The Use of AuRIX 100 Resin and Gekko Systems Technology for the Recovery of Gold. M.J.Virnig -Cognis Corporation Jurgen Picardo- Cognis Corporation

Transcription

1 The Use of AuRIX 1 Resin and Gekko Systems Technology for the Recovery of Gold M.J.Virnig -Cognis Corporation Jurgen Picardo- Cognis Corporation J.M.W. Mackenzie- Consultant Nick Katsikaros- Gekko Systems Pty Ltd Sandy Gray- Gekko Systems Pty Ltd 1

2 1. Introduction Both Cognis ion exchange technology and Gekko gravity and leaching technology for the recovery of gold are specialized systems which suit certain types of gold orebodies. There are some cases where synergy exists between the Gekko and Cognis technologies. These have already been discussed in a paper by Gray et al (1) In this paper the application of these technologies will be discussed, both individually and in combination to develop novel flow sheets for certain gold orebodies. 2. Cognis AuRIX 1 Ion Exchange Technology NR RNH HNR R = H, Alkyl, Resin Figure 1. General chemical structure of LIX 79 and AuRIX resin AuRIX 1 is a cross linked polystyrene resin functionalized with a guanidine group. The chemistry of the protonation and de protonation of AuRIX 1 resin and the concomitant extraction and elution of anionic species has been described in a number of papers ( 2 ) (3 ) ( 4 ). Loading RG (org) + H 2 O RGH 1+ OH 1- (org) RGH 1+ OH 1- (org) + Au(CN) 2 1- RGH 1+ Au(CN) 2 1- (org) + OH 1- (aq) Elution RGH 1+ Au(CN) 2 1- (org) + OH 1- (aq) RG (org) + H 2 O + Au(CN) 2 1- (aq) G = Guanidine, R = Resin, Alkyl. Figure 2. Guanidine chemistry The appropriate Chemistry for the extraction of aurocyanide by AuRIX 1 is shown in Figure 2. 2

3 AuRIX 1 has undergone extensive pilot testing. Two of these pilot tests have been reported at a previous International Gold Symposium in Peru and will be referred to only briefly here. 3. Resin in Solution (RIS) Pilot Plant Trials 3.1 Trial at Anglo American s Isabella Mine- Zimbabwe The plant had 5 extraction stages and one Zadra elution stage. Each stage contained 12 kg (wet) or 6 Kg (dry) resin. The upflow contactors were.8 metre diameter. The trial results are summarized below: PLS upflow at 6.9 m 3 /hr PLS grade 1.13 g/t Au Duration of trial 137 days This trial Number demonstrated of elutions the robustness 36 of the technology and the stability of the AuRIX resin. Gold recovery 6-8% which was equivalent to a parallel CIP circuit treating the same PLS RIS Pilot Total Trial gold in a Gold produced Mine 17 in Mexico kgs Average gold on resin 99 g/t Equipment Minimum Description supervision and Test was Parameters. required 3.2 Trial at a Mexican Gold Mine The extraction circuit consisted of five 6.35 cm. diameter by 117 cm tall columns arranged for downflow mode of operation (See Figure 3). Each column was filled with 1.1 L of wet resin. Wedge-wire screens at the top and bottom of the columns held the resin in place. The pilot plant was operated in a carousel mode and the lead column was manually taken out of the extraction circuit and placed in the elution circuit. The resin remained in the column at all times. PLS solution was taken from a tap in the main PLS line to the CIS plant. A paddle wheel type digital flowmeter / totalizer was used to measure and record flowrate, prior to entering the extraction circuit. Extraction was carried out at 7 BV/hr (1.3 L/min) in downflow flow mode. The elution circuit consisted of the resin column, electrowinning cell, recirculation tank, and diaphragm pump in a closed loop. The eluate was maintained at 6 o C. Elution was at 3 BV/hr. The polypropylene electrowinning cell contained 4 stainless steel cathodes and 4 stainless steel anodes. Operating voltage was and the current was 25-3 amps.. 3

4 immersion heater Eluate Recirculation Tank rectifier Diaphram pump sample valve TC deg C Electrowinning Cell eluate column EXTRACTION valve Flowmeter Totalizer PLS Line to CIS Plant Elution Circuit Raffinate to Pond Figure 3. Pilot Plant Layout Extraction Test Extraction was carried out in downflow mode at 7 BV/hr. After a break in period, the lead column was taken to elution every 24 hours, and the eluted column placed in the last position. Profiles taken at the 24 hour mark are shown in Figure 4 for a five column operation. As indicated, average solution concentrations, in ppm Au, were.319 (PLS),.224 (col#1),.125 (col#2),.44 (col#3),.18 (col#4), and.5 (col #5) Extraction Column Profiles after 24hr 5 column extraction mg/l Au in solution Days 1 7days 13 PLS Col #1 Col #2 Col #3 Col #4 Col #5 Figure 4. Extraction column profiles after 24 hours period of operation 4

