] The present invention aims to at least partially solve one of the technical problems in the related art.

[0008] Therefore, embodiments of the present invention propose a device for the cascade recovery of valuable metals from fly ash. Through an integrated furnace structure, efficient molten slag and molten salt are first separated in a molten state. Then, through multi-stage controllable voltage electrolysis, high-purity valuable metals are selectively recovered from the molten salt in a cascade manner. At the same time, high-quality purified molten salt and harmless molten slag are produced, realizing energy saving, short process and high value of all components in fly ash disposal.

[0009] The apparatus for cascade recovery of valuable metals from fly ash according to an embodiment of the present invention includes: The slag-salt separation unit is equipped with a fly ash inlet, a molten slag outlet, and a molten salt outlet. The internal structure of the slag-salt separation unit is designed to receive and melt fly ash, and to separate the two into a molten slag layer and a molten salt layer in the molten pool by utilizing the density difference between the molten slag and the molten salt. At least two stages of molten salt electrolysis purification units are connected in series. The inlet of the first stage of the molten salt electrolysis purification unit is connected to the molten salt outlet through a molten salt flow channel. Each stage of the molten salt electrolysis purification unit is equipped with an independent electrode pair, a power supply system for controlling the electrolysis voltage, and a purified molten salt outlet. The purified molten salt outlets of each stage are connected in sequence so that the molten salt can flow sequentially through each stage of the molten salt electrolysis purification unit for electrolysis purification. The slag-salt separation unit is physically connected to at least the first-stage molten salt electrolysis purification unit to form an integrated furnace structure.

[0010] In some embodiments, the slag-salt separation unit and the molten salt electrolysis purification unit, as well as two adjacent stages of the molten salt electrolysis purification unit, are separated by partition walls, which are provided with arched openings for the flow of molten material.

[0011] In some embodiments, the slag outlet of the slag-salt separation unit is located at the bottom of its molten pool or the lower part of its sidewall to correspond to the slag layer, and the molten salt outlet is located at the upper part of its sidewall to correspond to the molten salt layer.

[0012] In some embodiments, the slag-salt separation unit is provided with heating electrodes for heating and maintaining the molten pool at 750°C to 1350°C.

[0013] In some embodiments, each stage of the molten salt electrolytic purification unit is provided with a heat replenishment component for maintaining the molten salt at 700°C to 850°C.

[0014] In some embodiments, the molten salt electrolysis purification unit has three stages, and the power supply system for each stage is configured to provide an increasing electrolysis voltage.

[0015] Embodiments of the present invention also propose a method for the cascade recovery of valuable metals from fly ash using the apparatus described in the above embodiments.

[0016] The method for cascade recovery of valuable metals from fly ash according to embodiments of the present invention includes: S1, the pretreated fly ash is sent to the slag-salt separation unit for melting, the temperature of the molten pool is controlled, and the molten slag and molten salt formed by melting are separated into layers due to their different densities and discharged from the molten slag outlet and molten salt outlet respectively. S2, the separated molten salt is sequentially introduced into a series of multi-stage molten salt electrolysis purification units. A specific electrolysis voltage is applied in each stage for electrolysis, so that the valuable metal ions in the corresponding precipitation potential range are reduced and precipitated at the cathode, thereby achieving the stepwise enrichment and recovery of valuable metals while purifying the molten salt. S3 collects the slag discharged from the slag-salt separation unit, the purified molten salt discharged from the last stage molten salt electrolysis purification unit, and the metal enrichment obtained from the cathodes of each stage of the molten salt electrolysis purification unit.

[0017] In some embodiments, in step S1, the average temperature of the molten pool in the slag-salt separation unit is controlled to be 1000℃±200℃, and the mass ratio of SiO2 to CaO in the fly ash is adjusted to be between 0.8 and 1.2.

[0018] In some embodiments, in step S2, a three-stage series electrolysis is used, controlling the cell voltage of the first stage electrolysis to be 0.8V to 1.5V, the cell voltage of the second stage electrolysis to be 1.8V to 2.5V, and the cell voltage of the third stage electrolysis to be 2.8V to 3.5V, with the electrolysis temperature of each stage maintained between 700°C and 850°C.

[0019] In some embodiments, in step S2, the first-stage electrolysis is mainly used to enrich Ni, Sb, and Cu elements; the second-stage electrolysis is mainly used to enrich Pb, Fe, Cr, and Zn elements; and the third-stage electrolysis is mainly used to enrich Al, Mn, Si, and Mg elements.

