BE1023752A1 - LIME BASED SORBENT COMPOSITION FOR REMOVAL OF MERCURY AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

The present invention relates to a sorbent composition containing core-shell type particles for removing heavy metals, in particular mercury, from gas, said core comprising a calco-magnesium compound having the formula aCaC03.bMgC03.xCa0.yMgO. zCa (OH) 2.tMg (OH) 2.ul, where I represents the impurities, a, b, x, y, z and t are each mass fractions> 0 and <100%, u is a mass fraction> 0 and <20% by weight, and being coated with a coating having a thickness in the range of 50 nm to 20 μm, and comprising at least one metal salt and a sulfur-based compound and its method of preparation.

Description

"LIME BASED SORBENT COMPOSITION FOR THE REMOVAL OF MERCURY AND PROCESS FOR PRODUCING THE SAME

The present invention relates to a sorbent composition containing particles for removing heavy metals, in particular mercury, from flue gases.

Sorbent compositions containing solid particles are well known in the art.

US7923397B2 discloses a modified activated carbon sorbent (with elemental sulfur powder) for removing heavy metals from the flue gas. Although activated carbon is the best sorbent technique available to date for mercury uptake (the amount of mercury absorbed per gram of sorbent), coal is susceptible to combustion within the current. flue gas, which is one of the main disadvantages of this sorbent (see also US Patent 2002/0035925).

In addition, the use of activated carbon sorbent increases the total organic carbon content in the dust present during the evacuation of these flue gases, which carbon content is nowadays extremely regulated.

In addition, coal black colors the gypsum produced from a wet desulphurization process while the whiteness of this material determines its valuation value when sold to a gypsum manufacturer.

In addition, activated carbon is naturally retained in the fly ash waste stream. However, the activated carbon may interact undesirably with the additives used in the cement and concrete formulations, thereby excluding the use of said fly ash containing activated carbon as an additive in the cement industry which must therefore rather be eliminated at a relatively high cost.

For these reasons, it is necessary to provide alternatives to the use of active carbon, which are more cost-competitive, for their acquisition, but also for the treatment of by-products and the recovery of the residual material resulting from the treatment of flue gas.

US2014 / 0050641A1 relates to an aqueous mercury sorbent composition. Such a composition is produced by mixing (a) an aqueous solution of a precursor containing silica (such as silicic acid, colloidal silica); (b) an aqueous solution of a metal species (such as copper salts); and (c) a solution of a sulfur-based species (such as (poly) sulfide salts or dithiocarbamates) with (d) the process water.

Unfortunately, such an impregnated silica involves a fairly complex manufacturing process and is sold on the market as a rather expensive specialty product, particularly when the silica has to be very porous for good absorption properties, whereas on the other hand, it is intended to treat waste. Therefore, even if this solution is fairly well accepted on the market, in particular because, until now, no other sufficiently effective mineral sorbent composition has been proposed, there is still a problem for industrial actors with regard to the use of an expensive silica for treating flue gas which is waste.

In addition, according to this document, the sorbent composition is intended to be used in aqueous suspension in two different applications. In the first application, the aqueous suspension is designed for use in wet flue gas desulphurization units, where it is sufficiently stable. In the second application, the aqueous suspension is injected into the dry, hot smoke gas. However, in the latter case, the aqueous suspension is dried in the hot flue gas, probably causing in such a case, a loss of efficiency due to a sensitivity to thermal decomposition and a lack of reproducibility since the process of Sorbent drying is only undergone and uncontrolled. Indeed, tests in our study demonstrated that spray-dried colloidal silica particles tend to trap active chemical compounds in the particle core during the spray-drying step, thereby reducing overall efficiency.

In addition, the lack of reproducibility from one installation to another, as well as the difficulties related to the handling and injection of a liquid additive in a process treating a hot gas, are other problems encountered with this type. of product.

WO2014 / 164975A1 also describes a sorbent for removing mercury or sulfur from a gas stream. Such a composition is described as being a solid state composition of (a) an inorganic base (such as calcium hydroxide, sodium sesquicarbonate, sodium (bi) carbonate, potassium carbonate and / or calcium carbonate); and (b) a sulfide (such as ammonium sulfide, alkali metal sulfide, alkaline earth metal sulfide and / or transition metal sulfide); and (c) optionally a carrier (such as silicate, aluminate, aluminosilicate, and / or charcoal) carrying the mixture. Other mixtures are also described in patent WO2014 / 164980 containing silicate sorbents.

US2013 / 089479, US2011 / 012422, WO2015 / 057420, US6719828 or US 7288499 also disclose sorbent compositions based on clays or other known silicate sorbents.

Unfortunately, all commercially available mercury removal sorbent compositions have disadvantages because either they consist of an organic sorbent material and thus have a high efficiency in terms of mercury removal, but present risks of Inflammation, or they are generally mineral, but have a low efficiency in terms of mercury removal and / or are typically expensive.

There remains therefore a need to provide a mineral sorbent composition, effective for the removal of mercury flue gas, chemically stable, affordable from a cost point of view and compatible with the valuation of fly ash.

The present invention more specifically relates to a sorbent composition containing particles for removing heavy metals, in particular mercury, from flue gases, wherein said particles are core-shell particles in order to solve at least a portion of disadvantages mentioned above by providing a composition for oxidizing sorbent material used in the absorption of heavy metals, and more particularly ionic and metallic mercury, and its method of manufacture. The heavy metals can be removed from a fluid, preferably a gaseous fluid, especially flue gas, where the heavy metals are generally gaseous, in coal-fired power plants, municipal solid waste incinerators, and / or cement kilns and / or other industrial exhaust gases.

To solve this problem, the present invention relates to a lime sorbent composition, wherein the core comprises a calco-magnesium compound having the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul, where I represents the impurities, a, b, z and t are each mass fractions> 0 and <100%, x and y are each mass fractions> 0 and <50%, u is a mass fraction> 0 and <20%, the total a, b, x, y, z, t and u is equal to 100% by weight, relative to the total weight of said at least one calco-magnesium compound, characterized in that said core is coated with a shell comprising a metal salt and a sulfur compound, said sulfur compound being present in an amount sufficient to provide a protective effect on the composition, preferably the core and having a thickness of less than 20 μm and greater than 50 nm.

According to the present invention, the contents of CaCO 3, MgCO 3, CaO, MgO, Ca (OH) 2 and Mg (OH) 2 in calco-magnesium compounds can be easily determined by conventional methods. For example, they can be determined by an X-ray fluorescence analysis, the procedure of which is described in the EN 15309 standard, coupled with a measurement of loss on ignition and a measurement of the CO 2 volume according to EN 459-2: 2010 EN.

