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FLOTATION A SUPPLEMRNT TO MINERAL PROCESSING TECHNOLOGY OCT. 2003 FLOTATION Frank F. Aplan Mineral Processing Section The Pennsylvnia State University University Park, PA 16802 Froth flotation is a process for separating finely ground valuable minerals from their associated gangue. The process is based on the affinity of properly prepared surfaces for air bubbles. A froth is ed by introducing air into a pulp of finely divided ore in water containing a frothing or foaming agent. Minerals with a specific affinity for air bubbles rise to the surface in the froth and are thus separated from those wetted by water. In preparation, the ore must first be ground to liberate the intergrown valuable mineral constituent from its worthless gangue matrix. The size reduction, usually to about 208 mm 65 mesh, reduces the minerals to such a particle size that they may be easily levitated by the bubbles see Extractive metallurgy; Flocculating agents; Gravity concentration; Sedimentation; Size classification; Size reduction. Froth flotation is usually used to separate one solid from another, for solid-liquid separations, as in dissolved air flotation, and for liquid-liquid separations, as in foam fractionation. The process also has the potential to make a particle size separation since fine particles are more readily flocculated and floated than are coarse ones. Froth flotation is the principal means of concentrating copper, lead, molybdenum, zinc, phosphate, and potash ores, and a host of others. In the United States, nearly 400 million metric tons of ore are treated per year by this unit operation. Its chief advantage is that it is a relatively efficient operation at a substantially lower cost than many other separation processes. Separations by flotation also include widely divergent applications such as the separation of ink from repulped paper stock, peas from pea pods, oils from industrial wastes, and metal ions, bacteria, proteins, and colloidal particles from water. The first and most thoroughly studied and used application, however, is in the field of mineral engineering, and therefore, it is appropriate to consider the subject from this aspect. It is essential to choose reagents to coat the desired minerals selectively in the presence of many other mineral species. This is a difficult undertaking since a pure mineral is rarely found in nature. Extensive atomic substitution in the crystal lattice, adsorbed and occluded ions, and intimate interlocking of various minerals usually occur. In addition, the same chemical compound may be found in several crystal s often with varying surface properties. The same mineral may have a different chemical composition depending on its environment of ation. These complications require ingenuity of the mineral engineer and the applied surface chemist. Flotation was largely developed in the early part of the century as an empirical art, but in the past few decades great strides have been made toward understanding the basic physicochemical principles underlying the process. Flotation, as applied to the concentration of minerals, is used in conjunction with other mineral engineering operations, such as comminution, classification, gravity concentration, thickening, and filtration qv. The recently published FlotationA. M. Gaudin Memorial Volume 1 is an invaluable reference book as are several older but still useful books on the subject 2-7. Several other books and industrial bulletins given in the list of general references are also helpful. Professional journals, such as those published by the worlds major mining and metallurgical societies, and governmental publications are useful sources of information. Fundamentals The flotation separation of one mineral species from another depends on the relative wettability of surfaces. Typically, the surface free energy is lowered by the adsorption of heteropolar surface-active agents. The hydrophobic coating then acts as a bridge so that the particle may be attached to an air bubble. The basic principles of surface chemistry have contributed substantially to understanding flotation phenomena 8-11. Many of these physicochemical surface phenomena may be directly correlated with flotation. The contact angle is used as an investigative tool and the electrokinetic properties of surfaces and the adsorption density of certain ions are known to correlate well with flotation response 12. Contact Angles. An air bubble when brought into contact with a clean mineral surface usually does not adhere to the surface. However, if a suitable reagent is added, the mineral acquires a hydrophobic coating and an air bubble may be attached quite readily. This is illustrated in Figure 1. A schematic diagram of this phenomenon is shown in Figure 2, which may be represented as the well-known Young equation, γSA γSL γLA COSθ whereγSA, γSL, andγLA, are