Characteristics of droplets generated by bubble bursting from chromic acid solutions

The effects of electrolyte concentration and gas flow rate on the characteristics of droplets generated from bubbles bursting on the surface of CrO3 solution were studied with an experimental bubbling system. The experimental conditions included two electrolyte concentrations, 125 and 250 g l-1 of CrO3, and three flow rates of sparging air in the range of 4–8 l min-1. A cascade impactor collected droplet samples for chemical analysis. A laser aerosol spectrophotometer and an aerodynamic particle sizer were employed simultaneously to measure the number concentration and size distribution of the droplets. A layer of foam formed on the liquid surface under all experimental conditions studied except at the gas flow rate of 4 l min-1 in 125 g l-1 CrO3 solution. Foams had a significant effect on the characteristics of droplets generated from bursting bubbles. At identical gas flow rate and electrolyte concentration, the formation of foams led to a reduction in number concentration of droplets larger than 10 μm in aerodynamic diameter and a lower concentration of airborne Cr(VI). In the ranges of gas flow rate and electrolyte concentration tested, the results showed that the airborne Cr(VI) mass concentration increased significantly with gas flow rate and slightly with electrolyte concentration in the presence of foams. The results obtained in the present study should have applications in the emission control of Cr(VI)-containing droplets in chromium electroplating processes.