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Author vers. Cite: J. Dispersion Sci. Tech. 37(6), 836-845. DOI: 10.1080/01932691.2015.1065504

1 Breakup of Coagulated or Flocculated Clusters of Cellulosic Fines and CaCO3 Particles 2 Exposed to Hydrodynamic Stress 3 4

Martin A. Hubbe,a,* Miguel A. Sanchez,a Duangkamon Baosupee,b and Mousa Nazhad c 5 6 North Carolina State Univ., Department of Forest Biomaterials, Campus Box 8005, 7 Raleigh, NC 27695-8005, USA; Asian Inst. Technol., Klongluang, Pathumthani, 8 Thailand; and University of British Columbia, Pulp and Paper Center, Vancouver, BC, 9

Canada 10 11 ----------- 12 * To whom correspondence should be addressed: E-mail: [email protected] 13 a: North Carolina State University 14

b: Asian Institute of Technology 15

c: University of British Columbia 16 ----------- 17

18

Abstract 19 20

The capacity of fine particles to remain clustered together after being agglomerated by 21

polyelectrolytes plays an important role in papermaking and in the treatment of 22

wastewater. Tests were carried out with agglomerated suspensions of calcium carbonate 23

and primary cellulosic fines in neutral buffer solution. Agglomeration was induced either 24

by a high-charge cationic polyelectrolyte (a coagulant) or by sequential treatment with a 25

coagulant and a very-high-mass anionic acrylamide copolymer (a flocculant). Particle 26

size analysis, based on diffraction of laser light, showed that the coagulated suspensions 27

were susceptible to being redispersed by hydrodynamic shear. By contrast, flocculated 28

suspensions were only partly broken up. In a flocculated mixture of CaCO3 and 29

cellulosic fines, only the cellulosic fines could be separated from each other. The 30

intensity of shear was more critical than its duration. Conventional shear stress was more 31

effective for the breakup of the polyelectrolyte-induced agglomerates versus extensional 32

flow or intense ultrasonic vibrations. 33

mailto:[email protected]


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Key words: Polyelectrolyte-induced agglomerates; Redispersion; Hydrodynamic shear; 35

Particle size distributions; Cellulosic fines; Precipitated calcium carbonate 36

37

I. Introduction 38

39

1.1. 40

The manufacture of paper involves a delicate balance between efforts to achieve a highly 41

uniform distribution of fibers in the sheet as it is being formed, while at the same time 42

achieving efficient retention of fine particles.1-3 Excessively low retention efficiency of 43

fine materials such as minerals, hydrophobic sizing agents, and cellulosic fines, can lead 44

to decreased production rates, partial decomposition of certain chemical additives, and a 45

two-sided character of certain paper products.5-6 In order to improve the efficiency of 46

retention of fine particles, it is well known that one can add a sufficient dosage of a 47

highly charged cationic polyelectrolyte to approximately neutralize the negative charges 48

present on the surfaces of solids in a typical papermaking system.7-8 In addition, most 49

paper machines in current operation employ very-high-mass copolymers of acrylamide, 50

i.e. “retention aid” polymers, to keep the retention efficiency at the desired levels.9 51

However, one of the potential adverse consequences of such polyelectrolyte addition, 52

especially if it is excessive, can be a reduced uniformity of the paper due to an increased 53

level of fiber flocculation.3,10 54

55


One promising approach, in an attempt to achieve a suitably high level of retention of the 56

fine particles while still avoiding excessive flocculation of the fibers with each other, 57

involves the strategic use of hydrodynamic shear, which is inherent in the unit operations 58

leading to the forming of a sheet of paper.11-14 By judicious selection of the points of 59

addition for different chemical additives, the papermaker can influence the level of 60

hydrodynamic shear that will subsequently act upon that additive, up to the point where 61

the paper sheet has been fully formed. The choices are limited, however, since the levels 62

of hydrodynamic shear associated with such devices as fan pumps, hydrocyclones 63

