The heterogeneous reaction of Gibbsite powder with aqueous inorganic acid solutions; kinetics and mechanism

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Gibbsite powders (surface area 0·4–1·2 m.2/g.) react with well stirred aqueous inorganic acid solutions (0·5–10 g.-ion/l.) according to a four-thirds reaction order with respect to powder weight (i.e., second order in powder surface area). The rate constants (in excess of acid) vary linearly with acid concentration and with the function a±aw2; for perchloric acid (at a±= 1, aw= 1), the rate constants k1 for Gibbsite D at unit proton activity, are at 35°, 0·071; at 50°, 0·115; and at 65°, 0·195 g.–1(m.2/g.)–2 hr.–1. Hydrochloric acid solutions dissolve Gibbsite about three times faster than perchlorc acid solutions of equal concentration; chloride ions alone do not (complex Al and) dissolve Gibbsite. Hydrogen bisulphate ion solutions dissolve Gibbsite even in the absence of excess of hydrogen ion and also enhance hydrogen-ion reactivity. Overall, sulphuric acid solutions dissolve Gibbsite 15–30 times faster than perchloric acid of equal molar concentration. The energy of activation for the reactions (at 35–65°) varies from 10,600 to 12,500 cal./mole: the enhanced rates with the complexing ions correspond to increase in pre-exponential terms. The rate constants vary slightly with the particle size of the powder. The rate-determining step appears to be a bimolecular reaction between highly hydrated hydrogen ion adsorbed on the powder surface and adjacent reactive Al–OH sites.