Gold Operations, Gold Mining, Gold Producers Australia
Operations
Ground Fl, 17 Ord Street West Perth
WA 6005 Australia
PO Box 117 West Perth
WA 6872 Australia
Operations
GOLD OPERATIONS
Results from the recently completed feasibility study indicate the following operational and financial metrics.- Stage 1 Mining Ore Reserve of 10.45 Mt at 1.7g/t containing 567,000 ounces, using a cut-off grade of 0.5g/t.
- Initial mine life of 7 years, producing 504,000 ounces from CIL process plant.
- Process Plant and Infrastructure development capital cost $63 million.
- Owners cost $6.3 million, rehabilitation and closure $8 million (deferred).
- Native Title completed, final permitting largely complete.
- Inferred resource areas and depth extensions provide upside.
- Stage 2 scoping study to evaluate heap leach production from year 3.
OPERATIONAL STRATEGY
Mine design and scheduling is driven by a desire to maximise gold production in the first four years of the project life. The base case plan shows milling throughput rates of 1.50Mt per annum generating 72,000 ounces of gold.The production plan recognises that upside exists in bringing other Inferred Mineral resources into the Measured and Indicated categories, allowing the capacity for Reserve status assessment and the additional potential to exploit mineralised waste and other material through a lower cost heap leach operation.
In addition, depth and strike extension potential exists that will require further exploration to define. These upside factors require assessment, and if their potential is realised, brought into the project well before Year 4 with the potential of producing an increase in gold output.
MINE OPERATIONS AND PROCESS FLOW SHEET DESCRIPTION
The General layout of the planned process plant is shown in Figure 1.
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TREATMENT PLANT SITE
The treatment plant site is located approximately 10 kilometres south of the Nullagine town in Western Australia. The plant site is located immediately south east of a 60 metre high natural ridge - the topography being relatively flat with minimal vegetation. Occasional rock outcrops are present across parts of the area, however, surface conditions primarily comprise clayey sands and gravels.Ground conditions have been assessed as suitable to support loads typically associated with plant infrastructure. The proposed size cuts and fills of the site will be carried out to provide the required leveling for construction. The ROM pad base course will be constructed from waste and will grow to the full designed size, using low grade ore as it becomes available from mining operations.
As the area is one of the hottest in Australia, work areas and equipment conditions will be designed to take into account addressing these adverse climatic issues.
ROM PAD OPERATION
The mining workforce will mine and deliver ore from the Project's various open pits, which will be hauled to the 150,000 tonne capacity ROM stockpile area located adjacent to the crusher. A Caterpillar 980 size Front End Loader (FEL) will rehandle the ore into the ROM crusher feed bin for crushing.Ore stockpiles will generally be situated within a one hundred metre radius of the crusher ROM bin. Separated "fingers" or smaller stockpiles of ore will be developed to account for varying material hardness and ore grade, allowing for blending and throughput optimisation.
CRUSHING
The primary crusher will be fed from a 90 tonne live capacity ROM bin with a 600 mm nominal feed size control by a fixed grizzly. Any oversize rock will be broken in situ by a rock breaker mounted adjacent to the ROM bin, with feed rate controlled by a variable speed vibrating feeder, scalping off the minus 100mm fraction to bypass the jaw.The feeder will discharge the plus 100 mm fraction into a jaw crusher. The jaw crusher will operate with a closed side setting of 100 mm, which will result in 80% of the feed passing 125 mm. A five tonne monorail hoist will be mounted above the primary crusher to allow for the replacement of the jaw liners during maintenance.
Crushed material will be transported using conveyor belts to a 50 T live capacity mill feed surge bin and overflow onto a secondary surge stockpile. One variable spread apron feed will draw from the surge bin feeding the SAG mill. Additionally the mill may be fed from the surge stockpile using a front end loader during periods of crusher maintenance, ensuring a constant ore supply.