(“cleaners”), pressure screens, and headboxes in a paper machine system are mainly a 64

function of the design of the equipment and the rate of throughput.13 Papermakers can 65

select to add retention aid polymers either before or after a set of pressure screens.15 Pre-66

screen addition is often favored by papermakers who are placing priority on the 67

uniformity of the product and/or optimization of drainage/retention systems that employ 68

either colloidal silica or bentonite (sodium montmorillonite) products.16-17 Post-screen 69

addition of a retention aid is often favored by papermakers whose priority is to achieve 70

the desired efficiency of retention at the lowest cost of retention aid polymer. In either 71

case, it makes sense to fine-tune the types and dosages of the chemical additives used in 72

the retention program so that they match well to the levels of hydrodynamic shear present 73

in the paper machine system under consideration.18 74

75

While past studies have dealt with the effects of hydrodynamic shear on different aspects 76

of fine-particle retention, it has been less common to consider just the fine particles in 77

isolation, apart from the fiber portion of a papermaking furnish. Notably, Liimatainen et 78


al. carried out work in which calcium carbonate particles were allowed to interact with 79

stirred suspensions of cellulosic fines.19 In the absence of retention aid, the observations 80

could be fit very well to a Langmuir adsorption isotherm. Though the authors noted that 81

the Langmuir isotherm often implies a dynamic equilibrium between attachment and 82

detachment, they did not confirm whether or not particles were being detached in their 83

system. 84

85

The hydrodynamic forces required to detach colloidal particles from surfaces, after 86

various treatments with polyelectrolyte coagulants and flocculants, have been considered 87

in previous studies.3,11,20 A study based on turbulent flow in the annulus of coaxial 88

cylinders showed that the shear stress required to detach TiO2 particles from glass or 89

cellulose surfaces could be greatly increased by treatment with polyelectrolytes.21 In 90

particular, treatment with a very-high-mass cationic acrylamide copolymer was able to 91

increase the required shear stress for detachment of the particles from glass from about 92

0.8 Pa in a neutral buffer solution to about 320 Pa following treatment of the glass with 93

flocculating polymer. An important principle arising from the same series of work was 94

the finding that, when other factors are kept constant, a larger particle size implies a 95

lower shear stress required for detachment.22 In related work, Pelton and Allen showed 96

that highly flexible polymeric bridges could account for attachment of polystyrene 97

spheres to a surface, rendering them resistant to detachment when later exposed to flow.23 98

99

The present work was undertaken in an effort to better understand the effects of 100

hydrodynamic stress following two contrasting types of polyelectrolyte treatment – 101


coagulation and flocculation. These two types of treatment were examined in a previous 102

article, which was concerned with the joining of particles together into agglomerates.24 103

The cited article showed that the coagulation treatment – based on addition of a high-104

charge cationic polyelectrolyte – was only marginally effective in bringing about 105

increased agglomeration. On the other hand, flocculation, such as can be achieved by 106

sequential addition of high-charge cationic polymer followed by anionic acrylamide 107

copolymer (aPAM), resulted in large agglomerates that involved all of the particles 108

present in the suspension. The present work was aimed at finding out how different kinds 109

of hydrodynamic treatment affected polyelectrolyte-induced agglomerates of CaCO3 110

particles, cellulosic fines, and their combination in terms of particle size distribution and 111

microscopic appearance. In order to examine a broad range of flow types that might 112

affect the state of agglomeration, ordinary shear flow with a stir-bar or impeller was 113

compared with a more intensive action of a blender, exposure to an ultrasonic probe, and 114

the extensional flow induced by rapid jetting of suspensions through a syringe. 115

116

2. Experimental 117

The experimental system employed in this work was in key aspects identical to what has 118

been described in two previous articles by the authors.24-25 The descriptions that follow 119

will therefore emphasize details that are unique to the present article. 120

2.1. Materials 121

2.1.1 Cellulosic primary fines 122

Cellulosic fine matter, largely consisting of delignified parenchyma cells, was isolated 123

from unrefined bleached hardwood kraft pulp from a mill in the US southeast.25 After 124


dispersing the baled pulp in water, the suspension was passed through the last stage of a 125

Bauer-McNett classifier fitted with a 200-mesh screen. Rinsing was continued for at 126

least 10 minutes to allow most of the fines to pass through the screen openings and to be 127

collected in a barrel. After over-night sedimentation and collection, the primary fines 128

obtained in this manner were thickened by additional settling to reach a suitable solids 129

level in the range of 2-5%. 130

131

2.1.2 Mineral particles 132

Calcium carbonate particles were of the scalenohedral calcite type (PCC, Albacar® 5970 133

from Specialty Minerals Co.) with diameters of about 2-3 µm. 134

135

2.1.3 Water-soluble chemicals 136

Deionized water was used for initial dilution of the following polyelectrolytes to either 137

the 1% level (for coagulants) or the 0.1% level (for flocculants). A high-charge cationic 138

coagulant, poly-diallyldimethylammonium chloride (Aldrich cat. no. 40,901-4, having a 139

nominal molecular mass of 100,000 to 200,000 Daltons) was called “poly-DADMAC”. 140

