Method for reducing iron iii oxides contained in calcined clay

Abstract

The invention relates to a method for reducing iron III oxides contained in calcined clay, the method comprising, in particular, a step of introducing a calcined clay containing iron oxides III into a reduction reactor (100), a step of reducing the calcined clay in the reduction reactor (100), said step comprising an operation O1) of bringing the calcined clay into contact with a reducing gas containing hydrogen atoms, a step of separating reduced clay particles suspended in the reducing gas stream, a step of cooling, via a first cooling means, at least one portion of the reducing gas stream separated from the cooled reduced clay particles, and a step of cooling at least one portion of the reduced clay particles, said step comprising an operation O3) of bringing the reduced clay particles into contact with the cooled reducing gas.

Description

PROCESS FOR REDUCING IRON III OXIDES CONTAINED IN CALCINED CLAY

[0001] The invention relates to a method for reducing iron III oxides contained in a calcined clay. In particular, the invention relates to a method for reducing iron III oxides contained in a calcined clay for the production of a binder having desired color characteristics. Technical background

[0002] In the following, we are particularly interested in the reduction of iron III oxides contained in a calcined clay intended for use in the manufacture of cement, although the calcined clay can be used for other manufacturing purposes.

[0003] Cement manufacturing uses for the most part a fired material, clinker, which is produced from minerals whose essential constituent is calcium carbonate.

[0004] Clinker is obtained from a raw material composed of a mixture of minerals, including clay, a source of aluminosilicates, and limestone, a source of calcium carbonate. These minerals are successively mixed, dried, ground, preheated, decarbonated, then fired and partially melted in a rotary kiln to a temperature of approximately 1500°C, then the clinker thus formed is cooled.

[0005] Cement is obtained by finely grinding a mixture composed mainly of clinker.

[0006] In the clinker manufacturing process, in addition to the CO2 emissions from the combustion of the fuels used, calcium carbonate, which comes mainly from limestone, is decarbonated to obtain lime capable of recombining with silicon, aluminum, and iron oxides in the rotary kiln to form clinker. This decarbonation step releases a significant amount of carbon dioxide into the atmosphere.

[0007] National legislation on carbon dioxide emissions is becoming stricter and requires stakeholders to reduce the quantities released.

[0008] In addition to clinker, cement contains gypsum, which helps regulate the setting time of mortars and concretes. Cement also increasingly contains, and in ever-increasing proportions, materials commonly called "cement additives," which replace clinker in order to reduce the environmental impact and cost of cement manufacturing.

[0009] For example, the most commonly used clinker substitutes currently include limestone, blast furnace slag, fly ash from coal-fired power stations and natural pozzolan.

[0010] Apart from limestone, which simply acts as a "filler", an anglicism commonly used to designate fillers, these clinker substitute materials have a pozzolanic reactivity allowing them to participate in the hydraulic setting reaction. This pozzolanic reactivity contributes to maintaining the desired mechanical properties of mortars and concretes when the clinker content decreases.

[0011] Clays, especially those containing kaolinite, acquire pozzolanic reactivity when calcined and then become excellent substitutes for clinker in cement manufacturing. They are also called "artificial pozzolans."

[0012] Unlike clinker production, calcined clay production emits little CO2.

[0013] By judiciously decreasing the proportion of clinker while increasing the proportion of calcined clay, it becomes possible to produce a cement with desired properties.

[0014] Due to the iron III oxides, Fe2O3, it contains, clay has a reddish tint. If left untreated, adding calcined clay to cement will result in a cement with a pinkish tint.

[0015] Cement producers and end users want a gray colored cement.

[0016] Several processes have been implemented to modify the natural color of the calcined clay and make it gray.

[0017] Among these techniques, we can cite a process that consists of chemically reacting iron III oxide molecules Fe2O3 to obtain triiron tetroxides Fe3O4. This is an oxidation-reduction reaction. After an initial calcination step of the clay, the clay is sent to a reduction zone where a liquid reducing agent is injected directly onto the clay, in particular diesel. The diesel creates the conditions that allow a reduction of iron III oxides to obtain triiron tetroxides and iron oxides FeO.

[0018] Although this process produces a grey clay, it has two major drawbacks:

[0019] - a significant quantity of diesel for the reduction of iron III oxides is injected, impacting the cost of clay on the one hand and the environment on the other,

[0020] - the reduction reaction must be carried out at high temperature, impacting the energy efficiency of this process.

[0021] The invention aims in particular to remedy these drawbacks.

