Chemistry · Chemistry of the elements · Grade 8

Sulfuric acid and the contact process

How does the gas from copper smelting in Almalyk become sulfuric acid?

The situation

At the Almalyk Mining and Metallurgical Combine, when copper ores — sulfides — are smelted, a great deal of sulfur(IV) oxide is released. If this gas were let into the air, acid rain would fall; instead the combine makes hundreds of thousands of tonnes of sulfuric acid from it — for fertilisers, batteries and metallurgy. How does a poisonous gas become an acid, and why is the temperature in the contact converter held at exactly 400–450 °C? Why does concentrated acid turn sugar into charcoal, and why, when diluting it, must you pour “the acid into the water”?

Topic

Sulfuric acid and its production. Almalyk Mining and Metallurgical Combine sulfuric acid shop: SO₂ from copper smelting is turned into H₂SO₄ by the contact process; concentrated acid turns sugar into carbon

Key ideas

Contact process (V₂O₅)
Add acid to water
Sulfate ion — BaSO₄

Formulas

2 SO_{2} + O_{2} ⇌_{\underset{}{420 - 450_{}^{}_{}^{\circ} C}}^{\overset{}{V_{2} O_{5}}} 2 SO_{3}

In the contact converter: an exothermic reversible reaction, ΔH ≈ −198 kJ (per 2 mol of SO₃).

4 FeS_{2} + 11 O_{2} \overset{}{\to} 2 Fe_{2} O_{3} + 8 SO_{2}

Making SO₂: roasting pyrite (or $S + O_{2} \overset{}{\to} SO_{2}$ , or copper-smelting gases).

H_{2} SO_{4} + SO_{3} \overset{}{\to} H_{2} S_{2} O_{7}

SO₃ is absorbed not in water but in 98 % acid — this gives oleum, which is then diluted.

Ba^{2 +} + SO_{4}^{2 -} \overset{}{\to} BaSO_{4} ↓

The qualitative test for the sulfate ion: a white precipitate that does not dissolve in acids.

c = \frac{m (BaSO_{4})}{M (BaSO_{4}) V}

Gravimetry: the sulfate concentration of the solution from the mass of the precipitate ( $M$ = 233.39 g/mol).

Theory

Sulfuric acid: properties (§ 32)

Sulfuric acid $H_{2} SO_{4}$ is a heavy, oily, colourless liquid; the concentrated acid is ≈ 96–98 % and has a density of 1.84 g/cm³. It is a strong dibasic acid and gives two kinds of salt: sulfates (Na₂SO₄) and hydrogensulfates (NaHSO₄). The dilute acid, like other acids, reacts with metals above hydrogen ( $Zn + H_{2} SO_{4} \overset{}{\to} ZnSO_{4} + H_{2}$ ), basic oxides, bases and salts. The concentrated acid is different: (1) it strongly draws water out — it dries gases and chars sugar, paper and wood: $C_{12} H_{22} O_{11} \overset{}{\to} 12 C + 11 H_{2} O$ ; (2) it is a strong oxidising agent — it even dissolves copper, but SO₂ is released rather than hydrogen: $Cu + 2 H_{2} SO_{4} \overset{}{\to} CuSO_{4} + SO_{2} + 2 H_{2} O$ (on heating), and with carbon: $C + 2 H_{2} SO_{4} \overset{}{\to} CO_{2} + 2 SO_{2} + 2 H_{2} O$ ; (3) in the cold it passivates iron and aluminium — that is why it is transported in steel tankers.

Dilution: “acid into water”

When sulfuric acid mixes with water a great deal of heat is released: ≈ 67 kJ when 1 mol of acid dissolves in 10 mol of water. If we pour water into the acid, the light water (1.0 g/cm³) stays on top of the heavy acid (1.84 g/cm³) and all the heat is released in a thin layer — the water boils at once and spatters the acid around. That is why you always pour the acid into the water — in a thin stream, down a glass rod, stirring and cooling. Battery electrolyte is a sulfuric acid solution with a density of ≈ 1.27–1.28 g/cm³ (≈ 37 %); in preparing it too, the acid is poured slowly into the water, with cooling.

