DUANGKAMON BAOSUPEE , ASHLEY J. MASSEY MARTIN A. HUBBE · 2017-06-14 · 65 Experiments carried out in the presence of cellulosic fibres in aqueous suspension have 66 demonstrated

1 Author version. Citation: APPITA J. 63(3), 237-245.

1 Polyelectrolyte Coagulant and Flocculant Effects on Heteroagglomeration of Cellulosic 2 Fines and CaCO3 Particles 3 4

DUANGKAMON BAOSUPEE a, ASHLEY J. MASSEY b, MOUSA NAZHAD c and 5 MARTIN A. HUBBE b,* 6 7 a: Asian Inst. Technol., Klongluang, Pathumthani, Thailand 8 b: North Carolina State Univ., Department of Forest Biomaterials, Campus Box 8005, 9

Raleigh, NC 27695-8005, USA 10 c: University of British Columbia, Pulp and Paper Center, Vancouver, BC, Canada 11 12 ----------- 13 * Corresponding author. Tel. +1-919-513-3022 14

E-mail address: [email protected] (M.A. Hubbe) 15

----------- 16 17

18 Effects of a high-charge cationic polyelectrolyte (a coagulant), a very-high-mass cationic 19

polyelectrolyte (a flocculant), or the combination of a cationic coagulant followed by an 20

anionic flocculant were evaluated relative to the particle size distributions in suspensions 21

of cellulosic fines and CaCO3 particles. Laser diffraction particle size analysis for the 22

mixed suspension showed the virtual disappearance of signal corresponding to unattached 23

CaCO3 particles upon addition of cationic flocculant. Charge effects related to the 24

coagulant had less influence on the particle size distribution compared to the polymer 25

bridging effects of the flocculant. Microscopic images revealed differences in the 26

structure of agglomerates in the polymer-treated systems. The polymer-induced 27

attachments between CaCO3 and cellulosic fines can be interpreted as an additional stage 28

of heteroagglomeration, supplemental to those attachments that had already been formed 29

between CaCO3 and cellulose surfaces prior to addition of the polyelectrolytes. Resulting 30

structures depended not only on the polyelectrolytes but also on the characteristic shapes 31

of cellulosic fines and the fibrillation of their surfaces. 32

mailto:[email protected]


---------- 33

Key words: Cationic acrylamide copolymer; Heteroagglomeration; Cellulosic fines; 34

Calcium carbonate; Colloidal interactions; Particle size analysis 35

36

37

Introduction 38

In a previous article [1] the authors showed that, even in the absence of polyelectrolytes, 39

CaCO3 particles in aqueous suspension with cellulosic fines readily became agglomerated 40

in such a manner that the mineral became attached as clusters of particles on slender 41

fibrils of the cellulose. A laser diffraction method was used to evaluate changes in the 42

particle size distribution. When the proportion of CaCO3 was sufficiently high, the 43

particle size distributions showed maxima corresponding to each of the components of 44

the mixture. However, that work did not consider what happens in the presence of 45

polyelectrolyte coagulants and flocculants. Such macromolecules, which bring about 46

sticking collisions among various particles in aqueous suspensions, play an important role 47

in the manufacture of paper [2-5]. Cationic polyelectrolytes of moderate molecular mass 48

(about 10,000 to 500,000 g/mole) and high charge density are widely used as coagulants 49

to reduce or reverse the negative surface charge of typical cellulosic fibres, fine particles, 50

and related soluble anionic polymers present in a fibrous suspension used for the 51

preparation of paper [2]. Such treatment is understood to bring about agglomeration of 52

suspended particles either by neutralization of surface charges [6-8] or by attraction 53

between “charged patches” and uncovered surfaces of the respective particles or fibres [7, 54


9-12]. Likewise, polyelectrolytes of very high molecular mass (e.g. 3 to 20 million 55

g/mole) are widely used as bridging flocculants (“retention aids”) [4, 13]. 56

57

Often papermakers employ a combined approach in which treatment of the fibre 58

suspension with a coagulant is followed by treatment with a flocculant [14-15]. The 59

desired result of polyelectrolyte treatment includes holding fine particles onto the 60

surfaces of cellulosic fibres and/or agglomerating fine particles together as clusters, 61

giving a higher efficiency of retaining the fine particles as the fibres are being formed 62

into a sheet of paper. 63

64

Experiments carried out in the presence of cellulosic fibres in aqueous suspension have 65

demonstrated various effects of polyelectrolyte addition. For instance, Goossens and 66