[0020] In embodiments of the present invention, the molten slag and molten salt are first separated efficiently and cleanly by utilizing density difference in a molten state at 1000℃±200℃. This fundamentally eliminates the high energy consumption drawbacks of traditional high-temperature volatilization methods and avoids the complex process and secondary pollution of water washing. The separated molten salt then enters a three-stage series electrolytic purification system. By precisely controlling the incremental cell voltage (e.g., 0.8-3.5V), heavy metal ions (e.g., Ni / Cu, Pb / Zn, Al / Mn, etc.) with different precipitation potentials in the molten salt are selectively and separately reduced and enriched at the cathode, thereby converting the mixed heavy metals into valuable metal raw materials with clear refining value. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a device for the cascade recovery of valuable metals from fly ash according to an embodiment of the present invention.

[0022] Figure label: 1-Separation unit; 11-Heating electrode; 101-Fly ash inlet; 102-Slag outlet; 103-Molten salt outlet; 104-Slag layer; 105-Molten salt layer; 106-Fume exhaust port; 2-Electrolytic purification unit; 21-Electrode pair; 201-Purified molten salt outlet; 3-Partition wall; 301-Archway. Detailed Implementation

[0023] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0024] The apparatus for the cascade recovery of valuable metals from fly ash according to an embodiment of the present invention is described below with reference to the accompanying drawings.

[0025] like Figure 1 As shown, the device for cascade recovery of valuable metals from fly ash according to an embodiment of the present invention includes a slag-salt separation unit 1 and at least two stages of molten salt electrolytic purification units 2 connected in series.

[0026] The slag-salt separation unit 1 is equipped with a fly ash inlet 101, a molten slag outlet 102, a molten salt outlet 103, and a flue gas outlet 106. The internal structure of the slag-salt separation unit 1 is designed to receive and melt fly ash, and to use the density difference between molten slag and molten salt to separate the two into a molten slag layer 104 and a molten salt layer 105 in the molten pool.

[0027] The slag-salt separation unit 1 is a high-temperature melting reactor, whose core design is based on the density stratification principle in physicochemistry. After fly ash is heated to a molten state (about 1000°C) in the furnace, its main components will form two immiscible liquid phases.

[0028] The slag layer 104 is mainly composed of silicates and aluminates, with a relatively high density (approximately 2.8 t / m³), and settles at the bottom of the molten pool. The molten salt layer 105 is mainly composed of potassium chloride, sodium chloride, and a large amount of soluble heavy metal chlorides, with a relatively low density (approximately 1.8-2.0 t / m³), and floats on the upper layer of the molten pool.

[0029] Fly ash inlet 101 is used to directly feed pre-treated (dehydrated) powdered fly ash, eliminating the granulation step and simplifying the front-end process. Slag outlet 102 is located at the bottom or lower side of the molten pool, used for continuous or intermittent discharge of the lower layer of high-temperature glassy molten slag. This product has achieved solidification and stabilization of heavy metals and can be directly recycled (e.g., in building materials). Molten salt outlet 103 is located above the side of the molten pool, corresponding to the upper molten salt level, used to draw out molten salt rich in heavy metals for the next process. Flue gas outlet 106 is used to discharge the small amount of flue gas generated during the melting process.

[0030] The inlet of the first-stage molten salt electrolysis purification unit 2 is connected to the molten salt outlet 103 through the molten salt flow channel. Each stage of the molten salt electrolysis purification unit 2 is equipped with an independent electrode pair 21, a power supply system for controlling the electrolysis voltage, and a purified molten salt outlet 201. The purified molten salt outlets 201 of each stage are connected in sequence so that the molten salt can flow sequentially through each stage of the molten salt electrolysis purification unit 2 for electrolysis purification.

[0031] Molten salt electrolysis purification unit 2 is a modular purification system designed based on the principle of electrochemical potential sequence. Each unit is essentially an independent molten salt electrolysis cell.

[0032] Electrode pair 21 forms an electrolytic circuit in high-temperature molten salt. Specific metal ions gain electrons at the cathode and are reduced, forming metals or alloys that adhere to the cathode.

[0033] An independent power supply system is crucial for achieving cascaded recycling. By independently setting and controlling different electrolysis voltages (cell voltages) for each electrolytic cell, the reduction potential of the cathode can be precisely controlled. According to the Nernst equation, metal ions with more positive potentials (easier to reduce) will preferentially precipitate within their corresponding voltage range. By connecting multiple stages in series with progressively increasing voltages, the selective and sequential precipitation of different metals can be achieved.

[0034] The flow of molten salt is synchronized with the electrolytic purification process. The purified molten salt, after some metallic impurities have been removed in the previous stage of electrolysis, flows into the next stage for further purification. This series design enables segmented processing and step-by-step purification of molten salts with complex compositions.

[0035] The slag-salt separation unit 1 is physically connected to at least the first-stage molten salt electrolysis purification unit 2 to form an integrated furnace structure. The integrated furnace chamber, constructed of refractory material, aims to achieve closed-loop, gravity-fed transport of high-temperature molten salt, preventing solidification and pipe blockage due to air exposure during transfer, and avoiding significant heat loss. This structure ensures the continuity of heat and materials from the separation to the electrolysis process.

