A Decade of Gravity Gold Recovery G Wardell-Johnson1, A Bax2, W P Staunton3, J McGrath4 and J J Eksteen5 ABSTRACT Since 2001, the AMIRA P420 gold processing technology project series has been studying the recovery of gold using gravity devices, with particular emphasis on the centrifugal units, Knelson and Falcon concentrators. The work has entailed the surveying of operating gravity circuits in Australia, Africa and North and South America. This paper reviews some of the results and lessons of these studies, including common circuit operating and design factors, which limit gravity recovery; pointers towards effective modelling of gravity circuits; and the contribution of gravity to overall gold recovery.
INTRODUCTION The AMIRA P420 series of gold technology research projects has incorporated a significant component of research into gravity recovery of gold since 2001. Plant survey work has been an integral component of this research. The aims of this work have been to generate data suitable for development of process models as well as to evaluate the effectiveness of gravity operation and provide optimisation guidance at the specific sites. The quantity of data obtained for a range of gravity devices and installations allowed comparisons to be drawn between size recovery performance for different devices and for common design and operating factors that impact on plant performance to be identified. Aspects of plant survey design and data processing are discussed in this paper along with a summary of circuit studies undertaken and results generated. Non-plant survey related aspects of P420 gravity gold research such as model development and improvements in gravity gold characterisation are not specifically addressed in this paper.
PLANT SURVEY BACKGROUND AND DESIGN The typical sampling plan for the survey of classification and gravity circuits included characterisation of classification efficiency, primary unit recovery and gold room performance. In most cases, a bulk mill feed sample would also be collected on the day of the survey to permit the determination of ore gravity recoverable gold (GRG) content by a single-stage characterisation method. The survey method usually involved sampling of multiple streams concurrently over a single concentration cycle of a batch centrifugal concentrator, or, in the case of continuous gravity concentration units, a period covering at least five residence times (typically one hour). With respect to batch
centrifugal concentrators, it was found that best results are obtained when the feed, tail and concentrate for a particular concentration cycle are sampled directly and reliably. Preferably the full concentrate dump pertaining to the surveyed concentration cycle is collected and representatively subsampled after the full dry weight is determined. Cuts of the Knelson concentrate stream are a valid alternative, but intensive cyanidation (typically Gekko ILR or Consep Acacia Reactor unit) feed samples are not, because the concentrate from these devices does not pertain to the specific recovery cycle from which feed and tailing samples were obtained (Staunton, 2004; Wardell-Johnson et al, 2008). For plant sampling, full-stream cross-cut sampling is required to minimise sampling error; however in larger circuits this is often difficult and hazardous. Also, in many smaller circuits, inadequate access is available to obtain the required streams. Compromise can thus be necessary in the form of sampling from flushing or drain valve spigot points, dip samples into turbulent hoppers or distribution vessels and partial cuts of violently flowing streams. In these cases, larger errors must be assigned when mass balancing the circuit data. The sample cutters used in most of the survey work are depicted in Figure 1. All samples were processed at the university laboratories, generally after site-based weighing, filtering and drying. Samples were processed using three protocols: 1. For support samples whose gold content was not critical (eg cyclone feed or screen oversize), size distribution only was determined (down to 25 µm). 2. For high-grade samples in the gold room, sized fractions were assayed for gold content by total gold analysis with gravimetric finish.
