Igor Ametov

Rheological investigations of gibbsite particles in synthetic Bayer liquors

Abstract Rheological studies of colloidal gibbsite suspensions in caustic aluminate solutions (synthetic Bayer liquors) were undertaken to elucidate the particle interaction behaviour during the crystallisation step of Bayer processing. The influence of suspended particle volume fraction, temperature and aluminate supersaturation on both the static and dynamic rheology and its time-dependency is reported. Freshly prepared suspensions are near Newtonian, but upon aging, exhibit increased apparent viscosities, pseudoplasticity, yield stresses, viscoelasticity and thixotropy. The highly shear sensitive gibbsite particle networks which form in supersaturated liquors are not observed in saturated or under-saturated aluminate solutions. The rate of structure formation has been shown to depend strongly on the initial degree of supersaturation, but only weakly on temperature. These findings reflect the condition-dependent particle interaction behaviour which is influential in controlling the flocculation and cementation steps of gibbsite agglomeration.

Cation effects during aggregation and agglomeration of gibbsite particles under synthetic Bayer crystallisation conditions

Abstract Rheological methods have been used to study the influence of the liquor cation (sodium versus potassium) on the time-dependent gibbsite particle interactions that occur during Bayer process crystallisation. The temperature, supersaturation and seeding levels investigated simulate those experienced in industrial crystallisers. Gibbsite agglomeration was shown to occur by reversible aggregation followed by irreversible cementation. These two sub-steps were individually characterised by careful choice of seed surface area and liquor supersaturation during batch crystallisation. At seed loading levels less than 10% w/w aggregates are rapidly cemented into agglomerates, this is more pronounced in sodium- than potassium-based liquors. These suspensions were Newtonian and the extent of agglomeration correlated with their viscosity. At seed loading levels greater than 20% w/w particle aggregation resulted in extensively time-dependent and non-Newtonian rheology. However, the aggregates did not undergo cementation into agglomerates and no irreversible size enlargement was evident. Yield stress development with time was used to probe the kinetics of aggregation and quantify the particle interaction behaviour. The rate and extent of the particle network formation is more pronounced in sodium rather than potassium-based liquors, supersaturation dependent, alkali concentration dependent, but only weakly temperature dependent. These findings are discussed with respect to the chemical and physical mechanisms of agglomeration in Bayer crystallisation and the role of cation.

Naphthalene sulfonate functionalized dendrimers at the solid-liquid interface: influence of core type, ionic strength, and competitive ionic adsorbates.

The adsorption of naphthalene disulfonic acid surface-functionalized dendrimers (generation 4) on to colloidal alumina particles is reported, considering the role of dendrimer core type (ammonia vs benzylhydrylamine-polylysine) and electrolyte addition on the adsorption affinity and interfacial packing and competitive adsorption. Irrespective of the dendrimer core type, the maximum adsorbed amount increased with increasing ionic strength. The adsorption affinity of a benzylhydrylamine-cored SPL-7013 increased with increasing ionic strength, whereas a decrease was observed for the ammonia-cored SPL-2923. At high ionic strengths (>or=10(-1) M NaCl) dendrimers close pack at the interface as an array of equivalent hard spheres, whereas at lower ionic strengths both dendrimers occupy a lower area than theoretically predicted for either cubic or hexagonal close packing, based on double layer repulsion. The additional attraction between dendrimers is attributed to the intercalation of the neighboring dendrons. Adsorption of SPL-2923 is enhanced by the presence of Ca2+ ions and depressed by the presence of HCO3- and HPO4(2-) ions, whereas SPL-7013 adsorption is only depressed by the presence of HPO4(2-) ions, suggesting a dendrimer-specific competitive adsorption process. This work clearly demonstrates the role of dendrimer architecture on adsorption at an interface, a process of fundamental importance to a wide range of dendrimer applications.

Dendrimer Assembly in Solution and at Interfaces

Dendrimers are multi-functional nano structured polymers with a wide range of potential pharmaceutical applications. In contrast to the behavior of typical colloidal particles, dendrimer aggregation in aqueous solution is more pronounced at low ionic strengths, attributed to the interpenetration of dendrimer branches on neighboring molecules at low ionic strengths. Similarly, adsorption isotherms for dendrimer adsorption on alumina particles were strongly influenced by ionic strength and pH and were described by the Langmuir model, enabling the calculation of free energy of adsorption (DeltaGads), surface affinity constants and dendrimer area per molecule. (DeltaGads values were in the range -46 to -58 kJ mol-1, significantly greater than observed for the adsorption of monovalent species. Here we report the solution and interfacial properties of dendrimers and show that dendrimer association is strongly influenced by solution conditions.