Dustiness behaviour of loose and compacted Bentonite and organoclay powders: What is the difference in exposure risk?

Single-drop and rotating drum dustiness testing was used to investigate the dustiness of loose and compacted montmorillonite (Bentonite) and an organoclay (Nanofil^®5), which had been modified from montmorillonite-rich Bentonite. The dustiness was analysed based on filter measurements as well as particle size distributions, the particle generation rate, and the total number of generated particles. Particle monitoring was completed using a TSI Fast Mobility Particle Sizer (FMPS) and a TSI Aerosol Particle Sizer (APS) at 1 s resolution. Low-pressure uniaxial powder compaction of the starting materials showed a logarithmic compaction curve and samples subjected to 3.5 kg/cm² were used for dustiness testing to evaluate the role of powder compaction, which could occur in powders from large shipments or high-volume storage facilities. The dustiness tests showed intermediate dustiness indices (1,077–2,077 mg/kg powder) in tests of Nanofil^®5, Bentonite, and compacted Bentonite, while a high-level dustiness index was found for compacted Nanofil^®5 (3,487 mg/kg powder). All powders produced multimodal particle size-distributions in the dust cloud with one mode around 300 nm (Bentonite) or 400 nm (Nanofil^®5) as well as one (Nanofil^®5) or two modes (Bentonite) with peaks between 1 and 2.5 μm. The dust release was found to occur either as a burst (loose Bentonite and Nanofil^®5), constant rate (compacted Nanofil^®5), or slowly increasing rate (compacted Bentonite). In rotating drum experiments, the number of particles generated in the FMPS and APS size-ranges were in general agreement with the mass-based dustiness index, but the same order was not observed in the single-drop tests. Compaction of Bentonite reduced the number of generated particles with app. 70 and 40% during single-drop and rotating drum dustiness tests, respectively. Compaction of Nanofil^®5 reduced the dustiness in the single-drop test, but it was more than doubled in the rotating drum test. Physically relevant low-pressure compaction may reduce the risk of particle exposure if powders are handled in operations with few agitations such as pouring or tapping. Repeated agitation, e.g., mixing, of these compacted powders, would result in reduced (app. 20% for Bentonite) or highly increased (app. 225% for Nanofil^®5) dustiness and thereby alter the exposure risk significantly.