1934
Ind. Eng. Chem. Res. 2000, 39, 1934-1943
MATERIALS AND INTERFACES Elucidation of Sulfidation Mechanisms of Zinc Ferrite in a Reductive Gas Environment by in Situ X-ray Diffraction Analysis and Mo1 ssbauer Spectroscopy Makoto Kobayashi,* Hiromi Shirai, and Makoto Nunokawa Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry, Nagasaka 2-6-1, Yokosuka 240-0196, Japan
A difference in reactivity of zinc and iron in zinc ferrite during sulfidation in a reductive gas environment is important to explain sulfur removal performance of the double oxide. We focused on their sulfidation characteristics under relatively lower sulfur concentration to determine the factors for a superior performance on sulfur removal. A series of sulfidation tests of the double oxide were performed in reductive gas environments at mainly 550 °C in an in situ X-ray diffraction instrument that identified products of reduction and sulfidation. Complementary information on valence and magnetic properties of iron in the products was obtained by 57Fe Mo¨ssbauer spectroscopy analysis. Reduction products of zinc ferrite were wustite, partly zincbearing magnetite, and pure magnetite, depending on the strength of the reductive environment. Sulfidation of the reduced zinc ferrite yields sulfides of both zinc and iron, when the concentration of hydrogen sulfide was sufficiently high. Zinc sulfides, however, were predominantly produced in sufficiently low sulfur concentration, typically less than 80 ppm. The iron component at the low sulfur condition remained in the reduced chemical forms or as pure magnetite. These findings revealed that a higher performance during sulfur removal at lower sulfur concentration is maintained with zincite in the reduced zinc ferrite. Introduction High-temperature gas purification has various advantages in energy-conserving use in the potential technology such as integrated gasification combined cycle power plant. Zinc ferrite is a potential candidate for high-temperature sulfur removal from a reductive environment such as a coal-derived gas. Because the double oxide has a relatively higher sulfur capacity and an extreme performance for deep sulfur removal, purified coal gas may be used in a high-temperature fuel cell such as a molten carbonate fuel cell. The sorbent, however, includes an iron element that may also act as a trigger substance of carbon deposition1 that is harmful to the desulfurization performance of the sorbent. When the sorbent suffers from carbon deposition, the desulfurization performance may incur significant damage. Our previous study2 on iron oxide-based desulfurization sorbents tells us that a silica additive to the iron oxide preparation retards carbon deposition in carbon oxide containing gas. The same additive is effective in retarding carbon deposition on a zinc ferrite based desulfurization sorbent that is highly durable to multicycle desulfurization.3 Because the oxide has two metal elements that contribute to the sulfidation reaction in a reducing environment, differences in their sulfidation characteristics have an effect on the overall sulfidation * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81-468-56-2121. Fax: +81468-56-3346.
performance of the oxide. Though their chemical forms at sulfidation are important on the sulfur removal characteristics, the determination of products in the reducing gas was not established because precise in situ measurements are required to distinguish reaction products of zinc and iron in the double oxide. Zinc ferrite is a well-investigated material because of its importance in the field of magnetic application. Carefully prepared and well-characterized zinc ferrite exhibits antiferromagnetism below a Ne´el temperature, TN, of 10.5 K.4 Above the Ne´el temperature, zinc ferrite exhibits paramagnetism while other spinel ferrites derived from the reduction of zinc ferrite, such as magnetite, exhibit ferrimagnetism at the temperature. We considered that this property is useful to distinguish the iron status in products of reduction and sulfidation whose magnetism may differ from zinc ferrite. This work describes on the first application of in situ X-ray diffraction (XRD) analysis the reduction and sulfidation of zinc iron double oxide in a reducing environment. Products of those reactions were determined by in situ X-ray diffraction analysis and 57Fe Mo¨ssbauer spectroscopy. These two-step analyses were required to distinguish reaction products of zinc and iron in the double oxide. The main purpose of the work is reveal a reaction scheme of sulfidation on zinc ferrite in a reductive environment by applying those complementary methods.
10.1021/ie990456t CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1935 Table 1. Conditions for Reduction and Sulfidation Tests condition name
Figure 1. Optical geometry and reactor schematic of the in situ XRD spectrometer.
