By LUDWIG MOND.
[Footnote 1: A paper read at the annual general meeting of the
Society of Chemical Industry, London, July 10, 1889.]
As exemplifying to a certain extent the application of methodical
research to an industrial problem, I propose to bring before you
to-day an account of the work I have been engaged in for many years in
relation to the procuring of new and abundant supplies of ammonia, and
to investigations connected therewith.
Through the classic researches of Lawes and Gilbert, who proved, in
opposition to no less an authority than Liebig, that ammonia is a most
valuable manure which enables us not only to maintain, but to
multiply, the yield of our fields, and thus to feed on the same area a
much larger number of inhabitants, the immense importance of an
abundant supply of ammonia, more particularly for the Old World, with
its teeming population and worn-out soil, has been apparent to every
one.
For many years Europe has paid to South America millions upon millions
of pounds for ammonia in the shape of guano, and more recently, since
the supply of guano practically ceased, for nitrate of soda, which
effectually serves the same purpose as ammonia. During the past year
South America exported 750,000 tons of nitrate, of which 650,000 went
to Europe, representing a value of not less than 6,500,000l.
The problem of saving this immense expenditure to Europe, of making
ourselves independent of a country so far away for the supply of a
material upon which the prosperity of our agriculture--our most
important industry--depends, by supplying this ammonia from sources at
our own command, is certainly one of the most important which our
science has to solve.
It is more than 100 years since Berthollet ascertained that ammonia
consists of nitrogen and hydrogen, two elements which we have in great
abundance at our command, and innumerable attempts have been made
during this century to produce this valuable product by the direct
combination of the elements, as well as by indirect means. It has been
equally well known that we are in possession of three abundant sources
of nitrogen:
(1.) In the shape of matter of animal origin.
(2.) In the shape of matter of vegetable origin.
(3.) In the atmosphere, which contains no less than 79 per cent.
of uncombined nitrogen.
In olden times ammonia was principally obtained from animal matter,
originally in Egypt by the distillation of camel dung, later on from
urine, and from the distillation of bones and horn. The quantity so
obtained was very small and the products very expensive. The
introduction of coal gas for illumination gave us a considerable and
constantly increasing supply of ammonia as a by-product of the gas
manufacture, and until recently all practical efforts to increase our
supply of ammonia were directed toward collecting and utilizing in the
best possible manner the ammonia so obtained. The immense extension of
the coal gas industry all over the world has in this way put us into
possession of a very considerable amount of sulphate of ammonia,
amounting in Europe now to 140,000 tons per annum. In recent years
this has been augmented by the ammonia obtained by the distillation of
shale, by the introduction of closed ovens for the manufacture of
coke, combined with apparatus for condensing the ammonia formed in
this manufacture, and also by the condensation of the ammonia
contained in the gases from blast furnaces working with coal. But all
these new sources have so far added only about 40,000 tons of
sulphate of ammonia to our supply, making a total of 180,000 tons per
annum, of which about 120,000 are produced in the United Kingdom,
while we still import 650,000 tons of nitrate of soda, equivalent to
500,000 tons of sulphate of ammonia, to make up our requirements.
Many processes have from time to time been proposed to obtain ammonia
from other sources. The distillation of turf, which contains upward of
3 per cent. of nitrogen, has received much attention, and a large
number of inventors have endeavored to produce ammonia from the
nitrogen of the air; but none of these processes has to my knowledge
been successful on a manufacturing scale.
My attention was called to this subject at an early part of my career.
Already, as far back as 1861, I undertook experiments to utilize, for
the production of ammonia, waste leather, a waste material of animal
origin at once abundant and very rich in nitrogen, containing from 12
per cent. to 15 per cent. of this element. Distillation in iron
retorts yielded about half the nitrogen of this material in the form
of ammonia, the carbon remaining in the retorts containing still from
6 per cent. to 8 per cent. Distillation with a moderate quantity of
hydrate of lime increased the yield of ammonia only by 1 per cent. to
1½ per cent. A rather better result was obtained by distilling the
ground residual carbon with hydrate of lime, but this operation
proceeded very slowly, and the total yield of ammonia still remained
very far below the quantity theoretically obtainable, so that I came
to the conclusion that it was more rational to utilize the leather,
reduced to powder by mechanical means, by mixing it directly with
other manures.
