CA1043535A - Process for the preparation of purified phosphoric solutions from phosphoric acid in a wet way - Google Patents

Abstract

PROCESS FOR THE PREPARATION OF PURIFIED PHOSPHORIC
SOLUTIONS FROM PHOSPHORIC ACID IN A WET STATE

ABSTRACT OF THE DISCLOSURE
Process for the preparation of a purified solution of phosphoric acid, which may be partially neutralized, from wet process phosphoric acid, by liquid-liquid extraction by means of a substantially water-immiscible organic solvent.
The phosphoric acid is treated in the selected solvent, pre-ferably isobutyl alcohol, in the presence of sulphuric acid, by means of an aqueous solution containing phosphate ions and calcium ions in solution, the sulphate ions are precipitated in the form of calcium sulphate, and suspended in said aqueous solution which is separated from the phosphoric solution. The process is advantageously employed for the continuous prepar-ation of concentrated solutions of monosodium phosphate.

Description

~0~3535 The present process concerns the preparation o puri-fied solutions of phosphoric acid, which may be partially neu-tralized, from wet process phosphoric acid, by liquid-liquid extraction by means o~ an organic solvent, with a high extrab-tion yield, Processes for extraction by means of organic solvents are known which, when used for wet process phosphoric acid, pro-vide for the extraction of phosphoric acid to the exclusion of its impurities, primarily cationic impurities such as iron, chromium and aluminum compounds. In these processes, the sol-vents used are generally C4 to C8 alcohols which are charged with water to a greater or lesser extent and which are partly water-miscible. It is known also that the addition of a strong acid, in particular sulphuric acid, to the phosphoric acid, pro-motes extraction o~ the phosphoric acid. m is thus increasesthe extraction yield. However, sulphuric acid is at least par-tially extracted together wi~h the phosphoric acid, and requires removal.
The removal of ~he introduced sulphate ions is subject to various difficulties.
It has been proposed that the operation of removing the sulphate ions can be carried out by forming insoluble sul-phates of alkaline-earth metal salts such as barium sulphate, but this process is expensive. Other alkaline-earth salts have been proposed, particularly tricalcium phosphate which is par-tîally solubilized, as in U. S. patent No. 2,885,265. Carrying out the process described in the above-mentioned patent requires bringing into contact with the solvent, all of the slurry re-sulting from the sulphuric acid attack on the phosphate-bearing 3 raw material; the residual gypsum is thus removed with most of the impurities in the ore; the organic phase containing the phosphoric and sulphuric acids i9 then contacted with a second slurry resulting from attack on the ore, by means of a defi~
ciency of sulphuric acid. Ihe reactions in solvent phase, which lead to the formation of calcium sulphate, are neither complete nor rapid under these conditions; nor can the degree of purity achieved be satisfactory. In order to overcome these difficult-ies, the above-mentioned U. S. patent provides for controlling ~he attack on the ore with as small an amount of sulphuric acid as possible, thereby resulting, in known manner, in only moder-ate solubilization of the ore, and an only moderate solvent ex-traction yield.
The extra~tion of slurries also suffers from technolog-ical difficulties; the combined effect of these deficiencies means that the process set out in the U. S. patent cannot be applied continuously in industrial practice.
Generally, it has not been possible to ac~ieve, at the sa~e time, a high extraction yield and a phosphoric acid in a high degree of purity.
m e research, which lead to the making of the present invention, made it possible to provide a re-extraction process which, in a single, rapid and complete operation, removes a substantial excess of sulphuric acid w~ich is added, in the operation of extracting impure phosphoric acid by means of a solvent.
The process, according to the invention, provides for the preparation of very pure phosphoric solutions from impure wet pxocess phosphoric acid. In this process, the impure phos-plloric acid is e~tracted in counter-flow in the presence of sul-3 phuric acid and water, with a very high extraction yield, by means of an organic solvent which is substantially water-immis-cible. An aqueous phase, containing the major part of the im-purities, and an organic solvent phase are separated, the phos-phoric acid being re-extracted from the organic solvent phase.
m e process comprises the following sequenee of steps: in a first step, the impure phosphoric acid is extracted in counter-flow in the presence of sulphuric acid and water by means of an organic solvent which is substantially immiscible in water; in a second step, which characterizes the process, the phosphoric acid is treated in the organic solvent in the presen~e of sul-phuric acid, by contact by means of an aqueous solution contain-ing phosphate ions and calcium ions in solution, prepared from caleium phosphate, phosphoric acid and water, in respective amounts such that the content of calcium ions in solution is from 2% to 6% expressed-as CaO, so as to precipitate the sulphate ions in ~he form of calcium sulphate and to put them in suspen-sion in said solution; in a third step, water is used to wash the phosphoric acid in the organic solvent; and, in a fourth step, the organic solventis separated from a purified phosphoric ~aqueous solution which constitutes the product of the process.
The phosphoric acid which enters the extraction step advantageously has a sulphuric acid content which is at least equal to 2% and preferably is of the order of 4%.
It is of advantage for the second atep to be carried out as a eontinuous operation within a recycled flow of aqueous solution containing phosphate ions and calcium ions in solution, t~e flow rate of ~hich is from 0.2 to twice approximately the flow rate of the organic solvent phase. Ihe flow rate of the aqueous solution is preferably so controlled that it is substan-tially equal to the flow rate of the solvent phase.

