Los Filos Gold Mine — Technical Report 2017

A heap leach operation processes oxide, intrusive, carbonate, endoskarn, and sulphide ore types from three mining areas, with medium-to-high-grade material crushed and agglomerated before leaching and low-grade material placed directly as run-of-mine.

Report context

This technical report is dated December 31, 2017, and covers the Los Filos Gold Mine in Guerrero State, Mexico. The processing information describes year-end 2017 operating conditions. Processing details have evolved since operations began in 2007, although the basic design remains that of the original plan based on a heap leach operation using multiple-lift, single-use heap leach pads.

Processing route

Ore sources and classification

Ore is sourced from three areas: Los Filos Open Pit, Bermejal Open Pit, and Los Filos Underground. Ore types include oxides, intrusives, carbonates, endoskarn (altered intrusives), and sulphides. All ore types are processed for gold recovery except sulphide-rich material containing over 1% sulphur, which is currently stockpiled for future use.

Mineralization is classified as either low-grade or high-grade. Medium- to high-grade ore is defined as material with gold grades greater than 0.5 g/t Au for Bermejal or Los Filos Open Pit ores and all Los Filos Underground ore. Low-grade ore is material with gold grades between 0.198 to 0.5 g/t Au for Bermejal Open Pit and 0.241 to 0.5 g/t for Los Filos Open Pit ores. No ore from Los Filos Underground is classified as low grade.

Open pit ore is further classified into three categories: SLG (sub-low-grade), ROM (run-of-mine), and Crush (crushed) ores. All underground ores are classified as Crush. For Los Filos Open Pit, SLG ore has head grades of 0.244 to 0.315 g/t Au, ROM ore 0.315 to 0.433 g/t Au, and Crush ore greater than 0.433 g/t Au. For Bermejal Open Pit, SLG ore has head grades of 0.236 to 0.303 g/t Au, ROM ore 0.303 to 0.395 g/t Au, and Crush ore greater than 0.395 g/t Au.

Crushing circuit

Medium- to high-grade ore is sent to a crushing system consisting of a primary jaw crusher and two secondary cone crushers. The crushing circuit has a maximum operating capacity of 1,500 t/h, but depending on ore supply, crusher feed size distribution, and moisture content, the circuit normally operates at 890 t/h for 16 h/d.

The crushing plant principal components are a JM 311 Sandvik jaw crusher set at a 100 mm opening, and two Metso HP-800 cone crushers operating in parallel. A scalping screen with 100 mm openings precedes the jaw crusher. Ore is reduced to a target particle size of 80% passing minus 19 mm.

Open pit ore is end-dumped from 136 t capacity mine haulage trucks near the primary crusher into a 200,000 tonne stockpile without significant blending of ore types. Underground ore is end-dumped in a separate stockpile by 20 t capacity dump trucks. The ore is delivered to the jaw crusher feeder hopper, which is equipped with a 400 mm grizzly and fed by a dedicated loader from the 200,000 tonne stockpile. Underground ore is fed to the crusher as a weekly campaign, one day per week.

Los Filos Open Pit ores are described as generally very fine and clay-rich, with surveys suggesting particle sizes of P80 passing 100 mm and P50 passing 18 mm. Recent data collected by Los Filos indicates that ROM ore is 60% passing 25 mm.

Crusher tonnage is measured by a weightometer on the No. 1 conveyor, calibrated monthly by Los Filos technicians and checked annually by the manufacturer. A cross-belt sampling system at the discharge end of one of the intermediate conveyors cross-cuts the discharge stream every 15 minutes, with the final sample weighing 150 kg and taken to the laboratory for gold, silver, and moisture content analysis.

Agglomeration and transport

The Crush ore, known on site as “Crush” ore, is mixed with cement, lime, and water on the conveyor belt system for agglomeration. The current process adds 6 kg/t of cement and 8 kg/t of lime to the Crush ore on the transport conveyors and adds moisture by way of barren solution at six conveyor drop points, increasing moisture from approximately 3 to 5% up to 9%. The report notes that success of the agglomeration method is limited.

