• Chakravarty, Somik

Abstract

Handling and processing of granular material release fine solid dust particles, which in an occupational setting, can severely affect worker health & safety and the overall plant operation. Dustiness or the ability of a material to release dust particles depends on several material and process parameters and is usually measured by lab-scale dustiness testers. Dustiness tests remain mostly experimental studies and lack reliable predictive ability due to limited understanding of the dust generation mechanisms and the complex interactions between the particles, wall and fluid, occurring simultaneously during dust generation. In the framework of EU ITN project T-MAPPP, this thesis uses an experimental approach to understand the dust generation mechanisms by studying: a) the effects of key bulk and particle properties on powder dustiness; b) the nature and magnitude of inter-particle, particle-wall and particle-fluid interactions; c) the evolution of dustiness and generation mechanisms for long duration powder applications. The results indicate that the dust generation mechanisms differ based on particle size and size distribution of the powder. For the given test samples and experimental conditions, the differences in powder dustiness and dust emission patterns can be characterized by three different groups of powders; powders containing fine cohesive particles, bi-modal (consisting of fine and large particles) powders and lastly, powders consisting of only large particles. While bulk cohesion, especially that stemming from van der Waals forces (measured using shear testers) determines the level of dustiness for the fine powders (in such a way that higher bulk cohesion leads to lower dustiness), both the fraction of fine particles and cohesion determine the dustiness of bi-modal powders. The large particles can emit dust only through attrition of the primary particles into smaller aerosolizable fine particles.Analysis of a traced particle motion inside a cylindrical tube agitated by a vortex shaker dustiness tester shows the cyclic nature of the particle motion. The motion (position and velocity) is symmetric and isotropic in the horizontal plane with lowest radial velocities close to the tube centre and highest at the boundary wall of the test tube. The particles tend to rise up slowly in the middle of the tube while descending rapidly close to the wall. The highest values of the velocity are found at the highest heights and close to the wall of the test tube, where the population densities are lowest. Increasing particle size and vortex rotation speeds tends to increase particle velocity whereas increase in powder mass leads to a decrease in particle velocity for rotation speeds up to 1500 rpm. For the given samples (silicon carbide, alumina and acetylene coke) and the experimental conditions, the initial dustiness is determined by the fraction of fine respirable particles present in the powder but the long-term dust generation patterns and levels are influenced by the material attrition behaviour. Dust is generated by the fragmentation and/or abrasion of primary particles, which may lead to the production and emission of fine daughter particles as dust. The samples with large irregularly shaped particles are likely to show high dustiness by shedding angular corners through inter-particle and particle-wall collisions, thus becoming more spherical in shape. On the contrary, the smaller particles are more resistant to abrasion and generate relatively less dust. While the vortex shaker dustiness tests show similar trends as an attrition tester, our study using alumina and acetylene coke indicate that the results are not interchangeable. Results from this thesis help understand the influence of powder and process parameters which may be manipulated to reduce dust generation. Furthermore, experimental results can be used to develop and validate numerical models to predict dustiness.

Topics

• impedance spectroscopy
• laser emission spectroscopy
• carbide
• Silicon
• isotropic