~.043535 It is of advantage for the recycled flow o~ aqueous 'solution to contain from 5% to 20% by weight of calcium sulphate in suspension, and, preferably, an amount of the order of 10%.
use is preferably made of a reaction vessel which pro-vides for continuous opera~ion, The aqueous solution is preferably prepared by react-ing tricaleium phosphate and impure wet process phosphorie acid in t~e presence of water. Use is advantageously made of a por-tion which is taken from the impure phosphoric acid introduced into the process. The water used is preferably water which has ~een employed for the water-washing of the organic solvent ex-traet and whieh eontains a small proportion of phosphoric acid.
In the preparation of the-aqueous solution containing phosphate ions and calcium ions in solution essentially in the form of monoealcium phosphate, use is made advantageous~y of a ealcium phosphate ore whieh has preferably been calcined before-hand.
The reLative proportions of the components of the aqueous solùtion are` preferably so eontrolled as not to modify the composition of the recycled suspension, at the outlet from ~h-e con~act reaction *essel.
At the outlet of the contact reaction vessel, a part of the recycled flow of aqueous solution which i9 calcium sul-phate-bearing is removed from the aqueous s~lution which is re-cycled with phosphoric acid in the solvent, and the remainder of the solution to which ~he monocalcium phosphate solution is added, in an amount equi~alent to the re~oved portion of aqueous solution eontaining the ealcium ions in solution is recycled in a eontinuous flow.
m e removed portion of aqueous suspension is contaeted with a part ~t least of the impure phosphoric acid which enters into the process, and with sulphuric acid, in a suitable device comprising a mixing section and a decantation section. There is thus formed a supplementary amount of calcium sulphate which is r~moved at the same time as the previously formed calcium sul-phate, and the phosphoric acid, which is virtually free from calcium sulphate, is introduced into the extraction reaction vessel, in the presence of excess sulphuric acid.
m e amount o sulphuric acid added is calculated so as to have the desired content o S04--~ ions, in phosphoric acid, at the inlet of the extraction reaction vessel.
A supplementary amount of sulphuric acid can possibly be added at any point whatever of the extraction apparatus, as is known in particular from French Patent No. 1,531,487.
m e selected organic solvent is introduced at the same time. Ad~antageously, this solvent is generally collected after the last step and is possibly subjected to purification treat-ments by methods ~nown per se, beore being recycled for use in the extraction step.
~he organic solvent which is substantially immiscible in water is generally an aliphatic alcohol containing from 4 to 8 carbon atoms. It is o advantage for isobutyl alcohol to be used.
After the water wa~shing step, an organic extract con-taining phosphoric acid from which the S04-- ions have been removed, is collected.
In an advantageous embodiment, this organic extract is treated with water, and an aqueous solution of phosphoric acid which constitutes the product of the process, is co~llected.
In a preferred embodiment, the organic extract is treated with a neutralizing compound, such as a compound of an alkali metal such as sodium or potassium, or an ammoniacal com-pound, and a concentrated aqueous solution of alkali phosphate, which forms the product of the process, is collected.
m e neutralization operation is advantageously per-formed by introducing the neutralizing compound into a recycled flow of concentrated partially neutralized aqueous solution of phosphoric acid, in accordance with the process already described in the applicants' French patent application ~o. 1,553,095.
This reçycled flow is divided after the outlet from the neutralization reaction vessel, and a portion thereof is recycled, a controlled amount of neutralizing compound being added to the recycled portion. Ihe remaining portion of recycled flow of partially neutralized, concentrated and purified phosphoric acid, which forms the product of the pracess,-is then collec~ed.
It is noted that extraction by means of the organic solvent is performed with an extraction efficiency of the order of approximately from 95% to 98%, and most generally of the order ~ 96%.
It is also found that using the solution containing phosphate ions and calcium ions in solution, in accordance with the mode of operation descr~ed hereinbefore, provides for vir-tually complete removal of the excess of sulphuric acid cont~ined in the phosphoric organic extract, this being effected in a single reaction vessel.
- m is solution is prepared rapidly; it is noted in fact that under the above-described conditions,the ore is rendered soluble virtually instantaneously. In this way, there is pro-duced a solution which contains a large amount of calcium ions in solution; moreover, and this is o~ advantag~, this amount is ~ 043S35 kept virtually constantO
Regulating the amount o gypsum in suspension permits easy handling of the recycled solution, with a high flow xate.
In this way, the reaction medium in the contact reaction vessel is homogeneous and the multiple contacts between the phases promote the speed of the reaction, In the event that phosphoric acid is produced in the form ~f relatively.concentrated solutions, a high degree of purity is achieved, coupled with a very high extraction yield.
In the event that the phosphoric organic extract is - treated in the neutralization step, in accordance with the ad-vantageous mode of continuou:sly introducing neutralizing com-pound, as described hereinbefore, the same advantages are achieved, and moreovér, the phosphates are collected in a highly cQncentrated form which is economically very attractive.
mus, monosodium phosphate is readily obtained in the orm of a 40% concentrated solution, or even better, which re-duces the cost of subsequent dryi~g operations.
- . By virtue of their degree of purity, the products ob-2Q- ta~ne-~ in performing the process of the present inventicn can be use.d directIy, in prrticular in the detergent in-dustry, or in the foodstuffs industry.
Moreover, the process of the invention, which makes it possible to remove metal impurities such as iron, chromium ~5 and aluminum, originatin~ from the raw material, as well as the excess S04 -- ions, can easily be integrated into a proce~s com-prising ot~er operations intended to remove other impurities : which may be present.
Figure 1 shows the diagram of an installation for con-tinuously carrying out the process o~ the invention.

