Improvements in the Production of Double Superphosphates.
Year: 1898

page_text: Date of Application, 18th Jan., 1898
Complete Specification Left, 18th Oct., 1898-Accepted, 3rd Dec., 1898
Improvements in the Production of Double Superphosphates.
I, GEORG OTTO JULIUS SCHÜLER, of No. 31, Oberwiek, Stettin-on-the-Oder, in the
German Empire, Doctor of Philosophy, do hereby declare the nature of this invention
to be as follows:-
The present invention relates to improvements in the production of double
superphosphates.
Double superphosphates, that is to say, superphosphates containing from 38 to 45
per cent. of soluble phosphoric acid has heretofore been obtained by mixing
phosphoric acid of 1.565 specific gravity with finely powdered rich phosphates and
treating the mixture thus prepared like ordinary superphosphate, namely by drying,
disintegrating and sifting. This process, however, offers various inconveniences,
owing to the fact that only rich and therefore dear phosphates can be employed in
preparing superphosphates and, moreover, that this treatment of phosphates
remains still imperfect, so that in the final product from 4 to 6 per cent. of insoluble
phosphoric acid is generally contained. As the latter is not valued it enhances the
cost of the material and lowers its percentage.
Finally it has been also tried to improve the treatment of the phosphate by
stirring the same in hot phosphoric acid, but even this procedure has not proved a
success.
An entirely different result however is obtained if the powdered phosphate is
boiled with the acid, a method which apparently has not yet been employed for fear
of the formation of pyrophosphoric acid. In carrying on this process the mixture is
expediently evaporated, with the aid of steain heat, at a temperature of about 110° C.
and then allowed to cool in leaden cooling pans; thus hard cakes are obtained
which contain no trace of pyrophosphoric acid and yield a double superphosphate
having less than per cent. of insoluble phosphoric acid and only from 3 to 4 per
cent. of free phosphoric acid. It must, however, be observed that the phosphate
should be very finely pulverized, and sifted, but I prefer the so-called "filter meal "
settled in dust-collectors as best suited for the purpose stated. If no sufficiently
powdered phosphate meal is on hand the coarser particles of phosphate may be
removed or separated from the mixture of phosphate meal and acid by settling before
the said mixture is poured into the evaporating pan or pans.
Any phosphate, even those of low percentage and carbonaceous ones may be used,
also even carbonate, hydrate or powdered oxide of calcium may be employed for
neutralising the acid.
The phosphatic chalks, such as cipolin are also applicable.
It is immaterial whether the ground or powdered phosphate is added to the
phosphoric acid in the dilute condition in which this is obtained from the leaching
process, thus combining the procedure of concentrating the phosphoric acid and

page_text: converting the phosphates into superphosphates, or the phosphoric acid is first
concentrated in the usual manner and then heated with the phosphate; only in the
latter case however, the double-superphosphate becomes free of the calcic sulphate or
gypsum.
In
I may also proceed by using only the carbonate of calcium in the phosphate.
this case the phosphate is stirred into the phosphoric acid in a cold state, the muddy
solution of monocalcium phosphate is then siphoned off from the quickly settling
crude phosphate, whereupon the water is evaporated at 110° C. and the residue
treated as before. The latter method is however only advantageous when the residual
phosphate of high percentage can be disposed of at a correspondingly high rate.
The described mode of manufacture renders it possible to use any phosphate even
those of low grade as an addition, the grade of the double superphosphate, however,
being raised thereby only in so far as the formation of insoluble phosphoric acid is
reduced below per cent.
Instead of phosphoric acid a solution of monocalcium phosphate containing phos-
phoric acid, such as is obtained by leaching ordinary superphosphates, may be used.
As the latter contains but about from to of its soluble phosphoric acid as free
phosphoric acid, only from 3 to of the phosphate meal used in the first method,
will be necessary for the same quantity of total phosphoric acid. As, however, every
phosphate contains a certain portion of useless dust, flouride of calcium etc., the latter
method will yield obviously a somewhat higher percentage. It is easy to obtain a
double superphosphate of from 50 to 54 per cent.
The superphosphate after having been mixed with water or a dilute lye may be
washed out in a filter press or it may be extracted by being allowed to deposit or by
filtering it in suitable filter-vats. The most advantageous course is however to obtain
the strong lyes by settling or depositing and to wash out the more dilute lyes of
about 11 spec. gravity by passing the same through a filter press. By appropriate
washing or lixiviation a lye of from 1.21 to 1.27 specific gravity may be easily
obtained. In using more dilute lyes of course more fuel is required for the evapora-
tion; these dilute lyes, moreover, contain inore or less gypsum. Of course a suitable
solution of monocalcium phosphate may be obtained in any other manner, for
instance, by mixing or stirring an easily decomposable phosphate with a small
quantity of sulphuric acid.
The lye may be freed from iron by ferrocyanides, ferrocyanide of calcium for
instance; or by merely heating it nearly to its boiling point, a part of the ferric
oxide and alumina may be separated as diphosphate, an operation which may be done
with the filtrate as it runs from the filters, or after it has been partially neutralised.
Diphosphate of calcium is precipitated by this operation only to a slight extent.
The evaporation with the ground crude phosphate is effected as in the method first
described at about 110° C., as in general the latter process is only a modification of
the former and all the manipulations stated for the said former process may be applied
in carrying out the latter or modified process and vice versa. The most suitable
temperature must be determined for each phosphate by an experimental process
carried out on a small scale.
For the evaporation it is best to employ pans with heating tubes or flues embedded
in their walls, in which case it is convenient to provide an automatic means for
emptying the pan, either by providing a discharge gate or manhole of known con-
struction in the bottom, or by mounting the whole pan on trunnions so that it is
possible to tilt the pan and to pour the contents into the suitably arranged cooling
pans.
The solidified cakes are broken between corrugated or spiked rollers, dried in the
usual way, then disintegrated and finally sifted.
The carbonates or hydroxides of the alkalies or of ammonium may be used for
neutralising the acid.
The lye of monocalcium-phosphate may also be evaporated with the addition of
a quantity of phosphate, lime, ammonia or alkali smaller than that required for the
formation of a monobasic phosphate. The addition of the substance named may,

page_text: on the other hand, be entirely dispensed with and then a very rich product is
obtained which, however, will be also very acid and on this account hygroscopic.
It may be mentioned that the sufficiently concentrated mass can be dried
directly in the same pan, that is to say without solidifying the mass by cooling.
In this case the pressure of the steam must be reduced to about half an atmosphere.
This drying process may be effected in the boiling or evaporating pans as well as in
separated drying pans like nitrate driers, that are provided with strong agitators.
Dated the 18th day of January 1898.
PHILLIPS & LEIGH,
Agents for the Applicant.
I, GEORG OTTO JULIUS SCHUELER, of No. 31, Oberwiek, Stettin-on-the-Oder, in the
German Empire, Doctor of Philosophy, 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 present invention relates to improvements in the production of double super-
phosphates.
Double superphosphates, that is to say, superphosphates, containing from 38
to 45 per cent. of soluble phosphoric acid has heretofore been obtained by mixing
phosphoric acid of 1.565 specific gravity with finely powdered rich phosphates and
treating the mixture thus prepared like ordinary superphosphate, namely by drying,
disintegrating and sifting. This process, however, offers various inconveniences, owing
to the fact that only rich and therefore dear phosphates can be employed in preparing
the superphosphates and, moreover, that this treatment of phosphates remains still
imperfect, so that the final product generally contains from 4 to 5 per cent. and even
more, of insoluble phosphoric acid. As the latter is not valued it enhances the cost
of the material and lowers its percentage.
Finally, it has been also tried to improve the treatment of the phosphate by stirring
the same in hot phosphoric acid, but even this procedure has not proved a success,
since the amount of 4 to 5 per cent. of insoluble phosphoric acid could not be reduced,
nor products of higher percentage than 45 per cent. obtained.