Experimental Section Nature of a Specimen of Zinc Iron Double Oxides. Zinc iron double oxide powder was supplied as a pigment-grade synthesized oxide (manufactured by Titan Kogyo Co., Ltd. and supplied by Nikki Chemical Co., Ltd., TAN-T10). Analysis with powder XRD allowed determination of its crystal structure and crystallite size by the Scherrer method. The crystal structure of the oxides was identified from XRD patterns between 2θ ) 10 and 100° obtained with an XRD instrument (MAC Science Co., Ltd., MXP18VAHF22-SRA). Analysis of the magnetism of the double oxide was performed by 57Fe Mo¨ssbauer spectroscopy at 293 K to confirm its magnetic purity. The bulk composition of metallic components in the prepared oxides was determined by chemical analysis of aqueous solutions of the oxides. The iron concentration of the dissolved sample was determined by an inductively coupled plasma atomic emission spectrometer (Shimadzu, ICPS-2000) at a wavelength of 259.9 nm. The zinc contents were obtained by an atomic adsorption spectrometer (Hitachi, Z-8000) at a wavelength of 213.8 nm. In Situ XRD Analysis for Identification of a Reduced or Sulfurized Phase of the Oxides. Crystallographic structures of the samples were observed with the in-situ XRD instrument equipped with a hightemperature sample stage for humid corrosive gas. The main components around the optical system of the instruments are schematically shown in Figure 1. The sample holder and gas tubing of the instrument are specially constructed and are carefully heated to introduce a corrosive reducing gas including steam at ambient pressure. The sample holder is made of quartz that has a geometry of 20 mm width, 30 mm length, and 4 mm thickness. The sample holder was manufactured to a fine tolerance of 1 µm for thickness and flatness to attain an accurate sample position. The shape of the sample cavity is square with sides of 18 mm length and 0.2 mm depth. The sample cavity was filled tightly with the powder that was ground with a mortar made of agate. The temperature applied to the sample stage was between 50 and 650 °C, which was attained by the sample heater installed beyond the sample holder shown in the schematic figure. The reaction gas was also preheated by the other heating element installed around the inlet gas tube. The reaction gas was then introduced perpendicularly to the sample surface. Reduction and sulfidation were performed on the basis of both coal gas composition and a gas mixture of hydrogen-steamnitrogen. The coal gas condition simulated a dry coal fed and air-blown type gasifier as shown in Table 1. The concentration of hydrogen sulfide in coal gas depends on the fed coal and gasification condition; the concentration usually varies between one hundred and one
temp, °C pressure, MPa gas composition, vol % H2 H2O CO CO2 N2 H2S, ppm flow rate, L/min (NTP) sample weight, g
A
B
C
550 0.10
550 0.10
550 0.10
8.0 5.0 0.0 0.0 balance 0-500 0.50 0.20
20.0 5.0 0.0 0.0 balance 0-500 0.50 0.20
8.0 5.0 20.0 5.0 balance 0-500 0.50 0.20
thousand parts per million, while high sulfur coal yields coal gas that bears sulfur compounds up to percent order. In this study, the inlet concentration of hydrogen sulfide was chosen between the typical sulfur concentration in a coal-derived gas and the achievable concentration with the zinc ferrite sorbent; the range of sulfur concentration enables us to simulate various sulfur concentrations along the stream of the gas when the sorbent was used in a fixed-bed reactor. The selected sulfur concentration range, 0-500 ppm, shown in Table 1 focuses on the thermodynamic aspect and experimental restriction. As previously reported,3 the sorbent containing zinc ferrite has the potential to achieve under 1 ppm level of hydrogen sulfide. The lower limit of the test condition was determined from this fact. Although the upper limit, 500 ppm, does not cover the upper range of the actual sulfur concentration, the limit value represents the condition that is thermodynamically equivalent with the actual coal gas environment. When the temperature and major gas composition are given as in Table 1, thermodynamically stable phases remain as ZnS and FeS irrespective of the sulfur concentration between 500 ppm and 2 vol %. This estimation ensures that we can cover various sulfidation products available in the actual coal gas by applying sulfur up to 500 ppm. The higher limit of the sulfur concentration was also restricted by the durability of the X-ray window made of nickel foil and the scanning speed applicable to each measurement. We have determined the sulfur concentration as shown in Table 1 from these various environments. The reason to select the range of hydrogen sulfide concentration is mentioned precisely in the Results and Discussion section. Determination of the Chemical Form of Iron Compounds in a Reacted Zinc Ferrite. Specimens of zinc ferrite and its reaction products were analyzed by 57Fe Mo¨ssbauer spectroscopy using 57Co radiation at room temperature. The velocity scale was calibrated by the inner four peaks of a sextet of pure iron foil obtained at room temperature. The spectrum was also referred to determine the isomer shift. Information obtained from the analysis of spectra determined chemical forms of substances containing iron in the reacted zinc ferrite. Mo¨ssbauer parameters are obtained by general methodology described elsewhere.5 Those Mo¨ssbauer parameters, such as the isomer shift, quadrupole splitting, and magnetic splitting, were applied in the usual method to estimate electronic and magnetic properties of the sample. Because the raw Mo¨ssbauer spectra may contain absorption from several components of iron compounds, the spectra were resolved to elemental spectra by assuming a Lorentz function. The nonlinear leastsquares method was applied to curve fitting of raw spectra. The fitting procedure separates the spectrum into individual curves that are used to estimate the
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simplified reaction schemes for sulfidation for each reduction product are summarized as in eq 2. Basic
ZnFe2O4 ZnO + Fe3O4 ZnO + FeO ZnO + Fe Zn + Fe
Figure 2. Phase diagram of a Zn-Fe-O system at 800 K.
