|Of the total annual output of nearly 2½ million tons of Sulphur Extraction, almost the whole is performed by the United States and Sicily. The production figures for 1928 were as follows: |
|United States||1,981,873 tons.|
|Italy (including Sicily)||322,481 tons|
|Spain, sulphur rock||74,632 tons|
|Spain, refined sulphur||19,491 tons|
|Japan, sulphur rock||13,109 tons|
|Japan, refined sulphur||68,962 tons|
|United Kingdom and Irish Free State, spent oxide (average sulphur content 48.8 per cent.)||174,000 tons|
The remarkable rise in recent years of the industry in the United States is largely due to the introduction of new methods in mining the subterranean deposits. In Sicily and Italy, where the extraction is an old industry, the " ore " rarely contains much above 40 per cent, of sulphur and is considered rich if above 30 per cent. The process of extraction is based on the liquation of the element. Although attempts have been made to supply the necessary heat, directly or indirectly, by means of ordinary fuels, no system has proved so cheap as that utilising the heat of combustion of a part of the sulphur itself. The old calcarone method, which is now rapidly becoming obsolete, consisted in building up, usually against the side of a hill and on a sloping floor, a large heap (calcarone) of sulphur ore in such a manner as to give a mass containing vertical air channels; the outside of the heap was protected by a coating of earth or other mineral substance. The mass was then fired near the base, and by carefully regulating the course of the combustion, which lasted for several weeks, the proportion of sulphur lost by atmospheric oxidation (and by the reduction of calcium sulphate to sulphide) could be limited to approximately one-third, the remainder collecting on the floor, whence it was periodically run off into rectangular moulds. The product thus obtained contained 85 to 90 per cent, of sulphur.
The foregoing process, besides being uneconomical, suffered great disadvantage owing to the liberation of immense volumes of sulphur dioxide into the atmosphere. This evil is mitigated to a considerable extent in the more modern process. The sulphur ore is placed in a group or cycle of small kilns (fornelli), built of brickwork and so arranged that the hot gases generated in the first kiln pass successively through the remainder of the kilns, the full number of kilns frequently being six. In some cases the kiln is in the form of an oven covered by a cupola, inside which is a smaller cupola containing a coke fire. By the time fusion is complete in the first oven, the contents of the second are already heated to burning temperature, so that the heat of combustion of the sulphur is more completely utilised and a smaller proportion of sulphur dioxide produced. The pollution of the atmosphere relative to the output of sulphur is thus diminished; a further diminution in the deleterious influence of the sulphur dioxide on vegetation can be effected by restricting the process to certain periods of the year, although with the newest types of plant this is not necessary. The yield of sulphur obtained by these later methods is much larger (sometimes by 50 per cent.) than by the calcarone method, and the working is much more rapid, the reaction in the kilns generally being complete after a few days.
The most important deposits of sulphur in the world are those of Louisiana and Texas, U.S.A., where H. Frasch's method of extraction is followed. This obviates the formation of sulphur dioxide and at the same time yields a product of such a degree of purity (in some wells as high as 99*9 per cent.) as to be suitable in most cases for direct use. A boring is made in the earth down to the sulphur stratum so that a continuous pipe can pass thence to the surface; the pipe consists of three concentric tubes (see fig.). Superheated water, e.g. at 150° C., is forced down the annular spaces A, A, between the outer tubes, in order to melt the sulphur in the neighbourhood of the end of the boring. A blast of heated air down the inmost tube, B, then causes the molten sulphur to be carried up between the two inner tubes to the surface, where it is collected.
| Lower End of Pipe Used in Frasch Process |
Smaller quantities of Sulphur Extraction can be done in the preliminary roasting of certain sulphide ores where the desulphurised mass is required for further operations. Iron pyrites itself is sometimes treated in this way, the residual ferrous sulphide being applied to purposes such as the manufacture of sulphuric acid.
The iron oxide used in the purification of coal gas gradually becomes richer in sulphur and can repeatedly be revivified by exposure to air until the sulphur amounts to about 40 per cent, of the whole; the mixture is then of greater value as a source of sulphur. The " spent oxide," as it is commonly termed, is frequently applied to the production of sulphur dioxide or sulphuric acid, but the extraction of its sulphur by carbon disulphide has been effected as a successful commercial process; using the system of " counter-currents " the mass is exhausted of sulphur, whilst a saturated solution of sulphur is obtained, from which the solvent can be distilled and Sulphur returned to the extraction process.
In many coal- and oil-gas works in the Western States of America the Koppers process for the removal of hydrogen sulphide from the gas is employed. This consists of passing the gas up towers filled with wooden hurdles over which a dilute sodium carbonate solution is sprayed. The following reaction occurs:
Na2CO3 + H2S = NaHCO3 + NaHS;
the sulphur is then recovered from the solution by agitation in intimate contact with air in an emulsifying machine, a catalyst also being present:
NaHCO3 + NaHS + ½O2 = S + Na2CO3 + H2O.
The catalyst used is colloidal nickel sulphide. The sulphur is run off from the surface, and after filtering and washing is obtained as a paste containing 55 per cent, of water. After drying, a product is obtained which is more toxic than most other forms of sulphur and is very effective in agricultural work as a fungicide; the particles are almost of colloidal size, most being less than 15 μ.
A further method of obtaining sulphur from sulphide ores consists in heating the latter under high vacuum; the sulphur vaporises and is condensed in a suitable receiver.
Sulphur Extraction can be performed from alkali waste.