GB1576367A

GB1576367A - Purification of phosphoric acid

Info

Publication number: GB1576367A
Application number: GB4878777A
Authority: GB
Original assignee: Toyo Soda Manufacturing Co Ltd
Current assignee: Tosoh Corp
Priority date: 1976-12-27
Filing date: 1977-11-23
Publication date: 1980-10-08
Also published as: JPS5380389A; JPS5426519B2; FR2375143B1; DE2753820B2; DE2753820A1; FR2375143A1; BE862267A; IT1089147B; DE2753820C3

Description

(54) PURIFICATION OF PHOSPHORIC ACID (71) We, TOYO SODA MANUFACTURING CO., LTD., of No. 4560, Oazo-tonda, Shin-nanyo-shi, Yamaguchi-ken, Japan, a Japanese company, 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 relates to a purification of a wet process phosphoric acid. More particularly, it relates to a process for producing purified phosphoric acid from the wet process phosphoric acid by selectively removing organic impurities contained in the wet process phosphoric acid by a solvent extracting process.

The wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid and separating calcium sulfate usually contains impurities such as iron, aluminum, calcium, sodium, magnesium, titanium components and sulfuric acid, fluorine and silica components which are derived from the raw materials of the phosphate rock and the sulfuric acid.

Accordingly, such wet process phosphoric acid has been mainly suitable only for the preparation of fertilizers.

However, there have been developed and worked processes for producing purified phosphoric acid which is suitable for industrial uses, pharmaceutical uses and food additive uses, by extracting the wet process phosphoric acid with an organic solvent which dissolves phosphoric acid and has low miscibility with water, for example an alcohols e.g. n-butanol, isobutanol and isoamyl alcohol; a ketone e.g. methylethyl ketone and methyl isobutyl ketone; an ether e.g. isopropyl ether; a phosphoric acid ester e.g. tributyl phosphate or an gamines and back-extracting phosphoric acid with water.

An economical uncalcined phosphate rock is usually used for producing the wet process phosphoric acid. Accordingly relatively large amounts of organic impurities are included in the wet process phosphoric acid to give it a brown or black-brown color. When the wet process phosphoric acid is used for fertilizer, the organic impurities do not cause any trouble.

However, when purified phosphoric acid for industrial uses, pharmaceutical uses or food additive uses is produced by purifying the wet process phosphoric acid by the solvent extracting process, if a large amount of the organic impurities contaminated the resulting phosphoric acid, the following disadvantages are caused.

a) The phosphoric acid extraction efficiency and the impurity separation efficiency are lowered.

b) The phase separation speed for separating two liquid phases is remarkably slow whereby the separation of the two liquid phases is not easily attained for example using the conventional mixer-settler type extractor and it is necessary to use expensive equipment such as a centrifugal extractor.

c) Sometimes, a stable emulsion is formed whereby the separation of the two liquid phases is impossible.

d) A solid organic impurity is formed at the interface between the two liquid phases. This causes a scale which may hold up the operation for a long time.

Accordingly, it has been difficult to use the wet process phosphoric acid containing organic impurities in the solvent extracting process.

Various processes for purifying the wet process phosphoric acid by selectively removing the organic impurities before or during the acid extracting step have been proposed. For example, the process for treating the wet process phosphoric acid containing organic impurities with a column filled with granular active carbon, before feeding the wet process phosphoric acid to the acid extracting step has been proposed. However, in this process, clogging of the column filled with granular active carbon is caused by fine insoluble materials contained in the wet process phosphoric acid and a precipitate of fine gypsum and silicofluoride. These fine insoluble materials and the precipitates cover the surface of the active carbon resulting in deterioration of the activity and the recovery coefficient. Accordingly, the process could not be employed for industrial purposes.

When powdery active carbon is used instead of the granular active carbon, a large amount of the expensive active carbon is needed, a separate step of separating the active carbon is needed and the treatment of the wasted active carbon is troublesome from the viewpoint of environmental pollution. Accordingly, the process is not economical for industrial purposes.

The process for removing organic impurities from the acid extracting step by distilling the recycling solvent of the solvent phase containing substantially no phosphoric acid component, or by washing the recycling solvent with an aqueous solution of an alkali metal hydroxide, or by contacting it with an active carbon or an anion exchange resin, have also been proposed.

