Blackwater Gold Project — Pre-Feasibility Report

The proposed processing plant will treat 15,000 tonnes per day through a conventional crushing, grinding, leaching, and carbon-in-pulp flowsheet supported by testwork and prior feasibility study data.

Report context

The Blackwater Gold Project NI 43-101 Pre-Feasibility Report documents a process design based on testwork described in Section 13 of the report, together with information from a 2014 feasibility study. The proposed flowsheet is similar to many gold plants worldwide, with design details adapted from successful operation at Atlantic Gold's gold ore leach plant in Nova Scotia.

Processing route

Overall flowsheet

The proposed plant will consist of three-stage crushing; grinding in a single ball mill with gravity concentration incorporated into the grinding circuit; leaching with pre-oxidation; carbon-in-pulp adsorption of gold and silver; elution and electrowinning; smelting to gold silver doré; and cyanide destruction using an SO₂ air system.

Primary crushing

Ore will be loaded into a 360-tonne capacity feed bin of the primary crusher either directly from the ore truck or from a stockpile using a wheel loader. An apron feeder, with a fines conveyor installed beneath it, will feed the ore into a 250 kW jaw crusher (1.27 x 1.52 m) over an inclined vibrating grizzly to remove fines prior to the crusher. A hydraulic hammer will be installed to allow breaking of any oversize blocks.

Crushed ore will be fed to a 1.8 x 4.2 m vibrating screen, with minus 19 mm undersize material reporting to the product conveyor. Oversize will be conveyed to a secondary cone crusher (300 kW) with a closed side setting of 44 mm. Product from the crusher will be conveyed to a 2.4 x 6 m vibrating screen where minus 19 mm material will report to the product conveyor. Oversize will be conveyed to a pair of tertiary cone crushers (300 kW each, the same model as the secondary crusher except for the liner) with a closed side setting of 19 mm. The crushed product will be conveyed to the final 2.4 x 6 m screen, with undersize reporting to the product conveyor. Oversize will be conveyed back to the tertiary crushers. The product conveyor will transport the minus 19 mm ore to a covered stockpile where a tripper on the conveyor will deposit the material. A concrete tunnel under the stockpile will house the mill feed conveyor, and the crushed ore will be fed onto the mill feed conveyor at a controlled rate by three vibrating feeders.

All crushers and screens are to be installed in closed buildings to accommodate low temperatures and heavy snowfall at site. The circuit layout was designed to accommodate the three screens in one building and all cone crushers in another. These buildings will allow the use of dry fog for dust control. All conveyors will be fitted with walkways and the belts will be covered.

Milling

Crushed ore will be fed at a design feed rate of 680 tonnes per hour into a 7.3 x 12.5 m ball mill with a double pinion drive (two x 7 MW motors). The mill discharge will report to a pump box from which the slurry will be pumped to the cyclones and to the gravity recovery circuit. Both streams will have one duty pump and one standby. The ball mill will be fed with 75 mm balls and 0.5 kg/t of lime.

A cyclone cluster (four operating, two standby) will separate the P80 150 µm product, with cyclone underflow flowing directly to the ball mill. A gravity concentrator feed rate of 800 tonnes per hour is specified, with minus 2.0 mm feed to the concentrators. Concentrate to intensive leach is designed at 3.4 tonnes per day, with 30% gold recovery by gravity.

Leaching and carbon-in-pulp

Pre-leach thickening is not required. Solids feed to leach is 48%. Pre-aeration residence time is 2 hours, based on testwork. Leach residence time is 48 hours, based on testwork. Eight leach tanks are specified. Sodium cyanide consumption is 0.6 kg/t and lime consumption (quick lime) is 0.8 kg/t.

CIP residence time is 3 hours, with carbon concentration of 30 kg/m³ and carbon loading of 1,250 g/t gold.

Cyanide destruction

Feed solution CNWAD is 350 mg/L, with a discharge target of less than 1.0 mg/L. Residence time is 1 hour in two tanks. SO₂ consumption is 0.67 g/g CNWAD, lime consumption (quick lime) is 7.8 g/g CNWAD, and copper sulfate concentration is 15 mg/L.

Key reported parameters

Technical qualifications

The process design criteria were developed based on testwork described in Section 13 of the report, together with information from the 2014 FS. The crushing work index value of 0 kWh/t is sourced from the 2013 FS. Bond Ball Mill Work Index and Bond Abrasion Index values represent the 75th percentile from 2019 testwork. Certain values, including crusher ore stockpile live capacity, are identified as approximate calculations or design considerations. The overall flowsheet and each unit process are summarised in Figure 17-1 of the report.

Source: Blackwater Gold Project NI 43-101 Pre-Feasibility Report, Section 17 Recovery Methods (pages 195–200).