Beneficiation of plasma display panels

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BENEFICIATION OF PLASMA DISPLAY PANELS

by

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A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science

(Metallurgical and Materials Engineering).

Golden, Colorado Date . Signed: . Matt Esquibel Signed: . Dr. Patrick Taylor Thesis Advisor Golden, Colorado Date . Signed: . Dr. Michael Kaufman Professor and Head, Department of Metallurgical and

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ABSTRACT

A literature review, thermodynamics, and experimental testing were used to develop a beneficiation method for plasma display panels. The proposed process includes cutting the adhered glass, separating the front and back glass, acid leaching the glass panels separately, precipitating indium hydroxide, and precipitating rare earth oxalates. This process has the advantage of not requiring the glass to be crushed. Combining the front and back glass into one leaching stage followed by a two-stage precipitation is an option, but further test work is

required.

It was found that the best leach conditions for the ITO powder during testing was a temperature of 90°C, agitation rate of 600 rpm, 2.0054 g/L, 1M H2SO4, and 4 hours of leaching time. 99% of the indium was extracted at a grade of 96%. Leaching results suggest the process is controlled by the chemical reaction. The indium hydroxide precipitation was found to be optimal at a pH of 6, agitation rate of 400rpm, temperature of 25°C, 84 g/L NaOH, and a precipitation time of 90 minutes.96% of the indium was recovered as a hydroxide at a grade of 97%.

The acidic leaching of the rare earth phosphor powder was found to be best at a

temperature of 70°C, agitation rate of 600 rpm, 1M H2SO4, 2.5g/L, and 4 hours of leaching time. 99% of the REES were extracted with these parameters. Leaching results suggest the process is controlled by the chemical reactions. The parameters were then used on the phosphor glass and extracted 69% of the REEs at a grade of 10%. Rare earth oxalate precipitation results showed that it was possible to recover 98% of the REEs.

Combined leaching experiments showed promise in extracting both indium and the REEs, but further work is required. A two-stage precipitation was attempted on the combined

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leach solution. The first stage is an indium hydroxide precipitation and the second stage is a rare earth oxalate precipitation. The precipitation results were encouraging, but a high indium

concentration is required for the first precipitation stage to be selective. Further work is required on the two-stage precipitation parameters.

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TABLE OF CONTENTS

ABSTRACT ... iii

LIST OF FIGURES ... xi

LIST OF TABLES ... xiii

ACKNOWLEDGEMENTS ... xv

CHAPTER 1 INTRODUCTION ... 16

CHAPTER 2 LITERATURE REVIEW ... 18

2.1 The Plasma Display Panel ... 18

2.1.1 PDP Production ... 19

2.2 Primary Production of In, and REE ... 20

2.2.1 Indium Production ... 20

2.2.2 Rare Earth Production ... 22

2.3 Recycling PDPs ... 25

2.3.1 Current Recycling Strategies ... 25

2.3.2 ITO Processing... 25

2.3.3 Recovery of REEs from Rare Earth Phosphors ... 26

2.4 Review of Applicable Patents ... 28

2.5 Analytical Techniques ... 28

2.5.1 Inductively Couple Plasma ... 29

2.5.2 Environmental Scanning Electron Microscope ... 30

CHAPTER 3 PROCESS DEVELOPMENT ... 32

3.1 Acidic Leaching of ITO ... 32

3.1.1 Proposed Chemical Reactions ... 32

3.1.2 Thermodynamics of Acidic Leaching of ITO ... 33

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3.2 Acidic Leaching of Rare Earth Elements... 36

3.2.2 Thermodynamics of Acidic Leaching of Rare Earth Phosphors ... 37

3.2.3 Pourbaix Diagrams... 40

3.3 Precipitation of Indium Hydroxide ... 42

3.3.2 Thermodynamic Data... 43

3.4 Precipitation of Rare Earth Oxalates ... 45

3.4.2 Thermodynamic Data... 46

CHAPTER 4 EXPERIMENTAL PROCEDURES ... 48

4.1 Sample Preparation ... 48

4.1.1 Adhered Glass Fragments Sample Preparation... 49

4.1.2 Front Glass Sample Preparation... 50

4.1.3 Back Glass Sample Preparation ... 50

4.1.4 ITO Powder Sample Preparation ... 51

4.1.5 Phosphor Powder Sample Preparation ... 51

4.2 Thermal Delamination Experimentation... 52

4.3 Leaching Experimentation ... 53

4.3.1 ITO Powder Batch Leaching ... 53

4.3.2 Phosphor Powder Batch Leaching ... 54

4.3.3 Combined Powder Batch Leaching ... 54

4.3.4 ITO Powder Kinetic Leaching ... 55

4.3.5 Phosphor Powder Kinetic Leaching ... 56

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4.4.1 ITO Thin Film Removal ... 57

4.4.2 Phosphor Thin Film Removal ... 58

4.4.3 Simultaneous Thin Film Removal ... 59

4.5 Precipitation Testing ... 59

4.5.1 Indium Precipitation Testing... 59

4.5.2 Rare Earth Precipitation Testing ... 60

4.6 Two-Staged Precipitation Testing... 61

4.7 Analytical Techniques ... 62

4.7.1 Sodium Peroxide Fusion ... 62

4.7.2 Lithium Borate Fusion ... 63

4.7.3 Free Acid Titration ... 64

CHAPTER 5 RESULTS AND DISCUSSION ... 65

5.1 Thermal Delamination ... 65

5.2 Alkaline Leaching for Indium Extraction ... 66

5.3 Acid Leaching of ITO Powder for Indium Extraction ... 67

5.3.1 Effect of Varying Temperature ... 68

5.3.2 Effect of Varying Acid Type ... 69

5.3.3 Effect of Varying Leaching Time ... 70

5.3.4 Effect of Varying Solid Liquid Ratio... 71

5.3.5 Effect of H2O2 Addition ... 72

5.4 Acid Leaching of Phosphor Powder for Rare Earth Extraction ... 73

5.4.1 Effect of Temperature ... 73

5.4.2 Effect of Sodium Hydroxide Roast ... 74

5.4.3 Effect of Varying Leaching Time ... 74

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5.5 Combined ITO and Phosphor Powder Leach ... 76

5.6 Thin Film Removal ... 76

5.6.1 Indium Extraction from Front Glass ... 77

5.6.2 Rare Earth Extraction from Back Glass ... 77

5.6.3 Simultaneous Extraction of Indium and Rare Earths from Glass Mix ... 80

5.7 Indium Precipitation... 81

5.7.1 Determining Significant Effects ... 82

5.7.2 Statistical Analysis ... 85

5.7.3 Main Effects ... 87

5.7.4 Indium Recovery and Model Equation ... 88

5.8 Rare Earth Precipitation ... 88

5.8.1 Determining Significant Effects ... 89

5.8.2 Statistical Analysis ... 92

5.8.3 Main Effects ... 93

5.8.4 Rare Earth Recovery and Model Equation ... 94

5.9 Two-Staged Precipitation... 95

5.9.1 First Stage ... 95

5.9.2 Second Stage ... 96

CHAPTER 6 LEACHING AND PRECIPITATION KINETICS ... 98

6.1 Nernst Boundary Layer ... 98

6.2 Rate Controlling Step for Leaching ... 99

6.3 Precipitation Basics ... 100

6.4 Precipitation Kinetics ... 101

CHAPTER 7 PROCESS FLOW SHEETS AND ESTIMATED COSTS ... 103

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7.1.1 PDP Separate Streams Flow Sheet with Laser Cutting ... 103