Luner [9] and Horn and Melzer [16] showed that high-charged cationic polyelectrolytes 67

tended to maximise the state of agglomeration, as well as bringing about maximum 68

dewatering rates, at additional levels corresponding to an approximate net neutralization 69

of the surface charges in the system. It has also been noted in such studies that the 70

effectiveness of the cationic polymer treatment increases strongly with increasing 71

molecular mass [17-18]. Such behaviour has been explained in terms of the charged 72

patch mechanism [19-20]. Interaction between the covered and uncovered areas may 73

account for the more strongly accelerated dewatering and agglomeration observed in such 74

cases. 75

76


Recent articles have focused attention on interactions between cellulosic fines and other 77

finely divided materials such as calcium carbonate or clay filler particles. Liimatainen et 78

al. [21] studied the kinetics of CaCO3-cellulose interactions, as affected by cationic 79

polyelectrolytes. A turbidimeter was used in that work to evaluate the extent of retention 80

of CaCO3 on the cellulosic surfaces. Results were found to be consistent with a bridging 81

flocculation mechanism and a Langmuir adsorption isotherm. Results also were found to 82

depend on the relative surface area of the cellulose, such that there were differences when 83

comparing systems with primary (from unrefined pulp) and secondary (from refining of 84

pulp) cellulosic fines, as well as the fibre portion. Image analysis showed that self-85

agglomeration of the CaCO3 took place, in addition to bridging flocculation of CaCO3 86

particles onto the cellulosic surfaces. The already-cited work of Baosupee et al. [1] dealt 87

with a similar experimental situation, but with the innovative application of laser 88

diffraction particle size analysis to this system, as well as emphasis on the structure of the 89

agglomerates. However, no polyelectrolytes were considered in that part of the work. 90

Even in the absence of polyelectrolytes there was substantial adhesion in the wet state 91

between CaCO3 and cellulose surfaces, with preferential attachment to very thin fibrils 92

extending outwards from the cellulosic fine particles. 93

Laser diffraction particle size analysis also has been well demonstrated by Rasteiro et al. 94

in studies of the flocculation of CaCO3 particle agglomeration by cationic 95

polyelectrolytes [22-23]. For instance, results were found to depend on the degree of 96

branching of cationic flocculants having similar charge and molecular mass [23]. Related 97

studies also have been carried out using backscattered light [24-25]. A general finding 98


from these cited studies has been that the extent of flocculation depends on the type and 99

dosage of cationic flocculant, as well as the time of mixing. 100

101

Taking advantage of test protocols reported earlier [1], involving the use of laser 102

diffraction particle size analysis, the present article considers the effect of polyelectrolyte 103

treatments in fibre-free suspensions of cellulosic fines, with or without the further 104

addition of calcium carbonate particles. Stirred suspensions were treated with either a 105

cationic coagulant poly-diallyldimethylammonium chloride (poly-DADMAC), or a very 106

high mass cationic copolymer of acrylamide and a cationic monomer (cPAM flocculant), 107

and combinations of poly-DADMAC and a very high mass anionic copolymer of 108

acrylamide and acrylic acid (aPAM flocculant). In each case, attention was paid to the 109

particle size distribution and the microscopic appearance of agglomerates. 110

111

EXPERIMENTAL SYSTEM 112

Many aspects of the experimental system have been described earlier [1], for instance, 113

details of fractionating the pulp or refining followed by fractionation. Also, the 114

procedures used for particle size analysis and microscopy will not be repeated in as much 115

detail as earlier. 116

Materials 117

The cellulosic fines were isolated from bleached hardwood kraft pulp obtained from a 118

mill in the southeastern US [1]. Two classes of cellulosic fines were obtained for further 119

evaluation. “Primary fines,” were obtained from the unrefined pulp by use of the final 120

stage of a Bauer-McNett classifier fitted with a 200-mesh screen. The fibres within the 121


device were rinsed for at least 10 minutes to allow most of the fines to pass through the 122

screen openings and to be collected in a barrel, followed by over-night sedimentation and 123

collection. The primary fines thus obtained were further thickened by settling until a 124

suitable solids level in the range of 2-5% was obtained. 125

126

“Refined fines” were obtained by classifying whole bleached hardwood kraft pulp that 127

had been refined to 400 mL Canadian Standard Freeness (CSF) [1]. Such fines will 128

include not only parenchyma cells (i.e. refined primary fines) but also material released 129

from cells walls of fibres as a consequence of refining (i.e. secondary fines). These fines, 130

which had passed through the 200-mesh screen, were collected by settling in a barrel, as 131

already described. 132

133

Precipitated calcium carbonate (PCC, Albacar® 5970 from Specialty Minerals Co.) was 134

used as a representative filler component for papermaking. This product consists of 135

rosette-shaped (scalenohedral) particles having diameters of 2-3 micrometers. 136

137

Low-mass poly-DADMAC was Aldrich cat. no. 40,901-4, a solution polymer having a 138

nominal molecular mass of 100,000 to 200,000 Daltons. The very-high-mass cationic 139

acrylamide copolymer (cPAM) was Percol® 175 from Ciba Specialty Chemicals, having 140

a monomer molar content of 10% cationic groups; solutions having a concentration of 141

0.1% solids were freshly prepared from the dry-bead product, using deionized water and 142

one hour of gentle stirring. The very-high-mass anionic acrylamide copolymer (aPAM) 143

was similarly prepared from the dry product Floerger AN 934 (from SNF Floerger), 144


having a molar content of 30% anionic groups. The sodium sulfate and sodium 145

bicarbonate were of reagent grade. 146

147

Equipment 148

149

The size distributions of agglomerates were measured with a Horiba LA 300 particle size 150

analyzer. This approach had been shown earlier to provide useful information regarding 151

the state of agglomeration in the absence of polyelectrolytes [1]. A Lazer Zee 152

microelectrophoresis analyzer from PenKem was used to obtain zeta potential 153

information. Images of suspended and agglomerated samples were obtained with a light 154

microscope, Olympus BH2 UMA, using vertical illumination. 155

156

Tests of the State of Agglomeration of Cellulosic Fines – CaCO3 Mixtures 157

158

As in the earlier reported work [1], the filterable solids content was kept constant at 0.5% 159

by mass in pH 7 buffer solution (0.1 mM NaHCO3 added) of conductivity 1000 µS/cm 160

