Fine solid particles in industrial aqueous suspensions are difficult to separate gravimetrically and have been shown to aid transportation of radionuclides. Effective management of such suspensions is a critical decommissioning priority for high-risk facilities such as the legacy waste storage facilities at Sellafield, UK. Problems arise due to the complex heterogenous composition of suspensions (also referred to as sludges), particle size distributions and poor settling properties. Sites like Sellafield Ltd are interested in novel approaches to nuclear waste separation to reduce the impact of particle fines on downstream processing, with polymer flocculation emerging as a promising method to address these issues, enhancing the separation of fine solid particles from aqueous suspensions. However, the turbulent conditions typical of waste retrieval and transport operations through use of in line mixing pose significant barriers to maintaining stable floc structures and effective separation. This study investigated real-time monitoring of floc size under multi-cycle turbulent conditions using focused beam reflectance measurement (FBRM) in both laboratory and pilot scale systems. Three commercial polyacrylamide (PAM) flocculants; non-ionic, anionic, and cationic; were dosed at concentrations of 5-20 ppm with radwaste test material (calcite) to evaluate their robustness under variable shear conditions.
Laboratory-scale tests in an agitated baffled vessel demonstrated the capabilities of FBRM to monitor flocculation dynamics in real-time during various shear cycles. We revealed that reflocculation performance is strongly influenced by polymer charge and its interaction with the calcite surface. It was found that anionic PAM (A-PAM) at 20 ppm displayed superior regrowth via electrostatic attachment. Contrarily, the bridging flocculation of non-ionic (N-PAM) and cationic PAM (C-PAM) systems showed limited reflocculation due to polymer chain scission at elevated shear rates. Initial flocculation strength did not guarantee long-term stability; for instance, 10 ppm A-PAM exhibited high initial shear resistance but degraded significantly after multiple shear cycles compared to 20 ppm. Pilot-scale pipe flow experiments highlighted the challenges of achieving stable floc sizes under high-shear conditions – we found that 15 min intervals were too short to reach sufficient size distribution stabilization. N-PAM formed progressively stronger flocs under repeated shear, whereas A-PAM produced larger initial flocs which were more prone to size reductions at high shear, potentially due to initial underdosing. Incremental polymer dosing was less effective than single-dose applications, particularly for small particle suspensions. Future optimization efforts should explore lower impeller speeds and higher polymer doses to enhance reflocculation.
This work provides valuable insights into the interplay between polymer properties, hydrodynamic conditions, and particle characteristics, informing the development of efficient flocculation strategies for challenging radwaste suspensions treatment, transportation, and storage.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)