5 Extraction profiles at the 24 hr period prior to changing the lead column out to elution for a four column system are shown in Figure 4. Averaged values for Au solution concentrations, in ppm, were.35 (PLS),.255 (col #1),.154 (col#2),.7 (col#3), and.25 (col#4). Elution Testwork Initially, all resin elution was performed with an eluate composition of 4 g/l NaOH, 7 g/l sodium benzoate, and 1 ppm free CN. Total eluate volume in the circuit was 64 liters. Sodium benzoate was found in bench scale testing to accelerate elution kinetics. Elution was carried out at 6 o C for 6 hours. At termination, the difference between the gold concentration in the eluate entering and exiting the column was less than 1 ppm Au. Elution profiles (eluate exiting the column) typical of the sodium benzoate containing eluate are shown in Figure 5. Loaded and eluted resin analysis for a sample using this eluate is given in Table 1. Rich Eluate Au (ppm) time (min) Figure 5. Rich Eluate profiles versus time Sample Au mg/kg of dry resin Ag mg/kg of dry resin loaded eluted Table 1. Loaded and eluted resin analysis 5

6 Elutions were also carried out without sodium benzoate. Figure 6 shows the effect of sodium benzoate in altering the elution profile. At the end of 6 hours, the same differential between the eluate entering and exiting the column (less than 1 ppm Au) was obtained. Therefore, sodium benzoate was not a necessary addition for elution. Elution Comparison ppm Au time, minutes 2/3/2 typical w/ benzoate 3/3/2 no benzoate Figure 6. Rich Eluate Profile Comparison Dore analysis Dore analysis indicated that selectivity of the AuRIX resin for gold and silver over copper (the major base metal present) was favorable compared to the production plant using CIS at the mine in Mexico. During this campaign, the average head grade was.326 ppm Au. A total of 16.1 gm of dore was recovered. Approximately 5, L of solution was processed in this campaign, with each column undergoing 5 cycles of extraction-elution. The dore produced by the AuRIX 1 resin analyzed: 9.1 % Au, 9.37 % Ag, and.62% Cu compared to 68.7% Au, 25.9% Ag, and 5.5% Cu produced in the CIS production plant. 4. Capital and Operating Cost Comparison of AuRIX 1 RIS and CIS Plants Treating Heap Leach Solutions Case Study Parameters Based on the success of the Isabella Mine and the Mexican pilot plant trials, it was decided to make an economic comparison of CIS and RIS for the recovery of gold from heap leach solutions. The engineering study used in this work was carried out by Lycopodium Pty. Ltd, Perth Western Australia. (5). In order to make an economic evaluation between these two technologies, two operating conditions representing the extremes in solution flow and grade generally experienced in heap leach operations (See Table 2) were selected as the basis for the CIS and the RIS circuit designs. Case Circuit T PLS G d PLS Fl Annual Au P d ti 6

7 Type Grade (mg Au/L) Flow (m 3 /hr) Gold Symposium Lima, Peru, May 24 Production (oz/yr) 1 CIS.5 4 5, 2 CIS , 3 RIS.5 4 5, 4 RIS , Table 2. Circuits Selected for Comparison The parameters used in the RIS circuit designs have been based on the results of preliminary resin testwork and knowledge of conventional ion exchange circuits. The parameters used in the CIS circuit designs have been based on industry standard values and CIS plant operating data. For the RIS circuit design the following parameters were measured in the laboratory: Resin hydraulics, Resin extraction isotherm Resin extraction kinetics. Resin elution kinetics 4.1 Resin Bed Hydraulics The low bulk density of the resin and the regular spherical shape results in a low specific fluidization velocity limiting the upflow flowrate that can be passed through a resin contactor. Initial bed fluidization occurred at an approximate specific upflow velocity of 3.5 m 3 /m 2 hr. Specific upflow flowrates up to 2 m 3 /m 2 /hr can be used if sufficient provision is made in the contactor design to allow for the resulting bed expansion to prevent resin carryover. In a downflow configuration, the pressure drop across the resin bed was very low (< 1.5 kpa/m at 1 BV/hr) allowing the circuit to be designed for a high specific downflow flowrate with a low pressure drop and minimal bed compaction. Based on these results, the preliminary RIS circuit design incorporates downflow for extraction and upflow for elution. The downflow configuration for extraction allows the RIS circuit to operate over a wide range of flowrates without the concern over bed expansion limitations. The upflow configuration for elution will permit the flushing of any trapped particulate material from the resin column and partially expand the bed to prevent compaction and potential short-circuiting. 4.2 Freundlich Extraction Isotherm Measured volumes of high grade (2 ml) and low grade (1, ml) pregnant leach solutions (PLS) were contacted with five different masses of pre-conditioned resin for a period of 48 hours to ensure equilibrium was attained. The resin had been preconditioned with dilute caustic solution and pre-loaded to ~6 g Au/t dry resin to 7

