The recovery methods at the Almas gold project in Brazil use a conventional flowsheet of crushing, grinding, gravity concentration, and cyanide leaching. A recent expansion to eleven leach tanks has restored recovery near design levels, and a further expansion targeting 3 Mtpa is in detailed engineering.
The Almas Project, owned and operated by Aura Minerals Inc., is an operating gold mine located in the Tocantins State of Brazil. The project has been in production since June 2023 and is currently in an expansion phase. The process plant was commissioned with an original design throughput of approximately 1.3 Mtpa of run-of-mine (ROM) ore. A series of phased expansions have since lifted throughput, with a Phase 2 expansion completed in December 2025 that brought the plant to a nominal capacity of 2.35 Mtpa. A Phase 3 expansion, currently in detailed engineering, targets a further increase to 3 Mtpa.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Nominal Plant Throughput (Original Design) | 5,479 | tpd | Design value |
| Head Grade (Design) | 1.13 | g/t Au | Design value |
| Crushing Plant Availability | 72 | % | Design value |
| Mill Availability | 92 | % | Design value |
| Bond Ball Mill Work Index (BWi) | 10.1 | kWh/t | Test result |
| Primary Grind Size (P80) | 75 | µm | Design value |
| L-CIL Residence Time | 17.4 | h | Design value |
| L-CIL Extraction | 92.5 | % | Design value |
| Gold Recovery (2023) | 90.3 | % | Test result |
| Gold Recovery (2024) | 90.7 | % | Test result |
| Gold Recovery (2025) | 88.5 | % | Test result |
| Pre-leach thickener settling rate | 48.4 | t/d/m² | Design value |
| Detox Residence Time | 101 | minutes | Design value |
| Phase 3 Target Throughput | 3 | Mtpa | Design value |
| Phase 3 Crushing Capacity | 4.0 | Mtpa | Design value |
| Phase 3 Cyclone Circulating Load | 2,583 | t/h | Design value |
| UG Recovery Assumption | 85.17 | % | Engineering estimate |
Overview
The process design is based on test work completed for a 2021 feasibility study and historical testing. The flowsheet selected for that study uses typical industry unit operations for gold processing plants. The original flowsheet includes primary crushing followed by grinding to achieve a particle size distribution of 80% passing 75 µm. Part of the cyclone underflow is processed in a gravity circuit. The cyclone overflow feeds a pre-leach thickener. Thickener underflow is processed through a leach-CIL circuit. CIL tailings are treated for cyanide destruction. Loaded carbon from CIL goes to elution, and the eluate solution goes to electrowinning in the gold room followed by refining.
As part of the Phase 2 Expansion, four additional leach tanks were installed. This modification increased the available leach volume and maintained the residence time required for the higher plant feed rate. Two high-frequency trash screens were installed to replace the linear screen previously used on the cyclone overflow. A secondary hydrocyclone cluster is planned for installation. Under the modified configuration, the cluster is designed to receive the primary cyclone overflow, with underflow directed to the leach circuit and overflow routed to the thickener. These changes are intended to support a nominal plant throughput of 2.35 Mtpa.
Key Process Stages
The process plant has not reached the design throughput of 5,479 tpd. In 2024 the average throughput was 4,300 tpd. The process design gold leach extraction is 92.5%. The operating plant averaged 90.8% gold leach extraction since commissioning in 2023, which is 2% below design. Aura implemented a series of incremental debottlenecking measures to increase plant throughput. Operating data show that as throughput increased above the original 1.3 Mtpa, the effective CIL residence time decreased and overall gold recovery declined.
In August 2023, throughput was increased to approximately 1.5 Mtpa by upgrading the crushing plant conveyor motors. Average overall recovery decreased to about 90.8%. In August 2024, during what is referred to as Expansion Phase 1, throughput was further increased to about 1.8 Mtpa by upgrading the tailings pumping system with higher capacity pumps and a larger pipeline diameter. Average overall recovery decreased to around 89.6%. In April 2025, additional incremental changes, including installation of a dilution line to the pre-leach thickener, upgrading the grinding feed conveyor motor, and upgrading the thickener underflow pump, allowed throughput to reach approximately 2.0 Mtpa. At this rate, the average overall recovery further decreased to about 88.7%.
To restore gold recovery to design levels at higher throughput, Aura developed and implemented an expansion of the leach/CIL circuit. The key elements of Expansion Phase 2, completed in December 2025, were the installation of four additional CIL tanks, increasing the CIL circuit from seven to eleven tanks in series. Existing linear screen on the cyclone overflow was replaced with two high-frequency screens for trash removal to the pre-leach thickener. Provision was made for a second hydrocyclone cluster receiving the primary cyclone overflow, with underflow reporting directly to the CIL circuit and overflow reporting to the thickener. The expanded CIL circuit was engineered to support a nominal throughput of approximately 2.35 Mtpa, restoring an effective leach/CIL residence time comparable to the original 1.3 Mtpa design. Following commissioning of the additional tanks, plant data indicates that overall recovery increased to approximately 92%.
