Beneficiation of High Alumina Indian Iron Ore Slimes by Magnetic Carrier Technology
B. Das, S. Prakash, B. K. Mishra
institute of Minerals and Materials Technology, (Formerly Regional Research Laboratory),
Council of Scientific and industrial Research (CSIR), Orissa, India
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ABSTRACT
The separation of iron values from a typical Indian iron ore slimes containing 57.2% Fe, 5.81% SiO2 and 7.05% Al2O3 by wet magnetic separation and selective magnetic coating is investigated. The iron phases, which are mostly paramagnetic in nature, have been coated with magnetite or colloidal magnetite in presence of surfactant by manipulating the surface properties of the minerals. The studies have indicated that an iron concentrate containing 62.32% Fe with 66.5% iron recovery can be obtained at medium magnetic intensity of 7.8 kG which is much less compared to normal magnetic intensity of 13 kG to 14 kG required for hematite-goethite separation. The performance of this technique with respect to both grade and recovery is superior to conventional magnetic separation or flotation of iron ore slimes.
Keywords: Iron ore slimes; Beneficiation; Colloidal magnetite; Magnetic carrier technology; Selective coatingINTRODUCTION
A large quantity of iron ore in India is being washed to prepare the raw material feed for blast furnace and sintering plants. During the preparation of the iron ore, large quantities of superfines—relatively of lower grade, are being generated and discarded into the tailing ponds. The accumulation of these slimes in tailing ponds is increasing day by day due to regular production of iron ore for steel industries. The processing of these slimes has posed a challenging task for beneficiation and utilization in iron and steel making due to finer size, low iron content and complex mineralogy. The biggest problem is that it does not meet the raw material requirements of either the blast furnace or the sintering plant. In India roughly 10 million tons (MT) of these materials are being generated every year containing around 48%—60% iron causing environmental and ecological imbalances.
The importance of utility of these vast accumulated super-fines for sinter or pellet making has engaged many R&D organizations to recover iron values from these rejected tailings. In the last two decades, many processes for the recovery of iron values have been claimed and reported in the form of technical reports, papers and reviews (Reddy, et. al., 1985; Maulik et al., 1998; Despandy et al.,l997; Das et al., 2001). The efforts made to reduce the alumina and silica content in iron ore slimes by flotation and selective flocculation technique could upgrade the iron content to an acceptable limit but the alumina level could not be brought down to the desired level (Hanumantha Rao and Narasimhan, 1985; Das, et. al., 2005). The work carried out by density separator could only separate the siliceous gangue effectively (Pan et al., 2001). Studies conducted by high intensity magnetic separator has shown that an iron concentrate with 64.6% Fe, 2.8% SiO2 and 2.7% Al2O3 could be obtained after desliming the feed at 45 microns (Prasad et. al., 1988). Test results on slimes with high gradient magnetic separator (HGMS) have indicated that an iron concentrate of 67% Fe with an overall recovery of 78%-88% at a magnetic intensity of 3—4.4 kG could be achieved.
The tailings generated at Barsua iron ore mines of India stands out as an exception to conventional treatment. The iron content in the slime is very low compared to the other tailings of India and the gangue constituents containing alumina and silica are very high. In view of enormous amount of iron lost in processing which is essentially paramagnetic in nature, several investigations have been carried out to ascertain whether selective magnetic coating can be made effective to recover the iron lost to an acceptable concentrates. The concept is about application of magnetic carrier technology (MCT) which is an innovative way of selectively manipulating fine particles in suspensions by coating them with a magnetic species. The main factors contributing the selective attachment of magnetite particles on the surface of the desired particles are electric charge of the minerals, concentration of magnetite, pH, presence of adsorbed surfaetant, and electrolytic concentration of the pulp (Parsonage, 1988). The application of MCT by selective magnetic coating with colloidal and natural magnetite is an alternate method for the beneficiation of Indian iron ore slimes. In the process of selective magnetic coating the magnetic response of the target species are being enhanced by the addition of a small amount of magnetite or colloidal magnetite in conjunction with specific surface active agents allowing separation by magnetic separation methods (Prakash et al., 1999, 2000). The purpose of the paper is to illustrate the potential of MCT to beneficiate a natural iron ore slimes with a view to upgrade the levels of iron and reduce the gangue constituents to an acceptable limit.
