Selection Sizing and Developing the Optimum Gravity Gold Circuit for Your Project D Connelly1

ABSTRACT Gravity gold recovery has been practiced for thousands of years, making it one of the oldest forms of . Gold is 19 times heavier than water although the barren rock is generally only 2.7 times heavier. Any potential gold needs to be characterised to assess its amenability to a gravity recoverable gold (GRG) process based on well planned and executed test work. Scale up considerations need to be considered in designing a gravity circuit. The economic impact of a gravity circuit should be justifi ed based on the installed cost and likely benefi ts. Maximising gravity gold recovery results in a faster cash fl ow, lower operating cost per ounce and higher overall gold recoveries compared to the several weeks required to recover gold adsorbed onto carbon. The advent of centrifugal concentrators such as the Knelson and Falcon, coupled with intensive of concentrates (ILR), has revolutionised gravity gold circuit designs. Considerations associated with the sizing of the centrifugal machines and the percentage of the circulating load (bleed) that should be processed is discussed in this paper. The trends with gravity circuit layout, engineering, as well as optimum dumping cycles, are important areas which require close consideration. Key aspects of these are presented in this paper. There are a number of circuit confi gurations to be considered, including the integration of different gravity concentrators. Other gravity process design considerations such as fewer modules, the effect of equipment size and enhancement of process effi ciency using preconcentration, together with factors that affect OPEX and CAPEX are addressed. Practical considerations of maintenance, security from theft, minimisation of risk and water balance considerations are discussed using different equipment and circuit confi gurations. The trend towards using gravity recovery in fl otation plants is also reviewed. A number of examples are cited and good design aspects highlighted.

INTRODUCTION The history of gold recovery is as old as mineral processing Operationally it was learnt that the cleaning cycle frequency itself. Gold is well suited to gravity treatment and early plants time was important, as was keeping the Knelson bowl holes used jigs, strakes, shaking tables and drums. The clean because the fl ow rate of water was more critical rather mineralogy of gold with respect to nature and occurrence, than the water pressure. particularly liberation, dictates the applicability of gravity Two examples of high gravity gold recovery process plants processing. were Callie (approximately 60 per cent) and Bronzewing Johnson drums were phased out (Central Norseman) and (70 to 80 per cent) of the gold recovered to a gravity the Knelson concentrator became the industry standard. This concentrate. These are exceptional high gravity gold has been signifi cantly improved over time and automated recoveries. It is believed a Kelsey Jig was specifi ed for the to be self-emptying. The Dutch State Mines (DSM) screen Wallaby project, one of the few gold applications. aperture ahead of the Knelson and the feed split is important Gravity gold has many advantages which should be seriously in maximising gravity gold recovery. considered in any fl ow sheet. The introduction of centrifugal The Knelson concentrator treated a portion of the cyclone concentrators such as the Knelson, Falcon and Kelsey Jig have underfl ow (20 per cent) stream, however at Bronzewing a revolutionised gravity gold recovery. Gold fl ow sheets usually vibrating screen was used to treat the cyclone underfl ow have gravity as the primary step followed by other downstream prior to feeding the Knelsons. During the 1980s there was a processes such as carbon-in-pulp (CIP). compelling technical argument concerning higher recovery, The processing of concentrates was fi rst carried out using faster cash fl ow, lower operating cost considerations and strakes and amalgam drums, this was discontinued in the CAPEX. Some companies used spirals (Ok Tedi, Granites) and 1970s due to occupational health and safety issues and gravity traps before the Knelson as a common design feature. environmental issues with mercury. Mercury was phased out

1. MAusIMM(CP), Director/Principal Consulting Engineering, Mineral Engineering Technical Services Pty Ltd (METS), PO Box 3211, Perth WA 6832. Email: [email protected]

GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 69 D CONNELLY after payouts of compensation to employees and it is now only and some companies postpone installing to a later stage of used as a laboratory tool. Operators who suffered the effects of project development. mercury poisoning failed to recover their health. The OPEX is rarely a problem because usually this is the Gemini tables were common (Figure 1). However, many lowest cost gold to produce. operators returned to half size Wilfl ey tables, although The process risks with gravity gold are high; a number of some operators use gold wheels. The removal of tramp iron projects have miscalculated in terms of the percentage of continues to be a problem at some operations. gravity gold recovery.

