Aluminothermic Reduction: From Chrome Oxide Green to Metallic Chromium
Metallic chromium is made the way it has been made for over a century: set fire to a mix of chrome oxide green and aluminium powder, and let chemistry do the rest.
The reaction
Cr₂O₃ + 2Al → 2Cr + Al₂O₃ releases ≈ 536 kJ per mole of oxide — enough to drive the charge to roughly 2400–2700 °C adiabatically, far above the melting points of chromium (1907 °C) and the alumina slag. The reaction is self-sustaining once ignited: no external fuel, no furnace.
Separation by density
Liquid chromium (≈ 6.3 g/cm³) settles below the molten alumina slag (≈ 2.7 g/cm³). A clean slag line means a clean metal block on solidification — and the oxide particle gradation decides how clean that line is. Too fine and the charge dusts out; too coarse and reduction is incomplete.
Charge design
The smelting-grade oxide is screened to a controlled window that stays mixed before ignition and reacts completely through the burn. See the full process notes and typical specs on the smelt metals series page.
CONTINUE READING
The charge arithmetic
| Item | Typical value | Why it matters |
|---|---|---|
| Reaction enthalpy | ~2700 kJ/kg of Cr₂O₃ | Self-sustaining — no external heat once ignited |
| Peak reaction temperature | > 2000 °C | Metal and slag separate cleanly |
| Cr₂O₃ stoichiometric demand | ~2.4 t per tonne of Cr metal | Purity in → purity out |
| Impurity carryover (Fe, Si) | Near-total | Oxide impurities become metal impurities |
Aluminothermic reduction looks simple — mix two powders, light a fuse — and that simplicity hides the industry's central risk: the reaction is violent, exothermic and unforgiving of pre-blending mistakes. VerdeChrome deliberately does not supply or pre-mix aluminium powder with the oxide: a mixed Cr₂O₃/aluminium charge is a thermite composition, regulated as hazardous and too dangerous to ship. The professional workflow is the one our smelter customers use — oxide and aluminium sourced separately, blended only at the furnace under the smelter's own safety system. What we do supply is the oxide side of the charge: DELIKEOXIDE grades with controlled D50, because particle size of the oxide, not just its purity, decides reaction completeness and metal yield.
The thermite that built a metal industry
Metallic chromium is won by burning aluminium with chromium oxide: Cr₂O₃ + 2Al → 2Cr + Al₂O₃, a reaction that reaches over 2 000 °C and delivers molten chromium into a slag of alumina. It looks violent and is — which is exactly why industrial practice keeps the two reactants separate until the crucible, and why responsible suppliers never pre-mix them. The chemistry is unforgiving in another way: everything in the oxide lands in the metal or the slag. There is no refining step after the thermite; the metal's purity is decided by the oxide's purity before the charge is lit. That single fact turns oxide purchasing into metallurgy.
Why oxide purity sets the metal specification
Chromium metal grades are defined by impurity ceilings — iron, silicon, carbon, sulphur, aluminium, nitrogen — and every one of them enters through the oxide. Iron rides along in the Cr₂O₃ lattice; silicon and carbon come from the raw-material chain; sulphur from process residues. A 99.0 % oxide yields metal that must be sold into a lower grade; a 99.5 % oxide opens the aerospace and superalloy tier where chromium is alloyed into the materials that hold jet engines together. The price difference between oxide tiers is small; the price difference between chromium metal tiers is enormous. The smelter that buys oxide by specification, lot after lot, is buying its margin.
| METAL GRADE | TYPICAL Cr % | KEY IMPURITIES | OXIDE REQUIREMENT |
|---|---|---|---|
| Standard chromium | ≥ 99.0 | Fe, Si, C, S | ≥ 99 % Cr₂O₃, controlled Fe |
| Refined grade | ≥ 99.5 | tight Fe/Si | high-purity oxide, low alkali |
| Superalloy / aerospace | ≥ 99.9 | ppm-level everything | ultra-pure oxide, full traceability |
From oxide to aerospace: the chain of custody
The supply chain reads like a purity audit: oxide lot → thermite charge → metal ingot → vacuum refining → superalloy melt → turbine blade. At every link the certificate travels with the material, and a gap anywhere voids the downstream certification. Aerospace material specifications — AMS, ASTM, customer S-specs — all trace back to the oxide's chemistry. This is why serious smelters demand more than a CoA: batch traceability to production date, raw-material intake records, and a stable supplier relationship that outlives any single order. The oxide supplier's discipline is the foundation the whole chain stands on; when it is solid, nobody notices — which is the point.
The thermite reaction does not refine anything — it only reveals what the oxide brought in. Every smelter knows this; the successful ones buy their oxide accordingly, and treat the CoA as the real bill of materials.
