An X-Ray Photoelectron Spectroscopy (XPS) Study of

Vol. 12, No. 8

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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 1 — Experimental Work

S

lag is one of the key components that determine the quality of iron produced during the smelting process. The slag purifies iron by absorbing undesirable elements such as sulfur, phosphorus and silicon. The capacity of the slag to absorb each of these elements is controlled mainly by its composition. For instance, high basicity of the slag increases its sulfide capacity. Further, the slag chemistry has a strong impact on the activity of FeO in the slag, which impacts the yield at the end of ironmaking and steelmaking. It is important to understand how the slag chemistry and structure affect slag-metal reaction equilibria as well as slag properties such as viscosity, density and conductivity. Ironmaking slags contain silica and other complex forming components, and the structure of the silicate has a significant effect on the structure and behavior of the slag. This paper discusses the effects of CO/CO2, H2/ H 2O, and CO/CO2/H 2/H 2O gas atmospheres on the chemistry and structure of ironmaking slag. 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.1–6 The solubility of water in various slags has been studied by many researchers.7–17 In this work, the effects of the water vapor on the equilibrium distribution of AIST.org

elements such as sulfur,2,18 phosphorus1,18 and manganese,19 in addition to the effects on iron and magnesium oxides activities, were investigated.3,20,21

Silicates in Slag In ironmaking, silica is the major component in the slag, with concentrations ranging from 27 to 45 wt. % depending on the type and origin of the iron ore.3 Silica content is the independent variable from which the flux amount and slag composition are determined to control the physicochemical properties of the slag. Therefore, these properties of slags are controlled by the structure of silicate melt. Molecular and ionic theories are put forward to interpret the structure of silicate melts and their properties. The molecular theory, the stoichiometric approach, is based on the assumption that a liquid slag is composed of individual oxides, fluorides, etc., such as SiO2, CaO, Al2O3 and CaF2, and that these components can combine to form CaSiO3 (for instance) and other more complex compounds. This theory led to the use of the activity of a component such as SiO2 to characterize the effects of the addition of other components. This theory has been used widely in the study of metallurgical slags.18,21,22 However, earlier electrical conductivity studies

As part of the development of a novel ironmaking process, the chemistry and structure of selected slag systems were investigated. This novel ironmaking process aims at producing iron from iron oxide concentrates in a flash reactor using gaseous fuels and reductants, which will help reduce energy consumption and minimize greenhouse gas emissions.

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]

Brian Van Devener Surface Science Laboratory, University of Utah, Salt Lake City, Utah, USA

