EP0195031A1

EP0195031A1 - Method for purifying phosphoric acid prepared by a wet process

Info

Publication number: EP0195031A1
Application number: EP85904464A
Authority: EP
Inventors: Gilbert Bonel; Jean-Claude Heughebaert; Mohamed Chaabouni; Hassine Ayedi
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 1984-09-14
Filing date: 1985-09-13
Publication date: 1986-09-24
Also published as: FR2570363A1; FR2570363B1; MA20527A1; WO1986001792A1; US4692323A

Abstract

Procédé de séparation d'impuretés à base de magnésium, contenues dans un acide phosphorique préparé par voie humide. Ce procédé consiste à ajouter à l'acide phosphorique un composé d'ammonium R (NH4) soluble dans ledit acide et éventuellement un composé de fluor et un composé d'aluminium, de façon à engendrer une réaction de formation d'un complexe insoluble (NH4)x (Mg)y (Al)z (F,OH)6; ce complexe cristallisé en grains de grosse taille est ensuite facile à séparer par filtration et décantation.Process for the separation of impurities based on magnesium, contained in a phosphoric acid prepared by the wet process. This process consists in adding to the phosphoric acid an ammonium compound R (NH4) soluble in said acid and optionally a fluorine compound and an aluminum compound, so as to generate a reaction forming an insoluble complex ( NH4)x (Mg)y (Al)z (F,OH)6; this complex crystallized in large grains is then easy to separate by filtration and decantation.

Description

Method for purifying wet phosphoric acid.

The invention relates to a process for separating impurities based on magnesium and optionally fluorine and aluminum contained in a phosphoric acid prepared by the wet process, that is to say by acidification of a phosphate ore by means of of a mineral acid, generally sulfuric acid. In most industrial installations implementing the wet process, the acid obtained is a solution having a P ₂ O ₅ content by weight close to 30% (generally between 25 and 35%), the weight content being defined by the weight ratio in grams of P ₂ O ₅ per 100 grams of phosphoric acid.

The phosphoric acid solutions prepared by the wet method contain several impurities and in particular magnesium in content depending on the starting ore; this element constitutes an extremely annoying impurity, because it leads in the long run to the appearance of precipitates causing the formation of deposits or sludge in the tanks and the conduits of the installations: in addition to the practical difficulties encountered, these sludges or deposits are accompanied by significant losses of P ₂ O ₅ . In addition, the impurities in Mg lead to difficulties well known to specialists during most of the subsequent treatments of phosphoric acid (manufacture of fertilizers, manufacture of superphαsphoric acid).

In addition, the phosphoric acid solutions prepared by the wet method also mostly contain impurities based on aluminum and / or fluorine which, although less troublesome than the previous ones, can however cause parasitic precipitation.

The quantities of phosphoric acid produced by implementing the wet process being considerable (37 million tonnes of phosphoric acid manufactured worldwide in 1975, 84% of which was wet), numerous studies have been carried out to carry out its purification which is currently carried out mainly by three types of methods:

- solvent extraction which is a recent method in development, the defect of which is to require complex installations which considerably increase investments,

- fixation on ion exchange resins, which is an older method, the defect of which is to be very expensive and also to require complex installations,

- precipitation in the form of insoluble complexes and separation by filtration or decantation.

The invention relates to a method of the latter type. There are currently known essentially two chemical reactions used to precipitate magnesium and optionally aluminum and fluorine.

FR Patent No. 2,065,265 (ALLIED CHEMICAL CORP.) Describes a process for removing magnesium, aluminum and fluorine in the form of a phosphate-magnesium-aluminum fluoride precipitate, obtained by adding to the acid phosphoric fluoride and aluminum ions in appropriate proportions to lead to such a compound.

FR Patent No. 2,419,253 (AGRICO CHEMICAL Cy) describes a process for the separation of magnesium and aluminum impurities, precipitating them in the form of the compounds M _g Al ₂ F ₈ - M _g Al F ₅ by addition to phosphoric acid of fluoride ions and, where appropriate, aluminum ions in appropriate proportions to lead to these compounds.

We can refer to these two patents for more details on the reactions involved and the operating conditions; the implementation of these reactions has the advantage of requiring very little additional equipment in phosphoric acid manufacturing facilities. However, the precipitates obtained in both cases are in gelatinous form and lead to a long and relatively delicate physical separation (filtration or decantation); moreover the gelatinous texture traps and adsorbs phosphoric acid and causes a significant reduction in the P ₂ O _{5 content} and, therefore, a reduction in the overall acid production yield. Furthermore, these two precipitation reactions are slow and require several tens of hours to obtain effective precipitation, which lengthens the phosphoric acid production cycle all the more and increases the volume of the necessary settling tanks.

