Study on solid-liquid transition behaviors of cohesive powders and predict the critical arch-breaking gas flow rate

Abstract

Cohesive powders cannot flow freely in the silo and even behave like solid, which brings a great challenge to the powder handling process. Aeration is an effective method used to break the arch structure formed above the silo outlet and to promote the powder flow. However, it is difficult to determine the solid-liquid transition behaviors and to predict the critical arch-breaking gas flow rate in the aerated discharge systems. In this paper, cohesive powders including alumina (Al2O3), pulverized coal (COAL) and calcium carbonate (CaCO3) were used as experimental materials. Powder flow properties were characterized by conducting shear test, permeability test, and aerated discharge experiment. Unconfined yield strength of powders under zero pre-consolidated stress was obtained based on using a linear extrapolation procedure of shear parameters. Aerated discharge characteristics of cohesive powders were determined and, the critical condition corresponding to the transition from solid-like to liquid-state and the stable condition distinguishing the funnel flow pattern and mass flow pattern were defined, respectively. The Brown and Richards model, and the flow/no-flow criterion of Jenike were modified to describe the powder flow state and to predict the solid mass flow rate under the action of aeration by introducing the positive gas pressure gradient. A logical algorithm was proposed to predict the critical arch-breaking gas flow rate by assuming the same gas distribution coefficient under different aeration conditions. As a precondition, the critical solid mass flow rate was calculated by an iterative algorithm giving the same order of magnitude of that from the experiment, which was further related to the stable solid mass flow rate and the powder permeability index. This simplification provided us with satisfactory results for all the powders producing errors generally less than 30%, a more easy-to-operate method to determine the critical arch-breaking gas flow rate.