Preparation of phosphoric acid

Abstract

The invention relates to a method for the industrial preparation of phosphoric acid from an aqueous suspension comprising water and particles of at least one phosphate material dispersed in the presence of at least one additive of the anionic polymer type. Phosphoric acid is obtained by treating this suspension with at least one strong acid. The polymer is obtained by polymerization reaction of at least one acid selected from acrylic acid, methacrylic acid and salts thereof. The invention relates to the use of this anionic polymer as well as the method for improving the rheology of the suspension and the hydrodynamics of the reaction medium in which the reactions for the manufacture of phosphoric acid take place.

Claims

1. A method for preparing an aqueous phosphoric acid solution comprising treating at a temperature ranging from 40 to 100° C., an aqueous suspension (A) with a strong acid to form an aqueous phosphoric acid solution and phosphogypsum particles, wherein the aqueous suspension (A) comprises water and particles of at least one phosphate material whose size is between 10 and 400 μm dispersed in the presence of at least one anionic polymer with a molecular mass by weight (M.sub.W) of between 1,000 and 90,000 g/mol and obtained by polymerization reaction of at least one acid chosen from acrylic acid, methacrylic acid and salts thereof.

2. The preparation method as claimed in claim 1, wherein the Brookfield viscosity of the aqueous suspension (A), measured 90 s after preparation of the suspension, at 25° C., at 100 rpm and at a concentration of more than 45% by weight of phosphate material.

3. The preparation method as claimed in claim 1, wherein the strong acid used is selected from sulfuric acid, nitric acid, hydrochloric acid, and a mixture of these acids.

4. The preparation method as claimed in claim 1, wherein the strong acid is sulfuric acid.

5. The preparation method as claimed in claim 1, wherein the phosphoric acid has a P.sub.2O.sub.5 titer of more than 25%.

6. The preparation method as claimed in claim 1, wherein the aqueous suspension (A) is prepared by stirring, a mixture of water, particles of phosphate material and at least one anionic polymer obtained by polymerization reaction of at least one acid selected from acrylic acid, methacrylic acid and salts thereof.

7. The preparation method as claimed in claim 1 further comprising (c) separating the aqueous solution of phosphoric acid from the phosphogypsum particles.

8. The preparation method as claimed in claim 7, wherein the separated phosphogypsum particles are oblong in size about 250 μm or 220-350 μm, have a ratio between the radius of the inscribed circle of the particle and the radius of the circumscribed circle of the particle close to 1.

9. The preparation method as claimed in claim 7, wherein the phosphogypsum particles are separated by filtration.

10. The preparation method as claimed in claim 1, wherein the aqueous phosphoric acid solution has a concentration by weight of prepared phosphoric acid of between 20 and 45%.

11. The preparation method as claimed in claim 6, wherein the particles of phosphate material are provided in dried form.

12. The preparation method as claimed in claim 1, wherein the phosphate material particles are provided in an aqueous suspension (B).

13. The preparation method as claimed in claim 12, wherein the aqueous suspension (B) also comprises at least one anionic polymer with a molecular mass by weight (M.sub.W) of between 1,000 and 90,000 g/mol and obtained by polymerization reaction of at least one acid chosen from acrylic acid, methacrylic acid and salts thereof.

14. The preparation method as claimed in claim 13, wherein the aqueous suspension (B) has a concentration of phosphate material greater than 50%.

15. The preparation method as claimed in claim 13, wherein the Brookfield viscosity of the aqueous suspension (B), measured 90 s after preparation of the suspension, at 25° C., at 100 rpm and at a concentration of more than 45% by weight of phosphate material, is less than 1,500 mPa.Math.s.

16. The preparation method as claimed in claim 1, wherein the polymer is partially or totally neutralized.

17. The preparation method as claimed in claim 1, wherein the anionic polymer is obtained by a polymerization reaction also using at least one ester of an acid selected from acrylic acid and methacrylic acid.

18. The preparation method as claimed in claim 1, wherein the anionic polymer has a molecular mass by weight (M.sub.W) of from 2,000 to 90,000 g/mol.

19. The preparation method as claimed in claim 1, wherein the amount by weight (dry/dry) of anionic polymer used is between 0.1 and 5%, based on the amount of phosphate material.

20. The preparation method as claimed in claim 1, wherein the strong acid has a pKa of less than 4.

21. The preparation method as claimed in claim 1, wherein the aqueous suspension comprises a foamy phase whose volume is reduced to 40% of the total volume of the suspension.

