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Molten salt

Molten salt is an ionic compound, typically a mixture of inorganic salts, that transitions from a solid to a liquid state upon heating to its melting point, often at elevated temperatures ranging from approximately 300°C to over 1000°C depending on the composition. These liquids consist of dissociated cations and anions, enabling them to function as solvents, electrolytes, or heat transfer fluids with properties such as low viscosity (e.g., 0.71 × 10⁻⁶ to 0.96 × 10⁻⁶ m²/s at 1013 K), high electrical conductivity, wide thermal stability, and large heat capacities comparable to water on a volumetric basis.^[1]^[2] Common examples include pure salts like sodium chloride (NaCl, melting at 801°C) and eutectic mixtures such as lithium fluoride-beryllium fluoride (FLiBe, 67-33 mol%, melting at 459°C) or nitrate salts like Hitec (7-49-44 mol% NaNO₃-NaNO₂-KNO₃, melting at 142°C).^[1]^[3] The thermophysical properties of molten salts make them versatile for high-temperature applications, including density (e.g., 2413 – 0.488T kg/m³ for FLiBe, where T is in K), specific heat (e.g., 2397 J/kg·K for FLiBe at 973 K), and thermal conductivity (e.g., 0.63 + 0.0005T W/m·K for FLiBe).^[3] Unlike molecular liquids, they exhibit low vapor pressure and high boiling points (often >700°C), allowing operation in extreme conditions without significant evaporation, while their ionic nature provides superior dissolution capacity for metals and compounds.^[2] These characteristics also contribute to their stability as single-phase liquids, though corrosion on container materials like stainless steel or Hastelloy can occur at rates up to 1 mm/year depending on the salt and temperature.^[3] Molten salts have diverse industrial and energy applications, prominently in thermal energy storage for concentrating solar power (CSP) systems, where nitrate mixtures store heat at 565°C to enable dispatchable electricity generation beyond sunlight hours.^[4] In nuclear technology, fluoride and chloride salts serve as coolants, fuels, or heat exchange media in molten salt reactors (MSRs), such as the historical Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory, operating at 650°C with low neutron absorption.^[5]^[1] Other uses include electrolytic metal production (e.g., aluminum via the Hall-Héroult process), high-temperature fuel cells with carbonate electrolytes, and advanced battery systems capable of operating up to 700°C.^[2] Ongoing research focuses on chloride salts for cost-effective, high-temperature (>600°C) storage to improve efficiency in next-generation CSP and nuclear systems.^[6]^[7]

Definition and Properties

Definition

A molten salt is defined as an ionic compound that has been heated above its melting point to form a liquid state, consisting of dissociated cations and anions without the presence of a solvent. In this liquid form, the ions are free to move, imparting high ionic conductivity characteristic of these systems.^[8] This distinguishes molten salts from their solid crystalline forms, where ions are fixed in a lattice, and from ionic liquids that remain fluid at or near room temperature.^[1] The term "molten salt," also known as fused salt, originated in the context of early 20th-century electrochemistry, where it described heated ionic compounds used as electrolytes in high-temperature electrolytic processes.^[9] Unlike aqueous solutions, in which ions are hydrated by water molecules and limited by the boiling point of water (100°C at standard pressure), molten salts are anhydrous, enabling operations at elevated temperatures—often exceeding 300°C—without hydrolysis or decomposition of reactive species.^[10] This solvent-free nature allows for the study and application of electrochemical reactions that would be impractical or impossible in water-based media.^[11] The ionic nature of molten salts is fundamental to their behavior; upon melting, compounds such as sodium chloride (NaCl) dissociate into Na⁺ and Cl⁻ ions, facilitating electrical conduction through ion migration rather than electron flow. To lower melting points and achieve liquidity at more accessible temperatures, binary or multicomponent mixtures are commonly employed, forming eutectic compositions where the overall melting temperature is minimized compared to the individual components.^[12] These eutectics maintain the dissociated ionic structure while enhancing practicality for various electrochemical and thermal applications.^[13]

Physical Properties

Molten salts demonstrate exceptional thermal stability, enabling their use in high-temperature environments without significant decomposition. For many inorganic salts, this stability supports operating temperature ranges from approximately 300°C to 1400°C, depending on the specific composition.^[14] Their high heat capacity, typically on the order of 1.5 J/g·K, allows efficient storage and transfer of thermal energy, making them suitable for applications requiring sustained heat retention.^[15] The density of molten salts generally ranges from 1.5 to 2.0 g/cm³ and decreases with increasing temperature due to thermal expansion. For example, molten NaCl exhibits a density of about 1.5–1.6 g/cm³ near its melting point.^[16] Viscosity in these systems is low and also diminishes with temperature, often falling to values around 1–3 mPa·s at operational temperatures, which enhances fluidity and heat transfer efficiency.^[16] Electrical conductivity in molten salts arises from ionic conduction, where mobile cations and anions carry charge under an applied field. This is described by the relation