[0022] To this end, there is proposed firstly a method for reducing iron III oxides contained in a calcined clay, the method comprising the following steps: introduction of a calcined clay containing iron III oxides into a reduction reactor, reduction of the calcined clay in the reduction reactor comprising an operation O1) of bringing the calcined clay into contact with a reduction gas containing hydrogen atoms to obtain reduced clay particles containing iron II oxides and / or triiron tetroxides suspended in a reduction gas flow, separation of the reduced clay particles suspended in the reduction gas flow to obtain: a reduction gas flow separated from the reduced clay particles, and reduced clay particles, cooling, via a first cooling means, at least part of the reduction gas flow separated from the reduced clay particles to obtain a cooled reduction gas,andcooling at least part of the reduced clay particles comprising an operation O3) of bringing the reduced clay particles into contact with the cooled reduction gas to obtain a gaseous suspension of cooled reduced clay particles.,

[0023] This process advantageously makes it possible to obtain a gray-colored clay using a small amount of reducing agent, significantly lower than the amount used with liquid reducing agents. Furthermore, this process makes it possible to improve the reduction of iron III oxides because the contact of the calcined clay with the reducing gas is maintained for a prolonged period during each of the process stages.

[0024] Various additional features may be provided alone or in combination:the reducing gas comprises dihydrogen and / or carbon monoxide,the reducing gas contains:- dihydrogen and carbon monoxide, or

[0025] - dihydrogenthe reduction gas contains an amount of between 0.1 and 2 moles of reactants per mole of iron III oxide contained in the calcined clay introduced, preferably between 0.3 and 0.7 moles of reactants per mole of iron III oxide contained in the calcined clay introduced,the method further comprises a step a0) of calcining a raw clay at the end of which the calcined clay is obtained,the calcination step a0) is carried out at a calcination temperature of less than 950°C,the calcined clay introduced in step a) is at a temperature of between 600°C and 950°C, preferably between 750°C and 850°C,during the cooling step d), the temperature of at least part of the reduction gas flow is lowered below a threshold temperature of between 200°C and 450°C, during cooling step e),the temperature of at least part of the reduced clay particles is lowered below a threshold temperature of between 400°C and 600°C,the method further comprises, between step c) of separation and step e) of cooling, a step b') of additional reduction, in a complementary reduction reactor, of the reduced clay particles resulting from step c) of separation, step b') of additional reduction further comprising an operation O2) of bringing the reduced clay particles into contact with a complementary reduction gas containing hydrogen atoms to complete step b) of reduction of the calcined clay,the method further comprises a step f) of separation of the gaseous suspension of cooled particles of reduced clay to obtain:a gaseous phase separated from the cooled particles of reduced clay, the gaseous phase containing hydrogen atoms, andcooled particles of reduced clay,the method further comprises a step g) of introducing, into the reduction reactor, at least part of the gas phase separated from the cooled particles of reduced clay so that the reduction gas brought into contact with the calcined clay during operation O1) is at least partly made up of the gas phase separated from the cooled particles of reduced clay, the implementation of such an introduction step g) inducing the creation of a reduction loop,the method further comprises, after step d) of cooling at least part of the reduction gas flow separated from the particles of reduced clay, a step h1) of extracting from the reduction loop part of the cooled reduction gas to maintain a depression in the reduction loop,the method further comprises, after step h1) of extraction, a step h2) of sending the extracted part of the cooled reduction gas into a calciner,the method further comprises,a step h3) of heat input into the reduction loop to compensate for the heat losses resulting from the implementation of the reduction loop, the step h3) of heat input being carried out either in the reduction reactor or between step f) of separation of the gaseous suspension of cooled particles of reduced clay and step g) of introduction of at least part of the gaseous phase separated from the cooled particles of reduced clay, the step h3) of heat input into the reduction loop consists of introducing a predefined volume of hot gas, the step h3) of heat input into the reduction loop consists of heating indirectly during a step i) of heating via a heating means,the method further comprises a step j) of exchanging heat energy between the first cooling means and the heating means by circulation of a heat transfer fluid. Secondly, a use of the reduction method as described above is proposed for producing calcined clay having desired color characteristics.,

[0026] Thirdly, an installation for reducing iron III oxides contained in calcined clay is proposed, the system being capable of implementing the process as described above and comprising:

[0027] - a reduction reactor configured to contact a calcined clay with a reduction gas containing hydrogen atoms, the reduction reactor comprising an outlet of reduced clay particles containing iron II oxides and / or triiron tetroxides suspended in a reduction gas flow,

[0028] - a first gas / particle separation device configured to separate reduced clay particles suspended in the reduction gas flow, the first gas / particle separation device comprising:a gas outlet of a reduction gas flow separated from the reduced clay particles, anda particle outlet of reduced clay particles,- a first cooling means configured to cool at least part of the reduction gas flow separated from the reduced clay particles, the first cooling means comprising an outlet of a cooled reduction gas,

[0029] - a second cooling means configured to cool at least a portion of the reduced clay particles, the second cooling means comprising:a gas inlet for at least a portion of the cooled reduction gas, the gas inlet of the second cooling means being connected to the outlet of the first cooling means, andan outlet for a gaseous suspension of cooled reduced clay particles.

