Abstract: A method and apparatus for the concentration of an alkaline solution to an approximately anhydrous melt using the falling-film principle under atmospheric pressure wherein the solution to be concentrated is conducted along the internal surface of evaporator tubes (1). Each tube (1) is surrounded by a heating jacket (2). The heating jackets (2) are fed with heat-transfer fluid from a conduit (6) which is common to all jackets. An annular collecting trough (8) is associated with the lower end of each evaporator tube (1). The metal running off from the tube (1) is collected by the trough (8) separately from the vapors. All of the troughs (8) are connected to a melt discharge line (9) which is also common to all troughs. The vapors descending in the tube (1), in contrast thereto, pass through the opening (8a) of the collecting troughs (8) into a collecting duct (10) which is common to all troughs.

Abstract: An alkaline solution film flowing from the top toward the bottom in falling-film evaporator tubes (1) is heated by a heat-transfer fluid flowing from the bottom toward the top in a narrow heating jacket (5). Since large temperature differences occur between the heat-transfer fluid and the alkaline solution film in the upper heated portion of the evaporator tube (1), in contrast to the lower portion, especially at high flow rate as it occurs in a narrow heating jacket (5), the heat flow density can at that location lead to a lack of wetting of the evaporator tube. To avoid such critical heat flow densities, the upper section (5a) of the heating jacket (5) is conically widened and thus the flow rate in this section is reduced to such an extent that a critical heat flow density is not reached at that point.