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High-intensity autogenous liberation of diamonds from kimberlite in the hicom mill
Minerals Engineering, Vol. 10, No. 3, pp. 265-273, 1997
Pergamon Plh S0892--6875(97)00003-4
© 1997 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved o892~6875/97 $17.oo+o.oo
HIGH-INTENSITY AUTOGENOUS LIBERATION OF DIAMONDS F R O M K I M B E R L I T E IN T H E H I C O M M I L L
D.I. HOYER§ and D.C. LEEr § M.D. Research Company, 8 Khartoum Rd, North Ryde NSW 2113, Australia 1" Ashton Mining Limited, PO Box 962, West Perth WA 6872, Australia (Received 25 September 1996; accepted 3 December 1996) ABSTRACT Pilot plant tests with a 25 kW Hicom mill have shown that the Hicom mill successfully liberates diamonds from kimberlite in the presence of significant quantities of barren dolomite and chert. The Hicom mill is a high-intensity tumbling mill which uses a nutating action to generate acceleration fields 40 to 50 times stronger than gravity, producing rapid and intense breakage of ores and other materials. It can be used either autogenously or with steel or ceramic grinding media. Open circuit autogenous Hicom grinding tests were completed on two separate samples of a diamond-bearing kimberlite containing a quantity of barren dolomite and chert. A specially modified grinding chamber with 40 mm diameter discharge ports allowed the rejection of large barren lumps of dolomite and chert while maintaining sufficient residence time to liberate the diamonds from the softer kimberlite. Diamond and ceramic tracers were added during each test to determine whether any damage might occur, and to obtain residence time distributions. It was shown that diamonds are not damaged in the mill and that almost all of the l~mberlite was reduced to finer than 4 mm to achieve full liberation of valuable diamonds. ©1997 Published by Elsevier Science Ltd Keywords Comminution; autogenous grinding; diamonds; liberation
INTRODUCTION The Merlin kimbe,dite field in the Northern Territory of Australia was discovered by the Australian Diamond Explorat!ion Joint Venture. The kimberlites are being sampled for diamond content and scoping studies are in progress to assess the options for mining. The project is managed by Ashton Mining Limited. There was a need to investigate methods for liberating diamonds from an ore consisting of kimberlite with barren dolomite and chert. The dolomite and chert are significantly harder than the kimberlite. The valuable diamonds are considered to be fully liberated when all the kimberlite has been reduced to finer than 4 mm. Two feed streams are of particular interest: a primary jaw crusher product and a tailings stream (after gyratory mill and scrubber). It was desirable to break the kimberlite to below 4 ram, while at the same time allowing larger material to be ejected from the mill, both in order to eject large diamonds and so that large barren lumps could be rejected from the mill without wasting energy grinding them. Presented at Minerals Engineering '96, Brisbane, Australia, August 26-28, 1996
265
266
D.I. Hoyerand D. C. Lee
The Hicom mill is a high intensity grinding mill in which strong acceleration fields result in rapid breakage of material, and it has been demonstrated in previous test work to be effective in treating diamond-bearing materials. Residence times can be kept short, and with appropriate discharge ports this favours the rapid breakage of kimberlite and ejection of material before significant damage is done to the diamonds. A pilot plant test program was initiated in order to determine how the Merlin ore responds in the Hicom mill. The three major objectives were to establish to what degree the diamonds are liberated from the kimberlite, whether any damage occurs to the diamonds in the Hicom mill, and what energy consumption is required.
THE HICOM M I L L The Hicom mill is a commercial implementation of the nutating mill [1-5]. Strong acceleration fields impart an intense tumbling and agitation motion to steel, ceramic or rock particles to produce very rapid breakage rates of mineral ores. In operation it can be thought of as a high-intensity ball or autogenous mill with a small, rapidly moving grinding chamber. Figure 1 shows a schematic diagram of the Hicom mill. The grinding chamber is a truncated cone with a rounded base, open at the top and with a grate discharge at the bottom. A specially developed rolling bearing and mechanical drive arrangement cause the mill axis to nutate about a fixed point at a nutation angle of 4.75 deg. The motion is similar to swirling a conical flask in the wrist: the top wobbles and the bottom moves in a circular motion. The eccentricity e is the radius of this circle, and increases linearly from the top down to the base of the grinding chamber. Both the mill chamber diameter D and the eccentricity e increase down the mill axis. Hence the centrifugal acceleration also increases down the nutation axis.
CHUTE
SPRING
0R'VEHD"II'
ELECTRIC MOTOR--~
i
I--
I
-,.,,,..
SERVICES
i
GRINDING ~CHAMBER
,
,--
1
1 \
tilillll BASE FRAME
L_ TORQUE RESTRAINT
] ~
RUBBER VIBRATION MOUNT
""
t" MILL DISCHARGE
Fig. 1 A schematic diagram of the Hicom nutating mill. The Hicom mill presents a stationary feed throat which enables it to be fed from a convemionai feed belt while maintaining the advantage of high acceleration fields and high power densities within the mill. Grinding results from the intense tumbling motion of the media inside the grinding chamber, either autogenously or with steel or ceramic balls. The mill geometry also gives rise to a net acceleration in the direction of flow of the slurry, so that in continuous operation material is drawn through the mill in near plug flow [1]. The small physical size and the intense grinding action of these mills make them suitable for a wide range of industrial applications from fine and ultrafme grinding of hard or soft materials to rapid autogenous reduction of fine to medium sized material.
