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Lime chemistry in the Bayer process

Xu, Bingan (1991) Lime chemistry in the Bayer process. PhD thesis, Murdoch University.

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Abstract

A fundamental study of the kinetics and mechanism of lime chemistry in the Bayer process has been undertaken. To simplify the study and avoid interference from impurities, experiments were carried out in relatively pure systems. Calcium hydroxide and calcium oxide, the solid reactants, were used in two different forms: rotating discs and powders. The former gives the most useful mechanistic information, while the latter most closely approximates industrial practice. The structure and composition of the surface was also investigated using a variety of techniques including scanning electron microscopy.

The dissolution of calcium hydroxide in water has been investigated under a variety of conditions. A study of the dissolution of rotating calcium hydroxide discs at low rotation speeds (<250 rpm) showed that the reaction rate increased with increasing rotation speed and temperature but decreased when the solution contained either calcium ions or hydroxide ions. It was concluded that the dissolution of calcium hydroxide under these circumstances is controlled by the diffusion of calcium ions and hydroxide ions away from the calcium hydroxide surface. Using the Levich equation for mass transfer from a rotating disc and assuming that the dissolution step prior to slow diffusion from the surface is at equilibrium, it is possible to estimate the dissolution rate. Rates calculated in this way agree well with the measured rates.

At high rotation speeds the reaction becomes controlled by the speed of the dissolution process itself.

For powdered calcium hydroxide samples, it was found that the dissolution process can be approximated by the "shrinking sphere" model and that the associated rate constant behaves similarly to that for a rotating disc under conditions of diffusion control.

An investigation of the slaking of calcium oxide discs in water and solutions containing Ca2+ or OH- ions showed that the slaking process has the same behaviour as that for calcium hydroxide the diffusion discs, i.e. of Ca2+ or OH- ions away from the surface is also the rate-determining step when the reaction is slow. Therefore, the theory proposed for the dissolution of Ca(OH)2 discs applies to the slaking reactions. However , when the reaction rate is fast enough, the entire slaking process becomes chemically controlled. The similarity of the results obtained using both Ca(0H)2 discs and CaO discs indicates that the reaction of calcium oxide with water to form calcium hydroxide is not the rate-determining step.

It was found that the slaking of powdered calcium oxide also follows the shrinking sphere model proposed for the dissolution of calcium hydroxide particles. The rate of slaking depended on the size of the calcium oxide particles and the calcination conditions (temperature and time) under which the calcium oxide was formed from calcium carbonate. The presence of Ca2+ ions and OH- ions causes a reduction in the slaking rate similar to that observed for rotating disc samples under the same conditions, suggesting that the reaction mechanism is the same for both powdered and disc samples.

The slaking of both powdered CaO and rotating CaO disc samples in sodium carbonate solution is controlled by the deposition of calcium carbonate on the dissolving surface. This results in a reduction in the dissolution rate of the calcium hydroxide or calcium oxide, and a reduction in the rate of precipitation of calcium carbonate on the surface. Before the blocking layer of calcium carbonate is formed, the loss of carbonate ions from solution is at least partially diffusion controlled.

A study of the slaking of CaO in sodium aluminate solution was also conducted. The experimental results show that calcium aluminate precipitates on the reacting CaO surface, leading to a reduction in the dissolution rate of Ca(OH)2. The experimental results also suggest that the precipitation of calcium aluminate is chemically controlled while the slaking reaction remains diffusion controlled. Under competitive reaction conditions, the rate of CaCO3 formation is faster than that of Ca3Al2O5.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Mathematical and Physical Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Ritchie, Ian and Giles, Dion
URI: http://researchrepository.murdoch.edu.au/id/eprint/52725
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