Sulphide anions and compounds

During the formation of a sulphide mineral deposit there is usually the development of a primary sulphide halo in the host rock. Subsequent to ore formation, sulphide anions and compounds may be dispersed in the aqueous phase in groundwater and the gas phase in air-filled pore spaces to form secondary dispersion patterns. In exploration for sulphide mineral deposits, the simple determination of mobile sulphur species originating from sulphide ore deposits has a clear attraction. In order to understand the way in which such sulphur species occur in soils, a number of in-vitro experiments were performed. On the basis of the results, suitable methods of sampling and analysis have been chosen and tested in soils overlying 30 different mineral deposits. 

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Many metalliferous mineral deposits formed at depth are in the reduced state. Where they interface with the near-surface oxidising environment, there is considerable chemical reactivity. This typically takes the form of sulphide oxidation, which includes the generation of several meta-stable sulphur gases that have been shown to be useful in mineral exploration (Chapter 8). Incompletely oxidised sulphide anions and compounds are transported away from mineral deposits at depth by the groundwater, and can be mapped at surface as dispersion patterns of HjS (Chapter 9). 

The experimental investigations show that H2S does not migrate in the gas phase through soil. However, it migrates through water-saturated soil, probably in solution as sulphide anions and compounds. These are subsequently adsorbed onto soil particles, from which they can be released as H2S. This can be determined in concentrations as low as 0.01 mg using lead acetate paper. 

Two methods have been devised for the detection of the dispersion patterns of sulphide anions and compounds in soil (a) the shallow hole method and (b) the container method. In the first method, a 18 mm diameter steel rod is driven into the soil to make a hole that acts as the sample container. After the rod has punched a hole in the soil, it is removed and a device comprising an acid sprayer and gas extraction tube is inserted. (Fig. 9-3). The spray head must be at least 40 cm below the surface and the mouth of the hole must be thoroughly sealed by the spray device. Then the outlet tap of the spray reservoir is opened to allow 8 ml of 30% HCl to reach the spray head and to spray onto the walls of the hole below 40 cm... 

Surveys for dispersion patterns of sulphide anions and compounds determined as acid-released HjS in soil have been carried out over 30 mineral deposits in China, including skams, porphyry copper deposits and porphyritic iron deposits (sulphur contents higher at the margin of the ore body), altered-brecciated gold deposits and lead-zinc deposits in volcanic breccia. Three successful case histories are summarised here. Of the remainder, only four failed to yield anomalies over mineralisation. 

A method for detecting sulphide anions and compounds as acid-released H2S has been shown to yield good-contrast anomalies in soils overlying mineral deposits. Satisfactory results have been obtained in different overburden materials of varying thickness, under which mineral deposits are concealed at considerable depths. The equipment and procedure for measuring acid-released H2S in soil are simple, rapid and efficient, can be widely used and are easily adapted. 

The slow (rather than rapid) coloration of the suspended paper is consistent with the slow rate for the above reaction. For the lead acetate paper in contact with the warmed solutions, that in the more oxidising solution was coloured only slightly while that in the more reducing solution was extensively coloured. These results indicate the presence of sulphide anions and/or compounds in both bottles, but in amounts inversely proportional to the Eh of the solutions. 

From this experiment it can be inferred that, in the vicinity of oxidising sulphides, sulphide anions and/or compounds occur mainly in reducing solutions. Only very small amounts of H2S occur in the gas phase, even over solutions of low Eh. Near to the point of oxidation, COS is formed. These findings confirm that, in moist soils, any H2S will be in solution and thus gas-phase migration of H2S will not be possible. 

The inherent hydrophobicity once thought to be typical of sulphides (Ravitz and Porter, 1933) is now thought to be restricted to sulphides such as molybdenite (Chander et al., 1975) and other minerals or compound with special structural feature (Gaudin et al, 1957b). Common commercial sulphide minerals, which are needed to recover in flotation, are normally composed of anion (S ) and heavy metal ions such as Cu, Cu, Pb, Zn, Hg, Sb, Bi transitive metal ion such as Fe, Co, Ni and noble and rare metal ions such as Ag, Au, Mo. On the basis of structural pattern or mode of linkage of the atoms or polyhedral imits in space, Povarennyk (1972) introduced a crystallochemical classification of sulphide minerals, which have six major patterns as shown in Table 1.1. 

NaOH.Aq = Na2 S.Aq + H20. Here it must be supposed that the hydrogen of the hydrosulphide is present as an anion, and that it reacts with the hydroxyl of the caustic soda, forming water, while the sodium sulphide remains in solution in an ionised form, and can be recovered on evaporation in crystals with 9H20. Similar compounds exist with potassium. 

Gas chromatography has, of course, been used extensively in the analysis of many types of organic compounds with boiling points up to about 250°C, also to the analysis of organic compounds of lead, mercury, selenium, tin, manganese and silicon. Derivitisation of these compounds to produce compounds sufficiently volatile to be amenable to gas chromatography is frequently practised. Gas chromatography has also been applied to the determination of arsenic, antimony, selenium, tin, beryllium and aluminium and the common anions such as sulphate, nitrate, phosphate, sulphide, cyanide and thiocyanate. 

After an introductory discussion of such misfit structures, various terms that have previously been applied are reviewed, and degrees of incommensurability are used as the basis for a systematic nomenclature. The known structures of specific examples are then discussed graphite intercalates minerals with brudte-like layers as one component (koenenite, valleriite, tochilinites) silicates heavy metal sulphides (cylindrite, incaite, franckeite, cannizzarite, lengenbachite, lanthanum-chromium sulphide) anion-excess, fluorite-related yttrium oxy-fluorides and related compounds. 

The immediate source of acid-released H2S in soil is sulphide anions, for example,, HS, Me(HS)n , and/or mobile sulphur compounds, such as HjS, COS, CSj. At depth, circulating groundwater provides a medium through which sulphide anions are readily transported in aqueous solution. The results of in-vitro experiments indicate that, in the open pore spaces of overburden and soil, it is unlikely that H2S exists in the gas phase. This and the other sulphur gases are discussed further in Chapter 8. 

Some compounds absorbing close to 231 nm may be present in raw samples and thus may interfere with sulphide spectra. Several compounds, such as, for example, p-chlorophenol (absorption at 227 nm), anionic surfactant, RBS commercial product (absorption at 223 nm) and 1-propanthiol (absorption at 239 nm), were tested, and the results show low interference values to the studied compounds [34]. The error of restitution by deconvolution is 5% at maximum with the highest interference associated with anionic surfactant (leading to an error of 4.3%). 

Many of the oxides, sulphides, selenides and tellurides of the alkali metals (e.g. Li20, Li2S, etc.) have the so-called anti-fluorite structure, i.e. a fluorite structure in which the positions of the anions and cations are interchanged. Most of these can be regarded as essentially ionic compounds. The co-ordination is 4 8. 

Sigmatropic rearrangements of allyl sulphonium allyl ylides (75) provide the basis of a repetitive one-pot ring-expansion sequence" for 2-vinyl thiocyclic compounds (Scheme 50). A paper" and a section in a review article" have described the utilization of allyl thiol dianions and allyl sulphide anions in carbonyl group umpolung . 

Aromatic nitro compounds are a much more accessible class of substances, and a wide choice of methods exists for their reduction to amines. The choice of reducing agents includes (i) a variety of metals, usually employed in acidic or intly acidic media, (ii) metallic compounds involving a low valency state of the metal, stannous chloride being frequently used and (iii) anionic sulphur compounds such as ammonium and sodium sulphides and polysulphides, and... 

The common gangue material quartz (silica) is naturally hydrophilic and can be easily separated in this way from hydrophobic materials such as talc, molybdenite, metal sulphides and some types of coal. Minerals which are hydrophilic can usually be made hydrophobic by adding surfactant (referred to as an activator ) to the solution which selectively adsorbs on the required grains. For example, cationic surfactants (e.g. CTAB) will adsorb onto most negatively charged surfaces whereas anionic surfactants (e.g. SDS) will not. Optimum flotation conditions are usually obtained by experiment using a model test cell called a Hallimond tube . In addition to activator compounds, frothers which are also surfactants are added to stabilize the foam produced at the top of the flotation chamber. Mixtures of non-ionic and ionic surfactant molecules make the best frothers. As examples of the remarkable efficiency of the process, only 45 g of collector and 35 g of frother are required to float 1 ton of quartz and only 30 g of collector will separate 3 tons of sulphide ore. 

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