Changes of physicochemical and transport characteristics of ion exchange membranes in the process of operation under demineralization of wastewater production of nitrogen-containing mineral fertilizers

Abstract

Physicochemical and transport characteristics of heterogeneous cation-exchange (Ralex CM (H)- Pes) and anion-exchange (Ralex AM (H)-Pes) membranes differing in time of using in an industrial electro- dialysis vessel under desalting and concentration of waste ammonium and nitrate-containing water (condi- tioned membranes, membranes after one and six years of use) were investigated. The strength characteristics of the test samples were measured by the RMI-60 discontinuous machine, the density of ion-exchange mem- branes was determined by the pycnometric method. The estimation of the conducting properties of mem- branes was carried out by the contact-difference method for solutions of ammonium nitrate in the concentra- tion range 0.01-0.3 mol/dm³ at 1 kHz frequency of an alternating current. The diffusion permeability of the investigated ion-exchange membranes was measured in a flow cell for a sodium chloride / membrane / water system. An analysis of the obtained results showed that partial destruction of the heterogeneous ion-exchange membranes is observed at their prolonged use in the electrodialysis apparatus, and such destruction leads to an increase in the diffusion permeability and electrical conductivity, however at the same time their total ex-change capacity decreases. In the electrodialysis of a solution of ammonium nitrate with the concentration of 0.01 mol/dm³, the fluxes through the anion exchange membrane are much less than fluxes through the cation exchange membrane, since insoluble precipitation forms in the anion exchange membrane. Electrodialysis with a current density exceeding the limiting value is accompanied by irreversible dissociation of water mo- lecules at the interface with the solution, and the ion-exchange membrane starts to be freed from precipita- tion, as a result of which its electrochemical regeneration takes place. The obtained results testify to insignifi- cant deterioration of operational characteristics of ion-exchange membranes in the course of electrodialysis, which proves the prospects of its use for purification of waste nitrogen-containing water produced during the production of mineral fertilizers.