IL47397A - Purification of phosphoric acid - Google Patents

Description

ητιπττ nxam "I Ί D ' τ Purification of phosphoric acid / Produits Chimiques Ugine Kuhlmann - * The present invention provides a procedure for purifying vet-process phosphoric acid. As is veil known, in the vet process, phosphoric acid is made by leaching phosphate rock vith sulfuric acid. The resulting acid is relatively impure. While it is not necessary to purify the crude vet-proces phosphoric acid for making fertilizer grades of triple super phosphate or fertilizer ammonium phosphate, for most other purposes it must be purified, particularly if it is to compete vith the purer grades of thermal process phosphoric acid prepared by the burning of elemental phosphorus* BACKGROUND OF THE INVENTION The vet-process for manufacture of phosphoric acid is described in KIRK-OTHMEB "ENCYCLOPEDIA OP CHEMICAL TECHNOLOGY" (1964) Vol. 9, page 87 et seq.j Vol. 15, page 261 et seq.

Common impurities include sulfate, fluoride, chloride, iron, aluminium, vanadium, chromium, calcium, sodium and ammonium.

In viev of the fact that specific details depend to a great extent on the original rock and the degree of purity desired, a detailed description of any one operation vould not be typical of the industry. The prior art has in general considered it a necessity to employ a succession of precipitations folloved by filtering or other forms of mechanical separation.

Thus, for example, gradual increase in pH by addition of soda ash can successively precipitate the phosphates of iron, aluminium and finally alkaline earths. According to some approaches, sodium silicate has been advocated to remove fluorine, or alternatively, super heated steam is blown through concentrated phosphoric acid to reduce the fluorine content. agents, during or after the treatment with solvent, but such treatments are relatively complicated, are rarely quantitative, and do hot eliminate all the above mentioned impurities* It has also been proposed to pu ify wet-process phosphoric acid by two successive extraction stages both using amines as solvents, but with such a procedure only a small part of the H3P04 is recoverable as pure aqueous extract.

SUMMARY OP THE INVENTION Present applicants have now found a two-part extraction method which is capable of preparing in good yield an extremely pure grade of phosphoric acid from a concentrated aqueous solution of wet-process phosphoric acid. In the first part, the cationic impurities are separated by extracting the phosphoric acid and anionic impurities into a water-immiscible solvent, leaving the cationic impurities behind. The phosphoric acid and anionic impurities are then re-extracted by water and in the second part, the anionic impurities are separated by extracting them into a water-immiscible alkaline liquid medium and leaving a surprisingly pure grade of queous phosphoric acid.

In one embodiment of the new method, the crude wet-process phosphoric acid is first treated with a reducing agent such as a sulfide or a metal-like iron. In this way, elements such as chromium and vanadium which can initially be present in anionic complexes of high oxidative levels, such as chromate or vanjlate, are converted to eationic forms of lower oxidation levels which can then be removed in the first extraction directed to cationic impurities.

Thus, the present invention is a purification method comprising (I) contacting an initial aqueous phase of a crude wet- - di-isopropyl ether, di-n-butyl ether, di-isobutyl ether, di- ^ secondary butyl ether, di-tertiary butyl ether, n-buty-tertiary butyl ether methyl amyl ether, methyl hexyl ether, methyl cyclohexyl ether, methyl heptyl ether, ethyl butyl ether, ethyl isobutyl ether, ethyl propyl ether and the like* A particularly convenient source of alcohols suitable for this purpose are those obtained singly or in mixtures from the Oxo process of hydro-formylating olefins, making use also of veil known processing steps such as cracking, a dol condensation, hydrogenation and the like* A product known commercially as C^-Oxo-aleohol is particularly suitable* After the solvent, exemplarily an alcohol or ether such as above described, is charged vith phosphoric acid in an extraction train of one or more steps, it is vashed one or more times vith water and the phosphoric acid is then re-extracted by water to form an aqueous phosphoric acid solution. Throughout the operation, adjustments are made in the respective flov rates of the aqueous and non-aqueous phases and the number of steps used for each liquid-liquid contact, in a manner depending on the relative amounts of eationie impurities present, so as to optimize the efficiency of removal and obtain a phosphoric acid substantially free of cations, although still containing a certain quantity of anionic impurities* The procedure can be adapted to a counter-current operation by adoption of any of the procedural advances known to those trained in the art* (See the summary of "Liquid-Liquid Extraction" in IR -OTHHER, "ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY" (1964) Volume 8, Page 719 et seq.).