7.1.2 PDP Separate Streams Flow Sheet with Crushing and Sorting ... 105

7.1.3 PDP Combined Streams Flow Sheet with Laser Cutting ... 106

7.1.4 PDP Combined Streams Flow Sheet with Crushing ... 108

7.2 Material Flows and Assumptions... 109

7.3 Equipment Costs ... 111

7.3.1 Equipment Costs for PDP Separate Streams Flow Sheet with Laser Cutting ... 111

7.3.2 Equipment Costs for PDP Separate Streams Flow Sheet with Crushing... 112

7.3.3 Equipment Costs for PDP Single Stream Flow Sheet with Laser Cutting ... 113

7.3.4 Equipment Costs for PDP Single Stream Flow Sheet with Crushing ... 113

7.4 Total Fixed Capital Costs... 114

7.4.1 Total Fixed Capital Costs for PDP Separate Streams Flow Sheet with Laser Cutting ... 115

7.4.2 Total Fixed Capital Costs for PDP Separate Streams Flow Sheet with Crushing ... 116

7.4.3 Total Fixed Capital Costs for PDP Single Stream Flow Sheet with Laser Cutting ... 116

7.4.4 Total Fixed Capital Costs for PDP Single Stream Flow Sheet with Crushing ... 117

7.5 Labor Costs ... 118

7.6 Operating Costs and Estimated Revenue ... 118

CONCLUSIONS AND RECOMMENDATIONS ... 121

REFERENCES CITED ... 123

APPENDIX ... 125

10.1 ITO Powder Alkaline Leaches ... 125

ITO Powder Alkaline Leaches Cont. ... 126

10.2 ITO Powder Acidic Leaches ... 126

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Rare Earth Powder Acidic Leach Data and Calculations Cont. ... 133

10.4 Combined Leaching Data and Calculations ... 137

Combined Leaching Data and Calculations Cont. ... 138

10.5 ITO Precipitation Optimization ... 139

ITO Precipitation Optimization Cont... 140

10.6 REE Precipitation Optimization Data and Calculations ... 146

REE Precipitation Optimization Data and Calculations Cont. ... 147

REE Precipitation Optimization Data and Calculations Cont. ... 148

10.7 Two-Stage Precipitation Data and Calculations ... 165

Two-Stage Precipitation Data and Calculations Cont. ... 166

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LIST OF FIGURES

Figure 2.1. PDP Structure (Uchidoi)... 19

Figure 2.2. Indium Recovery from Residues(Alfantazi and Moskalyk) ... 21

Figure 2.3. Bastnasite Processing Flow Sheet (EPA 2013) ... 23

Figure 2.4. Recovery of REEs from Monazite Concentrate (EPA 2013) ... 24

Figure 2.5: Schematic of ICP Torch (Manning and Grow) ... 29

Figure 2.6. Shematic of the SEM (Reimer 1998) ... 30

Figure 3.1. In-H2O System at 25°C ... 35

Figure 3.2. Sn-H2O System at 25°C ... 36

Figure 3.3. Y-S-H2O System at 25°C ... 40

Figure 3.4. Eu-S-H2O System at 25°C ... 41

Figure 3.5. Gd-S-H2O System at 25°C ... 41

Figure 3.6. Tb-S-H2O System at 25°C ... 42

Figure 4.1: Glass Fragment with Adhesive... 49

Figure 4.2: Front Glass Sample ... 50

Figure 4.3: Tube Furnace used for Thermal Delamination Experimentation ... 52

Figure 4.4: Photograph showing the experimental set up used during the ITO powder kinetic leaching test. The thermometer used for temperature readings is not shown. ... 56

Figure 5.1: Indium and Tin Extraction at Varying Temperatures with 1g/L of ITO Powder Concentration ... 68

Figure 5.2: Indium and Tin Extraction at Varying Temperatures with 2g/L of ITO Powder Concentration ... 69

Figure 5.3: Indium and Tin Recovery over Time in 1M H2SO4 at 90°C ... 71

Figure 5.4: Indium and Tin Extraction at Varying Solid Liquid Ratio ... 72

Figure 5.5: Rare Earth Extraction over Time in 1M H2SO4 at 70°C ... 75

Figure 5.6: Phosphor Back Glass before Leaching ... 78

Figure 5.7: Phosphor Back Glass after Leaching ... 78

Figure 5.8: Phosphor Back Glass after Washing and before Scraping ... 79

Figure 5.9: Phosphor Back Glass after Leaching, Washing, and Scraping ... 80

Figure 5.10: Half-Normal Plot for Indium Recovery ... 83

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Figure 5.13: 3D Surface Plot for Indium Recovery ... 87

Figure 5.14: Half-Normal Plot for Rare Earth Recovery ... 90

Figure 5.15: Pareto Chart for Rare Earth Recovery ... 91

Figure 5.16: 3D Surface Plot for Rare Earth Recovery ... 94

Figure 7.1. PDP Process Flow Sheet Based on Separate Streams and Laser Cutting ... 104

Figure 7.2. PDP Process Flow Sheet Based on Separate Streams, Crushing, and Sorting ... 106

Figure 7.3. PDP Process Flow Sheet Based on One Stream, Two-Stage Precipitation, and Laser Cutting... 108

Figure 7.4. PDP Process Flow Sheet Based on One Stream, Two-Stage Precipitation, and Crushing ... 109

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LIST OF TABLES

Table 3.1. Thermodynamic Data for Reaction 3.1 ... 33

Table 3.10. Thermodynamic Data for Rare Earth Oxalate Precipitation ... 46

Table 5.1: Results of Thermal Delamination Experiments on Adhered Glass Fragments ... 66

Table 5.2: Alkaline Leach Tests ... 67

Table 5.3: Indium Precipitation Experimental Design ... 82

Table 5.4: Indium Recovery ANOVA Table ... 85

Table 5.5: Indium Recovery Statistical Parameters ... 86

Table 5.6: Rare Earth Precipitation Experimental Design ... 89

Table 5.7: Rare Earth Recovery ANOVA Table ... 92

Table 5.8: Rare Earth Recovery Statistical Analysis ... 93

Table 7.1. Required Equipment and Associated Costs for PDP Separate Streams Flow Sheet with Laser Cutting ... 111

Table 7.2. Required Equipment and Associated Costs for PDP Separate Streams Flow Sheet with Crushing ... 112

Table 7.3. Required Equipment and Associated Costs for PDP Single Stream Flow Sheet with Laser Cutting ... 113

Table 7.4. Required Equipment and Associated Costs for PDP Single Stream Flow Sheet with Crushing ... 114

Table 7.5. Factors used to Determine Total Fixed Capital Costs (Mular 2002) ... 115

Table 7.6. Total Fixed Capital Costs for the Separate Streams Flow Sheet with Laser Cutting 115 Table 7.7. Total Fixed Capital Costs for the Separate Streams Flow Sheet with Crushing ... 116

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Table 7.9. Total Fixed Capital Costs for the Single Stream Flow Sheet with Crushing ... 117 Table 7.10. Estimated Labor Costs ... 118 Table 7.11. Estimated Operating Costs and Revenue ... 120

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ACKNOWLEDGEMENTS

I would like to thank Dr. Taylor for his help and patience throughout this thesis. His guidance not only helped me with school, but also with the transition of going into industry for work. I would also like to thank Dr. Corby Anderson who embodies what it means to be from Butte, America.

I was privileged to share an office with Caelen Anderson and Joseph Grogan who helped me immensely with my project and kept me from getting too stressed. It was extremely valuable to have two people that had already been through a master’s thesis and were willing to give me advice on how to complete mine.

