KTH Materials Science
and Engineering
Properties of Ugandan minerals and fireclay
refractories
John Baptist Kirabira
Doctoral thesis
Stockholm, Sweden 2005
Akademisk avhandling som med tillstånd av Kungl Tekniska Högskolan
framlägges till offentig granskning för avläggande av teknologie
doktorsexamen fredagen den 3 juni 2005 kl 10.00 vid Institutionen för
Materialvetenskap,
Kungl
Tekniska
Högskolan,
föreläsningssal
B2,
Brinellvägen 23, Stockholm.
ISRN KTH/MSE ISRN KTH/MSE--05/46--SE+MEK/AVH
ISBN 91-7178-083-1
© John Baptist Kirabira, June 2005
ii
Abstract
Development of products which can be produced from a country’s natural
resources is very important as far as the industrialization of a nation and
saving foreign exchange is concerned. Presently, industries in Uganda and
the other states in the Lake Victoria region import all refractory-relatedconsumables, as the demand cannot be met locally.
Based on the
abundance of ceramic raw materials for high temperature applications in the
region and the demand for refractories by industries it is pertinent to develop
and manufacture firebricks by exploiting the locally available raw materials.
This thesis thus, concerns the characterisation of ceramic raw mineral
powders from the Lake Victoria region, more particularly, Uganda, with the
aim of developing firebrick refractories from the minerals. Two main deposits
of kaolin and a ball clay deposit were investigated to assess their potential in
the manufacture of refractory bricks. Raw- and processed sample powders
were investigated by means of X-ray diffraction (XRD), thermal analysis
(DTA-TG) and Scanning Electron Microscopy (SEM). In addition, the chemical
composition, particle size distribution, density, and surface area of the
powders were determined.
A comprehensive study on beneficiation of
using mechanical segregation of particles.
explore other potential applications like in
beneficiation process improves the chemical
pure, the major impurity being iron oxide.
Mutaka kaolin was carried out
The aim of the study was to
paper filling and coating. The
composition of kaolin to almost
A general production process scheme for manufacturing fireclay bricks
starting with raw powder minerals (Mutaka kaolin and Mukono ball clay) was
used to make six groups of sample fireclay brick. Experimental results from
the characterization of formulated sample bricks indeed revealed the viability
of manufacturing fireclay bricks from the raw minerals. Based on these
results, industrial samples were formulated and manufactured at Höganäs
Bjuf AB, Sweden. Kaolin from the Mutaka deposit was used as the main
source of alumina while ball clay from Mukono was the main plasticizer and
binder material. The formulated green body was consolidated by wet
pressing and fired at 1350°C in a tunnel kiln. Characterization of the sintered
articles was done by X-ray diffraction, scanning electron microscopy, and
chemical composition (ICP-AES). In addition, technological properties related
to thermal conductivity, thermal shock, alkali resistance, water absorption,
porosity, shrinkage, permanent linear change (PLC), linear thermal
expansion, refractoriness under load (RUL), and cold crushing strength were
iii
determined. The properties of the articles manufactured from the selected
naturally occurring raw minerals reveal that the produced articles compare
favourably with those of parallel types. Thus, the raw materials can be
exploited for industrial production.
Keywords: kaolin; clay; Ugandan minerals; fireclay; refractory; powders
characterization; beneficiation; ceramics; mullite.
iv
To my family;
for their love and pride in me
v
List of Publications
Paper I:
John Baptist Kirabira, Stefan Jonsson, and Joseph Kadoma
Byaruhanga, State of the Art Paper on development and
manufacture of firebrick refractories from locally available
alumina-rich clays in Uganda. Published in KTH report series as:
ISRN KTH/MSE—03/71—SE+MEK/TR.
ISRN KTH/MSE--05/47--SE+MEK/ART
Paper II:
John Baptist
Byaruhanga,
Ceramic raw
Industriels, in
Kirabira, Stefan Jonsson, and Joseph Kadoma
Powder Characterization of High Temperature
materials in the Lake Victoria Region. Silicates
press.
ISRN KTH/MSE--05/48--SE+MEK/ART
Paper III:
John Baptist Kirabira, Stefan Jonsson, and Joseph Kadoma
Byaruhanga, Production of firebrick refractories from kaolinitic
clays of the Lake Victoria region. Published in J. Australasian
Ceram. Soc. 40, (2004) 12—19.
ISRN KTH/MSE--05/49--SE+MEK/ART
Paper IV:
John Baptist Kirabira, Stefan Jonsson, and Joseph Kadoma
Byaruhanga, Laboratory beneficiation and evaluation of Mutaka
kaolin from the Lake Victoria Region, Uganda. Submitted for
publication to Appl. Clay Sci.
ISRN KTH/MSE--05/50--SE+MEK/ART
Paper V:
John Baptist Kirabira, Gunnar Wijk, Stefan Jonsson, Joseph
Kadoma Byaruhanga, Fireclay refractories from Ugandan
kaolinitic Minerals. To be submitted for publication.
ISRN KTH/MSE--05/51--SE+MEK/ART
vi
Contributions of the author
I. Literature search, review and writing.
II. Powder sample collection, performing experiments, characterization of
sample powders, evaluation of the results, and writing manuscript.
III. Planning experiments, performing experiments, characterization of sample
powders, evaluation of the results, and writing manuscript.
IV. Planning and design of experiments, characterization, evaluation of results,
and writing the manuscript. Hydrocycloning carried out at Southern and
Eastern Africa Mineral Centre (SEAMIC), Dar es Salaam, Tanzania.
V. Planning experiments, characterization by XRD, LOM and SEM, analysis of
results and writing manuscript. Production and testing of industrial sample
bricks was carried out at Bjuf Höganäs AB, Sweden.
vii
Contents
Abstract ......................................................................................................iii
List of Publications ........................................................................................vi
Contributions of the author............................................................................ vii
1. Background .............................................................................................. 1
1.1 Refractory raw materials........................................................................ 2
1.2 Manufacture of refractories .................................................................... 2
1.3 Materials and manufacture of fireclay refractories...................................... 3
1.4 Application of refractories ...................................................................... 4
1.5 Presentation of the thesis....................................................................... 5
2. Experimental techniques ............................................................................ 7
3. Summary of results and discussion .............................................................13
3.1 Characterization and processing of powders.............................................13
3.2 Beneficiation of Mutaka kaolin ...............................................................21
3.3 Manufacture and characterization of fireclay refractories............................25
3.4 Manufacture of industrial samples and characterization .............................32
4. Conclusions .............................................................................................37
4.1 Suggestions for future work .....................................................................38
Acknowledgments ........................................................................................39
References..................................................................................................40
Appended papers .........................................................................................42
viii
1. Background
Refractories are the backbone of industry because they are essential for all
thermal and chemical processing worldwide. Uganda, located in the Lake
Victoria region has an area of about 240,000 km2 and a population of 25
million people. Uganda and her neighbors in the Lake Victoria region are
known to be endowed with natural resources and offer a wide range of
investment opportunities in mining, fishing and agriculture. The present work
is concerned with ceramic refractory raw minerals and a survey by the
Department if Geological Survey and Mines, Entebbe, Uganda shows that
the following ceramic refractory minerals exist in abudance in various parts
of the country: silica raw materials, fireclay, alumina raw materials (kyanite,
andalusite, sillimanite, and corundum), magnesia raw materials (magnesite),
and forsterite raw materials (talc, pyrophyllite, serpentine asbestos). Other
typical refractory minerals may be available but of poor grade, or in small,
uneconomic deposits, or may as yet be undiscovered. Most of these raw
materials have not been exploited for their industrial applications. The ball
clays in particular have been restricted to manufacture of bricks, roofing
tiles, and pottery products. On small scale application, kaolin is used in
manufacture of chalk, insulation material in institutional and domestic
stoves, as a filler material in paint making and also in pottery ware.