[0030] Various additional features may be provided alone or in combination:the installation further comprises a second gas / particle separation device configured to separate at least a portion of the cooled particles from the gaseous suspension of cooled particles of reduced clay, the second gas / particle separation device comprising:- a gas outlet of a gaseous phase separated from the cooled particles of reduced clay, and- a particle outlet of cooled particles of reduced clay,the installation further comprises a complementary reduction reactor arranged between the first gas / particle separation device and the second cooling means,the gas outlet of the second gas / particle separation device is connected to a gas inlet of the reduction reactor so that the reduction gas is at least partly made up of the gas phase separated from the cooled particles of reduced clay in order to produce a reduction loop, the reduction reactor comprises an inlet configured to introduce therein a predefined volume of hot gas to compensate for the thermal losses which result from the implementation of the reduction loop, the installation further comprises a heating means arranged either on one or more of the walls of the reduction reactor, or between the gas outlet of the second gas / particle separation device and the gas inlet of the reduction reactor, the heating means is fluidically connected to the first cooling means, the first cooling means being configured to transfer heat energy to the heating means,the heating means and the first cooling means are contained in a heat exchanger in which a heat transfer fluid is capable of circulating to transfer the heat energy from the first cooling means to the heating means, the installation further comprises a device for ventilating the cooled reduction gas arranged between the first cooling means and the second cooling means to ensure the circulation of the gas flows in the reduction loop, the first cooling means comprises a means for extracting a portion of the cooled reduction gas to maintain a depression in the reduction loop.,

[0031] Other features, details and advantages will become apparent upon reading the detailed description below, and upon analyzing the attached drawing, in which:

[0032] This is a schematic representation of an installation according to the invention.

[0033] The drawing and description below contain, for the most part, elements of a certain character. They may therefore not only serve to better understand this disclosure, but also contribute to its definition, if necessary.

[0034] More specifically, in the following, in a non-limiting manner for the invention, it is considered that the installation for reducing iron III oxides contained in a calcined clay is part of a general process for manufacturing a binder, for example cement. In particular, it is considered that the reduced calcined clay is intended to be used in the manufacture of a cement as a substitute for part of the clinker or as a reactive filler material incorporated directly into the concrete or as a component of a geopolymer cement.

[0035] A plant for reducing iron III oxides contained in calcined clay is shown. The calcined clay is produced by implementing a step a0) of calcination, in a calciner (not shown in the figure), of a raw clay containing iron III oxides.

[0036] For the purposes of the present invention, raw clay means any type of clay mineral, including schists.

[0037] Preferably, the raw clay contains kaolinite.

[0038] In one example, before implementing the calcination step a0), the raw clay is ground and dried, then preheated in a preheater, and then sent to the calciner. Generally, the calciner comprises a combustion chamber in which hot gases are generated and a calcination reactor in which the clay from the preheater is brought into contact with the hot gases for calcination. The calcination reaction is preferably carried out in an oxidizing atmosphere, with an excess of oxidant. The flue gas outlet from the calciner may be connected to the preheater and possibly to other devices conventionally used in general binder manufacturing processes, such as a raw clay grinding and drying workshop, one or more flue gas decontamination and filtration devices.

[0039] In order to activate the pozzolanic power of the calcined clay and make it usable in the manufacture of cement, step a0) of calcination of the raw clay is carried out at a temperature below 950°C. Step a0) of calcination is carried out under stoichiometric or oxidizing conditions.

[0040] Then, a step a) of introducing calcined clay into a reduction reactor 100, in particular through a material inlet 1008 of the reduction reactor 100, is carried out. It should be noted that the calcined clay introduced in step a) is at a temperature between 600°C and 950°C, or preferably at a temperature between 750°C and 850°C. The calcined clay is then directed from the combustion unit, in particular from the calciner, to the material inlet 1008 of the reduction reactor 100. The reduction reactor 100 is a suspension reduction reactor and the reduced calcined clay suspended in a gas stream leaves it through the outlet 1002.

