Study On The Effect Of Fixatives In Papermaking System With High Conductivity

Closing whitewater system is widely adopted in morden papermaking systems in order to save fresh water and raw materials. However, it raises issues such as build-up of the dissolved and colloidal substances (DCS) including inorganic ions. The system with high amount of DCS and high conductivity is formed thereby, which deteriorates the machine runnability and the paper quality, and increases production cost. Fixing agents is widely used to control DCS by fixing it onto fibers and take them out of system with the result paper. Under high conductivity conditions, it is well-known that that the effect of retention aids will be reduced since they have long molecular chains which are easily affected by high conductivity. Compared to retention aids, fixing agents have higher charge densities and lower molecular weights, therefore it can be expected to have different performances with those of retention aids under high conductive conditions. This phenomenon is worthy of being confirmed.Therefore, in this stusy, we selected four fixing agents, polyamine (PA), polyethyleneimine (PEI) polydiallyldimethylammonium chloride (PDADMAC) and starched-based fixative (SBF) to study their performances under high and low conductivity conditions. Firstly, the intrinsic viscosities of the fixing agents were investigated under high conductivity conditions formed by Na+and Ca2+respectively. It was found that the intrinsic viscosities of PA, PDADMAC and SBF were decreased under high conductivity, while that of PEI was maintained.Then, the effects of fixing agents were investigated by measuring the turbidity, colourity, suspend solid content, particle size, cationic demand and chemical oxygen demand (COD) under both high and low conductivity conditions in whitewater and pulp system. The results showed that under high conductivity formed by Na+, the effects of PA and PDADMAC decreased in both whitewater and pulp system; the effect of SBF decreased when it was used in whitewater, but increased in pulp system, it can be attributed to the hydrogen bond between SBF and fibers; the effect of PEI was almost not influenced by conductivity. But under high conductivity conditions formed by Ca2+, the effects of all fixing agents were increased because of the double electric layer of the small particles was condensed by Ca2+, which lead to the destabilization of these particles, and cause easier agglomeration.At last, the promotion effects of fixing agent on the retention effect of cationic polyacrylamide (CPAM) were studied. It was shown that the above four kinds of fixing agents were still able to improve the retention effect of CPAM under such conditions. To sum up, in this study, the influences of Na+and Ca2+on the effects of fixing agent used both in whitewater and pulp system were studied in detail. The results will be helpful in applying fixing agents in paper mills and be beneficial in improving the clean production technics for papermaking.