Confocal Scanning Lar Microscopy Studies of Crystal Growth During Oxidation of a Liquid FeO-CaO-SiO2Slag
ANNA SEMYKINA,JINICHIRO NAKANO,SEETHARAMAN SRIDHAR,
VOLODYMYR SHATOKHA,and SESHADRI SEETHARAMAN
The oxidation of FeO in30wt pct FeO-35wt pct CaO-35wt pct SiO2slag was investigated as
part of a wider study on the recovery of Fe units through magnetic paration.A confocal
scanning lar microscopy(CSLM)technique was ud to visualize the oxidation of FeO in the
liquid slag.The formation event was obrved in situ under the CSLM and the ont of pre-
cipitation on a surface of the slag liquid was recorded at various temperatures in an oxidizing
atmosphere.A Time-Temperature-Transformation(TTT)diagram was constructed bad on
the CSLM results.Samples obtained from the CSLM heating chamber were analyzed by a
圣诞快乐英文怎么写
scanning electron microscope(SEM)equipped with an energy-dispersive spectrometer(EDS).
DOI:10.1007/s11663-011-9505-6
ÓThe Minerals,Metals&Materials Society and ASM International2011
广州留学I.INTRODUCTION
2013辽宁高考英语T HROUGH the production of iron and steel,large amounts of industrial wastes and by-products are generated.Efficient recycling of the materials is of increasing interest worldwide as a result of increasing sustainability in process with respect to increasing raw material costs and waste reduction.
For example,for the Swedish and Ukrainian steel industry,the steelmaking slag contains up to30wt pct wustite(FeO).It is important to maximize the amount of iron that is recovered from this slag so that the remaining portion can be ud for civil engineering purpos.Tofind a practical solution,joint efforts are currently underway at the Royal Institute of Tech-nology,Sweden;National Metallurgical Academy of Ukraine;and Carnegie Mellon University.
A sustainable approach to u steelmaking slag components bad on the transformation of nonmag-netic iron monoxide(or wustite)to magnetite by oxidation has been propod by the current
authors.[1] This allows the lective recovery of iron-bearing and non–iron-bearing slag constituents for specific purpos. From the technological point of view,pretreated slag is procesd using a magnetic method wherein iron oxides transformed to a magnetic form are parated for subquent u.The rest of the slag(nonmagnetic) could be ud effectively in the production of cement binder or in other applications.Magnetic products may be ud as a component for sintering mixture or for pelletizing iron ores.
The pha equilibria in the slags containing iron oxides have been reported by veral authors,including Turkdogan,[2]Bodsworth and Bell,[3]and Muan and Osborn.[4]Pownceby et al.[5]focud on the pha equilibria of Fe2O3-CaO-SiO2in air at1513K to 1573K(1240°C to1300°C)with the effects of basicity on the phas indicated.Lin-nan Zhang et al.[6]analyzed the oxidation mechanism in CaO-FeO x-SiO2slag with high iron content,using pure oxygen.阻抑
In our previous work,[7]the kinetics of oxidation of the FeO-CaO-SiO2system in air at the temperature range of 1623K to1773K(1350°C to1500°C)was investigated by Termogravimetric technique application.The current work investigates the mechanism of oxidation of Fe2+to Fe3+in molten slags by using a confocal scanning lar microscopy(CSLM)technique at the temperature range of1530K to1600K(1257°C to1327°C).The slags investigated were synthetic and of the30wt pct
FeO-35wt pct CaO-35wt pct SiO2system.
II.EXPERIMENTAL
A.Materials and Sample Preparation
CaO powder with a purity of99.9pct and SiO2 powder with a purity of99.5pct were supplied by Sigma Aldrich Chemie(Munich,Germany).SiO2and CaO powders were dried prior to weighing.FeO(wustite)was synthesized and examined by X-ray diffraction(XRD). The details of the synthesis were described in the previous work.[8]Platinum crucibles for holding the slags were99.99pct in purity and had dimensions of 5mm in diameter95mm in height90.4mm in thickness.Argon and air were supplied by Valley
ANNA SEMYKINA,Senior Rearcher,is with the Division of Materials Process Science,Royal Institute of Technology,SE-10044 Stockholm,Sweden,and with the National Metallurgical Academy of Ukraine,Dnipropetrovsk49600,Ukraine.Contact e-mail:anna@ kth. JINICHIRO NAKANO is with the Department of Materials Science and Engineering,Carnegie Mellon University,Pittsburgh,PA 15213and is Principal Rearch Scientist with National Energy Technology Laboratory,1450Queen Ave.,Albany,OR97321. SEETHARAMAN SRIDHAR,POSCO Professor,is with the Department of Mat
erials Science and Engineering,Carnegie Mellon University.VOLODYMYR SHATOKHA,Professor and Vice Rector,is with the National Metallurgical Academy of Ukraine. SESHADRI SEETHARAMAN,Professor,is with the Division of Materials Process Science,Royal Institute of Technology.
