The present application claims priority of French patent application No. 1356160, filed on 2 6 June 2013, the complete contents of which are incorporated here by reference for all purposes. Field of technology The invention relates to a method for preparing alkali bicarbonate particles and to the alkali bicarbonate particles obtainable by said method. The invention also relates to a method carried out for the purpose of controlling the bulk density of powders consisting of said particles, and/or carried out for the purpose of controlling the size of said particles. Prior art The alkali metal bicarbonates, such as sodium bicarbonate (NaHCC>3) and potassium bicarbonate (KHC03) , are products that have numerous properties, and are of interest in wide areas of use, extending from the pharmaceutical industry, to human and animal nutrition, passing through detergents, treatment of acid flue gases, and treatment of nonferrous metals. The commonest means for manufacturing these bicarbonates in the form of solid particles consists of crystallization of the bicarbonates by carbonation, with carbon dioxide, of a solution of carbonate of the corresponding alkali metal (sodium carbonate or potassium carbonate for example) or of a solution of hydroxide of the corresponding alkali metal (sodium hydroxide or potassium hydroxide for example). Generally these solutions are aqueous. However, water/alcohol solvent mixtures such as water/ethanol mixtures are also often used. The bicarbonates are often also crystallized by controlled cooling of solutions of bicarbonates, or by evaporation of such solutions. Most of the uses of alkali bicarbonates in the form of particle powders require control of their bulk density (designated 'BD' in the present text). This control is important so as to be able to control the weight that can be put in given packaging (bags, big-bags , for example) or to control the weight loaded in bulk transport trucks that have fixed standard volumes. Moreover, the industrial users of such powders often have volumetric metering apparatus which has to be adjusted if the density of the powders delivered changes, and this causes loss of time if using different batches or batches from various production sites. It is therefore desirable to have a simple means in production of such bicarbonates for adjusting their BD. Furthermore, the end users are interested in defined granulometric fractions that are optimized for their applications as a function of the required properties: for example dissolution rates, capacity for mixing with other powders homogeneously without segregation, flowability of the powders for ease of handling (measured for example by the slope angle, or by the flow rate through a calibrated orifice), etc. It is therefore important to have a simple and effective means so as to be able to adjust the granulometric fraction of the bicarbonate particles manufactured. In the existing methods, several parameters can be adjusted in order to control the density of the powders obtained as well as their granulometries. However, these two characteristics are not in a one-to-one relationship. In fact, the bulk density depends on several factors, among which we may mention: the size of the particles, the granulometric distributions (distributions by weight of the particles in different granulometric fractions, unimodal or multimodal distributions, wide or narrow granulometric distributions), the form factors of the particles (roundness, robustness, size of greatest concavity, surface concavity index), etc. Once again it is useful to have a simple means in production of such bicarbonates, for adjusting the size of the particles and notably their oversize or undersize on sieves with different cutoff diameters. Summary of the invention The present invention relates to a method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate (solution designated hereinafter: 'the solution') in the presence of an additive in the solution, characterized in that: the additive is selected from the following compounds: • sulfates, in particular sodium sulfate and organosulfates; • sulfonates; • polysulfonates; • amines, in particular primary amines, cyclic amines, quaternary amines comprising at least one carboxylic acid group; • hydroxysuitaines; • polycarboxylates; • polysaccharides; • polyethers and ether-phenols; • sulfosuccinates; • amidosulfonates; • aminosulfonates; • alkali metal hexametaphosphate in particular of sodium or potassium; • phosphates, in particular organophosphates, and phosphonates; - and the additive is present in the solution at a concentration of at least 1 ppm, advantageously at least 5 ppm, more advantageously at least 10 ppm. The inventors in fact found, surprisingly, that the additives listed above, used during crystallization of the alkali metal bicarbonate particles, made it possible to obtain particles having a synergy between their