Efficient fluoride recovery from poly(vinylidene fluoride), poly(vinylidene fluoride-co-hexafluoropropylene) copolymer and poly(ethylene-co-tetrafluoroethylene) copolymer using superheated water with alkaline reagent

Abstract

Mineralization of poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) [poly(VDF-co-HFP)] copolymer, and poly(ethylene-co-tetrafluoroethylene) copolymer (ETFE) in superheated water in the presence of an alkaline reagent was investigated with the aim of developing a technique to recycle the fluorine element. These polymers underwent almost complete defluorination to form fluoride ions (F–) in the reaction solution at a relatively low temperature (250 °C) under Ar atmosphere. When PVDF was reacted with aqueous KOH (1.0 M) for 6 h, the amount of which corresponds to 10 times the molar amount of fluorine content (as atoms) in PVDF, the yield of F– released into the reaction solution reached 95 %. This transformation was accompanied by the formation of carbon rich reside, which consists of amorphous carbon. In contrast, for a treatment performed with O2 instead of Ar, significant differences were observed in the products: while F– ions efficiently produced in the reaction solution, carbon rich residue did not form and the major species that composed the total organic carbon content in the reaction solution was oxalate. Four consecutive runs (that is, after one reaction at 250 °C under Ar was complete, new PVDF was charged to the reaction mixture and then reacted again) caused no decrease of the F– yield. When treated with aqueous KOH (1.0 M) under Ar at 250 °C for 6 h, poly(VDF-co-HFP) and ETFE copolymers also completely mineralized to form F– ions with 100 and 98 % yields, respectively. Introducing Ca(OH)2 to the resulting reaction solutions of these (co)polymers after superheated water treatment produced pure CaF2, i.e., artificial fluorspar.