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Abstract

Cooking liquors of significantly different sulphidity (e.g. one having a sulphidity about 30-40% and the other a sulphidity of above 40%, e.g. about 60-70%) may be produced in a simple energy-efficient manner. Black liquor is treated to recover chemicals, and the chemicals are dissolved to produce green liquor, which is clarified, as is conventional. The temperature of a first portion of the green liquor is decreased to effect crystallization of sodium carbonate in the green liquor by expanding the green liquor in at least two (and typically more than three) stages with vapor being produced during expansion. The sodium carbonate crystals produced are separated from the first portion of green liquor to produce a green liquor with high sulphidity (greater than about 40%) and this high sulphidity liquor may be heated by bringing it into direct heat exchange relationship with at least part of the expansion vapor (in a number of different stages). The separated sodium carbonate crystals are dissolved to produce a low sulphide content alkaline solution, which may be heated by bringing it into indirect heat exchange relationship with a second part of the expansion vapor. Heat recovered from the last expansion stage may be used to assist in dissolving the sodium carbonate crystals, as by using a heat pump system. The second portion of the clarified green liquor is used to produce a second green liquor having a sulphidity at least about 10% lower than the high sulphidity green liquor.

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First Claim