Effluent treatment process for effluents containing metallic sulfates

Abstract

The present invention relates to a method for treating an effluent comprising at least one metal sulfate, said method comprising at least the following successive steps: Adding a chemical alkaline agent to said effluent, said chemical agent being chosen from: magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), 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), Optionally, repeating step a) to achieve a pH between 1 and 9.5 higher than that chosen in step a), In the remaining effluent, adding lime to raise the pH to an optimal value between 10.5 and 12, depending on the effluent temperature (ambient or controlled) and the alkaline agent(s) added in steps a) and possibly b), to produce a basic solution containing precipitated white gypsum. Separate the precipitated white gypsum from the basic solution in step c) to produce a purified basic aqueous solution. This process yields high-purity white gypsum, selectively or mixedly precipitated metal hydroxides, and purified water that can be discharged into the receiving environment, the wastewater system, and / or recycled.

Description

Title of the invention: Process for treating effluents containing metallic sulfates. Technical field

[0001] Industrial effluents containing metal sulfates must undergo treatment before being discharged into the environment, the sewage 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 in order to significantly reduce the amount of final residues requiring landfill disposal. PREVIOUS TECHNOLOGY

[0002] On a global scale, wastewater treatment is a public health issue. Wastewater is any 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 various 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, and which have negative effects on human health at excessive concentrations.

[0003] Numerous solutions have been developed to separate metals from solutions containing sulfuric acid while reducing sulfate concentrations before discharge or reuse: - 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). - By adsorption: this technique uses activated carbon or ion exchange resins to remove metal ions from wastewater by adsorption. - By electrocoagulation: in this technique, an electric current is used to precipitate the 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. - 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. - 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. - By advanced oxidation treatment (including the use of ozone, UV or Fenton processes (hydrogen and iron peroxide)).

[0004] 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.

[0005] A combined treatment incorporating different methods can often be the most effective and economical solution for treating wastewater loaded with metallic sulfates.

[0006] The most commonly used 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, particularly gypsum (calcium sulfate dihydrate, CaSO4-2H2O), which are then separated by sedimentation (High-Density Sludge (HDS) process) or filtration.

[0007] Gypsum is a by-product of the reaction of sulfuric acid or other sulfates present in wastewater, in the presence of lime.

[0008] 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, etc.) from the metal sulfates (Mem(SO4)n) in the form of metal hydroxides (Me(OH)c). These metal hydroxides, which are obtained mixed with gypsum, pose two problems for recovery. 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 raw state, by the metal hydroxides. Pure gypsum can compromise its usability, 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 disposed of in a landfill. Therefore, the pollution is simply shifted from a liquid effluent to a solid residue.

[0009] 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 makes it possible to produce water of the desired quality, for example water compatible with the environmental standards in force locally, particularly in France.

[0010] 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 which can be separated and reused in order to significantly reduce the amount of final residues that must be sent to landfill.

[0011] These various objectives are achieved by means of 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), 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, in order to be potentially recycled in the chemical industry.

[0012] The alkaline chemical agent(s) and the pH(s) at which the metal hydroxides precipitate will be judiciously chosen according to the metals present in the effluent. Certain metal ions (Zn2+, Cu2+, Co2+, Ni2+, etc.) 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 alkaline agents—magnesium oxide (MgO) 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.

[0013] 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 (CaSO4-2H2O) 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 recycling. Unlike known prior art processes, this step is carried out at a pH between 10.5 and 12, depending on the temperature of the effluent, ambient or controlled, and depending on the alkaline agent(s) added to precipitate the metals in the form of hydroxides, in order to allow simultaneously the formation of gypsum (solid), the formation of magnesium hydroxide (solid) and / or the conversion of the ammonium ion (NH4+) into dissolved ammonia gas (NH3), which will be selectively separated.

[0014] The present invention therefore proposes a process for treating effluents containing at least one metallic sulfate, said process comprising the steps of:

[0015] - Add to said effluent an alkaline chemical agent chosen from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), 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 selectively precipitate the metal hydroxide(s) formed without precipitating gypsum, then separate it / them from the effluent so that it / they can / them be recycled,

[0016] - If necessary, repeat this step one or more times depending on the precipitation targeted selective removal 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) and magnesium hydroxide (Mg(OH)2), ammonia (NH3), ammonium hydroxide (NH4OH), to achieve 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,

[0017] - Once the metal hydroxide(s) has / have been separated from the effluent, Add lime to raise the pH to a value between 10.5 and 12, depending on the effluent temperature (ambient or controlled) and the alkaline agent(s) used to precipitate metals as hydroxides, to transform sulfate ions present in the effluent into solid calcium sulfate dihydrate (CaSO4·2H2O) that is uncontaminated or slightly contaminated by metals (i.e., usable white gypsum), and to precipitate magnesium (Mg2+) present in the aqueous solution, in the form of magnesium hydroxide (Mg(OH)2) and / or to convert the ammonium (NH4+) present in the aqueous solution, into dissolved gaseous ammonia (NH3).

