This report details the processing design for the Goldboro Gold Project, based on a conventional leach and carbon-in-pulp route with supporting metallurgical testwork.
Report context
The Goldboro Project, located in the Eastern Goldfields District of Nova Scotia, is the subject of a 2021 NI 43-101 Technical Report and Feasibility Study. This section of the report covers the recovery methods, process plant design, and associated infrastructure proposed for the project, drawing on testwork and engineering analysis completed for the feasibility study.
Processing route
Overall Process Design
Analysis of the provided testwork led to the selection of a conventional leach and carbon-in-pulp (CIP) process route, considered the most suitable for the deposit and project economics. The proposed flowsheet uses standard processes and technologies typical for gold recovery.
Key operating criteria include:
- Nominal throughput of 4,000 t/d or 1.46 Mtpa
- Crushing plant availability of 64%
- Plant availability of 92% for grinding, leach plant, and gold recovery operations
Mill Process Plant Description
The process design comprises several integrated circuits, including three-stage crushing, a covered crushed ore stockpile, a ball mill with cyclone classification, gravity concentration with intensive leaching, trash screening, a leach and carbon adsorption circuit, elution and electrowinning, carbon regeneration, cyanide destruction, arsenic precipitation, and tailings thickening.
Primary Crushing and Stockpiling
The crushing circuit is designed for an annual operating time of 5,631 hours (64% availability) at a capacity of 4,000 tpd. ROM ore is delivered by haul trucks to a ROM bin with a static grizzly. A vibrating grizzly feeder ahead of the primary jaw crusher screens out fine material. The primary crusher product combines with grizzly undersize and is conveyed to a secondary screen; oversize proceeds to a secondary cone crusher, with the product fed to a tertiary screen in closed circuit with a tertiary shorthead cone crusher. Combined screen undersize is conveyed to the mill feed stockpile with 80% passing 10 mm.
The stockpile is designed with a live capacity of 12 hours (2,000 tonnes), providing buffer capacity between crushing and grinding. Two belt feeders regulate feed at 181 t/h to the ball mill via a feed conveyor, with each feeder capable of independently supplying design capacity.
Grinding Circuit
The grinding circuit consists of a ball mill in closed circuit with hydrocyclones, sized for a ball mill feed size of 80% passing 10 mm and a product of 80% passing 100 µm. The mill is equipped with a trommel for scat removal; oversize material from the gravity scalping screen and hydrocyclone underflow are returned to the mill feed chute. The hydrocyclone classification circuit operates at a nominal circulating load of 400%.
Process water is added to the cyclone feed pumpbox to achieve the target density. Cyclone overflow reports to a trash screen ahead of leaching. The grinding circuit includes a 3,500 kW, 5.2 m diameter x 7.9 m EGL ball mill, cyclone feed pumpbox, classification cyclones, and associated pumps.
Gravity Circuit
A side stream of ball mill discharge is pumped to a gravity sizing screen, with screen undersize fed to a centrifugal gravity concentrator. A batch of concentrate is produced every 45 minutes and fed to the intensive cyanidation unit (ICU) holding tank; gravity tails are returned to the ball mill by gravity.
In the ICU cone, gravity concentrate is deslimed with process water for approximately 30 minutes. A reagent mixture containing sodium hydroxide, sodium cyanide, and leach aid is pumped through the cone bottom to create a fluidized bed. Pregnant leach solution is pumped to the gold room for electrowinning and refining; barren solids are pumped back to the cyclone feed pumpbox.
Leach and Adsorption Circuit
The leach-adsorption circuit comprises three leach tanks and six CIP tanks. Trash screen undersize flows to a pumpbox and then to the leach circuit; barren solution from electrowinning is periodically transferred to leaching. Total circuit residence time is 36 hours at 44% solids (w/w).
Hydrated lime slurry maintains a pH of 10.5 to 11, and cyanide solution is added to the first leach tank. Fresh and regenerated carbon is added to the last CIP tank and advanced counter-currently. Intertank screens retain carbon while allowing slurry flow; loaded carbon from the lead tank is sent to acid wash. Recessed impeller pumps transfer slurry between tanks.
Key equipment includes:
- Leach tanks: 15.3 m diameter x 16.7 m height, 100 kW agitator
- CIP tanks: 7.2 m diameter x 7.8 m height, 15 kW agitator
- Loaded carbon screen, intertank carbon screens, carbon sizing screen
Cyanide Destruction
CIP tailings at 44% solids flow by gravity to cyanide destruction tanks operating in parallel. The feed, including acid rinse and carbon transfer water, is estimated at 43% solids. Total residence time is approximately 120 minutes, designed to reduce weak acid dissociable cyanide (CN WAD) from 100 mg/L to less than 0.5 mg/L; total cyanide (CN TOT) is expected to be 0.5 mg/L.
