On the Flocculation and Agglomeration of Ceria Dispersion

Abstract

The ceria dispersions displayed a maximum in the yield stress at the isoelectric point (pI) at pH ∼ 7. At pH below pI, the flocculated dispersions at solids concentration of 30 wt% and above produced a smooth and homogeneous appearance. However, at pH above pI, phase separation was observed to occur quite quickly forming a clear layer of liquid supernatant. Upon examination, relatively large discrete aggregates had formed. This behavior can be explained by the surface chemical model proposed by Nabavi et al.^{[ 1 ]} According to Nabavi et al., nitrate anion, normally an indifferent electrolyte, is both adsorbed and bounded covalently to the surface of the ceria particles at pH below pI. A steric layer is formed preventing particle agglomeration. Such agglomeration was also prevented at high pH with the adsorption of pyrophosphate additive added at 1dwb% concentration. The model proposed by Nabavi et al also explained the agglomeration of ceria particles at pH > pI. Both the covalently bounded and adsorbed nitrates were displaced by the hydroxyl group forming this surface ‒OH group that on the interacting particles condenses to form ‒Ce‒O‒Ce‒ bridging bond and caused particle agglomeration. The yield stress-DLVO force model was obeyed by the ceria dispersion at pH below pI. A critical zeta potential of 56 mV was obtained and this gives an estimate value for its Hamaker constant in water of 82 zJ.

Keywords:

• Agglomeration
• bound nitrate
• ceria
• DLVO forces
• yield stress
• zeta potential

Acknowledgments

The authors wish to acknowledge CMAC of UWA for the usage of the SEM and Ms. E. Jen Teh for taking the SEM image.