[0041] The reduction reactor 100 is preferably a suspension reactor in which the calcined clay is maintained within it for a period of less than 10 seconds. The reduction reactor 100 comprises in particular a reduction gas inlet 1006 through which reduction gas is injected. Thus, during a following step, a step b) of reducing the calcined clay, in the reduction reactor 100, is carried out. This reduction step b) comprises in particular an operation O1) of bringing the calcined clay into contact with the reduction gas injected into the reduction reactor 100, the reduction gas containing hydrogen atoms. This makes it possible to reduce at least a portion of the iron III oxides of the calcined clay to Fe3O4 (triiron tetroxides) and / or FeO (iron II oxides).

[0042] The reducing gas containing hydrogen atoms allows the reduction of iron III oxides according to the following formulas to obtain mainly triiron tetroxides and possibly iron II oxides FeO:

[0043]

[0044]

[0045] and according to the following formulas, carbon monoxide, when present, reacts with iron III oxides:

[0046]

[0047]

[0048] Using reducing gas containing hydrogen atoms rather than diesel or any other liquid fuel as a reducing agent significantly reduces the amount of reducing agent used in reduction step b). Grey calcined clay is therefore produced at a significantly lower cost. In addition, the environmental impact of producing grey calcined clay is reduced.

[0049] Advantageously, the reducing gas comprises dihydrogen and / or carbon monoxide.

[0050] Even more advantageously, the reducing gas contains:

[0051] - a mixture containing dihydrogen H2 and carbon monoxide CO, or

[0052] - dihydrogen H2.

[0053] For example, dihydrogen H2 and carbon monoxide CO may come directly from a storage means or result from a dissociation reaction of a hydrocarbon in a reactor dedicated to the generation of the reduction gas. The dedicated reactor may be an endogas or exogas generator from which the reduction gas injected into the reduction reactor 100 is derived.

[0054] It should be noted that the above-mentioned reducing gases may be mixed with other gases, mainly nitrogen and / or carbon dioxide and / or gaseous hydrocarbons of generic formula CnHm which have not been dissociated into dihydrogen H2 and carbon monoxide CO.

[0055] Advantageously, the quantity of reducing gas injected into the reduction reactor 100 and with which the calcined clay is brought into contact corresponds to a quantity substantially between 0.1 and 2 moles of reactants per mole of iron III oxide contained in the calcined clay introduced. By respecting this dosage, a calcined clay with desired gray tones is obtained while minimizing the gas input, which makes it possible to reduce the production costs of the clay and its impact on the environment. Preferably, the quantity of reducing gas injected into the reduction reactor 100 and with which the calcined clay is brought into contact corresponds to a quantity substantially between 0.3 and 0.7 moles of reactants per mole of iron III oxide contained in the calcined clay introduced. Such a range allows an efficient reduction reaction while optimizing the energy consumption necessary for the production of the reducing gas.

[0056] After passing through the reduction reactor 100, reduced clay particles containing iron II oxides and / or triiron tetroxides suspended in a reduction gas flow are obtained at the outlet 1002 of the reduction reactor 100. The reduced clay particles suspended in the reduction gas flow are then sent, and conveyed via a fluid conduit, to a material inlet 1102 of a first gas / particle separation device 110 configured to, during a separation step c), separate the reduced clay particles from the reduction gas flow.

[0057] For the purposes of the present application, reduced clay particles are understood to mean calcined clay particles which have undergone the reduction reaction in the reduction reactor and have reached a desired color.

[0058] At the end of this separation step c), we then obtain: a reduction gas flow separated from the reduced clay particles which leaves a gas outlet 1104 of the first gas / particle separation device 110, and reduced clay particles which leave via a material outlet 1106 of the first gas / particle separation device 110.

[0059] In the exemplary embodiment illustrated in, the first gas / particle separation device 110 is of the cyclone type.

[0060] After this separation step c), the reduction gas flow separated from the reduced clay particles emerging from the first gas / particle separation device 110, for example via its upper outlet, can then be transported to a first cooling means 130 to which it is fluidically connected by a pipe.

[0061] Thus, a step d) of cooling, via the first cooling means 130, the reduction gas flow separated from the reduced clay particles is implemented.

[0062] The first cooling means 130 comprises, on the one hand, an inlet 1302 fluidly connected to the upper outlet, or gas outlet 1104, of the first gas / particle separation device 110 and, on the other hand, an outlet 1304 through which a cooled reduction gas exits. The first cooling means 130 is then configured to obtain a cooled reduction gas. For example, the first cooling means 130 may be a recovery boiler or an air cooler.

