Energy & Fuels 1995,9, 534-539
534
Viscosity and Structural State of Iron in Coal Ash Slags under Gasification Conditions Jan W. Nowok Materials Technologies, University of North Dakota Energy and Environmental Research Center, P.O. Box 9018, Grand Forks, North Dakota 58202-9018 Received November 21, 1994. Revised Manuscript Received February 2, 1995@
The effect of Fe3+/Fe2+redox equilibrium in ferrosilicate melts and ash slags on their viscosities is discussed. Viscosity experiments were carried out on two ash slags with low (Rochelle) and high (Illinois No. 6) iron content in COdCO (40/60) and air atmospheres as a function of temperature and as a function of time at a constant temperature. The lower values of viscosities measured in both ash slags under reducing conditions are assigned to the transformation of network-forming ferric iron to the network-modifying ferrous iron. Anomalies in viscosity are apparent in the reduced form of Illinois No. 6 ash slag during its oxidation in air at 1370 "C (slightly above the temperature of critical viscosity of the oxidized form of slag). First the viscosity significantly increases and then decreases to that of the oxidized form. The Fe3+/CFe ratio increases from 0.12 to 0.84. The conclusion is that changes of local configuration in the melt (coordination number and charge of iron ions and stability and size of crystalline phases such as hematite and quartz) determine the viscosity in Illinois No. 6 ash slag. Redox ratios of iron and structural positions of Fe3+ and Fe2+ions in quenched slags were determined with Mossbauer spectroscopy.
Introduction
librium at crystallization temperatures and how this may affect slag viscosity.
This paper is a follow-up of previous work on the viscosity and structural changes in coal ash slags in Newtonian and Bingham plastic regions.1!2This paper discusses the effect of redox ratios of iron, resulting from the reduction of network-forming Fe3+ t o networkmodifying Fe2+,on the viscosity change in slags under gasification conditions. The Fe3+/Fe2+redox equilibrium is an important parameter that depends on Cod CO and H20/H2 equilibria. The second multivalent element in slags, titanium, is usually ignored since its concentration is below 2 wt %. Despite the important role played by slags in gasifiers, no serious attempt has been made to study Fe3+/Fe2+ redox equilibrium in molten ash slags and its effect on slag viscosities. Prior attempts have been made to determine the oxidation states of iron in ash after fusion tests in air and reducing atmospheres, and it was found that ferrous iron may appear both in amorphous and crystalline phases such as fayalite, hercynite, and FeAl spinel^.^ In this paper, we review the equilibrium conditions of Fe3+ Z- Fez+ in CO2/CO in model ironbearing silicates based on data taken from literature. Furthermore, we extend this concept to coal ash slags to show how instability of melt-oxygen equilibrium under reducing conditions may affect solid-melt equi-
Iron Distribution in Silicate Melts and Its Structural Role on Viscosity of Melts. Various structural investigations of alkali and iron-bearing silicate and aluminosilicate melts have indicated that ferric iron (Fe3+)is tetrahedrally coordinated by ~ x y g e n ~and -~ that the charge deficiency of the Fe3+ion is compensated by an alkali ion. There is a general tendency for the Fe3+/Fe2+ratio to increase with alkali on tent.^ In alkaline iron-bearing silicate and aluminosilicate melts, the coordination number for iron is less ~ l e a r . The ~,~ ferric iron may occur both as a network former and as a network modifier.lOJ1 In silicate melts with magnesium, such as in diopside melt (CaMgSizOs),some Fe3+ ions replace Mg2+ ions in octahedral sites, and some Fe3+ ions replace Si4+in tetrahedral sites.12 Addition of up to 5 wt % Fez03 to calcium silicate melts results in increased viscosity. Additional Fez03 causes decreased viscosity. In contrast, the viscosity of CaOAl203-Si02 melt continuously increases with increasing alumina content up to CaO/Al203 = 0.5 (molar ratio).13 This likely results from the different structural role of Fe3+ and A13+ in alkaline-bearing silicate and aluminosilicate melts. This unusual viscous behavior of alkali
@Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1)Nowok, J. W.; Hurley, J. P.; Stanley, D. C. Energy Fuels 1993, 7, 1135. (2) Nowok, J. W. Energy Fuels 1994,8 , 1324. (3) Huffman, G . P.; Huggins, F. E.; Dunmyre, G . R. Fuel 1981,60, 585. (4)Fox, K. E.; Furukawa, T.;White, W. B. Phys. Chem. Glasses 1982,32, 169. (5) Goldman, D. S. Phys. Chem. Glasses 1986,27, 128. (6) Dingwell, D. B.; Brearley, M. Geochim. Cosmochim. Acta 1988, 52, 2815.
