Particle formation from single droplets of aqueous solutions of lead nitrate

The thermal evolution of droplets of aqueous solution of lead nitrate was studied in a drop-tube furnace, which simulates typical conditions for material synthesis, through spray pyrolysis, and for the thermal destruction of liquid-containing waste. The processes of droplet evaporation, precursor precipitation within the droplet and thermolysis of the precipitated particles were followed by means of the spectral analysis of the ultraviolet light scattered by the aerosol produced during the heating of aqueous droplets (100 mu m) of lead nitrate, with different salt concentrations, from ambient temperature up to 1200 K. Dimensions and physico-chemical properties of the droplets/particles were obtained in situ by means of ultraviolet spectra of light scattering (UVSLS) and compared with scanning electron microscopy (SEM) of the sampled material. A plasma generated in the air by an optical breakdown induced by a Nd:YAG laser was employed as the light source in the wavelength range 200-400nm, thus allowing an exceptionally high photon flux in the ultraviolet region where intense and species-specific interactions with metal species take place. The spray drying process was followed by measuring the light transmitted by the droplets in the backward region. As the drying process progresses, the surface concentration reaches a saturation value and solute is deposited as a solid phase forming a surface crust, which grows steadily. At this point in the process of droplet drying, information was retrieved from the light reflected by the particle interface. Two spectral scattering behaviors were detected at temperatures above the salt precipitation within the droplet. On the basis of Mie calculations and SEM measurements, these behaviors were attributed to lead nitrate particles with typical diameters of the residual droplets (about 50 mu m) and to micrometer-sized lead oxide particles. The effect of salt concentration on the drying process and the thermolysis of lead nitrate to oxide was investigated by changing the salt concentration from very dilute conditions up to almost the saturation concentration.