[0018] - Separate the gypsum that has precipitated from the aqueous solution thus obtained and possibly recycle it in part in the process of the invention and / or valorize it.

[0019] - Separating magnesium hydroxide (Mg(OH)2) and / or gaseous ammonia (NH3) dissolved and possibly recycle it or them in the process of the invention and / or valorize it or them.

[0020] In one embodiment of the invention, carbon dioxide can be added to the resulting aqueous solution to desaturate it with soluble calcium sulfate (CaSO4) 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 be separated from the solution by conventional means to obtain clarified water and can optionally be recycled.

[0021] 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.

[0022] Thanks to the process of the invention, gypsum is recovered predominantly in the form of white gypsum, the purity of which (CaSO4,2H2O) is sought to be high (> 50%), or more broadly as gypsum that can be used in various types of industry, particularly in the cement or plaster industry. The process according to the invention also makes it possible to obtain selectively precipitated or mixed metallic hydroxides, depending on the potentially targeted selective precipitation, which can also be used, as well as purified water that can be discharged into the receiving environment, the wastewater system and / or recycled. Description of the invention

[0023] According to a first aspect, the present invention relates to a process for treating an effluent comprising at least one metallic sulfate, said process comprising at least the following successive steps: a. Add an alkaline chemical agent to said effluent, said chemical agent being chosen from: magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), 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 separate from the effluent the solids containing the precipitated metal hydroxide(s), b. Optionally, repeat step a) to achieve a pH between 1 and 9.5 higher than that chosen in step a), c. In the remaining effluent, add lime to raise the pH to an optimal value between 10.5 and 12, depending on the effluent temperature, ambient or controlled, and 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 white gypsum precipitated in step c) from the basic solution so as to produce a purified basic aqueous solution.

[0024] This process is applicable 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 (“fine gas desulfurization” or FGD)) - this list is not exhaustive.

[0025] 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). In particular, they contain high concentrations of metal sulfate(s) such as copper(II) sulfate (CuSO4), iron(II) sulfate (FeSO4), zinc sulfate (ZnSO4), aluminum sulfate (Al₂SO₄), magnesium sulfate (MgSO₄), nickel sulfate (NiSO₄), manganese sulfate (MnSO₄), 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.

[0026] 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:

[0027] [Tables 1] Hypothesis simulations Noble acids AN Secondary acids AN + AS Flow rates LLL: 11111'OIIIBI lll / ïwlll pH (simulation) / 11 / 0, Ss / LL 1.4 1110 / 6111 MES mg / L 1.00 20000 12040 Sulfates mg / L 125000 25 000 65000 Çhlowes <100 4140 2524 3 / / / / / / / / / 111111111 / .X1111 / / / / / / / / Calcium mg / L 50 / 1111 / 5111111 50 Magnesium 50 / lllll / ^so / llllll / 50 Sodium WgO / / / / <1352 <1352 <1352 / mg / L / 11 <80 <00 11 / / <80111 / Iron to tal mg / L 14000 10000 11600 Iron (il) 14000 10 000 .ii^illB / ii Ee (lin ^mg / LslL llloilll 0 llliollll Aluminum mg / L <700 <200 <400

[0028] 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.

[0029] 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.

[0030] 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 selectivity required 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 present 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 present in the effluent - this list is not exhaustive.

[0031] [Tables2] 7 ' fe (HJ wrlpâle (3y3y(â: <?p^(<(123y3<<<3 <2 fr^'L àpH Ô 3 (3 <^ 33(:( <0.01 OfigrL(â:0:W^^^ (777((3O$^ (((((740:0:43(((( 0,1 thcpL â pH 4x2 Amt'tï'ifi ium S.tî •£ pH < ?,:>:> 74ÔO jYïrj / LO <'3 rn-æ'L pH S < 0,1 mgJL à pH ù * 65 < 0,3 rTKp'L apH Z.5 WÇW 3 :llX:pH(< 3377:<(ÈOiW^Oiï ((000040044: •ÿ : • 37:((7(31313^0^^^ Ms ■ ■ ■ ■ (( ( 7((((((((((^^(:(((((((((((((((((((((((((^^( ((((OiïktVl^33 ((((((^7p4ya (((((< P ü.^'L ctpH G < 2 trapL à pH $ <7(3:0 3 (7'7:73:O^ (33((:3((0(0.0^ 3(3<(L2(^ Csj-pr^i'îpiiaw^f! (::::::::::^7:-:4::::::::::::::::::::::) nxjZL à pH g :333(Q 7::3:(37fôù$p(fâ^ 37:3(37(0;$^ (:((((77:S0040(:((s Cw 00 ' ph. L :((((((u: '< pH < 12 < 3 pOÇp'L à pH ? 055 mcuL à pH 8

[0032] It is known that ferric iron hydroxide precipitates optimally at a pH of approximately 4.5. The process of the invention therefore makes it possible, when the chosen pH is between 4 and 5, to separate and recover any ferric iron hydroxide that may be 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 so as to obtain precipitated ferric iron hydroxide and a step for separating the precipitated ferric iron hydroxide from the rest of the effluent.

