This technical report presents the proposed processing route for the GSS Project based on metallurgical testwork conducted at SGS.
Report context
The 2018 technical report for the GSS Project describes a proposed mineral processing plant design with a nominal throughput of 6.0 Mtpa. The design is based on metallurgical testwork conducted at SGS Vancouver and Lakefield during 2017 and 2018. The report includes the process design criteria, unit process operations, infrastructure requirements, and plant control philosophy for the proposed treatment facility.
Processing route
Overview
Metallurgical testwork confirmed that GSS and Wadi Doum material is amenable to gold and silver recovery via conventional cyanidation techniques and carbon adsorption across a wide range of ore domains and lithologies. A trade-off study comparing hybrid CIL, leach CIP and leach pumpcell circuits showed that the leach pumpcell circuit had the lowest operating costs and the best economics over the life of mine, despite hybrid CIL having the lowest capital cost. Therefore, a leach pumpcell circuit has been adopted for design.
The process plant design is based on a robust metallurgical flowsheet designed for optimal precious metal recovery and removal of deleterious elements such as mercury. The flowsheet includes unit operations that are well proven in the industry.
Crushing and ore stockpile
The crushing circuit consists of single stage primary crushing with a gyratory crusher to produce a crushed product size P80 of approximately 110 mm. A vibrating grizzly jaw crusher installation was considered high risk due to the abrasive and competent nature of some fresh ores, so a primary gyratory crusher with an open side setting of 140 mm has been selected.
A crushed ore stockpile with a nominal live capacity of 10,000 t will be provided, equivalent to 13 hours of mill feed at 6 Mtpa. Three apron feeders will be installed underneath the stockpile in the reclaim tunnel to deliver ore to the milling circuit. Quicklime for pH control will be added directly onto the SAG mill feed conveyor.
Grinding and classification
The grinding circuit is configured as a two stage circuit with a SAG mill and closed circuit ball mill (SAB). The SAG mill will be a 9.14 m x 5.30 m EGL variable speed mill fitted with a 9.3 MW motor. The ball mill will be a 6.70 m x 11.3 m EGL fixed speed overflow ball mill fitted with a 9.3 MW motor.
The circuit will produce a P80 grind size of 53 µm for GSS oxide and Wadi Doum ores, approximately 75 µm for GSS transition ore and 90 µm for GSS fresh ores. In later years, pebble crushing will be required due to increasing ore competency; provision for this equipment has been allowed in the design. Pebble crushing will only be required for the more competent ores processed towards the end of the mine life. Cyclones of 500 mm in diameter have been selected for classification duty.
Pre-leach thickening
A high rate pre-leach thickener ahead of the leach circuit has been selected to thicken trash screen undersize to the design leach circuit feed density of 55% w/w solids. Rheology testwork confirmed that this density is achievable even for the oxide blends, as there are only trace amounts of clay in the ore.
Leach and CIP circuit
Head assays and metallurgical testwork indicated that the Block 14 ores do not show preg-robbing characteristics, therefore a leach CIP circuit can be considered for design. A dedicated pre-aeration tank has been included for passivation of cyanide consuming minerals, as testwork indicated that four hours of pre-aeration reduced cyanide consumption for a number of ore types.
The leach circuit incorporates 10 leach tanks in series to provide 45 hours leach residence time. Testwork indicated that for fresh and transition ores, leaching is generally complete within 32 hours, but oxide and Wadi Doum ores show benefit from longer leach times. Each leach tank will have a live volume of approximately 4,000 m³. The design is based on air addition rather than oxygen, as metallurgical testwork demonstrated no gold recovery benefit from using oxygen compared with air.
A Kemix Pumpcell CIP circuit has been selected for recovery of gold onto carbon, to minimise carbon inventory, gold in circuit and operating costs. The CIP circuit will operate as a carousel system in which the feed and discharge points of each tank can be changed, and carbon will only be moved when transferring loaded carbon to the elution circuit or returning barren carbon from regeneration. Carbon loading testwork indicated that good CIP performance can be expected, with a design carbon loading for gold and silver of 6,133 g/t Au + Ag used for sizing the CIP and elution circuit.
Elution, electrowinning and gold recovery
A 12.5 t split AARL elution circuit has been selected, including optional cold cyanide strip for removal of copper from carbon prior to elution. Based on calculated carbon movements, a total of seven elution cycles are required each week. Three parallel 33 cathode electrowinning cells are proposed for the gold room. Due to the high silver content of the ore, the electrowinning capacity is relatively large for the plant throughput.
A mercury retort will be provided for removal of mercury prior to smelting to produce doré. Electrowinning cell sludge will be filtered in a pressure filter prior to transfer to the mercury retort. Off-gases from electrowinning cells will be passed through an activated carbon filter for mercury removal. Barren carbon will be regenerated in a horizontal rotating kiln incorporating scrubbing of off-gases for mercury removal.
Tailings thickening and pumping
A high rate thickener has been selected to thicken the CIP tails to maximise process water recovery and reduce the volume of tailings. The tailings thickener also allows residual free cyanide in the process water to be recovered for re-use in the leaching circuit. Tailings thickener underflow will be pumped to the tailings storage facility using two stages of centrifugal pumping.
