Surfactants in flotation

The key stage of the entire process is a selective wetting of one of the components of the ore pulp with its subsequent concentration in the foam layer ( direct flotation) or in the chamber product ( inverse flotation). The physicochemical basis of flotation were formulated in the thirties-fifties of the last century [59—60] and further subjected to more specific investigations. Plenty of chemical and colloid-chemical processes take place simultaneously in the flotation pulp during a rather short period of time (usually several minutes), the main of which are  

1) adsorption of surfactants at 1/g and 1/si, l/s2 and I/S3 interfaces, meaning that solid phase particles (si, S2, S3, and so on) having a dispersity from 5 to 500 pm differ substantially in their nature. 

2) Dispersion of air and solid particles as a result of simultaneous mechanical and interfacial tension forces at 1/g and 1/s interfaces. 

3) Interaction of surfactants with ions present in the flotation pulp. 

4) Contact of a hydrophobic solid particle with an air bubble, sticking to the bubble and transportation of mineral particles into the foam layer. 

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Most of the theories on interactions of surfactants with minerals are closely related to their solution chemistry. For example, the ion-exchange adsorption theory proposed by Gaudin (1932, 1934) and Wark (1938) and the molecular adsorption theory proposed by Cook and Nixon (1950) are based on the dissociation equilibria and states of the collectors in water. More recently, Somasundaran (1976) observed that ion-molecule complexes of long-chain surfactants in flotation systems can have high surface activity depending upon the association equilibria of the surfactants in solutions (Ananthapadmanabhan et al., 1979 Kulkarni and Somasundaran, 1980). Also the cationic flotation behavior of salt type minerals is closely related to the formation of alkyl amine salt (Hu and Wang, 1990). In this chapter, solution equilibria of reagents relevant to selected flotation systems are examined. 

Fluorinated surfactants are used in ore flotation processes. Fluorinated surfactants are stable in concentrated acids and accelerate wetting of ore and the removal of the oxide layer. Perfluoroalkanoates, such as CF3(CF) COOK (n = 2,4, 6, 8) are more effective flotation agents than the corresponding hydrocarbon-type surfactants. The collecting power of the fluorinated surfactant in flotation of AI2O3 increases with increasing n (chain length) [155]. 

Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

Contact angle is proportional to (ysv - 7sl), therefore addition of a surfactant that adsorbs at the solid-solution interface should decrease ysL d therefore increase the quantity above and make 6 smaller. Yet such addition in flotation systems increases 6. Discuss what is inconect or misleading about the opening statement. 

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. 

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. 

Example In flotation, for a solid particle to float on a liquid surface, the upward pull of the meniscus around it (reflected in 6) must at least balance the weight of the particle. Any natural tendency of the particles to float or not float, depending on 6, can be modified by adding oils or surfactants to alter the interfacial properties. A mineral particle that does not float (Case 1) can be floated by adding surfactant (Case 2) as follows. 

Schramm, L.L. Stasiuk, E.N. MacKinnon, M. Surfactants in Athabasca Oil Sands Slurry Conditioning, Flotation Recovery, and Tailings Processes in Surfactants, Fundamentals and Applications in the Petroleum Industry, Schramm, L.L. (Ed.), Cambridge University Press, Cambridge, UK, 2000, pp. 365 130. 

The study of adsorption kinetics of a surfactant on the mineral surface can help to clarify the adsorption mechanism in a number of cases. In the literature we found few communications of this kind though the adsorption kinetics has an important role in flotation. Somasundaran et al.133,134 found that the adsorption of Na dodecylsulfonate on alumina and of K oleate on hematite at pH 8.0 is relatively fast (the adsorption equilibrium is reached within a few minutes) as expected for physical adsorption of minerals with PDI H+ and OH". However, the system K oleate-hematite exhibits a markedly different type of kinetics at pH 4.8 where the equilibrium is not reached even after several hours of adsorption. Similarly, the effect of temperature on adsorption density varies. The adsorption density of K oleate at pH 8 and 25 °C is greater than at 75 °C whereas the opposite is true at pH 4.8. Evidently the adsorption of oleic acid on hematite involves a mechanism that is different from that of oleate or acid soaps. 

A simple and largely applied method for foam formation is dispersion of gas through porous plates (filters) placed at the lower parts of foam generation apparatus [5-10], This method is employed in flotation, in gas adsorption and dust collection in set-up with turbulent gas emulsion, and in the equipment for foam separation. The dispersity of a foam thus obtained depends on filter pore size or capillary diameter, hydrophility of the material used in the dispersion device construction, physicochemical properties of the foaming solution (surface tension, viscosity, surfactant concentration, etc.) and conditions of the dispersion process. 

