EXTRACTION - BR 2018 Conference – Red Mud



EXTRACTION OF IRON FROM RED MUD: LOW TEMPERATURE REDUCTION TO MAGNETITE AND MAGNETIC SEPARATIONSumedh Gostu1*, Brajendra Mishra1, Gerard Martins21Department of Material Science and Engineering, Worcester Polytechnic Institute2Department of Metallurgy and Materials Engineering, Colorado School of MinesAbstract: Red-Mud (Bauxite residue) is a byproduct generated during the Bayers processing of bauxite. Iron is a major constituent in most of the world’s reserves of bauxite residue (3 billion tons). The utilization and value addition of iron in red-mud is a critical measure for economically valorizing the waste and it eases the process of extracting other valuable non-ferrous components in the waste. A gas based reduction involving a mixture of gases CO(g), CO2(g) and N2(g) (diluent) is proposed in this paper to convert hematite in red-mud to magnetite. The optimal conditions for reduction were determined to be temperature of 540 + 10 oC, PCO/PCO2 = 1 (0.070 atm (bar) + 0.001 atm (bar)) for a reaction time of 30 minutes. Magnetic classification of the reduced magnetite was performed by employing dry (Frantz separator) and wet (Davis tube separator) means. Approximately 98% of magnetite could be recovered in the magnetic fraction and the maximum grade of the magnetite achieved was ~60 %. The lower grade of magnetite was attributed to the presence of nanometer scale entities of agglomerated particulates which probably comprised of cation lattice-substitutions for Fe+3 by Al+3 and Ti+3 as evidenced in the STEM images and M?ssbauer spectroscopic analyses. Keywords: Red-mud, Iron extraction, magnetite, Low-temperature reduction, Magnetic-classification, Nano-size crystallites.Introduction: The Bayers process for chemical dissolution of bauxite is associated with a selective precipitation of iron oxide. This has been referred to as red-mud due to the color of its primary constituent iron (III) oxide (hematite). Iron oxide in conjunction with the Desilication product (DSP) constitute the red-mud (or Bauxite residue/BR) [1-4]. For every ton of primary aluminum produced by the Hall Heroult process, 0.8 to 2.5 tons of Bauxite Residue is generated in the Bayers process depending on the mineralogy of bauxite ore used [5]. The current worldwide accumulation of BR is estimated at 3 billion tons. The past half a century saw numerous research focused at utilization of BR. The use of BR as synthetic soil was one such measure. The high basicity of red-mud would only favor it being a revegetation measure [6, 7]. The utilization of red-mud in the construction industry has been researched upon due to the increasing demand of conventional materials for infrastructure. Bricks, constructional aggregates, roof decking and red-mud based ceramic materials have been manufactured using red-mud [8, 9]. Cement and geopolymer production from red-mud has also been intrigued upon but these products cannot compete with the commercially available conventional products from a quality perspective. Some other applications of red mud such as waste water treatment for toxic elements [10], cation exchanger [11], filter medium [12], flocculants [13], a pigment in paints [14] and a filler material for pesticides and insecticides [15] also prove to be uneconomically, qualitatively unfeasible for commercial utilization with respect to red-mud’s basicity, complex mineralogy, and presence of fine particulates. Extracting iron from red-mud could facilitate the utilization of red-mud for some of the above applications. Metallurgically, the removal of iron could favor the extraction of Aluminum, Titanium [16] and also concentrate the highly valued rare earth metals to extractable limits [17, 18]. Iron was recovered as a valuable byproduct pig iron by utilizing optimum fluxing and reductants [19-21]. The slag generated in these processes were utilized to extract Aluminum, Titanium and other value additions. Enormous research has been done to simultaneously extract metallic iron, alumina as sodium aluminate [19, 22-26] and aluminum silicates [27] by employing a soda ash/sodium carbonate and carbon based roasting. Magnetic separation was performed to separate the metallic iron and sodium aluminate was recovered through water leaching. Carbothermic reduction of red-mud was performed to produce metallic iron which was separated by magnetic separation [28-30], yet a clear separation between the ferrous and non-ferrous components was not achieved in all the reported cases. A similar result was observed by Regina et al in the separation of paramagnetic Hagg carbide reduced from red-mud. High Gradient magnetic separation was also tried to separate the iron from red-mud sample, but a lot of weak magnetic particles were discovered in the magnetic