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Phase compatibility in the system CaO-SiO-AlO-SOFeO and the effect of partial pressure on phase stability 2
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Isabel Galan, Theodore Hanein, Ammar Elhoweris, Marcus N. Bannerman, and Fredrik Paul Glasser Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03470 • Publication Date (Web): 01 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017
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Phase compatibility in the system CaO-SiO2-Al2O3-SO3Fe2O3 and the effect of partial pressure on phase stability Isabel Galana, †,*, Theodore Haneinb,‡ , Ammar Elhowerisa, Marcus Bannermanb, Fredrik P. Glassera a
Department of Chemistry, University of Aberdeen, AB24 3UE, United Kingdom
b
School of Engineering, University of Aberdeen, AB24 3UE, United Kingdom
* corresponding author:
[email protected],
[email protected] †Current affiliation: Institute of Applied Geosciences, Graz University of Technology, Rechbauerstrasse 12, 8010 Graz, Austria ‡Current affiliation: Department of Materials Science and Engineering, University of Sheffield, S1 3JD, United Kingdom
ABSTRACT: The compatibility of phases within the CaO-SiO2-Al2O3-SO3-Fe2O3 system is revisited. The influence of the SO2 partial pressure on the formation of phases and the stability of phase assemblages is addressed by means of new experimental methods (performed under controlled atmospheres) and thermodynamic calculations. Existing data from the literature are compiled and compared with the results obtained from this work. The results obtained are linked to real clinker formulations and practical implications are discussed.
pheric composition adds a new dimension to controlling clinker composition.
INTRODUCTION. Phase relations in the system CaO-SiO2-Al2O3-Fe2O3 (C-S-A-F) are of fundamental significance to a range of industrial processes including Portland cement manufacture and have been extensively investigated 1,2. Four component systems are relatively difficult to study and it has required a century of work to determine experimentally the phases in the C-S-A-F system and accumulate sufficient thermodynamic data to permit its complete description. The effects of small additions of other components, e.g. sulfate, has also largely been determined; however, calcium sulfoaluminate (C$A) clinkers, which have numerous advantages over Portland cement clinkers 36 , contain large amounts of SO3 and at least three additional solid phases must be considered: CaSO4 (anhydrite), C5S2$ (ternesite) and C4A3$ (ye’elimite). Understanding the stability of these new solid phases is complicated by the persistence of metastable phases, the formation of solid solutions, and polymorphic transitions. Melting relations are also complicated by the widespread immiscibility of silicate-rich and sulfate-rich melts. As these phases exert a significant SO3 vapor pressure at clinkering temperatures, the atmos-
In the title study, the phase relations in the five component C-S-A-$-F system are explored at subsolidus temperatures, typically ≈1200-1250°C. Sintering is sufficiently rapid to obtain complete reaction in reasonable times (hours to weeks); however, it is found that gas-solid reactions play a greater role in clinkering than hitherto appreciated. Vapor pressure data for the main phases relevant to C$A cements were previously reported by Choi and Glasser 7 and were recently re-assessed with new data and recalibrated by Hanein et al 8. Figure 1 presents the latest dataset graphically from which it can be seen that the vapor pressure curves of S-containing phases relevant as functions of 1/T differ considerably in slope. The slope is strongly linked via the ClausiusClapeyron equation to the latent heat of decomposition (and to a lesser extent the volume change), thus the decomposition reactions leading to a release of SO3 must be understood to allow a thermodynamic interpretation of this data.
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Figure 1. Vapor pressures of various sulfur species including anhydrite, ye'elimite and ternesite. This graph is digitized and 7 corrected from ; the correction is done using a single multiplicative scaling factor of 0.733863. Note that the SO2 partial pressure is self-generated so that O2 partial pressure is also constant and half that of SO2. Assumed decomposition reactions: C4A3$ -> 1/5C12A7 + 8/5CA + SO2 + 1/2O2 and C5S2$ -> 2C2S + C +SO2 + 1/2O2.
As all of these phases exert a partial pressure of SO3 they must decompose incongruently. CaSO4 dissociates at high temperatures according to the reaction:
CaSO 4 → CaO + SO 2 +
1 O2 2
experimental proof. The knowledge that reaction (1) is rapidly reversible means that, depending on the choice of conditions, transfer of sulfur from the vapor to the solid can also occur. Thus, while loss of sulfur may handicap laboratory experiments, under industrial conditions where sulfur-rich fuels are burnt for thermal energy, sulfur gain by the solid phase may also occur. Thus the assumption of constant solid elemental composition in the course of experiments may be unwarranted.
