Researchers in Japan have developed a process for converting commonly used fluoropolymers into fluorite (calcium fluoride), a useful starting material for making nearly all fluoride compounds.
‘Recently the depletion of fluorite, has been a problem,’ explains Naohisa Yanagihara of Teikyo University, who developed the method alongside Takahiro Katoha. ‘Not only is fluorite an exhaustible resource, but the fact that fluorite production is unevenly distributed between two countries (China and Mexico) is a problem. It is very important to establish an alternative method for sourcing fluorite, as well as a recycling technology to prepare for unforeseen events that may occur in the near future.’
Fluoropolymers, such as Teflon, are renowned for their non-stick properties. But they are notoriously difficult reprocess due to their extreme chemical resistance and inertness. Recent attempts to chemically recycle fluoropolymers have been limited by a requirement for high-pressure reaction vessels, long reaction times and harsh oxidising agents. Aiming to improve the sustainability and feasibility of fluoropolymer recycling, Yanagihara explains that their uses only general-purpose chemicals ‘and the reaction is performed under atmospheric pressure using an electric furnace at 400–500°C for three hours.’
The method begins by decomposing the fluoropolymers in molten sodium hydroxide to produce sodium fluoride and sodium carbonate salts, before dissolving this mixture in water. The next step involves acidifying the mixture with nitric acid and adding calcium chloride to form fluorite as a precipitate that can be easily removed by filtration.
‘This is a sustainable method for transforming fluoropolymers, one of the main global warming potential materials, into non-hazardous materials,’ comments Sabu Thomas, an expert in polymer recycling at Mahatma Gandhi University, India. ‘The method has good potential in terms of applications and in being extended to an industrial scale. A highlighting feature is the absence of exhaust gas generation during the process.’
The main limitations of the process relate to the use of molten sodium hydroxide. ‘Polymers that decompose at temperatures below the melting point of sodium hydroxide will either not be mineralised or the efficiency of mineralisation will be low,’ explains Yanagihara. ‘In addition, ordinary metals such as stainless steel are not suitable as reaction vessels because of the strong oxidising power of sodium hydroxide – reaction vessels may be limited to ceramics or other materials.’
N Yanagihara and Takahiro Katoh, Green Chem., 2022, 24, 6255 (DOI: 10.1039/d2gc00797e)