Investigation of the role of interfacial chemistry on particle interactions, sedimentation and electroosmotic dewatering of model kaolinite dispersions

The influence of pulp chemistry on particle interactions and dewatering behaviour of colloidal kaolinite dispersions has been investigated under coagulation conditions. The dispersion shear yield stress, settling rate and consolidation showed strong dependence upon pH and ionic strength, indicating a maximum at ∼pH 3.2 which was established as the isoelectric point (iep) by particle zeta potential analysis. A “gel point” solid concentration at which the dispersion began to be significantly networked and gravity-driven consolidation of the pulp was completely suppressed, occurred at 13 vol.% (∼28 wt.%). The dewatering rates due to coagulation were significantly lower than those commonly achieved by polymeric flocculation, however the sediment consolidation was ∼25% higher when compared with flocculated pulps. Electroosmosis was found to be effective in consolidating pre-sedimented pulps to spadeable pastes (∼30 vol.%) at pH values away from the iep where zeta potential was higher and ionic strength low (10^− 3 M). This pulp consistency or markedly improved consolidation behavior is not achievable under coagulation and/or flocculation conditions.     

Destabilization of model silica dispersions by polyelectrolyte complex particles with different charge excess,hydrophobicity, and particle size

Nonstoichiometric polyelectrolyte complex dispersions (PECs) with different charge excess and hydrophobicity as well as different average hydrodynamic particle size were used to induce flocculation of oppositely charged silica dispersions. PECs were formed with poly(diallyldimethylammonium chloride) (PD) as polycation and poly(styrene‐p‐sodium sulfonate) (NaPSS) and poly(acrylamide‐co‐sodium acrylate) (PR2540) as polyanions. PD was used as single polymer flocculant too. Flocculation process was characterized by optical densities of supernatants OD₅₀₀, by UV/VIS spectrometry, and determination of average floc sizes D(v; 0.5) as well as volume distribution of floc sizes by laser diffraction and determination of sedimentation velocity s by means of LUMiFuge? 114. It was found that the reaction process between silica and the used flocculants could be divided into three intervals (destabilization, flocculation optimum, and restabilization) as it is known for all other polymer flocculants. For an effective flocculation of a charged substrate, both electrostatic as well as hydrophobic interactions play an important role. The interval up to the beginning of the flocculation optimum is mainly ascertained by electrostatic interactions (the charge density of the flocculant) but the broadness of flocculation optimum depends largely on hydrophobic interactions. Hydrophobic interactions also play an important role for shear stability of the formed flocs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3776–3784, 2007     

Rheology of aging aqueous muscovite clay dispersions

In hydrometallurgical mineral processing, time and pH dependent rheology of dispersions comprising reactive particles can exert a striking influence on material processability and handleability. Rheological studies of 0.32 solid volume fraction (57 wt% solid) aqueous dispersions of muscovite clay mineral aging at pH 7 and 1 for 4 h were undertaken to investigate the temporal, pulp chemistry-mediated particle interactions. In particular, the dispersions’ flow and deformation behaviour in tandem with viscoelastic structure changes over time were probed. Dispersions at both pH 7 and 1 displayed non-Newtonian, Bingham plastic behaviour and strong time-dependent viscosities and yield stresses. Fresh dispersions showed a weak rheology at pH 7 initially, accompanied by thixotropy which transformed to anti-thixotropic and rheopectic behaviour upon aging. Furthermore, the viscosities, yield stresses and both the viscous (G″) and elastic (G′) moduli accentuated dramatically with time, reflecting an elastic to a more sol-like viscoelastic gel transformation. In contrast to pH 7, the particle interactions at pH 1 initially led to the formation of a highly networked gel structure which displayed thixotropic and predominantly elastic behaviour. Upon aging however, systematic attenuation of thixotropy, suspension viscosity, shear yield stress and viscoelasticity occurred. Above certain shear stresses, viscoelastic gel to sol structure transformation occurred, the creep behaviour being distinctly pH and time dependent. Differing pH-mediated pulp chemistries which prevailed at pH 7 and 1 upon aging were responsible for the contrasting, temporal particle interactions and microstructure evolution.     