Thanks are well deserved for the members of my CR3 focus group especially Don Lipkin of GE and George Martin of Veolia. Industry support was extremely helpful during this process.

Lastly, I would like to thank my fiancée Rebekah Gibson. Her patience and support throughout this project gave me the strength to finish.

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CHAPTER 1 INTRODUCTION

Flat panel displays (FPDs) are the predominant technology for televisions, computers, microdisplays, medical devices, and industrial instruments. Plasma display panels (PDPs) have begun to become more prominent in the FPD industry due to the increase in high definition technology. PDPs contain valuable constituents such as critical materials that industry leaders would like to recycle if it were economically feasible. The materials of interest are indium and rare earths.

The indium is present in the panel as indium tin oxide (ITO). The ITO is used as a transparent electrode on the surface of the front glass substrate. The amount of ITO in a panel depends on the panel size, but there is approximately 0.5 grams of ITO in a 44-inch panel. The potential steps for indium recovery include removal of the ITO from the glass substrate, recovery of indium from the ITO, and purification of indium.

The rare earth material is contained in the phosphors used to create color on the display. The rare earths present in the phosphors are yttrium, terbium, europium, and gadolinium. The phosphor coating is white and has been deposited onto the back glass substrate. The main problem with recovering rare earth elements (REEs) is removing the phosphor coating from the glass substrate and then recovering the rare earths from the phosphors. Lead solder is used to hold the phosphors together in some of the manufacturers’ panels. The lead contained in the solder could cause health and safety issues as well as contamination issues during processing.

In order to access the various valuable materials in the PDP, the panel will have to be delaminated or cut apart. The two glass panels are adhered with a low melting glass frit. The glass panels not only need to be separated from the metal frame, but the valuable materials need to be extracted and recovered from each of the glass panels.

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The PDP market has been declining since 2010, but there are millions of panels still available for recycling (NPD Group). End-of-life module waste can provide feed for a recycling process for years to come.

The objective of this research was to evaluate and develop PDP recycling technologies. The project tasks to be evaluated by this investigation were to separate the glass panels, remove the ITO and rare earth phosphor thin film, recover indium from the ITO thin film, and to recover REEs from the rare earth phosphors.

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CHAPTER 2 LITERATURE REVIEW

A literature survey of plasma display panels and recycling strategies was conducted in order to provide the researcher proper background information. The information provided valuable knowledge on the subject and was used to design experimental work.

2.1 The Plasma Display Panel

A PDP is a flat panel display where light is created by the excitation of phosphors by a gas discharge between two panels of glass. The gas discharge is produced by a mixture of neon and xenon. This mixture of noble gases is inert (King County Solid Waste Division). The PDP was invented by Professor Donald L. Bitzer, Professor H. Gene Slottow, and their graduate student Robert H. Wilson at the University of Illinois. They designed it to meet the requirements for a full graphics display for the Programmed Logic for Automatic Teaching Operations

(PLATO) which was a program started at the University of Illinois to research the use of computers in education (Weber).

In the 1970s and 1980s, many US companies took out licenses from the University of Illinois to research and produce plasma displays. Companies such as IBM, RCA, Zenith, and General Electric were among these companies. Japanese companies also took out licenses from the University of Illinois. Japanese companies included Hitachi, Matsushita, Sony, Fujitsu, LG Electronics, Samsung Electronics, Sanyo Electric, and Sharp. By the late 1980s, most of the US manufacturers had removed themselves from the PDP industry (King County Solid Waste Division).

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2.1.1 PDP Production

The PDP manufacturing process is usually divided into three different sub processes. The subprocesses are the front plate process, the rear plate process, and a panel process. Each of the subprocesses has multiple process steps and each step has different methods

Figure 2.1. PDP Structure (Uchidoi)

The front plate process constructs the front plate structure of the PDP. The first step is the formation of the transparent electrode on the surface of the front plate glass. The transparent electrode is the ITO thin film and can be deposited by vacuum deposition or patterning. A metal bus electrode and the dielectric layer are then placed onto the glass panel. The MgO protective layer is put on last through deposition.

The rear plate glass starts out with the address electrode, dielectric layer, and the rib structure. The rare earth phosphor layer is formed inside of the ribs. After completion of both

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glass panels, they are adhered together with a low melting glass frit. An exhaust pipe is installed, the space between the glass panels is filled with inert gas, and the aging process is started

(Uchidoi).

Panel reliability includes mechanical strength, operating stability, and stability of electrical contact of interconnect pads. The panel lifetime is based upon the operating lifetime and luminous lifetime. The operating lifetime is determined by the discharge voltage after long-term operation. The discharge voltage will increase until it exceeds the applied driving voltage which leads to failure. The luminance lifetime is determined by the phosphor degradation. Ions that come into contact with the phosphor surface will degrade the phosphors to the point of failure (Uchidoi).

2.2 Primary Production of In, and REE

It is important to have an understanding of the primary production of the materials

involved with PDPs. The materials of interest are indium and the REE (Y, Eu, Gd, and Tb). The following sections describe the primary production of these constituents.

2.2.1 Indium Production

Indium is an important by-product of zinc metal processing. Indium represents a very minor percentage of the earth’s crust comparable to silver. The average indium content in zinc ores ranges from less than 1ppm to 100ppm. Indium does not occur in the native state and is regarded as a semi-precious metal. The three major end uses of indium come from indium metal, alloys, and ITO. The largest use of indium is in thin film coatings in liquid crystal displays (LCDs) and FPDs.

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Indium is most commonly associated with zinc-bearing ores, the most common of which is Sphalerite. There are also relatively high levels of indium associated with some tin deposits.

Figure 2.2 shows a flow sheet for the recovery of indium from residues. In a process at Akita, the zinc residue undergoes sulfidation roasting followed by a sulfuric acid leach. The indium containing solution is then neutralized using calcium carbonate and is then processed using solvent extraction. Aluminum plates are placed into the solvent extraction solution to produce an indium sponge which is then electrorefined to produce indium metal.

Figure 2.2. Indium Recovery from Residues(Alfantazi and Moskalyk)

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Another process to recover indium from zinc oxide involves dissolving the indium in dilute sulfuric acid. The tin is removed first through neutralization followed by the indium. The indium residue is leached with sodium hydroxide to produce indium hydroxide which is leached with hydrochloric acid. The solution is purified through cementation with iron followed by cementation with aluminum to recover the indium from solution (Alfantazi and Moskalyk).

2.2.2 Rare Earth Production

There are various methods for processing REEs. The following section is only a brief discussion on some of the methods used to process REEs due to this investigation centering on rare earth phosphors and not rare earth bearing ores.

REEs do not naturally occur as metals, but only as part of a host mineral’s chemistry. Because of this, rare earth recovery requires a complex processing method to chemically break down the rare earth containing minerals. Three ores are considered the most feasible for rare earth extraction: bastnasite, xenotime, and monazite. (“Rare Earth Elements: A Review of Production, Processing, Recycling, and Associated Environmental Issues - P100EUBC.pdf”)

Mountain Pass Mine produced a concentrate from a bastnasite ore. The concentrate is calcined to allow cerium to be removed first. An acid digestion is then conducted and the

solution is treated via multi-stage solvent extraction to produce rare earth compounds. Figure 2.3 shows the bastnasite process flow sheet employed at Mountain Pass Mine.

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Figure 2.3. Bastnasite Processing Flow Sheet (EPA 2013)

A common method of processing monazite and xenotimes involves a NaOH digestion to produce rare earth hydroxides. This is then leached with hydrochloric acid and the chloride solution is processed via a multi-stage solvent extraction process to produce rare earth oxides.

