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Separation of Chromite from Serpentine in Fine Sizes using Magnetic Carrier a

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Yasar Ucbas , Volkan Bozkurt , Kemal Bilir & Halil Ipek a

Eskisehir Osmangazi University, Faculty of Engineering and Architecture, Department of Mining Engineering, Eskisehir, Turkey Accepted author version posted online: 31 Dec 2013.Published online: 14 Apr 2014.

To cite this article: Yasar Ucbas, Volkan Bozkurt, Kemal Bilir & Halil Ipek (2014) Separation of Chromite from Serpentine in Fine Sizes using Magnetic Carrier, Separation Science and Technology, 49:6, 946-956, DOI: 10.1080/01496395.2013.869602 To link to this article: http://dx.doi.org/10.1080/01496395.2013.869602

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Separation Science and Technology, 49: 946–956, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0149-6395 print / 1520-5754 online DOI: 10.1080/01496395.2013.869602

Separation of Chromite from Serpentine in Fine Sizes using Magnetic Carrier Yasar Ucbas, Volkan Bozkurt, Kemal Bilir, and Halil Ipek

Downloaded by [Eskisehir Ozman Gazi Universitesi] at 02:01 15 April 2014

Eskisehir Osmangazi University, Faculty of Engineering and Architecture, Department of Mining Engineering, Eskisehir, Turkey

In this study, possible separation of chromite from serpentine in fine sizes using a magnetic carrier was investigated as a function of pH, dodecylamine (DDA) dosage, and amount of magnetite. First, the zeta potentials of minerals were determined in the absence and presence of DDA, and then magnetic carrier experiments were performed in conditions based upon the zeta potential results. Experiments revealed that chromite could be separated from serpentine with a 96.8% recovery under the experimental conditions of 4 x 10−4 M DDA, 25 mg magnetite, and at pH 12. In the case of artificial mixtures of chromite and serpentine minerals, chromite concentrate containing 49.7% Cr2 O3 was obtained with 82.8% recovery from a feed containing 27.00% Cr2 O3. As a result of FTIR studies, physical adsorption of DDA on minerals was confirmed. It was also found that DDA adsorption on chromite was higher than those of serpentine and magnetite. FTIR studies performed with chromite+magnetite and serpentine+magnetite mixtures revealed that DDA adsorption on serpentine declined significantly while DDA adsorption on chromite slightly increased. Keywords magnetic carrier; zeta potential; chromite; serpentine; dodecylamine

INTRODUCTION Separation of minerals in fine particle sizes is becoming an important challenge in the mineral processing industry. Processing of fine particles by conventional mineral separation methods is not satisfactory due to the problems associated with particle sizes. Studies aimed at finding new methods on concentration of fine and ultrafine particles began three decades ago (1). Most of these methods are based on the combination of aggregation and flotation processes. These methods gave successful results on laboratory scale but their application in larger scale was doubtful due to the difficulty of controlling the involved parameters.

Received 25 March 2013; accepted 21 November 2013. Address correspondence to Volkan Bozkurt, Eskisehir Osmangazi University, Faculty of Engineering and Architecture, Department of Mining Engineering, Eskisehir, Turkey. E-mail: [email protected]

Magnetic carrier technology as a method of fine particle separation has received some attention (2–8). The principle of the process is the selective adsorption of the magnetite particles on to a mineral surface in a mixed suspension, and then the removal of the magnetite coated mineral particles from the suspension by magnetic separation techniques. Separation of minerals such as chalcocite from silica, sphalerite from gangue, and coal from ash using magnetic carrier technique has been described (4). The adsorption of magnetite on titanium and iron bearing impurities in kaolin in the presence of fatty acids prior to magnetic separation has also been described by several investigators (3, 5, 6). Magnetic carrier technology has also been applied to the water treatment process (2). The factors that influence the adsorption of the magnetite include electrical charge of minerals and magnetite, and the presence of adsorbed surfactant at the mineral and magnetite surfaces. The control of these parameters has led to a separation of many minerals such as apatite, barite, seelite, magnesite and iron minerals from calcite, dolomite, serpentine, quartz, and corundum minerals has been performed in laboratory and pilot scales (7–12). Chromite is one of the essential raw materials used in metallurgical industry, and chromite used in this industry should contain at least 48% Cr2 O3 . Therefore, concentration of chromite is often needed. Concentration of chromite is usually performed by gravity concentration methods based on differences between specific gravities of chromite and gangue minerals, especially serpentine. Considerable amount of fine chromite is lost to the tailing in the conventional gravity separation plants as a result of the process and/or equipment inefficiency in the fine particle size range. Considering the fact that Turkey is one of the leading chromite producers in the world, it should be expected that the amount of fine chromite to tailings could be quite large throughout Turkey. Separation of chromite from serpentine in fine sizes is a still challenging subject with the advances in mineral processing techniques. Magnetic carrier technology would well be an alternative method to concentrate chromite in fine sizes. Therefore, the aim of this study was to investigate possible separation of chromite from serpentine in fine sizes using magnetic carrier.

