Mechanical flotation cell

Froth flotation is an application of foams that is used to separate mineral components from each other based on their having different surface properties, typically their wettability and surface electrical charge. For example, froth flotation is the classic process used to separate copper from lead ore. The process involves having hydrophobic particles attach to gas bubbles which rise through a turbulent suspension to create a surface foam called a froth. Figure 10.2 shows an illustration of a mechanical flotation cell. This is the classic flotation device [53,91,625], First, the flotation feed particles are well dispersed into a particle suspension. Together with chemical flotation aids, such as collectors and frothers, this constitutes what is called the flotation pulp. In a mechanical flotation cell, air is fed in the form of fine bubbles and introduced near the impeller (see Figure 10.2). In addition to mechanical flotation cells, there are also pneumatic cells and cyclone flotation cells. Pneumatic... 

Conventional mechanical flotation cells have been widely used in the industry for many years and remain the primary means of cleaning coal particles smaller than 100 mesh. These units consist of a number of cells (usually 4-6) connected in series, each cell containing a mechanical agitator and aerator. The feed slurry, pretreated with frother and, usually, collector reagents, passes from one cell to the next with the most floatable coal being recovered in the first cell and the least floatable coal in the last cell. 

Lima OA, Deglon DA, Leal FUho LS. (2009) A comparison of the critical impeller speed for solids suspension in a hench-scale and a pilot-scale mechanical flotation cell. Miner. Eng., 22 1147-1153. 

Miskovic S. (2011) An investigation of the gas dispersion properties of mechanical flotation cells an in-situ approach. Ph.D. Thesis, Virginia Polytechnic Institute and State University, USA. 

Vmnett L, Contreras F, Yianatos J. (2012) Gas dispersion pattern in mechanical flotation cells. Miner. Eng., 26 80-85. 

Equipment Flacculator + flotation + pressurizing 4 in-line mechanical flotation cells ... 

Water Clarification. Process water that aeeds to be clarified comes from several differeat sources ia the recycling mill rejects from screeas and mechanical cleaners rejects from washers, thickeners, and flotation cells water that drains from the pulp as it is converted iato paper oa the paper machine (white water) and water from felt washers. These waters contain different dissolved chemicals and suspended soflds and are usually processed separately. 

The flotation process usually iuvolves three steps (/) the conditioning of the coal surface iu a slurry with reagents, (2) adhesion of hydrophobic coal particles to gas bubbles, and (J) the separation of the coal-laden bubbles from the slurry. In the conventional flotation process, when the coal particles become attached to air bubbles, the particles ate allowed to rise to the top of the flotation cell and form a stable froth layer (9). A mechanical scraper is used to remove the froth layer and separate the clean coal product from the refuse-laden slurry. 

Flotation is an important technique in mineral processing, where it is used to separate different types of ores. When used to separate solid-solid mixtures, the material is ground to a particle size small enough to liberate particles of the chemical species to be recovered. The mixture of solid particles is then dispersed in the flotation medium, which is usually water. The mixture is then fed to a flotation cell, as illustrated in Figure 8.12a. Here, gas is also fed to the cell where gas bubbles become attached to the solid particles, thereby allowing them to float to the surface of the liquid. The solid particles are collected from the surface by an overflow weir or mechanical scraper. The separation of the solid particles depends on the different species having different surface properties such that one species is preferentially attached to the bubbles. A number of chemicals can be added to the flotation medium to meet the various requirements of the flotation process ... 

The mechanical strength of the surface layers of many of the oxide copper minerals is weak. Therefore, flotation of oxide copper ores using sulphidization method, can improve by reducing turbulence and attrition within the flotation cell [3],... 

Air is most commonly dispersed with mechanical agitation. Figure 19.8 illustrates a popular kind of flotation cell in which the gas is dispersed and the pulp is circulated with impellers. Such vessels have capacities of 300-400 cuft. Usually several are connected in series as in Figure 19.8(b). The froth is removed from... 

Mechanical cell flotation cell that uses mechanical agitation of a pulp by means of an immersed impeller (rotor) and stator stirring mechanism (ASTM D-5114). 

As discussed in Section 1.2.2 the bubble shapes in fairly dry foams and froths (4 gas > 0.83, approximately) are not spheres or distorted spheres, but polyhedrons. In practice there will be distributions of both gas-cell sizes and shapes. In addition to the gas bubbles, froth contains the floated particles, pulp liquor, and a fraction of (hydrophilic) particles that did not float due to bubble attachment, but which were mechanically entrained in the froth. The pulp liquor and these latter particles all have to be allowed to drain back out of the froth. The rate of this drainage will be greatest at the froth-pulp interface (i.e., the bottom of the froth layer) and slowest near the top of the froth layer. Froth drainage equations are discussed elsewhere [53]. The froth needs to be a stable enough foam that some time can be allowed for these drainage processes, and also so that the upper layer(s) of the froth can be swept out of the flotation cell. On the other hand, the froth should not be too stable as a foam so that it will break easily after collection. In addition to the role of the frother, froth stability is also promoted by increasing liquid viscosity. 

Fig. 1. A Principle of flotation in a mechanical-type cell. The rotor and stator (which is here omitted for simplicity) keep the mineral particles and air bubbles in dispersion for adhesion. B Formation of hydrophobic and hydrophilic adsorption layer on solid in quartz-fluorite system...

Mechanical Cells. Figure 19-73 presents a schematic representation of a typical mechanical device commonly known as a flotation cell. It is characterized by a cubic or cylindrical shape, equipped with an impeller surrounded by baffles with provisions for introduction of the feed slurry and removal of froth overflow and tailings underflow. The machines receive the supply of air through a concentric pipe surrounding the impeller shaft, either by self-aeration due to the pressure drop created by the rotating impeller or by air injection by means of an external blower. In a typical installation, a number of flotation cells are connected in series such that each cell outputs froth into a launder and the underflow from one cell goes to the next one. The cell design may be such that the flow of slurry from one cell to another can either be restricted by weirs or unrestricted. 

These units generally consist of four flotation cells (Figure 6), each of which is equipped with a motor-driven self-aerating rotor mechanism. The following describes the principle of operation (8) ... 

Condition the solids to alter the wettability of the mineral and the gangue. The fundamental surface wettability for sulfide ores is different from oxides, silicates, and salt-type minerals. pH is a critical variable. Typical conditioning chemical additions include collector about 0.01 to 0.1 kg/Mg solids frother about 0.01 to 0.5 kg/Mg solids activator about 1 to 4 kg/Mg solids depressant about 0.02 to 2 kg/Mg solids. Allow 6-min contact for conditioning. Bubble size about 1000 pm. Flotation rate constant is 0.2 to 1 min sink rate constant is 0.005 min-. Flotation cells mechanical cell for fast float, sequential separation, and relatively coarse particle diameter 1.6 to 2.4 kW/m cell volnme. Pneumatic cell for relatively dilute feed concentrations and smaller particle diameters. Air blower 0.5 kW/m cell volume. Typical solids throughput 0.4 to 0.8 kg/s m feed concentration 10 to 40% w/w. Air escape velocity 0.02 m/s. Float times 6 to 20 min. Feed concentration to rongher or scavenger 30% w/w to cleaner 10% w/w. 

Yianatos JB. (2(X)7) Fluid flow and kinetic modeling in flotation related processes columns and mechanically agitated cells—a review. Chem. Eng. Res. Des., Trans. IChemE, Part A, 85(A12) 1591-1603. 

---