[0063] Advantageously, during step d) of cooling the reduction gas flow separated from the reduced clay particles, the temperature of the reduction gas flow is lowered below a threshold temperature of between 200°C and 450°C. This is referred to as indirect cooling since the reduction gas flow separated from the reduced clay particles is not in direct contact with the cooling fluid. It is thus possible to connect, to the outlet 1304 of the cooling means 130, a ventilation device 140 without the latter being damaged by excessively hot gas flows. The ventilation device 140 promotes the circulation of the gas flow in the aforementioned elements of the installation and also in those which will be described below.Furthermore, since the reduction gas has been cooled during the cooling step d), the opportunity is left to use a wide range of ventilation devices 140 which would then be suitable for such gas flow temperatures. Furthermore, the step d) of cooling the reduction gas flow separated from the reduced clay particles makes it possible to facilitate the implementation of the subsequent step e).

[0064] Advantageously, the reduced clay particles obtained in step c) which then exit through the material outlet 1106 of the first gas / particle separation device 110, are, for their part, sent to a complementary reduction reactor 120.

[0065] The complementary reduction reactor 120 is configured to maintain the reduced clay particles for residence times greater than 10 seconds and up to 10 minutes, preferably up to 5 minutes or even up to 2 minutes, under optimal reduction conditions. For example, the complementary reduction reactor 120 may be of the static reactor type, a fluidized bed reactor or even a rotating reactor. The complementary reduction reactor 120 makes it possible to treat calcined clays defined by high iron contents or to work at lower temperatures while reducing the consumption of reduction gas.

[0066] The inlet 1202 of the complementary reduction reactor 120 is then connected to the material outlet 1106 of the first gas / particle separation device 110. In this complementary reduction reactor 120, a step b') of complementary reduction is then implemented, further comprising an operation O2) of bringing the reduced clay particles into contact with a complementary reduction gas A which contains hydrogen atoms. This step b') of complementary reduction makes it possible to complete the step b) of reducing the calcined clay. The complementary reduction gas A is injected into the complementary reduction reactor 120 via a gas inlet 1206. Preferably, the complementary reduction gas A is identical to the reduction gas A injected into the reduction reactor 100. In another exemplary embodiment, the reduction gas A and the complementary reduction gas A are different but are both configured to reduce the calcined clay.

[0067] It should be noted that during the additional reduction step b'), a portion of the additional reduction gas A injected into the additional reduction reactor 120 can naturally flow towards its inlet 1202. According to an exemplary embodiment, the portion of the gas which flows towards its inlet 1202 is then introduced into the first gas / particle separation device 110 via its material outlet 1106 to mix with the suspension which is separated during the separation step c). Thus, additional reduction gas A from the additional reduction reactor 120 can then mix with the reduction gas introduced into the first gas / particle separation device 110 via its inlet 1102, this mixture then being sent to the first cooling means 130.According to another exemplary embodiment, the part of the gas which goes towards its inlet 1202 is sent upstream of the first gas / particle separation device 110, in particular into the reduced clay particles suspended in the reduction gas flow leaving the reduction reactor 100. According to yet another exemplary embodiment, the part of the gas which goes towards its inlet 1202 is sent into a complementary gas / particle separation device, of the cyclone type, the separated part of the particles which comes out is sent into the reduction gas flow separated from the reduced clay particles leaving the first gas / particle separation device 110, while the particles are sent into the reduced clay particles suspended in the reduction gas flow leaving the reduction reactor 100.

[0068] A step e) of cooling the reduced clay particles which exit via the material outlet 1204 of the complementary reduction reactor 120 is also implemented, via the use of a second cooling means 160 configured to cool the reduced clay particles.

[0069] The complementary reduction reactor 120 is then arranged between the first gas / particle separation device 110 and the second cooling means 160.

[0070] Preferably, as can be seen in the, the ventilation device 140 of the cooled reduction gas arranged between the first cooling means 130 and the second cooling means 160. In this way, the circulation of the gas flows in the reduction loop is easily maintained.

[0071] Advantageously, this step e) of cooling the reduced clay particles comprises an operation O3) of bringing the reduced clay particles into contact with the cooled reduction gas. This makes it possible to reduce, in an economical and ecological manner, the temperature of the reduced clay particles in order to avoid the reoxidation of the iron II oxides and / or the triiron tetroxides of the reduced clay particles which leave the reduction reactor 100 via its outlet 1002.

[0072] Advantageously, the operation O3) is carried out using direct contact of the cooled reduction gas with the reduced clay particles to the extent that the cooled reduction gas contains less than 2% oxygen by volume.

