PhD Thesis

Acknowledgements

3

Contents

9

List of Figures

13

Abbreviations and Notations

19

Introduction

21

1

Electrostatic Separation Processes: Control Variables and Measurement Techniques

25

1.1

Electrostatic separation: a multifactor process

26

1.1.1

Roll-type corona-electrostatic separation

26

1.1.2

Free fall tribo-electrostatic separation

28

1.2

Measurement techniques for electrostatic processes

32

1.2.1

Charge measurements

33

1.2.2

High voltage measurements

34

1.2.3

Surface potential measurement

35

1.3

Virtual instruments

38

1.3.1

Data acquisition techniques

39

1.3.2

Graphical programming

39

1.3.3

Developing virtual instruments in LabVIEW

40

2

Charge Measurements of Finely Divided Matter

43

2.1

Charge of insulating particles

43

2.1.1

Experimental set-up for measuring the charge of insulating particles in roll-type corona separators

46

2.1.2

Virtual instrument for single particle charge measurement

47

2.1.3

Discharge of insulating particles on the surface of the grounded roll electrode of a corona separator

54

2.2

Charge of insulating powders

59

2.2.1

Experimental setup for measuring the tribocharge of insulating powder

59

2.2.2

Virtual instrument for powder charge measurements

62

2.2.3

Variables that express the charge of insulating powders

64

2.3

Statistical control of tribocharging processes

67

2.3.1

Capability of a tribocharging device

69

2.3.2

Charts for the statistical control of a tribocharging process

72

2.3.3

Monitoring tribocharging processes - perspectives

76

3

Surface Potential Measurement of Insulating Granular Layers

79

3.1

Surface potential of insulating granular layers

80

3.1.1

Experimental set-up for surface potential measurements of charged insulating granular layers

80

3.1.2

Virtual instrument for surface potential decay measurement

83

3.1.3

Surface potential decay of charged insulating granular layers

84

3.2

Charge decay characteristics of insulating granular layers at the surface of grounded electrodes

86

3.2.1

Discharge of insulating particles in contact with a grounded electrode

86

3.2.2

Charge decay as function of the nature of the granular material

89

3.2.3

Charge decay as function of ambient conditions

92

3.3

Method for evaluating the feasibility of corona-electrostatic separation of granular insulators

94

3.3.1

Experimental procedure

95

3.3.2

Surface potential decay of HDPE and PVC granular layers

96

3.3.3

Corona-electrostatic separation of PE and rubber

101

4

High Voltage Measurement in Corona Electrostatic Separators

105

4.1

High-voltage energizing of corona electrodes

106

4.1.1

Experimental set-up for measuring the high voltage in corona separators

107

4.1.2

Spark discharge monitoring

108

4.1.3

High-voltage supply response to spark discharges

109

4.2

High-voltage sensing of the metal content in granular mixtures

112

4.2.1

Virtual instrument for high voltage measurements

114

4.2.2

Statistical analysis of high-voltage measurement data

115

4.3

Measurement of low metal contents in granular mixtures using a high-voltage sensor

123

4.3.1

High-voltage signal processing

124

4.3.2

Monitoring metal content in granular mixtures processed by electrostatic separation

127

Conclusions to Chapter 4

133

5

Modelling Metal / Insulator Separation Process

135

5.1

A linear-interaction model for electrostatic separation processes

136

5.1.1

Problem formulation using Taguchi methodology

137

5.1.2

Experimental implementation of Taguchi methodology

144

5.1.3

Interpreting the linear-interaction model

147

5.2

Robust control of electrostatic separation processes

150

5.2.1

Taguchi's approach to robust design

151

5.2.2

Experimental setup and results

154

5.2.3

Model for a robust electrostatic separation process

157

5.3

Effect of ambient humidity on the outcome of electrostatic separation processes

161

5.3.1

Formulation of the robust design problem

162

5.3.2

The classic analysis approach

166

5.3.3

The interaction analysis approach

168

Conclusion and Contributions

177

Appendix A: Charge measurements: LabVIEW instrument implementation

197

Appendix B: Discharge graph identification: results for insulating particles on a grounded plate

203