Peñasquito Runs a 124,000 t/d Lead-Zinc Concentrator With Twin SAG Mills, an HPGR and Carbon Pre-Flotation

Goldcorp's 2018 technical report on Peñasquito in Zacatecas, Mexico describes a sulphide plant with gearless SAG mills, pebble crushers working with an HPGR, sequential lead and zinc flotation, and a separate oxide heap leach with Merrill-Crowe recovery.

Peñasquito is an operating open pit polymetallic mine in the Concepción del Oro district of northeast Zacatecas State, Mexico, about 200 km northeast of the city of Zacatecas. In 2018 Goldcorp Inc. owned it outright through its subsidiary Minera Peñasquito. The mine works the Peñasco and Chile Colorado deposits. Open pit mining began in 2010 and full production in 2011. The technical report, effective 30 June 2018, describes a site where the pit feeds two process routes. A sulphide concentrator treats a nominal 124,000 t/d and produces lead and zinc concentrates carrying gold and silver. A heap leach handles a nominal 25,000 t/d of oxide ore. In the first half of 2018 the concentrator treated about 18.0 Mt.

Critical Data

Overview

The sulphide plant uses a large-scale concentrator flowsheet. A gyratory crusher feeds a covered coarse ore stockpile. Two SAG mills, each with two ball mills, grind the ore to 125 µm. Three pebble crushers and an HPGR handle the SAG mill pebbles and the product of an augmented crusher. The flotation area has carbon, lead and zinc circuits in that order. A carbon pre-float takes out carbonaceous material before lead flotation. Lead and zinc rougher concentrates are reground in vertical mills to 30 µm and cleaned in three stages. Both concentrates are thickened, pressure filtered and trucked to inland smelters or to ports.

On the oxide side, haul trucks place run-of-mine ore in 10 m lifts on the heap leach pad after adding lime. Cyanide solution leaches the ore. The pregnant solution is clarified, filtered and de-aerated before zinc dust precipitates the gold and silver. The precipitate is pressure filtered and smelted to doré.

Key Process Stages

Crushing and stockpiling

Rear-dump haul trucks tip into the gyratory crusher, which can support the plant's 92% utilization around the clock. A near-pit sizing conveyor was added to support higher throughput by helping with waste removal. The crusher discharges into a 500 t surge pocket, and an apron feeder sends the product to the coarse ore stockpile. Ten apron feeders in two lines of five reclaim the ore. Nine feed the SAG mills and one feeds an augmented crusher.

Grinding and pebble crushing

Each of the two gearless SAG mills runs in closed circuit with pebble crushing and the HPGR, and each feeds two ball mills. A Raptor XL 1100 augmented cone crusher with an 820 kW motor takes ore straight from the stockpile, and its product is dry screened. Augmented crusher screen oversize and SAG trommel oversize feed three Sandvik CH880 pebble crushers. Pebble crusher product and the augmented crusher screen fines go to a bin that feeds the HPGR or, when needed, the SAG mills directly. SAG trommel undersize below 19 mm joins the ball mill discharge in a common sump. Twenty-four G-max 33 cyclones in four banks send underflow back to the ball mills and overflow to flotation.

Carbon pre-flotation

Two banks of two 630 m3 Outotec rougher cells float carbon. A bank of three 300 m3 cleaners and a 5.5 m by 14 m column recleaner upgrade the concentrate, with three 300 m3 cleaner-scavengers. The report notes that recleaner concentrate goes to final tails until a planned tertiary precious metals recovery circuit is installed.

Lead flotation

Thirty 250 m3 rougher cells, in six rows of five, float the lead. Rougher concentrate is reground to 30 µm in two 485 kW Metso vertical mills with cyclones, or bypassed straight to the cleaner conditioner. Three cleaning stages follow, with seven 42.5 m3 first cleaners and smaller second and third cleaners. Collectors include SIPX and Aerofloat 7310 to boost gold and silver recovery.

Zinc flotation

Lead rougher tailings go to one conditioner per zinc bank, where copper sulphate activates the sphalerite and isopropyl ethyl thionocarbamate collects it. Thirty-six 250 m3 self-aerating tank cells in six banks of six float the zinc. Zinc rougher tailings go to the tailings pond. The zinc concentrate is reground to 30 µm in two vertical mills and cleaned in three stages.

Dewatering and tailings

Two 10 m high rate thickeners handle the lead concentrate and two 14 m units the zinc, each pair followed by 325 m3 storage tanks and three 14-plate Pneumapress filters. Two cyclone towers, each with two clusters of fifteen G-max 20 cyclones, classify the tailings.

Additional Interesting Data and Summary

Sulphide plant performance improved steadily after start-up. In 2010 gold recovery was 48%, silver 58%, lead 60% and zinc 65%. By the first half of 2018 they had risen to about 70% for gold, 84% for silver, 75% for lead and 83% for zinc, on feed of 0.48 g/t gold, 22.2 g/t silver, 0.19% lead and 0.59% zinc. The plant made about 56,000 t of lead concentrate and 154,000 t of zinc concentrate in that half year.

Power comes from the Mexican Federal Electricity Commission grid. Water comes from the tailings facility, well fields, pit dewatering wells and recycle streams, and fresh water is limited to reagent make-up and pump gland service. Reagent stocks cover mine usage plus three to seven days of supply.

Key Processes

• Gyratory crushing to a 91,800 t live stockpile
• Two gearless SAG mills, each with two ball mills
• Pebble cone crushing and augmented crushing combined with an HPGR
• Carbon pre-flotation with a column recleaner
• Lead rougher flotation, vertical mill regrind and three-stage cleaning
• Zinc activation, rougher flotation, regrind and three-stage cleaning
• Thickening and pressure filtration of concentrates
• Run-of-mine heap leaching with Merrill-Crowe zinc precipitation

Source: Peñasquito Polymetallic Operations, Zacatecas State, Mexico, NI 43-101 Technical Report, 30 June 2018.

Project website: Peñasquito Polymetallic Operations