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Figure 2.4. Recovery of REEs from Monazite Concentrate (EPA 2013)

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The recovery of rare earth metals from compounds such as oxides and chlorides can be difficult. The three primary methods to do this are reduction of anhydrous chlorides or fluorides, reduction of rare earth oxides, and fused salt electrolysis of rare earth chlorides or fluorides. Some less common processes include electrolysis, gaseous reduction, and vacuum distillation (EPA 2013).

2.3 Recycling PDPs

Recycling PDPs could result in creating a resource for indium and REEs. The US is lacking in rare earth deposits and rare earth recovery from PDPs could be a potential source.

2.3.1 Current Recycling Strategies

This investigation into the recycling of PDPs was focused on the removal of the thin film from the glass panels. There is currently no industrial processes in place for the recovery of indium and rare earths from the thin films in PDPs. Recycling strategies for PDPs are centered around precious metals contained in televisions, but not the material on the glass panels. There are methods to refurbish plasma televisions as well. Due to the lack of established processing methods for the glass panels, a different approach had to be taken in the literature review.

2.3.2 ITO Processing

For the processing of the ITO thin film, processes used to treat ITO sputtering targets were used. One process was to dissolve the ITO in an acid solution and recover an indium

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sponge using zinc as a replacement agent. The indium is purified using hot immersion to remove tin (Hsieh, Chen, and Say).

Another process included acid leaching of the ITO target with sulfuric acid, removing tin by sulphide precipitation, and recovering indium sponge by zinc cementation. The indium sponge produced by this method was 99.9% indium (Li et al.).

Methods for recovering indium from ITO contained in LCD panels were also considered. A process was developed using ITO powder to simulate the ITO in LCDs. The powder was leached with varying concentrations of HCl, H2SO4, and HNO3 for 18 to 24 hours. The indium was then recovered via solvent extraction.(Virolainen et al.)

2.3.3 Recovery of REEs from Rare Earth Phosphors

The rare earth phosphors in PDPs have similar chemistries to those in fluorescent lamps. To gain a better understanding of potential processing methods, strategies used to process

fluorescent lamp phosphors were investigated.

Porob et al (2012) US Patent US 8,137,645 B2 describes a method to recover rare earths from fluorescent light phosphors. The patent is based on 4 steps. The first step for rare earth recovery is done by firing the phosphor material with an alkali material such as sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. The firing temperature can range from 150°C to 1600°C depending on the temperature at which the phosphor will decompose. The second step is to take the decomposed mixture and wash it with either water or an acid. The acid can be something like hydrochloric acid or an organic acid like acetic acid. The washing step is designed to put the non-rare earth constituents into solution and remove them from the solid residue. The residue contains a mixture of rare earth oxides. The

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third step is to perform and acid digestion on the material. The digestion occurs between 80°C and 300°C depending on what temperature the residue will go into solution. The acid used needs to be strong and can be nitric acid, concentrated hydrochloric acid, or concentrated sulfuric acid. The solution will contain individual rare earth salts. The fourth step is to separate the rare earth constituents from solution. The individual rare earth salts or elements are then separated by standard methods that have already been developed such as ion exchange, solvent extraction, fractional crystallization, or a precipitation process. There can also be a pre-treatment where applicable. If there are acid-soluble rare earth oxides in the phosphor, then the rare earth oxides can be leached away from the phosphor before the firing step. Nitric acid, hydrochloric acid, sulfuric acid, or the like can be used for this step.

Otto et al. (2009) US Patent Application US 2009/0162267 A1 describes a method to recover rare earths from fluorescent lamp phosphors. The patent lists 6 individual process steps that must be done in order to recover rare earths as a usable product and recover them at a high efficiency.

 Mechanical separation of coarse parts

 Separation of halophosphate

 Extraction of RE fluorescent materials readily soluble in acids (mainly Y and Eu oxide)

 Extraction of RE fluorescent materials insoluble in acids (RE phosphates)

 Digestion of the remaining components containing RE (RE aluminates)

 Final treatment

The first step is to remove the coarse particles above 20µm and keep the fine particles. The second step is the separation of the halophosphate. This can be done by cold leaching with hydrochloric acid and saving the residue for further processing. It can also be done by hot

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leaching with HCl, removing calcium with sulfate, and keeping the filtrate for further processing. The readily dissolved RE oxides go into solution. The insoluble RE compounds are treated with concentrated sulfuric acid at high temperature or by an alkali. The rare earths are obtained in ionic form in the filtrate or as insoluble double sulfates depending on the method used. The fifth step can be carried out by the use of an alkali digestion. The material is treated with 150°C 35% strength sodium or potassium hydroxide under pressure. The sixth step is carried out by

precipitating out the rare earths by either using oxalic acid or ammonia.

2.4 Review of Applicable Patents

Chen et. al. (2006) US Patent Application 2006/012539 A1 describes a method to check PDPs for potential recycling and then a method to reuse the display panel. A new glass ring and extracting pipe replace the old parts and the PDP can be reused.

Iwamoto et al. (2010) US Patent Application US 2010/0022152 A1 describes a method to recover the glass panels for recycling. The glass panels are separated by cutting, and the outside glass is pulverized. The pulverized glass is then blasted at the front and back glass panels to remove the thin films. The glass panels can then be recycled in a normal glass recycling process.

Noma et al. US Patent US 6,632,113 B1 describes multiple methods to separate the glass from the frame and from each other. The glass can then be reused. They describe a method for cutting, crushing, and chemical dissolution of the glass frit.

2.5 Analytical Techniques

This section focuses on the analytical equipment and procedures that were used during this project.

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2.5.1 Inductively Couple Plasma

Inductively coupled plasma (ICP) is a radiofrequency induced plasma produced from energy supplied by a magnetic field. The magnetic field is produced by an induction coil which is wrapped around the ICP torch and has water flowing through it. The components that create the plasma make up the ICP torch. The sample is then injected into the plasma through the nebulizer. The most commonly analyzed sample is cations in solution. The nebulizer converts the liquid stream into an aerosol. (Manning and Grow) Figure 2.5 shows a schematic of a typical ICP torch.

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2.5.2 Environmental Scanning Electron Microscope

The environmental scanning electron microscope (ESEM) was used to determine the thickness of the rare earth phosphor thin film, gather a better understanding of both thin films, and to determine the presence of the ITO thin film after leaching.

Scanning electron microscopy (SEM) can gather an image and analyze bulk specimens simultaneously. Electrons emitted from a thermionic, Schottky, or field emission cathode are accelerated by the electrical potential difference between the cathode and the anode. An electron probe current is formed at the specimen surface by a series of electron lenses that de-magnify the source beam. Variations in probe current, aperture, and probe diameter are conducted by

changing the excitations of the first condenser lenses and the aperture limiting diaphragm in the final lens. Figure 2.6 shows a schematic of the fundamentals of a SEM (Reimer 1998).

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The ESEM uses a high vacuum at the electron gun and in the electron optical column. An environmental stage cell is created to maintain a higher working pressure while also maintaining high vacuum. The pressure in the environmental chamber will cause additional electron scattering. The back scatter electrons and characteristic x-rays are not significantly impacted by electron scattering which only occur at small angles in the environmental chamber (Reimer 1993).

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CHAPTER 3 PROCESS DEVELOPMENT

This chapter describes the chemical reaction and thermodynamics that were used for process development. Topics include proposed chemical reactions for leaching, leaching thermodynamics, pourbaix diagrams, proposed chemical reactions for precipitation, and precipitation thermodynamics.

