Minimum Ignition Temperature of layer and cloud dust mixtures

The prevention of dust explosions is still a challenge for the process industry. Ignition, in particular, is a phenomenon that is still not completely understood. As a consequence, safety conditions pertaining to ignition suppression are rarely identified to an adequate level. It is well known that, in general, the ignition attitude of a dust depends on several factors, such as the nature of the chemical, the particle size, moisture content, etc., but there is still a lack of knowledge on the effect of the single variables. This paper aims has the aim of providing data on the minimum ignition temperatures of dust mixtures obtained from a mixing of a combustible dust (flour, lactose, sucrose, sulphur) and an inert dust (limestone, extinguishing powders) as well as from the mixing of two different combustible dusts. Various mixtures with different weight ratios have been tested in a Godbert Greenwald (GG) furnace and on a hot plate in order to measure the effect of mixture composition on the minimum ignition temperature (MITL) of the layer and on the minimum ignition temperature (MITC) of the cloud. In order to further verify the effects of inert dust particle size, inerts sieved to different size ranges have been tested separately. Generally, both MITL and MITC increase as the inert content is increased. MITC is poorly affected by inert particle size when limestone is used. The MITL of pure flour is higher than the MITL of mixtures containing up to 40% of 32-75 micrometers of limestone. This was probably due to the behaviour of pure flour during the test, which demonstrated strong tendency to produce char, cracks in the layer and detachment from the hot plate.