| Date post: | 05-Aug-2015 |
| Category: |
Science |
| Upload: | rigoberto-carbajal-valdez |
| View: | 32 times |
| Download: | 1 times |
RecCathDiplom
DAV
DepartmIndustriCHALMGötebor
cyclinhode
ma of Scien
VID PRE
ment of Cheial MaterialMERS UNIVrg, Sweden,
ng PhoRay Tnce Thesis
ENTICE
mical and Bl Recycling VERSITY O, 2009
osphoTubess in Chemi
E
Biological E
OF TECHN
rescens and ical Engin
Engineering
NOLOGY
nt PowFluor
neering
wder rescen
from nt Tubbes
Recycling Phosphorescent powder from CRT’s and Fluorescent tubes DAVID PRENTICE © David Prentice, 2009 Department of Chemical and Biological Engineering Industrial Materials Recycling Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone + 46 (0)31-772 1000 Göteborg Sweden 2009
Diploma work 30 higher education credits
Recycling Phosphorescent Powder from Cathode Ray Tubes and Fluorescent
Tubes
David Prentice
Industrial Materials Recycling Department of Chemistry and Biological Engineering
CHALMERS UNIVERSITY OF TECHNOLOGY
Göteborg, Sweden 2009-06-18
Examiner: Professor Christian Ekberg Supervisor: Dr. Mark Foreman
Abstract The aim of the project is recycling phosphorescent powder from scrap cathode ray tubes (CRT)
and fluorescent tubes (FT). The CRT powder that all experimental work was carried out with
was separated from the glass and metal of the cathode ray tube by Stena Metall AB before it was
supplied to us. The fluorescent tubes that were used for the study were standard Aura, Ultimate
Long life, 18W 830 bulbs.
Initially the composition of the CRT and tube powder was investigated. The powders were both
suspected to contain a high concentration of lanthanides and it was hoped that these metals
would be recyclable. The presence of these metals was investigated using X-ray powder
diffraction, ICP-OES and Scanning Electron Microscopy.
Once the composition of the powders was determined the focus of the project then turned to
identifying the optimal leaching conditions for both the CRT powder and the fluorescent tube
powder.
Table of Contents 1. Introduction 1
1.1 Background 2
1.2 Purpose of Study 5
2. Experimental Materials 6
3. Experimental Techniques 7
3.1 X-ray powder diffraction 7
3.2 ICP OES 10
3.3 Electron Microscopy 11
4. Experimental Procedure and Theory 13
CRT Powder 13
4.1 XRD 13
4.2 Matrix Analysis 13
4.3 Aqua Regia Leaching 15
4.4 Nitric Acid Leaching 16
4.5 Citric Acid and TLCP Pre-Digest Room Temperature Leaching 17
4.6 Heated Leaching 18
Fluorescent Tube Powder 19
4.7 Nitric Acid Digest 19
5. Results and Discussion 21
CRT Powder 21
5.1 XRD 21
5.2 Matrix Analysis 22
5.3 Aqua Regia Leaching 24
5.4 Nitric Acid Leaching 27
5.5 Citric Acid and TLCP Pre-Digest Room Temperature Leaching 31
5.6 Heated Leaching 34
iv
Fluorescent Tube Powder 37
5.7 Nitric Acid Digest 37
6. Conclusions 40
7. Future Work 42
8. Acknowledgements 42
9. References 43
1
1. Introduction
In the world today it is estimated that 1 billion people own a television set, the majority of which
are cathode ray tube televisions. The growing popularity and the superior image quality produced
by a liquid crystal display (LCD) television means cathode ray tube televisions are becoming
obsolete and people are starting to replace them [1, 2]. Recycling processes are already in
operation in Germany and Poland at Stena Metall AB plants but the plants only recycle the glass
from the CRT so there is potential to expand the operation to include the recovery of precious
metals from the phosphorescent powder.
In modern society today almost every house hold is illuminated by incandescent light bulbs.
However the increasing scientific advancements regarding compact fluorescent tubes (CFT)
mean more and more people are converting to the fluorescent lighting. Compact fluorescent
tubes are superior to incandescent bulbs because they consume less electricity and generally have
a longer life expectancy [3]. With the ever growing popularity of the fluorescent lighting there is
a real need for an effective recycling process.
Recycling is an essential process nowadays due to the immense consumer demand for products
and services. In the world today both cathode ray tubes and fluorescent tubes are currently
recycled, however the processes used simply recycle the glass from the sources and there is real
potential to increase the effectiveness profit production of the recycling processes. This project
experimented with the potential of recycling phosphorescent powder from CRT’s and the
fluorescent powder from fluorescent tubes in the hope determining a viable and profitable
process that could be scaled up to an industrial process.
2
1.1 Background
CRT
The cathode ray tube was invented in 1897 by an English scientist named William Crookes and
has come a very long way since its invention. The tube invented by Crookes formed the basis for
all future cathode ray tubes. In 1926 a Scottish scientist named John Logie Baird successfully
produced an image on a television screen using a CRT in front of 40 persons at the Royal
Institution in London and he was the first ever person to do so [2]. NB John Logie Baird invented
the first TV but there were others who produced image production devices before 1926.
The cathode ray tube of a television uses the thermionic effect. Electrons generated under
vacuum conditions by the cathode are accelerated using electric fields and then deflected by
magnetic fields to strike the phosphorescent powder covering the screen. In summary, a beam of
electrons is fired at phosphorescent powder creating light which the human eye sees an image
[1].
Figure 1 diagram of cathode ray tube from a television set [4]
The phosphorescent powder is found on the screen on the screen of the CRT and it is responsible
for creating the images. Phosphorescent powders often contain lanthanides as they posses the
3
light emitting quality required for a CRT [1, 2]. The phosphorescent powder used in CRT has
developed throughout the years but has not changed much in comparison to the significant
electronic advancements of the cathode ray tubes. The progression from black and white
televisions to the high definition colour screens of today is a result of electronic advancements
rather than the effect of the phosphorescent powder. Different phosphors produce different
colour of light, for example europium glows red when bombarded with electrons.
