Harper Creek Copper Project — 2012 Technical Report

This feasibility study describes a conventional large-tonnage copper concentrator designed to process 70,000 t/d of VMS ore from the Harper Creek deposit.

Report context

This technical report and feasibility study for the Harper Creek Copper Project was prepared for Yellowhead Mining Inc. and dated March 29, 2012. The proposed process plant shows a conventional flowsheet for a large tonnage, low grade copper deposit in British Columbia, with modifications based on metallurgical testwork carried out on core samples specifically procured for that purpose. The ore is described as a VMS hosted by a sedimentary series of sandstone and siltstones.

Processing route

Comminution

The proposed concentrator will handle ore through a conventional primary gyratory crusher (1,524 mm x 2,261 mm / 60" x 89") at an average rate of 4,167 t/h, reducing run-of-mine feed to 80% passing 200 mm. Crushed material will be transported by overland conveyor to a coarse ore stockpile with a live capacity of 70,000 t. Ore is reclaimed by three apron feeders (two operating, one standby) at a nominal rate of 3,170 t/h for the grinding circuit.

The primary grinding circuit is a SAG-ball (SAB) circuit comprising one SAG mill (11.6 m diameter x 6.7 m long, 20 MW) and two ball mills (7.3 m diameter x 12.8 m long, 13 MW each). The SAG mill discharges onto a vibrating screen; screen undersize is pumped to two independent ball mill circuits operating in closed circuit with hydrocyclone clusters. The hydrocyclone overflow advances to flotation at a particle size of 80% passing 180 μm, containing approximately 35% solids by weight. The circulating load in the ball mill circuit is approximately 250%.

The report notes that the option of HPGR to replace the SAG mill was rejected because of the relatively low metric Bond ball mill work index (13.2) and the modest-to-high abrasion index (~0.35 on the most significant ore types). Comminution testwork carried out by FLS suggested that pebble crushing in the SAG circuit would not be required; this conclusion was reviewed by an independent consultant and appears to be related to the soft to moderately soft sedimentary lithology of the ore deposit. The circuit has been designed for future installation of a pebble crusher should it be required.

Flotation and regrinding

The hydrocyclone overflow feeds two rougher/scavenger flotation banks, each containing six 300 m³ tank cells (twelve cells total). The rougher/scavenger circuit operates at an elevated pH of 11.0, compared with earlier testwork carried out at pH 9.0. Lime is added to the SAG mill feed belt to raise the pH and aid selectively in flotation. Flotation reagents include PAX as collector and MIBC as frother, with provision for a second collector.

The rougher/scavenger concentrate is reground to 80% passing 20–25 μm using two IsaMill™ M10000 horizontal grinding mills. The feed slurry is densified through a hydrocyclone cluster of ten 250 mm cyclones before entering the IsaMills. Slurry lime may be added to the regrind hydrocyclone feed pumpbox to maintain pH at approximately 11 for downstream cleaning.

The reground concentrate reports to two 175 m³ column cells operating in parallel as first cleaner columns, followed by a single 175 m³ column cell as the second cleaner. The tailings from the first cleaner pass through two 50 m³ cleaner scavenger tank cells, with concentrate returned to the regrind circuit. The report notes that pyrite rejection in cleaning can be achieved after fine regrind without the use of special depressants such as cyanide.

Concentrate dewatering and handling

Final cleaner concentrate is thickened in an 18 m diameter high-rate thickener to 60% solids, then fed to two 100 m² filter presses. The filter cakes contain less than 8% water by weight, enabling shipment by bulk ocean freight without thermal drying. Filtered concentrate is stockpiled before trucking to an off-site concentrate handling facility in Vavenby.

Tailings and water

Rougher/scavenger and cleaner scavenger flotation tailings are pumped separately to the tailings storage facility. Process water from the tailings pond is reclaimed by pumps on a water barge and delivered to the process water pond for distribution. Fresh water from local sources is used for gland seal service, mill cooling, reagent preparation, and process water make-up.

Reagent handling

Reagents include PAX, MIBC, lime, flocculant, and anti-scalant. Most lime is added dry to the SAG mill feed belt; some quicklime is slaked on site and added as slurry to grinding and regrinding circuits to depress pyrite flotation. PAX is prepared at 20% solution strength by weight; milk of lime at 15% by weight; flocculant as a dilute solution of less than 0.5% strength.

Key reported parameters

Technical qualifications

This report presents a feasibility study flowsheet based on metallurgical testwork on core samples specifically procured for the program. The proposed primary grind of 180 μm represents a change from earlier PEA studies (105 μm), and testwork showed superior results at the coarser grind. The elevated pH of 11.0 in rougher/scavenger flotation achieved copper recovery equal to or better than earlier testwork at pH 9.0 while reducing weight recovery from 12% to 6.5%. Comminution testwork by FLS and independent review concluded pebble crushing is unlikely to be required, a conclusion related to the sedimentary lithology. The regrind mill sizing is based on XStrata applying an efficiency factor estimated from similar IsaMill™ installations. Flotation cell sizing used retention times from laboratory tests with typical scale-up factors. The flowsheet does not report achieved metallurgical performance, operating data from a commercial plant, or economic outcomes.

Source: Technical Report and Feasibility Study for the Harper Creek Copper Project, March 29, 2012, Section 17: Recovery Methods, prepared for Yellowhead Mining Inc.