MINERALOGY Mineralogy determines the amenability to gravity processing particularly the particle size, degree of liberation, density differential, particle shape, composition and hydrophobicity. Characterising the feed is the fi rst basic step in determining the most applicable separating equipment and developing the optimum fl ow sheet. Polished sections are useful, but old technology is being replaced by Qemscan. Sink Float analyses is useful because the major problem with gold is to locate suffi cient particles and be confi dent these particles represent gold occurrence. The scanning electron microscope and gold analyses combined with diagnostic leaching are useful in understanding the nature and occurrence of the gold present. Metallic gold is common and the size can vary from colloidal gold through to FIG 1 - Gemini Table (Downer EDI supplied image). nugget gold and as alloys with other metals or within sulfi des. (see Figure 3) The Inline Pressure Jig (Figure 2) has found application with some operations (Big Bell). Heavy media separation (HMS) has been looked at but not applied for some low-grade orebodies. ADVANTAGES OF GRAVITY At New Celebration and Granny Smith gravity concentration In the case of gravity recovered gold, faster cash fl ow, higher of the sulfi des in the CIP stream was employed, overall recovery and lower cost per ounce are all signifi cant including fi ne grinding of the gravity concentrate recovered advantages. Gravity gold recovery is environmentally friendly from the tailings. as no reagents are required. Other benefi ts include reducing Piloting has rarely been used for greenfi eld projects. the CIP feed grade, recovering coarse gold that would Generally companies go from bench scale to full scale but otherwise be slow to leach, improving CIP leach kinetics, piloting has been used where upgrading or retrofi tting was reducing carbon loadings, reducing gold in circuit lock up and considered for an existing operation. reducing cyanide consumption. The trend over the last twenty CAPEX is a major consideration with fully installed gravity years has been to maximise gravity gold recovery for the above circuits adding some two to fi ve per cent additional CAPEX reasons.

FIG 2 - InLine pressure Jig Cross-section (Gekko Systems image).

70 GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 SELECTION SIZING AND DEVELOPING THE OPTIMUM GRAVITY GOLD CIRCUIT FOR YOUR PROJECT

to evaluate various scenarios based on and ore recovery. Test work should target samples widely dispersed throughout the oxide, transition and primary zones. Samples need to be obtained with greater defi nition such as laterite, coffee rock, pisolites, saprolite etc. The impact of recovery with depth would be useful to understand if this can be tested in the primary core. Applying geometallurgical modelling techniques can directly reduce the risks associated with meeting production targets. Geometallurgy has the potential to act on both the consequences and likelihood axes to decrease risk. Areas of key focus include:  concentration of deleterious elements,  hardness,  grindability, FIG 3 - Locked Gold (METS - internal source).  mineral species and ‘mineral grade’, SAMPLE SELECTION AND REPRESENTIVITY  mineral liberation,  metallurgical recovery, Proper sample selection and planned laboratory test work form the basis of process selection based on gravity recoverable  mining recovery, gold (GRG). Interpretation of test results and translating to  drillability, an operable fl ow sheet is critical. Test plans should include  fragmentation, gravity test work on samples selected spatially throughout  reagent consumption, and the orebody to confi rm the range of recoveries encountered  smelter enabling characteristics. (see Figure 4). Composites are good at hiding variability. The There are a number of critical factors that need to be in place samples need designated coordinates so the recovery can be if a geometallurgical approach is to be successful. The fi rst and assigned to a location in the block model and be used in the mine schedule. This data will be used in conjunction with the foremost factor is to breakdown some of the traditional barriers variability data and support the recovery predictions used for between various professional disciplines. The geologists, a particular orebody. Data stored electronically can be used engineers and metallurgists involved in a project must truly want to work together with a shared goal and jointly recognise METALLURGY SAMPLES the value they can each contribute to the project. This approach DDH CROSS SECTION requires geologists, engineers and metallurgists to develop signifi cant understanding of the mining and ore treatment DDH3 DDH2 DDH1 Surface fl ow charts. Technical professionals must become ‘multi-

v1 v2 v3 Oxide lingual’ (capable of speaking and understanding geological, v 4 metallurgical and business language and concepts).

Transition GRAVITY TEST WORK Primary The typical GRG test work methodology proposed by the late Include Dilution Waste Include dilution waste Andre Laplante is to grind a 25 to 100 kg sample through three successively fi ner sizes (P80 850 μm to 75 μm) and between each step passing the tailings reground sample through the Oxide Ore Zone Knelson concentrator to recover gold liberated (Laplante, VariabilitySamples Transition Ore Xone 1 metre sections 2000) (see Figure 5). A single P80 75 micron GRG can also v1, v2, v3 Primary Ore Zone be undertaken but this is not as accurate and does not provide FIG 4 - METS internal representative sampling example. any liberation data.