(7) Carmichael, I. S. E; Nicholls, J. J . Geophys. Res. 1967, 72, 4665 (Figure 4). ( 8 )Mysen, B. 0;Seifert, F. A,; Virgo, D. Am. Mineral. 1980,65,867. (9) Virgo, D.; Mysen, B. 0. Phys. Chem. Mineral. 1985,12, 65. (lO)Virgo, D.; Mysen, B. 0.; Danckwerth, P. A,; Seifert, F. A. Carnegie Instit. Washington Year Book 1982,81,349. (11)Virgo, D.; Mysen, B. 0.;Danckwerth, P. A. Carnegie Inst. Washington Year Book 1983.82. 305. (12)%ng, Y. X.; Cable, M.; Ping, Z. Z.; Williams, J. M. J . Mater. Sci. 1992,27, 1137. 113) Mysen, B. 0. Reu. Mineral. 1986,12, 180.
0887-0624/95/2509-0534$09.00/0
Review of the Literature
0 1995 American Chemical Society
Energy & Fuels, Vol. 9, No. 3, 1995 535
Iron in Coal Ash Slags and alkaline iron-bearing silicate and aluminosilicate melts may be partly understood in terms of the configurational entropy of the smallest cooperatively rearranging units.14 It is suggested6 that ferric iron may form an iron-rich substructure such as Cao.sFeO2which, in turn, may have a significant impact on viscosity. Relative variations of the ferric-ferrous ratio in silicate and aluminosilicate melts depend on the fugacity of oxygen ( f T 0 2 ) ) . The proportion of ferric to ferrous species decreases both with the decreasing AO2) a t constant temperature and with increasing temperature The structural position of ferrous at constant f102).15 iron in the melts differs significantly from that of ferric iron. The Fe2+ ions are strongly ordered on the octahedral positions. However, a complex structural reorganization in iron-rich CaO-FeO-Fe203-Si02 melts (in the compositional vicinity of fayaliteI6 and low iron content (below 4 w t %)16 suggests that Fe2+ may also occupy tetrahedral positions. Viscosity, however, follows a nonlinear decrease with reduction of iron in melts.17 The concentration of Fe3+ ions usually increases with alkali and alkaline-earth oxide content in the melt.ls Equilibria in Silicate Melts under Reducing Conditions. The solubility of oxygen in a slag melt is proportional to its fugacity, flO2). Assuming that the pressure of 0 2 above a slag is in equilibrium with the oxygen in the slag, we may write the following equilibria:
where 0- and Oorepresent a nonbridging oxygen (NBO) which is conventionally taken to have a -1 charge and bridging oxygen (BO),respe~tive1y.l~The thermodynamic equilibrium constant for these reactions can be expressed either by oxygen fugacity QT02))1'2 or by partial pressure of oxygen Q 1 ( 0 2 ) ) " ~ and oxygen activity in a melt ( a d . In real gases, fugacity fi is used rather than partial pressure. Fugacity is the thermodynamically effective partial pressure of gas expressed in atmospheres, fi = ypi, where yi is the fugacity coefficient.20 One electron from reaction 1 is transferred into the multivalent ions such as ferric iron and forms ferrous iron: Fe3+
+ e- z z Fe2+
(3)
In this redox equilibrium, the melt participates in electron transfer. The redox equilibria of reactions 1 and 2 are controlled by the gas composition. Thus the fugacity/partial pressure of oxygen in equilibrium with iron-bearing slag is a constant characteristic of the redox equilibria of Fe3+/Fe2+. (14)Mysen, B. 0.; Virgo, D.; Scarfe, C. M.; Cronin, D. J. Am. Mineral. 1985,70,487. (15) Mysen, B. 0.;Virgo, D. Am. Mineral. 1989,74,58. (16) Dyar, M. D.; Naney, M. T.; Swanson, S. E. Am. Mineral. 1987, 72,792. (17) Dingwell, R. D. Am. Mineral. 1991,76,1560. (18) Baucke, F. G. K.; Duffy, J. A. Phys. Chem. Glasses 1993,34, 158. (19) Gurman, S. J. J. Non-Cryst. Solids 1990,125, 151. (20) Atkins, P. W. Physical Chemistry; Freeman: San Francisco, 1982; p 169.