The SO2/air method is used, requiring air, lime, copper sulphate, and sodium metabisulphite (SMBS). Each tank is equipped with air addition points and an agitator. The detoxification tanks discharge by gravity to the arsenic precipitation tank.
Arsenic Precipitation
Ferric sulphate is added at a design ratio of 8:1 iron to arsenic by weight, with design arsenic concentration in solution of approximately 6.3 mg/L. The precipitation tank provides residence time in excess of the 10-minute design. Tailings flow by gravity to a carbon safety screen; recovered carbon is returned to the CIP circuit, while screen undersize is pumped to the tailings thickener.
Main equipment includes an arsenic precipitation tank (6.5 m diameter x 7.2 m height, 30 kW agitator) and a carbon safety screen.
Tailings Thickening
Detoxified slurry flows to the tailings thickener, where underflow is dewatered to 60% solids (w/w). Thickener overflow is recycled to the process water tank; underflow is pumped to the tailings storage facility. Flocculant is added to improve settling; excess water from the process water tank is sent to effluent treatment.
Carbon Acid Wash, Elution, and Regeneration
Loaded carbon is treated with a weak hydrochloric acid solution to remove calcium, magnesium, and other salt deposits. The acid wash column has a 3-tonne capacity; acid is rinsed and discarded to the cyanide destruction tank. Acid-washed carbon is transferred hydraulically to the elution column.
Gold stripping uses the Pressure Zadra process. A high-cyanide, caustic solution is recirculated through a pressure elution column at 140°C. The pregnant solution passes through electrowinning cells to deposit gold and silver on cathodes before recycling.
Key elution equipment includes:
- Carbon elution column (3-tonne capacity)
- Propane-fired strip solution heater with heat exchangers
- Strip eluate and pregnant solution tanks
Stripped carbon is dewatered on a screen, fed to a kiln feed hopper, and metered into the carbon regeneration kiln.
Carbon reactivation uses a propane-fired rotary kiln, heating carbon to 650° to 750°C in superheated steam. Quenched carbon is screened to remove undersized fragments; new carbon is added via the quench tank. The reactivation circuit includes a dewatering screen, kiln with feed hopper and screw feeder, and quench tank.
Gold Room
Gold and silver sludge from electrowinning cells is washed off cathodes, filtered, dried, mixed with fluxes, and smelted in an electric induction furnace to produce doré bars. The gold room includes electrowinning cells with rectifiers, a sludge pressure filter, drying oven, flux mixer, smelting furnace with cascade doré moulds, doré vault, and dust collection. Security is provided by access control, intruder detection, and closed circuit television.
Reagent Handling and Storage
Reagent systems are located within curbed containment areas to prevent mixing of incompatible reagents. Storage tanks include level indicators, instrumentation, and alarms, with ventilation, fire and safety protection, eyewash stations, and MSDS stations provided. Sumps and sump pumps handle spillage control.
Required reagent systems include hydrated lime, sodium cyanide, hydrochloric acid, copper sulphate pentahydrate, sodium metabisulphite, sodium hydroxide, sulphamic acid, flocculant, activated carbon, smelting fluxes, and ferric sulphate.
Hydrated Lime is delivered in bags and slurried to 25% solids (w/w) in process water. The slurry is pumped through a ring main to leaching and cyanide destruction. An extraction fan removes reagent dust.
Sodium Cyanide is dissolved in freshwater to a 20% (w/v) dosing concentration, with an extraction fan over the bag breaker/mixing tank. Dedicated dosing pumps deliver solution to leaching and elution circuits.
Copper Sulphate is delivered as a 60% solution in intermediate bulk containers (IBCs) for the arsenic precipitation tank.
SMBS is delivered as solid flakes in bulk bags, dissolved to 20% (w/v), and dosed to the detoxification circuit. A ventilation fan removes SO2 gas generated during mixing.
Sodium Hydroxide (50% caustic solution) is delivered in IBCs and dosed to the elution, electrowinning, and cyanide mixing circuits.
Hydrochloric Acid is diluted to 3% w/v via inline mixing and delivered to the acid wash circuit.
