Patent Publication Number: US-2018050916-A1

Title: Process for producing sodium carbonate/bicarbonate

Description:
This application claims priority to French application No. 1463086, filed on Dec. 22, 2014, the whole content of this application being incorporated herein by reference for all purposes. 
     TECHNICAL FIELD 
     The invention relates to an improved process for producing sodium carbonate with ammonia and/or for producing sodium bicarbonate, such as a process for producing refined bicarbonate. The invention pertains more particularly to a production process featuring reduced emission of carbon dioxide (CO 2 ), of a process for producing sodium carbonate with ammonia, or of a process for producing refined bicarbonate. 
     In the present specification, a process for producing sodium carbonate with ammonia, also referred to as the Solvay process, means a process utilizing sodium chloride (NaCl), ammonia (NH 3 ) and carbon dioxide (CO 2 ) for the production of sodium bicarbonate (ammoniacal crude sodium bicarbonate) according to the following reactions: 
       NaCl+H 2 O+NH 3   NaCl+NH 4 OH  (1)
 
       NaCl+NH 4 OH+CO 2   NaHCO 3 (solid)+H 2 O+NH 4 Cl  (2)
 
     The sodium bicarbonate (ammoniacal crude sodium bicarbonate) may be subsequently calcined to give sodium carbonate (light soda ash) according to the following reaction: 
       2NaHCO 3 (solid)→Na 2 CO 3 (solid)+H 2 O(gas)+CO 2 (gas)  (3)
 
     In a first variant of the Solvay process, the ammonium chloride (NH 4 Cl) is regenerated to gaseous ammonia by reaction with an alkali, generally lime or caustic soda, followed by distillation. For example, with lime, according to the following reaction: 
       2NH 4 Cl+Ca(OH) 2 →CaCl 2 +2NH 3 +2H 2 O  (5)
 
     and the ammonia (gaseous) is recovered, generally by distillation. 
     The lime is generally produced by calcining limestone with coke, to produce quicklime, according to the following reaction: 
       CaCO 3 →CaO+CO 2 (gas)  (6)
 
     and the quicklime is then hydrated in the form of milk of lime to produce calcium hydroxide (Ca(OH) 2 ). 
     In a second variant of the process, when the preference is for utilizing the ammonium chloride in the form of a finished product, the ammonium chloride is crystallized in a fourth step (4) by addition of solid sodium chloride and by cooling; in this way, ammonium chloride is precipitated, and can be used, for example, as a fertilizer. This is accompanied by a net consumption of ammonia, according to the molar amount of ammonia extracted from the process which is not regenerated and not recycled. This variant of the Solvay process with ammonia is generally referred to as the dual process or Hou process. 
     The present invention may be applied to either of the two variants, the basic reactions in which are described above. 
     In one or the other variant, the production of “refined sodium bicarbonate” (“refined” in contrast to the ammoniacal crude bicarbonate) is carried out in general from solid sodium carbonate dissolved in aqueous solution, and the solid sodium bicarbonate is recrystallized and purified according to the following reaction: 
       Na 2 CO 3 (solution)+H 2 O(gas)+CO 2 (gas)→2NaHCO 3 (solid)  (7)
 
     Refined sodium bicarbonate may also be produced from sodium carbonate obtained by other processes, such as a sodium carbonate monohydrate process or a sodium sesquicarbonate process, these processes generally being supplied with trona or nahcolite minerals. 
     PRIOR ART 
     The Solvay process for producing sodium carbonate (also called soda ash) has undergone numerous developments and optimizations over 150 years, since its creation by Ernest Solvay. These developments have included in particular its energy optimization and the improved management of CO 2 . 
     The process for producing sodium carbonate with ammonia, and/or for producing refined bicarbonate, requires energy: of the order of 9.7 to 13.6 GJ/t of soda ash (sodium carbonate) produced. The energy required is primarily in the form of thermal energy, which is supplied by a steam generator integrated in the process for producing carbonate or bicarbonate. The source of energy most frequently used by the steam generator is a carbon fuel of coal, fuel oil or natural gas type. The boiler of the steam generator produces a flue gas (combustion gas) which contains in general from 3% to 18% of CO 2  by volume on a dry gas basis (generally from 3% to 10% for natural gas boilers and from 8% to 18% for coal or fuel oil boilers). 
     An example of the process for producing sodium carbonate according to the ammonia process, and of the production of refined bicarbonate, is described in Ullmann&#39;s Encyclopedia of Industrial Chemistry (“Sodium carbonate” chapter, 2002 edition, Wiley-VCH Verlag GmbH &amp; Co., 24 pages, in paragraphs 1.4.1 and 1.4.2). 
     The production of sodium carbonate and/or of sodium bicarbonate, like many industrial processes, emits carbon dioxide:
         by the emission of the steam generator flue gases,   and by the emission of CO 2 -depleted process gases, especially at the outlet of the lime kilns sector in transitory phase, for example during starting or stopping of a lime kiln (70 to 150 kg CO 2 /t soda ash), or at the outlet of the CO 2 -depleted soda plant column sector (40 to 100 kg CO 2 /t soda ash),   and, for the production of refined bicarbonate, at the outlet of the refined bicarbonate column (50 to 300 kg CO 2 /t sodium bicarbonate).       

