PROCESS FOR PURIFYING PHOSPHORIC ACID

Abstract: The invention relates to a process for removing iron from phosphoric acid prepared by the wet process. The Fe<3+> ions present as an impurity in the phosphoric acid are reduced to Fe<2+> ions. After the reduction the Fe<2+> ions can be removed from the phosphoric acid by cationic ion exchange.

Description: Process for purifying phosphoric acid The invention relates to a process for removing iron from phos¬ phoric acid prepared by the wet process. Phosphoric acid is prepared by the wet process by allowing a mineral acid, most commonly sulfuric acid, to react with a calcium phosphate concentrate, whereby a dilute phosphoric acid, containing approx. 30 % P 2 0 5 , and a calcium sulfate pre¬ cipitate are formed. After filtration, the acid contains many kinds of anionic impurities, such as sulfate and fluorine com¬ pounds, and cationic impurities, the most important of which are iron, aluminum, magnesium and calcium, as well as organic impurities. The quantity and type of the impurities are depend¬ ent primarily on the crude phosphate used as the raw material. The dilute acid is concentrated by evaporation to a concentra¬ tion of 50-60 % P z 0 5 , whereby the volatile impurities such as fluorine compounds are removed along with the evaporating water vapor, but the concentrations of other impurities increase. Many kinds of purification processes have been developed for the purification of wet phosphoric acid so that the concentra¬ tions of the impurities could be reduced to such a level that the acid could be used for even other purposes than the pro¬ duction of fertilizers. It is known to use an electrothermal purification process. By the electrothermal process it is possible to prepare a phos¬ phoric acid having a low iron content, but the process consumes a high amount of energy and is therefore expensive. From GB patent publication 1 588 413 there is known a process for removing iron from wet phosphoric acid, wherein the iron is precipitated by using an alkali ferrocyanide or ammonium fer- rocyanide. However, owing to the toxicity of cyanides, and especially of hydrocyanic acid, the process is not very attrac¬ tive. It is further known to extract the phosphoric acid by means of organic solvents, from which it is re-extracted into an aqueous phase and is concentrated. Most of the iron remains in the aqueous phase, from which it can be removed together with the raffinate. However, sufficiently low iron concentrations have not been achieved by extraction; an additional purification step has been required, such as the removing of iron from an organic phase by the use of anionic ion exchange resins (US patent publication 5 006 319), or it has been necessary to carry out other procedures (EP application publication 327 234). Another disadvantage of the process is that it is expensive. The produced extract is dilute with respect to phos¬ phoric acid, so concentration is necessary. The solvent has to be recycled by re-extraction or by distillation. The organic phase often has to be purified by an additional treatment. The purification of the raffinates is costly, and furthermore, organic residues must be removed from the product so that the product obtained will be water clear. In addition, the handling of organic solvents may result in problems of industrial hy¬ giene or in a fire hazard. It is also known that the removing of iron from an aqueous solution of phosphoric acid by means of a cationic ion exchange resin will not succeed, since Fe 3"* forms in phosphoric acid complexes so stable that they will pass through the ion ex¬ change treatment unchanged and the removing of iron will not be successful. The object of the present invention is indeed to produce phos¬ phoric acid which contains virtually no iron and which can be produced at moderate cost, without environmental hazards. It was observed, surprisingly, that the Fe 3* ion forms stronger complexes with the fluoride ion which is present as an impurity in phosphoric acid than does the Fe 3* ion. Likewise, in phos¬ phoric acid the Fe 3* ion may be present as a phosphate complex. Since the iron ions are bound in complexes, they will not as such become adsorbed to a cationic ion exchange resin. The removing of iron by means of an anionic ion exchange resin is also not efficient. According to the method of the invention, the Fe 3* complexes are broken up by reducing the iron ions to a valence of two. The complexes of the Fe 2* ion with a fluoride and phosphate ion are more unstable than are the corresponding Fe 3* complexes. After reduction, the Fe 2* ions can be removed by cationic ion exchange. The principal characteristics of the process according to the invention are indicated in the accompanying patent claims. The reduction of the Fe 3* ions can be carried out by electro¬ lytic reduction. Electrolytic reduction has an advantage in its purity; additional chemicals need not be introduced into the acid to be purified. This increases the available ion exchange capacity. Further advantages are the easy control of electro¬ lytic reduction and the fact that the reduction can be brought virtually to completion. It is also possible to use chemical reduction. Electrolytic reduction takes place in a cell in which the cathode and anode chambers are separated from each other by a diaphragm or an ion exchange membrane. This is done in order to prevent the re-oxidation, on the anode, of the Fe 3* ion which has been reduced on the cathode. Electric current is led to the electrodes, the materials of which can be selected freely so as to minimize the share of secondary reactions and energy losses. The acid which contains the impurity to be reduced serves as a catholyte which is circulated through the cathode chamber. Fe 3* ions are reduced to Fe 2* ions on the cathode when the voltage is switched on between the electrodes. The anolyte used may be either phosphoric acid similar to the acid to be reduced on the cathode, or it may be some other electrically conductive solu¬ tion. What is most important is that secondary reactions are minimal. The progress of the reduction can be monitored either by means of chemical analysis or a redox electrode. When the desired degree of reduction has been reached, the acid is transferred to the ion exchange stage. When necessary, the reduction and ion exchange stages can be repeated. The ion exchange following the reduction is carried out by using a cationic ion exchange agent. The acid to be purified is caused to flow through an ion exchange pulp which is in the form of an acid, the cations adhering to the pulp. When the pulp has been saturated with cations it is regenerated with a suitable regeneration solution and, when necessary, any resi¬ dues of the regeneration solution are washed out of the pulp before a new ion exchange cycle. The cationic ion exchange resin may also be in a Na * or K * form, in which case the purified phosphoric acid is suitable for the preparation of pure alkali phosphates. The concentration of the acid to be purified may be varied within a wide range. Likewise, the concentration of the Fe 3* ion which is present as an impurity may vary. The temperature at which the reduction and/or ion exchange is carried out can be selected according to the materials used. In general, electrolytic reduction and ion exchange are more rapid at an elevated temperature than at room temperature. A few embodiment examples are presented below with the purpose only to illustrate the present invention. Example 1. Reference example A dilute phosphoric acid was prepared from Siilinjarvi phos¬ phate. Before the ion exchange treatment the concentration was 27.3 % and the Fe concentration was 0.22 %. The phosphoric acid was pumped at room temperature through a macroporous strong cation resin Amberlite 200, at a rate of 100 ml/min. The dimensions of the ion exchange column were: diameter 0.044 m, height 2.93 m. After the ion exchange treatment the P 2 0 5 con¬ centration was 27.3 % and the Fe concentration was 0.17 %. 23 % of the iron had been removed. Example 2. A dilute phosphoric acid was prepared from Siilinjarvi apatite. Before the ion exchange treatment the P 2 0 5 concentration was 25.5 % and the Fe concentration was 0.27 %. The Fe 3* was reduced in an electrolytic cell packed with graphite, the electrode chambers being separated from each other by an ion exchange membrane. The solution to be reduced was circulated on the cathode side. A sulfuric acid of the same concentration was used on the anode side. The nominal electrode surface area of the cell was 8 dm 2 , and the current density calculated from it was 250 A/m 2 . The reduction was discontinued when the Fe 3* conversion was >99 %. The phosphoric acid was pumped through the ion exchange column mentioned in Example 1, at room temperature, at a rate of 100 ml/min. After the ion exchange treatment the P 2 0 5 concentration was 25.0 % and the Fe concentration was 0.007 %. Example 3 . A dilute phosphoric acid was prepared from Siilinjarvi apatite. Before the ion exchange treatment the P 3 0 5 concentration was 35.0 % and the Fe concentration was 0.37 %. The reduction of Fe 3* was carried out in accordance with Ex¬ ample 2. Phosphoric acid was pumped through the ion exchange column mentioned in Example 1, at a temperature of 70 °C, at a rate of 100 ml/min. After the ion exchange treatment the P 2 0 5 concentration was 33.7 % and the Fe concentration was 0.0168 %. Example 4. A dilute phosphoric acid was prepared from Siilinjarvi apatite. Before the electrolytic reduction treatment the P 2 0 5 concentra¬ tion was 26.2 % and the Fe concentration was 0.10 %. The Fe 3* ions were reduced to Fe 2* ions in an electrolytic cell the electrode surface area in which was 0.5 m 2 . The electrode cham¬ bers were separated from each other by a ceramic diaphragm. The current density used was 180 A/m 2 . At the end of the reduction the conversion of Fe 3* was 99.8 %. After the reduction the acid was introduced, at room temperature, at a rate of 200 ml/min, to an Amberlite 200 ion exchange column having a diameter of 0.1 m and a height of 1.5 m. After the ion exchange treatment the P 2 0 5 concentration was 24.5 % and the Fe concentration was 0.0015 %.