the interfacial tensions at the solid-air, solid-liquid, and liquid-air interfaces, respectively. This may be combined with the Dupre’ equation for the work of adhesion at the solid-air interface, wsa γLA γSL -γSA to give wsa γLA 1 - COSθ Considering that γLA remains essentially constant for the concentrations of surfactant usually used in flotation, it is apparent that the contact angle is a good measure of the work to create the solid-air interface. For this reason the contact angle has proved to be a simple yet very important tool for flotation research. The Electrical Double Layer. Particulate matter in aqueous solution invariably has an electrical charge because an excess of cations or anions exists at the solid surface. This may be the result of, for example, the dissociation of surface groups, unequal dissolution of lattice ions, atomic defects occurring in the natural solid, or broken bonds between atoms resulting from comminution. Figure 3 is a representation of the electrical double layer existing at a mineral surface showing the potential-determining ions. Attracted to this surface and extending out into the solution for some distance are counterions, in this case hydrated cations which maintain electroneutrality. The center line of closest approach of the counterions is approximately 0.3 nm from the surface. The diffuse double layer extends into the bulk of the solution for varying distances, depending largely upon the concentration and valence of the counterions. In a lO-5 molar solution of a monovalent electrolyte, the median distance may be about 100 nm, but this distance is reduced sharply with increasing concentration. The potential frequently measured in moving a particle through a solution is the potential at the slipping plane, called the zeta potential,ζ. The zeta potential is not the surface potential,ψ, but is rather the potential at some distance from the surface. For this reason its qualitative significance in determining, for example, the sign of the charge on the surface is often of greater value than is its exact quantitative value. Solid a b c Figure 3. a Schematic representation of the double layer; b potential diagram; c adsorption of flotation collector 11. Courtesy American Institute of Mining, Metallurgical and Petroleum Engineers. The surface potential,ψ0, and the concentration of potential- determining ions in solution C or C-re related as follows 8 ψ0 kT/νe ln C/C0 -kT/νeln C-/C-0 where k is the Boltzmann constant, T the absolute temperature, ν the valence of the potential-determining ion, e the electronic charge, and C0 and C0- the concentration of potential-determining ions at the point of zero charge. The concentration of potential-determining ions controls the sign and magnitude of the surface potential. The valence of the ion also makes a significant contribution. The ability of the potential-determining ions to change the surface charge is very important in flotation. An excellent, concise summary of double-layer computations is given in ref. 13. The classic example of potential-determining ions is that of Ag and Cl- or a silver chloride sol Table 1. With a point of zero charge, PZC isoelectric point at pAg 4 and a solubility product constant Ksp of 2 10-10, silver chloride is positively charged in solutions containing greater than 10-4 mol Ag/L and negatively charged in solutions containing Cl- in excess of 2 10-6 mol/L. In like manner, for other slightly soluble minerals Ksp ca 10-10, the lattice ions are also potential determining. Barite, BaS04, is an excellent example of this class of minerals, and the concentration of Ba2 and S042- determines its sign of charge and potential in solution. The PZC varies for each specific mineral. For the insoluble oxide minerals, H and OH- are the potential-determining ions, and these minerals are positively charged at pH values below their PZC and negatively charged at higher pH values. As shown in Table 2, the PZC is highly variable for this class of minerals. The same mineral type eg, chromite or asbestos may show more than one PZC, depending on compositional or morphological differences, or a series of minerals with a common cation eg, goethite, hematite, or magnetite may have a common PZC. Ref. 16 presents a comprehensive review of the PZC values for heavy metal oxides and hydroxides and s for the prediction of their PZC. Other ions may also alter the surface charge of a mineral in solution. Polyvalent ions such as Al3, Fe3, and Co2, near their pH of precipitation in dilute solution, may become specifically adsorbed and decrease the potential or reverse the sign of the charge of a mineral in solution. This charge reversal process 17 has been applied to flocculation 18 and to activation phenomena 14 see below. Heteropolar surfactants of the colloidal electrolyte class are also known to adsorb onto oppositely charged minerals through