Recent Gravity Improvements at the Porcupine Joint Venture

Transcription

1 Recent Gravity Improvements at the Porcupine Joint Venture Tony Y. Chong 1, B.Sc., ARSM, Chief Metallurgist, PJV Don Strickland 1, B. A. Sc., Project Metallurgist, PJV J.A. Folinsbee 1, P.Eng., Plant Manager, PJV Ron Millions 1, General Foreman, PJV Michael Fullam 2, P. Eng., Knelson Gravity Solutions Ishwinder Grewal 2, M. A. Sc., P. Eng., Knelson Gravity Solutions 1 Porcupine JV, Timmins, Ontario 2 Knelson Gravity Solutions th Ave Langley, BC V1M 2X5 (604) michaelfullam@knelson.com; ishwindergrewal@knelson.com Keywords: Gravity Recovery, Intensive Leaching

2 ABSTRACT Since milling began in 1912, the Dome mill, now part of the Porcupine Joint Venture (PJV), has been expanded several times, the most recent increasing the milling rate from 3,500 to 11,500 tonnes per day. Dome uses conventional crushing and grinding followed by gravity concentration to recover free milling gold. A cyanidation/cip process recovers the remaining gold in the gravity tails. Over the past few years, continuous gravity improvements have increased the gravity recovery from 20% to nearly 50% of the plant feed gold. This paper describes some of the factors that have contributed to this dramatic increase, and suggests how gravity recovery can be further improved. Gravity circuit availability has been improved to >98% with the implementation of a preventative maintenance program. More Knelson concentrators have been added to the circuit, and in 2002, an Acacia Intensive Leach Reactor was installed to replace the shaking table. In 2003, the entire gravity circuit was audited to evaluate and quantify the circuit performance. The metallurgical and mechanical operating conditions of the gravity circuit are now well understood, and areas for further improvement are well defined. INTRODUCTION The Porcupine Joint Venture was formed in 2002 between Placer Dome Inc. and Kinross Gold Corporation. One of the goals of the JV was to use the milling capacity of the Dome mill to process ore from Placer Dome and Kinross mines in the Timmins Camp. The PJV Dome Mill utilizes gravity concentration, cyanide leaching and carbon in pulp gold circuits to recover gold. Gravity recovery has been used in some form at the Dome Mill for most of its existence, first using conventional jigs, these being replaced in 1993 by two Knelson Concentrators, one for each of the two grinding circuits in the Dome mill. Over the years more concentrators have been added to improve the gravity recovery, most recently in 2002 for a current total of five. Also in 2002, an Acacia Intensive Leach Reactor was added to process the Knelson concentrate, replacing the shaking table. DOME MILL CIRCUIT DESCRIPTION Ore is fed to the Dome mill from three main sources; Dome s open pit and underground mines, and Hoyle Pond Mine. Ore is crushed in 3 stages to produce a product size of 80% passing ½. Primary and secondary crushing is achieved in a 400hp 42 x 65 gyratory and 400 hp 7 standard cone crusher, respectively. The latter feeds a 20 x 24 double deck screen in a closed circuit with a HP700 cone crusher. The screen undersize reports to two 4000 tonne fine ore bins and the oversize is conveyed to a 75 tonne tertiary surge bin feeding the HP700 cone crusher. Minus 1/2 material is fed to a grinding circuit that consists of two parallel grinding lines. Circuit A consists of a 10.5 in diameter x hp rod mill and 13.5 x hp ball mill and circuit B of a single 16 x hp ball mill. Gravity gold is recovered by the use of five Knelson CD-30 Concentrators fed from the cyclone underflow. In December 2002, a ConSep CS6000 Acacia Reactor was commissioned to intensively leach the Knelson concentrate. The Acacia loaded solution has a dedicated electrowinning circuit. Gravity recovery accounts for up to 45% of the recovered gold, depending on ore type.

