Method for treating effluents containing metal sulphates

Abstract

The present invention relates to a method for treating an effluent comprising at least one metal sulphate, the method comprising at least the following successive steps: a) adding an alkaline chemical agent to the effluent, the chemical agent being selected from: magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2) and magnesium carbonate (MgCO3), in order to adjust the pH to a value of between 1 and 9.5, thereby causing the formation and precipitation of at least one metal hydroxide, and separating the effluent from the solids comprising the one or more precipitated metal hydroxides; b) optionally repeating step a) in order to reach a pH of between 1 and 9.5 that is higher than that selected in step a); c) adding lime to the remaining effluent to increase the pH until an optimum value of between 10.2 and 12.5 is reached, depending on the temperature of the effluent, which may be ambient temperature or a controlled temperature, and on the one or more alkaline agents added in steps a) and optionally b), so as to produce a basic solution comprising precipitated white gypsum; d) separating the white gypsum precipitated in step c) from the basic solution so as to obtain a purified basic aqueous solution. This method makes it possible to obtain white gypsum of high purity, metal hydroxides precipitated selectively or in a mixture, and purified water that can be discharged into the receiving environment or the sanitation network, and / or recycled.

Description

[0001] DESCRIPTION

[0002] Title of the invention: Process for treating effluents containing metallic sulfates

[0003] TECHNICAL FIELD

[0004] Industrial effluents containing metal sulfates must undergo treatment before being discharged into the environment, the wastewater system, or reused. The present invention relates to a process for treating such effluents, which leads to the production of valuable white gypsum and purified water. This process can also produce hydroxides of metals or other minerals that can be separated and reused to significantly reduce the amount of final residue requiring landfill disposal.

[0005] PREVIOUS TECHNOLOGY

[0006] Globally, wastewater treatment is a public health issue. Wastewater is all water containing various elements originating from the population and / or commercial and / or industrial activities, which, in its current state, would pollute the environments into which it is discharged. Therefore, to protect these environments, these effluents undergo treatment before being released into the natural environment or a wastewater treatment system. Among the elements that must be treated are metals such as iron and aluminum, which are significant contaminants of ecosystems and the food web, having negative effects on human health at excessive concentrations. Numerous solutions have been developed to separate metals from sulfuric acid solutions while reducing sulfate concentrations before discharge or reuse:

[0007] By coagulation-flocculation: this procedure allows the precipitated particles to agglomerate into larger flakes that can be easily separated from the water. By neutralization: by adding bases or acids to adjust the pH of the water to be treated, the pH directly affecting the solubility of metals and the formation of metallic precipitates at the same time as the precipitation of gypsum (case of conventional High Density Sludge or HDS treatments).

[0008] By adsorption: this technique uses activated carbon or ion exchange resins to remove metal ions from wastewater by adsorption.

[0009] Electrocoagulation: In this technique, an electric current is used to precipitate metals out of the solution. The metal ions are converted into solid forms and can form metal flakes that accumulate at the bottom of the reactor, from where they can be removed.

[0010] By filtration using filtration membranes and techniques such as reverse osmosis, nanofiltration or ultrafiltration. These techniques can be used to separate metal ions from water.

[0011] By evaporation or crystallization of sulfates, which are then removed by filtration. By biological treatment using sulfate-reducing bacteria or other microorganisms or plants capable of precipitating metals or immobilizing them in biological matrices.

[0012] By advanced oxidation treatment (including the use of ozone, UV or Fenton processes (hydrogen and iron peroxide)).

[0013] The specific choice of methods to be used depends on the chemical composition of the wastewater, the concentrations of metals and sulfates, local regulatory standards for the discharge of treated water, and economic costs.

[0014] A combined treatment incorporating different methods can often be the most effective and economical solution for treating wastewater loaded with metallic sulfates.

[0015] The most common method for treating wastewater containing metal sulfates is chemical precipitation using precipitating agents such as sodium hydroxide (NaOH) or lime (CaO or Ca(OH)2), which are added to the wastewater to precipitate the metals as hydroxides and / or to insolubilize the sulfates. The reaction forms solids, notably gypsum (calcium sulfate dihydrate, CaSO4·2H2O), which are then separated by sedimentation (High-Density Sludge (HDS) process) or filtration.

[0016] Gypsum is a by-product of the reaction of sulfuric acid or other sulfates present in wastewater, in the presence of lime.

[0017] However, this type of process has the major drawback of also causing the precipitation of metals or metalloids (Me) (for example, at least one of the following metals: Fe, Ti, Al, Mn, Zn, V, Cr, Pb, Zr, Ni, Co, As, Sr, Ba, Mg, Ca, Na...) from the metal sulfates (Me m (SC>4)n) in the form of metallic hydroxides (Me(OH) cThese metal hydroxides, obtained by mixing with gypsum, pose two problems for recycling. First, the content of polluting metals in the gypsum makes its recovery very difficult given the very low metal concentration thresholds imposed at the input of recycling streams. Second, the potential discoloration of the gypsum, which is white in its pure state, by the metal hydroxides can compromise its recovery, as the whiteness index can be a key factor depending on the application. In other words, the gypsum obtained from this process is colored gypsum—red when the contaminant is ferric iron (Fe(III))—which cannot be used in industry and must be stored in a landfill. Therefore, the pollution is simply shifted from the liquid effluent to a solid residue.

[0018] The objective of the present invention is to present a process for treating industrial effluents containing metallic sulfates, which does not present the disadvantages associated with the production of non-recoverable products, and which allows the production of water of the desired quality, for example water compatible with the environmental standards in force locally, particularly in France.

[0019] In particular, the objective of the present invention is to provide a process for obtaining usable white gypsum and purified water, which can be easily implemented in industry. This process can also produce one or more hydroxides of metals or other minerals that can be separated and reused in order to significantly reduce the amount of final waste requiring landfill disposal. Advantageously, these metal hydroxides are directly usable, meaning they are of sufficient purity to be marketed without further processing. Finally, the process of the invention makes it possible to produce, under certain conditions, metal sulfates, metal chlorides, and / or metal chlorosulfates in usable forms, meaning they are of sufficient purity to be marketed without further processing.

[0020] These various objectives are achieved through the present invention, which provides a process for treating effluents containing at least one metal sulfate. This process comprises the steps of adding to the effluent an alkaline chemical agent judiciously chosen from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCOs), ammonia (NH3), or ammonium hydroxide (NH4OH), in order to progressively increase the pH of the effluent and transform the metal sulfate(s) present therein into metal hydroxide(s). As the pH increases, the metal hydroxide(s) precipitate and can be separated from the effluent, possibly selectively, in at least one step, for potential recycling in the chemical industry.

[0021] The alkaline chemical agent(s) and the pH(s) for precipitation of the metal hydroxide(s) will be carefully chosen according to the metals present in the effluent. Certain metal ions (Zn 2+ Cu 2+ , Co 2+ , Neither 2+ ...), can form metal complexes with ammonia, which are highly soluble at basic pH values ​​in excess ammonia. The formation of these complexes can limit precipitation or solubilize the corresponding metal hydroxides formed. Magnesium-based alkali reagents—magnesium oxide (MgO), magnesium carbonate (MgCO₃), and magnesium hydroxide (Mg(OH)₂)—may therefore be preferred to ammonia or ammonium hydroxide when metal complexation reactions that compete with metal hydroxide formation occur.

[0022] Then, once all or part of the metallic hydroxide(s) have been removed from the effluent, lime (CaO or Ca(OH)₂) is added to transform the sulfate ions present in the effluent into solid calcium sulfate dihydrate (CaSO₄FO₄) that is uncontaminated or very slightly contaminated by metals, i.e., into usable white gypsum. This precipitated gypsum is then separated from the aqueous solution for advantageous use. Preferably, no additional chemical treatment step is required to increase the purity of the gypsum obtained in this step of the invention. However, washing / rinsing steps may be carried out to remove residues contained in the precipitates, particularly any interstitial water that may still be present in the gypsum.

[0023] Unlike prior art processes, this step is carried out at a pH between 10.2 and 12.5 or between 10.2 and 12.0, depending on the effluent temperature (ambient or controlled) and the alkali agent(s) added to precipitate the metals as hydroxides. This allows for the simultaneous formation of gypsum (solid), magnesium hydroxide (solid), and / or the conversion of the ammonium ion (NF₄) into dissolved ammonia gas (NH₃), which are then selectively separated. In particular, when the alkali agent is a magnesium-based agent, it acts as a buffer, preventing an excessively rapid increase in pH upon lime addition, which could hinder optimal gypsum precipitation. By maintaining the solubility of the lime, more gypsum can precipitate, thus preventing the formation of undissolved lime deposits.Thus, preferably, when the alkaline agent used is a magnesium agent, the gypsum precipitation step can ideally be carried out at a pH of 10.8 or 11, and up to a pH of 11.5 to 12 or 12.5. From a pH of 12.5, the lime precipitates in turn, there are no longer enough Ca ions. 2+ in solution, and the pH then increases sharply, slowing down and stopping the precipitation of gypsum.

[0024] In a particularly advantageous embodiment, the metal hydroxides formed during the first stages of the process of the invention can also be treated and valorized by transforming them into metal sulfates, metal chlorides or metal chlorosulfates.

