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CHAPTER 2 LITERATURE SURVEY
Introduction
Many countries in the world, including Zambia, largely depend on copper as the backbone of their economies. Copper is the most widely mined mineral in Zambia though a few other minerals such as gold, cobalt and silver are also exploited. Copper producing companies have acknowledged the importance of producing high quality copper in order to satisfy the market demands and requirements. Copper refining by electrochemical reaction (electrolysis) is one of the very important components in the production of high quality metals in the mining industry and other small-scale ventures that contribute to sustainable development. There are two methods of electrochemical refining. Electrorefining is one method that produces high quality copper from 98% to 99.98% copper while electro-winning is used for the recovery of copper from dilute solutions. The methods make use of electrolytic reactors that contain the cathodes and anodes which are connected to electrical power at appropriate level, acting as the driving force, to allow for the electrochemical reaction to proceed smoothly. Various models have been developed to describe the behaviour of the electrodes (Rousar et al., 1986). Many authors have presented mathematical models for such reaction systems. Bisang (1997) developed the mathematical model of the parallel plate reactors, which take into account the diffusion layer adjacent to the working electrode and the convective layer in bulk solution. The results for copper electro-winning showed agreement with the transient equation for the diffusion layer in the convective region. The parallel plate model is widely used for copper recovery particularly in the copper mining companies. The objective of this chapter is to cover the introductory topics of electrochemical process, the main objectives, the benefits of the study, approach, thermodynamics of cells and the properties of electrolytes that enable the electrochemical process to proceed.
Electrochemical Process
The electrochemical process uses electrodes, power supply and electrolyte solution,which may be heated by an external source such as steam in a heat exchanger or directly by steam flow through jackets around cell baths. One pair of electrodes forms a cell,which acts as an electrochemical reactor. The electrochemical reactor uses the voltage as the driving force for the chemical reactions at the electrodes. If the electrodes that form the cell are made of different materials then the cell deposits metal ions from solution onto the cathode. The anode does not dissolve. This method can use solutions of fairly low concentration. Such a process, called electrowinning, is usually used to make the starting sheets which are then used in other processes. If the electrode pair is made of the same material then the cell formed creates an electrorefining process. The electrorefining process is where the anodes release the metal ions into solution and are later deposited onto the cathode producing efficiencies of above 92% (Gana et al.,1993). The electrolyte releases oxygen atoms on the anode (Bellino et al., 2001). The potential applied across the reactors results in polarization of electrodes to anode(positive) and cathode (negative). Polarization is caused by various factors: shape,composition of electrodes, type of electrolyte, temperature, flow rate, level of current and type of species in electrolyte. Concentration polarization is the formation of ions around the cathode or anode created by potential. The film of ions allows the dissolution or deposition of copper on anode or cathode if the potential changes. If concentration polarization sets in, the deposition of copper ceases until the voltage increases. The film then allows the deposition of metal again. Polarization results in the development of voltage drop across two electrodes, which obeys Ohms Law as described in Koryta and Dvorak (1987:90-92). The Law is given by equation (2.1).Where j is the current density (mA/m2), F (%) is the current efficiency and b is the constant given by the ratio zFka/ RTl . At low levels of current, the voltage-current relationship is linear while the resistance remains constant. At high values of current there is a marked departure from linearity (Skoog and West, 1976). Polarization results in the deposition of metal and evolution of gas at cathode and anode respectively as shown in anode and cathode reactions in equations (2.2) and (2.3) and the overall reaction in equation (2.4). If the anode is soluble, then the reaction is as given by equation (2.5).
ITEM PAGE
ABSTRACT
LIST OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
ABBREVIATIONS
CHAPTER 1: INTRODUCTION
1.0 Introduction
1.1 Approach
1.2 Objectives
1.3 Methodology
1.4 Thesis Outline
CHAPTER 2: LITERATURE SURVEY
2.0 Introduction
2.1 Electrochemical process
2.2 Thermodynamics of Cells
2.3 Study Model
2.4 Benefits of the Study
2.5 Summary
CHAPTER 3: REVIEW OF ELECTROCHEMICAL ENGINEERING
3.0 Introduction
3.1 Electrochemistry
3.2 Kinetics of cells
3.3 Temperature and Energy
3.4 Diffusion of Ions in Cells
3.5 Convection of Ions between Electrodes
3.6 Migration of Ions between Electrodes
3.7 Mass Transfer in Cells
3.8 Current Density and Overvoltage
3.9 Current Efficiency
3.10 Operation of Electrochemical Reactors
3.11 Smootheners and Metal Recovery
3.12 Nodulation
3.13 Impurities
3.14 Summary
CHAPTER 4: PROBLEM IDENTIFICATION
4.0 Introduction
4.1 The Problem and its Setting
4.1.1 Passivity
4.1.2 Temperature Changes
4.1.3 Distance between Electrodes
4.1.4 Variation of Concentration
4.1.5 Presence of Impurities
4.1.6 Inadequate Electrode-active Area
4.1.7 Development of Nodules
4.2 Assumptions
4.2.1 Assumption 1
4.2.2 Assumption 2
4.2.3 Assumption 3
4.2.4 Assumption 4
4.2.5 Assumption 5
4.2.6 Assumption 6
4.2.7 Assumption 7
4.3 Summary
CHAPTER 5: METHODOLOGY
5.0 Introduction
5.1 Study Design
5.2 Preparation of Reactor Baths
5.3 Measurements
5.3.1Effect of Electrode-active Area
5.3.2Effect of Current Density and Overvoltage
5.3.3Smootheners and Additives
5.3.4Temperature
5.3.5Impurities
5.3.6Distance Between Electrodes
5.4 Data Analysis
5.5 Summary
CHAPTER 6: RESULTS AND OBSERVATIONS
6.0 Introduction
6.1 Effect of Overvoltage on the Current and Current Density
6.2 Effect of Overvoltage on the Rate of Deposition
6.3 Effect of Temperature on Current Density and Efficiency
6.4 Effect of Current Density
6.5 Effect of Distance Between Electrodes
6.6 Effect of Concentration on Current Density and Current Efficiency
6.7 Effect of Impurities on the Current and Current Efficiency
6.8 Effect of Electrode-active Area on the Current Efficiency
6.9 Effect of Smootheners on the Current Density and Current Efficiency
6.10Summary
CHAPTER 7: DISCUSSION, CONCLUSION AND RECOMMENDATIONS
7.0 Introduction
7.1 Discussion
7.2 Environmental Considerations
7.3 Economic Benefits
7.4 Conclusion
7.5 Recommendations
BIBLIOGRAPHY
APPENDICES
