Effects of electrolytes and curing on freezing of ferric hydroxide sludge

This work elucidates how the electrolytes and curing time affect freezing and thawing treatment of ferric hydroxide sludge. The sludge is frozen at a freezing temperature of −16.5°C with addition of one of the following four electrolytes: sodium sulfate (Na2SO4), potassium nitrate (KNO3), sodium nitrate (NaNO3) and sodium chloride (NaCl). Although the eutectic temperatures for the former two and latter two electrolytes are higher than and lower than the freezing temperature, respectively, improvements of sludge filterability and settleability are independent of the electrolyte species as well as the additional amount. Differential scanning calorimetry (DSC) tests imply that ferric hydroxide particles might suppress the formation of salt/water eutectic, leading to a negligible effect of electrolyte species. On the other hand, curing of 2–24 h profoundly affects sludge performance, indicating the incorporation of mass transfer limitation into the moisture transfer.