However, in the acid extracting step, it is usual to use 4 to 20 times by volume of the organic solvent to the wet process phosphoric acid whereby large size equipment is needed, a large amount of the treating agent is needed, the amount of the organic solvent required is large and loss of the expensive organic solvent is substantial. Accordingly, such processes are not economically advantageous.

No economical and effective process for overcoming the problem of the organic impurities in the purification of the wet process phosphoric acid by the solvent extracting process has yet been found.

The inventors have studied the behaviour of the organic impurities in the solvent extraction process in order to develop a process for purifying the wet process phosphoric acid by solvent extraction, selectively removing the organic impurities. As a result, it has been found that most of the organic impurities contained in the wet process phosphoric acid are easily extracted in the organic solvent; the organic impurities which are easily extracted into the solvent phase, cause the above-mentioned troubles; the organic impurities are substantially not back-extracted with water so as to accumulate in the solvent phase in the acid extracting step during a long period of operation. Moreover, it has been found that the separation of the two phases is not adversely affected under certain specific conditions even though a large amount of the organic impurities are present.

The present invention provides a process for purifying a wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid, which process comprises; a first organic impurity extracting step wherein the phosphoric acid is contacted with an organic solvent to extract selectively organic impurities into the organic solvent phase, the chloride ion concentration being maintained at a level at which substantially no iron impurity is extracted and a second acid extracting step wherein the phosphoric acid obtained in the first step is contacted with the same organic solvent as used in the first step, and back extracted with water after removal of inorganic impurities therefrom by contacting with water or an aqueous solution of phosphoric acid of high purity to obtain an aqueous solution of purified phosphoric acid, wherein a part of the organic extract phase from the second step is recycled for use as the organic solvent in the first step, the organic impurities are removed from the organic solvent phase obtained from the first step and at least part of the resulting organic solvent phase is returned to the second step.

The accompanying drawing is a block diagram of one embodiment of the purification of a wet process phosphoric acid in accordance with the present invention wherein the reference letters and numerals indicate as follows: A: first organic impurity extracting section; B: second acid extracting section; C: column filled with active carbon; 1: wet process phosphoric acid; 2: phosphoric acid organic impurities are removed(; 3: pure water; 4: purified phosphoric acid; 5: raffinate; 6: organic solvent; 7: extract phase; 8: organic solvent phase containing organic impurities; 9: organic solvent phase (organic impurities are removed); 10: purified extract phase.

As a result of the study of the behavior of the organic impurities in the acid extracting step, the organic impurities in the wet process phosphoric acid are, with control of the chloride ion concentration extracted to the organic solvent phase in preference to the phosphoric acid component. In conventional processes part of the extracted organic impurities is moved out of the extracting step together with phosphoric acid component. However, most of the extracted organic impurities remain in the organic solvent phase, whereby the organic impurites are gradually accumulated in the solvent phase during the recycling of the solvent phase.Accordingly, even if a wet process phosphoric acid containing less organic impurities compared with the content of the organic impurities in the original wet process phosphoric acid is fed to the acid extracting, the concentration or the organic impurities in the solvent phase must increase to a high level during a long continuous operation whereby it may be difficult to continue the operation.

From the above-mentioned findings, the inventors have found that it is diffuclt to attain an industrial purification of a wet process phosphoric acid by the solvent extracting process without removing a substantial amount of the organic impurities before the extraction of the acid or selectively removing the organic impurities from the acid extracting step.

The separability of the two phases in the case of a contamination of the organic impurities is dependent upon the concentration of phosphoric acid and the concentration of metal compound impurities. When the concentrations of phosphoric acid component and the metal compound impurities are kept at high levels such as more than 20 wt. %and P2 5 and more than 2 wt.% of R203 (R: metal element) in the aqueous solution phase as the original wet process phosphoric acid, the separation of the two phases can be easily attained without the deterioration of the separability of the two phases found when using conventional extractors such as a rotary disc type extracting tower or a mixer-settler type extractor.