fraction [32]. Hydrothermal leaching studies to selectively separate iron from the red-mud were also not successful [33-36]. The research reported in this paper proposes a probable techno-economically feasible way to create value from iron in red-mud. A low temperature reduction process to reduce hematite in red-mud to produce magnetite and eventually separate it is presented. There has been limited research on this aspect of iron removal strategy. Magnetite finds its market avidly in computer drives and loud speakers in view of its ferromagnetic property [37, 38] while the black color of magnetite makes it a major constituent in paint pigments [39] and the iron content of magnetite makes it a viable fertilizer for crops [40]. Prior to the reported study, Yanyan Liu et al devised a method to produce magnetite by anaerobically coroasting pyrite (FeS2) with red-mud. The elemental sulfur produced helped in this reduction. A wet magnetic separation method was employed to separate magnetite but the grade of the separated magnetite was not mentioned by the authors [41]. A similar observation was put forward by the researchers at University of Missouri Rolla [42]. Experimental:Dewatered red-mud from the Jamaican refinery of Aluminum Corporation of America (ALCOA) was used for our study. The as received sample contained about 27 % water (L.O.I.). The sample was dried in a box furnace at 150 oC for 6 hours. The dried material was grounded and de-agglomerated in a roll crusher with a P80 of 212 ?m. A representative sample of red-mud was generated from the bulk using cone and quarter method. The representative sample was analyzed for in ICP OES, AAS, XRF, XRD, M?ssbauer spectroscopy, TGA, QEMSCAN, SEM and TEM as a preliminary characterization. The red mud used contained 32 % Iron in the form of Hematite and goethite, 8 % Aluminum as Aluminum Hydroxides and Alumina, 4 % Titania, 6 % Calcium Carbonate and Desilication product (DSP): Sodium Alumino Silicates. Red-Mud was reduced in a tube furnace in an atmosphere of CO, CO2 and N2. Stability diagrams for the reduction system were generated using the thermodynamic data generated from HSC 5.1. The outlet gases emerging from the tube furnace were passed through an online CO and CO2 monitor. Mass balance for the process was monitored and % reduction was measured using M?ssbauer spectroscopy. Magnetite reduced from red-mud using the optimum reduction parameters was subjected to magnetic separation. Dry and wet magnetic separation routes were tried in Frantz and Davis tube magnetic separator respectively. Operational parameters related to sample mass flow rate per magnetic field exposure, magnetic field intensity were varied and optimized. ICP-OES and M?ssbauer spectroscopy were used to analyze the elemental and phase compositions of the separated fractions. Theoretical Aspects: Phase stability diagram Thermodynamic analysis of the red-mud Carbon system was constructed utilizing the data from the HSC chemistry 5.1 software. Gibbs free energy for formation as a function of temperature was interpreted for different phases plausible during reduction reaction: hematite, magnetite, wüstite and cementite. The equilibrium thermodynamic diagram is modified to represent relative partial pressures of Carbon monoxide and Carbon dioxide respectively (reactants for the reaction system). The reactions among various components in our study are represented in equations 1-4. 3Fe2O3 (s) + CO(g) = 2Fe3O4(s) + CO2(g)(1)Fe3O4(s) + CO(g) = 3FeO(s) + CO2(g)(2)Fe3O4(s) + 6CO(g) = Fe3C(s)+ 5CO2(g)(3)3FeO(s) + 5CO(g) = Fe3C(s) + 4CO2(g)(4)The equilibrium representation for the reaction 1 is given by the Van’t Hoffs equation:ΔGo = -RTlnK(5)Where R is the universal gas constant and K is the equilibrium constant for the reaction. For eq 1,ΔG = -RTln[(aFe3O42*PCO2)/(aFe2O33*PCO)](6)Activities of Fe3O4 and Fe2O3 are assumed to be 1 which leads to equation 7. The values of ΔG are obtained at various Temperatures by inputting the reaction into the reaction equation tool box in HSC chemistry 5.1.ΔG = -RTln(PCO2/PCO)(7)This exercise is repeated for all the other reaction equilibriums. Then a graph is plotted between PCO/PCO2 on a decade scale and Log(PCO/PCO2) v/s Temperature and 1/Temperature respectively in Figure 1. Reduction experiments are conducted in the stability zone of Magnetite (Fe3O4) varying CO/CO2 ratios and Temperatures. 