1. Senior Research Metallurgist, Western Australian School of Mines, Curtin University, Perth WA 6000. Email:
[email protected] 2. MAusIMM(CP), Metallurgical Consultant, Western Australian School of Mines, Curtin University, Perth WA 6000. Email:
[email protected] 3. MAusIMM, Adjunct Professor, Western Australian School of Mines, Curtin University, Perth WA 6000. Email:
[email protected] 4. Metallurgical Technician, Western Australian School of Mines, Curtin University, Perth WA 6000. Email:
[email protected] 5. Professor/Manager Gold Technology Group, Western Australian School of Mines, Curtin University, Perth WA 6000. Email:
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TABLE 1 Gravity survey site and device summary. Sites/unit processes assessed Number of sites surveyed
7
5
13
25
Number of circuits surveyed
8
5
17
30
Ore gravity recoverable gold content determined
2
3
12
17
Hydrocyclone packs
8
6
16
30
Gravity feed screens
4
3
15
22
Knelson
6
3
11
20
Falcon
0
1
7
8
Jigs
1
1
1
3
Spiral concentrators/sluices
1
0
1
2
0
1
4
5
Batch centrifugal concentrators
FIG 1 - Gravity survey manual sample cutters. 3. For samples that required GRG content determination, a stream GRG test was undertaken to determine the content of already liberated gold in the particular stream. This test involves passing 8 to 12 kg of -850 µm sized material through a laboratory Knelson concentrator (KC-MD3), and assaying of the GRG (concentrate) and non-GRG (tailings) size fractions down to 25 µm. The lab Knelson is fitted with a custom built dry feeder system for enhanced feed rate control during GRG determination. Assay, sizing and stream density data obtained from laboratory work is used in conjunction with available stream flow data extracted from plant distributed control systems (DCS) systems, to derive a mass balance for the circuit using commercially available software. The derivation of a balance by both particle size and assay, facilitates the calculation of size recovery performance for each of the gravity unit processes. Partition performance for various species in classification and screening can also be derived. Some assumptions are usually required to complete a circuit mass balance, depending upon circuit design factors and data availability. For example, gravity feed flow where the target stream is cyclone underflow, can often be estimated from cyclone feed flow and proportionality between the number of gravity and non-gravity directed cyclones. For cyclone feed and mill discharge bleed gravity circuits, flow must generally be estimated/calculated from manifold orifice diameters and water balance data.
SITE AND DEVICE SUMMARY Project sponsors nominated sites for the purposes of gravity circuit survey work, with the number of target sites varying as a function of P420B, P420C and P420D project structure. Table 1 summarises the number of sites and circuits surveyed as well as the type of gravity units investigated over a ten-year period. Of the 25 sites surveyed, 15 were in Australia, eight in Africa and one each in North and South America. Four sites were renominated for survey work for a second, and in one case a third occasion after a multi-year gap, with most having undergone some interim modification or upgrade. There was considerable variability in the circuit flow sheets studied. Some recently commissioned circuits were surveyed, but most were mature, and in some cases only parts were accessible for gravity audit purposes. Early in the audit period, table-based gold rooms were more common, but a rapid uptake in the use of intensive cyanidation technologies meant that the opportunity to obtain size recovery data for secondary gravity recovery declined significantly. 226
P420B P420C P420D Overall
Flash flotation Gravity tabling
Mechanical Contact cell
1
1
1
3
3
0
2
5
RESULTS Survey results with major lessons and conclusions are discussed subsequently in sections dedicated to the various classification/gravity unit processes assessed over ten years of survey activity.
Ore gravity recoverable gold GRG has been defined as the gold content of an ore that can be recovered by gravity into a very small mass, typically less than one per cent (Laplante, Woodcock and Huang, 2001). A bulk sample of mill feed (or open-circuit semi-autogenous grinding (SAG) discharge) is necessary to determine GRG content by the single-stage test method developed under project auspices (Laplante and Staunton, 2005). In this test, 20 kg of sample is crushed and ground to a P80 of 75 µm, and passed through a laboratory Knelson. Concentrate and a subsample of tailings are screened and size assayed to derive GRG content. The test provides a good estimate of overall GRG content, but limited information on liberation characteristics. It is useful for broad gravity recovery estimates at study stages of a new project or when the gravity gold content of a new ore source at an existing operation is required. A wide range of GRG responses from low and finely distributed, to high and coarse has been revealed in the ore GRG testing undertaken. Three examples are shown in Figure 2, in which cumulative GRG is plotted as a function of particle size. The site S1 result is an example of a low and fine GRG response, S12 exhibits an average GRG content and size distribution and S8 an above average and coarse GRG response. The S1 result, if obtained in the design stage should rule out gravity as a process option, whilst the S8 response is most strongly suited to gravity gold recovery. Ore GRG test results do not always reconcile well with the ore GRG for the survey period calculated from the mass balance because dayto-day variations in feed GRG content can be very significant. Good repeatability of total GRG content and distribution for the same ore is generally observed over time, however. The main value of GRG characterisation of the ore lies in the understanding it provides of the maximum potential for gravity recovery. The better operated and more intensive the gravity recovery effort, the closer plant gravity recovery World gold CONFERENCE / Brisbane, qld, 26 - 29 september 2013
A Decade of Gravity Gold Recovery
Cumulative Recovery (% GRG)
100 90
S8
80
S12
70
S1
60 50 40 30 20 10 0 10
100
1000
Whilst there is significant variability in classification performance, it was found that there is a very consistent relationship between the partition curves for ore and GRG (Laplante and Staunton, 2005). This means that for greenfield projects or for cyclones, where the partition curve of GRG has not been measured, a highly reliable GRG partition curve can be extracted from that of the ore, using Plitt’s modified RosenRammler partition curve (Plitt, 1976). This is particularly useful in modelling to predict overall gravity recovery in an operating or greenfields circuit. In practice, the relationship can be quantified by the ratio of ore d50c to GRG d50c. This ratio and other measures of cyclone performance derived from survey data are summarised in Table 2.