Fluorescent Tube
Antoine-Henri Becquerel is thought to be the first person to ever produce a fluorescent lamp
when he produced a crude lamp in 1867. Thomas Edison, famous for inventing the incandescent
bulb along with several other inventions is credited with the invention of the fluorescent tube as
he was the first man to produce a commercially viable working tube [3]. The bulb invented by
Edison worked on the same principles as modern day tubes but the fluorescent coating he used
was calcium tungstate phosphor compound containing no lanthanides. However Edison’s
fluorescent light was not put into production and it was not until 1933 that fluorescent tubes
became available commercially [3, 5].
A fluorescent tube is a sealed under vacuum conditions and works by exciting gaseous vapour,
usually mercury, with electricity. When the mercury atoms are electrically excited they produce
short wavelength ultra violet rays, which in turn react with the fluorescent powder coating the
bulbs to produce light [3]. Scientifically speaking the excited ultra violet photons in the gas are
absorbed by the atoms of the fluorescent powder coating the tube resulting in the electrons of the
powder attaining an excited state. The electrons then return to there original state in a sudden
jump and it is during this emission stage that another photon is released which produces visible
light. Modern day tubes are shifting away from mercury vapour and are instead being filled with
inert gases such as argon and xenon [3].
4
Figure 2 basic diagram depicting a fluorescent bulb
5
1.2 Purpose of Study
The purpose of this study was to determine the optimal leaching conditions of both CRT powder
and fluorescent tube powder. The leaching process was specifically tailored to maximise the
recovery of the precious lanthanide metals. It was important to design an economically viable
and efficient leaching process so that it could be potentially implemented in industry. The
recovery of rare earth metals has the potential to be very lucrative as well as environmentally
friendly. The following figure (figure 3) shows the global price trend for rare earth metals.
Figure 3 Global Price trend for Rare Earth lanthanides [6]
0
2 000
4 000
6 000
8 000
10 000
12 000
1985 1990 1995 2000 2005 2010
Global Cost of Rare Earth Metals
Global Cost of Rare EarthMetals
Year
Cost Rare Earth Metals ($/t)
6
2. Experimental Materials
The phosphorescent CRT powder analysed in this project was received from Stena Metall AB.
The phosphorescent powder was separated from the rest of the cathode ray tube before it was
delivered but it is noteworthy that the powder came from a variety of sources including
televisions and computer monitors.
The fluorescent powder analysed in the report came from Aura, Ultimate Long life, 18W 830
fluorescent tubes. The powder was separated from the glass as explained later in the report.
7
3. Experimental Techniques
3.1 X-ray diffraction
Theory
X-ray powder diffraction (XRD) is an instrumental analysis technique used to distinguish
structures not specific compounds. Nominally XRD is used as a tool for providing approximate
information on a sample composition but it can also be used to provide structural information
such as the size and the degree of crystallation [7].
An XRD machine works by firing a beam of monochromatic X-rays at a sample. The x-rays are
produced under vacuum conditions inside a sealed tube. The tube is a high powered diode which
is controlled by adjusting the temperature of the cathode [7, 8]. When the X-rays make contact
with the sample they react in several different ways. Some X-rays are transmitted through the
sample, some are absorbed by the sample and some are diffracted by the sample [8]. It is the X-
rays that are diffracted by the sample that the machine interprets. X-rays can be produced using
two well known methods, the “Bremsstrahlung” and characteristic radiation. “Bremsstrahlung”,
meaning “to brake radiation” in German, is electromagnetic radiation and it works by producing
a beam of charged particles (electrons or protons) and accelerating them before deflecting them
with charged particles. During the deflection period X-rays are emitted if the bombarding
particles posses enough energy [9]. Characteristic radiation is formed by bombarding an atom
with electrons so that some electrons from the atoms are ejected from their shells. The electrons
then leave an electron hole which is filled by another electron belonging to the atom and it is
during this stage when the electron drops to a lower energy level that X-rays are emitted [10].
8
Figure 6 XRD analysis diagram
Sample
d-spacing
X-rays
Diffracted X-rays
)θ
Incident X-rays
Transmitted X-rays
Beam of charged
particles, typically
electrons
Charged
particles, either
similar or
opposite X-rays
Deflected Particles
e-
e-
e-
X-rays
Figure 5 Diagram of Characteristic Radiation
Figure 4 Diagram depicting Bremsstrahlung
9
When X-ray beam is diffracted by the sample the XRD machine can measure the distances
between the planes of the atoms by applying Bragg’s Law (equation1).
Equation 1
Where: n = order of the diffracted beam
λ = wavelength of the x-ray beam
d = distance between the atom planes
θ = angle of incidence for the x-ray beam
It is possible to rearrange Bragg’s law in order to determine the distance between the atom
planes, d. Every crystalline material has a characteristic set of d spacing’s. This means that by
determining the d value for any unknown sample it is possible to determine the structure of the
sample. [8].
All XRD analysis carried out in this report was carried out on a Siemens D5000 model with the
following specifications: Göbel multilayer mirror on primary side, long Soller slits and Sol'X
solid state energy dispersive detector on secondary side.
n λ = 2d sin θ
Figure 7 Diagram depicting the layout and theory behind the X-ray powder diffraction machine
[7]
10
3.2 ICP-OES
Theory
ICP OES stands for Inductively Coupled Plasma Optical Emission Spectroscopy. It is an
analytical technique used to determine the concentration of trace metals in liquid samples. Liquid
samples are drawn up through a capillary tube and enter into a nebulizer. Inside a pneumatic
concentric nebulizer samples interact with an argon gas supply to form mixture of fine droplets.
The mixture is then compressed and released into a spray chamber. When in the spray chamber
the aerosol mixture is heated by a plasma torch which supplies atoms with thermal energy so that
they can enter excited states [11, 12].
When an atom is supplied with enough thermal energy, it is possible for an electron to rise from
a low energy level, “ground state”, to one with a higher energy known as an “excited state”. This
stage is known as the “absorption” stage. When an electron is in an excited state it is unstable so
it returns to the ground state in order to obtain a more stable form [11]. When the electron returns
from a higher energy state to a lower energy state it is known as “emission”. It is the intensity of
the emission stage that the ICP OES measures and records.
