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Red Mud from Brazil: Thermal Behavior and Physical Properties Maria Lucia Pereira Antunes,† Sara Jane Couperthwaite,*,‡ Fabiano Tomazini da Conceic-~ao,§ Carolina Petrisin Costa de Jesus,† Pedro Kunihiko Kiyohara,|| Antonio Carlos Vieira Coelho,^ and Ray Leslie Frost‡ †
Universidade Estadual Paulista, UNESP, Campus de Sorocaba, Av. Tr^es de Marc-o, 511, CEP: 18087-180, Sorocaba, S~ao Paulo, Brazil Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia § Instituto de Geoci^encias e Ci^encias Exatas, Universidade Estadual Paulista, UNESP, Avenida 24-A, 1515, CEP: 13506-900, Rio Claro, S~ao Paulo, Brazil Instituto de Física, Universidade de S~ao Paulo, S.P., CEP: 05508-090, Brazil ^ Departamento de Metalurgica e de Materiais, Escola Politecnica, Universidade de S~ao Paulo, S.P., CEP: 05508-090, Brazil
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‡
ABSTRACT: The main constituents of red mud produced in Aluminio city (S.P., Brazil) are iron, aluminum, and silicon oxides. It has been determined that the average particle diameter for this red mud is between 0.05 and 0.002 mm. It is observed that a decrease in the percentage of smaller particles occurs at temperatures greater than 400 C. This observation corresponds with the thermal analysis and X-ray diffraction (XRD) data, which illustrate the phase transition of goethite to hematite. A 10% mass loss is observed in the thermal analysis patterns due to the hydroxideoxide phase transitions of iron (primary phase transition) and aluminum (to a lesser extent). The disappearance and appearance of the different phases of iron and aluminum confirms the decomposition reactions proposed by the thermal analysis data. This Brazilian red mud has been classified as mesoporous at all temperatures except between 400 and 500 C where the classification changes to micro/mesoporous. geopolymer,16,17 as composite materials,1820 as a catalyst,2124 as a coagulant,25,26 as a Fenton reagent,27 for the capture of CO2,2831 and as a metal,32 organic,33 and dye adsorbent.34,35 In order for applications of this residue to be devised, the thermal and physical properties of red mud need to be understood. This investigation has characterized this particular Brazilian red mud in the hopes that applications can be elucidated. The chemical and mineralogical composition of red mud is of great importance for the utilization of red mud in industrial applications. In the literature, there are limited references to the characteristics of Brazilian red mud. Red mud produced in the northern region of Brazil has been described by Snars36 and Mercury,37 while Costa27 and Ribeiro8 have reported the characteristics of red mud produced in Minas Gerais. To the best of the authors’ knowledge, there is no information available about red mud produced in the S~ao Paulo State. The aim of this investigation is to characterize the Brazilian red mud produced in the S~ao Paulo region, before and after heat treatment. The knowledge obtained from this investigation on red mud’s thermal behavior will allow for possible applications of this waste residue to be determined.
1. INTRODUCTION The ore which produces metal aluminum is bauxite. The composition of bauxite ore consists of a range of different hydroxide and oxides of aluminum, iron, and silicon.1 The majority of the world’s bauxite reserves are located in tropical and subtropical areas, and are called Lateritic bauxites.2 Brazil produces a third of the world’s bauxite, being surpassed only by Australia and China. In Brazil, metallurgy-bauxite ore sources are found in the northern and in the southeastern regions. The production of alumina occurs mainly in the northern (84.7%) and southeastern states. Bauxite refinery residues (red mud) are derived from the Bayer process by the digestion of crushed bauxite in concentrated caustic (NaOH) at elevated temperatures.1 The process results in the dissolution of gibbsite (Al(OH)3) and boehmite (AlO(OH)) to a sodium aluminate solution, while the remaining insoluble residue (45% liquor and 55% solid mud commonly known as red mud) is removed by means of flocculation and decantation.1,3,4 Red mud varies in physical, chemical, and mineralogical properties due to the different bauxite ore sources and refining processes employed. The bauxite ore composition varies from each mine, and thus, the refining processes employed need to be modified for each type of bauxite ore. Therefore, the composition of red mud around the world is different. The general consensus of the composition of red mud, however, has been found to be largely composed of iron oxides, primarily hematite (Fe2O3), goethite (FeOOH), boehmite (AlOOH), other aluminum hydroxides, calcium oxides, titanium oxides (anatase and rutile), and aluminosilicate minerals such as sodalite.3 In recent years, many studies have investigated different applications of red mud: as construction materials,57 as raw cement products,811 in the production of ceramics,1215 as a r 2011 American Chemical Society
2. MATERIALS AND METHODS Red mud samples were provided from the alumina plant located at Aluminio City, S~ao Paulo State (Brazil). The only pretreatment of red mud undertaken in this study, before thermal activation, was a drying stage that required the red mud to be Received: August 8, 2011 Accepted: November 19, 2011 Revised: November 18, 2011 Published: November 19, 2011 775
dx.doi.org/10.1021/ie201700k | Ind. Eng. Chem. Res. 2012, 51, 775–779
Industrial & Engineering Chemistry Research
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Table 1. Chemical Composition of Red Mud and Red Mud Heated at 500 C oxide
RM (%)
RM500 (%)
Table 2. Chemical Composition of Red Mud from Australia (Kwinana), Spain (Alcoa), and Germany (Boke)36 oxide
Australian RM
Spain
Germany
Al2O3
22.87
24.32
Al2O3
24.0%
21.2%
16.2%
Fe2O3 SiO2
