Influence of shape and surface charge on the sedimentation of spheroidal, cubic and rectangular cuboid particles

Abstract

This study investigated the complex settling behaviour of colloidal particles with varied surface charge and shape factors, of specific relevance to nuclear waste processing. Caesium phosphomolybdate (CPM), zirconium molybdate (ZM) and zirconium citromolybdate (ZMCA) were firstly synthesised, producing spheroidal, cubic and rectangular cuboidal morphologies respectively, and compared to agglomerated titania. While zeta-potential measurements indicated all simulant particles attained low isoelectric points, surface group leaching rendered suspensions very acidic, with CPM around its IEP, and ZM/ZMCA stable and positively charged. In sedimentation tests at various concentrations in water and 2 M HNO3, CPM and titania were found to settle with extremely high hindered settling exponents, consistent with aggregated structures. Exponents for ZM and ZMCA in water were both also well above values for spherical particles; however, this was assumed to be due to heightened drag effects from relative shape factors, rather than aggregation. ZMCA in particular showed a very high exponent of ~11.4, due to the propensity for the rod-like particles to settle in a flat conformation. For ZM in acid, double layer compression from the high electrolyte aggregated the dispersions, correlating to a significant increase in the settling exponent. An extended Stokes relationship was additionally used to understand theoretical limits of shape and aggregation on particle size prediction from hindered settling curves. Importantly, calculated sizes were consistent for the stable ZM and ZMCA in water, despite their non-sphericity and enhanced drag. The relationship failed however with agglomerated suspensions, highlighting its application as a general stability test for sedimenting dispersions.

Synthesis and Physical Property Characterisation of Spheroidal and Cuboidal Nuclear Waste Simulant Dispersions

“…For the ZM-a simulant, the cuboidal particles are shown to be around thirty times larger than the CPM with a D50 value of 10 μm. Unsurprisingly, when comparing SEM images ( Figure 1 d), ZM-b displays a larger D50 value than ZM-a with a value of 14 μm, although caution must be taken with light scattering estimations of any non-spherical particles [ 22 ]. In comparison to the literature, the CPM synthesised at 50 °C particle size is within the expected range, [ 21 ] while the D50 for the ZM-a is slightly larger than previously reported [ 33 ].…”

“…ZM has been the focus of several studies within literature, where it has been synthesised for various applications, such as a Technetium-99m generator [ 4 , 5 , 6 , 7 ], a precursor for the formation of the negative thermal expansion material ZrMo 2 O 8 [ 8 , 9 ] and predominately for nuclear waste based studies [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 ]. Clearfield et al [ 24 ] were the first to publish a synthesis route for ZM to characterise its ion exchange properties.…”

“…Several studies have looked into both CPM and ZM, specifically relating to the issues they cause within the HASTs, the challenge they pose to the Waste Vitrification Plant (WVP) and the in situ conversion of CPM to ZM [ 1 , 2 , 21 , 22 , 23 , 31 , 32 , 33 ]. Given the complexity of CPM and ZM dispersion behaviour, there is a critical need to study their synthesis and physical behaviour under a wide range of conditions.…”

Effect of morphology and hydrophobization of MoS2 microparticles on the stability of poly-α-olefins lubricants

“…Generally, it is well known that the drag force increases as function of projected area which is more important for irregular than spherical shape, especially if this projected area oriented in the direction of fall [41,45]. Thus, the drag force applied to the platelet-like particles is higher than that applied to the spheres, In other words, the displacement of the platelet-like particles is more affected by the drag forces of the medium, and thus their sedimentation rate is reduced with respect to spherical particles.…”

Wood pellet milling tests in a suspension-fired power plant

“…To provide control over the fineness (i.e., the cut size) of the comminuted product conveyed to the burners, coal roller mills apply static or dynamic classifiers [8,20], which classify particles based on their shape, size, and density [21]. The cut size is based on Stokes' law [22] that is only valid for the drag force of a spherical particle [23]. Williams et al [17] found that the classification system of a ring-roller mill for the comminution of densified biomasses followed the Stokes' law, indicated by an increasing sphericity with decreasing particle size.…”