~043535 Reference numeral 1 represents the flow of the phosphoric acid inthe wet way to be treated, a portion 2 of which acid enters a reaction ves-sel 3 which receives, at 4, sulphuric acid and at 5 an aqueous suspension resulting from the subsequent desulphatation step. The products from the reaction in the reaction vessel 3 are passed into a filtration zone 6 in which the insoluble products are separated at 7, while the liquid is passed by way of conduit 8 into an extraction contacting vessel 9 into which a sol-vent, which may possibly be mixed with water, is also introduced, at 10.
There is the possibility of introducing at any stage whatever of the contact-ing vessel 9, a portion of sulphuric acid from 4, which is shown at 11. Theresidual aqueous phase is discharged by way of the outlet 12.
At the outlet 13 of the extraction contacting vessel, there is collected an organic phosphoric acid extract which is passed into a desul-phatation contacting vessel 14 which also receives, at 15, an aqueous sol-ution of calcium ions, which was prepared in the reaction vessel 16. This solution is prepared by means of an amount of tricalcium phosphate introduced at 17 and contacted with a portion 18 of the wet process phosphoric acid introduced into the process. The flow of the aqueous solution of calcium ions is combined with a flow 19 of calcium suspension, which is drawn from the product issuing at 20 from the contacting vessel 14.
At the outlet 22 of the contacting vessel 14, there is collected an organic extract which is passed into the washing contacting vessel 22, where washing is effected by means of water introduced at 23, and which issues from the vessel 22 at 24 in the form of an aqueous solution, while the organic extract, A~