10-
FeSCFe= O 121
For log f,, = -9 00 08-
-
I
'HZ+~CO
06Fe"/ZFe = 0 234 0 4 - 'co
Fe"/ZFe= 0 000
02
-
'H2
0.0 1000
1200
1400
1600
Temperature, "C
Figure 1. Relation of fractional pressure of hydrogen and
+
+
carbon monoxide in H2 H2O and CO C02 gas mixtures with temperature a t constant oxygen fugacity. The Fe3+/ZFe ratios were taken from Mysen and others14for alkali silicate with basdacid = 0.47. The dotted lines correspond to fractional pressure of hydrogen and carbon monoxide as it exists in gasification systems with H2 = 27.5%, CO = 37.5%,C02 = 18.3%, and H2O = 16.7%. Also, dotted lines perpendicular to the temperature coordinate represent the constant values of the Fe3+/ZFeratio.
Reduction of ferric iron to ferrous iron depends on oxygen fugacity MOz)),the concentration of iron in the melt, the base composition of the melt, and the temperature.21 However, quantification of the relation between ferric-ferrous redox and its equilibrium oxygen fugacity requires a complete knowledge of the anionic structure of iron-bearing silicate melts. Difficulties may particularly arise during measurement of the viscosity of a slag because the equilibrium position is disturbed by a transporbcontrolled process where mass transfer through the melt decreases with decreasing rotation rate. It seems very appropriate t o evaluate the relationship between measured redox equilibria of Fe3+/Fe2+and oxygen fugacity using thermodynamic equations. To accomplish this, the following reactions should be considered:
co + 1 / 2 ~ 2 H,
CO,
+ 1/202H 2 0
(gas reaction 1)
(4)
(gas reaction 2)
(5)
Using the concept of experimental chemical metal1urgy,22the following equations are applied to calculate equilibrium constants: for gas reaction 1 In K p = 282400/8.3144T - 10.440 (6) for gas reaction 2 In Kp= 239500/83144T - 8.14 In T/8.3144 1.112 (7)
+
where T represents temperature in kelvin. Figure 1 shows the temperature dependence of hydrogen and carbon dioxide fractional pressures in H20 H2 and C02 CO mixtures, respectively, at constant oxygen fugacity. It appears that constant oxygen fugacity can be maintained if pressures of hydrogen and carbon monoxide decrease with temperature. The de-
+
+
(21) Schreiber, H. D. J . Geophys. Res. 1987,92,9225. (22) Gaskell, D. R. Introduction to Metallurgical Thermodynamics; Hemisphere: Washington, DC, 1981; p 241.
