CHARACTERIZATION OF FAYALITE FROM COPPER SLAGS

Transcription

1 Journal of the University of Chemical I. Mihailova, Technology D. Mehandjiev Metallurgy, 45, 3, 2010, CHARACTERIZATION OF FAYALITE FROM COPPER SLAGS I. Mihailova 1, D. Mehandjiev 2 1 University of Chemical Technology and Metallurgy 8 Kl. Ohridski, Sofia 1756, Bulgaria irena@uctm.edu 2 Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg. 11,Sofia 1113, Bulgaria Received 05 March 2010 Accepted 12 June 2010 ABSTRACT The aim of the present study was to characterize the fayalite crystal phases in two types of copper slag flash smelting furnace slag (FS) and converter (CS) slag, given their possible use in adsorption and catalytic processes. Two typical slag compositions different in chemical composition and way of production were selected, in order to clarify how these differences affect fayalite crystallization and composition. The main mineral phase found in copper slags is fayalite. The samples of the slag were characterized using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), Light optical microscopy (LOM) in transmitted and reflected light. Keywords: fayalite, copper slags, flash smelting furnace slag, converter slag. INTRODUCTION The chemical, phase and granulometric composition of metallurgical slags has always attracted a great deal of interest in view of their reuse in the process of production or use for other purposes, such as the extraction of certain metals from them or for the production of construction materials. Being a waste product, the slag recovery or disposal in designated places is closely related to the issue of environmental protection. During the cooling of the slag, crystallisation processes occur in it and certain phases are formed. The main phases formed during each crystallization process are known, and they depend on the cooling conditions of the metallurgical slag at certain stages in the production process. Since slag-forming additives (fluxes) are based on silicon, aluminium, calcium and magnesium, compounds of mineral analogues are formed, a process similar to the process of crystallization in natural silicate melts - volcanic lava or magma. For example, the slag from processing and production of ferrous metals is characterized by phases of the melilite group - gehlenite and akermanite. Some studies, however, have shown that these slags have adsorption properties for heavy metals, such as lead, copper, etc., and the increased content of these phases enhance the slag adsorption properties [1, 2]. In such cases, slag can be successfully used as an adsorbent in purification of water containing heavy metals [3]. It was also found that metals adsorbed on the slag surface, for example copper, make it catalytically active in processes of complete oxidation and catalytic removal of carbon monoxide and harmful organic substances [4]. The flash smelting furnace slag and converter slags in copper production contain mainly fayalite due to the nature of the process of production. Fayalite is a common member of the olivine group of minerals. Olivine is a name for a series of silicate minerals with the formula M 2, where M is most commonly Fe or Mg. Fayalite (Fe 2 ) and forsterite (Mg 2 ) form a substitutional solid solution where the iron and magnesium atoms can be substituted for each other without significantly changing 317