\sigma = n q \mu

where \sigma is the conductivity, n the ion density, q the ion charge, and \mu the ion mobility; typical values range from 0.1 to 2 S/cm, with molten NaCl showing around 0.87 S/cm at elevated temperatures.^[17]^[16] In binary or multicomponent mixtures, the melting point can be depressed relative to pure components through the formation of eutectic compositions, where the mixture has a minimum melting temperature lower than the individual salts, often by 100–200°C or more. This is determined by the phase diagram of the system.^[18]

Chemical Properties

Molten salts are characterized by high corrosivity, arising primarily from dissolved oxides, halides, or hydrolysis-derived species that aggressively attack metallic materials. In chloride-based melts, such as those composed of alkali metal chlorides, chloride ions (Cl⁻) play a key role in corroding steels by generating chlorine gas at the salt-metal interface through oxidation reactions, which diffuses inward and depletes protective elements like chromium and iron from the alloy.^[19] This mechanism leads to intergranular attack and pitting, with corrosion rates accelerating in the presence of oxidative impurities that form during salt preparation or exposure to air.^[20]^[21] For instance, in molten NaCl-KCl eutectics, stainless steels like 316 exhibit severe degradation due to Cl⁻-mediated dissolution of the passive oxide layer.^[21] The redox stability of molten salts is notable for their wide electrochemical windows, often spanning more than 2.4 V, which enables stable operation for reactions such as the electrodeposition of metals like aluminum or uranium without electrolyte decomposition.^[22] This broad stability range stems from the ionic nature of the melt, where the redox potential can be precisely tuned—through additives like reducing metals or gas sparging—to prevent excessive oxidation or reduction that could exacerbate corrosion.^[23] Such control is vital for maintaining the integrity of electrochemical processes in the melt.^[24] Hydrolysis presents a major risk when molten salts encounter moist environments, as water reacts with the salt ions to generate acidic species that heighten corrosivity. In nitrate melts, for example, exposure to humidity triggers the hydrolysis of salts like NaNO₃ to form nitric acid (HNO₃) and sodium hydroxide, with the acid component aggressively dissolving metal oxides and bases.^[25] Similarly, in halide systems, hydrolysis yields hydrochloric acid (HCl) or hydrofluoric acid (HF), which attack containment alloys by protonating and dissolving protective scales.^[3] These reactions are particularly problematic during storage or transfer, where even trace moisture can initiate cascading degradation.^[2] Purity is a critical requirement for molten salts, as contaminants like water or oxygen profoundly influence their chemical stability and ionic conductivity. Water impurities, even at parts-per-million levels, promote hydrolysis and reduce conductivity by forming less mobile species, while dissolved oxygen fosters oxide inclusions that accelerate corrosion through redox shifts.^[26]^[27] Rigorous purification—targeting impurity levels below 100 ppm—is thus essential to preserve the melt's electrochemical window and prevent instability from impurity-driven reactions.^[28]^[29]

Types and Examples

High-Temperature Inorganic Salts

High-temperature inorganic molten salts are ionic compounds that typically melt at temperatures exceeding 200°C and are predominantly employed in industrial processes involving elevated thermal conditions. These salts, often eutectic mixtures to achieve lower melting points than their pure components, exhibit good thermal stability and ionic conductivity suitable for high-heat applications. Common classes include chloride, nitrate, and fluoride-based salts, each tailored for specific operational demands. Chloride-based molten salts, such as the NaCl-KCl eutectic (50 mol% NaCl–50 mol% KCl), have a melting point of approximately 657°C and are utilized in electrolytic processes due to their electrochemical properties.^[30] Nitrate-based salts, exemplified by solar salt (60 wt% NaNO₃–40 wt% KNO₃), melt at 221°C and offer a balance of low melting temperature and thermal stability up to around 600°C.^[31] Fluoride-based mixtures, like FLiNaK (46.5 mol% LiF–11.5 mol% NaF–42 mol% KF), possess a melting point of 454°C and are applied in nuclear contexts for their low neutron absorption and high-temperature performance. Another example is FLiBe (67 mol% LiF–33 mol% BeF₂), which melts at 459°C and is used similarly in molten salt reactors.^[32]^[33] Cryolite (Na₃AlF₆), used in aluminum production, has a pure melting point above 1000°C but, with additives such as AlF₃ and CaF₂, the electrolyte mixture achieves a practical melting range of 950–980°C, enabling efficient dissolution of alumina.^[34]^[35]

