Nitric acid is predominantly manufactured through the catalytic oxidation of ammonia, a highly efficient industrial process often referred to as the Ostwald process. This multi-step chemical process converts ammonia into nitric acid, a vital chemical used across various industries.
The manufacturing process typically involves three primary stages:
1. Ammonia Oxidation (Catalytic Combustion)
This is the initial and most critical step, occurring at high temperatures. Ammonia gas is mixed with air (oxygen) and passed over a catalyst, usually a platinum-rhodium alloy, in the form of fine gauze.
- Reactants: Ammonia ($\text{NH}_3$) and Oxygen ($\text{O}_2$)
- Catalyst: Platinum-rhodium alloy gauze
- Temperature: High, typically between 800-950 °C (1472-1742 °F)
- Reaction: $4\text{NH}_3(\text{g}) + 5\text{O}_2(\text{g}) \xrightarrow{\text{Pt/Rh catalyst}} 4\text{NO}(\text{g}) + 6\text{H}_2\text{O}(\text{g})$
- Outcome: Produces nitric oxide ($\text{NO}$) and water vapor. The reaction is highly exothermic, meaning it releases a significant amount of heat.
2. Nitric Oxide Oxidation
The nitric oxide produced in the first stage is then cooled and allowed to react further with excess oxygen (from the air). This reaction is typically carried out at lower temperatures, as it is more favorable under those conditions.
- Reactants: Nitric Oxide ($\text{NO}$) and Oxygen ($\text{O}_2$)
- Temperature: Relatively lower, around 25-50 °C (77-122 °F)
- Reaction: $2\text{NO}(\text{g}) + \text{O}_2(\text{g}) \longrightarrow 2\text{NO}_2(\text{g})$
- Outcome: Forms nitrogen dioxide ($\text{NO}_2$), a reddish-brown gas.
3. Absorption
In the final stage, the nitrogen dioxide is absorbed in water within an absorption tower. The absorption process involves a series of reactions that ultimately yield nitric acid. Any remaining nitric oxide from this stage is usually recycled back into the process to maximize efficiency.
- Reactants: Nitrogen Dioxide ($\text{NO}_2$) and Water ($\text{H}_2\text{O}$)
- Reaction: $3\text{NO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \longrightarrow 2\text{HNO}_3(\text{aq}) + \text{NO}(\text{g})$
- Outcome: Produces an aqueous solution of nitric acid ($\text{HNO}_3$). The nitric oxide ($\text{NO}$) regenerated in this step is typically sent back to the second stage for re-oxidation, improving overall yield. The concentration of the nitric acid produced in this manner is usually around 50-70% by weight. To achieve higher concentrations (e.g., fuming nitric acid), further purification and dehydration steps are required.
Summary of the Nitric Acid Manufacturing Process
| Step | Main Reactants | Catalyst/Conditions | Primary Product | Key Reaction |
|---|---|---|---|---|
| 1. Ammonia Oxidation | Ammonia ($\text{NH}_3$), Oxygen ($\text{O}_2$) | Pt/Rh gauze, 800-950 °C | Nitric Oxide ($\text{NO}$) | $4\text{NH}_3 + 5\text{O}_2 \longrightarrow 4\text{NO} + 6\text{H}_2\text{O}$ |
| 2. Nitric Oxide Oxidation | Nitric Oxide ($\text{NO}$), Oxygen ($\text{O}_2$) | Cooling, 25-50 °C | Nitrogen Dioxide ($\text{NO}_2$) | $2\text{NO} + \text{O}_2 \longrightarrow 2\text{NO}_2$ |
| 3. Absorption | Nitrogen Dioxide ($\text{NO}_2$), Water ($\text{H}_2\text{O}$) | Absorption tower, various pressures | Nitric Acid ($\text{HNO}_3$) | $3\text{NO}_2 + \text{H}_2\text{O} \longrightarrow 2\text{HNO}_3 + \text{NO}$ |
Industrial Significance
The efficient production of nitric acid is crucial for numerous industries. It serves as a key raw material for:
- Fertilizers: Primarily for the production of ammonium nitrate.
- Explosives: Such as nitroglycerin, trinitrotoluene (TNT), and ammonium nitrate.
- Dyes and Pharmaceuticals: Used in various organic synthesis reactions.
- Metallurgy: For etching and pickling metals.
- Rocket Propellants: As an oxidizer.
Modern nitric acid plants are designed with energy recovery systems to capture the heat generated during the exothermic reactions, making the process more sustainable and cost-effective.
