Effect of a and gamma polymorphs of alumina on the preparation of MgAl_2O_4-spinel-containing refractory cements

摘要

The use of aluminous refractory materials containing MgAl_2O_4-spinel has led to a major breakthrough in the service life of refractory coatings applied in the industries and in the quality of the products. Many researches have been conducted to improve the synthetic procedures in order to reduce the production cost of these materials. In this way, refractory cements involving in situ generated spinel phase have been obtained from mixtures of active alumina and dolomites. Other investigations have demonstrated that gamma alumina is advantageous in comparison with the a polymorph in the synthesis and sintering properties of pure MgAl_2O_4-spinel. In this article, the performance of both polymorphs of alumina, used as raw materials in the preparation of the refractory cements, along with dolomite proceeding from Olavarria, in the centre of Buenos Aires Province (Argentine), is compared. The thermal and structural changes which take place during the firing of the batches up to 1450 °C were studied by the combination of diffractometric and infrared spectroscopy data, at the most remarkable reaction steps. According to these results, the study of phase changes within the investigated thermal range allowed to establish the main differences in the composition of both mixtures in each firing step. Independently of the type of alumina used, a mixed phase product consisting of spinel, as a major phase, accompanied by CaAl_2O_4 (CA) and CaAl_4O_7 (CA_2), as secondary phases, was obtained. In addition, it was found that the formation of these phases at lower temperature is favoured by gamma-Al_2O_3.