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Author(s)

David Nestor Urquizo Valdívia

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2003-05-05

Examining board members

Arthur Pinto Chaves; Áurea Maria Dias Gomes; Jose Renato Baptista de Lima; Luciana Maria Souza de Mesquita; Gil Anderi da Silva

• Advisor: Arthur Pinto Chaves

Abstract

The increase in mining activity is leading to the exhaustion of the rich and easy-to-process mineral deposits. As a result, ores have now decreased in grades and increased in mineralogical complexity. Also, conscience of environment care is growing. All of this results in that mining and mineral processing costs are increasing. A great number of ore deposits is associated to sulphide minerals, especially arsenopyrite. In many cases, gold (and other precious metals) is inclosed inside the sulphide grains or as part of the crystal lattice. In such a situation they are not accessible to leaching agents - they are called \"refractory ores\". Fine grinding is not enough to provide satisfactory recoveries of gold. The techniques available to process ores of this kind are expensive, given the processing complexity. As a result, most of the times, low grade ores and/or small deposits are not economically feasible. So, increasing the recovery of ores more and more refractory and at the same time reducing production costs becomes a critical job. On the other hand, conventional oxidation processes, specially roasting, may lead to poor results in precious metal recovery, due to the formation of slags (from coal ashes) or of cianicyde compounds. Pressure leaching is a sophisticated process, demanding high pressures, corrosion-resistant materials and an oxygen factory. Biotechnology is an alternative and innovative proposal. It uses bacteria to break down the mineralstructure through oxidation and to free the metals - which will be recovered by conventional techniques. Biological oxidation processes are used to extract metals from sulphide concentrates. Other contaminats, like As, Sb, Cd and others, are also selectively solubilized. So are basic metals like Cu and they must be recovered through solvent extraction and electrowinning. Key to this tecnology is understanding the exact role of the bacteria which oxidizes Fe and ) S. Bio-oxidation mechanisms (direct and indirect) are important to describe the sulphide mineral solubilization. By the direct mechanism, the bacterium Acidithiobacillus ferrooxidans strongly adheres to the surface of the mineral, promoting bioleaching. By the indirect mechanism, the leaching of the sulphide occurs via reduction of the ferric ion in solution, which is reoxidized by the Acidithiobacillus. The contribution of each of these mechanisms depends on the mineral\'s characteristics and on operational conditions. Cell and mineral surface interaction depends on many physical and biochemical parameters. Bacterial adhesion to each mineral depends on the kind of growing substratum, substratum concentration and mineral\'s characteristics. It also depends on microorganism biochemical properties and growing conditions. On the other hand, the bacterium surface composition changes according to culture substratum. Some of the external membrane components, like proteins, may have an importantrole in adhesion and oxidation. In this thesis, the variables affecting the oxidation rate of a refractory gold pyrite concentrate have been studied. To begin with mineral the characteristics\' influence have been analyzed, especially, how mineral substratum concentration interferes on the metabolic activity of Acidithiobacillus ferrooxidans. Selected parameters effect have also been studied: pH, presence of ferric ions and flotation chemicals on bioleaching course. The metabolical characterization of the microorganism used makes easier the evaluation of the nechanisms. The most important variables found in a 12 L Airlift reactor are size of the inocullum, air flow, percent solids and residence time. Leaching time, cellular mass concentration, pH and gold extraction were the answer variable. Their detailed study contribute to a better comprehension of the oxidation kinetics. Statistical planning has been used to evaluate the effect of these variables and existing relations among then. (AU)