Abstract:
The invention relates to an installation comprising a combustion chamber ( 15 ) provided with means ( 53 ) which are used to introduce the mineral load and are connected to a pre-heater ( 27 ), combustion means ( 55 ) for maintaining the chamber ( 15 ) at a temperature of between 700° C. and 900° C., means ( 57 ) for introducing a treatment gas having a controlled carbon dioxide content in order to oppose the dissociation of the carbonate in the chamber ( 15 ), and means ( 61 ) for removing the calcinated mineral load guided into a cooler ( 31 ). The chamber ( 15 ) comprises means ( 51 ) for forming a fluidised bed. The means ( 57 ) for introducing treatment gases are supplied at least partially by a line ( 91 ) or deriving part of the combustion flue gases of the chamber ( 15 ) emitted by means for discharging ( 21 ) said flue gases into the atmosphere. The invention can be used for the production of a cement-type hydraulic binder.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a 35 U.S.C. §§ 371 national phase conversion of PCT/FR2005/002360 filed 22 Sep. 2005, which claims priority of French Application No. 0411103 filed 19 Oct. 2004. The PCT International Application was published in the French language. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an installation for calcining a mineral load containing a carbonate in order to produce a hydraulic binder, of the type comprising at least one calciner, the installation successively comprising a preheater, at least one calciner for producing a hydraulic binder comprising a combustion chamber and a cooler; the combustion chamber having:
         means for introducing the mineral load into the chamber, connected to the preheater;   combustion means for maintaining the chamber at a temperature between 700° C. and 900° C., the combustion means having means for introducing an oxidizing gas for combustion coming from the cooler into the chamber;   additional means for introducing a treatment gas having a controlled carbon dioxide content into the chamber in order to oppose the dissociation of the carbonate in the chamber; and   means for removing the calcined mineral load that open into the cooler;
 
the installation comprising means for discharging the flue gases coming from the or each calciner into the atmosphere.
       

     The term “hydraulic binder” is understood to mean a powdery material composed of fine particles, which, in contact with water, react by forming a solidified block and develop mechanical strength properties. Examples of hydraulic binders are cements, limes, slags, pozzolans and ash from fossil fuel power stations. 
     The aforementioned installation is intended to produce an artificial hydraulic binder denoted by the term “kalsin”, as described in application EP-A-0 167 465. 
     Kalsin-type binders are products based on clay phases and on at least one carbonate, preferably a calcium carbonate with optionally a magnesium carbonate. The carbonate is activated by dehydroxylation and by calcium compounds, without forming free lime. 
     The term “carbonate” denotes a salt resulting from the combination of carbonic acid with a base. This salt comprises a carbonate anion and a metal cation, preferably an alkali or alkaline-earth metal cation. 
     The installation may also produce, at the same time as the kalsin, cement clinker. The term “cement clinker” is understood to mean the material exiting a fuel-fired rotary kiln, said material having formed balls or granules by partial melting at high temperature, for example around 1500° C., and by chemical compounds of various oxides such as calcium, silicon, aluminum and iron oxides. The clinker thus obtained is, after grinding with suitable additives, capable of producing a cement. A known installation for producing clinker is described in EP 0 754 924. 
     Known from the article “High energy savings through the use of a new high performance hydraulic component” by M. Paliard and M. Makris, in the work “Energy efficiency in the cement industry” edited by J. Sirchis, from Elsevier publishers (1990) is an installation for producing kalsin of the aforementioned type, which comprises a calciner having a fuel combustion region, into which an oxidizing gas for combustion that comes from the cooler is introduced, and a region for containing the load to be calcined, into which a gas having a controlled carbon dioxide partial pressure is introduced. 
     The carbon dioxide partial pressure is high in the containment region in order to oppose the dissociation of the carbonates. 
     However, the carbon dioxide that is introduced into the containment region is expelled into the atmosphere, which contributes to increasing the pollutant emissions in the atmosphere. 
     SUMMARY OF THE INVENTION 
     One object of the invention is therefore to provide a calcining installation of the aforementioned type that has reduced pollutant emissions. 
     To that end, one subject of the invention is an installation of the aforementioned type, characterized in that the chamber comprises means for forming a fluidized bed, and in that the means for introducing the treatment gas are supplied, at least partially, by at least one bypass duct with some of the flue gases, the bypass duct coming from the means for discharging the gases. 