Figure 8 photon absorption and emission diagram for gas atom
S3
Photon Absorption,
Occurs when atoms are
heated by plasma torch.
Photon Emission,
Occurs when electrons
lose thermal energy and
return to original state.
S2
S1
Ground State, S0
S4
11
All elements have unique wavelengths so the ICP OES machine is able to measure the intensity
of each wavelength. Therefore it is able to determine the concentration of the elements giving a
clear indication of the sample composition [11, 12].
When using ICP OES analysis it is important to create standards if you desire to make
quantitative measurements. Standards are solutions containing known concentrations of a
specific metal known or thought to be in a sample. Without standards there would be no basis for
comparison when analysing data from the ICP OES but it would be possible to [11].
All ICP OES analysis in this project was carried out on an iCAP 6000 series ICP Emission
Spectrometer.
3.3 Electron Microscopy
Theory
Scanning Electron Microscopy (SEM) is an analytical technique that takes photographic images
of samples and can also be used to obtain information regarding the quantity of elements present
within a sample. SEM works by producing electrons with kinetic energy within the range of 0.1-
30 keV depending on the magnification of the desired image [13]. When the electrons obtain the
desired energy level they pass through the final lens and enter the specimen chamber where they
interact with the sample. Signals are then generated that enable the image of the sample to be
formed [13].
12
Figure 9 Diagram of SEM [13]
When observing an image produced by the SEM it is important to note that a dark image
indicates the presence of light weight elements but also implies the sample is not very dense.
Whereas a light coloured image indicates the presence of heavy weight elements and also implies
the sample is quite dense. It is also possible using an electron microscope to carry out energy
dispersive X-ray (EDX) analysis. EDX is another analytical method that can determine the
composition of a sample. It works by firing a high energy beam of charged electrons at a sample
which on impact excite the atoms of the sample [14]. Some electrons in the sample then jump
from ground state to an excited state leaving a space in the electron orbital known as an “electron
hole”. The hole is then filled by an electron from a higher energy level and during this energy
drop X-rays are emitted. The energy of the produced X-rays can be measured and it is this
unique value that identifies the composition of the sample [13, 14]. It is possible to apply
Moseley’s law (equation 2) to identify the atomic number, z, value of the X-ray emission spectra
thus identify the composition of a sample.
13
Equation 2
Where,
f = frequency of the X-ray emission line
z = the atomic number of the element
k1 and k2 = constants that depend on the line type
All Electron Microscopy analysis was carried out on a Quanta 200 ESEM FEG.
4. Experimental Procedure and Theory
CRT Powder
4.1 XRD
To obtain an overview of the CRT powder experimental work began by carrying out X-ray
powder diffraction analysis. The intention was to use the X-ray diffraction data to obtain an
overview of the sample and gain some insight into the composition of the powder. The raw CRT
powder received from Stena Metall was taken for XRD analysis without any form of
pretreatment.
4.2Matrix Analysis
Theory
When using an analytical method like ICP OES there is a distinct possibility that the matrix of a
sample can invalidate readings because of interference effects. The composition of a sample and
the structure the compounds form what is known as the matrix [15].
√ ( )
14
4.2.1 Procedure
To check the validity of the results it was important to run a matrix analysis test. The test
reviewed the pre-digest experiment when the CRT powder was first digested in citric and TLCP
before being leached with 2M nitric acid at room temperature.
TLCP (S1) and 0.2M citric acid (S2) were diluted with both 1M nitric acid (D1) and 1M nitric
acid with uranium and ruthenium internal standard (D2). Each of the four solutions were mixed
together to form four new solutions which were S1 + D1, S1 + D2, S2 + D1 and S2 + D2. The
four solutions were then diluted to the following degrees 1:1, 1:10 and 1:100. When the solutions
were being diluted it was D1 and D2 that were used to dilute them (e.g. S1+D1 1:10, was made
from 1ml S1 and 10ml D2).
A metal standard was composed of 10 metals that were most relevant to the CRT powder was
produced which was then used to spike the 4 solutions. The standard contained: Ca, Eu, Pb, Y,
Zn, Cu, In, Pd, Se and Fe.
The standard was then mixed with the 4 experimental solutions to produce 1ppm solutions and
5ppm solutions. This meant 8 solutions were produced containing varying concentrations of the
metal standard. The following 8 mixtures were made: S11, S15, S21, S25, D11, D15, D21 and
D25. For example S11 was S1 containing 1ppm metal standard and S15 was S1 containing 5
ppm metal standard. Once again for purposes of the ICP OES each mixture was diluted with 1M
super pure nitric acid (D1) according to the following ratios 1:1, 1:10 and 1:100.
15
S1 D1 S2 D2
S1 S2
S1+D1 S1+D2 S2+D1 S2+D2
1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100
Ca, Eu, Pb, Y, Zn, Cu, In, Pd, Se and Fe
100ppm Solution
S1 S2 D1 D2
S1-1ppm S1-5ppm S2-1ppm S2-5ppm D1-1ppm D1-5ppm D2-1ppm D2-5ppm 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100 1:1 1:10 1:100
Figure 10 flow diagram depicting the matrix analysis method
Ideally the matrix analysis will reveal little or no interference between the solutions and the ICP
OES. A perfectly linear relationship between the concentration of the solutions and the intensity
of the standard would imply there is no matrix effect.
4.3 Aqua Regia Leaching
Aqua regia is an acid formed by mixing concentrated nitric acid and concentrated hydrochloric
report in a volume ratio of 1:3. It is an exceptionally potent mixture that is widely known for its
ability to dissolve the majority of all metals. It received the name “aqua regia”, meaning king’s
water in Latin, because of its ability to dissolve gold.
The aqua regia was thought the aqua regia would offer a means of totally dissolving the
phosphorescent powder allowing the quantity of all elements present to be determined.
1M HNO3
16
4.3.1 ICP OES
The aqua regia was produced by mixing concentrated nitric acid with concentrated hydrochloric
acid in a volume ratio of 1:1 (5+5 ml). 0.5 g of CRT powder was then digested in the aqua regia.
Immediately when the aqua regia came into contact with the CRT powder a considerable volume
of NO2 gas was produced. This suggests the nitric acid was being reduced during the digestion.