GRINDING
The grinding circuit will comprise a single stage 3,500 kW SAG (Semi Autonomous Grinding) mill in closed circuit with hydrocyclones. The SAG mill (7.0 m long and 6.1 m in diameter) will be operated with a ball charge of between 5% and 12% - consisting of 125 mm and 94 mm diameter steel balls. A grate discharge arrangement will prevent oversize material from passing through the mill without grinding.Mill discharge slurry will pass through a 15.5 mm by 15.5 mm aperture trommel, into a discharge launder and feed the mill discharge pumps. Plug valves in the mill discharge launder will enable the process operator to direct the launder flow to either of the mill discharge pump hoppers. Pneumatically controlled knife gate valves will be used to control and isolate pump feed allowing for pump maintenance while the circuit continues to operate.
Slurry from the SAG mill will be pumped up to a cluster of six hydrocyclones where it will be split into two streams, the overflow and the underflow. The cyclone overflow will have a P80 of 75 mm and will gravity feed onto two vibrating trash screens with 0.6 mm aperture to remove any oversized material. Oversize material from the trash screens will be returned to the mill, via a flop gate chute arrangement.
The cyclone underflow will be returned to the mill to be reground via a static screen to a CD30 Knelson centrifugal gravity concentrator to remove any 'free' gold. The screen oversize will be directed back to the mill feed chute, while the Knelson concentrator tails will be returned to the mill discharge launder.
Knelson concentrate material will be drawn off and pass to a storage cone situated in the goldroom. The storage cone will be mounted over an Acacia intensive leach reactor and batch treated daily.
LEACHING
The cyclone overflow from the trash screens will pass into the CIL circuit. This circuit will consist of six 10 m high by 12 m diameter tanks each holding approximately 1,000 m3 of slurry with mechanical agitation to prevent sanding.A plant control room will be located at the same level as the top of the tanks, providing views to the feed conveyor, mill, discharge and tailings pumps as well as being in close proximity to the trash screens, cyclones and gravity circuit.
Sodium Cyanide solution will be added to the circuit through a rotameter installed in the cyanide distribution pipe ring main.
The slurry flow from each tank to the next, pass through a 1.0 mm woven wire intertank screen flowing via a launder to the following tank.
Tank two will be the first of the five absorption tanks that will contain activated carbon. These five tanks are arranged in series and will operate in a counter current manner with the slurry passing from tank two to tank six and the carbon pumped forward from tank six to tank two using recessed impeller pumps. A five tonne portal crane will be mounted above the tanks to facilitate removal of the intertank screens, agitators and trash screens for maintenance.
Once the carbon reaches tank two, it will be pumped over a vibrating recovery screen with an aperture of 1.0 mm. The clean carbon will fall into an elution column and the slurry will be returned to tank two.
The first three tanks will be fitted with oxygen injection to accelerate absorption of gold. Provision for a three tonne per day PSA oxygen plant has been made in the design, located adjacent to tank four.
The slurry, once it has passed through the intertank screen in tank six, will gravity feed to a vibrating tails screen with 1.2 mm aperture panels. The screen will recover any carbon that may have passed through the final intertank screen.
Diluted water and flocculant will be added to the slurry before it enters a 17 metre diameter high rate thickener, where it will be thickened to > 55% solids. The overflow water will be recycled and pumped directly to the process water dam for reuse in the plant. Two pumps located on the thickener underflow will operate in duty / standby mode to deliver the slurry to the TSF.
Process plant tanks will be constructed on concrete ring beams within a concrete bunded containment structure with three sumps and pumps returning water, overflow and spillage back into the circuit.
The elution column will have a volume of 8 m3 and be capable of holding in excess of three tonnes of carbon. It will be constructed of stainless steel suitable for acid washing and elution.
The dilute acid wash solution will be prepared prior to stripping by mixing concentrated hydrochloric acid and raw water into a dilute acid tank. During acid washing the dilute solution will be pumped into the bottom of the column to remove contaminants from the carbon. After soaking, the carbon will be rinsed with water to displace any residual acid - with the acid and rinse water being disposed of back into the circuit;
Sodium hydroxide and cyanide solutions will be combined with raw water to achieve a strip solution. The strip solution will be circulated through a heat exchanger, prior to entering the base of the column. After soaking, heated water will be pumped through the column to remove the remaining gold from the loaded carbon. The pregnant eluate and rinse water will be directed to an eluate tank for gold recovery by electrowinning.
