Patent GB-1001296-A (GB)
Title: Method of and apparatus for producing a deionized liquid product by electrodialysis
IPC: C02F1/469
Filed: 1963-04-09
Abstract: <PICT:1001296/C6-C7/1> <PICT:1001296/C6-C7/2> <PICT:1001296/C6-C7/3> <PICT:1001296/C6-C7/4> A portion of the solvent component of an ionic solution is separated from the remainder of the solution by moving by an electric current ions of one polarity and solvent from a first chamber through a first membrane into a second chamber and moving the same numbers of ions of the same polarity by the same current from the second chamber through a second membrane preferentially permeable to said ions and of smaller pore size than the first membrane into a third chamber, whereby solvent accumulates in and is independently withdrawn from the second chamber.  Preferably a pressure gradient is applied across the membranes to oppose diffusion movement of the liquids in the direction of ionic movement.  Both membranes may be selectively permeable to ions of the same polarity and first or filter membrane preferably has pores sufficiently small to prevent the passage of bacteria.  The filter membrane may be of cellulose or cellulose derivatives, e.g. cellulose acetate, with or without nylon reinforcing filaments, or if organic solvent-containing liquids are to be treated a polythene filter membrane may be used.  Such a membrane, which is hydrophobic, treated to graft styrene sulphonate on to the polythene matrix (see Division C3) or quaternized to render it hydrophobic, may be used and when so treated has cationic or anionic exchange properties also respectively.  Another filter membrane referred to is a terpolymer of vinyl chloride, vinyl acetate, and acrylonitrile.  Suitable conically selective membranes are sulphonated or quaternized polythene-styrene copolymers, sulphonated- or vinyl pyridine-styrene-divinyl benzene exchange resins, or a rigid phenolsulphonic acid-formaldehyde resin.  Ionic membranes normally have a pore size of 2-10 mm  and are used together with filter membranes of pore size 100-450 mm .  A filter membrane of pore size 10 mm  can be used with an ionic membrane of pore size less than 7 mm , while filter membranes of pore size up to 10m  may also be used. Examples are given of removing salts and impurities including proteins from brackish water, natural or artificially muddied aqueous solutions, tap water, boiler feed water, aqueous solutions of salts, methyl alcohol containing sodium chloride, and a solution of potassium sulphate in equal parts of ethyl alcohol and benzene, the bulk of the benzene being removed also.  Water low in silica may be produced from tap water by employing anion permeable and filter membranes. As shown in Fig. 1, a typical three compartment cell has platinum electrodes 27, 28 and membrane supports or spacers 37, 40, e.g. of nylon or a vinylidene chloride copolymer, each having vertical ribs 38 and horizontal ribs 39 (see Fig. 2).  Using a cation permeable membrane C adjacent the cathode 28 and a filter membrane F adjacent the anode 27, liquid to be treated is introduced at 29, overflowing from the anode compartment at 30, and is transferred with cations to the space between the membranes F, C.  Cations with a smaller quantity of liquid are transferred to the cathode compartment which is supplied with liquid at 31 which overflows at 32.  The liquid retained between the membranes is extracted at 35, the outlet levels of ducts 32, 35, 30 being successively lower so that the flexible membrane C is pressed against the membrane F, hydrostatic transfer of liquid from the centre compartment to the cathode compartment is prevented, and leakage of liquid from the anode compartment to the centre compartment is avoided to ensure that transfer of liquid depends on cation movement. To maintain free passage for liquid in the centre compartment, the cationic membrane C has an embossed rectangular grid pattern.  If desired the non-contacting marginal portions of the membrane C may be coated with an impermeable lacquer.  Alternatively the spacers 37, 40 may be dispensed with and membranes 22, 23, e.g. based on polythene, stretched drum-tight within or over a frame, may be employed when they appear in use as shown in Fig. 6.  A multicompartment cell may be employed as shown in Fig. 8 comprising alternating cation permeable and filter membranes C, F, alternate compartments being fed with raw liquid over lines 52 with extraction at 54 while liquid product is withdrawn over line 53.  The spaces between membranes may accommodate spacers or filler material.  If membranes become clogged they may be regenerated by briefly reversing the liquid flow and hence the pressure gradient and electrode polarity.ALSO:A polythene sheet filter membrane of pore size 1.5 m  is made hydrophilic and cation exchanging by grafting on to its surface a thin layer of styrene sulphonate by (1) irradiating the sheet with U.V., (2) soaking the irradiated sheet in styrene, and, after air-drying, (3) immersing the sheet in chlorosulphonic acid, followed by washing in aqueous sodium chloride.