GB1019948A - Electrodialysis liquid treatment systems - Google Patents

Abstract

In electrodialysis apparatus, hydrogen introduced at an anode is converted to acid. As shown in Fig. 1, H2 liberated at a cathode 21 is pumped from a gas collector 28 to catalysing anode 37 where it is converted into hydrogen ions. The cell comprises end blocks 10, 11, the cathode 21, frame members or gaskets 15, 18, a connecting tube 26, a cation selectively <FORM:1019948/C6-C7/1> permeable membrane 14, an anion selectively permeable membrane 17 and the catalysing electrode 20 which itself comprises a cation selectively permeable membrane 35 and wire mesh 37 bearing finely divided Pt. Salt water is supplied to the cell through tubes 24, 27 and, as a result of the electrodialysis together with the pumping of H2 from the cathode to anode, aq. HCl and NaOH are produced which are removed through tubes 25 and 31 respectively. The cell may alternatively be used to generate H2SO4 from Na2SO4 and to generate CH3COOH from CH3COONa. The catalyzing electrode 20 may be made by coating an ion selectively permeable membrane on one side with Pt black powder and bonding to it with heat and pressure a Pt gauze (or a Ti or Ta gauze which has been coated with Pt), the gauze having a coating of sponge Pt. The sponge Pt may be applied by immersing the gauze as cathode in chloroplatinic acid for one minute at a current density of 100-150 ma/sq.cm. Alternatively, the electrode 20 may be made by coating a porous polyethylene sheet with Ag conducting paint, allowing the paint to dry and immersing the sheet in a solution of chloroplatinic acid when Pt replaces the Ag. The sheet may be connected as cathode. Fig. 5 (not shown) illustrates an electrodialysis stack wherein acid generated as in the Fig. 1 construction tends to prevent the formation of harmful precipitates within the stack. The stack comprises a cation selectively permeable catalytic anode 44 and a cathode 45 separated by alternate anion and cation selectively permeable membranes 47, 48. Liquid to be treated enters the stack at 49 and 50, H2 released at the cathode 45 is pumped to the anode 44, base generated at the cathode is removed through tube 64, an acid concentrated stream is removed through tube 58 and a diluted product stream through tube 61. Electrode washing streams are indicated at 62, 63, and 51. Fig. 6 (not shown) illustrates an electrodialysis stack wherein cathodic H2 is pumped to a catalysing anode as in Fig. 5 but wherein there is parallel flow of liquid through the stack compartments rather than series flow. In this case acid generated in a compartment 55 is pumped direct to the cathode compartment where it neutralizes the base formed at the cathode and neutralized liquid from a gas collector 52 is returned to the compartment 55. In the embodiments, the catalyzing electrode functions in the same manner as does one-half of a fuel cell, and the power requirements are reduced. Operation at temperatures up to 150 DEG C. can be achieved by using perhaloethylene instead of polyethylene membranes. In some embodiments, the hydrogen generated at the cathode is allowed to escape and hydrogen from an external source is introduced at the anode.