Chemistry · Chemistry of the elements · Grade 9

Making cast iron and steel

How is iron obtained from brown ore, and how does cast iron become steel?

The situation

At the metallurgical works in Bekabad, scrap metal is melted in an electric arc furnace and turned into steel. Most of the world's steel, however, comes by another route: reddish-brown iron ore is reduced with coke in a blast furnace more than 30 metres high to give pig iron, and the pig iron is turned into steel in a converter by blowing oxygen through it. At Almalyk copper and zinc are produced, and at Navoi gold. In what form do metals occur in nature, how are they extracted from the ore, and how does cast iron differ from steel?

Topic

Making metals: cast iron and steel. Bekabad Metallurgical Plant: steel from scrap metal in an electric arc furnace; pig iron from ore in a blast furnace

Key ideas

Blast furnace
Converter and electric arc furnace
Metallurgical methods

Formulas

Fe_{2} O_{3} + 3 CO \overset{}{\to} 2 Fe + 3 CO_{2}

In a blast furnace the iron oxide is reduced by carbon(II) oxide; the CO is formed from the burning of coke and from $CO_{2} + C \overset{}{\to} 2 CO$ .

CaCO_{3} \overset{}{\to} CaO + CO_{2} ↑ CaO + SiO_{2} \overset{}{\to} CaSiO_{3}

The flux (limestone) turns the waste rock into an easily melted slag.

m (CaCO_{3}) = m (SiO_{2}) \cdot \frac{M (CaCO_{3})}{M (SiO_{2})}

Calculating the flux: 1 mol of CaCO₃ for every 1 mol of SiO₂ (60.08 g → 100.09 g).

2 C + O_{2} \overset{}{\to} 2 CO Si + O_{2} \overset{}{\to} SiO_{2}

In the converter the oxygen oxidises the excess carbon, silicon, manganese and phosphorus in the pig iron — heat is released.

Δ G^{\circ} = Δ H^{\circ} - T Δ S^{\circ}

The Ellingham diagram: if the line of the reducing agent lies below the line of the oxide, it can reduce that oxide (university mode).

Theory

Metals in nature and how they are obtained

Unreactive metals (gold, platinum, sometimes silver and copper) occur in nature in the native (free) state, and the rest occur as compounds: oxides (haematite Fe₂O₃, magnetite Fe₃O₄, bauxite Al₂O₃·nH₂O), sulfides (chalcopyrite CuFeS₂, galena PbS, sphalerite ZnS), chlorides (halite NaCl, sylvite KCl, carnallite) and carbonates (siderite FeCO₃). A rock from which it is worthwhile to obtain a metal is an ore; it is first dressed (crushing, magnetic separation, flotation). The methods of extraction are: pyrometallurgy — reduction at high temperature with coke, CO, H₂ or a more reactive metal ( $Fe_{2} O_{3} + 3 CO \overset{}{\to} 2 Fe + 3 CO_{2}$ , $WO_{3} + 3 H_{2} \overset{}{\to} W + 3 H_{2} O$ , aluminothermy $Cr_{2} O_{3} + 2 Al \overset{}{\to} 2 Cr + Al_{2} O_{3}$ ); hydrometallurgy — bringing the ore into solution and recovering the metal from it ( $CuO + H_{2} SO_{4} \overset{}{\to} CuSO_{4} + H_{2} O$ , $CuSO_{4} + Fe \overset{}{\to} FeSO_{4} + Cu$ , cyanidation of gold); and electrometallurgy — electrolysis of a melt or a solution (Na, Mg, Al; refining of copper, zinc). Reactive metals (Li … Al) are obtained mainly by electrolysis of the melt, and those of middle reactivity (Zn … Pb, Cu) mainly by reduction.

Metallurgy in Uzbekistan and the blast furnace

Almalyk Mining and Metallurgical Complex — from the copper-porphyry ore of Kalmakyr it produces copper (electrolytic cathode copper since 1964), molybdenum, gold and silver; zinc and sulfuric acid are also made; Navoi Mining and Metallurgical Complex — Muruntau gold (cyanidation and sorption on ion-exchange resin); Bekabad — the Uzbekistan Metallurgical Plant (since 1944), which melts steel mainly from scrap in electric arc furnaces; Chirchik — tungsten and molybdenum. Into the top of a blast furnace (more than ≈ 30 m high) are charged in alternate layers ore, coke and flux (limestone), while from below air heated to 1000–1200 °C (the hot blast) is blown in through tuyeres. In front of the tuyeres the coke burns ( $C + O_{2} \overset{}{\to} CO_{2}$ , ≈ 2000 °C), and on the white-hot coke the CO₂ is converted into CO ( $CO_{2} + C \overset{}{\to} 2 CO$ ). The rising CO reduces the iron oxides step by step: Fe₂O₃ → Fe₃O₄ → FeO → Fe. The iron dissolves carbon (≈ 4 % C) and becomes pig iron, melting at ≈ 1150–1200 °C; the waste rock forms slag with the flux: $CaO + SiO_{2} \overset{}{\to} CaSiO_{3}$ . The slag is light — it floats on the pig iron, and the two are tapped through separate holes; the blast-furnace gas is burnt in the air heaters (Cowper stoves).