A very-high-mass cationic co-polymer of acrylamide (Percol® 175 from Ciba Specialty 141

Chemicals, having a monomer molar content of 10% cationic groups) was identified as 142

“cPAM”, a cationic flocculant. The initial solutions of flocculants were allowed to stir 143

gently for an hour before being employed in experimentation. A very-high mass 144

copolymer of acrylamide (70%) and acrylic acid (30%), Floerger AN 934 (from SNF 145

Floerger), was called “aPAM”. Reagent-grade sodium sulfate and sodium bicarbonate 146

were used to prepare a pH 7 buffer solution having a NaHCO3 concentration of 10-4 M 147


and an electrical conductivity of 1000 µS/cm at 20 oC, which was used as the suspending 148

medium for the suspensions described in this work. 149

150

2.2 Equipment 151

152

A Horiba LA 300 particle size analyzer was used to evaluate particle size distributions.25 153

Zeta potentials of suspended particles were evaluated with a Lazer Zee 154

microelectrophoresis analyzer from PenKem. An Olympus BH2 UMA microscope was 155

used to obtain optical images of particles after suspensions had been allowed to sediment 156

onto glass slides. 157

158

2.3 Preparation of agglomerated systems 159

Three combinations of suspended matter employed in a previous study were selected for 160

the present work: a suspension of just CaCO3, a suspension of just primary cellulosic 161

fines, and a mixture of the two, usually 80% CaCO3 and 20% fines, unless noted 162

otherwise. For preparation of agglomerated systems, the percent solids was 0.5%. 163

Destabilization of suspensions by "coagulation" entailed addition of the poly-DADMAC 164

at a dosage (dry-mass basis) of 0.025%. Destabilization of suspensions by "flocculation" 165

entailed sequential addition of poly-DADMAC (0.025%) followed by aPAM (0.015%) 166

with selected tests done at other combinations of dosages. 167

168

2.4 Exposure of agglomerated systems to flow conditions 169


A set of contrasting flow conditions was selected in order to assess the ability of 170

agglomerated particles to remain intact. The conditions were as follows: 171

1. Gentle Impellor Stirring: This level of shear was established by placing the beaker 172

containing the sample to be tested on a stir plate (Thermolyne, Barnstead Intl., 173

Nuova brand, model SP18425, 120 V, 7.3 amps), adding a 2 cm long magnetic 174

stir bar, setting the stir plate to a level of 3, and then stirring for 30 s. 175

2. Intermediate stirring: This level of shear was accomplished by the same method 176

as for Gentle Impellor Stirring, but with the stir plate set to 8 (where the 177

maximum was 10). This level of shear was maintained for 60 s. 178

3. Blending: This condition was accomplished with a Waring Commercial Blender 179

(model 51BL32, 120 V, 3 A). This blender allowed for high speed blending at one 180

speed. Fifty milliliters of sample test solution were briefly transferred to the 181

blender’s 100-mL container (Fisher cat. no. 14-509-18B) before the top was 182

covered with its elastic seal and the container placed on the blender’s stand. The 183

device was allowed to run for 60 s. 184

4. Low Ultrasonic Treatment: An ultrasonic homogenizer, the OMNI-Ruptor 250 by 185

OMNI International, Inc., was set to a 4 magnitude POWER setting at a PULSER 186

of ~30-32%. The timer was also set to just above three minutes. A probe with a 187

diameter of 10 mm was then inserted into the 50 mL test sample, about a 188

centimeter deep into the solution held within a 50 mL Erlenmeyer flask. At 60 189

seconds, the RESET button was hit to stop the treatment, and the probe was 190

removed from the sample. 191


5. High Ultrasonic Treatment: This condition was the same as the previous, but with 192

the POWER set to a magnitude of 6. 193

6. Syringe & Erlenmeyer Flask- This test regime involved jetting 50 mL of test 194

solution in and out of an Erlenmeyer flask using a 60 mL syringe, continuously, 195

for 10 minutes. The inner diameter of the tip of the syringe was approximately 196

1.8 mm, and each “in and out” cycle of squirting was completed in about 2 197

seconds. 198

199

In a further set of experiments, an impeller stirrer was used to provide a range of different 200

shear environments, as specified later. Those tests were carried out in a Dynamic 201

