Ind. Eng. Chem. Res. 2004, 43, 5221-5224
5221
Effect of Aluminum on Na5P3O10 (Form-II f Form-I) Thermal Transformation Regina Kijkowska,* Zygmunt Kowalski, Danuta Pawłowska-Kozinska, and Zbigniew Wzorek Institute of Inorganic Chemistry and Technology, Crakow University of Technology, Warszawska 24, 31-155 Krakow, Poland
The presence of Al3+, as an impurity, on the temperature of the polymorphic transformation of pentasodium tripolyphosphate Na5P3O10-II (low-temperature Form-II) into Na5P3O10-I (hightemperature Form-I) has been investigated using X-ray powder diffraction, IR spectroscopy, and scanning electron microscopy methods. The effect of Al3+ on the phase transformation was recorded between ignition temperatures of 350 and 450 °C. Al-free powders, ignited up to 450 °C, did not contain Form-I, while powders containing 0.5 or 1.0 wt % of Al were mixtures of Form-I/Form-II indicating that the presence of Al acted in favor of Form-I formation. Phase composition at temperatures of ignition higher than 500 °C was not dependent on Al presence. Na5P3O10 in all of the products ignited at 500-550 °C, whether Al-free or containing Al, was in Form-I. The presence of Al caused lower crystallinity and inhibited the grain growth of pentasodium tripolyphosphate while powder was ignited. Both phenomena are believed to act in favor of phase transformation at lower ignition temperatures. 1. Introduction Condensed sodium phosphates, among them pentasodium triphosphate (PST), and their structure and properties have been extensively studied during the 1950s.1-17 Dehydration of monosodium and/or disodium phosphates and their condensation yielding tetrasodium diphosphate or PST have also been discussed.18-24 PST is the most widely used key detergent component. It performs efficiently in all washing conditions. Anhydrous PST (Na5P3O10) exists in two polymorphic, monoclinic forms known as high-temperature Form-I and low-temperature Form-II.1-4,25 The transformation temperature of Form-II to Form-I, depending on the heating rate, is estimated to be in the range of 450500 °C and, according to Van Wazer, is not complete even at temperatures higher than 500 °C.4 Typically, both anhydrous crystalline phases which practically are not separable are the main constituents of the commercial PST, while tetrasodium diphosphate (Na4P2O7), metaphosphates (NaPO3)n, and hydrated PST (Na5P3O10‚ 6H2O) appear as byproducts. The weight ratio of FormI/Form-II in the product obtained during the technological process is usually controlled by the temperature of ignition. As opposed to Form-II, Form-I, having a higher hydration rate, exhibits “lumping” properties and after water addition cements together and sometimes solidifies. Lumping properties cause serious problems in the manufacturing of washing powders with the use of spray dryers; thus, producers in the past made a washing powder of PST with a high content of Form-II. In modern technology, elimination of solidification of the detergent slurries makes possible the use of the PST grades with a higher content of Form-I. That involves shorter hydration time and optimization of the laundry detergent process. To produce PST of diversified grades * To whom correspondence should be addressed. Fax: (48) (12) 628-2036. E-mail:
[email protected].
according to the requirements of washing powder producers, their attention has been turned from spray drying technology before ignition to dry mixing methods of final detergent components. By mixing Form-I with Form-II, it is possible to adjust their weight ratio, grain size, color, etc. to the requirements of buyers. Standards, so far, for example in Poland (Polish Standard PN-C84033), determine only the content of water-insoluble components (0.1-0.3 wt % in different grades), without determining the Form-I/Form-II ratio; some PST grades produced contain 90% of Form-I, and some others contain 80% of Form-II. For PST production, thermal or purified wet-process phosphoric acid has been used as a raw material.4,26-28 From a technological and also a scientific point of view, it is interesting to learn how some impurities, usually present in wet-process phosphoric acid, may affect the temperature of Form-I formation and the weight ratio of the two forms in PST. The aim of the investigations carried out in our laboratory is to determine the effect of aluminum, usually present in wet-process phosphoric acid as an impurity, on the content of Form-I in PST ignited at different temperatures. The aluminum content in a concentration up to 77-80 wt % of H3PO4 wet-process phosphoric acid, such as that obtained from Kola apatite, does not exceed the level of 0.5 wt %.29 That amount is equivalent to about 0.5 wt % of Al in dry Na5P3O10, if obtained from that acid. 2. Experimental Section Reagent grade chemicals phosphoric acid (15 M H3PO4), NaOH (POCH S.A. Poland), and AlPO4‚H2O (RdH Lab. GmbH & Co., Germany) were used without purification. PST (Na5P3O10) was prepared by evaporation of a solution containing sodium and phosphate ions with a molar ratio of Na/P equal to 5/3. To prepare a solution with the Al ion, an appropriate amount of AlPO4‚H2O
10.1021/ie030815r CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004
5222 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
Figure 1. XRD patterns of Al-free Na5P3O10 powders (B f F) ignited at different temperatures in comparison to (A) Na5P3O10II (Form-II), (G) Na5P3O10-I (Form-I), and (H) Na4P2O7 (sodium pyrophosphate).