27.04 19.19
28.97 20.76
Fe2O3 SiO2
28.5% 18.8%
37.5% 4.4%
44.8% 5.4%
TiO2
2.98
3.22
CaO
2.17
2.33
Na2O
8.01
7.97
MnO
0.16
0.17
MgO
0.04
0.04
of red muds produced in Brazil.36 This demonstrates not only that the Brazilian bauxite ore sources differ in composition but also that different refinery processes are employed for the different bauxite ores. Therefore, different red mud compositions are produced in Brazil. 3.2. Granulometric Analysis. Granulometric analysis allowed for the identification of the medium diameter of the particles that make up red mud samples. These results are presented in Table 3. Most particles have a diameter between 0.05 and 0.002 mm. At temperatures greater than 500 C (except RM700), the percentage of particles that have a diameter less than 0.002 mm decreases, while an increase in particles with a diameter between 0.05 and 0.002 mm is observed. The percentage decrease and increase for particles of less than 0.002 and between 0.05 and 0.002 mm, respectively, are approximately equal. Therefore, increasing the temperature causes the particle size to increase due to the dehydration of goethite to hematite. This observation corresponds well with the XRD data, which shows a large reduction in goethite and an increase in hematite. Overall, more than 90% of particles have a diameter less than 0.05 mm. The increase in smaller particles (less than 0.002 mm) at 700 C is believed to be caused by intermediate products in the formation of magnetite, which results in larger particles at 800 C. Using soil texture classification,38 red mud is identified as a loam silty clay. It presents dry cohesion, intense capillarity properties, some plasticity, and adsorption capacity. 3.3. Specific Surface Area. Heat treatment of RM causes an increase in the red mud surface area up to temperatures of 500 C (Table 4). However, heating to temperatures above 500 C results in a decrease of surface area. The pore diameter observed for RM, RM600, RM700, and RM800 samples are between 3 and 4 nm, and are classified as mesopores. RM400 and RM500, which have the highest surface areas, have pore diameters between 1 and 4 nm and are classified as micro and mesopores. These results correspond well with those observed in the granulometric analysis. 3.4. Thermal Analysis. Figure 1 shows the thermogravimetry (TG) and differential thermogravimetry (DTG) curves of RM. The combined mass loss from 120 and 1000 C is 18.29%. There are three phase transitions in this thermal analysis pattern: (1) Between 120 and 430 C, three decomposition steps occur involving the dehydroxylation of compounds found in red mud. This mass loss is believed to be due to the decomposition of goethite to hematite (based on the XRD results) and also due to the decomposition of gibbsite to a phase of alumina, most likely gamma.39 A combined mass loss of 10.66% was observed for these phase transitions. The following decomposition steps are proposed:
placed in an oven at 80 C overnight (RM). Heat treatment involved the heating of the red mud samples in a furnace for 3 h at different temperatures: 400, 500, 600, 700, and 800 C (these materials are called RM400, RM500, RM600, RM700, and RM800, respectively). The pH of the red mud samples were determined to be 10.0 ( 0.5, using a YSI 85 meter calibrated with standards solutions pH 7.00 (7.005 ( 0.010 in 25 ( 0.2 C) and 10.00 (7.000 ( 0.010 in 25 ( 0.2 C). The chemical composition of red mud (RM) and red mud heated to 500 C (RM500) was determined using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at Acme (Analytical Laboratories LTDA, Vancouver Canada). The major components (SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5) were analyzed by an ICP-AES after fusion using lithium metaborate/tetraborate. Particle size analysis of the components of RM and heat treated RM was obtained by the granulometric method. It was obtained by sieving methods and by sedimentation analysis. The specific surface areas of all samples were determined by BET/N2 adsorption methods using a Micromeritics ASAP 2010 instrument. Mineral composition was determined by X-ray diffraction (XRD), using a Philip X-Pert wide-angle X-ray diffractometer operating at 40 kV and 40 mA with CuKα radiation. The morphology of RM and heated RM samples were observed under a Phillips CM200 Transmission Electron Microscopy (TEM), attached with an energy dispersive X-ray spectrometer unit (EDS). Thermal decomposition of the natural red mud sample was carried out in a TA Instruments thermogravimetric analyzer (series Q500) in a flowing nitrogen atmosphere (80 cm3/min). Approximately 30 mg of sample was heated in an open platinum crucible at a rate of 2.5 C/min up to 1000 C at high resolution.
3. RESULTS AND DISCUSSION 3.1. Chemical Composition. The compositions of untreated red mud and red mud heated at 500 C are presented in Table 1. Iron and aluminum oxides are the main components of RM and RM500, making up half of the chemical composition of the red mud samples. The results show that no significant changes in the concentrations of iron and aluminum oxide occur. Comparing the three main components of red mud, the composition of Brazilian red mud is similar to Australian red mud (Table 2).36 When compared with Spanish and German red mud, there is a smaller amount of iron oxide and a greater quantity of silicon oxide in this Brazilian red mud (Table 2).36 Comparing the composition of RM with other red mud produced in different regions of Brazil shows the variability
243C : 2FeOðOHÞ f Fe2 O3ðsÞ þ H2 OðgÞ ðgoethite f hematiteÞðrefs 39 and 40Þ 776
dx.doi.org/10.1021/ie201700k |Ind. Eng. Chem. Res. 2012, 51, 775–779
Industrial & Engineering Chemistry Research
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Table 3. Granulometric Analysis of Red Mud at Different Temperatures weight percent (%) diameter 0.05>d >
diameter
diameter >0.05 mm
0.002 mm
