CHALLENGING THE RECOMMENDED COMPARATIVE DESIGN RATIOS FOR DMS CYCLONES

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1 CHALLENGING THE RECOMMENDED COMPARATIVE DESIGN RATIOS FOR DMS CYCLONES IGT Smith and D Lincoln Multotec Process Equipment (Pty) Ltd, Spartan, South Africa, 1619 ABSTRACT Cyclone efficiency was monitored while different vortex finder lengths were tested. The main purpose was to address the issue regarding vortex finder length and the insertion of a barrel. Preliminary tests with tracers show that the vortex finder length may be increased, whether a barrel is included or not. Compared to the exclusion of a barrel, the separation efficiency doesn t decrease when a barrel is inserted at the standard vortex finder length for the C360. INTRODUCTION Are you getting the most out of your dense medium cyclone? An on-going test program is pushing the boundaries of the Dutch State Mine (DSM) recommendations to maximize the efficiency of dense medium cyclones. The program is investigating the hypothesis that, by simply changing some critical dimensions at the correct ratios, it is possible to improve the overall efficiency of the cyclone and ultimately reduce the cost per ton treated. The vortex finder length is the first variable to be tested. The general recommendation by industry is that the length of the vortex finder should change when a barrel is added to the cyclone, while standard practice is to keep the length of the vortex finder the same. The test work is aimed to explore this principal further. The motivation behind this endeavour is to see whether separation efficiency was impaired by increasing cyclone capacity in a milieu where the quality of feedstock is ever decreasing as demand increases. The barrel is included to increase capacity and residence time to achieve better separation efficiency in more difficult to separate material. The test program is a lengthy process due to the many variables. However, the learning process is expected to have a significant impact on the understanding of dense medium cyclone design. BACKGOUND In September 1945, Driessen proposed a way to segregate fine coal (minus 3.2 mm at that time) by using heavy liquids and suspensions in cyclone washers, which introduce a centrifugal force rather than using gravity (Van der Walt, 1950). The Dutch State Mines then developed the heavy medium washer where the employed centrifugal force sped up the separation process and also improved on the overall efficiency (Van der Walt, 1950). These washers are now known as dense medium separating cyclones.

2 When the Dutch State Mines developed the dense medium cyclone washers, they gave a few recommendations on the cyclone geometry. Table 1 shows the recommended comparative design ratios by Dutch State Mines and those for Multotec s C360 cyclone (cyclone diameter (D C ) = 350 mm). Note that the spigot on the C360 cyclone during the test work had a diameter of 100 mm, to be in line with the recommended ratio. DSM Multotec s C360 Inlet Diameter [D I ] 0.20 D C 0.20 D C Vortex Finder Diameter [D V ] 2.15 D I 0.43 D C 0.42 D C Spigot Diameter [D S ] Max 0.85 D V 0.37 Dc Max 0.85 D V 0.37 Dc Cone Angle Standard 20 o Standard 20 o Cylindrical Part Length 0.66 D C 0.57 D C Barrel Length 1.84 D C 0.66 D C Combined Length 2.50 D C 1.23 D C Table 1 Comparative design ratios: Dutch State Mines vs. Multotec The barrel The geometries regarding the inlet, vortex finder and spigot diameters are on par with the DSM recommendation, and the only discrepancy is the length of the cylindrical part prior to the conical section. The C360 cyclone is a bit shorter in this regard whether a barrel is present or not. The Dutch State Mines doesn t differentiate between the inclusion and exclusion of a barrel, but increases the cylindrical part to a total of 2.50 of the cyclone diameter when the near density material in the feed is 30% or more. The Dutch State Mines defines near density material as the and increment from the cut density. Multotec on the other hand includes a barrel to increase cyclone capacity and also allow for larger residence time in the cyclone to achieve a better separation efficiency and less misplaced material in more difficult to separate material. The vortex finder In the beginning of centrifugal washers short circuiting of coarse particles to the overflow occurred in hydro cyclones. This is due to the close proximity of the inlet and overflow opening as described in Figure 1. The vortex finder was implemented to prevent this shortcircuiting from happening. On the other hand, coarse particles may also be entrained to the overflow when the vortex finder tip extends into the conical section. This is due to pressure turbulence and swirls forming near the vortex finder inlet when it is extended too much into the cyclone (Martinez et al., 2008). The sole purpose of the vortex finder was to counter the short-circuiting effect, and no clear recommendation was given on the length of the vortex finder. This was also evident in test work done by Van der Walt (1950), where by-pass of heavy material decreased by

