OA21589A - Accelerated carbonation process and its implementation in a process for recovering concrete waste and industrial gaseous discharges. - Google Patents

Abstract

The invention relates to an accelerated carbonation process comprising the following steps: a) recycled concrete aggregates with a particle size less than or equal to a VI value of between 1 mm and 6 mm, in other words a Ô/Vl sand, are provided; b) a separation step is carried out on the 0/V1 sand by defining a particle size cut-off with a V2 value of between 0.1 mm and 0.2 mm so as to obtain: - a 1st fraction with a particle size less than V2, and - a 2nd fraction with a particle size between V2 and VI; c) the 2nd fraction is subjected to an accelerated carbonation step in a dynamic carbonator (11) so as to obtain carbonated recycled concrete aggregates. The invention also relates to a method for recovering concrete waste and industrial gaseous discharges using the accelerated carbonation process, in particular cement plant gaseous discharges.

Description

ACCELERATED CARBONATION PROCESS AND ITS IMPLEMENTATION IN A PROCESS FOR RECOVERING CONCRETE WASTE AND GASEOUS DISCHARGES

INDUSTRIAL

Accelerated carbonation process and its implementation in a process for the recovery of concrete waste and industrial gaseous discharges

[0001] The present invention relates to a process for the accelerated carbonation of recycled concrete aggregates, as well as the implementation of this process in a process for recovering these aggregates and greenhouse gases emitted by an industrial installation, for example a cement plant.

[0002] In France, the demolition of buildings and structures constructed since the 1950s is now booming and generates 300 million tonnes of waste per year, of which approximately 36% is concrete-based materials. In this regard, it should be remembered that concrete is a mixture comprising, by mass, approximately: 80% of inert mineral materials, i.e. aggregates (in different forms: gravel, chippings and sand), 15% of a binder (essentially cement) and 5% of water.

[0003] However, with approximately 2 tonnes per person per year, concrete is the most consumed manufactured material in the world. Concrete manufacturing currently represents approximately 40% of total aggregate consumption. This is why, for economic and environmental reasons, in order to avoid drawing on natural resources as much as possible, it is essential to recover demolition waste materials, and in particular those based on concrete, in order to obtain recycled concrete aggregates that can perfectly replace “natural” aggregates.

[0004] In addition to the waste from concrete-based demolition materials, there is also the waste from concrete manufacturing, or in other words, non-compliant concrete because it has defects, as well as excess concrete not used on a construction site.

[0005] Therefore, in the context of the present invention, the term “recycled concrete aggregates” means concrete aggregates originating from: the demolition of structures or buildings containing concrete elements, concrete production waste, as well as excess concrete from construction sites. Recycled concrete aggregates are obtained by crushing and screening existing concrete which may be in the form of blocks and/or rubble. Recycled concrete aggregates are therefore composed of the old natural aggregate which is attached to the old cement paste. In addition, in the granular skeleton of this concrete, the following two types of fractions are distinguished:

- the “coarse” fraction which contains aggregates with dimensions greater than 6 mm (or in other words aggregates with a particle size greater than 6 mm);

- the “sand” fraction which contains aggregates with dimensions less than or equal to 6 mm (or in other words aggregates with a particle size less than or equal to 6 mm).

[0006] Thus, waste from concrete-based materials constitutes an important raw material that the construction industry is currently seeking to exploit to manufacture new materials for construction, in a virtuous ecological circle.

[0007] However, for recycled concrete aggregates to constitute interesting substitutes for natural aggregates, it is essential that the concrete comprising recycled concrete aggregates has equivalent mechanical properties, or even better than those of concrete obtained from natural aggregates. However, recycled concrete aggregates have a higher porosity in their microstructure, which constitutes a technological obstacle to their use as substitutes for natural aggregates. Indeed, this high porosity results in greater water absorption; which reduces the mechanical properties of the resulting concrete.

[0008] It is certainly known to overcome this porosity problem by subjecting recycled concrete aggregates to accelerated carbonation, namely by subjecting them to a gas flow containing carbon dioxide.

[0009] In the context of the present invention, the term “accelerated carbonation” means a carbonation process implemented by a device (in particular a laboratory or industrial device) which is therefore distinct from natural carbonation as defined below.

[0010] During the manufacture of concrete, the cement reacts with water in such a way that silica and calcium hydrates are formed. These hydrates give the concrete its mechanical strength. However, these hydrates are not perfectly stable over time: they naturally carbonate by slowly absorbing atmospheric carbon dioxide and are transformed back into limestone and silica gel. However, this natural carbonation phenomenon occurs very slowly (over several decades) and mainly on the surface of the concrete-based construction material. In addition, the quantities of carbon dioxide captured remain limited, since they do not exceed the 15 to 20% of carbon dioxide emissions that occur during the production of concrete.

[0011] Since recycled concrete aggregates are partly composed of cement paste which therefore contains these hydrates formed during the hydration of the cement, it is these hydrates which have the capacity 2 to carbonate when said recycled concrete aggregates are subjected to accelerated carbonation.

