Guanaco Mine: Heap Leach and Agitated Leach Recovery Circuits in Chile’s Antofagasta Region

The Guanaco Mine operates in the Antofagasta Region of Chile, producing gold and silver through cyanide leaching. Austral Gold Limited owns the operation, which began mining in 1993 under Amax and was later rehabilitated by Austral Gold between September 2009 and March 2010. Heap-leach processing restarted in September 2010 with the first gold pour in December of that year. A milling and agitated leach circuit was commissioned in 2017. Heap loading stopped that same year, and all ore went to the agitated leach circuit until February 2020. Amancaya ore continued through the mill until 2025. In 2023, reprocessing of existing heaps began, and studies are ongoing for the Inesperada mine and Dumbo clusters, with routing determined by gold head grade.

Critical Data

Overview

The Guanaco processing facility combines two recovery routes. A permanent heap leach operation uses static pads with carbon-in-column adsorption, while a milling and agitated leach circuit treats higher-grade feed. A conventional three-stage crushing plant feeds both circuits through a reversible conveyor. The quaternary HPGR stage was added in 2023 to improve gold and silver extraction from reprocessed heap material.

Ore routing depends on feed characteristics and gold head grade. Material can pass through all four crushing stages, through the quaternary stage only, or be fed directly to the crushed ore stock heap without further crushing. Sampling determines whether ore goes to heap leach or agitated leach. The heap leach circuit includes static Phase III and Phase IV pads with pregnant and intermediate solution ponds. The agitated leach circuit comprises a closed-circuit ball mill, hydrocyclone cluster, milling thickener, three leach tanks, and a three-stage CCD wash circuit. Solution processing uses both CIC-ADR and Merrill-Crowe systems.

Key Process Stages

Conventional Crushing

Ore arrives by truck and discharges into a coarse ore bin. A grizzly above the bin prevents material larger than 750 mm from entering, and a hydraulic rock breaker handles oversized blocks. An apron feeder delivers ore to the primary jaw crusher. Discharge goes to a double-deck vibrating screen, with oversize reporting to two secondary cone crushers. Tertiary screens feed two short-head cone crushers in closed circuit. Combined screen undersize forms the final conventional crushing product, conveyed by a reversible belt to either circuit. For heap leach feed, lime is added to the conveyor ahead of the truck-loading bin.

Quaternary Crushing

The quaternary stage uses a banana double-deck screen and an HRC-800 high-pressure grinding roll in open circuit. The circuit requires a nominal feed F80 of 5.6 mm, so Heap III material can be fed directly. Heaps I and II material and fresh ore need conventional crushing first. Feed enters through a dedicated external bin loaded by front-end loader or via a transfer conveyor from the secondary and tertiary undersize line. Lime is added from a silo beside conveyor CV07.

Heap Leach and ADR

Crushed ore is trucked to the static pad and stacked in strips to a maximum lift height of six metres, then levelled by dozer. Barren solution applies through drip irrigation. Solution percolates through the heap and drains by gravity through underdrain piping to the PLS pond. The ADR plant pumps PLS through five CIC columns in series at about 350 m³/h. Loaded carbon moves counter-current to solution flow, transferred batchwise from column 5 to column 1. Fully loaded carbon goes to elution, where acid washing removes inorganic foulants. The AARL split elution uses hot sodium cyanide and caustic soda, with high-grade eluate sent to electrowinning and lower-grade eluate recycled to the next elution. Stripped carbon regenerates in a gas-fired kiln. Three electrowinning cells operate in parallel, depositing metals on steel wool cathodes. Cathode sludge is washed, filtered, dried, blended with fluxes, and smelted in an induction furnace to produce doré bars.

Agitated Leach

Ore reclaims from the covered stock heap through two vibrating feeders onto a belt conveyor to the ball mill. The single-stage mill operates in closed circuit with a hydrocyclone cluster. Cyclone underflow returns for further grinding; overflow is the circuit product at a target P80 of 80 to 150 μm. Overflow feeds the milling thickener, with underflow at 50% solids by weight going to three agitated leach tanks providing at least 48 hours retention. Cyanide addition starts in the milling circuit. Leach discharge enters the three-stage CCD wash circuit, where wash solution flows counter-currently to maximise recovery. CCD thickener 1 overflow goes to the rich solution tank and Merrill-Crowe plant. CCD thickener 3 underflow goes to filter feed, then three plate-and-frame filter presses dewater and wash tailings to below 20% moisture. Freshwater serves as filter wash water and is the primary freshwater addition.

Merrill-Crowe

The refurbished Merrill-Crowe plant treats rich solution from CCD. It is preferred when silver grades exceed 15 g/t and silver recoveries top 60%; otherwise the CIC-ADR circuit should be evaluated. Solution passes through clarification filters, then a vacuum de-aeration tower reduces dissolved oxygen. Zinc dust addition precipitates gold and silver as metallic cement, retained in plate-and-frame filter presses cleaned manually. Precipitate is dried, fluxed, and smelted. Barren solution recycles as CCD wash water.

Tailings

Filter cake discharges to a concrete-lined containment area. A front-end loader transfers tailings to haul trucks for the dry stack storage facility. Residual cyanide degrades naturally through solar ultraviolet radiation and the arid climate. Tailings are spread, irrigated, and tilled to promote degradation.

Additional Interesting Data and Summary

The processing facility began as a heap leach operation under Amax in 1993, with three-stage crushing and a plant combining Merrill-Crowe and CIC-ADR circuits. After care and maintenance from 1997, Austral Gold modernised the operation. The Amancaya acquisition led to construction of the milling and agitated leach circuit in 2017. The existing Merrill-Crowe circuit was refurbished to handle higher-silver ore from Amancaya. Heap reprocessing starting in 2023 involves excavating and re-crushing material from Heaps I, II, and III.

Several design values stand out. The agitated leach plant is designed for 1,500 tpd. The ADR plant handles approximately 350 m³/h. The HPGR delivers a final product P80 of 4.1 mm from a nominal feed F80 of 5.6 mm. The milling circuit targets a grind P80 between 80 and 150 μm depending on ore leach characteristics. Leach retention is a minimum of 48 hours. Tailings filter cake moisture stays below 20%.

The facility comprises crushing, heap leach, agitated leach, and solution processing units. Crushing includes primary jaw, secondary and tertiary cone crushers with vibrating screens, plus the HRC-800 quaternary stage. The heap leach uses static pads with PLS and ILS ponds. The agitated leach circuit has a covered ore stock heap, closed-circuit ball mill, thickener, three leach tanks, CCD wash, and filter presses. Solution processing includes the five-column CIC circuit, acid wash column, AARL split elution, electrowinning cells, and the Merrill-Crowe system with zinc precipitation. An emergency solution storage pond contains solution during extreme precipitation or abnormal conditions.

Key Processes

• Three-stage conventional crushing with primary jaw, secondary and tertiary cone crushers
• Quaternary HPGR crushing with HRC-800 high-pressure grinding roll
• Static heap leach with drip irrigation and gravity drainage to PLS pond
• Five-column CIC adsorption with counter-current carbon movement
• AARL split elution with acid wash and carbon regeneration kiln
• Electrowinning with steel wool cathodes and induction furnace smelting
• Closed-circuit ball mill with hydrocyclone classification
• Three-tank agitated leach with minimum 48-hour retention
• Three-stage counter-current decantation wash circuit
• Merrill-Crowe zinc precipitation with clarification and de-aeration
• Plate-and-frame filter press tailings dewatering
• Dry stack tailings storage with natural cyanide degradation

Source: Technical Report on the Guanaco Mine, July 21, 2026. Project website: Guanaco Mine