According to MacDonald, 1966, the Ugandan kaolins are associated with
tertiary laterization over extensive area of precambrian terrain. Deposits
suitable for industrial use occur in a number of localities but the main ones
are Mutaka, Namasera, Migadde, Kisai, Kilembe and Buwambo. On the other
hand, clay deposits are widely distributed in swamps and valleys throughout
Uganda. Some studies concerning mineralogy, chemical and phase
transitions have recently been carried out on kaolins and clays in central
Uganda (Nyakairu et al, 1998; Nyakairu, et al, 2001(a); Nyakairu et al,
2001(b)). Although these studies show that Uganda is rich in mineral
resources traditionally used in the refractory industry, little has been done to
use them as precursors in development of quality refractory products. The
present study, therefore, is aimed at the development of high-quality
refractories based on domestic alumino-silicate raw materials.
In this work, raw materials were collected from three deposits for
investigation: Kaolin from Mutaka and Mutundwe, and ball clay from Mukono.
The powders were characterized for their chemical and physical properties.
In addition, processing of these minerals with the goal of upgrading their
quality was done. Emphasis was put on a detailed beneficiation of Mutaka
1
kaolin. Results of characterization reveal that the raw kaolins are of good
quality and compare well with those marketed in Europe. With a good Al2O3
content, and its abundance from the deposit, Mutaka kaolin was found to be
a good precursor for manufacture of fireclay refractories. From Mutaka kaolin
and Mukono ball clay powders, fireclay refractory bricks were formulated,
and manufactured. The refractory properties of the manufactured bricks
were characterized following international standard procedures and found to
compare favorably with those of parallel types.
1.1 Refractory raw materials
A refractory is a material that retains its shape and chemical identity when
subjected to high temperatures and is used in applications that require
extreme resistance to heat. Almost all raw materials used by refractory
manufacturers occur naturally. But they are prepared or processed in several
ways to lower the fluxes, unwanted oxides as much as possible. In general,
refractory materials are based on six main oxides: SiO2 (Silica), Al2O3
(Alumina), MgO (Magnesia), CaO (Calcia), Cr2O3, and ZrO2 (Zirconia). Of
recent date there are also refractories based on Carbon and a combination of
carbon with other elements e.g. B4C, and SiC.
1.2 Manufacture of refractories
There are four basic forms in which refractories are manufactured: shaped
products (bricks), unshaped products (monolithics), functional products and
heat insulating products. The bricks are used to form the wall, arches, and
floors of various high-temperature equipment while the unformed
compositions which include mortars, gunning mixes, castable (refractory
concretes), and ramming mixes are cured in place to form. Functional
products are used mainly as tap and gas purging systems in steel
manufacturing while heat-insulating are products for the refractory lining of
thermal plants. The present work concentrates on brick products and the
international standard brick shapes are 230x114x(64 or 76) mm and
250x124x(64 or 76) mm.
Refractory manufacturing like any other conventional ceramic product goes
through several stages. The major technical goals of manufacturing a given
refractory are embodied in its properties, performance of the component
intended application as well as shape and size requirements. The main
aspects of manufacturing consist of choices among raw materials, processing
2
methods and design parameters. The major insights of manufacture have to
do with the features of phase composition and microstructure, technically
known as material character. These are developed through processing and
are themselves responsible for product properties and behaviour.
The fabrication process for refractories also depends on the particular
combination of chemical compounds and minerals used to produce specified
levels of thermal stability, corrosion resistance, and thermal expansion,
among other property requirement. Refractory fabrication of shaped products
involves five major processes: raw material preparing/processing,
forming/shaping, drying, firing and sorting and packaging. Raw material
processing involves crushing or grinding raw materials, classifying and/or
grading by size, calcining, and drying. The materials are then mixed with
other materials and formed into shapes (for shaped refractories) under moist
or wet conditions. Bricks are formed by mixing raw materials with water
and/or other binders and pressing the mixture into a desired shape. After
forming and drying, refractories are fired. Firing sometimes referred to as
sintering or thermal treatment, involves heating the dry-formed material to
high temperatures in order to achieve a ceramic bond. Firing of refractory
materials results into a densified thermally stable structure, and bond
development through partial vitrification, sintering and/or crystallization. This
final process of firing gives raw materials their refractory properties and
hence the properties of the final product.
1.3 Materials and manufacture of fireclay refractories
Fireclay refractories, also known as chamotte bricks, belong to the aluminosilicate group with alumina content between 25-45 wt%. The others in this
group are the semi-acid (≤25 wt% Al2O3) and high alumina (>45 wt%
Al2O3), (Budnikov, 1964; Didier, 1982; Routschka, 2004). The precursor raw
material for making fireclay refractories is kaolin. Kaolin is the main source
of alumina in the manufacture fireclay refractories among other industrial
minerals. The higher the alumina content of a kaolin, the higher the
refractoriness. Raw materials are thus classified to be of high refractory
value as the amount of alumina in them increases. Hence, the obvious
application of the high-temperature portion of the Al2O3-SiO2 phase diagram
is in the refractory industry where the steady climb in the liquidus
temperature is seen to depend on the alumina content.
Ball clays are generally used in varying proportions to particular ceramic
bodies during manufacture. Ball clays are the binder material in ceramics and
3
more specifically in fireclay refractories manufacture. The clay being highly
plastic, facilitates the forming process and contibutes to the dry-strength, or
green strength, of the product. In addition, it enhances the sintering process
by providing a glassy-phase which bonds the aggregates together.
Fireclay refractories, like other ceramic products are processed through three
main stages: raw materials preparation, consolidation to compacts and
densification by sintering. The main constituents of fireclay refractories are
alumina (Al2O3) and silica (SiO2). These systems are normally based on
kaolinitic clays which generally present substantial shrinkage when fired. In
consideration of shrinkage and cracking of the product, raw materials are
fired, crushed and size graded into stable grog (calcined fireclay) and mixed
with ground clay slip. The grog promotes drying and limits dry and firing
shrinkage whereas the clay promotes sintering and bonding during firing.
