Note: Descriptions are shown in the official language in which they were submitted.
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The subject matter of the present invention is a con-
tinuous process for purifyiny crude phosphoric acid by extracting
the phosphoric acid with an organic solvent, which is infinitely
miscible with water and phosphoric acid, ln the presence of
alkali metal or ammoni~m ions, and recovering the organic solvent
dissolved in the phosphoric acid by distilling the solvent or
adding an alkali metal or ammonium compound and separating, in
an aqueous phase, the mono-, di- or tri-ortho-phosphate formed.
The purification of phosphoric acid by means or organic
solvents, which are infinitely or at least partially miscible
with water and phosphoric acid is already known and, like the
extraction of phosphoric acids with solvents which are not misci-
ble with water, it is the subject matter of patents. However,
it has been found that the phosphoric acids purified by means of
known processes do not have the degree of purity required in
specific fields of application, such as food chemistry and pharma-
ceutical chemistry.
The new process is characterized in that the crude
phosphoric acid, which is prepurified if required, and the organic
solvent are fed into an extraction vessel, wherein they are
mixed with one another in the presence of alkali metal or ammonium
ions while an organic phase and an aqueous phase are formed,
that the organic phase is brought into contact with an aqueous
` phase in a countercurrent in a series of several extraction
vessels, while the organic hase or the
aqueous phase in at least one of these extraction vessels is
mixed with aluminium ions and the aqueous phase in at least the
last extraction vessel:ismixed with alkali metal and/or ammonium
ions, whereupon the solvent phase which contains phosphoric acid
30 and water and is obtained in ~he coun~ercurrent extraction is
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separated in a manner known per se into phosphoric acid or salt
solution of phosphoric acid and solvent. The separation is
carried out preferably by distilling the solvent or by adding
metallic salts, oxides or hydroxides, particularly alkali metal
and ammonium compo~nds, while the corresponding mono-, di or
tri-ortho-phosphates are formed in an aqueous phase and a solvent
phase. The water-containing organic solvent separated from the
purified phosphoric acid or from the pure phosphate solution by
overflow is returned to the puriEication process and is used
again for extracting the crude phosphoric acid from its aqueous
solution. This process is particularly suitable for the removal
of fluorine as well as for the removal o heavy metals.
The aqueous phase separated in the first extraction
vessel contains the inorganic compounds separated iErom the crude
phosphoric acid in a dissolved form as phosphates, fluorides,
etc. Its P2O5 content is between approximately 8 and 16% by
weight, relative to the P2O5 content of the phosphoric acid
~ applied. Said aqueous phase can be processed in the fertilizer
;~ industry by means of known processes but according to a preferred
embodiment of the present invention it is processed-upon libera~
ting the phosphoric acid bonded in the phosphates - by means of
` sulphuric acid in a second series of extraction vessels by adding
; solvents which are iniEinitely miscible with water and phosphoric
acid, particularly isopropanol. In this afterextraction - as
in the first vessel of the main extraction - the phosphoric
acid is extracted fxom its agueous or sulphuric-acid solution
while forming a phase. The separated aqueous phase, which then
contains only 2 to 4% by weight of P~O5, relative to the P2O5
content of the starting acid, is removed from the purification
cycle and, iiE necessary, it is further processed to fertilizers.
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In the other extraction vessels of the second series
(afterextraction) the organic phase formed-which contains
phosphoric acid, sulphuric acid and water is brought into contact
with the aqueous, alkali metal- or ammonium-ions-containing
phase from the main extraction (extraction vessel 2), washed
and finally returned to the first vessel of the main extraction.
In the afterextraction the aqueous, alkali metal- or ammonium-
ions-containing phase also is in a countercurrent to the organic
phase. It is returned with advantage to the main extraction
process, for example, along with the crude phosphoric acid. As
mentioned hereinbefore, if required the crude phosphoric acid
is subjected to a prepurification, which is known per se and
can be carried out, for example, in such a way that the crude
phosphoric acid along with the recycled aqueous, alkali metal-
or ammonium-ions-containing phase is mixed with an amount of
calcium ions which is equivalent to that of the sulphate ions
as well as with an amount of SiO2 which is at least equivalent
to the fluorine content. If necessary, active carbon and sodium
sulphide are added for the removal of the organic impurities~
The sulphate ions absorbed from the organic phase are precipitated
as calcium sulphate like those from the crude phosphoric acid~-
suitably by adding phosphate ores. If necessary, the precipitated
gypsum is separated from the phosphoric acid by filtering and
decantingO
The subject matter of the invention is explained
hereafter in greater detail with reference to the attached draw-
ing, which represents the course of the process diagrammatically.
