| Summary: | 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. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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