Energy & Fuels 2003, 17, 1591-1594
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Improvement of the Binary Phase Diagram Na2CO3-Na2S Mathias Ra˚berg,* Dan Bostro¨m, Anders Nordin, Erik Rose´n, and Bjo¨rn Warnqvist Energy Technology and Thermal Process Chemistry, Umea˚ University, SE-901 87 Umea˚ , Sweden Received July 3, 2003. Revised Manuscript Received September 17, 2003
Gasification of black liquor is an attractive alternative to the traditional recovery boiler. However, in process modeling of gasification, thermodynamic data for the key components are quite uncertain, which will reduce the reliability of the modeling of the chemical processes in a gasifier. The objective of this work was to experimentally re-determine and improve data on the binary phase diagram Na2CO3-Na2S, especially on the Na2CO3 side of the system, which is the region of interest concerning black liquor combustion and gasification, and also the region with the most significant uncertainties. Measurements were carried out in a dry inert atmosphere at temperatures from 25 to 1200 °C, using high-temperature microscopy (HTM) and hightemperature X-ray powder diffraction (HT-XRD). To examine the influence of pure CO2 atmosphere on the melting behavior, HTM experiments in the same temperature interval were made. This paper presents new data complementary to earlier published data on the binary phase diagram Na2CO3-Na2S. These include re-determination of liquidus curves, in the Na2CO3-rich area, melting points of the pure components, as well as determination of the extent of the solid solution, Na2CO3(ss), area.
Introduction Black liquors, the byproduct of chemical pulping, contain almost all of the inorganic cooking chemicals, as well as the lignin and the other organic materials separated from the fibers during cooking in the digester. The two main functions of black liquor combustion and gasification processes, chemical and energy recovery, make the recovery process design and operation more complex than other combustion processes. Today, however, the recovery of chemicals from spent cooking liquor and generation of energy as well as the environmental aspects are all important parts of the chemical pulp mill recovery process. Gasification of black liquor is an attractive alternative to the traditional recovery boiler. Pressurized black liquor gasification is a new process which may be used both as a significant complementary process (booster concept) and in the future to completely replace the recovery boiler. This new technology has the potential to improve overall pulping economy, energy, and environmental performance of a pulp mill. The erection of a pilot scale Chemrec Oxygen-Blown Black Liquor Gasifier1 is under way in Piteå, Sweden, adjacent to the Kappa kraftliner mill. This process is presently under development, but plans for full-scale projects are being developed both in Scandinavia and North America. For engineering of this new process technology, development of accurate process models including chem* Corresponding author. Phone: +46-90-7865943. Fax: +46-907869195. E-mail:
[email protected]. (1) Lindblom, M. In Proceedings from the 2003 Colloquium on Black Liquor Combustion and Gasification, Park City, UT. (Proceedings available at: www.blackliquor.com.)
istry is crucial. Knowledge of the speciation of inorganic elements and their phase stabilities are important for understanding their behavior in the gasification process. The smelt of a kraft recovery boiler consists of approximately two-thirds Na2CO3 and one-third Na2S. Knowledge of how the melting point of these compounds varies with the composition of the smelt in the gasifier is thus of technical interest. Thermodynamic data for these key components are quite uncertain, which cause problems in modeling the chemical processes in a gasifier. The melting point of pure Na2CO3 found in the literature varies between 845 °C 2 and 858 °C.3 The corresponding range of melting point data for pure Na2S was found to be 1040 °C 4 to 1215 °C.5 Previously published data of the binary phase diagram Na2CO3Na2S also differ, not only on the melting temperature of the pure compounds, but also on the indication of a solid solution, Na2CO3(ss), as well. The first published investigation on this system was performed by Courtois6 in 1939. No indication of a solid solution was given and the melting point of pure Na2CO3 was determined to be 852 °C and 1180 °C for pure Na2S. The second publication on the system, studied with differential thermal analysis (DTA), was performed by Ovetchkin et al.7 in 1971. No solid solution was indicated and the (2) Ståhlstro¨m, A.; Skrifvars, B.-J.; Backman, R. REPORT 95-7, A° bo Akademi, Department of Chemical Engineering: A° bo, Finland, 1995. (3) Tegman, R.; Warnqvist, B. Acta Chem. Scand. 1972, 26, 413414. (4) Smirnov, M. P.; Strelnikova, L. N. Tsventnykh Metallov 1966, 23, 67. (5) Chiotti, P.; Markuszewski, R. J. Chem. Eng. Data 1985, 30, 197201. (6) Courtois, G. Compt. Rend. 1939, 208, 277-278.
