Four low rank coals were oxidized in 1N Na_2CO_3 aqueous solution at 85℃, into which atmospheric oxygen gas was bubbled. Coal peroxides formed were decomposed by an acid hydrolysis. The carbon conversion to volatiles such as CO_2 was at most 4%. The oxidation-hydrolysis depolymerized the coals and introduced oxygen functionalities such as -COOH and -OH into them. The treatment also dramatically increased the extraction yield of the coals with DMF, DMSO and THF/methanol mixture up to 90% or more. The extractability was correlated to the solubility parameters of the solvents much better than to their donor numbers. The increment of the extractability per amount of consumed oxygen strongly depended on the extent of C-C and C-O bond breaking.
