Turquoise Ridge Complex — 2024 Technical Report

This report describes the processing methods for oxide and refractory ores at the Turquoise Ridge Complex, including the Juniper oxide mill, Sage autoclave facility, and heap leach operations.

Report context

This technical report, dated March 15, 2024, presents the recovery methods for the Turquoise Ridge Complex, an operation that processes both oxide and refractory ore on-site. The processing facilities remain fundamentally unchanged from the original design and are considered suitable for processing the ores envisaged in the life-of-mine plan.

Processing route

Juniper Oxide Mill

Run-of-mine higher-grade oxide ore from Turquoise Ridge Surface sources is blended for gold grade, hardness, and carbonate content and fed to the Juniper oxide mill. The mill consists of a variable speed 671 kW semi-autogenous grind (SAG) mill operating in closed circuit with a discharge screen. The SAG mill product is fed to an 846 kW mill operating in closed circuit with cyclones. The final product grind size is 85% passing minus 200 mesh.

Cyclone overflow product is fed to the neutralization circuit, where the carbonate in the oxide ore is used to neutralize the acidic autoclave discharge slurry. The combined oxide slurry and autoclave discharge slurry are further neutralized with lime before treatment in the carbon-in-leach (CIL) circuit. The CIL circuit concurrently leaches gold from the ore and adsorbs it onto activated carbon. Final tailings slurry is pumped to the tailings storage facility. Gold loaded carbon is stripped, acid washed, kiln reactivated, and recycled back to the CIL circuit. Gold stripped from the carbon is electrowon and refined into doré for shipment to an offsite refinery.

Plant throughput can reach 108 tph (120 short tons per hour) depending on ore hardness, augmented when limestone is added. The plant is permitted for running 226 tph (250 short tons per hour) or 5,443 tpd (6,000 short tons per day).

Major equipment at the Juniper Mill includes a Marcy 5.5 m by 2.0 m (18 ft by 6.5 ft) SAG mill with 662 kW (900 hp) and a Marcy 3.5 m by 5.0 m (11.5 ft by 16.4 ft) primary ball mill with 846 kW (1,150 hp).

Sage Autoclave Facility

The Sage Mill consists of SAG milling followed by two-stage ball milling. The final grinding product is 80% passing minus 200 mesh. Cyclone overflow reports to a thickener. Thickener underflow reports to an acidification circuit where sulfuric acid is added as necessary to ensure adequate autoclave free acid solution levels. Free acid concentration for Turquoise Ridge Complex ore must be maintained at greater than 18 g/L. Thickener overflow solution is returned to the milling circuit. After acidification, ore slurry is added to two identical autoclaves operated in parallel. Two stages of flash heat recovery are used. Autoclave discharge is cooled before reporting to the lime neutralization circuit. Autoclave waste gas is cooled and scrubbed before discharging to the atmosphere.

Oxide ore and acidic oxidized sulfide ore slurry are combined in the neutralization circuit. After neutralization with the carbonate oxide ore and supplemental lime, the ore slurry reports to a CIL circuit where the ore is leached in cyanide solution to extract gold. Final tailings slurry is pumped to the tailings area where tailings settle and decant solution is reclaimed and reused in the grinding circuit.

Loaded carbon from the CIL circuit is transferred to the recovery plant. After acid washing to remove inorganic contaminants, the carbon is transferred to the pressure Zadra stripping circuit. Gold is stripped from the carbon using caustic and cyanide solution at elevated temperature and pressure. Pregnant solution from the stripping circuit is pumped to an electrowinning circuit where precious metal is removed from the solution as sludge. The sludge is filtered, dried in a mercury retort, mixed with fluxes, and refined into doré bars. After carbon stripping, barren carbon reports to the kiln regeneration circuit and returns to the CIL circuit.

Major equipment at the Sage autoclave includes a Koppers 8.5 m by 3.0 m (28 ft by 10 ft) SAG mill with 2,942 kW (4,000 hp), a Svedala 6.1 m by 9.2 m (20 ft by 30 ft) primary ball mill with 5,516 kW (7,500 hp), two Dominion 5.0 m by 8.8 m (16.5 ft by 29 ft) secondary ball mills each with 2,942 kW (4,000 hp), a 61.0 m by 7.6 m (200 ft by 25 ft) thickener with four 3.7 kW motors (5 hp), two autoclaves measuring 5.0 m by 22.3 m (16.5 ft by 73.3 ft) with 338.8 m³ (89,500 gal) each, and an Air Products ASU oxygen plant producing 95% O₂ at 1,233 tpd (1,360 tons per day).

