The invention relates to a method for recycling mixed alkali liquor in
indigo production, and the method comprises the following steps: feeding
indigo into a
filter press so as to be filter-pressed after an
oxidation process, concentrating a filter-pressing filtrate to obtain caustic
sludge, and feeding a filter-pressing concentrated liquor into a reaction still; washing the
indigo filter-pressed in the step (1), adding the caustic
sludge of the step (1) into washing water, warming and stirring until the caustic
sludge is completely dissolved, naturally cooling to
room temperature, filtering, concentrating the filtrate as a concentrated alkali liquor, and then feeding the concentrated alkali liquor into the reaction still; washing filter residue by using saturated Na2CO3, filtering to obtain a Na2CO3
solid, concentrating the filtrate as the concentrated alkali liquor, and then feeding the concentrated alkali liquor into the reaction still, introducing CO2 into the reaction still, stopping introducing CO2 when the
reaction temperature is at 60-70 DEG C, cooling to
room temperature, stirring for 1-2 hours, crystallizing, filtering to obtain
mother liquor and crude product Na2CO3, concentrating the filtrate as the concentrated alkali liquor, and then feeding the concentrated alkali liquor into the reaction still in the step (3). By adopting the method of circularly recycling, the filter-pressed filtrate, the washing water and the KOH in the caustic sludge are effectively recycled.