Diamba Sud Process Design Balances Oxide Start, Fresh Ore Future

Fortuna Mining Corp. plans a single-stage SAG milling circuit with pebble crushing and a conventional CIL gold recovery train for its Diamba Sud project in Senegal. The plant is designed to treat a blend of oxide and fresh ore, with a layout that allows for a future SAG-ball mill conversion.

Opening Background

The Diamba Sud Gold Project, held by Fortuna Mining Corp. through its indirectly owned Senegalese subsidiary Boya S.A., is a proposed open pit gold mining operation in the Kédougou Region of Senegal, about 665 km southeast of Dakar and 83 km from the town of Kédougou. The project, which contains seven gold deposits in the northern DS1 block, is the subject of a feasibility study based on Mineral Resource and Mineral Reserve estimates with an effective date of April 10, 2026. Boya applied for an exploitation permit in February 2026. Its retention period for the exploration permit was extended 60 days to August 21, 2026, to allow filing of the feasibility study. Once an exploitation permit is granted, the State of Senegal will receive a 10% free carried interest in the new operating entity, with Fortuna holding the remaining 90%.

Critical Data

Overview

The process plant at Diamba Sud is a conventional free-milling gold flowsheet built around crushing, SAG milling with pebble crushing, gravity recovery, and carbon in leach (CIL) with elution, electrowinning, and smelting. The design was developed from a metallurgical testwork program managed by Maca Interquip Mintrex and completed by ALS Metallurgy in Perth, supported by comminution circuit modelling by Orway Minerals Consultants.

Testwork showed the mineralization is free-milling with a low proportion of fine gold locked in sulfides, and the material is amenable to standard cyanidation. Comminution testing indicated the ore is moderate to hard, with some variability between oxide and fresh domains. The circuit selection process compared a single-stage SAG mill with pebble crushing against a SABC configuration, with the single-stage option chosen as the base case because of life-of-mine ore characteristics and operational flexibility. The plant layout includes space for a future ball mill that would allow conversion to a SABC circuit if needed.

Feed will come from open pit mines, blending oxide and fresh ore over the life of mine. The LOM schedule shows oxides as the dominant feed in Year 1, then fresh ore taking over from Year 2 onward.

Key Process Stages

Primary Crushing. Ore from the open pit is delivered by 100 t haul trucks to a ROM pad, where it is stockpiled by mineralization type and grade for blending. A front-end loader feeds a 130 m³ ROM bin, which discharges via an apron feeder into a single toggle jaw crusher rated at 185 kW. The crusher accepts nominal minus 800 mm rock, with a rock breaker handling oversize. Crushed product moves on a 1,500 mm wide conveyor to a surge bin with a live volume of approximately 103 m³, providing about 30 minutes of residence time. Excess material flows to a dead stockpile with a live capacity of roughly 7,500 dry tonnes, enough for approximately 24 hours of mill feed. Surge bin and reclaim feeders are controlled through a weightometer on the SAG mill feed conveyor.

Grinding and Classification. An 8.53 m diameter by 5.93 m long SAG mill performs primary grinding duty, with an installed power of 7.2 MW and a duty ball charge of 8–15%. The mill has a variable speed drive to adjust for changes in ore characteristics. During the initial oxide treatment period, the mill runs at about 50% of critical speed with a reduced ball charge of roughly 8.4%. As feed transitions to predominantly fresh ore, the mill speed increases to about 75% of critical speed with a ball charge around 10.1%. Mill discharge passes over a vibrating screen to separate pebbles, with undersize slurry reporting to a discharge hopper. Cyclone feed pumps in a duty/standby arrangement send slurry to a 20-pack classifying cyclone cluster with 13 operating cyclones. Cyclone overflow achieves a P80 of 106 µm and reports to the leaching circuit. A nominal 265% recirculating load is designed for oxide operation and 450% for fresh material.

Pebble Crushing. Oversize from the mill discharge screen reports to a 132 kW HP200 pebble crusher at a design rate of 144 dry t/h. The crusher operates at a closed side setting of 12 mm, with product returned to the SAG mill feed conveyor. A belt magnet and metal detector protect the crusher from tramp metal. If metal is detected, a flop gate bypasses the material back to the SAG mill.