The following reaction depicts the reduction of nitric acid.
e- + H
+ + HNO3 NO2 + H2O (reaction 1)
The powder was left to digest for 48 hrs before being filtered and thoroughly diluted. The digest
was filtered using a syringe filter and was then diluted with 1M super pure nitric acid. Two
dilutions were prepared for the ICP OES, x10 dilution and x100 dilution.
4.4 Nitric Acid Leaching
4.4.1 ICP OES
In an attempt to determine the optimal strength of nitric acid to use for the leaching process five
different strengths where tried. Approximately 0.5 g of the powder was digested in 1M, 2M, 4M,
8M and 16M nitric acid. During the initial stages of the leaching the stronger 8M and 16M nitric
acid digestions were creating substantial amounts of NO2 indicating the samples were being
oxidized. The XRD analysis only supplied an overview of the sample so the entire composition
of the powder is still unknown making it difficult to identify a specific reaction mechanism.
Once the samples of CRT powder had successfully digested for 48 hrs in the varying strengths of
HNO3 the samples were prepared for the ICP OES machine. First of all the samples had to be
filtered to remove any large particles. A large amount of solid was removed from the solutions
with the use of syringe filters. The ICP OES can only work with dilute samples in the region of
0.1to 5 ppm (NB for a metal like europium the concentration should be in the region of 1x10-6
M
to 3.5 x10-6
M) and requires homogeneous samples to avoid fouling the plastic tubes or glass
parts [12]. When filtration was completed the samples where ready to be diluted. Each sample
17
was diluted 4 times to the following degrees x10, x100, x 1000 and x10 000 dilution using 1 M
super pure HNO3.
Four standards were created containing 25 elements in total. The elements included not only
lanthanides but also some transition metals as well as a few selected elements of interest.
4.4.2 Electron Microscopy
In an attempt to improve the leaching process electron microscopy analysis was carried out on
the leached samples. This was done in order to obtain an idea of what metals remained in the
powder so that the reasons behind some compounds resistance to the leaching could be
determined.
The residues from the five nitric acid digestions were collected by filtration and dried in
preparation for the electron microscope. This was done using glass fiber filters in Buckner
funnels. The filters were left in the oven for 24 hrs to ensure the samples were entirely dry. A
small quantity of the each dry sample was then mounted onto a viewing stub and the samples
were analysed using SEM.
4.5 Citric Acid and TLCP Pre-Digest Room Temperature Leaching
4.5.1 ICP OES
Approximately 0.5 g of CRT powder was digested in 0.2M citric acid and TLCP for 48hrs at
room temperature. The TLCP digest mixture consisted of sodium acetate and acetic acid. After
48hrs the aqueous extract was removed by pipetting carefully avoiding too withdraw any solid.
To ensure all citric acid and TLCP was removed approximately 10 ml of milliQ water was added
to each vial and left over night. The milliQ water was then removed using the same pipetting
method. 2M super pure nitric acid was then added to each vial. The nitric acid digestions were
also left for 48hrs before they were filtered and diluted in preparation for the ICP OES.
18
The solution extract from the citric acid and TLCP digests were prepared for ICP OES analysis
along with the 2M nitric acid extracts. The samples were filtered to ensure they did not contain
any solid material capable of fouling the ICP OES and were then diluted with 1M super pure
nitric acid to the following order: x10 dilution and x100 dilution.
4.5.2 Electron Microscopy
The digest powder was separated using a Buckner filter and the sample was collected for SEM.
Due to the powder being digested twice the powder was only collected after the 2M HNO3
digestion and not after the citric acid or TLCP pre-digest.
4.6 Heated Leaching
4.6.1 Theory
It is well known that the rate of a chemical reaction can be increased by increasing the
temperature or increasing the surface area of an interface at which the reaction occurs. As the
CRT powder was already a very fine powder it was not practical to decrease the particle size
further, therefore it was decided to experiment with heating the leaching process. The data from
the previous digestions indicated that the two most successful digestions were the 2M HNO3 at
room temperature and the pre-digestion of the CRT powder in citric acid followed by 2M HNO3
at room temperature. Both of these digestions were boiled under reflux so the effects could be
observed. The samples were boiled on a Barnstead electrothermal heating mantle, EMA
0250/CEB.
4.6.2 ICP OES
0.78 g of phosphorescent CRT powder was digested in 60 ml of 2M HNO3 and boiled under
reflux. The sample was left for 24 hrs to ensure as complete a digestion as possible.
The solid residue was collected and examined by SEM. The dissolution of the metals and nitric
acid was collected and analysed using ICP OES.
19
20.31 g of monohydrate citric acid powder (Monohydrate powder, SIGMA, C-7129 [5949-29-1])
was mixed with 197 g of milliQ water. This produced a 0.485M citric acid solution.
1.14 g of CRT powder was added to a round bottom flask which was then filled with 150 ml of
the citric acid stock solution. A magnetic stirring bar was also added to the flask before it was
placed in the heating mantle. The mixture was boiled under reflux for 24 hrs to ensure as
complete a digestion possible occurred. NB during the heated citric acid digestion H2S was
produced, the distinct smell was noticed and to be certain it was H2S a small quantity of lead
nitrate powder was exposed to the gas. The powder instantly turned black proving H2S was being
emitted. Some of citric acid solution was collected so that it could be analysed before the rest of
the solution was poured into a Buckner filter allowing the powder to be collected on a fiber glass
filter. The powder was then put back into the flask and it was filled with 100 ml of 2M HNO3.
The flask was then once again boiled under reflux for a further 24 hrs. The nitric acid solution
was then collected for using a pipette so that it could be examined with the ICP OES and the
powder remaining from the digestion was collected for electron microscopy analysis.
Fluorescent Bulb Powder
4.7 Nitric Acid Leaching
4.7.1 X-ray Powder Diffraction
The seal on four Aura 18W 830 tubes were broken using a drill and left for 1hr to vent. Once the
hazardous mercury vapour was removed the end of one of the tubes was broken off. A fraction of
the fluorescent powder was removed from the tube surface using a spatula to scrape and acetone
to improve the recovery. The powder sample was then dried and prepared for XRD analysis. The
three remaining tubes were carefully wrapped individually in paper and then crushed in
preparation for ICP OES analysis. The purpose of the paper was to keep all the glass in place and
to protect lab users from the shards of glass.
