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The measurement of the shear rheology of concentrated particulate suspensions is important to a range of mixing, pumping and flow operations. The use of a four or six bladed vane attached to a rheometer in an open cup is a popular technique to achieve a rheological characterisation. A problem occurs in the use of automated software with a number of rheological devices for yield stress materials. A torque auto zero default causes the torque at the start of a test to be ignored, and can result in significant errors and underestimation of the yield stress or rheological response of the suspension. The potential effect of using a torque auto zero default is demonstrated for a concentrated particulate suspension of nickel laterite.

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]

Solid-liquid separation aims to increase the solids concentration. The dewatering extent can be parameterized through the compressive yield stress, py(ϕ) whereby to further consolidate a suspension, higher stress has to be applied to the material. In a variety of dewatering devices, shear (in addition to unilateral compression) is applied to the material and is demonstrated to be of benefit to the process, albeit without a basis for optimisation. To address this fundamental issue, two novel measurement techniques, namely the Vane-in-a-Filter (ViaF) and the Vane-under-Compressional-Loading (VuCL), are presented, allowing measurement of the shear stress response with different normal loads under fixed solids concentrations. Furthermore, high solid concentration shear yield stress measurements are possible. The results demonstrate that the shear stress response increases with normal load (at a fixed state of compression) and the peak shear stress has a minimum at shear rates of 0.01–0.1 s−1. [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]

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]

Silica sand and quartz represent the second most common mineral in the earth's crust. The silica-water interface is central to the processing of a large number of mineral systems and the flow properties of particulate suspensions. In particular, these flow properties define the upper limit to processing of tailings suspensions in the mining industry and, ultimately, the recovery of water. In some regions of the world, the scarcity of fresh water has pushed this industry to use seawater, either as is or partially desalinated. Here we analyze the impact of monovalent alkali metal chlorides that are typical of seawater on the viscoelastic behaviour of flocculated silica sediments at concentrations typical of thickening systems. The sediment samples were subject to creep-recovery tests. All sediments exhibit nonlinear viscoelastic behaviour, deduced from the compliance-applied stress relationship. Interestingly, creep recovery was observed to be related to the Hofmeister series, Cs + < K + < Na + < Li + , with Cs + producing lower recoveries than Li + . The yield stress, critical strain, and critical strain energy of the sediments were observed to be higher in maker salts (Na + and Li + ) compared with breaker salts (Cs + and K + ), due to stronger bonds in the particle networks. Our model of viscoelastic behaviour (Goñi et al., 2015) satisfactorily reproduces all experimental strain-time curves. The work provides novel input to decisions about the use of seawater or partially desalinated seawater in minerals processing with direct implications for tailings transportation, handling and water recovery. [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]

Abstract: The presence of coarse particulate components in tailings streams poses significant challenges to the mineral processing industry. Suspensions with fast-settling coarse fractions make traditional rheological measurements tedious and inaccurate. These measurements are critical to the design and operation of mixing, dewatering, pumping and tailings equipment and operations. A methodology for predicting the gel point, close packing fraction and shear rheology of particulate slurries containing a binary mixture of fine and coarse material is presented. The model has validity subject to the constraint that the particle size of the coarse material is sufficiently distinct from that of the fine material. The model enables calculation of the gel point and close packing fraction of the mixture based on the gel points and close packing fractions of the pure fine and coarse materials in the suspension. The model predicts that the gel point (and close packing fraction) of the fine material increases with increasing coarse fraction until a maximum value is attained before decreasing to the gel point (or close packing fraction) of the coarse material. This relationship provides a subsequent basis for a yield stress model to accurately predict the yielding behaviour of binary mixtures of fine and coarse material. The model is useful in determining whether a sample exhibits yield stress behaviour through knowledge of the gel point and close packing fraction of the constitutive fine and coarse components. The model is exemplified using particulate mixtures containing coagulated aluminium oxide, calcium carbonate and coarse sand. [Copyright &y& Elsevier]