The Effect Of Filling With Pre-Flocculated Lime Mud On Paper Properties

Lime mud coming from the alkaline recovery process of black liquor of a non-woodpulping process is a solid waste in the pulp and papermaking industry. Its main chemicalcomponent is calcium carbonate. The use of non-wood lime mud as a paper filler canpromote its resource utilization. However, some problems like poor alkyl ketene dimer(AKD)sizing efficiency or poor paper strength are caused when the lime mud is added to paper. Inorder to solve the above-mentioned problems, the lime mud was pre-flocculated with cationicpolyacrylamide(CPAM) or CPAM/cationic starch(CS)/AKD before it was added to thepaper stock for handsheet preparation in this study, and the corresponding effects of thepre-flocculation of lime mud on paper properties were studied.First, CPAM with different molecular weights and charge densities were used topre-flocculated the lime mud before it was added to the paper stock for handsheet preparation.The results showed that the particle size and Zeta potential of the filler were significantlyimproved after pre-flocculation. Compared to paper filled with native lime mud in traditionalprocess, the paper filled with pre-flocculated lime mud had better hydrophobicity, tensilestrength, burst strength, paper formation as well as filler retention, comparable opticalproperties and poorer air permeability. In addition, the 2-CPAM with a high molecular weightand low charge density was more effective in improving the hydrophobicity of handsheetsand filler retention when used as a filler flocculant, but less effective in the improvement ofpaper formation than 1-CPAM and 3-CPAM.Then, in order to simplify the process of the approach flow system, a multi-componentsystem of CPAM/CS/AKD was used to pre-flocculate the lime mud before filling. The effectsof pre-flocculation of lime mud with CPAM/CS/AKD on sizing and the mechanism were firststudied. It was found that the pre-flocculation of lime with CPAM/CS/AKD couldsignificantly lower the Cobb60 value and increase the contact angle of handsheets incomparison with native lime mud. What’s more, the sizing reversion of handsheets was alsoalleviated effectively. This was because the BET surface area and BJH pore volume ofpre-flocculated lime mud were lower than native lime mud and the Zeta potential was higherthan native lime mud, which led to a much less adsorption of AKD. Thus, the AKD sizingefficiency was significantly improved.At last, the effect of pre-flocculation of lime mud with CPAM/CS/AKD on fillerretention and paper properties were investigated. Compared to handsheets filled with nativelime mud, the filler retention, tensile strength, burst strength and paper formation ofhandsheets filled with pre-flocculated lime mud were evidently improved. The opticalproperties nearly remained the same while the air permeability decreased. The concentrationof the lime mud, the stirring speed, and the stirring time during the pre-flocculation processwere found to have a great influence on the paper formation. The optimized conditions ofpre-flocculation process were: 0.1% concentration of lime mud suspension, 500 rpm ofstirring speed, and 10 minutes of stirring time. By comparison of handsheets filled withCPAM and CPAM/CS/AKD pre-flocculated lime mud, it could be concluded that thepre-flocculation of lime mud with CPAM/CS/AKD was more promising in actual production.