3.1 Acidic Leaching of ITO

Leaching is the process of extracting a soluble constituent from a solid by means of a solvent. The objective of the acidic leaching of ITO was to extract In and minimize the amount of Sn extracted into solution. The following sections describe the fundamental chemistry used to develop the acidic leaching process.

3.1.1 Proposed Chemical Reactions

The first step for developing the acidic leaching process was to understand the chemical reactions taking place during leaching. The proposed chemical reactions for leaching ITO with sulfuric acid are as follows:

In2O3 + 3H2SO4 = In2(SO4)3 + 3H2O (3.1)

SnO2 + 2H2SO4 = Sn(SO4)2 + 2H2O (3.2)

As mentioned, the desired reaction is 31 in which In is extracted into solution. It was also desired to minimize reaction 3.2.

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3.1.2 Thermodynamics of Acidic Leaching of ITO

HSC Chemistry 7.1 was used to evaluate the thermodynamics of reactions 3.1 and 3.2. The thermodynamic data for both reactions can be seen in Table 3.1 and Table 3.2.

It can be seen from the tables that both reactions are thermodynamically favorable throughout the entire temperature range. As the temperature of reaction is increased, the free energy of reaction becomes more positive. Based on the enthalpies being less than zero, the reactions are assumed to be exothermic. A negative enthalpy shows that heat is given off by the reaction which is the definition of an exothermic reaction. A positive enthalpy shows that heat is required for the reaction to take place thereby making the reaction endothermic. Although both reactions are more thermodynamically favorable at lower temperatures, reaction kinetics is likely a key factor in both reactions.

Table 3.1. Thermodynamic Data for Reaction 3.1

T deltaH deltaS deltaG K Log(K)

°C kcal cal/K kcal

0.000 -114.900 -189.170 -63.228 3.925E+050 50.594 10.000 -115.554 -191.521 -61.325 2.176E+047 47.338 20.000 -116.229 -193.862 -59.398 1.933E+044 44.286 30.000 -116.922 -196.187 -57.448 2.626E+041 41.419 40.000 -117.633 -198.494 -55.474 5.238E+038 38.719 50.000 -118.360 -200.780 -53.478 1.482E+036 36.171 60.000 -119.102 -203.041 -51.459 5.759E+033 33.760 70.000 -119.858 -205.277 -49.417 2.993E+031 31.476 80.000 -120.627 -207.485 -47.353 2.030E+029 29.307 90.000 -121.407 -209.663 -45.268 1.758E+027 27.245 100.000 -122.197 -211.810 -43.160 1.908E+025 25.281

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34 Table 3.2. Thermodynamic Data for Reaction 3.2

3.1.3 Pourbaix Diagrams

Figure 3.1 and Figure 3.2 show the Eh and pH dependence of the stable phases of indium and tin respectively. Both indium and tin are soluble in acidic conditions. Both diagrams detail the stable phases present at particular Eh and pH ranges at 25°C.

The attempts at an alkaline leach were developed from these diagrams. At a pH of 7, the stable phase of indium is the aqueous ion In(OH)2 and the stable phase of tin is solid SnO2. It appears that a separation can be made by leaching at a higher pH (around 7). If only indium is being extracted, the thin film may not be removed from the glass. Due to this issue, it was decided that an acidic leach would be used.

°C kcal cal/K kcal

0.000 -44.535 -95.037 -18.575 7.302E+014 14.863 10.000 -44.937 -96.484 -17.618 3.975E+013 13.599 20.000 -45.355 -97.933 -16.646 2.574E+012 12.411 30.000 -45.786 -99.380 -15.659 1.950E+011 11.290 40.000 -46.231 -100.823 -14.658 1.701E+010 10.231 50.000 -46.687 -102.258 -13.643 1.688E+009 9.227 60.000 -47.155 -103.682 -12.613 1.883E+008 8.275 70.000 -47.632 -105.095 -11.569 2.338E+007 7.369 80.000 -48.119 -106.494 -10.511 3.201E+006 6.505 90.000 -48.615 -107.877 -9.439 4.798E+005 5.681 100.000 -49.118 -109.244 -8.353 7.815E+004 4.893

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35 Figure 3.1. In-H2O System at 25°C

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36 Figure 3.2. Sn-H2O System at 25°C

3.2 Acidic Leaching of Rare Earth Elements

The objective of the acidic leaching of the phosphor thin film was to remove the thin film from the glass and to extract the rare earth elements (REE). The following sections describe the fundamental chemistry used to develop the acidic leaching process.

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The proposed chemical reactions for the acidic leaching of the phosphors were not as straight forward as those for indium and tin. There are impurities such as Al, Mg, and Ca that are associated with the phosphors that will also leach into solution. These reactions have been left out of the proposed reactions. Instead, only the reactions with the rare earth oxides were of concern. The rare earths are present in the phosphors in either an oxide form or are used as a dopant in another oxide. The proposed chemical reactions are as follows:

Y2O3 + 3H2SO4 = Y2(SO4)3 + 3H2O (3.3)

Eu2O3 + 3H2SO4 = Eu2(SO4)3 + 3H2O (3.4)

Gd2O3 + 3H2SO4 = Gd2(SO4)3 + 3H2O (3.5)

Tb2O3 + 3H2SO4 = Tb2(SO4)3 + 3H2O (3.6)

3.2.2 Thermodynamics of Acidic Leaching of Rare Earth Phosphors

HSC Chemistry 7.1 was used to evaluate the thermodynamics of reactions 3.3, 3.4, 3.5, and 3.6. The thermodynamic data for leaching Y2O3, Eu2O3, Gd2O3, and Tb2O3 can be seen in Table 3.3, Table 3.4, Table 3.5, and Table 3.6 respectively.

As can be seen in the tables, all reactions are thermodynamically favorable throughout the entire temperature range. As the temperature of the reaction is increased, the free energy of reaction becomes more positive. Based on the reaction enthalpies being less than zero, the reactions are exothermic. Although all reactions are thermodynamically favorable at lower temperatures, reaction kinetics is likely to be a key factor in all of the reactions.

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C kJ J/K kJ 0.000 -683.165 -816.736 -460.073 9.717E+087 87.988 10.000 -685.944 -826.729 -451.856 2.312E+083 83.364 20.000 -688.804 -836.655 -443.539 1.092E+079 79.038 30.000 -691.740 -846.500 -435.123 9.567E+074 74.981 40.000 -694.744 -856.251 -426.609 1.466E+071 71.166 50.000 -697.814 -865.899 -417.999 3.731E+067 67.572 60.000 -700.943 -875.435 -409.292 1.508E+064 64.178 70.000 -704.127 -884.853 -400.490 9.293E+060 60.968 80.000 -707.362 -894.145 -391.595 8.433E+057 57.926 90.000 -710.644 -903.307 -382.608 1.092E+055 55.038 100.000 -713.967 -912.334 -373.529 1.960E+052 52.292

Y2O3(s) + 3H2SO4(l) = Y2(SO4)3(a) + 3H2O(l)

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Gd2O3 + 3H2SO4(l) = Gd2(SO4)3(a) + 3H2O(l)

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3.2.3 Pourbaix Diagrams

The Eh and pH dependence of the stable phases of Y, Eu, Gd, and Tb are shown in Figure 3.3, Figure 3.4, Figure 3.5, and Figure 3.6 respectively. The diagrams were made in HSC 7.1.

The pourbaix diagrams show that the REE present in the rare earth phosphors will go into solution in acidic conditions. Gadolinium requires the lowest pH (around 0.5) to go into

solution. These diagrams show promise for the acidic leaching of the rare earth phosphors because all four of the rare earth elements have a stable aqueous phase at acidic conditions.

