The Effect of Bulk Crystals on Sodium Salt Scaling in Black Liquor Evaporators

Black liquor evaporation is an essential unit operation in the chemical recovery cycle of a pulp mill. The high concentrations of sodium carbonate and sodium sulphate in black liquor cause crystallization fouling, or scaling, in the concentrator effect, which affects the operation negatively.The focus of this work was to evaluate the effect the content and size of the bulk crystals had on the scaling rate of the sodium salts. This was done experimentally by evaporating both pure sodium carbonate solution (to crystallize anhydrous sodium carbonate) and black liquor.It was found that bulk crystals were required to suppress rapid scaling during primary nucleation (when passing the metastable limit), which was tenfold higher than scaling during continuous crystallization under steady-state operation. The amount of bulk crystals did not, however, affect the scaling rate during continuous crystallization. It was also found that the scaling rate increased when the largest crystals were removed, in this case by a hydrocyclone. The results were similar for both sodium carbonate solutions and black liquor, although the latter had a finer crystal size and scale structure.