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CHARACTERISATION AND REUSE OF SOLID WASTES
By
Wentao Liang
Principal Supervisor: A/Prof. Cheng Yan
Associate Supervisor: Dr. Sara Couperthwaite
Submitted in fulfilment of the requirements for the degree of
[Master of Engineering (Research)]
Faculty of Science and Technology
Queensland University of Technology
2013
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© 2013 Wentao Liang 1
Keywords
Acid stability, adsorption, bauxite residue, fluoride, red mud
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© 2013 Wentao Liang 2
Abstract
The removal of fluoride using red mud has been improved by acidifying red mud
with hydrochloric, nitric and sulphuric acid, while affected by temperature. This
investigation shows that the removal of fluoride using red mud is significantly
improved if red mud is initially acidified, and thermally activated. The acidification
of red mud causes sodalite and cancrinite phases to dissociate, confirmed by the
release of sodium and aluminium into solution as well as the disappearance of
sodalite bands and peaks in infrared and X-ray diffraction data. The dissolution of
these mineral phases increases the amount of available iron and aluminium
oxide/hydroxide sites that are accessible for the adsorption of fluoride. The removal
of fluoride is dependent on the charge of iron and aluminium oxide/hydroxides (Fe2+
or Fe3+
/AlOH2+
or Al (OH) 2-
) on the surface of red mud. Acidifying red mud with
hydrochloric, nitric and sulphuric acid resulted in surface sites of the form ≡ SOH2+
and ≡ SOH. Optimum removal is obtained when the majority of surface sites are in
the ≡ SOH2+
as the substitution of a fluoride ion doesn’t cause a significant increase
in pH. Meanwhile, the minerals of red mud are changed through the thermally
treatment. This investigation shows the importance of having a low and consistent
pH, and the appropriate temperature for the removal of fluoride from aqueous
solutions using red mud.
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Table of Contents
Keywords ..................................................................................................................... 1
Abstract ........................................................................................................................ 2
Table of Contents ......................................................................................................... 3
List of Figure ................................................................................................................ 5
List of Tables................................................................................................................ 7
List of Publications ...................................................................................................... 8
Statement of Original Authorship ................................................................................ 9
The Proposed Supervisors and Their Credentials ...................................................... 10
Acknowledgments ...................................................................................................... 11
CHAPTER 1: INTRODUCTION .......................................................................... 12
Thesis outline .................................................................................................... 16 1.1
Significance ...................................................................................................... 18 1.2
CHAPTER 2: LITERATURE REVIEW .............................................................. 19
Bayer process review ........................................................................................ 19 2.1
2.1.1 Bauxite ore .............................................................................................. 19
2.1.2 Bayer process .......................................................................................... 22
2.1.3 Limitations of the Bayer process ............................................................. 25
Red Mud applications ....................................................................................... 26 2.2
2.2.1 Red mud issues ........................................................................................ 26
2.2.2 Red mud applications in materials .......................................................... 27
2.2.3 Red mud applications in water treatment ................................................ 29
Red mud treatment comparision ....................................................................... 33 2.3
2.3.1 Acid activated red mud ........................................................................... 33
2.3.2 Thermally activated red mud................................................................... 34
2.3.3 Combination treatment ............................................................................ 35
Fluoride applications ........................................................................................ 36 2.4
2.4.1 Fluoride issues ......................................................................................... 36
2.4.2 Red mud applications with fluoride ........................................................ 36
CHAPTER 3: EXPERIMENTAL AND CHARACTERISATION ..................... 39
Experimental techniques ................................................................................... 39 3.1
3.1.1 Fluoride adsorption ................................................................................. 39
3.1.2 Characterisation techniques..................................................................... 39
Characterisation techniques .............................................................................. 41 3.2
3.2.1 X-ray Diffraction (XRD) ......................................................................... 41
3.2.2 Infrared Spectroscopy (IR) ...................................................................... 43
3.2.3 Surface Area and Pore Size Analysis ...................................................... 44
CHAPTER 4: ACID ACTIVATED PROCESS ................................................... 51
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Experimental procedure .................................................................................... 51 4.1
4.1.1 Acid activated red mud ........................................................................... 51
Results and disscussion .................................................................................... 52 4.2
4.2.1 Elemental and mineralogical composition .............................................. 52
4.2.2 Leachants (ICP and reactions)................................................................. 55
4.2.3 Infrared spectroscopy .............................................................................. 58
4.2.4 Removal of fluoride using acid treated red mud ..................................... 64
CHAPTER 5: THERMALLY ACTIVATED RED MUD .................................... 68
Experimental procedures .................................................................................. 68 5.1
5.1.1 Thermally activated red mud................................................................... 68
Results and disscussion .................................................................................... 68 5.2
5.2.1 Red mud characterisation ........................................................................ 68
5.2.2 Thermally analysis .................................................................................. 73
5.2.3 Removal of fluoride using thermally activated red mud ......................... 77
CHAPTER 6: OVERALL CONCLUSIONS ........................................................ 83
CHAPTER 7 FUTURE WORK .............................................................................. 84
BIBLIOGRAPHY .................................................................................................... 85
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List of Figure
Figure 1: World bauxite reserves
Figure 2: Schematic representation of The Bayer process
Figure 3: Composition of red mud in different parts of the world
Figure 4: X-ray diffraction between planes of atoms
Figure 5: Classification of NDDT adsorption isotherm
Figure 6: Mineralogical composition of red mud washed with DI water
Figure 7: XRD patterns of washed and acid treated red muds (0.5 M)
Figure 8: SEM image of red mud reacted with concentrated HCl
Figure 9: ICP-MS of acid treated red mud filtrate
Figure 10: Infrared spectrum of washed red mud
Figure 11: Infrared spectrum of red mud treated with various concentrations of HCl
Figure 12: Infrared spectrum of red mud treated with various concentrations of
HNO3
Figure 13: Infrared spectrum of red mud treated with various concentrations of
H2SO4
Figure 14: Removal of fluoride (%) using different masses of acid treated red mud
and the associated changes in pH from the original 100ppm fluoride solution with pH
4.75
Figure 15: Removal of fluoride (%) using different masses of acid treated red mud
and the associated changes in pH from the original 100ppm fluoride solution with pH
7.99
Figure 16: X-ray diffraction pattern of red mud and corresponding reference patterns.
Figure 17: Infrared spectrum of red mud
Figure 18: DTG curve of red mud
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Figure 19: X-ray diffraction patterns of thermally activated red mud
Figure 20: Infrared spectra of thermally activated red mud
Figure 21: Fluoride adsorption (mmol/g) using red mud thermally activated to
1000°C for a 100mg/L fluoride solution with an initial pH of 4
Figure 22: Fluoride adsorption (mmol/g) using red mud thermally activated to
1000°C for a 100mg/L fluoride solution with an initial pH of 8
Figure 23: Fluoride adsorption (mmol/g) using red mud thermally activated to
1000°C for a 100mg/L fluoride solution with an initial pH of 4
Figure 24: Fluoride adsorption (mmol/g) using red mud thermally activated to
1000°C for a 100mg/L fluoride solution with an initial pH of 8
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List of Tables
Table 1: World bauxite mine production, reserves and reserve base
Table 2: Major mineralogical compositions of bauxites
Table 3: Major minerals composition of bauxite
Table 4: Different conditions for removing phosphorous by red mud
Table 5: d- values of iron oxides
Table 6: Types of isotherms, characteristics and examples
Table 7: Major elemental composition represented as (Al,Na,Si,Ti):Fe
Table 8: Oxide composition of red mud
Table 9: Physical properties of red mud, red mud heated to 500 and 1000°C
Table 10: ICP-OES of red mud after being immersed in water
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List of Publications
1. Wentao Liang, Sara J. Couperthwaite, Gurkiran Kaur, Cheng Yan, Dean W.
Johnstone and Graeme J. Millar, Effect of strong acids on red mud structural and
adsorption properties. Submitted to the Journal of Colloid and Interface Science
on the 13th September, 2013.
2. Wentao Liang, Sara J. Couperthwaite, Dean W. Johnstone, Cheng Yan, Gurkiran
Kaur and Graeme J. Millar, Adsorption properties of thermally activated red mud
for fluoride. Submitted to the Journal of Thermal Analysis and Calorimetry on the
11th October, 2013.
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QUT Verified Signature
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The Proposed Supervisors and Their Credentials
Principle Supervisor: Associate Prof. Cheng Yan
Dr. Cheng Yan is an associate professor in the School of Chemistry, Physics
and Mechanical Engineering at QUT. He is an established expert in synthesis and
characterization of various materials. His current research interests include
nanomaterial for renewable energy, structural and functional Nano composites,
biomaterials and biomechanics, Nano mechanics and modelling at micro and Nano
scales. He has been awarded 2 competitive ARC fellowships, 10 ARC projects and
granted over $5 m research funds. He has generated more than 200 publications and
co-chaired several international conferences.
Associate Supervisor: Dr. Sara Couperthwaite
Dr. Sara Couperthwaite is a Vice-Chancellor Research Fellow in the School of
Chemistry, Physics and Mechanical Engineering at QUT. She established a
reputation in the recycling of industrial waste residues for water purification
techniques, with a focus on the alumina industry and coal seam gas water. Her
current research interests include the optimisation of a layered hydroxide compound
that is generated from alumina waste and seawater for the removal of heavy metal in
other mineral processing industries, and the removal of selected cations and anions in
coal seam gas water using commercially available ion exchange media. She has been
awarded an ARC DECRA grant and generated close to $0.5 million in commercial
research projects. She has generated more than 130 publications, presented at a
number of alumina refinery focused conferences and has numerous industrial
contacts in the alumina and coal seam gas industries.
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Acknowledgments
The financial and infra-structure support of the Science and Engineering Faculty and
the Institute of Future Environments, Queensland University of Technology is
gratefully acknowledged.
I would like to thank the following people and organisations without who this thesis
could not have been completed.
They include:
(i) My QUT supervisors: Associate Professor Cheng Yan and Dr Sara
Couperthwaite for providing a challenging research project and assistance
in the editing.
(ii) The entire Couperthwaite and Millar group, with a special thanks to
Mitch De Bruyn and Kenneth Nuttall for training and assistance in
numerous instrumental techniques.
(iii) The postgraduate community and staff of the Science and Engineering
Faculty who provided much needed support.
(iv) Mr. Anthony Raftery for advice and technical support with the XRD
instruments and preparation methods.
(v) Dr. Llew Rintoul for his assistance with the vibrational spectroscopy
instruments.
Finally, I would like to give a special thanks to my family and friends for their love,
support and encouragement, especially my parents.
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1CHAPTER 1: INTRODUCTION
At the end of 2010, around 3 billion tonnes of bauxite refinery residue (red mud) had
been produced globally using the Bayer process to convert aluminium oxides and
hydroxides in bauxite ore to alumina [1]. It is estimated that at least an additional
120 million tonnes is produced each year [1]. The magnitude of waste being
generated by this industry clearly demonstrates the need for future developments in
technologies that utilised this raw material. Due to the chemical complexity and
classification (hazardous material under the Basel Convention) [2] of bauxite residue,
numerous researchers are trying to utilise the waste residue [3-14].
