778 



NA TURE 



[December 9, 1922 



sources for its whole supply of fixed nitrogen. No 

 more eloquent testimony to our unpreparedness in 

 this direction could be given than the fact that, while 

 a whole volume is devoted to the manufacture of 

 nitric acid from nitre and sulphuric acid, there is no 

 corresponding report on the supply of fixed nitrogen 

 from the air, for the all-sufficient reason that there 

 never was a supply to describe. Since, however, the 

 two factories of Gretna and of Queen's Ferry alone were 

 making during the war 1300 tons of nitric acid per 

 week, and since, moreover, the loss by submarines of 

 nitre ships from Chile was the cause of incessant anxiety, 

 lest the whole output of explosives should be arrested 

 by even a temporary stoppage of supplies, it was of 

 very great importance that the utilisation of the nitre 

 should be carried out with the highest degree of 

 efficiency. 



As usual, detailed attention resulted in economies 

 which, in an earlier stage, would have appeared to be 

 almost impossible. Striking evidence of the elaborate 

 care that was called for in this very large-scale pro- 

 duction is afforded by the fact that twenty pages of 

 the seventh report are devoted to a description of 

 the plant and process used for washing the sacks in 

 which the nitre was brought from Chile ! This resulted, 

 not only in the saving of a substantial percentage of 

 the precious nitre and in the elimination of a very 

 serious fire risk, but also gave a higher value to the bags 

 themselves ; thus, whereas an unwashed bag could 

 be sold for 2d., there were obtained, after washing, 

 60 per cent, of sound bags at 4jd. each, 39 per cent, 

 of slit bags at 3d., and 1 per cent, of ragged bags at 

 9-25/. per ton of 1400 bags. 



A more obvious source of loss arose from the decom- 

 position by heat of a certain proportion of nitric acid 

 into water, nitrogen peroxide, and oxygen, especially 

 towards the end of the distillation. It is this factor 

 which gives rise to the one important complication of 

 the plant, namely, the provision of towers in which 

 the nitrous fumes can be reconverted into nitric acid 

 by contact with oxygen and water. Since this oxida- 

 tion is relatively slow, it is essential to provide adequate 

 space in the towers, in order that the gases may not 

 pass through them too quickly. Another important 

 point in manufacture is to secure as large a proportion 

 as possible of nitric acid of high strength, since, as the 

 distillation proceeds, more and more water comes over 

 with the acid. In practice the acid was collected in 

 two batches, the receiver being changed when the 

 density of the distillate fell to 1-465, while the fire was 

 extinguished when the density fell to 1-340, although 

 a little more acid distilled over from the hot charge 

 before the retort was tapped. In a typical case, a 

 series of six charges gave 5-05 tons nitric acid in the 

 form of 90 per cent, acid and 3-55 tons in the form of 

 83-3 per cent, acid, giving a total yield of 86-6 per cent, 

 recovered by condensation ; to this must be added, 

 however, an estimated recovery of 5-5 per cent, in the 

 absorption towers, giving a total yield of 92 per cent. 

 The balance of 8 per cent, is due mainly to loss of gases 

 from the towers, especially during the brief period of 

 violent interaction which takes place at an early stage 

 of the distillation ; there is also a small loss of nitric 

 acid in the nitre-cake, from which the last traces of 

 acid cannot profitably be removed. In some instances, 



NO. 2771, VOL. I IO] 



however, a yield of more than 97 per cent, was reached, 

 the total loss being therefore less than 3 per cent. 



An interesting problem arose from the production 

 as a by-product of vast quantities of nitre-cake. This 

 was sometimes thrown away, e.g., by dumping in the 

 sea. since it was difficult to find a commercial outlet 

 for it. It was therefore a profitable process, during 

 an early period of the war, to neutralise nitre-cake 

 with the poorer qualities of caustic soda, and to sell 

 the product to the glass-makers as a substitute for 

 salt-cake. At a later stage, however, manufacturers 

 were persuaded to make more and more use of nitre 

 cake in place of sulphuric acid, and the cake gradually 

 acquired a market value, except at the more outlying 

 factories. On the other hand, the direct-conversion 

 process for the manufacture of ammonium nitrate 

 from sodium nitrate and ammonium sulphate led to 

 the production of vast quantities of sodium sulphate 

 as a by-product, for which no sufficient outlet existed, 

 with the result that two vast glistening pyramids 

 were accumulated as a new object of interest to be 

 seen by travellers on the G.W.R. just before reaching 

 Swindon. As a result of these two factors, the neutral- 

 isation of nitre-cake was changed abruptly from a 

 commercial operation, on which a useful profit might 

 be earned, into one in which the product was of less 

 value than the raw material. Under these conditions 

 the infant industry was abandoned as abruptly as if 

 the ashes of Vesuvius had fallen upon it and converted 

 the plants into a modern Pompeii. 



Sulphuric Acid. 



The manufacture of sulphuric acid involved a 

 two-fold problem : first, the provision of sufficient 

 supplies of chamber-acid, the manufacture of which 

 was as well established as that of nitric acid 

 from sodium nitrate ; and, second, the manufacture 

 of oleum, a far more difficult operation, which might 

 indeed be compared with the fixation of nitrogen, 

 except that the production of oleum had been carried 

 on during many years (although on a restricted scale 

 corresponding with the small normal demand for this 

 material), while the fixation of nitrogen was an 

 altogether novel enterprise in this country. Although 

 several new chamber plants were constructed, they 

 have not formed the subject of a report, perhaps 

 because the production of chamber-acid was very 

 largely left to contractors. On the other hand, new 

 capacity for the manufacture of oleum on a large scale 

 was provided m several Government factories, and 

 the experience gained in constructing and working 

 these plants is described in the fifth report of the 

 series. 



The oleum plants were of two principal types. The 

 first plants (e.g. that at Oldbury) were constructed on 

 the Mannheim system, in which the oxidation of 

 sulphur dioxide to the trioxide is effected by the use 

 of ferric oxide and of platinum in series. In the later 

 plants platinum alone was used as a catalyst. The 

 plants at Queen's Ferry, Gretna, and Avonmouth were 

 constructed on the Grillo system, in which the platinum 

 is supported on a base of calcined magnesium sulphate ; 

 but a plant on the Tentelew system, which is in some 

 respects intermediate between the other two systems, 

 since it employs platinum as the only catalyst, but in 