[0036] The entire unit follows a continuous operation logic of physical separation followed by chemical purification. Dehydrated fly ash enters the slag-salt separation unit 1 through the feed inlet and rapidly melts at high temperature. Inside the molten pool, the molten slag and molten salt automatically separate into layers due to their density difference. The upper layer of molten salt flows out through an overflow or controlled discharge port; the lower layer of molten slag is discharged from the bottom. This separation is completed in a liquid state, which is far more efficient than gaseous volatilization.

[0037] The high-temperature molten salt flowing from separation unit 1 flows sequentially into a series of first-stage, second-stage…N-stage electrolytic purification units 2. In the first-stage unit, a relatively low voltage (e.g., 1V) is applied, primarily to preferentially precipitate metals with the highest reduction potential (e.g., Ni, Cu). Subsequently, the molten salt flows into the second-stage unit, where the voltage is appropriately increased (e.g., 2V), causing metals with slightly lower potentials (e.g., Pb, Zn) to precipitate. This process continues, with each stage acting like a chemical sieve, capturing a specific group of metals at a specific voltage. After multi-stage purification, the heavy metal impurity content of the molten salt itself is significantly reduced, making it a relatively pure industrial salt raw material. The entire process simultaneously produces three products: harmless molten slag, purified molten salt, and metal concentrates adhering to the cathodes of each stage.

[0038] The fly ash cascade recovery device of this invention uses molten density separation (approximately 1000°C) instead of high-temperature forced volatilization separation (>1400°C), significantly reducing the temperature required for the reaction and thus greatly reducing energy consumption. Simultaneously, the rapid liquid separation eliminates the need for prolonged residence time to promote volatilization, improving the fly ash processing capacity (efficiency) per unit time.

[0039] The integrated furnace structure combines slag-salt separation and electrolytic purification into a compact refractory system, enabling short-process, continuous operation. It avoids the complex steps of molten salt transportation, heat preservation, and remelting between multiple devices, simplifying the system and reducing investment and operational complexity.

[0040] By using at least two stages of series connection and independently controlling the electrolysis voltage, the refined and tiered recovery of various valuable metals from fly ash is achieved. Through precise voltage control, different metals are separately enriched on different cathodes, providing higher-grade and relatively homogeneous raw materials for subsequent metallurgical refining.

[0041] In some embodiments, such as Figure 1As shown, the slag-salt separation unit 1 and the molten salt electrolysis purification unit 2, as well as the adjacent two-stage molten salt electrolysis purification units 2, are separated by partition walls 3. The partition walls 3 are provided with arched openings 301 for the flow of molten material, through which molten salt or slag flows to adjacent chambers.

[0042] Partition wall 3 is a solid wall constructed of refractory material. Its main function is to physically divide a large furnace space into functionally independent but closely connected process chambers. For example, it can separate the slag-salt separation chamber from the first-stage electrolytic purification chamber, or separate two adjacent electrolytic chambers, ensuring that each chamber can maintain its independent process environment.

[0043] For slag-salt separation unit 1, the core process involves high-temperature melting and static stratification, requiring a stable thermal field and a relatively calm molten pool surface. For electrolytic purification unit 2, the core process involves an electrochemical reaction under the influence of an electric field, requiring a stable current distribution and liquid flow conditions around the electrodes. The presence of partition wall 3 effectively prevents violent fluid disturbances between different chambers, mutual interference of temperature fields, and electric field diffusion, ensuring the stable operation of each core process.

[0044] The chambers are physically connected and heat is directly transferred through the melt. High-temperature molten salt flows from separation unit 1 (1000℃) into electrolytic purification unit 2 (750℃) through arch 301, carrying a large amount of sensible heat, which can effectively preheat the molten pool of electrolytic purification unit 2 and reduce the external heating energy consumption of electrolytic purification unit 2. The presence of partition wall 3 prevents disordered convection and diffusion of heat, allowing each unit to be more precisely controlled near its optimal process temperature.

[0045] In some embodiments, such as Figure 1 As shown, the slag outlet 102 of the slag-salt separation unit 1 is located at the bottom of its molten pool or the lower part of its side wall to correspond to the slag layer 104, and the molten salt outlet 103 is located at the upper part of its side wall to correspond to the molten salt layer 105.

[0046] The slag discharge outlet 102 corresponds to the high-density slag layer 104 (lower layer) formed after separation. By placing the discharge outlet here, the slag can be naturally collected at the lowest point of the molten pool under its own gravity, and discharged from this point in the smoothest and most thorough manner.

