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A novel laboratory procedure for predicting continuous centrifugal gravity concentration applications: The gravity release analysis
International Journal of Mineral Processing 154 (2016) 66–74
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
A novel laboratory procedure for predicting continuous centrifugal gravity concentration applications: The gravity release analysis G. Sakuhuni a, N.E. Altun b,⁎, B. Klein a, L. Tong a a b
University of British Columbia, NBK Institute of Mining Engineering, Vancouver, BC, Canada Middle East Technical University, Mining Engineering Department, 06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 20 March 2016 Received in revised form 8 July 2016 Accepted 21 July 2016 Available online 22 July 2016 Keywords: Centrifugal gravity concentration Knelson CVD Scale-up Gravity recoverable gold Gravity release analysis
a b s t r a c t A novel procedure, Gravity Release Analysis (GRA), is introduced for performance prediction of continuous centrifugal concentration, using lab-scale batch tests. Also, linked with the GRA, the Gravity Release Index (GRI) was developed for ranking the ore amenability to centrifugal gravity concentration. Ore samples from the flotation circuit of Myra Falls concentrator were subjected to Multi Pass Test and Gravity Amenability Test for comparison with GRA. Recovery kinetics of batch centrifugal concentration was analysed to establish the Gravity Release Index (GRI). Both Gravity Amenability Test and Multi Pass Test failed to cover the broader mass yield range of continuous centrifugal concentration and this was the main drawback in performance prediction of larger units from lab-scale data. GRA covered a wider mass yield range, yielding better performance prediction of larger continuous units. From the gravity recovery kinetics of the batch unit, varying GRI values for Au, Fe, Zn, Fe and S were obtained, with Au having the highest- GRI and amenability to centrifugal gravity concentration. Varying amenabilities for different Au forms (free Au particles, Au bearing sulphides) to centrifugal gravity concentration could also be distinguished. SEM analysis on recovered gold entities justified the prediction by the GRI. Overall, GRA could effectively predict continuous centrifugal concentration using small amounts of feed with lab-scale batch units. Determination of the GRI for targeted metals would provide further precision for bench-marking and scaling-up of continuous centrifugal concentration. © 2016 Published by Elsevier B.V.
1. Introduction Continuous discharge centrifugal concentrators, introduced in the 1990′s, brought a niche application for gravity concentration which could not be realized with the low mass yield batch or semi-continuous concentrators. A typical use of these units is scavenging Au-associated sulphides from flotation tails. This application increases productivity by reducing gold losses and lowering grinding costs by using a coarser grind size (Klein et al., 2010; Altun et al., 2015). However, full potential of this technology has not been exploited, partly due to the lack of reliable small sample procedures that demonstrate its applicability. Gravity recovery by centrifugal concentration is a probabilistic process dependent on several factors including specific gravity, degree of liberation, particle morphology, particle size and machine parameters. Like floatability, amenability to gravity concentration is not an intrinsic property. The best case of centrifugal separation is what is achievable as separation stages are increased, i.e. for a given ore sample the more the number of batch testing cycles, the sample undergoes the higher the gravity recovery, such that by running an infinite number of batch cycles the maximum gravity recoverability can be determined. Evaluation
⁎ Corresponding author.
http://dx.doi.org/10.1016/j.minpro.2016.07.004 0301-7516/© 2016 Published by Elsevier B.V.
of ore amenability to concentration precedes application of the technology and availability of reliable techniques capable of determining maximum recoverability is crucial for bench-marking process performance. For determining the amenability of a given ore to gravity concentration sink-and-float test is mostly used. Pratten et al. (1989) argued that a lab procedure to characterize fundamental response from a specific method/device should be based on the same separation mechanism. Thus, if the ore is to be concentrated by heavy media separation then sinkand-float approach will be appropriate, but for centrifugal concentration application, the laboratory procedure should be based on centrifugal concentration. Besides, other limitations of the sink-and-float approach exist, such as the inability to handle fines, toxicity of heavy liquids used and lack of higher specific density heavy liquids suitable for mineral separations. Several laboratory procedures were developed for assessing the ore amenability to centrifugal concentration and to evaluate the process. For testing of centrifugal gravity concentration in lab-scale, using small-batch units is universal, since there are no lab-scale continuous centrifugal concentrators. Batch Knelson MD3 is particularly suited and widely-accepted for characterizing centrifugal gravity concentration (Banisi et al., 1991; Laplante and Shu, 1992; Woodcock and Laplante, 1993; Laplante et al., 2000; Ghaffari, 2004; Clarke, 2005; Xiao, 2008). Generally, the concentration mechanism in a lab-scale
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Nomenclature CVD EDX GAT GRA GRG GRGS GRI MD3 SEM
Knelson Continuous Variable Discharge concentrator Energy Dispersive X-ray Spectroscopy Gravity Amenability Test Gravity Release Analysis Gravity Recoverable Gold Gravity Recoverable Gold bearing sulphides Gravity Release Index Laboratory scale 3-in. Knelson batch concentrator Scanning Electron Microscopy
centrifugal concentrator, such as the Knelson MD3, is similar to that of the continuous units, such as the Knelson Continuous Variable Discharge (CVD) concentrator. In both, particles are separated in a fluidised bowl rotating at high centrifugal force and the main separation mechanism is differential particle sedimentation in the centrifugal force field. The critical difference is, the CVD has pinch valves to discharge the concentrate continuously, while the MD3 has to be stopped to remove the concentrate. Also, past researchers confirmed the reliability of the labscale Knelson unit for predicting gravity recoverable gold (GRG) content (Banisi, 1990; Laplante et al., 1994; Woodcock, 1994; Clarke, 2005). The GRG procedure, which is by far the most widely accepted technique for characterizing ore amenability to centrifugal gravity concentration (Koppalkar, 2009) offers the upper limit of gravity recovery for the ore (Clarke, 2005). However, the cost, labour and feed (40–100 kg) requirement for the GRG test limits its use. A simplified GRG test using less feed (20 kg) and involving a comminution step which targets 80% passing 75 μm, followed by single pass in the MD3 was proposed (Clarke, 2005). However, the simplified GRG test recovered less gold than the standard GRG test and was, therefore, not adequate for characterizing ore amenability. Instead, it provided a better measure of the expected plant performance, as the industrial units are known to recover about 66% of the GRG predicted by the standard test. Another concern is, since the CVD is mainly used for scavenging, the feed to the CVD is already ground. Thus, the size liberation data obtainable for the standard GRG test can only be obtained from recovery by size instead of stage wise comminution. In addition, Subasinghe (2012) argued that GRG is machine dependent, i.e. it is a function of the feed size and the force acting on the particles and, is therefore, different for the batch MD3 and the continuous