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Energy & Fuels 1996, 10, 1241-1244
1241
Mg2Ca(SO4)3: Correct Stoichiometry of the Compound Previously Reported as Mg3Ca(SO4)4 Duane H. Smith,*,† Michael R. Close,‡ and Ulrich Grimm† Morgantown Energy Technology Center, U.S. Department of Energy, Morgantown, West Virginia 26507-0880, and Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506 Received March 26, 1996. Revised Manuscript Received July 24, 1996X
By elemental analyses of extracts from industrially produced material, X-ray diffraction studies on single crystals and powders, and critical evaluations of the previous literature, it is shown that the compound variously reported or assumed to be Mg3Ca(SO4)4 in previous studies is actually Mg2Ca(SO4)3 and that this finding is not only reasonable, but was to be expected.
Introduction In 1967 a compound (CAS Registry No. 16349-90-9) that was concluded to be Mg3Ca(SO4)4 was reported,1 apparently for the first time. In this initial study various proportions of MgSO4 and CaSO4 were repeatedly ground together and then heated while open to the atmosphere, at 750 to 850 °C for 10 days. The evidence for the new compound was primarily the development of a new X-ray diffraction pattern by the powders that had been so treated. In 1977 this same X-ray powder pattern was found in a bench-scale study of the sulfation of dolomite [MgCa(CO3)2].2 Recently the compound Mg3Ca(SO4)4 has been reported3,4 in the ashes of coal-fired power plants; but, as in the 1977 study, the sole evidence for its formation was the occurrence of a powder pattern very similar to that in the original report1 of the compound. Thus, the compound in question has been found to form under a variety of circumstances, but the stoichiometry originally reported never has been independently confirmed. Very recently we found this same X-ray powder pattern for samples of “ash” taken from the filter vessel of a pressurized fluidized bed combustion (PFBC) electrical power plant, where, in finely divided mixtures with CaSO4, MgO was sulfated by SO2 and O2 at about 750 °C.5 However, a careful reading of the 1967 and 1977 studies1,2 convinced us that the compound responsible for the powder pattern common to all of these studies1,2,4-6 almost certainly was not Mg3Ca(SO4)4 but probably Mg2Ca(SO4)3. Two kinds of evidence that the correct formula for this compound is indeed Mg2Ca(SO4)3 are presented here: First, we extracted the compound in question from the PFBC filter-vessel samples, showed that the char* Author to whom correspondence should be addressed. Also affiliated with the Department of Physics, West Virginia University. † U.S. Department of Energy. ‡ West Virginia University. X Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Rowe, J. J.; Morey, G. W.; Silber, C. C. J. Inorg. Nucl. Chem. 1967, 29, 925. (2) Hubble, B. R.; et al. J. Air Pollut. Control Assoc. 1977, 27, 343. (3) Chinchon, J. S.; Querol, X.; Fernandez-Turiel, J. L.; Lopez-Soler, A. Environ. Geol. Water Sci. 1991, 18, 11. (4) Havlicek, D.; Pribil, R. Atmos. Environ. 1993, 27A, 655. (5) Grimm, U.; Haddad, G.; Smith, D. H. Proceedings of the 1995 International Ash Utilization Symposium, Lexington, KY, Oct 23-25, 1995. (6) Turkdogan, E. T.; Rice, B. B. Metall. Trans. 1974, 5, 1537.
acteristic powder pattern was present before the extraction but absent from the solid residue after the extraction, and showed by elemental analyses that the stoichiometry of the material extracted was Mg2.0(0.1Ca1.0(0.03(SO4)3.0(0.2. Second, we grew single crystals, determined the unit cell dimensions, and showed that the densities calculated from these data are reasonable for Mg2Ca(SO4)3 but not for Mg3Ca(SO4)42; we then obtained X-ray powder patterns for the ground single crystals and showed that these powder patterns were consistent with the X-ray powder patterns in the literature1,2,4 for the material stated1,2 to be Mg3Ca(SO4)4. In short, the density of “Mg3Ca(SO4)4” showed it to be Mg2Ca(SO4)3, not Mg3Ca(SO4)4. Structure determinations for these single crystals subsequently showed them to be Mg2Ca(SO4)3. Background information about the PFBC plant and its chemistry5 and details about the crystal structures and their determinations7 may be found elsewhere. Experimental Section Particulate samples were collected from the filter vessel of the Tidd PFBC Clean Coal demonstration plant in Brilliant, OH. Portions of this material were variously used as received, heated (while open to the atmosphere) at 750 °C, or similarly heated at 925 °C. Leachates from these samples were prepared by extraction with water at 22 °C. The insoluble residues left by these aqueous extractions also were analyzed. Powder samples also were prepared by repeatedly rollermilling and heating (Aldrich) ACS reagent grade CaSO4 and MgSO4 together at 750 °C, similar to the method previously reported.1,2 Single crystals were grown by heating CaSO4 and MgSO4 together in sealed, evacuated containers of fused silica at the rate of 600 °C/h up to 1200 °C and holding the sample at that temperature for 6 h, followed by cooling at 30 or 20 °C/h. Slightly better samples were obtained by using the milled and heated material as the starting material in the single-crystal growing procedure. Powder patterns were obtained with a Philips PW 1800 diffractometer and Cu KR radiation; single-crystal X-ray diffraction data were collected with a Siemens P4 sealed-tube source, four-circle goniometer diffractometer. Both types of X-ray data were obtained at room temperature. Ground single-crystal powders were loaded onto a zero-background (7) Close, M.; Smith, D. H. Unpublished results.
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1242 Energy & Fuels, Vol. 10, No. 6, 1996
Figure 1. X-ray powder pattern of filter cake from a pressurized fluidized bed combustion system;5 also, powder patterns8 of the material previously reported1 as Mg3Ca(SO4)4; of water-insoluble, orthorhombic CaSO4 anhydrite; and of SiO2. quartz disk by dispersal of the powders in methanol followed by deposition of the dispersion on the disk surface. Aerodynamic particle-size distributions were measured with an Amherst Process Instruments Aerosizer. Elemental analyses for magnesium and calcium were performed with a Philips 9950 X-ray fluorescence (XRF), PerkinElmer 5000 atomic absorption (AA) spectrometer, and Dionex ion chromatograph (IC). Sulfur was determined by XRF and by combustion anaylyses with a Leco SC-432 DR instrument. Because of the importance of the Mg/Ca ratio, repeat XRF and AA measurements were made independently by two other laboratories (Coors and Galbraith, respectively), and special quality assurance precautions were taken for the IC measurements.
Results Extractions and Elemental Analyses. Figure 1 shows the X-ray diffraction pattern of a sample of filtercake particulates taken from the Tidd filter vessel.5 Figure 1 also illustrates the powder-pattern lines8 of a compound reported1 to be Mg3Ca(SO4)4, as well as the lines of CaSO4 anhydrite and SiO2. Comparison of the various patterns in Figure 1 shows that the filter cake contained significant amounts of the compound previously reported to be Mg3Ca(SO4)4, of SiO2, and of waterinsoluble, orthorhombic CaSO4 anhydrite, but no detectable MgSO4. (Because the origin of the calcium and magesium was dolomite, the presence of excess CaSO4 was to be expected, but the absence of MgSO4 is noteworthy.) The presence of “Mg3Ca(SO4)4” in the filter cake also is shown by Table 1. Table 1 lists positions (2Θ) and intensities of X-ray powder-pattern lines for “Mg3Ca(SO4)4,” as found in the listing of the Joint Committee for Powder Diffraction Studies8 (columns 1 and 2); for ground single crystals that we had grown (columns 3 and 4); for the filter-cake sample (columns 5 and 6); and for magnesium calcium sulfate that we synthesized in a manner similar to the original preparation1 of “Mg3Ca(SO4)4” (heated powder). For reasons described elsewhere,5 the magnesium calcium sulfate is believed to have been formed by the reaction nMgO + nSO2 + (n/2)O2 + CaSO4 ) MgnCa(8) Joint Committee for Powder Diffraction Studies, International Centre for Diffraction Data, Swarthmore, PA, 1987.
Smith et al.
(SO4)n+1. The small size of the particles (∼5 mm), the random mixing of the particles in the cake, and the high melting points1 of MgSO4 (1136 °C), CaSO4 (1462 °C), and their 3:1 mixture (1201 °C) all suggest the occurrence of a gas-solid reaction, with little if any melt present, in the formation of the filter-vessel material.5 As discussed below, the reported value of n (n ) 3)1 never was corroborated by further measurements.2 Because of the presence of “excess” CaSO4, the value of n could not be determined from measurements of the Mg/Ca ratio in the whole filter-cake sample. However, the absence of (water-soluble) MgSO4 and the low water solubility of the orthorhombic CaSO4 encouraged us to attempt an aqueous extraction4 of the magnesium calcium sulfate from the water-insoluble compounds. The extraction at 22 °C (at which the aqueous solubility of orthorhombic CaSO4 is only 0.2 g/100 mL)9 successfully removed essentially all of the MgnCa(SO4)n+1. This result is shown by Table 2, which lists the atomic ratios of magnesium and calcium, as measured by various techniques. The average of seven determinations of the Mg/Ca ratio was 1.99, and the measured S/Ca ratio was 3.0. Thus, the results of Table 2 indicate the stoichiometry was Mg2Ca(SO4)3, not Mg3Ca(SO4)4. This finding was supported by the X-ray diffraction pattern of the dried residue left after the extraction, which indicated the presence of no MgSO4 or magnesium calcium sulfate. For greater sensitivity, the extraction residue also was analyzed for magnesium by ion chromatography. As listed in Table 2, the Mg/Ca ratio in the residue was only about 0.01. Without this analysis it might have been supposed that the Mg/Ca ratio in the leachate were 2 rather than 3, because a substantial amount of the magnesium present did not dissolve during the extraction. However, the very low amount of magnesium in the insoluble residue shows that this was not the case. As shown by Kosolov,10 no double salt of MgSO4 and CaSO4 separates from aqueous solution at 25 °C. Hence, we made no attempt to analyze the water-soluble material for magnesium calcium sulfate. Further information about the properties of the sample from which Mg2Ca(SO4)3 was extracted and about the Mg2Ca(SO4)3 itself comes from a sample that was heated (open to the atmosphere) at 925 °C before the aqueous extraction was performed. Because MgSO4 is unstable at this temperature, one might suspect that the “MgSO4” in Mg2Ca(SO4)3 would be, also. The results shown in Table 2 for the Mg/Ca ratios in the 925 °C leachate and residue indicate that this is true. The MgO formed by the thermal decomposition of MgSO4 was essentially insoluble in water; hence, the Mg/Ca ratio in the leachate was only about 0.01. However, the Mg/Ca ratio in the insoluble residue was about 1.0. This ratio is hardly surprising, because the original source of the magnesium and calcium was dolomite. Hence, before the aqueous extraction, the original particulate sample contained essentially equal numbers of moles of Mg2Ca(SO4)3 and of CaSO4. (9) Hodgman, C. S., Ed. Handbook of Chemistry and Physics; Chemical Rubber Publishing: Cleveland, OH, 1959. (10) Kolosov, A. S. Tr. Khim.-Metall. Inst., Akad. Nauk SSSR, Zapadno-Sib. Fil. 1958, 12, 29; Izv. Sib. Otd. Akad. Nauk SSSR 1959, 3, 67.
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Correct Stoichiometry of Mg2Ca(SO4)3
Energy & Fuels, Vol. 10, No. 6, 1996 1243
Table 1. Powder-Pattern X-ray Diffraction Lines and Intensitiesa single crystald
Mg3Ca(SO4)4c db
filter cakee
heated powderf
(Å)
hkl
2Θ
I
2Θ
I
2Θ
I
2Θ
I
4.1208 3.6562 3.5908 3.4732 3.2094
110 002 111 020 012
21.411 24.318 24.591 25.301 27.616
8 16 100 2 16
21.548 24.325 24.775 25.628 27.775
3.8 6.4 100 0.5 6.9
21.465 24.340 24.695
4.9 11.3 100
21.345 24.210 24.640
3.8 3.6 100
27.690
7.4
27.635
7.2
29.813 31.129 32.572 35.236 37.488 38.334 38.937 39.542 41.339 42.944 44.846 47.240 49.857 50.239 50.536 51.452 52.338 53.457 54.663 55.561 56.338 57.256 58.245 58.856 59.860 60.293 61.670 62.360 63.939 64.548
2 2 55 4 16 2 2 10 10 20 6 2 8 10 20 2 2 2 2 2 2 4 2 8 8 6 2 6 6 6
32.602 35.300 37.668 38.358
45.8 3.0 10.0 2.5
31.338 32.478 35.198
5.9 19.2 1.9
38.342
1.6
39.628 41.430 42.898 44.835
9.1 14.3 7.9 4.7
39.488 41.318 42.890 44.798
9.0 9.0 4.1 3.4
50.190 50.49 51.328 52.430
13.2 13.4 1.3 10.8
50.112 50.525
5.6 16.9
54.348
6.4
59.105 59.982
5.4 10.3
62.388 63.862 64.458
5.2 7.0 8.9
2.8667 2.7384 2.5350 2.3848 2.3407 2.3113 2.2669 2.1770 2.1028 2.0182 1.9640 1.8330 1.8150 1.8017 1.7759 1.7470 1.7091 1.6762
112 022 030 031 212 113 023 131 004 123 222 014 132 041 114
1.6327 1.6084 1.5803 1.5698 1.5426 1.5273 1.5035 1.4984 1.4555 1.4446
223
31.175 32.675 35.38 37.690 38.428 38.935 39.730 41.445 42.978 44.875 47.190 49.700 50.225 50.622 51.412 52.328 53.578 54.715
0.65 53.7 1.7 9.3 2.6 2.0 7.4 9.0 9.3 5.0 1.1 8.8 9.6 14.0 2.0 1.9 0.8 0.7
56.300 57.230 58.345 58.772 59.912 60.575 61.640 61.870 63.908 64.450
2.6 2.8 2.05 2.5 5.5 2.17 1.0 1.3 6.3 3.2
a Hexagonal unit cell dimensions R ) 8.2984(6) Å, c ) 7.3761(6) Å, and Cu KR radiation. b Based on 2Θ values of column 5. c Filter cake from Tidd filter vessel.5 d Ground single crystals after determination of unit cell. e Reference 8, based on personal communication from authors of ref 1. f Sample prepared by mixing and heating 2:1 molar ratio of MgSO4 and CaSO4 at 750 °C, similar to ref 1.
Table 2. Elemental Analyses of Leachate and Insoluble Residue from Aqueous Extractions at 22 °C from Tidd Filter Cake Mg
Ca
S
Ta (°C)
Leachate 0.01 1 925 2.0 ( 0.02 1 ( 0.02 2.8 ( 0.13 750 2.1 1 750 1.9 ( 0.03 1 ( 0.03 750 2.0 ( 0.08 1 3.27 750 1.99 1 3.04 750 0.09 0.98 a
1 1
Residue 750 925
method ion chromatography X-ray fluorescence ion chromatography ion chromatography sulfur analyzer av for 750 °C samples ion chromatography ion chromatography
Highest temperature to which the sample was subjected.
Single-Crystal Studies. An attempt to grow single crystals of MgCa(SO4)2 from an equimolar ratio of MgSO4 and CaSO4 produced an opaque material with no observable crystals. An attempt to grow single crystals of Mg3Ca(SO4)4 from a 3:1 molar ratio of MgSO4 and CaSO4 produced only small, unusable crystals. Attempts to grow single crystals of Mg2Ca(SO4)3 from mixtures of KBr, MgSO4, and CaSO4 produced slightly yellowed, multigrained crystals. Single crystals grown from a 2:1 molar ratio of MgSO4 and CaSO4 (no KBr) tended to be twinned. The best crystals were obtained by intermittent roller-milling and heating of a 2:1 molar ratio of MgSO4 and CaSO4 at 750 °C until the characteristic X-ray powder pattern devel-
oped and then heating of this material in an evacuated, sealed tube at 600 °C/h up to 1200 °C, followed by cooling at 20 °C/h. For these single crystals we obtained the hexagonal unit cell dimensions a ) 8.2984(6) Å, c ) 7.3761(6) Å, and unit cell volume 439.89(6) Å3. Thus, for Mg3Ca(SO4)4, the densities calculated from this unit cell volume are d ) 1.878 g/cm3 for Z (the number of formula units per unit cell) ) 1 and d ) 3.756 g/cm3 for Z ) 2, whereas for Mg2Ca(SO4)3 and Z ) 2 the calculated density is 2.846 g/cm3. As discussed below, the densities of MgSO4 and CaSO4 are both close to 2.9 g/cm3; thus, the expected density of their double salt (regardless of its formula) also is about 2.9 g/cm3. Hence, the densities also indicate our material was not Mg3Ca(SO4)4 but was Mg2Ca(SO4)3. Having confirmed the existence of the compound Mg2Ca(SO4)3, we wished to determine if the compound reported to be Mg3Ca(SO4)4 existed also or if (as we suspected) “Mg3Ca(SO4)4” were actually Mg2Ca(SO4)3; that is, we sought to determine if our Mg2Ca(SO4)3 single crystals gave a different, or the same, X-ray powder pattern as the pattern originally attributed1 to Mg3Ca(SO4)4. Hence, we obtained powder diffraction patterns for two different samples of ground single crystals the unit cells of which had given the density d ) 2.846 g/cm3. Data were collected from 2Θ ) 20° to 2Θ ) 90°; the powder patterns from the two different samples were almost identical. Figure 2 illustrates the
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1244 Energy & Fuels, Vol. 10, No. 6, 1996
Figure 2. Powder pattern of ground single crystals of Mg2Ca(SO4)3.7 Comparison of these lines with those of “Mg3Ca(SO4)4” show that “Mg3Ca(SO4)4” is actually Mg2Ca(SO4)3.