0-635Figure 1: Stability diagram of Fe-C-O system at total pressure of 0.8 atm. Results and DiscussionRed mud is reduced to magnetite in a carbon based reducing atmosphere. Precursor reduction using petroleum coke was tried on red mud. Incomplete reduction of hematite in red mud to magnetite and slower kinetics of reaction were an indication to use of gaseous based reductant [43]. A mixture of CO, CO2 was used at a constant ratio to fix the reduction potential of the reaction while N2 was used as a buffer for the experiments. Reductions were performed using optimized parameters and the reduced magnetite so produced is subjected to magnetic separation in a dry and wet magnetic separator. The results of some of the experiments are presented in the following sections: Reduction ExperimentsReduction of Jamaican red mud was carried out in the presence of CO(g), CO2(g) and N2(g).The atmosphere was tailored to lie in the stability zone of magnetite (Figure 1). Experiments were carried out in an alumina sample boat (50mm (L) * 15mm (W) * 10mm (depth)). After the reduction, sample is cooled in the natural atmosphere and latter weighed. The reductions were studied varying CO:CO2 ratio (1:1.5, 1:1, 1:1), temperature (475 oC, 500 oC, 550 oC and 600 oC), time (10 min, 20 min and 30 min). Elemental, phase analysis and quantification were conducted in ICP, XRD and M?ssbauer spectroscopy respectively. 3D contour plots are generated for three CO:CO2 ratios varying % Magnetite (calculated using M?ssbauer spectroscopy) converted from hematite v/s time and temperature (Figures 2-4). Figure 2: 3D contour plot (% Magnetite converted v/s time and temperature) at CO:CO2 = 1:1Figure 3: 3D contour plot (% Magnetite converted v/s time and temperature) at CO:CO2 = 1:1.5Figure 4: 3D contour plot (% Magnetite converted v/s time and temperature) at CO:CO2 = 1.5:1For all the CO/CO2 ratios, as temperature increase the % conversion of hematite in red mud to magnetite increases. A similar trend is observed on increasing the CO/CO2 ratios. Reduction at 550 oC, CO: CO2 = 1:1 and a reduction time of 20 min were chosen as optimum. In addition the presence of some cementite peaks were seen (3-5 wt %). Reductions were performed using the optimized parameters obtained using 7 g of sample to generated precursors for next stage magnetic separation. Magnetic Separation The samples generated under the optimized conditions were subjected to magnetic separation in a dry and wet magnetic separator. The objective of the experiments was to separate the magnetite from the non-magnetic fractions of the reduced sample. Dry and wet magnetic separation were carried out in a Frantz and Davis tube separator respectively. Flow rate and magnetic field intensity were varied in both the magnetic separation experiments. Elemental mapping of the magnetic and nonmagnetic portions emanating from the separator. Recovery of magnetite separated the magnetic fraction v/s grade (purity) of the magnetically separated magnetite plots were generated for the separation experiments. Locii of all the experiments conducted in both the magnetic separators are presented in Figure 5 and 6. Figure 5: Grade v/s recovery plot of magnetite recovered in the magnetic fraction in a Frantz dry magnetic separator. Figure 6: Grade v/s recovery plot of magnetite recovered in the magnetic fraction in a Davis tube wet magnetic separator.Figure 5 and Figure 6 present an interesting inference about the magnetic classification. In some of the experimental parameters for magnetic separation (wet and dry), ~ 90 % recovery of magnetite in the magnetic fraction is obtained but the grade (purity) of the magnetite classified remained at 55- 60 %. The presence of very fine particulate agglomerates and/or lack of phase liberation between various phase constituents in the reduced magnetite could be attributed to the inefficiency in magnetic classification. Phase LiberationThe purpose of this section is to address the phase liberation if present in red mud precursor or reduced magnetite which was cited as a plausible reason for a decrease in magnetic classification efficiency in the previous section. Red mud precursor was sonicated in distilled water and subjected to study under the Transmission electron microscope. This study was conducted to check if particulate agglomeration was an issue. Figure 7 (a-d) shows the TEM image of the red mud precursor. a)b)c)d)Figure 7: TEM images of sonicated Jamaican red mud sample It is observed that the sample consists of numerous nanocrystals of sizes ranging from 40-120 nm. It can be concluded from this study that red mud essentially is composed of small nanocrystals and the larger particles are essentially composed of small nanoparticles. The crystal habitat was determined to be orthorhombic, these particles might be inferred as being lepidocrocite or goethite. It is unknown if the nano particulates are liberated mineralogically.center615315Fine particlesFe (Fe+3, Fe+2)Cementite3 octahedral, 2 tetrahedral sextetsSuper paramagnetic hematite and AluminogoethiteMag 39: Sample with highest % of residual hematiteMag 36: Sample with highest % of magnetiteFine particlesFe (Fe+3, Fe+2)Cementite3 