Flocculation Of Precipitated Calcium Carbonate Research Articles

Enzymatic nanocellulose in papermaking – The key role as filler flocculant and strengthening agent

Nanocelluloses have been increasingly used in composites since their reduced size, high aspect ratio and stiffness confer great strength to the materials. In papermaking, it has been proved that harsh and expensive chemical pre-treatments to generate nanofibrils, such as TEMPO-mediated oxidation, are not the most favourable and therefore the use of cellulose microfibrils (CMF) have gained extra attention, especially those produced with the aid of enzymatic hydrolysis. In the present work, strategies to improve filler flocculation and the papermaking properties, by using enzymatic CMF, are provided. The CMF degree of polymerization was found to be directly related to precipitated calcium carbonate flocculation, leading to higher retentions in the fibre matrix. Besides, the paper dry and wet strengths were much improved, allowing in return the production of high-filler loaded handsheets with reduced requirements for common paper additives.

Utilizing Cellulose Nanofibril as an Eco-Friendly Flocculant for Filler Flocculation in Papermaking

Cellulose nanofibril (CNF) is a type of nano-sized cellulosic material with a high aspect ratio and a large specific surface area, which makes it a promising additive to flocculate particles such as fillers. In this study, CNFs were prepared by wet disk-milling with three different degrees of fibrillation. The effect of the CNF on the flocculation of precipitated calcium carbonate (PCC) was elucidated. A photometric dispersion analyzer (PDA) investigated the flocculation phenomena resulting from the interactions between the PCC and the CNF, or polymers, in real-time. The results clearly showed that CNF could be used to control the flocculation behavior of the PCC particles. Scanning electron microscopy (SEM) images showed that the CNFs functioned as bridges between the two PCC particles. A significant reflocculation capability was observed in one- and two-component systems that consisted of CNFs. The addition of cationic starch (C-starch) could induce more efficient flocculation within a two-component system.

Flocculation kinetics of precipitated calcium carbonate (PCC) with sterically stabilized nanocrystalline cellulose (SNCC)

The interactions between precipitated calcium carbonate (PCC) and sterically stabilized nanocrystalline cellulose (SNCC) have been studied. SNCC is produced by periodate oxidation of cellulose fibres and consists of a crystalline cellulose region from which dialdehyde modified cellulose chains are protruding from both ends. The morphology of SNCC can be controlled by the degree of oxidation, which depends on the amount of periodate and the reaction time. In this study, three SNCCs with different reaction times were prepared and their interactions with PCC particles were examined. These SNCC particles have similar diameters (5⿿10nm) and their length is reduced with increasing reaction time, whereas the length of the protruding chains is increased. Adsorption isotherms of three SNCCs and dialdehyde modified cellulose (DAMC) on PCC particles were determined. It was found that DAMC had a higher affinity than SNCCs, and the affinity of SNCC increased with reaction time. This indicates that DAMC chains adsorb stronger than the crystalline region of SNCC on PCC. The PCC flocculation induced by three SNCCs was monitored by photometric dispersion analysis, which showed that SNCC particles could bridge PCC to induce flocculation at low dosages (above 1mg/g). SNCC induced the maximum flocculation when its fractional coverage is above 50%, implying that PCC particles coated by SNCC continue to flocculate, as observed experimentally. Further addition of SNCC to PCC suspensions results in slower flocculation rates and smaller floc sizes.

Flocculation kinetics of precipitated calcium carbonate induced by electrosterically stabilized nanocrystalline cellulose

