Dynamic behavior of dilute bentonite suspensions under different chemical conditions studied via magnetic resonance imaging velocimetry 

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Rheology

Rheology is a science that deals with the deformation of matter under applied stress. The ‘matter’ in this definition is a liquid or a ‘soft solid’ (alias ‘soft matter’). According to Doi (2013), soft matters are defined as matters that consist of large molecules or molecular assembles which are moving collectively, giving a large and non-linear response to the applied force or a slow and non-equilibrium response; these include polymers, colloids, surfactants, liquid crystals, some biological materials, granular solids, glassy materials, gels, foams. Pulps, slurries and suspensions examined in the present work fall into the ‘soft matter’ category.
Rheological measurement can be conducted in different kinds of rheometers (e.g., rotational rheometer, pipe or capillary rheometer) with various geometries (e.g., cone and plate, parallel plate, bob and cup). For characterization of stirred mineral slurries often the rotational rheometer is used. There is a verity of advantages suggested by rotational rheometers, including: ability to be utilized in the laboratory and on the site (coupled with industrial settings), relative compactness, convenient sample volumes required for measurement and considerable simplicity of associated measurement procedures, easy data analysis (Fisher et al. 2007).
The “gap” of the geometry is a distance between two surfaces, filled with the sheared sample under the study. For example, in the parallel plate geometry, the gap is the distance between the fixed and the rotating plates; in the concentric cylinder geometry, the annular gap is the radial distance between the two cylinders. Since industrial suspensions often contain non-colloidal particles, it is impractical to use narrow-gap geometries (e.g., cone and plate, parallel plates) for their characterization. It is suggested that the gap size has to be ten times the biggest particle in the suspension under the measurement (Fisher et al. 2007). Thus, a Couette concentric cylinder (Fig. 5a) or the vane (Fig. 5b) geometries are used (Fisher et al. 2007; Boger 2009). Utilization of the vane allows to eliminate the slip on the rotating element of the geometry, reduce the sample disturbance upon the rotating element immersion and directly measure the yield stress (Dzuy and Boger 1985; Benna et al. 1999; Fisher et al. 2007; Boger 2009; Tadros 2010; Farrokhpay 2012); it is also less susceptible to the artifacts arising from the large particles (Fisher et al. 2007), which is why it is often preferred to the bob and cup geometry. It was shown (Keentok et al. 1985; Yan and James 1997) that an assumption of a uniform stress distribution over a cylindrical yield surface around the rotating vane is valid for various kinds of yield stress fluids. The vane geometry may be used in both, infinite medium or in a cup (Barnes and Carnali 1990; Barnes and Dzuy Nguyen 2001; Fisher et al. 2007). In a latter case, the slippage may occur on the wall of the cup or external cylinder of the geometry that is in contact with the fluid. To prevent this kind of slippage, the inner surface of the cup may be roughened or grooved.

Clays

Beneficiation processes suitable for fine particles, such as flotation, are highly dependent on physico-chemical and mechanical properties of the pulp. In majority of cases the mechanics of the mineral pulp differs from the mechanics of Newtonian fluid and depends on the interactions between the suspended particles. Behavior of swelling clays is of a particular interest. In suspensions, these clays build various structures depending on the chemical environment of solvent. Apart from high aspect ratio and spatial chemical inhomogeneity of each particle, suspensions of swelling clays possess thixotropic properties (Abu-Jdayil 2011), often with the yield stress (Tombácz and Szekeres 2004; Zhang and Peng 2015). In flotation process, clays frequently appear to be problematic.
Indeed, beneficiation experts refer to clays as “flotation nightmare”, as this group of minerals possesses all the negative properties of the fine particles, including:
• Changing the froth stability (Farrokhpay et al. 2016; Leistner et al. 2017).
• Changing the bubble size (Shabalala et al. 2011).
• Increasing the pulp viscosity (and causing the turbulence damping) (Zhang and Peng 2015; Taner and Onen 2016).
• Cavern development in the flotation cell (Bakker et al. 2010; Shabalala et al. 2011; Taner and Onen 2016).
• Over-consumption of reagents (Leistner et al. 2017).
• Slime coating (Leistner et al. 2017).
• Mechanical entrainment (Taner and Onen 2016; Leistner et al. 2017).
Processing issues related to high clay contents are not exclusively encountered in the flotation process. Significant amount of problems up and down the material stream were also reported (Connelly 2013; Ndlovu et al. 2013; Gräfe et al. 2017). Efficiency of crushing and screening is often related to the presence of clays. Moist ores with high clay contents become sticky; that causes reduced efficiency of sieving due to the plugged grid apertures (phenomenon, also referred to as “blinding”). Bridging takes place in the cone crushers; dead zones of stagnant (non-flowing) material are formed in the tanks, hoppers and bins designed for storage and feeding. Presence of clay in the ore often justifies installation of the conveyer washers. Functionality of mills and classification devices is highly affected by the increased pulp viscosity (caused by clays). Kinetics of the grinding media (e.g., balls in a ball mill) is reduced in the conditions of high slurry viscosity. Increased sedimentation time in the spiral classifier and accumulation of the material on the spiral provides less sharp separation. Decreased separation sharpness due to the high clay contents was also reported in the hydrocyclones. Affecting the settling rates of other minerals particles (through increased viscosities and establishment of heterocoagulated particle network), the clays complicate thickening, dewatering in ponds and tailing drainage. Compressibility of clay suspensions is known to negatively affect filtering. Lower water recoveries and clay-contaminated water overflows cause problems with water recirculation in the mineral processing plants. High clay contents in the ores put limitation on the choice of the pumping devices and often cause inadequate pumping capacities. Efficiency of piping is entirely dependent on the mechanical properties of slurries with the high clay content; knowing the slurry rheology for this transporting method is a must. Presence of clays is also known to cause low percolation in the heap leaching. Fine clay particle size yields an unwanted entrainment with the liquid phase in the separation processes, such as dense media, magnetic and high intensity magnetic separation, causing media contamination and decreased concentrate grades.

