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. No attempts have .30 far been made to explain whether this is a misfit in the 3 •
particular phase diagram or proper experimental conditions were not set up by the previous worker.
Other investigations in this field give scanty infor- mation about the mineralogical changes of mullite in the presence of flumes at high temperatures, and leave no doubt that a thorough investigation on the subject is necessary.
Thus, in order to conduct a systemvtic investigation on the stability relationship of mullite, reactions between the mineral and different amounts of additives like, Li20' Na20 , Mg0 and Ca0 at various temperatures have been carried out. As one of the first essentials of these studies, interpretation of the phase equilibrium diagrams of the systems, Li20-A1203-
Si0 Na 0-Al 0 Mg0-Al 0 -Si0 and Ca0-A1203-Si021 2' 2 2 3-Si02' 2 3 2 were made.
The phase diagrams refer only to systems which are in equilibrium at the temperatures specified and define the relative amounts of different phases under equilibrium conditions. They give no indication, however, as to the velocity of reactions. Therefore, the progress of reactions, and consequently, tae attainment of equilibrium in each system were also studied. 4.
2 - Survey of the state of knowledge:
Chemistry of Mullite
2.1 General.
Mullite is a kind of alumino-silicate mineral and has a
chemical formula of 3 A1203.2Si02. It crystallises in the
orthorhombic system, usually in the form of elongated needle-
shaped crystals. Mullite is so-called because it was first
noted in the 'Isle of Mull', Scotland.
For a long time, it was thought that the needle-like
crystals found in hard•-fired fireclay and procelain were the
mineral sillimanite (Al 0 asiO ), but it was subsequently shown 2 3 3 and is now accepted that these needles are in fact the mineral
mullite 2.1). Although rare in nature, mullite is very common
in artificial products; porcelain, fireclay products, high- alumina refractories, etc. all contain this compound as one
of their most valuable constituents. It is considered as the
0 -Si0 and is produced most stable compound of the system, Al2 3 2' from every alumino-silicate compound at high temperatures and
under atmospheric pressure or at a pressure exceeding it by a
few hundred. This implies that the phase field of mullite is diminished by pressure/ and this is why, mullite is so common in artificial products and rare in natural rocks that are formed usually under high pressure conditions (2.2). The other alumino- 5. silicate minerals, e.g., andalusite, kyanite and sillimanite
(Al2 03 .SiO2') which are formed under high pressure and are only found in nature, transform on heating at varying temperatures above 1000°C to mullite. According to Bowen and Greig (2,3) mullite melts incongruently at 1810°C. Incongruent melting point of mullite was revised by Morris and Scholas (20.4) and found to be 1827°C. Toropov and Galakhov (2.5, 2.6) found a melting point of congruent nature at 1910°C. The most recent phase diagrams show a melting point of 1850°C as determined by Aramaki and Roy (2.7). Some of the important and characteristic properties of mullite as compiled by Barta and Bartuska (2.8) and Searle and
Grimshaw (2.9) are reproduced below:
(i) Cryetal aystein: Orthorhombic. (ii) Tr habit: Acicular.
(iii) Optical character. Positive. Birefringence: 0.012. Refractive index: 1.65 Specific gravity: 3.03 g/c:c. Hardness: 7,5 mohs. 4 Bending Strength: 4.2 x 10 kg./cm2. Modulus of 506 x 105 kg./cm2 at 0°C. shearing stress: 2.5 x 105 kg./cm2 at 1200°C 6.
2 (x) Modulus of elasticity: 3.51 x 105 kg./cm .
(xi) Heat conductivity: 0.01 cal./sec. cm.° C. 6 (xii) Specific electric 10 cm. at 600°C. 4 resistance: 10 cm. at 1400°C. (xiii) Characteristic X-ray
lines at (d a): 5.36, 30.41, 2.2 etc.
2.2 Mullite in the system, A1203-Si02
It was Shepherd, Rankin and Wright (2010) who first
investigated the physico-chemical properties of the system
(Al203-Si02) as the only crystalline compound to appear in this two-component system. The compound was reported to have
a congruent melting point of 1811 1- 10°C. According to these authors, there is a first eutectic invariant between sillimanite C with a composition of 10% A1203 and and SiO2 at 1600° 90%Si02•, the second eutectic exists between sillimanite and
Al203. At a later stage, in 1924, Bowen and Greig (2.3) published the results of their investigation on the system
Al 0 -Si0 and put forward a completely different theory. 2 3 2' According to them mullite (34A1 0 0.2Si0 ) is the only crystalline 2 3 2 phase that appears in this binary system. This compound was
found by them to melt incongruently at 1810°C. They placed in their diagram (as shown in figure 2.1A) the eutectic between
Si02 and mullite at 1545°C with an A1203 content of 5.5%.
According to them, there is no eutectic between mullite and
but a peritectic invariant. A1203 Although Bowen and Greigts observation that mullite is the only stable compound of this system is appreciated even to- day, their diagram has been criticised for a long time.
Numerous scientists have since been interested in the problem.
It was in the year 1951-53, when Toropov and Galakhov (2.5,
2.47) discovered the congruent behaviour of mullite and published their first diagram of the Al 0-SiC system as shown 23 2 in figure 2.1 B. According to them, mullite melts congruently
°C and forms a eutectic with Al 0_ at 1845°C with the at 1910 2 .7 composition of 77.5% A1203 and 22.5% SiO2. The congruent melting point of myllite has been confirmed by Budnikov et al
(2.19) and others (2.6,2.7) at a later stage (but contrary views have also been expressed). The possibility of the existence of solid solution in the mullite phase has also been the subject of much investigation. The solid solution between mullite
does not exist as indicated by Bowen and Greig and and SiO2 disclaimed by Nahmias (2.11) and McAtee and Milligan (2.12).
The absorption of alumina by the mullite lattice in the form of solid solution was first presumed by Posnjak and Greig (2.13), 8. and corroborated experindentally by Sosman (2.14). Following
Posnjak and Greig's investigation, Rooksby and Partridge (2.15) and other scientists at a later stage have observed the formation of solid solution between mullite and A1203. Rooksby and
Partridge, in fact, observed three distinct forms of mullite: a-form containing 71.8% A1203 and 28.2% Si02; p-form with an
2 in the form of solid solution; and y-form excess of A1 03 containing iron and titanium as solid solution. The relevant phase diagrams showing the solid solution range in the mullite phase were first published by Shears and Archibald (2.16) in
1954 and by Konopicky et al (2,17) in 1956.
Toropov and Galakhov (26) investigated the problem concerning the solid solution range in the mullite phase and revised their earlier diagram as shown in figure 2.1C.
According to these authors, mullite (3:2) has a congruent melting
o 20 point of about 1910 C and forms solid solution with A1 3 to a maximum composition of 78% A1203 and 22% Si02. A at eutectic exists between mullite solid solution and Al203 about 1840°C with a composition of 79% A1203 and 21% Si02.
Aramaki and Roy (2.7) published the resalts of their
0 -Si0 system in Their extensive studies on the Al2 3 2 1959. diagram as shown in figure 2.1 D is remarkably similar to that by Toropov and Galakhov (2.6). According to them, mullite 9 melts congruently at about 1850°C and forms solid solution with
A1203 with a maximum range of 75% a1203 and 25% Si02. The mullite solid solution forms a eutectic with A12 at about 03 1840°C with a composition of 78% A1203 and 22% Si02.
At a later stage, in 1960, however, Welch (2.18) emerged with a new interpretation of the mullite problem.
According to him, mullite is just one member of an extensive solid solution series, and the composition, 2A12030Si02 is more appropriate for the material. Thus, mullite of 3:2 composition has no special significance. He reported mullite as mplting incongruently at 1880°C. The solid solution range extends to an Al 0 -;:ontent of 75%. 2 3
2.3 Structure of mullite.
There are four types of anhydrous alumino-silicates to be found in nature. Three of these, namely, andalusite, kyanite and sillimanite are only naturally occurring minerals and have a general formula of A1203.bi02. The fourth mineral, mullite, is rarely found in nature but is very common in ceramic products.
It has a chemical formula of 3.A1 0 .2Si0 , and the other three 2 3 2 compounds transform on heating to it. In view of the importance of these minerals, their crystal structures have been thoroughly investigated and found to be cicse1.7 related.
The striking similarity of sillimanite and mullite in 10.
many respects has however made their differentiation very
difficult. Both these crystal phases have the same optical
orientation, character and crystal habits. Only a carefal
determination of the refractive indices makes a distinction
possible. Again, these refractive indices may also become the same, if ferric oxide and titanium dioxide are present in the crystalline solution. From the chemical point of view, however,
there is a significant difference between mullite and sillimanite.
The great difference in chemical composition of the two mi4erals and their almost identical X-ray patterns indicate the substi-
tution of silicon by aluminim. This has beer adequately dealt with by Tayloi (2.20) and Warren (2.21), according to
whom, mullite has a defect structure. Scholze (2.22) has also investigated the problem of the defect structure of mullite.
As the present studies do not concern the structural chemistry of the mineral, reference may be made to the original work by
these authors.
2.4 Formation of mullite.
All alumino-silicates, aluminium hydrosilicates, artificial mixtures or mixed gels of A120 + etc., 3 SiO2' transform into mullite at high temperatures. According to the phase diagram, the final product of all mixtures of the system,
All 0 -SiO at ordinary pressure below the melting point is 2 3 2 11. mullite. The equilibrium of the pure system is however influenced by foreign substances. specially by alkalies and other fluxes.
The bibliographyon investigations of the formation of mullite phase in fired clays and industrial, more or less sintered, ceramic bodies is numerous (2.23, 2.24.). Norton
(2.25) and Greig (2.26) were the. first to study the transfor- mation of the three AJ2Si05 alumino-silicates into mullite.
They found that kyanite transforms most easily, and traces of mullite can be detected even after a firing at 1200°C.
Andalusite and sillimanite also transform into mullite but only above 14000 and 155G C. respectively. Brindley and Nakahira
(2.27, 2.28) made a comprehensive study of the thermal effects of kaolinite and its conversion into mullite. The overall process of the dehydroxylation of kaolinite may be summarized as follows:
(1) Al2S1205(011)4 Al2S1207 (meta-kadinite) + 2H20.
(ii)2A1 Si 0 Al Si 0 (spinel phase) SiO 2 2 7 4 3 12 2' (iii)Al 4Si3012 2 Al2S105 (mullite-type phase) + SiO 2' (iv)3Al 2Si35 A1ei20,3 (mullite) + Si02.
Attempts have also been made to determine the most favourable conditions for the synthesis of mullite from its ingredients, both by sintering and fusion methods. Horte and 12.
Wiegmann (2.29) found that the A1203.Si02 ratio of 1:1 was the most suitable composition; whereas, West and Gray (2.30) and
Demediuk and Cole (2.31) described the ratio 2:1 and
Goncharov (2.32) described the ratio 3:2 as the most favourable composition. According to Neuhaus (2.33), these different observations are due to the fact that the methods of sintering and fusion do not give the same product even though the starting mEtterial is the same. The starting material of the composition of 3:2 A1203;Si02 ratio, for example, yields mialite
(3:2) if sintered, and a compound of the composition, 2A1203.,
if fusea. SiO2 Successful attempts have also been made to grow single crystals of mullite, These have been produced by Bauer et al.
(2.34) in a verneuil kiln by using oxyhydrogen flame.
2.5 Importance and the manufacture of mullite refractories.
The refractories that are high in crystalline mullite combine a good load-bearing resistance at high temperatures, good resistance to thermal spalling and excellent volume stability. The properties and uses of alumino-silicate refractories having mullite as a constituent mineral are well known. Barrett (2.35) has, however, reviewed the recent advances in this field with reference to the merits and demerits of various refractories. 13.
The initial use of mullite-type refractories made from
Indian Kyanite was based on their superior performance over the typical superduty and high-alumina grades when in contact with
molten metals, slags, alkaline volatiles, etc. Thus, the evaluation and application of synthetic mullite refractories have been based predominantly on the physical and chemical properties of the Indian kyanite-type refractories.
The trend toward high-temperature work in many industries has recognised the value of mullite refractories. In the metallurgical field, mullite is used to some extent in electric furnace roofs, hot metal mixers, heating furnaces, low frequency induction furnaces and for tap-holes and blest furnace stoves. In glass industry, mullite is used in the upper structure of the glass tank and for constructing the drawing chambers of the furnace.
Dependent on the application of mullite refractories, three types of synthetic mullite are usually manufactured:
1. Fused synthetic mullite, prepared by melting Bayer alumina and silica or banxite and kaolin in the electric furnace.
2. High-tediperature sintered synthetic mullite, prepared
by sintering mixtures of alumina and kaolin; alumina, kaolin o and kyanite; or banxite and kaolin above 1750 C.
3. Low temperature sintered synthetic mullite, prepared by sintering siliceous bauxite or mixtures of bauxite and kaolin above 1550°C. 14.
2.6, The stability of mullite in presence of different fluxing
materials.
According to widely accepted hypothesis, the fluxing
materials have great influence on the quantitative phase distribution
of fired alumino-silicate products. Several authors claim to have
succeeded in increasing the mullite-contents of some ceramic materials by
the use of certain fluxing agents. Many others, on the contrary have
observed the adverse effect on the mullite phase. The instability of
mullite in presence of various additives has long been known but few
studies seem to have been made in this respect.
McVay and. Harsh (2.36) in 1928 noticed. that, after long
service, the refractory bricks made of diaspore got enriched in alkalies
and vanadium pentoxide on the surface. The infiltration of some
coal-ash shifted the mullite-zone, and the quantity of mullite
crystals increased. In the hot zone, however, mullite decomposed
into corundum and glass. Budnikov and Khizh (2.37) also came to the
same conclusion by tLe microscopic investigation of a fireclay wall
after six months' service. The mullite crystals, they investigated,
contained a number of inclusions filled with Wit stite.
Yoshioka and Isomachu (2.38) made a comparative study on the
effect of materials like B203, Steatite, orthoclase, talc, Fe203' Ca0 and CaF on the formation of mullite. They observed that, 2 if added in a quantity of 1%, the materials influence the formation 15.
of mullite. In higher quantities, on the other hand, adverse effect is
observed.
Skola (2.39) observed corundum crystals in glass preparcd in
a tank lined with cast mullite-blocks. Crystals of nephelite were
also occasionally observed. He described these effects as the decom-
pobition of mullite in contact with glass. In order to explain this
behaviour of mullite- Skola carried out some laboratory experiments,
and found that a sample of technical mullite was dissociated by
1% Na2G03 to the extents of 30 to 40% and by 2% 11a2CO3 to 60 to 8pg
at temperatures 1400°-1600°C.
While studying the mineralogical effect of different fluxing
agents on the formation of mullite by fusion-cast processes, it as
perhaps Kraner (3.11) who first observed that the formation of mullite
was completely inhibited by the addition of Li20, 4.5% Na20 and
11.0% CaO, Ile tried to explain nis results in the light of the relevant phase diagrams, and inferred that the formation of mullite would also be checked by the addition of about 22% MgO, It may be
mentioned here that although Kraner furnished very valuable information in this field, he used the compatibility trianges in a wrong sense, i.e., above the melting range of the compositions. However, the fact that Ca0 prevents the formation of mullite was later confirmed by Nemetschek (2.40),
Wright and Wolff (2.41) on examination of refractory checkerwork in a regenerative furnace noticed that a granular residue of mullite and/or corundum was left after carbon-attack. According to them, carbon reduced the free or combined silica of the refractory to a volatile form, and this caused disintegration of mullite. Ford and
Rees (2.42) also observed the break-down of mullite in the presence of chromium oxide by long period of heating at 1600°C.
Filonenko and Lavrov (2.43) investigated. glass tank blocks microscopically and observed that, by heating refractories containing mullite in bulk and 4 to Wo Na20 to a temperature of 1820°C, corundum crystals were obtained. In other words, mullite was decomposed to corundum and siliceous glass. The effect of alkali vapours on high- alumina bricks has also been studied and explained by Brisbane and
Segnit (2.44) in the Light of Bowen and Schairerts (5.4) phase diagram of the system, K20-A1203-Si02. The temperature of melt- formation depends, he maintained, on the A1203:Si02 ratio, and it is advantageous to increase the Al 0 -content of the raw-materials. 2 3 Moore aid Heeley (4.11) and Skthner et al (2.45) investigated the mineralizing effect of TiO Fe 0 and alkalies on the formation 2' 2 3 of mullite. They observed that TiO2 is a very good mineralizer.
Fe 0 is a good mineralizer only at low concentration. Alkalies do 2 3 not influence the quantity of mullite but, on the contrary, spoil the mineralizing effect of TiO and Mopre ari Prasad (2,46) 2 Fe203. also studied the effects of the oxides of titanium, iron, sodium, 17. potassium, lithium, magnesium, and calcium on kaolin-alumina mixtures
corresponding to the sillimanite and mullite compositions. They carried
out experiments at temperatures, 15000-16000C for 24 hours and
observed that the formation of mullite was influenced by practicaly
any additive of low concentrations. Higher quantities of these foreign
materials, however, acted as retarders.
Rigby and Hutton (5.11) studied the effect of alkali and vanadium oxide slag on alumina silica refractories. They observed that mullite was decomposed by Soda giving rise to corundum and nepheline.
Gad (1,1) investigated the problem of the dissociation of mullite !n some detail, and found that the mineral was completely decomposed by the addition of 3% Li 0 and 40 Na20 at 1400°C. He also 2 carried out experiments in presence of MgO and Ca0 but did not observe any dissociation. These results are surprising, as according to the relevant phase equilibrium diagrams, mullite should also be unstable in presence of lime e.,nd magnesia.
In order to make a comprehensive study on the dissociation of mullite, therefore, experiments have been carried out in the present investigation with mullite and additives, like, Li20, Na20, MgO and
CaO. The relevant phase diagrams have been taken as a guide in these
stud4.es so that any discrepancy between the work of previous investi- gators and the interpretation of phase diagrams could possibly be 18.
explained. A brief discussion on different phase diagrams and their interpretation in the mullite region are presented in Chapter 3 below. °C °c melt- 1900- Si02+ A1203 1900 melt
1700' mullite -}- melt cor. 1700 -F mullite 1500 1500 5102 mullite 0 20 40 60 80 100 0 20 40 60 80 . S 102 Wt. % A1203 s102 Wt. % A1203 Fig.2.IA By Bowen And Greig. Fig, 2.1 B (Toropov -Galokhov-1953).
s.s. s.s. melt C °C melt 2000-° Liquid. liquid 1900 Cor. liquid 1800 - mullite 18'40° 1700 mullite s.s. liquid Cor.4- 1600 mullite s.s. 1595° 1500 5102+ mullite mullite 0 1400 20 40 6 0 86 I00 0 20 40 600 80 I
Si 02 Wt. % Al2 03 S102 Mole % A1203
Fig,2.1 C (Toropov -Galakhov4958). Fig. 2.1 D (Aramaki - Roy .1959 ).
Fig. 2.1 The System, A1203 — S102. 20.
Relevant Phase Equilibrium Diagrams and
their Interpretation in Mullite Region
3'1 Introduction.
The present investigation is not aimed at studying and composing phase equilibrium diagrams but at studying reactions between mullite and additives like Li20, Na20, Mg0 and Ca0 in the light of the interpretation of existing phase diagrams. A comprehensive phase diagram indicates the solids in equilibrium with liquid or with one another, the temperatures at which the first liquid appears and the liquidus curves.
It has, therefore, been considered as one of the first essentials of the present studies to have some knowledge of the phase equilibria of the systems in question, e.g.,
0 - SiO Na20 - A1203 - Si02, MgO - A1203 - Li20 - A12 3 2' Si02 and Ca0 - A1203 - Si02. Elaborate discussions of the phase relationships in ceramics and the interpretation of phase diagrams have been made by various authors in the past
(301 - 3.6).
Tho main difficulty encountered by earlier workers in this field was that they could not utilize the full advantage of the availability of phase diagrams, .and this probably led them to unsatisfactory conclusions. It was, trerefoa►e decided to attempt to interpret the phase diagrams first, and 21.
consequently, some preliminary calculations have been made from them in the mullite region. These are described below under the specific systems.
As the present studies concern the dissociation of mullite in presence of different additives, it was considered feasible to take only mixtures of mullite and each of the additives, as reaction compositions, instead of taking, in the conventional way, mixtures of the three constituent oxides for working in a three-component system. For carrying out reactions in each system, the percentage amounts of the additives have been calculated on the assumption that they form binary systems with mullite, as it were. In fact, however, none of these additives and mullite can remain as separate entities in a reaction system at high temperatures except in cases where solid solutions are formed, if any. Consequently, some reactions would take plee causing the break-down of mullite, and as soon as this break-down occurs no miter to
SiO and what extent, specific ternary systems with Al203, 2 each of the additives will be formed. Phases in equilibrium in diff.,:r.ent reaction products would then necessarily follow the prediction of the relevant phase equilibrium diagrams including the triangular sub-divisions called the compatibility triangles or the solidus triangles. 22.
According to the definition of the compatibility
triangle of a system (3.7), all the three components which.
constitute such a triangle should exist as solid phases in
equilibrium in a reaction composition within the triangle,
at a temperature below the melting range, i.e., in solid-state
reactions. Above the melting range of the composition,
however, one or more phases will disappear depending on
temperature and composition of the system. The idea of the
beginning of melting range of compositions inside a compati-
bility triangle can be obtained from a ternary invariant,
(a ternary eutectic or peritectic) in the relevant portion of
the phase diagram; in the case of compositions lying on
compatibility joins, haever, melting range may start at the
eutectic temperature of the two co-existing phases. Further,
from a reaction composition on the compatibility join of any
two components of a compatibility triangle, the third
component will completely disappear at a temperature below
the melting range of the composition, by solid-state
reaction. For example, in any composition, inside the compati-
billty triangle, 'mullite-anorthite-corundum' (figure 307),
all these three phases will be pesent in equilibrium below
the melting range, i.e., below 1512°C, which is the ternary
peritectic temperature of mullite, corundum and anorthite. 23.
Above the melting range, on the other hand, one or more phases including mullite will disappear from such reaction systems depending on temperature and composition. Reaction composition having 88.3% mullite and 11.7% CaO on the composition-join of anorthite and corundum will be completely free of mullite at a temperature below the melting range.
The existing phase equilibrium diagrams in three- component systems have, therefore, been taken as a guide to the present studies on the dissociation of mullite both in solid-state and in solid-liquid reactions, i.e., both below and above the melting range.
211 Method of Calculation of different phases in a ternary phase diagram.
3,201. When one of the phases in equilibrium is liquid.
With a very simple example, Kingery (3.8) has shown how to calculate the amounts of phases in equilibrium. In figure 3.1, the composition A falls in the primary field of
X. On cooling the liquid A, X begins to crystallise from the melt when the temperature reaches T1. The composition of the liquid changes along AB, because of the loss of X. Along this line, the lever principle applies so that at any point the percentage of X present is given by 24.
100 (N).
On further cooling, when the temperature reaches T2 and the crystallisation path reaches the boundary representing equilibrium between the liquid and the two solid phases, X and Z, Z begins to crystallise also; the liquid changes in composition along the path CD. At L, the phases in equilibrium are a liquid of composition L and the two solids X and Z, whereas the overall composition of the entire system is A.
As shown in figure 3.2, the only mixture of L, X and
Z that will give a total corresponding to A is
xA Ryf (loo) % x
zA a (100) = id Z
lA Ty (100) = %
That is, the smaller triangle, XZL is a ternary system in which the composition of A can be represented in terms of its three constituents,
3.2.2. When all the phases in equilibrium are solids.
If reactions in a three-component system are being conducted in the solid state, the phases in equilibrium would 25.
uecessarily be three solid compounds, and the theoretical calculation of the amounts of these phases can be made by using the compatibilfty triangle of that part of the phase diagram°
The procedure is obviously the same as discussed above, the smaller triangle, XZL, being considered to be formed of the three probable solid phases instead of one liquid and two solids.