Heap Leach Operations

Run-of-mine lower-grade oxide ore from Turquoise Ridge Surface sources is delivered to the Izzenhood heap leach pad (L8) located south-east of the Vista Pit. Ore is normally stacked in 6 m lifts. Both the Snowstorm and Sonoma leach pads are classified as inactive. The Osgood leach pad is in closure.

When a cell is full and ripped, drip irrigation lines are installed to apply barren cyanide solution to the top surface. The cyanide solution leaches gold from the ore as the solution percolates through the stacked ore and collects on an impermeable liner at the bottom of the heap. Pregnant solution is fed to a carbon-in-column (CIC) circuit to adsorb gold onto carbon. CIC barren solution is recycled back to the top of the leach pad. Gold loaded carbon is sent to the stripping circuit, which is the same circuit used for loaded carbon from the CIL circuit. There the carbon is stripped, acid washed, kiln reactivated, and recycled back to the CIC circuit. Gold stripped from the carbon is electrowon and refined into doré for shipment to an offsite refinery.

Major equipment for the heap leach facility includes a Gould vertical turbine pump with 372 kW (500 hp) and two sets of carbon columns, each being a train of five tanks measuring 2.6 m by 2.7 m (8.5 ft by 9.5 ft).

Plant Modifications and Improvements

Various plant modifications for improved performance are periodically undertaken. The CIL inter-tank screen upgrades are currently underway with both throughput and recovery enhancements expected. A 3% gold recovery adjustment has been included this year to the predictive curves, based on partial completion of the project to date. The site has also worked on debottlenecking the mill by upgrading trash screens in the grind circuit, which has reduced downtime associated with plugged strainers in the autoclaves.

Energy, Water, and Consumables Requirements

The process facilities will require 363.1 gigawatt-hours per annum. The expansion of Mega Pit does not require additional power or processing capacity but increases the life-of-mine duration.

Process water is sourced from Twin Creeks dewatering wells DW-76, DW-77, and DW-80, with requirements averaging approximately 11.4 million cubic meters per annum. Potable water is sourced from potable water well GFW-1, with 6,300 cubic meters consumed per annum.

The Juniper Mill uses grinding media for the SAG and ball mills, lime, and cyanide. Average annual life-of-mine consumptions are: grinding media 225 tonnes per annum; lime and cyanide usage are accounted for in the Sage autoclave usage.

The Sage autoclave uses grinding media for the SAG and ball mills, sulfuric acid, oxygen, lime, cyanide, and activated carbon. Average annual life-of-mine consumptions are: carbon 140 tonnes per annum, cyanide 1.6 kilotonnes per annum, lime 94 kilotonnes per annum, sulphuric acid 36 kilotonnes per annum, and grinding media 4 kilotonnes per annum.

Major consumables in the gold heap leach facilities are cyanide and antiscalant. Average annual life-of-mine consumptions are: cyanide 900 tonnes per annum and antiscalant 120 tonnes per annum.

Key reported parameters

Note: tpd = tonnes per day; tph = tonnes per hour; tpa = tonnes per annum; ktpa = kilotonnes per annum; m³ = cubic metres.

Technical qualifications

All process facilities are constructed and operational. Facilities are suitable for processing the ores included in the life-of-mine plan, including the Mega Pit Cut 40 expansion. No major modifications to the existing flow sheets would be necessary to process Cut 40 ores. Some additional facilities, such as construction of a new tailings storage facility and expansion of the Izzenhood heap leach pad, will be required to support operations as envisaged in the life-of-mine plan, including the Cut 40 expansion. The expansion of Mega Pit does not require additional power, processing capacity, or process/potable water capacity on a per annum basis but increases the life-of-mine duration. The 3% gold recovery adjustment included in predictive curves is based on partial completion of the CIL inter-tank screen upgrade project to date. Actual throughput can vary depending on ore type and constituents.

Source: Turquoise Ridge Complex NI 43-101 Technical Report, March 15, 2024, Sections 17.1 through 17.7 (Recovery Methods).