The vashed solvent used in the first part is then recycled droxide or ammonia. In this vay certain organic impurities are ^ eliminated and acids which have accumulated in the solvent are removed* According to the alkali used, the aqueous layer then contains ammonium or, say. sodium phosphates which can be exploited as scdituD tripolyphosphate or as ammonium or sodium phosphate fertilizers. The alkaline aqueous treatment can be applied to the entire amount of tt*fcd solvent or to a part thereof.

The second part of the method of this invention is then directed to separation of the remaining anionic impurities in the aqueous phosphoric acid freed of cationic impurities. Since cationic impurities are substantially absent, the anionic impurities are essentially in the form of acids such as sulfuric acid, hydrofluoric acid and hydrochloric acid. It is advantageous to concentrate the cation-free acid, by evaporation to about 40 by weight ^0?* n *kis manner small amounts of the first solvent dissolved in the aqueous phosphoric acid are also eliminated* The second solvent used for removal of anionic (acid) impurities is an alkaline liquid medium with substantial insolubility in water or aqueous phosphoric acid and poor dissolving power for phosphoric acid. Substances which are particularly satisfactory in possessing such properties are* the secondary and tertiary amines having long water-insoluble hydrocarbon chains and in particular, the solutions of such amines in diluent liquids which has both high compatibility with the amine and substantially no compatibility with the phosphoric acid* The amines suitable for use as extractants for anionic impurities in the method of this invention are the secondary and tertiary amines having a total of from 18 to 60 carbon atoms* alcohols, palmityl alcohol, margaryl alcohol, steary! alcohol, octadecyl alcohols and in particular decylalcohols , undecyl "i, alcohols;' dodecylalcohols , lauryl alcohols, tridecyl alcohols, tetradecyl alcohols, including myristyl alcohol and pentadecyl alcohols. When such an adjuvant is added to the diluent it is advantageous to use from about 5% to 20S of the adjuvant based 7 on the weight of diluent. As to the concentration of amine used, 8 benefits of the instant invention can be obtained with solvents 9 containing from about 5% to 100% by weight of the total solvent.

Although it is possible to eliminate the major part 11 of the anionic impurities by treating in a discontinous manner, 12 in one or more passes, a volume of phosphoric acid with one or 13 more volumes of an amine-based solvent of the above described ' 14 types, it is advantageous to effect this operation with' a minimum 15 of solvent. It is therefore preferable to carry out this operation 16 in a counter-current manner thereby minimizing the quantity of 17 solvent required and at the same time also lowering the amount of 8 phosphoric acid extracted along with the anionic impurities. 19 The anionic impurities and small · fraction of phosphoric] 20 acid extracted by the amine are then re-extracted by an aqueous 21 solution of an alkaline substcince like sodium hydroxide, sodium carbonate or ammonia. The obtained solution contains a mixture of phosphate, sulfate, fluosilicate and chloride containing all the anionic impurities of the acid removed by the amine extractor. When ammonia is used as the re-e tiaci.invJ aocnt, this- solution I I 26 j! particularly useful as fertilizer. Alternatively, a portion of 27;! the phosphate ions contained in this solution mixture can. be !l ■ ' ' ■ ' r j! recovered by crystaJ.J. ?. v xen as ammonium ρϋ:».^.».·; ;>. e crystals,, 251: Be e it.;; recycling to ΐ; ?' extraction ti.-<..ir> , .ise ' . jj ;! egenerate amine-based solvent is washe *« with ^a" ■■::·: in' one r EXAMPLE 1 ^ A train of liquid-liquid extractors has six extracting steps, five washing steps, three steps for re-extracting with water and one step for regenerating the solvent prior to recycling it to the extracting steps. In this example, this train is used for the counter-current treatment of one liter of a wet-process phosphoric acid whose composition is given in column 1 of Table I using 12 liters of a oxo-alcohol as extracting solvent.

This phosphoric acid is an aqueous solution corresponding to 53 by weight ?2°5 vnoae organic matter is initially destroyed by a treatment with dichromate in the ratio of 3 grams of sodium dichromate per liter of the crude phosphoric acid* After this dichromate treatment, vanadium and the excess chromate are reduced by addition of 0.2 grams of iron powder per liter of phosphoric acid.

The solvent charged with phosphoric acid is then washed by a counter-current of ©.35 liters of water, next re-extracted by a counter current of 2 liters of water* The wash waters charged with phosphoric acid and impurities are recycled through the extraction stages* After re-extraction the solvent is regenerated by 0«1 liter of 0$ aqueous sodium hydroxide.

In the course of this operation there is obtained 2*5 liters of phosphoric acid containing 302 grams/liter of P2°5' free of cationic impurities and 0*43 liters of sodium phosphate containing 155 grams/liter of ρ2°5 vnose compositions are shown in columns (2) and (3) of Table I. At this stage, the purified acid still contains greater than negligible quantities of The residual phosphoric acid containing some anionic impurities and all the cationic impurities can be recycled after recovery of the dissolved alcohol to the train for sulfuric Initial Acid Sodium ACid Acid Purified Phosphate Purified Before By ByBy A1amine Alcohol Alcohol product s> o Ex. 1&2 Ex. 1&2 Ex. 1&2 Example 2 % P205 53 25.2 13.4 33.1 Impurities % on P2O5 4.1 3.1 1.4 0.0005. SO4 F 0.35 0.32 0.61 0.01 CI 0.15 0.16 0.09 0.0002 Fe 1.05 0.0005 0.0002 0.0005 Al 0.53 0.0004 0.0002 0.0004 V 0.03 0.0003 0.0002 0.0003 Cr 0.14 0.0003 0.0003 0.0003 Ca 0.08 0.0004 0.0005 0.0004 Na 0.11 0.001 0.001 ' - NH4 0.0005 to 00 o CD CO o r-i r-H <—i (.V. grcanr./j i cer P2O5 containing also ammonium sulfate, chloride and fluosilicate according to the composition figures presented in column - of Table I... The ammonia is used in quantity such as to attain a Ή between about 7 and 8. This solution has .value as a fertilizer. ·. 6 Before being recycled to the extraction, the regenerated 7 solvent, is · washed in two steps with 80 ml/hour of water, this wate 8 serving to dissolve the ammonium salts obtained in the solvent- 9 regeneration step..

. ^ The lightly-colored organic impurities in the amine-11 purified phosphoric acid are- then oxidized by treatment with suf12 ficient hydrogen peroxide. The acid is then concentrated to a 13 54% by weight P2O5 content. The last traces of fluoride ar 14 eliminated in the course of this concentration and the end-productj 15 is colorless phosphoric acid of very' high purity whose composition; le is shown in column 6 of Table I. 17 EXAMPLE ' 3 18 - This Example compares, modifications of the instant in19 vention using respectively a discontinuous and. a counter-curre t 20 procedure, both starting with phosphoric acid as purified and con 21 centrated in Example 1 to a 0'% 2O5 content. 22 Part A. Λ 500-ml sample of said acid is treated di.s- 3 continuously with 500 ml of a 0.5 molar solution of Amberlite LA2 in ke sene containing 10¾ by weight of pxo-alcoho].. The analytical ¾θ¾φοβϋϊ@θ§ of phosphoric acid before and after* ύ ΐί this amine treatment are- given in Table II. Those figures show- that the lev.-- Is of remaining sulfate and fluoride a e divided by- factors Of 5 a d 2.3 respectively. T.o ro Q£¾j[.g£i of tho b'^tal- phosp .. c acid extracted by the amine-- alorig with ¾«■ imparities s i0.2¾* TABLE II Compositions of Phosphoric Acid ^ Before & After Amine Treatment Before 40 3.1 0,3 After 37·5 0,61 0.13 Part B. A corresponding purification is carried out in counter-current fashion using the liquid-liquid extraction train described in Example 2. The proportion of the total phosphoric acid extracted by the Amberlite LA2 is not greater than 8.1$ vhereas the levels of anionic impurities in the purified acid are only 5 ppm and 150 ppm respectively for sulfate and fluoride, the ma$or part of this residual fluoride being subsequently eliminated during the concentration by evaporation of this pure acid* EXAMPLE 4 Using a liquid-liquid extraction train like that of Example a counter-current treatment by 10 liters of oxo-alcohol is given to one liter of vet-process phosphoric acid having a P2^5 content of 5 $ by weight, and having been freed of arsenic by the addition of sodium sulfide and having also had a preliminary treat ment by addition of sodium dichromate for the purpose of destroying organic matter. After contacting the acid, this solvent is vashed by 0.25 liters of the phosphoric acid obtained from the re-extraction steps then re-extracted by 1.4 liters of vater and finally regenerated by concentrated aqueous sodium hydroxide before its recycling to the extraction. There is thus obtained 1,52 liters of phosphoric acid containing 460 grams/liter of p2°5 free of cationic impurities but still containing 3*4$ sulfate* O V chloride and 0.2$ fluoride* This alcohol-purified phosphoric acid is then treated in by a 0.5 molar solution of Alamine 336 in kerosene contai ing .12% of Ci3 oxo-alcohol. The different flow rates are : 0.5 li .ci" per hour of phosphoric acid and of the amine solution of 0.1 liter per hour fox" the water washing the : ammonium-regenera ted solvent. After this treatment, elimination of traces of solvent by activated charcoal -and concentration by evaporation to 54% P205 content, the colorless phosphoric acid obtained contains less than 200 ppm of cationic plus anionic impurities.

It will be understood that it is intended to cover all changes and modifications of the Examples of the invention herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention. /

Claims (4)

1. 8 47397/2 VE CLAIM: 1. A method for purifying crude vet-process phosphoric acid which comprises: 1, contacting an initial aqueous phase of said crude acid vith a water-immiscible solvent for phosphoric acid and allowing phosphoric acid and anionic impurities to be ex- tracted or charged into said solvent and leaving the Anionic impurities in said initial aqueous phase; 2. physically separating the charged solvent from the initial aqueous phase and contacting the charged solvent vith water to allov phosphoric acid and anionic impurities to be re- extracted into a second aqueous phase; 3. contacting the second aqueous phase vith a water-immiscible alkaline liquid medium and llowing the anionic impurities to be transferred to said alkaline medium; and 4. physically separating the second aqueous phase containing pure phosphoric acid.

2. The method of Claim 1 vherein the crude phosphoric acid is first treated vith sufficient of a sulfide or of metallic iron to reduce substantially all of the anionic forms of metallic impurities to cationic forms*

3. The method of claim 1 wherein the solvent for phosphoric acid used in step (l) is a monoalcohol or ether having four to eight carbon atoms or any mixture of these*

4. The method of claim 4 vherein the solvent is an Oxo-alcohol. 5· The method of claim 3 vherein the solvent is an ether* 6. The method of claim 1 vherein the second aqueous phase is concentrated by evaporation to a I*205 βοη*βη* of a* least about 40 by veight before contacting the alkaline medium as 7. The method of Claim 1 wherein the alk l ne liquid medium contains from about 5% to 100% of a secondary or tertiary amine having from 18 to 60 carbon atoms. ^¾ 8. The method of Claim 7 wherein the alkaline medium contains the amine and a diluent ,which has both high compatibility with the amine and substantially no compatibility with phosphoric acid. 9. ~ The method of Claim 8 wherein the diluent is common kerosene. 10. The method of Claim 8 wherein the diluent contains also from about 5% to 20%, based on the weight of diluent, of one or more monoalcohols having from 7 to 18 carbon atoms. 11. The method of Claim 10 wherein the monoalcohol is an alcohol produced by oxo-sy thesis. 12. The method of Claim 1 wherein the charged solvent, after being physically separated as in step (2) but before being re--extracted by an aqueous phase, is first washed one or more times with water, and wherein the wash waters thus obtained are used as 'diluent for alkali or ammonia to be used for regenerating the extracting solvents of steps (1) and (3) respectively. 13. The method of Claim 1 wherein the solvent, after re-extraction of phosphoric acid into the second aqueous phase as in step (2) , is regenerated with aqueous concentrated sodium hydroxide solution to produce a solvent fcefficiently £o*: recycling in step (1) and to form a by~pr