Overview
The A31 mining project, technically identified in major feasibility studies as the Soto Norte Project (under the stewardship of Aris Mining Corporation), represents one of the most significant and technically complex mineral processing developments in South America. Located in the mining-rich department of Santander, Colombia, the A31 mine is positioned within a region known for its high-grade gold-copper-silver deposits. The project is specifically designed to address the unique geological and environmental challenges of the area, balancing high-capacity industrial production with stringent environmental protections, particularly concerning the nearby Páramo de Santurbán ecosystem.
The A31 operation is anchored by a nominal throughput of 7.0 million tonnes per annum (Mtpa) of run-of-mine (ROM) feed. This massive scale places it among the tier-one gold projects globally. The processing philosophy is built around a hybrid recovery system capable of treating both oxidized saprolite material and deeper sulphide ores. By utilizing a sophisticated multi-stage recovery circuit, the project aims to produce two distinct doré products alongside a high-value copper-gold flotation concentrate. The significance of A31 extends beyond its throughput; it serves as a benchmark for sustainable mining in sensitive regions, employing a zero-discharge water strategy and advanced tailings management to ensure minimal footprint and maximal resource efficiency. This project highlights a strategic shift in the industry toward complex, high-recovery circuits that can adapt to varying mineralogical domains while maintaining high metallurgical efficiency.
Key Process Stages
The mineral processing circuit at the A31 mine is a multi-faceted system designed for flexibility and maximum metal recovery across different ore types. The flowsheet transitions from high-capacity comminution to a specialized separation process that includes gravity concentration, leaching, and flotation.
- Primary and Secondary Crushing: The ROM feed undergoes primary crushing (typically utilizing a large-scale gyratory crusher) to reduce the ore to a manageable size, followed by secondary crushing to prepare the feed for the grinding circuit.
- Grinding Circuit (SABC): The comminution stage features a Semi-Autogenous Grinding (SAG) mill and a Ball Mill operating in a closed circuit with hydrocyclones. This SABC (SAG-Ball-Crusher) arrangement ensures a consistent grind size (P80 of approximately 150 µm for flotation and finer for leaching).
- Upfront Gravity Gold Recovery: To minimize the “locked” gold in the circuit, an upfront gravity concentration stage is employed. This circuit uses centrifugal concentrators to capture free gold, which is then processed through intensive cyanidation to produce the first doré product.
- Carbon-in-Leach (CIL): Following gravity separation, the slurry reports to a CIL circuit. Here, the gold and silver are leached using cyanide and simultaneously adsorbed onto activated carbon. This stage is critical for recovering the fine, non-gravity gold from the oxide and transition ores.
- Sulphide Flotation: Specifically for the sulphide feed types, the circuit includes a flotation stage. The tailings from the leaching circuit or the primary ground ore are sent to rougher flotation cells, followed by regrinding and cleaning stages, to produce a high-grade copper-gold concentrate.
- Cyanide Destruction and Tailings Treatment: Environmental safety is ensured via an SO2/Air cyanide destruction circuit. For the gravity and intensive cyanidation streams, hydrogen peroxide dosing is utilized to neutralize cyanide before the solution is recirculated, ensuring a safe and closed-loop process.
Critical Data
The following technical parameters outline the design capacity and metallurgical expectations for the A31 processing plant over its Life of Mine (LoM).
| Parameter | Value | Unit |
|---|---|---|
| Nominal Annual Throughput | 7.0 | Mtpa |
| Target Gold Recovery (Sulphide) | 93.0 | % |
| Target Silver Recovery (Sulphide) | 78.0 | % |
| Target Copper Recovery (Sulphide) | 88.0 | % |
| Target Gold Recovery (Oxide) | 97.0 | % |
| Grind Size (Primary Flotation) | 150 | µm (P80) |
| Regrind Size (Concentrate) | 30 | µm (P80) |
| Operational Availability | 91.3 | % |
| Construction Workforce | 1,292 – 1,520 | Personnel |
Technical Details and Sustainability
The technical sophistication of the A31 project is driven by the need for high metallurgical performance in a geographically and socially sensitive area. One of the most critical aspects of the project’s design is its ability to handle “saprolite” ores. These weathered, clay-rich ores can often cause handling issues and negatively impact flotation recoveries. To mitigate this, the A31 flowsheet includes dedicated campaigns or stand-alone gravity wash stages (as seen in similar regional projects like Alacran) to ensure that fines do not interfere with the sulphide flotation kinetics. The regrinding stage for the flotation concentrate, bringing the particle size down to a P80 of 30 µm, is essential for liberating fine gold particles associated with copper sulphides, thereby maximizing the concentrate’s marketability and value.
Sustainability is the core pillar of the A31 development strategy. Recognizing the environmental value of the surrounding highlands, the project has committed to a “dry stack” tailings management facility (WMF). Unlike traditional wet tailings dams, dry stacking involves filtering the tailings to a low moisture content (approximately 8-10%) and compacting them into a stable landform. This drastically reduces the risk of dam failure and minimizes the project’s footprint. Furthermore, the A31 mine utilizes a zero-discharge water strategy. Process water is reclaimed from the tailings thickeners and filtration units and sent back to the milling circuit. Any cyanide-bearing streams are treated using the industry-standard SO2/Air or hydrogen peroxide methods to ensure that water quality meets or exceeds regulatory standards before any potential discharge or reuse.
Social sustainability is equally emphasized through the integration of local artisanal mining partners. The plant design for associated Aris Mining projects (such as Marmato) often includes a dedicated portion of the throughput—up to 750 tpd—reserved for processing ore from local contract mining partners. This provide a safe, regulated, and mercury-free alternative for the community, fostering a collaborative “co-existence” model between large-scale industrial mining and traditional local miners. By removing the need for artisanal cyanide or mercury use in the community, the A31 project significantly reduces the regional environmental impact, specifically targeting the reduction of untreated tailings releases into local river systems. The future outlook for A31 is one of long-term stability, with a projected mine life extending over several decades, providing a steady supply of critical metals while setting a new standard for ESG (Environmental, Social, and Governance) excellence in the Colombian mining sector.
Source: A31 | Aris Mining Technical Data | Cordoba Minerals Alacran FS
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
