PROCESS FOR PRODUCING SODIUM CARBONATE/BICARBONATE

Abstract

Process for producing sodium carbonate with ammonia and/or for producing refined sodium bicarbonate, wherein: a low CO.sub.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.sub.2-enriched gas by using a CO.sub.2 concentration module, such as an amine-type or ammonia or PSA or TSA or cryogenic distillation or membrane-type CO.sub.2 concentration module, and said CO.sub.2-enriched gas has an increased CO.sub.2 content of: +10% (at least) to +90% (at most), by volume on a dry gas basis relative to the CO.sub.2 concentration of the low content gas, and the CO.sub.2-enriched gas is subsequently recycled to the unit for producing sodium carbonate with ammonia and/or to the unit for producing refined sodium bicarbonate, to produce sodium carbonate, or sodium bicarbonate, or to carbonate at least one part of effluent from the unit for producing sodium carbonate and/or from the unit for producing sodium bicarbonate.

Claims

1. A process for producing sodium carbonate with ammonia and/or for producing refined sodium bicarbonate, wherein: a low CO.sub.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.sub.2 enriched gas using a CO.sub.2 concentration module, and said CO.sub.2-enriched gas has an increased CO.sub.2 content of: +10% (at least) to +90% (at most) by volume on a dry gas basis relative to the CO.sub.2 concentration of the low content gas, and the CO.sub.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 group consisting of: sodium carbonate, ammoniacal sodium bicarbonate, and refined sodium bicarbonate, or to carbonate at least part of an effluent from the unit for producing sodium carbonate with ammonia and/or generated by the unit for producing refined sodium bicarbonate.

2. The process according to claim 1, wherein the CO.sub.2 concentration module is a Temperature Swing Adsorption-type CO.sub.2 concentration module.

3. The process according to claim 1, wherein the CO.sub.2-enriched gas has an increased CO.sub.2 concentration of at most +80% by volume on a dry gas basis relative to the CO.sub.2 concentration of the low CO.sub.2 content gas.

4. The process according to claim 3, wherein the CO.sub.2-enriched gas has an increased CO.sub.2 concentration of at most +60% by volume on a dry gas basis relative to the CO.sub.2 concentration of the low CO.sub.2 content gas.

5. The process according to claim 1, wherein the CO.sub.2-enriched gas has a CO.sub.2 concentration of not more than 80% of CO.sub.2 expressed by volume on a dry gas basis.

6. The process according to claim 1, wherein the CO.sub.2-enriched gas has a CO.sub.2 concentration of not more than 70% of CO.sub.2 expressed by volume on a dry gas basis.

7. The process according to claim 1, wherein the CO.sub.2 concentration module uses energy for the CO.sub.2 concentration of the low CO.sub.2 content gas, wherein at least part of the energy is steam with a pressure of less than 10 bar relative, generated by an apparatus of the unit for producing sodium carbonate with ammonia and/or of the unit for producing refined sodium bicarbonate, and wherein the steam with a pressure of less than 10 bar relative is a high pressure steam which has been expanded after giving up part of its heat energy in at least one apparatus of the unit for producing sodium carbonate with ammonia or of the unit for producing refined sodium bicarbonate.

8. The process according to claim 1, wherein the CO.sub.2 concentration module uses energy for the CO.sub.2 concentration of the low CO.sub.2 content gas, and wherein at least part of the energy is a condensate or a liquid effluent having a temperature of at least 35 C. and not more than 110 C., generated by at least one apparatus of the unit for producing sodium carbonate with ammonia or optionally of the unit for producing refined sodium bicarbonate.

9. The process according to claim 1, wherein the low CO.sub.2 content gas is a carbon fuel steam generator flue gas.

10. The process according to claim 1, wherein the low CO.sub.2 content gas comes from an ammoniacal bicarbonate precipitation column or from a scrubber of such a column.

11. The process according to claim 9, wherein the low CO.sub.2 content gas has a CO.sub.2 concentration of between 5 and 16 vol % on a dry gas basis.