An entirely different result however is obtained if the powdered phosphate is boiled
with the acid, a method which apparently has not yet been employed for fear of the
formation of pyrophosphoric acid. Nevertheless I have found this fear is groundless
when working in the following manner.
The usual phosphoric acid used ought to be of a specific gravity not higher than 1.5.
I prefer acid of 1.38 specific gravity; but the acid may also be used as it runs from
the filter-presses, it may also be stronger than 1.38, but, as heretofore mentioned, it
must not be beyond 15. The more concentrated it is, the less gypsum it contains,
and therefore the resulting double superphosphate produced is of a higher grade or
percentage, moreover, the more concentrated it is, the higher is its boiling point.
I found that double-superphosphate should not be heated above 115° C. if it is
desired to avoid the formation of pyro-phosphoric acid. Phosphoric acid of 1.5 sp. gr.
boils at about 115° C. and therefore it cannot exceed this temperature.
In performing my invention, I proceed as follows:-Phosphoric acid of 1.38 sp. gr.
contains per cubicmetre about 450 kilogrammes of phosphoric acid of which 405 kilo-
grammes may be free acid. I mix this quantity with 390 kilogrammes of finely
ground Algerian phosphate containing perhaps 20 per cent. of phosphoric acid. After
the scum has settled I heat the mixture in a leaden pan with steam or direct fire.

page_text: The contents begin to boil at 106-107° C. As soon as the boiling point has been
raised to 110° C. the treatment is completed and the boiling pan is emptied into leaden
or cooling pans or preferably into common densifyers, such as usually employed in
the manufacture of superphosphate. The solidified mass is then ground, dried and
sifted as common superphosphate, containing then 50 % of phosphoric acid soluble
in water and but 1 % insoluble in water and no trace of pyrophosphoric acid.
Instead of Algerian phosphate any other phosphate may be taken, as for example
"Peace River," "Florida Rock," "Carolina River" phosphates. Even hard
phosphates, such as apatite are easily decomposed by my boiling process. Lime
phosphates as Algerian or even Ciply chalk are particularly adapted for use in my
process. These low grade and cheap phosphates could not hitherto be used in the
manufacture of double-superphosphate. Carbonic acid which formerly impeded the
reaction of the phosphoric acid upon the phosphate, escapes during the boiling
procedure.
The quantity of the phosphate to be added should be calculated after the well-
known chemical rules in such a manner that the phosphoric acid and the line
contained in the mixture will nearly be in proper proportions to form monocalcium-
phosphate.
Of course, the grade or percentage of the product corresponds to the quality of the
phosphate added, so that with Peace River phosphate, 46-47 %; with Algerian or
Florida hard rock phosphate, about 50 % ; and with high grade apatite, about 52 %
of phosphoric acid may be obtained.
By working in the described manner I get profit from the phosphate of lime as
well as from the carbonate of lime contained in the phosphate used, even those
containing much of the latter are particularly adapted for my process as said
before. But it is also possible to proceed in such a way as to use only the carbonate
of lime in the phosphate. In this case the phosphate is mixed with the phosphoric
acid cold, the turbid solution of monocalciumphosphate is siphoned off from the
quickly settling raw phosphate, evaporated at 110° C. and treated as before. The
latter method is however only advantageous when the residual high grade phosphate
can be disposed of at a correspondingly high rate. Instead of phosphate, even
carbonate of lime or quick lime or slaked lime alone may be used for manufacturing
double superphosphate of high percentage by boiling them with phosphoric
acid. The quantity wanted of these additions is calculated as mentioned before so
that the phosphoric acid and the lime contained in the mixture will nearly be in
proper proportions to form monocalciumphosphate. It must be noted that the
additional phosphate or lime nust be ground as fine as possible. The so-called
filter meal which is collected from the dust separating place, is especially fit
for the process. The additions should have passed through at least a No. 80
sieve.
I find it to be profitable to boil the said mixture with lime or carbonate of lime or
phosphates containing much of it, to 110° C.; with ordinary phosphates to 112°, and
with apatite to 115° C.
