A Comparative Evaluation of Crossflow Microfiltration Membranes for Radwaste Dewatering

Summary

Pilot testing of five diverse types of dewatering unit has established crossflow microfiltration as the most appropriate for thickening of mineral waste slurries. A comparative testing programme has been carried out using stainless steel and ceramic membranes. The main experimental measurements made were permeate flux and clarity and retentate concentration. The experimental systems have been examined for fouling, wear, ease of control and suitability for use with radioactive material.

Membranes used were in tubular form ie Pall PSS (2.5 µm limit of separation), Fairey Microfiltrex FM4 (1µm) and APV Ceraver (1.4 µm). Feed slurries were generally at 5 w/o solids concentration with particle size distributions in the range 1–100 um and comprised mixtures of magnesium hydroxide, slate, filter precoat (Dicalite Speedplus), graphite, oil and water. Slurries were dewatered to 30–50 wz/o depending on the rheological properties. On-line backwash and off-line chemical cleaning were employed. Flux decline by irreversible fouling did not occurr during 80 h operation of one of the membrane modules. Operation has been batch or semi-batch using 0.2–1 m³ initial inventory and approximately 0.1 m² membrane area. Average velocities in the crossflow tubes were in the range 3–5 m s^-1. Average transmembrane pressures were in the range 0.8–1.9 bar (g). With partially thickened material permeate fluxes were found to be of magnitude 100 × 10^-6 m³ m^-2 s^-1.

Crossflow microfiltration has been found to provide effective dewatering of a range of simulant radioactive wastes. Performance data and various process constraints have enabled selection of an optimal system for the particular application. Pressure drop data has been analysed to predict slurry viscosity for comparison with other measurements and with a view to use as a parameter for control of thickening.