8 simulate an anticipated plant barren resin. The residual solutions were then assayed for metal values and free cyanide. For the Mexico mine leach solution, 4 resin masses were contacted each with 19. L of solution pumped through a small column for 4 hours. Residual solutions were assayed for metal values. The resultant resin and solution data was then used to determine the equilibrium isotherm, which can be described by the following Freundlich equation: [Au] Resin = a [Au] Soln b The Freundlich constants and loading capacity of the resin for the low grade, high grade and Mexico mine heap leach solution are summarized in Table 3. Data Mexico Mine solution (.42 mg Au/L) Low Grade (1.56 mg Au/L) High Grade (7.82 mg Au/L) Freundlich Constants Loading Capacity a b (gau/t) Dry Resin 1, ,364 2, ,97 2, ,55 Table 3. Summary of Freundlich Constants The AuRIX resin has very good loading characteristics at low gold solution grades (< 1. mg/l Au) resulting in high loaded resin grades (up to 7, g Au/dry tonne of resin). 4.3 Loading Kinetics The high grade and low grade PLS were contacted with a measured mass of preconditioned resin in bottle rolls. The solutions were sampled at.5, 1, 2, 4, 8 and 12 hours and assayed for Au, Ag, Cu and Zn. Loading kinetics were also done on feed solution from the heap leach operation in Mexico. The data was then plotted as; ln [Au] Resin versus ln t, resulting in a straight line from which the Nicol-Fleming rate constant, k, and the equilibrium loading factor, n, could be derived (Nicol et al, 1984). The Nicol-Fleming rate equation is commonly used to model carbon circuits. It is: [Au] Resin = k[au] Soln t n where [Au] Resin is the concentration of gold on the resin in g/t and [Au] Soln is the time weighted average gold concentration in solution in mg/l. The Nicol-Fleming rate constants for the resin at the two solution grades, actual measured rate constants for carbon derived from plant data and the typical values used in the design of CIS circuits are shown in Table 4. 8

9 Case k (h -1 ) n 1.2 g resin /L (high grade) resin/l (low grade) resin/l (Mexico Mine) Fairview CIP Data Activated C Design Table 4. Nicol-Fleming Rate Constants The low n values for the two resin bench scale solution tests indicate that the adsorption rate of gold onto the resin was limited by the equilibrium loading capacity of the resin (i.e. insufficient resin was used in the kinetic tests). Without consideration of any effect that the equilibrium loading capacity may have had on extraction kinetics of the resin, the results indicate that AuRIX has significantly faster extraction kinetics than activated carbon as shown in Figure g Au/t Mexico Mine Carbon Data Carbon Design Low grade (1.56 mg Au/L) t hr Figure 7. Resin versus Carbon Rate Curves 4.4 Resin Elution Loaded resin samples from the continuous RIS testwork were combined, placed in three Perspex columns and used for the elution testwork. The barren eluant used in the elution testwork contained 4 g/l NaOH, 7 g/l sodium benzoate and 1 ppm NaCN spiked to 1 mg Au/L as aurocyanide. The barren eluant was heated to 6 o C and pumped in an upflow direction through each column at target flowrates of 5, 1, and 2 BV/hr. The results are summarized in Figure 8. The test results indicated the following: High pregnant eluant tenors (< 4 mg Au/L) were observed at the lowest flowrate (5 BV/hr) during the initial 3 minutes of elution. No significant difference in elution rate was observed between 1 BV/hr and 2 BV/hr flow rates The elution results show that eluted resin values of below 4 gau/t can be obtained in 12 hours and below 15 g Au/t in 24 hours of elution. For a 9

