The dynamics of sluice and spiral separations

Minerals Engineering, Vol. 8, Nos 1/2, pp. 3-21, 1995

Pergamc~ 0892-6875(94)0O098-O

Elsevier Science Ltd Printed in Great Britain 0892-6875/95 $9.50+0.00

THE DYNAMICS OF SLUICE AND SPIRAL SEPARATIONS

A.B. HOLLAND-BATT Mineral Technologies Division, Clyde Industries Limited, P.O. Box 5044, Gold Coast Mail Centre, QLD 4217, Australia (Received 28 January 1994; accepted 1 June 1994)

ABSTRACT

A basic design problem for both sluices and spirals is to detelraine the required trough length. The present contribution examines sluice perfotrnance in relation to stratification length as a preliminary to the more complex issue of spiral length. Test results obtah~ed jS'om partillel sided sluices are analysed and compared with computer based petformance projectiot~ to detetTnine how reliable the simulations are in forecasting actual results. The general trends appear colTect and recent developments in computational fluid dynamics offer promise of quantitative simulators for both sluices and spirals. Test results obtained dS'om both coal and mineral spirals having varying numbers of turns are compared and it is found that separation continues down thefull length of the troughs though at a diminishing rate. It is shown that mineral separations can be revitalised by employing repulpers, which offer the same advantages as constructing multiple shorter troughs on one column in a simple and more cost effective solution. The sedimentation rates in coal separations suggest that five or six turn troughs may be needed and in addition /he reversed product locations compared with mineral spirals indicate that repulpers are likely to be ineffective. Test results are presented that support both premises ,and it is concluded that the longer troughs currently in use are the best choice for coal spirals. Keywords Spirals, sltuices, stratification, trough length, repulpers.

INTRODUCTION Spiral separators :have proved to be both metallurgically efficient and cost effective in the treatment of a variety of ore types since their introduction nearly 50 years ago. More recently, this success has been repeated in the treatment of coal fines, though the performance is perhaps not as closely matched to the ideal process requirements as in the ease of many metalliferous separations. Because o f their vertical orientation and the usual employment of gravity feed with the associated distribution systems located above the spirals~, there is a minimum height requirement that can be inconvenient particularly when retrofitting spiral,; into existing plants. It is therefore a matter of some importance to ensure that the number of turns employed in the spiral is no greater than is needed to achieve the required separation. It has been claimed that effective separation is at an end after 2 turns on mineral spirals [1], while a less draconian viewpoint suggests that after 2 turns the rate of recovery on coal spirals has fallen to the point

4

A.B. HOLLAND-BATT

where a revitalisation of the separation is needed [2]. Most modern spirals employ 4 or 5 turns and occasionally 6 or 7 turns to achieve the required levels of performance, yet thirty years ago spirals with between 2 and 3.5 turns were commonplace in Australia. [3,4] Evidence has been quoted to support both points of view and the object of the present work is to see if it is possible to reconcile these apparent contradictions. The bulk behaviour of slurries flowing on launders and spiral troughs can be predicted with reasonable accuracy at low slurry densities, though the estimates become less and less reliable as the slurry density rises. Experimental results relating to the separation of heavy mineral on parallel sided sluices of various lengths are available and this evidence will be reviewed and compared with predicted performance as a preliminary to considering the more complicated separation on spiral troughs. The results achieved on both mineral and coal spirals will be examined in relation to trough length and the role played by re-pulping devices will be considered.

FLOW BEHAVIOUR ON P A R A L L E L SIDED SLUICES A number of tests were carried out [5] on a parallel sided sluice to investigate the effect of stratification length as a preliminary to designing a new high capacity sluice. The sluice was 150 mm wide and was inclined at 17° to the horizontal: the feed box could be moved to different locations so as to vary the effective length and at the bottom end it was equipped with a horizontal bull nose type splitter of the type employed on Reichert trays and cones. [6] The feed material employed was an originally low grade mineral sand from Queensland that had been augmented in heavy mineral content by addition of concentrated material (Table 1). The specific gravities of the high density solids was taken as 4.0 and that of the low density solids as 2.65 and the estimated bulk density was rounded to 2.7. T A B L E 1 Test material Sieve size

(microns)

Mass% retained

+300 +150 +75

8.2 86.1 5.7

Total

I00.0

Two series of tests at different feed rates were carried out, each series covering three insert settings and six sluice lengths. Slurry density was maintained at 60 % solids throughout and the feed grade, which was nominally 10% heavy mineral (HM), actually varied between 7 and 10% HM. The first series, conducted at 2.5 t/h solids, showed that there was insufficient mineral on the sluice to fully load the splitter so the results of the second series carried out at 3.5 t/h have been employed in this study. The results have been plotted as points in Figure 1 in the tbrm of separation efficiency [7] against stratification length and it can be seen that the minimum insert setting of -4 nun was inadequate to permit recoveries in excess of about 35 % and a plateau developed at lengths in excess of 3 m. The intermediate and maximum openings did not constrain the performance at greater stratification lengths and at 4 m length the efficiency was continuing to increase, though at a decreasing rate that suggested an eventual asymptote at about 60-65 % efficiency. The anticipated behaviour tbr the test material was estimated using a Eulerian model based on a single column of ten vertical cells, with the bulk flow characteristics and primary velocity profile predicted by use of the Manning equation. In selecting an appropriate value for the Manning coefficient, the flow relationship established by Subasinghe and Kelly [8] for sluices of normal aspect ratio was reworked to

Sluice and spiral separations

5

yield a value of 203.4 (in cm-s units). The experimental data of Abdinegoro and Partridge [9], obtained on an unusual sluice with a pronounced alteration in aspect ratio, was then reviewed and for the two higher flowrates employed (670 and 1000 mils) values of 207.2 and 205.3 were obtained. The mean value of 206.3 was sufficiently close to the value found by Subasinghe and Kelly to be acceptable for the purpose of preliminary modelling.