Crush ore transport to Pad 2 currently employs a series of overland conveyors, mobile stackers (“grasshoppers”), and a radial stacker. This transportation system reaches the midpoint of Pad 1 but falls short of reaching Pad 2. Ore is discharged from the conveyor onto Pad 1 as a stockpile, then moved by loader and 136 t mine haul trucks to Pad 2. Once on Pad 2, ore is piled in 5 metre lifts using lower unit footprint weight loaders or excavators.

A new overland conveyor approximately 1,400 m long was under procurement and installation as of August 2017, to begin at the eastern edge of Pad 1 and extend to the eastern edge of Pad 2. An existing agglomeration drum measuring 3.7 m internal diameter by 8.0 m long, previously used at start of operations in 2007, was being re-installed. The agglomerator is powered by two 112 kW (150 hp) motors, and when inclined at 3.7 degrees is targeted to achieve 900 tph throughput based on a residence time of 60 seconds at 6 rpm, with potential to increase to 1,050 tph with additional motor power. Completion and commissioning of the new overland conveyor and agglomeration drum was anticipated by end of March 2018.

Heap leach pad operation

All ore is processed via heap leaching. Heap Leach Pads 1 and 2 are in operation, each with a separate leachate collection system. Pads 1 and 2 cover areas of 2,515,000 and 721,000 m² respectively, for a total of 3,236,000 m². As of end of 2017, approximately 230 Mt of ore had been stacked on both pads. The current remaining capacity is about 60 Mt.

Pad 1, the original heap leach pad, has been historically loaded with both Crush ore and ROM ore but is presently only loaded with ROM ore. Low-grade and SLG ores are stacked as ROM ore on Pad 1 for leaching. Pad 2, which became operational in 2013, was initially loaded with ROM ore for the first one to two lifts but currently is only loaded with Crush ore at 5 metre lift heights.

For ROM ore on Pad 1, lift thickness is 8 m, irrigation rate is 8 to 10 L/m²/hr, cyanide concentration is 350 mg/L, and irrigation time is 120 days. No ripping is performed during the leach cycle for ROM ore. For Crush ore on Pad 2, lift thickness is 5 m, irrigation rate is 12 L/m²/hr, cyanide concentration is 450 mg/L, and irrigation time is 120 days. After 60 days of leaching, drip lines are removed, the top of the lift is ripped to a depth of 3.5 m using a CAT D11 dozer, drip lines are reinstalled, and leaching resumes for an additional 60 days.

The pads are geosynthetically lined. The gold-rich leachate solution (Pregnant Leach Solution, PLS) is collected at the bottom via a network of solution collection pipes and channeled into separate ponds for Pads 1 and 2. An anti-scaling agent (Zalta MA11) is added to prevent scaling in the ADR processes and leach pad irrigation system.

Adsorption-Desorption-Recovery (ADR) plant

The ADR plant is a conventional Carbon in Column (CIC) recovery facility with a gold refinery producing a gold-silver doré product.

The adsorption component consists of seven trains: five trains of four columns each and two trains of three columns each. Each carbon column is 15 m³, filled with four tonnes of carbon, with a total design capacity of six tonnes of carbon per column. The amount of carbon was reduced from six to four tonnes as pregnant solution grades decreased due to lower grade ore being mined.

Total PLS flow to the seven trains is approximately 6,100 m³/h. The nominal design flow for each train was 835 m³/h while the maximum design flow was 950 m³/h; this is routinely exceeded. Average flow through the columns has exceeded 995 m³/h and has been reduced to 950 m³/h to reduce carbon abrasion.

Gold is stripped from loaded carbon using the Pressure Zadra Process with hot alkaline cyanide solution in three carbon stripping circuits. The concentrated gold strip solution is cooled, clarified, and circulated through four electrowinning cells where gold is removed into a sludge. The sludge is removed from electrowinning cathodes by high-pressure water.

