Sulfuric acid ($H_2SO_4$) is primarily manufactured using the Contact Process, an industrial method that converts elemental sulfur or sulfur-bearing minerals into concentrated acid through three distinct chemical stages.
Key Production Steps & Chemical Reactions
Step 1: Burning Sulfur to Produce Sulfur Dioxide ($SO_2$)
Solid sulfur is melted, atomized, and burned in excess dry air inside a furnace at approximately $1000^\circ\text{C}$.
$$\text{S}_{(s)} + \text{O}_{2(g)} \longrightarrow \text{SO}_{2(g)} \quad (\Delta H = -297 \text{ kJ/mol})$$
- Alternative Feedstock: In metal smelting operations, $SO_2$ is produced by roasting sulfide ores like pyrite ($\text{FeS}_2$):$$4\text{FeS}_{2(s)} + 11\text{O}_{2(g)} \longrightarrow 2\text{Fe}_2\text{O}_{3(s)} + 8\text{SO}_{2(g)}$$
Step 2: Catalytic Oxidation of Sulfur Dioxide to Sulfur Trioxide ($SO_3$)
The cleaned and cooled $SO_2$ gas is mixed with dry oxygen and passed through a multi-bed converter containing a Vanadium(V) Oxide catalyst ($\text{V}_2\text{O}_5$) with potassium promoters.
$$2\text{SO}_{2(g)} + \text{O}_{2(g)} \xrightleftharpoons[\text{400–450}^\circ\text{C}]{\text{V}_2\text{O}_5\text{ Catalyst}} 2\text{SO}_{3(g)} \quad (\Delta H = -198 \text{ kJ/mol})$$
- Process Conditions:
- Temperature: Maintained at $400–450^\circ\text{C}$. Lower temperatures favor equilibrium yield (since the reaction is exothermic), but higher temperatures are needed to maintain acceptable reaction rates.
- Pressure: Kept near atmospheric pressure ($1–2 \text{ atm}$), which is sufficient to achieve over $99.5\%$ conversion yield using multi-stage absorption design.
Step 3: Absorption of $SO_3$ and Formation of Oleum
Directly reacting $SO_3$ with water generates an extremely hot, uncontrollable acid mist that is difficult to condense. To manage this safety hazard, $SO_3$ is absorbed into concentrated sulfuric acid ($98\%$) to form Oleum (fuming sulfuric acid, $H_2S_2O_7$).
$$\text{SO}_{3(g)} + \text{H}_2\text{SO}_{4(l)} \longrightarrow \text{H}_2\text{S}_2\text{O}_{7(l)}$$
Step 4: Hydration of Oleum to Sulfuric Acid
The resulting Oleum is diluted with water under controlled conditions to produce standard concentrated sulfuric acid ($98\%$ $H_2SO_4$).
$$\text{H}_2\text{S}_2\text{O}_{7(l)} + \text{H}_2\text{O}_{(l)} \longrightarrow 2\text{H}_2\text{SO}_{4(l)}$$
$$\text{Overall Reaction: } 2\text{S} + 3\text{O}_2 + 2\text{H}_2\text{O} \longrightarrow 2\text{H}_2\text{SO}_4$$
Process Summary
| Stage | Process Name | Operating Temperature | Key Catalyst / Media |
| Stage 1 | Sulfur Combustion | $1000^\circ\text{C}$ | Air / Oxygen |
| Stage 2 | Oxidation to $SO_3$ | $400–450^\circ\text{C}$ | Vanadium(V) Oxide ($\text{V}_2\text{O}_5$) |
| Stage 3 | Oleum Absorption | Ambient / Warm | $98\%$ $H_2SO_4$ |
| Stage 4 | Dilution | Room Temperature | Water ($H_2O$) |