Red mud (generally slurry) is comprised of iron oxides (20%-65%), titanium oxides
(2%-8%), silicon oxides (10%-15%) and undissolved alumina, along with a wide
range of other oxides depending on the country of origin [15, 16]. Trace levels of
metal oxides, such as arsenic, cadmium, chromium, copper, gallium, lead, mercury,
nickel and in some cases thorium and uranium, are of particular concern. Apart from
heavy metal contamination the alkalinity of red mud also possess restrictions on
viable applications due to the cost of neutralisation. Alkalinity in the residue exists in
both solid and solution as: 1) entrained liquor (sodium hydroxide, sodium aluminate
and sodium carbonate), 2) calcium compounds, such as hydrocalumite, tri-calcium
aluminate and lime, and 3) sodalite ((NaAlSiO4)6(Na2X), where X can be SO4
2-,
CO32-
, Al(OH)4- or Cl
-) [17].
The potential environmental implications of seepage, dam failures and flooding can
have a large negative impact on the surrounding water bodies, including groundwater,
lakes and rivers, when soluble caustic chemicals are released [18]. Mining industries
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take huge precautionary measures to minimise environmental risks (lining dams and
below capacity limits), however during natural disasters such as flooding there are no
measures that can be taken to prevent spillage. Apart from the potential risks of
tailing dams, there also exists the problem that large areas of land are being
transformed into landfill to contain the red mud residue [18]. The discovery of a
viable application (in-expensive and uses large quantities) for the reuse of red mud
will significantly minimise environmental impacts caused by tailings dams and the
associated costs of storage facilities (more than $80 million a year) [19].
In recent years, many researchers have focused on utilising red mud as an adsorbent
material, and have had success in the adsorption of heavy metals [20-22], arsenate
[23-28], phosphates [29] and to a lesser extent fluoride [30, 31]. Fluoride is naturally
found in groundwater due to the dissolution of fluoride bearing minerals over long
periods of time [32]. However, elevated levels of fluoride in groundwater can
generally be traced back to a number of industries, including but not limited to, glass
and ceramic production, electroplating, coal fired power stations, brick and iron
works, and aluminium smelters [33]. It is estimated that more than 200 million
people rely on contaminated drinking water containing more than 1.5mg/L of
fluoride (World Health Organisation safe level) [34]. Continual and excess exposure
to fluoride results in diseases such as osteoporosis, arthritis, brittle bones, and cancer,
infertility, brain damage, Alzheimer and thyroid disorders in humans [35].
Traditionally, contaminate fluoride drinking waters have been treated using lime,
which results in the precipitation of fluorite. However, other precipitating and
coagulating reagents include iron (III), alum, calcium and activated alumina [35].
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More involved processes, such as ion exchange, reverse osmosis and electro dialysis,
have also been explored, but have subsequent waste disposal issues and high
operating and maintenance costs [35, 36].
Of particular interest, is the removal of fluoride using activated alumina and iron-
based materials, schwertmannite (Fe8O8(OH)6(SO4)•nH2O), granular ferric hydroxide
(Fe(OH)3), and goethite(α-FeOOH) [35, 36], as they are found in red mud and thus
advocates red mud as an adsorbent. Fluoride adsorption using iron-based adsorbents
are facilitated by exchange reactions involving F- and OH
- with FeOH surface groups.
A maximum Langmuir adsorption capacity of 7.0 mg/g of fluoride on granular ferric
hydroxide has been reported by Kumar et al. [37] in the pH range 6.0 to7.0.
Phosphate and sulphate (inner-sphere forming species) have been shown to have
negative effects on the loading capacity of fluoride, while outer-sphere forming
species (chloride and nitrate) improved fluoride removal slightly [35].
For activated alumina (amorphous gibbsite (Al(OH)3) or alumina (Al2O3)), Farrah et
al. [37] observed a maximum loading capacity of 9.72mg/g (Langmuir) in the pH
range 5.5 to 6.5. At lower pH, fluoride removal decreased due to the formation of
AlFx soluble species, while at higher pH, OH- displaced F
-. A study by Cengeloglu et
al.[30] investigated the adsorption capacity of untreated and hydrochloric treated red
mud. The maximum removal capacity was obtained using the acid treated red mud
and observed a Langmuir loading capacity of 3.574 mg/g at pH 5.5 after 2 hours.
Granular red mud prepared by Tor et al.,[37] obtained a Langmuir loading capacity
of 0.644 mg/g at pH 4.7 after 6 hours during a batch trial. However, this later study
showed an increased loading capacity for column studies (0.773-1.274 mg/g), which
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successfully regenerated using 0.2M NaOH. Previous studies have also shown that
the spent red mud satisfies the toxicity characteristic leaching procedure (TCLP)
used to classify inert wastes [37].
This investigation will assess the structural and resulting adsorption properties of
Australian red mud untreated and activated by acid and thermal methods. Particular
emphasis will be placed on determining the reactions involved during the activation
processes and how the remaining mineralogical composition correlates with the
fluoride loading capacities.
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THESIS OUTLINE 1.1
Red mud is a complex material that requires an understanding of different fields of
science, including physical chemistry, chemical engineering and material science, in
order to obtain an appreciation of the hurdles that need to be overcome for its reuse
as an adsorbent material. This thesis includes a chapter on relevant literature,
characterisation of red mud and subsequent changes in structural and adsorbent
properties for acid and thermally activated red mud.
I. Chapter 2: Literature Review
Chapter 2 presents a review of current literature on the development of red mud as an
adsorbent material. It will be divided into two main parts: 1) red mud and current
applications and 2) potential hazard of fluoride. A large portion of the review will
cover the most recent research on red mud as an adsorbent for heavy metals [20-22],
arsenate [23-28], phosphates [29] and fluoride [30, 31].
II. Chapter 3 : Experimental and Characterisation
Chapter 3 highlights the experimental procedures used throughout this investigation
and provides theoretical background on some of the physical characterisation
techniques, such as X-ray diffraction, infrared spectroscopy and surface analysis.
III. Chapter 4: Acid Activated Red Mud
Chapter 4 describes the process of acid activation. The main purpose of the acid
activation process is to remove species that do not participate in the adsorption
process as well as to reduce the influence of carbonate (CO32-
) [38]. All red muds
have been fully characterised and include untreated red mud and acid activated red
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mud using hydrochloric, nitric and sulphuric acid. Characterisation techniques
include infrared spectroscopy (IR), scanning electron microscopy (SEM) coupled
with energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). A
comparison of the structural changes and adsorption properties of the three acids
have been made.
IV. Chapter 5: Thermally Activated Red Mud
Chapter 5 presents the effects of temperature on the structural and adsorption
properties of red mud. Calcining red mud is used to reduce a number of the
mineralogical phases (aluminium oxy/hydroxides and sodalite) in order to increase
the surface area of hematite available for the adsorption of fluoride and therefore the
removal of sodalite and carbonate could be decomposed in a proper temperature
range. The composition of thermally treated red mud has been fully characterised,
and as a result structural and physical properties change. The impact on fluoride
removal varies significantly with the compositional changes that occur.
V. Chapter 6: Conclusions
This investigation has shown that the mineralogical phase involved in the adsorption
of fluoride is hematite. The activation of red mud through acid and thermal methods
has improved the adsorption capacity of red mud.
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SIGNIFICANCE 1.2
Due to the substantial amounts of red mud stored in tailing dams and their potential to
fail or release red mud waste during natural disasters, this investigation looks at the
potential of red mud as an adsorbent material for fluoride. The development of a suitable
application of red mud will not only minimise the potential environmental risks that
tailing dams pose but it will also reduce costs associated with the construction and
maintenance of these dams. In addition, the development of a material suitable for the
treatment of fluoride contaminated waters increase the significance of the work being
conducted. This thesis utilizes acid activated and thermally treated red mud to adsorb
fluoride, with an emphasis on understanding the molecular and structural changes that
occur during the treatment process.
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2CHAPTER 2: LITERATURE REVIEW
BAYER PROCESS REVIEW 2.1
2.1.1 Bauxite ore
Lateritic bauxites (silicate bauxites) are found mostly in the countries in the region of
the Earth surrounding the Equator (tropics). They are formed by lateralization
(tropical weathering) of various silicate rocks such as basalt, gneiss, granite, shale,
and syenite. It is a prolonged process of chemical weathering which produces a wide
range of ore compositions of varying thickness, grade, chemistry and mineralogy.
The formation of bauxites depends on intensive and long-lasting weathering
conditions of the underlying parent rock in a location with very good drainage.
Tropical weathering enables the dissolution of kaolinite (Al2Si2O5(OH)4) and the
precipitation of gibbsite (Al(OH)3). Zones with the highest aluminium content are
generally located in areas rich in iron and oxygen. Aluminium hydroxide in the
lateritic bauxite deposits is almost exclusively gibbsite in tropical regions, such as
Australia.
Bauxite contains aluminium in the form of three main minerals, 1) gibbsite
(aluminium trihydroxide), 2) boehmite (aluminium oxyhydroxides - γ-AlO(OH)),
and diaspore (α-AlO(OH)), with gibbsite and boehmite being the most dominate
aluminium compounds in all bauxite ores. Apart from aluminium, it also contains a
mixture of iron oxides (hematite - Fe2O3 and goethite - α-FeO(OH)), the clay mineral
kaolinite, and small amounts of anatase (TiO2). Currently, bauxite ores are the only
feasible resource for the production of alumina on a commercial scale; however
research into the extraction of alumina from clay is being undertaken.
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European and Chinese bauxite ores are composed almost entirely of the monohydrate
forms boehmite and diaspore, except for Greece which contains predominately
diaspore containing ores. Compared with Europe, the ores from the rest of the world
are composed primarily of gibbsite, with small amounts of boehmite and minor
amounts of diaspora [39].
In Australia, aluminium ores are predominantly composed of gibbsite, which are
consequently required lower digestion temperatures and caustic concentrations.
However, due to the poor quality of bauxites of Australia in terms of alumina content
and high organic content, the aluminium industries of Australia generally extract
aluminium using relatively mild conditions. Due to the fact that Australia has one of
the largest bauxite deposits in the world, Australian alumina industries are a
competitive supplier of alumina [40].
Australia is one of the top producers of bauxite, totalling almost one-third of the world's
production, followed by China, Brazil, India, and Guinea [41]. Although aluminium
demand is rapidly increasing, there are sufficient known reserves to meet the worldwide
demands for aluminium for many centuries. Resources of bauxites, the raw material
for aluminium, are not widespread throughout the world. There are only seven bauxite-
rich areas: Western and Central Africa (mostly, Guinea), South America (Brazil,
Venezuela, Suriname), the Caribbean (Jamaica), Oceania and Southern Asia (Australia,
India), China, the Mediterranean (Greece, Turkey) and the Urals (Russia) [41]. A table
outlining the current extractions of bauxite are summarised in Table 1, while Figure 1
summarised the bauxite world reserves.
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Table 1: World bauxite mine production, reserves and reserve base [41]
Country Mine production
2010 2011 (est.)
