WATER TREATMENT METHOD TO REDUCE THE DISSOLVED SILICA CONTENT OF THE WATER

Abstract

A water treatment method for reducing the dissolved silica content thereof, the method being characterized in that it comprises at least one step of adsorbing the dissolved silica, the step consisting of passing the water through a reactor containing a granular adsorbent material consisting of iron(III) hydroxide and / or iron(III) oxyhydroxide grains, and at least one step of regenerating the adsorbent power of the granular material, the step consisting of bringing the granular material into contact with a base and at least one chloride.

Description

The invention relates to a method of water treatment to reduce the dissolved silica content of the water. More specifically, the invention relates to said method in which dissolved silica is adsorbed onto a regenerable adsorbent material. State of the art Silicon (Si) is present in many rocks and sediments. Through various chemical alteration processes, this element is found in many types of water (seawater, brackish water, river and lake water, etc.). Although the chemistry of silicon in water is complex and poorly understood, it is known that silicic acid is found in soluble form (“dissolved silica”) in these waters at concentrations that vary depending, in particular, on the chemical species present and the water temperature. However, this dissolved silica has several disadvantages when the water is to be used for certain applications. In fact, this dissolved silica can precipitate under certain conditions and, therefore, cause malfunctions in certain equipment that uses this water. For example, its presence is undesirable in water intended for boiler feed, in water intended for treatment in evaporator-concentrators, in water intended for filtration in membrane devices, particularly those incorporating reverse osmosis membranes, etc. In such installations, dissolved silica can precipitate and form scale or clog the equipment. This dissolved silica can also be a problematic species in itself, not because it can precipitate, but because its presence will disrupt certain chemical or physicochemical reactions involved in water treatment. For example, the presence of silica generates colloidal impurities in certain liquid-liquid extraction methods, which greatly disrupt phase separation. Another example is the interfering nature of dissolved silica in chelation resin methods aimed at removing other compounds. There are several state-of-the-art techniques for reducing the dissolved silica content in water. Therefore, they can be coagulated or electrocoagulated, then clarified. However, these methods lead to the production of sludge that also needs to be treated, and involve the use of large quantities of coagulant, which increases costs. It is also known to coprecipitate dissolved silica in the presence of alkali metals. Furthermore, the fact that this technique also requires large quantities of chemical reagents, such as caustic soda or lime, to induce the formation of insoluble alkali metal salts, means that when the concentration of dissolved alkali metal in the water to be treated is low, large quantities of highly soluble alkali metal salts, such as magnesium chloride, must be added to the water. Therefore, this technique has also proven to be relatively expensive to implement. These different techniques also share the disadvantage of having to be implemented at high pH values, generally above 10. Therefore, there is a need for an alternative solution to these different state-of-the-art methods, which allows the reduction of dissolved silica content in water without having to use large quantities of chemical reagents, and which can be used at lower pH values. For this purpose, the use of ion exchange resins is not industrially feasible. In fact, dissolved silica is primarily ionized only at pH values ​​> 9.4, and its removal by this method would require adjusting the pH to an alkaline range. For water containing a certain degree of alkalinity and / or hardness, this would cause the undesirable precipitation of carbonate salts. Furthermore, since these anionic resins are not strictly selective with respect to dissolved silica, a large portion of their exchange capacity would be mobilized by other anions (Cl·, Br, SCU2-, etc.) present in the water being treated. This lack of selectivity of anion exchange resins makes this method industrially unsustainable for silica removal because it would require the use of excessively large quantities of resins. Presentation of the invention This need is satisfied by the invention, which relates to a water treatment method for reducing the dissolved silica content thereof, characterized in that it comprises at least one adsorption stage consisting of passing said water through a reactor housing an adsorbent granular material consisting of iron (III) hydroxide and / or iron (III) oxyhydroxide grains and at least one stage of regenerating the adsorbent power of said granular material consisting of putting said granular material in contact with a base and at least one chloride. Therefore, the invention proposes using this material to adsorb dissolved silica and, when its adsorption capacity with respect to these chemical species is insufficient, regenerating this adsorption capacity by contacting it with a basic solution and at least one chloride solution. The medium(s) containing these species can be used several times, until their silica content(s) is, or is estimated to be, too high. Therefore, the cost of the method in terms of chemical reagents is low compared to prior art techniques. To the inventors' knowledge, the combined use of a base and a chloride to regenerate the adsorbent power of iron(III) hydroxides or oxyhydroxides has never been proposed in the prior art. The inventors observed that the use of such solutions allowed the crystalline form of these compounds to be maintained. However, it is this crystalline form that enables the adsorption of silica onto this type of material. Preferably, at least part of said granular adsorbent material is in the form of akaganeite. Akaganeite is a form of iron(III) hydroxide mineral with the formula p-Fe3+O(OH,Cl). This material is known to adsorb silica (Naren et al., Adsorption Kinetics of silicic acid on akaganeite 2013 J. Colloid Interface Sci. 399 2013 87-91), but this property has never been implemented on an industrial scale because an efficient regeneration method that would maintain its adsorption performance was unknown. Preferably, said granular material contains at least 5% by weight of akaganeite. The inventors observed that the use, according to the invention, of a base and at least one chloride to regenerate the adsorbent power of the granular material made it possible to maintain, or even improve, this adsorbent power. According to the embodiments, the base and said at least one chloride can be used concomitantly or successively. Therefore, according to one variant, said regeneration stage is carried out with a regeneration solution containing said base and said at least one chloride. According to another variant, the regeneration step is carried out with two regeneration solutions, one containing the base and the other containing the chloride. The solution containing the chloride can then be used either before or after the solution containing the base. Advantageously, this base is selected from the group consisting of NaOH, KOH, NH₄OH, and LiOH. Preferably, NaOH is used. Also advantageously, said chloride is selected from the group consisting of NaCl, KCl, LiCl, NH4Cl, MgCl2, CaCl2, BaCl2, MnCl2. Preferably, NaCl is used. In practice, the base will be used with a molarity between 0.1 and 4 N, preferably between 0.5 and 2 N, and the