Precipitate Co-Flotation of Orthophosphate and Fluoride
DIBAKAR BHATTACHARYYA, Research Associate
ROBERT B. GRIEVES, Professor and Chairman
Department of Chemical Engineering
University of Kentucky
Lexington, Kentucky
JOHN D. ROMANS, Engineer
Department of Air and Water Resources
State of North Carolina
Raleigh, North Carolina
INTRODUCTION
The removal of a soluble ionic species from aqueous solution followed by
concentration in a foam or froth can be accomplished first by precipitating the ion, by
adding a surface-active agent to act as a collector-frother, and then by aerating the
suspension and floating the precipitate-surfactant particulates to the surface of the
suspension. The initial charge of the precipitate has a significant effect on the adsorption (or
exchange) of the surfactant on the particles, with the charge established either by desorption
of one of the ionic species of the solid or by adsorption of ions from solution onto the
surface of the crystal. The constituent ions of the precipitate present in solution are
preferentially adsorbed over other ions. The surfactant, added as a collector-frother in a
flotation process, serves three or more functions: the adsorption (or exchange) of the
surfactant on the surfaces of the particles makes the precipitate suitable for gas bubble
attachment (the surfactant may also promote aggregation of the precipitate, becoming
incorporated in the precipitate structure) interaction between surfactant adsorbed on the
particles and "free" surfactant adsorbed at the gas-liquid, bubble interfaces produces
bubble attachment to the particles; and "free" surfactant acts as a frother, producing a
stable foam, which may be further stabilized by the presence of particulates.
Precipitate flotation has been studied by several investigators, beginning with Skrylev
and Mokrushin (1) and Baarson and Ray (2). The most recent investigations include the
flotation of magnesium hydroxide (3), of chromium hydroxide (4,5), of silver, uranium,
gold, nickel, and zinc precipitated by a hydrophilic organic reagent (6,7), of zinc and lead
hydroxides (8,9), and of cyanide complexed with Fe (II) (10). Practically all of these studies
have involved the precipitation and flotation of a single ionic species, including ionic
strength effects and the competition of other species; different species have been
progressively precipitated and floated (stage-wise) by pH variation, etc. No investigations
have been carried out of the simultaneous precipitation of two ions by s single precipitant,
followed by the co-flotation of the precipitates with a single surfactant at constant pH.
The objective of this work is to investigate experimentally the simultaneous
precipitation of orthophosphate and fluoride by lanthanum (La (III)) over an acidic pH
range (pH 3.5 to 6.0), followed by co-flotation of LaP04 plus LaF3 plus crystals containing
both orthophosphate and fluoride, with a single surfactant. The effects of pH, lanthanum
concentration, and surfactant concentration are discussed in terms of flotation results, in
terms of calculated and experimentally measured solution concentrations of the ionic
species of significance, and in terms of the characteristics of the precipitate particles.
Lanthanum has been reported as an orthosphosphate precipitant superior to Al (III), Fe
(III), and Ca (II) salts (11,12,13). In particular, at the same cation to orthophosphate
ratios. La (III) has yielded lower soluble orthophosphate concentrations (by as much as
three orders of magnitude) over a broader pH range compared to Al (III) (II).
Precipitation with Ca (II) necessitates an alkaline pH. La (III) also yields a small solubility
270