FIG 5 - GRG Recovery as a function of eff ort.

GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 71 D CONNELLY

It must be also noted that there is a key fl aw in all Knelson gravity recovery tests. The higher mass recovery in a bench- scale test does not refl ect typical plant operation. A batch centrifugal concentrator pulls only a fraction of this amount. Therefore, the actual mass recovery and consequently gold and silver recovery is likely to be signifi cantly lower in the plant than these results indicate. This is not the case with the GRG test work. Intensive leaching of the concentrates is typically carried out and unless there are specifi c issues, very high gold recoveries are usually obtained (see Table 1).

TABLE 1 Intensive leach results (METS internal source).

Gold balance Silver balance (Au g/t) (Ag g/t) FIG 6 - Wilfl ey Shaking Table (METS internal image). Extracted grade 74.8 3 a distribution box along part of the upper edge and spreads Residue grade 0.31 3 out over the table as a result of the shaking action and the Calculated head grade 75.1 6 wash water. Product discharge occurs along the opposite edge and end. These are high maintenance units and achieve high- Assayed head grade N/A N/A grade at the expense of recovery. The presence of galena and Residue analysis 0.30 3 iron sands can complicate achieving clean separations. These shaking tables were superseded to some extent by Gemini 0.32 tables in the 1980s through to the 1990s. The introduction of the Intensive Leach Reactor (ILR) Interpreting gravity recovery results requires a person compared particularly to tabling where gravity was proven skilled in the art and able to relate the results to plant practice. to offer very low recoveries; often well below 70 per cent Consultation with the vendors is also very useful to ensure the and much lower than expected by the minesites operating correct conclusions are being drawn. these devices (see Figure 7). This outcome was true for both installations running IPJs and Knelson Concentrators WATER QUALITY AND MASS BALANCES producing concentrates. The research indicated that tabling Gravity circuits use large volumes of water and usually require was clearly a ‘below optimum’ process. As a result, the concept good quality water. This is a problem in Australia due the of fi nding a production version of a laboratory bottle roll was scarcity and poor quality of water at many locations. The born in the form of the ILR (Figure 7) where all of the gold gold industry needed to fi nd solutions to treating resources in the concentrate was leached. The fi rst ILR prototype was in hyper saline water that cause buffering problems achieving built in 1997 for a trial at Mt McClure Gold Mine, operated satisfactory pH levels (Paddington Process Plant) which may by Australian Resources (see Figure 7). At some sites the result in high hydrogen cyanide (HCN) gas levels above the retrofi tting of the ILR replacing tabling resulted in gravity tanks. This high 200 000 ppm TDS water also affects the gold recovery increases of between 20 per cent to 50 per cent. ability to dissolve cyanide and achieve satisfactory oxygen levels in the pulp. Plants with good quality water suffered FLOW SHEETS AND DESIGN with sulfi de reducing bacteria (SRBs), which caused severe The introduction of centrifugal concentrators was the single corrosion within tanks and necessitated the tanks being lined with epoxy glass linings. largest positive change in gravity fl ow sheets. The use of 50 and up to 200 ‘g’ forces resulted in higher recovery of gold at the Mass balances around the grinding circuit can be carried out fi ner sizes. This was followed by the change from a bleed of the using Excel spreadsheets or LIMN fl ow sheet software. The use of gravity concentrators introduces water which can be a discharge to a bleed of the cyclone underfl ow. At the same process limitation or require the addition of a thickener for time the machines became larger with a greater percentage of the cyclone overfl ow stream. Where tailings fi ltration is used feed able to be processed. There was also change from batch the water balance can be critical. Vendors have responded manually emptied machines to auto dump machines allowing with centrifugal concentrators that use less water. the concentrate to fl ow by gravity through to a hopper in the The poor quality water necessitated reverse osmosis plants gold room. At the same time the Falcon concentrator arrived be used for centrifugal concentrator fl uidising water and on the market with a higher ‘g’ force. The latest trend has been stripping water. Some plants also used vacuum distillation to treat cyclone feed rather than cyclone underfl ow because units utilising waste heat from power plants to produce of higher gold recovery. Consideration in the design must potable water from saline sources (Bounty and Paddington include restricting access and minimising the possibility of Process Plants). theft. High re-circulating loads of gold should be avoided as fl at fl akes can be produced in the grinding process and these PROCESSING CONCENTRATES are not amenable to recovery in a centrifugal concentrator. The Wilfl ey shaking table (Figure 6) was used almost Gold does not occur in nature as fl akes. universally for cleanup of gravity concentrates prior to . Shaking tables are gravity concentration devices EQUIPMENT that consist of a riffl ed fl at surface, inclined slightly from the horizontal, shaken with a differential movement in the The equipment one would typically consider in developing a direction of the axis of tilt and washed at right angles to the fl ow sheet is listed below with salient operating considerations direction of motion by a stream of water. Feed enters through (see Figure 8 for other types of equipment).

