Camino Rojo Gold Recovery via Heap Leach & Merrill-Crowe

Source: Orla Mining Ltd (2026)
Website: https://orlamining.com/asset/camino-rojo/

Critical Data

Overview

Orla Mining Ltd. operates the Camino Rojo open pit heap leach gold-silver mine in Zacatecas State, Mexico, which has been in production since late 2021. The processing facility was commissioned in Q4 2021, with the Merrill-Crowe Plant achieving first gold pour on December 13, 2021, and commercial production declared on April 1, 2022. The site employs a two-stage closed-circuit crushing system to reduce ore to a target P80 of 25 mm, followed by conveyor stacking onto leach pads in 10-metre lifts. Lime (CaO) is added at an average rate of 3.6 kg/t for pH control before dilute cyanide solution is applied to dissolve gold and silver. Pregnant solution flows by gravity to a pump box and is then transferred to the Merrill-Crowe Plant, where precious metals are precipitated via zinc cementation, dewatered, retorted to remove mercury, and smelted into doré bars. The operation processes an average of 19,200 tonnes per day of crushed ore, with a temporary campaign of 494,000 tonnes of run-of-mine (ROM) ore placed between May and August 2025 at a nominal rate of 21,700 tpd. Overall gold recovery from start-up through September 30, 2025 is approximately 62% (based on ounces recovered to doré), while silver recovery is 8.3%, with additional metal in solution and precipitate inventories. This efficient heap leach and Merrill-Crowe flowsheet highlights Orla Mining’s commitment to low-cost, scalable gold and silver production in central Mexico.

Key Process Stages

• Stage 1: Two-Stage Closed-Circuit Crushing – ROM ore from 100-tonne haul trucks is crushed to P80 25 mm using a primary jaw crusher and two secondary cone crushers fed by double-deck vibrating screens. Oversize is handled by a rock breaker, while grizzly undersize bypasses the primary crusher. Product reports to a surge bin before feeding secondary screens and cone crushers in closed circuit.
• Stage 2: Conveyor Stacking and Lime Addition – Crushed ore is reclaimed from a product stockpile and conveyed via an overland conveyor to the heap stacking system. Pebble lime (CaO) is added on the overland conveyor at an average of 3.6 kg/t (2025 rate) for pH control. Water sprays at all transfer points and a stockpile cover mitigate dust.
• Stage 3: Heap Leaching – Ore is stacked in 10-metre lifts using a conveyor system comprising index feed, horizontal index, and radial stacker conveyors. A dilute cyanide solution is applied to the heap to dissolve gold and silver. Once a lift is drained, the old lift is cross-ripped before new ore is stacked on top to break compacted or cemented zones.
• Stage 4: Solution Handling – Pregnant solution flows by gravity to a pump box and is pumped at a nominal rate of 1,000 m³/h (design 1,200 m³/h) to the Merrill-Crowe Plant. Two of three pressure leaf clarification filters (with diatomaceous earth precoat and body feed) remove suspended solids from the pregnant solution.
• Stage 5: Merrill-Crowe Precipitation and Refinery – Clarified, deaerated pregnant solution is pumped to precipitate filter presses where ultra-fine zinc powder precipitates gold and silver. Lead nitrate is added as needed to improve efficiency. Precipitate is dried in an electric mercury retort, then mixed with fluxes and smelted in a tilting induction furnace. Doré bars are poured, cooled, cleaned, and shipped to a third-party refiner. Slag is re-crushed and re-smelted to recover residual values.

Additional Interesting Data and Summary

Section 17 of the NI 43-101 Technical Report provides detailed operational performance data for the Camino Rojo Project. As of September 30, 2025, the heap leach operation had stacked approximately 494,000 tonnes of run-of-mine (ROM) ore during a temporary campaign (May 11 to August 9, 2025) at an assumed recovery of 50% for gold and 9% for silver, which was implemented to address production shortfalls from higher proportions of Ki and transitional material. The overall gold recovery to doré averaged 62%, while silver recovery was 8.3%, with additional values in Merrill-Crowe precipitate and leach solutions (1.6% gold, 0.4% silver). The booked metals inventory as of the effective date is estimated at approximately 29,800 ounces of gold and 507,000 ounces of silver, including metals in solution, partially leached ore, and unleached areas such as newly stacked ore and side slopes. Gold inventory peaked in June 2023, declined steadily, then increased from September 2024 through September 2025. Silver inventory peaked in April 2024 and has since remained relatively steady. The Qualified Persons (QPs) note that modeled versus actual gold recoveries are in good agreement with reasonable inventory levels, but silver recoveries show a larger variance, possibly due to slower leach kinetics, insufficient free cyanide, or the need for better calibration of silver recovery estimates. The QPs recommend maintaining current silver recovery estimates while closely monitoring monthly composite columns to determine if a write-down of silver inventory is warranted. Environmental controls include water sprays at all crushing transfer points and a cover for the crushed product stockpile to manage dust. Mercury removed during the retort process is classified as hazardous waste and transported off-site for disposal. The Merrill-Crowe plant operates at a nominal pregnant solution flow of 1,000 m³/h (design 1,200 m³/h), with two clarification filters online and one on backwash/clean/precoat cycle. Precipitate filter presses are cycled offline periodically for compressed-air drying and collection. The tilting induction furnace produces doré bars that are stored in a vault pending shipment to a third-party refiner. Future outlook includes ongoing monitoring of silver recovery and heap inventory trends to optimize process efficiency and reconcile model assumptions, particularly for ROM ore. The Camino Rojo operation remains a significant gold and silver producer in the Zacatecas region, leveraging a proven heap leach and Merrill-Crowe flowsheet that delivers robust metallurgical performance with manageable environmental impact.

Key Processes: Heap Leaching, Cyanidation, Gravity Separation, Crushing

Target Commodities: Gold, Silver, Zinc