3 The cyclone overflows reports to a 155 thickener where the slurry density is increased to 55-60% solids. The thickener underflow feeds eight leach tanks in series, which provide about 15 hours residence time. Lime is added to the mill discharge pump boxes, thickener feedwell and leach tank #1 to maintain a ph of 11.5 to 12.0 during cyanide leaching. Cyanide is added to the head of the leach circuit at a concentration of 225 ppm. Oxygen is injected in some of the leach tanks to maintain an oxygen level between 15 and 20 ppm. After leaching, the slurry is pumped to the CIP circuit where solution gold is absorbed by activated carbon contained in the CIP tanks. Loaded carbon is removed from the tanks and stripped. The elution process transfers the gold from carbon into solution. The solution is passed through electrowinning cells where gold attaches itself to a cathode in the form of high-grade sludge. The cell is cleaned by power washing the sludge off the stainless mesh, and the sludge is dried and then refined in an induction furnace. Gold bullion, assaying approximately 80% Au and 15% Ag is cast into bar form and shipped to Johnson Matthey Ltd. for refining. Some of the tailings from CIP report to the backfill plant (when operating) where thickened slurry is returned to the underground opening. Final tails are pumped to the tailings facility. 11,500 TPD 3.5 g/tonne Crushing Ore Sources Dome U/G Dome Pit Hoyle Pond solids process water Grinding Gravity Circuit 47% Au Production Cyanidation/CIP 53% Au Production Refinery Gold Dore 440,000 oz/year Tailings Effluent Treatment Environment Figure 1: Simplified Flowsheet of Dome Mill The Effluent Treatment Plant (ETP) operates each year between May & September, to treat excess water that needs to be discharged to the environment. The ETP uses sulfur dioxide and air to destroy any residual cyanide. Ferric sulfate & lime are added to precipitate metal ions. The treated solution is then pumped to a 105 diameter x 16-9 clarifier where the precipitated sludge

4 is removed from the underflow & pumped to the tailing pond. The clear overflow is sampled before it is discharged to the environment. EDTA & carbon dioxide are added to the overflow to polish the quality of the discharge. GRAVITY CIRCUIT DESCRIPTION Cyclone underflow is split off from each launder to the Knelson CD-30 Concentrators. Two concentrators are installed on A circuit, with three concentrators on B circuit. Each concentrator has a dedicated static screen to remove +6 mm oversize. The aperture of the screen is larger than optimum, but smaller apertures limit the feed to the Knelsons to such an extent that feed rate is severely impacted. Vibrating screens at perhaps 2 mm aperture, sized to allow the maximum rated tonnage to each concentrator would improve gravity circuit performance significantly. The Knelson tails flow by gravity to the cyclone feed pumpbox. Knelson concentrate is pumped to a concentrate storage tank in the refinery. Each Knelson is flushed about every 45minutes. Once per day, the entire stored contents of the concentrate tank is sent to the Acacia Reactor. The Acacia Reactor is a batch intensive leach system that uses a series of process steps to first leach, and then electrowin gold from gravity concentrates. The basic flow sheet is quite simple and involves 8 major process steps. Transfer of concentrate from the concentrate storage tank. Pre-washing of the concentrate to remove slimes. Mixing of leaching reagents. Leaching. Recovery of the pregnant solution and washing of the leach residue. Discharging of the leach residue. Electrowinning of the gold from the pregnant solution. Disposal of electrowinning tail. At the completion of the leach cycle, the solid residues are washed, and then discharged back to the grinding circuit. In this case, all solid residues go back to A circuit. After completion of electrowinning, barren solution goes to the main electrowinning cell for further recovery prior to discharging to the plant barren tank. GRAVITY CIRCUIT AVAILABILITY The gravity circuit currently operates at high availabilities, typically in the range of 98-99%. In the past, there have been times where availability has been poor, dropping below 90% on some occasions. This was mainly the result of a gravity circuit installation that was retrofitted into an existing mill, with the layout of the circuit being less than optimum. Issues that were addressed to bring availability up to more usual standards were as follows: The available headroom between the cyclone underflow launder and the mill discharge pumpbox was limited, and as such, the slope of the Knelson feed and tailings lines was less than optimum for good slurry flow. This caused blockages in the tails lines, and slurry spillage on the drive system of the Knelsons as slurry backed up in the tails launder. Knelson drive belt and bearing life was quite poor compared to industry averages.

