Ion-exchange membranes

An electro-dialysis apparatus has a plurality of ion-exchange membranes 3, 5 separated from one another by spacer members, 2, 4, 2a, said membranes and spacer members having fluid inlet manifold holes 10, 10a and fluid outlet manifold holes 11, 11a, said spacers further having a fluid-flow path area 12, 12a connected to the inlet and outlet manifold holes of said spacers by means of connecting channels whereby fluid is caused to flow along and in contact with a membrane side adjacent thereto, said apparatus also containing a pair of terminal electrodes, 1, 1a for passing a current through said membranes and the fluid-flow path area of said spacers, at least one side of at least one said membrane containing selected portions 20 (see Fig.2) having increased physical stiffness and increased electrical resistance as compared to unselective portions, said portions of increased stiffness and increased electrical resistance corresponding substantially to that membrane area defined and encompassed within the connecting channel of the spacer placed adjacent to said membrane side whereby said increased stiffness is sufficient to substantially reduce the tendency of that area to flex under differential pressures. Suitably, the treated portions comprise the membrane area covering the inside edges of at least one of said manifold holes and extend outwardly therefrom at least in a direction towards the fluid-flow path area of the membranes and for a distance sufficient to substantially reduce lateral flow of current. Thus the treated area may extend in a direction towards the flow path of the membrane for a distance of from 1/4 in. to 2ins. The entire area commencing from the inside edge of the manifold hole and extending outwardly to the oppositely disposed edge of the liquid flow path of the membrane may be so treated. Both sides of the membrane may contain substantially identically treated portions. Treatment is effected by contacting said areas with a liquid polymerizable monomeric or comonomeric material which is soluble in polar solvents, the contact time being sufficient to allow said material to diffuse within said membrane area and thereafter polymerizing said material in situ in the presence of a polymerization catalyst to form a solid cross-linked substantially non-leachable polymer filler therein. For details see B5 Abridgement. Figs. 3, 4 and 5 (not reproduced) illustrate alternative forms of spacer and membrane