This technical report describes the proposed processing route and tailings management design for the Baomahun Gold Project based on metallurgical testwork and feasibility-level engineering.
Report context
The Baomahun Feasibility Study NI 43-101 Technical Report, dated 28 June 2013, presents the process plant and tailings management facility design for the Baomahun Gold Project in Sierra Leone. The report documents the proposed recovery methods, design criteria, and ore characteristics derived from metallurgical testwork, client input, and industry standards.
Processing route
Comminution
A single stage SAG (SS SAG) mill was selected for the comminution circuit design, based on comminution testwork results, OMC modelling, and grind recovery trade-off analysis. The crushing circuit will utilise a heavy duty single stage jaw crusher treating oxide and sulphide ores. Ore reclaimed from a ROM bin by an apron feeder discharges onto a vibrating grizzly feeder, with oversize reporting to the jaw crusher for reduction to SAG mill feed size of 100% passing 236 mm (P80 of 140 mm). Crusher product and grizzly undersize will be conveyed to a 6,000 t live capacity stockpile.
The milling circuit includes an 8.53 m diameter by 6.42 m EGL SS SAG mill with installed power of 8,400 kW, capable of treating 255 tph of crushed ore, operating in closed circuit with a cyclone cluster. Cyclone overflow at 80% passing 75 μm for oxides and 80% passing 106 μm for sulphides gravitates to the CIL circuit. Cyclone underflow is recirculated through the SS SAG mill, with a fraction directed to the gravity circuit.
Gravity and In-Line Leaching
A gravity circuit was recommended ahead of CIL due to presence of free gold. Cyclone underflow to the gravity circuit will be screened ahead of a gravity concentrator to remove oversize material (+2 mm). Free coarse gold will be recovered using a Knelson centrifugal concentrator KC-XD30 or equivalent on a batch basis. Concentrate will be stored in a batch tank and offloaded every 24 hours into an in-line leach reactor (ILR1000BA) or equivalent, where cyanide at high concentrations will be added to leach gold, with hydrogen peroxide addition allowed to enhance kinetics.
Carbon-in-Leach
CIL was selected as the optimum route due to the presence of preg-robbers. The CIL circuit feed slurry (pulp flow rate of 383 m³/h) will be screened ahead of leaching. The circuit consists of one pre-leach tank and six CIL tanks, each with 1,700 m³ capacity giving 24 hours residence time. Two high shear reactors will be used on the pre-leach tank for oxygen injection to partially oxidise active sulphides prior to CIL feed, improving leach kinetics and overall gold dissolution. Provision will be made to inject oxygen into each CIL tank if needed. Cyanide solution will be added in the first three CIL tanks. Each CIL tank will be equipped with an interstage screen to prevent carbon migration. Carbon will be intermittently transferred upstream from the last CIL tank to the first using transfer pumps.
Cyanide Detoxification
A cyanide detox facility was included to comply with cyanide requirements for discharge at the tailings dam and to lower cyanide levels in return process water to prevent preg-robbing ahead of CIL. Tailings slurry from the last CIL tank will gravitate to the cyanide detoxification circuit through a vibrating screen recovering fugitive carbon. Hydrogen peroxide and copper sulphate will be added to the detox tank (480 m³, 1 hour residence time). Detoxified slurry will be pumped to the tailings dam. Detoxification will occur only if residual cyanide levels rise above 50 ppm. An additional cyanide detox facility will treat excess supernatant water from the pond prior to environmental discharge, using hydrogen peroxide, copper sulphate, and hydrochloric acid.
Elution, Electrowinning, and Gold Recovery
A 6-tonne batch of loaded carbon from the CIL will be washed with 3% hydrochloric acid solution and rinsed with water prior to elution. Gold will be stripped using the pressurised Zadra method over 14-24 hours, with heat provided by diesel fired heaters. Electrolyte from elution will be directed to an electrowinning cell with steel mesh cathodes for gold recovery by electrolysis. Gold sludge will be decanted, dried in an electric oven, mixed with fluxes, and smelted in a diesel fired furnace to produce gold bullion.
Reagents and Consumables
The process plant will use lime for pH control, sodium cyanide for gold leaching and elution, caustic soda for elution, hydrogen peroxide for cyanide detoxification, copper sulphate for cyanide detoxification, hydrochloric acid for washing loaded carbon, fluxes for smelting, activated carbon for gold adsorption, plant diesel for elution and regeneration kiln heating and smelting, and grinding media and mill and crusher liners for comminution.
