An Investigation On Effects Of Sodium Chloride On Bubble Stabilization And Fine Coal Flotation Behavior

Flotation is the main way to improve the quality of fine coal,and the types and contents of ions in flotation pulp have an important effect on the flotation behavior.With the development of economy,the resource of freshwater is increasingly scarce.Due to scarcity of fresh water and stringent regulations on the quality of discharged water,the washing water close-cycling practice has become normalized in coal preparation plant and special water resources,such as groundwater,sea water,is used in coal preparation in some areas.The complexity and diversity of the solution environment caused by the above-mentioned reasons lead to an increase in the uncertainty of the coal flotation.Based on those,the flotation behavior of coal in organic salt solution is proposed in this paper.Sodium chloride,which has simple structure and is ubiquitous in flotation pulp was chosen as the object of study to discuss its influence on bubbles behavior in solution,froth and flotation behavior in coal flotation so as to reveal stabilizing effect of sodium chloride on bubble and its effect on coal flotation behavior and present important references for a high efficiency separation of coal in complex chemistry environment of flotation solution.The behavior of the two-phase foams and three-phase foams in coal flotation in the presence of sodium chloride were studied.Research shows that sodium chloride,similar to the surfactant,plays a role in the froth stability in the two-phase foams and three-phase foams.Based on the relevant papers,stabilizing effect of sodium chloride on bubble and its effect on coal flotation behavior were identified as the main research contents.The main contents of the paper include single bubble generation,bubble coalescence on the liquid level,the macroscopic characteristics of bubble group and foam in solution,effects of sodium chloride on coal flotation behavior and non-selective flotation of coal.Observation system is setted up to study single bubble generation,the chase and coalescence of adjacent bubbles and bubble rupture on liquid level in MIBC,octanol,PEG and sodium chloride solution,and record the coalescence time of bubble rupture on liquid level.The results indicate that the departure bubble size is controlled by surface tension of solution and the chase and coalescence behavour of adjacent bubbles.The lower the surface tension of frother solution,the smaller departure bubble size in solution.The chase and coalescence behaviours between bubble and subsequent bubble can be inhibited at high concentration of frother and sodium chloride solution.Generally,time of the bubble coalescence can be considered as a sum of the time of the bubble bouncing and lifetime of the liquid film formed at the interface by the colliding bubble.The lifetime of the liquid film formed at the interface has a great impact on time of the bubble coalescence at liquid level to a certain extent.The Marangoni effect slows down the film drainage in octanol solution to prolong the coalescence time,while the drainage time of film is shortened by reducing the hydrophobic force in sodium chloride solution.Observation system is built to study single bubble motion and size distribution of bubble cluster in solution.The results of single bubble motion indicate that bubble shape and velocity are related: as shape becomes more spherical the bubble slows down and vice versa.With the increaseing frother concentration,an increasing aspect ratio(bubbles more ellipsoidal)results in an increasing velocity,while aspect ratio and velocity of rising bubble do not change in sodium chloride solution.Similar to frother solution,there is a critical coalescence concentration in sodium chloride solution.The frother and sodium chloride concentration have a certain synergistic effect on bubble coalescence in solution.The gas holdup in flotation column is measured by using liquild level rising method,and gas holdup is determined by size distribution of bubble group togethet with bubble rising velocity.An increase in gas holdup with increasing sodium chloride concentration in frother solution was observed and was attributed to a decrease in bubble size.The effect of sodium chloride on the surface tension of frother solutions was studied.The froth ability and stability were tested and evaluated by dynamic froth index(DFI).Water content of the froth was tested using Combined Foamscan-spin.The effect of sodium chloride on surface tension of polyethylene glycol solution is obviously greater than that of 2-octanol and methyl isobutyl carbinol.With the increaseing NaCl concentration,the DFI values of the three kinds of frother were increased and water content of the froth were decreased.It is worth noting that the froth stability is improved when water content of the froth decrease.Similarly to the change of surface tension,the effects of sodium chloride on DFI and water content of polyethylene glycol solution are also greater than that of 2-octanol and methyl isobutyl carbinol.A kind of high-rank anthracite from Henan Shen Huo Mining Group was chosen as the research object.The effects of sodium chloride on fine coal flotation were studied systematically.The particle size distribution was tested by Laser Particle Size Analysis and Focused Beam Reflectance Measurement.The addition of sodium chloride improves the flotation rate of coal particles as well as combustible recovery effectively,due to decreaseing bubble size and increaseing particle size.The function of sodium chloride on particle size in flotation lies in two following aspects.Firstly,sodium chloride creates conditions for particles agglomeration by reducing the surface potential of the particles.Secondly,sodium chloride promotes the hydrophobic flocculation between coal particles through the severe hydration.The increase of particle size due to the above two mechanism enhances collision and adhesion between particles and bubbles.Non selective flotation behavior of fine coal slime under the effect of sodium chloride was also studied in this paper.The concept of Contribution Rate of Water Entrapment to the ash of concentrate was proposed and its test method was also given in the paper.Combining the test of rheological properties of pulp and froth stability,water entrainment behavior of particles was discussed.Sodium chloride promotes the flotation of coal particles as well as a large amount of gangue minerals.With the increase of NaCl concentration,the viscosity of pulp and the stability of three-phase foam increases,and which result in entrainment of gangue minerals with water into the froth at last.The above mechanism can be responsible for the increase of concentrate ash with increasing sodium chloride concentration.