ABSTRACT
Electrokinetic potential of particles has been extensively studied in colloidal systems over the past century, while up to date, the influence of gas on electrokinetic behaviors of particles has not been fully understood yet. In this study, the electrokinetic response of particles to gas nucleation was systematically investigated with coal as the object. The results showed that the nucleation of gas (both on particle surfaces and in water) significantly changed the particle' electrokinetic behaviors. Higher gas content and particle's surface hydrophobicity normally trigger more intensive gas nucleation, thus inducing more significant reduction of particle zeta potential. After gas nucleation, numerous nanobubbles (NBs) appear in the suspensions mainly in two forms: NBs adhering onto solid surfaces (ANBs) and NBs stagnating in bulk solutions (BNBs). ANBs not only enhance the surface heterogeneity, but also cause the "steric hindrance" effect, and electric double layer (EDL) overlapping and associated ions shielding towards charged particles, which significantly decrease their electrokinetic potentials. Although BNBs can also reduce the zeta potential of particles by EDL compressing, their functions are rather limited.
ABSTRACT
Micro-nanobubbles (MNBs) generated during hydrodynamic cavitation (HC) have been extensively studied in mineral processing field in the past two decades. Many researchers have claimed that MNBs can effectively promote the collection of fine particles in flotation, while studies on MNBs assisted mineral separation are rare. In this study, the role of bulk MNBs in desorbing flotation reagent was investigated, with the aim of illustrating the potential effects of MNBs on minerals separation. The results showed that bulk MNBs could efficiently remove the sodium oleate (NaOl) from diaspore surfaces, reducing the residual concentration of NaOl on solids, which was more significant when the amount of NaOl pre-adsorbed was relatively small. Furthermore, lower residual concentration of NaOl on solids caused by MNBs cleaning made the particles less hydrophobic and flocs more friable. Given that gangue entrapment in flocs was one of the main limits for high-selective flotation, the roles of MNBs in enhancing reagent desorption and associated flocs breakup and reorganization probably contribute to higher separation efficiency of different minerals, which was confirmed by the flotation results of diaspore/kaolinite mixture.
ABSTRACT
To improve the efficiency of the gasification or combustion process of coal water slurry (CWS), it is significant to optimize the rheological properties and increase the solid concentration of CWS. Particularly, preparing CWS from low quality coal remains a peculiarly intriguing subject due to the large reserve and low price of low quality coals in China and very successful industrial applications of CWS all over the world. In this work, refined coal particles were obtained by applying an improved fine particle flotation method on a low quality coal. The refined coal used for CWS preparation had a much lower ash content and higher calorific value than those of the raw coal, which could hardly be utilized for preparing a qualified CWS in basic fluidity. The CWS derived from the refined coal had a good fluidity, with apparent viscosity of 1045.75 mPa s and solid concentration >60 wt% in dispersant free conditions. The effects of dispersants i.e., Naphthalene Sulfonate Formaldehyde condensate (NSF), Polyoxyethylene Polycarboxylic Acid ether (PPA), and Sulfonated Melamine-Formaldehyde resin (SMF), and their dosages on the rheological characteristics of CWS prepared with the refined coal had been investigated. Experimental results showed that slurry ability for CWS was obviously improved by using the refined coal. This was due to the decrease in both the porosity and hydrophilicity of coal particles as confirmed by SEM and FT-IR analyses. The apparent viscosity of CWS was decreased by 30%-60% by adding one of those dispersants with a dosage of 0.6 wt%. Through observation of the rheological behaviors, the CWS samples generally behaved as a shear thinning fluid, and the measured viscosity was well correlated by the Herschel-Bulkley equation. The PPA dispersant exhibited the best performance on reducing the viscosity and yield stress among the dispersants in this study. It could be attributed to the best improvement in wettability of the coal surface and the largest decrease in surface tension of deionized water by PPA. The electrostatic force might have little contribution to viscosity reduction of CWS in this suspension.
