Caracterização e estudo das propriedades eletrocinéticas dos minerais de ferro: Hematita, Goethita e Magnetita

Abstract

Iron oxides are the main minerals in Brazilian iron deposits, hematite, magnetite, and goethite being the most important. These ore-minerals present different chemical, physical, and electrokinetic properties. Aiming to better understand the mineralogical and electrokinetic properties of the main iron minerals the target of this investigation was: to characterize mineralogically hematite, goethite and magnetite samples collected in the Iron Quadrangle, Serra of Carajás region and pegmatites area; and to measure their electrokinetic properties by microelectrophoresis and the empirical method of Mular and Roberts. The main mineral phases identified by XRD, optical microscopy, SEM/EDS, and infrared spectrometry in the compact hematite sample were: hematite; in the specular hematite sample: hematite, and quartz; in the martite-hematite sample: hematite, and quartz; in the mixed Carajás hematite sample: hematite, goethite, and gibbsite; in the goethite sample: goethite, hematite, quartz, and gibbsite; and in the magnetite sample: magnetite, and hematite. The compact hematite showed granular crystals and botryoidal goethite was present in the goethite sample. The specular hematite, martite-hematite, mixed Carajás, and magnetite samples showed the martitic morphological variety of hematite. In the specular hematite and martite-hematite samples lamellar crystals were also observed. The variation of microcrystalline hematite was observed only in the mixed Carajás hematite sample. All samples showed mesoporous and macroporous range, and goethite showed the highest pore volume among the samples studied, followed by mixed Carajás hematite. The martite-hematite presented higher pore volume than magnetite, compact hematite and specular hematite. Consequently, the goethite presented the largest specific surface area (10.9m2/g) followed by mixed Carajás hematite (5.2m2/g). The specific surface areas of the remaining samples were lower: martite-hematite 1.8m2/g, magnetite 1.0m2/g, compact hematite 0.81m2/g, and specular hematite 0.80m2/g. The presence of quartz and silicates in the samples compact hematite, specular hematite, and martite-hematite left the isoelectric point, IEP, pH lower than the value reported in the literature (pH 6.7). The presence of the mineral gibbsite in the samples mixed Carajás hematite and goethite yielded values of the IEP pH higher than those reported in the literature. Magnetite presented the highest coefficient of variation between the values of the IEP pH obtained by the microelectrophoretic (12.0%) and Mular and Robrets (3.8%) methods due to difficulties in sample preparation caused by the ferrimagnetic character of the mineral resulting in a tendency to particles aggregation.