CA1041734A - Purification of phosphoric acid - Google Patents

Abstract

S P E C I F I C A T I O N

PURIFICATION OF PHOSPHORIC ACID

A B S T R A C T

Wet process phosphoric acid is purified by extraction with a solvent, e.g. an acyclic pentanone or hexanone and release from the solvent to give a partially purified acid which is further purified by raising the concentration to an H3PO4 content of at least 84.9% and subsequently cooling to produce crystals of H3PO4 1/2H2O. The crystals are separated from the mother liquor and either melted or dissolved in water to produce phosphoric acid of high purity.

Description

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The present invention relates to the purification of phosphoric acid obtained by the wet process by reaction of sulphuric acid with phosphate rock. Such acid will hereafter be termed "wet process phosphoric acid". In particular the present invention relates to purification of such acid by solvent extraction techniques.
Proposals for solvent extraction purification of wet process phosphoric acid date back at least 40 years. Initially the solvents suggested were water-miscible, for example alcohols and acetone~ which required distillation in order to release the purified acid; alternatively there were suggested water immiscible short chain alcohols requiring many stages for effecting a satisfactory extraction. ;
In more recent years attempts to put into practice solvent extraction processes for wet process phosphoric acid have been made. However, these have largely been restricted to processes (such as those described in BP 805517 using butanol and BP 953378 using alkyl phosphates), ln which either chloridé
ions are especially added to the acid or chloride ions are present as a result of using hydrochloric acid instead of sulphuric acid in the production of the phosphoric acid. The presence of chloride ion promotes a salting out effect which assists in promoting transfer of the phosphoric acid species to the organic phase.
A group of recent proposals is concerned with the use as solvents of certain ethers which extract the phosphoric acid present in aqueous solutions at concentrations above a certain threshold value, quoted as 35~ H3P04, but do not extract any acid at concentrations below the threshold value. '~he first of these proposals is contained in BP 1,112,033 which, in !-

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addition to the others referred to above, also mentions a number of other possibilities including ketones. The method disclosed required a substantial temperature differential between eætraction and release of the acid, the former being carried out at a relatively low temperature and the extract being heated, with or without the addition of a little water, ;
to effect release of the acid. BP 1,240,285, which is a patent of addition to BP 1,112,033 describes the same process but using a mixture of solvents, one such mixture being an ether and a ketone, of which cyclohexanone is specifically mentioned.
The third proposal in the group concerned with solvents having a threshold value quoted as 35% H3PO4 is USP 3,556,739 which ) describes the use of a wide range of aliphatic esters, aliphatic and cycloaliphatic ketones and glycol ~ethers. While the `
specific description is directed only to the same type of process as that of BP 1,112,033 in which the extract is heated in order to release the acid, reference is also made to the ~-possibility of effecting release of the acid by an isothermal route involving one stage contact of the solvent extract with water. However, no information is given as to which of the -) many solvents referred to in connection with the temperature - ~-raising process may be employed or as to the temperatureS
' appropriate to it.
A process has also been described in BP 1063248 for ~;
removing nitric acid from the aqueous phosphoric nitric acid mixture obtained by decomposing phosphate rock with nitric acid. -" ~ .:.~' In this process a ketone i5 used to extract the nitric acid, leaving the phosphoric acid in its impure form in the aquèous phase. Such acid could only be used for fertiliser production.
We have found that wet process phosphoric acid which has been purified hy solvent extraction can be further purified, eg. to render it suitable for use in foodstuffs, by concen~
trating the acid to an H3PO4 content of at least 84~9% and then

