Orthokinetic Conditions Research Articles

Multi-salt coagulation of soft pitch colloids

The dynamic coagulation of colloidal pitch was quantified under orthokinetic conditions. The effects of single salt, multiple salt and cation valency on the stability of the colloidal pitch were investigated as a function of salt concentration. Critical coagulation concentrations (CCCs) were determined for a range of individual cations. The CCC in the presence of a number of divalent or trivalent cations was investigated to gain an understanding of the effect of multiple salts normally found in industrial systems.Electrostatic destabilisation of wood resin colloids by a single salt is strongly influenced by salt valency (z) and mostly independent of the individual cation (at constant z). Addition of a second cation to solution resulted in a decrease in the CCC for both calcium and aluminium ions in the presence of sodium ions. The decrease in the CCC for the wood resin colloids was non-linear and showed restabilisation of the colloids above the CCC, unlike the effects observed for a single salt. Comparison of the experimental CCC results with the DVLO theory indicates that a higher Hamaker constant than reported for the interaction of abietic acid with talc in water (used here as a model interaction) is needed. This suggests that the aggregation process of the wood resin colloids involves a stronger interaction than abietic acid with talc. Experimental CCC values for divalent and trivalent ions were also lower than those predicted theoretically. To obtain agreement between theory and experimental results, reduced Stern potentials and higher Hamaker constants were required, suggesting specific ion adsorption of the multivalent cations is occurring to reduce surface charge of the colloid.

Effect of shear, temperature and pH on the dynamics of salt induced coagulation of wood resin colloids

The effect of pH, temperature and shear on the coagulation dynamics of a wood resin suspension destabilized by salt additions was investigated. Stability of wood resin colloids was measured as a function of time using a photometric dispersion analyser (PDA) upon the step addition of salt solutions. Colloidal stability upon salt addition was found to be a function of pH (directly related to colloidal wood resin charge). This follows the predictions from the Derjaguin, Landau, Verwey and Overbeek (DLVO) theory. In general temperature also affected colloidal stability according to expectations. However, the effects were found to be strongly related to salt type. The effect of shear on the stability of a colloidal suspension was more important than expected. At low shear, the critical coagulation concentration of colloidal wood resins scaled with salt valency to the −6 power. Deviations from the Shultz–Hardy rule become important as shear was increased and a critical coagulation concentration (CCC) proportional to the counter ion charge to the power of 7 (CCC∝z−7) relationship was measured at the highest shear. We suggest a relationship of the form: CCC∝(Ωz)−6τ or CCC∝[z−6τ+Ω] where Ω=f(G) and τ=g(G). An additional term would be required for the systems under orthokinetic conditions usually found in industry.

Structure and rheology ofSiO2nanoparticle suspensions under very high shear rates

High shear rate experiments have been performed with capillary microviscometers onto SiO2 nanoparticles dispersed in alcohol (so-called nanofluids). The aim of these experiments was to investigate the processes of aggregation and dislocation of the nanoparticles in a shear flow under perikinetic and orthokinetic conditions. Shear rates as high as 2x10(5) s-1 were obtained in pressure-driven microchannels laminar flows. All the nanofluids under test have displayed a Newtonian behavior but with a strong enhanced viscosity, that is, the consequence of an effective volume concentration higher than the real one. It was possible to determine the average size of the aggregates and to find a correlation between their structure and the range of the hydrodynamic Peclet number at which experiments were performed. These results display a strong evidence of the role of aggregates and support the recent conclusions about the controversy of the thermal properties of nanofluids.

