Preparation of an Aqueous Suspension of Phosphate Material

Abstract

The invention relates to a method for preparing an aqueous suspension of at least one phosphate material, comprising, dispersing, in water, particles of phosphate material in the presence of at least one additive of the anionic polymer of acrylic acid or of methacrylic acid type. The suspension according to the invention has a viscosity of less than 1500 mPa.Math.s. The invention also relates to the conditioning of the phosphate material associated with the anionic polymer, for its subsequent treatment with at least one strong acid, for the industrial preparation of phosphoric acid.

Claims

1. A method for preparing an aqueous suspension of at least one phosphate material, the aqueous suspension having a Brookfield viscosity, measured 90 s after preparation, at 25 C., at 100 rpm and at a concentration of phosphate material of more than 45% by weight, which is less than 1,500 mPa.Math.s, the method comprising the dispersion in water of particles of phosphate material whose size is between 10 and 400 m, 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 according to claim 1, wherein the dispersion is carried out after the phosphate rock has been crushed.

3. The preparation method according to claim 1, wherein the polymer is partially or totally neutralized, preferably partially or totally neutralized by means of a derivative comprising at least one element chosen from lithium, sodium, calcium, magnesium and mixtures thereof, more preferably chosen from sodium, calcium and mixtures thereof.

4. The preparation method according to claim 1, wherein the anionic polymer is obtained by a polymerization reaction also using at least one other acid chosen from acrylic acid, methacrylic acid, maleic acid, itaconic acid and salts thereof or at least one ester of an acid chosen from acrylic acid and methacrylic acid.

5. The preparation method according to claim 1, wherein the anionic polymer has a molecular mass by weight (M.sub.w) between 2,000 and 90,000 g/mol, preferably between 2,000 and 50,000 g/mol, more preferentially between 2,000 and 10,000 g/mol, and more preferably between 2,000 and 8,000 g/mol.

6. The preparation method according to claim 1, wherein the amount by weight (dry/dry) of anionic polymer used is between 0.05 and 5%, preferably between 0.1 and 2%, based on the amount of phosphate material.

7. The preparation method according to claim 1, wherein the anionic polymer is non-sulfonated.

8. The preparation method according to claim 1, wherein the concentration by weight of particles of phosphate material in the aqueous suspension is greater than 50%, preferably greater than 55%, more preferentially greater than 60% or 65% or greater than 70% or 75%.

9. The preparation method according to claim 1, wherein the Brookfield viscosity of the suspension, measured 90 s after preparation, at 25 C. and at 100 rpm, is less than 1,200 mPa.Math.s, preferably less than 1,000 mPa.Math.s, more preferably less than 800 mPa.Math.s or less than 500 mPa.Math.s or even less than 350 mPa.Math.s or less than 200 mPa.Math.s.

10. The preparation method according to claim 1, wherein the Brookfield viscosity of the suspension, measured 90 s after preparation, at 25 C. and 100 rpm and at a concentration of more than 60% by weight of phosphate material, is less than 1,500 mPa.Math.s, 1,200 mPa.Math.s or 1,000 mPa.Math.s, preferably less than 800 mPa.Math.s or less than 500 mPa.Math.s, 350 mPa.Math.s or 200 mPa.Math.s.

11. The preparation method according to claim 1, wherein the particles of phosphate material have a size of less than 200 m or less than 150 m or a size of more than 30 m.

12. The preparation method according to claim 1, wherein the dispersion in water of the phosphate material particles is carried out under stirring, preferably under mechanical stirring.

13. An aqueous suspension of at least one phosphate material, having a Brookfield viscosity, measured 90 s after preparation, at 25 C., at 100 rpm and at a concentration of phosphate material greater than 45% by weight, which is less than 1,500 mPa.Math.s, the aqueous suspension comprising water and particles of phosphate material having a size of between 10 and 400 m and 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.

14. The aqueous suspension according to claim 13, wherein the polymer is partially or totally neutralized, preferably partially or totally neutralized by means of a derivative comprising at least one element chosen from lithium, sodium, calcium, magnesium and mixtures thereof, more preferably chosen from sodium, calcium and mixtures thereof.

15. The aqueous suspension according to claim 13, wherein the anionic polymer is obtained by a polymerization reaction also using at least one ester of an acid chosen from acrylic acid and methacrylic acid.

16. The aqueous suspension according to claim 13, wherein the anionic polymer has a molecular mass by weight (M.sub.w) between 2,000 and 90,000 g/mol, preferably between 2,000 and 50,000 g/mol, more preferably between 2,000 and 10,000 g/mol, and more preferably between 2,000 and 8,000 g/mol.

17. The aqueous suspension according to claim 13, wherein the amount by weight (dry/dry) of dry anionic polymer used is between 0.05 and 5%, preferably between 0.1 and 2%, based on the amount of dry phosphate material.

