Alkali Halide and MIBC Interaction at Typical Flotation Interfaces in Saline Water as Determined by Molecular Dynamics Simulations

Abstract

The molecular structure of the liquid–vapor interfaces of aqueous solutions of alkali metal halides and methyl isobutyl carbinol (MIBC, (CH3)2CHCH2COCH3) is determined by using molecular dynamics simulations with polarizable force fields for the first time. The salts are chlorides, and iodides, some of which are found in raw and partially desalinated seawater increasingly used in flotation operations in regions affected by severe and prolonged drought. The density profiles at the interfaces show that all ions prefer the interface; however, with MIBC, non-polarizable ions, generally small ones, are increasingly pushed into the liquid bulk. A few ions of comparatively less ionic NaCl than KCl and CsCl, persist at the interface, consistent with spectroscopy observations. On the other hand, strongly polarizable ions such as I− always share the interface with MIBC. In the presence of chlorides, the frother chains at the interface stretch slightly more toward vapor than in freshwater; however, in the presence of iodides, the chains stretch so much that they become orthogonal to the interface, giving rise to a well-packed monolayer, which is the most effective configuration. The dominant water configurations at the interface are double donor and single donor, with hydrogen atoms pointing toward the liquid, consistent with studies with sum-frequency generation experiments and extensive ab initio simulations. This picture changes radically in the presence of MIBC and salts. Depending on the halide and MIBC concentration, the different molecular configurations at the interface lead to very different surface tensions. The structure and properties of these new salt-rich interfaces and their impact on the location and arrangement of frother molecules should serve the flotation practitioner, especially in the search for the best frother and dosing in poor-quality water.

Editorial for Special Issue “Hydrodynamics and Gas Dispersion in Flotation”

“…An option for increasing the formation of bubble-particle aggregates is the use of micro and nanobubbles: a review on the generation, detection, and applications of nanobubbles in flotation was included in this volume [8], along with a study on the potential of bulk micro-nano-bubbles to improve quartz recovery [9]. The use of pulps containing soluble ions and frother molecules at the same time has been common in current flotation practice: (i) the arrangement of adsorbed frother molecules on the bubble surface proved to be influenced by the presence of different ions in the pulp [10], and (ii) electrolytes proved to reduce bubble coalescence, favoring the formation of small bubbles; however, in some conditions, they also promoted the formation by breakup, resulting in larger bubbles [11].…”