A static light and X-ray scattering study of supersaturated caustic aluminate liquors

Characterization of the aluminum-containing species in freshly prepared, su persaturated caustic aluminate solutions has been carried out by performing static light scattering on three series of solutions: constant NaOH concen tration, 4.00 and 6.00 M, with varying Al(OH)(3) concentrations, 0.39-4.68 M; and constant molar ratio OH/Al = 4.37, with NaOH concentrations of 0.10- 6.00 M (Al(OH)(3) concentrations of 0.07-4.38 M). For the two sets of solut ions containing constant NaOH concentrations, the intensity of the light sc attered did not change over the concentration range of Al investigated. Thi s implies that the solution speciation is independent of the Al concentrati on and therefore, for these solutions, a conventional Hc/tau versus c analy sis to determine the weight average molecular weight or the second virial c oefficient of the Al-containing species present is not valid. Analysis of t he constant OH/Al ratio solutions revealed markedly different behavior for the dilute extreme of the concentration range. Up to an Al(OH)(3) concentra tion of 0.81 M (Al of 0.022 g cm(-3)) increases in the intensity of light s cattered were observed. Above this concentration the intensity of light sca ttered decreased slightly and then became insensitive to further increases in solution concentration. These results indicate that the nature of the so lution species in fresh caustic aluminate liquors are insensitive to Al con centration but are sensitive to the concentration of NaOH. On increase of t he concentration of NaOH in solution, the nature of the Al-containing speci es changes. In relatively dilute solutions the Al-containing species are re latively dense with respect to the background solvent and thus scattering l ight independently from the solvent. In more concentrated solutions the Al- containing species are present as large diffuse particles that are intimate ly a part of the solution and, hence, do not scatter light independently fr om the solvent. Two small- and wide-angle X-ray scattering experiments were conducted on the OH/Al = 4.37 solutions at Al(OH)(3) concentrations of 0.9 3 and 4.12 M (0.025 and 0.111 g cm(-3) Al, respectively). Each solution was allowed to age at room temperature, and time-resolved measurements were ta ken. Similar trends in the small-angle X-ray scattering (SAXS) data were ob served for both solutions on aging. The SAXS data observed have been explai ned by the following mechanism. Aluminum hydroxide "nuclei" of approximate diameter 7 Angstrom form in solution. The initial growth path for these nuc lei is via addition of small solution species, and hence a continuous parti cle size distribution is formed. Upon aging, the concentration of the nucle i in solution decreases and their growth path becomes dominated by agglomer ation. This process occurs more rapidly in the dilute solution as compared to the concentrated solution and may reflect the difference in the nature o f the particle interfacial region as implied by static light scattering; i. e., the less diffuse particles agglomerate more rapidly and hence particle growth in dilute solutions is more rapid. Additionally the WAXS data show t he presence of gibbsite diffraction peaks in the dilute solution after 17 h of aging, whereas these are not present in the concentrated solution even after 45 h.