Probing the interactions of hydroxamic acid and mineral surfaces: Molecular mechanism underlying the selective separation

Separation process facilitated by selective adsorption of chemical reagents can be modulated through an in-depth understanding of reagent-surface interaction, which is of both fundamental and practical importance in a wide range of industrial applications. Hydroxamate is a class of widely used reagents in chemical and mineral engineering that can selectively adsorb on certain minerals (e.g., malachite, wolframite and rare-earth minerals) to facilitate their separation from polymetallic ores by froth flotation. In this study, the hydroxamate-mineral interaction has been probed at nanoscale using atomic force microscopy (AFM), where the hydroxamate-functionalized tips were fabricated by self-assembly of a novel reagent S-[(2-hydroxyamino)-2-oxoethyl]-N,N-dimethyl-dithiocarbamate (HAOMDC) on Au tips. The adhesion energy between hydroxamate-functionalized tip and wolframite was found much higher than that with calcite and quartz, attributed to the stronger bonding between hydroxamate groups and surface Fe atoms of wolframite. Under alkaline condition, the deprotonated hydroxamate groups, due to stronger electron donating power, exhibited enhanced adhesion on wolframite and calcite as compared to that under acidic condition. X-ray photoelectron spectroscopy (XPS) results showed the adsorption of octyl hydroxamic acid (OHA) on wolframite possibly via the formation of five-membered-ring structure. The AFM force results support the froth flotation data that OHA exhibits superior selectivity for wolframite against calcite and quartz. This work provides a useful approach to evaluate the reagent-solid interaction in selective separation processes, with implications for the development of novel chemical additives in many engineering and environmental applications.