22. A method for improving the hydrodynamics of a reaction medium during preparation of phosphoric acid, the method comprising the step of treating at a temperature ranging from 40 to 100° C., with at least one strong acid, an aqueous suspension (A) of particles of at least one phosphate material having a size of between 10 and 400 μm, dispersed in the presence of at least one anionic polymer with a molecular mass by weight (M.sub.W) of between 1,000 and 90,000 g/mol and obtained by polymerization reaction of at least one acid chosen from acrylic acid, methacrylic acid and salts thereof.

Description

EXAMPLE 1: PREPARATION OF PHOSPHATE ROCK PULP

(1) From a phosphate rock pulp, which is an aqueous suspension comprising water and particles of phosphate material, the characteristics of this suspension are determined. The phosphate material comes from the Khouribga (Morocco) deposits. The pulp is prepared by mixing water and crushed and ground phosphate rock, and optionally the anionic polymer according to the invention.

(2) Particle Size of Phosphate Material:

(3) The particle size distribution of phosphate rock pulp is measured using a Malvern Mastersizer 2000 laser diffraction granulometer. The results are shown in Table 2.

(4) TABLE-US-00002 TABLE 2 median particle size size (μm) d.sub.10 13.33 d.sub.50 130.65 d.sub.90 280.67
Pulp Density:

(5) The density is determined at 25° C. using a pycnometer of size 1501/100 (Sheen S230729) with a volume of 100 cm3. The clean pycnometer is weighed empty. The homogenized phosphate rock pulp is introduced into the pycnometer; the air present is purged and the pycnometer is closed. The full pycnometer is weighed. The mass of the empty pycnometer is subtracted from the mass of the full pycnometer, the value of this difference is multiplied by 10 and the density of the phosphate rock pulp is obtained. The results are presented in Table 3.

(6) TABLE-US-00003 TABLE 3 solids content (% by weight) density polymer-free pulp 51 1.538 polymer-free pulp 58 1.709 pulp with polymer 70.4 1.915 (P4) 0.4% by weight

(7) The use of a polymer according to the invention makes it possible to significantly increase the phosphate rock solids content and the density of the pulp while allowing easy handling of this concentrated pulp.

(8) Viscosity of the Pulp:

(9) In a 250 mL beaker at 25° C., 300 g of phosphate rock pulp is introduced into a 250 mL beaker with mechanical stirring (600 rpm—for 2 min). If necessary, the polymer according to the invention is added in the dry/dry amounts of dry polymer relative to the amount of dry phosphate rock shown in Table 3. Stirring is stopped and, after 90 s, the viscosity is measured at 23° C. by means of a Brookfield viscometer equipped with a type S63 spindle at a rotation speed of 100 rpm. The results are shown in Table 4.

(10) TABLE-US-00004 TABLE 4 dry substance viscosity (% by weight) (mPa .Math. s) polymer-free pulp 70 not measurable (paste) pulp with polymer (P1) 0.2% by weight >70 230 pulp with polymer (P3) 0.3% by weight >70 260 pulp with polymer (P4) 0.4% by weight >70 229

(11) Whereas polymer-free pulp does not allow viscosity to be measured, the pulp comprising polymer according to the invention has a controlled viscosity which makes it easily handled and transportable, in particular by gravity.

EXAMPLE 2: PREPARATION OF PHOSPHORIC ACID AND CHARACTERIZATION OF ITS QUALITY

(12) The phosphate material particles of an aqueous suspension according to Example 1 are brought into contact with sulfuric acid in the dihydrate process for the manufacture of phosphoric acid. A slurry is obtained which is filtered to separate the phosphogypsum and obtain an aqueous solution of phosphoric acid. A solution of strong phosphoric acid is obtained. If necessary, it can be concentrated by evaporating water under vacuum. Washing the phosphogypsum with a dilute solution of phosphoric acid or with water or sulfate-rich water can produce medium or weak phosphoric acid solutions.

(13) A similar procedure is used for different suspensions prepared according to Example 1.

(14) The reaction for the preparation of phosphoric acid is characterized by different parameters. The filtration time provides information on the shape of the phosphogypsum crystals present in the phosphate material. The filtration time also provides information on the quality of the phosphoric acid produced. The density indicates the titer of the phosphoric acid produced and must be above 1.266 at 25° C. to reach a generally acceptable quality.

(15) The amount of free sulfate present in the acid (g/L) is estimated from the level of residual sulfuric acid that did not react during the treatment of the phosphate material particles in the suspension. It provides information on the evolution of the phosphoric acid preparation reaction. Preferably, the aqueous phosphoric acid solution contains residual sulfate ions in a weight concentration of 20 to 35 g/L. More preferably, the aqueous phosphoric acid solution comprises residual sulfate ions in a weight concentration of 22 to 26 g/L.

