PROCESS FOR THE PURIFICATION OF PHOSPHORIC ACID

Abstract: The invention relates to a process for the purification of an impure phosphoric acid prepared by the wet process. The acid to be purified is concentrated to 58-68 % P2O5 acid by heating. The impurities are removed from the acid by crystallization, as a compound M<2+> (H2PO4)2.n.H3PO4, where M<2+> is a bivalent metal ion impurity and n may vary within 2-5.

Description: Process for the purification of phosphoric acid This invention relates to a process for removing impurities from phosphoric acid prepared by the wet process. Phosphoric acid can be prepared by the wet process by reacting a mineral acid, most commonly sulfuric acid, with a calcium phosphate concentrate, whereupon a dilute phosphoric acid, containing approx. 30 % P 2 0 5 , and a calcium sulfate precipitate are formed- After filtration, the acid contains many kinds of anionic impurities, such as sulfate and fluorine compounds, and cationic impurities, the most important of which are iron, aluminum, magnesium and calcium, and organic impurities. The quantity and type of the impurities depend above all on the raw phosphate used as the raw material. The dilute acid is concentrated by evaporation to a concentra¬ tion of 50-70 % P 2 0 5 , whereby volatile impurities, such as fluorine compounds, are mostly separated together with the evaporating water vapor, but the concentrations of other im¬ purities are increased. Many kinds of purification processes have been developed for the purification of we -process phosphoric acid in order that the concentrations of the impurities could be lowered to such levels that the acid could be used for purposes even other than the preparation of fertilizers. Known purification processes include solvent extraction, solvent precipitation, indirect purification, and ion exchange methods. These known processes have the disadvantage of being expensive. Magnesium, which is present as an impurity, has been precip¬ itated in prior art by means of, for example, hydrofluoric acid or silicofluoric acid. US patent specification 4 248 843 discloses a process for the purification of phosphoric acid wherein the magnesium ions present as an impurity are pre¬ cipitated as a pyrophosphate. The process has a disadvantage in a high viscosity at higher concentrations, complicating the crystallization of the pyrophosphate and the separation of the crystals. The crystallizing magnesium pyrophosphate is not biologically active, and therefore it as such has no utility purpose; the compound requires hydrolysis in order to be usable as a fertilizer or in animal feeds. An object of the present invention is indeed to produce very pure phosphoric acid at economical cost. A further object of the invention is to cause the metal ions present as impurities to precipitate as compounds which are usable in industry. According to the invention it has now been observed that the impurities present in phosphoric acid can be precipitated as compounds which crystallize within even lower concentration ranges of phosphoric acid than in state-of-the-art processes. It has additionally been observed that, if the initial acid used in the process does not contain impurities which are re¬ garded as hazardous, such as cadmium, the process offers an additional advantage. Namely, the waste compound formed as an impurity can be utilized as a useful raw material in industry. Thus, the impurity compounds separated by the present process will not become environmental hazards. The main characteristics of the process of the invention are given in the accompanying claims. According to the invention, primarily a process has been pro¬ vided for the removal of bivalent metal ion impurities from a wet-process phosphoric acid. In the process, a phosphoric acid of a fertilizer grade or filter acid grade is concentrated by heating to a P 2 0 5 concentration of 58-68 %. By cooling the acid mixture, the bivalent metal ions which are present as impurities are caused to crystallize as a compound M 2+ (H 2 Pθ 4 ) 2 ' -H 3 Pθ 4 , wherein the metal ions are present as M 2+ , and the number n of associated phosphoric acid molecules is 2- 5. The initial acid used may be a we -process phosphoric acid the P 2 0 5 content of which may be approx. 