Improvements in the Manufacture of Phosphoric Acid.
Year: 1886

page_text: Date of Application, 16th Dec., 1886
Complete Left, 15th Sept., 1887
Complete Accepted, 11th Nov., 1887
Improvements in the Manufacture of Phosphoric Acid.
WILLIAM BRANTINGHAM GILES Manufacturing Chemist of 100 Warton Road,
Stratford, Essex & ARTHUR SHEARER Analytical Chemist of 8 Hamfrith Road,
Stratford Essex do hereby declare the nature of their invention for IMPROVEMENTS IN
THE MANUFACTURE OF PHOSPHORIC ACID to be as follows:-
In the Manufacture of Phosphoric Acid instead of first preparing Phosphorus &
then oxydising it so as to obtain Phosphoric Acid, taking any suitable Phosphate or
Phosphatic Material such as Phosphate of Lime in any form, Phosphate of Lead,
Phosphate of Alumina or Iron, or the Phosphoric Slag which is produced in Iron or
Steel making & having separated as far as possible the bases from the acid by any of
the usual well known processes, treating the so obtained crude Phosphoric Acid in
one or other of the following ways:-
1st. Distilling the crude Phosphoric Acid at an elevated temperature i.e. from dull
redness to bright redness in retorts or ovens of suitable material, and passing through
the said vessels during the distillation a current of air with or without an admixture
of steam.
2nd. Creating a partial or perfect vacuum in the apparatus during the distillation
by which means the boiling point of the acid is much lowered, the acid is obtained of
much greater strength, & less fuel is required. It is found in practice that acid can
be distilled by this improvement (i.e. distillation in vacuo) of a specific gravity as
high as 2.000. Preferring to pass into the retort or distilling vessel during the
operation a small amount of steam to keep the distillate sufficiently fluid so as not to
block up the condensing arrangement, while maintaining as perfect an exhaustion as
possible, by an air pump, ejector or any suitable means.
3rd. Concentrating the crude phosphoric acid to a specific gravity of 1-800
to 1.900 according to the amount of impurities it may contain, straining or filtering if
necessary from any suspended matter, then cooling it down to a temperature of
about 40° to 50° Fahrenheit or lower; for this purpose a freezing machine may be
used if desired, whereupon if the acid has been properly prepared a large quantity of
phosphoric acid crystallises out & the impurities such as sulphuric & hydrochloric

page_text: acid, iron, alumina, magnesia, soda & the like remain in the mother liquor. This
acid which generally forms well defined crystals is then drained or pressed or treated
in a hydro extractor or centrifugal machine to separate as far as possible the mother
liquor & is then nearly pure, if greater purity is desired it is dissolved up again in hot
water to a gravity of about 1-840 and recrystallized. The mother liquors may then be
distilled as described above & so the whole of the free phosphoric acid is obtained in a
pure state.
Having now described the nature and means of carrying out our above
mentioned invention, what we claim as novel and new in it is, the preparation of
a pure phosphoric acid by :-
1st. The distillation of crude phosphoric acid at an elevated temperature, from a
dull red heat to a bright red heat with the aid of a current of air which may be
sucked in or blown in during the distillation. Using an admixture of steam with the
air or not.
2ndly. Instead of blowing or sucking in air, maintaining a more or less perfect
vacuum in the still, retort, oven, or apparatus employed, which is kept at a red heat
during the operation by which means the time of the distillation is greatly shortened
& much stronger acid is obtained with a minimum of fuel expended. Preferring
to use with as perfect a vacuum as possible a slight injection of superheated steam
so as just to keep the acid fluid in the condensing arrangement & to exclude air or
other gases as much as possible.
3rdly. Utilising the property which phosphoric acid possesses of yielding under
suitable conditions a crystallized acid to obtain from the crude & impure acid a pure
or nearly pure acid.
Dated this Fourteenth day of December 1886 as witness our hands.

page_text: COMPLETE SPECIFICATION.
Improvements in the Manufacture of Phosphoric Acid.
We, WILLIAM BRANTINGHAM GILES, Manufacturing Chemist, of 100 Warton Road,
Stratford, Essex, and ARTHUR SHEARER, Analytical Chemist, of 8 Hamfrith Road,
Stratford, Essex, 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
1
In the Specification of a former Patent granted to us No. 715 in the year 1886 we
described distilling crude phosphoric acid or mixtures yielding phosphoric acid
by the aid of a current of steam or hydrochloric acid.
According to our present invention we distil crude phosphoric acid or such
mixtures as aforesaid by heating it in a retort or oven to an elevated temperature
say from a dull to bright red heat and by then passing a current of air through
the retort with or without an admixture of steam and with or without tho aid of a
vacuum in the condenser.
Or we create a partial or nearly perfect vacuum in the apparatus and admit either
alone or in conjunction with air a slight injection of superheated steam sufficient
to keep the acid fluid in the condensing vessel and pipes leading thereto.
We also in some cases utilise the property which phosphoric acid possesses of
yielding under suitable conditions by crystallization a pure crystallized acid
or acids.
For distilling phosphoric acid we use by preference apparatus such as shewn
in section in the drawing hereunto annexed.
A is a retort of platinum, fireclay, or other suitable material placed in an oven B
which is surrounded by flues through which the heated products from a fire C below
it are passed the front only being left without any flue outsido it.
In the top of the retort are two openings, into one of which a funnel D provided
with a suitable cover is fitted to allow the crude acid (to be distilled) being fed into
the retort and into the other opening is fitted a conical pipe or "head" which is
closed at the top and from which a pipe extends to the end of a long platinum
tube F in which the acid distilled off from the retort is condensed. The pipe F is
made to pass concentrically through a pipe of larger diameter through which a
current of cooling water is caused continuously to pass. This arrangement
constituting a Liebig's condenser.
The further end of the platinum pipe is made to dip down into a large glass
"Wolfe's bottle" G-any acid remaining uncondensed is led off from this jar to a second
jar H and so to a third jar I from which a long glass tube inclining upwards is led
away to a vacuum pump. Preferably one in which a vacuum is obtained by jets of
steam issuing from nozzles.
At the front end of the retort two small openings are also formed-one near the
top and the other near the bottom.
Into the one at the top is fitted a short pipe K which is carried out through the
front of the oven and has fixed into it the end of a stoam pipe L on which is a