60 A

A X

5O

°.

1

40 X

+

N 3o El L~

20 tO

/ /

/

/

+--4== A.O.

+

X - .~4 olin

0

1

2

3

STRATIFICATIONLENGTH (m)

Fig. 1 Measured HM efficiencies on a 150 mm sluice. The sedimentation model employed the drag coefficient-Reynolds Number relationship and shape coefficients utilis~l in previous work on spiral behaviour [10] and the same system of correction for volume concentration effects. Bagnold shear effects can become significant at coarse sizes and high volume concentrations [8] and this mechanism was introduced in the form of an acceleration as proposed in a study by McNamera [11]: a diffusion effect proportional to the concentration gradient was also introduced as proposed in the same study. A limiting volume concentration of 60 % was imposed for the solids particles and. diffusion coefficients of 0.1 and 0.3 were assumed for the high and low density solids components. The computed results showed a negligible Bagnold effect due to the relatively fine sizes involved, but the diffusion coefficients were found to be significant control parameters. The predicted recoveries and assays have been plotted in Figures 2 and 3 in cumulative form against flow depth. The form of the bull-nose splitters made it impossible to specify an exact cut height to be used for the concentrate product, so the required cut height was found by interpolation from the appropriate recovery curve in Figure 2 by use of the measured recovery value. This cut height was then used to fred the predicted concentrate assay from Figure 3 and the corresponding efficiency was then calculated (Table 2). The curves shown in Figure 1 are based on these predicted efficiencies and they demonstrate surprisingly good agreement. Admittedly the measured recovery was used to locate the cut height and hence the predictex[ assay, but the resulting projected cut heights are grouped fairly well and this indicates that modelling based on simple sedimentation theory can provide estimates of the effect of trough length on the behaviour of conventional sluices that are of the correct order of magnitude. The results obtained by Abdinegoro and Partridge represent a more complex environment: as noted by Sivamohan [12], the enormous change in aspect ratio over a short distance in his design ere,ate unusual flow conditions in respect of both boundary layer and velocity head changes that do not occur in conventional sluices. To accommodate the unusual flow profile shown in Figure 2 of reference 9, it was

6

A.B. HOLLAND-BATF

necessary to introduce a quadratic velocity profile in both the vertical and horizontal planes: the form of relationship is shown in equation I. V=9L]----~.

h"

yoJl,,

+ - 61 ell & - 122 Cl 163 CO X rn ,44c .

/ ~ J

R

0

~5 ,-- 4 "-r H

I

l

I

I

I

I

I

I

I

t0

20

30

40

50

60

70

BO

90

i00

CUNULATIVE RECOVERY (%1

Fig.2 Predicted HM recoveries on a 150 nun sluice.

4" & X In V •.

7

6

• • . . -

6| CI I n CO |03 C l 244 ca 305 Oil 3 U can

~4

~3 2 ! I

J0

~5

I

I

I

20 25 30 CUI4ULAEIVE ASSAY (7~fl4)

i

35

Fig.3 Predicted assays on a 150 mm sluice.

40

Sluice and spiral separations

7

TABLE 2 Predicted sluice performance

Insert Sluice setting length (m) (cm) 4

.,4

Measured recovery

Cut height

Predicted Assay Effy

(%)

(In=)

(XrIM)

(%)

61 122 183 244 305 366

14.33 23.11 28.89 38.43 40.93 40.91

1.02 1.17 1.25 1.51 1.58 1.57

26.17 29.83 31.39 31.28 31.04 31.14

9.84 17.07 21.87 29.05 30.83 30.86

122 183 244 305 366

52.78 61.04 66.90 69.91 75.30

2.85 2.67 2.60 2.56 2.71

19.47 23.17 25.19 26.29 25.90

28.53 38.56 44.82 48.14 51.37

61 122 183 244 305 366

53.55 69.06

4.00 4.09

13.25 15.37

14.60 26.80

78.69

4.07

16.86

35.58

81.91 82.57 85.37

3.68 3.34 3.35

19.26 21.72 22.12

43.76 49.51 51.96

The simulation model was modified to handle a pinched sluice by introducing nmltiple lateral cells and allowing the pinch effect to progressively modify the cross-sectional flow area. When the relationship shown in eqn. 1 was applied to predict the concentration profile reported by Abdinegoro and Partridge for a quartz distribution at high volume concentration, the results shown in Figure 4 were obtained.

1oo 90

+ • Ab{lineooro & - Nckiaer| X = Predicted

8o ~ 70

~ Bo

~

ao

20 lO O

I

0

5

I

I

I

t0

I5

20

I

25

I

30

I

35

40

VOLUME CONCENTRATION (%)

Fig.4 Predicted andmeasured concentrations on a pinched sluice (after Abdinegoro and Partridge [9]).

8

A.B. HOLLAND-BAT'I"

McNamera [11] attempted to repeat the experinaents, but obtained somewhat different results with no concentration reversal near the bottom of the trough. There were uncertainties as to the precise equipment geometry in the crucial region preceding the splitters, so some divergence in the results is excusible. The predicted concentrations show higher values in the upper zones of the flow but agree reasonably well in the middle and lower regions: they also fail to find a concentration reversal near the bottom of the trough. In the zone adjacent to the outer walls of the sluice, the simulations suggested a more severe concentration gradient with a very dilute upper zone, caused by the much longer transit times in this region. It is possible that the inward transfer of fluid resulting from the pinch effect caused more of the upper dilute phase to flow into the central zone and reduce the slurry density accordingly. It is clear that the simple assumption of a bi-directional quadratic velocity profile is inadequate to describe the complex and progressively changing fluid flow in this particular sluice, but the overall pattern is correct and suggests that accurate predictions will be possible if the fluid motion can be calculated with greater precision. This abbreviated review of sluice behaviour suggests that sedimentation theory can provide a useful guide to the gross dynamics anticipated in sluice separations, even if the fine detail is still obscure. It appears justifiable to proceed to examine the more complicated separation environment within a spiral trough on the same basis.