[0033] 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 makes it possible, when the chosen pH is between 5.5 and 7.5, to precipitate and separate any aluminum hydroxide that may be 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.

[0034] It is known that ferrous iron hydroxide precipitates significantly at a pH above 9, and maximally at a pH between 10 and 12. The process of the invention therefore makes it possible, when the chosen pH is between 9 and 9.5, to precipitate and significantly separate the ferrous iron hydroxide possibly present in the effluent, within a pH range compatible with optimal use of alkaline agents. Magnesium or ammonia. Preferably, the process of the invention includes a specific step of adjusting the pH to a value between 9 and 9.5 so as to obtain a precipitated ferrous iron hydroxide and a step of separating the precipitated ferrous iron hydroxide from the rest of the effluent.

[0035] 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.

[0036] If a prior oxidation step is carried out, and if it is desirable to obtain aluminum hydroxide not mixed 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.

[0037] Thus, the 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 the precipitation of this or these metal hydroxide(s) to be precipitated and the desired selectivity (in particular if one wishes to isolate a particular metal hydroxide or a mixture of several metal hydroxides).

[0038] The separation or, conversely, the co-precipitation of several metal hydroxides present in the starting 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 needs, 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).

[0039] 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), ammonia (NH3), and ammonium hydroxide (NH4OH), to adjust the pH to a value between 1 and 7.5, resulting in the production and the precipitation of at least one metallic hydroxide, and separating the precipitated metallic hydroxide(s) from the effluent, b. To the remainder of the effluent, add a chemical agent selected from magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), 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 metal hydroxide, preferably different from that obtained in step a), and separate this from the remainder of the effluent, c. In the remaining effluent, add lime to raise the pH to an optimal value between 10.5 and 12, depending on the effluent temperature (ambient or controlled) and the alkaline agent(s) added in steps a) and possibly b), so as to produce a basic aqueous solution containing 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.

[0040] In a preferred embodiment, the pH of step a) is between 5.5 and 7.5 to produce and recover aluminum hydroxide and the pH of step b) is between 9 and 9.5 to produce and recover ferrous iron hydroxide.

[0041] 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.

[0042] Filtration, typically by filter press, allows the filtrate, comprising water and still-dissolved metal hydroxides, to be separated from the precipitated hydroxides, which are then 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 being filtered.

[0043] The metal hydroxides precipitated during steps a) and / or b) can advantageously be dehydrated by conventional means (for example a filter press), before possibly being reused in industry.

[0044] 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 Mg2+ ions to Mg(OH)2 and / or the conversion of NH4+ ions into dissolved NH3 gas. The maximum precipitation of Mg(OH)2 and / or the maximum conversion of NH4+ to NH3 will be sought.

[0045] 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.

[0046] 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 the E313-20 standard 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 / b9c8874dl78b4960801bl9d6e464b85d / ASTM-E313-20.pdf) or equivalent.

[0047] Furthermore, the "usable white gypsum" produced by the process of the invention is preferably 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.

[0048] When the temperature of the reaction medium is cold, for example below 10 °C, the conversion of more than 99% of NH4+ 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 NH4+ 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.

[0049] In a particular embodiment, when the alkaline agent 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 NH4+ to NH3 conversion. The pH of step c) can then be between 10.5 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 NH4+ to NH3 in a pH range between 10.5 and 12, while respecting any other specific constraints that may exist, on a case-by-case basis.

[0050] Preferably, the pH of step c) is between 10.5 and 12, depending on the reaction temperature, ambient or controlled, and depending on the alkaline agent(s) added in steps a) and possibly b); even more preferably, this pH is between 11 and 12. It can, for example, be set at 11.8.

[0051] 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 type 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 simultaneously. The basic solution produced as a result of these steps then comprises water and soluble calcium sulfate (CaSO4).

[0052] In an alternative embodiment, steps c) and d) of the process of the invention are carried out separately, successively.

[0053] The various steps of the process of the invention can be carried out at ambient temperature (for example, at about 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 step 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.

[0054] 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 to promote the production of a maximum of solids.

[0055] 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.

