Calcined clay production process
The process addresses the inefficiencies of existing calcined clay production by using a hydrogen-containing gas for reducing iron(III) oxides at lower temperatures, achieving cost-effective and environmentally friendly gray-colored calcined clay for cement production.
Patent Information
- Application Number
- FR2023002813
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-03-24
- Publication Date
- 2026-02-20
- Estimated Expiration
- 2040-11-05
AI Technical Summary
Existing processes for producing calcined clay to replace clinker in cement production are energy-inefficient and costly due to the high temperature reduction of iron(III) oxides using liquid reducing agents, leading to a pinkish tint that requires significant diesel fuel injection, which impacts both cost and environmental sustainability.
A process involving a calcination step under stoichiometric conditions followed by a reduction step using a hydrogen-containing gas in a separate reduction system, reducing iron(III) oxides at lower temperatures to achieve a gray color, minimizing reagent use and energy consumption.
The process produces gray-colored calcined clay with reduced reagent and energy consumption, lowering production costs and environmental impact while maintaining desired mechanical properties for cement production.
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Abstract
Description
Title of the invention: Process for producing calcined clay technical field
[0001] The invention relates to a process for producing calcined clay. In particular, the invention relates to a process for producing calcined clay intended for use as a component of cement. Technical background
[0002] Cement manufacturing uses for the most part a baked material, clinker, which is produced from minerals whose essential constituent is calcium carbonate.
[0003] 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, crushed, preheated, decarbonated, then baked and partially melted in a rotary kiln to a temperature of approximately 1500°C, and the clinker thus formed is then cooled.
[0004] Cement is obtained by finely grinding a mixture composed mainly of clinker.
[0005] In the clinker manufacturing process, in addition to the CO2 emissions from the combustion of the fuels used, the calcium carbonate, primarily from limestone, is decarbonated to obtain lime that can recombine 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.
[0006] National legislation regarding carbon dioxide emissions is becoming stricter and is forcing actors to reduce the quantities released.
[0007] 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 production.
[0008] By way of example, the most commonly used substitute materials for clinker at present are limestone, blast furnace slag, fly ash from coal-fired power plants and natural pozzolana.
[0009] Apart from limestone, which has a simple role as a "filler," an anglicism commonly used to designate fillers, these clinker substitute materials have a re pozzolanic activity allows 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.
[0010] Clays containing kaolinite acquire pozzolanic reactivity when calcined and thus become excellent substitute materials for clinker in cement manufacturing. They are also called "artificial pozzolans".
[0011] Unlike clinker production, calcined clay production emits little CO2.
[0012] By judiciously decreasing the proportion of clinker while increasing the proportion of calcined clay, it becomes possible to produce a cement with desired properties.
[0013] Due to the iron(III) oxides, Fe2O3, it contains, the clay has a reddish tint. Without treatment, the addition of calcined clay to cement will result in the production of a cement with a pinkish tint.
[0014] Cement producers, and end users, want to have a grey-coloured cement.
[0015] Several processes have been implemented to modify the natural color of the calcined clay and make it greyish.
[0016] Among these techniques, one can cite a process that consists of chemically reacting iron(III) oxide molecules (Fe2O3) to obtain trifer tetroxides (Fe3O4). This is a redox reaction. After an initial calcination step of the clay, it is sent to a reduction zone where a liquid reducing agent, in particular diesel fuel, is injected directly onto the clay. The diesel fuel creates the conditions that allow the reduction of the iron(III) oxides to obtain trifer tetroxides and iron(III) oxides (FeO).
[0017] Although this process makes it possible to obtain a grey-colored clay, it has two major drawbacks: - A significant quantity of diesel fuel is injected for the reduction of iron(III) oxides, impacting both the cost of the clay and the environment. - the reduction reaction must be carried out at high temperature, impacting the energy efficiency of this process.
[0018] The invention aims to remedy this drawback. Summary of the invention
[0019] To this end, a process for producing calcined clay having desired color characteristics is proposed first, in which it comprises: - a calcination step of clay in a calciner, said calcination step being carried out under stoichiometric or oxidizing conditions, - a reduction step of iron(III) oxides present in the calcined clay in a reduction system separate from the calciner, this reduction step being carried out by injecting a reduction gas, containing hydrogen atoms and put in direct contact with the calcined clay.
