Investigation into the influence of droplet size on the stability of diesel emulsions based on multiple light scattering

Diesel oil emulsification is a common method to enhance the coal slime flotation, but difficult to apply industrially due to its unstable characteristics. In this paper, based on the multiple light scattering technique and the statistical Box-Behnken design (BBD) tests, the effect of the process parameters such as shear speed (A), shear time (B), and the mass ratio of SDBS to Span80 (C) on turbiscan stability index (TSI) value was systematically studied, and the relationship between emulsion stability and emulsion droplet size was also explored. The results showed that the destabilization of emulsified diesel (ED) was related to gravity migration and an increase in droplet size and was well fitted by the classical first-order kinetic model with R2>0.99. The significant order in which the three factors negatively affect the TSI value is, shear speed (A) > the mass ratio of SDBS to Span80 (C) > shear time (B), with shear speed ranging from 1000 to 16000 rpm, with shear time from 10 to 15 min, and with ratio of Span 80/SDBS from 0.45 to 1.48, respectively. Analysis of variance (ANOVA) revealed a high R2 value of the regression model equation (R2=0.9857) and fitted to a second-order polynomial equation using multiple regression analysis. The representative particle size (D50) has a positive linear correlation with the TSI value, indicating the smaller the particle size, the more stable the ED. The theoretical analysis using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and Stokes-Richardson-Zaki model demonstrated that the smaller ED droplets have the greater energy barrier to overcome before coalescing and lower rising velocity to move to the upper oil phase, and hence better stability.