[0012] However, the efficiency and optimization of accelerated carbonation (i.e. its carbon dioxide capture performance, in particular reducing the porosity at the microstructure of the recycled concrete aggregates, as well as its speed of implementation) depend on a significant number of parameters such as in particular the particle size of the aggregates, the device for bringing said aggregates into contact with the gas flow, the temperature, humidity and pressure that should be selected so that this efficiency is as good as possible. However, it is not at all easy to choose and combine the right parameters for optimal implementation of accelerated carbonation. This is why this known accelerated carbonation technique is still the subject of intense research in order to improve its efficiency in order to recover not only recycled concrete aggregates but also industrial gaseous discharges.

[0013] Indeed, many industries, including the cement industry, are subject to the emissions trading system established by the European Union as part of the fight against climate change. This system is based on a principle of capping and trading emissions rights. A ceiling is set to limit the total level of certain greenhouse gases emitted by the industries covered by this system. This ceiling gradually decreases over the years in order to reduce greenhouse gas emissions. Within the limits of this ceiling, industries receive or purchase emission quotas that they can exchange with other industries according to their needs.

[0014] For example, during the manufacture of cement, the cement industry emits greenhouse gases (namely carbon dioxide) at the outlet of the clinker manufacturing kiln. This is why, given this system of emission quotas of the European Union and the constant lowering of the emission ceiling, it is essential for the cement industry to valorize its greenhouse gas emissions by implementing technical and ecological solutions which revalorize these carbon dioxide emissions.

[0015] Thus, in view of these ecological issues set out above regarding the recovery of concrete waste on the one hand and greenhouse gas emissions on the other, it would be interesting to have an industrial process which could recover these two types of solid and gaseous waste simultaneously and efficiently.

[0016] In this regard, application WO 2019/115722 A1 proposes a technical solution that meets this need, since it describes a method for manufacturing an additional cementitious material that can replace cement from exhaust gases containing carbon dioxide and recycled concrete aggregates whose parameter D»c is less than or equal to 1,000 pm (or in other words 90% of these aggregates have a particle size less than or equal to 1,000 pm). This method consists in bringing said recycled concrete aggregates stored in the form of a pile or in a silo into contact with the exhaust gas so as to obtain a carbonated material which is then deagglomerated to obtain said additional cementitious material.

[0017] As explained in patent application WO 2019/115722 A1, it is entirely preferable, in order to improve the efficiency and speed of carbonation, that the D»c parameter of the recycled concrete aggregates be less than or equal to 100 pm. In addition, the product obtained at the end of the process described in this patent application WO 2019/115722 A1 is a substitute for cement. However, as explained above, since concrete production currently represents approximately 40% of total aggregate consumption, it would be interesting to transform recycled concrete aggregates into a substitute for natural aggregates in order to avoid drawing on natural aggregate resources.

[0018] The inventors of the present invention sought to optimize the parameters of accelerated carbonation to make it more efficient so as to recover recycled concrete aggregates to obtain directly, namely without an additional step such as a deagglomeration step described in patent application WO 2019/115722 A1, carbonated aggregates which are perfectly suitable as substitutes for natural aggregates. The inventors of the present invention further sought to integrate their accelerated carbonation process into a process for recovering recycled concrete aggregate waste and greenhouse gas emissions, in particular gaseous emissions from cement kilns for the manufacture of clinker.

[0019] The invention thus has as its first object an accelerated carbonation process which is characterized in that it comprises at least the following steps:

a) recycled concrete aggregates are available with a grain size less than or equal to a specific value Vi which is between 1 mm and 6 mm, in other words a 0/Vi sand;

b) a separation step is carried out on the 0/Vi sand by defining a granulometric cut-off of a determined value V2 which is between 0.1 mm and 0.2 mm so as to obtain:

- a ^first fraction whose particle size is less than V2, in other words a 0/V2 sand, and

- a ^2nd fraction with a grain size between V2 and V1, in other words a V2/V1 sand

c) the ^2nd fraction is subjected to an accelerated carbonation step in a dynamic carbonator by bringing said ^2nd fraction into contact with a gas flow containing carbon dioxide so as to obtain carbonated recycled concrete aggregates.

[0020] In the context of the present invention, the term "dynamic carbonator" means an accelerated carbonation device which is configured so that, during step c) of said method, the ^2nd fraction is in motion within said accelerated carbonation device (for example by means of a lifting and dispersion device) and/or said accelerated carbonation device is in motion.

[0021] In one embodiment of the invention, the dynamic carbonator comprises:

- a ^1st open end through which the ^2nd fraction is introduced,

- a ^2nd open end through which the gas flow containing carbon dioxide is introduced, said ^1st and ^2nd open ends are separated by a rotating section extending in a substantially horizontal longitudinal direction and within which the ^2nd fraction is advanced from the ^1st open end to the ^2nd open end and the gas flow circulates counter-current to the advancement of the ^2nd fraction.

[0022] The introduction of the ^2nd fraction through the ^1st open end can be carried out sequentially or preferably continuously.

[0023] In a preferred embodiment of the invention, the section has a generally cylindrical shape.

[0024] The recycled concrete aggregates from step a) are a 0/V1 sand or in other words recycled concrete aggregates whose particle size is between a value close to 0 and a VI value which can be between 1 mm and 6 mm, preferably between 1.5 and 4 mm, more preferably between 2 mm and 4 mm. As preferred examples of the present invention, the recycled concrete aggregates from step a) are a 0/2 sand or a 0/4 sand.

[0025] The ^1st fraction comprises the fraction of fines of the sand 0/Vi from step a). More precisely, said fines have a particle size less than V2, V2 being between 0.1 mm and 0.2 mm. In other words, the fines have a particle size between a value close to 0 and approximately V2, the value V2 being excluded.

