Caracterização mineralógica do minério fosfático da Mina de Tapira - MG

Abstract

This work aimed to perform a detailed mineralogical characterization of phosphate ore types from deep parts of the Tapira mine, located in Minas Gerais State, Brazil, that are rich in carbonates and silicates, with the purpose to get suitable information for their concentration process improvement. The Tapira mine shows a heterogeneous mineralogical association and low grade of apatite, which leads to a complex industrial beneficiation process. Nowadays, the industrial concentration plant is partially fed with ore containing higher proportions of carbonates, what has caused problems in the apatite concentration by froth flotation, whenfatty acid collectors are used. This study was conducted on 12 ore samples prepared by crushing, grinding, desliming and size separation. The analytical methods used were: X-ray diffraction, chemical analysis, optical microscopy, scanning electron microscopy, chemical X-ray microanalysis (EDS energy-dispersive spectrometry and WDS wavelengthdispersive spectrometry) and infrared spectrometry (FTIR). Purified apatite concentrates from each sample were obtained in the laboratory by flotation and heavy liquid and magnetic separations. The results show that, although the 12 samples are classified as clinopyroxenites, they have variable modal proportions of the essential minerals, what influence in the choice of a suitable flotation reagent, aiming to a better apatite concentrate. The samples studied are classified in four ore types, based on the mineralogical characteristics and alteration degree: slightly altered clinopyroxenites (samples 2, 5 and 10), micaceous clinopyroxenites (samples 1, 4 and 9), argillaceous clinopyroxenites (samples 11 and 12) and clinopyroxenites rich in magnetite and perovskite (samples 3, 6, 7 e 8). The essential minerals are diopside, apatite, carbonates, micaceous minerals, magnetite, perovskite, anatase, quartz and ilmenite. Garnet (melanite) occurs in sample 4 and smectite (nontronite) in samples 11 and 12. Very lowamounts of amphibole, titanite, hematite, goethite, zircon, pyrite, monazite, barite, zirconolite, Mn, Ba and Fe oxide (probable hollandite and/or romanechite), and secondary phosphates of the rhabdophane and crandallite groups are present. Micaceous minerals were identified mainly as mixed-layer phyllosilicates (phlogopite/vermiculite and locally biotite/vermiculite) and pure vermiculite. Pure phlogopite and chlorite are also observed. The carbonates appear mainly as veinlets or as inclusions in apatite, mainly as calcite and dolomite. The carbonatescontent varies from 1% (samples 1 and 9 micaceous clinopyroxenites and 11 and 12 argillaceous clinopyroxenites) to 32% (sample 10 slightly altered clinopyroxenite), although the contents between 11% and 26% predominate. The apatite in the 12 samples is classified ascarbonate-strontium-fluorine-hydroxiapatite, is essentially primary and occurs as clean crystals, although fluid inclusions and inclusions mainly of carbonates are commonly observed. The apatite characteristics suggest that these samples would have a good performance in the froth flotation process, with high recovery. However, the problem could be their selectivity in relation to gangue minerals. Among the main calcium-bearing minerals occurring in the 12 clinopyroxenite samples of the Tapira mine (diopside, carbonates, perovskite, apatite, melanite and nontronite), the carbonates are the major contaminants of the concentrates (contents varies from <1% to 6%), with quantities of less than 1% of diopside, monazite, barite, perovskite, micaceous minerals, melanite and nontronite. The textural relations between apatite and carbonates indicate that a totally clean apatite concentrate would be impossible to achieve, since an excessive grinding of the ore would be needed.