[0025] The present invention therefore proposes a process for treating effluents containing at least one metallic sulfate, said process comprising the steps of:

[0026] - Add to said effluent an alkaline chemical agent chosen from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCOs), ammonia (NH3) and ammonium hydroxide (NH4OH), to transform the metal sulfate(s) present in the effluent into at least one metal hydroxide and to increase the pH of the effluent to a value between 1 and 9.5 in order to possibly selectively precipitate the metal hydroxide(s) formed without precipitating gypsum, then separate it / them from the effluent so that it / they can possibly be recycled,

[0027] - If necessary, repeat this step one or more times depending on the targeted selective precipitation of one or more metal hydroxides possibly still present in the effluent remaining after the previous step, by adding again, to the remainder of the effluent, an alkaline chemical agent chosen from magnesium oxide (MgO), magnesium carbonate (MgCOs), magnesium hydroxide (Mg(OH)2), ammonia (NH3), and ammonium hydroxide (NH4OH), to reach a pH value between 1 and 9.5 higher than that chosen previously, in order to precipitate the metal hydroxide(s) formed, without precipitating gypsum, then separate it / them from the effluent so that it / they can possibly be recycled,

[0028] - Once the metal hydroxide(s) has / have been separated from the effluent, add lime to raise the pH to a value between 10.2 and 12.5, or between 10.2 and 12.5, depending on the effluent temperature (ambient or controlled) and the alkaline agent(s) used to precipitate the metals as hydroxides, to transform the sulfate ions present in the effluent into solid calcium sulfate dihydrate (CaSC4.2H2O) that is uncontaminated or slightly contaminated by metals (i.e., usable white gypsum), and to precipitate the magnesium (Mg 2+ ) present in the aqueous solution, in the form of magnesium hydroxide (Mg(OH)2) and / or to convert the ammonium (NF) present in the aqueous solution, into dissolved gaseous ammonia (NH3).

[0029] - Separate the gypsum, which has then precipitated, from the aqueous solution obtained, and possibly recycle it in part in the process of the invention and / or valorize it.

[0030] - Separate the dissolved magnesium hydroxide (Mg(OH)2) and / or gaseous ammonia (NH3) and possibly recycle it or them in the process of the invention and / or recover it or them.

[0031] In one embodiment of the invention, carbon dioxide can be added to the resulting aqueous solution to desaturate it with soluble calcium sulfate (CaSC₂) present in the saturated aqueous solution. This desaturates some of the calcium present in the aqueous solution, causing it to precipitate as limestone (CaCO₃) and thus lowering the pH of the aqueous solution according to the calcium-carbonate equilibrium specific to the effluent at this stage of the process. This precipitated limestone can then be separated from the solution by conventional means to obtain clarified water and can potentially be recycled.

[0032] In a preferred embodiment, it is possible to add hydronium ions combined with sulfate ions and / or chloride ions (e.g. sulfuric acid and / or hydrochloric acid and / or acidic metal chlorides and / or acidic metal sulfates) to at least one of the metal hydroxides produced to obtain metal sulfates, metal chlorides and / or metal chlorosulfates.

[0033] In another embodiment of the invention, it is possible to add to the effluent, prior to the steps described above, a strong alkaline agent, a metal hydroxide, gypsum colored by metal hydroxide contaminants, in particular red gypsum or blue gypsum, to achieve a pH value between 1 and 3, preferably between 1.5 and 2, to obtain a pre-neutralization of the effluent, and to convert the colored gypsum contaminated by metal hydroxides into white gypsum.

[0034] Thanks to the process of the invention, gypsum is recovered primarily in the form of white gypsum, for which a high purity (CaSO4·2H2O) is desired (>50%), or more broadly as gypsum suitable for various industries, particularly the cement and plaster industries. The process according to the invention also makes it possible to obtain selectively precipitated or mixed metal hydroxides, depending on the potentially targeted selective precipitation, which can also be utilized, as well as purified water that can be discharged into the receiving environment, the wastewater system, and / or recycled. The process of the invention also makes it possible to generate metal sulfates, metal chlorides, and / or metal chlorosulfates (e.g., ferrous or ferric sulfate, ferric chlorosulfate, etc.).

[0035] DESCRIPTION OF THE INVENTION

[0036] According to a first aspect, the present invention relates to a method for treating an effluent containing at least one metal sulfate, said method comprising at least the following successive steps: a) Adding an alkaline chemical agent to said effluent, said chemical agent being selected from: magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCO3), ammonia (NH3), and ammonium hydroxide (NH4OH), to adjust the pH to a value between 1 and 9.5, resulting in the production and precipitation of at least one metal hydroxide, and separating from the effluent the solids comprising the precipitated metal hydroxide(s); b) Optionally, repeating step a) to achieve a pH between 1 and 9.5 higher than that selected in step a); c) Adding lime to the remaining effluent to raise the pH to an optimal value between 10.2 and 12.5, preferably between 10.2 and 12, depending on the temperature of the effluent, ambient or controlled, and depending on the alkaline agent(s) added in steps a) and possibly b), so as to produce a basic solution including precipitated white gypsum, d) Separate the precipitated white gypsum in step c), from the basic solution so as to produce a purified basic aqueous solution.

[0037] In the process of the invention, the lime is therefore added after the magnesium or nitrogen alkaline agent of the invention, and it is not planned to add lime prior to step a). Furthermore, the process of the invention preferably does not contain any step involving the application of an electric or magnetic field on the effluent to be treated.

[0038] The chemical agent used in steps a) and / or b) is preferably used as the sole neutralizing reagent, eliminating the need to combine multiple alkali / base agents. This ensures simplicity in pH control and by-product management.

[0039] In the process of the invention, gypsum is precipitated from calcium-containing lime without the external addition of any calcium counter-ion other than OH', for example, without the external addition of chloride counter-ions, so as not to exchange sulfate salts for another salt. The salinity of the resulting effluent is therefore advantageously reduced.

[0040] This process applies to any liquid medium, in particular to any aqueous effluent containing at least one metallic sulfate, including flue gas scrubbing effluents from non-hazardous or hazardous waste incineration plants, leachates from non-hazardous waste storage facilities, effluents from surface treatment, mining effluents, effluents from coal or gas-fired power plants (flue gas desulfurization effluents or FGD) - this list is not exhaustive.

[0041] The effluents to be treated by the method of the invention may be highly acidic due to a potentially high concentration of sulfuric acid (pH less than 2). They contain, in particular, high concentrations of metal sulfate(s) such as copper(II) sulfate (CuSC₂), iron(II) sulfate (FeSC₂), zinc sulfate (ZnSC₂), aluminum sulfate (A₂SC₂), magnesium sulfate (MgSC₂), nickel sulfate (NiSC₂), manganese sulfate (MnSC₂), etc. The sulfate concentration in these effluents is typically greater than 1.7 g / L. The metal concentration may, for its part, be greater than 0.2 g / L.

[0042] The quantity and composition of the starting effluent can, for example, be such as those described in the table below, resulting from the mixing of two effluents of spent acids - called noble acids (NA) and secondary acids (SA) in the particular case of this example: [Table 1]

[0043] The process according to the invention makes it possible to obtain, at the end of the process, a concentration in each of the metals corresponding to their solubility limit under the optimal reaction conditions of selected pH.

[0044] For example, the process according to the invention makes it possible to achieve a residual iron concentration in the purified water of less than 0.1 mg / L.

[0045] In a particular embodiment of the invention, the objective is also to produce one or more isolated and unmixed metal hydroxides, which can be recovered separately in different industrial sectors. To this end, the precipitation of the metal(s) present in the effluent will preferably be carried out selectively in several successive stages by adjusting the pH of the effluent so as to selectively precipitate the metal hydroxide(s) present in the effluent after the addition of the alkaline chemical agent(s), depending on the nature of the metal(s) present in the effluent and the desired selectivity to potentially recover the precipitated metal hydroxide(s).In this regard, a person skilled in the art knows at what optimum pH levels metal hydroxides from metals potentially contained in the effluent can precipitate and can therefore identify how many steps are required to carry out step a) between pH 1 and 9.5. The table below indicates the solubility of metal hydroxides and their optimum pH range for precipitation, for the main metals potentially contained in the effluent - this list is not exhaustive.

[0046] [Table 2]

[0047] It is known that ferric iron hydroxide precipitates optimally at a pH of approximately 4.5. The process of the invention therefore allows, when the chosen pH is between 4 and 5, the separation and recovery of any ferric iron hydroxide present in the effluent (particularly if an oxidizing agent has been added to the effluent before, during, or after step a). Preferably, the process of the invention includes a specific step for adjusting the pH to a value between 4 and 5 to obtain precipitated ferric iron hydroxide, and a step for separating the precipitated ferric iron hydroxide from the remaining effluent.

[0048] It is known that aluminum hydroxide precipitates significantly at a pH between 5.5 and 7.5, and maximally between 6 and 6.5. The process of the invention therefore allows, when the chosen pH is between 5.5 and 7.5, the precipitation and separation of any aluminum hydroxide present in the effluent. Preferably, the process of the invention includes a specific step of adjusting the pH to a value between 5.5 and 7.5 or between 6 and 6.5 so as to obtain precipitated aluminum hydroxide, and a step of separating the precipitated aluminum hydroxide from the rest of the effluent.

[0049] It is known that ferrous iron hydroxide precipitates significantly at a pH above 9. The process of the invention therefore makes it possible, when the chosen pH is between 9 and 9.5, to precipitate and significantly separate any ferrous iron hydroxide present in the effluent, within a pH range compatible with the optimal use of magnesium or ammonia alkaline agents. Preferably, the process of the invention includes a specific step for adjusting the pH to a value between 9 and 9.5 so as to obtain precipitated ferrous iron hydroxide, and a step for separating the precipitated ferrous iron hydroxide from the rest of the effluent.

[0050] If the starting effluent contains iron and aluminum, and if it is desirable to obtain aluminum hydroxide not mixed with ferrous iron hydroxide, then the process of the invention will advantageously include at least two different steps of pH adjustment and precipitate separation, one at a pH between 5.5 and 7.5 to precipitate and separate the aluminum hydroxide, the other at a pH between 9 and 9.5 to precipitate and separate the ferrous iron hydroxide.

[0051] If a preliminary oxidation step is carried out, and if it is desirable to obtain aluminum hydroxide unmixed with ferric iron hydroxide, then the process of the invention shall include at least two different pH adjustment and separation steps, one at a pH between 4 and 5, preferably between 4 and 4.2, to precipitate and separate the ferric iron hydroxide, the other at a pH between 5.5 and 7.5, preferably between 6 and 6.5, to precipitate and separate the aluminum hydroxide.