72 GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 SELECTION SIZING AND DEVELOPING THE OPTIMUM GRAVITY GOLD CIRCUIT FOR YOUR PROJECT

FIG 7 - Intensive Leach Reactor (ILR) (Gekko Systems image).

Solubility

Colour, Appearance Sorting

Density(Heavy Media) Cyclones Cones Drums

Circular Jigs Coal (Coal) Cyclones (Coal) Slimes Tables Wet Tables Coal (Ores) Air Tables (Coal) Density & Size Spirals Cone Bartles-Mozley Bartles Crossbelt Vanner

Froth Flotation Coal Surface Wettability Selective Flocculation

Wet Magnetic Separators Magnetic Susceptibility Wet High Intensity Magnetic Separators Dry Magnetic Separators Cobbing

Electrostatic Electrical Conductivity

1μm10μm100μm1mm 10mm100mm Particle Size FIG 8 - Equipment applicability with size (METS internal example). Knelson Concentrator are universally used for gravity gold recovery in grinding This was the fi rst centrifugal concentrator (50 gs) developed circuits with a capacity of 300 to 1000 t/h. Water use is a in Canada. The spinning motion of high speed centrifugal major consideration (see Figure 10). force against fl uidisation water causes separation of the gold Kelsey Jig particles; it can operate in a batch or semi batch sequence. The The Kelsey Jig is a cylindrical spinning jig that uses ragging main components consist of a riffl ed concentrating cone, drive and pulsation of the bed to effect concentration of heavy motor, water chamber and fl uidisation water unit. These are mineral into the jig. The ragging is supported by a laser cut universally used for gravity gold recovery in grinding circuits screen. The Kelsey Jig has been used for gold recovery but is with a capacity of 300 to 1000 t/h. Water use is a major more commonly applied to mineral sands, tin and tantalum consideration (see Figure 9). (see Figure 11). Falcon concentrator Inline pressure jig These were developed after the Knelson and the main Separation occurs based on relative density as well as difference is the 200 gs developed by the spinning bowl. These particle shape and size. High sg particles are drawn into the

GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 73 D CONNELLY

FIG 9 - Knelson Concentrator cross-section (Knelson image). FIG 10 - Falcon Concentrator cross-section (Falcon Concentrators image).

FIG 11 - Kelsey jig cross-section (Downer EDI mining image). concentrate hutch during the suction stroke. The screen is used for processing gravity gold concentrates. It is preferred pulsed vertically by a hydraulically driven shaft. Length of over tabling, which leads to higher overall gravity gold stroke and speed can be varied as well as screen aperture recovery. AR and Intensive Leach Reactor (ILR) comparison and ragging dimensions. Both concentrate and tailings are is common (see Figure 7). discharged under pressure. Used specifi cally for very coarse gold even ahead of the grinding circuit. Helix Spiral The actual ‘working surface’ of the concentrator is made up of Acacia reactor a series of leads (pronounced ‘leeds’) running lengthwise ‘in Used for processing gravity gold concentrates and is preferred reverse’ in a critical tight spiral pattern along the inner barrel. over tabling which leads to higher overall gravity gold The leads have a 45 degree angle on one side and a 90 degree recovery. AR and ILR comparison is common (see Figure 12). angle on the other and are larger in height and width at the rear of the barrel than at the front. During operation, the decreasing Intensive Leach Reactor height and width of the leads create the classifying effect as Essentially a large ‘bottle roll’ with high cyanide concentration the orepasses through the barrel. A set of angled spray heads and an oxidant to accelerate leach kinetics. The unit is modular continuously ‘wash back’ the lighter material while the and includes a pump and solid liquid separation cone. It is gold and heavy concentrate continue moving forward, falling

74 GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 SELECTION SIZING AND DEVELOPING THE OPTIMUM GRAVITY GOLD CIRCUIT FOR YOUR PROJECT

FIG 12 - ACACIA reactor fl ow sheet (Knelson image).