Low-Temperature Molten Salts

Low-temperature molten salts encompass a class of ionic materials that maintain liquidity at or near ambient conditions, typically with melting points below 100 °C and often under 25 °C, distinguishing them through organic cations, asymmetric structures, or eutectic formulations that suppress crystallization.^[36] These salts, including ionic liquids and deep eutectic solvents, enable operations under milder conditions compared to refractory inorganic melts.^[36] Ionic liquids (ILs) form a key subclass, defined as salts composed solely of discrete ions that liquefy below 100 °C due to disrupted lattice formation from ion asymmetry and delocalization.^[37] A representative example is 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), featuring an imidazolium cation and tetrafluoroborate anion, with a melting point of approximately 15 °C.^[38] Air-stable variants, such as those derived from the 1-ethyl-3-methylimidazolium cation paired with robust anions like tetrafluoroborate or hexafluorophosphate, resist hydrolysis and oxidation in humid environments, broadening their handling feasibility.^[39] Deep eutectic solvents (DESs) constitute another vital group, arising from eutectic mixtures of a hydrogen bond acceptor (typically a quaternary ammonium halide) and a hydrogen bond donor (such as a urea or alcohol), where intermolecular hydrogen bonding depresses the melting point far below that of the individual components.^[40] For example, a 1:2 molar ratio of choline chloride (melting point 302 °C) and urea (melting point 133 °C) produces a DES with a melting point of 12 °C, attributed to strong chloride-urea and choline-urea hydrogen bonds that stabilize the liquid phase.^[41]^[40] Relative to high-temperature molten salts, these low-temperature counterparts exhibit lower volatility, minimizing evaporative losses and enhancing environmental safety during processing.^[42] Their properties are highly tunable by varying cation-anion combinations or mixture ratios, allowing customization of viscosity, density, and solvation behavior for targeted needs.^[42] Low-temperature molten salts are categorized as protic or aprotic ionic liquids based on their formation mechanism and hydrogen-bonding potential. Protic ionic liquids (PILs) result from proton transfer between a Brønsted acid and base, yielding ions capable of donating or accepting protons for enhanced hydrogen bonding in applications like acid-base catalysis.^[43] Aprotic ionic liquids (AILs), the more prevalent type, involve no such proton exchange and feature stable organic cations (e.g., imidazolium or pyrrolidinium) with weakly coordinating anions, providing greater thermal and chemical inertness.^[43] Post-1990s innovations, particularly the introduction of task-specific designs by incorporating functional groups into ions, enabled ILs tailored for selective solubility or reactivity, such as in metal extraction or CO₂ capture.^[44]

Preparation and Handling

Melting Techniques

Molten salts are commonly melted through direct heating techniques, such as electric furnaces or induction heating, which are suitable for both pure salts and mixtures in laboratory and small-scale applications. Induction heating, for instance, is used to prepare eutectic melts like Flibe (67 mol% LiF–33 mol% BeF₂) in nickel crucibles by applying power at frequencies around 328 kHz to initiate melting at approximately 470°C, slightly above the theoretical eutectic point of 459°C.^[45] Electric furnaces provide precise temperature control via embedded heaters, enabling melts to reach operational temperatures up to 700°C while monitoring with type K thermocouples to prevent thermal decomposition of the salt components. These methods ensure uniform heating but require careful regulation to stay below decomposition thresholds, typically achieved through automated temperature feedback systems. Eutectic mixtures, which lower overall melting points for practical use, are prepared by blending constituent salts in proportions determined from phase diagrams, followed by co-melting. For the ternary chloride system NaCl–KCl–ZnCl₂, components are mixed stoichiometrically based on the diagram's eutectic composition (8.1 wt% NaCl, 31.3 wt% KCl, 60.6 wt% ZnCl₂; molar 13.8–41.9–44.3%, melting at ~229°C) and heated gradually in a furnace to form a homogeneous liquid phase without intermediate solidification.^[46] Similarly, systems like LiCl–KCl are optimized using thermodynamic models of binary and ternary phase behaviors to select blends that minimize melting temperatures while maintaining stability.^[47] This approach leverages the eutectic point's depressed temperature, often verified experimentally by differential scanning calorimetry during preparation. To mitigate oxidation and contamination, melting processes frequently employ vacuum or inert atmospheres, particularly for reactive salts. Operations in argon-purged gloveboxes reduce moisture and oxygen levels to below 250 ppm, preventing reactions that could alter salt purity during heating.^[45] Vacuum systems or inert gas covers are applied in electrochemical setups to maintain redox stability, avoiding oxide formation in chlorides or fluorides.^[48] Such conditions are essential for high-purity melts, with inert atmospheres like argon ensuring the process aligns with safety protocols for handling reactive media. At industrial scales, particularly in metallurgical applications, continuous flow systems facilitate large-volume molten salt processing by integrating melting with downstream operations like electrolysis or purification. These setups pump pre-heated salt through heated conduits or reactors, achieving steady-state temperatures (e.g., 475–525°C for chloride mixtures) with convective flow to enhance heat transfer efficiency.^[49] Energy efficiency is improved through optimized flow rates and minimal thermal losses, as seen in molten salt electrolysis for metal production, where continuous circulation reduces energy input by up to 20% compared to batch methods.^[50] Safety considerations, such as inert gas blanketing, are integrated to manage risks during sustained operations.

