Overview
The Midas mine, historically known as the Ken Snyder mine, stands as a cornerstone of precious metals production in the Elko County region of Nevada. Currently owned and operated by Hecla Mining Company following its acquisition of Klondex Mines, the Midas operation is a premier example of high-grade epithermal vein processing. Located in the Midas mining district, the facility serves not only as a primary extraction site for its own underground reserves but also as a critical regional processing hub for third-party and satellite ores, most notably from the Fire Creek mine. The project’s significance lies in its sophisticated metallurgical flowsheet, specifically designed to handle the complex, high-silver-to-gold ratios characteristic of epithermal deposits in the Great Basin.
The Midas mill was originally commissioned in the late 1990s and has undergone several phases of optimization to maintain its efficiency as head grades have evolved over decades of operation. With a nominal throughput capacity approaching 1,200 to 1,500 tons per day (tpd), and a historical ability to scale toward 4,000 tpd depending on ore blend and grinding requirements, the plant utilizes a combination of gravimetric separation and chemical leaching. The project is situated within a volcanic-hosted environment where gold and silver occur primarily as electrum and selenides. This mineralogical composition necessitates a precise approach to grinding and recovery, balancing the high throughput demands of a modern mining operation with the meticulous retention times required for maximum silver dissolution. As a center for innovation in Nevada’s “Gold Trend,” Midas continues to demonstrate how aging assets can be revitalized through technological integration and strategic ore blending.
Key Process Stages
The Midas processing circuit is a conventional yet highly optimized arrangement involving crushing, grinding, gravity concentration, and a Merrill-Crowe recovery plant. Each stage is tailored to manage the high silver content, which often precludes the use of standard Carbon-in-Leach (CIL) circuits due to the high carbon volumes that would be required. The primary stages include:
- Crushing and Feed Preparation: The Run-of-Mine (ROM) ore is subjected to a multi-stage crushing circuit, typically involving a primary jaw crusher followed by secondary cone crushing. The goal is to reduce the material to a size suitable for ball mill feed (approximately 80% passing 10mm).
- Grinding Circuit: The facility utilizes a single-stage or two-stage ball milling circuit operating in closed circuit with hydrocyclones. The target grind size is approximately 75 to 80% passing 200 mesh (74 microns), ensuring sufficient liberation of fine-grained electrum.
- Gravity Concentration: Within the grinding circuit, a portion of the cyclone underflow is diverted to centrifugal gravity concentrators (such as Knelson or Falcon units). This stage is critical, often recovering between 30% and 42% of the total gold content before it enters the chemical leach circuit.
- Agitated Leaching: The cyclone overflow is thickened and directed to a series of agitated leach tanks. Here, sodium cyanide is added to solubilize the precious metals. Due to the silver content, residence times are carefully managed to ensure complete silver extraction, which is kinetically slower than gold.
- Counter-Current Decantation (CCD): Following leaching, the slurry passes through a CCD circuit. This series of thickeners washes the pregnant solution from the solids, providing a high-clarity pregnant leach solution (PLS) for the recovery stage.
- Merrill-Crowe Recovery: The PLS is clarified, de-aerated to remove dissolved oxygen, and then mixed with zinc dust. The zinc precipitates the gold and silver from the solution—a process preferred over carbon adsorption when silver concentrations are high.
- Refining: The resulting zinc precipitate is filtered, dried, and smelted in an induction furnace to produce doré bars for final refining off-site.
Critical Data
The following data highlights the performance of the Midas mill over nearly a decade of operation under Hecla Mining and Klondex, reflecting shifts in throughput and recovery efficiency based on ore characteristics.
| Parameter | Value (2022-2023 Avg) | Unit |
|---|---|---|
| Annual Throughput | 1,200 – 1,441 | kt (dry) |
| Operating Throughput Rate | 184 – 193 | tph |
| Grind Size (P80) | 77 – 78 | % passing 200 mesh |
| Average Gold Head Grade | 2.55 – 3.16 | g/t Au |
| Gravimetric Recovery | 32.0 – 42.5 | % |
| Total Mill Gold Recovery | 85.2 – 87.2 | % |
| Cyanide Consumption | 0.51 – 0.52 | kg/t |
| Mill Availability | 91.3 – 93.0 | % |
Technical Details and Sustainability
The technical evolution of the Midas mill provides a roadmap for processing high-grade epithermal deposits. One of the most significant technical challenges managed at the site is the fluctuation in arsenic and selenium content, which can interfere with traditional cyanidation. To mitigate this, the Midas metallurgical team employs advanced recovery models that calculate gold recovery based on arsenic content. For instance, the “Low Arsenic” recovery model utilizes a regression equation (e.g., Recovery % = [0.9753 × Au Head Grade]) to predict mill performance and adjust reagent dosing accordingly. This level of predictive analytics allows the mill to maintain recoveries above 85% even as head grades have decreased from 5.96 g/t in 2015 to roughly 2.55 g/t in recent years.
The decision to utilize a Merrill-Crowe circuit instead of CIL is a defining technical feature of Midas. In many Nevada gold mines, CIL is the standard; however, the high silver-to-gold ratio at Midas (often exceeding 10:1) would lead to “carbon robbing” or require an excessively large carbon stripping and regeneration plant. The Merrill-Crowe process, which relies on zinc precipitation, is significantly more efficient for high-silver ores as it can process large volumes of pregnant solution without the limitations of carbon loading capacity. Furthermore, the gravity circuit’s ability to capture over 40% of the gold reflects the coarse nature of the electrum in certain vein segments, reducing the burden on the downstream leaching and precipitation stages.
From a sustainability and environmental perspective, Midas has made significant strides in reagent management and tailings disposal. Cyanide consumption has been optimized through automated titration and feedback loops, dropping from 0.82 kg/t in 2015 to approximately 0.52 kg/t today. This reduction not only lowers operating costs but also minimizes the environmental footprint of the cyanide destruction circuit. The mill’s high availability—consistently above 90%—is maintained through a rigorous preventative maintenance program, which is essential for a facility that operates 24/7 in a remote Nevada landscape. The integration of Fire Creek ore into the Midas mill also exemplifies “brownfield sustainability,” utilizing existing infrastructure to process new discoveries rather than constructing additional disruptive facilities. Looking forward, the Midas operation is positioned to incorporate more advanced sorting technologies and potentially transition toward more automated grinding controls to further enhance energy efficiency and metal liberation.
Source: Midas | Hecla Mining Company Technical Reports
Source: NI 43-101 Technical Report
This article provides an overview of mineral processing developments based on publicly available technical reports and industry analysis.