(using sodium sulfate). Impeller stirring was done at ~ 400 rpm. 161

162

The fines and fillers suspension were mixed in the mass proportions of: 100% cellulosic 163

fines, 50% fines and 50% PCC, 20% fines and 80% PCC, and 100% PCC. Cationic 164

polyelectrolytes were added at the same stirring speed as 1% (poly-DADMAC) or 0.1% 165

(polyacrylamide flocculants) solutions, where the additions were calculated as the dry 166

mass of polyelectrolyte on a given dry mass of suspended material. Mixing was carried 167


out under gentle impeller stirring (approx. 400 revolutions per minute), with subsequent 168

analysis (particle size tests or microscopy) carried out within several minutes. 169

170

The size distributions and appearance resulting from mixing of fines and fillers at various 171

mass ratios in the suspension, followed by polyelectrolyte treatments, were monitored by 172

measuring the aggregate sizes by laser diffraction scattering (LDS) and microscopy, 173

using the equipment specified earlier. For the microscopy tests, droplets of suspension 174

were placed on a glass slide, followed by a glass cover slip, and the observations were 175

carried out within several minutes of preparation. Particle size distributions were 176

evaluated using a Horiba LA 300 device, for which the standard recirculating flow was 177

employed, and the optional ultrasonification was not used. Distributions were reported 178

on a mass basis. 179

180

RESULTS AND DISCUSSION 181

182

Effects of High-charge Cationic Coagulant 183

Zeta potential 184

As shown in Figure 1, by treating suspensions of either CaCO3 by itself or its combination 185

(in two different ratios) with cellulosic fines with increasing levels of poly-DADMAC, it 186

was possible to progressively reverse the zeta potential from negative to positive. As was 187

noted earlier [1], even though pure calcite CaCO3 can be expected to acquire a positive 188

surface charge when placed into pure water [26-28], commercially prepared CaCO3 189

particles often have a negative charge due to treatment with phosphate or other dispersants 190


[29]. The present analysis revealed the CaCO3 to have a weak negative charge when 191

dispersed in the neutral aqueous solution. The cellulosic fines likewise were negative in 192

charge in the absence of polyelectrolyte treatment. Regardless of the starting zeta potential, 193

The data in Figure 1 show that dosages in the range 0.015 to 0.04% poly-DADMAC on a 194

dry mass basis were sufficient to reverse the sign of charge of the suspended particles in 195

the present systems. When the added dosage was higher, the zeta potential of both the 196

CaCO3 and its combinations with the cellulosic fines exhibited positive zeta potential 197

values, depending on the CaCO3 proportion and the polymer dosage. Others have likewise 198

observed that strong adsorption of polyelectrolytes over-compensates the initial surface 199

charge and reverses the sign of zeta potential [16, 30]. 200

201

Figure 1. Average zeta potential of suspensions of precipitated calcium carbonate, or its 202

mixtures with primary cellulosic fines, as a function of the dosage of low-mass poly-203

DADMAC (dry mass on dry mass). 204

25

20

15

10

5

0

-5

-10

-15

-20

Ze

ta P

ote

nti

al (

mV

)

0 0.025 0.05 0.075

Low-mass Poly-DADMAC (%)

100% CaCO3

80/20 CaCO3/cellulose



Particle size distribution and zeta potential 205

As shown in Table 1, when the dosage of poly-DADMAC was varied the changes in 206

particle size distribution were minor or below the level of random variations in the system. 207

Based on preliminary work, the standard deviation of replicate measurements was 208

estimated to be about 5% of the mean value. This finding of no significant change in 209

particle size distribution is tentatively attributed to the fact that the CaCO3 already tended 210

to attach itself to cellulosic surfaces before polymer addition [1]. This prior attachment 211

may have diminished the potential influence of poly-DADMAC as a coagulant in the 212

present experiments. Another possible explanation that was considered was that the 213

hydrodynamic shear forces associated with impeller stirring might have redispersed some 214

of the coagulant-induced agglomerates, due to the relatively weak nature of van der Waals 215

forces and/or patch-wise electrostatic attractions [31]; such mechanisms will be considered 216

further in a companion article. 217

218

Table 1. Effect of poly-DADMAC addition on the average size of agglomerates (µm, mass 219

basis) in systems with CaCO3 or its combinations with bleached hardwood kraft primary 220

fines. 221

222

Poly-DADMAC dosage

(%)

0 0.025 0.05 0.075

CaCO3 alone (µm) 3 3 3 -

80/20

CaCO3/cellulose (µm)

42 43 44 -


50/50

CaCO3/cellulose (µm)

55 56 54 55

223

224

Microscopy 225

As shown in Figure 2, the microscopic images support the particle size distribution results 226

in showing relatively little change in the extent or the nature of agglomerates upon addition 227

of poly-DADMAC. In the absence of CaCO3 particles, the treatment of primary fines with 228

