South Railroad Gold Heap Leach Recovery Methods and Process

Source: Orla Mining Ltd (2026)
Website: https://orlamining.com/asset/south-carlin-complex/south-railroad-project/

Critical Data

Overview

Orla Mining Ltd. operates the South Railroad Project located in the Bullion mining district of the southern Carlin trend in Nevada, USA. The project utilizes a conventional heap leach recovery circuit to extract gold and silver from the Pinion and Dark Star ore deposits. This state-of-the-art facility, detailed in the Feasibility Study Update, processes ore through a combination of run-of-mine (ROM) truck stacking and two-stage crushing. Approximately half of the ore is truck-stacked as ROM directly onto the pad, while the other half undergoes size reduction to a P80 of 1 inch before stacking. The heap leach operation employs dilute cyanide solution to dissolve precious metals, which are then recovered via a carbon adsorption circuit. Gold and silver are stripped from loaded carbon using a pressure Zadra hot caustic desorption process, followed by electrowinning to produce a precipitate sludge. The sludge is retorted for mercury removal and then smelted to produce gold and silver doré bars. With total proven and probable reserves of 73.4 million tons and a planned mine life of 10 years plus an additional 3 years of solution application, the South Railroad heap leach recovery method is designed for high efficiency and environmental responsibility. The process achieves an overall life-of-mine gold recovery of 70.8% and silver recovery of 12.4%, with Dark Star ore showing higher recoveries than Pinion ore. Key infrastructure includes a two-stage crushing plant with jaw and cone crushers, a multiple-lift single-use leach pad with composite liner system, and an ADR plant equipped with carbon columns, acid wash, desorption, and electrowinning circuits.

Key Process Stages

• Stage 1: Ore Preparation and Crushing – Run-of-mine ore is delivered to either the ROM stacking area or a two-stage crushing plant. The crushing circuit includes a stationary grizzly (36″ spacing), an apron feeder, a vibrating grizzly feeder (3″ apertures), a primary jaw crusher (Metso C130, 5.0″ CSS, 187 kW), primary discharge conveyor, secondary screen feeders, dual-deck vibrating screens (8 ft x 24 ft, 1.325″ bottom deck), secondary cone crushers (Metso HP450e, 1.20″ CSS, 336 kW each), and conveyors to the crushed ore stockpile. The target product size is P80 of 1 inch (25.4 mm).
• Stage 2: Truck Stacking and Lime Addition – ROM ore is hauled by 200-ton trucks to the heap leach pad at an average rate of 14,000 tpd. Pebble quicklime (CaO) is metered into each truck at a rate of 1.3 lb/ton ore to maintain alkaline pH. Trucks direct-dump ore on the current lift, and a dozer pushes it over the edge. Ore is stacked in 30 ft high lifts to a maximum heap height of 300 ft. The top of each old lift is cross-ripped before stacking a new lift to break up compaction.
• Stage 3: Heap Leaching with Cyanide – After stacking and ripping, drip irrigation emitters apply dilute cyanide solution at 0.0033 gpm/ft² to the ROM ore. Barren solution (pH ≥10) is delivered by high-head pumps at 5,000 gpm via steel pipelines and sub-headers. The leach cycle for ROM ore is 100 days. Pregnant solution percolates through the heap and is collected by perforated pipes at the base, flowing by gravity to a pregnant solution tank. The leach pad uses a composite liner system (80-mil HDPE geomembrane over compacted clay, permeability ≤10⁻⁶ cm/sec) with an overliner layer and leak detection system.
• Stage 4: Carbon Adsorption (CIC) – Pregnant solution (4,500 gpm nominal flow) is pumped to the ADR plant where it enters two trains of five cascade-style carbon-in-column (CIC) units (each 12.5 ft diameter, holding 6 tons of activated carbon). Gold and silver adsorb onto the carbon counter-current to solution flow. Once the lead column reaches target load, carbon is advanced to the elution circuit. Barren solution exits the last column through a vibrating screen and flows to the barren tank for cyanide make-up and re-circulation to the heap.
• Stage 5: Desorption, Electrowinning, and Smelting – Loaded carbon undergoes acid wash (dilute nitric acid, pH 1.0-2.0, ~4 hours) to remove scale. Desorption uses a pressure Zadra hot caustic circuit at 300°F and 100 psig for 10 hours, with pregnant eluate cooled to 175°F and fed to electrowinning cells. Gold and silver precipitate as sludge, which is dried in a retort oven for mercury removal and then smelted with fluxes (silica sand, borax, soda ash) to produce doré bars. Carbon is thermally reactivated in a rotary kiln for reuse.

Additional Interesting Data and Summary

The South Railroad Project’s heap leach recovery method incorporates extensive reagent handling and environmental safeguards. Sodium cyanide (NaCN) consumption is estimated at 0.70 lb/ton (0.35 kg/tonne) for both Pinion and Dark Star ore, with liquid 30% NaCN stored in a 30,000-gallon double-contained tank and distributed via double-containment pipes. Lime (CaO) is added at 1.3 lb/ton to maintain alkaline pH, stored in two 60-ton silos with variable speed feeders for precise metering onto haul trucks. Activated carbon (6×12 mesh) is used in the CIC circuit, with 3-4% replacement due to fines losses. Sodium hydroxide (50% caustic) is stored in a 15,000-gallon tank for strip solution preparation. Nitric acid (57%) is diluted to 7% for carbon acid wash. Fluxes (silica sand, borax, soda ash) are consumed at 2 lb per lb of gold+silver produced. Antiscalant is added at 6 ppm for leach solutions and 10 ppm for strip solutions to prevent scaling. The leach pad is constructed in four phases with an 80-mil HDPE composite liner over compacted clay (permeability ≤10⁻⁶ cm/sec), a 18-inch overliner layer for drainage and protection, and a leak detection system with perforated pipes for visual monitoring. Two lined solution ponds are sized: a process pond (9.6 million gallons) for 24-hour drain-down capacity and an event pond (25.5 million gallons) designed for a 100-year, 24-hour storm plus snowpack accumulation. The ADR plant includes two trains of five carbon columns (12.5 ft diameter, 6 tons carbon each), a pressurized Zadra desorption circuit (300°F, 100 psig, 10-hour cycle), electrowinning cells, a retort oven for mercury removal, and a smelting furnace to produce doré bars. The facility is designed to operate continuously with nominal solution flow of 4,500 gpm. Environmental considerations include double-containment for cyanide lines, leak detection under the leach pad, and lined ponds to prevent groundwater contamination. The project’s economic impact is supported by low operating costs from heap leaching and high gold recoveries, particularly from Dark Star crushed ore at 82.1%. The mine life of 10 years with an additional 3 years of solution application ensures maximum metal recovery. Future outlook includes potential expansion or optimization of crushing circuit throughput based on ore blending. Overall, the South Railroad heap leach recovery method represents a modern, efficient, and environmentally responsible approach to gold and silver extraction in the Carlin trend.

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

Target Commodities: Gold, Silver