Coal-based solid waste calcium-magnesium ion selective separation method and separation device thereof

Abstract

The present application belongs to the field of coal-based solid waste resource utilization and electrochemical separation technology, and discloses a coal-based solid waste calcium-magnesium ion selective separation method and a separation device thereof.The device is integrated by organically integrating an acid leaching unit, a solid-liquid separation unit, a pretreatment unit and a bipolar membrane electrodialysis unit, and the pH value of the alkaline chamber of the bipolar membrane electrodialysis unit is accurately controlled, the ion directional migration selectivity of the electrochemical method itself is utilized, the characteristics that H + and OH ‑ are generated by in-situ hydrolysis of the bipolar membrane under the action of an electric field are combined, the selective separation of calcium-magnesium ions is realized in the alkaline chamber by accurately controlling the pH value, H + generated by the bipolar membrane combines with anions in the acid chamber to generate regenerated acid, which can be returned to the acid leaching unit for recycling.The whole process does not need to add chemical precipitants or pH regulators, and realizes the selective separation of calcium-magnesium ions and the resource recovery of acid.

Description

Technical Field

[0001] This invention relates to the field of coal-based solid waste resource utilization and electrochemical separation technology, and in particular to a method and apparatus for selective separation of calcium and magnesium ions in coal-based solid waste. Background Technology

[0002] Coal-based solid waste (such as fly ash and coal gangue) is one of the largest industrial solid wastes emitted in my country. Its stockpiling not only occupies a large amount of land resources but also causes serious pollution to the ecological environment. However, coal-based solid waste contains abundant valuable metal elements such as calcium, magnesium, aluminum, and iron, and has important resource utilization value.

[0003] Currently, the main methods for extracting calcium and magnesium ions from coal-based solid waste include acid leaching, alkaline leaching, and combined leaching methods. While these methods can achieve the dissolution of calcium and magnesium ions to a certain extent, they have the following prominent problems: First, in traditional wet extraction processes, the acid leaching solution contains multiple metal ions such as calcium, magnesium, aluminum, and iron. Since the solubility products of these metal hydroxides are similar, traditional chemical precipitation methods are prone to generating mixed hydroxide precipitates due to local supersaturation during the one-time addition of alkali. This makes it difficult to achieve selective separation of calcium and magnesium ions, resulting in low purity of the target product and difficulty in subsequent separation.

[0004] Secondly, traditional precipitation processes require the addition of large amounts of chemical reagents (such as sodium hydroxide, sodium carbonate, etc.) as precipitants or pH adjusters, which not only increases production costs but also introduces new impurity ions, affecting product purity. At the same time, the consumption of added reagents is non-renewable, resulting in low resource utilization and generating a large amount of saline wastewater, increasing the environmental burden.

[0005] Third, in traditional precipitation processes, pH values ​​mainly rely on manual offline measurement and intermittent control, lacking real-time feedback adjustment methods. This makes it difficult to accurately distinguish the precipitation behavior of calcium and magnesium ions, resulting in severe co-precipitation and making it difficult to guarantee product purity.

[0006] Fourth, existing equipment lacks integrated means for synchronous, online, and selective monitoring of the concentrations of multiple key ions in the acid leaching solution and separation process. It is even more difficult to use the monitoring data for real-time closed-loop control of the leaching endpoint and precipitation separation process, resulting in process control relying on human experience and making it difficult to achieve precise and automated operation. Summary of the Invention

[0007] The purpose of this invention is to provide a method and apparatus for selective separation of calcium and magnesium ions from coal-based solid waste, which solves the technical problems of traditional methods such as long process flow, need for external chemical reagents, calcium and magnesium co-precipitation, and lack of process monitoring.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for selective separation of calcium and magnesium ions from coal-based solid waste, comprising the following steps: S1. In the acid leaching unit, coal-based solid waste is mixed with acid solution to carry out acid leaching reaction, and a slurry containing solid residue and ionic solution is obtained. S2. The slurry containing solid residue and ionic solution is fed into the solid-liquid separation unit to separate the first ionic solution and solid residue. S3. The first ion solution is fed into the pretreatment unit, and the pH is adjusted to 5.0-5.5 to allow Fe to... 3+ And Al 3+ The mixture is co-precipitated and then sent to a solid-liquid separation device to separate the second ionic solution and the solid precipitate. S4. Send the second ion solution into the raw material chamber of the bipolar membrane electrodialysis unit and apply DC power to the bipolar membrane electrodialysis unit. Driven by an electric field, cations in the feed chamber migrate towards the cathode and pass through the cation exchange membrane, where Ca... 2+ Mg 2+ The solution is moved to the alkali chamber; anions in the feed chamber migrate towards the anode, passing through the anion exchange membrane to the acid chamber; the residual liquid retained in the feed chamber is discharged; the first and second bipolar membranes dissociate H2O in situ into H+. + and OH - OH generated by the first bipolar membrane - Upon entering the alkali chamber, H2O generated by the second bipolar membrane... + Enter the acid chamber; Adjust the output intensity of the DC power supply to control the pH of the alkali chamber to 10.0~10.5, so that Mg... 2+ After precipitation and solid-liquid separation, solid Mg(OH)2 and liquid containing Ca are separated. 2+ Clear liquid, containing Ca 2+ The supernatant is returned to the alkali chamber; the pH of the alkali chamber is further increased to ≥12.0 or CO2 is introduced into the alkali chamber to reduce the Ca2+ content. 2+ After precipitation and solid-liquid separation, solid Ca(OH)2 or CaCO3 is separated, and the separated liquid is returned to S3 as regenerated alkali solution for pH adjustment. H generated by the second bipolar membrane + It enters the acid chamber and combines with the anions migrating in the raw material chamber to obtain regenerated acid, which is then returned to the acid leaching unit for recycling. The acid dissolution reaction in step S1, the pH control in step S3, the DC power supply in step S4, and the pH control in step S4 are all regulated by a PLC control unit.

[0009] Furthermore, in the method, the acid solution in step S1 includes hydrochloric acid and / or nitric acid; the concentration of the acid solution in step S1 is 0.5~2.5 mol / L; and the volume ratio of the acid solution in step S1 to the mass of the coal-based solid waste is 1~25 mL:1 g.