10.1021/ef0340256 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/04/2003
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melting point of pure Na2CO3 and Na2S was reported to be 856 °C and 1160 °C, respectively. Tegman and Warnqvist3 performed the third and latest published study of this system in 1972. The phase diagram was investigated with high-temperature microscopy (HTM) and DTA. They reported indications of a solid solution and the melting point of pure Na2CO3 was determined to be 858 °C and 1175 ( 10 °C for pure Na2S. Tegman and Warnqvist also made an attempt to study this solid solution with X-ray powder diffraction (XRD) measurements, with an exposure time of 30 min. They used samples of appropriate compositions, which had been quenched to room temperature within a few seconds. However, they were unable to determine the extent of the Na2CO3(ss) by this method. The previous investigations, mentioned above, on the Na2CO3-Na2S system were all made in an inert atmosphere. The objectives of this work were therefore to experimentally re-determine and improve data on the binary phase diagram Na2CO3-Na2S, especially on the Na2CO3 side of the system, which is the main region of interest concerning black liquor combustion and gasification, and also the region with the most significant uncertainties. The present work comprises re-determination of the binary phase diagram Na2CO3-Na2S in inert and in CO2 atmosphere. The phase diagram studies were carried out with HTM and high-temperature X-ray powder diffraction (HT-XRD). To examine the effect of pure CO2 atmosphere on the melting behavior, HTM experiments at compositions of XNa2S e 0.2 were performed. Experimental Section Preparation of Na2CO3 and Na2S. To achieve accurate results in the phase diagram study, completely anhydrous and pure chemicals (Na2CO3 and Na2S) are needed. Drying and purification of these were therefore necessary. Na2CO3 (Merck, p.a.) was purified as described by Tegman and Warnqvist.3 It was dried under vacuum at 200 °C for 3 h and then dried in a tube furnace at 500 °C with CO2 atmosphere for 8 h. The resulting product, pure anhydrous Na2CO3, was a white crystalline powder. Na2S is extremely hygroscopic and needs treatment in several steps. Pure anhydrous Na2S was prepared as described by Rose´n and Tegman.8 Na2S‚xH2O (Merck, p.a, x ) 7-9) was recrystallized in distilled water and then dried under vacuum in a desiccator with concentrated sulfuric acid, H2SO4 (J. T. Baker) as drying medium. The Na2S was then gradually heated to 300 °C during a period of 6 h under vacuum in a flask. The Na2S was further dried and purified in a tube furnace which was slowly heated to 900 °C for 8 h under a reducing atmosphere (N2/H2 ) 9:1). A graphite crucible was used for transporting the chemicals in and out of the furnace. The resulting product, pure anhydrous Na2S, was a white crystalline powder. HTM Measurements. To determine the melting points in the Na2CO3-Na2S system, a Leitz high-temperature microscope was used. A LINKAM TS 1500 hot stage was used for heating samples. The temperature range of the TS 1500 is up to 1500 °C and it is equipped with a gastight chamber for atmospheric control. Calibration of the HT 1500 hot stage was performed repeatedly during the period of measurement using melting points of selected calibration chemicals (Au, NaCl, and (7) Ovechkin, E. K.; Shevtsova, L. N.; Voitsekhovskii, A. E.; Kuznetsova, L. V. Zh. Neorg. Khim. 1971, 16 (11), 1672. Russ. J. lnorg. Chem. (Engl. Transl.) 1971, 16 (11), 1672. (8) Rose´n, E.; Tegman, R. Acta Chem. Scand. 1971, 25, 3329-3336.
Ra˚ berg et al. AgI) in the temperature interval of interest for the melting point study. To measure the sample temperature, a TMS 93 temperature programmer was used. The programmer is operated from a digital panel, and the sensor’s signal is digitally linearized to ensure an accurate temperature display of 1 °C for the TS 1500 stage. To carry out the phase diagram study, well-crystallized, completely anhydrous, pure chemicals were needed. Even small impurities, as small as 1%, cause lowering of the melting points. Due to the fact that Na2S is extremely hygroscopic, all handling of chemicals, e.g., mixing, weighing, sample preparations, and sample mounting in the hot stage were carried out in a glovebox. The model used was a Mecaplex GB 3111-C, evacuable glovebox made of acrylic glass. To obtain a dry atmosphere in the box, it was evacuated to pressures less than 1 mbar followed by introduction of Ar. To minimize moisture, containers with concentrated H2SO4 (J. T. Baker) and silica gel, SiO2 (Kebo) were used as drying agents in the box. Further, all tools used, such as mortar and spoons, were carefully dried at 200 °C and stored in desiccators. The hot stage was also stored in a desiccator. Melting point determinations were initially performed on Na2CO3 and Na2S to confirm that pure chemicals were used. Further, stoichiometric amounts of Na2CO3 and Na2S were weighed to achieve the right compositions and carefully ground to attain a good mixing. The sample compositions used were XNa2S ) 0, 0.1, 0.15, 0.2, and 1. Sample mixes were then pressed into small disks with dimensions of