0.025% poly-DADMAC appeared to increase the extent of flocculation. The fact that no 229

significant enhancement of agglomeration was evident from the particle size analysis is 230

tentatively attributed to the continuous stirring and recirculation employed in those 231

measurements. Results of earlier work suggest that additional attachments formed between 232

cellulosic fines as a result of van der Waals forces or patch-like electrostatic attractions can 233

be more easily broken by hydrodynamic shear in comparison to attachments that are 234

reinforced by polymer bridging [12, 31]. The lack of other effects of poly-DADMAC 235

treatment is tentatively attributed, once again, to pre-existing attachments between CaCO3 236

particles and cellulosic fibrils even before addition of the cationic polymer. This was 237

despite the fact that the amount of poly-DADMAC employed was more than enough to 238

reverse the sign of zeta potential of the surfaces in some cases. If the system had been fully 239

in equilibrium, then one might have expected the CaCO3-cellulose attachments to be 240

broken once the zeta potential of all of the free surfaces had been switched to strongly 241

positive. However, the microscopic images show that such particles remained attached. 242

This finding supports a view that the initial attachments were somewhat permanent and 243


thus not susceptible to redispersion upon addition enough coagulant to induce charge-244

charge repulsion between individually suspended particles. 245

246

247

Figure 2. Light-microscopic images of cellulosic fines (mixture of primary and secondary 248

fines) alone (top) or with calcium carbonate particles at two ratios. Middle column: no 249

polyelectrolyte added. Right column: after 0.025% poly-DADMAC treatment. 250

251

Effects of Very-high-mass Cationic Flocculant 252

Particle size distribution 253

The curves in Figure 3 show how the particle size distribution in a suspension of primary 254

fines was generally shifted to higher values by the addition of cPAM. As indicated by the 255

curve rising to the highest level in the figure, an untreated suspension displayed a modal 256

equivalent spherical diameter of about 40 µm. Most of the signal fell within a range 257


between about 10 and 300 µm, which is consistent with reported sizes of cellulosic fines 258

studied by other means [32-34]. The kink in the distribution at ~ 170 µm is tentatively 259

attributed to a characteristic size and shape associated with parenchyma cells, which make 260

up a large proportion of the type of fines being considered here [1, 35]. 261

262

Figure 3. Effect of cPAM dosage on the particle size distribution of primary cellulosic 263

fines in aqueous suspension 264

265

Zeta potential measurements were not completed for the systems represented in Fig. 3. 266

Preliminary tests yielded relatively large agglomerates, and these made it difficult to 267

interpret the test results. An estimate of the charge-neutralization point can be made by 268

reference to Fig. 1 and by noting that the charge density of the cPAM was about 0.21 times 269

that of the coagulant on a mass basis. Based on the molecular masses of the monomers 270

comprising the two kinds of polyelectrolytes, it can be estimated that an addition of about 271

Fre

qu

en

cy (

no

rma

lize

d)

Diameter (µm)

1 2 5 10 20 50 100 200 500

Zero cPAM

0.01% “

0.025% “

0.05% “

0.1% “

Primary fines alone


0.12% cPAM would be needed to achieve charge neutrality. In other words, only the 272

highest dosage represented in Fig. 1 approached neutral surface charge. 273

274

With the addition of cPAM, the right-hand edge of the distribution, representing the largest 275

particles in the distribution, was progressively shifted to somewhat higher values, 276

consistent with agglomeration of the cellulose fines. Only at the highest dosage of 0.1% 277

cPAM solids on a dry fibre basis was there also a significant rightward shift of the left-278

hand edge of the distribution, indicating a significantly decreased population of singlet 279

cellulosic particles. In other words, there was sufficient polymer bridging between 280

cellulosic surfaces such that most of the cellulosic fines were attached to at least one other 281

cellulosic fine particle. Most notably, at the highest cPAM dosage of 0.1%, the modal 282

particle size was increased by a factor of about 133/33 = 4. A dosage of 0.1% cPAM based 283

on solids would represent a relatively high retention aid dosage in a commercial 284

papermaking system. However, given the high relative surface area of cellulosic fines [36-285

37], it makes sense that a somewhat higher dosage than is typically used in paper mills 286

would be required to bring about full flocculation of a suspension of cellulosic fines. 287

288

Figure 4 shows corresponding data for a combined system with 80% by mass of CaCO3 289

and 20% of primary cellulosic fines. Here the most noticeable effect of adding cPAM, 290

even at the lowest level, was an almost complete disappearance of the peak centered at 291

about 4 to 5 µm, i.e. the particle size range corresponding to the CaCO3. There are two 292

aspects to consider when interpreting such a change. First, it may be a function of the 293

cPAM to flocculate any freely suspended CaCO3 so that it becomes bound to cellulosic 294


surfaces in such a way that its individual effects on light scattering essentially disappear. 295

This can happen because the cellulosic particles are so much larger than the CaCO3 [1]. 296

Thus, deposition of a layer of CaCO3 onto the cellulose does not have a substantial effect 297

on the light scattering characteristics of those cellulosic particles. Secondly, it is expected 298

that a cationic flocculant treatment will create polymer bridge attachments between the 299

main surface of the cellulosic fines and any microfibrils extending outwards into the 300

solution phase. As was noted in the earlier article [1], the contribution to the signal 301

corresponding to about 10 µm in Figure 4 is probably due to the observed clusters of CaCO3 302

associated with extended microfibrils. Thus, disappearance of that part of the curve upon 303

addition of cPAM can best be explained by a flocculation effect in which all of the tethered 304

materials become matted down onto the main cellulosic surfaces. In addition, as shown in 305

