Soluble mineral flotation paradox: Improved recovery with no sign of collector adsorption on minerals signifies colloidal attraction between bubble-bound collector colloids and mineral particles

Froth flotation uses air bubbles to separate mineral particles based on the difference in mineral surface hydrophobicity. Collectors are needed to selectively hydrophobize the surface of targeted minerals to augment the difference in surface hydrophobicity between wanted minerals and unwanted ones. Here, we showed that this classical (well-established) principle fails to describe the flotation of water-soluble minerals. Our systematic micro-flotation tests using soluble minerals (NaCl and KCl crystals) in brines and three popular collectors (dodecylamine hydrochloride, sodium dodecylsulfate, and sodium laurate) showed a substantial flotation recovery of the salt minerals without detected collector adsorption at the mineral surfaces by X-ray photoelectron spectroscopy (XPS). Complementary particle size distribution measurement indicated the occurrence of sub-micron collector colloids in saturated brines. Moreover, the flotation recovery agreed well with the measured contact angle, suggesting the attraction between bubble-bound collector colloids and salt crystal surfaces. These paradoxical results signify a new principle underlying the flotation of soluble minerals, i.e., the selective colloidal attraction between bubble-bound collector colloids and salt particles enables the flotation separation of soluble minerals.