20
4.7.2 Electron Microscopy
An extract of the powder from a broken tube was mounted onto a viewing stub and taken for
SEM.
4.7.3 ICP OES
Once the tubes were thoroughly crushed they were transferred into 1L plastic bottles. The bottles
were filled with all the glass from the tubes but the filaments and tube ends were removed. In an
attempt to determine the optimal concentration of nitric acid for the leaching process the glass
was digested in 2M, 4M and 16M nitric acid. Since the majority of the bottles contained inert
glass a large volume of nitric acid was added to each, approximately 150 ml. This was to ensure
as much of the fluorescent powder as possible was removed from the glass.
21
5. Results and Discussion
CRT Powder
5.1 XRD
The phosphorescent powder was predicted to contain high concentrations of rare earth
lanthanides. The XRD analysis was carried with the intent of proving the presence of valuable
lanthanides.
Figure 11 X-ray powder diffraction results for raw CRT powder
Key:
Raw Untreated CRT powder
Ceranite-(Ce), syn – Ce02 – Cubic – S-Q 49.5%
Dysprsosium Sulphide Oxide - Dy2SO2 – Hexagonal – S-Q 49.4%
Cerium Gadolinium Sulphide – (Ce0.8Gd0.2)S – Cubic – S-Q 1.1%
22
The graph above (figure 11) indicates the presence of several lanthanide compounds. NB the
identification of the dysprsosium sulfide oxide and the cerium gadolinium sulfide presence was
important. The presence of sulfide compounds created some problems when it came to leaching
the CRT powder because of a tendency to form insoluble sulfate compounds. It was thought that
the calcium, strontium and lead present in the powder were forming the compounds. The XRD
anlaysis is in no way quantatitive but served as a useful starting point for the remainder of the
experimental work.
5.2 Matrix Analysis
When analyzing the matrix effect the desired result is a linear relationship between concentration
and intensity as this means there is no interference effects from the matrix [15]. Also ideally the
four lines would overlap implying there is no interference what so ever [15].
Figure 12 Matrix analysis data for europium wavelength 3930 showing minimal matrix effect,
depicts the 4 solutions each spiked with 1ppm metal standard
The Eu 3930 graph (figure 12) line shows an almost ideal result and all elements of interest were
analysed in the same way to ensure only valid wavelengths were used for data. The following
graph (figure 13) depicts an element that suffers from notable interference at a certain
wavelength.
y = 92796x0,9364 R² = 0,9966
100
1000
10000
100000
0,001 0,01 0,1 1
S11
S21
D11
D21
Power (S11)
Power (S21)
Power (D11)
Power (D21)Concentration (ppm)
Intensity (Ct/s)
23
Figure 13 Matrix analysis data for lead wavelength 2614 showing potential matrix effect
All elements of interest were analysed in the same way meaning all wavelengths that suffered
from interference could be discarded. Thus all ICP OES experiments could be organized so that
only valid wavelengths were used and none susceptible to interference would be referenced.
-20
0
20
40
60
80
100
120
140
0 0,05 0,1 0,15 0,2 0,25
S11
S21
D11
D21
Linear (S11)
Linear (S21)
Linear (D11)
Linear (D21)
Intensity (Ct/s)
Concentration (ppm)
24
5.3 Aqua Regia Leaching
5.3.1 ICP OES
The purpose of the aqua regia digest was to obtain a complete dissolution of the CRT powder
allowing data on the exact composition of the powder to be gathered.
The aqua regia digest identified the main constituents of the powder to be calcium, europium,
lead, yttrium and zinc. It is clear to see that the aqua regia digest was quite ineffective and it
recovered a very small quantity of the 5 elements of interest in relation to the total mass of
sample digested.
Figure 14 ICP OES data for aqua regia digest of CRT powder at room temperature
0
5
10
15
20
25
30
Ca Eu Pb Y Zn
Aqua Regia digest
Aqua Regia digest
Mass of element in dissolution (mg)
25
Table 1 ICP OES data from aqua regia digestion of CRT
Element
Mass of
Sample
Digested
(mg)
Mass of
Element
in
dissolution
(mg)
Ca 546.7 0,597431
Eu 546.7 0,83156
Pb 546.7 0,286964
Y 546.7 13,64608
Zn 546.7 24,29002
Aqua regia is optimized for the digestion of transition metals such as palladium this is because
these metals form chloro complexes. However, it is very unusual that the aqua regia failed to
dissolve as much zinc and calcium.
5.3.2 Electron Microscopy
SEM was carried out on the digest residue so that the composition of the digested powder could
be determined. Knowing the contents of the residue meant the leaching process could be
optimized to improve the recovery of the more valuable elements.
26
Figure 15 SEM image of CRT powder leached with AR
The image (figure 15) is very light in colour which suggests a strong presence of heavy weight
elements. The light colour of the picture also indicates the digest residue is very dense and
contains a lot of metal. This supports the ICP OES results which showed less than 50mg of the
546 mg of CRT powder was digested in entered into dissolution.
Figure 16 EDX data of Spectrum 1 related to figure 15
The spectrum (figure 16) identifies the presence of yttrium and europium in the leached CRT
powder. Considering that they are the most valuable of the elements known to be present in the
powder it was decided the aqua regia digestion was ineffective and as result the experimentation
with aqua regia was stopped and a new approach developed.
27
5.4 Nitric Acid Leaching
5.4.1 ICP OES
The intention of the nitric acid leaching was to improve the recovery of the valuable lanthanides
from the phosphorescent powder and also to attain a greater dissolution of the powder.
Figure 17 ICP OES data from nitric acid digestions of CRT powder
The main constituents of the CRT powder were aluminium, calcium, europium, yttrium and zinc.
The 2M HNO3 leached large amounts of the 5 elements of interest and even extracted the highest
proportion of calcium of all the acid digestions. It was noteworthy that the stronger acids did not
leach the highest concentration of metals and the following table (table 2) will show the distinct
difference in the quality of leaching between the 2M HNO3 and 8M HNO3.