Australia 68,400 67,000
China 44,000 46,000
Brazil 28,100 31,000
India 18,000 20,000
Guinea 17,400 18,000
Jamaica 8,540 10,200
Greece 2,100 2,100
Other 22,880 27,080
Figure 1: World bauxite reserves[41]
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2.1.2 Bayer process
The one of methods of production of aluminium is the Bayer processes, which is
used for refining bauxite ore to smelting grade alumina (Al2O3). The process was
developed by Karl Josef in 1888, and has become the cornerstone of the aluminum
production industry. Depending on the quality of the ore, between 1.9 and 3.6 tonnes
of bauxite [39]. An investigation by Ostap [42], reported the major mineralogical
composition of bauxites over a large geographical area, including Africa, Australia,
India, Jamaica, Guyana and Brazil. This data is summarized in Tables 2 and 3.
Table 2: Major mineralogical compositions of bauxites[42]
Mineral Formula
Gibbsite Al2O3⋅3H2O
Boehmite Al2O3⋅H2O
Hematite Fe2O3
Aluminium goethite (Fe,Al)2O3⋅H2O
Anatase TiO2
Rutile Ti2O
Kaolinite, Halloysite Al2O3⋅2SiO2 ⋅2H2O
Quartz SiO2
Table 3: Major minerals composition of bauxite[42]
Constituent Percentage Composition
Caustic soluble Al2O3 30%-60%
Fe2O3 1%-20%Ti
TiO2 0.3%-10%
SiO2 (Clay) 0.1%-5%
SiO2 (Quartz) 0.1%-10%
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The Bayer process involves two distinct stages responsible for the several phase
transformations that enable alumina to be extracted from bauxite ore. The first
process requires increased temperatures that are dependent on the quality of the
bauxite ore (145°C for high-grade gibbsitic ores and 270°C for low-grade boehmitic
ores). Bauxite ore is crushed before the digestion process in concentrated sodium
hydroxide (NaOH). This part of the process transforms aluminium oxy/hydroxides
into a solution of sodium aluminate, while iron oxide compounds, quartz, sodium
aluminosilicates, calcium carbonate/aluminate and titanium dioxide remain insoluble.
After the dissolution of the aluminum oxy/hydroxides, the insoluble compounds are
removed by settling/filtration techniques. The clarification process is dependent on
the form of the aluminium present in bauxite ore. This waste residue is commonly
known as red mud in the alumina refining industry due to the distinct red colour
caused by the large iron concentration [43].
The next stage of the Bayer process involves the precipitation of gibbsite (Al(OH)3),
which is achieved by cooling the sodium aluminate solution under controlled
conditions. In order to achieve the desired gibbsite particle distribution, seeding
(addition of undersized gibbsite) is sometimes used to encourage the growth of
particles. The final stage of the process is to convert gibbsite to alumina by
calcination at temperatures of around 1000-1100°C in rotary or fluid flash calciners.
Chemically, the reactions of the Bayer process are depicted by the following
equations:
Extraction:
Gibbsite: Al(OH)3(s) + NaOH (aq) → Na+Al(OH)4
- (aq)
Boehmite: AlO(OH)(s)+ NaOH (aq)+ H2O Na+Al(OH)4
- (aq)
(1)
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Precipitation:
Na+Al(OH)4
- (aq) Al(OH)3(s)+ NaOH(aq) (2)
Calcination:
2Al(OH)3(s) Al2O3(s)+ 3H2O(g) (3)
A summary of all the processes involved in the Bayer process is illustrated in Fig.2
[39].
Fig.2 Schematic representation of The Bayer process [39]
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2.1.3 Limitations of the Bayer process
The two main chemical reactions that can limit the productivity and efficiency of
alumina refineries are; 1) the precipitation of gibbsite (Eq. 2) and 2) the
crystallization of sodium oxalate (Eq. 4). The co-precipitation of sodium oxalate with
gibbsite in the Bayer process results in the production of lower quality alumina due
to the incorporation of sodium as an impurity during calcination. Therefore,
numerous researches have been undertaken to minimize the amount of sodium
oxalate in the alumina stream through the use of materials such as lime or magnesia
that react with alumina to form layered cationic structures that capture and remove
oxalate [44].
Oxalate crystallization:
2Na+
(aq) +C2O42-
(aq) Na2C2O4(s) (4)
Precipitation of gibbsite from the pregnant liquor is a critical phase of the Bayer
process, which requires good crystal growths and purity. Precipitation is effected by
cooling the solution to 58°C, thus causing supersaturation and by introducing small
gibbsite particles as seeds. The crystalline gibbsite precipitate is separated into a fine
and a coarse fraction, whereby the fine fraction is recycled to the precipitators as
seed, while the coarse fraction represents the product to be calcined [40, 42].
Optimising the precipitation stage of the Bayer process is of great importance in
terms of productivity, however in terms of environmental issues and public relations
the optimisation of red mud chemistry and disposal is becoming a hot topic in the
scientific community.
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RED MUD APPLICATIONS 2.2
2.2.1 Red mud issues
The greatest downside of the alumina process is the large amount of waste that is
produced. For every ton of alumina produced, between 1-1.5 tonnes of red mud
needs to be treated and disposed [45]. The major chemical constituents of red mud
range from 20-65% Fe2O3, 10-27% Al2O3, 5-25% TiO2 and 2-8% Na2O, with
specific values depending on the source of the bauxite ore and digestion conditions
used [46].
According to Kumar et al. in 2006 [15], red mud had approached 90 million tonnes
annually, with the majority of waste residue comprising of high alkalinity with a pH
ranging from 10-13 [20]. The combination of the chemical complexity and alkalinity
of red mud poses difficulties in the safe disposal of the waste residue. There was a
major dam failure accident in Ajka, Hungary in October 2010 that caused significant
damage and loss of life because the red mud slurry was not treated before its disposal
[18]. Several environmental impacts were investigated after this disaster, with the
main focus on metal bioavailability and toxicity of contaminated soil and waterways.
The extent of environmental damage, the death of 10 people and countless more that
suffered caustic burns [47]. This was an unprecedented accident in the history of the
Bayer process. The accident proved that untreated red mud can pose a significant risk
to living organisms and the environment. Meanwhile, it is essential to note that the
cost for disposal can be up to 5% of alumina production costs [46].
The risks and costs associated with the large disposal areas for the storage of red mud
has received much attention, with many applications focusing on the utilizing of red
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mud as a secondary material in building materials and water purification. Pilurzu et
al. [48] successfully made bricks using red mud from the southwest of Sardinia,
however there are numerous innovations in the areas of adsorbents, ceramic glazes,
catalyst, pigments, and polymers [18]. However, due to the vast differences in the
composition of red mud, each application has to be designed with the original red
mud composition in mind. According to Wang et al. [49], the main composition of
red mud in Australia is crystalline hematite (Fe2O3), which makes up about 40% of
red mud, while in China, crystalline hematite (Fe2O3) is just less than 10%. The
Chinese red mud has a greater portion of quartz (SiO2), which makes up around 14%.
This vast chemical difference presents a huge challenge in the development of
applications.
2.2.2 Red mud applications in materials
I. Iron recovery
According to Liu et al. [20] , iron can be recovered from red mud by magnetic
separation, with the aim of using the recovered iron for building materials. They
analysed red mud with different chemical compositions, for the recovery of iron
based on the addition of additives followed by roasting at 1300°C for 110 minutes.
The study determined that the appropriate proportion of carbon powder to red mud to
be 18:100 and an additive ratio of 6:100. This method resulted in an iron recovery
rate of 81.40% depending on the initial iron content. The recovery of iron can be
explained by the following reactions:
Fe2O3 + 3C 3Fe + 3CO (5)
Fe3O4 + 4C 3Fe + 4CO (6)
FeO + C Fe + CO (7)
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II. Land fill and land reclamation
Red mud as a waste product should be disposed with as many precautions in place to
minimise the environmental impact. Although some alumina refineries implement a
neutralisation process before disposal, a lot of refineries still dispose without any
manipulation. Due to its very high alkalinity, it is essential that the residue is
neutralised prior to its use in alternative applications. It has been successfully done
by CVG-Bauxilum and Atomaer-KD Engineering Co., Inc. [50], which reduced the
pH of red mud from 12.20 to 7.60. The latest direction for the utilisation of red mud
is using it for filling mined or quarried areas, landfill cover [51], road-base and levee
material [48]. According to Brown and Kirkpatrice [51], red mud can be used as a
road-base and levee material, which is stacked and compacted by heavy equipment.
The red mud roads have a good condition even after a long period of time.
There is a prospective research field for agricultural land neutralization or
composting. Due to high alkalinity of red mud it can be mixed with manures to
neutralize the acidic agricultural land soil [48]. Meanwhile, according to Goldstein
and Reimers [52], red mud is a good disinfectant and stabilizing agent for sewage
treatment. It can be an organic composting material for agricultural uses after it has
been used to treat sewerage. It means it not only can reduce the water pollution, but
also can be a new organic composting material for sustainable agricultural
development.
III. Building materials and ceramic industry
Red mud has been used as bricks for building materials by Pilurzu et al. [53] in the
southwest of Sardinia. Smirnov et al. [54] treated red mud using sulphuric acid,
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which deactivates it along with the addition of Fe-Al coagulants before heat
treatment to produce red mud building materials meet specifications. The reuse of
red mud in the ceramic industry is also plausible. Balasubramanian et al. [55] mixed
red mud and fly ash to prepare glass-ceramic products that can be used to decorate
the exteriors in the building industry.
Red mud has been widely used in material industry and shows a lot of versatility.
However, the percentage of red mud used in metal recovery, building and ceramic
materials is limited, and in the scheme of the things makes no real impact on the
removal of red mud from tailing dams. For bulk use, landfill might be considered,
however extensive studies on the possible environmental implications long term need
to be further understood.
2.2.3 Red mud applications in water treatment
Red mud is not limited to landfill or building materials, but it can also be used to
produce valued materials for water treatment. In the past decade, there have been a
lot of applications of red mud in the water treatment industry, with some of the
studies showing promising results.
I. Coagulant
The composition of red mud from different parts of the world are summarised in
Figure 3 [48]. The typical composition of red mud has high contents of Fe and Al in,
which aligns well with coagulant applications that generally involve iron and
aluminium chemical compounds. According to Orešcanin et al. [56], red mud was
treated using sulphuric acid (30% wt), centrifuged, and then neutralised to pH with
waste caustic. This particular product could be used as a coagulant for heavy metal
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and turbidity removal in industrial wastewaters. The study revealed that 1 g of the
red mud coagulant could remove 99.3 mg of Cu2+
; 95.0mg of Zn2+
and 98.7mg of
Pb2+
from an initial solution containing 100 mg of each metal. The study proved that
it could be a plausible coagulant for this particular application.
