GB1571949A

GB1571949A - Purification of phosphoric acid

Info

Publication number: GB1571949A
Application number: GB1454677A
Authority: GB
Original assignee: RTL Contactor Holding SA
Current assignee: RTL Contactor Holding SA
Priority date: 1978-04-05
Filing date: 1978-04-05
Publication date: 1980-07-23

Description

(54) IMPROVEMENTS RELATING TO THE PURIFICATION OF PHOSPHORIC ACID (71) We, RTL CONTACTOR HOLDING S.A., a Swiss Company of 1A Terrassering, 6300 Zug, Switzerland, formerly ROHRTIL S.A., a Swiss Company, of Riva Caccia 1, Lugano, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the purification of phosphoric acid, especially phosphoric acid obtained by the so-called "wet process" by acidulation of phosphate rock.

Many proposals have been submitted for the solvent extraction purification of wet process phosphoric acid, for example, involving extraction with alcohols, such a butanol, alkyl phosphates, certain ethers, ketones, such as acetone and other unsubstituted acyclic dialkyl ketones, mixtures of ethers and ketones and esters, such as aliphatic esters, glycol esters and cycloaliphatic esters. All processes involving these solvent extraction techniques involve three separate stages of activity.

1. The extraction of phosphoric acid from the crude wet process phosphoric acid to the solvent by means of one at least, and generally many more, mixer-settlers or their equivalents in terms of columns.

2. The removal of impurities by a scrubbing operation on the extract (i.e. solvent laden with phosphoric acid) by use of a scrubbing liquor of phosphoric acid with an impurity content sufficiently low to enable the extrac tion of impurities from the extract. This scrub bing stage is carried out by means of many mixer-settlers, (as many as five or six are known known), or their equivalent in columns.

3. The extract (i.e. solvent laden with phos phoric acid) is then contacted with water or an aqueous base to release the phosphoric acid from the solvent to the aqueous phase free of the undesired impurities. This operation is by means of at least one, and generally more, mixer-settlers or their equivalents in terms of columns.

Such a multiple-stage extraction process is clearly cumbersome and costly, as well as having a limited efficiency.

According to the present invention we provide a process for the purification of phosphoric acid comprising supplying a substantially waterimmiscible extractant for phosphoric acid to the upstream end (with respect to extractant flow) of a liquid-liquid contactor, withdrawing extractant from the downstream end of the contactor, supplying crude phosphoric acid to the contactor at a first zone spaced from the upstream end of the contactor, supplying a back extraction medium to the downstream end of the contactor, withdrawing impurities from the upstream end of the contactor and withdrawing purified phosphoric acid from a second zone, downstream of the first zone but spaced from the downstream end of the contactor.

The contactor may suitably be a modification of the type described in British Patent Nos.

972,035 and 1,037,573 except that the impure phosphoric acid is introduced, and the purified phosphoric acid is withdrawn, at zones spaced from the ends of the contactor. Such a contactor is suitably provided with rotating buckets which ensure that the higher density liquid phase is continuously lifted and caused to fall through the lower density phase, while the lower density phase is carried down and allowed to rise through the higher density phase, thus ensuring efficient contacting.

The invention will now be described by way of example with reference to the accompanying flow diagram.

In the diagram 1 represents a liquid-liquid contactor, which is a modification of the type described in British Patents Nos. 972,03 5 and 1,037,573. The contactor is intended to contain substantially immiscible liquids normally meeting at a liquids interface shown as a dotted line 2. The contactor is divided into interconnecting compartments by partitions carrying rotating buckets to lift the liquid of higher specific gravity and allow it to fall back through the liquids interface, and also take the liquid of lower specific gravity down to pass back up through the liquid of higher specific gravity.

Crude phosphoric acid, suitably wet process phosphoric acid containing impurities such as iron and magnesium salts, enters contactor 1 from feed line 3 at a position "A" in the shell of the contactor which is spaced from the ends 4 and 5 of the contactor but nearer the "upstream" end 4. A suitable substantially water immiscible extractant for the phosphoric acid for example methyl isobutyl ketone, is introduced from line 6 at position C in the upstream end 4 of the contactor. Extractant is withdrawn from the downstream end 5 and recycled via line 7.

Water or aqueous base, capable of back extracting phosphoric acid from the extractant, is introduced at position J. The desired product, i.e. purified phosphoric acid, is withdrawn from position H via line 8. Position H, like position A, is spaced from the ends 4 and 5 of the contactor but is nearer the downstream end of the contractor. The waste raffinate, containing the impurities, is withdrawn at position B in the upstream end 4 via line 9.

The way in which this single stage purificatior is believed to occur is best illustrated by considering the contactor as divided into three theoretical zones a, b, and c by dotted lines 10.

It must be emphasised that these zones have been drawn for the purpose of illustration and the lines 10 do not represent any physical divisions in the contactor 1.