[0056] 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 then facilitate the separation of the two types of solids produced and present in the effluent, during step d), in particular when the alkaline agent used during steps a) and b) is a magnesium alkaline agent.

[0057] Indeed, when a magnesium alkali 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 separated from The optimal method for obtaining maximum gypsum purity (and therefore a minimum amount of Mg(OH)2 in cases where the target gypsum for future use must have high purity) and for maximizing the purity of the Mg(OH)2 (i.e., containing a low amount of CaSO4·2H2O) is required, either for advantageous recycling in steps a) and possibly b) of the process of the invention, or for valorization 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 µm and 15 µm.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 strictly 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.

[0058] Thus, it is preferable to limit as much as possible the production of small gypsum particles in the precipitation reactor, in step c).

[0059] 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.

[0060] By adding these gypsum nuclei along with the lime, the crystallization of calcium sulfate will be catalyzed, promoting and accelerating crystal growth by agglomeration on said nuclei, producing final crystals with a particle size larger than that of the nuclei. In addition, the competitive gypsum nucleation reactions (production of nuclei or very fine gypsum grains, on 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 induce sulfate leakage and an increased risk of post-precipitation of gypsum downstream of step d)).

[0061] 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 strictly greater than 15 µm, so as not to risk them mixing with the Mg(OH)2 particles.

[0062] To obtain gypsum seeds of a suitable 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 by using ultrasound.

[0063] The maximum targeted particle size of these gypsum nuclei will further 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.

[0064] The particle size of the gypsum seeds possibly added to the reactor in step c) is therefore ideally between 20 pm and 30 pm.

[0065] In addition, these calibrated gypsum seeds can be limed, that is to say mixed with lime, so as to make them more active.

[0066] 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).

[0067] These steps are illustrated in [Fig.4].

[0068] When an ammoniacal alkaline agent is chosen in steps a) and possibly b), these additional steps are illustrated in [Fig. 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 determined primarily by the optimum gypsum crystal growth desired in the reactor in step c).

[0069] 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 carried out in the process of the invention. In order to obtain better efficiency of the process of the invention, it may include a preliminary step aimed at increasing the pH of the starting effluent so that it reaches a pH of approximately at least 1. Such a preliminary step of increasing the pH may be carried out in various ways known to those skilled in the art.

[0070] In particular, it is possible to add a strong alkaline agent, preferably free of calcium oxide and / or calcium hydroxide (so as not to risk creating gypsum with a calcium input, leading to supersaturation of the effluent with calcium sulfate d). Such a strong alkaline agent can be, for example, sodium hydroxide or potassium hydroxide.

[0071] 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, for example red or blue (see also Example 6 in this regard). These hydroxides or this gypsum can be added to the starting effluent, and thus purified and recovered.

[0072] When red or blue gypsum is added, the calcium contained in the gypsum is already in the form of dissolved gypsum or calcium sulfate, and therefore will not present the disadvantages of calcium (hydr)oxide. In this preferred embodiment, the blue or red gypsum will be pre-washed to recover the dissolved metals present in the pore water.

[0073] The wash waters loaded with dissolved metals can be recycled in the treatment process, in step a), to precipitate the metals dissolved in said wash waters, in the form of metallic hydroxides which can thus potentially be recovered.

[0074] 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.

[0075] By "red gypsum" is meant 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).

[0076] By "blue gypsum" is meant precipitated calcium sulfate dihydrate (gypsum) containing iron in ferrous form (for example, 80% by mass percentage of gypsum CaSO4,2H2O and 20% by mass percentage of ferrihydrite (Fe(OH)2).

[0077] 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 of separating the white gypsum obtained from the red / blue gypsum is advantageously added before step a) of the process of the invention.

[0078] In another preferred embodiment of the process of the invention, an oxidizing agent is added to the starting effluent to oxidize all the metals in the solution that can be oxidized, in particular ferrous iron (Fe2+) to ferric iron (Fe3+). This oxidizing agent is, for example, dioxygen (O2), hydrogen peroxide (H2O2), or ozone. This oxidizing agent can be added before or simultaneously with the agent alkaline chemical of the invention, i.e. 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 metals (and in particular Iron) contained in the red or blue gypsum have been dissolved by the acidic effluent, i.e. during step a).

[0079] Thus, in this particular embodiment, the process of the invention includes, prior to or during step a), an oxidation step aO) carried out by adding to the effluent at least one oxidizing agent as described above.

[0080] Washing steps for precipitated products may be added to facilitate their recycling.

[0081] At the end of the process, the aim 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 the discharge of wastewater 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 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).

[0082] To eliminate the risk of post-precipitation of soluble calcium sulfate (CaSO4) still present, for example at saturation, in the basic solution after 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 (CaSO4) in the aqueous solution by precipitating some of the calcium present in the aqueous solution as limestone (CaCO3), and to lower 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 below 9.5. This will make it possible to obtain a With a CaSO4 saturation of less than 100%, the resulting limescale 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.