Abstract The interactions between precipitated calcium carbonate (PCC) and electrosterically stabilized nanocrystalline cellulose (ENCC), dissolved carboxylated cellulose (DCC), and a mixture of DCC and ENCC have been studied. ENCC can be produced by periodate, chlorite and TEMPO oxidations of cellulose fibers. It has a large surface area, a high carboxylated content and can be used as a high-performance reinforcement material. It is likely to effectively bind and flocculate with PCC to improve the properties of filled paper. Dissolved carboxylated cellulose (DCC) is produced together with ENCC in the oxidation reactions, and also has a high charge density. The flocculation of PCC induced by ENCC/DCC was measured by photometric dispersion analysis (PDA). It was demonstrated that ENCC or DCC adsorbed first on PCC particle surfaces due to electrostatic attractions, and subsequently ENCC or DCC induces flocculation between PCC. The repulsive electrostatic force reduces to zero when the isoelectric point of PCC particles is reached, allowing attractive van de Waals forces to dominate, resulting in maximum PCC flocculation. After the charge neutralization point was reached, more dissolved carboxylated cellulose adsorbed on the PCC particles, resulting in net negative charges on PCC surfaces, and dispersion of flocs due to electrostatic repulsion. For mixtures of dissolved carboxylated cellulose (DCC) and ENCC, DCC adsorbed faster on PCC than ENCC. Excess ENCC resulted in the exchange of DCC with ENCC, with ENCC particles able to bridge PCC. This mechanism was proved by photometric dispersion analysis and dynamic light scattering experiments.

Population balance modeling of precipitated calcium carbonate (PCC) flocculation induced by cationic starches

In order to predict and control the flocculation process, it is necessary to develop quantitative model which can describe aggregation, breakage and restructure phenomenon. In this paper, population balance equation was employed to model the PCC flocculation process. Energy dissipation rate was calculated to evaluate the floc strength. It was found, compared to the low charge density starch, the high charge density starch resulted in lower collision efficiency, lower restructure rate and stronger flocs (indicated by higher energy dissipation rate). The higher the temperature, the lower the energy dissipation rate was needed to break the flocs. The addition of NaCl did not show effect on the collision efficiency for both high and low charge density starches, but weaker flocs were formed and more prominent restructure rate was observed at higher NaCl concentration. The collision efficiency decreased with the increase of the shear rate for both starches. At higher starch dosage, lower energy dissipation rate could break the flocs and higher restructure rate was observed.

Effect of charge balance and dosage of polyelectrolyte complexes on the shear resistance of mineral floc strength and reversibility

We evaluated the effect of polyelectrolyte complexes (PEC) with varying balance of charges on the flocculation of precipitated calcium carbonate (PCC) particles. PECs composed of polyacrylamides carrying opposite charges (A-PAM and C-PAM) were investigated in terms of PCC floc shear resistance and re-flocculation effects. Light transmission was used in real time to monitor the dynamics of flocculation under shear fields. Compared to the single polyelectrolytes, PECs greatly enhanced particle re-flocculation while minor differences in shear resistance were observed. Shear resistance and re-flocculation depended strongly on the molecular weight and charge ratio of the PEC components. In order to achieve floc stability and re-flocculation conditions a minimum concentration of charge-asymmetric PEC should be applied.

Correlating Aggregates Structure with PEL Characteristics Using an Experimental Design Methodology

Aggregation of particles is fundamental for improving the performance of many solid/liquid processes. Aggregation can be induced by different means, one of the most common being based on the addition of polymeric additives, namely polyelectrolytes (PEL). In this work we have studied the flocculation of precipitated calcium carbonate (PCC), used as filler in papermaking, induced by a range of cationic polyacrylamides with different structures, varying in molar mass and degree of branching. The flocculation process was monitored continuously using the LDS technique. Using an experimental design strategy it was possible to produce a model which allows us to relate the aggregates structure, described by the scattering exponent (SE), with the PEL characteristics and concentration (R2 around 79%). The Partial Least Squares (PLS) regression method was selected to perform the regression, given the significant collinearity among the input variables. The results obtained lead us to conclude that the polymer intrinsic viscosity (related with the PEL molar mass and charge density, and supplying information about the volume of the polymer molecule in the solution) and the degree of branching, are the parameters with a stronger influence on the resulting flocs structure. The PLS model developed was successfully validated using an independent data set, which provides confidence on its consistency and prediction accuracy.