The impurities I include in particular those found in limestones and natural dolomites, such as silico-aluminate clays, silica, impurities based on iron or manganese, or those derived from the process. of manufacture of the calco-magnesian compound.

Tests carried out in our study have demonstrated, when testing a mixture of hydrated lime with a sulfur-based compound, that the lime present either on the surface or in the mixture with the sulfur-based compound when in contact with flue gases, has a strong tendency to reduce the ionic mercury to elemental mercury, thus giving a loss of efficiency in terms of mercury removal because the mercury must be in ionic form to react with the sulfur compound. In this document, hydrated lime means an industrial calcium-magnesium compound consisting essentially of calcium dihydroxide Ca (OH) 2 with impurities.

Surprisingly, according to the present invention, due to the core-shell-like structure of the coalescoated magnesium-coated particles having a thickness greater than 50 nm and less than 20 μm, the negative effect of the lime is reduced, which greatly increases the mercury removal capacity of the composition according to the present invention.

Indeed, in the product of the present invention, the core of the calcium-magnesium particles is protected from the mercury compounds by the shell layer of the sulfur compound.

The core of the calco-magnesian particles is coated with a shell comprising a sulfur-based compound in an amount sufficient to provide a protective effect on the composition, preferably the core. The term "protective effect of the composition" does not necessarily mean that 100% of the particles composing the composition are completely covered by the shell layer of the sulfur compound. The amount of particles which are covered by the shell layer of the sulfur-based compound may be 30% by weight, advantageously 35% by weight, preferably 40% by weight, more preferably 45% by weight. weight, in particular 50% by weight, advantageously 55% by weight, preferably 60% by weight, more preferably 65% by weight, in particular 70% by weight, still more preferably 75% by weight, particularly preferably 80% by weight, preferably 85% by weight, more preferably 90% by weight, advantageously 95% by weight, in particular 100% by weight, based on the composition based on lime.

The coating of the particles by the shell layer of the sulfur-based compound can be total or partial, provided that a protective effect of the composition, in particular of the core, is obtained. This means that some particles coated with the composition may have some parts of their surface that are coated while other parts of their surface have no coating, while avoiding a reduction of elemental mercury ionic mercury and thus providing the protective effect of the composition according to the invention.

Therefore, the lime sorbent composition can contain particles that are effectively and completely coated by the shell layer of the sulfur-based compound, particles that are partially coated by the shell layer of the compound-based compound. sulfur, particles which are not coated by the shell layer of the sulfur-based compound and which therefore consist solely of the calco-magnesium compound and particles which consist entirely of the sulfur-based compound. Particles that consist entirely of the sulfur compound are the result of the precipitation and agglomeration of the sulfur compound.

In addition, the presence of an alkaline lime core (calco-magnesium compound) helps to improve the stability of the sulfur-based compound in the envelope layer in its most reactive form which is S2 'while at the same time preventing time H2S emissions due to acidic conditions.

Therefore, the sulfur-based compound of the ring can react with the ionic mercury present in the flue gas and form HgS.

In a preferred embodiment, in the lime sorbent composition according to the present invention, said sulfur compound has the formula AaSβOv where α, β and y are each a mass fraction with β * 0 and where A is selected from the group consisting of calcium, magnesium, potassium, sodium and their mixture. In particular, said sulfur-based compound is selected from the group consisting of sulphide salts, such as calcium sulphide, dithiocarbamates, sulphate salts, such as calcium sulphate, dithiocarbamate-based polymers, polysulfide salts, such as calcium polysulfide, and mixtures thereof.

Therefore, the lime-based composition according to the present invention is mainly of inorganic nature, thus helping to reduce the overall carbon content in the fly ash.

In a particular embodiment of the present invention, in the lime-based sorbent composition according to the invention, said metal salt is chosen from the group consisting of the salts of titanium, vanadium, manganese, iron, nickel, copper, zinc and their mixture, preferably copper.

In a preferred embodiment, said metal salt is a copper sulfide or a copper polysulfide.

In another embodiment according to the present invention, the lime-based sorbent composition according to the invention further comprises a doping agent selected from the group consisting of alkali metal halides such as sodium or potassium halides, alkaline earth metal halides such as calcium or magnesium halides, ammonium halides and mixtures thereof.

Furthermore, in another preferred embodiment according to the present invention, the lime sorbent composition according to the invention further comprises a dispersing agent selected from the group consisting of (poly) sulphates, such as dodecyl sulphate. sodium (SDS), (poly) sulfonates, (poly) phosphates, (poly) phosphonates, such as diethylenetriamine-penta (methylene phosphonic acid) (DTPMP), polyols and mixtures thereof.

In an advantageous lime sorbent composition according to the invention, in the calco-magnesium compound corresponding to the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul, z + t > 60%, preferably> 70%, preferably> 80%, more preferably> 90%, in particular> 93% by weight, relative to the total weight of said at least one calcium-magnesium compound.

In another advantageous lime sorbent composition according to the invention, in the calco-magnesium compound corresponding to the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul, z> 60%, preferably> 70%, preferably> 80%, more preferably> 90%, in particular> 93% by weight, relative to the total weight of said at least one calcium-magnesium compound.

As a result, most of the core particles in the lime sorbent composition consist of hydrated lime, also known as slaked lime, which means that some particles may have a core that consists entirely of slaked lime particles or a mixture of slaked lime and quicklime in the same core or even particles of partially extinguished lime; at a rate such that the amount of slaked lime relative to the calco-magnesium particles is greater than 60% by weight.

Said at least one calco-magnesian compound according to the present invention is therefore at least formed of slaked lime, dolomitic lime extinguished, magnesium lime extinguished or quicklime from the calcination of natural limestones or natural dolomites.

The composition according to the invention may therefore also include calcium or magnesium carbonates, such as unburned materials from the calcination of natural limestones or natural dolomites or other products from the re-carbonation of calco-magnesium compounds. Finally, it may also comprise oxides of calcium or magnesium which are due to the partial hydration (extinction) of calco-magnesium compounds.

According to another variant according to the present invention, the calc-magnesium compound of the lime-based sorbent composition has a particle size distribution wherein di0 is in the range of 0.5 to 2 μm; d90 is in the range of 2 to 50 μm, preferably 5 to 40 μm, d50 is in the range of 0.5 to 50 μm, preferably 1 to 30 μm.

The notation dx represents a diameter expressed in μm, relative to which X% by mass of the particles measured are smaller or identical.

