Desulphurization of Ferronickel Alloy Using a Waste
Material from Alumina Production
by
Xinqiang Men
A thesis submitted in conformity with the requirements for the
degree of
Master of Applied Science
Graduate Department of Materials Science and Engineering
University of Toronto
Copyright by Xinqiang Men, 2012
ii
Desulphurization of Ferronickel Alloy Using a Waste
Material from Alumina Production
By
Xinqiang Men
Master of Applied Science
Department of Materials Science and Engineering
University of Toronto
2012
ABSTRACT
Red mud is a waste product of alumina production and has an
adverse effect on environment.
About 90 million tonnes of red mud are produced annually
throughout the world and little is
recycled for useful applications. The world nickel reserves
consist of approximately 30%
sulphide ores and 70% oxide ores. Despite the relative abundance
of oxide ores, 55% of
nickel and nickel alloys produced today are derived from
sulphide ores. However, with the
production of nickel and its alloys from low-grade oxide ores
becoming increasingly
important, a major concern is high sulphur level in the
resultant alloy. For this reason,
desulphurization of the ferronickel becomes an important
consideration. In the present study,
experiments were conducted to determine if red mud could be used
as a major ingredient of
custom designed fluxes for the desulphurization of ferronickel
alloy. Factors investigated
included desulphurization rates, contact angle measurements and
flux-refractory interactions.
iii
ACKNOWLEDGEMENTS
I would like to express my sincerest appreciation to my
supervisor Prof. A.McLean, Dr. Y.D.
Yang, Dr. Soda, Prof. M. Barati for their continual guidance,
support and encouragement.
I would like to thank for the help from Paul, Karim, Yuki and
other members of our research
group. Appreciation is also expressed to Allan from Ryerson
University for his help in
preparing materials for me.
I express my thanks to Process Research Ortech (PRO) for in-kind
support and the Centre for
Chemical Process Metallurgy (CCPM) together with NSERC for
financial support through a
CRD Grant. In addition, I would like to thank the Department of
Materials Science and
Engineering, and University of Toronto for support.
Finally, I would like to thank my family and friends in China
for their unending support and
encouragement throughout my study.
iv
TABLE OF CONTENTS
ABSTRACT
.............................................................................................................................
ii
ACKNOWLEDGEMENTS
.....................................................................................................
iii
TABLE OF CONTENTS
.........................................................................................................
iv
LIST OF TABLES
.................................................................................................................
viii
LIST OF FIGURES
.................................................................................................................
ix
LIST OF SYMBOLS
...............................................................................................................
xi
LIST OF APPENDICES
........................................................................................................
xiv
CHAPTER ONE:
INTRODUCTION.......................................................................................
1
1.1 Background
.....................................................................................................................
1
1.2 References
.......................................................................................................................
4
CHAPTER TWOLITERATURE REVIEW
.........................................................................
5
2.1 The Production of Alumina and Red Mud
......................................................................
5
2.1.1 Bayer
Process............................................................................................................
5
2.1.2 Sintering Process
......................................................................................................
9
2.1.3 Combined Process
..................................................................................................
12
2.2 Physical and Chemical Properties of Red Mud
.............................................................
16
2.2.1 Physical Properties of Red Mud
.............................................................................
16
2.2.2 Chemical Properties of Red Mud
...........................................................................
16
2.3 Negative Influence Caused by Red Mud
......................................................................
17
2.4 Means to Recycle Red Mud
..........................................................................................
18
2.4.1 Utilization of Red Mud as Building Materials
....................................................... 18
2.4.1.1 Producing cement
.............................................................................................
18
2.4.1.2 Producing glass-ceramic
..................................................................................
19
2.4.1.3 Producing
bricks...............................................................................................
19
2.4.1.4 Producing pigment
...........................................................................................
20
2.4.2 Recovery of Valuable Elements from Red Mud
..................................................... 20
2.4.2.1 Recovery of Al2O3 and Na2O
...........................................................................
21
2.4.2.2 Recovery of Fe2O3
............................................................................................
22
2.4.2.3 Recovery of
TiO2..............................................................................................
23
v
2.4.3 Utilization of Red Mud as Environmental Materials
.............................................. 24
2.4.3.1 Water treatment
................................................................................................
24
2.4.3.2 Gas cleaning
.....................................................................................................
28
2.4.3.3 Soil amelioration
..............................................................................................
28
2.5 Problems in Recycling Red Mud
..................................................................................
29
2.6 References
.....................................................................................................................
30
CHAPTER THREE: EXPERIMENTAL ASPECTS
..............................................................
36
3.1 Materials
........................................................................................................................
36
3.1.1 Solid Materials
........................................................................................................
36
3.1.2 Gas
Material............................................................................................................
38
3.1.3 Liquid Material
.......................................................................................................
38
3.2 Scanning Electron Microscopy (SEM)
.........................................................................
38
3.2.1 Basic Principles of SEM
.........................................................................................
38
3.2.2 Sample Preparation
.................................................................................................
39
3.3 X-ray Diffraction (XRD)
...............................................................................................
39
3.3.1 Basic Principles of XRD
.........................................................................................
39
3.3.2 Sample Preparation
.................................................................................................
41
3.4 X-ray Fluorescence (XRF)
............................................................................................
41
3.4.1 Basic Principles of XRF
.........................................................................................
41
3.4.2 Sample Preparation
.................................................................................................
42
3.5 Thermogravimetric Analysis (TGA or TG)
...................................................................
42
3.5.1 Basic Principles of TGA
.........................................................................................
42
3.5.2 Sample Preparation
.................................................................................................
43
3.6 Desulphurization Experiments
......................................................................................
43
3.6.1 Experimental Equipment
........................................................................................
43
3.6.2 Sample Preparation
.................................................................................................
44
3.6.3 Experimental
Procedure..........................................................................................
45
3.6.4 Sulphur Analysis of Samples
..................................................................................
45
3.7 Contact Angle
Experiments...........................................................................................
45
3.7.1 Experimental Equipment
........................................................................................
45
3.7.2 Sample Preparation
.................................................................................................
48
vi
3.7.3 Experimental
Procedure..........................................................................................
48
3.7.4 Measurement of Contact Angle
..............................................................................
49
3.8 Corrosion Experiment
...................................................................................................
49
3.8.1 Experimental Equipment
........................................................................................
49
3.8.2 Sample Preparation
.................................................................................................
50
3.8.3 Experimental
Procedure..........................................................................................
50
3.8.4 Corrosion Effect Analysis
.......................................................................................
50
3.9 References
.....................................................................................................................
51
CHAPTER FOUR: RESULTS AND DISCUSSION
.............................................................
52
4.1 Characterization of Red Mud
........................................................................................
52
4.1.1 XRF Study of Red Mud
..........................................................................................
52
4.1.2 XRD and TG Study of Red Mud
............................................................................
53
4.1.2.1 XRD and TG Study of Red Mud S1
................................................................
53
4.1.2.2 XRD and TG Study of Red Mud B1
................................................................
56
4.1.2.3 XRD and TG Study of Red Mud B2
................................................................
58
4.2 Desulphurization Experiments
......................................................................................
60
4.2.1 Evaluation of desulphurization results
...................................................................
60
4.2.2 Effect of some factors on the desulphurization reaction
........................................ 61
4.2.2.1 Effect of nickel content in the alloy
.................................................................
61
4.2.2.2 Effect of aluminum addition in the flux
........................................................... 63
4.2.2.3 Effect of iron oxide addition
............................................................................
64
4.2.2.4 Effect of temperature variation
........................................................................
66
4.2.2.5 Effect of carbon content
...................................................................................
67
4.3 Result of Contact Angle
Experiment.............................................................................
68
4.3.1 Hemispherical Temperature of Refining Flux
........................................................ 68
4.3.2 Contact Angle Measurements
.................................................................................
69
4.3.2.1 The Change of Contact Angle as a Function of Time and
Temperature .......... 71
4.3.2.2 The Change of Contact Angle as a Function of Nickel
Content ...................... 74
4.4 Result of Corrosion Test
................................................................................................
75
4.5 References
.....................................................................................................................
79
CHAPTER FIVE: CONCLUSIONS
......................................................................................