valence of the iron element and to identify the chemical form of the iron. The isomer shift was mainly used to estimate the valence of iron. Electric quadrupole splitting was referenced to evaluate the divalent nature of the concerned iron component. We referred to the strength of the internal magnetic field for magnetism of the substance. It is assumed that the peak area of the elemental curve is directly proportional to the atomic ratio of the iron element. Thus, the procedure gives quantitative information of the produced iron compounds. Results and Discussion Thermodynamical Estimation of Stable Phases in the Reduction and Sulfidation of Zinc Ferrite. Reduction and sulfidation of zinc ferrite is complicated because zinc and iron in the double oxide have individual characteristics for reduction and sulfidation. To predict the behavior of the oxide in reducing gas containing sulfur, the phase diagram was prepared by thermodynamic calculation using data of Barin et al.6 Figure 2 shows the typical diagram expressed on the partial pressure of oxygen versus partial pressure of disulfur at 800 K. The diagram indicates possible reduction schemes for zinc ferrite. When the partial pressure of S2 is practically zero, the reduced zinc ferrite derives the series of reduction products according to the partial pressure of oxygen. Five related products are expected in the sequence of reduction, which include single oxides of zinc and iron and metallic iron and zinc. These schemes are summarized in eq 1 where the
{
ZnO f Zn ZnFe2O4 f Fe O f FeO f Fe 3 4
(1)
reducing agent and the stoichiometric coefficients between reactants and products are omitted. The reduction sequences are branched into two paths as zinc ferrite initially decomposed to zinc oxide and magnetite. Once the double oxide is separated into two single oxides, their boundaries between neighboring reduction products differ. Thus, the phase diagram is separated into four related regions including corresponding reduction products. Sulfidation products in the reducing environment are expressed in the diagram. Because only three sulfide phases are stable, which are ZnS, FeS, and FeS2,
} }
}
f ZnS + Fe3O4 f ZnS + FeO f ZnS + Fe
f ZnS + FeS f (2) ZnS + FeS2
trends of the sulfidation are summarized as prior formation of zinc sulfide followed by iron sulfides. Zinc sulfide emerges prior to iron sulfide, as shown in each reaction scheme expressed in eq 2. These sulfidation products of zinc and iron form five new regions on the diagram. The above discussion implies that the difference in sulfidation characteristics of zinc and iron may be shown in the effect of sulfur concentration on sulfidation products. Although the concentration of sulfur compounds in unpurified coal gas ranges depending on coal, thermodynamically expected sulfidation products of zinc ferrite may not alter. If a sufficient amount of the gas in the range is equilibrated with zinc ferrite, the stable phase consists of ZnS and FeS. This is the basis for the upper limit of the hydrogen sulfide concentration of the test condition described in the Experimental Section. Thus, the sulfidation condition in Table 1 is sufficient to evaluate the possible sulfidation reaction of the double oxide in a typical coal gas condition. A stronger affinity of zinc to oxygen and sulfur causes the existence of zinc oxide and zinc sulfide at the region of lower partial pressure of oxygen and disulfur, where iron solely exists as metal. These behaviors in stable phases among zinc and iron may affect the sulfidation reaction in a reducing environment at the temperature. The behaviors that are suggested from thermodynamic calculation are referenced to understand the results obtained in the in situ measurements of the reduction and sulfidation of the double oxide. Determination of the Crystal Phase and Chemical Composition of the Double Oxides. Analysis with powder XRD of the zinc iron double oxide determined that its crystal structure was a franklinite, spinel structure, with formula ZnFe2O4. The chemical analysis of the double oxide also supported its chemical formula by showing an atomic ratio of Zn:Fe ) 1:2. The oxidation state of the iron in the double oxide was analyzed by Mo¨ssbauer spectroscopy. An obtained spectrum at room temperature was single doublet, indicating paramagnetism of the oxide, which agrees with the nature of the normal spinel ZnFe2O4 as reported by Schiessl et al.4 They found that partial inversion causes a broadened magnetic transition of the spinel. The transition of the spinel is complete only at 30 K; the spinel does not exhibit antiferromagnetism at room temperature. Partial inversion, however, is the most likely cause of superposition of doublet peaks in the Mo¨ssbauer spectrum recorded at room temperature. The oxidation number that was determined with the isomer shift of the spectrum was trivalent as expected from the chemical form of zinc ferrite. These facts also agree with the property of zinc ferrite with the normal spinel structure reported in the literature.7,8 The analysis confirmed that the double oxide is pure zinc ferrite. Estimation of the Effect of Temperature and Sample Volume on the Diffraction Pattern during in Situ XRD Measurements. Optics of the XRD instrument was selected so that the X-ray beam irradi-
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1937
Figure 3. Error in the diffraction angle due to offset of the sample surface.
ates within the sample surface at any diffraction angle applied in each measurement. This confirms that the diffraction pattern attributes to the sample by eliminating the possibility of overlap with a broadened diffraction pattern from the cell material, quartz. High-temperature XRD measurements are not the issue of hardware limitation for the purpose of this work.9 As the sample temperature is attained up to 700 °C in the reductive condition, in situ measurements can cover the entire condition that is practically applicable to the sorbent containing the double oxides. Identification of reaction products of the multiple phase oxides, however, may have difficulty arising from substantial shifts or error in diffraction patterns. Shifts of the diffraction peak due to a temperature increase might be caused by lattice expansion whose effect is not uniform to the temperature and diffraction angle. Thermal expansion whose effect on the peak position is a function of the diffraction angle may be altered by the property of the material. The error due to thermal expansion of the crystal lattice was estimated from the density data of zinc iron oxide at higher temperature. Thus, the effect does not have a severe effect on the peak attribution by assuming a linear dependency of the lattice expansion on temperature variation. The other uncertainty of in situ measurements is caused by movement of the sample surface due to a specific volume change of the sample during reaction, which will bring an error in the diffraction angle. This is a typical problem in the reduction of oxides that may have a shrinking effect on the sample. Actual reduction occasionally caused exfoliation and a bend of the sample resulting in swelling of the surface. Figure 3 shows the error as a function of the diffraction angle for a specific offset value of the sample surface from its accurate position. The sample thickness on the quartz cell was always 0.2 mm. If we assume the perturbation of the surface position is within the thickness of the sample, which is empirically correct, its effect on the error of the peak position is around (0.06° at 2θ ) 90°. This value is acceptable to identify the reaction products of the double oxide. While the above discussion focuses only on this effect during reduction, it is true that an effect similar to that of the diffraction pattern is expected during sulfidation that may cause dilation of the sample. Determination of Reduction Products of Zinc Ferrite by in Situ XRD Measurements. Determina-
Figure 4. In situ XRD profiles of pure zinc ferrite powder obtained in temperature-programmed reduction up to 650 °C.
