Sulfuric acid, historically known as “oil of vitriol,” was first discovered by Jaber ibn Hayyan. This colorless, odorless, and conductive substance is a highly potent mineral acid that is miscible with water in any proportion. When diluting sulfuric acid, water should never be added to the acid, as this causes a rapid and intense reaction, leading to boiling and spillage. The correct method is to pour the required amount of water into a container and then slowly add the acid. Note that the reaction of sulfuric acid with water is highly exothermic and easily dissolves in water.
Sulfuric acid is a strong corrosive and forms a significant part of acid rain. Raindrops react with air pollutants to produce acid. This substance is somewhat hazardous and reacts with metals (except mercury and lead), with the intensity of the reaction increasing at higher temperatures.
Production Method Sulfuric acid is produced through various methods. One of the most common methods involves four stages:
- Extraction of sulfur
- Conversion of sulfur to sulfur dioxide
- Conversion of sulfur dioxide to sulfur trioxide
- Conversion of sulfur trioxide to sulfuric acid
The primary sources of sulfur are petroleum and natural gas. Sulfur can also be extracted from ores such as copper, zinc, and nickel. In petroleum and gas extraction, sulfur-containing compounds (organic compounds or hydrogen sulfide) are removed. In mineral extraction, sulfur is obtained as sulfur dioxide during metal refining. In many metal ores, sulfur exists as sulfide compounds, which, when heated, produce metal oxides and sulfur dioxide.
After extracting high-purity sulfur, it is melted using indirect heat (via steam coils) at 150°C and transferred to a combustion furnace using a specialized pump. The amount of molten sulfur transferred to the furnace must be controlled, either through a return path to the sulfur pool or by adjusting the furnace burner needle.
In the combustion furnace, sulfur is converted to sulfur dioxide at temperatures of 950–1000°C. The required oxygen is supplied through compressed air, which is preheated to 150°C via a heat exchanger to maintain furnace temperature.
In the catalytic tower, sulfur dioxide is converted to sulfur trioxide using a vanadium pentoxide catalyst arranged in four layers with varying percentages (fourth layer: 70%, third layer: 20%, second layer: 7%, first layer: 3%). Approximately 200 kg of catalyst is needed per ton of 98% sulfuric acid. The gas produced has a very high temperature and must be cooled before entering the catalytic tower. For this, the gas is first passed through a steam boiler to reduce its temperature to 500°C. To prevent dust, silica, and impurities from entering the catalytic tower, the gas is filtered through a special silica filter. Dry, cool air is injected to lower the gas temperature to 450°C, suitable for the catalytic tower.
The heat generated from converting sulfur dioxide to sulfur trioxide can damage the catalyst. To prevent this, the sulfur trioxide gas is passed through a heat exchanger to reduce its temperature to 450°C. The produced sulfur trioxide gas may still contain some sulfur dioxide, which is fully converted to sulfur trioxide by passing through the third and fourth catalyst layers. The gas exiting the fourth layer is then cooled to 240°C in another heat exchanger.
In the absorption tower, sulfur trioxide gas and acid move in opposite directions—acid is sprayed from the top downward, while the gas moves upward. During this process, sulfur trioxide is absorbed by the acid, producing fuming sulfuric acid (acid with a high sulfur trioxide content).
Applications of Sulfuric Acid Sulfuric acid has numerous applications across various industries, including:
- Agriculture: Its primary use is in producing chemical fertilizers.
- Chemical Industry: Used in manufacturing antifreeze, insecticides, and pharmaceuticals.
- Paper Industry: Applied in producing tissue paper, newspapers, and cardboard boxes.
- Detergents: Acts as a solvent for fats and lipids.
- Leather Industry: Used to remove hair, grease, and impurities.
- Oil Industry: Employed in acidizing oil wells and refining petroleum to remove harmful compounds that damage refinery equipment.
- Food and Water Treatment: Dilute edible sulfuric acid solutions are used in food and to prevent bacterial growth in stored products.
- Battery Production: The liquid in batteries is made from diluted sulfuric acid mixed with distilled water.
- Textile Industry: Used in producing synthetic fibers like polyamide, polyester, and acrylic, and for removing impurities from woven fibers.
- Oil Refining: Applied in producing motor oil, grease, and refining used oils (acid washing and treatment with activated clay), as well as in vegetable oil plants to prepare seed residues for animal feed and remove grease from machinery.
- Military Industry: Used in nitration processes and producing explosives like nitroglycerin and trinitrotoluene.
Safety and Environmental Considerations Sulfuric acid is not flammable, but it can cause fires in the presence of flammable materials. Dry powder should be used to extinguish fires caused by this acid, not water, as the reaction with water is dangerous. When diluting, the acid must be slowly added to water, as adding water to acid produces intense heat and can be hazardous.
During storage, physical impacts to sulfuric acid containers must be avoided, and contact with water or flammable materials should be prevented. Inhaling sulfuric acid vapors can cause throat and nasal irritation, headaches, sneezing, reduced breathing, or lung issues. Acute or chronic skin contact may cause severe irritation, injury, burns, or blisters. Exposure to its vapors can seriously damage eyes and even lead to blindness. Ingestion causes severe poisoning, burns to the mouth and esophagus, and, in severe cases, vomiting and bloody diarrhea. Therefore, protective clothing, gloves, shoes, safety goggles, and respiratory masks are mandatory when handling this substance.
Conclusion The increasing use of sulfuric acid in various industries has led to a global rise in its production to meet societal demands driven by population growth. North America accounts for about 25% of global production, making it the largest producer, while the Middle East contributes only about 4%. In Iran, the growing use of chemicals and industrial metals in producing fertilizers, detergents, and metal extraction has increased the demand for sulfuric acid. In recent years, domestic production has significantly grown to reduce reliance on imports and prevent currency outflow.