Mössbauer analysis of iron ore and rapidly reduced iron ore treated by micro-discharge using carbon felt

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1 J Radioanal Nucl Chem (2015) 303: DOI /s Mössbauer analysis of iron ore and rapidly reduced iron ore treated by micro-discharge using carbon felt Kiyoshi Nomura Paulo de Souza Shoji Hirai Norimichi Kojima Received: 6 August 2014 / Published online: 31 August 2014 Ó Akadémiai Kiadó, Budapest, Hungary 2014 Abstract Several samples of iron ores produced in Australia and Brazil were analyzed by Mössbauer spectrometry. Almost all iron ores are composed of hematite, goethite, and fine grains of the oxides, of which the ratios are different among production area. The amount ratio of hematite and goethite, determined by Mössbauer spectrometry, were almost consistent with those obtained by a conventional chemical analysis especially for iron ore with less goethite. Iron ores were treated by a micro discharge using carbon felt (MD/CF). It is found that FeO, Fe(CO 3 ), and Fe 3 O 4 are produced by MD/CF. The treatment by MD/ CF is efficient in rapid reduction of iron ores. Keywords Iron ore Hematite Goethite Mössbauer analysis Chemical analysis Micro discharge using carbon felt (MD/CF) Introduction Many iron ores are produced in sedimentary deposit, whereas the non-iron metals are more often included in igneous deposit. It is known as chemical property of iron K. Nomura (&) N. Kojima Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo , Japan dqf10204@nifty.com P. de Souza CSIRO Computational Informatics, College Road, Sandy Bay, Hobart, TAS 7005, Australia S. Hirai Nuclear Safety Engineering, Faculty of Engineering, Tokyo City University, Tamazutumi Setagaya-ku, Tokyo , Japan that ferrous ions are easily dissolved and moved in acidic solution, and that ferric ions are not so dissolved and easily deposited. The high contents of iron are included in sedimentary deposit. Iron ores are composed of several kinds of iron oxides such as hematite (a-fe 2 O 3 ), and magnetite (Fe 3 O 4 ), and iron oxy-hydroxides such as goethite (a-fe- OOH) and lepidocrocite (c-feooh) [1]. A chemical analysis is used as a Japanese Industrial Standard (JIS) method for determining the components of iron ores in industry [2]. Mössbauer spectrometry is not cited as a JIS method yet although it can determine many iron oxides. Using several tens of mg/cm 2 of some ores produced in Australia and Brazil, we determine the amount of iron compounds by Mössbauer spectrometry, and compare the iron contents with those obtained by a chemical analysis. On the other hand, dielectric materials easily absorb microwave energy, and the microwave applied to minerals and inorganic products are reviewed by Haque [3]. Carbon is quickly heated up to 1,000 C for 0.2 min. Hematite is not so quickly heated by simple irradiation of microwave. We have shown that Mn ferrite compounds could be synthesized from the mixed iron and manganese oxides by a micro discharge (MD) using carbon felt (CF) for a short time [4]. We tried to treat iron ores by MD/CF for rapid reduction of iron ores, and found that the irradiation of microwaves using carbon felt for several minutes is efficient for a reduction of hematite and iron ores. Experiment The content ratios of Fe 2 O 3, FeOOH, and Fe 3 O 4 were determined for five iron ore samples (A and B from Brazil, C, E and M from Australia) by JIS methods (JIS M8211, M8212, and M8213 [5] ) as follows. About 10 g of samples

