A Comparative Study of Iron Concentrates by Mössbauer Spectroscopy and Optical Microscopy

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1 Hyperfine Interactions 134: , Kluwer Academic Publishers. Printed in the Netherlands. 187 A Comparative Study of Iron Concentrates by Mössbauer Spectroscopy and Optical Microscopy N. M. TORÍBIO 1,G.M.DACOSTA 2,,K.G.DESÁ 1 andc.b.vieira 3 1 Samarco Miineração SA, Mariana, MG, Brazil 2 Departamento de Química, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil; magela@iceb.ufop.br 3 Departamento de Engenharia Metalúrgica, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil Abstract. Five samples of iron-ore concentrates used for the production of pellets were heated at different temperatures under air atmosphere and characterised by chemical analysis, optical microscopy and Mössbauer spectroscopy. In all samples it was observed the presence of specular hematite, martite, and goethite in different proportions, whereas one sample also contains 15% (w/w) of magnetite. The presence of residual goethite was observed in the samples heated at 300 C, whereas the magnetite was only converted to hematite after heating at 900 C. However, the results of the optical microscopy pointed to the existence of large amounts of goethite even in the samples heated at 900 C, which is obviously incorrect. Therefore, it can be concluded that the reflectance of hematite derived from goethite is similar to that of the original goethite, limiting the use of optical microscopy for these types of samples. Key words: optical microscopy, Mössbauer spectroscopy. 1. Introduction The identification and quantification of the different mineralogical phases present in most iron ores are of crucial importance for the quality control, mine planning, processing, etc. In recent years the use of optical microscopy is becoming the most usual technique to access the mineralogical composition of these ores. An experienced operator is able to identify by reflected light goethite, magnetite, martite and specularite. The quantification is done by counting at least 500 particles of each phase, and then converting the volumetric percentage into weight using the theoretical densities of each phase [1]. Being aware that Mössbauer spectroscopy is a very useful technique that can directly furnish the relative proportions of several iron oxides/hydroxides, we present in this work the results of a comparative study of some iron ores using Mössbauer spectroscopy and optical microscopy. Corresponding author.

2 188 N. M. TORÍBIO ET AL. 2. Results and discussion Five iron-ore concentrates from the Alegria Mine (Mariana, MG, Brazil) with different proportions of the goethite, hematite and magnetite were obtained in a pilot plant. Chemical analysis of total and ferrous iron was made by titration with potassium dichromate, the other elements were determined by ICP, and the loss on ignition was determined after heating the samples at 1000 C during 2 hours (Table I). Each ore was heated in a furnace under air atmosphere at 300 C, 600 C, 900 C and 1100 C during approximately 30 minutes. Mössbauer spectra were obtained in a spectrometer operating at constant acceleration and with a triangular signal. The relative areas were calculated considering that both ferrous and ferric ions have the same recoil fraction. As the amount of magnetite is small, no serious errors were introduced with this approximation [2]. A light-reflected microscope (Leica, model MPS30) with a magnification of 200 times was used for the particles counting. Fractions below and above 325 # were used, and at least 500 particles of each phase were identified and counted. The volumetric percentage was converted into weight percentage using the theoretical densities of each mineral [1]. Mössbauer spectra at room temperature of the original and heated samples are shown in Figure 1, from which it can be seen that the goethite content is higher in sample NO5, whereas the content of magnetite is higher in sample NO2. The spectra for the samples heated at 300 C showed that most of the goethite was converted to hematite, and above this temperature no traces of this phase was found in any sample. The relative areas obtained from the numerical analysis of the spectra are listed in Table II. All these samples were also investigated by optical microscopy, and the results obtained according to the procedure described above are listed in Table II. As can be seen from Table II there is a good agreement between the results obtained by optical microscopy and Mössbauer spectroscopy for non-heated samples, but an enormous difference was observed in the heated samples. For instance, according to the Mössbauer results the original amount of goethite in sample NO5 was 69%, and only 9% remained after heating at 300 C. On the other hand, the results from optical microscopy indicated the existence of 75% of goethite in NO5 Table I. Partial chemical analysis of the iron-ore concentrates Sample NO1 NO2 NO3 NO4 NO5 FeT FeO SiO Al 2 O LOI

3 A COMPARATIVE STUDY OF IRON CONCENTRATES 189 Figure 1. Mössbauer spectra at room temperature of the original samples (left) and samples heated at 300 C (right). Table II. Comparative results between optical microscopy (OM) and Mössbauer spectroscopy (MS) Sample Mineral Original samples (%) Heated at 300 C(%) OM MS OM MS NO1 Hematite Goethite Magnetite 4 n.d. 5 n.d. NO2 Hematite Goethite n.d. Magnetite NO3 Hematite Goethite n.d. Magnetite 1 n.d. 2 n.d. NO4 Hematite Goethite Magnetite 6 n.d 3 13 NO5 Hematite Goethite Magnetite n.d n.d n.d. n.d. n.d.: not detected.

4 190 N. M. TORÍBIO ET AL. Figure 2. Temperature dependence of the hyperfine fields of hematites derived from goethitic iron ores. and of 63% in the heated sample. The same discrepancy was observed for the samples heated at 600 C and 900 C, i.e., goethite was supposedly present in these samples. This result is beyond any doubt wrong because goethite is converted to hematite at about 300 C [3]. Thus, it can be concluded that the reflectance of hematite derived from goethite seems to be similar to that of the original goethite, and care must be taken when interpreting data from optical microscopy. Another important result derived from the Mössbauer spectra of the heated samples is shown in Figure 2, from which it is seen that there is a steady increase in the hyperfine fields of the hematite phase with the firing temperature. Furthermore, the hematite produced from those samples with larger amounts of goethite (NO5, for example) systematically has a lower hyperfine field, which means that the crystallinity of the particles is lower [4, 5]. Thus, either higher temperatures or longer heating times must be used to obtain a bulk hematite from these ores. This result may have some implications in the production of pellets from highly goethitic ores. 3. Conclusions The mineralogical analysis of iron ores made by optical microscopy showed a good agreement for all iron phases with the results obtained by Mössbauer spectroscopy. On the other hand, an enormous difference in the goethite contents was observed for the samples heated above 300 C, and thus some care must be taken when interpreting data from optical microscopy. It can also be concluded that the reflectance of hematite derived from goethite seems to be similar to that of the original goethite.

5 A COMPARATIVE STUDY OF IRON CONCENTRATES 191 Acknowledgements This work was partially funded by Samarco Mineração S.A., CNPq and Fapemig (Brazil). References 1. Ferreira, C. M., Anais do Workshop em Geologia e Estratigrafia de Minério de Ferro, SBG Bol. 12 (1993), De Grave, E. and Van Alboom, A., Phys. Chem. Minerals 18 (1991), Cornell, R. M. and Schwertmann, U., The Iron Oxides, VCH Publishers Inc., New York, Verbeeck, A. E., De Grave, E. and Vandenberghe, R. E., Hyp. Interact. 28 (1986), da Costa, G. M., Galvão da Silva, E. and de Jesus Filho, M. F., Hyp. Interact. 67 (1991), 501.