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E_h-pH diagrams were constructed from mass-balanced, computer calculations for the copper-sulfur-water system involving different Cu/S ratios that pertain to chalcocite, djurleite, anilite and covellite. Calculations were completed for cases where oxidation of the sulfur proceeded to i) elemental sulfur, ii) thiosulfate, iii) sulfate and iv) destabilized sulfate. Stability regions for each copper sulfide were shown to be dependent on both the Cu/S ratio in the system and the sulfur oxidation state. E_h-pH diagrams were also constructed for chalcocite oxidation to metastable copper sulfides, both with and without xanthate. Stability regions for copper xanthates were also shown to be dependent on the sulfur oxidation state. As oxidation proceeded from elemental sulfur to sulfate, the copper xanthate stability region extended to lower potentials, directly dependent on the sulfide ion concentration. IGP experiments at pH 1.1 suggested that chalcocite oxidation produced metastable nonstoichiometric copper sulfides while cyclic voltammetry indicated they formed at pH 1.1, 4.6, 6.8 and 9.2. XPS implied that copper sulfides may be solid solutions of chalcocite with variable amounts of copper disulfide: CU₂S·xCUS₂. The presence of djurleite in the chalcocite samples was confirmed by X-ray diffraction and may be responsible for the reduction reaction which occurred just prior to the reduction of chalcocite to metallic copper. Reinterpreting cyclic voltammograms from a previous study indicated chalcocite reacted with xanthate to form cuprous xanthate and a nonstoichiometric copper sulfide near 0 mV. Chemisorbed xanthate formed at -295 mV which correlated well with the lower flotation edge determined in this and other studies. The standard free energy of the chemisorbed xanthate was determined to be -13.08 kcal/mole. / Master of Science

A study was conducted of CuₓS and its interaction with the substrate and ambient. The goals of these CuₓS on CdS and Zn_yCd_1-yS substrates were to find the differences in materials related properties, if any. Cadmium and zinc compositions in CuₓS formed on Zn_yCd_1-yS films (O < y < 0.25) by means of ion exchange were measured using Auger Electron Spectroscopy (AES), Atomic Absorption Spectroscopy (AAS), and Electron Spectroscopy for Chemical Analysis (ESCA). Net concentrations of Cd and Zn in as-formed Cu₂S are generally in the 10¹⁸-10¹⁹ cm⁻³ range. Heat treatments in both oxidizing and reducing ambients raise the concentrations by over an order of magnitude, with the Zn concentration increasing more so than those of Cd. Large increases in Zn at or near the CuₓS surface were measured subsequent to heat treatment, accompanied by increased oxygen. Following heat treatments, Cd and Zn concentrations in the CuₓS"bulk" are found to be less than 10¹⁹ and 10²⁰ cm.⁻³, respectively, for all substrate compositions used. It is proposed that the presence of Cd and Zn can adversely effect the minority carrier lifetimes. These effects would tend to reduce the light generated current. The effects of heat treating CuₓS/Zn_yCd_1-yS and CuₓS/CdS in reducing and various oxidizing ambients are also reported. Structural changes taking place in CuₓS as a result of these heat treatments were monitored by using x-ray diffraction. The principal physical mechanism responsible for phase changes in CuₓS appears to x be copper diffusion through the copper sulfide layer to the top surface as well as into the substrate. Changes in CuₓS stoichiometry were correlated with the sheet resistance of the CuₓS layer. Results indicate that heat treatment in a hydrogen atmosphere causes an increase in resistivity (corresponding to an increase in stoichiometry) while heat treatment in air causes the reverse effect. Wet air heat treatment tended to decrease the resistivity much more as compared to dry air. It was observed that CuₓS formed on Zn_yCd_1-yS tended to degrade in stoichiometry much faster as compared to CuₓS formed on CdS. The resistivity of evaporated CuₓS on plain glass seemed to be linked to the amount of free copper and sulfur present in the as-deposited film. Argon heat treatment tended to decrease the resistivity by an order of magnitude. Heat treatment tended to react the free copper and sulfur, giving CuₓS. Free copper and sulfur can increase the resistivity by acting as neutral impurity scattering centers. As-deposited films were always Cu rich as evidenced by x-ray diffraction and EDAX. Argon heat treatment tended to decrease the amount of free copper present. X-ray photoelectron spectroscopy (XPS) was applied to the surface chemical characterization of chemiplated CuₓS on Zn_yCd_1-yS and CdS. CuₓS was also vacuum evaporated onto glass substrates for this purpose. The effects of ambient (oxygen and water vapor in particular) on chemical species present at or near the CuₓS surface were investigated. Subsequent heat treatments of CuₓS/Zn_yCd_1-yS and CuₓS/CdS promoted migration of Zn and Cd toward the Cu₂S surface. When formed on CdS, the CuₓS surface was found to contain CdO (or Cd (OH)₂) CuO, CuSO₄.nH₂O and CdSO₄.nH₂). Cu₂S formed on Zn_yCd_1-yS was found to contain ZnO as the predominant chemical species, with the Cu and Cd compounds present in lesser amounts. Some interesting characteristics of powder standards used in the XPS studies, some of which have not appeared in the literature, are presented in Appendix 2. The above effects can account for key differences in the properties of CuₓS formed on Zn_yCd_1-yS and CdS films. This provides information on the possible degradation mechanisms for these types of junctions. / Ph. D.