536 Energy & Fuels, Vol. 9, No. 3, 1995
Nowok
iI n "
-
A log fo2= -9 B log fo2= -8 0 8 - c 1ogfot=-7 - D log fo,= -6
Fe"EFe= 0 121
-
04= 0 234
02-
oo+ 1000
: 1200
1400
1600
Temperature, "C
Figure 2. Carbon monoxide pressure in COz
+
CO mixture a t various oxygen fugacities as a function of temperature. The Fe3+EFeratio were taken from Mysen and others.I4
crease of H2 or CO content in appropriate gas mixtures coincides with the increase of the Fe3+/CFeratio. To calculate fractional pressure, it was assumed that p(CO2) + p ( C 0 ) = 1atm andp(H20) p(H2) = 1atm.22 Consequently, similar plots of p(H2) and p(C0) as a function of temperature can be calculated for lower and/ or higher oxygen fugacities. The general conclusion remains the same, however; the ratio of Fe3+/CFemay either decrease or increase, respectively. For comparison, the pressures of hydrogen and carbon monoxide usually observed under gasification conditions are also included in Figure 1 (dotted lines parallel to the temperature coordinate). It is assumed that the total pressure of the reducing atmosphere is unchanged at all temperatures over the ash slag. It is apparent that increasing CO2 and H2O content in appropriate gas mixtures tends to shift the equilibrium (eqs 4 and 5 ) toward higher ferric-to-ferrous ratio. In studying the hydrogen-oxygen reaction in a melt, two more equilibria should be ~ o n s i d e r e d : ~ ~ - ~ ~
+
H20 + 0- zOHH20
-
+
Fe"EFe = 0 000
06-
Fd'EFe
the reduction of Fe3+ to Fez+ with CO takes place in the absence of solid carbon, so we may disregard the Boudourd reaction (C C 0 2 2CO). If the Boudourd reaction takes place, the oxygen fugacity will be decreased causing a further reduction of ferrous iron to metallic iron (see further text). In our consideration, we ignore the gas-shift reaction since it does not change the total reduction activity of the gas mixture (CO H2O COS Hz). The temperature range (1000-1600 "C) considered here corresponds with that in slag melts in boilers. Thermodynamic Stability of Crystalline Phases in Slags Formed under Gasification Conditions. For purposes of thermodynamic calculations involving liquid-crystal equilibrium in the multicomponent oxide system, one needs free energy composition diagrams of liquid solutions of major components in ash slag near the temperature of critical viscosity, the mixing properties in the liquid at temperatures corresponding to the saturation state, and free energy of solid phases at various temperatures.2 When crystallization takes place, the crystal-liquid interface becomes an important factor. If crystallization into one or more stable crystalline phases occurs under reducing conditions, the crystal-liquid equilibrium may be controlled by the liquidgas equilibrium which, in turn, is controlled by oxygen fugacity. For the general redox calculations, we will use partial pressure rather than oxygen fugacity since oxygen fugacity coefficients for most of the considered minerals and their melts are unknown in the literature. The partial pressure of oxygen over the melt can be simply calculated with the aid of the equilibrium constants of the dissociation reaction28at temperature
+ OH
+ 0' = 2 0 H
(8)
+
+
T -AG"dT)/RT = log Kf
With this definition, the equilibrium constant of the dissociation reaction of silicate into other components on evaporation, such as Fe,SiO,
(9)
The hydroxyl groups so formed may reduce the viscosity of silicate (aluminosilicate) melts, particularly, in the transformation range,26 but water vapor may enhance the crystallization of melts.27 The use of carbon dioxide-carbon monoxide mixtures may be regarded as a measure of oxygen fugacity since these gases do not introduce any complexities in chemical equilibria. Figure 2 shows the various univariant equilibria curves for CO in a mixture with C02 which controls the oxygen fugacity over slags. In other words, CO mixture, at increased CO content in the C 0 2 constant temperature, causes the decrease both of oxygen fugacity and the Fe3+/CFeratio in NaO-A1203SiO2-Fe-0 melts.14 Also, the oxygen fugacity decreases with temperature a t constant CO pressure in the C02 CO mixture. So far we have assumed that
+
+
(23)Bartholomew, R. F. In Treatise on Materials Science and Technology; Tomozowa, M., Doremus, R. H., Eds.; Academic Press; New York, 1982; p 75. (24) Silver, L.; Stolper, E. J . Petrol. 1989,30, 667. (25) Stolper, E. Geochim. Cosmochim. Acta 1982, 46, 2609. (26) Spray, J. G. J . Geophys. Res. 1993,98, 8053. (27) Boulos, E. N.; Kreidl, N. J. J . Can. Ceram. SOC.1972, 41, 83.
