Thermal decomposition of coal fly ash by concentrated sulfuric acid and alumina extraction process based on it

The industrial applications of acidic leaching processes for alumina extraction from coal fly ash (CFA) have been limited due to its excessive usages of acid and fluoride. This limitation can be lifted by calcining the CFA with the concentrated sulfuric acid at high temperature. When CFA was mixed with the concentrated sulfuric acid of same amount and calcined at 300 °C, most of its aluminum components were transformed into aluminum sulfate. Excessive sulfuric acid was recycled by collecting sulfur trioxide produced during heating process. Morphological and X-ray diffraction evidence indicated that mullite inside CFA was thermally decomposed. Alumina and mullite were transformed into aluminum sulfate. A practical process of alumina extraction from CFA was developed based on this observation. The adoption of calcination process significantly reduced the sulfuric acid usage, shortened the process cycle, and eliminated the use of fluoride. The alumina extraction efficiency was improved up to 85%.