Process for treatment of separator for sodium hydrosulfite membrane cell

#This invention relates to a separator for use between an electrode and a membrane in an electrochemical membrane cell. More particularly the present invention relates to the coating and the process used to treat the polymer surfaces of the separator to make it hydrophilic.

#Many unsuccessful attempts have been made at developing a process for manufacturing alkali metal hydrosulfites, such as sodium hydrosulfite or potassium hydrosulfite, electrochemically that can compete commercially with conventional zinc reduction processes using either sodium amalgam or metallic iron. The electrochemical process for making hydrosulfite involves the reduction of bisulfite ions to hydrosulfite ions. For this process to be economical, current densities must be employed in a cell which are capable of producing concentrated hydrosulfite solutions at high current efficiencies.

#Control of undesirable electrochemical byproduct thiosulfate formation in hydrosulfite produced for use in the paper and pulp industry as a strong reducing agent or bleaching agent is essential. At the high concentrations of hydrosulfite previously produced by the electrochemical route, however, this byproduct reaction is more difficult to control.

#The objects, features and advantages of the invention will become apparent upon consideration of the following detailed disclosure of the invention, especially when it is taken in conjunction with the accompanying drawing wherein:

#FIG. 1 is a diagrammatic exploded view of the electrolytic cell in which the coated separator of the present invention is used, showing the electrolyte flow paths and the ion flow paths.

#As seen in the exploded and partially diagrammatic illustration in FIG. 1, a filter press membrane electrolytic cell, indicated generally by the numeral 10, is shown consisting of an anode backplate 11, separator means 21, cation selective membrane 25, a porous cathode plate 26, and a cathode backplate 28.

#The anode backplate 11 and cathode backplate 28 form the opposing sides of the bipolar electrode, which can be machined from a stainless steel plate or can be cast stainless steel. The stainless steel plate can, for example, be formed of 304L or 316 stainless steel as thick as 11/4" which is resistant to corrosion and is simply fabricated by machining the flat plate to create chambers through which the anolyte and catholyte fluids can pass into their respective anolyte and catholyte chambers. The thickness of the stainless steel plate provides stiffness and an extremely precise flatness to the structure. The cathode plate 26 is mounted to the cathode backplate 28 by screws (not shown) which are screwed into cathode support pedestals 31, while the anode rods 12 may be welded, such as by TIG welding, in place without warping the stainless steel plate.

#The anode structure includes a plurality of parallel positioned, vertically extending anode rods 12 welded at the top and bottom portions of the rods to the anode backplate 11. These rods 12 extend across the entire width of the anode backplate 11. These rods are, for example, 1/8" diameter nickel wire rods spaced apart from each other to create an anode inter-rod gap (not shown) of approximately 1/16" between adjacent rods. These anode rods 12 can be formed from nickel 200, or any other corrosion resistant composition providing low overvoltage characteristics. The vertical positioning of the anode rods 12 with the anode inter-rod gap provides clear flow channels from the bottom of the anode backplate 11, where the anolyte fluid enters via anolyte entry ports 18 into an anolyte distribution groove (not shown), to the top. Anolyte fluid flows vertically upwardly in the anode inter-rod gaps to the anolyte collection groove (not shown), before the liquid exits the cell through the anolyte exit ports 19. The vertical positioning of the anode rods 12 provides even current distribution across the anode and avoids gas blinding that can occur from the buildup of gas bubbles, which can consequently reduce the current density in the operating cell.