Sulfur metal sulfides

Sulfur is one of the more common constituents of the earth s crust and can be ranked as the 16th or 17th most abundant element [68]. The mean sulfur content-of the rocks forming the crust of the earth is estimated to be about 400 ppmw. Sulfur naturally occurs as elemental sulfur, metal sulfides in coal and mineral ores, sulfates, hydrogen sulfide in natural gas, and complex organic sulfur compounds in crude oil and coal. All of these various forms of sulfur are used as sulfur sources, but the most important sources are elemental sulfur, hydrogen sulfide in natural gas, and iron pyrites. 

Total Sulfur and Sulfide Sulfur. Total sulfur is predominately in the form of metal sulfate, and because sulfates act as inerts, these materials have htde impact on the process. Sulfide sulfur compounds, on the other hand, react and leave the furnace as a sulfur vapor, which may deposit in the gas handling system. A possible mechanism for this is the partial reaction of SO2 to H2S, followed by... 

Roasting ofSulRdes. Most nonferrous metals occur in nature mainly as sulfides. These cannot be easily reduced directly to the metal. Burning metallic sulfides in air transforms them into oxides or sulfates which are more easily reduced. The sulfur is released as sulfur dioxide, as shown by the foHowing typical reaction for a divalent metal, M ... 

This reaction is strongly exothermic and proceeds spontaneously from left to right for most common metallic sulfides under normal roasting conditions, ie, in air, because P q + Pq = - 20 kPa (0.2 atm) at temperatures ranging from 650 to 1000°C. The physical chemistry of the roasting process is more complex than indicated by equation 3 alone. Sulfur trioxide is also formed,... 

Sulfur constitutes about 0.052 wt % of the earth s cmst. The forms in which it is ordinarily found include elemental or native sulfur in unconsohdated volcanic rocks, in anhydrite over salt-dome stmctures, and in bedded anhydrite or gypsum evaporate basin formations combined sulfur in metal sulfide ores and mineral sulfates hydrogen sulfide in natural gas organic sulfur compounds in petroleum and tar sands and a combination of both pyritic and organic sulfur compounds in coal (qv). 

Occurrence. The metal sulfides, which are scattered throughout most of the world, have been an important source of elemental sulfur. The potential for recovery from metal sulfides exists, although these sources are less attractive economically and technologicaky than other sources of sulfur. Nevertheless sulfide ores are an important source of sulfur in other forms, such as sulfur dioxide and sulfuric acid. 

Pyrite is the most abundant of the metal sulfides. Eor many years, until the Erasch process was developed, pyrite was the main source of sulfur and, for much of the first half of the twentieth century, comprised over 50% of world sulfur production. Pyrite reserves are distributed throughout the world and known deposits have been mined in about 30 countries. Possibly the largest pyrite reserves in the world are located in southern Spain, Portugal, and the CIS. Large deposits are also in Canada, Cypms, Einland, Italy, Japan, Norway, South Africa, Sweden, Turkey, the United States, and Yugoslavia. However, the three main regional producers of pyrites continue to be Western Europe Eastern Europe, including the CIS and China. 

Hydrometallurgical Processes. Recovery of sulfur in the processing of nonferrous metal sulfides has been in the form of SO2 and/or H2SO4 when smelter (pyrometallurgical) operations are employed. However, there have been accounts of processes, mainly hydrometallurgical, in which sulfur is recovered in the elemental form (see Metallurgy, extractive). 

The H2S sulfanes are the subject of several reviews (129,133). Except for hydrogen sulfide these have no practical utiUty. Sodium tetrasulfide [12034-39-8] is available commercially as a 40 wt % aqueous solution and is used to dehair hides in taimeries, as an ore flotation agent, in the preparation of sulfur dyes (qv), and for metal sulfide finishes (see Leather Mineral recovery and processing). 

With hot metals, sulfur dioxide usually forms both metal sulfides as well as metal oxides. In aqueous solution, sulfur dioxide is reduced by certain metals or by borohydrides to dithionites. 

Other method thiosulfates have been prepared by reaction of suspensions of the metal sulfide with sulfur dioxide. However, these thiosulfates are... 

