Technical Insights into the Aquila Mineral Processing Circuit: Optimizing Polymetallic Recovery at the Don David Gold Mine

Overview

The Aquila mine, widely recognized as the flagship asset of the Don David Gold Mine (DDGM) complex under the stewardship of Gold Resource Corporation (GRC), represents a cornerstone of polymetallic mineral processing in the Oaxaca region of Mexico. Specifically located within the San Jose de Gracia district, the Aquila project (often referred to in technical literature as ‘El Aguila’) has evolved from a high-grade open-pit operation into a sophisticated underground mining complex that targets the Arista and Switchback vein systems. This project is significant not only for its consistent production of precious metals—gold and silver—but also for its critical output of base metals, including copper, lead, and zinc.

The significance of the Aquila processing plant lies in its versatile “differential flotation” flowsheet, which was specifically designed to handle the complex mineralogy of the Arista deposit. Since declaring commercial production on July 1, 2010, the facility has processed over 5.9 million tonnes of ore, yielding substantial quantities of gold and silver. The technical success of Aquila is rooted in its ability to adapt to varying ore types, transitioning seamlessly from oxide-rich surface material to the deeper, sulfide-rich polymetallic ores encountered in the underground levels. The mine serves as a model for resource optimization, utilizing a dual-recovery approach that combines gravity concentration, intensive leaching, and multi-stage flotation to maximize the economic return from every tonne of processed ore.

From an industrial perspective, the Aquila mine is a testament to the longevity and potential of epithermal vein systems in the Mogollon Transition. With ongoing exploration revealing underexplored extensions of known veins, the processing circuit continues to undergo technological upgrades to maintain high recovery rates even as head grades fluctuate. The operation’s focus on operational excellence, environmental stewardship, and technical innovation makes it a pivotal study in modern mineral processing.

Key Process Stages

The Aquila mineral processing circuit is a comprehensive facility designed to produce three separate base metal concentrates and high-purity gold-silver doré. The circuit is characterized by its flexibility, allowing for the differential separation of sulfides while simultaneously capturing coarse precious metals. The primary stages of the processing circuit include:

• Primary Crushing: Run-of-Mine (ROM) ore is reduced using a high-capacity jaw crusher. The station typically features a fixed grizzly and a vibrating grizzly feeder to ensure a consistent feed size of 80% passing (P80) approximately 75 mm.
• Grinding and Classification: The secondary reduction stage utilizes a SAG and ball mill configuration (SAB circuit). The grinding circuit is designed to achieve a target grind size of 80% passing 106 µm, which is essential for liberating the fine-grained sulfides prior to flotation.
• Gravity Concentration: Integrated into the grinding circuit is a gravity recovery loop featuring batch centrifugal concentrators (such as Knelson or Falcon units). This stage captures “free gold” grains that are too heavy for standard flotation.
• Intensive Leaching: The gravity concentrate is treated in a Gekko intensive cyanide reactor (Sepro Leach Reactor). This high-efficiency system rapidly dissolves gold and silver from the gravity-concentrated material.
• Differential Flotation: This is the heart of the polymetallic circuit. It consists of three sequential flotation banks:
  • Copper Circuit: Utilizes specific collectors and pH control to separate chalcopyrite from other sulfides.
  • Lead Circuit: Recovers galena from the copper tails.
  • Zinc Circuit: Employs copper sulfate activation to recover sphalerite.
• Sulfur Flotation (2022 Upgrade): A modern addition to the circuit, this stage recovers remaining gold associated with sulfur from the zinc flotation tailings. The resulting bulk sulfide concentrate is then sent to an agitated leach circuit.
• Agitated Leaching and Merrill-Crowe: The bulk sulfide concentrates and intensive leach residues undergo cyanide leaching in agitated tanks. Gold and silver are then recovered from the pregnant solution using the Merrill-Crowe zinc precipitation process, leading to the production of high-grade doré bars.

Critical Data

The following table summarizes the nominal technical parameters and performance data for the Aquila mineral processing facility based on recent operational reports.

Technical Details and Sustainability

The Aquila mine represents a sophisticated intersection of metallurgy and environmental engineering. One of the most technically challenging aspects of the Aquila circuit is the management of the differential flotation sequence. Because the Arista vein system is polymetallic, the circuit must precisely balance reagent dosages—such as xanthates for collection and zinc sulfate for depression—to prevent the misplacement of minerals. For instance, the copper circuit must suppress galena and sphalerite while successfully floating chalcopyrite. Failure to maintain these balances can lead to “penalties” at the smelter due to concentrate impurity. The recent addition of a sulfur flotation cell system in 2022 exemplifies GRC’s commitment to “scavenging” every possible ounce of gold, ensuring that minerals previously lost to tailings are now captured in a bulk sulfide concentrate for agitated leaching.

Sustainability is a core component of the Aquila operational philosophy. In the water-scarce environment of Oaxaca, the plant employs a rigorous water recycling program, recovering up to 85% of process water from tailings thickeners and filtration systems. The move toward dry-stack tailings technology is a significant environmental milestone for the project. By filtering tailings to a low moisture content and stack-placing them, the mine minimizes the risk associated with traditional tailings dams and reduces the overall footprint of the waste management facility. Furthermore, the use of cyanide destruction circuits (SO2/Air or similar) ensures that any residual cyanide in the leach tailings is neutralized to levels well below international environmental safety standards before final disposal.

Looking to the future, the technical legacy of the Aquila mine continues to influence the company’s development pipeline, including the Back Forty project in Michigan. The metallurgical lessons learned at Aquila—particularly in the recovery of precious metals from complex sulfide matrices—are directly applicable to the VMS (Volcanogenic Massive Sulfide) deposits found in the Great Lakes region. As the Aquila mine continues to deepen its underground operations, the processing circuit remains a dynamic entity, constantly optimized through real-time monitoring and advanced metallurgical testing at partner laboratories like ALS. This dedication to technical refinement ensures that Aquila remains a premier producer of high-grade concentrates for years to come.

Source: Aquila

Source: NI 43-101 Technical Report

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