ELG Mine Complex — 2018 Technical Report

This report details the process design, current operation, and recent modifications at the ELG Mine Complex processing plant.

Report context

This technical report, dated 4 September 2018 (Revision 0), describes the ELG Mine Complex (Morelos Property) processing facilities. The ELG Process Plant has been in commercial operation since March 2016. The design basis is 14,000 tonnes per day at 90% mill availability, with current operation at approximately 13,000 tpd (approximately 90% of design). The reported bottleneck in the grinding circuit is under optimization to balance workload between the SAG mill, ball mill, and pebble crusher.

Processing route

Crushing and grinding

Two identical crushing systems process run-of-mine ore from the El Limón and Guajes pits. Each crusher location has a 200-tonne capacity feed hopper fed directly by 100-tonne rear dump haul trucks. Primary gyratory crushers (1.067 m by 1.651 m) produce a 150-mm product for the SAG mill circuit. The El Limón pit uses a RopeCon conveyor system, installed in 2015 and operational since 2016, to transport crushed ore over approximately 1 km horizontal and 300 m vertical distance to the coarse ore stockpile. The Guajes pit uses a belt conveyor (1.219 m wide by 149 m long) to the same stockpile, which has a live capacity of 14,000 tonnes.

Crushed ore is reclaimed by two feeders onto a 1.22 m wide by 200 m long conveyor belt feeding the SAG mill. Primary grinding uses a 9.15 m diameter by 4.15 m effective grinding length SAG mill with a 7,000 kW motor, in closed circuit with a discharge screen and pebble crusher. Secondary grinding uses a 7.3 m diameter by 12.65 m effective grinding length ball mill with two 7,000 kW motors in closed circuit with hydrocyclones. Final product size averages 80% minus 83 microns. Cyanide addition to the grinding circuit has been discontinued as sufficient cyanide is present in recycled process water.

Leaching and carbon-in-pulp

A 32 m diameter high-rate thickener thickens cyclone overflow to 50% solids for the leach circuit. Leaching occurs in four to eight of eleven installed tanks (15.55 m diameter by 21.34 m high, each providing 3,815 m³ working volume at 20.1 m slurry level). Operating four or six leach tanks provides approximately 18 to 26 hours retention time at plug-flow for cyanide leaching at 50% solids. The pH is maintained above 10.5, and cyanide addition follows a recipe dictated by copper content, maintaining a cyanide-to-copper molar ratio above 3.5.

Gold and silver are adsorbed onto activated carbon in the CIP circuit, consisting of six 250 m³ AAC Pump Cell tanks in carousel configuration. Each CIP tank nominally contains 50 g/L of 6 by 12 mesh granular activated carbon, with 12 tonnes of carbon per cell. Carbon harvesting occurs once daily. Slurry from the last CIP tank passes over a carbon safety screen with 0.5 mm slotted panels before reporting to the cyanide recovery thickener.

SART plant

In late 2016, a SART (sulfidization-acidification-recycling-thickening) plant was added to address elevated copper in the ELG ore during ramp-up. The SART plant was expected to be in full operation by July 2018. It removes approximately 45% of copper per cycle from the cyanide recovery thickener overflow through sulphide addition at acidic pH of about 4.0, producing a saleable high-grade (>50%) Cu₂S precipitate. Design criteria include 15,600 tpd feed with 34% copper dissolution and approximately 1,800 ppm copper in process solution.

Tailing detoxification, dewatering, and disposal

CIP tailings are pumped to a 32 m diameter high-rate cyanide recovery thickener. Thickener underflow reports to cyanide detoxification, where WAD cyanide is oxidized by the SO₂/air process, typically using MT-2000 (ammonium metabisulphite) with oxygen injection. Two detoxification reactors (9.7 m diameter by 11.6 m high, each 803 m³ working volume) operate in parallel providing approximately 2 hours residence time.

Detoxified tailings are filtered using seven Diemme pressure filters and two Delkor horizontal belt filters. Each horizontal belt filter has a 162 m² bed surface, with throughput of 100 tonnes per hour producing cake at 18% moisture. Maximum filter plant throughput is estimated at up to 17,840 tpd. Filter cake at approximately 20% moisture is transported by conveyor to a filtered tailings storage facility.

Carbon stripping and regeneration

Loaded carbon undergoes cold washing with 50,000 ppm NaCN solution (stripping 50-80% of copper), followed by acid washing with dilute hydrochloric acid to remove inorganic contaminants. Carbon stripping uses a pressure Zadra process at 140°C, circulating approximately 12 bed volumes at 2 bed volumes per hour. Thermal reactivation occurs every second or third cycle in a horizontal rotary kiln at 700-750°C.

Refining

Pregnant solution is pumped at 13.71 m³/h through four 6 m³ electrowinning cells. Gold sludge is filtered, retorted at 600-700°C to remove mercury, then mixed with fluxes and melted in a diesel-fired furnace to produce doré bars.

Key reported parameters

Technical qualifications

The SART plant and additional filter capacity were in the final stages of construction and commissioning at the time of this report; the SART plant was expected to be in full operation July 2018. The SART plant is bypassed during maintenance. The grinding circuit is identified as a current bottleneck, with optimization ongoing. Horizontal belt filter capacity figures are based on operational test results. The copper sulphate catalyst for detoxification was anticipated but found unnecessary due to sufficient copper in plant solution. Design criteria are summarized in Appendix B of the source report.

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Source: M3-PN170117, Morelos Property, Form 43-101F1 Technical Report, 4 September 2018, Revision 0, Section 17: Recovery Methods.