     These emissions are for the most part a result of the physicochemical NaCl—NH 3 —CO 2  balances of the soda plant columns or of the Na 2 CO 3 —NaHCO 3 —CO 2  balances of the refined bicarbonate columns (cf. Ullmann&#39;s Encycl., loc. cit. paragraph 1.4.1). 
     There are various known techniques for concentrating CO 2 . 
     WO2011/112069 describes a process for capturing CO 2  from flue gases, using a PSA (Pressure Swing Adsorption) adsorption module based on hydrotalcite and zeolite, generating a gas enriched with CO 2  to more than 88% and up to 99.9% by volume on a dry gas basis; the enriched CO 2  is subsequently used in an ammoniacal brine (H 2 O, NaCl, NH 4 OH) for producing ammoniacal sodium bicarbonate, which is subsequently calcined to give sodium carbonate, and using caustic soda to regenerate ammonia. A disadvantage of this process is that caustic soda is most frequently produced by electrolysis of a sodium chloride (NaCl) brine, thereby co-generating gaseous chlorine (Cl 2 ), which must be utilized elsewhere. 
     US2014/0199228 describes a process for producing sodium carbonate by integration of a CO 2  capture module under flue gas pressure, with a process for producing sodium carbonate, in which the CO 2 , concentrated to more than 80% and up to 99.95%, is used to produce ammoniacal sodium bicarbonate. A disadvantage of the process is the partial operation under pressure, during the desorption of the enriched CO 2  between 8 and 25 bar, thereby giving rise to problems of corrosion and strength for the steels used. 
     On the other hand, CO 2  concentration processes have the drawback of being highly energy consuming: for example, a coal boiler steam generator self-consumes up to 30% of the energy produced for the capture of its CO 2 . 
     Over the 150 years of development and improvement of the Solvay process for producing sodium carbonate with ammonia, suppressing CO 2  emissions, or at least greatly reducing these emissions, has always stumbled on the energy cost of concentrating the low-concentration CO 2  resulting from the production processes, that is accompanied by an overall increase in fossil fuels (gas, coal, fuel oil) to be taken into account in the overall balance of CO 2  emitted or emissible from such manufacture. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The inventors of the present invention have found, surprisingly, that limiting the increase in CO 2  concentration of low-content gases obtained from production of sodium carbonate with ammonia and/or from production of refined bicarbonate, as for example a limited increase in the CO 2  level of +10 to +90%, advantageously of +10 to +80% or of +10 to +70%, without seeking to have a highly concentrated CO 2  gas (to obtain a gas comprising, for example, less than 80% by volume, or less than 70% by volume, of CO 2 , on a dry gas basis), irrespective of the CO 2  concentration technique used (amine process, ammonia process, PSA, TSA, cryogenic or membrane process, etc.), and with recycling of these gases to the production of sodium carbonate in order to produce ammoniacal sodium bicarbonate, and/or to the production of sodium bicarbonate, to produce refined bicarbonate, permitted a particularly advantageous synergy. The reason is that this approach makes it possible: 
     1. to limit greatly the additional energy consumption, especially by using the low-temperature heat energy of the process for enriching low CO 2  content gases, 
     2. to improve the particle size of the ammoniacal crude bicarbonate and/or of the refined bicarbonate thus produced, by lowering its humidity at the exit from the filter section or from the suction section, and thereby lower the energy consumption for the drying/calcination of the product (to light soda ash) or to dry refined bicarbonate, thereby releasing energy which can be used for the concentration of the CO 2  hitherto emitted to the stack, and allowing it to be recycled/used for the production of sodium carbonate or bicarbonate. 
     This limited enrichment allows a strong decrease in the overall CO 2  emitted by a soda plant of this kind and/or by a unit for producing refined bicarbonate, and/or in the CO 2  emitted by the power plant and/or the steam boiler supplying utilities to this soda plant. 
     This limited enrichment exhibits a large number of further advantages. 
     It makes it possible: 
     3. to decouple the production of sodium carbonate with ammonia from a refined bicarbonate which would be attached to it, and so to increase the ratio of sodium bicarbonate produced starting from sodium carbonate, by supplying more available CO 2  for the production of sodium bicarbonate than the limited excess generated by the combustible carbon from lime kilns and the CO 2  yields from ammonia soda-plant bicarbonation columns. 
     4. to increase the flexibility of the units for producing sodium carbonate by partially decoupling the lime kiln sections (producing CaO and CO 2 ): the CO 2  re-used does not require the corresponding calcination of limestone to quicklime. 
     5. to allow, alternatively, the use of fuel with a lower carbon content than the coke used in the soda-plant lime kilns (composed primarily of carbon and inerts): anthracite fuels or fuels comprising agricultural or forestry residues, which contain more hydrogen and other molecules, give rise to a drop in the CO 2  soda plant level (lime kiln ‘lean gas’) from 40 vol % to 25-35% (in other words, even leaner). 
     6. to decouple, alternatively, the operation of the lime kilns for obtaining quicklimes of optimized quality (allowing the production of more reactive quicklimes, or of less viscous milks of lime in order to increase their concentration) without having to operate the kilns for simultaneous production of a lime kiln lean gas with as high as possible a concentration in terms of CO 2  level (40 to 43 vol % on a dry gas basis). 
     7. to use, alternatively, horizontal lime kilns for the calcination of small limestone (particle size of less than 100, or even less than 50 or 15 mm), for which the CO 2  content of the calcination gases is again lower: 15 to 35%, or 15 to 30% by volume on a dry gas basis. 
     8. to use, alternatively, an excess of lime (CaO) produced relative to the CO 2 , for other production processes. 
     9. to reduce the consumption of NaCl-type raw materials, by increasing the bicarbonate precipitation yield. 
     10. to reduce the costs of compressing CO 2 /NH 3  with reduced volumes when the CO 2  is enriched. 
     11. to allow the use of a richer CO 2  (acidic carbon dioxide) for the treatment of the alkaline solid and liquid discharges from the soda plants and/or for production of refined bicarbonate, by reducing the size of the neutralizing apparatus, in order to improve the environmental footprint of the soda plants. 
     Consequently, the invention relates to a process for producing sodium carbonate with ammonia and/or for producing refined sodium bicarbonate, wherein:
         a low CO 2  content gas generated by a unit for producing sodium carbonate with ammonia and/or generated by a unit for producing refined sodium bicarbonate,   is enriched into a CO 2  enriched gas using a CO 2  concentration module, such as an amine-type or ammonia or PSA (Pressure Swing Adsorption) or TSA (Temperature Swing Adsorption) or cryogenic distillation-type or membrane-type CO 2  concentration module,
 
