摘要
浮选作为一种高效快速的固液分离技术,广泛应用于污水处理、水质净化、选矿和石油开采等诸多工业领域。在浮选过程中,空气以小气泡的形式分散在浮选系统中,作为载体,将粘附在其上的悬浮物带到液面。浮选过程通常采用表面活性剂来调节气泡尺寸及稳定性以提高浮选效率。因此,深入认识在表面活性剂存在下的气泡动力学特性,对提高浮选效率具有重要的理论价值。
论文在总结前人研究的基础上,针对目前研究较少的1～5mm的中尺度浮选气泡,采用高速摄影技术,在一个方截面浮选柱中实验研究了表面活性剂对单气泡和气泡团动力学特性的影响,并探讨了其影响机理。实验中,单气泡和气泡团采用不同气泡发生装置生成,液体使用蒸馏水和不同浓度的三种表面活性剂溶液,气泡的瞬时运动利用MS75K(Mega Speed Corp)高速摄影仪实时记录,气泡的形状、大小、轨迹、速度和分布均由图像处理软件(AVI Player,Mega Speed Corp)分析获得。
研究结果表明,除了喷嘴直径与进气流量外,表面活性剂的存在对单个气泡的运动特性也有非常重要的影响。实验发现,由于Marangoni效应,适量的表面活性剂可以有效地减小气泡体积,抑制气泡变形,降低气泡上升速度和增强气泡上升轨迹的规则性。相比于纯水,曲拉通100的浓度为0.15×10-3mOl/L时,气泡高宽比的振荡幅度缩减了57%,终端上升速度减小了35%,体积平均减小40%,气泡的运动轨迹更有规则,偏离中心位置较纯水中小。实验还发现,气泡的形状振荡影响着气泡的瞬时速度,气泡形状越扁,瞬时上升速度越大。在实验条件范围内,不同表面活性剂对气泡的影响程度差异较大,曲拉通100的影响程度强于聚乙二醇和正戊醇。
研究结果还表明,不同进气流量下,表面活性剂的存在均显著影响了气泡团的运动速度和尺寸分布特性。实验发现,相比于纯水中,由于Marangoni效应限制了气泡的聚并行为和增强了气泡的稳定性,表面活性剂溶液中,气泡团的上升速度减小,但气泡团的上升速度大于单个气泡的上升速度。表面活性剂溶液中,气泡团的尺寸分布范围变窄,气泡团的尺寸也更均匀。纯水和浓度为0.05×10-3 mol/L曲拉通100溶液中,气泡团的Sauter当量直径相比于纯水中减少了22.9%。实验还发现,相同流量相同浓度条件下,曲拉通溶液中气泡的尺寸分布范围最窄,气泡团的当量直径最小,正戊醇次之,聚乙二醇最大。另外,相同表面活性剂浓度下,增加进气流量,气泡分散度增大,尺寸分布不均匀,同时增大气泡团的平均直径和Sauter当量直径,但降低气泡团的比表面积。
Froth flotation is an efficient solid-liquid separation process, widely used in various industries such as wastewater treatment, water purification, mineral process and oil recovery. In flotation process, gas is normally introduced in the form of small bubbles into a flotation column and acts as carriers transporting particles to the surface of the continuous liquid phase. A proper amount of surfactants are generally added to the system in order to control bubble size and promote formation of a stable froth in practical flotation operation. Therefore, it is significantly essential to deeply understand bubble dynamic characteristics in the presence of surfactants in order to improve the efficiency of the flotation process.
After literature review on the flotation process and the role of surfactant, this thesis focused on meso-scale bubbles in a range of 1～5 mm which had not been studied extensively in flotation process at present. Experiment was carried out to investigate the effect of surfactants on bubble dynamic characteristics in a square plexiglass flotation column using high-speed photography technique. Effect mechanism was also represented. In this experimental study, single bubble and bubble swarms were released through the nozzle and a porous ceramic sparger at the bottom of column, respectively. Distilled water and three types of surfactant solutions with different concentrations were used for test liquid. The bubble motion was monitored and recorded by a high speed camera (MS 75K, Mega Speed Corp.). The sequences of the recorded images were then analyzed using the image analysis software (AVI View, Mega Speed Corp.) to obtain bubble trajectory, dimensions, velocity and distribution.
Experimental results show that the presence of the surfactant has a significant effect on single bubble behaviors in flotation column besides nozzle diameter and air flow rate. The right amount of surfactant was found to reduce bubble size, dampen bubble deformation, slow down the bubble rising velocity and improve bubble trajectory stabilization significantly due to the Marangoni effect. Compared to pure water, Triton X-100 of concentration 0.15×10-3mol/L can reduce the oscillating amplitude of the aspect ratio of the bubble by 57%, slows the terminal velocity of the bubble by 35% and reduces the volumn of the bubble by 40%. In addition, compared to motion in pure water, bubble trajectory takes more regular pattern and smaller deviation off center position in surfactant solutions than in pure water. It is also found that the bubble shape oscillations influence the bubble transient velocity:the more oblate the bubble, the faster it rising. Under the experimental conditions, surfactant effect on meso-scale bubble dynamics depends on surfactant type. The effect of Triton X-100 is stronger than Polyethylene glycol and n-Pentanol.
Experimental results also indicate that the presence of the surfactant has a dramatic effect on the rising velocity and size distribution of bubble swarms for various gas flow rates in flotation column. When bubbles rise in surfactant solutions, surfactant molecules can be adsorbed over the bubble surface, which stabilizes the bubbles and yields the repulsive force between the adsorbed surfactant layers of bubbles, resulting in preventing the coalescence of bubbles. As a consequence, the rising velocity of bubble swarms in surfactant solution is less than that in pure water. At same time, bubble swarms have more uniform and narrower size distribution in surfactant solutions than in pure water. In the cases of 0 (pure water) and 0.05x10-3mol/L Triton X-100 solutions, Sauter mean diameters of bubble swarm are 2.80 and 2.16 mm, respectively. Compare to pure water, Sauter mean diameters decreasse by 22.9%. Moreover, the narrowest bubble size distribution and the least Sauter mean diameter of bubble swarms can be observed in Triton 100 solution. Another finding is that the bubble sizes become nonuniform and the bubble size distribution is widened slightly with increasing gas flow rate. An increase in gas flow rate can increase the average bubble diameter and Sauter mean diameter, but decrease the specific surface area of bubble swarms.
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