Liberation of diamonds from kimberlite
267
The motion of the Hicom mill is conceptually similar to that of a centrifugal mill [6-8], except that the diameter and acceleration intensity vary down the length of the mill. There is no critical speed in these mills - they can be operated at any speed, limited only by the mechanical strength of the mill. The magnitude of the acceleration field in which the mill contents tumble is equal to the p r o d u c t o f the eccentricity and the square of the mill speed: A = o.2 ~
[11
where ~ is the mill speed and e is the eccentricity, or radius of nutation at any point along the axis. It has been shown [3] that the power consumption for nutating mills increases with the cube of the mill speed. At constant acceleration intensity and mill chamber geometry the power increases with the mill diameter raised to the power 3.5
EXPERIMENTAL DETAILS
Sample Preparation The feed material was a kimberlite/dolomite/chert ore, supplied in four 200 litre drums representing two drums each of two distinct sample types. Drums 1 and 2 were described as scrubber tallings. The material was substantially finer than 30 mm, with rounded shapes indicating some abrasive handling or treatment had occurred. Drums 3 and 4 were much coarser, containing approximately 30% between 30 and 100 mm, and were describexl as scrubbed crusher feed material. Most of the larger lumps had relatively rounded edges. The samples in these drums (3 and 4) were crushed in a hammer mill to finer than 35 mm for the pilot plant tests. Each drum was treated as a separate test sample, and each was blended and sampled by cone & quartering to obtain a representative sample for feed size analysis.
Pilot Plant Test Details The test program was conducted in the Hicom 90 wet grinding pilot plant at the Sydney laboratory of M.D. Research Company. The Hicom 90 is rated at 20 kW for autogenous grinding tests of this nature, and the tests were all run in open circuit. The conical grinding chamber had a volume of just 13 litres, and was fitted with smooth rubber liners (no ribs or lifter bars). The chamber was also modified for these tests by cutting two discharge ports of 40 mm diameter each into the sides of the chamber, about one third of the chamber length up from the bottom. The intention was to allow the discharge of all material, including relatively large barren lumps, but only after maintaining a sufficient residence time to break the kimberlite. There were no other discharge slots or ports - all the material discharged from the two 40 mm ports. Due to the limited sample size and the high feed rates required the material was fed manually by tipping pre-weighed buckets of ore at timed intervals into a specially constructed chute which distributed the material evenly onto the feed conveyor. This ensured that the desired feed rate was accurately maintained throughout each test. At the start of each test, as the feed material began to enter the mill, an additional quantity of material was added to simulate the steady state mill load. There was sufficient material for about 10 to 15 minutes running with each of the four samples. The average mill residence time was less than 60 seconds, and it was reasoned that this feeding arrangement would ensure steady running conditions by the end of each test. When three minutes of grinding time was left for each test a small tissue bag containing ceramic and diamond tracers was added to the mill. The mill discharge was sampled into separate buckets at 15 second intervals, commencing immediately the tracers were added. This procedure was used in each test to determine residence time distributions for the added tracers. The tracers were recovered manually from each bucket after the test and inspected for any damage that may have occurred. Each bag of tracers contained 20 small[ ceramic tracers of 6 mm diameter, three large ceramic tracers of 12 mm diameter, and
268
D.I. Hoyer and D. C. Lee
one or two 3 mm diamonds.
Summary of Test Conditions Table 1 lists the overall test conditions for each of the four runs. The tests were all done in open circuit at controlled feed rates between 1500 and 1800 kg/h, giving average mill residence times of around 50 seconds for the ore. T A B L E 1 Summary of Test Conditions
Test number Source
A1
A2
Crusher feed
B1
B2
Scrubber tailings
Sample mass, kg
377
368
262
258
Feed rate, kg/h
1500
1500
1500
1800
% solids in feed
80
74
74
77
Test duration, min
15
15
10
10
Net power, kW
11.9
12.5
13.1
14.4
Net energy, kWh/t
7.9
8.3
8.7
8.0
The first test was run at 1500 kg/h and 80% solids using the crusher feed material. The slurry discharge had the consistency of a thick paste, and while this did not hinder the passage of material through the Hicom mill itself, there were problems experienced in the slurry sticking to the discharge chute. Water sprays were installed on the discharge chute for subsequent tests, and this corrected the problem. Test A2, also with the crusher feed material, was done with increased water addition to the mill to give a slurry density of 74% solids. The primary effect of this was to increase the net energy requirement from 7.9 to 8.3 kW.h/t. Tests B1 and B2 used the scrubber tailings material at feed rates of 1500 and 1800 kg/h, and slurry densities of 74 and 77 %. Once again the effect of reduced slurry density is an increase in the net energy requirement, this time from 8.0 to 8.7 kW.h/t.