     The invention may comprise one or more of the following characteristics, taken individually or according to any technically possible combination:
         the means for discharging the gases comprise a duct for extracting the flue gases coming from the combustion chamber, the bypass duct being tapped off the extraction duct;   the extraction duct is connected to the preheater;   it comprises an additional calciner for producing clinker that is different from the calciner for producing a hydraulic binder, the additional calciner comprising a flame combustion furnace and the means for discharging the gases comprise an additional duct for extracting the flue gases from the flame combustion furnace, the bypass duct being tapped off the additional extraction duct;   the additional extraction duct is connected to an additional preheater, the additional preheater emerging into the additional calciner;   the combustion means successively comprise, between the means for forming the fluidized bed and the means for introducing a gas having a controlled carbon dioxide content:
           fuel supply means; then   means for injecting the oxidizing gas for combustion coming from the cooler into the chamber;   
           the cooler is at least partially supplied by a secondary bypass duct with some of the flue gases, coming from the means for discharging the gases;   the cooler comprises secondary means for forming a fluidized bed in order to cool the calcined mineral load;   it comprises recirculation means connecting a downstream region of the combustion chamber, located downstream of the means for introducing the treatment gas, to an upstream region of the chamber, located upstream of the combustion means;   it comprises secondary means for removing some of the recycled mineral load circulating in the recirculation means, the secondary removal means opening into the cooler;   the means for removing the calcined mineral load emerge between the means for introducing the treatment gas and the means for forming the fluidized bed;   the combustion means comprise a secondary combustion chamber having:   secondary means for introducing some of the mineral load coming from the preheater; and   means for injecting an oxidizing gas for combustion coming from the cooler into the secondary chamber; the secondary chamber being connected to the combustion chamber by an outlet duct opening between the means for introducing the mineral load and the additional means for introducing the treatment gas.       

     Another subject of the invention is a process for calcining a mineral load containing a carbonate in order to produce a hydraulic binder, of the type comprising at least one calcining phase, the process successively comprising a phase for preheating the mineral load in a preheater, at least one phase for calcining the preheated mineral load in a combustion chamber and a phase for cooling the calcined mineral load in a cooler; 
     the calcining phase comprising the steps of:
         introduction of the mineral load coming from the preheater into the chamber;   combustion of a fuel in order to maintain the chamber at a temperature between 700° C. and 900° C., the combustion step comprising the introduction of an oxidizing gas for combustion coming from a cooler into the chamber;   introduction of a treatment gas having a controlled carbon dioxide content into the chamber, in order to oppose the dissociation of the carbonate in the chamber; and   removal of the calcined mineral load in order to introduce it into the cooler;       

     the process comprising a phase of discharging the flue gases produced during the or each calcining phase into the atmosphere; 
     characterized in that the calcining phase comprises a step of forming a fluidized bed in the chamber, and in that the step of introducing a treatment gas comprises at least partially bypassing some of the flue gases discharged during the gas discharge phase and supplying the chamber with said bypassed flue gases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiment examples of the invention will now be described with reference to the appended drawings, in which: 
         FIG. 1  is a schematic diagram representing a first installation according to the invention; 
         FIG. 2  is an enlarged view of an upstream part of the installation from  FIG. 1 ; 
         FIG. 3  is an enlarged view of a downstream part of the installation from  FIG. 1 ; 
         FIG. 4  is a view similar to that from  FIG. 3  of a second installation according to the invention; 
         FIG. 5  is a view similar to that from  FIG. 3  of a third installation according to the invention; and 
         FIG. 6  is a view similar to that from  FIG. 2  of a fourth installation according to the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The installation  11  for calcining a raw mineral load represented in  FIG. 1  comprises a unit  13  for producing kalsin, equipped with a fluidized-bed combustion chamber  15 , and at the same time, a unit  17  for producing clinker, equipped with a fuel-fired rotary kiln  19 . 
     The installation  11  also comprises means  21  for discharging the flue gases into the atmosphere comprising a duct  23  for extracting the flue gases generated in the fluidized-bed chamber  15 , and an additional duct  25  for extracting the flue gases generated in the fuel-fired rotary kiln  19 . 
     Each duct  23 ,  25  is equipped with a fan  23 A,  25 A, and with a device  23 B,  25 B for adjusting the fan, for example a damper, or a variable speed fan for separately adjusting the respective flow rates of the gases circulating in the respective installations  13  and  17 . 
     In everything that follows, the terms “upstream” and “downstream” are understood relative to the circulation of the mineral load in the installation. 
     The unit  13  for producing kalsin successively comprises, from upstream to downstream, a preheater  27 , a calciner  29  comprising the fluidized-bed combustion chamber  15  and a cooler  31 . 