[0010] Furthermore, in the method, the reaction temperature of the acid dissolution reaction in step S1 is 25~120℃, and the dispersion rate is 100~600r / min.

[0011] The present invention also provides a selective separation device for calcium and magnesium ions in coal-based solid waste, which operates using the method described above. The device includes an acid leaching unit, a solid-liquid separation unit, a pretreatment unit, a bipolar membrane electrodialysis unit, a DC power supply, and a PLC control unit. The acid leaching unit has a solid waste inlet, an acid inlet, and a discharge outlet; the acid leaching unit includes a stirring device, a heating device, a temperature controller, a stirring speed controller, and a first online monitoring component; the temperature controller is electrically connected to the heating device; the stirring speed controller is electrically connected to the stirring device; the first online monitoring component is data-connected to the PLC control unit; The solid-liquid separation unit has an inlet, a liquid outlet, and a solid outlet; the inlet of the solid-liquid separation unit is connected to the outlet of the acid leaching unit. The pretreatment unit has an inlet, an alkali inlet, a liquid outlet, a solid outlet, a stirring device, a pH electrode, and a solid-liquid separation device; the inlet of the pretreatment unit is connected to the liquid outlet of the solid-liquid separation unit; the pH electrode of the pretreatment unit is connected to the PLC control unit for data transmission. The bipolar membrane electrodialysis unit includes a membrane stack, electrodes, electrode chambers, and a second online monitoring component. The electrodes include an anode and a cathode. The electrode chambers include an anode chamber and a cathode chamber. The membrane stack includes a first bipolar membrane, a second bipolar membrane, a cation exchange membrane, and an anion exchange membrane, arranged sequentially from cathode to anode as the first bipolar membrane, cation exchange membrane, anion exchange membrane, and second bipolar membrane. An alkali chamber is located between the first bipolar membrane and the cation exchange membrane; a feed chamber is located between the cation exchange membrane and the anion exchange membrane; and an acid chamber is located between the anion exchange membrane and the second bipolar membrane. The feed chamber is connected to the liquid outlet of the pretreatment unit. The second online monitoring component is connected to the PLC control unit for data transmission. The DC power supply is electrically connected to the electrodes of the bipolar membrane electrodialysis unit; the DC power supply is also data-connected to the PLC control unit.

[0012] Furthermore, in the device, the solid-liquid separation unit is a plate and frame filter press, a centrifuge, a vacuum filter, or a microporous filter.

[0013] Furthermore, in the device, both the first online monitoring component and the second online monitoring component independently include an ion-selective electrode, a conductivity sensor, and a pH electrode; The ion-selective electrode includes a cation-selective electrode and an anion-selective electrode; the cation-selective electrode independently includes one or more of the following: calcium ion-selective electrode, magnesium ion-selective electrode, potassium ion-selective electrode, sodium ion-selective electrode, iron ion-selective electrode, aluminum ion-selective electrode, and ammonium ion-selective electrode; the anion-selective electrode independently includes a chloride ion-selective electrode and / or a nitrate ion-selective electrode.

[0014] Furthermore, in the device, the alkali chamber, the raw material chamber, or the acid chamber each independently includes a circulation unit; the circulation unit independently includes a circulation pipeline and a circulation pump.

[0015] Furthermore, in the device, in the first online monitoring component, the ion-selective electrode is disposed at the outlet of the acid leaching unit or on a pipeline connected to the outlet, the conductivity sensor is disposed inside the acid leaching unit, and the pH electrode is disposed inside the acid leaching unit.

[0016] Furthermore, in the device, the second online detection component is disposed in each compartment and / or on the pipeline communicating with each compartment, each compartment including the alkali compartment, the raw material compartment and the acid compartment.

[0017] Furthermore, in the device, the anode material includes titanium coated with ruthenium, titanium coated with iridium, or titanium coated with ruthenium-iridium; the cathode material includes stainless steel or nickel.

[0018] As can be seen from the above technical solution, compared with the prior art, the present invention has the following beneficial effects: (1) Integrated design: The core functional units of acid leaching, solid-liquid separation, pretreatment and precise pH control of the bipolar membrane electrodialysis alkaline chamber are organically integrated into one, realizing a short-process conversion from coal-based solid waste to high-value-added magnesium products, and significantly shortening the process flow. This effect is determined by the sequential connection and collaborative work of each unit.

[0019] (2) In-situ selective separation of calcium and magnesium in the alkali chamber: taking advantage of the approximately 10 difference in solubility product between Mg(OH)2 and Ca(OH)2. 6 A significant difference of several times was achieved by precisely controlling the pH of the alkali chamber within the range of 10.0–10.5, allowing Mg to... 2+ Preferential complete precipitation while Ca 2+Residing in solution, the selective separation of calcium and magnesium is directly achieved within the alkaline chamber of a bipolar membrane electrodialysis system. Compared to traditional multi-stage separation processes requiring co-precipitation, redissolution, and stepwise precipitation, the separation process of this invention can be completed within a single alkaline chamber, significantly simplifying the process. This effect is determined by the inherent difference in Ksp between Mg(OH)₂ and Ca(OH)₂ and the precise control of the pH in the alkaline chamber.

[0020] (3) Electrochemical directional migration to achieve ion selective enrichment: using an electric field to drive cations to selectively pass through the cation exchange membrane, so that Ca 2+ / Mg 2+ Separation from anions achieves inter-group separation of target ions and impurity ions. This effect is jointly determined by the electric field driving and the selective permeation characteristics of the ion exchange membrane.

[0021] (4) Bipolar membrane in-situ alkali supply without the need for external chemical reagents: The core innovation of this invention lies in the use of a bipolar membrane to dissociate water molecules into H+ in-situ under the action of an electric field. + and OH - Its characteristics allow it to directly convert Mg into Mg in the alkali chamber. 2+ Selective precipitation provides OH - The precipitant is used, and the pH of the alkali chamber is precisely controlled by adjusting the current density. This design fundamentally avoids the problem of requiring the addition of chemical reagents such as NaOH in traditional processes, eliminating the operating costs of reagent procurement, storage, and addition, and avoiding the need for the addition of cations (such as Na+). + The negative impact on product purity is determined by the molecular structure of the bipolar membrane and its water dissociation function under an electric field.

