Study On The Mechanism Of Stickies Deposition And Its Control In Secondary Fibers Recycling

The recycled fibers have become an important raw material for papermaking, and more and more deinked pulps were used in many newsprint mills in recent years. A lot of contaminants such as pressure-sensitive adhesive, hot melt adhesive, coating adhesive, ink binding adhesive from waste paper as well as the lipophilic wood resins are released and dispersed in the pulp suspension during waste paper recycling. The contaminants could form the deposit on the forming wire, press section and dryers or the surface of paper sheets, which was harmful to the production efficiency and the product quality. The deleterious stickies problem became more and more serious due to more and more secondary fibers recycling in paper mills.The deposit obtained from a paper mill was analyzed and the origin of the deposit was determined. Some substances were chosen as the model substance to study the factors that influence the stickies deposit and the interactions of the components in the deposit. Basing on the understanding of the mechanism of stickies deposition, some novel methods for the control of stickies deposit were put forward.According to the analysis of the deposit from the pressurized screen before the wire section in Nanping Paper Mill, polyvinyl acetate (PVAc), polyacrylate (PA) and the wood lipophilic extractives were taken as the representatives of the deposit, but their content in the deposit was only about 5%. Most of the substances in the deposit were the nonsticky calcium carbonate and fines that could adhere to the sticky contaminants and deposit together, resulting in a great deal of the deposit.Tall oil and a pressure-sensitive adhesive whose component was polymethacrylate wererespectively chosen as the model substances of the wood resin and the synthetic adhesives to study the factors influencing the pitch deposit and stickies deposit. The results showed that calcium ions concentration had a significant effect on the pitch deposit. The pitch deposit was correlative with both the amount of the calcium soaps and the degree of dispersion of the calcium soaps. The fines adsorbed more pitch/calcium soaps particles compared with the fibers, and the adsorption of pitch/calcium soaps particles on the fines and fibers prevented them from further aggregation and deposition.Cationic polymers addition was one of the significant causes that led to destabilization of secondary stickies and the stickies deposit. Cationic polyacrylamide(CPAM) and cationic starch led to the destabilization of secondary stickies by interparticle bridging mechanism and polyethylene imine (PEI) led to the destabilization of secondary stickies by charge patch mechanism. Both CPAM and cationic starch resulted in more stickies deposit as the secondarystickies was partly destabilized at low level addition, but PEI resulted in more deposit as the secondary stickies was nearly completely destabilized. The stickies deposit decreased with excessive cationic polymer addition because of the adhesion of the destabilized stickies to the fibers of but not the restabilization of the secondary stickies. In addition, high concentration of calcium ions exacerbated the stickies deposit.The fillers such as heavy calcium carbonate, precipitated calcium carbonate and talc could led to the increase of the deposit because they could adhere to the sticky stickies and deposit together with them, especially as the fillers and the destabilized secondary stickies were not well adsorbed to the fibers at low cationic polymers addition. The increased amount of stickies deposit was much more than that of fillers in the deposit after fillers addition. The effect of different fillers on the stickies deposit was different. Comparing with talc, precipitated calcium carbonate addition resulted in less deposit at high dosage of cationic polymers.The effect of DCS and DS in the mechanical pulp on the stabilization and the stickies deposit was studied. The results showed that the synergism of anionic trash from mechanical pulp and the stickies with high stickiness and high concentration calcium ions from the deinked pulp was the cause that led to the more deposit as the furnish of mixed DIP and mechanical pulp was used instead of only DIP or mechanical pulp. More anionic trash led to the lower efficiency of cationic polymers and high stickiness contaminants from waste paper tended to aggregate with other contaminants to deposit. In addition, high concentration of calcium ions could exacerbate the stickies deposit because they could lead to the destabilization of stickies and decreased the efficiency of the cationic polymers simultaneously.DCS especially DS from the mechanical pulp decreased the retention efficiency of CPAM, and the higher fines retention was found after the enzymatic degradation of pectic acid that was the main substance causing anionic trash. The increase of cationic polymers efficiency could reduce the stickies deposit and the treatment of DCS or the pulp (mechanical pulp or DIP) with pectinase was an effective method for the stickies control.According to the influencing factors and the mechanism of the stickies deposit, some methods for control of stickies deposit were put forward.(1) The concentration of calcium ions in pulp suspension should be controlled. The pulp should be properly refined to a higher beating degree and higher content of fines, and the retention of fines and fillers should be increased.(2) The addition of ultrafine talc in the pulp suspension would reduce the stickiness of pitch so as to achieve the control of pitch deposit(3) The cationic polymers with high charge density and low molecular weight addition could neutralize the anionic trash in the pulp suspension and enhance the efficiency of retention aids to reduce the stickies deposit.