[0073] At the end of step e) of cooling the reduced clay particles, a gaseous suspension of cooled reduced clay particles is obtained.

[0074] To implement this step e) of cooling the reduced clay particles, the second cooling means 160 comprises a gas inlet 1602 through which the cooled reduction gas is introduced, this gas inlet 1602 being indirectly connected to the outlet 1304 of the first cooling means 130, via the ventilation device 140, and a material inlet 1606, connected to the material outlet 1204 of the complementary reduction reactor 120. The second cooling means 160 further comprises an outlet 1604 through which a gaseous suspension of cooled reduced clay particles exits. It should be noted that during this cooling step e), the cooled reduction gas heats up in contact with the reduced clay particles which are hotter than the cooled reduction gas.

[0075] Advantageously, during step e) of cooling the reduced clay particles, the temperature of the reduced clay particles is lowered below a threshold temperature of between 400°C and 600°C, this making it possible to guarantee non-reoxidation of the iron II oxides and / or triiron tetroxides in subsequent steps during which the reduced calcined clay is brought into contact with air, for example during final air cooling and handling and storage.

[0076] Advantageously, a step f) of separating the gaseous suspension of cooled particles of reduced clay is carried out after step e) of cooling the particles of reduced clay. This step f) of separating the gaseous suspension of cooled particles of reduced clay is implemented using a second gas / particle separation device 170 configured to separate cooled particles from the gaseous suspension of cooled particles of reduced clay. The gaseous suspension of cooled particles is then introduced into the second gas / particle separation device 170 via an inlet 1702 connected to the outlet 1604 of the second cooling means 160.The second gas / particle separation device 170 further comprises, on the one hand, a gas outlet 1704 for a gas phase containing hydrogen atoms separated from the cooled particles of reduced clay and, on the other hand, a particle outlet 1706 for cooled particles of reduced clay. This second gas / particle separation device 170 may, for example, be of the same type as the first gas / particle separation device 110. The cooled particles of reduced clay exiting through the particle outlet 1706 of the second gas / particle separation device 170 may be cooled during a final cooling step, for example in air, to obtain cooled particles of reduced clay at a temperature between 100°C and 150°C. At this temperature, the particles are easily stored and packaged. They may then be, for example, mixed with clinker for the production of cement.This use makes it possible to produce a grey cement, by reducing the proportion of clinker in the cement by replacing it with calcined clay.

[0077] Advantageously, a step g) of introducing, into the reduction reactor 100, the gas phase separated from the cooled particles of reduced clay is implemented. Thus, the reduction gas brought into contact with the calcined clay during operation O1) is at least partly made up of the gas phase separated from the cooled particles of reduced clay.

[0078] It should be noted that introducing into the reduction reactor 100, via the gas inlet 1004, the gas phase separated from the cooled particles of reduced clay induces the creation of a reduction loop.

[0079] In the presence of such a reduction loop, it is advantageous to guarantee thermal equilibrium and pressure equilibrium so that the reduction loop is sustainable.

[0080] Thus, it is advantageous, after step d) of cooling at least part of the reduction gas flow separated from the reduced clay particles, to implement a step h1) of extracting from the reduction loop part of the cooled reduction gas to maintain the pressure in the reduction loop at a value lower than ambient pressure, i.e. to maintain a vacuum in the reduction loop. The implementation of such an extraction step h1) also makes it possible to compensate for the volume of gas introduced into the reduction loop, in particular into the reduction reactor 100, during the injection of the reduction gas through the reduction gas inlet 1006 of the reduction reactor 100, during the implementation of step a) of introducing the calcined clay, and during the introduction into the reduction loop of false air which is introduced through possible sealing defects in the pipes or equipment.

[0081] The extraction flow rate from the reduction loop of a portion of the cooled reduction gas during step h1) is regulated via the use of a valve-type member 150, so as to maintain a constant depression in the loop.

[0082] Preferably, this valve-type member 150 is configured to extract cooled reduction gas at the outlet of the ventilation device 140, in the direction of circulation of the gas flows.

[0083] Then, a step h2) of sending the part of the cooled reduction gas extracted into the calciner used to obtain the calcined clay before its introduction into the reduction reactor 100, is possibly carried out. Thus, the extracted cooled reduction gases are incinerated and the energy released during the combustion of gases such as dihydrogen, carbon monoxide or even gaseous hydrocarbons contained in the reduction gas is recovered.

[0084] It should be noted that, since the calcination temperature is generally greater than or equal to the reduction temperature, there is a primary thermal input into the reduction reactor 100, therefore into the reduction loop, which is inherent in carrying out step a) of introducing the calcined clay into the reduction reactor 100.

