Interaction of calcium with sugar type ligands in solutions related to the Bayer process

Abstract

The calcium ion (particularly in the form of lime) is one of the most useful processing aids available to alumina refinery operators. In some cases the benefits provided arises through the actions of some soluble form of the calcium cation. Sugar derivatives increase the concentration of the calcium in the liquor phase, therefore investigating their complexation with compounds relevant to the Bayer process (i. e. Ca2+, Al(OH)4−) is important to the industry. Investigating the H+/Gluc– system shows that NMR spectroscopy is suitable to derive precise stability constants (log Ka= 3.23 ± 0.01), accordingly the Ca2+/Gluc– system has been treated analogously. The complex formation equilibria between Ca2+ and Gluc– has been characterized via multinuclear NMR (1H, 13C and 43Ca) measurements and the formation constants of the 1:1 complex as a function of the ionic strength (1 M ≤ I ≤ 4 M) and pH < 11 have been determined. The structure of the complex species has been identified via two-dimensional NMR spectra and the existence of a peculiar binding isomerism (carboxylate−C2 alcoholic OH carboxylate−C3 alcoholic OH) has been established. The complexation of Ca2+ with a set of sugar derivatives has also been investigated. These were the glucose, sorbitol and the anions of mucic acid, arabic acid and the heptagluconic acid. The complexes were characterized (formation constant and structure) via multinuclear NMR measurements supplemented with molecular modeling calculations. Structural investigation of these complexes revealed a novel class of scorpionate complexes. A home-made apparatus suitable to establish equilibrium solubilities over a wide temperature range (20–80 oC) has been constructed and tested. An experimental protocol to accurately determine Ca2+ concentrations via ICP−AES has been elaborated. Using the apparatus, the solubility of the CaO in NaOH (up to ~15 M) was determined at various temperatures (25.0, 50.0 and 75.0 °C). The solubility product of Ca(OH)2 and the stability constant of CaOH+ have been determined. Processing the solubility data with the aid of the ZITA suite of computer programs showed that beside Ca2+, CaOH+ is the only water soluble species present in these systems. The conditional stability constant of the high-pH calcium gluconate complex was determined via solubility measurements in 1.00 M NaOH medium and was found to be logKCaGluc = 4.02 ± 0.06. The Ca2+/Gluc– system in caustic solutions was studied by various techniques. The temperature dependence study of this system showed that chemical exchange between the various forms of Gluc– is slow at low temperatures (0−15 °C), but above 40−45 °C it becomes relatively fast on the NMR time scale. Regarding the structure of the high-pH calcium gluconate complex, results strongly indicate that the alcoholate groups on both the C2 and the C3 carbon atoms participate in Ca2+ binding. Potentiometric titrations were done using a home-built platinized-platinum hydrogen electrode. The evaluation of the titrations showed, that beside the CaGluc+ and CaGlucH–10 mononuclear complexes (and the GlucH–12– as well) the Ca2Gluc2H–30 bi- and the Ca3Gluc2H–40 trinuclear complexes are present in these systems. The geometry of the Ca3Gluc2H–40 polynuclear complex was optimized by molecular modeling calculations. The 1H and 27Al NMR measurements confirm the complexation between Al(OH)4– and Gluc– unambiguously. The pH potentiometric titration curves are practically identical to those performed in solutions containing Gluc– only. This fact suggests that the binding of aluminate to gluconate in these strongly alkaline solutions (pH > 12) is a pH−independent process. The stability constant of the complex was estimated from NMR spectroscopic measurements as new peaks emerge on the 1H NMR spectra of the Gluc– with increasing Al(OH)4– concentration. The Ca2+/Al(OH)4–/Gluc– ternary system was studied by multinuclear NMR spectroscopy and potentiometric titrations. NMR measurements showed that the presence of the CO32– had no effect on the Ca2+/Al(OH)4–/Gluc– ternary system even at high carbonate concentrations. To describe the titration curves of the ternary systems containing Ca2+, Al(III) and Gluc–, the stability constants of the particles describing the Ca2+/Gluc– binary systems were held constant. The solution species containing Al(III) only were Al(OH)2+ and Al3+. Beside the [Al(OH)3Gluc]– , [Al(OH)2Gluc]0 and [Al(OH)Gluc]+ binary complexes the formation of the [CaAl(OH)4Gluc]0, [Ca3Al(OH)6Gluc2]+ and [Ca3Al(OH)3Gluc3]3+ ternary complexes has been established.