Research on the preseparation space in hydrocyclones

International Journal of Mineral Processing, 31 ( 1991 ) 1-10 Elsevier Science Publishers B.V., Amsterdam

Research on the preseparation space in hydrocyclones Xu Ji Run, Luo Qian and Qiu Ji Cun Department of Mineral Engineering, Northeast University of Technology, Shenyang, P.R. China (Received October 3, 1989; accepted after revision July 4, 1990)

ABSTRACT Xu, J.R., Luo Q. and Qiu, J.C., 1991. Research on the preseparation space in hydrocyclones. Int. J. Miner. Process., 31: 1-10. The preseparation space in a conventional hydrocyclone, i.e., the space between vortex finder and column wall, has little effect on the separation process, thus it is presented in this paper to minimize the space by increasing the outer diameter of the vortex finder. As a result, the advantages of the improved hydrocyclone over the conventional one have been discovered as follows: ( 1 ) cutting down the short-circuit flow significantly; (2) increasing the energy contributed to the main separation space; ( 3 ) diminishing the internal pressure loss in the hydrocyclone; (4) changing the conventional zero vertical velocity locus into a wedge zone and making the separation more effective. By laboratory experiments, it is justified that the hydrocyclone with a narrow preseparation space not only improves the separation performance but also reduces the internal loss.

INTRODUCTION

According to the different functions, the internal space in a hydrocyclone can be divided into three parts (see Fig. 1 ), i.e., the preseparation space between vortex finder and column wall, the main separation space under the vortex finder and the space occupied by the air core. Generally speaking, the main separation process would be carded out in the main separation space, while the separation in the preseparation space is negligible. In fact, the threedimensional velocities in the preseparation space are not developed to the full, especially the tangential velocity is not obviously changed (Luo et al., 1989). The preseparation space, therefore, is mainly used to cut down the short-circuit flow (Kelly and Spottiswood, 1982), so that, to narrow down the preseparation space by thickening the vortex finder wall will have, at least, no unfavourable effects on the performance of the hydrocyclone. On the contrary, as illustrated by this paper, it can eliminate the short-circuit flow sig0301-7516/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

XUdlRIJNE'IAI

Moin Sep. Spoce \

/ presep.spGce

Fig. t. The different spaces in a conventional hydrocyclone.

Fig. 2. Cross-section of the hydrocyclone with a narrow preseparation space.

nificantly, increase the energy contributing to the main separation space, reduce the internal loss, improve the flow pattern and raise the separation

efficiency.

The cross-section of the hydrocyclone with a narrow preseparation space is shown in Fig. 2 where the width of the space is the same as the diameter of the feed inlet so as to minimize the sudden enlargement loss at the entrance and to maintain a constant throughput. The main geometry parameters of the hydrocyclones used here are given in Table 1: D is the diameter of the hydroTABLE 1

Structural parameters of hydrocyclones tested O (mm)

df (ram)

d, (mm)

do (mm)

a (°)

L (mm)

82

10

6;8; 10

12; t6;20

20

26; 10

RESEARCH ON PRESEPARATION SPACEIN HYDROCYCLONES

3

cyclones; dr, ds, do are the diameters of the inlet, apex and vortex finder, respectively; a is the cone angle and L the width of the preseparation space. P R E S E P A R A T I O N SPACE A N D S H O R T - C I R C U I T F L O W

The so-called short-circuit flow in a hydrocyclone is the part of feed which, being not processed, reports directly to the overflow across the vortex finder wall. A narrow preseparation space can diminish greatly the short-circuit flow because the flow has a longer road to go owing to the wider wall of the vortex finder and is more difficult to get into the overflow than that in a conventional hydrocyclone. On the basis of the boundary layer theory in fluid mechanics, Bloor and Ingham (1983) derived two formulae with regard to the thickness of the roof boundary layer J and the short-circuit flow rate QL through the layer:

W)Rc6, QL =2rt(#/p)W/t'+w)( V~nRc)'/t~+W)RcQ'L

6 = (I.t/p ) w/ ( m+ w) ( V i n R c )

