The present invention is about a process for treating all the ashes produced by a coal dust boiler (1), able to reduce the total unburnt matter content, to increase the combustion efficiency of the boiler (1), and to have the light ashes as the only waste arising from the coal combustion. In particular, said process provides for the extraction of the heavy ashes (4) from the boiler bottom (23), the ashes coming from the hoppers of the economizers (5) and the fraction of light ash richer of unburnt matter coming from the filters (11) used to collect the dust from the flue gas; said ashes are mixed in a silo (15), proportioned and transferred in one or more feeders (17) of the coal mills (18), and reintroduced in the boiler (1) after being mixed with the coal through the burners (2).