Controle de qualidade de bauxita: otimização do método via úmida para RxSiO2 e método alternativo via WDXRF para AvAl2O3

Abstract

In the metallurgical bauxite industry the quality control of the ore is performed by two chemical parameters, namely: available alumina (AvAl2O3) and reactive silica (RxSiO2), determined according to a procedure that simulates Bayer digestion in laboratory scale. They are parameters for prospective control, mining, processing and ore feeding in the Bayer process. For gibbsite bauxites, which are of interest in this work, AvAl2O3 is associated with the mineral-gibbsite and RxSiO2 to the mineral kaolinite, which is undesired and controlled in the Bayer process because it reacts with NaOH (leaching agent) to form Bayer sodalite, causing irreversible losses of NaOH - which means a considerable increase in the production cost of alumina. Bayer sodalite is a sodium aluminosilicate, insoluble in Bayer digestion conditions and therefore discarded along with the other insoluble bargain minerals in the red mud (insoluble solid residue of the Bayer process). In this context, we sought to: (1) optimize the Alcan method, aiming acid economy by proposing new acid dissolution conditions varying acid type (HCl, H2SO4 and HNO3), acid volume (20, 25 and 30 mL) and (2, 2.28 and 3.8 g), one of the steps for the determination of RxSiO2 and (2) proposing an alternative method for the determination of AvAl2O3 by WDXRF (X-Ray Fluorescence Spectrometry by Length of Dispersive Wave) through mass balance in digestion. Secondary patterns of red mud were properly characterized (XRD, DTA and XRF) and calibrated bauxite and red mud analytical curves with optimal response for linearity and reproducibility. The acid dissolution study of sodalite Bayer showed potential savings in the volume of acid for 25 mL of HCl for ores with up to 45% RxSiO2 and 20 mL for ores with up to 9% of RxSiO2. The HNO3 and H2SO4 presented discrepancies between the contents dissolved in different contents of RxSiO2, whose main influence was the salt used for the effect of the common ion, since NaCl is used in the Alcan method for HCl. Therefore it is necessary to investigate the best salt for dissolution tests with these acids. The mass balance by mineralogical estimation proved more efficient for the future development of the alternative proposal for AvAl2O3 determination studied in this work.