The materials used for making grog are generally more refractory than the
bonding material. The grog is crushed into different granulometry fractions;
course (1 to 3mm), middle (0.25 to 1mm) and fine (≤0.25mm). The crushed
grog is mixed in different batches and then bonded with a slip normally made
of a clay/kaolin mixture (0.125mm). The grog/slip mixture is then shaped by
press-forming. The formed shapes are dried and fired i.e. sintered at
temperatures between 1200 and 1500ºC for 6 to 24 hours. The higher the
alumina content, the higher the firing temperature. On sintering, the main
mineral phases for fireclay refractories are mullite (3Al2O3⋅2SiO2),
cristoballite (SiO2), and a glassy phase.
1.4 Application of refractories
Refractories are by far the major consumables to numerous industries
wherever heat is used and abrasion and acid resistance is required. The
principle user markets for refractory products include the following
industries: iron and steel, copper, gold, platinum, ferro-alloys, aluminium,
cement, lime, glass, ceramics, foundries, and gas plants, chemical plants,
petroleum plants, incinerators, sugar refineries and power stations. The iron
and steel industry is considered to be the largest consumer, estimated
between 50-80% of the refractories produced worldwide. Refractories are
normally used to serve the following purposes (Routschka, 2004):
•
Lining of plants for thermal processes (melting, firing, and heat
treatment furnaces and transport vessels).
4
•
Heat insulation
•
Heat recovery (regenerators and recuperators)
•
Construction of design components (functional products)
Fireclay refractories are generally characterized by low thermal expansion,
coefficient, low thermal conductivity, low specific gravity, low specific heat,
low strength at high temperature, and less slag penetration. Fireclay
refractories are generally applied in ladles, runners, sleeves, rotary cement
kilns, non-ferrous metal furnaces, glass tanks, waste incinerators, hit blast
stoves, low temperature zones in a blast furnace, coke ovens, annealing
furnaces, blast furnace hot stoves, reheating furnaces, soaking pits, etc.
The first paper thoroughly covers details concerned with manufacturing and
application of fireclay refractories, only a short discussion has been covered
in this introductory section.
1.5 Presentation of the thesis
The present thesis deals with the minerals, naturally occurring in the Lake
Victoria region in Uganda, specially suited for manufacturing of refractory
fireclay refractories. It includes the following papers:
1. “State of the Art Paper on development and manufacture of firebrick
refractories from locally available alumina-rich clays in Uganda”, John
Baptist Kirabira*, Stefan Jonsson**, Joseph Kadoma Byaruhanga*
2. “Powder Characterization of High Temperature Ceramic raw materials
in the Lake Victoria Region” , John Baptist Kirabira*, Stefan Jonsson**,
Joseph Kadoma Byaruhanga*
3. “Production of firebrick refractories from kaolinitic clays of the Lake
Victoria region”, John Baptist Kirabira*, Stefan Jonsson**, Joseph
Kadoma Byaruhanga*
4. “Laboratory beneficiation and evaluation of Mutaka kaolin from the
Lake Victoria Region, Uganda”. John Baptist Kirabira*, Stefan
Jonsson**, Joseph Kadoma Byaruhanga*
5. “Fireclay refractories from Ugandan Minerals”. John Baptist Kirabira*,
Gunnar Wijk***, Stefan Jonsson**, Joseph K. Byaruhanga*
5
*Department of Mechanical Engineering, Faculty of Technology, Makerere University, P.O.
Box 7062, Kampala, Uganda, email: [email protected]
**Department of Materials Science and Engineering, Royal Institute of Technology (KTH),
Brinellvägen 23, SE-100 44 Stockholm, Sweden, email: [email protected]
***Höganäs Bjuf AB, Box 502, SE-267 25 Bjuv, Sweden, email:
[email protected].
The first paper is a “state of the art” paper and covers the classification of
ceramics, definition of refractories and their classification, their applications
and their characteristics, the major refractories, the raw materials, the major
mineral sources and how they are formed and transported in nature, the
characterisation methods for raw materials, the high-temperature reactions
during sintering including discussions on the Al2O3-SiO2 phase diagram,
manufacturing of refractories, and finally, the benefits of exploiting Ugandan
ceramic deposits.
The second paper covers the characterisation of minerals from two kaolin
deposits (Mutaka and Mutundwe) and a ball-clay deposit (Mukono) in
Uganda. Both raw- and processed minerals are characterised with respect to
chemical composition, morphology, density, particle size distribution, surface
area, and finally, weight changes and phase transformations on heating. In
addition, the mineral constitution of the raw powders is investigated by XRD.
The third paper investigates the properties of six formulated and fired
sample bricks. The bricks are characterised with respect to dry shrinkage,
firing shrinkage, true porosity, apparent- and real density, water absorption,
microstructural and phase constitution after firing at 1250, 1300, 1400 and
1500°C, respectively, and finally, the cold crushing strength. Sieve analyses
of the Mutaka kaolin and Mukono ball clay are also given.
The fourth paper is concerned with investigation of Mutaka kaolin
beneficiation by hydro cycloning. Beneficiation is carried out with a hydro
cyclone in a laboratory environment. The beneficiated product is
characterized with respect to chemical composition, mineralogy, morphology,
density, particle size distribution, surface area, and finally, weight changes
and phase transformations on heating. A small sample of the beneficiated
kaolin is leached with oxalic acid in order to demonstrate the possibility of
iron oxide removal. The raw, beneficiated and acid leached powders are
characterized with respect to chemical composition, particle size distribution,
phase analysis, SEM, whiteness index and FTIR.
6
The fifth paper is concerned with manufacture of industrial fireclay bricks
from the studied minerals. Bricks of standard sizes are formulated, formed
and sintered in an industrial tunnel furnace. They are characterized with
respect to chemical composition, XRD, LOM, SEM, density, apparent porosity,
cold crushing strength, refractoriness, under load, thermal shock resistance,
thermal conductivity and alkali resistivity and their properties are compared
with parallel types.
2. Experimental techniques
The raw mineral samples were colleted from local deposits in Uganda as
shown in Figure 1, and in order to assure representative samples, not less
than 300kg were collected from each deposit. In paper 2, the collected
material was sub-divided into two parts for further processing. One part,
referred to as "raw", was ground and homogenised while the other part,
referred to as "processed", was mixed with water to form a homogeneous
slip. The slip was passed through various sieves to remove course particles,
stones, humus and sand. Then it was passed over a magnetic separator to
remove iron and, finally, it was dried.
In paper 3, the sample bricks were directly prepared from the raw minerals
since the previous processing was unsuccessful.
In paper 4, four tonnes of new materials were collected and 100 kg was
beneficiated by hydro cycloning. Although the beneficiation proved
successful, the raw minerals were used because of their purity, in Paper 5 to
produce fireclay refractories by an industrial process. The collection process
and investigations of powders and sintered bricks are shown in Figure 2 and
Figure 3.