granulometry, their form factor, or even their surface electric charge potentials, allowing either a large, adjustable increase in bulk density of the particles obtained, or a decrease in density. They also make it possible to modulate their granulometries, and their widening or narrowing of the granulometric distributions. The invention also relates to the alkali metal bicarbonate particles thus obtained, and/or obtainable by the present method. Description of the figures Fig. 1 is a light microscope photograph of sodium bicarbonate particles obtained by carbonation without addition of additive. Fig. 2 is a light microscope photograph of sodium bicarbonate particles obtained by carbonation with addition of additive (alginic acid 50 ppm. Example 3, Test 11(b)). Definitions In the present specification, selection of an element from a group of elements also describes explicitly: - selection of two or selection of several elements of the group, - selection of an element from a subgroup of elements that consists of the group of elements from which one or more elements have been removed. Moreover, it has to be understood that the elements and/or the characteristics of an apparatus, a method or a use, described in the present specification may be combined in all possible ways with the other elements and/or characteristics of the apparatus, method, or use, explicitly or implicitly, while remaining within the scope of the present specification, and must be considered as forming an integral part of the present specification. In the passages of the present specification given below, different embodiments, or variants of the invention are defined in more detail. Each embodiment or variant thus defined may be combined with another embodiment, or with another variant, and this for each embodiment or variant unless stated otherwise or manifestly incompatible when the range of one and the same value parameter is unconnected. In particular, any variant indicated as being preferred or advantageous may be combined with another variant or with the other variants indicated as being preferred or advantageous. In the present specification, the description of a range of values for a variable, defined by a lower limit, or an upper limit, or by a lower limit and an upper limit, also comprises the embodiments where the variable is selected respectively from the range of values: excluding the lower limit, or excluding the upper limit, or excluding the lower limit and the upper limit. In the present specification, the description of several successive ranges of values for one and the same variable also comprises the description of embodiments where the variable is selected from any other intermediate range included in the successive ranges. Thus, for example when it is described that "the quantity X is generally above 10, advantageously above 15", the present description also describes the embodiment where; "the quantity X is above 11", or also the embodiment where: "the quantity X is above 13.74", etc.; 11 or 13.74 being values between 10 and 15. The term "comprising" includes "consisting essentially of" as well as "consisting of". In the present specification, the use of "a" or "one" in the singular, also comprises the plural ("some"), and vice versa, unless the context clearly indicates the opposite. As an example, "an additive" denotes one additive or more than one additive. If the term "about" is used before a quantitative value, the latter corresponds to a variation of ±10% of the nominal quantitative value, unless stated otherwise. The term "average" denotes a number average, unless stated otherwise. The term "ppm" denotes parts per million expressed by weight (for example 1 ppm = 1 mg per kilogram). The term 'granulometric spread', or 'span', of a granulometric distribution of particles corresponds in the present specification to the following ratio: (D90-D10J/D50, the term D90 denoting the diameter for which 90 wt% of the particles have a size less than or equal to D90, the term D50 denoting the diameter for which 50 wt% of the particles have a size less than or equal to D50 (average size by weight) , the term D10 denoting the diameter for which 10 wt% of the particles have a size less than or equal to D10, Detailed description of the invention The present invention relates to a method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate (solution called hereinafter: 'the solution') in the presence of an additive in the solution, for modifying: - the granulometric distribution of the particles, in particular the fraction of 'fine' particles under 125 pm, or the fraction of 'coarse1 particles above 250 um, or the 'granulometric spread' (or 'span') - or their BD, - or their dissolution times, - or their slope angles, - or their flow rate, - or their abrasion index. In fact the inventors discovered that the various additives mentioned hereunder are modifiers of