The development of a semi-empirical electrowinning model to predict process performance

Tucker, Mandy (2019-12)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Electrowinning is the final step in the hydrometallurgical production of high purity copper and comprises passing an electric current through a copper-containing electrolyte to plate solid copper onto a cathode. Key electrowinning performance indicators are current efficiency, specific energy consumption, yield and metal quality. The high energy demand and associated cost make performance determination critical during operation, but online measurement is impractical due to the delayed nature of the measurements and corrosive environment caused by acid mist. The current manual approach to process control in industrial tankhouses requires improvement, through the shift towards a pre-emptive approach to attaining plant performance data. The development of a semi-empirical electrowinning model to predict process performance was considered in this research as a first step towards a dynamic model and the implementation of control in electrowinning practice. The objectives were to develop a model to predict electrowinning performance, to develop a parameter fitting approach to calibrate the model to bench-scale experimental data, and to apply the model to an industrial operation. The scope entailed a steady state model to predict current efficiency, specific energy consumption and solid copper yield based on operational and geometrical input variables. Model development constituted the design of a conceptual circuit diagram of an electrowinning cell consisting of up to hundreds of parallel pairs of electrodes, hardware and electrolyte resistances and a current loss parameter. The electrochemical reactions incorporated were copper reduction, water evolution and the cyclic reduction and oxidation of ferric and ferrous ions as an impurity. The model was coded in MATLAB through a first principles approach, combining a series of reaction rate and mass transfer kinetics, mass balances, electrochemical and thermodynamic equations and property correlations. The parameter fitting approach comprised the design of bench-scale experiments in which the input copper, sulphuric acid and iron concentrations, and current density were varied. The experimental data were used to calibrate parameters (for reaction and mass transfer rates and current loss) to the model through nonlinear regressions. The experiments revealed a constant rate of plating over time which validated the steady state assumption. Average current loss over the bench-scale experiments was 0.145 A (about 1 - 5% of total current), accounting for current loss due to stray currents, ineffective electrode contact and possible side reactions. The rate kinetics parameters fit relatively well to the experimental data, with an R2adj of 0.864 for copper reduction, 0.739 for water oxidation, 0.724 for iron reduction and 0.661 for iron oxidation. While the performance data for different industrial tankhouses were scattered, the electrowinning model accurately predicted the performance of the bench-scale setup, demonstrating the potential of the model to accurately predict performance in an electrowinning system with specifically fit parameters. The average absolute errors between the model and experimental data were 3.2% for current efficiency, 3.0% for specific energy consumption and 7.0% for copper plating rate. The model could be used directly for operator training or combined with the parameter fitting approach as a first step towards process control in an industrial electrowinning tankhouse.

AFRIKAANSE OPSOMMING: Elektroherwinning is die finale stap in die hidrometallurgiese produksie van hoë suiwerheid koper en behels die vloei van ʼn elektriese stroom deur ʼn elektroliet wat koper bevat om vaste koper op ʼn katode te plateer. Sleutel elektroherwinning werkverrigting aanwysers is stroom doeltreffendheid, spesifieke energie verbruik, opbrengs en metaal kwaliteit. Die hoë energie vereiste en meegaande koste maak die bepaling van doeltreffendheid krities gedurende bedryf, maar aanlynmeting is onprakties as gevolg van vertraging van die afmetings en korroderende omgewing veroorsaak deur suurmis. Die huidige handbenadering om die proses te beheer in industriële tenkhuise vereis verbetering, deur die skuif na ʼn voorkomende benadering om data van aanlegdoeltreffendheid te verkry. Die ontwikkeling van ʼn elektroherwinningmodel om prosesdoeltreffendheid te voorspel is oorweeg in hierdie navorsing as ʼn eerste stap na ʼn dinamiese model en die implementasie van beheer in elektroherwinningpraktyk. Die doelwitte was om ʼn model te ontwikkel wat elektroherwinning se doeltreffendheid voorspel, om ʼn parameter-passing-benadering te ontwikkel om die model met banktoetsskaal eksperimentele data te kalibreer, en om die model op ʼn industriële bedryf toe te pas. Die omvang het ʼn bestendige toestand model bevat om stroomeffektiwiteit, spesifieke energie verbruik en vastestof koper opbrengs op bedryfs- en geometriese inset veranderlikes te voorspel. Modelontwikkeling het die ontwerp van ’n konsepsionele stroombaandiagram van ʼn elektroherwinningsel behels, wat bestaan uit tot en met honderde parallelle pare elektrodes, hardeware en elektroliet weerstande en ʼn stroomverlies parameter. Die elektrochemiese reaksies geïnkorporeer was koperreduksie, waterevolusie en die sikliese reduksie en oksidasie van ferri- en ferro-ione as ʼn onsuiwerheid. Die model is gekodeer in MATLAB deur ʼn eerste beginsels-benadering, wat ʼn reeks reaksietempo en massa-oordragkinetika, massa-balanse, elektrochemiese en termodinamiese vergelykings en eienskap korrelasies, kombineer. Die parameter-passing-benadering het die ontwerp van banktoetsskaalekperimente behels, waarin die inset koper-, salpetersuur- en ysterkonsentrasies, en stroomdigtheid gevarieer het. Die eksperimentele data is gebruik om parameters te kalibreer (vir reaksie en massa-oordragtempo’s en stroomverlies) na die model deur nie-liniêre regressies. Die eksperimente het ʼn konstante tempo van platering oor tyd bekend gemaak, wat die bestendige toestand aanname valideer. Gemiddelde stroomverlies oor die banktoetsskaaleksperimente was 0.145 A (omtrent 1–5% van totale stroom), wat verantwoording doen vir stroomverlies as gevolg van swerfstrome, oneffektiewe elektrode kontak en moontlike newereaksies. Die tempokinetika parameters pas relatief goed met die eksperimentele data, met ʼn R2adj van 0.864 vir koperreduksie, 0.739 vir wateroksidasie, 0.724 vir ysterreduksie en 0.661 vir ysteroksidasie. Terwyl die doeltreffendheiddata vir verskillende tenkhuise onreëlmatig was, het die elektroherwinningmodel die doeltreffendheid van die banktoetsskaalopset akkuraat voorspel, wat potensiaal vir die model om doeltreffendheid akkuraat te voorspel in ʼn elektroherwinningsisteem met spesifieke gepaste parameters, demonstreer. Die gemiddelde absolute afwyking tussen die model en eksperimentele data was 3.2% vir stroomdoeltreffendheid, 3.0% vir spesifieke energie verbruik en 7.0% vir koperplateringtempo. Die model kan direk gebruik word vir bedryfsopleiding of gekombineer word met die parameter-passing-benadering as ʼn eerste stap na prosesbeheer in ’n industriële elektroherwinning tenkhuis.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107241
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