The only point to be remembered in this case is the fact that only at certain temperatures below the melting range of a reaction composition, such calculations will hold good.
This i$ particularly because the progress of reaction in the solid-;state is rather slow, and gives increased rate of reaction at increasing temperatures. Equilibrium may thus eventually be attained below the melting range only under optimum conditions, when the experimental results may follow the theoretical calculations made on thebasis of a particulir compatibility triangle. Therefure, when eal.;ulating the amounts of different phases from a compatibility triangle of a system, no specific temperature of the equilibrium state could possible be mentioned.
Conrequently, the terms, "below the melting range and above the melting range", have been used to demarcate the solid-state and solid-liquid reactions. 26.
3.3 Interpretation of relevant phase diagrams. 3.341. The System, Na20 - A120 - Si020
One needs be very cautious when interpreting the phase equilibrium diagrams, particularly those containing alkalies.
SiO phase diagram, as shown In the most recent Na20 - A1203 - 2 in figure 3.4, the inferred compatibility triangle, mullito- albite- corundum, predicts that the composition point having about 4.9% Na20 on the composition-join of Albite and Corundum will not contain any mullita; in other words, mullite will compltely disappear from a reaction-composition of 95.1% mullite; acid 4.9% Na20, below the melting range of the system, i.e., below 1104°C. Gad (1.1), as well as the present author lid not, however, observe dissociation of mullite to any appreciable extent under these conditions. It, therefore, implied three alternative calculations in order that a systematic investigation could be carried out0.
These were based on (as shown in figure 303): 1) the inferred compatibility triangle, Albite Mullite
- Corundum. ii)the supposed compatibility triange, Nepheline
Mullite - Corundum, and, iii)the supposed compatibility triangle, Nepheline solid solution - Mullite - Corundum. 27.
Although the phase diagram does not show the presence of the second or the third compatibility triangle, such a triangle has been assumed to exist on the consideration of the disagreement between the experimental evidence and the predic- tion of the first compatibility triangle, i.e., through Albite, mullite and corundum. Nepheline (or carnegieite) is, however, known to be the most stable ternary compound in this system, and is usually observed in decomposition products of alumino- sthcate refractories in service in alkaline atmosphere (3.10).
It can also be seen that the phase diagram contains inferred compatibility joins of nepheline and corundum and of nepheline solid solution (maximum range) and corundum which gives additional ground for assuming the existence of the compatibility triangle, either through nepheline-mullite- corundum, or through nepheline solid solution-mullite- corundum.
According to the second triangle, i.e., through nepheline-mullite-corundum (the triangle NMC in figure 3.3) about 12.7% Na20 would be necessary for complete dissociation of riullite below the molting range. Reaction compositions having less than 12.7% Na20, inside the triangle, would however contain all the three phases, i.e., mullite, corundum and nepheline below the mlting range. Above the melting range, of course one or more phases will disappear depending on temperature and composition of the reaction system. 28.
The third probable compatibility triangle (the triangle
NMC in figure (3.3) predicts however that 9 Na 0 would % 2 completely decompose mullite below the melting range, Due to the formation of solid solution of nepheline with silica, the existence of this compatibility triangle as an alternative to tho previous two may be possible. But as there can be only one such triangle in this region of the diagram, it :.s necessary to conduct careful experimental investigations to find out which one of these three is the relevant compatibility triangle.
It is seen in the diagram (figure 3,4) that the fields of stability of mullite and nepheline (or carnegieite) are separated from each other by that of corundum, and as such, mullite cannot form binary mixtures with nepheline; in other words, no eutectic can exist between the two, Yepheline
(carnegieite), however, forms eutectic with corundum at about
1475°C, but in the absence of any ternary eutectic or peritectic between these three phases, I.e, mrllite, corundum and nepheline, it is not possible to specify the beginning of melting range in this section of the diagram. As the phase diagram shows the presence of solid solution in the mpheline (carnegieite) phase, the system may get complicated, 29,
and in practice, the melting range may eventually start ac a
temperature much below 1475°C, this again depending on the amount of silica present in the system,
Theoretical calculations based on the three compatibility triangles as mentioned above are shown in tables 3.1 - 3.3.
As the application of compatibility triangles can only be made below melting range, amounts of different phases predicted above the melting range are necessarily the same in all the three compatibility triangles that have been considered here. Thus, although such theoretical calculations made far compositions having 2, 3 and 4% Na20 at 1500° and 1600°C and for
4.9, 7 and 9% Na20 at 140000 have bsen shown in tables 3.2 and 3,3, respectively, are equally applicable in all cases above the melting range,
3.3,2 - The system, Li 0-Al -SiO 2 2 3 2
In the absence of sufficient information about the phase-relationships in the system, Li00-AL203.Z102, straight- 30.
forward interpretation of the phase diagram (figure 3.5) in the mullite region could not be made. This was particularly so, as there is no compatibility triangle in the high- alumina suction of the diagram on the basis of which dissociation of mullite could be predicted.
The presence of the partial system, Si02-Li20.A1203,
very long solid-sclution range in the eucryptite-Si02 section of the.diagram, and the formation of the three ternary
counpounds, eucryptite, spodumene and petalite, have made
the probable location of the relevant compatibility triangle
extremely difficult. Gad (1.1) however, observed complete
disscciation of mullite in presence of 3% Li20 at 1400°C, and Kraner (3.11) also reported the suppression of mullite
with 2% Li20 at higher temperatures. Now , of the three ternary compounds, eucryptite starts melting with considerable
decomposition below its melting point (1380°C); Petalite
also decomposes at about 1000°C giving spodumene and siliceous
glass; spodumene has, however, been reported to be the most
stable phase at high temperature. These facts may be accepted
as the grounds on wnich assumption has been made for the
compatibility triangle, mullite-corundum-spodumene (the
triangle S-M-C in figure 3.6).
This supposed compatibility triangle would predict,
as in other systems, the complete dissociation of mullite in 31.
below the melting range of the presence of 3.4% Li20' reaction composition. But as the phase diagram shomthe presence of inferred liquidus curves between eucryptite and corundum, and between eucryptite and spodumene, the formation of binary mixtures of corundum and spodumene is not possible; thus no information about the eutectic temperature between corundum and spodumene is available in the literature. It is not possible therefore, to get any theoretical idea about the temperature when. the melting range of compositions would start. In aosence of such data, it can be said that melting range will start in this part of the diagram much below 1426°C, the melting point of spodumene. Calculations made on the assumption of the compatibility triangle, spodumene-ccrundum-mullite, below melting range are shown in table 3,4. No calculation of the amounts of different phases above melting range could be possible in absence of the necessary isotherms.
It thus depends on the experimental investigation to see where the melting range approximately starts in the range of compositions specified, and also, whether the compatibility triangle, on the basis of which calculations have been made is correct.
3.33. The system, Ca0-A1203-Si02
In the relevant section of the most recent phase diagram of the system, Ca0-A1203-Si02, which is shown in 32.
figure 3..7, there is a compatibility triangle of interest through mullite, anorthite and corundum. Obviously, this triangle infers that mullite will disappear from a reaction- composition having about 11.7% CaO on the composition join of anorthite and corundum, below the mlting range of the composition. Mullite may also disappear from reaction- mixtures, inside this compatibility triangle,containing less than 11.7%, above the mlting range; below the melting range, however, all the three solid phases, e.g., mullite, anorthite and corundum, will be present in such reaction products.
It is noticed in the phase diagram that mullite cemnot form binary mixtures or eutectic with anorthite, as the composition join of mullite and anoraite crosses the field of stability of corundum; no information about the eutectic temperature between anorthite and mullite is, therefore, available. The eutectic temperature between corundum and anorthite and the ternary peritectic temperature between o anorthite, mullite and corundum are reported to be 1547 C and 151200 respectively. In absence of the ternary eutectic of these three phases, the peritectic temperature of 1512°C may however be taken as the beginning of the melting range of compositions in this region of the diagram.
Theoretical calculation of the phases in equilibrium in the mullite region of the diagram is shown initable 3.5. 33.
3.31+. The system, Mg0-A1203-SiO2
The mullite region of the diagram, Mg0-A1203-Si02 is
complicated, and thus, no simple calcultions could be mad,: in
order to find how much Mg0 would be necessary for complete
dissociation of mullite. Consequently, the following
possibilities were examined.
Composition-join between corundum and cordierite does
not exist, and as such, there is no compatibility triangle
through mullite, cordierite and corundum. This moans
mullite is stable in that region of the diagram below the
melting range of the compositions having up to 7.0% MgO; above the melting range, haever, mullite may disappear,
partly or completely, depending on experimental conditions.
In the most recent Mg0-A1203-3i02 phase diagram
(figure 3,8 ) the only relevant compatibility triangle, in
the mullite region, seems to be the one through mullite,
cordierite and sarphirine. That this might give an insight into the investigatior on the dissociation of mullite in
presence of MgO, theoretical calculations of the probable
phases in equilibrium were made.
According to this compatibility triangle, about 19.3%
Mg0 would be necessary for complete dissociatipn of mullite,
'oelow the melting range of the system. Reaction-mixtures 34. containing mullite and less than 19.5% Mg0 would normally contain, under equilibrium conditions, all the three phases, e.g., mullite, cordierite and sapphirine; below the melting range. Above the melting range, on the other hand, on3 or more phases including mullite may disappear depending on temperature and composition of the reaction-mixtures. It is obvious from the phase diagram that the lowest temperature where mullite, cordierite, sapphirine and liquid can be present in a reaction-system is 1460°C,. This means the melting range of reaction-compositions in this region would not probably start below 1460°C.
Now, however, the sapphirine-composition falls in the primary phase field of spinel and has a very small field of stability adjacent to the primary phase fields far mullite and cordierite. Thus, in a reaction product where sapphirine is the probable phase, spinel may eventually appear at the expense of sapphirine. Reaction-compositions falling in the primary phase field of corundum, on the other hand, may contain corundum as one of the phases under equilibrium conditions. As boai cordierit' and sapphirine start eating with decomposition much below their melting points, the amount and nature of the phases in equilibrium are entirely dependent on the experimental conditions. 35.
It does not, however, necessarily follow that a reaction-composition falling in the primary field of a component will always contain that component as one of the phases in equilibrium. This has been observed by Foster (507) who cited, as an example, the case of cordierite. Thus, although the compcsition-point of cor•dierite falls in the primary field of mullite, the former can be quantitatively synthesized from its ingredients by solid-state reaction.
Cordierite, however, melts incongruently giving rise to some mullite. These phenomena can probably be explained in the light of the compatibility triangle in a particular system. Thus, any composition inside such a triangle, no matter what primary field it belongs to, will contain all the three components as solid phases in equilibrium below the melting range. It can, thorefam, be said, on the basis of the prediction of the compatibility triangle. cordierite- sapphirine-mullitc, that even a composition which falls in
the primary field of corundum, will not contain any corundum below melting range, but cordierite, sapphirine and mullite. Theoretical calculations are shown in table 3. 6. Cal:ulation of phases from the Mg0-A1235-Si02 diagram, as presented in table 3. 6, shows that the amounts of mullite in compositions having 7% and 5% MgO, under 36. equilibrium conditions at 1500°C, are higher than those below melting range, ice., below 1460°C. Further, spinel is seen to appear in place of sapphirine and cordierite. Now, as mullite should normally decompose more, the higher the temperature of these reactions, the following explanation may be put forward.
.0 It may be possible that the melting range of the compositions belonging to the corundum primary field does not start until 1578°C, which is the periteetic temperature of spinel, corundum and mullite. In this case, a temperature of 1500°C is below the melting range, and the phases present, in equilibrium, would therefore, be the same as in the case of 'below the melting range!.
2) As an alternative explanation, it may be said that sapphirine melts incongruenUy at about 1475°C, and spinel is the solid phase formed by this reaction. In the temperature range between 1450° and 1475°C (at about 14600), the reaction (3.12, 3.13).
sa:pphirine + corundum spinel + mullite. takes plue; at higher temperature, the association of the left side is incompatible, and at lower temperature, that on the right. Further, cordierite also melts incongruently giving mullite below its melting point. Thus, the amount 37.
of mullite, though lower below melting range of the compositions mentioned above, may be higher at a little higher temperature due to the decomposition of sapphirine and cordierite. This observation will not hold good, however, at considerably higher temperatures when mullite, spinel, sapphirine, cordLcrite all will disappear leaving only corundum and liquid as the phases in equilibrium 38.
TABLE 3.1 Calculation of Phases from Bowen and Schairerfs - SiO Diagram. Na20 - Al203 2
n. Reaction Composit Reaction Temp o C % Phases Predicted. % Mullite % Na20 Mullite Corundum Albite Glass
95.1 4.9 Below melting 0.0 60.7 39.3 0,0 range, i.e., below 1104.
96.0 4.0 (a)bellw melting 18.6 53.3 28.1 000 rang-.1 (b)aLove melting Same as in table 3.3 range
97.0 3.0 (a)below melting 40,0 40,3 19.7 0.0 range (b)above melting Same as in table 3.3 range
98.0 2,0 (a4 below melting 58.1 1 26,1 1 15.8 0.0 range 0) above melting Same as 4..n table 3,3 range. 39. TABIE 3.2 Calculation of Phases with reference to Nepheline Mullite - Corundum triangle (supposed) Reaction Com . ition Reaction % Phases predicted °o Temperature Mullite % Na2 o C Corun- Nephe- Mullite dum line. Glass
87.3 12.7 Below melting 0.0 42.0 58.0 0.0 range 91.0 9.0 (a)below melting 28.0 30.0 42.0 0.0 range (b)above melting 0.0 48.7 0.0 51.3 range at 1400° (a)below melting 43.8 23.6 32.6 0.0 93.0 7.0 range. (b)above melting Same as in table 3.3 range. I (a)below melting 60.8 I 16..4 22.8 1 0.0 95.1 4.9 range. (b)above melting Same as in Table 3.3 _singe. 1 (a)below melting 68.8 i 12.5 1 18.7 1 0.0 96.0 4.0 range (b)above melting Same .as in Table range. 3.3 1 • i (a)below melting 76.8 19.1 I 14.1 I 0.0 97.0 3.0 range 1 (b)above melting range. Same as in Table 3.3
(a)below melting 84,3 1 6.3 1 9.4 I 0.0 98,0 2.0 range. (b)above melting range. Same as in Table 3.3 i 40 .
TABLE 3.3 Calculation of Phases with reference to the triangle, Nepheline s.s. - Mullite - Corundum (supposed) Reaction Composition Reaction % Phases predicted. Temperature Corun- Nephe- o Mullite Glass % Mullite % Na20 C dum line.
Below melting 0.0 50.6 49.4 0.0 91.0 9.0 range.
(a)below melting 38.7 38.7 0.0 93.0 7.0 range. 22.6 (b)above melting 0.0 0.0 47.1 range - 1400 52.9
(a)below melting 27.4 26.4 0.0 95.1 4.9 range. 46,2 (b)at 1400 0.0 58.1 0.0 41.9
(a)below melting 21.4 22.4 0.0 96.0 4.0 rang?. 56.2 (b)al; :0..400 10.3 50.7 0.0 39.0 (c)at 1500 7.9 51.3 0.0 40.8 (d)at 16C0 0.0 56.6 0.0 43.4
97.0 3.0 (a)below melting 67.5 16.2 16,3 0.0 range (b)at 1400 32.3 39.4 0.0 28.3 (c)at 1500 30.5 39.1 0.0 30.4 (d)at 1600 26.7 41.4 0.0 31.9
(a)below melting 11.9 0.0 98.0 2.0 range. 78.0 10.1 (b)at 1400 56.4 24.9 0.0 18.7 (c)at 1500 55.3 24.0 0.0 20.7 (d)at 1600 5103 26.1 0.0 22.6
La.
TABLE 3.4
Calculation of Phases from Li 20-Al203-Si02 Diagram.
Reaction Composition Reaction % Phases predicted Temperature 11 % Mullite % Li 0 oC Spodu- 2 Mu itel mene Glass1Corun- dum.
96.6 3.4 below melting 0.0 42.0 0.0 1 58.0 range
97.0 3.0 (a)below melt- 12,1 35.6 0.0 52.3 ing range. (b)above melt- Could not be calculated ing range in absence of isotherms.
98.0 2.0 below melt- 41.7 1 2402 0.0 64.1 ing range. above melt- Could not be calculated ing range. in absence of isotherms.
99.0 1.0 below melt- 70.0 1 11.9 0.0 118.1 ing range. above melt- Could not be calculated ing range in absence of isotherms. 42.
TABLE 3.5
Calculation of phases from Ca0-A1203-Si02 Diagram.
% Phases predicted. Reaction Reaction Composition Temperature oC Mullite ICorun- Anor- Glass % Mullite % CaO. dum thite. 88.3 11.7 below melting 0.0 42.2 57.8 0.0 range, i.e., below 1512°C
93.0 7.0 (a)below melting 39.4 25.6 35.0 0.0 range (b)at 1600°C 6.8 4101 0.0 52.1
95.0 5.0 (a) 'below melting 57.7 17.6 24.7 0.0 range (b) at 16000 33.8 29.6 0.0 36.6 43.
TABLE 3.6
Calculation of phases from Mgo-A1203-Si02 diagram.
Reaction Reaction i % Phases Predicted. 1 Composition Temp. (Cordi- Sapph- Spinel Corun- % Mullite %Mg0 °C iMullite erite. irine. dum. Glass
80.7 19.3 below melting 0.0 21.1 78.9 0.0 0.0 0.0 range
93.0 7.0 (a)below 62.8 10.1 27.1 0.0 0.0 0.0 melting range (b)at69.0 0.0 0.0 17.0 0.0 14.0 1500° (c)at 16.0 0.0 0.0 0.0 54.0 50.0 1600° 95,0 5.0 (a)below 73.1 9.0 17.9 0.0 0.0 0.0 melting range. (b)at 0 77.5 0.0 0.0 12.5 0.0 10.0 150o- (c)at 4o.o o.o 0.0 0.0 22.7 37.3 1600°
44
X
Fig. 3.1 Illustration Fig. 3.2 Application
Of Crystallisation Path. Of The Lever Principle
In A Ternary System.
Na20
Fig. 3.3 The Inferred And Supposed Compatibility Triangles Of The System, NE120sA120304102.
45
ATrOrS10.,,— NaTifFeT-Tat.-0147 (I. J. F. Schairer and N. L. Bowen, 'The System Nae0-A1:01-SiOs," Am. J. Sri., 251, 129.9611Y861. Liberto De Fablo-Calan and Wilfred R. Foster, "Investigation of Rola of Beta Alumina in the System Nas0-AlsOrSiOs." J. Am. Ceram. See., 491.98 (1959).
\ \ , \
\ • CORUNDUM .S
\
\ , \‘' Nau Atv2S:O. 1+'•\;h. , %.\ . .s. k‘. • , _ ..+. 1, It. N. 'Sp , \\.\ k\:,.. , I , \ ‘ . ‘ ‘.•\1.„ \\ • k \ 1 N . .::„,,;:•, `s,‘, \ • \. • 4. \." • \ \ 1 \ \ I 1‘. \ I. ik i .„`• 30 1 \ 1. . ' • • 3A12032S102 . \ • \ . •••., "JIM ..1. •1' . ' • . .. \ !I.•••1 ... \ • • \ N. .1. '1 • • •\ •:____:,,'.1'. \ t••1, . , ' \ s, . ,„ , • • \ • • A ' Ai . __.% .... , 0+11140 * * --"k. • \ • . ... ,,,,,,,..: •1'. .4.• ‘ ., \ - , , ... ---.1,----- or -A----'- 20 . .k. /...112,, . \ BOA-AL,UM(NA ••\•\ •‘\ • ... \,...\\ • "a .. .1 . ' ' •r . \ . •S‘...\ ‘N .' C.- • ... . • • .% " \ • • • • • •
.1. ;• 1 ..... •,1:. . . , \'Z\A \ 10 .... .t. . \ / NeS \ • ‘, • . 1 .1. \ . \ 7 \ • ,..;
/ •••\• '— At, ,s 1 . , • /' ...... \ .11 \ ' , • + . \ \ 'I' v , ' 0 70 ° Na20.A1203 80 90 nucerpancrarr iNa20.11A1203 A1203
0-Al 0 -Si0 diagram Fig. 3.4 Section of Ea2 2 3 2 ( Ref. 3.9 ) 46 Si02
Li20.2 SiO 420.A1203.8 Si02(13)
Li20. 8102 420.AV03.4 Si02(s)
2 Li20.SiO Li20.A1203.2 Si02(E)
3 A1203.2 Si02
Li20 A1203
Fig. 3.5 The System, 1420-.0203-Si02 ( Roy And Osborn. Ref. 3.14 )
8102
Li20.A1203.4 Si02
M 3A1203.2 Si02
1420 A1203 Fig. 3.6 The Supposed Compatibility Triangle, Mullite - Corundum - Spodumene. 47
____:.:4.- • _____A 1 \ 1 1 1 1512 \ ...... 1, , 1
1\ • ' \ \1 \ 1 \ X
3A1203.2Si02 .•.+1850
1510 20
10
"..1595 70 550 14°0 60 80"'"" Ca0.7A1203 CaO.A1203 Ca0217A501203 Ca0.6A1203 A1203 1155 ••••1605 li'ERCENT
Section of Ca0-Al 0 -SiO diagram Pig. 3.7 2 3 2
( Ref. 3.9 )
48
1 a. z. AMA a. Y. acrsanmr. ane OIZOMITTIVICI 171317prnnorm MW oys•ern ‘ 1 Kg0-A1201-13103„" J. Geol., go, 181-88 (1952). 1 t 1 \ \ Shigeo Aramaki and Rust= Roy, "The Mullite-Corundum Boundary in the % M ULLIVE 1 1 System Ng end CaO-Me03-SiOs," J. Am. Ceram. Soc., 41. A \ Rt 11 ,tA \ 844-45 (1969). 1 A % 1 1 \ \ W. Schreyer and .1. F:Seltairer "Anhydrous Cordierites and'the System lig0- , . A 1 AlsOrSIOL" .1. of Petrology (In prises). 1 1 --- .:-46 \ 1 214130.2A1203.5 kti 02 \ 1 \ 's,.. \ , 1 I.% 1 \ .%,...,` 1 'k,. 145 I .PHIRINE \\` \\ 111 41! \.. % \A A ,\ +.?,. \ \ 1 1 \ \ ‘ \ \ '''''`,.• \ \ ••• I \ \ \\A1576, "ks,,,,., \ \ ____ _ 1 __ ...... \ 1 \ gyp` •"1,. 1 ...... N.;.:. \ '1-•-•‘ \ .., ‘''`i.,, \ '''• "...... : I \ ,_ 40 \ \' 11\ .., 1 -:....., A A1 1 A \A A \ ." ------..„. 1 , AA \ / N...... •ZN..:,....., 1\‘ \ X11 , % \ / ..-,.. ..., I ***.• .•s‘..N. A ..4c-....A A \ 41 ...• ‘ ‘ IA\ cs, \ i WII ‘ \ % ••,....\ / N. ‘s*,..
i • •••\ - • \ \ .,...... :„ •,-;,`,,, ‘ 1 --\-- , \\'\,.. ._ '- i N 30 ,\ A \\...... \ ,,./ , ‘. 3A1203.2S102 111 11 • •••• ' , 1 \ f \ 1 ' \ \\ '''‘,\/ Y \ ...11150 ----4, t s. 1 • ...\.... II t / •. ..-./..e \I -s• i , ,,,,,,,, . . i .I. .., 1`,. \\ \ • %-....i,, ,... / SPitIEL \ . ‘ • % .... ,,. ciaily UM ...... ‘ \ .....- / / 4840 ‘ 1 A , . '... ./ . ' \..eX r • ....)e..,... . i. 20 • • . 1\ ‘ ,, %\ .,.....,._,,_ ...... „__ , ,.. ., \ - / . r • \ 'T--- v ,••• • / ... - / •I • , • *, -./. .i. \ \ -*< . I ' • . . / .`,N' I. 4VgO.DA1203.2610..... , i . . 4 . A IA ' **•• • • • • Z:••,,' I A A 1 .1 / '''... . \ 1 - - i L \ • / • • ''•••.,: / • . 1 1 .. • ...... ,, j . \Ar___._A__•• , I0 A 1. 1 i / . I A . . At . I. // .1. \ ... / ... - ,... \ • • A • 'A I '/ . \ • ' .. •f• - - I - \•.... I/ . \ . . . f/ . . t • --...'e\SI • C-1- ...... 1r1`....1 . • A • \ • I • • 4 • - .1. /‘ _I ....'....,\ . s .1 . . ../ . A. A / 1 1 •••4,._ j I • • • ,,,,, • • •. • • -,,.. . • • • • -1.• • . .• -- .- \ . \t • • • N.),. • • i • ' — '''• k • g - • - \ /.' I 4.42020 1850 60 70 80 90 MgO.A1203 ^44925 A1203 •••-• 2135 •
0 -SiO diagram Fig. .2 Section of Mg0-Al2 3 2 ( Ref. 3.9 ) 49.
