Stornoway Diamond Corp. recently announced the results of plant-design optimization at the Renard diamond project that is expected to allow the immediate addition of a large diamond recovery capacity to the plant for no additional net capital cost. The project is located in north-central Quebec, Canada.

The Renard diamond plant is being designed and constructed under an EPCM agreement between Stornoway and SNC-Lavalin Inc., with subcontracted services being provided by AMEC Americas Ltd. and DRA Americas Inc. (DRA) for the specialized engineering and field support services relating to the plant’s crushing, material handling, and diamond recovery circuits. Detailed plant engineering is ongoing, with the first concrete pour for the plant’s foundation scheduled for April 2015.

However, the work completed to date has already indicated that sufficient design efficiencies and cost savings can be implemented to allow for a Large Diamond Recovery circuit (LDR) to be fully integrated into the plant’s primary flow-sheet within the existing capital budget and with no change of scope in the existing ore processing capacity of 2.2 million mt/y.

Stornoway’s January 2013 Optimized Feasibility Study for the Renard Diamond Project contemplated a diamond plant flow-sheet geared to the recovery of diamonds up to 30 mm in diameter (equivalent to a 200-carat round octahedral stone), with room for a LDR circuit to be retrofitted later as required. The direct capital cost for this feasibility plant design, without LDR, was estimated at that time to be C$162.7 million, excluding capitalized operating costs. The modified plant design that will be built contemplates the recovery of diamonds up to 45 mm in diameter (equivalent to a 600-carat round octahedral stone), with a corresponding direct capital cost estimate of C$147.1 million.

The optimized Renard diamond plant flow sheet, developed by DRA, will maintain the three-stage crushing regime established in the January 2013 Optimization Study. Run-of-mine ore will be broken down with a primary jaw crusher utilizing a 230-mm top size cut-off and fed to a 4-m-diameter rotary scrubber for disaggregation. Direct feed to Dense Media Separation (DMS) circuits will be +1 mm to –19 mm (compared with +1 mm to –30 mm previously) so as to improve efficiency and reduce diamond breakage risk. Diamond recovery from concentrate will be through magnetic separation and X-ray technology supplied by DebTech, with no grease table.

Recent bench tests on Renard diamonds at DebTech suggests a 98% recovery efficiency with X-ray sorters. Direct feed from the scrubber product screen to the LDR circuit will be +19 mm to –45 mm, with the top size cut-off adjustable to 60 mm. Oversize material between +45 mm and –230 mm will be fed to a cone crusher under choke feed, before returning to the DMS and LDR circuits via a sizing screen. The non-diamond bearing material from the LDR circuit and the DMS tails between +6 mm and –19 mm will be conveyed to high pressure grinding rolls, with the resulting cake returning to the scrubber for disagglomeration. Fine tails at -1 mm will be recovered via thickener and degrit cyclone before dewatering by centrifuge and combination with the +1 mm –6 mm DMS tails for disposal by dry-stacking as previously.

Nameplate ore processing capacity will remain at 6,000 mt/d (2.2 million mt/y) at an overall plant utilization (OPU) of 78%, expandable to 7,000 mt/d (2.6 mt/y) assuming an OPU of 83% and further operational optimization. The Renard mine plan contemplates 6,000 mt/d of ore feed from the Renard 2 and Renard 3 open-pit and underground mine, supplemented by 1,000 mt/d from the Renard 65 open-pit.