Steelmaking and alloying

Pig iron is brittle: it contains ≈ 4 % C, silicon, manganese, sulfur and phosphorus. Making steel means burning out (oxidising) the excess carbon and the impurities: $2 C + O_{2} \overset{}{\to} 2 CO ↑$ , $Si + O_{2} \overset{}{\to} SiO_{2}$ , $2 Mn + O_{2} \overset{}{\to} 2 MnO$ , and the phosphorus passes into the slag with lime. Basic oxygen converter: pure oxygen is blown through a water-cooled lance into ≈ 300 t of molten pig iron; no fuel is needed — the heat of oxidation raises the metal to above 1600 °C, and scrap absorbs the excess heat; one heat takes ≈ 40 minutes. Electric arc furnace: scrap is melted in the arc between graphite electrodes (a temperature of several thousand degrees), using ≈ 0.4 MW·h of electricity per 1 t. The open-hearth furnace is an old method (shut down at Bekabad in 2014). In 2024 about 70 % of the world's steel was made by the blast furnace–converter route and about 29 % in electric arc furnaces. At the end the FeO in the steel is reduced (with Mn, Si, Al) and the steel is alloyed: Cr ≥ 10.5 % gives stainless steel (18 % Cr + 10 % Ni — kitchen utensils), 11–14 % Mn gives Hadfield steel (railway crossings), and W, V, Mo give high-speed tool steels.

Lab work 17: cast iron and steel

The iron–carbon diagram: if the carbon content is below 2.14 % it is steel, if above, cast iron; the alloy with 4.3 % C melts at the lowest temperature, 1147 °C (the eutectic), and steel with 0.8 % C recrystallises in the solid state at 727 °C (the eutectoid: austenite → pearlite). Compare the samples: the hammer — cast iron breaks (brittle), steel bends (ductile); the fracture surface — grey cast iron shows coarse grey grains (graphite), white cast iron shiny crystals (cementite Fe₃C), and steel fine grains; sparks on the grinder — low-carbon steel gives long yellow sparks, high-carbon steel branching “stars”, and the sparks of cast iron are short and red; the file — white cast iron is very hard. The magnet attracts both — it cannot tell them apart. Cast iron is used for cast articles (cauldrons, radiators), and steel for rolled products, reinforcing bars and tools.

University: fundamentals of chemical technology

The Ellingham diagram plots $Δ G ° = Δ H ° - T Δ S °$ for oxidation reactions (per 1 mol of O₂). The lines of metal oxides slope upwards (the gas O₂ is consumed, $Δ S < 0$ ), while the line of $2 C + O_{2} \overset{}{\to} 2 CO$ slopes downwards ( $Δ S > 0$ ). The element whose line lies lower reduces the oxide lying above it: carbon reduces FeO above ≈ 750 °C and Cr₂O₃ above ≈ 1200 °C, and it could reduce Al₂O₃ and MgO only above ≈ 2000 °C, but in practice this is useless (carbides form and the energy cost is huge) — they are obtained by electrolysis (see the “Electrolysis and corrosion” experiment for details: electrolysis of table salt, aluminium). Dressing the raw material: mechanical (crushing, screening, gravity), thermal (roasting), electromagnetic (magnetite) and physico-chemical — flotation: hydrophobic particles stick to air bubbles and rise into the froth; coal is naturally hydrophobic — with kerosene (a collector) and a frother its ash content (the mineral part) is reduced; coke is made from the dressed coal. Copper: a poor ore (less than 1 % Cu) → flotation concentrate → smelting (matte) → converter (blister copper) → anode → electrolytic refining (99.99 % Cu at the cathode, Au, Ag, Se and Te in the anode slime); sulfuric acid is made from the SO₂ — the integrated use of the raw material. Gold: the ground ore is dissolved in an alkaline cyanide solution by the oxygen of the air: $4 Au + 8 NaCN + O_{2} + 2 H_{2} O \overset{}{\to} 4 Na [Au (CN)_{2}] + 4 NaOH$ , and then recovered with resin or zinc.

Safety and the model

Safety: in a metallurgical shop — protective clothing, a hard hat and a face shield; if molten metal and slag meet water there is a danger of explosion; the CO in blast-furnace gas is an odourless poison. In the laboratory: wear goggles at the grinder, beware of fragments flying off when the hammer strikes; do not taste copper sulfate or acids; cyanides are extremely poisonous — they are handled only at the plant, in an alkaline medium (pH > 10.5, so that no HCN is released). Model: in the blast furnace the masses of ore and flux per 1 t of pig iron are taken from stoichiometry (94 % Fe in the pig iron, 450 kg of coke), and the temperature profile is schematic; the changes of carbon, silicon, manganese and temperature in the converter are a simplified typical model; in the electric arc furnace 0.4 MW·h per 1 t; the Ellingham lines are straight lines with ΔH° and ΔS° at 298 K (phase transitions, for example the boiling of zinc and magnesium, are not taken into account); flotation follows first-order kinetics; time is sped up.

Did you know?

The Uzbekistan Metallurgical Plant in Bekabad started up in 1944. Today it melts steel mainly from scrap in electric arc furnaces; the old open-hearth furnaces were shut down in 2014.
In 2024 about 70 % of the world's steel was made by the blast furnace–basic oxygen converter route, and about 29 % in electric arc furnaces (worldsteel data).

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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