Goldstrike Mine — 2019 Technical Report

This report describes the mineral processing facilities at the Goldstrike Mine, which provide dual capability to treat single refractory and double refractory ores through either roasting or pressure oxidation.

Report context

This technical report, dated March 22, 2019, and identified as Project # 3078 for Barrick Gold Corporation, documents the recovery methods and mineral processing facilities at the Goldstrike Mine. The report describes process facilities that were in operation at that time, including the conversion from POX-CIL to POX-CaTS-RIL commissioned in 2014.

Processing route

Autoclave circuit (pressure oxidation)

The autoclave circuit consists of primary crushing, two parallel SAG Mill-Ball Mill grinding circuits with pebble crushing, five parallel autoclaves capable of alkaline or acid POX, two parallel CaTS leaching circuits including RIL and electrowinning for gold recovery, and a refinery producing doré bullion from both autoclave and roaster circuits.

The grinding circuit was constructed in two phases. Total installed grinding circuit capacity is approximately 17,500 stpd (dry). The Phase I grinding circuit is fed by a 50 in. by 60 in. jaw crusher discharging to a primary crushed ore stockpile. Ore is withdrawn by reclaim feeders and fed to a 22 ft diameter SAG mill operating in closed circuit with a pebble crusher. SAG mill discharge is pumped to secondary ball mills in closed circuit with six 20 in. diameter cyclones. Two ball mills operate: one 12.5 ft diameter by 14 ft long and the other 12.5 ft diameter by 18 ft long. Cyclone overflow feeds a tertiary 16 ft diameter by 23.5 ft long ball mill operating in closed circuit with six 30 in. diameter cyclones. Cyclone overflow feeds dewatering with one 100 ft diameter thickener and one 125 ft diameter thickener, providing ability to operate grinding circuits separately on alkaline or acid POX feed blends. A third 100 ft diameter thickener recycles grinding circuit process solution make-up water.

The Phase II grinding circuit is fed by a 42 in. by 65 in. gyratory crusher followed by a crushed ore stockpile. Ore feeds a 24 ft diameter SAG mill operating in closed circuit with a pebble crusher. SAG mill discharge screen undersize is pumped with ball mill discharge to twelve 20 in. diameter cyclones. Cyclone underflow returns to a 16.5 ft diameter by 30.5 ft long ball mill.

When treating an acid ore blend (carbonate to sulphide ratios below 7), grinding circuit thickener underflow feeds a series of acidulation tanks where sulphuric acid is added if required to digest carbonate content, limiting carbon dioxide gas generation in the autoclaves.

Five autoclaves operate in parallel, all configured for acid POX, while three lines can also be configured for alkaline ore POX. Milled, acidified slurry is fed to preheaters where hot steam from autoclave discharge flash tanks preheats the slurry. Pressure oxidation is carried out under elevated pressure and temperature using high purity oxygen. The exothermic reaction requires temperature control through addition of water for cooling or steam for heating. Autoclave discharge progresses through flash vessels with additional cooling in tube and shell slurry heat exchangers. Discharge slurry is neutralized to pH 8.0 with slaked lime prior to thiosulphate leaching.

Three autoclaves (#4, #5, #6) can operate under alkaline conditions for ores with higher carbonate levels. The grinding circuit product feeds a thickener dedicated to alkaline POX operation. Thickener underflow is directed to the acidulation circuit for storage, but no acid is needed. Due to higher carbonate concentration, the autoclave reaction does not generate excess acid. Alkaline POX discharge is also adjusted to pH 8.0 with slaked lime before thiosulphate leaching and RIL.

CaTS-RIL circuit (thiosulphate leaching)

Slurry from alkaline and acid autoclave circuits is pumped to parallel CaTS-RIL circuits, each comprising seven reactor tanks. Cyanide has been replaced with on-site production of calcium thiosulphate (CaTS) for gold dissolution. Resin is pumped counter-current to slurry, with a portion of new or recycled resin returned directly to the first RIL tank. Loaded resin from the first tank is transferred to elution and refining. Slurry exiting the final tank is sent to a tailings thickener then pumped to dedicated tailings storage facility (TSF3) to avoid comingling thiosulphate and cyanide solutions and to recycle a portion of calcium thiosulphate reagent in process water.

Gold bearing resin is processed in a multi-stage elution circuit, including copper elution and gold elution using trithionate. Pregnant solution containing gold is forwarded to dedicated electrowinning cells within the gold refinery. Stripped and regenerated resin is returned to the RIL circuit. Electrowinning cells contain stainless steel anodes and cathodes. Soluble gold deposits onto cathodes or forms sludge in cell bottoms. Cathodes and cells are cleaned periodically; gold-bearing sludge is filtered, heated in a retort to capture mercury, then fluxed and smelted in an induction furnace to produce doré bullion.

