SMALL MW HYDROXY- AND OXOCARBOXYLATE TYPE ORGANICS RELEVANT TO THE BAYER PROCESS - ACID-BASE PROPERTIES AND CALCIUM-COMPLEXATION IN MODERATELY TO HIGHLY ALKALINE MEDIUM
In the current study, the solution chemistry of organic ligands relevant to the Bayer process was studied in neutral and highly alkaline aqueous solutions in presence of calcium. Their deprotonation in pH > 12 region was also examined by pH-potentiometry, NMR spectroscopy and UV-Vis spectrophotometry. It was shown that these ligands underwent deprotonation in these hyperalkaline solutions; the deprotonation occurred on an OH group (hydroxycarboxylates) or on a CH2 group (oxocarboxylates). In certain cases, two deprotonation steps were detected. The deprotonation constants of these ligands were found to be rather different, between pKa = 13–15.5, which is explained in terms of the difference in their structure and/or that in the functional groups they contain. The complexation interactions with calcium were studied in neutral as well as in highly alkaline medium by Ca-ISE (ion selective electrode) and pHpotentiometry (H2/Pt electrode), respectively. In neutral solutions, complexes with 1:1 (ML) and in certain cases 1:2 (ML2) compositions were identified. In caustic solutions mono- and/or multinuclear calcium complexes were detected, and in these complexes the ligand contained deprotonated alcoholic hydroxide moiety. Multinuclear complex formation was found in systems containing certain sugar carboxylates as well as α- and b-ketoglutarate. The unexpectedly high stability of some high pH calcium complexes found highlights the importance of these organics in the Bayer process, where the prevailing physico-chemical conditions (high pH and temperature) may facilitate their formation.