Comprehensive thermodynamic calculations have been carried out on the chalcocite-KEX-water system based on complete mass balance equations which include both soluble and insoluble species. The calculations have yielded i) E_h-pH stability diagrams for different KEX additions, ii) equilibrium concentrations and amounts of all the dissolved and insoluble species including those of CuX and CuX₂, iii) two- and three-dimensional plots showing the effect of E_h and pH on the formation of selected species, and iv) minimum xanthate additions required to form CuX and CuX₂. These information can be used as a guide in predicting the optimum conditions for flotation and leaching of chalcocite. The upper limiting potentials predicted from the thermodynamic calculations are in excellent agreement with those determined from the microflotation tests. The lower flotation edges, on the other hand, are found to be dependent on the sequence of reagent additions. When xanthate is added after the addition of a reducing or oxidizing agent to control the potential, they are in reasonable agreement with predicted values. When the collector is added prior to the potential control, however, the lower flotation edges are significantly higher than the predictions. The flotation experiments carried out at several different concentrations show that the minimum amount of the collector is required between 0 to 200 mv, as predicted by the thermodynamic calculations. Voltammetry experiments carried out in the absence of a collector at pH 9.2 and 6.8 suggest that the anodic oxidation of chalcocite results in the formation of Cu₂O, Cu(OH)₂ and S^O. At potentials below -400 mv, Cu₂S is reduced to Cu^O and HS⁻. When xanthate is added, several adsorption peaks are observed. There are indications that the peaks appearing between -200 and -100 mv may involve the reaction between xanthate and Cu^O. However, at potentials above -40 mv, xanthate may adsorb directly on chalcocite without involving Cu^O. / M.S.

Adsorption mechanisms of modified thiol collectors on gold, silver, and gold-silver alloys have been studied and compared with those of ethyl xanthate (EX). The modified thiol collectors include dicresyl monothiophosphate (DCMTP), dialkyl dithiophosphinate (DTPI) and monothiophosphinate (MTPI). In general, the adsorption mechanisms on silver and gold-silver alloys can be explained by the EC-mechanism involving an electron transfer step and a chemical reaction step. Thus, the adsorption should be controlled by the Eh of the electrochemical oxidation of the electrode involved and the pK of the metal collector complex. According to this mechanism, DCMTP should adsorb on silver and gold-silver alloys at a lower potential than DTPI and MTPI since the pK of silver-DCMTP complex is larger than those of silver-DTPI and silver-MTPI. This has been verified to be the case by voltammetry, FTIR and contact angle studies. Likewise, EX adsorbs on silver at a lower potential than the modified thiol collectors because the pK of silver-EX is larger than those of the silver-modified thiol collectors. Both EX and the modified thiol collectors adsorb on silver at lower potentials than on the gold-silver alloys, which can be attributed to the lower activity of silver on the alloy surface. For the same reason, the potential for the onset of collector adsorption on alloys decreases with increasing silver content. / Ph. D.

A study of the electrochemistry of the PbS-H₂O and PbS-KEX-H₂O systems has been made by carrying out thermodynamic calculations, electrochemical experiments and microflotation tests. Particular attention has been paid to how well this system can be described by equilibrium thermodynamics. The thermodynamic calculations are more comprehensive than previous ones of this type since they are based on a mass balance which includes both insoluble and soluble species. The data they provide include equilibrium concentrations of all dissolved species at any E_h and pH and an E_h-pH stability diagram for each collector addition. Also, two- and three-dimensional plots showing the effect of E_h and pH on xanthate uptake by the galena surface have been presented for the first time. These are particularly useful because they can be directly compared to observed flotation data. The results of voltammetry, IGP and potential-step experiments suggest that the oxidation of galena at pH 6.8 and 9.2 begins at a potential below the value predicted by bulk thermodynamics with the electrosorption of OH⁻ and the formation of a metal-deficient sulfide and a surface lead oxide. When oxidation becomes extensive enough, bulk products, Sº and PbO, begin to nucleate. Thiosulfate is detected at pH 9.2, but only becomes significant at high potentials. The electrochemical experiments indicate that xanthate adsorbs onto galena via a one-electron transfer chemisorption reaction in the first monolayer and via the formation of PbX₂ in subsequent layers. It also appears that galena oxidation and xanthate adsorption are competitive processes that tend to inhibit each other. Ground galena exhibits natural floatability at pH 9.2 as long as oxidation extends to the formation of a metal-deficient sulfide, but not to bulk PbO. When 10⁻⁵ M xanthate is added, the upper potential limit for flotation agrees well with the value predicted from thermodynamics for the decomposition of PbX₂. The lower limit, on the other hand, is at least 200 mv lower than any of the predicted values. PbS dissolves anodically at pH 1.1 and 4.6 to form Pb²⁺ and Sº first by a random surface process and then by a nucleation and growth mechanism once oxidation becomes extensive enough. At pH 0, the relation between the open-circuit potential and mineral solubility, as predicted by the thermodynamic calculations, agrees quantitatively with that determined experimentally. However, as the pH is increased to 1.1 and 4.6, the system becomes increasingly less reversible. / Ph. D.