Solving Calcium Scaling Problems in Black Liquor Evaporators, 1998 International Chemical Recovery Conference Proceedings

The most common scaling problem in modem evaporators is the so-called hard scale, caused by precipitation of inorganic salts. The precipitation forms crystalline build-up on the evaporator heating surfaces. The hard scale can be further divided into two broad classes; water-soluble and water insoluble.

Water soluble scaling, such as sodium carbonate or burkeite (a double salt formed by sodium carbonate and sodium sulfate), used to be a major problem but can be easily handled by the latest evaporator technology. Water insoluble scaling is often still a major problem.

The most common scale forming compound is calcium carbonate (CaCO3 ), although different silica (Si) compounds are also a problem, especially in non-wood pulping. Modem evaporator technology has been applied to non-wood liquors with positive impact on the observed silica scaling /1/.

Calcium scaling remains a problem in many modem evaporator plants. Especially with the increase of modem batch pulping technologies the reported calcium scaling problems in the industry have increased in recent years. Calcium scaling is not a new phenomenon. It has been extensively studied, and several ways of decreasing it have been proposed. This paper discusses calcium scale formation and ways to reduce it.

One method of eliminating calcium scaling is Liquor Heat Treatment (LHT) for calcium deactivation. In this method, black liquor is treated thermally to generate inert calcium carbonate in the deactivation reactor. The effect is a reduction or complete elimination of calcium scaling in the evaporators. The theory behind calcium deactivation is discussed. Two calcium deactivation systems have started up, and two are currently (early 1998) under construction. This paper discusses in detail the scaling problems, calcium deactivation system design and system performance for UPM-Kymmene Corporation Kaukas mills in Lappeenranta, Finland. This system has been in operation since early summer 1997.