Carbonate.Since the quantity of added sulfate (sulfuric acid) is known, the amount of lime needed to precipitate the sulfate can be nearly stoichiometric. An additional amount is required to precipitate the sulfate ions that may be present in the Stretford solution initially. Solubilities of the various calcium salts are important because this determines the extent of the S042-/S032-removal and the amount of calcium which will be returned to the Stretford process. The solubility products of these salts in terms of pK,, at 25 “C are listed in Table I (11, 12). For easy comparison, their solubilities are also listed in the table. Because of the low solubility of calcium sulfite, this salt will precipitate preferentially on neutralization of the acid-treated Stretford solution with lime. Thus, it is important to steam strip SO2 well. From the solubility products of Ca(OH)2 and CaS04, the concentrations of Ca2+ and S042- ions can be calculated as a function of pH. The theoretical curve for 25 “C is shown in Figure 3. High pH favors low calcium content in the filtrate. This would be highly desirable because calcium ions returning to the gas absorption unit will see a carbonate environment, precipitate as insoluble calcium carbonate, and be removed as a contaminant in the recovered sulfur. d. Processing of Used Stretford Solution. T o test the complete decomposition and recovery scheme, 300 g of 20% H2SO4 was added dropwise with stirring to 500 g of a used Stretford solution. The initial p H of the solution was 8.7. As the pH decreased, COS evolved; then a t about p H 5 sulfur started to form and SO2 was evolved. Finally, the mixture a t p H 2.8 was evacuated (-25 in. vacuum) and held for 1.5 h to remove SO*. The mixture was filtered to remove sulfur; the filtrate was weighed, and the sulfur washed with water and
methanol, and dried. An amount of CaO stoichiometric to the sulfate concentration was added to the filtrate. The pH rose t o 13.2. The material balances are shown in Table 11, and the Ca2+ and concentrations in the filtrate are plotted on Figure 3. The data in Table I1 show that acid decomposition of thiosulfate is nearly complete (99%),and removal of sulfate and calcium is excellent, 93.5 and 97.6%, respectively. Recovery of ADA was 91 w t %. Vanadium recovery was not determined, but solubility data lead us to estimate that about 60% recovery is possible.
Literature Cited
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(1) Naber. J . E.. Wesse1inp.h. J. A.. Groenendall. W.. Chem. Eng. Prog , 69,29-34 (1973). (2) Ellwood, P., Chem Eng ( N Y ), 71,128-30 (1964). (3) Moyes, A. J., Wilkinson, J. S., Chem Eng (London),282,84-90 (1974). (4) Espenscheid, W. F., Yan, T. Y., U.S. Patent 3 959 452, May 25, 1976.
( 5 ) A i r e s , G. H., “Quantitative Chemical Analysis”, Harper & Row. New York, 1958, pp 202, 460. (6) Schmidt, M., Z. Anorg. Allg. Chem., 289,141-57 (1957). (7) Davis, R. E., J . A m . Chem. Soc., 80, 3565 (1958). (8) Nickless, G., “Inorganic Sulfur Chemistry”, Elsevier, New York, 1968, p 529. (9) Schmidt, M., “Reaction of the Sulfur-Sulfur Bond”, Meyer, B., Ed., Wiley-Interscience, New York, 1965. (10) Benson, H., Field, J., U.S. Patent 2 886 405, May 1959. (11) Smith, R. M., Martell, A. E., “Critical Solubility Constants, Vol. 4: Inorganic Complexes”, Plenum Press, New York, 1976. (12) Stumm, W., Morgan, J., “Aquatic Chemistry”, Wiley-Interscience, New York, 1970.
Received for reuieu: April 19, 1979. Accepted March 10, 1980
Correction In the article “Cadmium Bioaccumulation Assays. Their Relationship to Various Ionic Equilibria in Lake Superior Water” (Enuiron. Sci. Technol. 1979, 13, 701-706), by John E. Poldoski, on page 705, Table V, the calculated free cadmium for case no. 8 should be 7 X M instead of 1 x 10-8
M.
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