2 1260 J Radioanal Nucl Chem (2015) 303: Table 1 Content ratios of various elements in iron ore by fluorescence X-ray analysis Sample Element Contents (2r) Ore-A Fe 99.3 % (0.22) (Brazil) Mn 0.25 % (0.06) Zn 0.14 % (0.03) Ba 0.06 % (0.03) Ore-B Fe 99.1 % (0.23) (Brazil) Mn 0.39 % (0.07) Ba 0.1 % (0.03) Ni 0.08 % (0.04) Ore-C Fe 99.3 % (0.21) (Australia) Mn 0.08 % (0.04) Ba % (0.003) Ti 0.08 % (0.04) Ni % (0.032) Ore-M Fe 99.3 % (0.23) (Australia) Mn 0.12 % (0.06) Ba 0.09 % (0.03) Ti 0.09 % (0.04) Ore-E Fe 99.0 % (0.22) (Australia) Mn 0.22 % (0.06) Zn 0.72 % (0.07) Ba 0.11 % (0.03) Ti 0.16 % (0.05) were dried by heating at 100 C for 8 h to desorb the absorbed water, and weighted precisely at room temperature. The samples were heated at 400 C for 1 h, and weighted again. The difference of weights between before and after heating is corresponding to the weight of crystal water released from FeOOH. 2FeOOH! Fe 2 O 3 þ H 2 O: The content ratios of Fe and impurity of iron ores are shown in Table 1. Almost all iron ores contain more than 99 % Fe and a small amount of Mn, Zn, Ba, Ti, and Ni as impurity. The standard sample (99.9 % a-fe 2 O 3 ), is used for reference. The contents of Fe 3 O 4 were calculated by the analysis of FeO because in this case, FeO is not wustite but one part of Fe 3 O 4. The Fe contents of Fe 2 O 3 were calculated by subtracting the Fe contents of FeOOH and Fe 3 O 4 from total Fe in iron ores. It is reported that several ppm of U and Th is also included. Especially iron ores with a large amount of FeOOH contain more Th, and iron ores with a small amount of FeOOH contain less Th than U contents [2]. MD/CF treatment is as follows; about 100 mg powder was set between two species of carbon felt with the space of around 1 mm. Carbon felt fiber is 1 lm in diameter, which is corresponding to the penetration depth of microwave. Microwave by 500 W was applied to samples. Mössbauer spectra of the same samples were measured at room temperature by using 57 Co(Cr) source and conventional transmission mode, and the velocity was calibrated by using a-fe. The area intensity of some components was determined by Mosswinn program [6] using Lorentzian peaks. Results and discussion Analysis of iron ore by Mössbauer spectrometry Mössbauer spectra of Australian iron ores (C, E, and M) are shown in Fig. 1. Mössbauer spectra of samples E and M are composed of the magnetic sextets and paramagnetic doublet peaks. The outer sextet with a large magnetic field (B hf = 51.5 T, isomer shift (IS) = 0.37 mm/s, quadruple splitting (QS, 2e) =-0.20 mm/s) is assigned to hematite (a-fe 2 O 3 ), and the inner sextet with a small magnetic field (B hf = 37 T, IS = 0.37 mm/s, QS (2e) =-0.30 mm/s) is to goethite (a-feooh) with a needle crystal shape. The broad sextet is due to fine grains of iron ores or to isomorphic compounds of Fe 3? substituted by such as Al 3?. However, Al 3? was not detected clearly by fluorescent X-ray analysis. Thus, the ore-e sample contains the more fine grains of hematite than the ore-m sample. The content ratios of a-fe 2 O 3 and a-feooh are almost consistent with those obtained by a chemical analysis especially for iron ores with a small amount of a-feooh. Fe 3 O 4 was missing in almost all iron ores because the Mössbauer spectrum of Fe 3 O 4 gives characteristic two sextets due to the site A (tetrahedral site: Fe 3? ) and the site B (octahedral site: Fe 2? and Fe 3? ). The broad doublet with IS of about 0.35 mm/s was observed in center of Mössbauer spectra, which suggests the fine particles of Fe 2 O 3 and FeOOH although the intensity was small. The Fe content ratios of Fe 2 O 3, FeOOH, and Fe 3 O 4 obtained by the chemical analysis described in the above experiment are shown in Table 2. It is found from the chemical analysis that most of iron ores contain only 1 % or less than 1 % of Fe 3 O 4. It was hard to recognize the presence of Fe 3 O 4 by Mössbauer analysis. For the samples with relatively large amounts of goethite, Mössbauer spectra were decomposed into the hyperfine distributions because goethite is a fine crystal with needle shape. The goethite does not give a single sextet with sharp peaks, but the broad sextets with tailing peaks. The Fe content ratios of Fe 2 O 3 and FeOOH were calculated from the hyperfine distributions of the magnetic components, and also from fitting Mössbauer spectra, assuming that the outer broad sextets of a-fe 2 O 3 consist of two sextets and the inner broad sextets consist of three sextets. The each area intensity of the Mössbauer sub-

3 J Radioanal Nucl Chem (2015) 303: Fig. 1 Mössbauer spectra of Austrian iron ores (C, M, and E) and hyperfine distributions of Austrian iron ores (M and E) Table 2 Fe content ratios of iron compounds in various iron ores Sample name Chemical analysis Mössbauer analysis Fe 2 O 3 (%) FeOOH (%) Fe 3 O 4 (%) a-fe 2 O 3 (%) a-feooh (%) Super paramag. Fe 3? comp. (%) a Area intensity from hyperfine distributions b Assumption of two sextets of Fe 2 O 3 and three sextets of FeOOH Sample A Sample B Sample C Sample E a 58.9 a b 63.2 b 7.8 Sample M a 33.9 a b 39.8 b 6.1 spectra decomposed is shown in Table 2. The Fe contents are a little deviated from that obtained by a chemical analysis. It is considered that the difference is produced mainly due to a fitting procedure and to different Debye Waller factor of oxides and oxy-hydroxides (0.95:1.0) [7]. Comparing with a chemical analysis, Mössbauer analysis can give the steady identification of some oxides from at least three Mössbauer parameters although the quantitative analysis contains much error, depending on fitting procedures. Mössbauer spectrometry is suitable for qualitative analysis. For instance, a-feooh can be distinguished from c-feooh and b-feooh, and further from ferrous compounds, and a-fe 2 O 3 also can be distinguished from the different crystals of b-, c-, and e-fe 2 O 3 [1]. Reduction treatment of iron ore by micro-discharge (MD) using carbon felt (CF) 2 A sample can be heated up to 1,000 C by MD/CF after about 10 s. Mössbauer spectra of pure a-fe 2 O 3, treated in alumina crucible with a cup by MD/CF with 500 W for

