Foam separation of complex anions: Silver thiosulfate

An experimental study is conducted of the ion flotation of Ag(S₂O₃)– and Ag(S₂O₃)₂³– from an aqueous solution at pH 4.5 with a cationic surfactant. For initial solutions 2.0 × 10⁻³ to 8.0 × 10⁻³ M in total silver, optimum silver flotation at foam cesssation is achieved at a molar thiosulfate to silver ratio (Th/Ag) of 0.75 and surfactant to silver ratio (EHDA/Ag) of 0.5, at Th/Ag = 1.0 and EHDA/Ag = 1.1, and at Th/Ag = 2.0 and EHDA/Ag = 2.2. The silver flotation is 99⁺, 98⁺, and 92⁺ % at the three sets of ratios, respectively. At Th/Ag > 2.0, highly efficient flotation cannot be achieved due to decreased particle size, and perhaps by competition with free S₂O₃²⁻ for the surfactant. The stoichiometry of the ion flotation product is established from relative silver and surfactant flotation rates. The stoichiometry is independent of foaming time, but is a strong linear function of EHDA/Ag and Th/Ag, in contrast to the stoichiometry of flotation products of other simple and complex anions. Rate data can be fit reasonably by a first‐order reversible model. The rate constant is an inverse function of EHDA/Ag, indicating the desirability of pulsed surfactant addition. Results are discussed in terms of the average ligand number of silver, particle size, and surface potential measurements.