5 Understanding of correct maintenance practices was limited, with a wide variety of personnel working on the concentrators. Non-OEM parts were being used in the concentrators that were not specifically designed for them. The feed screens were not a standard design, and therefore parts were not interchangeable between them. Regular machine inspections and routine maintenance was not being carried out. Access to the concentrators and feed screens for routine maintenance was difficult. Fluidisation water pressure was quite variable. A joint preventative maintenance and circuit improvement program was implemented between Knelson and PJV, as follows: Knelson carried out several maintenance-training sessions with PJV operations and maintenance personnel. These sessions included both classroom instruction and hands on maintenance training. Knelson s Manager of Maintenance and Training now carries out annual maintenance inspections. The slope of the Knelson feed and tailings lines was optimised. Where the slope of the feed line was steep and the tailings line shallow, the machine was raised up to allow for better slurry flow characteristics. OEM parts were kept in stock. The feed screens were standardized, and the design modified to include some additional replaceable wear parts. A service platform was installed to all Knelsons for ease of daily inspection and monthly maintenance. A new water supply system dedicated to all the Knelsons that consistently controls water supply pressure. This system further improves utilization as Knelson tripping due to high bowl pressure is significantly reduced. The outcome of the program was that the concentrators were now operating correctly, and increased knowledge of correct maintenance practices kept them that way. The result of this program was improved gravity circuit availability, and a significant increase in overall gravity recovery. THE 50% GRAVITY GOAL When the PJV was formed in 2002, the gold production from the Dome mill increased from 315,000 ounces per year to 440,000 ounces per year. This was mainly due to the higher grades of the Hoyle Pond ore. As the Dome circuit was expected to become CIP limited, a goal of 50% gold recovery by gravity was set to reduce leach feed grades to an acceptable level. The installation of the Acacia Reactor was one of the projects implemented to achieve that goal. The Acacia Reactor was first put on line in December Over the course of the next few months, gravity recovery settled in the low to mid 40% range, an improvement from about 38%, but not as high as expected. The obvious question was why, as typically gravity recovery after the installation of an Acacia has improved substantially at other locations (Watson and Steward, 2002, Lethlean and Smith, 2000).

6 One partial explanation for the lower than expected recovery was the relatively low average electrowinning efficiencies over the first few months of operation. In general, EW efficiency was very high but there were a few process upsets from time to time, such as electrowinning onto already fully loaded cathodes in the EW cell, or low caustic addition. This brought the average EW efficiency for the first few months of operation to about 96%. Even so, this would have only impacted gravity recovery by about 1.5% overall. A program was put in place between PJV and Knelson to evaluate the Acacia recovery, and to characterize the overall gravity circuit performance. Factors affecting the gravity circuit performance that were examined included the following: Acacia leach and electrowin performance Losses due to Knelson concentrate tank design Cyclone performance Knelson Concentrator feed rate ACACIA PERFORMANCE The Acacia Reactor performance from January 2003 to mid April 2003 indicated average leach recovery of 98%. Initially EW efficiency was low at 96%, but this was due to operating issues that were resolved within the first few months. Since that time, EW efficiency is quite consistently around 99%. The Acacia Reactor has automatic sampling of solid residues, loaded solution, and barren solution built into the automation that comes with the system, but at the time of writing, the solid residues sampling valve for the system had not been installed. One challenge with sampling of the residues of intensive cyanidation is how to get a representative sample. Early in the commissioning phase only grab samples were being taken, and as both the size distribution and slurry density of the discharging residue stream changes from start to finish of the discharge cycle, it is very difficult to get a representative sample of the residues. The best one can do is to sample the discharge regularly during the solids discharge step and collect a reasonably large sample. This was done on several occasions, and the samples were then run through a Knelson MD-3 Laboratory Concentrator. The concentrate from the MD-3 is assayed to extinction, and the MD-3 tails are sampled for assay. A mass balance is performed and the feed grade and GRG are back calculated. This is equivalent to assaying the GRG in the sample to extinction. The metallurgical balance for the May 6 th Acacia run are shown in Table 1A and B, below. Table 1A Acacia Recovery for May 6 th Run Heads Residues Loaded Solution Weight (kg) Grade (g/tonne) Distribution (%)