[0052] Thus, pH adjustment can be done in one or more steps (preferably in one, two, three, four or five steps) depending on the nature of the metal hydroxide(s) contained in the effluent after the addition of the chemical agent, the optimal pH for precipitation of this or these metal hydroxide(s) to be precipitated and the desired selectivity (especially if one wishes to isolate a particular metal hydroxide or a mixture of several metal hydroxides).

[0053] The separation or, conversely, the co-precipitation of several metal hydroxides present in the initial effluent can be achieved by adjusting the number and pH and / or temperature conditions of the steps of the invention, on a case-by-case basis, according to the pH precipitation ranges of the metal hydroxides of the metals present, the experimenter's final requirements, and the desired final quantities of metals. It is important that the calcium sulfate contained in the effluent does not precipitate in step a) (nor possibly in step b) if it is carried out).

[0054] According to one embodiment of the invention, the treatment process of the invention comprises at least the following successive steps: a) Adding a chemical agent to said effluent, said chemical agent being selected from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCOs), ammonia (NH3), and ammonium hydroxide (NH4OH), to adjust the pH to a value between 1 and 7.5, resulting in the production and precipitation of at least one metal hydroxide, and separating the precipitated metal hydroxide(s) from the effluent; b) To the remainder of the effluent, adding a chemical agent selected from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCOs), ammonia (NH3), and ammonium hydroxide (NH4OH), to adjust the pH to a value between 1 and 9.5 higher than that of step a), resulting in the production and precipitation of at least one metallic hydroxide preferably different from that obtained in step a), and separate this from the rest of the effluent, c) In the remaining effluent, add lime to raise the pH to an optimal value between 10.2 and 12.5, preferably between 10.2 and 12, depending on the temperature of the effluent, ambient or controlled, and depending on the alkaline agent(s) added in steps a) and possibly b), so as to produce a basic aqueous solution comprising valuable precipitated white gypsum, d) Separate the precipitated white gypsum from the basic solution produced in step c) so as to produce a purified basic aqueous solution.

[0055] In the process of the invention, lime is added after the magnesium or nitrogen alkali of the invention, and no lime is added prior to step a). Furthermore, the process of the invention preferably does not contain any step involving the application of an electric or magnetic field to the effluent to be treated. The chemical agent used in steps a) and / or b) is preferably used as the sole neutralizing reagent, eliminating the need to combine several alkali / base agents. This ensures simplicity in pH control and by-product management. In a preferred embodiment, the pH of step a) is between 5.5 and 7.5 for producing and recovering aluminum hydroxide, and the pH of step b) is between 9 and 9.5 for producing and recovering ferrous iron hydroxide.

[0056] During steps a) and / or b) of the process of the invention, the metal hydroxides precipitated after pH adjustment are separated from the rest of the effluent by separation means such as filters, centrifuges, hydrocyclones, or by differential settling, press or belt filtration, or by centrifugation.

[0057] Filtration, typically by filter press, separates the filtrate, comprising water and dissolved metal hydroxides, from the precipitated hydroxides, which are retained in the filter cake (in the case of a filter press). It is also possible to filter the solution / effluent, for example, through sand and activated carbon, as described in the prior art. The solution / effluent can also advantageously be decanted before filtration.

[0058] The metal hydroxides precipitated during steps a) and / or b) can advantageously be dehydrated by conventional means (e.g. a filter press), before possibly being reused in industry.

[0059] In step c) of the process of the invention, the lime added is calcium hydroxide Ca(OH)2 or calcium oxide CaO. The amount of lime added is adjusted to satisfy the proportions required for the reaction to produce calcium sulfate dihydrate and to allow the precipitation of Mg ions. 2+ into Mg(OH)2 and / or the conversion of NH4 ions + into dissolved NH3 gas. Maximum precipitation of Mg(OH)2 and / or maximum conversion of NH4 + NH3 will be sought.

[0060] In the context of the present invention, the term "usable white gypsum" refers to precipitated (i.e., solid) calcium sulfate dihydrate that can be used in any type of industry, depending on user needs. It is defined in particular by its whiteness index and its purity as calcium sulfate dihydrate.

[0061] The "usable white gypsum" produced by the process of the invention preferably has a whiteness index greater than 50, preferably greater than 60, preferably greater than 70, preferably greater than 80, preferably greater than 85, preferably greater than 90, depending on the desired applications (the plaster industry uses white gypsum with an index greater than 80, but the cement industry uses white gypsum with a lower whiteness index). This whiteness index can be determined using a spectrophotometer or a colorimeter, according to the relevant standard standards, in particular standard E313-20 on the calculation of the whiteness index (point 7.2.1. and table 2 of the standard available at https: / / cdn.standards.iteh.ai / samples / 106215 / b9c8874d178b4960801b19d6e464b85d / ASTM-E313-20.pdf) or equivalent.

[0062] Furthermore, the "usable white gypsum" produced by the process of the invention is preferably only slightly contaminated by metals, that is to say, it contains, for example, a minimum of 70%, 75%, 80%, 85%, 90%, or even 95%, 97%, or 98% by mass percentage of precipitated calcium sulfate dihydrate. The purity of the gypsum can be measured by X-ray fluorescence, ICP (Induced Coupled Plasma), or by any other conventional means.

[0063] This valuable white gypsum is obtained from step d) of the processes of the invention without the need for any additional chemical treatment step. The processes of the invention therefore make it possible to produce gypsum of excellent quality, in large quantities, with a minimal number of steps.

[0064] When the reaction medium temperature is cold, for example below 10 °C, the conversion of more than 99% of NF to NH3 may require adjusting the pH to a value above 12. Heating the reaction medium to a temperature, for example, of 20 to 30 °C, increases the efficiency of the conversion of NF to NH3 at a pH below 12, typically between 11 and 11.7. It is therefore recommended to heat the reaction medium to at least 20 °C when the alkaline agent used is ammoniacal.

[0065] In one particular embodiment, when the alkali used is ammoniacal, the heat released during an NH3 stripping step or by another heat source can be recovered to heat the reaction medium and increase the efficiency of NF to NH3 conversion. The pH of step c) can then be between 10.2 and 12 (closer to 12 when the temperature reaches up to 30-35°C). Thus, the reaction medium of step c) can be heated to an optimal temperature chosen to maximize the conversion of NF to NH3 within a pH range of 10.2 to 12, while respecting any other specific constraints that may exist, on a case-by-case basis. Preferably, the pH of step c) is between 10.2 and 12.5, depending on the reaction temperature, ambient or controlled, and on the alkali agent(s) added in steps a) and possibly b). Even more preferably, this pH is between 10.2 and 12, preferably between 10.5 and 12, preferably between 10.8 and 12, preferably between 10.8 and 11.8, preferably between 11 and 11.8. It is possible not to heat the effluent at this stage. In this case, the temperature of stage c) of the process of the invention is preferably ambient temperature. It is also possible to heat the effluent to accelerate the gypsum precipitation process. In this case, the temperature of stage c) of the process of the invention is ideally below 80°C, preferably below 60°C, to avoid any unnecessary energy expenditure.

[0066] In steps c) and d) of the process of the invention, the precipitation and extraction of the precipitated gypsum can be carried out by any means commonly used for this purpose, for example, on a conventional High Density Sludge (HDS) thickening decanter, on an Actina™ or equivalent pellet reactor, or on a Saphira™ contact reactor. When such means are used, steps c) and d) of the process of the invention can be carried out concurrently. The basic solution produced as a result of these steps then comprises water and soluble calcium sulfate (CaSC₂).

[0067] In an alternative embodiment, steps c) and d) of the process of the invention are carried out separately, successively.

[0068] The various stages of the process of the invention can be carried out at ambient temperature (for example, at approximately 20°C), without heating the effluent or the added alkaline agents. However, in the variant using an ammoniacal alkaline agent, the heat from the possible ammonia stripping stage can be advantageously used to heat the effluent upstream of step c), within acceptable temperature limits determined on a case-by-case basis, as explained above.

[0069] Steps a), b) and c) of the process of the invention are preferably carried out in the presence of a mixer in order to homogenize the solution and promote the production of a maximum of solids.

[0070] The quantities of chemical agents and / or effluent and / or lime to be used in the process of the invention are easily determinable and / or adjustable by a person skilled in the art. In a preferred embodiment, the process of the invention further includes a step of seeding the precipitation reactor of step c) with gypsum seeds whose particle size has been calibrated, in order to promote the precipitation of gypsum in the reactor, and subsequently facilitate the separation of the two types of solids produced and present in the effluent, during step d), particularly when the alkaline agent used during steps a) and b) is a magnesium alkali.

[0071] In a preferred embodiment, the process of the invention further includes a step of separating the two types of solids (gypsum and magnesium hydroxide) which are produced simultaneously during step c).

[0072] Indeed, when a magnesium alkali-aggregate agent is chosen in steps a) and possibly b) of the process of the invention, two types of solids (gypsum and magnesium hydroxide) are produced simultaneously in step c) and must be optimally separated to obtain maximum gypsum purity (i.e., a minimum amount of Mg(OH)2 in cases where the target gypsum for recovery must have high purity) and to maximize the purity of the Mg(OH)2 (i.e., containing a low amount of CaSO4·2H2O), either for advantageous recycling in steps a) and possibly b) of the process of the invention, or for recovery outside the process of the invention. However, the separation of gypsum and magnesium hydroxide relies primarily on the difference in particle size of the two solids. The Mg(OH)2 particles present in the reactor after step c) will typically have particle sizes between 0.1 pm and 15 pm.To facilitate the separation of gypsum particles from Mg(OH)2 particles in step d) and prevent their mixing, it is therefore necessary to ensure that the gypsum particles produced in the reactor in step c) have a particle size greater than 15 µm. In other words, maintaining a targeted particle size of the gypsum precipitated in step c) that is distinct from that of the Mg(OH)2 is crucial for achieving optimal separation of the two solids contained in the effluent after step c) of the process of the invention.

