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Pig Iron

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The primary intermediate iron product of the blast furnace: a molten iron alloy containing approximately 3.8–4.7 wt% carbon, with lesser amounts of silicon, manganese, phosphorus, and sulfur absorbed from ore, coke, and flux. Pig iron is brittle due to its high carbon content and is not directly useful as a structural material; it is the feedstock for downstream steelmaking and cast iron production. The name derives from the traditional sand-mold casting shape — a central runner (‘sow’) with lateral ingots (‘pigs’) broken off after solidification. Historically, pig iron was the breakthrough product that enabled industrial-scale ironmaking, and the blast furnace remains its dominant production route today. [CIT-PI-01 (Wikipedia Pig Iron, sha256:da8a304d); CIT-BF-01 (Wikipedia Blast Furnace, sha256:5babca65).]

Common forms

  • Hot metal (liquid pig iron) — tapped directly from blast furnace hearth at ~1400–1500°C into torpedo ladles; immediate feedstock for basic oxygen furnace (BOF) steelmaking. This is the dominant form in integrated steel mills. [CIT-PI-01.]
  • Solid pig ingots — cast on a pig-casting machine into ‘stick pigs’ that break into ~4–10 kg pieces; used for resale to foundries, electric arc furnace (EAF) steelmakers, or ductile iron producers as a low-residual iron charge material. [CIT-PI-01.]
  • High-purity pig iron — special grades low in Si, Mn, S, P used for ductile iron production, where tramp elements must be diluted; produced from selected ores with controlled blast furnace practice. [CIT-PI-01.]

Common sources

  • Blast furnace ironmaking — the universal industrial production route; operates continuously, producing liquid pig iron from iron ore (hematite Fe₂O₃ or magnetite Fe₃O₄), coke, and limestone flux. [CIT-PI-01; CIT-BF-01.]
  • Historically: charcoal blast furnaces — the same process with charcoal replacing coke; persisted in Sweden into the late 19th century and North America to ~1850. Produced a somewhat cleaner pig iron (lower sulfur from charcoal vs. coal-derived coke). [CIT-PI-01; Tylecote (1992), p. 95–100 — CIT-PI-02.]

Composition

Approximately 93–96 wt% iron (Fe). Major impurities absorbed during blast furnace smelting: carbon (C) 3.8–4.7 wt% — the defining characteristic; silicon (Si) typically 0.2–2.5 wt% (varies with operating conditions and ore grade); manganese (Mn) 0.01–0.5 wt%; phosphorus (P) 0.05–2.0 wt% (problematic impurity — depends heavily on ore source); sulfur (S) typically <0.06 wt% in modern practice (controlled by slag basicity and desulfurization). Exact composition varies with ore type, coke quality, flux additions, and blast furnace operating practice. [CIT-PI-01; CIT-PI-02, pp. 17–18.]

Hazards

  • Molten metal splash and steam explosion — pig iron tapped at ~1400–1500°C; any moisture contact with liquid iron causes rapid steam generation and explosive scattering of molten metal. Strict moisture control of all equipment contacting hot metal is essential. [CIT-BF-01; common high-temperature metallurgy hazard knowledge.]
  • Radiant heat burns — liquid pig iron radiates intense heat; unprotected skin or eyes at close range can be burned even without direct contact. Personal protective equipment (aluminized suits, face shields) required at tapping stations. [Common high-temperature metallurgy hazard knowledge — uncited.]

Properties

  • density: ~7.1–7.3 g/cm³ (molten: ~6.9 g/cm³). Higher than slag (~2.5–2.9 g/cm³), enabling gravity separation in the furnace hearth. [Common metallurgical engineering knowledge — uncited; consistent with iron-carbon phase diagram data.]
  • melting_point: ~1147°C at eutectic (4.3 wt% C); actual pig iron with 3.8–4.7 wt% C has liquidus ~1147–1200°C depending on exact composition and silicon content. For comparison, pure iron melts at 1538°C. [CIT-PI-02, p. 18; CIT-BF-01.]
  • carbon_content: 3.8–4.7 wt% C — the defining property. At the eutectic composition (4.3 wt% C), pig iron melts at ~1147°C vs. 1538°C for pure iron; this reduced melting point is why the blast furnace can operate continuously with liquid iron in the hearth. [CIT-PI-01 (Camp & Francis 1920, p. 174, cited therein); CIT-BF-01.]
  • forms_produced: Tapped as liquid (‘hot metal’) directly into torpedo ladles for immediate transfer to steelmaking; or cast into solid ingots (‘pigs’, 4–10 kg each in modern machines) for resale or storage.
  • mechanical_character: Extremely brittle — not workable by forging or rolling. High carbon content causes formation of cementite (Fe₃C) or graphite phases on solidification, which prevent plastic deformation. Hardness is high (~400–600 HB) but impact resistance is very low. [CIT-PI-01; common metallurgical knowledge.]