12. The process according to claim 1, wherein the low CO.sub.2 content gas comes from a refined bicarbonate precipitation column.

13. The process according to claim 12, wherein the low CO.sub.2 content gas has a CO.sub.2 concentration of between 15 and 30 vol % on a dry gas basis.

14. The process according to claim 1, wherein the low CO.sub.2 content gas comes from a lime kiln.

15. The process according to claim 14, wherein the low CO.sub.2 content gas has a CO.sub.2 concentration of between 20 and 45 vol % on a dry gas basis.

16. The process according to claim 1, wherein the CO.sub.2-enriched gas is subsequently used in a unit for producing refined sodium bicarbonate.

17. The process according to claim 1, wherein the CO.sub.2 concentration module is an amine-type or ammonia or Pressure Swing Adsorption or Temperature Swing Adsorption or cryogenic distillation type or membrane-type CO.sub.2 concentration module.

18. The process according to claim 7, wherein the at least one apparatus of the unit for producing sodium carbonate with ammonia or the unit for producing refined sodium bicarbonate is a light soda ash dryer, a dense soda ash dryer, an ammonia distiller, or an electricity-generating steam turbine.

19. The process according to claim 1, wherein the low CO.sub.2 content gas is: a flue gas from a steam generator of the unit for producing sodium carbonate with ammonia or from the unit for producing refined bicarbonate, with a CO.sub.2 concentration between 5 and 16 vol % on a dry gas basis; an exit gas from a scrubber (LCL) of a precipitation column of crude bicarbonate, with a CO.sub.2 concentration between 5 and 16 vol % on a dry gas basis; an exit gas from a refined bicarbonate (BIR) precipitation crystallizer or column, with a CO.sub.2 concentration between 15 and 30 vol % on a dry gas basis; an exit gas from horizontal lime kiln (FCH), with a CO.sub.2 concentration between 15 and 30 vol % on a dry gas basis; or an exit gas from vertical lime kilns (FCH), with a CO.sub.2 concentration between 30 and 45 vol % on a dry gas basis; and wherein the CO.sub.2-enriched gas is a lean gas (FCH gas) used in the intermediate part of crude bicarbonate precipitation columns or BIR gas for the production of refined bicarbonate, with a CO.sub.2 concentration between 40 and 45 vol % on a dry gas basis.

20. The process according to claim 1, wherein the low CO.sub.2 content gas is: a flue gas from a steam generator of the unit for producing sodium carbonate with ammonia or from the unit for producing refined bicarbonate, with a CO.sub.2 concentration between 5 and 16 vol % on a dry gas basis; an exit gas from a scrubber (LCL) of a precipitation column of crude bicarbonate, with a CO.sub.2 concentration between 5 and 16 vol % on a dry gas basis; an exit gas from a refined bicarbonate (BIR) precipitation crystallizer or column, with a CO.sub.2 concentration between 15 and 30 vol % on a dry gas basis; an exit gas from horizontal lime kiln (FCH), with a CO.sub.2 concentration between 15 and 30 vol % on a dry gas basis; or an exit gas from vertical lime kilns (FCH), with a CO.sub.2 concentration between 30 and 45 vol % on a dry gas basis; and wherein the enriched gas is a gas used for the crystallization of refined bicarbonate (BIR) in a crystallizer (CR), with a CO.sub.2 concentration between 90 and 100 vol % on a dry gas basis.

Description

BRIEF DESCRIPTION OF THE FIGURE

[0066] FIG. 1 is a block diagram of various embodiments of the invention, using CO.sub.2 enrichment modules, which are referred to in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

[0067] The present invention relates to a number of embodiments of the process, which are detailed below.

