# Vanadium recovery from steel slag — the real process Reference flowsheet behind the **Slag Run** game. Every game mechanic maps to a real unit operation; this document is the ground truth so the game teaches the actual hydrometallurgy rather than invented steps. Chromium safety is treated as a first-class concern throughout. > Scope: BOF/LD converter steel slag and vanadiferous slags (the Dutch > Tata Steel IJmuiden / SmartSlag / VANELEX context). Vanadium in fresh steel > slag is typically **~1–2 wt% as V₂O₅**, locked in spinel/oxide phases as > V(III), which is **not** directly leachable — it must first be oxidised to > soluble V(V). Vanadium is an EU/US critical raw material. --- ## Flowsheet at a glance ``` steel slag │ 1. comminution (crush + mill) → liberate V-bearing phases ▼ sized feed │ 2. roast/activation (oxidise V(III)→V(V)) │ ├─ salt (sodium) roast Na₂CO₃/NaCl, 750–850 °C → water-soluble NaVO₃ │ └─ calcification roast CaO/CaCO₃, ~850 °C → Ca-vanadate (acid-leach) ▼ activated feed │ 3. leaching → pregnant V solution │ ├─ water leach (after Na roast) │ ├─ acid leach (H₂SO₄) │ ├─ alkaline leach (NaOH, high-Ca slags) │ └─ pressure / autoclave leach (higher T/P, faster, higher recovery) ▼ pregnant leach solution (PLS) ── + Cr(VI) hazard to manage │ 4. purification / separation → clean V solution │ ├─ Fe/Si/P/Al removal (pH, precipitation, settling) │ ├─ solvent extraction (amines / D2EHPA) — selective V, separates Cr │ └─ ion exchange (anion resins for vanadate) ▼ purified vanadate │ 5. precipitation → ammonium (poly)vanadate │ NH₄⁺ addition → NH₄VO₃ (AMV) / APV "red cake" ▼ AMV / APV │ 6. calcination (~450–550 °C) → V₂O₅ "black flake" (product) ▼ V₂O₅ ──► 7. electrolyte: dissolve in H₂SO₄, reduce to V³·⁵⁺ → VRFB electrolyte (this is the Speciation Quest redox ladder) ``` Leach residue (Ca/Fe/Si-rich) is valorised to cement/aggregate — the circular tail of the process. --- ## 1. Comminution Crushing and milling liberate the fine V-bearing spinel phases and create surface area. **Two levers the game models directly:** - **Recovery ∝ fineness** — more exposed surface leaches more V. - **Leach time ∝ particle size** — dissolution is surface-controlled, so coarse feed leaches far slower. The game uses *1 week per 0.1 mm at 20 °C*. Equipment ladder: hand breakage → hammer mill (~mm) → ball mill (sub-mm, with grinding media). Trade-off: capital + grinding energy vs. downstream leach speed and recovery. ## 2. Roasting / activation — oxidise V(III) → V(V) The key chemical unlock. Fresh-slag vanadium is V(III) in spinel and insoluble; oxidative roasting converts it to soluble V(V): - **Salt (sodium) roast** — with Na₂CO₃ / Na₂SO₄ / NaCl in air at 750–850 °C: `V₂O₃ + Na₂CO₃ + O₂ → 2 NaVO₃ + CO₂`. Product is **water-soluble sodium metavanadate**. Cheap and classic, but NaCl variants release chlorine and the oxidising roast also mobilises **Cr(VI)** (hazard, §7). - **Calcification roast** — with CaO/CaCO₃ at ~850 °C → calcium vanadates (Ca(VO₃)₂ / Ca₃(VO₄)₂), then recovered by acid leach. Na-free, lower emissions, favoured environmentally; needs an acid-leach step. - **Salt-free / direct routes** — skip roasting via aggressive acid or alkaline **pressure** leaching (see §3, autoclave). Lower emissions, higher capital. ## 3. Leaching Dissolve the activated vanadium into solution (the **PLS**, pregnant leach solution). Rate and recovery depend on **particle size, temperature, reagent concentration, liquid/solid ratio, and agitation** — all game levers. - **Water leach** after sodium roast → NaVO₃ solution (simplest). - **Acid leach** (H₂SO₄) — direct or post-calcification; also dissolves Fe, Ca, Mn (impurities to remove in §4). - **Alkaline leach** (NaOH) — selective for V, leaves Fe/Ca in residue; good for high-Ca steel slags. - **Pressure / autoclave leach** — elevated T (≈120–200 °C) and pressure sharply accelerate kinetics and lift recovery, and can leach directly without roasting. In the game this is the **€7000 autoclave**: a capital route that beats the fine-mill-plus-manual-agitation path on both time and recovery. **Agitation.