ISSN 00360295, Russian Metallurgy (Metally), Vol. 2013, No. 1, pp. 33–37. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.A. Raspopov, V.P. Korneev, V.V. Averin, Yu.A. Lainer, D.V. Zinoveev, V.G. Dyubanov, 2013, published in Metally, 2013, No. 1, pp. 41–45.
Reduction of Iron Oxides during the Pyrometallurgical Processing of Red Mud N. A. Raspopov, V. P. Korneev, V. V. Averin, Yu. A. Lainer, D. V. Zinoveev, and V. G. Dyubanov Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, 119991 Russia email: [email protected]
Received February 27, 2012
Abstract—The results of experiments on the use of red mud in traditional pyrometallurgical processes and plants are presented. The red muds of the Ural Aluminum Plant (UAZ, KamenskUral’skii) and the Alyum Plant (Tul’chiya) are shown to have similar phase and chemical compositions. The morphology of the iron oxides in red mud samples taken from mud storage is studied by Mössbauer spectroscopy. It is found that the metallic (cast iron) and slag phases that form during the pyrometallurgical processing of red mud by melting with a carbon reducer in the temperature range 1200–1500°C are clearly separated. Cast iron can be used in steelmaking, and the slag can be used for hydrometallurgical processing and extraction of nonferrous metals and for the building industry after correcting its composition. DOI: 10.1134/S0036029513010114
At present, the production of aluimina by the Bayer process, which is accompanied by the formation of a large amount of a residual alumina processing product (red mud), has the maximum scale in nonferrous met allurgy. Hundreds of millions of tons of red mud have been accumulated because of the absence of cost effective methods for its recycling.
We studied the red muds of the Ural Aluminum Plant (UAZ, KamenskUral’skii) and the Alyum Plant (Tul’chiya). At the first stage, we analyzed the phase compositions of the iron oxides present in the muds. The investigations were carried out at room tempera ture on am Ms110Em Mössbauer spectrometer under constant acceleration conditions using a Co57 source in an Rh matrix. To analyze the Mössbauer spectra of the red mud, we used the UnivemMS software package to decompose a general spectrum into the spectra of individual ironcontaining phases. To calculate these Mössbauer spectra, we used the procedure of identify ing complex spectra described in .
As shown in , red mud can be used to produce nonferrous metal (Sc, Zr) concentrates, and the resi due can be used in the building industry as a strength ening addition to cement clinker during the produc tion of cement, road stone, and clinker bricks and tiles [2, 3]. However, the high content (on average, 40– 50%) of iron oxides in red mud makes it possible to consider them mainly as a source of raw materials for ferrous metallurgy .
To refine the model of processing of the Mössbauer spectra of the red mud, we performed a semiquantita tive Xray diffraction investigation of mud samples.
It is difficult to perform the extraction of iron from red mud in traditional pyrometallurgical processes and plants because of the necessity of preliminary mud agglomeration and the high content of alkali com pounds in mud.
To find the characteristics of the equilibrium con ditions and compositions of the phases during the interaction of carbon with red mud, we used the Terra software package , which was successfully used ear lier to study the reduction of iron oxides and zinc in electric furnace mud .
Although technologies for processing red mud using alternative metallurgical processes were developed, they have not gained commercial acceptance [5, 6].
The reduction of iron oxides from the red mud was studied on a laboratory coal resistance furnace (Tam mann furnace). The metal and slag formed during experiments were analyzed by Xray diffraction (XRD) and chemical methods.
The purpose of this work is to study the possibility of recycling of red mud not subjected to specialpur pose preparation using traditional pyrometallurgical processes. 33
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line 4), which is likely to belong to iron hydroxide of a complex composition. To refine the model of processing of the Mössbauer spectra of the red mud, we performed a semiquantita tive XRD investigation of mud samples on an ULTIMA IV (Rigaku) Xray diffractometer using CuKα radiation. Allowing for XRD data, we detected the following iron distribution over the ironcontaining phases of the Alyum mud: 45% hematite (αFe2O3) and 55% lepidocrocite (γFeOOH). Apart from hematite and lepidocrocite, XRD analysis also revealed low con tents of gibbsite (Al(OH)3) and goethite (αFeOOH). According to the Mössbauer data, the iron distribution over the ironcontaining phases in the UAZ mud is as follows: 80% hematite (αFe2O3), 15% iron–alumi nosilicate (divalent iron), and 5% complex iron hydroxide. XRD analysis also detected grossular (alu minosilicate), boehmite (aluminum hydroxide), and complex iron hydroxide. Thus, the initial iron oxides of red mud consist of partly hydrated hematite with a various lattice param eter. The broad lines of the doublets of hydroxides and the low fields of the spectra of hematite indicate a fine grained structure of iron particles, which increases the reactivity of the material.
