The solubilities of Fe, Ni, V and Na salts in concentrated aqueous H2SO4 solutions at temperatures between 400 and 460 K

When residual fuel oil, which contains up to 4 wt% S, is burned in boiler plant, H2SO4 is formed which condenses as aqueous solutions (65–90 wt% H2SO4) on surfaces in the cool back-end. The oil contains traces of other elements, in particular Na, V, Fe and Ni, which also deposit on these surfaces as either sulphates (Na, Fe, Ni,) or oxides (V). When designing techniques to control acid deposition and the corrosion which it subsequently causes, account must be taken of the degree to which the acid properties can be modified by taking these compounds into solution. A series of measurements of the solubilities of relevant compounds in acid solutions within the appropriate ranges of concentration and temperature (400–460 K) have been made. Sodium sulphate has by far the highest solubility (55 wt% in a solution originally containing 85 wt% H2SO4 at 423 K). Of the other compounds considered, only V2O5 exhibits a solubility of more than 2 wt%; for example, at 463 K, in 75 wt% H2SO4, solutions containing up to 16 wt% V2O5 can be formed. Solutions with up to 23 wt% V2O5 can be stabilised by the presence in solution of small amounts (∼0.5 wt%) of Fe3+ ions. The structure of these solutions is discussed.