Molten Salt Electrolysis Research Articles

Study on the Synergistic Extraction of Lithium from Spent Lithium Cobalt Oxide Batteries by Molten Salt Electrolysis and Two-Step Precipitation Method

With the continuous development of society, the number of spent lithium-ion batteries has also increased, and the recovery of valuable metals such as Ni, Co, and Li has become the main research direction of many scholars. In this paper, the extraction process of lithium that enters the molten salt after LiCoO2 electrolysis is studied. Oxalic acid and phosphate are added to molten salt containing lithium ions to realize the two-part precipitation method to extract lithium. The influence of pH value, temperature, reaction time, and oxalic acid (or phosphate) addition on the process of oxalic acid calcium removal and phosphate lithium precipitation is analyzed. The results show that the calcium removal rate of oxalic acid has reached 99.72% (Initial conditions: PH = 7.0, T = 70 °C, t = 1.5 h, n(H2C2O4):n(Ca2+) = 1.2:1). The precipitation of Li3PO4 obtained in the phosphate extraction experiment of lithium is as high as 88.44% (Initial conditions: PH = 8.0, T = 70 °C, t = 1.5 h, n(actual dosage of Na3PO4):n(theoretical dosage of Na3PO4) = 1.2:1). The obtained lithium phosphate crystals show regular spherical particles, which can be seen by SEM.

Stereolithographic Additive Manufacturing of Zirconia Electrodes with Dendritic Patterns for Aluminum Smelting

Zirconia electrodes with dendritic patterns were fabricated by stereolithographic additive manufacturing (STL-AM). A solid electrolyte of yttria-stabilized zirconia (YSZ) was selected for oxygen separation in the molten salt electrolysis of aluminum smelting without carbon dioxide excretion. Thereafter, 4, 6, 8 and 12-coordinated dendritic structures composed of cylindrical lattices were designed as computer graphics. The specific surface area of each structure was maximized by changing the aspect ratio. The spatial profile and surface pressure of the hot liquid propagation in the dendrite patterns were systematically visualized by computational fluid dynamics (CFD). During the fabrication process, a photosensitive resin containing zirconia particles was spread on a substrate, and an ultraviolet (UV) laser beam was scanned to create a two-dimensional (2D) cross-section. Through layer laminations, three-dimensional (3D) objects with dendritic structures were successfully fabricated. The ceramics were obtained through dewaxing and sintering.

The electrochemical dissolution mechanism and treatment process in the molten-salt electrolytic recovery of WC-Co two-phase scraps

The use and scrap of cemented carbide, especially WC-Co series cemented carbide, are increasing every year. Recycle of cemented carbide scrap is paid attention to more and more by researchers. In this paper, the molten salt electrolysis method was employed to extract tungsten and cobalt. The focal points of this study were the electrochemical dissolution mechanism of WC-Co two-phase scraps and the treatment process of multiple pieces of WC-Co scrap. Electrochemical impedance spectroscopy indicated that the dissolution mechanism of WC-Co composite had the anodic potential dependence. The equivalent circuit model of WC-Co two-phase composite can adopt the “Rct parallel model” of cobalt and tungsten carbide. At the case of surface area evolution or multiple anodes electrolysis, the equivalent circuit model can also adopt the “Rct parallel model”, which built a bridge between single WC-Co anode and multiple WC-Co anodes. Consequently, we designed and achieved the recovery process of multiple WC-Co anodes. The XRD analysis and elemental analysis showed that the products were pure cobalt and pure tungsten, respectively.