- w/ ( l +

( 1) (2)

where/~ and p are the viscosity and density of the fed slurry, respectively, Vin is the entering velocity, Rc the radius of column part, w a parameter describing the flow pattern, with a value of 1 for laminar flow and 1/4 for turbulent and J' and Q[ are dimensionless thickness and flow rate, respectively, both being the function of the outer diameter of the vortex finder. In our experiments, Rc is 41 mm, the outer diameter of the vortex finder is 31 mm for the narrow preseparation space and 15 m m for the conventional one, corresponding values of 6' and Q[ are given in Table 2 according to Bloor's study. When p is 1.19 g cm -3,/~ is 0.012 P and V~, is 598 cm sec -1, the values of J and QL for different flow pattern of the roof boundary layer and different width of the preseparation space, calculated from eqs. 1 and 2, are listed in Table 3 (the actual short-circuit flow rate will be measured and discussed below). Because the actual throughput is only 470 ml sec-1, the flow pattern of the roof boundary layer should be laminar. It is seen that the TABLE 2 Values of J' and Q [ (Re=41 ram) L (mm)

J' J' Q[ Q[

For For For For

laminar pattern turbulent pattern laminar pattern turbulent pattern

26

10

2.4 0.64 0.92 0.125

2.6 0.48 0.53 0.055

4

xlJ Jl RIJN El AL

FABLE 3

Values o f 6 a n d QL(Rc=41 ram) L (ram)

QL Q~

For For For For

laminar pattern (/Lm) turbulent pattern (#m) laminar pattern (ml sec- ~) turbulent pattern (ml s e c - ' )

26

10

199 2196 118 661

216 1647 68 291

short-circuit flow of the hydrocyclone with a narrow preseparation space is only 14.5% of the total throughput, but it is high up to 25,1% of the total discharge in the conventional cyclone. P R E S E P A R A T I O N SPACE A N D E N E R G Y D I S T R I B U T I O N

A narrow preseparation space in a hydrocyclone will increase the energy distributed to the main separation space. Suppose that the energy distribution density within the whole preseparation space is even and does not vary with the enlargement or reduction of the space, then the energy used the area is proportional to the volume of the space. Let the widths of the preseparation space be Lo and L before and after narrowing the space and corresponding energy E0 and E respectively and the diameter of the cyclone D, the equation correlating E and Eo is derived as follows:

E/Eo = L ( D - L ) /Lo( O-Lo)

(3)

For example, if D = 8 2 mm, L = 10 mm and L0=26 mm, the energy contributed to the preseparation space will be cut down by 50%, which will increase the energy of the main separation space thus benefit the separation process under the same feed conditions. P R E S E P A R A T I O N SPACE A N D I N T E R N A L P R E S S U R E LOSS

The internal pressure loss in a hydrocyclone with a turbulent flow pattern may be described by eq. 4:

Ani =~V2,,/2g

(4)

where AHi is the pressure difference between inlet and overflow outlet, V~nis the average entering velocity, g is the gravity acceleration and ~ is called as resistence coefficient. Generally, the internal loss in a hydrocyclone is resulted from the wall surface friction, the turbulency and the viscosity etc. and difficult to be changed (Arato, 1984). However, as shown in this paper, the

RESEARCHON PRESEPARATIONSPACEIN HYDROCYCLONES

5

3.5

3,0

do = 16 mm ds = 8 rnm

L2.5 2.0 .i T .

1.5 U

0

~ 1.0

~ L =10 mm

E 0.5i c .

.

.

.