[0020] This process advantageously allows for obtaining a gray-colored clay using a small amount of reagent, significantly less than the amount used with liquid reagents. Furthermore, this process increases energy efficiency because the reduction step is carried out at lower temperatures compared to a process using liquid reagents.
[0021] Various additional features may be provided alone or in combination:
[0022] - process in which; - the reducing gas contains gaseous hydrocarbon-type reactants, or - the reducing gas contains dihydrogen and carbon monoxide type reactants, or - the reducing gas contains a dihydrogen-type reagent, or - the reducing gas contains gaseous hydrocarbon type reagents, dihydrogen and carbon monoxide;
[0023] - the reduction step includes a cooling operation of the calcined clay taking place simultaneously with the reduction of iron(III) oxides;
[0024] - during the cooling operation in the reduction chamber, the temperature the temperature of the calcined clay is lowered below a threshold temperature to avoid the re-oxidation of iron II oxides and / or trifer tetroxides contained in the calcined clay, said threshold temperature being substantially between 300°C and 600°C;
[0025] - the quantity of reducing gas injected into the reduction step corresponds to a quantity approximately between 0.01 and 2 moles of reactants per mole of iron(III) oxide present in the calcined clay;
[0026] - the process includes an intermediate cooling step between the cal step calcination and reduction stage, this intermediate cooling stage allowing the calcined clay to be cooled to a reduction temperature between 300°C and 800°C;
[0027] - the reduction step is carried out at a reduction temperature between 300°C and 800°C;
[0028] - the reducing gas contains reactants of the gaseous hydrocarbon type CnHm made up of n carbon atoms and m hydrogen atoms;
[0029] - the quantity of reducing gas injected into the reduction step corresponds to a quantity approximately between 0.01 and 1 mole of reactants per mole of iron(III) oxide present in the calcined clay;
[0030] - the reducing gas is a dihydrogen type reactant;
[0031] - the quantity of reducing gas injected into the reduction step corresponds to a quantity approximately between 0.2 and 2 moles of reagent per mole of iron(III) oxides present in the calcined clay;
[0032] - the reduction gas contains a mixture of dihydrogen-type reactants and carbon monoxide;
[0033] - the quantity of reducing gas injected into the reduction step corresponds to a quantity approximately between 0.2 and 2 moles of reactants per mole of iron(III) oxide present in the calcined clay;
[0034] - the reducing gas contains a mixture of gaseous hydrocarbon type reactants C nHm, dihydrogen and carbon monoxide;
[0035] - the quantity of reducing gas injected into the reduction step corresponds to a quantity approximately between 0.05 mole and 2 moles of reactants per mole of iron(III) oxides present in the calcined clay;
[0036] - the clay contains kaolinite;
[0037] - the calcination temperature in the calcination step is less than 950°C;
[0038] - calcined clay is suitable for being mixed with clinker for the production of cement.
[0039] Secondly, a use of a process such as previously described is proposed to produce calcined clay for the production of cement. Brief description of the drawings
[0040] Other features, details and advantages will become apparent from reading the detailed description below and from analyzing the accompanying drawings, in which: Fig. 1
[0041] [Fig.1] [Fig.1] is a schematic representation of an installation according to the invention. Fig. 2
[0042] [Fig.2] [Fig.2] is a schematic representation of a method according to the invention. Detailed description of the invention
[0043] The drawings and description below contain, essentially, elements of a definite nature. They may therefore not only serve to better understand this disclosure, but also contribute to its definition, if necessary.
[0044] Figure 1 shows a unit 1 for the production of calcined clay.
[0045] Unit 1 comprises: - a crusher 2, - a preheater 3, - a filter 4, - a combustion chamber 5, - a calciner 6, - a reduction system 7, - a final cooler 8, - a first cooling system 9, - a second cooling system 10.
[0046] The flow path of the gases and clay in the production unit 1 shown in [Fig. 1] is first described. Air 11 and a fuel 12 are injected into the combustion chamber 5. A burner 13 ignites this mixture in the combustion chamber 5. Hot gases exit the combustion chamber 5 and are sent to the calciner 6, then to the preheater 3, and finally to the mill 2. These hot gases are discharged through the filter 4. Simultaneously, the clay moves from the mill 2 to the final cooler 8, passing respectively through the preheater 3, the calciner 6, and the reduction system 7. The air 16 used in the final cooler 8 is at least partially routed to the combustion chamber 5.