tion of the temperature at which further reduction tests were performed was carried out by obtaining temperature program reduction of the fresh zinc ferrite. The target temperature of zinc ferrite operation of previous works varies in a relatively wide range. Our study concerning a zinc ferrite-silica composite powder was investigated at mainly 450 °C and below.3 U.S. DOE investigated zinc ferrite aiming at an operation temperature of 650 °C, at which a molten carbonate fuel cell is operated. Thus, we considered our initial consideration on reduction characteristics of zinc ferrite in the temperature range between 250 and 650 °C. The pure zinc ferrite, however, was not reduced significantly up to 450 °C; the diffraction pattern remained unchanged at this temperature in the gas mixture with hydrogen up to 20 vol %. Typical results of temperature-programmed reduction in condition A of Table 1 are shown in the Figure 4. The only alteration observed in the diffraction pattern at 450 °C was the peak position shifts due to thermal expansion of the crystal lattice. Magnetite formation, however, cannot be excluded from the maintained diffraction pattern. Zinc ferrite and magnetite, which are both spinel structures, exhibit very close diffraction patterns for powder XRD measurements. When the temperature was increased to 550 °C, zincite, ZnO, was obviously identified as a reduction product that is distinguishable from franklinite by the XRD pattern. The remaining part of zinc ferrite from which the zincite phase was separated should exist as iron-rich new phases. Though any new phase containing iron was not revealed at this temperature, diffraction patterns of magnetite and other spinel compounds might be hidden behind the diffraction peaks of zinc ferrite. Similarity of the diffraction pattern among such possible phases makes it difficult to identify each phase from powder XRD analysis. Magnetism of those phases, however, distinguishes magnetite from zinc ferrite, which will be discussed in the following sections. At 650 °C, diffraction from the zinc oxide phase is strengthened more than the lower temperatures. The iron oxide phase was finally observed as wustite, FeO, which is an appropriate reduction product at the temperature. Further reduction tests were performed at 550 °C to determine the effect of the partial pressure of hydrogen on reduction products. The gas composition was deter-
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Figure 5. Effect of the hydrogen partial pressure on reduction products determined by in situ XRD at 550 °C.
mined so that we can easily control the partial pressure of oxygen of the reaction gas; a hydrogen, steam, and nitrogen gas mixture was mainly used. The ratio of hydrogen to steam was varied to attain specific reductive conditions by changing the partial pressure of hydrogen against steam whose concentration was maintained to 5 vol % throughout the reduction tests. Simulated coal gas composition was also used to examine possible variance in the reduction products. The coal gas condition simulated the gas composition obtained in a dry coal fed, air-blown gasifier. Figure 5 shows diffraction patterns obtained for reduction conditions A and B, respectively. The scanning speed of 2θ was set to 1.2°/min to sweep between 10 and 82° for just 1 h. Because a succeeding scan for the next measurement was performed continuously, each peak on the diffraction pattern was measured repeatedly at 1-h intervals. The finite scanning speed causes a time lag between the peaks at diffraction angles on each scan. Because the diffraction angle between 29 and 44° covers distinctive peaks to identify the product phases, the maximum time lag that emerged in the range of angle is only 12.5 min. The delay is short enough to identify emerging phases during a reaction time of 5 h. When the hydrogen concentration was 8 vol %, zincite was an apparent product identified by XRD. Although the figure shows initial 5-h results, an extended reaction time up to 20 h did not produce a distinguishable iron oxide phase. The peak intensity of the zincite phase gradually increased within 20 h. When the hydrogen concentration increased from 8 to 20 vol %, wustite was detected as the apparent iron oxide product within 5 h. The variation of the diffraction peak intensities that are identified as zincite, wustite, and the double oxide is summarized in Figure 6. The ordinate axis indicates the intensity of each peak normalized by the initial intensity at the peak position. The initial intensity reflects the summation of background and incremental counts due to progress of the reduction during the X-ray scan reaching the peak position. This quantitative analysis clearly shows the difference in wustite formation due to the gas condition. It is noticeable that the diffraction peak of franklinite (3 1 1) decayed slowly in condition A as well as in condition B. We cannot attribute the decay to merely decomposition of franklinite, because the peak
position coincides with magnetite (3 1 1) diffraction. The decay, however, indicates obvious transformation of the double oxide. Reduction products of the double oxide in a simulated coal gas condition (a 20% CO, 5% CO2, 8% H2, 5% H2O, and N2 balance gas mixture) were also determined by in situ XRD. Trends of change in the diffraction peak intensity that correspond to FeO and related products are displayed in Figure 7. Though the increment in the peak area of zinc oxide was the same degree as condition A, a small increment was observed for wustite peaks. The additional reductant, carbon monoxide, probably acted to produce wustite. These results show that pure franklinite is reduced to component oxides, zincite and oxide containing the iron element, in reductive gas at 550 °C. The rate of wustite formation at 550 °C seems to depend on the gas composition. The gas condition B produced the FeO phase rather rapidly, while the gas condition A did not derive the FeO phase. Although the wustite phase was observed in the simulated coal gas condition at 550 °C, the rate of wustite formation was slower than the condition B. The ZnO phase, however, was observed in all of the conditions described above, which implies the existence of corresponding oxide-containing iron, such as Fe3O4. Sulfidation in the reductive environment was conducted at this temperature to consider the reactivity of zinc ferrite in a reductive environment. Comparison of zincite and wustite was our particular concern as the latter oxide is reported to have inferior sulfidation kinetics.10 Thus, sulfidation was conducted at 550 °C after prereduction at the same temperature throughout this section. Sulfidation Products of Zinc Ferrite in a Reductive Environment. Sulfidation products of the reduced zinc ferrite were determined by applying reactant gas with various concentrations of hydrogen sulfide. A relatively higher concentration of hydrogen sulfide (500 ppm) was introduced to the reduced products to grasp final sulfides obtained by complete sulfidation at the condition. Thermodynamical calculation indicates that the applied concentration is sufficiently high to produce sulfides of both zinc and iron. Phase equilibrium shows that the sulfidation products at this condition are FeS and ZnS, respectively. Zinc sulfide exists in many polytypic forms such as a zinc blend structure that exhibits diffraction pattern similar to that of the cubic iron sulfide. Thus, the identification of the sulfides is rather complicated to distinguish the compounds of zinc from that of iron. Figure 8 shows the in situ observation of variation of the diffraction pattern during sulfidation of pure zinc ferrite. The scanning speed of 2θ was also set to 1.2°/min to sweep between 10 and 100° in 75 min. The other methodologies in repeated measurements were the same as the reduction tests. Thus, time intervals of each peak measurement are 75 min. The front-most diffraction pattern expresses the products that were obtained during heating to 550 °C in the reaction gas that does not contain hydrogen sulfide. While FeO and ZnO were observed as reduction products, a substantial amount of franklinite remained unreduced as shown in the pattern. Hydrogen sulfide was introduced into the reaction gas with a concentration of 500 ppm when the first measurement came closer to 2θ ) 100°. Measurements of XRD were repeated continuously during the sulfidation reaction. Because the second diffraction pattern was measured at an
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1939
Figure 6. Trends of the diffraction intensity of the double oxide and its reduction products at 550 °C.