4 - Experimental Technique
4.1 Introduction to the determination of high-temperature Phase equilibria.
Numerous investigations 'lye been carried out on the determination of phase equilibria in the past, and full description of different experimental methods are available in the literature, Kingery (4.1) has, however, given a comprehensive review of various techniques of the measurement of high-temperature equilibria.
Of the two generally classified methods of such investigations, the dynamic ones are most satisfactorily used in systems where equilibrium is reached quickly, such as, in systems of metallic components. These dynamic methods employ changes ill properties of a system as the system is cooled or heated (or pressure changed) so that new phases appear. The thermal and differential thermal analysis are two of such widely used methods.
In systems, where equilibrium is sluggish because of high viscosity in the liquids or of low "crystallisation
Potential" of the solids, the ciynamic methods are unusable and it is necessary bo resort to the static methods. The sLtic methods employ the technique of holding a sample under specific conditions until equilibrium is reached and then determining the number and composition of the phases present 50. by measurement at the temperature (as with high-temperature x-ray or hot-wire microscope) or by quenching the system so rapidly that the high-temperature phases are maintained for examination at room temperature.
The most useful of the static methods is the quenching method. since it permits the use of small samples and the workability at room temperature. This procedure was developed for use in the silicate systems at the Geophysical Laboratory
(4,2). A small sample of homogeneous character having a known composition within the system under consideration is enclosed in a suitable container and heated at the desired temperature until equilibrium is attained The sample is then rapidly quenched from that temperature by dropping it instantaneously into a cold liquid; the equilibrium conditions prevailing at the temperature of emperiment are thus arrested.
Solid phases present at high temperatures are probably retained metastably at ordinary temperatures, and the phases which are liquids at high temperatures exist as glasses at low temperatures. Phases are then identified by the use of microscopy and/or by X-ray diffraction technique. 51.
4.2. Method of investigation on the dissociation of mullite. JIL
Dissociation of mullite in presence of different additives, like Li20, Na20, Mg0 and CaO, is, in fact, the process of changing the mineralogical composition and the physical properties of the material. These changes are obviously brought about by different chemical reactions under different experimental conditions. Thus, in order to obtain consistent results in the progress of reactions, it was decided to use, as reaction-charges, small pellets made by pressing intimate mixtures of finely ground mullite powder and each additive. The usual procedure of heating the samples and studying the reaction-products was then followed.
As the present investigation concerns the silicate systems only, where the attainment of equilibrium is rather slow, the quenching method of studying phase equilibria was adopted. On the consideration of the fact that recrystalli- sation or precipitation of phases in reaction-products of such systems is not so fast, only air-quenching was given to the heated samples. The extent of the dissociation of mullite was follo-Jed from the measurement of different phases, including any unreacted mullite, in each reaction-product. Identification of crystalline phases was done by Debye-Scher:er's X-ray photographic technique. Chemical method of analysis was used for the determination of the glassy phase, and for the 52.
determination of crystalline phases including any unreacted mullite, both the x-ray and the chemical methods of analysis were conjointly employed, wherever possible.
4.3. Measurement of different phases in the decompostion
products of mullite.
To ensure proper identification and precise determin- ation of the constituents of the material under investigation, it is advisable to employ more than one analytical technique.
Many authors have preferentially used either mixoscopy or the X-ray diffraction method cr both for detecting and measuring the amount of different phases in ceramic materials.
On the other hand, chemical method of allalysis has been widely used as on3 of the means of determining different phases°
The choice of analytical methods depend to a great ext3nt on the kind of reaction and the type of materials to be studied. In the present investigation, sampl,,sfired at higher temperatures were very hard, and it was rather difficult to make thin sections or polished-sections for petrographic analysis. On the other hand, samples fired at lover temperatures appeared in rather finely divided fog, where the application of microscopy would be unsuitable°.
Gad (101) has shown, however, that petrographic methods could 53. not be used for analysing the decomposition products of mullite due to the very small particle size of the crystalline phases and also due to the coating of crystals by the glassy phase. X-ray diffraction technique was thus preferably employed for identification and determination of crystalline phases in the present studies.
Some of the earlier investigators in this field, who employed X-ray diffraction techniques as the only means of analysis, ignored the presence of the glassy phase, and only determined the percentage amount of crystalline phases, e.g., corundum farmed and the unreacted mulliteo In the present inliestigation, however, attempt has been made to determine both the glar_sy phase and the crystalline materials in the decomposition products of mullite. The methods are described belowo
4,5.1. Determination of the glassy 2haseo
Different investigators have used different mathods for the determination of the glassy phase in ceramic materials, the most widely used of which is probably the microscopy (4.3-4.8). Several authors (4.9, 4.10) have also tried to use the X-ray diffraction technique for such purposes.
These processes seem to be rather tedious and round-about, particularly in the case of the X-ray photographic method,. 54.
In recent years, however, many .attempts have been made to use chemical means of selective dissolution of the glassy phase in ceramic materials. Quite a number of years ago, Line and Aradine (4.11) devised a method for the deter- mination of glass by using hydrofluoboric acid; Telvitie (4.12) advocated the phosphoric acid method. Unfortunately neither of these methods found satisfactory use for their insufficient selectivity and diminishing rate of disolution of some forms of giass (4013, 4.14). Demidova and Goucharov (4015) used
20% hydrofluoric acid for determining the glass-contents of alumino-silicate refractories and checked this method by
X-ray diffraction technique, but it was found to be of inadequate accuracy.
Konopicky and K8hler (4.16) made a comprehensive study of the chemica) attacK of fire-clay refractories and other ceramic products by hydrofluoric acid of different concentrations fo..- various periods of time. They observed in their experiments +that the glassy phase was best separated by treatment of the fired samples with 1% hydrofluoric acid solution at 0°C foi two hours. As the acid. of higher concentration attacked some of the crystalline phases including mulUte, they calibrated the method and fixed the variables as follows: amount of sample taken = 0.5 gm.; quantity of acid taken = 200 cc. of 1% hydrofluoric acid 55.
solution; and time allowed for reaction = two hours.
Amin Taha (4.17) found the method quite satisfactory but the calibration performed by Konopicky, et. al, was not acceptable to him; he had, therefore, his own calibration of the method which was as follows: amount of sample taken =
0.5 gm.; quantity of acid = 400 cc. of 1% hydrofluoric acid solution; time allowed for reaction = 4 hours; particle size of the sample = kolo mesh, B.S.S. The method, thus, appears to be quite encouraging, and it was decided to use this technique for the determination of the glassy phase in the present investigation. It was, however, found necessary to conduct an independent calibration of the mthod, keeping in mind, the practical difficulty of having samples of particle sizes finer than -300 mesh, B.S.S., and also about the eccnomy of time. Samples used for standardizing the method wero:
(a) Na0-Ca0 SiO , glass,
(b) Ca0 - A120 SiO glass: 3 - 2 (c)Sapphire single crystal, gruund and sieved through
300 mesh sieve, B.S.S.,
(d)commercial mullite (by courtesy of Cawood, Wharton and Co).
(e) Pure mullite, 3A1203.2Ei02e (f) Mullite of 2A1 0 2 3.Si02 composition. (g) Mullite of sillimanite composition. 56.
4.3.1,1 Calibration of the method of determination
of the glassy phase.
All the materials to be studied were ground and sieved
through 300 mesh sieve, B.S.S., dried and kept in a desiccator.
Analar hydrofluoric acid (40g) was used for making 1% solution,
and also, analar sulphuric acid was used for making N/10 solution.
Samples for analysis were accurately weighed into polythene
beakers and treated with required quantities of 15 hydrofluoric acid
solution. Sufficient time was allowed for reaction, the contents
being shaken at times in order to keep uniformity in concentration
of the acid. After the specific reaction period, the solution was
diluted with N/10 sulphuric acid solution and filtered through
special type of porcelain crucibles, The crucibles and the residues
were washed well with distilled water, dried and heated at about
8000C, cooled and weighed. The difference between the amounts of
samples taken and the residues was, after conversion into percentage, reported• as leachable materials (or glass at the temperature of
the formation of glass).
In order to fix the variables of the method, a few
preliminary experiments were carried out to see if there was any difference in dissolution at room temperature and at 00C. The results obtained at both of these temperatures, which are shown in table 4.1, were quite the same, but as it was found convenient to 5'? . carry out experiments at room temperature, all the measurements of the glassy phase were made at room temperature
TABLE 4.1
Applicability of the dissolution technique at
different working temperatures _ . Kind of sample Amount Quantity Time Temp. studied of of of of disso- Sample 1% RF Reaction Reaction lution. gm, cc/gm hours
Na C-Ca0Si0 0.25 800 2 0°C 72,4 2 2 glass. 0,25 800 2 room 72.5
0,25 800 2 0°C 71.9 Ca0-Al203-Si02 glass 0.25 800 2 room 71,8
Reaction product o ,/ 0,25 800 2 0 C of mullite and 470 34.0 °C Na00 at 1400 0.25 800 2 room 34,0 under equilibrium conditions 0.25 800 24 room 33.8
Now keeping the particle sizes of the materials fixed, i.e., - 300 mesh, the extent of dissolution by 1% hydrofluoric acid solution was studied with respect to: 58.
(i)varying amounts of samples.
(ii)varying quantities of TA HF-solution. and (iii) different periods of treatment. Thus, starting with the amount of sample and the quantity of acid, used by Amin Taha, under optimum conditiona, experiments were carried out to find suitable conditions, for the present investigation, under which complete dissolution of the glass could be obtained.
Variables of the method were consequently fixed as follows:
Amount of sample taken 0.125 gm. Quantity of acid used R 100 cc. (i.e. 800 cc/gm.),
Time of reaction allowed = 2 hours.
Particle size of samples = 300 mesh.
Reaction temperature Room temperature.
4.3e1.2. Reslts and discussion. Results of the dissolution of Na 0 - CaO - SW glass by 2 2 Vo hydrofluoric acid solution under varying conditions are shown in figures 4,1, 4,2, 4.3. The efficiency of the method was tested on CaO - A1203 - Si02 glass, as well as, on some of the experimental samles, and tlie results are shown in figure 4.3 and in table 4.2 respeatively. Several experimens were also performed to see the
effect of i% hydrofluoric acid solution on standard crystalline
materials under similar conditions. The results are given in Table 4.3. 59.
It is obvious from the results shown in table 4.3 that neither corundum nor mullite is attacked by 1% hydrofluoric acid solution under these conditions. The amounts of dissolution in the cases of different types of mullite are attributed to the amounts of the glassy phase, sapposedly present in each sample, and not to the crystalline material itself. This conclusion was arrived at from further treatment of these materials by i% hydrofluoric acid solution for longer periods of reaction (results of the treatment of a few samples for 24 hours have been given along with other relevant data in tables 4.2 and 4.3), when no more dissolution was observed. It is alsu noticed that neither Na 0 - Ca0 - SiO glass 2 9 nor CaO - Al20 - SiO glass showed 10 dissolution; this is 3 2 probably because they contained a little insoluble crystalline material that did not enter into the formation of glass. 6o.
TABLE 4.2
Application of the dissolution technique on experimental samples.
Kind cf sample Amount Quantity Time of studied of of Reaction disso- sample 210 HF, hours lution. gm. cc/gm.
Mixture of 66% 0.125 800 2 31.4 mullite (3:2) and 32% Na 0 -Ca° -S1.02 2 0.125 800 24 31.5 glass.
Reaction-product 0.125 400 2 34.0 cf mullite (3:2) and 0 Na20 at 0.125 400 24 33.9 1400°C, under ecluilibrium 0,125 800 2 34.0 conditions. 0.125 800 24 34.0.
TABLE 4.3 Effect of 110 HF solution on crystalline materials
Kind of Amount Quantity Time material of of of disso- Material acid Reaction lntion, gm. cc/gm. hours Corundum 0.125 800 2 0.0 Mullite (3:2) 0.125 800 2 0.4 Mullite (3:2) 0.125 800 24 0.4 Mullite (2:1) 0.125 800 2 0.2 Mullite (1:1) 0,125 800 2 10.1 Mullite (1:1) 0,125 800 24 10,1 Mullite (commercial) 0.125 800 2 803 Mullite (commercial) 0,125 800 24 803 ... 61.
Accuracy of the method.
There are several obvious factors on which the accuracy of the results would depend. Some error would necessarily be introduced as a consequence of the loss in weight of the filtering crucibles due to their being attacked by the hydrofluoric acid solution during filtration. Although the error under these conditions did not exceed Leo,. it was, howevers eliminated by taking into consideration the difference in weights of the filtering crucibles plus the residues and of the properly cleaned empty crucibles after filtration. This gave the actual amounts of the glassy phase in different samples.
Different variables, e,g,, time allowed for reaction, quantity of acid and the aaount of sample taken, variation in the concentration of the hydrcfluoric arid solution, all have their individual part to play and thus some error could possibly be added, -)rovideu tha- calibration of the method is fairly maintained, such errors would be negligible.
Another obvious error would be due to the personal factor, i.e., the manipulation. In order to assess this error, a few experiments were ca2ried out in triplicate but tho results, as shown in table 4„42 were so close as to be within ± Ov2. 62.
TABLE 4.4 Accuracy of the dissolution technique
Kind of sample Amount Quantity Time Number 0 studied. of of of of disso- sample 10 HF„ Reaction, Expts. lution. gm. cc/gm. Hours.
Nato - 800 2 1 Ca0 - SiO2 0.125 97.8 glass, 0.125 800 2 2 97.8 0.125 800 2 3 97.7
Reaction product of 0.125 800 2 1 42.3 mullite and 4.9% o 2 2 42.2 Na20 at 14C0 C under 0.125 800 equilibrium conditions. 0.125 800 2 3 42.4
Reaction product of 0,125 800 2 1 25.7 mullite and 4,9% Na20 at 1200°C 0,125 800 2 2" 25.8 under equilibrium • conditions. 0,125 800 2 3 25.7 63.
4,3,2, X-ray method of quantitative estimation of
crystalline phases.
Extensive studies have been made by various investigators on the quantitative assessment of different phases in clays, fire-clay refractories and other alumino-silicate materials by the use of
X-ray diffraction techniques (4.9, 4.10, 4,18- 4.22). Principles of the X-ray methoa.s and their application have been described in detail by various authors and reference may be made to their standard works (4.23 - 4.26).
For the application of the X-ray technique in quantitative analysis, use has been made of the fact that the intensity of an
X-ray line, which is characteristic of the particUlar substance under examination, is proportional to tne quantity of the material present (4,23), This of course depends on several factors like the cluali'6y off' the X-ray film2 conditions of the preparation of the sample, photographic developing processes, etc. Provided these experimental variables are kept fairly constant and the method of analysis is properly calibrated, measurement of the intensity of a particular X-ray line would give the amount of the material taken. In order to overcome the 4.nterfering factors, however, several Investigators have advocated the use of "internal
Ltandards" like NH c1 NaCl,etc. (4,10; 4.27 — 4029). Johnson 4 ' and Andrews (4.9), in their method of the quantitative determination 61+.
of mullite, corundum and the silica phases by X-ray diffraction procedure, found that the use of internal standard gave much scattering and that direct measurement of intensities for the respective phases gave much better results. They, of course, calibrated their method with known quantities of the relevant materials
Moore and Heeley (4.10) used Debye-Scherrer photographic technique for determining corundum and mullite in .alumino-silicate refractories. They calibrated the method by plotting ratios of densities of the X-ray lines for corundum and mullite against their percentage amounts in standard mixtures.
In thL: present investigation, as well, the application of the X-ay photographic proced'ne was made, but the ratios of intensities instead of densities of the characteristic X-ray lines were, however, used. as a measure of the amounts of materials presents As the use of the X--ray technique necessitated calibration of the metho:L with known quantities of standard materials, it was employed for the Quantitative determination of oefundum and mullite only. Due to difficulty in obtaining standard samples of sapphirine, cordierite, (spinel), nephelina, spodumene and anorthite, calibration of the X-ray method fJr the determination of individual minerals was not possible. Moreover, such determinations would require setting up a large number of calibration curves for each of these substances, using known quantities of either or lioth of the standard materials of corundum and mullite, in order to determine the solid phases C5.
present in a system. Considering all these factors, advantage was
taken of the chemical method of analysis as a supplementary technique,
where possible. For the determination of corundum and mullite,
however, the X-ray technique was calibrated with known quantities of
the two materials as described below.
4,302.1. Calibration of the X-ray method of analysis.
Pure samplea of mullite (3:2)7 prepared in the laboratory,
and corundum (sapphire single crystal) were finely ground to below
350 mesh, and P. series of standard mixtures were prepared in Which
the amounts of corundum and mullite were varied from 0 to 10 0 in
incretents of 5%. The two materials were weighed out with sufficient
accuracy to give uniform mixtures of the intended corundum:mullite ratios. X-ray patterns were obtained of all mixtures and the
intensities of salteble lines given by corundum and mullite were
then me as urerl . The ratios of intensities, thus obtained; were
plotted against the quantities of ,orundum and mullite and a calibration curve was drawn. This is ghoWn in figure 4,4,
In determining thie amounts of cor,indum and mullite in
experimental samples, intensity ratios of the same two lines in the diffraction patterns given by various reaction-products were determined and by referring to the calibration curve, the proportions
of corundum and mullite were obtained.
X-ray powder photographs were prepared by using a crystal- lographic X-ray unit of R4MAX-100. Debye Scherrer 9 cm. cameras 66. and Cu-Kararliation at a filament current of 15 milliamps and a voltage of 40 Kv. were used; the exposure time given in each case was 45 minutes. Only one kind of film, ILFORD Industrial G, was always used. The techniques for preparing the powder specimens
(by using gum tragacanth) and for developing and fixing the photo- graphic films and other experimental variables were standardised as closely as possible.
The lines chosen for the quantitative measurements of the amounts of corundum and mullite were those corresponding to the lattice spacings of d = 1,37 2 and d = 2.20 RI respectively. These lines were quite of high intensity and not masked by adjacent lines.
4.3.2.2, Measurement of intensities of the X-ray lines. Although some authors are reported to have made visual measurement of intensities of the X-ray lines in powder photographs taken of experimental samples, by comparison with photographs of standard samples, such procedures would probably introduce an error of about 10-15% (4.23). In cases where much accaracy is desirable, it is, hcwever, necessary to mea sure the intensities by micro- photometers. With a superior type of photometers, even a small quantity of crystalline materials can be determinel.
In the present investiation, therefore, all the X-ray patterns were photometered by a spectro-photometer (Joice-' Loebl double beam recording microdensitometer, mark 3), when the intensity- 67. peaks corresponding to the X-ray lines of the photographs were automa- tically recorded on a chart; a sample of these microphotemeter readings is shown in figure 4.6. As very sharp narrow and vertical peaks were obtained in each case, and as, also, all the
X-ray patterns were made under similar experimental conditions, it was decided to mea sure the maximum (height) intensities of the peaks instead of the integrated intensities. Any small amount of probable error arising out of the use of height intensities would possib7.y 'be eliminated by taking the ratio of the intensities. Due to certain obvious disadvantages of the cutting out of the peaks from the charts and weighing them, the idea of measuring the integrated intensities was given up.
Cullity (4.26) has, however, shown in his determination of the quartz-content of dusts that satisfactory accuracy was obtained by simply measuring the maximum intensities rather than the inte- grated intensities of the X-ray lines. He maintained that, as long as all the patterns are made under identical experimental conditions, a constant proportionality exists between the maximum and the integrated intensities, and as such, the measurement of maximum intensities gives satisfactory results. Klug and Alexandar
(4.24), have also shown their preference for the measurement of height intensities rather than for the integrated intensities. 68.
4,3.2.3. Accuracy of the method
Factors on which the accuracy of the method would depend are:
a)Particle sizes of the samples and the technique of the preparation of specimens. The particle sizes in all cases were however standardised as below 350 mesh, and the specimens were prepared under similar conditions (using the same amount of gum tragacanth), keeping the length and thickness of specimens almost the same in each case. Consequently, any error due to such factors would be negligible.
b)Photographic evaluation. In all experiments, the same kind of X-ray films (ILFORD - 'Industrial G' ) and identical developing processes were used. Any error arising out of slight variation in these conditions would, however, be eliminated by taking the ratio of intensities of.the selected X-ray lines.
c)Voltage and milliamps of the X-ray unit, Care was taken to keep these factors fairly constant. In this case, as well, any error would, possibly, be eliminated by taking the ratio of intensities.
d) Intensity measurements. This is probably the most obvious source of error. Error varies from method to method; thus, in the case of visual estimation, about 15% error may be introduced.
Different types of microphOtometer offer different amounts of accuracy. In the present studies, however, a micro-densitometer 69.
fitted with automatic recording device (Joice-Loebl double beam recording micro-densitometer, mark 3) was used, and in order to test its accuracy, one particular photograph was photometered several times (the results being given in table 4.5 below), when the same ratio of intensities was obtained in each case.
TABLE 4.5 Determination of the accuracy of microphotometer readings
Kind of sample Intensity Intensity Ratio of % amounts studied of of Intensities determined from Corundum Mullite calibration lino at line at Ic/Im curve a . 1,30. I d = 2.2 R T c Im Corundum Mullite
Standard (1)10.3 1.6 6,44 84.5 15.5 mixture of corundum = 85% (2)10,33 1,6 6.45 84.5 15.5 & mullite =15%
Decomposition produot of (1)23.6 6.2 3.8 76 24 mullite and 4% Na20 at 14000C. under (2)23.4 6.16 3.8 76 24 equilibrium conditions.
e) Overall error due to arwunwscranclationia experimental conditions. In order to assess this error, a few samples both of known and unknown compositions were photographed in duplicate; on 70.
photometering these photographs, absolute intensities were found to vary to some extent though their ratios agreed to within .t 0.2% which is
small enough to be neglected. 4.3.3. Chemical method of the determination of crystalline phases.
The principle utilised in the application of the chemical
method of dstermining the crystalline phases, in the present studies
has been to determine silica-contents of the residues of reaction
produces, after the removal of the glassy phase by treatment with to
hydrofluoric acid solution„ Due to certain practical difficulties,
complete chemical analysis could .tot be conducted, and thus, only
the estimation of silica in residues was performed by standard
procedure (4.30),
Each reaction product was first treated with IS hydrofluoric
acid solution to dissylve out the leachable material, and the residue
was accurately weighed into a pla tinum crucible. Required quantities
of concentrated sulphuric and hydrofluoric acids were added in and
allowed to react at a lower temperature on a sand bath. The
platinum crucible and the contents were then cooled in a desiccator
and weighed to get the difference as the silica-content of the
residue. This method was standardised by using it for the deter-
mination of known silica-contents of standard materials like mullite
samples of different compositions, when the results were found to
agree within ± 0.1 71.
