The Mineração Serra Grande Project is an operating gold mine located in Goiás, Brazil, owned by Aura Minerals Inc. The Serra Grande Metallurgical Plant began operations in October 1989 and currently processes an average of 4,200 tpd of ore (1,500,000 tpa) with a reported metallurgical recovery of 95%. The plant operates on a schedule of three 8-hour shifts.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Plant name | Serra Grande Metallurgical Plant | Not applicable | Located at Mineração Serra Grande Project |
| Start of operations | October 1989 | Not applicable | Not stated |
| Current processing capacity | 4,200 | tpd | 1,500,000 tpa |
| Metallurgical recovery | 95 | % | Reported current value |
| Work schedule | 3 x 8-hour shifts | Not applicable | Not stated |
| Number of leaching tanks | 16 | tanks | 202 m³ each; 8 also used for adsorption |
| Additional tanks | 4 | tanks | 404 m³ each |
| Mill 1 feed rate (dry basis) | 120 | tph | Circulating load 400% |
| Mill 2 feed rate (dry basis) | 45 | tph | Circulating load 300% |
| Mill feed particle size (F80) | 4.7 | mm | 100% smaller than 8.0 mm |
| Final crushing product target | 100% smaller than 1/4 | inch | Design specification |
| Silo 1 capacity | 1,800 | t | Not stated |
| Silo 2 capacity | 1,100 | t | Not stated |
| Pre-lime contact time | 3.5 | hours | At 58% solids |
| Leaching residence time | 22 | hours | Not stated |
| Activated carbon concentration in CIL | 25 | g/L | Average |
| Sodium cyanide solution concentration | 24 | % w/v | Added at tank 4 |
| Free cyanide in tank 4 | 400 | ppm | Controlled |
| Free cyanide in last tank | 100 | ppm | Not stated |
| pH in last tank | 10 | Not applicable | Not stated |
| Dissolved oxygen required | 12 | mg/l | Maintained by hydrogen peroxide addition |
| Dissolved oxygen in leaching pulp | 4 to 7 | mg/l | Range |
| Elution temperature | 95 | °C | Not stated |
| Sodium hydroxide concentration (elution) | 2 | % | Pre-prepared solution |
| Hydrated ethyl alcohol concentration | 2.2 | % | Added to pump suction |
| Acid wash HCl concentration | 3 | % | Cold solution |
| Acid wash duration (active) | 80 | minutes | Not stated |
| NaOH neutralization duration | 40 | minutes | 20% solution |
| Water wash duration | 60 | minutes | For chloride removal |
| Elution cycle duration | 12 | hours | Approximate |
| Regeneration furnace temperature | 750 | °C | Oxygen-free atmosphere |
| Final ingot purity | 85 to 94 | % | After smelting |
| Final gold purity after electrorefining | 99.9 | % | At Asahi Refining Canada |
Overview
The Serra Grande Metallurgical Plant uses a conventional gold recovery flowsheet built around crushing, grinding, gravity concentration, and carbon-in-leach (CIL) technology. Ore comes from multiple mines and is blended in the crushing area before processing. The plant produces gold concentrate from two separate circuits. A gravity circuit captures free gold using centrifugal concentrators. The CIL circuit recovers dissolved gold through adsorption onto activated carbon. Final gold production comes from smelting cathodes produced in the electrodeposition cells.
The flowsheet includes crushing in three stages, single-stage ball milling, gravity concentration, thickening, pre-lime treatment, cyanidation, CIL adsorption, acid scrubbing, elution, electrolysis, and carbon regeneration. Solid tailings go to filters and are stacked dry.
Key Process Stages
Crushing begins with trucks unloading ore into several piles in the crushing square. This arrangement allows blending of different ore types and feed control. A loader feeds a vibrating feeder hopper equipped with a frequency inverter that controls the primary crusher feed rate.
Primary crushing uses a single-shaft jaw crusher in open circuit. Material passes through an electromagnet to remove uncrushable items before going to a vibrating screen with two decks at a 2-inch and 1/4-inch opening, set at a 20-degree incline. Oversize material from both decks feeds the secondary crusher. Undersize from the 1/4-inch screen takes one of two routes. The dry route goes to two silos with capacities of 1,800 tonnes and 1,100 tonnes. The wet route feeds a spiral classifier. Classifier overflow pumps to the thickener. Classifier underflow feeds the silos.