Once the elution process is complete, the pregnant solution will be pumped into a closed circuit for electrowinning cells contained in the goldroom. DC electric current will be passed through stainless steel anodes and steel wool cathodes causing the gold in solution to plate out on the cathodes until the solution is barren. This will take approximately 16 hours after which the barren solution will be pumped back into the circuit.
Barren carbon will be returned either directly to the circuit or to the regeneration kiln. The horizontal kiln will operate at 6500C, at a rate of 200 kg per hour regenerating the carbon to > 90% of its original activity over approximately 15 hours. Discharge from the kiln will be cooled and released directly back into the circuit.
The gold plated steel wool will be removed from the electrowinning cells then calcined in an oven prior to smelting in a gas fired furnace to produce the gold bullion (dore`) bars.
Tailings will be pumped via a single 250 mm pipeline from the plant to a junction point at the crest of the dam. This junction point will allow diversion of the tailings and flushing of the lines. A pipeline with multipoint spigots will be located on the three constructed TSF walls, where the tailings will be deposited along the walls.
Surface water recovered from the tailings slurry will be collected into a rock walled decant pond where it will be pumped back through a single 160 mm OD pipeline to the plant process water dam for reuse.
Cyanide deliveries will be in bulk by iso-tankers holding 22.5 tonnes of solid cyanide briquettes containing 98% sodium cyanide. As part of a facility leasing agreement, the supplier will provide two iso-tankers, a sparging facility and 45,000 litre storage tank. The solid cyanide will be sparged by an automated system as required into the storage tank. Solution in the tank will be pumped to the leach tanks where it will be manually regulated to maintain the desired cyanide levels.
Grinding media will be delivered in 50 tonne loads by road transport in 200 litre drums. A change of various steel ball sizes will be added via the rehandle feed bin to maintain optimum ball loading in the mill. It is anticipated that 125 mm and 94 mm balls will be the main sizes utilised.
Carbon will be delivered by road transport in 600 kg bulka bags. A store of bags will be held on site with carbon added directly into the leaching circuit on a regular basis to maintain carbon levels.
Oxygen will be generated by a three tonne per day PSA plant provided and maintained by the supplier. Approximately 60 m3 of oxygen will be produced an hour, which will be sparged as required into the first three leach tanks to maintain enhanced dissolved oxygen levels.
Deliveries of caustic soda will be by road train tanker in 50 tonne loads of 50% concentration bulk liquid. The liquid will be discharged into a 30,000 litre storage tank ready to be pumped for direct use in the elution circuit.
Hydrochloric acid will also be delivered by road train tanker in 24 tonne loads of 32% concentration bulk liquid. The liquid will be discharged into a 30,000 litre storage tank ready to be pumped to the dilute acid tank for use in the elution circuit.
LPG deliveries will be by road train tanker in bulk liquid and discharged into six 7,500 litre storage tanks provided by the supplier under a facility agreement for use in the carbon regeneration kiln and gold room smelting furnace.
Diesel will be delivered by road train tanker into ten 55000 litre storage tanks. An automated pumping system will maintain fuel supply to a day storage tank at the powerhouse. A bowser will also be provided for use in refuelling vehicles.
Overhead power lines will run from the power house to the earthmoving contractor's yard, borefield, camp and the tailings storage facility for pumping return water.
The plant water demand based on a thickened slurry density of 55% to 60% solids and an instantaneous throughput of 125 tph will be 80 m3/h - 100 m3/h. Based on a mill availability of 95%, total water needs will range from 700,000 m3 to 850,000 m3 per annum. With minimal return water to the process water dam from the tailings storage facility the average make up water consumption of 0.7 tonne of water per tonne of ore milled is estimated.
Potable water will be stored in a separate enclosed water tank with filtration and purification installed as required.
