Iron ore is the second-most traded mineral commodity on the market, mainly for the manufacture of cast iron and steel (98% of the world production) [1
]. It corresponds to 15% of the products exported by Brazil, which stands as the third-largest iron-producing country and holds 12% of the world reserves, located mainly in the provinces of Quadrilátero Ferrífero, Minas Gerais (MG) and Carajás, Pará (PA) [4
The exhaustion of high-grade iron ore deposits from the main producing regions, located in Brazil, Australia, India and others, coupled with increased demand in the world market and increasingly severe environmental restrictions, have imposed a great challenge for the mineral industry. This current situation implies the development of beneficiation routes for marginal ores, aiming at greater metallic recovery and the minimization of tailings disposal, as well as the reprocessing of tailings deposited in dams, with Fe grades greater than 30%, to obtain products within the specifications for the steel industry [6
Mineralogical compositions of slimes from desliming operations of industrial flotation circuits of Brazilian iron ores have a high proportion of goethite (in some cases, >50%), followed by martitic hematite; quartz and smaller proportions of magnetite, kaolinite and gibbsite. The Fe grades of this material are between 30–53%. Silva and Luz [8
] carried out the magnetic concentration of a slime thickener underflow sample (100% −150 µm, with grades of 32.9% Fe, 37.7% SiO2
, 5.3% Al2
and 5.7% loss on ignition—LOI)) from a mine located in Quadrilátero Ferrífero. The magnetic field intensities tested were of 0.6, 0.9 and 1.2 T. The best result after a cleaner step was obtained with a magnetic field of 1.2 T: 64.6% Fe, 5.5% SiO2
and a mass recovery of 62.8%. For an underflow sample of slime thickener from the Brucutu mine (46% Fe, 15% SiO2
, 10% Al2
and 10% LOI), there was obtained a concentrate with 66.8% Fe, 0.8% SiO2
, 0.97% Al2
and 2.4% LOI for a magnetic field of 1.45 T. However, the mass recovery was very small (12.7%), due to the fine size distribution of the sample (d80
= 10 µm) [9
Different processes of metamorphism and weathering of iron formations in the different geographical regions of Quadrilátero Ferrífero led to the formation of different typologies of ores, which are classified as compact, semi-compact and friable itabirites, according to the percentage retained in a given mesh. The grades of Fe in these ores vary between 30% and 60%. At the western edge of Quadrilátero Ferrífero, compact itabirites comprise a percentage retained >55% +6.3 mm, semi-compact (between 30% to 55% +6.3 mm) and friable (<30% +6.3 mm) [10
]. At the central and eastern portions, there are a predominance of friable ores, and among them, there are the amphibolitic (~1.2% Al2
> 0.14% and LOI > 5%) and aluminous itabirites (Al2
> 3.0% and LOI > 3%) of Alegria’s (60% −0.15 mm) and Brucutu’s (80% −8 mm) deposits [11
]. These ores have a high proportion of goethite, generating large amounts of slimes, which cause problems both in the concentration by flotation and in the dewatering stages (thickening and filtration). For this reason, they are considered marginal or, depending on their grade, they are used as a natural fine sinter feed [11
In this study, a characterization (physical, chemical and mineralogical) was carried out on a sample of amphibolitic itabirite (marginal ore) from the eastern region of the Quadrilátero Ferrífero, which corresponds to 15% of the current reserves (263 million tons) of Brucutu [13
], aiming at the development of an adequate processing route to obtain a concentrate of this typology of ore to be incorporated into the industrial pelletizing process.
2. Materials and Methods
The amphibolitic itabirite sample used in this study was obtained by the composition of two subsamples: A1 (28 kg, 55.8% Fe) and A2 (52 kg and 41.2% Fe). These were collected in two different regions of amphibolitic itabirite in Brucutu’s deposit (Figure 1
), aiming at obtaining a Fe grade (~46%), compatible with ore grades current feed in industrial concentration plants in Quadrilátero Ferrífero [6
], since the friable ores with grades higher than 50% Fe can be used as a natural fine sinter product [13
]. As seen in Figure 1
, the ore has an ocher color and a clay appearance.
