ARTICLE pubs.acs.org/IECR
Analysis of Crystalline Structure of Sodium Tripolyphosphate: Effect of pH of Solution and Calcination Conditions F. Sadeghi*,† and A. Fayazi‡ † ‡
Chemical Engineering Department, Faculty of Engineering, University of Isfahan, P.O. Box, 81746-73441, Isfahan, Iran Setarvan Chemical Co., Razi Industrial Zone, Isfahan, Iran ABSTRACT: Phosphoric acid and caustic soda were reacted in a controlled environment to obtain a solution of orthophosphates. Three solutions were prepared at three different pHs. The solutions were dried and calcinated to produce STP (sodium tripolyphosphate). The calcination process was performed at two temperatures. The evolution of STP formation (phase characterization) with time during calcinations was studied. The type of crystalline phase as well as impurity formation during the calcination process were investigated using WAXD (Wide-angle X-ray diffraction) and FTIR (Fourier transform spectroscopy) techniques. At lower pH of 6.48 STP could be formed purely in either phase I or II by setting the calcination temperature at 560 °C for the former and 340 °C for the later. Impurities such as Na4P2O7 and NaPO3 in product were developed as the pH of the solution was increased. At high temperature such as 560 °C, STP formation was fast, and therefore the phase development was less time dependent. At a lower temperature of 340 °C and specially lower pH, the phase development for STP was noticeably time dependent. At pH of 6.57 and 6.77 both phases I and II were observed for low calcinations temperature of 340 °C; however, at higher calcination temperature of 560 °C only phase I was observed. Although WAXD is usually used as a conventional method to study the phase characterization, it was found that FTIR can be considered a reliable technique to study such phase development.
’ INTRODUCTION Sodium tripolyphosphate (STP) is an inorganic compound of polyphosphates with sodium salt (Na5P3O10). STP is widely used as an additive, and its application covers food preservative agents to lubricants for the ceramic industry. It is also consumed extensively in the meat industry as moisture binding agents. Mahon et al.1 were the first to show addition of STP to fish flesh prior to freezing inhibits the loss of moisture, soluble protein, minerals, and vitamins of frozen fish flesh during thawing and cooking.1 Sodium polyphosphates also possess antimicrobial properties and could be used as antimicrobial agents.2,3 Although some environmental limitations were introduced by some countries for using STP in detergent formulation, Banach et al.4 discussed that the life cycle analysis showed the smaller harmfulness of sodium tripolyphosphate for the environment when compared to its substitute Zeolite A. STP is also added to kaolin solutions in the ceramic industry to reduce the viscosity of solution and facilitate pumping.5 Papo et al.5 showed that addition of STP could modify the rheological behavior of kaolin solution such that attractive forces among particles are greatly reduced. From the rheological point of view STP induces a shear thinning behavior that improves dispersion and maximizes the solid loading of kaolin suspensions in water. STP is commercially produced by reaction of phosphoric acid and an alkaline compound such as sodium hydroxide or sodium carbonate in an aqueous solution such that the mole ratio of sodium to phosphorus is about 1.67. The produced solution is a mixture of 2 mols disodium phosphate (Na2HPO4) and 1 mol monosodium phosphate (NaH2PO4). In following the solution is dried and heated to 400500 °C.6 During heating (calcination) two main crystalline structures form for STP. STP phase I is the high temperature phase and thermodynamically stable phase, r 2011 American Chemical Society
whereas form II is the low temperature phase.6 Phase II can be converted to phase I when it is heated above 450 °C (transition temperature), but the reverse reaction is very slow.7 The structures of two forms differ in ionic coordination of cations. Form I is more rapidly hydrating form of STP and has a higher potential solubility than the low temperature form II. Both phases form a hexahydrate structure when absorbing water. Dissolving occurs relatively less readily for phase II in water, but supersaturated solutions containing 35% or more of STP II can be prepared as crystallization speed is low. Thus, for both forms of anhydrous STP, maximum solubility is evaluated by two opposing factors, the rate of dissolving of the anhydrous salt and the rate of crystallization of hexahydrate.8 Dissolving and hydration processes are the most important properties of STP to be considered. Fast-dissolving polyphosphates are desirable in many applications such as food industry where STP need to be dissolved before being applied to food products.9 Hydration and dissolving properties of STP are controlled mostly by the physical properties of the particles and phase crystalline structure of the compound.9 Drying process and calcination parameters influence phase formation during production of STP. Dick et al.9 introduced a method for producing a fast-dissolving, noncaking, food grade STP. They stated that STPP phase I was formed at temperature as low as 225 °C, and at higher temperatures it was converted to form II. Fast dissolving STP is desirable for many applications but is especially essential in the food industry. Reduction in dissolving Received: September 9, 2011 Accepted: December 14, 2011 Revised: December 11, 2011 Published: December 14, 2011 1093
