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Showing posts with label Alumina - Chrome - Silica system. Show all posts
Showing posts with label Alumina - Chrome - Silica system. Show all posts

Refractory Formation in Alumina - Chrome - Silica (Al2O3 - Cr2O3 - SiO2) System along with the Ternary Phase Diagram

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9-Dec-2009

Alumina - Chrome - Silica (Al2O3 - Cr23 - SiO) Refractory System

Most of the work and so, data are limited to the binary systems forming the edges of the Alumina - Chrome - Silica ternary system. Very few data are available on the phase relations in this ternary refractory oxide system. At first a partial liquidus diagram for this system was published by Born [V.A. Born, Transactions. 5th Conference Exper. Techn. Min. Petr. Publ. Acad. Science, USSR, 1958, p.479]. Solacolu [S. Solacolu in “Proceedings of the 8th conference on the Silicate Industry, (SiliConf.)”, Hungary, 1965, p.777] proposed an equilibrium diagram (see adjacent Fig.) for the Alumina - Chrome - Silica (Al2O3 - Cr23 - SiO) ternary refractory oxides system in which he divided it into two subdivisions:
(I) Subsystem SiO2 - 3Al2O3.2SiO2 - Cr2O3 contains three binary eutectic points e1, e2, and g3; and one ternary eutectic point E1, melting at 1580OC,
(II) Pseudo-subsystem Al2O3 - 3Al2O3.2SiO2 - Cr2O3which contains no ternary eutectic point. 
 Fig. - Thermal Phase Equilibria in the Alumina - Chrome - Silica (Al2O3 - Cr23 - SiO) system (After Solacolu)
Fig. - Phase equilibrium diagram for the system Alumina - Chrome - Silica (Al2O3 - Cr23 - SiO). Heavy lines are boundary curve, dashed lines are liquidus isotherms in degree Centigrade, and the two-liquid region is outlined by the zone of dots. (After Roeder et al.) 
From his observations Solacolu concluded that the body composition should be chosen from subsystem (II), especially in hatched quadrangle, where melting temperatures are above 2000OC. It may be mentioned here that the phase diagrams for different bounding binary systems as were adopted by Solacolu, are those given by Bowen and Grieg [N.L. Bowen and J.W. Grieg, Journal of American Ceramic Society, 7(4), 1924, p.238], Schairer [J.F. Schairer, Journal of American Ceramic Society, 25, 1942, p.241] and Bunting [E.N. Bunting, J. Res. Natl. Bur. Std. 5(2), 1930, p.325].
Bunting’s binary phase diagrams were also accepted by Roeder, Glasser, and Osborn [R.L. Roeder, F.P. Glasser and E.F. Osborn, Journal of American Ceramic Society, 51(10), 1968, p.585], who later on published a phase diagram for Alumina - Chrome - Silica system (see adjacent Fig.). For Al2O3 - SiO2 (alumina - silica system) Roeder et al. adopted the diagram of Aramaki and Roy. The major differences between these two phase diagrams of Alumina - Chrome - Silica system are that Solacolu omitted the two liquid region and he assumed that ternary liquids are in equilibrium with pure chromium (Cr2O3) crystals rather than with corundum solid solutions  (Alumina - Chrome solid solution). Roeder et al. concluded that at 1580OC (ternary eutectic), the eutectic liquid (6Al2O3 - 1Cr2O3 - 93SiO2) coexists with a mullite solid solution (19Al2O3 - 81Cr2O3), and crystoballite (SiO2). They presented also the diagrams to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800O, 1700O, and 1575OC.
Murthy and Hummel [M. Krishnamurthy and F.A. Hummel, Journal of American Ceramic Society, 43(5), 1960, p.267] presented data suggesting maximum solubility of Cr2O3 in mullite of 8 to 10% at 1600OC, while the beneficial influence of chromium on the resistance of alumino - silicate refractories like, mullite, sillimanite to the action of ferruginous slags also estimating the maximum solubility of chromium (Cr2O3) in these refractories under equilibrium conditions at 1600OC, were pointed out by Chadeyron et al. [A.A. Chadeyron and W.J. Rees, Transactions of British Ceramic Society, 42, 1942, p.163] and Ford and Rees. Under equilibrium conditions at 1600OC, mullite can take into solid solution up to 8% by weight of chromium. Further addition of chromium results in dissociation of mullite, most of the alumina (Al2O3) forming a solid solution with chromium (Cr2O3), while the remainder of the alumina (Al2O3) melts with the silica precipitated as a result of the dissociation. They also showed that a marked increase in the resistance of sillimanite to ferruginous slag was effected by incorporation of up to 15% of chromium.
Herabi and Davis studied the effect of varying amount of chromium (Cr2O3) and addition of mullite on densification of modified corundum ((Alumina - Chrome solid solution) [A. Herabi and T. Davis, Journal of Euro Ceramics, 2, 1989, p.2576]. On the basis of their studies these authors concluded that mullite modified corundum refractories show better Microstructural states and mechanical strength.
Sintering behaviour in the Alumina - Chrome - Silica (Al2O3 - Cr23 - SiO) Refractory System (Mullite - Chrome) in the reducing atmosphere was investigated by Yamaguchi [A. Yamaguchi, Ceramic International, 12(1), 1986, p.19]. In Yamaguchi’s experiment mullite was not formed from alumina and silica in the presence of chromium (Cr2O3) at high temperatures from 1300OC to 1500OC, and it was even thought to decompose to alumina (Al2O3), gaseous SiO, CO2 and CO.

Despite these contradictory reports, the author of this article (Dr. Abhijit Joardar), and Yang and Chan [Proceedings of the International Symposium of on Refractories”, Nov. 15-18, 1988, Hangzhou, China] found mullite to grow at the expense of corundum and silica phases better in Chromium - containing high alumina refractories than Chromium - free refractories.