An investigation into the viscosity of heavy medium suspensions.

The effect of a locally available dispersant (DPOO 1), on the viscosity of the heavy medium suspensions, was also investigated. Media particle size distribution has been shown to affect the viscosity of heavy medium suspensions.

Conclusions

Recommendations

Raw Data and Experimental Results

A1.3 Experimental Series B: Effect of Particle Size and Shape on the 312 Viscosity of Heavy Medium Suspensions. AI,4 Experiment Series C: Effect of DPOOI Particle Size and Media and Shape 328 on Separation Efficiency in a Dense Medium Cyclone.

Experimental Procedures 392

Equipment Diagrams and Photos

Tetrabromoethane Safety Data Sheet

Journal Article Published from this Work

LIST OF TABLES

Effect of DP001 on contaminated ferrosilicon suspensions (sp.gr. 2.4) l37 Figure 6.32 Effect of DPOOl on contaminated ferrosilicon suspensions (sp.gr. 196 Figure 7.2 Regression of magnetite #1 data at specific gravity 2.6 198 Figure 7.3 Yield stress versus specific gravity as predicted by the Bingham model for 199.

Figure 6.40 Effect of DPOOI at 500 s-1 shear rate (sp.gr.2.7) 143 Figure 6.41 Effect of DPOOI at 500 s-1 shear rate (sp.gr

NOMENCLATURE

LIST OF ABBREVIATIONS

Introduction

What is Heavy Medium Separation?

Stages in heavy media separation

MAGNETI~O

Parameters involved in heavy medium separations

The two most important rheological parameters in the separation of heavy media are the viscosity and stability of the suspensions [Collins et al. In the context of heavy media separations, this refers to the resistance of the fluid to particle penetration.

Objectives of the Investigation

Literature Review

Heavy Media

Solutions of salts in water
Organic Liquids
Suspensions of solids in water

At low solids volume concentrations, the non-Newtonian behavior is due to the non-Newtonian nature of the continuous phase. Suspensions are said to be stable when the settling rate of the medium particles is low.

Table 2.1 lists some the heavy media able to meet some of the above specifications

The Rheology of Heavy Medium Suspensions

Effect of particle size / size distribution
Effect of particle shape

They found that the viscosity and stability of the suspensions increased as the media size distribution became finer. They found that as the particle size distribution decreased, there was an increase in the stability of the suspensions.

Figure 2.1 Rbeograms for various types of suspensions [Aplan et al. (1964)]

Rheology modification of heavy medium suspensions

Surface forces acting between colloids/clays in aqueous media

Van der Waals forces
Electrical double layer forces
Structural ('hydration') forces
Steric and electrosteric forces

Surface active agents

1999) states that the nature of the clay contaminant strongly influences the increase in viscosity of magnetite suspensions. Flotation, on the other hand, requires that the solution show a tendency to settle due to the agglomeration of particles in the medium.

Effect of demagnetisation

In heavy medium separation plants, passage through magnetic separators in the medium recovery circuit induces a residual magnetization which causes flocculation or aggregation of the magnetized particles. Agglomeration also retains part of the suspension in the formed flocs, reducing the amount of free suspension or the continuous phase in the medium. These operating conditions include selection of the correct intermediate class (size distribution and shape); control of medium density;.

The design of the coil with regard to damping properties and field strength is of great importance. Therefore, the capacitance of the magnetizing coil can be reduced to achieve the same effect.

Modelling Slurry Rheology

The viscosity of spherical-particle suspensions

Suspensions with a small concentration of the solid phase
Suspensions with a high solid-phase concentration
Effect of pressure and temperature

The liquid represents the continuous phase of the suspension, and the solid particles the disperse phase. If a certain value of the solid particle concentration is exceeded, the behavior of the suspension becomes non-Newtonian. At relatively large concentrations of the solid phase, the non-Newtonian behavior is accentuated by the effect of particle interaction.

This fact is very important for the practical use of the model in plant control. The effect of temperature on the solid phase of the suspension is common only in suspensions containing compressible particles (eg latex, asphalt).