     As illustrated in  FIG. 2 , the preheater  27  comprises a plurality of preheating cyclones  33  mounted as a cascade, in order to bring the mineral load descending toward the calciner  29  into contact with the gases extracted from the calciner  29  that rise up toward the discharge means  21 . In the example illustrated, three cyclones  33 A,  33 B,  33 C are mounted in cascade. 
     The preheater  27  comprises an upper inlet  35  for introducing the raw mineral load, a lower outlet  37  for discharging the preheated load that emerges into the calciner  29 , a lower inlet  39  for introducing the gases coming from the calciner  29  and an upper outlet  41  for discharging the cooled gases that emerges into the extraction duct  23 . 
     Each cyclone  33  comprises a tangential inlet  43  for supplying gas and material, a lower outlet  45  for discharging the material that is equipped with a non-return valve and an upper outlet  47  for discharging the gas. 
     The upper inlet  35  for introducing the raw mineral load is connected to the gas duct leading to the tangential inlet  43 A for supplying the upper cyclone  33 A. 
     The lower outlet  37  for discharging the preheated load consists of the lower outlet  45 C of the lower cyclone  33 C. 
     The lower inlet  39  for introducing the gases coming from the calciner leads into the tangential inlet  43 C of the lower cyclone  33 C. 
     The upper outlet  41  for discharging the cooled gases consists of the upper outlet  47 A of the upper cyclone  33 A. 
     The tangential inlet  43 B of the intermediate cyclone  33 B is connected, on the one hand, to the lower outlet  45 A of the upper cyclone  33 A, and, on the other hand, to the upper outlet  47 C of the lower cyclone  33 C. 
     The upper outlet  47 B of the intermediate cyclone  33 B is connected to the tangential inlet  43 A of the upper cyclone  33 A. The lower outlet  45 B of the intermediate cyclone  33 B leads into the tangential inlet  43 C of the lower cyclone  33 C. 
     As illustrated in  FIG. 2 , the fluidized-bed combustion chamber  15  extends approximately vertically. It comprises, from upstream to downstream, from the bottom to the top in  FIG. 2 , means  51  for forming a fluidized bed, an inlet  53  for introducing the preheated mineral load, combustion means  55 , an inlet  57  for introducing a treatment gas having a controlled carbon dioxide content, and means  59  for recirculating the calcined mineral load. The chamber  15  is, in addition, equipped, between the means  51  for forming the fluidized bed and the inlet  53  for introducing the mineral load, with an adjustable side outlet  61  for removing the calcined mineral load. 
     The means  51  for forming the fluidized bed comprise a compressor  63  connected, on the one hand, to a gas source  65 , and, on the other hand, to a plurality of nozzles  67  for injecting the fluidizing gas. 
     The source  65  contains, for example, carbon dioxide or a gas comprising a mixture of carbon dioxide and oxygen, for example air mixtured with flue gases or gases coming from a reactor producing carbon dioxide, such as for example the rotary kiln  19 . 
     The volume content of carbon dioxide in this gas is for example between 10% and 40%. The volume content of oxygen in this gas, if it contains any, is for example between 3% and 21%. 
     The nozzles  67  are placed in the bottom of the combustion chamber  15 . Each nozzle  67  is connected to the compressor  63  by a duct  69  equipped with a regulating valve  71 . 
     The inlet  53  for introducing the mineral load is connected to the discharge outlet  37  of the preheater  27 . It leads sideways into the combustion chamber  15 . 
     The combustion means enable fuel, with or without oxidizing gas for combustion, to be introduced into the chamber  15  by means of burners. 
     The combustion means  55  comprise an inlet  73  for supplying fuel and an inlet  75  for injecting an oxidizing gas for combustion. 
     The inlet  73  for supplying fuel is placed approximately at the same level as the inlet  53  for introducing the mineral load. It is offset sideways relative to that introduction inlet  53 . 
     The inlet  73  is connected to an installation for storing, measuring out and transporting fuels (not shown) that contains for example low quality fuels. 
     The term “low quality fuel” is understood to mean, for example, waste or by-products such as petroleum coke, used tires, plastic residues, sawdust, used oils, sludges or animal meal, that have a low calorific value and that are difficult to burn. As these low quality fuels are generally available on the market at low cost, their use therefore represents a significant economic advantage. 
     The inlet  75  for injecting the oxidizing gas for combustion comes out downstream of the inlet  73  for supplying fuel, in the vicinity of this inlet  73 . 
