Metallic Chromium Quality: How Oxide Purity Sets Metal Specs
In aluminothermic smelting there is no refining step. The metal that solidifies under the slag is the oxide you charged, minus the oxygen. Impurity control is therefore a feedstock problem — and it is solved before the burn starts.
Impurities follow the oxide
Carbon, silicon and sulphur in the oxide survive the reaction and report to the metal almost quantitatively. Chromium-metal buyers specify ≤ 0.03 % C and ≤ 0.02 % S for aerospace-grade alloys — numbers that begin with low-C, low-S oxide lot selection at the oxide plant.
Silicon and aluminium balance
Silicon is controlled by charge stoichiometry: under-charging aluminium leaves unreduced oxide; over-charging leaves aluminium in the metal. The typical target table on the smelt series page shows the reference window for each element.
Traceability from ore to pallet
Blockchain-enabled QR tracking follows every lot from Cr ore (HS 261000) through processing to the final pallet — because in a no-refining process, the certificate of analysis is the specification.
CONTINUE READING
Oxide impurities → metal specs
| Impurity in Cr₂O₃ | Fate in aluminothermic reduction | Metal spec impact |
|---|---|---|
| Fe₂O₃ | Reduces to iron — 100 % carryover | Fe limit of the Cr metal |
| SiO₂ | Partially reduces to Si | Si limit, slag viscosity |
| S (sulphur) | Carries into metal | S spec of aerospace alloys |
| Na / K (alkalis) | Volatilize, contaminate furnace | Process upset, inclusions |
Inside the burn: what the aluminothermic reaction keeps
Aluminothermic chromium making is a chemistry problem disguised as a furnace operation. The reaction Cr₂O₃ + 2Al → 2Cr + Al₂O₃ releases roughly 536 kJ per mole of oxide — enough to take the charge to 2200-2400 °C without a single unit of external heat, and hot enough that metal and slag are both fully molten when the burn ends. Chromium (melting point 1907 °C, density near 6.3 g/cm³) sinks; the alumina slag (density near 3.2 g/cm³, melting point 2072 °C) floats above it, thinned by CaO or CaF₂ flux so it separates cleanly. Pour, cool, break the slag — and the metal you find is exactly the oxide you charged, minus its oxygen, plus whatever the chemistry decided to keep.
Nothing gets refined out after the burn. There is no AOD converter, no vacuum treatment, no slag-metal stirring step on a standard aluminothermic chromium line. The whole story in one sentence: every impurity in the oxide either reduces into the metal, stays in the slag, or volatilises — and each impurity chooses its path by thermodynamics, not by the smelter's wishes. Iron oxide reduces first and reports to the metal at essentially 100 % carryover, because iron is easier to reduce than chromium. Silicon dioxide reduces partially, splitting between metal and slag depending on how much aluminium remains in the charge. Sulphur and carbon carry into the metal nearly quantitatively. Sodium and potassium boil off at burn temperature and foul the furnace atmosphere instead — better than landing in the metal, but still a process upset with inclusion consequences.
The smelter's two control levers are therefore the charge recipe and the oxide certificate. Charge stoichiometry manages the aluminium balance — under-charge and unreduced Cr₂O₃ reports to the slag as chromium loss; over-charge and aluminium dissolves into the metal and blows the Al spec. The oxide certificate manages everything else. In a no-refining process, the CoA is the only gate between a contaminated ore lot and a contaminated metal heat.
Metal yields run 88-92 % of theoretical chromium on a well-run burn; the lost 8-12 % is mostly Cr₂O₃ trapped in slag and fine metal prills, recoverable only by re-melting at a cost that exceeds their value. Every impurity point the oxide gives away is therefore not just a spec problem — it is yield, energy and re-melt arithmetic folded into one number.
Charge-to-metal transfer matrix
| Impurity in Cr₂O₃ | Entry route | Fate in the burn | Metal consequence | Control at the oxide plant |
|---|---|---|---|---|
| Fe₂O₃ | ore gangue, mill wear | reduces to Fe, ~100 % carryover | Fe limit of the Cr metal | ore selection, magnetic separation |
| SiO₂ | ore gangue | partially reduces to Si | Si spec, slag viscosity | gravity beneficiation, lot selection |
| S | sulphates in processing | carries into metal | S ≤0.02 % aerospace spec | sulphate-free route, per-lot combustion test |
| C | carbonates, organic residues | carries into metal | C ≤0.03 % spec | calcination control, process hygiene |
| Na / K | process water, reagents | volatilises at burn temperature | furnace fouling, inclusions | low-alkali washing, water quality |
| Moisture | drying gaps | flash steam, charge disturbance | yield loss, erratic ignition | kiln drying, ≤0.2 % moisture cert |
Reading an oxide CoA as a metal specification
Every line of an oxide certificate maps to a line of the metal spec, and the mapping is arithmetic. Chromium is 68.42 % of Cr₂O₃ by weight: a tonne of 99.0 % oxide carries 684 kg of theoretical chromium, and at 90 % metal yield delivers about 616 kg of metal. Drop the assay to 97 % and the same tonne delivers 604 kg — ten kilograms of metal that must be replaced by higher-priced, higher-purity feedstock somewhere in the smelter's month. The CoA is not paperwork; it is the smelter's cost model printed on one page.
