Approximation of hindered zonal settling rates for flocculated inorganic/organic composite suspensions in inertial flow conditions

Abstract

Inorganic/organic composite nuclear wastes have poor settling properties which hinder major UK decommissioning operations. Improving the settling properties of these wastes and the accurate prediction of settling rates is therefore key. However, constricted access and limited monitoring capability in radioactive environments limits the use of primary material, necessitating the use of surrogate test materials. Herein, an organic laden nuclear waste test material was characterised by examining the surface chemistry, morphology and settling behaviour. A large molecular weight polyacrylamide polymer was deployed to aggregate the organic laden nuclear waste test material. The polyacrylamide successfully flocculated the test material, increasing the zonal settling rate and decreasing the turbidity by one and two orders of magnitude respectively at optimum polymer dose conditions. Whilst displaying steric stabilisation beyond the performance maxima, reductions in flocculant performance were small with no indication of permanent stabilisation at five times the optimum dose. To mitigate risk, it is critical to understand the dynamics of the settling process. Given the porous, fractal nature of the agglomerates, fractal modified hindered settling models were assessed in order to improve predictions at low solids concentrations. In particular, predictive models using drag coefficients compatible with creeping and inertia flow regimes were utilised in tandem with structural and size data to quantify the impact of neglecting inertia drag as favoured by previous literature. It was found that at low solids concentrations, the inter-particulate spacing was significant and that inertial flow conditions were integral considerations to achieve close first-order approximations of zonal settling rate.