In yet another advantageous embodiment of the lime sorbent composition according to the present invention, said shell has a thickness of less than 10 μm, preferably less than 5 μm, in particular less than 2 μm, advantageously less than at 1 μm, more preferably less than 700 nm, and greater than 75 nm, preferably greater than 100 nm, in particular greater than 150 nm.

It is understood that the smaller the kernel particle, the larger the external surface, thus making it possible to have a larger quantity of sulfur additive in the outer layer, for a sulfur / calco-magnesium compound ratio. (core) given. This should improve the overall mercury absorption.

In a particularly advantageous embodiment according to the present invention, the ratio between the calco-magnesium compound (core) and the sulfur in the sulphide form is in the range from 15: 1 m / m to 1: 1 m / m, preferably from 10: 1 to 2: 1 and preferably is 5: 1, advantageously 4: 1, more preferably 3: 1, preferably 2.5: 1 m / m, relative to the lime-based sorbent composition in the lime-based sorbent composition.

The amount of the sulfur compound relative to the composition of the present invention should be sufficient to provide a protective effect of the composition, preferably the core of the particles.

In another advantageous embodiment, the ratio between the calco-magnesium compound (core) and said metal salt is in the range of 15: 1 m / m to 1: 1 m / m, preferably 10: 1 at 2: 1, advantageously from 8: 1 to 3: 1, preferably from 7: 1 to 4: 1 and preferably is 5: 1 m / m with respect to the lime-based sorbent composition, the sorbent composition based on lime according to the invention. Other embodiments of the lime sorbent composition according to the present invention are mentioned in the appended claims.

The present invention also relates to a process for producing a lime sorbent composition, comprising the steps of: i) feeding an aqueous suspension of calco-magnesium compound having the formula aCaC03.bMgC03.xCa0.yMg0.zCa (0H ) 2.tMg (OH) 2.ul, where I represents the impurities, a, b, z and t are each mass fractions> 0 and <100%, x and y are each mass fractions> 0 and <50% , u is a mass fraction> 0 and <20%, the total a, b, x, y, z, t and u is 100% by weight, based on the total weight of said at least one calco-magnesium compound in an atomizer dryer ii) supplying a solution of calcium magnesium polysulfide in said atomizing dryer, said feeding of said aqueous suspension of the calcium-magnesium compound and the feeding of the calcium-magnesium polysulfide solution being carried out separately or together, optionally in the form of a premix of said solution calcium-magnesium polysulfide and said aqueous suspension of the calcium-magnesium compound; iii) forming a spray-dried lime-based composition having particles which are core-shell type particles, wherein the core comprises said calcium-carbonate compound; magnesium, said core being coated with an envelope having a thickness greater than 50 nm and less than 20 μm, which makes it possible to provide a protective effect to the core with respect to the medium of the flue gases and comprising at least one compound based on sulfur, said method further comprises the step of bringing an ammonia-metal complex in the form of a solution (1) to said solution of calcium-magnesium polysulfide or to said aqueous suspension of the calcium compound -magnesian or premixing said calcium-magnesium polysulfide solution and said aqueous suspension of calco-magnesian compound, said dry lime-based composition formed atomization member having particles which are core-shell particles further comprising at least one metal salt in the shell or (2) said spray dryer separately or together with said calcium-magnesium polysulfide solution or with said aqueous suspension of the calco-magnesian compound or with the premix of said calcium magnesium polysulfide solution and said aqueous suspension of the calcium-magnesium compound, said formed spray-dried lime-based composition having particles which are core-shell type particles further comprising at least one metal salt in the shell or, (B) the spray-dried lime-based composition having particles which are core-shell particles, said shell consisting of a first layer comprising said sulfur-based compound and a second layer comprising at least one metal salt than.

The atomizing dryer used in the process according to the invention has the advantage of allowing the formation of a dry composition recovering most of the components introduced during the process. Therefore, the amount of the various components relative to the dry composition can be approximately determined by the amount of these components initially introduced during the process.

As said above, the protective effect of the spray dried lime composition does not necessarily mean that 100% of the particles composing the composition are fully coated by the shell layer of the sulfur compound. The amount of particles which are coated by the shell layer of the sulfur-based compound can be 30% by weight, advantageously 35% by weight, preferably 40% by weight, more preferably 45% by weight. weight, in particular 50% by weight, advantageously 55% by weight, preferably 60% by weight, more preferably 65% by weight, in particular 70% by weight, still more preferably 75% by weight, weight, particularly preferably 80% by weight, preferably 85% by weight, more preferably 90% by weight, advantageously 95% by weight, in particular 100% by weight with respect to the composition of the composition. spray dried lime base.

The coating of the particles by the shell layer of the sulfur compound can also be total or partial. This means that some coated particles of the composition may have some parts of their surface that are coated while other parts of their surface are not coated.

Therefore, the spray-dried lime-based composition may contain particles that are effectively and completely coated with the sulfur compound-based shell layer, particles that are partially coated by the shell layer of the compound to be coated. sulfur base, particles which are not coated by the shell layer of the sulfur-based compound and which therefore consist solely of lime and particles which consist entirely of the sulfur-based compound. Particles that consist entirely of the sulfur compound are the result of the precipitation and agglomeration of the sulfur compound.

Preferably, said ammonia-metal complex is obtained by mixing a metal salt with an ammonia solution, wherein the ratio between said metal salt and said ammonia solution is in the range of 1: 2 m / m at 1:10 m / m and preferably 1: 4 m / m, advantageously 1: 5 m / m, preferably 1: 5.5 m / m, more preferably 1 6.

Advantageously, said calcium-magnesium polysulfide solution is obtained by mixing a sulfur-based compound with a calco-magnesium compound corresponding to the formula pCa0.qMg0.rCa (OH) 2.sMg (OH) 2. where I represents impurities, u is a mass fraction> 0 and <20%, p, q, r and s are mass fractions> 0 and <100%, with p + q + r + s> 60% by weight, by relative to the total weight of said at least one calco-magnesium compound for forming said calcium magnesium polysulfide solution.

In particular, in the process according to the present invention, said aqueous suspension of the calco-magnesian compound, also called milk of the calco-magnesian compound, has a solid content of between 30 and 45% by weight relative to the total weight of the suspension of the calco-magnesium compound.

Advantageously, in the process according to the present invention, the aqueous suspension of the calco-magnesium compound comprises particles having a particle size distribution in which d50 is in the range from 0.5 to 20 μm, preferably 0.5 at 10 μm and more preferably 1 to 5 μm.