[0047] The molten salt outlet 103 corresponds to the low-density molten salt layer 105 that floats on the upper layer after separation, ensuring that almost all of the outflowing material is molten salt from the upper layer. This achieves separation of the two melts at the outlet and avoids cross-contamination.

[0048] In some embodiments, such as Figure 1 As shown, the slag-salt separation unit 1 is equipped with heating electrodes 11 for heating and maintaining the molten pool at 750°C to 1350°C.

[0049] For example, a plasma torch electrode (non-consumable arc electrode) consists of a water-cooled metal (such as copper) gun body with a cathode (such as hafnium or tungsten) at the center. The gun body serves as the anode, or a separate anode is provided to ground. After a working gas (such as nitrogen or argon) is introduced, a high-temperature plasma arc is excited between the anode and cathode.

[0050] Graphite electrodes (consumable arc electrodes) are rod-shaped electrodes made of high-purity graphite, usually arranged in pairs or three phases, inserted into or suspended above the molten pool. When energized, an electric arc is generated between the electrode and the molten pool (or between the electrodes), causing heating.

[0051] Submerged arc electrodes / resistance electrodes are typically rod-shaped, plate-shaped, or spherical electrodes made of metals (such as copper or steel) or conductive refractory materials such as silicon carbide, and are partially or completely immersed in conductive molten charge. Current passes directly through the melt itself, utilizing the melt's resistance to generate Joule heat for heating. This is a common heating method in molten salt electrolysis and glass furnaces.

[0052] Optionally, in embodiments of the present invention, a composite or adaptive electrode system may be employed. For example, a plasma torch may be used during the start-up phase or when rapid heating is required; during the normal melting and maintenance phase, embedded metal or special refractory electrodes may be used for resistance insulation, because the molten fly ash (especially the formed molten salt layer 105) at this time already has good conductivity.

[0053] The melting of fly ash is a strongly endothermic process. The core function of the heating electrode 11 is to efficiently convert electrical energy into heat energy, providing the large amount of sensible heat and latent heat of fusion required to transform fly ash from a room-temperature solid into a high-temperature melt. Through power adjustment, the heating electrode 11 can form and control a relatively stable hot zone within the furnace that meets the temperature range of 750°C to 1350°C.

[0054] In some embodiments, each stage of molten salt electrolysis purification unit 2 is provided with a heat replenishment component for maintaining the molten salt at 700°C to 850°C.

[0055] For example, a second electrode system, installed inside or on the side wall of the electrolytic cell, independent of the anode-cathode pair used for electrolysis, can use inert metals resistant to molten salt corrosion (such as Inconel alloys or platinum) or special refractory materials (such as molybdenum disilicide) as electrode materials, immersed in molten salt or embedded in the furnace lining. This auxiliary electrode is connected to a power source, and the current generates Joule heating (i.e., resistance heating) through the resistance of the molten salt itself, directly supplementing the heat of the melt.

[0056] The ionic conductivity of molten salt is temperature-sensitive. At excessively low temperatures (<700℃), the molten salt viscosity increases, ion migration slows, and conductivity drops sharply, leading to increased electrolytic cell voltage, decreased current efficiency, increased energy consumption, and potentially even localized solidification and blockage. At excessively high temperatures (>850℃), while conductivity increases, it exacerbates molten salt volatilization, electrode and furnace lining corrosion, and does not improve electrolytic selectivity. Therefore, maintaining a stable temperature between 700-850℃ ensures high current efficiency and reasonable energy consumption during the electrolysis process.

[0057] In some embodiments, the molten salt electrolysis purification unit 2 has three stages, and the power supply system for each stage is configured to provide an increasing electrolysis voltage.

[0058] Commonly found valuable / hazardous heavy metals in fly ash exhibit a range of different standard reduction potentials (relative to Cl2 / Cl) in chloride molten salt systems. - (Refer to).

[0059] First stage (lowest voltage, e.g., 0.8-1.5V): Targets the metals with the most positive recovery potential and the easiest reduction. These metals are thermodynamically most readily precipitated and are typically noble metals or metalloids, such as nickel (Ni), copper (Cu), and antimony (Sb). Applying a lower voltage in the first stage allows for the selective and preferential extraction of these high-value metals, preventing them from being contaminated by other metals in subsequent steps.

[0060] Level 2 (medium voltage, such as 1.8-2.5V): Targets common heavy metals with moderate recovery potential. After easily reducible metals are removed, the voltage is increased to cause the next group of metals to precipitate, such as lead (Pb), iron (Fe), chromium (Cr), and zinc (Zn). These metals are usually present in high concentrations and are the main components of heavy metal pollution in fly ash.

