Surface charge flotation

Fig. XIII-9. The dependence of the flotation properties of goethite on surface charge. Upper curves are potential as a function of pH at different concentrations of sodium chloride lower curves are the flotation recovery in 10 M solutions of dodecylammo-nium chloride, sodium dodecyl sulfate, or sodium dodecyl sulfonate. (From Ref. 99.)...

Soluble Salt Flotation. KCl separation from NaCl and media containing other soluble salts such as MgCl (eg, The Dead Sea works in Israel and Jordan) or insoluble materials such as clays is accompHshed by the flotation of crystals using amines as coUectors. The mechanism of adsorption of amines on soluble salts such as KCl has been shown to be due to the matching of coUector ion size and lattice vacancies (in KCl flotation) as well as surface charges carried by the soflds floated (22). Although cation-type coUectors (eg, amines) are commonly used, the utUity of sulfonates and carboxylates has also been demonstrated in laboratory experiments. 

The retention time in the flotation chamber is usually about 3 to 5 min, depending on the characteristics of the process water and the performance of the flotation unit. The process effectiveness depends upon the attachment of air bubbles to the particles to be removed from the process water.57 The attraction between the air bubbles and particles is primarily a result of the particle surface charges and bubble size distribution. The more uniform the distribution of water and microbubbles, the shallower the flotation unit can be. 

With a chapter on particle-particle interaction (coagulation) the characteristics of particles and colloids as chemical reactants are discussed. Since charge, and in turn the surface potential of the colloids is important in coagulation, it is illustrated how in simple cases the modelling of surface complex formation permits the calculation of surface charge and potential. The role of particle-particle interaction in natural water and soil systems and in water technology (coagulation, filtration, flotation) is exemplified. 

Surfactant adsorption on solids from aqueous solutions plays a major role in a number of interfacial processes such as enhanced oil recovery, flotation and detergency. The adsorption mechanism in these cases is dependent upon the properties of the solid, solvent as well as the surfactant. While considerable information is available on the effect of solid properties such as surface charge and solubility, solvent properties such as pH and ionic strength (1,2,3), the role of possible structural variations of the surfactant in determining adsorption is not yet fully understood. 

Figure 10.3 The dependence of the flotation properties of goethite (FeO(OH)) on surface charge. Lower curves show the flotation recoveryin 10 5 M solutions of dodecylammonium chloride, sodium dodecyl sulphate, and sodium dodecyl sulphonate. From Fuerstenau [623], Copyright 1962, American Institute of Mining, Metallurgical and Petroleum Engineers.

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. 

Microorganisms have a complex cell envelope structure. Their surfaces charge and their hydrophobicity cannot be predicted, only experimentally determined [131]. Several microorganisms are not hydrophobic enough to be floated. They need collectors, similar to ore flotation. In cultivation media proteins which adsorb on the cell surface act as collectors. The interrelationship between cell envelope and proteins caimot be predicted, only experimentally evaluated. The accumulation of cells on the bubble surface depends not only on the properties of the interface, proteins and cells, but on the bubble size and velocity as well [132]. On account of this complex interrelationship between several parameters, prediction of flotation performance of microbial cells based on physicochemical fundamentals is not possible. Therefore, only empirical relationships are known which cannot be generalized. Based on the large amount of information collected in recent years, mathematical models have been developed for the calculation of the behavior of protein solutions and particular microbial cells. They hold true only for systems (e.g. BSA solutions and particular yeast strains) which are used for their evaluation. In spite of this, several recommendations for protein and microbial cell flotation can be made. 

Figure 10.35. Flotation of geothite with 10" M solutions of dodecylammonium chloride (open squares), sodium dodecyl sulfate (open circles) and sodium dodecyl sulfonate (filled circles), illustrating the dependence of flotation recovery on surface charge (after ref. (8))...

In mineral foam or froth flotation the sign and magnitude of the surface charge will influence the adsorption of additives (collectors) onto the mineral surface that will determine whether a specific mineral will float or sink, and therefore the efficiency of ifs separafion. The process of flofafion is based on fhe interacfions af fhe solid-liquid and solid-liquid-air inferfaces. The addi-fion of fhe proper collecfor defermines whaf mineral fraction will become... 

The significance of surface charge in alkali flotation systems was very controversial when it was first introduced in 1967 by Roman et al. [4]. Critics argued that... 

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