The role of foams and foam films depends significantly on the flotation type. From all methods only the foam adsorptive accumulation of surfactants and their complexes with ions, is based entirely on the regularities of the process of substance accumulation by adsorption in the stable foam. That is why further on the main target will be the analysis of the regularities of the adsorptive accumulation and separation of surfactants in the polyhedral foam. 

Ions of precipitate particles are adsorbed or attached at the surface of bubbles rising through a liquid, and are thereby separated. A substance which is not surface-active itself can be made so through union with, or adherence to, a surface-active agent (surfactant). Froth flotation involves separation (pre-concentration) by frothing. If an insoluble product is formed in interaction between the ion to be separated and a surfactant, the process is called ion flotation. If the ion is first precipitated and the precipitate is then floated with or without the addition of a surfactant, the process is called precipitate flotation. Flotation is accomplished in a special cylindrical vessel provided with a sintered glass disk at the bottom to break the gas (nitrogen, air) stream into small bubbles [92]. 

Interaction between two surfactants in aqueous solution producing synergism in foaming and decreased adsorption onto solid surfaces has been used to advantage in the separation of minerals. An alkyl sulfosuccinate-POE nonionic mixture that shows synergism in foaming and whose interaction results in decreased adsorption onto scheelite and calcite surfaces produced enhanced selectivity and recovery of scheelite by the flotation process (von Rybinski, 1986). 

A more plausible explanation for anionic surfactant adsorption on silica gel is found in the presence of about 0.2 wt.% alumina in the silica gel. The alumina pz-c is about pH=8.0, so it would be positively charged at the pH of the experiments. If the alumina is uniformly distributed through the silica, all of the adsorption could be accounted for provided a close-packed mono-layer of surfactant is formed on the alumina. This circumstance would also be consistent with the shape of the toe of the isotherm. Gaudin and Fuerstenau (25) advanced the idea of hemimicelle formation (two-dimensional micelles on a surface) to account for similar observations in flotation processes. 

Constants, A and B, for some nonionic surfactants used in flotation are shown in Table 2.9. 

In some cases, it is observed that the adsorption isotherm undergoes a sharp increase in slope at a particular surfactant concentration and this is attributed to lateral chain interactions between the adsorbed surfactant molecules (Somasundaran and Fuerstenau, 1966). A detailed thermodynamic treatment of this process is included in the next section due to the dominant role played by it in flotation systems. 

Various associative interactions of hydrolyzable surfactants in aqueous media can play a significant role in determining the adsorption behavior of these surfactants. For example, existence of ionomolecular complexes has been shown to have a significant effect on the adsorption of oleic acid on hematite as indicated by the flotation results (Xiao, 1990). Evidence for high surface activity of mixed acid-soap was obtained by surface tension measurements of oleate solutions (Ananthapadmanabhan, 1980). The surface activity of acid-soap was estimated to be larger than that of both the corresponding acid molecule and ionic soap. Similarly maximum flotation of quartz with alkylamine observed around pH... 

The basic interfacial process in flotation is selective hydrophobization (or lepophiliza-tion) and hydrophilization of particulate matter. The role of the solution chemistry is very important in flotation as it is determined by the dissolution behavior of mineral particles in the aqueous solution (pulp) and subsequent dissociation, hydrolysis and precipitation of the soluble species the dissolution, association, dispersion and emulsion behavior of various flotation reagents in the pulp and interactions among reagents with both soluble and surface species of minerals. The efficiency of flotation and separation of mineral particles and consumption of reagents are thus controlled by the solution chemistry of the pulp. As other processes such as oil displacement are also governed by such interactions and in turn by the wettability of the solid surface, the study of solution chemistry of surfactant/mineral/additive systems become very important for the development of many technologies. 

P. Somasundaran, The Role of lonomolecular Surfactant Complexes in Flotation, nl. J. Mineral. 

The effective enrichment of ores and minerals by flotation, including the possibility of separating materials with similar chemical properties, is achieved by employing a variety of different surfactants which selectively hydrophobize the surface of minerals that undergo flotation, and hydrophilize the surface of those that do not (or the other way around). Due to the relatively small surface area of the minerals treated, the consumption of surfactant-based flotation aids can be as low as 100 grams per tonne, which allows one to use rather complex and expensive surfactants in fine tuning the surface properties of the minerals separated. 

If a exceeds this value, a decrease of bubble size will only decrease the collision efficiency. Large bubbles with the size of the order of a 0.5mm are obtained in usual pneumatic dispersions formed in flotation machines. It is much more diflflcult to obtain bubbles 10 to 50 times smaller, which can be achieved by using completely different methods of bubble generation, electroflotation and air-dissolved flotation. Since a decrease of bubble dimensions is connected with substantial complication and a rise in price of the technology, it is necessary to forecast sufficiently accurately the required size of bubbles as a function of surfactant... 

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