[0026] The ^2nd fraction comprises the coarse fraction of the sand 0/V1 from step a). In other words, the recycled concrete aggregates of the ^2nd fraction have a particle size between V2 and VI, the values V2 and VI being included. The value V2 is between 0.1 mm and 0.2 mm and the value VI is between 1 mm and 6 mm, preferably between 1.5 and 4 mm, more preferably between 2 mm and 4 mm.

[0027] The inventors have in fact discovered that by subjecting the ^2nd fraction of recycled concrete aggregates, or in other words the fraction which is free from the fines of the 0/V1 sand from step a), to an accelerated carbonation step in a dynamic carbonator, said accelerated carbonation was very effective with a high carbon dioxide capture percentage and thus made it possible to obtain carbonated recycled concrete aggregates which were perfectly suitable for use as substitutes for natural aggregates in the manufacture of concrete.

[0028] The high percentage of carbon dioxide capture has the following effects:

- a reduction in the porosity of the microstructure of the recycled concrete aggregates thus carbonated; which makes it possible to limit the absorption of water which is harmful to the quality of the concrete in which these aggregates are incorporated;

- a limitation of the drop in the pH of the concrete in which these aggregates are incorporated (or in other words a limitation of the acidity of the concrete); which makes it possible to limit the corrosion of the steel in reinforced concrete structures.

[0029] The concrete obtained with these aggregates thus has excellent mechanical properties. More specifically, the carbonated recycled concrete aggregates obtained with the accelerated carbonation process according to the invention give the concrete in which they are incorporated good compressive strength, good durability properties, in particular good resistance to corrosion of the steel reinforcements contained in the concrete, said corrosion being caused by chloride ions and carbon dioxide.

[0030] The inventors discovered quite surprisingly that the selection of a particle size cut-off at a determined value V2 between 0.1 mm and 0.2 mm so as to extract the part of the fines from the sand 0/Vl, promoted the exchanges between the remaining recycled concrete aggregates (in other words the ^2nd fraction) and the gas flow during the accelerated carbonation step within the dynamic carbonator and thus improved the capture of carbon dioxide. This discovery of the extraction of the fines portion to accelerate the carbonate of recycled concrete aggregates goes against general knowledge in the technical field considered, and in particular the teaching of the aforementioned patent application WO 2019/115722 A1, which recommends favoring fractions of recycled concrete aggregates with the smallest particle size (in other words fine fractions) to implement accelerated carbonation.

[0031] Thus, during the accelerated carbonation step c) during which the selected recycled concrete aggregates (namely the ^2nd fraction) are brought into contact with the gas flow containing carbon dioxide, the carbonation reaction of the residual cement paste in these aggregates occurs.

[0032] The exchanges between the recycled concrete aggregates and the carbon dioxide are promoted when the dynamic carbonator comprises a ^1st open end and a ^2nd open end which are separated by a rotating section extending in a substantially horizontal longitudinal direction as described above. In this embodiment of the invention, said rotating section advantageously has a downward inclination oriented in the direction of advancement of the ^2nd fraction which is between 0.5° and 8°, more preferably between 1° and 5°. Most preferably, the downward inclination is 2°.

[0033] The rotating section may be a substantially inclined cylinder having a ^1st open end for the introduction, sequentially or preferably continuously, of the ^2nd fraction of recycled concrete aggregates and a ^2nd open end for the introduction of a gas flow containing carbon dioxide. The cylinder is rotated to allow the mixing of the bed of material consisting of the ^2nd fraction, as well as its advancement in the cylinder. This bed of material is swept by the gas flow which therefore advances counter-current to the advancement of the bed of material.

[0034] Advantageously, in order to increase the exchange surface between the ^2nd fraction of recycled concrete aggregates and the gas flow, the cylinder is equipped at its internal surface with a device for lifting and dispersing the recycled concrete aggregates within said cylinder. This device is perfectly within the reach of a person skilled in the art.

[0035] Other technical characteristics of the dynamic carbonator and the accelerated carbonation step are described below.

[0036] The dynamic carbonator of step c) may consist of a rotary drum dryer (i.e. a device that is perfectly known and used in many sectors of industry, including that of building materials) that has been adapted for the implementation of accelerated carbonation as described just above. In other words, the present invention may be implemented with a rotary drum dryer that has been adapted so as to obtain a dynamic carbonator having the technical characteristics described just above, as well as those described below. Examples of rotary drum dryers that may be used in the context of the present invention are in particular those of the TSM range marketed by the company Marini-Ermont or those with a jacket of the TTD range marketed by the company Allgaier.

[0037] The invention thus relates to the use of a rotary drum dryer for implementing accelerated carbonation on recycled concrete aggregates with a gas flow containing carbon dioxide.

[0038] The recycled concrete aggregates from the 0/Vi sand of step a) may come from the demolition of structures or buildings containing concrete elements, from concrete production waste or from excess concrete from construction sites. They may thus be concrete blocks and/or rubble, possibly reinforced with a steel structure, which are subjected to various successive (or sometimes simultaneous) stages of:

- sorting to extract steel materials,

- crushing, and

- screening, until obtaining the desired 0/Vi sand of recycled concrete aggregates.

