ACID

TECHNICAL FIELD

[0001] The present disclosure relates to the regeneration of acid from a stream of industrial spent acid. More specifically, the present disclosure is directed to a method and apparatus for controlling the regeneration of acid by extracting protons and anions through ion exchange membranes from spent acid to produce regenerated acid.

BACKGROUND

[0002] Acids are employed in a multitude of industrial processes. In some processes the acid itself is consumed as part of the basic chemical process. In other cases the acid may be an agent employed in processes such as ion exchange resin regeneration and metals leaching. During use in this second class of processes the acid, while not necessarily chemically consumed, becomes polluted with multivalent metal cations that render it unsuitable for further use in the intended application, and specifically unfit for further use as a leaching agent. The resulting acid is generally referred to as "spent acid".

[0003] Diffusion dialysis is one known method that has been researched and piloted for regenerating such polluted acid streams. Diffusion dialysis employs ion diffusion across a membrane, the process being driven by a concentration difference across the membrane. The phenomenon is governed by Fick's Law, which presents the diffusion flux of ions across the membrane as being proportional to the ion concentration gradient across the membrane. The diffusion flux of ions is generally slow, and hence, in order to improve that flux, it is usually necessary to decrease the membrane thickness and increase the membrane area. Given that membrane surface area is expensive, there is every merit in increasing the ion flux without resorting to any increase of the membrane area.

[0004] Diffusion dialysis employs inherently "passive" electrochemical stacks comprising anion exchange membranes separating spent acid chambers from regenerated acid chambers. Hence the process is not driven via external influence and is completely dependent on the ion concentration gradient between the spent acid and the regenerated acid. As a result, only dilute regenerated acids may be produced, otherwise acid recovery diminishes as the concentration gradient is reduced. The process is therefore inherently self-limiting. While it is conceptually possible to apply an electric potential across the membrane and the two ionic chambers in question, such as might be done in electrodialysis, this merely results in ions being forced across the anion exchange membrane without key ionic species being appropriately blocked. This does not result in suitable acid generation.

[0005] There is therefore considerable merit in seeking an acid regeneration process that can generate a more concentrated acid as compared with diffusion dialysis in order to thereby facilitate greater acid re-use and a commensurate reduction in hazardous waste.

SUMMARY

[0006] According to a first aspect, there is provided a method for regenerating a spent acid, the method comprising directing a solution low in ion concentration through a regenerated acid channel bounded on a first side by a cation exchange membrane selected from one of a monovalent cation exchange membrane and a proton permselective cation exchange membrane, and bounded on an opposing second side by an anion exchange membrane, wherein the cation exchange membrane also bounds a first spent acid channel and separates the regenerated acid and the first spent acid channels and wherein the anion exchange membrane also bounds a second spent acid channel and separates the regenerated acid and the second spent acid channels; directing the spent acid into the spent acid channels; and applying an electric field across the regenerated acid channel and the spent acid channels, wherein the electric field causes protons to move into the regenerated acid channel from the first spent acid channel across the cation exchange membrane and causes anions to move into the regenerated acid channel from the second spent acid channel across the anion exchange membrane.

[0007] The anion exchange membrane may be a monovalent anion exchange membrane and the spent acid may comprise monovalent and multivalent anions.

[0008] The anion exchange membrane may be an acid block anion exchange membrane.

[0009] The anion exchange membrane may have acid-blocking and monovalent- permselective properties. [0010] The electric field may be generated by a pair of electrodes comprising part of an electrodialysis stack, the electrodialysis stack further comprising the regenerated acid channel, the spent acid channels, and the anion and cation exchange membranes.

[0011] The solution low in ion concentration may flow through the regenerated acid channel, and the method may further comprise recirculating the solution through the regenerated acid channel after the solution has exited the regenerated acid channel.

[0012] The spent acid may flow through the spent acid channels, and the method may further comprise recirculating the spent acid through the spent acid channels after the spent acid has exited the spent acid channels.

[0013] The regenerated acid channel and the spent acid channels may comprise three of a series of electrodialytic channels and alternating anion exchange membranes and cation exchange membranes may be disposed along the series of electrodialytic channels to separate the electrodialytic channels from each other.

[0014] The method may further comprise, after applying the electric field, reversing the direction of the electric field; and exchanging the flows of the spent acid and regenerated acid in the channels.

[0015] According to another aspect, there is provided a method for regenerating a spent acid, the method comprising providing between two electrodes a first electrodialytic chamber bounded on a first side by a cation exchange membrane separating the first electrodialytic chamber from a second electrodialytic chamber and bounded on a second opposing side by an anion exchange membrane separating the first electrodialytic chamber from a third electrodialytic chamber, said cation exchange membrane selected from one of a monovalent cation exchange membrane and a proton permselective cation exchange membrane; directing solution low in ion concentration into the first electrodialytic chamber; directing a first portion of the spent acid into the second electrodialytic chamber; directing a second portion of the spent acid into the third electrodialytic chamber; applying an electrical potential difference between the two electrodes to transfer to the solution low in ion concentration protons through the cation exchange membrane from the spent acid and anions through the anion exchange membrane from the spent acid; and extracting regenerated acid from the first electrodialytic chamber. [0016] The anion exchange membrane may be a monovalent anion exchange membrane and applying an electrical potential difference between the two electrodes to transfer anions may comprise applying an electrical potential difference between the two electrodes to transfer monovalent anions through the monovalent anion exchange membrane from the spent acid.

[0017] The anion exchange membrane may be an acid block anion exchange membrane.

[0018] The anion exchange membrane may have both acid-blocking and monovalent- permselective properties.

[0019] According to another aspect, there is provided a method for regenerating a spent acid, the method comprising directing solution low in ion concentration between and in contact with a first planar surface of an anion exchange membrane and with a first planar surface of a cation exchange membrane selected from one of a monovalent cation exchange membrane and a proton permselective cation exchange membrane; directing the spent acid in contact with a second planar side of the cation exchange membrane and in contact with a second planar side of the anion exchange membrane; applying an electric field perpendicular to the planes of the membranes across the two membranes, the spent acid and the solution low in ion concentration; transferring protons from the spent acid through the cation exchange membrane to the solution low in ion concentration; transferring anions from the spent acid through the anion exchange membrane from the spent acid to the solution low in ion concentration; and extracting regenerated acid from a region between the two membranes.

[0020] The anion exchange membrane may be a monovalent anion exchange membrane and the transferring of anions from the spent acid through the anion exchange membrane may comprise transferring monovalent anions from the spent acid through the monovalent anion exchange membrane.

[0021] The anion exchange membrane may be an acid block anion exchange membrane.

[0022] The anion exchange membrane may have acid-blocking and monovalent- permselective properties. [0023] According to another aspect, there is provided a method for regenerating a spent acid solution, the method comprising supplying to an electrodialysis stack the spent acid solution and solution low in ion concentration; in the electrodialysis stack transferring to the solution low in ion concentration protons and anions from the spent acid; and extracting regenerated acid from the electrodialysis stack, wherein the electrodialysis stack comprises electrodialysis chambers alternately separated by anion exchange membranes and cation exchange membranes selected from one of a monovalent cation exchange membrane and a proton permselective cation exchange membrane.

[0024] The anion exchange membranes may be monovalent anion exchange membranes.

[0025] The anion exchange membrane may be an acid block anion exchange membrane.

[0026] The anion exchange membrane may have acid-blocking and monovalent- permselective properties.

[0027] The transferring of the protons may comprise transferring the protons through the cation exchange membranes under an action of an electric field, and the transferring of the anions may comprise transferring the anions through the anion exchange membranes under the action of the electric field.

[0028] The method may further comprise recirculating the regenerated acid from a first output conduit of the electrodialysis stack to a first input conduit of the electrodialysis stack.

[0029] The method may further comprise recirculating the spent acid solution from a second output conduit of the electrodialysis stack to a second input conduit of the electrodialysis stack.

[0030] The method may further comprise reversing a voltage applied to the electrodialysis stack; and exchanging the flows of the spent acid and regenerated acid in electrodialysis chambers.

[0031] In any of the methods presented herein, the anion exchange membranes may be acid block anion exchange membrane so that the leakage of protons through the anion exchange membrane is retarded. Alternatively, the anion exchange membranes may be monovalent anion exchange membranes so that the transferring of anions through the anion exchange membranes is more specifically the transferring of monovalent anions through the monovalent anion exchange membranes.

