The proposed gold recovery flowsheet for the Karma Project is based on heap leach technology with separate soft and hard rock crushing circuits.
Report context
This technical report, dated 2023, describes the proposed recovery methods and process plant design for the Karma Project. The plant is designed to process 4 Mtpa of oxide and transition ore delivered from five open pit mines: GGI, GGII, Kao, Rambo and Nami. Only a minimal amount of sulphide material from Rambo and Nami will be processed through the heap leach plant. Sulphide material from GGI, GGII and Kao is considered refractory and cannot be treated via heap leaching.
Processing route
Process plant overview
The proposed Karma process plant design will be based on gold heap leach technology. The plant will consist of two crushing circuits (for soft and hard ore respectively), agglomeration and stacking, heap leaching with cyanide solution, adsorption of the pregnant solution, elution and gold smelting. Services to the process plant will include reagent make-up, storage and distribution, water and air supply.
Crushing
Two separate crushing stations are proposed to treat soft and hard ore. The soft rock circuit will be partially skid-mounted and moveable, supplied as a complete package from a vendor. It will produce a final product with a top size of 50 mm using a two-stage crushing circuit employing a feeder-breaker and a double roll crusher. The feeder-breaker will size the run-of-mine material, which is then conveyed to a roller screen. Oversize material reports to the double roll crusher; screen undersize and roll crusher product discharge onto the agglomeration feed conveyor.
The mobile hard rock crushing circuit will be required intermittently when more competent ore is delivered from the pit. It will produce a final product of 80% passing 1.70 mm using a three-stage crushing circuit supplied as a complete package from a vendor, comprising a jaw crusher, secondary cone crusher and tertiary hammer mill, both in closed circuit with dedicated triple deck screens. The hard rock product is transferred to the agglomeration feed conveyor via a feed hopper and belt feeder.
Agglomeration and stacking
An agglomeration drum will be used to eliminate fines in the ore prior to stacking on the heap leach pad. Cement will bind fine particles and solution will be added to maintain critical ore moisture for optimum agglomeration. Agglomerated ore will be discharged onto an overland conveyor that delivers ore onto the heap leach pad. The pad design provides for a stack height of 20 m, with cell lift heights of 10 m.
Heap leaching
A leach period of 120 days has been allowed for normal solution flow. Barren solution will be sprayed across the stacked cell via the irrigation system, dissolving gold as it percolates through the heap to the pad floor. Gold-enriched solution is collected and gravitates to either the intermediate leach solution (ILS) pond or pregnant leach solution (PLS) pond. Excess solution overflows to the storm water pond.
While one cell is being irrigated, the second cell will be stacked. Gold-enriched solution from the ILS pond is pumped to the second cell for irrigation, further enriching the solution before it is pumped to the adsorption column.
Adsorption
Pregnant solution is pumped into the bottom of the adsorption column, which contains six stages of activated carbon. Solution discharges at the top as barren solution and gravitates through sieve bends to remove carbon before returning to the barren pond for reuse on the heaps. Loaded carbon is extracted from the first stage and hydraulically transferred to the elution circuit. Carbon is transferred between stages using eductors; fresh or reactivated carbon is added to stage 6.
Cyanide detoxification and storm water control
In high rainfall events, runoff from the pads and pond overflow gravitate to the storm water pond. Excess solution from the storm water pond overflows into the detox pond, where detox reagents (hydrogen peroxide and copper sulphate) are pumped to destroy residual cyanide. Hydrochloric acid is used to neutralise the alkaline solution prior to environmental discharge.
Scavenging column
Barren solution is pumped into the bottom of a two-stage scavenging column containing activated carbon. Solution discharges at the top and gravitates through a sieve bend to remove carbon before returning to the barren pond. Loaded carbon from the first stage is transferred to the acid wash circuit.
Acid wash
A cold acid wash circuit capable of handling a full 6 t batch is included. Every batch of loaded carbon is acid washed before elution by circulating 3% hydrochloric acid through the acid wash column. Loaded carbon is back-washed with raw water to remove trash, then acid washed. After neutralisation, the pre-treated carbon batch is hydraulically transferred to the elution column.
Elution
The elution method is Pressurized Zadra. A cyanide-caustic solution is pumped through the elution column while being heated via heat exchangers. Pregnant solution is directed to electrowinning cells once the column outlet temperature reaches 130°C. Spent electrolyte from electrowinning flows back to the elution area. This cycle continues until contamination levels become unacceptable.
Electrowinning
Eluate from the elution column is directed to the electrowinning feed tank and distributed to electrowinning cells. Gold is plated onto stainless steel cathodes as sludge. Loaded cathodes are removed periodically; the sludge is washed off, collected in the gold sludge tank, manually tapped off into a bucket and taken to the drying oven.
Regeneration
Eluted carbon is hydraulically transferred to an eluted carbon holding tank. Carbon is withdrawn by a screw feeder and discharged to a diesel-fired rotary kiln for thermal regeneration. Regenerated carbon is quenched, passed over a screen to remove fine carbon, and gravitates to the eductor tank.