Industrial production: the contact process (§ 35)

1. Making SO₂: sulfur is burned ( $S + O_{2} \overset{}{\to} SO_{2}$ ), pyrite is roasted ( $4 FeS_{2} + 11 O_{2} \overset{}{\to} 2 Fe_{2} O_{3} + 8 SO_{2}$ ), or non-ferrous metallurgy gases are used (Almalyk — gases from copper smelters and the zinc plant). 2. Purification: dust is caught in an electrostatic precipitator, and the gas is washed and dried with concentrated H₂SO₄ — arsenic compounds and dust “poison” the catalyst. 3. Contact converter: $2 SO_{2} + O_{2} \overset{}{⇌} 2 SO_{3}$ , the catalyst is vanadium(V) oxide V₂O₅, the pressure is close to atmospheric (≈ 100–200 kPa). The reaction is exothermic: a low temperature favours the yield of SO₃ (Le Chatelier's principle), but below ≈ 400 °C the catalyst hardly works — the reaction becomes far too slow. So the gas passes through several catalyst beds, and before each bed it is cooled to ≈ 420–450 °C. 4. Absorption: SO₃ is absorbed not in water (that would form an acid mist) but in 98 % acid: $H_{2} SO_{4} + SO_{3} \overset{}{\to} H_{2} S_{2} O_{7}$ (oleum), which is then diluted. With intermediate absorption (removing SO₃ after the 3rd bed) more than 99.7 % of the SO₂ is converted.

The sulfate ion (lab work 8) and gravimetry

The sulfate ion is detected by barium ions: $Ba^{2 +} + SO_{4}^{2 -} \overset{}{\to} BaSO_{4}$ — a white crystalline precipitate ( $K_{s}$ = 1.1·10⁻¹⁰). Carbonate and sulfite ions also give a white precipitate with barium, but BaCO₃ and BaSO₃ dissolve in hydrochloric acid with the release of a gas, while BaSO₄ does not — so the solution is acidified with HCl. Gravimetry: the precipitate is filtered, dried and weighed: $n (SO_{4}^{2 -}) = m (BaSO_{4}) / 233.39$ g/mol, $c = n / V$ . The problems of practical 2: molar concentration from the mass fraction and density of an acid, dilution, calculation from a reaction equation (theoretically ≈ 3.06 t of H₂SO₄ from 1 t of sulfur).

Uses, and Uzbekistan

Sulfuric acid is one of the most produced chemicals in the world (≈ 260 million t in 2022). A large part of it goes into mineral fertilisers: phosphate fertilisers (superphosphate, ammophos) and ammonium sulfate $2 NH_{3} + H_{2} SO_{4} \overset{}{\to} (NH_{4})_{2} SO_{4}$ . Also: metallurgy (bringing ores into solution, the electrolyte for copper electrolysis), batteries, oil refining, dyes and detergents. In Uzbekistan sulfuric acid is made, among other places, at the Almalyk MMC from metallurgical gases (the 2025 plan is ≈ 690 thousand t) — for the combine's own needs and for fertiliser plants.

Safety and the model

Safety: concentrated acid burns skin and eyes — wear safety goggles, gloves and an apron; pour the acid into the water, never the other way round; if it touches the skin, wash at once with plenty of water; SO₂ and SO₃ are poisonous, work in a fume cupboard; SO₂ is also released when sugar is charred. Model: the contact converter has 4 adiabatic beds, a gas with 10 % SO₂ and 11 % O₂, ≈ 120 kPa, the equilibrium is from NIST/CRC thermodynamic data, the rate is schematic (the catalyst “lights off” from ≈ 390 °C); the plant model is not to scale. Dilution: the integral heat of solution (NBS, 25 °C), the heat capacity of the solution is approximate, the vessel cools to the surroundings by Newton's law (faster in an ice bath); density from the table at 20 °C. In gravimetry the precipitate is taken to be complete and pure.

Did you know?

The Almalyk MMC planned to produce ≈ 690 thousand tonnes of sulfuric acid in 2025; once the new workshop being built at the zinc plant is running, it is expected to capture up to 79 thousand m³ of process gas per hour without releasing it into the atmosphere, giving up to 180 thousand tonnes more acid per year (kun.uz, July 2025).
The contact process was patented in 1831 by the English vinegar merchant Peregrine Phillips; at first the catalyst was platinum, which was later replaced by vanadium(V) oxide, cheaper and less easily “poisoned” by arsenic impurities.

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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