     This inlet  75  is formed from a plurality of peripheral openings leading into the chamber  15 . These openings are spread along an upstream coil  77  for injecting the gas, encircling the combustion chamber  15 . 
     This upstream coil  77  is directly connected to an upper outlet  79  for discharging the gases from the cooler  31 . The oxidizing gas for combustion introduced by the injection coil  77  is relatively rich in oxygen. The volume content of oxygen in this gas is for example between 3% and 21%. 
     Advantageously, this gas is relatively low in carbon dioxide. The volume content of carbon dioxide in this gas is for example between 0% and 5%. 
     The region  81  located between the injection nozzles  67  and the inlet  75  for injecting the oxidizing gas for combustion defines a region of dense fluidized bed, in which the mineral load is contained. The region  83  downstream of the inlet  75  defines a region of expanded fluidized bed. 
     The height of the dense region  81 , that is to say the distance that separates the injection nozzles  67  from the inlet  75  for supplying the oxidizing gas for combustion is chosen at the time of the design of the installation  11 , depending on the nature of the fuel that will be used in this installation  11 . More specifically, this height is increased in the case where low quality fuels are used and decreased if the fuels used are easy to burn, like fuels having a high calorific value, such as fuel oil, natural gas or some coals. 
     The inlet  57  for introducing a gas having a controlled carbon dioxide content is formed by a plurality of peripheral openings leading into the chamber  15 , spread along a downstream coil  85  for introducing the gas. 
     The downstream coil  85  is placed downstream of the upstream coil  77 , above this coil  77  in  FIG. 2 . 
     The introduction inlet  57  is supplied by first and second bypass ducts  91  and  93  coming from the means  21  for discharging the flue gases. 
     As illustrated in  FIG. 1 , the first bypass duct  91  is tapped off the duct  23  for extracting the flue gases generated in the combustion chamber  15 . 
     The second bypass duct  93  is tapped off the additional duct  25  for extracting the flue gases generated by the fuel-fired kiln  19 . 
     Each bypass duct  91 ,  93  is equipped with instrumentation comprising, successively from the extraction duct  21 ,  25  respectively, a damper  95  for adjusting the flow rate, a blower  97 , a flowmeter  99  and a sensor  101  for measuring the volume content of carbon dioxide in the duct  91 ,  93 . 
     Each component contained in the instrumentation is electrically connected to a central unit  103  for adjusting the carbon dioxide content in the gas having a controlled content that passes through the inlet  57 . 
     The gas having a controlled content of carbon dioxide is relatively rich in carbon dioxide. The volume content of carbon dioxide in this gas is for example between 20% and 40%. 
     Advantageously, this gas is, in addition, relatively low in oxygen. The volume content of oxygen in this gas is for example between 0% and 5%. 
     With reference to  FIG. 2 , the recirculation means  59  comprise successively from upstream to downstream, from the top to the bottom in  FIG. 2 , a duct  111  for discharging the material and the gas and a cyclone separator  113 . 
     The discharge duct  111  leads transversely to the upper end of the combustion chamber  15 . 
     The cyclone  113  comprises a tangential inlet  115  connected to the discharge duct  111 , a lower outlet  117  for discharging the material, and an upper outlet  119  for discharging the gas that forms the lower inlet  39  for introducing gases into the preheater  27 . 
     The outlet  117  leads into a recirculation duct  121  that opens sideways into the combustion chamber  15  between the gas injection nozzles  67  and the inlet  53  for introducing the mineral load, in the vicinity of this inlet  53 . The opening of the duct  121  into the chamber  15  is preferably located on the same side as the inlet  53 . The recirculation duct  121  is provided with a non-return valve  121 A. 
     With reference to  FIG. 3 , the outlet  61  for removing the material leads into an inlet  122  for supplying the material from the cooler  31 . It is equipped with a valve  123  for controlling the amount of mineral load removed. 
     The control valve  123  comprises, for example, a frustoconical flow passage  125  and a movable conical piston  127  for closing this passage  125 . 
     The closing piston  127  is mounted on the end of a rod that can be moved in translation between a position that closes the passage and a position that completely opens the passage. 
     The outlet of the valve  123  is equipped with a non-return valve. 
     The cooler  31  comprises a plurality of cooling cyclones  133  mounted in cascade, of the same structure as the preheater  27 , and a screw  135  for removing the final product. 
     However, unlike the preheater  27 , the tangential inlet  137 A of the upper cooling cyclone  133 A is connected to an outlet of the removal valve  123 . 
