Evaluation of the Flocculation and Reflocculation Performance of a

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Evaluation of the Flocculation and Re-Flocculation Performance of a System with Calcium Carbonate, Cationic Acrylamide Co-Polymers and Bentonite Microparticles Elisabete Antunes, Fernando Garcia, Angeles M Blanco, Carlos Negro, and M. G. Rasteiro Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 17 Dec 2014 Downloaded from http://pubs.acs.org on December 18, 2014

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Industrial & Engineering Chemistry Research

Evaluation of the Flocculation and ReFlocculation Performance of a System with Calcium Carbonate, Cationic Acrylamide CoPolymers and Bentonite Microparticles E. Antunesa, F. A. P. Garciaa, A. Blancob, C. Negrob and M. G. Rasteiroa,* a

CIEPQPF, Chemical Engineering Department, Coimbra University, Pólo II, Pinhal de Marrocos, 3030-790 Coimbra, Portugal c

Chemical Engineering Department, Faculty of Chemistry, Complutense University, 28040 Madrid, Spain

* Corresponding author: Tel: +351239798700; Fax: +351239798703. e-mail: [email protected]

ABSTRACT Flocculation, flocs resistance and reflocculation capacity of a PCC suspension with CPAMs varying in charge density and degree of branching were investigated in previous studies and correlated with retention and drainage performance in papermaking. In this study, the re-flocculation performance of the PCC suspension was investigated using the same C-PAMs in combination with microparticles, since microparticle retention systems have been widely used in papermaking to improve re-flocculation of the suspension. Moreover, flocculation and re-flocculation performance evaluated using the LDS technique was compared with that obtained using FBRM to monitor the process. The coupling of these two techniques allows a detailed analysis of the re-flocculation process, including evaluation of the flocs structure before and after the re-flocculation stage.

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KEYWORDS Flocculation;

re-flocculation;

LDS;

bentonite;

papermaking;

polyacrylamide;

polyelectrolyte 1. INTRODUCTION In papermaking, the evaluation of the flocculation process is of great importance to control the wet-end stage because both process efficiency (e.g. retention, drainage and runnability) and the quality of the final product (e.g. formation, strength and porosity) depend on the flocculation extent and on the flocs characteristics1-6. Studies on the wet-end chemistry have established that depending on the retention aid systems used, aggregation of the particles can occur by charge neutralization, patching, bridging or complex flocculation mechanism1,4. When polyelectrolytes are used to induce aggregation of a suspension, the flocs strength and the reflocculation capacity depend on the predominant flocculation process3,7-11. Studies of flocs strength based on micromechanics have shown that this property is also dependent on the shear rate prevailing during the flocculation process, which conditions the structure of the aggregates12,13. The stronger the flocs are initially the more difficult is reflocculation when the aggregate breaks14. Of course higher shear rates are needed when dealing with more resistant flocs and, when the shear force increases, the tails and loops of high molecular weight polymers are broken. Therefore, when the shear force decreases thereafter, the possibility of reflocculation by bridging decreases and reflocculation takes place rather through the patch mechanism3,14. In the case of patching, the effect of shear forces on the polymer is lower but, if the polymer is re-conformed within the diffuse layer, the


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interactions with other particles decrease. Hence, re-flocculation, though easier, may also happen to a lower extent than the original flocculation degree15. It is essential to produce flocs resistant to high shear forces because, in papermaking, too small flocs can reduce retention of fines and filler particles and reduce dewatering ability. To reduce the effect of shear on flocs size, microparticle retention additives have been widely used in papermaking since the microparticles help reflocculation of the suspension. In fact, previous studies12,13 have shown that flocs resulting from re-attachment of previously broken flocs produced only with PCC and polymer are very weak. The microparticle system is a type of dual retention aid in which highly anionic submicron particles (montmorillonite or colloidal silica) are used along with a cationic polymer such as polyacrylamide or starch. The cationic flocculant is generally added first causing particles’ aggregation. Then, the flocs formed are broken during a shear stage and the microparticles are added afterwards to induce reflocculation of the system. The reflocculated flocs formed are smaller and denser than the original ones16-18. The advantages of the microparticle system are numerous and well reported in literature. Several authors1,19-21 demonstrated that the re-flocculation capacity of microparticle aids significantly improved fines and filler retention induced by cationic polyacrylamides. In the same way, other authors have demonstrated that these systems improve simultaneously retention and drainage without overflocculation (formation of too large flocs) which can damage sheet formation6,20-22. In previous studies, it was shown that the branched structure of cationic polyacrylamides affects the flocculation process and the flocs structure10,23,24. In this case, the flocs obtained are small with an open structure resulting in the the improvement of the performance of these polymers as retention aid when comparing to


