H to infinity optimal control of a counter-current process
Master Thesis
1991
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University of Cape Town
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Abstract
The extraction of gold from ore that has been mined is the most important part of the process which eventually produces gold bullion. The process most commonly used today is that of carbon-in-pulp gold extraction (CIP). One of the main reasons for this is that it is the most economically efficient method of extracting gold from ore. The process uses activated carbon to absorb gold from a cyanide leach solution. Slurry containing the gold bearing ore and the activated carbon flow in a counter-current fashion. This counter-current flow enables a high percentage of the gold to be recovered. Gold can then be recovered through an elution process. Large amounts of activated carbon are used in the process and a formal multi variable control study of the adsorption section of the CIP process could provide further economic savings by extracting more gold with controlled amounts of carbon. A study was performed to identify the chemical mechanisms involved in the adsorption section of a CIP plant. It was felt that the workings of the process could best be established by designing a simple simulator of the process. The simulator was designed with four reactor tanks, in which the carbon absorbs gold from the leached slurry. The simulator uses a continuous transfer of carbon. In order to fully understand the operation of the HERIG, a simulation study was performed. This simulated model was a simplified version of the actual rig. The level changes of COLD water in the tanks were assumed to be instantaneous and the heat transfer coefficients were assumed to be the same for all four of the tanks. The calculation of the heat transfer coefficients was investigated thoroughly and care was taken to obtain accurate values. The simulator designed was a lumped parameter model. The pipes containing the HOT stream were divided into many small section, in each of which a constant temperature was assumed. A sum of the contributions of all the sections submerged under the COLD water was used to calculate the heat transferred into the COLD water. The COLD water in the tank is stirred continuously and is assumed to be at a constant temperature. The level of COLD water in each tank on the HERIG represents the mass of carbon in each of the tanks on a CIP plant. A change in the HOT water pipe temperature (concentration of Au in slurry) was examined as a function of a change in the level of COLD water in a tank (mass of carbon in a reactor). A steady state and dynamic analysis verified that trends observed from the CIP model were in fact mimicked by the trends observed on the HERIG. It was then decided to perform a formal control study of the HERIG, since the numerous similarities found between the CIP and HERIG enabled relevant conclusions to be drawn about the control of CIP from the control of the HERIG.
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Bibliography: leaves 156-160.
Reference:
Olivier, B. 1991. H to infinity optimal control of a counter-current process. University of Cape Town.