crystallization of the crystals of alkali metal bicarbonates that may be used in a small amount in a solution of alkali metal carbonate and/or bicarbonate, in which the crystals of alkali bicarbonate are crystallized. In the present specification, 'alkali metal bicarbonate particles' means particles comprising at least 60%, advantageously at least 75%, more advantageously at least 85%, even more advantageously at least 90% or at least 95% or at least 99% of alkali metal bicarbonate. Generally in the present invention, the alkali metal bicarbonate particles comprise at most 40%, advantageously at most 25%, more advantageously at most 15%, even more advantageously at most 10% or at most 5% or at most 1% of alkali metal carbonate. In the present invention, the alkali metal particles may also contain small contents of ammonium bicarbonate, in general at most 2%, or at most 1% or at most 0.7% of ammonium bicarbonate. This is the case notably when the particles are crystallized in solutions containing ammonia, such as in the crystallization of sodium bicarbonate in the Solvay ammonia process, for manufacture of sodium carbonate. The alkali metal bicarbonate particles obtained in this method are generally denoted by the expression 'crude bicarbonate1 (cf. Qllmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH Verlag GmbH & Co, Weinheim, "Sodium carbonate", Vol. 33 page 307). In the present invention, the alkali metal bicarbonate may be bicarbonate of: lithium, sodium, potassium, rubidium or cesium. Advantageously the alkali metal bicarbonate is sodium bicarbonate or sodium bicarbonate. More advantageously the alkali metal bicarbonate is sodium bicarbonate. In particular, in the present invention, the alkali metal bicarbonate particles are advantageously particles comprising at least 99% of sodium bicarbonate and less than 1% of sodium carbonate or water. The present invention is based on the discovery of the effects of the additives mentioned in the present specification, for which the different variants of the method and/or of the product obtained by said variants of the method are described in more detail below. Item 1. Method for preparing, by crystallization, alkali metal bicarbonate particles starting from a solution of alkali metal carbonate and/or bicarbonate (solution designated hereunder: 'the solution') in the presence of an additive in the solution, characterized in that: the additive is selected from the following compounds: • sulfates, in particular sodium sulfate and organosulfates; • sulfonates; • polysulfonates; • amines, in particular primary amines, cyclic amines, quaternary amines comprising at least one carboxylic acid group; • hydroxysultaines; • polycarboxylates; • polysaccharides; • polyethers and ether-phenols; • sulfosuccinates; • amidosulfonates; • aminosulfonates; • alkali metal hexametaphosphate in particular of sodium or of potassium; • phosphates, in particular organophosphates, and phosphonates; - and the additive is present in the solution at a concentration of at least 1 ppm, advantageously of at least 5 ppm, more advantageously of at least 10 ppm. Item 2, Method according to item 1, in which the additive is an organosulfate and is selected from the following compounds: • alkyl sulfates comprising from 4 to 20 carbons, preferably from 8 to 16 carbons, or from 8 to 14 carbons such as lauryl sulfate (dodecyl sulfate), • alkyl ether sulfates (such as Rhodapex ESB- 70/A2); Item 3. Method according to item 1, in which the additive is a sulfonate and is selected from the following compounds: • alkyl sulfonates; • alkylbenzene sulfonates or alkylnaphthalene sulfonates; in particular disodium dodecyl (sulfonatophenoxy) benzene-sulfonate (CAS 28519-02-0, such as Rhodacal DSB), disodium 2,2' (or 3,3')- oxybis[5(or 2)-dodecylbenzenesulfonate (CAS 25167-32- 2), sodium alkyl naphthalene sulfonate (such as Rhodacal BX-78), isopropylamine dodecylbenzene sulfonate (such as Rhodacal 330); • polysulfonates, in particular polyvinylsulfonic acid and alkali metal salts thereof; • aminosulfonates in particular: N-methyltaurine, sodium N-methyltaurinate, isopropylamine dodecylbenzene sulfonate; amidosulfonates in particular those with more than 6 carbons, in particular; sodium 2-[methyloleoylamino]ethane-1-sulfonate (such as Geropon 1/11); • sulfosuccinates in particular sodium docusate (such as Geropon SDS, Aerosol OT, Constonate, Diomedicone, Clestol, Complemix, Defilin, Dioctlyn, Dioctylal, Diosuccin); Item 4. Method according to item 1, in which the additive is an amine, and is selected from the following compounds; • primary amines, in particular hexamethylenediamine; • quaternary amines comprising at least one carboxylic acid group, in particular the quaternary amines of formula: (H3C) 3-N-R-COOH with R = (CH2)n and n = 1 to 4, in particular trimethylglycine (or betaine); • quaternary amines comprising at least one sulfonate group, in