What a serious smelting-grade CoA contains: Cr₂O₃ assay, Fe₂O₃, SiO₂, S, C, Na₂O + K₂O, moisture, particle-size distribution and bulk density — all by named method, on a defined lot, with retained samples. Assays run by ICP-OES after fusion are the norm; sulphur by combustion-infrared; carbon by combustion or infrared absorption. Sampling follows ISO 14488 discipline: the certificate describes the lot, not a hand-picked scoop.
Particle size is the forgotten spec line. Burn kinetics are set by the oxide's surface area: powder milled too fine ignites violently and dusts the charge; granulate too coarse leaves unreduced Cr₂O₃ locked in the slag. A controlled smelting grade ships with a PSD certificate and a granulation window tuned to the customer's furnace — the same chemistry, the same burn, week after week.
And the discipline behind the paper: lot definition, segregation between smelting and pigment grades, retained samples for three years, and third-party verification available on request. In a no-refining process, a smelter that cannot reconstruct the chemistry of a three-year-old lot from retained samples cannot reconstruct the chemistry of its own metal. The oxide CoA chain is the only audit trail the metal will ever have.
Why aerospace audits the oxide, not just the metal
Walk an aerospace audit backwards and it ends at the oxide plant. The spec chain runs: aero-engine material specification → superalloy composition → virgin chromium metal spec (C ≤0.03 %, S ≤0.02 %, tight Fe and Si windows) → smelting-grade oxide CoA. Every auditor knows the metal can only repeat what the oxide supplied, so the audit spends its time on the oxide: ore-source change control, lot definition, SPC records, retained samples, third-party cross-checks, and segregation between smelting-grade and pigment-grade material in the same warehouse.
The questions that decide a supplier approval are boring ones. What changed when the ore source changed, and who approved it? How is a lot defined — furnace batch, day, silo? Where is the SPC chart for Fe₂O₃ over the last 24 months, and what are the control limits? Show the retained samples from lot 12 of last year. Which mill grinds smelting grade, and can it also grind pigment grade? The answers separate a controlled oxide operation from a blender with a certificate printer.
The arithmetic the auditor is protecting: one contaminated oxide lot becomes one bad metal batch, which poisons an entire alloy heat — hundreds of kilograms of superalloy, six figures of value, scrapped or down-blended. The oxide saving that motivated the gamble was a few hundred euros. Aerospace purchasing does not price per kilogram; it prices per risk-adjusted kilogram, and the cheapest oxide on the market is the most expensive one to audit.
Aluminium balance deserves its own line on the audit checklist: the smelter's stoichiometry discipline decides whether the metal leaves the burn inside its Al window, and the oxide supplier's job is to make that discipline possible with stable chemistry. Under-charge loses chromium to the slag; over-charge leaves aluminium in the metal — both are spec failures with the same root cause: an oxide whose chemistry moved. A flat 24-month chemistry chart is the product. The metal is just its shadow.
A final word on the charge sheet itself: granulation, moisture and bulk density are chemistry's quiet partners. A lot with perfect impurities but 1 % moisture flashes steam through the burn and disturbs metal-slag separation; a lot that dusts loses oxide to the hood before the reaction ever starts. The smelter that pre-dries to ≤0.2 % and granulates to a defined window gets the same burn from the same recipe — and the same metal from the same oxide. When the CoA covers chemistry, moisture, PSD and bulk density on one page, the burn has no surprises left.
Sulphur and carbon deserve one more line: both are invisible to the eye and unforgiving in the metal. A single sulphate-contaminated batch can put a whole campaign of metal outside its S window, and the only detection that counts is the per-lot combustion test before the pallet leaves the oxide plant. Cheap oxide skips the test; certified oxide makes it the cheapest thing you buy.
Buyers negotiate cents per kilogram on smelting-grade oxide while the downstream failure costs euros per kilogram of metal. The productive question is not what this lot tests, but what the last 24 months of CoAs look like — a single good certificate is a sales document, while a flat SPC chart is a process, and only the second one survives an aerospace audit. Two things we will not do, on principle: we do not supply aluminium powder, and we do not pre-mix thermit charges. The blend is the smelter's job and the blast risk is real; keeping that boundary clean keeps responsibility unambiguous. What we sell instead is the boring thing — oxide whose chemistry does not move, documented from ore to pallet.
FAQ
What Cr₂O₃ purity does aerospace-grade metal require? Smelting grade at ≥99.0 % Cr₂O₃ with Fe₂O₃ and SiO₂ controlled, but the binding constraints are usually the traces — C and S in the oxide decide whether the metal can meet C ≤0.03 % and S ≤0.02 %. Why does carbon in the oxide matter? Carbon enters through carbonate residues and organic contamination in processing and carries into the metal almost quantitatively; calcination control and process hygiene are the fixes. Do you supply the aluminium powder or pre-mixed thermit? No. We supply only the oxide; premixing aluminothermic charges is the smelter's responsibility, and the hazard is not negotiable. How is sulphur kept low? Sulphate-free processing, ore selection, and per-lot combustion-infrared sulphur testing — the S certificate ships with every lot. What particle size suits the burn? It depends on the furnace and charge design; we ship granulated smelting grade with a PSD certificate, because too fine dusts and ignites erratically while too coarse leaves unreduced oxide in the slag.