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Ferrochrome Production

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Raw Materials
Sourcing chromium, nickel, and other alloying elements
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Ferrochrome Production
Reducing chromite to ferrochrome alloy

Ferrochrome is made by reducing crushed chromite with charcoal or coke and a silica flux at 1,600-1,800 °C, which in practice means an electric arc furnace and about 4,000 kWh per tonne of alloy. A charcoal-and-bellows furnace does not reach that range, and the alloy that comes out is saturated with carbon, so it cannot by itself give the low-carbon stainless the later steps assume.

Part of Stainless Steel Production

Smelting chromite ore with carbon into ferrochrome, the 60-70% chromium alloy that carries chromium into stainless steel.

The Furnace Is The Barrier, Not The Chemistry

The carrier gives the reaction as FeCr₂O₄ + 4C → Fe + 2Cr + 4CO. The oxygen count balances (4 O in the ore, 4 in the CO) and the carbon count balances (4 on each side). The carrier’s recipe is: crush the ore below 10 mm, mix with charcoal or coke and a silica sand flux, charge an electric arc furnace or a large crucible, heat to 1,600-1,800 °C, and tap molten alloy of 60-70% chromium from the bottom.

The temperature is the issue. The corpus’s own charcoal furnaces top out well below it: Metalworking gives a charcoal forge reaching 1,200-1,400 °C, and Bloomery Furnace sustains 1,100-1,300 °C. No article here shows a bellows furnace holding 1,600-1,800 °C, so the carrier’s “large crucible” option is unsupported by anything else in the corpus. Treat an electric arc furnace as a requirement, and ferrochrome as work for the same late stage as the arc furnace of EAF Operation for Stainless.

What You Need

Method: A Reduction Campaign

Step 1: Charge. Mix crushed ore, carbon and flux. The ratios are not given in the carrier; the stoichiometric carbon below is only a floor.

Step 2: Heat to 1,600-1,800 °C in the furnace. You should see a molten bath with a slag layer on top and a heavier metal layer beneath.

Step 3: Tap the heavy alloy from the bottom into dry, preheated iron or sand moulds, and cast into ingots as the carrier directs.

Step 4: Assay. Break a piece. Ferrochrome is silvery and very brittle (my recall; not in the corpus). Analyse for chromium before using it.

Worked Example

Carbon floor. Per mole of chromite, 4 mol of carbon. With C about 12.0 g/mol and chromite 223.8 g/mol (atomic masses from standard values, recalled), 4 × 12.0 = 48.0 g of carbon per 223.8 g of ore, or 48.0 / 223.8 = 0.21 kg of carbon per kg of pure chromite. Real charges use more, because carbon also dissolves into the alloy and burns.

Gas produced. Each 223.8 g of ore yields 4 mol of CO. At roughly 24.5 L/mol (room temperature), 4 × 24.5 = 98 L, so one kilogram of pure chromite makes 98 / 223.8 × 1,000 = 438 L of carbon monoxide.

Power for a campaign. At the carrier’s 4,000 kWh per tonne of alloy, 1 kg of ferrochrome takes 4 kWh. A 100 kg stainless melt at 17% chromium needs 17 kg of chromium; from alloy at 65% chromium (the midpoint of the carrier’s 60-70%) that is 17 / 0.65 = 26 kg of ferrochrome, or 26 × 4 = 104 kWh. On a continuous 15 kW supply (chosen as an example) 104 / 15 = 7 hours of furnace time at 100% efficiency, and more once losses are counted. The 4,000 kWh/t figure is from the carrier and I do not know whether it is per tonne of alloy or per tonne of ore.

The Alloy Comes Out Carbon-Rich

Reducing with carbon at these temperatures dissolves a lot of carbon into the metal. I recall carbon-reduced ferrochrome at about 6-8% carbon, which is a recalled figure and not in the corpus. Its consequence is worked through in Carbon Control: adding even a modest fraction of such alloy to steel brings carbon far above the carrier’s 0.08% limit. Low-carbon ferrochrome is made by a different process that I cannot source for a village, so plan the product around what carbon-rich ferrochrome allows.

Common Mistakes

  • Trusting the 1,600-1,800 °C range from a bellows furnace. Measure; Pyrometric Cones gives a way to check kiln temperature.
  • Charging damp ore or carbon. Water in a molten bath flashes to steam.
  • Assuming the metal is stainless’s chromium source without assay.

Safety

Carbon monoxide, molten metal and arcs

The reaction releases about 438 L of carbon monoxide per kilogram of pure chromite (arithmetic above). Run the furnace outdoors or under a flue that carries gas away; see CO Risk. Tapping releases metal at 1,600 °C and over; moulds and tools must be dry and preheated, because water or damp in a cold mould flashes to steam and throws molten metal. An arc furnace carries currents that can kill through a single slip; see EAF Operation for Stainless for the electrical stops.

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