The experimental work carried out in the present thesis covers a long range
of techniques. The specific details are given in the appended papers and only
a short summary is given here covering the types of investigations,
method/equipment used and where the investigations were performed. The
investigations on powders and on bricks are listed in Table 1 and Table 3.
7
2
3
1
1 – Mutaka kaolin deposit
2 – Mutundwe kaolin deposit
3 – Mukono ball clay deposit
Figure 1 Map of Uganda showing locations of the three mineral
deposits investigated.
8
Mutaka
kaolin
Mutundwe
kaolin
300 kg
300 kg
150 kg
150 kg
Ground
+
homogenized
Paper 2:
Ground,
wet sieved
+
magnetic
separation
+ dried
300 kg
150 kg
150 kg
150 kg
150 kg
Ground
+
homogenized
Ground,
wet sieved
+
magnetic
separation
+ dried
Ground
+
homogenized
Ground,
wet sieved
+
magnetic
separation
+ dried
”raw”
”processed”
”raw”
”processed”
”raw”
Mukono
ball clay
Wet ball milling
70%
Wet ball milling
30%
80%-0%
Grog fired at 1250°C
Course
30%
”processed”
20%-100%
Six binding slips
Middle
Fine
10%
40%
20%
6 sample bricks fired at 1250°C
Paper 3:
Fired at
1250°C
Fired at
1300°C
Fired at
1400°C
Fired at
1500°C
Figure 2 Collection, preparation and investigation of powders and
sintered bricks in Papers 2 and 3.
9
Mutaka
kaolin
Mukono ball
clay
100 kg
hydro
cycloning
200 g
Paper 4:
”beneficiated”
Oxalic acid
”acid leached”
700 kg
300 kg
Ball milled
Grog
fired at
1350°C
Course
30%
Paper 5:
Middle
20%
Fine
30%
20%
Industrial bricks fired at 1350°C
Figure 3 Collection, preparation and investigations of powders and
sintered bricks in Papers 4 and 5.
10
Table 1 Investigations on powder minerals, carried out in the present
work using various techniques/equipments at various sites.
Investigation
Chemical composition
Sieve analysis
Sandness
Plasticity
Phase constitution
Particle size distribution
Texture and morphology
Specific surface area
Density
Weight change and phase
transformation on heating
Beneficiation
Iron oxide separation
Whiteness index
* KTH:
Method/equipment
ICP-AES
Sieves
Sieve, mesh 48
Manual
XRD, FTIR
BI-90 particle sizer
SEM
BET
Pycnometer
TG-DTA
Performed at*
Analytica AB
MAK
MAK
MAK
KTH
KTH
KTH and KIMAB
KTH
KTH
KTH
Hydrocyclone
Acid leaching
Reflectometer
SEAMIC
KTH
STFI-Packforsk
Royal Institute of Technology, Materials Science and Engineering,
SE-100 44 Stockholm, Sweden
MAK:
Makerere University, Faculty of Technology, P.O. Box 7062, Kampala,
Uganda
Analytica AB:
Analytica AB, Aurorum 10, SE-977 75 Luleå, Sweden
KIMAB:
KIMAB, Drottning Kristinas väg 48, SE-114 28, Stockholm, Sweden
SEAMIC:
Southern and Eastern Africa Mineral Centre (SEAMIC), Dar es Salaam,
Tanzania
STFI-Packforsk:
STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm, Sweden.
11
Table 2: Investigations on sample fireclay refractories, carried out in
the present work using various techniques/equipments at various
sites.
Investigation
Dry- and fire shrinkage
Loss on ignition
Colour after firing
Bulk density
Real density
Water absorption
True porosity
Phase constitution
Cold crushing strength
Method/equipment
Measuring dimensions
Measuring mass
Naked eye
Measuring mass and
dimensions
Measuring mass and
volume of ground
powder
Measuring mass of dry
brick and saturated
with water
Calculated
XRD
Compression testing
Performed at
MAK
MAK
MAK
MAK
MAK
MAK
MAK
KTH
MAK
Table 3 Investigations on industrial fireclay refractories, carried out
in the present work using various techniques/equipments at various
sites.
Investigation
Bulk density
Apparent porosity
Cold crushing strength
Water absorption
Thermal shock
resistance
Refractoriness under
load
Permanent linear
change
Phase constitution
Method/equipment
PRE/R9, 78, p1.
PRE/R9, 78, p1.
PRE/R14—2, 90, p1.
PRE/R9, 78, p1.
PRE/R5.1, 78, p.1
Performed at
Bjuf Höganäs AB
Bjuf Höganäs AB
Bjuf Höganäs AB
Bjuf Höganäs AB
Bjuf Höganäs AB
PRE/R4, 78, p.1.
Bjuf Höganäs AB
PRE/R19, 78, p.1.
Bjuf Höganäs AB
Thermal conductivity
Chemical analysis
XRD, FEG-SEM, Thin
section/LOM
PRE/R32, 78, p.1.
Wet ICP-AES
Alkali test
Crucible method
SIMR-KTH, Bjuf
Höganäs AB
Bjuf Höganäs AB
Larfage Svenska
Höganäs AB
Bjuf Höganäs AB
12
3. Summary of results and discussion
3.1 Characterization and processing of powders
The results from the chemical analysis of the raw and processed (milling,
sieving and magnetic separation) mineral powders are given in Table 4. The
results of beneficiation of Mutaka kaolin by hydro cycloning and acid leaching
will be discussed in section 3.2. The most important components are Al2O3
and SiO2, since they have a decisive influence on the refractoriness and
strength of the final product. As can be seen, the impurities which are in
small amounts include Fe2O3, K2O, MgO, CaO, TiO2, MnO, N2O, and P2O5.
Table 4 Chemical compositions of raw- and processed powder
samples. Weight % of dry substance.
Substance Mutaka kaolin
raw
proc.
SiO2
48.800
50.100
Al2O3
36.000
35.500
CaO
<0.090
0.158
Fe2O3
0.238
0.323
MgO
0.038
0.117
K2O
1.140
1.100
MnO2
0.028
0.025
Na2O
0.048
0.053
TiO2
0.004
0.006
P2O5
0.009
0.011
LoI1
12.600
12.700
Mutundwe
raw
46.400
38.700
<0.090
0.791
<0.020
0.214
0.004
<0.040
0.039
0.043
13.800
kaolin
proc.
50.900
34.000
0.144
1.090
0.073
0.206
0.019
<0.040
0.053
0.055
12.600
Mukono
raw
67.200
18.200
0.306
2.830
0.363
0.975
0.026
0.185
1.380
0.049
8.100
clay
proc.
72.500
13.900
0.401
2.220
0.279
0.872
0.027
0.202
1.320
0.049
7.100
Generally, high alumina content is desired since both strength and
refractoriness are increased. As seen, the alumina content is decreasing after
processing, thus reducing the refractoriness of the powder raw material.
