Sulfides of the metallic elements

Many of the most important naturally occurring minerals and ores of the metallic elements are sulfides (p. 648), and the recovery of metals from these ores is of major importance. Other metal sulfides, though they do not occur in nature, can be synthesized by a variety of preparative methods, and many have important physical or chemical properties which have led to their industrial production. Again, the solubility relations of metal sulfides in aqueous solution form the basis of the most widely used scheme of elementary qualitative analysis. These various more general considerations will be briefly discussed before the systematic structural chemistry of metal sulfides is summarized. 

In the presence of hydrogen sulfide produced by anaerobic bacterial activity, particularly sulfate reducers, conditions are created whereby sulfides of copper and zinc could be formed. The partition of these metals between the sulfide phase and the organic phase depends on the relation between the stability constants of the complexes and the solubility product of the sulfides of these metals. Elements with small solubility products of their sulfides and low stability constants of their chelates would be expected to go into the sulfide phase when hydrogen sulfide is present. Copper is typical of such elements. Chalcocite has a solubility product of about 10" ° and covellite about 10"44, whereas the most stable chelates of copper have stability constants of about 10" Consequently, copper could be expected to be accumulated as the sulfide. Zinc sulfide has a much larger solubility product however, the stability of its chelates is lower. From the fact that zinc appears to be completely associated with the inorganic fraction of coal, it can be assumed that the relation between the solubility product of any of its sulfides and its chelates favors formation of the sulfide. Iron could be expected to follow a similar pattern. 

Metals always occur in their oxidized state in ores, often as the oxide or sulfide of the metal. In order to convert an ore to its elemental state, therefore, it must be reduced. Reduction is a chemical reaction that is the opposite of oxidation. Metals can be reduced in a variety of different ways. 

Like the refining of the PGMs, the analysis is compHcated by the chemical similarity of the metals. The techniques used depend on the elements present and their concentration in the sample. For some low grade samples, analysis is preceded by a concentration stage using fire assay with collection into a lead or nickel sulfide button. The individual metals can then be determined. 

MSH + MOH). Accordingly, solubilities depend sensitively not only on temperature but also on pH and partial pressure of H2S. Thus, by varying the acidity. As can be separated from Pb, Pb from Zn, Zn from Ni, and Mn from Mg. In pure water the solubility of Na2S is said to be 18.06g per 100 g H2O and for Ba2S it is 7.28 g. In the case of some less-basic elements (e.g. AI2S3, Cr2S3) hydrolysis is complete and action of H2S on solutions of the metal cation results in the precipitation of the hydroxide likewise these sulfides (and SiS2, etc.) react rapidly with water with evolution of H2S. 

Sulfur is widely distributed as sulfide ores, which include galena, PbS cinnabar, HgS iron pyrite, FeS, and sphalerite, ZnS (Fig. 15.11). Because these ores are so common, sulfur is a by-product of the extraction of a number of metals, especially copper. Sulfur is also found as deposits of the native element (called brimstone), which are formed by bacterial action on H,S. The low melting point of sulfur (115°C) is utilized in the Frasch process, in which superheated water is used to melt solid sulfur underground and compressed air pushes the resulting slurry to the surface. Sulfur is also commonly found in petroleum, and extracting it chemically has been made inexpensive and safe by the use of heterogeneous catalysts, particularly zeolites (see Section 13.14). One method used to remove sulfur in the form of H2S from petroleum and natural gas is the Claus process, in which some of the H2S is first oxidized to sulfur dioxide ... 

Hydrogen sulfide dissolves in water to give a solution of hydrosulfuric acid that, as a result of its oxidation by dissolved air, slowly becomes cloudy as S8 molecules form and then coagulate. Hydrosulfuric acid is a weak diprotic acid and the parent acid of the hydrogen sulfides (which contain the HS ion) and the sulfides (which contain the S2 ion). The sulfides of the s-block elements are moderately soluble, whereas the sulfides of the heavy p- and d-block metals are generally very insoluble. 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Biological activity can be used in two ways for the bioremediation of metal-contaminated soils to immobilize the contaminants in situ or to remove them permanently from the soil matrix, depending on the properties of the reduced elements. Chromium and uranium are typical candidates for in situ immobilization processes. The bioreduction of Cr(VI) and Ur(VI) transforms highly soluble ions such as CrO and UO + to insoluble solid compounds, such as Cr(OH)3 and U02. The selenate anions SeO are also reduced to insoluble elemental selenium Se°. Bioprecipitation of heavy metals, such as Pb, Cd, and Zn, in the form of sulfides, is another in situ immobilization option that exploits the metabolic activity of sulfate-reducing bacteria without altering the valence state of metals. The removal of contaminants from the soil matrix is the most appropriate remediation strategy when bioreduction results in species that are more soluble compared to the initial oxidized element. This is the case for As(V) and Pu(IV), which are transformed to the more soluble As(III) and Pu(III) forms. This treatment option presupposes an installation for the efficient recovery and treatment of the aqueous phase containing the solubilized contaminants. 

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