units. In addition to the GRG method, procedures such as the Multi Pass Test (Ghaffari, 2004) and the Gravity Amenability Test (GAT) are remarkable, but up to date no laboratory technique could be standardized for continuous centrifugal concentrators. This implies that none of the available laboratory procedures could best mimic such applications. The main reason for this is, these techniques cannot adequately characterize samples for the industrial scale, high-mass yield applications, such as those using Knelson CVD's: Lab-scale test procedures were developed for low mass yields (b 1%) which target liberated heavier particles, while continuous centrifugal concentrators recover heavy minerals and middlings. More than half of the commercial CVD units are used for Au-associated sulphides with mass yields N5%. To represent larger tonnage applications in lab-scale, adjusting the feed mass of the batch unit is critical to simulate the mass yield of the continuous units. By decreasing the feed per run for the MD3, thereby increasing the mass yield, it is anticipated that the MD3 performance should simulate the CVD performance. Also, the well-known GRG procedure can distinguish gravity recoverable and non-recoverable gold. However, there is no technique to quantify the extent of liability to gravity concentration from one mineral type to the next. While many minerals are gravity-recoverable, ease of recovery depends on the machine characteristics, density, mineralogy,
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liberation degree, particle size and shape, and no two mineral particles have exactly the same response to gravity. Unlike in flotation, where the rate constant can be quantified based on lab-testing and used in process design, no strategy to quantitatively measure mineral amenability to gravity concentration exists. This work investigates the suitability of the batch Knelson MD3 unit for predicting CVD applications; assesses suitability of existing lab procedures for predicting CVD performance; and aims at developing a new lab procedure for predicting CVD performance. Linked with these, a novel procedure code, Gravity Release Analysis (GRA) was presented. GRA was developed from Dell's flotation release analysis (Dell, 1953), which has become one of the standards in testing flotation efficiency (Dell et al., 1972). Suitability of this procedure for predicting CVD performance was assessed based on determining the maximum release of Au-bearing sulphides, using a relatively small amount of feed. By providing a measure of maximum gravity recoverable Au-bearing sulphides, a basis for bench-marking operating CVDs is provided. The Gravity Release Index (GRI) is also introduced to rank the degree of gravity recoverability for different minerals, based on a quantity similar to the flotation rate constant. This allows identifying the amenability to centrifugal gravity concentration, i.e. ranking the easily recoverable and hard-to-recover particles by gravity. The GRI is based on the GRA, that allows for fractional removal of concentrates, similar to the flotation tests described by Kelsall (1961). 2. Experimental 2.1. Ore samples and characterization Synthetic quartz/magnetite ore mixture and polymetallic ore from an operating mill were used for the test program. For the synthetic ore mixture, sized silica (LM # 20–30 and LM # 70) from Lane Mountain was blended in the ratio 1:3 and mixed with sized magnetite to constitute a 5% magnetite content. The magnetite used was passed through Davis Tube instrument to entirely remove non-magnetic impurities. The silica and magnetite in the synthetic mixture had similar particle size distributions, as presented in Fig. 1. Polymetallic ore samples, collected from Copper rougher tails, zinc concentrate and tailings streams of the flotation circuit at the mill of Myra Falls operation, Victoria, BC, Canada, were subjected to the same test conditions as the synthetic ore. The samples were riffled into one 2 kg sample, one 1 kg sample, two 500 g samples and two 250 g samples for the tests. All assays were done at the International Plasma Lab (IPL) in Richmond, British Columbia. For ore characterization, X-ray diffraction of representative ore samples with phase quantification using the Rietveld Topas 4.2 program was completed at the Earth, Ocean & Atmospheric Sciences, UBC. Scanning electron microscopy was also used to identify the gold species and
Fig. 1. Particle size distributions of silica and magnetite in the synthetic ore mixture.
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Samples weighing 5, 2, 1 and 0.5 kg were prepared and used in the tests. Each sample was run through the MD3 once (Fig. 2). The tailings and concentrate products were assayed for magnetite content using the Davis tube to separate the magnetite from silica, then by drying and weighing the different fractions and using the results for metallurgical balance. Table 1 shows the MD3 parameters used for the tests in this phase. 2.3. Multi pass test and gravity amenability test
Fig. 2. Procedure of the Single Pass Variable Mass Yield Test.
Table 1 Test conditions for single pass variable mass yield tests. Test parameter
Level
Centrifugal field, g Fluidisation flow rate, l/s Solids feed rate, kg/s
60 0.22 0.0083
gold associations recovered by the centrifugal gravity concentration tests. A Hitachi S3000 N VP-Scanning electron microscopy with EDX was used for detailed elemental analysis and to identify the gold species and gold associations for samples recovered by the centrifugal gravity concentration tests.
2.2. Mass yield test A single pass variable mass yield procedure in Fig. 2 was developed and used to assess the effect of mass yield on lab-scale batch centrifugal gravity concentration, using a Knelson MD3 concentrator. The main motivation in developing this was to design a test procedure which best simulates a typical plant scale CVD application, in which ore passes only once through the concentrator (open circuit) and which is capable of handling high mass yields.
In order to assess the adequacy of available procedures for predicting CVD application, the Multi Pass Test (Fig. 3) and Gravity Amenability Test (GAT, Fig. 4) procedures were applied on Myra Falls copper rougher tails. For both tests a feed sample of 5 kg, obtained representatively from the Myra Falls copper rougher tails stream, were used for testing gravity amenability and the results were compared to CVD6 performance. The amounts of the metals of interest in the copper rougher tails stream are presented in Table 2. 2.4. Gravity release analysis To improve on the deficiencies of existing methodologies for high yield applications, the Gravity Release Analysis (GRA) procedure in Fig. 5 was designed. The procedure consists of a series of rougher-scavenger-cleaner centrifugal gravity tests using the MD3. In this procedure particularly the cleaner stages are less dependent on operator skills compared to panning. One of the objectives of this procedure is giving an indication of how much metal or mineral of value can be recovered by a continuous centrifugal concentrator in a mineral processing circuit. Hence, the procedure was specifically designed to best predict continuous centrifugal concentration and to determine applicability of the CVD technology to recover gold associated with sulphides in a processing stream using small sized samples but it can also be applied to any centrifugal concentration application. As presented in Fig. 5, the starting sample size is 5 kg (similar to previous test procedures used for predicting continuous centrifugal concentration). The sample is rifle split into 1 kg sub samples and each sub sample is run into the MD3 twice, collecting the concentrate after each run to constitute 200 g of rougher & scavenger concentrate. These concentrates are then
Fig. 3. Procedure of the Multi Pass Test (Ghaffari, 2004).