[0073] Therefore, it is preferable to limit the production of small gypsum particles in the precipitation reactor as much as possible, in step c).

[0074] To do this, part of the gypsum crystals separated in step d) of the process of the invention can advantageously be recycled in step c), to introduce into the precipitation reactor calibrated gypsum seeds having a targeted particle size.

[0075] By adding these gypsum nuclei along with the lime, the crystallization of calcium sulfate will be catalyzed, promoting and accelerating crystal growth through agglomeration on the nuclei, producing final crystals with a larger particle size than the nuclei. Furthermore, competitive gypsum nucleation reactions (production of nuclei or very fine gypsum grains, at the nanoscale) will be limited, reducing the undesirable production of fine gypsum particles that cannot be separated from the Mg(OH)2 and / or the treated effluent (which could lead to sulfate leakage and an increased risk of post-precipitation of gypsum downstream of step d)).

[0076] In this context, it is therefore advantageous to introduce gypsum nuclei into the precipitation reactor in step c) to obtain final gypsum crystals with a particle size larger than that of the Mg(OH)2 particles (i.e., greater than 15 µm). The particle size of the gypsum nuclei introduced into the reactor in step c) will therefore be carefully chosen for this purpose. In particular, the average particle size of the gypsum nuclei introduced into the reactor in step c) will preferably be greater than 15 µm, to avoid the risk of them mixing with the Mg(OH)2 particles.

[0077] To obtain gypsum seeds of the appropriate particle size, grinding of the gypsum obtained in step d) or from another source will often be necessary to achieve a particle size greater than 15 µm. This grinding can be carried out by any conventional means, for example with a roller mill, a roller mill, or even using ultrasound.

[0078] The maximum targeted particle size of these gypsum nuclei will also be advantageously limited to 30 pm, so as to promote the growth of gypsum crystals to typically reach a particle size of mature gypsum crystals between 80 and 120 pm.

[0079] The particle size of the gypsum seeds possibly added to the reactor in step c) is therefore ideally between 20 pm and 30 pm.

[0080] Furthermore, these calibrated gypsum seeds can be limed, that is to say mixed with lime, in order to make them more active.

[0081] The process of the invention therefore preferably includes an additional step of grinding the precipitated white gypsum separated during step d) to obtain a calibrated particle size of gypsum greater than 15 pm, and optionally a step of activating the white gypsum thus ground by liming, and the addition of this ground and optionally activated white gypsum to the effluent during step c).

[0082] These steps are illustrated in Figure 4. When an ammoniacal alkaline agent is chosen in steps a) and possibly b), these additional steps are illustrated in Figure 5. The expected benefits of seeding the precipitation reactor with calibrated gypsum seeds, ground and mixed with lime, will be those described above, with the exception of the solids separation constraints in step d). The particle size of the gypsum seeds to be introduced into the reactor will thus be primarily determined by the optimum gypsum crystal growth desired in the reactor in step c).

[0083] Raw effluents containing one or more metal sulfates may have a very acidic pH (less than 1), which could disrupt the equilibrium of the chemical reactions involved in the process of the invention. To improve the efficiency of the process, it may include a preliminary step to increase the pH of the starting effluent to a pH of approximately at least 1. This preliminary pH increase step may be carried out in various ways known to those skilled in the art.

[0084] In particular, it is possible to add a strong alkaline agent, preferably free of calcium oxide and / or calcium hydroxide (to avoid the risk of creating gypsum with a calcium input, leading to supersaturation of the effluent with calcium sulfate). Such a strong alkaline agent could be, for example, sodium hydroxide or potassium hydroxide.

[0085] In a preferred embodiment of the invention, it is also possible to use alkalizing metal hydroxides as a strong alkaline agent, for example those contained in colored gypsum, such as red or blue (see also Example 6 on this subject). These hydroxides or this gypsum can be added to the initial effluent, and thus purified and recovered.

[0086] When red or blue gypsum is added, the calcium in the gypsum is already in the form of dissolved gypsum or calcium sulfate, and therefore will not have the drawbacks of calcium (hydr)oxide. In this preferred embodiment, the blue or red gypsum will be pre-washed to recover dissolved metals present in the pore water.

[0087] The wash waters loaded with dissolved metals can be recycled in the treatment process, in step a), to precipitate the metals dissolved in the said wash waters, in the form of metallic hydroxides which can thus potentially be recovered.

[0088] Thus, in a particular embodiment, the process of the invention includes, prior to step a), a step of increasing the pH of the effluent to a value of at least 1. This preliminary step is preferably carried out by adding a strong alkaline agent as described above, an alkalizing metal hydroxide or colored gypsum, for example red or blue.

[0089] By "red gypsum" we mean here precipitated calcium sulfate dihydrate (gypsum) containing iron in ferric form (for example, 80% by mass percentage of gypsum CaSO4,2H2O and 20% by mass percentage of ferrihydrite (Fe(OH)3).

[0090] By "blue gypsum" we mean here precipitated calcium sulfate dihydrate (gypsum) containing iron in ferrous form (for example, 80% by mass percentage of gypsum CaSC>4,2H2O and 20% by mass percentage of ferrihydrite (Fe(OH)2).

[0091] When red or blue gypsum is added to the effluent prior to step a) of the process of the invention, the iron contained in the gypsum is dissolved and transformed into ferrous or ferric sulfate. White gypsum is then obtained, which can be separated from the effluent by filtration or any other separation method conventionally used for this purpose. In this case, a preliminary step for separating the white gypsum obtained from the red / blue gypsum is advantageously added before step a) of the process of the invention.

[0092] In another preferred embodiment of the process of the invention, an oxidizing agent is added to the starting effluent in order to oxidize all the metals in the solution that can be oxidized, in particular ferrous iron (Fe 2+ ) in ferric iron (Fe 3+ This oxidizing agent is, for example, dioxygen (O2), hydrogen peroxide (H2O2), or ozone. This oxidizing agent can be added before or simultaneously with the alkaline chemical agent of the invention, that is, before or during step a). In the particular case where red or blue gypsum has been added to the effluent during a preliminary step as described above, the oxidizing agent will preferably be added after the metals (and in particular iron) contained in the red or blue gypsum have been dissolved by the acidic effluent, therefore during step a).

[0093] Thus, in this particular embodiment, the process of the invention comprises, prior to or during step a), an oxidation step a0 carried out by adding to the effluent at least one oxidizing agent as described above. In another preferred embodiment of the process of the invention, a reducing agent (such as sodium bisulfite or sodium metabisulfite) is added to the starting effluent in order to reduce all the metals in the solution that can be reduced, in particular ferric iron (Fe 3+ ) in ferrous iron (Fe 2+ ).

[0094] Thus, in this particular embodiment, the process of the invention includes, prior to or during step a), a reduction step aO) carried out by adding to the effluent at least one reducing agent as described above.

[0095] Washing / rinsing steps for the precipitated products can be added to remove the last unagglomerated residues, in order to increase purity and facilitate the recycling of the precipitated products.

[0096] Furthermore, effluent washing steps may take place prior to the process of the invention, to remove salts such as MgSC, Na2SO4 or NaCl, by conventional techniques, such as washing centrifuges, vacuum belt filters with washing, or filter presses with cake washing.

[0097] At the end of the process, the goal is to obtain high-quality treated water that can potentially be recycled, discharged into the wastewater network, or released into the natural environment. In France, environmental standards for wastewater discharge into the natural environment are strict and defined by several regulations. These standards are primarily stipulated in the Environmental Code, and discharge permits are issued by the prefectures, in accordance with the requirements of the local Water Agency. In Europe, the Water Framework Directive (2000 / 60 / EC) is the main instrument of water policy, which aims to protect and improve the quality of surface and groundwater. However, this directive does not establish specific limits for sulfates, but it does require Member States to establish environmental standards based on the characteristics of each river basin.In France, for example, there is no nationally specified limit value for sulfates discharged into surface waters in general. Limit values ​​are determined on a case-by-case basis, taking into account the carrying capacity of the river or receiving environment and the water quality objectives set for that environment. Thresholds may also depend on the sensitivity of the receiving area (for example, drinking water catchment areas, protected areas) and other water uses (agriculture, industry, human consumption).To eliminate the risk of post-precipitation of soluble calcium sulfate (CaSC₂) still present, for example at saturation, in the basic solution at the end of step d) of the process of the invention, it is recommended to add carbon dioxide (CO₂) to this basic solution to reduce the ionic product of calcium sulfate (CaSC₂) in the aqueous solution, by precipitating some of the calcium present in the aqueous solution, as limestone (CaCO₃), and to reduce the pH of the aqueous solution, according to the calcium-carbonate equilibrium and to a value conforming to the discharge standards in force on a case-by-case basis, for example a pH less than 9.5. This will make it possible to obtain a CaSC saturation of this solution of less than 100%.The limescale produced can easily be separated from the solution using conventional methods such as an HDS-type thickening clarifier, an Actina™-type contact reactor (pellet reactor) or equivalent, or a Saphira™ contact reactor or equivalent. Following this step, the water is purified and can be discharged into the environment without risk of pollution.

[0098] Thus, the process of the invention further optionally includes a step e) in which CO2 is added to the basic aqueous solution from step d) to precipitate some of the calcium present in the aqueous solution and to decrease the pH of the aqueous solution to a value between 4 and 11, preferably between 4 and 9.5 or even between 5.5 and 9.5, and then a step f) consisting of separating the limestone then formed from the aqueous solution.