[0061] The third stage (higher voltage, such as 2.8-3.5V): targets metals with more negative recovery potentials and are more difficult to reduce, as well as some amphoteric metals, such as aluminum (Al), manganese (Mn), silicon (Si), and magnesium (Mg). These elements may exist in complex ionic forms in chloride molten salts, requiring higher energy (voltage) to drive reduction.

[0062] The power supply system is configured to provide incremental voltages, serving as the direct means of achieving selective electrolysis. The voltage range for each stage is carefully set to cover the deposition potential range of the target metal group while minimizing the co-deposition of non-target metals. This makes the entire electrolytic purification process resemble a series of voltage-adjustable chemical filters: the first stage filters out metals of group A, the second stage filters out metals of group B, and the third stage filters out metals of group C. After these three stages of treatment, the total amount of heavy metal impurities in the molten salt is significantly reduced.

[0063] The following describes a method for recycling valuable metals from fly ash using the apparatus described in the above embodiments of the present invention.

[0064] The method for cascade recovery of valuable metals from fly ash according to embodiments of the present invention includes: S1, the pretreated fly ash is sent to the slag-salt separation unit 1 for melting, the temperature of the molten pool is controlled, and the molten slag and molten salt formed by melting are separated into layers due to their different densities and discharged from the molten slag outlet 102 and the molten salt outlet 103 respectively.

[0065] The fundamental differences in the physicochemical properties of the main components in fly ash (the slag phase of oxides such as silicon, calcium, and aluminum, and the chloride molten salt phase of heavy metals such as potassium and sodium) in the high-temperature molten state, especially in density.

[0066] By inputting external energy (heating electrode 11), fly ash is transformed from a multiphase heterogeneous solid powder into a homogeneous, flowable melt. In the molten pool, thermodynamic forces drive the system to evolve toward the lowest energy state—the denser phase sinks, the less dense phase floats, and a clear interface is spontaneously formed.

[0067] S2, the separated molten salt is sequentially introduced into the multi-stage molten salt electrolysis purification unit 2 connected in series. A specific electrolysis voltage is applied in each stage for electrolysis, so that the valuable metal ions in the corresponding precipitation potential range are reduced and precipitated at the cathode, thereby achieving the stepwise enrichment and recovery of valuable metals while purifying the molten salt.

[0068] Molten salt flows sequentially through multiple electrolytic cells, a temporal and spatial sequential process that ensures that each metal ion is removed in its designated stage.

[0069] At each specific voltage level, thermodynamically reducible metal ions (i.e., those with a reduction potential higher than the cathode potential) preferentially gain electrons at the cathode and precipitate as elemental metals. Increasing voltage means the cathode potential becomes progressively more negative, gradually expanding the range of reducible metal ions. The mixed metal ions are then eluted and concentrated on different cathodes according to their potential, from highest to lowest.

[0070] S3 collects the slag discharged from the slag-salt separation unit 1, the purified molten salt discharged from the last stage molten salt electrolysis purification unit 2, and the metal enrichment obtained from the cathodes of each stage of the molten salt electrolysis purification unit 2.

[0071] Collect molten slag to produce harmless and recyclable basic building material raw materials. Collect and purify molten salt to produce industrial salt products with significantly reduced impurity content. Collect metal concentrates at various levels to produce valuable metal raw materials that are classified and enriched.

[0072] Optionally, in step S1, the average temperature of the molten pool in the slag-salt separation unit 1 is controlled to be 1000℃±200℃, and the mass ratio of SiO2 to CaO in the fly ash is adjusted to be between 0.8 and 1.2.

[0073] The lower limit of the temperature was determined to be 950℃. This temperature is higher than the complete melting temperature of the chloride molten salt system (mainly KCl-NaCl) in fly ash (its eutectic point is about 660℃, but to achieve good fluidity, it usually needs to be >800℃). Simultaneously, it ensures that the silicate slag phase has sufficient fluidity. For fly ash slag with complex composition, 950℃ is the critical temperature to ensure that both phases can fully melt, significantly reduce viscosity, and thus quickly achieve density stratification and smooth discharge.

[0074] The upper temperature limit is determined (1350℃). When the temperature exceeds a certain range (usually >1300-1400℃), the saturated vapor pressure of potassium chloride, sodium chloride, and heavy metal chlorides will increase sharply, leading to their large-scale volatilization. Within this range, molten separation is the dominant mechanism, while the volatilization of harmful substances is suppressed to a low level, thereby achieving energy saving, emission reduction, and a reduction in flue gas purification load.

[0075] The optimal value of 1000℃ represents an optimized equilibrium point. At this temperature, the viscosities of both the slag and molten salt reach ideal ranges, the density difference is significant, the stratification rate is rapid, and the interface is clear. The slag exhibits good chemical stability, which is beneficial for the solidification of heavy metals, while the chloride volatilization rate remains at a controllable low level, resulting in a relatively acceptable erosion rate on refractory materials.

