Flotation cell

Fig. XIII-4. Schematic diagram of a froth flotation cell. Note the mineralized bubble shown in the inset. [Reprinted with permission from P. Somasumdaran, Interfacial ChemisUy of Particulate Flotation. AIChE Symp. Sen, 71(150), 2 (1975) (Ref. 58). Reproduced by permission of the American Institute of Chemical Engineers.]...

The actual flotation phenomenon occurs in flotation cells usually arranged in batteries (12) and in industrial plants and individual cells can be any size from a few to 30 m in volume. Column cells have become popular, particularly in the separation of very fine particles in the minerals industry and coUoidal precipitates in environmental appHcations. Such cells can vary from 3 to 9 m in height and have circular or rectangular cross sections of 0.3 to 1.5 m wide. They essentially simulate a number of conventional cells stacked up on top of one another (Fig. 3). Microbubble flotation is a variant of column flotation, where gas bubbles are consistently in the range of 10—50 p.m. 

The processes that occur in a typical flotation cell are schematically shown in Figure 5 and consist of agitation, particle—bubble coUision and attachment, flotation of particle—bubble aggregates, collection of aggregates in a froth layer at the top of the cell, removal of mineral-laden froth as concentrate, and flow of the nonfloating fraction as tailings slurry. 

Fig. 5. Processes that occur in a flotation cell A, air supply E, slurry inlet F, froth overflow L, froth layer inset, mineralized bubbles P, flotation pulp R,...

Up to 0.4 g/L of the iodine stays in solution and the rest precipitates as crystallized iodine, which is removed by flotation (qv). This operation does not require a flotation agent, owing to the hydrophobic character of the crystallized element. From the flotation cell a heavy pulp, which is water-washed and submitted to a second flotation step, is obtained. The washed pulp is introduced into a heat exchanger where it is heated under pressure up to 120°C to melt the iodine that flows into a first reactor for decantation. From there the melt flows into a second reactor for sulfuric acid drying. The refined iodine is either flaked or prilled, and packed in 50- and 25-kg plastic-lined fiber dmms. 

The solution leaving the flotation cell, containing about 0.4 g/L iodine, is sent to a kerosene solvent extraction process to recover the dissolved product. After neutralization with soda ash to the initial incoming alkalinity, the solution is returned to the nitrate lixiviation process. The iodine-chaiged kerosene is contacted with an acidic concentrated iodide solution containing SO2, which reduces the iodine to iodide. 

Pulp exiting the conditioners is diluted by usiag process brine to a soHds content of 30—35 wt % for use as feed for the flotation cells. In some plants, the coarse- and fine-fraction flows are floated separately. In most plants, the two fractions are recombiaed and the flotation is conducted ia a common operation. 

Fine screens Fine screens Flotation cells Optional Dilution... 

Forward cleaners Thickener Forward cleaners flotation cells Forward cleaners... 

Vtfesher Flotation cells Vtfesher Forward cleaners Fractionation... 

Bleach tower Reverse cleaners Forward cleaners Vtfesher Bleach tower Thickener Dispersion unit Flotation cells Reverse cleaners Vtfesher Bleach tower Vtfesher Bleaching Thickener Dispersion unit... 

Flotation. Flotation (qv) is used alone or in combination with washing and cleaning to deink office paper and mixtures of old newsprint and old magazines (26). An effective flotation process must fulfill four functions. (/) The process must efficiently entrain air. Air bubble diameter is about 1000 p.m. Typically air bubbles occupy 25—60% of the flotation cell volume. Increa sing the airRquid ratio in the flotation cell is said to improve ink removal efficiency (27). (2) Ink must attach to air bubbles. This is primarily a function of surfactant chemistry. Air bubbles must have sufficient residence time in the cell for ink attachment to occur. (3) There must be minimal trapping of cellulose fibers in the froth layer. This depends on both cell design and surfactant chemistry. (4) The froth layer must be separated from the pulp slurry before too many air bubbles coUapse and return ink particles to the pulp slurry. 

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. 

Water from screens, cleaners, washers, thickeners, and flotation cells contain relatively high levels of ink. These waters also contain valuable chemicals, ie, sodium hydroxide and surfactants. Recycle of this water can save up to 10% ia chemical costs. 

A large reserve of caUche ore bearing iodine is being processed in the Atacama Desert. Production of iodine there is relatively inexpensive. About 40% of the world supply of iodine is made from these Chilean deposits. The process consists of leaching the caUche with water. Brine is stripped of iodine using an organic solvent. The iodine is then removed from the solvent to form a slurry. SoHd-phase iodine is separated from the slurry in conventional flotation cells, dried, and packaged. Details of the process are proprietary. 

Two cocrystallization processes employ dibasic crystals as intermediates. The PPG process (199—202) is discussed under commercial processes. The PPC process (203) forms dibasic crystals from lime and recovered filtrates. The dibasic crystals are separated from thek mother liquor by decantation, slurried in caustic solution and chlorinated to produce a cocrystalline slurry of Ca(OCl)2 and NaCl. The slurry is sent to a flotation cell where the larger salt crystals settle out and the smaller hypochlorite crystals float to the top with the aid of ak and flotation agent. The hypochlorite slurry is centrifuged the cake going to a dryer and the centrate to the flotation cell. The salt-rich bottoms from the flotation cell are centrifuged and washed with dibasic mother Hquor. The centrates are recycled to the precipitation step. 

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. 

In 1981, a novel flotation device known as the air-sparged hydrocyclone, shown in Figure 3, was developed (16). In this equipment, a thin film and swid flotation is accompHshed in a centrifugal field, where air sparges through a porous wall. Because of the enhanced hydrodynamic condition, separation of fine hydrophobic particles can be readily accompHshed. Also, retention times can be reduced to a matter of seconds. Thus, this device provides up to 200 times the throughput of conventional flotation cells at similar yields and product quaHties. 

Flotation. The slurry of ground ore leaving the grinding circuit may be separated from part of the water in thickeners or may go directly to the flotation cells. The latter are rectangular tanks into which air is injected or drawn via impellers. Flotation is based on producing a water-repellent chemical film on the exposed sulfide minerals in the ground ore. The sulfide minerals collect on the surface of the air bubbles and rise to the top of the flotation cell, where they can be removed from the froth. The froth overflows the cells in collector troughs called launders. 

The pH of the pulp to the flotation cells is carefliUy controlled by the addition of lime, which optimizes the action of all reagents and is used to depress pyrite. A frother, such as pine oil or a long-chain alcohol, is added to produce the froth, an important part of the flotation process. The ore minerals, coated with an oily collected layer, are hydrophobic and collect on the air bubbles the desired minerals float while the gangue sinks. Typical collectors are xanthates, dithiophosphates, or xanthate derivatives, whereas typical depressants are calcium or sodium cyanide [143-33-9] NaCN, andlime. 

Nearly all the energy used in the concentrators is for electric motors driving ball mills, cmshers, pumps, and the agitators of flotation cells. About half the energy is used for grinding the ore to the proper size. 

The pulp density is adjusted to a consistency of 15 to 35% solids and the resulting feed is fed to the flotation cell. 

Figure 2.20 A simplified sketch of a flotation cell, showing its basic features.

The galena-bearing froth is removed from the top and the impoverished tailing is drawn away from the bottom of the cell. A device in which all these processes take place is called a flotation cell (Figure 2.20). 

The study of flotation kinetics relates to a number of mass transfer processes and these are listed in Table 2.8. The term, entrainment which figures in the mass transfer process statements made in Table 2.8 may be elaborated. It is the process by which particles enter the base of a flotation froth and are transferred up and out of the flotation cell suspended in the water between bubbles. Entrainment should be distinguished from true flotation, whereby particles come out of the cell attached to bubble surfaces. True flotation is chemically selective, while the entrainment process recovers both gangue and valuable minerals alike. Entrainment harms the product grade since recovery of the more abundant gangue mineral reduces the quality of the concentrate. This is especially true in the processing of fine ores. Much flotation research has dealt with reducing entrainment in order to improve... 

Figure 2.31 One way in which a number of flotation cells can be arranged into a flotation system.

It may be added that continuous developments are taking place in flotation cell design. A reader wishing to have a detailed information concerning a particular type of cell is better served by brochures published by the manufacturer than by a book. 

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