TABLE 2 P420 project classification efficiency data summary.
Particle Size (µm)
FIG 2 - Example ore gravity recoverable gold test results – single-stage test. will come to the ore GRG response. The GRG result is also a critical input into the P420 gravity model, which has been utilised to predict gravity recovery based on ore GRG and measured cyclone performance at a number of operations. The model also provides circuit reconfiguration or optimisation guidance when there is a significant shortfall between actual and predicted gold recovered by gravity.
Parameter
Separation sharpness
1.01
3.78
Classification
Circulating load ore
70
1014
Mass balance data around cyclones were used to calculate partition curves for ore, GRG and total gold. A representative selection of cyclone partition curves for ore over ten years is depicted in Figure 3 and for GRG in Figure 4.
Circulating load gravity recoverable gold
190
8300
Circulating load ratio
2
27.8
% gravity recoverable gold in cyclone overflow
6.4
63.5
Feed % Reporting to Underflow
100
80 70 60
C1
C2
40
C3
C4
30
C5
C6
20
C7
C8
10
C10
C12
0
10
100
1000
Particle Size (µm)
FIG 3 - P420 project cyclone partition curve summary (ore). 100
Feed GRG % Reporting to Underflow
Low
High
d50c ore
48
222
d50c gravity recoverable gold
1
47
d50c ratio (ore:gravity recoverable gold)
4
68
A wide range of cyclone performance has been observed, but in general cyclone efficiency in the gold industry is poor. Only nine hydrocyclone packs out of 30 surveyed yielded separation sharpness for ore of greater than 2. Good classification is indicated by values in the range of 2 to 2.5 and very sharp classification is indicated by a value of 4. Many grinding circuits are pushed well beyond design capacity with progressively higher densities necessary to accommodate the increasing throughput as volumetric capacity becomes limiting. Poor solids classification is generally accompanied by poor GRG classification in accordance with the partition relationship discussed above.
90
50
Measured value range
90 80 70 60
The average ore:GRG d50c ratio is approximately 10. Lower values indicate reduced likelihood of GRG being retained in the circulating load and therefore greater risk of GRG reporting to downstream cyanidation leach or flotation circuits. Gold losses to tailings can thus occur, particularly if leach residence time is limited. Higher d50c ratio values are a positive indicator for gravity recovery because greater GRG retention increases the probability of GRG capture by gravity devices tapping the grinding circulating load. An inverse correlation can be discerned between the d50c ratio and the percentage of GRG in cyclone overflow. There is also an inverse correlation between the GRG circulating load and cyclone overflow GRG content, ie the higher the circulating load of GRG, the lower the GRG content of cyclone overflow.
50
C1
C2
40
C3
C4
30
C5
C6
20
C7
C8
Gravity feed screens
10
C10
C12
The gravity feed screen is an important component of a gravity circuit in that it determines both the size distribution as well as the feed rate of sized material presented to the primary concentration device. Of the 22 screen installations studied, the majority (15) were of horizontal vibrating design
0
10
100
Particle Size (µm)
1000
FIG 4 - P420 project cyclone partition curve summary (gravity recoverable gold). World gold CONFERENCE / Brisbane, qld, 26 - 29 september 2013
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but inclined vibrating (three), horizontal static (one) and DSM style (three) screens were also encountered. The aperture size varied between 1 mm and 5 mm, with 2 - 3 mm most common. Partition curves for a representative sample of the screens studied over ten years are shown in Figure 5.
