Improved Process for Producing Oxalic Acid.
Year: 1897

page_text: Date of Application, 28th Jan., 1897-Accepted, 5th June, 1897
Improved Process for Producing Oxalic Acid.
I, GEORG FRIEDRICH ZACHER, of No. 27 Steindamm, Hamburg in the Empire
of Germany, Merchant and Manufacturing 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 :-
In the usual method of producing oxalic acid, sawdust or other material
containing cellulose, is digested with caustic alkalies in an open vessel, stirred, and
then to complete the reaction, spread out on hot plates to be there oxidised by the air
which is freely admitted to it.
This process is known to be very difficult and indirect, the result depending a great
deal on the quickness and trustworthiness of the men in charge, as the formation of
oxalic acid from cellulose is liable to marked variation according to the temperature
used (200-250° C.), and the thoroughness of the mixing, the oxidation above
mentioned is also very unreliable. In close relationship with the formation of
greater or smaller amounts of oxalic acid in the mixture, there are also formed
such substances as formic acid, acetates, phenol like bodies, empyreumatic
combinations of a most complicated kind, and the like. It is the formation of
these combinations, which the oxalic acid manufacturer tries to prevent as far as
possible, because they impart a brown colour, more or less deep, to the liquor
and to the solutions formed therefrom for subsequent treatment; and their
undesired presence has an exceedingly bad influence upon the crystallisation of the
alkaline oxalate and further involves a whole series of subsequent tedious and costly
processes necessary to obtain a commercially pure oxalic acid. The above mentioned
substances as well as the acetic and other combinations, are the direct cause of the
formation of a quantity of greasy and amorphous products from the solutions in
addition to properly crystallised alkaline oxalates; which formation during the
purification of the raw alkaline oxalate and the lyes resulting therefrom, causes a
considerable loss of oxalic acid. The finished mixtures must be dissolved in water,.
then from these solutions the alkaline oxalate must be allowed to crystallize and
separate from the mother lye.
These mother lyes still contain even after further concentration, considerable
amount of alkaline oxalate, which must be for the greater part lost, as the mother
lyes, which are heavily charged with the said substances, must be calcined for the
purpose of destroying them. Thereby the oxalic acid which is in solution, is
naturally also burned, while the calcined mother lyes produce potashes. The latter
must be first dissolved again, treated with caustic, and then concentrated. Therefore
a lot of work, money, and time have to be spent, to get a caustic lye from the mother
lyes capable of being used in the arts, and a considerable amount of alkaline oxalate
is lost at the same time. The alkaline oxalate obtained after separating the mother
lye, is again dissolved in water and converted into calcium oxalate and alkali by
means of milk of lime. The alkaline lyes then separated, are again treated as above
described. The calcium oxalate is decomposed by sulphuric acid and the
resultant hydrated oxalic acid is evaporated to crystallization in the usual.
manner.
All these disadvantages and difficulties it is the object of the present invention
to obviate, the entire digesting operation being carried out in vacuo in order
to get a clear or uncolored liquor and to prevent the formation of the above •
mentioned substances.

page_text: Furthermore, the vacuum permits of the use of a low temperature with its
consequent economy of fuel. Cellulose (sawdust) is used, preferably, for forming the
mixture to be digested, such cellulose baving been previously desiccated and:
deprived of its accompanying air, in vacuo in a comparatively warm vessel, which
vessel afterwards serves to receive the mixture for digestion. The result is that after
the desiccation of the cellulcse, the hot, concentrated alkaline lye, which has been
sucked into the vessel by the vacuum, is readily absorbed by the sawdust which is
free from air and dry, and that this absorption and the reaction of the lye upon the
cellulose, which contains no cold air, takes place without any fall of temperature and
without forming a scum, and without the results of a free access of atmospheric
oxygen to the mixture.
For carrying the invention into effect, a vessel which possesses a steam jacket is
preferably used. This vessel is in connection with a condenser or an air pump; and,
according to the circumstances, is heated by steam or cooled by water. Into this
comparatively hot vessel, first the saw dust or other cellulose containing material for
one digestion, is introduced, and the vessel having heen closed, is exhausted during
the stirring operation and a gradual elevation of the temperature to about 70° C.
to drive the air and water out of the sawdust. Hereupon the alkali lye, which
has been previously brought to a certain temperature and strength, (preferably,
temperature 70° C., strength 40° to 50° Baume) is sucked into the vessel by the
vacuum, the stirring operation being maintained. The proportions being 201 parts
concentrated lye to 100 parts dried cellulose. The vacuum is kept as constant as
possible during the subsequent operation, and the said temperature of 70° C. is brought
at length to at most 180° C., according to the progress of the digestion, which
progress is controlled by testing samples. For the purpose of destroying or
expelling the small amount of humus matter which would have collected in the
mass, near the end of the digestion, an oxygenating substance, such as hydrogen
peroxide, sodium peroxide, or the like, can be introduced, or atmospheric air which
has been enriched in a known manner with oxygen, is slowly sucked into the
apparatus, the latter still in vacuo. The object of performing this oxygenation slowly,
is to keep the temperature of the mass as low and as constant as possible. On this
latter condition depends the success of the operation, for, should the temperature be
allowed to exceed 200° C., in all probability it would decompose and destroy the
oxalic acid. The entire digesting operation is finished in a few hours and is
throughout even and safe.
Such a process of digestion, being prepared in vacuo and at a temperature
below 180° C., is distinguished from other methods particularly in respect of the
following points-that it contains hardly any of the before-mentioned substances
which colour the liquor, that the amount of alkaline carbonate is much smaller, and
that it contains a much higher percentage of oxalic acid than any other digestion
conducted in an open vessel and without a vacuum, as the conversion of the cellulose-
containing raw material is a more complete one, such material being nearly all.
converted into oxalic acid. Besides the important direct advantage of the greater
yield of oxalic acid, the new process allows an important shortening and simplification
of the following procedures. The finished hot liquid is dissolved in water in the
vessel which has been used for its production. This solution is forced into an open
stirring cylinder and thinned there, whereupon by adding the necessary amount of
lime, the conversion of all the oxalic acid which is to be found in the liquid into a
nearly white, very pure calcium oxalate takes place directly. Thus, the many tedious
and expensive operations formerly necessary for producing an alkaline oxalate as pure
as possible, upon which operations the success of the entire process depends, is
obviated.
The calcium oxalate produced by this method and which, in consequence of the
minimum presence of alkaline carbonate in digested mass, contains no calcium
carbonate worth mentioning, is separated from the lime in any known manner by
filtration and is decomposed by means of sulphuric acid; and, as the result of the
purity of the calcium oxalate, a good white oxalic acid is produced by this improved

page_text: process with only one crystallization. The alkaline lyes which are filtered from the
calcium oxalate, can be immediately used again for a new digestion. Thus
the whole difficult and expensive operation of calcining and treating with caustic,
are obviated.
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 :-
1. A process for producing oxalic acid from raw material containing cellulose,
consisting in digesting it in an alkaline lye in vacuo and at a low temperature, the
said raw material having been, preferably, previously desiccated and deprived of the
air accompanying it.
2. The process for producing oxalic acid, consisting of heating desiccated material
containing cellulose, to a degree not exceeding 180° C., in vacuo, digesting it in an
alkaline lye, in vacuo, oxygenating it in vacuo and finally precipitating the calcium.
oxalate from its solution, substantially as herein described.
D
تيمم
1000
PHILLIPS & LEIGH,
No. 22 Southampton Buildings, Chancery Lane, London, W.C.,
Agents for the Applicant.