E X P E R I M E N T A L RESULTS
Power and Energy Consumption Table 1 lists the net power and energy data for the four tests. Test 1 showed that running the mill at very high solids content (80% by weight) gave the most efficient grinding for the four tests, at 7.9 kWh/t. The main difference between Test 2 from Test I was that the feed water was increased to reduce the feed solids concentration from 80% to 74%. The result was a 5% increase to 8.3 kW.h/t in energy consumption. Test 3 was a different ore type to Tests 1 and 2, so conditions were kept the same as Test 2 for a comparison. The energy increased to 8.7 kWh/t, indicating that this ore type was slightly harder to grind than that of Tests 1 and 2. Test 4 was the same ore type as Test 3, and was run at the same conditions except that the feed rate was increased by 20% to 1800 kg/h and the percent solids was reduced to 77% (midway between Tests 1 and 2). The result was an 8% reduction in net energy requirement to 8.0 kWh/t. From Tests 1 and 2 it is reasonable to conclude that 2.5 % of the energy reduction was due to the increase in percent solids, and
Liberationof diamondsfromkimberlite
269
the remaining 5.5 % can therefore be attributed to improved efficiency from running at a higher throughput. No additional samples were available to try to further improve the energy efficiency. However, the following general observations can be made:
a)
Grinding is more efficient at higher solids densities, at least up to 80% solids, subject to adding wash water subsequent to grinding in order to facilitate slurry transport.
b)
Grinding is more efficient at higher feed rates. Assuming this trend continues to the maximum feed rate: an additional improvement in energy consumption is clearly possible. It is likely that substantial increases in feed rate will be possible by increasing the number of discharge ports, and by adding discharge slots in the base of the grinding chamber to increase the discharge of fine material.
Kimberlite content in the product The product samples were examined visually and found to contain only a very small proportion of kimberlite in the +4 mm size range. A few rounded nodules of kimberlite, possibly calcreted, remained. The remainder of the +4 mm product consisted of mainly chert and dolomite with a minor assortment of rock fragments derived from the country rocks surrounding the kimberlite pipes. Thus the objective of reducing all kimberlite to below 4 mm without substantially milling the chert and dolomite was largely achieved. A higher throughput rate may be possible, perhaps by installing more discharge ports, while still liberating virtually all of the diamonds from the kimberlite matrix. Size Distributior~ Figure 2 shows the size distributions of the feed and product material from each of the two ore samples. The size distributions are averages for several samples taken on each of the two runs with each ore type. Ore type A, the crusher feed material, produced a much finer product than ore type B, the scrubber tailings material, indicating that the scrubber product had a higher proportion of dolomite and chert.
1 O0
¢-
(3.
•- - 0 - - -
10
",, =
1
10
Particle size, mm Fig.2 Size distributions of the feed and product material from each of the two ore samples. MINE 10-3-8
A. Feed (crusher) A. Product B. Feed (scnJbber) B. Product
100
270
D.I. Hoyer and D. C. Lee
Tracer weights A tissue bag containing ceramic and diamond tracers was added to the mill about three minutes before the end of each test. At that time the mill discharge was sampled continuously until the end of the test, changing the sample collection bucket every 15 seconds. The purpose was firstly to see whether any damage occurred to the diamonds and tracers, and secondly to determine residence time distributions (RTDs) for the tracers. Table 2 lists the diamond and ceramic tracer weights before and after each test. The tracers added for each test included three 12 mm zirconia tracers, twenty 6 mm alumina tracers, and one or more diamonds. Three 3 mm diamonds were supplied by Ashton Mining Limited. The tracers were recovered by manually screening and washing the contents of the discharge collection buckets. No significant damage was recorded for any of the recovered ceramic and diamonds tracers. Note that different ceramic tracers were used in each test, which is why the initial weights vary between tests. Microscope images of the three tracer diamonds were recorded before and after the test program, and no damage was detected on any of the recovered diamonds. Diamond D2 lost 0.006 g in test 3. This was attributed to the presence of moisture during the initial weighing, as the microscope images did not show any physical change in the shape of the diamond. Diamond D3 was added in test B1 and not recovered. It was the smallest of the diamonds added, and was assumed to have escaped detection in the post-test screening and washing. The possibility of its having fractured cannot be eliminated, though it is felt to be unlikely given that none of the larger tracers suffered any significant damage.
TABLE 2 Diamond and ceramic tracer weights before and after each test.
Test number
A1
A2
B1
B2
(Tracer weights in g) 3 x 12 mm ceramic, before
12.78
12.57
12.94
12.61
3 x 12 mm ceramic, after
12.77
12.52
12.92
12.60
20 x 3 mm ceramic, before
8.37
8.14
8.18
8.21
20 x 3 mm ceramic, after
8.36
8.16
8.16
8.20
Diamond D1, before
0.0466
0.0466
0.0466
0.0466
Diamond D1, after
0.0466
0.0466
0.0466
0.0466
Diamond D2, before
0.0689
0.0683
Diamond D2, after
0.0683
0.0683
Diamond D3, before
0.0198
Diamond D3, after
lost
Tracer Residence Time Distributions Figure 3 shows the residence time distributions for individual diamonds and ceramic tracers recovered from the sample collection buckets for tests A1 and A2. The times indicated on the horizontal axes are the residence times for each tracer. The columns represent the number of 6 mm ceramic tracers recovered at each collection time. The circles and diamond shapes represent the 12 mm ceramic and 3 mm diamond tracers recovered. The results for Test A1 are not considered accurate as the slurry was too thick to be
Liberation of diamonds from kimberlite
271
satisfactorily transported down the discharge chute. Sprayers were installed for subsequent tests to help with the slurry collection. Most of the tracers had a residence time of less than one minute, though some can stay in the mill :for up to three minutes. The tracers shown in the fight-most column labelled 180 see represent tracers which were still in the mill after three minutes.