0
50
100
150
200
250
300
350
400
Al Ba Ca Cd Eu Fe Mg Mn Se Sr Y Zn
1M HNO3
2M HNO3
4M HNO3
8M HNO3
16M HNO3
Mass of element in dissolution (mg)
28
Table 2 ICP OES data, comparison from 2M and 8M HNO3 digestion of the CRT powder
2M HNO3
8M HNO3
Element
Mass of
Sample
Digested
(mg)
Mass of
Element
in
dissolution
(mg) Element
Mass of
Sample
Digested
(mg)
Mass of
Element in
dissolution
(mg)
Al 639 25,2479 Al 433 14,07932
Ca 639 361,4439 Ca 433 80,17661
Eu 639 2,97752 Eu 433 4,232105
Y 639 45,93068 Y 433 6,750997
Zn 639 139,0169 Zn 433 97,16342
Total weight of metals leached from powder with 2M HNO3: 577.2mg
Total weight of metals leached from powder with 8M HNO3: 204,4mg
Comparison of the 5 elements of interest in the two tables again highlights that the 2M nitric acid
leached far more of the powder than the 8M. The 2M digest digested 577.2 mg of metal from
639 mg of phosphorescent powder. The 90% recovery of the metals seemed quite impressive but
the 8M acid digest leached a higher quantity of the more valuable metals indicating that perhaps
the calcium was consuming all of the nitric acid during the 2M HNO3 digestion.
Material Balance (2M HNO3 and Calcium):
There are 361 mg of calcium present in dissolution. Calcium has a molecular weight of 44 g
mols. The powder was digested in 10 ml, 0.020 mols, of 2M HNO3.
a present in dissolution
29
Since 2 moles of nitric acid are required for every mole of Calcium it is possible to determine
that the calcium contained within the powder consumes 0.016 mols of acid. Based on this
calculation it is a real possibility that the calcium is consuming all of the nitric acid which
explains the poor recovery of the more valuable elements. The 8M digest leached 4.29mg of Eu
compared the 2.9mg digested by the 2M HNO3.
The most valuable of the 5 elements is europium with a current market value of 7647.58 SEK for
50g of europium (III) chloride hexahydrate [16]. Yttrium is also a valuable material with a
market value of 7646.62 SEK for 50g of yttrium (III) chloride hexahydrate [17] (Approximately
£600 British Sterling). Considering the 3mg of europium in the 2M nitric acid dissolution the
approximate value of 0.5g of phosphorescent CRT powder in terms of Eu alone is close to 1
SEK. Knowing that europium and yttrium were the two most expensive constituents of the
powder it was decided to that the leaching process should be tailored to enhance their recovery
from the powder. Considering the possibility that the calcium was consuming all of the nitric
acid it was decided that the calcium should be removed from the powder before it was digested
to obtain an improved recovery of the valuable lanthanides.
5.4.2 Electron Microscopy
SEM and EDX were carried out on the nitric acid digest residue.
Figure 18 SEM Image of CRT powder leached
with 2M HNO3 at room temperature
30
Figure 19 EDX data of Spectrum 1 related to figure 18
Figure 18 is quite a light image with very few dark patches. This indicated that the vast majority
of the powder consisted of heavier elements, which was confirmed by the spectrum (figure 19) as
it shows the repeated presence of both yttrium and europium. This also supports the ICP OES
data which indicated the 2M HNO3 failed to digest as much of the rare earth elements as the
stronger concentration acids.
Figure 20 Electron Microscope Image of CRT powder leached with 8M HNO3
31
Figure 21 EDX data of Spectrum 1 related to figure 20
Figure 20 is a very dark image indicating the residue contains a lot of light weight elements. This
supports the ICP OES data which showed the 8M HNO3 was unable to leach large quantities of
calcium.
There is a notable contrast between the electron microscope images for the 2M HNO3 and 8M
HNO3 digests. It is clear to see that the 2M digest residue contained the desired europium and
yttrium.
5.5 Citric Acid and TLCP Pre-Digest Room Temperature Leaching
5.5.1 ICP OES
Two potential reasons for the poor recovery of lanthanide metals were that the calcium present
was consuming all of the nitric acid or the nitric acid was reacting with the sulfate to produce
some very insoluble compounds. The original XRD scan indicated the CRT powder contained a
couple of lanthanide compounds containing. It was reasoned that the sulfur was reacting to create
very insoluble sulfate compounds, potentially lead sulfate, calcium sulfate and strontium sulfate.
In an attempt to circumvent the sulfate production and to remove the calcium from the powder
the CRT powder was pre-digested in citric acid and a TLCP digestion mixture. Then once these
32
elements which form insoluble sulfates had been removed the powder would be digested in nitric
acid.
Table 3 ICP OES data from CRT powder digest in citric acid at room temperature
Element
Mass of
Sample
Digested
(mg)
Mass of
Element
in
dissolution
(mg)
Ca 637,7 0,573437
Eu 637,7 0,040608
Pb 637,7 0,819411
Y 637,7 0,586113
Zn 637,7 1,776972
Table 4 ICP OES data from CRT powder digest in CA followed by 2M HNO3 at room
temperature
Element
Mass of
Sample
Digested
(mg)
Mass of
Element
in
dissolution
(mg)
Ca 633,9 3,698623
Eu 633,9 5,324791
Pb 633,9 0,912095
Y 633,9 85,07216
Zn 633,9 122,951
33
Pre digesting the CRT powder in citric acid before digesting it in 2M nitric acid at room
temperature did not remove the calcium as desired. However, the recovery of both europium and
yttrium was noticeably better than when the CRT was pre digested in citric acid.
Figure 22 Comparison of 2M HNO3 digest and Citric acid pre-digest followed by 2M HNO3
digest of CRT powder at room temperature
Figure 22 illustrates that when the CRT was first leached with the citric acid before being
leached with 2M HNO3 the recovery of the europium and yttrium compounds was improved.
The improved the recovery of the valuable metals meant this was the most efficient leaching
process determined so far in the project.
0 100 200 300 400
Ca
Eu
Y
Zn
2M HNO3
CA.2M HNO3
Citric Acid
Mass of element in dissolution (mg)
34
5.6 Heated Leaching
5.6.1 ICP OES
It was predicted that heating the two most favourable leaching processes would further improve
the dissolution of the valuable lanthanides.