Consequently, the raw powder was used directly without beneficiation in
manufacture of sample fireclay refractories in the present work.
As seen from Table 4, the impurity levels, i.e. amount of low melting fluxes
are low for the investigated minerals. A comparison to commercially available
minerals is made in Figure 4. As seen, the Ugandan minerals are generally
1
LoI is Loss on ignition at 1000oC
13
closer to the Al2O3-SiO2 side than the rest of the minerals which is beneficial
for the refractoriness of produced bricks. However, the alumina content is
falling on the lower side in the diagram, thus reducing the potential
refractoriness.
Uganda
Figure 4 Ternary of SiO2-Al2O3-Other oxides with compositions
indicated for Ugandan kaolins, investigated in the present work and
in Nyakairu, et al, 2001, and European commercially marketed
kaolins Ligas et al, 1997.
The particle size distribution is illustrated in Figure 5 as accumulated volume
fraction. Often, the so-called “equivalent diameter” of 90% volume fraction is
reported in the literature. This diameter is given by the intersection with the
dashed line. The particles with smaller diameters than the equivalent
diameter represent 90% volume of the material. As seen in the figure,
processing decreases the equivalent diameter readily for the Mutaka kaolin
(1148 to 745nm) but only moderately for the Mutundwe kaolin (1420 to
1151nm). The equivalent diameter for the Mukono ball clay, however,
increases by processing (436 to 598nm). The behaviour is understood by
comparing with the morphology of the minerals as observed in SEM. It is
also consistent with changes in density and surface area during processing.
14
0.5
log[-log(1-Vacc)]
0.0
-0.5
Mutaka raw
-1.0
Mutaka proc.
Mutundwe raw
Mutundwe proc.
-1.5
Mukono raw
Mukono proc.
-2.0
2.4
2.6
2.8
3.0
3.2
log(d), [nm]
Figure 5 Avrami plot of particle size distributions expressed as
accumulated volume fraction, Vacc, and particle diameter, d, in nm.
The morphologies of the raw and processed minerals are shown in Figure 6Figure 11. As can be seen, Mutaka kaolin shows a well developed lamellar
structure which is broken down by processing thus reducing the particle size
and the volume measured with the BET apparatus leading to a decreased
density. The Mutundwe kaolin shows the same behaviour. The raw kaolin
minerals exhibit a pseudo-hexagonal plate like shape which is similar to
other kaolinitic minerals reported else where (Murray, 2000; Murray, et al,
1993; Ekosse, 2000). The Mukono ball clay, on the other hand, is much finer
from the beginning showing a very fine structure which is likely to form
aggregates during processing, leading to an increased equivalent diameter
and an increased density. This is what has been shown experimentally.
15
Figure 6 SEM image of raw Mutaka kaolin
Figure 7 SEM image of raw Mutundwe kaolin
16
Figure 8 SEM image of raw Mukono ball clay
Figure 9 SEM image of processed Mutaka kaolin
17
Figure 10 SEM image of processed Mutundwe kaolin
Figure 11 SEM image of processed Mukono ball clay
The DTA analyses, Figure 12, show clear peaks at about 980ºC in the
kaolinite-rich samples. It is around this temperature that kaolinite transforms
to metakaolinite and this does not happen in the Mukono ball clay because of
the low kaolinite composition as shown by XRD scans. Similar studies have
shown the transformation of kaolin to metakaolinite and mullite (spinelphase) before melting (Chen, et al, 2000; Castelein, et al, 2001; Sonuparlak,
et al, 1987; Pask, 1988; Schneider, et al, 1994).
18
200
Heating
150
heat flow, [µV]
100
Mutaka kaolin
50
80/20
0
70/30
-50
Mukono ball clay
-100
-150
700
900
1100
1300
1500
1700
Temperature [°C]
Figure 12 DTA-signals of heat flow during heating. For clarity, the
curves are displaced as follows: Mutaka kaolin +100, 80/20 +50,
70/30 ±0 and Mukono ball clay -50
X-ray diffraction scans for the raw Mutaka kaolin and raw Mukono clay are
illustrated in Figure 13 and Figure 14, respectively. The X-ray scan for the
raw Mutaka kaolin shows that the mineral is predominantly composed of
kaolinite with some muscovite and halloysite while the raw Mukono ball clay
is mainly composed of quartz with some kaolinite and microcline.
19
800
K
K
600
Counts
K Kaolinite
H Halloysite
M Muscovite
K
M
400
HKK
K
200
K
K
H K
K
MM
M
K
K
K
MM K
KK
K
KKKK
0
10
20
30
40
50
60
o
2 Theta [ ]
Figure 13 XRD of raw Mutaka kaolin
800
Q
K Kaolinite
Q Quartz
M Microcline
Counts
600
400
Q
200
Q
Q
K
K
K
KKQ KQ
K Q QQ Q
M
M
Q
0
10
20
30
40
o
2theta [ ]
Figure 14 XRD of raw Mukono ball clay
20
50
60
3.2 Beneficiation of Mutaka kaolin
Supplementary to the initial processing of Mutaka kaolin by washing, another
investigation aimed at upgrading the raw kaolin was carried out. In this
investigation, a hydro cyclone was used to beneficiate the kaolin. In a similar
manner as described in the preceding paragraph, the beneficiated powder
product was characterized. The chemical composition of the raw and
beneficiated samples as determined by ICP-AES is summarized in Table 5
and Figure 15. The major oxides detected are the components of the
kaolinite formula i.e. Al2O3, SiO2 and H2O (LoI). Only small amounts of other
oxides, predominantly Fe2O3 and K2O were detected. It can easily be seen
that the beneficiation process improves the quality of the kaolin to almost
pure kaolin. The beneficiated sample has a deficit of only 0.30% Al2O3,
1.35% SiO2 and 0.25% H2O. The sum of the main impurities, K2O, Fe2O3 and
CaO is limited to 1.3%, only. It is interesting to note that the composition of
Fe2O3 increases from 0.238 to 0.417%. The reason for this may be that the
iron oxide is part of the beneficiated sample and can not be removed
mechanically. Another reason could be that Fe2O3, seems to concentrate in
the fine fraction of the clay mineral, which in turn, presents difficulties in the
separation process. Other researchers have found that sometimes iron
containing minerals could be part of the kaolinite structure (Hu, et al, 2003).
Admittedly, the beneficiated kaolin was not used in the fabrication of fireclay
bricks in this work because of the original purity of the raw minerals.
In order to improve the quality of the beneficiated kaolin for more
demanding applications, it was leached with oxalic acid in order to eliminate
Fe2O3. As seen from Table 5, the acid removes 64% of the iron oxide which
could also be clearly seen as an increased whiteness index. See Table 6 .
FTIR-spectra on the other hand, showed no differences when comparing,
raw, beneficiated and acid leached samples. See Figure 15.