~.043S35 after washing, is introduced at 25 into contacting vessel 26.
A neutralizing compound 28 is introduced at 27 into the con-tacting vessel 26, by means of a flow of partially neutralized phosphoric acid, which is drawn off at 29 from the outlet flow 30 of the-contacting vessel 26, m e partially neutralized phos-phoric acid which constitutes the product of the process is collected at 31.
Reference numeral 32 denotes recovery of the solvent which is recycled at 10, after purification and possibly dehydra-tion treatments, by conveIltional means (not shown)~
me following examples are given by way of illustra-tion, without limitation,of the various embodiments of the pro-cess of the invention.
EX~MPLE 1 In this example, a solution with a high calcium con-tent is prepared by a reacticnbetween some ore and a weak phos-phoric solution comprising the solvent washing water. m e product of the reaction, in which all the calcium of the ore introduced is rendered soluble, is mixed with a recycled por-tion of the product of the desulphatation operation, containing calcium sulphate, and the resulting suspension is introduced into the desulphatation reaction vessel.
In an installation as shown diagrammatically in the accompanying drawing, impure wet process phosphoric acid, con-tainin~ by weight P205 250/~ CaO 0.25%, and S04 1.5%, F 0.2%, MgO 0~2%, Fe2o3 + A1203 = 0.3%, is introduced at a rate of 1 t/h into the reaction vessel shown at 3.
Also added into the reaction vessel is 40 kg/h of sul-phuric acid, and also the slurry resulting from the desulphata-tion operation in the reaction vessel 16.

_g_ m e insoluble compounds are filtered off, and a solu-tion is produced whose weight composition is-P2o5 25%, S04 -~
total 3%, of which 2% is in the form of free sulphuric acid.
m is solution is introduced at ~he first s~age of a multi-stage contacting vessel comprising 5 theoretical stages, at the fifth stage.of which ~here is introduced a mixture of 92 weight % of isobutanol and 8we~ght % water; in a flow rate of 8 200 kg/h, and 30 kg/h of sulphuric acid, by way of the conduit 11.
At the outlet of the fifth stage, there is an aqueous effluent in a flow rate of 316 kg/h, which contains 4% of un-extracted phosphoric acid, and total S04 ~ ~ 15%, of which 10%
is free sulphuric acid. : ~
At the outlet 13 of t~e first..stage, ther.e is an or-ganic extract which is passed into a desulphatation contacting vessel comprising one theoretical stage, at the same time as a flow of a suspensiom prepared, as described herein~ft.er, in the reaction vessel 16.
A flow rate of 35 kg~h of phosphate-bearing ore whose composition by weight is P205 35%, and CaO 50% is passed into the reaction ~essel 16, at ~he same time as 147 kg of an aqueous solution of dilute phosphoric acid resùlting from the washing treatment with water in the contacting vessel 22, to.gether with a suspension which is introduced at l9 and whose weight composi-tion is P205 21%, insoluble caS04 14%, and CaO in soluble form

2%. ~he pr~ducts of the reaction pass into the desulphatation reaction vessel at the inlet denoted at lS.
- . The suspension issuing from the contacting vessel 14 is divided into two flows, one being recycled and feeding the reaction vessel 16, and the other being used to feed the re-action vessel 3.

The organic extract issuing from the reac~ion vessel 14, by way of the conduit 21, is introduced at the first stage of a washing contacting vessel comprising two theoretical stages, into which washing water is introduced at a flow rate of 230 kg/h.
The a~ueous solution issuing from the vessel is used to feed the above-described reaction vessel 16, while the purified organic extract is passed to the regeneration contacting vessel compris- ;
ing one theoretical stage, as indicated at 26.
Also introduced into this contacting vessel are an aqueous solution of approximately 40% by weight of monosodium phosphate, at a flow rate of 8 t/h, and sodium carbonate at a flow rate of 200 kg/h.
At the outlet from this c~ntacting vessel there is collected an approximately 40% r,loosodium phosphate aqueous sol-ution, a part of which is récycled at a flow rate ~f 8 t/h whilethe remainder, whose flo~ ~a~-~is l,lOQ kg~h, i5 collected and forms the product of the process.
Ihe product contains 96 X of the total P~05 passing into the installation Its composition, related to the P205, - 20 is as follows:
S04 : 0-2 ~/P20s - F : 0.5 CaO : O . 01 MgO : 0.01 Fe23 Y o.ol A1203 : 0.002 In this example, 50% of the sulphuric acid, added to promote the phosphoric a~id extraction, was used for the prepara-tion of a supplementary amount of phosphoric acid which is puri-fied in this process.

