Nexa Resources S.A. – Vazante Polymetallic Operations, Project #3245 Technical Report NI 43-101

This report details the processing facilities and recovery methods at the Vazante Polymetallic Operations as described in the NI 43-101 Technical Report dated January 29, 2021.

Report context

The Vazante Operation processing facilities have a nominal design processing capacity of approximately 4,400 tpd or 1.6 Mtpa at 96% utilization. The facilities produce approximately 350,000 tpa to 370,000 tpa of zinc concentrate and approximately 4,000 tpa to 5,000 tpa of lead concentrate that contains small amounts of silver. The operations consist of two adjacent plants, Plant C and Plant W, that are interconnected at various points. In 2020, the Vazante Operation processed 1.62 Mt at an averaged head grade of 10.4% Zn and 0.36% Pb.

Processing route

Plant C

Plant C is an older plant with an approximate capacity of 47 tonnes per hour (tph) that was historically used for the treatment of calamine ore and was subsequently converted to treat willemite ore. Plant C processes approximately 25% of the total processed tonnage.

Run of mine (ROM) ore is crushed in a three-stage crushing circuit in closed circuit with screens to 98% passing 9.5 mm. ROM ore is dumped into a silo and fed to the primary jaw crusher, which discharges onto a primary double deck vibrating screen. The top deck has a 38 mm aperture screen, and the bottom deck is a 16 mm x 14 mm aperture screen. Material retained on the top deck reports to two secondary jaw crushers, while material retained on the bottom deck feeds the tertiary cone crusher. The product of both secondary jaw crushers is returned to the primary vibrating screen. The primary vibrating screen bottom deck undersize is combined with the tertiary crusher discharge, which then forms the overall product from the Plant C crushing plant. This crushed ore is stored in a covered conical pile of up to 4,000 t capacity.

Crushed ore is reclaimed by FEL and fed to the Plant C grinding circuit, which consists of a 2.4 m diameter x 11 m effective length ball mill in closed circuit with three hydrocyclones. The target grind size is approximately 88% passing 150 μm. Grinding circuit product is pumped to Plant W for sulphide flotation. Sulphide flotation tails are then split, and a portion returns to Plant C where they are pumped to a conditioning tank where a sodium sulphide activator, dispersant, and amine collector are added to prepare willemite for flotation.

The conditioned slurry is then processed in the zinc rougher circuit, which consists of four 8.5 m³ forced air flotation cells. Rougher concentrate is processed in the Plant C zinc cleaner cells. Cleaner concentrate is combined with the Plant W rougher concentrate to form the final zinc concentrate, while cleaner tails report to the cleaner scavengers. Cleaner scavenger concentrate is either returned to the cleaner feed or reports to the final zinc concentrate. The rougher tail is directed to the zinc scavengers, which have four 8.5 m³ cells. The scavenger concentrate recirculates to the rougher feed. Scavenger tails are then combined with the Plant W rougher scavenger tails and pumped to the tailings thickener.

Plant W

Plant W is a modern plant, commissioned in 2003, with a capacity of approximately 145 tph. ROM ore is crushed in a four-stage crushing circuit in closed circuit with screens to 98% passing 9.5 mm. Crushed ore is homogenized on a stockpile with a capacity of approximately 6,000 t by a stacker-reclaimer before being fed to the Plant W grinding circuit, which consists of a ball mill in closed circuit with a high frequency vibrating screen. The screen undersize is approximately 88% passing 150 μm (P80 of approximately 125 μm) and is directed to two conditioning tanks in series prior to feeding the Plant W sulphide flotation circuit.

The sulphide (lead–silver) flotation circuit uses Methyl isobutyl carbinol (MIBC) frother, sodium sulphide activator, and potassium amyl xanthate collector (PAX). The Plant W circuit includes lead roughing and scavenging using three and two 70.0 m³ FLSmidth tank cells, respectively. Scavenger concentrate is recycled to rougher feed, while lead scavenger tails are split and the larger portion reports to the Plant W zinc flotation circuit while the remainder reports to the Plant C zinc flotation circuit.

Rougher concentrate is treated in the first cleaner stage, which consists of two 10.0 m³ FLSmidth tank cells. The first cleaner tail reports to the lead first cleaner–scavenger, with the lead first cleaner–scavenger concentrate returning to the lead first cleaner and lead first cleaner–scavenger tailings returning to lead rougher feed. The lead first cleaner is followed by a second cleaner stage, which consists of one 5.0 m³ FLSmidth tank cell in closed circuit with the first cleaner. There are two additional cleaning stages in closed circuit with each other, the third and fourth cleaners, which each consist of one 0.5 m³ FLSmidth tank cell. Third cleaner tailings report to a two-stage lead third cleaner–scavenger consisting of six 0.5 m³ FLSmidth tank cells. The first two cells produce a third cleaner–scavenger concentrate that returns to the third cleaner feed, while the last four cells recirculate their concentrate to the first two cells. Tailings from the third cleaner–scavenger return to the lead first cleaner. The fourth cleaner produces the final lead concentrate.

