this invention the at least one volatile component volatile component being volatile at the temperature at which the preconcentrated weak black liquor enters the evaporation zone and at the pressure in the evaporation zone that is evaporated from the preconcentrated weak black liquor to form the strong black liquor is mostly if not all water other ingredients of the preconcentrated weak black liquor may be evaporated although that is not most desirable the evaporation step relies on the mechanism wherein the energy absorbed by the preconcentrated weak black liquor during the heating step is preserved by maintaining a back pressure on the preconcentrated weak black liquor to prevent vaporization of the preconcentrated weak black liquor in the inlet 26 to the evaporation zone 24 and is subsequently released in the evaporation zone causing evaporation the number and size of the orifices in the die 28 affect the back pressure that is maintained upstream the back pressure must be sufficient to keep the preconcentrated weak black liquor from boiling before it enters the evaporation zone 24 a multi orifice die may be preferred particularly because the weak black liquor is to be concentrated or evaporated in a single pass through the.