An experimental design methodology to evaluate the importance of different parameters on flocculation by polyelectrolytes

Aggregation of particles is fundamental for improving the performance of many solid/liquid processes. Aggregation can be induced by different means, and one of the most common is based on the addition of polymeric additives, namely polyelectrolytes. In this work we have studied the flocculation of precipitated calcium carbonate (PCC), which is used as a filler in papermaking, and is induced by a range of cationic polyacrylamides with different structures, varying in molar mass and degree of branching. The flocculation process was monitored continuously using the LDS technique (laser diffraction scattering), which provides information about the size distribution and structure (fractal dimension) of the aggregates, for each sampling instant and, thus, about the kinetic curve for the flocculation process. This information has then been used to produce a model, based on an experimental design strategy, which allows us to relate flocculation efficiency with the PEL characteristics and concentration. The Partial Least Squares (PLS) regression method was selected to perform the regression, given the significant co linearity among the input variables. The results obtained lead us to conclude that the polymer intrinsic viscosity (related with the PEL molar mass and charge density, and supplying information about the volume of the polymer molecule in the solution) and the degree of branching are the parameters with a stronger influence on flocculation efficiency. The PLS models developed were also successfully validated using an independent data set, which provides confidence on their consistency and prediction accuracy. Surface plot of flocculation efficiency (y) against intrinsic viscosity‐IV (X 3 ) and degree of branching of the polyelectrolyte chain (X 1 ). IV and degree of branching are the polymer characteristics with stronger influence on flocculation efficiency. ► Flocculation of calcium carbonate induced by cationic polyacrylamides was studied. ► LDS allowed the construction of the kinetic curves for the flocculation process. ► An experimental design strategy was used to obtain a flocculation efficiency model. ► The model response is mostly dependent on polymer intrinsic viscosity and branching.

Flocculation of precipitated calcium carbonate (PCC) by cationic tapioca starch with different charge densities. II: Population balance modeling

A population balance model to describe the flocculation of PCC by two cationic tapioca starches is presented. The model takes into account aggregation, floc breakage and floc restructure. Floc strength as indicated by energy dissipation rate was also evaluated. It was found that the high charge density starch relates to lower collision efficiency, lower restructure rate and higher floc strength (higher energy dissipation rate) compared to the case with the low charge density starch. Lower energy dissipation rate was needed to break the flocs at higher temperature for both starches. On the other hand, the high charge starch was more likely to be negatively affected by the background electrolyte NaCl. The collision efficiency decreased with the increase of the shear rate for both starches. The difference of charge, starch morphology on PCC surface and the charge suppression effect were employed to interpret the results.

Flocculation of precipitated calcium carbonate (PCC) by cationic tapioca starch with different charge densities. I: Experimental

The dynamics of the flocculation of precipitated calcium carbonate (PCC) with two commercial cationic tapioca starches (S858 and S880) with different charge densities were studied by small angle static light scattering. The effects of temperature, polymer dosage, ionic strength, and shear rate on the floc size and structure were investigated by employing a response surface methodology involving the central composite statistical design and analysis of variance (ANOVA). The evolution of floc size and structure was observed and it was found that larger and more compact flocs were produced by the high charge density starch S880. The floc's mass fractal dimension indicates the floc's compactness and it was found that the compactness increases as the flocculation proceeds. The ionic strength plays a significant role on the floc size with S880. Finally, the negative effect of shear rate on floc size was more pronounced for the low charge density starch S858.

Effect of cationic polyacrylamide on precipitated calcium carbonate flocculation: Kinetics, charge density and ionic strength

We quantified the adsorption kinetics of cationic polyacrylamide (CPAM) of three charge densities but constant molecular weight and their effects on the dynamics of a polydisperse calcium carbonate (CC) flocculation at various ionic strengths. The adsorption kinetics of CPAM onto PCC can be explained by the balance of the electrostatic and van der Waals interactions, hydrogen bonding and steric hindrance between the adsorbed and dissolved CPAM molecules and CC. While the amount of adsorbed CPAM onto CC increased as a function of the CPAM charge density in the absence of salt, this relationship reversed at increased ionic strength (I=0.1). Decreasing charge on CPAM molecules allows more amide groups available for hydrogen bonding although reducing electrostatic attraction of CC. So the lower-charge CPAM benefits from the additional hydrogen bonding at higher ionic strength when the contribution of electrostatic attraction is reduced because of charge screening by salt. Discrepancies in quantity between the adsorbed and the polymer predicted from the monolayer surface coverage theory for flocculation rate were apparent. The polymer adsorption kinetics and its conformation were important variables in determining particle flocculation in this study, more than the quantity of adsorbed polymer.

