This report describes the proposed carbon-in-leach processing route for the Bissa mine, with design throughput increased from 3 Mtpa to a target of approximately 4 Mtpa.
Report context
The Bissa Asset NI 43-101 Technical Report, dated July 2012, was prepared for High River Gold Mines Ltd. The operation envisages open pit mining at an annual rate of approximately 3 Mtpa, utilising drill and blast with a truck and shovel approach to ore and waste haulage. A CIL process route was proposed for the Bissa mine at a nominal rate of 3 Mtpa, although the new design target throughput is nearer to 4 Mtpa.
Processing route
Crushing
Run of mine ore will be delivered to the plant by truck where it will be discharged into a hopper equipped with an 800 mm trash screen. Oversize will be broken up using a hydraulic breaker. From the hopper, material will be fed using an apron feeder onto a 150 mm aperture grizzly with all oversize material reporting to a single toggle primary jaw crusher. The jaw crusher will crush this oversize to a P80 of 170 mm before being recombined with the undersize. The material will then either be conveyed to the grinding circuit supply silo which will have an operating retention time of 4 hours or to an emergency stockpile which will be used to feed the grinding circuit in the event of breakdown or maintenance to the crushing circuit.
Grinding
Crushed ore will initially be ground using an 8.5 m diameter by 4.9 m long (6,000 kW motor) semi-autogenous (SAG) mill operating in closed circuit with 10 mm aperture vibrating screen. Screen oversize material will be conveyed back to the SAG mill feed conveyor, with provision for pebble crushing, whilst all undersize material will go for further grinding using a 6.1 m diameter by 9.1 m long (6,000 kW motor) ball mill operating in closed circuit with hydrocyclones. The hydrocyclones will operate at a target P80 of 75 µm with all underflow returning to the feed of the ball mill. Cyclone overflow will gravitate through a trash screen to remove any grit or remaining organic material after which it will be pumped to the leach circuit.
Carbon-in-Leach Circuit
Leaching will be performed using a standard circuit consisting of six agitated tanks which will provide a total circuit residence time of 24 hours. Cyanide will be added to the first tank at a rate of between 0.2–0.6 kg/t ore and allowed to fall as the pulp passes through the remaining five leach tanks. Each tank will contain an air injection system to, if necessary, allow the levels of dissolved oxygen to be increased in order to promote leaching. In order to adsorb the gold, barren activated carbon will be added to the sixth tank from where it will be advanced using a system of pumps against the flow of the pulp. Carbon is prevented from leaving each tank using inter-tank screens. The loaded carbon will then be recovered from the pulp using a screen and sent for elution in order to recover the gold. The leached tailings material will be pumped, at a density of 40% solids, to the tailings management facility.
Elution
Gold will be stripped from the activated carbon using an AARL (Anglo American Research Laboratory) elution circuit. The elution circuit is designed to take the gold content of the activated carbon from a loaded value of circa 2,200 g/t Au to a barren concentration of 100 g/t Au. This will be achieved initially by subjecting the loaded carbon to an acid wash using a 3% hydrochloric acid solution after which it will undergo pre-soaking using a 2% sodium cyanide/3% sodium hydroxide solution. Finally, the gold will be stripped from the carbon using fresh water heated to a temperature of 130 °C. Each elution cycle will be able to treat 6 tonnes of activated carbon and will take approximately 12 hours to complete. One complete elution will be performed each day. Following elution, the pregnant eluate will be sent for electrowinning in order to recover the gold within solution whilst the barren carbon will be sent for carbon regeneration.
Electrowinning
The gold contained within the pregnant eluate solution will be recovered onto stainless steel cathodes by means of electrowinning. The electrowinning circuit will consist of two 3.5 m³ electrolytic cells each containing a total of twenty two 1 m² stainless steel cathodes. An electrical current will be applied to the cathodes in order to remove the gold from solution where it will become plated onto the surface of the cathodes whilst the barren solution will be returned to the elution circuit for reuse. In order to minimise the total elution time, the pregnant eluate will only undergo a single stage of electrowinning of between 4 and 6 hours in length with the circuit designed to produce approximately 20 kg of metal each day.
