Thiosulfate, formation

Some metal thiosulfates are inherently unstable because of the reducing properties of the thiosulfate ion. Ions such as Fe " and Cu " tend to be reduced to lower oxidation states, whereas mercury or silver, which form sulfides of low solubiUty, tend to decompose to the sulfides. The stabiUty of other metal thiosulfates improves in the presence of excess thiosulfate by virtue of complex thiosulfate formation. 

Reaction with Thiosulfate-Formation of Bunte Salt. 

The direct-oxidation systems are specific to hydrogen sulfide other sulfur species are apparently not attacked. Solution degradation problems may be caused by thiosulfate formation as well as thiocyanate formation (if HCN is present in the gas to be treated). Solution regeneration techniques have been developed to attempt to minimize the impact of these effects. The systems must be operated with caution, in some cases, the solutions contain species that are considered toxic or environmentally hazardous. 

In addition to the reactions that produce elemental sulfur, competing reactions also occur that produce undesirable by-products such as sodium thiosulfate. This is detrimental, because the thiosulfate remains in solution, and its concentration can generally be reduced only by bleeding off a portion of the solution inventory. This solution purge waste stream is hazardous, largely because it also contains vanadium compounds. The key to reducing the metal content of the waste stream is to reduce the rate of thiosulfate formation. 

The rate of thiosulfate formation also increased with increasing temperature and pH. Many Stretford plants employ a sulfur melter to separate the elemental sulfur from Stretford solution. This subjects the solution to 150°C temperatures, which enhance generation of thiosulfate. 

J ADA Isomer Selection. Limited attention is often given in refineries to the isomer of ADA used (Lorton 1988). 2,6-ADA is a commonly used isomer, although it has been found inferior to 2,7-ADA in converting vanadium to its pentavalent form. If this conversion is not performed efficiently, elemental sulfur production rate will fall, and thiosulfate formation will increase. More attention to procuring only 2,7-ADA could augment the efficiency of the Stretford process. 

U.3.2.5 Use of Filter Instead of Melter. Significant thiosulfate formation can occur due to the high temperatures in a sulfur melter. Replacing the melter with a filter can eliminate this problem, and also provide a way of efficiently removing commercially salable sulfur from the solution (Lorton 1988). 

The formation of sulfur intermediates is both chemically and biologically mediated. Oxidation of hydrogen sulfide with oxygen and Fe(Mn) oxyhy-droxides and sulfate reduction are the main processes responsible for sulfur intermediates formation in the euxinic water columns, except for elemental sulfur, which is formed only during hydrogen sulfide oxidation. Chemically mediated reactions of thiosulfate formation are elemental sulfur(polysulfides)... 

TABLE 7-4. Origins of Thiosulfate Formation during Sulfite Pulping ... 

A Garner. Sources of thiosulfate in paper machine white water. Part II Thiosulfate formation during sodium hydrosulfite brightening. J Pulp Paper Sci 10(3) 51-57, 1984. 

Besides the main reactions, several side reactions (mostly leading to the formation of undesirable sulfur compounds) occur in the process. These side reactions depend on the operating conditions and the composition of the gas to be treated. Usually a certain amount of thiosulfate formation is inevitable. In some cases it may even be desirable to operate these processes so that hydrogen sulfide is quantitatively converted to thiosulfate according to the following equations ... 

Design and Operation. The absorber used in Ferrox installations has a lower section, or saturator, and an upper section, the absorber proper, llie saturator contains a continuous liquid phase, several feet high, through which the raw gas is bubbled before it enters the upper section. The function of the saturator is to provide sufficient contact time to complete the reaction between sodium hydrosulfide and ferric oxide before regeneration of the solution. If essentially complete reaction is achieved, thiosulfate formation in the regenerator is kept at a minimum. The upper part of the absorber contains sprays and wooden hurdles similar to those used in the Seaboard process and usually has a total height of 60 ft (Sperr, 1926). 

