PURPOSE: To enhance the recovery ratio of coal and to miniaturize an apparatus, by preparing purified slack coal by mixing high grade slack coal low in ash content respectively separated by primary and secondary electrostatic separation treatments by corona discharge.

CONSTITUTION: Slack coal is charged in the electrostatic separation tank of a first electric separator 2 along with transport air and primary electrostatic separation treatment is applied to said coal by corona discharge of high voltage of about 20W30KV to separate high grade slack coal which is, in turn, fed out to a duct 3. Low grade slack coal is charged in the electrostatic separation tank of a second electric separator 5 along with transport air and secondary electrostatic separation treatment is applied to said coal by low voltage corona discharge of about 2W3KV to separate and recover high grade fine particulate coal which is, in turn, supplied to the duct 3 to be met and mixed with the high grade slack coal separated and recovered by primary electrostatic separation treatment to recover coal at high yield.