Livengood Gold Project – Pre-feasibility Study (2021)

This report summarizes the processing design for the Livengood Gold Project, based on laboratory-scale testwork, equipment supplier information, and the study team's experience on similar projects, detailing a comminution, gravity, and carbon-in-leach flowsheet for the planned 65,000 t/d operation.

Report context

This pre-feasibility study for the Livengood Gold Project was completed in December 2021, under a National Instrument 43-101 technical report format. The processing-related recovery methods described are based on laboratory-scale testwork referenced in the report’s Chapter 13, equipment information from suppliers, and experience on similar projects. The resulting flowsheet shows this testwork and forms the basis for plant design, capital costs, and operating costs developed in the study.

Processing route

Primary Crushing

The primary crushing system is a single-stage, open-circuit gyratory crusher (60 × 89 in) with installed power of 1,000 hp (746 kW). The crusher is designed for a feed size (F80) of 31.5 in (800 mm) and a product size (P80) of 5.4 in (138 mm), with an expected utilization of 65% at the nominal throughput of 65,000 t/d (59,000 mt/d). The live capacity of the feed and discharge hoppers is designed for slightly over two truckloads, assuming a nominal payload of 320 t (291 mt). The gyratory crusher’s instantaneous throughput is 4,167 t/h (3,780 mt/h).

Crushed Ore Stockpile

The crushed ore storage pile is designed for a live capacity corresponding to approximately 12 hours of crushing, or 34,946 t (31,703 mt). The total capacity of the storage pile (live and dead) is 113,961 t (103,384 mt). The coarse ore stockpile is covered by a dome.

Secondary Crushing (Pre-Crushing)

Ore is reclaimed from the stockpile through a reclaim tunnel equipped with three apron feeders that feed a secondary cone crusher installed in open circuit. Two screens, 12 ft × 27 ft (3.7 m × 8.2 m), receive the gyratory crusher product, directing oversize material to a cone crusher (1,250 hp, 932 kW) that crushes to a P80 of 1.65 in (42 mm). The screen undersize and secondary crusher product are subsequently fed to the SAG mill. The secondary crusher is equipped with a by-pass chute to maintain high plant availability.

Grinding and Pebble Crushing

The grinding circuit uses a SAG mill and ball mill in a SABC configuration, with the SAG mill operating in closed circuit with a pebble crusher. The SAG mill (36 ft × 20 ft) has an installed power of 20,115 hp (15,000 kW) and is equipped with pebble ports that evacuate critical-size pebbles to the pebble crusher before being returned to the SAG mill. The ball mill (26 ft × 40.5 ft) has an installed power of 20,115 hp (15,000 kW) and operates in closed circuit with hydrocyclones. The required SAG mill power is estimated at 4.7 kWh/t (5.2 kWh/mt), and the required ball mill power at 4.9 kWh/t (5.4 kWh/mt), for a combined total of 9.6 kWh/t (10.6 kWh/mt) at the pinion, excluding pebble and secondary crusher power. The grinding circuit product is designed to a P80 of 250 µm.

The product from the SAG mill falls onto a classification screen, with oversize conveyed to a single cone (pebble) crusher (1,250 hp, 932 kW), with the product returned to the SAG mill. The scalping screen undersize (P80 2,800 µm) discharges into the cyclone feed pumpbox, from which slurry is pumped to two hydrocyclone clusters. Cyclone underflow feeds the ball mill. The ball mill product discharges into the gravity feed pumpbox.

Gravity and Intensive Leaching

The gravity circuit consists of two parallel lines composed of four Knelson concentrators each, fed by a portion of the ball mill discharge. Based on testwork and simulations, the design gold recovery of the gravity circuit is estimated at 30% for an average feed blend. The feed to the gravity circuit (93,600 t/d; 84,910 mt/d) goes to a distributor feeding eight gravity screens (one per concentrator), with oversize and tails returned to the cyclone feed pumpbox. Gravity concentrate, approximately 0.05 wt% mass pull, is sent to an intensive cyanidation (ILR) system. A batch intensive cyanidation system with two units processes the gravity concentrate, with gold extraction designed at 98%. The pregnant solution is pumped to a tank in the gold room, followed by electrowinning in a dedicated cell.

Carbon in Leach

Hydrocyclone overflow is pumped to a trash screen before discharging into the pre-leach high-rate thickener (213 ft, 65 m diameter). The thickener underflow at 60 wt% solids is diluted to 50% solids in the pre-conditioning tanks. Pre-conditioning with oxygen and lead nitrate is conducted in four large tanks with a designed retention time of four hours. Lead nitrate is added based on the antimony concentration in the feed; when sufficiently low, no lead nitrate is added.