Effect of Fe2O3 on Electro-Deoxidation in Fe2O3-Al2O3-NaCl-KCl System

The reduction of Fe2O3-Al2O3 is one of the important reactions in the resource utilization of iron-containing oxide waste. Fe2O3-Al2O3 was electro-deoxidized in the NaCl-KCl system by molten salt electrolysis to prepare FeO/Al2O3. The effect of the Fe2O3 content on the electro-deoxidation reaction process was studied. The results show that under the conditions of 850 °C, 2.3 V, and electro-deoxidation for 4 h, FeO/Al2O3 could be obtained by controlling the content of Fe2O3. The deoxidation process was divided into three stages: electric double layer charging, Fe2O3 electro-deoxidation to Fe3O4, and Fe3O4 electro-deoxidation to FeO. With the increase in the Fe2O3 content, the deoxidation reaction rate increased, and the low-valence iron oxide particles obtained by electro-deoxidation became larger. The mechanism of the influence of Fe2O3 on the electro-deoxygenation process was determined by analyzing the experimental results. The increase in the Fe2O3 content increased the concentration of activated molecules in the system, while it reduced the resistance of electro-deoxidation. The migration of active particles in the cathode was smoother, which increased the percentage of deoxygenation of activated molecules, thereby shortening the process of the deoxidation reaction.

Ionic Conductivities of Molten Alkali Metal Chloride Binary Systems by Equilibrium Molecular Dynamics Simulation: Composition and Temperature Dependence

Conductivity of molten alkali metal chlorides is the foundation of molten-salt electrolysis process. In the study, equilibrium molecular dynamics was performed to investigate the ionic conductivities of the molten alkali metal chloride binary systems. The calculated values were in good agreement with the experimental results. Simple databases for conductivity have been built based on the verified algorithm. Mathematic expressions of conductivity vs. temperature & composition have also been obtained according to calculated data. Ionic conductivities of these melts exhibited positive dependences on temperature. Adding a smaller alkali metal cation into molten alkali chlorides could improve the conductive performance. With the gradual addition of LiCl, ionic conductivities of molten LiCl-NaCl, LiCl-KCl, LiCl-RbCl, and LiCl-CsCl all increased monotonously. The conductivities of molten NaCl-KCl, NaCl-RbCl, and NaCl-CsCl also increased with the continuous addition of NaCl. The improvement effects of LiCl on ionic conductivities of molten KCl, RbCl, and CsCl are more significant than those of NaCl.

One-step controllable fabrication of 3D structured self-standing Al3Ni2/Ni electrode through molten salt electrolysis for efficient water splitting

Exploring more efficient and low-cost electrocatalysts to replace platinum (Pt) is highly desired to promote the practical hydrogen production through water splitting. Herein, a facile and effective strategy is proposed to fabricate self-standing Al3Ni2/Ni electrode with controlled phase composition and surface morphology, which is obtained by one-step electrochemical reduction of Al3+ on commercially available nickel in eutectic NaCl-KCl melt. Different from previously reported approaches, uniform Al3Ni2 monolith catalyst can directly grow onto Ni substrate. The deposit possesses unique three-dimensional (3D) cauliflower-like morphology comprising of nano- and microparticles due to the rapid nucleation rate during molten salt electrolysis. The as-fabricated Al3Ni2/Ni electrode can be directly used as the cathode to catalyze Hydrogen evolution reaction (HER). Impressively, it exhibits remarkable HER activity comparable to commercial Pt, including a low overpotential of 83.4 mV for a current density of 10 mA cm−2, a small Tafel slope of 40.7 mV dec-1, and excellent long-term stability over 36 h of continuous HER operation in 0.5 M H2SO4 solution. The intrinsic catalytic ability of Al3Ni2 with the unique hierarchical structure of nano/microsized grains can offer multiple effects, including massive exposed active sites, enhanced charge transfer and mass transport, and fast gas releasing that synergistically contribute to improving the electrocatalytic performance of HER. This work represents a highly promising approach to the design and one-step controllable fabrication of efficient and self-standing base metal electrode for electrocatalytic hydrogen production.