In a particular embodiment according to the present invention, said sulfur compound has the formula AaSβOv where α, β and y are each a mass fraction with β * 0 and where A is selected from the group consisting of calcium, magnesium, potassium, sodium and their mixture. In particular, said sulfur-based compound is selected from the group consisting of sulphide salts, such as calcium sulphide, dithiocarbamates, sulphate salts, such as calcium sulphate, dithiocarbamate-based polymers, polysulfide salts, such as calcium polysulfide, and mixtures thereof.

Advantageously, said metal salt is selected from the group consisting of the salts of titanium, vanadium, manganese, iron, nickel, copper, zinc and their mixture, preferably copper.

In a preferred embodiment, said metal salt is a copper sulfide or a copper polysulfide.

Preferably, said ammonia-metal complex is an ammonia-copper halide complex, preferably an ammonia-copper chloride complex.

In an alternative embodiment of the process according to the present invention, the process further comprises a step of adding a doping agent selected from the group consisting of alkali metal halides such as sodium or potassium halides, alkali metal halides, and the like. such as calcium or magnesium halides, ammonium halides and mixtures thereof.

Such a doping agent may be added to the spray dried lime composition, which means after spray drying, or to the calcium magnesium polysulfide solution, preferably to the calcium magnesium polysulfide solution.

In another variant of the process according to the present invention, the process further comprises a step of adding a dispersing agent selected from the group consisting of (poly) sulfates, such as sodium dodecyl sulphate (SDS), poly (poly) sulfonates, (poly) phosphates, (poly) phosphonates, such as diethylenetriamine-penta (methylenephosphonic acid) (DTPMP), polyols and mixtures thereof.

This dispersing agent is preferably added to the aqueous suspension of the calc-magnesium compound, before, after or simultaneously with the ammonia-metal complex.

Preferably, in the calco-magnesium compound corresponding to the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul, z + t> 60%, preferentially> 70%, preferably> 80 %, more preferably> 90%, in particular> 93% by weight, based on the total weight of said at least one calco-magnesium compound.

In another advantageous embodiment of the invention, in the calco-magnesium compound corresponding to the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul, z> 60%, preferentially > 70%, preferably> 80%, more preferably> 90%, in particular> 93% by weight, relative to the total weight of said at least one calco-magnesium compound.

More particularly, in the process according to the present invention, said shell has a thickness less than 10 μm, preferably less than 5 μm, in particular less than 2 μm, advantageously less than 1 μm, preferably less than 700 μm, and greater than 75 nm, preferably greater than 100 nm, in particular greater than 150 nm.

In a preferred embodiment of the process according to the present invention, the ratio of calco-magnesium compound to sulphide sulfur is in the range of 15: 1 m / m to 1: 1 m / m, preferably from 10: 1 to 2: 1 and preferably 5: 1, advantageously 4: 1, more preferably 3: 1, preferably 2.5: 1 m / m, in the solution of calcium magnesium polysulfide.

In another preferred embodiment of the present invention, the ratio of said calco-magnesium compound to said metal salt is in the range of 15: 1 m / m to 1: 1 m / m, preferably 10: 1 to 2: 1, advantageously from 8: 1 to 3: 1, preferably from 7: 1 to 4: 1 and preferably from 5: 1 m / m with respect to the solids content in the suspension of calcium-magnesium particles on which the metal is dispersed.

Preferably, the calc-magnesium compound has a specific surface area measured by manometry with nitrogen adsorption after degassing under vacuum at 190 ° C. for at least 2 hours and calculated according to the BET multipoint method as described in the ISO 9277: 2010e standard. between 5 m2 / g and 50 m2 / g. Other embodiments of the method according to the present invention are mentioned in the appended claims. Other features and advantages of the present invention will be drawn from the following nonlimiting description and with reference to the examples and drawings.

FIG. 1 represents a SEM image of the particles of the sample "product 1" (obtained from example 1). The particles have an average diameter of approximately 5 μm, the particle size being typically between 1 and 10 μm.

Figure 2 is a schematic presentation of the device used to measure the mercury absorption with the different samples according to the invention and the samples of the comparative examples.

Figure 3 shows the results of Example 1.

Figure 4 shows the results of Example 2.

Figure 5 shows the results of Comparative Example 1.

Figure 6 shows the results of Comparative Example 2.

Figure 7 shows the results of Comparative Example 3.

Figure 8 shows the results of Example 3.

In the drawings, the same reference numbers have been assigned to the same elements or similar elements. The invention relates to a sorbent in the form of a lime-based composition for cleaning flue gas laden with gaseous heavy metals, in particular mercury, comprising: a) a calco-magnesium compound which is a carrier (eg example a calco-magnesian compound at least formed with slaked lime, dolomitic lime extinguished, magnesium lime extinguished, but which may also include carbonates or oxides of calcium or magnesium), b) a metal salt ( such as salts of titanium, vanadium, manganese, iron, nickel, copper, zinc and their mixture, and preferably copper sulphide or copper polysulfide), and c) a sulfur-containing compound (such as sulphide, dithiocarbamates, sulphate salts, a dithiocarbamate-based polymer, polysulfide salts and mixtures thereof, preferably calcium sulphide, calcium polysulfide, calcium sulphate and mixtures thereof and more preferably calcium polysulfide).

Advantageously, a doping agent (such as alkali metal halides, alkaline earth metal halides, ammonium halides and mixtures thereof, preferably sodium, potassium, calcium or magnesium halides) may be added to the spray-dried lime-based composition or the calcium magnesium polysulfide solution, preferably the calcium magnesium polysulfide solution.

Optionally, a dispersing agent may be mixed with the suspension of calco-magnesium compound having the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul (such as (poly) sulfates (poly) sulfonates, (poly) phosphates, (poly) phosphonates, polyols, and mixtures thereof, preferably diethylenetriamine-penta (methylenephosphonic acid) (DTPMP) or sodium dodecyl sulphate).

The sorbent is a powder, in particular synthesized by spray drying and comprises in particular spherical particles having a core-shell structure in which the calcium-magnesium compound forming the support is the core, and in which the metal salt and the compound Sulfur make up the envelope.

The particle size distribution of this sorbent shown in FIG. 1 is preferably as follows: di0 = 1 μm and d90 = 10 μm, with d50 equal to 5 μm.

