Surface floatability

Surfaces that do not have strong surface chemical bonds that were broken tend to be nonpolar and are not readily wetted. Substances such as graphite and talc are examples that can be broken along weakly bonded layer planes without rupturing strong chemical bonds. These solids are naturally floatable. Also, polymeric particles possess... 

The particles consist of a bead with an exterior surface that is at least partially coated with a material capable of accelerating the oxidation of organic compounds floating on water, under illumination, and in the presence of air. The coated bead is water-floatable and has a diameter of less than 2 mm. The bead consists of a plastic material coated with an intermediate layer of a material that will not accelerate the oxidation of the plastic material by air or by itself, oxidized under illumination and in the presence of air by the outer coating material. 

Activators are those reagents which act in a manner converse to the action of depressants, i.e., they render those minerals floatable which either have been temporarily depressed or would not float without their assistance. They are generally soluble salts which ionize in the aqueous medium. The ions then react with the mineral surface, providing a monomolecular coating and thereby making the mineral surface favourably disposed to the collectors. Sphalerite (ZnS) is essentially not floatable with common collectors. The addition of Cu2+ to the solution, however, alters the mineral surface to CuS, which can adsorb collector. This feature is described elaborately in a later section. 

Native gold and its alloys, which are free from surface contaminants, are readily floatable with xanthate collectors. Very often however, gold surfaces are contaminated or covered with varieties of impurities [4], The impurities present on gold surfaces may be argentite, iron oxides, galena, arsenopyrite or copper oxides. The thickness of the layer may be of the order of 1-5 pm. Because of this, the flotation properties of native gold and its alloys vary widely. Gold covered with iron oxides or oxide copper is very difficult to float and requires special treatment to remove the contaminants. 

Fuerstenau (1980) found that sulphide minerals are naturally floatable in the absence of oxygen. Yoon (1981) ever attributed the natural floatability of some sulphide minerals to their very low solubility. Finkelstein et al. (1975) considered that the natural floatability of sulphide minerals are due to the formation of elemental sulphur and related to the thickness of formation of elemental sulphur at the surface. Some authors reported that the hydrophobic entity in collectorless flotation of sulphide minerals were the metal-deficient poly sulphide (Buckley et al., 1985). No matter whichever mechanism, investigators increasingly concluded that most sulphide minerals are not naturally floatable and floated only under some suitable redox environment. Some authors considered that the natural floatability of sulphide minerals was restricted to some special sulphide minerals such as molybdenite, stibnite, orpiment etc. owing to the effects of crystal structure and the collectorless floatability of most sulphide minerals could be classified into self-induced and sulphur-induced floatability (Trahar, 1984 Heyes and Trahar, 1984 Hayes et al., 1987 Wang et al., 1991b, c Hu et al, 2000). 

The most controversial and contradicting problem is, perhaps, the natural and collectorless floatability of sulphide minerals. Gaudin (1957) classified the natural hydrophobicity of different minerals according to their crystal structure and showed that most sulphide minerals were naturally hydrophobic to some extent, which had been fiirther proved based on van der Waals theory by Chander (1988, 1999). Lepetic (1974) revealed the natural floatability of chalcopyrite in dry grinding. Finklestein (1975, 1977) demonstrated that orpiment, realgar and molybdenite were naturally floatable, and that pyrite and chalcopyrite had natural floatability at certain conditions due to the formation of surface elemental sulphur. Buckley and Woods (1990,1996) attributed the natural floatability of chalcopyrite... 

Heyes and Trahar (1984) leached pyrite with cyclohexane and compared the extract with a sulphur-containing solution of cyclohexane in a UV spectra photometer as shown in Fig. 1.4, indicating that sulphur was present at the mineral surface. Therefore, the inherent hydrophobicity and natural floatability once thought to be typical of sulphides is now thought to be restricted to sulphides such as molybdenite and other minerals or compound with special structural features. The collectorless floatability that most sulphide minerals showed came from the self-induced or sulphur-induced flotation at certain pulp potential range and certain conditions. 

The cathodic reaction is oxygen reduction in Eq. (1-1). Because Fe(OH)3 and metal oxy-hydroxide species of iron, lead and zinc formed will coat the cathodic mineral surface, affecting its floatability. 

The presence of oxygen enhances the formation of the surface coating and depresses the flotation of pyrrhotite. It appears, therefore, that although the floatability of individual mineral may be controlled by pulp potential, the presence of several sulphide minerals, particularly when they are ground with steel media, leads to galvanic interaction among them and the alteration of certain mineral surfaces may be accelerated. Then the pulp potential dependence of their floatability may not follow those of individual mineral. 

The h-pH diagrams of surface oxidation of arsenopyrite and pyrite are shown in Fig. 2.16 and Fig. 2.17, respectively. Figure 2.16 shows that jBh-pH area of self-induced collectorless flotation of arsenopyrite is close to the area forming sulphur. The reactions producing elemental sulphur determine the lower limit potential of flotation. The reactions producing thiosulphate and other hydrophilic species define the upper limit of potential. In acid solutions arsenopyrite demonstrates wider potential region for collectorless flotation, but almost non-floatable in alkaline environment. It suggests that the hydrophobic entity is metastable elemental sulphur. However, in alkaline solutions, the presence of... 

Figure 2.17 shows that although the metastable elemental sulfur maybe present at pyrite surface, pyrite does not exhibit self-induced collectorless floatability except in very strong acidic media and a narrow oxidized Eh range. Such behavior may be similar to that of arsenopyrite in alkaline solutions due to the formation of hydroxides, thiosulphate. 

