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KTH Materials Science and Engineering
Properties of Ugandan minerals and fireclay refractories
John Baptist Kirabira
Doctoral thesis
Stockholm, Sweden 2005
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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
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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-related-consumables, 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 Mutaka kaolin was carried out using mechanical segregation of particles. The aim of the study was to explore other potential applications like in paper filling and coating. The beneficiation process improves the chemical composition of kaolin to almost pure, the major impurity being iron oxide. 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
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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.
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To my family; for their love and pride in me
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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 Kirabira, Stefan Jonsson, and Joseph Kadoma Byaruhanga, Powder Characterization of High Temperature Ceramic raw materials in the Lake Victoria Region. Silicates Industriels, in 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
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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.
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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
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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
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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
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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 alumino-silicate 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
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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).
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• 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*
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*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.
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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.
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Figure 1 Map of Uganda showing locations of the three mineral deposits investigated.
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2 3
1 – Mutaka kaolin deposit 2 – Mutundwe kaolin deposit 3 – Mukono ball clay deposit
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Figure 2 Collection, preparation and investigation of powders and sintered bricks in Papers 2 and 3.
Mutaka kaolin
Mutundwe kaolin
Mukono ball clay
300 kg 300 kg 300 kg
150 kg 150 kg 150 kg 150 kg 150 kg 150 kg
Ground +
homog-enized
Ground +
homog-enized
Ground +
homog-enized
Ground, wet sieved
+ magnetic separation
+ dried
Ground, wet sieved
+ magnetic separation
+ dried
Ground, wet sieved
+ magnetic separation
+ dried
”raw” ”raw” ”raw” ”processed” ”processed” ”processed”
Wet ball milling
Paper 2:
Wet ball milling
70% 30%
Grog fired at 1250°C
Course Middle Fine
30% 10% 40%
6 sample bricks fired at 1250°C
Fired at 1250°C
Fired at 1300°C
80%-0% 20%-100%
20%
Six binding slips
Fired at 1400°C
Fired at 1500°C
Paper 3:
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Figure 3 Collection, preparation and investigations of powders and sintered bricks in Papers 4 and 5.
Mutaka kaolin
Mukono ball clay
100 kg
hydro cycloning
”beneficiated”
200 g
”acid leached”
Oxalic acid
700 kg 300 kg
Paper 4:
Ball milled
Grog fired at 1350°C
Course Middle Fine
30% 20% 30% 20%
Industrial bricks fired at 1350°C Paper 5:
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Table 1 Investigations on powder minerals, carried out in the present work using various techniques/equipments at various sites.
Investigation Method/equipment Performed at* Chemical composition ICP-AES Analytica AB Sieve analysis Sieves MAK Sandness Sieve, mesh 48 MAK Plasticity Manual MAK Phase constitution XRD, FTIR KTH Particle size distribution BI-90 particle sizer KTH Texture and morphology SEM KTH and KIMAB Specific surface area BET KTH Density Pycnometer KTH Weight change and phase transformation on heating
TG-DTA KTH
Beneficiation Hydrocyclone SEAMIC Iron oxide separation Acid leaching KTH Whiteness index Reflectometer STFI-Packforsk
* KTH: 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.
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Table 2: Investigations on sample fireclay refractories, carried out in the present work using various techniques/equipments at various sites.
Investigation Method/equipment Performed at Dry- and fire shrinkage Measuring dimensions MAK Loss on ignition Measuring mass MAK Colour after firing Naked eye MAK Bulk density Measuring mass and
dimensions MAK
Real density Measuring mass and volume of ground powder
MAK
Water absorption Measuring mass of dry brick and saturated with water
MAK
True porosity Calculated MAK Phase constitution XRD KTH Cold crushing strength Compression testing MAK
Table 3 Investigations on industrial fireclay refractories, carried out in the present work using various techniques/equipments at various sites.