As the result of further study based on the findings on the behavior of the organic impurities in the acid extracting step, the inventors have found that the wet process phosphoric acid containing the organic impurities can be easily purified by the organic solvent extracting process by employing a step of selectively extracting the organic impurities from the wet process phosphoric acid (hereinafter referred to as the first section) and a step of extracting phosphoric acid for purifying the wet process phosphoric acid (hereinafter referred to as the second section) discharging a part of a phosphoric acid-organic solvent phase (hereinafter referred to as the extract phase) which is obtained by contacting the phosphoric acid with the organic solvent in the second section, using the extract phase as the solvent for extracting the organic impurities in the first section and purifying and recycling the solvent from the first step to the second.

The second section comprises an extraction battery (B 1) for extracting the phosphoric acid component from the aqueous phase (organic impurities having been removed) to the solvent phase, and a scrubbing battery for removing the inorganic impurities, extracted together with the phosphoric acid component into the solvent phase, by contacting with water or an aqueous solution of phosphoric acid having high purity and a stripping battery recovering the phosphoric acid component by contacting with water. The extracting method can be a known method. In the specification, the term "extract phase" means the solvent phase containing the phosphoric acid component and the inorganic impurities obtained from the extraction battery.

The extract phase from the second acid extracting section passed to the first section has a composition in substantial equilibrium with the wet process phosphoric acid with respect to the concentrations of phosphoric acid component and the metal compound impurities, whereby the phosphoric acid component and the metal compound impurities of the wet process phosphoric acid remain in the aqueous phase and only the organic impurities are selectively extracted into the organic phase, and the separation of the two phases can be attained at a high enough separating speed when the wet process phosphoric acid is contacted with the extract phase in the first section.

At the same time, because the part of the extract phase passed to the first section from the second section is purified before returning to the second section, the accumulation of organic impurities in the second section can be prevented, thus avoiding the problems caused by such organic impurities.

In the process, a part of the extract phase is discharged from the second section whereby it is necessary to maintain the flow rate ratio of the two phases in constant by adding the organic solvent in order to keep the extraction coefficient and the purification coefficient constant.

The organic solvent containing no organic impurity which is obtained by removing the organic impurities from the organic solvent phase containing the organic impurities obtained in the first section, is returned for use as the additional organic solvent. A loss of the expensive organic solvent can be substantially prevented by this method.

In the process of the present invention, it is necessary to remove the organic impurities from the organic solvent phase obtained in the first section and to recycle it to the second section.

It is also necessary to extract the organic impurities in the first section under conditions which allow the extraction of substantially no iron impurity.

The metal compound impurities in the wet process phosphoric acid are not easily moved to the organic solvent phase. However, when enough hydrochloric acid is included, the iron component forms a chloride complex which is easily extracted into the organic solvent phase.

It is substantially impossible to remove iron impurity in the second section under the conditions used in the first section because a large amount of hydrochloric acid is moved from the first organic impurity extracting section to the second section.

Moreover, if the iron impurity were extracted in the first section, a) the separability of the two phases in the first section would deteriorate; b) the separation of the iron impurity from phosphoric acid component would be difficult, with loss of phosphoric acid component in the separation of the iron impurity; and c) the iron impurity would be precipitated as iron phosphate and iron hydroxide in the separation of the organic impurities to form a large amount of scale.

Thus, it would be impossible to produce purified phosphoric acid suitable to use for industrial uses, food additive uses, and pharmaceutical uses, in an industrial process from a wet process phosphoric acid.

As is clear from the above-mentioned facts, the troubles caused by the organic impurities in the acid extracting step can be overcome by combining the first section with the second section and operating the first section under conditions for extracting no iron impurity whereby an easy extracting operation can be attained and the purified phosphoric acid can be easily and economically obtained from a wet process phosphoric acid containing organic impurities by the extracting process. The purified phosphoric acid obtained by the process of the present invention has high purity and can be used for industrial uses, food additive uses and pharmaceutical uses because it does not substantially contain any of the impurities, particularly organic impurities which are contained in the wet process phosphoric acid.

The first step in the process of the present invention can be a batch process or a continuous process. It is preferable to employ a counter-current continuous extracting process.