and said CO 2 -enriched gas has an increased CO 2  content of: +10% (at least) to +90% (at most) by volume on a dry gas basis relative to the CO 2  concentration of the low content gas, and
   the CO 2 -enriched gas is subsequently recycled to the unit for producing sodium carbonate with ammonia or optionally to the unit for producing refined sodium bicarbonate, in order:
           to produce at least one product selected from the following: sodium carbonate, or ammoniacal sodium bicarbonate, or refined sodium bicarbonate,   or to carbonate at least part of effluent from the unit for producing sodium carbonate with ammonia and/or generated by the unit for producing refined sodium bicarbonate.   
               

     Definitions 
     In the present specification, the term “a low CO 2  content gas generated by a unit for producing sodium carbonate and/or sodium bicarbonate” denotes a gas with low CO 2  content that is generated by: at least one of the equipments of the unit for producing carbonate or of the unit for producing bicarbonate, including optionally, among the ‘at least one equipment’: the steam production boiler of the unit for producing sodium carbonate or sodium bicarbonate, and producing a flue gas comprising CO 2 . 
     In the present specification, the term “a CO 2  concentration module of . . . type” denotes a module operating “a CO 2  concentration process of . . . type”. 
     In the present specification, the term “amine-type CO 2  concentration process” denotes any process for separating and concentrating carbon dioxide by CO 2  absorption/desorption cycle in a solution comprising an amine. 
     In the present specification, the term “ammonia-type CO 2  concentration process” denotes any process for separating and concentrating carbon dioxide by CO 2  absorption/desorption cycle in a solution comprising ammonia. 
     In the present specification, the term “PSA process” denotes any process for gas separation by pressure swing adsorption, employing cyclical variation of the pressure between a high pressure, called the adsorption pressure, and a low pressure, called the regeneration pressure. 
     In the present specification, the term “TSA process” denotes any process for gas separation by temperature swing adsorption, employing cyclical variation of the temperature between a low temperature, called the adsorption temperature, and a high temperature, called the regeneration temperature. 
     In the present specification, the term “membrane process” denotes any process for gas separation, or for separating gas dissolved in solution in ionic form, that employs a synthetic membrane. The molecules retained by the membrane constitute the retentate, whereas those which pass through the membrane give rise to a permeate. 
     In the present specification, the term “cryogenic distillation” denotes any process for gas separation, comprising a stage at temperature below ambient temperature of the unit place, and wherein at least part of CO 2  gas is either liquefied and/or freezed at solid state, including in that case a freezing-in and freezing-out cycle to provide an enriched CO2 gaz. 
     In the present specification, the term “recycled . . . to produce . . . ”, as in: “the CO 2 -enriched gas is subsequently recycled into the unit for producing sodium carbonate with ammonia and/or into the unit for producing refined sodium bicarbonate to produce at least one product selected from the following: sodium carbonate, or ammoniacal sodium bicarbonate, or refined sodium bicarbonate, or to carbonate at least a part of effluent from the unit for producing sodium carbonate with ammonia and/or generated by the unit for producing refined sodium bicarbonate”: refers to the fact that the CO 2  is recycled for its at least partial absorption (i.e. consumption) in one of the products (sodium carbonate, sodium bicarbonate) or in one of the effluents listed. 
     In the present specification, the term “soda plant” refers to a unit for producing sodium carbonate by the ammonia process. 
     In the present specification, ammoniacal crude bicarbonate, also called “crude bicarb”, refers to a compound comprising by weight on dry basis: at least 75% of sodium bicarbonate, not more than 25% of sodium carbonate, and at least 0.2% of ammonia (expressed as total NH 4   −  ion). Crude bicarb after precipitation column, and after separation of mother liquor, has a typical humidity from 8 to 20% water by weight expressed on humid product. 
     In the present specification, refined bicarbonate refers to a compound comprising at least 97% of sodium bicarbonate, advantageously at least 98% of sodium bicarbonate. 
     In the present specification, the choice of an element from a group of elements also explicitly describes:
         the choice of two or the choice of several elements from the group,   the choice of an element from a subgroup of elements consisting of the group of elements from which one or more elements have been removed.       