November 2015 ✦ 179

Table 1 have shown that the conduction Materials Used in the Present Study mechanism is predominantly Material Purity (%) Supplier ionic in nature rather than elecAluminum oxide (Al2O3) 99.99 Acros Organics (Morris Plains, N.J.) tronic, where the latter becomes Calcium oxide (CaO) 99.95 Alfa Aesar (Ward Hill, Mass.) significant only in slags with 70 Silicon(IV) oxide (SiO2) 99.995 Alfa Aesar (Ward Hill, Mass.) wt. % or more FeO or MnO.22 That result led Herasymenko23 to Magnesium oxide (MgO) 99.99 Acros Organics (Morris Plains, N.J.) formulate the basics of the ionic Iron(II) oxide (FeO) 99.95 Sigma-Aldrich (St Louis, Mo.) theory. This finding has been supManganese(II) oxide (MnO) 99 Sigma-Aldrich (St Louis, Mo.) ported by the chromatographic Calcium pyrophosphate separation of different polymeric ≥99.9 Sigma-Aldrich (St Louis, Mo.) (Ca2P2O7) silicate anions dissolved in aqueIron (Fe) ≥99.99 Sigma-Aldrich (St Louis, Mo.) ous solutions. 24,25 According to that theory, liquid slags are Iron(II) sulfide (FeS) 99.98 Alfa Aesar (Ward Hill, Mass.) composed of: (1) cations such as Water (H2O) 99.9997 Sigma-Aldrich (St Louis, Mo.) Ca2+ and Fe2+, (2) anions such as Hydrogen (H2) 99.999 Airgas (Denver, Colo.) O2- and S2-, and (3) anion comCarbon monoxide (CO) 99.999 Airgas (Denver, Colo.) plexes such as SiO44, AlO45- and PO43-.22 Therefore, unlike silicate Carbon dioxide (CO2) 99.999 Airgas (Denver, Colo.) minerals, which usually contain Nitrogen (N2) 99.999 Airgas (Denver, Colo.) only a single type of anion (monodisperse), e.g., SiO44- in olivine, silicate melts are composed of a distribution of different polymeric silicate anions of different molecular weights. preparation of slag samples for the experiments, will Consequently, silicate melts are polydisperse systems be discussed in this section. with statistical distribution of various molecular weights.26 Sample Preparation and Experimental Conditions In summary, the ionic theory can be presented — Table 1 presents a list of the chemicals used for mainly through the following equilibria: sample preparation. CaO was calcined in platinum crucibles at 1,200°C for 12 hours to decompose any MnO – nM(2/n)+ + O2- [Oxide dissociation in the slag] hydroxide and carbonate present. It was then stored in a desiccator with dry powders of SiO2, Al2O3, MgO, (Eq. 1) MnO, FeO, Ca2P2O7, FeS and Fe. Water of high purity (99.9997%) was used to prevent any scale formation i(SiO4)4- = (SiiO3i+1–j)2(i+1–j)– + (i–1+j)O2in the system. To prepare the master slag, predeter[Si anion polymerization] mined amounts of dry powders of CaO, SiO2, Al2O3, (Eq. 2) MgO and MnO were added to 4-cm-OD x 7.6-cmhigh x 0.3-cm-thick magnesia crucibles. The mixture where was premelted and held for 5 hours at 1,873 K under ambient atmosphere for magnesia presaturation. The i = the number of Si atoms in the polyanion and synthetic slag was then ground to a particle size of less j = the number of cycles. than 40 μm. Then dry powders of FeO, Ca2P2O7 and FeS were added and mixed for 36 hours in a tumbler Equation 2 produces chain or cyclic polyanions mixer. Before being presaturated with magnesia, the when j = 0 or ≥1, respectively.27 slag composition for the two experiments has the following composition in wt. %: CaO (29), SiO2(29), MgO (10), Al2O3 (15), MnO (2), FeO (15), FeS (0.9), Experimental Ca2P2O7 (1.5), CaO/SiO2 = 1.0. The partial pressure of oxygen (pO2) was controlled Experiments were conducted to determine the effect at ~1.5×10 -10 atm (1.52×10 -8 kPa) by a CO/CO2, H2/ of gas composition on the chemistry of the slag. X-ray H2O or CO/CO2/H2/H2O mixture. Table 2 shows photoelectron spectroscopy (XPS) was adopted to the equilibrium gas partial pressures as calculated explore the effect of gas composition on the chemby HSC software. The temperature was controlled at istry of the ironmaking slag. This technique, and 1,823 K.