Drainage/Retention Jar device, Paper Research Materials, Inc., http://www.brittjar.com. 202

203

After each of the applications to flow, as listed above, the sample was immediately tested 204

for particle size using the LA-300 Horiba Light Scattering Device. The Horiba LA-300 205

main compartment was filled with approximately 300 mL of the 1000 µS/cm buffer 206

solution, or to the line in the main compartment. Drops of the newly tested sample were 207

then added to the LA-300 Horiba main compartment via pipette until the dispersion T% 208

reached an optimum level at approximately 85.3%, within the ideal range of 95-75% for 209

the dispersion T%. The optional ultrasonication treatment, provided with the Horiba 210

device, was not employed. 211

212

2. Calculation of shear stress 213


Estimates of the hydrodynamic stress applied to the suspensions under the described flow 214

systems were obtained by reference to equations developed by others, as follows. 215

216

To estimate the typical shear stress experienced by suspended matter under the conditions 217

of turbulent shear flow (for instance when using a blender), the following equation was 218

used:26-27 219

220

)0.5 (1) 221

222

In this equation is the density of the fluid (taken to be 1.00 g/cm3), is the rate of 223

energy dissipation (estimated as 360 W per 100 g of fluid, based on the rating of the 224

blender and the amount of fluid employed), and is the kinematic viscosity (taken to be 1 225

cSt, or 10-6 m2s-1). 226

To estimate the extensional stress applied to suspended agglomerates passing through the 227

center of a contracting nozzle (i.e. the syringe), the following equation was used to 228

calculate the rate of extension:28 229

(2) 230

In this equation, Q is the flow rate, D is the smallest diameter of the nozzle, and is the 231

half-angle of the cone. The function f() can take on values in the range between zero 232

and 1.3. Because the syringe employed in the present work had a half-angle of 60 233

degrees, an f() value of 1.3 was used (see Table 1 of the cited work). The extensional 234

viscosity was estimated as three times the Newtonian viscosity of water.29 For the sake 235


of comparison, the shear rate and stress in the syringe system were calculated from the 236

equations for flow through a capillary: 237

238

and (3) 239

Here the term R refers to the inner radius at the outlet of the syringe. In using these 240

equations a dynamic viscosity of 1.002 mPas, corresponding to water at 20 oC, was 241

assumed. The flow rate calculated based on the release of approximately 60 mL of 242

aqueous solution in one second. The ultimate diameter of the syringe was 1.8 mm. 243

3. Results and Discussion 244

245

3.1. Zeta potential vs. poly-DADMAC treatment 246

As shown in Fig. 1, addition of poly-DADMAC to suspensions of either the precipitated 247

calcium carbonate or the primary cellulosic fines resulted in a stable positive zeta 248

potential, once the dosage exceeded about 0.3% on a dry-mass basis. The initial negative 249

zeta potential of the cellulosic fines is consistent with the presence of carboxylic acid 250

groups.2,30 The weakly cationic zeta potential of the untreated CaCO3 suspension is 251

consistent with expectations for that material.31 Interestingly, the present results for the 252

CaCO3 suspension were somewhat different from what was measured earlier,24 and the 253

differences might be related to minor variations in handling and dilution. 254


255

Figure. 1. Effect of high-charge cationic polymer dosage (dry mass basis) on the zeta 256

potential of suspensions of precipitated calcium carbonate and or primary cellulosic fines. 257

258

3.2. Breakup of agglomerates after coagulation with poly-DADMAC 259

Figure 2 shows optical micrographs of CaCO3 suspensions collected on glass slides. 260

Significant agglomeration is apparent in Part A of the figure, which corresponds to a 261

suspension that had just been treated with poly-DADMAC at the 0.025% level. Part B of 262

the figure shows a corresponding image after the suspension had been exposed to 30 s of 263

intense shear in the blender. Though the latter image still shows some particles that have 264

the appearance of being attached together, it can be concluded, in general, that the 265

hydrodynamic shear had been effective in dispersing the particles from each other. 266

267

0 1 2 3

30

20

10

0

-10

-20

Dosage of Poly-DADMAC (% on solids)

Ze

ta P

ote

nti

al (

mV

)

-30

Precipitated CaCO3

Cellulosic fines


A.

B.