FAQ
Do you supply the aluminium powder for thermite charges? No — we supply chromium oxide only. The reactants are never pre-mixed; thermite mixtures are the smelter's own regulated process. What oxide grades suit thermite? Metallurgical-grade Cr₂O₃ with controlled particle sizing and low C/S — discuss your charge design with us. Can you match a specific impurity ceiling? Yes — tell us the metal grade you must make, and we engineer the oxide to it. What documentation ships with the oxide? Full CoA, batch traceability and SDS, every lot.
Charge design: the metallurgist's levers
The thermite charge is a designed system, not a recipe. The oxide's particle sizing controls reaction speed — too fine and the reaction runs away, too coarse and it stalls; the aluminium powder grade and ratio set the heat balance; and the charge geometry decides whether the metal pools cleanly under the slag. Metallurgical-grade Cr₂O₃ is sized for controlled, complete reaction, and its carbon and sulphur levels are held low because both ride straight into the metal. The smelter's expertise is in balancing these levers; the oxide supplier's contribution is delivering a raw material whose chemistry and sizing are stable enough that the charge design stays valid lot after lot. Charge design is expensive to redo; a stable oxide is the cheapest way to never redo it.
The impurity ledger, line by line
Every impurity in the oxide has a destination. Iron follows chromium into the metal and caps the achievable grade; silicon and carbon do the same and additionally embrittle the product; sulphur is a steel-poison that downstream customers measure in ppm; alkali metals volatilise into the fume and complicate furnace operations. The oxide specification is therefore a metal specification written one step earlier: Cr₂O₃ content sets the chromium ceiling, Fe/Si/C/S set the alloy floor, and alkali control protects the process. Buyers who read the oxide CoA as the metal's birth certificate make better grades and better margins; buyers who buy on price discover the ledger in the metal analysis instead, where corrections are no longer possible.
From lot certificate to aerospace audit
Modern aerospace procurement audits the oxide, not just the metal. The audit trail reads: oxide CoA with named methods → charge log → metal analysis → remelt and vacuum-refining records → superalloy certificate → component history. A gap in the oxide documentation is a gap in the audit, and aerospace auditors are paid to find gaps. This is why the strongest smelters partner with oxide suppliers who treat traceability as a product feature: batch records linked to production dates, retained samples available for years, and certificates that name the methods instead of just the results. The oxide supplier becomes, in effect, the first page of the aerospace file — and the smartest smelters choose that page carefully.
The thermite reaction is honest to a fault: it reports everything the oxide contained. Buy the oxide like you are buying the metal — because chemically, you already are.
FAQ
What sizing suits our charge? We engineer the D50 to your charge design — share your reaction-speed target and we match it. Can you hold a specific C/S ceiling? Yes, written into the specification and verified per lot. Do you offer trial quantities for charge tests? Yes — 25 kg across three lots. Is the oxide itself hazardous? No — Cr(III) oxide, not classified under GHS; the SDS documents the full profile.
Why we do not sell thermite mixtures — and the safety logic behind it
A recurring question deserves a clear, permanent answer: VerdeChrome supplies chromium oxide — not aluminium powder, and never pre-mixed thermite. The reaction between Cr₂O₃ and aluminium is intensely exothermic, self-sustaining once initiated, and its mixtures are regulated and hazardous by nature; blending them is the smelter's own controlled operation, performed under its permits and process controls. The safe division of labour is the industry standard: the oxide supplier delivers a pure, well-sized reactant with full documentation, and the smelter runs the charge. Anyone offering a ready-mixed thermite powder is offering a hazard, not a service — and the responsible buyers know the difference instantly.
The market outlook for chromium metal and its oxide
Chromium demand runs on three engines: stainless steel (the volume driver), aerospace superalloys (the value driver), and specialty alloys and plating (the steady base). The oxide sits at the front of all three, because aluminothermic reduction remains the standard industrial route to chromium metal. As aerospace build rates climb and superalloy specifications tighten, the premium for high-purity, traceable oxide rises with them — the metal tier that demands ppm control grows faster than the commodity tier. For oxide suppliers and smelters alike, the strategy is the same: invest in purity, documentation and lot-to-lot stability now, because the customers who pay for those things are the ones who will still be ordering when the next cycle turns. The thermite reaction has not changed in a century; the paperwork around it has changed everything.
Chromium metal's future is written in its certificates. The oxide that arrives with its chemistry proven and its traceability intact is the raw material that the aerospace tier will build on — everything else stays in the commodity pile.
FAQ
What oxide purity do thermite smelters typically need? ≥ 99 % Cr₂O₃ for standard metal; higher tiers for refined and superalloy grades. What particle sizing works best? Engineered to the charge — reaction speed is set by the D50 and span; share your target and we match it. Do you ship internationally? Yes, with full export documentation and SDS per destination regulations. How is lot traceability maintained? Every lot is linked to production date and raw-material intake, with retained samples. Can we visit the production site? Yes — qualified customers are welcome; audits are how trust is built in this chain.
In chromium metal, chemistry and paperwork are the same product. Buy the oxide that arrives with both in order, and the metal will take care of the rest.