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We study the viscoelastic solid properties of cohesive particulate suspensions using creep and constant rate tests in a vane-in-large-cup geometry. A cup-to-vane diameter ratio larger than 4 is used to ensure that wall effects are minimized. In both the creep and constant rate tests, the modulus becomes nonlinear at strains consistent with scaled interparticle bond distances. Yielding and subsequent flow do not occur until strains of order 1, corresponding to a cage melting or particle crowding failure mechanism. In between the bond and cage melting strains, the modulus shows power-law softening with an index of approximately −0.8, corresponding to progressive bond breakage. The observed behavior in creep correlates with constant rate experiments and demonstrates that strain softening of the modulus prior to yielding is an important component of the rheology of particulate gels and suspensions. The assumption of linear behavior up to yielding appears to be inappropriate. We observe that the transient creep behavior initially shows power-law or Andrade creep that transforms to an exponential decay at long times. For creep stresses that show time-dependent yield, the break time decreases exponentially with stress. [ABSTRACT FROM AUTHOR]

In some batch filtration processes, highly permeable suspensions dewater fast compared to the rest of the process. Here, the impact of fast-filtering compressible materials on the throughput of fixed-chamber filter presses is explored. The dewatering properties for a compressible yet highly permeable minerals processing slurry are used as inputs to a filter press model to explore the effects of operating and design parameters. For fast-filtering materials, maximum throughput is achieved with wide cavities and minimal handling time, while membrane resistance can be significant. Pressure affects the maximum achievable concentration, as given by the strength of the material. Overall, this work demonstrates the combined use of material characterization and device modeling for filter press optimization. [ABSTRACT FROM AUTHOR]

An experimental system has been found recently, a set of coagulated CaCO 3 suspensions, which shows very variable yield behaviour depending upon how it is tested and, specifically, at what rate it is sheared. At Péclet numbers (Pe) > 1 it behaves as a simple Herschel–Bulkley liquid, whereas at Pe < 1 highly non-monotonic flow curves are seen. In controlled stress testing it shows hysteresis and shear banding and in the usual type of controlled stress scan routinely used to measure flow curves, it can show very erratic and irreproducible behaviour. All of these features appear to arise from a dependence of the solid phase, or yield stress, on the prevailing rate of shear at the yield point. Stress growth curves obtained from step strain-rate testing showed that rate-dependence was a consequence of Péclet number dependent strain softening. At very low Pe, yield was cooperative and the yield strain was order-one, whereas as Pe approached unity, the yield strain reduced to that needed to break inter-particle bonds, causing the yield stress to be greatly reduced. It is suspected that rate-dependent yield could well be the rule rather than the exception for cohesive suspensions more generally. If so, then the Herschel–Bulkley equation can usefully be generalized to read σ = σ 0 g ( γ ̇ ) + σ iso + k γ ̇ n (in simple shear). The proposition that rate-dependent yield could be general for cohesive suspensions is amenable to critical experimental testing by a range of means and along lines suggested. [ABSTRACT FROM AUTHOR]

The measurement of the local concentration of suspended particles within settling multiphase systems is important to many engineering applications; particularly for example in gravitational thickeners used extensively in minerals processing and water treatment. The ability to monitor concentration and stratification in situ may greatly aid in developing efficient separators and allow more accurate modeling of these processes. Acoustic backscatter systems (ABS) are a practical and relatively inexpensivecharacterization technique, with the ability to profile particle concentration as a function of depthin operationalenvironments. Currently, such instruments are only used for sedimentology studies in dilute estuarine flows as their application in concentrated industrial suspensions is significantly more complicated and largely unstudied. A novel application is reported herein investigating the use of an ABS to characterize a model mineral separation system, utilizing a bespoke continuous flow laboratory scale thickener with inline pipe flocculator. A 1 MHz probe mounted in the near-bed region of the thickener was used to measure consolidated bed build-up and segmented density changes from the signal decay. Importantly, the influence of a scraper-rake on solids densification was observed by analyzing systems at two different scraper rotation speeds (0.1 and 1.1 rpm). An optimal rotation speed was observed that led to enhanced underflow solids fractions. Higher speeds led to an assumed fluidization of the bed from secondary flows, which reduced bed densities in the vicinity of the scraper. [ABSTRACT FROM AUTHOR]