The preferred embodiment of the process according to the present invention is as follows: i) a solution of calcium polysulfide is prepared separately. This chemical compound is well known for several years. Its production requires a basic mixture of lime and elemental sulfur, both of which are dissolved in boiling water (80 ° C-100 ° C), stirred at 300 rpm for 2 hours. The ratios between lime and sulfur can vary (from 1: 1 m / m to 1: 2). Additional chemicals may also be used (a polyphosphonate for example). For this stage of the process, any hydrated lime or quicklime can be used (standard hydrated lime or with a large surface and / or a large pore volume); ii) the copper chloride is dissolved in water with ammonia (1: 3 to 1: 6 m / m) to form a stable ammonia-copper complex at high pH. This solution is then mixed with a milk of lime (aqueous suspension of hydrated lime in water) for several hours to ensure a total dispersion of the copper on the lime particles. For this stage of the process, it is advantageous to use fine lime milk (which means with a particle size distribution where di0 is between 0.5 and 1.5 μιτι, d50 is between 1 and 4 μm and d90 is between 2 and 10 μm) and highly concentrated with a solids content of between 30% by weight and 45% by weight relative to the total weight of the suspension of lime milk; when both preparations are complete, both are mixed and stirred with high shear for a short time (to limit the sulfur-copper reactions), then introduced into an atomizer dryer. During spray drying, there is a chemical reaction between the ammonia-copper complex and the calcium polysulfide, presumably leading to the formation of copper (poly) sulfide molecules with a portion of the calcium polysulfide which has not reacted. Ammonia is removed during the drying process. Part of the chloride counterions is also present in the final product.

Prior to spray drying, the future shell material consists mainly of Cu (NH 3) 4 and CuCl 2 / CaS x in excess of the amount of copper. After spray drying, the envelope consists mainly of CuSx and CaSx (the remaining amount of excess) and CuCl2 / Cu (OH) 2 (possible traces) as well as CaCl2 (possible traces). The invention will now be described using non-limiting examples.

EXAMPLES

In the following examples, the calcium polysulfide solutions are prepared according to the following general procedure: lime and elemental sulfur, both of which are dissolved in boiling water (80 ° C-100 ° C), are mixed together and stirred at 300 rpm for 2 hours. The ratios between lime and sulfur can vary (from 1: 1 m / m to 1: 2). Additional chemicals may be optionally used (eg polyphosphonate). For this stage of the process, any hydrated lime or quicklime can be used (standard hydrated lime or with a large surface area and / or a large pore volume).

Example 1.-

About 523 g of a lime milk according to the patent WO2014 / 064234 with a solids content of 45% are mixed with 50 g of copper chloride (grade 97% Alfa Aesar) and 300 ml of ammonia (solution with 25% VWR) and stirred at 300 rpm for 3 hours to form the suspension of lime milk No. 1.

A solution of calcium polysulfide is synthesized using the above-mentioned proposed method with a lime / sulfur ratio of 1: 1.5 m / m until the water is completely saturated with the polysulfide. Sulfur powder (99%) is purchased from VWR Chemicals. The solution is then filtered. 487.5 ml of this solution of saturated calcium polysulfide are then mixed with the suspension of No. 1 milk of lime and the mixture is spray-dried to obtain the product 1.

The atomizer dryer is the Atomizer Model MOBILE MINOR, from Brand GEA. The tank has a capacity of about 500 dm3, the air pressure can vary from about 0 to 5.5 bars. The injection speed depends on the peristaltic pump used.

The parameters of the atomizer dryer are as follows:

Mode: countercurrent,

Inlet pressure: 1 bar, inlet flow: 25 ml / min,

Inlet temperature: 210 ° C The carbon-sulfur analysis of this product (carried out on an Eltra CS 2000 using the manufacturer's recommendations with a high temperature oven at 1450 ° C, using 100 mg of product 1 + 100mg iron phosphate to improve combustion) shows a total sulfur content of 11%, which is close to our expectations (11 to 14%).

The absorption capacity of this product is then evaluated on a mercury bank shown in FIG. 2.

Figure 2 gives a schematic representation of the mercury bank used to evaluate the performance of sorbents.

The mercury bank 1 used and illustrated in Figure 2 consists of a few devices connected together. An exhaustive list is given below: - the mercury and flue gas generator (illustrated by number 2) is the central equipment of the bench designed to regulate the flow of gases 3, 4, 5, 6 (respectively N2 , 02, HCl and SO2). It also regulates the flow of mercury solution 7 (aqueous solution of HgCl 2 diluted in HCl) to the evaporator 8 with a peristaltic pump, - the evaporator 8 is an essential device and the beginning of the circuit, designed to convert the liquid solution of mercury 7 to vapor in the gas stream consisting at this stage of N2 (illustrated by number 3) and 02 (illustrated by number 4), - the mercury reduction unit 9 , 9 'is a piece of equipment similar to the evaporator 8, loaded with a catalytic material to reduce the ionic mercury metal mercury, - the furnace (not shown) is the heating unit of the reactor 10. The temperature at the interior of the reactor 10 is set at about 180 ° C, the reactor 10 is a metal cylinder of small width. It is connected to a T-fitting allowing access to the gas flow and a thermocouple for accurate recording of the temperature inside the reactor 10. It ends with a 2-μm metal filter located at the exit of the reactor 10, - the bypass 11 is located between the central valves. It helps to stabilize the levels of metallic and ionic mercury before the start of the test, - the coolers (illustrated by the number 12) are dedicated to the elimination of water in the gas flow which is a mandatory operation in because of the sensitivity of the analyzers 13 vis-à-vis the water. Their temperature is set at 1 ° C, the flowmeters 14 are devices used to measure and regulate the flow of gas. Their function is to ensure that the flow is divided equally between the two lines, - the mercury analyzers (illustrated by the number 13) are analyzers (one on each line) to detect the metallic mercury only in the stream of gas after the flow meters. The first (main line, equipped with the mercury reduction unit 9 ') shows the total mercury concentration (since the ionic mercury was reduced just before). The second shows the metal mercury concentration only, which leads to the concentration of ionic mercury by a simple subtraction.

The mercury bank is used to measure mercury removal according to the following experimental procedure.

The sorbent tested is first mixed with purified sand (washed with HCl, triple rinsing with demineralised water, size between 125 μιη and 250 μm) and poured into a fixed bed cylindrical reactor. Then, a flue gas having the following composition is injected at a total flow rate of 5.times.10.sup.-5 Nm.sup.3 / s so as to cross this bed:

Mercury: 800 μg / Nm3 Sulfur Dioxide: 70 ppm Hydrogen Chloride: 60 ppm Dioxene: 11%

The rest being dinitrogen

With two mercury analyzers, it is possible to measure the rates of both ionic and metallic mercury at the reactor outlet. To do this, the gas flow is also divided into two lines. Before arriving at the detector / analyzer, the gas flowing in the first line passes through a mercury reduction unit so as to convert, in metallic mercury, the possible fraction of mercury present in ionic form. In this way, all the mercury is measured. On the other line, only the metallic mercury is detected, which allows the calculation of the ionic mercury rate by a simple subtraction. With this device, it is possible to evaluate the mercury reduction capacity by a solid by applying the principle of the breakthrough curve. The reduction capacity is expressed in (μg Hg) / g of solid.