[0047] In what follows, a method 14 according to the invention will be described.
[0048] With reference to the drawings, the process 14 according to the invention comprises a first step El in which raw clay containing iron(III) oxides Fe2O3 is ground and dried in the mill 2.
[0049] In the drawings, solid lines represent the path of the clay and dotted lines indicate the path of the gas flows.
[0050] The crushed and dried clay is sent to the preheater 3, except for the finest fraction which is carried to the filter 4 by the flow of hot gases.
[0051] The process includes a second preheating step E2. In the preheater 3, the crushed and dried clay is preheated by hot gases from the calciner 6 to a temperature between 300°C and 800°C.
[0052] The preheated clay is then directed to the calcinerator 6. Thus, the process includes a third calcination step E3. In order to activate the pozzolanic properties of the calcined clay and make it usable in cement production, the calcination is carried out at a temperature below 950°C. Preferably, the calcination is carried out at a temperature between 700°C and 900°C. The calcination is carried out under stoichiometric or oxidizing conditions. In other words, the gas flow in the calcinerator 6 is generated in the combustion chamber 5 by the combustion of a mixture of an oxidizer and a fuel in stoichiometric proportions for complete combustion, or with an excess of oxidizer. for oxidizing combustion. In the embodiment shown in the drawings, the combustion chamber 5 is separate from the calciner 6, with air primarily serving as the oxidant. As previously mentioned, the hot gases produced in the combustion chamber 5 are sent to the calciner 6 to calcine the clay.
[0053] The calcined clay is directed to the reduction system 7. Thus, the process includes a fourth step E4 for reducing the iron(III) oxides contained in the calcined clay. The reduction is carried out by injecting a reduction gas 15 containing hydrogen atoms, which is brought into contact with the calcined clay. The reduction gas 15 is then sent to the combustion chamber 5.
[0054] The reduction gas 15 containing hydrogen atoms allows the reduction of iron(III) oxides according to the following formulas to obtain mainly trifer tetroxides and possibly iron(II) oxides FeO:
[0055] 3Fe2O3 + H2 <^2Fe3O4 + H2O
[0056] Fe3O4 + H2 → 3FeO + H2O
[0057] and according to the following formulas, carbon monoxide, when present, reacts with iron(III) oxides:
[0058] 3Fe2O3+CO ^2Fe3O4+CO2
[0059] Fe3O4 + CO ^>3FeO + CO2
[0060] Using a gas containing hydrogen atoms, rather than diesel fuel or any other liquid fuel, as a reducing agent significantly reduces the amount of reducing agent required in the reduction step E4. The resulting gray calcined clay is therefore produced at a significantly lower cost. Furthermore, the environmental impact of producing gray calcined clay is reduced.
[0061] The grey calcined clay is then directed to the final re-cooler 8, where the calcined clay is cooled with air. The process thus includes a fourth final cooling step E5.
[0062] Advantageously, the reducing gas 15 contains: - reagents of the gaseous hydrocarbon type with the generic formula CnHm, or - a mixture of reagents such as dihydrogen (H2) and carbon monoxide (CO), or - a reagent of the dihydrogen type H2, or - a mixture of reagents of the type gaseous hydrocarbons of generic formula CnHm, dihydrogen, and carbon monoxide.
[0063] It should be noted that these gases can be mixed with other gases, mainly nitrogen and / or carbon dioxide.
[0064] Advantageously, the fourth reduction step E4 includes a cooling operation 01 implementing the first cooling device 9. The cooling operation 01 takes place simultaneously with the reduction of iron(III) oxides. This allows the temperature of the calcined clay to be reduced during the fourth reduction step E4 in order to avoid the reoxidation of iron II oxides and / or trifer tetroxides after said fourth reduction step E4 and at the outlet of the reduction system 7.