(10)
-
2Fe
+ Si + 20,
(11)
can be expressed by
Kf= p(Fe)2p(Si)p(02)2/u(Fe2Si0,)
(12)
In general, the activity of the condensed phase, in our case FeaSiOl, is assumed to be 1. Since nonequivalent amounts of iron, silicon, and oxygen are formed during the dissociation process of fayalite, the partial pressure of each species will be equal to (~i/5)P,where vi is the stoichiometric coefficient of i and P is the total pressure of the species formed from the fayalite. Kfin terms of the total pressure is
Kf = (2/5P)2(1/5P)(2/5P)2
(13)
The determination of ~ ( 0 2 follows ) from the relation between oxygen partial pressure and total pressure P: p ( 0 2 ) = 2/5P.
Calculated partial pressures of oxygen for melt (solid)oxygen equilibrium at a given temperature, for selected minerals, are listed in Table 1. Any instabilities in (28) Barin, I. Thermochemical Data of Pure Substances; VCH: Weinheim, Germany, 1993.
Iron in Coal Ash Slags
Energy & Fuels, Vol. 9, No. 3, 1995 537
Table 1. Partial Pressures of Oxygen over Selected Minerals at Various Temperatures mineral composition CaSiO3 FezSiO4 CaAlzSizOe CaMgSizO6 Ca&Si07 NaA1Si04 FesO4 FeO Si02
FeAl204 a
mineral p-wollastonite Fayalite anorthite diopside gehlenite nepheline magnetite iron oxide silica hercynite
-logp(Oz) at 1400K 1817 9.3 13.3 7.6 1490 6.9 1823 9.2 13.3 12.9 1665 10.1 13.4 1863? 9.5 11.6 1525 10.3 1870 2.8 4.9 1650 3.7 4.7 12.3 1996 7.2 NAa NA 10.7
TI&
T,
280
1200K 16.2 9.4 16.2 15.8 16.4 14.3 6.2 5.8 15.1 13.2
Not available.
oxygen partial pressure over the melt(mineral1, above or below equilibrium, will cause changes in the solidliquid equilibrium and, furthermore, decomposition of the crystalline phase into new phases. The best example is to consider the stability of fayalite-oxygen equilibrium under reducing conditions. This phase is only stable in a narrow oxygen partial pressure, near atm; below that pressure (higher reducing conditions) crystallization of this phase may lead to growth of highly nonstoichiometric crystals, in metastable state, such as Fez+,Si04 which may finally decompose: t 2Fe0
+ xFe + SiO,
(14)
Since the oxygen partial pressure over FeO is higher than atm (Table 11, iron oxide is not expected to crystallize from the melt but rather to reduce to metallic iron.29 Reaction 14 may be controlled by a buffer in the = atm, the fluid phase such as H20.30 Above ~(02) decomposition of fayalite is subject to the oxidation eq~ilibrium:~~ Fe,SiO,
+ '/,O,
-
+
Fe203 SiO,
(15)
Both examples indicate that changes in oxygen partial pressure (fugacity) over the melt may cause crystallization. In multicomponent silicate systems derived from coal ashes, the solid-melt equilibrium may also be affected by the activity of oxygen in the slag, particularly when the oxygen partial pressure is reduced to the level below the melt-oxygen equilibrium. This may cause a deficit of oxygen in the silicate network structure referred to as the formation of oxygen vacancies, VO,in structural units (clusters) such as SiO3-2(V0-2).~~The important feature is that the defective clusters are short-lived complexes so that, upon breakup, the framework around the defect site can be reconfigured. The mobility of oxygen ions as well as structural units in (29)Massieon, C. C.; Cutler, A. H.; Shadman, F. Ind. Eng. Chem. Res. 1993, 32, 1239. (30)DeCapitani, C.; Brown, I. H. Geochim. Cosmochim. Acta 1987, 57, 2639.
a,
.-
-
240-
Y)
g
200-
.$
160-
8
120-
.9
In
> 80 40
-
01 1160
1240
1320
1400
Temperature, "C
Table 2. Composition of Ash Slags (weight percent expressed as equivalent oxide) Si02 A1203 Fez03 Ti02 CaO MgO NazO KzO 5.8 1.5 26.9 6.1 1.8 0.3 Rochelle 37.5 20.1 IllinoisNo. 6 46.6 23.1 18.3 1.2 5.6 1.6 0.5 3.1
Fe,+,SiO,
3204
(31)Muan, A. In The Evolution of the Igneous Rocks; Yoder, Jr., Ed.; Princeton University: Princeton, NJ, 1979; p 108. (32)Lau, P.; McMillan, P. W. J.Muter. Sci. 1982, 17,2715.