The present invention proposes to provide a new process for separating impurities of magnesium and, where appropriate, of aluminum and fluorine from phosphoric acid by carrying out a new precipitation reaction leading to the formation of a new insoluble complex. The invention aims to benefit from the advantages of this type of precipitation method (ease of implementation and low cost of materials), while eliminating the above-mentioned defects of known methods of this type. An objective of the invention is in particular to lead to the formation of a crystalline precipitate consisting of crystals of sizes greater than

0.1 micron, easy to separate by filtration or decantation and having a very low adsorption capacity of phosphoric acid.

Another objective is to reduce the precipitation times to reduce them to durations of the order of a few tens of minutes to a few hours.

To this end, the method of the invention consists:

. in the case where the atomic ratio of impurities rf = - (from fluorine to magnesium) is less than 3, to add to the phosphoric acid a fluorine compound to bring this atomic ratio to a value at least equal to 4,. in the case where the atomic ratio of impurities ra = - (from aluminum to magnesium) is g less than 0.5, to add to the phosphoric acid an aluminum compound to bring this atomic ratio to a value at least equal to 0.5,. to add to phosphoric acid a ammonium compound R (NH ₄ ) soluble in said acid, so as to generate a reaction for the formation of an insoluble complex of ammonium, magnesium, fluorine and aluminum,. then perform a physical separation of said complex.

Analysis of the precipitate obtained has shown that it contains a complex having the following general formula:

(NH ₄ ) _x (Mg) _y (Al) _z (F, OH) ₆ with 0.15 ≤ ≤ 1

and 1.0 ≤ ≤ 1.5

The values of x, y and z of this complex depend on the initial contents of Mg, Al and F of the phosphoric acid treated and on the operating conditions (relative proportion of the ammonium compound added, temperature); these parameters also influence the possible substitution

F> OH.

The precipitate is obtained, in all cases, in a few tens of minutes and the compound is formed of crystallites of size between 0.1 and 1 micron which are extremely easy and quick to separate. This separation can be easily carried out by filtration or decantation or a combination of the two operations. A negligible decrease in the P ₂ O _{5 content} of the acid has been observed after precipitation and washing, which is explained by the very low absorption power of the large crystals of the compound.

It should be noted that US Pat. No. 3,328,123 uses an ammonium compound to remove impurities based on iron and aluminum. However, the process described in this patent does not make it possible to obtain an insoluble complex of the type of that obtained in the invention since, even if the medium contained magnesium, it would not contain the fluoride ions necessary for the formation of this complex ; thus any magnesium present would remain in phosphoric acid. This explains why, despite the age of the aforementioned US patent (1964), industrial processes for elimination magnesium use other techniques much less advantageous than that of the invention, the elimination of magnesium currently remaining a crucial problem poorly resolved.

Furthermore, according to a preferred embodiment, the ammonium compound R (NH ₄ ) is added to the phosphoric acid in proportion as a function of the Mg content of this acid and such that the molar ratio of the medium obtained is substantially between 0.5 and 4.

In the case where the phosphoric acid treated does not contain or contains little aluminum or fluorine impurities, the process according to the invention leads to adding aluminum or fluorine compounds, preferably so as to reduce the rf and ra atomic ratios in the following ranges:

. 4 ≤ rf ≤ 20. 0.8 ≤ ra ≤ 2

The above proportions allow the formation of the above-mentioned ammonium complex to be favored to the detriment of other complexes which may also form, in particular Mg Al (F, OH) ₅ . It should be noted that, even in the ranges of molar and atomic ratios indicated above, the latter complex can be obtained but in a small proportion compared to the ammonium complex; this “parasitic” obtaining can be avoided by increasing the quantity of ammonium compared to aluminum so that the atomic ratio rn = is greater than 1. It has been noted that the ammonium complex precipitates with water retention in variable amounts depending on the operating conditions, the initial content of impurities, the initial dilution of the acid and the temperature of the treatment.

Preferably, the process is carried out at a temperature between 50 ° and 70 ° C, in particular close to 60 ° C; this temperature is close to the natural temperature at which the phosphoric acid is found at the end of its manufacturing cycle, so that no additional heating or cooling installation needs to be provided. Furthermore, to promote the crystallization of the above-mentioned ammonium complex, the phosphoric acid can be seeded by addition of insoluble ammonium complex previously obtained. This seeding can be carried out by means of a proportion by weight of approximately 1% to a few percent of the total mass of the precipitate to be obtained.

Phosphoric acid is generally prepared for the production of fertilizers. In this case, the ammonium compound used is preferably ammonium nitrate so as to pre-enrich the phosphoric acid with nitrate which remains in solution therein; it is also possible to use ammonia as a compound, this body being available near industrial fertilizer manufacturing facilities.