Fig.3 Composition of red mud in different parts of the world [48]
I. Adsorbent
Beside fluoride adsorption, there are others studies on the removal of a number of
pollutants using red mud. Phosphate is one of the main focuses for its removal from
effluent [57-60] and 80-90% of phosphate could be adsorbed using activated red mud
in the pH range 3.2 to 5.5 [60]. In addition to the removal of phosphates, a lot of
studies have also been conducted on the removal of nitrates using raw red mud and
activated red mud. Acid activated red mud (10g with 200ml of 20% wt HCl) had an
adsorption capactiy of 123 and 380 mg/g for raw red mud and activated red mud,
respectively [61]. Studies focused on arsenic removal have been shown to have
adsorption efficiencies 35% higher for acid active red mud than raw red mud [62,
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63]. Heavy metals contamination occurs in many aqueous wastewaters, and red mud
has been widely used for surface precipitation and adsorption of heavy metals, such
as Pb2+
, Cd2+
and Zn2+
[64].
i. Phosphorus
Phosphorus is a very important index for water quality, if it is discharged in excess
to waterways it may cause eutrophication, which leads to algal and hydrophytes
blooms. Compared with activated carbon, which is a commonly used adsorbent that
is relatively cost-effective [65], however red mud has been shown to have a more
effective role in the adsorption process. Robert et al. [66] added red mud into a
commercial peat for removing phosphorus, and the result showed that phosphorus
removal was raised from 17% to 21%, with a 95% red mud mixture.
Sulphuric acid treated red mud has been investigated by Mohanty et al. [67], found
improvements in the adsorption capacity of red mud especially when the pH is
around 4.5. Investigations into the effects of heat treatment and the resultant
adsorption capacity of red mud has reported by Li et al.[68], which found red mud
had higher adsorption efficiencies when heated to 700oC for 2 hours. The adsorption
rate increased to around 99%.
Red mud as an adsorbent for removing phosphorous are depended its source and
treatment methods used. In order to obtain sufficient removals, past investigations
show that acidification and heat treatment are required. Table 4 shows the different
treatments used for removing phosphorous by red mud [49]
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Table 4: Different conditions for removing phosphorous by red mud [49]
Adsorbent Temperature (oC) P capacity (mg g
-1)
RM-HCl 30 23.2
RM - 40-45
Bauxite 25 0.67-0.82
Heated Bauxite 25 0.98-2.95
RM-H2SO4 40 7.4
RM 25 114
RM-HCl 25 162
Heated RM 25 345
Bauxsol 23 6.5-14.9
ii. Nitrate
Nitrate pollution has increased globally due to increased use of fertilizers and
livestock farming. It may cause high nitrate concentration in ground water, and high
nitrate-related problems in the source of drinking water, which could pose significant
risks to public health. Due to the increasingly nitrate pollution, Çengeloğlu et al. [61]
used red mud and HCl-activated red mud for the removal of nitrate. Compared with
the original adsorption capacity of red mud (1.86mmol nitrate g-1
), while an
adsorption capacity of 5.86 mmol nitrate g-1
was obtained for HCl-activated red mud.
The increased adsorption capacity is shown in the following reaction mechanisms:
RMOH + H+
↔ RMOH2+
(8)
RMOH2+
+ NO3- ↔ RMOH2-NO3 (9)
RMOH2+
+ NO3- ↔ RMNO3 + H2O (10)
(RM represents Red Mud- Fe, Al or Si of red mud surface)
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RED MUD TREATMENT COMPARISION 2.3
2.3.1 Acid activated red mud
Acid treatment has been tested on red mud to increase its adsorption capability [30,
58, 61, 62, 69]. For the purpose of removal arsenic, 0.25- 2.0M HCl activated 50g
dry red mud by stirring for 2 hours. The treated red mud was separated from the
solution by filtration, and then washed and dried at 105℃ for 4 hours. The adsorption
efficiencies of acid activated red mud for As (III) was 35% higher than raw red mud,
with a relationship between the concentration of HCl and adsorption capacity being
observed. The number of As (III) adsorption efficiency was 70, 88, 77 and 75% for
red mud activated by 0.75, 1.0, 1.5 and 2.0 HCl, respectively, with can be seen the
best concentration was 1.0M HCl. The reason of adsorption decrease may be due to
the adsorbent was blocked by sodalite leakage. As a result, it can be observed that
adsorption ability of red mud can be increased by acid treatment [62]. Another study
on acid activated red mud was reported by Çengeloğlu,et. al. [61], which involved
batch adsoprion techniques for the removal of nitrate using red mud. All the red mud
was from Kenya, with an average composition of 18.7% Al2O3, 39.7% Fe2O3 and
14.5% SiO2. It was found the adsorption capactiy of the activated red mud was
approximately 3 times better than untreated red mud.
A red mud study by Worsley Alumina, Australia, with a compositions of 60% Fe2O3
16% Na2O, 15% Al2O3, 5% TiO2 and 5% SiO2, also showed increased removal
capacities using acid activated red mud. This investigation used 2M HCl and 2M
HNO3 at liquid/solid ratios of 20ml/g. The study also included the characterisation of
the acid activated red muds (RM-HCl and RM-HNO3), using surface analysis
techniques (BET) and X-ray diffraction (XRD). The BET surface area showed raw
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red mud had a surface area of 22.7m2/g, but acid active red mud had a larger surface
area; 28.48m2/g for RM-HCl and 38.15m
2/g for RM-HNO3. The total pore volume
was found to be 0.0566, 0.0779 and 0.0658cm3/g for raw red mud, RM-HCl and RM-
HNO3, respectively. All the results showed that acid activated red mud by HCl and
HNO3 increased the adsorption capacity of red mud, with HCl treated red mud
performing better than HNO3 [59].
Most applications focused on acid treatment by hydrochloric [61, 62], phosphoric
and nitric acid [59], with few reports on the use of sulphuric acid as an acid activator.
Heavy metal removal using red mud generally follows four steps: 1) surface
precipitation, 2) flocculation in the form of hydrolytic products, 3) chemical
adsorption and 4) ion exchage. Based on surface-complex formation, the metal ions
are removed by –OH group on suface or as synthetic cation exchanger [69].
2.3.2 Thermally activated red mud
Heat treatment is another widely used method to increase the adsorption capacity of
red mud. Previous investigations focused on phosphate removal using thermally
activated red mud have shown increases in adsorption capacities [57-59]. The study
by Li et al. [57], looked at the adsorption capacities of seven thermal activation
temperatures (ambient, 200, 500, 600, 700 800, 900 and 1000°C) for phosphate
removal and reported capacities of 9.19, 12.00, 14.27, 16.50, 19.05, 14.38, 14.13,
13.75 mg/g, respectively. Increases in adsorption capacities was reported as an
increase in surface area of red mud started towards 500°C, due to the expulsion of
water leading to the formation of a more porous structure [70]. The maximum
adsorption capacity of red mud was achieved at 700°C (19.05mg/g), which was more
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than double untreated red mud with an adsorption capacity of 9.19mg/g. Above
700°C, phosphate removal became less efficient, which has been explained as the
decomposition of some oxy/hydroxyl minerals and to a certain extent the
decomposition of calcite.
Another application has shown the thermal behavior of red mud and the chemical
components changes between 120 and 1400°C [71]. Various complementary
techniques, including X-ray diffraction analyses and thermal analyzer (gas-
chromatographic/mass spectrometer) were used in this investigation, which found
that red mud continued to undergo chemical changes up until 900°C. At this
temperature, H2O vapour and CO2 are lost due to the decomposition of aluminum
hydroxides and silico-alumino-carbonates, respectively. There were more changes
between 900 and 1100°C, assigned to Ca3Al2O6 and NaAlSiO4 due to the alkaline
oxides such as CaO and Na2O. Ferric iron (Fe3+
) was reduced to Fe2+
, and combined
with –TiO2 phase at these elevated temperatures.
2.3.3 Combination treatment
Literature has shown that the combination of acid and heat treatment influence the
adsorption capacity of red mud [58, 59]. However, a comprehensive review of the
effect of different acid and heat treatments on the removal of fluoride does not exist.
An investigation that activated red mud by treating it at 700°C for 2 hours washed
with 0.25M HCl (RM-HCl-700°C). A comparison of thermally activated red mud at
700°C (RM-700°C) with acid and thermal activation (RM-HCl-700°C) showed that
the combination of activation techniques had a higher loading capacity (26.4mg/g
phosphate compared to 19.1mg/g) [58]. While an investigation by Liu et, al. [58],
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reported the absorption capacity for phosphate as: RM-HCl > RM-HNO3-700 °C ≈
RM-HNO3 > RM-HCl-700°C > RM. Therefore, it is apparent that the concentration
of acid used in the acid activation requires more focus, as a number of studies report
the optimum calcination temperature to be at around 700°C [20, 57, 64, 72].
FLUORIDE APPLICATIONS 2.4
2.4.1 Fluoride issues
Fluoride is found naturally as sellaite (MgF2), fluorspar (CaF2), cyolite (Na3AlF6) and
fluorapatite (3Ca3(PO4)2Ca(F,Cl2)) [73]. Over time the weathering of these minerals
results in fluoride existing naturally in groundwater [74]. However, elevated levels of
fluoride in groundwater can generally be traced back to a number of industries, including
but not limited to, glass and ceramic production, electroplating, coal fired power stations,
brick and iron works, and aluminium smelters [75, 76]. It is estimated that more than 200
million people rely on contaminated drinking water containing more than 1.5mg/L of
fluoride (World Health Organisation safe level) [77]. Continual and excess exposure to
fluoride results in diseases such as osteoporosis, arthritis, brittle bones, cancer, infertility,
brain damage, Alzheimer and thyroid disorders in humans [76].
2.4.2 Red mud applications with fluoride
There are many different technologies that have been used to remove fluoride from
aqueous systems on a laboratory scale. Granular ferric hydroxide (GFH) has been
shown to be effective, with 95% fluoride removal being reported and a maximum
adsorption capacity of 7.0mg/g [78]. Factors that cause a reduced adsorption capacity
for fluoride included ionic strength, pH value, surface loading and a number of co-
existing anions. Ionic strength had a limited impact on fluoride adsorption in
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comparison to the pH, which shows optimum fluoride removal between 3- 6.5. Based
on batch methods, major anions reduced fluoride adsorption in the following order:
Cl < SO42-
< HCO3- < H2SO4 [79]. Metal oxides have also been shown to have good
adsorption capacity for fluoride. An Al-Ce oxide hybrid has been reported to have an
adsorption capacity of 91.4mg/g at 25 ℃ and pH 6.0. Aluminium titanate (AT) and
bismuth aluminate (BA), have been reported to have a fluoride loading capacity of
0.85mg/g and 1.55mg/g, respectively at 30 °C [80, 81]. More recently, a potassium
manganese oxide (KMnO4), iron oxide-hydroxide nanoparticles and activated
alumina have been investigated for their effectiveness in the adsorption fluoride from
aqueous media [82-84].
Due to the vast quantity of red mud available numerous studies have been conducted
on the potential application of red mud for a range of wastewater treatment options.
Commonly, red mud has been activated prior to use through either acid and/or
thermal treatment methods. Acid activated red mud significantly enhances fluoride
removal from aqueous solutions, demonstrated in the work by Çengeloğlu et.al., [61]
who used 20 wt% hydrochloric acid (HCl) as the acid activator. The results clearly
showed the activated form had a higher loading capacity than untreated red mud,
with maximum removal of fluoride being obtained at pH 5.5 [30].
In the case of phosphate adsorption, HCl activated red mud has also been report to
increase the amount of phosphate that can be removed compared to untreated red
mud with the highest adorsption capacity reported as 26.4mg/g (HCl contration at
0.25mol/L) [58]. Except for acid activated, temperature also influences the activation
of red mud. The maximum adsorption capacity of phosphate by calcined red mud can
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achieve 19.1mg/g, compared with other samples under different temperatures [58].
Another study relates thermal red mud, which heated red mud at 700°C for 2 hours,
and 99% phosphate removed [57]. Most reports showed activated red mud is better
than untreated red mud for the remove of number of anions such as fluoride [30],
nitrate [61], and phosphate [57, 58].
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3CHAPTER 3: EXPERIMENTAL AND CHARACTERISATION
EXPERIMENTAL TECHNIQUES 3.1
3.1.1 Fluoride adsorption
Red mud from an Australian alumina refinery was dried over a period of 2 days at
105°C before being crushed to a fine powder using a Fritsch agate ball mill. Using a
Retsch sieving stack consisting of 10 sieves, ranging from 4mm to 64µm, red mud
was processed to give a size fraction < 250µm. A known amount of red mud was
then placed in a SEM furnace that had been previously heated to the desired
temperature. The sample of red mud was placed ion the furnace and heated for 2
hours. After heating, it was allowed to cool to room temperature before being re-
weighted so that the mass loss could be calculated. The red mud was then placed in a
desiccator until it was used for adsorption studies.
3.1.2 Characterisation techniques
Fluoride analysis was obtained using a TPS uniPROBE fluoride (F-) ion selective
electrode (ISE). A fluoride ISE buffer was prepared using 1 M of sodium chloride
(NaCl) and 1 M sodium citrate dehydrate (Na3C6H5O7·2H2O) dissolved in
approximately 1.5 L deionised water. Sodium hydroxide (NaOH) was use to adjust
the solution pH to 5.5 before making the solution up to 2 L. Calibration standards of
1, 10, and 100 mg / L fluoride stock solutions were prepared from AR grade sodium
fluoride (NaF). Standards and samples were analysed by combining 10 mL of the
calibration standard solution (or sample) and 10 mL of the buffer solution whilst
being stirred. The concentration of fluoride was then measured using the fluoride
ISE.
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The oxide composition of red mud (RM) and red mud heated to 500ºC (RM500) was
determined using XRF. Particle size analysis of the components of RM and heat
treated RM was obtained by the granulometric method. It was obtained by sieving
methods and by sedimentation analysis. The specific surface areas of all samples
were determinate by BET/N2 adsorption methods. The pH of the red mud samples
was determined by mixing dry red mud (2g) in water (10mL) with a TPS WP40 pH
meter calibrated with pH 7.00 and 10.01 buffers.
X-Ray diffraction patterns were collected using a Philips X'pert wide angle X-Ray
diffract meter, operating in step scan mode, with Co K radiation (1.7903 Å).
Patterns were collected in the range 5 to 90° 2 with a step size of 0.02° and a rate of
30s per step. Samples were prepared as Vaseline thin films on silica wafers, which
were then placed onto aluminium sample holders. The XRD patterns were matched
with ICSD reference patterns using the software package HighScore Plus. The
profile fitting option of the software uses a model that employs twelve intrinsic
parameters to describe the profile, the instrumental aberration and wavelength
dependent contributions to the profile.
Samples of the residual acid solutions, after centrifugation, were analysed using an
Agilent ICP-MS 7500CE instrument. The samples were diluted by a factor of 20
using a Hamilton dilutor with 10 and 1mL syringes. A certified standard from
Australian Chemical Reagents (ARC) containing 1000ppm of aluminium,
magnesium, calcium, iron and sodium were diluted to form a multi-level calibration
curve and an external reference that was used to monitor instrument drift and
accuracy of the results obtained. Results were obtained using an integration time of
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0.15 seconds with 10 replications. Calibration curves had an r2 value of 0.998 or
higher.
Infrared spectra were obtained using a Nicolet Nexus 870 Fourier Transform infrared
spectrometer (FTIR) with a smart endurance single bounce diamond ATR
(attenuated total reflectance) cell. Spectra over the 4000-525 cm-1
range were
obtained by the co-addition of 64 scans with a resolution of 4 cm-1
and a mirror
velocity of 0.6329 m/s.
CHARACTERISATION TECHNIQUES 3.2
3.2.1 X-ray Diffraction (XRD)
The crystal structure of each material is unique and depends on how its atoms are
ordered in planes and axes. The intercept of these planes with the symmetry axis
determines their position within the crystal lattice. The atoms interact with the
electromagnetic properties of the X-rays causing them to scatter. The majority of the
waves are made extinct through destructive interference: however, some are
enhanced by constructive interference. The direction of these latter waves is related
to the distance between the atomic planes, known as d-values, and the Bragg angle
(Figure 4). is the angle at which the waves hit the crystal producing the maximum
interference. The Bragg equation links this angle of scattering to the d-values as
follows [85]:
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Fig.4: X-ray diffraction between planes of atoms
= (11)
Where λ= wavelength used (nm)
= order of reflection
The output is the observed intensity plotted against Bragg angle, from which the d-
values are calculated. The type of radiation usually used for iron samples is either
CoK or CuK . If CuK is used, the observed intensity is much lower, as iron
absorbs much of the radiation.
I. D-values of common iron oxides
Each crystalline product has characteristic d-values that can be found on reference
cards in the Joint Committee on Powder Diffraction Standards (JCPDS) mineral
powder diffraction file. A compound can be identified by comparing the calculated
d-values against the JCPDS diffraction file of iron oxides summarised in Table 5.
The values are in order of intensity; the greatest is listed first
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Table 5: d- values of iron oxides[85]
Mineral Most intense d-values (Å)
Hematite 2.70, 3.68, 2.52
Magnetite 2.53, 1.49, 2.97
Goethite 4.18, 2.45, 2.69
Akaganeite 3.33, 2.55, 7.47
Lepidocrocite 6.26, 3.29. 2.47, 1.94
Ferrihydrite 2.54, 2.24, 1.97, 1.73, 1.47
Feroxyhyte 2.54, 2.22, 1.69, 1.47
3.2.2 Infrared Spectroscopy (IR)
Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the
infrared region of the electromagnetic spectrum, that is light with a longer
wavelength and lower frequency than visible light. It covers a range of techniques,
mostly based on absorption spectroscopy. As with all spectroscopic techniques, it can
be used to identify and study chemicals. A common laboratory instrument that uses
this technique is a Fourier transform infrared (FTIR) spectrometer.
The infrared portion of the electromagnetic spectrum is usually divided into three
regions; the near-, mid- and far- infrared, named for their relation to the visible
spectrum. The higher-energy near-IR, approximately 14000–4000 cm−1 (0.8–2.5 μm
wavelength) can excite overtone or harmonic vibrations. The mid-infrared,
approximately 4000–400 cm−1 (2.5–25 μm) may be used to study the fundamental
vibrations and associated rotational-vibrational structure. The far-infrared,
approximately 400–10 cm−1 (25–1000 μm), lying adjacent to the microwave region,
has low energy and may be used for rotational spectroscopy. The names and
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classifications of these subregions are conventions, and are only loosely based on the
relative molecular or electromagnetic properties.
Infrared spectroscopy exploits the fact that molecules absorb specific frequencies
that are characteristic of their structure. These absorptions are resonant frequencies,
i.e. the frequency of the absorbed radiation matches the transition energy of the bond
or group that vibrates. The energies are determined by the shape of the molecular
potential energy surfaces, the masses of the atoms, and the associated vibronic
coupling.
In particular, in the Born–Oppenheimer and harmonic approximations, i.e. when the
molecular Hamiltonian corresponding to the electronic ground state can be
approximated by a harmonic oscillator in the neighbourhood of the equilibrium
molecular geometry, the resonant frequencies are associated with the normal modes
corresponding to the molecular electronic ground state potential energy surface. The
resonant frequencies are also related to the strength of the bond and the mass of the
atoms at either end of it. Thus, the frequency of the vibrations is associated with a
particular normal mode of motion and a particular bond type.
3.2.3 Surface Area and Pore Size Analysis
The specific surface area is a key functional parameter for an adsorbent. Summaries
of adsorption theory, Langmuir and Brunauer–Emmett–Teller (BET) equations are
provided in subsequent sections. The resulting isotherms from nitrogen adsorption
can be classified and the pore size distribution extracted from the data. The types of
isotherms and hysteresis loops are outlined, in addition to the various methods used
for pore size distribution determination.
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I. Adsorption Theory
When a disperse solid is exposed to a gas in an enclosed space at a known pressure,
the solid will absorb the gas. After a period of time, the pressure will become
constant. The extent, of gas adsorption on a porous solid depends on the temperature,
pressure and the molecular size of the gas and available surface area. The amount of
gas adsorbed can be calculated from the change in pressure, if the volume of the
vessel and solid are known. Hence for a known gas held at a specified temperature,
the amount of gas adsorbed is solely dependent on the pressure and therefore an
adsorption isotherm can be constructed (volume adsorbed vs. relative pressure,
(pressure/ saturated vapour pressure, / )). The surface free energy is reduced by
adsorption, hence entropy is reduced. It is an exothermic process, with the heat of
physical adsorption being similar in magnitude to the heat of condensation [86].
a) Langmuir equation
Langmuir [87] was the first to present a kinetic approach to adsorption was based on
a flat surface with zero accumulation on the surface. i. e. the rate of adsorption is
equal to the rate of desorption. It is assumed that the energy of adsorption is constant;
implying that the surface is uniform and monolaver coverage is reached at saturation.
b) BET equation
The BET equation (12) is based on kinetics and is a modified version of the
Langmuir isotherm with multilayer, rather than monolayer, adsorption [88]. The
previously adsorbed layer serves as adsorption sites for the next layer and so on.
Generalising the Langmuir equation maintains the disadvantage of the assumption
that the surface is energetically uniform, i.e. all adsorption sites are equivalent and
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that there is no interaction between adsorbed molecules. It is also assumed that at
saturated pressure the number of layers is infinite.
The BET equation is restricted in its validity to a relative pressure ( / ) in the range
0.05-0.35 [89]. The lower limit is set so as not to include the heat of adsorption for
the first layer of molecules, which is much greater than subsequent layers. The BET
equation breaks down at relative pressures > 0.35 and over-predicts adsorption by
computing a high entropy value [90].
=
+
∙
(12)
Where = adsorption capacity (mmol/g)
= monolayer adsorption capacity (mmol/g)
= pressure (Pa)
= saturation pressure (Pa)
= BET contant
The specific surface can be calculated if the ‘knee’ (point B in Figure 5) on the
isotherm is well defined. This would be characterised by a high value of the BET
constant , represents the monolayer adsorption capacity and is calculated from
the BET equation (12), assuming one molecule of nitrogen occupies 16.2 Å2.
II. Isotherm classification
The isotherm produced from nitrogen adsorption can usually be identified as one of
five classes proposed by Brunauer, Deming, Deming and Teller (BDDT) [91], in
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addition to a stepwise isotherm (see Figure 5). Table 6 provides a summary of the
characteristics of each isotherm, degree of porosity and an example system.
The Type IV isotherm which describes the adsorption for ferric hydroxide can be
used to calculate the specific surface of the material and the pore size
distribution[91]. Type IV is characteristic of a material possessing a pore size
distribution in the range of up to hundreds of angstroms. The definition of pore size
as recommended by IUPAC is an arbitrary classification, developed using nitrogen
as an adsorbent at its normal boiling point [92]. Pores widths than 20 Å are given the
term micropores, between 20 and 500 Å are mesopores and greater than 500 Å are
macropores. Each classification has a particular significance in terms of the
adsorption isotherm (see Table 6).