The influence of flocculant adsorption kinetics on the dewaterability of kaolinite and smectite clay mineral dispersions

Polymer mediated flocculation is central to efficient water recovery from and impoundment volume reduction of mineral waste tailings, particularly where clay minerals comprise a significant proportion. In this study, the adsorption kinetics of four high molecular weight polymeric flocculants onto colloidal, negatively charged kaolinite and smectite clay substrates under orthokinetic conditions has been investigated. For both kaolinite and smectite surfaces, the adsorption rate decreased in the sequence: non-ionic polyacrylamide homopolymer (PAM N) > anionic polyacrylamide-acrylate copolymer (PAM A) > non-ionic polyethylene oxide (PEO) > anionic polyacrylamide 2-acrylamido 2-methylpropane sulphonate copolymer (PAM S). Upon flocculation under laminar flow conditions, optimum subsidence rates for both clay types decreased according to flocculant type in the order PAM S > PEO > PAM A > PAM N, implying slower polymer adsorption rate facilitated higher floc settling behaviour. Calculation of first order rate constants for the adsorption of PAM N, PAM A and PAM S onto kaolinite at 25 °C and 50 °C and subsequent estimation of an activation energy in the range 14–22 kJ mol−1 polymer suggest that the overall energy is distributed among the transition state hydrogen bonds which are PAM polymer structure independent. Rationalisation of the results suggests that under industrially relevant conditions, flocculants with relatively slower tails’ particle attachment kinetics facilitate improved bridging performance and markedly enhanced settling behaviour.

Influence of ionic calcium on stability of sodium caseinate emulsions

The stability of fine sodium caseinate emulsions (1 wt.% protein, 25 vol.% n-tetradecane, 20 mM imidazole, pH 7) containing various concentrations of calcium chloride has been investigated under perikinetic and orthokinetic conditions by measuring time-dependent changes in droplet-size distribution. Under quiescent storage conditions at 20°C, samples containing at least 10 mM ionic calcium added after emulsification were found to exhibit an increasing average droplet size with time and a developing bimodal droplet-size distribution. Under turbulent conditions of intense shearing, these same emulsions exhibited time-dependent flocculation and coalescence. This interpretation was confirmed by light microscopy. Emulsions prepared with up to 6 mM Ca 2+ present during emulsification were stable in the presence or absence of flow, but satisfactory emulsions could not be prepared containing more than 6 mM ionic calcium. The results show that the emulsion stability is sensitive to whether the calcium ion content is adjusted before or after homogenization.

Perikinetic and Orthokinetic Aggregation/Fragmentation of Hairy Colloids

The aggregation of spherical colloids partially stabilized by a thin hairy surface layer was investigated under peri- and orthokinetic conditions. The weight and number average sizes of the aggregates were derived from the aggregate size distribution which was provided by particle counting. Perikinetic aggregation conditions were obtained in batches where polystyrene latex particles were suspended in a deuterium oxide/water mixture in order to prevent sedimentation. Periodically, small samples were taken from the medium and immediately analyzed. Orthokinetic aggregation conditions were established by flowing the suspension through a 1.25-m-length capillary of various internal diameter. Samples were collected at the outlet of the tube and immediately analyzed. At any time the aggregate size was found to decrease with the colloid concentration. Aggregate aging over periods of the order of the average time elapsed between two successive collisions was found to exert a major effect on the aggregate sizes in peri- and orthokinetic processes. This effect was increased by the particle concentration showing typical threshold effects such as a fast aggregation in dilute suspensions, a reduced influence of the concentration in the intermediate domain and a strong inhibition of the aggregate growth in more concentrated systems. Orthokinetic aggregation of hairy colloids was found to produce a narrower aggregate size distribution than perikinetic aggregation.

Coagulation rate of silica dispersions investigated by single-particle optical sizing

Abstract Using a single-particle optical sizer, the kinetics of the rapid coagulation of monodisperse silica particles of radius 265 nm in KCl and CaCl2 solutions was studied. The rate constant k11 of the process is 1.7 × 10−12 cm3 s−1, which is three times smaller than the Smoluchowski value for the overall rate constant. This discrepancy is explained by the retardation effect of viscous interactions and the existence of long-range attractive forces between particles. For suspensions with an initial particle concentration between 2.5 × 108 and 2.4 × 109 cm−3, the k11 values are independent of the pH value (in the range 7–10) and of the particle number concentration. The ratio between the coagulation concentrations of monovalent and bivalent counterions is 50, under both perikinetic and orthokinetic conditions, which indicates that this silica suspension is electrostatically stabilized. This is in contrast to literature data for very small SiO2 particles. We propose that the so-called “structural” forces are much less important for large silica particles.