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Showing posts with label Binary Phase Diagrams. Show all posts
Showing posts with label Binary Phase Diagrams. Show all posts

Mullite and Other Alumino-Silicate Refractories vis-à-vis Alumina - Silica (Al2O3 - SiO2) Binary Phase Diagram



Alumino-Silicate Refractories
Aluminosilicate or Alumino-Silicate minerals are the naturally occurring compounds mainly composed of aluminium, silicon and oxygen. These minerals are the major constituents of Kaolin and clay minerals. Besides Fireclay, Kyanite, Sillimanite, Andalusite and Mullite are some alumino-silicate minerals which constitute the main raw materials for Alumino-Silicate refractories. By themselves, these minerals especially sillimanite and Andalusite have a high melting point, low coefficient of expansion after heating and excellent resistance to alkaline melts. Kyanite, Sillimanite, Andalusite and all other sillimanite group of minerals break down at or below 1545OC and yield Mullite. Mullite is 3Al2O3.2SiO2 (71.8% Alumina, 28.2% Silica by weight), and is found naturally or is formed by firing combinations of alumino-silicate raw materials or aluminous raw materials. From the Alumina - Silica (Al2O3 - SiO2) phase diagram below, it is clear that a mullite material with more than 73 wt% Alumina (Al2O3) will consist of mullite and alumina. Below 70 wt% Alumina (Al2O3), the material will contain mullite and silica. In these two cases, the temperatures at which the liquid will first appear are radically different.
By varying the alumina and silica ratio in alumino-silicate refractories, a wide range of properties can be realized. Low-alumina, high-silica refractories are used in areas where high strength at service temperature is required such as steel ingot soaking pits. Alumino-Silicate refractories with 30 - 45% alumina (Fireclay refractories) have high-temperature volume stability and strength, excellent resistance to thermal spalling etc. because of which fireclay refractories are widely used in various metallurgical and non-metallurgical industries, furnace back-up lining and so on.    
Fig: Versions of the binary phase diagram of the Alumina - Silica (Al2O3 - SiO2) system proposed from time to time (a) Bowen and Grieg, Schairer (b) Aramaki and Roy (c) Aksay and Pask

A Review of Previous Work on Alumina - Silica (Al2O3 - SiO2) Phase Diagram in relation to the Formation of Mullite  
The Alumina - Silica (Al2O3 - SiO2) refractory oxides system has been the subject of several investigations in the past. Though many papers were presented on the melting relations and range of composition of mullite, it remained a matter of controversy about the diagram in the region of the compound mullite (3Al2O3 : 2SiO2) as has been discussed by Aramaki and Roy [Journal of American Ceramic Society, 45(5), 1962, p.229]. The first equilibrium diagram for the Alumina - Silica (Al2O3 - SiO2) system presented by Bowen and Grieg [Journal of American Ceramic Society, 7(4), 1924, p.238] as shown in the adjacent phase diagram fig (a), shows incongruent melting of mullite. Other significant features which could not be explained on the basis of this phase diagram, e.g. deviation from stoichiometry of the composition of mullite found in refractory bricks, and the crystallization of mullite from a melt of its compositional range, could now be understood with the modifications introduced in the phase diagram by Aksay and Pask [Journal of American Ceramic Society, 58(11 - 12), 1975, p.507] as shown in fig (c). According to this phase diagram showing the stable phases (solid line) and two meta-stable versions (dashed and dot-dashed lines) in the Alumina - Silica (Al2O3 - SiO2) system, mullite melts incongruently at the peritectic 1828OC on the International Practical Temperature Scale of 1968 (IPTS-68) to a liquid containing about 53 wt% Alumina, which is far from the compositional range of stable mullite solid solution (70.5 - 74.0 wt% Alumina). In the phase diagram of Aramaki and Roy as shown in fig (b), mullite is shown to melt congruently at 1850OC on the International Temperature Scale of 1948 (ITS-48), the second eutectic between mullite and corundum, 1840OC, is located at 77.5 wt% Alumina. The major changes introduced in the latest diagram (fig. c) are:
(i) The eutectic temperature was raised to 1595OC by Schairer in 1942; the eutectic composition was also shifted.
(ii) Mullite was found to have a narrow but stable range of solid solution among other by Aramaki and Roy, around the stoichiometric composition of mullite, 3Al2O3.2SiO2, determined by Bowen and Grieg.
Because of the excellent load bearing capacity, volume stability, high resistance to glass, molten metal and slags, mullite refractories find wide spread applications in the glass and metallurgical industries. They are also used as kiln furniture.

