44
ACID
AND
ALKALI
away with the lead chambers entirely. The pumping of the acids up to the top of the towers is now always performed by means of compressed air, either in the old “acid eggs,” or more economically in “pulsometers.” Most of the sulphuric acid manufactured is not required to be of higher strength than is furnished by the vitriol chambers, either directly (65 to 70 per cent.), or after a passage through the Glover tower (78 to 80 per cent.) This, for instance, holds good of the acid employed in the manufacture of sulphate of soda and hydrochloric acid from common salt, and in the manufacture of superphosphates. But for many purposes more highly concentrated acid is required. Until quite recently all such acid was made by boiling down the dilute acid, for which purpose a great variety of apparatus has been invented. The first question is always that of material. Lead can be used for the purpose only when the boiling-point of the acid is reduced by means of a vacuum—a plan which has not met with much success. Formerly glass vessels were generally employed and they still survive in England, but elsewhere they are not much used. Porcelain, enamelled iron, for high concentrations even cast-iron without any protection, are also in use. On the Continent platinum vessels have been for a long time almost universal, and they have been recently greatly improved by an internal lining of gold. The second consideration is the form of the vessels; these may be open pans or dishes, or closed retorts, or combinations of both. We also note the Faure and Kessler apparatus, which consists of a platinum pan, surmounted by a double-walled leaden hood, in such a manner that, while the hood is constantly cooled from the outside by water, the thin acid condensing on its inside is carried away without being allowed to flow back into the pan. The majority of acid makers, however, prefer retorts made entirely of platinum, preferably provided by the Heraeus process with a dense, closely adherent coating of gold, including the top or “ dome.” The new Kessler furnace is a very ingenious apparatus, in which the fire from a gas-producer travels over the sulphuric acid contained in a trough made of Volvic lava, and surmounted by a number of perforated plates, over which fresh acid is constantly running down; the temperature is kept down by the production of a partial vacuum, which greatly promotes the volatilization of the water, whilst retarding that of the acid. This furnace is also very well adapted for impure acids, unsuitable for platinum or platinum-gold stills on account of the crusts forming at the bottom of the retorts; and it is more and more coming into use both in Great Britain and on the Continent. A third consideration is the condensation of the vapours formed in the concentrating process ; the further the concentration proceeds the more sulphuric acid they contain. Condensation is a comparatively easy task in the case of platinum apparatus, but with glass or porcelain beakers or retorts it presents great difficulties. In this respect the Kessler furnace has also proved to be very efficacious. The highest strength practically attainable by boiling down is 98 per cent. H2S04, and this is only exceptionally reached, since it involves much expenditure of fuel, loss of acid, and wear and tear of apparatus. The usual strength of the O.V. of commerce, mostly designated by its specific gravity as 168° Twaddell, is from 93 to 95, or at most 96 per cent. H2S04. When attempts are made to push the process beyond 98 per cent., it is found that the acid which distils over is as strong as that which remains behind. Heal “monohydrate” or acid approaching 100 per cent, can be made by Lunge’s process of cooling strong O.V. down to - 16° C. when H2S04 crystallizes out, or by the addition of anhydrous S03 in the shape of fuming acid. Fuming or Nordhausen oil of vitriol, a mixture or
MANUFACTURE
chemical compound of H2S04 with more or less S03, has been made for centuries by exposing pyritic schist to the influence of atmospheric agents, collecting the solution of ferrous and ferric sulphate thus formed, boiling it down into a hard mass (“ vitriolstein ”) and heating this to a low red heat in small earthenware retorts. Since about 1800 this industry had been confined to the north-west of Bohemia, and it survived just till 1900, when it was entirely abandoned—not because its product had become any less necessary, but, quite on the contrary, because the enormously increasing demand for fuming sulphuric acid, arising through the discovery of artificial alizarine and other coal-tar colours, could not possibly be supplied by the clumsy Bohemian process. Other sources of supply had accordingly to be sought, and they were found by going back to a reaction known since the first quarter of the 19 th century, when Doebereiner discovered the combination of SO, and O into S03 by means of spongy platinum. This reaction, now known by the name of the catalytic or contact process, was made the subject of a patent by Philips, in 1831, and was tried later in many ways, but had been always considered as useless for practical purposes until 1875, when it was simultaneously and independently taken up by Clemens Winkler in Freiberg, and by Squire and Messel in London. Both these inventors began in the same way, viz., by decomposing ordinary sulphuric acid by a high temperature into SO.,, O, and PLO (the last of course being in the shape of steam), absorbing the water by sulphuric acid, and causing the S02 and O to combine to S03 by means of moderately heated platinum in a fine state of division. Winkler showed that this division was best obtained by soaking asbestos with a solution of platinum chloride and reducing the platinum to the metallic state, and he described later a specially active kind of “ contact substance,” prepared from platinum chloride at a low temperature. This revival of the synthetical production of S03, at a period when this article had suddenly become of great importance, caused the greatest excitement among chemists and led to numerous attempts in the same direction, some of which were at once sufficiently successful to compete with the Bohemian process. It was soon found that the production of a mixture of S02 and O from sulphuric acid, as above described, was both too troublesome and costly, and after a number of experiments in other directions inventors went back to the use of ordinary burnergas from pyrites and sulphur burners. For a good many years the further development of this industry was surrounded by great mystery, but it is now known that a satisfactory solution of the difficulties existing in the above respect was attained in several places, for instance, at Freiberg and in London, by the labours of the original inventors, Professor Winkler and Dr Messel. These difficulties were mostly caused by the solid impurities, contained in the burner-gases in the shape of flue-dust, especially the arsenic, which after a short time rendered the contact substance inactive, in a manner not as yet entirely understood. Another difficulty arose from the fact that the reaction S02 + O = S03 is reversible, the opposite reaction, S03 = S02 + O setting in but little above the temperature required for the synthesis of S03. As far as is known (so much secrecy having been observed) the best results obtained in various places, save one, did not exceed 67 per cent, of the theoretical quantity, the remaining 33 per cent, of S02 having to be converted into' sulphuric acid in the ordinary lead chambers. As is now known, the exception (undoubtedly the only one until 1899) was the process discovered as early as 1889 by Dr Knietsch, of the Badische Anilin-und-Soda-Fabrik, at Ludwigshafen, but kept strictly secret until 1899, when
