Vol. 12, No. 9
AIST Transactions
An X-Ray Photoelectron Spectroscopy (XPS) Study of the Effect of Water Vapor on Slag Chemistry and Structure Related to a Novel Flash Ironmaking Process: Part 2 — Calculation of the Degree of Polymerization
I
n Part 1, the chemistry and structure of selected ironmaking slags were investigated using the x-ray photoelectron spectroscopy (XPS) technique.1 In this part, further analysis is conducted to complement the XPS analysis presented therein, and the effects of CO/CO2, H2/H2O, and CO/CO2/H 2/H 2O gas atmospheres on the chemistry and structure of ironmaking slag will be discussed. The results of this work are important to the development of an eco-friendly novel flash ironmaking process with the potential for steelmaking in a single, continuous process.2–7 The solubility of water in various slags has been studied
by many researchers.8–18 In this work, however, the effects of the water vapor on the equilibrium distribution of elements such as sulfur, 3,19 phosphorus2,19 and manganese, 20 in addition to the effects on iron and magnesium oxides activities, were investigated.4,21,22
Slag Structure Silicate anions are comprised of silicon cations surrounded by four oxygen anions, forming tetrahedral units. These tetrahedra are joined together in chains or rings by bridging oxygen (BO), as shown in Figure 1. Cations
Figure 1
In Part 1, the chemistry and structure of selected slag systems were investigated using x-ray photoelectron spectroscopy analysis. In this part, the experimental results are further analyzed by performing non-bridging oxygen per tetrahedral calculations, and the results of scanning electron microscopy analysis of the samples are also presented. The results of this study will provide new insights into the slag chemistry under H 2O-containing atmosphere.
Authors Yousef Mohassab Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah, USA
[email protected]
Hong Yong Sohn Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah, USA
[email protected]
Models of silicate melt structural entities with their Q-notation, non-bridging oxygen (NBO)/Si ratios and structural units, as well as the bridging oxygen (BO) and NBO, respectively. AIST.org
December 2015 ✦ 185
Table 1 are classified into two categories with respect to their impact on the silicate chains or rings (silicate polymer). The first type is the network breaker, in which these cations tend to break the BO bond to form non-bridging oxygen (NBO), O - and free oxygen, O2-. This process is called depolymerization of silicate melt, which is usually expressed by the ratio of NBO atoms to the number of tetrahedrally coordinated atoms, or NBO/T ratio.23 It is worth noting that the rates and equilibria in slag-metal reactions as well as the physical properties are strongly dependent on the NBO/T ratio.24,25 Examples of the network-breaker cations are Ca2+, Mg2+ and Fe2+. In addition to that ratio, Si anions are frequently classified by the term structon or Q-notation (Qn where n = 0, 1, 2, 3, 4), which is defined as single atom (or ion or molecule) surrounded by others in a specified manner.26,27 Figure 1 shows examples of silicate ions with their Q-notation and NBO/T ratios in addition to other distinguishing features. An Si polyanion may have more than one isomer with different Q values, as will be shown later in the text. The second type of cation is the network former, such as Al3+ and P5+, which form AlO45- and PO43units. To balance the electrical charge, cations such as Ca2+, Mg2+ or Fe2+ are required. The cations involved in charge-balancing duties do not participate in network breaking. Thus Al2O3 increases the polymerization of the silicate melt.25 Like Al2O3, P2O5 increases polymerization, since it forms phosphate complexes that have greater affinity for cations, which are mostly of the network-modifier type, than silicate ions. This higher tendency toward cations accounts for the ability of phosphate to increase silica polymerization, since phosphate consumes the silicate network modifiers and hence decreases the attack of these cations on the silicate polymers. It is noted that P2O5 is less likely to form P-O-Si bonds compared with the aforementioned phosphate complexes that are based on the PO43- units.25 The silicate melts comprise various three-dimensionally interconnected units coexisting in the melt, such as SiO2, Si2O52-, Si2O64-, Si2O72- and SiO44-.25 The nature of these units is affected by the nature of the cation in the silicate melt. The SiO2 and SiO44units are stabilized by small cations of high valences, e.g., Mg2+ > Ca2+ > Na +. In the presence of these basic metal oxides, silicate polymers are broken according to the following pattern: Si6O1812- · Si4O128- · Si3O96- · Si2O76- · SiO4 -4. This explains the absence of isolated SiO2 molecules in industrial slags. Instead, SiO2-based slags are known to consist of polymers of tetrahedral units (SiO4 -4).24,28,29 In summary, the common equilibria among silicate anions can be presented mainly through the following equilibria:
186 ✦ Iron & Steel Technology
Instrument Operating Conditions for Spectro Genesis® SOP (ICP-AES) Parameter
Instrument operating conditions
Plasma power
1,400 W
Plasma (coolant) gas flow
12 Lmin-1
Auxiliary gas flow
1 Lmin-1
Nebulizer gas flow
1 Lmin-1
Optic flush Pump speed
0.5 Lmin-1 2 step (normal), 4 step (fast)
Wash time
25 seconds
Read delay
25 seconds
Replicate readings
3
MnO = nM (2/n)+ + O 2- [oxide dissociation in the slag] (Eq. 1) i(SiO 4)4- = (SiiO 3i + 1 – j)2(i + 1 – j)- + (i – 1 + j)O 2[Si anion polymerization] (Eq. 2) where i = number of Si atoms in the polyanion and j = number of cycles. Equation 2 produces chain or cyclic polyanions when j = 0 or ≥1, respectively.30
Experimental The preparation of slag samples and most of the experimental details were described in Part 1.1 The degree of depolymerization (NBO/T) of the slag was obtained by quantitatively analyzing its composition after equilibriation with molten iron under each gas atmosphere. In order to accurately analyze the slag samples, inductively coupled plasma-optical emission spectroscopy (ICP-OES) coupled with a Spectro Genesis® SOP ICP-AES spectrometer was used. Prior to the analysis, the samples were digested in a closed Savillex® vessel, which is made of a translucent and chemical-resistant material (Teflon™ PFA®) that can withstand temperatures up to 513 K. This digestion method was developed in this laboratory.4 The spectrometer was equipped with an autosampler and controlled by Smart Analyzer Vision software. The ICP operational conditions are given in Table 1. For calibration, several sets of multi-element standards A Publication of the Association for Iron & Steel Technology
Table 2 Wavelengths and Analytical Details Used for Each Element Element
Wavelength Method detec(nm) tion limit (ppm)
Standard error (ppm)
Correlation coefficient
Al
309.271
0.113
0.716
0.99826
Ca
183.801
0.331
0.446
0.99799
Fe
238.204
8.00
0.579
0.99805
Mg
280.270
0.003
0.713
0.99828
Mn
259.373
0.006
0.913
0.99718
P
178.287 213.618 214.914
0.319 0.373 0.400
0.420 0.488 0.386
0.99822 0.99878 0.99936
S
180.731 182.034
0.800 0.202
0.659 0.594
0.99903 0.99911
Si
251.612
0.800
0.369
0.99987
analytical working distance specified by EDAX. The tilting was set to zero in all the samples.