20-60 %, typically 30 % P 2 C> 5 or 50 % P 2 0 5 . The sulfate content of such a phosphoric acid typically varies within 2-4 % S0 4 . Wet-process phosphoric acid contains a variety of anionic, cationic and organic impurities, the concentrations of which vary depending on the origin of the phosphate concentrate from which the acid was prepared. A P 2 0 5 acid (concentration 52 %) prepared from the Siilinjarvi phosphate concentrate by the wet process typically contains as cationic metallic impurities approx. 0.1 % Al, 1.0 % Mg and 0.5 % Fe, whereas a P 2 0 5 acid (concentration 55 %) prepared from a concentrate obtained from Morocco typically contains approx. 0.8 % Al, 0.6 % Mg and 0.8 % Fe. By the process according to the invention it is also possible to purify a phosphoric acid which has been used for the surface treatment of metals, since such a phosphoric acid contains, for example, bivalent metal ions as impurities. By the process according to the invention it is possible to lower the cationic metal ion concentrations in phosphoric acid almost completely, or at least to an acceptable level. By the process the concentrations of, for example, the following metal ions have been lowered: Mg, Fe, Ca, Mn, Zn, Cd, Co, Cu, Pb, and Ni. According to X-ray diffraction spectra, the formed crystalline compound is neither magnesium pyrophosphate nor anhydrous monomagnesium phosphate. When the crystals are washed with ethanol, some of the phosphoric acid dissolves, and Mg(H 2 P0 4 ) 2 and Fe(H 2 P0 4 ) 2 -2H 2 0 were identified as the compounds in an X-ray diffraction spectrum. On the basis of analyses, the crystalline orthophosphate, M 2+ (H 2 P0 4 ) 2 -n-H 3 P0 4 , contains mainly bivalent cations, and phosphoric acid molecules (n=2-5) are associated with it. The crystallized metal compound crystals can be separated from the phosphoric acid by methods known per se. such as centrifugation, pressure filtration, vacuum filtration, or sedimentation. According to the invention it is advantageous to remove the sulfate from the acid before the concentration of the acid. However, the removal of the sulfate is not indispensable. The sulfate can, for example, be removed directly from the phos¬ phoric acid by precipitation as gypsum by means of a calcium salt. It has, however, been observed that a lowering of the sulfate concentration enables lower temperatures to be used in the acid concentration step than without a lowering of the sulfate concentration. The process also enables the waste compound crystallized out from phosphoric acid to be used as a raw material in industry. Magnesium phosphate crystallized out from a phosphoric acid prepared from a crude phosphate which has a low heavy metal content can be used, for example, in animal feeds and as a raw material for the fertilizer industry. In terms of use in animal feed it is preferable that the concentrations of detrimental ions (F, As, Al and Cr) in the crystals are reduced and the concentrations of useful trace elements (Fe, Mn, Zn and Cu) increase in proportion to the concentrations of the feed acid. It has also been observed that, when purified acid is stabil¬ ized, for example by allowing it to stand for a few days or by dilution, after-crystallization can be prevented. The invention is described below in greater detail with the help of working examples. It is, however, clear that the various applications of the invention are not limited to those presented below by way of examples but may be varied within the accompanying claims. Example 1 A fertilizer-grade phosphoric acid (Siilinjarvi) was con¬ centrated (150 °C, 65 mm Hg) to 67.5 % P 2 0 5 . The feed acid contained 2.76 % S0 4 . At 53 °C, seed was added at a rate of 2 % of the mass of the concentrated acid. After one day, when the slurry had cooled to 22 °C, the crystals were separated in a centrifuge, and the compound was ascertained by means of an X- ray diffraction spectrum. The quality of the feed and product acids are presented in Table 1. Table 1 P 2 0 5 % F % Mg % Fe % Feed 67 . 5 0 .21 1 .5 0 . 58 Product 66 . 9 0 .20 0 . 