5282
Ind. Eng. Chem. Res. 1997, 36, 5282-5290
Manufacturing of Pyro- and Tripolyphosphate Complexes of Micronutrients in the Process of Phosphates Condensation Barbara Grzmil† Institute of Inorganic Chemical Technology, Technical University of Szczecin, Pułskiego 10, 70-322 Szczecin, Poland
The aim of effected research was to appoint the possibility of manufacturingsin the polycondensation process of potassium and sodium phosphates at the temperature 400 °Csof solid, neutral pyro- or tripolyphosphate complexes of micronutrients, as (K,Na)4n-2Me(P2O7)n and (K,Na)5n-2Me(P3O10)n, where Me2+ means Cu2+, Zn2+, Mn2+, or Co2+. The influence of substrates mixture composition and of the mole ratio P2O74-/Me2+ or P3O105-/Me2+ on the properties of obtained products was defined. The condensed phosphates, which were manufactured, were typified by differentiated water solubility with reference to P2O5, K2O, Na2O, and Me2+ alike. The presence of assumed complex compounds in the manufactured condensed phosphates was not confirmed by means of X-ray diffraction analysis, because of the existence of different crystalline phases on the one hand and lack of suitable standard substances on the other, and also because of the possibility of their occurrence in amorphous form. Only in the case of the pyrophosphate complex with zinc, the existence of the phase, corresponding to K6Zn(P2O7)2 was found, and in the case with copper, the possibility of the occurrence of K6Cu(P2O7)2 was suggested. Therefore, for manufacturing of liquid fertilizers, the more suitable solution will be the complexing of micronutrients in water solution, with the utilization of alkali metals pyro- or tripolyphosphates, previously obtained in the condensation process. Introduction In the process of mixed, multicomponent fertilizers manufacturing, the micronutrients are introduced most often in the form of inorganic compounds at various degrees of water solubility, as for example sulfates, oxides, carbonates, or silicates (Hignett and Mc Clellan, 1985; Mortvedt, 1985). Minerals, slags, ashes and the other industrial wastes could be their sources (Mineeva et al., 1985, Wolski and Gawecki, 1989). During the granulation of such fertilizers, water-soluble forms of micronutrients can suffer change into insoluble forms. However, the presence of micronutrients complexes can prevent this unfavourable phenomenon (Glikes, 1977; Potatueva et al., 1985; Hignett and Mc Clellan, 1985). One of the methods of the manufacturing of solid phosphate fertilizers, containing micronutrients in complexed with polyphosphates form, is the low-temperature NH4H2PO4 or KH2PO4 condensation process, in the presence of these micronutrients. Potassium phosphate is produced as a result of the decomposition of potassium chloride with phosphoric acid, and the product is socalled-modified potassium metaphosphate at various properties (Bronnikov and Imhanickaia, 1960; Buhałva and Rabkina, 1975; Moore, 1994; Zaripov and Begłov, 1990). It was proved that according to the dehydration parameters, such as temperature and mole ratio Me2+/ P2O5 of parent substances, water-soluble products at a high content of hydropolyphosphates can be obtained (Glabisz and Grzmil, 1991; Grzmil and Glabisz, 1991). One can also carry on the polycondensation process of phosphoric acid in the presence of Me2+, e.g., ZnO, the dissolution of the obtained product and then neutralization of the suspension by means of NH3 or CaCO3 (Lavrov and Bykanova, 1975; Ray et al., 1993). There are also several well-known papers, in which thermal processes of the production and properties of neutral heavy metals pyrophosphates are described (Selivanova †
Telephone: +48 91 340843. Fax: +48 91 330352. E-mail:
[email protected]. S0888-5885(97)00305-9 CCC: $14.00