     Furthermore, the upper outlet  139 A of the upper cyclone  133 A is connected to the upstream injection coil  77 . 
     The tangential inlet  137 C of the lower cyclone  133 C is supplied by a duct  140  for introducing fresh air, equipped with a fan  141  that adjusts the intake of fresh air and a flowmeter  143  downstream of the fan  141  in the flow direction of the fresh air. 
     A tap  145  is provided between the tangential inlet  137 C and the removal screw  135 , under the introduction duct  140 , in order to recover the mineral material coming from the lower outlet  147 B of the intermediate cyclone  133 B that would not be carried away by the current of fresh air coming from the air introduction duct  140 . 
     The outlet  147 C for discharging material from the lower cyclone  133 C also leads into the removal screw  135 . 
     The removal screw  135  is placed in a cooling chamber  149  whose walls are cooled by water circulation. Thus, the material coming from the tap  145  and from the outlet  147 C is cooled, by indirect exchange, without contact nor mixing with the cooling water. The chamber  149  leads into a lower outlet  151  for discharging kalsin. 
     The unit  13  for producing kalsin is free from grinding means. 
     With reference to  FIG. 1 , the unit for producing clinker is, for example, of the type described in application EP 0 754 924. It comprises, from upstream to downstream, a meal preheater  161 , an additional calciner  163  for the preheated meal, equipped with a fuel-fired rotary kiln  19 , a cooler  165  for the calcined meal forming the clinker and a device  167  for grinding the cooled clinker. 
     The unit  13  comprises a duct  169  for discharging the flue gases generated in the rotary kiln  19  that extends between the calciner  163  and the preheater  161 . 
     Furthermore, the additional extraction duct  25  is connected to the preheater  161 . The flue gases produced in the kiln  19  rise through the preheater  161  and are discharged via the duct  25 . 
     The means  21  for discharging the gases comprise at least one filter  171  into which the extraction ducts  23 ,  25  emerge, and a fan  173  for discharging the gases into the atmosphere, connected to an outlet for discharging the gases from which the dust has been removed by the filter  171 . 
     The process for producing kalsin in the installation  11  will now be described. 
     This process comprises a step of preheating the mineral load, a step of calcining the preheated mineral load and a step of cooling the calcined mineral load. 
     In the preheating step, the raw mineral load or “raw meal” is introduced into the preheater  27  via the inlet  35 . 
     The meal is obtained from a mixture, called a “raw mix”, of calcium carbonate, with or without magnesium carbonate and clays or marls, containing silicon, aluminum and/or iron oxides. 
     The raw mix is ground in a known way in vertical roller mills or in ball mills to a fineness characterized by a weight quantity of particles of less than 200 microns of around 98% and a weight quantity of particles of less than 100 microns of around 80% to 90%. 
     With reference to  FIG. 2 , the raw mineral load flows successively from the top to the bottom in the cyclones  33 , countercurrent to the flue gases coming from the calciner  29  via the inlet  39 . 
     The mineral load is thus preheated in the preheater  27  by the flue gases to a temperature approximately between 650° C. and 800° C. level with the outlet  37 . 
     During this preheating, dehydroxylation reactions of the clays occur when the temperature is between 500° C. and 700° C. 
     In the combustion step, the preheated mineral load is introduced into the combustion chamber  15  through the inlet  53  for introducing the material. 
     Under the influence of the injection, through the nozzles  67 , of the fluidizing gas coming from the source  65 , the mineral load forms a dense fluidized bed in the region  81 . 
     The concentration of material in the dense region  81  is approximately between 50 kg/Sm 3  to 200 kg/Sm 3  of gas considered at standard temperature and pressure conditions (0° C. and 100,000 Pa). The velocity of the gases in the region  81  is between 0.6 m/s and 0.8 m/s, considered at actual temperature and pressure conditions. 
     Above the dense region  81 , the material suspended in the form of a rising stream is then picked up by the gases coming from the gas supply inlet  75  and the gas introduction inlet  57 . Thus, in the expanded region  83 , a fluidized bed expanded by the dilute phase is obtained, in which the velocity of the gases is greater than 2 m/s and preferably between 3 m/s to 5 m/s, and the material concentration is decreased relative to that of the dense phase in the dense region  81 . 
     Simultaneously, the fuel is introduced into the region  81  via the feed inlet  73 . The fuel is brought into close contact with the load by the mixing produced by the fluidization phenomenon produced by the means  51 . 
