Sulfuric AcidSulfuric 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. It is also used to make caprolactam, which is converted into polyamide 6 and in the manufacture of titanium dioxide, used, for example, as a pigment. Amongst its many other uses is in the manufacture of hydrofluoric acid and phenol with propanone all of which are used in many industries. Sulfuric acid is also obtained from ammonium sulfate. These contain sulfur compounds. used) sulfuric acid. a by-product in the manufacture of poly(methyl 2-methylpropenoate) and also recovered from 'spent' (i. for example. as plants which traditionally passed the sulfur dioxide to atmosphere are recovering it as sulfuric acid.6 million tonnes1 (as oleum) Data from: 1.e. an iron sulfide ore. Federal State Statistics Service: Russian Federation 2011 Manufacture of sulfuric acid The process for producing sulfuric acid has four stages: a) extraction of sulfur b) conversion of sulfur to sulfur dioxide c) conversion of sulfur dioxide to sulfur trioxide d) conversion of sulfur trioxide to sulfuric acid (a) Extraction of sulfur Easily the most important source of sulfur is its recovery from natural gas and oil. In particular. Many metal ores occur as sulfides and are roasted to form an oxide and sulfur dioxide. both organic and hydrogen sulfide both of which must be removed before they are used as fuels or chemical feedstock. . Worldwide about 35% of the sulfur is obtained as sulfur dioxide from sulfide ore roasting and this is increasing. Another important source of sulfur is as sulfur dioxide from metal refining.8 million tonnes 8. nickel and zinc.Annual production of sulfuric acid World Europe US Russia 200 million tonnes 19 million tonnes 35. in the manufacture of lead: Other metals manufactured from their sulfide ores include copper. China makes most of its sulfuric acid from pyrites. (c) Conversion of sulfur dioxide to sulfur trioxide (The Contact Process) A typical plant contains one cylindrical vessel which acts as a fixed bed reactor with four separate beds of catalyst. 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. vanadium(V) oxide on silica. known as a converter. one tonne of high pressure steam is also produced. by volume. is generally in the form of small pellets. In manufacturing one tonne of sulfuric acid. . heated to 700 K. The gases are cooled to about 700 K and the water in the surrounding boiler pipes is converted into steam.(b) Conversion of sulfur to sulfur dioxide If sulfur is the feedstock. to which caesium sulfate has been added as a promoter (Figure 2). The gases are very hot and so are passed through heat exchangers (waste heat boilers). 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. through which the sulfur dioxide and air pass: The catalyst. The function of the promoter is to lower the melting point of vanadium(V) oxide so that it is molten at 700 K. mainly sulfur trioxide. the conversion of sulfur trioxide to sulfuric acid. as low a temperature as economically possible is used. As shown above. Sulfur trioxide reacts with water and the reaction can be expressed as: . Based on diagram given by Mike Sellars (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 sulfuric acid.5% conversion is needed). However. for a satisfactory yield of sulfur trioxide (above 99. flow to the next stage. By kind permission of Haldor Topsøe Figure 3 A flow diagram of the Contact Process. The sulfur trioxide produced is removed between the third and fourth beds and flows to the next stage. Thus. it is an exothermic reaction so. heat is removed from the gas leaving each bed using heat exchangers.Figure 2 Vanadium(v) oxide catalyst used for the manufacture of sulfuric acid. a small amount of the sulfur dioxide is not converted and is passed through the fourth bed of catalyst and the resulting gases. The gas inlet duct can be seen in the middle of the picture. To keep the temperature at about 400 K. which is difficult to handle. The gas stream is filtered to remove any traces of sulfuric acid mist and is returned to the atmosphere using a high stack. The gases not absorbed contain about 95% nitrogen. and a sulfuric acid mist is formed. water itself cannot be used for absorption as there is a large temperature rise. Instead. the heat is removed by heat exchangers.However. This is kept at this concentration by addition of water and removal of acid at that concentration. and traces of sulfur dioxide. 5% oxygen. Date last amended: 20th December 2013 . Figure 4 A line diagram illustrating a heat exchanger used in the manufacture of sulfur trioxide. Figure 4. sulfuric acid of about 98% concentration is used.