The global shift toward decarbonisation, driven by the energy transition and increasing electrification of transport, is fuelling unprecedented demand for battery-grade metals such as lithium, nickel, and cobalt, as well as vast amounts of copper for conducting that electricity.
These developments have intensified the need for efficient, environmentally responsible metals extraction and recovery processes, particularly hydrometallurgy. It usually involves dissolving the metals in the ore in an acid, commonly sulfuric, and at elevated temperatures. It is a demanding process, and its pressure vessels and large process reactors must be made of materials that stand up to the test.
This is why Metso, a frontrunner in sustainable technologies, end-to-end solutions and services for the aggregates, minerals processing and metals refining industries globally has collaborated with Outokumpu to integrate high-performance duplex stainless steels into state-of-the-art hydrometallurgical equipment. Offering superior mechanical and corrosion properties, duplex stainless steels provide a cost-effective and technically robust solution.
The duplex advantage
Duplex stainless steels have a unique ferritic-austenitic microstructure composed of approximately 50% ferrite and 50% austenite. This imparts a combination of high mechanical strength, enhanced corrosion resistance, and good weldability. The presence of nitrogen as an alloying element further strengthens resistance to pitting and crevice corrosion while boosting mechanical performance.
Duplex stainless steels, particularly Outokumpu’s lean duplex Forta LDX 2101®, Forta DX 2205 and the super duplex Forta SDX 2507, have proven especially effective in demanding applications such as atmospheric leaching, pressure leaching autoclaves, solvent extraction systems, and battery recycling reactors.
Duplex versus competing materials
While carbon steel remains an inexpensive and widely available option, it is susceptible to corrosion in acidic and oxidising environments. To overcome this, it is typically coated or lined with fiberglass, rubber, or even lead-backed brick, which adds maintenance overheads and increases the risk of failure. Also, when operating in high-pressure or high-temperature environments, the low design stress of carbon steel necessitates thick wall sections, leading to heavy, cumbersome constructions.
In contrast, duplex stainless steels exhibit almost double the yield strength of typical carbon steels. This allows for thinner walls, reducing equipment weight as well as fabrication and transportation costs. Their inherent corrosion resistance often removes the need for linings or coatings, resulting in lower lifecycle costs.
Fiber-reinforced polymers (FRPs) offer excellent corrosion resistance in some chemical environments and are lighter than metals, but are mechanically limited. FRP constructions for load-bearing applications require additional support structures, often made from carbon steel, which reintroduces corrosion concerns. FRP’s vulnerability to high pressure and mechanical shock make them unsuitable for harsher, higher-temperature processes.
By contrast, stainless steel (especially duplex grades) is self-load-bearing and excels in both static and dynamic loading applications. In Metso’s modular VSF® X solvent extraction systems, stainless steel offers superior integrity, especially under seismic conditions, when compared to FRP alternatives.
While traditional austenitic steels like 316L/1.4404 offer good corrosion resistance, they require higher alloying content with nickel and molybdenum, which subjects them to significant price volatility. Practical experience has proven that in many hydrometallurgy applications, lean duplex performs at a similar level compared to 316L, with 1.5% nickel compared to 10.1% in 316L, and 0.3% molybdenum versus 2.1%.
Duplex in action for Metso
Duplex stainless steel plays a critical role in several of Metso’s hydrometallurgical equipment technologies. A typical example is the OKTOP® reactor line used for a variety of solid-liquid and gas-liquid reactions, including lithium hydroxide production and battery black mass recycling. In these tall, jacketed tanks, corrosion from hot sulphuric acid and abrasion from agitated slurries are key engineering challenges.
To meet these demands:
- Lean duplex Forta LDX 2101 is specified for general use due to its good corrosion and wear resistance in weak acid environments.
- In highly corrosive applications such as zinc direct leaching (~103°C, >100 g/l sulfuric acid), Metso has employed Forta SDX 2507. Its high pitting resistance (PRE of 43) and superior mechanical strength allow for thinner constructions without compromising durability.
Another example is the autoclaves used in pressure leaching at up to 280°C and 60 bar. For these conditions, vessel wall thickness and corrosion resistance are critical. Equipment using high-strength Forta SDX 2507 benefits from reduced wall thickness, simplifying offsite fabrication and transport while maintaining durability.
In one case at Impala Base Metals Refinery in South Africa, a brick-lined carbon steel autoclave was replaced with one constructed from Forta DX 2205, eliminating the need for brick and lead linings, thereby saving on operational costs, maintenance, and potential failure risks.
An electric future
As critical metal demand accelerates due to electric vehicles, grid storage, and broader decarbonisation efforts, the need for high-performance, sustainable materials in hydrometallurgical processing is clear. Duplex stainless steels offer a unique combination of mechanical strength, corrosion resistance, and lifecycle cost advantage.
Author note:
Mikko Palosaari, Senior Application Manager — Process Industry at Outokumpu.