2 Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010 the crystal structure. The complete solid solution between these two elements is observed in olivine. Compositions in the Mg Fe series commonly are identified by the molar percentages of forsterite (Fo) and fayalite (Fa) (e.g., Fo 90 Fa 10 ), or in shortened form, by just their forsterite number, where Fo # = Mg/(Mg+Fe) 100. The crystal structure of Fe 2 has been refined using X-ray methods [5-7]. Olivine has an orthorhombic structure. The structure consists of isolated 4- tetrahedra, which are held together by M cations occupying two types of octahedral site (M1 and M2). The isolated tetrahedra point alternately up and down along rows parallel to the z-axis. The magnetic spin structure has been determined using powder neutron diffraction at 4 and 58 K by Santoro et al., who have also measured the powder magnetic susceptibility between 4 and 300K [8]. Olivine with up to one formula unit of calcium (CaMg ) is referred to as monticellite. The iron analogue, kirschsteinite (CaFe ), is rare in nature. There also is a manganese end member, tephroite (Mn 2 ). The metal cations occupy two positions in the crystal structure, M1 and M2, which are defined by their symmetry; commonly, Fe 2+ displays a preference for the M1 site. The members of the olivine group are presented in Table 1. M. W. Schaefer has reported large amounts of Fe 3+ ions in fayalites (members of the olivine group) from various localities. Such data are contrary to the generally held belief that fayalite is incapable of accommodating much Fe 3+ ions in its crystal structure [9]. Members of the olivine group (olivines) are important rock-forming minerals in terrestrial, planetary, Table 1. Olivine End-Members. Mineral names Chemical composition Forsterite Mg 2 Fayalite Fe 2 Tephroite Mn 2 Liebenbergite Ni 2 Co-olivine Co 2 Ca-olivine Ca 2 Monticellite CaMg Kirschsteinite CaFe Glaucochroite CaMn and astronomical geomaterials. Olivines are the major component of Earth s mantle, they are common to many types of meteorites, and have been identified on the surfaces of planetary bodies and in the spectra of astronomical targets. Olivine phases are indicators of low-silica environments, they crystallize at high temperatures, and they generally break down readily in the presence of weathering agents such as water. As such, their identification and characterization is a subject of considerable interest to a wide variety of researchers [10]. Olivines are, quite literally, universally relevant minerals. On Earth, olivine compositions in the forsterite fayalite series are common in mafic to ultramafic rocks, in which their compositions typically range from about Fo Compositions between Fo 80 and Fo 50 are typical of gabbroic rocks. Olivine group minerals can be formed in natural and in technological processes - in certain mineral aggregates (rocks), in slags of the Blast furnace and agglomerates [11]. Fayalite also has considerable technological importance in copper metallurgical slags. Ivanov et al. carried out interesting research on phase composition and structure of copper slag from Bulgarian plants [12-16]. Our previous research [17, 18] on the phase composition and structure of flash smelting furnace slag and converter slag from copper production found that the chemical compositions of fayalite included Al, Mg, Zn, Cu, Al, Ca and other elements as isomorphic impurities. The phases in the samples were characterized by a variable and non-stoichiometric composition. However, the preference of the various elements contained in the slag to the different crystalline or amorphous phases was clearly manifested. The aim of the present study was to characterize the fayalite crystal phases in two types of copper slag flash smelting furnace slag and converter slag, given their possible use in adsorption and catalytic processes. Two typical slag compositions different in chemical composition and way of production were selected, in order to clarify how these differences affect fayalite crystallization and composition. EXPERIMENTAL Two kinds of copper slags were examined slag from flash-smelting furnace process and converter slag. Samples were taken from the slag fields used for crys- 318

3 I. Mihailova, D. Mehandjiev tallization of the liquid slag by spraying with water. The samples of copper flash smelting furnace slag and converter slag are labeled as FS and CS respectively. The samples of the slag were characterized using X-ray diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), Light optical microscopy (LOM) in transmitted and reflected light. Quantitative determination of elements in the samples was carried out by Gravimetric analysis, Complexonometry, Atomic absorption spectrometry (AAS) and Inductively coupled plasma (ICP-OES). The XRD characterization of the different samples of slag was conducted using a DRON 3M X-ray diffractometer with a CoKá radiation (40 kv, 25 ma) as an X-ray source (λ = nm). The samples were continuously scanned over an angular range of 2è 8 90 using a step size of PowderCell 2.4 software [19] was used to determine the unit cell parameters on the basis of experimental X-ray data. FTIR spectroscopy has been applied. Infrared transmittance spectra were recorded by using pressed pellet technique in KBr. The measurements were made with a FTIR spectrophotometer Bruker EQUINOX 55 in the frequency range from 4000 to 400 cm -1. In order to have a good evaluation about the structures and textures included of each sample as well as to make a comparison between different samples, light optical microscopy in transmitted and reflected light was used. In this study we have used polarizing microscopes Laboval pol u Carl Zeiss Jena. RESULTS AND DISCUSSION Chemical composition results Table 2 shows the chemical composition of samples of slag. X-ray diffraction Fig. 1 shows X-Ray diffraction data for samples of slag. Fayalite (Fe 2 ) and magnetite (Fe 3 O 4 ) are two dominant phases in the samples. The two presented X- ray diffraction patterns show amorphous halo at lower values of 2è, associated with the presence of amorphous phase. The converter slag sample contains larger amount of magnetite in comparison with the sample of flash smelting furnace slag. Divergence between experimental data and reference pattern data for the intensity of diffraction peaks may be observed. The reflections of the fayalite phase with the indices (120) (240) (020) (130) have a higher intensity than expected. This is due Fig. 1. XRD patterns of samples of copper slags 319