Manuscript submitted July13,2010.
Article published online March29,2011.
National Gas(Pittsburgh,PA).Alumina crucible-supports(8mm in diameter95mm in height90.5mm in thickness)for confocal microscopic studies were supplied by ULVAC(Kanagawa,Japan).
B.Apparatus and Procedure
1.Confocal scanning lar microscope
In the current study,the crystal formation event was measured optically in situ through a CSLM(Lartec 1LM21H,Yokohama,Japan),and the ont of obrvable precipitation on the surface of the liquid slag was imaged and recorded at various temperatures in air.The experimental asmbly has been described in detail in the previous publication.[8]It should be noted that at the be
ginning of the oxide formation,the gas oxygen potential had likely not reached its desired value and the slag would not be equilibrated with the atmosphere.The experiments were performed above the liquidus temperature of the slag.According to the Slag Atlas(2nd ed.),[9]the liquidus temperature of studied slag composition should be%1500K(1227°C). In the experimental tup ud,a cylindrical platinum crucible containing approximately0.030-g slag was placed on a high-density alumina crucible.The whole sample was then t on a platinum sampler pan(10mm diameter).In the beginning of experiment,the CSLM chamber was evacuated for10minutes and was purged with argon gas for20minutes at a rate of200mL/min. Each slag sample in the Pt-crucible was then heated at the CSLM hot stage in an Ar(purity99.999pct) atmosphere.To start from a completely molten slag, the sample was heated initially to1700K(1427°C)and maintained for20minutes until it melted completely. The sample was then slowly(5°C/min)cooled down to the desired aim temperature.The atmosphere in the heating chamber was then switched from Ar to air so that the designated oxygen partial pressure was obtained for magnetite/hematite precipitation.Before the exper-iments,the temperature calibration for the crucible holder was performed.The temperature of the samples was controlled by a proportional–integral–derivative (PID)controller unit and adjusted continuously to compensate for a heat change during oxidation reac-tions occurring over the atmospheric switch from Ar to air with aflow rate of200mL/min.
A temperature calibration for the sample holder was carried out using a two-thermocouple technique.The first thermocouple was attached directly to the sample holder while the cond one was introduced clo to the bottom of the platinum crucible.The reading from the latter was assumed to be the actual temperature,whereas the sample holder thermocouple was the t temperature. The sample(actual)temperature showed a negative deviation compared with the holder temperature.[8]
III.RESULTS
A.CSLM Studies
To study the formation event of crystals during the initial oxidation reaction,imaging through a CSLM technique was carried out.The precipitation behavior of the oxide particles during oxidation will be discusd first.
The crystal growth obrved by the CSLM is pre-nted in Figure1.In the beginning of the experiment, the powder sample(Figure1(a))was heated to1700K (1427°C)until it became completely molten in the argon atmosphere.
After melting,the sample was cooled slowly to the desired temperature and the argon gas was switched to
air, Fig.1—Crystals obrved by the CSLM for sample at1593K(1320°C)(oxidation in air):(a)sample as original powder form at room tempera-ture;(b)molten sample;(c)crystal formation after switching to air;(d)–(f)crystal growth and agglomeration with time.
while the temperature was maintained constant (Figure 1(b)).The time until the air reached the sample surface for the current gas flow rate was found to be approximately 40conds.This time wa
s estimated bad on the time when the temperature started to change becau of the oxidation reactions after the gas switch.The first crystals were obrved after 240conds of air introduction (Figure 1(c)).With time,the crystals grew (Figure 1(d))and agglomerated (Figures 1(e)and (f)).The obrved crystal size in the beginning was £4l m.With time,crystals grew and agglomerated with their sizes reaching 40to 50l m in some cas.
冗杂
The time until obrvable crystal precipitation took place was measured through in situ visualization using the CSLM and recorded at various temperatures.The crystal lengths was measured with time.A correspond-ing Time-Temperature-Transformation (TTT)diagram with average data was then constructed as shown in Figure 2,where two curves were identified.A node of the lower curve ems to be located slightly above 1530K (1257°C)where the fastest kinetics was found,whereas that of the upper curve ems to be located around 1570K (1297°C).As temperature was raid,slopes of the curves rapidly decread,indicating that the crystals required more time to form.