Safety and Stability Considerations

Molten salts present significant thermal hazards primarily due to their elevated operating temperatures, often exceeding 400°C, which can inflict severe burns on contact with human skin or materials.^[51] Exposure to these temperatures can also lead to rapid ignition of nearby combustibles or structural failures in containment systems.^[52] A particularly acute risk arises from interactions with water, where even small amounts can vaporize explosively upon contact, generating steam pressure that ejects large volumes of molten salt; industrial incidents have documented eruptions of over 4,500 pounds of salt from such reactions, resulting in fatalities and extensive damage.^[51] Corrosive and toxic risks are prominent with salts containing nitrates or fluorides, which can emit hazardous fumes like nitrogen oxides (NOₓ) from nitrate decomposition or hydrogen fluoride (HF) during processing.^[29] These emissions pose inhalation dangers and accelerate material degradation, with NOₓ having permissible exposure limits of 25 ppm TWA for NO and 5 ppm ceiling for NO₂, while HF is limited to 3 ppm TWA (2.5 mg/m³).^[53]^[54]^[55] Mitigation requires robust ventilation, such as hooded exhaust systems with scrubbers to capture fumes, alongside personal protective equipment (PPE) including heat-resistant gloves, full-face shields, long sleeves, and respiratory apparatus to prevent skin contact and airborne exposure.^[52]^[29] Stability is constrained by thermal decomposition thresholds, beyond which salts break down into oxides or other products; nitrate-based mixtures like Solar Salt remain stable up to approximately 600°C but decompose at higher temperatures, potentially forming insoluble phases that impair flow.^[56] Impurities, including moisture, carbon dioxide, or halides, exacerbate instability by promoting nitrite formation or carbonate precipitation, which shortens shelf life and heightens corrosivity over time.^[56] Safe handling protocols emphasize inert containment materials to withstand corrosive environments, with nickel-based alloys such as INOR-8 (containing 6-8% Cr and 15-18% Mo) exhibiting negligible corrosion at 650-700°C due to their resistance to fluoride or oxide formation.^[57] Graphite, particularly low-permeability grades, serves effectively as a structural or moderator material below 700°C, with minimal carbon pickup (less than 0.025% after extended exposure at 700°C).^[57] For spills, immediate isolation using barricades, followed by sweeping after solidification and flushing with water, is standard, while dry chemical or CO₂ extinguishers are mandated to avoid exacerbating reactions.^[52]

Applications

Metallurgical Extraction

Molten salts play a crucial role in metallurgical extraction through electrolytic processes that enable the reduction of metal oxides or halides at high temperatures, where aqueous methods fail due to thermal instability. One of the most prominent applications is the Hall-Héroult process for aluminum production, developed independently by Charles M. Hall and Paul Héroult in 1886. In this process, alumina (Al₂O₃) is dissolved in a molten cryolite (Na₃AlF₆) electrolyte, which lowers the operating temperature to about 950°C, and subjected to electrolysis using carbon anodes and cathodes. The electrolytic reduction deposits molten aluminum at the cathode, achieving a purity of over 99% while producing oxygen that reacts with the anode to form CO₂.^[58]^[59] For magnesium production, the Dow process, introduced in the early 20th century, utilizes molten magnesium chloride (MgCl₂) derived from seawater or brine as the electrolyte. The process involves electrolytic reduction at temperatures around 700°C, where Mg²⁺ ions are discharged at the cathode to form liquid magnesium metal, collected and cast, while chlorine gas is liberated at the anode for recycling in MgCl₂ regeneration. This method accounts for a significant portion of global magnesium output, providing a reliable source of the lightweight metal essential for alloys in aerospace and automotive industries.^[60]^[61] In the extraction of rare earth elements, chloride-based molten salts facilitate the separation and electrodeposition of individual metals such as neodymium (Nd) and lanthanum (La) from oxide or chloride feedstocks. These processes typically employ eutectic mixtures like LiCl-KCl or NdCl₃-LaCl₃ systems at 400–800°C, allowing selective reduction potentials to deposit pure metals or alloys on reactive cathodes, such as liquid bismuth or aluminum, for improved recovery yields. This approach is particularly valuable for recycling rare earths from electronic waste or magnets, enabling efficient purification without solvent extraction.^[62]^[63] The use of molten salts in these electrolytic extractions offers key advantages, including the production of high-purity metals (often >99%) free from aqueous contaminants and dross, as well as improved energy efficiency—typically 13–15 kWh/kg for aluminum versus higher consumption in alternative pyrometallurgical routes—due to the high ionic conductivity and stability of the melts at elevated temperatures.^[64]^[65]