4 Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010 Table 2. Chemical composition of slag samples. Elements/ Content, % mass Oxides Flash smelting furnace slag (FS) Converter Slag (CS) SiO CaO MgO K 2 O Na 2 O Fe Cu Pb Zn Ni Co As Sb Bi Cd Cr S Ó to textures in the samples related to fayalite crystals morphology. The presence of additional crystalline phases could not be ascertained from the presented XRD patterns. Peaks with negligible intensity remain after juxtaposing diffraction maxima with the reference patterns for fayalite and magnetite. Diffraction maxima at è (d = 3.00) in the XRD pattern of flash smelting furnace slag sample and è (d = 3.34) in the XRD pattern of converter slag sample are an exception. Their presence is likely due to copper sulphide phases. The cell parameters of fayalite and magnetite found in slag were calculated using the program Powder Cell 2.4. They are given in Table 3. As seen in Table 3, the values of the unit cell parameters of fayalite and magnetite in copper slags are very close to literature values for pure phases. FTIR analysis Fayalite is an especially important phase for different types of spectroscopy because, by definition, it contains an equal distri bution of Fe 2+ cations between the M1 and M2 octahedral sites. Thus, features associated with each of the two sites must represent equal numbers of Fe 2+ cations, removing the uncertainties associated with assumptions about order/disorder of Fe 2+ and other cations [24]. Thermal infrared band assignments for Mg Fe olivines were determined early on [25-26] from transmission spectra and many studies have resulted in refined and extended assignments. Symmetry analysis indicates that there are a total of 84 normal modes of vibration [27], 35 of which are infrared active (but which may not be individually observable in unpolarized spectra, or may be too weak to be observed). These modes represent internal stretching and bending modes as well as lattice modes (cation translations and rigid rotation translation) of the tetrahedra. In spectra of ran- Table 3. Cell parameters of the crystalline phases in the slags. Comparison with literary data. Sample Flash smelting furnace slag (FS) Converter slag (CS) Magnetite [20] Magnetite [21] Magnetite [22] Fayalite [5] Fayalite [23] Cell parameter of Cell parameters of fayalite (orthorhombic) magnetite (cubic) a a b c

5 I. Mihailova, D. Mehandjiev Fig. 2. FTIR spectra of the samples of copper slags domly oriented samples, the three principal bands between : 1000 and 850 cm -1 are the result of splitting of degenerate Si O asymmetric stretching vibrations of the í 3 mode in. A feature near 825 cm -1 results from the í 1 mode (symmetric stretch), and three or four features in the cm -1 region can be attributed to splitting of the degenerate í 4 asymmetric bending vibration. Features in the cm -1 region are attributable to the í 2 symmetric bending mode, rotations and translation of the tetrahedron, and translations of one of the divalent cations. In the far infrared ( cm -1 ), additional translational modes and combinations of and cation translations are observed [10]. Fig. 2 shows FTIR spectra of samples. As can be seen the presented spectra are mainly due to the fayalite phase in the samples. The vibrational modes in olivine series minerals vary slightly as a function of solid solution composition, leading to shifts in band position and changes in band strength, including the presence or absence of some features. Regardless of whether the measured olivines are natural or synthetic, their infrared spectra exhibit a Table 4. Olivine transmittance band positions, cm -1. Sample / Fo # FS (this study) CS (this study) Fo (417) Fo Fo Fo (407) Fo Fo (413) Fo (419) Fo Fo (418) 321

6 Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010 generally linear decrease in the wavenumber position of each band as the molar proportion of Fe increases, and this trend is attributed to the increasing radius and mass of the cation linking the tetrahedra. The linearity of band positions as a function of composition in the forsterite fayalite series indicates a nearly random distribution of Mg 2+ and Fe 2+ in the M1 and M2 sites and that any significant deviation from linearity would indicate ordering of the Mg 2+ and Fe 2+ ions [10]. In Table 4, experimental data for fayalite in copper slag is compared to literature data [10] for olivines with a forsterite number (Fo # ) in the range of There is a good correspondence between experimental data and literature data for olivines with Fo # below 11. Due to the low content of magnesium in slag, the forsterite number of the fayalite phase formed in slag should be small. It is interesting to make a theoretical estimate of the expected forsterite number on the basis of the available data for the content of the main components involved in fayalite structure formation. Magnesium, iron and silicon content in FS and CS samples are given in Table 1. On the basis of these data using the formula Fo # = a.100/a + 0,435.b, where a and b denote respectively the percentage content of magnesium and iron, the forsterite number Fo # was calculated, assuming that all magnesium and iron participate in the fayalite phase formation in both slags. The calculated values are as follows: Fo # = 4.5 (FS) and Fo # = 1.4 (CS). The forsterite number value of fayalite in flash smelting furnace slag is higher than that in converter slag, which should be expected given the higher content of magnesium in the phase. Experimental values for the forsterite number of fayalite phases may also be calculated using the linear relationship between the band position and the forsterite number. In Fig. 3 is shown the dependence of position of the band 1 at : 950 cm -1 on the forsterite number, according to literature data [28, 29]. In Fig. 3 with straight horizontal lines are also given the values derived by us for the same band of the fayalite phase in both slags. The intersection points of these lines with the straight line give the forsterite number value. Thus the calculated value for FS sample is 8.3, cm FS CS Fo # Fig. 3. The dependence of position of the band 1 at ~ 950 cm -1 on the forsterite number (The equation of the straight line is: Y= X. Then Fo # = 8.3 for the sample of flash smelting furnace slag (FS) and Fo # = 5.5 for the sample of converter slag (CS). Fig. 4. Microstructure of sample of flash smelting furnace slag. Elongate aggregates of fayalite and black magnetite crystals in a groundmass of glass. 120x optical magnification. Plane polarized (a) and cross polarized (b) transmitted light photomicrographs. 322