Fig.5: Classification of NDDT adsorption isotherm [91]
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Table 6: Types of isotherms, characteristics and examples
Type Characteristics Example System References
I. Microporous with small internal
surface
Oxygen on activated carbon [91]
II. Reversible non-porous or
macroporous
Nitrogen on iron catalyst [89]
III. Adsorben-adsorbate interaction
is weak
Water vapour on non-
porous carbon
[92]
IV. Hysteresis loop with a plateau,
mesporous
Benzene on iron oxide gel [89]
V. Asorben-adsorbate interaction is
weak porous
Water on charcoal [91]
VI. Stepwise multi-layer, non-
porous
Argon on graphitised
carbon black
[92]
During the adsorption process, the adsorbate diffuses through the pores of the solid.
Initially this is through the smallest, or micropores. These sites have the most energy
and have greatest polarity. As the pressure increases, the adsorbate molecules diffuse
to larger and therefore less energetic pores. In monolayer coverage, all the molecules
are in contact with the surface. For multilayer adsorption, only the lowest layer is in
contact with the surface [92].
III. Pore size distribution
There are several methods for determining the pore size distribution (PSD) from
isotherm data. Some have a mechanistic basis (e. g. Barrett, Joyner and Halenda
(BJH) and Dubinin-Stoeckli (D-S)), Horvarth-Kawazoe (11-K) uses a quasi-
thermodynamic approach and the density functional theory (DFT) is generated by
statistical thermodynamics. The advantages and disadvantages of these different
methods are outlined below.
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a) Mesopore determination
PSD based on the Kelvin equation, such as the BJH method [93] assume there are no
micropores and that the material exhibits a regular pore structure [92]. The BJH
method is only suitable for mesoporous size analysis and is incompatible with the H-
K, D-s and t-plot methods [94].
b) Micropore determination
Much of the work in this area of determining micropore volume was completed by
Dubinin and his co-workers [95, 96]. Other methods include the t-plot modification
and the H-K method [97]. However there has been some doubt as to their ability to
describe micropore adsorption realistically [98].
More recently interest has developed in a model using a density functional theory
(DFT) to calculate model isotherms. From this data a pore size distribution can be
generated using a deconvolution technique. A slit-like or cylindrical shape pore
geometry is assumed [99]. This has enabled a method to describe the entire pore size
range, rather than that limited by the Kelvin equation (13). For more details
regarding the DFT approach see [100].
+
=
)
(13)
Where = saturation vapour pressure (Pa)
= critical pressure (Pa)
= surface tension of liquid condensate (N m
-1)
= molar volume of the liquid condensate (cm3 mol
-1)
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= principle pore radii of curvature in the liquid meniscus (Å)
R = universal gas constant (J, mol-1
K -1
)
T= TEMPERATURE (K)
There is a characteristic drop in the PSD using DFT model between 8-10 Å, due to
the packing effect of adsorbate molecules in the pores. Initially a monolayer is
formed in the narrowest pores, a process known as volume filling. After a critical
pore value, a second layer is required to fill the pore. This leads to an. abrupt increase
in adsorbate uptake, indicating the onset of multilayer formation [99]
This model does not take into account the heterogeneity of the pore walls, namely
geometric and energetic differences [101]. In addition, electrostatic interactions are
not considered in the DFT model, which assumes spherical adsorbate molecules and
interaction solely by van der Waals forces [98]. This leads to an underestimation of
the PSD.
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4CHAPTER 4: ACID ACTIVATED PROCESS
The majority of researches investigating acid treated red mud with a focus on the use
of hydrochloric acid for the activation process. However, this could reduce the iron
content available for subsequent adsorption applications. Consequently, this chapter
of the thesis outline the structural changes and resulting adsorption capacities of
Australian red mud untreated and treated with hydrochloric, nitric and sulphuric acid.
Particular emphasis will be placed upon determining the reactions involved during
acid activation and how the remaining mineralogical composition correlates with
fluoride removal efficiencies.
EXPERIMENTAL PROCEDURE 4.1
4.1.1 Acid activated red mud
Red mud from an Australian alumina refinery was dried over a period of 2 days at
105°C before being crushed to a fine powder using an agate ball mill (Fig. 7 and fig.
8). Using a Retsch AS200 sieving stack (Fig.9) consisting of 10 sieves, ranging from
4mm to 64µm, red mud was processed to give a size fraction < 250µm. Known
concentrations of hydrochloric (HCl), nitric (HNO3) and sulphuric (H2SO4) acid were
prepared from concentrated AR reagents (Rowe Scientific). Red mud (12.5g) was
measured into 250mL Nalgene bottles and then reacted with 200mL of DI water and
each acid. Each bottle was placed on a Ratek rotary stirrer for 1 hour at 200rpm.
These samples were subsequently centrifuged at 400rpm for 10 min using a C2041
Centurion centrifuge. The red mud was placed in the oven to dry (90°C), while the
solution was stored for analysis.
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RESULTS AND DISSCUSSION 4.2
4.2.1 Elemental and mineralogical composition
The elemental composition of this Australian red mud comprised of predominately
iron (Fe2O3), titanium (TiO2) and aluminium (Al2O3 and AlO(OH)) mineralogical
compounds. Although the elemental analysis of red mud has been reported in
numerous papers, the composition of each red mud sample differs, due to the original
composition of the bauxite ore and the operating conditions used to extract alumina.
The elemental abundance in bauxite residues generally follow Fe > Si ~ Ti > Al > Ca
> Na [102]. The red mud used in this work has a particularly high content of
aluminium, suggesting that the operating conditions were not optimised. The phase
composition of untreated red mud (Figure 6) comprised of majorly hematite (Fe2O3),
gibbsite (Al(OH)3), boehmite (γ-AlO(OH)), sodalite (Na8(Al6Si6O24)Cl2 or
Na8(Al6Si6O24)CO3), TiO2 (anatase and rutile), quartz (SiO2) and possibly cancrinite
(Na6Ca2Al6Si6O24(CO3)2). These mineralogical phases agree with the elemental
analysis results (Table 6). The broadness of the peaks in the XRD pattern align with
the following remarks from Grafe et al., [102] that approximately 70% (by weight)
of bauxite phases are crystalline, while the remaining 30% are amorphous materials.
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Table 7: Major elemental composition represented as (Al,Na,Si,Ti):Fe
Red mud treatment Al:Fe Na:Fe Si:Fe Ti:Fe
DI water 1.27 1.04 0.73 0.22
Conc. HCl 0.98 1.04 0.71 0.21
0.5M HCl 0.58 0.27 0.22 0.22
Conc. HNO3 0.76 0.27 0.41 0.19
0.5M HNO3 0.96 0.57 0.40 0.22
Conc. H2SO4 1.07 0.97 0.73 0.00
0.5M H2SO4 0.82 0.37 0.32 0.18
A comparison of the washed and acid treated red mud XRD patterns (Figure 7)
shows some phase intensity changes, and suggests that a portion of the mineralogical
phases are unstable in acidic media. In particular, the sodalite peaks at around 16.2,
28.1 and 40.2 °2θ significantly decrease in intensity indicating the dissolution of this
phase. Multiple decreases in intensities are observed between 30 and 35 °2θ, and are
associated with quartz (HNO3 and H2SO4 predominately) and cancrinite. It appears
that 0.5M acid solutions have a limited effect on the overall mineralogical structure
of iron and titanium oxide phases. This is ideal, as a high iron content in red mud has
been found to be beneficial in the removal of fluoride [103].
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Figure 6: Mineralogical composition of red mud washed with DI water
Figure 7: XRD patterns of washed and acid treated red muds (0.5 M)
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4.2.2 Leachants (ICP and reactions)
I. SEM-EDX
A comparison of the amount of Al, Na, Si and Ti to the amount of Fe in the washed
and acid treated red muds is provided in Table 1. Interestingly, a higher
concentration of acid doesn’t necessarily mean that a greater amount of a particular
element will be removed from the solid phase. In the case of HCl and H2SO4, the
0.5M acids had a greater effect on the dissolution of compounds containing
aluminium, sodium and/or silica than their respective concentrated acid counterparts.
This can be explained by the formation of additional phases between the dissolution
species and excess Cl- ions when using concentrated HCl, for example that forms
NaCl. This is confirmed by XRD (Figure 7), which showed halite peaks (~ 36.9 °2θ).
The SEM image of red mud reacted with concentrated HCl (Figure 8) also clearly
shows the formation of an addition mineralogical phase, and based on the XRD
pattern it can be assumed that halite (NaCl) has coated the exterior of the red mud
particles. The formation of this phase is not expected to hinder the removal of
fluoride as it is highly soluble, however due to its solubility it will increase the
salinity of any treated solutions.
II. ICP-MS
After the treatment of red mud with each of the 1M acids, the filtrate was analysed
using ICP-MS to determine the concentration of major ions being released into
solution (Figure 9). It was found that sodium containing compounds are the most
susceptible to acidic solutions. An increase in Na+ is also observed for the washed
red mud sample indicating that about a third of sodium released into solution is due
to the dissolution of residual NaOH or NaOH trapped in sodalite aggregates. The
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general formula for sodalite is Na8 (Al6Si6O24) CO3, which has a Na:Al mole ratio of
1.3. By subtracting the amounts of Na and Al ions released by the dissolution of
NaOH (based on red mud washed values) from the concentrations of ions in the
leachant solution of HCl, H2SO4 and HNO3 gives the following Na:Al ratios: 1.52,
1.26 and 1.27, respectively. This indicates that the majority of sodium and
aluminium ions released in solution are due to the dissolution of sodalite. A lower
amount of sodium is released into solution for HCl treated red mud due to the
formation of NaCl. The release of calcium is proposed to be due to Ca substituted
sodalite and/or cancrinite (Na6Ca2Al6Si6O24 (CO3)2). The dissolution of sodalite is
predicted to form the following products [104]:
Na8 (Al6Si6O24) CO3 + 13H2SO4 →
4Na2SO4 + 3Al2 (SO4)3 + 6Si (OH) 4 + H2O + CO2↑ (14)
Na8 (Al6Si6O24) CO3 + 24HCl →
8NaCl + 6AlCl3 + 6Si (OH) 4 + H2O + CO2↑ (15)
Na8(Al6Si6O24)CO3 + 24HNO3 →
8Na(NO3) + 6Al(NO3)3 + 6Si(OH)4 + H2O + CO2↑ (16)
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Figure 8: SEM image of red mud reacted with concentrated HC
Figure 9: ICP-MS of acid treated red mud filtrate
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4.2.3 Infrared spectroscopy
I. Red mud
Infrared spectroscopy has been used to monitor changes in bonding environments of
the numerous components of red mud with the addition of different acids. These
changes include both shifts in band position (strength of bonds) and also
decreases/increases in intensity due to the dissolution/formation of phases. Washed
red mud (Figure 10) will be used as a baseline to any changes that have occurred
with the addition of different strengths of acid. A broad band between 3650 and 3000
cm-1
is associated with multitude overlapping hydroxyl-stretching bands, in
particular metal-OH groups and water. Based on the XRD pattern of red mud it is
believed that the multiple of bands between 3650 and 3300 cm-1
are due to the ν
(OH) stretching modes of gibbsite [105-107]. Boehmite peaks are observed at
around 3235 and 3124 cm-1
[105, 106]. Bands associated with surface hydroxyl
groups of hematite appear in the range 3700, 3635, 3490, 3435 and 3380cm-1
.[108]
In many cases these bands are not observed because the surface hydroxyl groups are
removed during the drying process. The overall broadness of the band is due to
multiple water stretching modes. Corresponding water bending modes are observed
as a low intensity broad peak centred at 1655 cm-1
.