[0099] Furthermore, the process of the invention may also include a step f) in which hydronium ions combined with sulfate ions (for example, by means of sulfuric acid (H2SO4) or acidic metal sulfates) are brought into contact with at least one of the metal hydroxides produced in steps a) and b), in order to generate a metal sulfate (MeSC) that can be used. For example, ferric iron hydroxide Fe(OH)3 can be brought into contact with FSC to produce ferric sulfate Fe2(SC>4)3, which can be used in water treatment or as an oxidizing agent.In addition to or instead of hydronium ions combined with sulfate ions, hydronium ions combined with chloride ions (Cl-) can be added (for example, by means of hydrochloric acid or acidic metal chlorides) to at least one of the metal hydroxides produced in steps a) and b), in order to produce metal chlorides and / or metal chlorosulfates such as ferric chlorosulfate (FeCISC), which can be used in various applications. The formation of these compounds depends on the reaction conditions, in particular the pH and the concentration of reactants. These conditions are well known to those skilled in the art. According to a first preferred embodiment, the chemical agent added in steps a) and b) is magnesium oxide (MgO), magnesium carbonate (MgCOs), or magnesium hydroxide (Mg(OH)2).

[0100] The chemical reaction that occurs when magnesium hydroxide Mg(OH)2 is added to the effluent is as follows:

[0101] Me a (SC>4)b + cMg(OH)2 --> aMe(OH) c + Mg c (SO4)b where Me is a metal, and where a, b, and c are natural numbers.

[0102] Upon the addition of calcium hydroxide Ca(OH)2, the compound MgSC reacts with it to produce white gypsum and magnesium hydroxide according to the reaction:

[0103] MgSÛ4 + Ca(OH)2 + 2H2O —> CaSO4 .2H2O( S )+ Mg(OH)2(s) where (s) symbolizes solid forms.

[0104] In this preferred embodiment, a preliminary oxidation step is recommended (e.g., with dioxygen) to convert ferrous iron to ferric iron, thus facilitating its separation from aluminum. The pH is then increased in step a) to a pH between 4 and 5.5 (preferably 4.2). At this pH, ferric iron hydroxide is in solid form and can therefore be separated from the effluent. The pH is then increased in step b) to a pH between 5.5 and 7.5 (preferably 6.5) to precipitate other metal hydroxides, particularly aluminum hydroxide. Finally, the addition of lime at a pH between 10.2 and 12.5 or between 10.2 and 12 (preferably 11.8) allows the production and precipitation of white gypsum and magnesium hydroxide, which can each be reused in the process of the invention, or elsewhere.

[0105] In this preferred embodiment, the process of the invention therefore comprises at least the following successive steps: a) Adding an oxidizing agent to said effluent, a) Adding a solution of magnesium oxide (MgO), magnesium carbonate (MgCOs) or a solution of magnesium hydroxide (Mg(OH)2) to the effluent to increase its pH to a pH between 4 and 5.5 resulting in the production and precipitation of at least one metal hydroxide present in the effluent (in particular ferric iron hydroxide), and separating the precipitated metal hydroxide from the effluent, b) Adding a solution of magnesium oxide MgO, magnesium carbonate (MgCOs) or a solution of magnesium hydroxide (Mg(OH)2) to the remainder of the effluent to increase its pH to a pH between 5.5 and 7.5 leading to the production and precipitation of at least one metal hydroxide (in particular aluminium hydroxide), and separate this from the remaining effluent, c) To the remaining effluent, add lime to raise the pH to an optimal value between 10.2 and 12.5, preferably between 10.2 and 12, producing a basic solution comprising precipitated white gypsum and precipitated magnesium hydroxide, and d) Separate the precipitated white gypsum and precipitated magnesium hydroxide respectively from the basic solution of step c).

[0106] The addition of lime and therefore the precipitation of gypsum during step c) can be done via a "High density Sludge" type process.

[0107] This step (c) also allows the dissolved magnesium to precipitate as Mg(OH)2. Step (c) therefore involves separating two solids (white gypsum and magnesium hydroxide) present in the basic solution obtained in step (c). This separation can be carried out using any conventional method, including a separator. The resulting solution contains water and soluble calcium sulfate residue. The Mg(OH)2 formed can either be recycled or used elsewhere.

[0108] When MgO or magnesium carbonate (MgCOs) are used as an alkali agent, the Mg(OH)2 produced is not recycled but is preferentially isolated for use in other applications, in solid or suspended form depending on the needs of industries, for example to decontaminate ship fumes, to stabilize waste, in microelectronics or to precipitate heavy metals.

[0109] The pH of step c) is between 10.2 and 12.5, depending on the reaction temperature, ambient or controlled, and on the alkaline agent(s) added in steps a) and possibly b); preferably, this pH is between 10.2 and 12, preferably between 10.5 and 12, preferably between 10.8 and 12, preferably between 10.8 and 11.8, preferably between 11 and 11.8. It is possible not to heat the effluent at this step. In this case, the temperature of step c) of the process of the invention is preferably ambient temperature. It is also possible to heat the effluent to accelerate the gypsum precipitation process. In this case, the temperature of step c) of the process of the invention is ideally below 80°C, preferably below 60°C, to avoid any unnecessary energy expenditure.

[0110] As described above, some of the white gypsum recovered during step d) is preferably crushed and activated (by liming, as described above) for recycling in step c) of this process.

[0111] The grinding of the gypsum obtained during step d) will ideally produce gypsum particles with a particle size greater than 15 pm, preferably between 15 pm and 30 pm, preferably between 20 pm and 30 pm.

[0112] The process described above therefore preferably includes, after step d), an additional step of grinding the precipitated white gypsum separated in step d) to obtain a gypsum particle size greater than 15 µm, for example, between 15 µm and 30 µm, preferably between 20 µm and 30 µm, and optionally a step of activating the ground white gypsum by liming. This ground and possibly activated white gypsum is then added to the reactor in step c) to promote gypsum crystallization and the formation of crystals with a particle size greater than 15 µm, as explained previously. These steps are illustrated in Figure 4.

[0113] The process of the invention advantageously allows the recovery of more than 85% of magnesium hydroxide initially introduced in steps a) and b). This magnesium hydroxide can be reused in the process of the invention, or recovered in other industrial sectors.

[0114] The magnesium hydroxide recovered during step d) can also possibly be reused in steps a) and b) upstream of the process, depending on the pH targets to be achieved at the different stages of metal hydroxide extraction.

[0115] Any excess magnesium hydroxide recovered during step d) can also be thickened and concentrated on a belt filter or centrifuge, for external valorization.

[0116] All the details provided above concerning the means to be used to carry out steps aO), a) to d) of the process of the invention apply to steps aO), a) to d) of this embodiment and do not need to be repeated.

[0117] Similarly, the optional preliminary and final steps described above can be added at the beginning or end of this process. In particular, this process may further include, prior to step a) or a0), a step of increasing the pH of the effluent to a value of at least 1.

[0118] This process may further include a step e) in which CO2 is added to the basic solution from step d) to precipitate some of the calcium present in the aqueous solution and to lower the pH of the aqueous solution to a value between 4 and 11, and then a step f) of separating the limestone from the aqueous solution.

[0119] Finally, this process may also include a step f) in which hydronium ions combined with sulfate ions (for example, sulfuric acid (H2SO4) or acidic metal sulfates) are contacted with at least one of the metal hydroxides produced in steps a) and b), in order to generate a metal sulfate (MeSC) that can be used. For example, ferric iron hydroxide Fe(OH)3 can be contacted with FSC to produce ferric sulfate Fe2(SC>4)3, which can be used in water treatment or as an oxidizing agent.In addition to or instead of hydronium ions combined with sulfate ions, hydronium ions combined with chloride ions (Cl-) can be added (for example, by means of hydrochloric acid or acidic metal chlorides) to at least one of the metal hydroxides produced in steps a) and b), in order to produce metal chlorides and / or metal chlorosulfates such as ferric chlorosulfate (FeCISC), which can be used in various applications. The formation of these compounds depends on the reaction conditions, particularly the pH and the concentration of reactants. These conditions are well known to those skilled in the art.

[0120] According to a second preferred embodiment, the chemical agent added in steps a) and b) is ammonia (NH3) or an aqueous solution of ammonia or ammonium hydroxide (NH4OH).

[0121] The chemical reaction that occurs when ammonia is added to the effluent is as follows:

[0122] Me a (SO4)b + CNH3 + dH2O -> Me a (OH) c + (NH4) C (SO4)b where Me is a metal, and where a, b, c and d are natural numbers.

[0123] Adding ammonia to the effluent produces ammonium sulfate (NH4)2SO4. Upon the addition of calcium hydroxide Ca(OH)2, the compound (NH4)2SO4 is converted into dissolved ammonia and precipitated gypsum according to the following reaction:

[0124] (NH4)2SO4+ Ca(OH)2CaSO4.2H2O + 2NH3+

[0125] In this second preferred embodiment, the process of the invention comprises at least the following successive steps: a) Adding ammonia (NH3) or an ammonium (NH4OH) solution to said effluent, to increase the pH until it is between 1 and 9.5, resulting in the production and precipitation of at least one metal hydroxide present in the effluent, and separating the precipitated metal hydroxide from the effluent; b) Optionally, repeating step a) to achieve a pH between 1 and 9.5 higher than that chosen in step a) to produce and precipitate at least one metal hydroxide different from that obtained in step a); c) Adding lime to the remaining effluent to increase the pH until it reaches an optimal value between 10.2 and 12.5 to produce a basic aqueous solution comprising precipitated white gypsum; d) Separating the precipitated white gypsum from the basic aqueous solution formed in step c).

[0126] During steps a) and b), ammonia or ammonium hydroxide will be added to raise the effluent pH to the optimum pH for precipitation of iron hydroxides (ferrous or ferric), metal hydroxides of other metals, or co-precipitation of other metals in the solution. The optimum pH adjusted by the addition of ammonia or ammonium hydroxide will be between 1 and 9.5.

[0127] As explained above, this increase in pH can be carried out in one or more steps within each step a) and b), depending on the optimal precipitation pH of the metal hydroxides to be precipitated and the desired selectivity.