21
Table 5 Chemical composition in wt % of raw-, of processed-, of
beneficiated- and of beneficiated and acid leached Mutaka kaolin
compared to pure, ideal kaolin. Data for raw and processed kaolin
are taken from Kirabira et al 2003, where information on accuracy
also can be found. LoI represents loss on ignition.
Oxide
Raw
Processed
Beneficiated
Beneficiated
and acid
leached
SiO2
48.800
50.100
45.200
47.600
46.550
Al2O3
36.000
35.500
39.200
39.100
39.500
CaO
<0.090
0.158
0.135
<0.100
-
Fe2O3
0.238
0.323
0.417
0.149
-
MgO
0.038
0.117
0.059
0.051
-
K2O
1.140
1.100
0.760
0.821
-
MnO2
0.028
0.025
0.012
0.011
-
Na2O
0.048
0.053
<0.040
0.058
-
TiO2
0.004
0.006
0.012
0.016
-
P2O5
0.009
0.011
0.022
0.023
-
12.600
12.700
13.700
13.500
LoI2[wt %]
Ideal
13.950
Table 6 Whiteness Index of Mutaka kaolin powders
Sample powder
ISO brightness
Y-value
(R 457)
Raw
84.53
89.44
Beneficiated
69.95
79.83
Beneficiated and acid leached
81.37
85.02
2
LoI is Loss on ignition at 1000oC
22
Figure 15 Chemical composition of beneficiated and of beneficiated
and acid leached Mutaka kaolin expressed as SiO2-Al2O3-other
oxides, in comparison with raw and processed Mutaka kaolin from
Kirabira et al (2003), and theoretical pure kaolin. Compositions refer
to dry substances..
23
3.6
3.2
Absorbance
2.8
2.4
Beneficiated + acid leached
2.0
1.6
1.2
Beneficiated
0.8
0.4
Raw
0.0
4000
3500
3000
2500
2000
1500
1000
500
-1
Wave number, [cm ]
Figure 16 FTIR spectra of Mutaka kaolin. Raw Mutaka kaolin (+0),
beneficiated Mutaka kaolin (+1) and beneficiated and leached
Mutaka kaolin (+2). Numbers in parentheses show the vertical
displacement of the curves.
The BI-90 analyses, Figure 17 , show that the beneficiation process has little
effect on the suspended particle distribution. A small increase in d90 from
1148 to 1329 nm has occurred. The slope is somewhat less indicating that
the size distribution has widened moderately. It is interesting to note that
the processing used by Kirabira et al (2003) reduced the d90 much more, to
745 nm. One may suggest that the small change in suspended particle size
distribution by the beneficiation process compared to the change by
“processing” is a result of the more gentle treatment. During processing, the
powder was milled while it was only attrition scrubbed during beneficiation.
As 90% of the concentrate has particles with a size less than 1.4 µm, the
results confirm the dominance of kaolinite in the concentrate, easily
fragmentizing.
24
0.5
log[-log(1-Vacc)]
0.0
-0.5
-1.0
raw
-1.5
processed
beneficiated
-2.0
2.6
2.7
2.8
2.9
3.0
3.1
3.2
log(d), [nm]
Figure 17 Avrami plot of particle size distribution (BI-90) of
beneficiated Mutaka kaolin compared to earlier results from Kirabira
et al (2003) on raw and processed Mutaka kaolin.
3.3 Manufacture and characterization of fireclay refractories
A laboratory production of fireclay was studied first. Six sample fireclay
refractories were formulated from a mixture of grog and a binder slip. The
same grog was used for all samples but the binder slip, composed of ball clay
and kaolin, had a clay composition varying from 20 to 100%. The resulting
gross compositions, calculated from the mixing proportions are given in
Table 7.
Sample 1, with the lowest clay content was selected for X-ray study. After
firing at 1250oC to 1500oC, specimens were crushed, ground and analyzed in
a powder diffractometer. The scans for the selected firing temperatures are
shown in Figure 18.
25
Table 7 Calculated chemical composition of formulated brick samples
(grog/slip = 80/20) produced from a 70/30 kaolin/clay grog bonded
with a kaolin/clay slip according to column 2.
Sample
1
2
3
4
5
6
Slip
%
clay
20
25
30
40
45
100
SiO2
Al2O3
CaO
Fe2O3
MgO
K 2O
MnO2
Na2O
TiO2
P2O5
%
61.69
61.80
62.03
62.32
62.49
64.39
%
35.04
34.88
34.60
34.20
33.39
31.56
%
0.17
0.17
0.18
0.18
0.18
0.21
%
1.10
1.13
1.16
1.22
1.25
1.56
%
0.15
0.15
0.15
0.16
0.17
2.05
%
1.25
1.25
1.25
1.24
1.25
1.21
%
0.03
0.03
0.03
0.03
0.03
0.03
%
0.10
0.10
0.10
0.11
0.11
0.12
%
0.45
0.46
0.48
0.51
0.52
0.69
%
0.02
0.02
0.02
0.03
0.03
0.03
700
Q
M
M
650
600
M
550
500
Counts
450
400
Q
M
Q
350
300
M
Q
200
150
M
Q
M
QM
Q
M
M M
50
0
10
20
M
M
Fired at 1400 C
M M
M
Fired at 1300 C
M M
Q QQM
Fired at 1250 C
M M
o
M
M M M
M M M
o
Fired at 1500 C
M
M
M M
M M M Q
M
M M Q
M
250
100
M M M
M
Q M
M
o
M
Q Q
M
QM M
30
o
M
40
50
60
o
2 theta, [ ]
Figure 18 XRD scan of sample 1 fired at 1250º (+0), 1300º (+70),
1400º (+140), 1500ºC (+210). Numbers in parenthesis show the
vertical displacement of the curves. Q=quartz, M=mullite.
The formation of a glass phase is evidenced by the bump in the XRD scan
between 12 and 38[º]. Mullite peaks are well developed at all sintering
temperatures. This is very satisfying sine the formation of mullite
(3Al2O3⋅2SiO2) is an important factor in the present study. Due to its
excellent high temperature stability, mechanical properties, low creep rate,
26
low thermal expansion coefficient and low thermal conductivity, mullite
products are widely used in heat insulation (Chen, et al, 2001).
Figure 19 shows the variation of dry and firing shrinkage with the amount of
clay in the binder slip. It is evident that both types of shrinkage increase with
increasing amount of clay in the slip. In all cases the firing shrinkage is
higher than the dry shrinkage because during firing the material sinters and
densifies. However, the difference is much more pronounced in the clay rich
region. It is interesting to note that both the dry- and firing shrinkage
become low at small amounts of clay in the binder slip.
