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The Structural Role of Fe and Zn in the Formation of Glasses Containing Electric Arc
Furnace Dust: an EXAFS study
F. Pinakidou, M. Katsikini, A. Mavromati, G. Kaimakamis, Th. Kehagias and E.C. Paloura *
Aristotle University of Thessaloniki, School of Physics, 54124 Thessaloniki, Greece.
*
paloura@auth.gr
1. Introduction
The volume of the solid waste that originates from the steel industry increases globally and thus its management must be
efficient and environmentally friendly. Electric arc furnace dust (EAFD) is the largest solid waste stream produced by steel
mills and it comprises mainly of heavy metals which are toxic when disposed to the environment. A simple and cost
effective method used to stabilize these metals is vitrification, which produces vitreous or glass-ceramic materials [1].
Application of this process immobilizes metallic Zn and Fe and thus leads to the formation of safe for disposal EAFD-rich
materials [2]. Since the structural rigidity and hence the chemical stability of the final products is strongly correlated to the
presence of the metals, it is crucial to study the structural role of Fe and Zn, i.e. the type of polyhedra that Fe and Zn
constitute the glassy matrix, in vitrified EAFD-rich industrial wastes.
2. Experimental Details
The studied samples are vitrified materials, produced by co-melting of the EAFD
with SiO 2 , Na 2 O and CaO and casting at 1400 o C. The EAFD mainly consists of ZnO
and ferric oxides (ZnFe 2 O 4 )). The EAFD concentration in the studied glasses
increases with a step of 5 wt% in the range 10 to 30 wt% while the SiO 2 /Na 2 O ratio is
equal to 11, i.e. in all samples the glassy network is a silica matrix. The Fe-K-and Zn-
K-EXAFS measurements were conducted at the synchrotron radiation facility BESSY
in Berlin using the KMC2 beamline. The spectra were recorded in the fluorescence
yield mode using a Si-PIN photodiode.
3. Results and discussion
The Fourier transforms (FT) of the k 3 -weighted χ(k) Fe-K edge EXAFS spectra
(k-range 2.8-9.5Å -1 ) of the studied samples are shown in Fig 1(a). In the FTs only the
1 st nearest neighbor (nn) shell is resolved and thus the samples are amorphous and
homogeneous. Therefore, the Fe-K-EXAFS spectra of all the EAFD-containing
glasses were fitted in the 1 st nearest neighbor shell that consists of oxygen atoms. The
EXAFS analysis results demonstrate that the local coordination of Fe is mixed, i.e. Fe
forms both FeO 4 tetrahedra and FeO 6 octahedra while the coordination environment of
Fe changes gradually from octahedral to tetrahedral when the EAFD content increases.
More specifically, as the EAFD concentration increases from 10 to 30 wt% the Fe-O
bond length (R Fe-O ) decreases from 1.93 to 1.88Å while the coordination number of Fe
(number of O atoms participating in the FeO x polyhedra N(O)) decreases from 4.9 to
3.4. The simultaneous decrease of the Fe-O bondlength and the coordination number
of Fe as a function of the waste concentration, are shown in Fig. 2. The intermediate
role of Fe, i.e. its mixed tetrahedral and octahedral coordination, in the glass has been
also reported previously on Fe and Pb-rich vitrified industrial wastes [3].
In order to determine the percentage of the different FeO x polyhedra in the glassy
matrix of the studied samples we fitted the Fe-K EXAFS spectra again using a mixed
model, which assumes that X% of the Fe atoms belong to octahedral sites while the
rest (100-X)% is bonded in tetrahedra. The spectra were fitted in the 1 st nn shell and
the fitting parameters were the percentage of the Fe tetrahedra and octahedra and the
value of the Debye-Waller (DW) factor. The fitting was performed simultaneously for
all the samples and the distance in the tetrahedral coordination was kept fixed to the
value derived from the previous EXAFS analysis (1.88Å).
The application of the mixed model discloses that in the glass with 10wt% EAFD
concentration, approximately 42 at% (±15%) of the Fe atoms are octahedrally
coordinated in the vitreous matrix, while the rest constitute tetrahedra. The same
percentage of octahedrally bonded Fe atoms (58±11 at%)) is also detected in the glass
with 15wt% EAFD. However, as the EAFD content increases further, the percentage
of the FeO 4 polyhedra increases significantly, i.e. in the glasses with 20 and 25wt%
EAFD, the percentage of the octahedrally coordinated Fe is approximately equal to
24at% (±6%), while it is drastically reduced to 10at% (±3%) when the EAFD content
reaches the value of 30wt%. Thus, the intermediate role of Fe is also verified by the
EXAFS results using the mixed model, i.e. Fe acts as a glass modifier and former
when the EAFD content ranges from 10 to 30 wt%. In particular, in the glass with the
highest waste concentration (30 wt%), the dominant role of Fe is that of network
former since the number of the FeO 6 octahedra is only 10 at%.
FT (arb. units)
FT (arb. units)
14
12
10
8
6
4
2
Fe-K edge
(a)
10 wt% EAFD
15 wt% EAFD
20 wt% EAFD
25 wt% EAFD
30 wt% EAFD
0
0 1 2 3 4 5 6
20
18
16
14
12
10
8
6
4
2
R (Å)
(b)
Zn-K edge
10 wt% EAFD
15 wt% EAFD
20 wt% EAFD
25 wt% EAFD
30 wt% EAFD
0
0 1 2 3 4 5 6
R (Å)
Figure 1: (a) The Fourier transforms
(FT) of (a) the k 3 ×χ(k) Fe-K edge
spectra and (b) of k 3 ×χ(k) Zn-K edge
spectra of all studied glasses
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