Separating Vessels
4.2 Dynamic separators
CHAPTER 4 SEPARATING VESSELS
CHAPTER 4 SEPARATING VESSELS
The ore is suspended in a very fine medium of either ferrosilicon or magnetite, and is introduced tangentially to the cyclone. The tangential inlet of the medium causes the medium to rotate rapidly, resulting in the formation of an air column, or vortex, in the centre of the column [Horsfall (1993)]. Most of the medium flows towards the centre of the vessel, and then out through the vortex fmder. The vortex finder is a tube extended into the body of the cyclone, and prevents short-circuiting of feed directly into the overflow. To create the vortex and cause particle separation, the DSM cyclone requires a certain pressure. The pressure is usually calculated as being equivalent to a certain static head [Horsfall (1993), Wills (1997)]. The minimum static head is usually taken as 9 x D, where D is the diameter of the cyclone at the inlet.
-
- INLET VORTEX FINDER
CONE·SHAPED PART
- ~
APEX
~
1
FEEO-
DISCARD The Scatel flow III Iht CyclOne
Figure 4.7 Dense medium Cyclone (Horsfall (1993»)
CHAPTER 4 SEP ARA TING VESSELS
Particles within the cyclone are subjected to two opposing forces: an outward centrifugal force and an inward acting drag force [Wills (1997)]. When particles of mixed density enter a the DSM cyclone, those with a density lower than the medium density do not pass through the medium, but are caught in the flow towards the vortex finder. Hence, they exit the cyclone through the vortex finder as overflow. On the other hand, particles with densities greater than the medium density, are able to penetrate the medium. These particles are thrown to the wall of the cyclone, and emerge at the apex of the cyclone, exiting as underflow.
One of the biggest challenges experienced in DSM cyclones is that the particles making up the medium are themselves affected by the centrifugal force. Segregation creates a density differential between the cyclone overflow and underflow. Factors that determine the extent of this differential are the cyclone diameter, pressure head, and medium grind [Horsfall (1993), Pryor (1960), and Wills (1997)]. As explained in Chapter 2, the media size distribution, particle shape, and slimes content determine the viscosity and stability of a suspension. To reduce the extent of particle segregation at low medium specific gravities, a medium with a fmer size distribution, rougher particle shape, and moderate slimes content is usually the preferred choice.
4.2.2 The Vorsyl Separator
Vortex finder --+-_-..1
Clean cool
Row cool
Separating chamber
Figure 4.8 Vorsyl separator (Wills (1997»)
CHAPTER 4 SEPARATING VESSELS
The Vorsyl separator is shown in Figure 4.8. This separator is mainly used in the coal industry to clean coal of small sizes up to about 30 mm [Wills (1997)]. Raw coal, together with a magnetite medium, is fed tangentially at the top of the column of the separating chamber under pressure.
Lighter material passes into the clean coal outlet through the vortex finder, while middlings and heavy material move to the wall of the vessel due to the centrifugal acceleration induced. The particles spiral downwards towards the base of the vessel where the drag caused by the proximity of the orifice plate reduces the tangential velocity of the medium, and creates a strong inward flow towards the throat. This inward flow results in regions of high centrifugal forces within the vessel, resulting in further separation of the middlings and heavier particles (shale). The shale, together with a small amount of the medium, are discharged through the throat into the shale chamber, which is connected to a second shallow chamber known as the vortextractor by a short duct. The function of the Vortextractor is to dissipate the inlet pressure energy, such that a large diameter outlet nozzle can be used without the passing of an excessive amount of the medium [Wills (1997)].
4.2.3 The Dyna whirlpool (DWP) Separator
Float discharge
Row feed
"
Sink discho'Ve
Figure 4.9 Dyna whirlpool rHorsfall (1993), Wills (1997)]
CHAPTER 4 SEPARATING VESSELS
The Dyna whirlpool (Figure 4.9) has become the most used separator for the cleaning of coal with a size range between 30-0.5 mm [Wills (1997)]. It differs from the cyclone in having a completely cylindrical barrel, with the medium inlet being separated from the feed coal. The floats product discharges from the bottom end, the sinks product from the upper end, which is also completely different to the cyclone [Horsfall (1993)]. The separator is operated at an inclined position, and medium of the required specific gravity is pumped under pressure into the lower inlet. The main advantage of this is that the feed does not have to be introduced under pressure since the vortex is created by the medium pumped in at the lower end of the barrel. The medium entering with the feed must only have enough pressure to wash the feed into the separator.
Particles with a specific gravity less than that of the medium pass through the vortex generated by the rotating medium. Particles with a much higher specific gravity are pushed to the outer wall of the barrel, and are discharged with the medium through the sink discharge pipe. Due to the proximity of the sinks discharge pipe and the feed inlet, the sinks are removed from the separator almost immediately, reducing wear on the outer walls of the unit. The reduction in wear, and since only the medium is pumped, decreases the maintenance costs, and also maintains performance of the unit. However, the Dyna whirlpool does not have the same sharpness of separation as the cyclone [Horsfall (1993)].
4.2.4 The Tri-Flo Separator
The Tri-Flo (Figure 4.10) separator can be regarded as two Dyna whirlpool separators bolted together [Horsfall (1993), Wills (1997)]. Floats from the first stage become the feed into the second stage. In this way a single unit gives three products.
The device can be operated with two media of differing specific gravities in order to produce sink products of individual controllable densities. Treatment using a single medium specific gravity produces a float and two sinks products which have slightly different separation densities [Wills (1997)]. Although the two-stage separation of the Tri-Flo separator increases the sharpness of separation, its main disadvantage is that only first stage floats can be re-treated. In most two-stage separations it is technically better to re-treat sinks [Horsfall (1993)].
CHAPTER 4 SEPARATING VESSELS
Sink I
f
Medium inlet
Float Medium inlet
Figure 4.10 Tri-Flo separator [Wills (1997)]
4.2.5 The Larcodems (LARge Coal Dense medium separators)
The Larcodems (Figure 4.11) is virtually a scaled-up Dyna whirlpool (section 4.2.3). The difference being that it can treat a wider range of particles, particularly large particles that cannot be treated in the Dyna whirlpool. Larcodems can treat particles with sizes as large as 100 mm [Horsfall (1993)]. The unit also has at its reject end, a Vortextractor. The function of the Vortextractor is to impose back pressure in the reject stream, in order to restrain the rate at which the sinks product flows.
CHAPTER 4
Cl.on cool
Me~i\lm Inlel I I
T
I
•
@ •
..
IFigure 4.11 Larcodems washer [Horsfall (1993)]