4 1262 J Radioanal Nucl Chem (2015) 303: (c) for 6 min. (b) for 4 min. (a) for 2 min. Fig. 2 Mössbauer spectrum of pure a-fe 2 O 3 treated by MD/CF for a 2 min, b 4 min, and c 6 min various minutes are shown in Fig. 2. Three sextets and one doublet were observed in addition to the outer sextet of the residual a-fe 2 O 3. The inner sextet with magnetic field B hf = 33.3 T is assigned to metal Fe, and the doublet with IS = 0.9 mm/s is to FeO. Two sextets with B hf = 48.9 T and IS = 0.30 mm/s, and with B hf = 45.7 T and IS = 0.72 mm/s come from Fe 3? in tetrahedral site (site A) of inverse-spinel, magnetite, and from the Fe 2? and Fe 3? species occupied in octahedral site (site B), respectively. Thus, it is clear that the atmosphere in alumina crucible with a cup becomes the reducing atmosphere because carbon fibers react with the lattice oxygen of iron ore by micro-discharge. The treated samples contained a lot of iron species produced. There remained a lot of a-fe 2 O 3 because the powder of pure a-fe 2 O 3 might not be uniformly sputtered on the carbon felt. With increase of the treatment time by MD/CF, the intensity of a-fe 2 O 3 decreased and the intensity of metallic Fe increased. Figure 3. Two or three sextets and two doublets were also observed in Mössbauer spectrum of sample ore-a, and ore- M, treated in alumina crucible with a cup by MD/CF for 2 min. The Mössbauer parameters of one doublet were IS = 0.96 mm/s, and small QS = 0.73 mm/s, due to Fe 2? species of wustite (FeO). Wustite has many iron (II) species with iron deficiency such as Fe 1-x O; Fe 0.9 O:A: IS = 0.91 mm/s, QS = 0.46, B: IS = 0.86 mm/s, QS = 0.78 mm/s [8]. Another doublets with IS = 1.21 mm/s and QS = 1.76 mm/s or large QS = 2.78 mm/s were observed due to formation of siderite (FeCO 3 ) although the QS values were the same as or larger than that expected for this mineral (IS = 1.24 mm/s, and QS = 1.80 mm/s [9]). The large QS values might be induced by dispersion of the fine FeCO 3 produced in iron oxide matrix. The a-fe 2 O 3 did not remain in the treated ore-a sample. Unquestionably, the micro-discharge method produces a noticeable fast reduction of all treated iron ores. Comparing with the treated iron ores, the treatment by MD/CF for pure a-fe 2 O 3 seems to be less effective. The impurity elements included in iron ores may play a role of catalysis for reduction although it is unclear what elements work very well. In addition, sample powders might not be set so well in uniform between carbon felts. In this step, some factors affect the intermediate products, but the treatment of MD/CF in closed system is very effective for reduction of iron ore. Fig. 3 Mössbauer spectra of iron ore-a (Brazil) and iron ore-m (Australia) treated by MD/CF for 2 min

5 J Radioanal Nucl Chem (2015) 303: Conclusions We have analyzed iron ores by Mössbauer spectrometry, and compared the results with those obtained by a chemical analysis method. Australian iron ores contain more goethite than Brazilian ones. For the iron ore with a few of goethite, the ratios of goethite and hematite analyzed by Mössbauer spectrometry were almost consistent with those obtained by a chemical analysis, but for the iron ores with much goethite, the ratios were deviated due to fitting of fine grains of goethite and the Mössbauer Debye Waller factor. However, many kinds of iron species can be identified by Mössbauer spectrometry. Many iron species such as the metal Fe, FeO, magnetite, fine Fe 3? oxides, and siderite are produced by treating pure a-fe 2 O 3 and iron ores by MD/CF in addition to the remained a-fe 2 O 3. Iron ores were reduced more rapidly than pure a-fe 2 O by MD/CF. Although some experimental conditions affect the intermediate products, the treatment by MD/CF for several minutes is very effective for rapid reduction of iron ores. References 1. Ujihira Y, Nomura K, Analyses of corrosion products of steel by conversion electron Mössbauer spectrometry, published by Research Signpost (India) 1996, ISBN: Yasutaka M, Hirai S (2011) Jpn J Iron Steel 97:70 3. Haque KE (1999) Int J Miner Process 57: JIS M 8211:Iron ores Method for determination of combined water content, Japanese Standards Association (1995). JIS M 8212:Iron ores Determination of total iron content, Japanese Standards Association, (1995). JIS M 8213:Iron ores Method for determination of acid soluble iron (II) content, Japanese Standards Association (2005) 5. Kurihara H, Yajima T, Nomura K (2009) J Powder Metallurgy Assoc 56: Klencsár Z, Mosswinn 3.1 program 7. Meisel W (1973) Proc. 5th Int. Conference on Mössbauer spectroscopy. Hucl M, Zemcvik T (ed), p Koch F, Cohen JB (1969) Acta Cryst B25: Ono K, Ito A (1964) J Phys Soc Jpn 19:899

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