80
vii
APPENDICES
........................................................................................................................
82
Appendix A: The results for desulphurization experiments
................................................ 82
Appendix B: The results for contact angle experiments
..................................................... 85
Appendix C: The photos of crucibles before and after corrosion
experiments ................... 86
Appendix D: The XRD and TG results of B3, B4 and B5
.................................................. 88
viii
LIST OF TABLES
Table 2.1 The main chemical composition of red mud......17
Table 3.1 The chemical composition of AISI 1117 low carbon
steel........36
Table 3.2 The chemical composition of nickel-based
alloy...36
Table 3.3 The chemical composition of stainless steel
substrates.....37
Table 3.4 The schedule of desulphurization experiment...44
Table 3.5 The schedule of contact angle experiment.....48
Table 3.6 The schedule of corrosion experiment...50
Table 4.1 The chemical composition of red mud......52
ix
LIST OF FIGURES
Figure 1.1 Flow chart of nickel oxide ores processing...2
Figure 2.1 Flow chart of Bayer process......7
Figure 2.2 Flow chart of sintering process....10
Figure 2.3 Flow chart of parallel process..13
Figure 2.4 Flow chart of serial process.....14
Figure 2.5 Flow chart of mixed process....15
Figure 3.1 EDX analysis of refractory brick.....37
Figure 3.2 Schematic diagram of a Scanning Electron
Microscope.....39
Figure 3.3 Bragg's Law.....40
Figure 3.4 Schematic diagram of desulphurization
experiment....43
Figure 3.5 Schematic diagram of contact angle
equipment......47
Figure 3.6 Image of the software to measure contact
angle..49
Figure 4.1 XRD result of red mud S1.......54
Figure 4.2 TG result of red mud S1..55
Figure 4.3 XRD result of red mud B1..56
Figure 4.4 TG result of red mud B1...,.57
Figure 4.5 XRD result of red mud B2..58
Figure 4.6 TG result of red mud B2.....59
Figure 4.7 Effect of nickel content in alloy on desulphurization
reaction...62
Figure 4.8 Effect of nickel content in alloy on apparent
desulphurization
rate constant ......62
Figure 4.9 Effect of aluminum addition in flux on
desulphurization reaction..63
Figure 4.10 Effect of aluminum addition in flux on apparent
desulphurization rate
constant...64
Figure 4.11 Effect of iron oxide addition in flux on
desulphurization reaction...65
http://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.html
x
Figure 4.12 Effect of iron oxide addition in flux on apparent
desulphurization rate
constant......65
Figure 4.13 Effect of temperature variation on desulphurization
reaction..66
Figure 4.14 Effect of temperature variation on apparent
desulphurization rate
constant......66
Figure 4.15 Effect of carbon content on desulphurization
reaction....67
Figure 4.16 Hemispherical temperature of refining flux.69
Figure 4.17 Contact angle with 5% nickel substrate as a function
of time.71
Figure 4.18 Contact angle with 10% nickel substrate as a
function of time...71
Figure 4.19 Contact angle with 15% nickel substrate as a
function of time...72
Figure 4.20 Contact angle with 5% nickel substrate as a function
of temperature.72
Figure 4.21 Contact angle with 10% nickel substrate as a
function of temperature...73
Figure 4.22 Contact angle with 15% nickel substrate as a
function of temperature...73
Figure 4.23 Contact angle as a function of nickel contents when
CaO content is
fixed...74
Figure 4.24 EDX result of refractory brick sample after
corrosion test.....76
Figure 4.25 The relation between CaO content in flux and
corrosion depth..76
Figure 4.26 SEM result of No. 1 test...77
Figure 4.27 SEM result of No. 2 test...77
Figure 4.28 SEM result of No. 3 test...77
Figure 4.29 SEM result of No. 4 test...77
Figure 4.30 SEM result of No. 5 test...77
Figure 4.31 SEM result of No. 6 test...77
Figure 4.32 SEM result of No. 7 test......78
xi
LIST OF SYMBOLS
aq: aqueous solution.
A : the total reaction area between molten slag and liquid
alloy.
AC: activated carbon.
A/S: the weight ratio between alumina and silicon oxide in
bauxite.
B1: the first kind of red mud from Bayer process in USA.
B2: the second kind of red mud from Bayer process in USA.
B3: the first kind of red mud from Bayer process in Canada.
B4: the second kind of red mud from Bayer process in Canada.
B5: the red mud from Bayer process in China.
2Cs : sulphide capacity of the slag.
CCD : counter current decantation.
d: the interplanar spacing.
EDX: energy-dispersive X-ray spectroscopy.
fs : the sulphur activity coefficient.
k : Etvs constant, which is valid for almost all substances and
its value is 2.1 x 107 [J K1
mol2/3
].
aK : the apparent desulphurization rate constant .
Ks : the sulfur mass transfer coefficient.
SL : sulphur distribution ratio between molten slag and liquid
metal.
xii
n: an integer.
OPC: ordinary Portland cement.
2PO : the partial oxygen pressure.
: the partial sulphur pressure.
r : the roughness ratio.
RMGA: red mud granular adsorbents.
iS][ : the initial sulphur content in the ferronickel alloy.
fS][ : the final sulphur content in the ferronickel alloy.
tS][ : the sulfur content in liquid metal at any sampling
time.
)%( Swt : the weight percent of sulphur in the slag.
]%[ Swt : the weight percent of sulphur in the alloy.
SEM: scanning electron microscopy.
t : time.
T : temperature.
TC : the critical temperature and corresponds to the temperature
at which the surface tension
is zero.
TBP: tri-butyl phosphate.
TGA: thermogravimetric analysis.
V : the molar volume of a substance.
V : the volume of liquid alloy.
WHIMS: wet high intensity magnetic separation.
xiii
XRD: x-ray diffraction.
XRF: x-ray fluorescence.
: surface tension.
ls : the interfacial tension between liquid and solid.
sg : the interfacial tension between gas and solid.
lg : the interfacial tension between liquid and gas.
S : desulphurization degree.
: the angle of incident X-rays.
* : the apparent contact angle which corresponds to the stable
equilibrium state.
: Youngs contact angle as defined for an ideal surface.
: the wavelength of X-rays.
xiv
LIST OF APPENDICES
Appendix A: The results for desulphurization experiments
................................................ 82
Appendix B: The results for contact angle experiments
..................................................... 85
Appendix C: The photos of crucibles before and after corrosion
experiments ................... 86
Appendix D: The XRD and TG results of B3, B4 and B5
.................................................. 88
1
CHAPTER ONE: INTRODUCTION
1.1 Background
Nickel is a very important metal that finds a variety of
applications including consumer
products, building, automotive and electronics. There are two
kinds of ores containing nickel,
namely oxide and sulphide ores. Anthony and Flett [1] reported
that about 55% of the
worlds nickel production originates from sulphide ores while the
balance is derived from
oxide ores. However, oxide ores count for about 70% of the
worldwide reserves. Apparently,
oxide ores will play a significant role in the production of
nickel in the future. Nickel oxide
ores can be classified two kinds --- limonite ore and saprolite
ores in terms of mineralogy.
Limonite ore usually contains high MgO content and low FeO
content which is treated by
pyrometallurgy to form ferronickel or matte. However, saprolite
ore contains high FeO
content and low MgO content which prefers hydrometallurgy to
recover nickel and cobalt.
The flow chart showing the treatment of nickel oxide ores is
shown in Figure 1.1.
2
Figure 1.1 Flow chart of nickel oxide ores processing [2]
Most of nickel produced using oxide ores is from
pyrometallurgical processes namely
ferronickel smelting or matte smelting. So, we can draw such a
conclusion that using nickel
oxide ores to produce ferronickel will become more and more
important in the future. Before
being used to make stainless steel or other materials, the
ferronickel needs pretreatment to
remove impurities especially sulphur and phosphorus that will
cause negative influence on
the properties of steel product.