The amount of solvent used in the first section is dependent upon the kind of organic solvent and more particularly the organic impurities in the wet process phosphoric acid. The amount of the organic impurities in the wet process phosphoric acid are dependent upon the producing district and the kind of the phosphate rock. Accordingly, the amount of the solvent used in the first section can not be precisely defined but is usually less than 3.0 vol. parts per 1 vol. part of the wet process phosphoric acid.

For example. when Morocco phosphate rock is used, the amount of the solvent is in a range of 0.2 to 0.5 vol. part per 1 vol. part of the wet process phosphoric acid and when Florida phosphate rock is used. the amount of the solvent is in a range of 0.4 to 1.0 vol. part per 1 vol.

part of the wet process phosphoric acid, whereby more than 80 wt.% especially more than 90 wt. % of the organic impurities contained in the wet process phosphoric acid can be removed.

In order to substantially prevent the extraction of the iron impurity in the first section, the concentrations of hydrochloric acid and chloride are controlled. The concentration of chloride ions is depending upon the kind of the organic solvent. The concentration can be easily decided by empirical tests. When the chloride ion concentration in the organic solvent phase is kept at less than 10g/liter as Cl, the iron impurity is not substantially extracted.

It is necessary to remove the organic impurities from the organic solvent phase obtained in the first section and to recycle the organic solvent to the second section. The method of removing the organic impurities can be any suitable conventional method. For example, the organic impurities can be removed by contacting the organic solvent phase obtained in the first section with a small amount of water to selectively recover phosphoric acid component from the organic solvent phase, and washing it with an aqueous solution of an alkali metal hydroxide or distilling it or treating it with active carbon.

The recovered aqueous solution of phosphoric acid is fed with the wet process phosphoric acid to the first section and the recovered organic solvent is recycled to the second section.

When the organic solvent phase containing the organic impurities is directly treated with active carbon. the organic impurities can be selectively adsorbed and removed and both of the phosphoric acid component and the organic solvent can be simultaneously recovered, advantageously. At least part of the recovered organic solvent is recycled to the second section and the remainder can be recycled to the first section. Accordingly, the amount of the extract phase recycled to the first section can be the same as the amount of solvent recycled from the first section to the second section and is usually in a range of 0.01 to 0.2 vol. part of total organic solvent.

The step of removing the organic impurities in the process of the present invention is effective and economical in comparison with the conventional processes because the volume of the treated solution is small and the solution contains a higher content of the organic impurities when treated.

The organic solvent used in the process of the present invention is the same in the first section and the second section. The organic solvent can be a conventional solvent for purifying phosphoric acid. From the viewpoints of the extraction of the organic impurities, the separability of the impurities from phosphoric acid, the cost, the availability, the stability and the handling, it is preferable to use alcohols especially, n-butanol and isoamyl alcohol and ketones, especially methyl isobutyl ketone.

In the present invention, the wet process phosphoric acids include the conventional wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid and separating calcium sulfate and the phosphoric solution acid obtained by concentrating the wet process phosphoric acid, the phosphoric acid solution obtained by removing excess sulfuric acid and hydrogen silicofluoride in a chemical method and the phosphoric acid solution obtained by removing a part of the organic impurities by treating the wet process phosphoric acid with an oxidizing agent.

The present invention will be further illustrated by the following examples wherein the terms "%" and "ppm" means "% by weight" and "ppm by weight".

EXAMPLE 1: A wet process phosphoric acid which was prepared by treating an uncalcined phosphate rock produced in Morocco with sulfuric acid was continuously purified by the process shown in Figure 1.

One vol. part of the wet process phosphoric acid was fed to the first mixer-settler of a first organic impurity extracting section (A) having 5 mixer-settlers, and 0.5 vol. part of a purified extract phase (7') discharged from the 10th mixer-settler of a second acid extracting section (B) having 40 mixer-settlers (of which the 1st to 10th are shown as B1, the 11th to 30th as B2 and the 31st to 40th as B3) was fed to the 5th mixer-settler in the first section (A) so as to contact them counter-currently, whereby aqueous phosphoric acid (2) having no organic impurity and having the composition given in the second column of Table 1 was obtained.