15 0 .47 Example 2 From Siilinjarvi filter acid (25.6 % P 2 0 5 ) and fertilizer acid (51.3 % P 2 0 5 ) , a mixture was prepared the sulfate level of which was lowered by adding calcium. This fertilizer-grade phosphoric acid, having a lowered sulfate level (0.2 % S0 4 ) , was concentrated to 64 % P 2 0 5 . Seeds (2 %) were added at 90 °C, and the crystals were separated after one day, the slurry hav- ing cooled to 25 °C. The analyses of the feed and product acids and the analyses of the crystals are shown in Table 2. Table 2 Filter Fertilizer Purified Mg phosphate acid acid acid p 2 o 5 % 25.6 51.3 63.6 64.8 F % 1.8 0.45 0.11 0.087 S0 4 % 1.5 1.6 0.09 0.96 Mg % 0.5 1.1 0.47 4.2 Fe % 0.28 0.56 0.67 0.72 Ca % 0.35 0.027 0.08 0.46 Al % 0.091 0.14 0.17 0.038 Mn ppm 230 490 2 2600 Zn ppm 25 49 12 220 Cd ppm 0.4 0.7 0.4 2 As ppm 2.8 2.9 0.2 <0.2 Cu ppm 4.6 4 <1 <1 Pb ppm <4 <4 4.4 <4 Ni ppm 1.2 3.1 <1 Example 3 Approx. 95 % of the iron present in a fertilizer-grade phos- phoric acid (40 % P 2 0 5 , Siilinjarvi) in which the sulfate level had been lowered (0.66 % S0 4 ) was reduced to bivalent iron and was concentrated to 57.9 % P 2 0 5 . At this concentration the acid contained 1.2 % Mg and 0.53 % Fe. After additional concentra¬ tions, approximately 80 % of the iron remained in the reduced state. The crystals were separated and the formed product acids were analyzed after one day and after one week. The results of the analyses are shown in Table 3. Table 3 82.5 C 90 C 95 C 100 C 105 C 110 C P 2 0 5 feed 57.9 60 61.3 62.8 63.8 64.4 Fe feed 0.53 0.58 0.59 0.6 Fe tot/l d 0.53 0.37 0.19 0.11 0.09 0.08 Fe 3+ /1 d 0.05 0.10 0.11 0.10 0.09 0.08 Mg/1 d 1.2 1.23 1.15 0.01 0.78 0.51 Fe tot/1 wk 0.53 0.23 0.14 0.11 0.1 0.08 Fe 3+ /1 wk 0.16 0.12 0.11 0.1 0.08 Mg/1 wk 1.2 1.18 1.05 0.81 0.55 0.32 Example 4 A fertilizer-grade phosphoric acid (Morocco) in which the sul¬ fate level had been lowered (1.3 % S0 4 ) was concentrated to 64 % P 2 0 5 . Seeds (1 %) were added at 5 °C, and the crystals were separated after one day, and the compound was ascertained by means of an X-ray diffraction spectrum. The analyses of the product acid and the crystals are shown in Table 4. Table 4 H 3 P0 4 M(2+) (H 2 P0 4 ) 2 -n-H 3 P0 4 MOROCCO MOROCCO yield % 93.6 6.4 PA % 64 58.4 F % 0.13 0.14 S0 4 % 0.9 7.8 * Mg % 0.44 2.4 Fe % 0.39 0.38 Ca % 0.21 3.1 * Al % 0.22 0.12 Mn ppm 13 84 Zn ppm 330 4400 Cr ppm 420 220 Cd ppm 10 250 As ppm 13.3 5.3 Cu ppm 34 240 Pb ppm <0.2 0.4 * The high concentrations of Ca and S0 4 are derived from gypsum Example 5 A fertilizer-grade phosphoric acid (Siilinjarvi) in which the sulfate level had been lowered (0.8 % S0 4 ) was concentrated to 64.2 % P 2 0 5 . Seeds (1 %) were added at 95 °C, and the acid was cooled in five hours to 50 °C. The feed acid contained 1.5 % Mg and the product acid contained 0.6 % Mg.

Claims: Claims 1. A process for the purification of an impure phosphoric acid prepared by the wet process, characterized in that the phosphoric acid to be purified is concentrated by heating to 58-68 % P 2 0 5 acid, from which acid the impurities are removed by crystallization as a compound M 2+ (H 2 P0 4 ) 2 'n-H 3 P0 4 , where M 2+ is a bivalent metal ion impurity and n may vary within 2-5. 2. A process according to Claim 1, characterized in that, before the concentration of the acid, sulfate ions are removed from the phosphoric acid to be purified. 3. A process according to Claim 1 or 2, characterized in that the concentration is carried out at a temperature of 90- 150 °C, the pressure being 65 mmHg. 4. A process according to any of the above Claims 1-3, characterized in that seed crystals are added to the con- centrated acid. 5. A process according to any of the above Claims 1-4, characterized in that bivalent metal ions, in particular Mg, Fe, Mn, Zn, Cd and Cu, are removed from the phosphoric acid by crystallization. 6. A process according to any of the above Claims 1-5, characterized in that the phosphoric acid to be purified is concentrated to at minimum 61 % P 2 0 5 acid. 7. A process according to any of the above Claims 1-6, characterized in that the phosphoric acid to be purified is concentrated to at maximum 65 % P 2 0 5 acid. 8. A process according to any of the above Claims 1-7, characterized in that the crystallization temperature of the impurity compound is 0-100 °C, preferably 20-90 °C. 9. Use of compounds crystallized as impurities by the process according to Claim 1 in fertilizers or in animal feeds.

Inventors: WECKMAN ANDERS, WECKMAN, ANDERS
Applicants: KEMIRA CHEMICALS OY, WECKMAN ANDERS, WECKMAN, ANDERS
Application year: 1995
Main classification: C01B25
IPC codes: C01B25