FLOW BEHAVIOUR ON SPIRALS Spiral troughs constitute a more difficult problem because the sedimentation behaviour only regulates the initial phase of the separation: transportation mechanisms and bed phenomena then modify the disposition of the particles in a complex manner prior to the recovery into final products. An approximate model of the free motion phase of the separation on a spiral was developed some time ago [10] but, as in the case of sluices, further development of the model has been awaiting the availability of more accurate information regarding the fluid behaviour. The experimental difficulties involved in measuring the behaviour within thin films has shifted the focus to the development of affordable computational fluid dynamic (CFD) techniques. Progressive reductions in hardware costs and a broadening of choice in the software area have created prospects of real progress in the near future. However, for present purposes the free motion model will used as the basis of the forecasts, with due recognition of the limitations of this approach. The lack of any satisfactory mechanism fbr the post deposition sorting and consolidation in the inner zone of spiral troughs in the existing simulator leads to anomalies in the predictions after the first few turns, particularly where the profile of the spiral is modified following product removal. Without algorithms to describe the bed forces and the sorting and dewatering processes that operate within the concentrate zone on spirals, particles that have already been recovered can be predicted to move out again and be lost, leading to an apparent reduction in recovery as the slurry moves further down the trough. For this reason, the simulations carried out to predict the effect of trough length have been restricted to the upper two or three tunas of the spirals. The same limitations in the model restrict the simulation output to the distribution of particle species across the trough. By selecting an appropriate location for product removal, it is possible to estinaate potential recoveries but not assays. Performance comparisons have mostly been based on separation efficiencies rather than recoveries, because the plotting area tbr the data remains the same regardless of the teed assay [7] and this is particularly helpful when dealing with the coal spiral data presented later in the paper. For the convenience of those unfamiliar with the definition of efficiency employed here, it is shown below in eqn. 2.

Sluice and spiral separations

E

=

9

__R-C

(2)

l-f It is clear from the form of the relationship that increases in efficiency (E) translate directly into increased mineral recovery (R) at a fixed mass take (C) when the feed grade (f) is low and is directly proportional to the recovery even at high feed grades. Previous studies It would be surprising if stratification length on spirals had not been studied in detail, but in Western literature very litde information has been published prior to the recent upsurge in interest. The first Humphreys spirals had four turns [14], but were increased in length to five turns comparatively quickly. Burt [ 13] summarised the length issue for sluices, but apart from a brief mention by Reaveley and Ritchie [15] the question of spiral length has been considered in detail only by Yashin et al [1]. The latter quoted results for the treatment of both tin ore and ilmenite, some of which are reproduced below. Table 3 shows a summary of the results achieved on spirals varying in length from 0.5 to 5 turns and both the recovery and efficiency were found to reach a performance plateau after 2 to 3 turns. TABLE 3 Low grade tin ore (after Yashin et al [1])

NO. TVRNS

FEED %Sn

0.5 1 2 3 4 5

0.26 0.29 0.28 0.27 0.27 0.24

CONCENTRATE %MASS %Sn 4.90 6.90 6.30 4.50 4.60 4.48

CRITERIA %RECY %EFFY

1.53 2.48 2.85 3.70 3.36 3.38

29.0 59.3 63.7 61.0 60.6 63.3

24.2 52.6 57.6 56.7 56.2 59.0

The same authors reported the results of a study in which ilmenite recovery was measured on spirals of 500 mm and 1000 mm diameter and ranging in length from 1 to 4 turns (Figure 5)" it can be seen that a plateau is again reached after 3 turns. This work was followed by a comparison (on 1000 mm diameter spirals) of 4 turn versus 2 x 2 turn units, the latter consisting of roughly two turns of separation followed by product isolation and retreatment on a further stage. The results are shown in Table 4 and it can be seen that a gain in efficiency of between 5 % and 15 % was found. T A B L E 4 llmenite treatment (after Yashin [1])

NO. TURNS 4 2x2

4 2x2

FEED

t/h 4.5 6.5 5.7 5.3

I

%Ilm. 32.8 30.7 28.4 34.8

CONCENTRATE %MASS %llm. 32.7 29.3 23.8 29.5

63.3 67.2 52.8 68.1

CRITERIA %EFFY %RECY 63.0 64.0 44.2 57.6

45.1 50.1 28.5 43.1

On the other hand, Reaveley and Ritchie compared 5 and 7 turn spirals on rougher duty treating mineral sand and concluded that there was a considerable improvement in performance with the 7 turn unit, with equivalent recoveries being attained in much lower mass takes. Also, apart from a few specialist applications, the spiral manufacturers have in the main stuck to 5 or more turns for roughing, cleaning and scavenging duties. There are cost arguments in favour of adopting a common trough length (i.e.

10

A, B. HOLLAND-BATr

height) for the design of frames and ancillaries: in the plant context, if five or more turns are needed for roughing duty, it would probably be counter-productive to attempt to introduce varying trough lengths in different circuits even if there was redundancy in the trough length.

100

90 ~

BO

>-

~ 7o u 60 ILl

!