electrostatic attraction and change the sign of the charge of a mineral in solution. These ions probably adsorb singly at dilute concentrations, but at greater concentrations ca lO-4-lO-5 mol/L 19 they may tend to in patches called hemimicelles. At this concentration they may cause the charge on the mineral to be reversed. Another class of ions, called indifferent ions, can influence the potential of a particle in solution. With increasing concentration and valence, these ions may adsorb onto the oppositely charged solid and reduce its zeta potential to a small value. They cannot, however, change the sign of the charge on the mineral as can the potential-determining ions. Their principal value is to cause spontaneously occurring flocculation when the zeta potential is reduced to some low value, eg, about 10 mV or below. Phases. In flotation, gas, liquid, and solid phases have to be considered. Gas Phase. The gas universally used is air, though often any gas will serve. Oxygen plays a special role in the flotation of the sulfide minerals 20-22. Theoretical analysis 23 indicates that the equilibrium adsorption density of the collector at the solid-gas interface exceeds its density at the solid-liquid interface. Experimental work with the adsorption of 1-hexanethiol by gold 24 and of sulfonate by mercury 25 appears to substantiate this thesis. Liquid Phase. The universal liquid for froth flotation is water, though in a few instances saturated brine 3 or seawater 5 is used. To this phase various reagents are added for selective control of the wettability of various mineral surfaces and to achieve the desired frothing. There are three general classes of reagents collectors promoters, modifiers, and frothers. Typical flotation collectors are shown in Table 3, though this is by no means an exhaustive list, and similar compounds often work equally well. For sulfide mineral flotation, both soluble and relatively insoluble sulfhydryl compounds are used. The hydrocarbon chain is relatively short typically C2-C5 and only a small amount of collector is usually required 0.01 to 0.1 kg/t. For nonsulfide mineral flotation, the soluble colloidal electrolytes are the generally accepted collectors except in a few instances where the nonionized fatty acid is used. The hydrocarbon chain length ranges from C12 to C18 and a larger quantity of collector is required ca 0.1 to 1 kg/t than that used for sulfide mineral flotation. The amount of collector required varies considerably and depends not only on the nature of the collector but also on the nature of the mineral to be floated. Chelation agents have also been given serious study as flotation reagents 11;however, their industrial use has been insignificant, largely because of their high cost. Modifying agents are classed as pH-regulating agents, activators, depressants, dispersants, and flocculants. They function in a wide variety of ways depending on the system. Their specific purposes are explained below. Frothing agents are used to provide a stable flotation froth, persistent enough to facilitate the mineral separation, but not so persistent that it cannot be broken to allow subsequent handling. The most commonly used frothing agents are short, branched-chain alcohols, such as MIBC methyl isobutyl carbinol, 4-methyl-2-pen-tanol, polypropylene glycol methyl ethers, pine oil, and cresylic acid and creosote. The froth serves a secondary purpose in providing a zone in which the unfloated mineral particles that have been mechanically entrapped by the rising air bubbles may drain away from the flotation concentrate. When the collector is an amine, soap, or sulfonate, an additional frother is often not needed see also Foams. Solid Phase. Crystal chemistry 3,11 plays an extremely important role in flotation, and the nature and structure of the solid phase are of critical importance. In addition to the direct influence of crystal structure, minor elemental substitution or chance adsorption of ions is sometimes the dominant feature. The solid phase may be roughly categorized into the five groups given in Table 4 sulfides, slightly soluble, insoluble oxides, soluble salts, and naturally floatable. Since each class of mineral is floated in a different manner, this categorization is a convenient way to assess the potential floatability of any mineral. The ease of floatability roughly correlates with the surface energy. Naturally floatable minerals have low surface energies, whereas those more difficult to float have much higher surface energies. The sulfides occupy a somewhat anomalous position because they have relatively high energies, though in practice they are so readily oxidized in aqueous solution that they present a much lower energy surface. In addition to the gross differences between minerals of these various classes, more subtle, but still important, changes in their floatability may come about because of