GB1601428A

GB1601428A - Process for purifying phosphoric acid

Info

Publication number: GB1601428A
Application number: GB21075/78A
Authority: GB
Original assignee: Rhone Poulenc Industries SA
Current assignee: Rhone Poulenc Industries SA
Priority date: 1977-05-23
Filing date: 1978-05-22
Publication date: 1981-10-28
Also published as: IT1105409B; IT7849488A0; DE2822303B2; JPS5413497A; CA1109635A; NL7805497A; PH13509A; BR7803244A; ES470068A1; DE2822303A1

Description

(54) PROCESS FOR PURIFYING PHOSPHORIC ACID (71) We, RHONE-POULENC INDUSTRIES, a French body corporate, of 22 avenue Montaigne, 75 Paris (8), France, 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 : The present invention concerns a process for purifying a wet-process phosphoric acid and, more particularly, concerns a defluorination process for producing a phosphoric acid of alimentary quality.

It is known that the acid attack of phosphate-bearing natural rocks provides, after filtration, a crude phosphoric liquid containing numerous anionic and cationic impurities, including metal ions, such as ions of Al and Fe, and anions, such as those from the attack acid and fluoride and fluosilicate, which result from impurities in the rocks. This impure acid is unsuitable for certain uses, and it is for this reason that different purification processes have been proposed. In view of Section 9 of the Patents Act 1949, attention is directed to the extraction and purification processes claimed in Patents Nos 1210446 and 1472970 respectively.In particular, the presence of the fluorine, irrespective of whether it is in the form of fluoride ion, fluosilicic ion, a metal-fluorine complex or otherwise, prevents it from being used in the foodstuff industry, which particularly requires a phosphoric acid whose fluorine content by weight, relative to the content of P2O5 of the aqueous solution, is not more than 10 parts per million (ppm).

Different processes for removing the fluorine have been proposed, which in particular comprise precipitating the fluorine from the crude phosphoric liquid in the form of alkali metal or alkaline-earth metal fluosilicate. Another type of process comprises entraining the fluorine from the crude acid by steam or a hot gas in the form of a volatile material (HF or SiF4). The best result which such processes can provide is a phosphoric acid having an F content of 500 ppm. based on P2O5, which stili renders it unsuited for use in the foodstuff industry.

The present invention provides a process for purifying wet-process crude phosphoric acid, comprising counter-current extraction of the liquor resulting from the acid attack on phosphate-bearing rocks by means of a solvent that has low miscibility with water, e.g. an alkyl phosphate or a suitable aliphatic alcohol, with the addition, before or during extraction, of concentrated sulphuric acid, whereby extraction of the phosphoric acid by the said solvent is facilitated and improved, if necessary desulphation of the resulting organic phase in counter-current by means of an aqueous solution containing phosphate and calcium ions, washing the organic solution by means of water or purified phosphoric acid, counter-current extraction of the phosphoric acid from the organic phase by means of water, whereby there is produced an aqueous solution of phosphoric acid that is substantially free from cationic impurities, and finally treatment of this purified aqueous solution by means of steam or other hot gas, whereby the final product obtained is a concentrated solution of defluorinated phosphoric acid whose F:P2O5 ratio by weight is not more than 10 ppm.

In one embodiment, phosphoric acid contained in impure solutions resulting from acid attack on natural rocks is continuously extracted in the form of a purified and relatively concentrated solution and in a proportion of higher than 90%, by treating an impure phosphoric acid solution in a first counter-current extraction apparatus or series of such apparatuses by means of an alkyl phosphate adding sulfuric acid at one or more points in the first extraction apparatus or series of extraction apparatus, extracting the solution of phosphoric acid in the solvent in the reverse direction by means of water in the form of an aqueous solution in a second counter-flow extraction apparatus or series of such apparatuses, the solution of phosphoric acid in the solvent being washed in a third counter-current extraction apparatus or series of such apparatuses by a fraction of the aqueous solution issuing from the second countercurrent apparatus or series, the said fraction, upon discharge from the third counter-current apparatus or series, being added in the first extraction apparatus or series to the flow of the initial impure phosphoric acid solution, returning to the first extraction apparatus or series the solvent issuing from the second extraction apparatus or series, collecting at the discharge from the first extraction apparatus or series a solution that is free of phosphoric acid and that contains the sulphuric acid added, and at the discharge from the second extraction apparatus or series the purified solution of phosphoric acid, whereby there is produced an aqueous solution of phosphoric acid that is substantially free from cationic impurities and is partially defluorinated, in which the aqueous phosphoric acid solution that has been purified in this way, which issues from the second series of counter-current extraction apparatus, is additionally subjected to a treatment by means of steam or other hot gas, whereby the final product collected is a concentrated solution of defluorinated phosphoric acid, in which the F:P2O5 ratio by weight is not more than 10 ppm.