PECULIARITIES OF ELECTROREDUCTION OF Li2CO3 IN THE EQUIMOLAR MELT OF SODIUM AND POTASSIUM CHLORIDES

Recently, preparation of carbon materials by molten salt electrolysis has been demonstrated as a promising way of carbon capture, utilization and storage. This paper discusses the results of studies on the mechanism of carbon electrodeposition reactions in molten carbonate salts, especially voltammetric investigations. A CV study of the electroreduction of Li2CO3 against the background electrolyte NaCl–KCl was carried out. It was found that the electroreduction of Li2CO3 to carbon occurs through the stage of a preliminary chemical reaction of acid-base type with forming of electrochemically active particles – CO2 and LixCO32-x at potentials of -0.8 and -1.7 V (against Pb|PbCl2 reference electrode), respectively. Both processes are irreversible, and the electroreduction of LixCO32-x takes place with diffusion control of the delivery of the depolarizer to the electrode surface. Electrolysis of melts have been carried out under potentiostatic conditions in an air atmo­sphere at different potentials (E = -1.0 and -1.8 V) at the temperature 750°C. The product yield was 40–60%. Compositions, morphological and structural peculiarities of carbon pro­ducts have been studied by XRD analyses, SEM and Raman spectroscopy. XRD study shows that a symmetrical halo occurs at 2θ ~21 and 43°, suggesting that obtained carbon materials are amorphous. The ratio of the intensities of D and G bands (ID/IG) on Raman spectra was > 1 that indicates the extent of defects and impurities in the electrolytic carbon or the pre­sence of ultrathin carbon. Agglomerated particles consist of degraded graphite structures with an approximate crystallite size of 30–40 nm. Their general appearance resembles the model of "crumpled paper".
 The electroreduction of Li2CO3 to carbon in molten salts presented in this study is environmentally benign and affordable, providing a promising route for the production carbon nanomaterials. Also, this work highlights the possibility of using Li2CO3 as a precursor of high-temperature electrochemical synthesis of various carbonaceous materials, in particular, tungsten carbides.

Recovery of cobalt from lithium-ion batteries using fluidised cathode molten salt electrolysis

The future need to recycle enormous quantities of Li-ion batteries is a consequence of the rapid rise in electric vehicles required to decarbonise the transport sector. Cobalt is a critical element in many Li-ion battery cathode chemistries. Herein, an electrochemical reduction and recovery process of Co from LiCoO2 is demonstrated that uses a molten salt fluidised cathode technique. For the Li-Co-O-Cl system, specific to the experimental process, a predominance diagram was developed to aid in understanding the reduction pathway. The voltammograms indicate two 2-electron transfer reactions and the reduction of CoO to Co at −2.4 V vs. Ag/Ag+. Chronoamperometry revealed a Faradaic current efficiency estimated between 70-80% for the commercially-obtained LiCoO2 and upwards of 80% for the spent Li-ion battery. The molten salt electrochemical process route for the recycling of spent Li-ion batteries could prove to be a simple, green and high-throughput route for the efficient recovery of critical materials.

Competitive Coordination of Chloride and Fluoride Anions Towards Trivalent Lanthanide Cations (La3+ and Nd3+ ) in Molten Salts.

Molten salt electrolysis is a vital technique to produce high-purity lanthanide metals and alloys. However, the coordination environments of lanthanides in molten salts, which heavily affect the related redox potential and electrochemical properties, have not been well elucidated. Here, the competitive coordination of chloride and fluoride anions towards lanthanide cations (La3+ and Nd3+ ) is explored in molten LiCl-KCl-LiF-LnCl3 salts using electrochemical, spectroscopic, and computational approaches. Electrochemical analyses show that significant negative shifts in the reduction potential of Ln3+ occur when F- concentration increases, indicating that the F- anions interact with Ln3+ via substituting the coordinated Cl- anions, and confirm [LnClx Fy ]3-x-y (ymax =3) complexes are prevailing in molten salts. Spectroscopic and computational results on solution structures further reveal the competition between Cl- and F- anions, which leads to the formation of four distinct Ln(III) species: [LnCl6 ]3- , [LnCl5 F]3- , [LnCl4 F2 ]3- and [LnCl4 F3 ]4- . Among them, the seven-coordinated [LnCl4 F3 ]4- complex possesses a low-symmetry structure evidenced by the pattern change of Raman spectra. After comparing the polarizing power (Z/r) among different metal cations, it was concluded that Ln-F interaction is weaker than that between transition metal and F- ions.

