Especially Adapted To The Manufacture Of Sulphate Of Ammonia. Inventor, Alex. Angus Croll
By George Anderson, of London.
The paper I have to lay before you describes the last product of the brain of one of your past presidents - Alexander Angus Croll - in connection with our industry. It may not be so well known to some of the younger as it is to many of the older members of the Institute that the fertile brain of Mr. Croll has done much for the improvement and the extension of the gas industry. I consider that he has been the most successful pioneer both in the cheapening and the purification of gas - two elements without which our industry would progress but slowly if at all; and the success which has crowned his efforts, to our advantage, has reflected itself favorably on himself, showing by his financial success that he has also been a good man of business. All these are conditions which enhance the value of this paper. In the present instance, I claim no other credit than that of being the mouthpiece of Mr. Croll, whose assistant I was for ten of the busiest and most important years of his eventful life; and having (with my son Bruce) taken part in the experiments, I have been asked to describe the process to the Institute.
The manufacture of sulphate of ammonia, as hitherto conducted, has consisted either in bringing together sulphuric acid and ammoniacal liquor or in distilling the liquor by external heat, or by the introduction of steam, and bringing it into contact with the acid in the form of gases and vapor of water. In either case a large volume of noxious gases is given off, the chief of which, being sulphureted hydrogen, has to be fixed by another method, in order to comply with acts of Parliament for the prevention of nuisances.
By the processes hitherto used, we sometimes get only 1¼ tons of salts to every ton of acid used; while in the more perfect forms of apparatus, we may get 1-1/3 tons of salts. By Mr. Croll's process, however, we get an increased yield of salts on the acid used, as follows: The experiments were made with sulphuric acid of the specific gravity of 1838, or nearly concentrated oil of vitriol; and the quantity used was 8 ounces in each experiment. The ammoniacal liquor was of uniform strength throughout all the experiments, being kept in a corked jar; and the solution of sulphate of ammonia was passed through filter paper before being crystallized. Thus we obtained a white salt. In each experiment the solution of sulphate was divided into four equal parts by weight, and one part filtered and crystallized to dryness over a spirit lamp; the weight in each experiment being as nearly as possible the same, or 3¼ oz. of salt to 2 oz. of acid - being in the proportion of 26 oz. of sulphate to 1 lb. of acid, or 32½ cwt. of salts to 20 cwt. of acid.
The results surprised me; and being uniform over a number of experiments, pleased me. Still, I preserved the character of a critic and said: "I should like to treat 8 oz. of acid in the ordinary way - saturating it with ammoniacal liquor, and then crystallizing it." "Oh!" Mr. Croll said, "we know what that will produce." I replied: "Yes; but I would like to do it with the precise acid and liquor we have been using, so that we may have the experiment on all fours with yours, barring your process." These experiments were made at his country residence. I was staying there for the night. So next morning I got down before him, went at my experiment, saturated 8 oz. of acid (and a nice smell I made) out in the grounds, treated it afterward by division into four parts, filtered and crystallized it, all as before, with the result that I obtained 2¾ oz., as against his 3½ oz. - or in the proportion of 27½ cwt. of salt to the ton of acid, as against his 32½ cwt.
I now thought of business. "What is the royalty to be?" I said, as we sat at breakfast. This we settled as we Scotch say "in a crack," or as an Englishman would say "in a jiffy." Mr. Croll decided to have the apparatus put up on a manufacturing scale here in Glasgow; and I determined to erect similar apparatus at one of my gas works.
I dare say that it will be uppermost in your minds, Whence comes the increased yield of salts? Well, I will state one fact, and leave you to ruminate on it, namely, by Mr. Croll's process we did not seem to produce any sulphureted hydrogen. The experiments were conducted in a room with ordinary doors and windows, but without a chimney; and we were not troubled with any offensive smell - a state of things that could not possibly have existed had we been experimenting with any other apparatus hitherto employed in the manufacture of sulphate of ammonia. The apparatus, which will presently be described, only substitutes, for the present mode of distillation, a new one, which forms the subject of Mr. Croll's patent. All other parts of present apparatus can remain as they now exist.
