Ind. Eng. Chem. Res. 2008, 47, 6821–6827
6821
Tripolyphosphate Made from Wet-Process Phosphoric Acid with the Use of a Rotary Kiln Regina Kijkowska,* Zygmunt Kowalski, Danuta Pawlowska-Kozinska, Zbigniew Wzorek, and Katarzyna Gorazda Institute of Inorganic Chemistry and Technology, Cracow UniVersity of Technology, Warszawska 24, 31-155 Krakow, Poland
The paper presents results on tripolyphosphate (STPP) preparation with the use of a specially constructed laboratory-scale rotary kiln. The kiln enabled agitation of the material calcined at the temperature range 400-430 °C. Wet-process phosphoric acid WPA-1, taken directly from one of the Polish plants, or WPA-2, imported from Finland, was neutralized with Na2CO3 to obtain a phosphate mixture with the molar ratio Na/P ) 5/3. To mimic industrial processing with recycling the product, some of the industrial STPP (formII) was added and the mixture obtained was fed into the rotary kiln through a vibratory feeder. The calcined product was tripolyphosphate in the crystalline form-II, and that did not depend on concentration of the WPA used, or on the amount of the STPP recycled into the kiln. When a similar phosphate mixture WPA + Na2CO3 + STPP-“recycled” was ignited at 400 °C in a stationary chamber oven, the product obtained was also STPP in form-II. In contrast, when a phosphate mixture WPA + Na2CO3 without STPP-“recycled” was calcined, the phase composition of the product obtained depended on the sort of WPA (WPA-1 or WPA-2) and, in the case of WPA-1, on its concentration. From concentrated WPA-2 the STPP formed more readily than from concentrated WPA-1. Lower H3PO4 concentration and recycling of the STPP into the phosphate mixture, before its ignition, creates favorable conditions for the STPP crystallization. 1. Introduction Sodium tripolyphosphate (STPP), Na5P3O10, is widely used in laundry, dishwasher, and industrial detergents. Although there are its substitutes, such as zeolites combined with polycarboxylates, STPP is still the most effective agent, performing several vital functions during the washing process. STPP exists in three crystalline monoclinic forms: the hexahydrate Na5P3O10 · 6H2O and two anhydrous modifications known as low-temperature Na5P3O10-II (form-II) and hightemperature Na5P3O10-I (form-I).1–8 It is widely accepted that the phase transition of form-II f form-I has been observed within the temperature range 450-500 °C. However, the transition may occur outside that range, when some impurities are present.9–11 In contrast to form-II, the high-temperature form-I exhibits “lumping” properties resulting from its extremely rapid hydration. Commercially sold STPP product contains either low-temperature form-II or a mixture of form-II and formI. The weight ratio of form-I/form-II is controlled either by the temperature of ignition or by mixing the final products of either form. Condensed sodium pyro- and polyphosphates, among them STPP, are produced mostly with the use of two-stage dehydration method.1,12,13 The first stage of this process consists in spray-drying the sodium orthophosphate solution obtained by mixing of phosphoric acid with soda ash or caustic soda. In the second stage, the dried orthophosphates are calcined (ignited) in the directly heated rotary kilns. Another type of the twostage processes is based on crystallization of orthophosphates in the first stage, and calcining in the rotary kilns in the second stage. For the production of different types of phosphates, such as acidic sodium pyrophosphate Na2H2P2O7, sodium pyrophosphate Na4P2O7, and tripolyphosphate Na5P3O10, appropriate molar ratios of Na/P and calcining temperatures are required. * To whom correspondence should be addressed. Tel.: (48) (12) 6333969. Fax: (48) (12) 6282036. E-mail:
[email protected].
In single-stage processes, instead of a spray-drying stage, sodium orthophosphate solution or a slurry, having a molar ratio of Na/P ) 5/3, is evaporated and the resulting dried intermediate is calcined in a single technological unit: a rotary kiln. While the material in the kiln is gradually heated to the desired exit temperature of 400-500 °C, the orthophosphates convert into tripolyphosphates.12 In industrial manufacturing procedures the STPP has been made from thermal or highly purified wet-process phosphoric acid (WPA). Taking in account the cost of WPA purification, the STPP production for the detergent industry (not as a food ingredient) could be based on a reasonable level of impurities in WPA. If such a solution is being considered, the process should be investigated first of all in terms of form-I/form-II composition of the final STPP product obtained from WPA from different sources and from different WPA technologies. The aim of the present work was to investigate STPP formation from industrial WPA by a single-stage method with the use of a laboratory-scale rotary kiln. To mimic the technological process, in which part of the final product is recycled, some amount of industrial STPP was added to an orthophosphate mixture, having a molar ratio of Na/P ) 5/3, before the material was put into the rotary kiln for calcining. 2. Experimental Section For the STPP preparation two sorts of industrial wet-process phosphoric acid were used: WPA-1, produced from Kola apatite, taken directly from one of the Polish plants; WPA-2, imported from Finland. Both acids contain impurities derived from raw material (Table 1). There is some difference in the impurity content: WPA-2 contains more Mg and significantly less Al. The acids were neutralized with Na2CO3 in order to make a mixture of orthophosphates with a molar ratio of Na/P ) 5/3; then some amount of industrial STPP (Na/P ) 5/3), denoted as STPP-“recycled”, was added, and the mixture obtained was fed
10.1021/ie800412q CCC: $40.75 2008 American Chemical Society Published on Web 08/16/2008
6822 Ind. Eng. Chem. Res., Vol. 47, No. 18, 2008 Table 1. Impurity Content in Industrial Wet-Process Phosphoric Acids: WPA-1 and WPA-2 no.
