GB733100A - Permselective membranes, their manufacture and use in electrodialysis - Google Patents

Abstract

Semi-permeable selective membranes are made of high molecular weight organic substances in which hydrogen atoms of reactive hydroxyl groups have been at least partly replaced by side chains of the formula <FORM:0733100/IV (a)/1> in which R represents an organic group containing at least one ionic group, i.e. an ionisable radical capable of dissociating in aqueous medium so that an electrically charged radical remains linked to the high molecular weight substance. A membrane of cellulose such as paper, transparent paper or regenerated cellulose, or of regenerated amylose, polyvinyl alcohol or similar high molecular weight substance with reactive hydroxyl groups may be reacted with a compound containing at least one N-methylol acid amide group with at least one ionic group or with a group which can readily be replaced by an ionic group in a second processing stage. A N-methylol compound of an acid amide wherein at least one hydrogen atom bound to a carbon atom has been replaced by halogen or by a trialkylamino, dialkylamino or sulphonic acid group may be used for introducing the desired groups into the substance of the membrane. For producing a positive membrane, the methylol compound of carbamidomethyl-trimethyl ammonium chloride or the methylol compound of chloro-acetamide may be used. For the production of a negative membrane from the compound thus obtained, the chlorine atom may be replaced by a sulphonic acid group, e.g. by the action of potassium sulphite. If the chlorine atom is replaced by dimethylamine, a positive membrane is obtained and the positive character may further be intensified by the action of methyl iodide. The membranes obtained preserve their selectivity in relatively strong salt concentrations in a liquid to be desalted and in the rinsing liquids. The selectivity of the membranes may be measured by determination of the concentration potential between solutions of potassium chloride of different concentrations separated by the membrane. A concentration potential of 55 mV, as compared with an ideal value of 58 mV, may be obtained when a membrane into which quaternary ammonium groups have been introduced is arranged as a partition between a 0.001 N KCl and a 0.01 N KCl solution. To reduce the swelling of the membranes in aqueous solutions, they may be reacted with compounds such as formaldehyde or the dimethylol compound of adipamide so as to form non-ionic bridge-bonds. Alternatively bridgebonds containing at least one ionic group may be formed, e.g. the cellulose or other membrane may be reacted with the methylol compound of a ,a 1-dibromoadipic diamide. In an example, a ,a 1 - bisdimethylamino - adipamide hydrochloride is dissolved in an aqueous solution of formaldehyde and then caustic soda is added. After one hour, oxalic acid is added and the precipitated sodium oxalate is filtered off. A regenerated cellulose membrane is soaked in the solution, dried, and then heated at 100 DEG C. In another example, a membrane of polyvinyl alcohol is impregnated with a solution obtained by heating a solution containing carbamido-methyl-trimethyl-ammonium chloride and adipamide with formaldehyde and potassium carbonate and subsequently acidifying with oxalic acid and hydrochloric acid to a pH of 2.0. The impregnated membrane is dried and then heated at 140 DEG C. for 10 minutes. The membranes have good ion-selective properties with respect to polyvalent ions and may be used for desalting brackish water containing magnesium sulphate. In an example, sea-water with a NaCl content of 30 gm./litre is placed in a three-chamber apparatus in which the anode compartment is separated from the dialysis compartment by a positive ion-selective membrane of regenerated cellulose which has been combined with the methylol compound of carbamidomethyl - trimethyl - ammonium chloride, and in which the cathode compartment is separated from the dialysis compartment by a negative ion-selective membrane of regenerated cellulose which has been combined with the methylol compound of a ,a 1-adipamide-disulphonic acid. The dialysis may be carried out at current densities of 2.5-25 m.Amp./sq. cm.ALSO:Semi-permeable selective membranes are made of high molecular weight organic substances in which hydrogen atoms of reactive hydroxyl groups have at least been partly replaced by side chains of the formula <FORM:0733100/IV (b)/1> in which R represents an organic group containing at least one ionic group, i.e. an ionizable radical capable of dissociating in aqueous medium so that an electrically charged radical remains linked to the high molecular weight substance. A membrane of regenerated amylose, or similar high molecular weight substance with reactive hydroxyl groups may be reacted with a compound containing at least one N-methylol acid amide group with at least one ionic group or with a group which can readily be replaced by an ionic group in a second processing stage. A N-methylol compound of an acid amide wherein at least one hydrogen atom bound to a carbon atom has been replaced by halogen or by a trialkylamino, dialkylamino, or sulphonic acid group may be used for introducing the desired groups into the substance of the membrane. For producing a positive membrane, the methylol compound of carbamidomethyl-trimethyl ammonium chloride or the methylol compound of chloro-acetamide may be used. For the production of a negative membrane from the compound thus obtained, the chlorine atom may be replaced by a sulphonic acid group, e.g. by the action of potassium sulphite. If the chlorine atom is replaced by dimethylamine, a positive membrane is obtained and the positive character may further be intensified by the action of methyl iodide. The membranes obtained preserve their selectivity in relatively strong salt concentrations in a liquid to be desalted and in the rinsing liquids. To reduce the swelling of the membranes in aqueous solutions, they may be reacted with compounds such as formaldehyde or the dimethylol compound of adipamide so as to form non-ionic bridge-bonds. Alternatively bridge-bonds containing at least one ionic group may be formed, e.g. the amylose membrane may be reacted with the methylol compound of a ,a -dibromoadipic diamide. In an example, a membrane of amylose is impregnated with a solution obtained by heating an aqueous solution containing carbamidomethyl-trimethyl-ammonium chloride, formaldehyde, and potassium carbonate and subsequently acidifying with oxalic acid to a pH of 2.0. The impregnated membrane is dried and then heated in air at 140 DEG C. for 10 minutes.