Claims: Claims 1. A process for removing iron from phosphoric acid pre¬ pared by the wet process, characterized in that the Fe 3* ions present as an impurity in the phosphoric acid are reduced to Fe 2* ions, after which reduction the Fe 2* ions can be removed from the phosphoric acid by cationic ion exchange. 2. A process according to Claim 1 for removing iron from phosphoric acid prepared by the wet process, characterized in that the Fe 3* ions present as an impurity in the phosphoric acid are reduced electrolytically. 3. A process according to Claim 2 for removing iron from phosphoric acid prepared by the wet process, characterized in that the electrolytic reduction takes place in a cell the cathode and anode chambers of which are separated from each other by a diaphragm or an ion exchange membrane. 4. A process according to any of the above Claims 1-3 for removing iron from phosphoric acid prepared by the wet process, characterized in that the cationic ion exchange is carried out with the help of a strong ion exchange resin in acid form. 5. A process according to any of the above Claims 1-4 for removing iron from phosphoric acid prepared by the wet process, characterized in that the cationic ion exchange is carried out with the help of an ion exchange resin in Na * or K * form. 6. A process according to any of the above Claims 1-5 for removing iron from phosphoric acid prepared by the wet process, characterized in that the reduction and/or the cationic ion exchange step may be repeated.

Inventors: KARI ESKO, KARJALAINEN TIMO, KARI, ESKO, KARJALAINEN, TIMO
Applicants: KARI ESKO, KARJALAINEN TIMO, KEMIRA OY
Application year: 1993
Main classification: C01B25
IPC codes: C01B25