[0039] The sorting step can be carried out with a de-ironing system comprising an electromagnet designed for the removal of steel elements combined with a magnetic de-ironing separation belt.

[0040] For example, the crushing may comprise a primary crushing step implemented by a crushing device which comprises:

- a receiving hopper, for example with a volume of around 15 ^m3 , fed by a loader with recycled concrete materials and a block size of less than or equal to 500 mm;

- a vibrating feeder;

- a primary jaw or impact crusher configured to produce recycled concrete materials with a block size less than or equal to a value between 60 mm and 200 mm,

- an output conveyor.

[0041] This crushing device may optionally be equipped with a de-ironing system as described above.

[0042] Then, the recycled concrete materials whose block size is less than or equal to a value between 60 mm and 200 mm thus obtained can be subjected to a secondary crushing step so as to obtain recycled concrete materials with a particle size less than or equal to 20 mm. This secondary crushing step can be implemented in a second crushing device which comprises for example:

- a sand removal system by screening;

- a gyratory crusher crushing concrete materials to obtain a particle size of less than or equal to 20 mm.

[0043] The recycled concrete materials thus obtained can be subjected to one or more screening steps until a 0/Vi sand is obtained. These screening steps are carried out in screening devices that are perfectly within the reach of those skilled in the art.

[0044] More generally, obtaining 0/Vi sand from recycled concrete aggregates from concrete blocks and/or rubble from demolitions and/or dismantling of homes or structures, from manufacturing waste or excess construction site waste, is perfectly within the reach of those skilled in the art.

[0045] Separation step b) can for example be carried out in a defiltering loop which consists of a flash drying system associated with a dynamic separator. The adjustment of the dynamic separator makes it possible to define the particle size cut-off at a determined value V2 which is between 0.1 mm and 0.2 mm.

[0046] Of course, step b) of separation of the accelerated carbonation process is perfectly within the reach of those skilled in the art and can also be carried out in a device suitable for separation other than the defiltering loop described above solely as an example of an embodiment of step b).

[0047] The residence time in the dynamic carbonator of the ^2nd fraction can be between 15 minutes and 12 hours. In an advantageous embodiment of the invention, this residence time is one hour.

[0048] In the embodiment of the invention in which the dynamic carbonator comprises a ^1st and a ^2nd open end which are separated by a rotating section extending in a substantially horizontal longitudinal direction as described above, the rotation speed of said rotating section is advantageously between 0.5 revolutions/minute and 10 revolutions/minute.

[0049] The temperature of the gas stream containing carbon dioxide is preferably between 15°C and 90°C.

[0050] The volume percentage of carbon dioxide in said gas stream is between 3% and 100%. In one embodiment of the invention, the gas stream contains only carbon dioxide.

[0051] The ^2nd fraction of recycled concrete aggregates can advantageously be moistened before step c) of the carbonation method according to the invention, preferably with a moisture content not exceeding 12%. In other words, if the ^2nd fraction is moistened, its moisture content is advantageously less than or equal to 12%. In the embodiment of the invention in which the dynamic carbonator comprises a ^1st and a ^2nd open end which are separated by a rotating section extending in a substantially horizontal longitudinal direction, the dynamic carbonator can comprise at the ^1st open end a water injection pipe configured to moisten the ^2nd fraction before implementing step c).

[0052] Furthermore, the relative humidity within the dynamic carbonator is advantageously between 50% and 100%.

[0053] The humidity level of the ^2nd fraction, as well as the relative humidity within the dynamic carbonator as described above are suitable for the optimization of step c) of accelerated carbonation, namely an improvement in the reaction kinetics and the percentage of carbon dioxide capture.

[0054] The gas stream containing carbon dioxide may consist of industrial gaseous discharges, preferably gaseous discharges from a cement industry.

[0055] This is why the present invention also relates to a process for recovering recycled concrete aggregates and industrial gaseous discharges which is characterized in that it implements the accelerated carbonation process according to the invention as described above and in that the gas flow containing carbon dioxide consists of industrial gaseous discharges, preferably gaseous discharges from a cement industry.

[0056] Thus, thanks to the recovery process according to the invention:

- recycled concrete aggregates are revalued into carbonated recycled concrete aggregates which are, as explained above, perfectly suitable as substitutes for natural aggregates to be implemented in concrete formulations;

- industrial gaseous discharges are revalued because they are used during step c) of the accelerated carbonation process.

[0057] Preferably, the industrial gaseous discharges are gases from a cement kiln, more preferably from a kiln for the manufacture of clinker.

[0058] In one embodiment of the invention, the industrial gaseous discharges are treated so as to obtain treated gases whose carbon dioxide content has been increased compared to that of the initial industrial gaseous discharges. The treated gases thus obtained can be stored, for example in a cement plant, before being used in the accelerated carbonation process according to the invention.

[0059] In one embodiment of the recovery method according to the invention, the ^1st fraction, in other words the 0/V2 sand, with a particle size less than V2 (i.e. the fines), V2 being between 0.1 mm and 0.2 mm, is used in the manufacture of a clinker. These fines are particularly suitable, because it is a decarbonated material which avoids the decarbonation of natural limestone.

[0060] In other words, the process for recovering recycled concrete aggregates and industrial gaseous discharges makes it possible to recover not only the ^2nd fraction with a particle size between V2 and VI to obtain carbonated recycled concrete aggregates following accelerated carbonation which are perfectly suitable as substitutes for natural aggregates to be used in concrete formulations, but also the ^1st fraction with a particle size lower than V2 which is a very suitable decarbonized material in the formulation of clinker.