[0065] Advantageously, the cooling operation 01 in the fourth reduction step E4 is carried out using a cooling fluid either without direct contact with the calcined clay or by direct contact with the calcined clay if the cooling fluid contains less than 10% dioxygen by volume, for example combustion gases taken from the outlet of the preheater 3 or the calciner 6. This makes it possible to avoid any significant reoxidation of iron II oxides and / or trifer tetroxides during and after said fourth reduction step E4.
[0066] Advantageously, during the cooling operation 01 in the reduction system 7, the temperature of the calcined clay is lowered below a threshold temperature between 300°C and 600°C, thus preventing the re-oxidation of iron II oxides and / or trifer tetroxides.
[0067] Advantageously, the quantity of reducing gas 15 injected in the fourth reduction step E4 corresponds to a quantity of approximately between 0.01 and 2 moles of reactants per mole of iron(III) oxides present in the calcined clay. By adhering to this dosage, a calcined clay with the desired grayish tints is obtained while minimizing the gas input, thereby reducing clay production costs and its environmental impact.
[0068] Advantageously, the process includes an intermediate cooling step Ei between the third calcination step E3 and the fourth reduction step E4, the intermediate cooling step Ei being implemented by means of the second cooling device 10. The intermediate cooling step Ei cools the calcined clay to a reduction temperature between 300°C and 800°C. This intermediate cooling step Ei is carried out before the calcined clay enters the reduction system 7. This intermediate cooling can be achieved by any suitable means, in particular by air. By cooling the calcined clay before the reduction system 7, it is possible to improve the energy efficiency of the production unit 1. The heat energy from the cooling is used for other applications, either in the same process or for another use.This is made possible by using a gas containing hydrogen atoms as a reducing agent, which allows the reduction of iron(III) oxides at lower temperatures than in a process using a liquid reducing agent, in particular.
[0069] Thus, it follows that the fourth reduction stage E4 is carried out at a reduction temperature between 300°C and 800°C. This reduction temperature makes it possible to improve the energy efficiency of production unit 1.
[0070] According to a first embodiment of the invention, the reduction gas 15 contains gaseous hydrocarbon-type reactants of the generic formula CnHm consisting of n carbon atoms and m hydrogen atoms. This gas has the advantage of being readily available and contains numerous hydrogen atoms, which makes it particularly useful.
[0071] The reduction gas 15 contains, for example, methane, propane, or butane. In this first embodiment, this reduction gas 15 undergoes partial dissociation reactions in the reduction system 7 in the presence of oxygen (O2), carbon dioxide (CO2), and water vapor (H2O) contained in the combustion gases. Indeed, the calcined material carries with it, between the particles, a small quantity of combustion gas. The dissociation reactions are as follows:
[0072] CnHm + f O H2 + n CO
[0073] CnHm + n CO2 H2 + 2“CO
[0074] cnHm + n H2O +n)H2 + n CO
[0075] In the first embodiment, the reduction gas 15 is therefore initially partially transformed into a mixture of dihydrogen and carbon monoxide.
[0076] The quantity of gaseous hydrocarbon-type reagents with the generic formula CnHm injected in the reduction step is substantially between 0.01 mole and 1 mole of reagents per mole of iron(III) oxides present in the calcined clay. The applicant has determined that this quantity makes it possible to obtain the desired grayish tints while minimizing this quantity in order to reduce the cost of producing the calcined clay and to reduce the environmental impact.
[0077] According to a second embodiment of the invention, the reduction gas 15 contains a dihydrogen-type reagent. This gas prevents the production of carbon oxides during the reduction step.
[0078] The amount of dihydrogen-type reagent injected in the reduction step is approximately between 0.2 moles and 2 moles of reagent per mole of iron(III) oxides present in the calcined clay. The applicant has determined that this amount makes it possible to obtain the desired grayish tints while minimizing this amount in order to reduce the production cost of the calcined clay and to reduce the environmental impact.
[0079] According to a third embodiment of the invention, the reduction gas 15 contains a mixture of dihydrogen and carbon monoxide type reagents. This gas has the advantage of being easy to synthesize in situ in a dedicated reactor, commonly called an "endogas generator".
[0080] The quantity of this mixture of dihydrogen and carbon monoxide type reagents The amount injected into the reduction step is approximately between 0.2 moles and 2 moles of reagents per mole of iron(III) oxides present in the calcined clay. The applicant determined that this quantity allows for obtaining the desired grayish tints while minimizing this amount in order to reduce the production cost of the calcined clay and minimize its environmental impact.