Investigation Method/equipment Performed at
Bulk density PRE/R9, 78, p1. Bjuf Höganäs AB Apparent porosity PRE/R9, 78, p1. Bjuf Höganäs AB Cold crushing strength PRE/R14—2, 90, p1. Bjuf Höganäs AB Water absorption PRE/R9, 78, p1. Bjuf Höganäs AB Thermal shock resistance
PRE/R5.1, 78, p.1 Bjuf Höganäs AB
Refractoriness under load
PRE/R4, 78, p.1. Bjuf Höganäs AB
Permanent linear change
PRE/R19, 78, p.1. Bjuf Höganäs AB
Phase constitution XRD, FEG-SEM, Thin section/LOM
SIMR-KTH, Bjuf Höganäs AB
Thermal conductivity PRE/R32, 78, p.1. Bjuf Höganäs AB Chemical analysis Wet ICP-AES Larfage Svenska
Höganäs AB Alkali test Crucible method Bjuf Höganäs AB
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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 Mutundwe kaolin Mukono clay raw proc. raw proc. raw proc. SiO2 48.800 50.100 46.400 50.900 67.200 72.500 Al2O3 36.000 35.500 38.700 34.000 18.200 13.900 CaO <0.090 0.158 <0.090 0.144 0.306 0.401 Fe2O3 0.238 0.323 0.791 1.090 2.830 2.220 MgO 0.038 0.117 <0.020 0.073 0.363 0.279 K2O 1.140 1.100 0.214 0.206 0.975 0.872 MnO2 0.028 0.025 0.004 0.019 0.026 0.027 Na2O 0.048 0.053 <0.040 <0.040 0.185 0.202 TiO2 0.004 0.006 0.039 0.053 1.380 1.320 P2O5 0.009 0.011 0.043 0.055 0.049 0.049 LoI1 12.600 12.700 13.800 12.600 8.100 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
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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.
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.
Uganda
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-2.0
-1.5
-1.0
-0.5
0.0
0.5
2.4 2.6 2.8 3.0 3.2log(d), [nm]
log[
-log(
1-V a
cc)]
Mutaka raw
Mutaka proc.
Mutundwe raw
Mutundwe proc.
Mukono raw
Mukono proc.
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 6-Figure 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.
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Figure 6 SEM image of raw Mutaka kaolin
Figure 7 SEM image of raw Mutundwe kaolin
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Figure 8 SEM image of raw Mukono ball clay
Figure 9 SEM image of processed Mutaka kaolin
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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 (spinel-phase) before melting (Chen, et al, 2000; Castelein, et al, 2001; Sonuparlak, et al, 1987; Pask, 1988; Schneider, et al, 1994).
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-150
-100
-50
0
50
100
150
200
700 900 1100 1300 1500 1700
Temperature [°C]
heat
flow
, [µV
]
Heating
Mutaka kaolin
80/20
70/30
Mukono ball clay
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.
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10 20 30 40 50 600
200
400
600
800
H
K KaoliniteH HalloysiteM Muscovite
KK
KKKK
K
KK
K
K
K
K
H
K
KMM
M
M
K
MK
KK
K
M
K
Co
un
ts
2 Theta [o]
Figure 13 XRD of raw Mutaka kaolin
10 20 30 40 50 600
200
400
600
800
K KaoliniteQ QuartzM Microcline
QQQQQKKQKKM
Q
Q
K
QK
M
K
Co
un
ts
2theta [o]
Figure 14 XRD of raw Mukono ball clay
21
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.
22
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 Pro- cessed
Benefi- ciated
Benefi-ciated
and acid leached
Ideal
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 -
LoI2[wt %] 12.600 12.700 13.700 13.500 13.950
Table 6 Whiteness Index of Mutaka kaolin powders
Sample powder ISO brightness
(R 457)
Y-value
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
23
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..
24
4000 3500 3000 2500 2000 1500 1000 500
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
Raw
Beneficiated
Beneficiated + acid leached
Ab
sorb
an
ce
Wave number, [cm-1
]
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.
25
-2.0
-1.5
-1.0
-0.5
0.0
0.5
2.6 2.7 2.8 2.9 3.0 3.1 3.2log(d), [nm]
log[
-log(
1-V a
cc)]
raw
processed
beneficiated
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.
26
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 Slip %
clay
SiO2
%
Al2O3
%
CaO
%
Fe2O3
%
MgO
%
K2O
%
MnO2
%
Na2O
%
TiO2
%
P2O5
% 1 20 61.69 35.04 0.17 1.10 0.15 1.25 0.03 0.10 0.45 0.02 2 25 61.80 34.88 0.17 1.13 0.15 1.25 0.03 0.10 0.46 0.02 3 30 62.03 34.60 0.18 1.16 0.15 1.25 0.03 0.10 0.48 0.02 4 40 62.32 34.20 0.18 1.22 0.16 1.24 0.03 0.11 0.51 0.03 5 45 62.49 33.39 0.18 1.25 0.17 1.25 0.03 0.11 0.52 0.03 6 100 64.39 31.56 0.21 1.56 2.05 1.21 0.03 0.12 0.69 0.03
10 20 30 40 50 600
50
100
150
200
250
300
350
400
450
500
550
600
650
700
MMMMM Q
MMMM QM
MM
MMMM
MMMQ Q
M
M
MMMM
MMQ
MMM
QMM
QQM
Q
MQ
M
M
M
MQ M MQQMM
MMQ
MMM
QM
MQM
Fired at 1500oC
Fired at 1400oC
Fired at 1300oC
Fired at 1250oC
Cou
nts
2 theta, [o]
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,
27
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.