** Stirring/lifting renews the boundary layer and speeds dissolution. The game models *manual lift-agitation* of the leach tank: it shortens leach time and adds recovery, but sloshing of a flexible tank causes **leachate loss + local contamination** — a real environmental trade-off (spill = cleanup + reputation cost). Tank fill is limited by liftable weight (mass = fill volume × solution density, checked against player strength under gravity). ## 4. Purification / separation Clean the PLS before making product: - **Impurity removal** — Fe by pH adjustment/precipitation (goethite/jarosite), Si by settling/coagulation, P and Al by precipitation. - **Solvent extraction (SX)** — extract vanadate with tertiary amines (e.g. Alamine 336) or D2EHPA from sulfate media; strip with ammonia/soda. This is also the main **V/Cr separation** step. - **Ion exchange** — anion-exchange resins load vanadate; elute concentrated. ## 5. Precipitation From the purified solution, add ammonium salts (NH₄Cl / (NH₄)₂SO₄) at controlled pH and temperature to precipitate **ammonium metavanadate (NH₄VO₃, AMV)** or **ammonium polyvanadate (APV, "red cake")**: `VO₃⁻ + NH₄⁺ → NH₄VO₃↓`. ## 6. Calcination Calcine AMV/APV at ~450–550 °C to drive off ammonia/water and form the product: `2 NH₄VO₃ → V₂O₅ + 2 NH₃ + H₂O`. Output is **V₂O₅** ("black flake", ~98–99%+; higher grades for battery use). This is the Slag Run end-product. ## 7. Chromium safety (cross-cutting) Oxidative roasting/leaching can mobilise **Cr(VI)** — chromate/dichromate, toxic and carcinogenic. Real plants must monitor Cr(VI), **reduce it to benign Cr(III)** (with Fe(II) sulfate, sulfite, or other reductants) and separate/immobilise the chromium. This is exactly the Speciation Quest **"Chroom onschadelijk maken"** level (Cr(VI) → Cr(III) → Cr₂O₃), and a scored safety constraint in Slag Run. ## 8. Electrolyte production (hand-off to the Speciation Quest) Dissolve V₂O₅ in sulfuric acid and reduce (chemically or electrochemically) to the mixed-valence **V³·⁵⁺** electrolyte (equimolar V(III)/V(IV)) used in the **vanadium redox flow battery (VRFB)**. Driving V down and up its oxidation ladder is the Speciation Quest redox puzzle — Slag Run makes the V₂O₅ that feeds it. --- ## Mechanic ↔ chemistry map | Game mechanic | Real unit operation | |---|---| | Foraging / ordering ekoliet 0/8mm | Feedstock sourcing (path/bank slag; graded aggregate) | | Mill tiers, output mm | Comminution / liberation | | (planned) roasting stage | Salt / calcification roast — V(III)→V(V) activation | | Recovery-by-size, time-by-size | Surface-controlled leach kinetics | | Leach-tank volume + liftability | L/S ratio, batch size, vessel handling | | Lift-agitation + sloshing loss/contamination | Agitation kinetics + spill/environmental cost | | €7000 autoclave | Pressure/temperature (hydrothermal) leaching | | (planned) SX / precipitation / calcination | Purification → AMV/APV → V₂O₅ | | Cr(VI) handling / Speciation Quest L2 | Chromium reduction & separation (safety) | | Speciation Quest redox ladder | V₂O₅ → VRFB electrolyte (V³·⁵⁺) | ## Sources & further reading Grounded in the standard vanadium-extraction literature (sodium/calcification roast–leach, SX, red-cake precipitation, calcination) and the ChemField KG source catalog (`data/chemfield-kg-sources.json`): Materials Project (Pourbaix/ oxide thermo), PHREEQC (aqueous speciation / log K), COD & RRUFF (slag/mineral structures), USGS MRDATA (vanadium resources). Numbers in the game are tuned for play, not process design.