12
Dry shrinkage of bricks
Firing shrinkage, 1250°C
Drying and firing shrinkage, [%]
10
Firing shrinkage, 1300°C
8
6
4
2
0
-2
0
20
40
60
80
100
Amount of clay in binder slip, [%]
Figure 19 Variation of shrinkage with amount of clay in the binder
slip
27
Table 8 shows physical and mechanical properties of the produced sample
bricks fired at 1250ºC. The properties obtained are within the range of
commercially produced fireclay refractories. The true porosity data scatter
around 2.63gcm-3 but show no trend on the percentage of clay in the binder
slip.
Table 8 Physical and mechanical properties of samples fired at
1250ºC
Sample
No.
Density
(gcm-3)
% Water
Absorption
Cold
crushing
strength
(MPa)
True
Porosity
(%)
1
1.85
13.1
39.7
29.90
2
1.83
13.1
34.0
30.60
3
1.90
11.9
37.7
26.90
4
1.89
9.0
41.2
24.40
5
1.83
8.2
39.7
29.60
6
1.82
5.1
32.2
28.90
Figure 20 shows the variation of average apparent density for all the six
samples with firing temperature. Experimental results show that the highest
density for the samples can be achieved at a firing temperature of 1300ºC.
The plot indicates a low spreading among the different clay compositions.
28
2,0
1,9
Apparent density, [gcm-3]
1,8
1,7
1,6
1,5
1,4
1,3
1,2
1,1
1,0
1200
1250
1300
1350
1400
1450
Firing Temperature, [°C]
Figure 20 Variation
temperature
of
average
apparent
density
with
firing
Microstructural analysis of the sintered brick samples was done using a field
emission electron microscope (FEG-SEM) LEO 1536 with GEMINI Column.
The samples for SEM analysis were cut into cubes of 1 cm and polished well.
To reveal the morphology of the mullite grains, an etching solution of
concentrated hydrofloride acid was used to remove the glass phase around
the mullite grains in the sintered samples. The cubes were coated with gold
to avoid charging effects. The SEM micrographs are shown in Figure 21. The
microstructure reveals mullite grains in form of needle-like shapes. The
needles are more distinct and resemble whiskers with increase in sintering
temperature (1300-1400ºC).
29
(a)
(b)
30
(c)
Figure 21 The morphology of the sintered sample brick specimens at
(a) 1250ºC, (b) 1300ºC and (c) 1400ºC for 6 hours.
31
3.4 Manufacture of industrial samples and characterization
The two materials used in this work were raw kaolin from Mutaka and raw
clay from Mukono. Kaolin is used as the main source of alumina and ball clay
as the binder material. A product mix was formulated and industrial samples
were produced at Höganäs Bjuf AB, Sweden. Thermal, mechanical, chemical
and physical tests have been carried out in conformance with international
standards.
A uniform batch mix of approximately 1000 kg were prepared by milling a
mixture of 70% Mutaka kaolin and 30% Mukono ball clay in a ball mill for 8h.
The ball mill was emptied and the mixture left to dry into a stiff mud. The
stiff mud was formed into bricks using hand moulds, dried and pre-calcined
at 1350ºC for 4h before being left to cool in the furnace. After cooling, the
calcined material, grog, was crushed and graded into three different sizes;
course (1-3mm), middle (0.25-1mm) and fine (< 0.25mm) using standard
sieves.
A mixture of 30% course, 20% middle and 30% fine of the graded grog was
mixed with 20% milled kaolin/clay binder of the same composition.
Accordingly, the resultant mixture was wetted to about 15% moisture,
kneaded, extruded and formed at about 400 MPa, using a hydraulic press, to
yield bricks of 248x123x66 mm. The brick samples were dried and fired in a
tunnel kiln at 1350ºC for 7-8 hours and slowly cooled during exit of the kiln.
The sintered bricks had an average dimension of 232x116x62 mm. Out of
them, cylindrical test-pieces of diameter approximately 50 mm and height 50
mm were cut and used for determining apparent porosity, bulk density, cold
crushing strength, permanent linear change in dimension, water absorption,
refractoriness under load, and thermal shock resistance.
The results of the chemical analysis of the brick raw powders and the final
brick are presented in Table 9. As seen, the present sample bricks are
composed of 30.6 wt% alumina, 64.7 wt% silica and 3.8 wt% fluxes.
The SEM micrographs of a chemically etched sample brick, Figure 22,
confirms that the microstructure consists of a network of mullite needles
evenly distributed and embedded in a glassy phase. The abundance of well
crystalline mullite confirms a good sintering and promotes a high cold
crushing strength and high corrosion resistance. The properties of the
sintered industrial bricks are summarized in Table 10.
32
Table 9 Chemical Composition of raw materials and final brick in
comparison with commercial types.
Raw Materials
Oxide
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
MnO2
Na2O
TiO2
P2O5
LoI [wt %]
Kaolin
[wt %]
48.800
36.000
0.238
<0.090
0.038
1.140
0.028
0.048
0.004
0.009
12.600
Ball clay
[wt %]
67.200
18.200
2.830
0.306
0.363
0.975
0.026
0.185
1.380
0.049
8.100
Final
Brick
Commercial types
[wt %]
64.700
30.600
0.880
0.110
0.350
1.970
0.030
0.070
0.390
<0.010
-
[wt %]
65-691
25-451
1.5-2.51
-
1
Refractories handbook, 1998.
Figure 22 SEM image of a sintered brick, polished and etched in conc.
HF for 30s.
33
Table 10 Properties of the prepared sample bricks in comparison
with commercial types.
Property
Bulk density, [kgm-3]
Apparent porosity, [%]
Water absorption, [%]
Shrinkage, [%]
• linear
• volumetric
Cold Crushing strength, [MPa]
Thermal shock resistance, [cycles]
Refractoriness under, 0.2 MPa
load, [°C]
Permanent linear Change,
(1400°C, 5h), [%]
Alkali resistance, [cm2]
1
Refractories handbook, 1998.
2
Didier, 1982.
3
Routschka, 2004.
4
Chesters, 1973.
Achieved
value
1938
24.6
12.6
Commercial
types
1900-20001,2,3
22-261,2
6.1
17.0
44,0±2.2
10±1
T05:1290
T1: 1350
T2: 1380
T5: 1420
-1.9±0.17
±34
1.2
1.7
>202
>101,2
≈12303
The X-ray scans of the raw minerals and of a sintered and crushed sample
brick are shown in Figure 23 and Figure 24. In order to depress the peak
heights, the logarithmic values of the counts are given in Figure 23. As seen,
the crystal phases of the raw minerals are replaced by mullite, quartz and a
glass phase. The latter is seen as a wide bump between approximately 15 to
35° 2θ. The normal diffractogram of the sintered bricks is shown in Figure
24.