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Industrial & Engineering Chemistry Research property could result in caking phenomenon that could hinder product handling. Form I cakes least and form II cakes the most; however, very high content of phase I is also subject to caking. Kijkowska et al.10 reported that because of the high hydration rate of form I lumping property was observed for this form and as the powder particles stick to each other. This is a critical issue for detergent industry when lumping can disrupt process of spray drying. For that in the past washing powders were produced with a high content of form II.11 With advancing technology of drying, lumping problem of detergent slurries has been resolved and STP of diversified grades according to the requirements of washing powder producers (if necessary with high amount of phase I) are manufactured.10 Hensler et al.11 compared two technologies for production of STP: Spray drying that is a fast single step production with a particle residence time in the range of minutes versus calcination in a rotary drum that possesses relatively longer residence time for particles. The former is aimed for higher production rate, while the later can be used to produce high purity (at least 90%) STP. Single spray drying process has been mostly applied for production of phase I rather than phase II. That is because production of phase II needs to be carried out at lower temperature that would cause heavy caking on the walls of spray dryer. Although few studies have been carried out on production and phase characterization of STP, very little has been reported on processing factors affecting STP phase structure. In this study a detailed investigation of structure evolution of STP by using WAXD (wide-angle X-ray diffraction) and FTIR methods is carried out with respect to applying different process conditions. The effects of three main processing factors: pH of solution (that is directly related to molar ratio of the components), temperature, and time of calcination on phase formation of STP are investigated. The results of this study can be a very useful source of information for industries that produce STP in large scale. The results can be used for the manufactures to optimize the feed flow ratio, calcinations time, and temperature to obtain STP with desirable crystalline structure.
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’ RESULTS AND DISCUSSION The solutions of orthophosphates were prepared at three different pHs of 6.48, 6.57, and 6.77 by reaction of caustic soda and phosphoric acid. The graph of pH solution versus Na/P mole ratio has been depicted in Figure 1 that reveals Na/P mole ratio of 1.34, 1.35, and 1.46 for pHs of 6.48, 6.57, and 6.77 respectively. After drying the solution in an oven, the obtained powder was calcinated at three different temperatures for 15, 30, and 60 min. The results of WAXD (wide-angle X-ray diffraction) experiments for the samples obtained from the solution with pH of 6.48 and calcinated at temperature of 340 °C are shown in Figure 2. During the short time of 15 min only phase II is formed. Increasing the calcination time to 60 min impurity of NaPO3 is formed and that means the overall percentage of STP is decreased in the product. The evolution of the peak of NaPO3 is in accordance with decreasing the intensities of the peaks attributed to STP phase II. It is important to note that the peaks 21.7 and 29° corresponding to the phase I of STP11 are absent for the sample of 15 min, while a slight peak at 21.7 appeared for the sample calcinated for 60 min. That could indicate that calcinations at longer times could lead to formation of a small amount of phase I for such a sample. When calcination temperature is increased to 560 °C for the sample with pH of 6.48, WAXD results reveal only phase I that is represented with two characteristic peaks of 21.7 and
’ EXPERIMENTAL SECTION Materials. Phosphoric acid (85 wt %) with a density of 1.685 g/cm3 and sodium hydroxide (97 wt %) with a density of 2.13 g/cm3 were purchased from Sigma-Aldrich. Sample Preparation. Phosphoric acid and sodium hydroxide were reacted in a controlled environment to produce a solution of orthophosphates. Three solutions with pHs of 6.48, 6.57, and 6.77 were prepared through addition of a solution of 0.5 M sodium hydroxide into a solution of 0.5 M phosphoric acid. The solutions were initially dried at 80 °C in an oven to obtain the orthophosphates powder. The dried powder was calcinated in a furnace to obtain STP compound. Calcination temperatures were set at 340 and 560 °C, and samples were collected from the furnace after 15, 30, and 60 min. Sample Characterization. For FTIR measurements, infrared spectra were recorded on a Nicolet Magna 860 FTIR instrument from Thermo Electron Corp. (Waltham, MA) with a resolution of 4 cm1 and an accumulation of 128 scans. For X-ray experiments, a Philips X’pert diffractometer was used. The generator was set up at 50 kV and 40 mA, and the copper CuKa radiation (λ = 1.542 Å) was selected using a graphite crystal monochromator.
