WO2019105599A1

WO2019105599A1 - Composition, method and apparatus for treating water containing fluorides

Info

Publication number: WO2019105599A1
Application number: PCT/EP2018/065863
Authority: WO
Inventors: Niki PEPE; Gianluca GIANSALVO
Original assignee: Brenntag Spa
Priority date: 2017-11-29
Filing date: 2018-06-14
Publication date: 2019-06-06

Abstract

composition for treating water contaminated by fluorides comprising an aluminum-containing compound, phosphoric acid and a flocculating polymer. The invention also relates to a method for treating water contaminated by fluorides by means of the composition and to an apparatus for treating water contaminated by fluorides according to the method.

Description

COMPOSITION, METHOD AND APPARATUS FOR TREATING WATER CONTAINING FLUORIDES

The present invention relates to a composition, a method and an apparatus for treating water, particularly for purifying water contaminated by fluorides.

Fluorides, also termed fluoride anions (F ), are present in the wastewater of some types of glassworks, particularly where glass is etched with hydrofluoric acid, in order to produce satin glasses. Moreover, fluorides are present in the waste of various types of metal working industries, such as for example steel plants, where hydrofluoric acid (or derivatives) is used for the surface treatment of the metals.

The content of fluorides in wastewater is regulated by law. For example, in Italy fluorides have the following legal limits (Legislative Decree 152/06) in wastewater: 6 mg/L of fluorides for discharges into surface water; and 12 mg/L of fluorides for discharges into the sewer system.

Currently, the industrial method commonly used to reduce fluorides in wastewater consists of two steps, i.e., the forming of a low-soluble salt of fluorides, i.e., calcium fluoride (CaF₂), and subsequent filtration on activated alumina.

In the first step, calcium hydroxide (slaked lime) is used as precipitating reagent, particularly in the form of so-called“lime water”, i.e., a solution having the concentration of approximately 50 g/L of lime, which is added to wastewater according to contact times that vary as a function of the concentration of fluorides to be reduced and the type of wastewater.

Adding lime water to the water containing fluorides causes the formation of a heterogeneous balance, i.e., the precipitation of calcium fluoride, which is scarcely soluble, in equilibrium with the aqueous phase, according to the following equation:

In an ideal solution of deionized water, at 20 °C calcium fluoride has a solubility of 17 mg/L. In stoichiometric terms, this means that the solubility of the fluorides is equal to approximately 8 mg/L, a value which is above the legal limit for discharge into surface water (6 mg/L). Actually, the concentration of fluorides in aqueous solutions can be even higher than that provided in ideal conditions, since the solubility of calcium fluoride varies as a function of some parameters, particularly the temperature, the effect of the common ion, the pH, the presence of extraneous ions, and the dimensions of the solid particles.

Since industrial wastewater to be purified is often constituted by very complex matrices, the result is that the solubility of calcium fluoride is further affected, resulting in concentrations of fluorides considerably higher than the legal limit, ranging between 15 and 30 mg/L.

Therefore, in order to be able to discharge industrial water within legal limits, it is necessary to perform a further treatment after precipitation with lime, with the use of a specific filtering mean, i.e., activated alumina (A1₂0₃).

In terms of apparatus, after treating the contaminated water in a reactor with lime water, the wastewater passes into a second reactor (flocculation reactor), where an anionic or non-ionic polyacrylamide with high molecular weight is dosed, which promotes flocculation in order to facilitate the separation of water from sludge (suspension of calcium fluoride CaF₂).

Subsequently, a sedimentation step occurs in which the sludge is separated from the water. Two lines depart from the sedimentation unit: the sludge line, which contains calcium fluoride that will be dehydrated and transferred to an authorized waste disposal facility, and the water line, which contains water with concentrations of fluorides that are still above the legal limits.

So far, the method most commonly used in order to comply with legal limits consists in making the wastewater, treated with lime and without the sludge, pass through a column of activated alumina (A1₂0₃).

The column of alumina is sized on the following parameters: hydraulic flow rate, concentration of fluorides in input, concentration of fluorides in output, time of exhaustion of the filtering material.

