The Kinetics of the Thermal Dehydration of the System Ca(H2po4)2.H2o-Cahpo4.2h2o

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1990 (EN)
The Kinetics of the Thermal Dehydration of the System Ca(H2po4)2.H2o-Cahpo4.2h2o (EN)

Vaimakis, T. C. (EN)

Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας (EL)
Vaimakis, T. C. (EN)

The thermal dehydration of an equimolar mixture of the Ca(H2PO4)2.H2O-CaHPO4.2H2O system was studied using a thermobalance up to 650-degrees-C. The weight loss takes place in two main steps, the first occurring at 90-160-degrees-C and the second at 270-450-degrees-C with weight losses of 13% and 8%, respectively. The dehydration was studied at heating rates of beta = 2.2, 3.7, 5.8, 8.0, 9.0 and 10.5-degrees-C min-1. From the experimental results the kinetic parameters of dehydration were studied by the methods of Friedman (F), Ozawa-Flynn-Wall (OFW), Kissinger (K) and Coats-Redfern (CR) as well as by the formal kinetics (FK). The results obtained with the first two methods (F and OFW) show that both the first and the second step occur in two stages. The second stage of the first step shows an activation energy substantially less than that of the first stage. This fact might be due to diffusion limitations of water removal during the late stages of dehydration. The same fact seems to be true also for the first and second stages of the second step treated by the F and OFW methods. The other three methods (K, CR and FK) do not distingish between the two stages noted above and the results are similar to those found for the first stage of each step. The calculated pre-exponential factors show a compensation effect of the activation energies. The results are discussed in relation to their physical meaning and in terms of the internal consistency of the methods used. (EN)

meaningful activation-energies (EN)

Πανεπιστήμιο Ιωαννίνων (EL)
University of Ioannina (EN)

Thermochimica Acta (EN)



<Go to ISI>://A1990ER73900013

Elsevier (EN)

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