Test A 1 . 8 0 % solids, 1500kg/h I
"O
8 7 6
> 0 0 0
5
(9 t._
L_
U) L_
0 0
I
i
I
i
i
[] ~ O
6 mmceramic 3 mmdiamond 12 mmceramic
I "
I
4 32~'~i~i~.,'.~i ~'':~'~ "~''~!!
0
30
60
0
~"{~'~~|'"~! ~
90
120
150
180
Test A2. 74% solids, 1500kg/h 8
"0
7
> O O
6
(9 %_ (9 (9 t.._
0
6 mmceramic 3 mm diamond 12 mmceramic
5 4
(9
3
!._
2
O
[]
1 0
0
30
60
90
120
150
180
Residence time, sec. Fig..") RTDs for individual diamonds and ceramic tracers for tests A1 and A2. Figure 4 shows the residence time distribution results for tests B ! and B2. It indicates that the higher feed rate of test B2 yielded a shorter residence time. However, the number of tracers involved is quite small, and at least some of the difference would be due to statistical variations. Figure 5 was constructed by adding the four RTDs for tests A1 to B2, to indicate the average RTD with a greater sample number.
272
D. 1. Hoycr and D. C. Lee
Test B1.74% solids, 1500kg/h 8
"13
2 0 > 0 0
[]
6mmceramic 3 m m diamond
0
12 m m c e r a m i c
7 6 5 4
P 0 0
3 2 1 0
30
0
60
90
120
150
180
Test B2.77% solids, 1800kg/h •
I
|
I
6 m m ceramic
6 0 0
5
(3)
4
ii~!!~ii?::
3 m m diamond 12 m m ceramic
0
_ !i::i
P
0
i
0
30
60
I
................
90
120
150
i
180
Residence time, sec. Fig.4 RTDs for individual diamonds and ceramic tracers for tests B] and B2,
20
15 10
2 o>
o o
o
0 0
30
60
90
R e s i d e n c e time,
120
150
180
sec.
Fig.5 The sum of the four RTDs for tests A1 to B2.
Liberation of diamondsfromkimberlite
273
CONCLUSIONS Four open circuit Hicom grinding tests were completed on two separate ore samples supplied by Ashton Mining Limited from the Merlin diamond field. The Hicom mill reduced a very high proportion of the kimberlite to below 4 mm to achieve 100% liberation of valuable diamonds, and this indicates that there was probably a degree of overgrinding. The net energy consumption was about 8 kWh/t, but it is likely that this figure can be significantly reduced by increasing the number of discharge ports and thereby reducing the average residence time. There was not sufficient material available to test this option. No damage was recorded to any of the diamond and ceramic tracers added during the tests. The residence time distributions for ~Iracers added during the tests indicate that the discharge is similar to a fully mixed vessel with a dead time of less than 15 seconds and an average residence time of about 45 seconds.
ACKNOWLEDGEMENTS This work was sponsored by Hicom International Pty Limited. The permission granted by that company to publish this paper is gratefully acknowledged. The experimental work was done by R. Oxley, J. Hunter and P. Samaras.
NOMENCLATURE A
CO
acceleration intensity, m/s 2 (Equation 1) eccentricity of mill axis at the base of the mill, m. This is the distance between the mill axis and the nutation axis, measured perpendicularly from the nutation axis. mill speed, rad/s
REFERENCES 1.
2. 3. 4.
.
6. 7. 8.
Boyes, J.M., High-intensity centrifugal milling - a practical solution. Int. J. Miner. Process., 22, 413-430 (1988). Hoyer, D.I. & Boyes, J.M., The High-Intensity Nutating Mill - A Batch Ball Milling Simulator. Minerak~ Engineering, 3(1/2), 35-51 (1990). Hoyer, D.I., Power Consumption in Centrifugal and Nutating Mills. Minerals Engineering, 5(6), 671-684 (1992). Hoyer, D.I., High-Intensity Autogenous Milling in the Hicom Mill - A Preliminary Simulation Model. iin Comminution - Theory and Practice, ed S.K. Kawatra, chap 25,339-354, SME-AIME, Littleton (1992). Hoyer, D.I. & Boyes, J.M., High-intensity fine and ultrafine grinding in the Hicom mill. XVth CMMI Congress, Johannesburg. SAIMM, 2, 435-441 (1994). Joisel, A., Planetary Mills. Rev. Mater. Constr. Trav. Publics, 493, 234-250 (1956). Lloyd, P.J.D., Bradley, A.A., Hinde, A.L., Stanton, K.H. & Schymura, G.K., A Full Scale Centrifugal Mill. J. S. Afr. Inst. Min. Metall., 82(6), 149-156 (1982). Hoyer, D.I., Particle Trajectories and Charge Shapes in Centrifugal Milling. Int. Conf. on Recent Advances in Mineral Sciences and Technology, Mintek, S. Africa, 401-409 (26-30 March 1984).
