Cerro Las Minitas Project — 2024 Technical Report

The conceptual processing facility design for the Cerro Las Minitas polymetallic skarn deposit is based on metallurgical test work, including a locked cycle test, and employs conventional sequential flotation followed by hydrometallurgical treatment of a pyrite concentrate.

Report context

The Cerro Las Minitas Project is a large polymetallic mineralized skarn deposit (silver, lead, zinc, copper) that will be mined as an underground mine. This NI 43-101 Technical Report Preliminary Economic Assessment is dated June 04, 2024. The conceptual design for the processing facility is based on recent metallurgical test work, specifically a locked cycle test (LCT) completed on Skarn Front Master Composite (PJ5287-LCT1) for the flotation circuit. The plant is designed to process 1,934,500 t of plant feed annually (5,300 t/d) to produce copper, lead/silver, zinc, and pyrite concentrates. Process design criteria are based on a 5,300 t/d (1,934,500 t/a) plant design, with the crushing circuit designed for 70% availability, filtration facilities for 80% availability, and the remainder of processing facilities for 92% availability.

Processing route

Crushing and grinding

The primary crushing circuit will be fed directly from haul trucks. A vibrating grizzly feeder will separate fines from coarse material entering the jaw crusher. Size reduction of mineralized material by a primary jaw crusher will reduce material size from an F100 of 400 mm to a P80 of 80 mm. Crushed material will be conveyed to a secondary screen with a bottom deck aperture of 15 mm. Screen oversize discharges to a secondary cone crusher; product from both primary and secondary crushers is returned to the screen. Undersize product from the secondary screen (P80 = 8 mm) will be conveyed to a crushed material stockpile with a live capacity of 3,786 t (12 hours live capacity at nominal feed rate).

Crushed mineralized material will be withdrawn from the stockpile using two feed reclaim belt feeders at a combined rate of 240 t/h. The grinding circuit is designed to reduce material from an F80 of 8 mm to a P80 of 100 µm in a single stage overflow ball mill. Grinding will be performed in a rubber lined ball mill, 5.0 m inside diameter x 8.1 m effective grinding length (EGL), powered by a fixed speed 3,510 kW motor, operated in closed circuit with hydrocyclones using stainless steel media. Hydrocyclone overflow (final grinding circuit product) flows to a trash screen and then by gravity to the copper flotation conditioning tank.

Sequential flotation

The flotation plant will consist of sequential selective flotation circuits to produce copper, lead, and zinc concentrates. Each circuit will consist of rougher flotation, rougher concentrate regrinding, and three stages of cleaner flotation.

Copper flotation and regrind: Cyclone overflow flows by gravity to a conditioning tank, then to a bank of rougher flotation cells. The copper flotation circuit consists of five 10 m³ rougher cells operated in series, followed by regrind and three stages of cleaner flotation (five 1.5 m³ first cleaner cells, two 1.5 m³ second cleaner cells, one 1.5 m³ third cleaner cell). Flotation is performed at natural pH. Reagents added include zinc sulfate, A-3894, sodium metabisulfite, and MIBC. Copper rougher tailing and copper first cleaner tailing flow by gravity to the lead rougher conditioning tank.

Lead flotation and regrind: Copper rougher tailing and copper first cleaner tailing flow to the lead rougher conditioning tank, then are pumped to lead rougher flotation cells. The lead rougher circuit consists of one row of five 20 m³ forced air cells operated at natural pH. After regrind, the circuit uses three stages of cleaner flotation (five 3 m³ first cleaner cells, three 3 m³ second cleaner cells, two 3 m³ third cleaner cells). Reagents include 3418A, sodium cyanide, zinc sulphate, and MIBC. Lead rougher tailing and lead first cleaner tailing flow by gravity to the zinc rougher conditioning tank.

Zinc flotation and regrind: Discharge from the zinc rougher conditioning tank is pumped to zinc rougher flotation cells. The rougher circuit consists of one row of five 50 m³ forced air cells operated at pH = 10 to 11. After regrind, the circuit uses three stages of cleaner flotation (five 8 m³ first cleaner cells, four 8 m³ second cleaner cells, three 8 m³ third cleaner cells). Milk of lime and copper sulfate are added to the zinc rougher conditioning tank to adjust pH. Milk of lime, SIPX, copper sulfate, and MIBC may be added to the zinc flotation circuit.

