Removal of Hardening Salts by Nanofiltration Composite Membranes

Author Name(s): Dinar D. Fazullin, Gennady V. Mavrin, Ilnar A. Nasyrov, Leysan I. Fazullina
Author Email: denr3@yandex.ru

Abstract

For removal of hardness salts, composite nanofiltration membranes with a “nylon-PS” polystyrene (PS) surface layer, a cellulose acetate (AC) surface layer on a nylon “nylon-AC” substrate, and a cellulose acetate surface layer on a polytetrafluoroethylene (PTFE) substrate were obtained “PTFE-AC”. To increase the retention capacity, several layers of polymer particles were applied to the surface of the substrate. The content of the composite membrane layer ranged from 9.2 to 18.6% by weight, depending on the number of layers. To stabilize the surface layer, the membrane was finally to heat treatment. According to scanning electron microscopy data, it was established that spherical polystyrene particles with sizes up to 10 μm are located on the surface and in the pores of the substrate. And the cellulose acetate particles of cellulose are located in the pores of the PTFE substrate. A decrease in the specific performance of the membranes with an increase in the number of surface layers has been established. The specific productivity of the dynamic membrane was 369 dm3 / m2 ∙ h, and for composite membranes with a surface layer of cellulose acetate higher than 2 times: 631 – 755 dm3 / m2 ∙ h at an operating pressure of 0.2 MPa, which is 3 to 7 times higher than with industrial nanofiltration membrane. The “PTFE-AC” and “nylon-PS” membranes showed high retention capacity in terms of overall hardness: 76.3% and 73.7%, respectively.

Introduction

Water hardness is a combination of chemical and physical properties of water associated with the content of dissolved salts of alkaline earth metals in it, mainly calcium and magnesium. Ions of calcium (Ca2+) and magnesium (Mg2+), as well as other alkaline earth metals, are present in all mineralized waters. Their source is natural deposits of limestone, gypsum, and dolomites. Hard water forms scale on the walls of heating boilers, batteries, which significantly impairs their thermal performance. Scale accounts for 90% of failures in water heating equipment. Therefore, water subjected to heating in boilers, boilers, etc. higher rigidity requirements are imposed. A thin layer of scale on the heating surface is not at all harmless, since the duration of heating through a scale layer with low thermal conductivity gradually increases, the bottom burns out faster and faster – after all, the metal cools more slowly and slowly each time, it remains in a heated state for a long time. In the end, it may happen that the bottom of the vessel does not survive and begins to leak. This fact is very dangerous in the industry where steam boilers are used.

Conclusion

Water with a high value of total hardness is not very suitable for technological needs and requires the removal of hardness salts. Of the membrane processes for the removal of hardness salts, the most effective is the process of nanofiltration. The retention capacity of the obtained composite membranes in terms of total hardness was 73-76%, with specific productivity of 370-755 dm3/ m2∙ h, which allows the use of these membranes for preliminary water treatment before the reverse osmosis process to obtain demineralized water.

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