Table 2.4 Rheological Models for heavy medium suspensions [Laskowski et al. (1999»)

Conclusion

Rheology Measurement

Tube viscometers

The boundary condition for equation 3.4 is that the velocity of the liquid is equal to zero at the capillary wall, i.e. For Newtonian fluids, the shear rate varies from a maximum near the wall to zero at the center of the capillary. Note the increase in wall shear rate and the increasingly plug-like nature of the flow as 'n' decreases.

The most common method is to connect holes in the capillary wall to pressure transducers. Usually, end effects (inlet and outlet effects) are avoided by ensuring that the capillary aspect ratio is greater than one hundred.

Figure 3.1 Ostwald viscometer (Blair (1969»)

Rotational visco meters

Concentric-cylinder viscometer
Double gap concentric-cylinder viscometer
High-shear viscometer
Cone-plate viscometer
Plate-plate viscometer

According to DIN 54453 standards, the radii and immersion length of the viscometer (Figure 3.5) must be limited under the following conditions [Slatter et al. For measuring suspensions, the value of the measuring gap 'h' should be large enough to accommodate the solid particles. Above this value, the requirements for calculating the viscosity of the liquid are no longer given [Barnes et al.

Unlike cone plate viscometers, the shear rate in plate plate viscometers is a function of the plate radius [Slatter et al. The shear rate increases linearly from zero at the axis to its maximum at the edge of the plate.

Figure 3.4 Cylinder measuring system according to (a) Searle's principle (b) Couette's principle rSlatter et al

Effect of stability on rheological measurement

To obtain meaningful results, when testing suspensions, the value of the measuring aperture 'h' must be at least ten times larger than the size of the aggregate particles.

On-line viscosity measurement

An industrial capillary on-line viscometer
An industrial on-line rotational viscometer

To reduce the rate of settling of common sludge under laminar flow conditions, the measuring tube is wrapped in vertical loops. The viscometer (Debex viscometer) is of rotary design and uses a specially designed coil that rotates in a sample of the suspension under investigation. The motor is found to be sensitive to temperature changes, which causes the speed of the coil to move.

The annulus, C, and the measuring chamber are both fitted with baffles to eliminate rotational movement of the liquid. The derivation of the shear stress equations assumes that the flow past the coil is laminar.

Figure 3.12 Debex on-line viscometer [Reeves (1986), Napier-Munn et al. (1996)]

Rotational measuring system according to DIN 53 019 and ISO 3219

Where CL is the correction factor and Gz is a constant depending only on the geometry of the measuring system. Note that in this gauge system the shear rate in the gauge gap is not constant at any location.

Separating Vessels

Static bath separators

Wemco cone separator
Drum separators
Drewboy Bath
Nowalt washer

The feed is fed onto the surface of the medium by free fall, allowing it to sink several centimeters into the medium. In drum separators, separation is achieved by the continuous removal of the basin product by lifters attached to the inside of a rotating drum [Pryor (1960), Wills (1997)]. In drum separators, however, the weir is on the opposite side of the drum from the feed chute.

Raw coal is fed into the center of the annular separation vessel, which is mounted with stirrer arms. The agitators carry the floating product and discharge it through a weir on the other side of the vessel, where it is carried out of the vessel by the medium flow.

Figure 4.1 Wemco cone separator [Wills (1997)]

Dynamic separators

The Dutch State Mines (DSM) Cyclone
The Vorsyl Separator
The Dyna whirlpool (DWP) Separator
The Tri-Flo Separator
The Larcodems (LARge Coal Dense medium separators)

Most of the medium flows towards the center of the vessel and then out through the vortex. The minimum static head is usually taken as 9 x D, where D is the diameter of the cyclone at the inlet. One of the biggest challenges faced by DSM cyclones is that the particles that make up the medium are affected by centrifugal force.

Particles with a density less than that of the medium pass through the vortex generated by the rotating medium. Particles with a much higher density are pushed to the outer wall of the barrel and discharged with the medium through the outlet pipe of the sink.

Figure 4.7 Dense medium Cyclone (Horsfall (1993»)

Efficiency of Heavy Medium Separations

The Partition Curve
Factors Affecting the Separation efficiency

Hydrodynamics of heavy medium separations
Effect of Viscosity and stability on Separation Efficiency
Effect of Cyclone design and operating conditions on Separation Efficiency

Thus, the ease or difficulty of separation depends on the amount of material present, which has a density close to that of the medium. On the other hand, the efficiency of a particular separation vessel depends on its ability to separate material with densities close to that of the medium. Under these conditions, viscosity plays no role in determining the magnitude of the drag on the particle [Napier-Munn (1990)].

