With polythionate solutions

An attractive modification of this method is to treat the neutralised polythionate solution with neutral hydrogen peroxide and a known excess of standard sodium hydroxide solution on the water-bath and to determine the quantity of acid formed by the amount of standard alkali neutralised during the oxidation.3 All polythionates except the dithionates are completely oxidised to sulphate by heating in a closed tube for one hour with iodine and sodium bicarbonate by titrating the excess of iodine the total polythionate present may be estimated.4... 

An alternative method is based on the interaction of mercuric chloride with polythionates in aqueous solution. A white precipitate of the mercuric thiochloride already mentioned under thiosulphuric acid 5 is formed the equation is... 

FIGURE 14.16. Intergranular corrosion of Type 347 stainless steel in polythionate solution at room temperature. (With permission from Sumitomo Metal Industries, Ltd.)... 

The liquid redox gas desulfurization processes grouped in tliis section are selected mainly for their historical value. They comprise three separate broad categories and are differentiated by the senibbing solution chemistry. These process categories are I) polythionate solutions, 2) iron oxide suspensions, and 3) iron cyanide solutions. Of the three types, only the iron oxide suspension processes were used extensively at one time. Except in very rare cases, most of these plants have since been shut down and replaced with more modem gas desulfurization units. 

The early development of liquid oxidation processes using polythionate solutions for the removal of hydrogen sulfide from gases derived from coal is to a latge extent identified with the work of Feld in Germany. F ld started his studies before the outbreak of World War 1, and his principal aim was to devise a process by which hydrogen sulfide and ammonia could... 

Thiosulfuric Acid. Thiosulfuiic acid [14921 -76-7] is relatively unstable and thus cannot be recovered from aqueous solutions. In laboratory preparation, a lead thiosulfate [26265-65-6] solution is treated with H2S to precipitate PbS, or a concentrated solution of sodium thiosulfate [7772-98-7] is treated with HCl and cooled to — 10°C to crystalline NaCl. Aqueous solutions of thiosulfuric acid spontaneously decompose to yield sulfur, SO2, and polythionic acids, H2S O. Thiosulfuric acid is a strong acid comparable to sulfuric acid. Dissociation constants, = 0.25, = 0.018, have been... 

These three methods are employed commercially. In addition, decomposition of polythionates in alkaline solution or their reaction with sulfide or sulfite gives thiosulfates ... 

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

If gaseous sulphur dioxide is passed into a solution of thiosulphate, a yellow solution is formed which on keeping becomes colourless it then yields a precipitate of sulphur when treated with formaldehyde and sodium hydroxide, but no polythionate can be detected. When the colourless solution is neutralised with sodium hydroxide, it is found to contain sulphite and thiosulphate, but sulphur is not precipitated. These results may be due to the formation of an additive compound such as that mentioned in (ii) above.5... 

Properties.—Trithionic acid is the least stable of the polythionic acids. The aqueous solution of the free acid, which is generally obtained from a cold concentrated solution of the potassium salt by the addition of a suitable acid, such as hydrofluosilicie or perchloric acid, which will remove the metal as a sparingly soluble salt, slowly decomposes, even at the ordinary temperature, with formation of sulphur, sulphur dioxide and sulphuric acid ... 

A major contribution of this paper was pointing out the importance of bioturbation and bioirrigation on chemical processes associated with carbonate dissolution. In the movement of sulfidic sediment from depth to near the interface by biological processes, oxidation of the sediment produces sulfuric acid which ends up titrating alkalinity, lowering pH, and thus lowers saturation state (e.g., Berner and Westrich, 1985). Actually this process is very complex, involving many reactive intermediate compounds such as sulfite, thiosulfate, polythionates, etc. Aller and Rude (1988) demonstrated an additional complication to this process. Mn oxides may oxidize iron sulfides by a bacterial pathway that causes the saturation state of the solution to rise with respect to carbonate minerals, rather than decrease as is the case when oxidation takes place with oxygen. 

Polythionates are obtained by reduction of thiosulfate solutions with S02 in the presence of As203 and by the reaction of H2S with an aqueous solution of S02, which produces a solution called Wackenroder s liquid. A general reaction reported to produce polythionates up to very great chain lengths is... 

The reaction of a strong solution of aqueous ammonia with the sulfide concentrate in a strongly agitated pressure vessel at a temperature between 160 and 190°F under an oxygen partial pressure of about 10 psi, either as pure oxygen or as compressed air, fulfills the optimal conditions for the above requirements. The iron present in the concentrate is oxidized to hydrated ferric oxide which, together with the silicates is insoluble in aqueous ammonia. The copper, nickel, and cobalt form their amines, while the sulfides are oxidized to sulfates, thiosulfates, and polythionates. 

The temperature and NHj coneentrations in the leach liquor have the most influence on the rate of leaching, followed by oxygen partial pressure and amount of agitation. A pregnant solution with sufficient thiosulfate and polythionate content must be produced to react with the copper present in the subsequent boiling stage. It must also be regulated to produce an iron oxide residue with very little absorbed nickel. 

Emulsions of very small droplets of liquid sulfur or of sulfur-rich compounds (e.g., polythionates) in water are called sulfur sols . Such mixtures can be prepared by a variety of methods. Hydrophobic sulfur sols are obtained on dilution of saturated solutions of elemental sulfur in organic solvents like ethanol or acetone with a large excess of water. These preparations are usually called Weimarn sols honoring the principal author who first re-... 

Historically, aqueous sulfur sols are related to the so-called Wacken-roder s solution which is obtained by reaction of H2S with SO2 in water at 0 °C and which contains polythionic acids as well as elemental sulfur besides other sulfur compounds (e.g., sulfate). For a detailed early investigation and vivid description of this mixture, see ref. [6] for more recent studies, see later. A compilation of 23 patents for the preparation of colloidal sulfur mixtures from the years 1931-1948 has been published by Meyer [7]. 

Thiosulfates also result from reduction of or HSOs" with elemental sulfur (p. 714), whereas reduction with H2S in Wacken-roder s solution (pp. 716-7) yields polythion-ates. It is also notable that the sulfite ion is involved in the 6-electron sulfite reductase reaction 503 -b bH" " -b 6e"... 

Pregnant solution from the leaching circuit contains typically 40-50 g/L nickel, 0.7-1.0 g/L cobalt, 5-]0g/L copper, 120-180 g/L ammonium sulfate. 5-10 g/L sulfor as thiosulfate and polythionates. and 85-100 g/L Tree ammonia.1 Copper removal requires inilia] reseoval of ammonia followed by ptacipilation of copper in the form of coppar sulfides. The unoxidized sulfur in solution assists in the precipitation process. Removal of ammonia by steam injection releases cupric ions to react with these sulfur compounds, as shown by the reactions ... 

Similar reactions occur with dilhionates and polythionates in solution, Final copper residuals are stripped using H3S under modest pressure. Aqueous effluent from copper removal is heated to 490 K (425°F and... 

Regeneration. The products formed by treating sulfur dioxide solutions with hydrogen sulfide depend in part on the pH. When hydrogen sulfide is added to a solution of sulfur dioxide in water, a complex mixture is formed that includes polythionic acids, thiosulfuric acid, and colloidal sulfur (in contrast to the crystalline sulfur obtained in the citrate process). This is known as Wackenroder s solution and has been extensively studied. The composition varies with the conditions used. With excess hydrogen sulfide the final product is ultimately approximately 100% sulfur (6). While the overall stoichiometry of the reaction is the same as the gas phase Claus reaction, the chemistry is more complex. 

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