[0068] Item 1. Process for producing sodium carbonate with ammonia and/or for producing refined sodium bicarbonate, wherein: [0069] a low CO.sub.2 content gas generated by a unit for producing sodium carbonate with ammonia and/or generated by a unit for producing refined sodium bicarbonate, [0070] is enriched into a CO.sub.2 enriched gas using a CO.sub.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.sub.2 concentration module, [0071] and said CO.sub.2-enriched gas has an increased CO.sub.2 content of: +10% (at least) to +90% (at most) by volume on a dry gas basis relative to the CO.sub.2 concentration of the low content gas, and [0072] the CO.sub.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: [0073] to produce at least one product selected from the following: sodium carbonate, or ammoniacal sodium bicarbonate, or refined sodium bicarbonate, [0074] 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.

[0075] Item 2. Process according to item 1, wherein the CO.sub.2-enriched gas has an increased CO.sub.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.sub.2 concentration of the low content gas.

[0076] Item 3. Process according to item 1 or 2, wherein the CO.sub.2-enriched gas has a CO.sub.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.sub.2 expressed by volume on a dry gas basis.

[0077] Item 4. Process according to items 1 to 3, wherein the CO.sub.2 concentration module is a TSA (Temperature Swing Adsorption)-type CO.sub.2 concentration module, preferably of CTSA (Continuous Temperature Swing Adsorption) type.

[0078] Item 5. Process according to items 1 to 3, wherein the CO.sub.2 concentration module is an amine-type CO.sub.2 concentration module.

[0079] Item 6. Process according to items 1 to 3, wherein the CO.sub.2 concentration module is an ammonia-type CO.sub.2 concentration module.

[0080] Item 7. Process according to items 1 to 3, wherein the CO.sub.2 concentration module is a PSA (Pressure Swing Adsorption) CO.sub.2 concentration module.

[0081] Item 8. Process according to items 1 to 3, wherein the CO.sub.2 concentration module is a cryogenic distillation-type CO.sub.2 concentration module.

[0082] Item 9. Process according to items 1 to 3, wherein the CO.sub.2 concentration module is a membrane-type CO.sub.2 concentration module.

[0083] Item 10. Process according to any one of items 1 to 9, wherein the CO.sub.2-enriched gas has a CO.sub.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.sub.2 concentration of the low CO.sub.2 content gas.

[0084] Item 11. Process according to any one of items 1 to 3, or to item 6, or to item 10, wherein the CO.sub.2-enriched gas has a concentration of not more than 80%, advantageously of not more than 70% of CO.sub.2, expressed by volume on a dry gas basis.

[0085] Item 12. Process according to any one of items 1 to 3, or to item 7, or to item 10, wherein the CO.sub.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.sub.2, expressed by volume on a dry gas basis.

[0086] Item 13. Process according to any one of items 1 to 12, wherein the CO.sub.2-enriched gas has a concentration of not more than 80% of CO.sub.2, expressed by volume on a dry gas basis.

[0087] Item 14. Process according to item 13, wherein the CO.sub.2-enriched gas has a concentration of not more than 70% of CO.sub.2, expressed by volume on a dry gas basis.

[0088] Item 15. Process according to any one of items 1 to 14, wherein the low CO.sub.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.sub.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-US-00001 TABLE 1 particularly preferred embodiments as per the present invention for enrichment of low CO.sub.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.sub.2 content gas, to give a gas enriched with CO.sub.2 and depleted in components other than CO.sub.2 (inerts, nitrogen, oxygen etc). Enriched gas & use Low CO.sub.2 content gas GP-GBIR GR BIR CR SOURCES % CO.sub.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.sub.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.sub.2 content gas): exit gas from scrubber (LCL) of precipitation column of crude bicarbonate (BIB or crude bicarb). CL-BIR (low CO.sub.2 content gas): exit gas from refined bicarbonate (BIR) precipitation crystallizer or column. FCH horiz. (Low CO.sub.2 content gas): exit gas from horizontal lime kiln (FCH) such as rotary lime kilns. FCH vertical (low CO.sub.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'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'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.

[0089] Item 16. Process according to any one of the preceding claims, wherein the CO.sub.2 concentration module consumes energy for the CO.sub.2 concentration of the low CO.sub.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.

[0090] 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.

[0091] 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.

[0092] Item 19. Process according to any one of the preceding items, wherein the CO.sub.2 concentration module uses energy for the CO.sub.2 concentration of the low CO.sub.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.