Models for rotary drum and disc filtration of compressible suspensions are developed using one-dimensional compressional rheology theory. The models account for cake formation while the drum or disc is submerged in the feed slurry, saturated cake consolidation upon surfacing due to capillary pressure, and cake desaturation. Desaturation does not occur for vacuum pressures below a critical value given by the material properties, or is incomplete if the formation and consolidation angles are too large. The disc filter model is formulated by applying the drum filter solutions to concentric annuli. The effect of different operating parameters such as submerged depth, rotational rate, slurry concentration, and vacuum pressure are investigated. The throughput varies linearly or quadratically with the drum or disc radius respectively, proportionally with the square-root of the angular rotational speed and linearly with the submerged angle. The angles for cake consolidation and desaturation are independent of the rotational rate. © 2010 American Institute of Chemical Engineers AIChE J, 2011 [ABSTRACT FROM AUTHOR]

The article presents a study on the one-dimensional modeling of various possible processes in the vacuum filtration of compressible flocculated suspensions. Mathematical models for such processes using the compressional rheology approach are described together with outlines of appropriate solution methods. The cake formation and consolidation components of the model have reportedly been validated, but experimental validation is required for the drainage model.

Shear stress versus shear rate and, hence, viscosity measurements for industrial materials are often difficult to measure using conventional rheometer geometries due the effects of particle size, particle migration, settling, and slip. It is demonstrated here that a four bladed vane is not only useful for yield stress measurement but is also a convenient device for the determination of the shear stress versus shear rate characteristics of industrial materials. The application considered is that of the vane in an infinite medium (a bucket) where the shear rate is precisely determined. Data for an alumina sample at two different states of dispersion and three industrial nickel laterite suspensions are presented to confirm the conclusion that a vane rotated in a bucket of fluid is a practical industrial rheometer. [ABSTRACT FROM AUTHOR]

Abstract: The surface of alumina, which is hydrophilic in general, was made hydrophobic either by adsorption of polymer (phys-adsorption) or by an alkylation reaction with alcohol (chem-adsorption) to enable dispersion in dodecane. Hypermer A70 (8.2nm) was used as the polymer and 1-octanol (1.2nm), 1-decanol (1.5nm) and 1-hexadecanol (2.5nm) were used as the alcohol (values in brackets are the approximate thickness of the steric barrier). Rheological measurements of ceramic suspensions indicate that it is possible to achieve a high solid loading (50vol.%) with relatively low viscosity (0.25Pas at 100s−1, the typical shear rate for pumping of liquids in pipes) as long as the stabilising molecule is large enough. The observed rheological behaviour fitted the Quemada viscosity model quite well when excluded volume effects were taken into account. Addition of 2.8wt.% of Hypermer A70 with respect to weight of alumina was enough to stabilise the particles. [Copyright &y& Elsevier]

Abstract: Mathematical models are potentially a valuable tool for the prediction of continuous gravity thickener operation. However, experience shows that existing mathematical models underestimate dewatering in thickeners for flocculated feed materials when predictions are made of either the underflow solids concentration for a given solids feed flux density or the maximum solids feed flux density achievable for a minimum underflow solids concentration set point. One reason postulated for this discrepancy is shear enhancement of sedimentation and bed dewatering as a result of aggregate densification. This process is not taken into account in conventional 1-D thickener models. A pilot scale column, operated at low bed heights without the addition of mechanical shear, produced results that compared well with 1-D model predictions. The effect of mechanical shear and/or greater bed height was to significantly enhance thickener performance relative to model predictions (as measured by underflow density or maximum solids flux density achievable for a nominated underflow density). An experimental method was developed that enabled shear to be incorporated into the suspension dewatering characterisation. The results suggest an order of magnitude increase in solid flux density can be expected under controlled shear conditions with polymer flocculated aggregates. The results also indicate that mechanical shear is not the only factor that can enhance dewatering, since higher beds, and hence longer residence times, also improve the achievable solids flux density. This is despite the fact that the thickener is operating in a regime that is predicted to be limited by the sediment permeability and not its compressibility. This suggests an additional mechanism must be at play in full scale operation and points a direction for further experimentation. [Copyright &y& Elsevier]

