Water Chemistry of Flotation Reagents

Long chain fatty acids such as oleic acid are among the commonly used reagents for the flotation of oxides, silicates and salt-type minerals. Flotation of these minerals using fatty acids is affected greatly by solution properties such as pH, since weakly acidic fatty acids undergo association interactions that can influence their adsorption and flotation properties. For example, oleic acid species will undergo dissocia-

Figure 1 pH of oleic acid precipitation. (From Morgan LJ, Anan-thapadmanabhan KP and Somasundaran P (1986) Oleate adsorption on hematite: problem and methods. International Journal ofMineral Processing 18: 39. Copyright: Elsevier Science.)

tion to form ions (Ol") at high pH values and exist as neutral molecules (HOI) at low pH value. In the intermediate region, the ionic and the neutral molecular species can associate to form ion-molecule complexes ((Ol)2H"). As the surfactant concentration is increased, micellization or precipitation of the surfactant can occur in the solution. In addition, surfactant species can associate to form other aggregates such as the dimer (Ol2) in premicellar solutions. Also, long chain fatty acids such as oleic acid have very limited solubility, which is a sensitive function of pH. The pH of precipitation of oleic acid calculated as a function of total oleate is shown in Figure 1.

The solution equilibria of oleic acid (HOl) are expressed as below:

pKsol = 7.6 (Ksol: solubility product)

pKa = 4.95 (Ka: acid dissociation constant)

pKd = — 3.7 (Kd: dimerization constant) HOl # Ol" = (Ol^H"

pKad =— 5.25 (Kad: acid-soap formation constant)

The species distribution of oleic acid as a function of pH based on the above equilibria at a given concentration is shown in Figure 2. It can be seen from this figure that:

1. The pH of the precipitation of oleic acid at the given concentration is 7.45.

2. The activities of oleic monomer and dimer remain almost constant above the precipitation pH and decrease sharply below it.

3. The activity of the acid-soap (Ol)2H" exhibits a maximum in the neutral pH range.

The surface activities of the various surfactant species can be markedly different from each other. It has been estimated that the surface activity of the acid-soap (Ol)2H" is five orders of magnitude higher than that of the neutral molecule (HOl) and about seven orders of magnitude higher than that of the neutral molecule (HOl) and about seven orders of magnitude higher than that of the oleate monomer Ol".

The existence of salt will also affect the surfactant-solution equilibria by changing the surface activities of the various surfactant species, the critical micelle concentration and the solubility of the surfactant, and the solvent properties of the solution.

Figure 2 Oleate species distribution as a function of pH. Total oleate concentration = 3x10"5mol L"1. (From Ananthapad-manabhan KP and Somasundaran P (1980) Oleate chemistry and hematite flotation. In: Yarar B and Spottiswood DJ (eds) Interfacial Phenomena in Mineral Processing, p. 207. New York: Engineering Foundation.)

Figure 2 Oleate species distribution as a function of pH. Total oleate concentration = 3x10"5mol L"1. (From Ananthapad-manabhan KP and Somasundaran P (1980) Oleate chemistry and hematite flotation. In: Yarar B and Spottiswood DJ (eds) Interfacial Phenomena in Mineral Processing, p. 207. New York: Engineering Foundation.)

It is clear that, to understand the adsorption of reagents on solids, the effects of concentration, pH, ionic strength and activities of the various possible reagent species on the adsorption process need to be taken into account.

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