¢'r

-, I---

50

g 40 _.J

~ 30 20 10 I

0

~

2

3

4

SPIRAL IURNS

Fig.5 Ilmenite recovery versus number of turns (after Yashin [1]). The opposing conclusions drawn by different workers may in part be explicable in terms of operating adjustments: if there are multiple splitters at various locations down the trough or if wash water is added, it is possible to modify the performance to some degree by adjusting these parameters. Also, uncontrolled variations in both feed rate and slurry density can affect the results to a significant extent. However, unless changes in the slurry environment or adjustments to controls are carried to extremes, the overall quality of the separation should not be affected sufficiently to reverse the results of trough length comparisons. The limited historical evidence available is insufficient to resolve the issue one way or the other and hence the present study was initiated. Mineral spirals Low grade spirals A prototype spiral designed for the treatment of low grade heavy mineral feeds at high feed rates was tested at the full design length of 6 turns and then shortened to 4 turns and finally 3.5 turns. The spiral had an inner wetted radius of 13 cm and an outer trough radius of 46 cm, these limits defining the working region of the trough surface. The potential distribution of heavy mineral (S.G. 4.4) over the inner zone of the spiral has been predicted for the first 3 turns of the spiral trough and plotted in Figure 6 in cumulative form against trough radius, and it can be seen that the projected recoveries stabilise after the second turn: the curves suggest the recovery increases by about 5 % per turn over the first two turns. The varying trough profile for this design permitted a progressive release of material towards greater radii of motion and the predictions made from the free motion model could be reflecting this feature The preliminary tests reported here covered a range of different feed rates and were thus not directly comparable, but the more definitive comparisons under fixed feed conditions that would normally have followed were not proceeded with because the design was considered unsatisfactory in certain respects and was extensively modified prior to the next phase of testing. However, the linear relationship found [7] between feed rate and recovery (or efficiency) fortunately makes it possible to interpolate for feed rate and estimate the perfbrmance at a fixed level. This has been done in Figure 7, where the points relate to the test runs and the curves represent the interpolated performance at 5.9 t/h for 3.5, 4 and 6 turns.

Sluice andspiralseparations

11

These results sug~:est a progressive improvement in performance even after 4 turns, with an average gain in efficiency of about 12% recorded over the last two turns at a fixed mass recovery of 10%. As noted above, if the mass take is constant and the feed grade is very low (about 0.75 % HM in this case), efficiency and milaeral recovery are equivalent to one another. The average gain in efficeney of about 6 % per turn is therefore directly comparable with the predicted recovery gain of 5 % per turn. The predicted stabilis~Ltion of the recovery results after two turns is therefore not substantiated by the test results, though it should be noted that this does not necessarily invalidate the argument that a revitalisation of the separation b y some means might restore the initially higher rates of recovery.

30 25 -- 20 >rlLLI

P~ t5 c,r -r-

lo -

D turns t ~

II1 . 3 t L r n l l

1t

10

12

J'3

J4

15

115

I

I

I

t7

tfl

lg

20

RADIUS (cm)

Fig.6 Predicted distribution of HM on the inner section of a high capacity, low grade prototype mineral spiral.

iO0 +- 6T A- 6T x - 4T 19= 4T V- 3.5T e - 3.5T

90 80 .,.- 70

3.59 fl.ll 9.47 5.18 5.93 11,2

t/h tin t/h t/h L/h t/h

+

'" 60

§

turns

>r.i z

50

,u_ 4o i

iii

30 20

~,

i

e+

0

~

X

~+

ae

iO

x

0

i

0

tO

I

20

i

30

i

~

i

40 50 60 MASS RECOVERY (X)

i

70

,

i ,.

80

i

90

J.O0

Fig.7 The effect of number of turns on a high capacity, low grade prototype mineral spiral.

12

A.B. HOLLAND-BA'I"r

High grade spirals Spiral separators designed to handle high grade feeds have to contend with the transport and recovery of much greater amounts of high specific gravity material and in consequence frequently operate at higher slurry densities than low grade or medium grade units. Both the pitch and the trough profiles are modified accordingly and it is relevant to enquire how the different separating environment affects the required trough lengths. The potential distribution of heavy mineral (S.G. 4.4) over the inner zone of an HG8 spiral has been predicted for the first 3 turns of the spiral trough and plotted in Figure 8 in the same format as Figure 6. The variations in the trough profiles for this design do not proceed as smoothly as for the low grade example and there is a markedly uneven distribution of recovery gains. However, if the total over the 3 turns is considered, recoveries of 40-50% are predicted.

100 90 80

70 "LI.I 60 m

~ 5o ~

m

40

"r

a

3O

X

20

÷ & X ~]

10 5

G

7

B

9

10

1~

12

-

0 I 2 3

tar~ ttc'n rot'as tuene

I

I

13

t4

15

RADIUS (cm)

Fig.8 Predicted distribution of HM on the inner section of an HG8 high grade mineral spiral. In Figure 9, the pertbrmance of a prototype 3 turn HG8 is compared with a 5 turn unit, the latter unit being equipped with a repulping mechanism after the first 3 turns. It can be seen that at a mass take of 40 %, efficiencies of 69 % and 85 % respectively were recorded. With a feed grade of 40 % HM, the relationship between efficiency and recovery becomes aR=.6AE, so these results are equivalent to recoveries of 81.4% and 91.0%. In order to obtain a predicted recovery of 81.4% after 3 turns, the concentrate band in Figure 8 needs to be extended to a radius of just under 12 cm. At this location, the first two turns are generating a recovery of about 47 %, so the incremental gain from 3 to 5 turns would be 47/100 x (100-81.4) = 8.7%. In arriving at this estimate, it is presumed that the separated material is removed and the remainder is redistributed across the trough through the agency of the repulping device. The predicted total after 5 turns is 90.1% compared with the measured value of 91.0%, which is of the correct order of magnitude. The effect of repulpers on mineral spirals Yashin et al [1] claimed that after two turns of motion the motion of the slurry had reached a steady state and suggested that effective separation had ceased. The evidence reported here on both low and high grade spirals suggests that although a stable flow may have been attained after two turns or so, the recovery of mineral is not complete and will continue for up to four more turns at a progressively lower

Sluice and spiral separations

13

rate. However, the removal of products followed by remixing might still be beneficial in that it could restore the initial higher rate of recovery, as noted recently by Edward et al [2]. This possibility was recognised over ~Ldecade ago by spiral manufacturers, along with an associated problem resulting from the elimination of wash water and intermediate splitters in an attempt to make spirals easier to control. The end effect on some designs was to create a bed of nearly static or sometimes totally stranded material at the inner edge of the trough, effectively altering the trough profile and inhibiting further separation.

