Improved process for leaching of ash from the recovery boiler electrostatic precipitator

摘要

The invention relates to a process for purifying recovery boiler electrostatic precipitator ash (via 19) from chlorides (Cl) and potassium (K), and recover useful pulping chemicals such sodium sulphate (Na2SO4). The inventive process uses two centrifugal separation stages (2a, 2b) in series with a leaching stage (1 a and 1 b) ahead of each centrifugal stage. The chlorides and potassium is bled out (26) with the first liquid fraction (LF1 ) from the first centrifugal separation stage and the final second dry matter substance (DM2) with enriched sodium sulphate content from the second centrifugal separation stage is sent (24) to the liquor recovery cycle, mixed into black liquor before the black liquor is being sent to the recovery process for reuse of the sodium sulphate in the pulping process. With the inventive process losses of sodium sulphate could be reduced significantly compared to a one stage leach-and-centrifuge process.