The secondary crusher is a hydrocone type with an eccentric shaft and a 5/8-inch opening, also in open circuit. Its product feeds two vibrating screens. The first screen has 2-inch and 1/4-inch openings. The second screen has 1/4-inch and 5/16-inch openings. These screens work in closed circuit with the tertiary crushing stage. Tertiary crushing uses crusher 03, a hydrocone with an eccentric shaft and a final chamber opening of approximately 15 mm, and crusher 03A, a hydrocone with an eccentric shaft and a final chamber opening of approximately 13 mm. Undersize from the 5/16-inch deck feeds the conveyor belt to the silos. Oversize from both decks returns to the crushers, closing the circuit. The final crushing product should reach 100% smaller than 1/4 inch.
Milling uses a single-stage, closed-circuit, wet grinding configuration. Two ball mills operate in parallel with classification hydrocyclones. Mill underflow is the circulating load. Overflow is the milling product. Mill 1 has a feed rate of 120 tph on a dry basis and a 400% circulating load. Mill 2 has a feed rate of 45 tph on a dry basis and a 300% circulating load. Typical ore feed particle size to the mills is 100% smaller than 8.0 mm, with an F80 of 4.7 mm.
The gravity concentration circuit uses two centrifugal concentrators, model Knelson XD30. These treat approximately 15% of the mill 01 circulating load mass and 40% of the mill 02 circulating load mass. Concentrator tailings return to the mills. Concentrate feeds the intensive leaching system.
Intensive cyanide leaching produces a rich solution that pumps to an electrolytic cell. Solubilized gold is recovered through electrorecovery. Overflow from the intensive leacher contains approximately 80% of particles passing a 150 mesh (106 µm) screen. This size ensures gold exposure and release. The overflow passes through a vibrating screen to remove undesirable material before going to a conventional thickener for solids percentage adjustment.
The thickener operates by gravity feed from two sources. Hydrocyclone overflow from the grinding mills comes in at 30% solids. Spiral classifier overflow from the crushing area also feeds the thickener. The thickened underflow pulp has approximately 58% solids and pumps to the leaching area. Thickener overflow returns to the grinding mill as process water.
The leaching area uses 16 tanks with a capacity of 202 m³ each. Eight of these tanks also serve for adsorption. Four additional tanks, each with a capacity of 404 m³, complete the circuit. All tanks use mechanical agitation with a propeller plus compressed air injection at the bottom.
Pre-lime treatment occurs in the first three tanks. This stage homogenizes pH and pre-oxidizes cyanicidal agents through peroxide addition. pH must be maintained at approximately 10.5. Control uses lime added to the mill feed. A pH meter in the first tank provides online analysis, and the operator makes corrections based on results. Dissolved oxygen concentration must be maintained at 12 mg/l through hydrogen peroxide addition, which passivates cyanicides in the ore. Contact time in pre-lime is approximately 3.5 hours for pulp with 58% solids.
Cyanidation begins in tank 4. A 24% w/v sodium cyanide solution is added here. Free cyanide concentration in this tank is controlled at 400 ppm. Free cyanide and pH decrease gradually through the tank series, reaching 100 ppm and pH 10 respectively in the last tank. Dissolved oxygen concentration in the pulp ranges between 4 and 7 mg/l. Residence time for leaching is approximately 22 hours.
The last eight tanks perform hybrid leaching and adsorption as a carbon-in-leach circuit. Pulp from leaching goes through a trash screen and a distributor feeding the CIL circuit. Activated carbon is present at an average concentration of 25 g/L. All tanks have rotating interstage screens. Pulp flows by gravity through interstage screens from the first CIL tank to successive tanks. Carbon flows in batch mode countercurrent to the ore pulp. Regenerated or new carbon is added to the last or penultimate tank. Carbon transfer between tanks uses vertical pumps installed in each tank, pumping carbon to the previous tank. Carbon moves daily from tank to tank until it reaches the first or second tank. Carbon in the first CIL tank reaches its maximum gold load. Interstage screens retain carbon while allowing pulp to flow between tanks. Compressed air is added in adsorption tanks to facilitate oxygen-driven gold leaching.
Loaded carbon from the first adsorption tank pumps to a loaded carbon screen that separates carbon from pulp. Pulp returns to the same tank. Carbon retained on the screen discharges by gravity into the loaded carbon tank with capacity for one day of storage.
