GB2078694A - Crystallizing phosphoric acid - Google Patents

Abstract

Wet process phosphoric acid containing 64-75% P2O5 and 50-1000 ppm Al and preferably with a total metals content of 0.3-0.7%, e.g. from Nauru rock is crystallized to form purified crystals of phosphoric acid in its anhydrous or hemi hydrate form. An integrated process with two crystallizations from the mother liquor, recycle of crystals produced to an earlier stage of the crystallization and a recrystallization of the product crystals gives a 60% yield of highly purified acid.

Description

SPECIFICATION Purification process This invention relates to the crystallization of phosphoric acid.

Phosphoric acid in industry is derived from "thermal" acid which is very pure and made from elemental phosphorus, or crude wet process phosphoric acid, made from phosphate rock and sulphuric acid. It is known to crystallize thermal phosphoric acid to form crystals of an hydros or hemihydrate acid, and many people have tried to crystallize wet process acid but very few have been successful and for a number of reasons no commercial process involving crystallization has been operated commercially for any significant period. The problems with the crystallization result from the impurities in the acid, which drastically increase the viscosity of the acid making the crystallization difficult to control and making the removal of mother liquor from the crystals difficult.Some of the impurities aiso tend to cocrystallize with the acid resulting in less purification and in some cases concentration of impurities in the crystals. Purification by crystallization has been described in association with other prior purification techniques i.e.-chemical purification as in Jap. Patent 44-14692 or solvent purification as in BP 1436115 or evaporation to at least 76% P205 concentration and separation of precipitated impurities, as in OLS 2815880. All these prior purification processes give substantially pure acids containing e.g. less than 1000 ppm total metals before the crystallization stage, so that again only an essentially already pure acid is crystallized. All these purification procedures suffer from disadvantages of too many stages of different types of procedure leading to uneconomic processes.

We acid. We have now found how to obtain phosphoric acid of substantial purity directly from wet process acid.

The present invention provides a process for preparing purified phosphoric acid, wherein wet process phosphoric acid containing 64-75% P205 50-1000 ppm Al and preferably 0.01-0.08% Mg, and 0.010.8% Ca and 0.010.8% Fe is cooled to produce crystals of purified anhydrous or hemi hydrate phosphoric acid and a mother liquor and the crystals are separated from said mother liquor.

Preferably said crystals are recrystallized at least once more e.g. 1-5 more times.

The wet process acid may contain 0.001-0.1% Ca such as 0.01-0.1% Ca e.g. 0.01--0.05% Ca, 0.01--0.6% Mg such as 0.10.5% Mg such as 0.10.35% Mg, 0.01--0.8% Fe e.g. 0.01-0.5% Fe such as 0.10.3% Fe, 50 ppm-1 000 ppm Al e.g. 50-500 ppm Al, 0.01--0.8% F such as 0.010.4% F e.g. 0.01--0.2% F and 1--3% SO4 or less than 1% SO4 e.g. 0.01-0.5% such as 0.10.5% 504. The acid may contain 0.010.8% Zn such as 0.01-0.2% Zn, as well as other impurities such as Cr, Ti, Si, Mn, Na usually in individual amounts of less than 800 ppm usually less than 200 ppm each and a total of less than 0.2%. Preferably the acid contains less than 1% of total metals e.g. 0.1-1% such as 0.30.85% especially less than 0.7% such as 0.30.7% total metals. Preferably the acid contains less than 0.1% organics (expressed as carbon) e.g. 1-500 ppm organics.

The wet process acid is usually derived from calcined phosphate rock particularly Nauru phosphate rock after calcination e.g. at 750-1 0000C to remove its organics and sometimes its carbonate content and reduce its F and Si content. Alternatively calcined phosphate rock from Zin in Israel Youssoufia, Khouribga, Bu Craa and Jordan may be used. The rock is treated with a mixture of sulphuric and phosphoric acid to form calcium sulphate and wet process acid, which are separated. The wet process acid is then concentrated e.g. from the 28-50% P205 acid made from the rock. The concentration may be by high temperature evaporation e.g. by vacuum evaporation to give 64-68% P2O5 acid or by submerged combustion or by evaporation by direct electrical heating, using the electrical cell as described in OLS 2815880.Any solids e.g. calcium sulphate separating during or after the concentration are removed before the crystallization. Advantageously the acid is concentrated to an intermediate level of concentration e.g. 50-68% P205 by e.g. vacuum evaporation and then concentrated further if needed e.g. by the direct electrical heating process. Usually the crystallization is carried out directly on the product of concentrating the wet process acid made directly from the rock calcined or otherwise with no extra purification steps to remove cation impurities (and preferably no extra purification steps to remove anion impurities) apart from separation of any solids forming during or after the concentration and of course separation of the solid comprising calcium sulphate in the production of the wet process acid.Thus preferably no solvent treatment is carried out to precipitate or otherwise remove impurities or to separate acid into an organic phase from which acid is recovered, and no addition of reagents causing precipitation of impurities in the wet process acid before, during or after concentration. The concentration of the wet process acid is raised to a level not greater than 75% P205.