In this example, the solution with a high calcium content is formed by the reaction betwe0n the ore and the entire amount of phosphoric acid, in the wet way, which is involved in the process.
The flow rate of the liquid at 2 is then zero. The flow rate in the conduit 1 is equal to that in the conduit 18, namely 1 ton per hour, of the s~e impure wet process phosphoric acid, as in the preceding example.
The liquid, which passes by way of the conduit 4, comprises H2S04, at a flow rate of 97 kg/h. The liquid in the conduit 8 is a solution titrating P2O5 22% soluble CaO 0.02%, free H4SO4 4%, at a flow rate of 1,100 kg/h. The contacting vessel 9 comprises 12 theoretical stages.
Isobutanol wi*h 8.3% water is introduced at 10, at a flow rate of 8 t/h.
The flow at ll is ~ero.
280 kg/h of a solution titrating P2O5 4% and H2S04 0.5% and con- ~`
taining all the metal impurities of the initial impure acid, is removed at 12.
The contacting vessel 14 comprises one theoretical stage. 1,080 kg/h of a slurry titrating P205 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, that is to say, 1,100 kg/h of slurry titrating P205 21%, insoluble gypsum 8% and soluble CaO 1%. The contacting vessel 22 comprises one theoretical stage. 210 kg/h of water is introduced at 23. A washing solution is introduced at 24 at a flow rate of 95 kg, the washing solution titrating P205 15% and containing the metal impurities which had .;

been extracted with the phosphoric acid.
Finally, a solution of monosodium phosphate, with approximately 40% of monosodium phosphate, containing 96% of the impure phosphoric acid which entered into the process, and in a state of purity equivalent to that of the product obtained in Example 1, except for the amount o sulphate which, expressed as P205, titrates 0.5%, is collected at 31~
In this example, 98% of the sulphuric acid used serves to prepare a supplementary amount of phosphoric acid which is purified in the process.
EX~MPLE 3 In this example, only a part of the impure phosphoric acid going into the process is used to prepare the calcium-rich solution which is then more highly concentrated in soluble cal-cium, then the solution in ~he preceding example.
Only the values which are changed in comparison with the preceding example are indicated hereinafter.
A solution of impure wet process phosphoric acid, titrating P20s 25%, CaO 0.25%, S04 1.5%, F Q.2%, MgO 0.2%, Fe203 + A1203 0.3%, is introduced at 1 in the accompanying draw-ing at a flow rate o 1 t/~. Ihis acid is divided` into two parts, one part of 360 kg/h being taken by way of 2 and the other of 640 kg/h being taken by way of 18.
A suspension titrating P20s 25%, soluble caO 3.5% and insoluble gypsum 5% is introduced at 15 at a ~low rate o~ 1,500 kg/h.
A suspension titrating P205 25%, soluble CaO 2% and insoluble g~psum 8.5% is introduced at 19 at a flow rate of 800 kg/h, while a flow rate of 750 kg/h o the same suspension is introduced at 5.

_13-Sulp~uric acid at a flow rat~ of 112 k~/h is intro-duced at 4.
The yield in respect of phosphoric acid recovered in the form of an approximately 40% monosodium phosphate solution is 96%. ~he state o purity of this monosodium phosphate is the same as that of the preceding examples, except for the amount of æulphate which, expressed as P205, has decreased from 0.2%
to 0.1%.
98% of the sulphuric acid used in the process provides or- the preparation. of purified phosphoric acid whi~h is produced in the form of monosodium phosphate.
EX~MPLE 4 In this example, the purified phosphoric solution.is prepared in the form of a phosphoric acid solution, instead of a monosodium phosphate solution.
At the outlet of the washing contacting vessel 22, under~the conditions of Example 1, there is recovered an organic extract 25 at a flow rate of 8.4 t/h. m e organic extract con-tains by weight P205 3%, H20 15% and H2S04 0.015%, and is passed 2~ ~ ~o~ ~he~irst stage of the multi-stage contacting vessel 26 which cn~r~wc~ ~g~t the~etical stages.
Wate~ is.~assed into the eighth stage of the .contact-ing vessel by way of the conduit 27, at a flow rate of 1.4 t/h.
An aqueous solution of purified phosphoric acid con-taining 96% of the acid introduaed and titrating 16% of P205 is collected at the first stage of the contacting ve~sel 26.
m e degree of purity is equivalent to that obtainèd in Example 1.
The solvent 32 is dehydrated by known means, before 3o being introduced into the contacting vessel 9 at 10.