After the homogenization of the amphibolitic itabirite sample ROM (run of mine), aliquots were removed for physical characterization: moisture determination, specific weight and size distribution; chemical: determination of FeTotal, SiO2, Al2O3, CaO, MgO, MnO, Fe3O4, P and LOI grades and mineralogical: mineral phases identification and determination of the quartz’s liberation. After determining the ore’s liberation mesh, a part of the sample was comminuted at −105 µm for exploratory concentration tests.
2.1. Physical Characterization
The natural moisture (wet basis) of the ore sample was performed in a furnace at 100 ˚C (±5). The specific weight (average of the values obtained by 3 scans) was determined by the Quantachrome Corporation pycnometer Ultrapyc 1200e/UPY-30 model (Boynton Beach, FL, USA) in accordance with the methodology of Silva et al [3
The determination of the ore’s size distribution, carried out in duplicate, was achieved through wet sieving (sieves from 8000 to 45 μm) and a laser particle size analyzer (fraction −45 µm), CILAS 1180 model, used under the following conditions: 60 s of ultrasound, 25% obscuration and the addition of 10 drops of sodium hexametaphosphate at 1% w/v to disperse the suspension.
2.2. Chemical Characterization and Loss on Ignition (LOI)
The ROM sample’s grades of FeTotal
, P, Al2
, MnO, MgO, TiO2
and CaO, by size fraction, as well as the concentration tests products, were determined by X-ray fluorescence—XRF (Rigaku X-ray spectrometer Simultix 14 model, Rigaku, Osaka, Japan). For this, fused pellets were made at 1000 °C of a mixture consisting of 1 g of each pulverized sample (−38 µm) and 5 g of lithium tetraborate/metaborate (67% Li2
). The Fe3
grade (1.3 g samples) was determined by Rapiscan’s Satmagan 135 equipment (Rapiscan, Skudai—Johor, Malaysia) according to the methodology described by Breuil et al. [15
] and Stradling [16
The experimental procedure for determining the loss on ignition (LOI) consisted of introducing the sample (150 g) in a muffle furnace, regulated at a temperature of 1000 °C, where it remained for 1 h, and the loss on ignition calculation was determined by the percentage of sample mass loss after calcination in relation to the initial mass.
2.3. Mineralogical Characterization
X-ray diffractometry—XRD (total powder method) was used to identify the mineral phases of the studied sample. For this, a PaNalytical model X’pert3Powder diffractometer (Malvern Instruments, Malvern, UK) equipped with a Cu tube (λCu = 1.5405 Å) and Ni filter was used. The operation conditions were: 45 kV and current of 40 mA and scanning angle (2Ɵ) from 5° to 90° counting time of 15 min. The data were collected by using the X-ray Data Collector software (version 5.4). Mineral phases identification in the X-ray diffraction pattern was performed by the software HighScore Plus (version 4.5), using the standard database X-ray patterns of the ICCD PDF-2, 2015.
Thermogravimetric analysis was used to confirm/identify the hydrated mineral phases present in the sample. The experimental procedure consisted of the introduction of a platinum crucible, containing the sample in the TA Instruments model TGA Q50 thermogravimetric analyzer (New Castle, DE, USA). Data collection was performed by the TA Instruments Explorer software (version 4.5A) under the following conditions: heating from 20 to 1000 °C, heating rate of 10 °C/min, isotherm of 5 min at 1000 °C and N2 flowrate = 100 mL/min (10 mL/min for cooling the thermobalance and 90 mL/min for purging the sample).
For textural studies of the amphibolitic itabirite, performed by optical microscopy (Leica optical microscope—DMLP) (Leica, Werzlar, Germany) and scanning electron microscopy (Hitachi SU3500 Tokyo, Japan) SEM, polished sections were made by inlaying 3 g of sample with a mixture of epoxy resin and the respective catalyst from the Epoxiglass brand (2:1 ratio). Afterwards, they were lapped (sandpaper: 180, 220, 320, 400, 600 and 800) and polished (diamond pastes of 6 µm, 3 µm, 1 µm and ¼ µm). The determination of the quartz’s liberation in regard to the iron minerals was performed by optical microscopy (Gaudin’s method) by counting 200 particles in each size fraction.