18. The aqueous suspension according to claim 13, wherein the concentration by weight of particles of phosphate material is greater than 50%, preferably greater than 55%, more preferentially greater than 60% or 65% or greater than 70% or 75%.

19. The aqueous suspension according to claim 13, the Brookfield viscosity of which, measured 90 s after preparation, at 25 C. and at 100 rpm, is less than 1,200 mPa.Math.s, preferably less than 1,000 mPa.Math.s, more preferably less than 800 mPa.Math.s or less than 500 mPa.Math.s, 350 mPa.Math.s or 200 mPa.Math.s.

20. The aqueous suspension according to claim 13, the Brookfield viscosity of which, measured 90 s after preparation, at 25 C., at 100 rpm and at a concentration of more than 60% by weight of phosphate material, is less than 1,500 mPa.Math.s, 1,200 mPa.Math.s or 1,000 mPa.Math.s, preferably less than 500 mPa.Math.s, 350 mPa.Math.s or 200 mPa.Math.s.

21. The aqueous suspension according to claim 13, wherein the particles of phosphate material have a size of less than 200 m or less than 150 m or a size of more than 30 m.

22. A method for conditioning a phosphate material, for its subsequent treatment at a temperature ranging from 40 to 100 C. by means of at least one strong acid, comprising contacting the phosphate material with 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.

23. The conditioning method according to claim 22, wherein the polymer is is partially or totally neutralized, preferably partially or totally neutralized by means of a derivative comprising at least one element chosen from lithium, sodium, calcium, magnesium and mixtures thereof, more preferably chosen from sodium, calcium and mixtures thereof.

24. The conditioning method according to claim 22, wherein the phosphate material is in the form of particles, preferably particles having a size of less than 400 m, more preferably less than 200 m or less than 150 m, or particles having a size of more than 10 m, preferably more than 30 m.

25. The conditioning method according to claim 22, wherein the aqueous suspension comprises, during subsequent treatment at a temperature ranging from 40 to 100 C. by means of at least one strong acid, a foamy phase whose volume is reduced to 40% or even reduced to 20% of the total volume of the suspension or, wherein the aqueous suspension has a bulk density, measured by means of a pycnometer and at a solids content of more than 60% by weight, ranging from 1.5 to 2, preferably ranging from 1.7 to 2.

Description

EXAMPLE 1

Preparation and Characterization of a Phosphate Rock Pulp

[0077] 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 three deposits near Khouribga (Morocco). The pulp is prepared by mixing water and crushed and ground phosphate rock, and optionally the anionic polymer according to the invention.

[0078] If necessary, the pulp can be concentrated by centrifugation or diluted by adding water. Particle size of phosphate material:

[0079] 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.

TABLE-US-00002 TABLE 2 median particle size size (m) d.sub.10 13.33 d.sub.50 130.65 d.sub.90 280.67

[0080] Pulp Density:

[0081] The density is determined at 25 C. using a pycnometer of size 1,501/100 (Sheen S230729) with a volume of 100 cm.sup.3. 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 shown in Table 3.

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 (P3) 0.3% by weight 70 1.905

[0082] The use of a polymer according to the invention significantly increases the phosphate rock solids content and the density of the pulp while allowing easy handling of this concentrated pulp.

[0083] Viscosity of the Pulp:

[0084] 300 g of phosphate rock pulp are introduced into a 250 mL beaker at 25 C. under mechanical stirring (600 rpmfor 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 25 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.

TABLE-US-00004 TABLE 4 solids content viscosity (% by weight) (mPa .Math. s) polymer-free pulp 70 not measurable pulp with polymer (P1) 0.2% by weight >70 230 pulp with polymer (P3) 0.3% by weight >70 260

[0085] 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.

[0086] Pulp StabilityPhase Shift Measurement:

[0087] 200 mL of phosphate rock pulp are introduced into a 250 mL beaker at 25 C. under mechanical stirring (2,050 rpmfor 2 min). If necessary, the polymer according to the invention is added in the dry/dry amounts relative to the amount of dried phosphate rock shown in Table 4. Then, 100 mL of pulp are poured into a graduated test tube and the volumes of the different phases that form are recorded over time. During sedimentation, two phases may appear. Phase 1, which is water and does not include particles of phosphate material due to the occurrence of a pulp separation phenomenon. Phase 2, which includes water and particles of phosphate material. The measured phase 1 volumes (mL) are shown in Table 5. This is a measure of the phase shift over time related to the sedimentation rate.

TABLE-US-00005 TABLE 5 60% pulp with polymer (P1) time (h) 50% polymer-free pulp 0.15% by weight 1 0.5 0 2 1 0 3 1.5 0 19 8 5 24 9 6

[0088] It can be seen that the presence of the polymer according to the invention makes it possible to obtain a suspension which is more concentrated in particles of phosphate material and which is more stable. In fact, the amount of phase-shifted pulp is nil or very much reduced in comparison with pulp containing no polymer.