Red mud is a by-product during alumina production. Usually,
about 1 to 1.5 tonnes of red
mud will be produced when we get 1 tonne of alumina. About 90
million tonnes of red mud
are produced every year in the world [3]. Red mud is deemed as a
toxic industrial waste
which needs treatment before discharge because of alkalinity and
chemical and mineralogical
properties. Until now, the main means to deal with red mud are
to store it in impoundments
or dump it in the sea depending on local environment. Nowadays,
how to recycle red mud
3
has become an important topic around the world. The last 40
years have witnessed a lot of
patent applications concerning the ways of red mud utilization.
However, a little amount of
red mud is consumed until now due to the limitations of
recycling red mud.
So, based on the information above, to use red mud to remove
sulphur in ferronickel seems
to be a promising method of recycling red mud. The objectives in
this study include:
1) To characterize several kinds of red mud.
2) To study desulphurization of ferronickel using fluxes made
from red mud.
3) To study the interfacial phenomena between fluxes made from
red mud and ferronickel.
4) To study whether fluxes made from red mud will attack the
refractory materials.
4
1.2 References
[1] M.T. Anthony, D.S. Flett. Nickel processing technology: A
review. Nickel: 1996
Commodity Meeting. Minerals Industry International. pp.
2642.
[2] T. Norgate , S. Jahanshahi. Assessing the energy and
greenhouse gas footprints of nickel
laterite processing. Minerals Engineering. 24, 2011, pp.
698707.
[3] S.B. Wang, H.M. Ang, M.O. Tade. Novel application of red mud
as coagulant, adsorbent
and catalyst for environmentally benign processes. Chemosphere.
72, 2008, pp.1621-1653.
5
CHAPTER TWOLITERATURE REVIEW
2.1 The Production of Alumina and Red Mud
Last decades have witnessed the prosperity of alumina industry.
The annual yield of alumina
across the world continues to go up. A lot of residue, red mud,
is also produced at the same
time. Usually, about 1 to 1.5 tonnes of red mud will be produced
with 1 tonne of alumina.
About 90 million tonnes of red mud are produced every year in
the world [1]. Nowadays,
most of alumina is produced from bauxite through three means
including Bayer process,
sintering process and combined process [2]. What kind of method
should be used to produce
alumina is mainly decided by the category of bauxite. In fact,
the criterion to evaluate the
quality of bauxite is based on its mineralogical and chemical
characteristics. We can classify
bauxites by their mineralogical characteristics so that there
are four kinds of bauxites in
nature including gibbsite bauxite, boehmite bauxite, diasporic
bauxite and combined bauxite
[3]. Usually, gibbsite bauxite is the easiest kind of bauxite to
digest compared to boehmite
bauxite and diasporic bauxite which is the most difficult kind
of bauxite to digest. Another
aspect influencing the quality of bauxite is the chemical
characteristics. We usually use A/S,
which means the weight ratio between alumina and silicon oxide
in bauxite, to judge the
quality of bauxite. The quality of bauxite goes up with the
increase of A/S. Nowadays, Bayer
process usually demands gibbsite bauxite or boehmite bauxite
whose A/S should be more
than 7. On the contrary, sintering process is used to deal with
diasporic bauxite whose A/S is
usually below 3.5.
2.1.1 Bayer Process
Bayer process was invented by Karl Josef Bayer, an Austrian, in
1889. Since then, it has been
the most popular method to produce alumina in that it is quite
suitable to deal with gibbsite
bauxite or boehmite bauxite which exists in most countries
producing alumina. In addition,
6
Bayer process also includes a variety of merits such as low
energy consumption, high-quality
product, excellent economic benefits, and relatively simple flow
chart. So, most alumina
nowadays is produced by Bayer process in the world. The flow
chart is displayed in Figure
2.1.
7
Figure 2.1 Flow chart of Bayer process
8
As can be seen from Figure 2.1, firstly, bauxite and lime will
be ground. After that, they will
be mixed with recycled alkaline solution in a wet ball mill to
form slurry. Next, the slurry is
pumped to digester where digestion takes place under suitable
pressure and temperature. The
main reaction in digestion is shown below.
Al2O3 (1 or 3)H2O+2NaOH+aq 2NaAlO2+aq (2.1)
In addition, SiO2 also reacts with alkali to form Na2O Al2O3 x
SiO2y H2O finally. The
slurry flowing out of the digester consists of sodium aluminate
solution and insoluble residue
including Na2O Al2O3 x SiO2y H2O and red mud. Sodium aluminate
solution will be
purified by filters to get rid of remaining residue. Next,
sodium aluminate solution will be
cooled and diluted. Then, it will be decomposed by mixing with
Al2O33H2O seed. The main
reaction in decomposition is shown below.
2NaAlO2 + 4H2O + aq Al2O33H2O + 2 NaOH + aq (2.2)
After decomposition, slurry flowing out of decomposers contains
recycled alkaline solution
and Al2O33H2O crystals that will be classified according to
size. The big Al2O33H2O
crystals will be changed to Al2O3 by roasting. However, the
small ones will return to the
decomposers as seed. Moreover, recycled alkaline solution is
evaporated to remove extra
water and used to digest new bauxite. During this process, some
Na2CO3H2O crystals will
form and react with lime milk to produce NaOH which will be
added into the recycled
alkaline solution to adjust its concentration. The separated red
mud is counter-current washed
using hot water several times in order to recover Al2O3 and Na2O
before disposed in landfills.
How many times the red mud will be washed is decided by the
utilization of red mud
produced. If the red mud will be sent to sintering process in a
combined process as a sort of
raw material still containing certain amount of alumina, it
usually will be washed for 3 or 4
times. However, if it will be deposited in landfills directly,
at least 6 times should be
employed to retrieve Al2O3 and Na2O as much as possible.
9
2.1.2 Sintering Process
Sintering process is mainly employed by a few countries to deal
with bauxite containing
diaspore. Compared with Bayer process, this method has a variety
of disadvantages such as
sophisticated flow chart, high energy consumption. However, the
most vital advantage of this
method is that it can use low-quality bauxite to produce
alumina. With the continuous
decrease of high-quality bauxite in the world, a conclusion that
sooner or later sintering
process will be more popular than now can be drawn. The flow
chart is displayed in Figure
2.2.
10
Figure 2.2 Flow chart of sintering process
11
Unlike Bayer process, sintering process uses soda ash and lime
to mix with ground bauxite to
form slurry. Then, the slurry is injected into rotary kiln to
form agglomerated material whose
main compositions are Na2O Al2O3, 2CaO SiO2, and Na2O Fe2O3. The
main reactions in the
rotary kiln are as follows.
Na2CO3 + Al2O3 Na2O Al2O3 + CO2 (2.3)
2CaO + SiO2 2CaO SiO2 (2.4)
Na2CO3 + Fe2O3 Na2O Fe2O3 + CO2 (2.5)
Next step is digestion in which agglomerated material will be
ground in mills with alkaline
solution to form sodium aluminate solution and red mud. In this
process, several reactions
take place as follows.
Na2O Al2O3 + 4 H2O + aq 2NaAl(OH)4+aq (2.6)
Na2O Fe2O3 + 4 H2O + aq 2NaOH + Fe2O33H2O+aq (2.7)
2CaO SiO2 + 2NaOH + aq Na2 SiO3 + 2Ca(OH)2+ aq (2.8)
2CaO SiO2 + 2 Na2CO3 + aq Na2 SiO3 + 2NaOH + 2Ca CO3+ aq
(2.9)
The result of digestion is that useful materials including Na2O
and Al2O3 are transferred into
solution but waste materials such as SiO2 and Fe2O3 will enter
solid residue named red mud.
At the same time, some SiO2 will reenter into the solution which
is the main impurity of
sodium aluminate solution that needs to be purified. During
desilication, CaO will be added
into sodium aluminate solution to get rid of most of SiO2. The
main reactions are shown
below.
xNa2SiO3+2NaAl(OH)4 + aq Na2O Al2O3 x SiO2y H2O+ 2xNaOH + aq
(2.10)
3Ca(OH)2 + 2 NaAl(OH)4 + aq 3CaO Al2O6H2O+ 2NaOH + aq (2.11)
3CaOAl2O6H2O + x Na2 SiO3+ aq 3CaOAl2OxSiO2yH2O+ 2xNaOH + aq
(2.12)
12
Then, sodium aluminate solution will be decomposed by CO2 gas to
produce Al2O33H2O
which will be changed to Al2O3 by roasting. The reactions are
shown below.