[0083] Thus, the process of the invention further includes optionally a step e) in which CO2 is added to the basic aqueous solution from step d) to precipitate part 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.

[0084] According to a first preferred embodiment, the chemical agent added in steps a) and b) is magnesium oxide (MgO) or magnesium hydroxide (Mg(OH)2 )•

[0085] The chemical reaction that occurs when magnesium hydroxide Mg(OH)2 is added to the effluent is as follows:

[0086] Mea(SO4)b + cMg(OH)2 -> aMe(OH)c + Mgc(SO4)b

[0087] where Me is a metal, and where a, b, and c are natural numbers.

[0088] Upon the addition of calcium hydroxide Ca(OH)2, the compound MgSO4 reacts with it to produce white gypsum and magnesium hydroxide according to the reaction:

[0089] MgSO4 + Ca(OH)2 + 2H2O -> CaSO4 .2H2O(s)+ Mg(OH)2(s)

[0090] where (s) symbolizes solid forms.

[0091] In this preferred embodiment, a preliminary oxidation step is recommended (for example, with dioxygen) to transform ferrous iron into ferric iron, thus facilitating its separation from aluminum. Then, in step a), the pH is increased to a pH between 4 and 5.5 (preferably 4.2). At this pH, the ferric iron hydroxide is in solid form and can therefore be separated from the effluent. Next, in step b), the pH is increased 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 10.5 and 12 (preferably 11.8) allows for the production and precipitation of white gypsum and magnesium hydroxide, each of which can be reused in the process of the invention, or elsewhere.

[0092] In this preferred embodiment, the process of the invention therefore comprises at least the following successive steps:

[0093] aO) Add an oxidizing agent to said effluent,

[0094] a) Adding a magnesium oxide (MgO) solution or a magnesium hydroxide (Mg(OH)2) solution 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,

[0095] b) Adding a magnesium oxide (MgO) solution or a magnesium hydroxide (Mg(OH)2) solution to the remainder of the effluent to increase its pH to a pH between 5.5 and 7.5, resulting in the production and precipitation of at least one metal hydroxide (in particular aluminum hydroxide), and separating this from the remaining effluent,

[0096] c) In the remaining effluent, add lime to raise the pH to an optimal value between 10.5 and 12, producing a basic solution comprising precipitated white gypsum and precipitated magnesium hydroxide, and

[0097] d) Separate respectively the precipitated white gypsum and the precipitated magnesium hydroxide from the basic solution of step c).

[0098] The addition of lime and therefore the precipitation of gypsum during step c) can be done via a "High density Sludge" type process.

[0099] 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 comprises water and soluble calcium sulfate residues.

[0100] Preferably, in this embodiment, the pH of step c) is between 10.5 and 12, or even between 11 and 12. It can for example be set at 11.8.

[0101] As described above, part of the white gypsum recovered during step d) is preferably ground and activated (by liming, as described above) for recycling in step c) of this process.

[0102] Grinding 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.

[0103] 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 optionally activated white gypsum is then added to the reactor in step c) to promote crystallization. of gypsum and the formation of crystals with a grain size greater than 15 pm, as explained previously. These steps are illustrated in [Fig.4].

[0104] The process of the invention advantageously allows 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.

[0105] 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.

[0106] Any excess magnesium hydroxide recovered during step d) can further be thickened and concentrated on a belt filter or centrifuge, for external valorization.

[0107] 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.

[0108] Similarly, the optional preliminary and final steps described above can be added at the beginning or end of this process.

[0109] In particular, this process may further include, prior to step a) or aO), a step of increasing the pH of the effluent to a value of at least 1.

[0110] Finally, 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) consisting of separating the limestone from the aqueous solution.

[0111] 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).

[0112] The chemical reaction that occurs when ammonia is added to the effluent is as follows:

[0113] Mea(SO4)b + cNH3 + dH20 -> Mea(0H)c + (NH4)C (SO4)b

[0114] where Me is a metal, and where a, b, c and d are natural numbers.

[0115] Adding ammonia to the effluent therefore makes it possible to produce ammonium sulfate (NH4)2SO4.

[0116] Upon the addition of calcium hydroxide Ca(OH)2, the compound (NH4)2SO4 is converted into dissolved ammonia and precipitated gypsum according to the reaction:

[0117] (NH4)2SO4 + Ca(OH)2 -> CaSO4.2H2O + 2NH3+

[0118] In this second preferred embodiment, the process of the invention comprises at least the following successive steps: a. Add 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 separate the precipitated metal hydroxide from the effluent, b. Optionally, repeat 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. In the remaining effluent, add lime to raise the pH to an optimal value between 10.5 and 12 to produce a basic aqueous solution containing precipitated white gypsum, d. Separate the precipitated white gypsum from the basic aqueous solution formed in step c).

