STUDIES ON THE DISSOCIATION OF MULLITE Thesis submitted for the degree of Doctor of Philosophy in the University of London by Naimuddin Ahmed, M.Sc. Department of Chemical Engineering and Chemical Technology, Imperial College of Science and Technology, London, S.W. 7. July, 1964. Abstract. Studies have been made on the stability relationship of mullite in four systems, namely, Na20-A1203-Si02, Li20- A1203-Si02, Ca0-A1203-Si02 and Mg0-A1203-Si02. Equilibrium reactions between mullite and each additive were first carried out in the solid state, following the principle of a compati- bility triangle. The results of the solid-state reactions have shown that: (i) In Bowen and Schairer's Na20-Al203-Si02 diagram, the compatibility triangle, mullite-corundum-albite, is not obeyed. An alternative triangle through mullite, corundum and nepheline solid solution has been put forward. (ii) In the Li 0-Al 0 -Si0 diagram, the relevant 2 2 3 2 compatibility triangle may be drawn through mullite, corundum and spodumene. (iii) The inferred compatibility triangles, mullite- anorthite-corundum, and mullite-cordierite-sapphirine, in the CaO-, Mg0-, A1203-Si02 systems, are confirmed. These triangles were not previously verified by reactions between mullite and the additives. Experiments were also performed at higher temperatures with mullite and different amounts of the additives in order to study the conditions under which mullite is dissociated. The observed and the calculated values were compared and any discrepancy explained. Contrary to earlier observations, lime and magnesia (besides soda and lithia) have been shown to cause dissociation of mulliteo Acknowledgements Grateful thanks are due to Mr. L.R. Barrett for his kind supervision, keen interest and constant encouragement shown throughout the progress of the work. The author is indebted to Mr. R.A. Lewis, his colleagues in the ceramics laboratory and to all those who extended their help and advice. The author also wishes to offer his gratitude to Dr.M.Q. Khuday Director, East Regional Laboratories, P.C.S.I.R., Dacca, East Pakistan, for hia kindly arranging the grant without which this work would not have been possible. DEDICATION TO NAZITEEF MY BELOVED WJJ "PATIENCE IS A VIRTUE" CONTENTS Title Page Abstract Acknowledgements Contents 1 Introduction 1 2 Survey of the state of knowledgeg chemistry of mullite. 4 2.1 General 4 2.2 Mullite in the system, A1203-Si02 6 2.3 Structure of mullite 9 2.4 The formation of mullite 10 2.5 Importance and the manufacture of mullite refractories 12 2.6 The stability of mullite in presence of different fluxing materials 14 Figure 2.1 19 3 Relevant phase equilibrium diagrams and their interpretation in the mullite region 20 3.1 Introduction 20 3.2 Method of calculation of different phases in a ternary phase diagram 23 3.2.1 When one of the phases in equilibrium is liquid 23 3.2.2 When all the phases in equilibrium are solids 24 3.3 Interpretation of relevant phase diagrams 26 3.3.1 The system, Na20-A1203-Si02 26 Title Page. 3.3.2 The system, Li2 0 Al203-S-O 2 29 3.3.3 The system, Ca0-A143-8102 31 3.3.4 The system, Mg0-A1203-5102 33 Tables, 3.1 - 3,6 38-43 Figures, 3.1 - 3.8 44-48 4 Experimental technique 49 4.1 IntrOduction to the determination of high-temperature phase equilibria 49 4.2 Method of investigation on the dissociation of mullite 51 4.3 Measurement of different phases in the decompoEition products of mullite 52 4.3.1 Determination of the glassy phase 53 4.3.1.1 Calibration of the method-of determination of the glassy phase 56 4.3.1.2 Results and discussion 58 4.3.1.3 Accuracy of the method 61 4.3.2 X-ray method of qianti:6ative estimation of crystalline phases 63 4.3.2.1 Calibration of the x-ray method of analysis 65 4.3.2.2 Measurement of intensities of x-ray lines 66 4.3.2.3 Accuracy of the method 68 4.3.3 Chemical method of the determina- tion of crystalline phases 70 4.3.4 Determination of phases in the system, Li20-A1203-S102 71 Title Page 4.3.5 Determination of phases in the system, Na20-Al2u3-S102 74 4.3.6 Determination of 1.hases in the system, MgO-A1203-Si 02 75 4.3.7 Determination of phases in the system, Ca0-A1203-Si02 78 4.3.8 Over-all accuracy of the deter- mination of different phases 81 Figures 4.1 - 4.6 84-88 5 Experiments on the dissociation of mullite in presence cf different additives 89 5.1 Materials and procedure:; 89 89 5.1.2 Prepe.liation of samples 91 5.1.3 Heating of samples 92 5.1.4 The mineralogy of reacruion- products 93 5.2 Experiments on commercial mullJte and the additives 94 5.2.1 Results and