Figure 1. pH of solution versus molar ratio (Na/P).
Figure 2. WAXD spectrum of the STP samples prepared at pH = 6.48 and T = 340 °C. 1094
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29° as shown in Figure 3. There is no sign of impurities or phase II even as calcinations time increased to 60 min. There is a small difference in the peaks intensities of WAXD spectrum of the samples with 15 and 60 min calcinations time. That could imply that at high temperatures the STP structure is developed at even a short residence time of 15 min. Although the transformation temperature of phase II to phase I is reported to be in the range of 450 to 500 °C, it has been discussed that the transformation process is a complex phenomenon and is frequently not complete even in the compound calcinated above 500 °C.12 However as it was discussed with applied conditions in our case pure STP with phase I is achievable. The FTIR spectrum of the samples calcinated at 340 and 560 °C have been shown in Figure 4 a and b respectively. Phase II of STP could be identified by the peak of 665 cm1.6 The spectrum of the sample with a calcination time of 60 min displays a shoulder at 710 cm1 that is likely due to the formation of a small amount of phase I10 as it was observed in the WAXD pattern (see Figure 2). For the sample calcinated at 560 °C two peaks of 710 and 756 cm1 are emerged that represent phase I.10 That confirms our WAXD results presented in Figure 3. pH was increased to 6.57, and WAXD results of the samples calcinated from such solution at 340 °C are demonstrated in Figure 5. The presence of both phases I and II of STP is confirmed as well as impurities of NaPO3 and Na4P2O7 in the compound. When the sample was calcinated at 340 °C for a longer time of 60 min, the intensities of the peaks attributed to
phase II of STP are increased in compensation for the intensities of the peaks corresponding to phase I (that are located at 21.7 and 29°) that could indicate formation of more phase II with time. With regard to the previous results, increasing the pH from 6.48 to 6.57 causes formation of phase I and some impurities in the compound for low calcinations temperature such as 340 °C. If the same sample is heated to 560 °C, the crystals of phase II are transformed to phase I as it is shown in Figure 6.
Figure 3. WAXD spectrum of the STP samples prepared at pH = 6.48 and T = 560 °C.
Figure 6. WAXD spectrum of the STP samples prepared at pH = 6.57 and T = 560 °C.
Figure 5. WAXD spectrum of the STP samples prepared at pH = 6.57 and T = 340 °C.
Figure 4. FTIR spectrum of the STP samples prepared at pH = 6.48, a) T = 340 °C and b) T = 560 °C. 1095
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Figure 7. FTIR spectrum of the STP samples prepared at pH = 6.57, a) T = 340 °C and b) T = 560 °C.
Figure 8. WAXD spectrum of the STP samples prepared at pH = 6.77 and T = 340 °C.
Figure 9. WAXD spectrum of the STP samples prepared at pH = 6.77 and T = 560 °C.