Figure 3. Viscosity-temperature relationship on the Rochelle ash slag.
the melt should increase as the concentration of oxygen vacancies increases, and consequently, viscosity should decrease.
Experimental Section Measurements of Viscosity. Slag viscosity was measured in air and COdCO (40/60) a t 1atm as the temperature dropped from 1400 to 1200 "C and also a t constant temperature (above the temperature of critical viscosity) as a function of time with a rotating bob v i ~ c o m e t e r . ~For ~ oxidizing and reducing atmospheres, the viscometer bobs were fabricated from platinum or molybdenum bar stocks, respectively. The oxidation kinetics of reduced ash slags were determined in air using a bob fabricated from platinum. The internal diameter of alumina crucible-to-bobwas 24.6/12.7 mm, which satisfies the linear shear strain of melt from crucible wall to bob as it is required by the Newtonian fluid flow. The viscometer was calibrated with National Bureau of Standards (NBS) silicate glass NBS 711. The precision was f 5 % . M6ssbauer Spectroscopy Tests. Mtissbauer spectroscopy was used to determine the oxidation state of iron ions and the structural positions of Fe3+ and Fez+ in quenched ash slags. The Fe3+EFe was calculated as the ratio of the area of the Fe3+ doublet relative to the total absorption envelope. The measurements were made a t room temperature. Materials. Rochelle and Illinois No. 6 coal ashes were studied, and their chemical compositions are listed in Table 2. Chemical compositions were determined after measurements of viscosity. X-ray diffraction (XRD) analysis was used to determine the crystalline phases in the slag samples after quenching them from 1000 "C.
Results and Discussion The viscosity-temperature relationships of the Rochelle and Illinois No. 6 ash slags measured both in air and in COz/CO (40/60)atmospheres are illustrated in Figures 3 and 4. In both ash slags, the viscosity is lower if measured under reducing conditions because of the reduction of tetrahedrally coordinated Fe3+ t o octahedrally coordinated Fez+. The change in coordination number of the iron ions is usually associated with depolymerization of the slag. Also, the decrease of viscosity under reducing conditions in comparison with that under oxidation conditions appears smaller for the Rochelle slag than for the Illinois No. 6, and this results from the lower ferrous iron content in Rochelle's ash slag. (33)Schobert,H. H.; Streeter, R. C.; Diehl, E. K. Fuel 1986,64,1611.
538 Energy & Fuels, Vol. 9, No. 3, 1995
Nowok Table 3. Isomer Shift (IS), Quadruple Splitting (QS),and FeS+/CFein Illinois No. 6 Slag A. Quenched in Water from Reducing Condition IS, mmis QS, "/s Fe3+EFe phase 2.09 Fez+in glass (84 wt % of Fe) 1.02 Fe3+in glass (12 wt % of Fe) 0.48 0.60 Total 0.12 B. Quenched in Water after Oxidation from Reduction Form phase
I
O J
1330
1290
1370
1.67 2.86
0.88 1.12
Fe3+/CFe
total 0.84
1490
1450
1410
hematite (70 wt % of Fe) Fe3+in glass (14 wt % of Fe) Fez+in glass (12 w t % of Fe) fayalite, FezSi04 (4wt% of Fe)
IS, mmis QS, " i s 0.35 -0.08 0.30 0.81
Temperature, "C
,
Hematite
Figure 4. Viscosity-temperature relationship on the Illinois No. 6 ash slag.
5
.-
_.
I'
101 100
I
I ,
-
99-
.-ln
5c
98-
I=
97 -
a9
8
E >
96 95-
95 -
90-
0
8
4
20
24
24
20
16
12
16 12 Time, min
4
8
-
Reduction 0 t Oxidabon
Figure 5. Variation of viscosity with time measured in Cod CO (40/60) and air on Rochelle ash slag at 1240 "C.
