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 ACID

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ALKALI

the patents were published. The principal features of this invention are, first, a much more thorough purification of the burner-gas than had been practised up to that time, both in a chemical and a mechanical sense, and second, the prevention of superheating of the contact substance, which formerly always occurred by the heat generated in the process itself. As the Badische process effects this prevention by cooling the contact apparatus by means of the gaseous mixture to be later submitted to the catalytic action, the mixture is at the time heated up to the requisite temperature, and a considerable saving of fuel is the consequence. Altogether this process has been brought to such a pitch of simplicity and perfection, that it is cheap enough, not merely for the manufacture of fuming oil of vitriol of all strengths, but even for that of ordinary sulphuric acid of chamber-acid strength, while it is decidedly cheaper than the old process in the case of stronger acids, otherwise obtained by concentration by lire. It should be noted that these are not the results of a few years’ working with an experimental plant, but of many years’ work with large plant, now equal to a capacity of 120,000 tons of pyrites per annum. It is therefore not too much to say that, in all probability, the contact process will ultimately be employed generally for concentrated acids. Still, for the reasons given in the beginning of this article, the revolution thus impending will require a certain time for its accomplishment. Since the Badische process has become known, several other new contact processes have come into the field, in some of which ferric oxide is employed as contact substance, but we must refrain from describing these in detail. Hydrochloric Add and Sulphate of Soda.—As long as the Leblanc process was paramount in alkali manufacture, the decomposition of sodium chloride (common salt) by sulphuric acid was in the first instance carried on for the purpose of obtaining sulphate of soda, commercially known as “ salt-cake.” Hydrochloric acid (muriatic acid) was a necessary by-product of the reaction : 21SraCl + H.,S04 = Na2S04 + 2HC1, but in some cases it was altogether treated as a waste-product, which at the same time constituted a great nuisance. Generally it was more or less perfectly recovered, but only exceptionally was it utilized to the fullest possible extent. The technical progress of the manufacture of hydrochloric acid (comp. Ency. Brit. vol. v. p. 678, and vol. xxii. p. 243) was promoted, to an extent perhaps unique in the history of legislative efforts, by the passing 0f the Alkali Acts, in 1863, 1874, 1881, and 1892, owing to the exemplary way in which the duties as inspector under these Acts were carried out by the late Dr R. Angus Smith and his successors, who directed their efforts not merely to their primary duty of preventing nuisance, but quite as much to showing manufacturers how to make the most of the acid formerly wasted in one shape or another. Hot merely Great Britain but all mankind has been immensely benefited by the labours of the British alkali inspectors, which were of course supplemented by the work of technical men in all the countries concerned. The scientific and technical principles of the condensation of hydrochloric acid are now thoroughly well understood, and it is possible to recover nearly the whole of it in the state of strong commercial acid, containing from 32 to 36 per cent, of HC1, although probably the majority of the manufacturers are still content to obtain part of the acid in a much weaker state, merely to satisfy the requirements of the law prescribing the prevention of nuisance. The principles of the condensation, that is of converting the gaseous HC1 given off during the decomposition of common salt into a strong solution of hydrogen chloride in water, can be summarized in a few words. The vapours of HC1, which are

MANUFACTURE

45 always diluted with air, sometimes to a very great extent, must be brought into the most intimate contact possible with water, which greedily absorbs the gas, forming ordinary hydrochloric acid, and this process must be carried so far that scarcely any HC1 remains in the escaping gases. The maximum escape allowed by the Alkali Acts, viz., 5 per cent, of the total HC1, is far above that which is now practically attained. For a proper utilization of this condensed acid it is nearly always imperative that it should be as strong as possible, and this forms a second important consideration in the construction of the condensing apparatus. Since the solubility of HC1 in water decreases with the increase of the temperature, it is necessary to keep the latter down—a task which is rendered somewhat difficult both by the original heat retained by the gases on their escape from the decomposing apparatus and by the heat given off through the reaction of HC1 upon water. Very different methods have been employed to effect all the above purposes. In Great Britain Gay-Lussac’s coke towers, adapted by Gossage to the condensation of hydrochloric acid, are still nearly everywhere in use, frequently combined with a number of stone tanks through which the gas from the furnaces travels before entering the towers, meeting on its way the acid condensed in the tower. This process is excellent for effecting a complete condensation of the HC1 as prescribed by the Alkali Acts, and for recovering the bulk of the acid in a tolerably strong state, but less so for recovering nearly the whole of it in the most concentrated state, although even this is occasionally attained. On the Continent, where the lastnamed requirement has been for a long time more urgent than in Great Britain, another system has been generally preferred, namely, passing the gas through a long series of stoneware receivers, and ultimately through a small tower packed with stoneware or coke, making the acid flow in the opposite direction to the gas. Great success has also been obtained by specially - constructed “ plate towers ” (already mentioned in connexion with their application to the manufacture of sulphuric acid), which allow both the coke towers and most of the stoneware receivers to be dispensed with. Improvements in the construction of the condensing apparatus had to go hand in hand with those in the construction of the “salt-cake furnaces” for treating the common salt with sulphuric acid. The ordinary way of effecting this treatment is to carry out the first reaction in cast-iron pans or “pots,” fired from below, and to complete it by applying a higher temperature in a “roastingfurnace.” If the latter is constructed as a reverberatory furnace, where the HC1 given off is mixed with the products of the combustion of the fuel, the acid obtained during this part of the operation is necessarily weak. It is therefore altogether preferable to construct the roastingfurnace as a “ muffle,” heated only on the outside, although the expenditure of more fuel is thus involved. The drawback formerly caused by leakages of acid gases into the fire-flues through cracks in the muffle has been removed by the construction of “ plus-pressure furnaces.” Many and often very costly attempts have been made to construct mechanical salt-cake furnaces. Of these Mactear’s furnaces have met with the greatest success, though this is limited by the fact that, apart from their great cost, they yield the hydrochloric acid in a weaker state than hand-wrought furnaces. For this reason they have been abandoned in some places, and are not likely to be generally introduced. The same holds good of the “direct salt-cake process” of Hargreaves and Robinson, in which salt is subjected to the action of burner-gases and steam at a low red heat, thus effecting the reaction: