The disposition of sulfur during the oxidation and subsequent leaching of retorted oil shale

The char and iron sulfides in retorted oil shale were oxidized at temperatures from 350 to 800°C. At temperatures above approximately 550°C, sulfur was retained by the shale as water-soluble sulfates, whereas oxidation at temperatures between 400 and 500°C resulted in a loss of as much as 22% of the sulfur as S02. The fraction of the sulfides converted to water soluble sulfates is proportional to the fraction of char burned. This implies a similar rate of combustion for char and iron sulfides. When retorted oil shale is oxidized at 650°C and below, the principal sulfur-fixing reaction appears to be between FeS, 02, and dolomite to produce approximately equal amounts of Ca and Mg sulfates. Oxidation at temperatures above 650°C yielded mostly CaS04. Oxidation at 800°C resulted in complete carbonate decomposition to CaO and MgO which, when mixed in water, resulted in a solution of high Ca concentration and a pH of 12.3. Exposed to air, the pH and Ca concentration of the solutions fell due to carbonate precipitation by reaction between CO2 dissolved from the air and Ca.