The phosphoric acid (2) was fed to the 10th mixer-settler in the second section (B). In the second section (B), phosphoric acid was extracted into the organic solvent phase in the 1st to 10th mixer-settlers (B1) and the inorganic impurities in that part of the resulting extract phase (7) not fed to section (A) were removed into an aqueous phase in the 11th to 30th mixer-settlers (B2). The 31st to 40th mixer-settlers (B 3) were used for recovering phosphoric acid component by aqueous extraction from the purified organic extract phase (10). Six vol.

parts of water saturated n-butanol (6) was fed to the first mixer-settler in the second section B and 2 vol. parts of pure water (3) was fed to the 40th mixer-settler and 0.6 vol. part of the purified phosphoric acid (4) was fed to the 30th mixer-settler whereby the purified phosphoric acid (4) was obtained from the 31st mixer-settler and the water saturated n-butanol phase (6) containing no phosphoric acid was obtained from the 40th mixer-settler.

The water saturated n-butanol was fed to the 1st mixer-settler in the second section (B).

The organic solvent phase (8) containing a large amount of organic impurities obtained in the first section (A) was treated by passing through a column (C) filled with Pittsburgh granular carbon CAL to remove the organic impurities and then recycled through the 11th mixersettler in the second section (B). As a result, most of the organic impurities contained in the wet process phosphoric acid were removed in the first section to obtain a colorless purified phosphoric acid.

The organic solvent phase in the second section was slightly colored at the initiation.

However, the accumulation of the organic impurities was not found. The separatability of the two phases in the first section and the second section was not changed whereby the operation was smoothly continued for a long time.

The purificed phosphoric acid was concentrated to obtain the conc. purified phosphoric acid having the following composition.

Table 1: Wet process Phosphoric acid Cone. purified phosphoric acid (organic impurities phosphoric acid are removed) P205 33 % 32% 61.5 % Ca 0.52 0.50 0.0003 Fe 0.16 0.16 0.0002 Al 0.25 0.24 0.0003 Ti 0.025 0.024 0.0003 Cl trace trace Color black-brown greenish blue pale yellow Reference 1: In accordance with the process of Example 1 except employing the first organic impurity extracting section (A) to directly feed the wet process phosphoric acid to the 10th mixersettler of the second acid extracting section (B) and treating the water saturated n-butanol phase containing substantially no phosphoric acid obtained from the 40th mixer-settler with a column (C) filled with active carbon and recycling the treated solvent to the 1st mixer settler, the purification of the wet process phosphoric acid was carried out.

The resulting purified phosphoric acid was concentrated under a reduced pressure to the concentration of 61.5 % as P205. The conc. purified phosphoric acid had brown color. The amount of organic impurity removed by the column filled with active carbon was about 1/5 in comparison with that of Example 1.

Reference 2: In accordance with the process of Example 1 except using a wet process phosphoric acid prepared by treating the uncalcined phosphate rock produced in Morocco with hydrochloric acid, the purification of the wet process phosphoric acid was carried out. As the result, the accumulation of the organic impurities in the second acid extracting section was not found and the operation was smoothly continued, however, a large amount of iron impurity was contained in the purified phosphoric acid and the purity was low because the iron impurity was extracted in the first organic impurity extracting section (A). The compositions of the phosphoric acids are shown in Table 2.

Table 2: Wet process Phosphoric acid Conc. purified phosphoric acid (organic impurities phosphoric acid are removed) P205 9.5 % 9.5 % 61.5 % Ca 8.6 8.6 0.001 Fe 0.05 0.03 0.25 Al 0.07 0.07 0.001 Ti 0.007 0.007 0.003 Cl 9.1 8.9 0.04 Color brown yellowish green yellow Example 2: In accordance with the process of Example 1 but using isoamyl alcohol instead of n-butanol and adding 35 % HC1 in the 1st mixer-settler of the second section so as to give the chlorine con. 5 to 8g/l as Cl. the purification of the wet process phosphoric acid was carried out.

As a result, the extraction of iron impurity to the organic solvent phase in the first section was not found. The result was the same with that of Example 1 Example 3: In accordance with the process of Example 1 but using methyl isobutyl ketone instead of n-butanol. the purification of the wet process phosphoric acid was carried out. The result was the same as that of Example 1.

