Creep in compression is described as plastic deformation of a refractory at high temperature with constant load for long soaking hours i.e. a measure of isothermal deformation of a refractory product under stress as a function of time. As with refractoriness under load, the ability of a refractory to withstand creep under compression depends on the softening point and the amount of glass phase in the refractory system. Andalusite-based refractories show extreme resistance to creep during thermal cycling between 1000°C and 1500°C. Because of impurities in bauxite, especially alkalis, the glassy phase forms at temperatures as low as 1100˚C which is responsible for low creep resistance of bauxite-based refractories despite the overall higher alumina content.
However, the situation has changed significantly in the past few years. The refractory industry has witnessed a dramatic increase in the cost of virgin raw materials. A couple of main reasons for this: (i) In China the introduction of environmental regulations and energy efficiency policies that resulted in the closure of highly polluting shaft and round kilns which were used for calcination of bauxite (Hutton, Yates, and Green, 2009). Although welcomed environmentally, this move led to shortages in refractory raw materials like bauxite and magnesia earlier, which were readily available for export even at lower costs (ii) Depletion of refractory quality bauxite and other refractory raw materials.This article presents a techno-commercial comparison between refractory raw materials Andalusite and Bauxite indicating the need for revival of Andalusite-based refractories in applications where they have been replaced by Bauxite.
Figure - RUL of alumina refractory bricks of various base raw materials (Source: HUBERT, P. Andalusite: a reactive mineral for refractories. Damrec’s internal document, vol. 1, 2001HUBERT, P. Andalusite: a reactive mineral for refractories. Damrec’s internal document, vol. 1, 2001)
Compared to bauxite as a refractory raw material, andalusite offers the following advantages:
Figure — Effect of slag penetration (Source: OVERBEEK, P.W. 1989. Andalusite in South Africa. Journal of the South African Institute of Mining and Metallurgy, vol. 9. pp. 157–171)
Refractoriness under load (RUL) is linked with the amount of glassy phase within the refractory system. Due to the low volume and high viscosity of the liquid phase formed and rigidity of predominant mullite phase, Andalusite-based refractories have a high refractoriness under load.
Andalusite-based refractories possess superior refractory properties than bauxite based-refractories and can thus be used in applications where bauxite-based refractories are currently used. Andalusite-based refractories exhibit better thermo-mechanical properties like hot strength, HMOR, refractoriness under load (RUL), creep resistance, and thermal shock resistance. Nevertheless, starting from 1990s many refractory manufacturers were giving preference to bauxite-based refractories over andalusite-based refractories mainly because of the availability of low-cost Chinese bauxite.
This trend started when alumina-bearing refractory raw materials such as andalusite, mullite, and in some instances even chamotte were being replaced by low-priced Chinese bauxite. At that time it was easy for refractory suppliers to provide their customers with refractories having higher alumina contents at a lower cost.
=> Refractories based on andalusite have an added economic advantage over bauxite-
based refractories in that unlike bauxite, which requires high-temperature calcination before use, andalusite is used in its raw state. Since andalusite does not require calcining it offers significant savings in terms of energy costs, transportation etc. See: Calcination of Refractory Raw Materials in different Kilns
=> The density difference between andalusite and bauxite refractories is typically 8–10%. Bauxite based linings therefore have a higher material demand compared to andalusite-based linings, a factor that needs to be considered in economic comparisons of lining concepts.
Andalusite-based refractories exhibit higher resistance to thermal shock. This is attributable to their typical network mullitic microstructure. The liquid glassy phase that is entrapped in the mullite crystal acts as a shock absorber that prevents crack initiation during thermal cycling.
Andalusite-based refractories show much superior slag corrosion resistance than refractories based on chamotte, bauxite, and bauxitic clays, in which even the smallest piece of material is still a composite of minerals between which the slag can penetrate. This is due to the dense, homogenous single-crystal structure of andalusite, in which there are virtually no channels of weakness along which slag can permeate and travel.