100

+ - 3 turn 1.76 t/h &- 3 turn 1.05 t/h X- 5 turn i.70 t/h

90 80 ~.-

70

"" 60

>-

?,5 50 ¢.3 D-a

~:

40

uJ

30 20 J0 0

i

0

10

i

20

i

30

i

|

i

40 50 60 MASS RECOVERY (~)

i

70

i

80

i

90

tO0

Fig. 9 The effect of number of turns on an HG8 high grade mineral spiral. The remedy was to reintroduce auxiliary splitters at intervals down the trough and to follow the product removal by a repulping action in which the more dilute material from the vortex zone adjacent to the outer wall of the spiral was sprayed back in a shallow fan over the inner part of the trough. The results achieved with an HG8 spiral equipped with a repulper have already been shown in Figure 9 and a comparison of the results achieved with and without repulping on an MG4 medium grade spiral treating mineral sand is shown in Figure 10. The adoption of a repuiper following the auxiliary splitter increases the efficiency at the same mass take by 5-6 % and similar gains have been recorded on many other spirals and feed types. There have to date been no instances recorded where the performance of spirals designed to operate withoul wash water has worsened with repulpers fitted, though occasionally there has been no discernable benefit. It seems clear that most of the claims put forward by previous workers have been correct to a degree within the sometimes restricted frames of reference employed in the individual studies. The fluid flow does appear to stabilise after a couple of turns, but with fully liberated materials the recovery of mineral does not suddenly cease on this account. The apparent cessation of separation in some studies may have more to do with Ihe degree of liberation existing in the ore than some limiting fluid dynamic condition: alternatively, it may have been a consequence of not extending the numbers of turns sufficiently to establish a clear t~,end, or it may have been an artifact of experimental error. Undoubtably the recovery rate does fall off progressively down a multi-turn spiral, but the removal of finished product f~llowed by the judicious application of repulpers can revitalise the separation without resorting to the coasiderable additional complexity and expense of having multiple shorter troughs on one column. HE 8-1/2-B

14

A.B. HOLLAND-BA'I'r

t00

+= No repulpers ! repulper

a =

90 80 z

70

"" 6 0

>.r..J z

ta 50 I-.-4

40 30 20 t0 i

0

10

I

i

20

30

i

i

i

40 50 riO HASS RECOVERY (l)

i

i

70

flO

i

90

:00

Fig. 10 The effect of repulpers on an MG4 medium grade mineral spiral. Coal spirals The first coal spirals were developed in 1945 by Humphreys [14] and these units had six turns operating within approximately the same height as the five turn mineral spirals developed previously. It seems likely that this trough length was adopted as a convenient way of standardising the dimensions of spiral modules after the necessary reduction in pitch had been established experimentally. After a couple of successful installations had been made, however, the market for coal fines dried up and the use of spirals in this field lay dormant for thirty years until interest was revived in Australia in the early 1980's. The first coal spirals developed by Wright and Mineral Deposits Limited (MDL) were of similar dimensions to mineral spirals [16] and had six turns, though both Vickers and MDL subsequently introduced high capacity coal spirals nearly a meter in diameter that had only 3.5 and 5 turns respectively. Recent claims that the separation dynamics on spiral separators would be better applied in multiple shorter troughs have been particularly directed at coal spirals [2] so it is an opportune time to review the basic design considerations for coal spirals. The issue of trough length will be examined both by simulation and experiment following the procedures applied earlier with mineral spirals. The most fundamental parameter in gravity separations is the comparative sedimentation rates of the particle species to be separated. Table 5 shows the predicted settling velocities for irregular shaped TABLE 5 Terminal velocities SIZE

VELOCITIES (ca/s)

(p-)

HM

REFUSE

1000 500

7.866142 3.994728 1.688531 0.552783 0.219830 0.055198 0.016181 0.004047

4. 142015 1. 937091 O. 741960 O. 219914 O. 083425 O. 020465 O. 0O5983 O. 001496

250

125 75 37

20 10

Sluice and spiral sepm'ations

15

particles (volume coefficient 0.3) ranging in size from 101~m to 1 ram, assuming a slurry density of 35 % w/w and specific gravities of 4.0 for the heavy mineral and 2.65 for the refuse. It can be seen that the heavy mineral particles settle twice as fast as the refuse at 500 I.tm, with the ratio increasing at finer sizes. If it is assumed that the bulk flow rates and slurry depths are similar on mineral and coal spirals (though in fact the coal spirals will exhibit slightly lower velocities due to the reduced pitch), in round terms the particles will require twice as long to fall the same distances on coal spirals as on mineral spirals. The lateral transportation of the settled particles should proceed at much the same rate, because the slightly lower fluid velocities acting on the particles will be offset by the lower specific gravity of the particles, but the controlling factor will always be be the sedimentation rate. Following this line of argument, if on a mineral spiral the separation rates have fallen to a point where they require revitalisation after two to three turns, on a coal spiral this condition should not arise until five to six turns have been traversed. This suggests that the use of repulpers or multiple short troughs might be counter-productive on a coal spiral, for if there are still substantial amounts of material being sorted and transported, the remixing effect may destroy a partial separation of the more difficult fractions that has taken some time to achieve. The issue of spiral length was addressed experimentally by Atasoy [17], who tested an LD4 coal spiral with one, two and five turns by fabricating a special mobile feed box that could be positioned at any desired location on the trough surface. Although this is a simple solution to the problem of altering the trough length, it does give rise to serious concerns about the distribution of the slurry across the trough surface. The geometry of the feed box and the first turn of most spiral separators incorporate features designed to dissipate excess fluid head and to distribute the slurry evenly across the trough. In the absence of these feattlres, direct discharge onto the trough only a short distance from the splitter region may well negate much of the separation potential. In fact, there were anomalies between the results reported by Atasoy at one and two turns, so the results have been averaged and reported as two turns or less. Table 6 shows the measured recoveries of particles having a specific gravity of 2.3 or greater for a full length (5 turn) trough and for the shortened trough. As can be seen, the recoveries of all sizes are subst:~atially greater at five turns, with an overall result about 16% higher. This margin appears substantial enough to offset possible weaknesses in the experimental approach and provides tangible evidence that the separation is not in stasis after two turns. TABLE 6 LD4 spiral recoveries (after Atasoy [17])