Specifically, a column of this type is sized typically with a specific flow rate of 6-9 m³/m²xh and a contact time between 9 and 12 minutes.

This method for removing fluorides, however, has some drawbacks, among which the main one is the high operating cost due to the treatment with alumina. This is particularly evident in large plants, such as steel plants and ironworks, where both the hydraulic flow rates and the concentrations of fluorides are high, with the consequence that the columns of alumina must be sized in order to retain high mass flows of fluorides.

This entails very onerous plant investments, for example for the construction of large activated alumina columns, and equally onerous operating costs, such as in particular the costs associated with the consumption of“fresh” alumina, with the disposal of the exhausted alumina, with the labor for emptying and filling the columns of alumina, and with any plant stops if a semi-continuous filtration system is not provided.

The aim of the present invention is therefore to eliminate said drawbacks of the process commonly used for treating water contaminated by fluorides, by providing a new chemical treatment that is more convenient than the known ones.

Within this aim, an object of the invention is to provide a composition and a method for treating water that allow to obtain concentrations of fluorides in treated water that are lower than the legal limits, without the need to perform filtration by means of a column of activated alumina.

Another object of the present invention is to provide an apparatus for performing the method according to the invention that is easy to implement, highly reliable and has competitive costs. This aim and these and other objects which will become better apparent hereinafter are achieved by a composition for treating water containing fluorides comprising:

10 to 15% by weight of an aluminum-containing compound selected from the group consisting of polyaluminum chloride, aluminum chloride, aluminum sulfate, polyaluminum sulfate, aluminum oxide and aluminum hydroxide;

15 to 30% by weight of phosphoric acid;

0.5 to 5% by weight of a flocculating polymer selected from the group consisting of poly(N,N-dimethyl-2-hydroxypropylammonium chloride), poly(dimethylamine-co-epichlorohydrin-co-ethylenediamine), poly(diallyldimethylammonium chloride), poly (3 -acrylamide propyl trimethylammonium chloride), dicyandiamide-formaldehyde resins and melamine-formaldehyde resins; and

water q.s. to 100% by weight,

on the basis of the total weight of the composition.

The aim and objects of the present invention are achieved also by a method for treating water containing fluorides, comprising the step of i) treating the water containing fluorides with a composition according to the invention as described above so as to obtain a suspension si.

Finally, the aim and objects of the present invention are achieved by an apparatus for treating water containing fluorides according to the method of the present invention, characterized in that it comprises a treatment reactor for obtaining the suspension si that comprises means for introducing the water containing fluorides and means for introducing the composition according to the invention as described above.

Further characteristics and advantages of the invention will become better apparent from the detailed description that follows and from the accompanying Figure 1, which is a view of the apparatus of the invention according to a preferred embodiment thereof. In a first aspect, the present invention relates to a composition for treating water containing fluorides which comprises, preferably is constituted essentially by, and more preferably is constituted by:

15 to 30% by weight of phosphoric acid;

water q.s. to 100% by weight,

on the basis of the total weight of the composition.

In the context of the present invention, the expression “water containing fluorides” or“water contaminated by fluorides” is understood to refer to water in which the content of fluorides is higher than the legal limits, particularly higher than 6 mg/L. For example, such water containing fluorides can derive from industrial waste of glassworks, in particular glassworks where the etching of glass with hydrofluoric acid is performed, and steel plants, in particular steel plants where the surface treatment of metals with hydrofluoric acid or its derivatives is performed.

The first component of the composition of the present invention is an aluminum-containing compound selected from the group consisting of polyaluminum chloride, aluminum chloride, aluminum sulfate, polyaluminum sulfate, aluminum oxide and aluminum hydroxide. However, it should be understood that other commercially available salts of aluminum can be used in the composition according to the invention, as long as they provide a content of A1₂0₃ that corresponds to that of the aluminum- containing compounds listed above when used in the quantities provided for by the invention.

In a preferred embodiment, the aluminum-containing compound is polyaluminum chloride.

