Comprehensive Analysis of the Ojuela Deposit Mineral Processing at La Encantada

Overview

The Ojuela deposit, often referred to as the “Ojuelas” complex within the La Encantada Silver Mine operation, represents a cornerstone of modern silver production for First Majestic Silver Corp. Located in the remote and rugged terrain of Coahuila, Mexico, specifically within the northern Sierra Madre Oriental, the project is a testament to the intersection of historical mining legacy and cutting-edge mineral processing. Historically, the broader region has been known for its significant polymetallic silver, lead, and zinc deposits, but the modern development of the Ojuela zone focuses on sustaining bulk silver production through advanced underground extraction and hydrometallurgical processing.

The La Encantada operation, which integrates the Ojuela deposit ore, is situated at an elevation that demands specialized equipment design, particularly for motors and crushing circuits. The deposit itself is characterized as a massive lens-type carbonate replacement zone (CRD), encircling skarn alteration. It is structurally complex, bounded by major fault systems such as the Dike Escondida, and contains mineralized zones that range from 1,425 to 1,675 meters in elevation. As traditional ore bodies at La Encantada approached depletion, First Majestic identified the Ojuela deposit as the next primary target. Its development required a transition to bulk mining methods, specifically sublevel caving (SLC), to handle the geometry and geotechnical conditions of the ore body. The significance of Ojuela lies not only in its resource size—estimated to support production of over 1 million tonnes over its initial three-year phase—but also in its role as a high-volume feed source for the La Encantada mill, ensuring the longevity of one of Mexico’s most prominent silver operations.

Key Process Stages

The processing of ore from the Ojuela deposit is conducted at the La Encantada mill, which is divided into two primary sections: Plant No. 1 (Comminution) and Plant No. 2 (Leaching and Recovery). The circuit is designed to handle a nominal capacity of 3,400 tonnes per day (tpd). The following stages outline the flow from run-of-mine (ROM) ore to the final silver doré product:

• Primary Crushing: ROM material is delivered to a 300-tonne coarse ore bin. Oversized rocks are broken by a hydraulic hammer on a 12″ x 12″ grizzly. A 24” x 36” primary jaw crusher reduces the material to approximately 3 to 3.5 inches.
• Secondary and Tertiary Crushing: The primary product is screened via vibrating screens with ⅜” apertures. Oversize material is processed through a Sandvik CH430 secondary crusher in a closed circuit, ensuring the final fine ore product passes 80–90% through a ¼” screen.
• Grinding: Fine ore is fed into a primary Metso ball mill (12’ diameter x 24’ length) powered by a high-torque 1,800 HP motor with a variable frequency drive (VFD). Two additional ball mills are available for operational flexibility, achieving a target grind size suitable for cyanide leaching.
• Cyanide Leaching: The ground slurry is directed to Plant No. 2, featuring 17 agitated cyanide leach tanks. The circuit provides a comprehensive residence time of approximately 72 hours (50 hours in the first stage and 22 hours in the second) to maximize silver dissolution.
• Counter-Current Decantation (CCD): A four-stage CCD system utilizes 125-foot thickeners to separate the silver-rich pregnant leach solution (PLS) from the solids.
• Merrill-Crowe Recovery: The PLS is clarified through autojet pressure filters and deaerated before zinc dust is added to precipitate the silver. This system has a capacity of approximately 550 kg of doré per day.
• Smelting: The resulting precipitate is dried and smelted in two induction furnaces to produce 23-kg silver doré bars with silver content ranging from 60% to 85%.
• Tailings Management: Final tailings are processed through three high-capacity press filters to recover water and create a dry-stackable material for storage.

Critical Data

The following table summarizes the key technical and operational parameters for the processing of Ojuela-sourced ore at the La Encantada facility.

Technical Details and Sustainability

Technical optimization at the Ojuela deposit is driven by the unique mineralogical characteristics of the ore. Metallurgical testwork has revealed that silver recovery from the Ojuela domain typically ranges between 60% and 70%. One of the primary technical challenges is the presence of manganese oxides, which can hinder silver dissolution by creating refractory conditions. To combat this, the operation has implemented specific process enhancements, such as the addition of lead nitrate and maintaining a higher pH range (7.2 and above) in the leach circuit, which has historically boosted recoveries by 1–2%. Additionally, the “Ojuelas” zone is part of a larger geometallurgical study that assigns specific recovery assumptions to different geological domains to ensure more accurate life-of-mine (LOM) planning.

Mining at Ojuela employs the sublevel caving (SLC) method, a bulk mining technique selected for its suitability in the geotechnical conditions of the skarn and replacement zones. This method involves the controlled collapse of the ore body, which naturally introduces internal dilution (estimated at 14% for Ojuela). To manage this, the mill’s comminution circuit is designed for robustness, capable of handling varying feed grades while maintaining consistent throughput. The use of a Metso ball mill with an 1,800 HP motor allows for the high-intensity grinding required to liberate silver from the complex mineral matrix.

From a sustainability perspective, La Encantada is a leader in water conservation within the Mexican mining sector. The facility utilizes a sophisticated tailings filtration system consisting of three press filters. This technology allows for the recovery of a significant portion of the water used in the leaching process, which is then recirculated back to the plant. The resulting “dry-stack” tailings are more stable and have a smaller environmental footprint compared to traditional slurry tailings dams. Furthermore, while the roasting circuit is currently on care and maintenance, First Majestic continues to evaluate its reactivation. The roasting process is capable of significantly improving silver recovery from manganese-rich refractory ores—increasing values from below 25% to as high as 70%. Future sustainability and efficiency initiatives include the potential commissioning of this roasting circuit to process historically stored tailings and more complex ore types from the deeper Ojuela levels.

The future outlook for Ojuela remains strong, with continuous refinements in the mine layout and development of over 7 km of new underground workings. As production continues to ramp up, the integration of real-time sampling and automated metallurgical balance tools ensures that the La Encantada plant remains at peak efficiency, maximizing the economic value of the Ojuela deposit while adhering to strict environmental standards.

Source: Ojuela / La Encantada Silver Mine Technical Reports 2024-2025

Source: NI 43-101 Technical Report

This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.