10 . carousal mode RIS plant operating with a 24 hour cycle there would be little advantage in restricting the elution time to 12 hours Resin Grade (mg/l Au) 9, 8, 7, 6, 5, 4, 3, 2, 1, Elution Time (hours) 5 BV/hr 1 BV/hr 2 BV/hr Figure 8 Resin Elution Rate 4.5 Resin Fouling/Poisoning and Mercury Issues. Resin fouling relates to the susceptibility of the resin to load or foul with inorganic salts (calcite or gypsum) or organic compounds (humic acid, diesel) that result in an apparent loss of loading capacity and extraction kinetics. In all laboratory and pilot testwork to date no evidence has been seen that the resin is susceptible to fouling of any type. The potential for fouling of AuRIX resin by organic compounds is expected to be significantly less than for carbon. Humic acids and other organic anions may load but will strip along with the aurocyanide in elution. For the purposes of this evaluation, an acid wash step was included in the resin elution cycle to address any potential fouling by inorganic salts. During the actual pilot plant experience, however, no requirement for acid washing was observed. Metal cyanide complexes such as Co(CN) 3-6, Fe(CN) 3-6, and Cu(CN) 2-3 can potentially bind to strong base ion exchange resins so strongly that they can not be removed resulting in poisoning of the resin. Data from the Isabella pilot plant trial indicates that the base metal cyanides will load to some equilibrium value but also are eluted. During the first 93 days of the trial (23 elution cycles), no increase in the level of base metals on the eluted resin was observed. Thiocyanate can also poison strong base resins. With AuRIX resin, the thiocyanate is eluted along with the gold. - Mercury cyanide anionic complexes such as Hg(CN) 3,Hg(CN) 2-4 will be extracted by AuRIX 1 and will elute with the gold. Because AuRIX 1, unlike carbon, does not require thermal reactivation no mercury fume problems will be encountered when using AuRIX 1. The neutral complex Hg(CN) 2 is unlikely to be extracted by AuRIX 1. While the co extraction of mercury with gold is a disadvantage, it does open the 1

11 possibility for the use of AuRIX 1 to clean up the washing solutions used in the remediation of depleted heaps which contain mercury as cyanide solution (2). 5. AuRIX 1 RIS Versus CIS Cost Study The key CIS and RIS design criteria used as a basis for the evaluation are summarized in Table 5. No allowance has been made for: Mining capital. Heap leach pad and pond construction. Power and water supply. Royalties and license fees. Owner s costs. Parameters Units Carbon-in-Solution Circuit Resin-in-Solution Circuit Case 1 Low Grade Case 2 High Grade Case 3 Low Grade Case 4 Low Grade PLS Flowrate Nominal m 3 /h Design m 3 /h PLS Grade Gold mg Au/L Silver mg Ag/L PLS Flow Direction Upflow Upflow Downflow Downflow Volumetric Flowrate BV/h Specific Velocity m 3 /m 2 h Number of Contactors No Design Gold Loading g Au/t 1,25 5, 4, 1, Elution Type Pressure Zadra Pressure Zadra Atmospheric Atmospheric Elution Batch Size dry tonnes wet tonnes Elution Frequency days Eluate Flow Direction Upflow Upflow Upflow Upflow Elution Cycle Time Hrs Number of Electrowinning Cells No Configuration of Cells Parallel Parallel Parallel Series Size of Cells mm 8 x 8 8 x 8 8 x 8 8 x 8 Number of Cathodes Table 5 Capital Cost Estimate Parameters 11

12 5.1 Capital Cost Estimate Capital costs were estimated for the adsorption, elution, gold room, regeneration, and reagent/services areas. The capital cost estimate for each circuit is summarized in Table.6 Cost Center Direct Costs Adsorption Elution/Gold room Regeneration Reagent/Services CIS Circuits Low US$ ('s) High US$ ('s) RIS Circuits Low US$ ('s) High US$ ('s) Total Direct 1,627 1, Indirect Costs EPCM Preproduction First Fill 1 Working Capital Spares Insurances Total Indirect , Contingency Grand Total 2,665 1,829 2,546 1,68 Note: "Low" refers to low grade PLS (.5 mg Au/L at 4 m 3 /hr) and "High" refers to high grade PLS (2. mg Au/L at 1 m 3 /hr). 1 Resin Cost =$3./wet Kg, Carbon Cost = $2.29/Kg. Table 6 Capital Cost Estimate Summary 5.2 Operating Cost Estimate Operating costs were estimated for the adsorption, elution, gold room, regeneration, and reagent/services area in terms of consumables, maintenance, labor, and power. The operating cost estimate for each circuit is summarized in Table 7 both as an overall operating cost and as the total operating cost per ounce of gold produced. 12