Flocculation and viscoelastic behavior of industrial papermaking suspensions

The effects of the surface charge type and density C496, C492 and A130LMW polyacrylamides (PAMs) on the rheological behavior of real industrial papermaking suspensions were quantitatively related to the degree of flocculation for the same industrial papermaking suspensions. The floc sizes were larger but less dense when anionic PAM was used, and this due to the repulsive forces between the anionic PAM and colloidal particles, leading to the development of open structure flocs of less density. On the other hand, rheological measurements showed that the papermaking suspension is thixotropic with a measurable yield stress. The results showed that the magnitude of the critical stress, τ _c , complex viscosity, η*, elastic modulus, G′, and viscous modulus, G″, depend on the number of interactions between the PAM chains and particle surface and the strength of those interactions. Cationic PAM showed higher values of η*, G′, G″ and τ _c compared to anionic PAM. This behavior is in good agreement with Bingham yield stress, τ _B , adsorption and effective floc density results. Similar to oscillatory measurements, creep measurements also showed that the deformation was much lower for the cationic PAM based suspensions than for the anionic PAM based suspensions. Furthermore, the results revealed that increasing the cationic PAM surface charge decreases the floc size but increases the adsorption rate, elasticity and effective floc density proposing differences in the floc structures, which are not revealed clearly in the Bingham yield stress measurements.     

PAM和粘土矿物的相互作用试验研究

选用不同电性、电量及分子量的PAM、两种典型的纯净粘土矿物样本,就PAM对分散的粘土颗粒的絮凝能力和机制、粘土矿物对PAM的吸附及解吸附进行了研究。结果表明,在没有阳离子桥的情况下,3种PAM对粘土矿物的絮凝能力依次为：阳离子PAM〉非离子PAM〉阴离子PAM,阳离子PAM为有效的絮凝剂,而阴离子PAM则为分散剂;粘土矿物对PAM的吸附量除与PAM的电性关系非常密切外,也受到粘土矿物本身特性的影响,尤其是所带电荷的性质与数量及比表面积的大小;PAM一旦被粘土颗粒所吸附,解吸附几乎是不可能的。PAM与粘土间复杂错综的了互作用姜系拘威了PAM在￡壤政虞、混水净化中应用的理论基础。     

Polyacrylamide induced flocculation of a cement suspension

The use of cellulose, instead of asbestos, in the fibre cement composites manufacture, using the Hatschek process, reduces cement retention and makes necessary to use a flocculant which is crucial for the plant productivity. The use of different types and doses of polyacrylamides (PAM) as well as the addition process, have been studied to obtain an in-depth knowledge of floc properties. A real-time methodology has been used to study size, shape, strength and reversibility of formed flocs, based on a focused beam reflectance measurement (FBRM) system. The results have been corroborated by particle vision and measurement (PVM) analysis. This paper shows that anionic PAM (A-PAM) are the most suitable to induce cement flocculation and to obtain optimal properties of the formed flocs. This is because the flocculation process is enhanced by the interaction of the Ca²⁺ ions, produced by the cement hydration, with the carboxylic groups of the polymer chains. Cations not only increase the stiffness of the chains, improving floc strength, they also enhance the importance of the patching aggregation mechanism when the initial bridges are broken. Higher molecular weight polymers improve initial aggregation but the effect of anionic charge is more important for a stable flocculation with time under variable shear conditions. Results show that the optimal dosage is between 100 and 200 ppm.     

Rheological behavior of a polymerically stabilized latex for use in water‐borne coatings

A unique urethane linkage that permits chemical grafting of poly(ethylene oxide) (PEO) linear chains to the surfaces of polystyrene (PS) latex particles has been developed. Chemically grafting the functionalized hydrophilic PEO macromers to the PS particle surface allows the latex to be polymerically stabilized in a water‐based medium. Advantages of the urethane linkage include the high yield of the macromer synthesis and the hydrolytic stability of the final latex. Rheological experiments are used to examine both processing behavior and interparticle interactions for latex systems with different amounts of grafted PEO. Dynamic rheological experiments reveal that, at high macromer concentrations, the grafted PEO layer is effective in shielding the attractive interactions of the core PS particles that lead to flocculation. However, at low macromer concentrations, strong interactions are seen even at low particle weight fractions, indicating the presence of a flocculated system. Steady shear rheological evaluations show that the latex systems possessed suitable flow behavior for coating applications, even at relatively high particle weight fractions. Experimental steady shear data is utilized in conjunction with the Krieger‐Dougherty equation to determine the size of the PEO stabilizing layer. The stabilizing layer thickness decreases as particle concentration increases, indicating a compressible system. Finally, the relationship between the strength of interparticle interactions and PEO graft density is gauged from the dependence of the power‐law exponent of the elastic modulus on particle concentration.     