2.4. Magnetic Concentration and Flotation Tests
The preparation of the sample studied for the concentration tests (magnetic and flotation) consisted of ROM classification/fragmentation in particle size −105 µm. For this, at first, the ROM was classified by wet sieving at −105 µm. After drying, only the fraction size +105 µm (79 kg) was dry-grinded by a laboratory rod mill for 1 min, followed by wet sieving −105 µm so as not to generate excessive slimes (−10 µm particles). This procedure was done until all sample reached −105 µm particle size. Finally, the prepared sample was dried, homogenized and splited into subsamples to perform the tests with non-deslimed and deslimed ore. For the desliming operation, 20 L of pulp with 25 wt% solids at pH 10.5 (adjusted with NaOH solution at 50% w/v) was stirred at 1200 rpm for 5 min. Then, the pulp was allowed to stand for 13 min to settle the 10 μm particles, and the supernatant (−10 µm particles) was removed. The initial volume was completed again, and this procedure was repeated 3 more times.
For magnetic concentration tests carried out with a non-deslimed sample, a magnetic carrousel concentrator (Minimag, Gaustec, Nova Lima, Brazil) with a 1.5 mm matrix gap was used. First, the equipment was switched on. Then, the magnetic field intensity was adjusted to the desired value (0.9 or 1.1 T), and the water tap was opened at 1 kgf/cm2 pressure. After, the magnetic field’s stabilization (20 min), the feed (rate of 5.7 kg/h) was continually fed in a closed circuit into the equipment. After the sample feeding finished, the flow of the feed and products magnetic and non-magnetic were simultaneously sampled. Then, they were filtered, dried, weighted and pulverized for chemical analysis in order to perform the mass and metallurgical balances. For the deslimed sample, the INBRAS L4 model magnetic concentrator with a 2.5 mm gap matrix was used. Firstly, the equipment was switched on, and the magnetic field was adjusted to 0.9 T (coil current of 15 A). After the magnetic field’s stabilization time (10 min), the washing water tap was opened at a flow rate of 20 mL/s, and the aliquot of the deslimed sample (25 g) was slowly added to the equipment’s feeder. After separation, the container holding the waste was removed. Another container was then inserted, the equipment turned off and the concentrate removed. This procedure was repeated 5 times. Finally, the magnetic separation products were filtered, dried and weighed, followed by homogenization, quartering and pulverization for chemical analysis of the products, which were later used in mass and metallurgical balances.
Reverse cationic flotation tests were carried out only with the deslimed sample under the following conditions: pH 10.5, pulp with 50 wt% solid and 500 g/ton of starch and amine with 50% neutralization degree (Flotigam 7100-Clariant): 170 and 200 g/ton. After adding the sample and water inside the flotation cell CDC—CFB 1000 EEPNBA model (1.2 L volume), the cell speed was adjusted to 1200 rpm, then gelatinized corn starch with NaOH was added and conditioned for 3 minutes, followed by the addition of amine, and conditioned for 1 min more. Finally, the air tap was opened, and flotation was carried out until the froth exhaustion. The float (tailing) and sunk (concentrate) products were filtered, dried and weighed, followed by homogenization, quartering and pulverization for chemical analysis, which were later used in mass and metallurgical balances.
Studies of physical, chemical and mineralogical characterization carried out with the sample of amphibolitic itabirite from the eastern region of Quadrilátero Ferrífero showed that the natural moisture of the ore is 10%, and the specific weight is 3710 kg/m3. These values are consistent with the mineralogical and microtextural compositions of the sample, which consist basically of 64.5% goethite (amphibolitic, alveolar, massive and earthy) and hematite (6.8%), mainly martitic, which are quite porous, and quartz (25.5%). For this reason, high proportions of slimes are generated in the comminution stage for the quartz liberation in 105 µm, which represent 33% of losses in the desliming stage, with Fe grades (~52%) higher than that of the ROM ore (46%) and deslimed ore (43%). Among the concentration tests (magnetic and flotation) performed with the studied sample, the magnetic concentration (0.9 T magnetic field) of the deslimed ore and subsequent blending of the concentrate obtained with the slimes provided a mass recovery of approximately 80% of the poor “pellet feed” to be added in the pelletizing process. However, further studies are needed to optimize the proposed circuit, in addition to the introduction of selective flocculation/magnetic separation of the generated slimes.