Table 5 ICP OES data collected from the heated 2M HNO3 digestion
Element
Mass of
Sample
Digested
(g)
Mass of
Element
in
dissolution
(mg)
Ca 0,78 6,76927
Eu 0,78 8,66269
Pb 0,78 5,58743
Y 0,78 142,625
Zn 0,78 215,639
Figure 23 Comparison of 2M HNO3 heated and room temperature digestions of CRT powder
0,1
1
10
100
Ca Eu Y Zn
2M HNO3room temp
2M HNO3heated
% of Element
recovered
from total
weight of
CRT powder
35
The graph (figure 23) above clearly highlights the fact that when heat is supplied the recovery of
all elements is improved. The recovery of the precious europium has increased by approximately
0.3%. This is a very respectable improvement considering the fact it constitutes little over 1% of
the entire powder. The recovery of calcium is far greater for the room temperature digestion than
it was for the heated digestion. It appears the heat suppressed the calcium dissolution.
Table 6 ICP OES data from the heated citric acid digestion of CRT powder
Element
Mass of
Sample
Digested
(g)
Mass of
Element
in
dissolution
(mg)
Ca 1,14 1,67134
Eu 1,14 0,93748
Pb 1,14 6,47591
Y 1,14 15,6691
Zn 1,14 8,33475
Table 7 ICP OES data from the heated 2M HNO3 digestion following the citric acid pre-
digestion
Element
Mass of
Sample
Digested
(g)
Mass of
Element
in
dissolution
(mg)
Ca 1,0557 10,5431
Eu 1,0557 12,8201
Pb 1,0557 1,68267
Y 1,0557 215,35
Zn 1,0557 344,039
36
The heated digestion was better able to recover the metals of interest, recovery of both zinc and
yttrium improved nearly 10% when compared to the 2M HNO3 room temperature digestion. The
recovery of europium was also improved by a distinct margin.
Figure 24 comparison of CRT powder digestion in citric acid followed by 2M HNO3 at room
temperature and heated
Again it is important to note that the dissolution of calcium is greatly reduced when the CRT
powder is pre-digested in citric acid and heated. This proves that pre-digesting the CRT powder
in citric acid whilst heating the solution followed by a heated 2M HNO3 digestion was the most
successful leaching process that was discovered in this project because it produces the biggest
dissolution of europium and yttrium.
0 10 20 30 40
Ca
Eu
Pb
Y
Zn
CA.2M room temp
CA.2M heated
% of Element
recovered from total
weight of CRT powder
37
Fluorescent Tube Powder
5.7 Nitric Acid Leaching
5.7.1 X-ray Powder Diffraction Analysis
To begin with the raw fluorescent tube powder was examined using XRD. It was predicted that
that results would highlight the presence of lanthanides.
Figure 25 XRD data for fluorescent tube powder
Key
Untreated Fluorescent Bulb Powder
Yttrium Europium Oxide – (Y0.95Eu0.05)203 – Cubic – S-Q 100 %
The data (figure 25) suggests that the powder is made up largely of yttrium europium oxide.
Considering the fluorescent nature of the powder and a large concentration of lanthanides was
expected. (NB yttrium is not a lanthanide but it does posses very similar characteristics).
Considering the presence of yttrium and europium it was decided that a nitric acid leach would
be employed.
38
5.7.2 Electron Microscopy
SEM and EDX analysis was performed on the untreated fluorescent tube powder.
Figure 26 SEM image of raw fluorescent tube powder
Figure 27 EDX data of Spectrum 1 related to figure 26
The SEM was carried out on untreated fluorescent powder. The data supported the X-ray powder
diffraction result which indicated the powder consisted of yttrium europium oxide. Figure 27
39
clearly shows repeated peaks for yttrium and europium confirming there presence in the powder.
The two tests proved very useful as the data was supportive rather than contradictive.
5.7.3 ICP OES
Once the fluorescent tube powder had been leached with the three different concentrations of
nitric acid the acid solutions were collected and analysed using ICP OES in a bid to identify the
composition of the dissolution.
Table 8 ICP OES data from the nitric acid digestions of the fluorescent tubes
Element
2M HNO3
Mass in
dissolution
(mg)
4M HNO3
Mass in
dissolution
(mg)
8M HNO3
Mass in
dissolution
(mg)
Ca 1,3059 0,340094 1,193355
Eu 40,81401 37,47075 21,83322
La 2,772652 10,45322 5,02619
Mg 0,12505 0,017333 0,433947
Pr 0,346428 0,331937 0,328586
Sm 0,153569 0,158265 0,157003
Y 326,8001 249,4205 21,41623
Total 372,3177 298,1921 50,38853
Each digestion contained all the glass from one fluorescent tube so the majority of the 76.4g
sample weight is inert glass. It was estimated that each tube contained approximately 1-3g of
powder but it was not possible to obtain an exact value. Table 9 shows that 2M nitric acid
digestion recovered 326 mg of yttrium and 40 mg of europium which is a lot better than the mass
recovered using the 8M nitric acid. Again it is clear that the weaker concentration of nitric acid is
far more suited to the leaching process. It is very noteworthy that the recovery of the metals
decreased as the concentration of nitric acid increased. Further experimentation was required to
ascertain the reason behind this.
40
Figure28 graphical representation of data from the nitric acid digestions of the F.T
6. Conclusions
Phosphorescent CRT powder
The aqua regia digest was designed to obtain a complete dissolution of the phosphorescent
powder but it failed to digest more than 40 mg of 546 mg of sample.
In terms of the nitric acid leaching it appears the most successful and effective strength of acid
was 2M HNO3. It produced 90% dissolution of the phosphorescent CRT powder and with a
concentration of only 2M it is far safer to use in industry than more concentrated nitric acid.
However the poor recovery of the valuable europium and yttrium is a disadvantage of using 2M
nitric acid because in order to make the recycling process economically viable the recovery of
the two elements needs to be high. The exceptionally high levels of calcium liberated by the 2M
HNO3 meant thought had to be put into the potential reasons for it and a solution determined.
