Solution chemistry of some dicarboxylate salts of relevance to the Bayer process

by Andrew John Tromans

Institution: Murdoch University
Year: 2001
Record ID: 1032090
Full text PDF: http://researchrepository.murdoch.edu.au/371/


These ions are either significant impurities in the concentrated alkaline aluminate solutions used in the Bayer process for the purification of alumina, or are useful models for degraded organic matter in industrial Bayer liquors. Such impurities are known to have important effects on the operation of the Bayer process. To develop a better understanding of the speciation of oxalate (the major organic impurity in Bayer liquors) in concentrated electrolyte solutions, the formation constant (Log Beta) of the extremely weak ion pair formed between sodium (Na+) and oxalate (Ox2-) ions was determined at 25 degrees C as a function if ionic strength in TMACl media by titration using a Na+ ion selective electrode. Attempts to measure this constant in CsCl media were unsuccessful probably because of competition for Ox2 by Cs+. Aqueous solutions of sodium malonate (Na2Mal) and sodium succinate (Na2Suc) were studied up to high (saturation) concentrations at 25 degrees C by dielectric relaxation spectroscopy (DRS) over the approximate frequency range 0.1 less-equal v/GHz less-equal 89. To complement a previous study of Na2Ox, formation constants of the Na+-dicarboxylate ion pairs were determined and they were shown to be of the solvent-shared type. Both the Mal2- and Suc2- ions, in contrast to Ox2-, were also shown to possess large secondary hydration shells. Apparent molal volumes (V[phi]) and heat capacities at constant pressure (Cp[phi]) of aqueous solutions of Na2Ox, Na2Suc, Na2Mal and K2Ox were determined at 25 degrees C up to their saturation limits using vibrating tube densitometry and flow calorimetry. These data were fitted using a Pitzer model. The adherence of V[phi] and Cp[phi] of various Na+ and K+ salts to Young's rule was examined up to high concentrations using the present and literature data. Young's rule was then used to estimate hypothetical values of CpV[phi] and V[phi] for the sparingly soluble Na2Ox at high ionic strengths, which are required for the thermodynamic modelling of Bayer liquors. The solubility of Na2Ox in various concentrated electrolytes was measured, at temperatures from 25 degrees C to 70 degrees C in media both with (NaCl, NaClO4, NaOH) and without a common ion (KCl, CsCl, TMACl). The common ion effect was found to dominate the solubility of Na2Ox. The solubility of calcium oxalate monohydrate (CaOx.H2O) was also determined. The solubilities of both Na2Ox and CaOx.H2O in media without a common ion increased with increasing electrolyte concentration, except in TMACl media, where they decreased. The solubility of Na2Ox was modelled using a Pitzer model assuming the Pitzer parameters for Na2SO4 and minimising the free energy of the system. The data were modelled successfully over the full concentration and temperature range of all the electrolytes, including ternary (mixed electrolyte) solutions.