Análise do teor de flúor em alumina do centro de tratamento de gases que alimenta as células electrolíticas da Mozal

Abstract

During the electrolytic reduction process, gases are generated, including fluorides, which are recovered by exhaustion and taken to Treatment Centers for Gas (CTG). Carrying out this treatment has environmental and economic impacts, as fluorides, both gaseous and particulate, are harmful. By recovering efficiently these gases, the supply of secondary alumina to the reduction, with 1.8% of fluorine, which allows to reduce the amount of trifluoride of additional aluminum, as around $2,000/kg AlF3 are needed for 19.7 kg AlF3/t Al produced. The present work focuses on the analysis of the fluorine content present in alumina secondary present in filters of a CTG, intending to analyze its causes, focusing on the impact of variations in gas flow and feed rate of alumina in the process. For this, the filters were operated at different volumetric gas flows, the primary alumina feed rate to one of the filters was doubled and analyzed the effect of these variations in fluoride concentration. This study was carried out at MOZAL, an aluminum foundry company located in the Industrial Park of Beluluane, in Mozambican province of Maputo. At MOZAL, gas treatment is carried out in CTGs, better known locally by the English acronym GTC (Gas Treatment Center), equipped reactors with Venturi configuration, connected to bag filters, where it is possible control the volumetric gas flow and alumina flow that enters the center. The analysis comprised 4 filters of CTG 1, namely, filters 4, 6, 8 and 13, with the first had a history of good percentages of fluoride and the remaining three showed values ​​below the target. Measurements were taken of temperature and pressure so that the calculation of the gas flow could be carried out, having been registered as maximum value 32.58 m 3 /s and minimum 13.51 m 3 /s. On the same filters, samples of fluorinated alumina and its spectroscopic characterization was carried out, in order to analyze the fluorine concentration, by X-Ray Fluorescence (FRX). Initially, the values ​​of percentage of fluorine in alumina ranged between 0.78% and 6.20%. At the end of the study, it was possible to maintain fluoride values ​​between 1.37 and 3.78%, and it was possible to observe that low gas flows and high feed flow rates negatively influence the concentration of fluorine in alumina (TRADUÇÃO NOSSA).