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Illumination and the adsorption of xanthate in the flotation of galena and marmatite Guarnaschelli, Claudio


The changes in the adsorption characteristics of potassium ethyl xanthate (KEtX) on galena (PbS) and marmatite [(Zn,Fe)S] due to illumination have been investigated. Studies by others have indicated that: (1) the amount of surfactant adsorbed by a semiconductor depends on its n-type or p-type character, (2) copper activation of sphalerite (ZnS) is required for flotation, (3) sphalerite is a semiconductor with an energy gap of 3.6 eV whereas galena is a semiconductor with an energy gap of 0.3 7 eV. The amount of xanthate adsorbed from aqueous solution on galena and marmatite was found to depend on both the intensity and photon energy (0.5 to 3.5 eV) of the incident light. In the galena system, increasing the light photon energy above the energy gap value increased adsorption of xanthate. The amount of xanthate adsorbed by the p-type galena was three times the amount adsorbed by the n-type galena. This suggested that the reaction may involve the transfer of an electron from the adsorbate to the adsorbent. The effect of the presence of an oxide film on the surfaces of galenas was also investigated and appeared to be less significant than the type of charge carrier originally dominant in the mineral. When copper-activated marmatite was illuminated by light with photon energies lower than the intrinsic gap (3.6 eV), adsorption of xanthate was less than when the mineral was kept in darkness. Similar "photodesorption" effects have been reported in the literature. These were explained by excitation of electrons from traps to the conduction band and subsequent recombination with holes in the valence band. Fewer charge carriers would then have been available to participate in the adsorption reactions. Flotation experiments agreed with the adsorption results above. Flotation recovery of activated marmatite dropped ca. 10% when the mineral was illuminated with a high intensity of 0.6 eV photons as compared to the recovery in daylight. A model that takes into account the surface concentration of electrons and the type and concentration of impurities is discussed. The activity and selectivity of surface reactions are explained in terms of the electrochemical potential, i.e. the Fermi energy level, the actual position and/or displacement of which is affected by the impurities present.

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