The structure and strength of depletion force induced particle aggregates

Aggregating fine particulate matter is common practice in many industrial solid-liquid separation processes. Data obtained in this work on dilute aqueous dispersions of model colloidal polystyrene latex spheres indicate that depletion flocculation, which uses non-adsorbing polymer, can yield very compact aggregates. Flocculation of the negatively charged latex particles was induced by the addition of a poly(acrylic acid) at pH 10. The structural compactness of the latex flocs formed in the dilute dispersions was characterised using small-angle static light scattering in terms of mass fractal dimensions. Rheological measurements on the concentrated latex dispersions in the presence of the non-adsorbing polyacid showed Bingham yield stress behaviour. Both the compactness and strength of the latex flocs were found to be significantly dependent upon the level of the polyacid, as well as the concentration of the initial particles. In particular, as the level of the polyacid was raised the floc compactness decreased, whereas its strength increased. They were both seen to level off at high polymer concentrations. Atomic force microscopy measurements were made at varying concentrations of the polyacid to provide a qualitative explanation of the observed floc structural behaviour of the dilute dispersions. By combining the fractal dimension and the Bingham yield stress we were also able to estimate the energy required to separate the flocs into single units in the concentrated dispersions. It was concluded that the interparticle interaction energy is the key to understanding the dependence of both the floc structure and strength on the polymer concentration.     

Flocculation of Kraft Lignin Sols with Polyethylene Oxide

Abstract

The flocculation of a specially prepared sol of kraft lignin by polyethylene oxide (PEO) was studied. The effects of polymer dosage, sol concentration, pH, and electrolyte addition (NaCl and CaCl₂) on the flocculation behavior were investigated. The efficiency of lignin removal increased with increasing electrolyte concentration, valency of the counterion, and decreasing pH of the sol. In all cases maximum flocculation occurred at an optimum polymer dosage (O.P.D.) which was found to be independent of pH and electrolyte concentration within the intervals studied. The O.P.D. (as percentage of the sol concentration) was found to be independent of the sol concentration. The mechanism of restabilization was studied by heating the stable dispersions to temperatures exceeding the θ-temperature of the PEO. The critical flocculation temperature was found to coincide with the θ-temperature of the PEO (enthalpic stabilization). The mechanism of flocculation is discussed and it is concluded that bridging occurs when the thickness of the electrostatic double layer is sufficiently reduced by addition of an electrolyte (sensitization flocculation) or by charge reduction on the particle surface (protonization).     

磷矿酸解渣与聚丙烯酰胺絮凝动力学

通过激光粒度仪在线监测搅拌槽内聚丙烯酰胺与磷矿酸解料浆的絮凝过程 ,研究了絮凝剂加入量、絮凝温度、搅拌速度和酸解渣原始平均粒径对絮凝动力学的影响 .用电泳试验测定酸解渣的表面电性 ,合成不同荷电类型的絮凝剂及通过絮凝剂加入量对絮凝过程的影响实验可以判定磷矿酸解渣与聚丙烯酰胺的絮凝过程符合架桥机理     

搅拌槽内絮凝动力学研究

采用激光粒度仪在线测定并研究了搅拌槽内聚丙烯酰胺与磷矿酸解渣的絮凝动力学。考察了不同浓度的酸解液，絮凝剂加入量，温度，酸解渣原始平均粒径和搅拌强度对絮凝动力学影响。在实验研究范围，絮凝初始速度随温度升高，絮凝剂加入量及搅拌强度的增加而提高，但随着酸解液的粘度增加和酸解渣原始平均粒径的增大而降低，体系中存在最佳的絮凝剂加入量。通过酸解渣的电泳试验，用自合成的阴离子，阳离子改性絮凝剂以及非离子型絮凝剂与酸解渣的絮凝试验发现，在中性的水中，酸解渣与不同荷电类型的絮凝剂的絮凝速度大小依次为：阳离子型＞非离子型＞阴离子型。而在磷酸酸解料浆中絮凝速度与絮凝剂的荷电类型无关。磷矿酸解渣与聚丙酰胺絮凝剂的作用主要表现为架桥机理。     