Plant Control
Plant drives and valves critical to production will be operable automatically from the control room via a Supervisory Control and Data Acquisition (SCADA) system. The plant will be controlled by Programmable Logic Controllers (PLCs). All PID loops will be monitored and controlled from the SCADA, with analogue values logged historically for one month.
Tailings Management Facility
The Tailings Management Facility (TMF) was designed to store up to 23.3 Mt of tailings generated from ore processing over a 12-year life. The facility will be located northwest of the process plant, east of Pujehun Village. The TMF starter embankment will be formed to elevation 171 mRL using compacted selected lateritic earthfill, with sequential raises in the upstream direction to ultimate embankment crest elevation of 193.5 mRL.
The starter embankment and subsequent interim lifts are designed with 1V:2.5H downstream slope, developing an overall embankment downstream profile of approximately 1V:3.5H at final elevation. The starter embankment raises will be formed downstream to elevation 175.5 mRL due to rapid rate of rise in the first two years. Subsequent lifts will have rates of rise between 1.5 m and 3.0 m per year.
Storm water will be managed by diversion channels designed for 1:100 year return flow, freeboard, and spillways. Emergency spillways will be provided for all construction phases. Seepage modelling indicates rates between 0.07 m³/h and 0.19 m³/h for starter and ultimate embankment elevations respectively. Slope stability analysis indicates the TMF embankment meets required safety factor criteria of 1.5 for static and 1.2 for pseudo-static conditions.
Water Supply
The greatest contribution to process water will be recycled water from the TMF, augmented by rain run-off and water from a dedicated Water Storage Dam (WSD) of 1.2 Mm³ capacity. The WSD will comprise a low permeability earthfill embankment constructed to a maximum height of 12 m above the valley invert.
Key reported parameters
| Parameter | Units | Oxide Ore | Sulphide Ore | Source |
|---|---|---|---|---|
| Throughput | Mtpa | 2 | 2 | Design criteria |
| Portion of Ore Body | % | 5-8 | 92-95 | Client |
| Gold Head Grade (Design) | g/t Au | 2.30 | 2.30 | Testwork |
| SS SAG Mill Specific Energy | kWh/t | Not tested | 27 | Testwork |
| Bond Ball Mill Work Index (Design) | kWh/t | 9.5 | 15.8 | Testwork |
| Leach Feed Size (P80) | μm | 106 | 75 | Design criteria |
| Leach Time | hours | 24 | 24 | Testwork |
| Gravity Recovery | % of head grade | 37 | 30 | Testwork |
| CIL Dissolution | % of CIL feed grade | 94.84 | 91.52 | Calculated |
| Total Overall Recovery | % of head grade | 95.95 | 93.24 | Testwork |
| SAG Mill Power | kW | 8,400 | 8,400 | Design |
| Plant Availability | % | 95 | 95 | Design criteria |
| Cyanide Detox Threshold | ppm | 50 | 50 | Design criteria |
| TMF Storage Capacity | Mt | , | 23.3 | Design |
Project website: https://www.theglobeandmail.com/investing/markets/stocks/CLUGF/
Technical qualifications
The process design criteria were derived from Amara input, metallurgical testwork, calculated data, vendor data or recommendation, SENET in-house expertise, industry standard or practice, engineering handbook, assumption based on experience, and external consultants. Plant recovery was determined using batch laboratory dissolution tests obtained for each ore body.
The TMF feasibility level design was completed in accordance with recommendations of international best practice, the EC Mine Waste Directive Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities (BREF), the Equator Principles, and other international standards.
AMEC recommended that kinetic testing of representative samples from the waste rock and sulphide tailings should be carried out to confirm whether the potential for acid generation will be ultimately realised. Ground conditions and foundation loadings should be validated further at detailed design. A programme of further field investigations, topographic survey and laboratory test work will be required prior to detailed design and construction.
The mine water management plan is a dynamic model intended to be updated throughout the mine development and operational phases. Data on tailings deposition, geotechnical and geochemical properties, vegetation, hydrology and meteorology will be collected throughout the deposition period to ensure appropriate closure strategy.
*Source: Baomahun Feasibility Study NI 43-101 Technical Report, 28 June 2013, Sections 17.1 and 17.2*
