Zygmunt KOWALSKI, Agnieszka MAKARA ? Institute of Inorganic Chemistry and Technology, Cracow University of Technology, Cracow, Poland

Abstract:
This paper presents by-products from the production of STPP using a one-stage method. Sodium tripolyphosphate Na5P3O10 (STPP) is condensed inorganic polyphosphate applied in various branches of the ndustry. STPP exists in three crystalline forms: two anhydrous forms defined as Phase I and Phase II and the hydrate form (Na5P3O10?6H2O). The methods of obtaining STPP in the form of Phase I or Phase I were developed and described. Data show that it is difficult to obtain only one of the phases which is not contaminated with at least a few percent of other phases and other sodium phosphates pyrophosphates and/or metaphosphates).

Please cite as: CHEMIK 2013, 67, 3, 198-205

Introduction

Sodium tripolyphosphate is among the most fundamental fillers used in cleaning and washing agents. STPP is used in detergents due to, among other things, its sequestration properties that make it possible to form soluble complex compounds with calcium and magnesium ions. Such complex compounds reduce water hardness. Binding heavy metals prevent the corrosion of washing appliances [1÷7]. Buffering properties of STPP can adjust food acidity. Condensed phosphates are also capable of forming complex compounds with proteins or pectins counteracting the dehydration process of food products (so called protein effect), whereas the formation of complex compounds with heavy metal ions inhibit the oxidation reaction, preventing the development of micro-organisms in food. STPP also stabilises water emulsions, fats and proteins [3, 8÷10].

Sodium tripolyphosphate can exist in two anhydrous forms, namely low and high-temperature forms defined as Phase I and Phase II, and in the form of hexahydrite. It is very difficult to obtain only one of STPP phases which is not contaminated with at least a few percentages of another phase and additionally with various quantities of sodium pyrophosphate and sodium metaphosphate. Sometimes, the product also contains significant quantities of amorphous forms [11÷16]. STPP can be also produced according to the classic spray-kiln technology (a two-stage dehydration method) or a single-stage method in which the process of drying and calcination occurs during the single technological process (a single-stage dehydration) [17÷19].

The selection of adequate calcination parameters and a type of raw material significantly influences the phase composition of the obtained product, that is, the presence of the main component and accompanying by-products. Commercial STPP usually contains 93% of Na5P3O10 in the form of Phase I and/or Phase II, whereas the remaining part consists of sodium pyrophosphate, sodium metaphosphate and hydrate form of STPP [17, 20÷24].

Methods of obtaining sodium tripolyphosphate

Thermal or purified wet-process phosphoric acid and sodium carbonate are usually used in the production of STPP. The mole ratio of Na2O/P2O5 (so called TM) of the used raw materials equals 1.67 [17, 18, 23÷28]. The 1st stage of STPP production consists in neutralising phosphoric acid with sodium carbonate or sodium hydroxide, then the obtained salts (sodium dihydrogen phosphate and disodium hydrogen phosphate) are condensed to tetrasodium diphosphate and disodium dihydrogen phosphate during drying process. Calcination resulting in condensing pyrophosphate salts into sodium tripolyphosphate [8] is the next stage. Neutralisation is the ion reaction whose rate increases along with the temperature increase. The neutralisation degree has to be strictly monitored, the content of Na4P2O7 in the end product increases at higher alkalisation degree, whereas NaPO3 can be formed at lower alkalisation degree [29, 30]. Sodium dihydrogen phosphate NaH2PO4 obtained from the neutralisation reaction is stable up to the temperature of 160?C; while above this temperature, it is dehydrated to Na2H2P2O7 which is condensed into (NaPO3)3 and Maddrell?s salt (the low-temperature form) [16]. The condensation process of disodium hydrogen phosphate (Na2HPO4) results in the formation of anhydrous Na4P2O7 [17, 31, 32].

The process of formation of anhydrous sodium tripolyphosphate starts at a temperature range of 200-250?C and its rate increases along with the temperature increase. At 300?C, condensation to anhydrous STPP lasts for a few minutes, while at ~400°C, condensed STPP assumes the form of Phase II. Within a temperature range of 450-500°C, Phase II is quite quickly transformed to Phase I which is not always complete and the product obtained at temperatures >500oC contains a few per cent of the low-temperature Phase II [16, 17, 19]. The further heating up to 865°C results in obtaining an alloy whose quick cooling and then holding at a temperature range of 500-620°C causes the reformation of Phase I of STPP, whereas slow cooling and holding at ~250°C causes the formation of Phase II [16, 17].

