Boron Nitride Crucibles Used to Make TiO2 From Molten Salt

In the quest for developing advanced materials and processes for various industries, researchers continuously explore new methods and materials to optimize their work. One such area of interest is the molten salt electrolysis of titanium dioxide (TiO2), a process that can be utilized in the production of titanium metal and in the field of nuclear materials. In a study by T. Nakamura, K. Sato, and K. Hiraoka, published in the Journal of Nuclear Materials in 2017, the authors showcased the development of a boron nitride (BN) crucible for this process. This article discusses the reasons behind the researchers' choice to use a boron nitride crucible, highlighting its advantages over other materials.

Properties of Boron Nitride Crucibles

Boron nitride is a versatile material with unique properties that make it particularly suitable for use in high-temperature applications, such as molten salt electrolysis. Some of these properties include:

High thermal stability: Boron nitride can withstand temperatures up to 3000°C in a non-oxidizing atmosphere, allowing it to maintain its structural integrity under the extreme conditions required for molten salt electrolysis. 

Low thermal expansion: BN has a low coefficient of thermal expansion, which means it undergoes minimal dimensional changes when subjected to temperature fluctuations. This property is crucial in maintaining the integrity of the crucible during the electrolysis process, as it minimizes the risk of cracking and failure.

Excellent chemical resistance: Boron nitride is resistant to most chemicals, including molten salts, and does not react with molten metal or its oxides. This property is particularly beneficial in the electrolysis of TiO2, as it prevents contamination of the end product and ensures a high-purity titanium metal.

Good electrical insulation: BN is an excellent electrical insulator, which is essential in the electrolysis process. This property allows the crucible to withstand the high voltages and currents involved in the process without short-circuiting or introducing electrical interference.

Low wettability: Boron nitride exhibits low wettability by molten metals and salts, preventing the formation of strong bonds between the crucible material and the molten salt. This property enables easy removal of the end product and prolongs the crucible's service life.

BN Compare with Other Crucible Materials

When comparing boron nitride crucibles to other materials commonly used in high-temperature applications, several advantages become apparent: 

Alumina (Al2O3) crucibles: Although alumina crucibles can withstand high temperatures, they tend to react with molten salts and metal oxides, leading to contamination of the end product. BN crucibles, on the other hand, offer excellent chemical resistance and prevent such contamination. 

Graphite crucibles: Graphite crucibles are also capable of withstanding high temperatures but are subject to oxidation at elevated temperatures, leading to structural degradation. Boron nitride crucibles, however, can maintain their structural integrity in non-oxidizing environments up to 3000°C. 

Zirconia (ZrO2) crucibles: Zirconia crucibles are chemically stable and can resist high temperatures. However, they have a higher coefficient of thermal expansion compared to BN, making them more susceptible to cracking during thermal cycling. Boron nitride crucibles, with their low thermal expansion, are a more suitable choice for molten salt electrolysis applications.

 On the other hands, as boron nitide is a machinable material, like grahite, it supports quick prototyping. After all, every household has knives that can cut a turkey, but you can't expect these utensils to handle a whole buffalo. 

Advantages of BN Crucibles in Molten Salt Electrolysis of TiO2

Using boron nitride crucibles for the molten salt electrolysis of TiO2 offers several benefits, including:

Improved process efficiency: The exceptional thermal and chemical stability of BN allows for a more efficient electrolysis process, as it minimizes the risk of crucible failure and the need for frequent replacements. 

Higher purity end product: Boron nitride's chemical resistance prevents contamination of the titanium metal by the crucible material, ensuring a high-purity end product, which is crucial in industries such as aerospace and nuclear materials. 

Extended crucible service life: The combination of high thermal stability, low thermal expansion, and low wettability contributes to a longer service life for BN crucibles compared to other materials. This property reduces downtime for crucible replacement and lowers the overall cost of the electrolysis process.

In conclusion, the use of boron nitride crucibles in the molten salt electrolysis of titanium dioxide offers numerous advantages over other materials. Its exceptional thermal stability, low thermal expansion, excellent chemical resistance, and low wettability make it the ideal choice for this high-temperature application. By employing BN crucibles, researchers can improve process efficiency, achieve a higher purity end product, and extend the service life of the crucible, ultimately lowering the overall cost of the process.