Ferric Sulphate is delivered as a 60% solution (w/w) and dosed to the arsenic precipitation tank.
Flocculant is delivered as powder in bulk bags, mixed to a 0.50% (w/w) solution, and dosed to the tailings high-rate thickener.
Reagent consumption and storage details for the process plant are provided in Table 17-2, including sodium cyanide (1.1 t/d), hydrated lime (11 t/d), activated carbon (58 t/y), copper sulphate (0.26 t/d), flocculant (0.2 t/d), hydrochloric acid (0.4 m³/day), sodium hydroxide (0.8 m³/day), SMBS (6.6 t/d), ferric sulphate (1.9 t/d), and ball mill media (3.0 t/d). Bulk storage is based on 6 weeks for activated carbon, 2 weeks for grinding media, and 10 days for other reagents.
Services and Utilities
Compressed air is supplied for plant and instrument air, with major consumers including leach and cyanide detoxification tanks and pneumatic instruments. High pressure air at 750 kPag is dried and distributed via air receivers.
Process water is supplied from a raw water storage tank, with a requirement of 7 m³/h of freshwater plus an additional 33 m³/h from freshwater or other influent sources such as site runoff. Overflow from the final tailings thickener is reused for process water. A dedicated gland water pump supplies all slurry pumps.
Power demand was estimated from the electrical load list and adjusted for nominal demand. The process plant's power requirements are summarized in Table 17-3, with total installed power of 9.3 MW, maximum demand of 7.0 MW, and nominal demand of 5.7 MW across all WBS areas.
Key reported parameters
| Parameter | Units | Design/Reported Value |
|---|---|---|
| Plant Throughput | t/d | 4,000 |
| Gold Grade – Design Mill Head | g/t | 2.58 |
| Crushing Plant Availability | % | 64 |
| Mill Availability | % | 92 |
| Bond Crusher Work Index (CWi), 75th percentile | kWh/t | 23 |
| Bond Rod Mill Work Index (BWi), 75th percentile | kWh/t | 17.6 |
| Bond Ball Mill Work Index (BWi), 75th percentile | kWh/t | 15.7 |
| SMC Axb, 25th percentile | – | 30.4 |
| Bond Abrasion Index (Ai) | g | 0.228 |
| Material Specific Gravity | t/m³ | 2.75 |
| Primary Grind Size (P80) | µm | 100 |
| Primary Crusher | – | Jaw, 1 m x 1.3 m |
| Secondary Crusher | – | Standard Cone, 1.32 m diam. |
| Tertiary Crusher | – | Shorthead Cone, 1.32 m diam. |
| Ball Mill Dimensions | m | 5.2 diam. x 7.9 EGL |
| Ball Mill Installed Power | MW | 3.5 |
| Leach Residence Time | h | 30 |
| CIP Residence Time | h | 6 |
| Gravity Gold Recovery (design) | % Au | 40 |
| Total Gold Recovery (life of mine) | % Au | 96 |
| Leach pH target range | – | 10.5-11 |
| Leach-CIP Operating Density | % solids (w/w) | 44 |
| Leach Sodium Cyanide Addition | kg/t | 0.5 |
| Leach Hydrated Lime Addition | kg/t | 1.0 |
| Leach & CIP Tanks | # | 3 + 6 |
| Tonnes of Carbon per Elution Column | t | 3 |
| Detoxification Residence Time | min | 120 |
| Detoxification Tanks | # | 2 (Parallel) |
| Detoxification SO2 Addition (as SMBS) | SO2:CN WAD ratio (w/w) | 10 |
| Detoxification Lime Addition | kg/t | 0.80 |
| Detoxification Discharge CN WAD, Design | mg/L | <0.5 |
| Detoxification Discharge CN TOT, Design | mg/L | 0.5 |
| Arsenic Precipitation Residence Time, Design | min | 10 |
| Ferric Sulphate Addition Ratio | Fe:As Ratio (w/w) | 8 |
| Thickener Underflow Density | % w/w solids | 60 |
Project website: https://nexgold.com/goldboro-gold-project/
Technical qualifications
The processing design is presented as a feasibility study level analysis, relying on metallurgical testwork, with specific parameters such as the Bond work indices and SMC value identified as design criteria. The report notes that circuit availability and recovery figures are design assumptions rather than demonstrated operational performance. The selected flowsheet shows a conventional approach considered suitable for the deposit but does not guarantee future metallurgical or economic performance. No pilot plant or demonstration scale data is provided; the design is based on laboratory testwork and engineering analysis.