[0076] By adjusting this ratio, the density of the slag can be moderately adjusted to optimize the density difference between it and the molten salt (which has a relatively fixed density of approximately 1.8-2.0 t / m³), promoting more thorough stratification. Within this ratio range, the slag easily forms a continuous, dense glassy network structure after cooling, effectively encapsulating, dissolving, or chemically bonding heavy metal ions, significantly reducing their leaching toxicity and meeting the requirements for harmlessness.

[0077] In some embodiments, in step S2, a three-stage series electrolysis is used, controlling the cell voltage of the first stage electrolysis to be 0.8V to 1.5V, the cell voltage of the second stage electrolysis to be 1.8V to 2.5V, and the cell voltage of the third stage electrolysis to be 2.8V to 3.5V, with the electrolysis temperature of each stage maintained between 700°C and 850°C.

[0078] Furthermore, in step S2, the first-stage electrolysis is mainly used to enrich Ni, Sb, and Cu elements; the second-stage electrolysis is mainly used to enrich Pb, Fe, Cr, and Zn elements; and the third-stage electrolysis is mainly used to enrich Al, Mn, Si, and Mg elements.

[0079] When a relatively low voltage (e.g., 1V) is applied in the first stage, the cathode potential is relatively positive. At this time, only metal ions with the most positive reduction potential (i.e., the easiest to reduce) (such as Ni²⁺) are present. + / Ni,Cu² + Cu can meet the precipitation conditions and is preferentially reduced. At this time, Pb², with a more negative potential, is... + Zn² + Plasma does not precipitate, thus achieving the first stage of selective enrichment.

[0080] After a significant amount of the first-stage target metal has been removed, the molten salt enters the second stage. At this point, the voltage is increased to 1.8-2.5V, the cathode potential becomes more negative, allowing the second group of metal ions (such as Pb²⁺) with slightly lower potentials to enter the second stage. + / Pb,Zn² + / Zn) satisfies the precipitation conditions. Similarly, the third-stage voltage is further increased to reduce the most difficult-to-reduce third group of metals (such as Al³⁺). + / Al).

[0081] Maintaining the electrolysis temperature at each stage between 700℃ and 850℃ ensures the high ionic conductivity of the molten salt, reduces ohmic voltage drop, and improves current efficiency; the good fluidity of the molten salt ensures mass transfer and replenishment of metal ions to the cathode; and inhibits molten salt volatilization and electrode corrosion (relative to higher temperatures).

[0082] Thus, through precise voltage control, precious metals such as Ni and Cu can be deposited almost independently on the first-stage cathode, forming high-grade enrichments (e.g., Ni mass fraction up to 56% in this embodiment), simplifying the subsequent refining process and increasing the value of the recovered metals. Similarly, the value of the second and third-stage products also increases due to their relative enrichment.

[0083] Staged electrolysis avoids the use of extremely high voltages in a single cell to reduce all metals. At extremely high voltages, side reactions (such as anodic chloride evolution, molten salt decomposition, and secondary metal reactions) are exacerbated, and current efficiency is severely reduced. After staging, each stage operates within its optimal and moderate voltage window, minimizing side reactions and increasing the proportion of electrical energy used for the reduction of the target metal, thereby reducing the energy consumption per ton of metal recovered.

[0084] Through three stages of deep sweeping at different voltages, heavy metal impurities in the molten salt are removed layer by layer. The total amount of harmful heavy metals such as Ni, Pb, and Zn in the final discharged purified molten salt can be reduced by more than 90%.

[0085] In summary, the following describes two specific embodiments of the present invention for the cascade recovery of valuable metals from fly ash.

[0086] First specific embodiment: The fly ash from waste incineration is dehydrated using steam, reducing the moisture content to 0.5%. The dehydrated powdered fly ash is directly sent to the slag-salt separation unit 1. Quartz sand and industrial NaCl salt are conveyed by a screw conveyor and sent to the slag-salt separation unit 1 in a certain proportion. The SiO2 / CaO ratio in the molten pool is 1.0.

[0087] The slag-salt separation unit 1 is heated by electrodes, with an average molten pool temperature of 1000℃. It utilizes the density difference between molten slag and molten salt (molten slag ρ≈2.8t / m³). 3 ) and molten salt (ρ≈1.9t / m 3 The two high-temperature melts in the furnace are separated into layers. The upper layer is molten salt 105, which is discharged through molten salt outlet 103, and the lower layer is molten slag 104, which is discharged through molten slag outlet 102, thus achieving preliminary slag-salt separation.

[0088] Molten salt enters the first-stage molten salt electrolysis purification unit 2. The average temperature of the molten pool is 750℃, the cell voltage is 1V, and the electrolysis time is 5h. The electrode plate consists of two sets of anode-cathode plate pairs. Ni, Sb, and Cu are removed from the molten salt under the electrolysis of the electrode plate to obtain first-stage purified molten salt.