The refinery includes the electrowinning cells, a filter to dewater the metal-rich sludge, a mercury retort, and an electric induction furnace that produces gold-silver doré bars of 500 to 1,000 oz.

There are five ponds with large storage capacities. Pad 1 PLS Ponds (two) total 109,000 m³, Pad 2 PLS Pond is 125,000 m³, the Recirculation/Intermediate Pond is 170,000 m³, Contingency Pond No. 1 is 455,000 m³, and Contingency Pond No. 2 is 980,000 m³, for total storage capacity of 1,839,000 m³.

Barren solution (barren of gold) is recharged with sodium cyanide and pumped back to the heap leach pads for distribution by a drip irrigation system.

Water and solution balance

The heap leach water balance model was updated in 2017 by Golder Associates, primarily incorporating historic operational data from December 2011 through November 2016. The 2017 updated model focused on infrastructure and operational logic addressing changes since 2011, primarily the addition of Heap Leach Pad 2, Process Pond 2, and Contingency Pond 2. Based on the water balance model, five evaporators were installed in mid-2017 to reduce excess water volume in the contingency ponds and to reduce overflow probability to less than 15% through 2027 with recirculation.

ADR plant performance

Average PLS treated per day is 156,000 m³, with total 56.93 million m³ treated in 2016. Gold recovery from PLS by carbon columns is 98.3%, averaging 650 oz/d Au.

Secondary gold recovery programs

A large-scale program was initiated in late 2016 and continued through 2017 to recover gold from the Heap Leach Pad Reserve on Pad 1. The program involved drilling 220 wells on a grid pattern of approximately 35 m centres to within 10 m of the underlying geosynthetic liner. Leaching solution (450 ppm cyanide) was injected at high pressure (1,200 kPa) and flow (up to 180 m³/h) into wells for gold extraction. Hydrated lime (milk of lime) was also injected to maintain pH of 9.0 or greater. Injection was applied for three to four hours per perforated zone, with each well having three to six zones. Surface re-irrigation was performed over the same area during high-pressure injection.

After the high-pressure injection phase, wells were re-injected with lower pressure rinse at approximately 40 to 50 m³/h for several weeks to months. After the low-pressure injection phase, the surface was scarified with a dozer and releached at approximately 8.5 L/hr/m² for approximately two months.

The first phase (Phase I) was completed in January 2016. Gold recovered by initial Phase I was estimated at about 1,370 ounces during injection and another approximately 250 ounces on rinsing, though this is not certain given it was based on flow rate and gold grades monitored in some wells. Phases II, III, and IV were initiated, and all wells had been injected with cyanide solution under high pressure conditions as of December 2017.

Leached ROM ore was excluded from the program as it was deemed too permeable for the pressure injection system.

In 2017, a program on Pad 2 involved turning over the top lift of leached Crush ore, then reapplying barren leach solution (350 ppm cyanide) for a secondary leach process. The turned ore was leached for an additional 30 days. Internal analysis reported 5% additional recovery of the gold.

Laboratory

The Los Filos assay and metallurgical testing facilities are in a secure compound including the ADR plant, ponds, and reagent mixing facilities. About 300 solid samples are processed daily from the open pits and 80 samples from underground. A 300 g sub-sample is cut from the 10 kg head sample using an automatic proportional sampler, then finely ground in one of two ring-and-puck pulverizers. Fire assay routine includes one duplicate, one blank, and one standard in a 24-sample batch. Two certified standards are used: 5.0 +/- 0.2126 g/t Au for fire assay/gravimetric assaying and 0.424 g/t Au for fire assay/AA assaying.

The assay laboratory is staffed with 21 persons and operates 24 h/d. The metallurgical test laboratory is staffed with 3 persons performing particle size analyses, bottle roll, and column leach testing. Two sets of column leach apparatus are used: 350 mm diameter x 3 m height and 150 mm x 2 m height columns. Leach test procedures are designed

Key reported parameters

Technical qualifications

The source describes laboratory and testwork facilities; reported test conditions are not equivalent to plant operating results.