Hydrophobic entity

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). 

Abstract This chapter first explains the natural flotability of some minerals in the aspect of the crystal structure and demonstates the collectorless flotaiton of some minerals and its dependence on the h and pH of pulp. And then the surface oxidation is analysed eletrochemically and the relations of E to the composition of the solutions are calculated in accordance with Nemst Equation. The E h-pH diagrams of several minerals are obtained. Thereafter, electrochemical determination such as linear potential sweep voltammetry (LPSV) and cyclic voltammetry (CV) and surface analysis of surface oxidation applied to the sulphide minerals are introduced. And recent researches have proved that elemental sulfur is the main hydrophobic entity which causes the collectorless flotability and also revealed the relation of the amount of sulfur formed on the mineral surfaces to the recoveries of minerals, which is always that the higher the concentration of surface sulphur, the quicker the collectorless flotation rate and thus the higher the recovery. 

Although the nature of the hydrophobic entity responsible for the self-induced flotation of sulphide minerals remains somewhat obscure, most reported results clearly show that it is only when the environment becomes slightly oxidizing that flotation is observed. The elemental sulphur and polysulphide-intermediates in the oxidation of sulphide to sulphur have ever been suggested to be of the hydrophobic species. Whatever it is, there is no doubt that sulphur can generate hydrophobicity and floatability. 

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... 

It may be concluded from the discussion above that the collectorless flotation was observed for many sulphides in moderately oxidizing potential region, but not in strongly reducing potential region. The available evidence suggests that the hydrophobic entity could be sulphur produced by superficial oxidation of the mineral. In general, flotation is observed in the potential-pH areas where elemental sulphin is metastable. 

Abstract The sodium sulphide-induced collectorless flotation of several minerals are first introduced in this chapter. The results obtained are that sodium sulphide-induced collectorless flotation of sulphide minerals is strong for pyrite while galena, jamesonite and chalcopyrite have no sodium sulphide-induced collectorless flotability. And the nature of hydrophobic entity is then determined through J h-pH diagram and cyclic voltammogram, which is element sulphur. It is further proved widi the results of surface analysis and sulphur-extract. In the end, the self-induced and sodium sulphide-induced collectorless flotations are compared. And it is found that the order is just reverse in sodium sulphide-induced flotation to the one in self-induced collectorless flotation. 

Abstract In the beginning, the mixed potential model, which is generally used to explain the adsorption of collectors on the sulphide minerals, is illustrated. And the collector flotation of several kinds of minerals such as copper sulphide minerals, lead sulphide minerals, zinc sulphide minerals and iron sulphide minerals is discussed in the aspect of pulp potential and the nature of hydrophobic entity is concluded from the dependence of flotation on pulp potential. In the following section, the electrochemical phase diagrams for butyl xanthate/water system and chalcocite/oxygen/xanthate system are all demonstrated from which some useful information about the hydrophobic species are obtained. And some instrumental methods including UV analysis, FTIR analysis and XPS analysis can also be used to investigated sulphide mineral-thio-collector sytem. And some examples about that are listed in the last part of this chapter. 

Pulp Potential Dependence of Collector Flotation and Hydrophobic Entity... 

Gaudin and Schuhmann (1936) and Harris and Finkelstein (1977) have shown that the most likely adsorbed hydrophobic entity is cuprous xanthate which could be formed by a combination of the oxidation of chalcocite and the reaction of the... 

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 ... 

In the case hydrophobic entity is assumed to be lead xanthate, lead xanthate would be formed by the reaction of the form (1-4) including ... 

However, the decomposition potential of zinc xanthate into dixanthogen is above 0.3 V according to reactions (4-33) or (4-34) and the upper potential limit of flotation of marmatite extends to 620 mV, which indicates the coexistence of dixanthogen on marmatite in this condition. The difference of flotation behavior and hydrophobic entity between sphalerite and marmatite may be due to the existence of iron in marmatite. 

The anodic scan section of cyclic voltammetry for pyrrhotite electrode, respectively, in pH=7, 8.8, 11, 12.1, 12.7 buffer solution with dithiocarbamate are presented in Fig. 4.28. The cyclic voltammograms curve at pH = 8.8 is also presented in Fig. 4.28. It can be seen that the anodic ciurent peak emerges at about O.IV. As pH increases, the peak moves to the left. This peak may correspond to the formation of disulphide. When the concentration of dithiocarbamate is 10 mol/L, the oxidation of dithiocarbamate forming disulphide is h = 0.22 V, which agrees with the results in Fig. 4.26. When the hydrophobic entity disulphide was formed, the flotation of pyrrhotite could be possible. 

Figure 4.35 presents the UV spectra of the cyclohexane solution extracted from marmatite after acted by DDTC. It can be seen that there exist three UV absorption peaks at 230 nm, 260 nm and 280 nm. The hydrophobic entity may be the mixture of diethyl dithiocarbamate and its dimmer on marmatite. 

From the flotation results in Fig. 4.21 and the voltammogram in Fig. 4.22, it is derived that the hydrophobic entity of collector on marmatite is mainly of disulphide of xanthate and dithiocarbamate, which is further confirmed by the above UV analysis. However, the UV analysis also suggested the coexistence of collector salts. We propose that the initial oxidation products of xanthate and dithiocarbamate on marmatite should be mainly of disulphide. At higher potential, the adsorbed disulphide may be decomposed and react with surface zinc species to form some parts of collector salts as in the following reactions ... 

For the case that the hydrophobic entity is disulphide, the mineral will be depressed when the reaction of the type (2-3) or (2-4) occurs before the reaction (1-3). Thus for the pyrite /diethyl dithiophosphate (DTP) system, pyrite will be depressed if the oxidation reaction... 

As is known to all, the flotation mechanism of sulphide minerals can be explained based on electrochemistry because sulphide minerals have the semiconductor character and a series of electrochemistry reaction occurring in solution. After these reactions, the surface of sulphide minerals changes and forms a new phase. We called it as self-corrosion of sulphide minerals. As before, the essence of the reaction between the collector and the minerals is the formation of the hydrophobic entity on the mineral surface, and then minerals can be floated. We can find that the reaction between the collector and the minerals is similar to the depression on mineral self-corrosion. In the corrosion, we called this effect as inhibition, and this kind of reagent is an inhibiting reagent. There are many studies on corrosion, especially its research method and theory. Thus, we can get some new information on the mechanism of sulphide flotation from corrosive electrochemistry. 

Buckley, A. N. and Riley, K. W., 1991. Self-induced floatability of sulphide minerals examination of recent evidence for elemental sulfur as the hydrophobic entity. Surf. Interface Anal, 17 655-659... 

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