Fugitive dust emissions caused by falling bulk solid masses

Control of fugitive dust emissions caused by bulk materials handling operations is becoming an increasingly important aspect of day to day operations in many industries. A particular class of materials handling process produces a disproportional large fraction of dust from parent bulk material; i.e. where bulk material undergoes free- fall or similar acceleration und subsequent impact either on a solid surface or on the top of further bulk material (e.g. stockpiling of material). Remarkable is that there has been little in the research literature about the dust generation mechanisms. Most of the research has been carried out on methods to determine the dustiness of a given material by the use of a "black box" method. In order to develop suitable control devices for minimizing dust emissions caused by falling bulk materials, the "black box" methods mentioned above will not bring usable results. For this purpose the basic physical mechanisms governing the down movement of the bulk solids and the particles must be understood and therefore detailed experimental investigations are necessary. Especially coarse bulk solids particles are often mixed with very fine abrasive ones, which then are the source of fugitive dust emission during the falling movement. An apparatus with corresponding measurement techniques to investigate the interaction between the coarse and fine particles and the resulting dust emission during the falling process, should be developed. Preliminary tests /40/ showed, that cluster formation in falling bulk material is an essential mechanism. It can be assumed, that different cluster formation behaviour will also capture or set free in a different way fine particles. This measurement technique should be able to analyse the cluster formation (number and size) of the falling material along the falling height. The influencing parameters and the mechanisms causing cluster formation together with the volume of entrained air and the generated dust should be investigated and understood. These results should enable to develope model derivations which can be used as the basis for further developments to minimize fugitive dust emissions caused by falling solids streams. In order to measure these parameters mentioned above an optical measurement device should be implemented in a bulk solids falling apparatus. The falling apparatus is feeded by a vibration conveyor. Pictures of the falling mass stream should be recorded with good resultion by a digital reflex camera connected with a cluster analysis software. The dust emission will be measured by a cascade impactor.

Fugitive dust emissions become an increasing meaning in many industrial fields, where bulk solids handling appears. Specifically when bulk solids undergo a falling movement, relatively high portion of fugitive dust emissions are released. It is remarkable that there has been carried out little on research in that field until today to clear up the physical mechanisms leading to that dust emissions. To characterize the "dustiness" of bulk solids so called "Dust Chambers" where developed. With these dust chambers, base on an "black box" method, it is impossible to clear up basics on physical mechanism of dust emissions during the falling movement of bulk solids. Due to this it is necessary to carry out detailed investigations to gain better knowledge of the falling process. Especially coarse particles are mixed with very fine (generated due to abrasion during primary movement of the bulk solids) ones. These fine particles can act as source of fugitive dust emissions during bulk solids handling. In the representational project a special test device, equipped with the appropriate measurement technique - was developed and build up. With this apparatus it is possible to investigate the interactions between coarse and fine particles. It was detected that the particle mass flow owns a "propelling force". This "propelling force" substantially depends on bulk solid properties, e.g. particle size distribution, particle density. The bulk solid mass flow induces a secondarily air stream. This leads to a circulating air movement inside of the test apparatus, which strongly influence the dust generation during the falling process. Especially if coarse particles mixed with fine ones this can be obtained. It is interesting how different operation and construction parameters - e.g. suction flow, falling length, cross section of the falling tube - influence the induced air stream. To reduce dust emissions it is important to describe the "propelling force" qualitatively and quantitatively. Due to experiments a "pressure skip" was found inside the falling tube. This "pressure skip" separates the soak in area und the upwards flow area. The location of the "pressure skip" is influenced by the suction flow. Inside the soak in area no dust is generated. In opposite to that dust is generated in the upward streaming area due to turbulent air flow.