The liquid is circulated at such a rate that a two- to threefold excess of ferric hydroxide over the stoichiometric quantity necessary for the complete reaction with hydrogen sulfide is present. Gollmar (1945) states that the process can be operated with less than the stoichiometric concentration of iron oxide and interprets the function of the iron as a catalytic oxygen carrier. Available historical data from several plants indicate that operation with an excess of iron oxide over the stoichiometric amount was commonly practiced. This excess seems to be required for complete removal of hydrogen sulfide and, also, to minimize thiosulfate formation in the thionizer. For a coal gas plant with a 10 MMsef/day capacity and a hydrogen sulfide removal rate of 400 grains/lOO scf, the chemical requirements are approximately 3,500 Ib/day of sodium carbonate and 2,800 Ib/day of iron. 

Process Operation. It is claimed that H2S is removed quantitatively and for the greater part converted to elemental sulfur. However, a certain amount of thiosulfate formation does take place, especially at high pH and low concentrations of blue. In some instances, it may even be desirable to operate the process so that the H2S is completely converted to thiosulfate. 

The Thylox process offered some economic advantages over processes previously discussed in this chapter. The consumption of alkali due to thiosulfate formation was reduced markedly, and the sulfur was produced in a much more valuable form. Estimated operating requirements for a plant treating 5 million cu ft/day of refinery gas containing 1,000 grains hydrogen sulfide/100 cu ft were reported by Dunstan (1938), and are shown in Table 9-4. 

Figure 9-19. Effect of temperature on thiosulfate formation in Stretford solution.

Figure 9-20. Effect of pH on thiosulfate formation In Stretford solution.

Nicklin and Holland, I963A). The effect of increased alkalinity on thiosulfate formation is rather significant above a pH of 8.8. This effect is more pronounced in the reactions of hydrosulfide with dissolved oxygen. For instance, at an oxygen partial pressure of 7 psia in the sour gas, a rise in pH from 8.3 to 8.8 triples the amount of sulfur converted to thiosulfate (from 3 to 9%) (Nicklin and Holland, l%3B). Likewise, increasing the concentration of dissolved solids in the liquor from 7 to 20% (by weight), doubles the sulfate formation rate. 

The conversion of elemental sulfur to thiosulfate (equation 9-50) increases as the temperature of the solution goes up, becoming quite significant above approximately I20°F. This route to thiosulfate formation is particularly burdensome in Stretford units that melt the sulfur directly without first separating the froth liquor (the froth contains only 5-8% sulfur) by filtration. 

In a properly operated plant, thiosulfate formation can be controlled at less than I % of the sulfur in feed gas (Ludberg, 1980). The maximum concentration of sodium sulfate and thiosulfate in the Stretford liquor is usually maintained between 25 to 30 grams per liter. Associated with each of these sulfur anions is an equivalent amount of sodium cation that must be added to maintain ionic balance. Thus, the formation of soluble sulfur species increases alkali makeup (usually soda ash). Also, the accumulation of sulfate, thio.sulfate, and sodium ions in the solution makes it necessary to discard part of the solution periodically to maintain control of the concentration of dissolved salts. This blowdown contributes to the loss of vanadium, ADA, and sodium carbonate. 

The LOCAT solution electropotential is also related to the solution Fe /Fe- concentration ratio. The higher the Fe- /Fe ratio at constant pH, the higher (less negative) the redox potential. At a constant Fe- /Fe ratio, the lower the pH, the higher the redox potential. To minimize thiosulfate formation and chelant loss, the LO-CAT process is monitored daily to ensure that operation stays within solution electropotential and pH limits (Eaton. 1992). 

The mechanism of thiosulfate formation in animals is still obscure. Three theories can actually be proposed for this problem according to one by Medes and Floyd (86), thiosulfate would be formed from sulfoxylic acid according to one by Fromageot and Royer (51), it would be formed by oxidation of hydrogen sulfide and finally one may think thiosulfate would result froifi the reaction between free sulfur and sulfite. 

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