Flotation efficiency

Figure 8.16 Hallimond tube used to measure flotation efficiency.

Switch off (or bypass) the gas flow snch that the flow-rate remains the same on re-opening the flow. Consistency of the flow-rate is necessary to obtain accnrate comparisons of flotation efficiency. Check this by re-measnring the flow-rate. 

Calculate the percentage flotation efficiency, that is % lected/original wt. 

Follow the procedure described above using 1 g portions in 10 , 10 , 10 , 10 " and 10 M CTAB solutions. Addition of CTAB may cause extensive frothing. Since the powder will be retained in this foam it must be broken up either mechanically using a Pasteur pipette or by the addition of a small amount of ethanol. The powder may also stick to the walls of the apparatus above the collection tube. This powder should be dislodged by gentle tapping on the walls of the apparatus. Calculate the mean flotation efficiency at each concentration and graph the results. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

Figure 11.9 I llustration of the connection between hot water flotation efficiency and measured chemical and physical properties, including bitumen zeta potential From Schramm and Smith [250]. Copyright 1987, Canadian Society for Chemical Engineering.

Flotation is a complicated process. Its separation efficiency depends not only on the properties of suspended substances the volume ratio of the gas bubbles to the suspended substances, and the retention time of the flotation tank, but also on the bubbles size. Usually, the smaller the bubbles are, the higher the flotation efficiency is. This is simply because smaller bubbles can provide larger specific surface area for particles attachment. Therefore, generation of finely dispersed bubbles is highly desired for any flotation processes. 

The purpose of the basin depth is for storage and removal without affecting flotation efficiency of floatable and settling particles. The basin efficiency is independent of basin depth. It is recommended that depth of floatation basin should not be less than 0.9 m, preferably 1.2 m. The practical maximum depth is 2.4-3 m (21). 

Increasing ionic strength decreases ndsotption, decreases thickness of double layer, decresses strength of attractive force between bobble (surfactant coated and floe panicle), decreeses effective size of floe panicles, and rednees flotation efficiency,... 

The process of approach of particles to the bubble surface can be described quantitatively by taking into account both the LRHI and the SRHI. For estimating the flotation efficiency, we introduce a dimensionless parameter of the collision efficiency. 

In addition dissolved air flotation experiments were carried out to obtain the flotation efficiency under the same experimental conditions as the zeta potential measurement. The values of particle capture efficiency were a maximum when the absolute values of zeta potentials of the bubbles and particles were at a minimum. 

The theory of Dukhin (1981) was generalised by Listovnichiy Dukhin (1986) where the effect of stabilisation is considered under arbitrary hydrodynamic flow conditions around a bubble and the effect of convective transfer of surfactant into the adsorption layer was taken into account. Numerical estimations of 0, were carried out and it was shown that the effect of the liquid interlayer stabilisation by a DAL decreases the flotation efficiency over a wide range of system parameters by more than an order of magnitude. Numerical estimations also point to the fact that the effects under consideration have a much smaller influence on flotation of spherical particles than of disk-shaped particles. 

A relatively new field is the use of flotation in wet textile processes [73]. The ( -potential of cotton fibres in aqueous solutions is negative, therefore they are effectively floated by cationics like quaternary ammonium salts, e.g. dodecyl trimethyl ammonium chloride. Sysilia et al. [74] have established, by measuring the electrokinetic potential, a clear rule between the positive surface charge of chromite and flotation efficiency. At low pH, chromite was effectively floated by fatty acid soaps, the ions of which are negatively charged under these conditions. The surfactant adsorption is reversible which is indicative of its physical nature. 

Separation of Cu(II) from an aqueous solution. Wang et al. [70] described a new method for separating heavy metal ions from dilute solution, and reported flotation of Cu(II) by CGAs. The effects of flow rate, amounts of CGAs introduced into the system, and surfactant concentration on the flotation efficiency have been investigated. The results show that the flotation efficiency at pH 5-6 has an optimum value for CGA flow rate and amount. When the pH is greater than 7, the flotation efficiency can be as high as 99% under optimum conditions. 

Increasing magnitude of ionic atmosphere ions reduces flotation efficiency through increased screening of electrical interactions. 

Increased particle size increased flotation efficiency. 

The optimum operating conditions for ion flotation vary widely from system to system. The flotation pH, bubbling time, and gas flow rate should be carefully optimized, because in most cases it is difficult to observe when the flotation is complete. The quantities of surfactants should be greater than the stoichiometric amounts, but large excesses may decrease the flotation efficiency. An increase in the quantity of... 

Increasing kmic strengih decreases adsorption, decreases thickness of double layer, decreases strength of attractive force between bubble (surfactant coaled and floe particle), decreases effective size of floe particles, and reduces flotation efficiency. 

Esso Port Jerome. Oily water flotation efficiency Parameters Upstream from DAP... 

FIGURE 2.15 Comparison of flotation efficiency obtained with IMPA depressants with conventional phosphoric acid depressant in the flotation of dolomitic rock phosphate. 

The relationship between flotation rate and particle size is complex [12]. The flotation efficiency decreases distinctly below a certain particle size (often below 1 pm). On the other hand, paper manufacturers prefer fine pigments, because many properties of the paper, such as degree of brightness, opacity and smoothness, are improved by decreasing particle size of the fillers. Therefore, a large amount of fillers with a particle size below 1 pm is expected in the waste paper. 

---