The aluminum-containing compound is present in the composition in a quantity of 10 to 15% by weight, preferably 10.5 to 14.5% by weight, more preferably 12 to 13% by weight, even more preferably 12.6% by weight, on the basis of the total weight of the composition.

In a preferred embodiment, the aluminum-containing compound can be in the form of an aqueous solution. Preferably, the composition according to the invention comprises polyaluminum chloride in the form of a 10 to 18% w/w solution in water. 10% and 18% w/w solutions of polyaluminum chloride in water are commercially available and are therefore preferred. Even more preferably, the composition according to the invention comprises polyaluminum chloride in the form of an 18% w/w solution in water.

In a preferred embodiment, the composition according to the invention comprises 60 to 80%, preferably 70%, by weight on the basis of the total weight of the composition of an 18% w/w solution of polyaluminum chloride in water.

The second component of the composition according to the invention is phosphoric acid, which is present in the composition in quantities of 15 to 30% by weight, preferably 18 to 19% by weight, even more preferably 18.75% by weight, on the basis of the total weight of the composition.

The phosphoric acid is preferably in the form of a 50 to 92% w/w solution in water. 50%, 75%, 85% and 92% w/w aqueous solutions of phosphoric acid are commercially available and are therefore preferred. More preferably, the phosphoric acid is in the form of a 75% w/w solution in water.

In a preferred embodiment, the composition according to the invention comprises 20% to 40%, preferably 25%, by weight on the basis of the total weight of the composition of a solution of at 75% w/w phosphoric acid in water.

The third component of the composition according to the invention is a flocculating polymer selected from the group consisting of poly(N,N- dimethyl-2-hydroxypropylammonium chloride), poly(dimethylamine-co- epichlorohydrin-co-ethylenediamine), poly(diallyldimethylammonium chloride), poly (3 -acrylamide propyl trimethylammonium chloride), dicyandiamide-formaldehyde resins and melamine-formaldehyde resins.

In a preferred embodiment, the flocculating polymer is poly(N,N- dimethyl-2-hydroxypropylammonium chloride).

The flocculating polymer is present in the composition in a quantity comprised between 0.5 and 5% by weight, more preferably between 2 and 3% by weight, even more preferably 2.5% by weight, on the basis of the total weight of the composition.

In a preferred embodiment, the flocculating polymer can be in the form of an aqueous solution. Preferably, the flocculating polymer is poly(N,N-dimethyl-2-hydroxypropylammonium chloride) in the form of a 5 to 75% w/w solution in water. Generally, solutions of poly(N,N-dimethyl-2- hydroxypropylammonium chloride) at a concentration between 5% and 10% w/w and between 25% and 75% w/w in water are commercially available and are therefore preferred, particularly those between 25% and 75% w/w. More preferably, the poly(N,N-dimethyl-2-hydroxypropylammonium chloride) is in the form of a 50% w/w solution in water.

In a preferred embodiment, the composition according to the invention comprises 1 to 10%, preferably 5%, by weight on the basis of the total weight of the composition of a 50% w/w solution of poly(N,N- dimethyl-2-hydroxypropylammonium chloride) in water.

In a preferred embodiment, the composition according to the invention comprises: 10 to 15% by weight of polyaluminum chloride;

15 to 30% by weight of phosphoric acid;

0.5 to 5% by weight of poly(N,N-dimethyl-2- hydroxypropylammonium chloride); and

water q.s. to 100% by weight,

on the basis of the total weight of the composition.

In another preferred embodiment of the composition according to the invention, the polyaluminum chloride is in the form of a 10 to 18% w/w solution in water, the phosphoric acid is in the form of a 50 to 92% w/w solution in water and the poly(N,N-dimethyl-2-hydroxypropylammonium chloride) is in the form of a 25% to 75% w/w solution in water.

In a particularly preferred embodiment of the composition according to the invention, the polyaluminum chloride is in the form of an 18% w/w solution in water, the phosphoric acid is in the form of a 75% w/w solution in water and the poly(N,N-dimethyl-2-hydroxypropylammonium chloride) is in the form of a 50% w/w solution in water.

