Agentes mineralizadores e seus efeitos nas características morfológicas e físico-químicas de aluminas calcinadas

Abstract

Calcination is an important step in the alumina production process, affecting the final properties of the resulting powder and the attained product. Over the previous decades, attempts have been conducted to understand this process aiming to decrease the calcination temperature and time, additionally to the improvement of the alumina properties. In this sense, the use of additives is promising as these mineralizers, like fluorides, act decreasing the transformation temperature from the transition alumina to the alpha phase. In this context, this study addressed the understanding of the influence of three different additives (solid fluoride, soluble fluoride and seeds) on the properties of the calcined alumina, considering the actual condition of the industry and focusing in technical aspects such as morphology, surface area, primary crystal size, alpha content and the soda decrease. There are several theories about the working reaction of these additives, however, no experimental procedure was carried out to prove those mechanisms. In this work, the AlF3 and the soluble fluoride's mechanism reaction were investigated by using DTA-TG-MS techniques. The results showed that the presence of fluoride in transition aluminas leads to the generation of gaseous compounds which are responsible for speeding up the mass transportation, thus, decreasing the temperature and energy required for obtaining the alpha phase. These mineralizers also influenced the alumina properties, such as an increase in the primary crystal size and a decrease in the surface area. The optimum properties can be attained by the right balance between temperature and mineralizer content. Other additives sources, such as NH4Cl, can be used for decreasing the soda content. This work highlights that their efficiency is directly related to a suitable gas confinement inside of the calciner.