Nexa’s Dry Stack Bet at Aripuanã: What the Operating Record Shows

Brazil’s deadliest mining disaster, the Brumadinho dam collapse of January 2019, killed 270 people and permanently altered how regulators, investors, and operators assess tailings risk. Nexa Resources’ Aripuanã mine, commissioned years later in the same country, was designed without a single conventional tailings dam.

Aripuanã is a producing underground zinc-lead-copper mine in Mato Grosso, Brazil, built from the outset around filtered dry stack tailings disposal and cemented paste backfill. This is not a pilot project or a retrofit. It is a full-scale, purpose-designed elimination of impoundment-based tailings storage at an operating mine.

What follows is an analysis of how the filtration system works technically, what the commissioning and operating record shows, where the system encountered real-world limits and how Nexa responded, and what the Aripuanã model signals about responsible tailings management as both an operational and investment consideration.

Why Aripuanã was built without a tailings dam from day one

The Brumadinho disaster did not just kill 270 people. It collapsed public trust in dam-based tailings storage across Brazil’s entire mining sector. Regulatory scrutiny intensified. Institutional investors flagged conventional tailings dams as a material liability. The reputational cost of operating upstream-raised impoundments in Brazil became, for many operators, unacceptable.

The scale of capital flows into Latin American mining reflects investor confidence that the region’s resource base justifies the regulatory and operational complexity, but the Brumadinho disaster fundamentally changed how that capital prices environmental liability when evaluating project structures.

The Global Industry Standard on Tailings Management, published in August 2020 by UNEP, the PRI, and the ICMM, establishes 77 requirements covering the full lifecycle of tailings facilities and provides the authoritative international framework against which dry stack designs like Aripuanã’s are increasingly benchmarked by institutional investors and regulators alike.

Nexa’s response at Aripuanã was structural, not incremental. The mine’s foundational engineering eliminated conventional slurry impoundment entirely, replacing it with a filtered dry stack facility and cemented paste backfill system designed into the project before the first tonne of ore was processed. At the corporate level, Nexa has pursued decommissioning or reconfiguration of legacy dams while mandating dry stack technology at new projects.

Felipe Lana de Almeida, Director of Projects, Engineering and Energy at Nexa Resources, has affirmed that dry stacking is a strategic priority and that outcomes at Aripuanã confirm the company is on a responsible and innovative path for sector transformation.

The distinction matters. Aripuanã was not an existing mine adapted to meet new standards. It was a greenfield project where the engineering decisions that define tailings risk were made at the design stage, before operational pressures or cost trade-offs could dilute the commitment.

How the filtration system converts wet tailings into stackable solids

Ore at Aripuanã is processed through flotation to produce three separate concentrates: copper, lead, and zinc. The tailings stream consists of sulphide flotation material and talc concentrate, which are thickened separately and then combined into a single stream before being routed to large pressure filter presses for dewatering.

The filtration process follows a defined sequence:

1. Flotation produces copper, lead, and zinc concentrates, each processed through its own thickening stage
2. Tailings streams (sulphide flotation tailings and talc concentrate) are thickened separately
3. Thickened tailings are combined into a single feed stream
4. The combined stream enters dedicated pressure filter presses, producing a filter cake targeted at approximately 10% moisture
5. Filter cake is temporarily stored in two stockpiles (each approximately 5,200 tonnes capacity)
6. Material is either trucked to the dry stack facility or directed underground as cemented paste backfill

All filtrate recovered from the process is pumped to dedicated recovered water ponds and reclaimed into the flotation circuit, minimising raw water intake.

The combined tailings filtration circuit is designed for a throughput of approximately 211 tonnes per hour on a dry basis. That capacity figure is central to understanding both the system’s capability and the production constraints that emerged during start-up.

Concentrate filtration versus tailings filtration: two separate circuits

Concentrate filtration and tailings dewatering operate as distinct circuits with different equipment. Publicly available project documentation identifies ANDRITZ SE1200 presses for copper and lead concentrate filtration and ANDRITZ A4F2500 presses, described as the supplier’s largest model, for tailings dewatering, with three units deployed in each application.

It should be noted that an earlier vendor case study attributed the tailings filtration technology to Metso (Larox® FFP presses), citing a 62-plate, 831 m² configuration with processing capacity of 150-200 tonnes per hour, sub-nine-minute cycle times, and 15% residual moisture. These figures are not reflected in publicly available Aripuanã technical reports, which consistently cite approximately 10% moisture and 211 tonnes per hour dry-basis throughput from the ANDRITZ-supplied system. The vendor case study metrics should be treated as supplier-reported figures rather than independently verified design parameters.

What the dry stack facility actually looks like on the ground

The tailings management facility (TMF) at Aripuanã is a double-lined dry stack with an engineered leak detection system and runoff collection ponds. It is not a pond retaining slurried material. It is not an embankment dam holding back a reservoir of saturated tailings.

The geotechnical distinction is fundamental. Filter cake deposited at approximately 10% moisture behaves as a compacted engineered fill. The failure modes that define the catastrophic risk profile of conventional tailings dams, specifically liquefaction of saturated material and overtopping of impoundment walls, are eliminated from the facility’s risk architecture entirely. Stability is managed using conventional geotechnical approaches for compacted fills, supplemented by liner and leak-detection systems designed for environmental protection.

A separate double-lined waste rock storage facility with equivalent runoff controls is part of the same containment philosophy. Process water and site runoff are collected and treated through engineered wetlands before any controlled discharge.

The liability profile that investors assume when holding equity in a mine with conventional dam infrastructure is categorically different from the profile at a dry stack operation. Aripuanã’s design eliminates the catastrophic tail risk that dam failures represent.