A few years later I became connected with a large animal charcoal
works, in which sulphate of ammonia was obtained as a by-product. Here
again I was met with the fact that the yield of ammonia by no means
corresponded with the nitrogen in the raw material and that the
charcoal remaining in the retorts contained still about half as much
nitrogen as had been present in the bones used.
From this time forward my attention was for many years given
exclusively to the soda manufacture, and it was only in 1879 that I
again took up the question of ammonia. I then determined to submit the
various processes which had been proposed for obtaining ammonia from
the nitrogen of the air to a searching investigation, and engaged Mr.
Joseph Hawliczek to carry out the experimental work.
These processes may be broadly divided into three classes:
(1.) Processes which propose to combine nascent hydrogen with
nitrogen at high temperatures or by electricity, with or without
the presence of acid gases.
(2.) Processes in which nitrides are first formed, from which
ammonia is obtained by the action of hydrogen or steam.
(3.) Processes in which cyanides are first formed and the ammonia
obtained from these by the action of steam.
We began with an investigation of those processes in which a mixture
of steam and nitrogen or of steam and air is made to act upon coke at
a high temperature, sometimes in the presence of lime, baryta, or an
alkali, sometimes in the presence of hydrochloric acid.
Very numerous patents have been taken out in this direction and there
is no doubt that ammonia has been obtained by these processes by many
inventors, but as I was aware that coke contains a considerable
quantity of nitrogen, frequently as much as 1.5 per cent., which might
be the source of the ammonia obtained, I determined to carry on the
investigation in such a way as to make quite certain whether we
obtained the ammonia from the coke or from the nitrogen of the
atmosphere, or from both. For this purpose we made for every
experiment carried on by a mixture of nitrogen or air with steam
another experiment with steam alone, carefully excluding nitrogen from
the apparatus. A very large number of experiments carried on at
carefully determined temperatures, ranging from 500° to 1,200°C., and
in which the directions given by the various inventors were most
carefully observed, all led to the same result, viz., that the
quantities of ammonia obtained were the same whether nitrogen was
introduced into the apparatus with the steam or whether steam alone
was used, thus proving conclusively that the ammonia obtained was
derived from the nitrogen contained in the coke.
Further, on carefully determining the nitrogen in the coke used, it
was found that the quantity of ammonia we had obtained in burning coke
in a current of nitrogen and steam very nearly corresponded with the
total nitrogen in the coke, so that we subsequently made our nitrogen
determinations in the coke by simply burning it in a current of steam.
A process belonging to this class, proposed by Hugo Fleck, in which a
mixture of carbonic oxide, steam, and nitrogen is made to pass over
lime at a moderate red heat in order to obtain ammonia, was also
carefully tried. It was claimed for this process that it produced
nascent hydrogen at temperatures at which the ammonia is not
dissociated, and for this reason succeeded where others had failed. We
found that a considerable amount of hydrogen was obtained in this way
at a temperature not exceeding 350°C., and that the reaction was
nearly complete at 500°C.; but although we tried many experiments over
a great range of temperatures, we never obtained a trace of ammonia by
this process.
Among experiments with processes of the second class, based upon the
formation of nitrides and their subsequent decomposition, the nitrides
of boron and titanium had received most attention from inventors. The
nitride of boron, which is obtained by treating boracic acid with
carbon in the presence of nitrogen, when acted upon by steam, forms
boracic acid again and yields the whole of its nitrogen in the form of
ammonia, but the high temperature at which the first reaction takes
place, and the volatility of boracic acid in a current of steam, make
it impossible to utilize this reaction industrially.
There seemed to be a better chance for a process patented by M.
