Are enhanced dust explosion hazards to be foreseen in production, processing and handling of powders consisting of nano-size particles?

Abstract

Based on experience with powders having particle sizes down to the range 1 - 0.1 μm one might expect that dust clouds from combustible nm-particle powders would exhibit extreme ignition sensitivities (very low MIEs) and extreme explosion rates (very high KSt-values). However, there are two basic physical reasons why this may not necessarily be the case. Firstly, the formation of clouds of well-dispersed primary particles from bulk powders consisting of nm-particles is extremely difficult to achieve, due to the comparatively very strong inter-particle cohesion forces. Secondly, should such a dispersion process nevertheless be fully successful, extremely fast coagulation processes in clouds within the explosive mass concentration range, would transform the primary nm-particles into much larger agglomerates within fractions of a second. Furthermore, for organic dusts and coal, the basic mechanism of flame propagation in the dust cloud is a further reason for not expecting extreme ignition sensitivities and explosion rates dust clouds from nm-particles. The overall conclusion is that dust clouds consisting of nm primary particles would not necessarily be expected to exhibit more severe MIE and KSt-values than those of dust clouds of μm primary particles. Recent experimental evidence confirms that this is in fact the case for K St, whereas for MIE the values for some metals are significantly lower for nm primary particles than for μm ones.