Discrete regulatory controllers will be located either in the control room, or in field mounted control panels. Details of these control loops are described below. Status information on selected process drives and process states will be collected by a Programmable Logic Controller (PLC) and displayed on control room computer monitors. Process and Instrument Diagrams (P & ID) to display the process and the status of all drives and main process conditions, as well as fault alarms, will also be displayed on control room computer monitors.
All major drives will have a field mounted control station to allow local start and stop, as well as remotely controlled from the appropriate control room. The control circuits for each starter will be hard wired with relay logic for any interlocking requirements. Sump pumps and intertank screens will be provided local electrical isolators to facilitate maintenance periods.
Diluted water and flocculant will be added to the slurry before it enters a 17 metre diameter high rate thickener, where it will be thickened to > 55% solids. The overflow water will be recycled and pumped directly to the process water dam for reuse in the plant. Two pumps located on the thickener underflow will operate in duty / standby mode to deliver the slurry to the TSF.
Process plant tanks will be constructed on concrete ring beams within a concrete bunded containment structure with three sumps and pumps returning water, overflow and spillage back into the circuit.
ELLUTION / GOLDROOM
The loaded carbon in the elution column will be stripped of gold by the AARL method of elution. This involves the following:- Acid washing of the carbon with dilute hydrochloric acid.
- Stripping of the carbon using a solution of cyanide and caustic heated to 120 C at 350 kPa.
- Electrowinning of the gold laden solution onto steel wool cathodes.
The elution column will have a volume of 8 m3 and be capable of holding in excess of three tonnes of carbon. It will be constructed of stainless steel suitable for acid washing and elution.
The dilute acid wash solution will be prepared prior to stripping by mixing concentrated hydrochloric acid and raw water into a dilute acid tank. During acid washing the dilute solution will be pumped into the bottom of the column to remove contaminants from the carbon. After soaking, the carbon will be rinsed with water to displace any residual acid - with the acid and rinse water being disposed of back into the circuit;
Sodium hydroxide and cyanide solutions will be combined with raw water to achieve a strip solution. The strip solution will be circulated through a heat exchanger, prior to entering the base of the column. After soaking, heated water will be pumped through the column to remove the remaining gold from the loaded carbon. The pregnant eluate and rinse water will be directed to an eluate tank for gold recovery by electrowinning.
Once the elution process is complete, the pregnant solution will be pumped into a closed circuit for electrowinning cells contained in the goldroom. DC electric current will be passed through stainless steel anodes and steel wool cathodes causing the gold in solution to plate out on the cathodes until the solution is barren. This will take approximately 16 hours after which the barren solution will be pumped back into the circuit.
Barren carbon will be returned either directly to the circuit or to the regeneration kiln. The horizontal kiln will operate at 6500C, at a rate of 200 kg per hour regenerating the carbon to > 90% of its original activity over approximately 15 hours. Discharge from the kiln will be cooled and released directly back into the circuit.
The gold plated steel wool will be removed from the electrowinning cells then calcined in an oven prior to smelting in a gas fired furnace to produce the gold bullion (dore`) bars.
TAILINGS DISPOSAL
The TSF will be located approximately 1 kilometre north west of the plant site and will be approximately 600 m x 600 m with a designed maximum embankment height of 17 m. The selected area is bordered by a ridge on its southern boundary with a nominal slope of 1:50 to the north and west. Initial construction will provide for the first two years production by building a starter embankment up to 11 m in height.Tailings will be pumped via a single 250 mm pipeline from the plant to a junction point at the crest of the dam. This junction point will allow diversion of the tailings and flushing of the lines. A pipeline with multipoint spigots will be located on the three constructed TSF walls, where the tailings will be deposited along the walls.
Surface water recovered from the tailings slurry will be collected into a rock walled decant pond where it will be pumped back through a single 160 mm OD pipeline to the plant process water dam for reuse.
