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In the paper, simultaneous removal of Al(III) and Cu(II) from dilute aqueous solutions by ion and precipitate flotation methods is investigated. Influence of the pH of the initial solution, the surface active collector concentration and the gas flow rate on the final removal ratio and the course of ion and precipitate flotations is presented. The results show that simultaneous flotations of Al(OH)3 and Cu(OH)2 insoluble species occur allowing to achieve their almost complete removal in the pH range between 7 and 9. An increase of the surface active agent concentration causes a decrease of the final removal ratio as well as of the flotation rate constant. An increase of the gas flow rate results in an increase of ion and precipitate flotation rates.

Gas hold-up and the oxygen transfer in the zones of the internal loop airlift reactor with rectangular cross-section was studied. It was found, that the downcomer to the riser gas hold-up ratio depends on the gas ow rate, the physicochemical properties of the system and on the reactor height. The ratio of the downcomer mass transfer coef®cient to the global mass transfer coef®cient was less than 6%. The ratio of the downcomer to the global mass transfer coef®cient slightly increased with increase of the gas¯ow rate and decreased with increase of the liquid viscosity. The proposed correlation for the global overall mass transfer coef®cient predicts the experimental data well within 16.6% deviation. It was con®rmed that the reactor height is the important parameter for a design and a scale-up of the airlift reactors. List of symbolsa m length A m 2 cross-section area A ± proportionality constant in Eq. (9) b ± exponent in Eq. (9) b m width c ± exponent in Eq. (9) c kg á m A3 oxygen concentration in the liquid c à kg á m A3 equilibrium liquid phase dissolved-oxygen concentration d ± exponent in Eq. (9) d R m equivalent riser diameter (= 4´¯ow area/wetted perimeter) D m 2 á s A1 diffusivity in liquid phase DTAR ± cylindrical draft-tube airlift reactor ELAR ± cylindrical external loop airlift reactor Fr u 2 GR g Á d R Froude number g m á s A2 gravitational acceleration h R m reactor height Dh m difference between the liquid level in the upper and the lower piezometric tube Dh m m distance between the manometer tube location k L a s À1 liquid-phase volumetric mass transfer coef®cient Mo g 4 L Á g q L Á r 3 Morton number RILAR ± rectangular inner loop airlift reactor Sc g L q L Á D Schmidt number Sh k L a Á d 2 R D Sherwood number t s time, residence time t C s circulation time u GR m á s A1 super®cial gas velocity based on the riser cross-section u C m á s A1 mean liquid circulation velocityGreek letters e ± gas hold-up g mPa á s liquid dynamic viscosity q kg á m A3 density r mN á m A1 surface tension s s time-constant f ± correction factor in Eqs (3, 4)