Marmato Lower Mine PFS Adds a 4,000 t/d SAG-Ball, Gravity and Leach-CIP Plant Beside the 1,200 t/d Flotation and Concentrate-Leach Upper Mine Mill

Aris Mining's 2022 prefeasibility study for the Marmato gold mine in Caldas, Colombia, describes two plants. The operating Upper Mine plant recovered 90.8% of the gold in 2021. The planned Lower Mine plant is designed for 95.4% gold recovery.

The Technical Report for the Marmato Gold Mine, Caldas Department, Colombia, Pre-Feasibility Study of the Lower Mine Expansion Project, was prepared by SRK Consulting and others for Aris Mining Corporation (formerly GCM Mining). Its effective date is June 30, 2022, and its report date is November 23, 2022. The Upper Mine plant has been expanded several times to its 1,200 t/d rated capacity. It uses three-stage crushing, ball milling, gravity and flotation, cyanide leaching of the reground concentrates, counter-current decantation (CCD) and Merrill-Crowe. The proposed Lower Mine plant, designed with Ausenco input and SGS Lakefield testwork, treats 4,000 t/d (1.46 Mt a year) by whole-ore leaching. It uses two-stage crushing, SAG and ball milling, gravity with intensive leaching, and leach and carbon-in-pulp (CIP).

Critical Data

Overview

At the Upper Mine plant, ore is crushed in three stages. It is ground in an open-circuit primary ball mill with gravity recovery and a secondary ball mill closed with cyclones. Rougher, scavenger and cleaner flotation follows. The gravity and flotation concentrates are reground and leached in two stages. Gold is recovered by CCD and Merrill-Crowe and smelted to doré. Tailings are detoxified, thickened and vacuum filtered for dry stacking, and part of the flotation tails is used as hydraulic backfill. The Lower Mine plant crushes ore in a jaw and a cone crusher and grinds it in a SAG mill and ball mill. Part of the cyclone underflow goes to gravity and intensive cyanidation. All ore is pre-aerated, leached and passed through CIP, and the tailings are detoxified, thickened and filtered.

Key Process Stages

Upper Mine crushing. 1.5 t rail cars dump into a hopper, and a slusher moves ore (500 mm top size) to a feed hopper and a vibrating grizzly ahead of a 25 x 40 inch jaw crusher. A double-deck screen closes the circuit with a 250 hp Nordberg Omnicone secondary and a 300 hp tertiary cone crusher. The product, minus 9.5 mm, goes to the fine ore bin. Grizzly undersize goes to a spiral classifier.

Upper Mine grinding, gravity and flotation. A 600 hp Allis Chalmers ball mill (9.5 x 14 ft) in open circuit discharges to a gravity concentrator. Cyclones at P80 150 µm close a 300 hp secondary ball mill. Rougher and two scavenger stages give 40 minutes, followed by one cleaner stage. The combined concentrates are thickened to about 55% solids and reground in a 200 hp Hardinge mill to about P80 44 µm.

Upper Mine leaching and Merrill-Crowe. The concentrate is conditioned at pH 10.5 and thickened. The first leach stage runs at 700 mg/L NaCN, and a thickener sends its overflow to Merrill-Crowe. A second stage of six tanks follows, and three CCD thickeners wash the residue. Total leach time is 30 hours. Pregnant solution is clarified, deaerated to below 1 mg/L oxygen and precipitated with zinc dust. The precipitate is smelted with a 1:1 weight of flux in an LPG-fired furnace. The flux is 40% borax, 30% sodium nitrate, 20% soda ash and 10% silica.

Upper Mine tailings. Leach residue is detoxified to below 1 mg/L weak-acid-dissociable cyanide and combined with flotation tails. The combined tails are thickened in a 30 ft thickener at Cascabel, filtered on a 10-disc vacuum filter to 18% moisture and trucked to dry stack storage.

Lower Mine crushing and grinding. A surface jaw crusher with a 600 mm static grizzly and a 100 mm vibrating grizzly feeds a secondary screen and cone crusher. The product is 100% passing 65 mm and goes to a stockpile with 24 hours of capacity. Two belt feeders deliver up to 181 t/h. The SAG mill trommel oversize returns by high-angle conveyor. The ball mill is closed with cyclones, and 40% of the underflow goes to a 2 mm scalping screen, a centrifugal concentrator and an intensive cyanidation unit.

Lower Mine leach and CIP. After a trash screen, a pre-aeration tank passivates reactive sulphides such as pyrrhotite and pyrite. Four leach tanks at 43% solids follow, with hydrated lime to pH 10.5 to 11. Six CIP tanks then give one hour each, for a total of 24 hours.

Additional Interesting Data and Summary

Upper Mine gold recovery rose from 85.5 to 87.1% in earlier years to 90.8% in 2021 and 92.3% in the first half of 2022, after plant upgrades. Power contracts with CHEC and EPM rise from 4.5 MW to 8.5 MW by early 2023. The Upper Mine uses about 600 m³/d of water from underground dewatering, plant recycle and tailings reclaim.

In summary, the 2022 Marmato report covers an existing 1,200 t/d plant that leaches only the gravity and flotation concentrates, and a planned 4,000 t/d Lower Mine plant. The new plant uses whole-ore SAG-ball grinding to 105 µm, gravity with intensive leaching, and leach-CIP, for a design gold recovery of 95.4%.

Key Processes

• Three-stage crushing (Upper Mine) and two-stage crushing (Lower Mine)
• Ball milling with gravity concentration; SAG and ball milling (Lower Mine)
• Flotation and concentrate regrind (Upper Mine)
• Two-stage concentrate cyanidation, CCD and Merrill-Crowe (Upper Mine)
• Intensive cyanidation of gravity concentrate (Lower Mine)
• Pre-aeration, leach and carbon-in-pulp (Lower Mine)
• Cyanide detoxification
• Tailings thickening, filtration and dry stacking

Source: Technical Report for the Marmato Gold Mine, Caldas Department, Colombia, Pre-Feasibility Study of the Lower Mine Expansion Project, prepared by SRK Consulting and others for Aris Mining Corporation, effective date June 30, 2022.