[0128] The lime added in step c) produces a basic aqueous solution comprising white gypsum (precipitated calcium sulfate CaSO4dihydrate uncontaminated by metals), and the ammonium ions present in the solution are converted into dissolved NH3 gas.

[0129] In this process, the addition of lime, and therefore the precipitation of gypsum, can be carried out via a high-density sludge process or equivalent. At the end of step d), the basic solution includes, in particular, water, dissolved NH3, and soluble calcium sulfate.

[0130] The ammonia NH3 formed during this reaction can, according to a particularly interesting preferred variant of the invention, undergo stripping to separate it from the effluent and purify the latter of this compound.

[0131] Thanks to this stripping step, the process of the invention advantageously allows the separation of more than 90%, preferably more than 95%, preferably more than 99% of the ammonia initially introduced in steps a) and b) from the aqueous solution. This ammonia can be recovered and reused in the process of the invention, or recovered in other industrial sectors.

[0132] According to a preferred embodiment of the invention, the ammonia NH3 recovered at the end of this stripping is then rerouted to the head of the process to be reused in step a) or b) of precipitation of the metals in the form of metal hydroxides.

[0133] Preferably, in this embodiment, the pH of step c) is between 10.2 and 12.5, preferably between 10.2 and 12, or even between 10.5 and 12. It can, for example, be set at 11.8. Preferably, the temperature of step c) of this process according to the invention is less than 80°C, 60°C or even 40°C (ambient temperature).

[0134] When the chemical agent used is ammonia or an ammonium solution, the stripping step required to separate the ammonia from the aqueous solution and the gypsum formed in step c) generates heat that can be recovered using a heat exchanger. This is particularly advantageous if the reaction medium is initially cold (e.g., below 10°C), as the conversion of NF to NH3 will not be optimal at a pH below 11. Heating the reaction medium to a temperature, for example, of 20°C to 30°C, increases the efficiency of the NF to NH3 conversion at a pH below 12.5, typically between 10.2 and 12.5 or between 10.2 and 12.

[0135] In a first, particularly preferred embodiment, the pH is increased in step a) to a pH between 5 and 7. At this pH, many metal hydroxides present in the solution, including aluminum hydroxide, are in solid form and can therefore be separated from the effluent. Then, the pH is increased in step b) to a pH between 8 and 9.5, so as to precipitate other metal hydroxides, notably ferrous iron hydroxide. Finally, the addition of lime at a pH between 10.2 and 12.5, for example between 10.2 and 12 (preferably 11.8), allows the production and precipitation of white gypsum, which can be reused in the process of the invention, or elsewhere.

[0136] In this first, particularly preferred embodiment, the process of the invention comprises the following successive steps: a) Adding ammonia (NH3) or an ammonium (NH4OH) solution to said effluent to raise the pH to between 5 and 7, resulting in the production and precipitation of at least one metal hydroxide present in the effluent (in particular, aluminum hydroxide), and separating the precipitated metal hydroxide from the effluent; b) Adding ammonia (NH3) or an ammonium (NH4OH) solution to the remaining effluent to adjust the pH to a value between 8 and 9.5, resulting in the production and precipitation of at least one metal hydroxide (in particular, ferrous iron hydroxide), and separating this from the remaining effluent; c) Adding lime to the remaining effluent to raise the pH to an optimal value between 10.2 and 12.5, or between 10.2 and 12, to produce an aqueous basic solution comprising precipitated white gypsum, d) Separate the precipitated white gypsum from the basic aqueous solution formed in step c).

[0137] Preferably, the pH of step a) is set at 6.5. At this pH, aluminum hydroxide is in solid form and can therefore be recovered by conventional separation methods.

[0138] Preferably, the pH of step b) is set at 9. At this pH, ferrous iron hydroxide is in solid form and can therefore be recovered by conventional separation means.

[0139] In a preferred variant, this process includes a final step d') consisting of separating the ammonia NH3 present in the basic solution from step d) by stripping.

[0140] In an even more preferred variant, the ammonia NH3 obtained in step d') is reintroduced into the effluent in steps a) and / or b) of the process.

[0141] In a second, particularly preferred embodiment, a preliminary oxidation step is carried out (for example, with dioxygen) to transform ferrous iron into ferric iron. Then, the pH is increased in step a) to a pH between 5 and 7. At this pH, many metal hydroxides present in the solution (notably ferric iron hydroxide and aluminum hydroxide) are in solid form and can therefore be separated from the effluent. Subsequently, the addition of lime at a pH between 10.2 and 12.5, for example, between 10.2 and 12 (preferably 11.8), allows the production and precipitation of white gypsum, which can be reused in the process of the invention, or elsewhere.

[0142] In this second, particularly preferred embodiment, the process of the invention comprises at least the following successive steps: a) Adding an oxidizing agent to said effluent, a) Adding ammonia (NH3) or an ammonium (NH4OH) solution to said effluent, to increase the pH until it is between 4 and 9.5, resulting in the production and precipitation of the metal hydroxide present in the effluent (in particular ferric iron hydroxide and aluminum hydroxide), and separating the precipitated metal hydroxide from the effluent, b) Optionally, repeating step a) to achieve a pH between 4 and 9.5 higher than that chosen in step a), c) Adding lime to the remaining effluent to increase the pH until an optimal value of between 10.2 and 12.5 is reached, preferably between 10.2 and 12, to produce an aqueous basic solution comprising precipitated white gypsum, d) Separate the precipitated white gypsum from the basic aqueous solution formed in step c).

[0143] Preferably, the pH of step a) is set at 6.5. At this pH, ferric iron hydroxide and aluminum hydroxide are in solid form and will precipitate and thus be recovered as a mixture.

[0144] Intermediate separation steps can be carried out, for example at a pH between 4 and 5, to separate the ferric iron hydroxide that precipitates at pH 4.2 and prevent it from mixing with the aluminum hydroxide. If it is desired to separate other metal hydroxides, intermediate separation steps can be performed at a pH below 4 or above 7.

[0145] All the details provided above concerning the means to be used to carry out steps aO), a) to d) of the process of the invention apply to steps aO), a) to d) of this embodiment and do not need to be repeated. Similarly, the optional preliminary and final steps described above may be added at the beginning or end of these processes.

[0146] In particular, these processes may also include, prior to step a) or aO), a step of increasing the pH of the effluent to a value of at least 1.

[0147] These processes may further include a step e) in which CO2 is added to the basic solution from step d) to precipitate some of the calcium present in the aqueous solution as limestone and to lower its pH to a value between 4 and 11, and then a step f) of separating said limestone from the solution.

[0148] Finally, this process may also include a step f) in which hydronium ions combined with sulfate ions (for example, sulfuric acid (H2SO4) or acidic metal sulfates) are contacted with at least one of the metal hydroxides produced in steps a) and b), in order to generate a metal sulfate (MeSC) that can be used. For example, ferric iron hydroxide Fe(OH)3 can be contacted with FSC to produce ferric sulfate Fe2(SC>4)3, which can be used in water treatment or as an oxidizing agent.In addition to or instead of the hydronium ions combined with sulfate ions proposed above, hydronium ions combined with chloride ions (Cl-) can be added (for example, by means of hydrochloric acid or acidic metal chlorides) to at least one of the metal hydroxides produced in steps a) and b), in order to produce metal chlorides and / or metal chlorosulfates such as ferric chlorosulfate (FeCISC), which can be used in various applications. The formation of these compounds depends on the reaction conditions, particularly the pH and the concentration of reactants. These conditions are well known to those skilled in the art.

[0149] BRIEF DESCRIPTION OF THE DRAWINGS

[0150] Figure 1 describes an example of a process according to a first embodiment of the invention. This process involves the addition of magnesium hydroxide and C>2 to the effluent to be treated.

[0151] Figure 2 illustrates an example of a process according to a second embodiment of the invention. This process involves the addition of NH3 or NH4OH to produce and precipitate the metal hydroxides. In this example, no dioxygen is added to a first reactor. The ammonia after stripping is advantageously reused at the beginning of the process in the first reactor.

[0152] Figure 3 describes a variant of the process in Figure 2, in which 1'O2 was introduced into the first reactor to oxidize metal ions such as Fe 2+ The ammonia after stripping can be reused at the beginning of the process in the first reactor.

[0153] Figure 4 describes the concept of gypsum precipitation in step c) of the process of the invention of Figure 1, in the presence of calibrated gypsum seeds.

[0154] Figure 5 describes the concept of gypsum precipitation in step c) of the process of the invention of Figure 2 or Figure 3, in the presence of calibrated gypsum seeds.

[0155] Figure 6 describes one of the preferred processes according to the invention, in which Mg(OH)2 is used as an alkaline agent to treat an effluent rich in metal sulfates, characterized in that the Mg(OH)2 is recycled while several valuable products of interest are advantageously formed: metal hydroxides, iron chlorosulfate, aluminum sulfate and limestone.

[0156] EXAMPLES

[0157] To better illustrate the process of the present invention, three different embodiments are given (Examples 1-3 and Figures 1-3). It is understood that these are merely examples and are not intended to be limiting. Furthermore, Examples 4 and 5 refer to Figures 4 to 5, which illustrate specific steps that can be added to each of the three processes according to the invention.

[0158] Example 1 (Figure 1): Process of the invention with Mq(OH)2 as the alkali agent

[0159] A first embodiment of the invention is schematically described in Figure 1.

[0160] In this example, the effluent to be treated contains 65 g / L of SC, 12 g / L of Fe(II), and 0.4 g / L of Aluminum. Its pH is 0.6, and its temperature is 20°C. The effluent is directed to a first reactor 1 equipped with an agitator, into which magnesium hydroxide is also added while the pH is controlled to maintain a value of approximately 4.2. Oxygen is then added.

[0161] The effluent exiting this first reactor 1 is directed to a first filter press 2 in order to separate the precipitates of ferric iron hydroxide Fe(OH)3 formed in the first reactor 1.