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Fig. 4. Procedure of the Gravity Amenability Test.
combined for each sub sample to constitute 1 kg feed for cleaner stages. This feed is run 4 times into the MD3 for cleaner stages. The concentrate after each cleaner is collected. The concentrates and the final tailings are assayed for the target elements. For the MD3, increasing bowl speed favours recovery and leads to dilution of concentrates by lower density species (Savas, 2016). It also favours recovery of finer particles. Increasing fluidisation flow rate favours concentrate grade, with rejection of fine heavy, and/or coarse lighter particles, thus compromising the recovery. The mass yield affects MD3 recovery and concentrate grade. By varying the feed weight, tests can be designed to either maximise recovery or grade. Thus, both bowl speed and fluidization flow rate can be maintained constant and the mass yield varied in line with each test objective. The laboratory scale results were compared to industrial scale results obtained using full- and pilot-scale CVD units. The industrial scale results were acquired from a rigorous optimisation study on the CVD units, including statistical experimental design and simulation optimisation to generate an operating curve. A more thorough description of this optimisation study is given elsewhere (Sakuhuni, 2014; Sakuhuni et al., 2015).
mass yield of w has been separated from the feed. k is the Gravity Release Index (GRI) which gives a measure of gravity amenability of the tested ore. C ¼ C0 expð−kwÞ
ð1Þ
w ¼ wð0Þ expð−k1 nÞ
ð2Þ
w ¼ wð0Þ expð−k2 tÞ
ð3Þ
w ¼ wð0Þ exp −k3 wp
ð4Þ
Three forms of the equation can be derived based on the GRA procedure. In Eq. (2), n is the number of concentration cycles, k1 gives a gravity concentration rate with unit 1/cycle, w(0) is the feed weight. In Eq. (3), t is the cycle time. It assumes that the feed rate of the MD3 is constant, so the cycle time can be calculated; k2 is the gravity concentration rate (1/min). In Eq. (4), wp is the weight percent of the sample recovered and the rate of gravity concentration (k3) can be calculated which is an indication of the amenability of the ore to gravity concentration.
2.5. Gravity recovery kinetics 3. Results and discussion For batch gravity recovery, a first order process is assumed. Accordingly, for gravity recoverable solid particles at a concentration of C =C0 when mass yield to concentrate is zero, integration leads to Eq. (1). In this equation w is the mass yield, C is the concentration of gravity recoverable mineral remaining in tailings after a concentrate accounting to a
Table 2 Amount of metals of interest in the Myra Falls copper rougher tails stream. Au (g/t)
Ag (g/t)
Cu (%)
Pb (%)
Zn (%)
Fe (%)
0.7
12.4
0.1
0.2
0.7
5.4
3.1. Assessment of suitability of MD3 for CVD performance prediction Fig. 6 presents the effect of increasing mass yield on MD3 performance showing that a recovery maximum exists as mass yield continues to increase. Both the recovery and upgrade ratio level off beyond a mass yield of 10%. Fig. 6 (a) shows particle size effect on recovery as a function of mass yield. At low mass yield batch type applications, the effect of particle size on magnetite recovery was not as significant as compared to coarser fractions. This was attributed to the effect of further upgrading in a fluidized concentrate ring, which allows for consolidated trickling to capture high-density fines. At higher mass
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Fig. 5. Procedure of the Gravity Release Analysis.
yields, though, particle size had significant effect on recovery, such that the coarser the magnetite the higher the recovery (Fig. 6a). The results suggest that high mass yield applications are more sensitive to differential settling, which is a function of both particle size and density. If no ample time is allowed for consolidation of the fines in the particle bed, the effect of particle size on recovery is amplified. These results have implications on CVD applications, which typically operate under high mass
yield and hence, which are more sensitive to feed particle size distribution than batch machines due to the dominance of differential settling as a separation mechanism. These machines may therefore require a closer particle size control for efficient operation, contrary to current practice where these units are mainly operated in scavenging circuits with no close control of feed size distribution. Also, Fig. 6b–d basically show that beyond a mass yield of 10% recovery gradually increases
Fig. 6. Effect of mass yield on MD3 performance: (a) Synthetic ore; Fe3O4 recovery w.r.t particle size (b) Synthetic ore; Fe3O4 recovery vs. upgrade ratio (c) Myra Falls Zn-concentrate; Au recovery vs. upgrade ratio (d) Myra Falls flotation tails; Au recovery vs. upgrade ratio.
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Fig. 7. Comparison of bench scale (GAT and Multi Pass Test) predictions and CVD performance.
whist the upgrade ratio levels off. This trend holds for both synthetic and real ore and was used as the basis of using a mass yield of 10% for the GRA procedure, with the aim of maximizing recovery whilst still upgrading. In addition, considering the challenge of obtaining a representative small sized sample and recovery benefit by increasing mass yield, a mass yield of 10% was considered suitable for designing the rougher stage for any lab scale procedure. It should also be noted that upgrade ratio decreases with increasing mass yield, since it follows the same trend as concentrate grade. 3.2. Assessment of existing laboratory procedures In order to assess the adequacy of existing laboratory procedures for predicting full-scale CVD applications, results obtained from the Multi Pass Test and GAT test were plotted to generate cumulative recovery vs. cumulative grade curves. These were then compared to the CVD operating line obtained through pilot scale testing and optimization at Myra Falls copper rougher tailings stream. Fig. 7 shows the results. It was seen that both bench scale tests, i.e. the Gravity Amenability Test (GAT) and Multi Pass Test do not cover the broader feasible mass yield range typical of CVD applications. The Multi Pass Test better predicted CVD performance than the GAT procedure. Furthermore the bench scale test results overestimate CVD performance and therefore provide an upper limit for CVD performance. The GAT predicts way above CVD performance and the Multi Pass Test, which has cleaning stages, better predicts CVD performance at low mass yield. Although a higher concentrate grade at the same mass yield would have been
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Fig. 9. Grade/Recovery curve for gold in Myra Falls flotation tails.
expected for the Multi Pass Test, results were just the opposite. The lower grades obtained for Multi Pass Tests indicates that some of the gold gets deported to the pan tails due to operator dependent panning procedure. Thus, the approach is unsuitable for characterizing fundamental response of centrifugal gravity concentrators, since it uses a different separation mechanism, as proposed by Pratten et al. (1989) and it is operator dependent. The CVD line exhibited inhibited gravity recovery at high mass yield (recovery above 15%), which was predictable by the Gravity Release Analysis (GRA) procedure (Fig. 8). The un-recoverable gold can either be attributed to un-liberated fine gold associated with sulphides and gangue or gold rendered non-recoverable by over grinding. Results obtained for the GRA procedure for Myra Falls final tailings are shown in Fig. 8. For purposes of comparing GRA results and CVD performance, final cleaner tailings were considered as the 5th concentrate. The results show that the GRA procedure better predicts CVD performance and covers a wider mass yield range. 3.3. Effect of mineralogy on gold recovery A model was developed to study the effect of mineralogy on the Au grade/recovery relationships and presented in Fig. 9. It was assumed that the feed was composed of four types of gold bearing particles; 1) Liberated gold, gravity recoverable 2) Liberated gold, gravity non-recoverable 3) Gold bearing sulphides 4) Gold bearing gangue. Mont Carlo simulation was used to generate weight percentage data of each mineral and gold grade in each mineral fraction and the recovery rate was
Fig. 8. Comparison of GRA and optimum CVD results based on NNREGA optimisation for Au: (a) Upgrade ratio w.r.t mass yield (b) Recovery w.r.t mass yield (Sakuhuni et al., 2015).