FIG 13 - Helix Spiral concentrator (Oro Industries image). out of the grooves and into the concentrate holding bin. This FIG 14 - Low G Horizontal Centrifuge (METS internal image). is a continuous process allowing the gold concentrate to be recovered. For larger volume processing, multiple Spirals can gravity gold recovery, but this is only available to sponsors. be installed in ‘series’ with the ore being feed automatically The model is more suitable for optimising gravity gold from one Spiral to the next. (see Figure 13). recovery with brownfi eld projects. The model requires a three stage GRG test, plant cyclone data and will predict optimum Low-G horizontal centrifuge gravity gold recovery. A single stage GRG at P80 75 can also be These devices are effectively shaking tables wrapped around used for predicting gravity recovery for a greenfi elds project to form a cylinder. The cylinder revolves subjecting particles but is not as accurate. The Parker Centre has a signifi cant to large forces at the same time the shaking enhances the reference database to draw upon. separation. Typically the feed size range 1 mm - 1 μm. There use is not widespread (see Figure 14). Falcon model specifi cations The model numbers and recommended capacities are listed SELECTION AND SIZING in Table 2. AMIRA P420 Knelson model specifi cations The Parker Centre Gravity model developed through the It is interesting to note that some conservatism is taken into AMIRA P420 project has developed a model for predicting account when sizing the equipment based on an average

GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 75 D CONNELLY

TABLE 2 Falcon capacity specifi cations (Falcon concentrators source).

Model L40 SB250 SB750 SB1350 SB2500 SB5200 Recommended solids capacity t/h 0 - 0.25 1 - 8 5 - 45 40 - 150 100 - 250 200 - 400 Max slurry capacity m3/hr 2.3 18 80 200 285 450 Concentrating surface area m2 0.03 0.17 0.53 1.08 2.14 3.37 G-force range Upper 200 200 200 200 200 200 Lower505050505050 Machine weight kg 37 365 1 135 3 175 4 679 6 784 Motor power kW(HP) 0.4 (0.5) 2.2 (3.0) 7.5 (10) 18 (25) 45 (60) 75 (100) Process water consumption m3/hr 0.24 - 1.2 1.8 - 2.7 6 - 9 8 - 15 15 - 28 25 - 35 Water supply pressure bar 2 - 3 2 - 3 2 - 3 2 - 3 2 - 3 2 - 3 Recommended max feed particle size mm 1.0 2.0 2.0 2.0 2.0 2.0 Maximum feed particle size mm 1.5 2.5 4.0 4.0 4.0 4.0 Maximum feed per cent solids % 55 - 70 55 - 70 55 - 70 55 - 70 55 - 70 55 - 70 Concentrate slurry volume Litre 1 50 100 150 250 350 Dimensions Width 0.49 1.04 1.22 2.24 2.67 2.99 Length 0.31 0.66 1.22 1.90 2.00 2.32 Height 0.51 1.02 1.73 2.07 2.27 2.73 circulating load fi gure typically 250 per cent but allowing for PROCESS RISKS AND PROBLEMS circulating loads up to 400 per cent. The trash screen sizing is typically 3.35 mm using a DSM which cuts at half the aperture Predicting gold recovery can be diffi cult to determine based on however larger plants have resorted to vibrating screens using a metallurgical test drill core sample composite with regards poly decks. The fi nal cut size depends on the assay by gold to representivity. GRG based on non-representative samples distribution and consultation with the vendor on top sizes is the major issue. Mt McClure miscalculated and needed to able to be processed by their machine (see Table 3). install a gravity circuit immediately while other plants with a gravity circuit didn’t use them (Resolute, Southern Cross). PILOTING Supergene enrichment and the transition to very high-grades This is rarely undertaken for greenfi elds gold project circuits, can be indicative of the need for a gravity circuit. The presence but has occurred for chrome, metallic copper or in additional of nugget gold at Bronzewing and Granites Gold made it specifi c applications where there was uncertainty. Piloting extremely diffi cult to predict likely gravity gold recovery using is common for brownfi eld projects or where retrofi tting of small test samples. equipment is being considered. For gold, the limit is batch testing using a Knelson or Falcon concentrator. Gekko may The presence of sulfi des (galena) and tramp iron can cause undertake testing using the Inline Pressure Jig (IPJ) where problems when cleaning up gravity concentrates for tabling very coarse gold is present. but is not an issue for the ILR. In a number of cases, GRG has over estimated the gravity gold recovery by some 30 per CAPEX cent because of the way the test work was undertaken or interpreted. For a gold project treating 1.2 Mta modern Knelson or Falcon concentrator gravity circuit installed in a Greenfi elds Another issue is allowing too little of the re-circulating load site including pipe work, trash screens, pumps, hoppers, to be bled to the primary recovery circuit, or the primary unit security cages, steelwork, civil and electrics can easily cost an failing to perform because feed is too coarse or too dense or additional $A 1M and for larger plants the fi gure is higher. As producing fl akes. a guide ILRs or Acacia reactors can cost in 2010 $A dollars between approximately $600 000 to $900 000 installed in TRENDS WITH GRAVITY GOLD a Greenfi eld site. The modular transportable nature of these Centrifugal concentrators have revolutionised gravity gold packages keeps the installed cost low. There are plants where recovery. The transition from batch concentrators to auto the additional CAPEX cannot be justifi ed based on the gravity gold recovery alone. The economic justifi cation current cut dump and continuous discharge concentrators has increased point for most projects is 20 per cent gravity gold recovery. effective gravity gold recovery. Retrofi tting of batch concentrators has occurred at a OPEX number of sites and older equipment such as Johnson Drums The OPEX for centrifugal concentrators has been calculated and continuous strakes and amalgam drums are relegated at between $0.08 to $0.12 /tonne of new feed. The operating to museum items. The introduction of larger units is a trend costs are very low being essentially power, water, spares and that currently sees no limitation. At the same time more of regular maintenance. The OPEX costs are very low and rarely the feed is being processed compared to a small bleed of a consideration against installing gravity equipment. the re-circulating load. Targeting gravity gold has become a