Abstract: Investigation of the response of plate and frame filter performance to changes in design and operating variables is complicated by the large matrix of potential variables and the inter-dependent kinetics of the fill, press and air blow stages. However, through some key simplifying constraints an integrated optimisation framework has been developed. An example application is presented for an iron ore fines material from the North West of Australia, which indicates an optimum filling duration exists for all conditions investigated and illustrates the throughput benefits of higher feed solids and pressing pressures and larger cavity thicknesses. Significantly, the optimum filling duration is not constant and depends on the ratio of pressing to filling pressure and cavity thickness employed, largely due to the important role of the pressing stage not from the perspective of compression, but rather from that of an increased rate of secondary filtration of unfiltered suspension remaining at the end of the filling stage. Although simplistic in the form presented and limited to incompressible systems, with the integration of more rigorous theoretical treatments of the individual filter stages, the applicability of the approach could be broadened to all types of substrates. [Copyright &y& Elsevier]

Abstract: This work describes an easy methodology for assessing the compressibility of particulate suspensions at relatively low pressures. The material used in this study was an anaerobically digested waste water sludge. Using a centrifuge, the equilibrium sediment bed height was recorded as a function of rotational speed during the centrifugation. This approach avoids errors due to expansion of the sediment bed after the centrifugation is stopped, a phenomenon typically seen with waste water sludges. A straightforward numerical method was used to solve a one-dimensional centrifugation model describing the equilibrium sediment bed height as a function of rotational speed. It has been shown that this approach yields realistic results. The one-dimensional model is able to describe the compressibility of the anaerobically digested sludge. Besides yielding a close fit for the measured sediment bed heights, extrapolating these results to a different initial solids volume fraction resulted in a good prediction of the bed height as well. Different constitutive equations for the relation between solids pressure and solids volume fraction were assessed. The typical power law functions [F. Tiller, W. Leu, Basic data fitting in filtration, Journal of the Chinese Institute of Chemical Engineers 11 (1980) 61–70], often used in filtration modelling, were the most adequate for describing the compressional behaviour of the sludge. A functional relation based on the osmotic pressure within the solids network, suggested for describing the phenomenology of filtration dewatering [K. Keiding, M. Rasmussen, Osmotic effects in sludge dewatering, Advances in Environmental Research 7 (2003) 641–645], could not be used to adequately describe the behaviour of the sludge at the low pressures generated during centrifugation. [Copyright &y& Elsevier]

Abstract: The role of natural organic matter (NOM) in changing the interaction between particles and subsequently the flow properties of concentrated particulate slurries has been modelled using electrokinetic and shear yield stress measurement data. Natural organic matter from soil, coal and peat sources as well as full and fractionated samples from aquatic sources has been examined. The data shows that the majority of the humic and fulvic rich samples adsorb in a flat configuration on a model alumina surface where the interaction is driven by a combination of electrostatics and chemical contributions to the adsorption free energy. Using a DLVO based model that assumes that the rheology response of a concentrated suspension of particles at low strain is the result of the summation of pair-wise particle interactions, the assumption of either a flat or molecularly dense adsorption configuration for most of the species at high additive addition rates was found to be valid. The model showed a consistent relationship between the measured rheology and electrokinetic data in terms of colloidal stability. The same modelling exercise for selected NOM samples at low dose showed a discrepancy between the electrokinetic and rheology data. This was for the NOM samples dominated by the neutral and hydrophilic fractions. The model indicated that the measured electrokinetics were poorly representative of the colloid stability possibly due to the presence of a poorly contiguous adsorbed layer that affects the hydrodynamic flow during electrokinetic measurements. [Copyright &y& Elsevier]