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modelled. The shape of the Au grade/recovery curve in Fig. 9 is determined by the assumed model inputs. Model 1 represents recovery of Gravity Recoverable Gold (GRG) for an ore consisting mainly of GRG and gangue. Model 2 represents recovery of gold existing as mineral type A and B; Mineral A has a very high gravity recovery rate and high gold content representing GRG and mineral B represents gravity recoverable gold bearing sulphides (GRGS). Model 3, which best tracks the gold results, shows gold distributed in all four species. Species A represents GRG, B represents GRGS, C represents particles with negligible gold and very low recovery rate which accounts for gold associated with gangue and D represents particles with high gold grade, and very low recovery rate accounting for non-GRG (rendered unrecoverable by gravity due to overgrinding). Selective gold recovery was achieved by gravity concentration on the Myra Falls flotation tailings sample. A high grade gold containing product was obtained in the first gravity concentration cycle. The gold grade decreases greatly after the first cycle due to the recovery of the low-grade mineral fraction. The results show that the gold in Myra Falls tailings is mainly non-GRG and there is very low GRG and significant GRGS. The model results are helpful to understand the bench scale tests. The results indicate that gold lost to the final tailings at Myra Falls is partly recoverable by centrifugal concentration. Based on characterization of the gold by particle size, only 30% of the gold in final tailings is above 38 μm, this could be the gold recoverable, whilst the 70% would have been rendered nongravity recoverable due to over grinding. The inhibited gravity recovery trends iron recovery and therefore, can be attributed to un-liberated gold locked in sulphides and gold rendered gravity non-recoverable due to over grinding.
concentrate and the tailings with no significant upgrading. The gold curve can be split into at least two phases, the first one with a high GRI and corresponding to easily recoverable and the second with a low GRI closer in magnitude to the base metal sulphides. The Gravity Release Index for gold decreases with each successive cleaner stage showing that the initial stages recover liberated gold whilst successive runs recover gold associated with sulphides and or gangue. Gold has the highest amenability to gravity concentration followed by iron then sulphur and lastly zinc, as expected. This shows the effect of specific gravity on centrifugal separation, where the highest density particles are more susceptible to gravity concentration. The shape of the gold line shows that there are at least two gold particle species being recovered, based on the two distinct gradients of the curve. From this plot, the relative proportions of the easily recoverable and more difficult to recover species can be quantified. For the flotation tails at Myra Falls, the gold species more amenable to gravity accounts for GRG whilst the less amenable species are gold bearing sulphides. Quantification of relative abundance of the species is useful to decide on whether to use batch or continuous concentrators for recovering the gold. When the gold bearing sulphides are more than the liberated gold species the CVD would be selected. The proportion of species is obtained from the y-intercept of the tangent to the low GRI species. In this case, it is estimated that above 90% of the gold in Myra Falls flotation tailings is associated with sulphides or rendered non-gravity recoverable. Fig. 11 shows the GRI ratio results for gold and sulphides obtained by dividing the gradients of the tangents to Fig. 10 by weight.
3.4. First-order kinetics for Myra falls flotation tails and gravity release index
To characterize the mineral species recoverable by the CVD concentrators, a Knelson CVD6 was integrated to treat the cyclone underflow at Myra Falls. Scanning Electron Microscopy (SEM) analysis was performed to identify the gold species recovered by the CVD. The results are shown in Fig. 12 (Sakuhuni et al., 2015) and Fig. 13. The SEM results show that gold exists as liberated electrum and unliberated and associated with base metal sulphides. This validates the two gold species predicted by the Gravity Release Index. Galena is the main constituent of the gravity concentrate from cyclone underflow because galena particles were still coarse enough to be recovered by gravity. Some of the liberated gold particles exhibit rod like shape, suggesting over grinding.
The weight remaining in tails can be plotted against concentration cycle, time or weight recovery. Fig. 10 is a plot of weight remaining in the tailings for each stage of concentration. A quantitative measure of gravity amenability, the Gravity Release Index (GRI) measured by the slope of the lines, is proposed. The slope measures gravity recovery per mass recovered, which translates to the ease at which the metal species are recovered. Except for gold, the recovery rate is constant, showing that gravity concentration kinetics is first-order. The base metal sulphides curves are straight lines, implying only one species of gravity recoverable particles exists for each base metal sulphides. These species will be the coarser fractions, which are mostly the middlings. The bulk of the sulphides, (above 70%) are just split between the gravity
3.5. Myra falls mineral association
4. Conclusion The Gravity Release Analysis procedure can predict CVD application by determining the maximum release of gold bearing sulphides from a
Fig. 10. Weight remaining vs. gravity concentration cycle as a means of measuring amenability to centrifugal gravity concentration.
Fig. 11. GRI ratio w.r.t gravity concentration cycle (dashed line shows base line).
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Fig. 12. SEM micrograph of Myra Falls gravity concentrate sample showing: Electrum labeled as Au; PbS, FeS2; Gangue grains labeled as Si; and Composite particles containing electrum and sulphides (Sakuhuni et al., 2015).
Fig. 13. EDX mapping of Myra Falls gravity concentrate SEM micrograph in Fig. 11 (Au = gold, Ag = silver, S = sulphur, Pb = lead, Si = silicates).
representative small sample. By providing a measure of maximum gravity recoverable gold bearing sulphides, the procedure also provides a basis for bench-marking operating CVDs. The procedure is anticipated to be applicable for other continuous centrifugal concentrators. It is a useful tool for process design and when applied it can boost application of the technology in mineral processing. Based on this iterative procedure the Gravity Release Index can be determined and used to predict the number of gravity recoverable particle species for gold and how amenable each particle type is to gravity recovery. With additional tests from other operations and calibration, the Gravity Release Index can be the basis of predicting potential applications for continuous centrifugal concentration by CVD devices.
Acknowledgements The authors would like to acknowledge the financial support from NSERC and FLSmidth-Knelson through the NSERC Project CRDPJ 120498809 and the permission to publish this paper.
