Optimization of Gravity Recovery of Gold at High Pressure Leach...
Transcript of Optimization of Gravity Recovery of Gold at High Pressure Leach...
Abstract—The study provides optimization data on the work
carried out on the Knelson Concentrator KC XD-20 at the High Pressure Leach (HPL) plant at Kansanshi Mining PLC (KMP),
Zambia. The concentrate sent to HPL plant contains approximately 8 g/t of gold. This study involved the optimization of various parameters and performance determination of KC XD-20. The parameters investigated include G-force, fluidization water flowrate, cycle time and particle size distribution. The feed flow rate and pulp density were kept constant. A series of optimization tests were carried out at KMP. The results of the tests showed that there was a significant increase in free gold recovery. The optimization tests
indicated an improvement of 12 – 25% of gold recovery under the conditions investigated. The optimum parameters obtained were 150 G, fluidization water flowrate of 13m3/h and cycle time of 30 min.
Keywords—Gold, Gravity recovery, Knelson Concentrator,
Optimization.
I. INTRODUCTION
A. Kansanshi Mining PLC
ANSANSHI Mining PLC (KMP), a subsidiary of First
Quantum, owns 80% and operates the copper and gold
mines while ZCCM‘S subsidiary owns the remaining
20%. Gold at KMP copper and gold mine exits in two
different forms namely gravity gold (nuggets) and refractory
gold, locked up in the pyrite and chalcopyrite matrix. The nugget gold is collected by the use of falcons with feed
coming directly from the mills. Gravity Gold (Nuggets) is
recovered from course material before any further processing
of the ore. The refractory gold is collected using Knelson
concentrators at the high pressure leach (HPL) plant after the
autoclave leaching liberates it from its complex lock up [1].
J. Siame is with the Copperbelt University, School of Mines & Mineral
Sciences, Department of Chemical Engineering, P.O. Box 21962, Kitwe,
Zambia. (Phone: +260 96 920 6022; fax: +260 21 222 8212; E-mail:
K. Muchima was with the Copperbelt University, School of Mines &
Mineral Sciences, Department of Chemical Engineering, P.O. Box 21962,
Kitwe, Zambia. (E-mail: [email protected]).
D. Chirwa is with Kansanshi Mining PLC, High Pressure Leaching Plant
(HPL) Solwezi, Zambia.
P. C. Magawa was with Kansanshi Mining PLC, High Pressure Leaching
Plant (HPL) Solwezi, Zambia.
B. Knelson Concentrator and its Basic Separation
Mechanisms
The first unit of batch Knelson Concentrator was introduced
into the mineral processing industry in 1978. Nearly 3 decades
of development and modification, different series Knelson
Concentrators for different application purposes have been
well manufactured and used in precious metals recovery
industries all over the world [2], [3], [4], [5]. The following is
a summary of the Knelson concentrator XD-20 presently used
in industry [5].
B.1. Extended duty series (XD)
The XD series, with its futures of compact design, stainless
steel construction and high quality components, is for hard
rock milling circuits to withstand severe operating conditions.
These features of the design have made the XD series the most
demanding application for precious metals recovery industry.
Centrifugal fields can be generated in two different ways.
Firstly, a fluid is introduced at a high tangential velocity into a
cylinder or conical vessel such as a hydro cyclone. Generally, the larger and heavier particles will be collected near the wall
of the separator, while the smaller and lighter ones will be
taken off through an outlet near the axis of the vessel. The
second way is the use of a centrifuge. In this case, a fluid is
introduced into a rotating bowl and it is rapidly accelerated.
All the fluid tends to rotate at a constant angular velocity,
and a forced vortex is established. The tangential velocity is
directly proportional to the radius at which the fluid is rotating
[6].
