In a kraft mill, large amounts of oxalate ions [C2O4]^-2, are formed in oxygen delignification, as well as in the D, Z and, to a lesser extent, in the E and P stages in bleaching. In the bleach plant, CaC₂O₄ scales can form in the acid stage. CaC₂O₄ solubility depends on the concentration of the Ca, and the pH range, and it increases with temperature. Upset conditions, such as high lime mud carry-over with white liquor, can cause CaC₂O₄ scaling in the bleach plant. While CaC₂O₄ scaling was not a problem in chlorine bleaching at low pH, chlorine dioxide stages with a higher pH have encountered scaling problems. Oxalate scaling is also associated with the closure of the bleach plant, and has been reported in various bleaching stages, including the Q stage.

CaC₂O₄ is not expected to precipitate in the black liquor evaporators, since CaCO₃ has a lower solubility in the evaporator environment and will precipitate instead. However, in a closed mill, where bleach plant filtrates are recycled to the recovery cycle, the possibility of Na₂C₂O₄ scaling exists. Solubility data are available for Na₂C₂O₄ and show that the solubility in black liquor depends on the temperature and the total Na concentration*. The recycle of bleaching effluent could double the amount of oxalate in the black liquor. Under normal operating conditions, when the dry solids content of black liquor are between 50-55%, burkeite (2Na₂SO₄.Na₂CO₃) starts to precipitate, reducing the concentration of Na and preventing the formation of sodium oxalate. However, when the amount of oxalate is increased due to the recycle of bleaching filtrates, the risk of Na₂C₂O₄ scaling is increased. Since the solubility of Na₂C₂O₄ increases with temperature, crystallization is expected to take place in the bulk solution and not on heat transfer surfaces. This could cause plugging problems in the evaporator tubes.

* Deposition of sodium oxalate in black liquor evaporation, P. Ulmgren and R. Radestrom, 2001, Int. Chem. Rec. Conf.

Calcium contributes to scaling in various operations in a pulp and paper mill. CaCO₃ is water insoluble, and has inverse solubility with temperature. Consequently, it forms on heat transfer surfaces such as digester heaters and black liquor evaporators. CaCO₃ scales are also seasonal, and not easy to deal with. Calcium enters the mill through wood/ bark, chemical make-up and water. Depending on the type of wood, the method of pulping used and the efficiency of clarification for the white liquor, the calcium concentration can be high in the digester. Scaling is initiated when the concentration of free Ca and free carbonate exceed the solubility limit, to form calcite (CaCO₃). In green liquor processing, where large amounts of Na₂CO₃ are present, pirssonite (Na₂CO₃.CaCO₃.2H₂O) forms.

Calcium can form water soluble compounds with components of black liquor. Many of these compounds are not thermally stable and break-up at high temperatures, resulting in supersaturation of CaCO₃ in black liquor, which leads to scaling. The scaling tendency is high in the temperature range 110-140 C, where most evaporators operate. There is a patented method for calcium deactivation, in which black liquor is heated and stored at high temperatures to break up the calcium compounds and reduce scaling tendency

Calcium is purged with tall oil soap, the dregs, the grits and the pulp in the recovery cycle. CaCO₃ scaling can also occur in bleaching in stages employing high pH. In the bleach plant, the acid sewer is rich in calcium and other metals. If acidic effluents are recycled to the recovery cycle, a metals removal system is required.