3 including the vortex finder in a dense medium cyclone treating coal. Van der Walt (1950) also tried to find the optimum vortex finder length, but found that the efficiency remained constant with the increase of the vortex finder up until a drastic decrease in cyclone efficiency when the vortex finder was increased too much (vortex finder length to cyclone diameter ratio of 1.74, where the next preceding setting was at a ratio of 1.00). The current ratio of vortex finder length to cyclone diameter used for the C360 cyclone is 0.42 D C. Figure 1 Illustrating how short circuiting was prevented by adding the vortex finder (Adapted from Martinez et al., 2008) Later Lotzien et al. (1978) investigated a number of cyclone geometries which included extensive vortex finder length and diameter test work as well. This research was primarily done to improve the separation performance of dense medium cyclones for the ultra-fine particles. Lotzien et al. (1978) found that the e.p.m. optimum was between the ratios of 0.7 and 0.8. Cyclone operation Normally when a cyclone isn t performing as it should, processes that influence the medium and material characteristics and behaviour are to blame when the cyclone geometries are correct and isn t worn or damaged. The operating pressure is also an important variable, but if the medium doesn t behave favourably, the cyclone will not perform adequately. The rheology of the medium plays an integral role, and when the viscosity is too high smaller particles cannot move through the medium (He and Laskowski, 1994). Multotec test for instability or viscosity problems by taking the feed density minus the overflow density, divided by the feed density, and the obtained figure should be between 3% and 12%. When it is above 12% the differential is too high and stability issues is encountered, and if it is below 3% viscosity problems are prominent. Another way to see whether the medium behaves as it should is to determine the extent of medium segregation by using the densities of the overflow and underflow. Here the

4 differential is the underflow density minus the overflow density. If the differential (medium segregation) is too large, particle hang-up can occur. This differential should be between 200 and 500 kg/m 3 (He and Laskowski, 1994). For the purpose of this paper the two types of differentials are distinguished between the differential described by the difference in overflow and underflow density (medium segregation extent Type 2) and the differential described by relating medium segregation to the feed (Type 1). EXPERIMENTAL The cast iron C360 cyclone was used during the test work. It has an inner diameter of 350 mm and was operated at 9D pressure (43 kpa). The un-scrolled inlet head was used during the test work, with the purpose of comparing the inclusion and exclusion of a barrel. The standard cone angle of 20 o and vortex finder diameter of 150 mm (0.43 D C ) was used with a 100 mm spigot diameter (0.29 D C 0.67 D V ). Subsequent test work will vary these geometries as well, and a large test work matrix will be completed. For the purposes of this paper the vortex finder was varied in increments of 50 mm, and the separation efficiency noted. Tests were also done at the standard vortex finder length for the C360 cyclone (which is 147 mm or 0.42 D C ), and vortex finder lengths that will correspond to the same distance from the conical section between the scenarios where a barrel is present and not. This means, that when a barrel is present, the vortex finder length was 415 mm and 35 mm from the cone, for example, and 177 mm in length to obtain the same distance from the conical section (35 mm) when no barrel is inserted. Refer to Table 2 for the entire vortex finder length range tested. Barrel Present No Barrel Present Correspondence Same distance from inlet mm From Cone mm From Cone mm Into Cone Table 2 Vortex finder lengths tested and explaining which correspond to the immediate cyclone geometry

5 The conical section was never entered (except for one length tested where no barrel was included). If the vortex finder is too long, it only reduces the capacity without any gain in efficiency. Each of the vortex finder adjustment runs were repeated five times, and previous work showed that the repeatability is extremely high. The standard deviation was also determined. Since the fields of interest are the commodities of coal and diamonds, the test work commenced with magnetite (FeSi is naturally the next step). The magnetite slurry was made up to a relative density of 1400 kg/m 3 and 85% material passed 45 µm. The particle size degradation of the magnetite was also monitored. 8 mm tracers from Tenova Mining & Minerals were used and 50 tracers of each density class were charged directly into the pipe system. There were 9 SG classes that ranged from 1.2 to 1.9 in increments of SG increments of 0.05 were requested, but weren t available. RESULTS AND DISCUSSION Separation efficiency The separation efficiency was measured by means of the Écart Probable Moyen (Ep) and obtained by the use of tromp curves. The Ep represents the error or misplaced particles and the lower the Ep the higher the separation performance. The individual Tromp curves were modelled by equation 1, and solved for the correct Ep and ρ 50 values, where is the partition number and is the density of the relative density interval. Typical partition curves are shown in Figure 2 and Figure 3 (examples of the actual tests), where the latter shows a sharper separation curve and hence, a lower Ep. [ ( ) ( ) ] (1) Equation 1 Partition curve (Tromp curve) model (Wills and Napier-Munn, 2006) By using the model and mathematical regression a fairly accurate estimate is made regarding the correct values for Ep and ρ 50 opposed to reading off the Ep and density cut-point. The Ep values and cut-point for the respective vortex finder lengths in Figure 2 and Figure 3 are and 1.455, and and respectively. It is interesting to note that the increase in separation efficiency is due to less misplacement of light material to the underflow (which was the case throughout) with extension of the vortex finder towards the conical section. The Ep values (actual) obtained for different vortex finder lengths are given in Figure 4 and Figure 6 for the exclusion and inclusion of a barrel respectively. A schematic of the vortex finder position within the cyclone is also given, and comprise of Figure 5 and Figure 7 (also for the exclusion and inclusion of a barrel respectively). The vortex finder lengths portrayed