In~the drawing the first series of extraction vessels
has the numbers 1 to 6 and the second series the numbers 11 to
15. The crude phosphoric acid and the water-soluble solvent,
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preferably isopropanol, which is recycled, are ~ed into the
extraction vessel 1. The overflow from the ext~action vessel
15, i.e., isopropanol, phosphoric acid and water, are added to
this mixture. Moreover, the aqueous phase from the extraction
vessel 13 is fed along with the phosphoric acid to said mixture.
As mentioned hereinbefore, crude phosphate is added in order
to precipitate the sulphate ion;, i.e., in an amount equivalent
to that of the sulphate ions to be removed. After the separation
; of the phases the organic phase in the extraction vessel 3 is
mixed with a compound yielding aluminium ions, preferably in
aqueous solution or phosphoric-acid solution, is passed through
several extraction vessels, i.e., the vessels 4, 5 and 6, and
brought into contact with an aqueous phase in a countercurrent.
Said aqueous phase contains alkali metal or ammonium compounds
and is formed in the extraction vessel 6 in the presence of one
or several water-soluble alkali metal or ammonium compounds,
particularly a sodium compound. Ortho-phosphates, particularly
acid ortho-phosphates are particularly suitable. However, the
corresponding carbonates, oxides, hydroxides, etc., can also
be used. Relative to the P2O5 content of the phosphoric acid,
the alkali metai compounds should be added in amount of 0.5
to 6% by weight (computed as alkali metal oxide)`, preferably
1.5 to 4% by weight.
For example, A12(SO4)3, Al(H2PO~3 and NaAlO2 are
suitable aluminium compounds. Finely divided aluminium oxides
and hydroxides are also suitable. They can be added in a
solid form or in a dissolved form. Amounts of 0.05 to 3.0% by
weight (computed as A12O3), preferably 0.2 to 1.5% by weight,
relative to the P2O5 content of the acid, were found to be
adequate.
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The puriEied phosphoric acid is separated from the solvent
either by distillinq the organic solvent or in the ~orm of
aqueous solutions as mono-, di- or tri-ortho-phosphates.
If the phosphoric acid is separated by adding alkali metal
or ammonium in the form of aqueous mono-, di- or tri-ortho-
phosphate solution, then this is carried out in the reaction
vessel 7, from which the water-soluble organic solvent is
returned to extraction vessels l and 11.
According to the embodiment shown in the Figure the aqueous
phase from the extraction vessel 2 is fed not into the extrac-
tion vessel l but directly into the extraction vessel 15. This
has the advantage that the particularly strongly polluted
aqueous phase separated out in the extraction vessel 1 can be
~- -fractionally separated and processed. The aqueous phase from
the extraction vessel l contains the impurities in the form of
' phosphates and, thereforer it is rendered soluble with mineral
acid, preferably with ~ SO4, and then fed into the extraction
; vessel 12, where it is extracted with the isopropanol coming
from the extraction vessel 11. The extract thus obtained
which contains phosphoric acid, water and isopropanol is passed
in an overflow through the extraction vessels 13, 14 and 15- and
washed in a countercurrent process with the aqueous phase,
which comes from the main extraction and is fed into the
extraction vessel 15. The impurities are discharged from the
extraction vessel 11 as salts, preferably sulphates, and are
rejected or processed to fertilizers. The aqueous phase
obtained in the extraction vessel 13 is passed into the
extraction vessel l along with the phosphoric acid. The
organic phase consisting of phosphoric acid, water and isoprop-
anol is passed into the extraction vessel l directly from the
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vessel 15. Suitable solvents which are miscible with water
are isopropanol, ethanol and acetone.
It is obvious that the subject matter of the invention
is not restricted to the embodiment shown in the Figure. The
number of series-connected extraction vessels is optional
and always depends on the degree of impurit~ of the starting
acid as well as on the desired degree of purity of the
phosphoric acid or of the ortho-phosphates.
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Exam~p~ (Cornparison Example)
20 cu m of crude phosphoric acid produced by rendering Morocco phos-
phate soluble with sulphuric acid and containing 52% by weight of P205 are
treated along with
4 cu m of aqueous phase from the extraction vessel 13
5 cu m of wash water
J 2.2 tons of Morocco sulphate
130 kg of silica
1.5 kg of sodium sulphide.
