Section 5.19 Froth Flotation

Pneumatic cell, less than mechanical cell. 

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Figure 19.8. The interaction of air and pulp in a froth flotation ceil and a series arrangement of such cells (a) Sectional schematic of flotation cell. Upper portion of rotor draws air down the standpipe for thorough mixing with pulp. Lower portion of rotor draws pulp upward through rotor. Disperser breaks air into minute bubbles. Larger flotation units include false bottom to aid pulp flow. (WEMCO Division, Envirotech Corp.). (b) A bank of three flotation cells. The floating concentrate is withdrawn continuously from each stage but the remaining pulp flows in series through the cells.

In contrast to conventional flotation, in which the desirable mineral is directly floated and collected from the produced froth, reverse (indirect) flotation aims to have the undesirable minerals preferentially floated and removed, leaving behind a slurry that has been concentrated in the desirable mineral. This method has been used for the purification of iron ore and the separation of salt from potash. The next section describes some other variations on conventional froth flotation. 

FIGURE 13.2 Cross section of a ball mill used for the wet grinding of copper ore preparatory to froth flotation. 

In industry, froth flotation processes are used to separate particles and/or droplets gas by attaching them to gas bubbles, which rise in a flotation vessel to form a product layer of foam teimed froth. The most common type of froth flotation is induced gas flotation (also termed scavenging flotation), in which gas bubbles are injected (sparged) into the flotation medium. Variations include dissolved gas flotation, in which gas is dissolved in water after which microbubbles come out of solution, attach to the dispersed species of interest and cause them to float (see also Section 8.3 and Chapter 10). 

In the flotation section, classification units may be used to split the minus 1.0 mm plus 0.1 mm feed into two size fractions for separate treatment. The coarser fraction (minus 1.0 mm to approximately plus 0.4 mm) can be concentrated by gravity (spirals) and froth flotation methods. The finer, minus 0.4 mm plus 0.10 mm, fraction is dewatered to 60%-75% solids. After being condi-... 

Plastic-Plastic Separation Both dry and wet processes can be used to separate one plastic from another in a recycling operation. The most common wet separation methods are based on differences in density between particles of different plastics [1, 2, 10, 49], Density is a bulk property of the plastic. Unfortunately, many factors beyond polymer type affect density. Plastic particles can also show differences in surface properties. Plastic-plastic separation methods based on surface property differences include triboelectric separation [75] (a dry process) and froth flotation [75] (a wet process). Density-based and surface-property-based methods for separating a stream of mixed plastics are covered in the following sections. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

Foams and emulsions may also be encountered simultaneously [114]. Figure 1.5 shows an example of an aqueous foam with oil droplets residing in its Plateau borders (see Section 5.6.7). In addition to containing gas, an aqueous phase, and an oleic phase, foams can also contain dispersed solid particles. Oil-assisted flotation of mineral particles provides one example (Chapter 10). Oil-sand flotation of bitumen provides another (Chapter 11). In the case of oil-sands flotation, an emulsion of oil dispersed in water is created and then further separated by a flotation process, the products of which are bituminous froths that may be either air (and water) dispersed in oil (from primary flotation) or air (and oil) dispersed in water (from secondary flotation). In either case, the froths must be broken and de-aerated before the bitumen can be upgraded to synthetic crude oil. (See Section 11.3.2). 

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. 

Emulsion flotation is analogous to carrier flotation. Here, small-sized particles become attached to the surfaces of oil droplets (the carrier droplets). The carrier droplets attach to the air bubbles and the combined aggregates of small desired particles, carrier droplets, and air bubbles float to form the froth. An example is the emulsion flotation of submicrometre-sized diamond particles with isooctane. Emulsion flotation has also been applied to the flotation of minerals that are not readily wetted by water, such as graphite, sulfur, molybdenite, and coal [623]. Some oils used in emulsion flotation include mixed cresols (cresylic acid), pine oil, aliphatic alcohols, kerosene, fuel oil, and gas oil [623], A related use of a second, immiscible liquid to aid in particle separation is in agglomeration flocculation (see Section 5.6.4). 

Figure 3.5 shows a cross section of a three-cell inductor dispersed gas flotation unit. Clean water from the effluent is pumped to a recirculation header (E) that feeds a series of venturi eductors (B). Water flowing through the eductor sucks gas from the vapor space (A) that is released at the nozzle (G) as a jet of small bubbles. The bubbles rise, causing flotation in the chamber (C), forming a froth (D) that is skimmed with a mechanical device at (F). 

Mechanical induced flotation units induce gas bubbles into the system by entrainment of gas in a vortex generated by a stirred paddle. Figure 3.43 shows a cross section of a dispersed gas flotation cell that utilizes a mechanical rotor. The rotor creates a vortex and vacuum within the vortex tube. Shrouds assure that the gas in the vortex mixes with and is entrained in the water. The rotor and draft inducer causes the water to flow as indicated by the arrows in this plane while also creating a swirhng motion. A baffle at the top directs the froth to a skimming tray as a result of this swirling motion. 

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