Sankey Diagrams & Sustainability Measures for Main Alloy Groups

Review · Nature 575, 64–74 (2019) · DOI: 10.1038/s41586-019-1702-5

Motivation

Iron, aluminium, nickel, and titanium are the four most-used structural metals. Together they underpin global infrastructure, transport, energy, and safety. Production is energy- and carbon-intensive, and demand is forecast to rise by up to 200% before 2050 — driving an urgent need for sustainability strategies across the full value chain.

Scale of the challenge

• Global metals market: ~€3,000 billion yr⁻¹
• Energy consumption: ~53 EJ yr⁻¹ ≈ 8% of global primary energy
• CO₂ emissions: 4.4 Gt CO₂eq yr⁻¹ (steel + Al combined)
• Tailings waste: 2,400 Mt yr⁻¹ from steel and Al; 160 Mt yr⁻¹ bauxite residue

Demand outlook

• Production growth predicted: up to +200% by 2050
• Urbanisation: >60% of population in cities by 2025
• Available post-consumer scrap falls short of demand by ~one-third until at least 2050
• Sustainability must address the entire value chain, not only primary production

Four Key Structural Metals — Snapshot (2017 data)

Source: Table 1, Raabe et al., Nature 575 (2019). Mfg. scrap = manufacturing/processing scrap fraction.

Material Flow Analysis: Sankey Diagrams of Metal Flow

The Sankey diagrams below trace each metal from mine through processing, manufacturing, and use to end-of-life. Flow width is proportional to mass. All figures from Raabe et al., Nature 575 (2019).

Key Findings from the Sankey Diagram for Iron & Steel

• Dual production route: blast furnace (pig iron) + electric arc (scrap)
• 45% of steel already produced from scrap
• Construction: 55% of end use — long in-use stock delays end-of-life scrap return
• New scrap and old scrap both recirculated; small disposal fraction persists

Key Findings from the Sankey Diagram for Aluminium

• Manufacturing scrap: 40% — highest of the four metals
• Transport and construction together: 51% of end use
• Old scrap loop visible but limited; Cu and Fe contamination restricts wrought-to-wrought recycling
• Electrolysis energy dominates primary CO₂ footprint

Key Findings from the Sankey Diagram for Nickel

• ~two-thirds of Ni used as alloying element in stainless steel
• Industrial machinery: 30% of end use
• 20% of post-consumer Ni scrap lost into carbon/copper scrap streams
• Landfill and cross-market losses clearly visible in the Sankey

Key Findings from the Sankey Diagram for Titanium

• Aircraft: 75% of end use — long service life delays scrap return
• Manufacturing scrap: 60% — highest loss fraction at processing stage
• Route: rutile → TiO₂ → TiCl₄ → sponge → ingot → mill product
• Virtually no post-consumer recycling; scrap diverted to ferrotitanium

Cross-cutting observation: For all four metals, demand will exceed available post-consumer scrap by approximately one-third until at least 2050. The in-use stock is still growing — end-of-life scrap will not close the gap without simultaneous reduction in primary production intensity.

Strategies Along the Value Chain

Pathways to Sustainability

1 — CO₂-Reduced Primary Production

• H₂-based direct reduction of iron ore (H₂-DRI): up to −50% CO₂ vs. conventional blast furnace; market entry ~2030
• H₂ injection into blast furnace gas mix: commercially implementable now as interim measure
• Electrolytic iron synthesis (molten oxide electrolysis): zero-CO₂ route; pilot scale, not before ~2040
• Aluminium: primary lever is switching Hall–Héroult electrolysis cells to renewable electricity
• Reducing manufacturing yield losses (steel 15%, Al 25%) offers near-term CO₂ savings

2 — Recycling and Scrap-Compatible Alloy Design

Scrap Sorting & Separation

• LIBS and XRF spectroscopy enable alloy-specific scrap identification at industrial throughput
• Within-alloy-family recycling minimises compositional downgrading
• Al: Cu and Fe contamination critical for wrought grades
• Ni: 20% of scrap lost to C-steel streams — improved separation essential

Recycling-Oriented Alloy Design

• Design alloys tolerant to multi-element scrap inputs
• Crossover (broadband) compositions: one alloy serves a wider application range
• Microstructure tuning instead of composition over-alloying
• Multi-component thermodynamics (up to 20 elements) required for reliable prediction

3 — Longevity and Re-use

• Corrosion costs: ~3.4% of global GDP (~US$2.5 trillion yr⁻¹)
• Protective coatings, sacrificial anodes, and self-passivating alloys extend service life
• Hydrogen embrittlement is a key risk for high-strength steels (>650 MPa) in H₂-economy applications
• Microstructure reset (heat treatment) enables remanufacturing and re-use of structural components

4 — Lightweighting and High-Temperature Efficiency

• ~12% of steel and ~27% of Al are used in transportation — key lightweighting sector
• Up to 30% mass reduction achievable via TRIP/TWIP steels, Al–Li alloys, Mg alloys
• Higher turbine inlet temperatures → better Carnot efficiency → less fuel per unit power
• Ni/Co-superalloys, Ti-aluminides, and Mo–Si–B alloys are the key high-temperature material systems

Priority Recommendations

Near-Term · Available Now

Implement Immediately

• Fossil-free / renewable energy in primary and secondary production
• Improved corrosion protection — largest longevity lever
• Reduce manufacturing scrap losses
• Waste heat harvesting for electricity

Medium-Term · ~2025–2035

Scale Up

• H₂-based direct reduction of iron ore at industrial scale (~2030)
• Automated scrap sorting (LIBS, XRF) across supply chains
• Within-alloy-family recycling as standard practice
• Thin-slab and thin-strip casting for steel and Al

Far-Term · Research Stage

Develop & Deploy

• Electrolytic iron synthesis — molten oxide electrolysis (not before ~2040)
• Crossover alloys for mixed/contaminated scrap streams
• Digital material traceability and re-use infrastructure
• Medium- and high-entropy alloys as impurity-tolerant systems

"Striving towards sustainability will become the next industrial revolution."

Nature 575 (2019)