Sulfuric acid is a strong mineral acid with the molecular formula H2SO4. Its historical name is vitriol. The salts of sulfuric acid are called sulfates. Sulfuric acid is soluble in water at all concentrations. Sulfuric Acid has many uses worldwide, they include lead-acid batteries for cars and other vehicles, ore processing, fertilizer manufacturing, oil refining and removing rust/oxidation.
A nation's sulfuric acid production is a good indicator of its industrial strength. World production in 2001 was 165 million tons, with an approximate value of US$8 billion. The major use for sulfuric acid is for the production of phosphoric acid which is used for the manufacture of phosphate fertilizers as well astrisodium phosphate for detergents. In this method, phosphate rock is used. Fluor apatite is treated with 93% sulfuric acid to produce calcium sulfate, hydrogen fluoride and phosphoric acid. The hydrogen fluoride is then removed as hydrofluoric acid. The overall process can be represented as:
Ca5F(PO4)3 + 5 H2SO4 + 10 H2O → 5 CaSO4•2 H2O + HF + 3 H3PO4
Sulfuric acid is also used in large quantities by the iron and steelmaking industry to remove oxidation, rust from automobiles and appliances. Used acid is often recycled via a Spent Acid Regeneration (SAR) plant. These plants combust spent acid with natural gas, refinery gas, fuel oil or other fuel sources. This combustion process produces gaseous sulfur dioxide and sulfur trioxide which are then used to manufacture new sulfuric acid. SAR plants are common additions to metal smelting plants, oil refineries, and other industries where sulfuric acid is consumed in bulk, as operating a SAR plant is much cheaper than the recurring costs of spent acid disposal and new acid purchases.
Ammonium sulfate, an important nitrogen fertilizer, is most commonly produced as a byproduct from coking plants supplying the iron and steel making plants. It is produced by reacting the ammonia produced in the thermal decomposition of coal with waste sulfuric acid. This allows the ammonia to be crystallized out as a salt and sold into the agro-chemicals industry.
Another important use for sulfuric acid is for the manufacture of aluminum sulfate, also known as paper maker's alum. This can react with small amounts of soap on paper pulp fibers to give gelatinous aluminum carboxylates, which help to coagulate the pulp fibers into a hard paper surface. It is also used for making aluminum hydroxide, which is used at water treatment plants to filter out impurities, as well as to improve the taste of the water.
Regarding the production of Sulfuric Acid, there are two major processes “lead chamber” and “contact” for production of sulfuric acid, and it is available commercially in a number of grades and concentrations. The lead chamber process, the older of the two processes, is used to produce much of the acid used to make fertilizers as it produces a relatively dilute acid (62%–78% H2SO4). The contact process produces a purer, more concentrated acid but requires purer raw materials and the use of expensive catalysts. In both processes sulfur dioxide is oxidized and dissolved in water. The sulfur dioxide is obtained by burning sulfur, by burning iron sulfides, by roasting nonferrous sulfide ores preparatory to smelting, or by burning hydrogen sulfide gas. Some sulfuric acid is also made from ferrous sulfate waste solutions from pickling iron and steel and from waste acid sludge from oil refineries.
In the lead chamber process, hot sulfur dioxide gas enters the bottom of a reactor called a Glover tower where it is washed with nitrous vitriol and mixed with nitric oxide and nitrogen dioxide gases. Some of the sulfur dioxide is then oxidized to sulfur trioxide and dissolved in the acid wash to form tower acid or Glover acid (about 78% H2SO4). From the Glover tower a mixture of gases (including sulfur dioxide and trioxide, nitrogen oxides, nitrogen, oxygen, and steam) is transferred to a lead-lined chamber where it is reacted with more water. Sulfuric acid is formed by a complex series of reactions; it condenses on the walls and collects on the floor of the chamber. There may be from three to twelve chambers in a series; the gases pass through each in succession. The acid produced in the chambers, often called chamber acid or fertilizer acid, contains 62% to 68% H2SO4. After the gases have passed through the chambers they are passed into a reactor called the Gay-Lussac tower where they are washed with cooled concentrated acid (from the Glover tower); the nitrogen oxides and un reacted sulfur dioxide dissolve in the acid to form the nitrous vitriol used in the Glover tower. Remaining waste gases are usually discharged into the atmosphere.
In the contact process, purified sulfur dioxide and air are mixed, heated to about 450°C, and passed over a catalyst; the sulfur dioxide is oxidized to sulfur trioxide. The sulfur trioxide is cooled and passed through two towers. In the first tower it is washed with Oleum (fuming sulfuric acid, 100% sulfuric acid with sulfur trioxide dissolved in it). In the second tower it is washed with 97% sulfuric acid; 98% sulfuric acid is usually produced in this tower. Waste gases are usually discharged into the atmosphere. Acid of any desired concentration may be produced by mixing or diluting the products of this process.
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