The test begins with a stabilization period of 10 minutes, then the gas is redirected to the reactor and the test begins. It ends when the total mercury level ("Hg tot" in Figures 3 to 7) is back to its base value. From this test, calculations are made by integrating the total mercury curve to access the mercury absorption value, in pg of Hg / g of sorbent used. In addition, a ratio of the reference value of the mercury content to the stable minimum mercury content is calculated (maximum elimination rate, in%).

About 100 mg of product 1 is mixed with 6.5 g of purified sand and poured into the fixed bed reactor. The absorption of mercury is 2000 μg / g. About 98% of the total mercury is eliminated during the first 15 minutes of the test. The results of the test are illustrated in Figure 3, where it can be seen that the product No. 1 shows mercury absorption kinetics, allowing the product 1 to reach its highest removal rate in a few seconds. After about two hours, the mercury is still absorbed in the gas phase but at a much lower rate, which is probably caused by a mercury diffusion process inside the sorbent particles.

Example 2

About 523 g of a lime milk according to the patent WO2014 / 064234 with a solids content of 45% are mixed with 12.5 g of copper chloride (97% Alfa Aesar grade) and 75 ml of ammonia (25% solution of VWR) and stirred at 300 rpm for 3 hours to form the suspension of lime milk No. 2.

A solution of calcium polysulfide is synthesized using the above-mentioned proposed method with a lime / sulfur ratio of 1: 1.5 m / m until the water is completely saturated with the polysulfide. Sulfur powder (99%) is purchased from VWR Chemicals. The solution is then filtered.

About 122 ml of this saturated calcium polysulfide solution is then mixed with the slurry of ne2 milk lime and spray-dried to obtain the product 2.

The absorption capacity of this product is then evaluated on a mercury bank according to the experimental procedure described in Example 1.

About 100 mg of product 2 are mixed with 6.5 g of purified sand and the mixture is poured into the fixed bed reactor. The mercury absorption value is 850 μg / g. About 95% of the total mercury is eliminated during the first 5 minutes of the test.

The test results are illustrated in Figure 4 where it can be seen that the product No. 2 has a behavior similar to that of the product No. 1 during the first 15 minutes of the test. After this period of time, the difference between the total value of mercury detected and the base value is much smaller and reduced faster than for the product No. 1, thus showing a limited diffusion due to the envelope layer thinner sulfur. The total absorption is interesting (850 pg / g vs 2000 pg / g, with 4 times less copper and sulfur).

Comparative Example 1, -

The absorption capacity of activated carbon according to the prior art is also evaluated on the same bank of mercury and according to the same experimental procedure as described in Example 1.

For this test, approximately 50 mg of Darco Hg-LH commercial product is mixed with 6.5 g of purified sand and the mixture is poured into the fixed bed reactor.

The mercury absorption value is 8000 μg / g. About 95% of the total mercury is removed for 70 minutes, after a stabilization period of 1 hour.

The results of the test are illustrated in Figure 5 where it can be seen, as expected, that the activated carbons have good mercury absorption values, with slower kinetics, however, because it takes almost 1 hour to reach the maximum absorption. There is no stabilization of the total mercury content below the baseline, which means that there is no diffusion, probably because of the highly porous structure of activated carbon and good accessibility of these pores.

Comparative Example 2.-

About 10 g of bentonite are mixed with 7.8 g of copper chloride and 40 ml of ammonia and stirred at 300 rpm for 5 hours. The suspension is then filtered using a Buchner filter. The solid phase is then mixed with a solution of 20.65 g of Na 2 S in 100 ml of H 2 O and stirred for 5 hours. After a second Buchner filtration, the solid phase is dried overnight to obtain a sample of bentonite impregnated with copper sulfide.

The absorption capacity of this product is then evaluated on a mercury bank according to the experimental procedure described in Example 1.

About 100 mg of copper sulfide impregnated bentonite is mixed with 6.5 g of purified sand and poured into the fixed bed reactor. The mercury absorption value is 100 μg / g. About 80% of the total mercury is eliminated during the first 5 minutes of the test.

The results of the test are illustrated in Figure 6, where it can be seen that bentonite, which is known to be a good cation exchanger, allows copper to be accessible and has good mercury oxidation capacity, but total mercury absorption is low.

Comparative Example 3.-

About 625 g of Ludox HS-40 (colloidal silica available from Sigma-Aldrich, 40% solids content) are mixed with 50 g of copper chloride and 300 ml of ammonia (25% solution) in 2 kg of ammonia. Deionized water and stirred for 3 hours at 300 rpm to form a colloidal silica suspension.

A solution of calcium polysulfide is synthesized using the proposed process with a lime / sulfur ratio of 1: 1.5 m / m until the water is completely saturated with the polysulfide. The solution is then filtered. 600 ml of this saturated calcium polysulfide solution are then mixed with said colloidal silica suspension and spray-dried to obtain the spray-dried colloidal silica product.

The absorption capacity of this product is then evaluated on a mercury bank according to the experimental procedure described in Example 1.

About 100 mg of spray-dried colloidal silica are mixed with 6.5 g of purified sand and the mixture is poured into the fixed bed reactor.

The mercury absorption value is 500 μg / g. About 93% of the total mercury is eliminated during the first 15 minutes of the test.

The results of the test are illustrated in FIG. 7, where it can be seen that the spray-dried colloidal silica sample has good kinetics and a good removal rate, but a low total mercury absorption compared to product No. because of the amount of impregnating compound trapped between the colloidal silica particles in the sorbent core.

Example 3.-

The test is performed to evaluate the protective effect of lime sorbent compositions according to the present invention.

Indeed, as mentioned above, lime, when in contact with flue gases, tends to reduce Hg2 + Hg, significantly complicating the task of mercury absorption in the flue gas.

The present invention makes it possible to significantly reduce this undesirable effect by producing core-shell type particles, in which the lime is protected by an envelope comprising a sulfur-based compound.

To evaluate this protective effect, a test was developed to measure and compare the propensity of three different sorbents to reduce ionic mercury present in a metal mercury gas. These three different sorbents consist of: • Sorbant 1: (SI) composition of slaked lime in conventional powder (comparative example 1). Sorbent 2: (S2) solid mixtures of conventional slaked lime and calcium polysulfide (Comparative Example 2). • Sorbent 3: (S3) composition comprising core-shell type particles whose core is slaked lime and the envelope is calcium polysulfide (according to the invention).

