Method for concentrating aqueous lye and apparatus suitable therefor

A method for concentrating dilute aqueous alkali comprising utilizing heat of reaction from the formation of 1,2-dichloroethane, wherein the dilute aqueous alkali is concentrated in a plurality of stages, at least part of heat required for concentrating the dilute aqueous alkali originates from a plant for preparing 1,2-dichloroethane, and at least a further part of heat required for concentrating the dilute aqueous alkali originates from at least one higher stage of the plant for concentrating the aqueous alkali and is used for partial heating of a first stage of the plurality of stages, the method comprising the following steps:

• producing a first aqueous alkali concentrate from a dilute aqueous alkali in a first concentration unit to which heat is supplied only by waste heat from one or more further concentration units,
• further concentrating the first aqueous alkali concentrate to give a second aqueous alkali concentrate in a second concentration unit to which heat is supplied,
• supplying at least part of the heat supplied to the second concentration unit in the form of heat originating from the plant for preparing 1,2-dichloroethane, and
• supplying at least a part of the heat supplied to the first concentration unit in the form of heat originating from the second concentration unit, and
• supplying heat to a heating unit located between the first and second concentration units to supply heat to the first alkali concentrate, at least part of the heat supplied to the heating unit being heat originating from at least one of:
• the plant for preparing 1,2-dichloroethane, the second concentration unit, and/or from a plant for concentrating aqueous alkali associated with a plant for chloralkali electrolysis,
• wherein the first and second concentration units each comprise a plate heat exchanger and a phase separator.