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
A novel laboratory procedure for predicting continuous centrifugal gravity concentration applications: The gravity release analysis G. Sakuhuni a, N.E. Altun b,⁎, B. Klein a, L. Tong a a b
University of British Columbia, NBK Institute of Mining Engineering, Vancouver, BC, Canada Middle East Technical University, Mining Engineering Department, 06800 Ankara, Turkey
a r t i c l e
i n f o
Article history: Received 20 March 2016 Received in revised form 8 July 2016 Accepted 21 July 2016 Available online 22 July 2016 Keywords: Centrifugal gravity concentration Knelson CVD Scale-up Gravity recoverable gold Gravity release analysis
a b s t r a c t A novel procedure, Gravity Release Analysis (GRA), is introduced for performance prediction of continuous centrifugal concentration, using lab-scale batch tests. Also, linked with the GRA, the Gravity Release Index (GRI) was developed for ranking the ore amenability to centrifugal gravity concentration. Ore samples from the flotation circuit of Myra Falls concentrator were subjected to Multi Pass Test and Gravity Amenability Test for comparison with GRA. Recovery kinetics of batch centrifugal concentration was analysed to establish the Gravity Release Index (GRI). Both Gravity Amenability Test and Multi Pass Test failed to cover the broader mass yield range of continuous centrifugal concentration and this was the main drawback in performance prediction of larger units from lab-scale data. GRA covered a wider mass yield range, yielding better performance prediction of larger continuous units. From the gravity recovery kinetics of the batch unit, varying GRI values for Au, Fe, Zn, Fe and S were obtained, with Au having the highest- GRI and amenability to centrifugal gravity concentration. Varying amenabilities for different Au forms (free Au particles, Au bearing sulphides) to centrifugal gravity concentration could also be distinguished. SEM analysis on recovered gold entities justified the prediction by the GRI. Overall, GRA could effectively predict continuous centrifugal concentration using small amounts of feed with lab-scale batch units. Determination of the GRI for targeted metals would provide further precision for bench-marking and scaling-up of continuous centrifugal concentration. © 2016 Published by Elsevier B.V.
1. Introduction Continuous discharge centrifugal concentrators, introduced in the 1990′s, brought a niche application for gravity concentration which could not be realized with the low mass yield batch or semi-continuous concentrators. A typical use of these units is scavenging Au-associated sulphides from flotation tails. This application increases productivity by reducing gold losses and lowering grinding costs by using a coarser grind size (Klein et al., 2010; Altun et al., 2015). However, full potential of this technology has not been exploited, partly due to the lack of reliable small sample procedures that demonstrate its applicability. Gravity recovery by centrifugal concentration is a probabilistic process dependent on several factors including specific gravity, degree of liberation, particle morphology, particle size and machine parameters. Like floatability, amenability to gravity concentration is not an intrinsic property. The best case of centrifugal separation is what is achievable as separation stages are increased, i.e. for a given ore sample the more the number of batch testing cycles, the sample undergoes the higher the gravity recovery, such that by running an infinite number of batch cycles the maximum gravity recoverability can be determined. Evaluation
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http://dx.doi.org/10.1016/j.minpro.2016.07.004 0301-7516/© 2016 Published by Elsevier B.V.
of ore amenability to concentration precedes application of the technology and availability of reliable techniques capable of determining maximum recoverability is crucial for bench-marking process performance. For determining the amenability of a given ore to gravity concentration sink-and-float test is mostly used. Pratten et al. (1989) argued that a lab procedure to characterize fundamental response from a specific method/device should be based on the same separation mechanism. Thus, if the ore is to be concentrated by heavy media separation then sinkand-float approach will be appropriate, but for centrifugal concentration application, the laboratory procedure should be based on centrifugal concentration. Besides, other limitations of the sink-and-float approach exist, such as the inability to handle fines, toxicity of heavy liquids used and lack of higher specific density heavy liquids suitable for mineral separations. Several laboratory procedures were developed for assessing the ore amenability to centrifugal concentration and to evaluate the process. For testing of centrifugal gravity concentration in lab-scale, using small-batch units is universal, since there are no lab-scale continuous centrifugal concentrators. Batch Knelson MD3 is particularly suited and widely-accepted for characterizing centrifugal gravity concentration (Banisi et al., 1991; Laplante and Shu, 1992; Woodcock and Laplante, 1993; Laplante et al., 2000; Ghaffari, 2004; Clarke, 2005; Xiao, 2008). Generally, the concentration mechanism in a lab-scale
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Nomenclature CVD EDX GAT GRA GRG GRGS GRI MD3 SEM
Knelson Continuous Variable Discharge concentrator Energy Dispersive X-ray Spectroscopy Gravity Amenability Test Gravity Release Analysis Gravity Recoverable Gold Gravity Recoverable Gold bearing sulphides Gravity Release Index Laboratory scale 3-in. Knelson batch concentrator Scanning Electron Microscopy
centrifugal concentrator, such as the Knelson MD3, is similar to that of the continuous units, such as the Knelson Continuous Variable Discharge (CVD) concentrator. In both, particles are separated in a fluidised bowl rotating at high centrifugal force and the main separation mechanism is differential particle sedimentation in the centrifugal force field. The critical difference is, the CVD has pinch valves to discharge the concentrate continuously, while the MD3 has to be stopped to remove the concentrate. Also, past researchers confirmed the reliability of the labscale Knelson unit for predicting gravity recoverable gold (GRG) content (Banisi, 1990; Laplante et al., 1994; Woodcock, 1994; Clarke, 2005). The GRG procedure, which is by far the most widely accepted technique for characterizing ore amenability to centrifugal gravity concentration (Koppalkar, 2009) offers the upper limit of gravity recovery for the ore (Clarke, 2005). However, the cost, labour and feed (40–100 kg) requirement for the GRG test limits its use. A simplified GRG test using less feed (20 kg) and involving a comminution step which targets 80% passing 75 μm, followed by single pass in the MD3 was proposed (Clarke, 2005). However, the simplified GRG test recovered less gold than the standard GRG test and was, therefore, not adequate for characterizing ore amenability. Instead, it provided a better measure of the expected plant performance, as the industrial units are known to recover about 66% of the GRG predicted by the standard test. Another concern is, since the CVD is mainly used for scavenging, the feed to the CVD is already ground. Thus, the size liberation data obtainable for the standard GRG test can only be obtained from recovery by size instead of stage wise comminution. In addition, Subasinghe (2012) argued that GRG is machine dependent, i.e. it is a function of the feed size and the force acting on the particles and, is therefore, different for the batch MD3 and the continuous units. In addition to the GRG method, procedures such as the Multi Pass Test (Ghaffari, 2004) and the