Electrodialysis including filled cell electrodialysis (electrodeionization)

A process for removing silica and other minerals from aqueous reverse osmosis permeate which permeate contains silica and said other minerals and has a pH substantially less than 9, said process comprising:

• increasing the pH of said permeate from substantially less than 9 to less than about 9.5 thereby obtaining pH-increased-permeate;
• passing said pH-increased-permeate through at least diluting compartments of a reversing or non-reversing electrodialysis stack which diluting compartments are bounded on the cathode side thereof by cation exchange membranes comprising as exchange groups, in at least those surface portions on the anode side thereof, a predominant amount of sulfonate moieties and a minor amount of weakly acid and/or weakly basic moieties and which diluting compartments contain uniformly-sized, short-diffusion-path anion exchange granules and uniformly-sized, short-diffusion path cation exchange granules;
• passing direct electric current through said stack which current is in excess of the limiting current of said stack;
• and recovering from said stack aqueous liquid depleted in silica and said other minerals as compared to said aqueous reverse osmosis permeate.

A process for removing silica and other minerals from aqueous liquid which liquid contains silica and said other minerals and has a conductivity less than about 10 micro-Siemens/cm, said process comprising:

• passing at least part of said aqueous liquid through diluting compartments of a reversing or non-reversing electrodialysis stack, said diluting compartments containing uniformly-sized anion exchange granules and uniformly-sized cation exchange granules;
• passing aqueous liquid through concentrating compartments of said stack;
• passing through said stack direct current in excess of the Cowan-Brown limiting current of said stack;
• controlling the conductivity of said aqueous liquid passing through said concentrating compartments to values pre-determined to make the sum of the electrical resistances of said concentrating compartments less than about 50% of the electrical resistance of said stack.

A process for removing silica and other minerals from aqueous liquid which liquid contains silica and said other minerals and has a conductivity less than about 10 micro-Siemens/cm and a pH substantially less than about 9, said process comprising:

• increasing the pH of at least part of said liquid to less than about 9.5 thereby obtaining pH-increased-liquid;
• passing said pH-increased-liquid through at least diluting compartments of an electrodialysis stack which compartments contain uniformly-sized anion exchange granules and uniformly-sized cation exchange granules;
• passing aqueous liquid through concentrating compartments of said stack;
• passing through said stack direct current in excess of the limiting current of said stack;
• and
• controlling the conductivity of said aqueous liquid which passes through said concentrating compartments to values pre-determined to make the sum of the electrical resistances of said concentrating compartments less than about 50% of the electrical resistance of said stack.

A process for removing electrolyte from aqueous fluid having a conductivity less than about 10 microsiemens/cm comprising:

• a) providing an electrodeionization stack characterized by:
• i) diluting compartments containing ion exchange granules and defining dilute stream flow paths;
• ii) concentrating compartments defining concentrate stream flow paths parallel to the dilute stream flow paths;
• iii) a Cowan-Brown limiting current density in the dilute compartments;
• iv) means for establishing an electrical field transverse to the flow paths;
• and
• v) an electrical resistance across the diluting compartments and an electrical resistance across the concentrating compartments, each resistance being a component of a total electrical resistance of the stack;
• b) directing said fluid into the dilute stream flow paths;
• c) passing a direct electrical current in excess of the Cowan-Brown limiting current density through the stack and in a direction transverse to the parallel flow of the flow paths;
• and
• d) maintaining a conductivity in the concentrate stream flow paths sufficient to ensure that the electrical resistance across the concentrating compartments is less than about 50% of the total electrical resistance of the stack.

A process for removing electrolyte from aqueous fluid having a conductivity less than about 10 microsiemens/cm comprising:

• a) providing an electrodeionization stack characterized by:
• i) diluting compartments containing ion exchange granules and defining dilute stream flow paths;
• ii) concentrating compartments defining concentrate stream flow paths parallel to the dilute stream flow paths;
• iii) a Cowan-Brown limiting current density in the dilute compartments;
• iv) means for establishing an electrical field transverse to the flow paths;
• and
• v) a summed electrical resistance across the diluting compartments and a summed electrical resistance across the concentrating compartments, each resistance being a component of a total electrical resistance of the stack;
• b) establishing a pH in said fluid in the range of 9 to about 9.5;
• c) directing the fluid into the dilute stream flow paths;
• d) passing a direct electrical current in excess of the Cowan-Brown limiting current density through the stack and in a direction transverse to the parallel flow of the flow paths;
• and
• e) maintaining a conductivity in the concentrate stream flow paths sufficient to ensure that the summed electrical resistance across the concentrating compartments is less than about 50% of the total electrical resistance of the stack.