EXPERIMENTAL METHODS
Iron ore slime sample
Representative iron ore slime samples from Barsua iron ore washing plant, India, is collected for the detail investigation studies. The complete chemical analysis of the samples is carried out by wet chemical and instrumental techniques. The size analysis of the as received sample up to 20 micron is carried out by wet methods using different standard sieves. Classified samples are examined under stereomicroscope by preparing the corresponding grain slides for identification of different minerals. The X-ray diffraction of selected samples are studied using a Philips model diffractometer with CuKα radiation.
Preparation of colloidal magnetite
The colloidal magnetite was prepared by mixing Fe (II) and Fe (III ) salts in a basic solution. After an initial brown precipitate, a black precipitate was formed (magnetite). The basic solution was added very slowly by using a dropper with constant stirring. The precipitated magnetite was allowed to settle and the supernatant liquid solution was removed. It was washed several times with distilled water to remove excess basic solutions and untreated salts. The oleate coated colloidal magnetite was prepared by adding l0-2M sodium oleate solutions at pH 11.0 and boiling the solution until the particles are completely dispersed.
Magnetic separation studies
All the magnetic separation studies were carried out by a wet high intensity magnetic separator with facilities for grid gaps and variable current. Prior to the magnetic separation studies the iron ore slime sample was conditioned for 5 minutes in a 2 liter glass beaker at a solid liquid ration of 1:10. Oleate colloidal magnetite or sodium oleate was added in desired quantity into the suspension. Sodium hexa-metaphosphate was used as the dispersant and added into the suspension during the time of conditioning. At the end of the conditioning time, the slurry was then passed through the magnetic separator slowly where the magnetic intensity was fixed previously. The magnetic particles retain inside the grooves while the non magnetic particles pass and eventually collected separately. Both types of particles were analyzed for silica, alumina and iron to determine the quality and recovery of iron values.
RESULTS AND DISCUSSIONS
Characteristics of iron ore slimes
The chemical composition of iron ore slime is given in Table 1. The sample contains 57.2% Fe, 5.81% SiO2, and 7.05 Al2O3. The size analysis and distribution of the as received sample is shown in Fig. 1. The 80% passing size of the sample is around 100 microns. The size analysis also shows that Al2O3and SiO2 content in the slime tend to concentrate in the finer fractions. The characteristics of silica and alumina of other iron ore samples of India also tend to concentrate in the finer fractions.
Table.1 Chemical Composition of Iron Ore Slimes
Fig. 1 Size analysis and distribution of iron, silica and alumina in different fractions
Mineralogy
The minerals identified in iron ore slime samples are hematite, goethite, gibbsite, quartz, and kaolinite. Granular hematite grains are of about 25 micron to 35 micron size. The free hematite particles consist of densely packed microplaty hematite (max length 20 micron) and microcrystalline hematite grains (<8 micron length) as shown in Fig. 2. Vitreous and ochreous goethites particles are abundant in the sample (Fig. 2A). Goethite replaces hematite in different degrees. Vitreous goethite frequently converts to ochreous goethite. Vitreous goethite is grey to dark grey in colour; ochreous/earthy goethite is black to dark grey and composed of ultrafine crystallites. Goethite replaces hematite in different degrees and fills up the voids and fractures in the iron ores. Hematite of varying sizes and shapes occur as inclusions within goethite (Fig. 2B and Fig. 2C). Kaolinite occurs in intimate association with ochreous goethite. Quartz grains are free in nature (Fig. 2D). The XRD pattern of different size fractions of Barsua iron slimes is illustrated in Fig. 3. The X-ray characteristics peaks of hematite show higher intensities in +75 micron size than that of other sizes, Conversely the peaks of gibbsite and goethite show relatively increase in +420 micron and +75 micron size. Quartz and kaolinite peaks are seen in -30 micron size indicating the presence of high silica and alumina contain in this finer size.