2 Na Al(OH)4 + CO2 + aq Na2CO3 + Al2O33H2O (2.13)
Al2O33H2O Al2O3 + 3H2O (2.14)
In addition, red mud produced will be deposited in landfills
after washed by hot water for 6
to 8 times to recover Al2O3 and Na2O.
2.1.3 Combined Process
Combined process, as its name shows, contains Bayer process and
sintering process at the
same time. If there exist gibbsite bauxite (A/S7) and diaspore
bauxite (A/S4)
simultaneously or bauxite whose quality is moderate (7A/S4) in a
region, combined
process is better to deal with these bauxites in comparison to
Bayer process or sintering
process separately. Generally, combined process contains three
various flow charts including
parallel process, serial process, mixed process.
Parallel process is employed to deal with gibbsite bauxite and
diaspore bauxite at the same
time. The flow chart for parallel process is displayed in Figure
2.3.
13
Figure 2.3 Flow chart of parallel process
As can be seen from Figure 2.3, there exist some advantages in
parallel process. First, it can
make full use of different bauxite resources simultaneously in a
region. Second, Na2CO3H2O
crystals produced from evaporation in Bayer process will be
directly used in sintering
process so that causticization reaction is not necessary in
Bayer process. In addition, some
organic impurities absorbed by Na2CO3H2O crystals will be burned
in sintering process. So,
the negative effects caused by organic impurities can be
eliminated in Bayer process. Third,
the loss of NaOH in Bayer process can be compensated by cheap
Na2CO3. However, there
14
also exist some disadvantages in parallel process. The first one
is that mixing green solution
will result in the increase of alkali circulation in Bayer
process which will influence some
technical conditions. Second, the production capacity of Bayer
process is dependant on that
of sintering process to some extent. Third, the flow chart of
parallel process is a little
complicated.
Serial process is good at dealing with moderate bauxite whose
A/S is located between 4 and
7. The flow chart of serial process is displayed in Figure
2.4.
Figure 2.4 Flow chart of serial process
In serial process, bauxite is firstly dealt with by Bayer
process. After that, the red mud
produced in Bayer process will be sent to sintering process as a
material to recover the
remnant Al2O3 and alkali. A part of green solution is sent to
Bayer process to decompose.
The merits in serial process are apparent. First of all, the
bauxite will be treated by two
15
processes, which will increase the overall alumina yield. Next,
most product is from Bayer
process so that the production cost is low. But, the demerits of
serial process cannot be
ignored. Firstly, the sintering temperature range of red mud
from Bayer process is quite
limited. This makes the operation of sintering kiln very tough.
Secondly, how to balance the
production of these two processes is a little difficult.
Sometimes, as long as the production of
Bayer process is not stable, which will affect the operation of
sintering process. Also, if the
amount of Fe2O3 in bauxite is not enough so that to compensate
the loss of alkali in Bayer
process becomes very difficult.
Mixed process makes some improvements of serial process and is
popular in China. The
flow chart of mixed process is displayed in Figure 2.5.
Figure 2.5 Flow chart of mixed process
16
In mixed process, the red mud produced in Bayer process will be
mixed with certain amount
of diaspore bauxite (A/S is around 4 ) to increase the sintering
temperature range. Thus, the
sintering process will produce enough green solution. Most of it
will be decomposed in
sintering process by CO2, the rest is sent to Bayer process to
compensate the loss of alkali
there. A big advantage of mixed process is that it is easy to
balance the production of two
systems. In addition, it also consumes some low-grade bauxite.
Its demerit is that the whole
flow chart is long and needs a lot of equipment.
2.2 Physical and Chemical Properties of Red Mud
Red mud widely differs in physical and chemical properties due
to various bauxites and
processing methods.
2.2.1 Physical Properties of Red Mud
Red mud is a powder material whose color presents red or gray
because of various contents
of iron oxide. Its specific gravity varies from 2840-2870 g/m3;
average particle size is below
75 m; specific surface area is 64.09-186.9 m2/g ; plasticity
index is 17.0-30.0; void ratio is
2.53-2.95 [4].
2.2.2 Chemical Properties of Red Mud
In red mudit mainly contains six kinds of oxides including CaO,
Al2O3 , Na2O, Fe2O3, SiO2
and TiO2. In addition, it also contains some quantities of MgO,
K2O and some trace elements
such as Zr, P, Sr, Cr, Ba, Mn [5]. Moreover, red mud is a highly
alkaline waste material with
pH 10-12.5.
The main chemical compositions of several kinds of red mud from
different countries are
displayed in Table 2.1.
17
Table 2.1 The main chemical composition of red mud (wt%)
CaO MgO Na2O K2O Fe2O3 TiO2 Al2O3 SiO2
USA 3.91 0.09 9.59 0.04 44.45 7.63 18.73 10.92
Canada
D1 10.07 0.18 7.35 33.98 5.86 17.37 12.21
Canada
D2 3.28 0.28 10.09 0.05 39.38 6.54 20.56 16.3
China 29.65 1.12 6.01 0.80 9.97 2.85 4.78 14.20
2.3 Negative Influence Caused by Red Mud
Red mud is deemed as a toxic industrial waste which needs
treatment before discharge
because of alkalinity and chemical and mineralogical properties
[6]. Until now, the main
means to deal with red mud are to store it in impoundments [7]
or dump it in the sea
depending on local environment [8]. However, these means will
result in a variety of
negative influences on environment including: 1) conventional
disposal needs a lot of areas
to hold it; 2) some chemicals in red mud, such as sodium
hydroxide and sodium carbonate,
will contaminate local ground water, soil and sea; 3) fine dust
will form during storage which
will be absorbed by animals and human beings; 4) it will also
cause adverse impacts on local
tourism due to esthetic and stigma problems [9]. So, to dump red
mud in the sea directly is
not permitted in many countries because of environmental
requirements. In addition, storing
red mud in impoundments also needs to meet a lot of
prerequisites including land and
technology availability, climatic and geographic conditions,
logistics and regulatory
requirements.
18
2.4 Means to Recycle Red Mud
Nowadays, how to recycle red mud has become a critical topic
around the world. The last 40
years have witnessed more than 285 patent applications
concerning ways for red mud
utilization [10]. However, little amount of red mud is consumed
until now.
2.4.1 Utilization of Red Mud as Building Materials
Nowadays, red mud is used for materials such as cement,
glass-ceramic, bricks and pigment.
2.4.1.1 Producing cement
Some papers focus on using red mud from Bayer process to produce
cement. For instance,
Tsakiridis et al. [11] studied the properties of ordinary
Portland cement (OPC) produced by
adding red mud. The result demonstrated that 3.5% (weight
percent) addition of red mud did
not negatively affect the quality of the produced cement. Singh
et al. [12, 13] described the
performance of three cements that were made from lime + red mud
+ fly ash; lime + red mud
+ bauxite; lime + red mud + bauxite + gypsum respectively. The
strengths of the cements
made from lime + red mud + bauxite and lime + red mud + bauxite
+ gypsum were
comparable or better to OPC. Vangelatos et al. [14] mixed red
mud that was dewatered by
means of a filter press with limestone and sandstone. They
produced various clinkers by
adding 1, 3 and 5 wt% red mud respectively. The results shown
that OPC they made
displayed good mechanical properties. Many years ago in China,
some companies used red
mud from sintering process to produce OPC. Due to the high
content of 2CaOSiO2 in this
red mud, it is suitable for producing cement. The addition of
red mud in process for
producing cement has some advantages. First, it can lower the
consumption of other raw
materials including limestone and iron powder. Second, it also
can improve the yield of kiln
[15]. However, no matter what kind of red mud will be used to
produce cement, there exist
several problems. First, the alkali in red mud will cause
negative influence on the
performance of cement. In addition, the chemical compositions of
red mud are quite difficult
19
to remain stable because of the fluctuation of alumina
production. Moreover, the total energy
consumption is very high because of the large amount of water in
red mud. Especially in
recent years, an increasing number of companies choose Dry
Process to produce cement. So,
the amount of red mud added to cement has decreased
significantly.
