What is molten salt used for?
Molten salts technology is attracting intensive R&D efforts and emerging in the circular economy. It is leading advancements in renewable electricity generation and heat storage as a safe, green alternative to conventional technologies.
The demand for molten salts for thermal energy storage alone was worth 8.6 billion USD in 2024 and is expected to have a CAGR of 9.4% in the same year. Molten salts technology is also being developed for recycling critical resources and hazardous tire waste, and its advantages are driving more usage across different industries, which we will cover in this article, along with:
- The many properties of molten salts technology,
- What the uses of molten salts in various industries are,
- And how molten salts are used to recycle important resources.
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What is a molten salts mixture?
Molten salts are simple inorganic compounds, such as fluorides, chlorides, and nitrates. A common example is sodium chloride or table salt. The standard molten salts mixture in industrial settings is 60 per cent sodium nitrate (NaNO3) and 40 per cent potassium nitrate (KNO3).
Molten salts are phase change materials that are solid at room temperature and atmospheric pressure. High temperatures, specific to the salt, melt them to produce stable liquids made of positively- and negatively-charged ions. For example, sodium nitrate, potassium nitrate, and sodium chloride all melt at different temperatures of 306.5°C, 334°C, and 801°C, respectively. The 60 per cent sodium nitrate + 40 per cent potassium nitrate mix remains a liquid only at high temperatures of 220-600°C.
How and what molten salts are used for depend on their following properties:
- Fluid stability: Molten salts have a viscosity similar to water at high temperatures and the ability to flow, which is useful in heat transfer applications. When the molten salts cool, they solidify and contract unlike water that expands when frozen and can burst pipes.
- High heat capacity: Molten salts have a higher latent heat capacity than conventional materials and store the heat applied to melt them. They can store heat over 700°C making them suitable as a heat transfer or storage medium.
- Electricity conductivity: In the liquid state where the chemicals are ionic in form, molten salts conduct electricity.
- Solvent: Molten salts act as solvents and can be used as alternatives to toxic volatile organic compounds (VOC). They can dissolve or dilute several organic and inorganic materials, such as metal oxides, or crystallise basic oxides at their freezing points.
- Catalysts: Some molten salts are catalysts and used in the synthesis of chemicals.
Molten salts are used in various applications, including direct heating, baths, and circulation. They are nonflammable and nonvolatile, making them ideal for industrial applications as a safe and environmentally friendly technology.
What are molten salts used for?
Several standard industrial processes use molten salts technology, such as nuclear reactors, heat transfer, electrochemistry, etc. The first use of molten salts was in 1950 to develop and test a nuclear-powered aircraft in the USA!
Molten salts as a heat transfer medium
Currently, one of the main uses of molten salts is as a heat transfer medium. Molten salts’ high heat capacity and viscosity are useful in transferring high temperatures in many energy systems for storing or producing energy, according to a 2022 review (Roper et al.).
A few examples are:
- Thermal energy storage: Renewable energy storage has been a challenge that molten salts address. Molten salts as thermal energy storage and heat transfer fluids are integral to new concentrating solar power (CSP) plants. Molten salts absorb heat from solar radiation that is focused by mirrors and lenses on a small receiver. Molten salts store the heat up to 600ºC for extended periods for later use. When required, the heat stored in molten salts is transferred using a heat exchanger to generate steam to turn a steam turbine for electricity production. Nitrate-nitrite molten salts are common in solar applications. Molten salts technology increases efficiency and storing capacity of solar power plants.
- Nuclear reactors: Molten salts cool solid fuels in nuclear reactors due to their heat transfer capabilities. Molten salts can also be used as fuel salts in nuclear reactors. Since the molten salts remain liquid even under low atmospheric pressures, it is an advantage that allows for use of systems that have relatively thin walls.
- Pyrolysis: The use of molten salts as heat transfer mediums has been further extended by integration into end-of-life tire (ELT) pyrolysis. Pyrolysis is a thermo-chemical process that uses high temperatures between 400-700ºC to break down the complex mix of substances in tires into simpler components that provide a range of secondary recycled products that can narrow the material loop to produce new tires, rubber, and paints.
Molten Salts Pyroprocessing of Non-Ferrous Metals
Pyroprocessing extracts non-ferrous metals by dissolving them in a molten salt bath. For example, metal ores like titanium oxide are combined with chlorine and carbon, and the resultant compound titanium tetrachloride (TiCl4) is smelted in molten salts. Once melted, the metal is boiled and then distilled to separate it from impurities to give pure TiCl4.
Using molten salt electrolysis for metal production is a more common method.