水动力学条件对絮体形成的影响

利用颗粒运动方程,分析颗粒运动导致的碰撞絮凝,提出涡旋惯性离心力和剪切力是颗粒接触絮凝的主要动力因素,其中涡旋剪切力是主导动力;湍流过渡区控制絮体的成长尺度和密度;并讨论了提高絮凝效率的技术途径。     

Comparison of two cationic polymeric flocculant architectures on the destabilization of negatively charged latex suspensions

Flocculation studies between cationic polymers and oppositely charged colloidal particles are reported in which both flocculation kinetics and floc structures are systematically investigated. The flocculation rate constant, stability ratio and kinetics laws are experimentally determined using particle counting for two polymer architectures; a cationic linear polymer and a two-branched polymer. Comparisons are also made using NaCl at different ionic concentrations for the destabilization of the colloidal particles. Detailed measurements of electrophoretic mobility and kinetics rate constants on varying the polymer dosage are reported. Results suggest that the polymer architecture plays important roles on the polymer dosage for the rapid destabilization of the colloidal suspension. The branched polymer at optimal dosage exhibits the highest flocculation rate constant, whereas on the other hand, the linear polymer concentration range of flocculation is larger. In both cases, polymer flocculation is more efficient by a factor of 5-6 than charge screening effects due to the presence of salt. Analysis of the stability ratio indicates that tele-bridging flocculation and electrostatic forces dictate the stability of the charged latex particle suspension. It is shown that the fractal concepts which are valid for aggregation processes are also applicable here and branched polymers as well as linear polymers yield to the formation of compact flocs in comparison to those obtained with salt.     

Effects of Polymer Additives and Dispersion State on the Mechanical Properties of Cellulose Nanocrystal Films

Films produced from colloidal and nanomaterial dispersions such as aqueous cellulose nanocrystal (CNC) dispersions tend to crack above a critical cracking thickness; this issue limits the ability to produce flow‐aligned CNC films at thicknesses above ≈50 μm. This research explores the effects of adding polyethylene oxide (PEO), flocculation with hydrochloric acid (HCl), and the combination of both methods on film cracking, mechanical properties, and mechanical anisotropy. Most research on CNC polymer composites focuses on composites where the polymer is the majority component. To the authors' knowledge there has been only one investigation of CNC films' mechanical anisotropy and no studies of using HCl flocculation to flocculate CNC dispersions prior to shear casting. PEO addition significantly reduces Young's modulus, tensile strength, and toughness, but enhances the Young's modulus anisotropy. Flocculation results in little property deterioration but nearly eliminates mechanical anisotropy. The combination of both techniques results in similar properties as flocculation alone. These findings highlight the trade‐offs between these two approaches and can be used to help guide further research on obtaining robust shear cast CNC films over a range of thicknesses.

     

Flocculation,Retention and Drainage in Papermaking: A Comparative Study of Polymeric Additives

Fibre and filler flocculation, filler retention and drainage, induced by several polymeric retention aids, were compared in laboratory experiments on mixtures of kraft fibres and two calcium carbonate fillers. Some experiments were also performed on thermomechanical pulp and de‐inked pulp fibres. Flocculation was measured by a focused beam reflectance measurement probe. It was found that flocs induced by polyethylene‐oxide (PEO) and cofactor broke up with time and shear and could not be reformed subsequently. Floc strength was the highest for PEO and the weakest for polyethylenimine and polyaluminium chloride. When comparing filler retention under optimal flocculation conditions, we found similar filler retention for all retention aids. Salt did not affect drainage for cationic retention aids, but reduced the drainage rate for PEO. Drainage with PEO was considerably slower than for other retention aids.     

助凝助沉剂在矿山酸性废水处理中的试验研究

以某铜矿山酸性废水为处理对象,综合运用Zeta电位、粒度分析等方法对助凝助沉剂加速絮凝物沉淀的机理进行研究.试验结果表明,废水pH=8.0,阳离子型高分子絮凝剂PAM的投加量为1.6mg/L,助凝助沉剂用量为2.5g/L时,可以取得较好的絮凝沉淀效果,沉淀速度较快,沉淀物粒径较大.     