chloride will also be used with a molarity between 0.1 and 4 N, preferably between 0.5 and 2 N. Also preferably, said granular material is used in a fixed or fluidized bed and said adsorption stage is carried out according to a volume loading rate comprising between 5 and 30 VL / h (“bed volume per hour”), i.e., between 5 and 30 cubic meters of water passing per cubic meter of granular material per hour. Preferably, these grains have a size between 0.2mm and 5mm. It will be observed that the method according to the invention can be implemented with a granular material in the form of a fixed or fluidized bed or even in an installation comprising an infinitely mixed reactor, followed by a settling tank, said installation being advantageously equipped with a pipe for recirculating the granular material deposited in the reactor and means for carrying a portion of the granular material extracted from the settling tank in contact with said base and said at least one chloride before returning the regenerated material to the reactor. Brief description of the figures The invention, as well as its various advantages, will be better understood through the following description of an embodiment thereof, given with reference to the drawings, where: Figure 1 is a schematic view of an installation for implementing the method according to the invention; Figure 2 is a graph showing the evolution of the adsorbent power of the granular bed of the installation according to Figure 1 after one regeneration stage according to the invention (upper curve) and two regeneration stages outside the invention (two lower curves). Description of achievements With reference to Figure 1, a water treatment installation comprises a tank 1 of water to be treated, a reactor 2 containing the granular adsorbent material 3, a treated water tank 4, and a regeneration dissolution tank 5. The water tank to be treated 1 is connected by a pipe 1a equipped with a valve 1b at the top of reactor 2, and the treated water tank 4 is connected at the bottom of reactor 2 by a pipe 4a equipped with a valve 4b. The regeneration dissolution tank 5 is connected at the bottom of reactor 2 by a pipe 5a equipped with a valve 5b and a pump 5c, and the top of reactor 2 is connected by a recirculation pipe 5e to this regeneration dissolution tank 5. Finally, this regeneration dissolution tank 5 comprises an evacuation pipe 5d. It will be observed that the height of the granular adsorbent bed 3 in reactor 2 may vary according to the embodiments and that, in general, it will be between 0.5 m and 2 m. Granular bed 3 is composed of 0.2 mm to 5 mm grains of iron oxyhydroxide containing at least 5% by weight of akaganeite. To treat the water contained in tank 1 to reduce the dissolved silica content, valves 1b and 4b are opened to allow this water to circulate through pipe 1a into reactor 2 in a downward flow and thus come into contact with the granular adsorbent material 3. The treated water is collected in tank 4 through pipe 4a. It will be noted that in other embodiments, it might also be possible to pass the water to be treated through the reactor in an upward flow. This adsorption stage is carried out with a volume loading rate of between 5 and 30 VL / h (“bed volume per hour”), that is, between 5 and 30 cubic meters of water passing per cubic meter of granular bed per hour. During the implementation of the adsorption stage, the granular adsorbent bed is gradually loaded with silica and its adsorbent power decreases. Based on the realizations, therefore, one can consider monitoring the concentration of dissolved silica remaining in the treated water collected in tank 4 or opt to limit the adsorption stage to a predetermined time. Once the adsorbent bed is deemed to have an adsorption capacity below a given threshold or has functioned for a sufficient time, valves 1 and 4b are closed so as to interrupt the flow of water to be treated in the reactor. Then a stage of regenerating the adsorbent power of the granular bed 3 can be implemented. For this purpose, a regeneration medium is produced. In the context of the present Example, a 1 N sodium hydroxide (NaOH) solution was used in which 50 g / L of 0.86 N sodium chloride (NaCl) were dissolved. Valve 5b is then opened and pump 5c is started to inject this regeneration solution into the granular adsorbent bed 3. The regeneration solution then passes through the granular bed and is recovered at the top of reactor 2 via recirculation pipe 5e, which allows IVIA / a / ZUZ I / UI 04 zz said regeneration solution be redirected to tank 5. During this regeneration stage, the silica adsorbed onto the iron oxyhydroxide grains, which contain at least 5% by weight of akaganeite, is transferred to the regeneration solution. At the end of this stage, pump 5c and valve 5b are turned off, then valves 1b and 4b are reopened to start a new adsorption stage. Therefore, several cycles can take place, each containing an adsorption stage and a regeneration stage, using the same regeneration solution. This regeneration solution is gradually loaded with silica, and when its silica concentration is deemed too high and / or at the end of a predetermined number of cycles, the consumed regeneration solution can be discharged through pipe 5d and completely or partially replaced with fresh regeneration solution. Therefore, the method according to the invention is not very demanding in terms of reagents (base and chloride). For comparison, a regeneration solution was used that did not contain chloride, but only contained 1 N NaOH, at 20 °C and 50 °C. In the graph of Figure 2, the volume loading in VL / h is represented on the abscissa and the adsorbent power of the granular bed of adsorbent in milligrams of S1O2 per gram of adsorbent material is represented on the ordinate. In this graph, the upper curve shows the adsorbent power of the granular bed after regeneration according to the invention with the regeneration medium described above prepared from 1 N NaOH and 50 g / l of NaCl at 20 °C, while the two lower curves reflect the adsorbent power of an identical granular bed after regeneration with a regeneration medium containing 1 N NaOH, but not chloride, at 20 °C (lower curve) and 50 °C (middle curve). According to this graph, the regeneration of the adsorbent bed is particularly better when, according to the invention, chloride ions are present in the regeneration solution. Even when heating the regeneration solution containing only NaOH to 50 °C, the regeneration performance of the adsorbent bed remains lower than that observed with the regeneration solution produced, according to the invention, from NaOH and NaCl. The method according to the present embodiment was carried out with water to be treated containing 300 mg of dissolved silica per liter. The amount of silica adsorbed onto the adsorbent material was measured after the first, seventh, and eighth regeneration cycles with the regeneration solution containing chloride and base. The results are given in Table 1 below. These results also show that, unexpectedly, the adsorbent capacity of the granular material increases with the number of regeneration cycles recommended according to the invention. These tests were repeated, and the same surprising result was obtained, which underscores a probable and continuous activation and / or development of new adsorption sites on the material, induced by this regeneration method.A method according to the invention applied to water to be treated containing a dissolved silica concentration between 70 mg(SiO2) / l and 100 mg(SiO2) / l also showed an increase in the adsorbent power of the granular material with the number of regeneration cycles according to the invention, with values ​​close to those presented in Table 1.

