Thermodynamic properties of calcium alkali phosphates Ca(Na,K)PO4

Abstract

Calcium alkali phosphates Ca(Na,K)PO 4 are main constituents of bioceramics and thermochemically produced phosphorus fertilizers because of their bioavailability. Sparse thermodynamic data are available for the endmembers CaNaPO 4 and CaKPO 4. In this work, the missing data were determined for the low-temperature phase modifications of the endmembers CaNaPO 4 and CaKPO 4 and three intermediate Ca(Na,K)PO 4 compositions. Standard enthalpy of formation ranges from-2018.3 ± 2.2 kJ mol-1 to-2030.5 ± 2.1 kJ mol-1 and standard entropy from 137.2 ± 1.0 J mol-1 K-1 to 148.6 ± 1.0 J mol-1 K-1 from sodium endmember b-CaNaPO 4 to potassium endmember b 0-CaKPO 4. Thermodynamic functions are calculated up to 1400 K for endmembers and the sodium-rich intermediate phase b-Ca(Na 0.93 K 0.07)PO 4. Functions above 640 K are extrapolated because of the phase transition from low-to high-temperature phase. Impurities in the synthesized intermediate phases c-Ca(Na 0.4 K 0.6)PO 4 and c-Ca(Na 0.35 K 0.65)PO 4 and one additional phase transition around 500 K impeded the determination of high-temperature thermodynamic functions. In general, data for phase transition temperatures agree with the previously reported phase diagrams.

Crystalline phase analysis and phosphorus availability after thermochemical treatment of sewage sludge ash with sodium and potassium sulfates for fertilizer production

“…Thus, phase modification of CaKPO 4 incorporated Na and has different reflexes (Fig. 3) as previous reported phase modification of CaKPO 4 [24,37].…”

“…If Na or K is preferably incorporated in phosphates [Ca(Na,K)PO 4 ] could not be deduced by laboratory-scale experiments. Synthesis of K containing Ca(Na,K)PO 4 indicates that Na is preferably incorporated in Ca(Na,K) PO 4 [37]. This supports the preferred formation Na-rich Ca(Na,K)PO 4 .…”

“…3). The reflexes in CT-K75-1050 are comparable to synthesized phase Ca(Na 0.4 K 0.6 )PO 4 [37] but shifted to higher angles indicating a lower K content (Fig. 3).…”

“…This is confirmed by an estimated phase composition of (Ca 0.90 Mg 0.10 ) (Na 0.57 K 0.43 )PO 4 (Table 2). The reflexes in CT-K100-1150 are located between samples Ca(Na 0.35 K 0.65 )PO 4 and high temperature modification of CaKPO 4 [37] and correspond to the estimated phase composition (Ca 0.86 Mg 0.14 )(Na 0.18 K 0.82 ) Fig. 2 Sections of diffractograms of samples CT-Na-1000 (black), CT-K25-1000 (green) and CT-K50-1000 (blue).…”

“…The data base of software HSC chemistry does not contain any Ca(Na,K) PO 4 species. Therefore, standard enthalpy of formation, standard entropy and coefficients for heat capacity for endmembers CaNaPO 4 and CaKPO 4 were taken from Herzel et al [37]. Thermodynamic data for intermediate Ca(Na,K) PO 4 were not considered because these data are incomplete.…”