Ion exchange resins have wide ranging laboratory and industrial applications. Ideally the resins should retain their exchange capacities and efficiencies for extended periods to be cost effective. Fortunately it is possible to regenerate the resin beds at predetermined intervals on the basis of online monitoring of parameters such as electrical conductivity and pH of output streams.
Regeneration involves three basic steps, namely, removal of solid deposits, removal of displaced ions with a concentrated solution of replacement ions and rinsing and flushing the resin bed.
Ion exchange resins suffer from loss in performance due to several reasons. The present article discusses some of the common factors that lead to such performance deterioration and suggests remedial measures.
Fouling of Ion Exchange Resins
An acid is commonly used for determining demineralization .Hard water can contain high levels of calcium which can form insoluble precipitate with sulphuric acid. This can lead to scale buildup and blocking of pipelines. In such situations replace the regenerating sulphuric acid with hydrochloric acid.
Underground supplies of water are mostly polluted with soluble ferrous ions which in contact with oxygen present in air get oxidized to ferric state. In this state it gets precipitated as insoluble ferric hydroxide which can clog the resin beds. It can be removed by introduction of a solution of sodium tripolyphosphate or sodium dithionite.Iron levels should be kept below 0.1 ppm in the feed water.
Organic matter is a common cause of water contamination. Such matter can adversely affect the exchange capacity of the resin beds. Such organics are removed prior to demineralization by flocculation with alum and ferric salts. Subsequent filtration removes the metal hydroxide floc and the co-precipitated organic compounds.
Resin beds should not be considered as filter media for bacteria and organics. Traces of organic matter often act as nutrients for bacteria and sustain their growth.This further contributes to the clogging of filter beds. Disinfectants such as peracetic acid and formaldehyde are effective in removal of bacterial contamination.
Chlorine treatment becomes necessary to eliminate microbial infections in drinking water. However, chlorine is known to damage resins. Dissolved chlorine is removed by passing chlorinated water through activated charcoal beds.
Resins need to be regenerated when the ions in the resin are fully replaced by ions present in solution. Continuous monitoring of outlet streams can automate initiation of regeneration cycles. Service runs between cycles can range from usually 12 – 48 hours. Carefully maintained resin beds can retain their exchange capacity for months if not years.