13 Low US$/a CIS Circuits High US$/a Low US$/a RIS Circuits High US$/a Consumables 33,926 96,848 55,456 26,78 Maintenance 129,24 85,93 73,82 51,59 Labor 343, ,125 29,625 29,625 Power 173,467 82,19 156,148 74,331 Total 976,758 67, ,49 443,255 US$/oz A Table 7 Operating Cost Summary While only at a preliminary level, the testwork was sufficiently detailed to provide a basis for the (± 25%) capital and operating costs estimates. The capital and operating cost estimates are summarized in Table 8 Case Plant PLS Grade Annual Production oz Au/a 5, 5, 5, 5, Capital Cost US$, s Operating Cost US$/oz Au CIS CIS RIS RIS Low High Low High 2,665 1,829 2,546 1, Table 8 Summary of Capital and Operating Costs Both the capital and operating cost estimates indicate a savings for a RIS circuit using AuRIX 1 (~ 4% to 12% in capital and 27% to 4% in operating costs) over conventional CIS circuits. In addition to the savings in capital and operating costs an RIS plant using AuRIX 1 offers significant advantages in terms of security. The resin would not be moved from the extraction and elution columns and there would be no access to the loaded resin. The only part of the RIS plant which would require special security would be the gold room. For installation and operation in remote areas where skilled operators are in short supply, an AuRIX 1 RIS plant will offer advantages of simple construction and operation. The plant could be constructed offshore and transported to site in a modular form. Once assembled, the main operating activities, excluding maintenance, would be restricted to switching of valves to operate the carousel and management of the gold room. For small orebodies, the plant could be readily disassembled and transported to a new location when the original heap was exhausted. 13

14 6. Gekko Systems InLine Leach Reactor and AuRIX 1. Gold Symposium Lima, Peru, May 24 The combination of the InLine Leach Reactor (ILR) and Aurix Resin offers some unique design advantages and synergies. In particular the intensification and modularization which Gekko Systems InLine Leach Reactor brings to gold processing is matched by the compactness and simplicity of the Aurix systems for loading and stripping. Furthermore Aurix provides a unique process route for treating gold concentrates which are difficult to treat for a number of reasons, including high copper and the presence of preg-robbers and carbon fouling agents. In a gravity only or gravity flotation plant Aurix improves the economics of treating the resulting medium grade concentrates by increasing and simplifying gold recovery from the pregnant solution without the complication and expense of a dedicated carbon stripping plant. 6.1 Treating High Grade Gold Copper Concentrates Brown s Creek Recovery of gold bullion from gravity or flotation concentrates produced in a copper concentrator is very attractive from both a cash flow and total gold revenue perspective. The vagaries of sampling a gravity concentrate containing free gold also lead to substantial uncertainties in correct payment terms for contained gold. A process developed by Gekko Systems and Hargraves Resources for the Brown s Creek copper concentrator is described in a paper by Gray et al. in which the economics were found to be very favorable. Unfortunately the mine has since been shut down due to flooding. The process involves intensive cyanidation under controlled conditions followed by gold recovery onto Aurix 1. The resin selectively recovers gold from the mixed gold copper leach solution. The loaded resin is stripped with caustic solution in a single step to provide a solution for electrowinning. From a concentrate containing about 15 ppm gold and 6% copper, over 95% of the total gold can be recovered into an 8% gold bar. Process Development A number of processes were investigated before the final flowsheet shown in figure 11 was decided on. Testwork was undertaken to prove leaching and demonstrate absorption and resin stripping. It was found that by using lead nitrate and LeachWell, a proprietary leach accelerant, as well as oxygen, gold recovery was accelerated and copper recovery reduced as shown in figure 9. Resin absorption kinetics and selectivity were demonstrated as shown in figure 1. It should be noted that very high levels of both gold and copper were present in the pregnant solution. The selectivity of gold over copper remained high with 95% gold and 5% copper being absorbed. In the worse case where all the loaded gold and copper is recovered into the final product this will result in a gold bar of 85% fineness. Aurix improves the electrowinning efficiency of the system in two ways. Firstly it reduces the heavy metal content and secondly it isolates any negative effects due to LeachWell which will reduce electrowinning efficiency if present in the cell. 14

15 1 % recovery Au leaching Cu leaching time (hours) Figure 9 Gold and Copper Leaching Kinetics 12 metal in solution (mg/l) Au Cu time (hours) Figure 1 Gold and Copper Absorption Kinetics and Selectivity. PLANT DESIGN The proposed plant flowsheet is shown in figure 11. The design is for an InLine Leach Reactor, model ILR1, followed by resin columns and a standard electrowinning cell. In the InLine Leach Reactor the concentrate is de-watered, mixed with fresh reagents and recycled stripped liquor from the resin columns, then leached in the rotating drum. 15

