Particle size distribution of airborne radioactivity in mineral sands separation plants

The results of an ongoing particle size characterisation program being undertaken by the Mines Department of W.A. in Western Australian mineral sands separation plants are reported. Twenty-seven determinations have been completed to date using two multistage cascade impactors. The results indicate the possible existence of a bimodal distribution (i.e. not log-normal) with a primary size mode between 20 and 50 μm and a secondary size mode of about 1 to 2 μm. AMADs measured over all test sites ranged from 3 to 24 μm (mean 9.0 ± 1.7) and GSDs ranged from 3.5 to 8 (mean 5.3 ± 0.4). Such large GSDs are outside the range of the ICRP Lung Model, invalidating the application of a single AMAD parameter to estimate internal radiation dose. Two alternative methods for the treatment of non-conforming data are proposed. Using these methods an effective AMAD of 4 μm has been found to be typical of mineral sands processing sites