The crystals obtained at different temperatures corre-sponding to the TTT diagram are prented in Figure 3.For the temperature range of 1570K to 1600K (1297°C to 1327°C),crystals 20–40l m in length were obrved (Figure 3(a))with geometry different from tho obtained at lower temperatures.At 1560K
rosas
(1287°C),obrved crystals were relatively disperd and somewhat angular (Figure 3(b)).At the tempera-ture range of 1550K to 1530K (1277°C to 1257°C),crystals were smaller (<5l m in some cas)and rather clustered (Figure 3(c)).The obrvations imply that nucleation occurred at a faster rate at 1533K (1260°C),where nucleation sites were saturated quickly,whereas crystals precipitated at 1593K (1320°C)could grow larger.This corresponds well with the TTT diagram shown in Figure 2.The TTT diagram shows a break in the temperature range of 1560K to 1570K (1287°C to 1297°C).A possible cau of this discontinuity (i.e.,the prence of two distinct curves)is a change in the precipitating pha.To study this phenomenon,three samples at three different temperatures were quenched in the CSLM chamber in Ar atmosphere at the maximum cooling rate with 74°C/s from 1580K to 1073K (1307°C to 800°C)and analyzed using a scanning electron microscopy (SEM)-energy-dispersive spectrometer (EDS).B.SEM-EDS Results商务英语词汇王
Quenched samples from the CSLM chamber were analyzed by SEM-EDS.SEM images of the quenched samples are prented in Figure 4.The resultant EDS analys were summarized in Table I .The EDS analysis has been done on the single spot marked as (1)in Figure 4.
According to Table I ,the iron content in the spot marked (1)in Figure 4(c)is lower than that in Figure 4(b).Becau of the SEM spatial resolution limit,the analyzed area in Figure 4(c)might be covering b
oth iron oxide and calcium silicates.
cutoff
IV.DISCUSSION
A.Thermodynamic Aspect
FeO in molten slags can be oxidized to magnetite or hematite,and if they are expod to an environment oxidizing enough to rai the oxidation degree of Fe.The following reactions may take place:
3FeO ðÞslag þ1=2O 2¼Fe 3O 4ðÞsolid
½1 2FeO ðÞslag þ1=2O 2¼Fe 2O 3ðÞsolid
½2
Fig.2—TTT diagram for the 30wt pct FeO-35wt pct CaO-35wt pct SiO 2
system.
Fig.3—The CSLM images for samples treated in air at (a )1593K (1320°C),(b )1560K (1287°C),and (c )1533K (1260°C).
To find out what phas can be obtained in the current oxidation atmosphere,a thermodynamic calcu-lation was performed by using FactSage 6.1(Thermfact Ltd.,Montreal,Canada and GTT Technologies,Aachen,Germany).Figure 5reprents a pha diagram,temperature vs partial pressure of oxygen in the system.As the experiment was conducted above 1500K (1227°C),the initial atmosphere was argon.The exper-imental conditions were confirmed to be located in the fully liquid region of the Figure 5.After switching to air,the FeO was oxidized,and depending on the temperature,crystals precipitated in accordance with the need to attain thermodynamic equilibrium as depicted in Figure 5.In air over a temperature range of 1560K to 1600K (1287°C to 1327°C),calcium silicate (CaSiO 3)can be obtained in the liquid slag (Figure 5),which might correspond to the crystals with different morphology (Figure 3(a)and 4(a)).
According to the pha diagram,hematite and cal-cium silicate can form in the liquid slag over a temperature range of 1530K to 1560K (1257°C to 1287°C)when expod to air.The CSLM investigation at 1560K (1287°C)shows commonly found morphol-ogy of the crystals (somewhat cubic shape;Figures 3(b)
and 4(b)).EDS analys of the quenched sample indicated the prence of calcium silicate and iron oxide.At a temperature of 1533K (1260°C)(which corre-sponds to the border between two regions:solid Ca 3Si 2O 7,hematite,CaSiO 3and liquid slag containing hematite and CaSiO 3),similar types of crystals were found by CSLM.
The TTT diagram shows a discontinuity in the temperature range of 1560K to 1570K (1287°C to 1287°C),which corresponds to conditions expected to cau the hematite formation when this slag is expod to air per Figure 5.The EDS analysis of the sample quenched from 1560K (1287°C)showed higher con-centration of iron oxide in precipitated crystals than tho obrved in the sample obtained at 1580K (1307°C).The sample quenched from 1580K (1307°C)mainly contained calcium silicate (CaSiO 3).From Figure 5,it can be also obrved that over a range of log 10(P O 2)=–3.0through 2.0,FeO in slag may be oxidized to magnetite [<1560K (1287°C)].To produce magnetite,it is advantageous to u a lower partial pressure of oxygen rather than air (e.g.,CO 2gas,CO/CO 2mixture or Ar).
The data obtained in the prent study and previous experience of the authors allowed offering the
following
Fig.4—SEM images for the samples quenched from:(a )1580K (1307°C);(b )1560K (1287°C);(c )1533K (1260°C).