Pergamon Plh S0892--6875(97)00003-4
© 1997 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved o892~6875/97 $17.oo+o.oo
HIGH-INTENSITY AUTOGENOUS LIBERATION OF DIAMONDS F R O M K I M B E R L I T E IN T H E H I C O M M I L L
D.I. HOYER§ and D.C. LEEr § M.D. Research Company, 8 Khartoum Rd, North Ryde NSW 2113, Australia 1" Ashton Mining Limited, PO Box 962, West Perth WA 6872, Australia (Received 25 September 1996; accepted 3 December 1996) ABSTRACT Pilot plant tests with a 25 kW Hicom mill have shown that the Hicom mill successfully liberates diamonds from kimberlite in the presence of significant quantities of barren dolomite and chert. The Hicom mill is a high-intensity tumbling mill which uses a nutating action to generate acceleration fields 40 to 50 times stronger than gravity, producing rapid and intense breakage of ores and other materials. It can be used either autogenously or with steel or ceramic grinding media. Open circuit autogenous Hicom grinding tests were completed on two separate samples of a diamond-bearing kimberlite containing a quantity of barren dolomite and chert. A specially modified grinding chamber with 40 mm diameter discharge ports allowed the rejection of large barren lumps of dolomite and chert while maintaining sufficient residence time to liberate the diamonds from the softer kimberlite. Diamond and ceramic tracers were added during each test to determine whether any damage might occur, and to obtain residence time distributions. It was shown that diamonds are not damaged in the mill and that almost all of the l~mberlite was reduced to finer than 4 mm to achieve full liberation of valuable diamonds. ©1997 Published by Elsevier Science Ltd Keywords Comminution; autogenous grinding; diamonds; liberation
INTRODUCTION The Merlin kimbe,dite field in the Northern Territory of Australia was discovered by the Australian Diamond Explorat!ion Joint Venture. The kimberlites are being sampled for diamond content and scoping studies are in progress to assess the options for mining. The project is managed by Ashton Mining Limited. There was a need to investigate methods for liberating diamonds from an ore consisting of kimberlite with barren dolomite and chert. The dolomite and chert are significantly harder than the kimberlite. The valuable diamonds are considered to be fully liberated when all the kimberlite has been reduced to finer than 4 mm. Two feed streams are of particular interest: a primary jaw crusher product and a tailings stream (after gyratory mill and scrubber). It was desirable to break the kimberlite to below 4 ram, while at the same time allowing larger material to be ejected from the mill, both in order to eject large diamonds and so that large barren lumps could be rejected from the mill without wasting energy grinding them. Presented at Minerals Engineering '96, Brisbane, Australia, August 26-28, 1996
265
266
D.I. Hoyerand D. C. Lee
The Hicom mill is a high intensity grinding mill in which strong acceleration fields result in rapid breakage of material, and it has been demonstrated in previous test work to be effective in treating diamond-bearing materials. Residence times can be kept short, and with appropriate discharge ports this favours the rapid breakage of kimberlite and ejection of material before significant damage is done to the diamonds. A pilot plant test program was initiated in order to determine how the Merlin ore responds in the Hicom mill. The three major objectives were to establish to what degree the diamonds are liberated from the kimberlite, whether any damage occurs to the diamonds in the Hicom mill, and what energy consumption is required.
THE HICOM M I L L The Hicom mill is a commercial implementation of the nutating mill [1-5]. Strong acceleration fields impart an intense tumbling and agitation motion to steel, ceramic or rock particles to produce very rapid breakage rates of mineral ores. In operation it can be thought of as a high-intensity ball or autogenous mill with a small, rapidly moving grinding chamber. Figure 1 shows a schematic diagram of the Hicom mill. The grinding chamber is a truncated cone with a rounded base, open at the top and with a grate discharge at the bottom. A specially developed rolling bearing and mechanical drive arrangement cause the mill axis to nutate about a fixed point at a nutation angle of 4.75 deg. The motion is similar to swirling a conical flask in the wrist: the top wobbles and the bottom moves in a circular motion. The eccentricity e is the radius of this circle, and increases linearly from the top down to the base of the grinding chamber. Both the mill chamber diameter D and the eccentricity e increase down the mill axis. Hence the centrifugal acceleration also increases down the nutation axis.
CHUTE
SPRING
0R'VEHD"II'
ELECTRIC MOTOR--~
i
I--
I
-,.,,,..
SERVICES
i
GRINDING ~CHAMBER
,
,--
1
1 \
tilillll BASE FRAME
L_ TORQUE RESTRAINT
] ~
RUBBER VIBRATION MOUNT
""
t" MILL DISCHARGE
Fig. 1 A schematic diagram of the Hicom nutating mill. The Hicom mill presents a stationary feed throat which enables it to be fed from a convemionai feed belt while maintaining the advantage of high acceleration fields and high power densities within the mill. Grinding results from the intense tumbling motion of the media inside the grinding chamber, either autogenously or with steel or ceramic balls. The mill geometry also gives rise to a net acceleration in the direction of flow of the slurry, so that in continuous operation material is drawn through the mill in near plug flow [1]. The small physical size and the intense grinding action of these mills make them suitable for a wide range of industrial applications from fine and ultrafme grinding of hard or soft materials to rapid autogenous reduction of fine to medium sized material.
