Floes

Heavy metals often can be removed effectively by chemical precipitation in the form of carbonates, hydroxides, or sulfides. Sodium carbonate, sodium bisulfite, sodium hydroxide, and calcium oxide are all used as precipitation agents. The solids precipitate as a floe containing a large amount of water in the structure. The precipitated solids need to be separated by thickening or filtration and recycled if possible. If recycling is not possible, then the solids are usually disposed of to a landfill. 

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]. 

The natural process of bringing particles and polyelectrolytes together by Brownian motion, ie, perikinetic flocculation, often is assisted by orthokinetic flocculation which increases particle coUisions through the motion of the fluid and velocity gradients in the flow. This is the idea behind the use of in-line mixers or paddle-type flocculators in front of some separation equipment like gravity clarifiers. The rate of flocculation in clarifiers is also increased by recycling the floes to increase the rate of particle—particle coUisions through the increase in soUds concentration. 

The type of floe requited depends on the separation process which foUows, eg, rotary vacuum filtration requites evenly sized, smaU, strong floes that capture ultrafines to prevent cloth blinding and cloudy filtrates. The floes should not be subject to sedimentation in the vat or breakage by the agitator. 

Such floes are not likely to cause localized air breakthrough, cake coUapse, shrinkage, or cracking in the dewatering stage. 

In gravity thickening, large and relatively fragile floes are needed to aUow high settling rates and fast coUapse in the compression zone. 

A fourth mechanism is called sweep flocculation. It is used primarily in very low soflds systems such as raw water clarification. Addition of an inorganic salt produces a metal hydroxide precipitate which entrains fine particles of other suspended soflds as it settles. A variation of this mechanism is sometimes employed for suspensions that do not respond to polymeric flocculants. A soHd material such as clay is deUberately added to the suspension and then flocculated with a high molecular weight polymer. The original suspended matter is entrained in the clay floes formed by the bridging mechanism and is removed with the clay. 

The effect of mechanical treatment on floe behavior is illustrated in Figure 5. In one work (40), identical slurries were treated with varying doses of the same polymer. At each dosage, it can be assumed that the same type of floe formed at the same rate. However, the dosage response was completely different depending on which parameter of the flocculated slurry was measured. Thus the term optimal flocculation caimot be appHed to any flocculant—substrate combination if the soHd—Hquid separation process or process parameter is not specified. 

Fig. 5. Effect of polymer dosage on different observed properties of flocculated slurry (40). Comparison of five parameters in a flocculation system (8%fluorite suspension + polyacrylamide Cyanamer P250). A, Rate of settling of floe boundary, in cm/s B, height of settled bed, cm C, height of consoHdated filter-cake, cm D, refiltration rate, arbitrary units and E, clarification, % optical transmission of 1 cm of supernatant Hquid after 3 min settling...

In the case of thickeners, the process of compaction of the flocculated material is important. The floes settle to the bottom and gradually coalesce under the weight of the material on top of them. As the bed of flocculated material compacts, water is released. Usually the bed is slowly stirred with a rotating rake to release trapped water. The concentrated slurry, called the underflow, is pumped out the bottom. Compaction can often be promoted by mixing coarse material with the substrate because it creates channels for the upward flow of water as it falls through the bed of flocculated material. The amount of compaction is critical in terms of calculating the size of the thickener needed for a particular operation. The process of compaction has been extensively reviewed in the Hterature (41,42). 

Eor vacuum filters, both the rate of filtration and the dryness of the cake may be important. The filter cake can be modeled as a porous soHd, and the best flocculants are the ones that can keep the pores open. The large, low density floes produced by high molecular weight polymers often coUapse and cause blinding of the filter. Low molecular weight synthetic polymers and natural products that give small but rigid floes are often found to be the best. 

Retention aid polymers are used in a very high shear environment, so floe strength and the abiHty for floes to reform after being sheared (43) is important. The optimum floe size is a compromise. Larger floes give better free drainage, but tend to produce an uneven sheet due to air breakthrough in... 

Dilution. In many appHcations, dilution of the flocculant solution before it is mixed with the substrate stream can improve performance (12). The mechanism probably involves getting a more uniform distribution of the polymer molecules. Since the dosage needed to form floes is usually well below the adsorption maximum, a high local concentration is effectively removed from the system at that point, leaving no flocculant for the rest of the particles. A portion of the clarified overflow can be used for dilution so no extra water is added to the process. 

Addition Point. The flocculant addition point in a continuous system can also have a significant effect on flocculant performance. The turbulence as the flocculant is mixed in and the floes travel toward the point where they enter the thickener or filter causes both the formation and breakup of floes. Usually there is an optimal addition point or points which have to be determined empirically. In cases where the same polymer is being added at two or more points, the relative amounts added at each point may also affect performance. Thus providing multiple addition points in the design of new installations is recommended (56). 

The slime, consisting of kaolin, fine quart2, and feldspar, is sometimes used as is after being dewatered. This material may be used in the manufacture of light-colored brick or may be further processed to produce a high grade ceramic kaolin used in the manufacture of dinnerware, electrical porcelain, or sanitary-ware (see Ceramics). Floes of kaolin may be sold in bulk from the drier or pulveri2ed and sold in a powdered form. 

Flocculants and surfactants (qv) are used frequently as filter aids, particularly when slimes are present or when the particles to be filtered are very fine and difficult to filter. Low molecular weight polymers are more commonly used. These form small, dense floes which provide higher cake porosity. 

The attenuation of ultrasound (acoustic spectroscopy) or high frequency electrical current (dielectric spectroscopy) as it passes through a suspension is different for weU-dispersed individual particles than for floes of those particles because the floes adsorb energy by breakup and reformation as pressure or electrical waves josde them. The degree of attenuation varies with frequency in a manner related to floe breakup and reformation rate constants, which depend on the strength of the interparticle attraction, size, and density (inertia) of the particles, and viscosity of the Hquid. 

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