Sulfide minerals are a major source of metals; however, certain sulfide minerals, such as pyrite and pyrrhotite, are less desirable. Froth flotation is a commonly used separation technique, which requires the use of several reagents to float and depress different sulfide minerals. Xanthate, a thiol collector, has gained immense usage in sulfide minerals flotation. However, some sulfides are naturally hydrophobic and may float without a collector. Iron sulfides, such as pyrite and pyrrhotite, are few of the most abundant minerals, yet economically insignificant. Their existence with other sulfide minerals leads to an inefficient separation process as well as environmental problems, such as acid mine drainage during mining and processing and SO2 emissions during smelting process. A part of the present study is focused on understanding their behavior, which leads to undesired flotation and difficulties in separation. The major reasons for the undesired flotation are attributed to the collectorless hydrophobicity and the activation with heavy metal ions. To better understand the collectorless hydrophobicity of pyrite, Electrochemical Impedance Spectroscopy (EIS) of freshly fractured pyrite electrodes was used to study the oxidation and reduction of the mineral. The EIS results showed that the rate of reaction increases with oxidation and reduction. At moderate oxidizing potentials, the rate of reaction is too slow to replenish hydrophilic iron species leaving hydrophobic sulfur species on the surface. However, at higher potentials, iron species are replaced fast enough to depress its flotation. Effects of pH and polishing were also explored using EIS. Besides collectorless hydrophobicity, the activation of pyrrhotite with nickel ions and interaction with xanthate ions makes the separation more difficult. DETA and SO2 are commonly used as pyrrhotite depressants; however, the mechanism is not very well understood. Contact angle measurements, cyclic voltammetry and Tafel studies have been used to elucidate the depressing action of DETA and SO2. It was observed that DETA and SO2 complement each other in maintaining lower pulp potentials and removing polysulfides. DETA also helps in deactivating pyrrhotite. Therefore, the combined use of DETA and SO2 leads to the inhibition of both the collectorless flotation and the adsorption of xanthate. The adsorption of xanthate on sulfide minerals is a mixed-potential mechanism, i.e., the anodic oxidation of xanthate requires a cathodic counterpart. Normally, the cathodic reaction is provided by the reduction of oxygen. However, oxygen can be replaced by other oxidants. Ferric ions are normally present in the flotation pulp. Their source could be either iron from the grinding circuit or the ore itself. The galvanic studies were carried out to test the possibility of using ferric ions as oxidants and positive results were obtained. Tafel studies were carried out to measure the activation energies for the adsorption of ethylxanthate on several sulfide minerals. Pyrite, pyrrhotite (pure and nickel activated), chalcocite and covellite were studied in 10-4 M ethylxanthate solution at pH 6.8 at temperatures in the range of 22 – 30 0C. The Tafel studies showed that xanthate adsorbs as dixanthogen (X2) on pyrite and pyrrhotite, nickel dixanthate (NiX2) on nickel-activated pyrrhotite and cuprous xanthate (CuX) on both chalcocite and covellite. However, the mechanism for xanthate adsorption on each mineral is different. The free energy of reaction estimated from the activation energies are in good agreement with thermodynamically calculated ones. / Ph. D.