Effect of physicochemical properties on critical sinking and attachment of respirable coal mine dust impacting on a water surface

Respirable coal mine dust (RCMD) inhalation is identified as the main cause of the resurgence of coal worker’s pneumoconiosis (CWP) since the mid-1990s. At present, the predominant dust control technology is the water spray system. However, in practice, the capture efficiency of RCMD by this technology is relatively low. To understand the capturing mechanism and develop improvement strategies, this research is focused on the surface chemistry study of RCMD and its impact on a water surface using a dynamic model. Proximate analysis, chemical, and mineral composition of a run-of-mine (ROM) coal sample from Appalachian region were analyzed using a proximate analyzer, Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and X-ray Diffraction (XRD), respectively. Contact angles were measured by capillary rise test using the Washburn equation. Based on the dynamic model, the effects of particle size, density, contact angle, and surface tension on the critical sinking were investigated. It was pointed out in this work that reducing surface tension, in turn, decreases contact angle, which has been neglected in the literature. Regime maps for different minerals were created and showed that organic matter has the highest critical velocity due to its low density and high contact angle. Reducing water surface tension to the critical solid surface tension of coal around 30 mN/m could maximize the attachment efficiency. Scaling laws, constructed by force balance, led to the criteria of critical sinking: $U_{\text{cr}}\sim \sqrt{\frac{6\left(1-\cos \theta \right)}{\left(2D+1\right)}}\sqrt{\frac{{\gamma }_{\mathrm{lv}}}{{\rho }_{l}R}}$, i.e., ${\text{We}}_{\text{cr}}\sim \frac{12\left(1-\cos \theta \right)}{\left(2D+1\right)}$. A semi-empirical formula for critical velocity was obtained by fitting the simulation data, $U_{\mathrm{cr}}=1.09\sqrt{\frac{6\left(1-\cos \theta \right)}{\left(2D+1\right)}}\sqrt{\frac{{\gamma }_{\mathrm{lv}}}{{\rho }_{l}R}}$. Attachment efficiency was defined and formulated as $P_a=\frac{U_0^2}{U_{\mathrm{cr}}^2}=\frac{W{e}_0}{W{e}_{\mathrm{cr}}}$, establishing relationships between attachment efficiency and the physicochemical properties of RCMD and water droplets.