(1)
Note that the stability of CaSO4 under atmospheric pressure is a function of temperature and the fugacity of two gases: SO2 and O2. This arises as SO O is more stable than SO3 at temperatures above ≈800°C and thus at high temperatures the solid/gas equilibria involve a re-oxidation of the main gas species, SO2, effectively to SO 3. This applies also to the decomposition reactions of ternesite and ye’elimite; however, the stoichiometry of these reactions is not well understood.
Cement kilns are normally operated under oxidizing conditions but oxygen partial pressures are typically much lower than in the standard atmosphere as oxygen is consumed in the course of combustion: in the title study two oxygen partial pressures are used for calculations: 21% and 1% oxygen. The low oxygen concentration is a close approximation to the oxygen partial pressures found in conventional cement kilns 9
A number of previous studies of the C-S-A-$-F system have treated it as involatile but without sufficient
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while the higher concentration facilitates comparison with laboratory work.
LITERATURE REVIEW. Phase relations in several of the limiting systems, such as CaO -SiO2-Al2O3 and CaO-SiO2-Al2O3-Fe2O 3, are well known; however, less is known about the five component system including SO3 and about the stability and persistence of phases like anhydrite, ternesite and ye’elimite. Previous studies have generally applied a closed-system model and determined phase relations using an experimental approach 10-15.
The implications of the points outlined above to experimental work and the production on an industrial scale of calcium sulfoaluminate cement clinkers is that the mineralogy is influenced by temperature, chemistry, and completeness of reaction as well as by the partial pressures of SO2 and O2. In some conditions, the bulk composition of the clinker batch may remain essentially constant in which case treating the system as involatile may be appropriate but in a wide range of conditions this will not be the case. The purpose of the title paper is to explore and delineate the sulfur regimes and to explain how the gas atmosphere affects the solid phase assemblages achieved. Isoplethal reactions are assumed at first before the impacts of changing gas conditions are explored. This new understanding is then used to predict, control, and enhance the efficiency of clinkering.
The main findings and the discrepancies of these previous works are summarized below:
C-$-A system: the compatibilities between C4A3$ and the aluminates CA, C3A and C12A7 were confirmed by Pliego-Cuervo and Glasser 14 and Sahu 12. Strigac and Majling 10 complemented this data by demonstrating the compatibility of C4A3$ with CA2, CA6 and A, as well as with C$ and C. These data are recast in Figure 2.
$
C$
C4A3$ C
A C3A
C12A7 CA
CA2 CA6
Figure 2. Isothermal subsolidus C-$-A diagram (weight %) according to literature below ~1250 °C and self-generating SO2 partial pressures at equilibrium.
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. The diagram is valid for temperatures
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Kapralic 13 conducted studies at temperatures ranging between 950 and 1150 °C in air so that “the reaction would occur in a reasonable time (1-6 days) without significant sulfur loss by volatilization (defined as >1%w/w SO3)”. These studies all lead to the construction given in Figure 2.
Sahu’s results on the compatibility of the phase pairs are based on experiments where tablets, previously encapsulated in small platinum crucibles with a welded cap, “were heated at different temperature intervals for various durations”. In some cases, where the identification of a binary relation between two phases was difficult from the experimental results, the reported results were based on “theoretical prediction”, but this approach was not described in their paper. In Strigac’s work, “the encapsulated samples were heated in an electric furnace in the temperature range of 1000-1200 °C for 5-50 hours depending upon composition of the mixtures”. 12
C-S-$ system: the role of ternesite, C5S2$, in this system is a cause of some dispute in the literature. Sahu 12 have stated that C5S2$ is not stable above 1150 °C and consequently not stable in reactions made close to the solidus. As a consequence, C2S and C$ were suggested to be a compatible pair; however, Pliego-Cuervo 14, Strigac 11, and Kapralic et al. 13, who experimented at temperatures from 950 to 1200 °C, have reported that C5S2$ is stable and that C2S and C$ are not compatible. Neither Pliego-Cuervo 14 nor Kapralic 13 specifically refer to the compatibility of C5S2$ with CS or C3S2. Furthermore, Gutt 15 and Strigac 11 report C5S2$ to be compatible with C3S2 but not with CS. In Figure 3, the resulting phase compatibility below the decomposition temperature of ternesite is given. C3S is considered to be unstable with respect to lime and dicalcium silicate stable at T