Liberation of diamonds from kimberlite
267
The motion of the Hicom mill is conceptually similar to that of a centrifugal mill [6-8], except that the diameter and acceleration intensity vary down the length of the mill. There is no critical speed in these mills - they can be operated at any speed, limited only by the mechanical strength of the mill. The magnitude of the acceleration field in which the mill contents tumble is equal to the p r o d u c t o f the eccentricity and the square of the mill speed: A = o.2 ~
[11
where ~ is the mill speed and e is the eccentricity, or radius of nutation at any point along the axis. It has been shown [3] that the power consumption for nutating mills increases with the cube of the mill speed. At constant acceleration intensity and mill chamber geometry the power increases with the mill diameter raised to the power 3.5
EXPERIMENTAL DETAILS
Sample Preparation The feed material was a kimberlite/dolomite/chert ore, supplied in four 200 litre drums representing two drums each of two distinct sample types. Drums 1 and 2 were described as scrubber tallings. The material was substantially finer than 30 mm, with rounded shapes indicating some abrasive handling or treatment had occurred. Drums 3 and 4 were much coarser, containing approximately 30% between 30 and 100 mm, and were describexl as scrubbed crusher feed material. Most of the larger lumps had relatively rounded edges. The samples in these drums (3 and 4) were crushed in a hammer mill to finer than 35 mm for the pilot plant tests. Each drum was treated as a separate test sample, and each was blended and sampled by cone & quartering to obtain a representative sample for feed size analysis.
Pilot Plant Test Details The test program was conducted in the Hicom 90 wet grinding pilot plant at the Sydney laboratory of M.D. Research Company. The Hicom 90 is rated at 20 kW for autogenous grinding tests of this nature, and the tests were all run in open circuit. The conical grinding chamber had a volume of just 13 litres, and was fitted with smooth rubber liners (no ribs or lifter bars). The chamber was also modified for these tests by cutting two discharge ports of 40 mm diameter each into the sides of the chamber, about one third of the chamber length up from the bottom. The intention was to allow the discharge of all material, including relatively large barren lumps, but only after maintaining a sufficient residence time to break the kimberlite. There were no other discharge slots or ports - all the material discharged from the two 40 mm ports. Due to the limited sample size and the high feed rates required the material was fed manually by tipping pre-weighed buckets of ore at timed intervals into a specially constructed chute which distributed the material evenly onto the feed conveyor. This ensured that the desired feed rate was accurately maintained throughout each test. At the start of each test, as the feed material began to enter the mill, an additional quantity of material was added to simulate the steady state mill load. There was sufficient material for about 10 to 15 minutes running with each of the four samples. The average mill residence time was less than 60 seconds, and it was reasoned that this feeding arrangement would ensure steady running conditions by the end of each test. When three minutes of grinding time was left for each test a small tissue bag containing ceramic and diamond tracers was added to the mill. The mill discharge was sampled into separate buckets at 15 second intervals, commencing immediately the tracers were added. This procedure was used in each test to determine residence time distributions for the added tracers. The tracers were recovered manually from each bucket after the test and inspected for any damage that may have occurred. Each bag of tracers contained 20 small[ ceramic tracers of 6 mm diameter, three large ceramic tracers of 12 mm diameter, and
268
D.I. Hoyer and D. C. Lee
one or two 3 mm diamonds.
Summary of Test Conditions Table 1 lists the overall test conditions for each of the four runs. The tests were all done in open circuit at controlled feed rates between 1500 and 1800 kg/h, giving average mill residence times of around 50 seconds for the ore. T A B L E 1 Summary of Test Conditions
Test number Source
A1
A2
Crusher feed
B1
B2
Scrubber tailings
Sample mass, kg
377
368
262
258
Feed rate, kg/h
1500
1500
1500
1800
% solids in feed
80
74
74
77
Test duration, min
15
15
10
10
Net power, kW
11.9
12.5
13.1
14.4
Net energy, kWh/t
7.9
8.3
8.7
8.0
The first test was run at 1500 kg/h and 80% solids using the crusher feed material. The slurry discharge had the consistency of a thick paste, and while this did not hinder the passage of material through the Hicom mill itself, there were problems experienced in the slurry sticking to the discharge chute. Water sprays were installed on the discharge chute for subsequent tests, and this corrected the problem. Test A2, also with the crusher feed material, was done with increased water addition to the mill to give a slurry density of 74% solids. The primary effect of this was to increase the net energy requirement from 7.9 to 8.3 kW.h/t. Tests B1 and B2 used the scrubber tailings material at feed rates of 1500 and 1800 kg/h, and slurry densities of 74 and 77 %. Once again the effect of reduced slurry density is an increase in the net energy requirement, this time from 8.0 to 8.7 kW.h/t.