In most practical cases, when a particle is moving in a fluid
under a centrifugal field, gravitational effects will be comparatively small, and can therefore be neglected. The
equation for the particles in the centrifugal field will be similar
to that for the motion in the gravitational field, except that the
gravitational acceleration ‗g‘ must be replaced by the
centrifugal acceleration [6]. The centrifugal acceleration is given by:
(1)
The main forces acting on a particle inside the Knelson
Concentrator are centrifugal force and axial drag force. For the
drag force, it is assumed that under laminar flow conditions,
the drag force on a spherical particle was entirely due to viscous forces within the fluid (Stock's equation), and can be
described as [7], [8]:
Optimization of Gravity Recovery of Gold at
High Pressure Leach Plant of Kansanshi Mining
PLC, Zambia
J. Siame, K. Muchima, D. Chirwa, and P. C. Magawa
K
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
165
(2)
where Fd: inward drag force, g.cm.s-2; Vr: velocity at radial distance r, cm.s-1; μ: the viscosity of the fluid medium, 0.01
g.cm-1.s-1 for water at 20 °C.
If the effects of the fluidization water and other forces on
the particle are not considered, the particle reaches its terminal
settling velocity when Fc equals Fd. Because we mainly
consider the behaviour of fine particles, Stokes equation could
be used to approximate the terminal settling velocity in a
centrifugal field by substituting for ‗g‘, as shown in Eq. (3) [6], [9].
( )
(3)
Why, in the centrifugal force field, can very fine particles
be more effectively separated, compared to the gravity field?
Eq. (4), answers this question, that is, as the centrifugal
acceleration increases, the size of the critical particle (the
finest particle that can be recovered) decreases [9].
√
(4)
Where, K, m, and c are coefficients, H is the thickness of
fluid film, and R is the average radius of the rotation drum.
From this equation, it is also easy to understand why the
Falcon Concentrator (or Super Bowl) can effectively recover
even finer particles than the Knelson Concentrator does, since
their centrifugal acceleration can reach 200 gs, compared to 60
gs of a regular Knelson Concentrator [7], [10].
The recovery of gold was calculated using the following
equation:
( )
( ) (5)
II. METHODOLOGY
The Knelson Concentrator XD-20 was used for the
optimization test works at KMP. The operating variables such
as G-Force, Fluidization water flow and cycle time were
selected using Knelson proprietary ―Independent Control
System‖ (ICS). The pilot test work was carried out over two
campaigns. A sample of 500g was collected on each campaign
per day from the sampling points as follows: Knelson
Concentrator 1 feed and Knelson Concentrator 1 tails; Knelson Concentrator 2 feed and Knelson Concentrator 2 tails
as shown in Figure 1, (a) and (b).
Samples from the two streams were drawn and filtered
using a pressure filter and then dried in the oven at 60 °C for 2
hours. The dried samples were then pulverized. Individual
samples of 50 g each were sent for chemical analysis. Gold
analysis was performed by KMP in Solwezi, Zambia. Fire
analysis of gold solid samples was also carried out. The tests
were repeated in triplicates.
For the particle size analysis, 2 kg sample was taken from
three sampling points: Autoclave discharge point (before the
thickener), Knelson Concentrator 1 feed, Knelson Concentrator 2 tails. The pulverized sample was then split into
1 kg batch each using the riffle splitter. Each portion of each
sample was then put on the 150 µm sieve with the 106, 75, 53
and 25 µm sieves arranged in descending order then placed on
a sieve shaker. The retained sample on each screen size was
then weighed and sent for gold analysis.
(a)
(b)
Fig. 1. Schematic flow sheet of the Knelson Concentrators XD-20: (a) Series configuration, (b) Parallel configuration.
TABLE I
OPERATING CONDITIONS OF THE KNELSON CONCENTRATORS
G-Force Cycle time (min) Fluidization water flowrate (m3/h)
100 30 -60 9 and 13
150 30-60 9 and 13
200 30-60 9 and 13
III. RESULTS AND DISCUSSION
A. Effect of G-Force
According to Cherest [11] the recovery is directly
proportion to the G-force. Increased G-force was found to be beneficial for the recovery. Figure 2 shows the effect of G-
force on Knelson Concentrator XD-20 unit recovery.