The intensities of the peaks corresponding to phase I increase with time representing progressive formation of phase I as calcination proceeds. In comparison to the sample prepared at 340 °C (Figure 5), the impurity of this sample is mainly Na4P2O7, and there is no indication of NaPO3 formation as the peak of 22.4° is absent. That could imply that with increasing temperature the NaPO3 is transformed into an STP compound. It is also important to note that the Na4P2O7 peak has relatively lower intensity with respect to the peaks of STP at higher temperature. The FTIR spectrum of the samples calcinated at 340 and 560 °C are shown in Figure 7 a and b respectively. The presence of two phases I and II in the sample calcinated at 340 °C confirms the results of WAXD patterns presented in Figure 6 where the intensity of the peak corresponding to phase II increases with time, whereas the sample calcinated at 560 °C shows only phase I. In the last step the pH of the solution was increased to 6.77. The WAXD spectrum of the calcinated samples at T = 340 °C are shown in Figure 8. At this pH the impurities of NaPO3 and Na4P2O7 are formed. Comparing the intensity of the peak attributed to Na4P2O7 (see Figures 8 and 6), it is found that for such pH the impurity of Na4P2O7 is formed to a larger extent as the intensity of the corresponding peak at 26.4° increases relative to the peaks corresponding to STP. In these conditions both STP phases I
and II are observed, and similar to our finding about pH of 6.57 longer calcination times (60 min) favor formation of phase II. It could be deducted that at low temperature such as 340 °C, the rate of STP formation (phase II) is lower and phase development could continue still beyond 60 min. That is the reason that a fast spray drying technique cannot be a good candidate for STP production with a high content of phase II since the drying needs to be carried out in a lower temperature range.10 However it has been discussed that with a precise control of off gas temperature (330 °C) and addition of a small amount of phase II (2 wt %) into liquor entering the spray dryer (seeding) and also incorporation of some sodium sulfate (2%) the amount of phase II could rise above 90%.8 The WAXD spectra for the sample prepared at pH of 6.77 and calcination temperature of 560 °C are shown in Figure 9. Similar to the sample prepared at pH of 6.57 there is no sign of NaPO3 and the main impurity is Na4P2O7. The amount of phase I increases over time as the intensities of corresponding peaks rise with time. Similar to the sample prepared with pH of 6.57, the sole impurity is Na4P2O7, but the intensity of the corresponding peak is relatively lower with respect to the STP peaks. One explanation is that the larger percent of STP is formed (larger conversion degree) at higher temperatures, while the amount of Na4P2O7 is constant. The FTIR spectra of the sample with pH of 6.77 calcinated at 340 and 560 °C are shown in Figure 10 a and b respectively. 1096
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Figure 10. FTIR spectrum of the STP samples prepared at pH = 6.77, a) T = 340 °C and b) T = 560 °C.
evolving with pH and its intensity is enhanced with pH where at pH of 8.4 a sharp individual peak in that area is observed that could represent the N4P2O7 compound. The effect of temperature on phase evolution has been demonstrated in Figure 12 where the WAXD spectrum for the samples prepared from the solution of 6.77 and calcinated for 60 min are presented. It is observed that with increasing temperature to 460 °C the sole impurity of NaPO3 and also all the phase II transform into phase I. As temperature increases the intensity of the corresponding peak to phase I improves. It is important to note that the impurity of NaPO3 is mainly formed in low temperatures such as 340 °C and disappears when the temperature increases.
Figure 11. FTIR spectrum of the samples prepared at pH = 6.48, 6.77, and 8.4 at T = 560 °C.
Figure 12. WAXD spectrum of the STP samples calcinated at pH = 6.77 for 60 min.