[0089] The cathode plate product of the first-stage molten salt electrolysis purification unit 2 is taken out, washed and filtered with water, and the mass fraction of Ni in the solid product is 56%, the mass fraction of Sb is 21%, the mass fraction of Cu is 8%, and the mass fraction of others is 15%.

[0090] The primary purified molten salt enters the secondary molten salt electrolysis purification unit 2. The average temperature of the molten pool is 750℃, the cell voltage is 2V, and the electrolysis time is 5h. The electrode plate consists of two sets of anode-cathode plate pairs. Under the electrolysis of the electrode plate, elements such as Pb, Fe, Cr, and Zn are removed from the molten salt to obtain the secondary purified molten salt.

[0091] The cathode plate product of the second-stage molten salt electrolysis purification unit 2 is taken out, washed and filtered with water, and the mass fraction of Pb in the solid product is 43%, Fe is 18%, Cr is 17%, Zn is 9%, and others are 13%.

[0092] The secondary purified molten salt enters the third-stage molten salt electrolysis purification unit 2. The average temperature of the molten pool is 750℃, the cell voltage is 3V, the electrolysis time is 5h, and the electrode plate consists of two sets of anode-cathode plate pairs. Under the electrolysis of the electrode plate, elements such as Al, Mn, Si, and Mg are removed from the molten salt to obtain the tertiary purified molten salt.

[0093] The cathode plate product of the third-stage molten salt electrolysis purification unit 2 is taken out, washed and filtered with water, and the solid product is found to have a mass fraction of Al of 22%, Mn of 20%, Si of 10%, Mn of 6%, and others of 42%.

[0094] In the final discharged molten salt, the total mass fraction of metallic elements such as Ni, Sb, Cu, Pb, Fe, Cr, Zn, Al, Mn, Si, and Mg decreased by 90.2%.

[0095] Second specific embodiment: Waste incineration fly ash is dehydrated using steam, reducing the moisture content to 0.5%. The dehydrated powdered fly ash is directly sent to slag-salt separation unit 1, where it is mixed with 5% (0.1%) of rare and precious metal Pt waste. Quartz sand and waste salt, mainly NaCl, are conveyed via screw conveyors and sent to slag-salt separation unit 1 in a certain proportion. The SiO₂ in the molten pool... 2 / CaO=1.0.

[0096] The slag-salt separation unit 1 is heated by electrodes, with an average molten pool temperature of 900℃. It utilizes the density difference between molten slag and molten salt (molten slag ρ≈2.8t / m³). 3 ) and molten salt (ρ≈1.9t / m 3 The two high-temperature melts in the furnace are separated into layers. The upper layer is molten salt 105, which is discharged through molten salt outlet 103, and the lower layer is molten slag 104, which is discharged through molten slag outlet 102, thus achieving preliminary slag-salt separation.

[0097] The molten salt enters the first-stage molten salt electrolysis purification unit 2. The average temperature of the molten pool is 750℃, the cell voltage is 1.5V, and the electrolysis time is 5h. The electrode plate consists of two sets of anode-cathode plate pairs. Ni, Sb, and Cu are removed from the molten salt under the electrolysis of the electrode plate to obtain first-stage purified molten salt.

[0098] The cathode plate product of the first-stage molten salt electrolysis purification unit 2 is taken out, washed and filtered with water, and the mass fraction of Ni in the solid product is 38%, the mass fraction of Sb is 26%, the mass fraction of Cu is 12%, and the mass fraction of others is 9%.

[0099] The primary purified molten salt enters the secondary molten salt electrolysis purification unit 2. The average temperature of the molten pool is 750℃, the cell voltage is 2.5V, the electrolysis time is 5h, and the electrode plate consists of two sets of anode-cathode plate pairs. Under the electrolysis of the electrode plate, elements such as Pb, Fe, Cr, and Zn are removed from the molten salt to obtain the secondary purified molten salt.

[0100] The cathode plate product of the second-stage molten salt electrolysis purification unit 2 is taken out, washed and filtered with water, and the solid product is found to have a mass fraction of 23% Pb, 28% Fe, 16% Cr, 11% Zn, 3% Pt, and 19% other components.

[0101] The secondary purified molten salt enters the third-stage molten salt electrolysis purification unit 2. The average temperature of the molten pool is 750℃, the cell voltage is 3.5V, the electrolysis time is 5h, and the electrode plate consists of two sets of anode-cathode plate pairs. Under the electrolysis of the electrode plate, elements such as Al, Mn, Si, and Mg are removed from the molten salt to obtain the tertiary purified molten salt.