[0032] This summary does not necessarily describe the entire scope of all aspects.

Other aspects, features and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In the accompanying drawings, which illustrate one or more example embodiments:

[0034] Figure 1 is a schematic view of an embodiment of an electrodialytic acid recovery stack (ED-ARS);

[0035] Figure 2 is a schematic view of an embodiment of an electrodialytic acid regeneration system; and

[0036] Figures 3 to 6 each depicts a schematic flow chart of an embodiment of a method for regenerating a spent acid.

DETAILED DESCRIPTION

[0037] As used in this disclosure:

A "monovalent cation exchange membrane" refers to a cation exchange membrane substantially permeable to monovalent cations including protons, less permeable to multivalent metal cations, and substantially impermeable to anions (whether multivalent or monovalent). "Substantially permeable" in the context of monovalent cation exchange membranes refers to the permeability ratio of monovalent cations to multivalent cations being greater than 1, and preferably being greater than 10.

A "monovalent anion exchange membrane" refers to an anion exchange membrane substantially permeable to monovalent anions, less permeable to multivalent anions, and substantially impermeable to cations (whether multivalent or monovalent). "Substantially permeable" in the context of monovalent anion exchange membranes refers to the permeability ratio of monovalent anions to multivalent anions being greater than 1 , and preferably being greater than 10.

A "proton permselective cation exchange membrane" refers to a cation exchange membrane substantially permeable to protons, less permeable to any other monovalent or multivalent cations, and substantially impermeable to anions (whether multivalent or monovalent). "Substantially permeable" in the context of proton permselective cation exchanges membranes refers to the permeability ratio of protons to other cations being greater than 10, and preferably being greater than 30.

An "acid block anion exchange membrane" refers to an anion exchange membrane that retards the migration of at least protons therethrough while permitting migration of monovalent and multivalent anions.

A "bipolar membrane" refers to a membrane comprising an anion-permeable layer and a cation-permeable layer. When used in conjunction with an electrical field, a bipolar membrane can efficiently dissociate a water molecule into a proton and a hydroxyl ion.

[0038] Embodiments described herein are directed to an ED process and system to regenerate spent acid. In a first aspect, the present disclosure provides an electrodialysis device that may be implemented to function under both forward and reverse applied electric potentials, to thereby be reversible. The term "reversible electrodialysis device" ("EDR") is employed in the present specification to describe an electrodialysis device that may function under both forward and reversed applied electric potentials. The EDR of the present disclosure achieves the desired end of increased ionic flux rates and greater acid concentrations during acid regeneration by employing spatially alternating anion exchange membranes (each an "AEM") with cation exchange membranes selected from one of monovalent cation exchange membranes (each an "m-CEM-m") and proton permselective cation exchange membranes, and by operating an acid recovery process under an applied electric potential. The AEM may be, for example, an acid block anion exchange membrane or a monovalent anion exchange membrane ("m-AEM-m") or an AEM having both acid- blocking and monovalent-permselective properties; that is, the AEM may be acid-blocking in that it retards H permeation and monovalent-permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0039] FIG. 1 shows an Electrodialysis Acid Recovery Stack (ED-ARS) 102 operating in its forward polarity configuration; in which electrode 106 is the negatively charged cathode and electrode 107 is the positively charged anode. The required voltage is supplied by direct current power supply 160. Alternating AEMs 108 and cation exchange membranes (each a "CEM") 109 selected from one of an m-CEM-m and a proton permselective cation exchange membrane separate a plurality of pairs of separation chambers 130 and 140. In the present specification the term "channel" is used to describe the plurality of chambers 140 and 130, each bounded on opposing sides by ion exchange membranes. The AEMs 108 are permeable to anions but are substantially impermeable to any cations. Suitable AEMs 108 include, but are not limited to, Astom™ AMX and Saltworks™ IonFlux™ AEM membranes. The CEMs 109 are preferentially permeable to protons as compared with metal cations, but substantially impermeable to any anions. Suitable CEMs 109 include, but are not limited to, Astom™ CMS, Saltworks™ IonFlux™ mCEM and pCEM membranes.