Gravity Amenability Test (GAT) are remarkable, but up to date no laboratory technique could be standardized for continuous centrifugal concentrators. This implies that none of the available laboratory procedures could best mimic such applications. The main reason for this is, these techniques cannot adequately characterize samples for the industrial scale, high-mass yield applications, such as those using Knelson CVD's: Lab-scale test procedures were developed for low mass yields (b 1%) which target liberated heavier particles, while continuous centrifugal concentrators recover heavy minerals and middlings. More than half of the commercial CVD units are used for Au-associated sulphides with mass yields N5%. To represent larger tonnage applications in lab-scale, adjusting the feed mass of the batch unit is critical to simulate the mass yield of the continuous units. By decreasing the feed per run for the MD3, thereby increasing the mass yield, it is anticipated that the MD3 performance should simulate the CVD performance. Also, the well-known GRG procedure can distinguish gravity recoverable and non-recoverable gold. However, there is no technique to quantify the extent of liability to gravity concentration from one mineral type to the next. While many minerals are gravity-recoverable, ease of recovery depends on the machine characteristics, density, mineralogy,
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liberation degree, particle size and shape, and no two mineral particles have exactly the same response to gravity. Unlike in flotation, where the rate constant can be quantified based on lab-testing and used in process design, no strategy to quantitatively measure mineral amenability to gravity concentration exists. This work investigates the suitability of the batch Knelson MD3 unit for predicting CVD applications; assesses suitability of existing lab procedures for predicting CVD performance; and aims at developing a new lab procedure for predicting CVD performance. Linked with these, a novel procedure code, Gravity Release Analysis (GRA) was presented. GRA was developed from Dell's flotation release analysis (Dell, 1953), which has become one of the standards in testing flotation efficiency (Dell et al., 1972). Suitability of this procedure for predicting CVD performance was assessed based on determining the maximum release of Au-bearing sulphides, using a relatively small amount of feed. By providing a measure of maximum gravity recoverable Au-bearing sulphides, a basis for bench-marking operating CVDs is provided. The Gravity Release Index (GRI) is also introduced to rank the degree of gravity recoverability for different minerals, based on a quantity similar to the flotation rate constant. This allows identifying the amenability to centrifugal gravity concentration, i.e. ranking the easily recoverable and hard-to-recover particles by gravity. The GRI is based on the GRA, that allows for fractional removal of concentrates, similar to the flotation tests described by Kelsall (1961). 2. Experimental 2.1. Ore samples and characterization Synthetic quartz/magnetite ore mixture and polymetallic ore from an operating mill were used for the test program. For the synthetic ore mixture, sized silica (LM # 20–30 and LM # 70) from Lane Mountain was blended in the ratio 1:3 and mixed with sized magnetite to constitute a 5% magnetite content. The magnetite used was passed through Davis Tube instrument to entirely remove non-magnetic impurities. The silica and magnetite in the synthetic mixture had similar particle size distributions, as presented in Fig. 1. Polymetallic ore samples, collected from Copper rougher tails, zinc concentrate and tailings streams of the flotation circuit at the mill of Myra Falls operation, Victoria, BC, Canada, were subjected to the same test conditions as the synthetic ore. The samples were riffled into one 2 kg sample, one 1 kg sample, two 500 g samples and two 250 g samples for the tests. All assays were done at the International Plasma Lab (IPL) in Richmond, British Columbia. For ore characterization, X-ray diffraction of representative ore samples with phase quantification using the Rietveld Topas 4.2 program was completed at the Earth, Ocean & Atmospheric Sciences, UBC. Scanning electron microscopy was also used to identify the gold species and
Fig. 1. Particle size distributions of silica and magnetite in the synthetic ore mixture.
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Samples weighing 5, 2, 1 and 0.5 kg were prepared and used in the tests. Each sample was run through the MD3 once (Fig. 2). The tailings and concentrate products were assayed for magnetite content using the Davis tube to separate the magnetite from silica, then by drying and weighing the different fractions and using the results for metallurgical balance. Table 1 shows the MD3 parameters used for the tests in this phase. 2.3. Multi pass test and gravity amenability test
Fig. 2. Procedure of the Single Pass Variable Mass Yield Test.
Table 1 Test conditions for single pass variable mass yield tests. Test parameter
Level
Centrifugal field, g Fluidisation flow rate, l/s Solids feed rate, kg/s
60 0.22 0.0083
gold associations recovered by the centrifugal gravity concentration tests. A Hitachi S3000 N VP-Scanning electron microscopy with EDX was used for detailed elemental analysis and to identify the gold species and gold associations for samples recovered by the centrifugal gravity concentration tests.
2.2. Mass yield test A single pass variable mass yield procedure in Fig. 2 was developed and used to assess the effect of mass yield on lab-scale batch centrifugal gravity concentration, using a Knelson MD3 concentrator. The main motivation in developing this was to design a test procedure which best simulates a typical plant scale CVD application, in which ore passes only once through the concentrator (open circuit) and which is capable of handling high mass yields.
In order to assess the adequacy of available procedures for predicting CVD application, the Multi Pass Test (Fig. 3) and Gravity Amenability Test (GAT, Fig. 4) procedures were applied on Myra Falls copper rougher tails. For both tests a feed sample of 5 kg, obtained representatively from the Myra Falls copper rougher tails stream, were used for testing gravity amenability and the results were compared to CVD6 performance. The amounts of the metals of interest in the copper rougher tails stream are presented in Table 2. 2.4. Gravity release analysis To improve on the deficiencies of existing methodologies for high yield applications, the Gravity Release Analysis (GRA) procedure in Fig. 5 was designed. The procedure consists of a series of rougher-scavenger-cleaner centrifugal gravity tests using the MD3. In this procedure particularly the cleaner stages are less dependent on operator skills compared to panning. One of the objectives of this procedure is giving an indication of how much metal or mineral of value can be recovered by a continuous centrifugal concentrator in a mineral processing circuit. Hence, the procedure was specifically designed to best predict continuous centrifugal concentration and to determine applicability of the CVD technology to recover gold associated with sulphides in a processing stream using small sized samples but it can also be applied to any centrifugal concentration application. As presented in Fig. 5, the starting sample size is 5 kg (similar to previous test procedures used for predicting continuous centrifugal concentration). The sample is rifle split into 1 kg sub samples and each sub sample is run into the MD3 twice, collecting the concentrate after each run to constitute 200 g of rougher & scavenger concentrate. These concentrates are then
Fig. 3. Procedure of the Multi Pass Test (Ghaffari, 2004).
