Extraction of magnesia from Egyptian serpentine ore via reaction with different acids .2. Reaction with nitric and acetic acids

The results of a kinetic study of the dissolution of magnesia from Egyptian serpentine ore with nitric and acetic acids are presented. The effects on the magnesia dissolution of (a) temperature ranging from 30 to 105 degrees C, (b) time ranging from 2.5 min to several days, and (c) molar ratios between one of the acids and MgO in the ore ranging from 4.42 to 3.86, are presented. From the analysis of the dissolution rate on the basis of a core model, it was found that dissolution rate of MgO could be well expressed by a rate equation based on the rate-limiting step of the surface chemical reaction. The apparent activation energies were ca. 17.4 and 8.3 kcal mol(-1) for nitric and acetic acid respectively. The high activation energy of reaction with nitric acid was found to be associated with relatively higher values of activation entropy (Delta S* = -19.5 cal mol(-1) K-1), whereas the relatively low activation energy of reaction with acetic acid is associated with negative values of entropy of activation (Delta S* = -44.3 cal mol(-1) K-1). The values of the latter parameter indicated that their values are related to the complexing affinity of magnesium cation towards acid anion. Thus, the values of entropy of activation are the predominant factor for the reaction between the ore and studied acids.