Experimental Results and Discussion

6.1.2.3.2Chemical / Elemental Specifications

CHAPTER 6 EXPERIMENTAL

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

6.2.1.1 Experimental Results and Discussion

Initially experiments were carried out on uncontaminated ferrosilicon suspensions at a specific gravity of 2.2. These results are illustrated in Figure 6.26.

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Shear rate (S-l)

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

•

800 900

The results show that DP001 has a negligible effect, if any. DP001 additions as high as 5 g / kg FeSi have little effect on the viscosity of the suspensions. This result is similar to the observations of Aplan and Spedden (1964), who stated that surface active agents have little effect on uncontaminated ferrosilicon suspensions, except for a minor amount of suspension of the finest particles, which is quite negligible in the case of the media used in this project.

From the above results, and the observations of Aplan and Spedden (1964), it was inferred that DPOO 1 would have no effect on uncontaminated ferrosilicon suspensions at all specific gravities.

It was then decided that the remainder of the experiments would be carried out on ferrosilicon suspensions contaminated with the slime. Figure 6.27 to Figure 6.37 show the rheograms for ferrosilicon media contaminated with the slime. Additions of DPOO 1 were made at 1 and 2 g DPOOllkg solids.

CHAPTER 6

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EXPERIMENTAL

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Figure 6.27 Effect ofDPOOl on contaminated ferrosilicon suspensions (sp.gr. 2.0)

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Figure 6.28 Effect of DPOOI on contaminated ferro silicon suspensions (sp.gr. 2.1)

CHAPTER 6 EXPERIMENTAL

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Figure 6.29 Effect of DPOOI on contaminated ferrosilicon suspensions (sp.gr. 2.2)

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Shear rate (S-l)

Figure 6.30 Effect of DPOOI on contaminated ferrosilicon suspensions (sp.gr. 2.3)

CHAPTER 6 EXPERIMENTAL

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Shear rate (S"l)

Figure 6.31 Effect of DPOOI on contaminated ferro silicon suspensions (sp.gr. 2.4)

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Shear rate (S"I)

Figure 6.32 Effect ofDPOOl on contaminated ferro silicon suspensions (sp.gr. 2.5)

CHAPTER 6

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EXPERIMENTAL

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Figure 6.33 Effect ofDPOOI on contaminated ferro silicon suspensions (sp.gr. 2.6)

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Shear rate (S·l)

Figure 6.34 Effect of DPOOI on contaminated ferrosilicon suspensions (sp.gr. 2.7)

CHAPTER 6

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EXPERIMENTAL

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Figure 6.35 Effect ofDPOOl on contaminated ferrosilicon suspensions (sp.gr. 2.8)

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Shear rate (S·l)

Figure 6.36 Effect ofDPOOl on contaminated ferrosilicon suspensions (sp.gr. 2.9)

CHAPTER 6 EXPERIMENTAL

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Figure 6.37 Effect of DPOOI on contaminated ferrosilicon suspensions (sp.gr. 3.0)

All the graphs show an increase in the viscosity of the suspensions upon addition of the slime. Table 6.4 gives a summary of the increase in viscosity due to the presence of slime. The values in the table were calculated by taking the average increase in the viscosity of the suspensions over the entire range of shear rates i.e across all shear rate values ..

Table 6.4 Percentage viscosity increase due to presence of slimes Specific gravity % Viscosity Increase

2.0 41.1

2.1 31.5

2.2 27.2

2.3 22.9

2.4 29.0

2.5 36.6

2.6 30.0

2.7 19.7

2.8 17.0

2.9 26.2

3.0 29.6

CHAPTER 6 EXPERlMENTAL

The table above shows that the highest increase in viscosity occurs at specific gravities 2.0 and 2.1. This is because of the higher percentage of clay added at these specific gravities in comparison to the others. The slime increases the viscosity of the suspensions in two ways.

Firstly, it was observed during experimentation that the slime had a clay-like nature to it. Clays, or colloidal particles, reduce the viscosity of suspensions by absorbing some water, thereby resulting in less water available to associate with the rest of solid particles. This results in apparent increase in the solid volume concentration [Eirich (1960), and He et al. (1999)].

Secondly, the non-magnetic particles introduced by the slimes increase the solid volume concentration of the suspensions. These finer particles increase the surface area of the suspension, meaning that there is a lot more association between the solids and the water. This also results in an increase the apparent solid volume concentration.

Table 6.5 Ferrosilicon viscosity reduction due to DPOOI addition

% Viscosity Reduction Specific Gravity 1 gDPOOllkg 2 gDPOOllkg

2.0 9.9 15.9

2.1 5.5 11.0

2.2 11.2 14.6

2.3 10.9 15.8

2.4 8.9 15.5

2.5 11.6 17.1

2.6 4.2 12.5

2.7 7.4 7.4

2.8 6.5 13.3

2.9 13.0 16.0

3.0 7.8 8.7

The graphs also show that the basic trend of the graphs does not change. This means that both the addition of slime and DPOOl do not change the behaviour of the suspensions from one form to another, e.g. from Newtonian to pseudoplastic. Most of the graphs do not show an appreciable presence of a yield stress. The reduction in viscosity shown in the graphs is thus probably due to a reduction in the apparent viscosity of the suspensions.