[0040] In certain applications, the AEMs 108 may be chosen to be an acid block anion exchange membrane that retards the migration of protons through the AEM. Suitable acid block anion exchange membranes include, but are not limited to, Saltworks™ IonFlux™ pAEM membranes.

[0041] In certain applications the spent acid solution comprises monovalent anions and multivalent anions. In order to selectively regenerate monovalent acids from this solution, including for example without limitation hydrochloric acid, hydrofluoric acid, and nitric acid, the AEM 108 may be chosen to be an m-AEM-m that is preferentially permeable to monovalent anions, including without limitation chloride, fluoride, and nitrate ions. Suitable m-AEM-m' s include, but are not limited to Astom™ ACS and Saltworks™ IonFlux™ mAEM membranes.

[0042] In certain applications the metal ions form metal complexes with anions, with an example complex being [CuC ] ^2" , and these metal complexes have the tendency to be transported across the AEM 108. In these applications to address the AEM 108 may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid-blocking in that it retards H ⁺ permeation and monovalent- permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0043] On each end of the EDR stack 102 are electrolyte chambers. In the forward polarity mode electrolyte chamber 105 is on the anode side and electrolyte chamber 104 is on the cathode side. An electrolyte solution is contained in an electrolyte tank (not shown) and pumped by electrolyte pump (not shown) through electrolyte distribution conduit 162 into electrolyte chambers 104 and 105 in parallel. The electrolyte solution flows back into the electrolyte tank shown in FIG.2 in a closed loop process via electrolyte return conduit 164. In an alternative embodiment (not shown) a series closed loop circuit may be used where the electrolyte solution flows in one direction through electrolyte chamber 105 and in the opposite direction through electrolyte chamber 104. Example electrolytes may include, but are not limited to, aqueous sodium sulfate, aqueous potassium nitrate, and other electrolytes known to those skilled in the art.

[0044] Adjacent to each electrolyte chamber 104, 105, and separated from it by a cation exchange membrane 110, is an optional rinse solution chamber 114, 115. In the embodiment shown in FIG.l the rinse solution chambers 114, 115 are separated from chambers 130, 140 by anion exchange membranes 108. In other embodiments, the rinse solution chambers 114, 115 may be separated from separation chambers 130, 140 by an acid block anion exchange membrane or by a monovalent anion exchange membrane (not shown). EDR stack 102 includes rinse solution chambers 114 and 115 that protect the electrolyte chambers 104, 105 from pollution with divalent scaling ions such as calcium or magnesium. Rinse solution is supplied via conduit 152 and may consist of conductive but non-scaling aqueous solution such as sodium chloride or hydrochloric acid. Rinse solution is removed from rinse solution chambers 114 and 115 via rinse solution return conduit 154.

[0045] Rinse solution chambers 114 and 115 are positioned next to each of the electrolyte chambers 104, 105 and the two rinse solution chambers 114 and 115 are both bounded by an anion exchange membrane 108 on the side furthest from the nearest electrode. This arrangement prevents cations, such as calcium and magnesium from entering the rinse solution chambers 114 and 115 from adjacent separation chambers 130,140. The fact that the rinse solution chambers 114 and 115 remain free of calcium and magnesium prevents the passage of such calcium and magnesium from the rinse solution chambers 114 and 115 to the electrolyte chambers 104 and 105 through the cation exchange membranes 110 that bound the electrolyte chambers 104 and 105. The rinse solution chambers 114 and 115 beneficially reduce the risk of electrode scaling, for example reduced calcium sulfate precipitation risk.

[0046] We observe that there is generally no significant concentration of monovalent metal cations such as sodium and potassium ions in typical industrial Spent Acid and that multivalent leached cations are the main industrial concern. CEM 109 is selected from one of m-CEM-m and proton permselective cation exchange membrane. With electrode 106 negative and electrode 107 positive to place EDR 102 in the forward polarity, protons (H ⁺ ) are forced through the CEM 109 by the electric potential, while multivalent cations (C ⁺ ) cannot permeate the same CEM 109. Simultaneously, all anions (A ) are blocked by CEM 109. The protons increase in concentration in chambers 130. The electric potential forces anions through anion exchange membranes 108, but anion exchange membranes 108 substantially block all cations. The result of the transit of anions through AEM 108 is that the concentration of anions increases in chambers 130 and decreases in chambers 140, thus constituting the regeneration of the anion component of the acid to be regenerated.