A model of floc strength under hydrodynamic forces

The strength of an idealised model of a floc has been examined with a view to calculating the effect of hydrodynamic forces on floc stability. Various spatial arrangements of spherical primary particles were considered joined by elastic polymer links, in accordance with the accepted bridging mechanisms of flocculation. An available structural frame analysis computer program was used to find the forces in the links as the flocs were considered stressed by simple shear fields. For a given shape of floc a dependence of maximum stable size on the shear gradient was established. There is general agreement with published experimental findings of the variation of the maximum size of an aggregate for a given shear gradient.     

Production and Characterization of Cellulose Nanofibrils from Different Chemical and Mechanical Pulps

In this study, mechanical fibrillation for the production of cellulose nanofibrils (CNF) from chemical and mechanical pulps with different chemical compositions was studied. To investigate the effect of nanofibrillation on wood pulps by the grinder, the nanofibrils obtained from grinded pulp were characterized with morphology, particle size distribution, apparent viscosity in aqueous solution, degree of crystallinity, and water retention capacity. The results showed that the low lignin-containing unbleached kraft pulp (UKP) exhibited good performance for fibrillation, resulting in CNF with high viscosity, high water retention value, and small particle size. However, the fibrillation of high lignin-containing chemi-thermomechanical pulp was the most inefficient which resulted in heterogeneous materials with relatively low viscosity, low water retention value, and large particle size compared to chemical pulps. Furthermore, bleached softwood pulp from radiata pine was found to be much faster and for easier fibrillation compared to the bleached hardwood pulp from acacia due to the more rigid structure of hardwood fibers.     

The rheology of aqueous alumina suspensions in the presence of hydroxyethylcellulose as binder

The rheological behaviour of aqueous colloidal alumina dispersions stabilised by the dispersant Darvan C, an ammonium salt of polymethacrylic acid, was investigated in presence of hydroxyethyl cellulose (HEC) at room temperature. Measurements were taken under both oscillatory and steady shear. It was found that the polymer induces flocculation of the particles probably due to a depletion mechanism. The flocculation process results in the creation of a particle network, which restricts particle movements and gives rise to a wide distribution of relaxation times. At high HEC concentrations, two sets of relaxation times can be distinguished, one at low and the other at high frequencies. At high solids content and high HEC concentrations, the response of the system becomes predominantly elastic at low frequencies. The behaviour is similar to an entangled network of high molecular weight uncross-linked polymer solution. Thus, HEC causes the formation of ‘topological restraints' comprised of chains of particles that give rise to a wider distribution of relaxation times with significantly lower terminal relaxation times. In addition, similarly to a polymer solution, complete superposition occurs of the apparent and dynamic viscosity in accordance to the Cox–Merz rule. Taking into account the combined data, a Cross type of formulation was found to fit the behaviour of all dispersions that can be represented in terms of a single dimensionless parameter. The behaviour of the adjustable parameters of this empirical model suggests a diffusion controlled cluster–cluster aggregation process that induces a more closed packed particle arrangement at high shear rates. ©     

EFFECTS OF DOSING REGIMEN AND AGITATION PROFILE UPON FLOC CHARACTERISTICS

The efficiency of solid-liquid separation processes that employ flocculation and sedimentation mainly depends upon the characteristics of aggregates produced by coagulation. Size and density are foremost, however, most flocculation processes are designed such that many larger floes are formed by floc-floc collisions resulting in the entrapment of large amounts of interstitial water. Thus, as particle size increases, floc density generally decreases. Not only does this affect the achievable rate of sedimentation, but it also contributes to the volume of process sludge that must be dewatered prior to ultimate disposal. The objective of the present work is to examine combinations of flocculant dosing and activation along with shear profile or history that can produce flocs of unusually compact structure, thereby increasing the efficiency of separation and reducing the volume of sludge produced. Four types of batch coagulation experiments were conducted, employing both single and intermittent polymer applications as well as periodic episodes of elevated shear to provide more compact constituent floc structures. A light obscuration method, in which the increase in diode phototube output during sedimentation was used to assess mean aggregate density, was employed throughout the study; it provided a convenient means for comparison of the effects of process modifications upon sedimentation.