THERMAL DEHYDRATION OF HYDROXIDES

The dehydration of Mg(OH)2 is particularly suitable for experimental investigation because of the existence of only two well-crystallized phases, Mg(OH)2 and MgO. The reaction mechanism has been studied in greater detail than has been found practicable for decomposition of most other hydroxides through a combination of complementaiy experimental approaches (kinetics, crystallography, microscopy and others). 

Freund et al. [3,4] have discussed the chemical changes which occur in the reaction zone. The transfer [3] of a proton from OH to a neighbour by a tunnelling process is only possible if there is a vacant acceptor level of equivalent energy available a 

The kinetics of dehydroxylation of Mg(OH)2 are sensitive to the prevailing pressure of water vapour, a factor which lead Sharp [6] to conclude that the value of should only be determined imder conditions of controlled atmosphere. From consideration of the ranges of values of which have been reported, 67 to 400 and 80 to 125 kJ mol fi om dynamie and from isothermal methods, respectively, it is concluded that more sophisticated analyses of the programmed temperature data are required. The most reliable value of Z , is identified [6] as 84 kJ mol, comparable with, or slightly greater than, the reaetion enthalpy. 

Stable ions, including silicate, sulfate, aluminate and phosphate ions, hinder the dehydration of Mg(OH)2. The influence depends on the particular ion present, its concentration and the crystallographic plane in which it is located. Both the induction and growth processes of the anisotropic water-elimination reaction are affected. 

Galwey and Laverty [13] concluded, from complementary kinetic investigations, that dehydration of Ca(OH)2 was satisfactorily represented by the first-order equation. The overall rate of H2O release was sensitive to the locally prevailing water vapour pressure and its distribution within the reactant mass. Kinetic characteristics were strongly influenced by the rates of intracrystalline and intercrystalline diffusive escape of product water which strongly depended on reactant compaction/dispersal within the heated zone. Measured values of and for Ca(OH)2 dehydration could not be identified with a single reaction step because 

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