The presence of two phases (I and II) are confirmed as depicted in Figure 10a. There is a small peak at 738 cm1 that still exists for the sample calcinated at 560 °C (see Figure 10b). The peak could be attributed to a Na4P2O7 compound. To investigate the evolution of the peak at 738 cm1 and evaluate its correspondence to the N4P2O7 compound, the FTIR results of the samples prepared from the solutions of three pHs, 6.48, 6.77, and 8.4, and calcinated at 560 °C for 60 min have been presented in Figure 11. It is observed that the peak at 738 cm1 is
’ CONCLUSION Sodium tripolyphosphate was developed in lab scale by reacting phosphoric acid with caustic soda followed by calcination of the solution at different time periods and temperatures. The effects of pH of solution, calcination time, and temperature on phase development of the final product were studied. Our findings can be summarized as follows: 1- At low pH of 6.48 pure STP can be formed. Setting calcinations temperature at a lower value of 340 °C yields mainly crystals of phase II, while calcination at a higher temperature range such as 560 °C results in production of only phase I. In such pH and low temperature range the composition of the compound was time dependent in a way that increasing calcinations time above 15 min results in formation of some NaPO3 impurity. However at high temperature such as 560 °C uniform STP was obtained at different calcinations times. 2- As the pH increase the impurities such as Na4P2O7 and NaPO3 were more readily produced. At pH of 6.57 and 6.77 both of these impurities were observed in the STP compound. However with increasing temperatures (above 460 °C) the sole impurity of Na4P2O7 was sustained. 3- At high pH of 6.77, the impurity of Na4P2O7 was extensively formed, and FTIR results also indicated progressive formation of such impurity with pH. 4- FTIR can be used as a complementary technique for phase characterization of STP specially for detection of impurity of Na4P2O7 in the STP product. ’ AUTHOR INFORMATION Corresponding Author
*Phone: +98 (311) 793-4014. Fax: +98 (311) 793-4031. E-mail:
[email protected]. 1097
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’ ACKNOWLEDGMENT The Chemical Company of Setarvan is greatly acknowledged for supporting this work. ’ REFERENCES (1) Mafaon, J. H. Preservation of fish, US Patent 3036923, 1962. (2) Zaika, L. L.; Kim, H. Effect of sodium polyphosphates on growth of listeria monocytogenes. J. Food Prot. 1993, 56, 577. (3) Zaika, L. L.; Scullen, O. J.; Fanelli, J. S. Growth Inhibition of Listeria monocytogenes by Sodium Polyphosphate as Affected by Polyvalent Metal Ions. J. Food Sci. 1997, 62, 867. (4) Banach, M.; Kowalski, Z.; Wzorek, Z.; Gorazda, K. A chemical method of the production of 00 heavy00 sodium tripolyphosphate with the high content of form I or form II. Pol. J. Chem. Technol. 2009, 11, 13. (5) Papoa, A.; Piania, L.; Ricceri, R. Sodium tripolyphosphate and polyphosphate as dispersing agents for kaolin suspensions: rheological characterization. Colloids Surf. 2009, 201, 219. (6) Kijkowska, R.; Kowalski, Z.; Pawzowska-Kozinska, D.; Wzorek, Z.; Gorazda, K. Effect of impurities (Fe3+ and Al3+) on the temperature of sodium tripolyphosphate formation and polymorphic transformation. Ind. Eng. Chem. Res. 2007, 46, 6401. (7) Kijkowska, R.; Kowalski, Z.; Pawzowska-Kozinska, D.; Wzorek, Z.; Gorazda, K. Tripolyphosphate made from wet-process phosphoric acid with the use of a rotary kiln’. Ind. Eng. Chem. Res. 2008, 47, 6821. (8) Kirk-Othmer. Encyclopedia of Chemical Technology; Wiley: NJ, 2007; Vol. 18. (9) Dick, C. C.; Schwartz, B.; McMunn, B. D.; Zeh, P. H. Fastdissolving non-caking food grade sodium tripolyphosphate. US Patent 4857287, 1989. (10) Kijkowska, R.; Kowalski, Z.; Pawzowska-Kozinska, D.; Wzorek, Z. Effect of aluminum on Na5P3O10 (form-II and form-I) thermal transformation. Ind. Eng. Chem. Res. 2004, 43, 5221. (11) Hensler, P. L.; Kelso, F. J.; Wolfe, G. E.; Zeh, P. H. Production of phase II sodium tripolyphosphate. US Patent 4656019, 1987. (12) Kijkowska, R.; Kowalski, Z.; Pawzowska-Kozinska, D.; Wzorek, Z. Quantitative determination of crystalline Na5P3O10 I (Form-I) in commercial tripolyphosphate using X-ray diffraction patterns. Cryst. Res. Technol. 2002, 37, 1121.
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