Spinel-Type Manganese Oxides Selectivity and Reaction Mechanism for Extraction of Lithium from Wairakei Geothermal Water

Abstract

The selectivity, of a number of spinel-type manganese oxides, for extraction of lithium from Wairakei geothermal water has been investigated. Lithium is found in Wairakei geothermal water at concentrations of 8-14 ppm and is considered a pollutant in the discharge water of the Wairakei geothermal power station. Geothermal water is thought to be an economically viable source of lithium. Spinel phase materials based on LiMn2O4 have been synthesised and acid washed to produce lithium selective ion sieves. The phases with the best lithium extraction efficiencies are materials of the LixMnyO4 and Li2yXMn3O8 series, where X = Mg, Sb, Al, Ni, with LiMn2O4 Li4/3Mn5/3O4 LiMn3O4 Li0.5Mn2O4, and Li2AlMn3O8 having the highest extraction efficiencies. Lithium is removed from the loaded sieve by washing in dilute acid. The spinel structure is maintained during the extraction/insertion reactions, however, there is an associated decrease and increase in the cell dimensions as lithium is extracted and reinserted respectively. Proposed mechanisms for the extraction/insertion involve three types of lithium site; 1) ion exchange; 2) redox; and 3) non-specific site. The solid state techniques of XRD, Raman spectroscopy MAS NMR, XPS, ESR and magnetic susceptibility, have been applied to the determination of the structures and reaction mechanism. XRD and MAS NMR studies show the reinsertion of lithium into the tetrahedral sites in the acid washed materials, with an associated increase in the lattice parameter dependant on the amount of lithium inserted. Shifts in the Raman bands, on lithium extraction and insertion, show changes in the vibrational states of the structure dependant on the amount of lithium extracted/inserted. The Raman results indicate changes in the bonding and/or chemical environment between spinel, sieve and loaded sieve samples. Significant resonance Raman enhancement is observed for the sieve materials at 514 nm, corresponding to electronic transitions observed in UV reflectance measurements. XPS, ESR and magnetic susceptibility measurements show changes in the manganese oxidation states of samples on acid treatment. XPS results show an increase in the manganese oxidation state on acid treatment, consistent with the oxidation of Mn3+ to Mn4+. Magnetic susceptibility measurements show a general increase then a decrease in the number of unpaired electrons on acid treatment and. treatment with geothermal water respectively.