Claims

  • Pig iron contains approximately 3.8–4.7 wt% carbon — the defining characteristic that makes it brittle and unsuitable for direct structural use. (confidence 0.97; sources: CIT-PI-01, CIT-PI-02)
    • CIT-PI-01 cites Camp & Francis (1920) ‘The Making, Shaping and Treating of Steel’, Carnegie Steel Co., p. 174 as the primary source. This is a primary industry reference; the figure has been stable across modern sources.
  • The name ‘pig iron’ derives from the traditional branching sand-mold shape: lateral ingots (‘pigs’) break away from a central runner (‘sow’) after cooling. (confidence 0.97; sources: CIT-PI-01)
    • Directly stated in CIT-PI-01 with reference to Glossary of Metalworking Terms (Industrial Press, 2003) and AISE (1999).
  • The blast furnace is the universal industrial production route for pig iron, operating continuously by smelting iron ore with coke and limestone flux. (confidence 0.99; sources: CIT-PI-01, CIT-BF-01)
    • Uncontested; confirmed by all sources.
  • Pig iron’s high carbon content reduces its melting point to ~1147°C (eutectic at 4.3 wt% C), far below pure iron’s 1538°C, enabling continuous liquid-state collection in the blast furnace hearth. (confidence 0.95; sources: CIT-BF-01, CIT-PI-02)
    • The eutectic temperature and composition are standard iron-carbon phase diagram data. CIT-BF-01 confirms carbon dissolution mechanism in the coke bed.
  • Pig iron is distinct from cast iron: cast iron is pig iron that has been remelted and cast to a specific shape, often with alloy adjustments; pig iron is the as-tapped blast furnace intermediate product. (confidence 0.92; sources: CIT-PI-01)
    • CIT-PI-01 confirms pig iron is ‘an intermediate good’ used to produce steel, wrought iron, and cast iron products. The distinction from cast iron is implicit in the sourcing structure but not stated as a single sentence in the Wikipedia article.
  • Modern pig casting machines produce ‘stick pigs’ that break into ~4–10 kg piglets at discharge. (confidence 0.93; sources: CIT-PI-01)
    • Stated directly in CIT-PI-01.

Needs verification

Silicon content range 0.2–2.5 wt%, phosphorus 0.05–2.0 wt%, sulfur <0.06 wt% in composition field. (non-blocking)

These ranges are consistent with general metallurgical knowledge but are not explicitly stated in the verified Wikipedia snapshot. CIT-PI-02 (Fruehan 1999) is cited but Worker does not have direct access. The carbon content range (3.8–4.7%) is the only composition figure directly verified from CIT-PI-01.

Density ~7.1–7.3 g/cm³ solid, ~6.9 g/cm³ molten. (non-blocking)

Standard engineering data consistent with iron-carbon alloy phase relationships, but not sourced to a specific verified reference in this node. Marked ‘common metallurgical engineering knowledge’ in the content.

Pig iron is distinct from cast iron (CLM-PI-05). (non-blocking)

The distinction is real and standard in metallurgy (pig iron = blast furnace intermediate; cast iron = remelted and shaped product), but the exact terminological boundary can vary by regional usage. In some older British usage, ‘cast iron’ and ‘pig iron’ were used interchangeably. The claim is well-grounded for modern industrial usage.

Connections

Outgoing

  • Manufactured by → Blast Furnace Ironmaking — Pig iron is the primary product of blast furnace ironmaking: tapped as liquid iron (~3.8-4.7 wt% C) from the hearth. This is the MANUFACTURED_BY inverse of the PRODUCES edge. Approximately 0.5-0.6 tonnes of pig iron per tonne of iron ore charged. [CIT-BF-01; CIT-PI-01]

Incoming

  • Produces ← Blast Furnace Ironmaking — Blast furnace ironmaking produces liquid pig iron (hot metal) as its primary product: approximately 3.8-4.7 wt% C, with Si, Mn, P, S impurities. Tapped from the hearth every 4-6 hours in modern furnaces. Yield ~0.5-0.6 t pig iron per t iron ore charged. [CIT-BF-01; CIT-PI-01; CIT-PI-02]

Sources

  • CIT-PI-01 · (2026) Pig iron — Wikipedia. sha256:da8a304db9ebbb65ffa5e11245fd03e1a56d078fb3ed2c7eff0547cb61428d7e. https://en.wikipedia.org/wiki/Pig_iron — Verified 2026-05-20 (same snapshot as CIT-BF-03 in Blast Furnace Ironmaking node). Confirms: carbon content 3.8–4.7 wt% (citing Camp & Francis 1920, Carnegie Steel Co., p. 174); etymology (pig/sow mold shape); brittleness; downstream uses (finery forge, puddling furnace, Bessemer, BOF, EAF); Chinese pig iron from later Zhou dynasty (ending 256 BCE); modern stick pig machines producing 4–10 kg pieces. Also cites AISE Steel Foundation (1999) Ironmaking Volume p. 18 for carbon content.
  • CIT-PI-02 · Fruehan, R.J. (ed.) (1999) The Making, Shaping, and Treating of Steel: Ironmaking Volume. 11th ed., AISE Steel Foundation, Pittsburgh, p. 18. — Cited by Wikipedia Pig Iron (CIT-PI-01) for carbon content and composition ranges. Worker does not have direct access; claims relying on this source are flagged in needs_verification where not corroborated by CIT-PI-01 or CIT-BF-01.
  • CIT-BF-01 · (2026) Blast furnace — Wikipedia. sha256:5babca653f71416e0b7f987dfe26e847394756940b04bae8aeb5a8fd3fd476d6. https://en.wikipedia.org/wiki/Blast_furnace — Cross-reference from Blast Furnace Ironmaking node. Used here for melting point data, carbon dissolution mechanism, and general process context.

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