     In the dense region  81 , the combustion of the fuel is initiated thanks to the oxygen contained in the fluidizing gas. This start of the combustion consumes all the oxygen coming from the gas source  65 , creating a gas atmosphere rich in carbon dioxide all around the mineral load particles. 
     On contact with the oxidizing gas for combustion coming from the cooler  31  through the coil  77 , the combustion of the fuel continues more rapidly. 
     The temperature is then between 700° C. and 900° C. in the combustion chamber  15 . 
     Calcium compounds are formed between the silicon, aluminum and/or iron oxides activated during the preheating step and the activated calcium carbonate, without releasing carbon dioxide. 
     To prevent the carbonates contained in the mineral load from dissociating after the introduction of the oxidize gas via the inlet  77 , the gas having a controlled carbon dioxide content, formed from some of the flue gases generated in the calciner  29  and from some of those generated in the calciner  163  of the unit  17  for producing clinker, is introduced into the chamber  15  through the downstream coil  85 . 
     The central unit  103  adjusts the flow rate of the gas injected depending on the levels of carbon dioxide measured by the sensors  101  in order to maintain the carbon dioxide level in the chamber  15 , measured by a sensor  199 , approximately between 25% and 40%. 
     The dissociation of the carbonates in the expanded region  83  is thus reduced, which decreases the production of carbon dioxide. 
     This result is also obtained when an external gas source having a controlled carbon dioxide content, such as a tank, is connected to the coil  85 , instead of the ducts  91  and  93 . 
     In the example shown, recycling some of the flue gases coming from the calcining chamber  15  and from the fuel-fired kiln  19  prevents additional carbon dioxide from being introduced into the installation  11 , which also contributes to reducing pollutant emissions into the atmosphere. 
     Furthermore, the gas introduced via the coil  85  has a carbon dioxide content that is adapted so as not to detract from the combustion of the fuel in the expanded region  83 , even if this fuel is of low quality. It is therefore possible to use a fluidized-bed chamber  15  to carry out the calcination, even with a low quality fuel. 
     In addition, the distance between the coils  85  and  77  is chosen to be greater when the fuel used is of low quality. As the dissociation of carbonates with the release of carbon dioxide is linked to the supply of heat generated by combustion of the fuel, it is possible, when low quality fuels that burn slowly are used, to have a greater reaction volume between the two coils  85  and  77 , without risking too fast a combustion that causes dissociation of the carbonates. This arrangement facilitates the use of low quality fuels. 
     The mineral load is then carried by the gases toward the upper end of the chamber  15 , then discharged through the duct  111  and the cyclone  113 . It is then reintroduced into the dense region  81  via the recycling duct  121 . The load thus carries out on average several calcining cycles in the combustion chamber  15 . 
     In the cooling step, according to  FIG. 3 , some of the calcined load is removed through the valve  123  and flows by gravity into the successive cooling cyclones  133 . The load is cooled by fresh air introduced by the duct  140  and flowing countercurrently to the load in the cooling cyclones  133 . 
     The thus cooled load has a temperature between 350° C. and 250° C. at the inlet of the screw  135 . It is then discharged via the screw  135 , while undergoing a final cooling. The end product discharged by the outlet  151  is a hydraulic binder denoted by the term kalsin. 
     This end product only requires a low grinding energy, because it is in the form of a fluid powder, only a few particles of which may sometimes agglomerate. 
     If necessary, the end product is partially mixed with the clinker produced in the unit  17  for producing clinker, in the amounts specified in EP 0 167 465 in order to constitute, after grinding, a hydraulic binder. 
     In one variant represented by dotted lines in  FIG. 2 , a material bypass duct  201  is tapped off between the material outlet  37  of the preheater  29  and the inlet  53  for introducing into the calcining chamber  15 , in order to bypass some of the preheated mineral load. This duct  201  leads into the recirculation duct  121 . 
     As a variant, the inlet  57  is only connected to the bypass duct  93  for the flue gases produced in the rotary kiln  19  of the unit  17 . 
     With reference to  FIG. 4 , the second installation  211  according to the invention is similar to the first installation  11 . 
     However, unlike the first installation  11  the cooler  31  is free from a cascade of cooling cyclones. 
     The cooler  31  comprises upstream and downstream fluidized bed channels  213  and  215 , arranged as a cascade. 
     The upstream channel  213  extends approximately horizontally between an inlet  217  for introducing the material, on the left on the drawing, and an outlet  219  for discharging the material leading into the downstream chamber  215 . It comprises a plurality of gas injection nozzles  221  and a downstream opening  223  for discharging the gas. 