The CIL circuit consists of two lines of seven tanks, each within concrete containment. Overflow from the pre-detox thickener is added to the first CIL tanks to adjust slurry percent solids to 50%. The designed retention time is 24 hours at 65,000 t/d (59,000 mt/d). The design carbon loading is 800 g/mt, to be confirmed by additional CIL testwork and simulation. Slurry flows counter-currently to carbon from tank 1 to tank 7. Fresh carbon is added to tank 7, advancing to tank 1 via carbon advance pumps. Slurry exits tank 7 over carbon safety screens before reporting to the pre-detox thickener. Loaded carbon from tank 1 reports to the carbon stripping system.

Adsorption, Desorption and Recovery (ADR)

Loaded carbon from the CIL tanks reports to the stripping circuit, assumed to require one strip per day. The ADR circuit includes an acid wash stage (two vessels) and a high-pressure, modified Zadra process for gold stripping. The stripping circuit (four vessels) operates with in-line electrowinning, with barren solution collected in two 20,236 gal (76.6 m³) tanks. The stripping cycle is two stages: copper stripped first, followed by gold. Stripped copper is converted to copper sulphate for use in the cyanide detoxification circuit.

Stripped carbon flows to two carbon regeneration kilns, with provision for 100% regeneration. The regenerated carbon is combined with fresh carbon to maintain supply to the CIL circuits. Pregnant solution from the Zadra circuit and the gravity ILR is split to feed seven electrowinning cells. Electrowinning sludge is filtered, dried, mixed with fluxes, and smelted in an induction furnace.

Pre-Detox Thickening and Cyanide Detoxification

Thickener overflow reports to the process water tank for upstream water needs and dilution prior to detoxification. The underflow of the pre-detox thickener (213 ft, 65 m diameter) is diluted to 50% from 60% solids. The selected cyanide detoxification process is the Inco SO₂/air process, with sulfur dioxide generated using a sulfur burner. The detoxification design has 1.5 hours retention time in two tanks. Tailings slurry is pumped at 50 wt% solids out of the detoxification unit. Water recovered by reclaim barge pumps from settled tailings is returned upstream. All cyanide process tanks are provided with secondary containment; process solution pipelines are contained within the mill complex.

Consumables and Reagents

Main consumables include grinding media (SAG mill: 5-in forged steel ball, 3,472 mt/y; ball mill: 3-in forged steel ball, 7,304 mt/y) and reagents. Table 17-2 provides the main reagents, usage areas, and annual consumption rates. Quicklime (CaO) is used for pH control in ball mills, CIL, and detoxification. Sodium cyanide is delivered in briquette form, dissolved in water, and distributed to process areas. Oxygen is produced by a vacuum pressure swing adsorption (VPSA) plant and bottom-sparged to pre-treatment and CIL tanks, with a liquid oxygen back-up system. Lead nitrate is used for pre-treatment, sodium hydroxide and hydrochloric acid for carbon stripping, and sulfur, sodium metabisulfite, and copper sulphate for cyanide detoxification. Flocculant is used in pre-leach and pre-detox thickeners.

Ancillary Facilities and Controls

The process plant building houses maintenance facilities, a centralized control room, metallurgical and sample preparation laboratory, change-rooms, lunchroom, offices, conference and training rooms. A plant control system with open architecture uses redundant Ethernet fiber optic cables, with vendor packages having standardized controllers. The control system includes operator workstations with historian software for plant data reporting and metallurgical optimization. An information system allows staff to monitor processes remotely. Closed-circuit television, calling and searching systems, and fire protection systems are included.

Process Water and Energy Requirements

Process water is distributed throughout the facility to dilute streams to required slurry densities. The majority is reclaimed from the CIL thickener overflow and tailings pond. Fresh make-up requirements are approximately 233 gpm (53 m³/h). Total operating power demand for the process plant is approximately 53 MW, with the crushing and grinding circuit representing approximately 57%. Liquefied natural gas will be used for heating within the process plant building.

Process Plant Personnel

The process plant requires a total of 140 employees, including 26 salaried staff and 114 hourly workers, across management, technical services, operations, and maintenance departments. The salaried manpower includes positions such as mill manager, metallurgists, and maintenance superintendents. The hourly workforce covers operators for crushing, grinding, gravity, leaching, stripping, refining, detoxification, tailings, reagents, and metallurgical technicians.

Key reported parameters

Technical qualifications

Specific limitations from the report include:

• The design carbon loading of 800 g/mt is to be confirmed by additional CIL testwork and simulation.
• Consumption rates for reagents are mostly based on bench-scale testwork, with reductions as deemed applicable to recycle streams and implementation of control strategies at industrial scale.
• The proper flux mix and quantity for refining will be established by the smelting flux supplier during the first months of operation.
• Power values cited are based on the motor output.
• Several equipment sizes and selections are based on preliminary design and require confirmation during detailed engineering.

Source: Tower Hill Mines Inc., NI 43-101 – Technical Report, Livengood Gold Project – Pre-feasibility Study, December 2021, Chapter 17 (Recovery Methods).