Polymer Centre members Dr Mark Ogden (senior lecturer, Dept. Chemical and Biological Engineering), Dr Robert Dawson (lecturer, Dept. Chemistry) and Tom Robshaw (CDT PhD Researcher) have published two research articles in the last six months. The papers are on the theme of treatment of spent potlining waste from the aluminium industry. This is a highly dangerous solid waste-form, on which there is no global consensus on correct disposal practice. It has been described as the most significant challenge for the future survival of the industry.
This however is only one side of the story. Spent potlining is a very rich source of labile fluoride. This is rapidly becoming a scarce resource, its parent mineral fluorspar (CaF 2 ) having made its way on to the EU “critical materials” list in 2014. There are estimated to be no more than 35 years-worth of geological fluorspar remaining in the world. This threatens future supply of fluorochemicals, which are needed for essentials such as non-stick coatings, transformers and circuit-breakers.
The researchers have conceptualised a complete treatment system for the potlining, involving crushing, chemical leaching, leachate treatment by ion-exchange for selective fluoride capture, fluoride elution, and finally precipitation and isolation of fluoride-bearing commodity chemicals. The two articles show development of the ion-exchange stage, for which a novel, lanthanum-loaded resin was chosen for the attempted extraction. The first paper (Journal of Hazardous Materials, 361, 200-209; DOI: 10.1016/j.jhazmat.2018.07.036) focusses on fundamental thermodynamic behaviour of the fluoride uptake, by way of loading isotherms, while the second (Chemical Engineering Journal, 367, 149-159; DOI: 10.1016/j.cej.2019.02.135) investigates the kinetics of the system and progresses to simulations of industrial conditions.
A number of key discoveries have added to the novelty of this work, principally that uptake of fluoride did not occur by the simple ligand-exchange process on the lanthanum centres, but by complexation of aqueous aluminium hydroxyfluorides. The chelating interaction allowed a lower energy uptake pathway, which was confirmed by an Arrhenius activation energy calculation. The use of X-ray photoelectron spectroscopy measurements, provided by Polymer Centre member Prof Graham Leggett, was instrumental in working out the unusual chemistry of the uptake process.
From a simulated ion-exchange column study, it was successfully argued that synthetic cryolite (Na 3 AlF 6 ) was a feasible recovery product and could be produced easily and at high purity. The system, if implemented industrially, would not only reduce the drain on global fluorspar reserves but also valorise the potlining and quickly offset any construction or conversion costs. A full technico-economic analysis on the proposed process is currently underway.
Article by Tom Robshaw; a 3rd year EPSRC Polymers, Soft Matter and Colloids CDT PhD student.