     In the present specification, the description of a range of values for a variable, defined by a bottom limit, or a top limit, or by a bottom limit and a top limit, also comprises the embodiments in which the variable is chosen, respectively, within the value range: excluding the bottom limit, or excluding the top limit, or excluding the bottom limit and the top limit. 
     The term “comprising” includes “consisting essentially of” and also “consisting of”. 
     The use of “one” or “a(n)” in the singular also comprises the plural, and vice versa, unless otherwise indicated. 
     If the term “about” is used before a quantitative value, this corresponds to a variation of ±10% of the nominal quantitative value, unless otherwise indicated. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         FIG. 1  is a block diagram of various embodiments of the invention, using CO 2  enrichment modules, which are referred to in Example 1. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a number of embodiments of the process, which are detailed below. 
     Item 1. Process for producing sodium carbonate with ammonia and/or for producing refined sodium bicarbonate, wherein:
         a low CO 2  content gas generated by a unit for producing sodium carbonate with ammonia and/or generated by a unit for producing refined sodium bicarbonate,   is enriched into a CO 2  enriched gas using a CO 2  concentration module, such as an amine-type or ammonia or PSA (Pressure Swing Adsorption) or TSA (Temperature Swing Adsorption) or cryogenic distillation-type or membrane-type CO 2  concentration module,
           and said CO 2 -enriched gas has an increased CO 2  content of: +10% (at least) to +90% (at most) by volume on a dry gas basis relative to the CO 2  concentration of the low content gas, and   
           the CO 2 -enriched gas is subsequently recycled to the unit for producing sodium carbonate with ammonia or optionally to the unit for producing refined sodium bicarbonate, in order:   to produce at least one product selected from the following: sodium carbonate, or ammoniacal sodium bicarbonate, or refined sodium bicarbonate,   or to carbonate at least part of effluent from the unit for producing sodium carbonate with ammonia and/or generated by the unit for producing refined sodium bicarbonate.       

     Item 2. Process according to item 1, wherein the CO 2 -enriched gas has an increased CO 2  concentration of not more than: +80%, advantageously of not more than: +70%, more advantageously of not more than +60%, even more advantageously of not more than +55%, even more advantageously of not more than +50% by volume on a dry gas basis, relative to the CO 2  concentration of the low content gas. 
     Item 3. Process according to item 1 or 2, wherein the CO 2 -enriched gas has a CO 2  concentration of not more than 95%, advantageously of not more than 90%, more advantageously of not more than 80%, more advantageously of not more than 70%, or even more advantageously of not more than 65%, or not more than 60%, or not more than 55%, or not more than 50%, or not more than 45%, of CO 2  expressed by volume on a dry gas basis. 
     Item 4. Process according to items 1 to 3, wherein the CO 2  concentration module is a TSA (Temperature Swing Adsorption)-type CO 2  concentration module, preferably of CTSA (Continuous Temperature Swing Adsorption) type. 
     Item 5. Process according to items 1 to 3, wherein the CO 2  concentration module is an amine-type CO 2  concentration module. 
     Item 6. Process according to items 1 to 3, wherein the CO 2  concentration module is an ammonia-type CO 2  concentration module. 
     Item 7. Process according to items 1 to 3, wherein the CO 2  concentration module is a PSA (Pressure Swing Adsorption) CO 2  concentration module. 
     Item 8. Process according to items 1 to 3, wherein the CO 2  concentration module is a cryogenic distillation-type CO 2  concentration module. 
     Item 9. Process according to items 1 to 3, wherein the CO 2  concentration module is a membrane-type CO 2  concentration module. 
     Item 10. Process according to any one of items 1 to 9, wherein the CO 2 -enriched gas has a CO 2  concentration of at least +15%, advantageously of at least +20%, more advantageously of at least +25%, even more advantageously of at least +30% by volume on a dry gas basis, relative to the CO 2  concentration of the low CO 2  content gas. 
     Item 11. Process according to any one of items 1 to 3, or to item 6, or to item 10, wherein the CO 2 -enriched gas has a concentration of not more than 80%, advantageously of not more than 70% of CO 2 , expressed by volume on a dry gas basis. 
     Item 12. Process according to any one of items 1 to 3, or to item 7, or to item 10, wherein the CO 2 -enriched gas has a concentration of not more than 85%, advantageously of not more than 80%, more advantageously of not more than 70%, of CO 2 , expressed by volume on a dry gas basis. 
     Item 13. Process according to any one of items 1 to 12, wherein the CO 2 -enriched gas has a concentration of not more than 80% of CO 2 , expressed by volume on a dry gas basis. 
     Item 14. Process according to item 13, wherein the CO 2 -enriched gas has a concentration of not more than 70% of CO 2 , expressed by volume on a dry gas basis. 
     Item 15. Process according to any one of items 1 to 14, wherein the low CO 2  content gas is a gas selected from the source gases indicated in Table 1 below (columns 1 and 2 of the table), and the CO 2 -enriched gas is an enriched gas according to Table 1 (columns 3 to 5 of the table) and used for the purpose stated in the same columns. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 particularly preferred embodiments as per the present invention 
               
