Solving Deposit Problems in the Smelt Dissolving Tank, 1996 Engineering Conference Proceedings

K. Salmenoja

Tampella Power Inc.

J. Kosonen

Enso-Gutzeit Oy

Serious availability problems were encountered since the start-up of the recovery boiler, RB6, in Enso-Gutzeit Oy, Imatra Mills, Kaukopää due to the scaling of the smelt dissolving tank agitators. The growth of the deposits on the impeller blades was so rapid that usually after a running period of only a couple of months the load of the impeller motors increased on a level that they had to be stopped. This lead to unscheduled shutdowns of the recovery boiler. The depositing problems revealed to be caused by pirssonite. Pirssonite (Na2 CO3 •CaCO3 •2H2 O) is the least soluble sodium salt in the green liquor and often causes deposit problems in the green liquor processing units. Total titrable alkali (TTA) level in RB6 green liquor was found to be around 140-145 g Na2 O/l on the average. This was about 5% higher than the solubility limit of pirssonite. TTA value in RB6 green liquor was reduced to 128 g Na2 O/l on the average, which is about 90% of pirssonite solubility limit. Since the lowering of the TTA level, boiler RB6 has been running continuously about six months without unscheduled shutdowns. No deposits have been found on the agitators. Only slight deposits have been occasionally formed on the bottom and walls of the RB6 dissolving tank.

Serious availability problems were encountered since the start-up of the recovery boiler, RB6, in Enso-Gutzeit Oy, Imatra Mills, Kaukopää due to the scaling of the smelt dissolving tank agitators. The growth of the deposits on the impeller blades was so rapid that usually after a running period of only a couple of months the load of the impeller motors increased on a level that they had to be stopped. This lead to unscheduled shutdowns of the recovery boiler. The depositing problems revealed to be caused by pirssonite. Pirssonite (Na2 CO3 •CaCO3 •2H2 O) is the least soluble sodium salt in the green liquor and often causes deposit problems in the green liquor processing units. Total titrable alkali (TTA) level in RB6 green liquor was found to be around 140-145 g Na2 O/l on the average. This was about 5% higher than the solubility limit of pirssonite. TTA value in RB6 green liquor was reduced to 128 g Na2 O/l on the average, which is about 90% of pirssonite solubility limit. Since the lowering of the TTA level, boiler RB6 has been running continuously about six months without unscheduled shutdowns. No deposits have been found on the agitators. Only slight deposits have been occasionally formed on the bottom and walls of the RB6 dissolving tank.