[0022] (5) In-situ regeneration and recycling of acid: H generated by bipolar membrane + In the acid chamber, it reacts with migrating Cl... - The anions combine to generate regenerated acid (such as HCl), which can be returned to the acid leaching unit for recycling, realizing the resource recovery of acid and reducing acid consumption and wastewater discharge. This effect is determined by the acid generation function of the bipolar membrane and the circulating connection between the acid chamber and the acid leaching unit.

[0023] (6) In-situ regeneration and closed-loop circulation of alkali for pretreatment: As a further preferred embodiment of the present invention, the NH4 introduced during the pretreatment stage is regenerated using the bipolar membrane electrodialysis system itself. + Or Na +As an "alkali carrier," it is regenerated in situ in the alkali chamber into a high-purity NH4OH or NaOH solution, which is then directly recycled to the pretreatment unit via pipeline. This design allows the entire process to require only a small amount of external alkali added during the initial startup, after which a closed-loop recycling of alkali can be achieved. This fundamentally eliminates the problem of continuous consumption of chemical reagents in traditional processes, further reducing operating costs, while avoiding the need for external cations (such as Na+). + The potential impact of the accumulation of these elements in the system on product purity has been mitigated, enabling green and efficient resource utilization.

[0024] (7) Full-ion online monitoring and intelligent control: By setting up Ca covering units in the acid leaching unit and each compartment 2+ Mg 2 + K + Na + Cl - Fe 3+ Al 3+ The selective electrode for multiple ions, combined with a conductivity sensor and a pH electrode, enables real-time multi-ion monitoring of the entire process, including acid leaching, ion migration, alkali chamber separation, and acid chamber regeneration. The PLC control unit automatically adjusts the current density based on feedback signals, achieving precise closed-loop control and automated operation of the alkali chamber pH. This performance is determined by the data connection between the online monitoring components and the PLC control unit, as well as the feedback control logic.

[0025] (8) High-efficiency utilization of resources: The solid residue obtained after solid-liquid separation can be further washed and recycled for residual valuable metals or used for building materials and other purposes; the iron-aluminum co-precipitate obtained after solid-liquid separation in the pretreatment stage can be recycled as raw material for polyaluminum iron flocculant; the Mg(OH)2 obtained by alkali separation can be directly used as a high-purity product containing Ca 2+ The clarified liquid can be further processed to obtain Ca(OH)2 or CaCO3 products, realizing the graded extraction and high-value utilization of multiple valuable elements in coal-based solid waste. This effect is determined by the product outlet settings of each unit and the step-by-step collection strategy.

[0026] (9) Precise monitoring of leaching endpoint and dissolution rate: By setting up a pH sensor and an ion monitoring device covering multiple ions in the acid leaching unit, the pH value of the slurry and the Ca content in the leachate are monitored in real time. 2+ / Mg 2+ The concentration change is automatically determined by the PLC control unit to identify the endpoint of the acid dissolution reaction, achieving an intelligent upgrade from "timed leaching" to "on-demand leaching." This effect is determined by the data connection and feedback control logic between the pH sensor, ion monitoring device, and PLC control unit. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0028] Figure 1 This is a schematic diagram of a bipolar membrane electrodialysis unit. Detailed Implementation

[0029] This invention provides a method for selective separation of calcium and magnesium ions from coal-based solid waste, comprising the following steps: S1. In the acid leaching unit, coal-based solid waste is mixed with acid solution to carry out acid leaching reaction, and a slurry containing solid residue and ionic solution is obtained. S2. The slurry containing solid residue and ionic solution is fed into the solid-liquid separation unit to separate the first ionic solution and solid residue. S3. The first ion solution is fed into the pretreatment unit, and the pH is adjusted to 5.0-5.5 to allow Fe to... 3+ And Al 3+ The mixture is co-precipitated and then sent to a solid-liquid separation device to separate the second ionic solution and the solid precipitate. S4. Send the second ion solution into the raw material chamber of the bipolar membrane electrodialysis unit and apply DC power to the bipolar membrane electrodialysis unit. Driven by an electric field, cations in the feed chamber migrate towards the cathode and pass through the cation exchange membrane, where Ca... 2+ Mg 2+ Anions migrate to the alkali chamber; anions in the feed chamber migrate towards the anode, passing through the anion exchange membrane to the acid chamber; residual liquid retained in the feed chamber is discharged; the first and second bipolar membranes dissociate H2O in situ into H+. + and OH - OH generated by the first bipolar membrane - Upon entering the alkali chamber, H2O generated by the second bipolar membrane... + Enter the acid chamber; Adjust the output intensity of the DC power supply to control the pH of the alkali chamber to 10.0~10.5, so that Mg... 2+ After precipitation and solid-liquid separation, solid Mg(OH)2 and liquid containing Ca are separated. 2+ Clear liquid, containing Ca 2+ The supernatant is returned to the alkali chamber; the pH of the alkali chamber is further increased to ≥12.0 or CO2 is introduced into the alkali chamber to reduce the Ca2+ content. 2+ After precipitation and solid-liquid separation, solid Ca(OH)2 or CaCO3 is separated, and the separated liquid is discharged as mother liquor. H generated by the second bipolar membrane + It enters the acid chamber and combines with the anions migrating in the raw material chamber to obtain regenerated acid, which is then returned to the acid leaching unit for recycling. The acid dissolution reaction in step S1, the pH control in step S3, the DC power supply in step S4, and the pH control in step S4 are all regulated by a PLC control unit.

[0030] In this invention, the coal-based solid waste includes one or more of fly ash, coal gasification slag, and coal gangue; coal gangue is a blackish-gray rock with low carbon content and harder than coal that is associated with coal seams during the coal formation process, and is a solid waste separated during coal mining and washing; fly ash is from coal-fired power plants and coal gangue and coal slime resource comprehensive utilization power plants, and is the fine fly ash and slag obtained after boiler flue gas is collected by a dust collector; coal gasification slag is from the coal gasification process, where the non-combustible substances in the coal melt into liquid ash slag after high-temperature gasification reaction in the gasifier, and are discharged after cooling and solidification.

[0031] In this invention, the particle size of the coal-based solid waste is preferably ≤0.074 mm. This particle size is obtained through crushing.

[0032] In this invention, the acid solution in step S1 preferably includes hydrochloric acid and / or nitric acid, and more preferably hydrochloric acid.