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Scanning electron microscopy (SEM)

Secondary electron (SE) SEM images of unique bentonite particles were obtained using MEB-FEG Hitachi S-4800. Kunipia-F bentonite powder was dispersed in ethanol (99.99% of C2H6O, Sigma Aldrich), the concentration of 0.1 vol% solid. A drop of suspension was placed onto the carbon-covered stage, which then was heated at 40 °C to provoke the evaporation of alcohol. In 30 min the sample was withdrawn from the heating plate and placed into the SEM sample holder. The technique allowed to provide high-quality images for the particles of the order of few microns and down to hundreds-tens of nanometers. Obtained images of individual bentonite particles can be found in the Fig. 26 below.

Materials and methods

Mineral powders utilized in this study were Kinipia-F sodium bentonite clay (Kunimine Industries Co., Ltd), silicon dioxide (quartz, Sigma-Aldrich), hematite (Brazil). Particle size distributions (PSDs) of the materials were measured using Mastersizer 3000 laser diffraction particle size analyzer (Malvern); the results are presented in the Fig. 41. Sodium bentonite and quartz were suspended in 1 × 10-2 M KNO3 aqueous solution at pH 10 to provoke repulsive interaction between the particles for better dispersion. Hematite was suspended in 1 × 10-2 M KNO3 aqueous solution at natural pH (approx. 6.3). Measuring the PSD of swelling clays can be challenging (Tan et al. 2017). Thus, the following steps were undertaken to avoid erroneous result: 1) the particles were soaked in the solution (1 × 10-2 M KNO3, pH 10) for 2 days prior to measurement, 2) soft ultrasound (US) was applied. Preference was given to the low US intensities during preparation to avoid possible particle exfoliation.
Dilute suspensions of this bentonite clay exhibit a wall slip in a Couette concentric cylinder geometry (Chernoburova et al. 2018). In the current study, a reduced slip geometry was thus used to perform the rheological measurements. The flow curve acquisition was conducted using the AR 2000 rheometer (TA Instruments), assembled with vane and cup geometry. Grooved cup was installed in the Peltier concentric cylinder temperature system (TA Instruments) and set to 20°C. Figure 42 displays the utilized geometry with dimensions; it was equivalent to a cylindrical Couette geometry with an internal cylinder equivalent diameter of 22 mm. The volume of specimen fed into the gap was 45 ml. Steady state measurements were conducted in the shear rate range from 0.01 to 100 s-1.
Suspensions for rheological measurements were prepared by dispersing the mineral(s) at desired volume percent solid in 1 × 10-2 M KNO3 aqueous solution and adjusting the pH to 4 (using 1 M and 0.1 M solutions of HNO3) or 10 (using 1 M and 0.1 M solutions of KOH). All bentonite-based suspensions were left to age for 48 h, with another pH-adjustment prior to rheological measurement. Same history of shear for all bentonite-containing specimens was achieved via fixed duration and speed of agitation applied at every conditioning step.
List of specimens prepared for rheological measurements is given in the Table 2. The preparation procedure is explained using specimen 1 as an example. The amount of quartz equal to 0.5 vol% solid was introduced in the 1 × 10-2 M KNO3 aqueous solution, followed by adjustment to pH 4. Suspension then was placed in the cup of the rheometer and the measurement was performed. Then, another 0.5 vol.% addition of quartz to the total 1 vol% solid was made and the pH was adjusted to maintain the pH 4. The rheological measurement was performed again. Same steps were repeated for the 2.5, 3, 5, 8, and 10 vol% solid. For the specimens 5, 6, 13 and 14 at each step both, quartz and hematite, were added at the same time in 1:1 volume proportion. Last column of the Table 2 contains the references to the figures depicting the results of rheological measurements that correspond to the given specimen conditions.

Table of contents :

Chapter I – A general overview
Introduction
I.I.Froth flotation
I.II.Inter-particle forces
I.III.Rheology
I.IV.Clays
I.V.Mix mineral systems
Materials and methods
Minerals
Scanning electron microscopy (SEM)
Reagents
Rheometry
Flotation
Bibliography
Chapter II – Dynamic behavior of dilute bentonite suspensions under different chemical conditions studied via magnetic resonance imaging velocimetry 
Résumé
Abstract
Introduction
Materials and methods
Results
Discussion
Conclusions
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Bibliography
Chapter III – Rheology of mineral mixtures
Résumé
Abstract
Introduction
Materials and methods
Results
Discussion
Conclusions
Funding
Acknowledgments
Conflicts of Interest
Bibliography
Conclusions
Bibliography
Résume élargi de la thèse en langue Français
Appendix
Appendix I: Images of the froth captured for 0.05, 0.1 and 0.3 vol.% bentonite in flotation of -40 μm quartz-hematite mixture
Appendix II: Images of the froth captured for 0.05, 0.1 and 0.3 vol.% bentonite in flotation of 40-75 μm quartz-hematite mixture
Appendix III: Full set of local rheological curves (6, 9, 12, 15, 18 and 21 rpm) for pH 8 in 1 × 10-2 M KNO3

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