16 Leached slurry overflows into the solution recovery section where the solids are recovered and clarified pregnant solution is pumped to the resin columns. The resin columns will be operated in split cycle; one column will load while the next is stripped and electrowon at elevated temperature. Concentrate reagents stripped leach solution feed hopper ILR1 settling cone solid tails solution bleed pregnant solution load (strip) Resin Columns strip (load) overflow solution reactor drum discharge sump to mill circuit to concentrate thickener Copper Precipitation and Detoxification Figure 11. Simplified Process Flowsheet. Electrowinning Economics The capital cost was estimated at about AUD$3, not including the electrowinning cells which are available on site. Operating costs are estimated at about AUD$3.5/oz. Operating costs are dominated by reagents. The following economic analyses are approximate with capital costs and benefits annualised over two years. At Brown s Creek gold recovered in flotation was about 4% of mill feed, however payment terms for the concentrate represent considerable operating and capital costs for the operation. Gold in concentrate attracts a 2% treatment charge and late payment which, at AUD$45/oz, (~USD33/oz) represents about AUD$29, /y treatment costs and AUD$1.8M in operating capital. This is a considerable economic incentive to maximise gravity gold recovery. The breakeven point for the project was an increase in gravity recovery to 6% of mill feed, with no increase in overall plant recovery which was very achievable based on mineralogy and the prevailing tabling practice. Further economic benefit was expected since it was extremely unlikely that a significant increase in gravity recovery could have no effect on overall plant recovery. The most 16

17 likely increase in gold recovery was considered to be between 2% and 7%. A 2% increase in recovery would give a further benefit of AUD7,/year at the then prevailing gold price of USD33/oz. 6.2 Gravity Only and Gravity Flotation Plants Gekko Systems has developed a number of flowsheets to treat high grade and difficult to treat ore bodies using advanced gravity concentration and flotation combined with intensive leaching of the resulting concentrates as discussed in a paper by Gray et al. The intensive leaching of gravity and/or flotation concentrates using high levels of cyanide and oxygen, when used in the proper application, can result in overall gold recoveries comparable to or greater to those seen with traditional whole ore leach circuits. Advantages include: Maximized return to shareholders with a combined reduction in capital and risk High security and less gold room labour Easy installation modular design A reduction in equipment lead time, Smaller plant footprint Reduced cyanide and carbon consumption Lower reagent handling Reduced tailings disposal and environmental costs Lower energy costs Aurix resin adds further synergy to this flowsheet by extending its application to lower grade concentrates, where solution recovery is particularly critical, and difficult to treat concentrates containing copper, heavy metals or carbon fouling material eg kerosene to inhibit preg robbing. RIS Recovery for a Complex Preg Robbing Gravity Flotation Concentrate A current project at Gekko Systems involves the recovery of gold from a highly complex orebody containing gold occurring as both free and sulphide associated gold of varying levels of refractoriness with a carbon rich preg robber. Past metallurgical test work had shown that gravity concentration could yield both a gold product suitable for smelting and gravity gold sulphide concentrate containing up to 8 percent of the gold. Flotation of the gravity tailings recovered a further 18 percent of the gold. Combined gold recoveries were reported to be up to 97 percent. However the recovery of gold from the concentrates by cyanide leaching in conventional CIP or RIP systems varied from 75% to 91% depending on oretype. This was identified as being caused by the presence of preg robbers and worsened by the fine grind sizes required to leach the gold under conventional leach conditions. Further testwork, by others, indicated that recovery could be improved through the use of high intensity gravity separation units, coarser grinds to minimise carbon liberation, kerosene blinding of the preg robbers and the use of the Gekko Systems ILR. The ILR allowed the use of a high effective liquid to solid ratio. So gold is made available for recovery from solution at low gold levels as soon as it is leached. This minimizes the 17