[0089] Pulp StabilityDetermination of Dilatancy:

[0090] The dilatancy of the phosphate rock pulp is measured with a Haake Rheostress 600 rheometer equipped with a CC20Ti cylindrical spindle. The dilatancy at 30 C. of a pulp sample (16 mL) is determined by measuring the viscosity regularly by increasing the speed of the spindle from 0 to 1,500 rpm (0 to 660 s.sup.1) in 120 s. The results obtained are shown in Table 6. Such a viscosity measured at varying shear rates makes it possible to evaluate the dilatancy of the pulp. Thus, if the viscosity increases as an increasing shear rate is applied, the slurry expands.

TABLE-US-00006 TABLE 6 pulp (solids viscosity (mPa .Math. s) content-% by weight) at 2 s.sup.1 at 600 s.sup.1 polymer-free pulp (>70) not measurable not measurable pulp with polymer (P3) 7,290 96 0.1% by weight (60)

[0091] Once again, while the pulp without polymer does not allow viscosity measurement, the pulp comprising polymer according to the invention has a controlled viscosity which makes it easily handled and transportable, in particular by gravity. Moreover, it can be seen that its viscosity does not increase with increasing shear rate; the suspension according to the invention is not dilatant.

EXAMPLE 2

Preparation of Phosphoric Acid and Characterization of its Quality

[0092] The phosphate material particles of an aqueous suspension according to Example 1 are treated in a manner known as such with sulfuric acid. A slurry is obtained which is filtered to separate the phosphogypsum and to obtain an aqueous solution of phosphoric acid. A solution of strong phosphoric acid is obtained. If necessary, it can be concentrated by evaporation of water under suction. Washing the phosphogypsum with an aqueous solution of phosphoric acid or with water or sulfate-rich water can produce medium or weak phosphoric acid solutions.

[0093] A similar procedure is used for different suspensions prepared according to Example 1. The quality 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.

[0094] 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. A high level means a low phosphoric acid titer and a high phosphogypsum filtration time. 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 ranging from 22 to 26 g/L. The filterability 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. Filterability should range from 5 to 7, preferably from 6 to 7, on a scale of 1 to 7. Filterability F is calculated according to the formula:

[00002]$F=\frac{A}{\left(1-\frac{B}{1.Math.0.Math.0}\right)*\sqrt{t.Math.1+t.Math.2+t.Math.3}}$

[0095] where [0096] A=17.80 measuring technique specific constant, [0097] B=moisture content of the phosphogypsum (% by weight), [0098] t1=filtration time of strong phosphoric acid (s), [0099] t2=average phosphoric acid filtration time (s), [0100] t3=filtration time of weak phosphoric acid (s).

[0101] The results obtained are shown in Table 7.

TABLE-US-00007 TABLE 7 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

[0102] 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.

[0103] Density of the Strong Acid:

[0104] After acid treatment and filtration of the phosphate material suspension, the density of the strong phosphoric acid is measured with a densimeter, graduated from 1,200 to 1,300 or from 1,300 to 1,400, and at a temperature of 25 C. The results obtained are shown in Table 8. Quality of the phosphoric acid produced: density and titer of the strong acid

[0105] After acid treatment and filtration of the phosphate material suspension, the density of the strong phosphoric acid is measured with a densimeter, graduated from 1,200 to 1,300 or from 1,300 to 1,400, 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 8.

TABLE-US-00008 TABLE 8 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 (P3) 0.3% 1.297 6.1 28.30 by weight (>70)

[0106] 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.

[0107] 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 objective) producing images processed with Imagej software. 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 m or 220-350 m in size (Q1), semi-oblong crystals of about 150 m or 125-160 m in size (Q2) and more compact or star-shaped crystals of about 50 m or 40-85 m in size (Q3). Type (Q3) crystals provide the best filterability results.

[0108] The relative amounts of type (Q3) crystals are increased and the filterability of phosphogypsum crystals is improved.

[0109] Evaluation of Losses of Phosphoric Acid Produced:

[0110] 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 losses results from the presence of acid in solution in the wash waters. The results are shown in Table 9.

TABLE-US-00009 TABLE 9 pulp (solids content- P.sub.2O.sub.5 losses (% by weight) P.sub.2O.sub.5 yield (% % by weight) A B C by weight) polymer-free pulp (60) 0.12 0.81 0.82 92 polymer-free pulp (>70) 0.12 0.67 0.67 94.3 pulp with polymer (P3) 0.08 0.68 0.25 95.3 0.3% by weight (>70)

[0111] 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 aqueous suspension of particles of phosphate material reduces the various losses of phosphoric acid. In particular, the acid losses resulting from the unattacked phosphate material during acid treatment are greatly reduced.

[0112] The conditioning method according to the invention therefore confers special properties on the particles of phosphate material as well as on the mixture of particles of phosphate material and polymer according to the invention.