It
The commercial phosphoric acid is not a pure substance in a chemical sense.
always contains soine monocalciumphosphate, a circumstance which I consider
advantageous, as by it the complete removal of the sulphuric acid is secured.
Furtherinore, with regard to the fact that the quantity of additional phosphate
required diminishes in the same ratio as the contents of lime in the phosphoric acid
increases, it is evident, that it is advantageous to get the phosphoric acid as much
saturated with lime as possible. For all natural phosphate contains a proportion of
sand, calcium fluoride etc., the less thereof is added to the mixture, the more easily
the products of double superphosphate is obtained of a high grade. For this reason
I prefer the following method.
Phosphoric acid very suitable for my process is obtained by leaching ordinary
superphosphate. Such lye containing but to of the phosphoric acid in a
free state, requires only. to of the quantity of additional phosphate required
when using ordinary phosphoric acid. It is easy to obtain a double-superphosphate

page_text: containing 50-54% and even more of phosphoric acid soluble in water, with
but 3 to 4% of free phosphoric acid and but -1% of insoluble phosphoric acid.
The superphosphate being mixed with water or a dilute lye may be leached in
filter presses or it may be extracted by decantation or in filter vats. By appropriate
leaching I may obtain a lye of 1.26-1.35 specific gravity, and thoroughly extracted
gypsum. I prefer to work with lyes of 1.28 sp. gr. Weaker lyes require inore coals
for boiling and obtain more gypsum, yielding a product of inferior value. Of course
weaker lyes may previously be evaporated to about 1.28 sp. gr. and then mixed and
boiled with phosphate or any other suitable addition. Also such lyes or lyes of
about 1.28 specific gravity may previously be evaporated to 1.38 sp. gr. or to 1.45 sp. gr.
ascertained in hot state, or even to the boiling point of 110° C. and then mixed with
phosphate, or lime, or carbonate of lime, and boiled to the end.
obtained good
results with all these modificatious, though the most advantageous manner of working
is that first described.
For instance, a lye of 1.28 sp. gr. obtained by leaching Algerian superphosphate,
contains about 260 kilogrammes of phosphoric acid per cubic metre of which but
about 68 kilogrammes are free acid. I mix with each cubic metre of this lye,
60 kilogrammes of a 291% Algerian phosphate, and after the scum has settled, I heat
the mixture. The contents of the pan begin to boil at 102° C.; from 105-107° C.
a further formation of scum is observed. When the boiling point has been raised
to 110° C. the treatment is completed and the boiling pan is to be emptied into the
cooling pans or otherwise condensed. The resulting product being dried, ground and
sifted contains 50-51 per cent. of phosphoric acid, soluble in water. By using Peace
River phosphate as addition, only 48% will be obtained, with high grade apatite or
guano, 52% or more is gained. Weaker lyes as of 1-28 sp. gr. contains gypsum and
yields in consequence weaker products. For example a lye of 1-18 sp. gr. allows of
obtaining but 46% of phosphoric acid soluble in water in the product, but if, on the
other hand, the lye is previously concentrated to 1-4 sp. gr. and separated from, the.
precipitate, 52% soluble in water will be gained with Algerian phosphate, and 56% or
more by using high grade apatite or guano.
Any phosphate may be used, as well as carbonate of lime, or quick, or slaked lime,
for saturating the phosphoric acid, also the carbonate of lime contained by the
phosphate alone may be used, as previously set forth. The quantity required of these
additions is to be calculated as stated, after the well-known rules of chemistry, so
that the surplus of the free acid in the lye and the surplus of base in the sub-
stance added, are sufficient to form monocalcium-phosphate. The most suitable
boiling point, to which the mixture is evaporated, depends somewhat upon the sub-
stance added. When using lime, carbonate of lime or phosphates containing much
of it, I prefer a heating temperature of 110° C., Peace River phosphate, pebble,
hard rock, apatite and similar hard phosphates are preferably heated to 112° C. or
even 115° C.
It may be noted that for the evaporation it is best to use pans with heating tubes
embedded in the leaden walls and emptying valve in the bottom.
