Study of phase transitions at pulp aeration with steam - air mixture

Introduction. Physical and chemical conditions for the formation of flotation complex at increasing the temperature of the entire volume of pulp and in the developed mode of flotation with increasing the temperature of the liquid phase only in the boundary layers of bubbles, in many respects similar, but operating energy costs in the second case is much less, because the volume content of air in the pulp is ~15%. Materials and methods. To study the enrichment of ores using the mode of flotation by steam - air mixture, a laboratory stand based on three column - type flotation machines was developed. Coupling of flotation machines provides jet motion of initial feed and rough concentrate; in the first two columns pulp aeration is carried out by pneumo - hydraulic aerator, in the third - by a nozzle providing mixing of air with water vapor. Field experiments on flotation are performed on samples of gold - bearing ores. Results. As a thermo - physical model of interaction of a vapor - air bubble with a cold carrying medium (pulp during flotation), a system is adopted in which water is separated from its vapor by a heat - insulating quasi - baffle. The two - phase vapor - liquid flow starts at the moment of time when the quasi - baffle is momentarily removed. The interaction is described by three conservation equations - liquid phase mass, bubble number and momentum. The equations of change along the surface of conjugate heat and mass transfer of vapor dryness and its temperature are written down. The influence of the gas not participating in phase transitions on heat and mass transfer during the interaction of the vapor - air phase with the supporting medium has been evaluated. Discussion. Using the developed thermo - physical model of interaction of hot vapor, enclosed in a bubble, with cold liquid, it is shown that, if vapor condensation prevails, as the bubble size decreases, the heat exchange between liquid and gas phase worsens, which prevents bubble collapse. Due to the compression of the bubble, the temperature in the bubble increases and, having reached its minimum value, the size of the bubble begins to increase due to overheating of the vapor and evaporation of the carrier liquid. Periodic oscillations of the bubble surface occur. Their damped character is connected with thermal dissipation of kinetic energy due to non - politropic or irreversible processes in steam: during condensation steam transfers more heat to the liquid than it receives from the liquid during its evaporation. The results of flotation of gold - bearing ores by steam - air mixture prove the technological efficiency of the developed mode of pulp aeration. Conclusion. Two - phase vapor - liquid flows are accompanied by phase transformations and associated mass transfer - condensation of the dispersed phase and evaporation of the carrier phase. The intensity of phase transformations is determined by the thermal conductivity of the vapor and liquid phases. If mass transfer (condensation - evaporation) is accompanied by wave oscillations of the bubble surface, heat transfer is accompanied by an increase in the water temperature in the bubble boundary layer. The thermal head and the behavior of steam as a gas are of subordinate importance. Resume. The results of the study can be used in flotation of gold - bearing ores with high content of fine gold.