component [%]
WPA-1
WPA-2
1 2 3 4 5 6 7 8
H3PO4 SO4 Fe Al F Mg Mn As
66.9 0.6 0.3 0.2 0.2 0.1 0.04 3 ppm
70.0 0.9 0.3 0.05 0.3 0.8 0.03 3 ppm
into the rotary kiln through a vibratory feeder for calcining. The aim of the STPP admixture was to mimic the technological process in which part of the final product is recycled and also to obtain a “half-dry” mixture, ready to feed the rotary kiln. The mixture without STPP-“recycled” is in the form of a wet paste and cannot be fed into the kiln. The STPP for “recycling” was taken directly from the same industrial plant where the concentrated (66.9% H3PO4) WPA-1 was produced. The samples, calcined in the rotary kiln and in some cases in a chamber oven, were identified by the X-ray diffraction (XRD) method with the use of Philips X’pert equipment with a graphite monochromator PW 1752/00 with Cu KR radiation, Ni filter, 30 kV, and 30 mA. A laboratory-scale rotary kiln (Figures 1 and 2), used for calcining, was constructed at our request. The parameters for calcining were selected after some preliminary tests, including temperature calibration in the kiln. 2.1. Adjustment of Parameters Required for Calcining in the Rotary Kiln. Material for calcining, after being fed into the kiln through the vibratory chute feeder (Figure 3) Laborette 24 (Fritsch) at the rate of 1 g/min-2.5 kg/min, slid down the tube tilted about 10° with respect to the horizontal position (Figure 2). Retention time of the material calcined in the kiln depends on the rotations and feeder setting as shown in Table 2. To adjust the temperature in the oven to the needs of the experiment, several measurements of temperature, using an additional set of thermocouples (Data Logger DO 9416) fixed at different lengths of the oven, have been performed. Considering the construction of the oven and the fact that the thermocouple recording temperature in the oven was located in the stream of warm air coming from the electric heater along the axis of rotation, while the calcined material was sliding along the bottom wall, the temperature characteristics were measured using additional, adhering to the bottom wall, thermocouples at different sites. The temperature was recorded after 0.5, 1, 1.5, 2, 2.5, and 3 h while the kiln was heated. Selected results are presented in Figures 4 and 5. The results presented in Figures 4 and 5 indicate that the temperature range of 400-450 °C, proper for the STPP (formII) formation, is maintained constant when the oven is preheated for 1-1.5 h before the calcining starts.The above results allowed the authors to select the following parameters for the STPP calcining in the rotary kiln: heating the oven before the calcining starts, at least 1 h; rotations of the kiln, 1 rpm; setting of the vibratory feeder, 20; temperature on control panel, 680 °C; temperature of the electric heater, 800 °C. 2.2. Preparation of the Mixture for Calcining with the Use of WPA-1. Some preliminary experiments with a mixture of Na/P ) 5/3, prepared by WPA-1 + Na2CO3 reaction, and ignited in the chamber oven revealed that STPP was not forming when concentrated (66.9% H3PO4) WPA-1 was used (Figure 6A). Dilution of WPA-1 with water down to a level of ∼44% H3PO4 enabled STPP formation (Figure 6E).
On the basis of the results obtained in the chamber oven (Figure 6E), diluted to a level of ∼44% H3PO4, WPA-1 was selected for the experiments to run in the rotary kiln. The mixture of phosphates with Na/P ) 5/3, obtained by the WPA-1 + Na2CO3 reaction, was in a form of a “wet paste”, when concentrated (66.9% H3PO4) WPA-1 was used, or in a “souplike” form, when diluted to ∼44 H3PO4 WPA-1 was used. That could be put in a crucible to ignite in a stationary chamber oven; however, such a wet material was technically not applicable to the vibratory feeder in the rotary kiln. To obtain a “half-dry” mixture, ready to be fed into the rotary kiln, industrial STPP, taken from the same plant, where WPA-1 was produced, was added before the mixture was put into the kiln. The amount of STPP-“recycled” was determined by the weight ratio of STPP/ WPA-1. To obtain a “half-dry” mixture, the weight ratio of STPP/WPA-1 ) 1.43 was determined after a few trials. To prepare a mixture for calcining, small portions of solid Na2CO3 were added into primarily diluted to 43.6% H3PO4 WPA-1, while the reagents were continuously mixed in a mortar. The amount of Na2CO3 was equivalent to a molar ratio of Na/P ) 5/3. Then, the industrial STPP (Na/P ) 5/3) in an amount adequate to the weight ratio of STPP/WPA-1 ) 1.43 was added. The mixing, after the ingredients were combined, continued for the next 4-5 min. The dry mixture obtained was subjected to sieving. The sample of ∼40 g (
