Metso, a global leader in sustainable minerals processing and metals refining technology, has partnered with Outokumpu to integrate high-performance duplex stainless steels into its advanced hydrometallurgical equipment. This collaboration comes at a time when the global push towards decarbonisation, driven by the energy transition and the increasing electrification of transport, is escalating demand for battery-grade metals like lithium, nickel, and cobalt, as well as copper for electrical conductivity. These trends are intensifying the need for efficient and environmentally responsible metals extraction and recovery processes, particularly hydrometallurgy.
Hydrometallurgy typically involves dissolving metals in ore using acids, often sulfuric acid, at elevated temperatures. This process demands robust materials for pressure vessels and large process reactors. Metso, a pioneer in sustainable technologies and end-to-end solutions for the minerals processing and metals refining industries, has turned to Outokumpu’s duplex stainless steels to meet these challenges. These steels offer superior mechanical and corrosion properties, providing a cost-effective and technically robust solution.
Duplex stainless steels have a unique ferritic-austenitic microstructure, composed of approximately 50% ferrite and 50% austenite. This composition imparts 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. Outokumpu’s lean duplex Forta LDX 2101®, Forta DX 2205, and the super duplex Forta SDX 2507 have proven particularly effective in demanding applications such as atmospheric leaching, pressure leaching autoclaves, solvent extraction systems, and battery recycling reactors.
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. In contrast, duplex stainless steels exhibit almost double the yield strength of typical carbon steels, allowing for thinner walls, reduced equipment weight, and lower 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 stainless steel plays a critical role in several of Metso’s hydrometallurgical equipment technologies. For instance, the OKTOP® reactor line, used for various solid-liquid and gas-liquid reactions, including lithium hydroxide production and battery black mass recycling, faces corrosion from hot sulphuric acid and abrasion from agitated slurries. 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