[0033] In this invention, the concentration of the acid solution in step S1 is preferably 0.5~2.5 mol / L, more preferably 1~2 mol / L, and even more preferably 1.5 mol / L.

[0034] In this invention, the volume ratio of the acid solution to the mass of the coal-based solid waste in step S1 is preferably 1~25mL:1g, more preferably 5~15mL:1g, and even more preferably 10mL:1g.

[0035] In this invention, the reaction temperature of the acid dissolution reaction in step S1 is preferably 25~120℃, more preferably 30~80℃, and even more preferably 50℃; the dispersion rate is preferably 100~600r / min, more preferably 200~500r / min, and even more preferably 400r / min.

[0036] In this invention, the alkaline reagent used to adjust the pH to 5.0-5.5 in step S3 preferably includes ammonia or sodium hydroxide. The amount of alkaline reagent used is not limited; adjusting the pH to 5.0-5.5 is sufficient.

[0037] In this invention, the alkaline reagent used in step S3 to adjust the pH to 5.0-5.5 (except for the initial addition) originates from the in-situ regenerated alkali solution in the alkali chamber of the bipolar membrane electrodialysis unit. The aim is to further reduce dependence on external chemical reagents and achieve closed-loop recycling of the pretreatment alkali. During the initial system startup, step S3 adds NH4+-containing solution to the first ion solution. + Or Na +Use an alkaline reagent (such as ammonia or sodium hydroxide solution) to adjust the pH to 5.0-5.5 to complete the Fe... 3+ And Al 3+ The coprecipitation removes the NH4+. At this point, the NH4+ in the solution... + Or Na + As an "alkaline carrier ion," NH4+ enters the feed chamber of the bipolar membrane electrodialysis unit along with the second ion solution. During the operation of the bipolar membrane electrodialysis, NH4+... + Or Na + Driven by an electric field, it passes through the cation exchange membrane into the alkali chamber. Because NH4OH and NaOH have extremely high solubility, they do not react with Mg in the alkali chamber as readily as Mg. 2+ Ca 2+ Instead of forming a precipitate, it exists stably in the liquid phase of the alkali chamber in ionic form, reacting with the OH- produced by the hydrolysis of the bipolar membrane. - They combine to form a regenerated alkali solution (NH4OH or NaOH solution). Because NH4... + / Na + During the circulation process, it is only used as a "carrier" and is hardly consumed. After the initial addition of a small amount of alkaline reagent, the entire system can achieve in-situ regeneration and recycling of alkali by relying on the bipolar membrane electrodialysis system. Only when a small amount of mother liquor is periodically discharged to remove accumulated impurity ions, a trace amount of fresh alkali solution needs to be added to maintain the ion balance of the system.

[0038] In this invention, the adjustment of the DC power supply output intensity in step S4 is not limited, as long as the pH value in the alkali chamber is 10.0~10.5.

[0039] The present invention also provides a selective separation device for calcium and magnesium ions in coal-based solid waste, which operates using the method described above. The device includes an acid leaching unit, a solid-liquid separation unit, a pretreatment unit, a bipolar membrane electrodialysis unit, a DC power supply, and a PLC control unit. The acid leaching unit has a solid waste inlet, an acid inlet, and a discharge outlet; the acid leaching unit includes a stirring device, a heating device, a temperature controller, a stirring speed controller, and a first online monitoring component; the temperature controller is electrically connected to the heating device; the stirring speed controller is electrically connected to the stirring device; the first online monitoring component is data-connected to the PLC control unit; The solid-liquid separation unit has an inlet, a liquid outlet, and a solid outlet; the inlet of the solid-liquid separation unit is connected to the outlet of the acid leaching unit. The pretreatment unit has an inlet, an alkali inlet, a liquid outlet, a solid outlet, a stirring device, a pH electrode, and a solid-liquid separation device; the inlet of the pretreatment unit is connected to the liquid outlet of the solid-liquid separation unit; the pH electrode of the pretreatment unit is connected to the PLC control unit for data transmission. The bipolar membrane electrodialysis unit includes a membrane stack, electrodes, electrode chambers, and a second online monitoring component. The electrodes include an anode and a cathode. The electrode chambers include an anode chamber and a cathode chamber. The membrane stack includes a first bipolar membrane, a second bipolar membrane, a cation exchange membrane, and an anion exchange membrane, arranged sequentially from cathode to anode as the first bipolar membrane, cation exchange membrane, anion exchange membrane, and second bipolar membrane. An alkali chamber is located between the first bipolar membrane and the cation exchange membrane; a feed chamber is located between the cation exchange membrane and the anion exchange membrane; and an acid chamber is located between the anion exchange membrane and the second bipolar membrane. The feed chamber is connected to the liquid outlet of the pretreatment unit. The second online monitoring component is connected to the PLC control unit for data transmission. The DC power supply is electrically connected to the electrodes of the bipolar membrane electrodialysis unit; the DC power supply is also data-connected to the PLC control unit.

[0040] The acid leaching unit is used to leach calcium and magnesium ions from coal-based solid waste into the liquid phase, forming a slurry containing solid residue and a first ion solution.

[0041] In this invention, the acid leaching unit is preferably an acid-resistant reactor.

[0042] The temperature controller in the acid leaching unit is used to control the acid leaching reaction temperature; the stirring speed controller is used to control the stirring rate. By coordinating the control of temperature and stirring rate, mass and heat transfer between the solid and liquid phases are enhanced, thereby improving the leaching efficiency of calcium and magnesium ions in coal-based solid waste. In this invention, the first online monitoring component or the second online monitoring component each independently includes an ion-selective electrode, a conductivity sensor, and a pH electrode.

[0043] In this invention, the ion-selective electrode includes a cation-selective electrode and an anion-selective electrode; The cation-selective electrode independently includes one or more of the following: calcium ion-selective electrode, magnesium ion-selective electrode, potassium ion-selective electrode, sodium ion-selective electrode, iron ion-selective electrode, aluminum ion-selective electrode, and ammonium ion-selective electrode. The anion-selective electrode independently includes a chloride ion-selective electrode and / or a nitrate ion-selective electrode.

[0044] In this invention, depending on the phase composition of coal-based solid waste, the cation selective electrode may also include selective electrodes for other corresponding ions, such as strontium ion selective electrodes, barium ion selective electrodes, manganese ion selective electrodes, etc.