A new approach to recover the valuable elements in black aluminum dross

Black aluminum dross (BAD) is a solid hazardous waste produced during the remelting of aluminum alloy scrap and contains a large amount of aluminum and other valuable elements. However, the extraction of its high-value components is challenging because of its complex phase and chemical composition. In this paper, a new process for recovering aluminum and other valuable elements from BAD is reported, which consists of three steps, i.e., catalytic hydrolysis of aluminum nitride, calcination of the obtained aluminum hydroxide, and molten salt electrolysis to prepare aluminum alloy. Under the established optimal conditions for the hydrolysis of aluminum nitride, 4 wt% NaOH, a temperature of 90°C, a liquid-solid ratio of 6 mL g−1, and a reaction time of 3 hours, the recovery rates of nitrogen and chloride salts are 88.8% and 97.57%, respectively. Bayerite and pseudo-boehmite generated by the hydrolysis of aluminum nitride present in BAD were converted into γ-Al2O3 when calcining the material at 800°C for 2 hours resulting in a multi-metal oxide with high alumina content. Aluminum alloy was prepared by electrolysis of the material in the Na3AlF6-AlF3-Al2O3 electrolyte system. This innovative technology can recover 97% of aluminum and other valuable metals (including silicon) in BAD and is a new route to achieve clean and high-value recycling of BAD.

Green preparation of vanadium carbide through one-step molten salt electrolysis

Vanadium carbide (VC) as excellent ceramic and functional material is usually prepared by carbothermal reduction of V2O5 which must be extracted from a typical V slag by complex processes. Pollutants, such as ammonia-nitrogen wastewater, NH3 and CO2 are inevitably discharged. A novel and green method for VC preparation was proposed by one-step co-electrolysis of soluble NaVO3 and CO2 in molten salt. It was found that VC with high purity was easily obtained by reducing electrolysis temperature and CO2 flow rate to 600 °C and 10 mL min−1 at 3.0 V. Besides VC with particles and layered stacking structure in products, a small amount of carbon and oxygen elements existed. The atomic percentage contents of C, V, and O elements in VC were about 50.0%, 44.5% and 3.8%, respectively. During electrolysis, CO32− and VO3− was reduced at about −0.55 V (vs. Ag/AgCl) and −1.38 V (vs. Ag/AgCl), respectively. CO32− ions were more easily reduced than VO3−, and was firstly reduced to CO22− and then converted to C. Then, VC was prepared by two routes from CO2 and NaVO3. One route is that VO3− ions are firstly electroreduced to VO2− ions and then are further electroreduced to VC with C. Another route is that VO3− ions are electroreduced to V which in-situ reacted with C to VC. Both VO3− and CO32− ions are electroreduced by two-step process. In final, VC is in-situ deposited on cathode. It provides a novel and green way to prepare VC and also achieves the high value-added utilization of vanadium slag and CO2.

Advances in Understanding of the Application of Unit Operations in Metallurgy of Rare Earth Elements

Unit operations (UO) are mostly used in non-ferrous extractive metallurgy (NFEM) and usually separated into three categories: (1) hydrometallurgy (leaching under atmospheric and high pressure conditions, mixing of solution with gas and mechanical parts, neutralization of solution, precipitation and cementation of metals from solution aiming purification, and compound productions during crystallization), (2) pyrometallurgy (roasting, smelting, refining), and (3) electrometallurgy (aqueous electrolysis and molten salt electrolysis). The high demand for critical metals, such as rare earth elements (REE), indium, scandium, and gallium raises the need for an advance in understanding of the UO in NFEM. The aimed metal is first transferred from ores and concentrates to a solution using a selective dissolution (leaching or dry digestion) under an atmospheric pressure below 1 bar at 100 °C in an agitating glass reactor and under a high pressure (40–50 bar) at high temperatures (below 270 °C) in an autoclave and tubular reactor. The purification of the obtained solution was performed using neutralization agents such as sodium hydroxide and calcium carbonate or more selective precipitation agents such as sodium carbonate and oxalic acid. The separation of metals is possible using liquid (water solution)/liquid (organic phase) extraction (solvent extraction (SX) in mixer-settler) and solid-liquid filtration in chamber filter-press under pressure until 5 bar. Crystallization is the process by which a metallic compound is converted from a liquid into a crystalline state via a supersaturated solution. The final step is metal production using different methods (aqueous electrolysis for basic metals such as copper, zinc, silver, and molten salt electrolysis for REE and aluminum). Advanced processes, such as ultrasonic spray pyrolysis, microwave assisted leaching, and can be combined with reduction processes in order to produce metallic powders. Some preparation for the leaching process is performed via a roasting process in a rotary furnace, where the sulfidic ore was first oxidized in an oxidic form which is a suitable for the metal transfer to water solution. UO in extractive metallurgy of REE can be successfully used not only for the metal wining from primary materials, but also for its recovery from secondary materials.