Pre-digesting the CRT powder in citric acid at room temperature in an attempt to circumvent the
calcium dissolution proved very successful as it increased the europium recovery by 2 mg as
well as improving the yttrium dissolution notably. In terms of an industrial process, pre-digesting
the powder in citric acid is very viable and economical and considering the improvements
0 50 100 150 200 250 300 350
Ca
Eu
La
Mg
Pr
Sm
Y
8M HNO3
4M HNO3
2M HNO3
Mass of element in dissolution (mg)
41
offered it is a very worth while step to include in the dissolution process. Monohydrate citric acid
powder is relatively cheap feedstock with a market value of 1 200 SEK for 5kg of 98% pure
reagent grade powder. The cost of the powder would be justified by the improvement in recovery
of the europium and yttrium.
The heated leaching experiments were proven to be the most successful of all leaching
techniques experimented with. The heated 2M HNO3 and the citric acid followed by 2M HNO3
digestion of the CRT showed significant improvement to the identical room temperature
digestion. Both of the heated digestions improved the dissolution of the europium by
approximately 0.6% (5-10 mg) but the heated digestion with the citrate pre-digest step recorded
the best recovery of all the leaching techniques experimented with. The 2M HNO3 citrate digest
achieved a dissolution containing 12mg of europium which is a vast improvement compared to
2M nitric acid digestion at room temperature which obtained 2.9 mg of europium in dissolution.
The improvements noticed were related directly to the suppression of the calcium dissolution. In
conclusion, the most successful leaching technique for the CRT powder was the heated 2M
HNO3 Citrate digestion.
Fluorescent Tube Powder
Similar to the CRT powder analysis the 2M HNO3 produced the most complete dissolution of the
fluorescent powder. The 2M HNO3 also boasted the highest concentration of the 3 most
prominent lanthanides (europium, lanthanum and yttrium) in dissolution. The poor recovery of
the lanthanides when leaching with the stronger concentrations of nitric acid was not expected
and required further investigation. However it was fortunate that the 2M HNO3 was the most
successful of the concentrations because it is far more applicable to industry.
42
7. Future Work
A lot of things can be done, beginning with experimentation with perchloric acid in an attempt to
achieve 100% dissolution of the phosphorescent CRT powder. It would also be worthwhile
experimenting with solvent extraction and attempt to identify the necessary reagents to recover
the dissolution of europium and yttrium. Finally further experimentation with the fluorescent
tubes would be advisable to try and determine the trend of decreasing dissolution when stronger
acids are used.
8. Acknowledgements
First of all I would like to thank my supervisor Dr Mark Foreman who was always available for
my questions and queries. I could not have done it without his help.
I would also like to thank my examiner Christian Ekberg for giving me the opportunity to come
to the Sweden and for all the help he has provided me during my stay.
I must also thank Kristian Larsson, Arvid Ödegaard and Joachim Holm for all there help with my
experimental analysis. It was much appreciated.
Thank you also to my fellow diploma students and friend’s Robert, Meritxell, Sina, Irena,
Sabrina and everyone else from the Department. I especially enjoyed the banter.
43
9. References
1. R.W Burns, “Television an international history of the formative years”, IEE History of
Technology series 22, Published by IET 1998, ISBN 0852969147.
2. Stephen Herbert. “A History of Early TV” Volume 2, Published by Taylor & Francis,
2004, ISBN 0415326672.
3. S. Perkowitz “Empire of light: a history of discovery in science and art”, Published by
Joseph Henry Press, 1998, ISBN 0309065569.
4. Website,[http://static.newworldencyclopedia.org/thumb/1/1d/Cathode_ray_tube.svg/782p
x-Cathode_ray_tube.svg.png] sourced for CRT image.
5. Julie A. Jacko and Andrew Sears, “The Human Computer Interaction Handbook”, 2nd
Edition revised, Published by CRC press 2008, ISBN 0805858709.
6. US government website sourced for global price trend of rare earth elements ,
[http://minerals.usgs.gov/ds/2005/140/rareearths.xls]
7. James R. Connolly “Introduction to X-Ray Powder Diffraction”, Published Spring 2007
for EPS400-002.
8. Marta J.K. Flohr, “X-Ray powder diffraction”, USGS, Published for US Geological
Survey, 1997.
9. Eberhard Haug, Werner Nakel, “The elementary process of Bremsstrahlung”, Published
by World Scientific 2004, ISBN 9812385789.
10. Norman Allen Dyson, “X-rays in atomic and nuclear physics”, Second Edition,
Published by Cambridge University Press 1990, ISBN 0521262801.
11. John R. Dean “Practical Inductively Coupled Plasma Spectroscopy”, Published by John
Wiley & Sons, Ltd 2005, ISBN 0-470-09348-X.
12. Thermo Electron Corporation, “iTEVA Software Manual for iCAP 6000 series ICP
Emission Spectrometers”.
13. Joseph Goldstein, Dale Newbury, David Joy, Charles Lyman, Patrick Echlin, Eric
Lifshin, Linda Sawyer and Joseph Michael, “Scanning Electron Microscopy and X-Ray
Microanalysis”, Third edition, published by Springer 2003, ISBN 0306472929.
14. Sami Franssila, “Introduction to microfabrication”, Illustrated edition, Published by John
Wiley and Sons 2004, ISBN 0470851058, 9780470851050.
44
15. Nigel T, Faithfull, “Methods in Agricultural Chemical Analysis a Practical Handbook”,
Published by CABI 2003, ISBN 0851996086.
16. Sigma-Aldrich website accessed to attain pricing information for europium chloride
compound [http://www.sigmaaldrich.com/sigma-aldrich/home.html].
17. Sigma-Aldrich website accessed to attain pricing information for yttrium cholride
compound [http://www.sigmaaldrich.com/sigma-aldrich/home.html].
18. From Royal Society of Chemistry web site, “Flat Panel Displays”. Discovered after
entering CRT into the search tool.
[http://www.rsc.org/ebooks/archive/free/BK9780854045679/BK9780854045679-
00001.pdf]
19. Shigeo Shionoya “Phosphor handbook”, Published by CRC Press 1998, ISBN
0849375606.