     The inlet  217  for introducing the material is connected, on the one hand, to a removal device  225  tapped off the recycling cyclone  113 , and, on the other hand, to the outlet of the removal valve  123 . 
     The injection nozzles  221  are distributed at the bottom of the channel  213  between the inlet  217  and the outlet  219 . They are suitable for producing a dense fluidized bed with the mineral load received from the calciner  29 , using compressed air or a mixture of air and carbon dioxide received from a compressor  226 . 
     The bottom of the upstream channel  213  is slightly tilted to promote the flow of the load from the inlet  217  toward the outlet  219 . 
     As a variant, the channel  213  is free from nozzles. The cooling air is injected via a plurality of orifices arranged in the bottom of the channel  213 . 
     The opening  223  for discharging gas is arranged at the upper downstream end of the channel  213 . The opening is connected to a tangential duct  227  for introducing into a discharge cyclone  229 . 
     The upper outlet  231  of the cyclone  229  supplies, with oxidizing gas for combustion, the upstream coil  77  of the inlet  75 , via a fan  232 A and a sensor  232 B for measuring the flow rate. 
     Furthermore, the lower material outlet  233  of the cyclone  229  leads into the outlet  219 , through a non-return valve. 
     The structure of the second channel  215  is similar to that of the first channel  213 . However, unlike the first channel  213 , a shell-and-tube heat exchanger  235 , supplied with water, is placed in the channel  215  opposite the injection nozzles  221 . Furthermore, the upper outlet of the discharge cyclone  239  of the second channel  215  leads into the means for discharging gases  21 , upstream of the filter  171  and downstream of the ducts  23  and  25 , via a fan  237 A. 
     The compressor  238  of the channel  215  is preferably supplied by ambient air. 
     The gases coming from the cyclones  229  and  239  are sucked up by the fans  232 A and  237 A. The flow rate of the gas circulating in the duct  231  is measured by the device  232 B, while its flow rate is controlled by the rotational speed of the fan  232 A or else using dampers whose position can be adjusted. The flow rate of the gas passing through the cyclone  239  is controlled by the speed of the fan  237 A so as to obtain a slightly negative static pressure in the duct  219 A connecting the outlet  219  of the first channel  213  to the second channel  215 . A device for measuring pressure is installed, for this purpose, in the duct  219 A. This device in the duct  219 A prevents the gas coming from the channel  215  from going back toward the channel  213  via the duct  219 A. 
     The device for extracting material  225  comprises a fluidizing chamber  251 , leading into the lower extension of the recirculation cyclone  113 , and a side outlet  253  for removing material that is closed off by a secondary removal valve  255 . 
     The fluidizing chamber  251  comprises, in the bottom, a plurality of nozzles  257  for injecting compressed air that comes from a compressor  259 . 
     The valve  255  has a structure similar to the removal valve  123 . It is placed between the outlet  253  for removing material and the inlet  217  of the upstream channel  213 . The outlet of the valve  255  is equipped with a non-return valve. 
     The operation of this installation  211  is, furthermore, similar to that of the installation  11  described with reference to  FIG. 1 . 
     However, unlike the installation  11 , the calcined mineral load is cooled by the fluidizing gas injected into the successive channels  213 ,  215 . 
     The installation  311  represented in  FIG. 5  differs from the first installation  11  by the fact that the inlet  57  for introducing the gas having a controlled carbon dioxide content is only connected to the second bypass duct  93  coming from the unit  17  for producing clinker. 
     Furthermore, the cooler  31  comprises a primary cooler  313 A and a secondary cooler  313 B placed under the primary cooler  313 A. 
     The primary cooler  313 A comprises two cooling cyclones  315 A,  315 B mounted as a cascade, as described previously. 
     The outlet of the removal valve  123  is connected to the tangential inlet  317 A of the upper cyclone  315 A of the primary cooler  313 A. Furthermore, the first bypass duct  91  is connected to the tangential inlet  317 B of the lower cyclone  315 B of the primary cooler  313 A. This tangential inlet  317 B is, furthermore, connected to a primary screw  319 A for product extraction. 
     The upper outlet  321 A of the cyclone  315 A leads into the upstream coil  77 . 
     The secondary cooler  313 B has a structure similar to the cooler  31  of installation  11 . However, the tangential inlet  329 A of the upper cyclone  331 A of the secondary cooler  313 B is connected to the lower outlet  333 B of the lower cyclone  315 B of the primary cooler  313 A. 