REAGENTS
Quicklime used to stabise Ph levels in the process plant will be delivered in bulk by double road train tankers in 58 tonne loads. The quicklime will be pneumatically discharged into two silos with 49 tonne and 34 tonne capacities.Cyanide deliveries will be in bulk by iso-tankers holding 22.5 tonnes of solid cyanide briquettes containing 98% sodium cyanide. As part of a facility leasing agreement, the supplier will provide two iso-tankers, a sparging facility and 45,000 litre storage tank. The solid cyanide will be sparged by an automated system as required into the storage tank. Solution in the tank will be pumped to the leach tanks where it will be manually regulated to maintain the desired cyanide levels.
Grinding media will be delivered in 50 tonne loads by road transport in 200 litre drums. A change of various steel ball sizes will be added via the rehandle feed bin to maintain optimum ball loading in the mill. It is anticipated that 125 mm and 94 mm balls will be the main sizes utilised.
Carbon will be delivered by road transport in 600 kg bulka bags. A store of bags will be held on site with carbon added directly into the leaching circuit on a regular basis to maintain carbon levels.
Oxygen will be generated by a three tonne per day PSA plant provided and maintained by the supplier. Approximately 60 m3 of oxygen will be produced an hour, which will be sparged as required into the first three leach tanks to maintain enhanced dissolved oxygen levels.
Deliveries of caustic soda will be by road train tanker in 50 tonne loads of 50% concentration bulk liquid. The liquid will be discharged into a 30,000 litre storage tank ready to be pumped for direct use in the elution circuit.
Hydrochloric acid will also be delivered by road train tanker in 24 tonne loads of 32% concentration bulk liquid. The liquid will be discharged into a 30,000 litre storage tank ready to be pumped to the dilute acid tank for use in the elution circuit.
LPG deliveries will be by road train tanker in bulk liquid and discharged into six 7,500 litre storage tanks provided by the supplier under a facility agreement for use in the carbon regeneration kiln and gold room smelting furnace.
Diesel will be delivered by road train tanker into ten 55000 litre storage tanks. An automated pumping system will maintain fuel supply to a day storage tank at the powerhouse. A bowser will also be provided for use in refuelling vehicles.
POWER SUPPLY
Power for the mine site will be supplied by a contractor owned and maintained multiple diesel generator power house. Six diesel generators rated for 1mW on a continuous basis will be installed with four operating at any one time. Under the supply agreement the contractor will supply the facility for the generators as well as the main switchboard and control panels housed in the switch room.Overhead power lines will run from the power house to the earthmoving contractor's yard, borefield, camp and the tailings storage facility for pumping return water.
WATER SUPPLY
Water for the milling circuit will be pumped from pit dewatering and production bores as well as return water from the tailings dam. Lined water dams will be constructed adjacent to the plant to store process and raw water. The raw water dam will be designed to overflow into the process water dam.The plant water demand based on a thickened slurry density of 55% to 60% solids and an instantaneous throughput of 125 tph will be 80 m3/h - 100 m3/h. Based on a mill availability of 95%, total water needs will range from 700,000 m3 to 850,000 m3 per annum. With minimal return water to the process water dam from the tailings storage facility the average make up water consumption of 0.7 tonne of water per tonne of ore milled is estimated.
Potable water will be stored in a separate enclosed water tank with filtration and purification installed as required.
PROCESS CONTROL SYSTEMS
System control has been deliberately designed for simplicity and where practicable manually operated with minimal automation. A main control room will be provided in the plant on top of the CIL tanks, with a smaller control cabin in the crushing area. Instrumentation will be provided within the plant to measure normal process control.Discrete regulatory controllers will be located either in the control room, or in field mounted control panels. Details of these control loops are described below. Status information on selected process drives and process states will be collected by a Programmable Logic Controller (PLC) and displayed on control room computer monitors. Process and Instrument Diagrams (P & ID) to display the process and the status of all drives and main process conditions, as well as fault alarms, will also be displayed on control room computer monitors.
All major drives will have a field mounted control station to allow local start and stop, as well as remotely controlled from the appropriate control room. The control circuits for each starter will be hard wired with relay logic for any interlocking requirements. Sump pumps and intertank screens will be provided local electrical isolators to facilitate maintenance periods.