               
                 for enrichment of low CO 2  content gases according to their source 
               
               
                 (row) and according to the use of the enriched gas (column). The 
               
               
                 intersection of the rows and columns expresses the enrichment of 
               
               
                 the low CO 2  content gas, to give a gas enriched with CO 2  and 
               
               
                 depleted in components other than CO 2  (inerts, nitrogen, oxygen etc). 
               
            
           
           
               
               
            
               
                   
                 Enriched gas &amp; use 
               
            
           
           
               
               
               
               
            
               
                 Low CO 2  content gas 
                 GP-GBIR 
                 GR 
                 BIR CR 
               
            
           
           
               
               
               
               
               
            
               
                 SOURCES 
                 % CO 2  vol. dry 
                 40-45% 
                 70-75% 
                 90-100% 
               
               
                   
               
               
                 GN, LCL- 
                  5-16% 
                 +24 to +40 
                 +54 to +70 
                 +74 to +95 
               
               
                 BIB 
               
               
                 CL-BIR, 
                 15-30% 
                 +15-30 
                 +45-60 
                 +65-85 
               
               
                 FCH horiz. 
               
               
                 FCH vertical 
                 30-45% 
                 +10-15 
                 +25-45 
                 +45-70 
               
               
                   
               
               
                 Key to abbreviations: 
               
               
                 GN (low CO 2  content gas): flue gas from the steam generator of the unit for producing sodium carbonate with ammonia or from the unit for producing refined bicarbonate. 
               
               
                 LCL-BIB (low CO 2  content gas): exit gas from scrubber (LCL) of precipitation column of crude bicarbonate (BIB or crude bicarb). 
               
               
                 CL-BIR (low CO 2  content gas): exit gas from refined bicarbonate (BIR) precipitation crystallizer or column. 
               
               
                 FCH horiz. (Low CO 2  content gas): exit gas from horizontal lime kiln (FCH) such as rotary lime kilns. 
               
               
                 FCH vertical (low CO 2  content gas): exit gas from vertical lime kilns (FCH). 
               
               
                 GP-GBIR (enriched gas): lean gas (as opposed to the “rich” gas below) (FCH gas) used in the intermediate part of the crude bicarb precipitation columns (cf. Ullmann&#39;s Encycl. FIG. 7) or BIR gas (for the production of refined bicarbonate). 
               
               
                 GR (enriched gas): rich gas in particular from the ammoniacal crude bicarb dryer gases, and used in the bottom part of the crude bicarb precipitation columns (cf. Ullmann&#39;s Encycl. FIG. 7). 
               
               
                 BIR CR (enriched gas): gas used for the crystallization of refined bicarbonate (BIR) in a crystallizer (CR) or in a column. 
               
            
           
         
       