268

Figure. 2. A: Before-shear appearance of precipitated calcium carbonate that had been 269

treated with poly-DADMAC. B: Same system after 30 s of shear in a blender 270

271

Figure 3 reports particle size distributions for a related system, based on diffraction of 272

laser light. The suspended solids consisted of 80% CaCO3 and 20% cellulosic fines, by 273

mass, which had been freshly coagulated by addition of poly-DADMAC at the 0.025% 274

level. The filled area (with the dashed line) represents the freshly coagulated system, 275

before application of strong hydrodynamic shear. The pre-shear distribution of particle 276

size had a modal value of about 40 µm, which was about the same as the value obtained 277

earlier for an individual suspensions of the primary cellulosic fines.24 A shoulder in the 278

pre-shear distribution, centered at about 8 µm, is consistent with the presence of clusters 279

of CaCO3 particles, possibly associated with very thin cellulosic fibrils.25 The rising 280

dashed line in the figure shows the cumulative distribution, indicating a median size of 20 281

µm. 282

283


The distribution represented by the solid line in Fig. 3 makes it clear that the distribution 284

became strongly bimodal after application of 30 s of intense shear in the blender. One of 285

the maxima in the post-shear distribution, centered at 4 µm, is consistent with the 286

presence of very small groups of CaCO3 particles, including single particles, doublets, or 287

triplets, etc.25 The other maximum in the distribution, centered at about 80 µm, is 288

consistent with the cellulosic fines by themselves. The fact that the latter value was 289

higher than what had been observed for a suspension of the cellulosic fines alone,25 290

provides evidence that the redispersion of the particles was not complete, or that the fines 291

tended to come back together again, due to the neutralization of charges. In general, 292

however, it is apparent that the hydrodynamic shear was sufficient to detach CaCO3 293

particles from the cellulosic fines. Also, most large clusters of CaCO3 particles appear to 294

have been substantially separated into either single particles or small clusters. 295

296

Fre

qu

en

cy

(no

rma

lize

d)

1 10 100 600

Diameter (µm)

0.025% poly-DADMAC

After

blender

Before

blender


Figure. 3. Particle size distribution of mixed suspension with 80% CaCO3 and 20% 297

primary cellulosic fines after having been treated with poly-DADMAC. Filled area: 298

Before application of shear. Solid line: Same, after subsequent exposure to 30 s of shear 299

in blender. Rising curves show the corresponding cumulative distributions. 300

301

Related tests with poly-DADMAC (not shown) were carried out with suspensions of 302

primary fines alone. In such cases was there insufficient agglomeration to justify 303

subsequent experiments with application of hydrodynamic stress. 304

305

3.2. Breakup of agglomerates after flocculation with poly-DADMAC & aPAM 306

307

Figure 4 shows corresponding results following flocculation of an 80:20 mixture of 308

CaCO3 particles and primary cellulosic fines by sequential addition of poly-DADMAC 309

(0.025% by mass) and then aPAM (0.015% by mass). Again, the pre-shear (gentle 310

stirring) distribution is represented by the dashed line and the filled area. The fact that 311

almost the entire distribution was larger than 10 µm is consistent with essentially all of 312

the CaCO3 particles having been incorporated into flocs that included cellulosic fines. 313

The modal value of the distribution was essentially the same as that of a well-dispersed 314

suspension of cellulosic fine particles, as reported earlier [ ].25 This result suggests that 315

the CaCO3 particles were mainly accumulating on the surfaces of the much larger 316

cellulosic particles. Due to the great difference in size between the two, the apparent size 317

of the agglomerates was close to that of the cellulosic fines by themselves. 318

319


The other two particle size distributions shown in Fig. 4 indicate the progressive effects 320

of two levels of hydrodynamic shear. The dotted line is for a system sheared at a higher 321

level of magnetic stirring (see Experimental). The solid line represents the effects of 322

intense shearing in a blender. These curves show results that were strikingly different 323

from the case where the same solids had been agglomerated by treatment with poly-324

DADMAC. In the flocculated system, the shape of the particle size distribution remained 325

almost constant after the application of shear, though there was a moderate shift towards 326

smaller agglomerate size in each case. Thus, the results are consistent with a splitting 327

mechanism of dis-agglomeration, in which the product consisted of smaller agglomerates, 328

but essentially no separation of individual CaCO3 particles, since the latter would have 329

been apparent in a size range of about 2 to 6 µm.25 In other words, the results tend to rule 330

out extensive erosion of individual CaCO3 particles from the agglomerated matter. 331