Using Light Scattering to Screen Polyelectrolytes (PEL) Performance in Flocculation

Flocculation of precipitated calcium carbonate (PCC) was monitored using light diffraction spectroscopy (LDS). Four cationic polyacrylamides of high molar mass and with different degrees of branching, all copolymers of acrylamide (AM) and acryloyloxyethyltrimethyl ammonium chloride (Q9), were tested. LDS supplied information about the kinetic curves for flocs growth and also for the flocs structure evolution. Flocculation kinetics, flocs size and structure, flocs resistance and reflocculation capacity could be correlated with the degree of branching of the polyelectrolytes (PEL). Furthermore, PEL with different degrees of branching corresponded to different values for the intrinsic viscosity, indicating differences in the polymer conformation, which explained well the performance differences in flocculation.

Understanding the Precipitated Calcium Carbonate Flocculation Mechanism Induced by Starch through Population Balance Modeling

The precipitated calcium carbonate (PCC) flocculation kinetics and floc structures induced by cationic tapioca starch were recorded by the Malvern Mastersizer 2000 (Malvern Instruments Inc, Malvern, UK). Of particular interest, a population balance model for PCC flocculation was employed to extract the flocculation constants, namely collision efficiency, magnitude of energy dissipation rate and restructuring rate. The model made an attempt to take aggregation, breakage and flocs restructuring into account simultaneously to describe the PCC flocculation by aggregation and breakage mechanism. Through a response surface methodology (RSM) involving a central composite design, the effects of temperature, polymer dosage, ionic strength, and shear rate on flocculation parameters were investigated in this paper.

Effect of Cationic Polyacrylamide Adsorption Kinetics and Ionic Strength on Precipitated Calcium Carbonate Flocculation

The effect of polymer adsorption kinetics and ionic strength on the dynamics of particle flocculation was quantified using a model system consisting of precipitated calcium carbonate (PCC) and cationic polyacrylamide (CPAM) at a low shear rate. All early flocculations detectable by a photodispersion analyzer (PDA) happened in nonequilibrium polymer adsorption regimes. We observed discrepancies in flocculation rates with the surface coverage theory, which is based on a simple monolayer adsorption model, in both early and late flocculation stages. For instance, the same amount of adsorbed CPAM reached at different polymer doses demonstrated different flocculating capabilities. This highlighted the importance of polymer adsorption kinetics upon flocculation. The transient conformation of the adsorbed CPAM during the kinetic process sometimes even superceded the adsorbed amount in the determination of PCC flocculation. Both antagonistic and synergetic effects of increased ionic strength on the CPAM-induced PCC aggregation were observed during early flocculation. However, late-stage PCC flocculation shared some similarities, irrespective of polymer dose and ionic strength. Despite the decreased amount of adsorbed polymer from the increased ionic strength, the combination of CPAM and salt, at certain concentrations, demonstrated a synergy to promote PCC aggregation more efficiently than the same amount of the respective components.

Evaluation of Polyelectrolyte Performance on PCC Flocculation Using the LDS Technique

Flocculation of precipitated calcium carbonate induced by polyelectrolytes was monitored using light diffraction scattering (LDS), which can supply the complete kinetic curve for the flocculation process. Cationic polyacrylamides of very high molecular weight with different charge densities and degrees of branching were tested. The effects of the charge density, degree of branching, and concentration of the flocculant on the flocculation process, as well as on floc resistance and reflocculation capacity, were investigated. Moreover, floc structure was evaluated through the calculation of the fractal dimension using the scattering matrix. The study demonstrates the benefit of using LDS for the evaluation, in a single test, of flocculant performance in the different stages of flocculation: aggregation, stabilization, deflocculation, and reflocculation.

