Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2000-06-07
2002-10-08
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S525000, C204S529000, C204S634000
Reexamination Certificate
active
06461491
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrodialysis apparatus and method used for transferring dissolved salts or impurities from a waste or other solution (commonly known as a diluate solution) into a concentrate solution, and more particularly, to a new and improved electrodialysis apparatus and electrodialysis process that enables the transfer of such dissolved salts or impurities from such a waste or other diluate solution into a concentrate solution without the precipitation of certain insoluble compounds adjacent the cathode of the electrodialysis apparatus or the liberation of chlorine gas at the anode of the electrodialysis apparatus or the liberation of ammonia at the cathode of the electrodialysis apparatus.
2. Background of the Invention
Electrodialysis (ED) is used in connection with the separation of dissolved salts or other impurities from one aqueous solution to another aqueous solution. The separation of these dissolved salts or other impurities results from ion migration through semi-permeable, ion-selective membranes under the influence of an applied direct current field that is established between a cathode (negative potential electrode) and an anode (positive potential electrode). The membranes may be selective for monovalent or multivalent ions depending on whether separation is desired between monovalent or multivalent cations and/or anions. The separation process results in a salt or impurity concentrated stream (known as a concentrate or brine) and in a salt or impurity depleted stream (known as a diluate). The concentrate and diluate streams flow in solution compartments in the electrodialysis apparatus that are disposed between the anode and cathode and that are separated by alternating cation and anion selective membranes. The outer most compartments adjacent the anode and cathode electrodes have a recirculating electrode-rinse solution flowing therethrough to maintain the cathode and anode electrodes clean. A schematic of one type of electrodialysis apparatus is illustrated in FIG.
1
.
The electrodialysis apparatus
20
shown in
FIG. 1
has a series of alternating cation semi-permeable, ion-selective membranes C and anion semipermeable, ion-selective membranes A disposed between a positive DC potential anode electrode
22
and a negative DC potential cathode electrode
24
. The cation-selective membranes C and the anion selective membranes A form compartments therebetween. As indicated in
FIG. 1
, concentrate and diluate solutions flow as indicated respectively by arrows
26
and
28
through adjacent compartments such that the concentrate and diluate solutions are separated from each other by the ion-selective membranes. The diluate solutions may contain salts (such as sodium chloride (NaCl)) or impurities (such as sodium chloride (NaCl) in acidic magnesium chloride (MgCl
2
) solutions or calcium chloride (CaCl
2
) and magnesium chloride (MgCl
2
) in sodium chloride (NaCl) and potassium chloride (KCl) solutions). Due to the potential maintained across each of the compartments and cation and anion selective membranes separating the compartments, cations (such as acid (H), sodium (Na), magnesium (Mg), calcium (Ca) and potassium (K)) and anions (such as chloride (Cl)) as well as water (hydration shell and osmosis) will tend to migrate from the diluate solution to the concentrate solution. Once these cations and anions are in the concentrate solution, they can be recovered and used for commercial purposes. Additionally, the purified or salt-depleted diluate solution also may have an increased commercial value.
As further indicated in the schematic of
FIG. 1
, an electrode rinse solution is circulated in an outer most compartment
30
adjacent to the anode
22
and an outer most compartment
32
adjacent to the cathode
24
. During the electrodialysis process, hydrogen tends to be evolved at the cathode
24
and oxygen tends to be evolved at the anode
22
. As a result, the pH level in the electrode rinse solution that is circulating through the compartment
32
adjacent the cathode
24
increases while the pH level of the electrode rinse solution that is circulating through the compartment
30
adjacent the anode
22
decreases. In view of the fact that the electrode rinse solution is mixed after flowing through the compartments
30
and
32
, the increase and decrease in the pH level of the electrode rinse solution used in the electrodialysis apparatus
20
tends to be neutralized.
The circulation of fluids through an electrodialysis apparatus, such as the electrodialysis apparatus
20
, is shown schematically in
FIG. 2
of the drawings. As shown therein, the diluate solution is pumped from and to a diluate tank
34
through the diluate compartments formed between the cation C and anion A selective membranes (labeled ED stack in
FIG. 2
) by a diluate pump
36
. In a like manner, the concentrate solution is pumped from and to a concentrate tank
38
through the concentrate compartments formed between the cation C and anion A selective membranes (ED stack) by a concentrate pump
40
. As the diluate solution flows through the diluate compartments, cations, anions and water from the diluate solution migrate through the cation C and anion A selective membranes to the concentrate solution. In addition, an electrode rinse solution is pumped by an electrode rinse solution pump
42
from an electrode rinse solution tank
44
through the compartments
30
and
32
adjacent respectively of the anode
22
and the cathode
24
. As previously indicated, any changes in the pH level of this electrode rinse solution is neutralized when the electrode rinse solution is mixed together as it flows from the compartments
30
and
32
to the electrode rinse solution tank
44
.
While the use of one electrode rinse solution has the advantage that changes to the pH level of the solution as it flows through the compartments
30
and
32
are neutralized, precipitates tend to form on the ion-selective membrane forming a side of the compartment
32
adjacent to the cathode
24
. In this regard, reference can be made to
FIGS. 3-8
of the drawings. These figures diagrammatically illustrate the types of precipitates that might be formed on a cation selective membrane that is disposed on a side of the compartment
32
adjacent the cathode
24
. It should be noted that the membrane arrangement in
FIGS. 3-8
is different from the arrangement in
FIG. 1
, but this has no bearing on the present invention.
FIG. 3
illustrates what might occur when an electrode rinse solution of sodium hydroxide (NaOH) is used in an electrodialysis apparatus in which is treated pickle liquors from the steel industries which liquors contain chloride, iron and/or manganese ions. The NaOH electrode rinse solution is generally a basic solution having a pH level of about 14. While the electrodialysis apparatus enables the removal of iron and/or manganese from those liquors, iron hydroxide (Fe(OH)
3
) and/or manganese hydroxide (Mn(OH)
3
) precipitates tend to form on the diluate side of the cation ion selective membrane on the side of the compartment
32
. The reason that such precipitates form is due to the migration of iron and/or manganese ions towards the cathode
24
that react with the hydroxide ions of the NaOH electrode rinse solution flowing through the compartment
32
. Such precipitates on the cation ion selective membrane are detrimental to the functioning of the electrodialysis apparatus
20
because the precipitates tend to block or interfere with the transfer of iron or manganese ions across that membrane.
FIG. 4
similarly illustrates what might occur when the NaOH electrode rinse solution is used in an electrodialysis apparatus in which is being treated effluents from the pulp and paper industry. Those effluents contain chloride and calcium ions. When these effluents are treated in an electrodialysis apparatus (such as the electrodialysis apparatus
20
) to remove chloride and calcium ions, calcium hydroxide (Ca(OH)
2
) precipitates
Hryn John N.
Sreenivasarao Kandipati
Pennington Joan
Phasge Arun S,.
The University of Chicago
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