[0045] In this invention, in the first online monitoring component, the ion-selective electrode is disposed at the outlet of the acid leaching unit or on a pipeline connected to the outlet. The conductivity sensor is located inside the acid leaching unit; The pH electrode is located inside the acid leaching unit.

[0046] In the first online monitoring component, the ion-selective electrode is used to monitor the Ca in the leachate during the acid dissolution reaction in real time. 2+ Mg 2+ K + Na + Cl - Fe 3+ Al 3+ and the concentration changes of other ions that may exist in the acid leaching solution of coal-based solid waste; the PLC control unit based on Ca 2+ and / or Mg 2+ The rate of calcium and magnesium dissolution was calculated based on the rate of change of concentration over time, and according to Ca... 2+ and / or Mg 2+ Whether the concentration reaches a stable value is used to determine the endpoint of the acid dissolution reaction, thus achieving automatic endpoint determination of the acid dissolution leaching process; K + Na + Cl - Monitoring is used to assess the background concentration of monovalent ions and anions in the leachate; Fe 3+ Al 3+ The monitoring is used to assess the leaching behavior of iron and aluminum during acid dissolution and the subsequent retention requirements; the conductivity sensor is used to monitor the change trend of total ion concentration in the solution in real time, as a supplementary monitoring method to the ion-selective electrode; the pH electrode is used to monitor the pH value of the slurry in real time during the acid dissolution reaction.

[0047] In this invention, the solid-liquid separation unit is preferably a plate and frame filter press, a centrifuge, a vacuum filter, or a microporous filter, and more preferably a microporous filter. There are no special limitations on the parameters of the solid-liquid separation unit; solutions well-known in the art can be used.

[0048] The solid-liquid separation unit is used to separate the slurry produced after acid dissolution into a clear first ionic solution and solid residue.

[0049] In this invention, the material of the anode preferably includes titanium coated with ruthenium, titanium coated with iridium, or titanium coated with ruthenium-iridium, and more preferably titanium coated with iridium.

[0050] In this invention, the cathode is preferably made of stainless steel or nickel, and more preferably stainless steel.

[0051] In this invention, the materials of the cation exchange membrane, the anion exchange membrane, the first bipolar membrane, and the second bipolar membrane are not particularly limited and can be commercially available products well known in the art.

[0052] Cation exchange membranes selectively allow cations (positively charged ions) to pass through while blocking anions (negatively charged ions); anion exchange membranes selectively allow anions (negatively charged ions) to pass through while blocking cations (positively charged ions); bipolar membranes are used to dissociate water molecules into H+ under the influence of an electric field. + and OH - , of which OH - Entering the alkali chamber, H + It enters the acid chamber and combines with anions to generate regenerated acid.

[0053] In this invention, adjacent membranes are separated by partitions, and the adjacent membranes and partitions enclose the alkaline chamber, the raw material chamber, or the acid chamber.

[0054] In this invention, the alkali chamber, the raw material chamber, or the acid chamber each independently includes a circulation unit; each circulation unit independently includes a circulation pipeline and a circulation pump. The raw material chamber also includes a raw material circulation pipeline and a raw material circulation pump connected thereto; the alkali chamber also includes an alkali circulation pipeline and an alkali circulation pump connected thereto; and the acid chamber also includes an acid circulation pipeline and an acid circulation pump connected thereto. The liquid in each compartment continuously circulates in its own independent circulation loop to enhance mass transfer and prevent concentration polarization at the membrane surface.

[0055] In this invention, the second online detection component is disposed in each compartment and / or on the pipeline communicating with each compartment, each compartment including the alkali compartment, the raw material compartment and the acid compartment.

[0056] In the second online monitoring component, for the ion-selective electrode, in the raw material chamber: Ca is monitored by calcium ion-selective electrode and magnesium ion-selective electrode. 2+ Mg 2+ Changes in concentration were used to assess ion migration efficiency and residue; K was monitored using a potassium / sodium ion-selective electrode. + Na + The migration behavior of Cl; monitoring Cl using chloride ion-selective electrodes and / or nitrate ion-selective electrodes. - NO3 - The migration behavior of Fe; monitoring Fe using iron ion selective electrodes and / or aluminum ion selective electrodes. 3+ and Al 3+ The remaining situation; In the alkali chamber: Ca2+ is monitored using calcium ion-selective electrodes and magnesium ion-selective electrodes, respectively. 2+ Mg 2+ Changes in concentration to determine Mg 2+ The degree of precipitation and Ca 2+ The retention status in solution was monitored to verify whether the pH control in the alkali chamber achieved the expected separation effect; NH4 was monitored using a potassium / sodium / ammonium ion selective electrode.+ K + Na + The accumulation of [acid / alkaline solution] is used to determine the timing of discharge of regenerated alkali solution and impurity mother liquor; Cl [acid / alkaline solution] is monitored using a chloride ion selective electrode and / or a nitrate ion selective electrode. - NO3 - Has reverse migration occurred? In the acid chamber: Cl- is monitored by a chloride ion-selective electrode and / or a nitrate ion-selective electrode. - NO3 - The concentration change was combined with the conductivity sensor to assess the generation efficiency of regenerated acid and the anion composition in the acid chamber; Conductivity sensors are used to monitor the changes in total ion concentration in solutions in each compartment in real time, serving as a supplementary monitoring method to ion-selective electrodes. pH electrodes are used to monitor the pH value of the effluent from each compartment in real time. Inside the alkali chamber: The pH electrode is the core sensor for achieving precise closed-loop pH control in this device. The PLC control unit has a built-in PID control algorithm that compares the real-time pH value of the alkali chamber with the preset target value (10.0~10.5) and sends a current adjustment command to the DC power supply based on the deviation. When the measured pH value is lower than the target value, the output current is increased to accelerate the dissociation of water in the bipolar membrane to generate OH-. - The rate at which the pH increases is adjusted; when the measured pH value is higher than the target value, the output current is reduced to lower the OH- ions. - The generation rate causes the pH to drop; through the closed-loop feedback control of real-time monitoring, deviation calculation, and current adjustment, the pH value of the alkali chamber is precisely stabilized within the target range of 10.0~10.5. In the acid chamber: The pH electrode is used to assist in assessing the H2 production generated by the bipolar membrane. + Efficiency and regeneration acid formation: When the pH value of the acid chamber continues to decrease, it indicates that H... + Normal production and increased regenerated acid concentration; when the pH value in the acid chamber rises abnormally, it indicates that the bipolar membrane is producing H+. + Efficiency decline or H + Reverse migration occurs; In the feed chamber: pH electrodes are used to assist in assessing ion migration status: during bipolar membrane electrodialysis operation, cations (Ca... 2+ Mg 2+ The migration of ) and anions (Cl) - The influx of ions can cause a shift in the pH of the raw material chamber, and the trend of this shift can help determine whether ion migration is proceeding normally.

