A furnace heat exchanger with multiple parallel flow passages wherein at least one flow passage has a cross-sectional shape which transitions from one form to another in a gradual manner so as to reduce the occurrence of stress therein. Specifically, the invention set forth demonstrates a method comprised of at least two steps in which up to 96% of potassium can be removed from an aqueous solution comprising potassium hydroxide. Pulses outputted from a shift register, etc, are used to execute ONOFF control of the operation of a current supply source to the level shifter of each divided unit.