Tessier du Mothay, who proposed to bring a mixture of nitrogen and
hydrogen into contact with titanium nitride and thus to form ammonia
continuously. Titanium is the only element of which we know at present
several combinations with nitrogen, and the higher of these does, on
being acted upon by a current of hydrogen at an elevated temperature,
produce ammonia and a lower nitride of titanium; but this lower
nitride does not absorb nitrogen under any of the conditions under
which we tried it, which explains the fact that if we passed a current
of hydrogen and nitrogen over the higher nitride, we at first obtained
a quantity of ammonia corresponding to the quantity which the nitride
would give with hydrogen alone, but that the formation of ammonia then
ceased completely.
Thus far we had quite failed to get the nitrogen of the air into
action.
With the third class of processes, however, based upon the formation
in the first instance of cyanides, we found by our very first
experiments that the nitrogen of the atmosphere can be easily led into
combination. A few experiments showed that the cyanide of barium was
much more readily formed than any other cyanide; so we gave our full
attention from this time to the process for obtaining ammonia by means
of cyanide of barium invented by MM. Margueritte and Sourdeval. This
process consists in heating a mixture of carbonate of barium with
carbon in the presence of nitrogen, and subsequently treating the
cyanide of barium produced with steam, thus producing ammonia and
regenerating the carbonate of barium. A great difficulty in this
process is that the carbonate of barium fuses at high temperatures,
and when fused attacks fireclay goods very powerfully.
We found that this can be overcome by mixing the carbonate of barium
with a sufficient quantity of carbon and a small quantity of pitch,
and that in this way balls can be made which will not fuse, so that
they can be treated in a continuous apparatus in which the broken
briquettes can be charged from the top, and after treatment can be
withdrawn from the bottom.
We found that the formation of cyanides required a temperature of at
least 1,200° C., and proceeded most readily at 1,400° C., temperatures
which, although difficult to attain, are still quite within the range
of practical working, and we found no difficulty in obtaining a
product containing 30 per cent. of barium cyanide, corresponding to a
conversion into cyanide of 40 per cent. of the barium present.
We found, however, that the cyanide when exposed to the atmosphere at
a temperature above 300° C. is readily destroyed under reformation of
carbonate of barium, so that it is absolutely necessary to cool it
down to this temperature before exposing it to the atmosphere, a fact
of great importance that had hitherto been overlooked.
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.
The quantity of steam thus required to obtain a good yield of ammonia
is rather considerable, and threatened to become a serious item of
expense. Only one-third of this steam is decomposed, in its passage
through the producer, and two-thirds remain mixed with the gases which
leave the producer. My endeavors were consequently directed toward
finding means to recover this steam, and to return it to the
producers, and also to utilize the heat of the gases which leave the
producers with a temperature of 450° to 500° C., for raising steam for
the same purpose. The difficulties in the way of attaining this end
and at the same time of recovering, in a simple manner, the small
amount of ammonia contained in the immense volume of gas we have to
deal with, were very great. We obtain from one ton of coal 160,000
cubic feet of dry gas at 0° C. and atmospheric pressure. The steam
mixed with this gas as it leaves the producer adds another 80,000
cubic feet to this, and the large amount of latent heat in this
quantity of steam makes the problem still more difficult. The
application of cooling arrangements, such as have been successfully
applied to blast furnace gases, in which there is no steam present,
and which depend upon the cooling through the metallic sides of the
apparatus, is here practically out of the question. After trying a
number of different kinds of apparatus, I have succeeded in solving
the problem in the following way:
The gases issuing from the producers are led through a rectangular
chamber partly filled with water, which is thrown up in a fine spray
by revolving beaters so as to fill the whole area of the chamber. This
water, of course, becomes hot; a certain quantity of it evaporates,
the spray produced washes all dust and soot out of the gases, and also
condenses the fixed ammonia. The water thus becomes, to a certain
degree, saturated with ammonia salts, and a certain portion of it is
regularly removed from the chamber and distilled with lime to recover
the ammonia.
[Illustration: Longitudinal Section of Plant for obtaining Ammonia
from Gas Producers.