On the basis of the fact that the silica thus determined in the residues of reaction products would not remain in free state in experimental samples and be attributed to the unreacted mullite and/or to any insoluble ternary compound present, calculation of the amounts of different phases was made. In cases, however, where both mullite and some insoluble ternary compound were present, determination of mullite by this procedure was not possible.
Consequently, the X-ray technique was employed as the means of determining crystalline phases in such reaction products.
Procedure of calculating different phas3s under different experiwental conditions are described below under each specific system,
4.3.4. Determjnation of phases in the system
Li,0 - A1203 - Si02.
In the range of compositions with which experiments were carried out in this system, spodumene was the most probable ternary compound below the melting range. The formation of spodumene in experimental samples was identified in the X-ray patterns (4 .31, 4.32) and its amount was determined chemically and checked wherever possibly oz; the
X-ray technique. Spodumene was prepared in the laboratory by heating its composition at about 1300°C for 15 hours, and its 72.
solubility determined by treatment with 1% hydrofluoric acid solution. The solubility was found to be 100%, and in order to confirm this data, X-ray photographs were taken of some of the experimental samples, both before and after their treatment by 1% hydrofluoric acid. It was observed that the characteristic X-ray lines for spodumene, which were present in the pattern taken with the sample before 1% HF-treatment, were missing in the X-ray photograph taken with the residue of the sample. X-ray photographs taken with the reaction-product of the composition, 96.6% mullite and 30.4% Li20 and fired at 1200°C, both before and after the dissolution of spodumene by 1% rydrcfluoric- acid solution are. show:1 in figure 4.6. The photograph of spodumene, prepared in the laboratory, is also shown in the same figure.
Thus, any leachable material obtained from reaction products in this system would be considered as either spodumene or glass depending on the temperature of reaction. If any characteristic lines for spodumene were detected in the X-ray pattern, the leachable material would be accounted for . spodumene, and on the other hand, absence of such ?.ines would let the leachable material be attributed to the glassy phase.
In cases where both spodumene and glass were present, however, their quantitative determination was not possible as both of these phases are soluble in 1% hydrofluoric acid solution. 73 •
Under these circumstances, an approximate amount of spodumene
(and consequently of glass) was determined by comparing the
X-ray patterns of experimental samples with these of corundum
and spodumene. Alternatively, a compromise was brought about
by taking into consideration the presence of the predominant
phase, thus ignaing the other phase appearing only in small
amounts. Such corciusions were arrived at from a close study
of the physical appearance of reaction products, the tempera-
ture of reaction and the X-ray photograph,
After the determination of soluble materials by
treatment with 1% hydrofluoric acid solution, the silica-
content was determined in the rosidile, and the amount of
unreacted mullite, if any, was calculated. The amount, in
percentage, of corundwil was obviously 100 - (% leachable
material % In this system, the chemical method
of analysis could be independently employed for the deter-
mination of different phases, but for identifying the phases,
however, X-ray method must have to be used.
The amounts of corundum and any unreacted mullite
were also determined by X-i'ay analytical procedure as
described before. The total amount of phases in a reaction
product, in percentage, was calculated by taking into account
the amount of the leachable material (either spodumene or
glass) determined chemically. 74.
4.3.5. Determination of phases in the system
Na 0 - A120 - SiO . 2 3 2
In this system, the probable ternary compound that could appear, according to the phase equilibrium diagram, under favourable conditions, is, either,
i) albite, Na20.A1207.6Si02, or
ii) nepheline (or carnegieite), Na20.A1203.23i02.
At no stage, however, was the formation of albite observed, and thus; the only ternary compound thet could probably be fo?med in the range of compositions tried in this system, below the melting range, was Nepheline (or cLrnegieite depending on temperature).
Just as, in the system, I, 20 - A1203 - SiO2, the amount of nepheline was determined by the chemical method of analysis and compared wherever necessary by the X-ray technique. volubility of nepheline was determined by treatment with 1% hydrofluoric acid solution and checked by taking X-ray photographs of some of the experimental samples, both before and after the treatment by 1% hydrofluori acid solution.
Absence of characteristic lines for nepheline in the X-ray pattern obtained from the residue confirmed complete solubility of the material in the solution of the acid used. Relevant
X-ray photographs are shown in figure 4.6. Tile procedure for 75. the determination of different phases in each reaction product was, obviously, therefore, the same as that followed in the system, Li20 - A1203 Si02.
4.3.6 Determination of phases in the system
Mg0 - A1203 - Si02
This system gave rise to a lot of complications, and detailed study in the mullite-region of the diagram was not, therefore, possible. An attempt was, howev,a.., made to simplify the analytical procedure for the determination of phases in reaction products.
According to the relevant compatibility triangle, as discussed in 3°34 before, the probable ternary compounds to appear i.1 the melting range of compositions with which experiments were carried out below melting range are cordierite and sapphirine. Under these conditions, however, the formation of cordierite and spinel in the reaction products was detected in the X-ray patterns. Peobabl:y: sapphirine, on decomposition partially or completely, gave rise to the spinel phase; moreover, due to the unavailability of any ASTM X-ray data for sapphirine, its identification in the X-ray patterns or its differentiation from spinel was not possible°
In order to determine different phases in this system, solubility of the compounds, spinel and cordierite, was 76.
determined by treatment of suitable experimental samples with
1% hydrofluoric acid solution. This was checked by examining
X-ray photographs of the experimental samples, taken both before
and after their treatment by 1% HF solution. As no character-
istic lines for cordierite were detected in the X-ray
patterns taken with residues, cordierite was supposed to be
completely soluble under these conditions; spinel, on the other
hand, remained unattacked. The following procedure for the
determination of phases in reaction products, under different
conditions, was thus adopted.
i) Cases where mullite cordierite and spinel were
identified in the X-ray pattern. Cordierita was determined
by dissolutich in 1% hydrofluoric acid solution, mullite was
calculated from the amount of silica determined in the residue
by heating with concen'Grated sulphuric and hydrofluoric acids,
and spinel by difference from 1004 If no cordierite was
detectea in the X-ray pattern, any leachable material would
be accounted for the glassy phase.
ii) Cases where cordierite, spinel and sapphirine
(but no mullite) were present. Cordieri',e was determined by
treatment with 1% hydrofluoric acid solution, and any silica
determined in the residue has attributed to the sapphirine
phase. The balance from 100 gave th- percentage of bpinul.
In absence of any cordierite and sapphirine, the leachable 77. material would be considered as the glass-content and the difference from 100 as spinel.
iii) Cases where mullite, cordierite and sapphirine were pre3ent. In such a case, quantitative determination of phases could not be made by the means employed in the present studies. As the silica-content of the residue, after the determination of cordierite by 1% HF, was supposed to be present in both the mullite and the sapphirine molecules, their determination was not possible. Fortunately, howeyer, such cases did not arise in the compositions studied, probably becuase sapphirine was decomposed giving rise to the spinel phase.
iv) Cases showing mullite, corundum and glass
(or cordierite).
Glass was determined by 1% hydrofluoric acid solution, and mullite and corundum by the X-ray technique. Mullite could also be determined by calculation from the amount of silica in the residue.
No cases were observ'd where corundum and cordierite
(and mullite) could be present, probably becauso these two substances are not compatible.
v) Reaction products containinE spinel corundum and glass. Exact determination of the amounts of crystalline phases in this case was not possible by the means employed. 78,
By comparison of the X.,-ray patterns given by such reaction products with standard patterns for corundum and spinel, approximate amounts of these two phases were however determined. The amount of glass was, as usual, determined by 1% hydrofluoric acid solution.
4.3.7 Determination of phases in the system,
Ca0 - A120 - SiO . 3 2
In the range of compositions with which experiments
were carried out in this system, the only probable ternary compound was anorthite. As th' X-ray technique could not be employed except for the identification of this compound, anorthite was Prepared in the laboratory by heating its composition at 1300°C for 15 hours, and the applicability of the chemical method was set up for its determination. The solubility of anorthite was determined by treatment with 1% hydrofluoric acid solution and found to be 66.6%, i.e.,
2/3rd. X-ray photographs of this artificially prepared compound. as well as, of some of the experimental samples, taken both before and after their treatment by 1% hydrofluoric acid solution, confirmed the dissolution of the greater part of the anorthite phase.
All attempts to determine silica-contents of the residues of this material were unsuccessful, as the addition 79. of sulphuric acid (and hydrofluoric acid) gave rise to the formation of CaSO4 (by reaction with anorthite, partly left in the residue) with consequent increase in the .weight of the residue. Determination of mullite by chemical means was not, therefore, possible. The following procedure was, however, adopted for the determination of different phases of reaction products in this system.
i)Cases where mullite, corundum and anorthite were present, c.v. below melting ran E2. The amount of the leachable material in each experimental sample, under these conditions, was determined by Treatment with 1% hydrofluoric acid, and this, on multiplication by 3/2, gave the total percentage of anorthite. Amounts of corundum and any unreacted mullite were, however, determined by the X-ray method with the help of the calibration curve already obtained. Percentage amounts of the three phases, in reaction products, below the melting range, were thus obtained.
ii) Cases where corundum, anorthite and liquid were the probable phases. The leachable materials obtained by treatment with 1% hydrofluoric acid solution, in the present case, contained total amount of liquid and 66.6% of anorthite present in the reaction product. Exact determination of these 80. two materials was not, therefore, possible. Such cases were, however, rare and their approximate determination was done either by comparing the Xeray patterns of experimental samples with those of corundum and anorthite, or by ignoring one phase, either anorthite or glass, which was present only in small quantity. This was decided by studying the physical appearance of the reaction products, the temperature of reaction and the X-ray patterns. The amount of corundum was fount. by subtracting the amount of leachable materials from
100.
iii) Cases where corundum, mullite and glass were present. This wa6 a straight-forward case in this systam.
Glass was determined by treatment with 1% hydrofluoric acid solution, and mullite and corundum by using the X-ray technique. Combining these results, the percentage amounts of all the three phases were obtained. In this particular case, mullite could also be determined by chemical means; the silica-content of the residue of each reaction product should obviously be attributed to any unreacted mullite.
Thus, attempts were made to simplify the procedures for the detmination of different phases, in different systems, under different experimental conditions. Results of the application of these methods are presented and discussed in the next chapter under the experiments on the dissociation of mullite. 81.
4.3.80 Overall accuracy of the determination of
different phases.
It has been shown before that errors arising out of the individual methods of the determination of different phases, e.g.,
1) the determination of glass by treatment with 1% hydrofluoric acid solution,
ii) the determination of mullite and corundum by the X-ray technique, and
iii) the determination of mullite and consequently, of corundum by the chemical method, was negligible.
Major errors, however, seemed to have been introduced in the following possible ways and attempts were made to minimise them by careful application of the methods.
(a) In samples, fired at lower temperatures; amounts of crystalline materials determined by the X-ray tochrilque were found to vary a little from those determined chemically. Samples fired at higher temperatures, however, showed very good agreement between the two methods of analysis. Scatter of results in such cases is shown in
Table 4.6. 82.
TABLE 4.6. Comparative results obtained by the two methods of analysis
Reaction Temp. Period % Phases determined in reaction- Composition of of products. Reaction Reaction Unreacted Corundum Nephe-'Glass Mullite Na 0 o Mullite Formed line Chem. c hours. 5;2 X-ray Chem X-ray Chem Chem.
95 5 1200 5 64.o - 11.8 24.2 0.0 95 5 1200 15 62.2 - 12.4 25.4 0.0 95 5 1200 50 65.8 60,,9 8.4 13.3 25.8 0.0 95 5 7200 150 65.8 60.2 8.4 14.o 25.8 0.0 95 5 1400 3 15.8 17.5 47.o 45.3 0.0 37.2 95 5 1400 5 7.1 9.5 52.6 50.2 o.o 40.3 95 5 1400 15 2.3 2.4 55.9 55.8 0.0 41.8 95 5 1400 3o o.0 - 57.7 - 0.0 42.3
Although the maximum variation in results obtained by the two methods of analysis was, in no case, more than 5,
the error was minimised by taking the average of the two sets of results. These average values have been used for the plotting of results. (b) In cases where the determination of any insoluble compound, other than corundum and mullite, in the reaction 83. products, could not be quantitatively made by the means employed, assessment of error was not possible. Such aases, which were found only in the case of the system, Mg0 -
A120 - were, however, rare, and thus, did not affect 3 SiO2' the overall studies on the disscciation of mullite.
84 100 a 90
LL: 80 70 "^
0 60 '
50 - By 40- tion
lu 30-
so 20 Dis 10 -
* 0 ' 0 200 400 600 800 Quantity Of Acid Used cc./gm.
Fig.4.1 Dissolution By 1% HF. Of Soda-Lime-Silica Glass As A Function Of The Quantity Of Acid.
100 90
80
F. 70- H
/0 _
0 60 1
50 By 40, ion t
lu 30 o s 20 Dis
10 * 1 . q.1?5 0 0.1 0.15 0.25 0.375 0.5 Amounts Of Sample Taken ( gm. ) Fig.4.2 Dissolution Of Soda-Lime-Silica Glass As A Funotion Of The Amounts • Of Sample. 85
100
90 U: 80
70 cEl 6o o50 I- A -- Sods"Lite-Silica Glass. 40 B Lime-Alumina-Silica Glass. 30
20
10
0 I
0 1 2 3 4
Time Of Dissolution In Hours.
FIG. 4.3 DISSOLUTION BY 1 % HF. OF :
A i SODA-LIME0-4SILICA GLASS. B LIME-ALUMINA-SILICA GLASS. AS A FUNCTION OF TIME. 86
100
0
HI 10
-P co 0 a) 0
(1-1 0 0 +31.0 0 0111) H
0.1 0 ' 10 20 30 40 50 60 70 80 90 100 Wt. % Corundum 100 90 80 70 60 50 40 20 10 0
Wt. % Mullite
FIG. 4.4 X-ray Calibration Curve 87
. . . . : .... FIG. 4.5 TYPICAL MICROPHOTOMETER TRACE
Fig: 4.6
Plate No. 1 91% mullite + 9% Na20.at 12006C, showing the formation of nepheline 2 residue of sample in plate 1 after HF treatment, showing no nepheline 3 Nepheline 96%-mullite+. 3.4%.T.420 at 1200°C, showing formation Of_spoqmene residue of sample in plate 4 after HF treatment, showing ne'sijodumen6 .6 Spodumene (artificial) 7 88-.3% mullite 11.7% CO at 140000:, showing the forMation of anorthite residue of sample in plata. 7 after HF treatment, showing anorthite partly dissolved 9 Anorthite (artificial) 10 80.7%.mulIite + 19.3% Mg0 at 14000C showing cordierite and saPphirine 11 residue of sample in plate 10 after HF treatment, showing no cordierite but sapPhirine (or spinel) 12 Spinel
89.
5 - Experiments on the dissociation of mullite in presence of different additives
5.1 Materials and .procedures.
5.1.1. Materials
A1203 "Analar" alumina powder supplied by M/S. Hopkin and Williams was used. This material was obtained in a very fine fcrm.
"Silester" supplied by Monsanto Chemicals was SiO2 used as a source of very pure Si02. This substance (ethyl silicate) is easily hydrolysed by dilute ammonium hydroxide.
Finely grcund Loch Aline Sand of superior quality, purified by treatment with hydrochloric acid was also used.
Mullite. For preliminary experiments, a commercial sample of mullite (courtesy by M/S. Cawood 'Wharton & Co. Ltd.) was used. This particular type of mullite (type I) contained various oxides as impurities together with a considerable amount of glass. The chemical analysis and X-ray photograph of the sample are given in table 5.1 and figure 5.1, respectively,
For the main part of the research, a very pure sample of mullite of 3:2 A1203 : Si02 mole ratio was prepared in the laboratory. An intimate mixture of finely ground alumina and silica powders of appropriate composition was pressed into discs 90. of 2" diameter and 1" thick (the calculated amount of "silester" was used as binder). The samples were heated very slowly at lower temperatures first and then kept at 1000°C for six hours. They were finally heated at about 1720°C for 24 hours in a tunnel kiln of M/S. Cawood Wharton & Co. Ltd. Mullite samples of 2:1 and 1:1 compositions were similarly prepared. As the fired samples of mullite were very hard, they were crushed and ground first in a rubber-lined ball mill with steel balls, and repeatedly treated with hydrochloric acid until the chemical analysis showed complete removal of iron. These were then ground more finely in a power-driven agate mortar to pass a 300 mesh sieve. The chemical analysis and X-ray photographs of these materials are shown in Table 5.1 and in Figure 5.1 along with those of the heat-treated samples.
Table 5.1 Chemical analysis of different samples of mullite iype of Mullite % A1203 % 4'1.02 % Free % Total % Glass by wt by wt. A1203 impurity (by 1% HF) (total) (total) (by X-ray) by wt. by wt.
1)Commercial 73.0 25.3 5.0 1.7 8.2 2)3:2 mole ratio 71.9 28.1 0.0 0.0 0.4 3)2:1 mole ratio 77.2 22.8 5.0 0.0 0.2 4)1:1 mole ratio 66.2 33.8 0.0 0.0 10.1 5)1:1 mole ratio 71.8 28.2 0.0 0.0 0.0 (HF-residue) 1 91.
Li20 Reagent quality Li2CO3 supplied by Hopkin &
TAlliams was used as a source of Li2O. (anhydrous) was used as a source Na20 "Analar" Na2CO3 of Na2CC3 . EE2 Although fairly pure sample of Mg0 could be made available, a sample of pure MgCO3 supplied by Hopkin and Williams was preferrably used as a source of MgO.
Ca0 "Analar" CaCO3 was used as a source of Ca0 in all the experiments.
5.1.2. Preparation cf samples.
A21 the materials were finely ground in an agate mortar, o dried at 110 C and kept in a desiccator for use. In order to prepare reaction mixtures, each ingredient was accurately weighed and intimately mixed in an automatic mixer by repeated rolling and stirring with intermediate grinding in agate mortar.
Similar procedure was also followed by Foster et al (5.1) in preference to the usual method of repeated firing and grinding of the reaction mixtures, where loss of alkalies could not be avoided. Approximately one gram sample of the reaction mixture was taken for each experiment. In order to achieve consistent results in the studies of reactions between mullite aad different additives, the powders were pressed into pellets of 1/2" 92e
diameter by using a perspex mould and a pressure of 2000 lbs/
sq. inch. Samples were then kept in a desiccator for at least
24 hours before heating.
54.1,3. Heating of samples
Pellets made of mullite powder and each of the additives
were heated at temperatures ranging from 1000° to 1600°C for
different periods of time depending on individual reaction
specifications. Usually,longer periods of heating at lower
temperatures and comparatively shorter pericds at higher
temperatures were allowed for the reactions until equilibrium
was reached. After the specific reqction periods, reaction-
products ware quenched in air. As tile crystallisation from melts
of the Lystems under invesagation was not very fast, only air-
quenching was given to the heated samples.
All the reactions were carried out under ordinary atmospheric conditions, and the furnaces usee were
a) platinum-wound horizontal electric resistance furnace for working up to 1400°C. This furnace had a 1.1/2" internal + diameter alumina tube an d a temperature dl.stribution of - 2°C over 1.1/2" at 1400°C. It was controlled by a "Sunvic" on-off
mechanism which gave a very steady temperature of 2° C.
b) a horizontal furnace with molybdenum tape wound on an impermeable alumina tube for use up to 1600°C. An inert gas 93 ,, atmosphere was maintained around the molybdenum winding by passing cracked ammonia. This furnace was controlled by a step- down transformer and gave a very stead.; temperature with a temperature distribution of 20C over 3" at 1600°C.
A platinum-platinum 13% Rhodium thermocouple was used for measuring temperatures up to 15000C (only spot readings at
1500C), The thermocouple terminals were connected to a potentiometer which allowed temperature readings with an accuracy of = i0C For measuring higher temperatures, Tinsley optical pyrometer was used with which an accuracy of 50C was obtained.
The furnaces were kept at the desired reaction-temperatures throughout the experiments / the specimens being pushed into position on permeable alumina boats, about 2" long. The time of reaction was taken from the moment the specimens reached the hot zone until the moment they were removed from it.
5c1r4. The mineralogy of reaction-pr.)ducts.
Theleated samples were crushed and ground to pass a 300
mesh sieve for chemical analysis and a 350 mesh sieve for X-ray analysis. X-ray powder photographs were taken with each of these reaction products in order to identify different phases, and
consequently, to determine the extent of dissociation of mullite in presence of different additi7os. quantitative det-irmination
of different phases were then performed by the procedures 94. described in chapter 4, under the experimental technique..
5.2. Experiments on commercial sample of mullite and the
additives.
Before carrying out experiments on pure samples of mullite, preliminary studies were made on the effect of additives, like, Li20, Na20, MgO and CaO, on the commerzial sample of mullite. Based on the experimental conditions set up by Gad
(1.1), reactions were performed between mulaite and
a) 2% Li20 at 1400°C for 5 to 15 hours.
b) 3% Li20 at 1200° anu 1400°C for 5 to 15 hours.,
c)4% Na20 at 1400°C for 5 to 15 hours. 1) 4% CaO at 1400° and. 1500°C for 5 to 15 hours
e) 4% Mg0 at 1400° and 1500°C for 5 to 15 hours..
Reaction prodaots were analysed in order to determine different phases present.
5,2.1. Results and discussions
The results of the dissociation of o,:,mmercial millite in presence of Li20, Na20, MgO and Ca0 are snown in table 5.2. It
0-content of as low as 2% completely is seen that an Li2 dissociates mullite at 1400°C. 3% Li20 decomposes most of it even at a temperature of 1200°C. Gad (14) obsnrved complete o dissociation of mullite (2:1) in presence of 3% Li20 at 1400 C, 950
0 showed complete In good agreement with his results, 4% Na2 break-down of this sample of mullite at 1400°C for a reaction
period of 5 hours. Contrary to Gad 7 s cbservation, however, 94% mullite was found to dissociate in presence of only 4% Ca0 at 1400°C, and at 1500°C the reaction was complete very quickly. 4% Mg0 did not noticeably decompose mullite at 1400°C but did so
at )500°Cr
As this sample of mullite contained various oxides as impunities together with some glass, an e' arced reaction rate was observed, and also the reaction temperature was rather
depressed. Thus, no conclusion could be drawn regarding the effect of different additives on the dissociation of mullite.
Detailed discussion will follow at a later stage under the
experiments on pure sample of mullite. These test experiments,
however, showed that mullite also gets dissociated in presence
of lime and magnesia; as opposed to Gad's observation. This
necesbitated further studies in this field. 96.
Table 5.2
Dissociation of commercial mullite
Reaction compositionlReaction Reaction Wt.% Phases as analysed J Temp. Period % Mullite % addition 0C. Hour, Mullite Corundum Glass
98 2% Li20 I 1400 5 0.0 62.0 38.0 97 3% Li.,o 1200 15 906 55.3 35.1 96 4% Na20 1400 5 ox 61.5 38.5 06 4% Ca0 1400 5 9,5 54.0 36.5 96 4% Ca0 1400 15 6,3 56.2 37.5 96 4% Ca0 1500 5 0.0 60.0 40.0 96 4% MgO 1400 15 89.3 C00 1 10.2
96 /!-% MgC 1f00 15 308 72.2 24.0
5.3 Experiments on p samnles of Mullite an different additiv)s,
In order i.o investigate the problem of the dissociation of mullite in the light of different phase equilibrium diagrams, studies were made on pure samples of mullite prepared in the laboratory, Mostof these experiments were ,:;aried out with mullite of 3:2 composition plus the additives. For the sake of comparison, several experiments were performed with mullite samples of 2:1 and 1:1 compositions. A few more experiments were carried out for studying the effect on the dissociation of 97• mullite (in presence of Na20) of the addition of free alumina or silica, thereby varying the overall alumina: silica ratio within the range of 2:1 to 1:1. Blank experimnts were also carried out by heating mullite alone.