Recovery of rare earths, lithium and fluorine from rare earth molten salt electrolytic slag via fluoride sulfate conversion and mineral phase reconstruction

To solve the problem of rare earths (RE) lithium (Li) and fluorine (F) recovery in rare earth molten salt electrolysis slag, a recovery method was developed based on magnetic separation, sulfuric acid leaching, HF recycling, water leaching, fluorination precipitation. First, RE, Li and iron-containing phases in a slag sample were separated using a magnetic separation method. Second, sulfuric acid-leaching was used to reconstruct the phases of RE and Li in the non-magnetic fraction, which were transformed into a rare earth sulfate easily dissolved in water. At the same time, the HF generated in the sulfuric acid leaching process is recovered by multistage adsorption. Finally, qualified fluoride rare earth products were obtained by HF cycle precipitation. The results show that the recovery of iron is 85.30% under the condition that the magnetic separation intensity of 0.668 T, particle size of 58–75 μm, and the main phase in the non-magnetic fraction is the rare earth phase, and the iron content is only 2.90%. Under the conditions of a concentrated acid concentration of 98.00%, a temperature of 633 K, a liquid–solid ratio of 2:1, a particle size of 58–75 μm, a constant stirring speed of 300 r·min−1, and reaction time of 3 h, the removal rate of F is 95.30%, and the transformation rates of Nd, Pr, Dy, and Li reached more than 95.00%. Through water immersion and fluorination precipitation, rare earth fluoride products can be obtained. The purity, particle size, and water content of the products met the standards required for industrial rare earth molten salt electrolysis.

Investigation of Electrowinning of Mg from MgO Using a Liquid Metal Cathode in MgF2-CaF2-NaF or MgF2-LiF Molten Salt

A novel magnesium (Mg) production process utilizing an electrolytic method was investigated for the direct reduction of magnesium oxide (MgO). Electrolysis of MgO was carried out with an applied voltage of 3.0 V using a copper (Cu), silver (Ag), or tin (Sn) cathode and carbon (C) or platinum (Pt) anode in magnesium fluoride (MgF2)−calcium fluoride (CaF2)−sodium fluoride (NaF) at 1273 K or MgF2− lithium fluoride (LiF) at 1083−1093 K. After the electrolysis of MgO in MgF2−CaF2−NaF molten salt, Mg alloys such as Mg2Cu, Cu2Mg, or Mg2Sn phases were produced with current efficiencies of 75.8−85.6% when the concentration of Mg in Mg alloys was 9.1−14.6 mass%. In addition, when the electrolysis of MgO was conducted in MgF2–LiF molten salt, Mg alloys such as Mg2Cu or AgMg phase were produced with current efficiencies of 76.2−81.7% when the concentration of Mg in the Mg alloys was 12.5−13.2 mass%. In addition, to produce high-purity Mg metal from Mg alloys, vacuum distillation was conducted. When vacuum distillation was conducted at 1100−1400 K for a duration of 5 h, the concentration of Mg in the Mg alloys feed decreased from 30.2−34.1 mass% to 0.64−1.75 mass%, and Mg metal with a purity of 99.998−99.999% was obtained under certain conditions. Therefore, the molten salt electrolysis using liquid metal cathode (MSE-LMC) process developed here is feasible for the direct reduction of MgO using an effective and environmentally sound method.