Thermal Energy Storage and Transfer

Molten salts serve as effective media for thermal energy storage and transfer in concentrated solar power (CSP) systems, particularly parabolic trough collectors, where they enable efficient heat capture, retention, and dispatchable electricity generation. In these setups, sunlight is concentrated onto receiver tubes containing the salt mixture, heating it to operational temperatures and allowing storage in insulated tanks for later use in steam production. This approach addresses the intermittency of solar energy by providing several hours of thermal output beyond daylight hours, enhancing grid reliability.^[66] A common formulation, known as solar salt—a eutectic blend of 60% sodium nitrate (NaNO₃) and 40% potassium nitrate (KNO₃)—operates effectively up to 565°C, at which point it transfers stored heat to a steam generator via heat exchangers, driving turbines for power output. This nitrate mixture, detailed further in the section on high-temperature inorganic salts, offers thermal stability suitable for direct heat transfer fluid (HTF) roles in advanced trough designs or as a storage medium paired with synthetic oil HTFs in conventional systems. The sensible heat storage mechanism exploits the salt's temperature-dependent enthalpy change, yielding capacities of hundreds to over 1000 MWh thermal per plant, for example 1100 MWh in the Crescent Dunes project, depending on plant size and delta-T (typically 290–565°C).^[67] The specific heat capacity of solar salt, averaging 1.5–1.6 J/g·K across its operating range, underpins its storage efficacy, enabling high energy density at costs of approximately $30/kWh thermal as of 2023—significantly lower than battery alternatives for long-duration applications. High convective heat transfer coefficients, often exceeding 1000 W/m²·K in forced-flow regimes, facilitate rapid and efficient heat exchange, permitting compact exchanger designs that reduce material use and system footprint in CSP plants.^[68]^[69] Historically, molten salt thermal storage gained commercial traction in California with the Solar Two demonstration plant in the 1990s, a 10 MW·e tower system that validated two-tank storage for over 15,000 hours of operation and paved the way for scaled deployments in trough-based facilities worldwide. Subsequent parabolic trough projects, such as those integrating solar salt storage, have demonstrated dispatchable output equivalent to 6–12 hours of full load, with overall plant efficiencies reaching 15–20% through optimized heat management.^[70]

Electrochemical and Nuclear Uses

Molten salt reactors (MSRs) utilize liquid fluoride salts, such as FLiBe (a eutectic mixture of lithium fluoride and beryllium fluoride), as both coolant and fuel carrier, enabling the dissolution of thorium or uranium fuels directly in the melt.^[71] These reactors typically operate at temperatures between 600°C and 700°C, allowing for high thermal efficiency and atmospheric pressure operation without the need for high-pressure containment systems.^[72] A key safety feature of MSRs is their large negative temperature coefficient of reactivity, arising from the thermal expansion of the molten salt fuel, which inherently reduces reactivity as temperature rises and helps prevent overheating or runaway reactions.^[73] In electrochemical applications, molten salts serve as electrolytes in high-temperature batteries and fuel cells. Sodium-sulfur (Na-S) batteries employ molten sodium as the anode and molten sodium polysulfide (Na₂S) as the cathode, separated by a solid β-alumina ceramic electrolyte that conducts sodium ions.^[74] These batteries operate at approximately 300°C to maintain the electrodes in molten states and ensure low resistance, providing high energy density suitable for grid storage.^[75] Similarly, molten carbonate fuel cells (MCFCs) use a ternary eutectic mixture of lithium carbonate (Li₂CO₃), potassium carbonate (K₂CO₃), and sodium carbonate (Na₂CO₃) as the electrolyte, which becomes molten at operating temperatures around 650°C.^[76] This configuration enables high electrical efficiency of up to 60% through direct electrochemical oxidation of fuels like natural gas or hydrogen, with the carbonate ions facilitating oxygen reduction at the cathode.^[77] Development of these technologies traces back to the 1960s, when Oak Ridge National Laboratory (ORNL) conducted the Molten Salt Reactor Experiment (MSRE), a 7.4 MWth prototype that operated successfully from 1965 to 1969, demonstrating the feasibility of thorium-fueled FLiBe systems and informing later designs.^[78] In the 2020s, renewed interest in Generation IV reactors has led to multiple MSR prototypes, including China's planned thorium-based demonstrators expected to start operations this decade. As of 2025, China's experimental TMSR-LF1 has achieved continuous refueling without shutdown and thorium-uranium fuel conversion.^[79]^[80] Na-S batteries have seen commercial deployment since the 1990s, with ongoing improvements in materials for lower-temperature variants, while MCFCs have progressed to megawatt-scale power plants, such as those by FuelCell Energy, integrating carbon capture capabilities.^[81]