was dissolved in a concentrated phosphoric acid solution, and then NaOH (20 wt %) was added to obtain the required molar ratio. The amount of Al was such that it resulted in 0.05-1.0 wt % of Al in the final product. The solution prepared was evaporated. The dry product obtained was ignited at temperatures of 350, 400, 450, 500, and 550 °C for 2 h. The powders obtained were identified using powder X-ray diffraction (XRD), IR spectroscopy, and scanning electron microscopy methods. For the XRD, Philips X’pert XRD equipment furnished with a graphite monochromator PW 1752/00, with radiation Cu KR, Ni filter, 2Θ from 10° to 60° at 30 kV and 30 mA was used. A Fourier transform IR spectrometer FTIR-FTS 175 (Bio-Rad) was used to record the IR spectra of the samples in KBr pressed pellets covering the wavenumbers 400-4000 cm-1. 3. Results and Discussion Polymorphic transformation of PST (Form-II f FormI) in powders ignited at different temperatures is illustrated by XRD patterns in Figures 1-3. For comparison, the XRD data of Form-I and Form-II and also of pyrophosphate are included in each figure. Al-free (reference sample in Figure 1) ignited at temperatures up to 450 °C is in Form-II, which is in agreement with the Van Wazer report.4 Similar XRD patterns, not included in the paper, were obtained for powders containing a low concentration of Al (0.05 wt %). Increased Al concentration (0.5-1.0 wt %) acts in favor of Form-I formation at temperatures lower than
Figure 2. XRD patterns of Na5P3O10 powders containing 0.5 wt % of Al (B f F) ignited at different temperatures in comparison to (A) Na5P3O10-II (Form-II), (G) Na5P3O10-I (Form-I), and (H) Na4P2O7 (sodium pyrophosphate). F1 indicates the appearance of XRD peaks characteristic of Form-I in the mixture of Form-I/FormII.
450 °C. The evidence is the presence of the XRD peaks at 2Θ ≈ 21.7° and at 2Θ ≈ 29.0° characteristic of Form-I recorded even at temperatures as low as 350 °C (Figure 2B, indicated as F1). The appearance of Form-I peaks is associated with a decrease in the intensity of those characteristic of Form-II. The intensity, taken as an integral (area) under the XRD peaks in question, ready from the computer system is increasing proportionally to the amount of Form-I in the mixture of Form-I and Form-II. Using the calibration method described earlier,30 the amount of Form-I has been estimated to be about 20 wt % in the powder ignited at 400 °C and about 50 wt % after ignition at 450 °C in contrast to Al-free PST, which when ignited at 450 °C is still in wellcrystallized Form-II. The characteristic is that the presence of Al acts in favor of Form-I formation only within the lower temperature range (350-450 °C). At temperatures of 500 °C and higher, the effect of Al disappears and all of the products ignited at 500-550 °C are in Form-I. The exception is some small amount of Na3PO4 recorded at the presence of a higher concentration of Al (Figure 3E,F). At lower ignition temperatures, some very small amount of sodium pyrophosphate, as a byproduct, has been shown at 2Θ ≈ 26.4°. IR spectra support the above results showing the effect of Al on Form-I formation at lower ignition temperatures. An example is presented in Figure 4. Powders ignited at 400 °C show the presence of a weak band at 712 cm-1 (Figure 4C,D f arrow), indicating the appearance of Form-I in the samples containing 0.5 and
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Figure 5. IR spectra of powders ignited at 500 °C, characteristic of Form-I: (A) Al-free Na5P3O10; (B) Na5P3O10 containing 0.5 wt % of Al; (C) Na5P3O10 containing 1.0 wt % of Al.
