The Research On Sulfate Leaching Properties Of Cation-exchange Resin Used In Power Plants

In the past few years, this problem that the concentration of SO₄^2- in steam generator (SG) water would increase after condensate polishing system had been put into operation had happened in several nuclear power plants at home and abroad. Research showed that the main reasons for high SO₄^2- content in SG water were SO₄^2- leaching of strong acidic cation-exchange resin and thermal degradation of broken cation-exchange resin which entered into thermodynamic system.Based on SO₄^2- leaching problem of strong acidic cation-exchange resin, the method of combining dynamic cycle dissolving-out and static soaking was used in research on SO₄^2- leaching properties of cation-exchange resin commonly used in condensate polishing system. Among them, dynamic dissolving-out experimental results showed that SO₄^2- dissolving-out quantity of cation-exchange resin increased with time prolonged, and individual resin appeared balance tendency (such as SP112H), and SO₄^2- dissolving-out velocity gradually decreased with time prolonged except 001×7. During the static soaking process, SO₄^2- dissolving-out quantity of cation-exchange resin also increased with time prolonged, and SO₄^2- total dissolving-out velocity appeared downtrend during long-term soaking (345d), although it had slightly fluctuated in short-term soaking period. By comparison and analysis of a series of leaching experiments, it was known that SO₄^2- leaching behaviors (including dissolving-out quantity and dissolving-out velocity) of different types cation-exchange resin were greatly different, and SO₄^2- dynamic cycle dissolving-out quantity and velocity were significantly more than static soaking, and SO₄^2- dissolving-out quantity of macroreticular cation-exchange resin was less than gel resin, and SO₄^2- dissolving-out quantity of spend resin was far less than new resin, and there was almost no SO₄^2- leaching in system of cation-exchange resin and action-exchange resin equivalent volume uniform mixing. Experimental results of high temperature and oxidized treatment showed that leachables of cation-exchange resin still contained several organic sulfosalt compounds except SO₄^2-, and they could continue to decompose and produce SO₄^2- at high temperature or oxidizing conditions. Furthermore, a large number of low-molecular-weight organic acids would have been produced after leachables had been treated at high temperature or oxidizing conditions.In order to understand SO₄^2- thermal degradation properties of cation-exchange resin, thermal degradation of cation-exchange resin has been carried out by autoclave experiments. Experimental results illustrated that feature of cation-exchange resin would obviously change after thermal degradation at 280℃and change have manifested that particle size minished and colour darkened. Contents of SO₄^2- generated by thermal degradation of cation-exchange resin increased with temperature rose, after temperature exceeded 200℃, contents of SO₄^2- would explosion and a great number of acidic materials would be produced. At high temperature of 280℃, contents of SO₄^2- increased with pyrolysis time prolonged, and contents of SO₄^2- produced by thermal degradation had relation to total exchange capacity of cation-exchange resin, and there was more SO₄^2- produced by thermal degradation if total exchange capacity of resin was large. Contents of SO₄^2- generated by thermal degradation of spend resin were slightly less than new resin, but spend resin could produce more acidic materials after thermal degradation. Furthermore, SO₄^2- determination results and infrared spectroscopy test results indicated that S200 resin almost had come up to complete decomposition after thermal degradation 24h at 280℃.All the experimental results obtained has led to the conclusion of leachables of cation-exchange resin and cation-exchange resin could generate SO₄^2- and acidic materials after they entered into thermodynamic system, and these SO₄^2- and acidic materials would seriously influence quality of water and steam, therefore they could induce corrosion of equipments and pipes of thermodynamic system. Consequently, it was suggested that strong acidic cation-exchange resin used in condensate polishing system should have less SO₄^2- leaching quantity, better strength of resistance to osmosis shock and better stability of resistance to oxidation. Furthermore, effective measures must be taken to prevent resin breaking and to remove broken resin in time.Finally, a mechanism of SO₄^2- leaching and thermal degradation of cation-exchange resin has been discussed. It was pointed out that SO₄^2- leaching process of cation-exchange resin was actually desulphonation reaction of benzenoid compounds in certain temperature environment. The mechanism that SO₄^2- was formed during resin's thermal degradation process was in fact irreversible"desulphonation"reaction, in other words, functional groups (—SO3H) of resin have dropped off from polystyrenic backbone because of the Carbon-Sulphur bond cleavage reaction.