Current detailed engineering work for Expansion Phase 3 is documented in the Phase 3 process design criteria and associated mass balance calculations. The objective of Phase 3 is to increase the sustainable plant capacity from 2.35 Mtpa to 3 Mtpa. Phase 3 includes the detailed engineering and construction of a new crushing plant with a nominal design capacity of 4.0 Mtpa ROM, intended to serve both the 3 Mtpa Phase 3 case and the conceptual 4.0 Mtpa Phase 4 case. The new crushing facility comprises a ROM feed hopper and truck dump with fixed grizzly and rock breaker, a vibrating grizzly feeder, a primary jaw crusher, a double-deck inclined vibrating screen, a secondary cone crusher, and a conical crushed ore stockpile with reclaim tunnel and three vibrating feeders.
Although the new crushing plant is designed for a nominal capacity of 4.0 Mtpa, the overall plant throughput in Phase 3 is constrained to 3 Mtpa by the capacities of the grinding and CIL circuits. The additional crushing capacity is a strategic provision for the potential Phase 4 expansion.
Additional Interesting Data and Summary
Internal technical documentation considers a scenario in which the plant is operated at throughputs of approximately 3.0 Mtpa. Based on the documented trend of decreasing recovery as throughput increased from 1.3 Mtpa to 2.0 Mtpa without additional CIL volume, it is reasonable to expect overall gold recovery at approximately 89% if the plant operates in the order of 3.0 Mtpa without a thermal carbon regeneration plant and without additional CIL volume. This expectation shows reduced effective CIL residence time at higher throughput, progressive carbon fouling in the absence of regeneration, and higher dissolved gold concentrations in solution across the CIL train.
The Phase 3 technical note indicates that installation of a thermal carbon regeneration furnace could significantly improve carbon activity. Under such improved conditions, it is technically plausible that overall recoveries at approximately 3.0 Mtpa could approach the low 90s. For the purposes of this technical report, 3.0 Mtpa operation with thermal regeneration should be regarded as a potential upside case until sufficient operating data are available to confirm sustained performance at that throughput.
The qualified person notes that underground material has not yet been the subject of a dedicated metallurgical test work program. All existing comminution parameters used in the current process design were developed from near surface open pit samples. For planning purposes, Aura applies an assumed gold recovery of approximately 85.17% for UG material. This figure is intended to be conservative. The QP considers it appropriate to declare Mineral Reserves that include UG material using this conservative recovery assumption so long as Aura commits to undertaking dedicated UG metallurgical test work in advance of UG production.
Key Processes
- Primary crushing via a mobile jaw crusher fed by a vibrating grizzly, with crushed material conveyed to a surge bin providing 1.6 hours of live storage.
- Single-stage SAG mill in closed circuit with hydrocyclones, with a SAG mill feed size F80 of 85 mm and product size P80 of 75 µm.
- Gravity concentration circuit using one semi-batch centrifugal concentrator fed from a portion of the cyclone underflow.
- Intensive leach reactor for gravity concentrate using a heated mixture of NaCN, NaOH, and LeachAid.
- Pre-leach thickening to increase solids concentration to 50% w/w ahead of leaching.
- Leach and adsorption circuit with one leach tank and ten CIL tanks providing 17.4 hours total residence time.
- Cyanide destruction using the SO2/air method with two tanks reducing CNWAD from 150 mg/L to less than 2.0 mg/L.
- Zadra elution for gold stripping operating at 140°C and four to six bar pressure.
- Electrowinning and smelting to produce doré bars in a secure gold room.
Source: NI 43-101 Technical Report, Almas Project, Tocantins State, Brazil, December 31, 2025. Project website: Almas Project
Consolidated report summaries
The following former standalone pages about this operation were consolidated here on October 25, 2026. They are retained for source traceability; dates and figures reflect each report’s own effective date and may be superseded by the current information above.
Almas Gold Project — 2021 Technical Report
This technical report presents a proposed gold processing plant design for the Almas Gold Project, based on feasibility-study testwork and historical testing.
Report context
The technical report is dated March 10, 2021, and describes a process design developed for a feasibility study. The design is based on results from several testwork programs, including testwork completed for the feasibility study and historical testing that evaluated different flowsheet options. The selected flowsheet is based on typical industry unit operations for gold processing plants.
Processing route
Overall process design
The flowsheet includes primary crushing followed by grinding to achieve a particle size distribution of 80% passing 75 µm. Part of the cyclone underflow will be processed in a gravity circuit and the cyclone product (overflow) will feed a pre-leach thickener, with the underflow processed through a leach/carbon in leach (CIL) circuit. CIL tailings will be treated for cyanide destruction. The carbon from CIL will go to elution, regeneration and the final solution will go to electrowinning and the gold room.