approximately 1.5 × 1.5 × 0.1 mm3 and with a weight of about 0.5 mg. The samples were then mounted on a sapphire plate in the TS 1500 hot stage for melting point observation at 100× magnification under a protective atmosphere of Ar and CO2, respectively. The initial heating rate was 50 °C/min, and near the melting point the heating rate was 1 °C/min. By observing the melting and growing behavior of the sample crystals in the vicinity of a phase change, during heating of the sample to the melting point and slowly cooling it until the first crystal appears, accurate determinations of the melting points were possible. This procedure was repeated several times. HT-XRD Measurements. The XRD instrument used to determine the extent of the solid solution in the Na2CO3-Na2S system, was a BRUKER AXS D8Advance, equipped with primary and secondary Go¨bel Mirrors and a Anton Paar HTK 16 high-temperature camera. Calibration of the HTK camera was performed using melting points of selected calibration chemicals (NaCl and AgI) in the temperature interval of interest for the study. To set up the measurement conditions and analyze the collected data the computer program suite DIFFRACplus 9 was used. Sample preparations were performed in the same way as for the melting point study, except that larger samples were used with dimensions of approximately 8 × 4 × 0.5 mm3 and with a weight of about 40 mg. To avoid exposure to moisture, the samples were transported in a desiccator to the XRD where it was put in a glovebag. The HTK camera was enclosed by the glovebag which enabled the sample mounting in a “safe” way considering the hygroscopic properties of the sample. The glovebag was evacuated and filled with N2 prior to mounting of the samples. A stream of He was maintained through the hot stage during sample mounting and measuring. The performed HT-XRD measurements can be summarized in two steps. First, measurements at 25 °C were made on pure Na2CO3 and on pure Na2S, respectively, to confirm that no impurities were present. Second, measurements on Na2CO3 were made in the temperature interval 25-800 °C in order to get the characteristics of its peak pattern. Finally, measurements were made on three different sample compositions of XNa2S ) 0.1, 0.15, and 0.2, in the same temperature interval. By tracking (9) DIFFRACplus, 1997-1999 Bruker AXS GmbH Analytical X-ray Systems, D-76181, Karlsruhe, Germany.
Binary Phase Diagram Na2CO3-Na2S
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Table 1. Melting Points for the Different Compositions in Ar and CO2 Atmosphere, Respectively composition
temp. of first melt (°C)
temp. of complete melting (°C)
XNa2S
Ar
Ar
CO2
0 0.1 0.15 0.2 1
795 ( 8 780 ( 7 757 ( 0
850 ( 0.5 842 ( 6 830 ( 7 817 ( 5 1190 ( 0.5
858 ( 0.5 850 ( 6 838 ( 7 825 ( 5
the strongest peak of Na2S to the temperature where it disappears from the diffraction pattern, i.e., the temperature of the boundary curve for Na2CO3(ss), the extent of the Na2CO3(ss) was determined. The time for the samples to stabilize between a new temperature and the start of the measurement varied between 2 and 24 h.
Results and Discussion The combined results from the HTM and HT-XRD methods have been used to make the binary phase diagram Na2CO3-Na2S more complete, especially in the region of interest for black liquor combustion and gasification. The result from the influence of CO2 atmosphere on the melting behavior is also shown. HTM Measurements. For each composition, three measurements were made, using different samples from the same mixture of Na2S and Na2CO3. Results from the melting point study are presented in Table 1, as average temperatures. The temperatures where the first melt appears represents the solidus curve Na2CO3(ss)/(l), see Figure 1. As shown in Table 1, the melting points of the pure Na2CO3 and Na2S are in good agreement with literature data,3 which indicates a successful drying and purification of the chemicals. There is an increase of 8 °C in melting point of both pure Na2CO3 and the sample mixes when Ar and CO2, respectively, are used as protective gases. The explanation of this deviation can be the thermal decomposition that Na2CO3 undergoes upon heating in Ar atmosphere. The decomposition of pure Na2CO3 has been reported by Kim and Lee10 to occur in two consecutive steps:
Na2CO3(l) h Na2O(s) + CO2(g)
(1)
Na2O(s) h 2Na(g) + 1/2O2(g)
(2)
If Na2O, as given by eq 1, has been formed using a protective atmosphere of Ar, this will cause a lower melting point. With an atmosphere of CO2 the equilibrium in eq 1 will be forced to the left and no Na2O will be formed and thus a correct melting point of Na2CO3 will be obtained. After the hot stage was calibrated it should, according to the manufacturer,10 have temperature accuracy and stability of less than 0.1 °C. With pure samples the uncertainties in the HTM measurements therefore consist mainly of the errors in weighing and different thickness of the sample disks used. The effect of weighing errors should be quite small due to the accuracy of the scales. The thickness of the sample gives a certain temperature gradient in the sample. A thicker (10) Kim, J.-W.; Lee, H.-G. Metall. Mater. Trans. B 2001, 32B, 1718.