A Review on Alumina - Chrome (Al2O3 - Cr2O­3) and Chrome - Silica (Cr2O3 - SiO2) Refractories along with their Binary Phase Diagrams

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Although refractories belonging to the Alumina - Chrome (Al2O3 - Cr23) refractory oxides system has found many applications in the industry yet not much research work on these two refractory oxides system have been done and therefore, data on the pure Alumina - Chrome (Al2O3 - Cr23) or Al-Chrome refractories are also either sparse or have scarcely been reported. Rather because of their improved properties, Alumina - Chrome or Al-Chrome refractories with some Silica (SiO­­2) content and other minor impurities have always been proposed by the refractory technologists for applications in numerous industries (areas). Nevertheless, this article is not to discuss the properties and applications of Mullite - Chrome refractories or those belonging to Alumina - Chrome - Silica (Al2O3 - Cr2O3 - SiO2) system which have been discussed in a separate article. Here we have tried to put together or review the past work done on the two (binary) refractory oxide systems.
Alumina - Chrome (Al2O3 - Cr23) Refractory System
A dilute solution of Cr2O3 in Al2O3 has been called as ‘Ruby’ since long. More concentrated solutions do not possess the desirable colour of the Ruby, but are of considerable interest because of their refractory properties. Dubion [Dubion, Compt. rend. 134, 1902, p. 840] reported that on heating mixtures of alumina (Al2O3) and chromia (Cr23) to red white state, 15 - 16% Cr23 united with alumina. Passerini [L. Passerini, Gazz. chim. Ital. 60, 1930, p.544] made an X-ray examination of mixtures and found that limited solution in the solid state occurred at 600OC. Bonthron and Durrer [K.J.A. Bonthron and E. Durrer, Z. anorg. u. allgem. chem.. 198(1), 1931, p.141] reported a eutectic at 30% Cr23. Barks et al. [R.E. Barks, D.M. Roy, and W.B. White, Am. Ceram. Soc. Bull. 44(4), 1965, p.317] showed that below 925OC, there exists a miscibility gap in the solid solution series between the two oxides. Bunting [E.N. Bunting, J. Res. Natl. Bur. Std. 6(6), 1931, p.947], whose work was accepted Wilde and Rees [W.T. Wilde and W.J. Rees, Trans. Br. Ceram. Soc. 42, 1943, p.123] and Roeder et al. [R.L. Roeder, F.P. Glasser, and E.F. Osboron, J. Am. Ceram. Soc. 51(10), 1968, p.585], made the attainment of equilibrium easier by precipitating chromium and aluminium hydroxides with ammonium hydroxide from the aqueous solution of Cr2(SO4)3.(NH4)2SO4.2H2O and Al2(SO4)3.(NH4)2SO4.24H2O. Their study of the Phase equilibria in the Alumina - Chrome system showed a complete solid solution series with no compound formation. The phase diagram for the Alumina - Chrome refractory oxides system sketched from the data of Bunting is shown in the adjacent phase diagram.
A patent [T.G. McDongal, A.H. Fessler, and K. Schwartzwalder, U.S. Pat. 2218584, Oct. 22, 1940; Ceram. Abstr. 20(1), 1941, p.21] assigned to the AC Park Company also confirmed the formation of complete solid solution between alumina and chromia. On the basis of microscopic examination Preston found the crystals of the Alumina - Chromia solid solutions, like hexagonal plates having the appearance of Corundum. He referred to these crystals as Chrome Corundum [F.W. Preston, J. Am. Ceram. Soc. 17, 1934, p.356].
An investigation into the sintering behaviour of this binary refractory oxide system without using any additive was made by Rai and Roy [H.L. Rai and P.B. Roy in “Proceedings of the symposium on the Sintering and Sintered Products”, Oct.29-31, 1979, BARC, Bombay, India, p.655]. Their results showed that Alumina-Chrome or Al-Chrome refractory can be made having higher cold and hot strength at relatively low temperature of 1650OC. While the Chromia (Cr2O3) content ranging 5-11%, is suitable for end use requirement of certain industries, it has also been found 15% Chromia (Cr2O3) addition and above with Alumina (Al2O3) can be even better in getting a better refractory material. However, the latter will need higher firing temperature.
In another paper Bogum and Faizullah [Proceedings of the International Symposium on Refractories, Nov. 1988, Hangzhou, China] have stressed on the important properties like high corrosion and thermal shock resistance properties of Alumina-Chrome or Al-Chrome bonded refractories. The vulnerable areas of glass melting furnace (e.g. overcoating refractory blocks at the metal line, throat assemblies, electrode blocks, doghouse corner refractory blocks and arches) can be upgraded with bonded Alumina - Chrome - Silica refractories for the remaining lining of the furnace. Chrome corundum phase containing refractories have been prime candidate for the working lining of slagging coal gasifier. Alumina - Chrome (Al-Chrome) refractories formed spinel reaction products at the slag-refractory interface which is one of the reasons for which they can be suitably applied in some areas of Slide Gate refractory assemblies. Fraser and many other workers reported exceptional resistance to corrosion by highly siliceous slag along with better results for Alumina - Chrome (Al-Chrome) refractories from service in coal gasifier, fiber glass tank and carbon reactors.                

Chrome - Silica (Cr2O3 - SiO2) Refractory System
Phase relations in the Silica - Chrome (Cr2O3 - SiO2) refractory oxides system as was determined by Bunting [E.N. Bunting, J. Res. Natl. Bur. Std. 5(2), 1930, p.325] are illustrated in the adjacent phase diagram. The liquid miscibility shown by him extend virtually across the phase diagram may actually be less extensive as suggested by the studies of Glasser et. al. [F.P. Glasser, I. Warshaw, and R. Roy, Phys. Chem. Glasses, 1(12), 1960, p.39] Due to the absence of sufficient additional data Bunting’s work was, however, accepted by many authors subsequently. The phase diagram of Chrome - Silica (Cr2O3 - SiO2) refractory oxides system shows no intermediate compounds; over most of the range of composition between chrome and silica there are two immiscible liquids at high temperatures and the primary crystalline phase is chromic oxide.

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