In this embodiment, the aqueous phosphoric acid solution issuing from the second extraction apparatus or series usually has a phosphoric acid concentration expressed as P205 of from 15 to 30%, preferably from 20 to 26%, a fluorine concentration expressed as the F: P2O5 ratio by weight of from 1000 ppm to 7000 ppm with substantially all the fluorine being present in the form of hydrofluoric acid and fluosilicic acid.

In a second embodiment, the phosphoric acid contained in the impure solutions resulting from the acid attack on natural rocks is extracted continuously in the form of a purified and relatively concentrated solution and in a proportion of higher than 95% by a method in which in a first step, the impure phosphoric acid is extracted in counter-current in the presence of sulphuric acid and water by means of an aliphatic alcohol containing less than 8 carbon atoms in the molecule; in a second step, the separated organic phase, which contains phosphoric acid and sulphuric acid, is treated by contact by means of an aqueous solution containing phosphate ions and calcium ions in a solution prepared from calcium phosphate, phosphoric acid and water, in respective amounts such that the content of calcium ions in solution is from 2 to 6% by weight expressed as CaO, so as to precipitate the sulphate ions in the form of calcium sulphate and to put them in suspension in the solution; in a third step the organic phase, which still contains the phosphoric acid, is washed by means of water; and in a fourth step, the phosphoric acid is extracted from the organic phase by means of water and the organic solvent is separated from a purified aqueous solution, which forms the product, whereby there is produced an aqueous phosphoric acid solution which is substantially free from cationic impurities and partially defluorinated, in which the aqueous phosphoric acid solution that has been purified in this way is additionally subjected to a treatment by means of steam or other hot gas, whereby the final product collected is a concentrated solution of defluorinated phosphoric acid in which the F:P2O5 ratio by weight is not more than 10 ppm.

In this second embodiment, the aqueous phosphoric acid solution resulting from re-extraction of the organic phase by water in counter-current generally has a phosphoric acid concentration expressed as P205 of from 15 to 30% and preferably from 20 to 26% by weight and a fluorine concentration expressed in the form of the F:P2O5 ratio by weight of from 1000 ppm to 7000 ppm, the fluorine being substantially present in the form of hydrofluoric acid and fluosilicic acid.

The treatment of the preceding substantially cation-free aqueous solution by means of steam or other hot gas is an important feature of the process of the invention. Taking into account the fluorine-bearing species present in the solution, under these conditions it is possible to entrain the fluorine in the vapour phase in the form of hydrofluoric acid and SiF4, which is separated from the aqueous solution, and to exhaust the solution in respect of fluorine until the value of the ratio F:P205 is lower than 10 ppm. When this is done, the aqueous solution of phosphoric acid, which is generally in the dilute condition, is concentrated to a P205 content that may be up to 55% and preferably 50% to 55% by weight.

In one method of defluorination in accordance with the invention, the aqueous phosphoric acid solution is subjected to a treatment with steam or hot gas in a single apparatus, whereby it is concentrated and defluorinated. The treatment may be a discontinuous operation utilizing an evaporator of known type into which the acid solution is introduced and heated to boiling or other predetermined temperature before being treated with steam or hot gas. The concentration-defluorination treatment may alternatively be formed continuously, the flows of aqueous acid solution and steam or other hot gas flowing in the same direction or opposite directions, preferably in counter-current. The continuous treatment apparatus may have one or more stages.A preferred apparatus of the continuous countercurrent type comprises a multi-stage column into which the aqueous solution to be concentrated and defluorinated is introduced at the top, while the flow of steam or other hot gas is introduced into the column at the bottom, to collect the concentrated and defluorinated phosphoric acid at the column bottom. Knowledge of the separation isotherms in respect of fluorine in the form of HF, H2SiF6, and SiF4, as between a phosphoric acid solution and its vapour makes it possible to determine the selection of the parameters of the apparatus and its mode of operation as regards the number of stages, temperatures, pressures and flow rates, for pre-established values of the final concentration of the acid and its degree of exhaustion in respect of fluorine.Other types of continuous apparatus, with a plurality of stages, comprise well-known gas-liquid contactor means such as a filled column, a column with plates or a spraying column.