Claims

1. A water treatment method for reducing the dissolved silica content thereof, characterized in that it comprises at least one step of adsorbing said dissolved silica, the step consisting of passing said water through a reactor housing a granular adsorbent material consisting of iron(III) hydroxide and / or iron(III) oxyhydroxide grains, and at least one step of regenerating the adsorbent power of said granular material, the step consisting of bringing said granular material into contact with a base and at least one chloride.

2. The method according to claim 1, characterized in that at least part of said granular adsorbent material is in the form of akaganeite.

3. The method according to claim 2, characterized in that said material comprises at least 5% by weight of akaganeite.

4. The method according to any one of claims 1 to 3, characterized in that said regeneration step is carried out with a regeneration solution containing said base and said chloride.

5. The method according to any one of claims 1 to 3, characterized in that said regeneration step is carried out with two regeneration solutions, one containing said base, the other containing said chloride.

6. The method according to any one of claims 1 to 5, characterized in that said base is selected from the group consisting of NaOH, KOH, NHMOH, LiOH.

7. The method according to any one of claims 1 to 6, characterized in that said chloride is selected from the group consisting of NaCl, KCl, LiCl, NH4Cl, MgCl2, CaCl2, BaCb, MnCl2. IVIA / a / ZUZ I / U1 04 zz 8. The method according to any one of claims 1 to 7, characterized in that said granular material is used in the form of a fixed bed and said adsorption stage is carried out with a volume loading of between 5 and 30 cubic meters of water passing per cubic meter of granular material per hour.

9. The method according to any one of claims 1 to 8, characterized in that said grains of iron (III) hydroxide and / or iron (III) oxyhydroxide have a size between 0.2 mm and 5 mm.