Abstract: The electrokinetics and rheology of a model alumina suspension has been investigated in the presence of a series of natural organic matter (NOM) fractions from soil and aquatic sources. The role of natural organic matter in changing both the dispersed state of the solids and the rheology of concentrated suspensions has been investigated. The data shows that at high surface coverage there is a good correlation between the nature of the adsorption layer and the electrokinetic and rheology responses. The data highlights key differences between the different fractions in aiding dispersion. At intermediate surface coverage, the correlation is less robust particularly for the more hydrophilic NOM fractions. The measured value of the electrokinetic potential is no longer a good indicator of colloidal stability as indicated by a DLVO type analysis. The reason may be due to the adsorbed layer showing a different response to particle movement in electrokinetic measurements than it does in static rheology measurements. In the latter case, the behaviour is dominated by particle interaction. [Copyright &y& Elsevier]

A new empirical three-parameter viscoelastic model is proposed for describing the rheological behavior of linear and nonlinear materials. In particular, the model succesfully reproduced experimental creep curves at various constant stresses for flocculated suspensions of alumina in the presence of different types of seawater salts. The new model is compared to the classical standard linear solid model and to an extension of this, also proposed here, to represent nonlinear materials. Based on the goodness of fit, the new model is found to be the most appropriate for the experimental system studied. A sensitivity analysis of the parameters to applied stress and type of salt in the sediments reveals that the new model, originally proposed as an ansatz, has three components, constant, linear and nonlinear, as inferred from the behaviour of each model parameter with the applied stresses. The linear contribution follows a correlation with the applied stresses for salts that enhance the hydrogen network of water, so called maker salts, and another for the salts that weaken the network, so called breaker salts. Even more interesting, the nonlinear contribution is characterized by large exponential decay lengths for breaker salts and small exponential decay lengths for maker salts. The existence of linear and nonlinear components may enable applying the model to a variety of systems. [ABSTRACT FROM AUTHOR]

A model is presented for design of a thickener for solid–liquid separation where the flocs or aggregates within the solid–liquid suspension undergo an aggregate densification process due to the action of rakes. This aggregate densification facilitates suspension dewatering. The novel feature of the model is that it manages to couple together a hindered settling zone (higher up in the thickener, where the flocs are separated from one another, and the suspension cannot bear weight) and a gelled suspension zone (lower down in the thickener, where the flocs are packed together, and the suspension is able to bear weight). The model determines solids fraction profiles throughout the hindered settling zone and the gelled suspension zone, and also gives zone heights and residence times. Parametric investigations using the model are carried out for different suspension fluxes (which influence the solids fluxes and underflow solids fractions attained), and also for different specified amounts of and rates of aggregate densification. [ABSTRACT FROM AUTHOR]

The behaviour in simple shear of two concentrated and strongly cohesive mineral suspensions showing highly non-monotonic flow curves is described. Two rheometric test modes were employed, controlled stress and controlled shear-rate. In controlled stress mode the materials showed runaway flow above a yield stress, which, for one of the suspensions, varied substantially in value and seemingly at random from one run to the next, such that the up flow-curve appeared to be quite irreproducible. The down-curve was not though, as neither was the curve obtained in controlled rate mode, which turned out to be triple-valued in the region where runaway flow was seen in controlled rising stress. For this first suspension, the total stress could be decomposed into three parts to a good approximation: a viscous component proportional to a plastic viscosity, a constant isostatic contribution, and a third shear-rate dependent contribution associated with the particulate network which decreased with increasing shear-rate raised to the −7/10th power. In the case of the second suspension, the stress could be decomposed along similar lines, although the strain-rate softening of the solid-phase stress was found to be logarithmic and the irreducible isostatic stress was small. The flow curves are discussed in the light of recent simulations and they conform to a very simple but general rule for non-monotonic behaviour in cohesive suspensions and emulsions, namely that it is caused by strain-rate softening of the solid phase stress. [ABSTRACT FROM AUTHOR]