[0093] Item 20. Process according to any one of items 15 to 19, wherein the low CO.sub.2 content gas is a carbon-fuel steam generator flue gas, advantageously having a CO.sub.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.

[0094] Item 21. Process according to any one of items 15 to 19, wherein the low CO.sub.2 content gas is from an ammoniacal bicarbonate precipitation column, or from a scrubber of such a column, and advantageously has a CO.sub.2 concentration of between 5 and 16 vol % on a dry gas basis.

[0095] Item 22. Process according to any one of items 15 to 19, wherein the low CO.sub.2 content gas is from a refined bicarbonate precipitation column or from a horizontal lime kiln, and advantageously has a CO.sub.2 concentration of between 15 and 30 vol % on a dry gas basis.

[0096] Item 23. Process according to any one of items 15 to 19, wherein the low CO.sub.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.

[0097] Item 24. Process according to preceding item wherein the low CO.sub.2 content gas has a CO.sub.2 concentration of between 15 and 45, or between 20 and 45, or between 30 and 45 vol % on a dry gas basis.

[0098] Item 25. Process according to anyone of item 22 to 24, wherein the low CO.sub.2 content gas is from a lime kiln in a tuning phase or in transitory regime, producing a low CO.sub.2 content gas with a CO.sub.2 concentration of at least 5 vol % on a dry gas basis, relative to its nominal operation.

[0099] Item 26. Process according to any one of items 22 to 25, wherein the low CO.sub.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.

[0100] Item 27. Process according to any one of items 23 to 26, wherein the low CO.sub.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.

[0101] Item 28. Process according to any one of the preceding items, wherein the concentration of the CO.sub.2-enriched gas is least 30%, advantageously at least 35%, more advantageously at least 40% by volume on a dry gas basis.

[0102] Item 29. Process according to item 21 or 22, or 28, wherein the CO.sub.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.

[0103] Item 30. Process according to the preceding item, wherein the CO.sub.2-enriched gas is recycled into an ammoniacal bicarbonate precipitation column.

[0104] Item 31. Process according to any one of items 23 to 28, wherein the CO.sub.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.sub.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.

[0105] Item 32. Process according to any one of the preceding items, wherein the low CO.sub.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.sub.4Cl is regenerated to NH.sub.3, such as, in particular, according to the process described in patent application EP 14188350.4.

[0106] Item 33. Process for producing bicarbonate according to item 32, wherein the low CO.sub.2 content gas is the exit gas from the refined bicarbonate crystallization reactor or column, and the gas enriched in CO.sub.2 by the CO.sub.2 concentration module comprises at least 40%, advantageously at least 60%, more advantageously at least 70% or even at least 80% of CO.sub.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.sub.2 in the precipitated refined bicarbonate beyond 70%, advantageously at least 80%, more advantageously at least 90%.

[0107] The energy consumption of different CO.sub.2 enrichments of low CO.sub.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.sub.2 concentration that are referred to in the present specification (amines, ammonia, PSA, TSA or CTSA, cryogenic, or membrane):

TABLE-US-00002 TABLE 2 average energy consumption of CO.sub.2 concentration processes according to CO.sub.2 enrichment (+80%, +30% or +10%). Case (CO.sub.2 concentration of the gases Use Energy consumption on vol % on dry basis) Example t Steam/t CO.sub.2 GN flue gases or LCL 10% .fwdarw. 90% BIR CR 1.30 t Ve/t CO.sub.2 GN flue gases or LCL 10% .fwdarw. 40% GP-GBIR 0.65 t Ve/t CO.sub.2 low CO.sub.2 FCH 30% .fwdarw. 40% GP-GBIR 0.30 t Ve/t CO.sub.2 Key to abbreviations: GN flue gases (low CO.sub.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.sub.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'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).

[0108] Among the various alternatives according to the present invention, that relating to the use of a TSA (and/or CTSA) CO.sub.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.sub.2, to decrease or even cancel additional generation of CO.sub.2 with combustion of fossil energy such as natural gas, coal or petroleum.