Fig. 2 A Particles types consisting of free hematite, free vitreous and ochreous goethite and locked grains; B) Hematite (white) inclusions in vitreous goethite (grey white) which grades to ochreous goethite; C) Hematite of varing sizes and shapes opaque as inclusions in ochreous goethite;
D) Irregular quartz grains showing opaque inclusions. Transmitted light partly crossed niclos. (Scale: 1 cm 84 micron)
Process synthesis
The beneficiation of Barsua iron ore slimes is problematic in comparison with the other Indian iron ore slimes due to the presence of more clayey materials in it. The studies carried out by different investigators have suggested that the iron concentrate could go up to 61.5 Fe with 50% recovery. The magnetic carrier technology was therefore employed to enhance the quality and recovery of iron concentrates. The surface properties of the associated minerals were studied as the coating mechanism is almost similar with that of the flotation studies. The variation of the electrophoretic mobility of hematite, quartz, oleate-magnetite, and colloidal magnetite as a function of pH is shown in Fig. 4. As can be seen in the figure, the PZC of hematite and colloidal magnetite occurs at about pH 6.8 and 7.13 respectively. The pzc of oleate- magnetite occurs at 4.8. Sodium oleate has a marked effect on the electrokinetic potential of hematite. The fact that significant adsorption of oleate on hematite surface occurs even at very low pH values and the adsorption of oleate on hematite is not purely physical but quite likely is a combination of chemisorption and physisorption (Shibata and Fuerstenau, 2002). The zeta potential of quartz was found to be negative through out the pH range studied. The experimental point of zero charge values is comparable with the values reported in literature. Due to the difference in zetapotential values, the interaction between oleate magnetite and hematite may take place due to electrostatic force of attraction. It has been also stated that oleate ions do not attach specifically with quartz but have high adsorption affinity for hematite at pH 71—7.5. Hence by controlling the pH of the suspension, the attachment between hematite and oleate magnetite can be achieved.
Fig. 3 X-ray diffraction studies of Barsua iron ore slimes (size fractions)
Fig. 4 Variation of zeta potential of various minerals with pH
In the experiments of selective magnetic coating, few runs were carried out at different magnetic fields. Sodium hexa metaphosphate (NaHMP) of 0.8 g/kg as the dispersing agent was added to keep the particles in proper dispersed conditions. No magnetic materials such as magnetite or oleate-magnetite were added. The results of the magnetic separation studies are shown in Table 2. It has been observed that maximum iron concentrate of around 61.2% Fe with 51.0% yield could be achieved. It was obtained by applying the magnetic intensity of 11 .4 kG. The yield has increased by increasing the current but the grade of iron decreases. It can be seen that only 61.0% Fe with 61.65% recovery could be obtained at 13.0 kG of magnetic intensity. Since the required grade of iron could not be achieved sodium oleate and oleate-magnetite were added as the coating materials to enhance the magnetic property of iron ore particles.
Table 2 Magnetic separation of iron ore slimes, (with NaHMP 0.8 g/kg)
The magnetic carrier technology needs some ferromagnetic substances as the carrier. As the slime sample contains some amount of fine magnetite, it was thought to take advantage of this phase. Therefore experiments were conducted only with sodium oleate so that the oleate will react with the magnetite phase already present in the slime sample. The results obtained are presented in Table 3. It has been observed that 62.12% Fe with 66% recovery could be achieved by using sodium oleate (0.8 g/kg) at pH 7.0 and magnetic intensity of 7.8 kG. Almost similar results were obtained by treating with oleate-magnetite. The silica and alumina content have been reduced to 2.0% and 4.2% respectively. It is seen from the results that by the addition of sodium oleate better concentration of iron could be achieved. Sodium oleate may be preferred over oleate-magnetite in view of cost and energy.
The effect of pH on the magnetic separation of iron ore slime is shown in Table 4. A magnetic intensity of 7.8 kG was applied while the concentration of sodium oleate or oleate colloidal magnetite was kept constant during the change of pH values. It has been observed that a concentrate of 62.13% with 66.5% recovery was obtained at a pH value of 7.5 but at other two pH values the grade and recovery values decreases.
Table 3 Effect of sodium oleate and oleate colloidal magnetite on the magnetic separation of shines
Table 4 Effect of pH on magnetic separation of slimes (magnetic intensity—7.8 kG)
CONCLUSIONS
A difficult-to-beneficiate iron ore slime collected from Barsua mines has been subjected to magnetic carrier technology. It has been found that selective magnetic coating of colloidal magnetic particles on hematite of iron ore slimes is possible. These coated particles (hematite) can be separated by medium magnetic intensity. The separation can be achieved either by coating the particles with colloidal magnetite. It has been found that the separation can be enhanced in presence of sodium oleate. The role of sodium oleate promotes adherence of colloidal magnetic particles onto the hematite surface. The best separation in presence of oleate is obtained at pH 7.5 and a magnetic intensity of 7.8 kG. A concentrate containing 62.32% Fe with 66.5% iron recovery can be achieved that can be used as sinter feed.
Acknowledgements The authors are thankful to all the colleagues of the Mineral processing department, IMMT, Bhubaneswar who have directly or indirectly supported for the completion of this piece of investigations.
REFERENCES
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