powder pattern for crystals grown by milling and heating MgSO4 and CaSO4 together before heating to 1200 °C. The positions (2Θ) and intensities of the X-ray powder-pattern lines for the ground single crystals are listed in Table 1 [along with these data for the corresponding lines of the filter-cake sample and of “Mg3Ca(SO4)4”].1, 8 (The theoretical powder pattern calculated from the single-crystal data has been published elsewhere.7) As summarized in Table 1, comparison of data for the compound previously reported1,8 as Mg3Ca(SO4)4, our ground single-crystal data, the filter-cake data, and data for magnesium calcium sulfate that we prepared in a manner similar to the initial preparation1 of “Mg3Ca(SO4)4” shows that all four sets of powder-pattern lines are essentially the same. Hence, each set of lines must have been produced by the same compound, and “Mg3Ca(SO4)4” and “Mg2Ca(SO4)3” must be one and the same compound. The elemental analyses and the densities both show this compound to be Mg2Ca(SO4)3. In other words, not only does the compound Mg2Ca(SO4)3 exist, but there is no X-ray diffraction evidence for the existence of Mg3Ca(SO4)4. As a further test of this conclusion, structure determinations for the single-crystal samples were performed, in which Ca and Mg were allowed to substitute for each other in the structure refinement.7 These structure determinations yielded the formula Mg1.9Ca1.1(SO4)3. Discussion In the 1977 study single crystals of both “Mg3Ca(SO4)4” and “Mg2Ca(SO4)3” were grown.2 Remarkably, the two allegedly different compounds were found to have the same (hexagonal) symmetry, similar cell
Smith et al.
dimensions, and essentially the same refractive indices. Intensity variations and line shifts were observed between the two X-ray powder patterns, but the patterns were “similar”.2 Moreover, similar to our findings, the density calculated for the “Mg3Ca(SO4)4” from the cell dimensions was either 1.89 or 3.78 g/cm3, depending on whether one or two formulas per unit cell were assumed. However, the density calculated2 from their cell dimensions for two formulas per unit cell of the “Mg2Ca(SO4)3” was about 2.95 g/cm3, similar to our 2.846 g/cm3 (calculated from our cell dimensions measured for a monoclinic space group). These latter densities compare favorably to the densities of MgSO4 (2.93 g/cm3) and CaSO4 (2.99 g/cm3); the densities calculated for “Mg3Ca(SO4)4” do not. Thus, the present study was motivated by the belief that the “Mg3Ca(SO4)4” and the “Mg2Ca(SO4)3”2 must be the same compound and that that compound almost certainly was Mg2Ca(SO4)3. If the reaction to form MgnCa(SO4)n+1 is carried out to completion for a series of samples with different starting ratios of MgSO4 and CaSO4, and if no thermal decomposition occurs, then the resultant X-ray diffraction patterns will exhibit only MgnCa(SO4)n+1 lines for the sample of the correct stoichiometric ratio of MgSO4 and CaSO4; however, samples with either excess MgSO4 or excess CaSO4 also will exhibit the lines of the excess compound. Thus, in principle, the method of the original report1 of Mg3Ca(SO4)4 could determine the stoichiometry correctly. However, if the reaction is incomplete, then one must guess whether more “excess” MgSO4 or CaSO4 is present. Moreover, X-ray diffraction patterns are not particularly quantitative, and small amounts of “excess” compounds may not be detected. We suspect that problems of incomplete reaction, thermal decomposition, and/or low X-ray pattern sensitivity were encountered in the original1 1967 study. The chance of a misinterpretation becomes even greater ifsas in the original papersonly two compositions are studied in the relevant composition range from n to n + 1. Conclusion We submit that the present study, the 1977 study,2 and the single-crystal structure determination7 corroborate a simple conclusion that is valid for several different papers1,2,4,5,7 in the literature: “Mg3Ca(SO4)4” and “Mg2Ca(SO4)3” are the same compound, and that compound is Mg2Ca(SO4)3. Acknowledgment. This work was supported by the Office of Fossil Energy, U.S. Department of Energy. We thank J. Petersen and the Department of Chemistry of West Virginia University for use of the X-ray diffractometer and associated equipment. EF9600523