[0085] However, this primary heat input may not be sufficient to compensate for the heat losses resulting from the use of the reduction loop and the implementation of the cooling step d) of the reduction gas stream separated from the reduced clay particles.

[0086] This is why it is advantageous to implement a step h3) of additional heat input in the reduction loop. The step h3) of additional heat input is carried out either in the reduction reactor 100, or between the step f) of separating the gaseous suspension of cooled particles of reduced clay and the step g) of introducing at least a portion of the gaseous phase separated from the cooled particles of reduced clay, i.e. between the second gas / particle separation device 170 and the reduction reactor 100.

[0087] In an exemplary embodiment, the step h3) of additional heat input consists of heating the gas phase separated from the cooled particles of reduced clay and is carried out via a heating means 180. The heating means 180 is preferably arranged at the level of the fluidic conduit which connects the gas outlet 1704 of the second gas / particle separation device 170 and the gas inlet 1004 of the reduction reactor 100. For example, as heating means, mention may be made, without being limited to this example, of an electric heating device which is capable of being arranged on the periphery of the fluidic conduit connecting the second gas / particle separation device 170 to the reduction reactor 100.

[0088] In an alternative embodiment not illustrated in the figure, the heating means is arranged on the periphery of the reduction reactor 100 to directly heat the interior volume of the reduction reactor 100.

[0089] In another alternative embodiment not illustrated in the figure, the reduction reactor 100 comprises an additional inlet configured to introduce a predefined volume of hot gas therein. In this case, the step h3) of additional heat supply consists of introducing a predefined volume of hot gas directly into the reduction reactor 100 via the additional inlet.

[0090] Advantageously, a step j) of exchanging heat energy between the first cooling means 130 and the heating means 180 by circulation of a heat transfer fluid is further carried out. Here, the heating means 180 is preferably fluidically connected to the first cooling means 130 then configured to transfer heat energy to the heating means 180. For example, this energy transfer can be induced by the movement of air which results from the implementation of step d) of cooling the reduction gas flow separated from the reduced clay particles, from the cooling means 130 in which step d) is implemented, to the heating means 180. In this way, the activation energy of the heating means 180 comes from an element of the reduction installation so as to avoid being supplied by external energy. This makes it possible to have an economical heating means.

[0091] Advantageously, the heating means 180 and the first cooling means 130 are contained in a heat exchanger 190 in which a heat transfer fluid is able to circulate to transfer the heat energy from the first cooling means 130 to the heating means 180. This makes it possible to optimize the heat exchanges between the first cooling means 130 and the heating means 180 in order to ensure, in an optimal manner, a bilateral transfer of heat between these two elements.

[0092] Preferably, the installation further comprises sensors configured to continuously or discontinuously measure the temperature of the gas flows circulating in the reduction loop, and the oxygen, hydrogen and carbon monoxide contents. The use of such sensors makes it possible in particular to monitor the oxygen content in the reduction loop and then to secure the reduction loop by minimizing the risk of explosion caused by the simultaneous presence of oxygen and reduction gas.

[0093] The installation and method described above advantageously make it possible to obtain a gray-colored clay using a small quantity of reducing gas because part of this reducing gas is recycled. Furthermore, this installation and this method make it possible to improve the reduction of iron III oxides because the contact of the calcined clay with the reducing gas is maintained for a prolonged period during certain stages of the process implemented in the installation described above.

