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Showing posts with label Mullite - Chrome. Show all posts
Showing posts with label Mullite - Chrome. Show all posts

Mullite - Chrome Refractory

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Mullite - Chrome refractory phase is a part of the Alumina - Chrome - Silica (Al2O3-Cr2O3-SiO2) ternary system. It won’t be irrelevant to mention here that I, the author of this blog, obtained my PhD in Refractories for my work on Mullite - Chrome refractories especially, study on the kinetics and mechanism of sintering, densification behaviour, and various physical, thermo-mechanical properties of compositions in the Alumina - Chrome - Silica (Al2O3-Cr2O3-SiO2) system, optimization of the various controlling parameters along with characterization of the sintered samples in terms of XRD, Microstructural analysis, Hot Modulus of Rupture (HMOR) etc. A review of the previous work done on the Alumina - Chrome - Silica system has been discussed in one of our earlier posts -
In a refractory the mullite phase can be developed by in-situ reaction sintering between the alumina (Al2O3) and silica (SiO2) containing raw materials or can be imparted directly by adding synthetic mullite grains. In the same way, Mullite - Chrome phase (with some Chrome Corundum solid solution) containing refractories can be formed using natural raw materials mainly calcined bauxite (of low iron, low impurity), calcined fireclay and green chrome oxide (ultrafine) with some sintering aid (?) through reaction sintering at a comparatively lower temperature around 1450 - 1500OC. Alternatively, such refractories can be made using synthetic raw materials such as fused alumina, calcined alumina or even synthetic mullite grains in suitable grading along with green chrome oxide (preferably high purity, ultrafine type) homogeneously dispersed throughout the refractory mix (powder). In the later case the firing temperature would be around 1600 - 1650OC with soaking time depending upon the various known factors.
The addition of Chrome (Cr2O3) to alumino-silicate and mullite refractories improves certain high temperature properties of these refractory products. Creep as well as slag corrosion resistances of high alumino-silicate and mullite containing refractories are considerably increased with the addition of Chrome (chromium oxide, Cr2O3) in them. The creep resistance enhancement of high alumino-silicate refractories is attributed to an increase in viscosity for the glassy phase in the bonding matrix due to the addition of Chrome while reasons for the better slag corrosion resistance of chrome - containing (Cr2O3 - containing) high aluminosilicate and mullite refractories are -

  1. Formation of a dense, Cr-spinel (Chrome-spinel) layer at the slag / refractory interface,
  2. Formation of an impermeable layer due to the crystallization of fibrous mullite facilitated by the presence of Cr2O3 in the refractory brick immediately adjacent to the interface which restricted the slag penetration,
  3. Formation of corundum solid solution (Alumina-Chrome corundum solid solution) which increases the inter-granular direct bonding near the interface at 1500OC to 1600OC reinforced the bonding matrix.
Because of these improved properties mullite-chrome refractories and alumina-chrome (Al-chrome) refractories have been found to perform better than the conventional refractories in furnace hearth areas of lead-zinc smelter and in secondary steel making processes such as in slide-gate refractory assemblies (nozzles, well blocks, porous plug seating blocks etc.) in the steel industry. Laboratory data have shown exceptional resistance to corrosion of these refractories to highly siliceous slag along with better results for these refractories containing mullite-chrome phase from coal gasifier, fiber glass tank furnace, and carbon reactors.  

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

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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.