High-gradient magnetic separation of colloidal uranium oxide particles from soil components in aqueous suspensions

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The separation of uranium oxide (UO2) particles from soil-surrogate particles in aqueous suspensions was achieved using filtration enhanced by a magnetic field. Enhanced attraction of paramagnetic UO2 colloids to a ferromagnetic stainless-steel filter placed in a strong magnetic field arises because of the positive magnetic susceptibility of the particles and the high-gradient field generated near ferromagnetic fibers. Enhanced uptake of smaller particles over larger ones occurs through Brownian motion that promotes the collision of particles with the ferromagnetic fibers of the filter. Hence, this work focused on UO2 particles in the colloidal size range. Experiments used a water-cooled electromagnet and an array of permanent magnets. Chemical analysis showed that the magnetic field increased the capture efficiency of uranium particles from a range of 27–53% with the magnet off up to 98% with the magnet on after a single pass of the suspension through the filter. The recovery of the UO2 particles from the filter, however, was more difficult to achieve. Small amounts of UO2, together with significant amounts of background SiO2 particles, were removed from the filter during a first flush with the magnetic field on. A much larger recovery of UO2 was not observed until a second out-of-field flush was performed, which also released some SiO2. The degree to which particle separation was enhanced through the use of multi-stage filtration compared to single pass-through filtration was also examined. A design was suggested that could be used to optimize the separation efficiency for a continuous process.