Pyrite flotation: Tailing from the zinc rougher tailing flows through four 30 m³ pyrite rougher flotation cells. Pyrite rougher concentrate is reground and sent to a single stage cleaner flotation to produce a pyrite concentrate. Pyrite rougher tailing and pyrite cleaner tailing (final tail) flow to the tailing thickener.

Concentrate dewatering

Final copper, lead, and zinc concentrates will be thickened, filtered, and loaded on trucks for shipment. Each concentrate stream is directed to a dedicated thickener. Concentrate thickener underflow is pumped to an agitated storage tank and then to a pressure filter. Filter cake discharges to a stockpile. Filter duty is shared for copper and lead concentrate.

Pyrite concentrate hydrometallurgical treatment

The pyrite concentrate from the concentrator will undergo processing in the hydroplant to recover precious metals. Pyrite concentrate from thickener underflow is diluted with process water and reground in an IsaMill to a product P80 of 10 µm.

The ultrafine grind slurry is leached in a cyanide leaching circuit consisting of four mechanically agitated leach tanks operating in series, each 5.8 m diameter by 6.3 m high. Sodium cyanide is added for gold and silver dissolution; milk of lime maintains operating pH between 10.5 and 11.0. Air is introduced to maintain oxygen levels at 7.75 nm³ oxygen per tonne solids. The leach circuit has a 24-hour retention time.

A four-stage CCD washing circuit using 8.0 m diameter thickeners recovers pregnant solution from the leached slurry. The washing ratio (flow rate of washing barren solution to solids in underflow) is 4.5:1 to achieve overall CCD washing performance efficiency higher than 99%.

Pregnant leach solution from the CCD circuit is treated in a Merrill-Crowe plant with a capacity of 85 m³/hr. The solution is clarified, deaerated, and zinc is added for precious metal precipitation. Wet filter cakes are transferred to retort pans, heated under vacuum to remove water and mercury, then dried filter cake is mixed with flux and smelted in an electric arc furnace to produce doré.

Cyanide destruction and tailings dewatering

Washed leach residue slurry from the CCD circuit is treated using a sulphur dioxide (SO₂)-O₂ process (INCO process) in two reaction tanks (3.8 m diameter x 4.3 m high) to reduce CN WAD cyanide concentration to less than 5 mg/L. Sodium metabisulphite, copper sulfate, oxygen, and milk of lime are added. Each tank provides 1 hour residence time.

Final flotation tailing (from pyrite rougher flotation) flows to a high rate tailing thickener. Thickener underflow is pumped to an agitated storage tank and then to two pressure filters operated in parallel. Filter cake is transported to the tailing storage facility or to the tailing paste backfill plant.

Key reported parameters

Technical qualifications

Reagent types and dosages were established in metallurgical programs conducted at Blue Coast Research between 2018 and 2020 and at Blue Coast Research Ltd. and Base Metallurgical Laboratories Ltd. between 2017 and 2023. The specific basis for the flotation circuit design was the results and conditions from a locked cycle test (LCT) completed on Skarn Front Master Composite (PJ5287-LCT1).

The grinding media consumption was calculated based on an estimated Bond abrasion index from similar process facilities and adjusted based on vendor recommendation for use of high chrome media. The same wear rate was assumed for both primary grinding and regrinding applications. This approach was taken since no Bond abrasion index data is presently available for the Cerro Las Minitas mineralization. For the grinding media and liner consumption, an estimated Bond abrasion index corresponding to the 50th percentile of abrasiveness of more than 2,000 samples tested at SGS was used, as no Bond abrasion index data is presently available for the SMSU and MSU mineralization.

Grinding media is assumed to be high chrome based on the use of stainless media during testing, in order to control Eh and ensure activation of chalcopyrite which was otherwise misplaced into the zinc concentrate.

Auxiliary systems such as reagent mixing and storage, maintenance and office requirements, and laboratory estimates were based on other similar projects. Reagent consumption was estimated from laboratory flotation tests and data from other properties. Grinding media consumption was estimated from comminution testing and data from other properties.

Source: Cerro Las Minitas Project NI 43-101 Technical Report Preliminary Economic Assessment June 04, 2024, Section 17 Recovery Methods.