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
166
Fig. 2. Effect of G-Force on Gold recovery.
B. Effect of cycle time
In general, for Knelson Concentrator application in a
secondary circuit or in other circuits with high levels of pyrite
in the ore, a phenomenon known as ―concentrate bed erosion‖
can occur, whereby gold already recovered on the surface of
the concentrating ring is removed due to the effect of high
specific gravity particles such as pyrite. As such, short
concentrating cycle times are generally of benefit. The test
work carried out proved this to be the case in this application.
Figure 3 shows more stability with the minimum cycle time
giving an experimentally accurate peak for all cycle time (30,
45 and 60 min) investigated. Initially, 60 minute cycles were used, but the results suggested shorter cycles would be of
benefit without causing significant dilution of the final copper
concentrate. Thus 30 minute cycles were adopted. Figure 3
shows the differences in unit GRG recovery between the 30
and 60 minute concentrating times.
Fig. 3. Effect of recovery related to cycle time.
C. Effect of fluidization water flowrate
The effect of using different fluidization water flowrate values on the gold recovery was investigated. Laplante et al.
[12] reported that the effect of fluidization water pressure was
minimal in their findings. Optimum fluidization occurs when
the force of the slurry against the cone is comparable to that of
the fluidization water flow rate.
Figure 4 shows gold recovery for two tests, one at low
fluidization water flow rate and another at high water flow rate
to understand the effect of fluidization water flow rate on gold
recovery. The tests conducted at low fluidization flow rate
(9m3/h) produced low recoveries in all sizes with an overall
recovery of 64% whereas the test performed at a high fluidization flow rate (13m3/h) produced high size by size
recoveries and an overall recovery of 71%. The low recoveries
are attributed to the combined effect of coarse size, and low
fluidization flow rate, 9m3/h, which act synergistically to
erode the concentrate collected in the riffles. Typically for an
operating Knelson, low recoveries have been reported due to
the suboptimal fluidization water flow rates [13].
However, the recovery trend is consistent for both high and
low fluidization water flow rates. However, as fluidization
water flows are increased beyond this range, a loss of recovery
of fine GRG is incurred.
Fig. 4. Effect of fluidization water flow rate on gold recovery.
D. Particle Size Analysis Table II (see appendix) shows the amount of gold contained
on each screen size. It is expected that a higher grade of gold
would be on the smaller size fraction as more gold would have
been liberated and there would be less gangue material.
Fig. 5. Cumulative % passing and retained as related to screen size
for the series test work.
0
20
40
60
80
1 2 3 4 5 6
Recovery %
Test runs
Recovery for 100G
Recovery for 150G
Recovery for 200G
0
10
20
30
40
50
60
70
80
1 2 3 4 5 6
Recovery
%
Test runs
recovery30
recovery45
recovery60
55
60
65
70
75
1 2 3 4 5 6
Recovery
%
Run number
recovery for 9recovery for 13
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
167
Furthermore, the results showed that gold grade was higher
on the 53µ screen and highest on the 25µ screen. The least
gold grade was found to be on the 106µ screen followed by
the +150µ screen. In Figure 5, the cumulative passing and
retained indicated that bulk of the feed material was a bigger
size, falling in the range of 150µm and above.
According to Grewal and Fullam [14], it is expected that that
the finer size particles would have the most gold losses.
However, optimization test work showed that the most gold
losses were in the courser size fractions (+53 µm). This could
have been due to gangue material being mostly in the courser
size fraction and less in the finer sizes.
IV. CONCLUSION
The objective of the work described in this paper was to
determine the optimum parameters and investigate the effects
of G-force, fluidization water flowrate, cycle time and particle
size distribution on the recovery of very fine gold in leach
tailings.
The optimum recovery of gold was achieved at different
conditions. The optimization results indicated an improvement
of 12-25% of gold recovery and this was achieved at 150G-
force, 13m3/h fluidization water flowrate and a 30min cycle
time. Replicate tests suggest that the G-force giving greater
than 150G-force do not offer additional gold recovery for
these feed materials.