For studying the progress of reaction and consequently the attainment of equilibrium in a reaction system, experiments were carried out at different temperatures for different periods of time. Reaction products at different stages of the progress of reactions were analysed to determine the amount and nature of phases under different experimental conditions. Sufficient time, in certain cases extending to one week, was allowed for the reactions, and the equilibrium was believed to have reached from the close agreement between the results of two or more successive analyses. In all the compositions studied, however, a period of one week at lower temperatures was found to be adequate for the apparent attainment of equilibrium in these solid-state reactions.
5.3.1. Experimental detail. Experiments on the dissociation of mullite were carried out as detailed below, choosing quantities of additives to suit each ternary system.
505.1.1. The system, Na20-A1203-Si02. Studies were performed in this system with the three alternative inter- 98. pretations in mind, and a range of compositions having mullite and 2,3,4, 4.9, 7, 9, and 12.7% Na20 was selected for conducting reactions. A. With a view to studying the dissociation of mullite in the light of the composition trianglo, albite-corundum- mullite (as shown in Bowen and Schairer's diagram), reactions between 95.1% mullite and 4.9% Na20 were conducted at 1000°, o 1100 and 1200°C (i.e., below melting range) for periods of 5 to 150 hours. Samples selected for analysis were of the reaction periods of 5, 15, 30, 50 and 150 hours. Reactions were also carried out in presence of 2, 3 and 4% Na20 at 1300°, 1400°, 1500° and 1600°C., i.e., above the melting range of these compositions. Be According to the interpretation of the compatibility triangle, !mph, line-corundum-mullite (assumed), experiments were carried out with the reaction-composition having 87.3% mullite and 12.7 % Na20 on the composition-join of nepheline- o o o corundum at 1200 , 1300 and 1350 C; this temperature range was selected in order to find the dissociation of mullite below the melting range. Reactions were also carried out between mullite and 4.9. 7, and 9% Na20 at 1200°: 1300°, 1350° and 1400°C until complete dissociation of mullite was observed. C. According to the compatibility .Griangle, nepheline solid solution-corundum-mullite, it was necessary to carry out 99. experiments with the reaction-composition having mullite and
9% Na20, on the composition-join of corundum and nepheline solid solution (maximum range), below the melting range. It was also necessary to carry out experiments in the presence of less than
9% Na20 above melting range in order to find the dissociation of mullite. Thus in fact, the range of compositions selected for experiments on the basis of the previous two compatibility triangles cover the reaction-compositions necessary for working in this compatibility triangle also.
5.3.1o2. The rystem, Li20-A1203-Si02. Experiments were carried out in this system with compositions having 3.4 and 3.7% o o Li20 at 1100 , 1200' and 2300 C. According to the assumed compatibi3ity triangle,spodmene-corundum-mullite, 3.4% L120 is necessary for complete dissociation of mullite below the melting range but considering the presence of solid solution range between spodumene and elcryptite, a composition having 3.7%
Li20 was also included in the study. Reactions were also carried out in presence of 3, 2, and 1% Li20 at 1200°, 1300°, 1400°, 1500° and 1600°C. in order to disco7er how far dissociation had proceeded. 5.3„.1.3. The system Ca0-A1203-Si02. Considering the relevant compatibility triangle, experiments were perfDrmed in presence of 11.7% Ca0 at the temperatures of 1200°, 1300°, 1400° 100. and 1500°C. But as this composition gave considerable liquid formation at 1500°C, this temperature was definitely above the melting range, and consequently, reactions were confined only up o to 1400 0. This was well below melting range. Studies were also made with compositions having mullite and 7 and 5% Ca0 at
1300°, 1400°, 1503° and 1600°C in order to sl,:udy the decomposition of mullite.
5.3.1.4. The system, Mg0-A1203-Si02. For carrying out experiments in this system, reaction-compositions having mullite ane. 19.3,7 and 5% MgO were selected. According to the requirements of the relevant compatibility triangle, e.g., through sapphirine, cordierite and mullite, e:zperiments were carried out with mullite and13% MgO by heating at 1400° and 1500°C, but since much liquid 0 o formation was observed at 1500 C and none at 1400 a temperature of 1400°C was taken as below the melting range. Thus, reactions were conducted at this temperature for periods of 5 to 70 hours. Experiments were also perfcrm9d in presence cf 7 and 5% Mg0 o at 1400 1 1500° and 1600°C.
5.4. Results and discussion on the dissociation of mullite.
The results and:discussion are dealt with as follows: A - Results and discussion of blank experiments on
mullite. 1010
B - Equilibrium results and discussion in general under
each ternary system followed by discussion in the light of the
relevant phase diagrams*
O - The influence of different factors on the dissociation
of mullite, e.g0 of the amounts of additives, and the temperature
and time of reactions.
D - Results and discussion of the comparative studies on
the dissociation of different types of mullite in presence of
various additives*
E - Results and discussion of the effect of free alumina
or silica on the dissociation of mullite in presence of Na20.
504010 Results and discussion of the blank experiments
on the stability of mullite.
The results of the heat-.reatment of mullite of 3:2
composition (in absence of any fluxing material) at temperatures,
1100°, 1500° and rNif 1800°C for periods of 3 months, 3 weeks,
and 3 hours respectively shoed no mineralogical changes of the material. The chemical analyses of these heated samples were
found to be exactly the same as that of the original material
(shown in table 54)0 The X-ray photographs of these heat-
treated samples have been shown along with that of the original
sample in figure 5.1. No lines for fre, corundum were identified 102. in these patterns. This shows that mullite of 3:2 composition is stable under these conditions., This observation refutes the validity of Skola's
(5.16, 5.17,5.18) statement that mullite was dissociated by heating on its own at about 1550°C for a prolonged period (5.19) Skola also observed that a sample of mullite prepared in his laboratory decomposed on heating at 1700°C (2.39),. This raises some doubt about the purity of the material prepared by him. Considering the results of the present investigation, Taylor's (5.2) thermodynamic hypothesis that mullite is unstable below 1200°C seems very surprising. According to him, mullite can only exist at temperatures above 1200°C. But the experiments carried out at 1100°0 (by the presen:: author) Fhowed no sign of the break-down of mullite. It is also wIteworthy that mullite of 2:1 composition, prepared under similar conditions, showed ther:esence of about
5% free corundum. On heat-treatment of this sample at 1500°C for 3 weeks, no change in the mullite-content was noticed. That is, the sample still contained about 5% free A1203. The X-ray photograph of the material has been shown in figure 5.1. On the ot'er hand, mullite of the sillimanite composition similarly prepared, gave an X-ray pattern (shown in figure 5.1) exactly similar to that of mullite of 3:2 composition. The 103.
material was chemically analysed and found to contain about
10% glass and a residue of 3:2 mullite. The results have been given in Table 501. The fact that the mixturra of alumina and silica of the sillimanite composition gave rise to 3:2 mullite plus glass is in accordance with the A1 0-Si0 diagram* These 2 2 observations as a whole indicate that mullite of 3:2 composition is the most stable alumino•silicate compound below the the
melting point of the material.
5.402. The system, Na20-A1203-S102*,
5.4.2.10 The equilibrium results and discussion in
general. The results of the dissociation of mullite in the 0 at various temperatures, presence of di±forent amounts of Na2 under equilibrium conditions, are shown in figures 5.2, 5.3, and 5*4. Detailed results of the dissociation of mullite under
different conditicns are shown in table 5.3. In figure 5.2, the percentage amounts of the three phases
in equilibrium, e.g., unreacted mullite, corLndum and nepheline
(and/or class, i.e., the percentage dissolution by 1% HF) have
been shown separately plotted against increasing amounts of
O. The same results have been plotted against increasing Na2 temperature of reaction and shown in figure 5.3. It is seen from figures 5.2 and 5.3 and also from table 5..3 that complete 101+. dissociation of mullite was not obtained with 9 or 12.7% o Both these compositions, however, showed complete Na20 at 1200 dissociation, as soon as the temperature of 1300°C was reached.
The temperature for complete dissociation of mullite in presence of 4.9 and 7% Na20 was found to be 1400°C, and that in presence of 4% Na20 to be 1500°. 2% and 3% Na20 did not decompose mullite completely at temperatures up to 1600°C..
The ternary diaGram shown in figure 5.4 is not a phase diagrEm but has been used only to represent the amounts of different phases in equilibrium under different conditions..
In fact, the results shown in figures 5.2 and 5.3 have been presented ia condensed form in this ternary diagram. The apices of the triangle represent the three phases present in a reaction system under equilibrium conditiona. In this figurel each continuous line has been drawn through points representing the amounts of Three phases obtained from a particular reaction- composition at different temperatures. There are therefore seven such curves corresponding to each of the compositions having mullite plus Na 0-contents of 2 to 12.7%. In the case 2 of the mixture having 12.7% Na20, the curve has been drawn through two points only just to show the experimental results.
The dotted lines show the amounts of the three phases that can be obtained at a particular temperature as a result of the dissociation of mullite in the presence of different 105.
amounts of Na O. There are four such curves corresponding tc 2 temperatures, 1200°, 1300°, 1400° and 1500°C.
The X-ray photographs taken of different reaction-
products under equilibrium conditions are shown in figures 5.8-5.9.
5.4.2.2. Discussion of results in the light of the
interpretation of the nhase diagram. The experiments were
carried out in this system with three alternative interpretations
of the relevant ph se diagram in mind (as discussed in chapter 3).
The discussions will thex.efore be made separately with reference
to each compatibility triangle.
A - The compatibility triangle, AJ.bite-Corundum-Mullite.
Experimental data obtained at temperatures of 1000°, 1100° and o 1200 C from the reaction composition of 95.1% mullite and 4.9%
on the compositun join of Albite and corundum, showed Na20, that mullite is remarkably stable under these conditions..
Accoraing to this compatibility triangle, corundum and albite
(glass at 1200°C) should have been obtained as the decomposition
products of mullite under these conditions, but even after reaction periods of three months at 1100°C and one leek at 1200°C,
no sign of considerable breakdown of mul.ite was observed. Gad
and Barrett (5.3) also did not observe any noticeable dissociation of mullite at 1200°C. 106.
In the X-ray patterns of these reaction products under equilibrium conditions (figure 5.8): the compounds identified were predominantly mullite, some nepheline and a little corundum.
No formaidon of albite was observed at any stage of these reactions.
The friable nature of the pellets also indicated the absence of glass in reaction products, i.e., these reactions proceeded in the solid state. The presence of the characteristic X-ray lines for nepheline, e.g., at d = 4421, 3.83, etc, and the absence of thP lines for albite, e.g., at d = 6.4 in the X-ray patterns were accepted as reasonable evidence of the formation of nepheline in the decompostion products of mullite.
The differen'e in the solubility of albite and nepheline in 156 hydrofluoric acid solution was also utilised in their differentiation X-ray photographs were taken both before and after the treatment of these experimental samples with 1%
HF. The absence of the lines for nepheline in the X-ray pattern taken of the residues confirmed the formation of nepheline in the reaction products.
Exact similarity of the X-ray patterns given by reaction products, both at 1000°C and at 1200°C, alEo corZirmed these observations. In the event of the formation of albite in these reaction products, albite would have formed liquid at 1200°C, thus showing only corundum (and mullite) in the x-ray patterns.
Consequently, the X-ray patterns of reaction products at 10000C 107. would be different from those at 1200°C. This was, however, not the case.
Contrary to the prediction of the compatibility triangle,
Albite-Corundum-mullite, complete dissociation of mullite was not observed in the presence of 409% Na 0 below the melting range. 2 Thus, experiments were carried out at higher temperatures in the presence of 2, 3, 4, and 409% Na20. Complete dissociation of mullite was observed in the presence of 4,9/.: Na20 at 1400°C, whereas a temperature of 1500°C was necessary for the reaction to complete in thelzesence of 4% Na20. Reaction mixtures having Na20-contents of 3 and 2% did not show complete break- down of mullite even after heating at 1600°C. This means that higher temperature of reaction is necessary for complete dissociation. Experiments could not be carried out at temperatures nigher than 1600° due to certain practical difficulties, e.g., segregation of liquid, probable loss of alkalies, etc.; this might give erroneous results.
In order to draw a brief comparison of the experimental results with those predicted by the phase diagram, calculated results have also been shown in figure 5.5. The following points seem worth mentioning.
i) Theoretically, 4% Na20 would dissociate mullite completely at 1600°C and to the extents of 92% and 90% at 1o8,
1500° and 1400°C respectively. Whereas Gad and Barrett (5.3) o observed complete dissociation at 1400 C, the present author 0 noticed that a temperature of 1500 C was necessary for the
O. reaction to go to completion in presence of 4% Na2 0 would dissociate ii) Theoretically, 3% and 2% Na2 mUllite to the extentsof 73% and 49% at 1600°C, and 70% and
45% at 1500°C. In the present investigation, the corresponding data have been found to be about 96% and 90% at 1600°C, and 80% aid 70% at 1500°C. Gad and Barrett performed most of their axperiments with mullite of 2:1 A1203:siiica ratio; moreover, they confined their studies to lower temperatures. Thus, their correoponding data are not available for comparison. They have, however, reported tnat the extent of the dissociation of mullite, in almost all cases, was less than that predicted by the diagram. They admitted that the equilibrium was not reached in these reactions, and this explains their results. Due to the unavailability of experimental data of similar studies at higher temperatures, in the literature, it is diffio.ult to make any conclusion. The present author, however, noticed that the dissociation of mullite in the compositions stuaied (except the case of 4.9% Na 0) was more than indicated by the diagram. 2 It seems therefore that in their studies, Bowen and Schairer
(5.4) could not avoid loss of alkalies due to repeated firing 109. and mixing of the reaction compositions.. This probably gave higher values of mullite, and thus, the phases calculated on the basis of the relevant isotherms and the liquidus curve in this part of the diagram might not represent the correct values.
As the determination of the liquidus curve or the isotherms do not come within the scope of the present investigation, no attempts can be made to explain these differences.
(iii) It is also surprising that the phase diagram predicts complete dissociation of mullite at :'1011.°C in presence of 4..9% Na20, while a temperature of 1600°C is necessary for the reaction to complete in presenoe of 4% Na20. Although the
0-contents of these two reaction-mixtures, i.e., 4.9% and Na2 4%, are rather close, the difference in the temperatures of complete dissociation of mullite in these two cases seems to be very higho. This ic a;!,.in probably due to the wrong placement of the primory phase boundary between mullite and corundum or some of the isotherms of this part of the diagram. The formation of nepheline in these compositions at lower temperatures indicates that the peritectic invariant point should exist between mullite, corundum and nepheline..
The present studies have however brought about a compromise in this respect by observing that,
0 does not dissociate mullite noticeably at a) 4.9% Na2 110.
1104°C, i.e., below melting range,
b) 4.9% Na 0 requires a temperature of 1400°C and k% 2 0 that of 1500°C for complete dissociation of mullite. Na2 Thus, the experimental results seem to disprove the existence of the compatibility triangle, albite-corundum- mullite (according to which mullite should have completely decomposed in presence of 4,,9% Na20 at about 1104°C). In order to find a reasonable solution to this controversial point, furth'r experiments were carried cut following the alternative interpretations of the phase diagram,.
_B. The compatibility triangle, nepheline-corundum- myllite. With a viiw to finding the experimental conditiIns of the complete dissociation of mullfte below melting range, reaction,: were carried oat in presence of 12.7, 9, and 7%
0. Results of these experiments at 1200°C une.er equilibrium Na2 conditions also showed reasonable stability of mullite, and confirmea the fact that complete dissociation of mullite is not possible in presence of 4.9% Na20 at about 1104°C. In presence of 12.7 and 9% Na20 at 1200°C, reactions showed only about 60% dissociation. Experiments were consequently carried out at higher temperatures. As shown in figures 5.2 to 5.4, the results of the experiments performed f.n presence of 12..7 and
0 at 1300 C gave complete break-down of mullite. Na2 ° Reaction composition having 7% Na20 did not show, however, complete decomposition of mullite at 13000C, but required a temperature of 1400°C for the reaction to complete. Theoretical amounts of phases have also been shown in figures 5..6-5.7. X-ray analysis of these reaction products at 1200°C showed the formation of nepheline which is in accordance with the interpretation of this compatibility triangle. Compositions
having 1207% Na20 showed trio presence of a little p-1!.l203 under these conditions, and as a result, the amounts of corundum in such reaction products were lower than expected.
This is probu.bly because the excess Na20 above that of 9% is not being completely used up in the process of the break-
down of mullize, and partly reacts with the free corundum to
form p-A1203. however, at 1300°1, the experimental sample containing 12,7% Ne„.0 gave .rise to the formation of carnegiete. In the compositions having 9 and 7% Na20, formation of aepheline was detected in the X-ray patterns (shown in figures
5.8-5.9). Although the inversion temperature of nepheline is
about 1254°C, formation of solid solution increases this
temperature, and thus, nepheline was still present in the samples containing 9 and 7% Na20 at 1300°C.
As was noted from the physical appearance of the reaction
products of compositions having 12.7 and 9% Na20, the temperature 112. of 1300°C is below melting range of these compositions. In the case of the composition having 7% Na20, this temperature is a little above melting range. In order to find the approximate temperature of the beginning of melting range of these compo- sitions, experiments were carried out at 1350°C for 15 hours.
On X-ray analysis of these reaction products, no nepheline was identified in the photographs taken with the sampl-as containing
7 and 9% Na20. This means the temperature of 1350°C is above the melting range of these two compositions; chereas, in the reaction product of the mix!'„ure containing 12.7% Na201 carnegieite was still present in equilibrium with ccrundum at that temperature. The composition having 7% Na20 did not show complete decomposition of mullite under these conditions; reactions were found to complete at 1400C even for a short period of heating. Although ex..Dariments- were not carried out with this composition for longer reaction periods at 1350° or at any temperature between 1350° and 1400°C, complete break-down of mullite would possibly be observed at this temperature range.
According to this compatibility triangle, mullite should completely dissociate in presence of 12.7% Na20 at temperatures below the melting range, i.e. below 1475°C. In fact, complete dissociation has been observed in the present studies at a reaction-temperature of 1300°C. This would have explained the 0-Al 0 phenomenon of the dissociation of mullite in the Na2 2 3- Si02 system, but complications arose due to the fact that both the compositions having 12.7 and 9% Na20 showel complete decomposition of mullite below the melting range. Such observations can probably be explained in the light of the solid solution range of nepheline. These are discussed below..
C. The ccmpatibilAty triangles Nepheline solid solution- corundum-mullite. This assumption is based on the maximum solid solution range in the nepheline comp.:si';ion. According to this triangle, 9% Na20 is necessary for complete dissociation of mullite below the melting range. The ea:perimental results as discussed in B above agree with this interpretation. This is possible because nepheline always takes up some extra amounts of silica in its molecule° Thus, 9% Na20 would accommodate all the silica obtainable from 91% mullite under the optimum conditions, giving rise to nepheline of maximum solid solution range. The increased amounts of leacha.21e materials in the reacticn products under equilibrium conditions indicated the formation of nepheline beyond stoichiometric composition.
It is a fact that there can be only one relevant compatibility triangle in association with mullite and corundum, that is, either (1) mullite-corundum-albite or
(2)mullite-corundum-nepheline or
(3)mullite-corundum-nepheline 114.
Similar observations have alscy.been maintained and discussed by
Foster (307) in connection with the system, Mg0-A1203-Si02. In the recent Na 20-Al203-SiO2 phase diagram, there are four inferred composition-joins between corundum and nepheline (including the solid solution range); in fact, however, only one of such joins can be used for drawing the probable compatibility triangle, if at all, through mullite, corundum and nepheline of certain composition. Once such a triangle has been drawn, the composition- join between corundum and albite will have 1;c, be withdrawn from the ternary phase diagram, es there cannot be two compatibility triangles present in association with mullite and corundum..
It has been explained by Foster (3,7) that the principle of compatibility triangles in a system is correct.. It is believed that this also holds good in the Na0-Alc. 20 3 -SiO 2 system. The ,resent investigation has not concentrated on the studies and composition of phase equilibrium diagrams.
Nevertheless, the experimental data that have been obtained from the studies on the dissociation of mullite can be useful for such purposes. Moreover, in alkali-alumino-silicate systems, where crystallisation is very difficult from viscous melts, this type of studies may be most valuable. Thus, on the basis of the discussion of results given above, a tentative shape may be
0-Si0 diagram.. given to the mullite-region of the Na20-Al2 2 115.
Thisis shown in figure 5.11. In favour of this proposition,
the following evidence may be put forward.
1) Several experiments were carried out with a composition of 4.9% Nap°, 68.3%A1203 and 26.8% Si02, o corresponding to a mixture of 95.1% mullite and 4.9% Na20, at 1000 and 1100°C (and also 1200°C) for reaction periods of 5 to 30 hours. On X-ray analysis, the reaction-products showed the presence of some nepheline (no albite), tridymite, traces of
mullite and mostly corundum. Such cases 7.:f non-equilibria may possibly be explained by considering the larger composition triangle through nepheline, corundum and silica.. In presence of
4.9% Na20' formation of mullite is probably inhibited under these experimental conditions, and the presence of free corundum and silica may be justified. Kraner (541) observed in his experiments that the formation of mullite is prevented by the presence of 4.5% Na2O. This supports these results.
The reaction-products were then treated with 1% hydrofluoric acid solution and X-ray photographs were taken with the residues, when the characteristic lines for only tridymite, corundum and very little mullite were identified. This shows that nepheline was formed in these reactions and was dissolved by 1% HF. The X-ray photographs are shown in figure 5.12.
The results of these experiments indicate that the 116.
conditions were not favourable for the formation of mullite, and consequently, for the attainment of equilibrium. If, however,
favourable experimental conditions are set up, e.g., by keeping
the heated samples at some suitable temperature, below the
melting range, for sufficiently long time, the formation of
mullite could possibly be effected. An example may be cited of
Bowen and Schairer's (5.5) effort of preparing albite by
"acclimating" the glass for months to years. The compatibility
triangle, mullite-corundum-nepheline (solid solution) would
then be applicable. If, on the other hand, reactions are
0 as the starting materials, carried out with mullite and Na2 such difficunies do not arise, and the applicability of this
triangle is easily understood.
2) In no stages of the reaction between 95.J% mullite o o and 4.9% Na20 at 1000 -1100 G was the formation of albite observed. The X-ray photographs of these reaction products
(shown in figure 5.8) showed, on the other hand, the formation
of nepheline. Had, however, any albite been formed as a result
of the dissociation of mullite, very little dissolution by 1%
HF would have been obtained. If any liquid of albite compo-
sition as an alternative to crystalline albite was formed at
this temperature range, the amount of dissolution would be about
40%. But in actual experiment, the amounts of dissolution 117. obtained under equilibrium conditions were 18.3% at 1000°,
21.4% at 1100° and 25.8% at 1200°C. On the contrary, if nepheline was formed in these reaction products, the amount of leachable material would be about 22..8 to 26.4% (depending on the solid solution range). This fairly agrees with the experimental data. These figures though not the decisive factors in this respect give good information about the formation of nepheline.