Flocculation of starch-coated solidified emulsion droplets and calcium carbonate particles

In papermaking, many colloidal particles are added to a pulp fiber suspension to improve paper properties. Given the right conditions, these different colloids can interact and flocculate. Examples of papermaking colloids are fillers and internal sizing agents, which improve opacity and hydrophobicity of paper, respectively. Internal sizing agents (added at the wet end of a paper machine) are commonly solidified emulsion droplets, stabilized by cationic starch and other stabilizers. We studied the interaction of a common internal sizing agent, alkyl ketene dimer (AKD), with calcium carbonate fillers. AKD is a liquid above 50–65 °C (depending on alkyl chain length), which can be emulsified above its melting point in the presence of a stabilizer, resulting, after cooling, in solid colloidal particles close to 1 μm in size. We investigated the interaction of AKD particles, stabilized by cationic starch, with precipitated calcium carbonate (PCC) particles. Pure PCC particles are positively charged, but they become negative in process waters. Flocculation experiments with positively charged AKD and negatively charged PCC were performed using a photometric dispersion analyzer. Instead of the expected heteroflocculation between AKD and PCC, we observed PCC homoflocculation and AKD homoflocculation, results confirmed by SEM. The results are explained by the transfer of starch from AKD to PCC, resulting in PCC flocculation by starch and AKD destabilization due to depletion of the stabilizer.

Interaction Modelling of PCC Flocculation Induced by Cationic Polymer for Water Clarification

The precipitated calcium carbonate (PCC) has been extensively used as additives in various industrial applications, particularly as fillers for papermaking. Due to the limited retention being achieved in application, it is necessary to clarify the PCC suspension after the application. Cationic polymers are often used as flocculants to induce the aggregation of PCC particles and facilitate the removal of PCC in water clarification. In this work, we focused on identifying the optimum dosage of the polymer and understanding the flocculation mechanism via the characterization of the zeta potential of the PCC suspension. Moreover, the free energy of interaction between PCC particles covered by polymer was investigated using a surface energy modelling that simulated the interaction under various polymer coverages. The modelling results indicated that the electrostatic free energy increased as the surface coverage increased from 33.7 to 81.8 %. The model also enables to predict the total free energy and the electrostatic energy as a function of the separation distance in spite of the lack of complete agreement with the experimental observation.

The influence of phenolic cofactors on the properties of calcium carbonate flocs formed with PEO

The tensile strength of flocs formed from the flocculation of precipitated calcium carbonate (PCC) by the addition of cofactor (poly(vinyl phenol-co-sodium acrylate)) followed by PEO was measured as a function of the cofactor/PEO mass ratio. The maximum floc strength corresponded to a cofactor/PEO ratio of 2–3. Two modes of floc rupture were observed. Weak flocs tended to give ‘stretchy’ behavior in which the flocs extended under tension leaving a long thin polymer tether with single embedded PCC particles. Strong flocs showed a ‘tough’ mode in which elongation at rupture was low. Stretchy flocs gave distances between neighboring embedded PCC particles in the tethers as great as 30 μm indicating that many PEO chains are involved in the bridges or tethers between two particles. We speculate that these PEO assemblies originate from the mechanical entanglement of the very long PEO chains supplemented with PEO-to-PEO binding from the small cofactor molecules.

Flocculation and Retention of Precipitated Calcium Carbonate by Cationic Polymeric Microparticle Flocculants

A series of cationic polymeric microparticles (CPMP) with different charge densities and particle sizes were synthesized using emulsion or microemulsion polymerization. The interactions among positively charged polymeric microparticles, negatively charged fibers, and precipitated calcium carbonate (PCC) particles were studied. It has been found that although CPMP could adsorb onto a positively charged PCC particle surface, effective flocculation of PCC in water could not be obtained by CPMP alone. However, the combination of CPMP with an anionic polyacrylamide (APAM) did significantly improve the flocculation of PCC. The flocculation mechanics of PCC in this dual flocculation system were that the CPMP first created effective patches on the PCC surface, then APAM formed interparticle bridges through positively charged CPMP patches. It has also been found that the particle size and the surface charge of CPMP are two major factors affecting the flocculation of PCC suspension. The retention of PCC on wood fibers using a dual-flocculant system of CPMP–APAM was studied, and the related mechanism is discussed here.