7 Table 1B Electrowinning Efficiency for May 6 th Run Loaded Solution Barren Solution EW Efficiency Weight (kg) Grade (g/tonne) Distribution (%) As a further verification of Acacia recovery, the entire stream of Acacia leach residue was tabled over a two-week period in June 2003, and the table concentrate was weighed and accounted for. Similar to the earlier Acacia residues sampling, a leach extraction of about 98% was found. First Acacia Deslime Characterization Clearly, the leach and electrowin performance of the Acacia were quite acceptable. The next area for study was the transfer of concentrate into the reactor, and characterizing the losses of the deslime step. The transfer of concentrate into the concentrate tank is discussed later in the paper. The Acacia reactor uses a deslime step to remove slimes from the Knelson concentrate prior to leaching. Knelson concentrate by its very nature does not contain large amounts of slimes, as the concentrate is recovered in a fluidized device that inherently removes most of the slimes. The deslime step serves two purposes. Later in the Acacia process, the loaded solution must be separated from the solids. This is accomplished by draining the loaded solution through the solids bed in the reactor, and as such, the presence of large amounts of slimes can slow down the recovery of loaded solution. The second is that eventually the loaded solution will end up in an electrowinning circuit, and the minimization of slimes in electrowinning is required. The deslime step is accomplished by fluidizing the concentrate using process water at a controlled flow rate. Slimes are allowed to overflow the reactor, and go back to the grinding circuit. The size of particle that exits the Acacia is related to its terminal velocity, and as such much finer gold particles will be retained in the reactor than gangue particles due to their higher sg. In general, gold deslime losses have been measured at other sites in the % range. The gold exiting the reactor is in the very finest size classes, generally sub 20 microns, and is the most easily leached gold. In most gravity recovery applications, we are not targeting gold in this size range, although the Acacia has been tested in a Biox application where flocculent addition was used to eliminate the deslime losses entirely (Peacocke, 2003). The deslime stream from the Acacia is directed to the solids discharge pump, as the existing sump was not able to cope with the flowrate. The pump discharge has a tee which could be used for sampling. The flowrate to a collection bucket then must be scaled to reflect the entire stream. The deslime was sampled on several occasions and surprisingly was found to be nearly devoid of gold. This was quite unexpected, and aroused suspicion that there were significant losses of fine gold in the concentrate storage tank overflow, and there was concern that much of the fine gold collected by the Knelsons was not actually getting to the Acacia.

8 Concentrate Storage Tank Losses Since there were virtually no deslime losses in the Acacia, the question was raised as to where the fine gold was going. Some anecdotal evidence was available that there were significant fine gold losses in the overflow of the concentrate storage tank, in particular the high gold grades in the circulating load of A circuit (compared to B), where this stream ended up. The concentrate tank design was such that as the Knelson concentrate was pumped to the concentrate tank, some of the fine gold would overflow back to A circuit. To make matters worse, the discharge of the concentrate to the tank was located very near the overflow. Panned grab samples from the overflow showed significant visible fine gold. Improved Concentrate Storage Tank Design The concentrate storage tank design was improved by raising the height of the tank and installing baffles to force the pumped concentrate away from the overflow. The baffle was installed at a 45 o angle to a distance of 3 feet below the overflow discharge. Fine gold entering the tank was now isolated from the overflow discharge and had to travel upward to exit the concentrate storage tank. Most fine gold particles caught in the up flow water tended to now settle in the tank, rather than exit via the overflow. Second Acacia Deslime Characterization The Acacia was again characterized for deslime losses, and now, as expected, fine gold losses were near the low end of the normal operating range, at 0.27%. It appeared that the fine gold being recovered in the gravity circuit was now in fact making it into the Acacia, and not overflowing the concentrate storage tank. GRINDING CIRCUIT GRAVITY AUDIT It was now apparent that the lower than expected gravity recovery was not the result of incomplete leaching. As this was suspected when the Acacia residue samples were collected, it was decided to also collect samples from the grinding circuit that could give some indication of the gravity performance in the grinding circuit. The grinding and gravity circuit was audited on May 5 th, The goal of the audit was to collect samples to determine the behavior of GRG in the grinding circuit. This data could be used in Knelson Concentrator s mathematical model, called KC Mod*Pro, and the effect of different variables on gravity circuit performance could be determined. KC Mod*Pro was developed in part based on earlier work at McGill University (Laplante, Woodcock and Noaparast, 1995) Sampling Methods The following samples from the grinding circuit were collected: Cyclone overflow from both circuits Cyclone underflow from both circuits

