What are molten salts?

What are molten salts?

Molten salts are a phase change material used to store thermal energy.

Phase change materials are solid at room temperature and atmospheric pressure and turn into fluids when heated. Molten salts store the energy applied to convert them into liquids as latent heat, which they can transfer to other materials. As phase change materials, molten salts have a higher latent heat capacity than conventional materials. Minimal temperature changes are needed to increase their heat capacity.

Molten salts are composed of nitrates, nitrites, carbonates, chlorides, and fluorides. Each compound has unique properties that make them useful for varying applications.

But the common feature of all molten salts is their thermal stability at high temperatures.

What are molten salts used for?

Many molten salts remain in the liquid state even at temperatures between 250 to 1000°C and have a low vapour pressure.

This property makes them suitable for applications where liquids at very high temperatures are necessary for heat storage or transfer.

Molten salts heated beyond their liquid temperature range degrade into gaseous components. Combining different salts can lower the melting points of the salts and increase the temperature range where they remain liquid. Different mixtures of salts are used depending on the temperatures required for specific applications.

The most common molten salt mixture used as a heat transfer medium is composed of 60 per cent sodium nitrate and 40 per cent potassium nitrate and melts when heated at 220°C. It remains liquid in the temperature range of 220-600°C and decomposes into nitrogen and nitrogen oxides at temperatures over 600°C.

The most widespread use of molten salts is to store thermal energy in solar power plants. During the day, the excess solar heat not used for making electricity is sent to molten salts for storage. Solar plants use the heat stored in molten salts to produce steam and generate electricity overnight. Molten salt systems can increase the capacity of solar plants by up to 70 per cent.

Molten salts are also used as heat transfer systems for processing waste tires and in pyrolysis for material recovery. They’re also commonly used for heating and quenching steel.

Waste heat recovery boiler

Molten salts technology

 the sole company employing molten salts as heat transfer agents in pyrolysis. To gain further insight into our unique technology, here are five critical facts to fully understand Molten® technology.

1.  Molten® technology is an innovation on a global scale.

Pyrolysis is a well-known technology, but Molten® technology offers a novel approach to solving the waste tire problem through pyrolysis. 

2. Molten salts are used as a heat transfer system.

There are five kinds of heat systems for pyrolysis:

1. Electricity
2. Flue gases
3. Microwaves
4. Molten Metal
5. Molten Salt

Each technology has unique features and is best suited for specific situations. Molten salt is an effective solution for pyrolysis because it’s sustainable, doesn’t harm the environment, has no risk of fire or pressure build-up, and has a high heat efficiency.

3. Molten salt usage significantly improves the heat transfer parameters.

Molten salt offers better heat control, which helps to maintain optimal process parameters. This makes achieving consistent quality products and higher safety standards easier than conventional solutions. Additionally, the system experiences less thermal stress.

4. There are three methods of using molten salts.

There are three types of molten salt systems: salt baths, direct heating, and circulating molten salts. In salt baths, molten salts are in an open vessel, and heat transfer occurs through natural convection. Direct heating uses metal assemblies with molten salts to heat materials directly. In circulating systems, the molten salts are kept in circulation as a heat medium for process heating or heat exchange.

5. Molten salts are better for the environment.

The process has a lower environmental impact. According to a NEUMAN & ESSER Climate Solutions report, the recovered Carbon Black produced is a sustainable option for certain grades of virgin Carbon Black and has a 20 per cent smaller carbon footprint.

Industrial applications of molten salts outside energy storage for solar power are gaining traction,  as is the application of pyrolysis on end-of-life tires to extract recovered Carbon Black, steel, oil, and gas.