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

Thesis (MEng)--Stellenbosch University, 2019.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.Master

Production Scheduling in Copper Electrorefining and Electrowinning Plants with Manufacturing Execution Systems

Copper electrolysis process is used to produce pure, over 99.99% copper with the help of electricity. Production dispatching and scheduling in copper electrolysis processes have been done mostly manually and without direct link to automation systems. Disruptions in the production processes usually require new decisions from production planners and changes to production schedules. Manual transfer of information is prone to errors and the transfer process usually takes longer time. In copper electrolysis production scheduling is put into practice by controlling the cranes, which are handling all the material transfers in the plant. One of the objectives of this thesis was to research how to implement new production dispatching features to the existing manufacturing execution system. Theoretical part of this thesis consists of introduction of suitable technologies and standards that could be considered as basis for the empirical research. The foundation for a fully automatic production scheduling and dispatching is a reliable communication between the manufacturing execution system and the automation system. This thesis work contains a review of multiple standards and ready-made commercial products that support these dispatching operations. The empirical part of this thesis consists of design, implementation and testing of a daily production schedule software for cranes. The implementation includes communication and data collection infrastructure but all user interface components are out the scope of this thesis. Also the high level fully automatic scheduling workflow was designed in a high level but was not implemented during this project. This implementation leveraged new products in the field of software technology. Suitability of these products for this and similar projects were evaluated. Most of the problems faced during the implementation were related to a commercial software platform selected for the implementation. Therefore it was suggested that the further use of the commercial product will be put on hold until these problems get fixed by possible software updates. Further development is related to the fully automatic scheduling and also for the development of the user interface

High-Efficiency Three-Phase Current Source Rectifier Using SiC Devices and Delta-Type Topology

In this dissertation, the benefits of the three-phase current source rectifier (CSR) in high power rectifier, data center power supply and dc fast charger for electric vehicles (EV) will be evaluated, and new techniques will be proposed to increase the power efficiency of CSRs. A new topology, referred as Delta-type Current Source Rectifier (DCSR), is proposed and implemented to reduce the conduction loss by up to 20%. By connecting the three legs in a delta type on ac input side, the dc-link current in DCSR can be shared by two legs at the same time. To increase the switching speed and power density, all-SiC power modules are built and implemented for CSRs. The switching waveforms in the commutation are measured and studied based on double pulse test. Four different modulation schemes are compared for high efficiency CSR considering the switching characteristics of different device combinations. The most advantageous modulation scheme is then identified for each of the device combinations investigated. A compensation method is proposed to reduce the input current distortion caused by overlap time and slow transition in CSRs. The proposed method first minimizes the overlap time and then compensates the charge gain/loss according to the sampled voltage and current. It is verified that the proposed method can reduce the input current distortion especially when the line-to-line voltage is close to zero. The dc-link current will become discontinuous under light load in CSRs, when the traditional control algorithm may not work consistently well. To operate CSR in discontinuous current mode (DCM), the CSR is modeled in DCM and a new control algorithm with feedforward compensation is proposed and verified through experiments. A protection scheme with fast response time is proposed, analyzed and verified to protect SiC devices from overvoltage caused by current interruption in CSRs. To deal with the harmonics and voltage sag in the input ac voltage, a new control algorithm is proposed. By adding ac current feedback control and proportional-resonant (PR) control, the proposed control algorithm can reduce the input current distortion and dc output voltage ripple under input voltage disturbance

Advances in Mineral Processing and Hydrometallurgy

This is a Special Issue of Metals devoted to aspects of Advances in Mineral Processing and Hydrometallurgy. This includes a global call for article submissions that also included Characterization along with Recycling and Waste Minimization. As such, both primary and recycled aspects will be considered. Possible specific topics included Mineralogy, Geometallurgy, Thermodynamics, Kinetics, Comminution, Classification, Physical Separations, Liquid–Solid Separations, Leaching, Solvent Extraction, Ion Exchange, Activated Carbon, Precipitation, Reduction, Process Economics and Process Control. Suggested application areas were in Gold, Silver, PGM’s, Aluminum, Copper, Zinc, Lead, Nickel, and Titanium. Critical Metals articles on topics such as Lithium, Antimony Tellurium, Gallium, Germanium, Cobalt, Graphite, Indium, and Rare Earth were also welcome. As such, this Special Issue of Metals was well supported by diverse submissions and the final publication of high-quality peer-reviewed articles

Technology 2004, Vol. 2

Proceedings from symposia of the Technology 2004 Conference, November 8-10, 1994, Washington, DC. Volume 2 features papers on computers and software, virtual reality simulation, environmental technology, video and imaging, medical technology and life sciences, robotics and artificial intelligence, and electronics