Emerging and Other Applications

Ionic liquids, a class of low-temperature molten salts, have found emerging applications in catalysis, particularly in biphasic systems that enhance reaction efficiency and reduce waste in fine chemical synthesis. For instance, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) serves as an immiscible phase for catalysts in biphasic reactions, allowing easy separation of products from the ionic phase and minimizing solvent use compared to traditional organic systems.^[82] This approach promotes greener processes by facilitating catalyst recycling and lowering environmental impact in organic transformations.^[83] Nitrate-based molten salts have emerged as promising media for CO₂ capture, offering reversible absorption at intermediate temperatures around 400°C. In these systems, alkali nitrate melts promote the carbonation of MgO sorbents, enabling efficient uptake of CO₂ from flue gases through the formation of magnesium carbonate, which can be regenerated by heating.^[84] The process demonstrates high cyclic stability, with capacities maintained over multiple adsorption-desorption cycles, positioning it as a viable option for post-combustion capture in industrial settings.^[85] Deep eutectic solvents (DES), another subset of low-temperature molten salts formed by hydrogen bond donors and acceptors like choline chloride and urea, are increasingly used for biomass processing, particularly lignin extraction from lignocellulosic materials. These bio-based solvents dissolve lignin selectively under mild conditions, yielding high-purity fractions suitable for valorization into biofuels or materials, while preserving cellulose integrity as an eco-friendly alternative to volatile organic solvents.^[86] Recent advances highlight tunable DES compositions that optimize delignification yields up to 90% from sources like wood or agricultural residues, advancing biorefinery sustainability.^[87] Beyond these, molten salts are explored as high-temperature lubricants in demanding environments, such as advanced engines or nuclear systems, where their low viscosity and thermal stability reduce friction and wear on ceramic-alloy contacts.^[88] Ionic liquids also function as non-volatile additives in base oils, improving lubricity and load-bearing capacity without compromising volatility.^[89] In photovoltaics, polymer-doped molten salt mixtures serve as solid-state electrolytes in dye-sensitized solar cells, enhancing ionic conductivity and long-term stability while avoiding leakage issues of liquid alternatives.^[90] Post-2010 research trends emphasize multifunctional molten salts in green chemistry, integrating catalysis, capture, and extraction to support circular economy principles.^[91]