Refining
At appropriate intervals, the gold plated cathodes will be removed from the electrowinning circuit and washed using a high pressure water jet in order to remove the deposited gold. This, along with an electrolytic sludge, will then be dried and calcined in order to remove impurities. Finally, the calcined product will be mixed with fluxes and smelted to produce doré bars. These will then be stored securely until they are transported off-site for further refining.
Carbon Regeneration
Following elution, barren carbon will be regenerated in an electric kiln. The carbon will be regenerated in batches of 500 kg by first heating to between 650–750 °C for approximately 15 minutes using the kiln after which it will be quenched in a tank of cold water. The quenched carbon will then be screened in order to remove any fines that may have been generated during either the leaching, elution or regeneration processes and returned to the CIL circuit.
Tailings Disposal
Tailings material from the CIL circuit will be pumped to the Tailings Management Facility at a density of 40% solids where it will be allowed to settle. Any supernatant that does not naturally evaporate will be recovered and returned to the plant to be reused.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Mining rate | Approximately 3 Mtpa | Proposed design |
| Process route | Carbon-in-leach (CIL) | Proposed design |
| Nominal plant throughput | 3 Mtpa | Proposed design |
| New design target throughput | Nearer to 4 Mtpa | Proposed design |
| Primary jaw crusher product P80 | 170 mm | Proposed design |
| Grinding circuit supply silo retention time | 4 hours | Proposed design |
| SAG mill dimensions | 8.5 m diameter x 4.9 m long | Proposed design |
| SAG mill motor power | 6,000 kW | Proposed design |
| SAG mill screen aperture | 10 mm | Proposed design |
| Ball mill dimensions | 6.1 m diameter x 9.1 m long | Proposed design |
| Ball mill motor power | 6,000 kW | Proposed design |
| Hydrocyclone target P80 | 75 µm | Proposed design |
| Number of CIL tanks | 6 | Proposed design |
| Total CIL circuit residence time | 24 hours | Proposed design |
| Cyanide addition rate | 0.2–0.6 kg/t ore | Proposed design |
| Loaded carbon gold grade | Circa 2,200 g/t Au | Proposed design |
| Barren carbon gold grade | 100 g/t Au | Proposed design |
| Acid wash concentration | 3% hydrochloric acid | Proposed design |
| Pre-soak solution | 2% sodium cyanide/3% sodium hydroxide | Proposed design |
| Elution temperature | 130 °C | Proposed design |
| Elution cycle capacity | 6 tonnes activated carbon | Proposed design |
| Elution cycle duration | Approximately 12 hours | Proposed design |
| Elution frequency | One complete elution per day | Proposed design |
| Electrowinning cells | Two 3.5 m³ cells | Proposed design |
| Cathodes per cell | Twenty two 1 m² stainless steel | Proposed design |
| Electrowinning duration | 4–6 hours single stage | Proposed design |
| Daily metal production | Approximately 20 kg | Proposed design |
| Carbon regeneration batch size | 500 kg | Proposed design |
| Carbon regeneration temperature | 650–750 °C | Proposed design |
| Carbon regeneration duration | Approximately 15 minutes | Proposed design |
| Tailings density | 40% solids | Proposed design |
Project website: https://ca.finance.yahoo.com/news/first-gold-poured-bissa-mine-141100232.html
Technical qualifications
All processing information presented in this report is based on proposed design parameters for the Bissa mine. No historical operating data or testwork results are included in the original recovery methods section. The report states that the CIL process route was proposed for the Bissa mine at a nominal rate of 3 Mtpa, although the new design target throughput is nearer to 4 Mtpa. No actual plant performance data, metallurgical recoveries, or economic outcomes are reported in the recovery methods section.
Source: Bissa Asset, NI 43-101 Technical Report, July 2012, Sections 17–17.9.