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EXPERIMENTAL Materials Chromite and serpentine samples were obtained from Turkish Maadin Company, Kavak Chrome Concentrating Plant. Fine-sized heavy media grade magnetite was used as a magnetic carrier and was from ELI Manisa Coal Preparation Plant. All samples were, if needed, first crushed and then further ground using ceramic fast mill (Gabrielli Fast Mill 2B) for 60 minutes. Dodecylamine (DDA) with purity of 99% was purchased from Merck used as a collector. Analar grade HCl and NaOH were used for pH adjustment and deionized water was used in all experiments. Methods Characterization of Samples Particle size analyses of samples were performed wet using particle size analyzer (Malvern Mastersizer 2000). Chemical compositions of the samples were determined using X-ray fluorescence spectroscopy (XRF, Spektro X-Lab). Mineralogical analyses of samples were performed using X-ray diffraction spectroscopy (XRD, Rigaku Miniflex, 1.54056-CuKα1). Zeta Potential Measurements Zeta potentials of chromite, magnetite, and serpentine were determined using Zeta-Meter System 3.0+ equipped with a camera system in the absence and presence of DDA. All the measurements were conducted in a dilute suspension of 0.05% solids by weight. The pH of the suspension was regulated either by the addition of dilute HCl or NaOH. The conditioning of the suspension was carried out for 10 min after maintaining the preset pH in the absence and presence of 1 x 10−4 M DDA. Thirty particles were counted at each pH value. Average zeta potential value was automatically calculated by apparatus and given by means of the Smoluchowski equation. Magnetic Carrier Tests The tests were carried out in a 100 mL beaker and a stirrer with a plastic impeller was used to maintain the particles in suspension. The pH was controlled with dilute HCl or NaOH. Initially the tests were performed separately for each mineral. For each test, 1 g of each mineral (chromite and serpentine), varying amount of magnetite (Mn, 0.5 g for the initial test) and 500 g/t kerosene were conditioned for 10 minutes at the desired pH in the absence and presence of DDA. After the conditioning period, stirring was stopped; magnetite and magnetite coated particles were removed by using high-intensity magnetic rod. Material remained (i.e., uncoated particles) in the beaker was filtered and dried in an oven before weighing. The weight was then noted and subtracted from the initial weight of the sample and the net weight of the magnetite coated particles was determined as % weight. This value was used as % recovery throughout the presentation of the results of magnetic carrier

tests. The attached magnetite was removed from the mineral particle surface after treatment with ethanol for confirming mass balance. The effects of pH, DDA dosage, and amount of magnetite were investigated. In the case of artificial mixtures of minerals (chromite and serpentine), equal weight (1 g) of each mineral was used while the procedure was the same as described previously. FTIR Measurements The infrared spectra were registered for all samples (chromite, serpentine and magnetite) prepared according to the zeta potential measurement procedure described above after air drying the minerals. The spectra of chromite and serpentine were also registered in the presence of magnetite (isolated in the same solution) for mimicking magnetic carrier tests. The FTIR spectra were obtained using a Perkin Elmer 2000 spectrometer with its own Diffuse Reflectance Infrared Fourier Transform (DRIFT) attachment. Typical spectrum was an average of 200 scans at 4 cm−1 resolution with a DTGS detector. Since the intensity of the bands with respect to adsorbed layers was low, the samples were not mixed with KBr. The untreated samples were used as a reference. The FTIR spectra of the samples treated with collector was measured with respect to the reference. The atmospheric water was always subtracted. RESULTS AND DISCUSSION Characterization of Samples Particles size analyses of the samples are given in Table 1. As can be seen, magnetite being the finest while serpentine being the coarsest. Chemical compositions of the samples are shown Table 2. Results confirm the purity of minerals especially, in terms of Cr2 O3 content for chromite, SiO2 and MgO contents for serpentine and Fe2 O3 and FeO contents for magnetite. Results of mineralogical analyses of samples studied and values of degree 2θ of characteristic peaks of pure minerals obtained from X-ray diffraction database are presented in Fig. 1 and Table 3, respectively, for comparison. The values of degree 2θ characteristic peaks were quite compatible to those mineral samples studied, confirming the purity of samples. TABLE 1 Particle size analyses of the samples Pass (%)

