31 P- and 65 Cu- solid state NMR studies of Cu(I) di-alkyl-dithiophosphates : complex Formation Mechanism on Synthetic Chalcocite Surfaces

Abstract: Alkyl substituted derivatives of dithiophosphoric acid are frequently used as collectors in froth flotation enrichment of copper sulfide ores. Due to interaction between Cu atoms at the surface and thiol collector molecules, a concomitant adsorption process occurs on the mineral surface. The presence of Cu(I) di-alkyl-dithiophosphate species on the Cu2S surfaces has been reported. However, to date, a more detailed description of the identification and composition of these species, has not been given and the processes leading to their formation are still not fully understood. In this work, polycrystalline Cu(I) di-alkyl-dithiophosphate complexes were synthesized and studied by 31P CP/MAS and static 65Cu NMR spectroscopy. Further investigation on crystal habits of these systems was carried out by powder X-ray Diffraction and Scanning Electron Microscopy. Copper(I) di-alkyl-dithiophosphate complexes exist only in the solid state and form tetra-, hexa- or octanuclear clusters with four or six bridging dithiophosphate ligands, respectively. A combined approach of the aforementioned techniques allowed the determination the type of Cu(I) dithiophosphate clusters. A cluster rearrangement process occurring during the recrystallization of these clusters in halogen-containing solvents is described. An equilibrium between different Cu(I) dithiophosphate (dtp) complexes and a solvent effect are discussed in terms of a specific donor- acceptor (D-A) interaction. Three different di-alkyl-dithiophosphates, potassium salts (KEt2dtp, KiPr2dtp and KiBu2dtp), were used for the investigation of the changes on synthetic chalcocite surfaces after collector treatment. The morphology of conditioned Cu2S surfaces was observed with SEM. A comparative analysis of the 31P chemical shift data estimated from the experimental NMR spectra of the polycrystalline Cu(I) dithiophosphate complexes was used for assigning the 31P resonance lines from the collector treated mineral surfaces. 31P chemical shift data (both isotropic chemical shifts and chemical shift anisotropy (CSA) data) of the species formed on the synthetic Cu2S surfaces allowed to assign these species to polycrystalline phases of: (i) Cu6L6 with L = Et2dtp¯, (ii) Cu4L4 with L = iPr2dtp¯ and (iii) Cu8L6(S) and Cu6L6 with L = iBu2dtp¯, respectively. A heterophase autocatalytic step process of the formation and growth of solid CuxLy (L= di-alkyl-dithiophosphate; x = 4, 6 or 8 and y=4 or 6) complexes on the surface of Cu2S is proposed. A possible role of the oxidized form of the dithiophosphate collector, bis[di(alkyl)thiophosphoryl] disulfane (disulfide), is discussed in terms of a specific self-redox interaction.