[0162] The remaining liquid is directed to a second reactor 3 also equipped with an agitator in which magnesium hydroxide is again added while controlling the pH to maintain it at a value of approximately 6.5.

[0163] The effluent exiting the second reactor 3 is directed to a second filter press 4 in order to isolate the aluminium hydroxide precipitates Al(OH)a formed in the second reactor 3.

[0164] The remaining liquid is directed to a third reactor 5, called the "gypsum precipitation reactor", also equipped with an agitator, in which lime is added while controlling the pH to maintain it at a value of approximately 11.8.

[0165] The effluent exiting the third reactor 5 is directed to a solids separator 6 in order to separate the magnesium hydroxide particles from the gypsum particles.

[0166] More than 85% of the magnesium hydroxide initially introduced in steps a) and b) can thus be recovered and reintroduced into the first and second reactors.

[0167] The liquid exiting separator 6 is then filtered on a third filter press 7 to separate the valuable white gypsum and remove any solid residue present in the treated effluent.

[0168] CO2 can be added to the liquid exiting the filter press 7 to desaturate the liquid with calcium sulfate (CaSC₂), precipitating some of the calcium present in the aqueous solution as limestone (CaCO₃). The limestone thus produced is separated from the solution using conventional methods 8 such as an HDS-type thickening decanter, an Actina-type contact reactor (pellet reactor) or equivalent, or a Saphira contact reactor or equivalent. A filter 9 can be added if necessary.

[0169] The water recovered from this process has a pH of 9 and contains less than 2 g / L of SO4, less than 0.5 mg / L of iron, and less than 0.5 mg / L of aluminum, and can therefore be discharged into the environment without risk of pollution. The iron and aluminum hydroxides produced in this process are advantageously contacted with sulfuric acid (H2SO4) or hydrochloric acid (HCl) to produce ferric sulfate and aluminum sulfate, or ferric chloride and aluminum chloride, respectively. Alternatively, these iron / aluminum hydroxides can be contacted with sulfuric acid and hydrochloric acid to produce ferric chlorosulfate and aluminum chlorosulfate.

[0170] In the process shown in Figure 1, Mg(OH)2 can be replaced by MgO or MgCOs. All other steps of the process, as described above, will then be identical to those shown here with Mg(OH)2 as the alkali metal.

[0171] It is possible to predict the quantity and yield of the various valuable and / or recyclable products that will be obtained by implementing this type of process by thermodynamic simulation using, for example, the Phreeqc software (https: / / www.usgs.gov / software / phreeqc-version-3).

[0172] In this example, the calculations were performed using the Phreeqc software.

[0173] The calculations were based on a stoichiometry of the reactions, without taking into account reaction times, which can range from a few minutes to several hours. They also do not consider the risk of salt inclusion or the performance of solid phase separation.

[0174] To perform these simulations, it was assumed that the incoming flow to the first reactor 1 contained the following chemical elements:

[0175] ■ Al: 40 kg / hr

[0176] ■ Fe: 1160 kg / hr

[0177] ■ SO4: 6501 kg / hr

[0178] Furthermore, the reagents required to carry out this type of process and the associated yields are as follows:

[0179] 1.1. Process 1 (Mg(OH)2) with formation of Mg(OH)2, valuable metal hydroxides or metal sulfates

[0180] 1.2. Process 1 (Mg(OH)2) with formation of Mg(OH)2, valuable metal hydroxides or metal chlorides

[0181] 1.3. Process 1 (Mg(OH)2) with recycling of Mg(OH)2 and formation of valuable metal hydroxides and metal sulfates 1.4. Process 1 (Mg(OH)2) with recycling of Mg(OH)2 and formation of valuable metal hydroxides, iron chlorosulfate and aluminum sulfate

[0182] 1.5. Process 1 (Mg(OH)2) with recycling of Mg(OH)2 and formation of valuable metal hydroxides, iron chlorosulfate, aluminum sulfate and limestone

[0183] In this process, the sulfate level was reduced by 92.4% (from 65000 mg / L to 1625 mg / L) and the total dissolved solids (TDS) was reduced by 92.3% (from 82309 mg / L to 6273 mg / L).

[0184] This particular process is described in Figure 6. The precise quantities and parameters of each step 1 to 26 are transcribed below:

[0185] When Mg(OH)2 is replaced by MgO in the first reactor, the reagents required to carry out this type of process and the associated yields are as follows:

[0186] 1.6. Process 1 (MgO) with formation of Mg(OH)2 and valuable metal sulfates 1.7. Process 1 (MgO) with formation of Mg(OH)2 and valuable metal chlorides

[0187] 1.8. Process 1 (MgO) with recycling of Mg(OH)2 and formation of valuable metal hydroxides, iron chlorosulfate and aluminum sulfate 1.9. Process 1 (MgO) with recycling of Mg(OH)2 and formation of valuable metal hydroxides and metal sulfates

[0188] When Mg(OH)2 is replaced by MgCOs in the first reactor, the reagents required to carry out this type of process and the associated yields are as follows:

[0189] 1.10. Process 1 (MgCO3) with formation of Mg(OH)2, valuable metal hydroxides and metal sulfates Example 2 (Figure 2): The process of the invention using ammonia as the alkaline agent, and with two precipitation / filtration steps

[0190] A second embodiment of the invention is schematically described in Figure 2.

[0191] In this example, the effluent to be treated contains 65 g / L of SC, 11.6 g / L of Fe(II) and 0.4 g / L of Aluminium. Its pH is 0.6, its temperature is 20°C.

[0192] The effluent is directed to a first reactor 21 equipped with an agitator in which ammonia is also added while controlling the pH to maintain it at a value of approximately 6.5.

[0193] The effluent exiting this first reactor 21 is directed to a first filter press 22 in order to isolate the precipitates of aluminium hydroxide Al(OH)3 formed in the first reactor 21.

[0194] The remaining liquid is directed to a second reactor 23 also equipped with an agitator into which ammonia is again added while controlling the pH to maintain it at a value of about 9.

[0195] The effluent exiting this second reactor 23 is directed to a second filter press 24 in order to isolate the ferrous iron hydroxide precipitates Fe(OH)2 formed in the second reactor 23.

[0196] The remaining liquid is directed to a third reactor 25 called the "gypsum precipitation reactor", also equipped with an agitator, in which lime is added while controlling the pH to maintain it at a value of about 11.8.

[0197] The effluent exiting this third reactor 25 is then directed to a third filter press 26 to separate the solid gypsum precipitated in the third reactor 25 and remove the solid residues from the treated effluent.

[0198] The ammonia still contained in the effluent is stripped in order to be recycled in reactors 21 and 23. Thanks to this stripping step, more than 98% of the ammonia initially introduced in steps a) and b) can be recovered and reused.

[0199] CO2 can be added to the liquid exiting the third filter press 26 to desaturate the calcium sulfate (CaSC₂), precipitating the calcium present in the aqueous solution as limestone (CaCO₃). The limestone thus produced is separated from the solution using conventional methods 27 such as an HDS-type thickening decanter, an Actina-type contact reactor (pellet reactor) or equivalent, or a Saphira contact reactor.

[0200] The water recovered at the end of this process has a pH of 9, and contains less than 2g / L of SO4, less than 0.5mg / L of Iron, and less than 0.5 mg / L of Aluminium and can therefore be discharged into natural environments without risk of pollution.

[0201] In this process, a heat exchanger 28 can optionally be introduced before the third reactor 25 in order to heat the effluent entering it and maximize the conversion of NF to NH3 at pH 11.8.

[0202] The iron and aluminum hydroxides produced in this process are advantageously contacted with sulfuric acid (H₂SO₄) or hydrochloric acid (HCl) to produce ferric sulfates and aluminum sulfates, or ferric chlorides and aluminum chlorides, respectively. Alternatively, these iron / aluminum hydroxides can be contacted with sulfuric acid and hydrochloric acid to produce ferric chlorosulfates and aluminum chlorosulfates.

[0203] It is possible to predict the quantity and yield of the various valuable and / or recyclable products that will be obtained by implementing this type of process by thermodynamic simulation using, for example, the Phreeqc software (https: / / www.usgs.gov / software / phreeqc-version-3).

[0204] In this example, the calculations were performed starting from an effluent containing:

[0205] The calculations were based on a stoichiometry of the reactions, without taking into account reaction times, which can range from a few minutes to several hours. They also do not consider the risk of salt inclusion or the performance of solid phase separation.

[0206] To perform these simulations, it was assumed that the incoming flow to the first reactor 1 contained the following chemical elements:

[0207] ■ Al: 40 kg / hr

[0208] ■ Fe: 1160 kg / hr

[0209] ■ SO4: 6501 kg / hr The reagents required to carry out this process and the associated yield are as follows:

[0210] Example 3 (figure 3): process of the invention with ammonia as an alkaline agent, and with a single precipitation / filtration step. A third embodiment of the invention is schematically described in figure 3.

[0211] In this example, the effluent to be treated contains 65 g / L of SO4, 11.6 g / L of Fe(II) and 0.4 g / L of Aluminium. Its pH is 0.6, its temperature is 20°C.

[0212] The effluent is directed to a first reactor 31 equipped with an agitator in which ammonia is added while controlling the pH to maintain it at a value of approximately 6.5. Dioxygen is added.

[0213] The effluent exiting this first reactor is directed to a first filter press 32 in order to isolate the precipitates of ferric iron hydroxide Fe(OH)3 and aluminium (Al(OH)3 formed in the first reactor.

[0214] The remaining liquid is directed to a second reactor 33 called the "gypsum precipitation reactor" also equipped with an agitator, in which lime is added while controlling the pH to maintain it at a value of about 11.8.

[0215] The effluent exiting this second reactor 33 is directed to a second filter press 34 to remove any residual solid gypsum precipitated in the second reactor 33 and present in the treated effluent. The ammonia still contained in the effluent is stripped for recycling into the first reactor 31. Thanks to this stripping step, more than 98% of the ammonia initially introduced in step a) can be recovered and reused.

