Floe flotation

Some modifications to the standard froth-flotation process have been developed in order to improve separation and recovery efficiency including carrier flotation, emulsion flotation, and floe flotation. These are all aimed at making very fine particles amenable to flotation, or at improving the efficiency of their flotation. 

Similarly, in floe flotation the small-sized particles become aggregated into floes that can attach to the air bubbles. Here, polymers may be used to induce the required level of flocculation. Examples include the floc-flotation of coal fines [631] or galena [632],... 

Flocculation of fine particles using polymeric materials (flocculants) and separation of such aggregates from particles of the other component(s) in the dispersed phase is known as selective flocculation [67]. The competition between different surfaces for the flocculant must be controlled in order to achieve adsorption on the targeted components). The aggregates of the polymer-coated particles, or floes, thus formed are separated from the suspension by either sedimentation-elutriation or floe flotation. 

Very finely divided minerals may be difficult to purify by flotation since the particles may a ere to larger, undesired minerals—or vice versa, the fines may be an impurity to be removed. The latter is the case with Ii02 (anatase) impurity in kaolin clay [87]. In carrier flotation, a coarser, separable mineral is added that will selectively pick up the fines [88,89]. The added mineral may be in the form of a floe (ferric hydroxide), and the process is called adsorbing colloid flotation [90]. The fines may be aggregated to reduce their loss, as in the addition of oil to agglomerate coal fines [91]. 

Flotation. Flotation (qv) is used to remove suspended soHds from wastes and for the separation and concentration of sludges (52,53). The waste flow is pressurized in the presence of sufficient air to approach saturation. When the pressurized air—Hquid mixture is released to atmospheric pressure in the flotation unit, minute air bubbles are formed. As they rise in the Hquor the sludge floes and suspended soHds are floated to the surface where the air—soHd mixture can be skimmed off. 

Flocculation. Flocculation (qv) uses chemical precipitation to effect separation (54). It is used to increase the rate of sedimentation and flotation. The resulting material known as floe can be removed by filtration, sedimentation, or flotation. 

The bubble size in these cells tends to be the smallest (10 to 50 Im) as compared to the dissolved-air and dispersed-air flotation systems. Also, very httle turbulence is created by the bubble formation. Accordingly, this method is attractive for the separation of small particles and fragile floes. To date, electroflotation has been applied to effluent treatment and sludge thickening. However, because of their bubble generation capacity, these units are found to be economically attractive for small installations in the flow-rate range of 10 to 20 mVh. Electroflotation is not expected to be suitable for potable water treatment because of the possible heavy metal contamination that can arise due to the dissolution of the electrodes. 

Flotation is certainly the major separation method based on the surface chemistry of mineral particles. It is, however, not the only method. Selective flocculation and agglomeration may be mentioned as other methods used commercially to a limited extent. The former is for hematite, while the latter is for coal and finely divided metallic oxide minerals. Both processes use the same principles as described for flotation to obtain selectivity. In selective flocculation, polymeric flocculants are used. The flocculants selectively adsorb on the hematite, and the hematite floes form and settle readily. Thereby separation from the sili-... 

Most of the pollutants may be effectively removed by precipitation of metal hydroxides or carbonates using a reaction with lime, sodium hydroxide, or sodium carbonate. For some, improved removals are provided by the use of sodium sulfide or ferrous sulfide to precipitate the pollutants as sulfide compounds with very low solubilities. After soluble metals are precipitated as insoluble floes, one of the water-solid separators (such as dissolved air flotation, sedimentation, centrifugation, membrane filtration, and so on) can be used for floes removal.911 The effectiveness of pollutant removal by several different precipitation methods is summarized in Tables 5.15-5.17. 

To obtain a suitable concentrate, the flocculated particles must be separated from the suspension. The usual method is sedimentation of the floes combined with elutriation of the dispersed particles. Flotation of the flocculated particles is a possible method to achieve that separation. The effect of polymers used as flocculants on the flotation of a few minerals has received... 

An alternative to sedimentation for removing suspended solids is flotation. This tends to be used for low-density particles that tend to float anyway during conventional sedimentation processes. Drinking water examples include algae and floes of humic and fulvic acids that result from the treatment of coloured waters [549]. Wastewater examples include fatty materials, pulp fibres, and oils that can be floated... 

Floes formed by EC are similar to chemical floe. But, EC floe tends to be much larger, contains less bound water, is acid resistant, and is more stable. In the chemical coagulation process, it is always followed by sedimentation and filtration. While in the electrocoagulation process, it can be followed by sedimentation or flotation. The gas bubbles produced during electrolysis can carry the pollutant to the top of the solution where it can be more easily concentrated, collected, and removed. 

Freshly formed amorphous Al(OH)3(s) sweep floes have large surface areas which is beneficial for a rapid adsorption of soluble organic compounds and trapping of colloidal particles. Finally, these floes are removed easily from aqueous medium by sedimentation or flotation. 

Water soluble polymers serve widely as flocculants, particularly in the water treatment, paper making, and minerals industries. The objective generally is to destabilize dispersions and cause large, strong, and compact floes to form quickly. The floes are often removed subsequently by sedimentation or flotation. Polymers perform efficiently without introducing the salt needed to... 

Polymeric forms of the coagulants (e.g., polyaluminum chloride) are rather useful because they require lower alkalinity consumption and the concomitant lower production of sludge. Once the colloids are destabilized, slow stirring can also help them merge and form relatively large aggregates called floes (hence the name flocculation) that are separated from water by settlement and filtration. In some cases, these floes can be attached to purposefully generated gas bubbles and separated by flotation. 

Pressurization could be carried out on the entire feed stream (full-flow pressure flotation) or a fraction of the feed stream while the remainder is introduced directly without aeration into the flotation tank (split-flow pressure flotation). The spht-flow system offers a cost saving over the full-flow units, since only a portion of the influent needs to be pressurized. In both cases, however, if the sohd particles in the feed stream are flocculated before introducing to the flotation tank, the high shear during pressurization, aeration, and pressure release can destroy the floes. Also, if the particle loading in the feed stream is high, both systems are susceptible to block e of the air release devices. To minimize these problems, recycle-flow pressure flotation is often practiced (Fig. 19-71). In this process, the feed stream, flocculated or otherwise, is introduced directly into the process vessel, and part of the clarified effluent is pressurized, aerated, and recycled to the flotation tank in which it is mixed with the flocculated feed. The air bubbles are released as they attach to the floes and float to the tank surface. The recycle-flow devices are found to offer the highest unit capacities. 

There has been very little publication/information on the treatment of palm oil refinery effluent. Osenga (41) introduced a treatment process consisting of a cross flow interceptor (CFl) for oil separation, physical and chemical treatment, and air flotation units to remove the floes followed by a batchwise activated sludge process for the liquid effluent treatment. This process also requires close supervision in order to achieve the desired treatment efficiency. 

Various methods for the removal of As from geothermal waste waters have been investigated at theoretical, laboratory, pilot plant and full plant scales. These include adsorption onto Fe-oxide floe and subsequent separation by dissolved air flotation (De Carlo and Thomas, 1985 Shannon et al., 1982) and co-precipitation with lime to form an As-rich calcium silicate (Rothbaum and Anderton, 1976). In both cases, effective removal was achieved only after oxidation of As" to As, For Fe-oxide floe treatment, competitive adsorption of silica inhibits As adsorption, particularly that of As" (Swedlund and Webster, 1999), suggesting that prior removal of silica would help optimise As removal efficiency. The use of ion selective chelating resins for As" removal from geothermal waters has also been successfully tried (Egawa et al, 1985). 

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