Abstract: There is disclosed a fly ash utilization system and process for the removal of sulfur dioxide from the flue gas of coal fired boilers. The principal features of the invention are the maintaining of high ionic concentration in a closed loop, water balanced system which is operated at a low pH, i.e. less than 4 and preferably less than 3. The ionic concentration of the scrubber slurry is maintained at at least 5,000 parts per million of alkali metal cations, preferably over 10,000 parts per million, at least a substantial portion of which has been leached from fly ash.The ionic concentration of the scrubber slurry is maintained at a high level by either providing small diked areas in a waste pond for settling of undissolved solids wherein the returned supernatant has a high ionic concentration. A thickener and filter system may also be used to recover high ionic concentration supernatant for return to the scrubber.