Claims

A method for reducing iron III oxides contained in a calcined clay, the method comprising the following steps:introducing a calcined clay containing iron III oxides into a reduction reactor (100),reducing the calcined clay in the reduction reactor (100) comprising an operation O1) of bringing the calcined clay into contact with a reducing gas containing hydrogen atoms to obtain reduced clay particles containing iron II oxides and / or triiron tetroxides suspended in a reducing gas flow,separating the reduced clay particles suspended in the reducing gas flow to obtain:a reducing gas flow separated from the reduced clay particles, andreduced clay particles,cooling, via a first cooling means (130), at least part of the reducing gas flow separated from the reduced clay particles to obtain a cooled reducing gas,andcooling at least part of the reduced clay particles comprising an operation O3) of bringing the reduced clay particles into contact with the cooled reduction gas to obtain a gaseous suspension of cooled reduced clay particles., The method of claim 1, wherein the reducing gas comprises dihydrogen and / or carbon monoxide. Process according to one of claims 1 or 2, in which the reducing gas contains an amount of between 0.1 and 2, preferably between 0.3 and 0.7 moles of reactants per mole of iron III oxide contained in the calcined clay introduced. Method according to one of claims 1 to 3, in which during cooling step e), the temperature of at least one part of the reduced clay particles is lowered below a threshold temperature of between 400°C and 600°C. Method according to claim 4, further comprising, between step c) of separation and step e) of cooling, a step b') of additional reduction, in a complementary reduction reactor (120), of the reduced clay particles resulting from step c) of separation, step b') of additional reduction further comprising an operation O2) of bringing the reduced clay particles into contact with a complementary reduction gas containing hydrogen atoms to complete step b) of reduction of the calcined clay. Method according to one of claims 1 to 5, further comprising a step f) of separating the gaseous suspension of cooled particles of reduced clay to obtain: a gaseous phase separated from the cooled particles of reduced clay, the gaseous phase containing hydrogen atoms, and cooled particles of reduced clay. Method according to claim 6, further comprising a step g) of introducing, into the reduction reactor (100), at least part of the gas phase separated from the cooled particles of reduced clay so that the reduction gas brought into contact with the calcined clay during operation O1) is at least partly made up of the gas phase separated from the cooled particles of reduced clay, the implementation of such an introduction step g) inducing the creation of a reduction loop. Method according to claim 7, further comprising, after step d) of cooling at least part of the reduction gas flow separated from the reduced clay particles, a step h1) of extracting from the reduction loop part of the cooled reduction gas to maintain a depression in the reduction loop. Method according to claim 8, further comprising, after step h1) of extraction, a step h2) of sending the extracted part of the cooled reduction gas into a calciner. Method according to one of claims 8 or 9, further comprising a step h3) of heat input into the reduction loop to compensate for the heat losses which result from the implementation of the reduction loop, the step h3) of heat input being carried out either in the reduction reactor (100), or between step f) of separation of the gaseous suspension of cooled particles of reduced clay and step g) of introduction of at least part of the gaseous phase separated from the cooled particles of reduced clay. Method according to claim 10, in which step h3) of heat input into the reduction loop consists of the introduction of a predefined volume of hot gas. Method according to claim 10, in which step h3) of heat input into the reduction loop consists of heating indirectly during a step i) of heating via a heating means (180). Method according to claim 12, further comprising a step j) of exchanging heat energy between the first cooling means (130) and the heating means (180) by circulation of a heat transfer fluid. Use of a reduction process according to one of claims 1 to 13 for producing calcined clay having desired color characteristics. Installation for reducing iron III oxides contained in a calcined clay, the system being capable of implementing a method according to one of claims 1 to 13 and comprising:- a reduction reactor (100) configured to bring a calcined clay into contact with a reduction gas containing hydrogen atoms, the reduction reactor (100) comprising an outlet (1006) of reduced clay particles containing iron II oxides and / or triiron tetroxides suspended in a reduction gas flow,- a first gas / particle separation device (110) configured to separate reduced clay particles suspended in the reduction gas flow, the first gas / particle separation device (110) comprising:a gas outlet (1104) of a reduction gas flow separated from the reduced clay particles, anda particle outlet (1106) of reduced clay particles,- a first cooling means (130) configured to cool at least a portion of the reduction gas flow separated from the reduced clay particles, the first cooling means comprising an outlet (1302) of a cooled reduction gas,- a second cooling means (160) configured to cool at least a portion of the reduced clay particles, the second cooling means (160) comprising:a gas inlet (1602) of at least a portion of the cooled reduction gas, the gas inlet (1602) of the second cooling means (160) being connected to the outlet (1302) of the first cooling means (130), andan outlet (1604) of a gaseous suspension of cooled reduced clay particles., The installation of claim 15, further comprising a second gas / particle separation device (170) configured to separate at least a portion of the cooled particles from the gaseous suspension of cooled particles of reduced clay, the second gas / particle separation device (170) comprising:- a gas outlet (1704) of a gas phase separated from the cooled particles of reduced clay, and- a particle outlet (1706) of cooled particles of reduced clay. Installation according to one of claims 15 or 16, further comprising a complementary reduction reactor (120) arranged between the first gas / particle separation device (110) and the second cooling means (160). Installation according to one of claims 15 or 17, in which the gas outlet (1704) of the second gas / particle separation device (170) is connected to a gas inlet (1004) of the reduction reactor (100) so that the reduction gas is at least partly made up of the gaseous phase separated from the cooled particles of reduced clay in order to create a reduction loop. Installation according to claim 18, further comprising a ventilation device (140) for the cooled reduction gas arranged between the first cooling means (130) and the second cooling means (160) to ensure the circulation of the gas flows in the reduction loop.