In a particularly more preferred embodiment, the composition according to the invention comprises, more preferably is constituted essentially, and even more preferably is constituted by:

- 60 to 80% by weight, preferably 70% by weight, of an 18% w/w solution of polyaluminum chloride in water;

- 20% to 40% by weight, preferably 25% by weight, of a 75% w/w solution of phosphoric acid in water; and

- 1 to 10% w/w, preferably 5% w/w, of a 50% w/w solution of poly(N,N-dimethyl-2-hydroxypropylammonium chloride) in water;

on the basis of the total weight of the composition.

In a second aspect, the present invention relates to a method for treating water containing fluorides which comprises the step of i) treating the water containing fluorides with a composition according to the invention as described above so as to obtain a suspension si. The above mentioned step i) has a duration comprised between 1 minute and 3 hours, preferably 30 minutes.

In a preferred embodiment, the method according to the invention, further comprises the step of i') pretreating the water containing fluorides with calcium hydroxide before step i).

The calcium hydroxide can be in the form of an aqueous solution having a concentration of at least 1 g/L, preferably 50 g/L. However, when the concentration of fluorides in the water to be treated is very high (10000- 15000 mg/L), the calcium hydroxide is added as is, without prediluting it with water in order to prepare the lime water.

In a preferred embodiment, the method according to any of the embodiments cited above further comprises the step of ii) treating the suspension si, obtained in step i), with an anionic, or cationic or non-ionic polyacrylamide, preferably an anionic polyacrylamide, so as to obtain a mixture comprising a flocculate and water.

In the present invention, the expression“anionic polyacrylamide” is understood to refer to a polymer obtained from the copolymerization of acrylamide with one or more unsaturated ethylene monomers containing at least one acid function, such as for example unsaturated ethylene mono- and dicarboxylic acids (C₃-C₆), among which mention can be made, purely by way of example, of acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid and their salts.

The expression“cationic polyacrylamide” is instead understood to refer to a polymer obtained from the copolymerization of acrylamide with one or more monomers having a positive net charge at a certain value of pH, such as for example quaternary ammonium salts of functionalized amine monomers, among which mention can be made, purely by way of example, of quaternary ammonium salts of dimethylaminoethyl acrylate, quaternary ammonium salts of dimethylaminoethyl methacrylate, and salts of diallyldimethylammonium. Finally, the expression“non-ionic polyacrylamide” is understood to refer to a polymer obtained by polymerization of acrylamide alone or by copolymerization of acrylamide with one or more additional non-ionic monomers, i.e., electrically neutral monomers, among which mention can be made, purely by way of example, of methacrylamide, N,N- dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-vinyl pyrrolidone and hydroxyethyl (meth)acrylate.

In the method according to the present invention, the anionic polyacrylamide, the cationic polyacrylamide and the non-ionic polyacrylamide are preferably with high molecular weight, particularly a molecular weight comprised between 5 x 10⁶ and 30 x 10⁶ daltons, more preferably between 20 x 10⁶ and 30 x 10⁶ daltons for the anionic polyacrylamide, and a molecular weight comprised between 1 x 10⁶ and 20 x 10⁶ daltons for the cationic polyacrylamide and the non- ionic polyacrylamide.

It has been observed that adding anionic, cationic or non-ionic polyacrylamide to the suspension obtained by treating the water containing fluorides with the composition according to the invention (and optionally with calcium hydroxide) leads to the formation of a flocculate which undergoes sedimentation up to three times faster than the sedimentation of the suspension si in the absence of other flocculants.

In a preferred embodiment, the method according to any of the embodiments described above further comprises the step of iii) removing the flocculate from the mixture obtained in step ii), so as to obtain water purified from the fluorides.

In a third aspect and with reference to Figure 1, the present invention further relates to an apparatus 10 for the treatment of water containing fluorides according to the method described above, comprising a treatment reactor 11 for obtaining the suspension si that comprises means 12 for introducing the water containing fluorides and means 13 for introducing the composition according to the invention.