The temperature measured in the apparatus is preferably from 105"C to 1350C and more particularly fom 1200C to 1300C, and the pressure may be a pressure equal to or lower than atmospheric pressure, generally from 0.6 bar to 1 bar, these two parameters being linked.

In a second defluorination method, the regenerated substantially cation-free aqueous phosphoric acid solution is concentrated in a first step to the final desired concentration by means of steam or other hot gas; when this is done, a part of the fluorine present in the solution passes into the vapour phase, taking into account the operating conditions, but the F:P2Os weight ratio produced after this concentration step does not attain the low value required in respect of an acid of alimentary quality. It is for this reason that the solution produced in this first step is then subjected to a final defluorination by means of steam or other hot gas.

The apparatus for concentrating the acid in the first step of this method comprises well known types of evaporators, operating discontinuously or continuously in cocurrent or in countercurrent. An apparatus of the multi-action countercurrent type is preferred, particularly triple-action since it uses less energy. The separation isotherms in respect of water as between an aqueous phosphoric acid solution and its vapour make it possible to determine the operating conditions for a final predetermined P2O5 concentration, starting from a given dilute solution.In general, operation is under conditions of an increasing pressure in the direction of the flow of phosphoric acid, the pressure preferably being from 0.07 bar to 1.3 bar. especially 100 to 760 mm of mercury, and under conditions of increasing temperature in the direction of the flow of phosphoric acid, of from 50"C to 145"C, especially 50"C to 1300C. Under these conditions, by introducing a dilute solution of a P2O5 concentration of close to 20neo by weight the resulting solution has a P2O5 concentration of close to 50% by weight.

In the second step of this second embodiment, the aqueous phosphoric acid solution, which has been concentrated to the desired value and which is produced as the result of the second step, is subjected to advanced defluorination by contact with steam or a hot gas. The apparatus used for carrying out this step is a known gas-liquid contacting means operating discontinuously or continuously, in co-current or in counter-current. It is preferred however to use a counter-current apparatus with a plurality of stages, such as a column with plates.

The phosphoric acid solution that enters the multistage countercurrent apparatus is generally pre-heated to the temperature at which it begins to boil at the pressure in the apparatus, and is maintained at that temperature. The flow of steam is also heated to that temperature so that there is no substantial variation in the P2O5 concentration of the solution. Knowledge of the separation isotherms in respect of the fluorine between an aqueous phosphoric acid solution and its vapour phase makes it possible to define the selection of the parameters as regards the number of stages of the apparatus and its operating conditions, in order to arrive at a predetermined degree of exhaustion in respect of fluorine, starting from a given initial acid.The temperature in the contacting means is generally higher than or equal to 1300C, preferably 135"C to 1650C, and the pressure is higher than or equal to 1 bar, preferably 1 to 2.5 bars.

The process of the invention makes it possible to remove the fluorine present in the aqueous regeneration solution of the phosphoric acid, to give an F:P70s weight ratio, in dependence on the selection of the operating conditions and the form of the apparatus. In particular an exhaustion in respect of fluorine such that the F:P2O5 ratio is not more than 10 ppm can easily be obtained.

The result obtained from the process is a phosphoric acid that can be used in the foodstuffs industry and in particular in human food.

The following examples are given by way of illustration of the invention. In the Examples, reference will be made to the accompanying drawings in which Figures 1 and 2 show installations for performing the first and second embodiments of the present invention respectively.

Example 1 The phosphoric acid solution to be purified results from filtration of the slurries produced by reaction of sulphuric acid on natural phosphate bearing rocks. The fluosilicic acid that it contains is partially separated by a treatment with sodium carbonate and another filtration operation.

The solution then has a P2Os concentration of 33%, and its content in respect of various impurities relative to the P2O5 content is H2SO4 3.9% Fe 1.56% V 247 ppm Mn 298 ppm Ca 4050 ppm Mg 1920 ppm F 9700 ppm Al 6600 ppm SiO2 4000 ppm Cr 208 ppm Rare earths and Y 250 ppm Reference will be made to Figure 1 of the accompanying drawings to follow the course of treatment of this solution as described below.