9 Mill discharge from both circuits Knelson concentrate from each of the five concentrators Concentrate tank overflow The cyclone samples were collected by taking cuts from each operating cyclone in the cluster on both circuits. Each sample was processed in the Knelson MD-3 Laboratory Concentrator, and a size-by-size gold analysis was performed on all products. The data from the cyclone sampling was also sent to Dr. Andre Laplante of McGill University for a more thorough analysis of the cyclone performance. Knelson concentrate is difficult to sample at the best of times. Typically, the entire flush volume of the machine should be collected, but this was simply not practical. The Knelson Concentrate is pumped to a concentrate storage tank in the refinery. It was decided to try to get a representative sample of the concentrate by taking regular cuts of the concentrate discharge during the flush cycle for each machine. Each sample was split into 12 size fractions, and each size fraction assayed to extinction. The concentrate tank overflow proved very difficult to sample. This stream drops directly from the tank and flows by gravity to A circuit. As such, any gold recovered in B circuit, which overflowed the concentrate tank, ends up in A circuit. The configuration of the overflow made it difficult to get a sample, and to do so would have required a very large volume, as the slurry density is very low, and the volume to the overflow quite high. Instead, a sample was collected off of a downward tee where heavy material was though to collect. This sample could be considered a rough grab, and not particularly representative, but at least indicative, and could confirm the visual results from panning the overflow. As explained earlier, the Knelson concentrate from each circuit is pumped to the concentrate tank, and this creates a constant overflow, which increases each time a machine flushes. The preferred way of transferring Knelson concentrate to a storage tank is to allow enough freeboard in the tank to accommodate the entire contents of the flush, and then after a settling period, decant enough water off of the top to accommodate the next flush. In fact, work is progress to install an auto-valve and modify the plant PLC program to allow for a decantation step. This will further reduce the probability of losing fine gold particles in the overflow stream. The concentrate tank sample was simply assayed to extinction in its entirety. AUDIT RESULTS Cyclone Characterization The results of the cyclone sampling proved interesting. The PJV generally tries to maximize mill throughput, even if there is a modest reduction in overall recovery. The idea is that lower overall recovery of more tonnage yields more units of gold at the end of the day than does higher recovery of less tonnage. One of the bottlenecks that limit the mill throughput is the capacity of the cyclone feed pumps, and as such, the cyclones are operated at high densities in order to get as much solids through the circuit as possible. This has a detrimental effect on classification. For predicting gravity recovery, we are more interested in the GRG partition curve, rather than