[0061] The invention will be better understood with the aid of the detailed description which is set out below with reference to the appended drawing representing, by way of non-limiting example, a diagram of an installation implementing the method for recovering recycled concrete aggregates and industrial gaseous discharges according to the invention, as well as experimental results implementing accelerated carbonation according to the invention.

[0062] [Fig. 1] Figure 1 is a schematic representation of an installation implementing the process for recovering recycled concrete aggregates and industrial gaseous discharges according to the invention.

[0063] [Fig. 2] Figure 2 is a histogram of the percentage of carbon dioxide capture obtained from 5 experimental samples comprising sands of different grain sizes.

[0064] In Figure 1 is schematically represented an installation 1 which implements the method of recovery of recycled concrete aggregates and industrial gaseous discharges according to the invention. A truck 2 delivers to the installation 1 a sand of recycled concrete aggregates of particle size less than or equal to 2 mm (in other words a 0/2 sand). This 0/2 sand is conveyed to a silo 3 by a ^1st conveying system 4.

[0065] Plant 1 has an annual recovery capacity of 15,000 tonnes of 0/2 sand. To do this, it is continuously supplied with 0/2 sand, at a rate of approximately 2 tonnes/hour.

[0066] The silo 3 comprises at its base an extraction system 5 which is configured to:

- feed 0/2 sand, via a worm screw 27, into a defiltering loop 6,

- as well as adjusting the flow rate of said defiltering loop 6 in 0/2 sand.

[0067] The defiltering loop 6 is broken down into a dynamic separator 7 and a flash drying system 8. The defiltering loop 6 is configured to:

- separate the 0/2 sand into a ^first fraction called 0/0.15 sand with a particle size of less than 0.15 mm and a ^second fraction, called 0.15/2 sand with a particle size of between 0.15 mm and 2 mm, and

- dry these two fractions.

[0068] More precisely, the adjustment of the dynamic separator 7 makes it possible to define a granulometric cut-off of a determined value V2 which is between 0.1 mm and 0.2 mm. In the present case, the value V2 was set at 0.15 mm.

[0069] At the end of the separation stage, the ^1st fraction represents, in mass percentages, approximately 20% (i.e. 0.4 tonnes/hour) and the ^2nd fraction 80% (i.e. 1.6 tonnes/hour).

[0070] The ^1st fraction thus obtained and which is therefore dry is then conveyed by a ^2nd pneumatic conveying system 9 to a kiln 10 of a cement plant for the manufacture of clinker. In this way, the fines of the 0/2 sand fraction are revalued in the kiln 10 of a cement plant as a decarbonized material very suitable in the formulation of clinkers.

[0071] The ^2nd fraction (which is therefore also dry) is humidified such that its humidity level is 4% before its introduction at the ^1st open end 12 of a dynamic carbonator 11 which also comprises a ^2nd open end 13. Said ^1st and ^2nd open ends 12, 13 are separated by a rotating section 31 which has a generally cylindrical shape with a length of 6.5 m and a diameter of 1.3 m. The humidification of the ^2nd fraction can be carried out with an injection pipe not shown in FIG. 1. By rotating the rotating section 31 of the dynamic carbonator 11 at a speed of 1.5 revolutions/minute, the ^2nd fraction advances from the ^1st end 12 to the ^2nd end ¹³ of said dynamic carbonator 11. This thus allows the mixing of the bed of material consisting of the ^2nd fraction and its advancement within the rotating section 31 of the dynamic carbonator 11.

[0072] The rotating section 31 further has a downward inclination of 2 which is oriented in the direction of advancement of the ^2nd fraction within said rotating section 31.

[0073] The residence time of the ^2nd fraction in the dynamic carbonator 11 is approximately one hour.

[0074] In addition, a gas stream containing a mixture which comprises, in volume percentages

23% carbon dioxide, 5% dioxygen, 65% dionitrogen and 7% water vapor, is injected at the ^2nd end 13 of the dynamic carbonator 11. Its origin is explained in more detail below. This gas flow is at a temperature of 55°C and has a flow rate of 2000 m ³ /h. The relative humidity within the dynamic carbonator 11 is 75%.

[0075] The ^2nd fraction is thus swept by the gas flow which circulates counter-current to the advancement of the ^2nd fraction within the rotating section 31 of the dynamic carbonator 11. In order to increase the exchange surface between the ^2nd fraction and the gas flow, the rotating section 31 is equipped at its internal surface with a device for lifting and dispersing the ^2nd fraction (not shown in FIG. 1).