Roaster circuit

Fluid bed roasters were constructed at site in 1999 to treat double refractory carbonaceous ores that could not be processed in the existing POX circuit due to elevated organic carbon content. The roasters use high purity (99.5% O₂) oxygen to burn off preg-robbing organic carbon and oxidize sulphide sulphur prior to conventional CIL processing.

The roaster facility includes primary and secondary crushing followed by two parallel dry grinding circuits and dual stage roasters with combined calcine quenching, dust and gas handling, neutralization, and CIL circuits. Loaded carbon is acid washed, pressure stripped, and regenerated at site.

Ore is reclaimed from roaster stockpiles and processed through two stages of open circuit crushing: a gyratory crusher, scalping screen, and cone crusher for screen oversize. Screen undersize and cone crusher product combine in a coarse ore stockpile.

Ore is reclaimed from the stockpile by apron feeders and conveyed to one of two parallel dry grinding circuits. Ore is heated with natural gas and progresses toward the centre of the mill as it is dried and ground, transported with air through screens, a static cyclone classifier, and a dynamic classifier for size separation. Oversize returns to the second stage of the grinding mill; undersize transfers to bag houses. Target grinding circuit product size to roasting is 80% passing 74 µm.

Material from the roaster silo feeds the top of the roaster by bucket elevator and fluidized feeder to the first stage (upper) bed of two parallel roasters. The exothermic reaction maintains first stage temperature between 524°C and 593°C with addition of coal and/or sulphur pellets as needed to maintain feed fuel value. Quench water controls internal roaster temperature as required. Solids flow by gravity to the second stage through an inter-stage solids transfer system where material bed temperature is maintained between 524°C and 561°C. Oxidation achieves approximately 99% sulphide sulphur oxidation and greater than 90% organic carbon oxidation. Calcine from the second stage discharges by gravity to the calcine quench system. High purity oxygen is injected at the bottom of the second stage, fluidizing material and supporting rapid oxidation.

Exhaust gas from each stage is classified using dry cyclones. Coarse material returns to the roaster; fine material is forwarded to gas quenching and final dust scrubbing. Off-gas from final dust scrubbers from both circuits is recombined for final off-gas cleaning.

The final gas cleaning circuit removes mercury through chlorine sparging to produce calomel. Sulphur dioxide gases are neutralized with lime. Carbon monoxide is oxidized to carbon dioxide in a carbon monoxide incinerator. Nitrous oxides are removed by passing off-gases through a mist stream of ammonia in the presence of an iron oxide-titanium oxide catalyst, exiting through a stack as nitrogen and water vapor.

Calcine from the roaster is cooled rapidly with recycled process water in quench tanks. Combined quench tank discharge feeds two neutralization tanks where milk-of-lime adjusts slurry alkalinity to pH 10. Neutralization circuit slurry is dewatered in a thickener with excess water recycled for reuse in quench tanks. Thickener underflow reports to the roaster CIL circuit.

Slurry from neutralization thickener underflow is pumped to a CIL circuit with eight agitated tanks. Cyanide is typically added to the first tank. Slurry flows through tanks 1 through 8. Activated carbon is transferred counter-current from tank 8 to tank 1 using recessed impeller pumps. Loaded carbon from tank 1 passes over a screen separating carbon from slurry, then transfers to a loaded carbon holding bin for transport to elution, acid washing, and regeneration in a carbon handling circuit within the autoclave facility. Slurry exiting the final CIL tank is sent to a cyanide destruction reactor before impoundment in the North Block Tailings Dam Facility (NBTDF).

Tailings management

Tailings impoundment and reclaim water from CIL versus RIL processing are kept separate. Roaster-CIL tailings are stored in the North Block Tailings Dam Facility (NBTDF). POX-CaTS-RIL tailings are stored in Tailings Storage Facility 3 (TSF3).

Key reported parameters

Technical qualifications

This report is based on the original technical report sections provided. Specific limitations from the source material include:

• The technical report section (Section 17) is focused on recovery methods and process description only; it does not include process performance data, economic analysis, or current operational status.
• The simplified process flowsheets referenced (Figure 17-1 and Figure 17-2) are not included in the sections provided.
• The report notes that the third stage grinding circuit is not indicated in Figure 17-1.
• The report does not provide design criteria for the CaTS-RIL circuit or autoclave circuit throughput rates.
• The report does not provide metallurgical balances or recovery data.
• The information presented shows the facility configuration as of the report date (March 2019); modifications or expansions after this date are not covered.

Source: Barrick Gold Corporation – Goldstrike Mine, Project # 3078, Technical Report NI 43-101 – March 22, 2019, Section 17 Recovery Methods.