In the lower wave number region, several bands are observed between 1550 and
1350 cm-1
, predominately due to carbonate ions in different bonding environments.
The only carbonate containing minerals identified in the XRD patterns are sodalite
and cancrinite, however some form of carbonate mineral may also be present in the
amorphous content of red mud. The most intense peak in the infrared spectrum (987
cm-1
) is believed to be due to stretching vibrations of Si(Al)-O in sodalite and
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cancrinite [109]. The small band at 696 cm-1
is also thought to be associated with the
Si-O-Al framework of sodalite [110]. It is also possible that nitrate is incorporated in
these cage structures due to the presence of bands at around 1430 cm-1
[111].
II. Acid treated red mud
The dissolution of sodalite (disappearance of the intense band at 987 cm-1
and the
bands between 1400 and 1500 cm-1
) is clearly observed in Figure 10 with the
addition of hydrochloric acid. These observations coincide with the interpretation of
the elemental analysis and the proposed dissolution reactions. In the absence of
sodalite, bands associated with Si-O vibrations (possibly quartz) are observed at
1036 cm-1
, which gradually decreases and is confirmed by XRD patterns. The band
profile also indicates the formation of SiO2·xH2O (formed from dissolved silica –
Si(OH)4), which has characteristic bands at 800 (w), 948 (w), 1090 (vs) 1190 (s,sh)
and 3330 cm-1
(m) [112] . This dissolution product appears to be relatively stable.
Minimal changes in the higher wavenumber region indicates that the major iron and
aluminium oxide/hydroxide components of red mud remain relatively unscathed until
the concentration of acid reached 1M (significant decrease in intensity suggesting the
initial stages of dissolution).
The infrared spectra of red mud treated with nitric acid (Figure 11) and sulphuric
acid (Figure 9) show many similar bands as those described for hydrochloric acid
treated red mud. Nitric acid treated red mud showed additional bands at 1406 and
1352cm-1
due to Al hydroxylated nitrate and ν3 NO3- co-adsorbed with H2O on the
red mud particles, respectively [111].Sulphuric acid treated red mud also showed an
additional band (broad shoulder on the 1074 cm-1
) ascribed to sulphate vibration
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modes found in the dissolution product thenardite (Na2SO4). The broadness of the
higher wave number region is proposed to be water adsorbed to thenardite.
Figure 10: Infrared spectrum of washed red mud
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.
Figure 11: Infrared spectrum of red mud treated with various concentrations of
HCl
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Figure 12: Infrared spectrum of red mud treated with various concentrations of
HNO3
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Figure 13: Infrared spectrum of red mud treated with various concentrations of
H2SO4
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4.2.4 Removal of fluoride using acid treated red mud
The effect of acid activated red mud and pH on the removal of fluoride from aqueous
solutions was investigated and found that red mud treated with sulphuric acid has the
greatest efficiency in fluoride removal (approximately 70% removed from an initial
concentration of 100ppm), independent of the initial solution pH (Figures 14 and 15).
It is clear from the results that the best removal percentages are achieved at low pH <
4.5. The study by Cengeloglu et al.,[103] reported similar removal percentages using
red mud, however maximum adsorption was reported to occur at a pH around 5.5.
Deviations in results between the studies are believed to be due to the different
activation processes used.
The removal mechanism of fluoride using red mud primarily involves neutral
(≡SOH) and protonated (≡SOH2+) sites on the oxide/hydroxide components (such as
hematite and gibbsite) when the pH is less than 7[113]. The increased removal
efficiencies of acid treated red mud compared to wash red mud are consistent with
the protonation of the surface hydroxyl groups (17). Increased removal efficiencies
for sulphuric acid treated red mud are due to 2 protons being available to protonate
the surface hydroxyl groups.
≡SOH + H+ → ≡SOH2
+ (17)
The benefit of the protonated sites in acid treated red mud is that the replacement of a
proton (H+) with fluoride (F
-) releases water (18), while the substitution of a proton
with a neutral site (19) releases a hydroxyl unit and in turn causes the pH to rise. This
is observed for the DI washed red mud sample (neutral pH and thus has ≡SOH sites)
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that showed promising removal efficiencies at low mass to volume ratios, however
with the continued release of OH- units, as F
- ions are adsorbed, the pH became
alkaline and resulted in the deprotonation of the surface sites (≡SO-), 20. This can be
further confirmed by comparing the removal efficiencies of washed red mud;
whereby the fluoride solution with an initial pH of 4.75 showed reasonable fluoride
adsorption (approximately 35%) followed by a sharp decrease as the pH rose above 6
(Figure 13), while a maximum of 10% fluoride removal is observed for the fluoride
solution with an initial pH of 8 (Figure 14). Fluoride adsorption is hindered when the
pH is greater than 6 because of the increasing repulsive forces between the
negatively charged surface (≡SO-) and fluoride ions.
≡ SOH2+ + F
- → ≡SF + H2O (18)
≡SOH + F- → ≡SF + OH
- (19)
≡SOH + OH- → ≡SO
- + H2O (20)
The efficiency of fluoride adsorption is highly dependent on the pH and any
fluctuations in pH. This is clearly observed in Figure 14 for the adsorption of fluoride
using sulphuric acidified red mud. This relationship shows that for consistent
fluoride adsorption, a constant pH needs to be maintained to avoid any sudden
shocks to the surface adsorption sites. It is also highly possible that the formation of
AlFx complexes, in particular aluminium trifluoride (AlF3), occurs when the pH of
solution is less than 4. The formation of AlF3 would account for some of fluctuations
in fluoride removal percentages. The study by Cengeloglu et al.[103] reported a
decline in fluoride removal at pH below 4 and did not report the formation of any of
these AlFx phases. This could be accounted for the difference in the quantity of
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gibbsite of the different red mud sources used, whereby this study had a greater
amount of Al2O3 (24.0%) than Cengeloglu (18.7%) [103]. The formation of this
phase is a result of gibbsite in red mud reacting with HF that forms under these
highly acidic conditions (21).
Al2O3 + 6HF → 2AlF3 + 3H2O (21)
Figure 14: Removal of fluoride (%) using different masses of acid treated red mud
and the associated changes in pH from the original 100ppm fluoride solution with pH
4.75
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Figure 15: Removal of fluoride (%) using different masses of acid treated red mud
and the associated changes in pH from the original 100ppm fluoride solution with pH
7.99
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5CHAPTER 5: THERMALLY ACTIVATED RED MUD
EXPERIMENTAL PROCEDURES 5.1
5.1.1 Thermally activated red mud
Red mud from an Australian alumina refinery was dried over a period of 2 days at
105°C before being crushed to a fine powder using an agate ball mill. Using a Retsch
sieving stack consisting of 10 sieves, ranging from 4mm to 64µm, red mud was
processed to give a size fraction < 250µm. A known amount of red mud was then
placed in a SEM furnace that had been previously heated to the desired temperature
(100, 200, 300, 400, 500, 600, 700, 800, 900, 1000°C). The sample of red mud was
heated for 2 hours. After heating, it was allowed to cool to room temperature before
being re-weighted so that the mass loss could be calculated. The red mud was then
placed in a desiccator until it was used for adsorption studies.
RESULTS AND DISSCUSSION 5.2
5.2.1 Red mud characterisation
The mineralogical composition of red mud and the corresponding reference patterns
are provided in Figure 16. The identified mineral phases in the sample are hematite
(Fe2O3), gibbsite (γ-Al(OH)3), boehmite (γ-AlO(OH)), sodalite (Na8(Al6Si6O24)Cl2),
anatase (TiO2), rutile (TiO2) and quartz (SiO4). The chemical composition of red
mud is presented in Table 1, with the main component of red mud being iron oxide
(27.77%). The combined oxide content of Fe2O3, Al2O3 and SiO2 in the red mud
sample makes up about 70%. Although the elemental analysis of red mud has been
reported in numerous papers, the composition of each red mud sample differs, due to
the original composition of the bauxite ore and the operating conditions used to
extract alumina. The elemental abundance in bauxite residues generally follow Fe >
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Si ~ Ti > Al > Ca > Na[114]. This particular red mud had a higher than usual
abundance of aluminium, suggesting that operating conditions were not optimal at
the time.
Figure 16: X-ray diffraction pattern of red mud and corresponding reference patterns.
Infrared spectroscopy has been used to monitor changes in bonding environments of
red mud after thermal activation. The infrared spectrum of red mud is provided in
Figure 17. The untreated red mud presents a broad band between 3650 and 3000 cm-
1, a number of overlapping medium strength bands between 1650 and 1300 cm
-1, a
strong relatively broad band centred at 968 cm-1
, and several weak bands ranging
from 800 to 400 cm-1
. The broad band centred at around 3300 cm-1
is assigned to a
multitude of overlapping hydroxyl-stretching bands, in particular metal-OH groups
and water. Based on the XRD pattern of red mud, bands associated with aluminium
species gibbsite, boehmite and hematite are expected. The multiple bands observed
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between 3650 and 3300 cm-1
are due to the ν(OH) stretching modes of gibbsite [105-
107] while the band at 3277 cm-1
is assigned to ν(OH) stretching modes of boehmite
[105, 106]. Bands associated with surface hydroxyl groups of hematite have been
reported in literature at around 3700, 3635, 3490, 3435 and 3380cm-1
[108]. In many
cases these bands are not observed because the surface hydroxyl groups are removed
during the drying process, and this appears to be the case in this investigation. The
broadness of the band indicates that the samples contain water, which is further
confirmed by water bending modes being observed as a low intensity band at around
1630 cm-1
.
Table 8: Oxide composition of red mud
Oxide RM (%)
Fe2O3 27.77
Al2O3 23.44
SiO2 19.00
Na2O 5.71
CaO 3.72
TiO2 3.05
MgO 0.24
MnO 0.09
Amorphous / other 17.01
The main band observed in the infrared spectrum at 968 cm-1
is assigned to Si(Al)-O
stretching vibrations in sodalite [109]. Weak bands at 709, 670 and 646 cm-1
are
assigned to vibrations associated with the Si-O-Al framework of sodalite [110]. A
number of bands between 1500 and 1300 cm-1
(1458, 1436, 1402 and 1318 cm-1
) are
associated with C-O, C-N and C-C vibration modes. Based on XRD, the only
carbonate or nitrate containing mineral identified is sodalite, however some form of
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carbonate mineral may also be present, such as calcium carbonate. The band at 1436
cm-1
indicates that nitrate is incorporated in sodalite [111].