[0081] According to a fourth embodiment of the invention, the reduction gas 15 contains a mixture of reagents of the type gaseous hydrocarbons of the generic form CnH₂m, dihydrogen and carbon monoxide. This gas has the advantage of being easy to synthesize in-situ in a dedicated reactor, commonly called an "exogas generator".
[0082] The quantity of this mixture of gaseous hydrocarbons, dihydrogen, and carbon monoxide-type reagents injected in the reduction step is substantially between 0.05 moles and 2 moles of reagents per mole of iron(III) oxides present in the calcined clay. The applicant has determined that this quantity makes it possible to obtain the desired grayish tints while minimizing this quantity in order to reduce the production cost of the calcined clay and to reduce the environmental impact.
[0083] Advantageously, the raw clay used in this production process 14 includes kaolinite. This type of clay can be used as a partial substitute for clinker in cement production.
[0084] Thus the calcined clay obtained by means of the process described above can be mixed with clinker intended for the production of cement.
[0085] The invention also relates to the use of the process 14 described above for producing calcined clay for cement production. Such use makes it possible to produce a grayish-colored cement by reducing the proportion of clinker in the cement by replacing it with calcined clay. The cement thus obtained has a desired grayish tint and mechanical properties substantially identical to those of a cement without calcined clay.
Claims
Demands
1. A process (14) for producing calcined clay having desired color characteristics, characterized in that the process (14) comprises: - a step (E3) of calcining a clay in a calciner (6), said calcination step (E3) being carried out under stoichiometric or oxidizing conditions, - a step (E4) of reducing iron(III) oxides present in the calcined clay in a reduction system (7) separate from the calciner (6), this reduction step (E4) being carried out by injecting a reduction gas (15), containing hydrogen atoms and brought into direct contact with the calcined clay, said process being characterized in that the reduction gas (15) is dihydrogen, or contains either a mixture of dihydrogen and carbon monoxide, or a mixture of gaseous hydrocarbons CnHm, dihydrogen and carbon monoxide.
2. A process (14) according to claim 1 in which the reduction step (E4) comprises an operation (01) of cooling the calcined clay taking place simultaneously with the reduction of iron(III) oxides.
3. A method (14) according to claim 2 wherein, during the cooling operation (01) in the reduction chamber (7), the temperature of the calcined clay is lowered below a threshold temperature to avoid the reoxidation of iron(II) oxides and / or trifer tetroxides contained in the calcined clay, said threshold temperature being between 300°C and 600°C.
4. A process (14) according to any one of claims 1 to 3 wherein the quantity of reducing gas (15) injected in the reduction step (E4) corresponds to an amount between 0.01 and 2 moles of reactants per mole of iron(III) oxide present in the calcined clay.
5. A method (14) according to any one of the preceding claims, wherein it comprises an intermediate cooling step (Ei) between the calcination step (E3) and the reduction step (E4), this intermediate cooling step (Ei) allowing the calcined clay to be cooled to a reduction temperature between 300°C and 800°C.
6. A method (14) according to any one of the preceding claims, wherein the reduction step is carried out at a temperature of reduction between 300°C and 800°C.
7. A process (14) according to claim 1 wherein, when the reduction gas (15) is dihydrogen or contains a mixture of dihydrogen and carbon monoxide, the amount of reduction gas (15) injected in the reduction step (E4) corresponds to an amount between 0.2 and 2 moles of reagent per mole of iron(III) oxides present in the calcined clay.
8. A process (14) according to claim 1 wherein, when the reduction gas (15) contains a mixture of gaseous hydrocarbons CnHm, dihydrogen and carbon monoxide, the quantity of reduction gas (15) injected in the reduction step (E4) corresponds to an amount between 0.05 mole and 2 moles of reactants per mole of iron(III) oxides present in the calcined clay.
9. A process (14) according to any one of the preceding claims, wherein the clay contains kaolinite.
10. A method (14) according to any one of the preceding claims wherein the calcination temperature in the calcination step (E3) is less than 950°C.
11. Use of a process (14) according to any one of the preceding claims for producing calcined clay for the production of cement.