Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/62

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ACID

AND

ALKALI

MANUFACTURE

pounds cannot be entirely separated. The manufacture of constructed for the purpose, very large quantities of sulacids and alkali, understood as above, is by far the most phuric acid are now made from blende, especially in important of inorganic chemical industries, and occupies Germany and Belgium, and to some extent also in Engthe great bulk of the capital and labour employed in that land and America. direction. It is, at the same time, the indispensable groundThe roasting of pyrites always takes place without work of all organic chemical industries, as well as of glass- using any extraneous fuel, the heat given off by the oxidamaking, paper-making, and many other branches of human tion of the sulphur and the iron being quite sufficient to activity. We can here give merely an outline of the carry on the process. If the ore is in pieces of the size of progress made in these manufactures during the last a walnut or upwards, it is roasted in plain “ kilns ” or twenty-five years, and of their actual state and their “ burners,” provided with a grating of suitable construction prospects for the immediate future. for the removal of the cinders, with a side door in the Sulphuric Acid (comp. Ency. Brit. vol. xxii. p. 636).— upper part for charging in the fresh ore on the top of the Nearly all sulphuric acid, in its various states of dilution partially burned ore, and with an arch-shaped roof, from with water, is still made by the old “ lead-chamber process,” which the burner-gas is carried away in a flue common to while all sulphuric anhydride, S03, together with its solu- a whole set of kilns. The latter are always set in a row tions in sulphuric acid, known as fuming or Nordhausen of twelve or more, and are one after another charged once oil of vitriol, is now made by the “contact process.” The or twice a day at appropriate intervals, so that a regular last-named process is probably destined to supersede the evolution of gas takes place all the day round. By emlead-chamber process in the near future for the manu- ploying suitable precautions, a gas of approximately facture of concentrated sulphuric acid, i. e., acid containing uniform composition is obtained, containing from 6 to 8 92 per cent. H2S04 and upwards. Acid, however, of less per cent. S02, with a little S03, and about 12 per cent, concentration (80 per cent, and below) has so far been of oxygen, which is more than sufficient for converting made more advantageously by the old process, and this later all the &i02 into S03 or H^SO^. The burning of state of affairs will probably continue, at least until the “ smalls ” or “ dust ” was formerly considered much more respective patents have run out, especially since the large difficult and incomplete than that of pieces, but this difficapital sunk in lead chambers is necessarily looking for culty has been entirely overcome in various ways, prinsome return, and is not likely to be abandoned without a cipally by the “shelf-burner,” originally constructed by determined struggle. Maletra, and by mechanical burners, which were formerly The first step is common to both processes, viz., the almost entirely confined to America, where the saving of manufacture of more or less dilute sulphur dioxide from labour is a primary consideration. Quite recently the elementary sulphur, or from such sulphur compounds as Herreshoff mechanical burner (developed from a burner readily act upon atmospheric oxygen with formation of constructed many years ago by M‘Dougal Bros., of Liversulphur dioxide (sulphurous anhydride, S09). The ele- pool) is making its way also in Europe. The roasting of mentary sulphur is generally native brimstone, though in blende is nothing like so easy as that of pyrites, since the exceptional cases use is made of the sulphur recovered as heat developed by the oxidation of the zinc sulphide a by-product in the manufacture of coal-gas, or from itself is not sufficient for carrying on the process, and exalkali waste, &c. The great bulk of the brimstone of ternal heat must be applied. It is now usually performed by commerce still comes from Sicily (comp. Ency. Brit. vol. a series of muffles, superposed one over another, so that the xxii. p. 634); in comparison with this source of supply, whole forms a kind of shelf-burner, with internally heated the brimstone obtained in Japan and a few other localities shelves (the “ Rhemania ” furnace). This operation is plays but a very secondary part, and the great deposits of both more costly and more delicate than the roasting of sulphur stated to exist in other localities, such as the pyrites, but it is now perfectly well understood, and gas is southern states of North America, Mexico, Daghestan, Ac., obtained from blende furnaces hardly inferior in quality to have not as yet been made available to any considerable that yielded by pyrites kilns. In America, and quite extent. The production of sulphur dioxide from element- exceptionally also in Europe, mechanical furnaces are used ary sulphur seems a very simple operation; yet room has for the roasting of blende. been left for many improvements made during the last Both kinds of gas, when issuing from the burner, hold decades by the introduction of continuously working in mechanical suspension a considerable quantity of “ flueburners, and by the utilization of the heat generated during dust,” which must be removed as far as is practicable the process. In most localities brimstone is too expensive before the gas is subjected to further treatment. Fluean agent for the manufacture of sulphuric acid; in Eng- dust contains principally ferric oxide, zinc oxide, arsenious land it is used to a very limited extent for the manufac- and sulphuric acids, and small quantities of the various ture of acid free from arsenic, and to a much larger metals occurring in the raw ore. All the thallium and extent in North America and Japan, but hardly at all in selenium on the market is obtained from this source. other countries. The great bulk of sulphuric acid is made Sometimes the burner-gas is employed directly for the by “roasting” metallic sulphides, principally pyrites and sake of the S02 which it contains, principally in the blende.' Pyrites in the pure state, FeS2, occurs in many manufacture of “ sulphite cellulose ” from wood. If it is countries in large quantities; most of that burned in the to be utilized for the manufacture of sulphuric acid, the United Kingdom comes from Spain, and contains a small practice up to the present has been, with few exceptions, quantity of copper which is extracted from the residues to carry it into “ lead chambers ” (vitriol chambers), which ( cinders ). Spanish pyrites is also used in Germany, are immense receptacles constructed of sheet-lead burned France, and America, together with the pyrites mined in together without any solder and suitably supported outthose countries. Good pyrites contains from 48 to 50 per side by wooden or iron framework. In these the sulphur cent., and exceptionally good, 52 per cent, of sulphur, of dioxide acts upon the oxygen contained in the same gas, which from 1 to 4 per cent, or upwards is left in the and upon water introduced as a spray or in the shape of cinders. Blende, the most important of zinc ores, contains steam, and the reaction S02 + O + H20 = H9S04 is brought only about half as much sulphur, which is, moreover, about. As, however, this reaction of its own accord takes burned off with much more difficulty; but as this has to be place only to a very small extent, an “ oxygen carrier ” is done in any case, in order to prepare the ore for the ex- always introduced in the shape of the vapours of nitric traction of the metal, and as suitable apparatus has been acid or the lower oxides of nitrogen. By the play of