Figure 4, cPAM treatment progressively moved the right-hand side of the distribution to 306

yet higher sizes, consistent with more agglomeration of cellulosic fines with each other. It 307

is worth noting that unlike Figure 3, the curves in Figure 4 represent systems in which 308

substantial amounts of CaCO3 were present on the cellulosic fines. Thus a key role of the 309

cPAM was to agglomerate CaCO3-encrusted fines into larger agglomerates. 310

311


312

Figure 4. Effect of cPAM dosage on the particle size distribution of an 80/20 (mass ratio) 313

mixture of CaCO3 to primary cellulosic fines in aqueous suspension 314

315

When comparing Figures 3 and 4, another striking difference is that the highest dosage of 316

cPAM (in the presence of both cellulosic fines and calcium carbonate) did not result in as 317

great a shift of the mean peak of the distribution. In other words, there was not as large of 318

an increase in the overall state of flocculation of the system as a whole. This difference 319

might be interpreted in two ways. First, individual CaCO3 particles have a low aspect ratio, 320

whereas cellulosic fines are somewhat fibrillar. The larger, more fibrillar cellulosic 321

particles can be expected to gather into larger, more extensive structures. A second 322

interpretation is based on an assumed higher surface area of the CaCO3 compared to the 323

cellulosic fines. A higher surface area constitutes a higher demand for an adsorbing 324

Fre

qu

en

cy (

no

rma

lize

d)

Diameter (µm)

1 2 5 10 20 50 100 200 500

Zero cPAM

0.01% “

0.025% “

0.05% “

0.1% “

80/20 mixture of

CaCO3 / cellulose


polyelectrolyte, thus spreading out and diluting its effect. Less of the flocculant would 325

then remain available to flocculate the cellulosic fines into larger structures. 326

327

Further supporting data, corresponding to CaCO3 alone and a 50:50 mixture of CaCO3 and 328

cellulosic fines, is provided in Figures 5 and 6. Figure 5 corresponds to a suspension of 329

only CaCO3 in the neutral buffer solution. Consistent with what is shown in Figures 3 and 330

4, addition of the cationic flocculant, regardless of its dosage, generally resulted in an 331

increase in the modal size of the suspended particles (singlets and agglomerates). 332

333

334 335

Figure 5. Size distribution in 100% CaCO3 suspension as a function of the dosage of 336

cationic flocculant based on the mass of solids. 337

338

Something worth noting in Figure 5 is the fact that there appeared to be an optimal 339

polyelectrolyte dosage to achieve the greatest size of agglomerates. The system treated at 340

Fre

qu

en

cy (

no

rma

lize

d)

Diameter (µm)

1 2 5 10 20 50 100 200 500

CaCO3 alone

Zero cPAM

0.01% “

0.025% “

0.05% “

0.1% “


the highest level of 0.1% cPAM (dry mass on dry mass) yielded a modal size of about 12 341

µm on the logarithmic axis, whereas the intermediate dosages of 0.025% and 0.05% 342

yielded a higher modal size of about 16 µm. Such findings suggest that the 0.1% 343

treatment level was above the amount needed to effectively cover half of the available 344

surface. Depending on the relative rates of polyelectrolyte adsorption vs. bridging 345

flocculation, such a situation can be expected to yield either inefficient flocculation or 346

various degrees of polymer-induced stabilization of the system. 347

348

Figure 6 is for the corresponding system with a 50:50 blend of CaCO3 and primary 349

cellulosic fines. Again, the main effect of the addition of the cationic flocculant was to 350

increase the degree of agglomeration. In this case the increase was monotonic, with a 351

progressive shifting of the modal size of suspended matter with increasing dosage of 352

cationic flocculant. 353

354

355


Figure. 6. Particle size distribution in suspension of a 50:50 blend of CaCO3 and primary 356

cellulosic fines at increasing levels of addition of cationic flocculant (on a mass basis). 357

358

The long tail of the distribution extending from approximately 10 µm down to about 2 359

µm may be tentatively attributed to CaCO3, either as single particles, as small clusters 360

freely suspended, or as clusters attached to very thin cellulosic fibrils [1]. Note that 361

related features in the tail of the distribution are shown much more prominently in Figure 362

4, a system in which there was a much higher mass ratio of CaCO3 to cellulose (80:20). 363

364

As shown by the results in Figure 7, the general effect of increasing dosage of cPAM was 365

to increase the mean size of particles (or agglomerates of particles) in the aqueous 366

suspensions. The effect was observed in each type of case considered – CaCO3 particles 367

alone, cellulosic fines alone, or when there was a combination, regardless of ratio. Notably, 368

the highest particle sizes were detected in mixed systems where both cellulosic fines and 369