2.4.1.2 Producing glass-ceramic
Some researchers studied the way to produce glass-ceramic using
such materials as fly ash
and blast furnace slag [16-18]. However, few papers were
concerned with glass-ceramic
based on red mud. Because red mud usually contains a certain
amount of CaO, Al2O3 and
SiO2, it is suitable to produce CaO-Al2O3-SiO2 glass-ceramics.
Peng et al. [19] used red mud
from a company in China to produce two nano-crystal
glass-ceramics through suitable
thermal treatment. Both glass-ceramics exhibited good mechanical
properties. H.Z. Yang et
al. [20] made double-layer glass-ceramic/ceramic tile using
bauxite tailings and red mud
whose amount reached 56% of the total weight. The product shown
good macroscopic
appearance, microstructure, and mechanical properties. J.K. Yang
et al. [21] studied the
feasibility of using red mud from sintering process and fly ash
collected from electrostatic
precipitator in a power plant to produce glasses and
glass-ceramic. His work demonstrated
that the total weight of red mud and fly ash could be up to 85%.
In addition, according to the
research of Erol et al. [22], the addition of red mud did not
affect the properties of glass-
ceramic made from fly ash and silica fume.
2.4.1.3 Producing bricks
In China, Yang and Xiao [23] employed red mud from sintering
process to make unsintered
bricks. They found the optimum proportion of red mud brick
(weight percent) could be 25-40%
red mud, 18-28% fly ash, 30-35% sand, 8-10% lime, 1-3% gypsum
and about 1% Portland
cement. After evaluation, the bricks reached the Chinese
criterion of first-class brick. Wu et
al. [24] made a variety of ceramic simple bricks containing
various amounts of red mud. First,
they mixed red mud from sintering process with other materials
such as fly ash and shale.
20
Then, the samples were fired at different temperatures. Next,
properties of samples were
analyzed. They found material containing 70% (weight percent)
red mud displayed the best
performance after firing at 1100C, which was capable of
substituting clay bricks in
construction. Kavas [25] used boron waste and red mud from
Turkey to make red mud bricks.
He added 5%, 10% and 15wt% boron waste respectively into the red
mud to make six bricks.
Then, these bricks were fired at 700C, 800C, 900C separately.
Next, samples were
analyzed in terms of mineralogical and mechanical properties.
The result demonstrated that
the adulteration of 15% boron waste not only decreased firing
temperature of red mud brick
but also increased mechanical performance of them.
2.4.1.4 Producing pigment
Pera et al. [26] used red mud from Bayer process to make
pozzolanic pigment. According to
their research, 11% burnt red mud could be mixed with other
materials to obtain the final
product.
Despite so many published papers related with using red mud to
produce building materials,
there does not appear to be any large-scale production in this
field. First, some pretreatment
steps of red mud including dewatering, drying, sintering are
necessary. All these processes
will increase the energy consumption and cost of final product.
Second, these new methods
are facing competition from existing processes. Third, some
uncertainties with health risk
about red mud brick still exist. For instance, Somlai et al.
[27] made a conclusion that the use
of red mud as building material was limited based on the study
of 226
Ra and 232
Th activity in
red mud from Hungarian bauxite.
2.4.2 Recovery of Valuable Elements from Red Mud
How to extract useful materials from industrial waste is a
popular topic because of the
exhaustion of natural resources. Red mud contains some valuable
materials such as Al2O3,
Na2O, Fe2O3, and TiO2. In addition, it also contains some
quantities of MgO, K2O and some
21
other trace elements such as Zr, P, Sr, Cr, Ba, Mn. So, it is
important to study the
possibilities to recover these materials.
2.4.2.1 Recovery of Al2O3 and Na2O
Cresswell et al. [28] used sulphur dioxide to digest red mud in
the presence of water. In this
step, soda, alumina and silica were dissolved in solution. Next
step was to selectively
precipitate silica to produce the liquor containing soda and
alumina. Then, the liquor was
causticized with lime to produce calcium sulphite, which was
separated firstly and then
calcined to regenerate lime and sulphur dioxide for recycle. The
purified sodium aluminate
was returned to Bayer process. However, this method has three
demerits. First, it has
complicated flow chart, which will increase the investment of
equipment and cost of product.
Second, the acid solution will cause corrosion of equipment.
Third, a lot of sulphur dioxide
will be consumed due to the high alkalinity of red mud. The
second way to recover Al2O3 and
Na2O of red mud from Bayer process is to send it to a sintering
process. This is combined
process mentioned in 2.1.3. In 1982, Cresswell and Milne [29]
described a hydrothermal
method to recover Al2O3 and Na2O of red mud from Bayer process.
They mixed red mud
slurry with a strong caustic solution and lime. The mixture was
heated to 290-310 C for 0.5-
2 hours under 4-9 MPa. The result was the recovery of typically
95% of soda and better than
70% of alumina into the solution phase. The crystallized sodium
aluminate was dissolved
and returned to Bayer process after desilication. Zhong et al.
[30] also studied the recovery
of Al2O3 and Na2O from red mud. They leached red mud with
different amounts of 40-60%
NaOH solution and lime milk at 120-210 C for 3.5 hours. Then,
the leaching liquor and
residue were separated by filtration. The residue was leached
with 7% NaOH solution and
lime milk at 170C for 2 hours. The result shown that 87.8% Al2O3
and 96.4% Na2O in the
red mud could be recovered. The measures mentioned above have
some disadvantages. First,
red mud must be treated under high temperatures and pressures,
which means a lot of energy
is needed. Second, strong caustic solution is needed which will
result in corrosion of
equipment.
22
2.4.2.2 Recovery of Fe2O3
Fe2O3 is another material in red mud that has attracted a number
of researchers. Until now,
there are three means to recover iron from red mud: smelting,
solid-state reduction and
magnetic separation. In smelting process, red mud is charged
into blast furnace or rotary
furnace with a reducing agent. Then, iron oxide in red mud is
reduced to generate pig iron
that can be used in steel production. Erag and Apak [31] mixed
red mud from Turkey with
dolomite and coke. After pelletized and sintered at 1100C for 1
hour in a furnace, the
mixture was finally smelted at 1550C to produce pig iron and a
slag that was used to
produce pigment-grade TiO2. However, smelting process has some
demerits. High energy
and capital costs are associated with blast furnace (BF)
operation because scale of operation
is high. Red mud must be mixed with some good-grade iron ore to
maintain the minimum
grade of the charge to BF. In addition, titanium reacts with
other constituents of the slag to
form multiple oxides that are difficult to leach [32]. In the
solid-state reduction process, the
mud is mixed with a reducing agent or contacted with a reducing
gas to produce metallic iron
[33]. The product can be an input either in a steel-making
furnace or a conventional blast
furnace. Compared to smelting process, solid-state reduction
process consumes less energy.
But, it also has some disadvantages. First, the metallic iron
produced is quite difficult to
separate from the rest of product. So, it is easily polluted by
gangue materials. Second, the
product is in a very fine form. Then, it needs agglomeration
before transportation [32].
Magnetic separation is also an attractive way to recycle iron
from red mud. According to the
study of Jamieson et al. [34], they employed wet high intensity
magnetic separation
(WHIMS) to get a magnetic fraction containing around 56% (weight
percent) Fe2O3 and a
non-magnetic fraction of less than 4% Fe2O3 (weight percent). In
China, Shandong
Aluminum Company also used WHIMS to produce iron ore containing
56%-75.92% (weight
percent) Fe2O3. The recovery rate of Fe2O3 was 45% (weight
percent) [35]. Another means is
to convert hematite or goethite in red mud to magnetite firstly,
which is followed with
magnetic separation. Obviously, this process is more complex
than magnetic separation
http://www.sciencedirect.com/science?_ob=RedirectURL&_method=outwardLink&_partnerName=27983&_origin=article&_zone=art_page&_linkType=scopusAuthorDocuments&_targetURL=http%3A%2F%2Fwww.scopus.com%2Fscopus%2Finward%2Fauthor.url%3FpartnerID%3D10%26%20
23
directly. But it also has some advantages. First, goethite is
easier to separate magnetically
and needs less energy to reduce compared to hematite. So, the
extra cost of reducing hematite
to magnetite can be compensated by the energy difference between
reducing hematite and
magnetite to metallic iron [32]. Xiang et al. [36] employed low
temperature reduction
(
24
Despite considerable work on extracting metals from red mud, no
large-scale process has
been fulfilled [40]. For instance, combined process in China is
facing a dilemma. Sintering
part in combined process can recover Al2O3 and Na2O present in
red mud from Bayer process.