Mr Croll has also introduced another mode of producing sulphate of ammonia, which dispenses with all the apparatus hitherto in use after the distillatory portion, and produces the salt in a state fit for the farmer, ready to be put on the land. This process consists in sending the products of distillation through a vessel filled with wood sawdust saturated with sulphuric acid. The ammonia becomes fixed and crystallized in the sawdust, and is ready for use. There are many works, both at home and abroad, to which the conveyance of sulphuric acid is both difficult and expensive, on account of the cost of carriage and the breakage which occurs; and thus in many such works the ammonia is not utilized. This saturated sawdust process will, I think, remove the difficulty; for I find that dry sawdust absorbs double its own weight of sulphuric acid, and this could be conveyed in the most ordinary casks in a damp state, and save all waste and annoyance from breakage of bottles. In this state it could be used by the farmer, or the sulphate of ammonia could be washed out, crystallized, and exported in the state of salt.
In the remainder of this paper I have been assisted by my son Bruce, who also assisted in the experiments that I have described. He has since been engaged on the trials on a manufacturing scale; and I ask you to permit him to read the concluding portion of the paper, in which he will describe the process, and what he has done.
The process referred to in the foregoing portion of the paper is a method employed for heating the liquor, whereby a chemical action is brought into play, with the results already mentioned. This method consists in passing the products of combustion of a furnace from a clear fire in a hot state through a still containing the ammoniacal liquor. The hot gases from the furnace impart their heat to the liquor, causing the volatilization of the condensed gases, and at the same time act chemically upon the liquor and evolved gases, so that ammonia and sulphuric acid are resulting products, in the compound state of sulphate of ammonia. The formation of the ammonia produced in the process is probably due to the decomposition of nitrogenous bodies contained in solution in the liquor - the sulphocyanide, for instance; the nitrogen being given off in the form of ammonia. Of the sulphuric acid produced, we look upon the sulphureted hydrogen as the source, also any sulphites existing in the liquor, which in their volatile state take up the atom of oxygen necessary for their conversion into sulphate.
The apparatus used in working the process consists of a tower still, containing a number of superposed trays about 3 or 4 inches apart, with a lipped hole through the bottom of each at the side. The trays are so placed in the tower that the holes are at alternate sides. The liquor passes into the top of the still, and zigzags down through the series of trays, as in an ordinary Coffey still. The bottom tray differs from the rest; being much deeper, and having holes through it connecting it with the furnace, which is set immediately below it. The products of combustion of the fuel are caused to pass from the furnace up through the holes in the trays in the still, and, together with the gases evolved from the liquor, are directed into the saturator, where the sulphate of ammonia is obtained either in solution or in the crystalline state.
Where the process is at present being worked, an exhauster is used to draw the furnace gases through the still; but it might be advantageous to use a blower.
A small plant has been put in action at the gas works in Kilkenny and another on a larger scale, and differing somewhat in detail, here in Glasgow at the Alum and Ammonia Company's works, where the liquor from the Tradeston Gas Works is converted. The trials on a working scale have only been made at both places within the past ten days; and, so far, nothing has appeared against the principle, though in certain of the details of construction some alterations are being made to improve it. The extra yield of salt from a given quantity of acid obtained in the experiments has been proved in practice, as also the absorption of the sulphureted hydrogen.
The other day, while ammoniacal liquor of about 9 oz. strength was being run at the rate of 70 gallons per hour through the still, 5 feet in diameter and 10 feet high, containing seventeen trays, no smell of sulphureted hydrogen was perceptible from the waste gases from the saturator, although on applying lead paper a slight trace of this impurity was noticeable, and it may be stated that the gases were being delivered at the ground level, where there was no difficulty in testing them.
In the Glasgow apparatus we have found it advisable to enlarge the pipe leading the gases into the saturator, as the volume of these is much greater than would be the case in the ordinary method of working. Further experience will probably indicate the desirability of increasing the height of the still, which, being only 10 feet, is not more than half the height that Coffey stills are ordinarily made.
Read at the recent meeting of the Gas Institute, Glasgow.