The F h-pH diagram for jamesonite is given in Fig. 2.18. It may be seen from Fig. 2.18 that in acid and neutral solutions, the reactions producing elemental sulphur may render jamesonite surface hydrophobic and jamesonite shows good collectorless flotation. In alkaline solution, the hydrophilic species Fe, Pb ", HSb02, SbOj, are produced, the collectorless floatability of jamesonite becomes weaker, it may be attributed to the presence of Fe(OH)3 and Pb(OH)2 at the same time on the jamesonite surface. The relative amounts of hydrophobic sulphur and hydrophilic Fe(OH)3 and Pb(OH)2 perhaps determine the collectorless floatability of jamesonite. 

The influence of pulp potential on the floatability of chalcopyrite is shown in Fig. 4.4 for an initial concentration of 2x 10 mol/L ethyl XMthate and butyl xanthate. The lower flotation potential is -O.IV for KBX and OV for KEX. The hydrophobic entity is usually assumed to be dixanthogen (Allison et al., 1972 Woods, 1991 Wang et al, 1992) by the reaction (1-3). The calculated potential in terms of reaction (1-3), are, however, 0.217 V and 0.177 V, respectively, for ethyl and butyl xanthate oxidation to dixanthogen for a concentration of 2 x lO" mol/L, which corresponds to the region of maximum recovery but not to the lower limiting potential for flotation, indicating that some other surface hydrophobicity to the mineral. Richardson and Walker (1985) considered that ethyl xanthate flotation of chalcopyrite may be induced by the reaction ... 

Therefore, at certain pH and H2O2 concentration, the complete decomposition of lead xanthate preadsorbed on galena renders galena surface hydrophilic and depression of galena, whereas the dixanthogen prefixed on chalcopyrite remains stable which confers chalcopyrite surface hydrophobic and floatable, which was proved by voltammogram method (Wang, 1992). 

The type and addition of frother are found to have a pronounced effect on the collectorless floatability of chalcopyrite (Heyes and Trahar, 1977). The recovery of collectorless flotation of chalcopyrite is much higher using PPG40 than amyl alcohol. The effects of several frothers on the collectorless flotation of some minerals have been tested and the results are presented in Table 10.1. It further provides the evidence that the type of frother produces a markable influence on collectorless flotation of sulphide minerals. The frothers with lower surface tension are more effective in enhancing the recovery of collectorless flotation of sulphide minerals. 

Chander, S. and Fuerstenau, D. W., 1975. Sulphide minerals with thiol collectors the chalcocite diethy dilhiophosphate system. 11th International Mineral Processing Congress, 1 583 - 603 Chander, S., Wie, J. M., Fuerstenau, D. W., 1975. On the native floatability and surface properties of naturally hydrophobic solids, hr P. Somasunfaran and R. G. Grieves(eds.), Advances in Interfacial Phenomena of Particulate/Solution/Gas Systems. AIME Symp., Ser., 150(71) 183-188... 

Often the minerals we want to float are hydrophilic, and surfactants (called collectors ) are added, which, at a specific concentration, adsorb onto the particle surface, making it hydrophobic and hence floatable. In a mixture of hydrophilic minerals, optimum flotation will occur where one of the minerals adsorbs collector but the others do not. 

Floatability is a- surface phenomenon and some solids are more easily floatable (such as sulfur, sulfides of the metals, graphite) than others (such as oxides, silicates, silica). 

The most widely applied activation procedure is that involving the use of copper(II) ions to enhance the floatability of some sulfide minerals, notably the common zinc sulfide mineral sphalerite.2 Sphalerite does not react readily with the common thiol collectors, but after being treated with small amounts of copper it floats readily owing to the formation of a surface layer of CuS." A similar procedure is often adopted in the flotation of pyrrhotite (FeS), pyrite (FeS2), galena (PbS) and stibnite (Sb2S3). In the context of coordination chemistry, the major contribution has been to the understanding of the chemistry involved in the deactivation of these minerals, a procedure often adopted in the sequential flotation of several minerals from a complex ore. 

Chander, S. Wie, J.M. Fuerstenau, D.W. On the Native Floatability and Surface Properties of Naturally Hydrophobic Solids in Advances in Interfacial Phenomena of Particulate/Solu-tion/Gas Syatems Applications to Flotation Research, Somasundaran, P. Grieves, R.B. (Eds.), American Institute of Chemical Engineers New York, 1975, pp. 183-191. 

Quite frequently the natural surface of a mineral requires preliminary chemical treatment before it will form the surface film required for collection One of the commonest instances of this is with sphalerite (zinc sulphide), which does not float properly when treated with xanthates. If, however, it is given a preliminary treatment with dilute copper sulphate solution, a very small amount of copper sulphide is deposited on the surface and the ore becomes floatable, the surface being now capable of reaction with xanthates. Such treatment is usually termed activation in general, an activating solution for a sulphide mineral should contain a metallic ion whose sulphide is less soluble than that contained in the mineral for zinc sulphides, silver, copper, mercury, cadmium, and lead salts are all effective activators. 

Scum. It consists of the floatable materials skimmed from the surface of primary and secondary settling tanks. Scum may contain grease, oils, fats, waxes, soaps, hair, paper, cotton, and plastic materials that have specific gravity less than 1. 

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