[0047] In certain applications, the AEM 108 may be replaced by an acid block anion exchange membrane so that the leakage of proton through the anion exchange membrane is retarded. Suitable acid block anion exchange membranes include, but are not limited to Saltworks™ IonFlux™ pAEM membranes.

[0048] In certain applications the spent acid solution comprises monovalent anions and multivalent anions. In order to selectively regenerate monovalent acids from this solution, including for example without limitation hydrochloric acid, hydrofluoric acid, and nitric acid, the AEM 108 may be chosen to be an m-AEM-m that is preferentially permeable to monovalent anions, including without limitation chloride, fluoride, and nitrate ions. Suitable m-AEM-m membranes include, but are not limited to Astom™ ACS and Saltworks™ IonFlux™ mAEM membranes.

[0049] In certain applications the metal ions form metal complexes with anions, with an example complex being [CuC ] ^2" , and these metal complexes have the tendency to be transported across the AEM 108. In these applications to address the AEM 108 may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid-blocking in that it retards H ⁺ permeation and monovalent- permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0050] In FIG.l and FIG.2 the following symbols have specific meanings: "E" represents Electrolyte, "R" represents Rinse liquid, "SA" represents Spent Acid, while "RA" represents Regenerated Acid. The symbol e ⁺ represents the cations of the electrolyte. The routing of the contents of separation chambers 130 and 140 may be controlled via suitable valve, conduit, and pump subsystems. For the sake of clarity, these are not shown in FIG.l and FIG.2, and are well known to practitioners in the art. The same is true of any recirculation of operating fluids through EDR 102.

[0051] FIG.2 is a schematic representation of an acid regeneration system 202 in which the SA channel (spent acid channel) 204 of acid regeneration system 202 is comprised of a plurality of chambers 140 of ED-ARS 102 in FIG.l, each bounded by an AEM 108 on a side facing electrode 106 and bounded by a CEM 109 selected from one of an m-CEM-m and proton permselective cation exchange membrane on a side facing electrode 107. The RA channel (regenerated acid channel) 206 of acid regeneration system 202 is comprised of a plurality of chambers 130 of ED-ARS 102 in FIG.l, each bounded by a CEM 109 on a side facing electrode 107 and bounded by an AEM on a side facing electrode 106. The electrolyte channel 208 of acid regeneration system 202 is comprised of the two electrolyte chambers 104 and 105 of ED-ARS 102 and the rinse channel 210 of the acid regeneration system 202 is comprised of the two optional rinse chambers 114 and 115 of ED-ARS 102. The structure and working of the electrolyte chambers 104 and 105 and of the optional rinse chambers 114 and 115 have already been presented above. The AEM 108 may be an acid block anion exchange membrane so that the leakage of proton through the anion exchange membrane is retarded. The AEM 108 may be an m-AEM-m so that substantially only monovalent anions are transferred through the anion exchange membranes. The AEM 108 may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid-blocking in that it retards H ⁺ permeation and monovalent- permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0052] The process associated with acid regeneration system 202 and (ED-ARS) 102 is initiated with a solution with low ion content and is supplied via input conduit 118 to RA channel 206 and spent acid being supplied to SA channel 204 via input conduit 119. The solution input via input conduit 118 will typically have at least some initial ionic content. The term "a solution with low ion content" as used in this specification is used to describe solution with conductivity less than 2 mS/cm, and preferably less than 1 mS/cm. The spent acid typically comprises protons, anions such as CI ^" , Br ^" , S04 ^" , N03 ^" and the like, as well as undesirable metal cations such as Ca ⁺⁺ , Mg ⁺⁺ and the like. The purpose of the system is to separate out metal cations from protons by keeping metal cations in the SA channel 204 of system 202, while concentrating the protons and their counter anions in the RA channel 206.

