The chemistry of flotation activation and depression of iron-containing sulphide minerals

Abstract: Thermodynamic calculations, electrochemical techniques, flotation and adsorption tests were used to study the physico-chemical principles involved in the flotation, activation and depression of the common ironcontaining sulphide minerals. Thermodynamic calculations on iron-xanthate-sulphur-water systems indicate that insoluble ferric hydroxo- xanthates can be obtained at a very low xanthate concentration in the neutral to weakly alkaline pH range, in which no liquid dixanthogen is formed. Dixanthogen is not the sole species for the flotation of iron-sulphides. Ferric (hydroxo-) xanthates play an important role in the flotation of the minerals. Good flotation of pyrite and other iron-containing sulphides occurs only when a monolayer of xanthate ion and/or dixanthogen is co-adsorbed on the ferric (di)hydroxo xanthate sites. Cyclic voltammetric investigations show that the initial oxidation of pyrite occurs simultaneously with the oxidation of hydroxyl ions. This facilitates the formation of iron hydroxide on the pyrite surface. The surface of pyrite oxidized at lower overpotentials is a sulphur-rich, surface but is covered with hydrophilic ferric hydroxide. The oxidation of pyrite at high overpotentials involves mainly sulphur and sulphide and is nearly independent of the previous history of the pyrite electrode surface. The initial oxidation of arsenopyrite surface produces ferric hydroxide and a realgar-like compound on the surface. At higher potentials, the oxidation of arsenopyrite results in elemental sulphur, arsenate and ferric hydroxide. Arsenate is adsorbed on the surface. The oxidation of arsenopyrite commences at lower potentials than that of pyrite. The separation of arsenopyrite from pyrite can therefore be achieved by selective oxidation of arsenopyrite. Flotation studies demonstrated that freshly ground arsenopyrite and pyrite are non-floatable in the absence of xanthate as a collector or strong iron-complexing agent: EDTA (hydrophilic). Good flotation was observed for both minerals with EDTA or ethyl xanthate. The floatability depends strongly on the pH value and concentration. With EDTA the minerals show good floatabilities in the pH range 6-10 for a concentrations <10-3M. Flotation is completely depressed for concentrations ≥10-2M. The principles for rendering the minerals hydrophobic by EDTA and xanthates are also discussed in this thesis. Studies of the activation of sphalerite, arsenopyrite, pyrite and pyrrhotite by Cu(II) ions by using adsorption tests and cyclic voltammetry showed that the activation reactions between the sulphides and copper(II) depend on the solution pH. The activation process in acidic media is essentially an electrochemical one. Sulphide activation by Cu(II) results in a sulphur-rich copper sulphide surface. In alkaline media, activation proceeds by a surface precipitation. The quantity of copper(II) adsorbed by the sulphides depends on the solution pH, Cu(II) concentration and oxygen content. In general, oxygen decreases the adsorption rate and the quantity of Cu(II) on the sulphide surface. The effect of oxygen on the activation of sulphide by Cu(II) decreases with an increase in pH value. Results from thermodynamic calculations for the cyanide-metalxanthate-sulphur systems and the literature show that the mechanisms of sulphide depression and deactivation by cyanide are extremely complex. Sulphide minerals are rendered hydrophilic and depressed by any of the following processes: the use of cyanide through adsorption of cyanometal complexes, dissolving metalxanthates and metal-sulphides, reducing the pulp potential to prevent chemical adsorption of xanthate ion and the formation and adsorption of dixanthogen, removal of heavy metal ions (activators) by forming cyano-metal complexes, and removal of elemental sulphur from the mineral surfaces.