Results and Discussion The bulk NBO/T was calculated for the three slags according to the method used by Mills,25,29 the description of which can be summarized as follows: NBO/T = wNB/xT (Eq. 3)
xT x SiO2 2x Al2O3 2x P2O5 (Eq. 4)
containing all the analytes of interest at eight different levels of concentration were prepared and measured at the suitable wavelengths. The emission lines used are shown in Table 2. After the analyses of the samples were completed, a single wavelength, which gave the best quantitative results compared with the reference samples, was selected for each element. The analysis reproducibility was maintained in the range of ±2 to ±10%. The final chemical compositions of the three slags after the equilibrium experiment (15 hours) as determined by ICP-OES are listed in Table 3. An FEI Quanta 600 FEG scanning electron microscope (SEM) was used to obtain micrographs of the slag samples. The SEM was equipped with an EDAX Genesis x-ray microanalysis system for energy-dispersive x-ray spectroscopy (EDS) analysis. The accelerating voltage was selected to be 15 kV to enable the quantitative analysis of all the elements of interest. Parts of the crushed crucible walls with slag pieces sticking to them were carefully mounted into epoxy, polished and coated with a thin layer of carbon using a carbon coater since the samples were not conductive. Also, fine powder of the synthetic slag was scanned to confirm its homogeneity before it was used in the experiment. The high-vacuum mode was employed to obtain high resolution and accurate EDS results. The working distance was selected to be within the
wNB = vNB – 2xAl2O3 (Eq. 5)
v NB 2 xCaO x MgO x FeO x MnO
(Eq. 6)
where wNB = net charge on the total network-breaking cations, xT = sum of mole fractions of network-forming cations, vNB = total charge of the total network-breaking cations and x = mole fraction of species. The calculation results are shown in Table 4. Based on the bulk NBO/T ratio presented in this table, the silicate polymers tend to become more depolymerized under the CO/CO2 atmosphere and more polymerized under atmospheres containing H2O. It is noted that the depolymerization ratio did not change significantly when H2/H2O was replaced by the CO/CO2/ H2/H2O atmosphere. Figure 2 shows a backscattered SEM image of a cross-sectioned magnesia crucible containing the slag after the equilibrium experiment. The image
Table 3 Equilibrium Chemical Composition of the Three Slags as Determined by ICP-OES (in wt. %) Gas mixture
FeO
MnO
CaO
Al2O3
MgO
SiO2
P2O5
S
CaO/SiO2
H2/H2O/CO/CO2
10.8
0.6
33.5
18.5
16.8
29.8
0.73
0.04
1.1
CO/CO2
16.9
0.6
26.3
15.0
26.2
23.6
0.69
0.07
1.1
H2/H2O
9.6
0.7
31.2
17.1
19.7
32.1
0.89
0.05
1.0
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December 2015 ✦ 187
Table 4
Figure 2
The NBO/T Values Under Different Gas Atmospheres CO/CO2
H2/H2O
H2/H2O/CO/CO2
xFeO
0.123
0.071
0.079
xMnO
0.004
0.005
0.004
xCaO
0.246
0.293
0.315
xAl2O3
0.077
0.088
0.096
xMgO
0.341
0.257
0.219
xSiO2
0.205
0.281
0.262
xP2O5
0.003
0.003
0.003
xS
0.001
0.001
0.001
xT
0.360
0.460
0.460
vNB
1.430
1.250
1.240
wNB
1.270
1.080
1.040
NBO/T
3.490
2.320
2.280
was mapped for Ca, Mg, Fe and Si elements by EDS. No pure oxides (isolated islands) were registered even close to the crucible wall. The slag sample was homogeneous and dominated by a dark matrix with light dendritic structures embedded in it, as shown in Figure 3. The sample was further magnified (the inset in Figure 3) and an EDS spot analysis was carried out. The dark matrix was found to be mainly (Si, Mg, Al, Ca, Fe)O phase (silicate anions with network modifiers) and the dendritic structure was dominated by (Fe, Mg)O phase (magnesiowüstite). This analysis was carried out on most of the samples in this study, based on which the absence of undissolved islands of any single oxide in the slag melt was confirmed. Thus, the degree of depolymerization (NBO/T) calculated in this work is representative of the slag samples, as will be discussed later. Table 5 summarizes the analysis results. The ratio of the more polymerized anions to the less polymerized anions, (Q2 + Q3)/(Q0 + Q1), in the H2O-containing atmosphere is at least twice that of CO/CO2 atmosphere, in agreement with the Fourier transform infrared spectroscopy (FTIR) results discussed elsewhere.4,31 In addition, the XPS and the NBO/T suggest that the studied slag is more depolymerized under CO/CO2 atmosphere and more polymerized under the H2/H2O atmosphere, as shown also in Figure 4. It is therefore notable that all the results consistently lead to the conclusion that the CO/CO2 slag has the lowest polymerization degree of all the three, whereas the two other slags have a similar degree of polymerization. This suggests that water affects the slag more significantly than CO2 when they coexist in the gas mixture.4 As mentioned in Part 1,1 the equilibria among Q-species in the studied slags are represented by 188 ✦ Iron & Steel Technology
(a)
(b)
(c)
(d)
(e)
Backscatter electron diffraction-scanning electron microscopy (BSED-SEM) micrograph of a cross-sectioned magnesia crucible containing slag (a) and energy-dispersive x-ray spectroscopy (EDS) elemental maps (b–e).