     
     Item 16. Process according to any one of the preceding claims, wherein the CO 2  concentration module consumes energy for the CO 2  concentration of the low CO 2  content gas, and at least part of the energy is steam with a pressure of less than 10, advantageously less than 5, more advantageously less than 3 bar gauge, generated by an apparatus in the unit for producing sodium carbonate with ammonia and/or in the unit for producing refined sodium bicarbonate. 
     Item 17. Process according to item 16, wherein the steam with a pressure of less than 10 bar gauge is a high-pressure steam expanded after having transferred part of its heat energy to at least one apparatus in the unit for producing sodium carbonate with ammonia and/or in the unit for producing refined sodium bicarbonate, such as: a light soda ash dryer, a dense soda ash dryer, an ammonia distiller, an electricity-generating steam turbine, steam recovery compressor. 
     Item 18. Process according to item 16 or 17, wherein the steam with a pressure of less than 10 bar gauge is a vapor or steam originating from the mechanical recompression of a steam or via an ejector of a steam or of a vapour from at least one apparatus in the unit for producing sodium carbonate with ammonia and/or in the unit for producing refined sodium bicarbonate, such as: the vapour from a quicklime hydrator, the vapour from a dissolver of quicklime to milk of lime, the vapour from a sodium carbonate monohydrate evaporator-crystallizer, the vapour from a light soda ash dryer, the vapour from a dense soda ash dryer, the vapour of any hot effluent. 
     Item 19. Process according to any one of the preceding items, wherein the CO 2  concentration module uses energy for the CO 2  concentration of the low CO 2  content gas, and at least part of the energy is a liquid effluent or a condensate having a temperature of at least 35° C. and not more than 110° C., generated by at least one apparatus in the unit for producing sodium carbonate with ammonia or in the unit for producing refined sodium bicarbonate. 
     Item 20. Process according to any one of items 15 to 19, wherein the low CO 2  content gas is a carbon-fuel steam generator flue gas, advantageously having a CO 2  concentration between 5 and 16 vol % on a dry gas basis, and wherein the carbon fuel is selected from the following: a coal, a charcoal, a gas, a lignite, a hydrocarbon, a fuel oil, a biomass, a carbon-containing household waste, a carbon-containing agricultural waste, a water treatment station residue, a carbon-containing industrial residue and mixtures thereof. The steam generator flue gas is advantageously in that case dedusted beforehand, and at least partly purified to remove NOx, and/or SOx, and/or HX. 
     Item 21. Process according to any one of items 15 to 19, wherein the low CO 2  content gas is from an ammoniacal bicarbonate precipitation column, or from a scrubber of such a column, and advantageously has a CO 2  concentration of between 5 and 16 vol % on a dry gas basis. 
     Item 22. Process according to any one of items 15 to 19, wherein the low CO 2  content gas is from a refined bicarbonate precipitation column or from a horizontal lime kiln, and advantageously has a CO 2  concentration of between 15 and 30 vol % on a dry gas basis. 
     Item 23. Process according to any one of items 15 to 19, wherein the low CO 2  content gas is from a lime kiln, advantageously a vertical kiln, advantageously a parallel flow regenerative lime shaft kilns, more advantageously a vertical mixed feed shaft kiln. 
     Item 24. Process according to preceding item wherein the low CO 2  content gas has a CO 2  concentration of between 15 and 45, or between 20 and 45, or between 30 and 45 vol % on a dry gas basis. 
     Item 25. Process according to anyone of item 22 to 24, wherein the low CO 2  content gas is from a lime kiln in a tuning phase or in transitory regime, producing a low CO 2  content gas with a CO 2  concentration of at least −5 vol % on a dry gas basis, relative to its nominal operation. 
     Item 26. Process according to any one of items 22 to 25, wherein the low CO 2  content gas is from a lime kiln operating with a carbon fuel other than coke, such as: an anthracite, or a carbon fuel from industrial or household residues, or from biomass. 
     Item 27. Process according to any one of items 23 to 26, wherein the low CO 2  content gas is from a lime kiln, and the lime kiln is selected from: a vertical shaft kiln, a vertical straight kiln, a mixed-feed vertical kiln, a vertical kiln with fuel feed through the wall, an alternating-cycle vertical kiln, or an annular vertical kiln. 
     Item 28. Process according to any one of the preceding items, wherein the concentration of the CO 2 -enriched gas is least 30%, advantageously at least 35%, more advantageously at least 40% by volume on a dry gas basis. 
     Item 29. Process according to item 21 or 22, or 28, wherein the CO 2 -enriched gas is recycled into an ammoniacal bicarbonate precipitation column, or refined bicarbonate precipitation column, and is used for the production of: ammoniacal bicarbonate, light soda ash, dense soda ash, or refined bicarbonate, or for the treatment of effluents. 
     Item 30. Process according to the preceding item, wherein the CO 2 -enriched gas is recycled into an ammoniacal bicarbonate precipitation column. 
     Item 31. Process according to any one of items 23 to 28, wherein the CO 2 -enriched gas has a concentration of at least 50%, advantageously at least 60%, more advantageously at least 70%, and preferably not more than 100% by volume on a dry gas basis, 
     and the CO 2 -enriched gas is recycled into an ammoniacal bicarbonate precipitation column, preferably at the bottom part of the ammoniacal bicarbonate precipitation column, or is recycled into a refined bicarbonate precipitation reactor or column, and is used in the production of: ammoniacal bicarbonate, light soda ash, dense soda ash, or refined bicarbonate. 
     Item 32. Process according to any one of the preceding items, wherein the low CO 2  content gas is generated by a unit for producing sodium carbonate with ammonia, and at least part of the filter liquid after separation of the ammoniacal crude bicarbonate is treated in an electrodialysis cell in which all or part of the NH 4 Cl is regenerated to NH 3 , such as, in particular, according to the process described in patent application EP 14188350.4. 
     Item 33. Process for producing bicarbonate according to item 32, wherein the low CO 2  content gas is the exit gas from the refined bicarbonate crystallization reactor or column, and the gas enriched in CO 2  by the CO 2  concentration module comprises at least 40%, advantageously at least 60%, more advantageously at least 70% or even at least 80% of CO 2  by volume on a dry gas basis, and is recycled to the refined bicarbonate crystallization reactor or column so as to increase the overall precipitation yield of CO 2  in the precipitated refined bicarbonate beyond 70%, advantageously at least 80%, more advantageously at least 90%. 
     The energy consumption of different CO 2  enrichments of low CO 2  content gas was simulated digitally and calculated by the inventors. The table below contains the average energy consumptions of the principal processes for CO 2  concentration that are referred to in the present specification (amines, ammonia, PSA, TSA or CTSA, cryogenic, or membrane): 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 average energy consumption of CO 2  concentration processes according 
               
               
                 to CO 2  enrichment (+80%, +30% or +10%). 
               