The battery of ten counter-current extractors 1 is supplied with the flow 4 of phosphoric acid solution to be purified, having the composition set out above, at a rate of 9.4 m3/h, with a flow 5 of pure tributyl phosphate at a rate of 75.6 m3/h. In addition, a flow 6 of 2040 kg/hour of 98% sulphuric acid is also supplied to the fifth stage of the battery 1. The tributyl phosphate, which is charged with phosphoric acid, issues at 7 and passes into the battery 3 of four extractors for the purposes of washing by means of a branch flow 8 of 4.8 m3/h of the solution of pure phosphoric acid taken from the discharge 9 of the ten-stage extractor battery 2.

The flow of tributyl phosphate charged with phosphoric acid and from which some entrained impurities are removed in the battery 3 passes at 10 into the battery 2 in which it is treated in counter-current by the flow of deionised water 11 at a rate of 13.6 m3/h.

After the branch flow has been drawn off at 8 from the discharge 9 of purified phosphoric acid solution, the remaining flow at 12 is a flow of 12.6 m3/h of that acid. the P205 content being 26%. The phosphoric acid solution used for the washing operation issues from the battery 3 at 13 and is joined to the solution 4 to be purified. The flow of tributyl phosphate which supplies the battery 1 issues from the battery 2. This flow is purified continuously in an extractor (not shown) taking off a flow of 3.2 m3/h, which is treated in counter-current by a flow of 1.6 m3/h of a solution of half-normal sodium hydroxide. An exhausted acid solution titrating 1.3% of P2O5, which also contains all the sulphuric acid added and the various metal impurities, issues from the battery 1 at 14. The purified phosphoric acid solution issuing at 9 from the battery 2 is of the following composition, the proportions of impurities being expressed relative to the P205 content.

H2SO4 0.5% Fe 60 ppm V 4 ppm Mn 1.8 ppm Ca 44 ppm Mg 37 ppm F 1500 ppm Al 16 ppm SiO2 1500 ppm Cr 8 ppm Rare earths and Y 40 ppm The degree of extraction of the phosphoric acid is 96%.

The purified phosphoric acid flow 12 is then treated in a decolorization column by activated carbon. The resulting solution is introduced at the top of a column with plates, which has four theoretical stages and which operates in countercurrent, the initial temperature of the acid being 110 C. A flow of steam heated at 3000C is introduced at the base of the column under a pressure of 1 bar, the ratio between the flow of steam and the flow of phosphoric anhydride being 40:1. At the bottom of the column there is collected a phosphoric acid solution at a temperature of 135"C, in which the P205 content by weight is 55% and the F:P205 ratio by weight is 10 ppm.

Example 2 Figure 2 of the accompanying drawings shows the general diagram of an installation for continuously performing the process in its second embodiment.

Reference numeral 1 denotes the wet-process phosphoric acid to be treated, a portion 2 of which passes into a reactor 3. At 4 the reactor receives sulphuric acid, and at 5 an aqueous suspension issuing from the subsequent desulfation step. The products of the reaction in 3 are passed into a filtration zone 6 in which the insoluble products are separated at 7, while the liquid is passed at 8 into an extraction contacting means 9 into which a solvent, possibly mixed with water, is also introduced at 10. It is possible for a portion of the sulphuric acid at 4, as shown at 11, to be introduced at any stage of the contacting means 9.

The residual aqueous phase is removed at the discharge 12.

An organic phosphoric acid extract is collected at the discharge 13 of the extraction contacting means, and passed into a desulphation contacting means 14 which also receives at 15 an aqueous solution of calcium ions, which was prepared in the reactor 16. This solution is prepared by means of an amount of tricalcium phosphate which is introduced at 17 and contacted with a portion 18 of the wet-process phosphoric acid introduced into the process. The flow of this aqueous solution of calcium ions is combined with a flow 19 of calcium-containing suspension taken from the product issuing at 20 from the contacting means 14.

An organic extract is collected at the discharge 21 of the contacting means 14 and passed into the washing contacting means 22 for washing by water introduced at 23 and which issues at 24 in the form of an aqueous solution, while the organic extract after washing is introduced at 25 into a contacting means 26. A flow of water 27 is passed to the last stage of the contacting means 26, and a solution 30 of aqueous purified phosphoric acid, containing 96% of the acid, is collected at the first stage.

In this example, the calcium-rich solution is formed by reaction between the ore and all of the wet-process phosphoric acid which is introduced into the process.