Figure 7. Reduction products of the double oxide in a simulated coal gas at 550 °C.
Figure 8. Variation of the XRD pattern of pure zinc ferrite during sulfidation in a reductive environment with a higher sulfur concentration (*1, wurtzite; *2, zinc blend).
earlier period in sulfidation condition, the peaks attributed to the reduction product phases were strengthened. This indicates that sulfidation of the reduction
products is still predominant reaction at the time when a sufficient amount of zinc ferrite remained on the surface of the sample. One zinc sulfide phase, wurtzite, was at least identified from the peaks appearing in the third and further patterns. The peak assigned to wurtzite (0 0 2) strengthened more rapidly than the other peaks attributed to the phase. This indicates superposition of the (1 1 1) diffraction from cubic sulfides; both iron and zinc are possible to form the cubic sulfides according to the peak. Another peak that was attributed to the (2 0 0) diffraction from the sulfide phase was recognized in the fifth measurement and afterward. It is natural that we assign this cubic sulfide to the iron sulfide because peaks attributed to wustite gradually decreased. We cannot still deny the coexistence of the cubic zinc sulfide from these facts because its major peak (1 1 1) may also stand at superposition of wurtzite (0 0 2). Exclusive assignments of the sulfides including the cubic phase will be discussed in the following section describing Mo¨ssbauer spectroscopy analysis of iron sulfide. After the third measurement, the peaks of the reduced phases as well as ones attributed to franklinite were weakened during progress of the sulfidation reaction. It is noticeable that the peaks specific to the zinc oxide phase almost disappeared in the third cycle of the measurement; the zinc oxide phase might have reacted with hydrogen sulfide within the third measurement. It can be seen from the figure that the wustite phase remained much longer than zinc oxide. This supports that wustite is an inferior sulfur sorbent with respect to the other oxides as reported in the literature.9 Though peaks assigned to franklinite were minor, the spinel phase still existed when it arrived at the tenth measurement. Because ZnO and FeO are invisible in terms of XRD measurement, reduction of zinc ferrite is considered to be the predominant reaction at this stage of reduction. These observations conclude that sulfidation products of franklinite are derived from the intermediate oxides, zincite and wustite, that are reduction products of zinc ferrite. Another possibility of the production path of zinc sulfide, however, is drawn from the phase diagram shown in Figure 2, which shows direct reaction of franklinite to form zinc sulfide and magnetite. Although this alternate path cannot be denied from the results, reduction of franklinite and succeeding sulfidation of the products
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Figure 9. Variation of the XRD pattern of pure zinc ferrite during sulfidation in a reductive environment with a lower sulfur concentration (*1, wurtzite; *2, zinc blend).
Figure 10. Variation of the intensity of the diffraction from zinc sulfide (0 0 2) and (1 0 0).
occur prior to other possible reaction paths as indicated in Figure 8. The sulfidation test performed in the same gas condition at 450 °C, at which reduction was not observed, was negative for direct production of both sulfides, ZnS and FeS. Because two intermediate oxides were observed in the prereduced franklinite, a difference in their sulfidation reactivity might cause sequential sulfide formation. Thus, the next issue to be determined was the sulfidation profile at a lower concentration of the sulfur compound. Figure 9 shows variation of the diffraction pattern of zinc ferrite during sulfidation of the oxide performed at a hydrogen sulfide concentration of 80 ppm. The conditions for XRD measurements were adjusted to the previous sulfidation test that enabled us to compare production rate of the sulfide phase with the intensities of corresponding peaks in each result. Time intervals of measurements for each peak are 60 min. Reduction products that are identical with those of the previous test, zincite and wustite, were identified from the diffraction pattern. The most significant difference from the high sulfur concentration test was unrecognized (2 0 0) diffraction of the cubic sulfide phases throughout the measurement. Intensities of peaks attributed to wustite remained constant during sulfidation measurements. Because the sulfidation stands at the phase boundary between FeO and FeS in the phase diagram, the condition enhances zinc sulfide production against iron sulfide production at a given reduction environment. The test results in this section indicate that the zincite phase was more reasonably sulfurized in the condition with a lower H2S concentration. Though the stable phase of the iron component in terms of chemical equilibrium was not clear, wustite seemed to remain unreacted in the condition. The above test results for high and low sulfur concentrations are summarized in Figure 10, which is a quantitative comparison of the sulfidation reaction of the double oxide with respect to the sulfur concentration. Because the intensity from the zincite (1 0 0) peak is increasing in both conditions, formation of wurtzite might be indisputable. The intensity ratios of (0 0 2) and (1 0 0) reflections of wurtzite are displayed to distinguish the presence of other sulfide phases on the emerging diffraction peaks. Because the ratio of (0 0 2) diffraction to (1 0 0) diffraction increased in the condition of higher sulfur concentration, formation of the cubic iron sulfide or zinc blend, ZnS,
