FACTORS AFFECTING THE SETTLING AND FILTERING PROPERTIES OF GREEN LIQUOR DREGS, 2023 PEERS Conference

Many kraft pulp mills have experienced persistent issues with green liquor dregs settling and filterability, resulting in poor liquor quality, poor lime mud quality and ultimately poor pulp quality. Dregs is made of mostly unburned black liquor char, along with some water-insoluble matter in smelt and lime mud in weakwash. As such, its settling behavior and filterability are likely affected by the conditions under which char is formed in the boiler, the impurities in the as-fired black liquor and the lime mud content in the weakwash. A laboratory study was conducted to examine the effects of black liquor burning conditions, char content and lime mud content on settling and filtering properties of the dregs. The results show that dregs with higher char concentrations or char that is formed by burning black liquor at lower temperatures and shorter time are more difficult to settle and filter. Adding lime mud accelerates char settling and filterability. The effect of lime mud assisted settling is more pronounced for char formed at a lower temperature or for a shorter time. These findings are consistent with mill experience that recovery boiler operating conditions and adding lime mud to the dissolving tank can have an impact on the settling and filterability of green liquor dregs.

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