3) Albite is a naturally occurring mineral, always which containing impurities like potassium, calcium, etc.,/probably favour its formation and staWlity. It is not normally formed in artificial mixtules of the three const:4.tuent oxides under ordinary experimental conditions. It can only be prepared by special technique, e.g., by hydro-thermal synthesis or in the presence of excess sodium silicate etc, It took Bowen and
Schairer (5,5), in certain cases, about 5 years to get a sample of albite from a mixture of and sodium silicate glass. A1203 It is, on the other hand, nepheline (carnegieite) that is usually found in the decomposition products of alumina-silicate refrac- tories in service in alkaline atmosphere and nct albite (3.10,
5.6-5.8)
Kenelm v. Gow (5.9) showed in his experiments on the action of vaporised sodium sulphate on aluminous refractories that the formation of nepheline is possible in reactions between n8.
mullite and 4.5% Na20. Gelsdorf and Schwiete (5.10) observed
the formation of nepheline (a-carnegieite) in mixtures of mullite
and Na 0 at temperatures as low as 1000°C. Pisty and Hutton 2 (5.11) studied the effect of alkali and vanadium oxide slags on
alumina-silica refractories. They noticed that in compositions
having alumina:silica ratio of 40:60, soda first reacted with
silica to form sodium silicate and then began to decompose
mullite giving rise to nepheline and corundum according to the
folloaing equation,
5A12030,2SiO2 + Na20 = Na20.A1203..2Si02 + A1203.
In compositions having A1203:Si02 ratio of 6o:4o or more, the soda reacted with excess alumina before decomposing mullite to
form nepheline. Thesa authors further showed that both albite and
jadeite decomposed to form nepheline at about 900°C. 4) Three experiments were performed with a mixture of the three constituent oxidJs of albite composition by
heating at
(a)1400 °C for half an hortr, then keeping at
1000°C for 15 hours, (b)1100 °C for 2 hours, then keeping at 900°C
for three days, and
(c) 800°C for 3 days. 119.
On X-ray analysis, no evidence of the formation of albite was found; on the contrary, the X-ray photographs showed the
presence of:
(a)nepheline and tridymite,
(b)nepheline and tridymite, and
(c)nepheline and quartz.
Two more experiments were carried out with compositions
0 2S_0 ) and jadeite corresponding to nepheline (Na200 Al2 3 2 ) by heating at 1200°C Tor 15 hours° The (Na20. Al2n3'- 4Si02 products on X-ray analysis were found to be all nepheline in both the cases. The X-ray photographs are shown in figure
5.12. It is, thus, seen that in a mixture contning the three oxides, Na20, A1203 and Si02, the most probable ternary compound is nepheline for carnegieite, depending on temperature).
It can be shown from thermodynamic considerations that the probability of the formation of nepheline in artificial mixtures is higher than that of albite. The values for the heats of formation of the compounds, Na20.A1203.3S102 and Na O. Al 0 .4SiO 2 2 3 2 have been reported in the literature (.5.12). These values were determined on the assumption that the heats of solution of dehydrated analcite (NaA1Si206. H20) and natrolite
(Na Al Si 0 .2H 0) are the same as those of compounds, 2 2 3 10 2 120.
respectively. The Na2 00A12 03 .-3SiO2 and Na20.A1 '2 03 .4SiO2 data quoted by Rossini et al (5.13) are found to be:
d H for Na20.A1203.3Si02 = 1180 k.Cal./mole, and -AH for = 1360 k.,Cal./mole,„ Na20.A1203.4Si02
The corresponding values for Na20.A1203,2Si02 (nepheline) and Na20.A1203.6Si02 (albite)are not available in the literature. If the assumption is made, however, that the energy liberated it adding the elements of Si02 to these structures is constant for each increment, then perhaps the approximate values for the heats of formation of nepheline and albite racy be obtained.
Thus: i) - 4 H for Na20.A120302Si02 = 1180-180 = 1000 k0Cal./mole. ii) - 4 H for Na20,1203.3Si02 = 1180 koCal./mole.. iii) - AH for Na20,A1203.4SiO2 = 1360 k.cal./mole. iv) - A H for Na20.A1203.6Si02 = 1360 + 360 = 1720 k.cal./mole
All these values are given at 25°C. Now, in order to find the probability of the foEmation of these compounds in artificial mixtures of the constituent oxides, the values for the }'eats of reaction need be evaluated. For a reaction to proceed, the heat ofibrJation of products must have to be lower than that of the reactants.
121.
That is,
(4 H products - L H reactants) 4, O.
On the basis of the values of the heats of formation as shown above, heats of reaction may be calculated as follows. The data for oxides are reported by :Latimer (544).
0.A1 07.2SiO i) Na20 + A1203 + 2SiO2 = Na2 2
H for Na20 = 99.45
4 H for Al205 = 399.1 4 H for 2SiO2 = -406.7 il H for reactants = - 905.25 k.cal./mole 4 H for product = - 1000 k.cal/mole.
That is 4 H of reaction . - 95 kcal/mole.
ii) Na20 + A1203 + 3Si02 = Na20.A1203.3Si02.
4 H for reactants = - 1108.6 kcal./mole.. H for product = - 1180.0 k.cal./mcle.
That is 4 H of reaction = - 71.4 kcal./mole.
0.A1 0 .I+SiO iii) Na20 + A1203 + 4SiO2 = Na2 2 3 2 d H for reactants = 1312 kcal./mole 6 H for product = 1360 k.cal./mole. That is 4 H of reaction = - 48 k.cal./mole. 122.
iv) Na20 + A1203 + 6Si02 = Na200A1203.6Si02
H for reactants = 1718.7 k.cal./mole.
4 H for products = - 1720.0 k.cal./mole.
That is, 4 H of reaction = - 1.3 k.cal/mole.
Although it is not known how far these values are correct, it may be noticed that their trend is towards a smaller probability of +he formation of albite in a mixture of the three oxides under ordinary conditionsc In order to explain such phenomena, it may be assumed that in reacticr mixtures of the three constituent oxides, e.g., of albite and jadeite compositions, the partial system, nepheline- silica, tegines to operate. Due +o the difficulty in crystal- lisation or formation of albtte under ordinary conditions, probably a pseudo-eutectic is formed between nepheline and
.silica. Thus, a mixture of the three constituent oxides in this partial system would normally give nepheline and glass, nepheline and silica, or silica and glass depending on temperature and composition G.°. the system,. Under special experimental conditions, however, if albite eventually appears in any of these compositionE, the results would then be interpretable in the light of the existing phase diagrams. 123.
5) In their studies of the binary system, corundum- albite, with compositions near the albite region, Schairer and
Bowen (505) observed the formation of mullite at certain stages of the r•:actions; this means, under favourable conditions, formation of mullite is possible in this region of the diagram below melting range. Li all their experiments, they noticed the appearance of 0-A1203; this p-A1203 could well, in fact, be mullite which, due to similar optical properties, was probably mista'cen as p-Al203. Bor(5.15) has proired thet mullite has very similar optical properties to those of p-A1 0 and these 2 3 are aometimec mistaken for each other. However, the formation of mullite and p-Al 0 in reaction mixtures of albite and corundum 2 3 indicates that: the composition-join of albite and corundum is rather doubtful. Schairer and Bowen did not carry out their experiments in this region of the diagram with mixtures of the three consuent oxides or of mullite and Na20; if they had done so, they would have probably observed the formation of mullite, nepheline and albite under suitable experimental conditions.
Corundum has a long primary field in the system,
and thus, appears as the most stable phase Na20-Al23-Si02' in all the compositions in this region of the diagram above melting range. It does not, however, necessarily mean that corundum will always be present in compositions within its 124. primary field below melting range, because the nature and number of phases in equilibrium will be governed by the relevant compatibility triangle or joins under these corditions. These facts have been explained and discussed by Foster (3.7) who cited the case of cordierite in the primary field of mullite as an example.
In their recent investigation of the system Na20-
Foster et. al. (5.1) observed the formation of mullite Al205-Si02' as tht„ major component in reaction composition having 5%
Na20, 48% A1203, and 47% Si02 at 1100°C. This result amply justifies the composition-join, mullite-albite. They also performed another reaction with a mixture of 7.5% Na20, 49%
at 1100°C for a period of 24 hours. A1203 and 43.5% SiO2 Corundum appeared as the major phase in the reaction-product butno albite was obseri-ed. This means corundum was in equili- brium with liquid, that is, the reaction was not carried out below the melting range. Thus, any conclusion made on the basis of these experiments would directly contradict Foster's idea about the compatibility triangle.. It appears, therefore, that if the experiments were carried out under controlled conditions, below the melting range, these authors would have probably observed the formation of mullite, nepheline and albite in that part of the diagram. This might eventually 125. prove the existence of the compatibility triangle, mullite- nepheline-albite. Such a triangle comes, however, as a conse- quence of the proposition of the compatibility triangle, nephelino s.s.-mullite-corundum , as shown in figure 5410. 6) If now the compatibility triangle, mullite- corundum-nephelina solid solution, is accepted, the whole studies on the dissociation of mullite in the presence of Na 0 can be 2 easily explained. According to the definition the compati- bilit7 triangle, mullite should completely dissoeiate in the
0 at a temperature below the melting range. Presence of 9% Na2 This has beer found to agree with the experimental findings of the present investigation. The fact that the previous investi- gators, as well as, the present author did not observe appreciable dissociation of mullite in presence of 4.9% Na20 or less at o c temperatures, 1000 -1200 C, is also explicable.
This compatibility triangle infers that any reaction composition within this triangle, that is, having mullite and
0-contents of less than 9%, would contain all the three Na2 phases, e.g., mullite, corundum and nepheline (solid solution) below the melting range. Thu3, it would not be surprising, if complete dissociation of mullite is not observed in these reaction compositions at temperatures of 1000°-1200°C. It may be mentioned here for clarification that the amounts of 126. phases calculated from such a compatibility triangel would agree with those determined experimentally only at a certain temper- ture below the melting range. For example, if a comparison is made of the experimental data given in figure 5.2 with the calculated values as shown in figure 507, the following observations can be made0
(i) 9% Na20 showed complete break-down of mullite at 1300° and not at 1200°C. Although both the temperatures are below the melting range, only 1300°C is the optimum temperature.
No experiments were carried out at any intermediate temperature but it might be possible that at temperatures a little below o 1300 C, mullite would also be dissociated completely.
(ii)7% Na.,0 should theoretically dissociate mullite to the extent of 78% below the melting range. Experimental data at 1200° and 1300° showed about 48% and 82% dissociation respectively. As 1300°C is a little above the melting range fir this composition, about 4% increase in dissociation has been observed. Thus the temperature at which the thecretical value would completely agree with the experimental result is a little below 1300°C.
0 (acccrding to the compatibility (iii)4.9% Na2 triangle, mullite-corundum-nepheline sos.) should show a dissociation of about 54% below the melting range. The 127. experimental values obtained at 1000°, 1100°, 1200° and 1300°C are 25%, 30%, 38% and 70%, respectively. 1300°C is a temperature above the melting range for this composition and 1200°C is below the melting range. Although no experiments were ,, performed at any intermediate temperature, it appears that the optimum temperature, should be higher than 1200° and considerably lower than 1300°C. Theoretical value would then agree closely with the experimental data.
(iv) Dissociation of mullite in p;7esew;e. of 4% Na20 or less, below the melting range, wculd be too insignificant to conduct ai,y experiments.
Above the melting range, on the other hand, mullite may compltely disappear from reaction compositions having less than
O. This would, however, depend on the temperature and 9% Na2 composition of the system and follow the general principles of the crystallisation :path, the liquidus curve and the isotherms. It has been found in the present investigation that,
0 require a temperature of 1400°C for whereas 7% and 4.9% Na2 complete dissociation of mullite, 4% Na20 needs 1500° and 3% 0 need higher than. 1600°C for the reaction to go to and 2% Na2 completion. As these values do not fully agree with those calculated from the existing phase diagram, it seems feasible to think that the peritectic point exists between corundum, mullite 128.
and nepheline.
(7) For the sake of comparison, compatibility triangles in the mullite region of the diagram, K20-A1203-Si02 have been shown in figure 5.10. Although not all the identical compounds
2 and Na have of the two systems, K20-A1203-Si0 20-Al205-Si02' been found to behave normally, the general picture of the two
phase diagrams is markedly similar. There exist two compati-
bility triangles in the mullite region of the diagram, one
through mullite, leucite and K-felspar, and the other through
mullite, leucite and corundum. Due to the unstable nature of
jadeite, the Na-compound identical with leucite, the relevant
compatibility triangle has been drawa through albite, corundum
and mullite in the Na20-A1203-SiO? diagram. It has, however,
been discussed above that the results of the present investi-
gation could not be explained in the light of the interpretation
of this compatibility triangle. Consequently, an alternative
compatibility triangle has been shown in figure 5.11. The
closer similarity of the mullite region of t'.9e Na20-A1203-Si02 diagram, thus brought about, with that of thE, 1:20-A1203-Si02
diagram makes the new arrangement more convincing.. 129..
5.403. The system, Li2O-A1203-Si02.
5.4.3.1. Equilibrium results and discussion in general.
The results of the dissociation of mullite under equilibrium conditions are shown in figures 5.13-5.15. Detailed results of the dissociation of mullite in presence of L120 under different conditions are shown in table 5.4 . In figures 5.13 and 5.14, the three phases in equilibrium, i.e., the unreacted mullite, corundum and spodumene (or glass), have been plotted separately against increasing amounts of Li20 and the temperature of reaction, respectively. It is seen from these figures that 3.4%
Li20 dissociates mullite completely at 1300°C, 3% Li20 at 1400° and 2% 71120 at 1500°C; whereas, 1% Li20 would require a temperature higher than 1600 for the reaction to complete. Due to probable loss of alkali, the dissociation was not perhaps 0 complete at 1600 alt'icugh about 96% conversion was observed. The triangulE.r diagram in figure 5.15 has been used to show these results in condensed form, the three apices of the triangle representing percentage amounts of the three phases in equilibrium. In this diagram, each continuous line shows the amounts of the phases obtained from a particular reaction composition at different temperatures, There are, therefore, 4 such curves corresponding to compositions having 1, 2, 3 and
3.4% Li2O. The percentage amounts of the phases obtained as a 130. result of the dissociation of mullite in presence of different amounts of Li 0 at a particular temperature have been shown by 2 the dotted lines. There are 4 such curves (not isotherms) 0 o o corresponding to temperatures, 1200 , 1300°, 14u0 and 1500 .
It may be noticed that the dissociation of mullite was complete in presence of 3.7% L120 at a temperature as low as 1200°C.
The X-ray photographs showing different phases in equilibrium in the decomposition products of mullite in presence of Li.,0 a2s given in figure 5.17.
5.4.3.2. Discussion of results in the light of the interpretation of phase diagram. According to the assumed compatibility triAngle through spodumene, mullite and corundum a few reaction compositions were selected for experiments on the dissociation of mulliL;,: the results of which have been shown in figures 5.i3 -
Dissociation of mullite in presence of 3.4% Li20 at
1100°C was rathersmal:_ and consequently, reactions were carried out at higher temperatures. At 1200°C, dissociation was quite considerable, and at 1300°C, was complete in presence of
0. Reaction was found to complete: in presence of 3.7% 3.4% Id2 C. For want of sufficient information in the le_20 at 1200° 0-Al 0 -Si0 phase diagram, theoretical mullita region of the Li2 2 3 2 knowledge of the melting range of these compositions could not 131.
be obtained. In th present investigation, however, the experi- o ments carried out at 1200 and 1300°C with reaction-compositions having 3.7 and 3.4% Li20 respectively did not show the formation of any liquid. Although no experiments were carried out at any
intermediate temperature in presence of 3.4% Li20, it might be
possible to observe complete dissociation of mullite at a
temrerature below 1300°, or a little above 1200°C.
Lower amounts of Li20, on the other hand, did not show
complete dissociation of mullite at these temperatures, ie. o et 1200 or 1300°C. Reaction products, under these experimental
conditions contained different amount of unreacted mullite,
corundum and spodamene. At temperatures, 1400° and 1500°C,
complete breakdown cf mullite occurred in presence of 3% and
0 respectively. 1% Li 0 dissociated mullite to an 2% Li2 2
extent of 96% at 16CO° Gad and Barrett (5.3) also observed complete decomposition of mullite with 3% L120 at 1400°C.
Kraner (3.11) observed complete supression of the formation of mullite by the fusion cast process in presence of
2% Li20. He suspected that there could be a compatibility triangle through petalite (Li20.A1203.8a02), Itullite and corundum. Perhaps, Kraner forgot that the conditions under which he carried out his experiments were high above melting range of this composition. As a matter of fact, the thfinition 132. of a compatibility triangle, in such cases, gets lost. Moreover, petalite decomposes below 1000°C giving spodumene(and glass).
In the present investigation, 3.,4 and 3.7% Li20 showed however complete break-down of mullite below the melting range.
0 lies on the supposed composition- The composition having 3.4% Li2 join of spodumene (stoichiometric) and corundum, while that having 3.7% Li20 indicate a composition-join of corundum and spodumene solid solution (with eucryptite). As both the compositions showed complete decomposition of mullite below
'he melting range, it seems more probable that the compatibility triangle would be drawn throug:i spodumere (stoichiometric composition), corundum and mullite. This triangle has beeL shown in figure 3.6 in Chapter 3. if the existence of this triangle is recognised, the whole phenomena of the dissociation of mullite in presencrs of Li20 which have been discussed above get automatically explained.
0-Al 0 -Si0 the following Similarly to the system, Na2 2 3 2' comparison can be made of the experimental data (shown in figure 5013) with those calculated from this compatibility triangle (figure 5.16).
(i) 3.4% Li20 should theoretically dissociate mullite completely at a temperature below the melting range. This has been observed experimentally at 1300°C. About 91% dissociation 133.
0 has been found at 1200 C. Both these temperatures are below the
melting range. No experiments were performed at any intermed- iate temperature. but it appears from figure 5,13 that a
temperature much below 1300°C and a little above 1200°C would be the optimum for complete dissociation.
(ii) 3% Li20 should theoretically dissociate mullite to the extent of about 88% below the melting range. The figure 5.13 shows about 90% dissociation at 1300° and So% at 1200°C.
Although the value at 1300°C fairly agrees with the calculated data, it seems that at a temperature, a little below 1300°C the observed and the theoretical values would agree completely.
(iii) 2% Li,0 shows a theoretical dissociation of about 58% below melting range, The corresponding experimental data are 68% at 1300° and 52% at 1200r-'C. It appears that the
theoretical value 1,ould completely agree with the experimental observation at a temperature below 1300° and above 1200°C; this would also be a temperature below the melting range.
0 shows a theoretical value of 30% dissocia- (iv)1% Li2 tion below the melting range and the experimental data of about
58% at 1400°C. 1400°C is a temperature above tne melting range, and consequently, the value obtained is much higher than the theoretical amount. As the dissociation of mullite at lower
temperatures was very small, experiments at 1200° and 1300° were 134. not carried out. It seems, however, that the calculated value would fully agree with the experimental data at a temperature below the melting range, possibly a little below 1300°C. Due to the absence of isotherms in the relevant phase diagram, theoretical data could not be made available for comparison with the observed values above the melting range. 5.4.4. ThP system, Ca0-A120f.Si02
5.4.4.1. Results and discussion in general. The results of the dissociation of mullite in presence cf Ca0 under equilibrium conditions are shown in figures 5.18 - 5.20. In figure 5.18 and 5.19, the perceatage amounts of the three phases unreacted mullite, corundum and anorthite (and/or glass), have been shown separately plotted against increasing amounts of CaO and the temperatures of reaction, respectively. In figure 5.20, the triangular diagram shows these results in condensed form. The continuous lines represent the percentage amounts of different phases obtained from a particular reaction composition at various temperatures. There are thus thre_: such curves corresponding to compositions having 5, 7 and 11.7% CaO. The dotted line shows the amounts of the three phases obtained from different compositions at 1400°C. Detailed results obtained under different conditions are also giVen in table 5.5. X-ray photographs showing phases in equilibrium are presented in figure 5.22. 135.
It is seen from figures 5.19 and 5.20 and also from table 5.5 that mullite gets complete]y in presence of
11.7, 7 and 5% CaO at 1400°, 1500° and 1600°C respectively.
Kraner (.';.11)- in his experiments on the formation of mullite by fusion cast processes discovered that 11.5% CaO completely checked the growtL of mullite in the reaction composition.
This, he said, was in line with the prediction of the relevant phase diagram. Gad (1.1) reported in his studies on the dissor.iation of mullite that he did not notice any break-down of mullite in presence of CaO. Although his experimental data are not available, it appears that either he did not investigate ir the light of the :Interpretation of the phase diagram or he failed to set up proper experimental conditions. The fact that
CaO prevents the form::tion of mullite was confirmed at a later stage by Nemetschek (2.•40) in 1958.
5.4.4.2. Discussion of results in the light of the interpretation of the_phase diagram. Experimental data shown in figures 5.18 - 5.20 indicate that 11.7% Ca0 completely decomposed mullite at 1/100°C, giving about 5505% anorthite and 44.5% corundum. The physical appearance of the reaction products (the friable nature of the pellets) showed complete absence of liquid at this temperature. This observation, therefore, is in harmony with the interpretation 13 6 .
of the compatibility triangle, Anorthite -mullite-corundum,
according to which mullite should disappear from a reaction-
composition of 88.3% mullite and 11.7% Ca0 on the composition-
join of anorthite and corundum at a temperature below 1547°C. The experimental results thus confirm the composition-join of anorthite and corundum.
Results of the reactions between mullite and 7% and 5%
Ca0 showed partial breakdown of mullite at 1400°C. The reaction
products contained different amounts of unreacted mullite, corundum and anorthite. This observation also fully agrees with the interpretation of the existlng compatibility triangle, according to which reaction-compositions having less than 11.7%
Ca0 would contain all thea.cee. phases, below the melting range.
The present investigation, therefore, fully justifies the exist- ence of the compatJ triangle in the relevant phase diagram.
By comparing the experimental data (figure 5.18) with the calculated values (figure 5.21), the following observations can be made:
(i) Theoretically, 11.7% CaO should completely dissociate mullite below melting range, giving 57.8% anorthite and 42.2% corundum. These values fairly agree with the experimental data of 55.5% anorthite and 44.5% corundum at 1400°C. No experiments were carried out at temperatures between 1300° ana 1400°C, but 137. it might be possible that this composition showed complete dissociation of mullite at a little below 1400°C.
(ii)Theoretically, 7% Ca0 should dissociate mullite to the exteAt of about 60% below the melting range. The experimental values are about 80% at 1400°C and 64% at 1300°C.
This indicates that 1300°C is a temperature completely below melting range for this composition.
(iii) 5% Ca0 should theoretically dissociate mullite to the extent of 42% below melting range. The experimental o value at 1400 shows a dissociation of 54%. This indicates that vile value obtained at a lower temperature, e.g., at 1300°C, wou_Ld possible agree with the calculated figure. No experiments were performed at a lower temperature, as the dissociation of mullite was small.
The resultr, of experiments carried out at temperatures above melting range show that mullite is completely dissociated in presence of 7% and 5% Ca0 at 1500° and 1600°C, respectively. These compositions showed considerable liquid formation at this temperature range. The reaction product obtained with 7% Ca0 at 1500°C contained corundum, anorthite and liquid under equilibrium condition. 5% Ca0 at 1600°C showed the presence of corundum in equilibrium with liquid. According to the relevant phase diagram, mullite should not completely dissociate until 138.
o above 1600 C in these compositions. The extents of dissociation at 1600°C in the presence of 7% and 5% Ca0 would be about 93% and 66% respectively. This apparent disagreement between the experimehtal and the theoretical results is probably due to the wrong placement of the primary phase boundary between mullite and corundum and also of the peritectic point between anorthite, corundum and mullite.