            
           
           
               
               
               
            
               
                 Case 
                   
                   
               
               
                 (CO 2  concentration of the gases 
                 Use 
                 Energy consumption 
               
               
                 on vol % on dry basis) 
                 Example 
                 t Steam/t CO 2   
               
               
                   
               
               
                 GN flue gases or LCL 10% → 90% 
                 BIR CR 
                 1.30 t Ve/t CO 2   
               
               
                 GN flue gases or LCL 10% → 40% 
                 GP-GBIR 
                 0.65 t Ve/t CO 2   
               
               
                 low CO 2  FCH 30% → 40% 
                 GP-GBIR 
                 0.30 t Ve/t CO 2   
               
               
                   
               
               
                 Key to abbreviations: 
               
               
                 GN flue gases (low CO 2  content gas): flue gas from the steam generator of the unit for producing sodium carbonate with ammonia or from the unit for producing refined bicarbonate. 
               
               
                 LCL (low CO 2  content gas): exit gas from column scrubber (LCL) for precipitation of crude bicarbonate (BIB). 
               
               
                 GP-GBIR (enriched gas): lean gas also called ‘weak gas’ (as opposed to the “rich” gas below) (FCH gas) used in the intermediate part of the crude bicarb precipitation columns (cf. Ullmann&#39;s Encycl. FIG. 7) or BIR gas (for the production of refined bicarbonate). 
               
               
                 BIR CR (enriched gas): gas used for the crystallization of refined bicarbonate (BIR) in a crystallizer (CR). 
               
            
           
         
       
     
     Among the various alternatives according to the present invention, that relating to the use of a TSA (and/or CTSA) CO 2  concentration module is particularly preferred when it uses, according to items 16 to 19, the excess low-temperature heat energy from the production of carbonate or from the production of refined bicarbonate, leading thus, by partial and limited concentration of CO 2 , to decrease or even cancel additional generation of CO 2  with combustion of fossil energy such as natural gas, coal or petroleum. 
     Moreover, in all of the cases where enrichment processes referred to in the present specification are used, the partial enrichment in CO 2  of low CO 2  content gases into gases with a CO 2  content of more than 40% concentration—for example at least 45%, or at least 50%, or at least 60%, or at least 70% of CO 2  by volume on a dry gas basis—makes it possible: to increase the particle size of the ammoniacal crude bicarbonate produced, or of the refined bicarbonate, reducing the amount of residual water in the steps of filtering or suctioning the crystallized solids and permitting a net gain in energy, which is in synergy with the use of a CO 2  concentration module, and so makes it possible to limit the overall energy consumption in the production of sodium carbonate by an ammonia process, or the production of a refined bicarbonate with reduced CO 2  discharge. 
     The examples that follow are intended for illustrating the invention. They should not be interpreted as limiting the scope of the claimed invention. 
     Example 1 
       FIG. 1  illustrates various modes of application of the present invention. The diagram elements in solid lines illustrate production of sodium carbonate by the ammonia process or production of refined bicarbonate. 
     The diagram elements in dashed lines (concentration modules and arrows in dashed lines) illustrate various modes of application of the present invention utilizing a limited-enrichment CO 2  enrichment module, in particular according to item 15. 
     Key to abbreviations in  FIG. 1 :
         AB: Ammonia absorber (production of ammoniacal brine)   CL: Bicarbonation column for precipitation of ammoniacal bicarbonate (crude bicarbonate).   FL: Filter for separating ammoniacal bicarbonate (crude bicarbonate) from the crystallization mother liquors (rich in dissolved NH 4 Cl and NaCl), referred to as filter liquid.   DS: Distiller for the crystallization mother liquors (for regeneration of ammonia), consuming lime and distillation steam and producing a distiller liquid rich in CaCl 2  in aqueous solution.   DV: Quicklime dissolver for the production of milk of lime, used for the distillation of the filter liquid (crystallization mother liquors of the ammoniacal bicarbonate).   SHT-SL: Light soda ash dryer (calcination of the ammoniacal bicarbonate to give light soda ash under the effect of heat, consuming steam).   FCH: Lime kiln   New lime kiln FCH: FCH operating with less carbon-rich fuel than coke (for example, partially hydrogenated organic material such as charcoal, anthracite, agricultural wastes, etc.).   GN: Steam generator of the unit for producing sodium carbonate with ammonia or of the unit for producing refined bicarbonate.   CO 2  enrichment: module for enrichment of low CO 2  content gas (of amine, ammonia, PSA, TSA, membrane or cryogenic type, etc.), preferably of TSA or CTSA type.       