For the same reasons (nitrate enrichment), if an aluminum compound is to be added, it will preferably be added in the form of aluminum nitrate. If a fluorine compound is to be added, it can be added in the form of hydrofluoric acid which is a common compound, inexpensive and which does not provide any impurity in the acid or possibly in the form of ammonium fluoride or ammonium bifluoride which participates in bringing ammonium ions.

Taking into account the tonnages of phosphoric acid manufactured, it is conceivable the considerable economic interest of the process of the invention which makes it possible to obtain phosphoric acid depleted in Mg and, where appropriate, in Al and F, in improved conditions of cost, time and ease of implementation. The process of the invention can be applied to traditional solutions of phosphoric acid which have a P ₂ O ₅ content by weight of between 25 and 35% at the outlet of the sulfuric acid attack reactor; it can also be applied to other solutions.

The following working examples are intended to illustrate the process of the invention.

EXAMPLE I For this experiment, we have reconstituted an industrial solution of phosphoric acid, as it most frequently occurs at the outlet of the manufacturing reactor (all the proportions, concentrations or contents supplied are by weight and related to the total mass):

30% P ₂ O ₅

0.25% Al ₂ O ₃ or 0.13% Al 0.5% Mg O or 0.30% Mg 0.2% Ca O

0.7% SO ₄

It should be noted that, in this example, the acid does not contain fluorine impurities (atomic ratio <- 0). The atomic ratio is equal to

0.38 therefore less than the preferred range indicated (0.8 - 2).

To this acid, placed in a beaker and brought to a temperature of 60 °, was added simultaneously with stirring for 1 hour solutions of hydrofluoric acid, ammonium nitrate and aluminum nitrate, so as to bring the weight concentrations at the following levels:

0.22% NH ₄

0.26% Al

2.35% F

So the molar ratio the middle is 1.0, the atomic ratio is reduced to 0.8 and the atomic ratio is reduced to 10. g

After 10 'of standing, a white crystalline precipitate is deposited, deposited at the bottom of the beaker, the supernatant solution being practically clear.

After an additional rest of 3 hours, this precipitate is separated in approximately 5 minutes by decantation and filtration in a funnel fitted with a paper filter, then the precipitate is washed. The solid precipitate, the liquid filtrate and the washing water were analyzed:

Precipitate

The composition was determined by chemical analyzes which led to the following formula for the compound obtained: (NH ₄₎ 0.48 (H _g) 0.96 Al _{1, 20} (F0,95 0H _0.05) ₆

The X-ray diffraction analysis of this body shows that it is a single-phase compound with a crystal structure comparable to that of the known compound NH ₄ Mg Al (F, OH) ₆ , nH ₂ O.

Infrared spectrometry shows the existence of OH bonds.

Liquid filtrate Analysis of this filtrate gave the following percentages (which are related to the concentrations of the element concerned in the initial phosphoric acid):

P ₂ O ₅ 94.95%

Mg O 47.44% AI ₂ O ₃ negligible

More than half of the magnesium was thus eliminated at the cost of a loss of approximately 5% of P ₂ O ₅ .

Wash water

The analysis of these waters gave the following percentages (always related to the concentrations of the element concerned in the initial phosphoric acid):

P ₂ O ₅ 4.96%

Mg O 10.56% Al ₂ O ₃ negligible These washing waters which can be used to pre-attack the ore therefore make it possible to recover almost all of the losses in P ₂ O ₅ , so that the overall yield is close 99.9% for P ₂ O ₅ . If we consider overall the washing water and the filtrate, the proportion of magnesium eliminated (expressed as a percentage in M g O) is therefore 42%, which represents a satisfactory separation on the industrial level, with a negligible loss in P ₂ O ₅ .

EXAMPLE 2.

The process was carried out on a solution identical to that of Example 1, under analogous conditions, by modifying only the added aluminum content, which rose to 0.53 corresponding to a ratio atomic = 1.6.

The precipitate obtained under the same conditions is in this case a two-phase system formed:. of the complex (majority: 95%)

(NH ₄ ) _x Mg _y Al _z (F, OH) ₆ , nH ₂ O

. and of the complex (in small quantity) Mg Al (F, OH) ₅ , nH ₂ O

The existence of these two phases is demonstrated by X-ray diffraction. In a series of similar experiments, the second phase was able to be eliminated by increasing the quantity of ammonium; this second phase disappears when the atomic ratio becomes greater than 1.

The analyzes show that the liquid filtrate contains 99.7% of the initial P ₂ O _{5 content} and 19.84% of the initial Mg O content; the wash water contains 0.1% of the initial P ₂ O _{5 content} and 2.16% of the initial Mg O content.