Figure 3. XRD patterns of Na5P3O10 powders containing 1.0 wt % of Al (B f F) ignited at different temperatures in comparison to (A) Na5P3O10-II (Form-II), (G) Na5P3O10-I (Form-I), and (H) Na4P2O7 (sodium pyrophosphate).
Form-I (Figure 5). The only difference is in the relatively lower intensity of the IR bands of Al-containing Form-I of the pentasodium tripolyphosphate (Figure 5B,C). Scanning electron micrographs give the evidence that Al ion inhibits grain growth of PST particles; the grains of powder containing Al are smaller than those of Alfree samples. An example is presented in Figure 6B. That may explain why Al-ion presence acts in favor of phase transformation at lower temperatures of ignition. The Al-containing Form-II with smaller particles (Figure 6B) should be characterized by a higher surface energy, which may be the driving force of the phase transformation that occurs at lower than the usually observed for Al-free PST temperature (Figure 2D). At higher than 450 °C, the phase conversion becomes less dependent on the grain size. From the intensity of the XRD peaks (Figure 1-3F), it can be noted that Al also lowers the crystallinity of the final product of PST. The lower crystallinity is also in favor of the rate of phase transformation, and it is possibly the reason of the lower intensity of the bands in IR spectra of the powders containing Al (Figure 5C). 4. Conclusion
Figure 4. IR spectra of powders ignited at 400 °C: (A) Na5P3O10II (Form-II); (B) Al-free Na5P3O10 (Form-II); (C and D) Na5P3O10 containing 0.5 and 1.0 wt % of Al, respectively (arrow at 712 cm-1 indicates a weak shoulder of the band of Form-I); (E) Na5P3O10-I (Form-I).
1.0 wt % of Al in contrast to the Al-free reference sample (Figure 4B). The IR spectra of all of the powders, whether Al-free or containing Al, ignited at 500 °C are characteristic of
Powders obtained by evaporation of impurity-free solutions, containing sodium phosphate with a Na/P molar ratio equal to 5/3, and that were ignited at 450 °C or lower temperatures were in the crystallographic Form-II of Na5P3O10 and did not contain Form-I. Introduction of Al3+ into solution acted in favor of Form-I formation in powders ignited at a lower (350-450 °C) temperature range. Results of ignition at higher than 450 °C were less dependent on Al-ion presence. All of the products ignited at 550 °C were identified by the XRD as Form-I of Na5P3O10 with a small amount of sodium pyrophosphate. In summary, the effects of Al are (i) lower crystallinity of the pentasodium tripolyphosphate and (ii) inhibition of the grain growth, while the powder is ignited.
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Figure 6. Scanning electron micrographs of powders ignited at 450 °C: (A) Al-free Na5P3O10 (Form-II f XRD shown in Figure 1D); (B) Na5P3O10 containing 0.5 wt % of Al (a mixture of Form-I and Form-II f XRD shown in Figure 2D).
The listed above phenomena act in favor of Form-II f Form-I transformation of pentasodium tripolyphosphate at lower ignition temperatures. Formation of Na5P3O1-I (Form-I) at lower ignition temperatures is less energy-consuming. Thus, the Al effect could be considered as one of the parameters included in the calculation of the production cost of Form-I. Acknowledgment This paper is a result of research financially supported by the State Committee for Scientific Research (KBN), Warsaw, Poland. Literature Cited (1) Van Wazer, J. R. Structure and Properties of the Condensed Phosphates. II. A Theory of the Molecular Structure of Sodium phosphates Glasses. J. Am. Chem. Soc. 1950, 72, 644. (2) Dymon, J. J.; King, A. J. Structure Studies of the two Forms of Sodium Tripolyphosphate. Acta Crystallogr. 1951, 4, 378. (3) Corbridge, D. E. C.; Lowe, J. E. Qantitative Infrared Analysis of Condensed Phosphates. Anal. Chem. 1955, 27, 1383. (4) Van Wazer, J. R. Phosphorus and Its Compounds; Interscience Publishers: New York, 1958; Vol. 1. (5) Van Wazer, J. R. Structure and Properties of the Condensed Phosphates. IV. Complex Ion Formation in Polyphosphate Solution. J. Am. Chem. Soc. 1950, 72, 655. (6) Van Wazer, J. R.; Karl-Kroupa, E. Existence of Ring phosphates higher than the tetrametaphosphate. J. Am. Chem. Soc. 1950, 72, 1772.
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Received for review November 3, 2003 Revised manuscript received May 5, 2004 Accepted May 6, 2004 IE030815R
