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|You are here: Home > Technique > Processes > Scientific report of the LGP2 > Papermaking and environment engineering > Interdependance of fibre flocculation and fine elements retention in a papermaking suspension||Update: July 20, 2011|
|Scientific report of the LGP2 (2006-2009)|
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|Researchers of the LGP2
This phD work has been done in the framework of an industrial research partnership (CIFRE grant) between the LGP2 and Centre Technique du Papier (CTP). Two departments of LGP2 have been involved in this study (namely "Papermaking & Environment Engineering" and "Paper Physics"), together with researchers from CTP (industry oriented) and the department of Chemical Engineering at the University of British Columbia in Vancouver (UBC), offering a unique opportunity to link basic approach to more applied results.
Sheet formation on the papermachine is mainly determined by fibre flocculation in the short circulation. Industrial furnishes consist of a mixture of cellulosic fibres, cellulosic fines, mineral filler, and chemical additives. All these elements undergo mechanical, as well as physico-chemical interactions in the suspension. Retention aids are necessary to retain fine elements in the fibre mat. These retention aids are efficient but not selective: they help attaching fine elements to fibres but also cause additional fibre flocculation, which is detrimental to optical and mechanical properties of the sheet. The objective of this study is to find strategies to minimise fibre flocculation, while ensuring an acceptable fine elements retention.
Firstly, we tried to describe "mechanical" fibre flocculation, without retention aids. Under those circumstances, flocculation is determined by fibre crowding (non-dimensional concentration accounting for fibre morphology) together with turbulence. Two methods are presented to calculate the crowding factor for suspensions having a distribution of fibre length. Then we used this approach to model experimental fibre flocculation data. The influence of flow speed and concentration on the flocculation of fibre furnish were studied (using the experimental setup shown in Figure 1. An expression, including the crowding factor and the flow speed was proposed to describe fibre flocculation [Figure 2]. These findings confirm the relevance of the crowding factor concept as a tool for fibre flocculation prediction. Over the same range of conditions, it was found that floc size distribution can be characterized by a Weibull law (even in the presence of filler and/or retention aids). Then corresponding sheet formation can be calculated using Farnood et al.'s model. It is shown that the flocculation/formation relationship is not expected to be linear : flocculation mainly affects formation in conditions of low fibre flocculation. Also, the calculated formation is mostly sensitive to flocculation at an inspection size of about 3 mm [Figure 3].
|Figure 1 - Experimental set-up||Figure 2 - Flocculation index prediction including the crowding factor
and the flow speed
Secondly, the impact of retention aids on fibre flocculation was evaluated, using furnishes typical of fine paper grades containing mineral fillers. Five retention systems were compared in terms of their relative flocculation/retention performance, namely: ATC+CS, ATC+CS+ASI, ATC+ASI+CS, ATC+CS+APAM and ATC+CS+CPAM (where ATC=fixing agent, CS=cationic starch, ASI=microparticle, APAM = anionic polyacrylamide and CPAM=cationic polyacrylamide). It was found that the studied retention systems exhibit different fibre flocculation responses. Microparticle system ATC+CS+ASI has the lowest sensitivity to fibre flocculation (desirable for fine paper grades). Nevertheless this interesting feature is obtained at the expense of cost. Polyelectrolyte system ATC+CS+APAM appears to be very cost-effective [Figure 4]. However, when advantageous conditions are sought for drainage, the flocculation/retention balance is seriously impaired, and the operating cost doubles.
|Figure 3 - Flocculation formation sensitivity
(in dark : highest sensitivity)
|Figure 4 - Cost/flocculation efficiency for different studied retention systems|
Eventually, to transfer the results to the short circulation, we modeled fine elements build-up in the circuits due to process waters recirculation. The influence of additional fines elements on fibre flocculation was investigated up to a level of 40 % of fibre mass. It is estimated, through a simple model of the short circulation, that such a level of fine elements can be observed in the headbox of a papermachine with a fine elements retention of 63 %. Fibre flocculation measurements on a pilot scale flow loop show that fine elements, consisting of a mixture of 1/3 fines and 2/3 filler, do not mechanically interact with fibres. However, they are found to drastically promote fibre flocculation in the presence of a retention system consisting of cationic starch and anionic polyacrylamide. It is proposed that fine elements can flocculate fibres by bridging. These findings show that sheet formation is likely to be affected by the level of retention through recirculation of fine elements in the white water. Consequently, maintaining a high retention is instrumental in optimising sheet formation, among other benefits.
The proposed fibre flocculation measurement technique, based on floc size distribution and virtually insensitive to light diffusion by filler, has proven to be very useful for studying industrial furnishes. Depending on the type of retention system, significant differences in fibre flocculation response were identified. This study validates some common industrial practice and offers a method to choose the most appropriate retention programme, depending on the desired flocculation/retention/drainage balance.
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