Gold room
Gold sludge collected in a bucket is loaded onto stainless steel trays and placed into a drying oven equipped with a mercury retort. The dried sludge, once cooled, is mixed with fluxes and smelted. The molten charge is poured into bullion moulds and cooled. The slag phase is broken away, leaving a relatively pure gold bar.
Heap leach pad design
All ore from the open pit will be delivered to the ROM pad, then crushed, agglomerated and conveyed to a single, centrally located heap leach pad situated 250 m south of the GGII pit. The pad is located on flat terrain with a low ridge running east to west across the north western corner. The pad has a slope of 1.0% draining north to south and 0.5% draining west to east.
The final heap leach pad will measure 1,620 m long by 810 m wide and will consist of 42 cells, each measuring 75 m wide by 405 m long at the base of the first lift, except the two easternmost cells which are 110 m wide by 405 m long. The pad will be stacked to a height of 20 m in two lifts of 10 m each. The heap leach operation is anticipated to have a life of 8.5 years.
The pad will be constructed in three phases: Phase 1 comprising 9 cells in the south east corner plus process ponds; Phase 2 comprising 9 north-eastern cells; and Phase 3 comprising the remaining 24 cells.
Layerworks under the pad consist of a sub-base of locally sourced or in-situ material compacted to 95% Mod AASHTO with a 600 mm layer of selected low permeability material forming the base. A 1.5 mm thick HDPE liner is laid directly onto the base to form a composite liner, with a protective layer of needle-punched geofabric. A 550 mm thick drainage layer of crushed aggregate with slotted pipes is placed over the liner.
Ponds
The PLS, ILS, barren, storm water, raw water and detox ponds are located in the south-eastern corner of the pad. All except the raw water pond are excavated into natural ground. The required capacity of the PLS, ILS and barren ponds is 29,600 m³ each, double lined with 1.5 mm HDPE. The storm water pond has a capacity of 1.25 million m³. The raw water pond has a capacity of 340,000 m³ and is constructed above natural ground level. The detox pond has a capacity of 15,000 m³.
Key reported parameters
| Parameter | Units | Design | Actual | Testwork Basis |
|---|---|---|---|---|
| Plant throughput | Mtpa | 4.0 | – | – |
| Soft rock crushing throughput | tph | 650 | – | – |
| Hard rock crushing throughput | tph | 200 | – | – |
| Agglomeration throughput | tph | 650 | – | – |
| Heap leach throughput | tph | 650 | – | – |
| Crushing product top size (soft rock) | mm | 50 | – | – |
| Crushing product P80 (hard rock) | mm | 1.70 | – | – |
| Leach period | days | 120 | – | – |
| Stack height | m | 20 | – | – |
| Lift height | m | 10 | – | – |
| Pad dimensions (final) | m | 1,620 × 810 | – | – |
| Number of cells | – | 42 | – | – |
| Pad life | years | 8.5 | – | – |
| Oxide gold head grade (GGI) | g/t | 0.95 | – | PEA |
| Oxide gold dissolution (lab, GGI) | % of HL feed grade | 92 | – | Testwork |
| Oxide plant leach dissolution (GGI) | % of HL feed grade | 90 | – | Calculated |
| Transition gold head grade (GGI) | g/t | 0.60 | – | PEA |
| Transition gold dissolution (lab, GGI) | % of HL feed grade | 75 | – | Testwork |
| Transition plant leach dissolution (GGI) | % of HL feed grade | 73 | – | Calculated |
| Sulphide gold head grade (Rambo) | g/t | 2.94 | – | PEA |
| Sulphide gold dissolution (lab, Rambo) | % of HL feed grade | 70 | – | Testwork |
| Sulphide plant leach dissolution (Rambo) | % of HL feed grade | 68 | – | Calculated |
| PLS/ILS/Barren pond capacity | m³ | 29,600 each | – | – |
| Storm water pond capacity | m³ | 1,250,000 | – | – |
| Raw water pond capacity | m³ | 340,000 | – | – |
| Detox pond capacity | m³ | 15,000 | – | – |
| Oxide moisture content | % | 15 | – | SENET |
| Transition moisture content | % | 10 | – | SENET |
| Sulphide moisture content | % | 5 | – | SENET |
| Soft rock crushing availability | % | 75 | – | PEA |
| Hard rock crushing availability | % | 75 | – | PEA |
| Agglomeration and stacking availability | % | 75 | – | PEA |
| Heap leach and ADR availability | % | 75 | – | PEA |
Project website: https://secure.northernminer.com/news/true-golds-karma-project-fully-funded-to-production/1003207343/
Project website: https://miningrecord.com/true-gold-receives-mining-permit/
Technical qualifications
The proposed design described in this report is based on heap leach technology. Ore characteristics were determined from selected comminution test results, comminution circuit simulations, head analyses and moisture content testing. Reagent consumptions were obtained from bench scale laboratory tests. When actual plant data become available, the design parameters may require adjustment. The heap leach pad design uses in-situ ferricrete as the base layer, subject to compaction to 95% Mod AASHTO to achieve sufficiently low permeability.
Source: Karma Project , 2023 Technical Report, Section 17.0 Recovery Methods.