octahedral, 2 tetrahedral sextetsSuper paramagnetic hematite and AluminogoethiteMag 39: Sample with highest % of residual hematiteMag 36: Sample with highest % of magnetiteM?ssbauer patterns of red mud, reduced magnetite produced under conditions producing least and maximum conversion efficiency, pure hematite and pure magnetite are presented in Figure 8. Figure 8: M?ssbauer patterns of select samplesThe presence of additional peaks in the red mud head sample when compared to the pure hematite sample suggests that the presence of very fine hematite particulates which show the presence of super paramagnetism and/ or the presence of aluminogoethite. Similarly in the reduced magnetite sample, the additional sextets present are attributed to the difference in the neighborhood of the M?ssbauer atom Fe57. This might be due to the occlusion of transition metal atoms (Al and Ti) in the hematite lattice. There are clear indications of decrease in magnetic classification efficiency owed due to lack of phase liberation as deciphered through the M?ssbauer spectrograms and (or) nano particulate agglomerations of various phases. A thorough phase classification needs to be conducted for the nano particulates to ascertain the mineralogical composition. ConclusionThe STEM photomicrographs of the sieved red-mud sample indicated the presence of “particulate” entities. Thus, assigning of a particle size to these entities, which are clusters of smaller- (nano)size crystallites, may well be an ambiguous (size) characteristic of this material. Reflecting on how red-mud is produced in the Bayer Process, this is not entirely surprising. In order to pursue this hypothesis, a sample of red-mud was sonicated in distilled water to “de-agglomerate” these clusters. Indeed, STEM photomicrographs reveal the nascent crystallites, which have sizes in the 40 120 nm range. Furthermore, the crystal-habit in some cases may well be associated with lepidocrocite or goethite, each of which has an orthorhombic crystal-structure. It has been demonstrated that “low” temperature (475oC to 600oC) gas-phase reduction of hematite in red-mud to magnetite is viable conversion-process that can be achieved with low partial-pressures of CO(g), and concomitant low partial-pressures of CO2(g). The low partial-pressures, which are required in order to avoid sooting (CO(g) disproportionation to C(s) and CO2(g)), require that N2(g) be employed to serve as diluent. The mass-loss associated with the reduced product (~1012 %) included that associated with the conversion of hematite to magnetite as well as decomposition of the aluminum-hydroxide phases also present in the red-mud. Solid-phase reduction-products obtained from the gas-phase reduction of red mud contained Fe3O4 (56.4 – 80.5 m%), Fe2O3 (0 20 m%), Fe3C (4.8 6.8 m %) and paramagnetic 2+ and 3+ phases (14 - 22 m%). A paramagnetic (M?ssbauer) resonance is most likely attributable to nano-size iron-oxide phases. The (preliminary) optimal-conditions for gas-phase reduction of the (Jamaican) red-mud investigated in the research, and reported in this thesis, are: processing temperature of 540oC ± 10C , partial pressures CO(g)and CO2(g) each of 0.070atm (bar) ± 0.001atm.(bar)/ inert diluent-gas: N2(g), for a conversion-time of 30min.Dry and wet magnetic-separation performed on the reduced samples did not achieve a magnetic (high-iron) fraction and a (low-iron) non-magnetic fraction as the desirable conversion-product property being sought. This result is most likely attributable to: either, 1) the cation substitution of, primarily, Al3+ and Ti4+/Ti3+ cations in the hydrated-oxide nanoparticles being converted to magnetite or, 2) nano-size particles of aluminum and titanium “oxides” occluded within the predominantly “large-particles/clusters” comprising the precursor red-mud and the subsequent magnetite product. AcknowledgementsThe authors thank CR3 (Center for Resource Recovery and Recycling) an NSF- IUCRC for funding this project. The authors also would like to give special commendation to Dr. Don Williamson, Professor Colorado School of Mines for helping with the M?ssbauer spectroscopy analysis which proved to be detrimental in the quantitative analysis of the phase constituents. ReferencesFathi Habbashi, “Textbook of Hydrometallurgy”, 1999; Métallurgie Extractive Québec.Edward, J., Frary, F., Jefferies, Z., “Aluminum and its Production”, 1930; McGraw-Hill Book Company, Inc. Robert. J. Anderson, “The Metallurgy of Aluminum and Aluminum Alloys”, 1925; Henry Carey Baird & CO., Inc. Burkin. A.R.,”Production of Aluminum and Alumina”, 1987, Society of Chemical Industry, John Wiley and Sons.Bauxite Residue Management: Best Practice, World Aluminum, European aluminum association, April 2013. 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