Mechanism of a Coal Chemical-Leaching Process and Recovery of Spent Chemicals: A Pilot-Scale Study

ABSTRACT

Methods for enriching organic macerals in coal are of great importance, but, due to the yield constraint, physical-beneficiation methods are less effective. Chemical beneficiation using alkali followed by acid has been studied at the pilot stage with a batch of 500 kg of feed coal for three different coal fractions of the physical-beneficiation process using dense media cyclone and froth flotation. The mechanism of silica and alumina reactions during the alkali-leaching process has been derived and found that 150°C is the critical point for precipitation of the unwanted sodalite compound. The composition of sodalite is calculated based on the experimental results. Due to the formation of sodalite, the acid-leaching step is critical for the removal of sodalite and, hence, regeneration of acid is of great importance. A process has been developed for acid regeneration in which the first major impurity, silica, is removed by a polycondensation process and the second major impurity, alumina, is removed by a reaction with sulfuric acid. Optimum operating conditions for the regeneration process have been identified where more than 99% of the silica and 80% of the alumina can be removed from the spent acid. Almost 60% of the spent acid can be regenerated for further use and is recycled.

KEYWORDS:

• Coal
• leaching
• regeneration
• hydrolysis
• polymerization