Monte do Carmo Project — 2023 Technical Report

The 2023 technical report presents a proposed processing route for the Monte do Carmo Gold Project, based on testwork and engineering design for a 1.92 Mtpa operation in Tocantins, Brazil.

Report context

The Monte do Carmo Gold Project, located in the state of Tocantins, Brazil, will be designed for processing 1,920,000 tonnes of ore per year (5,260.3 t/d). The process route was defined considering the results of process development tests described in Section 13 of the December 2023 Technical Report. The engineering design provided process design, process flow diagrams, mass and water balances, equipment lists, general arrangement drawings, preliminary piping and instrumentation diagrams, and single line electrical drawings.

Processing route

Three-stage crushing and primary crushing stockpile

The primary crushing plant functions on a continuous basis across two shifts, amounting to a total of 24 hours per day, for 365 working days a year. Ore transport to the plant occurs either via trucks or loaders, depositing ore into a hopper equipped with a static grid with an aperture allowing for a maximum Run of Mine dimension of 800 mm, with feed rate regulated by an apron feeder. Fines extraction is carried out in the vibrating grizzly, followed by the ore's introduction into the jaw crusher.

Subsequently, the material traverses a sacrificial belt conveyor before unloading onto conveyor 010-TC-02. The purpose of the sacrificial belt conveyor is to safeguard the primary conveyor against belt tearing. Conveyor 010-TC-02 directs the feed to the primary screen, where particles above 35 mm proceed to the secondary crusher. Both the oversize and the secondary crusher discharge are conveyed to belt conveyor 010-TC-03.

This conveyor discharges into a splitter, dividing the flow to two secondary screens. Particles above 9 mm are directed to conveyor 010-TC-04, while the undersize material is routed to belt conveyor 010-TC-05. Conveyor 010-TC-04 discharges into a feed bin equipped with belt feeders, supplying tertiary crushers. The tertiary crushers discharge onto conveyor 010-TC-03, completing the circulating load. Conveyor 010-TC-05 transports material to a stockpile in the milling area, with a live capacity of approximately 12 hours.

One stage ball mill grinding and classification

The ball mill receives various streams through a spout feeder: inlet dilution water, fresh ore, underflow from the classification circuit, gravity circuit and intensive leaching pulp, along with grinding media. A circulating load of 250% is expected with target grind size of P80 of 106 µm. The mill will discharge through a trommel into pump boxes and be pumped via pumps to the hydrocyclone cluster.

Gravity concentration and intensive leaching

The hydrocyclone overflow passes through two samplers and a trash screen. The screen underflow discharges onto vibrating screens with a 2 mm aperture size. The undersize of the screens will feed the hydrocyclone cluster, and the underflow of the hydrocyclone cluster will feed gravity concentrators. The oversize of the screens and the low-density fraction of the gravity concentrators proceed via gravity to the ball mill.

The concentrate (high-density fraction) from the gravity concentrators will be directed to an intensive leaching unit. The gravity concentrate will be batch processed in the intensive cyanidation unit in 24-hour intervals. The gravity concentrate will be leached to dissolve gold in a leach solution that includes sodium cyanide, caustic solution, and a leach accelerant. After the leach cycle is complete, the pregnant solution will be pumped to the electrowinning circuit while the intensive cyanidation unit residue will be pumped back into the mill.

Flotation of tailings from gravity concentration

Ahead of flotation, the milled ore in slurry will be pumped into the conditioning tank. Sufficient residence time is provided for conditioning for flotation chemicals including copper sulfate, amyl potassium xanthate, frother, and A-208. Lime will be added into the tank to adjust pH, if required.

In the conventional flotation circuit, the concentrate from the rougher cell is pumped to the cleaner stage for further upgrading, while the rougher tailings are directed to scavenger cells. The scavenger concentrate is recycled back to the rougher cell, and the scavenger tailings is transferred to the tailings thickener. Cleaner flotation concentrate will be pumped to the pre-leach thickener and cleaner flotation tailings will be pumped back to the rougher flotation feed box.

Bench test work has highlighted that the gold content and mass recoveries of both rougher and scavenger concentrates meet the conditions for direct integration into the CIL process. Subsequently, the flotation circuit will initially operate with only rougher and scavenger stages, whilst provision has been made for the cleaner circuit.