discussion 94 5.3 Experiments on pure samples of mullite and different additives 96 5.3.1 Experimental detail 97 5.3.1.1 The system, Na20-31203-Si02 97 -Si0 99 5.3.1.2 The system, Li20-Al203 2 5.3.1.3 The system, Ca0-Al203-S102 9 5.3.1.4 The system, Mg0-A1203-Si02 100 5.4 Results and discussion on tKe dissociation of mullite 100 Title Pape 5.4.1 Results and discussion of the blank experiments on the stab- ility of mullite 101 5.4.2 The system, Na20-A1203-Si02 103 5.4.2.1 The equilibrium results and discussion in general 103 5.4.2.2 Discussion of results in the light of the interpretation of the phase diagram 105 A The oompatibility- triangle, albite- corundum-mullite 105 B The compatibility- triangle; nepheline- corundum-mullite 110 0 The compatibility- triangle, nepheline s.s.- corundum-mullite 113 5.4.3 The system, Li20-A120,,,-S102 129 5.4.3.1 The equilibrium results and discussion in general 129 5,4.3.2 Discussion of :results in the light of the inter- pretation of phase diagram 130 5.4.4 The system, Ca0-A103-St 02 134 5.4.4.1 The equilibrium results and discussion in general 134 5.44.2 Discussion of results in the light of the inter- pretation of phase diagram 135 5.4.5 The system, MgO-A1203-S102 139 Title Pac,e 5.4.5.1 The results and discussion in general 139 5.4.5.2 Discussion of results in the light of the interpretation of phase diagram 139 5.4.6 The effect of increasing amounts of different additives on the dissociation of mullite 142 5.4.7 The effect of temperature on the the dissociation of mullite 144 5.4.8 Progress of reaction with tine and the consequent attainment of equilibrium 145 5.4.9 Resu]ts and discussion of the comparative studies on the dissociation of mullite of different compositions 148 5.4.10 Results and discussion of the effect of free alumina or silica on the dissociation of mullite in presence of Na20 150 Tables, 5.3 - 5.8 153-161 Figures, 5.1 - 5.40 162-200 6 General conclusion 201 7 Future work 203 References 204 1. 1 - Introduction Stability is an important factor in determining the suitability of refractory materials at high temperatures. For most purposes, especially for physics-chemical experiments, refractories should not, apart from other things, react exces- sively with the particular solid, liquid or gas with which they come into contact. No single refractory compound, however, satisfies all these requirements and the problem is to choose from the known compounds the one best suited to the particular operating conditions. The convenient method of investigation in the field of refractories is through the interpretation of phase equilibrium diagrams which are based on thermodynamic principles. These give very useful information about the nature and equilibrium states, of reactions in a system at different temperatures. During the past half-century, there have been extensive studies of the phase equilibrium relations in silicate systems (4.2). Many of these studies were, of course, undertaken on grounds of geological and petrological interests in the composition of rock-forming minerals and in their melting and crystallisation behaviour. Nevertheless, the results of these investigations have been of considerable theoretical and practical interests, not only in geology but also in physical chemistry and in the 2. chemistry of glasses, porcelains, refractories, cements and metallurgical slags. While studying reactions in different systems including stability relationship of various refractory materials in contact with different fluxes or slags, advantage must therefore be taken of the interpretation of relevant phase diagrams. Due to the wide application of muliite refractories, numerous investigators have studied the chemistry of the mineral mullite, but few worked on its stability relationship at high temperatures, particularly in the presence of alkalies and alkaline earths. Amongst the very few reeearchers who have carried out reactions involving the breakdown of mullite, Gad (1.1) observed that mullite is unstable in presence of alkalies but not in presence of the alkaline earths under similar conditions. This surprising result in their observation could be due to the fact that he did not investigate in the light of the interpretation of phase diagrams, according to which mullite should also be unstable in the presence of lime and magnesia. Further, mullite should, according to the Na 0-Al 0 - 2 2 3 phase equilibrium diagram, dissociate completely at about Si02 1104°C in presence of 4.9% Na2C, but Gad (1.1) observed that it had not noticeably done so even at 1200°C.