They found that viscosity and consistency had a direct influence on the separator density and Ep of the separators. The separation of the medium also has an effect on the relationship between the downstream density and the separation density.

Figure 4.12 Partition or Tromp curve (Wills (1997»)

4.3.2.3.1 Cone Inlet Area

A summary of the operating variables they investigated, and their effect on particle separation efficiency, is given below.

4.3.2.3.2 Cone Inlet Pressure

4.3.2.3.3 Cone ratio

4.3.2.3.4 Medium Split Ratio

4.3.2.3.4 Ore-to-Medium Ratio

4.3.2.3.5 Shape of Cone

Rheology Measurement

The Rotational Viscometer
Rheometer Modifications

The Dense Medium Cyclone
Sundry Analytical Equipment

Brookfield Viscometer
Malvern Sample Analyzer
Scanned Electron Micrograph (SEM)
Spectrophotometer

Heavy Media Suspension Properties

Determination of the Media particle size distribution
Media Particle Chemical and Physical Specifications

Analytical instruments used to determine the properties of the media used in these investigations are also described. The Malvern sample analyzer was used to obtain the size distributions of the media used in the investigations. Therefore, it is appropriate to plot the particle size distribution of the media used in this investigation.

Media particle densities were determined using the density bottle technique described by Wills (1997), included in Appendix A3. These differences in the size distribution of the media have a noticeable effect on their rheological properties.

Figure 5.1 Cross-section of DIN 53019 / 1803219 viscometer

6.1.2.3.1 Particle Shape

6.1.2.3.2Chemical / Elemental Specifications

Preliminary Rheological Tests

Aim of the Experiment
Experimental Method and Equipment
Experimental Results and Discussion

These results were plotted so that the shear stress of each of the suspensions was at a shear rate of 1200 S-I. The results show that the viscosity of suspensions increases with specific gravity. The viscosity of magnetite suspensions appears to be greater than that of ferrosilicon at the same specific gravity.

These graphs were plotted by taking the viscosity of each of the suspensions at a shear rate equal to 1200 S.l. At a specific gravity of 2.2, the behavior of the ferrosilicon-magnetite mixtures is close to Newtonian behavior.

Table 6.3 shows the solids volume percentage for ferrosilicon and magnetite #1 at the measured specific gravities

EXPERIMENTAL

Experiment Series A: The effect of DPOOI on the viscosity of heavy medium suspensions

The effect of DP001 on ferrosilicon suspensions
The Effect of DP001 on Magnetite Suspensions

Additions of DP001 up to 5 g/kg FeSi have little effect on the viscosity of suspensions. All graphs show an increase in the viscosity of the suspensions after the addition of mucus. The decrease in viscosity shown in the graphs is therefore probably due to a decrease in the apparent viscosity of the suspensions.

This reflects that there is a reduction of the viscosity of the suspensions even when the mucus content is increased. Above the specific gravity of 2.5, the viscosity of the magnetite suspensions becomes too high, leading to reduced efficiency of separation.

Figure 6.26 Rbeograms for uncontaminated FeSi suspensions at sp.gr .2.2

Experimental Results and Discussion
Ferrosilicon-Magnetite #1 Mixtures at a Ratio of 1:1
Ferrosilicon-Magnetite #1 Mixtures at a Ratio of 2:1
Ferrosilicon-magnetite #1 Mixtures at a Ratio of 1:2

Addition of an additional 1 gram and 3 g of DPOOllkg does not appear to reduce the viscosity of the suspensions much further. At a specific gravity of 2.3, the reduction in the viscosity of the suspensions by DPOO 1 additions corresponding to 2 g DPOO 11kg is not as pronounced as with 1g DPOO11kg. At a specific gravity of 2.4, the viscosity reduction for both Ig and 2g DPOOI additions is relatively high.

The figures above also show that the viscosity of the suspensions is reduced with some DPOO 1 additions. At a specific gravity of 2.2, the addition of Igram DP001 / kilogram reduces the viscosity of the suspensions by up to 10%.