180 ✦ Iron & Steel Technology

A Publication of the Association for Iron & Steel Technology

Table 2 slag. Moreover, XPS can be used to quantify the bridging oxygen (BO) and non-bridging oxygen pH2O pCO pCO2 pO2 pH2 (NBO) amounts through decon(atm) (atm) (atm) (atm) (atm) Gas mixture volution of its spectra to distincH2/H2O/CO/CO2 0.47 0.1 0.27 0.01 tive peaks that could be assigned 1.5×10-10 CO/CO2 0 0 0.81 0.04 to O°, O -, and O2- in the slag H2/H2O 0.71 0.14 0 0 system.29 All XPS measurements were made using a Kratos AXIS Ultra DLD multi-technique surface It is worth noting that the structural features of molanalysis instrument. The base pressure of the analysis ten silicates that can be elucidated with spectroscopic chamber during these experiments was 3×10 -10 Torr, techniques require a fast quenching rate (250–500 with operating pressures around 1×10 -9 Torr (1.3×10 -10 K/second) to freeze the chemical structures for the kPa). Spectra were collected using a monochromatic room-temperature spectroscopic measurements.11 Al Kα source (1,486.7 eV) and a 300×700 micron When the quench rate is slow, silicates crystallize and spot size. Both low-resolution survey scans and highthe slag partially loses its amorphous state. This crysresolution scans of energy ranges of interest were tallization produces structural details that may not performed. Some artifacts generated by charging be related to the high-temperature structure, thus were seen for all samples, resulting in peak shifts and adversely affecting the equilibrium interactions of broadening. Charging artifacts were minimized by slag in the molten state. One way to achieve such a flooding the sample with low-energy electrons and high quenching rate is by splashing the molten slag ions from the charge neutralizer system. To determine sample over a cold copper slab after fast discharge the absolute energy shift, spectra were referenced to from the reactor. In this work, however, the experithe C 1-second peak (284.7±0.2 eV) from adventitious ments were conducted in a horizontal furnace1–3 with carbon, which is a thin layer of carbonaceous material an alumina reactor tube. Although the alumina tube usually adsorbed on the surface of most air-exposed was lined with an alumina gutter, the time needed to samples.30 The mono Al source was operated at an withdraw the tray of samples out of the furnace was emission current of 12 mA with the target anode set (at fastest) 1 minute to avoid thermal shock on the aluto 15 kV for a resulting power of 180 W. For survey mina tube and thus destruction of the alumina tube. spectra, the data were collected using pass energy After that, the crucibles were quickly quenched in ice of 160 eV, a step size of 1 eV and a dwell time of 200 water. Therefore, some crystallization could not be ms. High-resolution regional spectra were collected avoided. However, a significant portion of the silicate using pass energy of 40 eV, a step size of 0.1 eV and a matrix maintained its amorphous status, as observed dwell time of 300 ms. Each high-resolution scan was by the scanning electron microscope (SEM), which averaged over three sweeps to improve signal-to-noise will be shown later. This finding was also observed by ratio. others.28 Another reason to reasonably draw conclusions from the room-temperature spectroscopic measurements and relate them to the high-temperature Results and Discussion structures is the consistent agreement among the results of multiple independent analyses by Raman, The raw data from the high-resolution O 1-second infrared, XPS and quantitative chemical analysis, spectra were fitted/deconvoluted to extract the relawhich is discussed in the following. tive bonding environments for the oxygen atoms. First, the backgrounds of the spectra were fitted using X-Ray Photoelectron Spectroscopy — XPS spectra Shirley-type backgrounds.31 Synthetic peak shapes are obtained by irradiating a material with x-rays were created using Gaussian/Lorentzian functions and detecting the kinetic energy and the number of with a fixed G-to-L ratio of 70-to-30.32 Constraints electrons escaping from the surface of the material were placed on the full width at half maximum within a depth of approximately 10 nm. Therefore, (FWHM) of all the peaks, letting them range from 0.6 XPS technique is commonly used for surface analysis to 2.0 and then constraining all three synthetic peaks applications rather than for bulk analysis as in infrato have the same FWHM, following some guidelines red (IR) and Raman. However, a deeper spectroscopic from Biesinger et al. and Crist.32,33 Once all peak analysis can be obtained when the surface is etched shapes and constraints were defined, a Marquardt with Ar ions. XPS is a valuable tool in detecting tiny algorithm for least squares fitting was applied until changes in the chemical environments of ions in the the best fit of the envelope function (the combined Partial Pressures of Gases as Calculated by HSC at 1,823 K (1 atm = 101.32 kPa)

AIST.org


Figure 1 6320

CO/CO2 9420

9300

H2/H2O

4320 3320 2320 1320

534

532

530

528

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

5320 7420 5420 3420 1420

Binding Energy (eV)

CO/CO2/H2/H2O

7300 5300 3300 1300

534

532

530

528

534

532

530

528

Binding Energy (eV)

Binding Energy (eV)

(a) (b) (c) X-ray photoelectron spectroscopy spectra of O 1 second under different gas atmospheres.

fit of the three synthetic components) to the raw data was achieved. Figure 1 shows the deconvoluted peaks of O 1 second. The three atmospheres showed the three states of oxygen assigned consistently with the literature as free oxygen (O2-) at ~530 eV, NBO (O -) at ~531.5 eV and BO (O°) at ~533 eV, as listed in Table 3.29,34 The different slags follow the order H2/H2O (16.7%) > CO/CO2/H2/H2O (11.6) > CO/CO2 (10.6%) with respect to the relative abundance of the BO, whereas they exhibit a reverse order based on the free oxygen; CO/CO2 (44%) > CO/CO2/H2/H2O (41%) > H2/ H2O (37%). However, using the ratio of the NBO/BO,

the order of the slags are as follows: CO/CO2 (4.3) > CO/CO2/H2/H2O (4.1) > H2/H2O (2.8). Based on XPS analysis, it is concluded that the studied slag is more depolymerized under CO/CO2 and more polymerized under the H2/H2O atmosphere. These results are consistent with Q-species distribution provided by Fourier transform infrared (FTIR) and Raman analyses. Based on the general polymerization reaction, Equation 2, combined with the area % assigned to the free oxygen (O2-) in the H2/H2O slag, the silicate anion should form ring and branched structures rather than chains. That explains the lower free