76 GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 SELECTION SIZING AND DEVELOPING THE OPTIMUM GRAVITY GOLD CIRCUIT FOR YOUR PROJECT

TABLE 3 Knelson Capacity specifi cations (Knelson source). Semi-continuous (Batch) Knelson concentrator specifi cations Knelson Solids feed Fluidisation Maximum Active Feed size Concen Concen Concen Motor ICS Concentrator capacity water volumetric capture volume weight net weight power or model required throughput area ICS jr avail Extended Duty (XD) Series KC-XD70 300 - 1000 45 - 80 m3/h 1360 m3/h 32 044 cm2 Max 6 mm 52 L 100 - 140 kg 18 455 kg 150 - 375 kW * t/h [200 - 350 [6000 [4.967 in2] [1/4”] recomm [3200 in3] [220 - [40 600 lbs] [200 - [3030 - US gpm] US gpm] 1.7 mm 310 lbs] 500 HP] 1100 t/h] [10 mesh] KC-XD48 200 - 400 41 - 61 m3/h 545 m3/h 13 941 cm2 Max 6 mm 20 L 45 - 50 kg 5680 kg 30 - 75 kW * t/h [180 - 270 [2400 [2161 in2] [1/4”] recomm [1220 in3] [100 - [12 500 lbs] [40 - 100 HP] [220 - US gpm] US gpm] 1.7 mm 110 lbs] 440 t/h] [10 mesh] KC-XD48VG 200 - 41 - 61 m3/h 545 m3/h 13 941 cm2 Max 6 mm 20 L 45 - 50 kg 5900 kg 45 - 75 Kw * 400 t/h [180 - 270 [2400 [2161 in2] [1/4”] recomm [1220 in3] [100 - 110 [13 000 lbs] [60 - 100 HP] [220 - US gpm] US gpm] 1.7 mm lbs] 440 t/h] [10 mesh] KC-XD40 125 - 250 27 - 45 m3/h 340 m3/h 11 130 cm2 Max 6 mm 17 L 38 - 56 kg 4100 kg 30 kW * t/h [138 - [120 - 20 [1500 [1725 in2] [1/4”] recomm [1000 in3] [85 - 125 [9000 lbs] [40 HP] 275 t/h] US gpm] US gpm] 1.7 mm lbs] [10 mesh] KC-XD40VG 125 - 250 27 - 45 m3/h 340 m3/h 11 130 cm2 Max 6 mm 17 L 38-56 kg 4270 kg 30 - 56 kw * t/h [138 - [120 - 20 [1500 [1725 in2] [1/4”] recomm [1000 in3] [85-125lbs] [9400 lbs] [40 - 75 HP] 275 t/h] US gpm] US gpm] 1.7 mm [10 mesh] KC-XD30 75 - 150 t/h 17 - 25 m3/h 205 m3/h 7115 cm2 Max 6 mm 12 L 21 - 27 kg 1724 kg 11 Kw * [83 - 165 [75 - 110 [900 US gpm] [1108 in2] [1/4”] recomm [730 in3] [47 - 59 [3800 lbs] [15 HP] t/h] US gpm] 1.7 mm lbs] [10 mesh] KC-XD30VG 75 - 150 t/h 17 - 25 m3/h 205 m3/h 7115 cm2 Max 6 mm 12 L 20 - 27 kg 1860 kg 11 - 22 Kw * [83 - 165 [75 - 110 [900 US gpm] [1108 in2] [1/4”] recomm [730 in3] [47 - 59 [4100 lbs] [15 - 30 HP] t/h] US gpm] 1.7 mm lbs] [10 mesh] KC-XD20 15 - 80 t/h 8 - 14 m3/h 109 m3/h 2982 cm2 Max 6 mm 5 L 7 - 10 kg 1000 kg 5.5 Kw [17 - 88 t/h] [35 - 60 [480 US gpm] [462 in2] [1/4”] recomm [300 in3] [15 - 22 [2200 lbs] [7.5 - 10 HP] US gpm] 1.7 mm lbs] [10 mesh] KC-XD20vg 15 - 80 t/h 8 - 14 m3/h 109 m3/h 2982 cm2 Max 6 mm 5 L 7 - 10 kg 1090 kg 5.5 Kw * [17 - 88 t/h] [35 - 60 [480 US gpm] [462 in2] [1/4”] recomm [300 in3] [15 - 22 [2400 lbs] [7.5 - 10 HP] US gpm] 1.7 mm lbs] [10 mesh] priority with fl ow sheet development using Knelson or Falcon are being proposed and used. At the same time continuous concentrators. Even recovering gold from copper concentrates ILRs are available including customised oxygen spargers and has been utilised at Copper Mines of Tasmania. Intensive multiple units capable of processing much larger tonnages of leaching of concentrates has taken over from tabling resulting ore. in signifi cantly higher gold recovery and more security with regards to potential theft. Using resin rather than CASE STUDIES is being tried in gold rooms as an alternative to electrowinning. Mina Sertão Gold Project The Mina Sertão deposit was discovered by a regional exploration Field Geologist working for WMC Resources DEVELOPMENTS (WMC) in 1997. WMC decided not to proceed with development The installation of internal scalping screens in the IPJ allows of the project because of its small size. The Sertão gold mine steel to bypass the jigging bed in the circulating load of a is part of the Goiás Velho gold project, located on mostly- milling circuit. At the same time automation of the IPJ to cleared grazing land in the State of Goiás, approximately 380 improve operability. Multiple and modular systems are also kilometres west of the Brazilian capital of Brasilia. Today the now available. The ‘Python’ Processing Plant in modular form Mina Sertão gold mine in the State of Goiás in Brazil is one of suitable for underground ore processing ‘gravity only’ plants the lowest cost gold mines in the world following its successful

GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 77 D CONNELLY commissioning and offi cial opening in March 2003. The Province. The current project reserves total 2.1 Mt at 5.06 g/t location of the mine is shown in Figure 15. gold and 137 g/t silver (see Figure 17). The Gravity Concentrator processes the screen undersize. This concentrator retains a gravity concentrate and produces a gravity tail. The gravity tail is returned to the comminution circuit via the SAG Mill Discharge Hopper for further liberation. When the gravity concentrate becomes suffi ciently enriched, it is transferred to the Intensive Leach Reactor, or ILR, for leaching under intensive conditions. The ILR is a specialist precious metals leach unit that uses an alkaline cyanide solution to leach gold and metallic silver from the high-grade gravity concentrates in batches. The concentrates are collected in the Concentrate Feed Tank. When suffi cient concentrate has been collected, the batch is transferred to a horizontal rotating drum together with barren solution and hydrogen peroxide. Cyanide solution, caustic solution and lead nitrate solution are added to the reactor via a solution tank and recirculated using the discharge hopper pump until the leaching reactions have completed. The pregnant solution is then pumped to the clarifi cation area to remove any solids FIG 15 - Mina Sertão location (Troy Resources NL supplied image). and recovery the gold and silver. The tailings are pumped to the SAG Mill Discharge Hopper for further liberation. Gravity gold recovery was mandatory for the oxide ore with 42.9 per cent of the gold recoverable into a gravity concentrate CONCLUSION of 0.76 mass per cent using a Knelson concentrator. The Gold mineralogy particularly particle size determines microscopic examination of the gravity concentrate revealed amenability to gravity gold recovery. There are large fi nancial abundant free gold up to 500 μm in size and down to 0.5 μm. drivers to maximise gravity gold recovery. Sample selection This was highly desirable that the gravity gold recovery was and representivity are risks that need to be managed in maximised as coarse gold entering the leach could result in assessing gravity gold recovery. The predictability of using GRG laboratory methods has improved outcomes, thus high tailings. The sulfi de ore achieved 53.5 per cent recovery reducing the previous high risks. The application of the of gold to a concentrate representing 3.9 mass per cent AMIRA P420 gravity model will also help in this regard. using a Knelson concentrator. A Gemini table was used for The introduction of centrifugal concentrators and intensive concentrate clean up before smelting. leaching has revolutionised the industry and equipment Andorhinas Gold Project enhancements are ongoing. These are low operating cost devices proving more effi cient on fi ner particles than any The Mina Sertao plant was moved to Andorhinas (Figure 16) other equipment previously available. The historical and at the end of the Mina Sertão project life and was re established technical importance of the ILR technology has not been duly to treat Legoa Seca and Andorhinas underground ore. recognised by the industry compared to the disadvantages of the previous tabling practices. The management of water balance issues can impact on the fl ow sheet and choice of equipment. The water quality for fl uidising also introduces limitations requiring the use of raw water not process water. The trend to process a larger mass of recirculating load and the use of ‘auto dump’ centrifugal concentrators gravitating concentrates direct to gold rooms is now common. From an engineering and operability perspective modern gravity circuits are far superior to what was the basis of design even ten years ago. Historically we have observed brownfi elds projects lifting gravity gold recovery from 17 per cent to 50 per cent by employing the previously mentioned techniques. There has also been a trend towards processing cyclone feed rather than cyclone underfl ow because of enhanced gold recovery results. Piloting of gravity circuits is not required unless the gold is particularly nuggetty or refractory gold is present. The FIG 16 - Andorhinas location (Troy Resources NL supplied image). size of centrifugal concentrators shows a consistent trend of larger machines and intensive leaching machines are moving towards continuous process units rather than batch units and The Legoa Seca ore gravity recovery was 20 per cent whereas fully automated. the Andorhinas gravity recovery was 60 per cent, an Acacia The use of all gravity modular plant underground where Reactor was installed to treat the gravity concentrates from cyanide cannot be used is also emerging for some projects. the Knelson concentrator. Casposo Gold Project ACKNOWLEDGEMENTS The Casposo process plant, as designated by Troy Resources The author would like to thank various companies, all NL, is situated in North-Western Argentina in the San Juan colleagues, engineers at various sites, METS’ staff and other