[0119] During steps a) and b), ammonia or ammonium hydroxide will be added to raise the pH of the effluent to the optimum pH for precipitation of iron (ferrous or ferric) hydroxides, metal hydroxides of other metals, or co-precipitation of other metals from the solution. The optimum pH adjusted by the addition of ammonia or ammonium hydroxide will be between 1 and 9.5.

[0120] 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 optimum pH for precipitation of the metal hydroxides to be precipitated and the desired selectivity.

[0121] The lime added in step c) produces a basic aqueous solution comprising white gypsum (calcium sulfate CaSO4 dihydrate precipitated uncontaminated by metals), and the ammonium ions present in the solution are converted into dissolved NH3 gas.

[0122] In this process, the addition of lime and therefore the precipitation of gypsum can be done by means of a "High density Sludge" type process or equivalent.

[0123] At the end of step d), the basic solution includes in particular water, dissolved NH3, and soluble calcium sulfate.

[0124] 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.

[0125] 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) from the aqueous solution and b). This ammonia can be recovered and reused in the process of the invention, or recovered in other industrial sectors.

[0126] According to a preferred embodiment of the invention, the ammonia NH3 recovered at the end of this stripping is then returned 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.

[0127] Preferably, in this embodiment, the pH of step c) is between 10.5 and 12, or even between 11 and 12. It can, for example, be set at 11.8.

[0128] 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 initial temperature of the reaction medium is cold (e.g., below 10°C), as the conversion of NH4+ to NH3 will not be optimal at a pH value below 12. Heating the reaction medium to a temperature, for example, of 20 to 30°C, increases the efficiency of the conversion of NH4+ to NH3 at a pH below 12, typically between 10.5 and 12. It is therefore recommended to heat the reaction medium to at least 20°C when the alkaline agent used is ammoniacal.

[0129] 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. Next, 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 10.5 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.

[0130] 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 increase the pH until it is 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. To the remaining effluent, add ammonia (NH3), or an ammonium solution (NH4OH), to adjust the pH to a value between 8 and 9.5, resulting in the production and precipitation of at least one hydroxide. metallic (particularly ferrous iron hydroxide), 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 10.5 and 12 to produce an aqueous basic solution containing precipitated white gypsum, d. Separate the precipitated white gypsum from the basic aqueous solution formed in step c).

[0131] Preferably, the pH of step a) is set at 6.5. At this pH, the aluminium hydroxide is in solid form and can therefore be recovered by conventional separation means.

[0132] 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.

[0133] In a preferred embodiment, this process includes a final step d') consisting of separating the ammonia NH3 present in the basic solution of step d) by stripping.

[0134] In an even more preferred embodiment, the ammonia NH3 obtained in step d') is reintroduced into the effluent in steps a) and / or b) of the process.

[0135] In a second, particularly preferred embodiment, a preliminary oxidation step is carried out (for example, with dioxygen) to transform ferrous iron into ferric iron, and 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 (in particular ferric iron hydroxide and aluminum hydroxide) are in solid form and can therefore be separated from the effluent. Next, the addition of lime at a pH between 10.5 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 second, particularly preferred embodiment, the process of the invention comprises at least the following successive steps:

[0137] aO) Add 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. If necessary, repeat step a) to achieve a pH between 4 and 9.5 higher than that chosen in step a). c. In the remaining effluent, add lime to raise the pH to an optimal value between 10.5 and 12 to produce an aqueous basic solution containing precipitated white gypsum, d. Separate the precipitated white gypsum from the basic aqueous solution formed in step c).

[0138] 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.

[0139] 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 carried out at a pH below 4 or above 7.

[0140] 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.

[0141] Similarly, the optional preliminary and final steps described above can be added at the beginning or end of these processes.

[0142] In particular, these processes may further include, prior to step a) or aO), a step of increasing the pH of the effluent to a value of at least 1.

[0143] Finally, 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) consisting of separating said limestone from the solution. Brief description of the drawings

[0144] Figure 1 describes an example of a process according to a first embodiment of the invention. This process involves adding magnesium hydroxide and O2 to the effluent to be treated.

[0145] Figure 2 describes 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, there is no addition of dioxygen in a first reactor. The ammonia after stripping is advantageously reused at the beginning of the process in the first reactor.

[0146] Figure 3 describes a variant of the process of Figure 2, in which O2 has been introduced into the first reactor to oxidize metal ions such as Fe2+. The ammonia after stripping can be reused at the beginning of the process in the first reactor.