[0216] CO2 can be added to the liquid exiting the filter press 34 to desaturate the calcium sulfate (CaSC₂), precipitating the calcium present in the aqueous solution as limestone (CaCO₃). The limestone thus produced is separated from the solution using conventional methods 35 such as an HDS-type thickening clarifier, an Actina-type contact reactor (pellet reactor) or equivalent, or a Saphira contact reactor. The water recovered from this process has a pH of 9 and contains less than 2 g / L of SO₄, less than 0.5 mg / L of iron, and less than 0.5 mg / L of aluminum, and can therefore be discharged into the environment without risk of pollution.

[0217] In this process, a heat exchanger 36 can optionally be introduced before the second reactor 33 in order to heat the effluent entering it and maximize the conversion of NF to NH3 at pH 11.8.

[0218] Example 4 (Figure 4): Gypsum recycling step to optimize the process of Example 1

[0219] Figure 4 describes an optional step of recycling some of the gypsum obtained in the process of the invention of Example 1 in order to facilitate the formation of gypsum in the precipitation reactor and the separation of the two solids formed there.

[0220] In this optional step, the precipitated white gypsum obtained in the process of Example 1 can be treated for reintroduction into the third reactor 45 (gypsum precipitation reactor) and to promote the growth of new gypsum in it.

[0221] A portion of the white gypsum isolated in the process after separation 46 is then ground in step 41. The ground gypsum then undergoes classification in step 42 to obtain a suspension of gypsum nuclei with a calibrated particle size of approximately 15 to 30 µm. These gypsum nuclei are added via pipe 43 to reactor 45, which contains the effluent from the second metal hydroxide separation filter. Lime can be added to the suspension of calibrated gypsum nuclei to enhance its effectiveness when introduced into reactor 45, where CaSO4·2H2O and magnesium hydroxide are precipitated. Gypsum grains with a particle size of less than 15 pm, separated from the calibrated grains in 42, are not introduced into reactor 45 but are used outside the process of invention 44.

[0222] Most of the white gypsum obtained after separation 46 can also be directly recovered without being recycled in the process, after possibly being filtered again 49.

[0223] Magnesium hydroxide after separation 47 is either reused in the first and second reactors 1 and 3 of Figure 1 to adjust the pH, or is used in another way 48.

[0224] The treated effluent is then conveyed to the CO2 addition stage and reactor 8 in Figure 1.

[0225] Example 5 (figure 5): gypsum recycling step to optimize the process of examples 2 and 3.

[0226] Figure 5 describes an optional step of recycling some of the gypsum obtained in the process of the invention of example 2 or 3 in order to facilitate the formation of gypsum in the precipitation reactor.

[0227] In this optional step, the precipitated white gypsum obtained in the process of Example 2 or 3 can be treated for reintroduction into the second reactor 33 or third reactor 25 (gypsum precipitation reactors), respectively, to promote the growth of new gypsum in them. These gypsum precipitation reactors correspond to reactor 55 mentioned below.

[0228] A portion of the gypsum isolated in the process after separation 56 is ground in step 51. The ground gypsum then undergoes classification in step 52 to obtain a suspension of gypsum nuclei with a calibrated particle size of approximately 15 to 30 µm, which is added via 53 to reactor 55 containing the effluent from the metal hydroxide separation filter presses. Lime can be added to the suspension of calibrated gypsum nuclei to enhance its effectiveness when introduced into reactor 55, in which CaSd₂FO₄ is precipitated and NF₄ is converted to gaseous NH₃. Gypsum grains with a particle size of less than 15 pm, separated from the calibrated grains in 52, are not introduced into the reactor 55 but are used outside the process of the invention 54. Most of the white gypsum obtained after separation 56 can also be used directly without being recycled in the process, after possibly being filtered again 59.

[0229] The ammonia produced in the gypsum precipitation reactor 55 is either reused in the first and / or second reactors to adjust the pH and precipitate the metal hydroxides, or recovered in another way 57.

[0230] An optional heat input can be provided upstream of reactor 55 to facilitate the conversion of NF to NH3 at pH 11.8 in reactor 55.

[0231] In this case, a heat exchanger 28 from Fig. 2 or 36 from Fig. 3 can be added.

[0232] The heat exchanger 50 in Fig. 5 can be used to cool the effluent heated by ammonia stripping, as needed, to comply with the permitted effluent discharge temperature, on a case-by-case basis. The treated effluent is then conveyed to the CO2 addition stage and the CaCO3 precipitation reactor 27 or 35, shown in Figures 2 or 3.

[0233] The heat exchanger 50 of Fig.5 can be positioned as described in Figure 5 or downstream of reactor 27 or 35 of Figures 2 or 3, depending on the desired CaCOs precipitation efficiency.

[0234] Example 6: Pre-neutralization step of the effluent by adding red gypsum or blue gypsum prior to the process of the invention.

[0235] The effluent to be treated contains 65 g / L of SO4, 11.6 g / L of Fe (II) and 0.4 g / L of Aluminium. Its pH is 0.6, its temperature is 20°C.

[0236] Red gypsum (pH 9, 20°C) containing 700 g / L of solids (80% CaSO4.2H2O and 20% ferrihydrite (Fe(OH)s) is added to the starting effluent, before step a) of the process of the invention.

[0237] The effluent entering this process therefore contains 61.4 g / L of SO4, 10.6 g / L of Fe(II), 6 g / L of Fe(III), 41 g / L of red gypsum and 0.4 g / L of Aluminium. Its pH is approximately 1, and its temperature is 20°C.

[0238] A first filter press is used to separate the gypsum added to this effluent before it is introduced into the first reactor 1, 21, 31 of the processes of examples 1 to 3.

Claims

DEMANDS 1. A process for treating an effluent, said effluent comprising at least one metal sulfate, said process comprising at least the following successive steps: a) Adding a chemical agent to said effluent, said chemical agent being selected from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), and magnesium carbonate (MgCOs), to adjust the pH to a value between 1 and 9.5, resulting in the production and precipitation of at least one metal hydroxide, and separating the precipitated metal hydroxide from the effluent; b) Optionally, repeating step a) to achieve a pH between 1 and 9.5 higher than that selected in step a); c) Adding lime to the remaining effluent to raise the pH to an optimal value between 10.2 and 12.5, so as to produce an aqueous basic solution comprising precipitated white gypsum, d) Separate the precipitated white gypsum from the basic solution produced in step c) so as to produce a purified aqueous basic solution.

2. A method according to claim 1, comprising, prior to step a), a step of increasing the pH of the effluent to a value of at least 1.

3. A process according to any one of claims 1 or 2, comprising, prior to or during step a), an oxidation step aO) carried out by adding at least one oxidizing agent to the effluent.

4. A process according to any one of claims 1 to 3, comprising a step e) in which CO2 is added to the basic solution from step d) to precipitate some of the calcium present in the aqueous solution and to lower its pH to a value between 4 and 11, and then a step f) consisting of separating the limestone from the solution.

5. A process according to any one of claims 1 to 4, comprising an additional step of separating the white gypsum and magnesium hydroxide precipitated during step c).

6. A process according to claim 5, comprising an additional step of grinding the precipitated white gypsum separated during this additional step, to obtain a calibrated particle size of gypsum greater than 15 pm, and optionally a step of activating the white gypsum thus ground by liming, and the addition of this ground and optionally activated white gypsum to the effluent during step c).

7. A process according to any one of claims 1 to 6, comprising an additional step of adding hydronium ions combined with sulfate ions and / or hydronium ions combined with chloride ions to at least one metal hydroxide produced in said step a) or b), to produce at least one metal sulfate, at least one metal chloride and / or at least one metal chlorosulfate.

8. A process according to any one of claims 1 to 7, comprising the following successive steps: a) Adding an oxidizing agent to said effluent, a) Adding a solution of magnesium oxide (MgO), magnesium carbonate (MgCOs), or magnesium hydroxide (Mg(OH)2) to the effluent after step a) to raise its pH to a value between 4 and 4.5, resulting in the production and precipitation of at least one metal hydroxide present in the effluent, and separating the precipitated metal hydroxide from the effluent, b) Adding a solution of magnesium oxide (MgO), magnesium carbonate (MgCOs), or magnesium hydroxide (Mg(OH)2) to the remainder of the effluent to raise its pH to a pH between 5.5 and 7.5, resulting in the production and precipitation of at least one metal hydroxide, and separating this from the remaining effluent. c) In the remaining effluent, add lime to raise the pH to an optimal value between 10.2 and 12.5, producing an aqueous basic solution comprising white gypsum and precipitated magnesium hydroxide, d) Separate the precipitated white gypsum and precipitated magnesium hydroxide from the aqueous basic solution from step c).

9. Process according to claim 8, wherein the magnesium hydroxide recovered during step d) is reused in steps a) and b) upstream of the process or otherwise recovered.

10. A process according to any one of claims 8 and 9, comprising a step e) in which CO2 is added to the basic aqueous solution from step d) to precipitate some of the calcium present in the aqueous solution as limestone and to lower its pH to a value between 4 and 11, and then a step f) consisting of separating said limestone from the solution.

11. A process according to any one of claims 8 to 10, comprising an additional step of adding hydronium ions combined with sulfate ions and / or hydronium ions combined with chloride ions to at least one metal hydroxide produced in said step a) or b), to produce at least one metal sulfate, at least one metal chloride and / or at least one metal chlorosulfate.

12. A method according to any one of claims 8 to 11, comprising, prior to step a) or a0), a step of increasing the pH of the effluent to a value of at least 1.

13. A method according to any one of claims 1 to 12, wherein in step c), the temperature of the effluent remains below 60°C.

14. A method according to any one of claims 1 to 13, wherein in step c), the effluent remains at ambient temperature.

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