In a preferred embodiment, the apparatus according to the invention further comprises a reactor 14 for pretreating the water containing fluorides upstream of the treatment reactor 11, which comprises means 15 for introducing the water containing fluorides and means 16 for introducing calcium hydroxide.

In a preferred embodiment, the apparatus according to any of the embodiments as described above further comprises a flocculation reactor 17 for obtaining the mixture comprising flocculate and water downstream of the treatment reactor 11 and comprising means 18 for introducing the suspension si and means 19 for introducing the anionic, cationic or non ionic polyacrylamide, preferably anionic polyacrylamide.

In another preferred embodiment, the apparatus according to any of the embodiments as described above further comprises a sedimentation unit 20 downstream of the flocculation reactor 17 and comprises means 21 for introducing the mixture of flocculate and water, means 22 for discharging the flocculate (sludge line) and means 23 for discharging the purified water (purified water line).

In a preferred embodiment of the apparatus according to any of the embodiments as described above, one or more of said treatment reactor 11 , said pretreatment reactor 14 and said flocculation reactor 17 further comprises one or more of: a stirrer 24, for example a screw stirrer; a pH measurement device 25; and a temperature measurement device 26.

It should be understood that even in the embodiments of the present apparatus that do not comprise a flocculation reactor it is possible to introduce a sedimentation unit downstream of the treatment reactor. In this case, the sedimentation unit will comprise means for introducing the suspension si, means for discharging the sludge (i.e., the minerals formed by reaction of the components of the composition of the present invention with the fluorides) and means for discharging the purified water. In a preferred embodiment the apparatus comprises:

- a reactor 14 for the pretreatment of the water containing fluorides, which comprises means 15 for introducing the water containing fluorides and means 16 for introducing the calcium hydroxide;

- a treatment reactor 11 for obtaining the suspension si downstream of the pretreatment reactor 14, comprising means 12 for introducing the water containing fluorides and means 13 for introducing the composition according to the invention;

- a flocculation reactor 17, for obtaining the mixture comprising flocculate and water, downstream of the treatment reactor 11, comprising means 18 for introducing the suspension si and means 19 for introducing the anionic, cationic or non-ionic polyacrylamide, preferably the anionic polyacrylamide; and

- a sedimentation unit 20 downstream of the flocculation reactor 17, comprising means 21 for introducing the mixture of flocculate and water, means 22 for discharging the flocculate and means 23 for discharging the purified water.

The invention will now be described with reference to the following non-limiting examples:

Example 1 : preparation of the composition

700 Kg (70% by weight) of an aqueous solution of polyaluminum chloride having a 18% w/w concentration are placed in a mixing tank provided with a screw stirrer. 250 Kg (25% by weight) of 75% w/w phosphoric acid in water are added and mixing is performed until a homogeneous solution is obtained. 50 Kg (5% by weight) of an aqueous solution containing 50% w/w of poly(N,N-dimethyl-2- hydroxypropylammonium chloride) are added to the solution thus obtained and mixing is performed until a homogeneous solution is obtained.

Example 2: treatment of wastewater containing fluorides

3000 liters of wastewater deriving from an acid washing process and having a content of fluorides of 75-90 mg F/L are introduced in a first reactor having a capacity of 5000 liters and provided with a screw stirrer, a pH measurement device, and a temperature measurement device.

30 liters of lime water (50 g/L of Ca(OH)₂) are dosed with stirring for each m³ of wastewater until a pH of 10.2-10.5 is reached. In this manner the precipitation of calcium fluoride and the forming of a suspension containing calcium fluoride in which the content of fluorides is 23-25 mg F/L are obtained.

The suspension is transferred from the first reactor to a second reactor having a capacity of 5000 liters and also provided with a screw stirrer, a pH measurement device, and a temperature measurement device. One then proceeds with the reduction of the fluorides still present in the suspension by dosing in the second reactor 2.25 kg (750 ppm) of the composition according to Example 1 and leaving it to react for approximately 30 minutes. The reaction between the fluorides of the suspension and the components of the composition of Example 1 causes the precipitation of fluorapatite and aluminum fluoride, which form according to the following reactions:

During the reaction that occurs in the second reactor, triggering of coagulation is ensured by the action of the poly(N,N-dimethyl-2- hydroxypropylammonium chloride) that is present in the composition of Example 1.