particular hydroxysultaines, such as cocamidopropyl hydroxysultaine (such as Mirataine CBS) ; • cyclic amines (such as Miranol DM); Item 5. Method according to item 1, in which the additive is a polycarboxylate, and is preferably selected from the following compounds: polyacrylates, in particular crosslinked polyacrylates, or copolymers of acrylic and maleic acids, or of sodium prop-2-enoate (such as: Polyco, Rhotex GS, Acrysol lmw-45N, Hiviswako 105); in particular polyacrylates or acrylic and. maleic copolymers, or alkali metal salts thereof, of average molecular weight below 8000 g/mol, or greater than or equal to 8000 g/mol; Item 6. Method according to item 1, in which the additive is a polysaccharide, and is preferably selected from the following compounds: • guar gums and their derivatives, in particular hydroxypropylguar (such as Jaguar HP-105); • alginic acid and its salts such as of sodium or of calcium or of copper (such as: Kaltostat, Calginat, Landalgine, Kalrostat, Kelacid, Vocoloid, Xantalgin); • carboxymethylcellulose (such as . Aquaplast, Carmethose, Cellofas, Cellpro, Cellugel, Collowel, Ethoxose, Orabase, Lovosa); Item 7. Method according to item 1, in which the additive is a polyether or an ether-phenol or a polyaromatic ester, and is preferably selected from the following compounds: • polyethylene glycol mono(tristyrylphenyl) ether; • ethoxylated oleyl alcohols (such as Rhodasurf ON/870-E); • branched octylphenoxy poly(ethyleneoxy)ethanols (such as Igepal CA630); • ethoxylated polyarylphenols, in particular ethoxylated tristyrylphenol (such as Soprophor CY/8); Item 8. Method according to item 1, in which the additive is a sulfosuccinate, and is preferably selected from the following compounds: • sulfosuccinates, in particular: sodium docusate (such as Aerosol OT, Constonate, Diomedicone, Clestol, Complemix, Defilin, Dioctlyn, Dioctylal, Diosuccin); Item 9. Method according to item 1, in which the additive is an organophosphate or a phosphonate, and is preferably selected from the following compounds: • polyoxyethylene alkylether phosphates, whose alkyl chain advantageously comprises from 6 to 20 carbons, in particular polyoxyethylene octadecenyl ether phosphate (such as Lubrophos LB/400-E); • poly(oxy-1,2-ethanediyl),.alpha.- (dinonylphenyl)-omega-hydroxy-phosphate (such as RHODAFAC RM-510); • alkyl-phosphonic or hydroxyalkyl-phosphonic acids of semi-structural formula ( (HO) 2OP) -R with R an alkyl group, or hydroxyalkyl group, in particular with R an alkyl group with 1 to 3 carbons, or R a hydroxyalkyl group comprising 1 to 3 carbons and 1 to 2 hydroxyl (-0H) groups, or salts thereof in particular of alkali metal such as sodium or potassium; • alkyl-diphosphonic acids, in particular those of semi-structural formula { (HO) 20P)-R-(PO(OH)2) and with R an alkyl group, or a hydroxyalkyl group, in particular with R an alkyl group with 1 to 3 carbons, or R a hydroxyalkyl group comprising 1 to 3 carbons and 1 to 2 hydroxyl (-OH) groups (such as etidronic acid { (HO) 2OP}- C(CH3)OH-(PO(OH)2) ) , or salts thereof in particular of alkali metal such as sodium or potassium; • nitrilo-alkyl-phosphonic acids of semi- structural formula ( (HO) 2OP-R) 3-N with R an alkyl chain with 1 to 5 carbons, or salts thereof in particular of alkali metal such as sodium or potassium, in particular nitrilo-tri-(methanephosphonic) acid or salts thereof in particular of alkali metal such as sodium or potassium. Item 10. Method according to any one of items 1 to 9, in which the additive is present at a concentration of at most 200 ppm, advantageously of at most 100 ppm, more advantageously of at most 70 ppm. Item 11. Method according to any one of items 1 to 10, in which crystallization of alkali metal bicarbonate particles is carried out by cooling the solution. Item 12. Method according to item 11, in which cooling of the solution is carried out between 70° and 30°C. Item 13. Method according to any one of items 1 to 12, in which crystallization of alkali metal bicarbonate particles is carried out by carbonation of the solution with carbon dioxide. Item 14. Method according to item 13, in which carbonation of the solution is carried out at a temperature of at least 20 °C, advantageously at least 30°C, and preferably of at most 95°C, more preferably of at most 90°C. Item 15. Method according to any one of items 1 to 14, in which crystallization of alkali metal bicarbonate particles is carried out by evaporation of at least a proportion of the solution. Item 16. Method according to any one of items 1 to 15, in which crystallization of alkali metal bicarbonate particles is carried out by concomitant cooling and carbonation of the solution; or by concomitant cooling and carbonation and evaporation of the solution. Item 17. Method according to any one of items 13, 14 or 16, in which carbonation of the solution with carbon dioxide is carried out with a