I, arb. units
0.88 (b) 1 43
8 v, mm/s
Fig. 1. Mössbauer spectra of the red mud of (a) Alyum and (b) UAZ plants. Systems of lines of (1) hematite, (2) lepi docrocite, (3) iron–aluminosilicate, and (4) complex iron hydroxide.
RESULTS AND DISCUSSION Phase Composition of the Iron Oxides in Red Mud An analysis of the spectra of the red muds of Alyum (Fig. 1a) and UAZ (Fig. 1b) indicates the presence of both magnetically ordered and paramagnetic com pounds. The magnetically ordered parts of the initial spectra of both types of mud consist of the sextets of the hyperfine structure of hematite (αFe2O3) with Heff = 503–506 kOe (Fig. 1, system of lines 1). Lower Heff as compared to the standard field (515 kOe) and broad lines point to a certain degree of hydration of hematite molecules. The following doublets appear in the paramagnetic part of the spectrum in the velocity range from –1.0 to +2.0 mm/s: the doublet with an isomer shift δEI = 0.36 mm/s and a quadrupole split ting ΔEQ = 0.56 mm/s for the Alyum mud (Fig. 1a, line 2) and the doublet with an isomer shift δEI = 1.14 mm/s and a quadrupole splitting ΔEQ = 2.61 mm/s for the UAZ mud (Fig. 1b, line 3). More over, the spectrum of the UAZ mud has a broad (Γ/2 = 0.77 mm/s) single line with δEI = 0.58 mm/s (Fig. 1b,
Thermodynamic Simulation of the Interaction of the Red Mud Components with Carbon in Heating The carbon reduction of iron oxides can occur over the wide temperature range that corresponds to solid and liquidphase interaction fields . Based on the reference data of the Terra software package, we deter mined the thermodynamically stable condensed phases at a given temperature and pressure for a mix ture of 100 g UAZ red mud and 8 g carbon (Fig. 2). As follows from the calculation of the equilibrium composition during pyrometallurgical processing of the iron oxides, initial hematite transforms into a more sta ble form (magnetite) in heating, and magnetite loses an oxygen atom to form three iron monoxide molecules according to the reaction at 870 K. The quantity of cementite formed according to the reaction 3FeO + 4C = Fe3C + 3CO begins to decrease smoothly when the temperature increases to 1030 K. The process then accelerates, cementite disappears at 1120 K, and metallic iron pre cipitates according to the reaction Fe3C + CO2 = 3Fe + 2CO. In the temperature range 1030–1120 K, a gas phase, iron, and cementite are in equilibrium, and the 2 equilibrium constant has the form K = p CO / p CO2 . We compared the data shown in Fig. 2 to the calcu lated data obtained by a combination of the reactions of formation of Fe3C, CO2, and CO and using the thermodynamic parameters from handbook  and found satisfactory agreement between them, which
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REDUCTION OF IRON OXIDES Wi, mass fraction
Wi, mass fraction Fe3O4(m.)
0.3 Fe3C(m.) 0.2 0.1
1600 T, K
Fig. 2. Change in the equilibrium compositions of the iron and carboncontaining condensed phases in the red mud of UAZ in heating of a mixture of 8 g carbon + 100 g mud at p = 0.1 MPa. Wi is the mass fraction of the phase composition.
Wi, mass fraction
Fig. 3. Change in the equilibrium compositions of the iron and carboncontaining condensed phases in the red mud of UAZ in heating of a mixture of 12 g carbon + 100 g mud at p = 0.1 MPa.
Fig. 4. Change in the equilibrium compositions of the iron and carboncontaining condensed phases in the red mud of UAZ in heating of a mixture of 18 g carbon + 100 g mud at p = 0.1 MPa.