-2
0
2
4
6
8
10
12
0 20 40 60 80 100
Amount of clay in binder slip, [%]
Dry
ing
an
d f
irin
g s
hri
nkag
e,
[%]
Dry shrinkage of bricks
Firing shrinkage, 1250°C
Firing shrinkage, 1300°C
Figure 19 Variation of shrinkage with amount of clay in the binder slip
28
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.
29
1,0
1,1
1,2
1,3
1,4
1,5
1,6
1,7
1,8
1,9
2,0
1200 1250 1300 1350 1400 1450
Firing Temperature, [°C]
App
aren
t den
sity
, [gc
m-3
]
Figure 20 Variation of average apparent density with firing temperature
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).
30
(a)
(b)
31
(c)
Figure 21 The morphology of the sintered sample brick specimens at (a) 1250ºC, (b) 1300ºC and (c) 1400ºC for 6 hours.
32
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.
33
Table 9 Chemical Composition of raw materials and final brick in comparison with commercial types.
Raw Materials Final Brick
Commer- cial types
Oxide Kaolin [wt %]
Ball clay [wt %]
[wt %]
[wt %]
SiO2 48.800 67.200 64.700 65-691 Al2O3 36.000 18.200 30.600 25-451 Fe2O3 0.238 2.830 0.880 1.5-2.51 CaO <0.090 0.306 0.110 MgO 0.038 0.363 0.350 K2O 1.140 0.975 1.970 MnO2 0.028 0.026 0.030 Na2O 0.048 0.185 0.070 TiO2 0.004 1.380 0.390 P2O5 0.009 0.049 <0.010 LoI [wt %] 12.600 8.100 - - 1Refractories handbook, 1998.
Figure 22 SEM image of a sintered brick, polished and etched in conc. HF for 30s.
34
Table 10 Properties of the prepared sample bricks in comparison with commercial types.
Property Achieved value
Commercial types
Bulk density, [kgm-3] 1938 1900-20001,2,3 Apparent porosity, [%] 24.6 22-261,2 Water absorption, [%] 12.6 Shrinkage, [%]
• linear • volumetric
6.1 17.0
Cold Crushing strength, [MPa] 44,0±2.2 >202 Thermal shock resistance, [cycles] 10±1 >101,2 Refractoriness under, 0.2 MPa load, [°C]
T05:1290 T1: 1350 T2: 1380 T5: 1420
≈12303
Permanent linear Change, (1400°C, 5h), [%]
-1.9±0.17 ±34
Alkali resistance, [cm2] 1.2 1.7 1Refractories handbook, 1998. 2Didier, 1982. 3Routschka, 2004. 4Chesters, 1973. 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.
35
20 30 40 50 602.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
K KKK
KKK K
KKK
Q
Q
Q
Q
QQ Q
mm
mm
KK
K
KK
KKKK
K
K
KK
K KKK
K
M
MM
Q
Q
Q
Q
MMM MMM MMM
M
M
MM
log
[co
un
ts]
2 Theta [o]
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.
20 30 40 50 600
2000
4000
6000
8000
10000
12000
M MulliteQ Quartz
Q
QM
MM
M
MM MM M
Q
M
MM
MM
Cou
nts
2 Theta [o] Figure 24 XRD scan of a powder prepared from a sintered brick.
36
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.
-7
-6
-5
-4
-3
-2
-1
0
1
2
200 400 600 800 1000 1200 1400 1600
Temperature, [oC]
Ch
an
ge i
n h
eig
ht,
[%
]
Expansion of Alumina tube
Corrected curve
Uncorrected curve
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
37
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.
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
400 600 800 1000 1200 1400Temperature, [oC]
Th
erm
al co
nd
uct
ivit
y,k
, [W
m-1
K-1
]
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
38
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.
39
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
40
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Appended papers