CaCO3 particles were present before addition of the cPAM. This enhancement of 370

agglomeration is tentatively attributed to the fact that the CaCO3 and cellulosic surfaces 371

had been found to adhere to each other even in the absence of polyelectrolyte addition 372

under aqueous conditions matching those employed in the current work [1]. Reversal of 373

the trend just mentioned in the absence of cPAM and at the highest level of treatment is 374

possibly due to the presence of some CaCO3 particle unattached to cellulose in those 375

systems, thus bringing down the calculated average. 376

377


378

Figure 7. Effect of very-high-mass cationic PAM dosage (mass basis) on the mean 379

agglomerate size (mass average) in suspensions of either CaCO3 alone, 50/50 380

CaCO3/cellulose, or cellulose (primary fines) alone. 381

382

It is recommended that future work be carried out to determine whether or not the 383

observed effects of cPAM dosage also can be explained more comprehensively in terms 384

of fractional coverage of the solid surfaces by oppositely charged areas. Such an analysis 385

was only touched upon in the present work due to the porous, irregular nature of the 386

cellulosic materials employed. Past studies have shown that the fractional coverage of 387

surfaces by polyelectrolyte can be used as a way to explain the kinetics of flocculation [7-388

9]. However, none of the cited works provided an independent evaluation of surface 389

area, relying instead on the fitting of data to a model. A further complication arises due 390

to the fact that the conformation of a linear polyelectrolyte, as used in the present work, 391

0 0.01 0.025 0.05 0.1

Cationic PAM dosage (%, dry basis mass)

160

140

120

100

80

60

40

20

0

Me

an

Ag

glo

me

rate

Siz

e (

µm

)100% CaCO3


100% primary fines


can change markedly during the course of its adsorption onto a surface [38-40]. To 392

further address such issues it would make sense to apply such methods as atomic force 393

microscopy (AFM) [10-11, 38], and a geometrically simple surface such as mica [38] to 394

address these issues. Non-porous model surfaces, such as glass fibers, could be used to 395

avoid uncertainties related to permeation of polyelectrolytes [41]. 396

397

Effects of High-charge Cationic Coagulant followed by Anionic Flocculant 398

It is known that one of the most effective ways to bring about flocculation of suspensions 399

of negatively charged particles involves their treatment with a high-charge cationic 400

substance followed by use of an anionic flocculant [14-15]. Accordingly, Figure 8 shows 401

the results from tests involving sequential treatment with low-mass poly-DADMAC, then 402

aPAM. As shown, this combination yielded strongly increasing mean particle size with 403

increasing aPAM dosage. 404

405


406

Figure 8. Effect of aPAM dosagage on the mean agglomerate size of 50/50 407

CaCO3/cellulosic fines suspensions that had been treated first with low-mass poly-408

DADMAC at the 0.05% level based on solids. 409

410

Something unexpected about the results shown in Figure 8 is that primary fines yielded 411

larger flocs than secondary fines (fines obtained after refining of the pulp) at the same 412

levels of CaCO3 usage and aPAM dosage. Based on an average deviation between 413

replicate tests of ca. 5 m, the difference in results for the two kinds of cellulosic fines 414

can be regarded as significant. The reason for this effect is not well understood. One 415

possibility is that the effect is due to a larger surface area per unit mass of secondary fines 416

compared to primary fines [42-43]. As was noted earlier, the higher specific surface area 417

would tend to dilute the effect of the aPAM. If flocculation is correlated with the ratio of 418

aPAM to surface area, then the two curves in Figure 8 might be brought into better 419

200

180

160

140

120

100

80

60

40

20

0

Me

an

Ag

glo

me

rate

Siz

e (

µm

)

0 0.025 0.05 0.075

Dosage of Anionic PAM (%)

Primary fines

Refined fines

All: 50/50 CaCO3/cellulosic fines

0.05% low-mass poly-DADMAC,

Then aPAM treatment as shown


agreement. Since surface areas were not determined in the present work, such an 420

explanation merits further study. 421

422

423

Conclusions 424

The combination of laser diffraction particle size analysis and light microscopy was 425

found to be useful for characterizing agglomeration phenomena in aqueous suspension of 426

CaCO3 and cellulosic fine particles. Treatment with a high-charge moderate-mass 427

cationic polymer (a coagulant) gave rise to subtle changes in the appearance of the solids, 428

but had no significant effect on the measured particle size distributions. By contrast, a 429

very-high-mass cationic polyelectrolyte (a flocculant) gave rise to significantly increased 430

agglomeration by all measures. Changes in the shape of a particle size distribution curve 431

indicated a matting down of cellulosic nanofibrils – together with their tethered clusters 432

of CaCO3 particles – as a result of treatment by the cationic flocculant. Charge-related 433

effects and a bridging mechanism were shown to play important roles relative to the state 434

of agglomeration of the suspensions. The polyelectrolyte-induced attachments involving 435

CaCO3 and cellulosic fine particles tended to supplement the effects of CaCO3-fines 436

attachments that had already been established before the polymer treatments. 437

438

Acknowledgments 439 440

The authors are grateful for the financial support from SCG Paper Public Company 441 Limited, Bangkok, Thailand, which allowed Duangkammon Baosupee to carry out the 442 research as a visiting scientist at North Carolina State University. The authors are also 443 grateful for North Carolina State University, which made laboratory resources available 444 for the research. 445 446


References 447 448 1. Baosupee, D., Massey, A. J., Nazhad, M. and Hubbe, M. A. – Heteroagglomeration as 449 a Mechanism of Retaining CaCO3 Particles on the Fibrils of Cellulosic Fines: A Study by 450