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cooling to produce crystals of H3PO4 ~iH20. The production of phosphoric acid crystals has been described in the past where a suggestion has been made to up-grade by crystallisation, wet process phosphoric acid which has prevlously been desulphated and defluorinated. This proposa:L is not in fact practicable as a method of producing pure phosphoric acid crystals since it results in inclusions in the crystals of for example, iron phosphates. Moreover the crystals are difficult to separate from the mother liquor since this is very viscous. Although it is known that by cooling aqueous acid of concentration below 92% H3PO4 it crystallises as H3PO4~H20 (see Thorpe s Dictionary of Chemistry, Fourth Edition, Volume IX, page 503) this knowledge relates to thermal acid and hence is only of academic interest, thermal acid requiring~ no purification.
The present invention provides a process for purifying wet process phosphoric acid, which comprises subjecting to extraction with an organic solvent to give a solvent extract containing phosphoric acid, releasing the acid in the extract, eg. with water to give an aqueous purified phosphoric acid containing less than lOOppm of metal impurities, and then concentrating the aqueous acid to give a concentrated acid of H3PO4 content of 85 - 92~i, eg 8a.9% - 88~ii, then cooling the concentrated acid, eg. to 8 - 12 C to bring about formation of crystals of the formula H3PO4,~H20 and a mother liquory and separating the crystals from the mother liquor.
Preferably the concentration of the aqueous purified acid from the solvent extraction is raised by use of vacuum evaporators such as forced circulation evaporators or alternatively, P205 obtained by burning elemental phosphorus may be dissolved in the solvent extracted purified acid or concentrated thermal phosphoric acid may be admixed with the purified acid to produce an acid of the desired concentration.

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Preferably the aqueous purified phosphoric acid obtained from the solvent extraction has a metal content of ~
less than 100 ppm, as can be the case when methyl isobutyl I -ketone is the solvent. Scrubbing of the extract containing impurity acid is needed to obtain this highly purified acid.
Since the acid used to produce the initial wet process phosphoric - acid from phosphate rock is sulphuric acid, the solvent extracted acid contains a substantial proportion of sulphuric acid, eg. of the order of 0.5 - 1.0% H2S04 on a 85% ~3P04 basis. We have found that the presence of this impurity does not interfere with the crystallisation. If, however, an unduly large amount of liquor, resulting in a high impurity content, adheres to the crystals when they are separated, eg. by ;
centrifuging from the mother liquor, it can readily be removed by washing.
The mother liquor from the crystallization after dilution with water to an appropria~e strength may ad-vantageously be used as the liquor fvr scrubbing the extract, integrating the solvent purification and crystallization processes. ~ ;
Any organic solvent capable of extracting phosphoric acid from wet process phosphoric acid may be used in the initial solvent extraction.~ Thus the solvent may be tributyl phosphate, - butanol, isoamyl alcohol or diisopropyl ether.
However preferably the solvent is an unsubstituted acyclic dialkyl ketone of 5 or 6 carbon atoms, eg. methyl isobutyl ketone, which is most preferred, diethyl ketone and methyl n propyl ketone.
Thus the present invention preferably comprises contacting wet process phosphoric acid with said acyclic ketone to give a ketone phase containing at least some of the phosphoric acid extracted from the wet process acid, `

scrubbing the ketone phase, then contacting it with water to produce aqueous phosphoric acid of increased purity having ¦

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regard to the feed wet process acid, and then raising the concentration of khe aqueous acid, crystallizing the hemi-hydrate crystals and separating them.
The solvent extraction with the acyclic ketone, scrubbing and release are described in our Application 170075 from which the present application has been divided.
With the acyclic ketone as solvent for the solvent extraction it is possible to carry out an efficient initial purification of wet process acid at the commercially available concentrations, operating both the extraction and release at a temperature which is at or relatively near ambient `~
temperature. This obviates the need either to refrigerate the acid for extraction step or raise the temperature of the extract for the release step, and also ensures that the acid/
solvent system is at a suitable viscosity for carrying out the process.
The preferred ketone is one unmentioned in USP
3556739 namely methyl isobutyl ketone. This is contrary to expectation, since this ketone is known to be useful for extracting from acid for analytical purposès the very metal ~
impurities which remain in the aqueous phase in this solvent `
extraction process. We have fcund however, that methyl -isobutyl ketone has the advantage of being able to extract substantially all the H3PO4 values from a commercial wet process phosphoric acid of about 70 - 85~ acidity as defined below. Ketones of higher molecular weight require a higher concentration of phosphoric acid in the feed before they will extract this amount of H3PO4 and it would therefore be necessary to institute a further difficult concentration stage of the acid prior to usin~ such a ketone. Methyl isobutyl ketone further has the advantage o having a higher flash point as compared to ketones of lower molecular weight but forming an azeotrope having a boiling point sufficiently low to enable easy solvent recovery by distillation. Further more methyl , . . ... , , . ~ , : ...................... . .

