Sulfuric Acid (H2SO4) is a strong mineral acid used in many industrial processes as well as batteries. It is one of the largest inorganic industrial chemicals produced by tonnage. The quantity of sulfuric acid produced has been used as an indicator of a country's industrial status. Sulfuric is formed from the combination of sulfur, oxygen, and hydrogen. Chemically, it is a molecule composed of SO3 and water (H2O). As more SO3 is absorbed in sulfuric acid, it can reach the point where there is more SO3 in solution than there is water to combine with it to form sulfuric acid. At this point, the solution is considered to exceed 100%. At concentrations over 100% it is commonly called fuming sulfuric or Oleum. Oleum has a number of industrial uses. As the term fuming sulfuric suggests, it readily releases SO3 which is considered a highly hazardous compound by the US EPA.

Sulfuric acid is used in batteries, primarily car batteries. It is also used in many industrial processes to refine crude oil, make nylon and other chemicals, produce phosphoric acid for fertilizer, industrial users, soft drinks, and animal feed. It s also used in water treatment systems to purify water and control pH.

Sulfuric Acid is a clear liquid which weighs about 1.8 times as much as water (15 pounds/gallon). It reacts violently with water, bases, and some organic materials, but is normally stable. Its properties are strongly dependent on temperature and concentration. At a common industrial strength of 95%, it freezes at -12 degrees F and boils at over 500 degrees F. It is soluble in water from 0% to over 100%.

Concentrated sulfuric acid reacts strongly with water. Mixing water and acid will generate tremendous heat, and can result in explosions if the water gets trapped. If water and acid are improperly mixed, then it can generate enough heat to melt plastics or shatter glass containers from thermal shock. The heat can cause the water to boil and generate high pressures.It will absorb water from air and organic materials. It can char wood, dry air to almost zero humidity, and destroy flesh. Depending on concentration and temperature, it can be extremely corrosive to metals.

Most sulfuric acid is manufactured by burning sulfur to form SO2, catalytically converting the SO2 to SO3 using vanadium pentoxide catalyst, and then absorbing the SO3 in concentrated sulfuric acid. The absorbed SO3 raises the acid concentration and water is added to bring the concentration back down. Each step of this process generates heat, and sulfuric acid plants are excellent energy sources. Steam from the plant can be used to provide energy for other processes or to generate power. The air used in the furnace is first pulled through a drying tower where sulfuric acid absorbs the water from the air so it cannot condense and form acid in the "dry' portions of the plant.

Recently, sulfuric acid produced from ore smelter gases has become more common. These plants get their SO2 feed from copper, iron, zinc, or other ore processing plants. They serve as pollution control systems to greatly reduce the amount of SO2 being released into the air by the smelter.

Originally plants were constructed largely from lead and brick because they resisted sulfuric corrosion. During the 1900s plants were built using cast iron and acid resistant brick lined carbon steel. New types of stainless steels have been developed and are being used in all parts of the plant. Anodic protection is usually used to keep stainless steel (Types 304 and 316) from corroding in acid heat exchangers. Teflon and other fluoropolymers are also becoming more common in sulfuric acid plants.

As energy becomes more valuable, energy recovery systems become more complicated. Heat recovery systems built of 310 stainless steel operating in very tightly controlled temperature and concentration ranges allow steam to be produced from heat formerly thrown away. Plants built or upgraded since 1990 can produce as much as 1.7 tons of steam for each ton of sulfuric acid produced. Power generation has become an important part of most sulfuric acid facilities.

SO2, SO3, and sulfuric acid are all harmful to the environment. While all occur naturally, particularly in volcanoes and geothermal vents, they harm plants and animals. Older plants were severe polluters, but tightening environmental regulations and technological improvements now result in over 99.7% of the SO2 produced being recovered as product (at least in developed countries). Newer technologies now appearing in the marketplace are resulting in even greater reductions.

Most sulfuric plants now recycle all water which becomes contaminated with acid so none, or only a small stream, which can be properlyy treated is released into the environment. Modern plants actually emit less SO2 per BTU of energy produced than a power plant burning fuel oil (without a stack scrubber). Users of sulfuric acid generally either neutralize it or return it for recycling. If disposed of untreated, it is a hazardous waste and requires special (and expensive) handling.