page_text: suitable tap for controlling the passage of steam--steam is supplied to the opposite
end of this pipe from a steam boiler-and as we have found that it is highly desirable
that the steain admitted to the retort. should be superheated to a high temperature we
make the steam pipe L to pass through the heated fino of the oven or embed it in the
wall of this flue whereby it becomes very highly heated.
The air which is to be introduced into and passed through the retort to convey
away from it the phosphoric acid vapours may be introduced into the retort through
the end of the short pipe K-the joint between it and the steam pipe not being made
perfectly air tight or it may be admitted through the joint between the funnel and
retort or between the funnel and its lid.
The small opening at the front of the bottom of the retort is closed by a plug M.
By withdrawing the plug any fluid residue which remains in the retort after all
phosphoric acid has been distilled off is allowed to escape and flow away at the front
of the oven.
As the process of distillation is carried on it is preferred to maintain as perfect a
vacuum as possible in the apparatus and to admit only sufficient steam to the
apparatus to keep the distilled acid when condensed in a liquid condition. If the
condensed acid passing off from the end of the platinum pipe F into the jar G is seen
to be in too thick or pasty a state then more steam is admitted whereas if the acid
is in too liquid a state then less steam is admitted.
Or we use a mechanical contrivance for discharging from time to time all solid
acid collected in the condensing pipe in this case little or no steam need be admitted
and only just sufficient air to convey the acid to the condenser.
To obtain pure phosphoric acid from the crude phosphoric acid by crystallization
we take any suitable phosphatic material as we have above described preferring to
use some form of phosphate of lime and having separated the acid from its accom-
panying base or bases as far as possible by any of the usual means we concentrate it
to a specific gravity which may vary from about 1.650 to 1-850 and after straining
or filtering it from the impurities which are usually thrown out, we bring it to a
suitable gravity in order that it may be crystallized.
This density will vary considerably with the amount of the still contained
impurities and must be different according to which particular crystallized phos-
phoric acid it is desired to form. If we wish to form the acid (H, PO, H, O) the
gravity must be about 1.700 to 1750 but if we wish to produce the acid (H, PO₁)
the gravity will be about 1-800 to 1.850. As wo have said above however we
find these particular gravities are greatly modified by the presence of other bodies
and that the exact density which is desirable in any particular case can only be
determined by experiment. It is also not a matter of indifference which of the
above acids should be formed we find that both can be applied and each has
special advantages when dealing with various kinds of raw phosphates. Having
obtained an acid of suitable density we cool if necessary down to about 30 to 60
degrees Fahrenheit and then add a few crystals of the particular acid which we desire
to form. If the acid is of a proper density and the temperature not too high a
large amount of the particular crystallised acid will form in the course of a short
time. These acids form fine large well developed crystals easily drained, while the
impurities such as sulphuric acid, hydrochloric acid, iron, alumina, magnesia, soda,
and the like remain for the most part in the mother liquor. We then drain, press,
or treat these crystals in a hydro extractor or centrifugal machine to separate as
far as possible the mother liquor. We then if it is desirable melt the crystals by heat,
adding water if necessary and repeat the crystallization until the requisite degree of
purity is attained.
In this manner we separate a considerable part of the phosphoric acid in a pure
form as a preliminary operation, and we then evaporate the mother liquors to a
syrupy or pasty condition and then distil them as we have described in the former
part of this specification. In this manner the whole of the free phosphoric acid is
obtained in a pure form the basic impurities being left in the retort or other
operating vessel in the form of acid phosphates or silicates.

page_text: Having now particularly described and ascertained the nature of our said inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
1. The distillation of crude phosphoric acid or mixtures yielding phosphoric
acid by heating it in a retort through which is passed a current of air or air with
admixture of steam and with or without a vacuum in the condenser.
2. The distillation of crude phosphoric acid or mixtures yielding phosphoric acid by
heating it in a retort to which a small quantity of air is admitted and which is in
connection with a condenser in which a vacuum is maintained.
3. The distillation of crude. phosphoric acid or mixtures yielding phosphoric acid
by heating it in a retort to which a slight injection of superheated steam is
admitted and which is in connection with a condenser in which a vacuum is
maintained.
4. The purification of crude phosphoric acid by crystallization and separation from
the mother liquor.
CARPMAEL & Co.,
Agents for the Applicants.