The wet process acid of 64-75% P205 concentration is agitated and cooled, preferably with addition of seed crystals of purified phosphoric acid or thermal phosphoric acid. The acid is cooled to a temperature usually 3-400C e.g. 1 0-400C less than the melting point of phosphoric acid of that P205 concentration; the crystallization temperature is lower the lower the P2O5 content of the acid and the higher the total metal impurity content. For 72% acid the acid is usually cooled to 0-400C and for 66% P205 acid the acid is usually cooled to 20C to + 1 50C. The wet process acid preferably has a concentration greater than 72.4% P205 if anhydrous acid is wanted though acids of 68-75% P205 e.g.

70-75% may be used for this purpose or 66-68% P205 if hemi hydrate acid is wanted. The crystallization is advantageously completed in 0.1-10 hr e.g. 0.3-2.0 hr. The magma is then separated in the minor impurities e.g. Zn, Ca and Al to contents of less than 100 ppm each.

Advantageously the crystals are remelted and are recooled to produce repurified crystals, again preferably in the presence of seed crystals and this procedure may be repeated 1-4 more times to give a very highly purified acid. Crystals from acids having 64-75% P205 and up to 0.4% Mg (e.g. for a 7075% P205 acid) often need recrystallization only once while those from corresponding acids with 0.1 0.8% Mg or 0.40.6% Mg (again e.g. for a 70--750Jo P205 acid) are often recrystallized twice to give similar purity acids. Advantageously the mother liquor from the or each recrystallization is recycled to the previous crystallization stage.

In order to maximize the yieid of purified acid, it is preferable to take an initial crop of crystals containing 3070% of the P208 in the acid being crystallized and then recover further successive crops of crystals from the mother liquor, preferably 1 or 2 such further crops. Each successive crop of crystals is preferably remelted and mixed with the mother liquor from the previous crystallization. Preferably also each crystallization stage is operated to give 3070% e.g. 45-65% of the P205 in the acid which is crystallized being recovered as crystals.Thus in an integrated process a concentrated wet process acid is separated into two fractions, one highly purified acid, which is two or three times crystallized, and an impure mother liquor from which 2 or 3 successive crops of crystals have been removed.

Preferably in the process, the crude acid is crystallized and separated into first purified crystals and a first mother liquor, which are separated, the first mother liquor is cooled further to produce second purified crystals which are separated from second mother liquor, said second purified crystals are melted to give a second acid which is mixed with the crude acid before crystallization, the first purified crystals are melted to give a first acid and the acid cooled to give third purified crystals which are separated from a third mother liquor, which is recycled to mix with the crude acid before crystallization, in each of the 3 crystallizations 3070% of the P205 values in the liquid acid being separated as crystals; advantageously the third purified crystals are melted to give a third acid which is cooled to give fourth purified crystals which are separated from a fourth mother liquor which is recycled to mix with the first acid before the crystallization of the third purified crystals.

While it is possible to obtain two successive crops of crystals from the mother liquor from the first crystallization, we have found that with some acids solids in particular a magnesium dihydrogen phosphate-phosphoric acid complex tends to be deposited easily in the second successive crystallization from the mother liquor and with difficulty and slowly in the first successive crystallization.

This problem becomes important when the Mg content of the acid being crystallized at that stage reaches 7700 ppm (e.g. for a 6875% P205 acid). With original acids of 6475% P205 content from which the first crystals were obtained in which the Mg content of the original acid was above 0.4%, one successive crystallization only is desirable and that should be done quickly within 3 hr; the original acids with Mg contents of up to 0.4% e.g. 0.1-0.35% Mg (e.g. for a 7075% P205 acid) as in acid from calcined Nauru rock can be submitted to the two successive crystallizations from the mother liquor usually without such deposition problems, at least when yields of 3060% of crystals are taken at each stage.

Final decolourization of the purified acids from the effects of any residual organic impurities may be carried out if needed or desired by treatment with activated carbon. In this way water white acid may be obtained by twice crystallization from the green crude acid.

Advantageously the invention is carried out as shown in the accompanying drawing which is a block diagram of an integrated process for converting dilute wet process acid into purified crystalline anhydrous acid.

