A Process for Recovering Cyanides from Coal Gas.
Year: 1891

page_text: Date of Application, 22nd Dec., 1891-Accepted, 13th Feb., 1892
COMPLETE SPECIFICATION.
A Process for Recovering Cyanides from Coal Gas.
I, WILLIAM LEE ROWLAND, of 4800 Chester Avenue in the City and County
of Philadelphia and State of Pennsylvania, one of the United States of America,
Chemist, do hereby declare the nature of this invention and in what manner
the same is to be performed, to be particularly described and ascertained in and
by the following statement :-
The object of my invention is to remove the cyanides from coal gas in the
ammoniacal liquor, and recover them therefrom as completely and economically
as possible to this end I add to the water to be put into the washers, scrubbers,
or other form of apparatus used for extracting the ammonia from coal gas, any
soluble salt of iron, as sulphate of iron, chloride of iron, etc. in sufficient quantity
to convert all the cyanides in the crude gas into ferro-cyanides while passing
through the ammonia scrubber in the usual manner for the removal of ammonia.
By this means all the cyanides can be converted into ferrocyanide of ammonia
and removed in solution in the ammoniacal liquor in one and the same operation
generally employed for the removal of ammonia.
The iron salt is at first changed in the scrubber into insoluble carbonate,
sulphide, or hydrate of iron, but it is washed along with the water through the
scrubber and gradually changed in to the soluble ferrocyanide of ammonia, thus
removing the cyanides from the gas.
This formation of sulphide of iron is but an incidental step in the process as
it is gradually and completely changed by contact with the cyanides in the gas
into ferrocyanide of ammonia if only the proper quantity of iron salt is used:
this quantity varies according to the amount of cyanide found by analysis to be
present in the gas but is usually less than five per cent. of that necessary to
remove the sulphides. As small an excess as possible in practice over the amount
theoretically required should be employed, as any considerable excess will form
enough sulphide of iron to clog or impede the action of the ammonia scrubber. The
solution of such a small amount of iron salt in the large amount of water necessary
to remove all the ammonia from the gas produces a very weak solution usually con-
taining not over one half per cent. by weight of the salt.
Owing to the long and intimate contact with the gas in the ammonia scrubber or
washer, that apparatus is especially adapted for the removal of cyanides by this
process, and aim to accomplish this economically without interfering with the
removal or subsequent recovery of the ammonia.
Insoluble iron salts, as the oxide, can also be used by keeping them in suspension
in the water by mechanical agitation, but the soluble salt is preferable as it requires
no agitation after being once dissolved, and the combined acid also assists in the
removal of the last traces of ammonia from the gas.
The addition of the iron salt serves not only to convert into ferrocyanide the
cyanides usually removed by the ammoniacal liquor when water alone is used, but
also fixes the volatile cyanogen compounds that usually pass on to the purifiers with
the gas, thus greatly increasing the yield.
I now distil in any form of ammonia still the ammoniacal liquor containing the
ferrocyanide, and the ferrocyanide may be recovered from the settled waste liquor
from the still by precipitation with an acid iron salt in the well known way for
producing blues, but I prefer to add a second portion of salt of iron or iron oxide
to the ammoniacal liquor just before or during the distillation, when by the boiling
off of the ammonia in the presence of an excess of the iron salt the soluble ferro-
cyanide of ammonia will be converted into an insoluble double salt, the ferrocyanide
of iron and ammonia, which may be separated afterwards by settling or filtration

page_text: along with the soluble lime salts when lime is used in the ordinary ammonia distil-
lation process. Lime is usually used by ammonia distillers to free the fixed
ammonia, but this lime being converted into insoluble carbonate, sulphate, etc., has
no bearing on this cyanide process, except that it acts as an impurity or waste
material in the settlings.
These settlings are now treated with an excess of lime or "milk of lime" which
decomposes the insoluble ferrocyanide into soluble lime salt, which may be removed
by filtration and washing.
I now boil the filtered liquor and add a potash or soda salt, as the chloride or
sulphate, but preferably the former, sufficient to precipitate all the ferrocyanide as the
double ferrocyanide of lime and the alkali used, or the liquor may be acidified and
boiled, when the double ferrocyanide of lime and ammonia will be precipitated. In
either case the precipitate is separated from the liquor, or settling and boiled with a
carbonate of the alkali, or the solution of ferrocyanide of calcium obtained as above
may be treated with an alkaline carbonate at once, forming an alkaline ferrocyanide,
but this method is not as economical and I therefore prefer the treatment with the
chloride. The resulting solution of alkaline ferrocyanide is separated from the
insoluble lime salts by filtration and may then be evaporated and crystallized by
any of the well known methods.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is
1st. The berein-described process for removing the volatile cyanogen compounds
from gas made from soft or bituminous coal, consisting in the addition of an iron
salt to the water for extracting the ammonia from the gas passing through the
scrubber substantially in the proportion to remove cyanides but insufficient to
remove sulphides, thus enabling the cyanogen compounds to be removed as soluble
ferrocyanide of ammonia along with the ammonia compounds usually recovered.
2nd. The herein-described process for recovering ferrocyanides from ammoniacal
liquor, consisting 1st, in adding a portion of iron salt to the liquor, boiling off the
ammonia in the usual manner and treating the settlings with lime to obtain a
solution of ferrocyanide of calcium, 2nd, adding an alkaline chloride or sulphate to
this solution to form the double salt, the ferrocyanide of calcium and the alkali,
and finally, decomposing this with an alkaline carbonate to form an alkaline ferro-
cyanide, which may be crystallized in the usual manner.
Dated this 21st day of December 1891.
H. B. BARLOW,
17, St. Ann's Square, Manchester, Agent for Applicant.