Condensation of aerosols in metallic fume

Abstract

Condensation studies in the heat-pulse cloud chamber are shown to be relevant to many practical situations including metal fuming. Particles nucleate in an inner zone, or boundary layer, and their subsequent growth depends on external conditions and the volatility of the metal.

Results are presented for Pb, Mn, Zn and Cd. Zn and Cd are volatile and low-melting, and particles collected from the outer zone where they circulate remotely from the generator are of two distinct types: prismatic and spherical. Prisms, favoured by low temperatures in the outer zone, take on dendritic forms in a certain range. Nearer the melting point, spheres predominate and become smoother and grow much larger. Dendrites and rough spheres are very persistent in suspension, but the smooth spheres roughen and become more buoyant if traces of oxygen are present. Pb is comparatively involatile and forms only small spheres. These aggregate to chains at low ambient temperatures, which keeps them in suspension. All precipitates are reactive to the atmosphere.

The results for metals are compared to those obtained previously with volatile salts. It is shown that the change from prisms to spheres may be expected theoretically for Zn and Cd.