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Studies on the Hall-Heroult aluminum electrowinning process
The additives increase the conductivity of the medium and lower the melting point of the cryolite from 1011 to 920-970oC resulting in decrease in energy ...
[36]
Ionic Liquids as Multidimensional Materials: A Review from ...
May 30, 2025 · Ionic liquids (ILs) are revolutionary materials composed entirely of ions and remaining liquid below 100 °C, often under ambient conditions.
[37]
Ionic Liquids and Their Interaction with Cellulose | Chemical Reviews
Ionic liquids appear to be highly polar due to their ionic character, resulting in their enhanced biopolymer dissolving capacity.
[38]
Surface chemistry of carbon electrodes in 1-ethyl-3 ...
The melting point of EMImBF4 (15 °C) is too high for practical applications, but can be lowered by the addition of other ionic liquids [1], organic solvents [2] ...
[39]
[PDF] Air and Water Stable 1-Ethyl-3-methylimidazolium Based Ionic Liquids
A series of novel air and water stable low melting salts based upon the 1-ethyl-3-methylimidazolium cation (EtMeim + ) have been prepared and characterized; two ...
[40]
Ternary Deep Eutectic Solvent Behavior of Water and Urea–Choline ...
Jun 4, 2019 · From the hydrogen bond lifetimes, it was found that a ratio of 1:2 between choline chloride and urea is necessary for a reasonable strength of ...
[41]
[PDF] 1 Structure of Deep Eutectic Solvents (DESs)
For example, the melting point of choline chloride:2 urea varies in the literature from 12 °C (0.652 wt% water)74 to 24 °C (0.200 wt% water).75 These.
[42]
Recent Advances in the Use of Ionic Liquids and Deep Eutectic ...
Jun 6, 2025 · Furthermore, they have intrinsic advantages over molecular solvents, including safer usage profiles and high tunability, which allow tailored ...
[43]
Protic ionic liquids for sustainable uses - RSC Publishing
Dec 8, 2023 · Definition of protic ionic liquids. PILs are formed by proton transfer, upon neutralisation of a Lewis base with a Brønsted acid (eqn (1)).
[44]
Ionic liquids: a brief history - PMC - NIH
The earliest of these was when Paul Walden was searching for molten salts that were liquid at temperatures at which he could use his equipment without special ...
[45]
[PDF] Enhanced Molten Salt Purification by Electrochemical Methods
The melt is initially prepared and tested using an induction heating system. The metallic crucibles (nickel) are initially chosen to better couple with the ...Missing: electric | Show results with:electric
[46]
Experimental Study of Eutectic Molten Salts NaCl/KCl/ZnCl2 Heat ...
Mar 18, 2022 · From the phase diagram of the ternary mixture of chloride salt NaCl ... The process and preparation to obtain the eutectic salt mixture ...
[47]
[PDF] Thermodynamic modeling of the KCl-LiCl-NaCl-UCl3 system for ...
The LiCl-UCl3 system is a simple eutectic phase diagram with no intermediate compounds. This system was optimized initially by Ghosh et al. [20] using the ...
[48]
Design and operation of a molten salt electrochemical cell
Most molten salt electrochemical cells are located within inert gas gloveboxes to avoid oxygen and moisture; therefore, a compact design is desired to conserve ...
[49]
[PDF] Continuous Purification of Molten Chloride Salt - NREL
The results in this study will aid in modeling and scaling a continuous electrochemical purification cell at industrially relevant scales. Experimental.
[50]
[PDF] Molten Salt Reactor Salt Processing – Technology Status
Aug 29, 2018 · continuous flow of molten salt and liquid bismuth through the cell. The cells are not proposed as batch operations. However, in such a ...
[51]
[PDF] Investigation Report - Chemical Safety Board
The potential hazards in the operation of molten salt bath furnaces can result in explosions, fires, or both, either inside the salt bath furnace or outside the ...
[52]
[PDF] Safety in Molten Salt Bath Operations - Kolene Corporation
Feb 4, 2020 · The equipment operator, maintenance personnel and others should wear long sleeves, gloves and eye protection whenever they are in the area of ...
[53]
[PDF] MOLTEN NITRATE SALT DEVELOPMENT FOR THERMAL ...
In this paper, common-anion additions to NaNO3-KNO3 mixtures (binary Solar salt) were investigated as a means to identify low melting (low liquidus temperature) ...
[54]
[PDF] chapter 13 construction materials for molten-salt reactors* 13-1 ...
Evaluation tests of various materials in fluoride salt systems have indicated that nickel-base alloys are, in general, superior to other commercial alloys for ...
[55]
[PDF] Chapter 2. Production and Processing of Aluminum
In the Hall-Héroult process, the electrolyte is molten cryolite (Na3AlF6) in which 2–8% of alumina (Al2O3) is dissolved. To lower the melting point, industrial ...
[56]
Hall Process Production and Commercialization of Aluminum
Charles Martin Hall succeeded in producing aluminum metal by passing an electric current through a solution of aluminum oxide in molten cryolite.History of Aluminum · Hall's Early Experiments with... · Aluminum Production by...
[57]
Magnesium Extraction from Seawater - American Chemical Society
Passing an electric current through molten MgCl2 breaks apart the salt into magnesium metal and chlorine gas. Dow's Freeport seawater magnesium plant began ...
[58]
Dow Process - an overview | ScienceDirect Topics
Figure 2.9.37 shows magnesium production based on the Dow process. An MgCl 2 solution is produced by adding hydrochloric acid to magnesium hydroxide extracted ...
[59]
Research Status of Electrolytic Preparation of Rare Earth Metals and ...
Mar 29, 2024 · This paper mainly reviews the production process of light rare earths (La, Ce, Pr, Nd) and their alloys prepared by fluoride molten salt ...
[60]
Recovery and separation of rare earth elements by molten salt ...
This review systematically summarizes the separation and electroextraction of REEs on various reactive electrodes in different molten salts. It also highlights ...