SIZE FRACTION

(~m)

FEED 1MASS

RECOVERY OF SG>2.3 5TURN

I <-2

+1700 +850 +300 +106

43.0 25.0 18.0 5.5 8.5

22.6 51.1 61.2 83.0

13.6 28.7 44.2 49.7

TOTAL +106/~m

I00.0

41.6

25.9

-3350 -1700 -850 -300 -106

The effect of trough length on the performance of an LD4 coal spiral has been explored by simulating the first three tunas of motion at a feed rate of 2.9 t/h and a slurry density of 35% solids w/w. The cumulative distribution of coal (S.G. 1.25) particles across the trough from the outer to the inner wetted limit has been plotted against radius of motion in Figure 11. From one turn onwards, there is little variation with radius because most of the material is located in close proximity to the outer wall. Taking the results at 45 cm as representative of the coal product, there is a projected gain in recovery of 18.8% over turns one to :three, or 9.4% per turn. This is an idealised result, however, because the actual coal particles would be of varying composition and would cover a range of specific gravities.

16

A. B, HOLLAND-BA'FI"

A shortened four turn version of the LD4 spiral was tested against the full length five turn version on a coal of medium difficulty containing roughly 30% Ash. A feed rate of 2.9 t/h was employed and the slurry density was 35 % solids w/w. The results have been plotted in Figure 12 in the form of the separation efficiency for coal against mass recovery to coal product. At a mass recovery (coal yield) of 80%, the difference in performance is about 4% efficiency, which translates to a very much smaller recovery gain of 1.3%. The simulation results are clearly very optimistic and provide a poor guide to actual performance. This is perhaps inevitable, because the known inadequacies in the modelling of the fluid dynamics will be magnified in effect as the specific gravity of the particles more closely approaches that of the fluid. It was clearly advisable to base the assessment of coal spirals on testwork rather than the currently inadequate simulation capability, so a programme of testwork was undertaken to compare the results from a full length LD4 spiral against those achieved with two sequential stages of two turns each mounted on a common column.

Ioo 90

B0 L

.

.

.

.

.

.

.

.

70 IT-

~O 6o N 5o ....I

~ 40 30 20

t . 0 tuPnl

t0 25

I

.

I

= 2

II

.

I

I

I

I

30

35

40

45

! ttmn

turnl

3 tu~nll

50

FIADIUS (cm)

Fig. 11 Predicted recoveries of SG 1.25 particles on an LD4 coal spiral. 60

+= 4T 2.g3 t/h m. 5T 2.B6 L/h

5O ..J

8 4o >-

,,,

31) 2O JO

i

tO

i

20

i

30

M

i

i

40 50 60 MASS RECOVERY (¢)

i

J

70 "~ 80

i

90

Fig. 12 The effect of number of turns on an LD4 coal spiral.

t00

SIuiee and spiral separations

17

The 2 x 2 turn spiral was made from the standard design by by fitting a feed box to the two turns preceding the auxiliary splitter and attaching a multi-compartment splitter box to the end. The lower trough consisted of a feed box fitted to the last two turns of the standard design complete with pivoting splitters and collector box. It was relatively easy to investigate alternative inter-stage flow logics by diverting the pipes from the multi-compartment box as required. The preferred flow logic was to remove a high ash reject on the first stage and to retreat the coal product on the second stage, because this would ensure a healthy feed rate to the lower stage at the correct slurry density. The alternative scheme of removing a high grade coal product from the top stage and retreating the refuse was les:s attractive due to the delivery to the lower stage of inadequate quantities of feed at too high a density. The feed material was a relatively easy to treat coal containing about 25 % Ash. Two tests were carried out on a standard LD4 spiral at 3.5 t/h and 35 % solids w/w with different mass takes to provide adequate coverage of the critical region adjacent to the peak efficiency. The results have been plotted in Figure 13 in the form of coal separation efficiency against mass recovery with a common curve fitted to the four points. A single two turn unit was tested first, with two tests again being performed but in this instance the feed rates were 2.94 and 3.90 t/h respectively. Fortunately, the linear variation in performance with feed rate [7] makes it possible to perform a simple interpolation to find the equivalent performance at 3.5 t/h, and this has ~:lso been included in Figure 13 as a dashed line along with the curves for the tests.

t00 90

~, 80

+= 2T x - 2T V= 5I @= 5T 2T - -

2.94 t/h 3.90 t/h 3.57 L/h 3.55 t/h 3.5 Lib (estllmtetl)

~i 70 "'

>... f..2~ 2:

60

u., 5 0

~: 40 hj

30 20

10

10

20

30

40 50 60 MASSRECOVERY (%)