gas comprising: 10 to 100% of CO2 by volume based on dry gas, advantageously at least 20% of C02 by volume based on dry gas, more advantageously at least 30% of CO2 by volume based on dry gas, even more advantageously at least 35% of CO2 by volume based on dry gas. Item 18. Method according to any one of items 1 to 17 carried out at a temperature between 20°C and 95°C, preferably between 30 and 90°C. Item 19. Method according to any one of items 1 to 18, in which addition of the additive is carried out for controlling or modifying: • the granulometry (such as D10, D50, D90 or the span), or • the undersize or oversize on a sieve at 125 urn, or • the undersize or oversize on a sieve at 250 um. of the alkali metal bicarbonate particles obtained after separation of the crystallization solution and of the alkali bicarbonate particles. Item 20. Method according to any one of items 1 to 19, in which addition of the additive is carried out for controlling or modifying: • the dissolution time, or • the bulk density (BD), or • the flow rate in a calibrated orifice, • the abrasion index, of the alkali metal bicarbonate particles obtained after separation of the crystallization solution and of the alkali bicarbonate particles, then drying of the alkali metal bicarbonate particles. Item 21. Method according to item 20 according to which addition of the additive makes it possible to increase the BD, and the increase in the BD of the alkali metal bicarbonate particles is at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 8 0%, or at least 90%, or at least 100% relative to the BD of alkali metal bicarbonate particles obtained in the same conditions of crystallization but without addition of additive. Item 22. Method according to item 21 according to which addition of the additive makes it possible to reduce the BD, and the reduction of the BD of the alkali metal bicarbonate particles is at least 10%, or at least 15% relative to the BD of alkali metal bicarbonate particles obtained in the same conditions of crystallization but without addition of additive. Item 23. Method according to item 19 carried out for controlling the size of the alkali bicarbonate particles, in particular for controlling the sieve undersize at 125 urn or sieve oversize at 250 urn of the alkali bicarbonate particles. Item 24. Method according to any one of items 19 to 21, in which addition of additive makes it possible to: • increase the granulometry (such as D10, D50, D90) or reduce the span, or • reduce the sieve undersize at 125 urn,, or • increase the sieve oversize at 250 pm, or • reduce the dissolution time, or • increase the bulk density (BD), or • reduce the abrasion index, of the alkali metal bicarbonate particles relative to one and the same preparation that would be .performed without additive, and in said method the additive is selected from the additives listed in item 6 {the polysaccharides), in particular alginic acid or carboxymethylcellulose. Item 25. Method according to item 24, in which crystallization of the alkali bicarbonate particles is carried out by cooling and/or by carbonation. Item 26. Method according to any one of items 19 to 21, in which addition of additive makes it possible to: • increase the granulometry (such as D10, D50, D90) or reduce the span, or • reduce the sieve undersize at 125 urn, or • increase the sieve oversize at 250 pm, or • reduce the dissolution time, or • increase the bulk density (BD), or • reduce the abrasion index, of the alkali metal bicarbonate particles relative to one and the same preparation effected without additive, and in which said method the additive is selected from the additives listed in item 5 (polycarboxylates) and selected from; polyacrylates, in particular crosslinked, or acrylic and maleic acid copolymers or alkali metal salts thereof, of average molecular weight greater than or equal to 8000 g/mol. Item 27. Method according to any one of items 19, 20 or 22, in which addition of additive makes it possible to: • reduce the granulometry (such as D10, D50, D90) or increase the span, or • increase the sieve undersize at 125 um, or • reduce the sieve oversize at 250 um, or • reduce the dissolution time, or • reduce the bulk density (BD), or • increase the abrasion index, of the alkali metal bicarbonate particles relative to one and the same preparation effected without additive, and in which said method the additive is selected from the additives listed in item 5 (polycarboxylates) and selected from: polyacrylates, in particular crosslinked, or acrylic and maleic acid copolymers or alkali metal salts thereof, of average molecular weight below 8000 g/mol. Item 28. Method according to item 26 or 27, in which crystallization of the alkali bicarbonate particles is carried out by cooling and/or by carbonation. Item 29, Method according to any one of items 19, 20 or 22, in which addition of additive makes it possible to: • reduce the granulometry (such as D10, D50, D90) or increase the span, or • increase the sieve undersize at 125 um, or • reduce the sieve oversize at 250 um, or • reduce the dissolution time, or • reduce the bulk density (BD), or • increase the abrasion index, of the alkali metal bicarbonate particles relative to one and the same preparation effected without additive, and in which said method the additive is selected from the additives listed in item 9 (an organophosphate or a phosphonate). Item 30. Method according to item 29, in which crystallization of the alkali bicarbonate particles is carried out by cooling and/or by carbonation. Item 31. Alkali metal bicarbonate particles obtainable by the method according to any one of items L to 30 and comprising at least 10 ppm, advantageously at least 20 ppm of the additive. - 16 - The additives are used in the present invention, in general, at a concentration in the solution of at least 1 ppm, advantageously at least 5 ppm, more advantageously at least 10 ppm. The amounts in ppm (parts per million) are expressed by weight and correspond to the amount of additive introduced into the solution if crystallization is effected in a batch (discontinuous) process or corresponds to the proportion, of additive introduced relative to the solution fed into the crystallization apparatus if crystallization is carried out continuously. They correspond to the mg of active substance of additive per kg of solution. A low concentration of these additives is sufficient to have a remarkable effect on the bicarbonate particles obtained. However, a concentration of additives that is too high adversely affects the purity of the alkali metal bicarbonates. In the present invention the additive is present at a concentration of at most 200 ppm, advantageously of at most 100 ppm, more advantageously of at most .70 ppm, even more advantageously of at most 50 ppm. Amounts of about (i.e. ± 10%) of: 10 ppm, or 20 ppm, or 50 ppm of additive in the solution are particularly advantageous. However, the amounts of additives may be adjusted in order to adjust the desired value of the parameter, for example BD, or the parameter of the granulometric sieve oversize, in particular at 125 urn or at 250 um. The solution of alkali metal carbonate and/or bicarbonate is preferably an aqueous solution comprising from 1 to 30 wt% of alkali metal carbonate and/or comprising 1 to 18 wt% of alkali metal bicarbonate. Crystallization of the particles according to the invention may be done with seeding or without seeding of crystals of alkali metal bicarbonate. In a preferred embodiment according to the invention, the solution is seeded. In the case of batch crystallization with seeding,, an amount of seeding of the order of 0.1 to 10% of bicarbonate crystals is introduced before the start of crystallization of the bicarbonate particles according to the invention. In the case of crystallization carried out continuously, the suspension density of the bicarbonate crystals executing production in the crystallization apparatus constitutes the seeding itself. In this case, suspension densities of alkali metal bicarbonate particles relative to the weight of the suspension of particles in the crystallization mother liquor are generally from 0.1 to 25 wt%. In a first embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by cooling the solution. In a second embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by carbonation of the solution with carbon dioxide. In a third embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by evaporation of at least a proportion of the solution. Each of the three embodiments mentioned above may be combined in the present invention. In particular, in a fourth embodiment according to the invention, crystallization of alkali metal bicarbonate particles is carried out by concomitant cooling and carbonation of the solution; or by concomitant cooling and carbonation and evaporation of the solution. In the case when crystallization is effected by carbonation with a gas comprising carbon dioxide, injection of the gas might not be saturated by the solvent of the solution at the temperature of the suspension of the crystallizing particles. In this case, injection of the gas is accompanied by partial evaporation of the solvent of the solution. This also causes cooling of the solution. In each of the above embodiments, it is preferred in the present invention when at least two liquid and gaseous phases are present during preparation of particles by crystallization, when the continuous phase is the liquid phase and when the gas phase is a dispersed phase in the liquid phase. In each of the above embodiments when there is carbonation of the solution, carbonation of the solution with carbon dioxide is carried out with a gas generally comprising: 10 to 100% of C02 by volume based on dry gas, advantageously at least 20% of CO2 by volume