Electrodialysis reversal process and apparatus with bipolar membranes
AssigneeYeda Research and Development Co. Ltd.
InventorOra Kedem
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Abstract
Water softening by a combination of electrodialysis and calcium carbonate precipitation. The hard feed water or alternatively recycled soft water is rendered alkaline and is then subjected to a treatment by which precipitation of calcium carbonate is induced. One way of bringing about such precipitation consists in passing the alkaline water across a cake of calcium carbonate crystals having a size not exceeding 100 microns, deposited on a filter and being in an at least fifty fold excess over the amount of calcium carbonate to be precipitated.
Claims12
A process of softening hard water by the precipitation therefrom of an amount of calcium carbonate and separate withdrawal of precipitated calcium carbonate and soft water, the process comprising the steps of rendering feed hard water alkaline by electrodialysis against a brine containing a water soluble alkali metal or calcium salt with periodical current flow reversal by passing said hard water and said brine through alternating product and brine compartments within a stack of asymmetric bipolar ion-exchange membranes so arranged that each brine compartment is bound by two cation-exchange layers and each product compartment is bound by two anion-exchange layers withdrawing said alkaline hard water from the stack;
- mixing withdrawn alkaline hard water with fresh hard water to produce an alkaline diluate and subjecting said diluate to a crystallization inducing treatment by which calcium carbonate precipitation is induced;
- and recovering mother liquor from said precipitation as product soft water.
Dependencies are inferred from the claim text.