Figure 3.3. Y-S-H2O System at 25°C

14 12 10 8 6 4 2 0 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Y - S - H2O - System at 25.00 C C:\sw\HSC7\EpH\YS25.iep pH Eh (Volts) H2O Limits Y(OH)3 YH3 Y(+3a) YSO4(+a) Y(SO4)2(-a)

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41 Figure 3.4. Eu-S-H2O System at 25°C

Figure 3.5. Gd-S-H2O System at 25°C

14 12 10 8 6 4 2 0 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Eu - S - H2O - System at 25.00 C C:\sw\HSC7\EpH\EuS25.iep pH Eh (Volts) H2O Limits Eu(OH)3 Eu2(SO4)3*8H2O Eu(+3a) Eu(+2a) EuO(+a) Eu(SO4)(+a) 14 12 10 8 6 4 2 0 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Gd - S - H2O - System at 25.00 C C:\sw\HSC7\EpH\GdS25.iep pH Eh (Volts) H2O Limits Gd(OH)3 Gd(+3a)

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42 Figure 3.6. Tb-S-H2O System at 25°C

3.3 Precipitation of Indium Hydroxide

The objective of the precipitation of indium hydroxide was to selectively recover indium from the sulfuric acid leach solution. The following sections describe the fundamental chemistry used to develop the indium precipitation process.

The first step in developing the precipitation of indium hydroxide was to understand the chemical reaction taking place. The proposed chemical reactions are as follows:

14 12 10 8 6 4 2 0 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Tb - S - H2O - System at 25.00 C C:\sw\HSC7\EpH\TbS25.iep pH Eh (Volts) H2O Limits TbH2 Tb2O3 TbO2 Tb7O12 Tb(+3a) TbO(+a) TbSO4(+a)

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In2(SO4)3(a) + 6NaOH = 2In(OH)3(s) + 3Na2SO4 (3.7) Sn(SO4)2(a) + 4NaOH = Sn(OH)4(a) + 2Na2SO4 (3.8)

H2SO4 + 2NaOH = Na2SO4 + 2H2O (3.9)

The objective of the precipitation was to perform reaction 3.7 and minimize reaction 3.8. Reaction 3.9 is the reaction between the sulfuric acid and sodium hydroxide and would occur along with both reactions.

3.3.2 Thermodynamic Data

HSC Chemistry 7.1 was used to evaluate the thermodynamics of reactions 3.7, 3.8, and 3.9. The thermodynamic data for the proposed reactions can be seen in Table 3.7, Table 3.8, and Table 3.9.

As can be seen in the tables, all of the proposed reactions are thermodynamically favorable throughout the temperature range. For all of the reactions, the Gibbs free energy becomes more negative as temperature increases. The enthalpies of the reactions are less than zero, suggesting that the reactions are exothermic.

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C kJ J/K kJ 0.000 -443.534 724.738 -641.496 4.831E+122 122.684 10.000 -443.262 725.718 -648.749 4.889E+119 119.689 20.000 -442.990 726.659 -656.010 7.951E+116 116.900 30.000 -442.711 727.595 -663.282 1.983E+114 114.297 40.000 -442.403 728.594 -670.563 7.276E+111 111.862 50.000 -442.072 729.635 -677.854 3.793E+109 109.579 60.000 -441.724 730.696 -685.155 2.721E+107 107.435 70.000 -441.365 731.758 -692.468 2.612E+105 105.417 80.000 -440.999 732.809 -699.790 3.275E+103 103.515 90.000 -440.632 733.835 -707.124 5.243E+101 101.720 100.000 -440.266 734.827 -714.467 1.051E+100 100.022

In2(SO4)3(a) + 6NaOH = 2In(OH)3 + 3Na2SO4

C kJ J/K kJ 0.000 -354.585 433.995 -473.131 3.053E+090 90.485 10.000 -361.292 409.883 -477.350 1.168E+088 88.068 20.000 -368.247 385.746 -481.328 5.919E+085 85.772 30.000 -375.445 361.605 -485.065 3.862E+083 83.587 40.000 -382.871 337.506 -488.561 3.167E+081 81.501 50.000 -390.529 313.436 -491.815 3.197E+079 79.505 60.000 -398.423 289.378 -494.829 3.899E+077 77.591 70.000 -406.558 265.321 -497.603 5.650E+075 75.752 80.000 -414.936 241.257 -500.136 9.586E+073 73.982 90.000 -423.561 217.176 -502.428 1.880E+072 72.274 100.000 -432.434 193.073 -504.479 4.212E+070 70.624

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3.4 Precipitation of Rare Earth Oxalates

The objective of the rare earth precipitation experiments was to selectively precipitate the REE as oxalates. The following sections describe the fundamental chemistry used to develop the rare earth precipitation process.

The first step in developing the rare earth precipitation process was to understand the reactions taking place. The proposed reactions are as follows:

Y2(SO4)3(a) + 3H2C2O4 = Y2(C2O4)3(s) + 3H2SO4 (3.10) Eu2(SO4)3(a) + 3H2C2O4 = Eu2(C2O4)3(s) + 3H2SO4 (3.11) Gd2(SO4)3(a) + 3H2C2O4 = Gd2(C2O4)3(s) + 3H2SO4 (3.12) Tb2(SO4)3(a) + 3H2C2O4 = Tb2(C2O4)3(s) + 3H2SO4 (3.13)

C kJ J/K kJ 0.000 -278.640 -6.585 -276.841 8.811E+052 52.945 100.000 -268.567 27.381 -278.784 1.067E+039 39.028 200.000 -262.638 41.555 -282.300 1.472E+031 31.168 300.000 -257.764 50.888 -286.930 1.419E+026 26.152 400.000 -261.798 43.358 -290.985 3.815E+022 22.582 500.000 -252.813 55.806 -295.959 9.930E+019 19.997 600.000 -243.998 66.530 -302.089 1.184E+018 18.073 700.000 -235.312 75.949 -309.222 3.973E+016 16.599 800.000 -226.705 84.368 -317.245 2.773E+015 15.443 900.000 -218.136 92.004 -326.070 3.308E+014 14.520 1000.000 -209.599 98.987 -335.624 5.903E+013 13.771

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The objective of the rare earth precipitation experiments was to perform reactions 3.10, 3.11, 3.12, and 3.13 and minimize the precipitation of impurities.

3.4.2 Thermodynamic Data

HSC 7.1 did not have a sufficient database to perform the required thermodynamic calculations. Dr. Hsin Huang used the program Stabcal to determine the thermodynamics of the rare earth oxalate precipitation reactions. The thermodynamic data can be seen in Table 3.9. For all of the reactions, the Gibbs free energy is leass than zero and, therefore, the reactions are spontaneous.

Table 3.10. Thermodynamic Data for Rare Earth Oxalate Precipitation

Chi and Xu described the distribution of oxalic acid precipitants. The two-step dissociation was expressed by(Chi and Xu):

H2C2O4 = H+ + HC2O4- (3.14)

HC2O4- = H+ + C2O42- (3.15) The predominant species at a pH of 2 is HC2O4- and the predominant species at a pH of about 4.5 or higher is C2O42-. The latter is the active species for oxalate precipitation.(Chi and

Reaction delta Greaction

kcal logK 2(Y3+ A) + 3(H2C2O4) = Y2(C2O4)3 + 6(H + A) -16.501 12.095 2(Eu3+ A) + 3(H2C2O4) = Y2(C2O4)3 + 6(H + A) -19.172 14.053 2(Gd3+ A) + 3(H2C2O4) = Y2(C2O4)3 + 6(H + A) -19.165 14.048 2(Tb3+ A) + 3(H2C2O4) = Y2(C2O4)3 + 6(H + A) -16.572 12.147 A = acid

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Xu). An equilibrium constant equation based upon the equations shown in section 3.4.1 was created to determine if the precipitation reaction will occur. Chi and Xu described the equilibrium constant equation as:

(3.16)

Where RE is the rare earth element and Ksp is the solubility product of the rare earth oxalate. Using equation 3.16, a large K3 value of 1.79 × 1010 is calculated signifying that the reaction is thermodynamically feasible. Chi and Xu determined that the solution stabilizes at a pH of around 2.