10 the ore partition curve, as the behavior of GRG is different from the ore (Banisi, Laplante, and Marois, 1991, Laplante, 2000). Gold grinds slowly, and reports to cyclone underflow in high proportion at much finer sizes than does the ore. The high gold circulating load yields multiple opportunities for gravity gold recovery, and allows recovery of a good portion of the GRG by treating only a fraction of the circulating load. The cyclone performance, however, can have a large effect on gravity recovery. If the GRG partition curve is lazy, then gold in the finer size classes will have lower residence time in the grinding circuit, and recovery of these size classes will be less than if the GRG partition curve is sharper. The ore, gold and modeled GRG cyclone partition curve for the Dome B Circuit as calculated by Dr. Andre Laplante are shown in Figure 2 (Laplante, 2003, personal communication). The ore partition curve in this case is somewhat lazy, and suffers from the effects of maximizing tonnage to the mill. The GRG partition curve also has lower than optimum sharpness, and this has a detrimental effect on overall gravity recovery. This should not be considered the fault of the cyclone, but more so the effect of operating at densities and throughput that do not maximize the performance of the cyclones, but allow the mill to recover more total ounces of gold at the end of the day. 100 % to U/F Particle Size, µm Solids Au SolCalc. Modelled GRG Knelson Concentrate Characterization Figure 2 B Circuit Cyclone Partition Curve The results of the Knelson concentrate sampling showed large variations in grade between different concentrators, but gold in all size classes, and with approximately the same gold distribution. Average gold distribution for Knelson operating in B circuit was finer than for A circuit. There were likely some sampling errors, as the concentrate sampling method was somewhat rudimentary. Figure 3 shows the average gold distribution of the Knelson concentrate for both A and B circuit. The sampling indicated that less fine gold was being recovered to concentrate than other operating sites. This was not unexpected considering the wide Knelson feed size distribution due to the 6 mm scalping screens, and the low residence time of the fine gold in the grinding circuit due to the cyclone performance.

11 Au Distribution (%) A Circuit Knelsons B Circuit Knelsons Size (microns) Figure 3 Average Gold Distribution of the Knelson Concentrate for A and B Circuit This data confirmed that the concentrators were performing reasonably well, and that the problems lay elsewhere in the circuit. Concentrate Storage Tank Overflow The concentrate tank overflow sample indicated a grade of 1,168 g/tonne, but as no mass balance was attempted, this figure simply indicated that some gold was being lost in the tank overflow. ORE CHARACTERIZATION Gravity Recoverable Gold To be able to model the grinding and gravity circuit, it is first required to know something about the nature of the gold in the ore. A standardized test has been developed to characterize the amount of gold in an ore theoretically recoverable by gravity (Woodcock and Laplante, 1993). A GRG test was performed on Dome ore in 1998 (Laplante, 1998). The ore is highly amenable to gravity recovery. At a grind p80 of 75 micron, 77% of the gold is gravity recoverable. The cumulative GRG distribution of the Dome ore is shown in Figure 4. The GRG liberation is sensitive to grind, however, and the 77% GRG value is not reached in the Dome mill, as the current p80 grind is much coarser than this. The PJV also processes ore from the Hoyle Pond mine, but a GRG test has never been performed on this ore. Prior to the formation of the PJV, the gravity circuit performance at Hoyle Pond was quite variable, with gold gravity recovery at times approaching 80%, with an apparent link to overall feed grade. It is thought that the Hoyle Pond ore is at least as high as Dome, and quite

12 likely higher, dependent on grade. In general, gravity recovery is higher on days where a larger proportion of Hoyle Pond ore is milled. As no data was available for this other ore source, the Dome GRG data was used for modeling purposes. This likely understates the result, but allows at least the relative effect of the factors affecting gravity recovery to be examined. Cumulative GRG (%) Gold Particle Size (microns) Figure 4 GRG Distribution of the Dome Ore MATHEMATICAL MODELLING OF THE GRAVITY CIRCUIT KC Mod*Pro KC Mod*Pro was developed at Knelson to be able to assist clients in the design and operation of gravity circuits. GRG entering a grinding circuit ends up in the gravity concentrate, exits the circuit via the cyclone overflow, or is turned into non-grg as it is ground finer. The model simply calculates the proportion of each. KC Mod*Pro is not limited to batch centrifugal concentrators, it can be used for any gravity process, providing the unit recoveries and cyclone performance are known or can be estimated. KC Mod*Pro is able to model the GRG recovery of Knelson and Falcon Concentrators, standard and In-line Pressure Jigs, shaking tables, flash flotation and contact cells (Grewal, 2001). Knelson uses two versions of KC Mod*Pro, the first is a simple version that does not characterize on a size-by-size basis. The second is a size-by-size version, which models up to 12 size classes independently. The size-by-size version was used for analysis of the gravity circuit at PJV. As the Dome mill has two circuits, each circuit had to be modeled independently, and the results combined to obtain an overall result. Modelling Results The results of the modeling are shown in Table 2. As can be seen, on the day the circuit was sampled it was predicted that 30% of the total gold entering the circuit would be recovered to the Knelson concentrate. In actual fact, the May 5 th mill results showed gravity recovery for that day