The flow of the liquid at 2 is then zero. The flow in the conduit 1 is equal to that in the conduit 18, namely 1 tonne per hour of an impure wet-process phosphoric acid containing by weight: P205 25%; CaO 0.25%; and S04 1.5%; F 0.2%; MgO 0.2%; Fe2O3+Al2O3 = 0.3% The liquid passed through the conduit 4 comprises H2SO4 at a flow rate of 97 kg/h. That at 8 is a solution titrating P2O5 22%; soluble CaO 0.02%; free H2SO4 4%, at a flow rate of 1100 kg/hour. The contacting means 9 comprises 12 theoretical stages.

Isobutanol containing 8.3% of water is introduced at 10, at a flow of 8 t/h.

The flow at 11 is zero.

280 kg/h of a solution titrating P205 4% and His04 0.5% and containing all the metallic impurities of the initial impure acid is removed at 12.

The contacting means 14 comprises one theoretical stage. 1080 kg/h of a slurry titrating P2O5 26%, CaO 3% and CaSO4 2% is introduced at 15. Ore titrating P205 35% and CaO 50% is introduced at 17 at a flow rate of 80 kg/h. The flow rate at 20 is equal to the flow rate at 5, namely 1100 kg/h of slurry titrating P2Os 21%, insoluble gypsum 8% and soluble CaO 1%. The contacting means 22 comprises one theoretical stage. 210 kg/h of water is introduced at 23. A washing solution titrating P205 15% and containing the metallic impurities that had been extracted with the phosphoric acid is introduced at 24 at a flow rate of 95 kg.

Water is passed to the eighth stage of the contacting means 26 by the flow 27 at a flow rate of 1.4 t/h.

A purified aqueous phosphoric acid solution containing 96% of the acid entering and titrating 15% of P205 is collected at the first stage of the contacting means 26.

The solvent 32 is dehydrated by known means before being re-introduced into the contacting means 9 at 10.

The second mode of the invention is then performed, comprising preliminary concentration of the aqueous phosphoric acid solution resulting from regeneration with water of the organic phase, jointly with partial defluorination, then a subsequent step of advanced defluorination, the P205 concentration of the acid remaining substantially unaltered.

In the first step of concentrating and partially defluorinating the solution of decationised phosphoric acid, the apparatus used is a triple-action contacting means operating in countercurrent mode, the first exchanger being a parallelipipedal stainless steel shell 7 metres in height, while the second and third exchangers are of the graphite shell and tube type and are 6.1 metres in height. Phosphoric acid from the preceding regeneration operation is introduced into the apparatus at a flow rate of 16.7 tonnes/hour, the concentration and the fluorine and silica contents of which are as follows: P205 content by weight .................................... 15% F/P2 C5 ratio by ........................... weight 6150 ppm SiO2/P205 ratio by weight ...................... 2300 ppm The steam is introduced in a ratio of 2.6 tonnes per tonne of P205 introduced.The operating parameters of the apparatus and the results achieved are set out in the following table.

Temp Absolute Content of the phosphoric OC Pressure acid solution mm Hg P205 content % F/P2O5 SiO2mgll Inlet Outlet Exchanger 3 50 100 15 20.5 6150 800 Exchanger 2 77 300 20.5 25 6050 1140 Exchanger 1 130 760 25 50.5 800 190 The acid solution issuing from exchange No. 1 of the triple-action contacting means is pre-heated to a temperature of 164 C and is then continuously introduced at the head of a plate column comprising five theoretical stages, while at the base of the column steam is introduced at a flow rate of 0.8 tonne/tonne of P205, at a temperature of 164 C, the pressure in the column being 2.5 bars. The product is drawn off at the bottom of the column. The solution produced contains 51% of P205, and its F:P2Os ratio by weight is 7 ppm.

WHAT WE CLAIM IS: 1. A process for purifying and defluorinating an impure aqueous solution of phosphoric acid produced by attack of a phosphate-bearing rock by means of sulphuric acid, that comprises the counter-current extraction of the impure aqueous solution using a solvent that has low miscibility with water with the addition, before or during extraction, of concentrated sulphuric acid, washing the resulting phosphoric-acid-containing organic phase by means of water or purified phosphoric acid, counter-current re-extraction of the phosphoric acid contained in the organic phase by means of water, separation of the resulting dilute aqueous solution of decationized and partially defluorinated phosphoric acid, and treatment of said solution by means of steam or other hot gas, whereby the final product collected is a concentrated and defluorinated aqueous solution in which the F:P2Os ratio by weight is not more than 10 ppm.

