Sulfuric acid is one of the most important compounds made by the chemical industry. It is used to make, literally, hundreds of compounds needed by almost every industry.
Uses of sulfuric acid
By far the largest amount of sulfuric acid is used to make phosphoric acid, used, in turn, to make the phosphate fertilizers, calcium dihydrogenphosphate and the ammonium phosphates. It is also used to make ammonium sulfate, which is a particularly important fertilizer in sulfur-deficient.
Figure 1 Uses of sulfuric acid.
It is widely used in metal processing for example in the manufacture of copper and the manufacture of zinc and in cleaning the surface of steel sheet, known as 'pickling', prior to it being covered in a thin layer of tin, used to make cans for food.
Annual production of sulfuric acid
The process for producing sulfuric acid has four stages:
(a) Extraction of sulfur
Easily the most important source of sulfur is its recovery from natural gas and oil. These contain sulfur compounds, both organic and hydrogen sulfide both of which must be removed before they are used as fuels or chemical feedstock.
Another important source of sulfur is as sulfur dioxide from metal refining. Many metal ores occur as sulfides and are roasted to form an oxide and sulfur dioxide, for example, in the manufacture of lead:
Worldwide about 35% of the sulfur is obtained as sulfur dioxide from sulfide ore roasting and this is increasing, as plants which traditionally passed the sulfur dioxide to atmosphere are recovering it as sulfuric acid. In particular, China makes most of its sulfuric acid from pyrites, an iron sulfide ore.
(b) Conversion of sulfur to sulfur dioxide
If sulfur is the feedstock, it must first be converted to sulfur dioxide. Molten sulfur is sprayed into a furnace and burnt in a blast of dry air at about 1300 K. The sulfur burns with a characteristic blue flame:
As excess air is used the emerging gas contains about 10-12% sulfur dioxide and 10% oxygen, by volume. The gases are very hot and so are passed through heat exchangers (waste heat boilers).
The gases are cooled to about 700 K and the water in the surrounding boiler pipes is converted into steam. In manufacturing one tonne of sulfuric acid, one tonne of high pressure steam is also produced.
A typical plant contains one cylindrical vessel which acts as a fixed bed reactor with four separate beds of catalyst, known as a converter, heated to 700 K, through which the sulfur dioxide and air pass:
The catalyst, vanadium(V) oxide on silica, is generally in the form of small pellets, to which caesium sulfate has been added as a promoter (Figure 2). The function of the promoter is to lower the melting point of vanadium(V) oxide so that it is molten at 700 K.
(d) Conversion of sulfur trioxide to sulfuric acid
The sulfur trioxide formed from the third bed (and the small amount from the fourth bed) are now converted to
However, water itself cannot be used for absorption as there is a large temperature rise, and a sulfuric acid mist is formed, which is difficult to handle. Instead, sulfuric acid of about 98% concentration is used. This is kept at this concentration by addition of water and removal of acid at that concentration.
Figure 4 A line diagram illustrating a heat exchanger used in the manufacture of sulfur trioxide.
The gases not absorbed contain about 95% nitrogen, 5% oxygen, and traces of sulfur dioxide. The gas stream is filtered to remove any traces of sulfuric acid mist and is returned to the atmosphere using a high stack.
Date last amended: 20th December 2013