[0053] In FIG.2 the RA, now comprising an increased concentrations of H ⁺ and anions, is output from RA channel 206 on RA output conduit 120 (see also FIG.l) and collected in regenerated acid (RA) tank 220. From there RA may be recirculated to RA channel 206. As explained above, the initial charge of RA for system 202 may be supplied via suitable valves and pumps to input conduit 118 as water with at least some nominal ionic content. The SA, comprising pollutant metal cations, is output on SA output conduit 121 (see also FIG.l) and collected in spent acid tank 221. From there SA may be recirculated to SA channel 204 for further proton and anion extraction. The initial charge of SA may, like the initial charge of SA, be supplied via suitable valves and pumps to input conduit 119. Electrolyte (E) is similarly output from electrolyte channel 208 on electrolyte output conduit 164 to electrolyte tank 264, and may be recirculated from there to electrolyte channel 208. Optional rinse agent (R) is similarly output from rinse channel 210 on rinse output conduit 154 to rinse tank 254, and may be recirculated from there to rinse channel 210. For the sake of clarity, the pumps and multiway valves required to route the SA, RA, E and R and are not shown in FIG.l and FIG.2 and are well known to practitioners in the art. They will not be further addressed here.

[0054] In a first embodiment, as described above, the CEM 109 is an m-CEM-m that preferentially lets through monovalent cations but stops all anions.

[0055] In a second embodiment CEM 109 may be chosen to be an even more selective cation exchange membrane in the form of a proton permselective cation exchange membrane that is preferentially permeable to protons over any other monovalent or multivalent cations.

[0056] In a third embodiment, a bipolar ion exchange membrane may be chosen to replace CEM 109. [0057] In a fourth embodiment, the AEM 108 may be an acid block anion exchange membrane so that the leakage of protons through the anion exchange membrane is retarded.

[0058] In a fifth embodiment, the AEM 108 may be an m-AEM-m so that substantially only monovalent anions are transferred through the anion exchange membranes.

[0059] In a sixth embodiment, AEM 108 may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid- blocking in that it retards H ⁺ permeation and monovalent-permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0060] The electrodialytic acid regeneration apparatus 102 and system 202 of the present specification may be described at the hand of FIG.l and FIG.2 as comprising two substantially parallel planar electrodes 106 and 107 disposed to produce an electric field between them; a direct current power supply 160 for applying an electrical potential difference between the two electrodes 106 and 107; a planar anion exchange membrane 108 and a planar cation exchange membrane 109 selected from one of an m-CEM-m and and a proton permselective cation exchange membrane, the two membranes 108 and 109 each disposed parallel to and between the electrodes 106 and 107 and bounding two opposing ends of a first RA electrodialytic chamber 130, for example the second RA chamber from the left in FIG.l; an input conduit 118 disposed to provide a solution low in ion concentration to the first electrodialytic RA chamber 130; and an output conduit 120 disposed to extract a high acid concentration solution from the first electrodialytic chamber 130; wherein the first electrodialytic 130 chamber is arranged for converting the solution low in ion concentration into the high acid concentration solution under the influence of the electric field by receiving protons from spent acid in the SA chamber 140 to its right in FIG.l through the cation exchange membrane 109 and anions from the spent acid in the SA chamber 140 to its left in FIG.l through the anion exchange membrane 108. The AEM 108 may be an acid block anion exchange membrane so that the leakage of proton through the anion exchange membrane is retarded. The AEM 108 may be an m-AEM-m so that substantially only monovalent anions are transferred through the anion exchange membranes. AEM 108 may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid-blocking in that it retards H ⁺ permeation and monovalent-permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0061] The apparatus may further comprise a second electrodialytic chamber, being a

SA chamber 140, on an opposing side of the cation exchange membrane 109 selected from one of an m-CEM-m and a proton permselective cation exchange membrane from the first RA electrodialytic chamber 130; a third electrodialytic chamber, being another SA chamber 140, on an opposing side of the anion exchange membrane 108 from the first RA electrodialytic chamber 130; second and third input conduits 119 arranged for supplying the spent acid to the second and third SA electrodialytic chambers 140; and second and third output conduits 121 arranged for extracting a spent acid from the second and third SA electrodialytic chambers 140. The second and third output conduits 121 may be arranged to direct spent acid to the second and third input conduits 119 for recirculation and the first output conduit 120 may be arranged to direct high acid concentration water to the first input conduit 118 for recirculation in order to increase the acid concentration yet further.