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isobutyl ketone has low solubility (of the order of 1-2%) in aqueous H3PO4 thus permitting the final product of the release stage to have only a low ketone content which may be removed with comparative ease as compared to other ketones as a result of the composition and boiling point of the azeotrope formed. `~
It further permits the production of an aqueous raffinate having an H3PO4 content in the range 55 - 65% which after remova~ of the small amount of methyl isobutyl ketone present is suitable for use as such in fertiliser production, unlike the raffinate produced by using ketones of lower molecular ¦
weight. i The fact that phosphoric acid of normal merchant grade acidity may have substantially all of its H3PO4 content extracted lnto methyl isobutyl ketone at temperatures at which it is normally convenient and desirable to work (i.e. in the range 25 - 50C, at which the system has a suitable viscosity) would normally require that a little water be added in order to ¦
produce a handlable raffinate. This may be accomplished either by adding water with or to the feed or more preferably by ! , .. ~ .
feeding a~ueous phosphoric acid, which has been employed in a scrubbing operation to reduce the impurity content of extract, ¦
into the initial operation where wet process acid is contacted with the ketone.
It is also preferred, contrary to the teaching of ¦
USP 3556739, that either or both of the extraction of H3PO4 -from the wet process phosphoric acid into the five or six ¦
carbon atom - containing ketone and the release of purified acid, if carried out into water, is carried out in more than ¦
one stage. The reason for this is that we have found a ¦
greater concentration of H3PO4 occurs in the raffinate for a given impurity content for a one stage extraction than for '" ' ' 4~3~

