A holistic approach to control and optimization of a PGM concentrate flash drying unit

Global trends stipulate the need to optimize existing assets while reducing environmental impacts. This paper illustrates the procedure that was followed to achieve such goals at one of the coal burning flash dryer units at an Anglo American smelter. An indepth study of the process and related literature led to the development and installation of a revised control philosophy. This system includes modifications to the existing regulatory control structure as well as a hybrid rule-based and model-predictive advanced process control (APC) layer. Since commissioning of the APC, this flash dryer’s average throughput has increased by more than 6%, despite higher feed moistures. Furthermore, even though coal consumption has increased slightly, operating efficiency has improved by close to 5%. This was made possible by improving stability of the drying column outlet temperature by approximately 40%, which in turn enabled selection of a more optimal setpoint. Recent data has shown that APC utilization now exceeds 95%. This is indicative of a successful controller installation with good site acceptance. The same philosophy has therefore since been rolled out to most of the other flash dryers in the group.http://www.saimm.co.za/ai201

Using MV overshoot as a tuning metric in choosing DMC move suppression values

Traditionally the tuning of dynamic matrix control (DMC) type multivariable controllers is done by trial and error. The APC engineer chooses arbitrary starting values and tests the performance on a simulated controller. The engineer then either increases the values to suppress movement more, or decreases them to have the manipulated variables move faster. When the controller performs acceptably in simulation, then the tuning is improved during the commissioning of the controller on the plant. This is a time consuming and unscientific exercise and therefore often does not get the required attention. This leads to unacceptable controller behaviour during commissioning and sub-optimal control once commissioning is completed. This paper presents a new method to obtain move suppression factors for DMC type multivariable controllers by using a Nelder Mead search algorithm to find move suppressions that will provide acceptable control behaviour. Acceptable behaviour is described by characterising the dynamic move plan calculated by the controller for each of the manipulated variables.http://www.journals.elsevier.com/isa-transactions/ai201

An investigation into lubrication and oxide breakdown during load-carrying capacity testing

Changes occurring in the friction and wear mechanisms during a load-carrying capacity test, lubricated with cetane containing a carboxylic acid, were investigated. The changes in wear scar/track appearance and oxide coverage/composition were analyzed during every load stage. The main conclusions were as follows: The breakdown in the protective oxide layer formed on the opposing steel surfaces was found to be the prerequisite for initiation of seizure. The seizure load achieved during load-carrying capacity testing quantifies the ability of the test fluid to prevent transition to the adhesive wear regime. The most severe surface damage was found to occur during the first few seconds after test initiation. Desorption of the adsorbed lubricant film and the subsequent removal of the naturally occurring thin oxide layer results in the initial period of adhesive wear. Partial recovery to a state of acceptable friction occurs after the period of initial seizure. During this period, the surface coverage by the adsorption lubricant molecules and the oxide coverage are sufficient to prevent severe adhesive wear from occurring. Wear is primarily a combination of oxidative, abrasive, and fatigue wear (all possible in the regions of mixed friction and boundary lubrication). Final lubricant breakdown and eventual seizure are initiated when the oxide removal rate exceeds the oxide formation rate resulting in severe adhesive wear followed by seizure.http://www.tandfonline.com/loi/utrb202015-09-30hb201

Failure analysis on retrieved ultra high molecular weight polyethylene (UHMWPE) acetabular cups

Retrievals obtained after revision surgery, provide valuable information on the rate of wear and the causes of wear that take place in vivo. The aim of this study was to perform a root cause failure analysis to determine the principle reason for mechanical failure on failed acetabular cups, retrieved during revision surgery, by making use of sound engineering failure analysis techniques. The bearing couples varied between steel on UHMWPE and ceramic on UHMWPE. The information gained during this analysis brought to the fore that the principle mode of mechanical failure for acetabular cups is overheating of the UHMWPE. The resulting overheating is mainly due to a lack of good and/or sufficient lubrication

Alkyl ammonium intercalation of Mozambican bentonite

Organo-bentonites containing quaternary ammonium surfactants were prepared using purified bentonite from the Boane deposit in Mozambique. The intercalation was effected by intimate mixing of the surfactants into a concentrated dispersion of soda ash activated bentonite. TG indicated organic contents that exceeded CEC expectations by up to 21 mass %. XRD results were consistent with: (i) paraffin-type extended chain intercalation at tilt angles of 27° to 33°; and (ii) interdigitated monolayer intercalation of the C14 single alkyl chain surfactant and bilayer intercalation of the C16 double alkyl chains and its 1:1 mixture with the single chain surfactant. However, FTIR analysis revealed disordered alkyl chain conformations. White oil dispersions containing 10 mass % organo-bentonite (inorganic basis) featured high viscosities. They showed strong shear thinning behaviour confirming the ability of the montmorillonite particles to exfoliate.Financial support for this research from the Institutional Research Development Programme (IRDP) and the South Africa/Mozambique Collaboration Programme of the National Research Foundation (NRF) is gratefully acknowledged