On separating the washed precipitate this phosphoric acid contains
45.5% by weight of P205 and the following impurities in p.p.M. relative to
P205:
Fe2037,200 V 720 alkali metal oxides 27,000
A12312,900 S03 18,000
MgO12,500 F 2,000
CaO12,600 As
1000 litres/h of this prepurified acid are treated in a mixer-settler
with 5000 litres/h of recycle isopropanol and 1000 litres of alcohol-water-
phosphoric acid phase from the processing stage of the aqueous phas~ with
sulphuric acid (from extraction vessel 15) at temperatures between 20 and
30C. In this operation 200 litres/h of aqueous phase containing the im-
purities of the phosphoric acid are obtained continuously. In a counter-
current in a set of 5 mixer-settler vessels the upper alcohol-water-phosphoric
acid phase is brought into contact and washed with 280 litres/h of a saturated
monoalkali phosphate solution, which is produced in the mixer-settler 6 of
the Figure by adding 40 litres/h of a 50% sodium hydroxide solution. The
purified alcohol-water-phosphoric acid phase is neutralized in a mixer-settler
7 with a 50% solution of caustic soda until the monosodium-phosphate stage is
` attained. The separated alcohol phase is cooled and reused for the purifica-
tion of the phosphoric acid.
200 litresih of the aqueous phase obtained by mixing the phosphoric
acid with alcohol in the extraction vessel 1 are mixed with 70 litres/h of a
75% sulphuric acid and treated with 1000 litres/h of recycled isopropanol in
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a system of two ~nixer-settler vessels. In this operation 116 litres/h of
an aqueous phase are obtained. The phase containing 3.0% by weight of the
P205 applied is rejected or processed to fertilizers. The alcohol-water-
phosphoric acid-sulphuric acid phase is washed in a countPrcurrent in a set
of three mixer-settler vessels with 280 litres/h of saturated monoalkali metal
phosphate solution in order to remove the sulphuric acid from the alcoholic
phase. During this treatment 200 litres/h of an aqueous phase are obtained.
Said aqueous phase is fed to the main extraction along with the crùde phos-
phoric acid. The alcohol-water-phosphoric acid phase, which still contains
a small amount of sulphuric acid, is also fed to the ~irst extraction vessel
of the main extraction.
The analytical results of the purified monosodium phosphate solution
in p.p.m., relative to P205 are:
Fe 20 Mg <20 F 150
Al C 50 V 3
Ca < 20 S03 ~ 400
Example 2
A crude phosphoric acid according to example l is processed analogous-
- ly to the process described but with the difference that 50 litres/h of
aluminium sulphate solution (80 g of A1203 per litre) are additionally fed
into the mixer-settler 2 of the washing set consisting of 5 mixer-settler
vessels in order to remove the fluoride ions still present despite the pre-
- purification.
The anlysis of the purified monosodium phosphate solution shows the
following result in p.p.m. relative to P205:
Fe 12 V 3
Al ~ 50 S03 ~ 500
Ca C 20 F 17
Mg ~ 20
Example 3 (Comparison Example)
` 20 cu m of crude phosphoric acid obtained by rendering calcined
North-Carolina phosphate soluble with sulphuric acid and containin~ 52.8/~
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of P205 are treated along with
4 cu m of recycled aqueous phase from the extraction vessel 13
5 cu m of wash water
3.0 tons of calcined North-Carolina phosphate
and 1.5 kg of sodium sulphide.
On separa~ing the precipitate this phosphoric acid contains 44.7%
by weight of P205 and the follo~ing impurities in p.p.m. relative to P205:
Fe203 26,500 so3 16,700
A1203 13,600 As
MgO 20,200 F 7,000
CaO 11,800 alkali metal oxides 29,000
The pretreated phosphoric acid was processed analogously to the process
described in example 1, but with the difference that 45 1itres/h of a 50%
solution of caustic soda were fed into the last stage of the washing set of
; 5 mixer-settler vessels and that the aqueous phase formed by mixing the phos-
phoric acid with alcohol is mixed with 130 litres/h of a 75% sulphuric acid.
The loss of P205 in the phosphoric acid, relative to the amount of P205
applied, is 3.4% by weight.
The analytical results of the purified monosodium phosphate solution
are shown hereafter in p.p.m. relative to P205:
Fe 21 S03 ~ lOOO
Al C 50 F 700
Mg 20
Ca 6
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Example 4
The same crude phosphoric acid as in example 3 is processed analogous-
ly to the process described in example 3 but with the difference that 55
litres/h of monoaluminium phosphate solution (Al203 content 7.5% by weight)
are additionally fed into the second.stage of the washing set of 5 mixer-
settler vessels.
The purified monosodium phosphate solution shows the following ana-
lytical results in p.p.m. relative to P~05:
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Fe 15 . 3
Al ~ 50 F 20
Mg 20
Ca C 10
The examples show that the fluorine content of phosphoric acid and
of the phosphates produced therefrom can be reduced in a non-predictable
manner by the process according to the in~ention.
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