When dry stack hit a production ceiling and what Nexa did about it

Start-up operations at Aripuanã revealed what the design specifications could not fully anticipate: the tailings filtration circuit became the critical production bottleneck. The constraint intensified during the rainy season, when higher ambient moisture made filter cake management more demanding and reduced the effective throughput of the system.

Nexa faced a choice. The cheaper option was to relax tailings standards and revert to some form of dam-based disposal to relieve the constraint. The company chose the opposite path.

Nexa acquired a fourth tailings filter press, allocating approximately US$8-9 million in sustaining capital to expand the filtration circuit. Installation was completed in the first half of 2026, with the full operational benefit being realised progressively through the remainder of the year.

Sustaining capital pressures are a consistent theme across base and precious metal mining operations, and the US$8-9 million fourth filter investment at Aripuanã illustrates how environmental architecture commitments generate capital calls that operate independently of commodity price cycles.

Management has framed this investment as central to achieving full nameplate throughput while maintaining the dry stack approach, a deliberate choice to solve the production problem by adding filtration capacity rather than abandoning the tailings architecture that defined the project.

Capital allocation decisions made under operational pressure are among the most reliable indicators of whether ESG commitments are durable or contingent. The fourth filter investment signals that Aripuanã’s dry stack approach is not dependent on convenient operating conditions.

Near-complete water recirculation and what it means for resource efficiency

The filtration architecture at Aripuanã produces a secondary benefit that extends beyond dam elimination. All filtrate streams, from both concentrate and tailings filtration circuits, are collected and returned to their respective flotation circuits, achieving near-complete water recirculation at the mine.

The specific water management mechanisms at Aripuanã include:

• Filtrate from concentrate filtration returned to dedicated recovered water ponds and recirculated to flotation
• Filtrate from tailings filtration collected and reclaimed into the process circuit
• Process water and site runoff captured via collection ponds and treated through engineered wetlands
• Controlled discharge only after passive treatment, minimising contamination risk to surrounding water systems

Conventional wet tailings disposal stores large volumes of process water in impoundments, where evaporation losses, seepage, and long-term treatment obligations accumulate. The dry stack approach at Aripuanã sharply reduces external water intake by recovering and recirculating the water that would otherwise be lost or stored.

In Brazil’s mineral-rich regions, where water access and contamination risk are live regulatory and community concerns, the operational water profile of a mine directly affects its social licence to operate. Mines that demonstrably minimise water draw and contamination risk face lower regulatory exposure and maintain stronger community relationships, both factors that affect long-term operating certainty.

The operational water profile of a mine directly affects its social licence to operate, and mining social licence failures in Latin America have repeatedly demonstrated how community health concerns and regulatory gaps compound into project-halting conflicts that no amount of capital can quickly resolve.

What Aripuanã tells the mining sector about the operational viability of dam-free design

The Aripuanã operating record addresses a question that has lingered over filtered tailings technology for years: whether dry stack disposal can function reliably at production scale in a tropical, high-rainfall environment, not only in the arid conditions where the technology has a longer established history.

Mato Grosso’s climate is one of the more demanding settings for dry stack operations. Seasonal rainfall increases moisture management complexity and, as the rainy season bottleneck demonstrated, can constrain filtration throughput in ways that arid-climate benchmarks do not predict.

The honest assessment of the record surfaces both achievement and cost:

• No conventional tailings dam was constructed or required
• Near-complete water recirculation was achieved across the filtration circuit
• A US$8-9 million fourth filter investment was required to maintain the approach under operational pressure
• The tropical operating environment imposed constraints that demanded additional capital and engineering response

These are not disqualifying limitations. They are the real costs of maintaining a dam-free tailings architecture, and the mining sector needs to price them into project economics when evaluating the replicability of the model.

Felipe Lana de Almeida has stated that outcomes at Aripuanã confirm Nexa is on a responsible and innovative path for sector transformation.

For mining and energy investors evaluating ESG claims across their portfolios, Aripuanã offers a reference point grounded in operating evidence rather than design-phase aspiration. The question is no longer whether filtered dry stack tailings can work at scale. It is whether operators are willing to bear the capital and engineering costs of maintaining it when production pressures arrive.

ESG risk pricing in resource equities remains inconsistent across the sector, with investors often applying broad-brush discounts to mining companies rather than distinguishing between operators with structural tailings risk and those, like Nexa at Aripuanã, that have engineered catastrophic tail risk out of their project architecture.

Aripuanã as a template, not just a project

Aripuanã is not a proof-of-concept. It is a producing mine with a live operating record that validates filtered dry stack tailings at full scale in a high-rainfall tropical environment, one of the more challenging settings for the technology.

The record is not without cost. Filtration capital intensity is real: the US$8-9 million fourth filter investment illustrates the ongoing capital commitment required to sustain a dam-free approach when operating conditions test the system’s throughput limits. Rainy season sensitivity imposes constraints that arid-climate dry stack operations do not face. These are genuine factors that any operator considering a similar approach must account for.

The clearest signal Nexa has provided, however, is the capital allocation decision itself. When the filtration circuit became a bottleneck, the company chose to add capacity rather than revert to dam-based disposal. That choice, made under real operational pressure, distinguishes a durable engineering commitment from a design-phase aspiration.

Investors seeking primary-source detail on the fourth filter commissioning and the Aripuanã tailings storage facility design should consult Nexa’s investor disclosures and technical reports directly. The broader literature on filtered tailings performance in tropical environments provides comparative context for assessing the model’s replicability across the sector.

This article is for informational purposes only and should not be considered financial advice. Investors should conduct their own research and consult with financial professionals before making investment decisions.