Figure 1. The binary phase diagram Na2CO3-Na2S.
sample has a larger temperature gradient that influences the melting behavior. Hence a thin sample disk is preferred. The overall errors in the melting points are summarized in Table 1. Previous published data of the melting points of pure Na2CO3 and Na2S show large differences. The main reason for this is probably impure chemicals. However, one example of how the measurement method can affect the result is found in the literature. The extremely high melting temperature of pure Na2S (1215 °C) reported by Chiotti et al.5 might be explained by the high heating rate of 50 °C/min they used during differential thermal analysis, DTA, measurements. During cooling, the Na2S crystallized at 1188 °C, which is a more likely temperature. Another cause of the differences in melting point is the handling of the chemicals after drying and purification. This is extremely important for Na2S as it easily undergoes humid air oxidation. One improvement of the HTM study in the present work, compared to Tegman and Warnqvist,3 is the sample mounting. They mounted the samples outside the glovebox, which means that the samples were exposed to air for a short period of time. In this work, the samples were mounted inside the glovebox and were never exposed to air. HT-XRD Measurements. The temperature of the boundary curve for Na2CO3(ss) at XNa2S ) 0.1 was 490 °C and for XNa2S ) 0.15 the temperature was 630 °C. Concerning XNa2S ) 0.2, Na2S was still detectable at 755 °C. The experimental results from both the HTM and HT-XRD studies are plotted in Figure 1 together with previously published data.3 The data points in the diagram correspond to measurement results in inert atmosphere except for the
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Figure 2. The difference in melting point of sample compositions of XNa2S ) 0, 0.1, 0.15, and 0.2 with Ar and CO2 atmosphere, respectively.
melting point of pure Na2CO3, which was chosen to 858 °C (CO2 atmosphere). This was motivated by the fact that pure Na2CO3 only exists in CO2 atmosphere. The dashed elongation of the Na2CO3(ss) boundary curve is an estimate for XNa2S < 0.1. In Figure 2, a magnification of the phase diagram has been made. This figure shows the difference in melting points when a protective atmosphere of Ar and CO2, respectively, was used. The influence of CO2 atmosphere is probably less significant at higher concentrations of Na2S in the sample, since the amount of Na2O formed (see eq 1) will be less. During the HT-XRD measurements, dry He was conducted through the HTK 16 camera and over the sample. Helium is preferable in HT-XRD measurements since its low mass reduces the absorption of the X-rays, compared to, for instance, N2 or Ar. The uncertainties in the HT-XRD data are the same as for the HTM concerning weighing errors and the like in the HTM
Ra˚ berg et al.
study, the samples were never exposed to air between the glovebox and the sample mounting. The obtained XRD diffraction patterns at 25 °C show no signs of impurities and show good agreement with the diffraction patterns of Na2CO3 and Na2S in the Powder Diffraction File, PDF-2, Data Base.12 The largest uncertainty in the HT-XRD data, when it comes to determine the exact extent of the solid solution, originates from the temperature interval between the measurements. The uncertainty for the sample of XNa2S ) 0.1 is 490 ( 10 °C, i.e., the solid solution is formed between 480 °C (the last temperature where Na2S was detectable in the diffraction pattern) and 500 °C (the next temperature where Na2S was undetectable in the diffraction pattern). Following the same line of argument, the uncertainty for the sample of XNa2S ) 0.15 is 630 ( 10 °C. The sample of XNa2S ) 0.2 has no uncertainty since not all Na2S forms a solid solution at this mole fraction. This is supported partly by the HTM measurements and by the phase diagram according to Tegman and Warnqvist.3 Conclusions The results of this work show how the two experimental methods, HTM and HT-XRD, have been used to improve the binary phase diagram Na2CO3-Na2S. The HTM result show good agreement with previous data3 on the system, and the HT-XRD experiments resulted in the determination of the Na2CO3(ss) boundary curve. The influence of CO2 atmosphere at compositions of XNa2S e 0.2 was also determined. Acknowledgment. Financial support by the Swedish Energy Agency is gratefully acknowledged. Senior research engineer Ragnar Tegman, Chemrec AB, is gratefully acknowledged for valuable discussions concerning Na2S purification. EF0340256 (11) Linkam Scientific Instruments Ltd. TMS 93 reference manual; 8 Epsom Downs Centre, Waterfield, Tadworth, Surrey, KT20 5HT, England. (12) Powder Diffraction File, PDF-2, Data Base. PDF Numbers: 370451 (Natrite) and 25-0815 (Gregoryite); The International Centre for Diffraction Data, ICDD, 1997.