The content of the second reactor is then transferred into a third reactor having a capacity of 3000 liters and provided with a screw stirrer, a pH measurement device and a temperature measurement device. Inside the third reactor, a flocculation is performed by dosing with stirring 0.024 kg of an anionic polyacrylamide with high molecular weight, so as to increase the rate of separation of the sludges (containing the minerals listed above that formed during the treatment of the wastewater) from the purified water, in which the content of fluorides is now equal to 3.5 mg F/L, therefore lower than the legal limit.

The content of the third reactor then passes into a sedimentation unit from which two lines, the purified water line and the sludge line, depart. The water can be discharged into a surface watercourse or a sewer, while the sludge is dehydrated, reduced in volume and transferred to the waste disposal plant.

Example 3: comparative analysis of costs

A comparison of operating costs deriving from the treatment of water contaminated by fluorides with the composition according to the present invention has been performed with respect to the costs deriving from the treatment by filtration on activated alumina (alumina method). The comparison is based on the cost in terms of consumption of alumina and of consumption of a composition according to Example 1 in order to perform the removal of 10 kg of fluorides.

The activated alumina has a maximum absorption capacity of 10 g F/kg and a commercial cost of approximately 2.50-3.00€/kg. Therefore, 1000 kg of alumina are necessary in order to reduce 10 kg of fluorides and the cost for reducing 10 kg of fluorides is therefore at least 2500€ in alumina.

Although exhausted alumina can be regenerated by means of chemical treatments aimed at removing the retained fluorides, so as to reduce the costs deriving from the consumption of alumina, in the long run this solution is not optimal because the fluorides removed during the regeneration of the exhausted alumina (in this example 10 kg) are brought back to the top of the treatment apparatus and thus are added to the normal flow of mass of the contaminated water to be treated.

On the other hand, based on the tests performed by the Applicant, in the worst scenario 1 kg of the composition of Example 1 is necessary to reduce 10 g of fluorides. The cost of the composition of Example 1 can vary between 0.80 and 1.00€/kg, depending on the cost variation of the raw materials required for preparation. Therefore, in order to reduce 10 kg of fluorides the cost in terms of consumption of the composition of Example 1 is 800-1000€.

One deduces from this that the operating cost for performing the treatment of water contaminated by fluorides by using the composition according to the present invention is reduced by 60% with respect to the state of the art treatment process with alumina.

In practice it has been found that the composition of the present invention, the method that uses it and the corresponding apparatus for treating the water contaminated by fluorides fully achieve the intended aim and objects, since they allow to perform an effective removal of the fluorides from the contaminated water with lower operating costs and reduced plant costs with respect to treatment with the alumina method.

In particular, for an equal amount of reduced fluorides the composition according to the present invention has a lower cost than“fresh” activated alumina.

Moreover, in the method according to the present invention there is no forming of exhausted alumina loaded with fluorides that has to be disposed with high costs as special waste, but only a mixture of insoluble minerals that can be transferred to the waste disposal plant.

In terms of apparatus costs, the apparatus according to the invention does not use a column of alumina and instead provides for the insertion of a reactor between the lime addition step and the flocculation step, the cost of which is considerably lower than the cost of a column of alumina (approximately 50% less, excluding the alumina), for an equal hydraulic flow rate and concentration of fluorides to be treated.

The composition, the method and the apparatus according to the present invention are susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; furthermore, all the details may be replaced with other technically equivalent elements.

In practice, the materials used, as well as the dimensions, may be any according to the requirements and the state of the art.

