Eliminating undesirable rheological properties of legacy waste sludge

Lead Research Organisation: University of Leeds
Department Name: Chemical and Process Engineering

Abstract

Recent research has shown that adding 0.5 vol% colloidal silica (~100 nm) can eliminate the yield stress of a 35 vol% alumina suspension (particle size ~550 nm; yield stress in absence of silica > 100 Pa). Due to the difference in particle size and particle density, the number ratio of the major and minor particle is approximately 1:5. It is hypothesized that elimination of suspension yield stress is via the lubricating action of the spherical additive, and through appropriate particle selection and design of the blending protocol it is envisaged that the blend ratio can be significantly reduced such that the additive has little impact on the overall waste volume.



Research aim: Demonstrate the approach of additive blending to eliminate the yield stress of high solids content legacy sludges.



Research objectives:

Characterize representative test sludges to understand variation in particle size, density, shape and particle/surface chemical composition. This work is crucial to understand the heterogeneous nature of test simulants that mimic sludges encountered in nuclear waste processing. The data will be used to inform of potential particle-types (of differing chemical composition) which could be used as an additive particle.

Test appropriate additive materials to eliminate yield stress of high solids content sludges. Previous research has shown the best performance is achieved when the major and minor component particle species are electrostatically repulsive, however the size difference between the major and minor component species has not been studied. The effect of particle size ratio between the major and minor component species will be considered, along with blending bi-disperse additives to treat polydispersed suspensions.

Study the long-term aging effects of particle additives on the stability, consolidation and rheology of high solids content suspensions. With the plan for legacy wastes to be stored for several decades, it is important that the physical properties of the suspension can be predicted over such time scales. To simulate long-term aging, suspensions with particle additives will be exposed to elevated conditions (centrifugation, temperature, aqueous pH and conductivity) to assess the possibility for the suspension yield stress to redevelop.

Develop the particle additive blending protocol so that the method can be easily scaled. Effective yield stress reduction is achieved when the particle additive is homogenously distributed throughout the yield stress sludge. Methods of adding the particle additive and mixing the particle additive into the yield stress sludge will be studied to demonstrate methods that require little energy and short time effect.

Scale-up the lab-scale method to pilot-plant scale (led by Sellafield Ltd. in partnership with NSG Environmental Ltd.). Working with project partners we will work to translate knowledge from the 100 mL to 500 L scale. Sample preparation and blending protocols will be revisited and optimized for large scale blending.

Project outputs. The research will be communicated with particle partners, industry experts and the wider academic community through reports, presentations and publications. The research student will engage with the relevant Sellafield CoE and work with industry experts to ensure knowledge transfer.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/S022295/1 31/03/2019 29/09/2027
2439263 Studentship EP/S022295/1 30/09/2020 29/09/2024 Olivia Laura Pickup