Experimental data from batch settling tests on polymer flocculated suspensions was used to determine the material properties that quantify dewatering behaviour and relate settling rate to solids concentration. These material properties were subsequently used to predict the performance of a pilot-scale gravity thickener, including the expected solids concentration profile in the thickener. Analysis of data from a novel laboratory fluidisation rig, used to simulate the hindered settling zone of a thickener, and the pilot-scale thickener, indicates that relative to simple batch settling tests, the dewatering behaviour and related material properties of flocculated aggregates change over time, even at concentrations less than the gel point of the suspension and in the absence of mechanical shear. The change is primarily attributed to the phenomenon of aggregate densification as a result of aggregate-aggregate buffeting and manifests as enhanced settling and increased thickener throughput. A fluidisation case study using calcite with no added mechanical shear (only that due to fluidisation) found that the aggregates densified to 86% of their original diameter over a period of order 4000 s. This was enhanced with the addition of mechanical shear with a further decrease in aggregate size to 76% of their original diameter. Almost identical results were observed for the pilot thickener. Laboratory data typically underestimates thickener performance, even at the pilot scale, and the agreement between laboratory and pilot results represents a novel outcome for the characterisation of material dewatering properties. The observations herein are an important step towards full-scale thickener modelling incorporating aggregate densification effects due to shear. [ABSTRACT FROM AUTHOR]

The impact of several electrolytes present in seawater on the rheological properties of flocculated alumina suspensions was assessed by using creep testing at pH conditions both above and below the isoelectric point. Flocculated suspensions were obtained by adding a high molecular weight anionic polymer flocculant to a cylinder of alumina suspension and aggregating the suspension using the plunger method. The suspension was allowed to sediment and the supernatant liquid decanted prior to tests of the viscoelastic properties of the resulting sediment. Salts analyzed were MgCl 2 , CaCl 2 and NaCl, known as water structure makers, and KI and KCl, known as water structure breakers. A high salt concentration of 0.5 M was used to approach the ionic strength of seawater. For both pH's studied, the sediments demonstrated viscoelastic non-linear behavior observed as a non-linear relationship between yield strain and applied stress. In the absence of flocculant, it has been observed that the viscosity and yield stress of a suspension increase in the presence of structure maker salts for the case of both silica and alumina suspensions. In the presence of flocculant, new results are found: when the solid and flocculant were of opposite charge, the shear yield stress of the alumina suspensions increased with the breaker character of the salts, however when the solid and flocculant were of similar charge, then the yield stress is independent of the character of the salts. Similar water structures on the interacting surfaces lead to strong particle networks. The work demonstrates the importance of salt-surface interaction to the rheology of suspensions in the presence of polymer flocculants, as is typical of many mineral processing operations world-wide. [ABSTRACT FROM AUTHOR]

Abstract: A knowledge of the properties that quantify the rate and extent of particulate suspension dewatering through sedimentation and consolidation is critical to the prediction of dewatering performance for industrial processes such as gravity settling, centrifugation and filtration. The strength of a network of particles in compression is quantified as a compressive yield stress or modulus, P y (ϕ). The rate of dewatering is inversely related to the hindered settling function, R(ϕ), which is a measure of hydrodynamic resistance. Dewaterability characterisation over a broad range of solids concentrations is a challenging task requiring tests including batch settling and pressure filtration. Unfortunately, for many systems, these tests are prohibitively slow and an alternative such as centrifugation is useful in accelerating the characterisation. This paper validates algorithms for the characterisation of dewaterability from centrifugal sedimentation data obtained in a laboratory centrifuge with interface height detection and provides guidelines for obtaining accurate results over a broad range of solids concentrations. Typical dewaterability data for a flocculated industrial mineral slurry and a biological sludge are used as inputs for the numerical prediction of centrifugal sedimentation behaviour. Equilibrium centrifugation at a range of rotation speeds has been simulated and P y (ϕ) data determined using a method described by Buscall and White (1987). The result is shown to be accurate to within 10% of the expected values. Transient centrifugation data has been simulated and R(ϕ) determination using a new algorithm is shown to be accurate over a broad range of solids concentrations extending beyond the gel point, provided a new data rejection criterion is utilised. In addition, errors at low concentration in R(ϕ) data, caused by dilution, are described. It is shown that currently available analysis techniques are unlikely to quantify these errors. [Copyright &y& Elsevier]