The disclosures in Italian Patent Application No. 102017000136929 from which this application claims priority are incorporated herein by reference.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A composition for treating water containing fluorides comprising:

- 10 to 15% by weight of an aluminum-containing compound selected from the group consisting of polyaluminum chloride, aluminum chloride, aluminum sulfate, polyaluminum sulfate, aluminum oxide and aluminum hydroxide;

- 15 to 30% by weight of phosphoric acid;

- 0.5 to 5% by weight of a flocculating polymer selected from the group consisting of poly(N,N-dimethyl-2-hydroxypropylammonium chloride), poly(dimethylamine-co-epichlorohydrin-co-ethylenediamine), poly(diallyldimethylammonium chloride), poly (3 -acrylamide propyl trimethylammonium chloride), dicyandiamide-formaldehyde resins and melamine-formaldehyde resins; and

- water q.s. to 100% by weight,

on the basis of the total weight of the composition.

2. The composition according to claim 1, comprising:

- 10 to 15% by weight of polyaluminum chloride;

- 15 to 30% by weight of phosphoric acid;

- 0.5 to 5% by weight of poly(N,N-dimethyl-2- hydroxypropylammonium chloride); and

- water q.s. to 100% by weight,

on the basis of the total weight of the composition.

3. The composition according to claim 2, wherein the polyaluminum chloride is in the form of a 10 to 18% w/w solution in water, the phosphoric acid is in the form of a 50 to 92% w/w solution in water and the poly(N,N- dimethyl-2-hydroxypropylammonium chloride) is in the form of a 25% to 75% w/w solution in water.

4. The composition according to claim 3, comprising:

- 60 to 80%, preferably 70%, by weight of an 18% w/w solution of polyaluminum chloride in water;

- 20 to 40%, preferably 25%, by weight of 75% w/w phosphoric acid in water;

- 1 to 10%, preferably 5%, by weight of a 50% w/w solution of poly(N,N-dimethyl-2-hydroxypropylammonium chloride) in water;

on the basis of the total weight of the composition.

5. A method for treating water containing fluorides comprising the step of i) treating the water containing fluorides with a composition according to any of claims 1-4 so as to obtain a suspension si .

6. The method according to claim 5, further comprising the step of i') pretreating the water containing fluorides with calcium hydroxide before step i).

7. The method according to claim 6, wherein the calcium hydroxide is in the form of a solution having a concentration of at least 1 g/L, preferably 50 g/L.

8. The method according to any of claims 5 to 7, further comprising the step of ii) treating the suspension si with an anionic or cationic or non ionic polyacrylamide, preferably an anionic polyacrylamide, so as to obtain a mixture comprising a flocculate and water.

9. The method according to claim 8, further comprising the step of iii) removing the flocculate from the mixture so as to obtain water purified from the fluorides.

10. The method according to any of claims 5 to 9, wherein the step i) of treating the water containing fluorides lasts between 1 minute and 3 hours, preferably 30 minutes.

11. An apparatus for treating water containing fluorides according to the method of any of claims 5 to 10, characterized in that the apparatus comprises a treatment reactor (11) for obtaining the suspension si that comprises means (12) for introducing the water containing fluorides and means (13) for introducing a composition according to any one of claims 1-

4.

12. The apparatus according to claim 11, further comprising upstream of the treatment reactor (11) a reactor (14) for the pretreatment of the water containing fluorides that comprises means (15) for introducing the water containing fluorides and means (16) for introducing the calcium hydroxide.

13. The apparatus according to claim 11 or 12, further comprising downstream of the treatment reactor (11) a flocculation reactor (17) for obtaining the mixture comprising flocculate and water that comprises means (18) for introducing the suspension si and means (19) for introducing the anionic or cationic or non-ionic polyacrylamide, preferably the anionic polyacrylamide.

14. The apparatus according to claim 13, further comprising downstream of the flocculation reactor (17) a sedimentation unit (20) that comprises means (21) for introducing the mixture of flocculate and water, means (22) for discharging the flocculate and means (23) for discharging the purified water.

15. The apparatus according to any of claims 11 to 14, wherein one or more of said treatment reactor (11), said pretreatment reactor (14) and said flocculation reactor (17) further comprises one or more of a stirrer (24), a pH measurement device (25), and a temperature measurement device (26).