The effect of ionic additives on aerosol coagulation

Abstract

The effect of ionic additives on particle size characteristics during gas phase production of fine powders is investigated. In aerosol manufacture of particulate commodities, electrolytes are sprayed into the process stream to control the phase and size characteristics of the product powders. These electrolytes dissociate into their constitutive ions, some of which preferentially adsorb onto the particle surface and give rise to electrostatic repulsion forces between particles that affect particle—particle interactions, and hence, particle growth by coagulation. The size distribution of these particles depends on the strength of charging at the particle surface, which is related to the ionization of the electrolytes. A thermochemical equilibrium calculation is performed to estimate the charging ability of the alkali metals (Na, K, and Rb). Ionic effects on the rate of aerosol coagulation are numerically evaluated using a sectional solution for the aerosol dynamics equation. It is found that high repulsive energy barriers can result in narrow size distributions and small average particle sizes, in agreement with experimental observations in the literature. In addition, ionic additives result in size distributions that are narrower than those predicted by the self-preserving theory for coagulation.