[0057] A DC power supply is used to provide a controllable DC electric field; the DC power supply is an adjustable DC power supply, and its output voltage, output current and energizing time are all continuously adjustable.

[0058] In this invention, a solid-liquid separation device is provided on the circulation pipeline of the alkali chamber to separate the Mg(OH)2 and Ca(OH)2 solid precipitates in the circulating liquid of the alkali chamber from the regenerated alkali liquid; The clear liquid outlet of the solid-liquid separation device is connected to the alkaline liquid inlet of the pretreatment unit through a reflux pipeline, and a flow control valve is installed on the reflux pipeline. The solid-liquid separation device is preferably a microporous filter, a hydrocyclone sedimentation device, or a membrane separator; The circulation pump on the circulation pipeline of the alkali chamber is located between the outlet of the alkali chamber and the solid-liquid separation device. The flow control valve is connected to the PLC control unit. The PLC control unit automatically controls the opening of the flow control valve according to the feedback signal from the pH electrode of the pretreatment unit, so as to realize the quantitative reuse of the regenerated alkali solution to the pretreatment unit. A solid-liquid separation device installed on the alkali chamber circulation pipeline separates Mg(OH)2 and Ca(OH)2 solid precipitates from the regenerated alkali solution. The precipitates are collected as product after solid-liquid separation, while the regenerated alkali solution is returned to the pretreatment unit through a reflux pipeline to replace externally added alkaline reagents for adjusting the pH of the first ion solution. The reflux flow rate is automatically controlled by the PLC control unit based on real-time feedback signals from the pH electrode in the pretreatment unit, by adjusting the flow control valve on the reflux pipeline to ensure that the pretreatment pH remains stable within the target range of 5.0~5.5. When the alkalinity of the regenerated alkali solution in the alkali chamber decreases due to reuse, the PLC control unit synchronously adjusts the output current of the DC power supply to accelerate the bipolar membrane water dissociation rate and replenish the consumed OH-. - To maintain the pH stability of the alkaline chamber.

[0059] The PLC control unit receives feedback signals from various online monitoring components, the pH electrode of the pretreatment unit, and the flow control valve, and automatically adjusts according to preset control logic to achieve precise closed-loop control.

[0060] Other parameters, conditions, methods, and devices of the present invention are not limited, and solutions well known in the art can be used.

[0061] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0062] In this embodiment, the coal-based solid waste used is coal gasification slag, which comes from a chemical plant in Shaanxi, China. The specific composition (wt%) is: SiO2 47.451%, Al2O3 13.869%, Fe2O3 17.562%, CaO 10.157%, SO3 3.143%, MgO 0.816%, K2O 2.71%, with the balance being other components.

[0063] Example 1

[0064] This embodiment provides a method for selective separation of calcium and magnesium ions from coal-based solid waste, including the following steps: S1. The acid leaching unit is an acid-resistant reactor equipped with a stirring device, heating device, pH sensor, temperature controller, stirring speed controller, and ion monitoring device. It has a solid waste inlet and an acid inlet at the top, and a discharge outlet at the bottom. Coal-based solid waste, after pre-treatment by crushing (particle size ≤ 0.074 mm), is added to the reactor through the solid waste inlet. Simultaneously, hydrochloric acid solution (concentration 1.5 mol / L, hydrochloric acid solution to coal-based solid waste ratio 10:1 (mL / g)) is added through the acid inlet. The stirring and heating devices are then started. The reaction temperature is controlled within 50℃ using the temperature controller, the stirring speed is controlled at 400 r / min using the stirring speed controller, the pH value change of the slurry is monitored in real time using the pH sensor, and the Ca in the leachate is monitored in real time using the ion monitoring device. 2+ Mg 2+ K + Na + Cl - Fe 3+ Al 3+ The concentration change of plasma is monitored during the acid dissolution reaction under stirring and heating conditions. The PLC control unit controls the Ca2+ concentration based on feedback from the ion monitoring device. 2+ and / or Mg 2+ The dissolution rate is calculated from the concentration change curve, when Ca 2+ and / or Mg 2+ The endpoint of the acid dissolution reaction is automatically determined when the concentration tends to stabilize within the continuous monitoring time, yielding the Ca-containing solution. 2+ Mg 2+ Al 3+ Fe 3+ A mineral slurry containing various metal ions and solid residues.

[0065] S2. After the acid dissolution reaction is completed, the slurry is discharged from the outlet and sent to a plate and frame filter press through pipeline. After filtration, the slurry is filtered at a pressure of 0.5 MPa and a temperature of ≤60℃. The resulting clear filtrate is an acid leaching solution containing calcium and magnesium ions, which is discharged from the liquid outlet. The resulting filter cake (acid leaching residue) is discharged from the solid outlet and can be further washed to recover residual valuable metals or used for building materials and other purposes.

[0066] Under the acid dissolution conditions of Example S1, the leaching rate of major metal ions in coal-based solid waste can reach: Ca 2+ 96.5%, Mg 2+ 85.7%, Al 3+ 84.3%, Fe 3+ 80.0%.

[0067] S3. The acid leaching solution containing calcium and magnesium ions is fed into the pretreatment unit, and 15wt% ammonia solution is added as a starting alkali. The PLC control unit regulates the pH electrode to precisely adjust the pH to 5.0~5.5, so that Fe... 3+ And Al 3+ The mixture is co-precipitated and then sent to a solid-liquid separation device to separate the aluminum and iron ions into an acid leaching solution, which is discharged from the liquid outlet. The resulting aluminum and iron solid precipitate is discharged from the solid outlet and can be recycled as a raw material for polyaluminum-iron flocculant.

[0068] After the pretreatment in S3 of this embodiment, the removal rate of iron and aluminum ions in the acid leaching solution can reach over 99%.

