Dynamic simulation of the leaching and adsorption sections of a gold plant
This dissertation describes the development of a dynamic simulator for the leaching and adsorption sections of a gold plant. In contrast to the milling stage which precedes the leaching and which is a purely mechanical process, leaching and adsorption are hydrometallurgical processes which are of particular interest to chemical engineers. Leaching is a well-defined chemical processes in which gold is dissolved out of the rock by reaction with cyanide ions. The leaching process occurs in a series of stirred tank reactors and is easily modelled. The adsorption process is far more challenging to model. The adsorption occurs on carbon particles which are mixed into the pulp and this gives rise to the name carbon-in-pulp (CIP). The actual adsorption of the gold cyanide complex on the lattice structure of the carbon particles is a surface phenomenon which, while it has not been totally defined, can yet be described by conventional rate processes. The adsorption also takes place in a cascade of stirred tank reactors, but the occasional pumping of carbon up the cascade and the resulting counter-current movement of carbon and pulp present modelling challenges. A dynamic simulator was regarded necessary for this process to determine what the short and long term effects of process disturbances are. While steady state models have been developed before, they are not able to describe the transient responses to such changes. Disturbances are all too common on an operating plant and as a result the plant never truly reaches a steady-state. Any control strategy for the plant must necessarily be developed by taking the transient responses into consideration. Another requirement was for the simulator to be flexible enough to be adapted quickly to various plants. It was also to be able to read in any applicable and easily available information from plant data files and to use the data to recreate reasonably accurate outputs. The simulator is written as a collection of ordinary differential equations each of which is a mole balance of one of the components (or state variables) in the system. The mole balances include the effect of chemical reactions between the various reactants describing the production and depletion of these components. The hydrodynamics of the bulk pulp phase are also accounted for by considering the amount of all components within process units and the movement of components between the units. Various factors affecting the two sections of the plant have been investigated, most of which have been considered in theory or were included in simulators by earlier investigators. Some aspects, such as attrition and screen overflows, have been included in a dynamic simulator for the first time. Attrition was found to have a major effect on the efficiency of the adsorption process by levelling out the gold solution profile and thereby reducing the rate of loading on coarse carbon. Other inefficiencies are the result of unsteady operation, especially of wildly fluctuating feed flowrates which make the addition of reagents difficult to control, and various process upsets in the CIP such as screen breakages and overflows, which allow loaded carbon to move downstream with the pulp.
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