based on dry gas, more advantageously at least 30% of C02 by volume based on dry gas, even more advantageously at least 35% of C02 by volume based on dry gas In the present invention, preparation of alkali metal bicarbonate particles by crystallization is generally carried out at a temperature between 20°C and 95 °C. It is advantageous for the temperature to be at least 30°C. It is also advantageous for the temperature to be at most 90°C. The bulk density of the alkali metal bicarbonate particles is measured by the following procedure: the particles are put in a smooth steel hopper of truncated shape (top diameter 53 mm, bottom diameter 21 mm, height 58 mm) closed at the bottom by a closing device consisting of a horizontal sheet of smooth steel. Then the bottom of the hopper is opened by removing the closing device, and the particles are collected in a 50 cm3 cylinder (inside diameter 37 mm, height 46 mm) situated (its top) less than 5 cm under the closing device. Then the excess of particles projecting above the cylinder is leveled, without tamping the powder, and checking that the whole cylinder is filled with powder to the brim, the weight of the powder contained in the 50 cm3 is weighed and the ("free") bulk density of the powder is expressed as the apparent weight of the powder referred to the volume of 50 cm3, expressed in kg/liter or kg/m3. The following examples are given to illustrate certain embodiments of the present invention. They are not limiting in any case. Examples Example 1 - Crystallization of sodium bicarbonate particles by cooling - Control of the free bulk density. Equipment used: 500-ml double-jacket beakers, stirred by a magnetic bar at 350 rev/min. Buchner filter and laboratory vacuum pump. The "BICAR" used as seed is a sodium bicarbonate of brand "BICAR® Codex" of Pharmacopeia quality, from the SOLVAY company sieved between 200 and 250 um. At the end of the step of crystallization by cooling, the suspension of bicarbonate particles is filtered on a Buchner and washed with 50 ml of ethanol saturated with sodium bicarbonate, to remove most of the crystallization mother liquor. The particles are then dried in open air overnight. The agglomerates due to drying are broken up on a stainless steel plate with a rubber stopper with light hand pressure, and the powder obtained is then sieved at 500 nm. The following table (Table 4. a) only gives the additives for which the BD is greater than the mean BD of the reference tests + 3 standard deviations (i.e. > 47 0 kg/m3) . Moreover, the ratio BD add / BD ref is the ratio of the BD of the sodium bicarbonate crystallized in the presence of additive divided by the mean BD of the sodium bicarbonate crystallized without additive (i.e. 427 kg/m3) : with additives of the polyacrylate type of different molecular weights (average molecular weights). Geropon T/77 Sodium hexametaphosphate. o BD > 700 kg/m3: Sodium dodecyl sulfate. - For 50 ppm of additive (ppm - mg active substance /kg initial liquid): o 470 < BD < 600 kg/m3 using Rhodacal DSB, Rhodacal BX-78 o 630 < BD < 898 kg/m3 using sodium polyacrylate of average molecular weight at least 8000 g/mol o 600 < BD < 700 kg/m3 using polyvinysulfonic acid/ Rhodacal 330 o BD > 700 kg/m3 using sodium dodecyl sulfate. The. following table (Table 4.c) only gives the additives for which BD is below the mean BD of the reference tests minus 1.4 standard deviations (i.e. ^ 407 g/L) . Table 4. c - BD of the particle powders with additive (organophosphates and phosphonates) and comparison with and without additive. Comments for the organophosphates and phosphonates: - For 20 ppm of additive (ppm = mg active substance /kg initial liquid): 369 < BD < 4 07 kg/m3 (factor 0.8 6 to 0.95) using organophosphates or phosphonates; i.e. decreases by 5 to 14% of BD relative to the reference BD. Example 2 - Crystallization of sodium bicarbonate. particles by cooling - Control of the sieve oversize.. Results of the tests with additives: The following table (Table 6) only gives the additives for which the undersize at 125 urn is greater than 38 wt% (> mean value of the reference, tests without additive + 3 standard deviations). Moreover, the ratio F add / F ref is the ratio of the undersize fraction by weight at 125 urn of sodium bicarbonate crystallized in the presence of additive divided by the average undersize fraction by weight at 125 urn of sodium bicarbonate crystallized without additive (i.e. 26 wt%) : The following table (Table 7) only gives the additives for which the oversize at 250 urn is greater than the average oversize at 2 50 pm + 3 standard deviations (30 wt%) of the sodium bicarbonates of the reference tests without additive. Moreover, the ratio F add / F ref is the ratio of the oversize fraction by weight at 250 urn of sodium bicarbonate crystallized in the presence of additive divided by the average oversize fraction by weight at 250 urn of sodium