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Fig. 4. Procedure of the Gravity Amenability Test.
combined for each sub sample to constitute 1 kg feed for cleaner stages. This feed is run 4 times into the MD3 for cleaner stages. The concentrate after each cleaner is collected. The concentrates and the final tailings are assayed for the target elements. For the MD3, increasing bowl speed favours recovery and leads to dilution of concentrates by lower density species (Savas, 2016). It also favours recovery of finer particles. Increasing fluidisation flow rate favours concentrate grade, with rejection of fine heavy, and/or coarse lighter particles, thus compromising the recovery. The mass yield affects MD3 recovery and concentrate grade. By varying the feed weight, tests can be designed to either maximise recovery or grade. Thus, both bowl speed and fluidization flow rate can be maintained constant and the mass yield varied in line with each test objective. The laboratory scale results were compared to industrial scale results obtained using full- and pilot-scale CVD units. The industrial scale results were acquired from a rigorous optimisation study on the CVD units, including statistical experimental design and simulation optimisation to generate an operating curve. A more thorough description of this optimisation study is given elsewhere (Sakuhuni, 2014; Sakuhuni et al., 2015).
mass yield of w has been separated from the feed. k is the Gravity Release Index (GRI) which gives a measure of gravity amenability of the tested ore. C ¼ C0 expð−kwÞ
ð1Þ
w ¼ wð0Þ expð−k1 nÞ
ð2Þ
w ¼ wð0Þ expð−k2 tÞ
ð3Þ
w ¼ wð0Þ exp −k3 wp
ð4Þ
Three forms of the equation can be derived based on the GRA procedure. In Eq. (2), n is the number of concentration cycles, k1 gives a gravity concentration rate with unit 1/cycle, w(0) is the feed weight. In Eq. (3), t is the cycle time. It assumes that the feed rate of the MD3 is constant, so the cycle time can be calculated; k2 is the gravity concentration rate (1/min). In Eq. (4), wp is the weight percent of the sample recovered and the rate of gravity concentration (k3) can be calculated which is an indication of the amenability of the ore to gravity concentration.
2.5. Gravity recovery kinetics 3. Results and discussion For batch gravity recovery, a first order process is assumed. Accordingly, for gravity recoverable solid particles at a concentration of C =C0 when mass yield to concentrate is zero, integration leads to Eq. (1). In this equation w is the mass yield, C is the concentration of gravity recoverable mineral remaining in tailings after a concentrate accounting to a
Table 2 Amount of metals of interest in the Myra Falls copper rougher tails stream. Au (g/t)
Ag (g/t)
Cu (%)
Pb (%)
Zn (%)
Fe (%)
0.7
12.4
0.1
0.2
0.7
5.4
3.1. Assessment of suitability of MD3 for CVD performance prediction Fig. 6 presents the effect of increasing mass yield on MD3 performance showing that a recovery maximum exists as mass yield continues to increase. Both the recovery and upgrade ratio level off beyond a mass yield of 10%. Fig. 6 (a) shows particle size effect on recovery as a function of mass yield. At low mass yield batch type applications, the effect of particle size on magnetite recovery was not as significant as compared to coarser fractions. This was attributed to the effect of further upgrading in a fluidized concentrate ring, which allows for consolidated trickling to capture high-density fines. At higher mass
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Fig. 5. Procedure of the Gravity Release Analysis.
yields, though, particle size had significant effect on recovery, such that the coarser the magnetite the higher the recovery (Fig. 6a). The results suggest that high mass yield applications are more sensitive to differential settling, which is a function of both particle size and density. If no ample time is allowed for consolidation of the fines in the particle bed, the effect of particle size on recovery is amplified. These results have implications on CVD applications, which typically operate under high mass
yield and hence, which are more sensitive to feed particle size distribution than batch machines due to the dominance of differential settling as a separation mechanism. These machines may therefore require a closer particle size control for efficient operation, contrary to current practice where these units are mainly operated in scavenging circuits with no close control of feed size distribution. Also, Fig. 6b–d basically show that beyond a mass yield of 10% recovery gradually increases
Fig. 6. Effect of mass yield on MD3 performance: (a) Synthetic ore; Fe3O4 recovery w.r.t particle size (b) Synthetic ore; Fe3O4 recovery vs. upgrade ratio (c) Myra Falls Zn-concentrate; Au recovery vs. upgrade ratio (d) Myra Falls flotation tails; Au recovery vs. upgrade ratio.
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Fig. 7. Comparison of bench scale (GAT and Multi Pass Test) predictions and CVD performance.
whist the upgrade ratio levels off. This trend holds for both synthetic and real ore and was used as the basis of using a mass yield of 10% for the GRA procedure, with the aim of maximizing recovery whilst still upgrading. In addition, considering the challenge of obtaining a representative small sized sample and recovery benefit by increasing mass yield, a mass yield of 10% was considered suitable for designing the rougher stage for any lab scale procedure. It should also be noted that upgrade ratio decreases with increasing mass yield, since it follows the same trend as concentrate grade. 3.2. Assessment of existing laboratory procedures In order to assess the adequacy of existing laboratory procedures for predicting full-scale CVD applications, results obtained from the Multi Pass Test and GAT test were plotted to generate cumulative recovery vs. cumulative grade curves. These were then compared to the CVD operating line obtained through pilot scale testing and optimization at Myra Falls copper rougher tailings stream. Fig. 7 shows the results. It was seen that both bench scale tests, i.e. the Gravity Amenability Test (GAT) and Multi Pass Test do not cover the broader feasible mass yield range typical of CVD applications. The Multi Pass Test better predicted CVD performance than the GAT procedure. Furthermore the bench scale test results overestimate CVD performance and therefore provide an upper limit for CVD performance. The GAT predicts way above CVD performance and the Multi Pass Test, which has cleaning stages, better predicts CVD performance at low mass yield. Although a higher concentrate grade at the same mass yield would have been
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Fig. 9. Grade/Recovery curve for gold in Myra Falls flotation tails.
expected for the Multi Pass Test, results were just the opposite. The lower grades obtained for Multi Pass Tests indicates that some of the gold gets deported to the pan tails due to operator dependent panning procedure. Thus, the approach is unsuitable for characterizing fundamental response of centrifugal gravity concentrators, since it uses a different separation mechanism, as proposed by Pratten et al. (1989) and it is operator dependent. The CVD line exhibited inhibited gravity recovery at high mass yield (recovery above 15%), which was predictable by the Gravity Release Analysis (GRA) procedure (Fig. 8). The un-recoverable gold can either be attributed to un-liberated fine gold associated with sulphides and gangue or gold rendered non-recoverable by over grinding. Results obtained for the GRA procedure for Myra Falls final tailings are shown in Fig. 8. For purposes of comparing GRA results and CVD performance, final cleaner tailings were considered as the 5th concentrate. The results show that the GRA procedure better predicts CVD performance and covers a wider mass yield range. 3.3. Effect of mineralogy on gold recovery A model was developed to study the effect of mineralogy on the Au grade/recovery relationships and presented in Fig. 9. It was assumed that the feed was composed of four types of gold bearing particles; 1) Liberated gold, gravity recoverable 2) Liberated gold, gravity non-recoverable 3) Gold bearing sulphides 4) Gold bearing gangue. Mont Carlo simulation was used to generate weight percentage data of each mineral and gold grade in each mineral fraction and the recovery rate was
Fig. 8. Comparison of GRA and optimum CVD results based on NNREGA optimisation for Au: (a) Upgrade ratio w.r.t mass yield (b) Recovery w.r.t mass yield (Sakuhuni et al., 2015).