Table I.EDS Results of the Quenched Samples (Marked (1)in the figures)from the CSLM Chamber,Corresponding
to Fig.4Element Atomic Pct Figure 4(a)(1)O 58.80Si 18.99Ca 20.19Fe 2.02Total
100.00Figure 4(b)(1)O 60.3Si 10.87Ca 10.81Fe 18.02Total
100.00Figure 4(c)(1)O 58.74Si 15.785Ca 16.785Fe 8.69Total
100.00
Fig.5—Temperature vs partial pressure of oxygen pha diagram.The line marked A corresponds to the partial pressure of oxygen in air.P O 2is prented in Pa.Calculated using FactSage 6.1.
strategies aimed at maximizing the yield of iron in the form of magnetite:
5diary(a)To perform oxidizing at lower temperatures with
subquent quenching in inert atmosphere as soon asfirst precipitates appear.
(b)To control a partial pressure of oxygen ,
additives of CO2in order to tailor precipitation within the area where Fe3O4is prent among the solid products.Our previous studies[10]have dem-onstrated this possibility.
(c)Simultaneous processing of steelmaking ,fer-
romangane slag.It was found by the current authors that with mangane oxide in the synthetic slag system, magnetite can be obtained as a spinel as well as man-gane ferrite.[8]Mangane in the slag stabilizes the magnetite and arrests subquent oxidation to hema-
tite,as it was found for the ternary system.
B.Kinetic Analys of the Crystal Growth
The oxidation process can be controlled by the following steps:
(a)Nucleation of magnetite/hematite during the initial
oxidation reaction
(b)Chemical reaction
(c)Oxidation due to diffusion of oxygen/iron through
the product layer
Once a crystal is nucleated,three possible ways for crystals to grow on the slag surface can be considered. First,the gas/solid interface can grow into the gas pha when iron cations diffu across the crystal pha to the interface where they react with oxygen.Second,when oxygen anions diffu across the crystal pha to the liquid/solid interface,this interface moves into slag by interacting with iron cations.In the third scenario,the triple point where gas,liquid,and solid meet can laterally grow without solid-state diffusion involved. The growth can occur when the sufficient amount of iron cations and oxygen anions interact at the interface. Provided that dissociation rates are comparable in all the three cas,growth at the triple point may occur most easily becau of(1)larger interfacial energy and (2)faster diffusion in the gas pha(gas-pha mass transfer)and in
the liquid pha(liquid-pha mass transfer).The three possible rate controlling steps for this growth are as follows:
(a)Gas-pha mass transfer control:If it is assumed
that Fe z+is always sufficiently available at the interface,diffusion of O2through the diffusion boundary layer controls the reaction rate.
(b)Liquid-pha mass transfer control:If Fe z+requires拜见岳父岳母
long range diffusion and its supply is slow,the gas flow rate and gas-pha mass transfer rate have a negligible effect on growth of crystals compared with the diffusivity of Fe z+.
(c)Mixed control of mechanisms1and2.
From the obtained CSLM images,the length of the crystals was measured with time.The average data were plotted In Figure6.
The growth rate of crystals incread with tempera-ture.Similar behavior was found during the studies of quaternary FeO-MnO-CaO-SiO2system.[8]This could be indicative of a faster rate of transport in the liquid pha as a result of a lower viscosity.
The current TTT diagram for the crystal formation at the gas/liquid slag interface(Figure2)showed that,with an increa in temperature,the start of the precipitation is delayed as well.This could be caud by lower degree of super saturation for crystal precipitation as the temperature is incread.
V.CONCLUSIONS
In the current work,the kinetic study of oxidation of FeO in the liquid slag has been conducted by CSLM.The real-time confocal analysis showed the crystal formation behaviors during oxidation of FeO in a30wt pct FeO-35wt pct CaO-35wt pct SiO2 liquid slag.With time,the crystals grew and agglom-erated,reaching,in some cas,40l m in length. Different shapes of crystals were obrved at different temperatures.At temperatures>1560K(1287°C),the crystals reached20–40l m in length.According to the thermodynamic calculations,this type of crystal cor-responds to the calcium silicate pha.In the temper-ature range of1530K to1560K(1257°C to 1287°C),crystals had somewhat a cubic shape and corresponded to the expected formation of hematite (Fe2O3).A TTT diagram was constructed bad on the CSLM results.
ACKNOWLEDGMENTS
The authors are thankful to Swedish Foundation for Strategic Environmental Rearch(MISTRA)for t
hefinancial support through the project Eco-Steel Production(Sub project no.:88035)administered by Swedish Steel Producers Association(Jernkontoret). Thefinancial support for Anna Semykina from the Swedish Institute is gratefully
acknowledged.
Fig.6—The measured growth rates of the crystals in air at different temperatures(average data).