whose strongest diffraction is close to the (0 0 2) reflection of wurtzite, is expected. The increasing ratio, however, cannot distinguish the presence of the iron sulfide and zinc blend. Thus, the formation of the cubic iron sulfide should be confirmed with further information from Mo¨ssbauer spectroscopy. One additional information of the intensity ratio of (0 0 2) to (1 0 0) is that the ratio is reverse as the pure wurtzite phase; an indexed database of XRD indicates that the sulfide exhibits a (0 0 2) peak intensity of 84% of (1 0 0). Coexistence of the zinc blend is suggested if the peaks were not attributed to iron sulfide. Estimation of the Reduction State of Iron in the Reduced Zinc Ferrite. Reduction of zinc ferrite in a relatively strong environment, condition B in Table 1, yields wustite and zinc oxide, which is confirmed by the in situ XRD. The other condition A, however, yields zinc oxide but wustite. This implies the existence of an ironrich phase that exhibits a diffraction pattern similar to zinc ferrite. Reduction states of iron in the reduced zinc ferrite were determined by Mo¨ssbauer spectroscopy of the sample obtained in each in situ XRD measurement. The results of Mo¨ssbauer spectroscopy identifying substances containing iron are summarized in Table 2. The fresh and unreacted zinc ferrite exhibit a sole doublet in the Mo¨ssbauer spectrum. The doublet peak in the Mo¨ssbauer spectrum is attributed to paramagnetism of the double oxide, which was able to distinguish the double oxide from its reduction products that exhibited sextet adsorption spectra. Results obtained for the sample reduced for 5 h in condition B presume the existence of wustite from paramagnetism and a relatively higher value of the isomer shift. The value of a quadrupole split also agrees with the value for the compound on the database. The sextets 2c and 2d suggest the existence of a considerable amount of iron compounds with magnetic properties that are not on the available database. The in situ XRD of the reduction exhibited a cubic structure and a spinel structure corresponding to wustite and zinc ferrite. The identification of the magnetic phase was well-explained by the literature on zinc-bearing magnetite. Because the emerged zincite phase was fairly confirmed by in situ XRD, the magnetic phase is expected to be derived by dislocation of the zinc portion from zinc ferrite. If zinc atoms at the tetrahedral sites were partially separated from the spinel structure, iron elements would substitute the vacant sites while partly remained zinc would
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1941 Table 2. Reduction Products of the Double Oxide Determined by Mo1 ssbauer Spectroscopya condition
no.
IS (mm/s)
QS (mm/s)
H (kOe)
% Fe
identified state of iron
unreacted reduction, B, 5 h
1a 2a 2b 2c 2d 3a 3b 3c 3d 3e 4a 4b 4c 4d 4e 5a 5b 5c
+0.35 +0.35 +0.86 +0.27 +0.63 +0.37 +0.67 +0.17 +0.57 +0.57 +0.37 +0.67 +0.17 +0.57 +0.57 +0.35 +0.30 +0.50
0.36 0.36 0.86 0.03 0.05 0.32 0.00 0.03 0.05 0.24 0.32 0.00 0.03 0.05 0.24 0.45 0.02 0.02
0 0 0 486 438 0 463 479 440 413 0 463 479 440 413 0 484* 437*
100 14 44 12 30 12 4 22 34 28 5 13 25 32 25 27 19 54
ZnFe2O4 ZnFe2O4 FeO ZnxFe3-xO4 [A] ZnxFe3-xO4 [B] ZnFe2O4 Fe3O4 [B] ZnxFe3-xO4 [A], Fe3O4 [A] ZnxFe3-xO4 [B] ZnxFe3-xO4 [unknown] ZnFe2O4 Fe3O4 [B] ZnxFe3-xO4 [A], Fe3O4 [A] ZnxFe3-xO4 [B] ZnxFe3-xO4 [unknown] Fe3+ [B] ZnxFe3-xO4 [A], X ) 0.6 ZnxFe3-xO4 [B], X ) 0.6
reduction, A, 5 h
reduction, A, 20 h
Zn-bearing magnetite11
a IS: isomer shift. QS: quadrupole splitting. H: hyperfine field. [A]: tetrahedral site. [B]: octahedral site. *: read from figure in the literature.11
