The process referred to as evaporation, is when the diluted black liquor leaves the washing section it contains about 15% dry substances (ds). This evaporation process must increase the dryness to at least 60% in order to make it combustible.

Through a series of evaporators – which are basically heat exchangers – that operate at progressively lower pressure, the steam from one stage is then used to heat the next stage by condensing there. Evaporation normally takes place between six or seven stages, frequently called effects. Live steam at pressure will enter the first effect thereafter the pressure and temperature drops from effect to effect. In the example shown opposite. The weak black is pumped into the fourth effect while the black liquor is removed from the first effect.

The condensate from the first effect is then clean and goes back to the boiler feedwater tank. The condensate from other stages can also be used in other parts of the process i.e. wash water. Non-condensable gases are also removed then extracted and burnt.

Weak black liquor which contains rosin soap, will then be separated and recovered for the production of tall oil and raw product which is used by the chemical industry.

Click image to enlarge.

Examples of NAF valves used in this process

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