Understanding the activation mechanism of Pb2+ ion in benzohydroxamic acid flotation of spodumene: Experimental findings and DFT simulations

Abstract

In recent years, benzohydroxamic acid (BHA) has been widely used as a selective collector in the flotation of oxide minerals such as scheelite, wolframite, cassiterite and ilmenite despite its moderate collecting ability. Lead nitrate (Pb(NO3)2, LN) is a commonly used activator for improving BHA flotation. According to the classical activation theory, the pre-adsorbed Pb2+ ion can increase the number of active site on oxide mineral surface, thus increasing the subsequent adsorption amount of BHA collector. In this study, BHA and LN were used, for the first time, in the flotation of spodumene, a valuable silicate and major source mineral for lithium battery materials. Interestingly, the adsorption test results show that the adsorption amount of BHA on spodumene surface was the same whether LN was added or not, while the micro-flotation test results show that the addition of LN helped increase the spodumene recovery from 0 to 80% using 120 mg/L BHA at pH 8. These results seem to be contradictory to the classical activation theory, yet by using density functional theory-based first-principle calculations, X-ray photoelectron spectroscopy and atomic force microscopy theoretically and experimentally, their underlying mechanism is therefore revealed. Those results show that in the absence of LN, a “flat” adsorption mode with the benzene ring of BHA parallel to the spodumene surface, is more inclined to a monodentate mode between O2− ion (attached to N) of BHA and surface Al3+ ion, leading to an adsorption energy of −113 kJ/mol. While with the presence of LN, a “vertical” adsorption mode with the benzene ring perpendicular to the spodumene surface, is favored in a bidentate mode between two O2− ions of BHA and Pb2+ ion of pre-adsorbed solvated Pb(OH)+ complex, leading to a much higher adsorption energy of −270 kJ/mol. Based on coordination chemistry analysis, differences in the adsorption mode and energy of BHA on Pb2+-activated and unactivated spodumene surfaces can be explained by differences in coordination number and bond dissociation energy. This study will provide valuable insights into the activation of Pb2+ ion in BHA flotation of valuable silicate minerals, and help to enrich understanding of activation mechanisms of metal ions in anionic collector flotation of minerals.