332

Based on the laser diffraction analysis, the application of shear to the flocculated system 333

of cellulosic fines together with CaCO3 particles appeared to have produced agglomerates 334

smaller than the cellulosic fines by themselves, whereas one would expect such 335

agglomerates to be at least as large cellulosic fines by themselves. Apparently this aspect 336

of the results is due to the way in which the system interacts with light, since the 337

algorithm used in the fitting assumes that only perfectly spherical, uniform particles are 338

present. This issue will be considered further when discussing the microscopic evidence 339

for the system. 340

341


342

Figure. 4. Particle size distribution of mixed suspension with 80% CaCO3 and 20% 343

primary cellulosic fines after having been treated sequentially with poly-DADMAC, then 344

aPAM. Filled area: Before application of shear. Dodded line: Same, after brisk stirring. 345

Solid line: After subsequent exposure to 30 s of shear in blender. Rising curves show the 346

corresponding cumulative distributions. 347

348

A tendency for hydrodynamic shear to result in a splitting mechanism of breakdown of 349

agglomerated particles is supported by past work.32-33 Lu and Spielman were among the 350

first to report a maximum stable floc size corresponding to a given level of hydrodynamic 351

shear.34 The fact that small CaCO3 particles were not dislodged during that process is 352

consistent with the known greater difficulty of detaching smaller particles from solids 353

exposed to a given level of shear stress.22 354

355

Fre

qu

en

cy

(no

rma

lize

d)

1 10 100 600

Diameter (µm)

0.025% poly-DADMAC

0.05% aPAM Gentle stirring

Brisk stirring

Blender


The next two figures provide some context for the particle size distributions of systems 356

treated with the two-component flocculent system. Figure 5 is from a suspension of just 357

the primary cellulosic fines, after sequential treatment with poly-DADMAC, then aPAM. 358

Part A shows a typical floc that was present before application of shear. Part B is from a 359

related sample after the application of shear in a blender. Inspection of images of this 360

type made it possible to conclude that though hydrodynamic shear was affective in 361

breaking up the flocculant-induced agglomerates, the redispersion was only partial. It is 362

worth emphasizing out that the polyelectrolytes are much smaller than what can possibly 363

be seen in an optical micrograph; rather, the apparent tethered linkages among different 364

cellulosic particles can be identified as cellulosic microfibrils attached to the surfaces.25 365

Presumably, the role of the poly-DADMAC and aPAM combination involved formation 366

of bridges between cellulosic surfaces, including the very slender fibrils.35-36 367

A.

B.

368

Figure. 5. Suspension of primary fines after having been treated sequentially with poly-369

DADMAC, then aPAM. A: Before-shear appearance. B: Same, after subsequent 370

exposure to 30 s of shear in blender 371

372


Figure 6 shows a related set of images, except that the solids consisted of 80:20 mixtures 373

of CaCO3 and primary fines. The image in Part A (pre-shear) shows a predominance of 374

spheroidal agglomerates of various sizes. By contrast, Part B shows that after the 375

application of shear in a blender the agglomerates tended to be elongated. Thus, it 376

appears that the shear flow had stretched at least some of the larger agglomerates into 377

narrower, longer clusters. 378

379

A tendency of hydrodynamic shear to distort and elongate agglomerates of flocculated 380

particles was predicted by Higashitani et al. based on a finite-element model.32 In the 381

present work, the presence of cellulosic matter, which is inherently fibrous, may be 382

expected to further facilitate the elongation of flocs. However, there is evidence that the 383

distribution of CaCO3 particles relative to the surfaces of cellulosic fines was not 384

uniform, with larger concentrations of CaCO3 present at different points along an 385

individual cellulosic fine particle. In addition to the somewhat non-uniform distributions 386

apparent in Fig. 6, such a distribution also can help explain why the modal particle size of 387

the distributions shown in Fig. 4 after high shear in a blender (ca. 20 µm) was 388

substantially lower than the corresponding distribution for a well-dispersed suspension of 389

primary fines by themselves (ca. 40 µm).25 Since the parenchyma cells that constitute 390

most of the cellulosic fines are too strong to be broken by the blending action, there has 391

to be a different explanation for the apparent reduction in size below that of the cellulosic 392

fine particles by themselves. It appears that uneven clusters associated with different 393

regions of a cellulosic fine particle can affect the diffracted light such that the instrument 394

reports the presence of particles of a somewhat smaller size. 395


396

A.

B.