7 I. Mihailova, D. Mehandjiev Fig. 5. Typical morphology of fayalite crystals in the sample of flash smelting furnace slag. 120x optical magnification. Plane polarized (a) and cross polarized (b) transmitted light Fig. 6. Inclusions of magnetite and silicate glass in the fayalite crystals from flash smelting furnace slag. 120x optical magnification. Plane polarized (a) and cross polarized (b) transmitted light photomicrographs. and that for CS sample is 5.5. There are certain differences between calculated and found values of the forsterite number, though the theoretical estimate suggesting a higher forsterite number of flash smelting furnace slag compared to that of converter slag was confirmed. However, it should be kept in mind that silicon also plays a determining role. Calculations based on its content show that fayalite formation phase, in which all magnesium and iron participate can not be expected. Taking into consideration the above limitation in the calculation of the forsterite number, forsterite number values obtained are 4.7 for flash smelting furnace slag and 1.6 for converter slag. The calculation assumes that all the magnesium participates as well as some iron, inasmuch as allowed by the presence of silicon. From this follows that the excess iron should form a new phase and presence of magnetite in both slags was actually observed. Since magnetite is clearly distinguished under X-rays as a separate phase, it appears that there is less iron participating in the fayalite phase. At the same time silicon-containing phases are additionally found that could increase the value of the forsterite number, which was actually observed. Some deviations from data for the isomorphic series forsterite - fayalite are inevitable and may be also due to possible inclusion in the olivine crystal structure of other cations present in the slag. Light optical microscopy Olivine group minerals crystallize in the same system, both in rocks and in slags, but their morphologies are different. The most frequently occurring forms of olivine in rocks have been analyzed by optical microscopy. These forms are crystal and rounded. Olivine group minerals from slags show skeletoidal or crystal 323

8 Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010 Fig. 7. Allotriomorphic fayalite crystal in the converter slag. 120x optical magnification. lane polarized (a) and cross polarized (b) transmitted light photomicrographs Fig. 8. Fayalite displaying skeletal crystal form in the converter slag. 120x optical magnification. Plane polarized (a) and cross polarized (b) transmitted light photomicrographs Fig. 9. Typical morphology of fayalite crystals in the sample of converter slag. 120x optical magnification. Plane polarized (a) and cross polarized (b) transmitted light photomicrographs. 324

9 I. Mihailova, D. Mehandjiev Fayalite crystals occurring in experimentally obtained copper converter slags with variable SiO2 contents are described [15]. The crystals differ in the degree of skeletal growth which is explained in terms of hindrances in heat transfer brought about by the increasing viscosity of the melt upon increase of the SiO2 content. The formation of tubular inclusions in crystals elongated along the b-axis is related to the influence of sulphide droplets at the surface of the crystals [15]. In Figs are given the typical structural relationships between phases present in the examined samples of slag. It was confirmed that fayalite is the dominant crystal phase, followed by magnetite and copper sulphide inclusions, with the glass phase present as a main mass. Fayalite crystals when viewed in transmitted light were pleochroit with shades ranging from green to pink. For FS sample were typical dendritic growth and subparallel orientation in space as well as not fully formed skeletal crystals. The relations between fayalite and the magnetite phase in the samples differed. In fayalite crystals themselves there were small grains of magnetite inclusions. In some cases the latter formed elongated zones in skeletal fayalite crystals and showed simultaneous crystallization of both phases. Heterogeneous xenomorphic crystals were typical for CS sample. They included many impurities of crystalline phases and glass within their boundaries (Fig. 9). Specific skeletal forms were also observed. The magnetite content was definitely higher compared to that in FS sample and the amorphous phase was more heterogeneous. CONCLUSIONS Fig. 10. Fayalite (1), magnetite (2), sulphide inclusions (3) and copper (4) in silicate glass a) and b) sample of flash smelting furnace slag; c) sample of converter slag. 120x optical magnification. Reflected light photomicrographs. form under microscope. Morphology of olivine crystal forms in slags differs from those in natural aggregates (rocks) [11]. From the present study of fayalite crystal phases in two types of copper slag - flash smelting furnace slag and converter slag the following conclusions can be drawn: Despite the complex composition of metallurgical slag and the possibility of inclusion of other elements as isomorphic impurities in the fayalite phase Fe2SiO4, the latter s crystal lattice was not substantially altered, according to XRD data. The values of the unit cell parameters of fayalite and magnetite in copper slags were very close to literature values for pure phases. This may have been due to isomorphic substitutions of Fe2+, 325