Figure 3
BSED-SEM micrograph showing the homogeneity of the slag, with an inset with higher magnification showing the slag matrix with the floating dendritic structures. A Publication of the Association for Iron & Steel Technology
Table 5
Figure 4
Comparison of Some of the Structure Analysis Results
Raman3
Q0
59.3
28.9
Q1
7.0
64.4
Q2
33.7
6.7
Q3
—
—
+
Q3)
2.0
13.9
+
Q1)
0.5
0.07
Q0
18.6
53.5
Q1
12.7
39.0
Q2
15.2
7.5
Q3
53.5
—
(Q0 + Q1)/(Q2 + Q3)
0.5
12.4
(Q2 + Q3)/(Q0 + Q1)
2.2
0.08
Q0
17.9
93.5
Q1
29.2
—
Q2
15.4
6.5
Q3
37.5
—
(Q0 + Q1)/(Q2 + Q3)
0.9
14.4
(Q2
1.1
0.07
(Q0 (Q2
+
Q1)/(Q2
+
Q3)/(Q0
NBO/T
3.49
140 120 100 80 60 40 20
H2/H2O
0 BO
17, 47
2.32
+
+
Q1)
ψ
NBO/T
Effect of the type of reductant, relative to BF gas composition, on BO, NBO/T and the ratio ψ = (Q0 + Q1)/(Q2 + Q3 ).
2Q2 = Q1 + Q3
CO/CO2/H2/H2O
Q3)/(Q0
CO/CO2 H2 H2/H2O/CO/CO2
160
CO/CO2
11, 46
180
Normalized Values
FTIR3
Species
XPS (BO, NBO)
(Eq. 12)
12, 48
2.28
Equations 7 and 8 for CO/CO2 atmosphere and Equations 9–12 for H H2/H2O and CO/CO2/H2/ H2O:
Under the CO/CO2 atmosphere, the higher-order Q-species tend to dissociate to form more stable, lower-order species. On the other hand, under the two H2O-containing atmospheres, the smaller Q-species tend to polymerize and form larger silicate polymers. The equilibrium constants of Equations 7–12, as written, are greater than 1. Based on this, it can be concluded that the type of gas atmosphere affects the real basicity of the slag. This is expected to impact the distribution ratios of S, P and Mn between slag and molten iron.20,32,33 This was also found to affect refractory lining and FeO content in the slag.21,22,34
Q1 + O2- = 2Q0 (Eq. 7) 2Q2 + Q0 = 3Q1 (Eq. 8) 2Q0 = Q1 + O2(Eq. 9) 3Q1 = 2Q2 + Q0 (Eq. 10) 3Q2 = 2Q3 + Q0 (Eq. 11)
AIST.org
Conclusions Based on the NBO/T calculations and spectroscopic analyses, H2O stabilizes the more polymerized silicates anions rather than the depolymerized monomers. The investigated slags at 1,823 K under H2/H2O and CO/CO2/H2/H2O exhibited NBO/T ratios of the first two cases about 35% lower than in the case of CO/CO2. In the slags under H2/H2O and CO/CO2/ H2/H2O atmospheres, the degree of depolymerization was 75% and 55% less than those under a CO/ CO2 atmosphere. Therefore, it can be concluded that the higher the water content in the gas atmosphere, the more polymerized the silicates in the slag. This difference in the degree of polymerization plays a critical role in the chemical and physical properties of the slag, especially in the distribution of elements between slag and molten iron and slag viscosity. Based December 2015 ✦ 189
on the spectroscopic and chemical analyses, it is concluded that H2O in the gas atmosphere increases the degree of polymerization of silicate slags and consequently the slag viscosity.
Acknowledgments The authors acknowledge the financial support from the American Iron and Steel Institute (AISI) through a Research Service Agreement with the University of Utah under AISI’s CO2 Breakthrough Program. This material also contains results of work supported by the U.S. Department of Energy under Award Number DE-EE0005751.
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