6 Partition Number Partition Number in Figure 5 and Figure 7 are the actual lengths used as given in Table 2, and the vortex finder length to cyclone diameter ratio is also included in Figure 5 and Figure 7 in brackets below the vortex finder length given in millimetres Relative Density 8mm U/F Model 8mm U/F Figure 2 Tromp curve: un-scrolled inlet head with barrel and VF L = Relative Density 8mm U/F Model 8mm U/F Figure 3 Tromp curve: un-scrolled inlet head with barrel and VF L = 350 mm (Ratio to D C = 1.0)

7 Ep Value Vortex Finder Length to Cyclone Diameter Ratio (D C = 350 mm) 8mm Tracers Standard: 0.42 on C360 Figure 4 Un-scrolled inlet head without a barrel: Ep values Figure 5 Schematic of vortex finder position within cyclone: Without barrel

8 Ep Value Vortex Finder Length to Cyclone Diameter Ratio (D C = 350 mm) 8mm Tracers Standard: 0.42 on C360 Figure 6 Un-scrolled inlet head with barrel included: Ep values Figure 7 Schematic of vortex finder position within cyclone: With barrel included

9 Cut-Density It is evident that separation efficiency is increased by extending the vortex finder towards the conical section. It is also noted that the cyclone performs poorer when the vortex finder is shorter than the standard length, but performs roughly the same as the scenario where no barrel is included at the standard vortex finder length. However, the cyclone performs better when a barrel is present with a longer vortex finder, especially when the vortex finder is situated close to the conical section. This is when the vortex finder is 65 mm from the cone with a barrel (corresponding to a 147 mm vortex finder with no barrel) and 35 mm from the cone with a barrel (corresponding to a 177 mm vortex finder with no barrel). The bottom-line is: according to the tracer tests, the separation efficiency for 8 mm tracers is increased with increase of vortex finder length. Material should also be tested to make this a definitive statement. Density cut-point Figure 8 and Figure 9 show the density cut-point as the vortex finder was extended through the cyclone. The 8 mm tracers show an initial steady increase at a vortex finder length to cyclone diameter ratio of 0.3 to 0.4. This is as expected, and happens due to the density shift within the cyclone. Density shift can be described as smaller particles separating lower down in the cyclone at a higher density band than larger particles which separate quickly and higher up in the cyclone at a lower density band. This is due to the drag force present in the cyclone that causes smaller particles to travel at a slower rate through the cyclone Vortex Finder Length to Cyclone Diameter Ratio (D C = 350 mm) 8mm Tracers Standard: 0.42 on C360 Figure 8 Un-scrolled inlet head without barrel: Cut-density

10 Differential [% related to feed] Differential [kg/m 3 ] Cut-Density Vortex Finder Length to Cyclone Diameter Ratio (D C = 350 mm) 8mm Tracers Standard: 0.42 on C360 Figure 9 Un-scrolled inlet head with barrel included: Cut-density Medium behaviour The medium behaviour of a cyclone is extremely important, and the differentials of a cyclone should be between 3% and 12% (Type 1). The differentials (the block figures read off on the left vertical axis as indicated by the arrow) varied from 5% to 7% and the medium behaviour was good at an average of 5.9% for the entire vortex finder length test work, refer to Figure % 11% 10% 9% 8% 7% 6% 5% 4% 3% Vortex Finder Length to Cyclone Diameter Ratio (D C = 350 mm) Type 1 Type 2 Standard: 0.42 on C Figure 10 Summarising the medium behaviour differentials by Type 1 and 2 (With barrel)