However, the high energy consumption increases the cost of
product significantly. Magnetic
separation to recover iron from red mud seems popular in China
now. But, the product
contains low Fe2O3 content which cant compete with virgin iron
ore. In addition, some
methods employed acid to leach red mud. Apparently, a large
amount of acid will be
consumed due to the high alkalinity of red mud and how to
recycle the acid is also a problem.
Generally speaking, an integrated process for recovering various
metals from red mud
simultaneously should decrease the waste output as much as
possible and show technical and
economic viability in the future.
2.4.3 Utilization of Red Mud as Environmental Materials
In recent years, people also have found some applications of red
mud in water treatment, gas
cleaning and soil amelioration due to its special properties
such as high alkalinity and high
content of oxide component.
2.4.3.1 Water treatment
The first impurity in water that has drawn much attention is
phosphate, which will result in
the algal and hydrophytic boom and rapid deterioration of water
quality. Li et al. [41] studied
the effect of acidification and heat treatment of raw red mud
and fly ash on the sorption of
phosphate in parallel experiments. First, red mud undergoing
acidification and heat treatment
displayed better result in removal of phosphate than red mud
without any treatment. Second,
result also displayed that red mud stirred with 0.25 mol/L HCl
for 2 hours performed the
maximum removal of phosphate (99% removal of phosphate) compared
to red mud treated
with 0.5, 1 and 2 mol/l HCl . The reason was that dilute HCl
improved the surface areas of
the red mud, which enhanced the phosphate removal. In addition,
another red mud sample
25
prepared by heating the red mud at 700 C for 2 h performed
better phosphate removal (99%
removal of phosphate) than other samples heated at 200, 500,
600, 800, 900, 1000 C. This
could be explained by the effect of temperature on water removal
and sintering. During
heating at 500 C, the surface area of red mud was enlarged
initially because the expulsion of
water resulted in the development of porosity. After this, the
phosphate removal process was
most efficient at 700 C and then became less efficient with the
increase of temperature or
time. This was because the decomposition of some hydroxyl
groups, calcite and the sintering
shrinkage of materials. According to their study, initial pH
would also affect the sorption
capacity significantly. Zhu et al. [42] produced adsorbents
based on red mud to remove
phosphate from aqueous solution. The results shown that
phosphate removal efficiency was
97 % with optimal reaction conditions initial phosphate
concentration 100 mg/ L, red mud
dosage 10 g/L, pH 2.0, respectively. The phosphate removal
efficiency of the red mud
adsorbents decreased with the increase of pH. Yue et al. [43]
studied the characteristics of red
mud granular adsorbents (RMGA) for phosphate removal from
aqueous solution. RMGA
were produced with different mass ratio of three raw materials
(red mud, bentonite and starch)
and sintered at various temperatures. Their performance was
evaluated by adsorption
capacity on phosphate. The adsorption experiments were directed
with the selected operating
parameters (stirring speed of 100 rpm, reaction time of 5 h,
adsorbent dosage of 4 g/L, initial
pH of 3.00 and initial phosphate concentration of 50 mg/L) under
certain aquatic
temperatures. The result displayed that red mud ratio and
sintering temperatures affected the
characteristics of RMGA greatly, and that the optimum
parameters, under which the largest
adsorption capacities could be achieved, varied with different
aquatic temperatures. This
phenomenon was caused by the chemical reactions that occurred
gradually in RMGA with
the increase of sintering temperature, which led to
diversification of surface characteristics
and a decrease of the amount of effective components for
adsorption, so the adsorption of
phosphate on RMGA was affected by these interactions.
26
Nitrate is also a pollutant material in water. Ho et al. [44]
studied the removal of nitrogen and
phosphorus from sewage effluent by passage through sand amended
with red mud. The result
demonstrated that the removal efficiency decreased with the drop
of red mud content in the
sand. For example, an average of 24% nitrogen removal was
obtained with 30% red mud, 9%
removal with 20% red mud and very little removal with 10% red
mud. At the same time, the
corresponding phosphorus removals were 91%, 63% and 50%
respectively. The reason was
that the drop of red mud content in the sand also decreased the
adsorption capacity of
amended sand and increased the infiltration rate. engelolu et
al. [45] employed the original
and activated red mud in batch adsorption technique to remove
nitrate from aqueous solution.
They found the red mud activated using 20% HCl for 20 minutes
was better than original red
mud. The nitrate saturation capacities of activated and original
red mud were 5.858 mmol
nitrate/g dry wt. of red mud and 1.859 mmol nitrate/g dry wt. of
red mud, respectively.
Another notable impurity in water is fluoride. The accumulation
of fluoride in human body
will damage the function of bones and cause fractures. In
addition, fluoride is also a
carcinogen [46]. engelolu et al. [47] studied removal of
fluoride from aqueous solution by
using the original and activated red mud in a batch
equilibration technique. Influence of pH,
adsorbent dose and contact time on the adsorption were
investigated. The fluoride adsorption
capacity of activated form was found to be higher than that of
the original form. The
maximum removal of fluoride ion was obtained at pH 5.5.
Activated red mud performed
better than original red mud. About 82% fluoride could be
removed by activated red mud
based on the experimental result.
Sometimes, wastewater contains some trace elements that are also
a potential danger to
human beings and environment. Orecanin et al. [48] produced
coagulant based on red mud
from an Alumina plant. First, the red mud was mixed with diluted
sulphuric acid (30%
weight) with solid /liquid ratio 1:10. Then, the liquor was
separated from the residual red
27
mud by centrifugation or filtration through the filter press.
Next, the acid red mud (pH 0) was
neutralized with waste base to pH 8. The coagulant was in
gel-like state and suitable for
heavy metals and turbidity removal from industrial wastewater.
They employed the coagulant
to treat waste water generated from pressure washing of boats
coated with antifouling paints
containing high amounts of Cu2+
, Zn2+
, Pb2+
, Ti4+
, and Cr3+
. The result shown that the
coagulant based on red mud could get rid of heavy metals
successfully. For instance, the
concentrations of Cu2+
, Zn2+
, Pb2+
, and Cr3+
decreased from
4260 ng/g, 10900 ng/g, 5350
ng/g, 3970 ng/g to 81.4 ng/g, 43.5 ng/g, 31 ng/g, 290 ng/g
respectively. Santona et al. [49]
investigated the heavy metal adsorption of red mud with and
without acid treatment. The
results shown that adsorption capacity of the red mud without
treatment for the three heavy
metals was ZnPb > Cd. Acid treatment with HCl decreased the
red muds capacity to absorb
the heavy metals by 30%. They believed that acid treatments
dissolved a portion of the
cancrinite which played a significant role in adsorbing heavy
metals and also varied the
structure of the red mud. Altundoan et al. [50] found acid
treatment could increase arsenic
adsorption capability of red mud. Batch adsorption studies have
shown that activated red
mud in dosages ranging from 20 to 100 g/l can be used
effectively to remove arsenic from
aqueous solutions. The process is pH dependent, the optimum
range being 5.87.5 for As (III)
and 1.83.5 for As (V). The maximum removals are 96.52% for As
(V) and 87.54% for As
(III) for solutions with a final pH of 7.25 and 3.50,
respectively, for the initial arsenic
concentration of 133.5 mol/ l (10 mg/ l), activated red mud
dosage of 20 g /l, contact time
of 60 min and temperature of 25 C. Gen-Fuhrman [51] investigated
the arsenic adsorption
capability of seawater-neutralized red mud through acid
treatment, combined acid and heat
treatment. The results shown that acid treatment increased the
removal efficiency. The
activated red mud pretreated by acid and heat could remove about
100% arsenate at pH 4.5
when the initial arsenate concentration is 2 mg/l. In addition,
no pollutants were released
after adsorption process.
