Pyritic sulfur, organic sulfur, and ash-forming minerals are removed from coal by a 2-stage alkaline treatment, using sodium carbonate or bicarbonate as the reagent. The first stage is an alkaline oxidation at moderate temperatures (130°-150° C.), and the second stage is a non-oxidizing alkaline treatment at a much higher temperature (250°-330° C.). The alkaline treated coal is extracted with an aqueous mineral acid, preferably hot aqueous sulfuric acid (H2 SO4) followed by washing with hot water. The resulting low-ash, low-sulfur coal can be used as a fuel in oil-firing boilers, and for similar applications where minimal-ash content is a basic requirement.
Iowa State University Research Foundation, Inc.
Wheelock, Thomas D., "Process for producing low-ash, low-sulfur coal" (1985). Iowa State University Patents. 311.