inhibit the complete substitution of A sites with iron. This results in production of zinc-bearing magnetite rather than pure magnetite. This argument corresponds well to the Mo¨ssbauer spectroscopic study11 on zincsubstituted magnetite by Kanzaki et al. They prepared magnetite, zinc ferrite, and substituted magnetite containing various portions of zinc from an aqueous suspension. They found that a doublet appears in addition to the two sextets that are typical of magnetite with a reverse spinel structure when the zinc concentration in the sample with a formula ZnxFe3-xO4 was increased from X ) 0.0 to 1.0. The sample showed a sole doublet at X ) 1.0, where the composition reaches that of the zinc ferrite formula, while the two sextets disappeared. We assumed that the variation in the Mo¨ssbauer spectrum may occur in our sample in the opposite direction; the Mo¨ssbauer spectrum of our zinc ferrite comes closer to that of magnetite as zinc is dislocated in the progress of reduction. The literature also mentioned that zinc addition decreases the isomer shift of the sextet for the B site (octahedral site) from 0.67 mm/s at X ) 0.0, which is the typical value shown for magnetite, to 0.50 mm/s at X ) 0.6. The Mo¨ssbauer parameter for zinc-bearing magnetite for Zn0.6Fe2.4O4 is cited in Table 2 for comparison with our results. There is remarkable similarity between the variation in the isomer shift of the sextet for the zinc-bearing magnetite (5b and 5c) and the observed isomer shift of the sextet for components 2c and 2d of the reduced magnetite as shown in Table 2. The slight difference in isomer shifts between 3d and 5c indicates that our reduced sample contains a smaller amount of zinc because of the progress of ZnO formation. This fact indicates that our observation on the sextet arises from the zinc-dislocated franklinite. Magnetic properties of zinc-dislocated franklinite are identical with those of the zinc-substituted magnetite in the literature, while derivation of the substance in terms of the zinc content is in the opposite direction. Reduction of the double oxide in a relatively week environment, condition A in Table 1, puts iron elements in various oxidation states. The Mo¨ssbauer parameter was determined for the sample reduced for 5 and 20 h to give information on reaction schemes. Zinc-dislocated franklinite was also produced in both analyzed samples as shown in the table (components 3c, 3d, 4c, and 4d). The isomer shift of components 3d and 4d was +0.57
mm/s, whose value was even smaller than the value observed for component 2d at condition B. This indicates that more zinc elements remained in the zinc-substituted magnetite. This is reasonably explained by the smaller amount of zincite production in condition A, which is concluded by the weaker peak intensity of zincite during in situ XRD measurements at the condition as shown in Figure 5. The most significant difference from the results in condition B was that the paramagnetic oxide, FeO, was not observed in the sample reduced either for 5 h or for 20 h. In contrast, magnetite, Fe3O4, was observed in this condition. While the condition falls in a thermodynamically stable region of FeO in the phase diagram, the weaker reduction may retard the production of FeO. The proportion attributed to the B site of magnetite (3b) was only 4% at 5 h, which indicates zinc dislocation is significantly slow at the given condition. Its content, however, increased 3-fold at 20 h. Component 3c was attributed to the A site of magnetite as well as the zinc-substituted magnetite. Component 4c exhibited a slight increase that includes a positive contribution due to production of magnetite and a negative effect of consumption of zinc-dislocated franklinite. The gross increase of component 4c was thus smaller than the value expected for the A site of magnetite from the increase of iron of component 4b. As a result, the portion of franklinite was reduced and replaced with magnetite at a reaction time of 20 h. The isomer shift values for 3d and 4d were the same within experimental error, while the amount of magnetite and zinc oxide increased as described above. This implies that the zinc content of zinc-dislocated franklinite is determined by the reduction condition. Further investigation is required to verify this hypothesis. These facts indicate that magnetite is the apparent final product of franklinite reduction in the condition. Mo¨ssbauer spectra also showed other sextets for 3e and 4e at both reaction times. Their Mo¨ssbauer parameters do not completely match those of zinc-bearing magnetite. Because their isomer shifts indicate their property of mixed valence, their origin is probably in iron of a similar spinel structure. The major points obtained in this section are briefly summarized in the following outline. The production of wustite, FeO, in reduction condition B as well as the existence of zinc-dislocated franklinite that was not identified by the in situ XRD measurements was
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Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000
Table 3. Sulfidation Products of Reduced Double Oxide Determined by Mo1 ssbauer Spectroscopya condition
no.
IS (mm/s)
QS (mm/s)
H (kOe)
% Fe
identified state of iron
unreacted sulfidation, B, H2S, 500 ppm
1a 2a 2b 2c 2d 2e 3a 3b 3c 3d
+0.35 +0.40 +0.72 +0.67 +0.27 +0.63 +0.86 +0.67 +0.27 +0.63
0.36 0.66 0.68 0.00 0.03 0.05 0.86 0.00 0.03 0.05
0 0 0 463 486 438 0 463 486 438
100 53 23 11 10 3 4 42 30 24
ZnFe2O4 unknown iron sulfide ZnxFe1-xS, sphalerite Fe3O4 [B] ZnxFe3-xO4 [A], Fe3O4 [A] ZnxFe3-xO4 [B] FeO Fe3O4 [B] ZnxFe3-xO4 [A], Fe3O4 [A] ZnxFe3-xO4 [B]
sulfidation, C, H2S, 80 ppm
a
IS: isomer shift. QS: quadrupole splitting. H: hyperfine field. [A]: tetrahedral site. [B]: octahedral site.