[0069] S4. The liquid outlet of the solid-liquid separation device in the pretreatment unit is connected to the mixed liquor inlet of the bipolar membrane electrodialysis unit via a pipeline. The clarified acid leaching solution enters the raw material chamber through the mixed liquor inlet and circulates under the drive of the raw material circulation pipeline and circulation pump. The bipolar membrane electrodialysis unit is as follows: Figure 1 As shown.

[0070] The anode material is titanium-coated iridium (Ti / IrO2), and the cathode material is 316L stainless steel. Under the drive of the electric field applied by a DC power supply, anions in the raw material chamber pass through an anion exchange membrane (AHA type, Astom Corporation, Japan; membrane thickness 0.22 mm; membrane surface resistivity 4.1 cm⁻¹). 2 The cations migrate towards the anode. The cations in the feed chamber pass through a cation exchange membrane (Astom CMB type, 0.21 mm thick, 4.5 cm resistivity). 2 ) migrates towards the cathode, Ca 2+ Mg 2+ and a small amount of K + Na + It selectively crosses the cation exchange membrane and migrates to the alkaline chamber.

[0071] The PLC control unit automatically adjusts the output current density of the DC power supply based on the real-time feedback signal from the online pH electrode in the alkali chamber, precisely controlling the pH of the alkali chamber within the range of 10.0 to 10.5. Under this pH condition, Mg... 2+ It preferentially reacts with OH generated by the bipolar membrane (Astom Corporation, Japan, cation exchange layer / anion exchange layer composite, membrane thickness 0.28 mm, water dissociation voltage 1 V).- It combines to form Mg(OH)2 precipitate, while Ca 2+ Within this pH range, it does not precipitate and remains in solution in ionic form. The Mg(OH)₂ solid generated in the alkaline chamber reacts with Ca... 2+ The mixed slurry of the solution circulates under the drive of the alkali circulation pipeline and circulation pump, and is continuously discharged through the magnesium slurry outlet. After solid-liquid separation, high-purity Mg(OH)2 product is obtained. Containing Ca... 2+ The pH of the supernatant is raised to above 12.0 or CO2 is introduced, and the supernatant is collected in the form of Ca(OH)2 or CaCO3.

[0072] A small amount of K in the solution + Na + Due to the high solubility of hydroxides, they will not precipitate within the controlled pH range and will eventually be discharged in the mother liquor, ensuring the high purity of the Mg(OH)2 product.

[0073] At the same time, the H atoms dissociate under the influence of the electric field in the bipolar film + Cl migrated from the acid chamber and the raw material chamber - Anions combine to form the corresponding acid. The liquid in the acid chamber circulates under the drive of the acid circulation pipeline and circulation pump. The generated acid can be returned to the acid leaching unit for reuse.

[0074] Iron ion selective electrodes and aluminum ion selective electrodes are configured on the raw material chamber pipeline for monitoring Fe. 3+ and Al 3 + Residual status; Ca was simultaneously monitored in the raw material room. 2+ / Mg 2+ Migration efficiency and K + / Na + / Cl - Migration behavior; Alkali chamber monitoring of Mg 2+ Precipitation degree, Ca 2+ Retention status and K + / Na + Accumulation level; Cl in the acid chamber - The concentration change was used to assess the efficiency of regenerated acid generation and the composition of anions in the acid chamber.

[0075] Example 2

[0076] The difference between this embodiment and Embodiment 1 is that the solid-liquid separation unit in S2 uses a microporous filter. The filtration parameters of the microporous filter include: filtration accuracy of 0.45 μm, filtration pressure of 0.3 MPa, and the material of the microporous filter membrane is polypropylene (PP) microporous filter membrane with the following properties: membrane pore size of 0.45 μm, pH tolerance range of 1~14, and maximum operating temperature of 80℃. The acid-soluble slurry passes through the microporous filter membrane under pressure to obtain an acid leaching solution with higher clarity, which is particularly suitable for precision membrane stack systems with high requirements for the cleanliness of the influent.

[0077] Example 3

[0078] The difference between this embodiment and Embodiment 1 is that the alkaline reagent used to adjust the pH in step S3 is derived from the regenerated alkaline solution in the bipolar membrane electrodialysis alkaline chamber. Specifically, during the initial system startup, a small amount of 15wt% ammonia solution is added in step S3 as a starting alkaline agent to adjust the pH to 5.0-5.5. During subsequent continuous operation, the regenerated alkaline solution (mainly NH4OH) separated by the solid-liquid separator on the alkaline chamber circulation pipeline is returned to the pretreatment unit via a reflux pipeline to replace the externally added ammonia solution for pH adjustment. The reflux flow rate is automatically controlled by the PLC control unit based on the real-time feedback signal from the pH electrode in the pretreatment unit, ensuring the pH remains stable within the range of 5.0-5.5. Testing showed that Embodiment 3 achieves comparable iron and aluminum removal efficiency to Embodiment 1, but with significantly lower operating costs and no need for continuous addition of external chemical reagents.

[0079] After processing according to the methods in Examples 1-3, the purity of Mg(OH)2 products can reach over 95%; the purity of Ca(OH)2 or CaCO3 products can reach over 95%.

[0080] In summary, the device of this invention organically integrates the acid leaching unit, solid-liquid separation unit, pretreatment unit, and bipolar membrane electrodialysis unit into one unit, and precisely controls the pH value of the alkaline chamber of the bipolar membrane electrodialysis. It utilizes the ion-directed migration selectivity inherent in electrochemical methods, combined with the in-situ water dissociation of the bipolar membrane under an electric field to generate H₂. + and OH - Its characteristics allow for the selective separation of calcium and magnesium ions in the alkaline chamber through precise pH control, while the bipolar membrane generates H₂. + In the acid chamber, the calcium and magnesium ions combine with anions to generate regenerated acid, which can be recycled back to the acid leaching unit. The entire process requires no external chemical precipitants or pH adjusters, achieving selective separation of calcium and magnesium ions and resource recovery of the acid.