70

80

90

t00

Fig. 13 Effect of number of turns on an LD4 coal spiral. The comparative results at 3.5 t/h for two and five turns show a difference of just over 12% efficiency, or an average of 4% gain per turn while traversing turns three, four and five. When four and five turn units were compared earlier in Figure 12, the difference for the last turn was 4% also, which indicates a steady and contJinuing accumulation rather than an asymptotic approach to some limiting value. The full two stage tests were carried out in pairs with different mass takes and the preferred flow logic was investigated first. The results of the tests are shown in Figure 14, together with the efficiency curve recorded at 3.5 t/h for the standard five turn unit on the same coal. The feed rates are generally lower for the two stage tests, which actually favours the latter because the removal of refuse proceeds more efficiently as the feed rate is reduced.J7] Figure 14(a) shows two tests at different splitter settings for the coal retreat option with maxinmm refuse recovery from the top stage, Figure 14(b) shows two further

18

A.B. HOLLAND-BATT

tests with a smaller refuse take on the top stage, Figure 14(c) repeats these conditions but at a higher feed rate, and finally Figure 14(d) shows the alternative flow logic of retreating the refuse on the lower stage. tO0

O0

+- 2,64 t/h Nlnilauia yield A" ~91 t/h Nidle! yield

90

90

~80

+- 2.48 t/ll 14|nll~e~ yle]d b- ~,~3 t/ll Nediull yield

80

~ 7o

70 60

ua 5 0 f_J

50,

,u_ 4O uJ

4111

311 211 10

10

0

0 lO

1:'0

30

40 50 60 BASS RECOVERY (g)

70

flO

SO

100

tO

20

30

40 50 60 I,L4SS RECOVERY (g)

(a) tO0 90

70

80

90

tO0

(b)

x - 3.06 tfn Nlniwan yield B- 2111 t / t Illdlu! yield

tO

'F, 3.M t / h Nlnlllm f i e l d b- 3.11| t ~ l Imdllla yield

30

~Bo

70 50

"" 611

5O ~ 4O uJ

40

3O

30

20

20 t0 0 10

20

30

4O 50 60 MASS RECOVERY (,t)

70

(c)

80

90

100

lO

20

30

4O 50 60 MASS RECOVERY ('tl

70

80

90

tO0

(d)

Fig. 14 Results of tests on a 2 x 2 turn LD4 coal spiral.

(a) Coal retreat (max. refuse 1); (b) Coal retreat (smaller refuse 1); (c) Coal retreat (as b, larger t/h); (d) Refuse retreat. In nearly all cases, a common curve could be drawn through the points for each pair of tests. Despite the feed rate advantage, in no case do the points offer a higher efficiency value than the standard five turn unit, though the peak values are similar in Figures 4(a) and 4(c). The refuse retreat logic showed very poor results as anticipated. It is appreciated that such technical comparisons are not always entirely appropriate to plant objectives such as maximising the yield to coal product at a fixed ash level, but these conditions merely translate into different comparison points on the efficiency curves. When the overall efficiency envelopes on the plot are at best equal or otherwise inferior to the standard one, however, there is no prospect of any metallurgical benefit to be derived. It is therefore concluded that in coal separations, employing multiple shorter troughs in place of a continuous trough of similar total length is an inferior design, because it offers no advantage in terms of metallurgical performance and would be considerably more expensive to fabricate. The effect of repulpers on coal spirals As noted in the preamble to this section, the relative settling rates of heavy mineral and refuse particles provide an argument against the use of shorter troughs or repulpers on coal spirals. The experimental

Sluice and spiral separations

19

evidence presented to date has uniformly favoured the standard five turn units, with shorter troughs providing inferior performance and multiple shorter troughs providing no advantage. In considering the role of repulpers, there are also significant differences between mineral and coal spirals. On a mineral spiral, the repulping action adds additional solids and water to the concentrate zone thereby potentially improving the recovery, while on a coal spiral the action of the repulper relocates slurry from the coal product to the refitse zone, which in the short term at least must decrease the recovery of coal. In view of this, il: would be very surprising if repulpers provided any benefit on coal spirals, but to complete the record two sets of comparisons have been carried out: one at low feed rates and densities and the other at normal levels. Each series included tests with no repulpers, one repulper after the second turn and two repulpers fitted after the second and fourth turns respectively, the splitters being adjusted to suit the altered conditions in each ease. At low feed rates and densities (Figure 15), there is a clear and progressive dq~rease in efficiency from no repulpers to two repulpers. At normal feed rates and densities (Figure 16), the curves for no repulpers and one repulper are close enough for any difference to be masked by experimental error, but there is again a significant reduction in efficiency with two repulpers. On this evidence, it is concluded that there is no benefit from fitting repulpers and in most cases they will have an adverse effect. t00

+ - 0 r | p u l p i r l 2.61 Lib A- o repulplr! Z.M t/l~ x - I rePulpor 2.43 t/h D- I rePulPer 2.80 t/h V- 2 repulp~m 2.78 t/h O- 2 rmlpsrs 2.64 t ~

~80 ..J

70 "" 60

>-

~ 4o

t0 i

i

i

i

|

t0

20

30

40

50

i

60 MASS RECOVERY (%)

i

70

i

80

i

90

t00

Fig. 15 The effect of repuli'~rs on an LD4 coal spiral (2.5 t/h, 26% solids). 100

+AxBVe-

90

~ec

I I 2 2 0 0

repulpor repulper relpUlpora rlpu]pm's repulp|rs repulp~s

3.43 3.50 3.57 3.46 3.57 3.55

tlh tin t/h t/h t/h t/h

7o

"'60

4O 3O 2O t0 0 0

iO

20

30

40 50 60 NASS RECOVERY (I)

70

80

90

tO0

Fig. 16 The effect of repulpers on an LD4 coal spiral (3.5 t/h, 35 % solids).