[0062] In a further consideration of FIG.l and FIG.2 the electrodialytic acid regeneration apparatus of the present specification may comprise a stacked plurality of electrodialysis chambers 130 and 140 disposed collectively between oppositely chargeable electrodes 106 and 107, wherein (a) adjacent chambers 130 and 140 in the plurality of electrodialysis chambers are alternately separated by an anion exchange membrane 108 and a cation exchange membrane 109 selected from one of an m-CEM-m and a proton permselective cation exchange membrane; (b) of two mutually adjacent chambers 130 and 140 of the plurality of electrodialysis chambers a first chamber 130 is disposed for receiving water of low ion concentration; (c) a second chamber 140 of the two mutually adjacent chambers is disposed for receiving a spent acid; (d) the first chamber 130 is configured for receiving through a cation exchange membrane (for example 109) under an action of an electric field between the oppositely chargeable electrodes 106 and 107 protons from the second chamber 140; (e) the first chamber 130 is further configured for receiving through an anion exchange membrane 108 under the action of the electric field between the oppositely chargeable electrodes 106 and 107 anions from a third electrodialysis chamber.

[0063] In yet a further consideration of FIG.l and FIG.2 the electrodialytic acid regeneration apparatus of the present specification may comprise: two oppositely chargeable substantially parallel planar electrodes 106 and 107 disposed for creating between the electrodes 106 and 107 an electric field; first and second pluralities of alternating electrodialytic chambers of respectively a first 140 and second 130 type bounded substantially parallel to the electrodes 106 and 107 by a plurality of alternating anion exchange membranes 108 and cation exchange membranes 109 selected from one of an m- CEM-m and a proton permselective cation exchange membrane disposed to be subject to the electric field; a first plurality of input conduits 119 disposed in liquid communication with the first plurality of electrodialytic chambers 140 and arranged for supplying to the first plurality of electrodialytic chambers 140 a spent acid; a second plurality of input conduits 118 disposed in liquid communication with the second plurality of electrodialytic chambers 130 and arranged for supplying to the second plurality of electrodialytic chambers 130 solution with a low concentration of ions; a first plurality of output conduits 121 disposed in liquid communication with the first plurality of electrodialytic chambers 140 and arranged for extracting from the first plurality of electrodialytic chambers 140 a solution comprising metal cations; and a second plurality of output conduits 120 disposed in liquid communication with the second plurality of electrodialytic chambers 130 and arranged for extracting from the second plurality of electrodialytic chambers 130 a high acid concentration solution.

[0064] In any of the above considerations presented, the anion exchange membrane

108 may be an acid block anion exchange membrane so that the leakage of proton through the anion exchange membrane is retarded. The anion exchange membrane 108 may be an m- AEM-m so that the transferring of anions through the anion exchange membranes is more specifically transferring of monovalent anions through the monovalent anion exchange membranes. AEM 108 may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid-blocking in that it retards H ⁺ permeation and monovalent-permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions.

[0065] In another aspect this disclosure presents at the hand of FIG. 3 a method for regenerating a spent acid, the method comprising: providing [310] between two electrodes a first electrodialytic chamber bounded on a first side by a cation exchange membrane separating the first electrodialytic chamber from a second electrodialytic chamber and bounded on a second opposing side by an anion exchange membrane separating the first electrodialytic chamber from a third electrodialytic chamber, said cation exchange membrane selected from one of an m-CEM-m and a proton permselective cation exchange membrane; directing [320] solution low in ion concentration into the first electrodialytic chamber; directing [330] a first portion of the spent acid into the second electrodialytic chamber; directing a second portion [340] of the spent acid into the third electrodialytic chamber; applying [350] an electric potential difference between the two electrodes to transfer protons to the solution low in ion concentration through the cation exchange membrane from the spent acid and anions through the anion exchange membrane from the spent acid; and extracting [360] regenerated acid from the first electrodialytic chamber.

