An aqueous precipitation—phase extraction route to hydrous zirconia particles

A novel process for ZrO 2· nH 2O particle precipitation and recovery from the aqueous phase is described, which utilises the hydrophobicity imparted by alkyl carboxylate surfactants. An aqueous solution containing zirconium nitrate was mixed with a pH-modifying solution (aqueous ammonia) in the presence of the surfactants which adsorbed on the growing particles by chemical reaction, rendering them oleophilic/hydrophobic. Contact angle measurements of oil (iso-octane) droplets on the surfactant-treated metal (hydr)oxide surface were used to screen the solution conditions required to achieve a 180° angle, corresponding to the complete wetting of such surfaces by the oil. In the pH range 7.5–8.5 with selected alkyl carboxylates, oleophilic/hydrophobic particles were extracted from the high ionic strength aqueous phase in which they were produced, into the immiscible iso-octane phase, in which no further true phase growth ( cf. aggregation) can occur; this also obviates the dewatering problems encountered in, for example, sol—gel processes. Particle size measurements after sonication of the organic dispersion indicated particles in the 200–1200 nm size range, though after washing with methanol and further sonication, sub-100 nm particles were produced. A continuous reactor system which was used to produce zirconia—yttria particles is also described and implications for ceramic powder processing are discussed.