E X P E R I M E N T A L RESULTS
Power and Energy Consumption Table 1 lists the net power and energy data for the four tests. Test 1 showed that running the mill at very high solids content (80% by weight) gave the most efficient grinding for the four tests, at 7.9 kWh/t. The main difference between Test 2 from Test I was that the feed water was increased to reduce the feed solids concentration from 80% to 74%. The result was a 5% increase to 8.3 kW.h/t in energy consumption. Test 3 was a different ore type to Tests 1 and 2, so conditions were kept the same as Test 2 for a comparison. The energy increased to 8.7 kWh/t, indicating that this ore type was slightly harder to grind than that of Tests 1 and 2. Test 4 was the same ore type as Test 3, and was run at the same conditions except that the feed rate was increased by 20% to 1800 kg/h and the percent solids was reduced to 77% (midway between Tests 1 and 2). The result was an 8% reduction in net energy requirement to 8.0 kWh/t. From Tests 1 and 2 it is reasonable to conclude that 2.5 % of the energy reduction was due to the increase in percent solids, and
Liberationof diamondsfromkimberlite
269
the remaining 5.5 % can therefore be attributed to improved efficiency from running at a higher throughput. No additional samples were available to try to further improve the energy efficiency. However, the following general observations can be made:
a)
Grinding is more efficient at higher solids densities, at least up to 80% solids, subject to adding wash water subsequent to grinding in order to facilitate slurry transport.
b)
Grinding is more efficient at higher feed rates. Assuming this trend continues to the maximum feed rate: an additional improvement in energy consumption is clearly possible. It is likely that substantial increases in feed rate will be possible by increasing the number of discharge ports, and by adding discharge slots in the base of the grinding chamber to increase the discharge of fine material.
Kimberlite content in the product The product samples were examined visually and found to contain only a very small proportion of kimberlite in the +4 mm size range. A few rounded nodules of kimberlite, possibly calcreted, remained. The remainder of the +4 mm product consisted of mainly chert and dolomite with a minor assortment of rock fragments derived from the country rocks surrounding the kimberlite pipes. Thus the objective of reducing all kimberlite to below 4 mm without substantially milling the chert and dolomite was largely achieved. A higher throughput rate may be possible, perhaps by installing more discharge ports, while still liberating virtually all of the diamonds from the kimberlite matrix. Size Distributior~ Figure 2 shows the size distributions of the feed and product material from each of the two ore samples. The size distributions are averages for several samples taken on each of the two runs with each ore type. Ore type A, the crusher feed material, produced a much finer product than ore type B, the scrubber tailings material, indicating that the scrubber product had a higher proportion of dolomite and chert.
1 O0
¢-
(3.
•- - 0 - - -
10
",, =
1
10
Particle size, mm Fig.2 Size distributions of the feed and product material from each of the two ore samples. MINE 10-3-8
A. Feed (crusher) A. Product B. Feed (scnJbber) B. Product
100
270
D.I. Hoyer and D. C. Lee
Tracer weights A tissue bag containing ceramic and diamond tracers was added to the mill about three minutes before the end of each test. At that time the mill discharge was sampled continuously until the end of the test, changing the sample collection bucket every 15 seconds. The purpose was firstly to see whether any damage occurred to the diamonds and tracers, and secondly to determine residence time distributions (RTDs) for the tracers. Table 2 lists the diamond and ceramic tracer weights before and after each test. The tracers added for each test included three 12 mm zirconia tracers, twenty 6 mm alumina tracers, and one or more diamonds. Three 3 mm diamonds were supplied by Ashton Mining Limited. The tracers were recovered by manually screening and washing the contents of the discharge collection buckets. No significant damage was recorded for any of the recovered ceramic and diamonds tracers. Note that different ceramic tracers were used in each test, which is why the initial weights vary between tests. Microscope images of the three tracer diamonds were recorded before and after the test program, and no damage was detected on any of the recovered diamonds. Diamond D2 lost 0.006 g in test 3. This was attributed to the presence of moisture during the initial weighing, as the microscope images did not show any physical change in the shape of the diamond. Diamond D3 was added in test B1 and not recovered. It was the smallest of the diamonds added, and was assumed to have escaped detection in the post-test screening and washing. The possibility of its having fractured cannot be eliminated, though it is felt to be unlikely given that none of the larger tracers suffered any significant damage.
TABLE 2 Diamond and ceramic tracer weights before and after each test.
Test number
A1
A2
B1
B2
(Tracer weights in g) 3 x 12 mm ceramic, before
12.78
12.57
12.94
12.61
3 x 12 mm ceramic, after
12.77
12.52
12.92
12.60
20 x 3 mm ceramic, before
8.37
8.14
8.18
8.21
20 x 3 mm ceramic, after
8.36
8.16
8.16
8.20
Diamond D1, before
0.0466
0.0466
0.0466
0.0466
Diamond D1, after
0.0466
0.0466
0.0466
0.0466
Diamond D2, before
0.0689
0.0683
Diamond D2, after
0.0683
0.0683
Diamond D3, before
0.0198
Diamond D3, after
lost
Tracer Residence Time Distributions Figure 3 shows the residence time distributions for individual diamonds and ceramic tracers recovered from the sample collection buckets for tests A1 and A2. The times indicated on the horizontal axes are the residence times for each tracer. The columns represent the number of 6 mm ceramic tracers recovered at each collection time. The circles and diamond shapes represent the 12 mm ceramic and 3 mm diamond tracers recovered. The results for Test A1 are not considered accurate as the slurry was too thick to be
Liberation of diamonds from kimberlite
271
satisfactorily transported down the discharge chute. Sprayers were installed for subsequent tests to help with the slurry collection. Most of the tracers had a residence time of less than one minute, though some can stay in the mill :for up to three minutes. The tracers shown in the fight-most column labelled 180 see represent tracers which were still in the mill after three minutes.