'

,

,

10 20 30 40 0 60 70 80 Average Entering Velocity Squared, V?n

0

Fig. 3. Width of preseparation space and internal loss.

internal loss can be reduced by narrowing the preseparation space. Figure 3 is one of the experimental results. When the width of the preseparation space is narrowed from 26 to 10 mm, the resistence coefficient lowers down by 17%. The variation of resistence coefficients with width L at different overflow and underflow outlets is given in Table 4 and Table 5 from which it can be calculated that the resistence coefficients decrease about 15-20%. TABLE 4

Resistence coefficient ~ and vortex finder diameter do (d, = 8 mm ) do ( m m )

for 1= 26 m m for L = 10 mm ,fig (%)

12

16

20

1.4470 1.2304 14.97

0.1807 0.6674 17.68

0.2823 0.2315 18.00

TABLE 5

Resistence coefficient ~ and apex diameter d,(do= 16 m m ) d, ( m m )

for L = 26 mm

~for L = 10 m m ,J~ (%)

6

8

10

0.8678 0.7245 16.51

0.8107 0.6674 17.68

0.7416 0.5907 20.35

0

X|! J l Rt rN I-IT A[

The causes of decrease of the internal pressure loss after narrowing the preseparation space may be analysed from the following three aspects: ( 1 ) the reduction of the volume naturally cuts down the energy consumption in the space; (2) the width of the space is just equal to the diameter of the inlet, minimizing the sudden enlargement loss at entrance; and (3) the eddy flow loss is reduced because the original two-ring eddy flow, as shown by the measurements of the flow field, has been substituted by a single-ring one ( Luo et al., 1989). It is very important for a hydrocyclone with subsequent operations, e.g., another cyclone in sequence, to decrease the internal loss and enhance the overflow pressure. Recently, some researchers devote themselves to the study of transforming the kinetic energy of overflow at the outlet into static pressure and have made certain progress (Arato, 1984; Boadway, 1984). But as there is no development so far on the decrease of the internal pressure loss in hydrocyclones, the method to cut down the internal loss described above may be a convenient approach. PRESEPARATION SPACE AND FLOW FIELD

Narrowing the preseparation space must have some effects on the flow field, especially the vertical velocity in the main separation area. It is well known that there is a zero vertical velocity locus as one of the characteristics of the flow field in a conventional hydrocyclone, while in the cyclone with a narrowed preseparation space a zero vertical velocity wedge zone takes the place of the original locus (see Fig. 4) because of the obstruction of the thicker vortex finder wall to the vertical flow. In theory, the zero vertical velocity locus, like a screen in radial, determines the separation size in a conventional cyclone. In fact, however, whether a particle fed into the cyclone is within or outside the locus, i.e., reports to overflow or underflow, depends not only on the nature itself such as size and density but also on the radial turbulence, in other words, the m o v e m e n t of the particle near the locus is to some extent governed by some accidental factors. Fortunately, the zero vertical velocity wedge zone in the hydrocyclone with a narrow preseparation space acts as a multiscreen, diminishing the accident of wrongly reporting to overflow or underflow for the separated particles and increasing the separation precision. In addition, the flow field in the preseparation space has some favourable changes as the space is thined. The distributions of vertical and tangential velocities in the area before and after narrowing are shown in Fig. 5 where the radial velocity, not being drawn, is always about zero and does not vary with the width of the space. It is clear in Fig. 5 that in the narrowed preseparation space the distribution of vertical velocity becomes simpler, the original tworing flow changes into a single-ring flow, the downward flow near the wall of

RESEARCH ON PRESEPARATION SPACE IN HYDROCYCLONES

7

Vortex Finder Walt

/ -.j

;/ (a)

(b)

Fig. 4. (a) Zero vertical velocity locus in a conventional hydrocyclone and (b) zero vertical velocity wedge zone in the hydrocyclone with narrow preseparation space.

~ (a)

o

(b)

Fig. 5. Flow field in preseparation space in (a) conventional and (b) improved hydrocyclones. O - vertical velocity; • - tangential velocity.

vortex finder gets smaller, showing the decrease of the short-circuit flow, and the tangential velocity varies from the original almost homogeneous distribution along the radius to a pattern similar to the forced vortex, being more beneficial to the dispersion of the feed because of the tangential shearing. EXPERIMENTAL SEPARATION RESULTS

In order to justify the hydrocyclone with a narrow preseparation space superior to the conventional one, some experimental examinations have been done at feed pressure of 0.9 kg c m - 2 in laboratory with a hematite slurry of