The increase in the aluminum content compared to Example 1 therefore led to an extraction of 78% of the magnesium (expressed in M g O) at the cost of a loss in P ₂ O ₅ of 0.2%.

EXAMPLE 2

The process was carried out on a solution identical to that of Example 1, under analogous conditions, by modifying only the proportion of ammonium nitrate added, to bring the percentage of ammonium to

0.11%, corresponding to an NH 0.5 molar ratio. The precipitate obtained under the same conditions as above is a two-phase system formed by the same phases as in Example 2.

The analyzes show that the liquid filtrate contains 91.39% of the initial P ₂ O _{5 content} and 42.2% of the initial M g O content. The washing water contains 8.52% of the P ₂ content Initial O ₅ and 3.20% of the initial Mg O content.

It should be noted that the reduction in the ammonium content compared to Example 1 has led to increased difficulties in achieving the physical separation of the precipitated, due to the formation of the very amorphous second phase.

This example shows that the second phase appears in greater quantities when the quantity of ammonium is reduced compared to aluminum.

EXAMPLE 4.

The process was carried out on a solution identical to that of Example 1 under analogous conditions, by modifying only the proportion of added hydrofluoric acid, the NH ₄ / A1 ratio and Al / Mg ratio to bring the percentage in F at 4.70%, corresponding to an atomic ratio = 20, the ratio to 2 and the compared to 2. The precipitate obtained under the same conditions is single phase.

The analyzes show that the liquid filtrate contains 96% of the initial P ₂ O _{5 content} and 2% of the initial M g O content; the wash water contains 5.65% of the initial P ₂ O _{5 content} .

The increase in the fluorine, aluminum and ammonium content compared to Example 1 therefore led to an extraction of 98% of the magnesium (expressed in Mg O) at the cost of a loss of 0.3% in P ₂ O ₅ . The results obtained in the preceding examples are to be compared with the average results obtained by the known precipitation methods which have been mentioned previously; these processes generally make it possible to extract approximately 50% of the magnesium at the rate of a loss of 2 to 3% in P ₂ O ₅ .

Claims

1 / - Process for the separation of impurities based on magnesium and optionally aluminum and fluorine, contained in a phosphoric acid prepared by the wet method, of the type in which said impurities are precipitated in the form of a crystallized complex, consisting of:

. in the case where the atomic ratio of impurities rf = (from fluorine to magnesium) is less than 3, to add a fluorine compound to the phosphoric acid in order to reduce this atomic ratio to a value at least equal to 4,

. in the case where the atomic ratio of impurities ra = (from aluminum to magnesium) is lower at 0.5, to add an aluminum compound to the phosphoric acid to bring this atomic ratio to a value at least equal to

0.5,

. adding to the phosphoric acid an ammonium compound R (NH ₄ ) soluble in said acid, so as to generate a reaction for the formation of an insoluble complex of ammonium, magnesium, fluorine and aluminum,

. then perform a physical separation of said complex.

2 / - Process according to claim 1, characterized in that the ammonium compound is added to the phosphoric acid in proportion as a function of the Mg content of the acid and such that the molar ratio of the medium obtained is substantially between 0.5 and 4.

3 / - P r o cé de s e l o n u n e s of claims 1 or 2, characterized in that a soluble fluorine compound is added to the phosphoric acid in an appropriate proportion to bring the above atomic ratio between 4 and

20.

4 / - Process according to claim 3, characterized in that hydrofluoric acid is added as the fluorine compound.

5 / - P r océ dé lon one of claims 1, 2, 3 or 4, characterized in that a soluble aluminum compound is added to the phosphoric acid in an appropriate proportion to bring back the atomic ratio ra between 0.8 and 2. 6 / - Process according to claim 5, characterized in that aluminum nitrate is added as the aluminum compound. 7 / - P ro cé dé according to one of claims 1, 2, 3, 4, 5 or 6, characterized in that the ammonium compound is added to phosphoric acid so that the atomic ratio r _n = - ^NH is greater than 1.

8 / - P r o c é de s e on one of claims 1, 2, 3, 4, 5, 6 or 7, characterized in that the phosphoric acid is seeded by addition of insoluble ammonium complex previously obtained.

9 / - P r o cé de s e on one of claims 1, 2, 3, 4, 5, 6, 7 or 8, characterized in that the precipitation is carried out at a temperature between 50 ° and 70 ° C.

10 / - Salt process on one of claims 1, 2, 3, 4, 5, 6, 7, 8 or 9, characterized in that ammonium nitrate or ammonia as the ammonium compound.

11 / - Pre ceded on one of the preceding claims, in which the physical separation of the complex consists of filtration and / or decantation.

12 / - P r océ dé according to uned es claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11, applied to the purification of a phosphoric acid solution having a content of P ₂ O ₅ approximately between 25 and 35%.