[0147] Fig. 4 describes the concept of gypsum precipitation in step c) of the process of the invention of Fig. 1, in the presence of calibrated gypsum seeds.

[0148] Figure 5 describes the concept of gypsum precipitation in step c) of the process of the invention in Figure 2 or Figure 3, in the presence of calibrated gypsum seeds. EXAMPLES

[0149] To better understand the process of the present invention, three different embodiments are exemplified here (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.

[0150] Example 1 ([Fig. 1]): process of the invention with Mg(OH)2 as the alkali agent

[0151] A first embodiment of the invention is schematically described in the [Fig.l].

[0152] 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.

[0153] The effluent is directed to a first reactor 1 equipped with an agitator in which magnesium hydroxide is also added while controlling the pH to maintain it at a value of about 4.2. Dioxygen is added there.

[0154] 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.

[0155] 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 about 6.5.

[0156] The effluent exiting the second reactor 3 is directed to a second filter press 4 in order to isolate the aluminum hydroxide precipitates A1(OH)3 formed in the second reactor 3.

[0157] 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 about 11.8.

[0158] 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.

[0159] More than 85% of magnesium hydroxide initially introduced in steps a) and b) can thus be recovered and reintroduced into the first and second reactors.

[0160] The liquid exiting the 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.

[0161] CO2 is added to the liquid exiting the filter press 7 to desaturate the liquid with calcium sulfate (CaSO4), precipitating some of the calcium present in the aqueous solution as limestone (CaCO3). The limestone thus produced is separated from the solution using conventional means 8 such as an HDS-type thickening decanter, an Actina™ type contact reactor (Pellets reactor) or equivalent, or a Saphira™ contact reactor or equivalent. A filter 9 may be added if necessary.

[0162] 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 the natural environment without risk of pollution.

[0163] Example 2 ([Fig.2]): process of the invention with ammonia as the alkaline agent, and with two precipitation / filtration steps

[0164] A second embodiment of the invention is schematically described in [Fig.2],

[0165] 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.

[0166] 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 about 6.5.

[0167] The effluent exiting this first reactor 21 is directed to a first filter press 22 in order to isolate the precipitates of aluminium hydroxide A1(OH)3 formed in the first reactor 21.

[0168] The remaining liquid is directed to a second reactor 23 also equipped with an agitator in which ammonia is again added while controlling the pH to maintain it at a value of about 9.

[0169] The effluent exiting this second reactor 23 is directed to a second filter press 24 in order to isolate the precipitates of ferrous iron hydroxide Fe(OH)2 formed in the second reactor 23.

[0170] 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.

[0171] 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.

[0172] 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.

[0173] CO2 is added to the liquid exiting the third filter press 26 in order to desaturate the calcium sulfate (CaSO4), precipitating the calcium present in the aqueous solution as limestone (CaCO3). The limestone thus produced is separated from the solution using conventional means 27 such as an HDS-type thickening decanter, an Actina™-type contact reactor (Pellets reactor) or equivalent, or a Saphira™ contact reactor.

[0174] 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 the natural environment without risk of pollution.

[0175] 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 NH4+ to NH3 at pH 11.8.

[0176] Example 3 ([Fig. 3D: process of the invention with ammonia as the alkaline agent, and with a single precipitation / filtration step

[0177] A third embodiment of the invention is schematically described in [Fig.3].

[0178] 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.

[0179] 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 about 6.5. Dioxygen is added.

[0180] 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 (A1(OH)3 formed in the first reactor.

[0181] 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.

[0182] The effluent exiting this second reactor 33 is directed to a second filter press 34 to remove any residue of solid gypsum precipitated in the second reactor 33 and present in the treated effluent.

[0183] The ammonia still contained in the effluent is stripped in order to be recycled in 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.

[0184] CO2 is added to the liquid exiting the filter press 34 in order to desaturate the calcium sulfate (CaSO4), precipitating the calcium present in the aqueous solution as limestone (CaCO3). The limestone thus produced is separated from the solution using conventional means 35 such as an HDS-type thickening decanter, a reactor a contact reactor (Pellet reactor) of the Actina™ type or equivalent, or a Saphira™ contact reactor. The water recovered from 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 0.5mg / L of Aluminum, and can therefore be discharged into the natural environment without risk of pollution.

[0185] 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 NH4+ to NH3 at pH 11.8.

[0186] Example 4 ([Fig.4]): Gypsum recycling step to optimize the process of Example 1

[0187] Figure 4 describes an optional step of recycling part 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.

[0188] In this optional step, the precipitated white gypsum obtained in the process of Example 1 can be treated to be reintroduced into the third reactor 45 (gypsum precipitation reactor) and promote the growth of new gypsum in it.