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CHAPTER 4 EXPERIMENTAL PROCEDURES

The experimental procedures that were used during this project are discussed in this chapter. The procedures are described in order to allow future researchers to better understand the testing methods that were used and to be able to reproduce the experiments.

4.1 Sample Preparation

Taking representative samples is an important part of scientific testing. The homogeneity of each of the glass panels in a PDP lowered the importance of sampling. On an industrial scale, the glass will be fed into the system at any number of different shapes and sizes. The goal for laboratory experimentation was to create glass samples that were close to a rectangular shape in order for the area of the sample to be determined easily and accurately. During experimentation, there were five types of samples that were tested:

 Glass fragments with adhesive holding the front and back glass together.

 Front glass fragments containing the transparent ITO thin film.

 Back glass fragments containing the rare earth phosphors

 Stock ITO powder used as a sputtering feedstock for ITO thin films.

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4.1.1 Adhered Glass Fragments Sample Preparation

PDPs were received from Veolia and were from a 42 inch Plasma TV. The two glass panels are adhered along the outside border of the panel. Samples were taken along this border using three basic steps:

 The adhered glass was physically pried away from the metal frame. The fragments broke off in different sizes and shapes. The border between the adhered glass and non-adhered glass on each of the fragments was then scored. A metal rod was placed under the score and pressure was applied to the edges of the glass to make a clean break along the score.

 Scores were then made across the adhered glass on both glass faces to make similar sized samples for testing. The scores were made to maintain a constant length for all of the samples. The same metal rod was used to make a clean break along the score line.

 Breaking was continued to make multiple samples of the same length.

Figure 4.1 shows the adhered glass after being pried from the metal frame. The fragment would then be scored and broke so that only the adhered glass was remaining. The sample will be the same width as the adhesive.

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4.1.2 Front Glass Sample Preparation

The front glass panel of the PDP contains the ITO transparent thin film and covers the front area of the PDP. Samples consisting of a much smaller area were required for laboratory sized experiments. The front glass was scored so as to outline samples that would be of similar size and rectangular shape. A metal rod was then placed underneath the score and pressure was applied to the edges of the front glass panel to break the glass along the score. Figure 4.2 shows a typical sample that was produced from the above method.

Figure 4.2: Front Glass Sample

4.1.3 Back Glass Sample Preparation

The back glass panel of the PDP contains the rare earth phosphors and covers the same area as the front glass panel. Samples for laboratory testing needed to have much smaller areas. The same scoring and breaking method that was used for the front glass samples was also used

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for the back glass samples. Scoring was done to outline acceptable sized samples and then the glass was broken along the scoring lines using a metal rod and applying pressure. Some of the back glass samples did not break evenly due to the back conductive layer being still attached. A few of these samples were used in testing, but their areas had to be accurately calculated before testing.

4.1.4 ITO Powder Sample Preparation

ITO powder was ordered from Alfa Aesar and consisted of 90% In2O3 and 10% SnO2. The particle size was between 45µm and 25µm which was deemed small enough for chemical testing. The powder was purchased to closely resemble the material that is sputtered onto the front glass of a PDP. Due to the ITO powder being bought from a chemical supply company, the particle size was already consistent throughout the powder and did not require sample

preparation to produce a uniform particle size distribution.

4.1.5 Phosphor Powder Sample Preparation

Phosphor powder was received from General Electric (GE) as well as purchased from Phosphor Technology. The powder from GE was from fluorescent lamps which have the same chemistry as phosphors used in PDPs. The purchased powder was phosphor powder used in PDP applications. The combined phosphor powder amounted to one blue phosphor, one green phosphor, and two red phosphors. The particle sizes of the powder were all below 45µm and above 5µm. The particle sizes were fairly consistent between the powder received from GE and the purchased powder, although the GE powder was slightly larger in average particle size. For laboratory testing, the powders were blended together to form one feed sample. The four powders were combined together in equal parts by weight for each test.

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4.2 Thermal Delamination Experimentation

In order for the front and back glass panels to be separated from each other and to be processed, the PDP had to be delaminated. A series of tests were performed in a tube furnace at varying temperatures and atmospheres. The experimental set up used for the thermal

delamination testing can be seen in Figure 4.3.

Figure 4.3: Tube Furnace used for Thermal Delamination Experimentation

Either air or nitrogen, depending on the test, was supplied to the furnace at a rate of 1 L/min. The furnace was heated to the desired temperature and the prepared adhered glass sample was placed into the center of the furnace. Inlet gas was run through a column containing

Drieright® to ensure proper moisture removal from the gas entering the furnace. Outlet gas was passed through a fiberglass filter to remove particulates and then was sparged into a dilute NaOH scrubbing solution. The ceramic openings of the tube furnace were kept cool by running cold water through copper pipe which was wrapped around the ceramic. This prevented the ceramic

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from overheating and cracking. The temperature inside the furnace was monitored during testing using a thermocouple. Samples were kept in the furnace for the desired test time (between 20 and 90 minutes) and were allowed to cool in the furnace. Two tests were run with the sample being immediately removed after the desired time had elapsed. Delamination was then attempted while the sample was still hot.

4.3 Leaching Experimentation

Two different types of leaching tests were used during the leaching experimentation. These consisted of batch leaches and kinetic leaches. The methods for each of these types of leaching tests are described in greater detail in the next sections.

4.3.1 ITO Powder Batch Leaching

Batch leaching tests were conducted on the ITO powder. These tests were conducted to determine initial leaching conditions. A known mass of ITO powder was measured based on the solid to liquid ratio of the test. The solid to liquid ration was varied between 1g/L to 30g/L. The sample was mixed into a leach solution (HCl, H2SO4, or NaOH) of known concentration and placed into a glass beaker containing a magnetic stirring rod. The concentration of the lixiviant was varied for each test, but the concentration was kept between 0.5M and 6M. The slurry was mixed at varying rotations per minute (RPMs) and for varying lengths of time. Agitation was varied between 200 RPM and 600 RPM while the time varied between 30min and 4hr.

Temperature was also test dependent and fluctuated between 20°C and 90°C depending on the test.

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4.3.2 Phosphor Powder Batch Leaching

Batch leaching tests were also conducted on the phosphor powder. Powder samples were prepared in the manner discussed in 4.1.5. A known mass of mixed powder was measured based on the desired solid to liquid ratio which was kept between 1g/L and 10g/L. The sample was mixed into an acid leach solution of known concentration. The acid concentration was kept between 0.5M and 4M. The slurry was mixed with a magnetic stirring rod which was running between 200 RPM and 600 RPM depending on the test. The test time was changed for each test with the shortest being 1hr and the longest being 4hr. Temperature was test dependent and was varied between 20°C and 70°C.

