Unique selective activating effect of lead ions on the flotation of NaCl crystals against KCl crystals with fatty acid collector colloids

In the flotation of water-soluble minerals (e.g., NaCl and KCl crystals), the high ionic-strength environments alter the typical interfacial interactions and challenge the applicability of conventional flotation theories. Here, we report an intriguing effect of Pb(II) ions on the flotation of NaCl and KCl crystals in brines using sodium/potassium laurate as collectors. We revealed contrary effects of Pb(II) ions on flotation recoveries of NaCl crystals and KCl crystals. Pb(II) ions strongly enhance the NaCl crystals recovery but show no considerable effect on KCl crystals recovery. Interestingly, such activation is observed exclusively for Pb(II), not for Ca²⁺, Mg²⁺, Ba²⁺, and Cu²⁺. Our analysis using XPS and XRD rules out the adsorption of lead on crystal surfaces but reveals a significant presence of lead on the collector colloids via ion substitution. In particular, we unveil a crucial role of interfacial hydrogen bonding between Pb(II) ions in the collector colloids and the outward-pointing OH groups of surface-bound water on NaCl crystals. These hydrogen bonds sustain a strong bubble-particle attraction for high recovery of NaCl crystals. No similar hydrogen bonding is possible for KCl crystals which causes a poor recovery of this salt. These insights provide hints to tailoring the flotation conditions to enable efficient separation of water-soluble minerals, which is the core interest of the potash industry.