Recovery and separation of rare earth elements by molten salt electrolysis

Recovery and separation of rare earth elements by molten salt electrolysis

Extraction of Cu and Ni from Low-Ni Matte by a Combined Chemical Precipitation and Molten Salt Electrolysis Approach

The extraction of Cu and Ni from low-Ni matte in an efficient and green manner is essential to harvest value-added metals with minimum environmental footprint. Herein, a combined approach of chemical precipitation and molten salt electrolysis was used to extract valuable metals (Cu and Ni) from the low-Ni matte. First, the low-Ni matte was converted and separated into CuS and NiS by ammonium chloride roasting, and then, it was water leached and precipitated. Fe was removed from the leachate solution by adjusting the pH value, and the separation of Cu and Ni was carried out by adjusting the temperature, S-to-Ni molar ratio, and precipitation time. The obtained CuS and NiS were then separately converted to sulfur gas at the anode and Cu and Ni at the cathode in molten NaCl-KCl. The reduction behaviors of CuS and NiS were studied using cyclic voltammetry (CV). This study proposes an eco-friendly way to extract valuable metals from low-Ni matte.

Process and performance of ultra-high-speed laser cladding corrosion-resistant layer on the surface of tungsten cathode

Ultra-high-speed laser cladding technology is used to clad the surface anti-corrosion layer of tungsten cathode for molten salt electrolysis. Using MSC. Marc finite element simulation software, optical microscope (OM), scanning electron microscope (SEM), energy spectrum (EDS), X-ray diffractometer (XRD) and microhardness tester and other analytical testing methods, respectively, the cladding process, the cladding layer microstructure morphology, phase composition, phase distribution and microhardness are simulated, and the structure is observed and analyzed. The simulation results show that the preheating process before cladding for the tungsten substrate can effectively avoid the cracking phenomenon caused by internal stress; the structure and performance observation results show that the cladding layer has a dense structure and no cracks, forming a good metallurgical bond with the substrate, the difference in the structure of the cladding layer affects the hardness of the cladding layer.

Correlation of Morphology and Crystal Structure of Metal Powders Produced by Electrolysis Processes

In this review paper, morphologies of metal powders produced by the constant (potentiostatic and galvanostatic) regimes of electrolysis from aqueous electrolytes are correlated with their crystal structure at the semiquantitative level. The main parameters affecting the shape of powder particles are the exchange current density (rate of electrochemical process) and overpotential for hydrogen evolution reaction. Depending on them, various shapes of dendrites (the needles, the two-dimensional (2D) fern-like, and the three-dimensional (3D) pine-like dendrites), and the particles formed under vigorous hydrogen evolution (cauliflower-like and spongy-like particles) are produced by these regimes of electrolysis. By decreasing the exchange current density value, the crystal structure of the powder particles is changed from the strong (111) preferred orientation obtained for the needle-like (silver) and the 2D (lead) dendrites to the randomly orientated crystallites in particles with the spherical morphology (the 3D dendrites and the cauliflower-like and the spongy-like particles). The formation of metal powders by molten salt electrolysis and by electrolysis in deep eutectic solvents (DESs) and the crystallographic aspects of dendritic growth are also mentioned in this review.

Scale-Up Study of Molten Salt Electrolysis using Cu or Ag Cathode and Vacuum Distillation for the Production of High-Purity Mg Metal from MgO

To produce high-purity Mg metal directly from MgO, the scale-up of a novel electrolytic process using a metal cathode and vacuum distillation was investigated. The electrolysis of MgO was conducted in MgF2–LiF molten salt using Cu or Ag cathode and graphite anode at 1053 K. The influence of the cathode metal and the concentration of Mg in the Mg alloy on the current efficiency was investigated. In addition, the interaction of Al2O3 in contact with the electrolyte and cathode was studied. When electrolysis was carried out with an applied current of 8.81–9.59 A for 75–95 h, Mg alloys such as Mg2Cu and MgAg were produced with a current efficiency of 82.2–88.0%. In addition, owing to the reaction between the Mg alloy and Al2O3, a dense layer of magnesium aluminum oxide was produced. Afterward, when the vacuum distillation of the Mg alloys was conducted at 1300 K, 99.9997% Mg metal was obtained.

Highly Efficient Synthesis of Silicon Nanowires from Molten Salt Electrolysis Cell with a Ceramic Diaphragm

Highly Efficient Synthesis of Silicon Nanowires from Molten Salt Electrolysis Cell with a Ceramic Diaphragm