4.3.3 Combined Powder Batch Leaching

After leaching parameters were determined for the ITO powder and the phosphor powder, combined batch leaching tests were conducted. Test parameters were taken directly from the individual powder tests. A known quantity of ITO powder and phosphor powder were combined and mixed into a H2SO4 solution of known concentration. The concentration was kept at 1M, which was predetermined from the ITO powder batch tests and the phosphor powder batch tests. The solid to liquid ratio was test dependent staying between 4 g/L and 20 g/L. The temperature was between 70°C and 90°C for most of the tests, with one test being run at ambient temperature. The slurry was mixed with a magnetic rod set to between 400 RPM and 600 RPM depending on the test. Leaching times were kept between 2 hr and 4 hr to ensure proper leaching of the ITO.

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4.3.4 ITO Powder Kinetic Leaching

A kinetic leach experiment was done by taking 0.5g of ITO powder and mixing it with 1M H2SO4. The solid liquid ratio was 1g/L for the experiment. The solution was stirred with a magnetic stirring rod spinning at 800 RPM. The solution was heated to 90°C and the experiment was run for 24hr. During the test, 10 samples of 2mL each were taken for chemical analysis. The samples were taken at the following times:

 Sample 1: 1minute  Sample 2: 5 minutes  Sample 3: 10 minutes  Sample 4: 30 minutes  Sample 5: 1 hour  Sample 6: 2 hours  Sample 7: 4 hours  Sample 8: 8 hours  Sample 9: 12 hours  Sample 10: 24 hours

The slurry was allowed to settle for one minute before each sample was taken to avoid the collection of solids in the 2 mL samples. Temperature was recorded at each sample interval for the first hour and then every hour after that. Figure 4.4 shows the experimental set up used during the experiments. The watch glass was used to minimize liquid loss through evaporation. The solution volume was taken before and after the experiments to determine the amount lost to evaporation.

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Figure 4.4: Photograph showing the experimental set up used during the ITO powder kinetic leaching test. The thermometer used for temperature readings is not shown.

4.3.5 Phosphor Powder Kinetic Leaching

A kinetic leaching experiment was also conducted on the phosphor powder. About 1g of mixed phosphor powder was mixed with 1M H2SO4. The solid to liquid ratio was 2g/L and the slurry was stirred with a magnetic stirring rod spinning at 800RPM. The solution was heated to 70°C and the experiment was conducted for 24hr. During the test, 9 samples of 2mL each were taken out for chemical analysis. The samples were removed in the following schedule:

 Sample 1: 1minute

 Sample 2: 5 minutes

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 Sample 6: 2 hours

For each sample, 10mL of slurry were removed and placed into a centrifuge tube. The tube was spun in the centrifuge for one minute to force the solid particles to settle. 2mL of the solution were taken from the tube for chemical analysis while the rest was re-mixed and put back into the test slurry. This procedure was done to avoid collection of any solids in the samples. Temperature was recorded at each sample interval for the first hour, and then every hour after. The solution volume was measured before and after the test to determine the amount of solution lost during the experiment due to evaporation.

4.4 Thin Film Removal

Batch leaching tests were conducted on the front glass and the back glass. Tests were run separately on each type of glass to determine the best parameters for removing each of the thin films. Combined tests were then run with both types of glass to determine if the thin films could be removed simultaneously. The methods for each of the thin film removal tests are discussed in greater detail in the following sections.

4.4.1 ITO Thin Film Removal

Batch leaching experiments were conducted on the PDP front glass for ITO thin film removal. Glass samples were prepared in the manner discussed in section Front Glass Sample Preparation. Samples were then mixed with either HCl or H2SO4 of known concentrations and stirred using a drop down propeller running at a pre-determined RPM based on the particular

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test. An average of 125g of glass was used per test with 500ml of solution being used for all tests. A magnetic stirring rod could not be used for agitation because it would collide with the chunks of glass in the beaker. The solution was heated to the desired test temperature (between 20°C and 70°C) and the test was run for varying lengths of time. Experiments were initial run to determine if the thin film could be removed, and then parameters were changed based on the results from the ITO powder leach.

4.4.2 Phosphor Thin Film Removal

The back glass containing the rare earth phosphors was also used in batch leaching experiments. Glass samples for the thin film removal experiments were prepared in the manner discussed in section Back Glass Sample Preparation. The prepared glass samples were mixed with known concentrations of either HCl or H2SO4. A drop down propeller spinning at pre-determined RPM was used to agitate the solution. The use of a magnetic stirring rod was attempted, but the rod had a tendency to collide with the glass samples which would stop its continuous rotation. An average of 60g of back glass and 300mL of solution was used for each test. The solution was heated to the desired test temperature (between 20°C and 70°C) and each test was run for varying lengths of time. Experiments were initially conducted to determine if the thin film could be removed, and then experimental parameters were changed according to the results from the phosphor powder batch leaching experiments. After batch leaching, the samples were washed, dried and the remaining thin film was scrapped off. Scraping was done with a metal spatula and required very little effort.

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4.4.3 Simultaneous Thin Film Removal

A thin film removal test was conducted on a combined feed of front and back glass. Front and back glass samples were prepared in the same manner as the individual film removal tests. The prepared glass samples were added to 1L of 1M H2SO4 and stirred with a drop down propeller spinning at 400RPM. The total glass weight was about 490g. The solution was heated to 90°C and the test was run for 4hr. The experimental parameters were determined from the ITO and rare earth phosphor individual thin film removal tests. After completion of the test, the back glass samples were washed and scraped with a metal spatula to remove any visible thin film still on the glass.

4.5 Precipitation Testing

Precipitation tests were conducted on the solutions after leaching tests had been completed. Precipitation tests were run on the ITO powder leach solutions to recover In from solution. Tests were also run on the phosphor powder leach solutions to recover Y, Eu, Tb, and Gd from solution. Initial tests were run to determine the best precipitation parameters for each of the elements. These parameters were then used to test a two-staged precipitation to precipitate In and the rare earths in separate steps from the combined powder leach solution. The methods for the precipitation tests are described in the following sections.

4.5.1 Indium Precipitation Testing

Precipitation tests were conducted to attempt at recovering In from solution while precipitating a minor amount of Sn. Solutions from the ITO batch leaching experiments were used as the feed solution for precipitation testing. 50mL of solution were put into a beaker and

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agitated using a magnetic stirring rod. The stirring rod was spun between 200RPM and 600RPM depending on the test. A known quantity of 3M NaOH was added to the solution until the

solution reached the desired pH. Precipitation was tested at pH values of 2, 4, 6, and 10. Temperature was also varied between tests and was kept between 20°C and 70°C. The test time started when the solution was at the desired temperature and pH. Experiments were run for 30 minutes, 60 minutes, and 90 minutes. After the experiments were finished, the solution was filtered and samples were taken for chemical analysis. Initial experiments were done to determine the feasibility of In precipitation. Later experiments were used in an attempt to optimize the process parameters.

4.5.2 Rare Earth Precipitation Testing

Precipitation experiments were conducted in an attempt to recover Y, Eu, Gd, and Tb from solution. The solutions from the phosphor batch leaches as well as the solutions from the phosphor thin film removal experiments were used as the feed solutions. 50mL of solution was added to a beaker and agitated with a magnetic stirring rod spinning between 200 and 600RPM depending on the test. A known quantity of oxalic acid was added to the solution and NaOH was then added until the desired pH was reached. Oxalic acid concentration was test dependent and was between 5g/L and 30g/L. Test pH varied between 2 and 6. The tests ran at a pH of 2 required little to no NaOH addition. The test time started when all of the oxalic acid was dissolved, the desired pH was achieved, and the desired temperature was reached. Test

temperatures were between 20°C and 70°C. Experiments were run for 30 minutes, 60 minutes, and 90 minutes. After the experiments were finished, the solution was filtered, the precipitate was tried, and samples were taken for chemical analysis. Initial tests were run to determine the