and Morosova, 1975; Gołoshchapov and Martynenko, 1976; Konstant and Limante, 1977; Morosova et al., 1978; Tananaev and Grunce, 1984; Levchik et al., 1995). It is also possible to obtain hydrated solid phases corresponding to the compounds, for example, MeR2P2O7‚nH2O, MeR3P3O10‚nH2O, or R6Me(P2O7)2‚ nH2O, by precipitation (in adequate conditions) of alkali metal (R+) pyro- or tripolyphosphates and micronutrient (Me2+) salts from the solution at adequate concentration (Basset et al., 1936; Van Wazer, 1961). The common opinion is that these condensed phosphates are in principle water-insoluble. However, sodium copper pyrophosphate Na6Cu(P2O7)2‚16H2O, at good water solubility, was obtained (Basset et al., 1936). In the present paper the investigations were undertaken, the aim of which was to define the possibilities of producingsin the polycondensation process of phosphatesssolid, neutral pyro- or tripolyphosphate complexes of micronutrients, as R4n-2Me(P2O7)n and R5n-2Me(P3O10)n, where R+ ) K+ or Na+; Me2+ ) Cu2+, Zn2+, Mn2+, or Co2+. As it is found in the publications, the complexes, which are characterized by higher values of stability constant in the solution, can be obtained by usingsas the sequestrating agentsnot hydrogen polyphosphates but neutral polyphosphates (Ringbom, 1963). As a substrate in low-temperature polycondensation of phosphates, the solid product from the process of lowchloride NPK fertilizers production was used (Glabisz et al., 1992). In this process, as a result of the conversion reaction of potassium chloride with ammonium dihydrogen phosphate (from the ammoniation process of wet phosphoric acid), a solid phase is obtained, with the following main components: KH2PO4 and NH4H2PO4. Because of the decomposition of ammonium dihydrogen phosphate and ammonia desorption during the dehydration process, it was assumed that the ammonium and hydrogen ions would be substituted with the sodium ion (as Na2CO3). The first stage of investigations included the preparation of potassium-sodium or potassium-calcium con© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5283
densed hydrophosphates (Grzmil and Kic, 1995). It was proved, that under the most favorable conditions of condensation reaction it is possible to obtain potassium-sodium polyphosphates which are totally watersoluble and which are suitable for liquid PK fertilizers’ production. However, under the other conditions, condensed potassium-sodium or potassium-calcium phosphates with differentiated solubility were obtained, which could make a group of so-called fertilizers with controlled solubility, i.e., slow release fertilizers. In the next stage of investigations, not only was the NH4+ ion replaced by the sodium ion (as Na2CO3) but also the H+ ion, to obtain neutral pyro- or tripolyphosphates, whichstogether with micronutrientsscreate complex compounds, with higher stability constants than those for hydropolyphosphates (Grzmil, 1997). Methodology Polycondensation of potassium, sodium, and ammonium hydrophosphates, in the presence of a specified micronutrient (as MeSO4) was carried on at a temperature of 400 °C and following molar ratios of the substrates: system K2HPO4- or Na2HPO-MeSO4 K2HPO4-KH2PO4- or Na2HPO4-NaH2PO4-MeSO4 KH2PO4-NH4H2PO4-Na2CO3-MeSO4
mole ratio of the substrates 4:1 (1) 5:1 6:1 4:2:1 (2) 5:2.5:1 6:3:1 4:2:4:1 or 6:3:4.5:1 (3) 3:3:4.5:1 or 4.5:4.5:5.25:1
It was assumed that the condensation processes in the investigated heterogeneous mixtures would take place according to the following reactions:
2nR2HPO4 + MeSO4 f R4n-2Me(P2O7)n + R2SO4 + nH2O (1) 2nR2HPO4 + nRH2PO4 + MeSO4 f R5n-2Me(P3O10)n + R2SO4 + 2nH2O (2) 3KH2PO4 + 3NH4H2PO4 + 4.5Na2CO3 + MeSO4 f (K3-x,Na9-y)10Me(P2O7)3 + (Kx,Nay)2SO4 + 7.5H2O + 4.5CO2 + 3NH3 (3) 4.5KH2PO4 + 4.5NH4H2PO4 + 5.25Na2CO3 + MeSO4 f (K4.5-x,Na10.5-y)13Me(P3O10)3 + (Kx,Nay)2SO4 + 11.25H2O + 5.25CO2 + 4.5NH3 (4) The molar ratio of the complexing agent (i.e., P2O74or P3O105-) and the micronutrient was changed from 2 to 3 for the first series of experiments (systems 1 and 2), while that ratio for the second one was equal to 3 (system 3). Because of NH4H2PO4 decomposition and ammonia desorption during the polycondensation process, it became necessary, in that case, to substitute NH4+ and H+ ions with the sodium ion (as Na2CO3), to obtain neutral polyphosphates. To reach the highreaction ratio, water was introduced to the substrates for preliminary, partial dissolving of the components to improve the condensation conditions. The liquid to solid phase weight ratio was equal to 0.15, which cor-