Sorbent 3 is prepared according to the following procedure. A solution of calcium polysulfide is synthesized using the above-mentioned method with a lime / sulfur ratio of 1: 1.5 m / m. Then, different amounts of this solution are mixed with a milk of lime (aqueous suspension of conventional lime in water) for 1 hour and then introduced into an atomizer dryer to obtain core-shell type particles having different amounts of calcium polysulfide. During spray drying, it is believed that most calcium polysulfide molecules are deposited on the surface of the lime, creating an envelope. Intentionally, a metal salt source is not added in this process so as not to distort the test results because of the oxidizing nature of this compound (which would then oxidize the Hg0 to Hg2 +).

Sorbent 2 (52) is prepared by spray drying the same calcium polysulfide solution alone to obtain spray-dried pure polysulfide particles. The resulting product is then mixed with conventional powdered lime (SI) to create a solid mixture with the same amounts of polysulfide as for sorbents 3 (S3).

The test of the protective effect is carried out on the mercury bank, described in Example 1, according to the following experimental procedure.

The tested sorbent is first mixed with purified sand (washed with HCl, triple rinsed with deionized water, size between 125 μm and 250 μm) and poured into a cylindrical fixed bed reactor. Then, a combustion gas having the following composition is injected at a total flow rate of 9.33 × 10 -5 Nm 3 / s so as to cross this bed:

Mercury: 800 μg / Nm3 Sulfur Dioxide: 500 ppm Hydrogen Chloride: 90 ppm Dioxene: 7%

The rest being dinitrogen

The test begins with a stabilization period of 10 minutes, then the gas is redirected to the reactor and the test begins. It ends when the metal mercury level is back to its original value. According to this test, calculations are made by integrating the metal mercury curve to obtain a propensity to reduce mercury, expressed in μg of Hg / g of sorbent used.

The results are presented in FIG. 8 where it can be seen that a composition of conventional powdered lime (sorbent 1) exhibits a propensity to reduce the ionic mercury to metallic mercury (Hg2 + -> Hg °) which is equal to 110 pg /boy Wut.

It can also be seen in this figure that core-shell particles (sorbent 3) have a lower propensity to reduce ionic mercury than conventional slaked lime, due to the protective effect generated by the polysulfide shell. calcium covering the lime core.

In addition, it can be seen that solid mixtures of conventional slaked lime and calcium polysulfide (sorbent 2) do not exhibit as good a protective effect as core-shell particles, since for a given amount of polysulfide calcium, the value obtained with the sorbent 2 is almost always higher than that obtained with the sorbent 3. In addition, such a solid mixture, depending on the amount of calcium polysulfide, may even have a propensity to further reduce the mercury ionic than conventional slaked lime since the value obtained with sorbents 2 may be greater than that of reference (sorbent 1).

It is obvious that the present invention is not limited to the embodiments described and that variations can be applied without departing from the scope of the appended claims.

Claims (28)

