Unveiling the Amphoteric Surface Reactivity of Montebrasite using Spectroscopic and First-Principles Methods

The amblygonite-montebrasite group of minerals is the most abundant lithium-bearing phosphate found in phosphorus-rich lithium pegmatites. The separation of montebrasite from spodumene—the primary lithium ore mineral—commonly by flotation, is necessary for effective downstream lithium extraction. Understanding how water molecules interact with the montebrasite surface is necessary to understand its separation from other minerals by flotation. This study examined the interaction between water and montebrasite using dissolution tests, surface charge analysis, and various spectroscopic techniques (UV, FTIR, XPS, and XANES), all supported by density functional theory calculations. We found evidence of amphoteric properties (hydroxylation of surface Al and Li, and protonation of phosphate group) with acid-base reactions, F/OH ion exchange (-142.61 kcal/mol), non-stoichiometric dissolution (F > Li > Al > P), and dissociative chemisorption (-187.06 kcal/mol) at the mineral–water interface. This surface reactivity could impact solution chemistry and influence collector adsorption on the mineral surface during flotation. Surface dissolution and ion exchange may lead to the release of Li to industrial process water, in addition to F, potentially forming LiF in solution. These findings are important in the froth flotation field because the change in solution pH caused by significant amounts of amphoteric minerals, such as montebrasite, can affect flotation results since flotation performance depends on pH, leading to unstable flotation circuits. However, this issue may be managed by achieving and/or maintaining an equilibrium pH through an appropriate conditioning stage. This work builds important knowledge of montebrasite surface properties and the montebrasite-water interface, essential to understanding many natural phenomena and engineering processes. Accordingly, we discussed the implications for montebrasite froth flotation, mineral alteration, environmental impact, and its potential as a promising functional material for heterogeneous catalysis and Li-ion batteries.