Browsing by Author "Richardson, P.E."

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    A comprehensive study of the electrochemistry and floatibility of pyrite in coal flotation
    Tao, Dongping (Virginia Tech, 1994)
    Pyrite (FeS₂) is the major source of sulfur in various coals, and its efficient removal has proven to be a more difficult task than expected. Flotation is generally considered to be the most practicable process for the preparation of coal fines. However, even this technique is usually unable to remove more than 50% of pyrite from a 65-mesh coal sample, which is the typical feed to flotation. There are three major reasons for the low separation efficiency of liberated pyrite from coal by flotation. They include self-induced hydrophobicity of pyrite caused by superficial oxidation, nonselective hydraulic entrainment of pyrite particles into froth product, and incomplete liberation of pyrite from coal that results in composite coal-pyrite particles, i.e., middlings. The present study was undertaken to address problems associated with these recovery mechanisms of pyrite and develop techniques to enhance pyrite rejection in coal flotation. To better understand self-induced hydrophobicity of pyrite, chronoamperometry and voltammetry on freshly fractured electrodes were used to explore incipient oxidation and reduction of the mineral. Voltammetry on rotating ring-disc electrodes (RRDE) was carried out to provide information on soluble species and kinetics of oxidation and reduction processes. X-ray photoelectron spectroscopy (XPS) was used for chemical identification of oxidation products. Galvanic coupling with sacrificial anodes was investigated as a practical method to cathodically protect pyrite and prevent its oxidation. Microflotation tests were conducted under controlled potentials at different solution pH's, and the results were correlated with electrochemical studies. The feasibility of improving pyrite rejection by controlling its surface chemistry was tested in flotation experiments conducted with a 2"-diameter microbubble flotation column and a conventional 5-liter Denver flotation cell. Effects of froth stability on the microbubble flotation of coal were studied with an objective of minimizing hydraulic entrainment of pyrite. The operating parameters were systematically varied to study their effects on water recovery which was used as a measure of froth stability. It has been demonstrated that the upgrading of coal in a flotation column can be significantly improved when froth stability is properly controlled. In an attempt to enhance the rejection of pyrite in middlings, various column circuits were experimentally examined and theoretically analyzed. The effect of circuit configuration on the overall circuit performance was evaluated by separation efficiency and separation curves. It has been shown that the overall separation efficiency of column flotation is rather insensitive to circuitry due to the unique characteristics of the unit flotation column, i.e., the addition of the wash water into the froth.
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    Thermodynamic and kinetic studies of galena in the presence and absence of potassium ethyl xanthate
    Pritzker, Mark David (Virginia Polytechnic Institute and State University, 1985)
    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 Eh and pH and an Eh-pH stability diagram for each collector addition. Also, two- and three-dimensional plots showing the effect of Eh 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.