As is seen from this reaction, the reduction of 3 moles hematite requires 10 moles free carbon. Since the hematite content in the charge is 50%, the stoichi Vol. 2013
1600 T, K
Fig. 5. Change in the partial pressure pi of the gases that form in heating of a mixture of 18 g carbon + 100 g UAZ red mud at p = 0.1 MPa.
indicates that the thermodynamic simulation method used in this work is sufficiently reliable. The computer program demonstrates the forma tion of complex oxides CaAl2SiO6, Na2SiO3, and NaAlO2 at high temperatures in the UAZ mud. It is interesting that sodium passes from sodium silicate to sodium aluminate, which is a stronger compound, at above 870 K and the active form of alumina takes place, whereas a pronounced affinity to the base oxides (CaO, MgO) manifests itself in silica. The quantity of carbon that is necessary for iron to come from hematite to cementite completely can be calculated by the reaction 3Fe2O3 + 10C(sol.) = 2Fe3C + 7CO + CO2.
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ometrically necessary quantity of carbon is 12.5 g per 100 g charge. As follows from the data in Fig. 3, metallic iron can form in a dispersed form in the charge volume at the solidphase reduction temperatures. To extract this iron, the cake should be fragmented and magnetic separation should be applied, which is difficult because of the possibility of substantial iron contami nation by particles of other oxides. When performing calculations with excess carbon in a charge (mixture of 18 g carbon + 100 g mud), we found excess carbon and cementite in this condensed system after oxygen is fully fixed (Fig. 4). At high tem peratures, cementite decomposes to form a carbon saturated iron solution (cast iron) and free carbon. The decomposition kinetics of iron oxides and cementite is shown in Fig. 5. The gasphase composi tion remains constant at above 1140 K. Sodium evap
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yellow gas, which is likely to be related to the intensi fication of sodium evaporation.
Table 1. Contents of the main components in red mud samples Component content (wt %) in the red mud of plant
The results of chemical analysis of the slag and metal obtained in experiments are presented in Tables 2 and 3, respectively. It follows from Tables 2 and 3 that the reduction of iron oxides under the experimental conditions is com plete. A high sulfur content in the metal is associated with the presence of sulfur in the reducer material. XRD analysis of the obtained slags shows that small quantities of wüstite (FeO), magnetite (Fe3O4), and calcium–silicon oxide (CaSiO3) are present in various UAZ mud samples against the background of an amorphous matrix. Iron is also present in the compo sition of magnesium–aluminum oxide, and other ele ments enter into the compositions of nepheline (NaAlSiO4) and perovskite (CaTiO3). CONCLUSIONS
Table 2. Results of elemental analysis of the cast iron formed after melting of the red mud of UAZ
(1) The chemical and phase compositions of the red mud from the Alyum and UAZ plants were stud ied. Iron in the mud of Alyum was shown to be present in the compositions of hematite αFe3O4 (45%) and lepidocrocite γFeOOH (55%). Iron in the UAZ mud is distributed over ironcontaining phases in as follows: 80% hematite (αFe2O3), 15% iron–aluminosilicate (divalent iron), and 5% complex iron hydroxide.
Element content*, wt % Heat C
* Iron for balance.
(2) The interaction of the red mud components with carbon was subjected to thermodynamic simula tion over a wide temperature range. It was shown that metallic iron can form in a dispersed form in the charge volume at the solidphase reduction tempera tures. It is difficult to extract this iron.
oration develops at high temperatures in a reducing atmosphere. Experimental Investigation of the Reduction of Iron Oxides from Red Mud
(3) The red mud from the Alyum and UAZ plants was subjected to reducing melting, and clearly sepa rated products, namely, a metal (cast iron) and a slag, were obtained.
In the state of natural humidity (up to 10%) with out additional drying and calcination, red mud was charged into a graphite crucible, heated to 1770 K, and held at this temperature for 10 min. At 1520 K, the mud melted to form a freerunning mass. At tempera tures above 1470 K, we observed the release of brown–
(4) Pyrometallurgical processing of red mud was shown to provide the formation of slags with a higher Al2O3 content as compared to that in the initial red mud.
Table 3. Results of elemental analysis of the slag formed after melting of the red mud of UAZ Element content*, wt % Heat Al
* Oxygen for balance RUSSIAN METALLURGY (METALLY)
REDUCTION OF IRON OXIDES
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Translated by K. Shakhlevich