Molten Salts in Electrolytics and Fuel Cells
Molten salts are popular for electrolysis because their electrical conductivity is several times higher than aqueous and organic electrolysis. Molten salts’ high temperatures support rapid electrode reactions, therefore a higher voltage, though this property can be a disadvantage at times.
Examples of molten salts in electrolytics include:
- Metal extraction: Molten salts with high melting points, electrical conductivity, and electrochemical stability are useful in extracting aluminium and titanium from raw ores.
- Critical resources recovery: Molten salts electrolysis can help in the recovery of critical resources and metals from waste/secondary resources such as abandoned rare earth metals, spent lithium batteries, waste cemented in carbide scrap, and spent fuel. With the rise of renewable energy, demand for critical metals is increasing. The metals are considered critical as they are essential to the security and economy of a country and their supply chains are fragile, since they are sourced from regions with less government control. Molten salts address challenges in conventional aqueous solution electrolysis. They provide anhydrous and oxygen-free conditions and inhibit hydrogen production that interferes with the electrodeposition of metals. Therefore, molten salt electrolysis is preferred for extracting, purifying, and resource recycling of rare earth metals, alkali, aluminium, and magnesium.
- Fuel Cells: Molten salts are used as electrolytes with other compounds in batteries called fuel cells that use electrochemical conversion to convert chemical energy to electrical energy. This process is used with carbon-containing fuels, including biofuels, to generate electricity. These Molten Carbonate Fuel Cells (MCFCs), can operate at high temperatures of 580-700oC. However, electrolyte vaporisation and corrosion can be disadvantages.
Molten Salts Cleaning for Remanufacturing
Cleaning secondary and reusable materials is essential during remanufacturing. Cleaning helps detect repair needs during processing and assembly. Molten salts combinations of sodium nitrite/nitrate baths are used to strip metals of impurities like carbon compounds, oil, and metal depositions. Cleaning with high quality molten salts uses their catalytic and oxidative properties and does not deform surfaces. However, corrosion must be tackled. For example, appliance manufacturers use molten salts baths to clean paint from items that fail quality tests in order to reuse materials again.
Molten Salts Oxidation (MSO)
The many uses of molten salts shows their versatility. Among thermal methods, molten salts oxidation (MSO) is a non-flame process that can destroy several kinds of wastes while retaining items of interest like inorganic or radioactive materials. MSO can oxidise several categories of waste, such as hazardous, mixed plastics, and medicinal wastes. It is also used to destroy biological and chemical weapons, munitions, explosives, and rocket fuel.
In this process, waste and air are sent to a molten sodium carbonate bath and the only emissions are steam, oxygen, carbon dioxide, and nitrogen. Conventional technologies use acidic gases that react with waste material, while MSO is stable and non-reactive.
MSO technology was pioneered for nuclear processing and applied for coal gasification initially. In the future, it could become a viable recycling method for challenging waste streams like plastics.
How are molten salts used in tire pyrolysis?
Contec is the only company in the world that uses molten salts as a heat transfer medium in ELT pyrolysis to produce circular secondary raw materials. Contec developed the patented technology after five years of R&D efforts in close collaboration with the Warsaw University of Technology and engineers.
Molten®, Contec’s proprietary technology, uses a commercial mix of sodium nitrate and potassium nitrate. The molten salts are heated, melted, and pumped into a jacket that keeps circulating them in a loop around the reactor containing ELT rubber granules. This thermal treatment of rubber is even and without hotspot formation due to heat transfer from the molten salts and an auger that rotates the rubber granules. As a result, the tire waste is broken down to yield high quality Recovered Carbon Black.
Contec has found various other advantages in using molten salts. The medium requires less energy to melt and retains its high temperature, considerably reducing the energy requirement and carbon footprint of pyrolysis. Moreover, using molten salts as a heat transfer medium prevents the buildup of pressure and avoids accidents and explosions, for which pyrolysis plants are notorious.
Molten® has helped to make Contec’s tire pyrolysis process safe, efficient, and environmentally friendly. Its pilot plant situated in Szczecin, Poland has two pyrolysis plants and the company aims to triple its capacity soon following successful fundraising in 2023.
Improving circularity with molten salts
Several new technologies are emerging to usher in the circular economy. Molten salt technology is one of them. Molten salts are used to produce and store renewable energy and help recover critical resources. The long list of molten salts’ properties is also increasing how, where, and what molten salts are used for in industrial waste reduction.
At Contec, we’re dedicated to advancing molten salt technology for the circular economy. We provide sustainable and circular products such as recovered Carbon Black (ConBlack®), recovered Tire Pyrolysis Oil (ConPyro®), and recovered Steel (ConWire®), applying molten salts.
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