Characteristics of droplets generated by bubble bursting from chromic acidsolutions

The effects of electrolyte concentration and gas flow rate on the character istics of droplets generated from bubbles bursting on the surface of CrO3 s olution 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 exper imental conditions studied except at the gas flow rate of 4 l min(-1) in 12 5 g l(-1) CrO3 solution. Foams had a significant effect on the characterist ics of droplets generated from bursting bubbles. At identical gas flow rate and electrolyte concentration, the formation of foams led to a reduction i n number concentration of droplets larger than 10 mu m in aerodynamic diame ter and a lower concentration of airborne Cr(VI). In the ranges of gas flow rate and electrolyte concentration tested, the results showed that the air borne Cr(VI) mass concentration increased significantly with gas flow rate and slightly with electrolyte concentration in the presence of foams. The r esults obtained in the present study should have applications in the emissi on control of Cr(VI)-containing droplets in chromium electroplating process es. (C) 1999 Elsevier Science Ltd. All rights reserved.