Open Chem., 2016; 14: 132–139 Review Article Open Access Efrosyni N. Peleka* and Kostas A. Matis Hydrodynamic aspects of flotation separation DOI 10.1515/chem-2016-0014 received January 8, 2016; accepted April 6, 2016. ii) Dissolved-air flotation (DAF), clarifies wastewaters by removal of suspended oil or solids. Abstract: Flotation separation is mainly used for removing Air is dissolved in the wastewater under pressure and particulates from aqueous dispersions. It is widely used for released in a flotation tank. The resulting tiny bubbles ore beneficiation and recovering valuable materials. This adhere to the suspended matter and float to the surface paper reviews the hydrodynamics of flotation separations where it is removed by skimming. DAF is widely used in and comments on selected recent publications. Units are treating effluents from refineries, petrochemical and distinguished as cells of ideal and non-ideal flow. A brief chemical plants, natural gas processing plants, paper mills, introduction to hydrodynamics is included to explain an similar industrial facilities, and general water treatment. original study of the hybrid flotation-microfiltration cell, Flotation is a very complicated process combining effective for heavy metal ion removal. fundamental hydrodynamics with many elementary physicochemical steps (bubble-particle interaction forces, Keywords: fine particles, flotation, flow pattern, particle-particle interaction forces etc.). Its modelling is hydrodynamics, metal ions removal, water, wastewater, important to understand the process; it is also a necessary minerals, recovery tool for equipment design and optimization. Modelling is difficult because of the many phenomena involved and the wide disparity of scales. The design and efficient operation of flotation cells 1 Introduction requires knowledge of processes inside the cell: how much time is allowed for the particles to mix with the air 1.1 The process and materials recovery bubbles; whether there are dead spaces where matter gets trapped, or bypassing, with either air or pulp leaving the Flotation separation originated with mineral processing cell too soon. All these malfunctions – termed non-ideal (froth flotation). Its importance to the global economy is flow – lead to poor performance. The hydrodynamics enormous. Without it many familiar metals and inorganic governing the aeration governing the aeration, collection raw materials would be exceedingly scarce and costly. It and separation steps of the flotation process have also has also found wide application to metal recovery from a been investigated by the equipment suppliers to develop wide variety of heavy metal bearing waste streams [1,2]. new technologies. There are two broad categories, depending on the The internal flow pattern is important in chemical method of bubble generation [3]: unit design. The liquid characteristics, the unit scale, i) Dispersed-air flotation, including electroflotation the baffles, and the impeller are among the factors that (electrolytic flotation), where air is introduced by propellers influence the flow pattern. Units are distinguished as cells or pumped through diffusers. Bubble size is relatively large of ideal and non-ideal flow [4]. In ideal cells the transport (100–1000 μm). High shear and high bubble rise rate limit and mixing processes can be described mathematically, this to processes such as ore benefication, and in contrast to real ones where the transport processes are only approximately known and must be modelled. A flotation tank can be thought of as consisting of two basic regions: a ‘noisy’ reaction zone and a ‘quiet’ flotation zone. In the reaction zone bubbles are brought in *Corresponding author: Efrosyni N. Peleka: Division of Chemical contact with incoming contaminated water and particles Technology, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, Greece, E-mail: [email protected] and bubbles attach. In the flotation zone, bubble/particle Kostas A. Matis: Division of Chemical Technology, Department of aggregates rise achieving separation. In a properly operating Chemistry, Aristotle University of Thessaloniki, Thessaloniki, Greece flotation tank no settling particles occur in both zones [5]. © 2016 Efrosyni N. Peleka and Kostas A. Matis, published by De Gruyter Open. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. Bubble size plays an important role in fine particle parameters acting at different length scales make the flotation [2]; electroflotation bubbles (both hydrogen and applied models exceedingly complicated, and new multi- oxygen) have a mean diameter near 50 μm under normal scale generalizations have been developed [12]. Despite operating conditions with horizontal stainless steel sheet the significance of turbulence in enhancing flotation electrodes [7]. Although much attention has been paid to there has been no unified approach to modeling flotation particle size, the role of bubble size has been comparatively in turbulent flow. disregarded and deserves more attention [1b]. However, Literature examining sustainable mineral and unconventional bubble generation methods utilize fine mining mostly concerns global and national scales [13]. bubbles, as in electroflotation. New economic activities are surprisingly dependent A large amount of valuable minerals has been on traditional raw materials. All electronic appliances discarded as fines and ultrafines because of inadequate use copper and require electrical power which may be technology to process them economically. On the other generated by coal and transmitted by aluminum and hand, fine particles often constitute a problem in chemical copper wire. A home PC typically contains around thirty industry, and raw materials processing, often as by- mineral ingredients [14]. Hence, recycling is important. products [3]. Recent flotation research is leading towards green Research in hydrodynamic cavitation for natural chemistry [15]. Sustainability in chemical and process resource recovery has been reviewed [8]. Interactions industry is essential, as the need for fresh drinking water between tiny bubbles and fine particles in an aqueous slurry sources is becoming urgent worldwide. Environmentally were analyzed based on particle surface properties and friendly biosurfactant flotation agents (produced by yeast types of gas nuclei. Tiny bubbles generated by cavitation or bacteria from sugars, oils, alkanes and wastes) could increased the contact angle of solids and attachment force, replace chemosynthetic surfactants [16]. Biosurfactants bridged fine particles to form aggregates, minimized slime can potentially be as effective as synthetic surfactants coating, removed surface oxidation layers, and reduced with some distinct advantages including high specificity, reagent consumption. biodegradability and biocompatibility. A complex model has been used to describe liquid- Heavy metal wastewaters usually derive from phase mixing in a typical flotation column (for mineral electroplating, plastic manufacturing, fertilizer, pigment, beneficiation), involving a fully-mixed zones-in-series mining and metallurgical processes. They are usually model which allowed both forward and backward toxic and metals accumulation through the food chain flow between the zones [9,10]. The model involved two poses a serious health hazard [17]. Thus, gradually stricter parameters: the number of zones and the ratio of backward regulations press for innovative removal treatments. to net forward flow. The degree of mixing in the gas- The perceived gap between dispersed-air (froth) liquid flotation column, important for performance, was flotation and dissolved-air flotation was a conference affected by the relative magnitude of the gas and liquid sub-theme [18]. The gap perhaps originates from the flows. separate traditions and the infrequent interaction among A computational fluid dynamic model incorporating the separate applications. The different foci (selective flotation kinetic expressions has been developed by separation or not), the difference in particle concentrations, Kostoglou et al. to simulate the performance of flotation and the use of chemical reagents may explain the gap. tanks [5]. The simulations revealed key dissolved-air Mineral processing may be lucrative; however, effluent flotation characteristics in determining local flotation treatment imposes costs. rates. This study focuses on tanks that operate without external flow mixing (impeller, etc.), such as DAF tanks where bubble buoyancy, particle settling and turbulence 1.2 Hydrodynamic background contribute comparably to local flotation rates; their relative contributions were examined. In Denver type dispersed-air “Flotation cells seem never to have been seriously flotation tanks turbulence is dominant due to the intense researched; admittedly, there are many ingenious mixing imposed by an impeller, so gravitational/buoyant inventions in the patent literature; but they were not effects can be ignored. the fruits of a deep knowledge of the hydrodynamics” In a properly operating flotation tank no settling [19]. The following is intended mainly for those not particles occur in both zones. Ill-functioning may close to chemical engineering, although a flotation cell occur if a tank is operated using bubbles larger than is not a typical chemical reactor. There is no reaction but it was designed for [11]. The interrelationships among separation and no density change. 134 Efrosyni