[0076] A gas flow at a flow rate of 6000 m ³ /hour is taken from the outlet 14 of a clinker manufacturing kiln 15 of a cement plant. The composition of this gas flow, in volume percentages, is as follows: 23% carbon dioxide, 5% dioxygen, 65% dionitrogen and 7% water vapor. The temperature of this gas flow is 350°C. This gas flow is conveyed with a ^3rd conveying system 29 to a cooling device which consists of an atomization system 16 comprising nozzles for atomizing air and water so as to be cooled to a temperature of 150°C. Then, this gas flow is conveyed with a ^4th conveying system 30 to a bag filter 17 to be dusted. This gas flow is then conveyed with a ^5th conveying system 18 to an intersection point 19 from which:

- a ^1st part of this gas flow having a flow rate of 4000 ^m3 /hour is conveyed with a ^6th conveying system 20 to the defiltering loop 6 to be injected there and

- a ^2nd part of this gas flow having a flow rate of 2000 ^m3 /h is conveyed with a ^7th conveying system 21 into a cooling device 22 consisting of an air-air heat exchanger to cool it to a temperature of 55°C. The gas flow thus cooled is then conveyed with an ^8th conveying system 23 to the ^2nd end 13 of the dynamic carbonator 11. This is the gas flow which is introduced into the dynamic carbonator 11 for the implementation of accelerated carbonation and which has been described above.

[0077] Then, the gases remaining at the end of the accelerated carbonation and the gases used in the defiltering loop 6 are collected at the outlet 24 of the dynamic separator 7 to be dedusted, then conveyed by means of a ^9th conveying system 32 to a cement kiln 33 to be reintroduced into the gases of said kiln 33.

[0078] In this installation 1, the flow rate of gas flow supplying the dynamic carbonator 11 is in excess compared to the maximum potential for capturing carbon dioxide by the recycled concrete aggregates 14. Thus, the installation 1 contributes to the recovery of part of the gaseous discharges from the cement kiln 15. Indeed, 470 kg of carbon dioxide are emitted per tonne of cement produced. If the cement plant produces one million tonnes of cement per year, the accelerated carbonation installation 1 as described, which is capable of carbonate 15,000 tonnes/year of 0/2 sand, contributes to a reduction of around 0.08% of the carbon dioxide emissions of this cement plant.

[0079] The major interest of installation 1 lies in the recovery of recycled concrete aggregates into carbonated recycled concrete aggregates which are perfectly suitable as substitutes for natural aggregates to be used in concrete formulations.

[0080] The recycled concrete aggregates obtained at the end of the accelerated carbonation are discharged at the ^2nd end 13 of the dynamic carbonator 11 to be conveyed, via an endless screw 27 to a bucket elevator 29, then introduced into a storage silo 26 via a connecting sheath 25.

[0081] The recycled concrete aggregates are conveyed via a worm screw 27 to a truck 28 to be transported outside the installation 1.

[0082] EXPERIMENTAL PART:

[0083] Experiments were carried out to demonstrate the impact on the percentage of carbon dioxide capture of samples made up of recycled concrete aggregates from which the fines were extracted following accelerated carbonation, as well as the properties of concretes obtained with these carbonated samples.

[0084] the ^first series of experiments

[0085] In a ^first series of experiments, a 0/2 sand obtained after crushing recycled concrete aggregates from the demolition of a building was subjected to separation steps consisting of 4 air jet sievings according to standard NF EN 993-10 with the following particle size cuts: 0.1 mm, 0.125 mm, 0.15 mm and 0.02 mm so as to prepare the following 5 samples:

- ^1st sample containing the initial 0/2 sand fraction;

- ^2nd sample containing a 0.1/2 sand fraction;

- ^3rd sample containing a sand fraction 0.125/2;

- ^4th sample containing a sand fraction 0.15/2;

- ^5th sample containing a sand fraction 0.2/2.

[0086] The mass of each of the samples was 500 g.

[0087] The 5 samples were subjected to accelerated carbonation for a period of one hour in a dynamic carbonator consisting of a sealed laboratory mortar mixer equipped with a gas circulation system and a heating system.

The conditions were as follows:

- the gas stream was a mixture of 25 vol. % carbon dioxide, 70 vol. % nitrogen, 4.7 vol. % oxygen, 0.3 vol. % nitrogen dioxide and 500 ppm sulfur dioxide at a temperature of 55°C;

- a humidity rate of the samples of 4.5%;

- a relative humidity within the mixer of 95%;

- a mixing speed of 10 revolutions/minute.

[0088] At the end of this accelerated carbonation, 5 carbonated samples were thus obtained. In other words, the capture of carbon dioxide by the 5 samples during this accelerated carbonation made it possible to obtain 5 carbonated samples.

[0089] The percentage of carbon dioxide capture of each of the 5 samples was determined with a carbonate bomb by carrying out an attack with hydrochloric acid on each of samples 1 to 5 before and after the accelerated carbonation and by measuring the pressure induced by the release of carbon dioxide resulting from this acid attack. By "percentage of carbon dioxide capture of a sample" is meant the ratio of the mass of carbon dioxide captured by said sample to the mass of said sample.

[0090] More specifically, the following protocol was implemented:

- 0.8 g of the sample to be analyzed was transferred into the reaction vessel of the carbonate bomb;

- 5 mL of a 5% volume calcium acetate solution was added to the container;

- 5 mL of a 37% by volume hydrochloric acid solution was gently poured into the reaction vessel;

- the carbonate bomb was stirred slowly for a period of between 1 and 10 minutes.

[0091] The carbon dioxide released during the acid attack of the carbonates increased the pressure within the carbonate bomb.

[0092] Table 1 below details the percentage of carbon dioxide capture for each of the 5 samples.

[0093] [Table 1]

N* Sample 1 2 3 4 5 % carbon dioxide capture 1.4 2 2.5 3.1 1.6

[0094] Figure 2 is a histogram of the carbon dioxide capture percentages of samples Nos. 1 to 5.

