Flotation technique: Its mechanisms and design parameters
Introduction
Flotation is a widely used, cost-effective separation technique for wastewater treatment [1], mineral beneficiation [2], micro-oxygenation of wine [3], fermentation [4], ink removal [5], plastic recycling [6] etc. Flotation techniques are used for the various chemical, mineral and biological industries. In this technique, components like fine particles, oil droplets, contaminants etc. are separated from the mixture based on their hydrophobic or hydrophilic surface properties. The essence of the flotation process depends on using the gas bubbles to capture the particles based on their surface hydrophobicity and hydrophilicity [7]. The gas bubbles produced in conventional flotation columns are in the size range of 1.0–1.5 mm in diameter [8]. The gas bubbles are used to adhere the hydrophobic particles selectively and carry them to the surface of the liquid, hence they form a froth zone where it can be separated while the hydrophilic particles are discharged from the bottom outlet as the tailing. The efficiency of the flotation process is a function of the probability of particle-bubble collision, particle-bubble attachment, and particle-bubble detachment. Froth flotation is an extensively accepted separation technique where the separation characteristics depend on narrow particle size range between approximately 10–100 μm. Beyond this particle size range, the separation efficiency of flotation process reduces notably because of difficulty to attachment of weak hydrophobic particles to gas bubbles [9]. Fine particles have large specific surface areas and low mass due to which the probability of bubble-particle collision is limited therefore results in slow separation rate [10]. Conventional flotation machines does not generate bubble size less than 600 μm for which its application is limited to separate the coarser particles only [11]. Very fine particles may float with smaller size bubbles and fine or mid-sized particles with the bigger size bubbles [12]. Flotation of fine particle (<37 μm) and ultrafine particle (<8–13 μm) is a key challenge. Feed in flotation consists of wide range of particle size, therefore, flotation column needs to have wider bubble size to target fine as well as coarse size particles [13,14]. So, flotation of fine particles is one of the major challenging tasks. Research is conducted to explore the mineral-bacteria interaction in order to understand the mechanisms of selectivity of microorganism towards specific particle [15]. Perez-Garibay et al. [16] investigated the effect of bubble size distribution, superficial gas velocity, surfactant concentrating and three different particle sizes such as coarse (100 μm), medium (39 μm) and fine (15 μm) on the flotation characteristics. Investigation reports that particular particle-size distribution needed an optimum bubble-size distribution profile, bubble size ranges between 150–1050 μm diameter and gas holdup ranging from 0.2% and 1.3%. The wide range of bubbles can be produced using dissolved air flotation technique in combination with a surfactant. This technique is able to produce coarse bubbles (400–800 μm) and nanobubble (200–720 nm) where nanobubbles are obtained by selective separation from the microbubble. The separation efficiency of the particles in presence of a combination of coarse and nanobubbles are compared with only coarse bubbles which reported that separation can be improved by 20–30% for the very fine particles (8–74 μm) while a slightly low separation is reported for coarse particles (67–118 μm) [17]. Lim et al. [18] proposed that integration of microbubble with macrobubble reduces the macrobubbles-oil attachment time by 82%, enhance the bubble-oil contact angle by 40.35 and interfacial surface area of attachment by 54.5%. Many studies are focused on the mineral-bacteria interaction in order to understand the mechanisms behind the floatability and mineral selectivity achieved in the beneficiation process. Recent studies demonstrate the microorganisms aided mineral beneficiation with the modern advancement in biotechnology. Microorganisms specifically involve to remove the gangue particles which are detrimental as responsive flotation reagents [19]. Flotation columns are simple in construction and mostly suited for carrying out the separation of the desired element from the liquid or solid mixture in a multiphase environment. Flotation columns are multiphase contacting device where the liquid is in continuous phase while the gas and particles are in dispersed phase. In counter current operation, the feed is introduced after conditioning with reagents, which enters into the flotation column approximately at 2/3 of the column height where it mixes with liquid and interacts with the swarm of gas bubbles that is introduced from the bottom of the column through a gas distributor [20].
The relative motion of particles and gas bubbles governs the probability of the bubble-particle attachment, bubble loading, and flotation rate. Counter current movement of feed and gas bubble results in reduction of rise velocity of the bubbles, which increases their retention time in the slurry, hence decreases the compressed air requirement and increases the specific throughput of the column. In the counter current operation, the probability of bubble-particle collision is high because of the large aerated mixture in the column, the long distance of the transport of the bubble and particle along the column length and low longitudinal slurry mixing [21]. In some cases, the co-current operation is useful during the treatment of coarse particle in which particle-laden bubble rise time is reduced and residence time of the particle increases [22]. The flotation column can be divided into three specific zones: recovery zone, cleaning zone and froth zone. Recovery zone or collection zone: the zone between feed slurry inlet and sparger. In the recovery zone, the downflow feed slurry interacts with upflow gas bubbles where the attachment of the particles to the bubble takes place. The zone controls the degree of the capacity of flotation columns because the capacity of the flotation column depends on the intensity of bubble-particle collision, the probability of attachment and surface area of the gas bubble. Cleaning zone: the zone above feed slurry to froth interface level. Froth zone: the zone above the interface. Dead zone: the zone below the sparger zone. The volume below the sparger (dead volume) does not help in flotation but is used to expel tailings. Hydrophobic solid particles attached to the air bubbles are collected by froth while the hydrophilic particles are left in the slurry. It is not always the case of making a solid particle hydrophobic, but in some cases, a solid particle is made to be hydrophilic using reagent (depressant) so that their attachment to the bubble does not take place and settle down in flotation column. Generally, four steps are required during froth flotation such as:
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conditioning of desired solid particles during which hydrophobicity imparted to the surface of the solid
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the introduction of feed slurry to the flotation column where collision and attachment of solid particles to gas bubbles take place
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stable froth formation on the surface of flotation column and
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the removal of mineral-laden froth or tailing from the flotation cell.
In industrial flotation column, the entrainment of gangue particle is common, so several stages of flotation columns are involved to meet the economically acceptable quality of the desired mineral in the product. In this review article, the details of various design parameters of flotation process and the effect of operating and geometric variables influencing the separation efficiency are described. The optimization of different hydrodynamic characteristics is also discussed, and some strategies are suggested for improving the working principle of flotation devices.
Section snippets
Type of flotation and its application
Numerous techniques of flotation processes based on different working principles are briefed as follows:
Design parameters of flotation process
In the subsequent section, the hydrodynamic characteristics like flow regimes, gas holdup, bubble size and its distribution, pressure drop, wetting phenomena, RTD, mixing characteristics, channelling, induction time, entrainment, settling velocity and interfacial area are described.
Importance and perspective of future research
For a long time, flotation process has been used as a successful way of separating fine particles, wastewater treatment, deinking of paper etc. in chemical process industries. In spite of this, still, there are certain issues that are yet to be addressed to optimize the flotation process. After going through the literature it is observed that there is a huge uncovered area of research in hydrodynamics and microbubble assisted flotation process. Design parameters like flow pattern, gas holdup,
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