IL42487A - Process for the extractive purification of phosphoric acid containing cationic impurities - Google Patents

Description

Process for the extractive purification of phosphoric acid containing cationic impurities a»»3^up cnnn ' am This invention relates to the purification of phosphoric add, and more particularly to a process for the purification of phosphoric acid using extraction techniques.

Phosphoric acid is generally produced 1n accordance with two processes. In one process, phosphoric add Is produced from phosphate ore by contact with a strong mineral acid, such as sulfuric add. Calcium in the phosphate ore 1s generally separated in the process as gypsum whereas the other constituents in the phosphate ore remain in the phosphoric acid as dissolved solids or soluble salts to the extent they cannot form slightly soluble compounds in the strongly acid medium with either sulfuric or phosphoric acid. In many Instances, these impurities Interfere with further processing of the phosphoric acid, and often the employment of the technical grade acid for many purposes is rendered Impossible by these impurities. Efforts have not been lacking to free the phosphoric acid of the Impurities to permit a broader utilization of the purified phosphoric acid. In one long-used purification method the phosphoric acid is neutralized with alkalis. Accordingly, the impurities dissolved in the raw acid are precipitated as slightly soluble salts or metal hydroxides. However, this method is suitable only 1f the phosphoric acid is to be used in the alkali salt form.

If compounds other than alkaline phosphates are to be made from the phosphoric acid, then purification methods are required whereby the free acid remains. In German Patent 648,295 there is described a method of purifying raw phosphoric acid by treating the phosphoric acid with an organic solvent in the presencd of an alkaline compound whereby Ionic impurities, such as calcium, iron, aluminum, chromium^ vanadium and fluoride ions are removed as a viscous mass. After the difficult separation of the viscous mass, phosphoric acid 1s recovered by dlstlllatlonof the organic solvent. Mentioned as organic solvents are methyl and ethyl alcohol, and acetone. Alkaline compounds used in such process Include free alkali and ammonium bases, the salts of sulfuric acid, oxalic acid, carbonic acid, nitric acid and the like. The amount of alkaline comply to be added is optimally set at 10 to 15% with respect to the raw phosphoric acid. It is clear that the process is only applicable to the purification of phosphoric add since a partial desaturatlon of the phosphoric acid 1s achieved as a result of the large amounts of alkaline compounds required for purification.

In accordance with co-pending application DOS 2050406 filed 14 October 1970, «s* gneel-to- the -same -ass-ignee- «s-the-presefit- nvefiti A number of other purification processes have been disclosed 1n which the impurities are separated by extraction techniques. All of such extraction processes extract the phesphoric acid by treatment with a water-insoluble or li itedly water-soluble organic solvent to transfer the phosphoric acid and in part also impurities, depending on the distribution coefficient, into the organic phase. By reverse extraction and distillation, respectively, a solvent-free phesphoric acid highly depleted of impurities is recovered. Organic solvents that are insoluble 1n water or miscible to a limited degree include butanols, amy(alcohols, ketones, ethers, esters, such as trial kyl phosphate, butyijaicetate , amy! acetate, etc. Organic amines have also been proposed as extraction agents and 1n some Instances 1n comb1nat1r^ with the aforementioned organic solvents.

It is an object of the present invention to provide an improved extraction process for purifying phosphoric acid.

Another object of the present invention is to provide an improved process for extracting contaminants from crude phosphoric acid.

These and other objects of the present invention are accomplished by addling to a crude phosphoric acid an organic solvent miscible in all proportions with water and phosphoric acid and thereafter contacting the resulting solution with an inorganic material in aqueous or solid form to effect the preferential passage of the impurities of the crude phosphoric acid/organic solvent solution into an aqueous inorganic phase with phosphoric acid remaining in the organic solvent phase from which purified phosphoric acid is recovered by distillation of the organic solvent. In a preferred embodiment, the organic solution is passed through a cationic exchange resin in the H+ form prior to distillation.