WHAT WE CLAIM IS: 1. A process for purifying a wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid, which process comprises; a first organic impurity extracting step wherein the phosphoric acid is contacted with an organic solvent to extract selectively organic impurities into the organic solvent phase, the chloride ion concentration being maintained at a level at which substantially no iron impurity is extracted and a second acid extracting step wherein the phosphoric acid obtained in the first step is contacted with the same organic solvent as used in the first step. and back extracted with water after removal of inorganic impurities therefrom by contacting with water or an aqueous solution of phosphoric acid of high purity to obtain an aqueous solution of purified phosphoric acid, wherein a part of the organic extract phase from the second step is recycled for use as the organic solvent in the first step, the organic impurities are removed from the organic solvent phase obtained from the first step and at least part of the resulting organic solvent phase is returned to the second step.

2. A process according to claim 1 wherein the chlorine concentration in the organic solvent phase in the first step is kept to less than 10 g/liter as Cl.

3. A process according to claim 1 or claim 2, wherein the organic solvent phase containing organic impurities obtained in the first step is treated with active carbon to remove the said impurities.

4. A process according to any preceding claim wherein the phosphate rock is an uncalcined phosphate rock.

5. A process according to any preceding claim wherein the organic solvent is n-butanol, isoamyl alcohol or methyl isobutyl ketone.

6. A process according to claim 1 substantially as herein described with reference to the Examples.

7. Purified phosphoric acid made by a process according to any preceding claim.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the purification of the wet process phosphoric acid was carried out. The resulting purified phosphoric acid was concentrated under a reduced pressure to the concentration of 61.5 % as P205. The conc. purified phosphoric acid had brown color. The amount of organic impurity removed by the column filled with active carbon was about 1/5 in comparison with that of Example 1. Reference 2: In accordance with the process of Example 1 except using a wet process phosphoric acid prepared by treating the uncalcined phosphate rock produced in Morocco with hydrochloric acid, the purification of the wet process phosphoric acid was carried out. As the result, the accumulation of the organic impurities in the second acid extracting section was not found and the operation was smoothly continued, however, a large amount of iron impurity was contained in the purified phosphoric acid and the purity was low because the iron impurity was extracted in the first organic impurity extracting section (A). The compositions of the phosphoric acids are shown in Table 2. Table 2: Wet process Phosphoric acid Conc. purified phosphoric acid (organic impurities phosphoric acid are removed) P205 9.5 % 9.5 % 61.5 % Ca 8.6 8.6 0.001 Fe 0.05 0.03 0.25 Al 0.07 0.07 0.001 Ti 0.007 0.007 0.003 Cl 9.1 8.9 0.04 Color brown yellowish green yellow Example 2: In accordance with the process of Example 1 but using isoamyl alcohol instead of n-butanol and adding 35 % HC1 in the 1st mixer-settler of the second section so as to give the chlorine con. 5 to 8g/l as Cl. the purification of the wet process phosphoric acid was carried out. As a result, the extraction of iron impurity to the organic solvent phase in the first section was not found. The result was the same with that of Example 1 Example 3: In accordance with the process of Example 1 but using methyl isobutyl ketone instead of n-butanol. the purification of the wet process phosphoric acid was carried out. The result was the same as that of Example 1. WHAT WE CLAIM IS:

1. A process for purifying a wet process phosphoric acid prepared by treating a phosphate rock with sulfuric acid, which process comprises; a first organic impurity extracting step wherein the phosphoric acid is contacted with an organic solvent to extract selectively organic impurities into the organic solvent phase, the chloride ion concentration being maintained at a level at which substantially no iron impurity is extracted and a second acid extracting step wherein the phosphoric acid obtained in the first step is contacted with the same organic solvent as used in the first step. and back extracted with water after removal of inorganic impurities therefrom by contacting with water or an aqueous solution of phosphoric acid of high purity to obtain an aqueous solution of purified phosphoric acid, wherein a part of the organic extract phase from the second step is recycled for use as the organic solvent in the first step, the organic impurities are removed from the organic solvent phase obtained from the first step and at least part of the resulting organic solvent phase is returned to the second step.

7. Purified phosphoric acid made by a process according to any preceding claim.