Touro Copper Project — 2018 Technical Report

This report presents the proposed processing flowsheet, design criteria, and testwork results for a two-phase copper concentrator at the Touro Copper Project.

Report context

The Touro Copper Project technical report, dated April 2018, describes a proposed conventional copper flotation concentrator designed to process open-pit sulfide ore in two phases. Phase 1 targets a plant throughput of 6.0 Mt/y, with a Phase 2 expansion to 10 Mt/y planned prior to year 8 of operation. The life of mine is 13 years, with potential extension if additional ore resources are identified. The previous operation on site closed in 1986 and most processing equipment has been removed; no re-use of the previous plant is envisaged.

Processing route

Flowsheet development

Flowsheet development was based on testwork reported by Wardell Armstrong International (WAI) to produce a concentrate grade of 27% Cu at 90% recovery. Locked cycle tests completed during the latest testwork program produced concentrates averaging 29.1% Cu at 87.0% recovery, indicating that the target is achievable.

The proposed process flowsheet uses a conventional SAG mill – ball mill (SAB) grinding circuit followed by a copper flotation recovery circuit. The initial concentrator includes primary crushing, primary and secondary grinding, rougher flotation, regrinding, three stages of cleaner flotation, and concentrate thickening and filtration. The concentrator and associated service facilities will process run-of-mine ore to produce dewatered copper concentrate and tailings slurry.

The preliminary mining schedule outlines annual ore movement ramping up to 5.1 million tonnes in the first year, reaching a peak of 6.0 million tonnes in the following two years, then ramping up again from year 8 to 10.0 million tonnes. To avoid three successive plant upgrades, only two design points are considered: an initial Phase 1 plant throughput of 6.0 Mt/y and a Phase 2 upgrade to 10 Mt/y prior to year 8.

Primary crushing and coarse ore stockpile

The primary crushing circuit is designed to meet Phase 2 production capacity at 75% utilization, equating to a nominal average capacity of 1,522 t/h. A 42 x 65 inch gyratory crusher with a conservative maximum capacity of 1,600 t/h has been selected. The crusher accepts a top size of 1,000 mm and produces a product P80 of approximately 125 mm at a closed side setting of 150 mm.

The ROM bin feeding the primary crusher is designed for direct truck dump from two dump points with a capacity of 200 t. Primary crushed ore is stored on an open conical stockpile of 63 m base diameter. The live capacity is 18,313 t, providing a 20-hour milling capacity buffer for Phase 1 and a 12-hour buffer for Phase 2. Stockpile reclaim is via two gravity reclaim apron feeders of 1,250 t/h capacity each.

Grinding circuit

A primary grind P80 of 125 µm was selected based on an economic analysis of grind sizes. While some ore types showed increased revenue at a finer P80 of 106 µm, payback periods ranging from 3.1 to 11.2 years were considered uneconomic for a project with capital cost constraints.

The SAB circuit was selected over other options. Multi-stage crushing with HPGR and AG milling with pebble crushing were rejected due to higher capital costs. The Phase 1 design uses a 6.0 Mt/y SAB circuit, expanding to a SABC circuit at 10 Mt/y with the addition of pebble crushing and a second ball mill.

SAG mill power is determined based on the Axb value. The SAG mill selected is 10.36 m diameter by 6.64 m effective grinding length (EGL) with 14,500 kW installed power, providing 18% contingency for charge fluctuations. In Phase 2, the grates are pebble ported to allow extraction of approximately 25% pebble recirculating load.

The ball mill selected is 6.25 m diameter by 9.8 m EGL overflow design with 7,250 kW motor power, providing 14% design contingency. A second identical ball mill is added for Phase 2. The SAG mill uses twin pinion drives powered by two 7,250 kW motors, common with the ball mill motor for critical spares reduction.

Two 5½ foot Symons cone crushers with 200 kW motors are selected for pebble crushing at 10 Mt/y. One crusher operates continuously choke fed while the second operates intermittently to regulate surge bin level.

Flotation circuit

A rougher flotation residence time of 10 minutes laboratory equivalent was selected based on economic analysis. Increasing from 6 to 10 minutes gave an average simple payback period of 1.7 years, considered economic. Further increase from 10 to 14 minutes gave an average payback of 3.8 years, considered too long.

For Phase 1, the flotation circuit includes six 130 m³ rougher tank cells, four 38 m³ Cleaner 1 cells (2+2 configuration), three 38 m³ Cleaner-scavenger cells, four 16 m³ Cleaner 2 cells (2+2 configuration), and three 16 m³ Cleaner 3 cells. Laboratory retention times are scaled up by a factor of 2.5 for plant design, with effective cell volume of 85%.

The regrind product size is selected at P80 20 µm. Regrind mill sizing uses a method based on benchmarking of other copper sulfide flotation concentrators. The regrind mill operating work index is 1.25 times the feed ore ball mill work index. A 4.27 m diameter by 6.83 m EGL regrind ball mill with 2,000 kW installed power is selected.

For Phase 2 expansion to 10 Mt/y, three additional 130 m³ primary rougher cells are installed ahead of the existing roughers. A single stage of column flotation (primary cleaner) is added to clean primary rougher concentrate to final concentrate grade. The column is 2.5 m diameter by 8.0 m high forced aeration design. This expansion, together with the reduced feed grade, allows the rest of the circuit to remain unchanged.

Concentrate dewatering

Final concentrate is thickened in an 11 m diameter high-rate thickener, selected based on standard industry values of 0.20 t/h/m² for flotation concentrates. Concentrate is thickened to nominally 59% w/w solids and stored in a 200 m³ agitated filter feed tank providing 12 hours surge capacity. Further dewatering uses a vertical plate pressure filter producing filter cake at nominally 9% moisture, with 18 plates selected for 97 m² filtration area, increasing to 104 m² with two additional plates for Phase 2.

Reagents

Flotation reagent consumption rates are nominal for both phases. Lime dosing at 2,000 g/t is added to the SAG mill feed chute. Collectors (PAX at 30 g/t and thionocarbamate at 40 g/t) and frother (Dowfroth DF250 at 40 g/t) are dosed to multiple flotation circuit points. Flocculant at 40 g/t is added to the concentrate thickener.

Key reported parameters

Technical qualifications

The reported values represent proposed design criteria based on testwork. Locked cycle test results averaged 29.1% Cu at 87.0% recovery, not the design 90% recovery target. The design grade for Phase 1 (0.66% Cu) includes a 15% margin above the highest yearly average of 0.57% Cu. Similarly, the Phase 2 design grade of 0.44% Cu is 15% greater than the highest yearly average of 0.38% Cu.

Several economic analyses used throughput values that have since changed: the primary grind size analysis used 5 Mt/y (changed to 6 Mt/y), the rougher residence time analysis used 5 Mt/y and 100 m³ cells (changed to 6 Mt/y and 130 m³ cells), and the comminution circuit trade-off study used 5 Mt/y expandable to 8 Mt/y (changed to 6 Mt/y expandable to 10 Mt/y). The report states these analyses remain valid despite the changes.

Tailings settled solids concentration of 75% w/w is estimated and requires verification by testwork. Tailings rheology also requires confirmation. Flotation reagent consumption rates are nominal design values. Flocculant dosing pumps are sized for twice the required dose rate to cater for process fluctuations. Recoveries of gold and silver in the copper concentrate are considered at or below the minimum level for smelter credits.

Source: Technical Report on the Mineral Resources of the Touro Copper Project, April 2018, Section 17 Recovery Methods.