Magnetite (μm)

Chromite (μm)

Serpentine (μm)

%10 %50 %90 %97

2.83 8.95 26.29 42.00

0.26 6.67 34.11 86.25

2.73 22.84 173.17 243.19

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TABLE 2 Chemical analyses of the samples Oxides (%)


Cr2 O3 Fe2 O3 FeO Al2 O3 SiO2 MgO CaO LOI

TABLE 3 2θ characteristic peaks of pure minerals

Magnetite

Chromite

Serpentine

0.08 67.16 31.21 0.44 0.11 0.02 0.38 −

54.00 16.79 − 12.50 0.02 14.95 0.14 1.14

0.51 8.23 − 0.02 46.00 31.00 0.09 13.65

Zeta Potential Measurements In the Absence of DDA Zeta potentials of chromite, magnetite, and serpentine were measured in the absence of DDA at different pH values (Fig. 2). Zero point of charge (zpc) of chromite, magnetite, and serpentine were determined as 7.2, 5.0, and 3.1, respectively. Similar results were observed in previous studies performed with these minerals (13–19, 9, 10, 12). In the Presence of DDA Zeta potentials of chromite, magnetite, and serpentine were measured in the presence of 1x10−4 M DDA solution at

Minerals Chromite Magnetite Serpentine

2θ

2θ

2θ

35.6 (100) 35.5 (100) 12.1 (100)

63.7 (90) 62.7 (85) 24.4 (70)

57.6 (90) 57.2 (85) 19.4 (50)

different pH values (Fig. 3). As can be seen from Fig. 3, in the presence of DDA zeta potentials of chromite, magnetite, and serpentine increased in the positive direction at all pHs, and zpc of those minerals shifted to the higher pHs. Changes in zeta potential values of minerals at pH below the zpc suggested that amine adsorption could take place possibly due to an exchange of amine ions for H+ at these acid pH values and overall positive surface charges. However, under such conditions, higher amine concentration needs for significant amine adsorption to take place on mineral surfaces. Zeta potential response of minerals at pH values above the zpc suggested typical physical adsorption systems where adsorption is controlled by mineralcollector electrostatic charge interaction plus association of hydrocarbon chains. DDA is partly ionized and molecular at these pH values and adsorptions on minerals are obtained by co-adsorption of ionized and molecular DDA. DDA adsorption at pH higher than 10 coincides with the pH of onset of DDA precipitation. (13–16).

FIG. 1. X-ray diffractograms of samples.


SEPARATION OF CHROMITE FROM SERPENTINE

FIG. 2. Zeta potentials of chromite, magnetite and serpentine in the absence of DDA.

FIG. 3. Zeta potentials of chromite, magnetite and serpentine in the presence of DDA.

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Magnetic Carrier Tests In the Absence of DDA From the zeta potential measurements of minerals in the absence of DDA (Fig. 2), it is observed that magnetite and serpentine are negatively charged while chromite is positively charged at pH 5–7. This could suggest the possible magnetite coating of chromite through electrostatic attraction. Magnetic carrier tests were conducted under these conditions. The results showed that minerals could well be coated with magnetite as predicted, but attachment of magnetite to minerals were so loose that even a gentle shaking of the beaker resulted in the detachment of the magnetite from minerals. This indicates that electrostatic attraction itself is not enough for successful magnetite coating. Similar results were obtained in literature related to the removal of serpentine and quartz from magnesite using magnetic carrier methods (9, 12). This was explained by the presence of a thin film of water between the hydrophilic surfaces of magnetite and minerals. Thin film of water could not be ruptured even though magnetite and mineral particles are very close to each other. Consequently, the particles were easily detached by even gentle stirring. In the Presence of DDA From the zeta potential measurements of minerals in the presence of DDA (Fig. 3), it is observed that magnetite and serpentine are negatively charged while chromite is positively charged at pH 7–9. This could suggest the possible magnetite coating of chromite through electrostatic attraction, as a result, its separation from serpentine. Magnetic carrier tests (1 g mineral, 0.5 g magnetite, 500 g/t kerosene) were carried out with chromite and serpentine minerals in the presence of 1x10−4 M DDA at pH 8. Chromite recovery of 20% was obtained. In order to increase the chromite recovery to target recoveries (95%