Primary crushing and stockpiling
The crushing circuit is designed for an annual operating time of 6,130 h or 70% availability at the capacity of 3,560 t/d. Material is hauled from the mine or stockpiles and fed by front-end loader into the mobile crushing system, which is composed of a run-of-mine (ROM) hopper, a vibrating grizzly feeder, a primary crusher and a discharge conveyor, along with auxiliary equipment. Provision for dumping on the ROM pad for blending and re-handling into the ROM hopper is provided. Material from the ROM hopper is crushed by a primary jaw crusher. ROM hopper material is reclaimed by a vibrating grizzly at 212 t/h to feed the jaw crusher.
A mobile rock breaker is utilized to break oversize rocks at the feed to the jaw crusher. The crushed material is conveyed to a surge bin that provides approximately 3 hours of live storage at the nominal processing rate. The bin has an overflow system, which forms an emergency stockpile next to the bin. Given the milling operation is designed for an annual operating time of 8,059 h or 92% availability, this will result in excess crushed material production when the crusher is operational. The mill feed surge bin is equipped with two vibrating feeders to regulate feed at 161 t/h into the SAG mill.
Grinding and classification
The grinding circuit includes a low-aspect SAG mill with trommel screen and cyclone classification. Pebbles from the SAG mill are fed to a recycle circuit via conveyor and discharged on the SAG mill feed conveyor to recycle to the SAG mill. The transfer point of the pebble recycle conveyor has a chute that allows purging of the pebbles as required.
Gravity recovery
Gravity recovery of the cyclone underflow slurry is performed by one semi-batch centrifugal gravity concentrator. The gravity circuit feed source is cyclone underflow slurry, with 25% of the cyclone underflow directed to the gravity circuit. Scalping screen undersize is fed to the centrifugal concentrator. The gravity concentrate is collected in the concentrate storage cone and subsequently leached by the intensive cyanidation reactor circuit. The tails from the gravity concentrator report to the gravity tails pumpbox.
Intensive leach reactor
Concentrate from the gravity circuit reports to the intensive leach reactor (ILR) to extract the contained gold by intensive cyanidation. The concentrate from the gravity concentrator is directed to the ILR gravity concentrate storage cone and de-slimed before transfer to the ILR. ILR leach solution (mixture of NaCN, NaOH and LeachAid – an oxidant) is made up within the heated ILR reactor vessel feed tank. The leached residue within the reaction vessel is washed, with wash water recovered to the reaction vessel feed tank, and then the solid gravity leach tailings are pumped to the CIL circuit. The ILR pregnant leach solution is pumped from the reaction vessel feed tank to the ILR pregnant solution tank located in the gold room. ILR pregnant solution is treated in the gold room for gold recovery as gold sludge using a dedicated electrowinning cell.
Pre-leach thickening
Trash screen undersize feeds the pre-leach thickener, which increases the solids concentration to 50% (w/w) prior to the leach-CIL circuit. Flocculant is added to the thickener feed to improve solids settling. The thickener overflow is reused as process water throughout the plant, mainly at the cyclone feed pumpbox.
Leach and adsorption circuit
The leach-adsorption circuit consists of one leach tank and six carbon-in-leach (CIL) tanks. The circuit is fed by the pre-leach thickener. The leach and CIL tanks are identical in size, with a total circuit residence time of 24 hours at 50% w/w density. Air is sparged to each tank to maintain adequate dissolved oxygen levels for leaching at 5-8 mg/L. Hydrated lime is added to adjust the operating pH to the desired set point of 10.5-11. Cyanide solution is added to the first leach tank. Fresh/regenerated carbon from the carbon regeneration circuit is returned to the last tank of the CIL circuit and is advanced counter-currently to the slurry flow by pumping slurry and carbon. Slurry from the last CIL tank gravitates to the cyanide detoxification tanks.
Cyanide destruction
CIL tails at approximately 50% w/w solids flow by gravity to the two cyanide destruction tanks. Each tank operates with a total residence time of approximately 60 mins to reduce weak acid dissociable cyanide (CNWAD) concentration from 150 mg/L to less than 2.0 mg/L to comply with environmental requirements prior to deposition in the TSF. Cyanide destruction is undertaken using the SO2/air method. The reagents required are air, lime, copper sulphate, and sodium metabisulphite (SMBS). From the detoxification tank, the tailings report to the carbon safety screen.
Carbon acid wash, elution and regeneration
Prior to gold stripping stage, loaded carbon is treated with a weak hydrochloric acid solution to remove calcium, magnesium, and other salt deposits. The acid-washed carbon is then hydraulically transferred to the elution column for gold stripping.