In 1915, Rankin and Wright (3.1 ) determined the prima2y phase boundary between mullite and corundum, indicating the incongruent nature of mulliteo They placed the peritectic point at 1512°C. Toropov and (3alakhov (2.6) in their investi- gation showed a boundary curve which is far too different from that of Rankin. and Wright. Aralaaki and Roy (2.7) determined this liquidus curve, showing mullite as a congruently melting compound. Th(,y did not seem to dispute about the peritectic point determined by Rankin and Wright. Welch (2.18), however, found this peritectic point to be different from that determined by the earlier investigators. It appears that much work is yet to be done regarding the final status of the boundary curve between mullite and corundum and also of the pe-7itectic point between mullite, anorthite and corundum. From the results of the present investigation, it seems that the peritectic point should be at a temperature of (or a little below) 1500°C. 139.
5.4.5. The system, Mg0-A1203-Si02
5.4.5.1. Results and discussion in general. The results of the dissociation of mullite in the presence of Mg0 under different experimental conditions are shown in table 5.6 and in figures 5.23 and 5.24. Due to strange behaviour of compounds like cordierite and sapphirine, it has not been possible to present the :results in the same fashion as in other systems. However, it is seen from table 5.6 and figures 5.23 - 5.24 that mullite is completely dissociated in presence of
19.3, 7, and 5% MgO at temperatures, 1400°, 1500° and 1600°C respectively. The experimental data obtained from a reaction composition of 93% mullite and 7% MgO at 1400°C show that mullite is quite stable under these conditions. Probably, this is why Gad did not observe any dissociation of mullite in the presence of MgO. 1500C, however, the dissociation of mullite is quite appreciable, even in low concentrations of MgO. For example, 5% MgC dissociated mullite to the extent of 64% at 1500°C . X-ray photographs showing differert phases in equili- brium are shown in figure 5.25. 5.4.5.2. Discussion of results in the light of the interpretation of the phase diagram. The experimental data shown in table 5.6 indicate that 19.3% Mg0 dissociated mullite 140, completely at 1400°C. The physical appearance of the reaction products showed complete absence of liquid at 1400°C (except on long heating). This observation fully agrees with the inter- pretation of the compatibility triangle, cordierite-mullite- sapphirine. According to this triangle, mullite should completely disappar from a reaction composition of 80.7% mullite and 19.3% Mg0 at a temperature below the malting range, i.e., below 1460°C. The experimental results therefore confirm the compatibility-join of cordierite and sapphirine. It may however be mentioned that the differentiation between spinel and sapphirine by X-ray analysis could not be effectively made due to the striking similarity of their strongest lines. The determination of sapphirine was performed by chemical analysis, based on she amount of silica estimated in the residue of reaction-products, as descriled in chapter 4. 7% Mg0 did not noticeably decompose mullite at 1400°c, below mating range. 12% MgO, however, showed dissociation to an extent of about 52% at 1400°C, giving rise to a little cordierite and sapphirine. These results amply justify the existence of the compatibility triangle, cordierite-mullite- sapphirine, as according to this triangle, in reaction-mixtures containing mullite and less than 19.3% MgO, all the three phases should be present at temperatures below the melting range. 141.
Thus, by comparing the theoretical (table 3.6 and figure 5.26) and the observed (Table 5.6) values of different phases in equilibrium, below the melting range. the following observations can be made.
(i) That 19.3% Mg0 completely dissociates mullite at
1400°C, i.e., below the melting range, fully agrees with the theory.
(ii) Theoretically, 7% Mg0 should show a dissociation of about 37%, below the melting range. Experimental data at o 1400 C did not however show much dissociation of mullite.
Although reactions were not performed at temperatures between
1400° and 1460°C (melting range starts at 1460C), it appears from the experimental data at 1400° and 1500°C that the theoret- ical value would agree with the observed value at an intermediate temperature ("below 1460°C). (iii)5% Mg0 should theoretically decompose mullite to an extent of 27% below the melting range. As the dissociation would be rather small at lower temperatures, it was not thought feasible to carry out such experiments.
Above the melting range, mullite was completely disso- ciated in presence of 7 and 5% MgO at 1500° and 1600°C respectively. The products were corundum and glass (together with some spinel). According to the phase diagram, mullite should 142. not completely dissociate until above 1600°C in these compositions. The extent of dissociation in presence of 7 and
5% MgO at 1600°C would be (theoretically) 84% and 60% respec- tively, the unreacted mullite remaining in equilibrium with corundum and liquid. Thus, it is seen that higher amounts of dissociation have been obtained in actual experiments than those predicted by the diagram. This apparent difference is possibly due to the incongruent behaviour of sapphirine (and corailrite) which on decomposition gave much liquid formation; mullite was not probably compatible with this liquid. The system is rather complicated and any other explanation cannot be put forward, particularly when the present studies do not concern the determination of tha liq•iidus curves (the primary phase boundaries) andloi the isotherms.
5,4.6, The effect of increasing amounts of different
aduitives on the dissociation of mullitel
in order to see how the dissociation of mullite was influenced by varying percentage amounts of Na20, Li20, Ca0 and
MgO, the results obtained under equilibrium conditions have been considered. Thus, reference may be made to figlres, 5.2, 5.13, and 5.18. In these figures, the percentage amounts of different phases in equilibrium at various temperatures have been plotted against the increasing amounts of the additives. 11+3.
Dut to the complex nature of the system, Mg0-A1203-Si02, it has not been possible to show the effect of Mg0 in a similar way, and reference may be made to table 5.6. For the sake of comparison, the theoretical results have also been shown in figures 5.5-5.7, 5.16, 5.21 and 5.26, under the specific systems.
The results of these studies show that the dissociation of mullite becomes more and more apparent at a particular temperature as the amounts of the additives are increased in veaction-compositions. This also reduces the time required for complete dissociaticn of mullite at a particular temperature. 11+4.
,5.4070 The effect of temperature on the dissociation
of mullite. With a view to studying the influence of temperature on the dissociation of mullite, equilibrium results in different systems have been taken into consideration. Thus, reference may be made to figures, 5.3, 5.14, and 5019. In these figures the percentage amounts of phases in equilibrium have been plotted against increasing temperature of reaction. For the results of the system, Mg0-A1203-Si02, refr:rence may be made to the table 5.6. It iu seen from these figures that the higher the temperature of reaction, the greater is the dissociation cf mullite in a particular reaction composition. This is true to both the cases where reactions are carried out under equilibrium conditions and also fcr a particular period of time. Thus, for example, a reaction-mixture of 95.1% mullite and 4.9% Na 0 showed 2 dissociation of mullite under equilibrium conditions to the extents of 40%, 70% and 100% at temperatures, 1200°, 1300° and o 1400 C respectively. Increasing temperature of reaction also reduces the amounts of additives required for complete dissociation of mullite. The reaction-mixture of 95.1% mullite and 4.9% Na20, for example, did not show complete o dissociation of mullite at 1200 or 1300° under equilibrium conditions; whereas, the reaction was complete at 1400°C. 145.
5.4.8. Progress of reaction with time and the consequent attainment of equilibrium.
The results of analyses performed at various stages of the progress of the dissociation of mullite in the presence of Na201 Li20, Ca0 and Mg0 are shown in figures, 5.27 5.33, 5.34 5.37, 5.38 5.40, and 5.23 - 5.24 , respectively. In these figures, the percentage amounts of unreacted mullite, and so also, of other phases in the reaction products have been plotted against time of reaction°
In order that equilibrium was fairly attained in each reaction sys-i-em, all the experiments were carried cut for different periods of time.. Although not many analyses covld possibly be performed in each set of reaction due to their time-consuming and tedious nature, it seemed from these results that equilibrium was nearly reached It is seen that the reactions ara very sluggish at lower temperatures, specially in presence of smaller amounts of the additives. This is particu- larly so in the case of samples containing Na20. At higher temperature, on the othnr hand, reactions proceed faster with the consequent attainment of equilibrium in a reascnable time. In samples containing Li20, Ca0 and MgO equilibrium was found to be attained fairly quickly, probably because crystallisation was not so difficult as in the samples containing Na20. 146.
The theory of phase equilibria, however, gives no information about the reaction velocity, and thus, an attempt was made to determine the rate of the dissociation of mullite. On application of the formulas for the first and second order reactions, the results of the dissociation of mullite were not found to follow these rate-mechanisms. The commonly used formula for ceramic reaction, e.g.,
(1 ) 2 ()KD t (S:20, 5.21) Y was also applied but linear relationship was not obtained by plotting (1 - 3/177-X )2 against time, t. It appears that these di2ferent rate-mechaaisms would hold good for reactions occurring during the first few hours (in some cases, minutes) only, In the preset studies, due to the long time required fur the attainment of equilibrium, reactions have been carried out for a much longer period thus ignoring the effect of the first few hours. It has not therefore been possible to concentrate on the kinetic studies of the reactions associated with the dissociation of mullite. The progress of reactions has only been expressed as a plot of percentage amounts of undissociated mullite (and so also of other phases) against log time in hours. 147.
Gad and Barrett (5.3) have, however, shown that the reactions involving the dissociation of mullite in the presence of additives like Na 0 are in fact diffusion-controlled processes. 2 According to these diffusion-controlled processes, the progress of reaction in a heterogeneous system necessarily depends on mass-transfer of cne reactant or both through the product layer.
That is, in order that reactions should proceed, the ions (or atoms) must leave their original positions to migrate to new ones. Such ionic migrations occur through lattice defects along the contact area. The most important rate-determining factor, besides the extension of the contact area, is the number of lattice defects, since such reactions cannot proceed without any defects being present. Lattice defects are always present above the temperature of absolute zero and they grow in number at higher temperatures. Therefcres the increasing temperature of a reaction system accelerates the progression of the diffusion layers by increasing the mobility of ions and the number of lattice defects.
The increasing temperatures of reaction also introduce more and more of liquid in a system, and this causes an increase in reaction-rate. In order to show these effects, an example may be given by determining the rate (slope) of the dissociation of mullite in presence of 4.9% Na20 at 3200°, 1300° and 1400°C 148. at a particular reaction period of 15 hours (from figure 5.30). Thus the rates of reaction at 1200°, 1300° and 1400°C are
0.1, 0.19 and 0.34% per hour respectively.
2.4.9. Results and discussion of the comparative studies on the dissociation of mullite of different
compositions. The results of the experiments performed under identical experimental conditions in the presence of different additives are shown in table 5.7. The percentage amounts of different phases in equilibrium only have been shown.
It is seen that the ex.6ent of the dissociation of mullite in a sample of sillimanite composition is less than that in the other two samples (i.e., of 3:2 and 2s1 composition) under identical conditions. The mullite of 2:1 composition is however dissociated more than the sample of 3.2 composition. These observations hardly need any clarification. As the sillimanite composition belongs to a different set of compatibility triangles in different systems, higher amounts of the additives would be necessary for complete dissociation of mullite (except in the case of Mg0-A1203 -Si02 system). It may be mentioned here that in reaction-mixtures containing mullite of sillimanite composition, melting range starts at a considerably lower temperature. Therefore, in industrial 149.
application of alumino-silicate refractories, where much liquid formation is not desirable, the composition of the refractory must be chosen from the high alumina section of the diagram.
On the other hand, although both the samples of 3:2 and
2:1 mullite belong to the same oompatibilitity triangle, the calculated amounts of the additives for the dissociation of mullite would be less in the case of 2:1 composition.
Consequently, a particular amount of an additive in a reaction mixture would give mere dissociation of mullite in a sample of 2:1 composition than that in the For example 4.9%
0 is necessary far complete dissociation of 3:2 mullite at Na2 1400°C, while 2:1 mullite needs only 4% Na20. It does not however mean that the refractory containing high alumina- ratios is unsuitable, as the melting range in both the cases starts at the same temperature.
Exception to the above observation is the system,
Mg0-A1203-Si02. According to the phase diagram, almost equal amounts of MgO (a little higher in the case of 3:2 and 2:1 sample) would be necessary for complete dissociation of mullite of all the three compositions. This is probably because more of the A120 molecules (rather than of SiO2' ) are taken away from 3 the reaction systems with the consequent formation of spinel or sapphirine. Reactions between mullite of sillimanite composition 150. and MgO were not carried out, and thus no conclusion could be made.
504.10. Results and discussion of the effect of free
&lumina or silica on the dissociation of mullite in
the presence of Na20
The results of the dissociation of mullite in the presence
0 plus extra amounts of A120 or SiO are shown in of Na2 3 2 table 508. The percentage amounts of different phases in equilibrium only have been shown.
It is seen that the adO.ition of extra A120 or SiO to 3 2 different reaction r3yctems did not affect the results of the overall system under equilibrium conditions. That is, the amounts of phases obtained from reaction compositions of mullite plus Na 0 plus free Al 0_ or SiO are very nearly the same as 2 23 2 those obtained fron. reaction mixtures of only mullite plus
0 of the same overall compositions. Na2 The addition of extra A120 or SiO to a mullite 3 2 composition only shifts the composition of the overall reaction system towards higher or lower Al 0 -region of the diagram. 2 3 Consequently, the amounts and the nature of phases in equilibrium would necessari]y be the same in both cases, provided the overall composition remains the same. For example, the reaction composition of 90% mullite (3:2) plus 6% A1203: plus 151.
4% Na 20 is in fact the reaction composition of 96% mullite (of 2:1 ratio) plus 4% Na O. Experimental data obtained from 2 both the compositions agree fully well,.
The presence of free Si02 gives, however, higher initial liquid formation, and this enhanced the reaction rate, since the presence of the glassy phase influences the diffusion- controlled processes. Kenelin v. Gow (5.9) also observed in his experiments that the reactions between Na2SO4 and high-alumina refractories were speeded more by the presence of SiG2 than by a-A1203. This is possible because silica gives more liquid formation at a lower temperature. in the present investigation, hDwever, it has beer. observed that the addition of either free
reduces the time of the attainment of equilibrium. A1203 or SiO2 This can be explained with the help of Rigby and Hutton's (5.11) studies on tha reaction between Na2SO4 and high-alumina and high- silica muliite refractories. According to them, the soda first
or SiO to form sodium aluminate or reacts with the free A1203 2 silicate, which, in turn, starts decomposing mullite. This probably enhances the reaction rate. In absence of any free
20 or soda alone takes longer time to aissociate A1 3 SiO2' mullite. This has, in fact, been observed in the present studies as shown in table 5,8.
Gad (1.1) noticed that the dissociation of mullite 152.
(both 3:2 and 2:1) in the presence of 4% Na20 was reduced to a great extent by the addition of extra 5% Si02 to the reaction compositions. He described this effect as being due to the form- ation of sodium silicate which "diluted" the reaction system, consequently, the rate of the dissociation of mullite was slowed down. The fact as, that on adding extra 5% Si02 to reaction mixtures of mullite (both 3:2 and 2:1) plus Na20, the over-all compositions were shifted towards higher Si02- region of the diagram, As such, a higher proportion of Na20 would be necessary for complete dissociation of mullite. This does not mead however that the samples with a Si02:A1203 ratic higher than 2:3 are more stable in presence of alkalies than those with a lower Si° :Al 0 ratio. The former would 2 2 3 give more liquid forLo.tion at a comparatively lower temperature and this would render the refractory unsuitable for high temperature use. 153. TABLE 5,3 Dissociation of Mullite in presence of Na2O
Reaction Conditions % Phases as analysed in reaction products. Mullite Na20 Temp. Time Mullite Corundum Nepheline Glass. % % oc hour unreacted 87.3 12.7 1200 5 44.7 3,1 52.2?. 11 II II 15 39.5 50 55.5 II II ti 50 37,2 50 56.0 - tit II it 150 36.4 7.4 56.2. - " " 1300 5 20.0 26,9 53,1 • II II II 15 6;4 -33;6 58.0 I' II II 30 200 38.1 59 9 - II " t, 50 0.0 39,9 60.1 II " 1350 15 0.0 37.5 part 62.5 91.0 9.0 1200 5 45,2 16.3 38.5 II II It 15 40.6 16,8 42.6 - I• t• a. 50 38,8 17.1 44.1 - II It 41 150 37.2 18.3 44.5 - tt " 1300 5 24.5 30.4 45.1 II It II 15 9.2 40.0 50;8 • It II II 30 3.4 44.6 52.0 - tt 41 41 50 0.0 47,8 52.2 - II ". 1350 15 0.0 44, 6 - 55.4 93.0 7,0 1200 -5 56.2 15.0 28.8 - It II II 30 48;8 19.9 31;3 - It II II 50 48.5 20,0 51.5 41 It It 150 47.8 20,4 31.8 - II " 130o 5 41.0 21.4 37.6 part II II I, tt 15 250 33.5 41.0 It It It ti 30 . 19.4 38.4 42.2 II II II 50 18.1 -39,5 42.4 it • II II 1350 15 15.2 39.8 - 45.0 ti tt 1400 1 15.0 39.2: 45.8 tt ,, n 2 2.3 49,5 48:2 II It It 3 0.0 51.5 - 48.5
(Contd/...) 154.
Table 5.3 (Contd.)
Reaction Conditions % Phases as analysed in reaction products. Glass Mullite Na2 0 Temp. Time NUlite Corundum Nepheline % % °C hour unreacted 95.1 4.9 1000 150. 75.2 6.5 16;3 fi II 1100 2150 70.6 8,0 21.4 -
II 11 1200 5 6400 11.8 24.2 II It It 15 62 0 2 12,4 25.4 - It II 11 50 60.9 1303 25.8 - II It It 150 60.2 24.0 25..8 .- 11 " 1300 5 37.6 30.3 32.1 It II II 15 310 34.0 34.2 II II fi 31.2 30 34.4 .- .34.4 It It 1400 3 16.6 46.2 37;2 II II II 5 8.3 51.4 - 40.3 11 II II 15 2.3 55.9 - 41.8 11 tt II 30 0.0 57.7 - 42.3 II It 1300 2 0.0 56.6 - 43.4 96.o 4.o 130o 5 46.4 27.9 • 25.7 It It II 15 43.2 30.6 • 26.2 11 II II 30 42 .5 31.0 .. 26.5 II It II 5o 42.3 31.2 - 26.5 II " 140o 5 22.0 43.8 - 34.2 it 1, II 15 18.5 45.7 ,... 35.8 It 11 11 30 16.4 47.1 - 36.5 II It II . 50 16;8 46.7 - 36.5 11 " 1500 1 7.0 54.5 - 38.5 11 tt It 2 3.1 56,9 40.0 It II 11 3 0.0 59.5 - 40.5 97.0 3.0 1400 5 40.5 34.0 25.5 II tt 12 15 32;5 39.4 - 28.1 11 II II; 30 31,7 39.4 - 28.9 1, It II 50 31.0 39.8 _ 29.2 II II 1500 5 24.0 46,0 - 30.0 II II 11 15 20,6 47;4 - 32.o 11 It 11 30 110,2 47.8 ' 32:0 tt It 1600 1 16.2 45;8 38.0 II II II 5 3;8 52,;7 - 43.5 It II It 15 4.7 52.7 - 42.6
(Contd/...) 155.
Table 5.3 (Contd.)
Reaction Conditions •% Phases as analysed in reaction cralucts. MUllite Na20 Tgmp. Time Corundum Nepheline Glass u0 hour unreacted 98.0 2,0 1400 5 48.5 32.7 - 18.8 il H II 15 43.6 34.6 - 21.8 It ” u 30 42.1 '3507 _ 22.2 H il 11 5o 42.5 35.0 - 22.5 H II 1500 5 33.5 39.7 - 26.8 It It II 15 31.8 40.2 - 28.0 H H H 30 30.8 41.0 - 28.2 it " 1600 1 17.8 45.7 - 36.5 II II II 5 8.6 50.8 - 40;6 I, H 15 9.2 50.6 - 40.2 156.
TABLE 5.4 Dissociation of mullite in presence of Li20
1 Reaction Conditions % Phases analysed in reaction products
NUllite Lio0 Temp. Time HUllite Glass "' °C hour unreacted Corundum Spodumene
96.3 3.7 1200 15 9.1 514 39.5 ii II I, 5o 2.3 57.7 0.0 - II It 11 150 0.0 59.4 40,6 - 96.6 3.4 110o 5 48.0 2308 28,2 - n n 15 4265 28.2 29.3 - 11 If It 50 40,2 29;6 30.2 - 1, fl If 150 38;5 30,4 31;1 - n " 1200 5 22;5 42.8 34.7 11 II 11 15 16,2 48;8 35.0 • It . II II 5o 10.7 53.1 36.2 - 11 11 tt 150 9.5 54:0 36.5 .1 A 1300 1 10,8 48.7 40:5 - 1, It If 3 5.9 52.3 41.8 - 11 11 II 10 0.0 57,7 42.3 - 97.0 3.0 1200 5 33.2 39;4 27.4 • It 11 It 15 28.3 42.1 29;6 It It II 50 21.8 46.4 31;8 - 11 II 11 150 20;8 467 32.5 - 11 " 1300 3 16.6 46.5 36.9 • II It II 10 11.2 50.5 38.3 II It It 30 10.1 51.6 380 - II 11 II 5o q.7 52.0 38.3 - It. " 1400 1 5.6 51,9 42.5 part It 11 It 3 1.8 54.2. 44.0 II II it It 5 0.0 558 44,2 11
(Contd 157.
Table 5.4 (Contd.).
Reaction Conditions % Phases analysed in reaction products
Mullite Li20 Temp. Time Mullite Corundum Spodumene Glass % % °U hour unreacted
98.0 2.0 1200 50 48,5 28(,0 23.5 it 11 11 150 48.2 280 23.5 - " " 1300 3 38.3 34.8 26.9 - „ n n. 10 35.5 37.0 27.5 - It It 11. 30 32 0 6 38.4 29.0 • n II. 11 50 32,6 38.4 -29.0 - 3 „ " 1400 18.4 49;4 32;2 - II II 11 • 5 17.6 50,2 '32.2 part it It II 15 1308 ... .6 33.6 n il II 11 H 30 1305 5207 33.8 Il " 1500 1 3.3 58.0 - 38.7 It II, 11 3 0.0 6100 - 39.0 • • 99.0 1,0 1400 3 51.4 27,8 20;8 part n et H 5 4.8.2 29,;7 22.1 ti It 11 tt 15 43.1 33.4 23.5 11 It 11 It 30 42.8 33.7 23.5 n n " 1500 1 28,8 43.0 28.2 II It II 3 25.2 45.3 • 29.5 n 11 II ao 23,5 46.7 29.8 It 11 It 15 23.2 47.0 •- 29.8 it " 1600 1 5,2 57.1 - 37.7 II It 11 5 4;5 57.7 37.8 II II 11 15 4.4 57.8 - 37.8 ,__ 158.
TABLE 5.1
Dissociation of mullite in presence of CaO
Reacts on Conditions 0 Phases analysed in reaction products Mullite Ca0 Temp. Time Mullite Corundum Anorthite Glass 0 0 0C hour unceaated 88.3 11.7 1200 5 40.5 24.3 35.2 R it u 15 38.0 25.4 36.6 - u 'I II 150 38.0 25.4 3.6.6 - II 130o 5 25.6 30.9 43.5 - ,t U u 15 22.2 32,3 45.5 u II It 30 22.2 32.3 45.5 - u " 1400 1 2.3 42.8 54.9 - u II u 3 0.0 44.5 55.5 - • 93.0 7.0 1300 5 42,5 31.7 25.8 u il, n. 15 3(%8 .35.0 28.2: - " ti it 30 35,6 35.6 28.8 it " 1400 3 23.2 4304 33.4 _ 1, u u. 1C 19;7 45.8 34.5 - u II U 30 19.7 45.8 34.5 11 " 1500 1/12 3.3 52,2. Traces 44.5 u u u 1/2 0.0 55.0 il 45.0 95.0 5,0 1400 3 53.2. 24;a: 22;6 - II It It. 20 45.6 30.3 24.1 - U ii n 30 45.6 30.3 24.1 _ it n 1500 1 22.-6 43.2 Traces .34.2' it u. H. 5 19.3 45.1 it 35.6 II u u 15 19;3 45.1u.._ 35.6 11 " 1600 1 0,5 49.5 50.0 u u 11 2 0.0 50.0 - 50.0 159.