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the traditional linear polymers6,25. However, when using these polymers, after a shearing stage the reflocculation capability of the broken flocs is very small because aggregation takes place mainly by the bridging mechanism. It is, therefore, interesting to evaluate the reflocculation performance of these branched polyelectrolytes combined with a microparticulate system. In this study, the re-flocculation performance of both linear and branched polymers, in combination with microparticles, was investigated using two techniques: laser diffraction spectroscopy (LDS) and focused beam reflectance microscopy (FBRM). The results obtained with these two complementary techniques will be compared. 2. EXPERIMENTAL 2.1 MATERIALS In order to perform the flocculation tests, a commercial scalenohedral PCC suspension, supplied by OMYA, was used in this study. The PCC suspensions were prepared at 1 % (w/w) in distilled water and, in order to obtain a good dispersion of the particles, the suspensions were first magnetically stirred for 20 minutes and then submitted to sonication at 50 kHz during 15 minutes. The PCC suspensions were prepared daily. After this treatment, the median size of the particles, as obtained by LDS, was approximately 0.5 µm and the suspension pH 7.5. The zeta potential of the particles was -30 mV in distilled water. Four new cationic polyacrylamides (C-PAM), emulsions of very high molecular weight, developed and supplied by AQUA+TECH, were used in this study. The main characteristics of the polyelectrolytes used are summarized in Table 1. The polymer


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content of the emulsions was approximately 40% (w/w). The cationic monomer in all the polymers is dimethylamino ethyl acrylate. Flocculant solutions were prepared with distilled water at 0.1% (w/w). In order to guarantee the effectiveness of the flocculants, the diluted solutions have to be prepared everyday. The re-flocculation tests were performed with the addition of a microparticulate, bentonite. The median size of the bentonite particles, measured by LDS, was 5.7 µm (surface area based diameter) and the bentonite suspension was prepared at 2% (w/w) in distilled water. Bentonite was added, after flocs breakage, to the PCC suspension in a concentration of 2.5 mg/g (mg bentonite/g PCC) (based on industrial plant information). Stirring in the vessel, during the re-flocculation stage, was the same as during the initial flocculation step. 2.2 METHODS PCC flocculation was monitored by measuring the aggregates sizes by LDS using a Malvern Mastersizer 2000 (Malvern Instruments, UK)10,28-30, and by FBRM, Focused Beam Reflectance Microscopy, using the FBRM M500LF manufactured by Lasentec, USA. LDS measurements The PCC suspension was added to 700 mL of distilled water in the LDS equipment dispersion unit until 70% obscuration (average PCC concentration around 0.5% (w/w)) and the tests were carried out setting the pump speed to 1400 rpm (312 s1

). During the whole process the flocculation vessel was stirred mechanically using the

sample unit facilities of the Malvern Mastersizer 2000 equipment. Obscuration was always kept above 5% to assure a good signal quality10,23. Flocs sizes were measured every minute during 14 min. The floc resistance evaluation was performed using two


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different types of shear forces. The first approach was to submit the flocs to sonication during 30 seconds at 20 kHz. The second method involved the increase, during one minute, of the recirculating peristaltic pump speed from 1400 rpm to 2200 rpm, which corresponds to increasing the shear rate from 312 s-1 to 708 s-1. After both shearing tests, the shear force was restored to the initial value to allow the reflocculation process to take place, which was monitored during 5 minutes. Even if these shearing conditions are milder than the ones attained in a papermaking machine, they will allow us to evaluate the re-flocculation ability of the aggregates as a function of the additives characteristics. On the other hand, flocculation with the microparticulate system was performed for flocs broken up 30s after the flocculant addition and at the end of the full flocculation process (14 minutes) which corresponds to attaining a constant flocs size. In both cases, the bentonite suspension was added after flocs breakage and after the initial shearing was restored (312 s-1), re-flocculation having progressed afterwards during at least 7 minutes. Moreover, the mass fractal dimension and the scattering exponent of the reflocculated flocs were calculated at the end of the re-flocculation process9,29. This mass fractal dimension provides a mean of expressing the degree to which primary particles fill the space within the nominal volume occupied by an aggregate: for solid non-porous particles dF =3 and for porous particles 1

Evaluation of the Flocculation and Reflocculation Performance of a

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