According to the process of the present invention, a crude phosphoric acid produced by any of the various digestion processes can be purified. Moreover, phosphoric acids which are utilized in many chemical industrial operations and which must be subsequently freed of foreign substances may be purified in accord aire with the present invention, for example in the preparation and maintenance of phosphoric acid etching baths of electrolytic polishing baths.

More specifically according to the present invention there is now provided a process for purifying a crude phosphoric acid by extraction which process comprise a) adding to said phosphoric acid an organic solvent miscible in all proportions with phosphoric acid and water and if need be filtering any precipitates present; b) contacting the homogeneous solution of crude phosphoric acid and organic solvent of step (a) with such a quantity of a concentrated aqueous solution of an material not forming , , . j iL ^ tl .. inorganic seH-net-tefld4n§ to form precipitates during extraction, that the liquid aqueous salt phase into which migrate the impurities of the phosphoric add 4s remains as a separate phase preserved in addition to the phosphoric acid solvent bhase; the c) separating ^ organic solution f om Art aqueous salt phase after a d1s^^ tribution equilibrium has set in and optionally after the extraction step has been repeated; and d) recovering phosphoric acid from said organic solution by distilling off the organic solvent.

In Israel Patent 36006 published Oct. 22, 1974 there is described a process for the upgradi g of crude phosphoric acid comprising the steps of adding a water soluble organic liquid and an inorganic salt to the crude acid to precipitate out solid impurities; and thereafter separating the organic liquid from the resulting purified phosphoric acid.

Both the process of the present invention and the process of said patent relate to the purification of crude phosphoric acid under addition of an organic solvent miscible with phosphoric acid and water in any proportion, and of an inorganic salt, the solution, however, being effected in different ways.

The known process according to Israeli Patent 36006 is a classical precipitation method, in which the impurities of the phosphoric acid are precipitated in a crystal! iser as a solid precipitate and subsequently filtered out in a centrifuge, after the solvent has been added to the mixture of crude acid and the inorganic salt.

The process according to thearesent invention on the other hand, is an extraction method in which the impurities are extracted from the phosphorioecid-solvent phase immiscible with the aid of a concentrated aqueous solut4on of an inorganic salt, the-twe liquid being formed remain es phases wh eh-de-He%-fflix,-be*H§-pFesePve€lT No preci itations form at this stage.

Indeed, the process must be arranged in such a way as to preclude precipitation the during the addition of the salt solution, as an extraction process will not function otherwise. There are always two liquid phases in contact, between which a state of establ ished equilibrium is formed. After this equilibrium has been attained, the organic and inorganic phases are separated, each phase being re-extracted with a fresh, OFPSS--pend4ng-phase7 organic solvent and inorganic salt solution respectively.

Thus, in the extraction process according to the present invention separation of the impurities from the phosphoric acid is effected with the aid of a liquid (inorganic salt solution) which, due to its dissolving capacity with respect to the impurities, becomes an extraction agent while in the known precipitation processes, on the other hand, the water-soluble solvent serves as precipitating agent^ for the impurities of the phosphoric acid, 1n the presence of inorganic salts.

As organic solvents suitable for use in the process of the present invention there are preferred alcohols and/or ketones which are misdble with water and phosphoric acid in all proportions.

The ratio of organic solvent to the P2<_»5 content of the phosphoric acid to be treated can fluctuate within a wide range, generally of from 1:1 to 20:1 with a preferred ratio of between about 5:1 to 10:1. With highly concentrated acids, the ratio is usually smaller than in the case of moderately concentrated acids. For a phosphoric acid - 4b - having a PgOg content of from 50 to 55%, a ratio of organic solvent to P205 of from 8:1 to 10:1 is sufficient.

The concentration of the phosphoric acid plays only a subordinate part in the purification process. Besides the standard, commercially available phosphoric acid with a P205 content of 50 to 55%, acids with higher concentrations can also be purified by the process of the present invention as well as dilute acids having roughly a PgOg content of 20%. The organic phase must be free of clouding substances prior to extraction in order to avoid difficulties during phase separation due to the formation of an intermediate layer on the phase boundary by precipitated deposits.

The concentrated aqueous inorganic solution added to the homogeneous solution of phosphoric acid and organic solvent are formed from inorganic materials selected from the group consisting of the inorganic salts, inorganic hydroxides, or mixtures of salts and hydroxides which are soluble in water and which do not tend to form precipitates during extraction. It has been found that the soluble salts of phosphoric acid are ideally suited since these phosphates do not lead the phosphoric add with foreign anions for a small portion of the ions will migrate into the organic phase while attaining equilibrium. In order to achieve an optimal P205 distribution between the phases, it is preferable to use the acid phosphates, however, salts, such as those of carbonic acid, sulfuric add, boric add, or of other acids may also be used. The salts of the multivalent cations, such as magnesium, zinc, calcium, or aluminum are especially suitable.

It is also possible in lieu of a concentrated aqueous salt solution to add and vigorously mix salts and/or free bases in solid form to the solvent-containing phosphoric add solution. A concentrated aqueous phase will form by dissolution of the salt and/or free base. This procedure is preferably always used if the organic phase has a relatively high water content. In this manner of operation, the water is removed from the organic - 5 - phase thereby leading to a more concentrated phosphoric acid.

It is also possible to form the concentrated aqueous i norganic solution from a mixture of salts . A combination of salts is advantageously used if it is desired to achieve selective stripping of the impurities of the crude acid by selection of the cations and/or anions comprising the aqueous salt phase.

The volumetric ratio of the aqueous salt solution to the organic solvent containing phosphoric acid has no influence on the process of the present invention and may vary within extremely broad l imits , e.g. between about 10: 1 to 1 :250 . Preferably for economic reasons a large volume of the organic solution wi l l be treated with a smal l volume of the aqueous salt solution. It has been demonstrated that a volumetric ratio, such as 80: 1 may be managed without difficulties even in multi-stage extraction units.

The extraction is carried out by Intimately contacti ng the phases unti l transfer of material has proceeded to the point of equil ibrium. Upon reaching the equi l ibrium, the impurities in the phospho ic add to be treated become concentrated in the aqejjous salt phase whi le the phosphoric acid preferential ly remains in the organic phase. The ions of the aqueous sal t solution desirably remain in the aqueous phase, since the salts are only slightly soluble 1n the organic solvent. As a resul t of the migration of the impuri ties from the phs^phoric acid/organic solvent phase i nto the aqueous phase, an excel lent purification of the phosphoric acid to be treated is achieved. Since an equi librium is always establ ished i n the event of formation of two phases , an increasingly greater depletion or stripping of impurities from phosphoric acid can be achieved by repeated extractions .

After completion of extraction, the phases are separated and the organic solvent disti l led from the phosphoric and/organic solvent phase to recover the organic solvent and purified ph^phoric acid. The aqueous sal t phase may be used for other purposes. - 6 - The separation of the phases presents no difficulties for there exists no tendency to form emulsions since the phases have significant differences in density, to wit 0.8 to 1.2 for the organic phase and 1.3 to 1.6 for the aqeuous salt phase.

The process of the present Invention can be carried out batch-wise in mixer-settler apparatus or continuously in extraction columns of various constructions.

Generally, a small proportion of the salt phase migrates into the organic phase and leads to a certain "misplacement" of the salt. When salts of phosphoric acid are used, misplacement is confined to the cations. The following examples will show that misplacement is Insignificant. However, in order to obtain a -free pure phosphoric add, the organic phase may be passed afterjseparatlon through a highly acidic cationic exchange resin in the H+ form, wherein the misplaced cations are bound. The organic phase is subsequently subjected to a distillation operation to distill the solvent and recover pure phosphoric add. The solvent may be recycled to the initial process operation.

The following examples are illustrative of conditions for the process of the invention and it is to be understood the scope of the invention 1s not to be limited thereby. All percentages are by weight unless otherwise designated. Weight ratios refer to the ratio of the weight or organic solvent to the weight of P2O,- and the volumetric ratios fefer to the ratio of the volume of organic solution to the volume of the aqueous solution unless otherwise specified.

The examples illustrate that a sphosphoric acid of high purity can be obtained by the extraction process of the present invention and that extrae-tion of the impurities from the raw acids is also effected for both cationic and anionic impurities. - 7 - EXAMPLE 1 ^ A phosphoric and/organic solvent solution of the following composi tion is continuously formed: Component Wt. % P2°5 9-° Isopropanol 71 .0 Fe 0.02 Al 0.02 H3 0.10 HF 0.03 H2S04 0.02 The solution is contacted with a saturated aqueous solution of monomagneslum phosphate in a volumetric ratio of 80:1 1n a 20-stage screen bottom column. An organic phase is withdrawn from the head of the column of the following composition: Component Wt. % P2°5 8.92 Isopropanol 70.20 Fe <0.001 Al <0.001 NH3 <0.OOl Mg 0.03 HF 0.002 H2S04 ^.001 The organic phase is passed through a strongly add cation exchanger in the H+ form and is Introduced into a distil lation column wherein the solvent is distilled as an azeotropic composition.

A phosphoric acid having the follow ng composition is recovered from the distil lation column: - 8 - A phosphoric acid having the following compositions is recovere from ■tho-distillation column: Component Wt. % 46.2 P2°5 0.001 Fe 0.001 Al 0.0Θ1 Mg 0.001 H2S04 0.003 HF 0.009 EXAMPLE II An iron pickle liquor on the basis of phosphoric acid had the following composition: P2O5 14.5 Fe 3.5 To the pickle liquor isopropanol is added in a weight ratio of 3:1 and the resulting solution freed by filtration of a clouding precipitate. - 9 - The alcohol ic phosphoric add solution had the fol lowing compo^ltlonf^'* Component Wt.¾ P205 2.4 Fe 0.08 Isopropanol 75.0 This resulting solution is extracted in a five-stage mixer-settler apparatus with an aqueous saturated monozlnc phosphate solution in a volumetric ratio of 70: 1 . An organic phase 1s withdrawn from the apparatus of the following composition: Component Wt.% P2°5 2·3 Fe <0.001 Zn 0.02 Isopropanol 74.5 The alcoholic phosphoric add solution is passed through a strongly acid cation exchanger in the H+ form and then freed of solvent (as an aieotrophic mixture) by distil lation. A phosphoric acid of the following composition is recovered: Component Wt.% P205 16.2 Fe 0.004 Zn 0.002 The thus recovered phosphoric acid isrepeatedly used as a pickl ing bath after reaching the required concentration.

EXAMPLE III An Industrial phosphoric add produced by the wet digestion process with sulfuric acid had the followi ng composition: Component Wt.% P205 55.1 H2S04 1.8 HF 0.7 Fe 0.2 ^ Al 0.2 Ca 0.08 The crude acid is admixed with isopropanol having a density of 0.813 in a ratio of 5: 1 and the resulting solution freed by fil tration of a clouding precipitate. The filtrate had the fol lowing composition: Component Wt.% P2°5 9 J H2S04 0.28 HF 0.10 Fe 0.008 Al 0..008 Ca 0.001 Isopropanol 72.5 The alcoholic phosphoric acid filtrate 1s continuously treated In an 18-stage extraction column (having rotating installations) with an aqueous saturated monoalumlnum phosphate solution 1n a volumetric ratio of 70: 1. The resulting organic phase had the following composition: Component Wt.% 9.0 P2°5 H2S04 0.002 HF <0.001 Fe 0.001 Al 0.090 Ca «0.001 Isopropanol 72.3 The organic solution is passed through a strongly acidic cation exchange resin in the H+ form and freed of solvent by distillation. A Phosphoric acid is recovered having the fol lowing compoiition: - 11 - Component Wt.% P205 54.8 H2S04 0.01 HF 0.003 Fe 0.002 Al 0.002 Ca 0.001 EXAMPLE IV An alcohol ic phosphoric add solution of the fol lowing composi tion: Component Wt.¾ P2°5 9-° H2S04 0.002 HF 0.001 Fe 0.001 Al 0.09 Ca 0.001 isopropanol 72.3 1s extracted I n an extraction col umn with an aqueous add se4d u sodium phosphate solution. The extraction col umn Is a 10-stage mixer-setter extraction apparatus wherein the extraction 1s carried out i n counter-current flow. The sodium ion quantity necessary to form the aqueous phase is a saturated , roughly 46%, aqueous mohosodlum phosphate sol ution which is Introduced Into the tenth mixer stage whereas the alcoholic phesphorlc acid solution to be treated is introduced Into the first mixer stage. An aqueous acid pft* phate phase rich 1n foreign substances is di scharged from the first mixer stage of the settler. The volume of the saturated, aqueous monosodi um phosphate solution introduced into the extractor 1s such that the two streams withdrawn from the extraction apparatus had a vol umetric ratio of 70: 1 .

The fol lowing is the composition of the alcoholic phosphoric acid sol ution: Component Wt.% P205 8.9 H2S04 0.005 HF 0.005 Na 0.04 Fe φ.ΟΟΟΊ AT 0.0001 Ca 0.0001 Isopropanol 72.3 In order to remove the still -present cations , the al cohol ic phosphoric add solution is passed through a strongly acidic cation exchange resi n in the H+ form. Isopropanol 1s separated by distil lation and recycled to the extraction apparatus . The recovered phosphoric acid had the followi ng composi tion: Component Wt.% P205 54.0 H2S04 0.003 HF 0.002 Na 0.001 Fe 0.0005 Al 0.0005 Ca 0.0005

Claims (6)

1. 42487/3 WHAT IS CLAIMED IS: ^ 1. A process for purifying a crude phosphoric add by extraction which process comprises : a) adding to said phosphoric acid an organic solvent mlscible 1n all proportions with phosphoric acid and water and if need be fi ltering any precipitates present; b) contacting the homogeneous solution of crude phosphoric acid and organic solvent of step (a) with such a quantity of a concentrated aqueous solution of an Inorganic material not forming precipitates during extraction, that the l iquid aqueous salt phase Into which migrate the impurities of the phosphoric acid remains in addition to the phosphoric-acid sol vent phase ; c) separating the organi c sol ution from the aqueous sal t phase after a distribution equilibrium has set i n and optionally after the extraction step has been repeated ; and d) recovering phosphoric acid from said organic solution by distilling off the organic solvent.

2. The process as defined in claim 1 wherein said i norganic material 1s a concentrated aqueous Inorganic solution formed from an Inorganic substance selected from the group consisting of inorganic sal ts , inorganic hydroxides , and mixtures thereof which are soluble in water and which do not form precipitates during step (b) .

3. The process as defined 1n claim 1 wherei n said inorganic material Is a solid Inorganic substance selected from the group consisting of inorganic salts , inorganic hydroxides , and mixtures thereof which are sol uble In water and which do not form precipitates during step (b) .

4. The process as defined in claim 1 wherein said inorganic material 1s an acid salt of phosphoric add.

5. The process as defined 1n claim 1 wherein said organic phase of step (c) 1s passed through a cation exchange resin 1n the H+ form prior to step (d) .

6. The process as defined In claim 1 wherein the organic solvent 1s selected from the group consisting of the aliphatic alcohol s and ketones. 9. The process as defined 1n Claim 8 wherein the aliphatic alcohol 1s Isopropanol. 10. The process as defined in claim 1 wherein the weight ratio of organic solvent to the weight ratio of P^O^ in the phosphoric add to be treated is between about 1:1 to 20:1. 11. The process as defined 1n Claim 2 wherein the volumetic ratio of organic solution to the concentre t4d aqueous Inorganic solution 1s between about 250:1 to 1:10. 12. The process as defined in Claim 9 wherein the ratio is preferably between 5:1 to 10:1. 13. The process as defined in claim 1 wherein steps (b) and (c) are repeatedly performed prior to step (d). 14. The process as defined in Claim 11 wherein the contact and separation steps are performed in an extraction column operated in countercurrent flow. 15. The process as defined in C§|i]im 11 wherein the process 1s carried out batch-wise in a plurality of mixer-settlers operated in countercurrent flow. For the Applicants Wolff, Bregman and Goller