[0102] The cathode plate product of the third-stage molten salt electrolysis purification unit 2 is taken out, washed and filtered with water, and the mass fraction of Al in the solid product is 26%, the mass fraction of Mn is 33%, the mass fraction of Si is 5%, the mass fraction of Mn is 5%, and the mass fraction of others is 31%.

[0103] In the final discharged molten salt, the total mass fraction of metallic elements such as Ni, Sb, Cu, Pb, Fe, Cr, Zn, Al, Mn, Si, and Mg decreased by 95.6%.

[0104] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0105] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0106] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0107] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0108] In this invention, the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0109] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. An apparatus for fly ash stepwise recovery of valuable metals, characterized by, include: The slag-salt separation unit is equipped with a fly ash inlet, a molten slag outlet, and a molten salt outlet. The internal structure of the slag-salt separation unit is designed to receive and melt fly ash, and to separate the two into a molten slag layer and a molten salt layer in the molten pool by utilizing the density difference between the molten slag and the molten salt. At least two stages of molten salt electrolysis purification units are connected in series. The inlet of the first stage of the molten salt electrolysis purification unit is connected to the molten salt outlet through a molten salt flow channel. Each stage of the molten salt electrolysis purification unit is equipped with an independent electrode pair, a power supply system for controlling the electrolysis voltage, and a purified molten salt outlet. The purified molten salt outlets of each stage are connected in sequence so that the molten salt can flow sequentially through each stage of the molten salt electrolysis purification unit for electrolysis purification. The slag-salt separation unit is physically connected to at least the first-stage molten salt electrolysis purification unit to form an integrated furnace structure.

2. The device for fly ash staged recovery of valuable metals of claim 1, wherein, The slag-salt separation unit and the molten salt electrolysis purification unit, as well as the two adjacent molten salt electrolysis purification units, are separated by partition walls, which are provided with arched openings for the flow of molten material.

3. The device for fly ash staged recovery of valuable metals of claim 1, wherein, The slag outlet of the slag-salt separation unit is located at the bottom of its molten pool or the lower part of its side wall to correspond to the slag layer, and the molten salt outlet is located at the upper part of its side wall to correspond to the molten salt layer.

4. The device for fly ash staged recovery of valuable metals of claim 1, wherein, The slag-salt separation unit is equipped with heating electrodes for heating and maintaining the molten pool at 750°C to 1350°C.

5. The apparatus for flyash staged recovery of valuble metals of claim 1 wherein, Each stage of the molten salt electrolysis purification unit is equipped with a heat replenishment component for maintaining the molten salt at 700°C to 850°C.

6. The device for fly ash staged recovery of valuable metals of claim 1, wherein, The molten salt electrolysis purification unit has three stages, and the power supply system for each stage is configured to provide an increasing electrolysis voltage.

7. A method for fly ash stepwise recovery of valuable metals using the apparatus according to any one of claims 1-6, characterized by, include: S1, the pretreated fly ash is sent to the slag-salt separation unit for melting, the temperature of the molten pool is controlled, and the molten slag and molten salt formed by melting are separated into layers due to their different densities and discharged from the molten slag outlet and molten salt outlet respectively. S2, the separated molten salt is sequentially introduced into a series of multi-stage molten salt electrolysis purification units. A specific electrolysis voltage is applied in each stage for electrolysis, so that the valuable metal ions in the corresponding precipitation potential range are reduced and precipitated at the cathode, thereby achieving the stepwise enrichment and recovery of valuable metals while purifying the molten salt. S3 collects the slag discharged from the slag-salt separation unit, the purified molten salt discharged from the last stage molten salt electrolysis purification unit, and the metal enrichment obtained from the cathodes of each stage of the molten salt electrolysis purification unit.

8. The method of fly ash staged recovery of valuable metals according to claim 7, wherein, In step S1, the average temperature of the molten pool in the slag-salt separation unit is controlled at 1000℃±200℃, and the mass ratio of SiO2 to CaO in the fly ash is adjusted between 0.8 and 1.

2.

9. The method of fly ash staged recovery of valuable metals according to claim 7, wherein, In step S2, a three-stage series electrolysis is adopted, with the cell voltage of the first stage electrolysis controlled at 0.8V to 1.5V, the cell voltage of the second stage electrolysis at 1.8V to 2.5V, and the cell voltage of the third stage electrolysis at 2.8V to 3.5V, and the electrolysis temperature of each stage maintained between 700℃ and 850℃.

10. The method of fly ash staged recovery of valuable metals according to claim 9, wherein, In step S2, the first-stage electrolysis is mainly used to enrich Ni, Sb, and Cu elements; the second-stage electrolysis is mainly used to enrich Pb, Fe, Cr, and Zn elements; and the third-stage electrolysis is mainly used to enrich Al, Mn, Si, and Mg elements.

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