A salt is an ionic compound formed by the strong electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). These compounds possess a neutral electric charge and typically exist as crystalline solids at room temperature. When heated above its melting point, the solid transitions into a liquid phase known as a molten salt or fused salt. This liquid state demands a significant energy input because the powerful electrostatic forces of the ionic bonds require extremely high temperatures to break. For instance, common table salt (sodium chloride) must be heated past 801°C before it becomes a liquid.
Fundamental Composition and Physical State
Molten salts exist as liquids composed entirely of mobile, electrically charged ions, lacking the discrete molecular structure found in common solvents like water or organic fluids. Upon melting, the rigid crystal lattice of the solid salt is disrupted, allowing the constituent ions to move freely. This movement gives the liquid the ability to conduct electricity.
To enable operation at lower temperatures, engineers often utilize eutectic mixtures, which are blends of two or more salts designed to have a significantly lower melting temperature than the individual components. Common chemical families used include nitrates, halides (such as chlorides and fluorides), and carbonates. For example, a common blend of 60% sodium nitrate and 40% potassium nitrate, often termed solar salt, has a melting point of approximately 222°C to 225°C.
Once liquefied, these compounds exhibit a relatively low viscosity, often comparable to water, allowing them to be pumped easily through complex piping systems. This liquid state is characterized by low volatility and a wide temperature range between the melting and boiling points. This wide liquid range makes them particularly suitable for processes requiring the transfer of high amounts of heat.
Distinctive Thermal and Chemical Properties
The utility of molten salts stems from their remarkable thermal characteristics, particularly their capacity to store and transfer thermal energy efficiently. They possess a high specific heat capacity, meaning they can absorb large amounts of heat energy relative to their mass without experiencing a proportional temperature increase. A commonly used solar salt mixture, for example, provides a substantial thermal reservoir for industrial applications.
These liquids feature extremely low vapor pressures, even when heated to temperatures exceeding 600°C. Unlike high-pressure water, molten salt remains a liquid at nearly atmospheric pressure even at high temperatures. This property simplifies system design by reducing the need for expensive, thick-walled piping and high-pressure containment vessels.
Many molten salts, especially fluoride and chloride mixtures, exhibit high thermal stability, remaining chemically unchanged even when operating up to 800°C. This stability allows them to function reliably in high-temperature environments that would cause organic fluids to quickly decompose. The potential for high-temperature operation directly supports greater efficiency when converting thermal energy into electricity.
A notable challenge associated with molten salts is their tendency to be chemically aggressive toward metallic containment materials at elevated temperatures. The ionic nature of the hot liquid enables it to leach components from standard steel alloys, such as chromium, weakening the structural material. Mitigating this corrosion requires the use of specialized, high-cost nickel-based alloys, like certain Hastelloys, or the application of protective coatings to the interior surfaces of tanks and pipes.
Key Industrial and Energy Applications
The unique combination of high heat capacity and low vapor pressure has made molten salts indispensable for modern Thermal Energy Storage (TES) systems. In concentrated solar power (CSP) plants, large arrays of mirrors focus sunlight to heat the salt, which can reach temperatures around 565°C. The heated salt is then stored in large, insulated tanks, functioning as a thermal battery that can retain heat for many hours.
This stored thermal energy is later used to generate steam for a conventional turbine, allowing the plant to produce electricity long after the sun has set. The use of molten salts in TES provides a reliable, dispatchable source of power from a variable renewable energy source. The two-tank storage system, where hot salt is kept separate from cold salt, is the most widespread configuration in commercial CSP facilities.
Molten salts are central to the design of advanced nuclear technology, specifically Molten Salt Reactors (MSRs), a class of Generation IV reactor designs. In these reactors, the salt mixture serves either as the primary coolant or, in some designs, as the solvent for the nuclear fuel itself. The low operating pressure of MSRs provides an inherent safety advantage compared to high-pressure water reactors, significantly reducing the risk of a high-pressure loss-of-coolant accident.
The liquid fuel designs allow for continuous chemical processing, which can improve fuel utilization and potentially reduce the volume of high-level nuclear waste generated. A common salt mixture used in these reactors is FLiBe, a combination of lithium fluoride and beryllium fluoride, which is thermally and radiolytically stable at high operating temperatures of 700°C or more.
Beyond large-scale energy production, molten salts have established roles in traditional industrial processes. They are used extensively in metallurgy as heat transfer baths for the heat treatment and tempering of specialized alloys. The ability of molten salts to conduct electricity when liquid also makes them suitable for high-temperature electrochemical processes, such as the Hall-Héroult process for producing aluminum metal.