The particle size distribution was found to be a contributing
factor to the overall yield. This was mainly due to either
interstitial trickling or composition of gangue material in the
overall courser materials. The recovery was best in the -75µm
size fraction. Extending the size analysis to -25µm
demonstrated that the Knelson Concentrator is concentrating
gold bearing silicate particles down to 25 µm. However,
particles finer than 25 µm are beginning to follow the water
flow into the tailings.
APPENDIX
TABLE II AMOUNT OF GOLD CONTAINED ON EACH SCREEN SIZE.
Screen size (µm) 150 106 75 53 25
Sample collection point
Weight %retained 49 24.5 23 2.5 1
Autoclave Discharge Cumulative % retained 49 73.5 96.5 99 100
Gold % contained 28 12.6 14.6 22.7 21.6
Weight % retained 76 17 2.5 3.2 1
Knelson Feed Cumulative % retained 76 93 95.5 99 100
Gold % contained 40.77 7.7 4.34 26.844 20.26
Weight % retained 52.5 25.5 13 6.5 2.5
Knelson Tails Cumulative % retained 52.5 78 91 97.5 100
Gold % contained 49.95 27.46 11.29 6.67 4.74
ACKNOWLEDGMENT
The authors would like to thank the Kansanshi Mining Plc, Solwezi, Zambia and the Copperbelt University, Kitwe,
Zambia.
REFERENCES
[1] Press release, ―Kansanshi fact, last update May,‖ Solwezi, 2007.
[2] B .V. Knelson, ―Centrifugal concentration and separation of precious
metals,‖ 2nd International Conference on Gold Mining, Vancouver, pp.
303-317, 1988.
[3] B. V. Knelson, ―Development and economic application of Knelson
concentrator in low grade alluvial gold deposits,‖ The AusIMIM Annual
Conference, Rotorua, New Zealand, March 18-21, pp. 123-128, 1990.
[4] L. Huang, ―Upgrading of Gold Gravity Concentrates. A Study of the
Knelson Concentrator. Ph.D. Thesis, pp. 294, 1996.
[5] B. V. Knelson, ―Batch Knelson Concentrator,‖ (Booklet), pp. 6, 2004.
[6] J. M. Coulson, J. F. Richardson, J. R. Buckhurst, and J. H. Harker,
―Chemical Engineering, vol. 2, 4th Ed., Particle Technology and
Separation Processes,‖ Pergamon Press, Oxford, New York, pp. 968,
1991.
[7] J. Sutter, ―Super Bowl nominal specifications‖, 1997 [Online]
http://www.rmsross.com/sb_spec.htm (Accessed on 13/10/2013).
[8] G. G. Stokes, ―Mathematical and Physical Papers‖. Trans. Cambridge
Phil. Soc., 9, Part II, 51ff, 1901.
[9] Y. Sun, ―Gravity Separation,‖ Metallurgy Press, Beijing, pp. 310, 1982.
[10] S. McAlister, ―Case study in the use of the Falcon gravity concentrator,‖
Proceedings of 24th Annual Meeting of the Canadian Mineral
Processors, Ottawa January, pp.20, 1992.
[11] A. Cherest, ―Performance of Knelson XD-20 concentrator at golden
giant mine under different operating conditions‖, Research PROJECT
306-410B, McGill University, 2000.
[12] A. R. Laplante, Y. Shu, and Marois, ―Experimental characterization of a
laboratory centrifugal separator,‖ Canadian Metallurgical Quarterly 35
(1), pp. 23–29, 1996.
[13] F. A. Vincent, ―A comparison of Knelson Concentrator and jig
performance for gravity recovery,‖ M. Eng. Thesis, McGill University,
pp. 131, 1997.
[14] I. Grewal, and M. Fullam, ―Gravity circuit optimization via
mathematical modeling by particle size classes,‖ [Online]. Available from www.knelsongravitysolutions.com [accessed January 2013].
Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg
168