Figure 17: Infrared spectrum of red mud
The differential thermalgravimetric curve for red mud shows a number of
decomposition steps (Figure 3).The initial mass losses (56 and 82°C) are due to
physio-sorbed water and account for a total mass loss of 3.30% of a total mass loss of
11.65%. Up to 600˚C, the observed weight loss is proposed to be related to the
dehydroxylation of AlO(OH) phases. The largest mass loss of 4.70% is due to the
dehydroxylation of gibbsite (22) and boehmite (23) [115, 116]. At higher
temperatures, up to 700˚C, the observed weight loss could be related to the
decomposition of sodalite group phases attributed to the evolution of CO2 and
formation of mixed oxide phases [117]. It has been reported the mass loss in this
temperature range is due to the decomposition of calcium carbonate [116, 118],
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however this mineralogical phase has not been identified in the XRD patterns (Figure
19) but is possible as XRF found 3.72% CaO exists in the samples (Table 8). The
XRD pattern also identified the formation of nepheline (NaAlSiO4) in red mud
activated above 800°C, which corresponds to a mass loss of 0.93% observed in the
DTG curve (Figure 18). Nepheline forms as a result of the decomposition of the
sodalite cage structure (24) [119].
Dehydroxylation of gibbsite
Al(OH)3(s) → AlO(OH)(s) + H2O(g) (22)
Dehydroxylation of boehmite
AlO(OH)(s) → Al2O3(s) + H2O(g) (23)
Formation of magnetite
Na8Al6Si6O24Cl2(s) → 6NaAlSiO4(s) + 2NaCl(g) (24)
Figure 18: DTG curve of red mud
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5.2.2 Thermally analysis
The mineralogical composition of red mud thermally activated up to 1000°C is
provided in Figure 19. The patterns are dominated by hematite (Fe2O3) peaks for all
temperatures investigated, with only a very weak magnetite (Fe3O4) peak observed at
1000°C. The major changes in mineralogy with increased temperatures is the
disappearance of sodalite and boehmite peaks and the appearance of nepheline,
potassium aluminium oxide and minor amounts of other mixed metal oxides.
Boehmite peaks are no longer visible at temperatures greater than 500°C, while
sodalite peaks are no longer observed at 1000°C. Upon the disappearance of sodalite
peaks the identification of nepheline is observed, indicating that sodalite has
transformed to nepheline as described in Equation 24 for the DTG curves.
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Figure 19: X-ray diffraction patterns of thermally activated red mud
The physical properties of red mud and red mud thermally activated at 500 and
1000°C are provided in Table 9. The mean particle size of the red mud particles
increase as the temperature increases from 26.8µm for washed red mud up to 34.2µm
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for thermally activated red mud at 1000°C. Apart from the disappearance and
appearance of mineralogical phases in the XRD pattern, the sharpness of the most
intense bands (predominately hematite) increases as the temperature increases. Thus,
thermal activation improves the crystallinity of these phases. A similar observation
has been reported by Wu and Liu.[116] The surface area of red mud increases to
around 500°C (increase of around 30m2/g), however reduces to almost the same
surface as originally observed for the washed red mud sample. The increase in
surface area correlates with the decomposition (Figure 18) of the aluminium
hydroxide phases gibbsite and boehmite. It is proposed that the increase in surface
area is due to an increase in small pores forming as free water is removed during the
decomposition of gibbsite and boehmite. The reduction in surface area for red mud
activated at 1000°C is believed to be related to an increase in the pore diameter[118]
and/or the change in morphology, whereby an increase in the average size of
particles is observed due to agglomeration formation[116]. The report by Wu and
Liu,[116] presents a graph that shows the increase in average particle size up to
1350°C. This data shows a gradual increase in particle size up to 600°C, with
minimal size changes observed between 600 and 750°C, and a more rapid increase in
particle size up to 1200°C (38-39 µm).
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Table 9: Physical properties of red mud, red mud heated to 500 and 1000°C
pH BET (m2/g) Mean diameter of particles (µm)
Red mud 10.19 14.51 25.8
RM 500°C 10.21 44.01 31.6
RM 1000°C 9.30 17.36 34.2
Infrared spectra of red mud thermally activated at 500 and 1000°C clearly show the
decomposition of aluminium hydroxide species such as gibbsite and boehmite
(Figure 20). The multiple bands observed between 3650 and 3000cm-1
and at around
1600cm-1
decrease significantly for RM 500°C and are no longer visible at 1000°C.
The decomposition of these phases has been confirmed by XRD (Figure 19). The
intense Si(Al)-O stretching vibrational band centred at 969cm-1
is present in all
spectra, however the nitrate bands (1400-1475cm-1
) decrease in intensity for RM
500°C and are absent for RM 1000°C. These observations provide additional
evidence of the transformation of sodalite (Na8Al6Si6O24Cl2) to nepheline (NaAlSiO)
because both compounds will show intense Si(Al)-O stretching vibrations. The
disappearance of nitrate species indicates that the sodalite structure has nitrate in its
cages. After red mud had been heated to 1000°C more detailed information at the
lower wavenumber region could be deduced. These bands are believed to be
associated with the Fe-O and Fe-O-Fe bending modes of hematite. Bands at around
400 and 650cm-1
are characteristic of the A2u out-of-plane modes, while the in-plane
modes of hematite are located at around 525 and 400cm-1
.[120] The 414cm-1
band is
observed for all red mud samples, but at increased temperatures the other hematite
bands become visible; 678, 620 and 539cm-1
.
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Figure 20: Infrared spectra of thermally activated red mud
5.2.3 Removal of fluoride using thermally activated red mud
The effect of thermal activation is shown in Figures 21 and 22, whereby washed red
mud and red mud activated to 1000°C performed better than red mud activated at
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500°C. Figure 21 also shows the different pH regions and relative fluoride adsorption
potentials of the different red muds. At higher solution pH, red mud activated to
1000°C had the greatest fluoride adsorption but decreases as the pH increases. This is
also observed for the other red mud samples. RM1000°C shows a greater resistance
to pH increases, which has also been shown in Table 9 to only have a residual pH of
9.30 compared to 10.20. It is also proposed that RM1000°C performed better than
RM500°C due to an increase in the ratio of hematite to other mineralogical phases
(Figure 19 shows hematite as the primary phase), which increased the probability of
fluoride adsorbing to its surface.
Figure 21: Dependence of fluoride adsorption (mmol/g) with pH using a 100mg/L
fluoride solution with an initial pH of 4.
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Figure 22: Dependence of fluoride adsorption (mmol/g) with pH using a 100mg/L
fluoride solution with an initial pH of 8.
The main removal mechanism for fluoride is through its adsorption onto the surface
of iron oxide/hydroxides, and as such it is pH dependent. The best adsorption occurs
in the lower pH region. This pH dependency is observed in Figures 23 and 24. For a
fluoride solution of 100mg/L with an initial solution pH of 4, a maximum fluoride
adsorption of 3.73 mmol/g for RM1000°C is obtained, however this rapidly declines
as the pH increases as the solution becomes more alkaline. Above pH 7, the amount
of fluoride removed remains between 0.07 and 0.05 mmol/g. The fluoride solution
with an initial pH of 8 shows minimal fluoride removal, however this is not
unexpected due to the strong competition that hydroxide ions have for the iron
surface. A maximum adsorption of 0.07 mmol/g is observed (Figure 24). The study
by Çengeloğlu et al, [30] on the removal of fluoride using red mud showed a
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maximum removal at a pH of around 5.5. Deviations in results between the studies
are believed to be due to the different activation processes used.
Figure 23: Fluoride adsorption (mmol/g) using red mud thermally activated to
1000°C for a 100mg/L fluoride solution with an initial pH of 4.
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Figure 24: Fluoride adsorption (mmol/g) using red mud thermally activated to
1000°C for a 100mg/L fluoride solution with an initial pH of 8.
The removal mechanism of fluoride using red mud primarily involves neutral
(≡FeOH) and protonated (≡FeOH2+) sites on the oxide/hydroxide components (such
as hematite and gibbsite) when the pH is less than 7 [113]. The increases in
adsorption at lower pH are due to the protonation of the iron surface sites The
increases in adsorption at lower pH are due to the protonation of the iron surface sites
(25).
≡FeOH + H+ → ≡FeOH2
+ (25)
Hematite neutral sites only show promising removal efficiencies at low mass to
volume ratios. However, at higher mass to volume ratios the continual release of OH-
units as F- ions are adsorbed causes the pH to become more and more alkaline. This
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© 2013 Wentao Liang 82
results in the fluoride adsorption process to be hindered due to increasing repulsive
forces between the negatively charged surface (≡FeO-) and fluoride ions (26).
≡FeOH + OH- → ≡FeO
- + H2O (26)
The relationship between pH and fluoride adsorption shows that for consistent
fluoride adsorption, a low, constant pH needs to be maintained to avoid any sudden
shocks to the surface adsorption sites. At pH levels less than 3, it is also highly
possible that AlFx complexes form, in particular aluminium trifluoride (AlF3). The
formation of AlF3 would account for some of the fluctuations observed in the
fluoride loading capacities. The formation of this phase is a result of gibbsite in red
mud reacting with HF that forms under these highly acidic conditions conditions
(27).
Al2O3 + 6HF → 2AlF3 + 3H2O (27)
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6CHAPTER 6: OVERALL CONCLUSIONS
Red mud is comprised of a number of mineralogical phases; however the most
important phases in the removal process are hematite and gibbsite. The treatment of
red mud with acid improved the adsorption properties of red mud in 2 ways: 1)
transformed ≡SOH / ≡SO- sites to ≡SOH2
+ and 2) increased the availability of metal
oxide/hydroxide sites through the removal of sodalite and cancrinite phases. In order
to achieve reasonable removal efficiencies for fluoride a pH < 4.5 needs to be
maintained, with sulphuric acid producing the best removal efficiencies. Red mud
treated with sulphuric acid gave the best removal efficiencies for fluoride due to 2
protons being available to protonate the surface hydroxyl groups. Sudden changes in
pH have shown to have negative effects on the removal efficiencies and thus need to
be controlled.
In the case of thermally activated red mud, the effect thermal activation has shown
washed red mud and red mud activated at 1000°C performed better than red mud
activated at 500°C. Meanwhile, at higher solution pH, red mud activated to 1000°C
had the greatest fluoride adsorption but decreases as the pH increases as observed for
the other red mud samples. RM1000°C shows a greater resistance to pH increases
once introduced to solution, but only have a residual pH of 9.30 compared to 10.20.
Hence, according the data analyse, keep in 1000°C and pH value around 4 can
achieve the greatest fluoride adsorption.
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7CHAPTER 7 FUTURE WORK
This study has shown that red mud as an adsorption material can be used to
successfully remove fluoride. Due to red mud is a by-product of Bayer Process,
brings high risks for environment, the research could be continually focused on
balancing thermally activated and acid treatment, trying to arrange a best point for
fluoride adsorption.
In this case, thermally activated should be treated as a single factor to test the
minerals changes of red mud, 100 °C to 1000°C temperatures are reasonable.
Meanwhile, the orders of treatment might have different results. Hence, acid
treatment is following thermally treated is another factor should be considered.
All the research should be expected that the red mud resources differences, as it
mentioned above, which will bring the different research story.
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