The operation for producing ammonia and regenerating the carbonate of barium by acting upon the cyanide with steam offers no difficulty whatever, and if the temperature is not allowed to exceed 500° C., the results are quantitative. The regenerated carbonate of barium acts actually better than the ground witherite used in the first instance, and if care is taken that no impurities are introduced by the pitch which is used to remake the briquettes and to replace the small amount of carbon consumed at each operation, I see no reason why it should not continue to act for a very long time.
The cyanide is not acted on by carbonic oxide, but carbonic acid destroys it at high temperatures, so that it is not possible to produce it by heating the briquettes directly in a flame free from oxygen, but containing carbonic acid. The process has, therefore, to be carried out in closed vessels, and I designed for this purpose the following apparatus:
Clay retorts of moderate dimensions and thin walls are placed vertically in a furnace, passing through the hearth as well as through the arch of the furnace. These are joined at the bottom to cast iron retorts of the same shape as the earthenware retort. Through a cast iron mouthpiece on the top of the retort the material was introduced, while in the cast iron retort below the material was cooled to the necessary temperature by radiation and by the cold nitrogen gas introduced into the bottom of it. The lower end of the cast iron retort was furnished with an arrangement for taking out from time to time small quantities of the material, while fresh material was in the same proportion fed in at the top. As a source of nitrogen I used the gases escaping from the carbonating towers of the ammonia-soda process. The formation of cyanide of barium from barium carbonate, carbon, and nitrogen absorbs a very large amount of heat—no less than 97,000 calories per equivalent of the cyanide formed—which heat has to be transmitted through the walls of the retort. I therefore considered it necessary to use retorts with very thin walls, but I did not succeed in obtaining retorts of this description which would resist the very high temperatures which the process requires, and for this reason I abandoned these experiments. I was at that time not acquainted with the excellent quality of clay retorts used in zinc works, with which I have since experimented for a different purpose. I have no doubt that with such retorts the production of cyanides by this process can be carried out without great difficulty.
I believe that the process will prove remunerative for the manufacture of cyanogen products, which, if produced more cheaply, may in the future play an important role in organic synthesis, in the extraction of noble metals, and possibly other chemical and metallurgical operations.
The process certainly also offers a solution of the problem of obtaining ammonia from the nitrogen of the atmosphere, but whether this can be done with satisfactory commercial results is a question I cannot at present answer, as I have not been able to secure the data for making the necessary calculations.
I am the more doubtful about this point, as in the course of our investigations I have found means to produce ammonia at small cost and in great abundance from the immense store of combined nitrogen which we possess in our coal fields.
Among the processes for obtaining ammonia from the nitrogen of the air which we investigated, was one apparently of great simplicity, patented by Messrs. Rickman and Thompson. These gentlemen state that by passing air and steam through a deep coal fire, the nitrogen so passed through is to a certain extent converted into ammonia. In investigating this statement we found that the process described certainly yields a considerable quantity of ammonia, but when we burned the same coal at a moderate temperature by means of steam alone in a tube heated from the outside, we obtained twice as much ammonia as we had done by burning it with a mixture of air and steam, proving in this case, as in all others, the source of the ammonia to have been the nitrogen contained in the coal. The quantity of ammonia obtained was, however, so large that I determined to follow up this experience, and at once commenced experiments on a semi-manufacturing scale to ascertain whether they would lead to practical and economic results.
I came to the conclusion that burning coal by steam alone at a temperature at which the ammonia formed should not be dissociated, although it yielded more ammonia, would not lead to an economic process, because it would require apparatus heated from the outside, of great complication, bulk, and costliness, on account of the immense quantity of raw material to be treated for a small amount of ammonia obtainable.
On the other hand, if the coal could be burned in gas producers by a mixture of air and steam, the plant and working of it would be simple and inexpensive, the gas obtained could be utilized in the same way as ordinary producer gas, and would pay to a large extent for the coal used in the operation, so that although only one-half of the ammonia would be obtained, it seemed probable that the result would be economical.
I consequently constructed gas producers and absorbing plant of various designs and carried on experiments for a number of years. These experiments were superintended by Mr. G. H. Beckett, Dr. Carl Markel, and, during the last four years, by Dr. Adolf Staub, to whose zeal and energy I am much indebted for the success that has been achieved. The object of these experiments was to determine the most favorable conditions for the economic working of the process with respect to both the cost of manufacture as well as the first cost and simplicity of plant. The cost of manufacture depends mainly upon the yield of ammonia, as the expenses remain almost the same whether a large or a small amount of ammonia is obtained; the only other item of importance is the quantity of steam used in the process. We found the yield of ammonia to vary with the temperature at which the producer was working, and to be highest when the producer was worked as cool as was compatible with a good combustion of the fuel. The temperature again depended upon the amount of steam introduced into the producer, and of course decreased the more steam increased. We obtained the best practical results by introducing about two tons of steam for every ton of fuel consumed. We experimented upon numerous kinds of fuel, common slack and burgy of the Lancashire, Staffordshire, and Nottinghamshire districts. We found not much difference in the amount of nitrogen contained in these fuels, which varied between 1.2 and 1.6 per cent., nor did we find much difference in the ammonia obtained from these fuels if worked under similar conditions. Employing the quantity of steam just named we recovered about half the nitrogen in the form of ammonia, yielding on an average 0.8 per cent. of ammonia, equal to 32 kilos, of sulphate per ton of fuel. In order to obtain regular results we found it necessary to work with a great depth of fuel in the producers, so that slight irregularities in the working would not affect results. Open burning kinds of slack do of course work with the greater ease, but there is no difficulty in using a caking fuel, as the low temperature at which the producers work prevents clinkering and diminishes the tendency of such fuels to cake together.