Arsenic pentasulfide (arsenic(V) sulfide), As S q, is stable in air up to 95°C, but at higher temperatures begins to dissociate into arsenous sulfide and sulfur. It is prepared by the fusion of arsenic with sulfur foUowed by extraction with ammonia and reprecipitation at low temperatures by addition of hydrochloric acid. Arsenic pentasulfide is precipitated at low temperatures from strongly acidic arsenate solutions by a rapid stream of hydrogen sulfide. It is hydrolyzed by boiling with water, yielding arsenous acid and sulfur. Salts derived from a number of thioarsenic acids are formed from arsenic pentasulfide and alkaH metal sulfides. 

Catalysis by Metal Sulfides. Metal sulfides such as M0S2, WS2, and many others catalyze numerous reactions that are catalyzed by metals (98). The metal sulfides are typically several orders of magnitude less active than the metals, but they have the unique advantage of not being poisoned by sulfur compounds. They are thus good catalysts for appHcations with sulfur-containing feeds, including many fossil fuels. 

In the second phase, performed at a maximum temperature of about 370°C, the sulfur and a portion of the coke are removed by combustion. The rate and exothermicity are controlled by limiting the flow of combustion gas through the catalyst. Spent base metal catalysts may have sulfur levels of from 6 to 12 wt % in the form of metal sulfides. A high degree of sulfur removal must be achieved in these first two regeneration steps to avoid the formation of sulfate on the support during the final combustion step. Such a formation causes a loss of catalyst activity. 

Chlorination of OCT with chlorine at 90°C in the presence of L-type 2eohtes as catalyst reportedly gives a 56% yield of 2,5-dichlorotoluene (79). Pure 2,5-dichlorotoluene is also available from the Sandmeyer reaction on 2-amino-5-chlorotoluene. 3,4-Dichlorotoluene (l,2-dichloro-4-methylben2ene) is formed in up to 40% yield in the chlorination of PCT cataly2ed by metal sulfides or metal halide—sulfur compound cocatalyst systems (80). 

Permanent coloration can also be achieved by exposing hair to certain metals copper, silver, and especially lead salts. Preparations containing aqueous solutions of lead acetate may include a source of sulfur, usually thiosulfate, which may react with cystine in the hair to produce some cysteine or may react directiy with the metal ion to form dark metallic sulfides. Preparations of this type, which darken hair gradually, are not universally considered safe. 

A variety of routes is available for the preparation of metal-thionitrosyl complexes. The most common of these are (a) reaction of nitride complexes with a sulfur source, e.g., elemental sulfur, propylene sulfide or sulfur halides, (b) reaction of (NSC1)3 with transition-metal complexes, and (c) reaction of [SN]" salts with transition-metal complexes. An example of each of these approaches is given in Eq. 7.1,... 

In type A reaction soluble sulfide ions combine with metal ions to form a precipitate of insoluble metal sulfide. Sulfur s oxidation state of minus two does not change in this reaction. The reaction is... 

Sulfur compounds, whether organic or inorganic in nature, cause sulfidation in susceptible materials. The sulfide film, which forms on the surface of much con-stmction materials at low temperatures, becomes friable and melts at higher temperatures. The presence of molten sulfides (especially nickel sulfide) on a metal surface promotes the rapid conversion to metal sulfides at temperatures where these sulfides are thermodynamically stable. High-alloy materials such as 25% Cr, 20% Ni alloys are widely used, but these represent a compromise between sulfidation resistance and mechanical properties. Aluminum and similar diffusion coatings can be of use. 

An alternative procedure for removing an ion from solution is to change its identity by changing its oxidation state. The metal ions in very insoluble heavy metal sulfide precipitates can be dissolved by oxidizing the sulfide ion to elemental sulfur. For example, copper(II) sulfide, CuS, takes part in the equilibrium... 

The solubility of a solid can be increased by removing one of its ions from solution acid can be used to dissolve a hydroxide, sulfide, sulfite, or carbonate precipitate and nitric acid can be used to oxidize metal sulfides to sulfur and a soluble salt. 

A transargononic structure for sulfur, with six bonds formed by sp3d2 hybrid orbitals, was suggested for sulfur in the octahedral molecule SF6 long ago, and also for one of the sulfur atoms, with ligancy 6, in binnite (Pauling and Neuman, 1934). Some transargononic structures of metal sulfides have been proposed recently by Franzen (1966). 

---