Sometimes Phase I of sodium tripolyphosphate is formed at a relatively low temperature of 235?C, then its content increases along with the temperature increase and disappears as a result of the transformation into Phase II. This is an example of the Gay-Lussac- Ostwald step rule, called the law of successive reactions, which applies to substances that can exist in a few forms, and states that an unstable form is obtained first, prior to the stable form [16, 17, 22]. Obtaining Phase I during the low-temperature transformation of orthophosphates and then the transformation into Phase II indicate that the transformation of Phase I into Phase II is catalysed by the presence of the amorphous form. Maddrell?s salt (NaPO3 II and NaPO3 III) and sodium trimetaphosphate (NaPO3 I) are two other crystalline forms which can exist at other temperatures and are clearly formed from the acidic amorphous phase (Na2H2P2O7) [16, 17]. STPP belongs to the group of compounds with incongruent melting point, i.e. while transforming into liquid, STPP also splits into another phase and liquid of different chemical composition than the starting compound [17, 33÷35]. According to [14], sodium tripolyphosphate is the intermediate phase between the twocomponent Na3P3O9-Na4P2O7 system which melts incongruently into the liquid form and crystalline Na4P2O7 at 620-622?C.

Description of by-products formed in the production process of STPP

Depending on the process conditions and a type of the used raw material, the production process of sodium tripolyphosphate is always accompanied with the formation of such by-products as sodium pyrophosphate and sodium metaphosphate [17]. Sodium pyrophosphate, similarly as STPP, belongs to condensed inorganic phosphates. Tetrasodium diphosphate, called neutral sodium pyrophosphate (Na4P2O7) and disodium dihydrogen diphosphate (Na2H2P2O7), called sodium acid pyrophosphate are the most important regarding the industrial significance [15÷17, 32]. Pyrophosphates tend to exist at various polymorphous phases, which can be caused by three types of structure arrangement [11, 15, 17, 32].

For obtaining Na4P2O7, the temperatures of drying and calcination are practically the same as for producing STPP. However, the mole ratio of Na/P, so called TM, is considerably higher (1.95-2.0). In the production process of Na2H2P2O7, TM equals 1.0 and the calcination temperature is within the range from 207 to 227?C [3, 8, 32]. As sodium pyrophosphate is capable of sequestering calcium and magnesium ions, it can be used as water softening agent, and similarly as STPP, it is an ingredient of soaps and detergents [15, 32]. Sodium pyrophosphate also acts as a crystal growth inhibitor (impedes the formation of dental calculus caused by saliva), thus it can be used in toothpastes and mouthwash products [17, 32, 36, 37]. In the meat industry, sodium pyrophosphate improves meat succulence, tenderness and colour [8, 17, 32, 38], and by sequestering heavy metal ions, it contributes to food preservation [3, 32, 39].

Sodium metaphosphate [11, 15, 16], NaPO3 II (high-temperature Maddrell?s salt), NaPO3 III (low-temperature Maddrell?s salt) and NaPO3 IV (Kurrol?s salt) belong to the 2nd group of by-products formed in the production process of STPP. General chemical formula of metaphosphates is (NaPO3)n, where n is assigned to high values. Sodium trimetaphosphate (NaPO3)3, known as NaPO3 I or Knorr?s salt, was the first cyclic compound. Sodium trimetaphosphate exists in three polymeric forms: Form I, II and III [15-17, 40, 41] which can be obtained from melted sodium polyphosphates at maintaining the adequate temperature conditions [15, 16, 40]. Sodium metaphosphate is an excellent water softening agent; it very effectively counteracts the release of calcium carbonate in the form of boiler scale from boiling spring or tap water [7, 17, 40, 42]. Metaphosphate is used in the meat industry due to its capacity of binding water, metal ion emulsification and sequestration [43], which stabilises the product shape, enhances its succulence, tenderness, colour and microbiological purity [8, 17, 40].

Experimental part

Materials
?Alwernia? wet-process phosphoric acid (WPPA) from Zakłady Chemiczne Alwernia S.A.[Alwernia Chemical Company, joint-stock company] and thermal phosphoric acid p.a. (purchased from POCH S.A.). Phosphoric acids were neutralised using soda ash (I grade) from Zakłady Chemiczne Alwernia S.A..

