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

December 16, 2009

Mullite - Chrome Refractory

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 [Refractory Formation in Alumina - Chrome - Silica (Al2O3 - Cr2O3 - SiO2) System along with the Ternary Phase Diagram].   
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.              
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November 23, 2009

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.
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