Table 3 Peak Parameters Used to Fit O 1-Second XPS Spectra Position (eV)

Corrected positiona (eV)

FWHM

Area

Corrected areab

Atomic %

∆Ec (eV)

Free (O2-)

528.4

530.5

1.6

10,659.7

6,173.4

43.8

—

(O-)

529.5

531.6

1.6

11,102.2

6,425.5

45.6

1.1

530.9

533.0

1.6

2,568.5

1,485.3

10.6

1.4

Peak CO/CO2 NBO

BO (O°) H2/H2O Free (O2-)

528.7

530.3

1.6

5,480.1

3,173.1

36.7

—

NBO (O-)

529.9

531.5

1.6

6,962.7

4,028.8

46.6

1.2

BO (O°)

531.2

532.8

1.6

2,488.1

1,438.6

16.7

1.3

Free (O2-)

528.3

530.4

1.6

8,693.6

5,035.1

40.9

—

(O-)

529.4

531.5

1.6

10,119.9

5,857.3

47.5

1.1

530.6

532.7

1.6

2,468.4

1,427.6

11.6

1.3

CO/CO2/H2/H2O NBO

BO (O°) aCorrected

position (eV) = Position (eV) + 2.1 (eV). bCorrected area = (Area/(RSF·T·MFP)); RSF: relative sensitivity factor, T: temperature in Kelvin, MFP: mean free path. c∆E (eV) = E (below) – E(above).



Table 4 oxygen content in this slag with the high polymerization tendency. Table 4 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 FTIR results discussed elsewhere.3,35 In addition, the XPS results suggest that the studied slag is more depolymerized under CO/CO2 atmosphere and more polymerized under the H2/H2O atmosphere. 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 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, as shown in Figure 2.3 Combining the analysis results, the equilibria among Q-species in the studied slags are represented by Equations 3 and 4 for CO/CO2 atmosphere and Equations 5–8 for H H2/H2O and CO/CO2/H2/H2O: Q1

+

O2-

=

2Q0 (Eq. 3)

2Q2 + Q0 = 3Q1 (Eq. 4)

Comparison of Some of the Structure Analysis Results FTIR3

Raman3

Q0

59.3

28.9

Q1

7.0

64.4

Q2

33.7

6.7

Q3

—

—

(Q0 + Q1)/(Q2 + Q3)

2.0

13.9

(Q2

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 + Q3)/(Q0 + Q1)

1.1

0.07

Species

XPS (BO, NBO)

CO/CO2

+

Q3)/(Q0

+

Q1)

11, 46

H2/H2O

17, 47

CO/CO2/H2/H2O

2Q0 = Q1 + O2-

12, 48

Figure 2 (Eq. 5) 180

3Q1

=

2Q2

+

Q0

CO/CO2 H2 H2/H2O/CO/CO2

160

(Eq. 6)

(Eq. 7) 2Q2 = Q1 + Q3 (Eq. 8) 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 bigger silicate polymers. The equilibrium constants of Equations 3–8, as written, are substantially greater than 1.

Normalized Values

3Q2 = 2Q3 + Q0

140 120 100 80 60 40 20 0

BO

ψ

Effect of the type of reductant on the bridging oxygen and the ratio ψ = (Q0 + Q1)/(Q2 + Q3 ). The results are normalized with respect to the CO/CO2 atmosphere.

Effect of Water Vapor on Slag Basicity — According to Flood and Förland36 and Lux,37 the acid-base AIST.org


reaction in an oxide system is defined as “the transfer of oxygen from a state of polarization to another,” which is consistent with the Lewis acid-base definition. Therefore, in aprotic solvents (relevant to silicate melts), O2- replaces H+. Thus, a basic oxide is O2donor and an acidic oxide is O2- acceptor. Base = Acid + O2-

the real basicity of the slag. This is expected to have an impact on the distribution ratios of S, P and Mn between slag and molten iron.19,40,41 In addition, this was found to affect refractory lining and FeO content in the slag.20,21,42

Conclusions (Eq. 9)

4-

2-

SiO4 = SiO3 +

O2(Eq. 10)