2. A process according to Claim 1 in which the organic phase resulting from the extraction of the crude aqueous acid is treated, before washing with water or pure phosphoric acid, with an aqueous solution containing phosphate ions and calcium ions in solution, this solution having been prepared from calcium phosphate, phosphoric acid and water in amounts such that the content of calcium ions in solution is from 2 to 6% by weight expressed as CaO, so as to precipitate the sulphate ions in the form of calcium sulphate and to put them in suspension in the solution.

3. A process according to Claim 1 or 2 in which the P2O5 concentration of the aqueous phosphoric acid solution substantially free from cationic impurities resulting from the re-extraction operation has a P2O5 content of from 15% to 30% by weight and an F:P2O5 ratio by weight of up to 7000 ppm, and the final acid obtained as the product has a P2O5 content of from 50% to 55 o by weight.

4. A process according to any one of Claims 1 to 3 in which the concentration operation and the defluorination operation are performed in a single apparatus, discontinuously, by injecting steam or other hot gases into the acid solution at a temperature of 135 C.

5. A process according to any one of Claims 1 to 3 in which the concentration operation and the defluorination operation are performed in a single counter-current continuous multi-stage liquid-vapour contacting means by means of steam or other hot gases at a temperature of 135 C and under a pressure of 1 bar.

6. A process according to any one of Claims 1 to 3 in which, in a first step, the acid is continuously concentrated and partially defluorinated in counter-current by steam

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. type and are 6.1 metres in height. Phosphoric acid from the preceding regeneration operation is introduced into the apparatus at a flow rate of 16.7 tonnes/hour, the concentration and the fluorine and silica contents of which are as follows: P205 content by weight .................................... 15% F/P2 C5 ratio by ........................... weight 6150 ppm SiO2/P205 ratio by weight ...................... 2300 ppm The steam is introduced in a ratio of 2.6 tonnes per tonne of P205 introduced. The operating parameters of the apparatus and the results achieved are set out in the following table. Temp Absolute Content of the phosphoric OC Pressure acid solution mm Hg P205 content % F/P2O5 SiO2mgll Inlet Outlet Exchanger 3 50 100 15 20.5 6150 800 Exchanger 2 77 300 20.5 25 6050 1140 Exchanger 1 130 760 25 50.5 800 190 The acid solution issuing from exchange No. 1 of the triple-action contacting means is pre-heated to a temperature of 164 C and is then continuously introduced at the head of a plate column comprising five theoretical stages, while at the base of the column steam is introduced at a flow rate of 0.8 tonne/tonne of P205, at a temperature of 164 C, the pressure in the column being 2.5 bars. The product is drawn off at the bottom of the column. The solution produced contains 51% of P205, and its F:P2Os ratio by weight is 7 ppm. WHAT WE CLAIM IS:

1. A process for purifying and defluorinating an impure aqueous solution of phosphoric acid produced by attack of a phosphate-bearing rock by means of sulphuric acid, that comprises the counter-current extraction of the impure aqueous solution using a solvent that has low miscibility with water with the addition, before or during extraction, of concentrated sulphuric acid, washing the resulting phosphoric-acid-containing organic phase by means of water or purified phosphoric acid, counter-current re-extraction of the phosphoric acid contained in the organic phase by means of water, separation of the resulting dilute aqueous solution of decationized and partially defluorinated phosphoric acid, and treatment of said solution by means of steam or other hot gas, whereby the final product collected is a concentrated and defluorinated aqueous solution in which the F:P2Os ratio by weight is not more than 10 ppm.

treatment in a multi-action contacting means and, in a second step, the resulting solution is defluorinated in a multi-stage counter-current contacting means by steam treatment.

7. A process according to Claim 6 in which the multi-action contacting means operates at a temperature ranging from 50"C to 1300C and at a pressure ranging from 100 to 760 mm Hg and the multi-stage contacting means operates at a temperature ranging from 135 to 165"C and at a pressure of from 1 to 2.5 bars.

8. A process as claimed in Claim 1 substantially as hereinbefore described in Example 1 or 2.

9. A process as claimed in Claim 1 carried out in apparatus substantially as hereinbefore described in Figure 1 or 2 of the accompanying drawings.

10. Phosphoric acid purified by a process in accordance with one of the preceding claims.