bicarbonate crystallized without additive (i.e. 15 wt%) : The following table (Table 7.b) gives, the specific results obtained with sodium polyacrylates of different average molecular weights between 1800 and 1 250 000 g/mol and with various amounts added: 10, 20, 30, 50 ppm. Table 7.b - Results of the tests with additives of the sodium polyacrylate type with different average molecular weights - Oversize on vibratory sieve for particle powders (F> 250 urn in wt%). The following table (Table 8) only gives the additives for which the fraction of particles between 125 and 250 urn is greater than the mean fraction 125-250 urn + 3 standard deviations of the reference tests without additive (i.e, 68 wt%) . Moreover, the ratio F add / F ref is the ratio of the 125-250 urn fraction by weight of sodium bicarbonate crystallized in the presence of additive divided by the average 125-250 urn fraction by weight of sodium bicarbonate crystallized without additive (i.e. 59 wt%): Table 8 - Results of the tests with additives Oversize on vibratory sieve for particle powders. - For 10 ppm of additive (ppm = mg active substance /kg initial liquid): o Production of fines with the following additives: ■ Sodium dodecyl sulfate - For 20 ppm of additive (ppm = mg active substance /kg initial liquid): o Production of fines (F<125 urn) with the following, additives: ■ Lubrophos LB/400-E ■ Rhodacal 330 ■ Rhodafac RM 510 ■ Sodium dodecyl sulfate ■ Repel-O-TEX SF2 ■ Magnesium ■ Rhodasurf ON/870-E ■ Igepal CA 630 ■ Antarox L-62 ■ Jaguar HP105 o Production of coarse particles (F> 250 urn) with the following additives: ■ Sodium hexametaphosphate ■ Sodium polyacrylate ■ Hexamethylenediamine ■ Polyvinysulfonic acid For sodium polyacrylate, it can be seen from Table 7.b that for the production of coarse particles there is a limiting average molecular weight for the homopolymers of polyacrylic acid. It is found in fact that starting from 8000 g/mol, the granulometric fraction above 250 pm (F > 250 um) is almost constant at the concentrations of additive tested. - For 50 ppm of additive (ppm = mg active substance /kg, . initial liquid): o Production of fines (F<125 urn) with the following additives: ■ Rhodacal 330 ■ Lubrophos LB/400-E ■ Sodium dodecyl sulfate o Production of a Bicar with reduced granulometric spread: ... ■ Polyvinysulfonic acid - For 20 ppm of additive (ppm = mg active substance /kg initial liquid) : it can be seen that the organophosphates such as: ■ Lubrophos LB/400-E ■ Rhodafac RM 510 significantly increase the number of fines passing through the 125 um sieve. Example 3 - Crystallization of sodium bicarbonate particles by carbonation. The present example illustrates continuous carbonation for manufacture of sodium bicarbonate with and without additives. Operating conditions: The preparation of sodium bicarbonate particles by crystallization by carbonation is carried out in a closed precipitation reactor with useful capacity of 5 liters, stirred at 800 (referenced 'a ' ) or 1070 revolutions per minute (referenced 'b' ) . A feed solution containing 100 grams of sodium bicarbonate per kilogram of aqueous solution and 130 grams of sodium carbonate per kilogram of aqueous solution is injected continuously at the center of the stirred reactor at a flow rate of 3.3 liters per hour. Carbon dioxide is injected into the reactor at a flow rate of 750 normal liters per hour via a perforated pipe placed at the bottom of the reactor under the stirrer. The suspension formed from sodium bicarbonate particles is withdrawn continuously at the bottom of the reactor at a flow rate of 3.3 liters per hour in order to maintain a constant level in the reactor. The reactor for crystallization by carbonation is placed under relative pressure of the gas cover 200 to 600 mbar relative. The additive is prepared as a solution of 1 wt% of active substance in water in such a way that the amounts of additive are expressed here as weight of active substance per weight of solution to be crystallized. The solution of additive is injected into the center of the stirred suspension. For the sodium polyacrylate, the average molecular weight of the additive used was 5100■g/mol- . (below 8000 g/mol). After establishment of continuous conditions, a proportion of the suspension of crystals is recovered and is filtered on a Buchner filter and filter paper. The filtered solution comprises about 125 grams of sodium bicarbonate per kilogram of aqueous solution and 65 grams of sodium carbonate per kilogram of aqueous solution. The sodium bicarbonate particles obtained on the Buchner are washed with 250 ml of ethanol and then dried in the same way as in example 1. The powder of..... particles thus obtained is characterized. The following measurements are performed: - Undersize on a sieve at 125 urn: (wt%) - Oversize on a sieve at 250 pm: (wt%) - Laser granulometry (D10, D50, D90 and span) (urn) - Bulk density