Oxidation
0
20
4
16
a
12
12
a
20
16 4
,
I
, -4
I
-8
,
,
,
0
, 4
,
,
,
12
8
Velocity, mmis Figure 7. 57Feresonant absorption Mossbauer spectra of Illinois No. 6 ash slag exposed to COdCO (40/60)and quenched in water from 1370 "C. 103 102
.
800
944 -12
101 100 99 98
O
Reduction t Oxidation
97 96 95 94 93
90
Time, min
Figure 6. Variation of viscosity with time measured in Cod CO (40/60)and air on Illinois No. 6 ash slag at 1370 "C.
Figures 5 and 6 compare the viscosity kinetics of Rochelle and Illinois No. 6 slags measured first under reducing and then under oxidation conditions at 1240 and 1370 "C, respectively. Distinct differences in the viscous behavior of both slags are apparent in reduced slags during their following oxidation in air. The small increase of viscosity in Rochelle slag may be related to the abundance of Fe2+ions; oxidation to Fe3+seems to require more oxidation time than 30 min. The major crystalline phase formed near the temperature of critical viscosity, both under oxidizing and reducing conditions, is clinopyroxine, Ca(Fe,Mg,Al)SizOs. In contrast, the viscosity of the reduced form of Illinois No. 6 slag significantly increases and then decreases with time (Figure 6) during oxidation. This suggests the occurrence of complex structural reorganization in this melt: Mossbauer results indicate an increase of
-7
-5
-3
-1
1
3
5
7
9
Velocity, mmls Figure 8. 57Feresonant absorption Mossbauer spectra of Illinois No. 6 ash slag after oxidation from the reduced form and quenching in water from 1370 "C. ferric iron content from Fe3+EFe= 0.12 t o Fe3+EFe = 0.84 (Table 3, Figures 7 and 8). Mossbauer and XRD tests showed the occurrence of hematite as a major crystalline phase, and XRD also showed quartz as a minor phase. Both phases may crystallize at temperatures at which the oxygen partial pressure is above the fayalite-oxygen equilibrium, ~ ( 0 2 >) atm (eq 15). XRD patterns obtained from quenched Illinois No. 6 ash slag from 1000 "C after measuring of viscosity in COdCO (40/60) indicate the crystallization of hercynite as a major phase and anorthite as a minor phase. The hercynite phase matches the low oxygen partial pressure over crystal-oxygen equilibrium (Table 1) well. Crystallization of this phase in iron-bearing ash slags
Iron in Coal Ash Slags
after measuring of viscosity under gasification conditions is also reported in the literature.34 The major feature of the viscosity increase in Illinois No. 6 ash slag during its oxidation from the reduced form seems to be the nucleation and crystallization of hematite and quartz (see eq 15). A further decrease of viscosity is less obvious, perhaps because of the partial dissolution of the smallest magnetite and quartz crystallites into the slag. About 14 w t % of ferrous iron from the total 100 w t % of iron in the slag appears to be in the amorphous phase (Table 3). Thus changes in the oxidation state of iron can affect not only the distribution of the elements in the slag, from the surface to the bulk, but also crystallization phenomena a t the surface and in the bulk. It was observed that in solid glasses, the Fe3+ ion may occur in two different coordination states: in tetrahedral (Fe3+)tand octahedral (Fe3+),,, and for this reason both the diffusion coefficient and mobilities of (Fe3+)t ions are lower than those of (Fe3+), ions.35,36Similar effects are expected to occur in glass melts. We may speculate that changes of coordination number of iron ions from octahedral to tetrahedral during oxidation of Fez+ t o Fe3+ may also cause an increase in slag viscosity. Mossbauer spectra, besides the determination of oxidation states of iron, allow information on the structural positions of Fe3+and Fez+in glasses from the hyperfine parameters (isomer shift [IS]) and electric (crystalline) field gradient at iron sites (quadruple splitting [&SI)to be obtained.15 In particular, IS of Fe3+ provides information about the number of oxygen ligands. In crystalline iron silicate, values of IS (Fe3+)