Parameter Number of installations
CD20
CD30
XD48
1
13
6
Nominal capacity range (t/h)
15 - 80
50 - 100
200 - 400
46
2 - 88
34.5 - 340
90
S1
S4
S5
Actual feed rate range (t/h)
80
S6
S7
S10
Cycle time range (mins)
60
21 - 90
30 - 60
70
S11
S12
S8
Concentrate yield (%)
0.03
0.05 - 3.1
0.007 - 0.08
60
Gravity recoverable gold recovery (%)
50
Concentrate grade (g/t)
46
12.3 - 83.6
6.5 - 41.1
9100
520 - 27 000
290 - 2569
40 30
100
20
90
10
80
0
Recovery to Concentrate (%)
Feed % Reporting to Screen Underflow
100
TABLE 3 P420 project Knelson concentrator summary.
C1 C2 C3
70
100
1000
Particle Size (µm)
10000
FIG 5 - P420 project gravity feed screen partition curves. In general, gravity feed screen separation efficiency was found to be satisfactory with reasonably sharp separation and low bypass of feed fines to oversize. There were, however, some notably poor screens surveyed and all of the DSM installations were found to be significantly undersized or overfed so that a very high proportion of feed fines were rejected to oversize. Some horizontal vibrating screens were also struggling under high feed rates and almost all screens produced insufficient undersize to feed downstream gravity concentrators at a rate close to nominal rated capacity. It seems that of all the unit processes in a gravity plant, the area most often targeted for minimisation of capital cost is feed screening. This results not only in underutilisation of the downstream primary gravity device but also reduces the potential to select an aperture size appropriate for the gravity separation duty. The finer the GRG size distribution in the ore, the finer the gravity feed size should be, in order to minimise the size difference between the top size of gangue particle and the target GRG. Indeed the screen is often targeted for debottlenecking by increasing screen aperture size, which acts to allow more oversize to report to the concentrator. This reduces machine recovery efficiency by blocking the passage of the much finer and lighter GRG particles and also increases the erosion rate of GRG captured on the surface of the concentrate bed within the centrifugal concentrator rings.
Knelson concentrators Knelson concentrator units featured in 20 of the circuits surveyed. In most instances these machines are fed with screened cyclone underflow or cyclone feed, but in some cases, centrifugal concentrators are used to scavenge GRG from flash flotation concentrate. Of the circuits audited, the majority employed 30 inch machines and the largest unit studied was the 48 inch model.
C4
60
C7
50
C9
40
C10
30
C12
20
C14
10 0
C13 10
100
1000
Particle Size (µm)
FIG 6 - P420 project Knelson concentrator size recovery curves. size recovery efficiency, but actual performance is extremely variable as indicated by the size recovery performance curves depicted in Figure 6. ‘Average’ size recovery pattern for the Knelson concentrator is difficult to assess. Circuit C10 could be said to display an average recovery pattern, but the majority of Knelson concentrators seem to exhibit some degree of mid-range dip in recovery (eg C3, C4, C7 and C9), which relates inversely to the size distribution of feed to the machine. This phenomenon is believed to be a function of the particle dynamic phenomena that apply in centrifugal concentrating devices. It can be speculated that after the initial instantaneous fill of the concentrate riffles, GRG particles in feed must either be large and heavy enough to displace the gangue particles already occupying the concentrate bed, or fine enough to percolate through and fill interstitial gaps. Intermediate-sized GRG particles of similar size to feed gangue may therefore have less probability of being captured. If this theory is correct, sites having a significant recovery/feed size inverse relationship may benefit by increasing fluidising water flow to expand the bed or, alternatively, reducing bowl rotation speed to increase the effect of fluidising water on bed expansion.
Knelson dry solids throughput rate was very often found to be below the nominal rated capacity for the size of unit installed. This is usually due to upstream constraints such as gravity feed screen capacity or low flash flotation concentrate yield. Below specification throughput rates should favour
The average concentrate yield for all Knelson concentrators surveyed (excluding those grossly underfed) was 0.054 per cent and the average concentration cycle time was 45 minutes. In general, higher concentrate yield equates to higher GRG recovery, albeit at some reduction in concentrate grade, since more gangue also reports to concentrate. Concentrate yield is reduced by longer concentration cycle times and poor bowl condition due to service wear. There appeared to be some increase in average concentrate yield over the ten-year assessment period as limitations in concentrate storage and treatment capacity became less common. This enables shorter concentration cycle times to be adopted.
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A summary of Knelson concentrator installations surveyed during the P420 project series is provided in Table 3.