397

Figure. 6. Mixed suspension of 80% CaCO3 and 20% primary fines after having been 398

treated sequentially with poly-DADMAC, then aPAM. A: Before-shear appearance. B: 399

Same, after subsequent exposure to 30 s of shear in blender 400

401

3.2. Comparing effects of intensity vs. duration of shear 402

The strength of polyelectrolyte-induced bridges and their ability to resist the effects of 403

flow can be expected to depend on such variables as polymer dosage, the intensity of the 404

flow event, and its duration. Figure 7 shows results of an experiment in which 405

agglomerates of primary fines were first treated with poly-DADMAC at the 0.05% level, 406

followed by different levels of aPAM treatment. Agitation was then applied by means of 407

an impeller stirrer at three speeds of agitation. The error limits in the figure show the 408

results of two replicate tests, which were averaged in each case. The most striking 409

conclusion arising from this set of experiments was that the duration of hydrodynamic 410

shear had no significant effect on the results. Rather, the key variables were the 411


flocculant dosage and the rotational speed of agitation. Higher dosages of aPAM yielded 412

larger agglomerate size, whereas higher shear yielded smaller particle size. 413

414

Figure. 7. Comparing the effects of agitation speed and duration on the mean diameter 415

of agglomerates of primary fines after sequential treatment with 0.05% low-mass poly-416

DADMAC and the indicated amounts of aPAM (30% anionic groups) at the impeller 417

rotational speeds and durations shown. 418

419

Based on the results just presented it is possible to draw some general inferences 420

regarding the polyelectrolyte-induced flocs. The fact that the agglomerate size, after 421

exposure to each of the shear levels, tended to increase with increasing aPAM dosage is 422

consistent with an increasing density of macromolecular bridge chains linking adjacent 423

surfaces. A higher density of bridging thus renders the system more shear-resistant. The 424

fact that the results did not depend on time, within the ranges of conditions tested, 425

200

180

160

140

120

100

80

60

40

20

0

Agitation: 500 1000 1000 1500 1500 RPM

Duration: 60 s 60 s 30 min 60 s 30 min

Me

an

Dia

me

ter

(µ

m)

ALL: 0.05% poly-DADMAC

Pink: 0.025% aPAM

Blue: 0.05% aPAM

Green: 0.075% aPAM


suggests systems in which detachment was initiated mainly by rupture of polymer chains 426

– an event that requires application of a force that exceeds a well-defined limit.37 For 427

instance, it has been estimated that a force of 7 nN is needed to snap a single 428

polyisoprene or polybutadiene chain,38 and that value would not be expected to depend 429

on the duration of application of the force. The strong dependency of the agglomerate 430

size on the agitation speed (Fig. 7) further supports this view. As predicted by Tomi and 431

Bagster, based on a model in which a floc elements are joined together by flexible 432

polymer bridges, the maximum particle size was strongly dependent on the intensity of 433

the applied hydrodynamic shear.39 It appears that a 60 s exposure was a sufficient length 434

of time to ensure that the polymer bridge attachments had been subjected to essentially 435

the full range of local flow events that could be experienced during 30 minutes of 436

agitation. 437

438

3.2. Comparing effects of different flow systems 439

Up to this point in the article, the only flow systems considered have been partially or 440

fully developed turbulent shear flow, induced by either a magnetic stir bar, an impeller, 441

or a blender device. Further tests were carried out to find out whether the 442

polyelectrolyte-induced flocs were vulnerable to certain other types of hydrodynamic 443

stresses, in addition to shear stress. The system under consideration consisted of an 80:20 444

mixture of CaCO3 and cellulosic fines that had been treated sequentially with 0.05% 445

poly-DADMAC followed by 0.05% aPAM. As shown in Fig. 8, the intense shear flow 446

imparted by the blender was clearly the most effective in terms of reducing the 447

agglomerate size. If one compares the first three groups of histogram bars, starting from 448


the left of the figures, then it becomes clear that agglomerate size decreased with 449

increasing intensity of shear. 450

451

452

Figure. 8. Comparing the effects of different flow systems imposed upon mixtures of 453

80% CaCO3 and 20% primary fines after sequential treatment with 0.05% low-mass 454

poly-DADMAC and 0.05% aPAM 455

456

The results in Fig. 8 corresponding to exposure of the flocculated suspensions to an 457

ultrasonic probe showed a somewhat surprising result – the largest (or equal) measured 458

diameters compared to any of the other flow systems, even including the gentlest stirring 459

with a magnetic stir-bar. This is despite the fact that the ultrasonic probe system expends 460

up to 250 Watts at its highest setting, which is in a similar range to the energy rating of 461

the blender system (360 W). The difference is tentatively attributed to a highly flexible 462

