Electroflotation

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

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. 

Both dissolved air flotation (DAF) and electroflotation have been successfully applied to the removal of contaminants from surface water as well as groundwater.62 64... 

Krofta, M. and Wang, L. K., Development of innovative flotation-filtration systems for water treatment, part C electroflotation plant for single families and institutions, Proc. American Water Works Association Water Reuse Symposium III, San Diego, CA, 3, 1251-1264, 1984. 

Kuhn et alS50> described the electroflotation process which represents an interesting recent development for the treatment of dilute suspensions. Gas bubbles are generated electrolyt-ically within the suspension, and attach themselves to the suspended particles which then rise to the surface. Because the bubbles are very small, they have a high surface to volume ratio and are therefore very effective in suspensions of fine particles. 

An electroflotation plant usually consists of a steel or concrete tank with a sloping bottom as shown in Figure 1.49 liquid depth may typically be about 1 m. Since the flotation process is much faster than sedimentation with fine particles, flotation can be achieved with much shorter retention times — usually about 1 hour (3.6 ks) — and the land area required may be only about one-eighth of that for a sedimentation tank. 

Some examples of electrochemical waste treatment are given in Table 8 (122,123,125—132). Other electrochemical processes such as electro dialysis (qv), electroflotation, and electrodecantation are also used in waste treatment. 

Electrochemical technique (also electrocoagulation) is a simple and efficient method for the treatment of potable water. This process is characterized by a fast rate of contaminant removal, a compact size of the equipment, simplicity in operation and low capital and operating costs. Moreover, it is particularly more effective in treating wastewaters containing small and light suspended particles, such as oily restaurant wastewater, because of the accompanying electroflotation effect. 

Unfortunately, up to date, no solid evidence was reported to support the hypothesis of the above adsorption mechanism. Moreover, the hydrogen gas produced at the EC cathode prevents the floes from settling properly on leaving the electrolyzer [34], In order to overcome this problem, an EC process followed by an electroflotation (EF) operation can be applied. In this combined process, the EC unit is primarily for the production of aluminum hydroxide floes. The EF unit would undertake the responsibility of separating the formed floes from water by floating them to the surface of the cell. 

In the electroflotation method, the bubble size is critical to the efficiency of the phase separation and depends on the electrode metal. The bubble sizes formed on cathodes with small overpotentials for hydrogen evolution (such as Pd, W, and Ni) are larger than those formed on high... 

Based on these fundamentals, many systems and apparatus have been built, being operative on an industrial scale. Different types of reactors have also been designed. The electrodes may be parallel plates [162, 163] or sacrificial Al pellets as anode [164,165]. The feeding of pressurized air has been implemented in many electrocoagulation-electroflotation systems [159,166-168]. Some plants have a press to remove water from the sludge [169,170] and a processing tank with a closed S-shaped one-way flow path [171]. 

Il in and Sedashova [179] have described an electroflotation method and a small-sized plant to remove oil products from effluents based on adhesion of pollutant particles to finely divided bubbles of H2 and 02 produced during the electrolysis of water. From an initial petroleum product concentration of 1000 mg L-1, a residual concentration of 1 10 mg L-1 was found following electroflotation. This concentration could be further reduced to about 0.01 mg L-1 by the addition of inorganic coagulants. 

Khosla NK, Yenkatachalam S, Somasundaran P. Pulsed electrogeneration of bubbles for electroflotation. J Appl Electrochem 1991 21 986-990. 

Schade H, Peter KProcess and device for separating solid and liquid matter from industrial wastewater by electroflotation. European Patent EP 686603 Al, 1995. 

Osasa K, Kawanami H, Tukamoto E, Sakai K. Performance of continuous electroflotation of an oil-in-water (O/W) emulsion using a packed-bed-type sacrificial electrode as an anode. Kagaku Kogaku Ronbunshu 1996 22 1450-1456. 

Osasa K, Miyazaki T, Sakai K. Performance test of a rectangular electroflotation cell with packed-bed-type sacrificial electrodes. Kagaku Kogaku Ronbunshu 1997 23 594-596. 

Ritter H. Purification of dirty water by electroflotation. German Patent DE 4416973 Al, 1995. 

Lee MS. Wastewater treatment by means of electroflotation and electrocoagulation. European Patent EP 794157 Al, 1997. 

Il in VI, Sedashova ON. An electroflotation method and plan for removing oil products from effluents. Chem Pet Eng 1999 35 480 481. 

Ibanez JG, Singh MM, Szafran Z. Laboratory experiments on electrochemical remediation of the environment Part 4. Color removal of simulated wastewater by electrocoagulation-electroflotation. J Chem Educ 1998 75 1040-1041. 

Khelifa, A., Moulay, S. and Naceur, A. W. (2005), Treatment of metal finishing effluents by the electroflotation technique. Desalination, 181(1-3) 27-33. 

At the turn of the twentieth century, some works focused on the study of these technologies have reported very promising results. The proper design of the electrochemical cell to promote electroflocculation and electroflotation processes, the... 

Parameters k and k2 can be easily related to the hydrodynamic conditions (flow rate, stirring rates) and to the current density by empirical equations. The influence of the current density can also be related to the reagent dose for parameter k and to the bubble generation for parameter k2 (the flow rate of cathodically generated hydrogen is proportional to the current density). Thus, this semiempirical model considers easily and simultaneously the gas-liquid mass transfer, the collections of solid particles in electroflotation processes, and the effect of the current density. 

Tsai, L.S., Hemlem, B. and Huxsoll, C.C. (2002) Disinfection and solids removal of poultry chiller water by electroflotation. J. Food Sci. 67,2160-2164. 

It is generally believed that there are three possible mechanisms involved in the process electrocoagulation, electroflotation, and electrooxidation. However, it can be suggested that, during the bipolar EC-EF process, bipolar aluminum electrodes are mainly responsible for electrocoagulation. 

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