a two s-tage extractlon. Thus less H3PO4 is lost in the raffinate if the extrac~ion is carried out in two stages. The two stage extraction is conveniently accomplished by introducing the feed acid into one mixer unit of a two stage counter current mixer - settler extractor unit and the ketone into the other.
If a scrubbing stage is employed the recycled scrub liquor iq fed into the same unit as the feed acid. The use of a multi- ~ -stage release permits an aqueous phosphoric acid of higher H3PO4 content to be obtained When an acid of lower concentration than merchant grade acid is available, e.g. acid of acidity in the range 50-65%, it may be preferred, rather than concentrating the acid to employ a pentanone such as diethyl ketone or methyl n propyl ketone as the solvent, using both a two stage extraction into the ketone of H3PO4 and a 2 stage release into water, normally after a scrubbing operation has been effected. The concentration of phosphoric acid which is used to obtain most beneficial results according to the process of the invention will vary depending upon the ketone employed. If pentanones are employed -~
an acid of an acidity as hereinafter defined of at least 40%
normally below 65% (preferably an acid of acidity from 50-55%
as for example is obtained by the process described in BP
1209911) is desirably employed.~ In such processes it is normal that instead of producing CaSOa -2H20 as the by product of the reaction of H25O4 on phosphate rock Ca SO4 ~ H2O i5 formed, either in one stage from the attack or two stages, e.g. as a result of recrystallising the slurry obtained from a con-ventional process. If hexanones, such as methyl isobutyl ketone, are employed the acid i5 desirably of a concentration of at least 65%, frequently in the range 70 to 85% preferably about 74 - 79% expressed as H3PO4. The acidity of the acid is herein defined as the total weight percentage of H3PO4 and H2SO4 species present therein, this simple addition being possible as a result of ~13PO4 and H2SO4 both having a molecular ` ~iL0~3~ ~:
weight of 98. Thus an acid having an H3PO4 total of 74% and an H2SO4 content of 5% would have an acidity of 79%.
Use of such solvents will permit the production of purified acids after one stage of release into water of about ;
33% H3PO4 if pentanones are employed as the extractants, and about 45% H3po4if hexanones are used. The strengths of the acid obtainable can be increased if multi-stage releases are employed. For example an acid of concentration 55 - 58% H3PO4 can be obtained from an extract in methyl isobutyl ketone if 10 - such techniques are employed. ; : -Phosphoric acid for purification according to the invention is obtained by filtration or other separation of -calcium sulphate from the slurry obtained by the attack of sulphuric acid on phosphate rock. In such a state it contains a wide range of impurities, some of which such as fluorides .~
and sulphate and dissolved organic material may if desired by removed from the acid prior to the solvent extraction of the present invention, other impurities such as iron, chromium, magnesium and other metallic cations, however, are not removable by such a pre-treatment.
Conventionally the acid from the attack stage is cooled as much as possible in order to induce post precipitation of the impurities present. The acid will normally be cooled to about 25 - 30.
The temperature at which the solvent extraction is optimally carried out may vary according to the concentration of acid. However, we have found that a temperature below 50C e.g. of 25 to 50C is normally convenient for all concentrations of acid and one in the range 35 - 40C for methyl isobutyl ketone using an acid of an acidity in the range 65 - 35% is particularly useful.
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The acid to be employed in the present process will either be obtained direct at the required concentration, or will be concentrated up to this level e.g. by vacuum evaporation before purification. Acid of an acidity in the range 74 - 79 is the most commonly available commercial acid at the present time. Thus, for example, when methyl isobutyl ketone is employed it is common to employ an acid of about 79~ acidity~
P 3 4 2 4 or 78% H3PO4and 1~ ~2SO
The wet process acid is contacted with a ketone having five or six carbon atoms, such as methyl n propyl ketone, - di~thyl ketone, and methyl isobutyl ketone. The extracting ketone is contacted with the aqueous wet process phosphoric ~. ...
acid in weight ratio of above 0.3 : 1 preferably in the range 0.5 to 2,0 : 1 more preferably in the case where the ketone is methyl isobutyl ketone in the range 1.0 to 1.5 : 1 optimally 1.2 to 1.4 : 1 for one stage extractions.
! A ratio greater than 2.0 : 1 may be employed but would result in a requirement for larger vessels. If a 2 stage extraction is employed a slightly higher ratio e.g.
1.3 to 1.6 : 1 of ketone feed may be employed. We have found that aqueous phosphoric acid of the highest purity in a plant of the minimum size per unit H3PO~ through puts may be obtained if the ketone phase contains from 28 to 40% by weight of ~3PO~. ;
The contacting of the acid with the ketone is carried out in conventional equipment such as mixer-settlers and counter current packed columns. Normally an extractor with two or ¦
three actual or theoretical counter current stage, preferably two stages is employed for the reasons explained previously, although a one stage operation is possible. ¦
Alternative apparatus for carrying out the contact such as seived slate, rotating disc or pulsed column extractors of the desired number of theoretical stages may be employed.
If a phosphoric acid of high purity is required the extract containing the H3PO~ is then scrubbed by repeatedly l;73~L :

contacting it with a small amount of scrub liquor which is preferably an aqueous solution of phosphoric acid of a high degree of purity in order to extract the cationic impurities ~
into the aqueous phosphoric acid phase. Alternatively water ;
can be used which re-extracts some H3POg in the first contact .
which may then be contacted with further impure extract in contact with which it acts as the phosphoric acid scrubbing liquor. The phosphoric acid used for scrubbing should have -; an impurity content sufficiently low to enable it to extract impurities from the ketone extract. It should also have an ~ , H3PO4 content of less than 45% for pentanones and less than 56~
for hexanones in order to avoid transfer of H3PO4 from the scrub liquor into the extract. It may either be acid obtained by the thermal process or by a wet process fulfilling the above . i i conditions. Conveniently it is recycled purified acid from the release operation of the process. The amount of phosphoric acid used or produced by the partial extraction of H3PO4 1nto water and employed as the liquor to scrub the organic extract ., . .~ .
- will normally be at least in amount of 10% of the amount of H3PO4 present in the organic phase. Preferably it is in the range 30 - 50%. That is to say the weight ratio of scrubbing liquor to extract is normally from 0.25 to 0.5 : 1 of the acid which has an H3PO4 content of about 56% if a hexanone such as methyl isobutyl ketone or about 45~ if a pentanone is employed.
After scrubbing the phosphoric acid i5 released from the organic phase by contacting it with water. , This may be accomplished in one stage if desired.
However, when water is used more commonly a multistage release, normally using 2 actual or theoretical stages is employed.
The amount of water employed should be such that substantially all of the phosphoric acid passes into the aqueous phase from the organic. Desirably no more than 3% preferably no more than 1% of the H3PO4 values will be retained in the organic phase. The H3PO4 content of the aqueous phosphoric acid in ,.

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; a one stage release operation will be in the range 44 to 48%.
H3PO4 iE the ketone employed is methyl isobutyl ketone depending -upon the temperature employed; for example using water introduced at a temperature of 20~C an acid of H3PO4 content 45~ may be obtained. Using a 2 counter current stage extraction, higher concentration e.g. in the range 55 - 59~ may be obtained, for example 56% using water at 20C. For pentanones such as diethyl ketone and methyl propyl ketone, a 2 stage release will provide an acid of about 43 - 48~ H3PO4. If desired water at other temperatures for example condensates obtained from a heat exchanger such as used for vacuum concentrators may be employed. Typically temperatures in the range 20 - 40C
for the release stop will be employed. FQr a 2 stage release the extract i5 fed to one mixer unit of a 2 stage counter current mixer settler unit and the water to the other mixer section.
Although acid of these concentrations may be used direct for a number of applications it will normally be desirable to concentrate it if it is to be transported. When employing methyl isobutyl ketone as the solvent the amount of water employed for satisfactory one stage release is normally in ¦
the range 0.2 to 0.4 : 1 based on the weight of extract, whereas for two stage release an amount of water in the range 0.10-0.3:1 ¦
may be employed. For pentanones a ratio of 0.4 - 0.6 : 1 for ¦
a one stage and 0.05-0.3:1 for two stage release suffices. It should be understood however that if a more dilute acid is desired more water may be employed. ¦
The final product obtained may contain a small amo~mt ¦
of the ketone solvent employed. This can be released for ¦
example by distillation if desired. The process of the ¦
invention thereEore normally produces two streams of phosphoric ¦
acid, one obtained via the solvent which contains approximately ¦

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50 - 70% of the H3PO4 present in the starting material and -having a total metallic content less than 100 ppm relative - -to H3PO4, and a less pure one containing phosphate not extracted into the organic phase which is suitable for use in fer~ilizer production. Alternatively a less pure purified stream containing eg. 95% of the initial H3PO4 content may be - produced by using a high concentration of feed acid and little or no scrubbing.
The process of the invention is illustrated by the , following Examples 1 and 2; Examples A - H describe solvent ~urifications which can be used to make the purified aqueous acid which can be concentrated and crystallized in an an- ~ ;
aloguous manner.
In each of the Examples 1, 2, A and B, the feed phosphoric acid employed was of the following composition.
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Specific gravity . . 1.66 ;
H3PO~ . . .-. . . . 77.4% , H2SO4 . . . . . . . 1.6 Fe . . . . . . . 0.23%
Mg . . . . . . . 0.25% r In examples 1, 2, A and B the extracting solvent was methyl isobutyl ketone.
Examples A, B and 1 (a) Example A is herein described with reference to the flow diagram Figure 1. This illustrates a process in which both .
the initial extraction of the H3PO4 into methyl isobutyl ketone and the final release into water are both accomplished using one stage mixer settlers (1 and 2 respectively in the figure). The solvent extract is separated from the aqueous raffinate and scrubbed by being passed through a series o~
mixer set~lers, 3, to extract cationic impurities from the solvent extract. The scrub liquor employed is the purified ,.
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acid obtained from the release mixer settler 2.
The feed acid is introduced to the first mixer settler at a temperature of about 25C. The temperature rises as a result of the release of heat of extraction of the H3PO4 into `
the ketone.
The exact details af the conditions pertaining at - each stage of the process are shown on the flow diagram.
The product had an analysis of H3PO4 45%
H2SO40.65~
Fe 25 ppm Al ~5 ppm Mg ~5 ppm CaC 5 ppm Na ~1 ppm V ~2 ppm Cr ~2 ppm Cu ~2 ppm -Ni ~1 ppm Pb Cl ppm Cd~1 ppm Example B is illustrated by Fig. 2.
In this example the 1 stage mixer settler employed for the release of the H3PO4 from the ketone phase into the water was replaced by a 2 stage opposition. The temperature of the feed acid was again 25C. The exact conditions per-taining at dif~erent stages and the compositions of the streams are shown in Fig. 2. ¦
The first stage of Example 1, Ex. 1 (a) is illustrated by Fig. 3. In this example 2 stage mixer settlers are ;
employed both for the extraction of the H3PO4 into the ketone and for the release of H3PO4 into water. Figures for the conditions pertai~s at various points and the compositions of the various streams are shown on the figure.
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Example 1 (b) Product phosphoric acid from the solvent extractio~
operation described in Example l(a) of 7.4 gph, is steam , , stripped countercurrently in a packed column yielding 7.0 gph acid at SG = 1.43, containing 42.0~ P2O5 and less than 50 ppm ketone, and recovering 0.2 gph each of ketone and water mutually saturated. This essentially solvent free phosphorlc ~ ~-acid is concentrated in a forced circulation evaporator at 80C ~
. . ., under reduced pressure to produce 4.0 gph containing 62.0% ~25 1.0% SO
40 ppm Fe less than 10 ppm Mg i less than 50 ppm F
SG = 1.70/20C
- which after cooling to 30C is fed to a continuous crystalliser , operating at ~ - 12C. The slurry produced is centrifuged to produce 31.5 lb/hr of crystalline phosphoric acid hemihydrate (2H3F04 H20) of analysis ::

less than 500 pm S03 less than 5 ppm each of Fe, Mg, F
and also 36 lb~hr of aqueous phosphoric acid of analysts 58~ P2O5 1.9~ S03 less than 100 ppm each of Fe, MG and F. ¦
The purity of the crystalline product can be further improved by washing it with a solution of a portion of the crystals in water. This of course reduced the yield of crystals. I
The crystals are melted by adding them to a stirred l;
vessel in which the contents are maintained above 30 C to provide a liquid phosphoric acid su.table for transporting.

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The acid forming the mother liquor is suitable for sale as produced.
~xample 2.
In this example the object was to recover the total ..
"upgraded" phosphoric acid in the form of crystals. Phosphoric acid produced as in Example 1 (a) from the solvent extraction - operation, 14.7 gph, was steam stripped and concentrated to produce 8.0 gph containing .

61.5% P2O5 1.55% SO3 .
25 ppm Fe .
10 ppm Mg .
less than 50 ppm F
SG = 1.70/20 C
which after cooling to 30 C was fed to a crystalliser comprising .
a stirred tank and a recirculation loop via a scraped surface - :
:
heat exchanger, a temperature of approximately lOnC being main- .
tained with brine circulating at -5C.
The slurry produced was.centrifuged yielding a 64 lb/hr ..
of phosphoric acid hemihydrate crystals and 72 lb/hr of aqueous phosphoric acid. 11 lb/hr of the crystals were dissolved in water to yield 12 lb/hr of aqueous phosphoric acid to be used .
for washing the crystals on the centrifuge . :
The washings were combined with the separated aqueous . ~:
phosphoric acid and diluted with 3.1 gph water to yield 8.1 .
gph acid to be used for scrubbing the solvent extract. This ,~
acid contained 41.0% P2O5 and 2.1% SO3.
The resultant 55 lb/hr of crystals were melted in a ;1,' , ' stirred tank, the contents of which were maintained at 30-40C, :~
to provide liquid phosphoric acid for sale containing 66~ P2O5 : :
less than 200 ppn SO3 and less than 5 ppm each of Fe, Mg, and F. ~.

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Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for purifying wet process phosphoric acid, which comprises subjecting it to extraction with an organic solvent to give a solvent extract containing phosphoric acid, scrubbing said solvent extract to increase the purity thereof, releasing the acid in the extract after said scrubbing to give an aqueous purified phosphoric acid containing less than 100 ppm of metal impurities, concentrating the aqueous acid to give a concentrated acid of H3PO4 content of 84.9 to 92%, cooling the concentrated acid to bring about formation of crystals of H3PO4 1/2 H2O and a mother liquor, and separating the crystals from the mother liquor.

2. A process according to claim 1, wherein the concentrated acid has an H3PO4 content of 84.9 to 88%.

3. A process according to either claim 1 or 2, wherein the crystals are melted to produce an aqueous phosphoric acid having a P2O5 content of 66%.

4. A process according to claim 1, wherein the concentrated acid is cooled to 8 to 12°C.

5. A process according to any one of claims 1, 2 and 4, wherein the concentrated acid is obtained by vacuum evaporation of the aqueous purified phosphoric acid obtained from the solvent extraction and release.

6. A process according to any one of claims 1, 2 and 4 wherein the acid in the extract is released by contacting it with water to give the aqueous purified phosphoric acid.

7. A process according to claim l, wherein the solvent is an unsubstituted acyclic dialkyl ketone of 5 or 6 carbon atoms, butanol, isoamyl alcohol, tributyl phosphate or diisopropyl ether.

8. A process according to claim 7, wherein the solvent is a dialkyl ketone of 5 or 6 carbon atoms.

9. A process according to claim 8, wherein the solvent is methyl isobutyl ketone.

10. A process according to claim 9, wherein the solvent extract is scrubbed with purified phosphoric acid before release to give the aqueous acid.

11. A process according to claim 10, wherein the purified phosphoric acid used for scrubbing is derived from the mother liquor obtained from the crystallization after addition of water.

12. A process according to any one of claims 1, 2 and 4, which comprises (a) contacting wet process phosphoric acid of acidity 65-85% with methyl isobutyl ketone in a solvent:acid weight ratio of 0.5:1 to 2:1 to form an organic extract-containing H3PO4 and an aqueous raffinate, (b) separating the extract from the raffinate, (c) scrubbing the extract, (d) contacting the scrubbed organic extract with water in two or more counter current stages to effect release to give the aqueous purified phosphoric acid and an organic layer, (e) separating the aqueous acid and the organic layer, (f) raising the contraction of the aqueous phase to give a concentrated acid of 84.9% to 92% H3PO4 content, (g) cooling said concentrated acid to bring about the crystallization of crystals of H3PO4 1/2H2O from the mother liquor and separating the crystals, and (h) recycling said mother liquor for use as the scrub liquor in stage (c) after addition of water thereto.

13. A process according to any one of claims 1, 2 and 4, which comprises:
(a) contacting wet process phosphoric acid with a pentanone to form an organic phase containing at least some of the H3PO4 content and an aqueous raffinate, (b) separating said organic phase and said aqueous raffinate, (c) scrubbing the organic extract, (d) contacting the scrubbed organic extract with water in two or more counter current stages to effect the release to give the aqueous purified acid and an organic layer, (e) separating the aqueous acid and organic layers, (f) raising the concentration of the aqueous phase to a P2O5 content to give a concentrated acid of 84.9% to 92%
H3PO4 content, (g) cooling said concentrated acid to bring about the crystallization of crystals of H3PO4 1/2H2O from the mother liquor, and (h) recycling the mother liquor for use as the scrub liquor in stage (c) after addition of water thereto.

14. A process according to any one of claims 1, 2 and 4, wherein the solvent is an unsubstituted acyclic dialkyl ketone of 5 or 6 carbon atoms, butanol, isoamyl alcohol, tributyl phosphate or diisopropyl ether, the solvent extract is scrubbed with purified phosphoric acid before release to give the aqueous acid, and the purified phosphoric acid used for scrubbing is derived from the mother liquor obtained from the crystallization after addition of water.