13 at 35.2% of total gold. This was a particularly low day for gravity recovery. The amount of Hoyle Pond ore milled that day was about 40% less than typical, so lower gravity recovery was to be expected. As the GRG value of Hoyle Pond ore has never been characterized, but is believed to be somewhat higher than Dome, this difference does not seem unreasonable. Increasing Knelson Feed Rate Table 2- Modeling Results Quite often, the most cost effective way to increase gravity circuit performance is to maximize the tonnage being treated by the gravity circuit. A visual examination of the concentrators as they were operating indicated severe underfeeding of some units. Estimated tonnages to one Knelson were as low as 15 mtph, and another as high as 60 mtph. Some of this was attributable to partly blinded and undersized screens, and part to the geometry of the circuit, as the concentrators are gravity fed. Since the Knelsons were retrofitted into the existing circuit, the installation had to make the best of a less than ideal layout. The Knelson CD-30 concentrator is generally rated at mtph, and has been fed up to 150 mtph on some occasions. In general, the feed rate limitation is not the concentrator itself, but getting the feed to the unit, or the slope and length of the tailings line to prevent feed from backing up in the tails launder of the machine. By simply increasing the feed tonnage to the Knelsons to the maximum rated tonnage of 100 mtph would increase the modelled gravity recovery on May 5 th from 30% to 40%, or a relative improvement of 33%. This, of course, would require both circuit geometry changes as well as the use of larger vibrating screens, and was not considered practical, at least in the short term. Practical Circuit Geometry Changes It was felt it was possible to increase the tonnage to several of the concentrators, and get the feed rate to each unit to 60 tonnes per hour without major circuit changes. Changes on A Circuit included modifying the cyclone underflow launder to allow for increased slurry flow. Several of the cyclones, as well as the launder were extended closer towards the concentrators to increase the slope of the line, whilst still allowing for good slurry flow in the Knelson tails lines. The elevation of the B circuit concentrators were optimized further to allow for a minor increase in flowrate. This increased feed rate would improve the modelled gravity recovery under the conditions of May 5th to a more modest increase, from 30% to 34%, a relative increase of 13%. Improving Cyclone Performance Circuit Gold In Gravity Gold Rec. (g/hour) (g/hour) (%) A Circuit B Circuit Total Improved cyclone performance at PJV was also modeled. As there is a current feasibility study to expand the circuit and improve the grind characteristics, the effect of improved cyclone performance is of particular interest, not only to gravity recovery, but to overall recovery as well. It was possible to model the effect of operating the cyclones at more optimum densities and conditions at current grind, in effect sharpening the curve. The current and improved GRG

14 cyclone partition curves are shown in Figure 5. By improving the cyclone performance, and increasing the tonnage to practical limits, modeled gravity recovery on May 5 th would have increased from 30% to 43% of total gold, for a relative increase of 43%. 100 % to U/F Particle Size, µm Current Cyclone Operation Improved Cyclone Operation Modelling Summary Figure 5 Current and Improved Cyclone Partition Curve for Dome B Circuit The modelling exercise shows the importance of examining the gravity circuit as a whole, not simply focusing on the operation of the concentrators. There are many factors that have an effect on gravity recovery, and each circuit will have a unique set of variables that affects the result. Due to the lack of GRG data on the Hoyle Pond ore, the results are thought to be understated, but the relative importance of one factor versus another can at least be surmised. Summaries of the modeling results are shown in Table 3. Table 3 KC Mod*Pro Mathematical Modelling Results Base Case Rated Tonnage Practical Tonnage Improved Classification and Practical Tonnage Circuit Gold In Gravity Gold Rec. Gravity Gold Rec. Gravity Gold Rec. Gravity Gold Rec. (g/hour) (g/hour) (%) (g/hour) (%) (g/hour) (%) (g/hour) (%) A Circuit B Circuit Total VIBRATING SCREENS The Knelson scalping screens in the Dome gravity circuit were installed at a time when the importance of maximizing feed rate was not as well understood as it is today. Generally, the goal with scalping screens is to provide quality tons to the Knelson Concentrator but also allow maximum feed rates. Increasing the aperture size of the feed screens will have a detrimental effect on single pass Knelson recovery, but will allow more tonnage to get to the Knelson. The effect of

