Ion foam flotation of neodymium: From speciation to extraction

Ion foam flotation of neodymium (Nd³⁺) was studied using a pH-sensitive surfactant, the nona-oxyethylene oleyl-ether carboxylic acid, which has the peculiarity of remaining water soluble in the presence of multivalent cations. The flotation efficiency factors were determined as a function of pH and for two different (surfactant/Nd) molar ratios, equimolar (1/1) or stoichiometric (3/1). An opposite trend was observed, showing low (<30%) and high (>95%) ion extraction yields respectively for equimolar and stoichiometric conditions. We show that i) the determination of the surfactant/Nd speciation solutions is essential to understand the very different behaviors between equimolar and stoichiometric experiments and ii) the ion extraction process is mainly driven by interfacial mechanisms. In terms of applications we show that once the ion extraction is efficiently performed for pH values between 5 and 7, the recovery of the surfactant is possible by performing a second flotation process at lower pH (2.5). Moreover ion foam flotation using this pH sensitive surfactant was shown to be highly efficient for the removal of traces and ultra-traces of metal ions. Therefore the results presented here suggest that the development of a continuous process, for example in the context of waste water treatment, is feasible. (authors)