Laser Light Diffraction and Microscopy, Colloids Surf. A 441: 525-531 (2014). 451 452 2. Horn, D. and Linhart, F. – Retention Aids, In: Roberts, J. C. (ed.), Paper Chemistry, 453 Second Ed., Blackie Academic and Professional, Chapman and Hall, Glasgow, p. 64-82, 454 1996. 455

456 3. Hubbe, M. A. and Rojas, O. J. – Colloidal Stability and Aggregation of 457 Lignocellulosic Materials in Aqueous Suspension: A Review, BioResources 3: 1419-458 1491 (2008). 459 460

4. Hubbe, M. A., Nanko, H. and McNeal, M. R. – Retention Aid Polymer Interactions 461

with Cellulosic Surfaces and Suspensions: A Review, BioResources 4: 850-906 (2009). 462 463

5. Svedberg, A. and Lindström, T. – Improvement of the Retention-Formation 464 Relationship using Three-Component Retention Aid Systems, Nordic Pulp Paper Res. J. 465 27: 86-92 (2012). 466

467 6. Arno, J. N., Frankle, W. R. and Sheridan, J. L. – Zeta Potential and its Application to 468

Filler Retention, Tappi 57(12): 97-100 (1974). 469 470 7. Sandell, L. S. and Luner, P. – Flocculation of Microcrystalline Cellulose with Cationic 471

Ionene Polymers, J. Appl. Polymer Sci. 18: 2075-2083 (1974). 472 473

8. Solberg, D. and Wågberg, L. – On the Mechanism of Cationic-Polyacrylamide-474 Induced Flocculation and Re-dispersion of a Pulp Fibre Dispersion, Nordic Pulp Paper 475

Res. J. 18: 51-55 (2003). 476 477

9. Goossens, J. W. S. and Luner, P. – Flocculation of Microcrystalline Cellulose 478 Suspensions with Cationic Polymers – Effect of Agitation, Tappi 59(2): 89-94 (1976). 479

480 10. Pfau, A., Schrepp, W. and Horn, D. – Detection of a Single Molecule Adsorption 481 Structure of Poly(ethylenimine) Macromolecules by AFM, Langmuir 15: 3219-3225 482 (1999). 483 484

11. Horn, D. – Exploring the Nanoworld of Interfaces and their Functions during Paper 485 Manufacturing and Upgrading, Wochenbl. Papierfabr. 129: 1589-1596 (2001). 486

487 12. Tripattharanan, T., Hubbe, M. A., Venditti, R. A. and Heitmann, J. A. – Effect of 488 Idealized Flow Conditions on Retention Aid Performance. 2. Polymer Bridging, Charged 489 Patches, and Charge Neutralization, Appita J. 57: 448-454 (2004). 490 491


13. Lindström, T., Söremark, C. and Eklund, L. – Kinetic Aspects of the Flocculation of 492

Cellulosic Fibre Dispersions with Polyacrylamides, Tech. Trans. CPPA 3: TR114-TR118 493 (1976). 494 495

14. Unbehend, J. E. – Mechanisms of ‘Soft’ and ‘Hard’ Floc Formation in Dynamic 496 Retention Measurement, Tappi 59(10): 74-77 (1976). 497 498 15. Wågberg, L. and Lindström, T. – Some Fundamental Aspects of Dual Retention Aid 499 Systems, Nordic Pulp Paper Res. J. 2: 49-55 (1987). 500

501 16. Horn, D. and Melzer, J. – Influence of Macromolecular Cationic Drainage Aids on 502 Electrokinetic Properties of Pulp, Papier 29: 534-541 (1975). 503 504 17. Alince, B. and Robertson, A. A. – Aggregation of Microcrystalline Cellulose with 505

Polyethylenimine, Colloid Polymer Sci. 252: 920-927 (1974). 506

507 18. Gruber, E., Grossmann, K. and Schempp, W. – Interactions of Synthetic Cationic 508

Polymers with Fibres and Fillers. Part 2: Effects on dDewatering and Retention, 509 Wochenbl. Papierfabr. 124: 102-105 (1996). 510 511

19. Kasper, D. R. – Theoretical and Experimental Investigations of the Flocculation of 512 Charged Particles in Aqueous Solutions by Polyelectrolytes of Opposite Charge, Ph.D. 513

Diss., CIT, Pasadena, CA, 1971, 201 p. 514 515 20. Gregory, J. – Rates of Flocculation of Latex Particles by Cationic Polymers, J. 516

Colloid Interface Sci. 42: 448-456 (1973). 517 518

21. Liimatainen, H., Haapala, A. and Niinimäki, J. – Retention of PCC and GCC Fillers 519 on Chemical Pulp Fines Surfaces, TAPPI J. 8(9): 38-42 (2009). 520

521 22. Rasteiro, M. G., Garcia, F. A. P., Ferreira, P., Blanco, A., Negro, E. and Antunes, E. 522

– The Use of LDS as a Tool to Evaluate Flocculation Mechanisms, Chem. Eng. 523 Processing 47: 1323-1332 (2008). 524

