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Carbonate and sulfate equilibria in highly concentrated aqueous solutions

Capewell, Stephen G. (1999) Carbonate and sulfate equilibria in highly concentrated aqueous solutions. PhD thesis, Murdoch University.

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Refining alumina from its ores by the Bayer process is one of Australia’s largest industries. Continual recycling of the process liquor results in the build up of simple inorganic anions such as carbonate and sulfate which can have major deleterious effects on alumina production. In order to develop improved processes for their removal, high quality thermodynamic data are sought by the alumina industry. This thesis describes a fundamental investigation of the chemical speciation and solubility of carbonate and sulfate in highly concentrated aqueous mixed electrolyte solutions.

Given that sodium is present at very high concentrations in Bayer liquors and plays an important role in the behaviour of these anions in industrial process solutions, the nature of very weak sodium ion-interactions has been quantitatively investigated. The association of Na+ (aq) with CO32- (aq) and SO42~ (aq) has been measured using high precision sodium ionselective electrode potentiometry in solutions of ionic strength ranging from 0.5 to 7.0 M in CsCl media and at 1.0 M in Me4NCl media. Dielectric relaxation spectroscopy has also been used to provide information on the structure of the ion-pairs present and to provide independent confirmation of the association constants.

Systematic measurement of the solubilities of Na2CC>3 and Na2S04 have been made in a variety of highly concentrated aqueous mixed electrolyte solutions. These include the ‘noninteracting’ media CsCl and Me4NCl, and the industrially relevant caustic sodium aluminate (synthetic Bayer liquor) solutions. A high precision solubility apparatus, capable of operating in the temperature range of 5 to 90°C (with a long term temperature stability of ± 0.003 K) was purpose built for these measurements. The apparatus produced solubility data with a routine precision of ± 0.05%, and agreement to within ± 0.1% of recommended IUPAC values for test salts.

The data for carbonate and sulfate obtained in the present work have been combined with carefully chosen thermodynamic data from the literature in an attempt to model the solubility of the sodium salts in concentrated mixed electrolytes and in alkaline aluminate solutions. These calculations were able to rationalise qualitatively the experimental observations. It is concluded that, despite the weakness of the interactions, sodium ion-association to carbonate and sulfate is a key factor in determining the solubilities of Na2CO3 and Na2SO4 in industrial Bayer liquors.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Hefter, Glenn and May, Peter
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