responded to the moisture content of the solid product from the potassium chloride conversion process. The reaction mass (about 12 g) of a definite composition was first thoroughly mixed, then placed in a glass crucible, and finally put into a furnace. The heating time of the reacting substances to the assumed temperature and the time of polycondensation at that temperature were equal to 1.0 h. The substrates were heated at the rate of about 6.3 °C/min, but without being mixed. The process was carried on in a periodic mode, in a vertical, laboratory tubular furnace, equipped with an automatic control system. The reactor was purged by the air at the rate of about 6 dm3/h., for a uniform carrying away of the gases, ammonia, carbon dioxide, and steam, emitting during the reaction. Total and water-soluble P2O5 content were determined in the products, as well as the fractions of particular forms, i.e. ortho-, pyro-, tripoly-, and higher condensed phosphates, determination of which has been made by means of the colorimetric vanadium-molybdenum method (Marczenko, 1979). Additionally, the fraction of the water-soluble forms of potassium and sodium in the obtained composition was determined by means of the flame photometric technique and the content of copper, zinc, manganese, and cobalt by means of the polarography technique. (Połektov, 1969; Milner, 1962). The ammonia content was determined with an ion-selecting ammonium electrode and the carbon dioxide content by means of gasometric analysis (Orion Research, 1979; Struszyn´ski, 1954). Diffraction analysis was applied for the identification of the phase composition of the crystalline condensation products. Results and Discussion In the first stage of investigations, the effects of the complexing agent cation’s type (K+ or Na+) and of the molar ratio of the P2O74- or P3O105- anion and the micronutrient were determined in relation to the following: (1) water solubility of obtained products, basing oneself on the determination of P2O5 content; (2) the fraction of the water-soluble form of the micronutrient; (3) the fraction of P2O5 particular forms: ortho-, pyro-, tripoly-, and higher condensed forms in relation to water-soluble phosphates; (4) the phase composition of obtained polyphosphate complexes. In the second stage the condensation process of the KH2PO4-NH4H2PO4 mixture at analytical purity, and of real products from the production process of lowchloride NPK fertilizers, at a different level of impurities, where the main phase was (NH4,K)H2PO4 (Table 1), was carried on. The influence of the initial mixture composition and of the impurities present in the system on the listed above properties of obtained products was determined. The results, gained from particular sets of experiments, referring to K2HPO4 dehydration in the presence of Cu2+, Zn2+, Mn2+, or Co2+ are set in Table 2. It was found that the water solubility of obtained products was differentiated and oscillated from about 80 to 100%. However, among the soluble phosphates, the main fraction composed pyrophosphates-in case of dehydration in the systems containing copper and manganese or tripolyphosphatessin the presence of zinc and cobalt, although the production only of pyrophosphate complexes together with the listed cations (as K4n-2Me(P2O7)n was the aim of the experiments, described above. Water solubility of particular microelements, included in the condensed phosphates, was also differentiated. The
5284 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 Table 1. Quantitative and Phase Composition of Conversion Products of Potassium Chloride with Ammoniated Wet Phosphoric Acid content no. P2O5 1 2 a
K2O
N
Cl-
% SO42-
F-
Fe
Mg
Al
ppm As Cd Cr Mn Mo Ni Pb Zn
45.58 20.77 5.23 5.32
1.42
0.158 0.520 0.20 0.29