[0109] Moreover, in all of the cases where enrichment processes referred to in the present specification are used, the partial enrichment in CO.sub.2 of low CO.sub.2 content gases into gases with a CO.sub.2 content of more than 40% concentrationfor example at least 45%, or at least 50%, or at least 60%, or at least 70% of CO.sub.2 by volume on a dry gas basismakes 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.sub.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.sub.2 discharge.

[0110] 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

[0111] 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.

[0112] 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.sub.2 enrichment module, in particular according to item 15.

[0113] Key to abbreviations in FIG. 1: [0114] AB: Ammonia absorber (production of ammoniacal brine) [0115] CL: Bicarbonation column for precipitation of ammoniacal bicarbonate (crude bicarbonate). [0116] FL: Filter for separating ammoniacal bicarbonate (crude bicarbonate) from the crystallization mother liquors (rich in dissolved NH.sub.4Cl and NaCl), referred to as filter liquid. [0117] DS: Distiller for the crystallization mother liquors (for regeneration of ammonia), consuming lime and distillation steam and producing a distiller liquid rich in CaCl.sub.2 in aqueous solution. [0118] 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). [0119] SHT-SL: Light soda ash dryer (calcination of the ammoniacal bicarbonate to give light soda ash under the effect of heat, consuming steam). [0120] FCH: Lime kiln [0121] 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.). [0122] GN: Steam generator of the unit for producing sodium carbonate with ammonia or of the unit for producing refined bicarbonate. [0123] CO.sub.2 enrichment: module for enrichment of low CO.sub.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)

[0124] Production of crude ammoniacal bicarbonate as described in Ullmann's encyclopedia (see above) in section 1.4.1.2 & 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.sup.3 of CO.sub.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.sup.3 of CO.sub.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.

[0125] 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)

[0126] The same crude ammoniacal bicarbonate production process as described in the preceding example is made up with a lime-kiln lean-gas CO.sub.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.sub.2 on a dry gas basis.

[0127] This lean gas is partially enriched by a TSA-type CO.sub.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.sub.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.

[0128] The same carbonation column is used as in Example 2, with a quantity of 37% lean gas (weak gas) readjusted in CO.sub.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.sup.3 of CO.sub.2 at 40 vol % on a dry gas basis, per ton of soda ash produced. The rich gas (strong gas) at 70% CO.sub.2 by volume on a dry gas basis is replaced by the rich gas enriched to 85 vol % CO.sub.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.sub.2 quantity identical to that corresponding to the flow rate of rich gas in Example 2.

[0129] 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.

[0130] 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.sub.2 concentration module.

[0131] The utilization yield of NaCl is increased from 73% (Example 2) to 76% (Example 3). The absorption yield (one pass) of CO.sub.2 is equivalent to that in Example 2.

[0132] 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.

[0133] This example shows the advantage of using partial CO.sub.2 enrichment: the overall capture yield of low-content CO.sub.2 (37%) is improved substantially. The capture of CO.sub.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.sub.2 and to increase significantly the overall fixation balance of CO.sub.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)

[0134] 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 XVThe manufacture of Refined Sodium Bicarbonate, pp. 196-197.

[0135] A lime kiln gas with 37 vol % of CO.sub.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.

[0136] In a second phase, the same unit for producing refined sodium bicarbonate is fed with CO.sub.2 gas from a mixture of bicarbonation column exit gas (at 20 vol % CO.sub.2 on a dry basis) and of lime kiln gas (at 37 vol % CO.sub.2 on a dry basis), this mixture being enriched with CO.sub.2 by an amine-type CO.sub.2 concentration module, to a CO.sub.2 concentration of 60 vol % CO.sub.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.

[0137] 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.

[0138] 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 m 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.sub.2.

[0139] The average degree of capture of the CO.sub.2 in the crystallized sodium bicarbonate goes from 70% to 88%.

[0140] Similar results may be obtained with a CO.sub.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.sub.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.

[0141] 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.