[0095] In view of Table 1 and Figure 2, we note that with identical accelerated carbonation conditions:

- Samples #2 to #5 have better carbon dioxide capture than sample #1. This demonstrates the impact on the efficiency of accelerated carbonation when fines with a particle size lower than a given V2 value between 0.1 mm and 0.2 mm were extracted from the 0/2 sand fraction.

- The best capture of carbon dioxide (3.1%) is obtained with sample no. 4, namely with 0.15/2 sand or in other words a fraction which was obtained from 0/2 sand by removing the fines with a particle size of less than 0.15 mm. This percentage of carbon dioxide capture is much higher than that of 0/2 sand which is 1.4%.

^2nd series of experiments

[0096] In a ^2nd series of experiments, the 0/2 sand was subjected to accelerated carbonation under the same conditions as for the ^1st series of experiments with the sole exception that the laboratory mortar mixer remained static. The accelerated carbonation was therefore carried out statically without mixing the 0/2 sand.

[0097] After one hour, the percentage of carbon dioxide capture was 0.8% and after two hours it was 1%.

[0098] Thus, this ^2nd series of experiments demonstrates the beneficial impact on the percentage of carbon dioxide capture when the carbonator is dynamic or in other words set in motion. This in fact promotes exchanges between the recycled concrete aggregates and the gas flow.

[0099] ^3rd series of experiments

[0100] In a ^3rd series of experiments, samples No. 1 and 4 containing respectively 0/2 sand and 0.15/2 sand were subjected to accelerated carbonation for a period of 28 days in the same laboratory mortar mixer as that of the ^1st and ^2nd series of experiments and which remained static. The conditions were as follows:

- the gas flow was a mixture of 3% vol. carbon dioxide and 97% vol. air at a temperature of 20°C;

- an initial humidity rate of the samples of 5%;

- a relative humidity within the carbonation enclosure of 65%.

[0101] The carbon dioxide capture percentage for sample #1 was 2.8% and that of sample #4 was 3.6%.

[0102] This ^3rd series of experiments also demonstrates the positive effect on the efficiency of accelerated carbonation when the fines were extracted from the 0/2 sand.

[0103] ^4th series of experiments

[0104] In a ^4th series of experiments, sample No. 6 and sample No. 7 were prepared. Sample No. 6 contained 0/4 sand obtained after crushing recycled concrete aggregates from the demolition of a building. This 0/4 sand was subjected to a separation step consisting of air jet sieving according to standard NF EN 993-10 with a particle size cut-off of 0.15 mm so as to obtain sample No. 7 with a mass of 500 g which contained 0.15/4 sand.

[0105] Samples Nos. 6 and 7 were subjected to accelerated carbonation for a period of one hour in the same laboratory mortar mixer as that of the previous series of experiments, and this under the same accelerated carbonation conditions as those of the ^first series of experiments.

[0106] The percentage of carbon dioxide capture for:

- sample no. 6 (sand 0/4) was 2.3%;

- sample no. 7 (sand 0.15/4) was 3.6%.

[0107] This ^4th series of experiments further demonstrates the beneficial effect on the percentage of carbon dioxide capture, when the fines (in other words the recycled concrete aggregates with a grain size of less than 0.15 mm) were extracted from the 0/4 sand.

[0108] ^5th series of experiments

[0109] During this ^5th series of experiments, the properties of a concrete prepared with carbonated sample no. 2 of the ^1st series of experiments (in other words a 0.1/2 sand which has been carbonated) were compared with those of a concrete prepared with a conventionally used natural sand.

[0110] More precisely, for this ^5th series of experiments, we used:

- carbonated sample no. 2 from the ^1st experimental series;

- a carbonated sample no. 2’.

[0111] Carbonated sample No. 2' was obtained from a ^2nd 0/2 sand different, in terms of its physicochemical properties, from that used for the ^1st series of experiments, because it was obtained after crushing recycled concrete aggregates from the demolition of a building of different origin.

[0112] This ^2nd 0/2 sand was also subjected to a separation step which consisted of sieving by air jet according to standard NF EN 993-10 with the 0.1 mm particle size cut-off so as to obtain a sample no. 2' containing a 0.1/2 sand fraction.

[0113] Sample No. 2' was subjected to accelerated carbonation under the same conditions as those of sample No. 2 and which are described in the ^first series of experiments so as to obtain carbonated sample No. 2'.

[0114] Preparation of concretes no. 1 to 3:

[0115] Three concretes (concrete no. 1, concrete no. 2 and concrete no. 3) were prepared using a conventional concrete manufacturing process.

[0116] More specifically, concrete No. 1 was prepared with, in particular, natural sands and aggregates. [0117] Concrete No. 2 was prepared with the same composition of natural sands and aggregates as concrete No. 1, except that 30% of the mass of natural sands was replaced by carbonated sand from sample No. 2.

[0118] Concrete No. 2 was prepared with the same composition of natural sands and aggregates as concrete No. 1, except that 30% of the mass of natural sands was replaced by carbonated sand from sample No. 2’.

[0119] Properties of concretes no. 1 to 3:

[0120] Considering the quantity of natural sands substituted in concretes No. 2 and No. 3 by carbonated sand, namely sand originating respectively from carbonated samples No. 2 and No. 2’, as well as the rate of carbon dioxide which was captured during the accelerated carbonation during the preparation of these carbonated sands of carbonated samples No. 2 and No. 2’, concretes No. 2 and No. 3 have a carbon footprint reduced by 10.8% and 8.5% respectively compared to that of concrete No. 1.