34
Q
7.0
6.5
QM
M
6.0
log[counts]
5.5
K
KK
5.0
Km
m
M
Q
M
Q
K
K
KK
KK
KK
mK
K
K
KK
MM MM
K
K
K
K
K
Q
3.5
3.0
M
Q
mK
Q
4.0
M
M MM
K
K
4.5
M M
M
K
K
Q
Q
K
K
Q
Q Q
Q
Q
2.5
2.0
20
30
Q
Q
40
Q
50
60
o
2 Theta [ ]
Figure 23 XRD scans of raw powders of the raw materials and a
crushed sintered brick powder. Mukono ball clay (+0), Mutaka kaolin
(+1.5), Sintered brick (+3). Numbers in parenthesis show the
vertical displacement of the curves. M, Q, K and m represent mullite,
quartz, kaolinite and muscovite respectively.
12000
Q
M Mullite
Q Quartz
10000
Counts
8000
6000
4000
M
Q
2000
M
M
M
M
M
M MM
M
M
Q M
Q
Q
M
M
0
20
30
40
50
60
o
2 Theta [ ]
Figure 24 XRD scan of a powder prepared from a sintered brick.
35
Refractoriness under load is a vital property of refractories since the time of
service of a refractory is determined by its deformation under load at high
temperature, finally leading to failure. The test serves to evaluate the
softening behavior of fired refractory bricks at rising temperature and
constant stress conditions. Results of the refractoriness under 0.2 MPa load
are summarized in the three curves shown in Figure 25. The effective
refractoriness under load curve (corrected) is obtained by adding the brick
subsidence curve (uncorrected) and the corresponding alumina tube
expansion curve. The corrected curve shows that on heating from 200-800°C
the brick slightly expands. It starts to shrink gradually between 900-1300°C
after which drastic subsidence starts. Deformations corresponding to 0.5%,
1.0%, 2.0% and 5.0% of the initial height of the test piece can be obtained
from the corrected curve and are given in Table 10. T0.5 corresponds to the
beginning of subsidence and T5 corresponds to beginning of failure i.e. the
brick cannot work above this temperature.
2
Change in height, [%]
1
0
-1
-2
-3
-4
Expansion of Alumina tube
-5
Corrected curve
-6
Uncorrected curve
-7
200
400
600
800
1000 1200 1400 1600
Temperature, [oC]
Figure 25 Refractoriness under a constant load of 0.2 MPa and a
heating rate of 5°Cmin-1.
The thermal conductivities of the bricks at nominal temperatures from 500 to
1250°C are shown in Figure 26. As seen, thermal conductivity increases
linearly with temperature which is typical to dense bricks, like the present
one. In a dense brick heat conduction through the brick material is more
36
important than radiation. On the other hand, in a light brick, like insulating
bricks, the heat conduction by radiation is much more important. Since the
latter follows the T4-law, one may conclude that it is of less importance in the
present brick.
Thermal conductivity,k, [Wm-1K-1]
2.0
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
400
600
800
1000
o
Temperature, [ C]
1200
1400
Figure 26: Thermal conductivity of a sintered brick
4. Conclusions
In the present study, kaolin and ball clay minerals from Ugandan deposits
have been studied with an aim of ascertaining their suitability for ceramic
products development with emphasis on fireclay refractories. Mineralogical,
chemical and physical characterization of the raw and processed minerals
has been done on Mutaka kaolin, Mutundwe kaolin and Mukono ball clay.
Kaolinite is the dominant mineral in the Ugandan kaolins followed by
muscovite, halloysite and montmorillonite. The chemical composition of the
kaolins includes mainly SiO2 and Al2O3, with minor impurities of Fe2O3, TiO2,
MgO, K2O, MnO, CaO and P2O5. On the other hand, the ball clay is
predominantly composed of quartz and kaolinite. The kaolins are quite pure
and a successful beneficiation of Mutaka kaolin has been achieved through a
37
mechanical process of particle separation based on a wet classification
method bringing its chemical composition close to that of ideal kaolin. On
leaching the beneficiated sample iron oxide is reduced by 64%. For chemical
and mineralogical composition, the minerals have been found to be suitable
for manufacture of fireclay refractories. Consequently, Mutaka kaolin and
Mukono ball clay were used to formulate industrial bricks. Results of the
technological properties of the manufactured fireclay bricks indeed compare
favorably with those of the parallel commercially produced bricks.
The achieved properties are attributed to the right choice of powder mixes
and a thorough thermal treatment leading to good sintering properties. The
sintering process is promoted by a reasonable amount of fluxing oxides
which contribute to formation of a glass phase that binds the mullite crystals.
The formulated bricks can be used as general purpose bricks for reheating
furnaces, rotary cement kilns, checkers, boilers, ladles, non-ferrous metal
furnaces, waste incinerators and for other processing industrial applications.
In light of the foregoing investigations, the kaolin from Mutaka and ball clay
from Mukono are the deposits recommended as viable and reliable supplies
of raw materials for the manufacture of fireclay refractory bricks.
4.1 Suggestions for future work
It has been proved technically that quality fireclay refractories can be
produced from the selected minerals. An economic feasibility study should be
investigated to ascertain the possibility, and eventual exploitation of the
deposits. This could be implemented through setting up a refractory
production line in Uganda or processing the minerals for export to refractory
manufacturers.
It has also been demonstrated that the main impurity after mechanical
beneficiation of Mutaka kaolin can be removed by acid leaching. It should be
possible to completely remove this impurity thus further improving the
whiteness of Mutaka kaolin and the possibilities of its application in the paper
industry. Hence, kinetically controlled acid leaching processes could be
investigated.
Regarding the improvement of Mutaka kaolin for applications other than
refractories, investigations of its possible alternative uses, in particular as a
coating and/or filler material could be tried.
38
Acknowledgments
I am delighted to express my sincere gratitude for the support,
encouragement, and friendship given to me by individuals, institutions, and
organizations whose contributions have helped me accomplish this thesis.
First and foremost, my warmest gratitude to my supervisor Associate Prof.
Stefan Jonsson. He has been a source of inspiration, support and
encouragement at arduous times as well as pleasant work. I am also grateful
to my Mak supervisor Dr. Joseph Byaruhanga who has supported and guided
me throughout this work.
Furthermore, I would like to thank my colleagues at the Department of
Materials Science, KTH and at the Faculty of Technology, Mak for their
camaraderie. And to all my friends, thanks for all the support, the valiant
comments and the companionship.
Financial support from the Swedish International Development Agency
through Sida/SAREC-Mak Research Collaborative Programme is gratefully
acknowledged. Special appreciation to the administration at the Faculty of
Technology, Mak and the School of Graduate Studies, Mak for all the
logistical and academic support.
At the same time I acknowledge the fruitful collaboration with Geological
Survey and Mines Department, Uganda Industrial Research Institute, the
Southern and Eastern Africa Mineral Centre (SEAMIC), Department of
Materials Science, UDSM, and Höganäs Bjuf AB, Sweden where some of the
experiments in this thesis were carried out.
Finally, am very grateful to my beloved family for giving me support and
encouragement at all times.
Stockholm, June 2005
John Baptist Kirabira
39
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Appended papers
42