[0189] A portion of the white gypsum isolated in the process after separation 46 is then ground in step 4L. 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 containing 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, in which CaSO4·2H2O and magnesium hydroxide are precipitated. Gypsum grains with a particle size less than or equal to 15 pm, separated from the calibrated grains in 42, are not introduced into reactor 45 but are used outside the process of invention 44.

[0190] 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.

[0191] Magnesium hydroxide after separation 47 is either reused in the first and second reactors 1 and 3 of [Fig.1] to adjust the pH, or is recovered in another way 48.

[0192] The treated effluent is then conveyed to the CO2 addition stage and reactor 8 of [Fig.1].

[0193] Example 5 ([Fig.5D: gypsum recycling step to optimize the process of examples 2 and 3.

[0194] 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.

[0195] 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.

[0196] 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 seeds 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 may be added to the suspension of calibrated gypsum seeds to enhance its effectiveness when introduced into reactor 55, in which CaSO42H2O is precipitated and NH4+ is converted to NH3 gas. Gypsum grains with a particle size of 15 µm or less, separated from the calibrated grains in 52, are not introduced into reactor 55 but are utilized outside the process of the invention 54.

[0197] Most of the white gypsum obtained after separation 56 can also be directly recovered without being recycled in the process, after possibly being filtered again 59.

[0198] 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 is recovered in another way 57.

[0199] An optional heat input may be provided upstream of reactor 55 to facilitate the conversion of NH4+ to NH3 at pH 11.8 in reactor 55.

[0200] In this case, a heat exchanger 28 of [Fig.2] or 36 of [Fig.3] can be added.

[0201] The heat exchanger 50 of [Fig.5] can be provided to cool the effluent heated by the stripping of ammonia, as required, to comply with the permitted discharge temperature of the effluent, on a case-by-case basis.

[0202] The treated effluent is then conveyed to the CO2 addition stage and the CaCO3 precipitation reactor 27 or 35, of Figures 2 or 3.

[0203] The heat exchanger 50 of [Fig.5] can be positioned as described in [Fig.5] or downstream of reactor 27 or 35 of Figures 2 or 3, depending on the desired CaCO3 precipitation efficiency.

[0204] Example 6: Pre-neutralization step of the effluent by adding red gypsum or blue gypsum prior to the process of the invention,

[0205] 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.

[0206] Red gypsum (pH 9, 20°C) containing 700 g / L of solids (80% CaSO4.2H2O and 20% ferrihydrite (Fe(OH)3) is added to the starting effluent, before step a) of the process of the invention.

[0207] 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.

[0208] 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), 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 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 10.5 and 12, 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 method 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 grinding the precipitated white gypsum separated in step d) to obtain a calibrated particle size of gypsum greater than 15 µm, and optionally an activation step of the white gypsum thus ground by liming, and the addition of this ground white gypsum and possibly activated in the effluent during step c).

6. A method according to any one of claims 1 to 5, comprising the following successive steps: a) Adding an oxidizing agent to said effluent, a) Adding a magnesium oxide (MgO) solution or a magnesium hydroxide (Mg(OH)2) solution 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 magnesium oxide (MgO) solution or a magnesium hydroxide (Mg(OH)2) solution 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) Adding lime to the remaining effluent to raise the pH to an optimal value between 10.5 and 12, 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).

7. A process according to claim 6, wherein the magnesium hydroxide recovered in step d) is reused in steps a) and b) upstream of the process.

8. A method according to any one of claims 6 to 7, 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.

9. A process according to any one of claims 1 to 5, comprising the following 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 less one metallic hydroxide present in the effluent, and separate the precipitated metallic hydroxide from the effluent, b) To the remainder of the effluent, add ammonia (NH3), or an ammonium (NH4OH) solution, to adjust the pH to a value between 8 and 9.5, resulting in the production and precipitation of at least one metallic hydroxide, and separate this from the remainder of the effluent, c) To the remaining effluent, add lime to raise the pH to an optimal value between 10.5 and 12 to produce an aqueous basic solution including precipitated white gypsum, d) Separate the precipitated white gypsum from the aqueous basic solution formed in step c).

10. A process according to any one of claims 1 to 5, comprising 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 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 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 between 10.5 and 12 is reached to produce an aqueous basic solution comprising precipitated white gypsum, d) Separating the precipitated white gypsum from the aqueous basic solution formed in step c).

11. A method according to claims 9 or 10, further comprising a final step d') consisting of separating the ammonia NH3 present in the basic solution of step d) by stripping.

12. A process according to claim 11, wherein the ammonia NH3 obtained in step d') is reintroduced into the effluent in steps a) and / or b).

13. A method according to any one of claims 6 to 12, comprising, prior to step a) or a0), a step of increasing the pH of the effluent to a value of at least 1.

14. A method according to any one of claims 9 to 13, further 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.

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