525 23. Rasteiro, M. G., Pinheiro, I., Garcia, F. A. P., Ferreira, P. and Hunkeler, D. – Using 526 Light Scattering to Screen Polyelectrolytes (PEL) Performance in Flocculation, Polymers 527 3: 915-927 (2011). 528 529

24. Taipale, T., Holappa, S. and Laine, J. – Isoluation and Characterization of Cellulosic 530 Pulp Fines and their Interactinos with Cationic Polyacrylamides, J. Dispersion Sci. 531

Technol. 32: 863-873 (2011). 532 533 25. Korhonen, M. H. J., Holappa, S., Stenius, P. and Laine, J. – Flocculation of fillers 534 with polyelectrolyte complexes, Nordic Pulp Paper Res. J. 28(2): 239-247 (2013). 535 536


26. Pierre, A., Lamarche, J. M., Mercier, R. and Foissy, A. – Calcium as Potential 537

Determining Ion in Aqueous Calcite Suspensions, J. Dispersion Sci. Technol. 11: 611-538 635 (1990). 539 540

27. Huang, Y. C., Fowkes, F. M., Lloyd, T. B. and Sanders, N. D. – Adsorption of 541 Calcium Ions from Calcium Chloride Solutions onto Calcium Carbonate Particles, 542 Langmuir 7: 1742-1748 (1991). 543 544 28. Madsen, L. – Surface Charge of Calcite, In Encyclopedia of Surface and Colloid 545

Science, Marcel Dekker, Vol. 4, p. 4982-4994 (2002). 546 547 29. Gill, R. and Scott, W. – The Relative Effects of Different Calcium Carbonate Filler 548 Pigments on Optical Properties, Tappi J. 70(1): 93-99 (1987). 549 550

30. Theodoly, O., Cascao-Pereira, L., Bergeron, V. and Radke, C. J. – A Combined 551

Streaming-potential Optical Reflectometer for Studying Adsorption at the Water/Solid 552 Surface, Langmuir 21: 10127-10139 (2005). 553

554 31. Hubbe, M. A. – Detachment of Colloidal Hydrous Oxide Spheres from Flat Solids 555 Exposed to Flow. 4. Effects of Polyelectrolytes, Colloids Surf. 25: 325-339 (1987). 556

557 32. Pelton, R. H., Jordan, B. D. and Allen, L. H. – Particle Size Distributions of Fines in 558

Mechanical Pulps and some Aspects of their Retention in Papermaking, TAPPI J. 68(2): 559 91-94 (1985). 560 561

33. Retulainen, E., Luuko, K., Fagerholm, K., Pere, J., Laine, J. and Paulapuro, H. – 562 Papermaking Quality of Fines from Different Pulps – The Effect of Size, Shape and 563

Chemical Composition, APPITA J. 55: 457-460 (2002). 564 565

34. Kang, T. and Paulapuro, N. – Characterization of Chemical Pulp Fines, TAPPI J. 566 5(2): 25-28 (2006). 567

568 35. Bäckström, M., Kolar, M.-C. and Htun, M. – Characterisation of Fines from 569

Unbleached Kraft Pulps and their Impact on Sheet Properties, Holzforschung 62: 546-552 570 (2008). 571 572 36. Marton, J. – The Role of Surface Chemistry in Fines–Alum Interactions, Tappi 573 63(2): 121-125 (1980). 574

575 37. Marton, J. – The Role of Surface Chemistry in Fines–Cationic Starch Interactions, 576

Tappi 63(4): 87-91 (1980). 577 578 38. Brotherson, B., Bottomley, L. A., Ludovice, P. and Deng, Y. L. – Salt Effect on 579 Polyacrylamide Conformation on Mica Studies by Single Molecule “Pulling” with 580 Scanning Probe Microscopy, J. Phys. Chem. B 112(40), 12686-12691 (2008) 581 582


39. Orblin, E. and Fardim, P. – Interactions between Cationic Polyelectrolyte and Pulp 583

Fines, BioResources 6(3), 2340-2355 (2011). 584 585 40. Peng, P. and Garnier, G. – Effect of Cationic Polyacrylamide on Precipitated 586

Calcium Carbonate Flocculation: Kinetics, Charge Density and Ionic Strength, Colloids 587 and Surfaces A – Physicochemical and Engineering Aspects 408, 32-39 (2012). 588 589 41. Wu, N., Hubbe, M. A., Rojas, O. J. and Park, S. – Permeation of polyelectrolytes 590 and other solutes into the pore spaces of water-swollen cellulose: A review, BioResources 591

4(3), 1222-1262 (2009). 592 593 42. Ramamurthy, P., Vanerek, A. and van de Ven, T. – Efficiency of AKD Sizing in 594 Mixed Hardwood-Softwood Furnishes, J. Pulp Paper Science 26(2): 72-75 (2000). 595 596

43. Chen, H., Park, A., Heitmann, J. A. and Hubbe, M. A. – Importance of Cellulosic 597

Fines Relative to the Dewatering Rates of Fibre Suspensions, Industrial and Engineering 598 Chemistry Research 48: 9106-9112 (2009). 599

600 601

Documents

DUANGKAMON BAOSUPEE , ASHLEY J. MASSEY MARTIN A. HUBBE · 2017-06-14 · 65 Experiments carried out in the presence of cellulosic fibres in aqueous suspension have 66 demonstrated