A Decade of Gravity Gold Recovery
Specific site and duty factors have an impact on machine performance. In the case of Site C13, the very low recoveries in all but the 600 µm to 850 µm size fraction were found to be a result of a maintenance error. Two of the bolts affixing the polyurethane bowl to the rotating bowl assembly were left out on reassembly, thus facilitating fluidising water bypass. This situation was repeated at a second site some time later (C1) and emphasises the advisability of involving the machine supplier/agent in maintenance auditing on a semi regular basis.
Falcon concentrators Falcon concentrators are an alternative centrifugal gravity concentration option to Knelsons and they may also be used to treat screened cyclone underflow or cyclone feed and flash flotation concentrate. Significantly less survey sites included Falcon semi-batch (SB) centrifugal concentrators than Knelson concentrators over the review period, but data from eight different gravity circuits incorporating these units is summarised in Table 4.
100 90
C6A
80
C6B
Recovery to Concentrate (%)
The average recovery of GRG to Knelson concentrate determined over the period was 32 per cent and sites achieving above 40 per cent such as C10 and C14 should be regarded as achieving excellent overall Knelson concentrator performance, subject to feed rate being within the nominal capacity range for the size of the unit.
70
C11A
60
C11B
50 40
C12
30 20
C2
10
C3
0
10
100
Particle Size (µm)
1000
FIG 7 - P420 project Falcon concentrator size recovery curves. between size recovery and feed sizing not being quite as obvious. It may be possible to explain such a trend by the difference in bowl designs. The Falcon concentrator has a smooth lower section where some preconcentration can occur before contacting the upper collection ring section rather than the full ring design in the Knelson. This is illustrated in Figures 8 and 9.
TABLE 4 P420 Falcon concentrator summary. Parameter
SB750
SB1350
SB2500
SB5200
Number of installations
2
1
4
1
Nominal capacity (t/h)
5 - 45
40 - 150
100 - 250
200 - 400
Actual feed rate (range – t/h)
41 - 43
5
7.1 - 90
103
30
30
20 - 45
60
Concentrate yield (%)
0.09 - 0.11
0.54
0.076 - 2.1
0.045
Gravity recoverable gold recovery (%)
22.4 - 41.7
57
2.5 - 78.2
36.7
Concentrate grade (g/t)
2200 - 3340
1525
139 - 2349
1329
Cycle time (range – mins)
FIG 8 - Knelson concentrator operation schematic (source: Laplante).
A number of features in the observed Falcon operating data reflected similar tends to the Knelson data. Dry solids throughput was typically below nominal design capacity, including extreme cases where the flash flotation concentrate (Falcon feed) production rate was well below design rate. Only one site surveyed was operating the machines at above the recommended minimum solids throughput rate. This is despite the fact that higher total gravity recovery can generally be achieved by overfeeding centrifugal concentrators and accepting some downgrading in recovery efficiency, rather than by underfeeding and thereby reducing the proportion of the circulating load treated. Observed cycle times for Falcon concentrators were, on average, shorter than for Knelsons at 32 minutes. This resulted in a higher average concentrate yield (excluding grossly underfed machines) of 0.08 per cent. Size recovery performance for the surveyed Falcon concentrators is depicted in Figure 7. As for Knelson concentrators, size recovery performance was extremely variable. There may be slightly reduced variability in recovery performance across the size classes when compared with Knelsons, with the inverse relationship World gold CONFERENCE / Brisbane, qld, 26 - 29 september 2013
FIG 9 - Falcon concentrator operation schematic (source: http://www.seprosystems.com). 229
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Overall there is little to choose between the two major centrifugal devices in terms of size recovery and overall recovery performance and selection is more likely to come down to cost, personal experience/preference and the level of back-up service provided by the manufacturers and their agents.
Flash flotation Flash flotation is a useful option to preconcentrate free gold associated with base metal sulfides or arsenopyrite when ore GRG content is low (< 25 per cent) or very fine, thus precluding direct gravity recovery methods (Laplante and Dunne, 2002). Five circuits operating mechanically agitated flash flotation tanks and three operating pneumatic (contact) cells with high shear, bubble-slurry contact systems were surveyed, most treating a split of cyclone underflow. Average mass yield to concentrate was found to be extremely low at 0.7 per cent, mostly due to difficulties with tank level control. Notwithstanding this difficulty, flash flotation is generally extremely effective in recovering fine GRG (