[0081] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for selective separation of calcium and magnesium ions from coal-based solid waste, characterized in that, Includes the following steps: S1. In the acid leaching unit, coal-based solid waste is mixed with acid solution to carry out acid leaching reaction, and a slurry containing solid residue and ionic solution is obtained. S2. The slurry containing solid residue and ionic solution is fed into the solid-liquid separation unit to separate the first ionic solution and solid residue. S3. The first ion solution is fed into the pretreatment unit, and the pH is adjusted to 5.0-5.5 to allow Fe to... 3+ And Al 3+ The mixture is co-precipitated and then sent to a solid-liquid separation device to separate the second ionic solution and the solid precipitate. S4. Send the second ion solution into the raw material chamber of the bipolar membrane electrodialysis unit and apply DC power to the bipolar membrane electrodialysis unit. Driven by an electric field, cations in the feed chamber migrate towards the cathode and pass through the cation exchange membrane, where Ca... 2+ Mg 2+ Anions migrate to the alkali chamber; anions in the raw material chamber migrate towards the anode, passing through the anion exchange membrane and migrating to the acid chamber. The residual liquid retained in the raw material chamber is discharged; the first and second bipolar membranes dissociate H2O in situ into H+. + and OH - OH generated by the first bipolar membrane - Upon entering the alkali chamber, H2O generated by the second bipolar membrane... + Enter the acid chamber; Adjust the output intensity of the DC power supply to control the pH of the alkali chamber to 10.0~10.5, so that Mg... 2+ After precipitation and solid-liquid separation, solid Mg(OH)2 and liquid containing Ca are separated. 2+ Clear liquid, containing Ca 2+ The clear liquid is returned to the alkali chamber; Continue to raise the pH of the alkali chamber to ≥12.0 or introduce CO2 into the alkali chamber to reduce the Ca2+ content. 2+ After precipitation and solid-liquid separation, solid Ca(OH)2 or CaCO3 is separated, and the separated liquid is returned to S3 as regenerated alkali solution for pH adjustment. H generated by the second bipolar membrane + It enters the acid chamber and combines with the anions migrating in the raw material chamber to obtain regenerated acid, which is then returned to the acid leaching unit for recycling. The acid dissolution reaction in step S1, the pH control in step S3, the DC power supply in step S4, and the pH control in step S4 are all regulated by a PLC control unit.

2. The method according to claim 1, characterized in that, The acid solution in step S1 includes hydrochloric acid and / or nitric acid; the concentration of the acid solution in step S1 is 0.5~2.5mol / L; the volume ratio of the acid solution in step S1 to the mass of the coal-based solid waste is 1~25mL:1g.

3. The method according to claim 1 or 2, characterized in that, The reaction temperature of the acid dissolution reaction in step S1 is 25~120℃, and the dispersion rate is 100~600r / min.

4. A selective separation device for calcium and magnesium ions in coal-based solid waste, operated using the method described in any one of claims 1 to 3, characterized in that, The device includes an acid leaching unit, a solid-liquid separation unit, a pretreatment unit, a bipolar membrane electrodialysis unit, a DC power supply, and a PLC control unit. The acid leaching unit has a solid waste inlet, an acid inlet, and a discharge outlet; the acid leaching unit includes a stirring device, a heating device, a temperature controller, a stirring speed controller, and a first online monitoring component; the temperature controller is electrically connected to the heating device; the stirring speed controller is electrically connected to the stirring device; the first online monitoring component is data-connected to the PLC control unit; The solid-liquid separation unit has an inlet, a liquid outlet, and a solid outlet; the inlet of the solid-liquid separation unit is connected to the outlet of the acid leaching unit. The pretreatment unit has an inlet, an alkaline inlet, a liquid outlet, a solid outlet, a stirring device, a pH electrode, and a solid-liquid separation device. The inlet of the pretreatment unit is connected to the liquid outlet of the solid-liquid separation unit; the pH electrode of the pretreatment unit is connected to the PLC control unit for data transmission. The bipolar membrane electrodialysis unit includes a membrane stack, electrodes, electrode chambers, and a second online monitoring component. The electrodes include an anode and a cathode. The electrode chambers include an anode chamber and a cathode chamber. The membrane stack includes a first bipolar membrane, a second bipolar membrane, a cation exchange membrane, and an anion exchange membrane, arranged sequentially from cathode to anode as the first bipolar membrane, cation exchange membrane, anion exchange membrane, and second bipolar membrane. An alkali chamber is located between the first bipolar membrane and the cation exchange membrane; a feed chamber is located between the cation exchange membrane and the anion exchange membrane; and an acid chamber is located between the anion exchange membrane and the second bipolar membrane. The feed chamber is connected to the liquid outlet of the pretreatment unit. The second online monitoring component is connected to the PLC control unit for data transmission. The DC power supply is electrically connected to the electrodes of the bipolar membrane electrodialysis unit; the DC power supply is also data-connected to the PLC control unit.

5. The apparatus according to claim 4, characterized in that, The solid-liquid separation unit is a plate and frame filter press, centrifuge, vacuum filter or microporous filter.

6. The apparatus according to claim 4, characterized in that, The first online monitoring component or the second online monitoring component each independently includes an ion-selective electrode, a conductivity sensor, and a pH electrode; The ion-selective electrode includes a cation-selective electrode and an anion-selective electrode; the cation-selective electrode independently includes one or more of the following: calcium ion-selective electrode, magnesium ion-selective electrode, potassium ion-selective electrode, sodium ion-selective electrode, iron ion-selective electrode, aluminum ion-selective electrode, and ammonium ion-selective electrode; the anion-selective electrode independently includes a chloride ion-selective electrode and / or a nitrate ion-selective electrode.

7. The apparatus according to claim 4, characterized in that, The alkali chamber, the raw material chamber, or the acid chamber each independently includes a circulation unit; the circulation unit independently includes circulation pipelines and circulation pumps.

8. The apparatus according to claim 6, characterized in that, In the first online monitoring component, the ion-selective electrode is disposed at the outlet of the acid leaching unit or on a pipeline connected to the outlet, the conductivity sensor is disposed inside the acid leaching unit, and the pH electrode is disposed inside the acid leaching unit.

9. The apparatus according to claim 6, characterized in that, The second online detection component is disposed in each compartment and / or on the pipeline communicating with each compartment, each compartment including the alkali compartment, the raw material compartment and the acid compartment.

10. The apparatus according to claim 4, characterized in that, The anode material includes titanium coated with ruthenium, titanium coated with iridium, or titanium coated with ruthenium-iridium; the cathode material includes stainless steel or nickel.

Images