Research Methodology

Charge blends were prepared by adding gradually soda ash to weighed quantity of ~10 g of phosphoric acid (?Alwernia? and thermal phosphoric acid) in the quantity required to maintain the stoichiometric mole ratio of Na/P = 1.67. The obtained mix was rubbed in a mortar until obtaining the uniform ?semi-dry? blend in the form of sand. The prepared blends were transported to evaporating dishes and calcinated in a chamber furnace within a temperature range of 250-550°C for 60 minutes. The phase composition of calcination products was determined in ex situ tests with Philips X?Pert diffractometer equipped with graphite monochromator PW W 1752/00 and using Cu Ka radiation and Ni filter. Table 1 presents the phase composition of products. The content of phosphate form in the obtained products was determined using the method of ionic liquid chromatography with the chromatography kit of the US company called DIONEX [17]. The content of ortho-, pyro-, tripoly- and higher forms of phosphates was determined by the chromatographic analysis the results of which, expressed as Na2HPO4, Na4P2O7, Na5P3O10, Na6(PO3)6, are listed in Fig. 1(A-D). The samples for determining phosphates were dissolved in water by preparing a 0.2% solution. Each sample was dissolved in water and then filtrated, and a filtrate containing watersoluble sodium phosphates was analysed. The content of insoluble substances in the analysed samples was determined by a gravimetric method (Fig. 2) [44].

The microscope photos were taken using JSM 5510LV scanning electron microscope at 5000x magnification for the products obtained at the calcination temperatures of 350°C, 450°C and 550°C (Figs 3 and 4).

The results from the chromatographic analysis of the products make it possible to state that the content of phosphate forms and insoluble substances varies depending on the calcination temperature and a type of the used raw material (Fig. 1). The sample obtained from thermal phosphoric acid and calcinated at 250°C contained 13.17% of STPP, while STPP content was increased to 86.28% by increasing the calcination temperature only by 50°C. At 350°C, STPP reached its maximum content of 95.44% and then, the further increase in temperature caused the gradual reduction in Na5P3O10 content. At 550°C, STPP content drastically dropped which was likely to be caused by the decomposition of its crystalline structure to, among other things, Na4P2O7. This can be confirmed by nearly a two-fold increase in the content of tetrasodium diphosphate at 550°C (20.31%) in comparison to its content at 500°C (10.54% of Na4P2O7). Sodium metaphosphates insoluble in water were present in the samples obtained from thermal phosphoric acid at a temperature range of 300-500°C (Fig.2). The maximum content of insoluble substances was observed at 450°C (5.59%). The content of tetrasodium diphosphate varied from 85.81% at 250°C, at which the formation rate of Na4P2O7 is the fastest, to 3.94% at 350°C. The content of sodium orthophosphates varied within the range of 0.17-2.86%.

In the samples obtained from ?Alwernia? WPPA, the formation of sodium tripolyphosphate was conducted in a similar way as in the samples obtained from thermal acid, whereas the lower content of STPP was observed. At 350°C, the highest content of Na5P3O10 was observed (79.96%) which was decreasing as the calcination temperature was increasing. At the temperatures of 500°C and 550°C, STPP content was reduced to 64.51% and 65.50% respectively and Na4P2O7 content was increased to 30.97% and 29.79%. Insoluble substances and higher forms of phosphates, similarly as for the samples with thermal phosphoric acid, exist within the temperature range of 300-500°C, and the highest content of 3.17% was also observed at 450°C.

The microscope photos illustrate the changes that occurred in the tested samples depending on the calcination time and a type of the used raw material. The microscope image of products without any impurities (Fig. 3) shows that the obtained product is more coarsely crystalline than the products from ?Alwernia? wet-process acid (Fig. 4). The impurities present in acid influenced the product crystallisation depending on their type and quantity in the analysed system.

Conclusions

Sodium pyrophosphates and sodium metaphosphates were observed in all analysed samples regardless of the type of acid and the parameters of the calcination process. The selection of the adequate parameters of calcination can reduce or increase the quantity of byproducts, while it is impossible to obtain the product containing only the phase of sodium tripolyphosphate because of the complexity of sodium phosphate transformations. The chromatographic analyses demonstrate that the samples from thermal acid are characterised by higher content of sodium tripolyphosphate and insoluble substances, whereas the products from ?Alwernia? WPPA usually have higher content of pyro- and meta- forms and they always contain orthoform. The differences in the quantitative composition of phosphate forms in the samples from thermal and wet-process acids, at the same calcination parameters, result from the differences in the chemical compositions of acids. The by-products were found in each of the analysed samples.

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Zygmunt KOWALSKI ? (Sc.D., Eng.) Profesor graduated from the Jagiellonian University in 1969. Currently, he is the Dean of the Faculty of Chemical Engineering and Technology at the Cracow University of Technology and Head of the Institute, Manager of the Department of Inorganic Technology and Environmental Biotechnology. Specialisation? technology of inorganic compounds and environmental engineering.
Contact details: Institute of Inorganic Chemistry and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155, Cracow.
e-mail: ,
Phone: + 01, fax: + 35,

Agnieszka MAKARA ? Ph.D. (Eng.) graduated from the Faculty of Chemical Engineering and Technology at the Cracow University of Technology in 2007. She is an [assistant professor] at the Department of Inorganic Technology and Environmental Biotechnology. Specialisation ? inorganic chemical technology.