18 time preg robbers are in contact with high grade gold solution. Gold Symposium Lima, Peru, May 24 1% Tabling Recovery Yield Curve 1 Au Leaching 9% 9 8% 8 7% 7 Recovery 6% 5% 4% % leached % 3 2% 2 1% 1 % % 2% 4% 6% 8% 1% Yield time (hours) Fig 12: Gravity recovery mass yield curve Fig 13: Concentrate leach recoveries From the test work results a simplified flow sheet was developed for the plant. This flow sheet incorporates InLine Pressure Jigs, Bowl Centrifugal Concentrators and tabling to produce a high yield gravity concentrate. Gekko Systems continuous ILR1 is then used to leach the concentrate, producing a clarified pregnant solution. The choice of the optimal process for gold recovery from the leach solution is not trivial, and has not been finalised. The solution has significant levels of heavy metals, cyanide and kerosene which affect the performance of carbon, conventional resins, zinc precipitation, direct electro-winning and Aurix. The evaluation of Aurix for this application included a pilot trial of a conventional resin in solution (RIS) system similar to the RIS systems previously piloted for Heap Leach operations. The pilot plant consisted of four columns in a carousel arrangement. The pregnant solution was passed through the four columns in series for 16 hours to load the resin. After 16 hours pregnant solution flow was stopped, the first column removed and stripped, then replaced at the end of the series of columns, becoming the final column in the sequence. Pregnant solution flow then recommenced and continued for a further 16 hours. A total of five loading and four stripping cycles were performed. The aim of the trial was to confirm design parameters for gold absorption and check for any reduction in resin performance due to a build up of silver, which was at high levels in the solution, or other elements. The solution used was derived from a bulk leach of the concentrate to be treated. The resin used was pre loaded to gold levels predicted from earlier test work. This was to accelerate the approach of the system to steady state and therefore minimize the number of cycles required. 18

19 Electro-winning was not practical at the scale of the test so a fresh strip solution was used throughout. The column geometry was designed to emulate the expected full scale resin hydraulics. Therefore the height of the column was as per full scale design and the column area was proportional to the solution flow to give the same superficial flow velocity and bed expansion. The major test parameters and results are summarized in table 9 below. Loading recoveries are the average over the last three absorption cycles. Stripping recoveries are for all four strips and are related back to the first four loading cycles. Column Design Result Summary Loading Design Gold Recovery 97.5 % Loading Aurix per Stage 92.5 ml Final Gold Recovery 98.3 % Stage Diameter 15 mm Gold Initial Grade 13.4 ppm Aurix Rest Height 523 mm Final Silver Recovery 31 % Cylinder Height 147 mm Silver Initial Grade 64.3 ppm Solution Flowrate 28 ml/min Residence Time per Stage 2.6 min Total Residence Time 1.6 min Stripping Load Cycle time 16 hours Total Gold Recovery 97 % Total Test Loading Time 73 hours Average Gold Grade 88 ppm Stripping Total Silver Recovery 18 % Strip Flowrate 4.5 ml/min Average Silver Grade 77 ppm Strip duration 15 hours Table 9 Summary of Pilot Test Design and Results. Gold recovery was high at over 98% and agreed well between the loading and stripping performance. Indicating the steady state was reasonably close to the initial conditions chosen. Silver recovery was lower, as had been expected, and more silver was loaded than stripped indicating the steady state loading of silver on the columns increased through the tests. This was to be expected since no pre-loading with silver was performed. The loading performance of the four column system over time is shown in Figure 14. It is seen that the recoveries of gold are high throughout the test and are gradually improving, indicating the steady state performance was slightly better than predicted in the design calculations. 19

20 Gold Recovery 1% 99% 98% 97% 96% 95% 94% 1% 9% 8% 7% 6% 5% 4% Silver Recovery 93% 92% 91% Gold Silver 3% 2% 1% 9% % Total Test Time (hours) Figure 14 Gold and Silver Recovery in Aurix RIS Pilot Test The silver recoveries can be seen to drop over the duration of the test as the silver loading on the columns built up. The silver recoveries also drop sharply over each loading cycle showing the columns are fully loaded with silver. Analysis of the final loaded resin showed no significant build up of minor elements and no signs of poisoning were observed during the test. There was no indication of silver levels interfering with gold absorption. The results demonstrated Aurix could be used to recover gold from a complex solution at design performance. RIP Recovery for a High Copper Gold Flotation Concentrate In a second project at Gekko Systems the properties of Aurix have been used in the design of a leach plant for the recovery of gold from a medium grade gold concentrate. In this project the aim is to economically recover bullion from a flotation concentrate with a gold grade of about 6ppm and a copper content of about 2%. Currently the concentrate is sold to a refinery and substantial refinery charges are suffered. Leaching tests established high gold recoveries were obtainable at economic cyanide addition rates and moderate levels of copper leaching, however direct electrowinning of the resulting pregnant solution was too slow to be effective and copper was recovered at high levels resulting in a very low gold content bullion. Experimental leach recoveries are shown in figure 15 and electrowinning results in figures 16a and 16b. Note the difference in solution concentrations between the two electrowinning curves. 2