     Example 2 (not Conforming to the Invention) 
     Production of crude ammoniacal bicarbonate as described in Ullmann&#39;s encyclopedia (see above) in section 1.4.1.2 &amp;  FIG. 7  is carried out. The amount of lean gas (‘weak gas’) injected at 2.5 bar in the middle of the carbonation column is 510 Nm 3  of CO 2  at 40 vol % on a dry gas basis, per ton of soda ash produced. The amount of rich gas (‘strong gas’) injected at 3.5 bar at the bottom of the column is 390 Nm 3  of CO 2  at 70 vol % on a dry gas basis, per ton of soda ash produced. The temperature profile along the column (exothermic carbonation reaction) exhibits a temperature maximum of 58° C., and the slurry leaves the carbonation column at 30° C. 
     The moisture content of the ammoniacal crude bicarbonate produced, at the exit from the rotary filter, is approximately 18%. 
     Example 3 (in Accordance with the Invention) 
     The same crude ammoniacal bicarbonate production process as described in the preceding example is made up with a lime-kiln lean-gas CO 2  enrichment module operating with a fuel having a lower carbon content. The lime kiln gas produced has a lean gas with 37% by volume of CO 2  on a dry gas basis. 
     This lean gas is partially enriched by a TSA-type CO 2  concentration module operating over a temperature range between 38° C. (adsorption) and 98° C. (desorption), to produce a gas enriched to 85% of CO 2  by volume on a dry gas basis, which concentration is measured on a calibrated infra-red Siemens Ultramat 23 analyser. The concentration module uses hot condensates from the distillation section as heating fluid. 
     The same carbonation column is used as in Example 2, with a quantity of 37% lean gas (‘weak gas’) readjusted in CO 2  level to 40% with the gas enriched to 85%, and injected at 2.5 bar, in the middle of the carbonation column. The quantity of lean gas injected is unchanged at 510 Nm 3  of CO 2  at 40 vol % on a dry gas basis, per ton of soda ash produced. The rich gas (‘strong gas’) at 70% CO 2  by volume on a dry gas basis is replaced by the rich gas enriched to 85 vol % CO 2  on a dry gas basis, injected at the same pressure of 3.5 bar, injected at the bottom of the column and in a 100% relative CO 2  quantity identical to that corresponding to the flow rate of rich gas in Example 2. 
     The temperature profile along the column (exothermic carbonation reaction) exhibits a temperature maximum of 61° C., and the slurry leaves the carbonation column at 30° C. 
     The moisture content of the ammoniacal crude bicarbonate produced at the column outlet is 14% water (average over 24 hours) at the exit of the rotary filter, requiring less steam in the light soda ash dryer (SHT-SL) and compensating the surplus of energy consumed by the CO 2  concentration module. 
     The utilization yield of NaCl is increased from 73% (Example 2) to 76% (Example 3). The absorption yield (one pass) of CO 2  is equivalent to that in Example 2. 
     The carbonation column production rate is subsequently increased gradually. An increase of +15% in the column capacity produces the same crude ammoniacal bicarbonate moisture content as in Example 2. 
     This example shows the advantage of using partial CO 2  enrichment: the overall capture yield of low-content CO 2  (37%) is improved substantially. The capture of CO 2  at the carbonation column exit and its reconcentration to a concentration of 50% to 85% would therefore make it possible to loop this CO 2  and to increase significantly the overall fixation balance of CO 2  produced in the lime kiln section to more than 70%: between 80% to 95%, depending on the possible recovery of the low-temperature heat energy from the unit for producing sodium carbonate. 
     Example 4 (in Accordance with the Invention) 
     A comparative test similar to Examples 2 and 3 is carried out in a refined bicarbonate production unit similar to that described by Te Pang Hou, Manufacture of Soda, American Chemical Society Monograph Series, Ed. The Chemical Catalog Company, Inc. New York USA, 1933, Chapter XV—The manufacture of Refined Sodium Bicarbonate, pp. 196-197. 
     A lime kiln gas with 37 vol % of CO 2  on a dry gas basis is used for the carbonation of the refined sodium bicarbonate. A sample is taken at the outlet of the carbonator every hour and is analysed for its particle size, over 24 hours. 
     In a second phase, the same unit for producing refined sodium bicarbonate is fed with CO 2  gas from a mixture of bicarbonation column exit gas (at 20 vol % CO 2  on a dry basis) and of lime kiln gas (at 37 vol % CO 2  on a dry basis), this mixture being enriched with CO 2  by an amine-type CO 2  concentration module, to a CO 2  concentration of 60 vol % CO 2  on a dry basis. The amine-type concentration module is supplied with energy by the 2 bar steam from the expansion of steam at the outlet of the SHT-SL. 
     A series of samples are taken from the outlet at the carbonator each hour, in the same way as above, over a duration of 24 hours, and the samples are analysed for particle size. 
     The change in the weight-average diameter of the sodium bicarbonate crystals produced, and measured by passing them through 500, 400, 355, 315, 250, 200, 160, 125, 100, 63 and 45 μm screens, is significant: +12%. The steam consumption found for the refined sodium bicarbonate dryer is a drop of 7% over the test period, relative to the use of unenriched CO 2 . 
     The average degree of capture of the CO 2  in the crystallized sodium bicarbonate goes from 70% to 88%. 
     Similar results may be obtained with a CO 2  concentration module of PSA (Pressure Swing Adsorption) type, or cryogenic distillation-type, or membrane-type, wherein advantageously at least part of the energy used by the CO 2  concentration module is steam at less than 10 bar gauge, or a hot condensate, from the unit producing refined sodium bicarbonate, such as steam or condensate exiting the sodium bicarbonate dryer. 
     Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.