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modelled. The shape of the Au grade/recovery curve in Fig. 9 is determined by the assumed model inputs. Model 1 represents recovery of Gravity Recoverable Gold (GRG) for an ore consisting mainly of GRG and gangue. Model 2 represents recovery of gold existing as mineral type A and B; Mineral A has a very high gravity recovery rate and high gold content representing GRG and mineral B represents gravity recoverable gold bearing sulphides (GRGS). Model 3, which best tracks the gold results, shows gold distributed in all four species. Species A represents GRG, B represents GRGS, C represents particles with negligible gold and very low recovery rate which accounts for gold associated with gangue and D represents particles with high gold grade, and very low recovery rate accounting for non-GRG (rendered unrecoverable by gravity due to overgrinding). Selective gold recovery was achieved by gravity concentration on the Myra Falls flotation tailings sample. A high grade gold containing product was obtained in the first gravity concentration cycle. The gold grade decreases greatly after the first cycle due to the recovery of the low-grade mineral fraction. The results show that the gold in Myra Falls tailings is mainly non-GRG and there is very low GRG and significant GRGS. The model results are helpful to understand the bench scale tests. The results indicate that gold lost to the final tailings at Myra Falls is partly recoverable by centrifugal concentration. Based on characterization of the gold by particle size, only 30% of the gold in final tailings is above 38 μm, this could be the gold recoverable, whilst the 70% would have been rendered nongravity recoverable due to over grinding. The inhibited gravity recovery trends iron recovery and therefore, can be attributed to un-liberated gold locked in sulphides and gold rendered gravity non-recoverable due to over grinding.
concentrate and the tailings with no significant upgrading. The gold curve can be split into at least two phases, the first one with a high GRI and corresponding to easily recoverable and the second with a low GRI closer in magnitude to the base metal sulphides. The Gravity Release Index for gold decreases with each successive cleaner stage showing that the initial stages recover liberated gold whilst successive runs recover gold associated with sulphides and or gangue. Gold has the highest amenability to gravity concentration followed by iron then sulphur and lastly zinc, as expected. This shows the effect of specific gravity on centrifugal separation, where the highest density particles are more susceptible to gravity concentration. The shape of the gold line shows that there are at least two gold particle species being recovered, based on the two distinct gradients of the curve. From this plot, the relative proportions of the easily recoverable and more difficult to recover species can be quantified. For the flotation tails at Myra Falls, the gold species more amenable to gravity accounts for GRG whilst the less amenable species are gold bearing sulphides. Quantification of relative abundance of the species is useful to decide on whether to use batch or continuous concentrators for recovering the gold. When the gold bearing sulphides are more than the liberated gold species the CVD would be selected. The proportion of species is obtained from the y-intercept of the tangent to the low GRI species. In this case, it is estimated that above 90% of the gold in Myra Falls flotation tailings is associated with sulphides or rendered non-gravity recoverable. Fig. 11 shows the GRI ratio results for gold and sulphides obtained by dividing the gradients of the tangents to Fig. 10 by weight.
3.4. First-order kinetics for Myra falls flotation tails and gravity release index
To characterize the mineral species recoverable by the CVD concentrators, a Knelson CVD6 was integrated to treat the cyclone underflow at Myra Falls. Scanning Electron Microscopy (SEM) analysis was performed to identify the gold species recovered by the CVD. The results are shown in Fig. 12 (Sakuhuni et al., 2015) and Fig. 13. The SEM results show that gold exists as liberated electrum and unliberated and associated with base metal sulphides. This validates the two gold species predicted by the Gravity Release Index. Galena is the main constituent of the gravity concentrate from cyclone underflow because galena particles were still coarse enough to be recovered by gravity. Some of the liberated gold particles exhibit rod like shape, suggesting over grinding.
The weight remaining in tails can be plotted against concentration cycle, time or weight recovery. Fig. 10 is a plot of weight remaining in the tailings for each stage of concentration. A quantitative measure of gravity amenability, the Gravity Release Index (GRI) measured by the slope of the lines, is proposed. The slope measures gravity recovery per mass recovered, which translates to the ease at which the metal species are recovered. Except for gold, the recovery rate is constant, showing that gravity concentration kinetics is first-order. The base metal sulphides curves are straight lines, implying only one species of gravity recoverable particles exists for each base metal sulphides. These species will be the coarser fractions, which are mostly the middlings. The bulk of the sulphides, (above 70%) are just split between the gravity
3.5. Myra falls mineral association
4. Conclusion The Gravity Release Analysis procedure can predict CVD application by determining the maximum release of gold bearing sulphides from a
Fig. 10. Weight remaining vs. gravity concentration cycle as a means of measuring amenability to centrifugal gravity concentration.
Fig. 11. GRI ratio w.r.t gravity concentration cycle (dashed line shows base line).
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Fig. 12. SEM micrograph of Myra Falls gravity concentrate sample showing: Electrum labeled as Au; PbS, FeS2; Gangue grains labeled as Si; and Composite particles containing electrum and sulphides (Sakuhuni et al., 2015).
Fig. 13. EDX mapping of Myra Falls gravity concentrate SEM micrograph in Fig. 11 (Au = gold, Ag = silver, S = sulphur, Pb = lead, Si = silicates).
representative small sample. By providing a measure of maximum gravity recoverable gold bearing sulphides, the procedure also provides a basis for bench-marking operating CVDs. The procedure is anticipated to be applicable for other continuous centrifugal concentrators. It is a useful tool for process design and when applied it can boost application of the technology in mineral processing. Based on this iterative procedure the Gravity Release Index can be determined and used to predict the number of gravity recoverable particle species for gold and how amenable each particle type is to gravity recovery. With additional tests from other operations and calibration, the Gravity Release Index can be the basis of predicting potential applications for continuous centrifugal concentration by CVD devices.
Acknowledgements The authors would like to acknowledge the financial support from NSERC and FLSmidth-Knelson through the NSERC Project CRDPJ 120498809 and the permission to publish this paper.
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