28
Besides the pollutants mentioned above, some researchers also
studied the possibilities of
using red mud to remove other pollutants such as dye stuff,
organics and bacteria in
wastewater [52-54].
2.4.3.2 Gas cleaning
A lot of gases including CO2, H2S, SOx and NOx are produced from
industries every day. All
these gases will cause negative influences on environment. Jones
et al. [55] employed an
investigation on carbon dioxide capture using raw and
neutralized red mud for carbonation. It
was found that hydroxide alkalinity consumption was 85% on raw
red mud and 89% on red
mud after only 5 min of carbonation. Based on the experiments,
they calculated that the
amount of carbon dioxide that could be removed annually at
aluminum refineries in Australia
is potentially 15 million tons.
2.4.3.3 Soil amelioration
Feigl et al. [56] studied the effectiveness of red mud in
stabilizing contaminated mine waste
and agricultural soil. The whole process lasted for two years.
First, the result shown that a 5%
(by weight) red mud addition decreased the highly mobile,
water-extractable amount of Cd
and Zn by 57% and 87%, respectively, in the agricultural soil
and by 73% and 79%,
respectively, in the mine waste. Second, addition of red mud did
not increase the toxicity of
the treated mine waste and soil and decreased the Cd and Zn
uptake of Sinapis alba test
plants by 1829%. These results indicated that red mud applied to
agricultural soil had no
negative effects on plants and soil microbes and decreased the
amounts of mobile metals,
thus indicating its value for soil remediation. Liu and Luo [57]
investigated effects of
granulated red mud addition on the fractionation of Pb and Zn in
soil and plants growth.
Results shown the concentrations of Pb and Zn decreased by
adding granulated red mud. 5%
granulated red mud addition decreased the concentrations of Pb
and Zn by 41.03% and 26.55%
respectively. Moreover, they found 1% addition of granulated red
mud addition could help
growth of leeks.
29
Although red mud has found some usage in environmental
materials, there still some obvious
demerits. First, red mud requires pretreatment such as
acidification or thermal treatment in
order to gain better performance. This will increase the cost of
using red mud. Second, the
amount of red mud used in environmental materials is very
limited.
2.5 Problems in Recycling Red Mud
Until now, a large number of papers have discussed various ways
to recycle red mud.
However, few of them are commercially implemented. This can be
explained by reasons
below [58].
1) The unique properties of red mud limit its recovery and
utilization. For example, the high
water content and high alkalinity of red mud usually require
pretreatment before it is used.
2) Red mud has found quite limited consumption in many aspects.
For instance, the addition
amount of red mud in cement production decreases significantly
due to the advance of
technology.
3) The safety of using red mud impedes its utilization. For
example, some researchers worry
about radioactive damage caused by red mud. In addition, people
still have a lot of work
to make sure whether recycling red mud will result in negative
impacts on environment.
4) The product based on red mud cant compete with those from
existing processes in terms
of quality. For instance, iron ore made by magnetic separation
of red mud contains less
Fe2O3 compared to traditional product.
Thus, researchers should make effort to find an increasing
number of measures to recover red
mud in the future. An ideal means to recover red mud should be
both technically and
economically viable. At the same time, high-quality product
should be produced by
consuming large amount of red mud and harmless to either
environment or human beings.
30
2.6 References
[1] S. Wang, H.M. Ang, M.O.Tade. Novel application of red mud as
coagulant, adsorbent and
catalyst for environmentally benign processes. Chemosphere. 72,
2008, pp. 1621-1653 .
[2] Y. Liu, C. Lin, Y. Wu. Characterization of red mud derived
from a combined Bayer
process and bauxite calcinations method. Journal of Hazardous
Materials. 146, 2007, pp.
255-261.
[3] R.K. Paramguru, P.C. Rath, V.N. Misra. Trends in red mud
utilization- A review, Mineral
Processing and Extractive Metallurgy Review. 26:1, 2005, pp.
1-29.
[4] X.L. Nan, T.A. Zhan, Y. Liu, Z.H. Dou. Analysis of
comprehensive utilization of red mud
in China, The Chinese Journal of Process Engineering. 10, 2010,
pp. 264-270.
[5] Na zhang , X.M. Liu , H.H. Sun , L.T. Li . Evaluation of
blends bauxite- calcinations-
method red mud with other industrial wastes as a cementitious
material: Properties and
hydration characteristics, Journal of Hazardous Materials. 185,
2001, pp. 329-335.
[6] A.R. Hind, S.K. Bhargava, S.C. Grocott. The surface
chemistry of Bayer process solids: a
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36
CHAPTER THREE: EXPERIMENTAL ASPECTS
3.1 Materials
3.1.1 Solid Materials
Three kinds of red mud S1, B1and B2, from sintering process and
Bayer process
respectively are used in experiments. Some other chemicals were
used to make various
fluxes. Among these chemicals, the purities of Al2O3, CaO, SiO2,
Fe2O3, Na2CO3 are 99.8%,
99.95%, 99.8%, 99% and 99.5% respectively. Pure nickel and
aluminum (purity greater than
99.9%) and AISI 1117 low carbon steel whose chemical composition
is shown in Table 3.1
were mixed to form an alloy. Another type of alloy was the
nickel-based alloy whose
chemical composition is shown in Table 3.2. The compositions of
stainless steel substrates
are shown in Table 3.3.
Table 3.1 The chemical composition of AISI 1117 low carbon
steel
Element C Mn P S
Weight(%) 0.14-0.20 1.0-1.3 0.04 0.08-0.13
Table 3.2 The chemical composition of nickel-based alloy
Element chemical composition (%)
C S Al As Ca Cr Fe Ni P Si Balance
Weight(%) 0.42 0.39 0.08 0.6 0.01 0.15 9.03 50.2 9.1 2.54
27.48
37
Table 3.3 The chemical composition of stainless steel
substrates
Stainless
Steel
chemical composition (%)
Cr Ni C Mg Cu Mo Si S P other
1 15-17.5 3-5 0-0.07 0-1 3-5 0 0-1 0-0.03 0-0.04
Cb and Ti
0.15-0.45
2 17-19 8-10 0-0.15 0-2 0-1 0-1 0-1 0-0.15 0-0.2 0
3 16-18 0-0.75 0.95-1.2 0-1 0-0.5 0-0.75 0-1 0-0.03 0-0.04 0
4 16-18 10-15 0-0.08 0-2 0-0.75 0-3 0-1 0.03 0-0.045 N 0-0.1
5 13.5-16 24-27 0-0.08 0-2 0 1-1.5 0-1 0-0.025 0-0.025
Al 0.15 ;
Ti 0-2.35
The size, roughness and thickness of substrates were about 2.54
x 2.54 cm, 0.3m and 5mm
respectively. Before use, substrates were degreased in acetone
and ultrasonically cleaned. For
corrosion experiments, crucibles made from refractory bricks
were used. Their composition
was analyzed by EDX and is shown in Figure 3.1, which shows the
main chemical materials
in refractory bricks are MgO and Cr2O3.
Figure 3.1 EDX analysis of refractory brick
38
3.1.2 Gas Material
The purity of Argon gas used for the experiments was 99.5%.
3.1.3 Liquid Material
Acetone was used to clean the substrates to get rid of grease on
the surface.
3.2 Scanning Electron Microscopy (SEM)
3.2.1 Basic Principles of SEM
Scanning Electron Microscopy (SEM) is widely employed to reveal
information of
samples such as texture, chemical composition, and crystalline
structure and orientation of
materials combined with EDX (Energy-dispersive X-ray
spectroscopy). SEM uses a beam
of accelerated electrons to scan the surface of a sample.