Tailings storage facility
The proposed tailings storage facility is located within a valley north of the unnamed Wadi running north of the process plant. The facility has been designed as a two cell paddock facility to allow alternate deposition and subsequent upstream raising. A partial HDPE basal liner will be employed over the base of the valleys with an extensive underdrainage system and subliner drainage. Seepage analysis and stability analysis were conducted on the embankments to support the design.
At the final stage of construction, the total embankment length of the East Cell would be approximately 2.8 km, with a maximum embankment height of 38 m. The total embankment length of the West Cell would be approximately 1.3 km, with a maximum embankment height of 31 m. The tailings beach surface at full capacity will cover an area of approximately 280 ha.
Water supply
The groundwater resource required to operate a 6.0 Mtpa plant is a constant water supply of an average of 145 l/s, or 4.6 million cubic metres of water per annum. Exploration drilling of 17 boreholes in Area 5 and four test production boreholes commenced in April 2017 and was completed in September 2017. Boreholes were drilled between 90 and 200 m deep, with groundwater intersected at an average depth of 75 m below surface. A numerical model was constructed using Visual Modflow Pro and calibrated to match pump tests. The aquifer contains 100 Mm³ in the Measured Resource category and a further 40 Mm³ in the Indicated Resource category.
Power supply
The power supply will be an on-site heavy fuel oil (HFO) fuelled generation plant under a Build Own Operate Transfer (BOOT) arrangement, transferred after five years. The load profile estimates a 24.3 MW peak and 23.5 MW average demand, serviced using eight 5.2 MW Anglo Belgian Corporation HFO generators. A power cost of US$0.136/kWh was used in the operational cost analysis based on a HFO380 price of $0.525/l.
Key reported parameters
| Parameter | Units | Oxide & Wadi Doum | Transition | Fresh | Source |
|---|---|---|---|---|---|
| Plant Capacity | tpa | 6,036,000 | 6,036,000 | 6,036,000 | Orca |
| Gold Head Grade (max) | g Au/t | 2.04 | 1.84 | 1.62 | Orca |
| Silver Head Grade (max) | g Ag/t | 3.45 | 4.33 | 2.44 | Orca |
| Design Gold Recovery (Leach) | % | 90.5 | 85.5 | 83.7 | Testwork |
| Design Silver Recovery (Leach) | % | 37.2 | 45.3 | 56.7 | Testwork |
| Plant Availability | % | 91.3 | 91.3 | 91.3 | Lyco |
| Crushing Work Index (CWi) | kWh/t | 4.4 to 6.7 | 5.0 to 11.2 | 6.8 to 11.3 | Testwork |
| Bond Ball Mill Work Index (BWi) | kWh/t | 8.5 to 12.7 | 8.4 to 13.3 | 12.3 to 19.3 | Testwork |
| Abrasion Index (Ai) | 0.04 to 0.11 | 0.11 to 0.34 | 0.40 to 0.91 | Testwork | |
| Grind Size (P80) | µm | 53 | 75 | 90 | Orca |
| Pre-aeration Time | hrs | 4 | 4 | 4 | Testwork |
| Leach Circuit Residence Time | hrs | 45 | 45 | 45 | Orca |
| Leach Slurry Density | % w/w | 55 | 55 | 55 | Orca |
| Number of Leach Tanks | 10 | 10 | 10 | Lyco | |
| Number of Adsorption Tanks | 7 | 7 | 7 | Kemix | |
| Cyanide Consumption – Plant | kg/t | 0.44 | 0.49 | 0.51 | Testwork / Lyco |
| Quicklime Consumption – Plant | kg/t | 1.31 | 1.09 | 0.72 | Testwork / Lyco |
| Elution Circuit Type | Split AARL | Split AARL | Split AARL | Lyco | |
| Elution Circuit Size | t | 12.5 | 12.5 | 12.5 | Kemix |
| Frequency of Elution | strips/wk | 7 | 7 | 7 | Kemix / Lyco |
| Tailings Thickener U/F Density | % w/w | 58 | 62 | 65 | Testwork / Lyco |
Source notes: "Orca" refers to advice from Orca Gold Inc. "Testwork" refers to metallurgical testwork conducted at SGS or vendor testwork. "Lyco" refers to Lycopodium calculations and modelling. "Kemix" refers to modelling by Kemix. "OMC" refers to advice from Orway Mineral Consultants.
Technical qualifications
The metallurgical testwork conducted at SGS has provided the basis for the plant design. The complete OMC Report No. 7889-02 Rev 2, March 2018, should be referenced for detail of comminution modelling and complete comminution data set. Reagent consumption in the plant includes an allowance for residual cyanide and quicklime supply at 90% active CaO.
The process plant design was based on a nominal capacity of 6.0 Mtpa of ore and head grade of 1.84 g/t Au and 4.33 g/t Ag, with overall gold recovery 86% and silver recovery 45%, showing the highest pregnant solution grade over life of mine.
The reported ranges for comminution parameters show variation across ore types and ore domains. Process design criteria were developed from multiple sources including metallurgical testwork conducted at SGS, Orca advice, comminution modelling by Orway Mineral Consultants, CIP modelling by SGS and Kemix, and Lycopodium calculations and modelling.
Source: GSS Project 2018 Technical Report, Sections 1.16, 1.17, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6.