The Plant W zinc circuit recovers a willemite concentrate from the lead scavenger tailings, or the grinding circuit screen undersize during periods when the lead circuit is bypassed. The zinc flotation feed is conditioned with a sodium sulphide activator, dispersant, and amine collector in a single conditioning tank. The first stage of flotation is the zinc roughers, which consist of nine 14.2 m³ forced air flotation cells. Rougher concentrate combines with Plant C zinc rougher concentrate to form the final zinc concentrate. Rougher tailings are pumped to the zinc first scavenger circuit, which has five 14.2 m³ forced air flotation cells. The first scavenger concentrate is recirculated to the zinc rougher feed. The zinc first scavengers are followed by the zinc second scavengers, consisting of an additional four 14.2 m³ cells. Second scavenger concentrate reports to Plant C flotation feed, while zinc second scavenger tailings are pumped to the tailings thickener for dewatering and disposal together with the Plant C rougher scavenger tails.

Lead concentrate handling

The lead cleaner circuit included a small lead regrind mill; however, this mill was decommissioned in 2016. The final lead concentrate is dewatered using a pressure filter to produce a filter cake with approximately 10% moisture. Lead concentrate is loaded into bulk bags at the Vazante Operation and transported approximately 900 km to the Port of Itaguai for shipment. Concentrate is sold in 200 t to 600 t shipments, depending on production quantities. Lead concentrate is loaded onto trucks for delivery to customers.

Zinc concentrate dewatering

Zinc concentrates from Plant W and Plant C are combined and stored in stock tanks. The combined rougher concentrates may be fed to a single G cell. The G cell, when operational, floats carbonate from the zinc rougher concentrates. This produces a low grade zinc bearing carbonate concentrate, which can sometimes be treated separately at the Três Marias smelter. When produced, this low grade concentrate is stored and dewatered in the low grade G cell pond. The final zinc concentrate is the combined Plant W and Plant C concentrates when the G cell is bypassed, or the G cell tailings when the G cell is operational. Operation of the G cell was suspended in early 2017 and is assumed not to operate in the LOM mill production plan.

Final zinc concentrate is thickened in a 16 m diameter Dorr Oliver-Eimco concentrate thickener. Thickener overflow is recycled to process water, while thickener underflow is pumped to hydrocyclones. The coarser underflow is dewatered in three vacuum drum filters, while the finer overflow is dewatered in one Andritz plate and frame pressure filter. The combined cake produced contains approximately 13% moisture.

Filtered zinc concentrate is stored in a zinc concentrate silo prior to being loaded into trucks for transport to the Três Marias smelter. Zinc concentrate in excess of the silo capacity can be stored in a covered stockpile and then reclaimed by FEL to be loaded into trucks.

Tailings dewatering

Combined Plant W and Plant C tailings are combined and pumped to tailings cyclones, thickener, and filters where the tailings are dewatered before being hauled to the Pilha Garrote DSTSF. Coarse cyclone underflow is hauled to the Pilha Garrote DSTSF, while cyclone overflow is thickened in the tailings thickener prior to being filtered. Overflow from the tailings thickener and filtrate are directed to the water pond within the Aroeira TSF and is then recycled in the grinding and flotation plants. Thickener underflow is filtered before being hauled to the Pilha Garrote DSTSF.

Concentrate sales and processing

Zinc concentrate produced at the Vazante Operation is sent to Nexa’s Três Marias zinc smelter approximately 250 km from Vazante. Zinc concentrate is trucked in bulk approximately 250 km to the Três Marias smelter. The Três Marias smelter circuit leaches concentrate from the Vazante Operation through an autoclave with recycled leach solutions to dissolve carbonates. This is primarily to remove MgO, which can affect electrolysis efficiency. Zinc silicate is then leached with sulphuric acid to zinc sulphate, with chemistry controlled to precipitate silica. The zinc bearing solution is then purified and zinc is recovered by electrolysis. Fluorine levels in leach solutions must be controlled to avoid problems with stripping zinc cathodes in the electrolytic cell house. The Três Marias smelter relies on treating a balance of sulphide concentrates that, when roasted, generate the sulphuric acid that is used to leach the concentrates. The Vazante Operation concentrate makes up approximately 70% of the feed to the Três Marias smelter. Overall zinc recovery from the Vazante Operation concentrate at Três Marias is 94.4%. The conversion costs of the Vazante Operation concentrate are projected at $474/t recovered zinc.

Energy, water, and process materials requirements

Power for the Vazante Operation is supplied from the national grid, which can provide up to 55 MW via two transmission lines. Power requirement for the operations is not expected to exceed 46 MW (for the mine and processing plants) for the remainder of the LOM.

Water from concentrate and tailings dewatering is recycled to the process via the Aroeira TSF, which is used as a water storage facility. Make-up process water is sourced from the Santa Catarina River and pumped to the processing plant. With the conversion to dry stack tailings disposal, the amount of water available for recycle to the processing plant is expected to increase.

Process consumables include:

• Forged grinding media for the ball mills
• PAX (sulphide flotation circuit collector)
• MIBC (frother)
• Sodium sulphide (willemite activator)
• Sodium carbonate (pH modifier)
• AGLP 250 (dispersant)
• Amine willemite collector
• Flocculant

Manpower

The processing plant personnel comprises management and supervisory staff, including metallurgical personnel, operators, and maintenance personnel, totalling 304 people. This number is made up of 215 permanent employees and 89 contractors and is not anticipated to change significantly in the foreseeable future.

Key reported parameters

Technical qualifications