TABLE 5,6 Dissoc:;ation of mullite in presence of MgO
Reaction Conditions % Phases analysed in reaction products , Mullite Mg0 Temp Time Mullite Corm- Spinel Sapph- Cordi- Glass % % °C hour unreacted dum urine erite
80,7 19.3 140o 5 Traces 0,0 33,8 40.0 26.2 it it " 15 11. 0,0 34.9 40.0 25.1 - U u u 70 0,0 0.0 68,0 12.0 0.0 20.0 93.0 7,0 1400 5 95.7 0,0 0.0 o.o 0.0 4.3 11 it it. 15 9400 o.o 0,0 0.0 0,0 6.0 11 H t1 70 92,7 0.0 traces 0.0 0.0 7.3 u it 1500 5 46.4 25,.0 0.0 0.0 0.0 28.6 u u H 15 2.7 51.3 0.0 o.o 0.0 46.0 u a I, 20 0.0 53.5 traces 0,0 (.0 46.5 95,0 5.0 1500 5 76.3 •8.5 - - 15.2 II It 11 15 35.7 35.7 - - 28:6 11 II It '(0 35,7 35.7 - - - 28.6 u u 1600 1 34,0 34.0 - - 32.0 II 11 H 5 2,8 52,7 - 44.5 u u u 15 0.0 54.4 traces - - 45.6 160. TABLE 5,7
Comparative studies on different samples of mullite
Reaction Conditions %o phases analysed in reaction prods-
MUllite Additive Temp Time Atallite Corundum Glass Kind % Kind % °C hrs. Dhreacted
3:2 96 Na20 4 1400 50 16.8 46.7 36.5 2:1 96 Na20 4 1400 50 0.0 66.8 33.2. m2:1 96 Na20 4 1400 .10 0.0 100,0 - 1:1 95.1 Na20 4,9 1400 .30 15.2 38.2 46,6 3r2, 95.1 Na20 4,9 1400 30 C.0 57.7 42.3
3:2 98 Li20 2 1400 30 13,5 52.7 33.8 + 2.1 98 1120 2 1400 30 5,6 63.9 30.5 12:1 98 Li20 3 1400 3 0,0 100,0 - 1:1 98 Li20 2 1400 30 35.8 29.3 34.9
3:2. 88.3 CaO 11.7 1400 3 0.0 44.5 55.5 ++ 2:1 88.3 'a0 11,7 1400 1 0,0 55.6 44.4 ++ 1:1 88.3 CaO 11,7 1400 10 10,7 32.9 56,4 ++
3:2, 93 MgO 7 1500 20 0.0 5.3.5 46.5 2:1 93 MgO 7 1500 20 0.0 73.6 26,4 (+ spinel)
m - Results obtained by Gad.
- Containing some spodumene. ++ - All Anorthite. 161.
TABLE 5.0
on the dissociation Effeot of free A7.203 or SiO2 of mullite.
Reaction Conditions % Phases as analysed in reaction products.
Mullite Na 0 Extra Time Mullite Corundum Glass 2 addition Temp. Unreacted %oC Hrs, Type % A1203/ d SiO 7° ----, 2 • . 3:2, 96 4 - - 1400 15 18.5 48.3 33.2 3:2 96 4 - - 1400 50 16,8 46.7 36,3 4 0 6 1400 15 0.0 64.4 35. 3:2 90 A12 3 6 3:2 95.1 4..9 - - 1400 15 2.4 55.8 448 3:2 95.1 4.9 - - 1400 30 0.0 57.7 42.3 3:2 90.1 4,9 Sio2 5 1400 30 14.5 40.0 45.5 a3:2 91 4.0 Si02 5 1400 10 Major Very little - 2:1 96 4.0 r. - 1400 50 0.0 66.8 33.2 2:1 90 A,0 Si02 6 1400 50 15.5 46.7 37.8 4.0 Sio 5 1400 10 61.0 - 12:1 91 2 39.0 1:1 95.1 4.9 - - 1400 30 15.2 38.2 46.6 1:1 90.1 4.9 A1203 5 1400 15 0.0 56.8 43.a
- Results obtained by Cad,
Fig. 5.1
Plate No.. 1 Mullite (3t2) 2 Heated:. at. I1011°C: for:- mentha
3 II U "' 15.0419-C for 3. weoke
it " 180.120-C• for 3- hours
5 Mullite (za)
6 Muilite (1:1) '7 Mullite (commezoia1)
9' Corundum
Fig. j. ray Photographs. 163 al 60 H .i 50' D 2 40 0 1200 C w i_ 30 d U %), 4 2.-n 0 w 0 cc 0 zD 10 C 0 0 O..,
60
50
M 40
30 UNDU R 20 O C I0 %/® 0 S
al S z_
.3 LA LU I a R G a.I Z /O
0 ND A 20 0 I 2 3 4 4.9 7 9 Wt•°/0 Na2_0
FIG. 5.2 DISSOCIAT ION OF MULLITE BY Naz0 164
70
E 60
LIT 50 UL 40 D M
E 30 CT 20 REA
N 10 U
% 0 M U ND ORU C
°/°
6o U) 5o
CD 40 CC
PHELI NE 30 NE 20 /0
0 < 15 12 13 14 15 16 • TEMPERATURE X iO0°C
FIG. 5.3 DISSOCIATION OF M UL LITE BY NcizO
165
Mullite 80 A-- 1100 °C . B-- 1200 °C C-- 1300 °C . D-- 1400 °C. 30 70 E-- 1500 °C . 9% Nay) F-- 1600 °C. 4-9 0\0 40 60
11, 50 50 fo
12 70/0Na 4 6o 9 oul 2 40 0 liderC QUO :5"- 2 co 7o 3 'Y
30/o Na20 80 0 7 ' 0 fl? I I 0 0 10 t0 I 2 '0 1-11 3 a 4 i9 12.7 9 7 4.9 4 10 20 30 40 50 6o 70 80 wt. % Corundum
Fig. 5.4 Equilibrium Results Of The Dissociation Of Mullite By Na10. 166
c) 60 -oe Ge •
cio-s G 0 50 00
. 40 a) •-o 4-, 0 -r4 o a) c14 30 a) U) a) cd ,szl PL' 20 10 0 0 2.0 3.0 4.0 •4.9 Wt. % Na20 Fig. 5.5 Theoretical Phases From The Na20-A1203-Si02 Phase Diagram (Reference To Compatibility Triangle, Albite-Mullite-Corundum) %Phase s As Predicted. % Phases As Predicted. 70 60 80 50 40 20 10 0 30 1 1 - - 2 344.9 Fig. 5.6TheoreticalPhasesWithRespectToSupposed Fig. 2 5.7 Corundum. Compatibility Triangle,Nepheline-Mullite- Wt. %Na20 3 Mullite-Corundum. Theoretical PhasesWithRespect ToSupposed Compatibility Triangle,Nepheline S.S.- Wt. % 4 Na20 4.9 7 9 7 12.7 9.0 167 F3g.. •5.:8 Tlate . 1 16.1% :mullite + 4.49% Na0 .at -1000°C, :showing nevheltue :and corundum 2 at I10000 3 .tr at :1)2:03d00 4 95.1% + 44.4_ Na20 at a30000, showing =Mite and corundum' 5 -95.1% mullite + 4.9% Na20 at 14,009C, showing corundum 6 93% mullite + 7% Na20 at 120000, showing mullite, nepheline And corundum If at 1S-00°C 8 93% mullite + 7% Na20 at 140000, showing corundum 9 91% mullite + 9% Na2O at 12000, showing -mullite, nepheline and corundum 10 91% mullite + 9% Na20 at 13000, showing corundum 11 87.3% =unite + 12.7% Na2D at 1200C, showing mullite, nepheline (and .AorUhdum) 12 87:3% mullite + 12;7% Nap() at 3.30400, showing Corundum 8 9 Fig. 5.9 nato No. 1 98% mullite + 2% Na20 at 14000C, showing nullite and corundum 2 at 15000C 3 at 16004/C 4 97% =unite +3%. 11a20 at 14000C showing mnllite and corundum 5 II at 150000 6 11 at 16009C '7 96% mullite + 4% Na20 at 1600°0 showing mullite and .corundum 8 n at 1400°C 96% mullite + 4% Na20 at 1800°C showing corundum 10 93% mullite + 7% Na20` at1350°C showing mullite and corundum 11 91% Dualite + 9% NA20 at 13500C showing - Corundum 12 874% mullite + 12.7% Na20 at 1350°C showing carnegieite and corundum I Fie.. 7 9 X-ray Photc orach Kai 0 Fig.5.10 Compatibility Triangles In The Mullite- Region Of The: %System, K20- A1203- Si02 Na20 Fig.5.11 Proposed Compatibility Triangles In The Mullite-Region Of The System, Na20-4203-Si02 ( Present Investigation rvr 1):164,e4 lip,. 1 iioptteline (natural) 2 (EatitttitA) 11 4Uailirte tqA0WA:a*11) Same as in Fig. 4.6. 4 Albite (nuturial) eptapoisiti tveatetl. it330C shows 'nepbe1ike.11 tPielytitite 6 Albite oempooltibn Milted &t §Ooed elpws nepholtAtyr titutrtz *I 4.9% NA.24' + 68,3% Ai O3 2& 31.0 heated. at 110007 Shove nepheine corundii* • quarts (+ mulatto ti.464-1) 9 reaidue, of sample lit "'late # Elfefif, -treatment mho* tatg-Eft4icoa Tim i Qparts 111. Ibrizittrite 5 6 7 • EL, A Fig. 5.12 X-ray Photographs. 173 50 40 rOo 4-1rl 30 o . 20 ti 41 10 0 6o 50 40 0 V. 30 ?5 45 40 35 30 25 20 0 1 3 3.4 3.7 Wt. % Liz() FIG. 5.13 DISSOCIATION OF MULLITE BY Liz°. 174 rat (16) 30 a)+) +.1 .1-4 20 cg 10 0 45 ;-1 C) 40 rd 35 .cg•C) • O m g 0 30 9(1)8 ' 25 0 p4 co 20 1000 1100 1200 1 300 1400 1500 1600 TEMPERATURE °C FIG. 5.14 DISSOCIATION OF MULLITE BY Li20. 175 D,,D1,D37-3.0:1d20 at 1100, 1200, 1300°C. A1,A2,A37- 1% Li2O at 1400, 0 -- 3.7% Liao at 1500, 1600°C. 1200°C. Bil,B21B37- 2% L 2.0 at 1300, ?5 1400, 1500°C. Co ,C21 Cely.., 3% Liz() at 1200, 1300, 1400°C. S 45 55 65 70 75 wt. % Corundum. Fig. 5.15 Equilibrium Results Of The Dissociation Of Mullite By Li20. %Phase s As Predicted. 1.0 Fig. 5.1 6 TheoreticalPhasesWithRespectToSupposed Compatibility Triangle,'Spodumene-Mullite - Corundum. . Wt. %Li20 2.0 3.0 3.4 176 Fig. 5.17 Plate No. 1 99% mullite + 1% Li20 at.15000C, showing mullite + corundum 2 at 1600°C 3 96% Mullite+ 2% 14120 at 1400°0 Showing mullite + corundum (4, spodumene. traces) • 4 98% mullite+ 2% 1,120 at 1E,0060-. showing only- Corundum 5 97% mullite + 3% Lig° a 130000 Showing mullite ,+ corunci. spodumene 6 97% mullite+ 3% 420 at 1400°C showing corundw20- gpodumene 7 96.6% mullite + 3.4% Li20 at 1204..E slowing mullite+ corunalmn+ spodumehe 8 96:6%)milI1te + 344%1E1.0 at 130000 showing corundum + spoiftmene . 9 96.3% =ante + L1 at 120000 8110-sing corundum +' spOdianene dIMINIIIMINIIII111111111111111MIMMI V 178 50 40 - rd r--1 30 C.) 0 Cd a) 20 10 - 0 60 1500 C. 7 1400 °C. 0 40 z 1300 °C. 30 5 . 7 11.7 wt. % Ca° OF FIG. 5.18 DISSOCIATION/ MULLITE BY. •Ca0 179 40 C a) -4-D rd 30 r 4-) H 0 0 20 () 10 11.7 0 50 7% . 40 11.7 % 5 % Ca° o 30 20 ti 50 0 40 0.) -P ra .r., ai r-I -P 30 TEMPERATURE °C FIG. 5.19 DISSOCIATION OF MULLITE BY Ca0 180 Mullite A1 ,A2,A3T- 11.7% CaO at 70 1200, 1300, 1400°C. 7%"CaOat-, 1300, 1400, 1500°C. 4,C21C3-- 5% CaO at 20 60 1400, 1500, 1600°C. o\c) 30 4 0 _-? 40 40 4:1 Q 0 30 ** S Co --L N 0 --Q.'0 tiCO \ 60 20 -_.7 70 10 3o 40 5o 6o 70. 80 90 Wt. % Corundum. Fig. 5.20 Equilibrium Results Of The Dissociation Of Mullite By CaO. 181 70 (JO 60 50 d. 0 te 0 40 c> dic Pre As 30 % Phases 20 .;°‘0 CS'A 1 10 ,t) ‘)* 0 V 0 3 5 7 9 % Ca0 ' Fig. 5.21 Theoretical Phases From The Ca0-A1203-Si02 Phase Diagram. kixr, Nitro. P. 90.:11 A0-14,Pla 4.- 5% 10. 1P)SeV) alictwAt•- rak1.41, t.P cAr..uv44 2:: 90 .%,01‘.0:. at ipop°c oholarg. zp‘P.-4:-te oqz‘WwF),_ 5-, Mg 141111-:be.- +- 51 Caq, at; 16519°C,., allowillg 91%17: QOM**, % tatilAtte cao, at 3.5000C , 4heidlig•114414.9*- c_o-MIZAPP1 +- 9414rthttP3 15i Q$ maklait.e.+, 7,1k CaQ at 1-500,0 4b.oviUg; o_ollytwidAmu axvrthte tyaope), 88.4%_. =pito 4.7% CaO at 1290°C, showing mi41aite cortinduzp..- anoithite at. I4QOPC 8. ,• • mi... ." rqtalite- shoyang eorupclups and alkorthitp Fig. 5.22 X-ray Photographs. % Phases As Analysed. 75 6o 45 30 ( 1 Fig. G1 5.23 9 . 58 ... 1 Dissociation OfMullite By log time.inhours O O C) 0 5 6 0 C . _ 1600 I C 15 Y Corundum-1500Corundum-I500 ' Glass -1500 60 • 5% MgO. ° C. ° 70 C. G-- 183 % Phases As Analysed. 50 0 10- 20" 70 60 40- 30- Fig. 5.24Dissociation OfMulliteBy 5 log timeinhours 0 Cord 10 1520 D G1 o ieri:te-I400 es9.15r1 60' se 1.5 0 I 0 ° - 't1 6 0 7,6 7 &19.3%MgO. tP cp 0 . ,6 `;$ y" -9 0 70 184 Fig. 5.25 Plate No. 1 95% mullite + 5% Mg° at 1500PC, showing mullite+ corundum. 2 95% mullite + 5% Mg0 at 16009C, showing corundum + spinal-, 3 93% mullite + 7% Mg0 at 14000C, showing only mullite 4 93% mullite + 7% Mg0 at 1500°C, showing corundum + spinel 5 88% mullite + 12%'40 at 1400°C, -=showing mullite + cordierite + sapphirine 80.7% mullite + 19.3% 140 at 1400PC 'for 5 hours) showing,cordierite+ saiiphirine 7 at 1400.91 (for 15 hours) 8 80.7% mullite + 19.3% MgO. at 1400°C (for 70 hours), showing spinel % Phases As Predicted. rk) kri ON co 0 0 0 0 0 0 0 0 O 0\ P P• el- m 0 • Pi OD 0 0 O (se P. 0 187 50 Corundum - 1600 41u112 to 7400 0. 40 .4. Corundum 1500°C. Glass 1 °° i Corundum-1400°C. it; a) Mullite co 30 15:1° *c. G G ,...... --G---- Glass - 1500°C. G C lass - 1400'C. m 20 .....G_._.----"------TGT 0 A 10 — 117 Mullite -160 ° . . 0 . . . . 2 3 4 5 10 30 log time in hours Fig.5.27 Dissociation Of Mullite By 2% Na2O. 188 6o Corundum . 1600°C. 50 COrundum - 1500°C. 40 • A600. d. ass° Corundum - 1400°C se M ly Ana 30 - Glass - 1500°C. As °C. Glass - 1400 ses Pha 20 % Mulllte - 1500"C. 10 Mnllite - 1600°C. 0 5 10 15 .30 50 log time in hours Fig.5.28 Dissociation Of Mullite By 3 % Na20. -.7 %Phas es As Analysed. • 50 60 40- 30 20. 10 1 Glass -1500°C. to Fig. 2 % f o 5.29 b 3 DissociationOfMullite By4%Na20. log timeinhours 5 Corundum -1400 Glass -1300 Corundum -.1300 Mullite -1300 Mullite -1400C. \ ° C. ° C. 30 ° ° C. C. 189 -M G AA 190 60 Mullite . 1200% °C. 1400 Co 50 Glass - 1400°C. d. se Gla s - 1 00°C. ly Ana 30- Mullite 1300°C. • As s Nepheline - 1200°C. Phase 20. % 10 - o ' 4 3 5 19 15 50 150 log time in hours Fig.5.30 Dissociation Of Mullite By 4.9 % Na20 191 60 50 ,aNSO Go Er Glass - 1400°C. Mullite - 1200°C. °C. 4 Nepheline - 1300 4 rundum - 1300°C. a) Cd 4 30 Nepheline - 1200°C. M a) Corundum 1200°C 44 20 -. Mullite - 1300°C. 1 2 3 .5 15 30 50 150 log 'time in hours Fig. 5.31 Dissociation Of Mullite By 7 % Na20. 192 60 50- Nepheline - 1300°C. Nepheline - 1200°C. 40- Mullite - 1200°C. cd 30 CO OD CO cd 20 P4 10- 0 10 15 30 150 log time in hours Fig. 5.32 Dissociation Of MUllite By 9 % Na20. ••• 193 • 60 arnegieite 1.300°C. Nepheline - 1200°C. Mullite is 1200°C. 40- e 10 7 'Corundum - 1200°C. 0 5 . 10 15 30 50 150 log time in hours Fig. 5.33 Dissociation Of Mullite By 12.7% Na20. 194 60 Corundum -.1600°C. 50 - orun um - 1500°C. Mullite - 1400 40- G G- Glass - 1600°C. dum - 1400°C. G a) Glass - 1500°C. 4r11211te 150 . Glass - 1400°C. Al 20- 10. NI Mullite - 1600°C. 0 3 5 10 15 30 log time in hours Fig. 5.34 Dissooiation Of Mullite By 1% 420. 1 , 195 6o A5004c. C coron6112' 50- 40. Glass 1500°C. Corundum M __-r1000 Mullite- 300°Cp Glasa ' (3 Spodumene:7--1300°O. Mullite 1400°C. 10- It to 0 I • 3 5 10 15 30 50 log time in hours Fig. 5.35 Dissociation Of Mullite By 2 % L120. 196 40 tv En ri 4 3o .41 4 20 NFL 3 5 10 15 30 50 150 log time in hours Fig. 5.36 Dissociation Of Milts By 3% Lit 0. 197 5 Spodumene - 1300 C. to _ 1100°c. a) ca Spodumene -.1200°C. 4 30- 0 Spodumene • 1100°C. 0 0 0 FL, 2Q. 4212/te 120000. 1 5 10 15 50 150 log time in hours \ • Pig. 5.37 Dissociation Of Mullite By 3.4% 420. 198 6o Glass - 1600°C. 50 Corundum - 1600°C. Mu221te 1400 C. G---- orundum- 1500°C. 40- Glass - 1500"C. 14 30- 1400°C. m m mw .--'------G----GAnorthite - 1400°C. Ai 20. M P4 Mullite -1500C.-C. 10. Mantel - 1600°C. om 2 5 10 15 30 log time in hours Fig. 5.38 Dissociation Of Mullite By 5% CaO. 7 ZT 199 60 C. C: c " 15 50, Corundum 1400°C. C; Glass - 1500°C. 40. 2/422.ite 130o , ft; orthi•tei':;;;.:1400°C. to .Ail rg C • 4 3°- 1-50 to Anorthite 1300°C 20- MUllite - 1400°C. 10 • 4rulilte 1500°C 0 0.1 0.5 1.0 3.0 5:0 10 15 30 log time in hours Fig. 5.39 Dissociation Of Mullite By 7 % CaO. : 200 60 Ct-----Anor;3ite:T40g"---- aC. 5co. Corundum 1400°C. orthite - 1300°C. 4o- Mullite - 1200°C. to Anorthite - 1200°C. co r•-1 corundum - 1300 °C. F3 3°- Corundum - 1200°C. to NI • to ° .10 20- Mullite - 1300 C. Pi Vk 10' 1400°C Fel 1 3 5 . 15 30 150 log time in hours Fig. 5.40 Dissociation Of Mullite By 11.7% CaO. • 201. 6 - General Conclusion From the studies of the dissociation of mullite in the presence of different additives, the following main conclusions can be drawn: 1) The experimental evidence on the dissociation'of mullite in the presence of Na20 have not supported the albite- mullite-corundum compatibility triangle in Bowen and Schairer's Na20-Al203-Si02 diagram. An alternative triangle through nepheline solid solution, mullite and corundum has been put forward. The presence of nepheline, mullite and corundum in some reaction products under equilibrium conditions shows that the peritectic invariant point snould exist between nepheline, mullite, corundum and liquid. and not between albite, mullite, corundum and liquid. 0-A1 0 2) The preoent studies in the system Li2 2 3-Si02' have suggested that the relevant ampatibility triangle should be drawn through spodumene, mullite and corundum. 3). The fact that both Mg0 and CaC dissociated mullite completely has shown Gad's observation to be wrong. 4) The pzsent investigation has confirmed the inferred compatibilitr triangles, sapphirine-cordierite-mullitc and anorthite-mullite-corundum, in systems, Mg0-A1203-Si02 and 202. Ca6-A1203-Si02, respectively. The results of the dissociation of mullite in the • presence of Ca° above the melting range have suggested that a precise determination of the primary phase boundary between corundum and mullite, and also, of the peritectic invariant point between ano:thite, mullite and corundum should be made. 5) In order to study the phase relationship in the mullite-region of different ternary diagrams (specially the alkal:.-systems), it is suggested that mullite should be used, as well as, the constituent oxides, aS the starting material. 203. 7 - Future Work. The present investigation on the dissociation of mullite has indicated that the phase equilibria studies in the mullite- region of different diagrams, specially the alkali-alumina- silica systems, should be made with mullite and each additive as the starting materials. For the sake of comparison, these reactions should be performed in two ways: (i) in the solid state (or in presence o:7 small amounts of liquid, i.e., a little above the malting range), and (ii) by mating the compositions, cooling very slowly and keeping at a desired temperature for sufficiently long time, Some of the many tossibilities of future work in field are: (a) re-investigation of the primary phase boundaries between mullite and the acljoininp; compounds in different systems, and (b) revision of some of the isotherms in the mullite region :if different diagrams. 204. REFERENCES 1,1 Gad, G. M.; Ph.D. Thesis, London University, 1948. 2.1 Cheaters, J,H,; "Steel Plant Refractories testing, Research and development", Sheffield United Steel Co., 1963. 2.2 Miyashiro, A.; J. Geol. Soc., Japan, Z., 218, 1949. 2.3 Bowen, N.L„, and Greig; J.W.; J. Am. Cer. Soo. 1, 238,1924. 2.4 Morris, L.D. and Scholar, S.R.; J. Am. Cer. Soc., 18, 359, 1935. 2.5 Toropov and Galakhov.; Bonn. Nemp. u MOH. 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