The same authors also introduced pO2- = –log aO2as a characteristic scale to measure the oxoacidity, which is analogous to pH scale in aqueous solutions. Equation 2 shows the silicate polymerization reaction that yields three oxygen types: singly bonded O (NBO), doubly bonded O0 (BO) and free oxygen.38 Therefore, that reaction can be simplified as: 2O- = O0 + O2(Eq. 11) Combined with Equation 1, the dissociation of metal oxide, one can expect the acid-base property of dissolved oxides to significantly affect the extent of silicate polymerization by producing or consuming free oxygen anions (O2-). As mentioned earlier, metal oxides are either network modifiers or formers, i.e., O2- acceptor or O2donor, respectively. Therefore, the relative abundance of basic (network modifier) and acidic (network former) oxides should give a general indication of the extent of depolymerization of the melt, and consequently should be a simple indicator of the trend of changes in oxide activities with slag composition. Hence the equilibrium constants for gas-slag-metal reaction and the element distribution ratios expressed in terms of slag components are often functions of slag basicity.39 Basicity according to any of the indices mentioned above, whether the ratio or the difference between basic and acidic slags, or even the optical basicity, is basically a measure of the free oxygen in the slag that can, for example, oxidize dissolved elements in the molten iron (e.g., S, P, Mn, Si), dissociate silicate polymers or control the lining wear. Thus, any of the aforementioned basicity indices could be used based on the bulk chemical analysis results of the slag constituents. From O 1-second XPS results, one can find that the order of the different atmospheres based on O2-% is CO/CO2 (44%) > CO/CO2/H2/H2O (41%) > H2/ H2O (37%), as mentioned earlier. Based on this, it can be concluded that the type of gas atmosphere affects 184 ✦ Iron & Steel Technology

Based on the 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 BO values of 55% and 9%, respectively, higher 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 polymerization degrees plays a critical role in the distribution of elements between slag and molten iron as well as the activity coefficients of oxides in the slag. In addition, the degree of polymerization controls the physical properties of the slag, in particular, its viscosity. Based 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. For the first time, it has been shown in this work that gas atmosphere has a significant effect on slag chemistry.

Acknowledgments The authors acknowledge the financial support from 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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AIST Transactions Key Reviewers Dr. Fred B. Fletcher principal research engineer, ArcelorMittal Global R&D Research and Development, Coatesville, Pa., USA

Dr. P. Kaushik senior research engineer, ArcelorMittal Global R&D Steelmaking Process Research, East Chicago, Ind., USA

Professor Patricio F. Mendez Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alta., Canada

Dr. Ronald J. O’Malley Missouri University of Science and Technology, Rolla., Mo., USA

AIST Transactions seeks to publish original research articles focusing on scientific or technical areas relevant to the iron and steel community. All articles are peer-reviewed by Key Reviewers prior to publication. The scope of the Transactions spans all fields of iron and steel manufacturing, from extractive metallurgy to liquid metal processing, casting, thermomechanical processing as well as coating, joining/welding and machining. Articles related to structure and properties, i.e., characterization, phase transition, chemical analysis and testing of creep, corrosion or strength are accepted. In addition, papers on process control, testing and product performance are also encouraged. The Transactions will publish papers on basic scientific work as well as applied industrial research work. Papers discussing issues related to specific products or systems can be considered as long as the structure follows a scientific investigation/discussion, but not for the aim of advertising or for the purpose of introducing a new product. Papers should be no longer than 5,000 words, or 12 pages including figures. Papers will either be accepted unconditionally, conditionally based on reviewers’ comments, or rejected. The review process should be completed in less than two months. Upon acceptance, authors will be asked to complete a form to transfer copyright to AIST. A PDF proof of the accepted manuscript will be sent electronically to the author(s) approximately one month prior to publication for final approval. Reprints of published manuscripts can be ordered through AIST. Please contact me with any questions regarding AIST Transactions.

Dr. Stefanie Sandlöbes scientist, Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf, Germany

Dr. Il Sohn associate professor, NeoMetallurgical Processing Lab, Department of Materials Science and Engineering, Yonsei University, Seoul, Korea

Professor Lifeng Zhang School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, China

Professor Hatem S. Zurob

P. Chris Pistorius POSCO Professor Carnegie Mellon University 5000 Forbes Ave. Pittsburgh, PA 15213 USA Phone: +1.412.268.7248 [email protected]

To submit a manuscript for consideration, an electronic copy (saved as an MS-Word document) should be sent to: AIST Transactions Email [email protected]

AIST Transactions are indexed through Chemical Abstracts Service, Columbus, Ohio.

McMaster University, Hamilton, Ont., Canada