     Furthermore, the upper outlet  335 A of the upper cyclone  331 A of the secondary cooler  313 B leads into the combustion chamber  15  by a secondary inlet  337  for supplying oxidizing gas for combustion. The secondary inlet  337  opens upstream of the upstream coil  77  in the vicinity of this coil  77 . 
     The mineral load exiting the lower cyclone  331 C of the secondary cooler  313 B leads into the secondary product extraction screw  337 B. 
     The mineral load extracted by the product extraction screws  319 A,  337 B respectively of the primary  313 A and secondary  313 B coolers forms the hydraulic binder in powder form, denoted by kalsin. 
     As a variant (not shown), the installation  11  is free from a unit for producing clinker. In this variant, the inlet  57  for introducing the treatment gas is only connected to the first bypass duct  91 . 
     The installation  411  represented in  FIG. 6  differs from that represented in  FIG. 2  in that the combustion means  55  comprise a secondary combustion chamber  413  connected to the fluidized-bed combustion chamber  15  via an outlet duct  415 , inclined downward. 
     The outlet duct  415  leads into the combustion chamber  15  between the inlet  53  for supplying the mineral load and the coil  85  for injecting the gas having a controlled carbon dioxide content, above the dense region  81 . The chamber  15  is thus free from an upstream coil  77  for introducing the oxidizing gas for combustion, the tilted duct  415  constituting means  75  for injecting oxidizing gas for combustion into the chamber  15 . 
     The secondary combustion chamber  413  comprises means  417  for supplying fuel that lead into the upper part  418  of the chamber  413 . The amount of fuel introduced into the secondary chamber  413  by the means  417  is adjustable relative to the total amount of fuel sent into the calciner  29 . 
     Furthermore, the secondary combustion chamber  413  comprises two ducts  419  and  421  for injecting oxidizing gas for combustion that open tangentially into the chamber  413 , respectively into the upper part  418  and into a middle part  422  of the chamber  413 . These ducts  419  and  421  are connected to the upper outlet  79  for discharging gases from the cooler  31 . The outlet  79 , unlike installation  11  represented in  FIGS. 1 to 2 , is not directly connected to the combustion chamber  15 . 
     The secondary combustion chamber  413  comprises, in addition, an inlet  423  for supplying mineral load that emerges between the tangential ducts supplying oxidizing gas for combustion  419  and  421 . This supply inlet  423  is connected to a distribution device  425  placed in the duct  427  that connects the outlet  37  of the preheater to the inlet  53  for introducing mineral load into the chamber  15 . The distribution device  425  is controlled in order to adjust the relative amount of mineral load introduced into the fluidized-bed combustion chamber  15  and into the secondary combustion chamber  413 . 
     In this installation  411 , the combustion of the fuel introduced by the introduction means  417  is initiated in the upper part  418  of the secondary combustion chamber  413 , in the absence of mineral load. 
     Thus, fuels that are extremely difficult to burn begin to catch fire in the upper part  418  of the secondary combustion chamber  413 . 
     The heat generated by the combustion of the fuel in this upper part  418  is then transmitted to the middle part  422  where the mineral load introduced by the supply inlet  423  undergoes an, at least partial, combustion that continues in the outlet duct  415 . The relative amount of mineral load introduced respectively into the combustion chamber  15  and into the secondary combustion chamber  413  is adjusted depending on the respective amount of fuel introduced into these chambers  15  and  413 . 
     Thanks to the invention that has just been described, it is possible to have an installation for producing a hydraulic binder that significantly reduces the carbon dioxide emissions expelled into the atmosphere, by the use of a fluidized-bed calcining chamber and by recycling the flue gases generated in the installation. 
     The injection of recycled flue gases into the downstream region of the combustion chamber furthermore limits the production of carbon dioxide by decarbonation in this downstream region. 
     The thermal energy consumed in order to implement the process according to the invention in the installation is reduced, on account of the low heat of reaction and the lower combustion temperatures relative to a unit for producing clinker. Furthermore, the use of kalsin as a hydraulic binder only requires a low electric power for grinding the product delivered downstream of the cooler. 
     The installation according to the invention also makes it possible to use a fluidized-bed chamber to calcine the mineral load, which allows the use of low quality fuels. 
     The installation may comprise a unit for producing kalsin connected to a unit for producing conventional clinker, in order to increase the overall hydraulic binder production capacity in the installation, while limiting the emissions of pollutant gases relative to a unit for only producing clinker of equivalent capacity.