\ l ~ Jl RI'N E | , k l

20% weight concentration. The structure parameters of the tested hydrocyclones are the same as that in Table 1, with only do= 16 mm and ds=6 mm. One of the experimental results of the conventional and improved hydrocyclones is given in Table 6 and Table 7, respectively, and Fig. 6 shows the corresponding performance curves. (The same experiments were repeated five times and similar results were obtained). It can be seen that the hydrocyclone with a narrow preseparation space has a less short-circuit flow (10%) than the conventional cyclone ( 18% ) and a higher separation precision for the finer particles less than 38/~m because the fraction of the particles to underflow is reduced. TABLE 6 Experimental results (for conventional hydrocyclone, L = 26 mm ) Particle size (,am)

Underflow mass (%) in size i

Overflow mass (%) in size i

Feed mass (%) in size i

Fraction to overflow (%) in size i

Fraction to underflow (%) in size i

+ 0 - 19 + 19 - 3 8 +38 - 4 5 + 45 - 55 + 55 - 65 +65 - 7 6 +76 - 100 Total

0.00 0.48 0.27 22.56 42.41 5.27 5.01 76.00

0.00 1.92 0.43 8.73 10.68 1.13 1.10 24.00

0.00 2.40 0.70 31.29 53.09 6.40 6.11 99.99

0.00 79.9 61.4 27.9 21.1 17.7 18.0

0.00 20.1 38.6 72.1 79.9 82.3 82.0

TABLE 7 Experimental results (for improved hydrocyclone, L = 10 mm ) Particle size (#m)

Underflow mass (%) in size i

Overflow mass (%) in size i

Feed mass (%) in size i

Fraction to overflow (%) in size i

Fraction to underflow (%) in size i

+0 19 + 19 - 3 8 +38 - 4 5 +45 - 5 5 +55 - 6 5 +65 - 7 6 +76 - 100 Total

0.00 0.49 0.25 21.29 47.94 5.35 5.68 81.00

0.00 2.21 0.26 8.79 6.53 0.58 0.63 19.00

0.00 2.70 0.51 30.08 54.47 5.93 6.31 100.00

0.00 81.9 51.0 29.2 12.0 9.8 10.0

0.00 18.1 49.0 70.8 88.0 90.2 90.0

-

RESEARCH O N P R E S E P A R A T I O N SPACE IN H Y D R O C Y C L O N E S

9

90

8C 70

60

50

q h

Z

~o 3o

o Normol cyclone

C3

• New cyclone

d

,o//

~ 2o

o~

' 2'0

' 4b

' 6'0

' 8'0

' 60

SIZE, .~m

Fig. 6. Performance curves of two kinds of hydrocyclones. CONCLUSIONS

Following improvements will be made in hydrocyclones if the width of the preseparation space is reduced properly: ( 1) diminishing the short-circuit flow significantly; (2) decreasing the energy contributed to the preseparation space and making the energy distribution within the whole cyclone more appropriate; (3) reducing the internal pressure loss and enhancing the pressure of overflow at outlet; and (4) improving the flow field in the main separation space, changing the zero vertical velocity locus in the conventional hydrocyclone into a corresponding wedge zone thus increasing the separation precision. The experimental results show that the hydrocyclone with a narrow preseparation space can certainly improve the separation performance. REFERENCES Arato, E.G., 1984. Reducing head or pressure losses across a hydrocyclone. Filtr. Sep., 21 (3): 181-182.

IO

XtJ Jl R U N Ei-AL.

Bloor, M.I.G. and Ingham, D.B., 1983. Theoretical aspects of hydrocyclone flow. Prog. Filtr. Sep., 3: 57-147. Boadway, J.D., 1984. A hydrocyclone with recover5' of velocity energy. Pap. d I, 2nd lnt Conf. Hydrocyclones, Bath, England. Kelly, E.G. and Spottiswood, D.J., 1982. Introduction to Mineral Processing. Wiley, New York, NY, pp. 214-216. Luo Qian, Deng Changlie, Xu Jirun, Yu Lixin and Xiong Guangai, 1989. Comparison of the performance of water-sealed and commercial hydrocyclones. Int. J. Miner. Process., 25: 297310.