(16) The filtrability of the phosphate material suspension (tons P.sub.2O.sub.5/m.sup.2/day) enables the production capacity of strong phosphoric acid to be evaluated. The filterability of phosphogypsum is linked to its crystallinity. Particular shapes of phosphogypsum crystals can lead to piling up which degrades the efficiency of filtration or to filter clogging. On a scale of 1 to 7, the filterability should range from 5 to 7, preferably 6 to 7. The filterability F is calculated according to the formula:

(17) $F=\frac{A}{\left(1-\frac{B}{100}\right)*\sqrt{t1+t2+t3}}$
for which A represents the specific constant of the measurement technology, B represents the moisture content of the phosphogypsum (% by weight), t1 represents the filtration time of the strong phosphoric acid (s), t2 represents the average phosphoric acid filtration time (s), t3 represents the filtration time of weak phosphoric acid (s).

(18) The filterability results obtained are presented in Table 5.

(19) TABLE-US-00005 TABLE 5 pulp (solids content-% by weight) filterability polymer-free pulp (60) 5 pulp with polymer (P1) 0.2 % by weight (>70) 7 pulp with polymer (P3) 0.3% by weight (>70) 6 pulp with polymer (P4) 0.4 % by weight (>70) 7

(20) The presence of polymer according to the invention in the aqueous suspension of particles of phosphate material makes it possible to obtain a filterability maintained or even improved while making it possible to increase the solids content. The efficiency of the method for producing strong phosphoric acid is improved.

(21) Performance of the phosphoric acid preparation reaction and quality of the phosphoric acid produced: density and titer of the strong acid.

(22) After acid treatment and filtration of the phosphate material suspension, the density of the strong phosphoric acid is measured with a density meter graduated from 1200 to 1300 or 1300 to 1400 and at a temperature of 25° C. The titration of the phosphoric acid solution is carried out in a manner known per se. The results obtained are shown in Table 6.

(23) TABLE-US-00006 TABLE 6 pulp (solids content-% by weight) density filterability P.sub.2O.sub.5 titer polymer-free pulp (60) 1.274 5.4 25.33 pulp with polymer (P4) 0.4% 1.289 6.7 29.67 by weight (>70)

(24) The presence of polymer according to the invention in the aqueous suspension of particles of phosphate material makes it possible to prepare a highly concentrated pulp with very improved properties. The P.sub.2O.sub.5 titer is improved. Similarly, the acid density is improved.

(25) When phosphoric acid is prepared by treating the aqueous suspension of phosphate material particles with sulfuric acid, the phosphogypsum (calcium sulfate) crystals must have controlled dimensions to improve their separation by filtration. The sizes and dimensions of the crystals of different filtration retentate are determined using an optical microscope (Olympus SZX-ILLD200, DF PLFL 1.6* PF lens) producing images processed with Imagej software.

(26) Different crystal forms are present: acicular (A), tabular (B) or compact crystals or polycrystalline aggregates (C). For these crystals of different shapes, several size ranges are present. Among these crystals are oblong crystals of about 250 μm or 220-350 μm in size (Q1), semi-oblong crystals of about 150 μm or 125-160 μm in size (Q2) and more compact or star-shaped crystals of about 50 μm or 40-85 μm in size (Q3). Type (Q3) crystals provide the best filterability results. The results are shown in Table 6.

(27) The relative amounts of type (Q3) crystals are increased and the filterability of phosphogypsum crystals is improved.

(28) Evaluation of Losses of Phosphoric Acid Produced:

(29) In the preparation of phosphoric acid expressed in P.sub.2O.sub.5 equivalent, the overall chemical yield of the phosphoric acid preparation may be reduced due to acid losses. Generally, the method of acid preparation leads to losses of acid in different forms. These losses can be identified and measured. Part (A) of the product losses corresponds to the phosphoric acid present in the phosphate rock which is not attacked during acid treatment. Part (B) of the phosphoric acid product losses is related to the acid trapped in syncrystallized form within the phosphogypsum crystals. Part (C) of the phosphoric acid product loss results from the presence of acid in solution in the wash waters. The results are presented in Table 7.

(30) TABLE-US-00007 TABLE 7 pulp (solids content-% by P.sub.2O.sub.5 losses (% by weight) P.sub.2O.sub.5 yield (% weight) A B C by weight) polymer-free pulp (60) 0.12 0.81 0.82 92.0 polymer-free pulp (>70) 0.12 0.67 0.67 94.3 pulp with polymer 0.03 0.2 0.1 98.6 (P4) 0.4% by weight (>70)

(31) In addition to a strong improvement in the overall efficiency of the phosphoric acid preparation reaction, the use of a polymer according to the invention in the reaction for the preparation of phosphoric acid from the aqueous suspension of phosphate material particles reduces the various losses of phosphoric acid. In particular, the acid losses resulting from the unattacked phosphate material during the acid treatment are greatly reduced.