[0121] Concretes no. 2 and no. 3 do not show the same percentage reduction in carbon footprint.

This is explained by the aforementioned different origins of the 0/2 sands that were used to obtain the carbonated samples No. 2 and No. 2’. The percentage difference of 2.3% is not surprising given the different origins of the two 0/2 sands and therefore their differences in physicochemical properties. It can be seen that sample No. 2 captured more carbon dioxide than sample No. 2’ during accelerated carbonation.

[0122] The compressive strength of concretes No. 1 to 3 was measured according to standard NF EN 12390-3 on test specimens measuring 11 x 2 2 cm after 7 and 28 days of wet curing at 20°C.

[0123] Table 2 below details the compressive strength (expressed in MPa) at 7 days and 28 days for concretes No. 1 to 3.

[0124] [Table 2]

Compressive strength at 7 days (MPa) Compressive strength at 28 days (MPa) I Concrete n*l 29 36,S Concrete n*2 29.1 36.4 I Concrete n'3 283 37.3

[0125] From Table 2, it can be seen that the compressive strength of concretes No. 2 and No. 3 is equivalent to that of concrete No. 1. After 7 days, the compressive strength of concretes No. 2 and No. 3 is very close to that of concrete No. 1 and after 28 days, the compressive strength of concrete No. 3 is slightly better than that of concrete No. 1.

[0126] Thus, these laboratory experiments show that concretes obtained from sands of which part of the natural sands have been substituted by carbonated sands obtained following accelerated carbonation carried out on sands with a grain size of 0.1/2 (in other words sands from which the fines with a grain size of less than 0.1 have been extracted) have mechanical properties equivalent to those of concretes obtained from natural sands.

[0127] These experiments demonstrate that the process of accelerated carbonation of recycled concrete aggregates is an effective solution for obtaining recycled concrete aggregates which are perfectly suitable as substitutes for natural aggregates to be implemented in concrete formulations.

Claims (12)

1. Accelerated carbonation process, characterized in that it comprises at least the following steps: a) recycled concrete aggregates are available with a grain size less than or equal to a specific value Vi which is between 1 mm and 6 mm, in other words a 0/Vi sand; b) a separation step is carried out on the 0/Vi sand by defining a grain size cut-off of a specific value V2 which is between 0.1 mm and 0.2 mm so as to obtain: - a ^1st fraction whose granulometry is less than V2, in other words a 0/V2 sand, and - a ^2nd fraction with a grain size between V2 and VI, in other words a V2/V1 sand c) the ^2nd fraction is subjected to an accelerated carbonation step in a dynamic carbonator (11) by bringing said ^2nd fraction into contact with a gas flow containing carbon dioxide so as to obtain carbonated recycled concrete aggregates. 2. Accelerated carbonation process according to claim 1, characterized in that the recycled concrete aggregates from the 0/V1 sand of step a) come from the demolition of structures or buildings containing concrete elements, from concrete production waste or from excess construction site concrete. 3. Accelerated carbonation process according to claim 1 or 2, characterized in that the residence time in the dynamic carbonator (11) of the ^2nd fraction is between 15 minutes and 12 hours. 4. Accelerated carbonation process according to any one of claims 1 to 3, characterized in that the temperature of the gas flow containing carbon dioxide is between 15°C and 90°C. 5. Accelerated carbonation process according to any one of claims 1 to 4, characterized in that the volume percentage of carbon dioxide in said gas flow is between 3% and 100%. 6. Accelerated carbonation process according to any one of claims 1 to 5, characterized in that the ^2nd fraction is humidified before step c) with a humidity level less than or equal to 12%. 7. Accelerated carbonation process according to any one of claims 1 to 6, characterized in that the relative humidity within the dynamic carbonator (11) is between 50% and 100%. 8. Accelerated carbonation method according to any one of claims 1 to 7, characterized in that the dynamic carbonator (11) comprises: - a ^1st open end (12) through which the ^2nd fraction is introduced, - a ^2nd open end (13) through which the gas flow containing carbon dioxide is introduced, said ^1st and ^2nd open ends (12, 13) are separated by a rotating section (31) extending in a substantially horizontal longitudinal direction and within which the ^2nd fraction is advanced from the ^1st end (12) to the ^2nd end (13) and the gas flow circulates counter-current to the advancement of the ^2nd fraction. 9. Accelerated carbonation method according to claim 8, characterized in that the rotating section (31) has a downward inclination oriented in the direction of advancement of the ^2nd fraction which is between 0.5° and 8°, more preferably between 1° and 5. 10. Accelerated carbonation method according to claim 8 or 9, characterized in that the rotation speed of the rotating section (31) of the dynamic carbonator (11) is between 0.5 revolutions/minute and 10 revolutions/minute. 11. Process for the recovery of recycled concrete aggregates and industrial gaseous discharges, characterized in that it implements the accelerated carbonation process according to any one of claims 1 to 10 and in that the gaseous flow containing carbon dioxide consists of industrial gaseous discharges, preferably gaseous discharges from a cement industry. 12. Recovery method according to claim 11, characterized in that the industrial gaseous discharges are gases from a cement kiln, preferably from a kiln for the manufacture of clinker.