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INTRODUCTION
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.
COMMERCIAL FACTORS
=> 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.
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.
Compared to bauxite as a refractory raw material, andalusite offers the following advantages:
- An andalusite crystal remains stable even after firing at high temperature and therefore, does not require any preparatory firing before use whereas raw Bauxite requires calcination at 1600-1650°C to make it suitable to be used as refractory raw material. Since andalusite does not require calcining it offers significant economies in that it saves energy, an advantage that is certain to be of importance in the light of increased energy costs.
- Andalusite can be used at its natural grain size as a refractory aggregate in the manufacture of refractory bricks.
- Andalusite-based refractories offer better scope for control of dimensional tolerances, warpage etc. which is very helpful in the manufacturing of complex refractory shapes.
- Andalusite has a higher chemical purity than other refractory raw materials, particularly bauxite and chamotte. The andalusite grains are consistent up to a few millimetres, each grain being a single monolithic crystal with a very low open porosity.
- Because of very low content of impurities (Na2O, K2O, MgO, CaO, Fe2O3, TiO2) we have a very small amount of glassy phase at high temperatures. This provides a high resistance to thermal shocks, high creep resistance, a high temperature of deformation. These parameters are much higher than those of bauxite-based refractories or corresponding alumina-containing refractories based on mixtures of bauxite with any other alumina-containing raw material.
- The ability of andalusite to form mullite phase at elevated temperatures, provide high strength with resistance to physical and chemical corrosion. In the fired state andalusite-based refractories contain mullite as the major phase along with glass, whereas bauxite-based refractories contain corundum as the major phase along with minor quantities of mullite and glass.
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)
A Techno-Commercial Comparison between Andalusite and Bauxite in their applications as Refractory Raw Material
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.
TECHNICAL FACTORS
Technical advantages of using Andalusite in place of Bauxite
Superior Refractory Properties of Andalusite-based Refractories than Bauxite-based Refractories
Thermal Shock Resistance
- The refractories industry is in general dependent on China for raw materials, for example 95% of refractory bauxite production is controlled by China. On the other hand, China changed its policy, and now discourages the export of raw materials. This has been brought about by limiting the volume of material by imposing a system of export licenses, fees, and taxes. None of these costs are borne by Chinese domestic producers and also, exports of finished products made from these raw materials are not taxed. Due to the licensing and taxation of exported raw materials, Chinese producers of refractories enjoy a cost advantage in export. The net result of these measures by China is that international demand for raw materials exceeds supply, and a steep rise in price of refractories in the global market which has rendered refractory suppliers in many countries at significant disadvantage compared to Chinese refractory producers.
- Refractory suppliers for their viability have to turn to alternative raw materials that can serve the same purpose. One such example is the use of andalusite refractories in applications where bauxite is currently being used. South Africa is the biggest producer - supplier of high quality refractory grade andalusite and has no export restrictions. This gives a guarantee for stability with respect to both availability and consistency in quality and price (unlike Chinese Bauxite which is subject to export restrictions), a reason that the refractory manufacturing industry cannot afford to ignore for their sustenance. Andalusite : Mineralogy, Properties, Occurrences and Deposits along with a glimpse of Andalusite Industry and its Market
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.
High Refractoriness Under Load (RUL)
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)
High Resistance to Chemical Attack and Penetration by Slag and Metal
Techno-Commercial
CONCLUSIONS
- For applications where refractoriness and hot mechanical properties are the key parameters to complete the appropriate performance, andalusite is the most convenient raw material for temperature above 1400°C. At lower temperature, chamotte 60% Al2O3 seems suitable. A good compromise could be a combination of andalusite and chamotte 60% Al2O3. In any case, this combination must be preferred to that with calcined bauxite only. Results compared after testing LCC made with refractory aggregates of andalusite, chamotte 45% Al2O3, chamotte 60% Al2O3, Chinese bauxite 90% Al2O3 showed andalusite-based low cement castables with more stable physical and mechanical properties in a large range of firing temperature. This result is explained by the andalusite mullitisation process which reinforces the microstructure of the castable because of Mullite bonding. Consequently, andalusite serves to improve high temperature mechanical or physical properties of high alumina low cement castables. (Hubert et al., Proceedings of Tehran International Conference on Refractories, 4-6 May 2004)