While the common phosphoric acid, produced in the usual manner is nearly free
from iron and alumina, even when made of phosphates, such as Peace River
phosphate containing a good deal thereof, the lye of such superphosphates contains
nearly the whole quantity of the iron and alumina of the phosphate, if freshly
employed. Therefore, when using such freshly made superphosphates the
iron and alumina should be removed from the lye before it is boiled
with lime, carbonate of lime, or phosphate, when. a high-grade double
superphosphate is required, for it is well known in industry, that the content
of iron and alumina is the cause of the retrogradation of superphosphates. Iron
can be precipitated by the addition of any ferrocyanide, calciumferrocyanide for
instance, but I prefer to neutralise the lye alinost completely and to precipitate iron
and alumina together by boiling it a short time.
For example, the lye of a superphosphate made of a 36% Maury County,
Tenessee, phosphate, (with 44% of iron and alumina) contains 72 kilogrammes

page_text: of free phosphoric acid per cubic-metre. This lye, when boiled with the necessary
quantity of Algerian phosphate, yields a double superphosphate containing 43% of
phosphoric acid, soluble in water, and 71% insoluble therein. After having boiled
the lye with 52 kilogramines of chalk, containing 54.3% lime (CaO), and having
separated the slimy precipitate, a 52% double superphosphate is obtained from said
lye by boiling it with the required quantity of Algerian phosphate.
Instead of boiling the lye of superphosphate, with carbonate of lime, caustic lime
or phosphate as described, the carbonates or hydroxides of alkali or of ammonium
may be used for manufacturing a kind of double superphoshate. The quantity of
these bases required is calculated as before, so as to form monobasic-phosphates. The
mixture is boiled exactly in the same manner as when working lime or phosphate,
also the lye may previously be cleaned in the described manner, if required. The
thus produced high-grade superphosphates contain about 50% of phosphoric acid
soluble in water and a small proportion of soda, potash, or ammonia, and are especially
fit for garden or tropical cultures.
I obtain the best results, as repeatedly stated, by boiling the lye of superphos-
phates with, phosphate, lime, or alkalı or ammonia in such proportions as to form
monobasic-phosphate, but by altering these proportions somewhat, products may be
obtained also fit for use. Particularly, it may sometimes be profitable to diminish
the quantity of additional basic substances, or the addition of such substances may
even be entirely omitted, and the pure lye of superphosphates may be evaporated.
Thus a very high-grade product is obtained, which contains a large amount of free
acid and therefore is very hygroscopic.
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. The herein described process for producing double superphosphates from phos-
phates consisting in boiling phosphoric acid with finely-ground phosphate, calcium
carbonate hydrate of calcium, or caustic lime, in about the proportion required for
forming mono-calcium phosphate, substantially as set forth.
2. The process for producing double superphosphates containing in boiling a solu-
tion containing mono-calcium phosphate and phosphoric acid with finely ground
phosphate, calcium carbonate, hydrate of calcium, or caustic lime, in about the pro-
portion necessary for saturating the free phosphoric acid contained in the mono-
calcium phosphate, substantially as set forth.
3. The process for producing double superphosphates, consisting in boiling a
solution of monocalcium-phosphate with ammonia, ammonium carbonate, potassium
carbonate, sodium carbonate, or with hydroxide of potassium or sodium, all in about
the proportion necessary to combine with the free phosphoric acid, substantially as
set forth.
4. The process for producing double superphosphates, consisting in boiling a solu-
tion of mono-calcium phosphate with calcium hydrate, or caustic lime, or ammonia,
ammonium carbonate, potassium carbonate, sodium carbonate, or with hydroxide of
potassium or sodium, in a proportion less than that required for combining with the
free phosphoric acid, or in merely evaporating the lye without adding the said or any
of the said substances substantially as set forth.
5. The double superphosphates obtained by the process substantially as described.
Dated the 18th day of October 1898.
PHILLIPS & LEIGH,
22, Southampton Buildings, Chancery Lane, London, W.C.,
Agents for the Applicant.