confirmed by Mo¨ssbauer spectroscopy. Major reduction products in condition A were similar to those of zincdislocated franklinite retaining a larger amount of zinc, while a minor portion was identified as magnetite. Estimation of the Variation of the Chemical Form of Iron during Sulfidation. When zinc ferrite was subjected to sulfidation at conditions shown in Figure 8, the product clearly confirmed with in situ XRD was wurtzite, ZnS. Though the presence of iron sulfide was indicated for the high sulfur condition from the peak area analysis, we could not confirm that the peaks were merely attributed to iron sulfide. The first objective of Mo¨ssbauer spectroscopy analysis of the sulfurized sample is to identify the chemical form of iron sulfide. The sulfurized sample was subjected to analysis by Mo¨ssbauer spectroscopy. Obtained Mo¨ssbauer parameters are summarized in Table 3. The parameter clearly shows the existence of iron sulfide in the sample suffered in the hydrogen sulfide concentration of 500 ppm. Though the doublet component 2a exhibited a closer Mo¨ssbauer parameter of pyrite, the sulfide was not identified from the in situ XRD results. The other doublet component 2b is identical with the cubic sulfide observed in situ XRD; the phase contributed to the increasing peak ratio of (0 0 2) over (1 0 0). Although the major gas composition was equivalent to the reduction condition where FeO was formed, identified iron oxides include zinc-dislocated franklinite and magnetite. In fact, the sulfidation test shown in Figure 8 clearly exhibited the formation of wustite during an earlier period of sulfidation. Thus, we concluded that sulfidation of FeO proceeded reasonably in the given condition. This fact seems to contradict the report10 that asserted a detrimental effect of wustite formation on the sulfidation of zinc ferrite. The quantitative relation among reactant and product, however, is affected by combinational kinetics of the reduction of zinc ferrite and the sulfidation of the reduction products. Therefore, it exceeds the objective of this study to describe the sulfidation kinetics of magnetite and zincite obtained in the reduction of zinc ferrite. The next objective is to examine the formation of iron sulfide under the low sulfur concentration. As we discussed in Figure 9, sulfidation of zincite proceeded predominantly under low sulfur concentration. The test condition for the figure, however, produced a significant amount of wustite. We tried to confirm the effect of low sulfur concentration in the milder reduction condition attained by condition C in Table 1. Because the major gas composition of the condition is the same as that applied to the reduction test of Figure 7, the reduction product may contain a substantial amount of FeO and magnetite. The Mo¨ssbauer parameters obtained from the sulfidation test, however, exhibit no evidence of iron
sulfide as shown in Table 3. Produced iron compounds remained in wustite, magnetite, and zinc-dislocated franklinite as expected. From this result, one can plausibly attribute the emerging peaks in 80 ppm of hydrogen sulfide to the zinc sulfide phases, wurtzite and zinc blend. Thus, we can conclude that the zinc ferrite produces zinc sulfides identified as wurtzite and zinc blend when the H2S concentration was as low as 80 ppm while increasing the H2S concentration up to 500 ppm produces an additional phase of iron sulfide. Conclusions Pure zinc ferrite that was identified as franklinite with the chemical formula of ZnFe2O4 was subjected to the series of reduction and sulfidation tests performed at mainly 550 °C in an in situ XRD instrument. Reaction products were determined by the analysis of in situ powder XRD measurements and complementary information on the iron compounds by 57Fe Mo¨ssbauer spectroscopy analysis of the products. Zincite, ZnO, was clearly shown by the in situ XRD observation. Mo¨ssbauer spectra of reduction products indicated that a substantial amount of iron-containing phases remained as zinc-dislocated franklinite whose magnetic properties are identical with those of zinc-bearing magnetite. Single oxides of iron such as wustite and magnetite depending on the reduction condition were also identified by Mo¨ssbauer spectroscopy. Thus, reduction products of zinc ferrite were identified as a mixture of zincdislocated franklinite, zincite, and a series of iron oxides depending on the strength of the reducing environment. Sulfidation of zinc ferrite in a reductive environment yields wurtzite, zinc blend, and iron sulfides, when the concentration of hydrogen sulfide was sufficiently high. Zinc sulfides, however, were predominantly produced in a sufficiently low sulfur concentration, typically less than 80 ppm. The iron component at the low sulfur condition remained in the reduced chemical form or as pure magnetite. These findings revealed that a superior performance of zinc ferrite during sulfur removal at a lower sulfur concentration is maintained with zincite in the reduced zinc ferrite. Literature Cited (1) Sasaoka, E.; Iwamoto, Y.; Hirano, S.; Uddin, M. A.; Sakata, Y. Soot Formation over Zinc Ferrite High-Temperature Desulfurization Sorbent. Energy Fuels 1995, 9, 344. (2) Shirai, H.; Kobayashi, M.; Nunokawa, M. Characteristics of H2S Removal of Mixed-Oxide Sorbents Containing Fe and Zn at High Temperature. Enerugi Gakkaisi 1998, 77, 1100. (3) Kobayashi, M.; Shirai, H.; Nunokawa, M. Investigation on Desulfurization Performance and Pore Structure of Sorbents Containing Zinc Ferrite. Energy Fuels 1997, 11, 887-896.
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1943 (4) Schiessl, W.; et al. Magnetic Properties of the ZnFe2O4 Spinel. Phys. Rev. B 1996, 53, 9143-9152. (5) Long, G. J.; Grandjean, F. Mossbauer Spectroscopy Applied to Inorganic Chemistry; Plenum Press: New York, 1984; Vols. 1-3. (6) Barin, I.; Sauert, F.; Schultze-Rhonhof, E.; Sheng, W. S. Thermochemical Data of Pure Substances; VCH Verlagsgesellschaft mbH: Weinheim, Germany, 1993; Parts I and II. (7) Battle, J.; Clark, T.; Evans, B. J. 57Fe Mo¨ssbauer Spectroscopy of Zinc Ferrite Prepared by a Variety of Synthetic Methods. J. Phys. IV: Fr. 1997, 7, 257. (8) Clark, T. M.; Evans, B. J. Enhanced Magnetization and Cation Distributions in Nanocrystalline ZnFe2O4: A Conversion Electron Mo¨ssbauer Spectroscopic Investigation. IEEE Trans. Magn. 1997, 33, 3745.
(9) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedure for Polycrystalline and Amorphous Materials; Wiley-Interscience: New York, 1974. (10) Underkoffler, V. S. Summary and Assessment of METC Zinc Ferrite Hot Gas Desulfurization Test Program, Final Report Volume I; U.S. DOE: Morgantown, WV, 1986. (11) Kanzaki, T.; Kitayama, K.; Shimokoshi, K. Mo¨ssbauer Spectroscopy Studies on Zn-Bearing Ferrite. J. Am. Ceram. Soc. 1993, 76, 1491.
Received for review June 22, 1999 Revised manuscript received November 29, 1999 Accepted February 22, 2000 IE990456T