In zone a the crude phosphoric acid stream entering at A and the extractant entering at C will flow in counter-current. The action of the rotating buckets results in phosphoric acid being taken up by the solvent along with some proportion of the iron and magnesium salts, for example about 5%, leaving 95% of the impurities in aqueous solution. These are withdrawn at B.

The extractant, laden with phosphoric acid and also still containing some, e.g. 5%, of the impurities, passes to zone b in which the remain ing impurities are removed by a "scrubbing" operation. This is believed to be effected by back flow of purified phosphoric acid from zone c (illustrated by arrow d), which purified phosphoric acid is flowing countercurrent to the phosphoric acid-laden extractant passing through zone b. Thus the remaining 5% impuri ties will be withdrawn from the extractant to the back flow of purified phosphoric acid, which subsequently passes to zone a and is withdrawn at B with the impurities already removed in zone a.

The extractant, laden with phosphoric acid, but free of impurities, passes to zone c where it flows countercurrent to the water or aqueous base stream entering at J. Pure phosphoric acid is thus transferred to the aqueous phase and withdrawn at H, leaving the released extractant free for recycle via line 7.

The entry and withdrawal positions A and H for the impure and pure phosphoric acid are selected for optimum process efficiency, being variable for each case, so long as they are spaced from end walls 4 and 5. Suitable length ratios for the spacing of position A from end wall 4 to the spacing between A and H to the spacing of position H from end wall 5 are 2:4:3 or 3:4:3.

The following Table gives the compositions of the various streams at positions A, B, C, H and J and in the back flow of purified acid indi cated by arrow d obtainable using a contactor as described above with a methyl isobutyl keton (MIBK) extractant entering at C and water entering at J.

Thus it can be seen that a high purification of at least about 90% can be obtained. Large flow rates can be used in the contactor while still maintaining extraction efficiency. Of course the flow rates can be varied to give optimum extrac tion. The positions A and H can be varied as TABLE

Flow % Composition % Impurities Position (gallons H3P04 MIBK HzO Fe Mg S03 perhour A 6.5 77.4 205 0.23 0.25 131 B 1.0 56.0 16 34.2 153 1.66 496 C 18.0 0.7 98.0 1.3 J 4.0 100 H 9.5 56.0 1.6 419 25ppm Sppm 0.5 Backflow at 4.5 56.0 1.6 419 25ppm Sppm 0.5 arrow d discussed above. By varying the rate of product take-off the raffinate can be reduced to enable maximum conversion of crude phosphoric acid to purified phosphoric acid.

WHAT WE CLAIM IS: 1. A process for the purification of phosphoric acid comprising supplying a substantially water-immiscible extractant for phosphoric acid to the upstream end (with respect to extractant flow) of a liquid-liquid contactor, withdrawing extractant from the downstream end of the contactor supplying crude phosphoric acid to the contactor at a first zone spaced from the upstream end of the contactor, supplying a back extraction medium to the downstream end of the contactor withdrawing impurities from the upstream end of the contactor and withdrawing purified phosphoric acid from a second zone downstream of the first zone but spaced from the downstream end of the contactor.

2. A process according to claim 1, wherein the crude phosphoric acid is crude wet process phosphoric acid.

3. A process according to claim 1 or 2, wherein the extractant is methyl isobutyl ketone.

4. A process according to claim 1, 2 or 3, wherein the back extractant is water or an aqueous base.

5. A process according to any one of claims 1 to 4, wherein contacting is carried out by causing the higher density liquid to be continuously passed through the lower density liquid and vice versa.

6. A process according to claim 5, wherein the contactor has rotating buckets for lifting the higher density liquid for its passage through the lower density liquid, and lowering the lower density liquid for its passage through the higher density liquid.

7. A process according to any one of the preceding claims wherein the length ratio of the distance of the inlet means to the first zone from the upstream end to the distance between the second zone to the distance of the outlet means from the second zone from the downstream end is 2:4:3 or 3:4:3.

8. A single-stage process for the purification of phosphoric acid substantially as described herein with reference to the accompanying flow diagram.

9. Purified phosphoric acid when obtained by the process of any one of claims 1 to 8.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. discussed above. By varying the rate of product take-off the raffinate can be reduced to enable maximum conversion of crude phosphoric acid to purified phosphoric acid. WHAT WE CLAIM IS:

1. A process for the purification of phosphoric acid comprising supplying a substantially water-immiscible extractant for phosphoric acid to the upstream end (with respect to extractant flow) of a liquid-liquid contactor, withdrawing extractant from the downstream end of the contactor supplying crude phosphoric acid to the contactor at a first zone spaced from the upstream end of the contactor, supplying a back extraction medium to the downstream end of the contactor withdrawing impurities from the upstream end of the contactor and withdrawing purified phosphoric acid from a second zone downstream of the first zone but spaced from the downstream end of the contactor.