[61]
Chloride Molten Salt Electrolysis Enables Integrated and Energy ...
Oct 9, 2025 · Additionally, the CMSE process as shown here exhibits superior coulombic efficiency (70%), very high metal purity (99.4 wt.%), and now ...
[62]
(PDF) Molten salt electrolysis for sustainable metals extraction and ...
Advantages of electrorefining are that a dross-free product is produced and, generally,. much less atmospheric pollution is generated than for pyrometallurgical ...
[63]
How solar thermal energy storage works with concentrated solar
Tower CSP: In tower CSP, a molten salt mix, like sodium nitrate and potassium nitrate, is heated by reflecting sunlight with mirrors onto a receiver atop a ...
[64]
Two-tank molten salt storage for parabolic trough solar power plants
The study concluded that the specific cost for a two-tank molten salt storage is in the range of US$ 30–40/kWhth depending on storage size.
[65]
(PDF) Properties and heat transfer coefficients of four molten-salt ...
Sep 11, 2017 · Molten salts used in high temperatures are more and more interested in the CSP for higher energy conversion efficiency. Thermal physical ...
[66]
Top 10 Things You Didn't Know About Concentrating Solar Power
The Solar One project near Barstow, California, paved the way for Solar Two, the world's first large-scale molten salt power tower facility, launched in 1996.
[67]
[PDF] Status of Molten Salt Reactor Technology
Negative temperature coefficient, salt flow rate; fissile/fertile additions. Approach to engineered safety systems. Intrinsic, passive, using natural ...
[68]
[PDF] Molten Salt Reactor Fundamental Safety Function PIRT - INFO
The molten salt could be either a fluoride or another halide operating at a temperature in excess of. 600°C. The FOM used was radioactive material release to ...
[69]
[PDF] Preliminary Assessment of Considerations for the Front-End ...
The fuel-salt mixtures used in fluid-cooled MSRs exhibit large negative temperature coefficients of reactivity due to their high thermal expansion; therefore, ...
[70]
Sodium Sulfur Battery - Zhang's Research Group
The discharge process produces roughly 2 Volts. Operation and Properties. Sodium sulfur batteries are typically operated at high temperatures between 300–350° C ...
[71]
[PDF] Dynamic Modeling of Sodium Sulfur Battery Grid Storage Units
Sodium sulfur batteries have an operating temperature between 300° and 360° C (572° and 680° F) to maintain Na and S at molten states and low resistance values.<|control11|><|separator|>
[72]
[PDF] 1- MOLTEN CARBONATE FUEL CELLS (MCFCs)
The molten carbonate fuel cell operates at approximately 650 °C (1200 °F). The high operating temperature is needed to achieve sufficient conductivity of the ...
[73]
Fuel Cell Efficiency Explained | FuelCell Energy
Mar 27, 2023 · High temperature fuel cells (such as carbonate fuel cells) can reach more than 60 percent efficiency and outperform similar-sized conventional energy sources.
[74]
Time Warp: Molten Salt Reactor Experiment—Alvin Weinberg's ...
Jun 1, 2016 · The Molten Salt Reactor Experiment achieved its first self-sustaining nuclear reaction on June 1, 1965. Three years later, on Oct. 8, 1968, ...
[75]
Molten Salt Reactors - World Nuclear Association
Sep 10, 2024 · FLiNaK (LiF-NaF-KF) is also eutectic and solidifies at 454°C and boils at 1570°C. It has a higher neutron cross-section than FLiBe or LiF but ...
[76]
[PDF] Technology Strategy Assessment - Sodium Batteries
In recent years, a lower temperature (< 150°C) NaS system that employs dissolved, rather than molten, polysulfides has also been developed (Enlighten ...
[77]
Ionic Liquids and Dense Carbon Dioxide: A Beneficial Biphasic ...
Review November 5, 2010 Ionic Liquids and Dense Carbon Dioxide: A Beneficial Biphasic System for Catalysis
[78]
[PDF] Recent progress in biphasic catalytic systems using ionic liquids
This mini-review showed the recent works in biphasic catalysis systems using functionalized ... ionic liquids. Chem Eng News 2021,. 99:8–8. [5] W. Hou, Q ...
[79]
Molten Salt-Promoted MgO Adsorbents for CO2 Capture
For the regeneration test, CO2 adsorption was conducted at 300 or 325 °C in the flow of pure CO2 (1 atm) for 55 min and desorption was performed at 400 or 450 ° ...
[80]
Enhanced CO2 capture and stability of MgO modified with alkali ...
Jul 23, 2024 · Molten nitrate facilitates the dissolution of carbonate into the molten salt, and the reaction of Mg2+ and CO2–3 (from CO2 and O2 ...
[81]
Structure–function relationships of deep eutectic solvents for lignin ...
Deep eutectic solvents (DESs) have been used for lignocellulosic biomass fractionation and lignin extraction due to their simple procedure, selective solubility ...
[82]
Recent advances in biorefineries based on lignin extraction using ...
This review aims to summarize the state-of-the-art in the use of deep eutectic solvent pretreatment to fractionate lignocellulose, with a focus on lignin ...
[83]
[PDF] Tribological Behavior of Ceramic-Alloy Bearing Contacts in Molten ...
Molten salts have great potential as high-temperature lubricants due to their low shear strength and exceptional mobility over an extreme range of operating ...
[84]
Ionic Liquids as Lubricant Additives: A Review - PubMed
Feb 1, 2017 · This work reviews the recent advances in developing ILs as additives for lubrication with an attempt to correlate among the cationic and anionic ...
[85]
Polymer-Doped Molten Salt Mixtures as a New Concept for ...
Oct 9, 2017 · A conceptually new polymer electrolyte for dye-sensitized solar cells is reported and investigated. The benefits of using this type of electrolyte based on ...
[86]
Ionic Liquid-Based Dye-Sensitized Solar Cells—Insights into ...
Jun 8, 2021 · Dye-Sensitized Solar Cells Using Ionic Liquids as Redox Mediator. Chem. Phys. Lett. 2018, 691, 373– 378, DOI: 10.1016/j.cplett.2017.11.035.Introduction · Results and Discussion · Conclusions · Supporting InformationMissing: post- | Show results with:post-