Diferenças nas respostas à sulfatação de óxidos e hidróxido de ferro (III) visando à extração seletiva de minérios estratégicos ricos em ferro

Abstract

Sulfation is a technique applied to the extraction of mineral commodities, such as rare earth, nickel, and titanium, as a preparation stage for leaching. Iron oxides and hydroxides are mineral constituents commonly present in these ores. Thus, the response of iron compounds to sulfation will directly impact acid consumption as well as the complexity and costs related to subsequent purification steps. The sulfation of three hematites (granular, specular and produced by roasting), magnetite and goethite samples were investigated over the increase in temperature (80 - 240 °C) and time (5 - 30 min). This study focused, in particular, on the influence of phase transformation, physical characteristics, and the crystalline structure of minerals on the response of iron dissolution. The samples showed different behavior. At 80°C, the fraction of solubilized iron varied from 2 to 70%, according to the sequence: specular hematite < calcine hematite < granular hematite < magnetite << goethite. The increase in temperature from 80°C to 160°C resulted in increased sulfation of magnetite (42.0 to 95.0%), calcine hematite (16.0 to 55.5%) and specular hematite (2.0 to 13.0%), with little or negative effect on the sulfation of goethite (70.0 to 76.0%) and granular hematite (33.0 to 25.0%). For all samples, the reacted fraction increased rapidly with the sulfation time (5 to 15 minutes) and stabilized on a plateau, which was attributed to the formation of the ferric sulfate layer. The removal of this layer allowed for the sulfation to proceed and increase in iron dissolution in 22% for rhe granular hematite and in 11% for the specular hematite in the subsequent stages. The phase transformations of goethite and magnetite affected sulfation rates. The reduction of iron dissolution from goethite was attributed to the formation of a superficial layer of hematite, a less reactive phase over sulfation. Magnetite, on the other hand, showed the highest reacted fraction (~95%) at 240°C. The formation of vacancies, favored by phase transformation to maghemite, combined with the presence of ferrous ion in the crystalline structure, were considered key factors in the increase of reacted fraction over temperature. The specular hematite showed reacted fractions below 13% in all experimental conditions, due the predominance of the less reactive (006) plane in a sulfuric medium. A detailed and thorough review of the thermodynamic data of the HSC software was also undertaken, to select data for the selective leaching process analysis of lanthanum and thorium in the presence of iron, in a medium containing phosphate and sulfate species. Overall, 1251 comparisons of thermodynamic data from 10 primary sources were evaluated, resulting in a revised database with 83 species. The significant inconsistencies found were attributed to probable typing problems when transposing the data, unit conversion, as well as the combination of data from different sources. The risks of erroneous conclusions in modeling and thermodynamic simulations, without previous careful investigation of the data used, were illustrated through Eh-pH diagrams of the selected species.