Elution and regeneration begin with the loaded carbon tank located above the acid scrubbing column. Carbon transfers by gravity. Acid washing occurs in batches lasting approximately 3 hours. A 3% cold hydrochloric acid solution recirculates for 80 minutes to remove precipitated carbonates. pH is then neutralized using a 20% sodium hydroxide solution for 40 minutes. Carbon is washed with process water for 60 minutes to remove chlorides. Loaded carbon then transfers to one of the elution columns through an ejector.
Gold desorption occurs in columns using a pre-prepared 2% sodium hydroxide solution at 95°C. Hydrated ethyl alcohol at 2.2% is added directly to the pump suction feeding the columns. The solution passes through a heater and injects into the column bottom, percolating upward through the carbon bed. The eluate overflows from the column top into a flow tank and pumps to the electrodeposition cell. Depleted solution from electrolysis transfers to a preparation and storage tank, then pumps back into the elution column, closing the circuit. The complete cycle lasts approximately 12 hours.
After elution, carbon transfers via ejectors to a dewatering screen in the regeneration area. Screen oversize falls into the regeneration furnace feed tank. Thermal reactivation uses a rotary kiln with external gas heating. Carbon is heated to 750°C in an oxygen-free atmosphere maintained by steam. Hot carbon discharges into a carbon cooling and storage tank. This tank also receives new carbon to replace carbon consumed in the circuit. Regenerated and new carbon is pumped by recessed rotor slurry pumps for adsorption. The water-carbon slurry passes through a static screen for dewatering before reaching the last adsorption tank. Dewatered carbon discharges by gravity into the last CIL tank.
The smelting process receives and melts cathodes from the gold concentrate obtained through adsorption and gravimetric circuits by electrolysis. Ingots from smelting have purity levels of approximately 85% to 94%. These are sent to Asahi Refining Canada in Brampton, Ontario, for electrorefining to produce gold with 99.9% purity.
Additional Interesting Data and Summary
The plant has undergone a series of optimizations since its original 1,300 tpd capacity. The current 4,200 tpd throughput represents roughly a threefold increase over the original design capacity. The flowsheet demonstrates multiple recovery pathways for gold, combining gravity concentration with intensive leaching for coarse gold and CIL technology for finer gold.
The crushing circuit includes a dual-route system with both dry and wet classification options. The wet route uses a spiral classifier whose overflow can bypass the milling circuit entirely by feeding directly to the thickener.
Mill 1 operates with a significantly higher circulating load at 400% compared to mill 2 at 300%. The gravity circuit treats a larger proportion of mill 2's circulating load at 40% versus 15% for mill 1.
The CIL circuit uses larger tanks for part of the circuit. Four tanks at 404 m³ each supplement 16 tanks at 202 m³ each. The pre-lime step at 3.5 hours is relatively short compared to the 22-hour leaching residence time.
The elution circuit operates in a closed loop with a 12-hour cycle. Carbon regeneration reaches 750°C in a reducing atmosphere. The final gold product achieves 85% to 94% purity at site before being sent to Canada for refining to 99.9% purity.
Key Processes
- Three-stage crushing (jaw, hydrocone, hydrocone) achieving final product 100% smaller than 1/4 inch
- Single-stage wet ball milling in closed circuit with hydrocyclones
- Gravity concentration using Knelson XD30 centrifugal concentrators treating 15-40% of circulating load
- Intensive cyanide leaching of gravity concentrate with electrorecovery
- Pre-lime treatment at pH 10.5 with peroxide addition for cyanicide passivation
- Cyanidation with 24% w/v NaCN solution, controlled at 400 ppm free cyanide
- Carbon-in-leach adsorption with 25 g/L activated carbon concentration
- Acid scrubbing using 3% HCl followed by NaOH neutralization
- Gold elution at 95°C with 2% NaOH and 2.2% ethyl alcohol
- Thermal carbon regeneration at 750°C in rotary kiln
- Electrodeposition for gold recovery from solution
- Smelting of cathodes to produce gold ingots at 85-94% purity
- Off-site electrorefining to 99.9% gold purity
Source: NI 43-101 Technical Report on Mineral Resource and Mineral Reserve Mineração Serra Grande Project, Goiás, Brazil, November 30, 2025. Project website: Mineração Serra Grande Project