The gold stripping (elution) circuit uses the Anglo-American Research Laboratory (AARL) process. The elution sequence commences with the injection of a set volume of water into the bottom of the elution column, along with the simultaneous injection of cyanide and sodium hydroxide solution. During the pre-soak, the caustic/cyanide solution is circulated through the column and the elution heater until a temperature of 95°C is achieved. Upon completion of the pre-soak period, additional water is pumped through the trim heat exchanger and elution heater, then through the elution column to the pregnant eluate tank at a rate of 2.0 Bed Volumes (BV)/h. At this stage, the temperature of the strip solution passing through the column is increased to 120°C.
Carbon is reactivated in a gas-fired rotary kiln. Dewatered barren carbon from the stripping circuit is held in a 3-t kiln feed hopper. A screw feeder meters the carbon into the reactivation kiln, where it is heated to 650° to 750°C in an atmosphere of superheated steam to restore the activity of the carbon.
Electrowinning and gold room
Gold is recovered from the pregnant solution by electrowinning and smelted to produce doré bars. The pregnant solution from both elution and the intensive cyanidation circuit is pumped through one electrowinning cell with stainless steel mesh cathodes. The gold-rich sludge is washed off the steel cathodes in the electrowinning cell using high-pressure spray water and gravitates to the sludge hopper. The sludge is filtered, dried, mixed with fluxes, and smelted in an electrical induction furnace to produce gold doré.
Key reported parameters
| Design Parameter | Units | Value |
|---|---|---|
| Plant Throughput | t/d | 3,560 |
| Head Grade – Design | g/t Au | 1.58 |
| Crushing Plant Availability | % | 70 |
| Mill Availability | % | 92 |
| Bond Crusher Work Index (CWi) | kWh/t | 17.1 |
| Bond Ball Mill Work Index (BWi) | kWh/t | 10.1 |
| JK Axb | – | 47 |
| Bond Abrasion Index (Ai) | g | 0.069 |
| Primary Crusher | – | Metso C116 or Equivalent |
| Material Specific Gravity | t/m³ | 2.79 |
| Angle of Repose | degrees | 37 |
| Moisture Content | % | 5.0 |
| SAG Mill Dimensions | – | 5.0 m dia. X 9.0 m EGL |
| SAG Mill Installed Power | MW | 3.75 |
| SAG Mill Discharge Density | % w/w | 70 |
| SAG Mill Ball Charge | % v/v | 21 |
| Primary Grind size (P80) | µm | 75 |
| Gravity Circuit Feed Source | – | Cyclone underflow slurry |
| Gravity Circuit Feed Rate | % of cyclone underflow | 25 |
| Gravity Circuit Recovery | Au (%) | 17.5 |
| Pre-leach thickener settling rate | t/d/m² | 34.4 |
| Pre-leach thickener diameter | m | 12 |
| L-CIL Residence Time | h | 24 |
| L-CIL Extraction | Au (%) | 92.5 |
| L-CIL Operating Density | % w/w | 50 |
| L-CIL Dissolved Oxygen Target | mg/L | 5-8 |
| L-CIL pH Target | – | 10.5 – 11.0 |
| CIL Carbon Concentration | g/L | 15 |
| L-CIL Sodium Cyanide Addition | kg/t | 0.8 |
| L-CIL Hydrated Lime Addition | kg Ca(OH)2/t | 0.26 |
| Leach & CIL Tanks | # | 1 + 6 |
| Elution Circuit Capacity | t | 3.0 |
| Detox Residence Time | minutes | 120 |
| Detox Oxygen Addition Rate (weight) | O2:SO2 | 3.0 |
| Detox Feed Cyanide Concentration | mg/L CNWAD | 150 |
| Detox Cyanide Discharge Target | mg/L CNWAD | <2.0 |
| Detox Copper Sulphate Addition | mg/L Cu+2 | 50 |
| Detox SO2 Addition (weight) | SO2:CNWAD | 5.5 |
| Detox Lime Addition (weight) | CaO:SO2 | 1.0 |
Project website: https://www.juniorminingnetwork.com/junior-miner-news/press-releases/585-nasdaq/augo/92498-aura-minerals-approves-development-of-almas-gold-project.html
Technical qualifications
The process design is based on the results of several testwork programs, including testwork completed for the feasibility study and historical testing. Historical testing evaluated different flowsheet options. The flowsheet selected for the feasibility study is based on typical industry unit operations for gold processing plants. The report does not provide actual operating data from a built plant, as the design is proposed. Key process design criteria are presented as design values, not confirmed operating results. The report states that a source for the flowsheet and layout drawings is Ausenco, 2020.
*Source: NI 43-101 – Almas Gold Project – March 10, 2021, Section 17 Recovery Methods*