80

60

40

20

0Magnetic Stir-bar Blender Ultrasonic Probe Syringe

(Level 3) (8) (high) (low) (high) squirt

Type of Hydrodynamic Shear

Dia

me

ter

Me

as

ure

d

(m

) Mean

Median

Mode


nature of the polymer bridges and to the relatively short ranges of motion imparted by the 463

shock waves of ultrasonification. Vasilev et al. cited evidence that the amplitudes of 464

cavitation events inherent in the use of ultrasonic probes fall in the range of 15 to 90 465

µm.40 Because the ultrasonic waves being considered are acting within a condensed 466

aqueous fluid, and the solids present are similarly quite resistant to compression, it could 467

be expected that the changes in relative distances between adjacent particles in an 468

agglomerate, due to ultrasonic vibrations, would be a minor fraction of the cited 469

distances. It is well known that ultrasonic waves can be very effective not only in the 470

detachment of particles from surfaces,41 but also in the disruption of biological cells.40,42-471

43 The fact that the present systems, agglomerated by poly-DADMAC and aPAM, were 472

not at all susceptible to ultrasonic disruption is further testimony to the flexibility and 473

toughness of the polymer bridging systems.23,44 474

The last remaining set of histogram bars, to the far-right in Fig. 8, correspond to 475

experiments in which the suspension was repeatedly jetted through a 60 mL syringe, over 476

the course of 10 minutes. The extensional shear stress exerted on particles that happen to 477

be in the center of flow can be estimated from Equation 2, by inputting the critical 478

dimensions of the outlet from the syringe and estimating the rate of flow. The 479

extensional stress was calculated as 102 Pa, whereas the corresponding shear stress at the 480

wall of the syringe opening was calculated to be 78 Pa (by Equation 3). Bałdyga et al. 481

established that a tensile rupture mechanism is likely to predominate in extensional 482

flow.45 Kobayashi found that increasing flow through a syringe-type system was 483

increasingly effective in breaking up flocs of polystyrene latex particles, leading to 484

smaller floc diameters.46 The fact that significant detachment was not found for the 485


"Syringe squirt" condition shown in Fig. 8 suggests that the exerted force was below the 486

level required to bring about tensile splitting of the agglomerates. 487

488

The magnitude of shear stress within the blender (not accounting for possible higher 489

values near to the tips of the impellers) was 320 Pa (from Equation 1), which was 490

considerably higher than what was calculated in the case of flow through the syringe 491

system. This result appears consistent with the fact that the blender was considerably 492

more effective in breaking up agglomerates in comparison to flow through the syringe. 493

But such a comparison cannot account for the fact that the agitation with the magnetic stir 494

bar, even at the gentle level of application, yielded substantially lower particle size in 495

comparison to passage through the syringe. It should be kept in mind, however, that use 496

of a magnetic stirring system creates an opportunity for colloidal materials to become 497

pinched and rubbed in the zone of contact between the stir-bar and the vessel, and that 498

effect could well explain the lower diameters recorded for those cases, in comparison to 499

the ultrasonic and squirting treatments. 500

501

IV. Conclusions 502

Detachment of particulate suspensions including CaCO3 particles, cellulosic primary 503

fines, and their combination, following coagulation or flocculation with polymers, could 504

be broken up to different degrees by application of hydrodynamic stresses. A high-505

charge density cationic polymer, poly-DADMAC (a coagulant), yielded increased 506

agglomeration of CaCO3 particles, but such agglomerates were readily dispersed by 507

application of shear flow. By contrast, a sequential treatment with the coagulant 508


followed by an anionic acrylamide copolymer (a flocculant) effectively agglomerated 509

CaCO3 particles onto the shift the distribution of agglomerate size in the direction of 510

smaller values, but individual CaCO3 particles failed to be released. One of the main 511

effects of hydrodynamic shear was to elongate the agglomerates, leading to a non-512

uniform distribution of CaCO3 particles associated with the cellulosic fines. The 513

flocculated systems were not at all susceptible to breakage by application of either 514

extensional flow or the action of an ultrasonic probe, even though the amounts of energy 515

imparted by ultrasonication were almost as large as that provided by the blender. 516

517

Acknowledgments 518 519

The authors are also grateful for North Carolina State University, which made laboratory 520 resources available for the research. 521

522

References 523

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