Through the interactions between
sample and electrons, signals including secondary electrons,
visible light, characteristic X-
rays, backscattered electrons, and diffracted backscattered
electrons will be generated. By
using various detectors, a variety of information about the
sample can be produced. For
instance, secondary electrons are mainly employed to display
morphology and topography
of the sample; characteristic x-rays can be used to determine
chemical composition. The
SEM system contains six parts including electron source (gun),
electron lens, sample stage,
detectors, display/data output devices, infrastructures that
consists of vacuum system,
cooling system and power system. A typical SEM system is shown
in Figure 3.2.
http://serc.carleton.edu/research_education/geochemsheets/electroninteractions.html
39
Figure 3.2 Schematic diagram of a Scanning Electron Microscope
[1]
3.2.2 Sample Preparation
Samples for SEM analysis were obtained from the corrosion
experiments. First, crucibles
used for these experiments would be cut in half. Then, small
pieces of samples around the
slag line were taken from each crucible. Next, each sample was
put into a mould, which was
charged with resin and hardener in the ratio of 2 to 1. After
several hours, the samples were
fastened in hard resin cylinders. The samples were dried and
cleaned and the surface coated
with a thin carbon film.
3.3 X-ray Diffraction (XRD)
3.3.1 Basic Principles of XRD
XRD is a non-destructive analytical technique to characterize
crystalline materials. When a
beam of X-rays whose wavelength is known passes through a
crystalline sample to be
examined, the interaction of the incident rays with the sample
results in constructive
http://en.wikipedia.org/wiki/Thermogravimetric_Analysis#cite_note-0#cite_note-0
40
interference when conditions satisfy Bragg's Law. This law
relates the wavelength of
electromagnetic radiation to the diffraction angle and the
lattice spacing in a crystalline
sample. These diffracted X-rays are then detected, processed and
counted. By scanning the
sample through a variety of 2 angles, all possible diffraction
directions of the lattice should
be attained due to the random orientation of the powdered
material. The material can be
identified by changing the diffraction peaks to d-spacings in
that each crystalline material has
a set of unique d-spacings, which is achieved through comparing
d-spacings with standard
reference patterns. The principle of this technique is based on
Bragg's Law (n = 2 d sin),
which could be explained by Figure 3.3.
Figure 3.3 Bragg's Law [2]
http://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.htmlhttp://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.htmlhttp://serc.carleton.edu/research_education/geochemsheets/BraggsLaw.htmlhttp://en.wikipedia.org/wiki/Thermogravimetric_Analysis#cite_note-0#cite_note-0
41
When the top beam strikes the top layer at atom z, the second
beam continues to the next
layer where it is scattered by atom B. The second beam must
travel the extra distance AB +
BC, which must be an integral (n) multiple of the wavelength ()
since the phases of the two
beams remain the same. So,
n = AB +BC (3.1).
In triangle ABz, we can relate d and to the distance (AB +
BC).
AB = d sin (3.2).
Because AB = BC, we have,
n = 2 d sin (3.3)
and Bragg's Law has been derived. Where n is an integer is the
wavelength of X-raysd
is the interplanar spacing and is the angle of incident
X-rays.
3.3.2 Sample Preparation
Each sample is ground to pass a 270 mesh sieve. After that,
about 0.4 g sample is charge into
a glass holder cavity. Next, sample is packed tightly and extra
material removed using a glass
slide. A Philips PW 1830 X-ray diffractometer is employed to
scan samples with CuK
radiation at 40 kV and 40 mA. The range of 2 is changed from 20
to 65 degree and software
is used to analyze the results.
3.4 X-ray Fluorescence (XRF)
3.4.1 Basic Principles of XRF
When a sample is illuminated by an intense X-ray beam, atoms in
the sample will gain
energy from the incident X-ray by ionizing so that electrons
located on lower energy orbital
42
will transfer to higher energy orbital. The positions left by
these electrons will be occupied
by electrons from higher energy orbital. During this process,
energy is released in the form of
characteristic X-rays because of the energy difference between
the inner electron orbital and
the outer one. Since each element has unique atomic structure,
the change of intensity of
characteristic X-rays emitted can permit determination of the
chemical composition of a
sample. The intensity of the energy measured by detectors in an
X-ray fluorescence (XRF)
spectrometer is proportional to the amount of the element in the
sample. The exact value of
this proportionality for each element is derived by comparison
with standards whose
composition is known from prior analyses by other
techniques.
3.4.2 Sample Preparation
Each sample is ground to pass 270 mesh sieve. During grinding, a
device called desk mill is
employed. Boric acid is used as a binder when the sample is
compressed with a pressure of 5
tons per square inch. Samples were analyzed using a Philips 2404
sequential X-ray
fluorescence spectrometer. Philips software was used to scan the
entire X-ray spectrum and
provide the final data results.
3.5 Thermogravimetric Analysis (TGA or TG)
3.5.1 Basic Principles of TGA
Thermogravimetric Analysis is a test to study changes in weight
in relation to change in
temperature. TGA has found a variety of uses in describing
characteristics of materials such
as water of crystallisation, degradation of materials, reaction
kinetics, oxidation and
reduction. The equipment has a high-precision balance with a pan
(generally platinum) to
hold the sample. Sometimes a quartz crystal microbalance is used
for measuring smaller
samples on the order of a microgram (versus milligram with
conventional TGA).[3] A
http://en.wikipedia.org/wiki/Weighthttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Thermogravimetric_Analysis#cite_note-0#cite_note-0
43
thermocouple is employed to accurately monitor the temperature.
The temperature in many
testing methods routinely reaches 1000C or greater. The
atmosphere may be controlled with
an inert gas to prevent oxidation or other undesired reactions.
The whole process can be
controlled by a computer.
3.5.2 Sample Preparation
Firstly, 40 mg sample is charged into a platinum crucible. The
sample holder is placed into a
high temperature oven. With the increase of temperature, changes
of sample weight are
measured continuously. Heating rate was 10 C per minute; the
temperature varied from
25C to 1100C; atmosphere for the test was dry air.
3.6 Desulphurization Experiments
3.6.1 Experimental Equipment
The experimental system used for desulphurization studies
included a 30KW high frequency
induction furnace, crucibles made of either graphite or
magnesium oxide, and two Pt/Pt -10%
Rh thermocouples. A diagram demonstrating this experimental
system is shown in Figure 3.4.
Figure 3.4 Schematic diagram of desulphurization experiment
http://en.wikipedia.org/wiki/Thermocouplehttp://en.wikipedia.org/wiki/Inert_gas
44
3.6.2 Sample Preparation
Before use, red mud and CaO are dehydrated by heating at 600 and
200 respectively.
The flux powder was pressed to a small cylinder before use so
that it was convenient to put it
into the crucible at high temperatures. The desulphurization
schedule is shown in Table 3.4.
Table 3.4 The schedule of desulphurization experiment
Exp Flux Ni content(wt%) Temperature(K)
1 45.5gS1+4.5gCaO+10%Al 0 1673
2 45.5gS1+4.5gCaO+10%Al 10 1673
3 45.5gS1+4.5gCaO+10%Al 20 1673
4 45.5gS1+4.5gCaO+10%Al 30 1673
5 45.5gS1+4.5gCaO+10%Al 40 1673
6 45.5gS1+4.5gCaO+10%Al 50 1673
7 40g B1 + 10% Al 10 1673
8 40g B1 + 5% Al 10 1573
9 40g B1 + 5% Al 10 1673
10 40g B1 + 5% Al 10 1773
11 40g B1 + 0% Al 10 1673
12 40g B1 + 8% Al 10 1673
13 30g
50 1673 CaO:Al2O3:SiO2=50:40:10
14
30g
50 1673 CaO:Al2O3:SiO2=50:40:10
20% Fe2O3
15
30g
50 1673 CaO:Al2O3:SiO2=50:40:10
40% Fe2O3
16 45.5gS1+4.5gCaO+10%Al
10 1673 C: 4%
17 45.5gS1+4.5gCaO+10%Al
10 1673 C: 2.57%
45
3.6.3 Experimental Procedure
For every experiment, certain amount of alloy is placed in the
reaction crucible, which is
placed in the induction furnace. The main function of two
thermocouples was to monitor the
temperatures of hot metal bath and graphite lining crucible. The
temperature difference
between these two thermocouples was 100C. When the hot metal
temperature reached the
required value, a suction tube that was made of quartz was used
to extract the first metal
sample which was to be analyzed to obtain the initial sulphur
level. After the first sample, the
flux was charged into the reaction crucible and this was taken
as