Adsorption of sulphite, oleate and manganese(II) ions by β‐manganese dioxide and its activation in flotation

A high-grade ore sample A and a synthetic sample B of β-MnO2 were characterised by a number of physical methods. The points of zero zeta potential were found at pH4.4 and 4.6 for A and B, respectively. Below the critical micellar concentration and at pH 7, the adsorption isotherms of oleate on to sample B followed the Freundlich equation, the adsorption density decreasing with rising temperature. Over a wider concentration range, the isotherm had a Langmuir shape and was displaced to higher adsorption densities in presence of NaCl. The adsorption density was slightly affected by pH, but it gradually decreased beyond pH 9. From a comparison of the adsorption and ζ potential results, the adsorption mechanism could not be electrostatic only but may also involve van der Waals and π interactions. The adsorption isotherm of sulphite ions exhibited a broad maximum and was shifted to lower adsorption densities in presence of oleate. The amount of Mn2+ ions released on conditioning sample A in sulphite solutions increased with increasing sulphite concentration. It could be assumed that sulphite ions were adsorbed prior to reaction with MnO2 producing Mn2+ ions, which were the activating species in flotation. Although there was an adsorption competition between sulphite and oleate ions, the adsorption of the latter was enhanced in presence of sulphites. The adsorption density of Mn2+ ions on to sample B increased linearly with increase in their equilibrium concentration. Assuming the low-coverage form of the Langmuir isotherm, the heat of adsorption was found to be 51.9 kJ/mole. The adsorption density of Mn2+ ions increased with increasing the pH of the medium from 5 to 6.5, in agreement with the more negative surface charge on the MnO2 particles. Above a certain Mn2+ concentration, the charge was reversed. The adsorption of Mn2+ ions is interpreted in terms of an Na+/Mn2+ exchange mechanism. Tentatively, the cations are assumed to be in the outer Helmholtz layer of the electric double layer of MnO2. The activating effect of Mn2+ cations was demonstrated by the increased adsorption density of oleate in presence of Mn2+ ions and by the corresponding increase in flotation recovery. Oleate adsorption was exothermic, i.e. similar to its adsorption in absence of activating cations. It is assumed that activation involves electrostatic attraction between the activating Mn2+ and the oleate ions.