15 this has never been fully quantified. Thus, the Knelson scalping screen often limits feed to the Knelson to the extent that overall gravity recovery is impacted, and there can often be a benefit to installing a larger aperture screen deck to increase the feed rate. This tug of war between feed rate and reduced stage recovery is usually, but not always, won by feed rate. Obviously, one area for further improvement would be the installation of vibrating screens with an aperture sized just larger than the largest GRG. This would have significant capital cost, and the benefit may not be worthwhile, but could be an area for further study. BEYOND 50% GRAVITY RECOVERY Figure 6 shows the gravity recovery (monthly and 3 month rolling average) at the PJV since the beginning of 1998 to the end of August The addition of more Knelson Concentrators over the last several years, the recent improvement to the circuit with the addition of the Acacia, and the optimisation due to the gravity circuit audit have pushed gravity near the 50% mark. Had the gravity circuit still only comprised the original two Knelson Concentrators and the shaking table, gravity recovery would still be in the low 20 s. More importantly, the increased gravity recovery appears to be having a significant effect on overall recovery, with year to date mill recoveries approximately 1% higher, although other projects within the mill have contributed to this as well. Gravity Recovery and 3 Month Rolling Average Gravity Recovery (% of total gold) Jan-98 Apr-98 Jul-98 Oct-98 Jan-99 Apr-99 Jul-99 Oct-99 Jan-00 Apr-00 Jul-00 Oct-00 Jan-01 Apr-01 Jul-01 Oct-01 Jan-02 Apr-02 Jul-02 Oct-02 Jan-03 Apr-03 Jul-03 Figure 6 Gravity Recovery at the PJV January 1998-August 2003 The improvements in gravity recovery depended on co-operation between the client and the supplier, with each contributing to the overall result. As the PJV considers yet another expansion, bringing the possibility of improved classification, as well as additional feed rate to the gravity circuit, the bar may yet be raised again. ACKNOWLEDGEMENTS The Authors would like to thank Dr. Andre Laplante of McGill University for his work in characterizing the cyclone performance, and Mark Melanson of the PJV who carried out much of the circuit sampling work.

16 REFERENCES Grewal, I., Effect of Various Variables on Gravity Gold Recovery in Grinding Circuits Results From Mathematical Modelling Grewal, Ishwinder, Knelson website. Laplante, A. R., 2003, personal communication Laplante, A.R., Report on the Characterization of Gravity Recoverable Gold in a Sample of Ore from the Dome Mine, 1998 Laplante, A.R., Ten Do s and Don ts of Gold Gravity Recovery, Randol Gold Silver Forum Randol International Limited. Laplante, A.R., F. Woodcock and M. Noaparast, Predicting Gravity Separation Gold Recoveries, Minerals and Metallurgical Processing, May 1995, pp Lethlean, W and Smith, L, Leaching of Gravity Concentrates Using The ACACIA Reactor, Randol Gold Silver Forum Randol International Limited. Peacocke, K., 2003, personal communication Watson, Barrie, and Steward, Grant, 2002, Gravity Leaching, The Acacia Reactor, Woodcock, F. and Laplante, A Laboratory Method for Determining the Amount of Gravity Recoverable Gold, Randol Gold Forum, Beaver Creek, September 1993, pp