21 % leached time (hours) Figure 15 Leach Kinetics of Gold and Copper Au Cu 14 3 Au in Solution (ppm) Experimental Model Cu in Solution (ppm) Experimental Model Time (min) Figure 16a Gold Electrowinning Time (min) Figure 16b Copper Electrowinning Aurix was recognized as an ideal solution for this application. Initial test work using actual leach solutions confirmed the isotherms, kinetics and selectivity of gold over copper were suitable. These are shown below in figure 17b and figure 17c. It was noted that between the 4 and 24 hour samples the loading of copper on the Aurix reduced as the gold crowded off some of the copper which had initially loaded. This demonstrates the strong selectivity of Aurix for gold under these conditions. Design criteria developed in previous projects were used to determine the contact times and loadings required. Mass balance calculations confirmed the gold to copper ratios would remain within established performance parameters. 21

22 Figure 17a Gold Absorption Isotherms 16 Figure 17b Copper Absorption Isotherms 16 Au Aurix Loading mg/wet L hr; ph hr; ph hr; ph11.5 Cu Aurix Loading mg/wet L hr; ph hr; ph hr; ph Au Solution assay ppm Cu Solution assay ppm It was then recognized that significant further simplification of the process could be achieved by using a Resin in Pump (RIP) absorption system. This greatly simplified solid liquid separation and reduced solution gold losses. To accomplish the RIP contacting Gekko Systems has developed a multistage countercurrent downflow column which allows the direct treatment of fine dilute slurries without the need for clarification. The dilute slurries, under 1% solids, produced by the InLine Leach Reactor are more amenable to being treated in an RIP column than more concentrated slurries. A simplified flowsheet of the process developed to treat this concentrate in shown in figure 18. The concentrate is fed to the ILR where it is added to recycled barren solution and make up reagents. The reacted slurry is then pumped to a safety screen to remove oversize material before being fed to the RIP column. The Barren slurry is withdrawn from the bottom of the column and dewatered in a thickener, where the bulk of of the solution is recycled to the ILR. The solids and a small amount of entrained barren solution is taken to a detoxification circuit prior to disposal. The resin in the contactor is advanced up the column counter current to the slurry until the highly loaded resin at the top of the column is withdrawn and transferred to the stripping circuit where it is stripped at elevated temperature with caustic and the gold simultaneously recovered by electrowinning. The stripped resin is returned to the base of the column for reuse. This flowsheet is currently in the final stages of evaluation for installation at full scale. Gekko Systems believes Aurix presents many opportunities to extend the application of the InLine Leach Reactor while significantly simplifying existing treatment routes. By applying a creative design approach based on solid laboratory results the future of Aurix in gold recovery is extremely promising. 22

23 water Resin Screen existing concentrate filter Safety Screen repulp sump pump leach peroxide leach cyanide ILR Tails Cone Resin Strip Column barren solution tank Fresh Resin Holding Tank InLine Leach Reactor Feed Surge Tank Feed Pump RIP Column Strip Solution Storage detox peroxide final tails Detox Holding Tank 2 off Detox transfer pump Detox Reaction Tank KEY Leach slurry continuous Eduction solution flow batch Resin transfer batch Strip and electrowin flow batch Gold Product batch Figure 18 InLine Leach Reactor, Aurix in Pulp and Detox Circuit to Treat a Low Grade, High Copper Flotation Concentrate Electrowinning gold filtercake 23

24 References. (1) Gray, AH, Katsikaros, N and Fallon, P. Gold Recovery From Copper Gold Gravity Concentrates Using The Inline Leach Reactor And Weak Base Resin. Oretest Copper Gold Symposium, Perth. November 1999 (2) Virnig, M.J., Mackenzie, J.M.W., and Adamson, C. The use of guanidine based extractants for the recovery of gold. Hidden Wealth, Johannesburg. South African Institute of Mining and Metallurgy pp (3) Virnig, M.J. and Mackenzie, J.M.W., Extractants for the Recovery of Gold. III International Gold Symposium Lima Peru (4) Kordosky,G.K.,Kotze,M.H.,Mackenzie,J.M.W., and Virnig,M.J. New solid and liquid extractants for gold. Proceedings XVIII, International Mineral Processing Congress. Sydney 1993, pp (5) Cognis AuRIX 1 Resin for Gold Extraction. Fisher, G.T., Lewis, R.G. Virnig, M.J. Mackenzie,J.M.W., and Davis, M.R. ALTA 2 SX/IX Technical Proceedings, 2. ALTA Melbourne Australia. (6) Gray AH,Abols J, McCallum A, Patrick G, Johansen G., CIP Who Needs It? Canadian Mineral Processor s Conference, Vancouver, January 23 24