"CLAIMS" A lime sorbent composition containing particles for removing heavy metals, in particular mercury, from flue gas, said particles being core-shell type particles characterized in that the core comprises a calco-magnesium compound having the formula aCaC03.bMgC03.xCa0.yMgO.zCa (OH) 2.tMg (OH) 2.ul, where I represents the impurities, a, b, z and t are each mass fractions> 0 and <100% , x and y are each mass fractions> 0 and <50%, u is a mass fraction> 0 and <20%, the total a, b, x, y, z, t and u is equal to 100% by weight , based on the total weight of said at least one calco-magnesium compound, characterized in that said core is covered with an envelope comprising a metal salt and a sulfur-based compound, said sulfur-based compound being present in a quantity sufficient to allow a protective effect of the composition, preferably of the core and having a thickness less than 20 μm and greater than 50 nm. A lime sorbent composition according to claim 1, wherein said sulfur compound has the formula AaSpOv where α, β and y are each a mass fraction with β * 0 and where A is selected from the group consisting of calcium, magnesium, potassium, sodium and their mixture. A lime sorbent composition according to claim 1 or claim 2, wherein said sulfur compound is selected from the group consisting of sulphide salts, such as calcium sulphide, dithiocarbamates, salts sulfate, such as calcium sulfate, dithiocarbamate-based polymers, polysulfide salts, such as calcium polysulfide, and mixtures thereof. A lime sorbent composition according to any one of claims 1 to 3, wherein said metal salt is selected from the group consisting of titanium, vanadium, manganese, iron, nickel, copper, zinc and their salts. mixture, preferably copper. A lime sorbent composition according to any one of claims 1 to 4, wherein said metal salt is a copper sulfide or a copper polysulfide. A lime sorbent composition according to any one of claims 1 to 5, further comprising a doping agent selected from the group consisting of alkali metal halides such as sodium or potassium halides, sodium halides, and the like. alkaline earth metals such as calcium or magnesium halides, ammonium halides and mixtures thereof. The lime sorbent composition according to any one of claims 1 to 6, further comprising a dispersing agent selected from the group consisting of (poly) sulfates, such as sodium dodecyl sulfate (SDS), poly (sulfonates), (poly) phosphates, (poly) phosphonates, such as diethylenetriamine penta (methylene phosphonic acid) (DTPMP), polyols and mixtures thereof. A lime sorbent composition according to any one of claims 1 to 7, wherein z + t> 60%, preferably> 70%, preferably> 80%, more preferably> 90%, in particular> 93 % by weight, relative to the total weight of said at least one calco-magnesium compound. A lime sorbent composition according to any one of claims 1 to 8, wherein the calco-magnesium compound has a particle size distribution wherein di0 is in the range of 0.2 to 3 μm, preferably from 0.5 to 2 μm; d90 is in the range of 2 to 50 μm, preferably 5 to 40 μm, d50 is in the range of 0.5 to 50 μm, preferably 1 to 30 μm. The lime sorbent composition according to any one of claims 1 to 9, wherein said shell has a thickness of less than 10 μm, preferably less than 5 μm, in particular less than 2 μm, advantageously less than at 1 μm, more preferably less than 700 nm, and greater than 75 nm, preferably greater than 100 nm, in particular greater than 150 nm. A lime sorbent composition according to any one of claims 1 to 10, wherein the ratio of calco-magnesium compound to sulphide sulfur is in the range of 15: 1 m / m at 1: 1 m / m, preferably from 10: 1 to 2: 1 and preferably is 5: 1, advantageously is 4: 1, more preferably is 3: 1, preferentially is 2.5 : 1 m / m, based on the lime-based sorbent composition. A lime sorbent composition according to any one of the preceding claims wherein the ratio of said calco-magnesium compound to said metal salt is in the range of 15: 1 m / m to 1: 1 m. preferably from 10: 1 to 2: 1, advantageously from 8: 1 to 3: 1, preferably from 7: 1 to 4: 1 and preferably from 5: 1 m / m to the sorbent composition based on lime. 13. A process for producing a lime sorbent composition comprising the steps of: i) supplying an aqueous suspension of a calco-magnesium compound having the formula aCaC03.bMgC03.xCa0.yMg0.zCa (OH) 2.tMg (OH) 2.ul, where I represents the impurities, a, b, z and t are each mass fractions> 0 and <100%, x and y are each mass fractions> 0 and <50%, u is a mass fraction> 0 and <20%, the total a, b, x, y, z, t and u is 100% by weight, based on the total weight of said at least one calco-magnesium compound in a atomizer dryer ii) supplying a solution of calcium magnesium polysulfide in said atomizing dryer, said supply of said aqueous suspension of the calcium-magnesium compound and the feeding of the calcium magnesium polysulfide solution being carried out separately or together, possibly under the form of a premix of said calcium-magnesium polysulfide solution e t of said aqueous suspension of the calco-magnesian compound iii) forming a spray-dried lime-based composition having particles which are core-shell type particles, wherein the core comprises said calco-magnesium compound, said core being coated an envelope having a thickness greater than 50 nm and less than 20 μm, which makes it possible to provide a protective effect to the composition with respect to the medium of the flue gases and comprising at least one sulfur-based compound, said method further comprises the step of using an ammonia-metal complex in the form of a solution (1) to said calcium-magnesium polysulfide solution or said aqueous suspension of the calco-magnesian compound or to the meadow mixture of said calcium magnesium polysulfide solution and said aqueous suspension of the calcium-magnesium compound, said spray-dried lime-based composition formed e having particles which are core-shell type particles further comprising at least one metal salt in the shell or (2) said spray dryer separately or together with said calcium magnesium polysulfide solution or with said aqueous suspension of the calcium-magnesium compound or with the premix of said calcium magnesium polysulfide solution and said aqueous suspension of the calcium-magnesium compound, said formed spray dried lime-based composition having particles which are particles of the type core-shell further comprising at least one metal salt in the shell or (3) the spray-dried lime-based composition having particles which are core-shell particles, said shell consisting of a first layer comprising said sulfur compound and a second layer comprising at least one metal salt. 14. The method of claim 13, wherein said aqueous suspension of the calco-magnesian compound has a solid content of between 30 and 45% by weight relative to the total weight of the aqueous suspension of calco-magnesian compound. The method of claim 13 or claim 14, wherein the aqueous suspension of the calco-magnesium compound comprises particles having a particle size distribution wherein d50 is in the range of 0.5 to 20 μm, preferably 0 to , 5 to 10 μm and more preferably 1 to 2.5 μm. The process according to any one of claims 13 to 15, wherein said sulfur compound has the formula AaSpOv where α, β and y are each a mass fraction with β * 0 and where A is selected from the group consisting of calcium, magnesium, potassium, sodium and their mixture. The process according to any of claims 13 to 16, wherein said sulfur compound is selected from the group consisting of sulphide salts, such as calcium sulphide, dithiocarbamates, sulphate salts, such as calcium sulphate, polymeric dithiocarbamates, polysulfide salts, such as calcium polysulfide, and mixtures thereof. The method of any one of claims 13 to 17, wherein said metal salt is selected from the group consisting of titanium, vanadium, manganese, iron, nickel, copper, zinc and their mixtures, preferably copper . The method of any of claims 13 to 18, wherein said metal salt is a copper sulfide or a copper polysulfide. The method of any one of claims 13 to 19, wherein said ammonia-metal complex is an ammonia-copper halide complex, preferably an ammonia-copper chloride complex. The method of any one of claims 13 to 20, further comprising a step of adding a doping agent selected from the group consisting of alkali metal halides such as sodium or potassium halides, metal halides. alkaline earth metals such as calcium or magnesium halides, ammonium halides and mixtures thereof. The method of claim 21, wherein said doping agent is added to the calcium magnesium polysulfide solution. The method of claim 21, wherein said doping agent is added to the spray-dried lime-based composition. The method of any one of claims 13 to 23, further comprising a step of adding a dispersing agent selected from the group consisting of (poly) sulfates, such as sodium dodecyl sulphate (SDS), ) sulfonates, (poly) phosphates, (poly) phosphonates, such as diethylenetriamine-penta (methylenephosphonic acid) (DTPMP), polyols and mixtures thereof with the aqueous suspension of the calc-magnesium compound corresponding to the formula aCaC03.bMgC03 .xCa0.yMg0.zCa (0H) 2.tMg (0H) 2.ul. 25. A process according to any one of claims 13 to 24, wherein in the calco-magnesian compound corresponding to the formula aCaC03.bMgC03.xCa0.yMg0.zCa (0H) 2.tMg (OH) 2.ul, z + t> 60%, preferably> 70%, preferably> 80%, more preferably> 90%, in particular> 93% by weight, relative to the total weight of said at least one calco-magnesium compound. 26. A method according to any one of claims 13 to 25, wherein said envelope has a thickness of less than 10 μm, preferably less than 5 μm, in particular less than 2 μm, advantageously less than 1 μm, more preferably less than 1 μm, more preferably less than 1 μm, more preferably less than 1 μm, more preferably less than 1 μm, more preferably less than 700 nm, and greater than 75 nm, preferably greater than 100 nm, in particular greater than 150 nm. 28. A process according to any one of claims 13 to 26, wherein the ratio of calco-magnesium compound to sulphide sulfur is in the range of 15: 1 m / m to 1: 1 m / hr. m, preferably from 10: 1 to 2: 1 and preferably from 5: 1, advantageously from 4: 1, more preferably from 3: 1, preferably from 2.5: 1 m / m in the solution of calcium magnesium polysulfide. The method of any one of claims 13 to 27, wherein the ratio of said calco-magnesium compound to said metal salt is in the range of 15: 1 m / m to 1: 1 m / m, preferably from 10: 1 to 2: 1, advantageously from 8: 1 to 3: 1, preferably from 7: 1 to 4: 1 and preferably from 5: 1 m / m to the solid content in the suspension of calcium-magnesium particles on which the metal is dispersed.