11 Differential [% related to feed] Differential [kg/m 3 ] Another key method to determine medium behaviour is by addressing Type 2 (the circle figures read off on the right vertical axis in indicated by the arrow). If the medium is segregated to a too large extent, material hang-up occurs. Hang-up was noticed and found to be imminent when the difference in overflow and underflow density reaches 450 kg/m 3. This was determined by observing no hang up for vortex finder lengths 0 mm and 50 mm, while at 100 mm particle hang-up occurred (also see Figure 10). This is the case for vortex finder length to cyclone diameter ratios of 0.29 and higher. 12% 11% 10% 9% 8% 7% 6% 5% 4% 3% Vortex Finder Length to Cyclone Diameter Ratio (D C = 350 mm) Type 1 Type 2 Standard: 0.42 on C Figure 11 Summarising the medium behaviour differentials by Type 1 and 2 (Without barrel) It is also noted that the difference in overflow and underflow density remained fairly constant up to a vortex finder length to cyclone diameter of 1.00 (Figure 10), and it may be assumed that normal cyclone operation can be expected when the vortex finder is extended up to this point. Further down the cyclone the medium segregation will be at a too great extent as well as the particle hang up. Particle hang-up of near density particles has the potential to rapidly increase cone wear. The same behaviour is seen when no barrel is present (Figure 11), but the differentials don t increase to the same extent as when the barrel is included stays around 500 kg/m 3. The mean density of hang-up material is roughly the same as the medium density, where mostly the 1.4 SG tracers hang-up. This is in accordance with the theory regarding how a dense medium cyclone operates. A balance between the centrifugal force radially outward and the drag force radially inward exists. Heavy particles tend to move outward to the cyclone wall and down to the apex, while lighter particles move toward the air column and up the vortex finder. The heavy particles either exit through the apex (spigot), or are recirculated to the vortex, penetrating the dense medium profile again and move towards the cyclone wall. This creates a density profile within the cyclone that may be higher in density

12 than the dense medium fed to the cyclone, Van der Walt (1950). Thus, it can be deduced that the material that gets hanged-up within the cyclone should be near in density to the medium density, and the cut-density should be higher (which is the case and common knowledge). CONLUSIONS When the vortex finder is at standard length (147 mm or 0.42 D C ) the same separation efficiency was obtained with and without a barrel. In both cases the separation efficiency is increased with increasing the vortex finder length, and the highest efficiency is obtained when the vortex finder was 35 mm from the conical section in the case without a barrel and 65 mm from the conical section when the barrel is present. This corresponds to a vortex finder length to cyclone diameter ratio of 0.5 (177 mm) and 1.1 (385 mm) with and without a barrel respectively. The down side, however, is that medium segregation becomes very high at these vortex finder settings, and material hang-up may be high. This in turn may cause wear in the cone. Medium segregation remains constant through vortex finder length to cyclone diameter ratios of 0.42 to 1.00, where 0.42 is the standard vortex finder length to cyclone diameter for the C360 cyclone. It can then be concluded that particle hang-up and cone wear will be the same within this ratio range. The best separation efficiency that was obtained within this range was at a vortex finder length to cyclone diameter ratio of 1.00, which relates to a vortex finder length of 350 mm this was for the case when a barrel is present. It must be stressed, however, that the differentials are influenced by a number of factors, which include: shape of inlet head (square or rectangular) inlet design (tangential or evolute) spigot size medium density cyclone diameter feed pressure medium viscosity and stability. With no barrel present, the vortex finder was increased to just before the conical section starts (0.5 vortex finder length to cyclone diameter ratio). It also seems that the differential at this vortex finder length is also acceptable at 500 kg/m 3. It is recommended to undertake another study that involve the explanation of the stepwise increase in separation efficiency as the vortex finder is extended through the cyclone by means of computational fluid dynamics (CFD).

13 REFERENCES He Y.B. and Laskowski J.S., 1994, Effect of dense medium properties on the separation performance of a dense medium cyclone, Minerals engineering, Lotzien R., Hober H. & Schneider F.U., 1978, Cleaning of ultra-fines in heavy liquid cyclones, Aufbereitungs-Technik, Martinez L.F., Lavin A.G., Mahamud M.M. & Bueno J.L., 2008, Vortex finder optimum length in hydrocyclone separation, Chemical engineering and processing, Van der Walt P.J., 1950, A study of the operation of the cyclone washer and its application to Witbank fine coal, Journal of the chemical, metallurgical and mining society of South Africa, none Wills B.A. and Napier-Munn T.J., 2006, Wills mineral processing technology, 7 th Elsevier Science & Technology Books, Burlington MA, p ed.,