CHAPTER 6 EXPERIMENTAL

The graphs also show that reduction in suspensions viscosity is higher at high shear rates. This is a promising result because the reduction in viscosity occurs at shear rates in the region industrial separators operate under (approximately 100 S·1 to 600 S-I). Upon addition of 1 g DPOOllkg FeSi, there is a reduction in the viscosity of the suspensions. Addition of a further 1 g / kg FeSi results in a further reduction for some of the suspensions. Table 6.5 is a summary of the average viscosity reduction in contaminated ferrosilicon suspensions with DPOOI at various specific gravities. The percentage viscosity reduction in the table was also calculated by taking the average values over the entire range of shear rates.

Rheological measurements were also carried out at constant speeds and different slimes additions to simulate build up in a heavy medium separation circuit. These tests were carried at specific gravities from 2.5 to 3.0, and DPOOI additions of 2 g / kg solids. The results are illustrated in Figure 6.38 to Figure 6.43, for a constant shear rate equal to 500 S·I.

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Figure 6.38 Effect ofDPOOI at 500 fl shear rate (sp.gr. 2.5)

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CHAPTER 6 EXPERIMENTAL

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Figure 6.39 Effect of DPOOI at 500 S-I shear rate (sp.gr. 2.6)

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% Slime

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

CHAPTER 6

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Figure 6.41 effect of DPOOI at 500 S·I shear rate (sp.gr. 2.8)

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Figure 6.42 Effect ofDPOOl at 500 s"1 shear rate (sp.gr. 2.9)

EXPERlMENT AL

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CHAPTER 6

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Figure 6.43 Effect of DPOOI at 500 S-I shear rate (sp.gr. 3.0)

EXPERIMENTAL

12 14

At a specific gravity of 2.5 it can be seen that the viscosities for suspensions with DPOOl are lower than that of plain suspensions at a given % slime. This reflects that there is a reduction the viscosity of the suspensions, even upon increasing the slime content. Even at high slime content, the effect ofDPOOl is still clearly visible. The same results were observed at a specific gravity of 2.6. At low slimes content the effect of DPOOl appears to be less defined than that at higher slimes content. As the slime content is increases the effect of DPOOl becomes modified, levelling off at slimes content from lO % upwards. What can be seen from the above figures is that the extent of viscosity reduction by DPOO 1 decreases as the specific gravity increases. The viscosity reduction at specific gravity 3.0 is much less than that at other specific gravities. This means that to reduce the viscosity at higher specific gravities, a higher dosage of DPOO 1 may be required.

The results above are very significant to the viscosity reduction in heavy medium separations where slimes can cause problems. The results show that a small amount of DPOO 1 is capable of reducing the viscosity of the suspensions even when there is an increase in slimes content. This means that if an operator observes an increase in slimes content, they can add the appropriate amount ofDPOOl based on the mass flowrate of the feed stream.

CHAPTER 6 EXPERlMENT AL

Rheological tests were also carried out at a constant shear rate of 240 S-I, and specific gravity from 2.9 to 3.4. The reason for carrying out these tests at a low shear rate was to investigate whether DPOO 1 could reduce the viscosity of ferrosilicon suspensions at shear rates similar to those at start up. When heavy medium suspensions have been shutdown for a while, the stored ferrosilicon suspensions sometimes cement and agglomerate, making pumping of the suspension to the separating vessels very difficult [Rodis et al. (1960)]. Figure 6.44 to Figure 6.49 show the results for these investigations.

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Figure 6.44 Effect of DPOOI at 240 fl shear rate (sp.gr. 2.9)

CHAPTER 6 EXPERlMENTAL

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Figure 6.45 Effect of DP001 at 240 S-1 shear rate (sp.gr. 3.0)

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Figure 6.46 Effect ofDP001 at 240 S-I shear rate 9sp.gr. 3.1)

CHAPTER 6 EXPERIMENTAL

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Figure 6.47 Effect of DPOOI at 240 ^S·l shear rate (sp.gr. 3.2)

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Figure 6.48 Effect of DPOOI at 240 5"1 shear rate (sp.gr. 3.3)

CHAPTER 6 EXPERIMENTAL

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Figure 6.49 Effect of DPOOI at 240 S·I shear rate (sp.g~. 3.4)

The results from this investigation are summarised in Table 6.6. The percentage viscosity reduction was taken as an average over the entire range of slimes and DPOO 1 additions i.e.not at any specific % slime, but over the entire range.

Table 6.6 Percentage viscosity reduction with DPOOI at 240 S·I shear rate Specific Gravity % Viscosity Reduction

2.9 32.1

3.0 20.5

3.1 23.4

3.2 18.4

3.3 16.6

3.4 19.1

All tested specific gravities showed a reduction in suspension viscosity on addition of DPOO 1.

This reduction at a low shear rate shows that viscosity reduction can be achieved in processes where there has been a long shutdown. Thereby, improving the pumping characteristics of the suspensions, resulting in reduced energy costs.

CHAPTER 6 EXPERIMENTAL

Aplan et al. (1964), using spherical ferrosilicon media suspensions contaminated with mangan~se

ore slimes, also found a reduction in viscosity upon the addition of a chemical dispersant. The chemical dispersant they used for their investigations was sodium hexametaphosphate. Between 0.01 and 0.03 %

(o.i

and 0.3 grams / kg solids) sodium hexametaphosphate additions, the viscosity reductions achievable were as high as fifty per cent. However, beyond a critical dosage, they observed a reduction in the stability of the suspensions. They showed that there is region between which a reduction in viscosity can be achieved without reducing the stability of the suspensions.