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Understanding the heap biooxidation of sulfidic refractory gold ores Bouffard, Sylvie

Abstract

An investigation has been conducted into the nature and rates of the physical, chemical, biological, and thermal processes involved in the heap biooxidation of pyrite from refractory gold ores. A heap-scale model of the ideal process was developed, aided by a systematic experimental approach, which accounts for the following phenomena. Grain-Scale Kinetics - The thermal and chemical functionals driving the oxidation kinetics of the pyritic ore sample were modeled from batch, potentiostatic stirredtank leaching tests using a pyrite concentrate prepared by flotation from the bulk ore. Particle-Scale Kinetics - The influence of diverse pyrite occurrences within ore particles classified into six size fractions were quantified from isothermal, potentiostatic, upflow, packed bed experiments. Bacterial Kinetics and Dynamics - Substrate (ferrous ions or elemental sulfur) oxidation and growth of iron- and sulfur-oxidizing cells were modeled over three specific temperature ranges with a dual, limiting-substrate Monod expression, coupled with temperature-dependent death rates. Reversible attachment of a predominantly attached population with few planktonic cells was modeled using a Langmuir isotherm. Biological parameters were either measured or estimated from small and large column leaching data, and were found to be in good agreement with published values. Solute Dynamics - The backbone structure of the heap model was represented as stagnant pores of uniform or variable lengths, which are connected at one end to plug flow channels, and which are also in intimate contact with a uniformly distributed gas stream. Volumetric proportions of solid, liquid, and gas were measured in unsaturated columns under several conditions (binder addition, agglomeration, particle size, column height, irrigation rate). Pore lengths were estimated from tracer residence time distribution curves. Heat Model - A published heat model, comprised of heat conduction, generation, and advection by liquid, dry air, and vapor, coupled with climatically-dependent boundary conditions, was grafted onto the main model framework. These elements were integrated into an unsteady-state system of non-linear partial differential equations, solved numerically with an explicit approach for chemical and biological reaction rates, and implicit finite difference approximations for concentrations. Small and large column tests were performed with the same pyritic ore to estimate unknown biological parameters, to validate the model at the small scale, and to ascertain the influence of several operating factors on depth and lateral profiles of conversion (pyrite and elemental sulfur), concentrations, and temperature. Excellent fits of several types of leaching indicators reveal the rate-limiting step to shift from particle kinetics to oxygen gas-liquid mass transfer with increasing temperatures, particle kinetics, and head grade, as well as decreasing mass transfer coefficient. According to the model simulations, large pellets made up of rapidly-oxidizable pyrite leach zone-wise as a result of the rapid consumption of oxygen in meager concentrations within the pellet pores. Shorter heaps and large irrigation and aeration rates are suitable conditions for homogeneous leaching in heaps, and for avoiding temperature segregation and the establishment of overheated dead zones.

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