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A fundamental study of the acidic pressure oxidation of orpiment and pyrite and high temperature Long, Hu

Abstract

The acidic pressure oxidation of pyrite (FeS2) and orpiment (As2S3), as a pretreatment of refractory gold ores, has been studied by investigating the reaction chemistry and kinetics. The effects of retention time, temperature, particle size, oxygen partial pressure, pulp density, sulfuric acid concentration, and cupric ions were evaluated for both minerals. The effect of ferric ions on the oxidation rate of As(III) to As(V) was also examined. Also, the effects of mole ratio of Fe/As on the oxidation of pyrite and orpiment mixtures were evaluated. During the acidic pressure oxidation of orpiment, most of the arsenic was found to be in the trivalent state after 2 hours oxidation at temperatures ranging from 170 to 230°C, and subsequent oxidation to As(V) with oxygen was very slow. Cupric ions do not affect the oxidation of As(III). However, the rate of oxidation of As(III) to As(V) is rapid in the presence of ferric ions. Sulfate becomes the predominant product species only at temperatures above 190°C. The oxidation kinetics are controlled by product layer diffusion with an Arrhenius activation energy of 22.2 kJ/mol (5.31 kcal/mol) over the temperature range of 170 to 210°C. However, as temperature increases above 210°C, the rate-controlling step switches to a surface chemical reaction with an Arrhenius activation energy of 50.0 kJ/mol (12.0 kcal/mol). The initial reaction rate is approximately -1/5 order with respect to pulp density, and 1/4 order with respect to oxygen partial pressure in the system studied. During the acidic pressure oxidation of pyrite, ferric, instead of ferrous, is the initial product at high temperatures. No elemental sulfur was found at temperatures above 190°C. An activation energy of 33.2 kJ/mol (7.94 kcal/mol) was observed over the temperature range 170 to 230°C. The reaction order with respect to oxygen partial pressure was found to be 1/2 at 210°C, which indicates an electrochemical mechanism of oxidation. The oxidation kinetics follow shrinking particle behavior. The surface chemical reaction is the rate-controlling step. The passivating shrinking sphere model was developed to represent the pressure oxidation of pyrite in the system studied at high temperature, based on the batch experimental data, which can be used to further develop a mathematical model to simulate continuous autoclave oxidation. During the acidic pressure oxidation of mixtures of pyrite and orpiment, As(III) generated from orpiment was oxidized rapidly to very low levels in the presence of pyrite or the Fe(III)/Fe(II) couple. However, the oxidation rate of ferrous decreased in the presence of orpiment. Iron arsenate was generated in all tests conducted for the mixture, which is of critical importance in view of the current controversy surrounding the environmental stability of these compounds. Elemental sulfur was produced only at low ratios of Fe/As. The pressure oxidation of orpiment is much slower than that of pyrite due to the formation of elemental sulfur. However, when the two minerals are mixed together, preferential leaching for orpiment is feasible due to galvanic effects. Thus orpiment oxidation is accelerated, while oxidation of the nobler pyrite is slowed.

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