20

A. B. HOLLAND-BATT SUMMARY

AND

CONCLUSIONS

This contribution has examined the dynamics of separation on both sluices and spirals from the viewpoint of both theory and experiment, with particular emphasis on the length of trough needed. The current simulation capability gives useful indicative predictions on sluices and mineral spirals, but the inadequacies in the modelling particularly of fluid flows prohibit quantitative performance predictions. The inaccuracies become too great when attempting to predict the behaviour in coal separations because the specific gravity of the particles is much closer to that of the fluid than in the ease of heavy minerals and any anomalies in the fluid flow calculations exert a proportionately greater effect. Recent progress in the field of computational fluid dynamics is expected to remedy many of these deficiencies in the near future. Studies of mineral spirals have shown that the diminishing rate of separation evident on the lower turns of the trough can be improved by removing finished grade material and redistributing the slurry across the trough. While this can be achieved by fitting sequential shorter troughs on one column with inter-stage feed boxes and product transfer systems, the introduction of repulpers after the auxiliary splitters has achieved the same end in a simple and more cost effective manner. When the relative sedimentation rates of heavy mineral and refuse particles were examined, it appeared unlikely that multiple shorter troughs on one column would offer any advantage in coal separations. Given that coal spirals operate with the products reversed compared with a mineral spiral, the use of repulpers to redistribute material from the coal product across to the refuse region did not appear to offer any potential advantage. Test results confirmed that both suppositions were correct. The adoption of 5 or 6 tums for the majority of spiral designs offered by manufacturers has been based on a semi-empirical body of knowledge built up over fifty years of experience. Recent progress in modelling the behaviour on sluices and spirals has provided some theoretical justification for traditional design rules and continuing progress in this area will lead to further improvements in the design techniques. In mineral separations, current spirals equipped with repulpers at appropriate locations are capable of meeting the process requirements of providing flexible operation and high upgrading capability at acceptable recoveries. Coal spirals present a more difficult design task and there is still considerable scope for improving the designs so that they more closely approach industry requirements, particularly in the area of cut densities. On the evidence presented here, the adoption of multiple shorter troughs or repulpers do not offer any potential benefits.

ACKNOWLEDGEMENT The author would like to thank the management and staff of the Mineral Technologies Division of Clyde Industries Ltd. for their interest and support. Particular thanks are due to Technical Services staff for their assistance with all phases of the testwork.

SYMBOLS C

Mass fraction of solids reporting to the product

f

Feed assay (fractional)

h

Height within the flow (cm)

hs

Total height of flow above datum (cm)

R

Recovery of component of interest to product (fractional)

V

Longitudinal velocity along sluice (cm/s)

Sluice and spiral separations

21

Mean longffudinal velocity along sluice (cm/s) R

Recovery of component of interest to product (fractional)

Y

Lateral distance from centerline across trough (cm)

Yo

Distance of sluice wall from centerline across trough (cm)

REFERENCES l.

2. 3. 4. 5. 6.

.

8. 9. 10. 11. 12. 13. 14. 15.

16. 17.

Yashin, A V., Aniken, M.F. & Skrpko, V.A., Spiral Separators. Nedra Press (Moscow), Part III, Chapter 6, (1984). Edward, D., Li, M. & Davis, J.J., Spiral research: technique development and use. In Davis J.J. (ed.), Proceedings of the Sixth Coal Preparation Cotference, Paper B2, 116 (1993). Pullar, S.S., Metallurgical practice in the beach sands industry. Prec. Aust. Instn. Mitt. Metall., 205, 77-104 (1963). Pullar, S.S., Development in separating equipment in the Australian heavy mineral sands industry. In ProceeaTngs of the eighth CMMC, Melbourne, 6, 1343-1357 (1965). Graves, R.A., Parallel sided sluice testwork. Internal memo. R87/80. Mineral Deposits Limited, (22/2/1980). Holland-Batt, A.B., Design of gravity concentration systems by use of empirical mathematical models. I~ Proceedings of the eleventh CMMC, Hong Kong 1978. Jones M.J. Ed. 133-143 (London IMM, 1979). Holland-Batt, A.B., Interpretation of spiral and sluice tests. Trans. Instn. Mitt. Metall., (Sect. C: Mineral Process. Extr. Metall.), 99, C11-20 (1990). Subasinghe, G.K.N.S. & Kelly, E.G., Modelling pinched sluice type concentrators. Conu'ol '84, 87-95 (1984). Abdinegor,:), S. & Partridge, A.C., Flow characteristics of a pinched sluice. Prec. Aust. Instn. Min. Metall. Cos~,rence, Western Australia, (Aug. 1979). Holland-Batt, A.B,, Spiral separation: theory and simulation. Trans. hsstn. Min. Metall., (Sect. C: Mineral[ Process. Extr. Metall.), 98, C46-60 (1989). McNamera, A., Modelling pinched sluices. Second interim report, Julius Krutschnitt Mineral Research Centre, submitted to Mineral Deposits Limited, 6 (Feb. 1987) Sivamohan, R., A study of gravity concentration with emphasis on surface phenomena. Doctoral Thesis, Lulea University of Technology, Ch. 2, 6 (1985). Burt, R.O., Gravi~ Concentration Technology. Volume 5 in Developments in Mineral Processing Series. (Elsevier, Amsterdam), 227 (1984). Thompson, J.V. & Welker, M., The Humphreys Companies: Development and application of Humphreys Spiral Concentrator. SkiUings' Mining Review, 4-15 (Feb. 24, 1990). Reaveley, B.J. & Ritchie, I.C., The development of high efficiency spiral separators. In Australia: a Worm Source of lhnenite, Rutile and Zircou. AuslMM (Perth Branch) Cotference, 87-97 (1986). Holland-Batt, A.B., Turner, J.H. & Hunter, J.L., The separation of coal fines using flowing-film gravity concentration. Powder Technol., 40, 129-145 (1984). Atasoy, Y , A study of particle separation in a spiral separator. M.Sc. Thesis (unpublished), Colorado School of Mines, 105-107 (1987).