[0066] The method for regenerating a spent acid associated with the apparatus and system of FIG.l and FIG.2 may also as per FIG.4 be described as comprising the steps of directing [410] solution low in ion concentration between and in contact with a first planar surface of an anion exchange membrane and with a first planar surface of a cation exchange membrane selected from one of an m-CEM-m and a proton permselective cation exchange membrane disposed parallel to and proximate the anion exchange membrane; placing [420] the spent acid in contact with a second planar side of the cation exchange membrane and in contact with a second planar side of the anion exchange membrane; applying [430] an electric field perpendicular to the planes of the membranes across the two membranes, the spent acid and the solution low in ion concentration; transferring [440] protons to the solution low in ion concentration through the cation exchange membrane from the spent acid and anions through the anion exchange membrane from the spent acid; and extracting [450] regenerated acid from a region between the two membranes.

[0067] The method for regenerating a spent acid solution associated with the apparatus and system of FIG.l and FIG.2 may as per FIG.5 comprise supplying [510] to an electrodialysis stack the spent acid solution and solution low in ion concentration; in the electrodialysis stack transferring [520] to the solution low in ion concentration protons and anions from the spent acid; and extracting [530] regenerated acid from the electrodialysis stack; wherein the electrodialysis stack comprises electrodialysis chambers alternately separated by anion exchange membranes and cation exchange membranes selected from one of an m-CEM-m and a proton permselective cation exchange membrane. The transferring of the protons comprises transferring [522] the protons through the cation exchange membranes under an action of an electric field; and the transferring [524] of the anions comprises transferring the anions through the anion exchange membranes under the action of the electric field. The method may further comprise recirculating the regenerated acid from a first output conduit of the electrodialysis stack to a first input conduit of the electrodialysis stack. The method may also further comprise recirculating the spent acid solution from a second output conduit of the electrodialysis stack to a second input conduit of the electrodialysis stack. The method may further comprise reversing a voltage applied to the electrodialysis stack; exchanging the solution low in ion concentration within the electrodialysis stack from a first plurality of electrodialysis chambers to a second plurality of electrodialysis chambers; and exchanging the spent acid solution within the electrodialysis stack from the second plurality of electrodialysis chambers to the first plurality of electrodialysis chambers.

[0068] The method for regenerating a spent acid associated with the apparatus and system of FIG.l and FIG.2 may also as per FIG.6 be described as comprising the steps of directing [610] a solution low in ion concentration through a regenerated acid channel located between first and second spent acid channels; directing the spent acid into the spent acid channels [620]; and applying an electric field across the regenerated acid channel and the spent acid channels [630]. The regenerated acid channel may be bounded on a first side by a cation exchange membrane selected from one of a monovalent cation exchange membrane and a proton permselective cation exchange membrane, and bounded on an opposing second side by an anion exchange membrane. The cation exchange membrane may also bound a first one of the spent acid channels and separate the regenerated acid and that first spent acid channel and the anion exchange membrane may also bound a second one of the spent acid channels and separate the regenerated acid and that second spent acid channel.

[0069] In all of the method descriptions above, the anion exchange membranes may be an acid block anion exchange membrane so that the leakage of protons through the anion exchange membrane is retarded. The anion exchange membranes may be m-AEM-m's so that the substantially only monovalent anions are transferred through the anion exchange membranes. The anion exchange membrane may be an anion exchange membrane that has acid-blocking and monovalent-permselective properties; that is, the AEM may be acid- blocking in that it retards H ⁺ permeation and monovalent-permselective in that it is substantially permeable to monovalent anions while being less permeable to multivalent anions. EDR systems can develop scale on the membrane surface over time. Membrane scale be can reduced by periodically reversing the polarity of ED-ARS stack 102, such that ions travel in opposite directions through the ion exchange membranes under forward or reverse polarity operating modes. In order to reverse polarity, the solutions in their respective circuits are exchanged. The details of this process are described in PCT Patent Application PCT/CA2012/000843 (publication no. WO2013/037047), entitled "Method, Apparatus and System for Desalinating Saltwater," which is hereby incorporated in full. For example, when the ED-ARS stack 102 is operating in forward polarity as shown in FIG. 1, a first plurality of chambers contains the regenerated acid and a second plurality of chambers contains the spent acid. To operate the ED-ARS stack 102 in reverse polarity, the flows of fluid through the first and second plurality of chambers are exchanged, and the polarity of the power supply 160 is reversed.

[0070] It is contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification.

[0071] While particular embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modification of and adjustments to the foregoing embodiments, not shown, are possible. It is contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification.