SULPHUR DIOXIDE. Sulphur dioxide (SO,) is found free in considerable quantities in Nature, in certain volcanic emanations, and being an exothermic compound it is readily formed together with a trace of SO, by burn ing sulphur in the air: S Os= SO, + 71.6 cals., and 1 kilo of sulphur develops 2,165 cals. on burning. The formation of SO, by burning solid sulphur occurs without change of volume, that is, from one volume of 0, one volume of SO, is obtained, and from this its composition may also be deduced, because if SO2 were to be formed then from three volumes o 0. one would obtain two volumes of SO.. Sulphur dioxide is manufactured in dustrially by roasting certain metallic sulphides, especially pyrites (sulphides of iron, copper, etc.), in special furnaces: CuS + 110= CuO + SO. and also: 2FeS, + 110= Fe.0, + 4S0. + 419.8 cals., that is, 105 cals. for each molecule of SO..
The SO, from large copper smelting plants is sometimes utilized for the manufacture of sulphuric acid in lead chambers, but cannot be used for the manufacture of catalytic sul phuric acid, because the catalyst soon becomes covered by powdered zinc oxide and the ex cessive amount of moisture renders the acid very dilute. Liquefied sulphur dioxide is now a regular article of commerce and it is generally transported in steel cylinders or flasks, holding approximately 100 kilos. It is also shipped in small cylinders holding about 10 pounds for use in laboratories and medical institutions. Within late years sulphur dioxide has been used quite extensively in the manufacture of ice, particularly in small ice machines manu factured for domestic purposes. This substance is also used extensively as a reducing agent, a bleaching agent, a disinfectant and as a fire extinguisher. Sulphur dioxide is a dry, color less gas of suffocating, disagreeable and pene trating smell; it gives white fumes in moist air; it is harmful to respiration and to vegetation. It is readily liquefied when cooled with ice and salt at a pressure of three atmospheres. In the liquid state it has a specific gravity of 1.53 at 0 F., boils at and solidifies at —76°.
History.— Sulphur dioxide was first lique fied by Monge in Clouet about the beginning of the 19th century. In 1878 R. Pictet prepared liquid sulphur dioxide for his ice machines by allowing a continuous stream of strong sul phuric acid to pass on to molten sulphur, which was placed in a cast-iron retort heated to 400°.
2H.SO. + S =3S0, + 2H.0. Regular develop ment of SO. may be obtained in the laboratory by heating one part of mercury or of copper turnings with three parts of strong sulphuric acid in a flask Cu + 2SO.H. = CuSO. + 211,0 + SO..
Commercial Methods of The general method of commercial manufacture of sulphur dioxide is in accordance with the Hanisch and Schroeder patent, dated 1883 and perfected later (German Patent 52,025 of 1889). Briefly this method consists of die burn ing of pure brimstone in a suitable furnace, the very hot gases from the burner passing through a suitable cooling apparatus and thence to an absorption tower where the SO, gases only are absorbed by cold water which flows to a digester where the solution is heated sufficiently to drive off the SO, gases, which after passing through drying chambers is com pressed into a liquid. One of the functions of the absorption tower just mentioned is to release from the SO. gases any nitrogen or oxygen which escapes from the top of the tower.
Commercial Uses of Formally most of the sulphur dioxide produced was used directly on the spot for the manufacture of sulphuric acid. However, its uses are now varied and greatly extended, and a considerable market is found for it for use in wood pulp factories, paper mills, sugar refineries, bleaching and dye works, ice manufacturing plants, chem ical and metallurgical processes, as well as a disinfectant and fire extinguisher. New fields are constantly appearing where it is found that sulphur dioxide can be used to great advantage. A unique use to which it is occasionally put is to freeze and solidify moist soil around coffer-dams where it is impossible to keep out the water by ordinary piling or sheathing methods.
Medicinal Uses.— Very recently it has been shown by medicinal men that dioxide when properly handled is of great value in stay ing the action of tubercular affection. This opens a very interesting field for the use of this substance, and without doubt other uses, in a medicinal way, will be found for it. As a powerful disinfectant the effects of sulphur dioxide gas are well known.
The manufacture and use of Sulphur Diox ide is described at length in Molinari's