Characterisation of Ugandan raw-minerals for 7654/FULLTEXT01.pdfCharacterisation of Ugandan raw-minerals…

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  • Characterisation of Ugandan raw-minerals for firebricks -before and after sintering.

    John Baptist Kirabira

    Licentiate thesis

    Royal Institute of Technology

    Department of Materials Science and Engineering Division of Mechanical Metallurgy

    Stockholm 2003

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    Akademisk avhandling som med tillstnd av Kungl Tekniska Hgskolan framlgges till offentlig granskning fr avlggande av teknisk licentiatexamen onsdagen den 10 december 2003 kl. 10.00 vid Institutionen fr Materialvetenskap, Kungl Tekniska Hgskolan, frelsningssal B2, Brinellvgen 23, Stockholm. Fakultetsopponent r docent Tommy Ekstrm, Lnnviksvgen 66, 178 90, Eker. ISBN 91-7283-645-8 ISRN KTH/MSE--03/60--SE+MEK/AVH John Baptist Kirabira, November 2003

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    Contents 1. Introduction ................................................................................................................... 1

    1.1 Presentation of the thesis .................................................................................. 1 1.2 Materials and manufacture of firebricks ...................................................... 2

    2. Experimental Techniques .......................................................................................... 2 3. Summary of results ..................................................................................................... 4 4. Conclusions.................................................................................................................. 18 5. Suggestions of future work ..................................................................................... 18 Acknowledgements............................................................................................................. 19 References ............................................................................................................................. 20 APPENDIX: PAPERS (I-III) ........................................................................................... 21

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    1. Introduction

    1.1 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 firebricks. It includes the following three 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*

    *Department of Mechanical Engineering, Faculty of Technology, Makerere University, P.O. Box 7062, Kampala, Uganda, email: gatsby@tech.mak.ac.ug **Department of Materials Science and Engineering, Royal Institute of Technology (KTH), Brinellvgen 23, SE-100 44 Stockholm, Sweden, email: stefan@mse.kth.se 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 beneficiated 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, phase constitution after firing at 1250, 1300, 1400 and 1500C, respectively, and finally, the cold crushing strength. Sieve analyses of the Mutaka kaolin and Mukono ball clay are also given.

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    Since the first paper thoroughly covers all details of concern in manufacturing of firebricks, a short introduction will be given here, only.

    1.2 Materials and manufacture of firebricks Firebricks, like other ceramic products are processed through three main stages: raw materials preparation, consolidation to compacts and densification by sintering. The main constituents of firebricks 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 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 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 1500C for 6 to 24 hours. The higher the alumina content, the higher the firing temperature. The fired samples generally are left to cool in the furnace.

    2. Experimental Techniques The mineral samples were colleted from local deposits in Uganda as shown in Fig. 1, and in order to assure representative samples, not less than 300kg were collected from each deposit and 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. 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 2 respectively.

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    Fig. 1 Map of Uganda showing locations of the three mineral deposits

    1

    2 3

    1 Mutaka kaolin deposit 2 Mutundwe kaolin deposit 3 Mukono ball clay deposit

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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 Coupled Plasma-

    emission Spectrometry (ICP-AES)

    SGAB

    Sieve analyse Sieves MAK Sandness Sieve, mesh 48 MAK Plasticity Manual MAK Phase constitution XRD KTH Particle size distribution BI-90 particle sizer KTH Texture and morphology

    SEM KTH and SIMR

    Specific surface area BET KTH Density Pycnometer KTH Weight change and phase transformation on heating

    TG-DTA KTH

    * 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 SGAB: SGAB Analytica, Lule Tekn. Universitet, SE-971 87 Lule, Sweden SIMR: Swedish Institute for Metals Research, Drottning Kristinas vg 48, SE-114 28,

    Stockholm, Sweden

    Table 2: Investigations on sample firebricks, 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

    3. Summary of results The results from the chemical analysis are given in Table 3. The most important components are Al2O3 and SiO2, since they have a decisive influence on the refractoriness and strength. Generally, high alumina content is desired since both strength and refractoriness are increased. As

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    seen, the alumina content is decreasing after processing, thus reducing the refractoriness of the powder. Consequently, the raw powder was used directly without beneficiation in the present work.

    Table 3: 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 48.8 50.1 46.4 50.9 67.2 72.5 Al2O3 36.0 35.5 38.7 34.0 18.2 13.9 CaO

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    Fig. 2. Ternary of SiO2-Al2O3-Other oxides with compositions

    indicated for Ugandan kaolins, investigated in the present work and in reference [1], and European comm