Gravity Recovery. A portion of the cyclone underflow, from three cyclones, reports to a gravity scalping screen with a 2 mm aperture. Oversize from the screen returns to the SAG mill feed. Undersize feeds a single Knelson KC-QS40 centrifugal concentrator operating at a nominal feed rate of 225 t/h with a maximum of 250 t/h. The concentrator runs on a batch cycle of 30–40 minutes, producing a concentrate mass pull of roughly 50–65 kg per cycle. Concentrate goes to a Gekko ILR1000BA intensive leach reactor capable of treating up to 3,000 kg/day. Tailings from both the concentrator and reactor return to the mill discharge hopper for retreatment.

Trash Screening and Pre-Leach Thickening. Cyclone overflow passes over a trash screen, with oversize reporting to a trash bin. The slurry can bypass the thickener and go directly to leaching, typically during oxide treatment, or pass through a leach feed thickener to achieve the required solids concentration during fresh ore operations. Thickener overflow reports to the process water tank for reuse, and underflow is pumped to a distribution box feeding either the first or second CIL tank.

Carbon in Leach Circuit. The CIL train consists of seven tanks of nominal 1,920 m³ each, providing a total slurry residence time of 24 hours at 45% solids by weight for fresh ore. All tanks have hollow shaft agitators for air injection to satisfy oxygen requirements. Carbon is held in all tanks except the first, where the inter-stage screen acts as a safety screen. Recessed impeller carbon transfer pumps advance carbon between tanks. Loaded carbon from tank 2 undergoes an acid wash before elution. A vibrating carbon safety screen adjacent to tank 6 collects any escaping carbon for manual reintroduction.

Elution, Electrowinning and Smelting. The elution circuit is a split AARL system with a strip capacity of 6 t per cycle, using two columns: an acid wash column and an elution column. The strip solution, prepared with sodium hydroxide and sodium cyanide, is preheated to about 95 °C and then to an operating range of 125–140 °C. The elution sequence uses one bed volume of strip solution, four bed volumes of hot elution, two bed volumes of rinse water, and two bed volumes of cooldown water. Total acid wash time is about three hours, and the elution cycle about nine hours. Pregnant solution circulates through electrowinning cells for 8–12 hours. Barren carbon transfers to a dewatering screen before a regeneration kiln operating at a nominal feed rate of 300 kg/h over a 20-hour period per day. Gold sludge from both the gravity and elution circuits is calcined and smelted in a diesel-fired furnace to produce doré bars, with gravity and CIL sludges refined separately for metallurgical accounting.

Tailings Disposal. Tailings are pumped to a tailings storage facility through an above ground pipeline, except where buried at road crossings. The pipeline is laid in a fully bunded and lined trench. Leaks are detected by comparing readings from two flow meters, one at the plant and one at the TSF.

Additional Interesting Data and Summary

The process design carries a few notable elements worth flagging. The surge bin and dead stockpile arrangement means the mill can keep running through crusher downtime, with the stockpile holding roughly 24 hours of feed. Lime is added directly to the mill feed conveyor to raise slurry pH for cyanidation, and quicklime consumption is estimated at about 3.3 kt per year for oxide and 0.5 kt for fresh mineralization.

Power for the process plant comes from a planned site-based heavy fuel oil power plant, with distribution at 11 kV and 415 V. The water supply strategy draws 70–92% of process make-up water from recycled water from the TSF supernatant pond, supplemented by a water harvesting facility, groundwater, and open pit dewatering.

The qualified person for the report observed that the flowsheet is conventional and well-established for free-milling gold, and no significant technical risks were identified with the selected process design. Testwork coverage for certain deposits, including Southern Arc and Moungoundi, is comparatively limited, but the available results were considered sufficient to support the selected flowsheet and recovery assumptions.

Key Processes

• Conventional free-milling gold processing route with crushing, SAG milling, gravity recovery, CIL, and gold recovery via elution, electrowinning, and smelting.
• Single-stage SAG milling circuit with pebble crushing as the base case, with layout provision for future conversion to a SABC configuration.
• Gravity recovery circuit treating 24–32% of cyclone underflow, with intensive leach of concentrate.
• Flexible CIL operation with one pre-leach tank and six leach tanks, providing approximately 24 hours of total residence time.
• Pre-leach thickening for fresh ore operations.

Source: Diamba Sud Gold Project Technical Report, July 13, 2026. Project website: Diamba Sud Gold Project