10 Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010 both by cations with smaller ionic radii, such as Mg 2+, and by cations with larger ionic radii. The obtained FTIR spectra of copper slag samples corresponded almost entirely to the spectrum of the fayalite phase. This made possible the use of FTIR to determine the composition of fayalite through the established dependency of spectrum bands positions on fayalite composition. Dendritic growth as well as not fully formed skeletal crystals were common features relevant to the morphology of fayalite in the examined samples of copper slag. Fayalite crystals included many impurities of crystalline phases and glass within their boundaries. Specific crystal forms were observed in both types of slag. On the base of presented data the copper slags may be used as adsorbents and support for catalysts. REFERENCES 1. S. Dimitrova, Wat. Res., 30, 1996, S. Dimitrova, D. Mehandjiev, Wat. Res., 32, 1998, S. Dimitrova, V. Nikolov, D. Mehandjiev, J. Mat. Sci., 36, 2001, S. Dimitrova, G, Ivanov, D. Mehandjiev, Comptes rendus de l Académie bulgare des Sciences, 55, 2002, J. R. Smyth, American Mineralogist, 60, 1975, R. M. Hazen, American Mineralogist, 62, 1977, F. Princivalle, L. Secco, Mineralogy and Petrology, 34, 1985, R. P. Santoro, R. Newnham, S. Nomura, Journal of Physics and Chemistry of Solids, 27, 1966, M. W. Schaefer, Nature, 303, 1983, V.E. Hamilton, Chemie der Erde - Geochemistry, 70, 1, 2010, S. Devic, L. Marceta, Journal of Mining and Metallurgy, 43 B, 2007, I. Ivanov, P. Bakardjiev, I. Grozdanov, Rudodobiv i metalurgia, 3, 1966, 11 16, (in Bulgarian). 13. I. Ivanov, P. Bakardjiev, I. Grozdanov, Ann. of UCTM Sofia, XIV, 4, 1967, (in Bulgarian). 14. I. Ivanov, P. Bakardjiev, I. Grozdanov, Rudodobiv i metalurgia, 9, 1967, (in Bulgarian). 15. G. N. Kirov, I. Ivanov. Kristall. und technik., 3, 4, 1968, Ivanov, K. Genevski, T. Genevska, P. Bakardjiev, Metalurgia, 1, 1990, 7 9 (in Bulgarian). 17. I. Mihailova, Ann. MGU, 52, Part 1 Geol, 2009, I. Mihailova, Ann. MGU, 52, Part 1 Geol, 2009, W. Kraus, G. Nolze, Powder cell 2.4, Federal Institute for Materials Research and Testing Rudower Chaussee Berlin, Germany. 20. B.A. Wechsler, D.H. Lindsley, C.T. Prewitt, American Mineralogist, 69, 1984, C. Haavik, S. Stolen, H. Fjellvag, M. Hanfland, D. Hausermann, American Mineralogist, 85, 2000, H.St.C. O Neill, W.A. Dollase, Physics and Chemistry of Minerals, 20, 1994, R.M.Hazen, American Mineralogist, 62, 1977, M.D. Dyar, E.C. Sklute, O.N. Menenzieses, P.A. Blandnd, D. Lindsndsndsley, T. Glotch, M.D. Lanene, M.W. Schaefer, B. Wopen., R. Klima, J.L. Bishopop, T. Hiroi, C. Pieters, J. Sunsnshinene, American Mineralogist, 94, 2009, B. Tarte, Spectrochim. Acta, 19, 1963, D. A. Duke, J.D. Stephens, American Mineralogist, 49, 1964, Chopelas, American Mineralogist, 76, 1991, J.W. Salisbury, L.S. Walter, N. Vergo and D.M. D Aria, Infrared ( ìm) Spectra of Minerals, The Johns Hopkins University Press, Baltimore and London, A.M. Hofmeister and K.M. Pitman, Phys. Chem. Miner., 34, 2007,