Salt-type minerals can be usually floated with either anionic or cationic collectors. In a number of systems, flotation has been reported to remarkably increase above the concentrations where precipitation of the collector salt is initiated. Some studies attribute this phenomenon to heterocoagulation of oppositely charged colloidal precipitate and mineral particles. In this study, ulexite, a semisoluble boron mineral, in the presence of various multivalent ions, i.e. Ba2+, Mg2+, Ca2+, and Al3+, was found to exhibit excellent flotation even when particles, colloidal precipitates, and bubbles acquire a similar charge, which indicates that attractive structural forces exceed the forces of electrostatic repulsion. (C) 1998 Academic Press.