Test A 1 . 8 0 % solids, 1500kg/h I
"O
8 7 6
> 0 0 0
5
(9 t._
L_
U) L_
0 0
I
i
I
i
i
[] ~ O
6 mmceramic 3 mmdiamond 12 mmceramic
I "
I
4 32~'~i~i~.,'.~i ~'':~'~ "~''~!!
0
30
60
0
~"{~'~~|'"~! ~
90
120
150
180
Test A2. 74% solids, 1500kg/h 8
"0
7
> O O
6
(9 %_ (9 (9 t.._
0
6 mmceramic 3 mm diamond 12 mmceramic
5 4
(9
3
!._
2
O
[]
1 0
0
30
60
90
120
150
180
Residence time, sec. Fig..") RTDs for individual diamonds and ceramic tracers for tests A1 and A2. Figure 4 shows the residence time distribution results for tests B ! and B2. It indicates that the higher feed rate of test B2 yielded a shorter residence time. However, the number of tracers involved is quite small, and at least some of the difference would be due to statistical variations. Figure 5 was constructed by adding the four RTDs for tests A1 to B2, to indicate the average RTD with a greater sample number.
272
D. 1. Hoycr and D. C. Lee
Test B1.74% solids, 1500kg/h 8
"13
2 0 > 0 0
[]
6mmceramic 3 m m diamond
0
12 m m c e r a m i c
7 6 5 4
P 0 0
3 2 1 0
30
0
60
90
120
150
180
Test B2.77% solids, 1800kg/h •
I
|
I
6 m m ceramic
6 0 0
5
(3)
4
ii~!!~ii?::
3 m m diamond 12 m m ceramic
0
_ !i::i
P
0
i
0
30
60
I
................
90
120
150
i
180
Residence time, sec. Fig.4 RTDs for individual diamonds and ceramic tracers for tests B] and B2,
20
15 10
2 o>
o o
o
0 0
30
60
90
R e s i d e n c e time,
120
150
180
sec.
Fig.5 The sum of the four RTDs for tests A1 to B2.
Liberation of diamondsfromkimberlite
273
CONCLUSIONS Four open circuit Hicom grinding tests were completed on two separate ore samples supplied by Ashton Mining Limited from the Merlin diamond field. The Hicom mill reduced a very high proportion of the kimberlite to below 4 mm to achieve 100% liberation of valuable diamonds, and this indicates that there was probably a degree of overgrinding. The net energy consumption was about 8 kWh/t, but it is likely that this figure can be significantly reduced by increasing the number of discharge ports and thereby reducing the average residence time. There was not sufficient material available to test this option. No damage was recorded to any of the diamond and ceramic tracers added during the tests. The residence time distributions for ~Iracers added during the tests indicate that the discharge is similar to a fully mixed vessel with a dead time of less than 15 seconds and an average residence time of about 45 seconds.
ACKNOWLEDGEMENTS This work was sponsored by Hicom International Pty Limited. The permission granted by that company to publish this paper is gratefully acknowledged. The experimental work was done by R. Oxley, J. Hunter and P. Samaras.
NOMENCLATURE A
CO
acceleration intensity, m/s 2 (Equation 1) eccentricity of mill axis at the base of the mill, m. This is the distance between the mill axis and the nutation axis, measured perpendicularly from the nutation axis. mill speed, rad/s
REFERENCES 1.
2. 3. 4.
.
6. 7. 8.
Boyes, J.M., High-intensity centrifugal milling - a practical solution. Int. J. Miner. Process., 22, 413-430 (1988). Hoyer, D.I. & Boyes, J.M., The High-Intensity Nutating Mill - A Batch Ball Milling Simulator. Minerak~ Engineering, 3(1/2), 35-51 (1990). Hoyer, D.I., Power Consumption in Centrifugal and Nutating Mills. Minerals Engineering, 5(6), 671-684 (1992). Hoyer, D.I., High-Intensity Autogenous Milling in the Hicom Mill - A Preliminary Simulation Model. iin Comminution - Theory and Practice, ed S.K. Kawatra, chap 25,339-354, SME-AIME, Littleton (1992). Hoyer, D.I. & Boyes, J.M., High-intensity fine and ultrafine grinding in the Hicom mill. XVth CMMI Congress, Johannesburg. SAIMM, 2, 435-441 (1994). Joisel, A., Planetary Mills. Rev. Mater. Constr. Trav. Publics, 493, 234-250 (1956). Lloyd, P.J.D., Bradley, A.A., Hinde, A.L., Stanton, K.H. & Schymura, G.K., A Full Scale Centrifugal Mill. J. S. Afr. Inst. Min. Metall., 82(6), 149-156 (1982). Hoyer, D.I., Particle Trajectories and Charge Shapes in Centrifugal Milling. Int. Conf. on Recent Advances in Mineral Sciences and Technology, Mintek, S. Africa, 401-409 (26-30 March 1984).