Vacuum flotation

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. 

In vacuum flotation, the process stream is saturated with air at atmospheric pressure and introduced to the flotation tank on which a... 

In contrast to vacuum flotation, dissolved-air flotation units can be operated on a continuous basis by the application of pressure. This consists of pressurizing and aerating the process stream and introducing it into the flotation vessel that is maintained at the atmospheric pressure. The reduction of pressure results in the formation of fine air bubbles and the collection of fine particulates to be floated and removed as sludge. 

Bubble sizes at formation generally increase with surface tension and orifice diameter. Prediction of sizes in swarms from multiple orifices is difficult. In aqueous solutions of low surface tension, Bubble diameters of the order of 1 mm are common. Bubbles produced by the more complicated techniques of pressure flotation or vacuum flotation are usually smaller, with diameters of the order of 0.1 mm or less. 

This is a unit operation process where air bubbles, as gas, are used to remove solid or liquid particles from the liquid wastewater. The air bubbles are often trapped in the morphology of the suspended particles and as a result of buoyant forces, the particles move up and float on the surface where they are skimmed out. The common flotation methods include dissolved air, air flotation, vacuum flotation, and chemical additives.3... 

In vacuum flotation, the influent process liquid to be treated is saturated with air at atmospheric pressure. There is an air-tight enclosure on the top of the flotation chamber in which a partial vacuum is maintained (75). The fine air bubbles (20-80 nticrons) are generated under laminar flow conditions by applying such a vacuum to the flotation chamber. The theory is that the lower the pressure, the lower the air solubility in water. The soluble air originally in water is partially released out of solution as extremely fine bubbles due to a reduction in air solubility caused by negative vacuum pressure. The... 

In contrast to vacuum flotation, dissolved-air flotation units can be operated on a continuous basis by the application of pressure. This consists of pressurizing and aerating the process stream and introduc-... 

Vacuum flotation—saturating the suspension with air at atmospheric pressure and transferring the suspension to an e ansion chamber where tire pressure is reduced to less than atmospheric the dissolved air rapidly comes out of its supersaturated solution, producing bubbles. 

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