Experimental Investigation of Bubble Size in Flotation: Effect of Salt, Coagulant, Temperature, and Organic Compound

Abstract

SummaryFlotation is an important segment of water treatment. Bubbles are generated and dispersed in water where they capture oil and solids residuals and lift them up to the surface. Bubbles are generated either mechanically by agitation or hydraulically by induced gas ejectors. The bubble size has a crucial role in this kind of separation. In general, the smaller the bubbles, the better separation. Water chemistry has an important role in bubble formation and size. A laboratory scale flotation unit was developed to generate the microbubbles with diameters of 10 to 100 µm. The effects of salinity, coagulant, temperature, and ethanol on bubble size were investigated. The addition of 5000 mg/L of sodium chloride (NaCl) affected mainly bubble size with 50 µm diameter, whereas bubble size reduced and diameter decreased by one-half with the addition of NaCl. The bigger bubbles (100 µm size) were affected only at a salinity of 40 000 mg/L, whereas the smaller bubbles (20 to 30 µm) remained stable and did not change with the further increase of salinity. Coagulant affected the water chemistry with a high effect on pH and oxidation-reduction potential (ORP), creating a more acidic and oxidant environment; however, the effect on bubble size was not very significant. Increasing water temperature from 20 to 55°C slightly decreased the bubble size, especially for larger bubbles (diameter of 100 µm). The effect of organic compounds such as ethanol on bubble size reduction was significant. The effect of the hydrostatic level of a water column on bubble size was also investigated. The bubble size did not change with increasing the water level; however, the frequency of bubble generation reduced and reached zero at high water elevation. The effect of flotation in oil removal was investigated, and it was observed that ratio of bubble to droplet size was an important factor. The oil coating of bubbles was the main mechanism of oil removal. The smaller bubbles were more readily coated with oil droplets, whereas bigger bubbles tended to attach to clusters.