Effects of Spray Surfactant and Particle Charge on Respirable Dust Control

Abstract

Great efforts have been taken in underground coal mines to prevent coal workers' pneumoconiosis by controlling respirable coal dust, but respirable coal dust levels in mines still often exceed occupational exposure limits. Water-based spray systems are one of the primary dust control methods in mines; studies have suggested a potential to improve spray dust collection efficiency by adding surfactants. This dissertation investigates the effectiveness of different surfactant-containing sprays in capturing lab-generated monodisperse polystyrene latex particles and polydisperse coal dust by analyzing the impact of particle diameter, aerosol charging condition, surfactant type, sign and magnitude of particle charge, spray solution surface tension, spray drop size, and sign and magnitude of spray drop charge on spray collection efficiency. In order to focus on how the electrical effects caused by adding surfactants to spray water impact spray collection efficiency, the spray surface tension and drop size were taken into account during statistical modeling. Results indicate that particle size had the most important impact on respirable dust capture by water-based spray. Most particles with a diameter greater than 2 �m can be removed by the spray regardless of other factors. The magnitude of particle charge affected spray efficiency in that highly-charged particles tended to be removed more efficiently than weakly-charged particles. The magnitude and sign of the charge on surfactant-containing spray drops depended on both surfactant classification and concentration. Pure water spray drops and spray drops with nonionic (Triton X-100) and cationic (DAH) surfactants tended to carry net positive charges on average, whereas spray drops with anionic (SDS) surfactant tended to carry a net negative charge on average. However, the magnitude of spray drop charge was independent from the concentration of surfactant in the spray water. After controlling the surface tension and drop size, test results indicated a positive correlation between the magnitude of spray drop charge and spray collection efficiency. Nonionic surfactant-containing spray performed better in capturing respirable dust than the other sprays tested here, especially for weakly-charged aerosols. This superiority may be due to the relatively low surface tension and high charge magnitude of drops containing nonionic surfactant as compared to the other sprays tested here. Although the predominant mechanisms for respirable dust capture by water-based spray are inertial impaction and interception, electrical effects are also an important factor, especially for highly-charged particles. Therefore, the electrical effects caused by adding surfactants to spray water should be a consideration for future research regarding surfactant effectiveness.