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Electrodialysis metathesis (EDM)


Similar to ED, an Electrodialysis metathesis (EDM) stack is comprised of alternating cation- and anion-exchange membranes. However, instead of a single diluting and single concentrating stream, EDM has two diluting streams and two concentrating streams. The combination of the four streams and associated membranes is called a quad; commercial EDM stacks include 100 or more quads. The figure below shows a single quad in detail and how that quad is incorporated in the rest of the EDM stack. Brackish water or desalination concentrate is fed to the EDM feed compartments. The applied voltage causes the cations to move to the right (towards the cathode) and anions to the left (towards the anode). Cations from the EDM feed combine with chloride from the NaCl to produce a mixed chloride salt concentrate stream (called Mixed Cl). Anions from the EDM feed combine with sodium from the NaCl to produce a mixed sodium salt concentrate stream (called Mixed Na). This metathesis, or changing of partners, produces highly soluble sodium and chloride salts such as calcium chloride and sodium sulfate from sparingly soluble salts. NaCl is added to the EDM at a rate equivalent to the amount of ions removed from the EDM feed. EDM acts like a kidney in the ZDD process by removing troublesome salts from the RO/NF concentrate. The desalinated EDM product (called diluate) can be returned to the RO/NF feed or blended directly with the RO/NF permeate, thus allowing for higher overall water recovery in the ZDD system.

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The first description of EDM appeared in US Patent 2,721,171, which was issued in 1955 and assigned to DuPont. This patent described production of H2SO4 and NaOH from Na2SO4. Winger described the use of EDM to generate NaOH from lime and NaCl (Winger, 1957). J. R. Ochoa et al. used an electrolytic cell with a single set of membranes in an EDM arrangement to convert a salt to an acid (Ochoa, et al., 1993). Another group described the use of EDM to convert magnesium chloride and sodium sulfate to magnesium sulfate and sodium chloride (Alheritiereet al., 1998). A more recent patent, US Patent 6,712,946, describes the use of EDM to produce 2-keto-L-gulonic acid from its calcium salt. In this, HCl is fed to the other depleting compartments, and CaCl2 is a byproduct. There is no indication that any of the aforementioned examples of EDM was ever practiced on a scale larger than laboratory scale. Indeed it is apparent that the ZDD (US Patents 7,459,088 and 7,083,730) process is the first commercial process to utilize EDM.

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