Dilute wet process phosphoric acid 1 is concentrated e.g. in an electrically heated graphite cell in 2 and the concentrated acid filtered if necessary to remove precipitate. The filtered concentrated acid is mixed with melted recrystallized acid 9 and mother liquor 22 from a recrystallization of the crystals first produced. The concentrated acid at 4 is cooled, seeded and allowed to crystallize to give crystals 5 and mother liquor 6, which are separated. Mother liquor 6 is mixed at 7 with melted crystals 8, cooled further, reseeded, and a further crop of crystals 9 is obtained with mother liquor 10; the crystals 9 and mother liquor 10 are separated. Crystals 9 are melted and passed to the initial crystallization stage while mother liquor 10 is cooled and seeded at 11 to give a further crop of crystals 8 and mother liquor 12, which are separated.Crystals 8 are melted and recycled to 7 while mother liquor 1 2 may be converted into fertilizers or industrial phosphates such as tripolyphosphates.

First crop crystals 5 are remelted and cooled and crystallized at 20 with addition of seed crystals of purified acid to give further purified crystals 21 and mother liquor 22 which is recycled to the first crystallization 4. The purified crystals 21 may be used in metal finishing solutions, or even in food, depending on their impurity content. The purified crystals may, in a process not shown, be melted, cooled and crystallized again to give trebly purified crystals, which are separated, the mother liquor being recycled to the first recrystallization at 20.

The invention is illustrated in the following Examples in which percentage are by weight. The crude acid for Ex. 1 and 2 was obtained as follows. Nauru apatite phosphate rock was calcined at 8500C to give a calcined rock, which was then treated with a mixture of sulphuric and phosphoric acids containing sulphuric acid in amount to react with 98% of the Ca content of the rock, to give gypsum and wet process acid which were separated.The wet process acid contained 33.4% P205,0.51 504, 0.31% F, 0.13% Fe, 0.13% Mg, 0.17% Ca, 680 ppm Na, 290 ppm Zn, 110 ppm AI, 13 ppm Cl, 55 ppm K, 1 70 ppm Si as well as other metallic and non metallic inorganic impurities in total amount of less than 300 ppm and less than 25 ppm organic impurities (expressed as carbon).

The wet process acid was concentrated by evaporation by direct electrical resistance heating in a cell as described in OLS 2815880, to give a concentrated acid containing 73.0% P205,0.22% Fe, 0.21% Mg, 640 ppm Zn, 190 ppm Ca, 96 ppm Al, 140 ppm Cr, 0.0785% F, 0.26% SO4, 5.4 ppm As and other metallic and non metallic impurities in amount of less than 700 ppm. A small amount of solids were separated from the hot concentrated acid.

The warm filtered concentrated acid was placed in a crystallizer fitted with paddle stirrer and cooled to 350C. Seed crystals of purified anhydrous phosphoric acid were added and after further cooling to 300C over 45 min, the crystals which had formed were separated from the magma in a centrifuge to give a 50% recovery of crystalline anhydrous phosphoric acid containing 380 ppm Fe, 350 ppm Mg, 88 ppm Zn, 60 ppm Ca, 39 ppm Al, 25 ppm Cr, 180 ppm F, 340 ppm SO4 and 0.9 ppm As.

The crystalline anhydrous acid obtained was melted and crystallized to 340C with added seed crystals of purer acid to give a magma from which centrifugation gave a 50% recovery of recrystallized anhydrous phosphoric acid containing 39 ppm Fe, 36 ppm Mg, 10 ppm Zn, 10 ppm Ca, 2 ppm Al, 4 ppm Cr, 35 ppm F, 40 ppm SO4 and 0.2 ppm As.

Two further crops of crystals were obtained from the mother liquor from the crystallization of the concentrated acid in amounts (expressed as recovery of crystals as a percentage of original concentrated acid) and with purities (in ppm) as given below. The crystallization conditions of temperature and time were 30oC for 50 min for the secdnd crop and 1-5 C for 100 min for the third crop.

Crop Recovery % Fe Mg Zn Ca Ai Cr F As S04 Second 25 1500 1400 350 260 52 82 435 5.3 1500 Third 12.5 4100 3800 1000 270 180 250 410 10.6 5100 EXAMPLE 2 An integrated crystallization process was carried out with process operations as shown in the accompanying drawing. The initial concentrated acid was as described in Example 1 and was mixed with remelted crystals from the second crystallization of mother liquor and with mother liquor from the recrystallization of the crystals from the first crystallization. The crystallization temperature for the first crystallization was 360C and the crystallization time was 40 min. Other crystallization temperatures and times were as described in Ex. 1. In each case seed crystals were added. A 50% split of P205 between the crystals and mother liquor at each stage was performed.A total recovery of 60% of twice purified crystals of anhydrous phosphoric acid was obtained. Analyses of the twice purified crystals (21) and the residual mother liquor (from 11) were as follows (in ppm).

Fe Mg Zn Ca Al Cr F As S04 Crystals 13 12 3 3 < 1 < 1 12 < 0.1 13 Mother Liquor 5485 5237 1595 470 240 350 1945 15 6480 EXAMPLE 3 In a similar manner to that of Ex. 1, a wet process acid was made from calcined Zin (Israel) rock, concentrated and any solids separated, then crystallized once at a 50% split of crystals to mother liquor, the crystals remelted and recrystallized again with a 50% split of crystals to mother liquor and the original mother liquor cooled and crystallized again. Analyses of the various fractions (amounts being expressed in part per million by weight apart from the P205 value) were as follows.

Acid %P205 Fe Mg Zn Ca Al Cr F S04 1 71.2 1400 4600 920 47 140 200 2270 20600 2 72.3 380 1000 240 40 40 62 740 5700 3 73.4 45 71 11 2 3 8 280 1000 4 70.7 1400 7700 1100 96 150 210 2130 28400 5 66.3 3200 7400 1800 36 290 430 3430 37600 The crystallization times and temperatures were similar to those in corresponding stages of Ex. 1 but generally 1-50C lower. In this table acid 1 is the crude concentrated acid, acid 2 is the first crystals, acid 3 is recrystallized acid from those crystals, acid 4 is the crystals from the first mother liquor and acid 5 is the second mother liquor from the crystallization of acid 4.

EXAMPLE 4 An identical operation to that of the process of Ex. 3 was performed with wet process acid made from calcined Youssoufia phosphate rock apart from a double recrystallization of the crystals of acid first obtained under conditions similar to thse of the first crystallization in Ex. 3. The results were as follows, analyses expressed being expressed as in Ex. 3.

Acid %P205 Fe Mg Zn Ca Al Cr F S04 1 70.1 2000 4200 1100 74 160 570 2300 25700 2 72.0 550 1000 270 51 40 170 780 5800 3 71.7 54 100 22 15 4 24 311 725 4 72.0 2 11 < 2 3 < 1 5 30 60 5 69.0 2480 5145 1300 25 140 600 2300 33250 6 65.0 5320 10050 2900 40 420 1300 2500 53350 In this table acid 1 is the concentrated acid, acid 2 is first crystals, acid 3 is the recrystallized acid from those crystals, acid 4 is the acid obtained by crystallization of the melted acid 3, acid 5 is the crystals from the first mother liquor and acid 6 is the second mother liquor from the crystallization of acid 5.

Claims (16)

1. A process for preparing purified phosphoric acid, wherein crude wet process phosphoric acid containing 61 75% P205 and 50-1000 ppm aluminium (expressed as Al) is cooled to produce crystals of purified anhydrous or hemihydrate phosphoric acid and a mother liquor, and the crystals are separated from said mother liquor.

2. A process according to claim 1 wherein the wet process acid contains 0.01-0.8% Mg.

3. A process according to claim 2 wherein the wet process acid contains 0.1-0.5% Mg.

4. A process according to claim 3 wherein the wet process acid contains 6875% P205 and 0.10.35% Mg.

5. A process according to any one of claims 1 4 wherein the wet process acid contains 0.010.8% calcium (expressed as Ca) and 0.010.8% iron (expressed as Fe).

6. A process according to any one of claims 1-5 wherein the wet process acid contains 50-500 ppm AI.

7. A process according to any one of claims 1-6 wherein the wet process acid contains 0.30.7% total metals (expressed as their elements).

8. A process according to any one of claims 1-7 wherein the separated crystals are melted and recrystallized to produce a recrystallized purified acid, which are separated from a second mother liquor.

9. A process according to claim 8 wherein the second mother liquor is recycled to the initial crystallization.

10. A process according to claim 9 wherein the crude acid is crystallized and separated into first purified crystals and a first mother liquor, which are separated, the first mother liquor is cooled further to produce second purified crystals which are separated from second mother liquor, said second purified crystals are melted to give a second acid which is mixed with the crude acid before crystallization, the first purified crystals are melted to give a first acid and the acid cooled to give third purified crystals which are separated from a third mother liquor, which is recycled to mix with the crude acid before crystallization, in each of the 3 crystallizations, 30-70% of the P205 values in the liquid acid being separated as crystals.

11. A process according to claim 10 wherein the third purified crystals are melted to give a third acid which is cooled to give fourth purified crystals which are separated from a fourth mother liquor which is recycled to mix with the first acid before the crystallization of the third purified crystals.

12. A process according to any one of claims 1-11 1 wherein the crude acid has been made by acidification of phosphate rock, separation of calcium sulphate from a phosphoric acid which is concentrated in at least one step and any insoluble solids separated.

13. A process according to claim 12 wherein the crude acid for crystallization has been made from a dilute phosphoric acid by concentration without any extra purification steps involving addition of any precipitating reagents or any organic solvent.

14. A process according to any one of claims 1-1 3 wherein the crude acid is derived from calcined Nauru rock.

1 5. A process according to claim 1 substantially as described in any one of the Examples.

16. Purified phosphoric acid prepared according to any one of claims 1-15.