78 GRAVITY GOLD CONFERENCE / BALLARAT, VIC, 21 - 22 SEPTEMBER 2010 SELECTION SIZING AND DEVELOPING THE OPTIMUM GRAVITY GOLD CIRCUIT FOR YOUR PROJECT

FIG 17 - Casposo Project Location (Troy Resources NL supplied image). consultants for their contribution and the management Lastra, R, 2005. Mineralogy of gold ores, gold extraction short of METS for their permission to publish this paper and course, COM. constructive criticism of various drafts. Lunt, D and Weeksm, T, 2005. Process Flowsheet selection, in Developments in Mineral Processing, volume 15 (Elsevier). REFERENCES Ryan, A, Johanson, E and Rogers, D 2005. Feasibility study plant Adams, M D, (ed) 2005. Advances In Gold Ore Processing (Elsevier design, in Developments in mineral processing, volume 15 B V: The Netherlands). (Elsevier). Angove, J, 2005. Metallurgical testwork: Gold processing Vaughan, J P, 2004. The process mineralogy of gold: The options, physical ore properties and cyanide management, in classifi cation of ore types, Journal of Metals, 56(7). Developments in mineral processing, volume 15 (Elsevier). Vaughan, J P and Kyin, A, 2004. Refractory gold ores in Archaean Deschênes G and Guo H, 2005. Cyanidation of Gold (Mining and greenstones, Western Australia: Mineralogy, gold paragenesis, Mineral Sciences Laboratories CANMET: Ottawa). metallurgical characterization and classifi cation, Mineralogical Laplante, A R, 2000. Ten do’s and don’ts of gold gravity recovery, Magazine, 68(2), pp 255-277. McGill University, Department of Mining and Metallurgical Yannopoulos, J C, 1991. The of Gold, (Van Engineering. Nostrand Reinhold: NewYork). Laplante, A R and Spiller, D E, 2002. Bench scale and Pilot plant testwork for gravity concentration circuit design, in Proceedings Mineral Processing Plant Design, Practice, and Control, 1:160-175 (Society for Mining, Metallurgy and Exploration: Littleton).

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