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Synthesis and sintering of chromium-free complex spinels in the MgO-Al2O3-FeOx-Me4+O2 systems Lodha, Rahul

Abstract

Magnesia-chrome refractory ceramics are used in non-ferrous industry because of their corrosion resistance against fayalite-type slags, rich in FeO. Unfortunately, Cr³⁺ may oxidize to Cr⁶⁺ during the smelting and converting processes, thus making their spent pot-lining hazardous. Two strategies are explored in this present work to develop chrome-free refractories sinterable at lower temperatures. The first is to replace Cr³⁺ with tetravalent cations (Me⁴⁺) that form complex spinel with MgO and Al₂O₃ at 1350-1550 ºC. These ions could promote simultaneous synthesis and sintering through formation of non-stoichiometric spinel at high temperatures and re-precipitation of the complex spinel during cooling. The second objective is to study the contribution of nano-size powders of the spinel formers to achieve both synthesis and sintering of Cr-free spinel solid solutions. The current research proved that synthesis and sintering of such spinels, or their solid solutions, could be performed at much lower temperatures (1350-1550 ºC) than the Cr³⁺-bearing version, which requires firing at 1750-1850 ºC to achieve equivalent properties. It is hypothesised that the defect super-structure due to Me⁴⁺ ions inducing the spinel phase formation at relatively low temperatures (1350-1550 ºC) results in the simultaneous synthesis and sintering of these complex spinels. The defect super-structures formed at lower temperatures have a higher lattice parameter than the final spinel solid solution phase. The volume expansion due to spinel formation that retards densification is also overcome due to the shrinkage in volume of the complex spinel phase. The results indicate that addition of Fe₂O₃ in MgO-Al₂O₃-Me⁴⁺O₂ systems results in improved reaction sintering with an open porosity of ~4% at 1450 ºC. However, the narrow particle size distribution for preparation of aggregates makes it necessary to fire the pressed specimens at higher temperature or in two stages in order to achieve similar open porosity to the co-clinkered aggregates for basic refractory ceramics. This research work contributes to the preparation of chrome-free aggregates and binding systems and show that synthesis and sintering of magnesia-spinel phase led to development of chromium-free binding phase for basic castables with comparable flexural and compressive strength to magnesia-chrome bricks with firing temperatures below 1500 ºC.

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