Flotation of monazite in the presence of calcite part II: Enhanced separation performance using sodium silicate and EDTA

Selective depression of calcite in a monazite-calcite flotation system was systematically studied using micro-flotation and electrokinetic tests as well as X-ray photoelectron spectroscopy (XPS) characterization and solution chemistry calculations. Monazite was found to be depressed by both sodium silicate and sodium hexametaphosphate when calcium ions appeared in solution. It was hypothesized that co-adsorption of calcium species (CaHCO₃ ⁺ and CaCO_3(aq)) and Si(OH)₄ as well as SiO(OH)₃ ^- via hydrogen bonding formed a compact layer over the monazite surface, thereby reducing octanohydroxamic acid adsorption by steric hindrance. The use of citric acid or EDTA formed soluble chelates (e.g. CaCit⁻ and CaEdta²⁻) with the calcium ions which negated the flotation suppression effect. An XPS study indicated that both EDTA and citric acid chemically adsorbed on the monazite surface, while the adsorption on the calcite surface was achieved mainly through hydrogen bonding. EDTA was more effective than citric acid due to two reasons: (i) citric acid itself presented stronger depression effects and (ii) citric acid has weaker chelating properties for calcium. Using 2.5 × 10⁻⁴ M octanohydroxamic acid, 0.05 g/L sodium silicate and 6 × 10⁻⁵ M EDTA, monazite recovery of more than 90% was achieved while recovering only 20% of the calcite.