.,. ; .

Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of pure phosphoric solutions, from impure wet process phosphoric acid, wherein the impure phosphoric acid is ex-tracted in counter-flow in the presence of sulphuric acid and water, with an extraction yield from about 95% to 98%, by means of an organic solvent which is substantially water-immiscible, and an aqueous phase containing the major part of the impurities and an organic solvent phase are separated, the phos-phoric acid being re-extracted from the organic solvent phase, the process being characterized by the following succession of steps: in a first step, the impure phosphoric acid is extracted in counter-flow in the presence of at least 2% by weight sulphuric acid and water by means of substantially water immiscible aliphatic alcohol containing from 4 to 8 carbon atoms; in a second step, the phosphoric acid is treated in the organic solvent in the presence of sulphuric acid, by contact by means of an aqueous solution con-taining phosphate ions and calcium ions in solution, in which the content of calcium ions in solution is from 2% to 6% expressed as CaO, so as to precip-itate the sulphate ions in the form of calcium sulphate which remains in suspension in said solution; in a third step the phosphoric acid in the organic solvent is washed with water; and, in a fourth step, the organic sol-vent is separated from a purified phosphoric aqueous solution which constitutes the product.

2. A process according to claim 1 in which the aqueous solution con-taining phosphate ions and calcium ions in solution is prepared by reaction of calcium phosphate, phosphoric acid and water.

3. A process according to claim 1, characterized in that the phosphoric acid which passes into the extraction apparatus has a sulphuric acid content of the order of 4% by weight.

4. A process according to claim 1, characterized in that the second step is carried out continuously within a recycled flow of aqueous solution containing phosphate ions and calcium ions in solution, the flow rate of which is approximately from 0.2 to twice the flow rate of the organic sol-vent phase.

5. A process according to claim 4, characterized in that the recycled flow of aqueous solution has a flow rate which is substantially equal to the flow rate of the organic solvent phase.

6. A process according to claim 5, characterized in that the recycled flow of aqueous solution contains from 5% to 20% by weight of calcium sul-phate in suspension.

7. A process according to claim 1, characterized in that a part of the recycled flow of aqueous solution bearing calcium sulphate, is taken off at the outlet from the contact reaction vessel from the aqueous solution with the phosphoric acid in the organic solvent in the presence of sulphuric acid, and the remainder of the solution is recycled, there being added there-to an amount, equivalent to the part removed, of aqueous solution containing phosphate ions and calcium ions in solution.

8. A process according to claim 7, characterized in that the removed part of aqueous suspension is contacted with a part at least of the impure phosphoric acid which enters into the process, and with sulphuric acid, a supplementary amount of calcium sulphate is formed, said supplementary amount being removed, and the phosphoric acid which is virtually free from calcium sulphate is introduced into the extraction reaction vessel, in the presence of excess sulphuric acid.

9. A process according to claim 8, characterized in that a supple-mentary amount of sulphuric acid is added at any point whatever of the ex-traction apparatus.

10. A process according to claim 1 wherein the aliphatic alcohol is isobutylalcohol.

11. A process according to claim 1, characterized in that after the third step of washing with water, the organic extract is treated with water and an aqueous solution of phosphoric acid is collected, to form the product.

12. A process according to claim 1, characterized in that after the third step of washing with water, the organic extract is treated with a neutralizing compound, and a concentrated aqueous solution of partially neutralized alkali phosphate is collected as the product.

13. A process according to claim 12 in which the neutralizing compound is selected from the group consisting of sodium, potassium and ammonium compound.

14. A process according to claim 12, characterized in that the neutral-izing compound is introduced into a recycled flow of partially neutralized alkali phosphate solution.