Carbon-in-leach of the flotation concentrate

Flotation concentrate will be pumped to a pre-leach feed thickener to increase slurry density for the downstream cyanidation process. The thickener overflow will report to a pre-leach thickener overflow tank which is then pumped to the process water tank.

The thickener underflow is pumped to the CIL circuit consisting of six tanks, equipped with agitators and interstage screens to retain the carbon in each tank for the necessary time. Sodium cyanide solution is added in the first tank, and if necessary, lime to maintain the target slurry pH. Carbon (new, eluted, or regenerated) is added in the last tank of the sequence, while loaded carbon will leave the CIL circuit from the first and second CIL tanks.

The pulp with carbon will be pumped countercurrent between tanks through indented rotor pumps. All CIL tanks will have air injection circuits for pulp oxidation. CIL tailings will pass over a safety screen and a vezin sampler before being pumped to neutralization.

Desorption, regeneration and gold room

The loaded carbon from the CIL circuit will be pumped and screened before being received in the rich carbon tank. For every 3 tonnes of accumulated carbon, the acid wash column will be fed by gravity. Hydrochloric acid solution will be circulated, followed by neutralization with caustic soda and washing before transporting loaded carbon to the elution column.

The elution circuit will be atmospheric using the Zadra Process. The eluate solution containing NaOH and NaCN will be prepared in tank 070-TQ-02. The eluate will be pumped through the carbon in the column, then circulated in the electrowinning cell and returned to the eluate tank.

After being stripped of gold, the carbon to be regenerated will pass through a rotating screen for dewatering before entering the silo and regeneration kiln. The regenerated carbon enters the quench tank from where it is pumped into the new carbon tank. Regenerated carbon returns to tanks in the CIL circuit.

The electrolysis and metallurgy area, located in the gold room, will have two electrodeposition circuits. The electrowinning cathodes will be manually transferred to the cathode washing tank where a high-pressure washer will be used to dislodge gold sludge from the cathode surface. The sludge will be filtered by a filter press. The resulting filter cake will be dried in a drying oven. The dried filter cake will then be transferred manually into the electric smelting furnace with flux materials where it will be batch smelted into gold doré bars and stored in a secure vault.

Cyanide detoxification, tailings thickening, filtration and disposal

The neutralization section consists of two tanks with agitators where hydrogen peroxide and copper sulfate will be added. The neutralized slurry is mixed with flotation tailings. All tailings are received in a transfer sump and pumped to the tailings thickener. The clarified overflow from the thickener gravitates to the process water tank.

Thickened tailings are pumped to agitated surge tanks before being pumped to the tailings filters. A filter cake with a target solid content of 78% (w/w) will be produced. The tailings filter cake will be transported by truck to Waste Pile 1, which contains co-deposited filtered tailings and mine waste rock. The filtrate will flow into the process water tank.

Fresh and reclaim water supply

The process water circuit incorporates water recovered in various thickening and filtration processes. Water make-up will be achieved using mine water accumulated in ponds and recovered by pumps. Raw water is delivered to the water tank using centrifugal pumps. It is also used as process water make-up, feeding the process water tank by gravity.

The main process water source comes from the overflows of the two thickeners. Other sources are filtrate from the tailings filtration process and drainage from the tailings dry stack decant reservoir. Process water can also be supplied from the raw water tank. Three process water pumps (two duty, one standby) supply process water to various consumers throughout the plant site, with the main consumer being the grinding circuit. The process water tank is constructed from mild steel and has a live volume ensuring 120 minutes of residence time.

Key reported parameters

Technical qualifications

The recovery methods section is based on the results of process development tests described in Section 13 of the Report. The engineering design provided process design, process flow diagrams, mass and water balances, equipment lists, general arrangement drawings, preliminary piping and instrumentation diagrams and single line electrical drawings. The reagent consumption rates are estimates based on testwork laboratory reports from 2018, 2021, 2022 and 2023. Detoxification tests using sodium metabisulfite have also been conducted, yielding positive results, and in the upcoming project phase, sodium metabisulfite may be considered as an alternative to hydrogen peroxide.

*Source: Monte do Carmo Project , 2023 Technical Report, Section 17 Recovery Methods, December 2023*